Development of sub-micro structured composites based on an epoxy ...

Order number: 2005-ISAL-00113

year: 2005

Thesis

DEVELOPMENT OF SUB-MICRO STRUCTURED COMPOSITES BASED ON AN EPOXY MATRIX AND PYROGENIC SILICA Mechanical behavior related to the interactions and morphology at multi-scale

Presented at the National Institute of Applied Sciences (INSA) of Lyon

To obtain the degree of

Doctor of Philosophy Ecole doctorale Matériaux de Lyon Specialty: Polymer and Composite Materials

By

Elodie BUGNICOURT Engineer in Materials of INSA of Lyon

Defense on December 19th, 2005 before the examination commission

Jury: Giovanna Costa

Senior Scientist, ISMAC- CNR, Genova, Italy

Referee

Roland Séguéla

Senior Scientist, Université de Lille

Referee

Herbert Barthel

Doctor, Wacker Chemie, Germany

Jocelyne Galy

Doctor, IMP UMR CNRS 5627, INSA Lyon

PhD supervisor

Jean-François Gérard

Professor, IMP UMR CNRS 5627, INSA Lyon

PhD supervisor

François Boué

Doctor, LLB, CEA Saclay

Invited member

Etienne Fleury

Professor, IMP UMR CNRS 5627, INSA Lyon

Chairman

Elodie Bugnicourt, PhD INSA Lyon, 2005

2

2005 SIGLE

ECOLE DOCTORALE

NOM ET COORDONNEES DU RESPONSABLE

CHIMIE DE LYON M. Denis SINOU

M. Denis SINOU Université Claude Bernard Lyon 1 Lab Synthèse Asymétrique UMR UCB/CNRS 5622 Bât 308, 2ème étage 43 bd du 11 novembre 1918 69622 VILLEURBANNE Cedex Tél : 04.72.44.81.83 Fax : 04 78 89 89 14 [email protected]

E2MC

ECONOMIE, ESPACE ET MODELISATION DES COMPORTEMENTS M. Alain BONNAFOUS

M. Alain BONNAFOUS Université Lyon 2 14 avenue Berthelot MRASH M. Alain BONNAFOUS Laboratoire d’Economie des Transports 69363 LYON Cedex 07 Tél : 04.78.69.72.76 [email protected]

E.E.A.

ELECTRONIQUE, ELECTROTECHNIQUE, AUTOMATIQUE M. Daniel BARBIER

M. Daniel BARBIER INSA DE LYON Laboratoire Physique de la Matière Bâtiment Blaise Pascal 69621 VILLEURBANNE Cedex Tél : 04.72.43.64.43 Fax 04 72 43 60 82 [email protected]

E2M2

EVOLUTION, ECOSYSTEME, MICROBIOLOGIE, MODELISATION http://biomserv.univ-lyon1.fr/E2M2 M. Jean-Pierre FLANDROIS

M. Jean-Pierre FLANDROIS UMR 5558 Biométrie et Biologie Evolutive Equipe Dynamique des Populations Bactériennes Faculté de Médecine Lyon-Sud Laboratoire de Bactériologie BP 1269600 OULLINS Tél : 04.78.86.31.50 Fax 04 72 43 13 88 [email protected]

EDIIS

INFORMATIQUE ET INFORMATION POUR LA SOCIETE http://www.insa-lyon.fr/ediis M. Lionel BRUNIE

M. Lionel BRUNIE INSA DE LYON EDIIS Bâtiment Blaise Pascal 69621 VILLEURBANNE Cedex Tél : 04.72.43.60.55 Fax 04 72 43 60 71 [email protected]

EDISS

INTERDISCIPLINAIRE SCIENCES-SANTE http://www.ibcp.fr/ediss M. Alain Jean COZZONE

M. Alain Jean COZZONE IBCP (UCBL1) 7 passage du Vercors 69367 LYON Cedex 07 Tél : 04.72.72.26.75 Fax : 04 72 72 26 01 [email protected]

MATERIAUX DE LYON http://www.ec-lyon.fr/sites/edml M. Jacques JOSEPH

M. Jacques JOSEPH Ecole Centrale de Lyon Bât F7 Lab. Sciences et Techniques des Matériaux et des Surfaces 36 Avenue Guy de Collongue BP 163 69131 ECULLY Cedex Tél : 04.72.18.62.51 Fax 04 72 18 60 90 [email protected]

Math IF

MATHEMATIQUES ET INFORMATIQUE FONDAMENTALE http://www.ens-lyon.fr/MathIS M. Franck WAGNER

M. Franck WAGNER Université Claude Bernard Lyon1 Institut Girard Desargues UMR 5028 MATHEMATIQUES Bâtiment Doyen Jean Braconnier Bureau 101 Bis, 1er étage 69622 VILLEURBANNE Cedex Tél : 04.72.43.27.86 Fax : 04 72 43 16 87 [email protected]

MEGA

MECANIQUE, ENERGETIQUE, GENIE CIVIL, ACOUSTIQUE http://www.lmfa.eclyon.fr/autres/MEGA/index.html M. François SIDOROFF

M. François SIDOROFF Ecole Centrale de Lyon Lab. Tribologie et Dynamique des Systêmes Bât G8 36 avenue Guy de Collongue BP 163 69131 ECULLY Cedex Tél :04.72.18.62.14 Fax : 04 72 18 65 37 [email protected]


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PREAMBLE This work was supported by the German company Wacker Chemie GmbH based in Burghausen. It was carried out in the Macromolecular Materials Laboratory (LMM / IMP, UMR CNRS #5726) at INSA of Lyon. The PhD was promoted and supervised by Dr Herbert Barthel from Wacker Co.; Dr. Jocelyne Galy and Prof. Jean-François Gérard from LMM. The interdisciplinarity and complementarily of the people involved made possible this project based on the study of materials at the boundary between organic and inorganic science, taking benefits of the experience of LMM in the field of thermoset resins and of the expertise of Wacker in the field of silica and surface modification.

ACKNOWLEGEMENTS First of all, I thank gratefully Giovanna Costa and Roland Séguéla who accepted to refer the work presented in this manuscript.

Then, I would like to thanks a great number of people who were involved in this work.

My first acknowledgements are for the 3 supervisors of this work: Herbert Barthel, JeanFrançois Gérard and Jocelyne Galy not only for their indispensable guidance but also for letting me a good balance of autonomy in my research, for the fruitful scientific discussions, and experimental advices. Thanks as well for giving me the opportunity of participating to various congresses and taking part to many external collaborations that resulted in a really enriching experience for me.

I’d like express my thanks to Wacker Co., not only for its financial support but also for its efficient scientific and technical assistance, kind attention and nice welcoming in Burghausen… I acknowledge all the people who attended to the meetings and made relevant comment about my work, especially Herbert Barthel, Torsten Gottschalk-Gaudig, Michael Dreyer, Rita Kelermann who I really enjoyed working with.

Then, I want to thank all the people who contributed in the realization of the experiments presented throughout this manuscript thanks to several cooperations with other laboratories in France and abroad:


4

−

The people from the Laboratoire Léon Brillouin, LLB CEA Saclay, who helped me in the field of neutron scattering, especially François Boué for the time spent in experiments and background discussions, and Vincent Thévenot for helping me in the technical problems.

−

Bruno Alonso and Dominique Massiot from the Centre de Recherche sur les Matériaux à Haute Temperature (CRMHT) for welcoming me in Orléans and realizing the high resolution NMR experiments, and analyzing the data with me.

−

Alberto Fina and Giovanni Camino from the Centro di Cultura per l'Ingegneria delle Materie Plastiche di Politecnico Torino, Sede di Alessandria, for assisting me in the characterization of the fire resistance by cone calorimetry. Special thanks to Alberto Fina for and great availability and for my pleasant stay in Italy.

−

Jean-Marie Letoffé and Catherine Sigala, form the laboratory Multimatériaux et Interfaces (LMI), UMR CNRS 5615 of UCB Lyon 1, for providing GCMS and high resolution TGA facilities, and useful advices for the analysis of the data obtained.

−

Stéphane Rouquette, responsible of Malvern Co. France in Orsay for giving me the possibility of using a new optical technique to characterize the suspensions microstructure.

−

Jean-Claude Bernengo, Yves Tourneur and Batoule Smatti, from the Centre Commun de Quantimétrie of Université Lyon 1, for assisting me in the image analysis and in the observation using confocal microscopy.

−

Anne Baudouin, from CPE, for carrying out the solid state carbon MNR experiments.

−

Jean Pascal Philibert for giving me a hand for the first dispersions that I realized at ITECH.

I also gratefully acknowledge all my colleagues at the LMM, PhD students, permanent researchers, technical and administrative staff, for pleasant time spent together as well as for experimental support, especially: −

Pierre Alcouffe for realizing the TEM observations for me.

−

Nathalie Issartel and Hervé Perrier Camby for their indispensable help in practical maters, and their availability

−

Robert Di Folco for taking time for adjusting different devices for me.

Furthermore, I have a special thought for the students whose training periods were associated to this PhD: Francine Lagarrigue, Paco Isoardi, Gilles Horvath and Rodolphe Lafargue.

I hope not to forget anyone who helped me directly to carry through this PhD.

Last but not least, I thank all the people who contributed indirectly in this PhD: those who crossed my way during these 3 years and offered me a bit of sunshine and those who don’t need a written clue here to know I really need their support and presence around me…


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CONTENTS1

GENERAL INTRODUCTION

9

I.

MATERIALS BACKGROUND, LITERATURE AND METHODOLOGY

13

I.1.

Pyrogenic silica

15

I.1.1. The different types of synthetic silica

16

I.1.2. Synthesis and multi-scale organization of pyrogenic silica

18

I.1.3. Chemical structure and surface properties

20

I.1.4. Surface modification

23

Generalities about epoxy reinforcement and sub-micro structured composites

25

I.2.1. Reinforcement of epoxy

25

I.2.2. Specific effect of nano-fillers

29

I.2.

I.3.

I.4.

Processing of sub-micro structured composites, filler effect on the rheology and reactivity of epoxy systems

33

I.3.1. Processing of sub-micro structured composites

33

I.3.2. Rheological behavior related to the dispersion state

38

I.3.3. Effect of fillers on the crosslinking kinetics of epoxy networks

46

Expected mechanical properties of epoxy / silica composites

49

I.4.1. Interactions between epoxy and silica, silica influence on the dynamics of polymer matrix

I.5.

49

I.4.2. Mechanical properties of epoxy / silica composites in the glassy state

52

I.4.3. Mechanical properties of epoxy / silica composites in the rubbery state

59

I.4.4. Applicative properties of epoxy / silica formulations

63

Methodology of the study

69

I.5.1. Summary of the state of the art and position of the material under study 69 I.5.2. Organization of the study

II.

II.1.

71

PROCESSING OF EPOXY / SILICA COMPOSITES, CONTROL OF THE INTERACTIONS AND MORPHOLOGIES DURING POLYMERIZATION

75

Formulation

77

II.1.1. Epoxy-amine matrices

78

II.1.2. Specifications of the silica used

81

II.1.3. Nomenclature

84

1

Furthermore detailed tables of content at the beginning of each chapter and annex


6

II.2.

II.3.

II.4.

Processing and characterization of silica suspensions

84

II.2.1. Optimization of the dispersion step

85

II.2.2. Characterization of the dispersion state

86

Characterization of the crosslinking step

96

II.3.1. Preparation and crosslinking of the epoxy / silica composites

96

II.3.2. Kinetics of epoxy-amine reactions in presence of silica

97

II.3.3. Evolution of the morphologies during the polymerization

105

Study of the interactions generated at the silica / epoxy interface

111

II.4.1. General approach for any silica surface chemistry

111

II.4.2. Investigation of the covalent bonding at the amino-modified silica / matrix interface

III.

III.1.

III.2.

III.3.

114

SOLID STATE PROPERTIES OF EPOXY / SILICA COMPOSITES: STUDY OF THE MECHANICAL BEHAVIOR DEPENDING ON THE MORPHOLOGY

125

Morphological study of epoxy / silica composites

127

III.1.1. Foreword on the transparency

128

III.1.2. Morphological study by transmission electron microscopy

128

III.1.3. Image Analysis of TEM micrographs

136

III.1.4. Structural study by small angle neutron scattering

145

Mechanical behavior of silica / epoxy composites

158

III.2.1. Mechanical properties of MDEA-based systems

158

III.2.2. Mechanical properties of Jeffamine-based systems

165

Relevant parameters for mechanical properties of epoxy / silica composites and relationships with the structure

173

III.3.1. Influence of silica content

173

III.3.2. Influence of the interfacial surface area and adhesion developed between silica and epoxy matrix

180

III.3.3. Discussion on toughening improvement in the glassy state

184

III.3.4. Discussion on the optimal morphology for the reinforcement

185

GENERAL CONCLUSIONS AND PERSPECTIVES


187

7

APENDIX

ANNEX A.

191

THEORETICAL BACKGROUND ON THE TECHNIQUES EXPERIMENTAL CONDITIONS

191

A.1.

Processing and characterization of the silica suspensions

193

A.2.


202

A.3.

Characterization of the interfacial interactions

204

A.4.

Morphological characterization

208

A.5.

Mechanical characterization

220

A.6.

Characterization of the thermal and combustion behavior

223

ANNEX B.

ADDITIONAL RESULTS FOR THE SECOND CHAPTER

225

B.1.

Complements on the materials

226

B.2.

Additional results for the characterization of the rheological behavior of the silica suspensions

B.3.

Complements on the influence of silica on the kinetics of epoxy-amine reactions

B.4.

229

232

Additional results for the study of the morphological evolution during crosslinking by SANS

238

Additional results for the study of the interfacial interactions

241

ADDITIONAL RESULTS FOR THE THIRD CHAPTER

249

C.1.

Morphological study of crosslinked systems

250

C.2.

Additional results of DMA

257

C.3.

Additional results of tensile test for Jeffamine-based systems

258

B.5.

ANNEX C.

ANNEX D.

THERMAL AND COMBUSTION BEHAVIOR OF EPOXY / SILICA COMPOSITES

259

D.1.

Residual internal thermal stresses

260

D.2.

Calorimetric measurements

262

D.3.

Thermal stability

266

D.4.

Fire retardancy

269

D.5.

Conclusion on epoxy / silica composites thermal properties

273

EXTENDED ABSTRACT IN FRENCH

275

BIBLIOGRAPHICAL REFERENCES

295


8

General introduction

GENERAL INTRODUCTION

Foreword on sub-micro structured materials

For the last few years, sub micro-structured materials have been the object of many scientific and technical studies, due to the large impact on a wide range of properties even at low filler content (mechanical, barrier, thermal properties …). Okada et al.’s work for Toyota about the introduction of montmorillonites into PA6, traditionally regarded as the initiator of the rise of socalled “nanocomposites”, was realized in the early 90’s. This class of materials does not belong anymore to the world of scientific model objects, they are applied in our everyday life in fields such as packaging, automotive… However, sub micro-structured materials are far from being a new topic. Indeed, the reinforcement of polymers by addition of fillers, exhibiting at least one dimension in the nanoscale (such as carbon back, precipitated or fumed silica for rubber reinforcement), has been a common practice for decades. The use of the term nanocomposites, following the current trends, in order to refer to these materials is new and can appear a bit misused… The concept “the smallest, the best”, at first widespread in the scientific opinion, which led to a kind of “nano-fashion”, is far from being checked each time [AVN05] and may induce some confusions. Often, the initial promises of nano-structured materials were not fully kept and they did not result valuable, at least in an industrial context, compared to the processing difficulties. For particles in the scale of 100 to 1,000 nm, that can be called sub-micronic fillers, there might be an optimal size in between these two limits to obtain the best results depending on the property targeted. Maybe, it would be proper in order to describe the behavior of the filled systems, to distinguish fillers not only depending on their size but also depending on their specific surface area, which is the first order parameter inducing their specific behavior compared with micro-fillers. Indeed, the incomparable role played by the interface make them “active” fillers. Besides, the security and safety issue tends to be more and more discussed about every new nano-technology developed, generating a phobia that could repel industry which has to face tough standards. It is thus worthwhile pointing out that pyrogenic silica is a well-known and controlled filler which is not toxic for human being (contrary to crystalline silica).


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Context and objective of the study

Even if the largest number of works still deals with layered fillers, the natures of inorganic particles used in the field of organic-inorganic sub-micro structured composites tend to diversify, with for example the use of colloidal silica. There is still a large magnitude of improvement possible in this field, especially in the scope of a possible industrialization in the near future.

