Journal of Colloid and Interface Science

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1 Journal of Colloid and Interface Science 394 (2013) Contents lists available at SciVerse ScienceDirect Journal of Colloid and Interface Science Facile fabrication of Pickering emulsion polymerized polystyrene/laponite composite nanoparticles and their electrorheology Young Jae Kim a, Ying Dan Liu a, Hyoung Jin Choi a,, Soo-Jin Park b a Department of Polymer Science and Engineering, Inha University, Incheon , Republic of Korea b Department of Chemistry, Inha University, Incheon , Republic of Korea article info abstract Article history: Received 23 August 2012 Accepted 11 December 2012 Available online 28 December 2012 Keywords: Pickering emulsion Polystyrene Laponite Electrorheological fluid Polystyrene (PS)/laponite composite nanoparticles were fabricated using a surfactant-free Pickering emulsion polymerization method, in which emulsions of styrene dispersed in water were stabilized by hydrophilic laponite modified with cetyltrimethylammonium bromide. The PS/laponite nanoparticles, of which their surface was covered compactly by laponite clay platelets, were observed by scanning electron microscopy. Fourier-transform infrared spectroscopy, X-ray diffraction, and thermogravimetric analysis confirmed their chemical composition, crystallographic structure, and thermal properties and weight loss percentage of the laponite located on the surface of the PS particle, respectively. When an external electrical field was applied, the chain-like structure of the laponite coated nano-sized PS particle exhibiting electrorheological characteristics was observed by optical microscopy. The electrorheological performance of the bulk properties was also examined using a rotational rheometer equipped with a high voltage generator. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction Polymer/inorganic nanocomposites have been studied extensively owing to their outstanding in mechanical, electrical, optical and rheological properties [1]. In particular, polymer/clay nanocomposite particles exhibit superb electrical properties due to the intrinsic electrical properties of clay when fabricated appropriately. These materials have been adopted as electrorheological (ER) materials [2 6]. The electrorheological characteristics lead to the polarization of particles when dispersed in a non-conducting medium, resulting in structural change under an applied electric field. The most common type of ER fluid is a colloidal suspension of solid dielectric or conducting particles dispersed in an insulating fluid, exhibiting Newtonian fluid behavior without an applied electric field [7]. On the other hand, when an electric field is applied to an ER fluid, the dispersed particles are polarized and aligned along the direction of an electric field resulting in increased shear viscosity. Therefore, its rheological behavior can generally be expressed by a Bingham fluid with a yield stress [8,9]. Recently, polystyrene (PS)/SiO 2 particles via surfactant-free emulsion were introduced using an emulsion polymerization technique to obtain polymer/clay nanocomposite particles in a range of polymeric systems, such as the interaction of hydrophobic silica Corresponding author. Fax: address: hjchoi@inha.ac.kr (H.J. Choi). nanoparticles and classical surfactants at the non-polar oil water interface [10,11]. In particular, when these particles are used to stabilize an emulsion system without conventional surfactants, the process is called a surfactant-free Pickering emulsion, in which the solid particles (<100 nm) are generally adsorbed strongly at the interface between the aqueous and organic liquids, and then produce both oil/water and water/oil emulsions with significant stability [12 14]. In this study, the PS/laponite core shell structured composite nanoparticles were fabricated as a new ER material based on the above surfactant-free Pickering emulsion approach, in which the laponite was modified with cetyltrimethylammonium bromide (CTAB) to produce a hydrophobic surface prior to its use. The synthesized PS/laponite nanoparticle with hydrophobic surface improves the dispersion stability and compatibility of the particles with organic oil medium [15,16]. Moreover, core shell-structured particles were reported several times as ER materials in previous study, showing various advantages than pure polymeric or inorganic particles [17 19]. Disc-shaped CTAB-modified laponite was used as a stabilizer to synthesize the PS nanospheres. Note that the laponite particles are synthetic clays, 1 nm in thickness and 20 nm in lateral diameter [12], composed of two tetrahedral silica sheets and a central octahedral magnesia sheet. The silicon and magnesium atoms are balanced by 20 oxygen atoms and four hydroxyl groups, with an empirical formula of 0.7Na + - [(Si8Mg5.5Li0.3)O 20 (OH) 4 ] 0.7 and a cationic exchange capacity of /$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.

