Effect of Surfactant on the Viscoelastic Behavior of Semidilute

Langmuir 2000, 16, 8611-8621

8611

Effect of Surfactant on the Viscoelastic Behavior of Semidilute Solutions of Multisticker Associating Polyacrylamides† Enrique Jimeńez-Regalado,‡ Joseph Selb, and Franç oise Candau* Institut Charles Sadron (CRM), 6, rue Boussingault 67083 Strasbourg Cedex, France Received February 7, 2000. In Final Form: June 5, 2000 The interactions between well-defined hydrophobically modified polyacrylamides (HMPAM) and the anionic surfactant SDS (sodium dodecyl sulfate) or the cationic surfactant DTAB (dodecyltrimethylammonium bromide) were studied in aqueous solution using steady-flow and oscillatory rheological experiments. The structure of the HMPAM used consists of randomly distributed blocks of hydrophobic monomers (N,N-dihexylacrylamide) in the polyacrylamide backbone. A thorough investigation of the rheological behavior of HMPAM/surfactant mixtures as a function of polymer concentration and of different molecular parameters (molecular weight, hydrophobe content, and hydrophobic block length) provides some novel insight into the surfactant binding properties of multisticker chains. The experiments were performed on series of HMPAM located both in unentangled and entangled semidilute regimes. In both regimes, the rheological behavior strongly depends on the level of surfactant addition, with first an increase and then a decrease in the values of the zero-shear viscosity η0 and of the terminal relaxation time TR without significant change in the plateau modulus G0. The latter observation that differs from most of the other reported conclusions suggests that the viscosity enhancement is not due to the formation of additional mixed plurifunctional aggregates, but to an increase in the lifetime of the preexisting cross-links resulting from surfactant binding. At high surfactant concentrations (J0.05 M), the solubilization of the individual hydrophobic blocks by the surfactant micelles leads to a decrease of G0, η0, and TR and the HMPAM lose their associative properties. The results also suggest that the maximum of viscosity corresponds to an optimum surfactant binding which is about the same, whatever the concentration of hydrophobic units in the HMPAM solution. SDS has a much larger effect on the η0 variation than DTAB, likely due to a different lifetime of the mixed aggregates.

Introduction The interactions of hydrophobically modified watersoluble polymers (HMWSP) with small surfactant molecules has become the object of extensive research in recent years.1-4 This interest stems from the need in industrial applications to accurately control the solution viscosity under a variety of shear conditions. These polymers consist of a long hydrophilic chain to which small amounts of hydrophobic substituents are incorporated as pendant chains, blocks, or terminal groups.5-13 Aqueous solutions † Part of the Special Issue “Colloid Science Matured, Four Colloid Scientists Turn 60 at the Millennium”. ‡ Present address: Centro de Investigacion en Quimica Aplicada (CIQA), 146 Blvd Enrique Reyna Hermosillo, 25100 Saltillo Coahuila, Mexico. * To whom correspondence should be addressed: Tel.: (33) 388 41 40 38. Fax: (33) 388 41 40 99. E-mail: [email protected], [email protected].

(1) Interactions of surfactants with polymers and proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (2) Hansson, P.; Lindman, B. Curr. Opin. Colloid Interface Sci. 1996, 1, 604. (3) Winnik, F. M.; Regismond, S. T. A. Colloids Surf. A, Physicochem. Eng. Aspects 1996, 118, 1. (4) Polymers-Surfactants Systems; Kwak, J. C. T.; Ed.; Surfactant Science Series, 77; Dekker: New York, 1998. (5) Polymers in Aqueous Media: Performance through Association; Glass, J. E.; Ed.; Advances in Chemistry Series 223; American Chemical Society: Washington, DC, 1989. (6) Polymers as Rheology Modifiers; Schulz, D. N., Glass, J. E., Eds.; ACS Symposium Series 462; American Chemical Society: Washington, DC, 1991. (7) Water-Soluble Polymers. Synthesis, Solution Properties and Applications; Shalaby, S. W., McCormick, C. L., Butler, G. B., Eds.; ACS Symposium Series 467; American Chemical Society: Washington, DC, 1991.

of HMWSP exhibit unusual rheological properties which arise from intermolecular reversible associations of the hydrophobic groups. In the presence of an ionic surfactant with a high affinity for the hydrophobic domains, the rheological behavior of these systems is governed by an intricate interplay between hydrophobic, hydrophilic, and ionic interactions. A demonstration of these interactions, common to all sorts of HMWSP investigated, is the presence of a maximum in the variation of the zero-shear viscosity as a function of surfactant concentration, occurring at a concentration close to the cmc of the pure surfactant solution.14-40 This effect has been ascribed to a strengthening of the hydrophobic interactions at low surfactant (8) Macromolecular Complexes in Chemistry and Biology; Dubin, P., Bock, J., Davies, R. M., Schulz, D. N., Thies, C., Eds.; Springer-Verlag: Berlin, 1994. (9) Hydrophilic Polymers: Performance with Environmental Acceptability; Glass, J. E., Ed.; Advances in Chemistry Series 248; American Chemical Society: Washington, DC, 1996. (10) Winnik, M. A.; Yekta, A. Curr. Opin. Colloid Interface Sci. 1997, 2, 424. (11) Rubinstein, M.; Dobrynin, A. V. Trends Polym. Sci. 1997, 5, 181. (12) Larson, R. G. The Stucture and Rheology of Complex Fluids; Oxford University Press: New York, 1999. (13) Rubinstein, M.; Dobrynin, A. V. Curr. Opin. Colloid Interface Sci. 1999, 4, 83. (14) Gelman, R. A. Int. Dissolving Pulps Conf. 1987, 159. (15) Sau, A. C.; Landoll, L. M. In Polymers in Aqueous Media: Performance through Association; Glass, J. E., Ed.; Advances in Chemistry Series 223; American Chemical Society: Washington, DC, 1989; Chapter 18, p 343. (16) Tanaka, R.; Meadows, J.; Phillips, G. O.; Williams, P. A. Carbohydr. Polym. 1990, 12, 443. (17) Lundberg, D. J.; Glass, J. E.; Eley, J. E. J. Rheol. 1991, 35, 1255. (18) Iliopoulos, I.; Wang, T. K.; Audebert, R. Langmuir 1991, 7, 617. (19) Dualeh, A. J.; Steiner, C. A. Macromolecules 1990, 23, 251. (20) Biggs, S.; Selb, J.; Candau, F. Langmuir 1992, 8, 838.

