The thin layers in the zone I are influenced by the properties of the substrate ..... entirely conducting region at the top of the film and entirely insulating region at ...
TURUN YLIOPISTON JULKAISUJA ANNALES UNIVERSITATIS TURKUENSIS
SARJA – SER. AI OSA TOM. XXX ASTRONOMICA – CHEMICA – PHYSICA - MATHEMATICA
Buildup, Structure and Physical Properties of Some Polyelectrolyte Multilayer Films
by Mikko Salomäki
TURUN YLIOPISTO Turku 2005
From the Department of Chemistry, University of Turku, Turku Finland and Gradute School of Chemical Sensors and Microanalytical Systems
Supervised by Professor Jouko Kankare Department of Chemistry, University of Turku, Finland. Professor Jukka Lukkari Department of Chemistry, University of Turku, Finland. Reviewed by Professor Olli Ikkala Department of Engineering Physics and Mathematics, Helsinki University of Technology, Finland Professor Heikki Tenhu Laboratory of Polymer Chemistry, University of Helsinki, Finland. Opponent Professor Vincent Ball Facuté de Médecine, Institut National de la Santé et de la Recherche Médicale, Strasbourg, France
ISBN
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Preface The experimental work in this thesis was carried out in the Laboratory of Analytical Chemistry in the University of Turku during 2000-2005. At the beginning I attended the Graduate School of Materials Research (GSMR). During the years 2002-2005 I attended the Graduate School of Chemical Sensors and Microanalytical Systems (CHEMSEM). I am grateful for the funding from the Academy of Finland, CHEMSEM graduate school and Palomaa-Erikoski fund. I wish to express my gratitude to my supervisors. Professor Jouko Kankare has put enormous time and effort to this work by designing and developing the quartz crystal impedance measurement system. At the early stages of the experimental work professor Jukka Lukkari encouraged me to continue with his enthusiasm towards the current research. I would also like to thank all past and present co-workers in the laboratory of analytical chemistry and those who have made a contribution to the included papers: Janne Ahonen, Timo Ala-Kleme, Sami Areva, Keijo Haapakka, Tatu Hellström, Iko Hyppänen, Kari Kleemola, Natalia Kocharova, Kirsi Laaksonen, Taina Laiho, Hanna Paloniemi, Janika Paukkunen, Piia Tervasmäki, Mikael Wasberg, Antti Viinikanoja, Igor Vinokurov and Timo Ääritalo. I would also like to thank Kari Loikas and Mauri Nauma for the top quality custom-made measurement instruments and the related highly sophisticated technical equipment. I am also thankful for the help that I received with the broken electronic devices and computers. I thank my wife Satu and my son Eetu for helping me to focus on the most important things in life.
Turku, November 2005
Mikko Salomäki
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Table of contents
PREFACE........................................................................................................................................ 3 LIST OF SYMBOLS AND ABBREVIATIONS .......................................................................... 5 LIST OF PUBLICATIONS ........................................................................................................... 6 1. INTRODUCTION....................................................................................................................... 7 1.1. LAYER-BY-LAYER DEPOSITION............................................................................................... 8 1.1.1. Buildup and structure of the films ................................................................................. 9 1.1.2. Applications................................................................................................................. 12 1.2 CHARACTERIZATION OF THE FILMS ....................................................................................... 13 1.2.1. Thickness shear mode resonator ................................................................................. 13 1.3. PERSPECTIVE OF THE STUDY ................................................................................................ 15 2. EXPERIMENTAL SECTION ................................................................................................. 16 3. BUILDUP AND STRUCTURE ............................................................................................... 19 3.1. BUILDUP OF RIGID-ROD TYPE POLYELECTROLYTES .............................................................. 20 3.2. LAYER INTERPENETRATION .................................................................................................. 23 3.3. THE HOFMEISTER EFFECT..................................................................................................... 28 3.4. EFFECT OF TEMPERATURE .................................................................................................... 32 4. PHYSICAL PROPERTIES ..................................................................................................... 37 4.1. ELECTRICAL AND OPTICAL PROPERTIES ............................................................................... 38 4.2. VISCOELASTIC PROPERTIES .................................................................................................. 40 4.3. LAYER MATRIX REPRESENTATION ........................................................................................ 44 4.4. EFFECT OF COUNTERIONS ..................................................................................................... 47 5. SUMMARY ............................................................................................................................... 51 6. REFERENCES.......................................................................................................................... 53
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List of symbols and abbreviations Polyelectrolytes used in the study. (See section 2 for the chemical structures) PSS PDADMA PXV P3TOPS P3TOPA PAH
Sodium poly (4-styrene sulfonate) Poly(diallyldimethylammonium chloride) Poly(p-xylyleneviologen bromide) Sodium 3-(3’-thienyloxy)propanesulfonate 3-(3’-thienyloxy)propylethylammonium bromide Poly(allylamine hydrochloride)
Symbols used in the equations β φ K Z T vp v
= = = = = = = ζ = ρ = G = J = m = ω = m or Г = δ = *
Growth exponent Galvani potential Association constant Bulk acoustic impedance Shear stress Phase velocity of the acoustic wave Particle velocity Local acoustic impedance* Density of the material Shear modulus of the material* Shear compliance of the material* Complex mass impedance of the material Angular frequency of the oscillation Areal mass density of the material Loss angle
Complex parameters are denoted as following: Re(G ) = G′ and Im(G ) = G′′
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List of publications
I
Lukkari, J.; Salomäki, M.; Viinikanoja, A.; Ääritalo, T.; Paukkunen, J.; Kocharova, N.; Kankare, J. Polyelectrolyte multilayers prepared from watersoluble poly(alkoxythiophene) derivatives J. Am. Chem. Soc. 2001, 123, 6083-6091.
II
Lukkari, J.; Salomäki, M.; Ääritalo, T.; Loikas, K.; Laiho, T.; Kankare, J. Preparation of multilayers containing conjugated thiophene-based polyelectrolyte. Layer-by-layer assembly and viscoelastic properties Langmuir 2002, 18, 8496-8502.
III
Salomäki, M.; Loikas, K.; Kankare, J. Effect of polyelectrolyte multilayers on the response of a quartz crystal microbalance Anal. Chem. 2003, 75, 5895-5904.
IV
Salomäki, M.; Tervasmäki, P.; Areva, S.; Kankare, J. The Hofmeister anion effect and the growth of polyelectrolyte multilayers Langmuir 2004, 20, 3679-3683.
V
Salomäki, M.; Laiho, T.; Kankare, J. Counteranion-controlled properties of polyelectrolyte multilayers Macromolecules 2004, 37, 9585-9590.
VI
Salomäki, M.; Vinokurov, I.; Kankare J. Effect of temperature on the buildup of polyelectrolyte multilayers. Langmuir 2005, 21, 11232-11240.
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1. Introduction Organic thin films have, for a multitude of reasons, attracted a lot of attention over the years. It is now more than 200 years since Benjamin Franklin observed that a drop of oil spreads wide over the surface of water, having calming influence on the surface.1 In an academic world, the surface modification by a thin film of organic molecules has been in the interest of surface scientists for decades. Nanoscale control in the organic film deposition by self-organization has been the one of the important aims to achieve. Similar nanolevel self-organization can be found in many of the nature’s own syntheses. By depositing thin organic film on to a solid surface, the material may exhibit properties which are not accessible with inorganic materials. A solid material covered with organic thin film might show remarkable optical, electrical, mechanical, surface protecting, hydrophobic, hydrophilic and anchoring properties. The properties can be selectively modified by the nature of the thin film on the surface, at the same time leaving the bulk properties of the substrate material unchanged. There are several different routes to accomplish a surface coating of required organic molecules by self-assembly. As early as 1917 Langmuir carried out the study of monolayers of amphiphilic molecules formed on the water-air interface.2 Later Blodgett developed a method of transferring the Langmuir monolayers on to a solid surface,3,4 providing a base for Langmuir-Blodgett (LB) monolayer depositing technique. In a conventional film deposition with the LB technique, a drop of volatile solvent containing amphiphilic molecules is dispensed on the water-air interface. The amphiphiles orient themselves so that the hydrophilic head groups are pointing towards the water phase. The LB film is then transferred on to a substrate by moving the substrate through the interface in a direction determined by the hydrophilic or hydrophobic nature of the substrate. The LB method provides a useful tool to build ordered ultrathin molecular assemblies. The method has high demand of the purity and the expertise of the operator. Besides, the amount of suitable materials is limited, which is because of requirement of amphiphility of the depositing molecule. The LB technique is considered to be an efficient and well studied method of making thin organic films and the interest towards it has been strong, but mostly academic. Self-assembly of ultrathin organized organic monolayers, provides a possibility to create a thin film of organic molecules, which is more strongly attached on to a surface than it is in the LB films.5,6 Self-assembled monolayers (SAM) are unidirectional layers formed on a solid surface by a spontaneous organization of molecules. A film of covalently or ionically bound small molecules is formed when a substrate is dipped in to a solution
7
containing particular amphiphilic molecule, for example silane, thiol or carboxylate. The spontaneous assembly has demands on the substrate surface material, since the covalent bond formation is specifically dependent on the substrate-molecule pair. Thiol compounds and gold is one of the well-established combinations. SAMs are nowadays well studied systems that hold great promise for applications in different areas including molecular recognition and surface modification. A step towards the multilayer system was taken when Iler described the formation of multilayer assemblies by spontaneous adsorption of oppositely charged particles.7 The technique included dipping a substrate in aqueous solutions containing particles of opposed charge. It was then couple of decades later when Decher and coworkers8,9 discovered the technique of layer-by-layer (LbL) deposition of polyelectrolytes to form a structure known as polyelectrolyte multilayer.10 The versatile and rather easily manageable polyelectrolyte multilayer films provide the base for this thesis.
