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Langmuir 1996, 12, 6184-6187
Rheological Studies of Hydrophilic and Hydrophobic Silica Suspensions in the Presence of Adsorbed Poly(N-isopropylacrylamide) Masami Kawaguchi,* Takashi Yamamoto, and Tadaya Kato Department of Chemistry for Materials, Faculty of Engineering, Mie University, 1515 Kamihama, Tsu, Mie 514, Japan Received February 19, 1996. In Final Form: September 30, 1996X The shear flow behavior of silica suspensions for five different silicas in aqueous poly(N-isopropylacrylamide) was investigated as a function of the volume fraction of the silica particles. The amount of polymer adsorbed on the silica surface was almost constant, irrespective of the silica surface nature, except for the colloidal silica, on which less polymer was adsorbed. The suspensions of the hydrophilic silicas showed shear thinning and pseudoplastic flow. The suspensions of the hydrophobic silica particles showed Newtonian flow at low silica contents, with viscosities comparable to those of the colloidal silica suspensions in the presence of the polymer and those of the suspensions of hydrophilic silica with the same size. With an increase in the silica content the hydrophobic silica suspensions changed from shear thinning to pseudoplastic flow.
Introduction For technologies as diverse as water treatment, recovery of minerals, soil stabilization, enhanced oil recovery, and transport of suspended coals, the proper control of watersoluble polymer adsorption is deeply required. Thus, the efforts to understand adsorption of water-soluble polymers have been mostly concerned with hydrophilic substrates such as various oxidized inorganic materials. By contrast, little attention has been paid to the interfacial behavior of water-soluble polymers at hydrophobic interfaces that are not wetted by water, for instance, water/low-energy solid surfaces. In a previous paper1 we reported that the amount of poly(N-isopropylacrylamide) (poly(NIPAM)) adsorbed on the silica surfaces from aqueous solution increased with temperature and above 19 °C more poly(NIPAM) adsorbed on the hydrophobic surface than on the hydrophilic one. The hydrophobic silica particles, separated and floated in water, were gradually dispersed and wetted by adsorption of poly(NIPAM) on the surfaces with an increase in adsorption time. Thus, we believe that the hydrophobic particles can be well suspended in the water phase by adsorption of poly(NIPAM). When inorganic particles are suspended in polymer solutions, their stabilization can be evaluated from measurements of the stability, dynamic properties, and rheological characteristics and interpreted in terms of polymer adsorption behavior.2-4 Rheology was very useful to understand the characteristics of suspensions, and several results have been reported for various inorganic particles in aqueous media.5-19 In this paper, to verify the dispersion properties of the hydrophobic silica particles in aqueous media, the effects X Abstract published in Advance ACS Abstracts, December 1, 1996.
(1) Tanahashi, T.; Kawaguchi, M.; Honda, T.; Takahashi, A. Macromolecules 1994, 27, 606. (2) Krieger, I. M. Adv. Colloid Interface Sci. 1972, 3, 111. (3) Tadros, Th. F. Adv. Colloid Interface Sci. 1980, 12, 141. (4) Tadros, Th. F. In Solid/Liquid Dispersions; Tadros, Th. F., Ed.; Academic Press, London, 1987; pp 1-16 and 225-274. (5) Iler, P. K. J. Colloid Interface Sci. 1975, 51, 388. (6) Rubio, J.; Kitchener, J. A. J. Colloid Interface Sci. 1976, 57, 132. (7) Eisenlauer, J.; Killmann, E. J. J. Colloid Interface Sci. 1980, 74, 108. (8) Eisenlauer, J.; Killmann, E.; Koren, M. J. J. Colloid Interface Sci. 1980, 74, 120. (9) Wang, T. K.; Audebert, R. J. Colloid Interface Sci. 1987, 119, 459.