2 Y.J. Kim et al. / Journal of Colloid and Interface Science 394 (2013) approximately 47 mequiv/100 g. The negative surface charge density was reported to be approximately e /Å. Laponite has attracted increasing attention for stabilizing emulsion systems due to the more uniform and smaller dimension of laponite than other clay species, such as montmorillonite and bentonite in addition to their surface characteristics [20 22]. On the other hand, note that ER property of pure laponite has been also reported [23,24]. 2. Experimental The PS/laponite nanoparticles were prepared using two processes. In first process, the CTAB modified laponite was synthesized by an ion exchange method. In the second process, PS/laponite nanoparticles were fabricated via Pickering emulsion polymerization using the modified laponite Characterization The morphology of the synthesized PS/laponite particle was observed by scanning electron microscopy (SEM) (S-4300 Hitach) and transmission electron microscopy (TEM) (Philips CM200). The molecular structure of the product particles was characterized by Fourier transform infrared spectroscopy (FT-IR) (Perkin Elmer System 2000). The mass amount of the component was measured by thermogravimetric analysis (TGA, TA instrument Q50, USA) with a heating rate of 10 C/min in a nitrogen gas. We investigated the size distribution of the synthesized composite particles using an electrophoretic light scattering device (ELS-8000, Otsuka, Japan). An optical microscope (OM, Olympus BX-51, USA) equipped with a DC voltage generator was used to observe the formed chain-like structure of the ER fluid. The ER behavior was examined using a rotational rheometer (Physica MCR 300, Stuttgart) equipped with a high voltage generator Preparation of modified laponite Synthetic laponite (grade: RDS, purchased from Rockwood additive Ltd.) is a trioctahedral clay with lithium substituting for magnesium in the octahedral layers with its cation exchange capacity (CEC) of 47 mequiv/100 g. The specific particle density was 2.65 g cm 3. CTAB (Sigma Aldrich) used was of analytical grade. The fabrication of hydrophobic clay was undertaken using the following procedure. First, 16.0 g of laponite was added to 800 ml of distilled water with constant stirring at 80 C. At the same time, a second solution of a stoichiometric amount (based on CEC) of CTAB was added to 200 ml of distilled water at room temperature. Both solutions were stirred for 10 h. Finally, a CTAB solution was added carefully to the clay solution, which resulted in the immediate formation of a milky color. The mixture was then stirred at 80 C for 24 h. Then the resulting solution was covered and cooled to room temperature overnight. The dispersion solution was filtered, redispersed in 500 ml of water for 1 h, and filtered again. This process was repeated several times to remove any free surfactant. The filter-treated clay was dried at 100 C in a vacuum oven for 1 day, crushed with a mortar and pestle, and then returned to the vacuum oven for 3 h at 100 C Synthesis of PS/laponite nanoparticle PS/laponite composite nanoparticles were synthesized by a Pickering emulsion polymerization method using CTAB modified laponite plates as a solid stabilizer. Styrene (Daejung, Korea) and 2,2 0 -azobis(2-methylpropionamidine) dihydrochloride (AIBA) (Aldrich, USA) were used as the monomer and cationic watersoluble initiator, respectively. One gram of CTAB modified laponite was added to 180 ml of distilled water and ultra-sonicated until a clear aqueous solution was obtained. After ultra-sonication, the laponite dispersion solution was stirred for 1 h. Subsequently, 9 ml of styrene was added to the laponite dispersion solution and agitated for 5 min using a homogenizer. The emulsion was mixed a few minute to prevent the formation of a visible organic layer. The resulting emulsion was poured into a 250 ml round-bottomed flask, which was sealed with a rubber seal and bubbled with nitrogen for 20 min. An AIBA solution (0.09 g/5 ml) was then added. After the reaction mixture was heated to 60 C for 12 h, the final product was centrifuged with both distilled water and methanol to remove the excess initiator, monomer and free laponite plates, and then dried in a vacuum oven at 65 C for 3 days. Finally, the product particle was dispersed in silicone oil and sonicated for 1 h to obtain a uniform ER fluid containing 10 vol% PS/laponite nanoparticles. 3. Results and discussion The mechanism of preparing PS/laponite core shell particles by a surfactant free Pickering emulsion polymerization is illustrated in Scheme 1. As a stabilizer, the CTAB modified laponite was adsorbed on the surface of the styrene monomer droplet to stabilize the O/W system. After adding a water soluble initiator (AIBA), the O/W mixture changes from cloudy to milky white and polymerization occurred in the styrene droplets with laponite adsorbed at the boundary surface. Fig. 1a c shows the SEM, particle size distribution and TEM images of PS nanoparticles stabilized by the CTAB modified laponite plates, in which the spherical nano-scaled PS/laponite particles exhibited an uneven surface with diameters ranging from 50 nm to 300 nm. SEM image of PS/laponite particles confirmed the absorbed PS core particles. The rough surface of the PS nanoparticles was attributed to the addition of CTAB modified laponite platelets, which act as a stabilizer in this Pickering emulsion polymerization. Fig. 1b represents the particle size distribution of PS/ laponite nanoparticles, estimated via a Stokes-Einstein equation with a hydrodynamic diameter. The measurement gives an average hydrodynamic diameter of the particles and the polydispersity index (PDI) of the particles. It was evident from Fig. 1b that 97.6 wt% of the particles was distributed in the range of nm and 2.4 wt% of the particles was distributed in the range of Scheme 1. Mechanism of surfactant-free Pickering emulsion polymerization for PS/ laponite nanoparticle.

