Langmuir 1995,11, 1807-1812
1807
Monitoring Flocculation in Situ in Sterically Stabilized Silica Dispersions Using Rheological Techniques Gautam S. Grover and Stacy G. Bike* Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136 Received April 12, 1994. I n Final Form: December 14, 1994@ Flocculation of sterically stabilized colloidal dispersions is a technologically important and complex phenomenon. Often, a state of controlled, weak flocculation is desired to slow the aging of dispersions due to irreversible flocculation and subsequent sedimentation. Using rheological techniques, we have characterized flocculationin situ in sterically stabilized fumed silica dispersions toward identifying those variables important to the control of the flocculated network strength. Both poly(methy1methacrylate) homopolymersand polystyrenelpoly(methy1methacrylate) block copolymershave been used as stabilizers. Dynamic oscillatory measurements have been used to quantify the elastic moduli and hence the strength of the flocculatednetworks as a function of polymer surface coverage, molecular weight, architecture, and solvency as well as particle volume fraction. The results show that the flocculated network strength is weaker for complete surface coverage and when diblock copolymers are used as stabilizers, as expected. In addition, a scaling of the elastic modulus with particle volume fraction demonstrates weaker flocculation with increasing polymer molecular weight. This work illustrates the utility of rheological techniques in studying the dynamic flocculation process.
Introduction Steric stabilization is widely used to control the flocculation of colloidal particles in applications a s diverse as coatings,l magnetic tapes,2inks,3and biological ~ y s t e m s . ~ Steric stabilization is especially important in nonaqueous dispersions, where the low dielectric constant of the medium renders electrostatic stabilization ineffective. Despite its wide use, however, the stabilization of colloidal particles by adsorbed macromolecules remains more of a n art than a science in many technological applications. This lack of quantitative understanding has resulted, in part, from the difficulty of accurately measuring interparticle forces in concentrated colloidal dispersions. Indeed, analytical techniques with the resolution of the surface forces apparatus5 do not exist to characterize interparticle forces in such dispersions. In addition, the complexity of many technologically relevant dispersionscomprised of polydisperse, nonspherical particles stabilized by adsorbed polymers of varying molecular weightsmakes quantitative analysis difficult. Both rheological techniques and osmotic pressure measurements have been used to indirectly assess interparticle forces in concentrated colloidal dispersions.6-21 Abstract published in Advance A C S Abstracts, April 15,1995. (1)Crowl, V. T.; Malati, M. A. Faraday Discuss. Chem. SOC.1966, 42,301. (2) Smith, T. A,; Bruce, C. A. J . Colloid Znterface Sci. 1979,72,13. (3) Wong, R.; Hair, M. L.; Croucher, M. D. J . Imaging Technol. 1988, 14,129. (4) Napper, D.H.J. Colloid Interface Sci. 1977,58, 390. (5)Israelachivili, J. N.; Adams, G. E. J . Chem. SOC.,Faraday Trans. 1 1978,74,975. (6) Hoffman, R. L. J . Colloid Interface Sci. 1974,46,491. (7)Firth, B. A,; Neville, P. C.; Hunter, R. J. J . Colloid Interface Sci. 1974,49,214. (8)Buscall, R.; Goodwin, J. W.; Hawkins, M. W.; Ottewill, R. H. J . Chem. SOC.,Faraday Trans. 1 1982,78,2889. (9) Goodwin, J. W.; Hughes, R. W.; Partridge, S. J.; Zukoski, C. F. J . Chem. Phys. 1986,85,559. (10)Patel, P. D.: Russel. W. B. J . Colloid Interface Sci. 1989.131, 201. (11)Buscall, R.; McGowan, I. J.; Mumme-Young, C. A. Faraday Discuss. Chem. SOC.1990,90,115. (12) Tadros, Th. F.; Hopkinson,A.FaradayDiscuss. Chem. SOC.1990, 90,41. (13)Chen, M.; Russel, W. B. J . Colloid Interface Sci. 1991,141,565. (14) Luckham, P.F.; Ansarifar, M. A,; Costello, B. A. de L.; Tadros, Th. F. Powder Technol. 1991,65, 371.