S0743-7463(96)00147-3 CCC: $12.00
of addition of poly(NIPAM) on the rheological characteristics have been investigated and compared with the behavior of hydrophilic silica particles in the presence of poly(NIPAM). Experimental Section Materials. A poly(NIPAM) sample was prepared by radical polymerization of freshly recrystallized N-isopropylacrylamide in an acetone/benzene solution using azobis(isobutyronitrile) (AIBN) as an initiator at 54 °C. The resulting polymer was diluted with acetone, purified by dropwise precipitation of the solution in a large amount of benzene, and dried under vacuum. The poly(NIPAM) was fractionated in a mixture of acetone and benzene by lowering the temperature. The molecular weights (Mw’s) of the fractionated poly(NIPAM) samples were determined from the intrinsic viscosity measurement in tetrahydrofuran at 27 °C.20 The molecular weight distributions (Mw/Mn) were determined using a GPC system of two gel GMH columns, a Toyo Soda RI-8 refractometer, and a Nippon Denshi Science U-228 chart recorder. The fluent used was a 10 mM LiBr/DMF solution. In this study, we used one fractionated poly(NIPAM) sample with Mw ) 3.67 × 105 and Mw/Mn ) 1.70. Water was purified with a Millipore Q-TM system. Pure grade quality benzene and acetone were used after distillation. Five silicas were used: colloidal silica of Snowtex-20 with a particle diameter (2d) of 15 ( 3 nm; fumed hydrophilic silica of Aerosil 300 with a surface area (As) of 300 ( 30 m2/g, 2d ) 7 nm, and a silanol density of 2.5/nm2; fumed hydrophilic silica of Aerosil 130 with As ) 130 ( 25 m2/g, 2d ) 16 nm, and a silanol density of 2.5/nm2; nonporous hydrophobic silica of Aerosil R-972, which is Aerosil 130 modified by dimethyldichlorosilane, with As ) 110 ( 20 m2/g, 2d ) 16 nm, and a silanol density of 0.2-0.4/nm2; nonporous hydrophobic silica of Aerosil R-812, which is Aerosil (10) Wang, T. K.; Audebert, R. J. Colloid Interface Sci. 1988, 121, 32. (11) Vaslin-Reimann, S.; Lafuma, F.; Audebert, R. Colloid Polym. Sci. 1990, 268, 476. (12) De Silva, D. P. H. L.; Luckham, P. F.; Tadros, Th. F. Colloids Surf. 1990, 50, 263. (13) Lafuma, F.; Wong, K.; Cabane, B. J. Colloid Interface Sci. 1991, 143, 9. (14) Liu, S. F.; Lafuma, F.; Audebert, R. Colloid Polym. Sci. 1994, 272, 196. (15) Otubo, Y. Adv. Colloid Interface Sci. 1994, 53, 1. (16) Kawaguchi, M. Adv. Colloid Interface Sci. 1994, 53, 103. (17) Kawaguchi, M.; Kimura, Y.; Tanahashi, T.; Takeoka, J.; Suzuki, J.; Kato, T.; Funahashi, S. Langmuir 1995, 11, 563. (18) Kawaguchi, M.; Naka, R.; Imai, M.; Kato, T. Langmuir 1995, 11, 4323. (19) Tadros, Th. F.; Taylor, P.; Bongnolo, G. Langmuir 1995, 11, 4678. (20) Fujishige, S. Polym. J. 1987, 19, 297.
© 1996 American Chemical Society
Hydrophilic and Hydrophobic Silica Suspensions 300 modified by trimethylchlorosilane, with As ) 260 ( 30 m2/g, 2d ) 7 nm, and a silanol density of 0.4-0.5/nm2. The Snowtex20 was kindly supplied by Nissan Chemical Industries, Ltd. (Tokyo, Japan), and the remaining silicas were gifts from Nippon Aerosil, Ltd. (Yokkaichi, Japan). The characteristics of the respective silica particles were cited from the manufacturer. The Aerosil 130 and Aerosil 300 silicas were dried in a vacuum oven at 130 °C. The solid content in the Snowtex-20 was determined to be 21.7 wt % by evaporation of the dispersed medium (water) in weighed Snowtex-20 and by drying the residue under vacuum. For preparation of the silica suspensions in aqueous poly(NIPAM) solutions, a weighed amount of silica was mixed in a glass bottle with an aqueous solution of poly(NIPAM) with a known concentration. The resulting suspensions were made homogeneous by mechanical shaking in an air incubator for 2 weeks at 30.0 ( 0.2 °C. The silica contents were 2.3, 3.6, 4.9, and 7.2 (7.5) vol %, where the figure in parentheses corresponds to the silica content of the Snowtex-20, and the corresponding concentrations of poly(NIPAM) were fixed at 0.71, 1.1, 1.5, and 2.4 g/100 mL. Adsorption of poly(NIPAM). The amounts of poly(NIPAM) adsorbed on the silica were determined as follows. A mixture of silica and aqueous poly(NIPAM) solution was prepared as described above, the silica was separated using a Kubota 6700 centrifuge, and the supernatant was removed. The poly(NIPAM) 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 for 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 in the shear rate range from 1 to 103 s-1 using a cone and plate geometry (plate diameter, 50.0 mm; cone angle, 0.04 rad) at 30.0 ( 0.2 °C.
