Clay-Based Polymer Nanocomposites: Research and ...

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Copyright © 2005 American Scientific Publishers All rights reserved Printed in the United States of America

Journal of Nanoscience and Nanotechnology Vol. 5, 1574–1592, 2005

Clay-Based Polymer Nanocomposites: Research and Commercial Development Q. H. Zeng,1 A. B. Yu,1 G. Q. (Max) Lu,2 ∗ and D. R. Paul3 1

School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia 2 ARC Centre for Functional Nanomaterials, The University of Queensland, Brisbane, QLD 4072, Australia 3 Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712, USA

This paper reviews the recent research and development of clay-based polymer nanocomposites. Clay minerals, due to their unique layered structure, rich intercalation chemistry and availability at low cost, are promising nanoparticle reinforcements for polymers to manufacture low-cost, lightweight and high performance nanocomposites. We introduce briefly the structure, properties and surface modification of clay minerals, followed by the processing and characterization techniques of polymer nanocomposites. The enhanced and novel properties of such nanocomposites are then discussed, including mechanical, thermal, barrier, electrical conductivity, biodegradability among others. In addition, their available commercial and potential applications in automotive, packaging, coating Delivered and pigment, materials, and in to: particular biomedical fields are highlighted. Finally, byelectrical Publishing Technology Purdue University Libraries IP: future 50.49.22.37 On: Tue, 20 Oct 13:11:08 the challenges for the are discussed in terms of 2015 processing, characterization and the mechScientific Publishers anisms governing the Copyright: behaviour ofAmerican these advanced materials.

Keywords: Polymers, Nanocomposites, Clay Minerals, Layered Silicates, Processing and Characterization, Materials Properties, Commercialization, Drug Delivery, Biomaterials.

CONTENTS 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Clay Minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Layered Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. General Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Surface Modification . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Polymer Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Processing and Characterization . . . . . . . . . . . . . . . . . . . . 3.2. Basic Properties of Nanocomposites . . . . . . . . . . . . . . . . 4. Applications and Commercial Development . . . . . . . . . . . . . . . 4.1. Applications in Automotive Components . . . . . . . . . . . . . 4.2. Packaging Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Nanocomposite Coatings and Pigments . . . . . . . . . . . . . . 4.4. Nanocomposites in Electromaterials . . . . . . . . . . . . . . . . . 4.5. Nanocomposites for Drug Delivery . . . . . . . . . . . . . . . . . 4.6. Applications of Nanocomposites in Sensors and Medical Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7. Miscellaneous Applications . . . . . . . . . . . . . . . . . . . . . . . 5. Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗

Author to whom correspondence should be addressed.

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1. INTRODUCTION 1574 1576 1576 1577 1577 1577 1577 1578 1584 1585 1586 1586 1586 1587 1587 1588 1588 1589

In the past decade, polymer nanocomposites have emerged as a new class of materials and attracted considerable interest and investment in research and development worldwide. This is largely due to their new and often much improved mechanical, thermal, electrical and optical properties as compared to their macro- and micro-counterparts. In general, polymer nanocomposites are made by dispersing inorganic or organic nanoparticles into either a thermoplastic or thermoset polymer. Nanoparticles can be three-dimensional spherical and polyhedral nanoparticles (e.g., colloidal silica), two-dimensional nanofibers (e.g., nanotube, whisker) or one-dimensional disc-like nanoparticles (e.g., clay platelet). Such nanoparticles offer enormous advantages over traditional macro- or micro-particles (e.g., talc, glass, carbon fibers) due to their higher surface area and aspect ratio, improved adhesion between nanoparticle and polymer, and lower amount of loading to achieve equivalent properties. One general approach to prepare polymer nanocomposites is to employ intercalation chemistry of layered inorganic solids in which polymer 1533-4880/2005/5/1574/019

doi:10.1166/jnn.2005.411

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Clay-Based Polymer Nanocomposites: Research and Commercial Development

Prof. Aibing Yu was specialized in ferrous metallurgy obtaining his B.Eng. in 1982 and M.Eng. in 1985 from Northeastern University (China) and Ph.D. in 1990 from the University of Wollongong (Australia). He then worked as a postdoctoral fellow with CSIRO Division of Mineral & Process Engineering (1990–1991) and a research fellow at the University of Wollongong (1992). Since June 1992, he has been with the School of Materials Science and Engineering, the University of New South Wales (Australia), as Lecturer (1992–1995), Senior Lecturer (1995–1998), Associate Professor (1998–2001) and Professor (2001). He is also Inaugural Director of a university multidisciplinary research centre “Centre for Simulation and Modelling of Particulate Systems”. His research expertise is mainly in particulate science and process engineering. He has published over 300 papers in refereed international journals and conference proceedings. He is an elected Fellow of Australian Academy of Technological Sciences and Engineering (ATSE).

Delivered by Publishing Technology to: Purdue University Libraries Prof. Gaoqing (Max) Lu is currently Federation and Director, Australian Research IP: 50.49.22.37 On: Tue, 20 Oct 2015Fellow, 13:11:08 Council Centre for Functional Nanomaterials. He holds Copyright: American Scientific Publishers the Chair of Nanotechnology in Chemical Engineering in the School of Engineering at University of Queensland. His area of interests include nanoparticles, layered materials, nanocomposites and nanoporous materials for chemical functional applications. Graduated from Northeastern University, China with B.Eng. (1983), M.Eng. (1986), Lu moved to Australia to do his Ph.D. in Chemical Engineering at the University of Queensland in 1987. Upon completion of his PhD in 1991, he became a lecturer in Nanyang Technological University Singapore till 1994. Since 1994, he has been senior lecturer, associate professor and professor in the Chemical Engineering Department of University of Queensland. He is Fellow of the Australian Academy of Technological Sciences and Engineering (ATSE) since 2002, and won many prestigious awards and honours including the Orica Award, Le Fevre Prize, RK Murphy Medal, and Top 100 Engineers in Australia. Prof. Donald R. Paul holds the Ernest Cockrell, Senior Chair in Engineering at the University of Texas at Austin and is also the Director of the Texas Materials Institute. He received degrees in Chemical Engineering from North Carolina State University (B.S.) and the University of Wisconsin (M.S. and Ph.D.) and then worked at the Chemstrand Research Center for two years. He joined the Department of Chemical Engineering at the University of Texas at Austin in 1967 where he served as department chairman during 1977–85. His research interests include the broad areas of polymer science and engineering and chemical engineering. Current research involves various aspects of polymer blends, membranes for separation, drug delivery, packaging, processing, and nanocomposites. His interests in nanocomposites are relatively new and include developing strategies for exfoliation of clays in polymers by melt processing and understanding the properties of the resulting materials. He has edited numerous books on blends and membranes and published over 520 papers in the areas mentioned above. He is a member of the National Academy of Engineering since 1988 and the Mexican Academy of Science since 2000. He has served as Editor of Industrial and Engineering Chemistry Research, published by ACS, since 1986. J. Nanosci. Nanotech. 5, 1574–1592, 2005

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Qinghua Zeng obtained his B.Eng. in 1989 from Southern Institute of Metallurgy (now Jiangxi University of Science and Technology), China. He worked as a research engineer and did his postgraduate studies at General Research Institute for Non-ferrous Metals (1989– 1995) and University of Science and Technology Beijing (1995–1999). In 1999, he moved to the University of New South Wales (UNSW), Australia, as a visiting fellow, where he completed his PhD in Materials Science and Engineering. He is currently a research associate with UNSW and ARC Centre for Functional Nanomaterials. His research interests include polymer nanocomposites, layered solid materials, molecular modeling and simulation, materials and surface chemistry. He has published over 30 scientific publications in refereed journals and conference proceedings.

Clay-Based Polymer Nanocomposites: Research and Commercial Development Table I.

Clay minerals used for polymer nanocomposites.

