Langmuir 1992,8, 2413-2416
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Viscoelastic Measurements of Silica Suspensions in Aqueous Cellulose Derivative Solutions Yoshitaka Ryo, Yasuhiro Nakai, and Masami Kawaguchi; Department of Chemistry for Materials, Faculty of Engineering, Mie University, 1515 Kamihama-cho, Tsu Mie 514,Japan Received March 16, 1992.I n Final Form: July 8, 1992 Steady-state shear stress (a) and storage (G') and loss (G")moduli of silica suspensions in aqueous hydroxypropyl methyl cellulose (HPMC)solutions have been measured using a coaxial cylinder rheometer as functions of both HPMC and silica concentrations. The silica aggregation of silica particles in water led to a gel-like material, and HPMC moleculesadsorbed on silica particles. In the steady-state experimenta a bump was observed at low shear rates in the plot of the steady-state shear stress for silica suspensions with higher silica contenta than 7.5 ?6 vs the shear rate due to the inhomogeneities in the flows. Dynamic experiments showed that when the silica content was low, showing a weak aggregated structure, the viscoelastic property of the silica suspension changed from a solidlike to a liquidlike character with an increase in HPMC concentration in the supernatant, whereas when the silica content was high, indicating a strongly aggregated structure, the silica suspensions showed a solidlike viscoelastic character.
Introduction Characteristics and structures of silica suspensions prepared from fumed silica such as Aerosil in polymer solutions should be typically determined by gelation due to aggregation of silica particles and by polymer adsorption behavior at the silica surfaces. Information of the microstructure of the silica suspension is expected to be obtained by using small-angle neutron scattering (SANS); however, there are few reports on the SANS measurements of the silica suspensions in polymer solutions. On the other hand, to understand the characteristics and interactions in the silica suspensions in terms of the rheological parameters of the steady-state shear stress, the corresponding apparent viscosity, and the storage and loss moduli, there are some systematic works.l-ll In particular, Otaubo and c ~ - w o r k e r s ~have - ~ J ~extensively measured the rheological properties of the silica suspensions in water and glycelin mixed solutions of polyacrylamides with molecular weights of (2-5) X lo6. They suggested that the shear-induced bridging process where the flocculate-flocculate bond is formed by adsorption of polymer extending from one particle to a particle in the other flocculate under shear flow can be explained by the bond percolation concept. In a previous paper12the viscosity measurements of the silica suspensions in aqueous hydroxypropyl methyl cellulose (HPMC) solutions were investigated by using a coaxial cylinder rheometer. Silica suspensions in the high molecular weight HPMC solutionsshowed rheopexy when they were preshearing at the high shear rate, while silica suspensions in low molecular weight HPMC solutions showed viscosities higher than that of the corresponding dispersion media in the entire shear rate range. The aim of this study is to investigate more precisely the dependence of silica content as well as HPMC (1) Eieenlauer, J.; Killmann, E. J. Colloid Interface Sci. 1980,74,108. (2) Eieenlauer, J.; Killmann, E.; Korn, M. J . Colloid Interface Sci. 1980, 74, 120. (3) Heath, D.; Tadros, Th. F. J . Colloid Interface Sci. 1983, 93, 320. (4) Otaubo, Y.; Umeya, K. J. Colloid Interface Sci. 1983, 95, 279. (5) Otaubo, Y. J. Colloid Interface Sci. 1986, 112, 380. (6) Otaubo, Y.; Watanabe, K. J . Colloid Interface Sci. 1988,122,346. (7) Otaubo, Y.; Watanabe, K. J . Colloid Interface Sci. 1989,127,214. (8) Otaubo, Y.; Watanabe, K. J . Colloid Interface Sci. 1989,133,491. (9) Otaubo, Y. Langmuir 1990, 6, 114. (10) De Silva, G. P. H.L.; Luckham, P. P.; Tadros, Th. F. Colloids Surf. 1990, 50, 263. (11) Otaubo, Y.; Watanabe, K. Colloids Surf. 1990, 50, 341. (12) Kawaguchi, M.; Ryo, Y.; Hada, T. Langmuir 1991, 7, 1340.
concentration on the flow behavior at the steady state for silica suspensions in aqueous HPMC solutions. Moreover, dynamic measurements were performed to clarify the viscoelastic property of the silica suspensions by focusing on the gelation character of the silica particles in water. Their viscoelastic properties will be discussed in terms of the ratio of the silica content to the initial HPMC concentration, which can be related to the amount adsorbed at the silica surfaces.
