Rheological studies on inverse microlatices

Rheological Studies on Inverse Microlatices FRANCOISE CANDAU, *'] PASCALE BUCHERT,* A N D IRVIN KRIEGER~ "'2 *Institut Charles Sadron (CRM-EAHP) CNRS-ULP, 6 rue Boussingault 67083, Strasbourg Cddex, France, and tLaboratoire de Spectrom~trie et d'Imagerie Ultrasonores, URA du CNRS 851, Universitd Louis Pasteur, 4 rue Blaise Pascal, 67070 Strasbourg Cddex, France Received December 13, 1989; accepted April 9, 1990 The rheological properties of inverse microlatices prepared by polymerization in nonionic microemulsions have been investigated. These microlatices consist of sterically stabilized particles ( d < 130 n m ) of polyacrylamide and polymethacrylate of trimethylaminoethyl chloride, swollen by high water contents (-~50% water, 50% polymer) and dispersed in an organic medium. The theological behavior of these materials follows quite closely that expected for hard-sphere suspensions. In particular, the data for the dependence on volume fraction of the high- and low-shear limiting viscosities are well fitted by the Krieger-Dougherty equation (I. M. Krieger and T. J. Dougherty, Trans. Soc. Rheol. 3, 137 (1959)). However, the study of viscosity versus shear stress shows that these microlatices have a Newtonian behavior extending up to a volume fraction of ca. 50%, in contrast to the limit of ca. 25% reported for conventional latices. Partition of the disperse phase into polymer particles and small surfactant micelles could account for this difference. © 1990AcademicPress,Inc. INTRODUCTION

The rheological behavior of concentrated colloidal dispersions has been the subject of numerous theoretical and practical investigations. Many authors have attempted to extend the classical theories established for dilute suspensions of rigid spheres to the case of concentrated latices ( 1-6 ). However, there are still problems in understanding the observed phenomena, due to interparticle interactions which are important even at relatively low concentrations. These interactions affect the flow behavior of the particles as well as their structures. Recently, we reported on the polymerization of water-soluble monomers in nonionic microemulsions, which produced stable microlatices consisting of polymer particles of high molecular weight dispersed in a continuous oil medium (7, 8). Microemulsions are

To whom correspondence should be addressed. 2 Permanent address: Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106.

dispersions of oil and water stabilized by an appropriate mixture of surfactant molecules (9). They form spontaneously and, in contrast with emulsions, they are thermodynamically stable. The characteristic scale of a microemulsion is much smaller than that of an emulsion (-_ 100 A against 1 ~m ). As a result, microemulsions are optically transparent. They are also usually Newtonian fluids of low viscosities (a few mPa-s). In the oil-rich regions, they are formed of water-swollen spherical droplets of uniform and small size dispersed in the oil medium. When the amount of the aqueous phase tends to be of the same order of magnitude as that of the organic phase, the description generally given is that of a bicontinuous structure formed of randomly interconnected oil and aqueous domains with the surfactant molecules located at the interface ( 10, 11 ). It should be noted that the stabilization of domains of so small a size requires a fairly large amount of surfactant (-~10-15%). Water-soluble monomers were polymerized either (i) inside water-swollen micelles stabi-

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Journalof Colloidand InterfaceScience. Vol. 140,No. 2, December1990