10.1021/la000168y CCC: $19.00 © 2000 American Chemical Society Published on Web 09/06/2000

8612

Langmuir, Vol. 16, No. 23, 2000

concentration, due to a noncooperative binding of the surfactant, while at high surfactant concentration, the transient polymer network is disrupted. The maximum in solution viscosity was found to occur at about the same ratio of bound surfactant to HMWSP hydrophobe for several systems.41,42 Still the detailed mechanism behind these phenomena is not well understood and can be dependent on the nature of the HMWSP investigated, despite the universality of the experimental observation on the zero-shear viscosity. Hydrophobically modified water-soluble polymers can be classified into two main classes: (i) telechelic polymers which are linear poly(ethylene oxide) (PEO) chains endcapped by long alkyl chains (∼C12-C18); (ii) multisticker polymer chains in which the hydrophobic groups are randomly distributed as isolated units or as small blocks along the hydrophilic backbone. Some features of the rheological behavior of telechelic polymers are reasonably well understood thanks to recent theoretical and experimental work.25,43-48 In particular, the respective contributions of the plateau modulus and the lifetime of the cross-links to the variation of the zeroshear viscosity with surfactant concentration have been unambiguously assessed.45,46 The understanding of the rheological properties of multisticker systems is not as advanced as that of telechelic systems. A case that has been considered theoretically is (21) Magny, B.; Iliopoulos, I.; Audebert, R.; Piculell, L.; Lindman, B. Prog. Colloid Polym. Sci. 1992, 89, 118. (22) Binana-Limbele, W.; Clouet, F.; Franç ois, J. Colloid Polym. Sci. 1993, 271, 748. (23) Chang, Y.; Lochhead, R. Y.; McCormick, C. L. Macromolecules 1994, 27, 2145. (24) Senan, C.; Meadows, J.; Shone, P. T.; Williams, P. A. Langmuir 1994, 10, 2471. (25) Huldeń, M. Colloids Surf. A, Physicochem. Eng. Aspects 1994, 82, 263. (26) Guillemet, F.; Piculell, L.; Nilsson, S.; Djabourov, M.; Lindman, B. Prog. Colloid Polym. Sci. 1995, 98, 47. (27) Guillemet, F.; Piculell, L. J. Phys. Chem. 1995, 99, 9201. (28) Hogen-Esch, T. E.; Amis, E. J. Trends Polym. Sci. 1995, 3, 98. (29) Nystro¨m, B.; Thuresson, K.; Lindman, B. Langmuir 1995, 11, 1994. (30) Xie, X.; Hogen-Esch, T. E. Macromolecules 1996, 29, 1734. (31) Ka¨stner, U.; Hoffmann, H.; Do¨nges, R.; Ehrler, R. Colloids Surf. A, Physicochem. Eng. Aspects 1996, 112, 209. (32) Aubry, T.; Moan, M. J. Rheol. 1996, 40, 441. (33) Thuresson, K.; Lindman, B.; Nystro¨m, B. J. Phys. Chem. B 1997, 101, 6450. (34) Petit, F.; Iliopoulos, I.; Audebert, R.; Szo¨nyi, S. Langmuir 1997, 13, 4229. (35) Kulicke, W. M.; Arendt, O.; Berger, M. Colloid Polym. Sci. 1998, 276, 617. (36) Kopperud, H. M.; Hansen, F. K.; Nystro¨m, B. Macromol. Chem. Phys. 1998, 199, 2385. (37) Seng, W. P.; Tam, K. C.; Jenkins, R. D. Colloids Surf. A., Physicochem. Eng. Aspects 1999, 154, 365. (38) Panmai, S.; Prudhomme, R. K.; Peiffer, D. G. Colloids Surf. A, Physicochem. Eng. Aspects 1999, 147, 3. (39) Kaczmarski, J. P.; Tarng, M. R.; Ma, Z. Y.; Glass, J. E. Colloids Surf. A, Physicochem. Eng. Aspects 1999, 147, 39-53. (40) Yang, Y.; Schulz, D.; Steiner, C. A. Langmuir 1999, 15, 4335. (41) Piculell, L.; Thuresson, K.; Ericsson, O. Faraday Discuss. 1995, 101, 307. (42) Piculell, L.; Guillemet, F.; Thuresson, K.; Shubin, V.; Ericsson, O. Adv. Colloid Interface Sci. 1996, 63, 1. (43) Tanaka, F.; Edwards, S. F. Macromolecules 1992, 25, 1516. (44) Annable, T.; Buscall, R.; Ettelaie, R.; Whittlestone, D. J. Rheol. 1993, 37, 695. (45) Annable, T.; Buscall, R.; Ettelaie, R.; Shepherd, P.; Whittlestone, D. Langmuir 1994, 10, 1060. (46) Annable, T.; Buscall, R.; Ettelaie, R. Colloids Surf. A, Physicochem. Eng. Aspects 1996, 112, 97. (47) Jenkins, R. D.; Silebi, C. A.; El-Aasser, M. S. In Polymers as Rheology Modifiers; Schulz, D. N., Glass, J. E., Eds.; ACS Symposium Series 462; American Chemical Society: Washington, DC, 1991; Chapter 13, p 222. (48) Tam, K. C.; Jenkins, R. D.; Winnik, M. A.; Basset, D. R. Macromolecules 1998, 31, 4149.

