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Associating Polyelectrolytes with Perfluoroalkyl Side Chains: Aggregation in Aqueous Solution, Association with Surfactants, and Comparison with Hydrogenated Analogues F. Petit,† I. Iliopoulos,*,† R. Audebert,† and S. Szo¨nyi‡ Laboratoire de Physicochimie Macromole´ culaire, CNRS URA 278, ESPCI-10, rue Vauquelin, 75231 Paris Cedex 05, France, and Laboratoire 3S, Centre de Recherche, 7, rue de l’Industrie, MC-98000 Principaute de Monaco Received January 2, 1997. In Final Form: May 7, 1997X Derivatives of poly(sodium acrylate) bearing a few mole percent of perfluoroalkyl side chains were synthesized. Their solution properties were investigated by rheology and compared to those of their hydrogenated analogues. As the hydrogenated modified polymers, these new materials display an associating behavior. In semidilute solution the modified polymer exhibits viscosities of several orders of magnitude higher than the unmodified poly(sodium acrylate). However, this viscosifying effect is more pronounced for the perfluorinated derivatives. By comparing the rheological behaviors we find that a polymer bearing C7F15CH2 side groups is as associative as a polymer containing the same fraction of C13H27 chains. This is in agreement with Ravey and Ste´be´’s1 conclusions concerning surfactant association that a CF2 is equivalent to 1.7CH2 as regards its hydrophobicity. Mixtures of the perfluorinated polymers with their hydrogenated analogues or with hydrogenated surfactants were successively studied. For low modification ratios (e7 mol %) and at concentration ranges close to the critical aggregation concentration the mixing is not ideal. This is in line with the nonideal behavior displayed by mixtures of perfluorinated and hydrogenated surfactants.
Introduction Over the last 20 years, hydrophobically modified water soluble polymers or so-called associating polymers have found an increasing number of practical applications. Because of their extraordinarily viscosifying properties, they are used as thickening agents in paints, in cosmetics, for enhanced oil recovery,2-4 etc. These new materials are water soluble polymers bearing a small amount of highly hydrophobic groups. In semidilute aqueous solution the hydrophobic moieties associate. Aggregates, presumably of micellar type, are formed and act as reversible cross-links between the polymer chains. Very viscous solutions or gels displaying a shear thinning behavior can thus be obtained. With hydrophobically modified polyelectrolytes the viscosity of the solution can also be enhanced by several orders of magnitude upon addition of salt, instead of the viscosity decrease classically observed with nonassociating polyelectrolytes. Until now most studies have been devoted to associating polymers bearing hydrocarbons as hydrophobic groups,5-12 and only a few have dealt with the synthesis and the properties of derivatives bearing perfluorinated hydro†
Laboratoire de Physicochimie Macromole´culaire. Laboratoire 3S, Centre de Recherche. X Abstract published in Advance ACS Abstracts, July 15, 1997. ‡
(1) Ravey, J. C.; Ste´be´, M. J. Colloids Surf. A, Physicochem. Eng. Aspects 1994, 84, 11. (2) Landoll, M. L. U. S. Patent 4 228 277, 1980. (3) Landoll, M. L. U. S. Patent 4 243 802, 1981. (4) Evani, S. European Patent Application 057 875, 1982. (5) 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; p 218. (6) Biggs, S.; Hill, A.; Selb, J.; Candau, F. J. Phys Chem. 1992, 96, 1505. (7) Chang, Y.; McCormick, C. L. Macromolecules 1993, 26, 6121. (8) Guillemet, F.; Picullel, L. J. Phys. Chem. 1995, 99, 9201. (9) Maechling-Strasser, C.; Franc¸ ois, J.; Tripette C. Polymer 1992, 33, 627. (10) Ringsdorf, H.; Venzmer, J.; Winnik, F. M. Macromolecules 1991, 24, 1678. (11) Sinquin, A.; Hubert, P.; Dellacherie, E. Langmuir 1993, 9, 3334.