It is well-known that epoxy networks present insufficient mechanical properties for certain applications, particularly due to their brittleness. This work was aimed at developing pyrogenic silica / epoxy composites in order to achieve a mechanical reinforcement of epoxy networks for the conventional arrays of applications of epoxy such as coatings, adhesives or composites. Filler addition also generally results in the enhancement of additional properties of epoxy matrices such as thermal and dimensional stability, fire resistance, surface hardness... The filler used in this work, i.e. pyrogenic silica, presents a multi-scale organization (fractal aggregates of a few hundreds nanometers formed of primary particle of tenths nanometers diameter). It has been extensively used in order to reinforce rubbers, as well as for rheological modifications, but only few studies deal with the use of fumed silica into thermosetting polymers.

In this work, the natures of epoxy and amine comonomers generating the matrix, as well as that of silica surface modification, were varied in order to design the interactions developed at the filler/ matrix interface (physical interactions vs. covalent bonding). An originality of this study is that, from changing the formulation of the network, the influence of silica could be investigated in both rubbery and glassy states at room temperature. The relationships between the morphology, interactions, and mechanical behavior of the materials developed were investigated in order to understand the reinforcement mechanisms involved. One of the main challenge in this work consisted in controlling silica dispersion state, therefore the different steps of the processing were carefully optimized. The morphology of the composites was characterized at multi-scale at various stages of the preparation and a particular attention was paid to the understanding of the interactions effectively developed at the interface between the filler and the matrix since both dispersion and adhesion of the filler were expected to be key parameters for the resulting properties.


10


The part I of this manuscript is devoted to the presentation of the background needed for this study, i.e.: i) the materials used are described, especially fumed silica, ii) the state of the art in the field of sub-micro structured epoxy / silica composites is reviewed with a particular focus on the mechanical behavior, and iii) the methodology followed in this work is explained in the basis of the literature study.

The experimental results obtained are then reported and discussed. The part II is focused on all the steps before and during the crosslinking of the epoxy / silica composites: i) the processing of the materials and monitoring of the rheological behavior related to the dispersion state, ii) the study of the crosslinking stage i.e. the investigation of the influence of silica on the reactivity of epoxy-amine systems, and of the evolution of the morphology during the network formation. A section of this second part deals with the study of the interactions generated at the interface between the silica and the matrix, depending on silica surface chemistry.

The part III is dedicated to the characterization of the properties of the final crosslinked epoxy / silica composites, especially the morphologies, the solid state mechanical behavior in the elastic and fracture regions, as well as the dynamic mechanical behavior. The relationships between the structure and properties are finally discussed and the leading parameters governing these properties are highlighted.

Besides, the reader is warned that, in order not to make the reading of the manuscript too “heavy” due to the large number of systems studied and of techniques carried out, the annexes of the manuscript include a fair part of the results. Complements on the theoretical background of the techniques used and experimental conditions are reported in annex A. Additional results, especially for other surface modifications of silica considered, not discussed in the body of the report, can be found in annex B for the steps before and during crosslinking (complements for chapter 2) and in annex C for the characterization in the solid state (complements for chapter 3). Furthermore, whole results and discussion concerning thermal properties and fire resistance of epoxy / silica composites are reported in annex D.


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12

Chapter I: Materials background, literature and methodology

I.

MATERIALS BACKGROUND, LITERATURE AND METHODOLOGY ...................15 I.1.

Pyrogenic silica ............................................................................................................ 15

I.1.1.

The different types of synthetic silica...................................................................... 16

I.1.1.a.

Silica sol.......................................................................................................... 16

I.1.1.b.

Precipitated silica ............................................................................................ 17

I.1.1.c.

Pyrogenic silica ............................................................................................... 17

I.1.1.d.

Silica obtained via other thermal processes ................................................... 17

I.1.1.e.

Comparison of physical properties.................................................................. 18

I.1.2.

Synthesis and multi-scale organization of pyrogenic silica..................................... 18

I.1.3.

Chemical structure and surface properties ............................................................. 20

I.1.3.a.

Specific surface area ...................................................................................... 20

I.1.3.b.

Surface chemistry ........................................................................................... 21

I.1.3.c.

Interaction with water ...................................................................................... 22

I.1.3.d.

Summary of properties.................................................................................... 23

I.1.4.

I.2.

I.1.4.a.

Principle .......................................................................................................... 24

I.1.4.b.

Grafting in solution .......................................................................................... 24

I.1.4.c.

Grafting in vapor phase................................................................................... 25

Generalities about epoxy reinforcement and sub-micro structured composites .......... 25

I.2.1.

Reinforcement of epoxy.......................................................................................... 25

I.2.1.a.

Fillers traditionally used to reinforce epoxy..................................................... 26

I.2.1.b.

Nano-fillers: the answer to reach a toughness / stiffness compromise?......... 28

I.2.2.

I.3.

Surface modification ............................................................................................... 23

Specific effect of nano-fillers................................................................................... 29

I.2.2.a.

Size effect, high specific surface area ............................................................ 29

I.2.2.b.

Ability for interactions / reactions of the nano-fillers with the medium ............ 30

I.2.2.c.

Multi-scale organization .................................................................................. 31

I.2.2.d.

Geometric characteristics ............................................................................... 32

Processing of sub-micro structured composites, filler effect on the rheology and

reactivity of epoxy systems..................................................................................................... 33 I.3.1.

Processing of sub-micro structured composites ..................................................... 33

I.3.1.a.

General routes for the processing of sub-micro structured composites.......... 34

I.3.1.b.

Processing of formulations based on epoxy and silica ................................... 36

I.3.2.

Rheological behavior related to the dispersion state .............................................. 38

I.3.2.a.

Rheological behavior of fumed silica suspensions ......................................... 38


13


I.3.2.b.

Effect of particle size and shape ..................................................................... 40

I.3.2.c.

Effect of the surface chemistry and medium nature........................................ 42

I.3.2.d.

Effect of the silica content ............................................................................... 43

I.3.3.

I.4.

Effect of fillers on the crosslinking kinetics of epoxy networks ............................... 46

I.3.3.a.

Filler effect on epoxy crosslinking via condensation mechanism.................... 46

I.3.3.b.

Filler effect on epoxy crosslinking initiated via ionic mechanism .................... 47

Expected mechanical properties of epoxy / silica composites ..................................... 49

I.4.1.

Interactions between epoxy and silica, silica influence on the dynamics of polymer

matrix ................................................................................................................................ 49 I.4.1.a.

Interactions of fumed silica with epoxy ........................................................... 49

I.4.1.b.

Effect on the glass transition temperature ...................................................... 50

I.4.2.

I.4.2.a.

Effect of conventional silica fillers (micron size).............................................. 53

I.4.2.b.

Effect of sub-micro silica particles................................................................... 56

I.4.3.

Mechanical properties of epoxy / silica composites in the rubbery state ................ 59

I.4.3.a.

General effect of fillers on a rubbery network ................................................. 59

I.4.3.b.

Effect of silica surface modification................................................................. 61

I.4.3.c.

A few examples of filler addition in rubbery epoxy networks .......................... 62

I.4.4.

I.5.

Mechanical properties of epoxy / silica composites in the glassy state .................. 52

Applicative properties of epoxy / silica formulations ............................................... 63

I.4.4.a.

For adhesives ................................................................................................. 64

I.4.4.b.

For coatings, surface properties ..................................................................... 65

I.4.4.c.

For matrices of conventional composite materials.......................................... 67

Methodology of the study............................................................................................. 69

I.5.1.

Summary of the state of the art and position of the material under study .............. 69

I.5.2.

Organization of the study........................................................................................ 71


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I. MATERIALS

BACKGROUND,

LITERATURE

AND

METHODOLOGY

This chapter is aimed at presenting the background of the materials used, the state of the art about epoxy / silica composites, as well as the methodology of the study carried out. In a first part of this chapter, the characteristics of different types of synthetic silica, especially pyrogenic silica, are presented. In a second part of this chapter, general background about the reinforcement of epoxy and the factors related to sub-micro fillers efficiency are reported. Then, the processing routes for sub-micro structured composites, and the general behavior of filled epoxy before and during crosslinking are presented for filled materials. Then, the expected final mechanical properties of epoxy / silica composites are reviewed in glassy and rubbery states, as well as the applicative properties depending on the fields targeted. In a last part of this chapter, the methodology of this study is detailed, i.e. the different steps leading to the obtaining of the epoxy / silica composites, the relevant parameters and the questions arising in this study.

I.1. Pyrogenic silica

The filler used in this study is a pyrogenic (or fumed) silica commercialized by Wacker company. First, different types of synthetic silica are introduced briefly because their effects are compared with those of fumed silica for the reinforcement of epoxy from literature results. The synthesis process leading to the specific multi-scale organization of fumed silica, at different scales, is then presented. The surface properties of pyrogenic silica determining the interactions developed with the organic medium are then detailed, as well as the possibilities of chemical modification of the surface.


15


I.1.1. The different types of synthetic silica Silicon is one of the most abundant components on the Earth surface, especially in the form of quartz. Silica, or silicon dioxide, can also be synthesized industrially. It can be crystalline (generally for natural origins) or amorphous (generally for synthetic origins). There are various synthesis routes for the manufacturing of synthetic amorphous silica (Figure I-1): −

wet route: sol/gel process, precipitation

−

thermal route: pyro-hydrogenation, arc, plasma

Figure I-1

I.1.1.a.

Different categories of synthetic silica

Silica sol

The sol-gel process is one of the most extensively described in the literature. It is based on the condensation of silanol groups to form a siloxane network according to the reaction:

Si

OH

+

HO

Si

Si

O

Si

+

H2O

Silicate of sodium or alcoxysilanes can be used as raw materials to get silanols species by hydrolysis. The hydrolysis and the condensation take place simultaneously in an aqueous solution, forming stable colloidal particles. The condensation reaction is influenced by the addition of an electrolyte or by changing the pH of the solution. The growth of particles or the bonding of particles leading to the formation of a network can be respectively favored depending on the conditions chosen. After the sol / gel transition, an elastic behavior appears, and a hydrogel is obtained in case the solvent is water, or respectively an alcogel in alcohol. When dried, a hydrogel provides a xerogel and an alcogel provides an aerogel. The porosity can be tailored and a subsidiary thermal treatment can be necessary to stabilize the material.


16


The advantage of this process is to lead to pure and homogeneous silica particles under low temperatures. Silica obtained by sol-gel process, so-called silica sol (or also sometimes silica gel), is generally used for the manufacture of films, fibers, powders, composite or porous materials [PAQ03].

I.1.1.b.

Precipitated silica

The precipitated silica was developed in the 1940’s as white reinforcing filler for rubbers, which is still its main application field [CON05]. It is obtained by acidification of a solution of silicate of sodium in the presence of sulphuric acid or of a mixture of carbon dioxide and hydrochloric acid according to the reaction: Na2OXSiO2 + H2SO4 → XSiO2 + Na2SO4 + H2O. Rather individual silica particles are obtained because the gelation is avoided during the process.

I.1.1.c.

Pyrogenic silica

The pyrogenic (or fumed) silica was prepared for the first time by the German chemist Klopfer, in 1941, with the same objective as for the precipitation process. The processing is detailed in the paragraph I.1.2. Pyrogenic silica consists of a nonporous, amorphous, fine, fluffy white powder and is totally amorphous. The main markets of fumed silica are: reinforcing filler mainly for silicone rubbers (application accounting for about 55% of the market of fumed silica), matting agent for paints and polymers, free flow agent for powders, rheological additive providing thickening or thixotropy to polymers, adhesives, paints, inks... The main suppliers of fumed silica world wide are Wacker (HDK®), Degussa (Aerosil®) and Cabot (Cabosil®), with an annual world market around 150,000 tones (2004). Note: All our experimental work was focused on this type of silica, hence the author will use the word “silica” for pyrogenic silica in the continuation of the text (unless otherwise specified).

I.1.1.d.

Silica obtained via other thermal processes

Silica can also be synthesized in an electric arc: from the reduction of quartz at 2,000°C in the presence of coke, silicon monoxyde is created and then oxidized by air to form silica. This


17


process is not widespread because of its high cost and of the poor properties of this silica compared with fumed silica. Finally, the plasma process is also rarely employed.

I.1.1.e.

Comparison of physical properties

The physical properties of the different types of synthetic silica such as the porosity, size and organization of the particles depend largely on their synthesis process. They are summarized in the Table I-1 and are detailed further in case of fumed silica in the following paragraph. Thermal processes

Table I-1

Wet processes

Physical properties of different synthetic silica depending on their process of synthesis [KAT87]

I.1.2. Synthesis and multi-scale organization of pyrogenic silica Pyrogenic silica is obtained via a high temperature process (Figure I-2). The hydrolysis of silicon tetrachloride in a flame of mixed hydrogen and oxygen, between 1,200°C and 1,500°C approximately, leads to the formation of silica and hydrochloric acid according to the reaction: SiCl4 + 2 H2+ O2 → SiO2 + 4 HCl A part of the hydrochloric acid formed is evaporated but the excess remains physically absorbed on silica surface explaining its slightly acid character (3.5G” δ low 10

10 1E-3

δ (°)

εmax 1E-2

0.1

Sol: G”>G’ δ ~90° 1

20 10 10

0

ε Figure II-13

Illustration of the parameters analyzed in a strain sweep experiment: ○ G’, ○G”, ○δ= f(ε)

These dynamic behaviors were measured for hydrophilic (Figure II-14) and hydrophobic (Figure II-15) silica at various contents into epoxy. Up to 10 wt. % of hydrophilic silica, a liquid behavior was observed in all the strain range tested, whereas, a sol-gel transition was observed from 5 wt. % of hydrophobic silica. Additionally, εmax increased as the silica content increased, i.e. the yield stress to apply to break the gel was greater as the silica content increased.. 10 N-D330 15 N-D330 1.000E6

1.000E6

1.000E5

1.000E5

10 N-D330 15 N-D330 90.00

10000

1000

1000

100.0

100.0

delta (degrees)

10000

70.00

G'' (Pa)

G' (Pa)

80.00

60.00 50.00 40.00 30.00 20.00 10.00

10.00 1.0000E-3

0.010000

0.10000

10.00 1.0000

0 1.0000E-3

0.010000

strain

(a)

0.10000

1.0000

strain

(b)

Figure II-14 Strain sweep experiments at 20°C for hydrophilic silica suspensions into epoxy with various contents (a) dynamic modulus 10N-D330: • G”, ○ G’; 15N-D330: • G”, ○ G’. (b) loss angle □ 10N-D330, □ 15N-D330.


93

Chapter II: Processing of the epoxy / silica composites, control of the interactions and morphologies during polymerization

10H-D330 5 H-D330 2.5H-D330

10H-D330 5 H-D330 2.5H-D330 1.000E5

90.00

1.000E5

80.00 70.00

1000

1000

100.0

100.0

10.00

10.00

delta (degrees)

10000

G'' (Pa)

G' (Pa)

10000

60.00 50.00 40.00 30.00 20.00 10.00

1.000 1.0000E-3

0.10000 strain

1.000 10.000

(a)

0 1.0000E-3

0.010000

0.10000 strain

1.0000

10.000

(b)

Figure II-15 Strain sweep experiments at 20°C for fully hydrophobic silica suspensions into epoxy with various contents (a) dynamic modulus 2.5H-D330: • G”, ○ G’; 5H-D330: • G”, ○ G’, 10H-D330: • G”, ○ G’. (b) loss angle □ 2.5H-D330, □ 5H-D330, □ 1OH-D330.

The variation of the storage modulus at low strain (G’0) as a function of the silica content is plotted in the Figure II-16 for hydrophilic and hydrophobic silica suspensions into epoxy. The stiffness of the gel first increases linearly as a function of the silica content then a threshold around 2.5 wt. % of hydrophobic silica and 7 vol. % of hydrophilic silica is reached. A fitting to the power law behavior described by the equation I-2 provides an exponent n=2 for hydrophilic silica suspensions before threshold, whereas a fitting to the equation I-3 with a threshold of 2.5 wt. % (~1.3 vol. %) provides a value of exponent n= 2.8 for hydrophobic silica suspensions after the threshold. Values are lower than those obtained by Khan et al. [KHA93], but the trends are the same depending on the type of interactions developed in the systems. Note that taking into account the fractal morphology of these silica, the theoretical percolation threshold is expected around 7.4 vol. %, in agreement with the rheological value for hydrophilic silica, calculated as follow [DOR95, PAQ05]: 3 - Dm

Equation II-3 ⎛R ⎞ φ* = ⎜ ⎟ ⎝a⎠ where a is the radius of the primary particles forming the fractal aggregate included in a sphere

of radius R, R>a (cf. III.1.3.b) with the values determined from small angle neutron scattering (SANS results in part III.1.4.): a ~ 5 nm, D ~ 2.25 and R ~ 160 nm (from dynamic light scattering).