RHEOLOGY

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OF INVERSE LATICES

lized with an anionic surfactant, Aerosol O T (AOT; sodium 1,4-bis(2 ethylhexyl)sulfosuccinate) ( 12 ) or (ii) in nonionic bicontinuous microemulsions (7, 8, 13). In both cases, the dynamic character of microemulsions produces structural changes during polymerization. In the case of AOT systems, one observes a notable increase in the particle size, so that each final latex particle is the result of the fusion of around 100 initial micelles. The final microlatices were shown to consist of two populations in equilibrium: (a) spherical polymer particles ( d ~ 40 n m ) with a narrow size distribution and (b) small hydrated AOT micelles (d -~ 3 n m ) which arise from the excess of surfactant caused by the particle fusion. In the case of nonionic microemulsions, it was shown that the bicontinuous structure breaks down at the very early stages of the polymerization and globules are formed as the reaction proceeds (7, 8 ). Polymerization in bicontinuous microemulsions requires a surfactant-topolymer ratio lower than that in the case of globular AOT microemulsions (-~0.5 to 1 versus -~3). As a consequence, the volume fraction of micelles is considerably reduced, but their presence cannot be neglected. The inverse microlatices prepared by microemulsion polymerization differ from the more conventional aqueous or nonaqueous colloidal dispersions by a lower particle size (d < 120 nm) and by a large swelling of the particles (e.g., 50% water, 50% polymer). In addition, the process can produce large volume fractions of the disperse phase (up to 60%) consisting of the polymer particles and possibly of small swollen micelles, as mentioned above. No rheological data have been reported to date on inverse microlatices or on inverse latices (i.e., prepared in inverse macroemulsions). In this study, we have investigated the dependence of the microlatex viscosity on both shear rate + and volume fraction ff for 0.02 < 4~ < 0.60. The experimental data have been compared with existing hard-sphere theories and with rheological equations of state.

EXPERIMENTAL

Materials Water was double distilled. Cyclohexane was distilled before use. Isopar M is a narrow isoparaffinic mixture (predominantly C13, Cl4, C15, boiling range 207-257°C) from Esso Chemie, which was filtered before use. Acrylamide (AM) from Fluka was recrystallized twice with chloroform. The methacrylate of trimethylaminoethyl chloride ( M A D Q U A T ) was supplied by Norsolor-Orkem as a 75% w t / w t aqueous solution. The emulsifier used for acrylamide polymerization is a blend of sorbitan sesquioleate (Arlacel 83, HLB = 3.7) and a polyoxyethylene sorbitol monooleate with 40 ethylene oxide residues ( G 1086, HLB = 10.2). In the case of MADQUAT, the emulsifier consists of a blend of sorbitan monooleate with 20 ethylene oxide residues (Tween 80, HLB = 15) and sorbitan sesquioleate (Arlacel 83). The emulsifiers were supplied by Atlas Chemical Industries N.V. and were used without further purification. The microemulsions were prepared by adding with stirring the aqueous m o n o m e r solution in requisite proportions to the mixture of emulsifiers, AIBN and oil. Optimization of the formulation taking into account the effect of m o n o m e r on the structural properties of microemulsions has been reported previously ( 14, 15). The optimum HLB value of the blend has been found to be 9.3 for acrylamide microemulsions and -~ 12.9 for M A D Q U A T microemulsions. In the case of microemulsions containing acrylamide (a neutral monomer), a small amount of sodium acetate was added prior to polymerization; we have shown elsewhere that addition of saltingout-type electrolytes strongly enhances the stability of the resultant microlatices (8).

Microemulsion Polymerization The polymerization experiments were carfled out in water-jacketed reaction vessels, after purified nitrogen was bubbled through the Journal of Colloid and Interface Science, Vol. 140, No. 2, December 1990

468

C A N D A U , BUCHERT, A N D K R I E G E R

microemulsion to eliminate oxygen. The m o n o m e r was initiated by irradiation at 20 °C from a source of ultraviolet light (mercury lamp) with AIBN as a photosensitizer (0.3% based on m o n o m e r ) . Total conversion to copolymer was achieved within less than 30 min. The final products, which consist of water-swollen polymer particles dispersed in oil, are clear and stable microlatices. Electron microscopy (EM) experiments show that the particles are spherical (13), and quasi-elastic light scattering measurements performed on dilute samples provide the hydrodynamic particle diameter. Tables I and II list the compositions of the different samples investigated together with values of the volume fraction q} of the disperse phase (water + m o n o m e r + emulsifier) and the hydrodynamic diameter DH.