3 110 Y.J. Kim et al. / Journal of Colloid and Interface Science 394 (2013) Fig. 1. SEM (a), size distribution (b) and TEM image (c) of PS nanoparticles stabilized by CTAB-modified laponite nm. Accordingly, the result of dynamic light scattering studies on the size of the PS/laponite nanoparticles is in a good agreement with the of SEM image. Fig. 1c shows a TEM image of the core shell structure for synthesized PS nanoparticles, where the grey spheres and dark strips are considered PS cores and laponite plates, respectively. The grey cores are surrounded by densely stacked laponite plates, confirming the role of laponite plates as a stabilizer. The PS/CTAB modified laponite nanoparticle size observed by TEM was similar to that determined by SEM. The chemical structure of the synthesized PS/laponite was examined by FTIR spectroscopy. Fig. 2 presents the FT-IR spectra Fig. 2. FT-IR spectra of pure PS, laponite, CTAB modified laponite, and laponitestabilized PS nanoparticles. of synthesized PS, laponite, CTAB modified laponite and PS/laponite nanoparticles. For pure PS in Fig. 2, the characteristic bands observed at 760 and 694 cm 1 confirm the presence of a monosubstituted aromatic group, while the absorption peaks at around 3000 cm 1 arise from the attachment of additional C H groups. The characteristic peaks of laponite at 1015 cm 1 and 470 cm 1 were assigned to Si O bending vibrations, whereas the peak at 660 cm 1 indicated an Mg O bond. The CTAB-modified laponite not only exhibited the laponite characteristic peak but also showed the C H stretching vibration and tertiary dimethyl amines, cm 1 and cm 1, respectively. From the FT-IR spectrum of PS, the peaks were detected around the aromatic ring-stretching vibrations that occur in the region cm 1. The bands observed in the infrared spectrum near 760 and 690 cm 1 confirm the presence of a mono-substituted aromatic group. In addition, the C H stretching vibration was detected at cm 1. Furthermore, the FT-IR spectra of the PS/laponite nanoparticles exhibited the typical chemical characteristics of both PS and laponite. Fig. 3 represents the thermal properties of laponite, CTAB-modified laponite, PS, and PS/laponite nanoparticles. The weight loss of laponite was approximately 2% when the temperature was increased from 700 C, which is in agreement with the amount of impurities. For the CTAB modified laponite, the weight loss at 700 C increased with the enhancement of CEC-relative CTAB substitution, which means the organic content in the organoclay also increased simultaneously. The complete weight losses for different CTAB modified laponite was 18.9%. For pure PS particles, there were two main temperature regions of weight loss. The weight loss below 390 C was attributed to the evaporation of physically absorbed water and residual solvent in the samples, and the weight loss beyond 390 C was assigned to the decomposition of PS. Virtually no residual material was observed at temperatures higher than

4 Y.J. Kim et al. / Journal of Colloid and Interface Science 394 (2013) Fig. 3. TGA graph of (a) laponite, (b) CTAB modified laponite, (c) PS, and (d) laponite-stabilized PS nanoparticle. 480 C. For PS/laponite nanoparticles, their decomposition temperature was increased to C due to the surface coating by laponite discs. From the measured mass of the residue, the weight ratio of laponite in the PS/laponite nanoparticles was approximately 8.85%. Using the density of PS (1.09 g cm 3 ) and laponite clay (2.65 g cm 3 ), we then calculated density of the PS/laponite nanoparticle to be 1.23 g cm 3. It was much lower than that of pure laponite clay. On the other hand, the density measured using a gas pyconometer (AccuPyc 1330, Micromeritics Instrucments Co., USA) was 1.19 g cm 3, which is