Langmuir 1996,11, 4323-4327
4323
Effects of Polymer Adsorption on Structures and Rheology of Colloidal Silica Suspensions Masami Kawaguchi,*?tRyoji Naka,? Masayuki Imai,t and Tadaya Katot Department of Chemistry for Materials, Faculty of Engineering, Mie University, 1515 Kamihama, Tsu,Mie 514 Japan, and Neutron Scattering Research Laboratory, Institute of Solid State Physics, University of Tokyo, Tokai, Naka, Ibaraki 319-11 Japan Received April 3, 1995. I n Final Form: July 21, 1995@ By a combination of small angle neutron scattering (SANS) and rheological measurements, characterization of colloidal silica suspensions in aqueous (hydroxypropy1)methylcellulose(HPMC)solutionsand their shear viscosities and dynamic moduli measurements were performed as functions of concentrations of silica and HPMC and temperature. From the SANS measurements, adsorption of HPMC induced no changes in particle packing in the silica suspensions,indicating no formation of aggregates. The amounts of HPMC adsorbed on the silica surfaces decreased with an increase in the silica content, due to the ordered structure within the silica suspensions. The shear viscosity increased with an increase in adsorptiontime as well as silica concentration due to an increase in the adsorbed amount, and it showed shear thinning. When the temperature was lowered, the shear viscosity increased and viscoelastic responses were enhanced because of an increase in the effective hydrodynamic layer thickness and a higher probability of occurring in the chain entanglements and polymer bridging under better solvent conditions.
Introduction In preparation of solid particle dispersion in polymer solutions, adsorption of polymer on the particles should play a role in the stability and rheological responses of the dispersions. Although many studies on various dispersions were reported concerning their stability, dynamic properties, and structural changes, the effluence of polymer adsorption on rheological characteristics of the dispersions was not well under~tood.l-~ Recently, several reports have been published that describe the rheological behavior of various silica suspensions by taking account of polymer a d s o r p t i ~ n . ~A- n~ extensive study on the effects of particle sizes on the rheological properties of flocculated silica suspensions by large molecular weight poly(acry1amides)was investigated by O t s ~ b o . *Tadros ,~ and co-workers6studied the rheology of silica suspensions in nonaqueous solutions of low molecular weight block copolymer of poly(Pviny1pyridine)/ poly(tert-butylstyrene) as a function of the adsorbed amount of the block copolymer. The silica particles used are classified as a fumed silica and easily aggregated to form like a gel, which owns relatively open structure in the dispersion medium. In contrast, for the suspensions of stable (colloidal)silica particles dispersed in polymer solution, several important investigations have been reported?-14 Audebert and cot
Mie University.
* University of Tokyo.
Abstract published inAduance ACSAbstracts, October 1,1995. (1)Krieger, I. M.Adu. Colloid Interface Sci. 1972,3, 111. (2)Tadros, Th.F.Adu. Colloid Interface Sci. 1980,12,141. (3)Tadros, Th.F.In SolidlLiquid Dispersions; Tadros, Th.F., Ed.; Academic Press: London, 1987;pp 1-16, 225-274. (4)Otsubo, Y.Langmuir 1990,6,114. (5)Otsubo, Y.J . Colloid Interface Sci. 1992,153, 584. (6)De Silva, D.P. H. L.; Luckham, P. F.; Tadros, Th. F. Colloids Surf. 1990, 50,263. (7) Kawaguchi, M.; Kimura, Y.; Tanahashi,T.; Takeoka, J.; Suzuki, J.;Kato, T.; Funahashi, S. Langmuir 1996,11, 563. (8)Liu, S. F.;Lafuma, F.; Audebert, R. Colloid Polym. Sci., in press. (9)Iler, P. K.J . Colloid Interface Sci. 1975,51,388. (10)Rubio, J.;Kitchener, J. A. J . Colloid Interface Sci. 1976,57,132. (11)Wang, T.K.; Audebert, R. J . Colloid Interface Sci. 1987,119, 459. (12)Wang, T. K.; Audebert, R. J.Colloid Interface Sci. 1988,121,32. (13)Vaslin-Reimann, S.;Lafuma, F.; Audebert, R. Colloid Polym. Sci. 1990,268,476. @