Rheological Measurements In order to cover a wide range of shear rates, several different viscometers were used. These were (a) a capillary viscometer of Ubbelohde type with varying pressure head; (b) a coaxial cylinder rheometer (Rheomat 30 Contraves) for low-shear rates + ( 1.6 × 1 0 - 2 S -1 < "y < 128 S--1 ) ; ( C ) a coaxial cylinder rheometer (Rheomat 30 Contraves) for intermediate-shear rates (6.24 s -I < -~ < 1800 s - l ) ; (d) a Weissenberg rheogoniometer (Carri-Med Ltd.). A coneand-plate geometry was used, mainly with a very small cone angle (0.25°), the imposed

TABLE II Compositions (wt/wt) and Particle Sizes of PolyMADQUAT Microlatices Microlatex PolyMADQUAT Water Cyclohexane Emulsifiers 0 (%) DH (A)

4

5

6

24.99 25.00 37.49 12.52

15.00 15.00 55.00 15.00

20.00 20.00 47.00 13.00

54.39 1380

37.26 874

44.77 1132

steady shear rate ranging from 10 -2 to 1.1 × 10 4 S - I . All measurements were made at 25°C. Rheological analyses of the data were performed by standard methods. We have checked for a given sample that the curves obtained by means of the various instruments superposed on a single curve, and that there was no degradation at very high shear rates. In the case of polyMADQUAT (PMAD) microlatices, evaporation of cyclohexane (bp = 80.7°C) precluded the use of the Weissenberg rheogoniometer. In spite of attempts to enclose the sample, evaporation was rapid, because of the small sample size (0.2 ml) associated with the small cone angle. As a result, only the first three instruments (a, b, c) were used for PMAD microlatices. RESULTS A N D DISCUSSION

Variation of Viscosity with Shear Rate TABLE I Compositions (wt/wt) and Particle Sizes of Polyacrylamide Microlatices Microlatex

1

2

3

Polyacrylamide Sodium acetate Water Emulsifiers Isopar M

19.90 4.04 27.67 18.01 30.38

10.01 2.00 17.00 18.00 52.99

19.90 4.03 27.67 15.02 33.38

61.63 800

39.02 648

58.18 904

(%) DH (A)

Journal of Colloid and lnterjace Science, Vol. 140, No. 2, December1990

We have studied the variation of viscosity with shear rate ~/for various volume fractions of the dispersed phase prepared by dilution of the latex with the corresponding continuous phase. Measurements were performed for shear rates ranging from 0.15 s -1 to 1.1 × 10 4 S 1. The variation of the relative viscosity ~r (~/r = n/~s, where ns is the viscosity of the continuous phase) with ~ is given in Fig. 1 for polyacrylamide (PAM) latices and in Fig. 2 for PMAD latices. One observes a Newtonian flow response up to volume fractions of about 50%

469

RHEOLOGY OF INVERSE LATICES

for PMAD latices and 53-55% for PAM latices. Beyond these volume fractions, one can distinguish three flow regimes: the regions I and III correspond to Newtonian limits at lowand high-shear rates, respectively; while region II corresponds to a shear-thinning flow behavior. There are indications of a fourth region of shear-thickening behavior in the two highest concentrations of PAM latices. Thixotropic loop measurements, in which increasing shear rates were followed by decreasing shear rates, showed no evidence of a dependence of viscosity prior shear history. Similar behavior has been observed by Krieger et al. on other systems, notably on polystyrene monodisperse sphere suspensions dispersed in various fluids (16) and on polymethylmethacrylate colloidal suspensions dispersed in silicon oils and sterically stabilized with a triblock copolymer (17). Hoffman has proposed a mechanism with accounts for the four regimes (18). The dominant forces which control the rheological behavior-of hard-sphere suspensions are Brownian forces and hydrodynamic forces. The low-strear Newtonian plateau represents the equilibrium or zeroshear structure of the dispersion under Brownian forces and interpartivle potentials. Under increasing shear, hydrodynamic forces

