Langmuir 1994,10,1421-1426
1421
Association and Thermal Gelation in Mixtures of Hydrophobically Modified Polyelectrolytes and Nonionic Surfactants Anne
Sarrazin-Cartalas,?
Ilias
Iliopoulos,*vt
Roland Audebert,t and Ulf Olssont
Laboratoire de Physico-Chimie Macromol6culaire, Universite Pierre et Marie Curie, CNRS U R A 278,ESPCI, 10 rue Vauquelin, F-75231 Paris cedex 05,France, and Physical Chemistry 1, Chemical Center, Lund University, P.O. Box 124,S-22100 Lund, Sweden Received August 28, 1993. I n Final Form: January 21, 1994@ We studied the association between hydrophobically modified poly(sodium acrylate) and a series of oligoethylene glycol monododecyl ether surfactants (Clz E, with n = 4 , 5 , and 8). The polymers contain a small fraction of long alkyl side groups (dodecylor octadecyl). The alkyl groups of the modified polymers associatewith the surfactant aggregates,which induces dramatic changes in the rheology of these systems. The viscosity of the mixture may be 4 orders of magnitude higher than that measured in mixtures of the nonmodified poly(sodiumacrylate) and the surfactant. The systems containing C12Ea as surfactant exhibit a very interesting thermal behavior. Their viscosity increases and a physical gel is formed by increasing the temperature. This thermal behavior is interpreted as due to the transformation of the surfactant aggregates from micelles to vesicles with increasing temperature. We argue that at least two different types of gel structures can be recognized in solutions containing hydrophobically modified polymers and surfactant aggregates. In gels where the surfactant forms small micellar aggregates,mixed micelles, including the hydrophobic side chains of the polymer, act as cross-linksof the polymer network. On the other hand, when the surfactant forms giant vesicle aggregates, the polymer adsorbed on the bilayer membrane can form bridgesbetween adjacent vesicles. This leads to an effective cross-linkingof the surfactant aggregates.
Introduction Complex water-based fluids containing polymers and surfactants find important practical applications in various domains such as detergency,paints, cosmetics, oil recovery, etc. They also play a key role in many biological systems. For example, they can control the functionality and the stability of cell membranes.' The interactions between model synthetic or natural polymers and surfactants were intensively studied during the last 30 years, and several reviews have appeared.2-" In most cases, a strong cooperative association is observed between polyelectrolytes and oppositely charged surfactants, and rather weak interactions are observed between nonionic polymers and ionic surfactants (especially anionics). The most studied system has been the couple poly(ethy1ene oxide) (PEO)/sodium dodecyl sulfate (SDS).596 Interactions between ionic polymers and nonionic surfactants have only been reported in cases where hydrogen bonding between the polymer and surfactant is effective.7 The detailed mechanisms behind these phenomena are not always well understood, but in many cases the hydrophobic interactions are suspected to be of great importance. In particular it has been found that the association between polymers and surfactants is strength-
* To whom correspondence should be addressed. + Universite de Pierre e t Marie Curie. Lund University. Abstract published in Advance ACS Abstracts, April 1, 1994. (1) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Int. Ed. Engl. 1988,27,113. (2)Goddard, E. D. Colloids Surf. 1986,19,255,301. (3)Hayakawa, K.; Kwak, J. C. T. In Cationic Surfactants Physical Chemistry;Rubingh, D., Holland, P. M., Eds.;Marcel Dekker: New York, 1991;p 189. (4)Lindman, B.; Thalberg, K. In Polymer-Surfactant Interactions; Goddard, E. D., Ananthapadmanabham, K. P., Eds.; CRC Press: Boca Raton, FL, 1992;p 203. (5)Cabane, B.; Duplessix, R. Colloids Surf. 1985,13, 19. (6)Cabane, B.; Duplessix, R. J. Phys. (Paris) 1987,48,651. (7)Saito, S.;Taniguchi, T. J. Colloid Interface Sci. 1973,44,114. @
0743-7463/94/2410-1421$04.50/0
ened if the polymer carries a low fraction of very hydrophobic groups.sz0 In some cases hydrophobic interactions overcome even the electrostatic repulsions between similarly charged polyelectrolytes and surfactants, and association phenomena are o b ~ e r v e d . ' ~ JOf~ course, ?~~ the association is greatly enhanced if the surfactant and the polyelectrolyte are oppositely ~ h a r g e d . ~ ~ ~ ~ ~ A particularly interesting feature of the polymer and surfactant systems is that in some cases a gel is formed upon heating. This has been observed in mixtures of ethyl hydroxylethyl cellulose (EHEC) with ionic surfactants.24 EHEC is a water-soluble derivative of cellulose exhibiting alower critical solution temperature (LCST).25If an ionic surfactant is added, a gelation occurs instead of the phase separation. EHEC has a clear hydrophobic character (8)Lundberg, D.J.; Glass, J. E.; Eley, R. R. Proc. ACS Diu. Polym. Mater. Sci. Eng. 1989,61, 533. (9)Gelman, R. A. Int. DissoluingPulps Conf., TAPPIProc. 