S0743-7463(97)00003-6 CCC: $14.00
phobic groups.13-17 However, such materials might be of great interest because of the very peculiar properties of fluorocarbons such as phobicity for hydrocarbons, low surface energies, high solubilization capacity for gases, especially for oxygen.18-22 Besides, it is well-known that fluorocarbon amphiphiles have a higher tendency to selfassemble than the corresponding hydrocarbons.1,19,23-26 Consequently, for perfluorinated modified polymers improved viscosifying properties should be expected. Moreover, it seems interesting to compare the differences in the associating properties of hydrogenated and perfluorinated polymers to the well known differences of solution behavior between hydrogenated and perfluorinated surfactants. These last years, we studied the synthesis and the solution properties of a family of associating polyelectrolytes, that is, hydrophobically modified poly(sodium acrylate) (HMPA) obtained by grafting a few mole percent of hydrogenated alkyl side chains onto a poly(sodium (12) Tanaka, R.; Meadows, J.; Williams, P. A.; Phillips, G. O. Macromolecules 1992, 25, 1304. (13) Zhang, Y. X.; Da, A. H.; Hogen-Esch, T. E. J. Polym. Sci. C: Polym. Lett. 1990, 28, 213. (14) Seery, T. A. P.; Yassini, M.; Hogen-Esch, T. E.; Amis, E. J. Macromolecules 1992, 25, 4784. (15) Hwang, F. S.; Hogen-Esch, T. E. Macromolecules 1995, 28, 3328. (16) Xie, X.; Hogen-Esch, T. E. Macromolecules 1996, 29, 1734. (17) Ka¨stner, U.; Hoffmannn, H.; Do¨nges, R.; Ehrler, R. Colloids Surf. A, Physicochem. Eng. Aspects 1994, 82, 279. (18) Lo Nostro, P. Adv. Colloid Interface Sci. 1995, 56, 245. (19) Mukerjee, P. Colloids Surf. A, Physicochem. Eng. Aspects 1994, 84, 1. (20) Riess, J. G. Colloids Surf. A, Physicochem. Eng. Aspects 1994, 84, 33. (21) Handa, T.; Mukerjee, P. J. Phys. Chem. 1981, 85, 3916. (22) Riess, J. G. New J. Chem. 1995, 19, 891. (23) Giulieri, F.; Krafft, M. P. Colloids Surf. A, Physicochem. Eng. Aspects 1994, 84, 121. (24) Shinoda, K. In Colloidal Surfactants, Some Physicochemical Properties; Shinoda, K., Nagakawa, T., Tamamushi, B. I., Isemura, T., Eds; Academic Press: New York, 1963; Chapter 1. (25) Szo¨nyi, S.; Watzke, H. J.; Cambon, A. Progr. Colloid Polym. Sci. 1992, 89, 149. (26) Trabelsi, H.; Szo¨nyi, S.; Gaysinski, M.; Cambon, A.; Watzke, H. J. Langmuir 1993, 9, 1201.
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Petit et al.
Table 1 modification ratio 3 7 10
alkyl side chain C8H17
C12H25
C14H29
CH2C7F15 CH2CH2C8F17
PA-3C8 PA-3C12 PA-3C14 PA-3C8F PA-7C12 PA-7C8F PA-7C10F PA-10C12 PA-10C8F
acrylate) (PA) backbone. In this paper, we report on the synthesis and the solution properties of perfluorinated analogues (HMPA-F) as studied by viscometry. Similarities and differences in the behavior of HMPA and HMPA-F are emphasised and compared to the available information on perfluorinated and hydrogenated surfactants. The mutual phobicity between hydrocarbons and fluorocarbons is eventually evidenced by the behavior of mixtures of perfluorinated and hydrogenated analogues and by the study of the association of these polymers with surfactants.
Figure 1. Viscosity as a function of polymer concentration for HMPA-F and for unmodified PA.