94


1000000 100000

G'0 / G'0(D330)

H 10000 N 1000 100 10 1 0

2

4 6 φv (silica vol. %)

8

10

Figure II-16 Variation of the relative modulus at low strain as a function of the volume fraction (20°C) for silica suspensions into epoxy with ♦ hydrophilic and ■ hydrophobic surface chemistry

This behavior was related to the microstructure. Indeed, the observation of a transition from a gel behavior to a sol behavior, implies that there is a percolated network of filler in the suspension at rest which is broken as the strain applied increases. This was corroborated by the transmission electron microscopy (TEM) observation of the final crosslinked system filled with 5 wt. % of hydrophobic silica (Figure II-17). In contrast with 5 wt. % of hydrophilic silica, no contact is observed between the aggregates, they are individually dispersed (TEM micrograph reported in III.1.2.a).

Skelton

Figure II-17 Morphology of the crosslinked MDEA-based system filled with 5 wt. % of hydrophobic silica showing a percolating network of fillers related to the gel-like rheological behavior Skelton obtained from image analysis (annex A.4.2)

Additional plots of the dynamic rheological behaviors of the suspensions at 10 wt. % of silica displaying other surface modification into epoxy and Jeffamine can be found in annex B.2.2. At this given filler content, the only other suspension that exhibited a sol-gel transition into epoxy was 10a10-D300. All Jeffamine-based suspensions presented liquid behaviors over all strain range applied. This behavior was again attributed to the screening of the possible particleElodie Bugnicourt, PhD INSA Lyon, 2005

95


particle interactions, responsible for the formation of the percolated network of silica, due to the Jeffamine segments strongly adsorbed on silica surface. To conclude on the rheological study of the suspensions, it is interesting to point out once again, how, at a same silica loading, the behavior of a suspension can vary depending on the interactions developed in the system (balance between particle-polymer vs. particle-particle), and differs in between epoxy and Jeffamine-based suspensions. From a fine selection of the silica surface chemistry according to the nature of the medium, newtonian behavior, yield stress, thixotropy can be displayed. The relationship between the rheological behavior and the dispersion state could be elucidated rather straightforwardly for the different suspensions.

II.3.


After obtaining the silica suspensions into epoxy-amine comonomers, the crosslinking is realized in order to prepare the final networks of which solid state mechanical properties are characterized later on. The modification of the kinetics of the epoxy-amine reactions due to silica addition, and the evolution of the dispersion state of the silica during this step, are the two topics under study in this paragraph.

II.3.1. Preparation and crosslinking of the epoxy / silica composites The crosslinking is realized in a programmable oven, the cooling down is slow in order to minimize the residual thermal stresses. The first step of the cure schedule is realized at intermediate temperature in order to reach the gelation. The post cure is realized at a temperature greater than Tg∞ (i.e. the glass transition temperature of a fully cured system) in order to complete the reaction and reach a conversion close to one. The cure schedules used enabled to reach the maximal conversion, as it was checked from successive differential scanning calorimetry (DSC) measurements showing no evolution of the glass transition temperature.


96


II.3.1.a.

Formulations based on the aromatic hardener

The desired amount of silica suspension into epoxy, and if needed additional neat epoxy, is weighted and mixed with the corresponding amount of MDEA. This blend is heated during a few minutes at 90°C, in order to melt the MDEA. Then it is stirred mechanically again and poured into an open mold. The thermal crosslinking is carried out according to a cure schedule given in the Figure II-18: 4 hours at 135°C, followed by a post-cure of 4 hours at 190°C [ZIN99]. Generally, the samples prepared were flat panels of thickness varying between 2 and 5 mm.

II.3.1.b.

Formulations based on the aliphatic hardener

The same kind of procedure is followed: the desired amount of silica suspension into epoxy and/or into Jeffamine, and if needed pure epoxy and/or Jeffamine, are weighted and mixed together mechanically and then poured into an open mold. The cure schedule used for Jeffamine-based systems is 4 hours at 120°C followed by a post-cure of 4 hours at 150°C (Figure II-18). It was adapted in order to have a crosslinking kinetics rather similar with that of MDEA-based systems at their respective cure temperature (gelation time in the same range).

200 Tg∞ (D330-M)

T (°C)

150 100 50

room temperature

0 Tg∞ (D330-J)

-50 0

2

4

6

8

10

12

14

t (hours)

Figure II-18

Cure schedules for ▲ MDEA- and ■ Jeffamine-based systems

II.3.2. Kinetics of epoxy-amine reactions in presence of silica As largely emphasized in the literature study, the large active surface of fumed silica is expected to modify the reaction kinetics of the epoxy-amine network formation. Two main methods were used to investigate these changes: chemio-rheology and Near Infra Red spectroscopy during crosslinking. Elodie Bugnicourt, PhD INSA Lyon, 2005

97


II.3.2.a.

Effect of silica addition on the kinetics of polymerization Conversion of epoxy groups

The crosslinking of epoxy-amine systems takes place according to the reactions previously reported (Figure II-4). Therefore during the polymerization, the advancement can be followed by the consumption in epoxy or amine groups as a function of the time. Evolutions of the conversion in epoxy, recorded by Infra-red spectroscopy, are plotted in the Figure II-19 for the neat MDEA-based system and for that filled with hydrophilic silica. The system filled with hydrophilic silica is found to be faster and the final conversion is close to 100 % for both systems at the end of this first segment of the polymerization.

1 0.8

x

0.6 0.4 0.2

D330-M 5N-D330-M

0 0

50

100

150

200

250

t (min) Figure II-19 Evolution of the conversion at 135°C for MDEA-based systems: ♦ neat system, ■ system filled with 5 wt. % of hydrophilic silica

Gelation time measurement by chemiorheology The gelation times, corresponding to the appearance of a three-dimensional chemical network, were measured by chemio-rheological experiments at the isofrequency point as described in annex A.2. The tests were performed during an isothermal curing process respectively at 135°C for MDEA-based systems and 120°C for Jeffamine-based systems (corresponding with the first segment of the cure schedule). An example of multi-frequency sweep curve obtained for a filled system is given in the Figure II-20, and indicates the gelation time, as well as the relaxation exponent ∆. Elodie Bugnicourt, PhD INSA Lyon, 2005

98


ω: rad/s 1 1.58 2.51 3.98 6.31 10 15.8 25.1 39.8 63.1 100

100

tan (δ)

10

gelation time ~4300s tan(δ)~2.5 i.e. ∆~0.76

1

0.1

0.01 3500

4000

4500

5000

t (s)

Figure II-20 Measurement of the gelation time and relaxation exponent ∆ by a frequency sweep experiment during the crosslinking of a filled epoxy-amine system at 135 °C (5N-D330-M)

Full results of the variation of the gelation time and relaxation exponent as a function of the silica type and content for MDEA- and Jeffamine-based systems are reviewed in the annex B.3. First, it has to be underlined that the addition of any type and content of silica led to reduction of the gelation time for both hardeners. The magnitude of the reduction depends significantly for the formulation and the effect of various parameters such as the silica surface area, content and surface modification are discussed later on. This effect is illustrated in the Figure II-21 for an MDEA-based system filled with 5 wt. % of hydrophilic silica of surface BET= 200 m²/g compared with the neat matrix and is in agreement with the conversion curves previously reported (Figure II-19). Results of chemio-rheology were correlated with those from NIR in annex B.3.2.

10 neat system tgel ~5900s

tan (δ)

5 wt.% hydrophilic silica tgel ~4300s

1

0.1 4000

4500

5000

5500

6000

6500

t (s)

Figure II-21 Effect of the addition of hydrophilic silica on the gelation time of MDEA-based systems at 135 °C: 5N-D330-M vs. D330-M


99


Then, even though values of relaxation exponent are not too accurate, most are close to Rouse percolation theory (0.7), except those of the systems filled with highly hydrophobic silica which are significantly lower (< 0.5). This result is consistent with the liquid state rheological behavior before crosslinking that pointed out the formation of a percolated network of filler from 5 wt. % of hydrophobic silica into epoxy. It is known that when the value of ∆ decreases, the stiffness of the gel increases [ELO96]. This might be due to an overlapping of the physical gel of fillers and the chemical network leading to a stiffer behavior of the gel at the gelation point.

II.3.2.b.

Effect of the specific surface area of hydrophilic silica

The effect of the silica specific surface area on the gelation time was investigated (Figure II-22). As the specific surface area increased, the gelation time decreased linearly up to 200 m²/g. The point at 300 m²/g does not follow the same trend anymore, which was attributed to the presence of micro-porosities on silica surface from this value of surface BET, and lower accessibility of a part of the silanols.

1.2 1

50% D / 50% N

tgel / tgel0

0.8

BET (m²/g) tgel (s)

S y = -0.001x + 1 2 R = 0.99

0.6 0.4 0.2 0 0

50

100

150

200

250

300

neat matrix T N S D 50%D / 50%N

0 300 200 125 50 125

5910 4360 4300 4820 5600 5010

BET (m²/g)

Figure II-22

Gelation time at 135°C for MDEA-based systems filled with 5 wt. % of hydrophilic silica as a function of the silica specific surface area

II.3.2.c.

Effect of the content of hydrophilic silica

For a given specific surface area (200 m²/g), a linear decrease is also observed as the silica content increases (Figure II-23).


100


1.2

tgel / tgel0

1 0.8

silica wt.% 0 5 11

0.6 0.4

y = -0.048x + 0.986 R2 = 0.99

0.2

tgel (s) 5910 4250 2750

0 0

2

4

6

8

10

12

silica wt. %

Figure II-23

Gelation time at 135°C for MDEA-based systems filled with various contents of hydrophilic silica of specific surface area 200 m²/g

Note on the behavior of Jeffamine-based systems: Generally, the same trends as a function of silica surface BET, content, surface modification and residual silanols, were observed for Jeffamine-based systems (table in annex B.3.2.), except for amino-modified silica that had a low catalytic influence on the kinetics for MDEAcrosslinked systems but a larger influence on Jeffamine-based systems. This special point concerning amino-modified silica and the competition between the reaction of the amino groups brought by silica and by the hardener are debated later on.

II.3.2.d.

Discussion on the particular case of reactive amino-modified silica Effect of the residual silanols content

The evolution of the gelation time of epoxy-amine systems filled with modified silica, expected to react with the matrix, was also investigated: for a same amino-silane at variable and controlled grafting content, the gelation times turned out to be related only to the content of residual silanols.


101


1.2 y = -0.003x + 1 2 R = 0.98

tgel/tgel0

1 0.8

neat matrix A a50 a10 T

0.6 0.4 0.2 0 0

20

40

60

80

% residual OH

tgel (s)

0

5910

15 50 90 100

5770 4900 4200 4360

100

residual silanols %

Figure II-24 Gelation times at 135 °C for MDEA-based systems filled with 5 wt. % of partially to fully amino-modified silica as a function of the content of residual silanols on silica surface, BET= 300 m²/g

Discussion The results for amino-modified based systems can appear somehow surprising because the conversion at gel should in principle decrease because amino-modified silica can be regarded as an additional multifunctional crosslinker, and it is well known from Flory’s theories, the conversion at gelation, xgel, decreases with an increase of the components functionality. An approximate calculation based on Macosko-Miller’s theory leads to an epoxy conversion at gelation around 40 % for a system filled with 5 wt. % of amino-modified silica (assuming that primary particle act crosslinker exhibiting a functionality of about 480, and a yielding of the reaction of reactive groups on silica surface of 50 %) instead of 58% for the neat system (as calculated in annex B.3.2.b). With fully amino-modified silica, A, the gelation time (Figure II-24, and annex B.3.) is only slightly reduced compared to neat MDEA-based system (tgel/tgel0 ~ 0.97 with 5 wt. % of silica), a higher decrease of gelation time could have been expected. The decrease is in a larger range for Jeffamine-based system (tgel/tgel0 ~ 0.74 for 5 wt. % of silica). Aliphatic amino-groups are known to be generally more reactive with epoxy groups than aromatic ones. Thus, regardless of the immobilization on silica surface leading to a decrease of their reactivity, the amino groups brought by silica surface should react faster than those brought by the aromatic hardener. In order to work out this point, the kinetics of the two systems filled with amino-modified silica were recorded at the same temperature of 120°C (Figure II-25), which is the standard cure temperature for the aliphatic hardener.


102


1

0.8

xgel th neat system

x

0.6

xgel th filled system

0.4

D330-M 7A-D330-M D330-J 7A-D330-J

0.2

0 0

20

40

60

80

100

120

140

160

180

200

220

240

t (min)

Figure II-25 Evolution of the conversion vs. time of reaction at 120°C for MDEA-based systems: x neat system and ■ system filled with amino-modified silica and for Jeffamine-based systems: ▲ neat system and ♦ system filled with amino-modified silica

The amino groups provided by silica only account for about 3 % of the total amino groups, so that the differences investigated are really close to the resolution of the method. For Jeffaminebased systems, the slope is the same at the beginning for filled and neat system, in consistence with the expected behavior since all amino groups are aliphatic. At the beginning, the kinetics of MDEA-based system is slightly accelerated in the presence of silica, maybe due to the reaction of the aliphatic groups on silica surface. Besides, the relative reduction of the time to reach the respective theoretical conversion at gelation due to amino-modified silica addition for Jeffaminebased system is larger than for MDEA-based system and in agreement with the ratio of gelation times obtained. Additionally, the fact that MDEA-based systems are normally cured at 135°C, might lead to the lowering of the difference between MDEA-based neat system and that filled with amino-modified silica, as it was observed from chemio-rheological experiments. An additional point to consider is that the reaction at the silica interface with epoxy may also already take place in the suspension prior the crosslinking step, as we will try to elucidate in a forthcoming section. This could be one of the reasons why the gelation time was not significantly decreased for MDEA-based system in the presence of amino-modified silica, because dispersion could only be realized into the epoxy prepolymer (curing agent is solid), so that amino-groups might already be “consumed” in the suspension before the crosslinking step. Accordingly, for Jeffamine-based systems, the gelation time decreased in a larger range in the presence of amino-modified silica because the reaction at the silica interface with the epoxy could take place during crosslinking…


103


II.3.2.e.

Conclusion on the catalytic effect of silica Overall number of silanols as relevant parameter for kinetics

So, straightforward correlations appeared between the gelation time of silica / epoxy-amine composites and the silica specific surface area, weight fraction and content of residual silanols on silica surface. The different results presented so far, plotted altogether in the Figure II-26, lead to the conclusion that this catalytic effect is in fact directly related to the total number of silanol groups in the sample.

1.2 1

tgel / tgel0

0.8 0.6 0.4 0.2 0 0

5E+18

1E+19 1.5E+19 2E+19 2.5E+19 3E+19 3.5E+19 4E+19 number of silanols per g of sample

Figure II-26 Summary of the gelation times at 135 °C for MDEA-based systems filled with various contents, specific surface area and surface modifications of silica, as a function of the overall number of silanols in the sample

The catalytic effect of silanols on epoxy-amine reaction, responsible for the reduction of the gelation time of filled systems, is in good agreement with the literature [ALT01, MAR05…] according to the mechanism reported in the part I.3.3.a. Discussion on the catalytic effect due to silica compared with that of water Two additional experiments were carried out to ensure that the surface silanols groups are actually responsible for the catalysis of the epoxy-amine reaction: water addition and use of a pre-dried silica. The corresponding chemio-rheological and NIR data are included in the annex B.3.3. The catalytic effect could as well be due to the water adsorbed on silica surface, which quantity is supposed to be also rather proportional to the silanol content on silica surface, the different impurities in the silica could also be involved.


