J. Phys. Chem. B 2008, 112, 23-28
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Controlled Electrophoretic Deposition of Uniquely Nanostructured Star Polymer Films Suseela Somarajan,† Saad A. Hasan,‡ Chinessa T. Adkins,§ Eva Harth,§ and James H. Dickerson*,† Department of Physics and Astronomy, Interdisciplinary Graduate Program in Materials Science, and Department of Chemistry, Vanderbilt UniVersity, NashVille, Tennessee 37235 ReceiVed: July 11, 2007; In Final Form: September 24, 2007
The controlled electrophoretic deposition of polystyrene/divinylbenzene (PS/DVB) star polymer films from a colloidal suspension is reported. Liquid suspensions, containing the PS/DVB star polymer, were prepared by injecting a dichloromethane (DCM) solution of the star polymer into a stratified liquid combination of hexane and DCM. A variety of hexane/DCM volume ratios were examined to identify the optimal solution conditions for electrophoretic deposition; thin films were produced from both unmixed and well-mixed hexane/ DCM suspensions. Unmixed suspensions yielded spatially separated thin films, deposited in a controlled fashion, that were dependent on the polarity of the corresponding electrode. Films on the positive electrode differed in thickness, microstructure, and appearance from those formed on the negative electrode. In contrast, films produced from well-mixed hexane/DCM suspensions deposited uniformly across only the negative electrode. Atomic force microscopy studies revealed nanostructured surface morphologies that were unique to each of these films. Additionally, these microscopy studies shed light on the possible conformations of star polymers adsorbed on a surface. By controlling the composition and the mixing state of the solution and by controlling the bias of electrodes, we achieved controlled deposition of star polymer films with a specific nanostructure. These nanostructured films may have broad use in optical and biological device applications.
1. Introduction Conjugated polymer films have been investigated for implementation in flexible electronic device applications, such as light-emitting diodes (LEDs), photocells, and field-effect transistors.1-7 Thin films of polymers can be fabricated easily by wet processes such as spin-coating and ink-jet printing.8,9 Recent work suggests that the specific morphology of polymer films may be critical to the performance of electronic and optical devices comprised of these films.10 Therefore, developing a casting technique that controllably engenders a particular morphology to the polymer films will be of substantial interest to the community. Unlike other wet casting techniques that cannot readily produce polymer films with combined low surface roughness, long-range film thickness homogeneity, and a distinct nanostructure, electrophoretic deposition (EPD) can produce casts of polymers with all of these characteristics.11 Polymer EPD incorporates the separate phenomena of colloid formation in suspension (solidification), colloid propulsion to the deposition sites (electrophoresis), and colloid aggregation and adhesion onto a substrate (deposition).9,12 These combined processes can facilitate the formation of nanostructure in thin polymer films. Additionally, EPD provides substantial control over the film thickness, marked enhancement of the deposition rate, and improved film composition homogeneity, compared to other wet casting-nanostructured thin film deposition methods.13-15 While investigations of the electrophoretic deposition of conjugated, linear polymers exists in the literature, EPD of star polymers has yet to be reported.9-11,16,17 * Corresponding author. E-mail:
[email protected]. † Department of Physics and Astronomy. ‡ Interdisciplinary Graduate Program in Materials Science. § Department of Chemistry.