Experimental Section Materials. Poly(acrylic acid) of average molecular weight 150 000 was purchased from Polysciences as a 25% solution in water. Solid PA in the acid form was obtained by ultrafiltration of the commercial product first with an aqueous 0.01M HCl solution then with a large excess of water followed by final freeze drying. 1H,1H-Pentadecafluorooctylamine (C7F15CH2NH2) was obtained from Fluorochem and was used without further purification. 1H,1H,2H,2H-Heptadecafluorodecylamine (C8F17CH2CH2NH2) was synthesized by reduction of 1H,1H,2H,2Hheptadecafluorodecylazoture. Details are given elsewhere.27,28 Potassium dodecyl sulfate (KDS) was purchased from Fluka Biochemika and was used as received. Pentadecafluorooctanoic acid was obtained from Aldrich and was neutralized by an aqueous 1 M NaOH solution before use. Other reagents were of analytical grade. Synthesis. HMPA-Fs were synthesized according to the same procedure as their hydrogenated analogues.5,29 The perfluorinated amine was grafted onto the PA backbone by using dicyclohexylcarbodiimide as a coupling agent. Thanks to this modification reaction we can obtain series based on the same PA backbone with various degrees of modification and different alkyl chain lengths. The resultant copolymers have the following structure: CH2
CH C O–
100 – x O Na+
CH2
CH C
x O
NHR
where x is the modification ratio and R is the hydrogenated or the perfluorinated side chain. For the hydrogenated copolymers a random structure was found.30 As the perfluorinated derivatives were obtained via the same procedure, a random structure is also expected. The modification ratio was checked with both 1H NMR and elementary analysis. It was found that under similar experimental conditions the yield of modification was about 70% for the perfluorinated alkylamines instead of 100% for the hydrogenated ones.5,29 In the following part of this paper a polymer containing 3 mol % of H-dodecyl chains is denoted as PA-3C12. With HMPA-F the same designation is used, except that C8F and C10F refer to CH2C7F15 and CH2CH2C8F17 groups, respectively. Table 1 displays the different polymers synthesized and used in this study. Apparatus and Conditions. Concentrated stock solutions of polymer were prepared at least 18 h before use. Final solutions of the desired polymer concentration, Cp, were prepared by proper (27) Svo¨nyi, S.; Guennouni, F.; Cambon, A. J. Fluorine Chem. 1991, 55, 85. (28) Trabelsi, H.; Svo¨nyi, F.; Michelangeli, N.; Cambon, A. J. Fluorine Chem. 1994, 69, 115. (29) Wang, T. K.; Iliopoulos, I.; Audebert, R. Polym. Bull. 1988, 20, 577. (30) Magny, B.; Lafuma, F.; Iliopoulos, I. Polymer 1992, 33, 3151.
Figure 2. Viscosity as a function of polymer concentration for HMPA and for the same HMPA-F as in Figure 1. PA-3C8 follows the same curve as PA; the corresponding experimental points are not displayed on the graph for the sake of clarity. dilution of the stock solutions followed, if necessary, by addition of solid NaCl. The final solutions were equilibrated at room temperature at least 18 h before the measurements. Mixtures of hydrogenated and fluorinated polymers were prepared by direct solubilization in deionized water of an equal amount of each solid polymer at least 24 h before use. Mixtures containing a given polymer concentration, Cp, and surfactant concentrations ranging from 10-5 to 10-1 mol/L were prepared from the stock solutions of polymer and surfactant. The mixtures were equilibrated at room temperature 18 h before use. Viscometric measurements were performed with a Contraves Low-Shear 30 viscometer thermostated at 25 °C. All viscosity values reported in this paper are given for a 0.06 s-1 shear rate.
Results and Discussions Viscometric Properties of HMPA-F and Comparison with HMPA. Figure 1 displays the variation of the viscosity as a function of Cp for the perfluorinated polymers and for the unmodified PA. In Figure 2, viscosity data for the hydrogenated analogues are also plotted. For the unmodified PA a sharp viscosity increase is found at low polymer concentration due to the high extension of the polyelectrolyte coil in salt free solution. Then, at higher polymer concentration the viscosity increase becomes less pronounced because of the progressive self-screening of the electrostatic repulsions. This is a typical polyelectrolyte behavior corresponding to the semidilute unentangled regime described by Dobrynin et al.31 In this regime, the viscosity of a salt free solution of polyelectrolyte should vary as Cp1/2, which is in good agreement with the viscosity curve obtained for PA. The modified derivatives have a very different behavior. As soon as their concen(31) Dobrynin, A. V.; Colby, R. H.; Rubinstein, M. Macromolecules 1995, 28, 1859.