1.1. Layer-by-layer deposition
Preparation of ultrathin polyelectrolyte multilayers by alternating adsorption of polyanions and polycations defeats the limitations that are recognized in the LB and SAM techniques. The basic principle of the multilayer deposition is presented in the Figure 1.1. A charged substrate of any size or shape can be immersed into a solution of polyelectrolyte with a net charge opposite to the surface charge. The polymer adsorbs on to the substrate because of the electrostatic interactions within a couple of minutes,11,12 resulting in the surface charge reversal.13,14 Rinsing removes the unbound polymer from the substrate, preparing it for the next deposition step of oppositely charged polyelectrolyte. This rather simple technique has several advantages. The film synthesis is straightforward, inexpensive, easily automatized and reproducible. No harsh solvents are needed, making this environmentally friendly “kitchen table” synthesis available to everyone, even without expensive laboratory facilities. There are no strict restrictions to the substrate size, shape or material. The polyelectrolyte layers can be even sprayed on the surface.15 There is also a huge amount of molecules that can be used in the multilayer formation. Incorporating different materials into the multilayer films is fairly simple. In principle, any macromolecule that is charged when dissolved in aqueous or organic solutions is potentially suitable. There is a considerable amount of materials that meet the earlier mentioned rather loose criteria. These are for example: conventional synthetic polyelectrolytes (referring to majority of the published papers), conjugated
8
polyelectrolytes,16,17 latex particles,18 inorganic nanoparticles,19 macromolecules of biological interest,20 and even relatively small molecules such as dyes.21
-
1.
-
+ +
+
+ + + +
2.
+ +
+ +
+ + +
-
+ +
-
+ ++ -
+ +
+
+ +
+ +
+
-
-
+
-
-
Figure 1.1. Layer-by-layer adsorption technique. 1. Immersion of a charged substrate in to solution of oppositely charged polyelectrolyte, followed by rinsing of the substrate to remove external material. The substrate charge will be reversed. 2. Immersion of substrate in to solution of again oppositely charged polyelectrolyte, followed by rinsing.
1.1.1. Buildup and structure of the films
The thickness of the deposited polyelectrolyte layer depends strongly on the ionic strength of the polyelectrolyte solution.22,23 High ionic strength induces polyelectrolyte charge neutralization by the counterions, leading to formation of extensively coiled and gradually more globular polyelectrolyte molecule. As a result of that more polyelectrolyte is deposited on to the oppositely charged surface. The deposited material intermingles extensively with previously adsorbed layers and forms loops and tails on the surface. Other important factors affecting the deposition of the multilayer are found to be: electrolyte type,24 polymer charge density,25,26 pH (especially for weak polyelectrolytes),27 deposition time,24 rinsing methods, quality of the solvent,24,28 polymer contour length,29 and temperature.30
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The individually deposited layers in the multilayer system are known to be highly entangled.31,32 Therefore, a single deposition step with flexible polyelectrolyte leads to mass and thickness increase of the particular film, but, in the most cases, the deposited polyelectrolyte will not form a discrete layer.10 After each deposition step, the surface charge must be reversed in order to deposit the next polyelectrolyte layer. In addition to neutralizing the surface charge, the amount of the deposited polyelectrolyte must be large enough for overcharging the surface by the charge carried by the polyelectrolyte. The loops and tails formed by the deposited polyion are attributed to take part in the charge overcompensation on the surface.13,33,34 The charge overcompensation on the surface has been detected in the zeta-potential measurement, and the potential of the surface varies symmetrically with each layer.35,36 The final structure of a multilayer film is assumed to consist mostly of 1:1 complexes of polyelectrolytes,14,24,37 but there are also reported deviations of that.38,39 The presence of counterions inside the multilayer film has been discussed controversially. Small counterions are claimed to be removed during the washing period, leading to a locally neutral polyelectrolyte complex.40,41 Nevertheless, it has been shown that the multilayer involves both polyions and small ions42 and up to 30% of the charged sites on polyions may be bound by oppositely charged fluorescent probes.43 According to a paper on the multilayer structure, the polyelectrolyte multilayer film is proposed to be composed of three rather different zones,44 as indicated in the Figure 1.2. The thin layers in the zone I are influenced by the properties of the substrate and it could be also treated as an adhesion material between the substrate and the bulk material of the film. The zone II forms the bulk of the film, where the charge is assumed to be compensated by polyelectrolyte complexes via intrinsic charge compensation. The zone III is in direct contact with the solution, with at least partial charge compensation by the counterions. The charge overcompensation is attributed to take place in the zone III. It has been proposed that the excess charge extends several layers inside the film.45 The zone model proposes that the zone III could be swollen in pure water because of the possible loops and tails reaching far away in to the solution, while the swelling of the zone II would require increasing ionic strength to partially break the bonds between the oppositely charged polyelectrolytes.45 The boundaries of the zones are rather diffuse. When the first couple of layers are adsorbed, the thin film can be apparently treated as a mixture of zones. At this stage the film has the film-solution interface (zone III) and filmsubstrate interface (zone I) with rather diffuse structure. If the number of layers is increased the growth in the multilayer is proposed to take place in the zone II. The growth also eventually sets the zones I and III completely apart. This three-zone model of the film structure has been taken as a guideline in this thesis.
10
III II III II I 1
2
I
3
Figure 1.2. The zone model of polyelectrolyte multilayers. 1. Very thin film comprised of few layers on the surface, showing features of all zones without clear zone separation. 2. A film with enough layers for the zones to separate. 3. Thick film, which is dominated by the zone II.
The sequential surface charge reversal is usually reproducible. For that reason, a monotonous buildup of the multilayer film could proceed for hundreds (Papers III and V) or even presumably for thousands of times. The regular buildup of the multilayer film is considered to be possible after the precursor layers in the zone I are formed.46,24 The amount of layers depends on the properties of the polyelectrolytes and the ionic strength of the solution. The linear buildup regime can be regarded as a steady state phase where the deposition of a polyelectrolyte, with a finite interpenetration range, generates a constant mass and thickness increments in the developing film. The nonlinear buildup regime in the polyelectrolyte multilayer assembly is a rather common feature. However, it has been demanding to explain the exact mechanism behind this phenomenon. Some studies support the surface roughness mechanism. In that model the nonlinear buildup is assigned to the continuous increase of the surface roughness, which leads to an increase of the physical surface area that is available for the adsorption. The exponential buildup that is assigned to the surface roughening is proposed to take place when slightly charged polymers are used.47,48 It is also observed under conditions where the charges of the highly charged polymer are compensated with high salt concentration.49 Some models explain the nonlinear buildup by the increased penetration length of the adsorbed polymer.45 The penetration length describes an active volume that is capable for intake of the depositing polymer. This active volume concept corresponds to the zone III
11
of the polyelectrolyte zone model. An alternative approach to the exponential buildup has been taken in the theory of the diffusion of polyions in and out of the film.50 In this theory the whole film can be realized as an active volume in which at least one of the polyelectrolytes can freely diffuse. The internal diffusion has been proposed and demonstrated for some polyelectrolytes of biological nature.51,52,53,54 It is noteworthy that the buildup regime is usually, for some reason, considered to be an inherent property of a distinct polyelectrolyte. Based on the published data one might get an idea that the conventional synthetic polyelectrolytes adopt mostly the linear buildup regime while the polyelectrolytes of biological nature might provoke the exponential buildup of the multilayer.
1.1.2. Applications
The number of patents that has been applied for the applications for polyelectrolyte multilayers is increasing all the time.55 The commercially available multilayer applications are so far: contact lens with a polyelectrolyte multilayer surface material, free standing electrically conductive elastomeric nanocomposite film (Metal Rubber), and a sheet that reduces ripening of fruits and vegetables (Yasa-sheet). Considering the short history of the technique, it is not surprising that there are, up to now, no more applications commercially available. However, there are numerous promising applications for polyelectrolyte multilayers which could have commercial potential. The layer-by-layer deposition technique is well suited for the construction of precise nanolevel architectures, which are required for the development of sensors for different purposes. Some examples of the sensor applications are: chemical sensors,56,57 pH sensors,58 biosensors,59,60 and humidity sensors.61 Also membranes for separation62,63 and selective permeation64 can be built with this method. The future electronics may include totally polymeric electrical components, which can be built with the multilayer technique. This group includes electrical components such as: conducting polymer films,65,66,67,68 light-emitting diodes (LEDs),69,70 transistors71,72 and nanowires.73,74 Deposition onto arteries might be useful in repairing damaged blood vessels.75 Hollow spheres, or multilayer microcapsules, provide an entirely independent field of study.76,77 The spheres are suggested to be used for example in drug delivery applications. Surface modification is needed in a variety of applications, for example surface protection,78,79 enzyme immobilization80,81 and electrode modification.82 In addition to the surface bound material, also freestanding films can be built with this
12
method.83 Patterned multilayers provide an external dimension, making available the possibility to build three-dimensional nanolevel structures.84,85,86
1.2 Characterization of the films
Various analytical methods can be used in the characterization of multilayer films. This thesis concentrates mainly on quartz crystal microbalance but also some other techniques are used. One of the most popular methods for studying the polyelectrolyte multilayers is simple UV-Vis spectroscopy. In this method the films must be prepared on a transparent substrate and the polyelectrolytes have to absorb in the operating wavelengths. Some of the most employed polyelectrolytes PDADMA and PAH, however, do not absorb in the UV-Vis range. Among the widely used methods are also atomic force microscopy (AFM)87,88,89 and ellipsometry.90,91,92 These methods give valuable information about the dimensions and the morphology of the film. Electrochemical techniques provide information about the electroactive species inside the film.93,94,95 The electrochemical techniques can also be rather easily incorporated with spectroscopy to achieve more specific information about the electrochemical reactions inside the film. The abovementioned methods are used in this thesis. In addition to those there are many efficient methods for studying polyelectrolyte multilayer films, including, for example, X-ray and neutron reflectivity,96,97 nuclear magnetic resonance,98,99 small angle neutron scattering,100 zeta-potential measurements,35 contact angle measurements,101 optical microscopy,102,103 luminescence methods104 and IR-spectroscopy.87 The list of the methods mentioned here is not comprehensive and the amount of compatible methods is increasing as the multilayer study proceeds.
