Molecular Weight Dependence of Structures and Rheological

Nov 15, 1996 - Solid State Physics, The University of Tokyo, 106-1 Shirakata, Tokai Naka,. Ibaraki 319-11, Japan. Received October 6, 1995. In Final F...
6 downloads 0 Views 224KB Size
Download PDF
Langmuir 1996, 12, 6179-6183

6179

Articles Molecular Weight Dependence of Structures and Rheological Properties for Fumed Silica Suspensions in Polystyrene Solutions Masami Kawaguchi,*,† Atsushi Mizutani,† Yushu Matsushita,‡ and Tadaya Kato† Department of Chemistry for Materials, Faculty of Engineering, Mie University, 1515 Kamihama, Tsu, Mie 514, Japan, and Neutron Scattering Laboratory, The Institute for Solid State Physics, The University of Tokyo, 106-1 Shirakata, Tokai Naka, Ibaraki 319-11, Japan Received October 6, 1995. In Final Form: April 19, 1996X Fumed silica suspensions in trans-decalin and trans-decalin solutions of polystyrene (PS) have been studied by a combination of rheology and small-angle neutron scattering (SANS) techniques as functions of the concentrations of silica and PS and the molecular weight of PS. When the fumed silica particles are suspended in the solvent, the storage modulus is higher and independent of frequency, indicating the formation of gel-like network structures through hydrogen-bonding of the surface silanol groups. The scattering experiments have demonstrated that the mass fractal dimension corresponding to the power law correlation was 2.0 ( 0.05. Rheological measurements at constant silica and PS contents reveal that the storage modulus drastically decreases with an increase in PS molecular weight due to partially breaking down the gel structures by an effective flocculation of the silica particles by absorption of larger PS chains, leading to an increase in the scattering intensity at lower wave vector ranges. At higher PS concentrations the storage modulus increases, suggesting that chain entanglements occur between the PS chains adsorbed on the silica surfaces and free PS chains.

Introduction The rheological behavior of the fumed silica suspensions in various media has been deeply investigated by many researchers,1-26 and it strongly depended on the structure of the aggregates formed in silica suspensions. In nonpolar †

Mie University. The University of Tokyo. X Abstract published in Advance ACS Abstracts, November 15, 1996. ‡

(1) Iler, R. K. The Chemistry of Silica; Wiley Interscience: Toronto, 1979. (2) Chahal, R. S.; Pierre, L. E. St. Macromolecules 1969, 2, 193. (3) Berrod, G.; Vidal, A.; Papirer, E.; Donnet, J. B. J. Appl. Polym. Sci. 1981, 26, 833. (4) Ziegelbaur, R. S.; Caruthers, J. M. J. Non-Newtonian Fluid Mech. 1985, 17, 45. (5) Kosinski, L. E.; Caruthers, J. M. J. Non-Newtonian Fluid Mech. 1985, 17, 69. (6) Kosinski, L. E.; Caruthers, J. M. J. Appl. Polym. Sci. 1986, 32, 3393. (7) Kosinski, L. E.; Caruthers, J. M. Rheol. Acta 1986, 25, 153. (8) Cohen-Addad, J. P.; Roby, C.; Sauviat, M. Polymer 1985, 26, 1231. (9) Viallat, A.; Cohen-Addad, J. P.; Pouchelon, A. Polymer 1986, 27, 843. (10) Khan, S. A.; Maruca, M. A.; Plitz, I. M. Polym. Eng. Sci. 1991, 31, 1701. (11) Aranguren, M. I.; Mora, E.; DeGroot, J. V., Jr.; Macosko, C. W. J. Rheol. 1992, 36, 1165. (12) Khan, S. A.; Zoeller, N. J. J. Rheol. 1993, 37, 1225. (13) Krieger, I. M. Adv. Colloid Interface Sci. 1972, 3, 111. (14) Rubio, J.; Kitchener, J. A. J. Colloid Interface Sci. 1976, 57, 132. (15) Tadros, Th. F. Adv. Colloid Interface Sci. 1980, 12, 141. (16) Tadros, Th. F. In Solid/Liquid Dispersions; Tadros, Th. F., Ed.; Academic Press: London, 1987; pp 1-16, 225-274. (17) Otsubo, Y. Langmuir 1990, 6, 114. (18) Otsubo, Y. J. Colloid Interface Sci. 1992, 153, 584. (19) Otsubo, Y. Adv. Colloid Interface Sci. 1994, 53, 1. (20) Kawaguchi, M.; Ryo, T.; Hada, T. Langmuir 1991, 7, 1340. (21) Ryo, Y.; Nakai, Y.; Kawaguchi, M. Langmuir 1992, 8, 2413. (22) Kawaguchi, M.; Ryo, T. Chem. Eng. Sci. 1993, 49, 393. (23) Nakai, Y.; Ryo, T.; Kawaguchi, M. J. Chem. Soc., Faraday Trans. 1993, 89, 2467.