Polyelectrolytes with Perfluoroalkyl Side Chains
Figure 3. Viscosity as a function of NaCl concentration for several HMPA-Fs and HMPA’s at different polymer concentrations (Cp ) 1% and Cp ) 2%).
tration exceeds a critical value, Cpc, their viscosity sharply increases above that of the unmodified PA. Both series of modified PA’s (perfluorinated and hydrogenated) display the same general trends: the higher the content of the hydrophobic groups or the longer the hydrophobic chains, the lower the Cpc and the more pronounced the sharpness of the viscosity curves above Cpc. These results are typical for associating polymers. They have been explained by a micellar type aggregation of the alkyl chains belonging to different macromolecules, which leads to a reversible cross-linking of the solution.5,29 This molecular mechanism responsible for the viscosity increase is supported by small-angle X-ray scattering and fluorescence spectroscopy measurements.32 This intermolecular association is also evidenced at a macroscopic level by the behavior of both series of modified polymers in the presence of increasing NaCl concentrations (see Figure 3). In a previous paper29 it was shown that upon addition of salt the viscosity of PA decreases as the electrostatic repulsions are progressively screened. Dobrynin et al.31 predict that in the semidilute unentangled regime the viscosity should be salt independent as long as the salt concentration, Cs, is much smaller than the polymer counterion concentration. At Cs much higher than the polymer counterion concentration, the salt ions control the screening of the Coulomb interactions and the viscosity decreases. With modified polymers two different behaviors are encountered according to the polymer concentration. At low polymer concentration, the viscosity decreases upon addition of salt until final phase separation.29 At higher polymer concentration the viscosity first increases and passes through a maximum, and then further addition of NaCl leads to a sharp viscosity decrease followed by phase separation. Those two behaviors have been ascribed to the balance between two effects:29 screening of the electrostatic repulsions, which leads to chain contraction and thus to a viscosity decrease, and salting out effect of NaCl, which leads to enhanced hydrophobic association. This second effect favors either a viscosity increase if the polymer concentration is high enough to allow intermolecular association or a viscosity decrease if intrachain interactions prevail. Despite all these similarities, perfluorinated derivatives have a much stronger tendency to associate and as a consequence they are more efficient viscosifiers than their hydrogenated analogues (see Figure 2). The same difference regarding self-association was observed in surfactant systems1 and was explained by the greater importance of the water structuring effects with fluoro(32) Petit, F.; Iliopoulos, I.; Audebert, R. J. Chim. Phys. 1996, 93, 887.
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carbons.19 Whereas PA-3C8 exhibits no evidence of association within the concentration range studied, PA3C8F has a viscosity curve which lies in between the viscosity curves of PA-3C12 and PA-3C14. In the same way the viscosity curves of PA-7C8F and PA-10C8F are very close to those of PA-7C12 and PA-10C12, respectively. These observations are to be compared to the results obtained by Ravey and Ste´be´1 on surfactants. They proposed that, as far as the critical micelle concentration (cmc) is concerned, a CF2 is equivalent 1.7CH2. Extending this equivalence to our polymers, we find that PA-3C8F, PA-7C8F, and PA-10C8F should behave as PA-3C13, PA7C13, and PA-10C13, respectively. PA-3C8F and PA7C8F have viscosity curves that are slightly above the curves of PA-3C12 and PA-7C12, respectively, as expected from the above considerations. However, it can be noticed that this rule is clearly not followed by the couple PA10C8F/PA-10C12. At this high content of hydrophobic groups intermolecular associations might be significantly counterbalanced by intramolecular associations, which would favor chain contraction and would lead to smaller than expected viscosities. Several authors13,33 have already shown that there is an optimal hydrophobic comonomer content, xopt, in order to obtain a maximum viscosifying effect due to the competition between interand intramolecular association. From the data of Figure 1 we estimate xopt to be close to 10% for HMPA-F. This xopt value is considerably higher than those reported