Results and Discussion Adsorption of Poly(NIPAM). For the respective silica surfaces, the adsorbed amount, A, of poly(NIPAM) is independent of the volume fraction of silica, φ0. For Aerosil 300, Aerosil 130, R-972, and R-812, A ) 0.13 ( 0.01 g/g, whereas for Snowtex A ) 0.05 ( 0.005 g/g for 0.02 e φ0 e 0.08. The number of adsorbed chains per particle can be roughly estimated to be 0.32 for Snowtex-20, 1.0 for Aerosil 130 and R-972, and 0.08 for Aerosil 300 and R-812, respectively. The resulting adsorbed amount of the polymer was smaller than that reported in a previous paper,1 where the added concentrations of silica and polymer were one order of magnitude lower than those in this study but the ratio of the silica to the polymer concentrations was the same as that in the present study. The difference in the adsorbed amount of the polymer could be mainly less effective surface areas of the silica particles in this study than those in the previous one, since aggregation of silica particles easily occurs at higher silica content. Viscosities of Silica Suspensions with No Added Poly(NIPAM). The steady-state shear viscosities of hydrophilic silica suspensions of the Snowtex-20, Aerosil 130, and Aerosil 300 silicas were investigated at various silica contents. For Snowtex-20, all the components display Newtonian flow; that is, the steady-state shear viscosity is independent of the shear rate. On the other hand, the suspensions of Aerosil 300 and Aerosil 130 show shear thinning, as shown in Figure 1, which shows typical plots of steady-state shear viscosity as a function of shear rate at various silica volume fractions. From the small-angle neutron scattering measurements,17 the Aerosil silica particles suspended in water show a fractal structure with the dimension of 2; namely, they form an open aggregated structure.21 Such aggregates are partially broken down on application of
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Figure 1. Steady-state shear viscosities, η, of the Aerosil 300 (open symbols) and Aerosil 130 (filled symbols) suspensions in the absence of poly(NIPAM) as a function of the shear rate, γ˘ , for various volume fractions of the silica content: squares, φ0 ) 2.3 vol %; triangles, φ0 ) 4.9 vol %; circles, φ0 ) 7.2 vol %. Table 1. Zero-Shear Viscosities of Snowtex-20 Suspensions zero-shear viscosity/Pa‚s φ0
with poly(NIPAM)
without poly(NIPAM)
3.6 vol % 4.9 vol % 7.2 vol %
0.0035 0.0050 0.0134
0.0015 0.0018 0.0022
shear flow and then give low viscosity values with an increase in shear rate, leading to the shear thinning behavior. The shear thinning is reversible, and it takes within 10 s to reach a steady state at the respective shear rates for all silica suspensions. The same trends are observed for the silica suspensions with added poly(NIPAM). The resulting zero-shear viscosity increases with an increase in silica volume fraction, and its magnitude is much larger than that for Snowtex-20. At the same silica content, the Aerosil 130 suspensions show stronger shear thinning than the Aerosil 300 ones, and the zero-shear viscosity of the Aerosil 130 suspension is larger than that of the Aerosil 300 suspension. However, the dynamic viscoelastic responses for the silica suspensions were too weak to obtain accurate storage and loss modulii. This suggests that the attractive interaction between the particles in an aqueous medium is less than that in nonpolar media, where silica particles form gels through interaction between the silanol groups on the silica surface. Viscosities of Silica Suspensions with Adsorbed Poly(NIPAM). The steady-state shear viscosities of the Snowtex-20 suspensions in the presence of poly(NIPAM) show a near-Newtonian behavior in the entire range of silica contents, and their zero-shear viscosities are much larger than those without poly(NIPAM), as shown in Table 1. This is mainly attributed to an increase in the effective hydrodynamic volume for the hydrodynamic poly(NIPAM) layer adsorbed on the silica surface. Furthermore, since the size of an isolated poly(NIPAM) chain is comparable to the silica diameter, the poly(NIPAM) chains probably do not form polymer bridging between the silica particles.22,23 We fit the data to the Dougherty-Krieger equation24 to estimate an effective hydrodynamic layer thickness, (21) Mandelbrot, B. B. The Fractal Geometry of Nature; Freeman: San Francisco, CA, 1982. (22) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: New York, 1988. (23) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993; and references therein. (24) Dougherty, T. J. Ph.D. Thesis, Case Institute of Technology, 1959. Krieger, I. M. In Surfaces and Coatings Related to Paper and Wood; Marchessault, M., Skaar, C., Eds.; Syracuse University Press: Syracuse, NY, 1967.