S0743-7463(95)00839-0 CCC: $12.00

media with no hydrogen-bonding ability, the fumed silica particles form network structures through hydrogenbonding of the surface silanol groups and they behave as gel-like materials. When the fumed silica particles are suspended in polar media with hydrogen-bonding ability, such media interfere with the formation of the fumed silica network by attaching themselves to the active silanol sites on the silica surface, indicating the silica suspensions to be essentially sol-like materials. For the media, in which the fumed silica particles are suspended, some high viscous oils have been used, and more recently, much attention has been devoted to the rheological properties of the silica suspensions in polydimethylsiloxanes.2-12 In these studies it was recognized that the resulting rheological responses can be qualitatively explained by the absorption interaction between the polymer and the silanol groups. Furthermore, the rheology of the fumed silica suspensions in aqueous polymer solutions has been extensively studied by several research groups,13-25 and it was found that the amount of polymer adsorbed on the silica surface caused changes in the rheological responses, indicating that polymer adsorption maintains their mechanical strength. In water the silica suspensions showed weaker elastic responses, since watr plays a role in the polar medium for the fumed silica and there is not a strong network structure in the silica suspensions. On the other hand, there were few studies on the rheology of the silica suspensions in nonaqueous polymer solutions. De Silva, Luckham, and Tadros have investigated the rheology of the silica suspensions in aromatic (24) Kawaguchi, M.; Kimura, Y.; Tanahashi, T.; Takeoka, J.; Suzuki, J.; Kato, T.; Funahashi, S. Langmuir 1995, 11, 563. (25) Kawaguchi, M. Adv. Colloid Interface Sci. 1994, 53, 103. (26) De Silva, G. P. H. L.; Luckham, P. F.; Tadros, Th. F. Colloids Surf. 1990, 50, 263.