Of these two, rheological measurements are less sensitive to the presence of impurities and require no special preparation of the dispersion. In addition, the rheological properties of a dispersion mirror the interparticle forces since these properties depend on both the Brownian motion of the particles and the hydrodynamic interactions of the particles with the surrounding fluid and with each other. The presence of an adsorbed macromolecular layer on the particles makes them “soft” and increases their effective volume fraction, correspondingly modifying the flow properties of the dispersion. It has been shown that rheological measurements probing the interactions between sterically stabilized colloidal particles qualitatively agree with direct force measurements between mica surfaces bearing adsorbed polymer chains.14-17 Often, a state of controlled, weak flocculation is desired to slow the aging of dispersions due to irreversible flocculation and subsequent sedimentation. Low-deformation rheological measurements, or dynamic mechanical measurements a t small amplitudes, in which the structure is not significantly perturbed from equilibrium, can be used to quantify the strength of a flocculated network. The applicability of low-deformation measurements in studying interactions in colloidal dispersions has been widely demonstrated.8-21 While several studies have assessed the viscoelastic properties of flocculated suspensions subjected to small d e f o r m a t i o n ~ , ~ - little ~ ~ J work ~-~~ has focused on quantifying the state of the dispersion during flocculation.22 We have used i n situ small amplitude dynamic oscillatory measurements to gauge the strength of network (15) Costello, B.A. de L.; Luckham, P. F.; Tadros, Th. F. Langmuir 1992,8,464. (16) Costello, B. A. de L.; Luckham, P. F.; Tadros, Th. F. J.Colloid Interface Sci. 1992,152, 237. (17) Tadros, Th. F.; Liang, W.; Costello, B.; Luckham, P. F. Colloids Surf. A 1993,79,105. (18)LianEc. W.: Tadros. Th. F.: Luckham, P. F. J . Colloid Interface Sci. 1993,lZ8,152. (19) Otsubo, Y.; Watanabe, K. J . Colloid Interface Sci. 1989,127, 214. (20) Otsubo, Y.; Watanabe, K. Colloids Surf. 1989,41, 303. (21) Otsubo, Y. Langmuir 1990,6, 114. (22) Frith, W. J.; Mewis, J.; Strivens, T. A.Powder Technol. 1987, 51, 27.
0743-746319512411-1807$09.00/00 1995 American Chemical Society
1808 Langmuir, Vol. 11, No. 5, 1995
Table 1. Polymer Molecular Weights and Polydispersity Indexes polymer MW polydispersity index PMMA 58 500 1.03 PMMA 88 000 1.04 PMMA 330 000 1.11 PSPMMA 47:53O 37 600 1.10 PSPMMA 4753 107 800 1.10 a
Ratio of the two polymers by weight in the diblock.
formed by flocculating silica particles bearing either a n adsorbed poly(methy1 methacrylate) homopolymer or a polystyrene/poly(methyl methacrylate) copolymer. The interparticle interactions were controlled by varying the particle volume fraction as well as the polymer concentration, molecular weight, and solvency. This work illustrates the utility of rheological measurements in assessing flocculation behavior in complex dispersions.
Experimental Section Materials. Cab-0-Si1(Grade L-901, an amorphous, nonporous fumed silica provided by Cabot Corp., was used for the experiments. This silica is prepared by the flame hydrolysis of Sic14 in amixture of H2 and 0 2 at about 1800"C. This process produces spherical primary particles of diameter 7-30 nm. The primary particles fuse into chainlike aggregates, which then entangle to form large agglomerates during cooling.23 The average size of these agglomerates was 800 nm, as determined by transmission electron microscopy, and the specific surface area was 105 m2/g. Two types of polymeric stabilizers were selected for study: poly(methy1 methacrylate) (PMMA) homopolymers and polystyrendpolflmethyl methacrylate) (PSPMMA)copolymers (Table 1). All polymers were obtained from Polymer Labs, Inc. Toluene was used as the solvent for the homopolymer-baseddispersions, while both toluene and xylene were used as the solvents for the copolymer-baseddispersions. Both solvents were obtained from Mallinckrodt and were used as received. The x parameter (at 25 "C) is 0.45 for PMMA in toluene and 0.50 for PMMAin xylene, while it is 0.41 and 0.40 for PS in the same respective solvents.24 Polymer Adsorption Studies. Polymer adsorption was quantified by IR spectroscopyaccording to the method of van der All IR studies were conducted on a Cygnus 100 Linden et Fourier transform infrared spectrometer (Mattson Instruments) using a liquid nitrogen-cooled mercury-cadmium-telluride detector and an interferometer mirror speed of 2.53 c d s . The studies were conducted in the middle infrared range (400-4000 cm-I) using a 25 mm transmission cell with zinc selenidewindows. The sample chamber was continuously purged with nitrogen to ensure a uniform C02 and water vapor-free background. Adsorption isotherms were constructed using dispersions of constant silica volume fraction with varying polymer concentration. These