104


i) In order to know if it was reasonable to attribute the large catalytic effect to the silica surface silanols, their effect was simulated by addition of water into a neat epoxy-amine network. In spite of the lower mobility of the silanols on silica surface as compared to water, it was found out that the addition of the same amount of hydroxyl groups provided by water as those supplied by fumed silica led to the same range for the resulting gelation time of epoxy-amine system. It was also confirmed by the NIR conversion plot. ii) A silica suspension into epoxy was prepared using a silica pre-dried during 24 hours above 120°C. This led to a gelation time only slightly longer than that of the system in which the silica was not pre-dried before (4,500 s, against 4,300 s without pre-drying for 5N-D330-M, anyway both much lower than that of the neat system of 5,900 s). So the kinetics was not significantly influenced, this could be a proof for the responsibility of silanols in the catalysis. But, the high kinetics of water sorption on silica surface might have made impossible to remove it all before the dispersion step, and the experiment would then turn out to be irrelevant to provide an answer to the initial interrogation. To conclude, silica addition leads to an acceleration of epoxy-amine reactions related to the content of silanols in the system. This effect has to be accounted for in an industrial context because the gelation time limits the time of processability. The processing condition and cure schedule (crosslinking time and temperature) has be to adapted if necessary.

II.3.3. Evolution of the morphologies during the polymerization The morphologies of the silica phase may change during the formation of the epoxy-amine network as the miscibility parameters of the system evolve. The nature of the chemical modification of the silica surface is expected to play an important role in this step. Theoretically, a segregation / demixtion of the fillers is likely to be induced by the polymerization, due to the decrease of the entropy of the system. However, other studies reported that, for example in case of epoxy filled with clays, the exfoliation can take place during the polymerization and especially at the gelation time [LEP02]. The final morphologies might thus result from a competition between the kinetics of polymerization and the filler structuration, and be influenced by the crosslinking conditions (time, temperature).


105


II.3.3.a.

Comparison between initial and final morphologies

The initial dispersion states were observed by standard optical microscopy and PVS technique and characterized by grindometer and rheology as it was presented in the paragraph II.2.2. The final morphologies are part of the topic of next chapter (III.1.), but we can henceforth precise that they were achieved by TEM and present a much higher resolution than the initial observations. The direct comparison between initial and final morphologies, i.e. before and after crosslinking, is not sufficient because it does not allow a quantification but it gives clues about the mechanisms of morphological evolution involved during polymerization. Let us at least withdraw the trends from these direct comparisons. Generally, the same kinds of morphologies were observed in suspension as in solid state, i.e.: −

Fine dispersion for hydrophilic silica from the beginning till the end of the process, fact that would contradict the assumption that phase separation occurs during polymerization

−

For partially hydrophobic silica, the same conclusion arises

−

Presence of agglomerates in the two stages for amino-modified silica, but it resulted impossible to quantify the amount of these agglomerates. The mean size of the agglomerates by volume was 7 microns before crosslinking according to PVS (II.2.2.a). This can be, a priori, consistent with the TEM observations in the solid state, or maybe a bit overestimated. However, the part of “ideally”-dispersed aggregates was only 36 % initially, which seems to be much less than the part of individually dispersed aggregates seen on the final morphologies (III.1.2.b.).

−

Percolated network of fillers were observed on the TEM micrographs for highly hydrophobic silica (III.1.2.a.), but big agglomerates, initially detected by PVS in the liquid state, were not obvious anymore in the final state. Nevertheless, the presence of macroscopic heterogeneity in the silica repartition was noted. It can be assumed that the silica-richer zones results from the “dissolution” of the initial agglomerates i.e. diffusion of the aggregates out from the initial clusters.

Two complementary techniques were carried out in order to try to elucidate more accurately the morphological evolution in situ, i.e. in real time during the polymerization: 1. Confocal microscopy was used but only in case of amino-modified silica (A) because the resolution is limited down to the size of the micronic agglomerates. Therefore, the individually dispersed aggregates, featuring mainly in the morphologies obtained with other types of silica, would not be visible through this technique. 2. Small angle neutron scattering (SANS) was used for the study of amino-modified silica and hydrophilic silica. In contrast, for this technique, the resolution is rather limited on the upper size because it does not allow studying agglomerates (limited around 300 nm).


106


II.3.3.b.

Real time confocal microscopic observation

Confocal microscopy was used in order to investigate the morphological evolution in real time during the polymerization of the epoxy-amine system filled with amino-modified silica following a given zone in the sample with a higher resolution compared to conventional microscopy. The experimental details of the set up and method developed for the confocal microscopic observation at the polymerization temperature are given in annex A.4.4.b. The only observations presented here were realized in transmission mode (Figure II-27). These photos suggest that no major evolution of the morphology takes place: the agglomerates are present from the beginning till the end of the polymerization. Additionally, it could be noticed from the end of the first hour of reaction (rather before gelation that is expected after approximately one hour and a half), the motion is really limited in the system. Unfortunately, the resolution was too low to make quantification and further conclusions, and would require succeeding in the confocal mode in fluorescence in order to improve the resolution (as discussed in annex A.4.4.b).

50 µm

50 µm

50 µm

50 µm

Increasing observation time at 135°C Figure II-27 Real time observation of the morphological evolution by confocal microscopy at 135°C: images after 20 min, 40 min, 1h, 1h20 (~ gelation time)for a MDEA-based system filled with 5 wt. % of amino-modified silica grafted with a fluorescent compound (5Afluo-D330-M, wt. %.of fluorescent probe vs. silica A = 0.1%).

II.3.3.c.

Real time neutron scattering experiments

The real time SANS kinetical experiments were performed on the anisotropic spectrometer PAXE at the Laboratoire Léon Brillouin (laboratoire commun CEA-CNRS) in Saclay. The fundamentals of the technique and the experimental conditions are presented in annex A.4.3.


107


Adjustments The basic assumption is that the gelation limits the morphological evolution. In order to allow recording spectra with enough resolution (at least 20 minutes for each), the polymerization temperature was reduced down to 110°C. In these conditions, the gelation time lies between 4 and 5 hours depending on the formulation considered (Figure II-28). The SANS monitoring of the kinetics were realized in a thermo-regulated oven (annex A.4.4.a.), a spectra was recorded after 0, 10, 30 minutes and then each 30 minutes during 5 hours.

12

5N-D330-M, hydrophilic silica, larger catalytical effect

10

D330-M, neat matrix, slowest system

t (h)

8 6 4 2 0 80

90

100

110

120

130

140

T (°C)

Figure II-28

Evolution of the gelation times (hours) as a function of the polymerization temperature for MDEA-based system: ■neat and filled with ♦5 wt. % of hydrophilic silica

Results So, the purpose of this experiment is to check if an agglomeration or desaglomeration occurs during the polymerization of epoxy / silica composites depending on silica surface chemistry. The whole SANS patterns recorded during the polymerization are reported in the annex B.4. The evolution of the signal in time is not obvious. In the large-q and middle-q regions, the curves are almost overlaid from the beginning till the end of the experiment. This means that the mass and surface fractal dimensions are not significantly modified, which is not surprising because the aggregates are rather stiff. The largest evolutions are expected at the large distances characterizing the aggregates and their interactions, i.e. in the low-q region, but it is also the region where the signal / noise ratio is the lowest. A zoom on this region is displayed in the Figure II-29 displaying one spectra at the beginning of the experiment, one at half time and one at the end. There might be a slight displacement of the maximum towards the smaller distance and a decrease of its intensity for the systems filled with hydrophilic silica. For amino-modified silica, no significant trend can be withdrawn. The patterns obtained for composites filled with hydrophilic and amino-modified silica are substantially different all along the experiment (respectively peak vs. increase at low q, different


108


slopes…). The complete analysis of the patterns and of their differences depending on the formulation is reported in the part concerning the study of the morphologies after crosslinking (III.1.4.c.) which gave rise to more satisfying results.

0.1

initially mid time end

initially mid time end

I (cm-1)

I (cm-1)

0.1

0.01 0.001

-1

0.01

q (A° )

(a)

0.01 0.001

0.01 -1

q (A° )

(b)

Figure II-29 Kinetical study by SANS for epoxy / silica composites: “Zoom” on the low-q region of the I(q) patterns at the beginning, mid-time and end of the polymerization at 110°C for MDEA-based systems filled with (a) 5 wt. % of hydrophilic silica and (b) 5 wt. % of amino-modified silica

What could be expected? Let us base our discussion on an example form the literature of SANS study of individual silica particles (Figure II-30) realized by Cousin et al. [COU03]. Upon agglomeration (i.e., in this case, lowering pH), no change is notable at high q values, whereas at low q values, the maximum of intensities shifts towards higher values at lower q values showing an increase of the size of the aggregates formed by the silica.

pH, Imax

Figure II-30

: aggregation

Dependence of SANS patterns I(q) on the pH for silica / latex nanocomposites before drying showing the aggregation phenomenon under acid conditions [COU03]


109


Obviously, the case under study is substantially more complicate due to the fractality of fumed silica aggregates, and their possibility to interpenetrate. The influence of agglomerates is discussed further in the paragraph devoted to the SANS analysis of the final morphologies. So, two conclusions can arise from these experiments: either the real absence of morphological evolution during crosslinking, or the irrelevancy of the scale reached by SANS in order to characterize the morphological evolution. The use of ultra-SANS can be proposed to allow going further in the low-q region (large distances) and check if the interactions between aggregates / agglomerates evolved during the polymerization.

II.3.3.d.

Discussion on the evolution mechanisms

Beforehand, let us just precise that it is rather difficult to find the right experiments, allowing a high temperature real time observation, with a sufficient resolution at the different scales of organization present in the system, to answer to the question of the morphological evolution during the polymerization of the epoxy / silica composites. We can eventually wonder if it is reasonable to think that there is an absence of morphological evolution during the polymerization as it seems to be suggested by these microscopic and scattering experiments. For any nature of silica surface, the competition with the polymerization might limit the morphological evolution. The catalytic effect of the fillers on the kinetics also has to be taken into account in case this competition occurs. It would have been interesting to change the cure schedule in order to tune the reactivity of the system and have additional information concerning the competition between the structuration of the fillers and the crosslinking. In case of hydrophilic silica, the optimal dispersion might remain unchanged from the beginning till the end. In absence of shearing force, it could be assumed that even at the crosslinking temperature, the viscosity of the medium is too high for the aggregates to diffuse. This assumption seems however contradicted for highly hydrophobic silica (H). In case of amino-modified silica, various additional hypotheses can be brought about: −

the reaction of the amino functions around the agglomerates might create a kind of shell and freeze the initial morphology from the beginning of the curing step, or even from the dispersion step

−

the agglomerates might be also preformed during the manufacturing of the silica…

It is believed that the leading phenomena involved is the competition between the structuration and reaction of the reactive groups on the surface of the initial agglomerates. The same mechanism can be responsible for the morphologies of the other types of silica, as discussed in


110


the annex C. This hypothesis was also proposed by Fiedler et al. [FIE05c] who obtained big agglomerates in case of amino-modified silica, they supposed that the reaction occurs from the beginning of the dispersion at room temperature.

II.4.

Study of the interactions generated at the silica / epoxy

interface

As mentioned in the literature part, the interfacial interactions are expected to influence substantially the final properties of the material, thus it is important to work out their nature and intensity. A simple calculation, assuming that the interphase thickness is only 1 nm thick (much lower even than for micro-composite, in which it can reach hundreds nanometers), the interphase for a sample filled with 10 wt. % of silica of surface BET = 300 m²/g accounts for a weight content greater than 3 wt. %, which influence can not be neglected on the overall behavior of the material. First, general elements are reported for epoxy systems filled with any type of silica, then potentially reactive silica with the matrix are studied by various techniques to ensure that the covalent bonding actually occurs.

II.4.1. General approach for any silica surface chemistry II.4.1.a.

Study of the interactions before and during crosslinking

The investigation on the one hand of the dispersion state, and on the other hand of the influence on the kinetics of the polymerization as a function of the silica surface modification already gave a good idea of the interactions developed in the system. As pointed out in the previous paragraph, these interactions evolve during the polymerization so we can wonder which the natures of the interactions in each step are. Initially, for the suspensions of unmodified silica into epoxy prepolymer or Jeffamine, the formation of hydrogen bonds can lead to a strongly adsorbed prepolymer layer on the silica surface silanols (Figure II-31). During the crosslinking, additional H bonds can take place between the silica surface silanols and the secondary amine, or hydroxyl groups formed. Note that the aromatic rings of epoxy prepolymer and aromatic hardener (MDEA) can also lead to strong adsorption on silica surface [DUC96], but the lower flexibility of the macromolecular Elodie Bugnicourt, PhD INSA Lyon, 2005

111


chains due to higher crosslink density also has to be taken into account in the case of MDEAbased networks. H C

H2C

H C

H2N

O

O

O

Si

Si

O

CH3

H

H

H2 C

H

H2 H C C

n CH3

NH2

H O

Si

Si O

(a) Figure II-31

(b)

Schematic representation of the interactions developed between pristine silica surface and (a) epoxy prepolymer, (b) Jeffamine in the initial silica suspension

In presence of amino-modified silica, a covalent bonding is expected, but the step and conditions in which it takes place has to be worked out, as discussed later on. The expected structure of the epoxy-amine network filled with amino-modified silica is illustrated in annex B.1.2.

II.4.1.b.

Interactions in the final stage

The final morphologies, reported and discussed in the third chapter, provide wide information concerning the interactions developed within the system; but many other properties can be used such as the residual thermal stresses, the glass transition temperature and heat capacity, the behavior after hydrothermal aging. The purpose of this paragraph is thus to characterize the interface through these techniques. Molecular mobility and residual stresses As described in the literature part (I.4.1), silica fillers are expected to influence the segmental motion of the polymer chains surrounding, especially because of the large surface of interaction, and because the size of the chains between crosslinks is in the range of the size of the primary particles (annex B.1.2.). It was found out that the residual stresses (annex D.1.) increase in presence of untreated silica (by 20 % for an addition of 5 wt. % of unmodified silica in the epoxy-amine system). The increase is a bit lower with amino-modified silica, maybe due to the organo-silanes allowing a


112


better stress transfer and as a consequence, a more homogeneous stress state after curing and cooling down of the sample. The chains mobility was investigated by the heat capacity variation and temperature of glass transition. The increase of Tg for MDEA-based systems in presence of silica suggested that the chain mobility is reduced. It is less reduced in presence of amino-modified silica. These results were completed by DMA analysis of the temperature of main mechanical transition, associated to the glass transition temperature, as a function of the silica content and nature, as presented in the chapter III.2. The theoretical thickness of the interphase accounting for the decrease of heat capacity was calculated according to a model developed by Lipanov and Theocaris. At a given content of silica, the thickness of the immobilized phase was found to be slightly larger in case of hydrophilic silica than in case of amino-modified silica, and it increases as the silica content increased. Values in the range of 0.2 nm were found for a composite filled with 10 wt. % of silica, but it can be assumed that an underestimation was achieved through this model according to the “non-physical” meaning of such low calculated thickness. Influence of hydrothermal aging In the field of fiber reinforced epoxy composites, hydrothermal aging tests are commonly carried out in order to characterize the quality of the adhesion between the glass fibers and the matrix. A low adhesion leads to the increase of water diffusion at the interface inducing a degradation of the overall mechanical properties after hydrothermal aging. In order to characterize the adhesion at the interface between fumed silica and epoxy matrix, depending on the surface modification of the silica, an hydrothermal aging was carried out during 72 h at 88 °C into hot water. The Young modulus of the samples was measured before and after aging by tensile tests according to the procedure developed in annex A.5.2.a. The values obtained are reported in the Table II-6. The matrix exhibits a constant value whereas the filled systems presented a slight decrease. Unexpectedly, the decrease is a bit more important in case of amino-modified silica in spite of the convalent bonding at the filler / matrix interface. This could be explained by the poorer dispersion state for epoxy-amine networks filled with amino-modified silica, even though agglomerates seem to occlude matrix and not air. Most likely, this is due to difference wetting states of silica surface by polymeric network, thus different local compactness in the interfacial region that can allow the diffusion of water molecules more easily in case of amino-modified silica. Same hypothesis seem to be involved in the changes of density of the systems depending on the formulation (results reported in annex B.5.1.).


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Einitial (GPa) Efinal (GPa) Efinal/Einitial 2,628 2,627 1,00 D330-M 2,651 2,536 0,96 5N-D330-M 2,649 2,495 0,94 5A-D330-M Table II-6 Young modulus for epoxy / silica composites measured before and after hydrothermal aging: MDEA-based neat system compared to filled either with hydrophilic or amino-modified silica

II.4.2. Investigation of the covalent bonding at the amino-modified silica / matrix interface II.4.2.a.