1.2.1. Thickness shear mode resonator
The quartz crystal microbalance has a central role in this thesis. The advantages of the technique are that it is a mass sensing method and there is no need for a labeling step in the deposition process. Layer formation and adsorption kinetics can be conveniently observed even in optically opaque solutions. Changes of viscoelastic properties can be observed in situ in the depositing film. Regardless of the fact that the QCM technique is
13
routinely used in thin film study, the fundamental theory of quartz crystal oscillator system is often considered rather complicated and the QCM-analysis is usually reduced to the level of the classical Sauerbrey equation.105 Since the basic theory of the quartz crystal oscillator forms an important part of the thesis, some aspects of the theory are clarified briefly in the later sections. The AT-cut quartz crystal acts as a thickness shear mode resonator (TSR), where the surface can be activated to oscillating shear deformation. The QCM technique is, at the moment, utilized routinely as a gravimetric probe in the fields of electrochemistry106 and biochemistry,107 for example, providing a base for different sensors.108,109 Characterization of thin rigidly coupled films on the surface of the quartz crystal resonator is the most well-known application. This aspect of QCM has been utilized nearly fifty years. Frequency changes of the oscillator can be directly related to the areal mass density using the classical Sauerbrey equation.105 The earliest experiments were done in vacuum and it was, for a long time, thought that utilization of the resonator under liquid was impossible because of the high damping effect of the viscous solution. This was until 1980, when Nomura et al.110 proved that the QCM oscillator could be used under liquid by increasing the power in the crystal circuit. This capability has given the opportunity to the wide range of in situ studies under liquid, including adsorption,111 desorption and reaction kinetics112,113 at the surface. Later it was realized that additional information could be obtained by recording the energy losses of the crystal oscillation.114 This crystal impedance technique gives valuable information on the deviations from rigidity of the substrate bound material. On the other hand, the physical properties of the substrate bound material can now be revealed if the system is analyzed correctly. Equivalent circuits are commonly used to describe the physical properties of the mass loaded quartz crystal resonator.115,116 These circuits allow the analysis of not only rigidly coupled, but also viscoelastic layers and liquids. The most commonly used equivalent circuit is called Butterworth-van Dyke circuit. The components of the circuit describe the mechanical properties of the loaded quartz crystal resonator. These equivalent circuits give rather simple picture of the properties of the loaded resonator. However, the precise interpretation of the elements and the parameters is rather questionable.117,118 The use of equivalent circuits is reasonable when they are applied for studying a change that takes place in a particular system. The comparison between two different systems by their component values, with no information of the physical behavior of the surface bound material, should be done with a great concern. The same applies to the characterization using merely the frequency change and the dissipation coefficient, obtained from the QCM-D119 equipment. It should be noted that all the parameters describe the basic physical oscillation behavior of the quartz crystal resonator, and can finally be brought into a common form, being therefore comparable.
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1.3. Perspective of the study
The number of research articles considering the buildup and the properties of the polyelectrolyte multilayers has had a nearly exponential increase since the invention of the layer-by-layer deposition method. Despite that, rather little is still known about the exact mechanism behind the multilayer film formation. To exploit fully the possibilities of this simple, yet effective, method, it is necessary to understand the process of formation and the factors that influence the buildup, the overall structure, and the physical properties of the particular films. This thesis concentrates on these basic and fundamental aspects of the polyelectrolyte multilayers. The experimental conditions affecting the abovementioned aspects of polyelectrolyte multilayer films are studied in detail utilizing well-established methods along with some new innovative approaches. The paper I concentrates on conducting polyelectrolytes presenting the first polyelectrolyte multilayer that is prepared solely from conducting polymers, using polythiophene derivatives as both polycation and polyanion. Also the extent of layer interpenetration in the polyelectrolyte multilayers is addressed in this paper. The paper II concentrates on the softness of thin polyelectrolyte multilayers and the factors relating to that. The role of rigidity and the hydrophobicity of the polyelectrolyte in the buildup process is also discussed in the paper II. The paper III focuses on new methods to calculate the elastic parameters from the polyelectrolyte multilayers. The effect of counterions is discussed in the papers IV and V. The Hofmeister effect in the buildup of the polyelectrolyte multilayers is discussed in the paper IV. The calculation methods presented in the paper III are used in the paper V to investigate the counterion effects on the elastic properties of polyelectrolyte multilayer films. The paper VI concentrates on the effect of temperature on the nature and general mechanism of the film buildup.
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2. Experimental section
The selection of polyelectrolytes that are used in this study includes mainly commercial polyelectrolytes: sodium poly(4-styrene sulfonate) (PSS), poly(diallyldimethylammonium chloride) (PDADMA) and poly(allylamine hydrochloride) (PAH). There are a number of studies that are carried out using these particular polyelectrolytes and therefore, the behavior of the polyelectrolytes can be easily referred to the literature. With the aid of characterization of the films, and for making novel functional films, we have synthesized two conjugated polythiophene derivatives namely sodium 3-(3’thienyloxy)propanesulfonate (P3TOPS) and 3-(3’-thienyloxy)propyltriethylammonium (P3TOPA). Poly(p-xylyleneviologen bromide) (PXV) is used as a model compound of a hydrophobic polyelectrolyte. The synthesis of the polyelectrolytes is described in the paper I. The chemical structures of the polyelectrolytes are presented in the Figure 2.1. Polyelectrolyte multilayers for the papers I and IV were prepared on the charged surfaces by sequential dipping. In the paper II the multilayers were prepared manually in the flowtrough cell. In the papers III, V and VI, the multilayer films were made by using an automated multilayer deposition system, which was build for this particular purpose. The fully automated LbL-machine (Figure 2.2) with QCM measurement system consists of a flow cell (Figure 2.3) and a computer controlled peristaltic pump with a multi-position valve for switching between the coating and the rinsing solutions. The flow-cell is placed in a container inside a thermostat bath (Lauda ultra-kryomat RUK-60) in order to achieve the accurate thermal stability needed in the quartz crystal measurements. The cell temperature is monitored with a calibrated thermistor during the measurement. The whole deposition and measurement system is controlled by a computer program written with the National Instrument’s LabVIEW general instrumentation utility. The crystal parameters are measured using a prototype crystal analyzer with the impedance detection, based on the principle of double sideband modulation and lock-in detection.120 The commercial instruments in the experimental setup are Stanford research systems SR830 lock-in amplifier and DS345 frequency synthesizer.
16
n N
+
N
+
n
Poly(p-xylyleneviologen bromide) (PXV)
SO 3-
Sodium poly(4-styrene sulfonate) (PSS)
SO 3O
n
n
S
+
Sodium 3-(3’-thienyloxy)-propanesulfonate (P3TOPS)
N
Poly(diallyldimethylammonium chloride) (PDADMA)
+
N
O
n
S
n
+ NH3
3-(3’-thienyloxy)-propyltriethylammonium bromide (P3TOPA)
Poly(allylamine hydrochloride) (PAH)
Figure 2.1. Chemical structures of the polyelectrolytes used in the multilayer deposition.
17
Frequency synthesizer
Computer
Modulated high frequency signal
Measurement probe with signal demodulation
Low frequency modulation source Lock-in amplifier
Quartz crystal Low frequency signal
Solution1 Solution2
Multi-position valve
Pump
Solution3
Figure 2.2. The fully automated LbL deposition and measurement system.
Figure 2.3. The flow-trough cell built for the QCM measurements. White arrows indicate the connection of the crystal and the lower part of the cell to the upper part of the cell. The spring holds the quartz crystal in place with a constant force. The black arrows show the direction of a liquid flow in and out of the liquid compartment of the cell.
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3. Buildup and Structure
To utilize completely the advantages of the layer-by-layer deposition method, it is necessary to understand the factors affecting the buildup and the structure of the films. The variation of ionic strength is the first discovered and the most widely used method to control the amount of deposited material in a single deposition step. In addition, there are factors that have caught rather little attention although they have proved to be relatively important. The charge screening of a polyelectrolyte increases if the ionic strength of the solution is increased, but the charge screening is also specifically dependent on the polyelectrolyte counterion pair, leading to a variation in the thickness of the deposited multilayer.24,121 The wide variety of accessible electrolytes provides a useful approach to fine-tune the thickness, among with other properties, of the polyelectrolyte multilayer. Properties of polyelectrolytes, for example, rigidity and hydrophobicity, have also a characteristic role in the buildup process. In addition, reasonably large effects on the buildup have been observed when the deposition temperature has been changed.30 In most cases the buildup adopts a linear pattern, which is easily understandable. Also nonlinear buildup pattern in polylectrolyte multilayers is a relatively common feature. However, it has been difficult to explain the mechanism behind this phenomenon and it has raised rather extensive discussion. The structure of the deposited multilayer has a profound effect on the properties of the multifunctional films. One of the key issues in the development towards multifunctional devices is the precise control and understanding of the multilayer nanostructure. Many applications require high dimensional accuracy, such as polymer LEDs, for example. The LED structure demands discrete layers of insulators, semiconductors and conductors.122 Therefore, in order to optimize the structure of the polyelectrolyte multilayer at the nanolevel, an accurate control of the layer interpenetration and information of the penetration depth of polyelectrolytes are needed.
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3.1. Buildup of rigid-rod type polyelectrolytes
Conjugated aromatic polyelectrolytes are an example of rigid-rod polymers. The rather hydrophobic backbone combined with pendant ionic groups makes them liable to aggregation.123 The polythiophenes utilized in the papers I and II are highly colored, which makes possible to follow the buildup of multilayers using UV-vis spectroscopy. It is shown in the Figure 3.1 that the amount of polythiophene oscillates regularly during the multilayer buildup. Considerable amount of polythiophene detected on the surface after the standard washing procedure is removed when the next layer of PSS or PDADMA is adsorbed. The average addition of polythiophene per layer is calculated to be 1.0 nmol/cm2 for P3TOPS and 2.8 nmol/cm2 for P3TOPA (with respect to monomer concentrations), while the average amount of polymer left on the surface is only less than half of that. Despite that, the amount of polythiophene that remains in the multilayer increases linearly with the number of bilayers in both cases. For that reason it is assumed that there is normal layer formation with charge overcompensation after each deposition step. The partial removal scheme suggests that part of the previously adsorbed polythiophene either forms a soluble complex with the added polyelectrolyte or is displaced in other way, such as, spontaneous desorption. The similar partial desorption step, leading a zigzag shaped graph, has also been observed with polyelectrolytes having medium a charge density.127,124 In all-thiophene multilayers (P3TOPS/P3TOPA), the average additions of P3TOPS and P3TOPA are calculated to be 0.95 and 1.7 nmol/cm2 per layer respectively, but the next polythiophene addition always removes part of the previous layer, leaving behind only up to half of the formerly adsorbed material also in that case. Based on the evidence shown here, the desorption is therefore attributed to an inherent property of polythiophenes. It should be pointed out that the thiophene coverage obtained after the desorption is estimated to be very close to the monomolecular film.