© 1996 American Chemical Society

6180 Langmuir, Vol. 12, No. 26, 1996

hydrocarbon solutions of poly(2-vinylpyridine)/poly(tertbutylstyrene) (P2VP/PBS) diblock copolymers as a function of the molecular weight of the block copolymers.26 In the study, the lowest molecular weight block polymer gave stable silica dispersions characterized by Newtonian flow and the pseudoplastic flows were observed for the silica suspensions in the presence of the other two high molecular weight block polymers. Moreover, the silica suspensions in the solvent showed the highly elastic nature of gel-like materials, and the storage modulus was larger than the loss one at the lowest silica volume fraction of 0.004. The aim of this work is to understand the effects of polymer adsorption on the rheological behavior of the fumed silica suspensions in a nonpolar medium. Rheological measurements of the fumed silica suspensions in trans-decalin and trans-decalin solutions of monodisperse polystyrene (PS) have been performed as functions of the concentrations of silica and PS as well as the molecular weight of PS. We also have investigated the structures of the silica suspensions by small-angle neutron scattering (SANS) measurements. Furthermore, the effects of the dispersion media on the rheological properties of the silica suspensions will be discussed in comparison with the previous rheological and SANS experimental results24 for the fumed silica suspensions in aqueous polymer solutions. Experimental Section Materials. Five monodisperse polystyrenes, having Mw ) 9.68 × 103 (PS-10), 96.4 × 103 (PS-100), 355 × 103 (PS-355), 706 × 103 (PS-706), and 8420 × 103 (PS-8420), were purchased from Tosoh Co. The polydispersities (Mw/Mn) of PS-10, PS-100, PS350, PS-706, and PS-8420 were determined to be 1.02, 1.01, 1.02, 1.05, and 1.17, respectively, using a Toyo Soda HLC-802A gel permeation chromatography (GPC) instrument with a UV-8 Model II detector. The wavelength was 254 nm, and the eluent used was tetrahydrofuran (THF). Spectra grade quality trans-decalin was used as a solvent for PS and a dispersion medium without further purification. THF used for an eluent of the GPC measurements was guaranteed reagent grade and used without further purification. A fumed silica, Aerosil silica 130, was kindly supplied by Japan Aerosil Co. (Yokkaichi, Japan). From the manufacturer of the Aerosil 130 silica, the average particle diameter is 16 nm, the surface area is 130 m2/g, and there are 2.5 silanol groups per 1 nm2. The purification of the Aerosil 130 silica was the same as described previously.20-24 For preparation of the fumed silica suspensions in trans-decalin or in trans-decalin solutions of PS, a weighed amount of Aerosil 130 silica was mixed with the solvent or a preprepared transdecalin PS solution with a known concentration in a glass bottle. The resulting suspensions were subjected to mechanical shaking to obtain a homogeneous mixture in a Yamato BT-23 water incubator attached a shaker for 1 week. The temperature of the water in the incubator was controlled to 27 ( 0.1 °C. The silica contents were 5.7 (2.2), 8.6 (3.3), and 11.5 (4.4) wt (vol) %, and in particular for the 8.6 wt % silica suspension the concentrations of PS examined were 0.57, 1.72, and 2.87 wt %, whereas for the other two silica suspensions the PS concentration was fixed at 1.72 wt %. Adsorption of PS. The amounts of PS adsorbed on the silicas were determined as follows. The silica suspensions prepared by the same procedure as described above were subjected to separation of the silica using a Kubota 6700 centrifuge in order to remove the supernatant. The PS concentration in the supernatant was determined with a UV spectrometer. SANS Measurements. SANS experiments were performed using the University of Tokyo SANS-U instrument.27 The wavelength (λ) was selected to be 0.70 nm using a velocity selector with variable speeds and pitches, and the wavelength resolution was ∆λ/λ ) 10%. The monochromatic beam was collimated by a series of circular apertures having a 20 mm diameter. The (27) Kawaguchi, M.; Naka, R.; Imai, M.; Kato, T. Langmuir 1995, 11, 4323.

Kawaguchi et al.

Figure 1. Double logarithmic plot of the scattering intensity (I(q)) against wave vector (q) for the 8.6 wt % silica suspension in trans-decalin. samples were transferred to quartz cells of path length 2 mm after agitation of the silica suspensions with a touch mixer. The sample to detector distances of 4 and 12 m correspond to the wave vector range from 0.005 to 0.677 nm-1. The scattering intensities, which were recorded on a two-dimensional detector, were rotationally averaged about the transmitted beam and background corrected with an empty cell curve. The resulting scattering intensity was also obtained by normalization to the isotopic scattering intensity of protonated water. The SANS experiments were performed at 27 ( 0.1 °C. Rheological Measurements. The dynamic modulus was measured using an MR-300 Soliquid meter produced by Rheology Co. Ltd. (Kyoto, Japan).24,27 The dynamic modulus measurements were performed in the frequency range from 0.025 to 5.03 rad s-1 using a cone and plate geometry (plate diameter, 32 mm; cone angle, 5°) at 27 ( 0.5 °C. The cone and plate rheometer was filled with each silica suspension after the gel structure in the suspension was broken with a touch mixer. In general, it is well-known that suspensions show nonlinear viscoelastic responses to large strains. The measurements were carried out at a strain of 1.0% in the linear range. The measurement system was modified with an aluminum cover to limit solvent evaporation.