Type of clay

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2:1 type MMT Hectorite Saponite Fluorohectorite Laponite Fluoromica (Somasif) 1:1 type Kaolinite Halloysite Layered silicic acid Kanemite Makatite Octasilicate Magadiite Kenyaite

Formula

Origin

Substitution

Layer charge

Mx (Al2−x Mgx Si4 O10 (OH)2 · nH2 O Mx (Mg3−x Lix Si4 O10 (OH)2 · nH2 O Mx Mg3 (Si4−x Alx O10 (OH)2 · nH2 O Mx (Mg3−x Lix Si4 O10 F2 · nH2 O Mx (Mg3−x Lix Si4 O10 (OH)2 · nH2 O NaMg25 Si4 O10 F2

N N N S S S

Octahedral Octahedral Tetrahedral Octahedral Octahedral Octahedral

Negative Negative Negative Negative Negative Negative

Al2 Si2 O5 (OH)4 Al2 Si2 O5 (OH)4 · 2H2 O

N N

— —

Neutral Neutral

Na2 Si4 O9 · 5H2 O NaHSi2 O5 · 7H2 O Na2 Si8 O17 · 9H2 O Na2 Si14 O29 · 10H2 O Na2 Si20 O4 · 10H2 O

N/S N/S S N/S S

Tetrahedral Tetrahedral Tetrahedral Tetrahedral Tetrahedral

Negative Negative Negative Negative Negative

M indicates exchangeable ions represented by monovalent ions. Symbols: N (nature) and S (synthetic).

between two silicon tetrahedron sheets. Stacking of the is inserted into the interlayer space. Such layered solids layers leads to a van der Waals gap between the layinclude graphite, clay minerals, transition metal dichalcoers. Isomorphic substitution of Al with Mg, Fe, Li in the genides, metal phosphates, phosphonates and layered douoctahedron sheets and/or Si with Al in tetrahedron sheets ble hydroxides, etc. Among them, clay minerals have been gives each three-sheet layer an overall negative charge, widely used and proved to be very effective due to their which is counterbalanced by exchangeable metal cations unique structure and properties. Such minerals include both residing in the interlayer space, such as Na, Ca, Mg, Fe, natural clays (e.g., montmorillonite, hectorite and saponite) Li. University Libraries and synthesized clays (e.g., fluorohectorite, laponite and Delivered by Publishing Technology to: and Purdue type: The clays consist of layers made up of one magadiite) as shown in Table I. them, montmorilIP:Among 50.49.22.37 On: Tue, 20 Oct1:1 2015 13:11:08 aluminium octahedron sheet and one silicon tetrahedron American Scientific Publishers lonite and hectorite are to date theCopyright: most widely used ones. sheet. Each layer bears no charge due to the absence of This review will highlight the recent advances in isomorphic substitution in either octahedron or tetrahedron the development and applications of clay-based polymer sheet. Thus, except for water molecules neither cations nor nanocomposites. It is designed to be connected with the anions occupy the space between the layers, and the layers previous reviews in the literature on either organoclays, are held together by hydrogen bonding between hydroxyl clay-polymer interactions or the processing aspects of groups in the octahedral sheets and oxygen in the tetraheclay-polymer nanocomposites.1–12 It will start with a brief dral sheets of the adjacent layers. introduction to the structure, properties and surface modiLayered Silicic Acids: The clays consist mainly of fication of clay minerals, followed by the processing and silicon tetrahedron sheets with different layer thickness. characterization techniques of polymer nanocomposites. Their basic structures are composed of layered silicate The various properties of nanocomposites in relation to networks and interlayer hydrated alkali metal cations. their structure and processing methods will then be discussed. Finally, the review will highlight some promising applications (some of course have been commercially available) in automotive, packaging, coating and pigments, electrical materials, and biomedical fields. Al-O octahedron Si-O tetrahedron

2. CLAY MINERALS 2.1. Layered Structure Clay minerals used for polymer nanocomposites can be classified into three groups as summarized in Table I. They are 2:1 type, 1:1 type and layered silicic acids. Their structures (Fig. 1) are briefly described as follows. 2:1 type: The clays belong to the smectite family with the crystal structure consisting of nanometer thick layers (platelets) of aluminium octahedron sheet sandwiched in 1576

Tetrahedral sheet

2:1 type (Montmorillonite)

Octahedral sheet

1:1 type (Kaolinite)

Layered silicic acid (Kanemite)

Fig. 1. Structure of clay minerals represented by montmorillonite, kaolinite and kanemite. They are built up from combinations of tetrahedral and octahedral sheets whose basic units are usually Si–O tetrahedron and Al–O octahedron, respectively.

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organic molecule reported for this purpose is amino acid adopted in the in-situ polymerization of nylon 6 nanocomposites. However, the most popular modification for clays is to exchange the interlayer inorganic cations (e.g., Na+ , Ca2+ ) with organic ammonium cations. Another key aspect of surface modification is to swell the interlayer space up to a certain extent (normally over 20 Å) and hence reduce the layer–layer attraction, which allow a favourable diffusion and accommodation of polymer or precursor into the interlayer space.

2.2. General Characteristics

3. POLYMER NANOCOMPOSITES The important characteristics pertinent to application of clay minerals in polymer nanocomposites are their rich3.1. Processing and Characterization est intercalation chemistry, high strength and stiffness and high aspect ratio of individual platelets, abundance in When layered clays are filled into a polymer matrix, either nature and low cost. First, their unique layered structure conventional composite or nanocomposite (Fig. 2) can be and high intercalation capabilities allow them to be chemformed depending on the nature of the components and ically modified to be compatible with polymers, which processing conditions. Conventional composite is obtained makes them particularly attractive in the development of if the polymer can not intercalate into the galleries of clay-based polymer nanocomposites. In addition, their relclay minerals. The properties of such composite are simatively low layer charge (x = 02–0.6) means a relatively ilar to that of polymer composites reinforced by microweak force between adjacent layers, making the interlayer particles. There are two extreme nanostructures resulting cations exchangeable. Therefore, the intercalation of inorfrom the mixing of clay minerals and a polymer providganic and organic cations and molecules into the intering a favor conditions. One is intercalated nanocomposite layer space are facile, which is an important aspect of their (I), in which monolayer of extended polymer chains is uses in polymer nanocomposite manufacturing. AmongTechnology the Delivered by Publishing to: into Purdue University Libraries inserted the gallery of clay minerals resulting in a well smectite clays, MMT and hectorite are IP: the 50.49.22.37 most commonly On: Tue, 20 Oct 2015 13:11:08 ordered multilayer morphology stacking alternately polyCopyright: American Scientific Publishers used ones while others are sometimes useful depending mer layers and clay platelets and a repeating distance of on the targeted applications. Moreover, although smectite a few nanometers. The other is exfoliated or delaminated clays are naturally not nanoparticles, they can be exfoliated nanocomposite (II), in which the clay platelets are comor delaminated into nanometer platelets with a thickness pletely and uniformly dispersed in a continuous polymer of about 1 nm and an aspect ratio of 100–1500 and surmatrix. However, it should be noted that in most cases the face areas of 700–800 m2 /g. Each platelet has very high cluster (so-called partially exfoliated) nanocomposite (III) strength and stiffness and can be regarded as a rigid inoris common in polymer nanocomposites. ganic polymer whose molecular weight (ca. 13 × 108 ) is much greater than that of typical polymer. Therefore, very Layered clay Polymer low loading of clays is required to achieve equivalent properties compared to the conventional composites. Finally, and importantly, they are ubiquitous in nature and therefore inexpensive. 2.3. Surface Modification One of the drawbacks of clays is the incompatibility between hydrophilic clay and hydrophobic polymer, which often causes agglomeration of clay mineral in the polymer matrix. Therefore, surface modification of clay minerals is the most important step to achieve polymer nanocomposites. Upon organic treatment, clays become hydrophobic and hence compatible with the specific polymers (thermoplastics, thermosets or elastomers). Such modified clays are commonly referred to as organoclays. The surface modification process is similar to the treatment of fiberglass with silane coupling agents to ensure a perfect compatibility or chemical bonding with polymers. The first J. Nanosci. Nanotech. 5, 1574–1592, 2005

III

Conventional composite

I

II

Nanocomposite

Fig. 2. Schemes of clay-based polymer composites, including conventional composite and nanocomposite with intercalated (I), exfoliated (II) or cluster (III) structure.