20
8
2
thiophene (nmol/cm )
7 6 5 4 3 2 1 0 0
2 4 6 8 number of polyelectrolyte layers
10
Figure 3.1. Spectrophotometric determination of the thiophene based polyelectrolytes in multilayers: Polythiophene on top (solid symbols), PSS or PDADMA on top (open symbols). ■,□ (solid line) PSS/P3TOPA; ○,● (solid line) P3TOPS/PDADMA; ●,■ (dotted line) P3TOPS/P3TOPA; ▲,Δ (dashed line) P3TOPS/PDADMA (assembly in DMSO-H2O, 80/20 v/v). Even layers correspond to polycation on top.
In case of flexible polyelectrolytes, the layer thickness can be easily adjusted by increasing the ionic strength of the deposition solution.125 In P3TOPS/P3TOPA deposition, however, the increase of ionic strength from 0.1 M to 0.5 M does not have a significant effect on the thickness. Similar polyelectrolyte layer buildup that is independent on the ionic strength has been observed for PDADMA-copolymer with less than 50% charged monomers.126,127 Nevertheless, the charge densities of P3TOPS, bearing sulfonate groups, and P3TOPA, bearing tertiary ammonium groups, are high and even though the charge density of P3TOPS is slightly reduced because of oxidation and the self-doping effect,128,129 it has been shown that the multilayer buildup proceeds regularly irrespective of the oxidation state of polythiophene.16,17 The independence on the ionic strength is therefore attributed partially to the short contour length (only about 30 repeating units) and partially to the rigidity of the polythiophene chain. It is certain that this kind of polyelectrolyte chains cannot easily form a coiled structure which is generally considered to be responsible for the increase in the layer thickness caused by the high ionic strength.130 Instead of coiling, these polythiophenes tend to form interchain aggregates.131,132 P3TOPS, for example, forms stable aggregates in the electrolyte
21
solutions with an average size of 200 nm (paper I). This is mostly because of the hydrophobic polymer backbone. Aggregates are held together with weak hydrophobic and stacking interactions. The aggregation of the polythiophene chains, followed by deaggregation on the surface, can be considered to be responsible for the partial removal of polythiophene layers upon adsorption of the next polyelectrolyte layer. Also, the utilized polythiophene chains are rather short, which has been attributed to the tendency to form soluble polyelectrolyte complexes.133 Further support for the aggregationdeaggregation assumption is obtained from the multilayer deposition from the dimethyl sulfoxide (DMSO)-water mixture. The DMSO medium has shown clear deaggregation behavior of polythiophene solutions (paper I). The average mass increase after the deposition of polythiophene is lower from the DMSO-water solution than from the water solution. Adding the next layer leads to the desorption of polythiophene from the film, but the desorbed amount is much smaller than in the multilayers deposited in water, which is a clear indication of the role of aggregates in the desorption process carried out in the multilayer deposition with rigid-rod type polyelectrolytes. The aggregates can also be rinsed out of the surface by careful washing. It is shown in the paper II that the extended washing period of 15 minutes was found to remove part of the P3TOPS layer and, in this case, the adsorption of the PDADMA layer does not lead to additional desorption of P3TOPS. Therefore, the adsorption of the next polyelectrolyte layer and the extended washing can be considered as alternative procedures leading to similar multilayer buildup. To summarize, the aggregation-deaggregation that is found using hydrophobic rigid-rod type polyelectrolytes, implies that only those chains which are electrostatically bound to the oppositely charged polyelectrolyte are expected to survive the extended rinsing or the tendency to form soluble complexes in the next assembly step.
22
3.2. Layer interpenetration
The polyelectrolytes that are capable of forming 1:1 polyelectrolyte complexes have been generally noticed to form complexes in the multilayer film even through several layers. It is therefore assumed that the layer interpenetration is a natural property of polyelectrolyte multilayers. The extent of interpenetration, however, is dependent on the experimental conditions. The single layer in a PAH/PSS multilayer has been found to penetrate through two bilayers11 and, as a result, the polyelectrolytes do not form strictly stratified layers but show an overlap about four neighboring layers in both directions.134 The nature of the separating layers has also an important role in the layer superstructure. One thick (ca. 5.7 nm) poly(acrylic acid)/PAH bilayer can block the energy transfer between the layers, but even eight thin PSS/PAH layers (ca. 5.3 nm) are not enough for total insulation.135 Four PAH/PSS bilayers are found to be enough to prevent the charge transfer between the electrode and the electroactive polyelectrolyte layer.136 The paper I deals with the interpenetration phenomenon on the basis of the charge transfer between the electrode and the conjugated polyelectrolyte layer through the insulating polyelectrolyte multilayer. In the paper I, the polyelectrolyte multilayers are prepared in a such way that the single layer of P3TOPS was adsorbed at the top of the insulating multilayer of varying thickness, as indicated in the Figure 3.2.
PDADMA P3TOPS PDADMA or PXV PSS PDADMA or PXV
e-
PSS PDADMA or PXV PSS PDADMA or PXV Au-substrate
Figure 3.2. The structure of insulating polyelectrolyte multilayer. The number of insulating layers varied.
23
It is advantageous to use ac-voltammetry instead of dc-voltammetry to study the charge transfer through insulating layers.137 The method presented in the paper I combines the ac voltammetry and spectroelectrochemical techniques to obtain steady state acelectroreflectograms, which are the magnitude of an ac reflectance signal 1/R(dR/dE) as a function of potential. The reflectance signal is created from the reversible changes in the oxidation state of the polythiophene layer on top of the insulating film. The advantage of the method is that the ac-reflectance signal is created only by electroactive polythiophene and the interference caused by other processes is eliminated. The peak values of electroreflectograms plotted vs. the frequency of the ac-signal show a sigmoidal shape (Figure 3.3). That is because the charge transfer between the surface and the polythiophene layer that occurs in low frequencies becomes impossible after a certain limiting frequency range because of a limited charge transfer rate.
normalised 1/R (dR / dE) peak
1.0 0.8 0 0.6 1 0.4
5 3 7
0.2 0.0 0.01
0.1
1 10 100 frequency / Hz
1000 10000
Figure 3.3. The electroreflectance response as a function of excitation frequency for the multilayers with different number of insulating PDADMA and PSS layers deposited in 0.2 M Na2SO4. Experimental points and fitted curves are shown.
As the number of the insulating PDADMA/PSS layers between the electrode and the P3TOPS layer increases, the sigmoidal reflectance curves move to lower frequencies, indicating a decrease in the charge transfer rate. There is up to four orders of magnitude
24
shift in the frequency when the number of layers between the substrate and polythiophene layer is increased. A complete break of connection between the substrate and the polythiophene layer is observed with nine insulating layers, showing no signal in the electroreflectance measurements. The charge transfer rate constants can be calculated from the measured data using a simple equivalent circuit method created for studying the charge transfer in monolayers.138 The rate constants obtained from the fit are presented in the Figure 3.4. The insulating PSS/PDADMA multilayers induce a considerable drop of charge transfer rate constant when going from zero to seven insulating layers. If the insulating layers are deposited at lower ionic strength, the rate constants are lower after three insulating layers, suggesting that the PSS/PDADMA layers deposited at lower ionic strength are better insulators than the layers deposited at higher ionic strength. At this point it should be noted that the distances are measured from dry films and polyelectrolyte multilayers are known to swell in liquid, but the measurement conditions were the same for each sample. A couple of additional measurements clarified the role of the ionic strength. The first multilayer of PSS/PDADMA was deposited at lower ionic strength and P3TOPS at higher ionic strength and the second multilayer was prepared conversely. In the first case the rate constants increased to the same number as with thick PDADMA/PSS layers. In the latter case, the rate constants experienced a considerable drop, not reaching the same value than with thin insulating layers, but close enough to have a clear picture of the behavior (Figure 3.4). A suggested mechanism for the behavior is that, when the thin insulating multilayer, prepared at low ionic strength, is exposed to a P3TOPS solution with high ionic strength, the ionic bonds within the polyelectrolyte multilayer are opened and the film becomes swollen enabling the polythiophene chains to penetrate more easily into the film. On the other hand, the permeable and swollen bulk of the PSS/PDADMA formed at higher ionic strength becomes more impermeable upon exposure to a P3TOPS solution of low ionic strength. Therefore, the adsorbing polyelectrolyte sees the previously adsorbed layers in a state that is determined by the ionic strength of the current solution. The apparent conclusion is that the ionic strength of the solution used for the adsorption of a particular polyelectrolyte layer is more important for the interpenetration of this layer than the ionic strength of the solutions used for the adsorption of the other layers, or their thickness.
25
0
1000 0
1 1 1
-1
k ct (s )
100
3 3 3
10
5
5
5
5
1
5
7 7 0.1 0
7
7
5 10 insulating layer thickness (nm)
15
Figure 3.4. The charge transfer rate constants calculated from the electroreflectance data for different multilayers as a function of the insulating layer thickness. PDADMA/PSS insulating layers : (■), all layers assembled in 0.2 M Na2SO4; (○), all layers assembled in 0.02 M Na2SO4; (Δ), insulating layers assembled in 0.02 M Na2SO4, P3TOPS in 0.2 M Na2SO4; (∇), insulating layers assembled in 0.2 M Na2SO4, P3TOPS in 0.02 M Na2SO4. PXV/PSS insulating layers: (+), all layers assembled in 0.2 M Na2SO4. The number of insulating layers indicated for each point. The lines shown are only guides to the eye.
The nature of the polyelectrolyte plays also an important role in the layer interpenetration.135 If the insulating multilayer consists of PXV/PSS, the layers are thin but insulate better than the PSS/PDADMA system deposited at the same ionic strength. This might be explained by the solvation effect, since the PXV/PSS-layers contain hydrophobic PXV chains and swelling in that multilayer is apparently less significant. The specific electron transfer mechanism in the multilayer system allows the charge transfer even through 15 nm of insulating layer. However, the electron transfer rate in the insulating media should decay exponentially if the distance between the donor and the acceptor is increased, with a decay constant of ca. 1 Å.139 A significant drop in the charge transfer rate in the studied multilayers takes place after adsorption of one insulating layer (ca. 1 nm). However, the drop is certainly much smaller than would be expected based on the distance dependence of electron transfer. Therefore it is assumed that the interpenetrated polyelectrolytes must carry the charges inside the insulating layer and the charge transfer rate can be regarded as a measure of the interpenetration inside the multilayer. The interpenetration depth varies from 7 to 15 nm and is smallest when the
26
isolating layer is made of PSS/PXV in 0.2 M Na2SO4, a little higher when the layer is made of PSS/PDADMA in 0.02 M Na2SO4, and highest when the layer is made of PSS/PDADMA in 0.2 M Na2SO4. In all cases the penetration depth is equal to seven (or less than nine) layers. This is in accordance with the reported data on the layer interpenetration.11,134,135,136 An important conclusion is also that the layer interpenetration in the PSS/PDADMA multilayer is more dependent on the number of layers than their thicknesses. On the basis of the layer interpenetration results, the studied multilayer can be realized as an insulator/conductor blend material. The multilayer film with 9 insulating layers has an entirely conducting region at the top of the film and entirely insulating region at the surface of the electrode. Further support for the assumed physical model is obtained from the value of reflectance signal maximum, presented in the Figure 8 in the paper I. The signal is growing with the number of insulating layers up to three insulating layers. This is mainly because of an increasing amount of polyelectrolyte per layer, attributed to the initial buildup of the multilayer. Notably, the reflectance signal does not settle to a steady level, as would be expected. A drop of signal is observed after four or more insulating layers on the surface. This is attributed to the partial loss of electrical contact between the polythiophene and the electrode.