Jime´ nez-Regalado

that of many associating sites randomly distributed along a chain at concentrations large enough that the chains are entangled. In that limit, the rheology of the systems is described by a sticky reptation model.49 The plateau modulus is then mainly controlled by entanglements and should be insensitive to the effect of surfactant. From the experimental point of view, the results do not lead to clearcut conclusions. It has generally been argued that the addition of surfactant promotes initially the formation of new cross-links resulting in an increase of both plateau modulus and zero-shear viscosity. However, this statement is often deduced from measurements of the storage modulus at one given frequency, which might lead to an erroneous conclusion as discussed later in this paper. A further difficulty which appears as for example in the case of HMWSP based on some cellulose derivatives (ethylhydroxyethylcellulose, EHEC) is linked to the nonnegligible binding of the surfactant onto the backbone itself.29,33 Recently, we have reported a study on the rheological properties of well-defined hydrophobically modified polyacrylamides (HMPAM) prepared by a free-radical micellar polymerization technique.50,51 This process gives copolymers in which the hydrophobic units are incorporated as small blocks randomly distributed along the polyacrylamide backbone.52-55 The viscoelastic behavior of semidilute solutions of various series of copolymers with variable molecular weights, hydrophobe contents, and hydrophobic block lengths was investigated as a function of polymer concentration. Two different regimes could be clearly identified: a semidilute unentangled regime where the viscosity is controlled by intermolecular interactions and a semidilute entangled regime where it is essentially dominated by entanglements. In the latter regime, the results are quite well accounted for by a hindered reptation model.49 Such systems are particularly well suited to a study of their interactions with ionic surfactants because their structural parameters can be varied in a controlled manner51 and also because of the absence of specific interactions between surfactants and the polyacrylamide backbone.56 In this paper, we report measurements of linear and nonlinear viscoelasticity of various series of HMPAM in the presence of the anionic surfactant SDS (sodium dodecyl sulfate) or the cationic surfactant DTAB (dodecyltrimethylammonium bromide). The results obtained allow us to provide a better understanding of the mechanism underlying the interactions of surfactants with multisticker polymers. Experimental Section The synthesis of the samples has been described in detail in previous papers.51,52,57 The associating copolymers are polyacrylamides hydrophobically modified with a small amount of N,Ndihexylacrylamide (DiHexAM). They were synthesized in aqueous solution by using the micellar technique of Valint et al.58 with (49) Leibler, L.; Rubinstein, M.; Colby, R. H. Macromolecules 1991, 24, 4701. (50) Candau, F.; Jimeńez Regalado, E.; Selb, J. Macromolecules 1998, 31, 5550. (51) Jimeńez Regalado, E.; Selb, J.; Candau, F. Macromolecules 1999, 32, 8580. (52) Hill, A.; Candau, F.; Selb, J. Macromolecules 1993, 26, 4521. (53) Ezzell, S. A.; Hoyle, C. E.; Creed, D.; McCormick, C. L. Macromolecules 1992, 25, 1887. (54) Branham, K. D.; Davis, D. L.; Middleton, J. C.; McCormick, C. L. Polymer 1994, 35, 4429. (55) Candau, F.; Selb, J. Adv. Colloid Interface Sci. 1999, 79, 149. (56) Sabbadin, J.; Le Moigne, J.; Franç ois, J. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 2, p 1377. (57) Volpert, E.; Selb, J.; Candau, F. Macromolecules 1996, 29, 1452.

Multisticker Associating Polyacrylamides

Langmuir, Vol. 16, No. 23, 2000 8613

Table 1. Polymer Characteristics samplea

Mw (×10-3)

[H]b (mol %)

NHc

5PAM 1M1D3.2 1M2D3.2 1M2D7 5M05D3.2 5M1D3.2 5M1D5 5M1D7 5M2D3.2 14M1D3.2

480 115 140 160 450 420 450 460 420 1 400

0 1.02 1.96 2.03 0.48 1.0 0.96 1.0 2.0 0.95

0 3.2 3.2 7 3.2 3.2 5 7 3.2 3.2

a The sample code refers to the molecular characteristics of the polymers (see Experimental Section). b Hydrophobe content in the final copolymer. c Number of hydrophobes per micelle = hydrophobic block length.

sodium dodecyl sulfate (SDS) as the surfactant and 4,4′-azobis(4-cyanovaleric acid) (ACVA) as the initiator. In this process, the high density of hydrophobic molecules in the micelles favors their incorporation as blocks randomly distributed in the polyacrylamide backbone.52-55 Note that the use of N,N-dialkylacrylamides such as DiHexAM instead of N-monoalkylacrylamides was shown to lead to samples homogeneous in composition (i.e., all polymer chains have the same composition whatever the degree of conversion).57,59 Due to the strong influence of the presence of surfactant on the rheological properties of HMPAM in aqueous solution, it is very important to carefully purify the copolymer samples in order to remove all the surfactant used in the micellar polymerization process. This was achieved by a repeated redissolution (in water)/precipitation (in methanol) process (2-3 times). The characteristics of the samples investigated are given in Table 1. The hydrophobe content [H] in the copolymers was 0.5, 1, or 2 mol %. The hydrophobe/surfactant molar ratio in the reaction mixture was adequately adjusted in order to get the initial number of hydrophobes per micelle, NH, ranging from 3.2 to 7.51 The length of the hydrophobic blocks in the copolymer is assumed to correspond roughly to NH.55 The molecular weight was varied from ≈105 to 1.4 × 106 by using mercaptoethanol as a chain transfer agent, which behaves like an ideal transfer agent for polyacrylamide (polydispersity index Mw/Mn ≈ 2).51 The number-average degree of polymerization N was taken as Mw/ 2m (m ) 71, molecular weight of the acrylamide). The weightaverage molecular weight Mw and the composition of the samples were determined by light scattering and 1H NMR as previously described.57 It should be noted that for copolymers with the strongest hydrophobic character ([H] > 1 mol % or NH > 3), NMR measurements in pure D2O do not give reliable results (DiHexAM contents lower than expected from the monomer feed composition). A similar behavior was previously reported for other amphiphilic polymers60-62 or for block copolymers in a poor solvent for one block,63 and was ascribed to a “freezing” of the poorly solvated component. This problem was overcome by performing NMR measurements on HMPAM in a 85/15 wt/wt DMSO-d6/ D2O mixture, which led to a DiHexAM content corresponding within the experimental error to that in the monomer feed. The sample code of the copolymers refers to the molecular weight Mw (1, 5, and 14 stand for ≈140 000 ((20 000), ≈450 000 ((20 000), and 1 400 000, respectively), to the content in hydrophobic comonomer [H], and to the NH value (i.e., = the length of the hydrophobic block). For example, 1M1D3.2 stands (58) Valint, P. L., Jr.; Bock, J.; Schulz, D. N. In Polymers in Aqueous Media: Performance through Association; Glass, J. E., Ed.; Advances in Chemistry Series 223; American Chemical Society: Washington, DC, 1989; Chapter 21, p 399. (59) Candau, F.; Volpert, E.; Lacik, I.; Selb, J. Macromol. Symp. 1996, 111, 85. (60) Renoux, D. The`se, Universite´ Louis Pasteur, Strasbourg, France, 1995. (61) Pabon, M. The`se, Universite´ Louis Pasteur, Strasbourg, France, 1997. (62) Yassini, M.; Hogen-Esch, T. E. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1994, 35 (1), 478. (63) Liu, Q.; Wilson, G. R.; Davis, R. M.; Riffle, J. S. Polymer 1993, 34, 3030.

