SOLUTION BEHAVIOR OF HYDROPHOBICALLY-MODIFIED SODIUM POLYACRYLATE

A number of hydrophobically modified sodium polyacrylates (HMPAA's) have been synthesized and their solution and rheological properties investigated by means of capillary viscometry, small deformation oscillation, and electron spin resonance (ESR) spectroscopy studies. In the presence of added electrolyte (0.1 mol dm-3 NaBr), capillary viscometry and ESR spin probe studies indicated the existence of intramolecular hydrophobic associations at low polymer concentrations, with a gradual transition to intermolecular associations occurring at higher polymer concentrations. In the absence of added electrolyte, only intermolecular associations were observed, and the critical polymer concentration necessary for the formatio-Ti of such associations was found to show little dependence on the hydrophobe chain length and content. In contrast to typical polyelectrolyte behavior, an increase in storage modulus, G', of the HMPAA solutions was observed upon the addition of relatively low concentrations of sodium chloride, although a decrease in G' was observed at higher electrolyte concentrations. The rheological properties of mixed polymer/anionic surfactant systems were found to be strongly dependent on the level of surfactant addition with both an increase or decrease in G' of the system being obtainable. The maximum achievable G' of the various HMPAA/surfactant systems increased with increasing alkyl chain length of added anionic surfactant but showed little dependence on the nature of the anionic head group. At ''optimum'' levels of surfactant addition, the originally viscous polymer solutions exhibited gel-like characteristics. The addition of relatively low concentrations of the cationic surfactant cetyltrimethylammonium bromide (CTAB) was found to induce phase separation of the anionic polyelectrolytes, in the form of a gelatinous precipitate. The amount of CTAB required to induce phase separation of the HMPAA's was greater than that required for phase separation of the precursor sodium polyacrylate. In all instances, the level of CTAB required to induce phase separation was at least an order of magnitude less than that required for a 1:1 stoichiometry of interaction between the cationic surfactant molecules and the carboxyl groups of the polymers.