Results and Discussion Adsorption of PS. For the 8.6 wt % silica suspensions in all 0.57 wt % PS solutions and in the 1.72 wt % PS-100, PS-355, PS-706, and PS-8420 solutions as well as for 11.5 wt % silica suspensions in the 1.72 wt % PS-100, PS-355, PS-706, and PS-8420 solutions, all added PS chains were adsorbed on the silica surfaces. For the 8.6 wt % silica suspensions in the 1.72 wt % PS-10 solution, the amount of PS adsorbed on the silica surfaces was determined to be 0.808 mg/m2, whereas the adsorbed amounts of PS-10, PS-100, PS-355, and PS-706 for the 8.6 wt % silica suspensions in 2.87 wt % PS solutions as well as for the 5.7 wt % silica suspensions in 1.72 wt % PS solutions were 0.808, 2.10, 2.45, and 2.45 mg/m2, respectively. Furthermore, the adsorbed amounts of the respective molecular weights of PS were found to be consistent with the plateau value in the corresponding adsorption isotherms.28 SANS Measurements. SANS is one of most convenient techniques for investigating the microstructures of condensed matter. The shape of the scattering curve, a plot of scattering intensity (I(q)) as a function of wave vector (q), gives a clue to information on the microstructures of the objects. Figure 1 shows typical scattering curves of the 8.6 wt % silica suspensions in decalin, and log(I(q)) scales linearly with log(q) with a slope of ≈ -2.0 below q ) 0.145 nm-1, above which a plot of log(I(q)) against log(q) yields a straight line with a slope of ≈ -3.2. The former slope corresponds to the mass fractal dimension (Dm) for the aggregated objects, while the latter slope gives the surface fractal dimension (Ds) according to the (28) Mizutani, A.; Kawaguchi, M.; Kato, T. Unpublished data.

Fumed Silica Suspensions in Polystyrene Solutions

Langmuir, Vol. 12, No. 26, 1996 6181

Figure 3. Frequency dependence of the storage modulus (G′) for the silica suspensions in trans-decalin with various silica concentrations (Cs): (0) Cs ) 5.7 wt %; (4) Cs ) 8.6 wt %; (O) Cs ) 11.5 wt %.

Figure 2. Double logarithmic plots of the scattering intensity (I(q)) against wave vector (q) for the 8.6 wt % silica suspensions in 1.72 wt % PS solutions of PS-10 (a), PS-100 (b), and PS-706 (c).