for a copolymer with a molecular weight of 115 000 containing 1 mol % of DiHexAM and synthesized with a initial number of hydrophobes per micelle equal to 3.2 (see Table 1). For the study of polymer/surfactant mixed systems, the solutions were prepared as follows. Stock aqueous solutions of pure polymers were first prepared at the desired concentration (0.5 wt % e C e 7 wt %) in deionized water, and then gently stirred for 2 days. Appropriate amounts of SDS (Acros 99%) or DTAB (Fluka 98%) were directly added to a fraction of the stock polymer solutions to obtain stock polymer/surfactant solutions with the highest surfactant concentrations, CS (typically CS ≈ 0.5-1 wt % = 20-35 mM), that were homogenized by stirring for 1 day. Solutions at various lower surfactant concentrations were prepared by mixing the required weighted amounts of surfactant-free and surfactant-containing stock polymer solutions, and the mixtures were allowed to equilibrate under mild stirring for one further day. Note that the aqueous solutions of HMPAM investigated with or without surfactant are in all cases perfectly homogeneous and transparent in the range of concentrations investigated. The critical micellar concentration (cmc) and aggregation number (Nagg) of the SDS and DTAB at 25 °C are cmcSDS ) 8 mM, Nagg,SDS ) 75 and cmcDTAB ) 16 mM, Nagg,DTAB ) 63, respectively.64,65 Linear viscoelasticity experiments were performed at 25 °C on samples that were viscous enough to provide a meaningful analysis with a Haake RS100 controlled stress rheometer equipped with a cone-plane geometry (angle 1°, diameter 20, 35, or 60 mm depending on the sample viscosity). Flow experiments were carried out with the same rheometer as above or with a Contraves LS30 low shear rheometer, depending on the sample viscosity. More details on the experimental procedures are given elsewhere.57,66

Experimental Results (a) Zero-Shear Viscosity. The zero-shear viscosity, η0, was determined from the zero-shear limit of the viscosity η obtained in the steady-shear flow experiments. Figure 1a-c shows some typical variations of η0 versus SDS concentration. The behavior is that already reported for many associating systems, namely a maximum of η0 at a surfactant concentration CS,max. A systematic investigation as a function of the polymer concentration, C, and of the different molecular parameterssthe molecular weights Mw, the hydrophobe content [H], and the hydrophobic block length NHsallows us to draw the following conclusions: (i) Upon increasing C, CS,max is shifted to higher values with a broadening of the peak (Figure 1a). (ii) The position of CS,max with respect to the cmc of the pure surfactant decreases upon increasing the molecular weight (Figure 1a,b, Mw ≈ 125 000 and 1c, Mw ≈ 440 000) but increases with the hydrophobe content (i.e., when the spacing decreases between hydrophobic blocks) (Figure 1b). It does not seem to vary significantly with NH (Figure 1c). (iii) The amplitude of the viscosity peak is an increasing function of polymer concentration (Figure 1a), hydrophobe content (Figure 1b), and hydrophobic block length NH (Figure 1c). As a matter of fact, in a number of systems that were already very viscous without SDS, the viscosity at CS,max was too high to be measured experimentally. This was the case for systems with molecular weights Mw g 5 × 105 and/or with NH ) 5-7 and/or [H] ) 2% at polymer concentrations larger than 2%. The same trends are observed in the presence of DTAB (Figure 2a-c). The maximum of viscosity occurs at a surfactant concentration smaller or equal to the cmc of (64) Mukerjee, P.; Mysels, K. Critical Micelle Concentration of Aqueous Surfactant Systems; NBS: Washington, DC, 1971. (65) Malliaris, A.; Le Moigne, J.; Sturm, J.; Zana, R. J. Phys. Chem. 1985, 89, 2709. (66) Volpert, E.; Selb, J.; Candau, F. Polymer 1998, 39, 1025.

8614


Figure 1. Effect of SDS concentration on the zero-shear viscosity of aqueous HMPAM solutions: (a) 1M1D3.2 at the polymer concentrations C indicated; (b) copolymers with different hydrophobe contents (C ) 3 wt %), (b) 1M1D3.2; (9) 1M2D3.2; (c) copolymers with different microstructures (C ) 1 wt %), (9) 5M1D3.2; (b) 5M1D5; ([) 5M1D7.

DTAB. One also notices that the enhancement of viscosity resulting from the addition of DTAB is much less pronounced than for SDS (≈1 to 3 orders of magnitude). These results confirm the conclusion of other studies stating that the binding mechanism onto the hydrophobic moieties depends strongly on the nature of the surfactant.33,37,38,67-69 The surfactant concentration range corresponding to the data of Figures1 and 2 is rather limited. Experiments have been performed on some typical systems in a much broader range. They show that the viscosity continues to

Jime´ nez-Regalado

Figure 2. As in Figure 1 except the surfactant is DTAB.

decrease upon increasing surfactant concentration, CS, and tends progressively toward the viscosity of the unassociated system. This is illustrated in Figure 3 showing the variation of η0 with SDS concentration for 5M1D5 sample. It can be seen that the viscosity is about the same as that without surfactant for CSDS ≈ 0.015 M and becomes close to that of the pure PAM analogue for CSDS J 0.05 M. The slightly higher viscosity for the copolymer could be due to a polyelectrolyte-like behavior (67) Goddard, E. D. Colloids Surf. 1986, 19, 255. (68) Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; Chapter 5, p 203. (69) Zhang, K.; Xu, B.; Winnik, M. A.; Macdonald, P. M. J. Phys. Chem. 1996, 100, 9834.


Figure 3. Effect of SDS concentration on the zero-shear viscosity of 1 wt % aqueous solutions of 5M1D5 copolymer sample (b) and of the homopolyacrylamide analogue 5PAM (O).

Figure 4. Storage (G′) and loss (G′′) moduli as a function of circular frequency for a 2 wt % aqueous solution of 5M1D3.2 at a low SDS concentration (CSDS ) 2 mM) and at CSDS ) CS,max ) 8 mM.

Langmuir, Vol. 16, No. 23, 2000 8615

Figure 5. Viscosity versus shear rate for a 5 wt % aqueous solution of 1M1D3.2 at the SDS concentrations indicated.