Among the nanostructured polymer materials that have been the subject of increased research attention, multi-arm star polymers have attracted interest because of their physical properties (controlled compositions, topologies, and functionalities) and their potential applications in nanotechnology.18-22 These macromolecules have a three-dimensional, compact shape with several polymer chains as arms linked to a core. Since the density of the polymer at the center of the star differs significantly from that of the outer regions, the conformational properties of star polymers are markedly dissimilar from those of homologous linear polymers.23,24 These star polymer systems have been proposed for use in several applications, such as nanoscale targeted drug delivery, conjugated bio-tagging systems, and bulk nanostructured luminescent materials.25-27 To facilitate their employment in various device architectures, more must be learned about the behavior of multi-arm star polymers in deposition schemes, like electrophoretic deposition. This paper presents the first observations of the controlled electrophoretic deposition of polystyrene/divinylbenzene (PS/ DVB) star polymer films, made possible by varying the preparation of the colloidal suspension. Our research concentrated on the electrophoretic deposition and characterization of nanostructured films, deposited from colloidal suspensions of PS/DVB star polymers, prepared via nitroxide-mediated polymerization (NMP).28 The star polymer suspensions were established by injecting a dilute dichloromethane (DCM) solution of star polymer material into a stratified, phaseseparated liquid, consisting of immiscible quantities of hexane and dichloromethane. Since this method is similar to postpolymerization purification and does not use additional surfactants or cross-linkers, which would mediate star polymer to star polymer binding, the suspended polymer and, therefore, the EPD films retain the purity of the original material.16 Atomic force
10.1021/jp075425n CCC: $40.75 © 2008 American Chemical Society Published on Web 12/11/2007
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microscopy (AFM) studies of the surface morphologies of the films reveal unique nanostructured features for each deposition site on both the positive and negative EPD electrodes. These characteristics vary as functions of the solution preparation and the corresponding EPD electrode. Exploration of the star polymer conformation within the films, based on the deposition location and the net charge of the star polymer, revealed a possible correlation among those parameters. 2. Experimental Methods Our approach to synthesize polystyrene/divinylbenzene NMP star polymers was adapted from the work of Bosman et al.28 A mixture of polystyrene (2.0 g, 0.23 mmol, Mn ) 8600, PDI ) 1.17, Sigma Aldrich), styrene (0.24 g, 2.3 mmol, Sigma Aldrich), and divinylbenzene (0.13 g, 0.81 mmol, Sigma Aldrich) was dissolved in chlorobenzene (2.5 mL, Sigma Aldrich), degassed by four freeze-pump-thaw cycles and sealed under argon. The polymerization mixture was then stirred at 124 °C for 24 h, after which it was allowed to cool. The addition of methanol (Sigma Aldrich) precipitated the star polymer (1.9 g, MW ) 76000, PDI ) 1.41) from the chlorobenzene. Electrophoretic deposition experiments were performed using dilute, all-dichloromethane and all-hexane solutions of the star polymer, as well as using a combined suspension, prepared as follows. Star polymer powder (20 mg) was dissolved in 3 mL of dichloromethane (DCM) (Fisher Scientific). A volume of 0.5 mL of the resulting solution was then slowly injected into a stratified, phase-separated liquid of hexane (Omnisolv) and DCM in an approximate 9:1 volume ratio. The total suspension volume was approximately 20 mL. Since hexane is not a good solvent for the star polymer, the injection produced a colloidal suspension of the star polymer in that region of the liquid. The DCM-rich layer resided in the upper layer, while the hexanerich layer existed in the lower layer. We define this preparation as the unmixed suspension. The well-mixed suspension was formed by vigorously mixing the previously injected 9:1 hexane/ DCM liquid. After mixing, the suspension was immediately used in an EPD experiment. This paper reports the electrophoretic deposition of star polymer film from the hexane-rich colloidal suspension layer, the DCM-rich dissolved layer, and the mixed hexane/DCM suspension. A standard nanoparticle electrophoretic deposition setup was employed. The vertically aligned, inward-facing electrodes were separated by approximately 2 mm in a parallel plate configuration. The electrodes consisted of commercially produced, silicon dioxide (SiO2)-passivated and indium tin oxide (ITO)coated polished float glass (CG-51IN, Delta Technologies, Ltd., Stillwater, MN). The ITO-coated glass was cut into 1.3 cm × 2.5 cm pieces; each electrode was subsequently cleaned via ultrasonication in isopropyl alcohol (Sigma Aldrich), acetone (Sigma Aldrich), and deionized water. We report on films deposited for 5 min using a constant applied voltage of 100 V, provided by a Keithley 6-1/2-digit model 6517A Electrometer/ High-Resistance Meter, which also measured the EPD current. To study the time dependence of film thickness, further depositions were performed with the deposition time varying from 30 s up to 5 min. At the end of each EPD run, the electrodes were extracted from the colloidal suspension and kept at 100 V for an additional 5 min while the film dried. This step enables further densification of the deposit using local eddy currents.12,29,30 Drop casts of the star polymers from a dilute DCM solution were prepared as a standard of comparison for infrared spectroscopy.