for HM-nonionic polymers bearing perfluorinated side groups.13,15,16 Obviously, electrostatic repulsion effects play here an important role, and as expected they are, at least partially, suppressed by adding salt (see below). Evidence of improved intramolecular association with PA-10C8F can also be found in the comparison of the viscosity curves upon addition of salt (see Figure 3). No viscosity curve is reported for 2% PA-10C12 because addition of salt to this sample gave rise to a huge viscosity increase that could not be studied with the Contraves Low-Shear 30 viscometer even at salt concentrations close to the final phase separation (obtained for Cs ) 2.5%). However, with PA-10C8F the viscosity decrease is recorded for Cs g 1%. Moreover, it is interesting to note that this viscosity decrease is also more pronounced than for the less modified PA-7C8F. These results clearly indicate that the role of the intramolecular association as compared to the intermolecular one is more important for PA-10C8F than for PA-10C12 or PA-7C8F. As a result xopt moves to lower values, below 10%, by increasing salt concentration. Phobicity between Fluorocarbons and Hydrocarbons as Studied in HMPA/HMPA-F Mixtures and in Their Association with Surfactants. It is wellknown that perfluorinated and hydrogenated surfactants do not mix ideally due to the mutual phobicity of hydrocarbon and fluorocarbon chains.19,21 Usually, the cmc of such surfactant mixtures is higher than the value corresponding to the ideal mixing.34,35 This nonideality of mixing can even lead to a microphase separation with the coexistence of fluorocarbon rich and hydrocarbon rich micelles.34,36-38 It was therefore interesting to check if similar behavior could be observed when mixing HMPA and HMPA-F. Figure 4 displays the variation of the viscosity as a function of Cp for PA-3C12, PA-3C8F, and a 1/1 (w/w) (33) Landoll, L. M. J. Polym. Sci.: Polym. Chem. 1982, 20, 443. (34) Funasaki, N.; Hada, S. J. Phys. Chem. 1980, 84, 736. (35) Shinoda, K.; Monura, T. J. Phys. Chem. 1980, 84, 365. (36) Mukerjee, P. J. Phys. Chem. 1976, 80, 1388. (37) Carlfors, J.; Stilbs, P. J. Phys. Chem. 1984, 88, 4410. (38) Aramoto, M.; Ikeguchi, M.; Takiue, T.; Ikeda, N.; Motomura, K. J. Colloid Interface Sci. 1995, 174, 156.
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Figure 4. Viscosity as a function of polymer concentration. Comparison between the pure polymers (PA-3C8F and PA3C12) and a 1/1 (w/w) mixture of these two polymers.
Figure 5. Viscosity as a function of polymer concentration for PA-7C10F, PA-10C12, and a 1/1 (w/w) mixture of these two polymers and for PA-7C8F, PA-7C12, and a 1/1 (w/w) mixture of these two polymers.
mixture of these two polymers. Had the mixture been ideal, one would have expected its viscosity curve to lie in between the curves of the two polymers. In fact, in the concentration range investigated, the viscosity curve of the mixture remains below those of both PA-3C12 and PA-3C8F and the Cpc for the mixture is higher than those of the two individual polymers. A higher total polymer concentration is thus required to achieve alkyl chain aggregation, as perfluorinated and hydrogenated side chains do not form mixed aggregates at least for polymer concentrations close to Cpc. However, one can notice that the discrepancy between the viscosity curve of the mixture and the hypothetic curve of the ideal mixing seems to reduce gradually at high polymer concentrations. We thus believe that the segregation effect vanishes and C8F/C12 cross-association becomes effective when the concentration of hydrophobic groups is high enough. More evidence of this phenomenon is given in Figure 5. It displays the variation of viscosity as a function of polymer concentration for PA-7C10F, PA-10C12, and a 1/1 (w/w) mixture of these two polymers. The two polymers as well as their mixture have the same rheological behavior. As before, we conclude that for these highly modified polymers the tendency to associate is so strong that no evidence for segregation is found. Data for the PA-7C8F/PA-7C12 system are also displayed on Figure 5. For this system an intermediate behavior between those of the PA-3C8F/ PA-3C12 and the PA-7C10F/PA-10C12 systems is obtained. Finally, we must note that all the HMPA/HMPA-F mixtures studied are clear and they do not exhibit any evidence of either macroscopic phase separation or turbidity.
Petit et al.
Figure 6. Viscosity as a function of sodium pentadecafluorooctanoate concentration for PA-3C8F and PA-3C12. Cp ) 3%.