C* is the overlap critical concentration of the polymer) exhibit the classical behavior of viscoelastic polymeric systems,51 that is, a Newtonian plateau followed by a shear-thinning behavior beyond a critical shear rate γ˘ c (Figure 5). In systems with C* e C e 5C*, the variation of viscosity vs shear rate, γ˘ , shows a classical signature of associating systems, that is, a pronounced shearthickening effect immediately followed by a shear-thinning. This is illustrated in Figure 6a,b, which shows the η(γ˘ ) variations for 1M2D7 sample (C ) 2 wt %) in the presence of increasing amounts of either SDS or DTAB. It can be seen that the amplitude of the shear-thickening is not strongly dependent on the surfactant concentration. On the other hand, the shear rate corresponding to the onset of shear-thinning first decreases until CS ) CS,max and then increases again. Obviously, the inverse critical shear rate γ˘ c-1 follows qualitatively the behavior of the zero-shear viscosity. This observation is quite general and applies to all the systems that we have investigated. Discussion

of the copolymer/ionic surfactant complex. The slight increase in viscosity for both samples at very high surfactant concentration is simply due to the contribution from the pure surfactant micelles (actually, decreasing specific viscosity values are obtained). (b) Linear Viscoelasticity. The main effect of the addition of SDS to the copolymer solutions of HMPAM investigated here is to slow the relaxation process. This is illustrated in Figure 4 that shows the comparison between the frequency dependence of the storage and loss moduli for two solutions of 5M1D3.2 sample at C ) 2% in the presence of respectively 2 mM SDS and 8 mM SDS (≡ CS,max). The shapes of the representative curves remain quite similar but there is a shift of the frequency scale of about 20-fold. In fact, a detailed analysis of the viscoelastic spectra shows that the relaxation is slowed progressively from CS ) 0 to CS ) CS,max and speeds up again at CS > CS,max. Qualitatively, this means that the terminal time variation follows that of the zero-shear viscosity.70 It must be stressed that this behavior is observed for all the systems investigated in the presence of both SDS or DTAB. (c) Nonlinear Viscoelasticity. The shear rate dependence of the viscosity η is strongly dependent on the location of the investigated system in the concentration diagram. Systems with C . C* (typically C J 5C*, where (70) Guillemet, F. The`se, Universite´ Pierre et Marie Curie (Paris VI), Paris, France, 1995.

The most characteristic feature of surfactant interactions with hydrophobically modified polymers is the presence of a maximum in the variation of the zero-shear viscosity as a function of surfactant concentration for a given polymer concentration.14-40 This effect has been ascribed to the formation of mixed micellar-type aggregates containing alkyl chains belonging to the polymer and the surfactant molecules and the scenario generally accepted is the following: (i) At low surfactant concentration (CS < CS,max), there is a noncooperative binding of the surfactant to the hydrophobic entities leading to the formation of both mixed unifunctional and plurifunctional aggregates. In a mixed unifunctional aggregate, only one hydrophobic entity or two entities belonging to the same polymer molecule are incorporated. These mixed aggregates do not contribute to the high-frequency elasticity of the system. A mixed plurifunctional aggregate contains several hydrophobic entities from different chains and therefore acts as a crosslink. The latter can be obtained either by a bridging effect of the surfactant onto several associating sequences initially isolated, or by binding of the surfactant onto a preexisting cross-link. In both cases, this leads to a strong enhancement of the viscosity of the system. (ii) At high surfactant concentration (CS > CS,max), there is a general consensus that the formation of mixed unifunctional aggregates and pure surfactant micelles are

8616


favored at the expense of plurifunctional aggregates, accounting for the large decrease in viscosity. These two opposite effects lead to the maximum in viscosity observed. Concerning the low surfactant concentration range, the data reported do not always support convincingly the above conclusions and the issue of whether the viscosity enhancement is due to the increase of cross-link density or to an increase in the lifetime of the cross-links is still open. As a matter of fact, a detailed description of the association mechanism is difficult to assess from the viscosity data as η0 ) G0TR depends on both the plateau modulus G0 and the terminal time TR, and the respective role of these two parameters is not clearly understood so far. Furthermore, the dynamics of the systems strongly depend on the concentration regime (entangled, nonentangled) and on the microstructure of the copolymers (telechelic, multistickers, ...). For aqueous solutions of telechelic associating polymers like the so-called HEUR (hydrophobically ethoxylated urethane) that are quasi-Maxwellian fluids, the respective contributions of G0 and TR to the viscosity behavior have been unambiguously characterized.45,46 For systems at concentrations close to the critical overlap concentration C*, both the plateau modulus and the relaxation time go through a maximum as a function of surfactant concentration. As the elasticity of these systems is mainly controlled by the cross-link density, this behavior indicates that at low surfactant concentration, new plurifunctional aggregates form by bridging dangling chains and loops but, as the surfactant concentration increases, the probability of finding simultaneously several associating end groups within the same aggregate decreases rapidly. This promotes the formation of a higher number of unifunctional aggregates and a decrease of the modulus. At high polymer concentrations (C . C*) where loops and dangling chains are rare to begin with, the plateau modulus decreases monotonically upon increasing CS whereas the relaxation time still exhibits a maximum. Multisticker associating polymers are generally made of much longer chains than telechelic systems and in the semidilute regime form entanglements that contribute significantly to the rheology of the solution. This was shown in a previous study for the systems investigated here.51 Rheology experiments allowed us to define three different regimes: (i) A dilute regime, C < Cη, where the chains are isolated and the viscosity is essentially controlled by intramolecular interactions. This regime does not significantly differ from that of unmodified polymers. (ii) A semidilute unentangled regime, Cη < C < CT. The concentration Cη is independent of NH and on [H] and corresponds approximately to the overlap concentration C* of the unmodified polymer analogues. In this regime, the associations are mostly intramolecular in the vicinity of C* and mostly intermolecular in the vicinity of CT where in addition the entanglements appear. The chains are likely to obey Rouse dynamics. (iii) A semidilute entangled regime, C > CT. The break CT occurs at a concentration close to the critical concentration Ce where the unmodified polymer chains are entangled. The asymptotic behavior obeys the reptation prediction with in particular a C4 dependence of the zeroshear viscosity. This indicates that in this regime the rheological behavior is totally dominated by entanglements. In the present study, we have investigated the effect of surfactant on the rheological properties of systems located