27
3.3. The Hofmeister effect
The classical Hofmeister series deals with the interactions of counterions with macromolecules and it has been observed already more than 100 years ago. The phenomenon has been named the Hofmeister effect after its inventor Franz Hofmeister.140,141 The experimental series, which was originally found in the experiments on the precipitation of egg white proteins in the presence of different salts, is presently known to play an important role in several biological phenomena.142 The anion effect has been selected to the focus of the study, instead of the cation effect, since the anions have a much larger difference in polarizability than typical cations, because of the larger variety in diameter. Therefore, it is expected that the anions have larger effect also on the forming polyelectrolyte multilayer. Furthermore, the Hofmeister effect of anions has been generally noted to be greater than the effect of cations.143 A cation effect of the polyelectrolyte multilayer buildup has been studied earlier by Dubas and Schlenoff.24 They concluded that the less solvated, stronger binding ion would drive the polyelectrolyte to the interface more effectively. In the paper IV, an intensive anion binding was found to take place in solutions containing NaClO4, NaSCN and NaI. These anions precipitated PDADMA from the solution. The precipitation can be understood on the basis of Hofmeister series (scheme 3.1), because the anions are found at the far side of the series. The PSS/PDADMA multilayers show clearly different thicknesses according to the anion used in the deposition (Figure 3.5). The film thickness order was found to have an apparent similarity to the classical Hofmeister series (scheme 3.1).
ClO4− > SCN − > I − > NO3− > Br − > Cl − > CH3COO− > HCOO− > F − > OH − > HPO42− > SO42− Scheme 3.1. An example of the Hofmeister series of anions, ordered by their ability of salting-in proteins.143 BrO3- and ClO3- are generally considered to be found in proximity of chloride.
28
Br
80
NO3
-
70 Thickness (nm)
-
90
-
ClO3
60
-
50
Cl BrO3
40 30
HCOO F
20
-
10 0 2
4
6
8 10 12 14 16 18 20
Number of layers
Figure 3.5. Thicknesses of dry PSS/PDADMA films determined with ellipsometry. Multilayers are deposited in 0.1 M sodium salt of corresponding anions.
Characterization of the film buildup merely on the basis of the Hofmeister series is not reasonable. That is because there is no universal parameter to quantify the series, although there have been numerous attempts to find one. Besides that, the position of some anions in the series varies depending on the method used to determine their properties.143,144 In the paper IV, the quantification of the experimental data is carried out using two parameters describing the ion-water interaction, namely the viscosity Bcoefficient of the Jones-Dole145 empirical expression146 and hydration entropy of the anion.147 The empirical nature of the Hofmeister series remains, while the mentioned measurable parameters rank the ions into series, which are comparable to the classical Hofmeister series. The viscosity B-coefficient and the hydration entropy of the anion are certainly different type of parameters although they have a fairly good linear relationship with each other.148 The B-coefficient describes the effect of the hydration shell on the surroundings of an anion in terms of the viscosity change of solution, and the hydration entropy describes the tendency of an anion to form or discard the hydration shell. Figure 3.6 reveals the connection between the parameters of the ion-water interaction and the deposited film thickness. Multilayer thickness vs. the B-coefficient of the depositing anion shows an exponential decrease whereas thickness vs. hydration entropy shows an exponential increase. The qualitative interpretation is that the hydration entropy of the anion is apparently more reliable in predicting the layer thickness. On the other hand, it is important to point out that the employed B-coefficient values are averages146 collected
29
from a great number of experimental results, sometimes with considerable variation. The hydration entropy, instead, can be determined quite accurately using an electrochemical method.147 Additional information about the charge screening affinity is obtained from the viscosity measurements (paper IV). Charge screening of polyelectrolytes by counteranions shows an apparent correlation with the viscosity of the polyelectrolyte solution. The counteranion that deposits the thickest polyelectrolyte layer produces the lowest viscosity of the polyelectrolyte solution. Strongly binding anion makes the polyelectrolyte to shrink into more dense form, inducing a lower viscosity. The same effect can be assumed to take place in the polyelectrolyte layer deposition, where the strongly binding anion produces a densely packed polymer layer that creates a large thickness increment in the film. The viscosity of the polyelectrolyte solution can be treated as a qualitative measure of the binding strength of the counteranion, a similar conclusion also being reported earlier.149 The viscosity of the polyelectrolyte solution correlates well with the hydration entropy and the B-coefficient of the counteranion, giving the apparent conclusion that the least hydrated counteranion binds strongest to the polyelectrolyte. Monoatomic ions have shown to have a correlation between the radius of hydrated anion and the polymer binding affinity.149 Since the experimental data in the paper IV included also other than monoatomic anions, the use of hydration radius of anion in the characterization of polyelectrolyte binding121 is not regarded reasonable. That is mainly because the size of the hydrated anion does not describe either the properties of the hydration shell separately, or the factors affecting the existence of the hydration shell. The hydration entropy, instead, has been found to describe the binding affinity between the polyelectrolyte and the counteranion regardless of the structure of the anion. It has been also shown that the Hofmeister series can be used as a guideline for evaluating the binding affinity of counterions in the polyelectrolyte multilayers. The Hofmeister series is typically used in connection with polymers of biological nature, such as, proteins and it has been rarely mentioned in connection with synthetic polyelectrolytes. This is the first time the Hofmeister trend has been related to the properties of polyelectrolyte multilayers.
30
Br
90 -
NO3
80
Thickness (nm)
-
70
-
ClO3
60
Cl
50
-
BrO3
40
HCOO
30
-
F
20 10 0 -0.04
0.00
0.04
0.08
0.12
B-coefficient (dm3 mol-1)
90
Br
80
-
NO3
70
-
ClO3
60 Thickness (nm)
-
Cl
50
-
-
BrO3
40 -
30
HCOO
-
F
20 10 0 -160
-140 -120 -100 -80 -1 -1 Hydration entropy (J K mol )
-60
Figure 3.6. Thickness of ten bilayer PSS/PDADMA multilayer vs. B- coefficient of the counteranion (upper graph), hydration entropy of the counteranion.(lower graph) Symbols: squares and solid line = ellipsometry, circles and dashed line = AFM. The lines are added only as a guide for eye.
31
3.4. Effect of temperature
Temperature has been shown to have an important role in the layer-by-layer buildup.30 Figure 3.7 (paper VI) shows the PSS/PDADMA multilayer deposition at different temperatures using NaBr as the electrolyte. The imaginary part of the local acoustic impedance (Im ζ0 , given in units Rayl=kg m-2s-1) has the first order proportionality to the areal mass density (Γ) of the deposited material in the “Sauerbrey regime”:
ΔΓ ≈
Δ Im ζ 0
ω
(3.1)
Here ω is the angular frequency of the oscillator. Apparently the general mechanism of the buildup process is changed because of the temperature increase. While the buildup observed for the depositions at 15 and 25 oC is mainly linear, the increasing temperature brings along a clear progression in the multilayer buildup. The buildup changes into exponential at higher temperatures. The nonlinearity of the buildup has been discussed extensively in the previously published papers.50,51,52,53 It is shown in the paper VI that the same trend is observed also in other polyelectrolyte systems. The PSS/PDADMA deposition in NaF exhibits a clearly linear buildup at 25 oC and exponential buildup at 55 o C. Because of the smaller mass increments per bilayer in this film than in the film deposited in NaBr, the exponential buildup regime consists of nearly one hundred bilayers. Even though the PSS/PAH -pair has been generally considered as a model example of linear buildup,60,150 this system also follows the same buildup trend. A common feature in all of the studied systems in the paper VI is the expansion of the exponential buildup regime upon increasing temperature.
32
250
o
55 C
Im ζ0 (kRayl)
200
150 o
45 C
o
35 C
100 o
25 C 50
o
15 C
0 0
5
10 15 20 25 Number of bilayers
30
Figure 3.7. PSS/PDADMA multilayers deposited in 0.1 M NaBr at different temperatures.
The reason for the temperature effect is not completely clear. Some papers emphasize the similarity between the effect of temperature and the effect of ionic strength,30,41 because increased temperature and increased ionic strength have been found to generate thick films. Another common feature is the smoothing the previously adsorbed polyelectrolyte multilayer films.151,152 It has been shown, both theoretically and experimentally, that increasing ionic strength reduces the radius of gyration of a polyelectrolyte.153,154 The similarity between the two factors is also supported by a study carried out in the paper VI. The measurements of polyelectrolyte solutions show that the increase in temperature decreases the reduced viscosity. The apparent conclusion is that the polyelectrolytes are liable to adopt a more globular form when heated and, therefore, do not give as great a contribution to the viscosity as at lower temperatures. It is supposed that the increased temperature as well as the increased ionic strength brings along interference in the electrostatic binding between the oppositely charged polyelectrolytes, causing breakage of the bonds inside the film. That allows the oppositely charged polymers to form new energetically more favored bond configurations.155,151 This reorganization produces increasingly entangled and interpenetrating polymer chains, indicating that the polymer chains would eventually reach configurations that are closer to the global energy minimum. This particular change in a polyelectrolyte multilayer is observed in healing and smoothing of the film surface, together with slight swelling of the film.151,152
33
The measurements of the PSS/PDADMA films in the paper VI show that the buildup is in the most cases divided into distinct steps (Figure 3.8.). First there is an initial buildup which can be related to the formation of zone I. The measurements carried out at five different temperatures show almost identical buildup during the first four bilayers. The buildup is exponential but it is independent of temperature. Then follows an exponential buildup regime, with a temperature dependent buildup rate. The basic assumption is that now there are zones I and III present and at least one of the polymers is rather freely diffusing within the polymer blend at the polymer-solution interface. It is also assumed that at the end of each deposition step there exists equilibrium between the polymer concentration in solution and the composition of the film. Within these conditions the buildup can be expressed using the growth exponent (β) as stated in the paper VI:
Γ k +1 = Γ1 exp ( β k )
(3.2)
Here Г is areal mass density of the film and k is the number of bilayers. Under these conditions the layer-by-layer deposition is exponential if the diffusion is able to carry polymer within the entire film during the applied contact time. Eventually, when the film becomes adequately thick, the complete mixing of the polymer blend within the contact time becomes impossible and the depositing polymer will have a finite blending depth. The buildup is no longer exponential and independent on the thickness of the underlying film. The zones I and III are now separated by the zone II and the multilayer adopts its final configuration, providing a base for the linear buildup.