come into play, and the system progressively transforms into a structure of ordered sheets of particles parallel to the flow planes. Thus region II corresponds to a transition regime, while region III corresponds to the fully ordered bidimensional structure. A shear-thickening region, IV, reported by Hoffman (18), sets in when the ordered structure is no longer stable at higher shear rates. It should be noted that significant shear thinning occurs in these systems only at volume fractions exceeding 50%, values much higher than those observed for conventional latices where shear-thinning behavior can be detected at volume fractions as low as 25% (19). The reason for this is probably connected to the content of surfactants higher than that in the case of conventional latices. One possible explanation is deformability of the microspheres due to the low interfacial tension and to the high water content of the dispersed phase. A more plausible explanation is that some of the surfactant is not incorporated into the effective microspheres, as assumed in calculating the volume fraction, but is present in micellar form in the continuous phase as discussed above. Unfortunately, it is not possible to calculate the number of polymer particles, due to the bicontinuous structure of the start-

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FIG. I. Semilog plots of relative viscosities of PAM microlatices versus shear rate: O, sample I; [Z, sample 2; A, sample 3.

Journal of Colloid and Interface Science, Vol. 140,No. 2, December1990

470

CANDAU, BUCHERT, AND KRIEGER

~r 100

• • • ~'<~v=54'S°/°

SO

•

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FIG. 2. Semilog plots of relative viscosities of PMAD microlatices versus shear rate: I , sample 4; e, sample 5; - , sample 6.

ing system and because of the possibility of partitioning of water between particles and surfactant micelles in the present system. In view of the presence of small micelles in the continuous phase, the particle size distribution is effectively bimodal, which might account for the discrepancies detailed above.

Rheological Equations of State Krieger ( 1 ) has shown that the relative viscosity of hard-sphere suspensions can be correlated to a dimensionless reduced-shear-rate stress ar given by O"r =

aa3/kT,

[1]

where a is the particle radius, k the Boltzmann constant, and T the absolute temperature. In terms of dimensionless variables, the wellknown Williamson equation (20) ~ = f ( ¢ ) subsequently derived by Krieger and Dougherty (21 ) becomes 17rO-- ~/r co nr =

~rov q-

1 + blorl

'

[21

where nm and ~/r~ are the low-shear and highshear limiting relative viscosities and b is an adjustable parameter of the order of magnitude of unity. Journal of Colloid and Interface &'ience, Vol. 140, No. 2, December 1990

Equation [2] has been established for monodisperse hard-sphere suspensions. In the present study, the particle size distribution is likely to be bimodal. We have however plotted in Fig. 3 the variation of nr = f ( a r ) for polyacrylamide microlatices, taking for a the radius measured for the polymer particles. For a given volume fraction, the points for the various particle sizes superpose on a single curve. The most significant results are obtained for q~ = 58%, where a large variation of ~/r is observed. This tends to prove that the effect of polymer particles is dominant. However, this should not be considered as a conclusive test of the corresponding states principle, since the available range of particle sizes was limited due to specific constraints inherent in the microemulsion polymerization process. Contrary to the case of conventional latices, the small particle size facilitates the determination of the low-shear Newtonian plateau while the high-shear Newtonian plateau is more difficult to attain. The data for a 61.6% volume fraction were fitted to Eq. [2] with the three adjustable parameters nrO, nr~, and b. Data in the shearthickening range were not included in the analysis. The result is shown in Fig. 4. The best fit was obtained for b = 1.5 (linear regression coefficient r = 0.994).