Experimental Section Materials. One HPMC sample of 65SH-4000 was kindly supplied by Shin-EstuChemical Co., Ltd. It was purified by the same method as described previously.12 The molecular weight of the samplewas determinedby intrinsicViscositymeasurements in aqueous 0.1 N NaCl solution at 25.0 f 0.05 O C using an Ubbelohde viscometer.'3 The molecular weight (M,)was 403 X lo3,the degree of substitution (DS)of the methyl group was 1.8, and the molar substitution (MS) of the hydroxypropyl group was 0.18. The values of DS and MS were obtained from the manufacturer. Nonporous Aerosil 130 silica (Nippon Aerosil, Yokkaichi, Japan) with a surface area of 141 m2/g,a particle diameter of 16 nm, and a silanol density of 3/nm2was used as received. It was dried under a vacuum at 200 O C . Water purified by a Millipore Q-TM system was used. Pure grade quality NaN3 was used as a preservative for HPMC. The preparation method of the silica suspensions from silica slurries and aquoeus HPMC solutionswas the same as described in detail earlier.12 The stability of the silica suspensions was roughly estimated by visual observation of how long the silica suspensionsremained suspended without any sedimentation.We used such a silica suspension in which no sedimentation was observed for over 1 month for the rheological measurements. The amountsof addedsilica and HPMC were expressedas weight percent in the final mixtures. Adsorption of HPMC. A 0.16-g sample of the Aerosil 130 silica was mixed with an aqueous HPMC solution with a given concentration in a stoppered glass centrifugation tube. The mixture was placed in an air incubator to allow equilibration with stirring by a magnetic chip for 1day at 27 f 0.1 "C,and the supernatant was removed to separate the silica using a Kubota BK-200 centrifuge. The HPMC concentration (C,) in the supernatant was determined by an Otauka Electronics DRM1020differential refractometer (Osaka,Japan) at the wavelength of 488 nm at 25 "C. (13) Kato, T.;Tokuya, T.; Takahashi, A. Kobunshi Ronbunshu 1982,
39, 293.
0743-746319212408-2413$03.00/0 0 1992 American Chemical Society
Ryo et al.
2414 Langmuir, Vol. 8, No. 10, 1992 c
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Figure 1. Adsorption isotherm of HPMC on the Aerosil 130 silica at 27 f 0.1 O C . Rheometer. Steady-stateshear stress-shear rate (steadyflow) and oscillatory (dynamic)measurements were performed using an MR-3 Soliquid Meter produced by Rheology Co. Ltd. (Kyoto, Japan).'* The steady flow measurements were carried out in the shear rate range of 0.01-148 s-l, and the dynamic measurements were performed in the frequency ( w ) range of 0.031-12.4 a-1 using a coaxial cylinder geometry. The outer and inner diameters were 39.90 and 36.97 mm, respectively,and the immersion length was 8.97 mm. The temperature of the sample chamber waa maintained at 27 f 0.5 O C . For the respective steady-statemeasurementa,the silica suspensionswere presheared at the highestshear rate for 5 min. Since the Lissajous figures for the oscillations of the outer and inner cylinders showed an elliptical shape for all silica suspensions studied, there is no correction for the values of the storage (G') and loss (G")moduli obtained from the Markovitz equation," unless statad otherwise.