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The silanol groups in the interlayer regions favor the organic modification by grafting organic functional groups in the interlayer regions. They are natural clay minerals except for octosilicate, but can be synthesized as well. Layered silicic acids are potential candidates for the preparation of polymer nanocomposites because they exhibit similar intercalation chemistry as smectite clays. Besides, they possess high purity and structural properties that are complementary to smectite clays.


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3.1.1. Processing Techniques

nanostructure can be studied by monitoring the position, shape, and intensity of the basal reflections of XRD patThere are several processes to make clay-based polymer terns of the materials. For instance, the extensive layer nanocomposites, including in-situ polymerization, solution separation in an exfoliated nanocomposite is reflected exfoliation and melt intercalation. As shown in Figure 3, in the disappearance of any coherent XRD whereas the each technique consists of several steps to achieve polymer finite layer expansion in an intercalated nanocomposite is nanocomposites and begins with organoclays or sometimes associated with the appearance of a new basal reflection pristine clays. In the in-situ polymerization, monomers are corresponding to the larger gallery height. XRD offers intercalated into layered clays and subsequently polymera convenient method to determine the interlayer spacing ized within the gallery via heat, radiation, pre-intercalated of original layered clays and intercalated nanocomposites. initiators or catalysts. In the solution exfoliation, layered Unfortunately, such technique can not provide information clays are exfoliated into single platelets using a solvent about the spatial distribution of the clay layers or structural in which the polymer is soluble. The polymer is then non-homogeneities in nanocomposites. It is also difficult mixed with the clay suspension and adsorbed onto the to study systematically systems having a broadening peak platelets. The solvent is finally eliminated from the clayand weakening intensity. From this point of view, inforpolymer complex through evaporation. In the melt intercamation from XRD patterns is not sufficient to reveal the lation, layered clays are directly mixed with the polymer formation mechanism and ultimate structure of nanocommatrix in the molten state. The formation of polymer posites. In contrast, TEM can provide direct qualitative nanocomposites is driven by different forces depending information of structure, morphology and spatial distribuon the technique used, and each technique has its advantion of the various components as well as the defect structages and disadvantages as summarized in Table II. The ture. However, one should be cautious about data from polymers studied so far cover almost the whole range of XRD when the layer spacing in intercalated nanocompospolymers, such as thermoplastics, thermoset plastics, elasites exceeds 6–7 nm or when the layers become relatively tomers, specific, and biodegradable polymers. As an examdisordered in exfoliated nanocomposites. In this case, the ple, we summarize briefly in Table III the various types of simultaneous use of small angle X-ray scattering (SAXS) polymers studied up-to-date for polymer nanocomposites with wide angle XRD can yield quantitative characterizain relation to the processing used. InTechnology particular, to: tion of the structure Libraries in polymer nanocomposites. Besides, Delivered method by Publishing Purdue University Table IV lists some conductingIP:polymers used so far.Tue, 20 Oct 50.49.22.37 On: 2015magnetic 13:11:08resonance (NMR) is another important nuclear Copyright: American Scientific tool Publishers for probing surface chemistry and coordination in exfoliated polymer nanocomposites, which may help quan3.1.2. Characterization Techniques tify the level of clay exfoliation. Fourier transform infrared Various techniques have been employed to study polymer (FTIR) and Raman spectroscopy can be used to understand nanocomposites as summarized in Table V. The structure the structural formation of polymer nanocomposites. of polymer nanocomposites is generally characterized by X-ray diffraction (XRD) and transmission electron micro3.2. Basic Properties of Nanocomposites scope (TEM). XRD, in particular wide angle XRD, is the Polymer nanocomposites exhibit enormously enhanced most commonly used technique for examining the strucproperties and higher performance as compared to both ture and occasionally for studying the process kinetics of their conventional polymer composites and pure polymers. polymer nanocomposites. Either intercalated or exfoliated In addition, such improvement is obtained without the increase of polymer density and the loss of its optical propCuring agent Monomer erties and recycling. For example, polymer nanocomposSwelling Polymerization Nanocomposite ites containing 2–8% of clay demonstrate great increase Organoclay in mechanical (tensile, stress, strain) properties together with the thermal (dimensional) stability. They also reduce Solvent Polymer the gas and liquid permeability. Moreover, they improve the flame retardancy while retaining optical clarity of pure Swelling Intercalation Evaporation Nanocomposite polymer. Finally, they can display interesting conductivOrganoclay ity properties and improved biodegradability when conductive polymers and biodegradable polymer are involved, Curing agent Thermoplastic respectively. Blending

Annealing

Nanocomposite

Organoclay

Fig. 3. Flowchart of three processing techniques for clay-based polymer nanocomposites: in-situ polymerization (upper), solution exfoliation (middle) and melt intercalation (bottom).

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Processing techniques for clay-based polymer nanocomposites.

Processing

Drive force

Disadvantages Clay exfoliation depends on the extent of clay swelling and diffusion rate of monomers in the gallery; Oligomer may be formed upon incompletely polymerization. Compatible polymer-clay solvent system is not always available; Use of large quantities of solvent; Co-intercalation may occur for solvent and polymer. Slow penetration (transport) of polymer within the confined gallery.

In-situ polymerization

Interaction strength between monomer and clay surface; Enthalpic evolvement during the interlayer polymerization.

Suitable for low or non-soluble polymers; A conventional process for thermoset nanocomposites.

Solution exfoliation

Entropy gained by desorption of solvent, which compensates for the decrease in conformational entropy of intercalated polymers. Enthalpic contribution of the polymer-organoclay interactions.

Prefer to water-soluble polymers.

Melt intercalation

Environmental benign approach; No solvent is required; Nanocomposites can be processed with conventional plastic extrusion and molding technology.

Examples nylon 6, epoxy, polyurethane, polystyrene, polyethylene oxide, unsaturated polyesters, polyethylene terephthalate. epoxy, polyimide, polyethylene, polymethylmethacrylate.

nylon 6, polystyrene, polyethylene terephthalate.

aspect ratio together with the good affinity between polywas not affected much by the exfoliation process used.139 mer and organoclay. For instance, stronger interface interIn the case of polypropylene (PP) nanocomposite,69 the actions significantly reduce the stress concentration point slight enhancement in tensile stress is attributed partially upon repeated distortion which easily occurs in convento the lack of interfacial adhesion between apolar PP and tional composites reinforced by glass fibers and thus lead polar clays, which may be improved by adding maleic to weak fatigue strength. Delivered by Publishing Technology anhydride modified PP to the matrix. The tensile stress to: Purdue University Libraries The unprecedented mechanical properties of nylon is even more decreased in PS intercalated nanocomposIP: 50.49.22.37 On: Tue, 20 Oct 2015 13:11:08 6-clay nanocomposite synthesized by in-situ polymerizaites due to the weak interaction at PS and clay interface.86 Copyright: American Scientific Publishers tion were first demonstrated by researchers at the Toyota In thermoset nanocomposites, the exfoliation of clay minCentral Research Laboratories.170 Such nanocomposites erals can also result in substantial property improvement, exhibit significant improvement in strength and modulus, including enhanced mechanical properties, dimensional namely, 40% in tensile strength, 60% in flexural strength, stability, thermal stability, chemical stability, resistance 68% in tensile modulus, and 126% in flexural moduto solvent swelling, excellent transparency, together with lus. The RTP Company has reported equivalent prophigh barrier property and reduced flammability of polymer erty enhancement of nylon 6-clay nanocomposites synnanocomposites.1–3 thesized by direct melt intercalation.171 The increase in modulus is believed to be directly related to the high 3.2.2. Thermal Properties aspect ratio of clay layers as well as the ultimate The thermal stability of polymer composites is generally nanostructure. Moreover, a dramatically increase was also estimated from the weight loss upon heating which results observed in exfoliated nanostructures such as MMTin the formation of volatile products. The improved therbased thermoset amine-cured epoxy nanocomposite and mal stability in polymer nanocomposites is due to the magadiite-based elastomeric epoxy nanocomposite.135 172 clay platelets which hinder the diffusion of volatiles and Figures 4 and 5 show the effect of clay loading on tensile assist the formation of char after thermal decomposition. modulus26 170 172 173 and yield strength20 of some polymer Blumstein174 first reported the thermal stability improvenanocomposites. In contrast, a relatively small increase ment in PMMA nanocomposite, which showed that interwas reported for the intercalated nanocomposites such as calated PMMA containing 10% clay degraded at about those from clay and polymethylmethacrylate (PMMA)114 40–50 C higher than unfilled PMMA. Recently, there and PS.85 have been a great deal of reports on the improved thermal The interaction strength at the interface would greatly stability of nanocomposites made with various organoclays affect the mechanical properties of polymer nanocomposand polymer matrix.119 175–177 For example, the improveites. For example, polar PMMA and ionic nylon 6 interment in thermal stability is reported for cross-linked acts with clay layers which may explain the stress increase polydimethylsiloxane exfoliated with 10% of orangofor intercalated PMMA nanocomposites and exfoliated montmorillonite136 and intercalated nanocomposites made nylon 6 nanocomposites, respectively. The impact properties measured for nylon 6 nanocomposites showed that it from the polymerization of methyl methacrylate,114 J. Nanosci. Nanotech. 5, 1574–1592, 2005