Strategy and procedure

The kinetical study, as well as the general considerations just presented, did not prove with full confidence, the occurrence of the covalent reaction in between the matrix and the aminomodified silica, thus the direct characterization of this bond was needed. Indeed, even if potentially reactive fillers are introduced in the matrix, the question of the reaction is not so trivial because other features have to be taken into account: −

This reaction can be submitted to the unfavorable competition with the reactions leading to the formation of the network and the immobilization of the groups brought by silica decreases their reactivity. Due to the aliphatic nature of the amino-silane, the reaction with epoxy is likely to take place initially into the suspension and hopefully not to be submitted to the competition with the polymerization. But it might also need an additional heating. In that case, the competition would be in favor of the amino-silane compared to the aromatic hardener MDEA, regardless of the steric hindrance of the amino groups, and for aliphatic amine Jeffamine, the intrinsic reactivity would be the same. This illustration of the competition was attempted by NIR study of these systems, but it was not obvious due to the low proportion of amino-groups on silica surface in the whole filled systems.

−

The presence of agglomerates could limit the part of amino-groups accessible to react.

−

The additional problem rising, when the amino-modified silica (A) is considered, is the possible back-bonding of the flexible amino-ended chains that could result in a screening of the amino groups on silica surface as already pointed out.

The most likely structure accounting for the excess of epoxy in the initial amino-modified silica suspensions (a/e ~ 0.018 for 5A-D330) is illustrated in the Figure II-32. But other configurations could also exist such as: i) no reaction at all, ii) one only amino-hydrogen reacted, if a limitation due to steric hindrance occurs, or iii) formation of a bridge between two neighboring silica particles by a DGEBA molecule (less likely to happen considering the excess of epoxy). This


114


last hypothesis could however explain the agglomeration of silica particles which appeared impossible to remove by any process.

Figure II-32

Most likely structure of the amino-modified silica after introduction into epoxy prepolymer due to the excess in epoxy groups

It resulted impossible to distinguish the eventual C-N bond between silica and epoxy, directly in the epoxy-amine networks filled with amino-modified silica, due to the low abundance of this bond (silica only provides a few percents of the amino-groups in the system), so a preliminary extraction of the silica from the mixture was indispensable. The procedure used to remove the largest part of adsorbed epoxy on silica surface is given in annex A.3.1.a. Four samples of silica were prepared from the amino-modified silica A according to different conditions allowing to minimize or maximize the expected reaction with epoxy (Table II-7) and extracted from the suspension into epoxy.

Symbol

Preparation conditions

Ai

Commercial amino-modified silica, in initial state as supplied by Wacker Co. (reference)

Ad Ar

Am

Amino-modified silica extracted from its suspension into epoxy after standard dispersion process (warming up to c.a. 80°C for at least 30 minutes) Amino-modified silica extracted after dispersion and post cure (15 hours at 150°C) in the epoxy prepolymer, maximal reaction expected Amino-modified silica extracted immediately after manual dispersion into epoxy prepolymer (contact time: 5 minutes at room temperature), no expected reaction, control of unextractable part of adsorbed epoxy

Table II-7

Conditions for the preparation of the 4 samples of amino-modified silica presenting various levels of reaction with epoxy

These samples were compared using various techniques: Infra-Red spectroscopy, elemental analysis, Thermogravimetric analysis (TGA), TGA coupled with gaz chromatography and mass spectroscopy (GC/MS), and solid state nuclear magnetic resonnace (NMR). Some of these techniques were sensitive to the overall quantity of epoxy in the sample (adsorbed and convalently bonded to silica), other allowed the direct characterization of the C-N covalent bond potentially formed between silica and epoxy. The results of these techniques are presented with


115


an increasing level of selectivity concerning the bond to evidence. The objective is to check if the reaction takes place during the dispersion stage, or otherwise the conditions leading to this bonding, as well as the yield of the reaction reached in each condition, i.e. proportion of aminogroups on silica surface reacted with epoxy (the maximal conversion would correspond to two DGEBA molecules per nitrogen atom on silica surface).

II.4.2.b.

Results of Infra-Red spectroscopy

The four samples of amino-modified silica were analyzed by Medium Infra-Red spectroscopy (Figure II-33) according to the conditions described in annex A.3.3.

Ai

Am

Am Ar Ad

Ar Ai Ad

Φ of D330 at 1515 cm-1

Figure II-33 IR spectra for the different samples of amino-modified silica reacted with epoxy and on the right: zoom on the band at 1,515 cm-1 attributed to aromatic ring of epoxy prepolymer

The assignment of the bands can be found in annex B.5.2. for the different samples. Unfortunately, the bands that could allow the separation of the groups involved in the reaction are convolute and therefore impossible to use. The band at 1,515 cm-1, attributed to the aromatic rings of epoxy prepolymer, is well resolved and was used in order to quantify the total DGEBA present in the system. The variations of the height of this peak, depending on the conditions of preparation, are consistent with the level of reaction expected: Ar is supposed to represent the maximal reaction reachable with epoxy (height: hmax), and Am the control for the part of unreacted epoxy adsorbed on the silica surface that could not be extracted (h0). The conversion in amino-hydrogens on silica surface during the dispersion step, reached for the sample Ad (hd) was calculated to reach 84.6 % of the maximal reaction [xd=(hd-h0)/(hmax-h0)].


116


II.4.2.c.

Results of Elemental Analysis

The rough values of the titration in carbon, oxygen, nitrogen and hydrogen by elemental analysis for the different samples are provided in annex B.5.4. They were used to calculate the number, d, of molecules of DGEBA bonded to each nitrogen on the silica surface assuming that all the epoxy had reacted (Table II-8). This led to a yield of the reaction of 84 % achieved during the dispersion step, and an epoxy content greater than the maximal reaction possible when a post-cure was performed due to either the part of epoxy still adsorbed or the occurrence of an etherification of the excess of epoxy on silica surface during this step.

Wt. %/ orga

Reaction

Sample

C

H

N

O

d (DGEBA/ N)

Yield (%)

Ai*

60.92

15.14

12.02

11.91

0

0

Ad

41.3

5.07

1.08

52.54

1.67

84

Ar

45.82

5.22

0.99

47.97

2.07

103

Table II-8 Elemental analysis of carbon, hydrogen, nitrogen and oxygen, scaled to the organic part in the sample; d: number of epoxy bonded to each N on silica surface, d=(c/n-5)/23.7, with: c: number of carbon /100 g of silica, and n: number of nitrogen per 100 g of silica, *theoretical structure of the organolayer on Ai: C5H14NO, i.e. 57.69 wt. % of C, 13.46 wt. % of H, 13.46 wt. % of N and 15.38 wt. % of O

II.4.2.d.

Results of Thermo-Gravimetric Analysis Standard TGA

First, the different silica samples were analyzed using standard TGA. The weight loss for each sample was related to the total quantity of organic part remaining. It increased when the sample was prepared after increasing contact time between silica and epoxy at high temperature. These results are reported in the Figure II-34, and allowed calculating the yield of the reaction reached in each condition compared to the maximal reaction (Table II-9). The total ratio of epoxy is not significantly modified during the post-cure performed on Ar compared to Ad: the max reaction might be already reached for Ad. These results are fully consistent with those obtained by FT-IR and Elemental Analysis, but all of them take into account both physisorbed and covalently bonded epoxy on silica surface.


117


100 90

90

Am

85 80

Ad

75

Ar

70

Ai

95

% wt.

% wt.

100

Ai

95

Am

85 Ad

80 75

Ar

70 65

65

60

60 50

150

250

350

450

50

550

150

250

(a) Figure II-34

350

450

550

T (°C)

T (°C)

(b)

Standard TGA scans for the different samples of amino-modified silica reacted with epoxy under (a) air and (b) helium (from 20°C to 550°C, at 10°C / min)

Flow

Air

Helium

Sample

Wt. %

Yield (%)

Wt. %

Yield (%)

Ai*

6.7

0

5.2

0

Am

16

42.7

14.6

46.5

Ad

31.5

84.0

30.4

96.7

A Table II-9

33.1

88.3

30.6

97.0

Weight loss at the final step of the TGA scan under oxidant and inert flow, and yield of the reaction of amino-modified silica with epoxy. *data provided by Wacker Co. for Ai: wt. % of C: 4.5-5.5 i.e. 7.8-9.5 wt. % of organic compounds, which means that the degradation of the organic part is not complete at 550°C

The curves of the derivative weight versus temperature (Figure II-35) evidenced different temperatures of interest depending on the sample considered: a first maximum around 310°C was observed for each sample, it was related the degradation of the epoxy physisorbed on silica surface, and the second maximum around 360°C only present for Ad and Ar, was related

0.25

0.3

Ar

0.2

Ad

0.15 0.1

Am

0.05

Ai

0 50

150

250

350 T (°C)

(a)

450

550

derivative weight vs T (%/°C)

derivative weight vs. T (%/°C)

to the degradation of the part of covalently-bonded epoxy on silica surface

Ad

Ar

0.25 0.2 0.15 0.1

Am

0.05

Ai

0 50

150

250

T (°C)

350

450

550

(b)

Figure II-35 Derivative weight lost versus temperature the different samples of amino-modified silica reacted with epoxy under (a) air and (b) helium (from 20°C to 550°C, at 10°C / min)


118


Max resolution TGA coupled with GC / MS A coupling between TGA in Max resolution mode under air flow and gas chromatography, mass spectroscopy (GC / MS) (conditions in annex A.3.4.) was used in order to know what the nature of the gas effluents was at each temperature of interest, and quantify the ratio of covalent vs. adsorbed epoxy. The only sample analyzed using this technique was that obtained under standard conditions, Ad (Figure II-36). In max resolution, three main temperatures of iso-thermal degradation could be isolated: 310°C, 370°C, and 560°C in agreement with the derivative curve obtained by standard TGA. The table of identification of the gas effluent at each sampling temperature is given in annex B.5.4. Basically, at 310°C, only light fraction are found in the gas effluents, whereas from 360°C, different types of silanes derivates are emitted from the recomposition of the degradation products. This means that from this second temperature the structures bonded to silica are affected and not only the adsorbed layer. We shall remind the reader that this peak at 360°C is absent in the scan of the sample Am, control for the unextracted adsorbed epoxy (Figure II-35). An approximate calculation assuming that the first temperature of isothermal degradation (360°C) is due to the adsorbed epoxy leads to 27% of epoxy adsorbed on silica surface. If we consider that the degradation above 360°C is that of the covalently bonded epoxy, this leads to 73% of the total epoxy which is covalently bonded, i.e. 63% of the maximal reaction reachable theoretically.

310°C, 8.9 wt. % Light fraction degradation 360°C, 5.6 wt. % From 360°C: silanes found in the gas effluents Up to 580°C, 18.1 wt. %

Figure II-36 Maxi resolution TGA for the sample Ad i.e. amino-modified silica extracted from its suspension into epoxy after standard dispersion process (form 20°C to 600°C, at variable heating rate depending on the kinetics of weight loss, under oxydant flow)


119


II.4.2.e.

Results of solid-state NMR spectroscopy

The results previously reported seem to attest of the occurrence, at least partially, of the aminogroup on silica surface with epoxy during the dispersion, however these previous techniques either did not allowed distinguishing covalently bonded epoxy on silica surface, or allowed a rough approximation. Consequently, to obtain finer information directly about the expected carbon-nitrogen chemical bond, solid state NMR experiments were carried out. Different types of solid state NMR were used in order to analyze the structure of the samples depending on the conditions of reaction first via the signal of the carbon and second via the signals of the nitrogen and proton besides. Solid state Carbon NMR Solid state

13

C NMR experiments were carried out in CP/MAS mode at 500 MHz, in order to

study the structures of the different samples of amino-modified silica reacted with epoxy in different conditions. An example of spectra with the assignment of the chemical shifts is presented in the Figure II-37. For the other samples studied, the spectra can be found in annex B.5.5.a. 31 ppm -C-

O-CH2-CH-CH2-O-

-C-

OH

CH3

OH

CH3

70 ppm

42 ppm

157 144 114 ppm ppm ppm 128 ppm

22 ppm CH2

156.9 144.0

15 ppm CH2 CH 3 Si CH 3

0 ppm

0.1

44 ppm CH2

O-CH 2-CH-CH2 n O

69.7

N

45 and 50 ppm

CH3

CH2-CH-CH 2-O-

127.6 114.2

H

CH3

49.7 41.8 30.5 21.9 15.2

60 ppm

O Si

190

170

150

130

110

90

70

50

δ13C(ppm)

30

10

-10

-30

-50

13 C solid state NMR spectrum obtained for the amino-modified silica extracted after Figure II-37 dispersion into epoxy prepolymer (Ad), and assignment of the chemical shifts on the expected structure


120


First, let us point out the trend expected from 13C NMR if the reaction occurs. The epoxy / amine junction should lead to: i) the appearance of a peak around 58-60 ppm (CH2 beside nitrogen), ii) an increase of the peak at 70 ppm for CH-OH groups formed during the first step of the epoxy / amine condensation (same chemical shift as CH2-O-Φ of DGEBA), iii) a decrease of the peaks characterizing epoxy ring at 45 and 50 ppm [EUS90]. A comparison of the different samples in the aliphatic zone of the spectra (Figure II-38) put in light the following differences as the expected reaction increased: compared to the peak at 0 ppm, the peaks at 70, 42 ppm and the shoulder at 60 ppm increased whereas the peak of epoxy group at 45 ppm decreased in agreement with the occurrence of the reaction epoxy/ amine. Note that the chemical shift at 77 ppm, especially high for the post-cured system, Ar, might be due to side reaction of etherification during a long time of reaction at high temperature in a large excess of epoxy (peak due to a carbon adjacent to an ether group at 76.5 ppm). However, it was impossible to obtain a quantification of the level of reaction from these results of solid state NMR due to the low resolution and wide convoluted peaks obtained. This is the reason why it was decided to use high resolution NMR.

Increasing reaction expected

Ar

Ad

Am

Figure II-38 Zoom on the aliphatic zone of the 13C solid state NMR spectra for the different samples of amino-modified silica reacted with epoxy

High resolution proton and nitrogen solid state NMR High resolution proton solid-state NMR at 750 MHz was performed at CRMHT Orléans. The spectra recorded for the different samples of amino-modified silica extracted from epoxy are displayed in the Figure II-39. The comparison with spectra obtained through standard solid state proton MAS NMR at 500 MHz (reported in annex B.5.5.b.), of which resolution was really low, highlights the interest of high resolution NMR [MAS02]. Unfortunately, the signal assigned to NElodie Bugnicourt, PhD INSA Lyon, 2005

121


H groups (6.5 ppm in the spectra of Ai) is overlaid with the signal of phenyl groups of DGEBA, preventing to check if this signal disappeared when a reaction was expected.

Increasing reaction expected

Ar

Ad

Am

* *

14.0

12.0

10.0

8.0

6.0

Ai

4.0

2.0

0.0

-2.0

-4.0

(ppm)

-6.0

-8.0

δ (ppm) Figure II-39

Proton solid state NMR spectra for the different samples of amino-modified silica reacted with epoxy, * residual ethanol

14

N NMR [750 MHz– static “echo”] was also attempted in order to get a straightforward answer

on the disappearance of the N-H groups, but it resulted unsuccessful due to an interfering artifact due to the boron nitride stator. In CP

15

N{1H}, the adjustments were complicated and the low abundance made the acquisition

too slow to get any significant signal. 1

H{14N} TRAPDOR

The TRAPDOR (transfer of population by double resonance) sequence is an extension of REDOR (rotational echo double resonance) in case at least one nuclei is quadripolar. This technique generally allows the determination of the distance between two nuclei by comparing the intensity of the signals recorded in closely related experiments [BRO05b, LAW02].


122


1

H{14N} TRAPDOR experiments were carried out at the CRMHT Orléans (750 MHz) to study the

initial amino-modified silica and that extracted from the dispersion into epoxy. The difference between the 1H signal in echo with and without coupling with14N was maximized by fine tuning of the pulses length and interval (spectra in direct irradiation compared to that coupled with nitrogen, and difference as a function of the dephasing time can be found in annex B.5.5.c.). The final substracted spectra for the samples Ai and Ad are reported in the Figure II-40. In the spectra of the initial silica Ai, it is assumed that the peak at 6.5 ppm is only due to the signal of the proton in first neighboring position from

14

N, i.e. N-H. Remember that it was convolved with

the signal of the proton of the aromatic ring of the DGEBA in direct irradiation. This peak is absent in case of silica Ad, showing an absence of residual amino hydrogen, and thus corroborates the occurrence of the reaction at the interface between amino-modified silica and epoxy during the dispersion state.