RHEOLOGY OF INVERSE LATICES

471

TABLE III With b fixed at 1.5, the Williamson equation was fitted to data obtained at ¢ = 58.3% (r Limiting Relative Viscosities ~o and ~ for Different = 0.995). The corresponding curve is reported Volume Fractions of PAM Microlatices~ in Fig. 5. The good agreement between preI, (%) m0 r/r~ dicted and experimental values confirms the independence of b with concentration, as pre61.6 208.0 30.1 viously shown by Kl-ieger and Dougherty on 58.3 78.0 19.1 53.5 24.2 -other systems ( 17, 21 ). *50.0 24.0 11.0 Values of ~/~0and ~/rc~ obtained for the shear*45.0 10.8 7.0 thinning samples by this procedure are given *40.0 6.25 5.03 in Table III together with the Newtonian rel39.0 5.0 5.0 ative viscosities, where nrO = n ~ *35.0 4.20 3.70 *30.0 3.17 2.90 Figure 6 shows the variation of the low-shear 25.0 2.4 2.4 and high-shear relative viscosities with volume 10.0 1.4 1.4 fraction for the various samples investigated. "8.0 1.25 -When data were available for different samples at the same volume fraction, the points plotted The entries indicatedby an asteriskare results obtained are the values reported in Table III. One ob- by Papir and Krieger on nonaqueous dispersions of serves a large divergence of the viscosity for a monodisperse polystyreneparticles of diameters from 0.15 to 0.43 #m (16). volume fraction above 60%. This behavior is similar to what is generally observed for conventional latices and can be well described by close-packing volume fraction, which is 64% the equation of Dougherty and Krieger (21 ), for randomly packed uniform spheres. The full lines in Fig. 6 are the best fits of Eq. [ 3 ] to the ~b ~-[n]'~p data, the two adjustable parameters being [ 7] [31 and Or. One finds where [~7] is the intrinsic viscosity (equal to 2.5 for hard-sphere suspensions), and q~pis the

Low-shear limit: [710 = 2.8 -+ 0.2

and

q ~ p 0 = 6 4 + 1%

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t

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I

I

1

10

.. ~rr

FIG. 3. Semilog plots of relative viscosity versus dimensionless shear stress for PAM microlatices: ©, sample 1; [], sample 2; A, sample 3. Journal of Colloid and Interface Science, Vol. 140, N o . 2, D e c e m b e r 1 9 9 0

472

CANDAU, BUCHERT, AND KRIEGER

r/r

~'r

200

15020010050 o~ooooooOO~o

150

10 10-a

i 10-3

; , 10"2

t 10"~

I 1

I dr 10

100

FIG. 4. Krieger-Dougherty fit of non-Newtonian viscosity data at 05= 61.6%. High-shear limit: [~]~o = 2.8 -+ 0.2

and

q~p~=78+3%.

The values of [ 7] are somewhat higher than that predicted by Einstein, which m a y suggest that some organic solvent is included in the adsorbed layer of surfactants surrounding the particles. The high-shear limit of the packing volume fraction is somewhat uncertain, due (i) to the fact that non-Newtonian behavior is not observed below 50% and (ii) to masking of the high-shear plateau by the onset of shear-thickening behavior. CONCLUSIONS The results of this rheological investigation, the first performed on water-swollen inverse

20C 150 100 50

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FIG. 5. Krieger-Dougherty fit of superposed non-Newtonian data at 05 = 58.3%.

Journal of Colloid and Interface Science, Vol. 140, No. 2, December 1990

i

i

i

,

i

i

0.1

0.2

0.3

0.4

0.5

O.b

FIG. 6. Test of Krieger-Dougherty volume fraction equation fitted to low- and high-shear limiting viscosities. Lines represent Eq. [3] with [~] = 2.8; 050o= 0.64 and 05p~= 0.78; O, valuesof Table III; A, sample 1; ×, sample 2; U], sample 3.

microlatices, differ in two respects from the hard-sphere-like behavior previously reported for conventional latices. The first difference is the range of Newtonian behavior, which extends up to a volume fraction of ca. 50% in contrast with the limit of ca. 25% previously reported. The second respect is the values of the packing fractions 4~p, here found to be 64 _+ 1% for the low-shear limit and 78 _+ 3% for the high-shear limit, whereas Papir and Krieger reported 52 and 68%, respectively (16). Note that our experiments were performed over a wider concentration range (q5 up to 61% against 50% in Ref. ( 16 )), which should allow a more accurate determination of q~p. To account however for these differences, one could invoke the fact that the volume fraction of the polymer particles is smaller than that of the initial aqueous phase and consider that the effective continuous phase consists of the oil and the small micelles. However, micelles are known to behave like hard spheres