Figure 1. (a) The unmixed, hexane/dichloromethane preparation, with the star polymer solution injected at the hexane/DCM interface without any further perturbation. The dichloromethane-rich region (Layer 1) and the hexane-rich region (Layer 2) are in a 9:1 volume ratio. (b) The hexane/dichloromethane preparation, with the star polymer solution injected at the hexane/DCM interface with additional vigorous mixing of the constituents.
Atomic force microscopy was performed with a Digital Instruments NanoScope III in tapping mode. Reflectanceabsorption infrared spectroscopy (RAIRS) was performed using a Bio-Rad Excalibur FTS-3000 spectrometer in single reflection mode. Optical micrographs were obtained through a Leitz Ergolux DIC photomicroscope fitted with an Angstrom Sun CFM-USB-2 digital microscope camera. Scanning electron microscopy was performed with a Hitachi S-4200 field emission scanning electron microscope, operating at 5 kV. Film thicknesses were measured with a Veeco Dektak 150 profilometer. 3. Results and Discussion 3.1. Colloidal Suspensions Suitable for Electrophoretic Deposition. Our first series of experiments on the PS/DVB star polymers consisted of EPD trials using only dichloromethane as the solvent. None of the deposition experiments using the all-DCM solution, in the entire phase space of dc voltages and EPD run times, yielded a polymer film. We expected this outcome since neutrally charged, well-solvated PS/DVB polymers do not readily polarize in electric fields less than approximately 30 kV/cm.31 In response to these results, we attempted to produce thin films from the star polymers suspended in hexane, a typical nonpolar, organic solvent used for the EPD of semiconductor nanocrystals.14,30 However, the star polymers were not well-solvated in hexane, readily precipitating from the solution. Subsequent attempts to prolong the suspension of the stars, through vigorous stirring and agitation to facilitate the deposition, also were unsuccessful. Our next step to reconcile the solution parameters for electrophoretic deposition in hexane and DCM involved exploring what effect combining the solvents would have on the successful deposition of the star polymer films. We decided to investigate the combination of hexane and DCM star polymer solvent over a range of volume ratios of the liquids. Since hexane and DCM are immiscible, their steadystate mixture is a phase-separated, bi-layered liquid with DCM at the top. We used this stratified liquid as the starting point for preparing all of our EPD solution samples. We introduced the PS/DVB star polymer solution into the stratified hexane/ DCM liquids by injecting a concentrated polymer solution into the DCM near the interface of the immiscible liquids. This produced an intermingling of the PS/DVB polymer-infused DCM solution both in the DCM layer of the beaker and across the hexane/DCM interface into the hexane area. This yielded a DCM-rich, polymer-infused region in the top portion of the stratified liquid and a hexane-rich, polymer-infused region in the bottom portion of the liquid. As seen in Figure 1a, the result was a stratified suspension with a clear, DCM-rich upper layer
Star Polymer Films
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Figure 2. (a) Photograph exhibiting the deposition of a thin film of the star polymers, indicated by the arrow, from the unmixed colloidal suspension onto negative (left) and positive (right) ITO electrodes (each 1.3 cm × 2.5 cm shown in full) with (b) optical micrographs and (c) SEM images of the films on their respective electrodes. The scale bar corresponds to 1 cm in the photograph, to 20 µm in the micrographs, and to 1.2 µm in the SEM images.
Figure 3. (a) Photograph exhibiting the deposition of a thin film of the star polymers, indicated by the arrow, from the well-mixed colloidal suspension onto negative (left) and positive (right) ITO electrodes (each 1.3 cm × 2.5 cm shown in full) with (b) optical micrographs and (c) SEM images of the films on their respective electrodes. No polymer film was found on the positive electrode. The scale bar corresponds to 1 cm in the photograph, to 20 µm in the micrographs, and to 1.2 µm in the SEM images.