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Figure 2. Steady-state shear viscosities, η, of the Aerosil 300 (open symbols) and Aerosil 130 (filled symbols) suspensions in the presence of poly(NIPAM) as a function of the shear rate, γ˘ , for various volume fractions of the silica content and poly(NIPAM) concentrations: squares, φ0 ) 2.3 vol % and C0 ) 0.71 g/100 mL; triangles, φ0 ) 4.9 vol % and C0 ) 1.5 g/100 mL; circles, φ0 ) 7.2 vol % and C0 ) 2.4 g/100 mL.
η/η0 ) [1 - (φ/φp)]-[η]φp
(1)
where [η] is 2.5 for hard spheres and φp is the so-called packing fraction that is equal to 0.64 for random packed spheres and 0.74 for hexagonal packed spheres. In the calculation, we should use η0 as a viscosity value of the supernatant solution of poly(NIPAM) with a concentration, Cp, in the dispersed medium of the corresponding silica suspension, instead of water viscosity, and we employ φp ) 0.64. From the so-calculated φ value, we can obtain the effective hydrodynamic layer thickness, dh, according to the following equation:
φ ) φ0[1 + dh/d]3
Figure 3. Log-log plot of the steady-state shear viscosity, η, of poly(NIPAM) solutions as a function of the polymer concentration.
(2)
where φ0 is the volume fraction of only solid particles. By using d ) 7.5 nm, the resulting dh values are 7.9, 6.9, and 6.3 nm for the 3.6, 4.9, and 7.5 vol % Snowtex-20 suspensions, respectively, which are less than the radius of gyration of a poly(NIPAM) chain in water, 17.5 nm, calculated from the Flory-Fox equation using the intrinsic viscosity. The adsorbed polymer chains are partially collapsed on the silica surface. In contrast, the flow curves for the Aerosil 300 and Aerosil 130 suspensions strongly depend on the silica volume fraction, as shown in Figure 2. At the lowest silica content the Aerosil 300 suspensions show shear thinning, and with an increase in the silica content the flow curve shows pseudoplastic flow with no zero-shear viscosity observed. Except for the highest silica content, the viscosities are almost the same as in the absence of poly(NIPAM). At the highest silica content the viscosity at the shear rate of 1 s-1 is one order of magnitude larger than that in the absence of the polymer. For the Aerosil 130 suspensions similar results were obtained, and their viscosities are larger than those of the Aerosil 300 ones at the fixed silica content. This rheological behavior may be explained by taking account of changes in both the aggregated structures of the silica particle and the viscosities of the suspended media. The size of the aggregated silica particles should be much larger than that of a poly(NIPAM) chain, and poly(NIPAM) chains can migrate into the fractal structure. The polymer adsorption occurs partially, breaking up the aggregated structure and leading to a decrease in the effective hydrodynamic volume. Whereas the viscosity of the suspended media is larger than that of water (0.797 mPa‚s at 30.0 °C) due to the presence of the unadsorbed poly(NIPAM), the concentrations of free poly(NIPAM) in the suspended media are less than 0.12 g/100 mL and
Figure 4. Steady-state shear viscosities, η, of R-972 (open symbols) and R-812 (filled symbols) suspensions in the presence of poly(NIPAM) as a function of the shear rate, γ˘ , for various volume fractions of the silica content and poly(NIPAM) concentrations: squares, φ0 ) 3.6 vol % and C0 ) 1.1 g/100 mL; triangles, φ0 ) 4.9 vol % and C0 ) 1.5 g/100 mL; circles, φ0 ) 7.2 vol % and C0 ) 2.4 g/100 mL.