relationship I(q) ∝ q-(6-Ds).29,30 Thus, at higher q regimes Porod’s law (Ds ) 2) did not hold, indicating that the surfaces of the primary particles are not smooth and fractally rough. However, for the fumed silica suspensions in water the SANS measurements gave different fractal dimensions:24 the mass fractal dimension was the same, and the surface fractal dimension was consistent with Porod’s law corresponding to a two-dimensional surface. The difference may be attributed to the polarity of the dispersion medium, and in water the fumed silica particles are partially flocculated to form aggregated clusters. Similar results were obtained for the mass fractal dimension analysis of the aggregation of the silica suspensions of various types of silicas in nonaqueous and aqueous solutions using different scattering methods, such as light,12,31-34 X-ray,31 and neutron,24 as functions of particle size and particle concentration. On the other hand, the mass fractal dimension for the fumed silica suspensions in mineral oil was obtained to be 1.75 using light measurements by Khan and Zoeller,12 and they showed that the mass fractal dimension was independent of the surface modification of the silica particles. Figure 2 summarizes typical scattering curves of the 8.6 wt % silica suspensions in 1.72 wt % PS trans-decalin solutions for various molecular weights of PS. The shape of plots of log(I(q)) as a function of log(q) is independent of molecular weight of PS with a distinct crossover between a first power-law region with an exponent of -2.0 and a second one with a higher exponent of -3.2. The respective exponents were similar to those of the silica suspensions (29) Freltoft, T.; Kjems, J. K.; Sinha, S. K. Phys. Rev. 1986, B33, 269. (30) Hurd, A. J.; Schaefer, D. W.; Martin, J. E. Phys. Rev. 1987, A35, 2361. (31) Schaefer, D. W.; Martin, J. E.; Wiltzius, P.; Cannell, D. S. Phys. Rev. Lett. 1984, 52, 2371. (32) Aubert, C.; Cannell, D. S. Phys. Rev. Lett. 1986, 56, 738. (33) Rouw, P. W.; de Kruif, C. G. Phys. Rev. 1989, A39, 5399. (34) Chen, M.; Russel, W. B. J. Colloid Interface Sci. 1991, 141, 564.

in trans-decalin mentioned above. The q value at the crossover point is 0.145 nm-1, independent of the dispersion media. From Figure 2, we noticed a stronger scattering intensity of the silica suspensions in PS solutions than that in the solvent for the lower q regimes beyond the crossover point. The difference in the scattering intensity between the PS solution and the solvent increases with an increase in molecular weight of PS (not shown here). This means that the adsorption interaction between PS and the silanol groups causes partial flocculation of the silica particles and formation of some flocs and that the larger PS chains more effectively and partially flocculate the silica particles. This fact will be discussed more by taking account of the rheological results described below. On the other hand, the fumed silica suspensions in aqueous polymer solutions showed the same scattering curves as in water,24 suggesting that there are no changes in the microstructure of the fumed silica suspensions in water by polymer adsorption. Rheological Measurements. Before discussing the effects of the concentrations of PS and silica on the rheology of the silica suspensions, we should address the rheological properties of the PS solutions and the silica suspensions in trans-decalin. The rheological responses of the PS solutions used for the dispersion media are much weaker and out of the reliability of our instruments, whereas the viscoelastic responses of the silica suspensions are strong due to the formation of the network structures through hydrogen-bonding of the surface silanol groups. In Figure 3, the storage modulus (G′) of the 5.7, 8.6, and 11.5 wt % silica suspensions in the solvent is shown as a function of frequency. The G′ value for the 8.6 and 11.5 wt % silica suspensions is independent of the frequency, and the 5.7 wt % silica suspension shows only a slight frequency dependency of G′. Such a frequency dependence of G′ looks very much similar to that of a cross-linked network, such as gel materials. Furthermore, the G′ value increases with an increase in the silica content, indicating the formation of stronger network structures. On the other hand, the loss modulus (G′′) is two orders of magnitude lower than the G′ value for the respective silica suspensions (not shown here). It was found that, even at low solid contents, three-dimensional gel structure is present to maintain the elastic responses through hydrogenbonding. Similar results were observed for the fumed silica suspensions in mineral oil10,12 and an aromatic hydrocarbon.26 The 2.0 wt % silica suspension in mineral oil and the 0.4 vol % silica suspension in an aromatic hydrocarbon showed no frequency dependence of G′. Figures 4-7 show the G′ curves for the silica suspensions in PS-10, PS-100, PS-355, and PS-706 solutions. In the presence of the lowest molecular weight PS, the G′ value is independent of frequency, whereas the G′ value shows

6182 Langmuir, Vol. 12, No. 26, 1996

Figure 4. Frequency dependence of the storage modulus (G′) for the silica suspensions in 1.72 wt % PS-10 solution with various silica concentrations (Cs): (0) Cs ) 5.7 wt %; (4) Cs ) 8.6 wt %; (O) Cs ) 11.5 wt %. The arrows from bottom to top displayed in the figure correspond to the plateau G′ values of the 5.7, 8.6, and 11.5 wt % silica suspensions in trans-decalin, respectively.