(Layer 1) and a cloudy, hexane-rich bottom layer (Layer 2). The cloudiness was indicative of the polymer-infused, hexanerich lower portion since the star polymer dissolves poorly in hexane. The DCM-rich region remained clear since the polymer dissolved well in this solvent. We monitored the stability of the hexane/DCM liquid suspensions at volume ratios ranging from 1:1 to 14:1 (hexane/DCM). We discovered that the optimal volume ratio for producing EPD films was approximately 9:1. This ratio was employed for all subsequent star polymer EPD experiments. We performed EPD experiments on two different preparations of the 9:1 volume ratio of hexane/DCM, injected with the PS/DVB star polymer-infused DCM solution. The first preparation involved the aforementioned, stratified hexane/DCM liquid, with the star polymer solution injected at the hexane/ DCM interface without any further perturbation. The second preparation involved the 9:1 phase/separated liquid with the star polymer injection, but with an additional vigorous mixing step of the fluid components, which yielded a cloudy suspension, as seen in Figure 1b. 3.2. Controlled Electrophoretic Deposition of Films. Beginning with the unmixed preparation, we performed the EPD experiment at 100 V for 5 min followed by 5 min of drying in the atmosphere with the voltage still engaged. An optimum voltage of 100 V was employed to locomote the star polymers and to minimize electrode surface chemistry simultaneously. Any extant, yet suppressed, electrochemical activity in the system may be attributed to the possible reactivity of the ITO electrodes with the DCM solvent.32 No appreciable degradation was observed in the overall surface roughness and integrity of the ITO electrodes, contrary to results found in the literature. These conditions produced films on both electrodes, with a film
appearing on the upper region of the positive electrode (corresponding to the clear layer of the colloidal suspension) and on the bottom part of the negative electrode (corresponding to the cloudy layer of the suspension). The arrowed regions in the images of Figure 2a indicate the regions with the star polymer deposits. The film on the positive electrode had a translucent appearance, due to its relative thickness, while the film on the negative electrode was opaque. The second preparation allowed us to determine whether mixing of the suspension would affect EPD and the resulting films. Using this well-mixed preparation, we conducted EPD runs also at 100 V for 5 min followed by 5 min of drying. For this sample preparation, we observed a uniform, opaque deposition of PS/DVB star polymer only on the negatively biased electrode. There was no visible deposition on the positive electrode as shown in Figure 3a. The electrodes upon which deposition occurred were indicative of the net charge on the colloidal star polymers. For the unmixed, stratified liquids, deposition on the upper part of the positive electrode implied that the colloids in the DCM-rich layer possessed a net negative charge. Deposition on the lower part of the negative electrode suggested that, in the hexanerich layer, the colloids possessed a net positive charge. This apparent spatial separation of charge in the unmixed suspension gives rise to the controlled deposition. Because the deposition of the star polymers was suppressed on the positive electrode for the well-mixed hexane/DCM liquids, such mixing appeared to have the effect of neutralizing the net negative charge on the star polymer colloids. 3.3. Film Characterization. The initial characterization of the electrophoretically deposited polymer films involved an assessment of the time dependence and solvent dependence of
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Figure 4. Graph of the thicknesses of the electrophoretic films as functions of time. The circles correspond to the well-mixed solutions; the stars correspond to the hexane-rich solution; and the triangles correspond to the DCM-rich solution.