dispersions were first equilibrated for 24 h at 25 "C under constant agitation. The silica particles bearing adsorbed polymer were then removed by centrifugation, leaving the supernatant containing the nonadsorbed polymer for IR analysis. The integrated absorbance of the characteristic PMMA peak at 1734 cm-l (C=O stretching) was measured in the supernatant, from which the polymer concentration was determined using the appropriate calibration curve constructed from polymer solutions of known concentration. The amount of polymer adsorbed onto the silica was then calculated by a simple mass balance. Rheological Studies. The small amplitude oscillatory measurements were conductedon a Bohlin VOR rheometer using a C14 concentric cylinder measuring system (rotating outer cap of diameter 15.4 mm and a stationary inner bob of diameter 14 mm). During the linear viscoelastic measurements, the cup was oscillated sinusoidally with a frequency w. The resulting (23) Cab-0-Si1Fumed Silica: Properties and Functions. Technical Bulletin, Cabot Corp., Tuscola, IL. (24) CRCHandbook ofpolymer-Liquid Interaction Parameters and SolubilityParameters; Barton,Allan, F. M., Ed.;CRC Press: Boca Raton, FL, 1990. (25) Van der Linden, C . ; van Leemput, R. J. Colloid Interface Sci.
1978,67,48.
Grover and Bike deflection of the bob was then measured and converted to a shear stress t,from which the following parameters were calculated:
G* = d y = G
+i G
(1)
G = G* cos 6
(2)
G = G* sin 6
(3)
where the complex modulus G* is comprised of the elastic modulus, G , and the viscous modulus, G , and y is the applied strain. The elastic or storage modulus G describes the solid (Hookean) nature of the dispersion and is analogous to Young's modulus, while the viscous or loss modulus G describes the dissipative (viscous)component ofthe dispersion. The loss angle 6, computed by measuring the time lag between the sine waves of the applied strain and the resulting stress, is a measure of the viscoelastic nature ofthe dispersion. This angle is 0" for a purely elastic solid and 90" for a purely viscous liquid, while viscoelastic dispersions lead to intermediate values of 6. The dispersions for the rheological studies were prepared with varying amounts of silica and polymer to study the effect of both particle volume fraction and surface coverage on the flocculation behavior. The silicavolume fractions rangedfiom 0.020 to 0.045. This range was limited at low volume fractions by the detection limit of the instrument and at high volume fractions by the agglomerated state of the silica. Three different polymer concentrations were selected for the rheological studies, providing surface coverages of 0.15,1.5, and 2.0 mg/m2;plateau adsorption for all dispersions was on the order of 2 mg/m2. All dispersions were prepared by adding the required amount of polymer and silica to the solvent, followed by equilibration under constant agitation for 24 h. Prior to the small amplitude oscillatory measurements, the samples were presheared at a constant shear rate of 29 for 500 s. This preshear provided uniformity of the sample microstructure and minimized any effects of sample shear history, which is known to affect rheological measurement^.^^,^^ In addition, this selected shear rate was low enough to prevent migration of the particles in the sample thus ensuring a uniform distribution of the silica particles. During the preshear, the dispersion viscosity decreased as the existing flocculated structure was broken down. Since there was a weak dependence of the dynamic moduli on the specific value of the shear rate selected for the preshear, all samples were subjected to the same preshear protocol. A dependence of G on the initial shear rate has also been observed by Frith et aLZ2 Following the preshear, the dispersions were allowed to flocculate within the sample cup, and small amplitude oscillatory measurements were taken at regular intervals over a period of 3 h at frequencies from 0.07 to 15 Hz. The sample temperature was held constant at 25 f 0.1 "C, and a solvent trap was used to minimize solvent evaporation. All measurements were conducted in the linear viscoelastic region, where the stress is linearly proportional to strain. This region was identified by conducting a strain sweep, in which the strain was increased from zero in small steps and the moduli were calculated from the loss angle. The largest strain for which G was independent of strain was selected for the experiment, thus maximizing the signal while staying within the linear region. Outside this linear region, G decreased and 6 increased with strain due to a breakdown of the flocculated structure. For the dispersions studied, the linear region was at strains below the detection limit of the instrument. According to the method of Tadros et u Z . , ' ~ we conductedthe dynamic tests at the lowest strains possible on the instrument. Note that dispersions typically exhibit linear regions far smaller than those of polymeric fluids.