Jime´ nez-Regalado

Figure 6. Viscosity versus shear rate for a 2 wt % aqueous solution of 1M2D7 at the surfactant concentrations indicated: (a) SDS; (b) DTAB.

both in unentangled and entangled semidilute regimes, with the aim of analyzing separately the contributions of G0 and TR. (a) Effect of Surfactant on the Elastic Behavior. The analysis of the viscoelastic data is difficult because the behavior of the solutions is non-Maxwellian. In particular, there is no marked plateau modulus. It is, however, possible to study qualitatively the effect of surfactant addition by comparing the viscoelastic spectra of systems at different surfactant concentrations. The comparison is illustrated in Figure 7a-c, where are superposed the viscoelastic spectra obtained for systems with a SDS concentration CS,max corresponding to the maximum in viscosity and those with a lower amount of SDS. The spectra have been shifted horizontally in a loglog scale to bring into coincidence the two crossing frequencies ωC, where G′(ω) ) G′′(ω). Figure 7a refers to the system 1M1D3.2 (C ) 6 wt % with C/Ce ≈ 1.2 and C/C* ≈ 7.5)51 that is an entangled solution whose viscoelasticity is described by the reptation model. In Figure 7b are reported the data for the system 1M1D3.2 (C ) 4 wt %, with C/Ce ≈ 0.8 and C/C* ≈ 5),51 that belongs to the semidilute unentangled regime but with a concentration close to the crossover between unentangled and entangled solutions. Finally, Figure 7c shows the results obtained for the system 5M1D5 (C ) 1 wt %, C/Ce ≈ 0.5 and C/C* ≈ 2),51 located in the unentangled regime but at a concentration close to C*. For the three systems, one observes a good superposition of the data at least in the frequency range ω e ωC despite a rather large effect of the


Langmuir, Vol. 16, No. 23, 2000 8617

Figure 8. Effect of SDS concentration on the terminal time TR (b) and plateau modulus G0 (O) for a 3 wt % aqueous solution of 1M2D3.2.

relaxation process up to CS,max, then to speed it up without affecting the elastic behavior. A similar conclusion was reached by Guillemet for cellulose derivatives.70 It must be stressed that a plot of the storage modulus at a fixed (low) frequency versus CS would lead to a maximum because of the shift of the relaxation frequency.24,29,31,37,42,71,72 This is what has been mostly reported in the literature with an erroneous conclusion in the elastic behavior of these systems. We have previously shown that for systems with terminal times short enough, the variations of G′(ω) and G′′(ω) in the low-frequency range were well described by the Maxwellian model; that is, they vary like ω2 and ω, respectively.51 In that case, it is possible to estimate from the low-frequency behavior the plateau modulus G0 and the terminal time TR:

TR ) lim ωf0

Figure 7. Superposition of the viscoelastic spectra under an horizontal shift for aqueous solutions of HMPAM in the presence of SDS (a) 6 wt % 1M1D3.2 (aω ) 14.7); (b) 4 wt % 1M1D3.2 (aω ) 1.5); (c) 1 wt % 5M1D5 (aω ) 7.5). The shift factor aω ) ωC/ωC,max is the ratio of the crossing frequency for the sample at the SDS concentration indicated to that at CS,max.

SDS content on the crossing frequency ωC. Deviations are only observed in the range ω > ωC where both local fluctuations and effects of the lifetime of the cross-links become operative. A good superposition of the viscoelastic spectra is similarly obtained in the whole SDS concentration range from CSDS , CS,max to CSDS . CS,max. In particular, the spectra of two systems with about the same viscosity but located on either side of the viscosity maximum almost superpose each other without any frequency shift (results not shown). These results indicate that the main effect of gradual addition of surfactant is first to slow down the

G′ (ω1 G′′ );

G0 )

1 G′′ lim TRωf0 ω

( )

Figure 8 shows the variation of these parameters as a function of SDS concentration, CSDS, for the system 1M2D3.2 (C ) 3 wt %) located in the semidilute unentangled regime. Within the experimental accuracy, G0 is independent or slightly increasing with CSDS whereas the terminal time goes through a maximum like the zeroshear viscosity, supporting thus the conclusion given above. These results are indeed expected for systems located in the reptation regime where the effect of entanglements is predominant over that of the plurifunctional aggregates (Figure 7a). Under the assumption of binary temporary cross-links and of a good solvent for the backbone, the sticky reptation model49 predicts for such systems:

G0 ≈ C9/4 TR ≈ C15/8N7/2[S]2τ(1 - 9/p + 12/p2)-1 where N is the number of monomers, [S] the molar ratio of stickers with respect to the total number of monomers, τ the lifetime of the aggregates, and p the fraction of stickers engaged into a cross-link. The plateau modulus only depends on the polymer concentration whereas the terminal time might be affected through the lifetime of the plurifunctional aggregates and/or through p. (71) Tanaka, R.; Meadows, J.; Williams, P. A.; Phillips, G. O. Macromolecules 1992, 25, 1304. (72) Deguchi, S.; Kuroda, K.; Akiyoshi, K.; Lindman, B.; Sunamoto, J. Colloids Surf. A, Physicochem. Eng. Aspects 1999, 147, 203.