their viscosities are on the order of 1 mPa‚s from a loglog plot of viscosity versus concentration of poly(NIPAM) solution, as shown in Figure 3. These two factors could compensate each other, leading to little changes in viscosity, except for the highest silica content. However, there are no chain entanglements between free poly(NIPAM) chains because the concentration is less than the overlapping concentration of poly(NIPAM); namely, the reciprocal of the intrinsic viscosity [η] ) 0.454 dL/g. As expected, the hydrophobic silica particles of R-972 and R-812 can be dispersed in water by adsorption of poly(NIPAM), as displayed in Figure 4. The flow curves for the R-972 suspensions show near Newtonian behavior, expect for the highest silica content, which shows pseudoplastic flow. Whereas the R-812 suspensions with φ0 < 3.6 vol % give near Newtonian flow, an increase in the silica content changes the flow behavior from shear thinning to pseudoplasticity. This indicates that adsorption of poly(NIPAM) on the surface of hydrophobic silicas gradually weakens the hydrophobic interactions between the silica particles and then the silica particles fully covered by the polymer tend to separate and disperse each other in the water phase. Thus, poly(NIPAM) chains play a role as a steric stabilization agent for the hydrophobic silica particles. Such an adsorption kinetic process is fairly long, and it takes 10 days to obtain a constant adsorbed amount of poly(NIPAM). From a comparison of the shear flow curves in Figure 4, poly(NIPAM) clearly shows the better dispersion power for the R-972 silica particles in water than that for the R-812 ones. The difference may be attributed to the more hydrophobic character of the R-812 particles than the R-972 ones, since the hydrophobic moiety in the former silica is more bulky than that in the latter particles. In
Hydrophilic and Hydrophobic Silica Suspensions
addition, a preliminary experiment shows that much more hydrophobic silica particles, such as Aerosil R-202 silica coated by silicone oil, cannot be well dispersed at 3.6 vol % silica contents for 2 weeks. Furthermore, to evaluate the dispersion ability of the hydrophobic silica particles by adsorption of poly(NIPAM), it is useful to compare the viscosity with that for the Snowtex-20 suspension. Since both Snowtex-20 and R-972 silica particles have a similar particle size, their viscosity data will be compared without taking account of the size effects. The viscosities of the hydrophobic silica suspensions at the silica content of 3.6 vol % are less or similar to that of the colloidal silica one. At the silica content of 4.9 vol % the hydrophobic silica suspensions have about twice as large a viscosity as that of the Snowtex-20 suspension. This discrepancy seems not to be large by consideration of the differences in their surface nature and their particle size distributions. Moreover, it is noticed that the viscosities of the hydrophobic silica suspensions are less than those of the hydrophilic ones for the same size particle. Sorting out the effects of hydrodynamic volume, solution viscosity, and colloidal interactions requires characterization of [η], the solution viscosity η at the concentration of free polymer in the suspended medium, and the size R of the flow unit. From these quantities, a plot of η0/η versus [η]c as well as a plot of η0/η versus ηR3γ˘ /kT is useful to interpret the rheological behavior of dispersions, where η0 is the zero-shear viscosity, γ˘ is the shear rate, and k is
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the Boltzmann constant. However, it is impossible to measure the value of R via the dynamic light scattering method, since the suspensions are very turbid. Conclusions Fumed hydrophilic silica particles suspended in water showed shear thinning or pseudoplastic flow, indicating an aggregated structure which probably partially breaks down under shear flow. The colloidal silica suspensions in the presence of adsorbed poly(NIPAM) showed Newtonian flow, and the viscosity was higher than that without the polymer. The fumed hydrophilic silica suspensions showed the same viscosities as those without the polymer, except for that with the highest silica content, indicating that polymer adsorption occurs partially, breaking the aggregated structure in the suspensions. When the hydrophobic silica particles were mixed with aqueous poly(NIPAM) solutions, they were able to be suspended in aqueous media by adsorption of the polymer on the silica surface. Shear flow measurements gave clear evidence for well dispersed suspensions of the hydrophobic silica particles in aqueous media; namely, the suspensions showed Newtonian flow, and the viscosity was the same as that of the colloidal silica suspension at lower silica contents. The hydrophilic silica suspensions showed a change from shear thinning to pseudoplastic flow with an increase in the silica content. LA960147X