Results and Discussion Overview. The presence of an adsorbed polymer layer on colloidal particles does not ensure stability to flocculation. Instead, the stability of the dispersion is a function of both the extent and the nature of the adsorp(26) Schreuder, F. W. A. M.; van Diemen, A. J. G.; Stein, H. N. J. Colloid Interface Sci. 1986,111, 35. (27) Leighton, D.; Acrivos, A. J.Fluid Mech. 1987,181,415.
Rheological Monitoring of Flocculation in Dispersions 2.0[
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Langmuir, Vol. 11, No. 5, 1995 1809
4 1 : k
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Figure 2. Time dependence of the elastic modulus G for a silica dispersion (4 = 0.04) stabilized by PMMA of MW 88 000 at full surface coverage in toluene.
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tion, which depend on the interactions between the macromolecule, the particle, and the medium. The strengths of these interactions influence the physical properties of the dispersion and ultimately the properties of the final product incorporating the dispersion. For example, any irreversible flocculation that occurs in coatings can affect the properties of the final coating film, including durability, color strength, and On the other hand, complete dispersion of the pigment does not ensure stability to sedimentation in the package.l These observations suggest that an intermediate state of dispersion-that of weak, reversible flocculation-is optimal, in which the colloidal particles bearing adsorbed polymer form a weak gel-like structure. The existence of this state of flocculation implies that polymer chains adsorbed onto adjacent particles are weakly entangled, forming a n extensive three-dimensional network that inhibits settling yet can be easily broken down by the application of shear. Such a weakly flocculated system resides in a shallow secondary minimum of the interparticle potential curve. Both the depth of this minimum, Vmin,and the particle volume fraction, 4, control the state of flocculation for a given particle size (and a t low free polymer concentrat i o n ~ ) For . ~ ~a given particle size and strongly attached polymer chains, Vmin is a function of the following parameter^:^^ (1)the fraction of surface sites occupied by the polymer (the surface coverage), 8; (2) the thickness of the adsorbed polymer layer, 6; and (3) the quality of the solvent, given by the Flory parameter, x. Vmindecreases with increasing surface coverage,increasing polymer layer thickness, and increasing solvency of the stabilizing moieties. Note that the polymer layer thickness is determined to a large extent by the molecular weight of the stabilizing moieties and to a smaller extent by the solvency ofthese moieties. In the results described below, we have used rheological techniques to assess the role of particle volume fraction in addition to polymer surface coverage, molecular weight, and solvency in controlling the state of flocculation. Adsorption Measurements. Shown in Figure 1 is the adsorption isotherm for PMMA of molecular weight (MW) 88 000 adsorbed onto silica from toluene. This isotherm shows the specific adsorbance A (mg of polymer adsorbed/m2 of silica surface) plotted against the equi~~
librium concentration of polymer in the dispersion medium, cp. All of the adsorption isotherms were found to demonstrate this same general behavior. The plateau adsorbance for all polymers in toluene was on the order of 2.0 mg/m2,which compares well with literature values for both PMMA and PSPMMA adsorption onto ~ i l i c a . ~ ~ , ~ ~ In xylene, the plateau adsorbance was approximately 2.1 mg/m2. There was no significant variation in the plateau adsorbance with either polymer molecular weight or architecture. Rheological Measurements. To characterize the structure formed in the flocculating silica dispersions, the elastic modulus was measured a t specific time intervals while flocculation was occurring in the rheometer sample cup. Figure 2 shows representative data illustrating the increase in the elastic modulus as flocculation occurs from time t = 0 min (immediately following the preshear) to t = 180 min. These data are for MW 88000 PMMA adsorbed onto silica (4 = 0.04) from toluene a t a specific adsorbance of 2.0 mg/m2. The elastic modulus G , which is a n indication of the strength of the flocculated network, increased by approximately 2 orders of magnitude over the 3 h period. With the exception of the data at t = 0, G exhibits a very