the thickness of the films. The results, evinced in Figure 4, show that the thickest films are formed from the well-mixed solutions and that the thinnest films are formed from the unmixed DCMrich solutions. These findings for the overall film thickness correspond to the relative volumes of the star polymer solutions and the relative total masses of the star polymers in the corresponding solutions. The time dependence of the film thickness for each of the solutions, particularly that of the wellmixed solutions, exhibits a sigmoid shape that is comparable to trends predicted by both the Hamaker theory for colloidal particle deposition and a recently derived, resistivity-based model of nanoparticle EPD.33-35 Our initial assessment of the integrity and appearance of the thin films entailed a survey of the topology of the films via optical microscopy. The micrographs revealed micrometer-sized aggregates of the stars dispersed within a thin film of tightly packed star polymers. For the unmixed suspension (Figure 2b), the left-hand image illustrates the film with large average diameter (0.5-5 µm), loosely packed star polymer aggregates on the negative electrode. The right-hand image shows relatively small average diameter (e1 µm) aggregates, also loosely dispersed in the film on the positive electrode. For the wellmixed suspension, the corresponding optical micrographs of the films (Figure 3b) show star polymer aggregates on the negative electrode of an average diameter (0.5-3 µm) that fell between the diameters of the aggregates on the positive and negative electrodes from the unmixed preparations. Because mixing has the effect of homogenizing the suspension, the well-mixed suspension yielded films with aggregates of this intermediate size. The right-hand image of Figure 3b confirms the absence of a star polymer film on the positive electrode. Scanning electron microscopy (SEM) verified the presence and the size of the aggregates on the three electrodes that exhibited polymer deposits, as well as the absence of film on the fourth electrode (Figures 2c and 3c). The hazy, whitish appearance of the films can be attributed to Mie scattering by the micrometer-sized aggregates. Whereas the regions between the aggregates appear transparent in the optical micrographs, the SEM images show that a tightly packed star polymer film with nanostructured morphology resides on the surface of the ITO electrodes as well. This motivated further investigation of the surface topology of the films. Atomic force microscopy (AFM) of the star polymer films revealed different nanostructures and polymer conformations that were unique to the specific suspension states and the
Figure 5. Atomic force microscopy images of the PS/DVB films deposited from the unmixed suspension onto (a) the negative electrode and (b) the positive electrode and (c) from the well-mixed suspension onto the negative electrode. (d) Three possible conformations for a star polymer adsorbed to a surfce.37
deposition sites. The length scale of the AFM images in Figure 5 (500 nm × 500 nm) provides insight on the conformation of the polymer film in the regions between the micrometer-sized aggregates. For the unmixed, stratified liquid, the film on the negative electrode possessed ellipsoid features with heights ranging from 10 to 15 nm and lateral dimensions approaching 50 nm (Figure 5a). The film deposited on the positive electrode possessed narrower, peanut-shaped features of a length of approximately 40 nm, of a width of approximately 20 nm, and of a height of approximately 10 nm (Figure 5b). The features on the positive electrode appear more tightly packed than those on the negative electrode. The film deposited onto
Star Polymer Films the negative electrode from the well-mixed second preparation also had ellipsoid features, slightly larger than the features of the aforementioned negative electrode from the unmixed liquid (Figure 5c). The feature sizes observed on each electrode reflect the colloid size in the liquid. A previous study of colloidal polymer deposition reported that colloids become larger as their concentration in the preferred solvent increases, allowing for more efficient solidification.9,11,36 In our experiments, less DCM is available to solvate the polymer in the unmixed hexane-rich and well-mixed phases than is available in the unmixed DCMrich phase. Hence, we observe larger structural features in the films deposited from the unmixed hexane-rich and well-mixed phases. A star polymer adsorbed to a surface may reside in one of three conformations: a strong adsorption regime for which the arms are fully adsorbed to the surface (Conformation 1), an intermediate adsorption regime for which some of the arms are adsorbed (Conformation 2), and a weak adsorption regime for which a modicum of the arms are partially adsorbed (Conformation 3) (Figure 5d).37 In a semidilute solution, the ends of a star’s arms are essentially free and behave as linear polymer chains.37 Once deposited into a film, the interaction of these arms with their surroundings determines the surface conformation. The AFM images suggest that the star