8618


The thickening mechanism for unentangled multisticker systems, that is for C < Ce, is more complicated and not completely understood. A model describing the RouseZimm dynamics of nonentangled semidilute solutions of associating polymers was recently proposed by Rubinstein end Semenov.73 The dynamical behavior of these systems involves two characteristic time scales: the effective lifetime of a strand of the network and the chain relaxation. The modulus G0 associated with the latter process is the Rouse modulus of the chain which is proportional to the concentration of chains C/N. This modulus is independent of the hydrophobic characteristics of the associating polymers and is the same as that of the unmodified homopolymer. Thus, the experimental observation that G0 is independent of SDS concentration is at first sight in accordance with the theoretical prediction. However, to ascertain this conclusion, it would be important to measure the plateau modulus for systems with different C/N and different hydrophobic characteristics. This was not possible for systems without surfactant because the terminal time was too short to be determined reliably.51 The slowing down produced by addition of surfactant brings the terminal time within the experimental frequency window. Therefore, we have been able to measure G0 for different systems at CS,max. The results are reported in Figure 9, a and b, where G0 is plotted as a function of C/N and C, respectively. They show a rather complex behavior. For systems with [H] ) 1 mol %, G0 varies approximately as (C/N)1.2 or C2.6, with however, a scattering of the data that does not allow us to ascertain which of C/N or C is the relevant variable. For the systems with [H] ) 2 mol %, G0 exhibits a much steeper dependence of C/N or C and furthermore seems to increase with NH. The latter observation suggests that G0 is not simply the Rouse modulus of the chain but that it depends on the density of cross-links. Therefore, the behavior of G0 in the unentangled regime might mean that the added SDS only decorates the hydrophobic blocks without modifying the fraction of blocks engaged into plurifunctional aggregates. The maximum of the zero-shear viscosity could then be assessed to a maximum in the lifetime of plurifunctional aggregates due to the SDS binding. This scheme only holds in a limited range of SDS concentrations. It can be seen in Figure 10 that for a molar ratio of CSDS/CH of about 6 (where CH is the molar concentration of hydrophobic units in the solution), one tends to recover the viscosity of associating polymers without SDS. With an excess of SDS, the viscosity drops to that of the unmodified polymers (cf. Figure 3). This effect which is ascribed to the solubilization of the individual hydrophobic blocks by the surfactant micelles is discussed in subsection b. Thus, the results show unambiguously that the maximum of zero-shear viscosity is due to a maximum of the terminal time without significant change of the plateau modulus. It is only at much higher surfactant concentrations that the shear modulus is likely to decrease together with the viscosity as the system loses its associative properties. (b) Surfactant Binding Mechanism. The correlation between the binding process and the maximum of viscosity is difficult to assess. In the limit of very low surfactant concentrations, CS , CS,max, the bound surfactant is in equilibrium with free surfactant. In the opposite limit, CS . CS,max, the surfactant forms mainly micelles either pure or containing solubilized hydrophobic blocks. What happens in between is not fully understood. One observation, at first sight puzzling, is that the maximum is located on (73) Rubinstein, M.; Semenov, A. N. Macromolecules 1998, 31, 1386.

Jime´ nez-Regalado

Figure 9. Variation of the plateau modulus as a function of (a) C/N for systems with different structural characteristics and located in the unentangled semidilute regime and (b) polymer concentration for the same systems.

Figure 10. Normalized zero-shear viscosity versus the molar ratio of surfactant concentration to the concentration of hydrophobic units.

either side of the cmc of the pure surfactant depending on the system (cf. Figures 1 and 2). In fact, as previously reported by Piculell et al., the relevant parameter controlling CS,max seems to be the hydrophobic content of the system.41,42 In Figure 11a,b are reported the variations of the molar ratio CS,max/CH as a function of CH for both types of systems (SDS and DTAB). In the same figure are reported the variations of cmc/CH of the pure surfactants as a function of CH. It can be seen that at low hydrophobe concentrations, CS,max is lower than the cmc, irrespective of the surfactant. In the limit of high values of CH, the variations of CS,max/CH and cmc/CH coincide for DTAB surfactant whereas CS,max is larger than the cmc in the case of SDS. The number of amphiphilic molecules per


hydrophobic unit does not strongly depend on the nature of the surfactant, despite the large difference between the cmc’s. It must be pointed out that CS,max represents the total surfactant concentration in the solution at the maximum and not the actual bound surfactant CS,bound which is CS,max - CS,free, where CS,free is the concentration of the free monomeric surfactant (CS,free e cmc). Therefore, it is only in the limit CS,max . cmc that CS,bound can be approximated to CS,max. This remark may explain the significantly higher values of CS,max/CH at low CH in the curves of Figures 11. From the asymptotic limit of these curves, it can be inferred that about 2 SDS molecules (1.5 DTAB) bind to one monomeric associating unit at CS,max, that is ≈6.5 SDS molecules (≈5 DTAB) for a block with NH ) 3.2. Then, a bifunctional aggregate would contain about 13 SDS molecules and about 10 DTAB molecules. These values are very close to that obtained from binding isotherms by Piculell et al. for hydrophobically modified hydroxyethyl cellulose (HMHEC) (ratio of bound surfactant to HMHEC hydrophobe at CS,max of around 3).41,42 Conductometry and fluorescence experiments performed on similar multisticker polymers have shown that the binding is progressive and noncooperative.20 One can imagine that the rheology of these systems can be described by the same approach with and without surfactant, but in the former case, the sticker is the surfactantdecorated-hydrophobic block. The initial addition of surfactant enhances progressively the binding, which is still limited at the maximum of viscosity to a few molecules per hydrophobic block (less than 6.5 for NH ) 3.2). The lifetime of the aggregates formed by these blocks should then be maximum. Beyond this concentration, as the hydrophobic blocks become enveloped by more and more surfactant molecules, the aggregates resemble more to classical micelles and the lifetime decays progressively toward that of a pure micelle. Eventually, when there is a large excess of surfactant, the latter forms mainly pure micelles, some of which solubilize individually a block, which then can no longer form cross-links. The system loses its associative properties as the hydrophobic blocks gain entropy by dispersing individually into micelles. The nature of the surfactant has a great influence on the viscosity enhancement of the solution. Figure 12 shows the variation of the ratio of the viscosity at the maximum ηmax over that at zero surfactant concentration ηS)0 as a function of the concentration of hydrophobic units in the solution, CH. There is a large scattering in the data meaning that CH is not the relevant parameter for the enhancement of the viscosity but that the microstructure of the copolymer plays also an important role. However, one clearly sees that SDS has a much larger effect than DTAB. This can be due to a lesser binding of DTAB as suggested by the results of Figure 11a,b or more likely to a different lifetime of the aggregates. Let us also note that a lesser binding of cationic surfactants with respect to anionic surfactants is generally observed.33,37,38,67-69 The size of the trimethylammonium headgroup of DTAB and the relative inability of cationic groups to interact with the hydration layer surrounding the polymer have been proposed to account for this behavior.68,74 In can also be remarked in Figure 12 that the enhancement of viscosity is not strongly dependent on the concentration of hydrophobic units at least in the high hydrophobe concentration range where the reptation scheme applies. This suggests that the maximum of viscosity corresponds to an optimum binding which is the same, whatever CH. The spacing (74) Witte, F. M.; Engberts, J. B. F. N. Colloids Surf. 1989, 36, 417.

Langmuir, Vol. 16, No. 23, 2000 8619

Figure 11. Variation of the ratio CS,max to the concentration of hydrophobic units CH as a function of CH for different series of HMPAM (C, wt %). The solid line represents the variation of cmc/CH of the pure surfactant. (a) SDS, (b) DTAB.