34
5
Im ζ0 (Rayl)
10
4
10
3
10
0
5
10 15 Number of bilayers
20
Figure 3.8. PSS/PDADMA deposition in 0.1 M NaBr at 45 oC. The straight line is the fit of the equation (3.2). The curved line is the fit of the linear buildup regime.
The growth exponent (β) values obtained are in line with the values previously reported for hyaluronan/chitosan multilayers using the quartz crystal microbalance.53 The growth exponent is the essential parameter for determining the temperature dependence of the multilayer buildup. The relationship between the growth exponent and the deposition temperature is derived in the paper VI:
dβ 2⎛ d ϕ +S − ϕ−S + ϕ−M − ϕ +M ≈ − ⎜⎜ ΔH SD+→ M + ΔH SD−→ M − n+ ΔH XD+ − n− ΔH XD− − F d (1/ T ) R⎝ d (1/ T ) T
⎞ ⎟⎟ ⎠
(3.3) Here n± is the Bjerrum formation function for the ion-macromolecule complexation, i.e.,
n± =
K ± ⎡⎣ X ± ⎤⎦
, where ⎡⎣ X ± ⎤⎦ is the counterion concentration and K ± is the 1 + K ⎡⎣ X ⎤⎦ ±
±
association constant of the counterion polymer complexation. The terms ϕ ±S and ϕ±M are the galvani potentials in the solution and multilayer during the cationic and anionic deposition respectively.
35
Based on the equation (3.3) the temperature dependence of the buildup rate can be divided into three different groups of terms. First there is the enthalpy for the transfer
(
)
process from solution to the multilayer ΔH SD+→ M + ΔH SD−→ M . Then there is the enthalpy for the counterion-polymer complexation
( n ΔH +
D+ X
+ n− ΔH XD− ) , which is, in fact,
dependent on the type of the salt or the salt concentration. These enthalpy terms can be considered to be nearly constant within this rather narrow temperature range. The third
⎛
term is the potential term ⎜ F ⎜
⎝
d ϕ+S − ϕ−S + ϕ−M − ϕ+M d (1/ T ) T
⎞ ⎟⎟ , which is probably small ⎠
because the interfacial potential is mainly determined by the small counterions due to their higher mobility. The transport numbers of counterions are close to one and not significantly dependent on temperature. The linearity of the β vs. 1/T plot in the paper VI supports the abovementioned assumptions. The net enthalpy change for the polyelectrolyte deposition, calculated from the equation (3.3), is 1.2 ± 0.2 kJ mol-1. The thickness of the zone III can be estimated form the total mass of layers in the exponential regime by assuming some density of the material. The zone III thickness values for PSS/PDADMA film, deposited in NaBr, show a strong temperature dependence. The estimated values are: 69, 88, 120, 360 and 6000 nm for deposition temperatures 15, 25, 35, 45, 55 oC respectively. It is noteworthy that the values increase exponentially as temperature increases, reaching an extremely high value at 55 oC. Also the bilayer mass density in the linear regime exhibit exponential increase as the temperature is raised. It can be stated that the effect of the deposition temperature in this polyelectrolyte system is enormous.
36
4. Physical properties
One of the central points of interest in this thesis is the viscoelasticity of the prepared thin films. Viscoelastic material has properties of both liquids and solids. It can undergo elastic deformation and viscous flow. There are two very simple mechanical models that have been used to describe the behavior of the viscoelastic material.156 The Maxwell element (spring and dashpot in series) is a simplified model of a viscoelastic liquid. A Maxwellian material behaves like a solid on short timescales and like a liquid on longer timescales. The Maxwell model describes the stress relaxation but does not adequately the creep. The Voigt element (spring and dashpot in parallel) is a representation of a viscoelastic solid. The Voigt model describes the creep but not sufficiently the stress relaxation. The polyelectrolyte multilayers have been assumed to behave as simple Voigt elements,182 but obviously the viscoelastic behavior of the multilayer films is more complicated. In order to use multilayered polyelectrolyte films in a variety of applications, for example in the surface modification, accurate information about the film behavior under liquid is required. However, there is not much precise information available on the viscoelastic properties of the polyelectrolyte multilayers. Polymers in general are exceptionally versatile materials that can form glass-, rubber-, and even gel-like materials. The versatility of the polymer coatings may offer nearly unlimited possibilities. In most cases it is advantageous to understand how the multilayer film behaves under a deforming stress. One of the great advantages in the layer-by-layer assembly is that functional macromolecules can be rather easily incorporated in the multilayer film to obtain desired new properties. A conducting polymer with pendant ionic groups provides an advantage of water solubility and the ability to build functionalized polyelectrolyte multilayers. Conducting polymers have a rather short history. In the late 1970s Heeger and MacDiarmid found that polyacetylene produced by Shirikawas’s method showed a 12 order of magnitude increase in electrical conductivity when exposed to oxidating agents.157 Since that discovery, a vast number of other conducting polymers have been synthesized. Conducting polymers are proclaimed as futuristic materials for the next generation of electronic and optical devices, because they have exceptional optical and electrical properties.158 The pioneering work on multilayer films with conducting polyelectrolytes has been made by Rubner et. al.66,68
37
4.1. Electrical and optical properties
The electrical properties of any material are the result of the electronic structure of the material. The conducting polymer can be assumed to form electron bands through extensive molecular orbital overlap and the electronic properties of conducting polymers can be explained by the band theory.159 Oxidation removes electron from the sp2-based carbon system, which is a common structure for most conducting polymers, forming a radical cation called polaron. Further oxidation removes the unpaired electron yielding a dicationic species called bipolaron. Polarons and bipolarons act as charge carriers in the polymer film and oxidation of the film should make it conductive. The measurement of the in-plane conductivity of a (PDADMA/P3TOPS)5 film resulted low conductivity (1.6 *10-5 S cm-1). It is generally noticed that the conductivities of poly(alkoxythiophenes) are low, but this value is markedly lower than reported for the bulk samples of similar water soluble polymers.160 Despite that similar low conductivity values are obtained for the polyelectrolyte multilayer containing PDADMA and polythiophene derivative.161 The low conductivity of PDADMA/P3TOPS multilayer is, therefore, partially attributed to the insulating PDADMA layers between the conducting P3TOPS layers. It was shown earlier in electroreflectance measurements that even one PDADMA layer between the electrode and the P3TOPS layer decreased the charge transfer rate significantly. The conductivity could be raised by simply replacing PDADMA with conducting P3TOPA. The conductivity of the (P3TOPS/P3TOPA)5 film is 40 times higher than in the earlier mentioned film, increasing to the value of 6.2 *10-4 S cm-1. This is not a very high value, but taking in to account the fast switching between the conducting and nonconducting states, it makes these kinds of polyelectrolyte multilayers promising for the sensor applications. The conductivity of poly(alkoxythiophene) can probably be improved by synthesizing more regioregular polymer,158,159 with head-to-tail bonding between the monomer units, meaning that the side groups are located in the 3’-3 -positions of each thiophene dimer units in the polymer. The employed polythiophenes have apparently a regiorandom structure with also head-to-head (3’-4) and tail-to-tail (4’3) structures in the dimer units inducing steric hindrance for the delocalized π-system, which will certainly reduce conductivity. Oxidation causes also radical changes in the optical properties of polythiophenes. Neutral polythiophene films are usually red to blue in color, while the oxidized polythiophene films are blue to gray. A broad variety of color changes, which can be structurally controlled, have been observed for polythiophenes in their respective redox states.162,163 These optical changes are a consequence of new electron levels existing in the band gap.
38
While the neutral polymer has only its characteristic π->π* -transition, several new transitions are possible between the orbitals in the oxidized state. The energies of these new transitions are lower and result in the polymer having absorptions at higher wavelengths. The spectra of oxidation of (P3TOPS/PDADMA)5 shows typical features of polythiophene films. The π->π* transition at 630 nm decreases during the oxidation. The new bands show up at 820-1050 and 1870 nm. The medium spectral range is more complex and there are transitions that are difficult to determine. The spectral behavior is in accordance with the generation on polarons and bipolarons as charge carriers upon oxidation.164,165 It should be pointed out that the color change of a five-bilayer film upon the oxidation is visible to the naked eye.
0.075
0.025 0.000 0.075 Δ absorbance
Δ absorbance
0.050
-0.025
0.050 0.025 0.000
-0.050
-0.5
500
1000
0.0 0.5 Potential (V vs SCE)
1500
2000
2500
wavelength (nm)
Figure 4.1. Differential absorption spectra of a (P3TOPS/PDADMA)5 multilayer in 0.1 M NaNO3 / D2O. Spectra taken at 0.100 V intervals from –0.50 V to +0.60 V (spectrum at – 0.60 V taken as reference). The inset shows the evolution of absorbance at 630 (•), 850 (+) and 1850 nm (Δ).
39
4.2. Viscoelastic properties
In the air polyelectrolyte multilayer films resemble rigid glass-like material.166 On the other hand, some polyelectrolytes show an apparent viscoelastic behavior under liquid.51 The viscoelastic properties of some polyelectrolyte multilayers can also be simply modified.167 The increase of viscosity and softness is generally attributed to water trapped inside the polymer material. The polyelectrolyte chains form loops and tails reaching out of the film,168 providing a possibility for the structural entrapment of water. As stated earlier in the introduction, the QCM analysis of viscoelastic properties of thin films is often made on the basis of equivalent circuits and using rather simple mechanical models. The equivalent circuits and mechanical models give an overidealistic picture of the material at the resonator surface. An alternative approach is taken in the papers II, III, V and VI, in which the analysis of the oscillation is done using the acoustic impedance to describe the behavior of the resonator system. The acoustic shear impedance (Z) is defined as the ratio of stress (T) to the phase velocity (vp) of the transverse acoustic wave.