similar to the calculated density from the TGA analysis. The density mismatch between PS/laponite particles and continuous oil (silicone oil, KF-96, 0.96 g cm 3 ) was obviously reduced compared with pure laponite-clay-based ER fluid. Thus, the suspension stability is expected to be improved including sedimentation stability and dispersion stability. The ER fluid was prepared by dispersing the CTAB modified laponite-stabilized PS nanoparticles in silicone oil (ShinEtsu, KF-96-50cS) and the microstructural change of this ER fluid was then observed directly by optical microscopy under an applied electric field with a DC high voltage source [25]. The CTAB modified laponite-stabilized PS nanoparticle-based ER fluid exhibited a typical ER chain structure. In the absence of an electric field, the particles were dispersed randomly in silicone oil, indicating a liquid-like state (Fig. 4a). When the electric field was present, the particles begin to move and form a chain structure with the adjacent particles. Finally, the particles aligned along the direction of the applied electric field forming a chain structure (Fig. 4b). Normally, this phenomenon for ER fluids forming fibril chains under an external applied electric field occurs and the structure remains as long as the field is applied. The ER fluid containing PS/laponoite nanoparticles was examined further using a rotational rheometer attached to a high voltage generator through both rotational and oscillation test modes. The ER fluid that was prepared by dispersing dried PS/laponite nanoparticles in the silicone oil with a particle concentration of 10 vol% exhibited typical ER behavior. Fig. 5 presents the flow curves of the ER fluids obtained from the controlled shear rate test under various electric field strengths. Without applying an electric field, the ER fluid behaves in a similar manner to a Newtonian fluid, in which the shear stress increases linearly with increasing shear rate. On the other hand, when exposed to the applied electric field, the shear stress curves exhibited an overall weak concave shape, reaching the minimum value and then increased rapidly with further increases in shear rate. This has been observed for many ER materials by both us and other researchers [23,26,27]. This phenomenon is considered to be the effect of the low particle volume fraction and the hydrophobic affinity between the particles and the medium of the ER fluid which all lead to low interfacial polarizability and sequentially low ER effect. Due to the polarization, the dispersed particles form chain-like structures that are similar to that observed by optical microscopy. The attractive forces generated between dipoles span the electrodes. This fibril structure will resist and reform, but flow finally starts until the shear rate approaches a critical value, after which the fibril structure is destroyed completely. On the other hand, the Bingham fluid model is the simplest rheological equation of state with two parameters originating from the yield stress s 0 and Newtonian viscosity g 0, and has been adopted widely as a model for ER suspensions [28]. On the other hand, the Bingham fluid model is unable to fit well, particularly in the low shear rate regime as shown in Fig. 5. Therefore, another model, Cho Choi Jhon (CCJ) model [29], was adopted to describe the unique shear stress behavior and yield stress, which has been already applied in previous studies [30]. This model provides a more appropriate explanation to the flow curves of ER fluids above the entire shear rate range, particularly for the low shear rate region. The equation is described as follows: Fig. 4. OM images of the ER fluid based on PS/laponite nanoparticles without an electric field (a) and with an electric field (b).