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Advantages

Clay-Based Polymer Nanocomposites: Research and Commercial Development Table III.

Selected polymer nanocomposites using structural polymers.

Polymer type Thermoplastics nylon 6 nylon 12 nylon 66 polyimide

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polypropylene polyethylene polystyrene

poly(-caprolactone)

Layered clay

Processing method

Ref.

MMT/Kaolinite MMT fluoromica MMT MMT/hectorite/ Saponite/fluoromica MMT MMT

in-situ polymerization melt intercalation in-situ polymerization melt intercalation solution exfoliation

[13–17] [15, 17–41] [42–44] [32, 38, 45] [46]

in-situ polymerization in-situ polymerization melt intercalation solution exfoliation in-situ polymerization melt intercalation solution exfoliation in-situ polymerization melt intercalation melt intercalation melt intercalation in-situ polymerization in-situ polymerization in-situ polymerization melt intercalation in-situ polymerization solution exfoliation solution exfoliation solution exfoliation solution exfoliation

[47–54] [55–60] [59, 61–66] [67–70] [71–75] [76–84] [85] [86–93] [94–100] [94, 101, 102] [94] [103, 104] [105] [106–108] [109, 110] [111] [112] [112] [113] [114, 115]

MMT MMT MMT MMT MMT fluorohectorite saponite MMT fluorohectorite hectorite MMT MMT MMT MMT MMT MMT

polyolefin polycarbonate polyethylene terephthalate high density polyethylene nitrile copolymer poly(p-phenylenevinylene) poly(methyl methacrylate) Thermoset Delivered Publishing Technology to: Purdue University Libraries diglycidyl ether of by bisphenol A MMT in-situ polymerization IP: 50.49.22.37 On: Tue,MMT 20 Oct 2015 13:11:08 polyester in-situ polymerization Copyright: American MMT Scientific Publishers polyurethane in-situ polymerization solution exfoliation melt intercalation Elastomer epoxy MMT solution exfoliation MMT/hectorite/ in-situ polymerization fluorohectorite/ fluoromica/magadiite poly(dimethylsiloxane) MMT melt intercalation nitrile-rubber MMT melt intercalation butadiene-acrylonitrile copolymer MMT solution exfoliation poly(styrene-b-butadiene) copolymer MMT melt intercalation

styrene85 87 and epoxy precursors.132 Another thermal behavior is the heat resistance upon external loading which can be measured from the heat distortion temperature (HDT). The HDT of nylon 6 nanocomposites reported by Toyota researchers is increased from 65 C of pristine nylon to 145 C. The increase in HDT has also been observed in clay-based nanocomposites for other polymer systems such as PP178 and polylactide (PLA).179 Such an increase in HDT is very difficult to achieve in conventional polymer composites reinforced by micro-particles. Finally, flame retardancy and mechanical properties are both improved in clay-based polymer nanocomposites while the mechanical properties are always degraded in polymer composites with conventional flame retardants. Such fire resistance of polymer nanocomposites is attributed to the carbonaceous char layers formed when burnt and the 1580

[116] [117–123] [124–129] [130] [131] [132] [133–135] [136, 137] [138, 139] [140] [141]

structure of clay minerals. The multilayered clay structure acts as an excellent insulator and mass transport barrier. Char formation and clay structure impede the escape of the decomposed volatiles from the interior of a polymer matrix.180 The flame retardancy has been recently reviewed by Gilman.175 Flame retardancy is normally evaluated by the reduction of the peak of heat release rate (HRR). 3.2.3. Barrier Properties Polymer nanocomposites have excellent barrier properties against gases (e.g., oxygen, nitrogen and carbon dioxide), water and hydrocarbons. Studies have showed that such reduction in permeability strongly depends on the aspect ratio of clay platelets, with high ratios dramatically J. Nanosci. Nanotech. 5, 1574–1592, 2005

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Selected polymer nanocomposites with some conducting

Polymer/type

poly(vinylpyrrolidone) Electronic conducting polyaniline polypyrrole polythiophene polyphosphazene

Synthesis method

Ref.

MMT hectorite MMT

solution exfoliation solution exfoliation melt intercalation

MMT

solution exfoliation

[142–147] [147] [82, 148–159] [160]

hectorite MMT MMT hectorite MMT MMT MMT

in-situ intercalation in-situ intercalation solution exfoliation in-situ intercalation in-situ intercalation solution exfoliation solution exfoliation

[161, 162] [162–165] [166] [167] [168] [166] [169]

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Ionic conducting polyethylene oxide

Layered clay

8

Tensile modulus (Gpa)

Table IV. polymers.


4 nylon 6 [26] 2

polypropylene [173]

0 0

5

10

15

20

Clay (%) Fig. 4. Effect of clay loading on tensile modulus for different claybased polymer nanocomposites.

enhancing gaseous barrier properties. The water permeability of exfoliated polyimide (PI) nanocomposites as shown in Figure 6 has been reported by Yano et al.46 Characteristics and properties through the use of organoclays with different aspect ratios. The best gas barrier properties would be obtained Degree of swelling and interlayer distance of clays Dispersion degree of clay platelets in polymer nanocomposites with fully exfoliated clay minMorphology (conventional, intercalated, exfoliated erals. Moreover, the aspect ratio of clay platelets was or mixed) observed to affect greatly the relative permeability coeffiKinetics of intercalation process cient for PI filled with 2% of organoclay. The permeabilSurface roughness and morphology ity toto: water vaporUniversity of exfoliated poly(caprolactone) (PCL) Delivered by Publishing Technology Purdue Libraries Dispersion degree of clay particles IP: 50.49.22.37 On: Tue, 20 Oct 2015 has 13:11:08 nanocomposites also been investigated which indiMorphology and its development Scientific Publishers Microstructure (intercalated vsCopyright: exfoliated) American cates a dramatic decrease in the relative permeability with Spatial distribution of clay platelets the increase of nanometer clay platelets.103 The effect of Structural heterogeneities clay loading on relative permeability coefficiency of some Defect structure and atomic arrangement polymer nanocomposites is shown in Figure 753 181–183 In Crystallization behavior of polymer addition, polymer nanocomposites have also shown better Surface roughness barrier properties against organic solvents such as alcohol, Particle size and distribution Morphology and microstructure (intercalated vs toluene and chloroform. exfoliated) The enhanced barrier properties of polymer nanocomComponent identification and analysis posites may be explained by the labyrinth or tortuous pathInterfacial interactions way model as proposed in Figure 8.46 When a film of Crystallization and orientation of polymer

Table V. Common characterization techniques for clay-based polymer nanocomposites.