1987,159. (10)Winnik, F. M. Langmuir 1990,6,522. Ringsdorf, H.; Venzmer, J. Langmuir 1991,7,905, (11)Winnik, F.M.; 912. (12)Hu,Y. Z.; Zhao,G.L.;Winnik, M. A.;Sundararajan,P.R.Langmuir 1990,6,880. (13)Peiffer, D. G. Polymer 1990,31,2353. (14)Dualeh, A. J.; Steiner, C. A. Macromolecules 1991,24,112. (15)Biggs, S.;Selb, J.; Candau, F. Langmuir 1992,8, 838. Williams,P.A. Carbohydr. (16)Tanaka,R.;Meadowa,J.;Phillips,G.0.; Polym. 1990,12,443. (17)Tanaka, R.; Meadows, J.; Williams, P. A.; Phillips, G. 0. Macromolecules 1992,25, 1304. (18)Wang, T. K.; Iliopoulos, I.; Audebert, R. In 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;Chapter 14,p 218. (19)Iliopoulos, I.; Wang, T. K.; Audebert, R. Langmuir 1991,7,617. (20)B. Magny, I. Iliopoulos, R. Audebert, L. Piculell, B. Lindman, Prog. Colloid Polym. Sci. 1992,89,118. (21)McGlade,M. J.;Randall,F. J.;Tcheurekdjian,N. Macromolecules 1987,20,1782. (22)Zana, R.; Kaplun, A.; Talmon, Y. Langmuir 1993,9,1948. (23)Binana-Limbel6, W.; Zana, R. Macromolecules 1987,20, 1331. (24)Carlsson, A.; Karlstrom, G.; Lindman, B. Colloids Surf. 1990,47, 147. (25)Carlsson, A.; Karlstrom, G.; Lindman, B.; Stenberg, 0. Colloid Polym. Sci. 1988,266,1031.
0 1994 American Chemical Society
1422 Langmuir, Vol. 10, No. 5, 1994
which is increased with increasing temperature, favoring the interactions with surfactants as was concluded from extensive studies by Carlsson and others.2P29 Increased interactions between EHEC and ionic surfactants have two consequences. The first is an increased solubility of the polymer in water: the bound ionic surfactant endows an apparent polyelectrolyte nature to the initially nonionic EHEC, resulting in an increased LCST of the system. The second is the thermal gelation phenomenon which is rather related to the heterogeneous distribution of the hydrophobic substituents (ethylgroups) along the polymer chain.24 The gelling is due to the formation of micelle-like clusters containing surfactant molecules and hydrophobic blocks belonging to more than one polymer chain. An increased LCST had been reported also for other nonionic polymers when mixed with ionic surfactants as, for instance, for poly(N-isopropylacrylamide).30 However, the latter polymer has a very regular structure without heterogeneities in the distribution of the hydrophobic groups (isopropyl), and subsequently it does not exhibit thermal gelation when mixed with ionic surfactants. It is very tempting to check if this behavior could occur also with other polymer/surfactant systems. For this reason we have decided to work with polymers having locally a strong hydrophobicity and with surfactants exhibiting phase transitions with increasing temperature. The polymers were hydrophobically modified derivatives of poly(sodium acrylate) (HMPA) bearing 1 or 3 mol% N-dodecyl(or octadecy1)acrylamide units. In water and in brine, they associate by formation of hydrophobic micellar-type clusters and they present intriguing rheological behavior: the viscosity of their aqueous solutions increases with the ionic strength.18p31As the best surfactant candidate was chosen the nonionic dodecyl ether of tetraethylene glycol, C12E4. In the binary C12E4/H20 system and for surfactant concentrations lower than 25 % , a sequence of several homogeneous phases is observed as a function of temperature.32 At very low temperature a liquid isotropic phase is found which separates upon heating (between 5 and 20 OC depending on the surfactant concentration) into two isotropic liquid phases (clouding). At these low temperatures C12E4 forms very large micellar aggregate^.^^ When the temperature exceeds approximately 20 "C, a dispersion of lamellae in water is obtained;32 under such conditions formation of vesicles is suspected.34 A t temperatures higher than 50 "C the L3 phase appears (liquid bilayer continuous). In the first part of this work we report the rheological evidence of association between HMPA and some nonionic surfactants (ClzE,, with n = 4, 5, and 8). In the second part we focus on the phenomenon of thermal gelation of the system HMPA/C12E4. (26) Karlstrom, G.; Carlsson,A.;Lindman, B. J. Phys. Chem. 1990,94, 5005. (27) Carlsson,A.;Lindman,B.;Watanabe,T.;Shirahama,K.Langmuir 1989, 5, 1250. (28) Carlsson,A.; Karlstrbm, G.; Lindman, B. J.Phys. Chem. 1989,93, 3673. (29) Zana, R.;Binana-Limbelb,W.;Kamenka,N.;Lindman, B. J.Phys. Chem. 1992,96, 5461. (30) Schild, H. G.; Tirrell, D. A. Langmuir 1991, 7, 665. (31) Wang, T. K.; Iliopoulos, I.; Audebert, R. Polym. Bull. 1988, 20, 577. (32) Mitchell, D. J.;Tiddy, G. J.T.;Waring,L.;Bostock, T.;McDonald, M. P. J. Chem. SOC.,Faraday Trans. 1 1981, 79,975. (33) Henriksson, U.;Jonstromer, M.; Olsson, U.;Saerman, 0.;Klose, G. J.Phys. Chem. 1991, 95, 3815. (34) Kunieda, H.; Nakamura, K.; Davis, H. T.; Evans, D. F. Langmuir 1991, 7, 1915.