5 112 Y.J. Kim et al. / Journal of Colloid and Interface Science 394 (2013) Fig. 5. Flow curves fitted to the Bingham model (dot lines) and CCJ model (solid line) of PS/laponite nanoparticle based ER fluid under various electric field strengths.! s s 0 ¼ 1 þ ðt 2 _c Þ a þ g 1 1 þ 1 _c ðt 3 _cþ b ð1þ Fig. 6. Yield stress as a function of the electric field strength for the ER fluid of PS/ laponite nanoparticles. where t 2 and t 3 are time constants, which used to describe the change in the shear stress and g 1 is the viscosity at a high shear rate. s 0 is a function of an electric field and a is related to the decrease in stress. The exponent b is related to the decrease in shear stress at the low shear rate region, whereas b in the range 0<b 6 1 is for the high shear rate region. The fitting result in Fig. 5 confirmed the utility of the CCJ model in fitting these kinds of flow curves. Table 1 lists the yield stresses and optimal parameters. As shown in Fig. 5, shear stress decreases slightly after achieving the yield value until the shear rate reaches a critical point ð _c crit Þ. This is followed by a gradual increase with further increases in shear rate at each electric field strength; at 1.0, 2.0, 3.0 and 4.0 kv mm 1 _c crit is 4.6, 10.4, 12.1 and 13.0 s 1, respectively. In Table 1, the yield stresses increase with increasing electric field strength. Similarly, the infinite shear stress (s) in the CCJ model increases slightly and was 4, 11, 23 and 29 Pa at electric field strengths of 1.0, 2.0, 3.0 and 4.0 kv mm 1, respectively. The tendency of growth is due to the increased electric field strengths. The other parameters in Eq. (1) (a, t 2, b, and t 3 ) were also adjusted to fit the flow curves optimally. The yield stress is a characteristic factor of an ER fluid, as measured by either a static or dynamic test, and is dependent on the electric field strength, volume fraction of the particles and the dielectric or conductive properties of the particle [31]. The correlation between the yield stress and electric field strength is represented by the following power law, s y / E m ð2þ The exponent m can be obtained by fitting the yield stresses over a broad electric field range on a logarithmic scale. Fig. 6 shows the correlation between the yield stress and electric field strength. The exponent parameter was expected to be 2.0 from the polarization model. However, the value of m varies due to different ER materials with the empirical distribution range of [24,30,32]. For the ER fluid of PS/laponite nanoparticles, m was estimated to be 1.2, indicating its lower field dependence. As shown in the TEM image, there is a near single layer of laponite coated on the surface of PS, which is the only part for ER effect because the PS core is a non-polarizable material. Moreover, the electrostatic interaction between the dielectric spheres is treated as a dipole in the polarization model. In ER fluids, a often differs from 2 due to the particle concentration, particle shape, nonlinear conductivity of oil and electric field strength, whereas the applied electric field induces electrostatic polarized interactions among the particles and also between the particles and electrodes [33,34]. As stated above, these might be important factors for the departure of the exponent from 2.0, as shown in Fig. 6. To perform dynamic oscillatory tests, it is important to determine the amplitude of oscillation in the linear viscoelastic region Table 1 Fitting parameters of Bingham and CCJ model equations to the flow curves of PS/ laponite nanoparticles based ER fluid. Model Parameter Electric field strength (kv mm 1 ) Bingham s g CCJ s t a g t b Fig. 7. Amplitude sweep (G 0 : closed symbols; G 00 : open symbols) of the PS/laponite nanoparticle-based ER fluid with a fixed angular frequency of 6.28 rad s 1 under various electric field strengths.