Figure 12. Normalized zero-shear viscosity at the maximum versus the concentration of hydrophobic units for the same series as in Figure 11 (same symbols).

between the hydrophobic blocks and their length are likely to play a role but we have no data in the reptation regime (because of experimental difficulties due to the excessively high viscosity) which permit to draw some conclusions. (c) Nonlinear Behavior. Shear-thickening followed by shear-thinning is a typical behavior of associating polymers in a concentration range close to C*.20,37,47,66,75,76 A shear-thickening can be accounted for by a non-Hookean (75) Bock, J.; Siano, D. B.; Valint, P. L., Jr.; Pace, S. J. In Polymers in Aqueous Media: Performance through Association; Glass, J. E., Ed.; Advances in Chemistry Series 223; American Chemical Society: Washington, DC, 1989; Chapter 22, p 411. (76) Tam, K. C.; Guo, L.; Jenkins, R. D.; Bassett, D. R. Polymer 1999, 40, 6369.

8620


Jime´ nez-Regalado

Figure 13. Zero-shear viscosity versus the reciprocal critical shear rate for two aqueous solutions of HMPAM in the presence of SDS at different concentrations (the data are taken from Figures 5 and 6a).

elastic behavior of the network strands in the high stretching limit. However, this mechanism can only provide a modest shear-thickening as that observed in telechelic systems as highly stretched strands pull out of the cross-link.12,44,47,48,77 In multisticker polymer chains, shear-thickening is thought to be caused by a shear-induced change in the balance between intramolecular and intermolecular associations.12,20,37,76,78,79 This process can only occur in the vicinity of C* where the probability of intramolecular associations is significant whereas this probability is almost zero in the entangled regime. Shearing flow stretches the molecules, which favors intermolecular associations with a resulting increase in viscosity. As for the shear-thinning, it can be attributed to shear-induced breakup of the gel structure. The onset of shear-thinning is expected to occur at a critical shear rate γ˘ c of the order of the inverse terminal time TR. Turning now to the effect of surfactant, one observes that the shear-thickening is not suppressed by addition of SDS (DTAB) and even it is rather enhanced, an indication that the intramolecular associations are not destroyed (cf. Figure 6a,b). One can also note in the same figures that the surfactant concentration dependence of γ˘ c-1 closely follows that of the zero-shear viscosity with a maximum at the surfactant concentration CS,max for which η0 is maximum. This is also observed for systems with C . C* that do not exhibit shear-thickening but only shear-thinning (see Figure 5). It that case, γ˘ c can be determined with a reasonable accuracy. It is found that η0 is proportional to γ˘ c-1 (Figure 13, 1M1D3.2 sample). This result, which is verified for all the samples investigated, confirms our previous observations that the variation of the zero-shear viscosity with surfactant concentration can only be accounted for by a variation of the terminal time TR ≡ γ˘ c-1 without a variation of the plateau modulus (given by the slope of η0 vs γ˘ c-1). As for the systems showing shear-thickening, γ˘ c has been taken arbitrarily at the maximum of the shear-thickening. We can observe that the variation of η0 with γ˘ c-1 is still linear (Figure 13, 1M2D7 sample), despite this uncertainty and the fact that the plateau modulus is somewhat dependent on γ˘ c because of the creation of new plurifunctional aggregates under shear. (77) Marrucci, G.; Bhargava, S.; Cooper, S. L. Macromolecules 1993, 26, 6483. (78) Witten, T. A., Jr.; Cohen, M. H. Macromolecules 1985, 18, 1915. (79) Ballard, M. J.; Buscall, R.; Waite, F. A. Polymer 1988, 29, 1287.

Figure 14. Schematic representation of the variation of the zero-shear viscosity, terminal relaxation time, and plateau modulus, as a function of surfactant concentration

Similar results are found for the systems in the presence of DTAB. Conclusion We have reported a detailed investigation on the rheological behavior of semidilute aqueous solutions of HMPAM-surfactant mixtures as a function of polymer concentration and of the molecular parameters (molecular weight, hydrophobe content, and hydrophobic block length). Analysis of the data leads to a consistent picture of the surfactant binding, which differs in some aspects from that commonly accepted for multisticker polymer chains. In particular, the results suggest that the strong increase in viscosity observed at surfactant concentration around its cmc is mainly caused by an increase of the terminal time correlated to an increase in the lifetime of the preexisting cross-links, or by the formation of new short-lived intermolecular associations. Unfortunately, the characteristic time of the hydrophobe pull-out of micelles cannot be measured in the frequency range experimentally available. The variation of the characteristic rheological parameters, the zero-shear viscosity η0, the terminal relaxation time TR, and the plateau modulus G0 as a function of surfactant concentration CS can be schematically represented by the curves of Figure 14. The most characteristic feature is that G0 remains independent of CS in a large range of surfactant concentrations whereas both η0 and TR go through a maximum within the same range. This is expected in the reptation regime which is dominated by entanglements. On the other hand, for unentangled systems, this behavior is more surprising; this suggests that the main role of the surfactant is to gradually decorate the hydrophobic blocks (isolated or associated) with increasing surfactant addition without inducing significant bridging of the unassociated blocks that would contribute to the modulus. Assuming a complete binding


of the available surfactant gives a value of about 2 SDS molecules/hydrophobic unit (slightly less for DTAB) for the optimum binding at CS ) CS,max corresponding to the maximum in viscosity. This value, although likely overestimated, agrees well with that proposed by Piculell et al. for a number of systems.41,42 In this respect, the determination of binding isotherms and fluorescence experiments would give useful information on the structure and aggregation numbers of the mixed aggregates at CS,max. The concentration of hydrophobes in the HMPAM solution seems to be the key parameter for the position of CS,max whereas the viscosity enhancement is rather independent of this parameter. A total disruption of the

Langmuir, Vol. 16, No. 23, 2000 8621

transient associating network is observed at high surfactant concentration (g0.05 M), which is ascribed to the “solubilization” of the hydrophobic blocks by the surfactant micelles. Nonlinear rheological experiments corroborate the conclusions inferred from linear viscoelastic measurements. Acknowledgment. The authors thank J-F. Joanny for important remarks concerning the interpretation of the results and S. J. Candau for helpful discussions. E.J.R. thanks the Mexican government for the financial support granted through the CONACyT. LA000168Y

Effect of Surfactant on the Viscoelastic Behavior of Semidilute

Recommend Documents