Polymer Gels and their Interactions with Ionic Surfactants

Abstract: The response on chemically crosslinked gels and polymer solutions on the addition of ionic surfactants and/or addition of salt at different temperatures have been studied. All polymers studied are water soluble and slightly hydrophobic or hydrophobically modified. Because of their hydrophobicity the polymers have a strong tendency to associate, both with themselves and with surfactants. Another important property of such polymers is the decreased water solubility on temperature increase. At a certain temperature called the lower consolute temperture (LCST) the polymer in solution precipitates and the polymer gel collapses. Most of the work deals with volume changes in chemically crosslinked polymer gels on ionic surfactant addition. To get a greater understanding linear polymer-surfactant interactions in solutions are also investigated. The main part of this work is on different cellulose ethers such as: ethyl(hydroxyethyl) cellulose (EHEC) and its hydrophobically modified analogue HMEHEC and a cationic hydrophobically modified cellulose called LM200. Other polymers investigated are the polyacrylamide based polymer gels: poly-N-isopropyl acrylamide (p-NIPA) and a polyacrylamide with pendant poly(ethylene oxide) chains. The surfactants used are anionic, cationic and nonionic. On addition of an ionic surfactant to the polymer gels the surfactants associate to the polymer at a certain concentration called the critical aggregation temperature (cac). At this concentration the gel has a fast swelling up to a concentration that corresponds to the critical micelle concentration (cmc) of the surfactant not bound to the polymer. The swelling is due to the contribution from the dissociated counterions to the osmotic pressure. The maximum swelling of the gel comes at a free surfactant concentration near the cmc. In a polymer solution the surfactant binding leads to an increased viscosity due to the extra physical crosslinkes from the mixed surfactant-polymer aggregates. When surfactants are added the LCST is increased. When salt is added to the surfactant-polymer gel system the cac, and the onset concentration of the swelling decreased. There is also a volume decrease of the gel over the whole surfactant concentration interval due to the screening of the charges from the aggregated surfactants. Binding isotherms for the surfactant-polymer systems have been made with different techniques (NMR self-diffusion, sodium flame-emission measurements and UV-absorbance).