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Langmuir 1997, 13, 4770-4771
Differences in Shear Viscosities between Structured and Destructured Silica Suspensions in the Presence of Adsorbed Poly(N,N-dimethylacrylamide) Masami Kawaguchi,* Takashi Yamauchi, Akihiro Ohkubo, and Tadaya Kato Department of Chemistry for Materials, Faculty of Engineering, Mie University, 1515 Kamihama, Tsu, Mie 514, Japan Received March 21, 1997. In Final Form: June 9, 1997
Introduction When fumed silica particles were suspended in polymer solutions, their rheological characteristics were very useful for understanding the stability of the resulting silica suspensions in terms of polymer adsorption behavior and the structure of the formed aggregates.1-8 In most silica suspensions, since the amount of polymer adsorbed on the silica surfaces was at the plateau of the adsorption isotherms for the corresponding polymer, the resulting silica suspensions were stable. Furthermore, the silica contents were not beyond 15 wt % because of the difficult preparation of the homogeneous silica suspensions at the high silica contents. However, it was found that it is possible to prepare the homogeneous fumed silica suspensions with relatively higher silica contents in water by a combination of mechanical mixing and shaking. Moreover, since some small aggregates are usually present in the fumed silicas even in air, after destructuring such aggregates, we can easily prepare relatively highly concentrated silica suspensions. When the fumed silicas are mixed with a solution of polymers, which can be adsorbed on the silica surface, to make a silica suspension, the adsorbed amount of polymer seems to influence the stability of the silica suspension. In the absence of free polymer chains in the dispersion media, the effects of polymer adsorption on the stability of the silica suspension can be understood without taking into account the interactions between the free and adsorbed polymer chains as well as between the free polymers. In this study, to verify the stability of the fumed silica suspensions with the relative high silica content in aqueous media, the effects of amount of poly(dimethylacrylamide) (poly(DMAM)) adsorbed at the silica surfaces on rheological characteristics have been investigated using structured and destructured silica particles. Their rheological behavior will be compared with those of some fumed silica suspensions with low silica contents reported before.5,6,8 Experimental Section Materials. A poly(DMAM) sample was prepared by radical polymerization of freshly distilled N,N-dimethylacrylamide in a benzene solution using azobisisobutylronitrile as an initiator at 55 °C. The resulting polymer was diluted with an acetone/ benzene mixture, purified by precipitation of the solution in a large amount of benzene/n-hexane mixture, and dried under vacuum. The molecular weight (Mw) of the poly(DMAM) sample (1) Eisenlauer, J.; Killmann, E. J. J. Colloid Interface Sci. 1980, 74, 108. (2) Eisenlauer, J.; Killmann, E.; Koren, M. J. J. Colloid Interface Sci. 1980, 74, 120. (3) De Silva, D. P. H. L.; Luckham, P. F.; Tadros, Th. F. Colloids Surf. 1990, 50, 263. (4) Otubo, Y. Adv. Colloid Interface Sci. 1994, 53, 1. (5) Kawaguchi, M. Adv. Colloid Interface Sci. 1994, 53, 103. (6) Kawaguchi, M.; Kimura. Y.; Tanahashi, T.; Takeoka, J.; Suzuki, J.; Kato, T.; Funahashi, S. Langmuir 1995, 11, 563. (7) Kawaguchi, M.; Mizutani, A.; Matsushita, Y.; Kato, T. Langmuir 1996, 12, 6179. (8) Kawaguchi, M.; Yamamoto, T.; Kato, T. Langmuir 1996, 12, 6184.