6 Y.J. Kim et al. / Journal of Colloid and Interface Science 394 (2013) that the sample would not be affected by the applied strain. Therefore, the amplitude sweep experiment was performed with a constant angular frequency of 6.28 rad s 1 in the varying strain window from 0.001% to 10%. The constant angular frequency was determined in the amplitude sweep, which was suggested by the test system. Fig. 7 plots the storage (G 0 ) and loss (G 00 ) moduli as a function of the strain amplitude under an applied electric field. G 0 was much higher than G 00 under the applied electric field. Therefore, the PS/laponite nanoparticle-based ER fluid is certainly much solid-like, showing its certain rigidity in Fig. 7. Both G 0 and G 00 curves showed a plateau regime at the small amplitude region, so-called linear viscoelastic region (c LVE ). The mean c LVE was approximately 0.003% and is a definite value for the frequency sweep. When the electric field strength becomes larger than 1.0 kv mm 1, the plateau regime of the G 0 and G 00 appears to broaden, revealing the enhanced elastic solid characteristics of the ER fluid. On the other hand, both the G 0 and G 00 moduli decrease rapidly due to an irreversible change in the structure of the ER fluid. Fig. 8a and b shows the frequency dependence of the G 0 and G 00 moduli, which were measured in the angular frequency range from 1 to 100 rad s 1. The G 0 represents the elastic response, whereas G 00 denotes the viscous part of the system. The frequency sweep also appears similar to an amplitude sweep, in which the G 0 values are higher than G 00 indicating the dominant solid-like behaviors over viscous behaviors in the structure of the ER fluid. In the frequency sweep test without applying an electric field, the storage Fig. 9. (a) Dielectric spectra (e 0 : closed symbols; e 00 : open symbols) and (b) Cole Cole plot of the ER fluids. The fitting lines are generated from Eq. (3). modulus showed similar behavior to liquid-like characteristics, in which G 0 increases linearly in proportional to the frequency. On the other hand, under an applied electric field, both moduli showed a wide plateau region, which were stable over the entire frequency range, and actually independent of the frequency. This suggests that the ER fluid possesses very strong solid-like behavior in an electric field, which is evidence of the dominating factor of elastic property over the viscous one, as discussed in the amplitude sweep tests. In addition, a stepwise increase in both G 0 and G 00 with the electric field strength was also observed. To further examine the ER properties of the PS/laponite nanoparticle-based ER fluid, the dielectric properties were examined using an LCR meter and the results are shown in Fig. 9a and b. The results show that the space charge polarizability (i.e. interfacial polarization) plays an important role in determining the ER effects. Both the permittivity (e 0 ) and dielectric loss factor (e 00 ) were measured as a function of frequency (x). The line in Fig. 9a was obtained by fitting a dielectric relaxation model, Cole Cole equation model [35]. The Cole Cole equation is useful for examining the relationship between the dielectric and ER properties of the ER fluid [36,37]. The model is described in terms of the complex dielectric constant as follows: Fig. 8. Storage modulus G 0 (a) and loss modulus G 00 (b) in a frequency sweep test under various electric field strengths. De e ¼ e 0 ie 00 ¼ e 1 þ ð1 þ ixkþ 1 a In Eq. (3), e is a complex dielectric constant and e 0 and e 00 are the dielectric constant and dielectric loss, respectively. De is the difference between the dielectric constant at 0 and infinite frequency (e 0 and e 1 ). They are the distribution curves over a broad frequency ð3þ

7 114 Y.J. Kim et al. / Journal of Colloid and Interface Science 394 (2013) range. k is the relaxation time at the frequency of which the dielectric loss reaches the maximum value. The exponent (1 a) determines the broadness of the relaxation time distribution. When a is zero, Eq. (3) reduces to Debye s single relaxation time model. De is the achievable polarizability in the ER fluids, which equals for PS/laponite particle-based ER fluid. The value of De was much lower than those reported in other studies [38 40]. One explanation is that PS/laponite nanoparticle contained approximately 8.85 wt% laponite and approximately 91 wt% non-polarizabile polystyrene. Because of the laponite content in the PS/laponite particles, De is a relative low value that leads to a weak ER effect (low yield stresses). 