Ad

Ai

10.0

8.0

6.0

4.0

2.0

0.0

-2.0

-4.0

δ (ppm)

Figure II-40 Solid state NMR spectra in TRAPDOR 1H{14N} presenting the difference S-S0 where S is the spectrum with and S0 without irradiation of 14N for the samples of initial amino-modified silica (Ai) and amino-modified silica extracted from dispersion into epoxy (Ad)

II.4.2.f.

Conclusion on the reaction of reactive silica with the matrix

First, it should be noticed that, evidencing the occurrence of the reaction at the silica surface is not a common practice in the literature especially in case of sub-micro structured composites. It seems often obvious for the formulator that if a potentially reactive filler is introduced, the reaction takes place straightforwardly with no further condition. It is indeed a hard topic due to the weakness of the signal to evidence. All the techniques, carried out complementarily, lead to


123


the conclusion that the reaction actually takes place, at least for the first amino-hydrogen (primary amine). This verification was important to us, in order to know the impact of the interfacial adhesion on the properties. Even if the silica account only for a few percents of the reactive groups, it was also important to ensure that the stoechiometric ratio has to be adjusted properly and distinguish the influence of a defect of stoechiometry from that of the modification of the silica surface chemistry. For the other silica potentially reactive with the matrix, the study of the reaction could not be performed systematically by lack of time. The silica Ap, exhibits a greater reactivity than the silica A (due to the short methylene spacer), so that the reaction is expected to take place furthermore than in the case of the silica A studied. In contrast, the intrinsic reactivity of the silica E grafted with an aliphatic epoxy silane is lower than that of the aromatic epoxy groups of DGEBA with the aromatic hardener. The achievement of the reaction for epoxy-modified silica with the MDEA-based matrix can thus be doubtful. It might be more likely to happen when the silica dispersion is previously realized into the aliphatic hardener. To conclude, in order to be sure that the reaction occurs, it is advisable to choose a silane more reactive than the hardener to overcome the reduction of intrinsic reactivity due to the steric hindrance and obtain a favorable competition. But, as seen in the previous paragraph, it might be problematic for the dispersion…

To summarize, in this second chapter, an optimized procedure was developed in order to process sub-micro composites based on an epoxy matrix and fumed silica with a complete mastering and understanding of each step. First, silica suspensions into epoxy and amine comonomers were prepared paying a special attention to the dispersion state obtained (which was characterized by rheological analysis), because it can affect the final properties of the materials. The crosslinking of the composites was then performed, during this stage both the catalytic effect of silica on the epoxy-amine reactions due to surface silanols and the evolution of the silica morphology were investigated. Additionally, the interactions developed between the silica and the epoxy matrix were studied in the different stages of the preparation depending on silica surface chemistry: it allowed showing good interactions between silica surface silanols and the polar epoxy matrix, and in case of reactive silica a large effort was invested to demonstrate that the potential reaction between amino-group on silica surface and epoxy matrix actually occurs. The characterization of solid-state properties of the epoxy / silica composites obtained according to this procedure is presented in the next chapter.


124

Chapter III: Solid state properties of the epoxy / silica composites: study of the mechanical behavior depending on the morphology

III.

SOLID STATE PROPERTIES OF EPOXY / SILICA COMPOSITES: STUDY OF THE

MECHANICAL BEHAVIOR DEPENDING ON THE MORPHOLOGY ...................................... 127 Morphological study of epoxy / silica composites ................................................. 127

III.1. III.1.1.

Foreword on the transparency.......................................................................... 128

III.1.2.

Morphological study by transmission electron microscopy ............................... 128

III.1.2.a.

Hydrophilic silica: influence of the specific surface area........................... 129

III.1.2.b.

Effect of the surface modification.............................................................. 130

III.1.2.c.

Effect of other parameters of the formulation and of the dispersion

process.............. ............................................................................................................ 134 III.1.2.d. III.1.3.

Summary of the morphologies depending on the formulation .................. 135

Image Analysis of TEM micrographs ................................................................ 136

III.1.3.a.

Shape and size characterization of the silica dispersed phase (euclidean

morphometric parameters) ............................................................................................ 136 III.1.3.b. III.1.4.

Analysis of the fractal geometry of the silica dispersed phase ................. 141

Structural study by small angle neutron scattering ........................................... 145

III.1.4.a.

Introduction on SANS experiments........................................................... 145

III.1.4.b.

Procedure for the structural analysis from SANS measurements based on

examples from the literature .......................................................................................... 146 III.1.4.c.

Results for epoxy / silica composites ........................................................ 150

Mechanical behavior of silica / epoxy composites ................................................ 158

III.2. III.2.1.

Mechanical properties of MDEA-based systems .............................................. 158

III.2.1.a.

Dynamic thermo-mechanical behavior...................................................... 158

III.2.1.b.

Mechanical properties by tensile tests ...................................................... 162

III.2.1.c.

Fracture resistance ................................................................................... 163

III.2.1.d.

Compromise stiffness - toughness............................................................ 165

III.2.2.

Mechanical properties of Jeffamine-based systems ......................................... 165

III.2.2.a.

Dynamic thermo-mechanical behavior...................................................... 165

III.2.2.b.

High strain mechanical properties by tensile tests.................................... 168

III.3.

Relevant parameters for mechanical properties of epoxy / silica composites and

relationships with the structure ............................................................................................. 173 III.3.1.

Influence of silica content ................................................................................. 173

III.3.1.a.

In the glassy state ..................................................................................... 174

III.3.1.b.

Comparison between the behavior of both systems in the rubbery state . 176

III.3.1.c.

Perspectives on mechanical modeling...................................................... 178


125


III.3.2.

Influence of the interfacial surface area and adhesion developed between silica

and epoxy matrix............................................................................................................... 180 III.3.2.a.

In the glassy state ..................................................................................... 180

III.3.2.b.

Comparison between the behavior of both systems in the rubbery state . 181

III.3.2.c.

Discussion on the necessity of chemical bonding..................................... 183

III.3.3.

Discussion on toughening improvement in the glassy state ............................. 184

III.3.4.

Discussion on the optimal morphology for the reinforcement ........................... 185


126


III. SOLID STATE PROPERTIES OF EPOXY / SILICA COMPOSITES: STUDY OF THE MECHANICAL BEHAVIOR DEPENDING ON THE MORPHOLOGY

This third chapter is devoted to the characterization of solid-state properties of epoxy / silica composites: more especially, it is focused on final morphologies and mechanical properties. Influences of various parameters were investigated such as the silica content, specific surface area and surface modification, the hardener nature, which controls the state of the sample (rubbery vs. glassy) and influences the interactions with the filler due to different polarities. The morphological study is carried out mainly through transmission electron microscopy observation coupled with image analysis and through small angle neutron scattering. The mechanical properties are studied via dynamic mechanical analysis, tensile test and fracture mechanics. The relationships between the mechanical behavior and the structure are finally discussed in details depending on the parameters of the formulation, dispersion state, interfacial adhesion, and the mechanisms of reinforcement involved in the stiffening and toughening of these epoxy / silica composites are commented.

III.1.

Morphological study of epoxy / silica composites

Various techniques were used in order to elucidate the final morphologies of the crosslinked epoxy / silica composites at multi-scale: transparency, transmission electron microscopy, image analysis, and small angle neutron scattering. The results are presented in this paragraph with an increasing accuracy.


127


III.1.1. Foreword on the transparency The first insight about the morphology of the systems was given by the macroscopic observation of the sample aspects. Indeed, the transparency of filled polymer is a qualitative criterion that appeared rather significant for the characterization of the dispersion state. Besides, the transparency can fulfill an industrial requirement for applications of these epoxy / silica formulations for instance in the field of clear coatings. In case of MDEA-based networks, the transparency was preserved only for the systems based on hydrophilic silica, partially-amino modified silica, piperazino-modified silica Ap and epoxymodified silica E (for sample of thickness typically ~5 mm). Due to the presence of a few large aggregates, the systems based on hydrophilic silica of low surface BET (50 m²/g) are not fully transparent either in spite of the perfect dispersion. In case of the Jeffamine-based systems, whatever the silica surface area, the transparency was conserved up to the highest content of hydrophilic silica dispersable i.e. 15 wt. % for a silica of surface BET= 200 m²/g (Figure III-1), and even up to 40 wt. % for a silica of BET= 50 m² /g. For surface-modified silica, the transparency follows the same trend as in MDEA-based systems even though it is not as sensitive to the presence of a few micronic agglomerates.

matrix 5 wt. %

Figure III-1

15 wt. %

Transparency of filled Jeffamine-based networks: neat matrix vs. systems filled with various contents of hydrophilic silica

III.1.2. Morphological study by transmission electron microscopy The morphologies of the crosslinked epoxy / silica composites were observed using transmission electron microscopy4 (TEM, specifications given in annex A.4.1) in order to characterize the level and uniformity of the particles dispersion in the polymeric continuous phase and discuss the effect of the formulation. Previously, the homogeneity of the neat matrices based on the two hardeners was verified up to the highest magnification of observation (Annex C.1.1.a.). 4

Other imaging techniques such as Scanning Electron Microscopy and Atomic Force Microscopy were attempted but pictures obtained were not resolved enough to characterize accurately the dispersion state.


128


For each filled system, one significant micrograph at low magnification and one at high magnification were chosen to be reported in this manuscript. The morphologies of every system presented in this chapter were obtained using the standard dispersion process (dissolver) and crosslinked using the aromatic hardener MDEA. The aim is to show the effect of the specific surface area and surface chemistry of the silica on the morphology. Further pictures are reported in annex C, in order to highlight the effect of other parameters of the formulation and processing such as: the nature of the hardener, the nature of the dispersion medium, the type of dispersing tool used, the silica content, as well as the results for other surface chemistries (additional epoxy- and amino-modified silica considered). They are commented at the end of this paragraph.

III.1.2.a. Hydrophilic silica: influence of the specific surface area The micrographs are presented in the Figure III-2 through Figure III-5 with decreasing specific surface area with a silica content of 5 or 10 wt. %. For epoxy networks filled with hydrophilic silica, an homogeneous, optimal dispersion state (i.e. at the level of the elemental aggregates), was obtained whatever the specific surface area (from 50 up to 300 m²/g). This morphology was attributed to the good interactions developed between the polar matrix and silica surface silanols as already pointed out. In these systems, particlepolymer interactions appear to be favored compared to particle-particle interactions. As already mentioned, and as it is clear from these pictures, the primary particles size and size distribution are larger for the lowest specific surface area (50 m²/g).

Figure III-2 TEM image for 10T-D330-M: crosslinked MDEA-based system filled with 10 wt. % of hydrophilic silica of specific surface area of 300 m²/g


129


Figure III-3 TEM image for 5N-D330-M: crosslinked MDEA-based system filled with 5 wt. % of hydrophilic silica of specific surface area of 200 m² / g

Figure III-4 TEM image for 10S-D330-M: crosslinked MDEA-based system filled with 10 wt. % of hydrophilic silica of specific surface area of 125 m²/g

Figure III-5 TEM image for 10D-D330-M: crosslinked MDEA-based system filled with 10 wt. % of hydrophilic silica of specific surface area of 50 m²/g

III.1.2.b. Effect of the surface modification The morphologies for unmodified silica has just been shown, those for modified silica are reported now distinguishing hydrophobic and reactive (i.e. epoxy and amino-modifed) silica. Elodie Bugnicourt, PhD INSA Lyon, 2005

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Hydrophobic silica The morphologies of MDEA-based systems filled with 5 wt. % of partially and fully hydrophobic silica of specific surface area 200 m²/g are presented. For partially hydrophobic silica (Figure III-6), the same type of fine, homogeneous dispersion state was achieved as with hydrophilic silica. The good interactions between the residual surface silanols (~ 50 %) and epoxy matrix are likely to be involved, and no particle-particle interaction is evidenced. In contrast, for the systems filled with fully hydrophobic silica (Figure III-7), the silica concentration did not appear evenly dispersed macroscopically. In the silica-richer regions, at high magnification, a percolation trend of the silica aggregates was noticed due to particleparticle interactions. This observation is in good agreement with the high viscosity and sol-gel transition for silica suspensions into epoxy prepolymer, as already reported in the previous part (pictures at higher magnification were shown in II.2.2.b).

Figure III-6

Figure III-7

TEM image for 5h-D330-M: crosslinked MDEA-based system filled with 5 wt. % of partially hydrophobic silica

TEM image for 5H-D330-M: crosslinked MDEA-based system filled with 5 wt. % of highly hydrophobic silica


131


Reactive silica The morphologies of MDEA-based systems filled with 5 wt. % of different amino- and epoxymodified silica, of specific surface area 300 m²/g, were observed. For fully amino-modified silica, A, the main part of the aggregates were well-dispersed, but some large (~ 5 µm) and relatively compact agglomerates remained (Figure III-8). The results of further tests carried out in order to solve this poor dispersion state and break the initial agglomerates are reported later on. For partially amino-modified silica, with a substitution of 50 % of the initial silanols using the same amino-silane, the dispersion obtained was almost uniform (Figure III-9), only a few small medium compact regions (< 1 µm) were noticed. Again, the dispersion turned out to be favored by the residuals silanols. A correct dispersion was also achieved for the other partially aminomodified silica (10 %) tested (annex C.1.1.f.). Contrary to fully amino-modified silica, A, and contrary to every other amino-modification (presented in annex C.1.1.f), a good dispersion state could be reached with fully piperazinomodified silica, Ap (Figure III-10), with no notable agglomerates larger than the micron scale.

Figure III-8

Figure III-9

TEM image for 5A-D330-M: crosslinked MDEA-based system filled with 5 wt. % of fully amino-modified silica

TEM image for 5a50-D330-M: crosslinked MDEA-based system filled with 5 wt. % of partially amino-modified silica


132


Figure III-10

TEM image for 5Ap-D330-M: crosslinked MDEA-based system filled with 5 wt. % of piperazino-modified silica

Finally, for epoxy-modified silica by a standard process, E, the dispersion obtained was correct (Figure III-11), with no compact region larger than the micron scale. In contrast, all the morphologies for other epoxy-modifications, reported in annex C.1.1.f., were characterized by the presence of micronic agglomerates (5 - 10 µm). For Em and Ead, the greater part of the silica aggregates were well-dispersed but for epoxy-modified silica by a solvent route, Esol, the dispersion was especially poor. Indeed, this modification resulted in a “micro-composite” type morphology: all the silica was dispersed as large agglomerates and no individually dispersed aggregate was found in the matrix-rich regions.

Figure III-11

TEM image for 5E-D330-M: crosslinked MDEA-based system filled with 5 wt. % of epoxymodified silica

The reasons for the presence of agglomerates depending on silica surface modification were already discussed in the part concerning the morphological evolution during crosslinking step (II.3.3.d.) : to sum up, good initial interactions in the suspensions (condition fulfilled for E and Ap, but probably not for A due to back bonding) are needed to prevent the formation of silica agglomerates otherwise the initial morphology can be frozen from the beginning of the reaction of the functional group (epoxy or amine) on silica surface during the dispersion or crosslinking. Elodie Bugnicourt, PhD INSA Lyon, 2005

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III.1.2.c. Effect of other parameters of the formulation and of the dispersion process The micrographs commented in this paragraph are reported in annex C. Effect of the silica content The effect of the silica content was observed for hydrophilic silica and amino-modified silica A. It was found out that the type of dispersion state obtained at lower silica content, previously presented, is conserved at higher silica loadings: i) optimal dispersion state without direct particle-particle interaction were achieved up to high content of hydrophilic silica (40 wt. % of silica of BET= 50 m²/g into Jeffamine-based system), ii) for amino-modified silica, bigger agglomerates (5 – 10 µm) occluding matrix were observed at higher loading (11 wt. % into MDEA-based systems). Effect of the hardener nature First, it was evidenced that the hardener nature is not a first order factor governing the type of morphology5: i) uniform dispersion for hydrophilic silica and partially hydrophobic were obtained into both MDEA- and Jeffamine-based systems, and ii) the presence of agglomerates for aminomodified silica A was observed with the two types of curing agents. Despite, for highly hydrophobic silica, the morphologies of MDEA- and Jeffamine-based systems were slightly different. For 5H-D330-J (annex C.1.1.d.), the percolation trend was not as obvious as for 5H-D330-M (Figure III-7) and a few residual agglomerates (< 5 µm) were still present after crosslinking. This might be connected to the stronger adsorption of the aliphatic hardener on silica surface screening the particle-particle interactions responsible for the formation of the percolating network and to its higher reactivity freezing the initial morphology. Effect of the dispersion medium and process Due to the problems faced to obtain a fine dispersion state using the standard procedure for amino-modified silica A, additional procedures were tested (morphologies in annex C.1.1.e.). Beforehand, it was checked that agglomerates were present for any dispersion medium i.e. if

5

Consequently, most observations were realized for MDEA-based systems only (the preparation of TEM samples by ultra-microtomy is easier when the glass transition temperature is above room temperature). Elodie Bugnicourt, PhD INSA Lyon, 2005

134


silica dispersion was carried out into either epoxy prepolymer, into Jeffamine, or into both apart. Then, additional dispersing tools were tested: butterfly dissolver, bead mill, 3-roll mill, preparation of a masterbatch followed by a dilution, ultrasonic bath, dispersion using a miniextruder or a kneader, combination of various devices, and eventually solvent-aided dispersion. In spite of the numerous attempts for optimization, agglomerates were always observed for amino-modified silica A. Thus apparently, morphologies are governed by initial interactions between silica and epoxy, and agglomerates are frozen due to the reaction of silica with epoxy.