workers have intensively performed polymer adsorption and rheologicalmeasurements to understand the stability of silica suspensions in aqueous m e d i ~ m . ~ JThey l - ~ ~found an unusual shear thinning behavior due to shear-induced bridging by adsorption of a large molecular weight poly(ethylene oxide) on the silica surfaces even if the volume fraction of particles is rather low.8 More recently we have measured the rheological properties of the silica suspensions prepared from two M e r e n t silica particles, such as the aggregated and stable (colloidal)silicas in aqueous solutions of (hydroxypropyl)methylcellulose (HPMC), which adsorbs on the silica particles, at a fixed silica concentration.' We noticed characteristic rheologicalproperties for the colloidalsilica suspensions in comparison with the aggregated silica suspensions. In the flow curve, a plot of steady-state shear stress against shear rate, at lower shear rates the shear stress almost linearly increased with increasing shear rate, and at higher shear rates it showed shear thinning. Thus, the shape of the flow curve resemble that of the HPMC solution. In the dynamic measurements, the loss modulus is larger than the storage one and the storage and loss moduli showed strong frequency dependencies. In this paper, in order to have additional and complementary information on the colloidal silica suspensions dispersed in HPMC solutions, small angle neutron scattering (SANS) and rheological measurements have been performed as a function of adsorption time in relation to the concentrations of silica and HPMC. Furthermore, the effect of lowering the temperature on the rheological properties of the silica suspensions will also be discussed in terms of changes in solvent conditions of polymer solution.
Experimental Section Materials. One HPMC sample of 65SH-400 was kindly supplied by Shin-Etsu Chemical Co., Ltd. It was purified by the same method as that described previously.16-18The molecular (14)Lafuma, F.; Wong, K.; Cabane, B. J . Colloid Interface Sci. 1991, 143,9. (15)Kawaguchi, M.;Ryo, T.; Hada, T. Langmuir 1991, 7, 1340. (16)Ryo, Y.; Nakai, Y.; Kawaguchi, M. Langmuir 1992,8,2413. (17)Kawaguchi, M.;Ryo, T. Chem. Eng. Sci. 1993,49,393. (18)Nakai,Y.;Ryo, T.; Kawaguchi, M. J . Chem. Soc.,Faraday Trans. 1993,89,2467.
0743-7463/95/2411-4323$09.00/00 1995 American Chemical Society
Kawaguchi et al.
4324 Langmuir, Vol. 11, No. 11, 1995 weight of the sample was determined by intrinsic viscosity measurements in aqueous 0.1 N NaCl solution a t 25.0 f 0.05 "C using an Ubbelohde viscometer.lg The molecular weight (MW) of 65SH-400 was 317 x lo3. The degree of substitution (DS)of the OCH3 group and the molar substitution (MS) of the OC3HsOH group were 1.8 and 0.15, respectively, according to the manufacturer. Water purified by Millipore Q-TM was used. Pure grade quality NaN3 was used as a preservative for HPMC. Colloidal silica particles, Snowtex-C, which were kindly supplied from Nissan Chemical Industries, Ltd. (Tokyo,Japan), are stable sols due to the electrostatic repulsion between the negative charges on the silica surface in the range pH = 8The Snowtex-C was used without further purification. The diameter of the silica particles measured by the manufacturer was 15 & 3 nm. The solid content in the Snowtex-C was determined to be 21.7 wt % by evaporation of the dispersed medium (water) in a weighed Snowtex-C and by drying the residue under vacuum. For preparation of the colloidal silica suspensions in aqueous HPMC solutions, a weighed amount of Snowtex-C was mixed with a pre-prepared aqueous HPMC solution with a known concentration in a glass bottle. The resulting suspensions were subjected to mechanical shakingto obtain a homogeneousmixture in a Yamato BT-23 water incubator attached to a sharker for 10 days. The temperature of the water in the incubator was controlled to 10 & 0.1 or 27 f 0.1 "C. The pH of the colloidal silica suspension, which was not adjusted, was around 8.2. The silica contents were 5, 10, and 15 wt %, and the concentrations of HPMC were fixed a t 0.5 and 1.0 wt %. Adsorption of HPMC. The amounts of HPMC adsorbed on the silicas were determined as follows. A constant amount of the silica was mixed with 20 mL of aqueous HPMC solution, the mixture was mechanically shaken for a desired period of 2, 5, and 10 days at 10 & 0.1 or 27 f 0.1 "C in the water incubator, and then the supernatant was removed to separate the silica using a Kubota 6700 