Results and Discussion Adsorption of HPMC. Changes in the HPMC concentration of the supernatant solution monitored by the gel permeation chromatography technique showed that adsorption equilibrium of HPMC on Aerosil 130 silica surfaces was attained within 5 h. Thus, 1day was enough to reach an equilibrium state.15 Figure 1 shows an adsorption isotherm of HPMC on the Aerosil 130 silica. In the low HPMC concentration the adsorbed amount of HPMC was relatively low and gradually reached the plateau with an increase in HPMC concentration. The shape of the isotherm was of the rounded type owing to the polydispersity (M,/M, * 10)of HPMC. We have measured the amount of polymer adsorbed at various ratios of the added silica amount (C,)and initial HPMC concentration (C,)for various molecular weights of HPMC.16 It was found that the plots of the ratio of the supernatant HPMC concentration (C,) to the value of C, as a function of the ratio of the values of C, and C, can almost be fitted on a master curve, irrespective of the HPMC molecular weight, where C,, C,, and C, are expressed in grams per deciliter. When the ratio of C,/C, was larger than 0.2, the adsorbed amount was well in the plateau region for the respective adsorption isotherms. Steady-StateShear Stress. All HPMC solutions used in the dispersion media show a shear thinning behavior, which is typically observed for relatively high molecular weight polymer solutions. After preshearing the silica suspensions at the shear rate of 124 s-l, the shear stresses for most silica suspensions increased sigmoidally with an increase in time and finally reached their plateau values, which are called the steady-state shear stresses. In general, the lower the shear rate was, the longer time dependence the shear stress showed, and with a decrease in both the values of C, and C,, the time dependence of the shear stress became shorter. A t this moment the shear rate dependence of the steady-state shear stress will mainly be (14)Markovitz, H.J . Appl. Phys. 1952,23, 1070. (15) Nakai, Y.; Kawaguchi, M. Unpublished data.
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Figure 2. Double-logarithmicplots of steady-stateshear stresses ' silica suspensionsas a function of shear rate at various of the 5% HPMC concentrations, C,,: 0, 0.75%;0 , 1.0%;0, 1.25%; 0, 1.5%;e, 1.75%;0,2.0%.
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Figure 3. Double-logarithmic plots of steady-state shear rate stresses of the 7.5% silica suspensions as a function of shear rate at various HPMC concentrations. The symbols are the same as in Figure 2.
conducted to study the stability of the silica suspensions under shear flow. Figure 2 shows double-logarithmicplots of the steadystate shear stress (a)as a function of shear rate (7)for the 5 % silica suspensions at various HPMC concentrations. All silica suspensions showed non-Newtonian flow. The value of u increased with increasing initial HPMC concentration. The value of u for the higher shear rate range increased with increasing shear rate along a straight line with a slope of 0.60 f 0.05. Figure 3 shows the shear rate dependence of Q of the 7.5% silica suspensions at various concentrations of HPMC. Above the 1.25% HPMC concentration there was observed a bump at a shear rate -0.1 s-l, followed by a short plateau, which corresponds to the yield stress, and finally the u value increased with an increase in the shear rate along a straight line with a slope of 0.50 f 0.05. The bump appeared to disappear with an increase in HPMC. Such a complex flow curve at lower shear rates may be related to the kinetics of aggregated silica cluster generation and breaking of the clusters under shear flow: for the shear rates below where the bump is observed, the homogeneous silica suspensions produced by preshearing at the highest shear rate preferentiallytended to aggregate to form clusters due to the gelation character of the silica particles, which leads to inhomogeneous suspensions. For the higher shear rates after the bump in the flow curve, breaking of the generated clusters proceeds, the cluster size becomes smaller, and the number of clusters becomes larger. Thus, an inhomogeneous system seems to lead to complex flow behavior. If the shear rate where the bump is observed in the flow curve is related to the mechanical strength of the cluster,it is expected that the bump position should shift to the higher shear rate and to the larger shear stress with an increase in silica concentration. This expectation will be realized in 10% silica suspensions (see next paragraph). Figure 4 displays the shear rate dependence of the Q value for the 10% silica suspensions at various HPMC concentrations. Except for the silica suspension in 1.0% HPMC solution, a bump was observed at shear rates of
Viscoelastic Measurements of Silica Suspensions
Langmuir, Vol. 8, No. 10, 1992 2415
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Figure 4. Double-Logarithmic plots of steady-state shear rate stresses of the 10%silica suspensions as a function of shear rate at various HPMC concentrations. The symbols are the same as in Figure 2. 0.2-0.3 s-l in the flow curve followed by a relatively long plateau. The bump was observed at the shear rates larger than the 7.5% silica suspensions as expected. For the silica suspension in 1.75% HPMC solution a clear bump disappeared and the shear stress was almost independent of the shear rate for 0.3 < y < 2 s-l, indicating the yield stress. Above the shear rates where the yield stresses were observed, the u value increased with an increase in shear rate and the shear rate dependence of the shear stress was related to the HPMC concentration: the slope of doublelogarithmic plot of the shear stress against the shear rate decreased with increasing HPMC concentration. Similar complex flow behavior at lower shear rates was recently observed for some systems: at low shear rates (