RHEOLOGY OF INVERSE LATICES

with respect to the rheological properties. Furthermore, for volume fractions of the disperse phase up to 50%, the values of n~0 are comparable to those found by Papir and Krieger on monodisperse nonaqueous dispersions (16), as shown in Table III. Except for the differences cited above, therefore, these inverse microlatices follow quite closely the theological behavior expected for hard-sphere dispersions, with respect to the influence of volume fraction and shear stress. ACKNOWLEDGMENTS The authors thank Drs, J. Francois, F. Kern, and D. Collin for their valuable help in some of the experiments and D. Quemada for stimulating discussions. One of us (P.B.) acknowledges (NORSOLOR) ORKEM Group Co. for financial support. REFERENCES 1. Krieger, I. M., Adv. Colloid Interface Sci. 3, 111 (1972). 2. Hoffman, R. L., in "Science and Technology of Polymer Colloids" (G. W. Poehlein, R. H. Ottewill, and J. W. Goodwin, Eds.), NATO ASI Series E, No. 68, p. 570. Nijhoff, Boston/The Hague/Dordrecht / Lancaster, 1983. 3. De Kruif, C. G., Van Iersel, E. M. F., Vrij, A., and Russel, W. B., J. Chem. Phys. 83, 4717 (1985). 4. Quemada, D., J. Theor. AppL Mech., Special Issue, pp. 267, 289 (1985).

473

5. Krieger, I. M., in "Future Directions in Polymer Colloids" (M. E1-Aasserand R. M. Fitch, Eds.), NATO ASI Series E, No. 138, p. 119. Nijhoff, Dordrecht/ Boston/Lancaster, 1987. 6. Russel, W. B., ibid., p. 131. 7. Candau, F., Zekhnini, Z., and Durand, J. P., J. Colloid Interface Sci. 114, 398 (1986). 8. Holtzscherer, C., and Candau, F., J. Colloidlnterface Sci. 125, 97 (1988). 9. See, for example, Bellocq, A. M., Biais, J., Bothorel, P., Clin, B., Fourche, G., Lalanne, P., Lemaire, B., Lemanceau, B., and Roux, D., Adv. Colloid Interface Sci. 20, 167 (1984). 10. Striven, L. E., Nature (London) 263, 123 (1976). 11. Friberg, S., Lapczynska, I., and Gillberg, G., J. Colloid Interface Sci. 56, 19 (1976). 12. Candau, F., Leong, Y. S., Pouyet, G., and Candau, S. J., J. Colloid Interface Sci. 161, 167 (1984). 13. Candau, F., and Buchert, P., Colloids Surf, in press. 14. Holtzscherer, C., and Candau, F., Colloids Surf. 29, 411 (1988). 15. Buchert, P., and Candau, F., J. Colloid Interface Sci. 136, 527 (1990). 16. Papir, Y. S., and Krieger, I. M., J. Colloid Interface Sci. 34, 126 (1970). 17. Choi, G. N., and Krieger, I. M., J. Colloid Interface Sei. 113, 101 (1986). 18. Hoffman, R. L., Trans. Soc. Rheol. 16, 155 (1972); 46, 491 (1974). 19. Woods, M. E., and Krieger, I. M., J. Colloid Interface Sci. 34, 91 (1970). 20. Williamson, R. V., J. Rheol. l, 283 (1930). 21. Krieger, I. M., and Dougherty, T. J., Trans. Soc. Rheol. 3, 137 (1959).

Journalof Colloidand InterfaceScience, Vol.140,No. 2, December1990