III.1.2.d. Summary of the morphologies depending on the formulation A mark from A to F was assigned to each system studied to qualify the dispersion state, i.e.: −

A: uniform, optimal dispersion at the level of individual aggregates

−

B: correct dispersion, major part of the silica evenly dispersed at the level of individual aggregates, a few medium compact regions < 1 µm

−

C: percolating morphology, no agglomerate

−

D: a few medium compact silica zones between 1 µm and 5 µm and the major silica part evenly dispersed

−

E: compact agglomerates typically in the range 5 µm, and the remaining silica part evenly dispersed at the level of individual aggregates

−

F: compact agglomerates typically in the range 10 µm, and no silica aggregates dispersed individually (micro-composite type morphology).

Values are summarized in the Table III-1. The same kind of ranking is presented in annex C.1.1.g for additional types of silica surface chemistries that were analyzed during this work. Hardener Silica wt.% N

5 A

MDEA 10 – 11 A

T

A

S

A

D

A

Comment

5 A

A also with 21 wt.%

A also with 40 wt.%

h

A

A

H

C

D

a50

B

A

E

Ap

B

E

B

Table III-1

E

E using any device or procedure

Jeffamine Comment

E

E using any device or procedure, D at low silica content

Classification of the morphologies of the epoxy / silica composites as a function of the formulation (type and content of silica, type of hardener)


135


III.1.3. Image Analysis of TEM micrographs The behavior of fumed silica and fumed silica filled-systems can generally be closely related to the nano-scale and meso-scale morphology of the silica phase [PAP00]. Many studies refer to the fractal character of fumed silica in order to complete the usual euclidean morphometric parameters. In this work, qualitative and quantitative information are needed concerning the silica morphology in the epoxy matrix to help us in the understanding of the relationships between the structure and the properties of the epoxy / silica composites. The research of relevant shape, size and fractal parameters for the analysis of the silica dispersion state was based on the literature. These examples are presented in this paragraph and followed by their application to the analysis of our own experimental results. First, the euclidean parameters resulting from image analysis are reported, and then those of fractal parameters characterizing the morphology of the silica into epoxy matrix.

III.1.3.a. Shape and size characterization of the silica dispersed phase (euclidean morphometric parameters) Methodology Since the morphology of fumed silica and carbon black aggregates are rather similar, the procedure for the analysis of silica morphology into epoxy matrix used in this work was partially inspired from that developed for the study of carbon black. Indeed, in the past, the morphology of carbon black, in the dry state, or in situ into filled rubber, has been largely investigated [HER92, 93], for example for tire production. More recently, studies were carried out for the same goal regarding precipitated silica in “green tires” allowing an enhancement of the rolling resistance and a decrease of fuel consumption [CON05]. Indeed, elastomers reinforcement is known to be strongly related to the dispersion state of the filler because: i) it defines the level of interactions potentially developed with the rubber and ii) the ability of the aggregates/ agglomerates to occlude polymer. Various morphological descriptors can be chosen in order to refer of the dispersion state of a filler. The criterion used by Costa, Conzatti et al. [COS03, CON05] was the area of the indivisible structure in the frame. The area over perimeter ratio can also be a significant parameter [FAL03, CON05]: it decreases as the dispersion improves. It also characterizes the intensity of the interactions with the matrix: for a same area, if the filler tends to favor the interactions with the matrix, the perimeter increases and the ratio over perimeter decreases.


136


The morphology is additionally described by the anisotropy and the bulkiness of the aggregate, which can give an idea of the quantity of matrix immobilized, as illustrated in the Figure III-12.

Figure III-12 Illustration of the parameters of anisotropy, bulkiness for carbon black aggregates [HER93]

Results for epoxy / silica composites In this work, image analysis of TEM micrographs provided values about the shape and dimensions of the isolated structure in the systems (aggregates or agglomerates): size, convex and standard perimeter and area, shape factor, aspect of the skeletonized pictures. Some of these parameters are illustrated and compared for different composite formulation later on. Detailed elements on the calculations of morphometric parameters used as well as the procedure used are reported in annex A.4.3. Frames presenting large enough number of undivisible structures were analyzed for the statistical study to be relevant. The main parameters investigated here are the influence of the silica surface chemistry and hardener nature. The networks analyzed were all filled at 5 wt. % with: hydrophilic silica of 200 m²/g (N), partially hydrophilic silica (h), highly hydrophobic silica (H), or amino-modified silica (A) and crosslinked with MDEA or Jeffamine (respectively Figure III-13 vs. Figure III-14 at the same scale of observation).

silica:

N

Figure III-13

h

H

A

View of the morphologies analyzed by image analysis for MDEA-based systems filled with 5 wt. % of different types of silica (frame length: 2 µm)


137


silica: Figure III-14

N

h

H

A

View of the morphologies analyzed by image analysis for Jeffamine-based systems filled with 5 wt. % of different types of silica (frame length: 2 µm)

The results are summarized in the Table III-2 and represented as histograms in the Figure III-15 to Figure III-17. At a first glance, two populations appear depending on the hardener nature for the following reasons: i) due to the interactions developed in the systems, and ii) because the microtomed cuts are thicker in case of rubbery systems at room temperature (~100 nm for Jeffamine systems against ~60 nm for MDEA-based systems) giving the illusion of a higher silica content. Thus, undivisible aggregates / agglomerates appear bigger on the 2D frame if a “multi-layer” of silica is projected. Additionally, it should be noted that the threshold can not take into account at the same time the well-dispersed aggregate and the big agglomerates in a satisfying way on the same image. These structures must be treated distinctly. In case of presence of agglomerates, i.e. for the silica A, results provided are for the evenly dispersed regions in the sample. The sizes are biased for these samples and the occluded matrix in the agglomerates could not be evaluated. System mean Feret (nm) area (nm²) aspect ratio mean roudness bulkiness P/A 77 3604 1.63 4.00 1.84 0.114 5N-D330-M 73 3141 1.55 3.69 2.01 0.118 5h-D330-M 136 7439 1.58 3.97 2.08 0.080 5H-D330-M 190 28008 1.56 3.45 1.85 0.037 5A-D330-M 165 19246 1.59 3.76 1.89 0.046 5N-D330-J 122 11399 1.62 3.29 2.00 0.055 5h-D330-J 198 35206 1.63 3.91 2.08 0.033 5H-D330-J 154 20089 1.61 3.46 1.85 0.041 5A-D330-J Table III-2 Comparison of the average parameters of dimensions and shape of the aggregates first for the MDEA-based systems, and then for the Jeffamine-based systems filled with different silica

The size of the individual structures was evaluated by the average Ferets length6 and area (Figure III-15). The hardener nature prevails for the size variations of the dispersed silica 6

The Ferets are the lengths intercepted in 8 directions on the object, their mean value was regarded as the size of the object in our calculations. Elodie Bugnicourt, PhD INSA Lyon, 2005

138


aggregates. For a given nature of hardener, the changes depending on silica surface modification are rather similar: finest dispersion for hydrophilic silica and largest structures for hydrophobic silica and amino modified silica. The variations seem fully consistent with the morphologies observed (Figure III-13 vs. Figure III-14).

Geometric mean of Ferets: F = L.W where L is the longest and W is the shortest Feret

M 33 0-

5h -D

-D 5N

-D 33 0J 5h -D 33 0J 5H -D 33 0J 5A -D 33 0J

5N

33 0M

33 0M

5A -D

33 0M

5H -D

33 0M

5h -D

33 0M 5H -D 33 0M 5A -D 33 0M 5N -D 33 0J 5h -D 33 0J 5H -D 33 0J 5A -D 33 0J

40000 35000 30000 25000 20000 15000 10000 5000 0

200 180 160 140 120 100 80 60 40 20 0

5N -D

Area A: A = N.a where N is the number of pixels covering the object and a is the single pixel area

(a)

(b)

Figure III-15 (a) Mean Ferets length (nm), (b) Mean area (nm²), first for the MDEA-based systems, and then for the Jeffamine-based systems filled with 5 wt. % of different types of silica

The mean aspect ratio does not change in a large range from one system to another, with a mean value around 1.6 for the different systems studied (Figure III-16(a)). The roundness parameter (Figure III-16(b)) is generally lower for amino-modified silica (tendency to form more spherical aggregates) than for highly hydrophobic and hydrophilic silica, characterizing a higher linearity of the aggregates. These variations are in agreement with those of the bulkiness showing more compact structures in case of amino-modified silica (Figure III-17 (a)). The quantity of immobilized matrix in these space filling structures can be evaluated by the bulkiness parameter which is in average c.a. 2, i.e. the apparent volume fraction of an aggregate occluding matrix is twice the real volume fraction of silica. For the perimeter over area ratio (P/A, Figure III-17 (b)), the interactions with the medium are favored for the system presenting a high value of this parameter. The same variations are observed for the two hardeners, that is to say: favored interactions in case of hydrophilic silica and partially hydrophilic silica due to silanols, unfavored interactions for amino-modified silica, that tends to agglomerate, and highly hydrophobic silica for which interparticle interactions prevail. This is consistent with the respective surface energy of the respective of the components: around 40 mJ/m² for epoxy-amine matrix [CHE91a, b], which presents thus a higher wettability on hydrophilic silica (~72 mJ/m²) than on highly hydrophobic silica surface (~33 mJ/m²) [BAR95]. Elodie Bugnicourt, PhD INSA Lyon, 2005

139


b

Aspect ratio (or shape factor): b/a for the equivalent ellipsoid

Roundness:

R=

P² 4 πA

R=1 for a sphere

a 4.5

1.64 1.62

4.0

1.60 1.58

3.5

1.56 1.54

3.0

1.52

5N

5N

-D 33 0M 5h -D 33 0M 5H -D 33 0M 5A -D 33 0M 5N -D 33 0J 5h -D 33 0J 5H -D 33 0J 5A -D 33 0J

2.5


1.50

(a) Figure III-16

(b)

(a) Mean aspect ratio, (b) Mean roundness, first for the MDEA-based systems, and then for the Jeffamine-based systems filled with 5 wt. % of different types of silica

Bulkiness: ratio convex area over area

B=

Ratio perimeter over area: P A

A A

c

\\\ Convex Area ■ ■ Area 0.120

2.10 2.05 2.00 1.95 1.90 1.85 1.80 1.75 1.70

0.100 0.080 0.060 0.040 0.020

(a)

33 0J

33 0J

5A -D

-J

-D

33 0

5H

33 0J

5h -D

-D

33 0M

5N

33 0M

5A -D

-M

-D

33 0

5H

5h -D

33 0M -D 5N

5N


-

(b)

Figure III-17 (a) Mean bulkiness, (b) Mean ratio perimeter over area P/A, first for the MDEA-based systems, and then for the Jeffamine-based systems filled with 5 wt. % of different types of silica

Furthermore, it was found out that the shape of the indivisible agglomerates / aggregates in a given system was independent of their size: the plot of the convex perimeter as a function of the perimeter and that of the convex area as a function of the area are linear for every aggregate analyzed. The average aspect ratio and roundness were also constant all over variation range of aggregates area (plots reported in annex C.1.2.).


140


III.1.3.b. Analysis of the fractal geometry of the silica dispersed phase The preamble of this paragraph is devoted to the presentation of a few basics about fractal geometry and the mathematical background needed for the study of the fractality of fumed silica by image analysis and SANS later on. The methodology and results are then reported. Introduction Fractal structures can be found everywhere in the nature (well-known example of the profile of the Brittany coast, but also mountains, vegetation, clouds…). However, theories about fractal geometry have been developed rather recently, mostly in the 70’s, in particular by Benoît Mandelbrot [MAN84] who introduced the concept of dilation symmetry or self similarity. In fact, Euclidean concepts allowed gathering structures depending on their degree and order of symmetry; however it could not afford for random, irregular objects. The dilation symmetry consists, in contrast, in the invariance of the structure under a change of scale, the objects presenting this character are self similar or fractal structures. Most materials display a fractal morphology at the microscopic level (silica - Figure III-18- and gold aggregates, coal, polymeric gel structure…). According to the definition, a fractal object should display the same structure over countless decades. However concerning real objects, the scaling range in which they present a fractal character is most of the time narrow, but the fractal notions can still be used in order to quantify the structure in this limited range [MAR02]. Physical techniques to investigate silica fractality are based either on gas adsorption (BET measurements for silica as a powder) or on scattering experiments (neutron, X-ray, light).

375 nm 8.5 µm Figure III-18

Images illustrating the fractality of fumed silica at different levels of organization (agglomerate, aggregate) [BAR98]

Mathematical background A simplified theory of fractal, needed for the obtaining of the parameter of fractal dimensions in this work, is presented here from Mandelbrot’s theories [MAN84, MAR87, GOU92].


141


For Euclidian objects, the ratio ρ =

P is constant (P: perimeter, A: area), whereas for fractal A 1/2

objects ρ is a non-intensive value. That is to say when the dimension of the “yardstick” tends towards 0, then the length of the object diverges (cf. length of the Brittany coast). Thus, for fractals objects, Mandelbrot defined the divergent ratio:

P A

1/Dp

ρ = D

=

1/2

P , i.e. P ∝ A Dp/2 A

Equation III-1

Dp /2

where DP is the fractal dimension in two dimensions, respecting the condition: 1 (circle) ≤ Dp ≤ 2 (infinitely rugged perimeter). The perimeter fractal dimension, Dp, defined for bi-dimensional structures, can be obtained from the slope of the log-log plot of the perimeter vs. the area. Following the same approach in 3D, the mass fractal dimension Dm can be obtained. The scaling behavior gives the following relation between the number N of primary particles of radius a included in a sphere of radius R, R>a (Figure III-19):

⎛R ⎞ N=⎜ ⎟ ⎝a⎠

Dm

Equation III-2

R

Figure III-19

Diagram representing a fractal object [DOR95]

Thus the mass of the structure is as follow:

M ∝ R Dm with

Equation III-3

1 (line) ≤ Dm ≤ 3 (compact object, sphere).

Note that the fractal dimension is always lower (or equal) than the Euclidean dimension. Consequently, the mass fractal dimension, Dm, can be obtained from the slope of the log-log plot of the mass vs. the radius. The value of Dm is related to the openness of a structure, the smaller Dm, the more open the structure. For real objects, the range of the fractal character is limited and above a certain length scale, called cut-off length, ζ, the density variation can no longer be observed, so that the fractal parameter is relative to the observation scale. Methodology The procedure for the analysis of silica fractality into epoxy matrix used in this work was Elodie Bugnicourt, PhD INSA Lyon, 2005

142


inspired from that developed by Herd et al about carbon black. From online automated image analysis of TEM micrographs on many different grades of carbon black, Herd et al [HER92] obtained values of the perimeter fractal dimension, Dp, included between 1.05 and 1.36, and mass fractal dimension, Dm, included between 2.28 and 2.84. These two parameters correlated inversely. Herd related the values of fractal dimensions to the parameters of Euclidean geometry traditionally used to classify carbon black in 4 categories as a function of the shapes of the aggregates (Figure III-20). The perimeter fractal dimension might appear somehow difficult to “conceptualize” physically; however, it is worth noting that, in case of carbon black, the values could be well related with the absorption of DBPA (dibutylphtalate). This method is traditionally used in order to evaluate the specific surface area of carbon black: an increased DBPA adsorption corresponds to an increased roughness of the surface.