S0743-7463(97)00307-7 CCC: $14.00
was determined to be 2.18 × 106 from the intrinsic viscosity measurement in methanol at 25.0 °C.9 Water was purified by a Millipore Q-TM system. Pure grade quality benzene, acetone, and n-hexane were used after distillation. Aerosil 200 silica with a surface area (As) ) 200 ( 20 m2/g, a diameter (2d) ) 12 nm, and a silanol density of 2.5/nm2 was used, and it was kindly supplied by Nippon Aerosil, Ltd (Yokkaichi, Japan). Aerosil 200 silica was treated at a constant speed of 65 rpm for 30 min in a ball mill, in which 250 g of the Aerosil 200 silicas was destructured with 5 kg of alumina ball with the diameter of 15 mm in a chamber of 7.0 L. This led to a higher apparent density of 125 g/L, whereas the apparent density of the untreated Aerosil 200 is ca. 50 g/L, according to the manufacturer. The original and treated silicas were called Aerosil 200-1 and 200-2, respectively, and they were dried in a vacuum oven at 130 °C. For preparation of the silica suspensions in aqueous poly(DMAM) solutions, a weighed amount of silica was mixed in a glass bottle with an aqueous solution of poly(DMAM) with a known concentration. The resulting suspensions were made homogeneous by mechanical shaking in an air incubator for 1 week at 27.0 ( 0.2 °C, and their pH values were around 4.0. The silica content was fixed at 20.0 wt %, and the concentrations of poly(DMAM) were varied from 0.5 to 2.0 g/100 mL. Adsorption of Poly(DMAM). The amounts of poly(DMAM) adsorbed on the silica were determined as follows. A mixture of silica and aqueous poly(DMAM) solution was prepared as described above, the silica was separated using a Kubota 6700 centrifuge, and the supernatant was removed. The poly(DMAM) concentration (Cp) in the supernatant was determined by an Ohtsuka Denshi DRM-1021 refractometer. To confirm the reproducibility of the experiments, we performed at least two measurements under the same conditions. The errors in the adsorbed amount were less than 10%. Rheological Measurements. Steady-state shear viscosities of the silica suspensions were measured using an ARES viscoelastic measurement system produced by Rheometrics Scientific Inc. (NJ). The measurements were carried out by changing from 103 to 0.03 s-1 using a cone and plate geometry (plate diameter, 50.0 mm; cone angle, 0.04 rad) at 27.0 ( 0.2 °C.
Results and Discussion Adsorption of Poly(DMAM). Since poly(DMAM) was not present in the supernatant solution for the respective silica suspensions, all poly(DMAM) chains should be adsorbed on the silica surfaces. The adsorbed amounts of poly(DMAM) were determined to be 0.025, 0.05, and 0.10 g/g for the dosing poly(DMAM) concentrations of 0.5, 1.0, and 2.0 g/100 mL, respectively. Thus, there is no free poly(DMAM) in the dispersion media. From the separate experiments, the plateau amount of poly(DMAM) adsorbed on the silica surfaces was determined to be ca. 0.17 g/g. Viscosities of Silica Suspensions without Poly(DMAM). Steady-state shear stresses of the silica suspensions of the Aerosil 200-1 and 200-2 silicas without poly(DMAM) are shown as a function of shear rate in Figures 1 and 2, respectively. The Aerosil 200-1 silica suspension shows pseudoplastic flow, indicating the presence of internal aggregated structure in the silica suspension. Similar rheological results were obtained for the Aerosil 130 silica suspensions.6 Furthermore, the small angle neutron scattering measurements showed that the Aerosil 130 silica particles suspended in water showed a fractal structure with the dimension of 2; namely, they formed an aggregated structure.6 Probably, Aerosil 200-1 particles can be expected to have a fractal structure in their aggregated silica suspensions. Such aggregates are partially broken down on application of shear, and then their viscosities decrease with an increase in shear rate, leading to the pseudoplastic flow behavior. The pseudo(9) Trossarelli, L.; Meirore, M. J. Polym. Sci. 1962, 54, 445.
© 1997 American Chemical Society
Notes
Figure 1. Steady-state shear stresses of the Aerosil 200-1 suspensions in the presence of poly(DMAM) as a function of the shear rate for various polymer concentrations (C0): b, 0.0 g/100 mL; O, 0.5 g/100 mL; 0, 1.0 g/100 mL; 4, 2.0 g/100 mL.
Figure 2. Steady-state shear stresses of the Aerosil 200-2 suspensions in the presence of poly(DMAM) as a function of the shear rate for various polymer concentrations. Symbols are the same as in Figure 1.