XRD/WAXRD

SEM TEM/HRTEM

AFM

FTIR

NMR

SAXS

TGA DSC Cone calorimetry Rheometry Mechanical test

Local dynamics of polymer chains Morphology and dispersion of clay particles Surface chemistry Dispersion of nanoscale clay platelets Morphology (intercalated, exfoliated or mixed) and its development Phase behavior and structure evolution Lamellar texture and thickness Thermal stability Melting and crystallization behavior Local dynamics of polymer chains Flame retardancy, such as heat release rate and carbon monoxide yield Thermal stability Nanorheology Young’s modulus Tensile strength Elongation at break Viscoelastic properties


100

Yield strength (MPa)

Techniques

90

80 LMW nylon 6 MMW nylon 6

70

HMW nylon 6 60 0

2

4 Clay (%)

6

8

Fig. 5. Effect of clay loading on yield strength for different clay-based nylon 6 nanocomposites. Reproduced with permission from [20], T. D. Fornes et al., Polymer 42, 9929 (2001). © 2001, Elsevier Ltd.

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1

Permeability

0.6

0.4

0.2

0

0

300

600

900

1200

1500

Fig. 8. Proposed model for the tortuous zigzag diffusion pathway of a gas through clay-based polymer nanocomposites.

Aspect ratio

interlayer cations and their mobility ensure a significant ionic conductivity of the system. Moreover, the intercalation of neutral species could affect the hydration shells of interlayer cations and therefore significantly modifies the ion mobility, electrical conductivity and other electrical parameters. The ionic conductivity of crown ether-clay polymer nanocomposites is formed, the sheet-like clay lay(Fig. 9) was reported to be several orders of magnitude ers orient in parallel with the film surface. As a result, gas higher than that of corresponding clay.145 184 In addition, molecules have to take a long way around the impermeable it increases with temperature up to a maximum value clay layers in polymer nanocomposite than in pristine depending on the nature of the intercalated crown ether. polymer matrix when they traverse an equivalent film Further improvement in conductivity is expected by interthickness. It is interesting to note that the enhancement of Delivered by Publishing Technology to: calating Purdue electroactive University Libraries polymers into clay minerals. barrier properties does not ariseIP: from the chemical 50.49.22.37 On:interacTue, 20 OctNanocomposites 2015 13:11:08 based on the intercalation of polymer tions since it does not depend on Copyright: the type of American gas or liquid Scientific Publishers electrolyte (e.g., polyethylene oxide, PEO) into clay minmolecules. erals are an attractive substitute of conventional polymersalt compounds. The ionic conductivity of the latter is 3.2.4. Electrical Conductivity strongly affected by the crystallinity of the material, the ion-pair formation as well as high mobility of counter-ions. Clay minerals exhibit unique electrical properties, which In contrast, the ionic conductivity of the former avoids the is mainly attributed to their ionic conductivity. Although the clay layers can be regarded as insulators, the hydrated Fig. 6. Effect of clay aspect ratio on relative permeability coefficiency of polyimide nanocomposites. Reproduced with permission from [46], K. Yano et al., J. Polym. Sci., Part A: Polym. Chem. 35, 2289 (1997). © 1997, John Wiley and Sons, Inc.

–4

1

Log [S], (S/cm)

PEO

0.8

Permeability

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0.8

hydrogen, polyimide [53] nitrogen, rubber [181] oxygen, polyimide [53] oxygen, polylactide [191] water, polyimide [53] water, polyurethane [183]

0.6

0.4

DB24C8 15C5 –6 18C5

0.2

–7

4

5

6

7

8

12C4

9

10

enthalpy, (KJ / mol)

0 0

3

9

6

12

15

Clay (%) Fig. 7. Effect of clay loading on relative permeability coefficiency of polymer nanocomposites. Data of rubber nanocomposite and polyurethane nanocomposite are derived from Refs. [181, 183] with the following assumed density: 0.92 g/cm3 (rubber), 0.97 g/cm3 (polyurethane) and 1.8 g/cm3 (organoclay).

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Fig. 9. Ionic conductivity of intercalated compounds obtained from various oxyethylene and Na-montmorillonite as a function of the formation enthalpy of the interlayer complexes. The represented crown ethers are: 12-crown-4 (12C4), 15-crown-5 (15C5), 18-crown-6 (18C6), dibenzo-24crown-8 (DB24C8), PEO: poly(ethilene oxide) of 105 molecular weight. Reproduced with permission from [145], E. Ruiz-Hitzky et al., Adv. Mater. 7, 180 (1995). © 1995, Wiley-VCH (Weinheim, Germany).


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reported the formation of PPR nanocomposites from Cu2+ fluorohectorite. Moreover, doping of these nanocomposites with iodine vapors produces a strong increase in the DC conductivity from 2 × 10−5 S/cm to 12 × 10−2 S/cm and a high anisotropic ratio (4 × 103 ) as well.

–4

–6

3.2.5. Biodegradability

Another interesting and exciting property is the significantly improved biodegradability of nanocomposites made from organoclay and biodegradable polymers. Tetto et al.187 first reported studies on the biodegradability of nanocom–8 100 80 60 40 20 posites based on PCL, which showed an improved T, C biodegradability over pure PCL. Such an improved biodegradability of PCL in clay-based nanocomposites Fig. 10. Arrhenius plots of ionic conductivity for LiBF4 /PEO and PEO Li+ -montmorillonite intercalated nanocomposite. Reproduced with permay be attributed to a catalytic role of the organoclay mission from [148], R. A. Vaia et al., Adv. Mater. 7, 154 (1995). © 1995, in the biodegradation mechanism. Lee et al.188 reported Wiley-VCH (Weinheim, Germany). (Fig. 11) the biodegradation of aliphatic polyester-based nanocomposites during compost. They attributed the retarmobility of the anions (negatively charged clay layers), dation of biodegradation to the improved barrier properties the anion-complexed cation interactions and the collapse of the aliphatic polyester nanocomposite based on clay. of clay layers. For instance, PEO-clay nanocomposites Ray et al.179 182 189 reported (Fig. 12) the biodegradability show higher ionic conductivities than the clays, increasing of PLA and the corresponding nanocomposites prepared with temperature until a maximum at around 600 K.145 with organoclay, along with a detailed mechanism of the Moreover, the maximum conductivity in the direction pardegradation. It was shown that the biodegradability of allel to the clay layer is in the 10−5 –10−4 S/cm range. PLA nanocomposite made from organoclay is significantly Similar nanocomposite wasDelivered also reported by direct melt by Publishing Technology to: Purdue Libraries enhanced. They University attributed such a behavior to the pres148 intercalation of PEO (40%) into Li-MMT. This nanoIP: 50.49.22.37 On: Tue, 2015 13:11:08 ence20ofOct terminal hydroxylated edge groups in the clay composite (Fig. 10) has shown to enhance the stabilCopyright: American Scientific Publisherstest was used to study the degradalayers. A respirometric ity of the ionic conductivity at lower temperature when tion of the PLA matrix in a compost environment around compared to more conventional PEO/LiBF4 mixture. Such 58 C.190 191 Unlike the weight loss, which reflects the stability enhancement is explained by the fact that interstructural changes, CO2 evolution provides the ultimate calation of PEO avoids its crystallization and thus elimibiodegradability of PLA in nanocomposites. The time nates the presence of non-conductive crystallites. dependence of the degree of biodegradation of pure PLA Nanocomposites with conjugated conducting polymers and nanocomposite indicated a significant enhancement in have also been reported including polymers such as biodegradability of PLA in nanocomposites. PANI,161 163 185 polypyrrole (PPR),166 186 and polythiophene (PTP).166 There are three different ways to pre100 pare such conducting polymer nanocomposites from clay APES minerals, including in-situ polymerization induced by APES/30B (97/3 wt%) APES/30B (95/5 wt%) interlayer transition metals (e.g., Cu2+ , Fe3+ ) or exter80 APES/30B (90/10 wt%) nal oxidizing agents (e.g., ammonium peroxodisulphate), APES/30B(80/20 wt%) or addition of polymer during the hydrothermal syntheAPES/30B(70/30 wt%) 60 sis of clay minerals. By using the first approach, PANI nanocomposites with an intercalated structure have been 40 successfully prepared from fluorohectorite161 and MMT163 as demonstrated by XRD and FTIR analyses. Moreover, a significant increase in the electrical conductivity was 20 observed, which attains high in-plane direct current (DC) conductivity (5 × 10−2 S/cm) with an anisotropic ratio 0 close to 105 .161 Similar materials have also been formed 0 5 10 15 20 25 30 35 from MMT by the second technique, which exhibited conTime (day) ductivity (10−6 S/cm) much lower than the former one. Fig. 11. Biodegradability of aliphatic polyester (APES) nanocomposites Such a difference is probably due to the nature of with different loading of organoclays (Cloisite 30B). Reproduced with PANI chains obtained under various conditions and the permission from [188], S. R. Lee et al., Polymer 43, 2495 (2002). © 2002, anisotropy of material conductivity. Mehrotra et al.186 also Elsevier Ltd. Li-MMT/PEO LiBF4/PEO