Sarrazin-Cartalas et al.
Experimental Section Materials. Poly(acry1ic acid) (PA) was purchased from
Polysciences Inc., and the average molecular weight, given by the supplier, was 150 OOO. The hydrophobically modified derivatives were prepared as described elsewhereal and used in the sodium salt form:
Xis the modificationdegree (mol % ) and n the number of carbon atoms ofthe alkyl chain (n= 12 or 18). An example of the sample designation is as follows: 1-Cl8 contains 1 mol % N-octadecylacrylamide units. The modified polymers have the same polymerization degree as the precursor PA and a random distribution of the alkyl groups along their chains.% Oligoethyleneglycol monododecylethers (C12E4, ClPES, C12Es) were purchased from Fluka (>98% purity) and used as received. Preparationof Polymer/Surfactant Mixtures. Mixtures containing the desired composition were prepared by mixing aqueous stock solutions of polymer and surfactant. The concentration of the two stock solutions was twice that in the final mixture, and they were stirred for 24 h before use. The mixtures were shaken vigorously many times and equilibrated for 1 or 2 days at room temperature. In the case of we tookparticular care to prepare the mixtures at sufficiently low temperature in order to have clear one-phase solutions. Apparatus. Most of the viscositymeasurements,except those reported in Figure 5, were performed with a Contraves LS-30 viscometer at low shear rates (between 0.06 and 1.28 8-l) corresponding to the Newtonian viscosity. The viscosity measurements reported in Figure 5 as well as the oscillatory experimentswere performed with a Carri-Medcontrolled-stress rheometer equipped with a cone and plate geometry. The shear storage, G', and the shear loss, G",moduli were recorded in the linear viscoelasticity range. In the temperature scan experiments(increasingtemperature), steps of 5 O C were applied (5 min per step),except for that reported in Figure 5 for which a continuous increase of 1 "C/min was applied.
Results and Discussion Evidence of Polymer/Surfactant Association. Figure 1shows the effect of surfactant concentration on the viscosity of a solution containing modified polymer (3C12) in water. The polymer concentration is kept constant a t 1% by weight, and the temperature is 25 "C. Small changes in the length of the polar group of the surfactant have enormous influence on the rheological properties of the system. With the more hydrophilic surfactant the viscosity curve presents a smooth maximum a t arather low surfactant concentration, =3 X 10-3 mol L-1. A different concentration dependence and a more drastic viscosity enhancement were found with C1zEb. Here, the viscosity increases continuously with surfactant concentration. Note that at the temperature of the experiment forms small spherical micelles in the (25 O C ) the entire range of surfactant concentration studied while the C12E5 forms large aggregates, the size of which increases with the surfactant concentration.36.37 Similar maxima in viscosity were found when anionic or cationic surfactants were added to HMPA s o l u t i 0 n s ~ ~and ~ ~ to 0 other hydrophobically modified polymer aqueous solutions.8~9J5J7 (35) Magny, B.; Lafuma, F.; Iliopoulos, I. Polymer 1992, 33, 3151. (36) Nilsson, P. G.; Wennerstrbm, H.; Lindman, B. J. Phys. Chem. 1989,87, 1377. (37) Lindman, B.; Jonstrbmer, M. In Physics of Amphiphilic Layers; Meunier, J., Langevin, D., Boccara, N., Eds.; Springer-Verlag: Berlin, 1987; p 235.
Langmuir, Vol. 10, No. 5, 1994 1423
Association and Thermal Gelation in Mixtures
POLYMER
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1
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1o - ~
1o-z
I
z
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SWACTANT CONCENTRATION(mol r')
v
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.
Figure 1. Variationof the viscosityof a hydrophobicallymodified polymer solution (3-Cl2) with the surfactant concentration (polymer concentration 1% ,temperature 25 "C, shear rate