polymers in each film reside predominantly in one of the three conformations. The ellipsoidal features, found in the film deposited onto the negative electrodes from both suspension states, indicate minimal interaction of the arms of the star polymers with underlying star polymer units (Conformation 3). The narrower peanut-shaped features, found in the film deposited on the positive electrode from the unmixed liquid, reflects some adsorption of the arms and some interaction between adjacent units (Conformation 2). The intriguing nanostructured conformation observed in the films prompted the question of whether said nanostructure was indicative of changes in the physical and chemical integrity and composition of the star polymers due either to the solvent or to the electrophoretic deposition. Infrared absorption spectroscopy, comparing each of the EPD films and the DCM-solution drop cast, confirmed that the physical and chemical integrity of the stars remained largely unchanged during the deposition. Figure 6 highlights specific absorption modes for each of the films and the drop cast, indicated by the arrows. The primary frequency modes for the drop cast polymer (A), for the wellmixed film (B), and for the hexane-rich film (C) are identical. On the contrary, the spectrum for the DCM-rich film (D) exhibited a small blue-shift in the primary symmetric, asymmetric, and aromatic vibrational mode energies. The shifts in the infrared absorption spectrum for this film, which suggest a constrained physical environment for the star polymers, are consistent with the proposed conformation (Conformation 2) of partially adsorbed arms. By simply varying the suspension conditions and electrode bias, the same PS/ DVB star polymer material demonstrates differing degrees of interaction on the surface of the deposited film. To our knowledge, these observations represent the first examples of the controlled electrophoretic deposition of PS/ DVB star polymers. We specifically chose polystyrene star polymers, made from shorter (9k) linear polymers, because they typically yield star polymers with a high degree of incorporated linear arms.28 These compact stars also allow for the observation of the solvent-mediated modulation of the conformation of the stars, both in solution and in thin-film form. Star polymer architectures, prepared from high-molecular-weight polymers,
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Figure 6. Infrared absorption spectra for the C-H vibrational stretching peaks for the PS/DVB star polymer films. The frequency modes for the drop cast polymer (A), for the well-mixed film (B), and for the hexane-rich film (C) [methylene (symmetric) ) 2852 cm-1; methylene (asymmetric) ) 2922 cm-1; aromatic sites ) 3026 and 3060 cm-1] were identical. In contrast, the spectrum for the DCM-rich film (D) exhibited a small blue-shift in each of the primary vibrational modes [methylene (symmetric) ) 2854 cm-1; methylene (asymmetric) ) 2925 cm-1; aromatic sites ) 3028 and 3061 cm-1], indicated by the arrows. The spectra are shifted vertically for clarity.
are markedly less desirable because the resulting structures comprise fewer linear arms and, therefore, the relative change in conformation, depending on the solvent, is not as pronounced. Future investigations will include the deposition of refined star polymer architectures containing, for example, electroactive features. 4. Conclusion In this study, we have demonstrated that the specific preparation of colloidal suspensions of polystyrene/divinylbenzene star polymers yields controlled deposition and unique polymer conformation when the polymers were cast into thin films via electrophoretic deposition. The resultant films exhibited unique nanostructured morphologies, depending on whether the preparation of the stratified hexane/dichloromethane liquid, injected with the PS/DVB in DCM solution, involved mixing of the liquid. The film effects we observesdeposition location, feature size, and polymer conformationsare all a function of the relative concentration of good solvent, DCM, in each phase of the suspensions. With their unique nanostructures, these films will be further investigated for use in optical applications. Because successful electrophoretic deposition requires charged, dipolar, or polarized particles in solution, we have shown that the preparation of PS/DVB colloidal suspensions transforms the neutrally charged polymer into colloids with both positive and negative net charges. Elucidating the mechanism of this charge generation, however, will require further study. Controlled deposition confirms our ability to manipulate the spatial distribution and charge of the colloids through the mixing state of the suspension. Additionally, the mixing state lends itself to selecting the nanoscale conformation of and interaction among the polymer units, as observed on the films. While this study utilized polymers in organic solvents, the production of watersoluble star polymers, whose polarity and conformation may be similarly manipulated in solution, and electroactive star
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