Biodegradability (wt%)

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Log [S], S/cm

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Clay-Based Polymer Nanocomposites: Research and Commercial Development after 32 days

after 50 days

after 60 days

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passage. Besides, scratch resistance is strongly enhanced by the incorporation of layered silicates.192 The polymer nanocomposites can be used in highly technical areas to improve the ablative properties in aeronautics.193

REVIEW

Neat PLA

4. APPLICATIONS AND COMMERCIAL DEVELOPMENT Polymer nanocomposites represent an exciting and promising alternative to conventional composites owing to the dispersion of nanometer clay platelets and their markedly improved performance in mechanical, thermal, barrier, optical, electrical, and other physical and chemiPLA-ODA4 cal properties. Thus, many companies have taken a strong interest and invested in developing nanoclays (Table VI) and polymer nanocomposites (Table VII). Increasing number of commercial products has become available. PLA-SBE4 The first commercial product was a nylon 6 nanocomposite reported by Toyota Central Research Laboratories Fig. 12. Photographs demonstrating the biodegradation evolution of and Ube in 1990.14 170 Unitika became the second producer neat polylactide (PLA) and various PLA/organoclay nanocomposites of nylon 6 nanocomposites in 1996 by using synthetic recovered from compost. The initial shape of the crystallized samples was 3 × 10 × 01 cm3 . Reprinted with permission from [189], S. S. Ray mica as the nanofillers. Shortly, commercial products by et al., Macromol. Mater. Eng. 288, 203 (2003). © 2003, Wiley-VCH direct melt intercalation were manufactured by the RTP Verlag GmbH. Company, Southern Clay Products, Nanocor and Honeywell Polymer among others. So far nylon nanocomposites 3.2.6. Other Properties are still the dominant commercial polymer nanocomposDelivered Publishing Technology Purdue University Libraries Polymer nanocomposites also by show significant improve- to: ites though others are being produced, such as PET-clay IP:properties. 50.49.22.37 Tue, 20 Oct 2015 13:11:08 ment in most general polymer ForOn: example, (Eastman and Nanocor, 1999) and PP-clay (GM, Montell, Copyright: American Scientific Publishers they have the transparency similar to pristine polymer Nanocor and SCP, 2000). These materials are mainly used materials because the clay platelets are about one nanomeas automotive parts and barrier packaging films, but also ter thickness. Thus, such clay platelets with size less as additives, compounding and filament. It is believed that than the wavelength of visible light do not hinder light’s future markets for polymer nanocomposites will expand PLA-MAE4

Table VI.

Selected commercial nanoclays.

Product Closite

Characteristics Organophilic

Applications

Producer

Additives to enhance flexural and tensile modulus, barrier properties and flame retardancy of thermoplastics Nylon, epoxy, unsaturated polyester, engineering resins Thermoplastic olefin and urethane, styrene-ethylene butylene-styrene, ethylene vinyl acetate

Southern Clay Products

Nanomers

Microfine powder

Masterbatches

Pellet

Bentone

With a broad range of polarity

Additives to enhance mechanical, flame retardancy and barrier properties of thermosets and thermoplastics

Elementis Specialties

Nanofil

Improve the mechanical, thermal and barrier properties

Thermoplastics and thermosets

Sud-Chemie

Planomers

Additive, enhance mechanical barrier properties, thermal stability and flame resistance Nanopigments, e.g., blue, red, green, yellow, high UV-stability Additive, excellent transparency and improved barrier properties

Electric and electronic, medical and healthcare, adhesive, building and construction materials

TNO

Decorative coloring, UV-stable coloring, heavy metal free coloring Transparent packaging materials, protective coatings, transparent barrier coatings

TNO

PlanoColors PlanoCoatings

Nanocor PolyOne Corporation, Clariant Corporation, RTP Company

TNO

Clariant Corporation: www.clariant.com; Elementis Specialities: www.elementis-spec.com; Nanocor: www.nanocor.com; PolyOne Corporation: www.polyone.com; RTP Company: www.rtpcompany.com; Southern Clay Products: www.nanoclay.com; Sud-Chemie: www.sud-chemie.com; TNO: www.tno.nl.

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Clay-Based Polymer Nanocomposites: Research and Commercial Development Selected commercial polymer nanocomposites.

Product

Characteristics

Producer

Automotive parts (e.g., timing-belt cover, engine cover, barrier fuel line), packaging (e.g., cosmetics, food, medical, electronics), barrier film Step-assist for GMC Safari and chevrolet Astro vans, heavy-duty electrical enclosure

Bayer, Honeywell Polymer, RTP Company, Toyota Motors, Ube, Unitika

Nylon nanocomposites

Improved modulus, strength, heat distort temperature, barrier properties

Polyolefin nanocomposites

Stiffer, stronger, less brittle, lighter, more easily recycled, improved flame retardancy

M9™

High barrier properties

Juice or beer bottles, multi-layer films, containers

Basell, Blackhawk Automotive Plastics, General Motors, Gitto Global Corporation, Southern Clay Products Mitsubishi Gas Chemical Company

Durethan KU2-2601 (nylon 6)

Doubling of stiffness, high gloss and clarity, reduced oxygen transmission rate, improved barrier properties Doubling of stiffness, higher heat distort temperature, improved clarity Highly reduced oxygen transmission rate, improved clarity Improved stiffness, permeability, fire retardancy, transparency and recycling

Barrier films, paper coating

Bayer

Medium barrier bottles and films

Honeywell Polymer

High barrier beer bottles

Honeywell Polymer

Catheter shafts and balloons, tubing, film and barriers, flexible devices Automotive, furniture, appliance

Foster Corporation

Aegis™ NC (nylon 6/barrier nylon) Aegis™ OX SET™ nanocomposite nylon 12 Forte™ nanocomposite

Improved temperature resistance and stiffness, very good impact properties

Noble Polymer

Basell: www.basell.com; Bayer: www.bayer.com; Blackhawk Automotive Plastics: www.blackhawkplastics.com; Foster Corporation: www.fostercomp.com; General Motors: www.gm.com; Gitto Global Corporation: www.gitto-global.com; Honeywell Polymer: www.honeywell.com; Mitsubishi Gas Chemical Company: www.mgc.co.jp; Noble Polymer: www.noblepolymers.com; RTP Company: www.rtpcompany.com; Southern Clay Products: to: www.nanoclay.com; Toyota Motors: www.toyota.com; Ube: www.ube.com; Delivered by Publishing Technology Purdue University Libraries Unitika: www.unitika.co.jp. IP: 50.49.22.37 On: Tue, 20 Oct 2015 13:11:08

