Structural and Rheological Properties of Hydrophobically Modified Polysaccharide Associative Networks Catherine Esquenet, † Pierre Terech, ‡ Franc ¸ ois Boue ´, § and Eric Buhler* ,†,| Centre de Recherches sur les Macromole ´ cules Ve ´ ge ´ tales (CERMAV), UPR-CNRS 5301, University Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France, Laboratoire Physico-Chimie Mole ´ culaire, DRFMC/SI3M, CEA-Grenoble, 17, rue des Martyrs, 38054 Grenoble Cedex 9, France, Laboratoire Le ´ on Brillouin (CEA-CNRS), CEA Saclay, 91191 Gif-sur-Yvette, France, and Groupe de Dynamique des Phases Condense ´ es, UMR-CNRS 5581, cc26, University Montpellier 2, 34095 Montpellier Cedex 5, France Received December 18, 2003. In Final Form: February 23, 2004 The phase behavior of hydrophobically modified chitosans (HMCs) in aqueous solution has been investigated using scattering and rheology experiments. We observed four regions on the phase diagram of the associative polymer: (i) a supernatant phase (unimers phase) at low polymer concentration; (ii) a dilute solution of intermolecularly bridged flowerlike micelles at intermediate concentration; (iii) an associative gel phase at high polymer content; and (iv) a phase separation. In the present paper, we discuss the structural and dynamical properties of the HMC associative networks (c > c*) at a fixed hydrophobic degree of substitution of 2% and fixed alkyl side chains (stickers) length C8 (domains iii and iv of the phase diagram). As the polymer concentration is increasing, a connecting network is formed from the percolation of bridges between micellar aggregates. In this regime, small-angle neutron scattering and light scattering measurements show that ∼50-nm flower aggregates are acting like junction points in the network. The effect of the concentration, the stress, and the shear on the structure of the network is discussed. In particular, we observe bridge-to-loop transitions and then the formation of microgels or a low-connected network under shear. Therefore, our results are compared to recent theoretical models and to the results reported for telechelic systems. 1. Introduction Over the past two decades, hydrophobically modified water-soluble polymers or so-called associating polymers have found an increasing number of applications. As a result of their remarkable thickening properties, they are used in paints, in cosmetics, for enhanced oil recovery, and so forth. 1-5 These new materials are water-soluble polymers bearing highly hydrophobic groups. 6-8 Also, some studies have been devoted to hydrophobically modified polyelectrolyte polysaccharides, that is, polysaccharides with low levels of hydrophobic groups (i.e., 1-5%). 9-12 Hydrophobically associating polyelectrolytes have shown unusual rheological features and high solubilization properties in aqueous media. These properties arise from the inter- or intramolecular interactions among hydro- phobic groups, providing hydrophobic microdomains in an isotropic aqueous solution. In the present work, we have examined the structure and the dynamics of hy- drophobically modified chitosans (HMCs) with a hydro- phobic substitution degree of 2%. The HMC consists of alkyl side chains (hereafter called stickers) covalently linked to the polyelectrolyte chitosan backbone. In acid conditions, chitosan is water-soluble as a result of the presence of protonated amino groups, and it exhibits a polyelectrolyte character. The length and the number of alkyl side chains along the polycationic backbone control the degree of hydrophobicity of HMC. In a previous study, we have examined the structure and the phase behavior of dilute aqueous HMC solutions in the presence of 0.3 M acetic acid and 0.05 M sodium acetate. 12 The aggregation process and the structure of the micelles in the dilute regime were discussed. Figure 1 shows the sequence of phase behaviors determined in the alkyl chitosan concentration-alkyl side chain length plane at fixed temperature T ) 25 °C and at a fixed degree of substitution of 2% (by 2% we mean that 2% of the monomers carry a hydrophobic graft). The excess salt concentration being equal to 0.05 M, the charges of the polyelectrolyte main chain are screened. The partial phase diagram was obtained for the polymer concentration varying from 0 to 10 -1 g/cm 3 and for an alkyl side chain * To whom correspondence should be addressed: E. Buhler, Groupe de Dynamique des Phases Condense ´ es (GDPC), UMR 5581, CC26, University of Montpellier 2, 34095 Montpellier Cedex 5, France. Phone: 33 (0)4 6714 3982. Fax: 33 (0)4 6714 4637. E-mail: buhler@gdpc.univ-montp2.fr. † University Joseph Fourier. ‡ CEA-Grenoble. § CEA Saclay. | University Montpellier 2. (1) Principles of Polymer Science and Technology in Cosmetics and Personal Care; Goddard, E. D., Gruber, J. V., Eds.; Marcel Dekker: New York, 1999. (2) McCormick, C. L.; Bock, J.; Schults, D. N. Encyclopedia of Polymer Science and Engineering; John Wiley: New York, 1989; Vol. 17, p 730. (3) Bock, J.; Varadaraj, R.; Schultz, D. N.; Maurer, J. J. In Macro- molecular Complexes in Chemistry and Biology; Dubin, P. L., Bock, J., Davies, R. M., Schultz, D. N., Thies, C., Eds.; Springer-Verlag: Berlin, 1994; p 33. (4) Polymers as Rheology Modifiers; Schultz, D. N., Glass, J. E., Eds.; Advances in Chemistry Series 462; American Chemical Society: Washington, D.C., 1991. (5) Hydropilic Polymer, Performance with Environmental Accept- ability; Glass, J. E., Eds.; Advances in Chemistry Series 248; American Chemical Society: Washington, D.C., 1996. (6) Petit, F.; Iliopoulos, I.; Audebert, R.; Szo ¨nyi, S. Langmuir 1997, 13, 4229. (7) Tanaka, R.; Meadows, J.; Williams, P. A.; Phillips, G. O. Macromolecules 1992, 25, 1304. (8) Biggs, S.; Hill, A.; Selb, J.; Candau, F. J. Chem. Phys. 1992, 96, 1505. (9) Lee, K. Y.; Jo, W. H.; Kwon, I. C.; Kim, Y. H.; Jeong, S. Y. Langmuir 1998, 14, 2329. (10) Kjøniksen, A. L.; Iversen, C.; Nystro ¨ m, B.; Nakken, T.; Palmgren, O. Macromolecules 1998, 31, 8142. (11) Kjøniksen, A. L.; Nystro ¨ m, B.; Iversen, C.; Nakken, T.; Palmgren, O.; Tande, T. Langmuir 1997, 13, 4948. (12) Esquenet, C.; Buhler, E. Macromolecules 2001, 34, 5287. 3583 Langmuir 2004, 20, 3583-3592 10.1021/la036395s CCC: $27.50 © 2004 American Chemical Society Published on Web 04/02/2004