Experimental range found:

Figure III-20

−

Dp=1.02-1.08 for spheroidal aggregates

−

Dp=1.14-1.21 for ellipsoidal aggregates

−

Dp=1.27-1.33 for linear aggregates

−

Dp=1.31-1.44 for branched aggregates

Different categories of shape of carbon black aggregates [HER92]

Results for epoxy / pyrogenic silica composites The perimeter fractal dimension (two-dimensional) was obtained thanks to the slope of the loglog plot of the projected surface as a function of the perimeter (after the equation III-1) as shown in Figure III-21(a). The aggregate mass was calculated according to a model used by Herd [HER92]: 8 A² M = ρ.Vagg = ρ. . 3 P

Equation III-4

where: A is the area and P is the perimeter of the TEM two-dimensional projected image of the aggregate, and ρ is the silica density (ρ = 2.2 g.cm-3) The mass fractal dimension was deduced from to the slope of the log-log plot of the mass as a function of the size of the aggregates (after the equation III-3) as shown in Figure III-21 (b).


143


6

Dp/2= 0,69 log (M) (AU: pixels3)

log A (AU: pixels²)

4 3 2 1 0 0

1

2

3

4

Dm = 2,46

5 4 3 2 1 0 0

0.5

log P (AU: pixel)

1

1.5

2

2.5

log (R) (AU: pixels)

(a)

(b)

Figure III-21 Determination of the fractal dimensions: example for the system 5A-D330-M (a) log-log plot aggregate area vs. perimeter, Dp: perimeter fractal dimension, (b) log-log plot aggregate mass vs. size, Dm: mass fractal dimension

Results of fractal dimensions are reviewed in the Table III-3, and compared to the model shape found out by Herd for carbon black (cf. Figure III-20). The variations from one silica to another are not too large and, again, two populations appear as a function of the nature of the hardener. Although conform to the expected trend, the model shape deduced from Herd’s study does not appear as a sufficiently sensitive selector for the description of the systems under study. System 5N-D330-M 5h-D330-M 5H-D330-M 5A-D330-M 5N-D330-J 5h-D330-J 5H-D330-J 5A-D330-J

Dp 1.33 1.41 1.42 1.39 1.25 1.28 1.28 1.26

Dm 2.49 2.3 2.35 2.46 2.6 2.55 2.59 2.62

Model geometry Linear / branched Branched Branched Branched Linear Linear Linear Ellipsoidal / linear

Table III-3 Comparison of the average fractal dimensions of the aggregates for the systems filled with different types of silica, and corresponding model shape deduced from Herd’s study [HER92]

However, the evaluation of the fractality of three-dimensional objects based on the analysis of the projection in 2D of a thin layer (typically 60 to 80 nm) leads to the question: “how to obtain reliable information and quantitative information about the 3D structuration?” [PAP00]. Rigorously, the determination of the fractal dimension from the 2D-projection is only valid for fractal dimension lower than two. Moreover in the fractal theories, some infinite objects are considered whereas the number of primary particles composing an aggregates is finite, this can bias the result. Jullien et al. [SIG01] compared the results obtained from a numerical model and the ones measured from image analysis of TEM micrographs, and concluded that image analysis leads to a large underestimation of the fractal dimension. Scattering technique was used to obtain complementary information about the size and fractal geometry of the aggregates without the drawback just underlined as it can give access to the three-dimensional morphology. SANS results are presented in the following paragraph. Elodie Bugnicourt, PhD INSA Lyon, 2005

144


III.1.4. Structural study by small angle neutron scattering As previously presented (II.3.3.c.), one of the objectives of small angle neutron scattering (SANS) experiments was the investigation of the evolution of silica dispersed phase morphology during the polymerization of the matrix. More especially, SANS was used for the characterization of the crosslinked epoxy / silica composites depending on silica specific surface area and surface chemistry. This technique allows analyzing the integrality of the sample in the reciprocal space, in contrast with direct local observation by TEM. Quantitative morphological parameters were deduced from SANS measurements, which results are reported and discussed in this paragraph. The theory of SANS and experimental conditions and detailed in annex A.4.4.a.

III.1.4.a. Introduction on SANS experiments The crosslinked samples analyzed were all based on the aromatic hardener filled with a silica content fixed at 5 wt. % (i.e. ~2.6 vol. %) to fulfill the semi-diluted conditions (c< c*, c* being the critical concentration for which direct interactions between neighboring aggregates exist) according to TEM frames. The silica used were hydrophilic silica of specific surface area of 50, 125, 200 and 300 m²/g, and reactive silica of 300 m²/g either amino-modified (A, a50, Ap) or epoxy-modified. SANS measurements were carried out at the LLB (Laboratoire Léon Brillouin: laboratoire commun CEA-CNRS) in Saclay. The patterns were recorded at room temperature mainly on the spectrometer (PAXE) devoted to anisotropic scattering, except those for the composites filled with the amino-modified silica A and the hydrophilic silica N that were recorded on the instrument dedicated to isotropic scattering (PACE) and that filled with partially amino-modified silica a50 that was recorded on the second anisotropic spectrometer (PAXY). The principle of the spectrometer is illustrated in the Figure III-22 and the features of the spectrometer are reported in annex A.4.4.b.


145


Figure III-22

Diagram of a small angle neutron scattering spectrometer with an anisotropic detector X,Y type of that used in this study (PAXE) [LLB05]

The multi-scale structures present in the medium – i.e. indivisible aggregates of about 200 nm, formed of elemental spherical particles of about 10 nm diameter, gathered in agglomerates measuring up to a few microns – required to use various configurations (λ, D) to cover a wide scattering vector range, as defined in the equation:

q=

4π 2π x ² + y ² . sin θ ≈ . λ λ D

Equation III-5

Where: (x, y) is the position of the cell on the detector, λ is the wave length of the neutron beam, and D is the distance between the sample and the detector. With: 2.5 m < D < 5 m and 6 < λ < 25 A°, the overall q-range covered varied from 2. 10-3 up to 0.15 A°-1, i.e. Bragg distances between 4 and 300 nm were accessible (annex A.4.4.b.). In order to obtain the absolute intensity (in cm-1), the raw patterns were corrected by subtraction of the incoherent background, normalized using water as calibration standard (equation B-1), and the different configurations were gathered on the same plot (procedure in annex A.4.4.c).

III.1.4.b. Procedure for the structural analysis from SANS measurements based on examples from the literature Colloidal and fumed silica has been widely studied in the literature as model objects presenting a multi-level structure or for the study of the aggregation phenomena [HAS95, HU01, COU03, BOT03, BER03]. For instance, Beaucage et al. studied the structuration of fumed silica in a rubbery matrix [BEA95]. Thanks to the combination of various scattering techniques (light, USAXS and SAXS), the pattern could be reconstituted in a large range of scattering vector (Figure III-23). Three regions were distinguished on the intensity pattern, in agreement with the well-known triple structure of fumed silica. The presence of micronic size agglomerates (Rg1 ~ 5 µm), aggregates (Rg2 ~ 300 nm), and primary particles measuring few nanometers (Rg3 ~ 9 nm) were evidenced. The radius of gyration were obtained by fitting the curve to the unified Beaucage equation with n = 3 structural levels (equation A-21 in annex A). Elodie Bugnicourt, PhD INSA Lyon, 2005

146


Figure III-23 Experimental data of Small angle light, Bonse-Hart X-ray, and pinhole X-ray scattering for precipitated silica (Hi-Sil®, surface BET= 150m²/g) in a rubber matrix [BEA95]

The approach used for the analysis of the SANS patterns was mainly inspired by that developed by Beaucage just reported. However, in the present study, the agglomerates potentially formed were not accessible (> 300 nm). An example of scattering pattern I(q), represented in log-log plot, with the features resulting from the analysis of the different regions is given in the Figure III-24. From low-q to high-q values, the primary information worked out from such a plot are: −

shape at low-q and position qmax in case a maximum is observed

−

−α slopes α1 and α2 of the two successive power law behaviors of the shape: I(q) ∝ q

−

the cross-over qc-o1 and qc-o2 between the different regimes of the plot. 1000

Aggregate

100

10 -1

I (cm )

Guinier regime

Primary particle

α1~ 2

1

Porod regime 0.1

0.01 0.001

α2 ~ 4 qmax qc-o1

q (A°

Figure III-24

qc-o2

0.01 -1

0.1

)

SANS pattern for a MDEA-based crosslinked sample filled with 5 wt. % of hydrophilic silica (5N-D330-M)


147


Then, from these values, structural parameters can be extracted as follow: −

The first slope α1 at lower q, is included between -2 and -3. This zone (of the type “Guinier-region”) characterizes the mass fractal distribution of the primary aggregates which mass fractal dimension is: D m = α 1 .

−

The second slope α2, at high-q, is included between -3 and -4. This zone (so-called “Porod- region”) characterizes the primary particles of surface fractal dimension: Ds = 6 - α2 .

−

In order to determine the size of the primary particles, an approximation used for example by Boyard et al. [BOY03] as an alternative to the fitting the patterns with Beaucage’s unified equations, the gyration radius of these particles, Rg2, was approximated by the inverse of the cross-over between the 2 linear regions, qc-o:

R = g2

1 q

Equation III-6

c −o 2

Additionally, for compact spherical morphology, such as silica primary particles, the gyration radius Rg is related to the real radius R by various authors [ESP90, HAJ99b]:

R² =

5 .R ² g 3

Equation III-7

This eventually allowed calculating the geometrical diameter of the spherical primary particles constituting the aggregate, φpp=2R, in A°:

φ = 2. pp

5 1 . 3 q

Equation III-8

c −o 2

Besides, the model used to determine the size of the primary particles, can be credited by papers quoted from the literature. For example, El Harrak et al. [ELH04] studied colloidal silica sol suspensions using SANS. They fitted the results to a log-normal distribution and found a mean size of Rg= 50.9 A° (Figure III-25). In first approximation, if the cross-over between the two regimes is determined, a value of qc-o = 2. 10-² A°-1 is worked out i.e. Rg= 5 nm (equation III-6). For a spherical morphology, it leads to a diameter of 13 nm (equation III-8) in good agreement with the diameter of 14 nm announced by the supplier.


148


Figure III-25 SANS pattern and size distribution for a colloidal silica suspension in dimethylacetamide (φSi = 0.25 %) [ELH04]

−

Sizes characteristic of the aggregates can be worked out in the left part of the “Guinier-region”. The gyration radius of the aggregates, Rg1, is obtained by determination of the cross-over with the low-q regime, corresponding to the inflexion of the curve:

R = g1

1 q

Equation III-9

c −o1

The geometric size (radius R) of the fractal aggregate can be determined after the equation III-8, using the size of the primary particles and the fractal dimension obtained from the spectra. Although the aggregate mass can be deduced from the maximum of scattering intensity, it would require a level of accuracy of the absolute value which can not be guaranteed here.

4 ⎛R ⎞ M = ρ. .π.a .⎜ ⎟ 3 ⎝a⎠

Dm

i.e.

3

.

⎛ 3.M ⎞ R=⎜ ⎟ ⎝ 4.ρ.π ⎠

− Dm

a

Dm − 3

Equation III-10 .

So that, the gyration radius was used to compare the aggregate sizes in the different systems assuming the geometric size (radius R) is proportional to the gyration radius since the fractal dimension are not too different from one system to another (coefficient α such as R = α.Rg, function of the aggregate fractal dimension, regarded as constant). −

Finally, the shape of the I(q) plot in the low-q region was reported (maximum, plateau or not accessible in the range observed). Indeed, it can provide further information on the organization of the scattering objects in the medium. The appearance of a peak at low-q, which was sometimes observed in the scattering profile, can be related to interparticle interactions (structure factor) leading to a long distance correlation between the scattering units [BAR98b, COT99]. In certain cases, a preferred spacing between aggregates can be calculated in first approximation according to Bragg law, calculated as:

d

aggreg

=

2π q

Equation III-11

max


149


Qiu et al. [QIU05] characterized the behavior of aqueous colloidal silica suspensions (particles of 25 nm, bare or coated with PEO). Contrast matching of the solvent to the adsorbed layer was realized in order to screen its contribution to scattering. Experimental data (Figure III-26) exhibited a peak of scattering at low-q and were well described by a structure factor based on a repulsive potential of the form:

⎡ κ( ω − d) ⎤ U( ω) = dU 0 . exp ⎢ − ω ⎥⎦ ⎣

Equation III-12

where d is the diameter of the particle, U0 is the potential depth, ω is the interparticle distance that could be deduced from the measurement by fitting the data. Stabilization by electrostatic repulsion was involved in case of the dispersion of the bare particles whereas steric interactions took place between the PEO for modified particles. The spacing between particles, deduced from the patterns, decreased as the concentration increased (Figure III-26).

(a)

(b)

Figure III-26 SANS patterns (a) for suspensions of bare silica particles at various concentrations: ∇: 1%, □: 6%, ○: 11%, ∆: 16%. (b) for suspensions of PEO stabilized silica at various concentrations ∇: 1%, □: 6%, ○: 11%, ∆: 16%. Data scaled to the same silica volume fraction [QIU05]

In the following tables summarizing SANS results, the features reported are: the shape of I(q) at low q, qc-o1, Rg1, Dm, qc-o2, φpp and Ds (as defined previously).

III.1.4.c. Results for epoxy / silica composites Structural information resulting from SANS patterns of crosslinked epoxy / silica composites were analyzed and discussed in the light of the TEM observations and of the results of image analysis previously presented.


150


Comparison between pristine silica and silica in the epoxy-based composite First, log-log plots of the scattering intensity versus the scattering vector for the pristine silica (as powder between glass plates) and for the epoxy network filled with 5 wt. % of the same hydrophilic silica (200 m²/g) are shown in the Figure III-27. For the former, the increase of scattering intensity at low-q is continuous, whereas for the latter, a noteworthy maximum of scattering is observed at about 2.4 10-3 A°-1. Although the silica concentration respected the semi-diluted conditions, a certain correlation between scattering units was expressed by the interaction peak. Indeed, it can be connected to the existence of a preferred distance of 210 nm between aggregates (calculated by the equation III-11). The peak is furthermore pronounced in linear scale and can not be considered as an artifact. Indeed, the absence of maximum was checked for really diluted silica suspension in deuterated THF (~1 vol. %) due to the absence of interaction in between the scattering units. In case of silica powder, the peak is also absent because no most probable inter-particular distance exists, no long distance organization is present in the sample. The other features of the two plots for the pristine silica and for the filled epoxy network are hopefully rather similar because the scattering object is the same, the numerical results can be found in Table III-4. Those of the crosslinked filled network are also commented afterwards in comparison with other types of silica.

5N-D330-M

10000

N (powder) 1000

-1

I (cm )

100 10 200nm

1 0.1 0.01 1E-3

1E-2

1E-1

1E+0

-1

q (A° )

Figure III-27 SANS patterns I(q) for: ♦ crosslinked epoxy-based composite filled with 5 wt. % of hydrophilic silica and ■ pristine silica (as a powder). On the right: reminder of the composite morphology


151


Sample Shape at low-q qc-o1 (A°-1) 2 Rg1 (nm) Continuous increase N 0.0040 50 5N-D330-M Max: daggreg= 210nm

Dm 2.28 2.23

qc-o2 (A°-1) 0.027 0.024

φpp (nm) 9.9 10.9

Ds 2.3 2.19

Table III-4 Structural features obtained from the analysis of SANS patterns of the pristine hydrophilic silica of surface BET 200 m²/g (powder) and of the crosslinked epoxy-based composite filled with 5 wt. % of the same silica

Let us just discuss briefly if the dimensions worked out are reasonable. The scattering peak observed for the epoxy filled system characterizes a repulsion-type behavior between scattering aggregates (cf. reference [QIU05] already quoted). This behavior was related to the favored particle-polymer interactions compared to particle-particle interactions responsible for the stabilization of the optimal dispersion state and appears fully consistent with the morphology reminded in the Figure III-27). Additionally, the distance found between scattering units of about 200 nm is reliable. The size of the primary particles (~10 nm) is in the range expected and is discussed in further details as a function of silica specific surface area in the following paragraph. The gyration diameter of the aggregates is known to be largely lower than the geometrical size especially for fractal open objects such as fumed silica aggregates. It is in the same range as half the mean Ferets length worked out by image analysis as illustrated in the Figure III-28. Consequently, it can be underlined that the values deduced from SANS patterns are rather consistent with the structure observed and the model used are validate.

Rg ~mean Ferets/2

Figure III-28 Correlation of the radius of gyration from SANS analysis and results of image analysis

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