Copyright: American Scientific Publishers from current automotive, packaging and containers, coatindustry because of their remarkable increase in HDT and ings as represented in Figure 13 to other industries such enhanced barrier properties together with their mechanias appliances and tools, electrical and electronic, buildcal properties. Apart from nylon 6, a thermoplastic olefin ing and construction, biomedical and bioengineering fields. nanocomposite with as little as 2.5% clay was employed Table VIII shows selected applications and some of them by General Motors for step-assist on Safari and Chevrolet are briefly discussed here. in 2002. It is now believed that polymer nanocomposites can be utilized as potential materials in various vehicles for external and internal parts such as mirror housings, door 4.1. Applications in Automotive Components handles and under-the-hood parts. The weight advantage Today, polymer composites are widely used in automoof polymer nanocomposites could have a significant impart tive industries. However, such composites are fabricated on environmental protection and material recycling. It is by adding large amounts of micro-particles, thermal stabilizers, chemical resistant and flame resistant additives into the polymer matrix. Therefore, their improved performance often comes with the increase in materials density and low fuel efficiency. In contrast, polymer nanocomposites offer higher performance with significant weight reduction and affordable materials for transport industries such as automotive and aerospace. The first commercial product of clay-based polymer nanocomposites is the timing-belt cover made from nylon 6 nanocomposites in Toyota Motors in early 1990s.138 Such timing-belt cover exhibited good rigidity, excellent thermal stability and no wrap. It also saved the weight by up to 25% due to the lower amount of clays used. Besides, nylon 6 nanocomposites have also been used as engine Fig. 13. Selected commercial products made from clay-based polymer cover, oil reservoir tank and fuel hoses in the automotive nanocomposites. J. Nanosci. Nanotech. 5, 1574–1592, 2005

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Applications

Clay-Based Polymer Nanocomposites: Research and Commercial Development Table VIII.

Potential biomedical and bioengineering applications of clay-based polymer nanocomposites.

Clay/Organoclay

REVIEW

Zeng et al.

Polymer

Behaviors/Applications

Ref.

MMT MMT TDTMA-MMT Cloisite 20A Somasif-MAE Somasif-MAE300

poly(N-isopropylacrylamide) polyacrylamide poly(N-isopropylacrylamide) poly(ethylene-co-vinyl acetate)

swelling-deswelling behaviors swelling-deswelling behaviors thermal response and controlled release rate drug delivery for dexamethasone

[228] [229] [230] [218]

Cloisite 30B AAPTMA-MMT Halloysite

poly(N-isopropylacrylamide)

drug release behaviors for caffeine, crystal violet and phenol red drug release for diltriazem hydrochloride and propranolol hydrochloride drug delivery for tetracycline drug carrier for 5-fluorouracil iontophoretic delivery for calcium ions drug loading and release of diphenydramine hydrochloride

Halloysite MMT MMT DDTMA-Kanemite and its microporous derivative

chitosan/polyethyleneimine chitosan/poloxamer 407

[219, 220] [227] [225] [221] [222] [224]

MMT: montmorillonite; AAPTMA: (3-acrylamidopropyl)trimethyl ammonium; DDTMA: dodecyltrimethyl ammonium; TDTMA: tetradecyltrimethyl ammonium.

predicted that widespread use of polymer nanocomposites would save 1.5 billion liters of gasoline over the life of one year’s production of vehicles and reduce related carbon dioxide emissions by more than 5 billion kilograms.194

PLA,182 190 191 196–201 poly(butylene succinate) (PBS),202 203 PCL,105 204–214 unsaturated polyester,132 polyhydroxy butyrate,215–217 aliphatic polyester.118 188 Among them, PLA is of increasing commercial interest because it is made from renewable resources and readily biodegradable.

4.2. Packaging Materials Nanocomposite Coatings and Pigments Delivered by Publishing Technology to: 4.3. Purdue University Libraries The excellent barrier properties of clay-basedOn: polymer IP: 50.49.22.37 Tue, 20 Oct 2015 13:11:08 nanocomposites would result in considerable Novel nanocomposite additives have been developed by Copyright: enhancement American Scientific Publishers of shelf-life for many types of packaged food. Meanwhile, TNO Materials based on a combination of layered natthe optical transparency of polymer nanocomposite film is ural or synthetic clays and block-co-polymers or surfacgenerally similar to their pristine counterparts, which is tants. The excellent adhesion between the clay layers impossible for conventional polymer composites. Thereand the polymer matrix induces remarkable enhancements fore, the above property advantages would make them in material properties. The PlanoColors or nanopigments acceptable widely in packaging industries as wrapping made from clays and organic dyes are believed as potential films and beverage containers, such as processed meats, environment-friendly substitutes for toxic cadmium (Cd) cheese, confectionery, cereals, fruit juice and dairy prodand palladium (Pd) pigments. The PlanoColors can be ucts, beer and carbonated drinks bottles. For example, readily dispersed on a nanoscale in bulk polymers as well Bayer has recently developed a new grade of plastic films as in coatings. Various colors of pigments are possibly profor food packaging which are made from nylon-6 exfoliduced by choosing suitable dyes from a wealth of organic ated nanocomposites.195 dyes. Moreover, materials dyed with PlanoColors remain On the other hand, enormous amounts and varieties of completely transparent since the size of these pigments plastics used today are produced mostly from fossil fuels is smaller than the wavelength of light. In addition, an in particular polystyrene and polyolefins and poly(vinyl improved oxygen, ultraviolet (UV) and temperature stabilchloride), and consumed and discarded in the environment. ity combined with high brilliance and color efficiency has These plastics do not degrade spontaneously and thus are been displayed which is probably due to the huge surface treated by incineration, resulting in large amounts of area of the nanopigments and their enhanced interaction carbon dioxide or even toxic gases. Therefore, there is an with light. urgent need for the development of environment-friendly polymer materials that would not involve the use of toxic 4.4. Nanocomposites in Electromaterials or noxious components in their manufacture and could allow degradation via a natural process specifically in Polymer nanocomposites open a promising route to some applications such as composting, packaging, agriculnovel organic–inorganic materials with peculiar electrical ture, and hygiene. To meet such a goal, various biodegradproperties. The remarkable electrochemical behavior able polymers have recently been reported to be used in of conducting polymers associated with clay minerals the manufacturing of polymer nanocomposites, including attracts potential applications such as modified electrodes, 1586


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intercalation compounds with controllable de-intercalation process. Recently, Deasy and coworkers225–227 investigated the potential applications of halloysite, a hollow microtubular clay mineral, for novel drug delivery systems, in particular diltiazem hydrochloride and propranolol hydrochloride for angina and hypertension, and tetracycline for periodontitis. 4.6. Applications of Nanocomposites in Sensors and Medical Devices