centrifuge. The HPMC concentration (CJ in the supernatant was determined by a gravimetric method: after sedimentation of silica particles at 15 000 rpm for 1day the solvent of a weighed amount of the supernatant was evaporated and the residue was dried under vacuum and weighed. SANS Measurements. SANS experiments were performed using the University of Tokyo SAN-U instrument. The wavelength (2) was selected to be 0.70 nm using a velocity selector with variable speeds and pitches, and the wavelength resolution was A M = 10%. The monochromatic beam was collimated by a series of circular apertures having a 20-mm diameter. The samples were transferred to quartz cells of path length 2 mm. The sample to detector distances 4 and 12 m correspond to the wave vector range from 0.005 to 0.677 nm-'. Data were corrected for empty cell background. The resulting scattering intensity was obtained by normalization to the isotopic scattering intensity of protonated water. The experiments were performed at 27 i 0.1 "C. Rheological Measurements. Steady-state shear viscosity and dynamic moduli were measured using an MR-300 Soliquid meter produced by Rheology Co. Ltd. (Kyoto,Japan). The steadystate shear viscosity measurements were carried out in the shear rate range from 0.01 to 148 s-l, and the dynamic moduli measurements were performed in the frequency range from 0.05 to 12.4 s-l using a cone and plate geometry (plate diameter, 30 mm; cone angle, 5") at 10 0.5 and 27 f 0.5 "C. In general, it is well-known that suspensions show nonlinear viscoelastic responses to large strains. The dynamic measurements were carried out at a strain of 10% in the linear range. The measurement system was modified with an aluminum cover to limit solvent evaporation.
*
Results and Discussion Adsorption of HPMC. Figures 1and 2 show plots of the amounts of HPMC adsorbed on the silica surfaces against the value of C, as a function of adsorption time and silica contents for the silica suspensions in the 0.5 (19)Kato, T.; Tokuya, T.; Takahashi, A.Kobunshi Ronbunshu 1982, 39, 293.
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and 1.0 wt % HPMC solutions, respectively. When adsorption time is longer and silica content is lower, the adsorbed amount of HPMC per unit mass of the silica increases for the respective HPMC solutions. Similar results for the adsorbent concentration dependence of the adsorbed amount of polymer have been reported for several systems.20-22In those studies it was found that there are no effects of a non-equilibrium state or of an artifact such as partial coagulation of dispersed particles. However, adsorption behavior observed in this study may be correlated with the separate distance between the colloidal silica particles, which can be determined from the SANS measurements described below, resulting in a loss of the effective surface area for adsorption with an increase in silica content. Slow adsorption kinetics and the silica concentration dependence of the adsorbed amount are quite different from the adsorption of HPMC on the surfaces of the aggregated silicas,' where the adsorbed amount of HPMC attained an equilibrium state within 1 day and it was almost independent of the adsorbent mass added. SANS Measurements. SANS is one of most useful techniques for investigating the structures of the condensed matter. The shape of the scattering curve, a plot of scattering intensity (&I) as a function of wave vector (q), gives a clue to information on the structures of the objects. Figure 3 shows typical scattering curves of Snowtex-C dispersions with various silica contents. A peak in Z ( q ) is caused by interference effects, and its presence indicates that the particles are not arranged at random but have some short-range ordering due to the electrostatic repulsion between the negative charges on the silica particles. Thus, the shift of the peak to higher q values with an increase in the silica content reflects a reduction in the equilibrium separation distance between (20) Greeland, D. J. J . Colloid Interface Sci. 1963,18, 647.
(21)Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Fleer, G. J. J . Polym. Sci., Polym. Phys. Ed. 1980,18, 559. (22) Koopal, K. L. J . Colloid Interface Sci. 1981,83, 116.