Figure 5. Frequency dependence of the storage modulus (G′) for the silica suspensions in 1.72 wt % PS-100 solution with various silica concentrations (Cs): (0) Cs ) 5.7 wt %; (4) Cs ) 8.6 wt %; (O) Cs ) 11.5 wt %. The arrows from bottom to top displayed in the figure correspond to the plateau G′ values of the 5.7, 8.6, and 11.5 wt % silica suspensions in trans-decalin, respectively.

Figure 6. Frequency dependence of the storage modulus (G′) for the silica suspensions in PS-355 solutions with various concentrations of silica (Cs) and PS-355 (Co): (0) Cs ) 5.7 wt % and Co ) 1.72 wt %; (4) Cs ) 8.6 wt % and Co ) 1.72 wt %; (O) Cs ) 11.5 wt % and Co ) 1.72 wt %; (2) Cs ) 8.6 wt % and Co ) 0.57 wt %; (b) Cs ) 8.6 wt % and Co ) 2.87 wt %.

a slight frequency dependency for the silica suspensions in the PS solutions of higher molecular weight than 100 × 103. A comparison of the G′ value in the presence of PS and that in the absence of PS should lead to an understanding of the effects of polymer adsorption on the rheology of the silica suspensions. We notice several unique features: (1) in the presence of PS-100 the respective G′ curves overlay those for the silica suspension in the solvent; (2) the G′ curve for PS-10 is a poorer fit with that in the solvent than that for PS-100; (3) for the silica suspensions in PS-355 and PS-706 solutions, the respective G′ values are one order of magnitude less than that in the solvent

Kawaguchi et al.

Figure 7. Frequency dependence of the storage modulus (G′) for the silica suspensions in PS-706 solutions with various concentrations of silica (Cs) and PS-706 (Co): (4) Cs ) 8.6 wt % and Co ) 1.72 wt %; (O) Cs ) 11.5 wt % and Co ) 1.72 wt %; (b) Cs ) 8.6 wt % and Co ) 2.87 wt %.

and the degree of depression in the G′ value from that in the solvent becomes larger with higher silica contents; (4) the 8.6 wt % silica suspensions in the 0.57 wt % PS-706 or 1.72 wt % PS-8420 solutions are not indicatve of viscoelastic responses for our instrument, since precipitation of the silica particles was observed for both silica suspensions. By taking account of the rheological properties mentioned above, it is interesting to consider the conformation of PS chains adsorbed on the silica particles. The size of an isolated PS chain with molecular weights less than 100 × 103 is smaller than the diameter of the individual silica particles, and such PS chains have smaller probability for the formation of polymer bridging between the silica particles. On the other hand, above the molecular weight of 355 × 103 their sizes are comparable or larger than that of the silica particle. The PS chains probably take a loop-train-tail conformation on the silica surface,35,36 and it will be expected that some portions of the loops and tails of the adsorbed PS chains behave as cross-linkers among the silica particles, leading to so-called polymerbridging.37 Such a polymer-bridging mechanism plays a role in the enhancement of the flocculation of silica suspensions, leading to changes in their rheological responses as described above. Therefore, we propose a model for PS adsorption onto the silica particles to interpret the SANS results and rheological properties of the silica suspensions as shown in Figure 8. For PS of lower mass than that of PS-100, a hydrogen-bonding gel structure in the silica suspension is almost not disrupted by PS adsorption, whereas PS chains of higher mass than that of PS-355 effectively play a role in polymer-bridging to partially break down the gel network, leading to the flocculation of the silica particles. The 11.5 wt % silica suspension in 1.72 wt % PS-10 solution resulted in the larger G′ value than that in the same concentration of PS-100. It is not easy to give a satisfactory interpretation of the difference at the present time. However, the free PS-100 concentration in the dispersion medium is nearly zero, while that of PS-10 is ca. 0.48 wt %, and thus the viscosity in the dispersion medium of the suspension could strongly influence the viscoelastic responses. This interpretation is confirmed by additional dynamic modulus measurements of the 11.5 wt % silica suspensions in 0.57 and 2.30 wt % PS-10 solutions: the G′ value surely increases with an increase (35) Kawaguchi, M.; Takahashi, A. Adv. Colloid Interface Sci. 1992, 37, 219. (36) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993. (37) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983.