Apart from drug delivery, some researchers investigated the possibility to improve the properties of some stim4.5. Nanocomposites for Drug Delivery uli responsive hydrogels by incorporating clay platelets. The property enhancement, especially on thermal responPolymer nanocomposites have recently been investisivity, swelling-deswelling rate and molecular diffusion, gated for controlled drug delivery as summarized in is expected to extend clay-based nanocomposites to Table VIII. Giannelis and co-workers218 reported that such applications as artificial muscles and rapid actuthe addition of organoclays not only reduced the rate ators shown in Table VIII. Recently, Messersmith and of drug (i.e., dexamethasone) release from the biocomZnidarsich228 investigated the feasibility of developing patible poly(ethylene-co-vinyl acetate) matrix but also polymer nanocomposites capable of altering their structure increased the Young’s modulus as compared to the pure and properties upon the application of external stimupolymer. Lee and co-worker219 220 fabricated a series of lus. They synthesized by in-situ polymerization the therclay-based poly (N-isopropylacrylamide) nanocomposite moresponsive nanocomposite hydrogels consisting of clay hydrogels and investigated their swelling and drug release layers dispersed within a poly(N-isopropyl acrylamide) behaviors. It was shown that the increase in the content of (PNIPAM), a thermally responsive polymer which exhibits either intercalated quaternary ammoniums in organoclays Delivered by Publishing Technology to: Purdue Libraries (LCST) of 32 C a lower critical University solution temperature or clay in nanocomposites led to the decrease in swelling IP: 50.49.22.37 On: Tue, 20 Oct 2015 13:11:08 in Scientific the presence of water. The resulting nanocomposite ratio of the nanocomposite hydrogels butCopyright: the increase in American Publishers with low loading level of clay (up to 3.5%) showed simthe gel strength. In addition, the drug release behaviors ilar shrinkage behavior to that of pure PNIPAM hydrogel of clay-based poly(N-isopropylacrylamide) nanocomposwhile there was no LCST in the nanocomposite hydroite hydrogels were examined for a few model drugs (i.e., gel with high level of clay (10.7%). In another work, neutral caffeine, cationic crystal violet and phenol red). Churochkina et al.229 reported that the intercalation of They found that the drug release largely depended on the negatively charged clay in neutral and slightly charged factors such as the content of clay and its intercalated polyacrylamide matrix improved their mechanical propagents, the charge of drug solute, interaction between the erties while keeping the superabsorbing characteristics of gel and drug solute, and ionic strength of the medium. polyacrylamide hydrogel. Later, Liang et al.230 reported a Lin et al.221 attempted to develop a composite as drug thermosensitive organoclay-PNIPAM nanocomposite with carrier and in-situ release for colorectal cancer therapy. enhanced temperature response which attributed to the Such a composite was made by intercalating 5-fluorouracil improved interfacial chemistry through the coupling agent. into the interlayer space of montmorillonite. Szántó and Biomedical poly(urethane urea)s (PUU) are used in a Papp222 223 investigated the iontophoretic delivery of calvariety of biomedical applications such as blood sacs in cium ions from bentonite to find a suitable drug for transventricular assist devices and total artificial hearts. Howdermal introduction of calcium ions into the body. They ever, such elastomers have relatively high permeability to evaluated in vitro the permeation of the calcium ions air and water vapor. One traditional approach to reducthrough pig skin in a diffusion cell. Such studies may ing permeability is to modify the chemistry of polylead to the application of clay minerals in the therapy mer. Recently, Manias and co-workers183 231 232 reported a of osteoporosis. Ambrogi et al.224 studied the drug inclumore efficient approach in which clay platelets were dission and in vitro release of intercalation compound and persed into polymer. The resulting polymer nanocomposmicroporous material from kanemite. Their results showed ites have showed significantly reduced (ca. fivefold) gas that the release of diphenydramine from the microporous permeability as well as the improvement in mechanical materials was initially very fast with 60% of the drug properties (e.g., stiffness, modulus). Such enhancements was released within 5 min. In the case of intercalation are beyond what can be achieved by chemical modificompound, the drug release was gradual but only 30% of cation of polyurethanes. However, their biocompatibility the drug was finally released. Therefore, it is possible to and high cycle fatigue resistance need to be confirmed develop microporous materials with smaller pore size and J. Nanosci. Nanotech. 5, 1574–1592, 2005

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biosensors, solid-state batteries, smart windows and other electrochemical devices. For instance, PPR nanocomposites can be developed for modified electrodes used as sensors or as devices for electrocatalysis. PEO nanocomposites could become novel electrolyte materials because of their relatively higher ambient conductivity and weak temperature dependence over conventional LiBF4 /PEO electrolytes as well as their single ionic conduction character. In addition, such a nanocomposite is an excellent model system to probe and understand the structure and dynamics at the interface.

REVIEW


for such polymer nanocomposites. O’Neil et al.233 studied the potential design of nylon 12 nanocomposites for catheter shafts which require varying mechanical properties along their length to allow for manipulation of the device. Their studies showed that clay-based nanocomposites from a polyether block amide copolymer could achieve a balance in flexibility and stiffness and thus eliminate the use of different materials in a single catheter shaft. 4.7. Miscellaneous Applications

Zeng et al.

However, in spite of these efforts and some successes in commercial development of clay-based polymer nanocomposites, their design, manufacturing and applications are often empirical, and large scale productions are still in its infancy. The reasons are mainly because of the limited theoretical knowledge on such novel nanostructure materials, such as a basic guideline for the selection of surfactants and the modification of clays for the purposes of targeted polymer matrix, the mechanisms of superior reinforcement observed as compared with their micro-counterparts, and the establishment of a simple processing-structureproperty relationship for such nanocomposites. Therefore, further development of polymer nanocomposite materials depends largely on our understanding of the above fundamentals in relation to their formation, processing, property prediction and design. In particular, future research is expected to address the following challenges and issues:

Polymer nanocomposites are applicable to not only the above mentioned areas but also many other areas. For instance, flame retardancy of polymer materials is required in many applications, such as in automotive parts, personal computers, and building materials. In this regard, polymer nanocomposites offer a promising alternative to the commonly employed flame resistant materials con• A simple but effective way to modify clay minertaining halogenous and phosphorous flame retardants. als and improve their compatibility with desirable polymer Moreover, the potential of using PANI nanocomposites matrix since the dispersion of clay platelets directly affects 234 as electrorheologically sensitive fluids, or the use of the final structure and thus the final properties of polydispersed layered clays in a liquid crystal medium is mer nanocomposites. Surface modification is even more another attractive application, which could result in a staimportant for a polar polymers. ble electro-optical device that is capable of exhibiting a • Effective experimental techniques to quantitatively bistable and reversible electro-optical effect between an characterize and analyze the microstructure, mechanical, opaque and transparent state.192 Therefore, more efforts thermal, thermodynamic, and rheological properties. Delivered by Publishing are needed to extend nanocomposites to such Technology a variety of to: Purdue University Libraries Processing-structure-property relations to facilitate IP: 50.49.22.37 On: Tue, 20 Oct•2015 13:11:08 applications. the design, manufacturing, and applications of clay-based Copyright: American Scientific Publishers polymer nanocomposites.

5. CONCLUDING REMARKS It is clear that clay minerals are cost-effective and versatile raw materials for polymer nanocomposites, due to their unique layered structure, rich intercalation chemistry, high aspect ratio, high in-plane strength and stiffness, as well as abundance in nature and availability at low cost. As presented in this review, clay-based nanocomposites from various polymers have been reported in the past decade. These polymer nanocomposites have demonstrated significantly improved properties, including mechanical, thermal, gas and liquid barrier, flame retardancy, optical, electrical and biodegradable properties. In addition, the very low level of clay loading makes the overall density similar to pure polymer and also greatly improves their processing capability for film or fibers, which is unlikely in conventional polymer composites. As a result, claybased polymer nanocomposites have been produced as energy-saving and environment-friendly automotive parts and packaging materials. Their future markets will further expand from current automotive, packaging and containers, coatings and pigments to other industries such as appliances and tools, electromaterials, building and construction. Moreover, clay-based polymer nanocomposites have shown promising applications in the biomedical and bioengineering fields. 1588

Abbreviations AFM DSC FTIR HDT HRR HRTEM MMT NMR PANI PBS PCL PEO PET PI PLA PMMA PP PPR PS PTP PUU SANS

Atomic force microscope Differential scanning calorimetry Fourier transform infrared Heat distortion temperature Heat release rate High-resolution transmission electron microscopy Montmorillonite Nuclear magnetic resonance spectroscopy Polyaniline poly(butylene succinate) poly(caprolactone) polyethylene oxide poly(ethylene terephthalate) Polyimide Polylactide Polymethylmethacrylate Polypropylene Polypyrrole Polystyrene Polythiophene Poly(urethane urea) Small-angle neutron scattering J. Nanosci. Nanotech. 5, 1574–1592, 2005

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Small angle X-ray scattering Scanning electron microscopy Transmission electron microscopy Thermal gravimetric analysis Wide angle X-ray diffraction X-ray diffraction

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Received: 5 April 2005. Accepted: 20 April 2005.

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