Z =−
T vp
(4.1)
The acoustic impedance is also related to the shear modulus of the material (G)
Z = ρ vp = ρG
(4.2)
Here ρ is the density of the material. In an oscillatory movement, the acoustic impedance becomes complex valued, resulting in the complex bulk acoustic impedance, a frequency dependent property of the material. In analogy to the bulk acoustic impedance the local (shear) acoustic impedance (ζ) can be defined as the ratio of stress (T) and particle velocity parallel to the surface (v).
ζ =−
T v
(4.3)
The local acoustic impedance has also been called mechanical impedance. The local acoustic impedance at the oscillator surface (ζ0) can be calculated from the electrical impedance,169 which is a measurable quantity of the quartz crystal oscillator system. It
40
has been shown that the local acoustic impedance for a homogeneous surface load can be represented in a form of a nonlinear Riccati equation.117 2
dζ ⎛ ζ ⎞ = ⎜ ⎟ −1 dm ⎝ Z ⎠
(4.4)
is defined as the complex mass impedance of the material, which is a cumulative Here m areal mass density between the surface and some defined distance from the surface. In the paper II, the viscoelastic properties of thin polyelectrolyte multilayers are analyzed, with apparent softness of the film, using the parameters derived from the equation (4.4). To estimate the viscoelastic properties of thin polyelectrolyte multilayers the following first order approximations of the changes in the real and imaginary parts of the local acoustic impedance can be done:117
Δζ ′ = Re (ζ 0 − Z l ) ≈ ω m f
⎛G ρl Im ⎜ l ⎜ Gf ρf ⎝
⎞ ⎟⎟ ⎠
⎡ ρ ⎛ G ⎞⎤ Δζ ′′ = Im (ζ 0 − Z l ) ≈ ω m f ⎢1 − l Re ⎜ l ⎟ ⎥ ⎜ G f ⎟⎥ ⎢⎣ ρ f ⎝ ⎠⎦
(4.5)
(4.6)
Here Zl is the acoustic impedance of the liquid, ω the angular frequency of the oscillator, mf the areal mass density of the film, and ρf and ρl are densities of the film and liquid, respectively. Gf and Gl are the shear moduli of film and liquid, respectively. Combining the equations (4.5) and (4.6) leads to elimination of the areal mass density.
1 ρf Δζ ′′ = − tan δ Δζ ′ Z l 2 J ′f
(4.7)
Here J’f is the real part of the shear compliance of the film (J = G-1), and δ is the loss angle defined as the ratio between the energy lost and stored in the oscillation (tan δ = G”f / G’f). The loss angle can be estimated and in the case of paper II the alteration of the angle from 10 to 80 does not cause a great error in the final values. Therefore, it can be stated the equation (4.7) contains only measurable parameters and a “softness parameter” J’f / ρf, which is independent of the mass of the film. The “softness parameter” can be used in evaluating viscoelasticity of thin films, because the larger is the shear modulus
41
(G) the stiffer is the film (J’f = G’f / |Gf|2), resulting in lower J’f / ρf . The values for thin multilayers consisting of both flexible PSS and PDADMA and rigid-rod type P3TOPA and P3TOPS are presented in the Figure 4.2. The values have been calculated on the basis of the equation (4.7) by assuming a loss angle of 45 degrees, which is considered to be close to reality. In the P3TOPS/P3TOPA and PSS/P3TOPA multilayers, the softness parameter shows an almost continuous decrease with increasing number of layers. The striking feature in the graph is the behavior of PDADMA. Each time PDADMA is the outermost layer there is a clear increase in the softness parameter leading to oscillation in the graph. The effect of PDADMA chainlength is also remarkable. Shorter chainlength leads to smaller values of J’f / ρf. Altering the length of PSS does not have a noticeable effect. The apparent conclusion from the measurement data is that there is a formation of a soft outer layer after each PDADMA deposition step. Flexible polyelectrolytes have been shown to have loops and tails reaching far away into the solution.11,170 High molecular weight PDADMA is hydrophilic and can also obviously form large loops on the film surface. It is therefore assumed that the oscillation in the film softness is attributed to water-like diffuse gel formed on the surface of the multilayer with high molecular weight PDADMA as the top layer. The effect can be understood as a predictable behavior of the zone III, that is, swelling in pure water. The effect of the last deposited layer is probably amplified by the layer interpenetration, considering that the excess charge extends several layers inside the film.45 It should also be pointed out that the deposition using nitrate as counteranion produces thick PDADMA layer with presumably intensively coiled polyelectrolyte material (papers IV and V), which could then swell in contact with pure water. The length of PSS has a much smaller effect. However, the adsorption of next layer of PSS or P3TOPS induces apparent shrinking and drying of the underlaying PDADMA layer, resulting in lower J’f / ρf values. The softness measurements of thin polyelectrolyte multilayers imply that the outermost layer is clearly responsible for the layer-to-layer variation of the apparent viscoelastic properties during the buildup. This phenomenon is strikingly present when the films are made of polyelectrolytes with a large difference in the hydrophobicity or length. Similar outer layer dependent swelling of polyelectrolyte multilayers has been attributed also to the surface potential.171 Were that the reason for the terminating layer effect, it would apparently not be dependent either on the polymer chain length or hydrophobicity.
42
-8
3
-1
-1
Jf' / ρf (m Pa kg )
10
-9
10
0
2 4 6 8 10 number of polyelectrolyte layers
Figure 4.2. The film softness parameter J’f / ρf for the polyelectrolyte multilayers in pure water at different stages of the assembly. Filled symbols, polythiophene on top, open symbols, PSS or PDADMA on top. □,■ (solid line) : PSS/P3TOPA; ●,○ (solid line) : P3TOPS/PDADMA; ● and ■ (dotted line) : P3TOPS and P3TOPA, respectively, in P3TOPS/P3TOPA; ∇ (dashed line) PSS(70 kDa)/PDADMA (400-500 kDa); Δ (dashed line), PSS(263 kDa)/PDADMA (low Mw). Even layers correspond to polycation on top. Stiffness increases downwards in this Figure. The values for protein layers calculated from the reference182 are indicated as dashed lines.
The polyelectrolyte multilayers exhibit rather high J’f / ρf values indicating very soft films, even when the film contains hydrophobic rigid-rod type polyelectrolytes. Actually, the J’f / ρf values of all of the films are in range of the values for soft protein layers.182 The energy dissipation in the protein films is attributed to the large amount of water trapped inside the film, in addition to the losses due to the deformation of proteins themselves.172 If the polythiophene containing films are compared to the electropolymerized polythiophene films,173 a clear difference in the softness can be found. Polyelectrolyte multilayers are much softer than the electropolymerized films. Despite the differences in viscoelasticity, the optical and electrical properties of the films are relatively similar (paper I). The capacity of absorbing large amounts of water, 174,23 combined with the fact that softness varies with the last deposited layer, implies that the outer hydrated zone has a large effect on the apparent softness of thin polyelectrolyte multilayers, i.e. the films without fully developed zone II .
43
4.3. Layer matrix representation
The studies of the elastic properties of the thin films are divided into two basic approaches: compress and shear stress methods. Since the dimensions of the studied materials are in nano to micro scale, there is no straightforward method to obtain the elastic properties of the material. The elastic properties of multilayer microcapsules are determined from collapse of the microcapsules under increased osmotic pressure175,176 or by applying a load on a microcapsule with an AFM-type device.177,178 The elasticity of the multilayer film on a substrate has also been studied by applying a force by AFM on the surface of the film,179 and by using the reflection interference contrast microscopy.180 The compression methods require a reliable physical model of the film behavior under pressure to obtain correct results. As an example of the shear methods, the elasticity of thin polymer and protein films on a solid surface have been estimated by using a quartz crystal microbalance (QCM) in a mode that records not only the frequency shift but also the energy losses of the crystal oscillation.181,182,183 The evaluation of the elastic properties of thin films using QCM is demanding and cannot be done with high accuracy.184 Therefore, a certain thickness of the film is needed. That is clearly shown in the paper II, in which the apparent softness of the five bilayer film was dominated by the last deposited layer (zone III). The method of calculating the elastic parameters of the polyelectrolyte multilayers, based on the layer matrix derived form the equation (4.4), is presented in the paper III. The layer matrix expresses the properties of layers in the film superstructure. The layer in this concept does not have to mean a single deposited layer. It can also be understood as a bilayer or a slice of ten layers etc. As mentioned earlier, the primary data from the QCM measurement can be represented with the complex local acoustic impedance. The local acoustic impedance is a continuous function at the no-slip interfaces. Therefore, if the layer on the resonator surface is cut into thin parallel slices, marking the cutting planes by 1 and 2, it can be shown by solving the Riccati equation (4.4) that:
ζ2 =
ζ 1 cos
ωΔm Z
− jζ 1Z sin −1
− jZ sin
ωΔm Z
ωΔm
+ cos
Z
ωΔm
(4.8)
Z
Here ζ1 and ζ2 are the local acoustic impedances at the cutting planes 1 and 2. Δm is the areal mass density of the slice. If we have a system with N layers, the outermost cutting
44
plane is the interface against the medium with acoustic impedance of the solution. Therefore, the slice at the top of the film can be defined as following:
ζ N −1 =
ζ N cos
ωΔmN ZN
+ jZ N sin
ωΔmN
ZN ωΔmN ωΔmN + cos jζ N Z N −1 sin ZN ZN
(4.9)
This kind of transformation can be used in iterative calculation of the whole film from the film-solution interface to the resonator surface. Equations (4.8) and (4.9) represent a Möbius transformation in the complex plane. The Möbius transformation can be conveniently represented as a matrix operation
z2 =
A11 z1 + A12 ⎛ A11 =⎜ A21 z1 + A22 ⎝ A21
A12 ⎞ ⎟ D z1 = A D z1 A22 ⎠
(4.10)
The major advantage of the matrix representation is that the applied Möbius transformations can be represented as a matrix multiplication, having the advantage of simplifying the multilayer representation. The apparent outcome is that the propagation of the acoustic wave in a stratified medium can be described by matrices in an analogous way to the propagation of light in stratified dielectric layers.185 Using the analogy to produce multilayer matrix representation, the local acoustic impedance at the resonator surface for a n-layer multilayer becomes then
ζ 0 = ( A1A 2 " A n −1A n ) D ζ N
(4.11)
Here ζN is the local acoustic impedance of the medium. The layer matrix Ai represents the physical properties of the corresponding layer or slice, and can be written according to the equations (4.9) and (4.10) as
ωΔmi ⎛ ⎜ cos Z i Ai = ⎜ ⎜ −1 ωΔmi ⎜ jZ i sin Zi ⎝
45
jZ i sin
ωΔmi ⎞
Zi ⎟ ⎟ ωΔmi ⎟ cos ⎟ Zi ⎠
(4.12)
Up to this point everything is still accurate and no simplifications to the physical model have been made. To use this representation in connection with polyelectrolyte multilayers an assumption of homogeneity of the film provides remarkable benefits. If the film is totally homogenous, meaning that the bulk acoustic impedance is the same everywhere within the film, the matrices in the equation (4.11) commute, and their product has the form of equation (4.12), with Δmi substituted by the total areal mass density of the layer. Then only three parameters are needed to describe the layer. There are the areal mass density, and the real and imaginary components of the bulk acoustic impedance. Based on the assumption of the layer homogeneity the properties of the multilayer can be iteratively calculated from the measured local acoustic impedance data. The requirement of the homogeneity of the film might be difficult to achieve in thin multilayers because of the possible variation in the material properties in the three different zones formed in the multilayer.44 Therefore, the number of layers must be increased to several hundreds in order to achieve reliable results from the multilayer films with thin individual layers.