Langmuir, Vol. 11, No. 11, 1995 4325
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the particles. Similar results have been obtained by Wong et al.23 The separation distance D can be calculated from the Bragg's relation, D = 2nIq: the values of D for the 5, 10,15, and 20 wt % silica suspensions are 43.6,33.2,29.9, and 25.6 nm, respectively. Thus, it can be expected that an HPMC chain is less easy to diffuse and attach to the silica surface with an increase in the silica content, leading to less adsorbed amount of HPMC. Typical scattering curves measured for the 10 wt % silica suspensions in 0.5 and 1.0 wt % aqueous HPMC solutions after 10 days mixing are displayed in Figure 4. Two scattering curves are superimposed on a curve, and they also are in agreement with the scattering curve of the 10 wt % Snowtex-Csilica slurry. Similar results were obtained for other silica suspensions with 5 and 15 wt % silica content (not shown here). This means that adsorption of HPMC on the silica surface does not influence the ordered structures of the silica particles, namely, no changes in the separation distance between the silica particles, suggesting that the particles do not directly interact and they form no aggregates. It is interesting to consider the conformation of HPMC chains adsorbed on the silica particles. Since the size of an isolated HPMC chain is comparable to the silica diameter, the probability of formation of polymer bridging between the silica particles is not higher than that for longer chains. Thus, the HPMC chains probably take a loop-train-tail conformation on the silica surface. However, it will be expected that some portions, such as loops and tails, of the adsorbed HPMC chains behave as crosslinkers among the silica particles, leading to so-called polymer bridging with an increase in the silica contents. Such a polymer bridging mechanism also plays a role in the viscosity enhancement of the silica suspensions mentioned below, (23) Wong, K.;Cabane, B.;Duplessix, R. J . Colloid Interface Sci. 1988,123,466.
Figure 6. Plots of the steady-stateshear viscosity against the shear rate of the Snowtex-C silica suspensions in a 0.5 wt % HPMC solution with various silica concentrations (C,) at 27 "C as a function of adsorption time: ( x ) at C, = 5 wt % for 2 days; (+) at C, = 5 wt % for 5 days; (*) at Cs = 5 wt % for 10 days; ( 0 )at C, = 10 wt % for 2 days; (A)at C, = 10 wt % for 5 days; (W) at C, = 10 wt % for 10 days; (0)at C, = 15 wt % for 2 days; (A) at C, = 15 wt % for 5 days; (0)at C, = 15 wt % for 10 days.
Rheological Measurements. Before discussing effects of adsorption time and concentrations of HPMC and silica on the rheological properties ofthe silica suspensions, we should address the shear viscosities of the 0.5 and 1.0 wt % aqueous HPMC solutions. The respective HPMC solutions show slightly shear thinning at higher shear rates, and the zero shear rate viscosity increases with an increase in HPMC concentration. The zero shear rate viscosity at 10 "C is larger than that at 27 "C for the same HPMC concentration because of a better solvent for HPMC in water at 10 "C than at 27 "C. However, storage modulus responses are much more insensitive for the aqueous HPMC solutions. Figure 5 shows plots of the steady-state shear viscosity against the shear rate for the 5, 10, and 15 wt % silica suspensions in 0.5 wt % aqueous HPMC solutions at 27 "C as a function of adsorption time. Irrespective of the silica content, the shear viscosity increases with an increase in adsorption time, leading to formation of a dense polymer layer and to an increase in the effective hydrodynamic volume of the particles. The 5 wt % silica suspensions show pseudoplastic flow that is indicative of a flocculated structure. This stems mainly from the shielding of negative charges on the silica surfaces due to the larger amounts of HPMC adsorbed on silica surfaces. The 10 and 15 wt % silica suspensions have the zero shear rate viscosity in the lower shear rates and the shear thinning is observed at lower shear rates than that for HPMC solutions, indicating rearrangements of the ordered structures of silica particles as well as some portions of the chain entangelements and of the polymer bridging under higher shear flows. Moreover, we found that shear thinning occurs at lower shear rates with increasing adsorption time, attributed to the weaker chain entanglements occurring between the adsorbed and free HPMC chains because of the less concentration of HPMC in the dispersion medium. Similar results are observed for the silica suspensions in 1.0 wt % HPMC solution at 27 "C, as shown in Figure 6, but their shear viscosities increase by 1 order of magnitude compared with those for the silica suspensions in 0.5 wt % HPMC solution. This is attributed to the larger adsorbed amounts of HPMC, which lead to an increase in the effective volume of the particles in the dispersion medium and also to the higher probabilities of polymer bridging as well as the chain entanglements occurring between the HPMC chains in the adsorbed layer and in the bulk solution. When the temperature is lowered to 10 "C, the shear viscosities of the silica suspensions increase by 2 orders
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of magnitude compared with those at 27 "C, as shown in Figures 7 and 8 for the silica suspensions in 0.5 and 1.0 wt % HPMC solutions, respectively. These suspensions show pseudoplastic flow, and their shear viscosities almost linearly decrease with the shear rate beyond the shear rates larger than 1 s-l, which corresponds to the yield stress, irrespective of the silica suspension. Lowering the temperature causes better solvent conditions for HPMC in water as described above. Though the lower amounts of polymers adsorbed under better solvent conditions were observed in many c a ~ e s ,the ~ ~amounts ,~~ of HPMC adsorbed on the silica surfaces at 10.0 "C are (24) Kawaguchi, M.; Takahashi, A. Adu. Colloid Interface Sci. 1992, 37, 219. (25) Fleer, G. J.; Cohen Stuart, M. A,; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993.