weak dependence on frequency (the scatter in the data a t t = 0 is due to the low values of G , which are a t the detection limit of the instrument and thus influenced by instrument noise). This behavior is typical of a highly structured elastic network. Relatively well-dispersed systems exhibit a dependence of G on frequency. At low frequencies, the hydrodynamic forces persist long enough to allow relaxation of the microstructure. This relaxation leads to viscous dissipation of the applied energy by microstructural rearrangement3zwithout any breakdown of the flocculated structure. As the oscillation frequency increases, the characteristic time for the oscillations is too short to allow for relaxation of the microstructure; instead, the particles oscillate about their equilibrium positions. This alternate relaxation and compression of the flocculated structure leads to an elastic energy storage and release, and consequently an increase in the elastic modulus. On the other hand, the weak frequency dependence of G shown in Figure 2 indicates the presence of a flocculated gel-like structure that is unable to relax. Microstructural rearrangement of the particles is thus
~
(28) Doroszkowski, A. In Paints and Surface Coatings: Theory and Practice; Lambourne,R., Ed.;Ellis Honvood Ltd.: Chichester,England, 1987. (29) Vincent, B. Croat. Chem. Acta 1983,56,623.
(30)Thies, C. J . Phys. Chem. 1966, 70,3783. (31) Thies, C. J . Poly. Sci. Part C 1971, 34, 201. (32) Strivens, T.A. Colloid Polym. Sci. 1983,261, 74.
Grover and Bike
1810 Langmuir, Vol. 11, No. 5, 1995 10
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37,000
0.1
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1 Frequency (Hz)
inhibited, shifting the characteristic time for the diffusive relaxation to longer times.33 Similar flocculation studies with the other dispersions demonstrated a difference in both the breakdown and the recovery behavior based on polymer architecture. Following the preshear, the initial values of G were a t least 2 orders of magnitude lower for the copolymer-based dispersions than for the homopolymer-based dispersions (at the same silica volume fraction and polymer surface coverage). After 180 min of flocculation, however, the values of G were within 1order of magnitude for all the dispersions. These results indicate that the preshear was more effective in destroying the flocculated networks that formed in the copolymer-based dispersions during sample equilibration; specifically, the flocculation that occurred in these dispersions was weaker and hence more reversible. These results are expected. When the copolymer is used for stabilization, acid-base interaction^^^,^^ promote anchoring of the basic PMMA block to the acidic silica surface; the PS block does not adsorb and instead forms the steric layer. In contrast, the PMMA homopolymer must both adsorb onto the silica surface and form the steric layer, with the tails of the adsorbed homopolymer contributing most to the steric protection. As a result, the diblock copolymer is a more effective stabilizer than the homopolymer and promotes weaker flocculation. The effect of surface coverage on the flocculation behavior was studied by varying the initial concentration of polymer added to the dispersion. The polymer concentrations selected provided two surface coverages at less than plateau adsorption, 0.15 and 1.5 mg/m2,and one surface coverage corresponding to plateau adsorption, nominally referred to as 2.0 mg/m2. Representative data illustrating the effect of surface coverage are shown in Figure 3 for dispersions containing MW 88 000 PMMA adsorbed onto silica ( 4 = 0.04)from toluene. The decrease in the elastic modulus with increasing surface coverage confirms that higher surface coverages lead to weaker flocculated networks and enhanced dispersion stability, as expected. At lower surface coverages, the interparticle attraction is stronger due to thinner steric layers and probable bare patches on the particle surfaces that promote bridging flocculation. As the surface coverage increases, (33) Ploehn, H.J.;Goodwin,J. W. FaradayDiscuss. Chem. SOC.1990, 90, 77.
(34) Howard, G.J.; Ma, C. C. J. Coatings Technol. 1979, 51 (651),
47. (35) Fowkes, F.M.;Mostafa, M. A. Ind. Eng. Chem. Prod. Res. Dev. 1978, 17, 3.
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Figure 3. Variation in the elastic modulus G with surface coverage for silica dispersions (4 = 0.04) stabilized by PMMA of MW 88 000 in toluene.