Fumed Silica Suspensions in Polystyrene Solutions

Langmuir, Vol. 12, No. 26, 1996 6183

Figure 9. Double logarithmic plots of the plateau storage modulus (G′) for the silica suspensions in trans-decalin (O), PS-10 (0), PS-100 (4), PS-355 (b), and PS-706 (2) against the volume fraction of silica (φ). The PS concentration is fixed as 1.72 wt %. The solid line has a slope of 4.3.

Figure 8. Schematic model for adsorption of low-mass PS (b) and high-mass PS (c) on gel-like network structured silica particles (a). Adsorbed polymer is represented by heavy lines, and free polymer, by light lines.

in PS concentration, and a 11.5 wt % silica suspension in the 0.57 wt % PS-10 solution, where no free PS-10 is present in the dispersion medium, gives the same G curve as that without PS. On the other hand, Figures 6 and 7 show a larger G′ value for the 8.6 wt % silica suspensions with an increase in PS concentration. This can be interpreted as chain entanglements between adsorbed PS chains and free PS chains, since 0.11 wt % PS remained in the dispersion medium for the added PS concentration of 2.8 wt %, whereas for other lower PS concentrations all PS chains were adsorbed on the silica surfaces. It is important to discuss the dependence of G′ on the volume fraction (φ) of the silica particles to understand what mechanism controls the formation of the gel network structure. We find the silica suspensions in the solvent and in the 1.72 wt % solutions of PS-10 and PS-100 to show a power law behavior, i.e. G′ ∝ φn, as shown in Figure 9. The silica suspensions in the solvent and PS-100 solutions have approximately the same exponent of 4.3 ( 0.2, whereas the silica suspensions in the PS-10 solution have an exponent of 5.6 ( 0.3. The difference in the

exponent n could be related to the amount of free PS chains in the dispersion medium, whereas the G′ values at the frequency of 0.1 rad s-1 for the silica suspensions in the 1.72 wt % solutions of PS-355 and PS-706 do not increase with φ according to a power law. The G′ values of the silica suspensions in PS-355 solutions level off at the higher silica concentration, and the G′ value in the presence of PS-706 decreases with an increase in silica content. Thus, it can be concluded that the lower the ratio of the concentrations of PS to silica, the smaller the storage modulus, due to more effective flocculation of the silica particles by polymer-bridging of adsorbed larger PS chains. Conclusions The fumed silica particles suspended in a nonpolar solvent showed the higher storage modulus independent of frequency, and the modulus scaled a power law slope of 4.3 with increasing silica volume fraction. Addition of the smaller molecular weight PS than 100 × 103 to the silica suspension in the solvent caused little changes in the storage modulus when the free PS concentration in the dispersion medium was low. At the constant silica contents the silica suspensions in the larger mass PS solutions had lower storage moduli than those in the lower mass PS solutions. SANS experiments were used to probe the effect of polymer adsorption on the microstructure of these silica suspensions. The plots of the scattering intensity against wave vector for the lower wave vector ranges showed a power law behavior with the exponent of -2. The scattering intensity at lower wave vector ranges increased with an increase in molecular weight of PS, suggesting that the large scattering objects are formed by flocculation. LA9508396