4. Conclusion PS/laponite nanoparticles were synthesized successfully by a Pickering emulsion method. Their morphologies were characterized by SEM and TEM, showing that modified laponite clay covered the polystyrene surface. TGA revealed the weight percent of the conductive laponite to be approximately 8.85%, indicating the deposition of the laponite on the PS core. An ER fluid based on PS/laponite nanoparticles experienced rotation and oscillation under an applied electric field. The ER performance of the PS/laponite fluid was lower than the typical ER behavior due to the low volume fraction of laponite in PS/laponite nanoparticle. This type of flow curve was fitted well by the suggested CCJ model. The yield stress of PS/laponite at variable electric field strengths was rearranged to a uniform line to understand the correlation between the yield stress and electric field strength. Furthermore, dielectric analysis revealed polarizability in the nanoparticle-based ER fluid, which is also consistent with their ER effects. Acknowledgment This study was supported by the Ministry of Knowledge Economy, Korea (2012). [2] Y. Méheust, K.P.S. Parmar, B. Schjelderupsen, J.O. Fossum, J. Rheol. 55 (2011) 809. [3] H. Yilmaz, H.I. Unal, B. Sari, J. Appl. Polym. Sci. 103 (2007) [4] Y.D. Kim, J.H. Kim, Colloid Polym. Sci. 286 (2008) 631. [5] Y.C. Cheng, J.J. Guo, G.J. Xu, P. Cui, X.H. Liu, F.H. Liu, J.H. Wu, Colloid Polym. Sci. 286 (2008) [6] M. Stenicka, V. Pavlinek, P. Saha, N.V. Blinova, J. Stejskal, O. Quadrat, Colloid Polym. Sci. 286 (2008) [7] J. Trlica, P. Saha, O. Quadrat, J. Stejskal, J. Phys. D: Appl. Phys. 33 (2000) [8] X.P. Zhao, J.B. Yin, Chem. Mater. 14 (2002) [9] P. Hiamtup, A. Sirivat, A.M. Jamieson, J. Colloid Interface Sci. 295 (2006) 270. [10] N.G. Eskandar, S. Simovic, C.A. Prestidge, J. Colloid Interface Sci. 358 (2011) 217. [11] D. Yin, Q. Zhang, H. Zhang, C. Yin, J. Polym. Res. 17 (2010) 689. [12] E. Balnois, S. Durand-Vidal, P. Levitz, Langmuir 19 (2003) [13] P. Wheeler, J. Wang, L. Mathias, Chem. Mater. 18 (2006) [14] G. Yin, Z. Zheng, H. Wang, Q. Du, J. Colloid Interface Sci. 361 (2011) 456. [15] M. Stěnička, V. Pavlínek, P. Sáha, N. Blinova, J. Stejskal, O. Quadrat, Colloid Polym. Sci. 289 (2011) 409. [16] M.J. Hato, K. Zhang, S.S. Ray, H.J. Choi, Colloid Polym. Sci. 289 (2011) [17] M. Sedlačík, M. Mrlík, V. Pavlínek, P. Sáha, O. Quadrat, Colloid Polym. Sci. 290 (2012) 41. [18] K. Shin, D. Kim, J.C. Cho, H.S. Lim, J.W. Kim, K.D. Suh, J. Colloid Interface Sci. 374 (2012) 18. [19] V. Pavlínek, P. Sáha, T. Kitano, J. Stejskal, O. Quadrat, Physica A 353 (2005) 21. [20] Y. Cui, M. Threlfall, J.S. van Duijneveldt, J. Colloid Interface Sci. 356 (2011) 665. [21] F.F. Fang, J.H. Kim, H.J. Choi, C.A. Kim, Colloid Polym. Sci. 287 (2009) 745. [22] S.A.F. Bon, P.J. Colver, Langmuir 23 (2007) [23] B. Wang, M. Zhou, Z. Rozynek, J.O. Fossum, J. Mater. Chem. 19 (2009) [24] K.P.S. Parmar, Y. Meheust, B. Schjelderupsen, J.O. Fossum, Langmuir 24 (2008) [25] S.G. Kim, J.W. Kim, W.H. Jang, H.J. Choi, M.S. Jhon, Polymer 42 (2001) [26] B.M. Lee, J.E. Kim, F.F. Fang, H.J. Choi, J.F. Feller, Macromol. Chem. Phys. 212 (2011) [27] Y.G. Ko, U.S. Choi, Y.J. Chun, Macromol. Chem. Phys. 209 (2008) 890. [28] D.K. Klingenberg, C.F. Zukoski, Langmuir 6 (1990) 15. [29] M.S. Cho, H.J. Choi, M.S. Jhon, Polymer 46 (2005) [30] F.F. Fang, Y.D. Liu, H.J. Choi, Smart Mater. Struct. 19 (2010) [31] D.J. Klingenberg, F. van Swol, C.F. Zukoski, J. Chem. Phys. 94 (1991) [32] J. Yin, X. Zhao, X. Xia, L. Xiang, Y. Qiao, Polymer 49 (2008) [33] X. Song, A. Hu, N. Tan, D. Ma, Y. Liu, Mater. Chem. Phys. 126 (2011) 369. [34] B.J. Park, H.J. Choi, J. Colloid Interface Sci. 345 (2010) 554. [35] K.S. Cole, R.H. Cole, J. Chem. Phys. 9 (1941) 341. [36] M.S. Cho, Y.H. Cho, H.J. Choi, M.S. Jhon, Langmuir 19 (2003) [37] J. Yin, X. Xia, X. Wang, X. Zhao, Soft Matter 7 (2011) [38] C. Liu, C. Li, P. Chen, J. He, Q. Fan, Polymer 45 (2004) [39] M. Pluta, J.K. Jeszka, G. Boiteux, Eur. Polym. J. 43 (2007) [40] J. Xu, C.P. Wong, Composites Part A 38 (2007) 13. References [1] J. Yin, X. Zhao, Nanoscale Res. Lett. 6 (2011) 256.