1
1
10
Frequency (Hz)
Figure 4. Dependence of the elastic modulus G on polymer molecular weight and architecture for silica dispersions (4 = 0.04) in toluene at full polymer surface coverage and at t = 180 min. the van der Waals forces between the particles are shielded by adsorbed polymer layers which also provide a physical barrier to flocculation. These results are consistent with the findings of de Silva et al.36 For given solvency conditions, changing the molecular weight of the polymer provides a means to vary the thickness of the steric layer. As the polymer molecular weight increases, a thicker steric layer and a corresponding increase in dispersion stability is expected, manifesting in a decreased elastic modulus. As Figure 4 shows, however, this was not observed for the dispersions studied. For a silica volume fraction of 0.04 and full surface coverage by the polymer, the largest elastic moduli (at t = 180 min) occur for the highest molecular weight polymers in both the homopolymer-based and the copolymer-based dispersions. For example, as the PMMA molecular weight increased from 58 000 to 88000, there was a small decrease in G , likely due to the increased stability provided by the thicker polymer layer; as the molecular weight was further increased to 330 000, however, there was a significant increase in G . These unexpected results may be due to an increased probability of entanglement of polymer chains on adjacent particles with the thicker steric layers, leading to stronger flocculation. The data suggest that high molecular weight polymers do not necessarily enhance dispersion stability and indeed may even promote flocculation. Finally, note that the elastic moduli for the copolymer-based dispersions were less than those for the homopolymer-based dispersions. In agreement with the results presented above, the copolymerspromoted weaker flocculation and greater dispersion stability. A qualitative measure of the strength of interparticle interactions can be obtained by measuring G as a function of particle volume Shown in Figure 5 is G as a function of frequency a t volume fractions from 0.020 to 0.045 stabilized by MW 330 000 PMMA. The data can be described by a power law relationship:
G = a4b
(4)
(36) De Silva, G.P. H.; Luckham, P. F.; Tadros, T . F. Colloids Sur$ imn. 50. 263. (37)Shih, W.-H.;Shih, W. Y.; Kim, %-I.; Liu, J.; Aksay, I. A. Phys. Reu. A 1990,42,4772. (38) Buscall, R.;Mills, P.A.;Goodwin,J. W.; Lawson, D. W. J . Chem. - - - - , - - I
SOC.,Faraday Trans. 1 1988,84,4249. (39) Otsubo, Y.;Nakane, Y. Langmuir 1991, 7, 1118. (40) Van der Aerschot, E.; Mewis, J. Colloids Surf. 1992, 69, 15.
Rheological Monitoring of Flocculation in Dispersions
Langmuir, Vol. 11, No. 5, 1995 1811 x
1
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Figure 6. Scaling of the elastic modulus G with silica volume fraction c$ for dispersions stabilized by PMMA of MW 88 000
at full surface coverage in toluene. Table 2. Power Law Indexes for the Homopolymer-BasedDispersions power law index b
10.3 7.6 5.6
The power law index b is a gauge of the strength of the interparticle forces, with higher values of b indicating stronger interparticle forces. Indexes ranging from 2 to 6 have been reported in various systems39as critical for the appearance of a n elastic response. The data shown in Figure 5 are replotted in Figure 6 as the elastic modulus (at 1Hz) as a function of volume fraction, giving a power law exponent of 7.6. The values of b for the homopolymer-based dispersions are summarized in Table 2. These relatively large values of b indicate strong interparticle attractions and compare well with the values obtained by de Silvaeta1.36for strongly flocculated Aerosil dispersions. The weak frequency dependence of the elastic moduli as shown in Figure 5 also confirms that the dispersions are highly flocculated. The b value decreases with increasing polymer molecular weight, indicating weaker interparticle forces. Note that this analysis suggests that the interparticle forces are weakest with the MW 330 000 PMMA; this result is in direct contrast to that drawn from the structure recovery studies a t 9 = 0.04 (Figure 4). Similar experiments with
1
I
10
Frequency (Hz) Figure 7. Effect of solvency of the anchor PMMA block on the elastic modulus G for silica dispersions (9= 0.035)stabilized by PSPMMA of MW 107 000 at full surfacecoverage in toluene (M)
~01.m MW 58 000 88 000 330 000
Toluene
0.1
at full surface coverage in toluene.