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Frequency / lis Figure 10. Storage ( G ) and loss ( G )moduli as a function of frequency of the Snowtex-C silica suspensions in a 1.0 w t % HPMC solution with various silica concentrations for the adsorption time of 10 days. The symbols are the same as those in Figure 9. not always smaller than those at 27.0 "C. The reason why there is such a temperature dependence is not clear at the present time. An increase in the shear viscosity with lowering of the temperature seems to be contributed to an extended conformation of the adsorbed HPMC chains in a good solvent condition. The followingthree factors are mainly considered to have the responsibility of the higher viscosity: (1)an increase in the effective hydrodynamic volume of the particles due to the adsorbed HPMC layer thickness; (2) chain entanglements easily occurring between the adsorbed and free HPMC chains; (3) different types ofpolymer bridging, such as direct bridging, bridging through entanglements of adsorbed chains, and bridging through entanglements of free chains, between the silica particles. The last two factors correspond to attractive forces between the particles. Silica suspensions at 10 "C show more viscoelastic responses than those at 27 "C. The storage and loss moduli are displayed as a function of frequency in Figures 9 and 10 for the silica suspensions in 0.5 and 1.0 wt % HPMC solutions with the adsorption time of 10 days, respectively. Both the storage and loss moduli increase with increasing frequency for the respective suspensions. At the same silica concentration the storage and loss moduli increase with an increase in HPMC concentration. Moreover, when the silica concentration increases, the crossover point where the storage modulus becomes larger than the loss modulus shifts to lower frequency and the storage and loss moduli show gradually weaker frequency dependencies. This means that the rheological properties of the silica suspensions change from liquidlike to solidlike viscoelasticity with an increase in silica content. An
Langmuir, Vol. 11, No. 11, 1995 4327
Colloidal Silica Suspensions increase in the storage modulus is a consequence of the thicker steric barriers for the adsorbed HPMC layer and of the polymer bridging. Moreover, we notice a unique viscoelastic response for the frequencydependenceof the storage modulus, though in general the Newtonian behavior can be coupled with a rapid drop of the dynamicviscoelastic functions at lower frequencies. The much smaller slope of the storage modulus than 2 probably stems from the presence of the ordered structures of silica particles, leading to the weak internal structure in the silica suspensions.
Conclusions The regular particle packing in the silica suspension induced slow polymer adsorption kinetics and a decrease
in the amount of polymer adsorbed on silica surfaces per unit area with an increase in silica concentration. SANS measurements gave clear evidence for no changes in the particle packing within the suspensions, although polymer adsorption on particle surfaces occurred, partially screening the electric surface charges on the silica surfaces. Thus, the resulting silica suspensions showed no aggregation of the silica particles, and they showed rheological responses, attributed to the hydrodynamic layer thickness, the entanglements of polymer chains, and the polymer bridging. Moreover, when concentrations of silica and polymer increased, chain entanglements and polymer bridging frequently occurred, leading to further enhancement of rheological responses of the silica suspensions. LA950262S