In order to confirm these results, we studied the association of HMPA or HMPA-F with hydrogenated and perfluorinated surfactants. Figure 6 displays the variation of viscosity upon addition of sodium pentadecafluorooctanoate in 3% PA-3C8F and 3% PA-3C12 aqueous solution. Upon addition of perfluorinated surfactant, PA3C8F exhibits the classical behavior. The viscosity rises to a maximum and then decreases. This behavior has been attributed to the interactions of the surfactant micelles with the polymer alkyl chains.39,40 For some optimum surfactant concentration (close to the cmc8) mixed micelles are formed with the surfactant and the polymer alkyl side chains. These mixed micelles act as reversible cross-links between the macromolecules and a viscosity increase is obtained. However, at large surfactant concentration, micelles are mainly composed of surfactant molecules: the probability of finding alkyl grafts belonging to two different polymer chains involved in the same micelle becomes low and therefore the viscosity of the system decreases. However with the hydrogenated analogue, PA-3C12, the viscosity remains constant, indicating that there is no interaction between this hydrogenated polymer and the perfluorinated surfactant. Similar results are obtained when the reverse experiments are performed, i.e., when a hydrogenated surfactant (KDS) is added to 3% PA-3C12 and 3% PA-3C8F (Figure 741). Although KDS associates with the hydrogenated polymer, there is no association with the perfluorinated analogue PA-3C8F. These results are therefore further evidence of the segregation between fluorocarbons and hydrocarbons exhibited by weakly modified polymers at polymer concentrations close to their Cpc. However, when a more hydrophobic polymer, PA-7C10F, is used this segregation vanishes: a viscosity maximum is found (Figure 7) and the perfluorinated alkyl side chains form mixed micelles with the hydrogenated surfactant. This observation is in line with our data on HMPA/HMPA-F mixtures (see Figures 4 and 5). A viscosity maximum was also observed by Ka¨stner et al.17 upon addition of sodium dodecylsulfate to hydroxyethylcellulose derivatives modified by perfluoroalkyl side groups. However, it is interesting to note that they observed an association (a viscosity maximum) in all the systems they studied, despite the mutual phobicity (39) Magny, B.; Iliopoulos, I.; Zana, R.; Audebert, R. Langmuir 1994, 10, 3180. (40) Biggs, S.; Selb, J.; Candau, F. Langmuir 1992, 8, 838. (41) In Figure 7 when no surfactant is added the viscosity of PA3C12 is slightly above that of PA-3C8F. To perform this experiment we used a different sample of PA-3C8F from the one previously studied. Its molecular weight distribution was slightly different.
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hydrocarbons entails a segregation only when the polymer is weakly modified and when its concentration is close to Cpc. Conclusion
Figure 7. Viscosity as a function of KDS concentration for PA-3C12 (Cp ) 3%), PA-3C8F (Cp ) 3%), and PA-7C10F (Cp ) 1.25%).
between the hydrogenated surfactant and the perfluoralkyl side groups of hydroxyethylcellulose (HEC). Similar conclusions have been drawn out recently by Xie and Hogen-Esch16 for mixtures of fluorinated (hydrogenated) hydrophobically modified poly(N,N-dimethylacrylamide) (PDMA) with perfluorinated or hydrogenated surfactants. This could be explained by the higher hydrophobicity of the nonionic HEC17 and PDMA16 derivatives compared to our hydrophobically modified polyelectrolytes, the backbone of which is highly hydrophilic and exhibits strong electrostatic repulsion toward the anionic surfactant. From all these experimental observations we conclude that the mutual phobicity between fluorocarbons and
Hydrophobically modified derivatives of poly(sodium acrylate) bearing perfluorinated pendent alkyl chains were synthesized and their solution properties were investigated by viscometry. As their hydrogenated analogues, these new materials display an associating behavior. However, because of the more hydrophobic nature of the perfluorinated groups, aggregation properties are enhanced. By comparing the rheological properties of HMPA and HMPA-F we conclude that CH2C7F15 exhibits a hydrophobic character similar to a C13H27 group. Mixtures of HMPA-F with HMPA and their mixtures with hydrogenated or perfluorinated surfactants were studied. It was shown that, despite the mutual phobicity between fluorocarbons and hydrocarbons, a highly modified polymer interacts with both perfluorinated and hydrogenated amphiphilic compounds (polymer or surfactant). On the other hand, we found that a weakly modified polymer associates more selectively with polymers and surfactants of the same chemical nature (perfluorinated or hydrogenated) as itself. This selectivity of association could also be modulated by varying the modified polymer concentration. Acknowledgment. We thank Mrs. L. Bon Nguyen for her valuable assistance in the experimental part of this work and the joint CNRS-industry program DIMAT “Polyme`res Hydrosolubles Associatifs” for support. LA970003Y