46
4.4. Effect of counterions
The previously presented iterative matrix fitting method has been used in the paper V to achieve information about the effect of anions on the polyelectrolyte multilayers. The obtained storage modulus values for the PSS/PDADMA multilayers deposited in the presence of different counteranions are presented in the Figure 4.3. The Figure shows that the multilayers are clearly divided in two categories by the values of the storage modulus. There is also an obvious dividing point in the graph, which is found near the location of chloride. The remarkable stiffening of the films follows the increasing hydration entropy of the counteranions and the magnitude of the change in stiffness is comparable to the glass transition of a bulk polymer.186 The dramatic increase in the multilayer stiffness is connected to the increased charge screening of the polycation by the counteranions. The extent of the charge screening was stated already in the paper IV. Such a great increase in the stiffness is an apparent indication of structural differences between the multilayers and it is attributed to the type of charge compensation in the polyelectrolyte multilayer. There are two ways of achieving charge compensation in the polyelectrolyte multilayer: intrinsic, referring to polyelectrolyte complexes, and extrinsic, referring to polyelectrolyte-counterion charge compensation.40 It has been shown that the oppositely charged strong polyelectrolytes favor the intrinsic compensation.40 To clarify this, the charge compensation is treated as ion pair formation in the equations (4.13 and 4.14). The corresponding association constants Ke and Ki describe the constants for extrinsic and intrinsic charge compensation in monomer units, respectively. Therefore, as an assumption, Ki for strong polyelectrolytes is apparently much higher than Ke leading to 1:1 polyelectrolyte complexes. Based on the correlation between the hydration entropy and counterion binding (paper IV) it can be proposed that the counterion with higher hydration entropy would cause a higher Ke value and a drop in the charge density of the polymer in the solution.
Ke ZZZX PDADMA+ + A− YZZZ PDADMA+ A−
(4.13)
Ki + − ZZZ X PDADMA+ + PSS − YZZ Z PDADMA PSS
(4.14)
47
9
10
Glassy state
-
NO3 8
10
Br
-
-
G' (Pa)
ClO3
7
10
-
F
HCOO
-
-
Cl
-
BrO3
Rubbery state 6
10
-160
-140 -120 -100 -80 -1 -1 Hydration entropy (JK mol )
-60
Figure 4.3. Storage modulus of bulk multilayer vs. hydration entropy of the counteranion used in the PSS/PDADMA multilayer deposition. The range of shear modulus of the glassy and rubbery state of some common polymers is shown on the graph.186 The comparison between the static shear modulus and the dynamic storage shear modulus can be done if the Voigt viscoelastic model is assumed.
The similar trend caused by the charge density can be found in the film deposition using weak polyelectrolytes (PAH and poly(acrylic acid)). The charge density of the polyelectrolyte in solution can be adjusted with the change of pH of the deposition solution.38 That induces dramatic changes in the layer thickness.27 It is shown in the paper V that comparable changes in the mass of the deposited film can be obtained by changing the counteranions used in the deposition (Figure 4.4).
48
-2
Bilayer mass density (μg cm )
6 Br
5
-
-
NO3 4 3 -
ClO3 2 -
-
1
-
F
HCOO
BrO3
Cl
-
0 -160
-140
-120
-100
-80 -1
-60
-1
Hydration entropy (JK mol )
Figure 4.4. Bilayer mass density of multilayers vs. hydration entropy of the counteranion used in the PSS/PDADMA multilayer deposition. The line is added as a guide for eye.
Further support for the differences in the type of the charge compensation is shown using x-ray photoelectron spectroscopy (XPS) in the paper V. The comparison between different multilayers is made on the basis of two films which represent different classes of the stiffness values, and are made in the presence of an anion that contains bromine. The measured films, deposited in NaBr and NaBrO3, were terminated with PSS to reveal realistic information about small ions inside the multilayer. The studied films revealed different surface compositions in the XPS spectra. No sodium ions were detected in either sample. The film deposited in NaBrO3 shows highly interpenetrated polyelectrolyte multilayer structure with the S/N (sulphur/nitrogen) ratio in the surface of ca. 1:1, with absence of bromine. However, the film deposited in NaBr reveals the presence of counteranions. The charge neutrality was detected on the surface, since the amount of positively charged groups was equal to the amount of negatively charged groups, giving the (Br+S)/N ratio of ca. 1:1. The results showed that in the film deposited in NaBr approximately one third of the ammonium groups are compensated with bromide. Based on the XPS measurements, the breakpoint in the Figure 4.3 can be attributed to the change in the charge compensation type. Therefore, it can be stated that by using ClO3-, NO3- or Br- as the counteranion the polymer charge density will be large enough to maintain the polymer solubility, but the deposited multilayer will most probably contain
49
partly extrinsically compensated polymer. For that reason, the PDADMA layers formed in presence of abovementioned counteranions are not only deposited in considerably higher amounts but are relatively stiffer and produce a plastic-like material with PSS. The evidence shown here opens a new approach to the counterions inside the polyelectrolyte multilayers, showing that the effect of counterions on the properties of the resulting film should not be disregarded.
50
5. Summary This thesis deals with very fundamental aspects of polyelectrolyte multilayers. These issues are related to the factors affecting the buildup, structure and properties. Despite the fact that there are a large number of published papers on the polyelectrolyte multilayers, relatively little is still known about the buildup process. The purpose of this thesis was to obtain more specific information about the earlier mentioned factors, taking advantage of the layer by layer deposition method. Polythiophenes are used in this thesis as colored and electroactive probes in the multilayer study. Polyelectrolyte multilayers were prepared from water soluble electrically conducting polythiophene derivatives. The electrical and optical properties of those multilayers in aqueous solutions are found to resemble polythiophene films in organic media. In addition, polythiophenes provide an example of rigid-rod type polymers with their own characteristic buildup behavior. The amount of polythiophene in the film oscillates during the multilayer buildup. This is attributed to the desorption of polythiophene from the adsorbed aggregates, leaving behind an electrostatically bound, nearly monomolecular layer. The polythiophenes can be used as electrochemical probes in the study of charge transfer through the insulating polyelectrolyte multilayer. The rate of charge transfer can be treated as a measure of the layer interpenetration. The extent of interpenetration is influenced by the nature of the insulating layers and it is dependent on the ionic strength of the solution used in the deposition of the interpenetrating polymer. The interpenetration depth is found to vary from 7 to 15 nm. The buildup of conventional synthetic polyelectrolytes has a well-known relation to the ionic strength of the depositing solution. However, the effect of the salt type is often neglected. There is a clear Hofmeister trend in the multilayer deposition. The characterization of the anion effects can be done using factors that describe the ion-water interaction in order to explain the changes in the multilayer buildup. The Jones-Dole viscosity B coefficient and the hydration entropy of the depositing anion have a strong correlation with the thickness of the deposited multilayer. The deposition temperature has been shown to have a significant effect on the character of the buildup and also on the general mechanism of the buildup of the polyelectrolyte multilayers. In addition to the increase of the bilayer mass, the temperature increase has been shown to change the buildup regime from an almost totally linear to exponential one. A simple phenomenological model was derived, based on the equilibrium between the polymer concentration in the solution and the composition of the layer. This model predicts that every layer-by-layer buildup process is inherently exponential, turning linear
51
whenever the diffusion rate is not fast enough to carry the polymer within the entire thickness of the layer. One important goal of this thesis was to understand the influence of the polyelectrolyte multilayer coating on the thickness shear mode resonator and to further use the data to obtain physical parameters of the film. The viscoelastic properties of thin polyelectrolyte multilayers can be estimated using the QCM impedance data, showing that thin polyelectrolyte multilayers immersed in water are similar in elastic properties to very soft hydrogels. If there are clear differences in the length and hydrophobicity between the deposited polyelectrolytes, the diffuse gel which is formed in the film solution interface can dominate the viscoelastic properties of the whole film. It is shown that method of using layer matrix representation for describing a homogenous multilayer reveals rather accurate and new data about the surface bound multilayer. The counteranions used in the deposition are found not only to produce films with different thicknesses but also with different elastic properties. By changing counteranion we have been able to increase the stiffness of the polyelectrolyte multilayer so significantly that the change is comparable to the rubber-glass transition of common polymers. The increase in stiffness is attributed to the change in the polyelectrolyte charge compensation type, which induces structural changes inside the polyelectrolyte multilayers. As a conclusion it can be stated that even though the polyelectrolyte multilayers are nowadays well studied systems, still rather simple polyelectrolyte systems may possess characteristic properties that have remained unsolved. There are multiple factors that affect the buildup and properties of the multilayer films. Those factors can also obviously amplify or reduce each other, making the analysis very complicated. One of the major problems in this kind of complex systems is the difficulty of adjusting only one factor at the time. In addition, even the fundamental theories of the buildup have some inconsistency. Therefore, considerable amount of work is needed to solve these remaining uncertain issues.
52
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