0.01
0.45 0.50
1
Frequency (Hz)
t
PMMA
. Toluene 0.41 . Xylene 0.40
b
Figure 5. Variation in the elasticmodulus Gwith silica volume fraction c$ for dispersions stabilized by PMMA of MW 330 000 10
- Solvent PS
-1
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(25.C)
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the copolymer-based dispersions led to lower values of b than with homopolymer-based dispersions, in support of the other data indicating weaker flocculated networks in the copolymer-based dispersions. In addition, we have observed even lower values of the power law index in our work with dispersions containing model silica particles stabilized by either adsorbed PMMA of lower molecular weight ( < 10 000) or grafted long-chain alcohols,41suggesting significant screening of van der Waals forces and hence weaker interparticle forces. Finally, we investigated the effect of varying the solvency of the anchor block on the flocculation behavior of the dispersions containing copolymers. For nonaqueous dispersions, the solvency can be controlled through either the dispersing medium or the temperature. We elected to vary the solvency by altering the dispersing medium, which allowed us to selectively alter the solvency of the anchor block without significantly affecting the solvency of the stabilizing block. The solvents chosen were xylene and toluene: xylene is a 0 solvent for PMMA (at 25 "C), while toluene is a good solvent; both are good solvents for PS. We consequently expect stronger anchoring of PMMA in xylene and hence enhanced stability with no significant effect on the solvency of the stabilizing PS block. As shown in Figure 7 for 9 = 0.035 and full surface coverage, the stronger anchoring of the copolymer in xylene leads to a lower elastic modulus, almost 1order of magnitude lower than that measured in toluene. Of course, we would expect even a greater change in the elastic modulus with a change in the solvency of the stabilizing block. Similar results were obtained a t other volume fractions. This work has illustrated the utility of rheological measurements in studying flocculation behavior in sterically stabilized systems. We have focused on the effect of polymer molecular weight, surface coverage, architecture, and solvency in addition to particle volume fraction on the strength of the resulting flocculated network. This work has demonstrated that in situ rheological measurements can provide information about interparticle interactions in concentrated dispersions without the need for special sample preparation. Of significance is that we used rheological techniques to study interparticle interactions in dispersions of highly aggregated, nonspherical particles. Commercial grade fumed silica was selected for this study because of its wide use in coating systems a s a thickener and an antisag agent (41) Grover, G. S.; Bike, S. G., to be published.
Grover and Bike
1812 Langmuir, Vol. 11, No. 5, 1995 among many other uses. The irregular nature of the aggregates prevented detailed quantitative interpretation of the results, however. To overcome these limitations, we are conducting further rheological studies on dispersions of monodisperse, spherical silica particles bearing either physically adsorbed or chemicallygrafked stabilizing chains. The screening study presented here has provided the experimental protocol for these future studies on model dispersions.
Summary We have used dynamic oscillatory measurements to study the flocculationbehavior of sterically stabilized silica dispersions in situ. These measurements have allowed us to investigate the effect of changing polymer molecular weight, surface coverage, architecture, and solvency in addition to particle volume fraction on the strength of the resulting network in these dispersions. The results show that the flocculated network strength is weaker for complete surface coverage by the polymer and when diblock copolymers are used as stabilizers, as expected. In addition, a scaling of the elastic modulus with particle volume fraction demonstrates weaker flocculation with
increasing polymer molecular weight; no definitive trend with molecular weight was observed with the absolute value of the elastic modulus, however. This work has illustrated the utility of rheological techniques in studying the dynamic flocculation process in a complex system. The complexity of the systems studied, based on highly aggregated silica particles, precluded theoretical modeling. Future work will be directed toward studying flocculation behavior in dispersions of spherical silica particles stabilized by monodisperse polymers. The work described here has provided an experimental protocol for quantifying interparticle interactions in these more well-defined systems.
Acknowledgment. We gratefully acknowledge the financial support of the National Science Foundation under Grant CTS-9058078. We also acknowledge the support of the Office of Research and Development, U.S. Environmental Protection Agency under Grant R-815750 to the Great Lakes and Mid-Atlantic Hazardous Substance Research Center. Partial funding for this research has also been provided by the Xerox Foundation. LA9301957