plastic flow is almost reversible, and it takes within 10 s to reach a steady state at the respective shear rates. The Aerosil 200-2 silica suspension, on the other hand, shows nearly Newtonian flow; that is, the steady-state shear stress almost linearly increases with the shear rate, indicating that the silica particles are much better dispersed than Aerosil 200-1. The resulting viscosity of ca. 0.01 Pa‚s is one order of the magnitude larger than that calculated from the Einstein equation for a dilute dispersion of rigid spheres,10 and it is also much larger than that of the colloidal silica particles,8 indicating that aggregation partially occurs in the Aerosil 200-2 silicas. The presence of the aggregated structure can be verified by an increase in turbidity when the Aerosil 200-2 is dispersed into water. However, the turbidity intensity was much lower than that for the suspension of Aerosil 200-1. Viscosities of Silica Suspensions with Poly(DMAM). The rheological behavior for the suspensions in the presence of polymer may be explained by taking into account changes in both the aggregated structures of the silica particle and the viscosities of the suspended media. The viscosity of the suspended media is almost equal to that of water because of the absence of the unadsorbed poly(DMAM). Thus, viscosity should be mainly related to changes in the aggregated structure of silica particles caused by applying shear. Since the adsorbed amount of poly(DMAM) is lower than the plateau adsorbed amount of poly(DMAM) and the molecular diameter of 61 nm for a poly(DMAM) chain estimated from the Flory-Fox relation in water is much larger than the size of the individual silica particle, poly(DMAM) chains should prefer to adsorb simultaneously on several particles, and the polymer bridging11 between the silica particles occurs at lower dosing poly(DMAM) (10) Einstein, A. Ann. Phys. 1905, 17, 459; 1906, 19, 271; 1921, 34, 591.
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concentrations. Adsorption of poly(DMAM) on the silica surfaces leads to changes in the shape of the flow curve of the Aerosil 200-1 suspensions, as shown in Figure 1: a clear bump in the flow curve appeared at the shear rate of ca. 20 s-1, and the shear stress at the bump decreases with the order of the amount of poly(DMAM) adsorbed at the silica surfaces. Therefore, the adsorbed poly(DMAM) chains play a role in reinforcements of the aggregated structures, and at the highest dosing polymer concentration the aggregated structure was partially broken up by the adsorbed polymer chains. Furthermore, the bump was observed at the same shear rate, irrespective of the adsorbed amount and it should be related to the kinetics of formation and breaking of the silica aggregates under shear: at a shear rate 20 s-1, breaking of the clusters generated occurred. Similar results were observed for some silica suspensions with 7.5 wt % silica contents,5,6 where the bump was observed around the shear rate of 0.1 s-1 and it appeared to fade out with an increase in adsorbed amount of polymer. Since the Aerosil 200-2 particles are better dispersed in water than the Aerosil 200-1 ones, it can be expected that adsorption of poly(DMAM) chains should more effectively influence the rheology properties of the Aerosil 200-2 silica suspensions than the Aerosil 200-1 ones by flocculation of the silica particles. In the presence of poly(DMAM), the Aerosil 200-2 silica suspensions indicate pseudoplastic flow, and the shear thinning exponent calculated from the slope of a double logarithmic plot of shear stress and shear rate12 increases with an increase in the amount of poly(DMAM) adsorbed on the silica surfaces, as shown in Figure 2. Moreover, the silica suspension in the 1.0 g/100 mL poly(DMAM) solution has a yield stress of ca. 2 Pa for the wide range of shear rates. At the highest shear rate, the shear stresses for the silica suspensions in the presence of poly(DMAM) are larger than that without poly(DMAM). This indicates that some parts of the flocculated structure formed by adsorption of poly(DMAM) remain without a complete breaking up. The magnitude of the shear stress at the highest shear rate increases with a decrease in the amount of poly(DMAM) adsorbed on the silica surfaces, and it is nearly equal to that of the Aerosil 200-1 silica suspension without poly(DMAM) under the same conditions. Conclusions Differences in the shear flow behavior of silica suspensions between the structured and destructured silicas could be much more clearly understood without taking into account effects of the free polymer in the dispersion media. For the suspensions of structured silicas, the presence of the internal aggregated structures could be clearly confirmed by polymer adsorption, whereas for the suspensions of destructured silicas, the smaller amount of polymer adsorbed on the silica surfaces leads to more mechanical strongly flocculated suspensions. The resulting flocculated structure showed a similar shear stress to that for the suspensions of the structured silicas at the highest shear rate, in which most structures in the silica suspensions should be broken up. Therefore, shear flow measurements gave some evidence for changes in the structures of the suspensions induced by polymer adsorption. LA970307K (11) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: New York, 1988. (12) Bird, R. B.; Armstrong, R. C.; Hassager, O. Dynamics of Polymeric Liquids; Wiley: New York, 1987; Vol. 1.
