LIGHT-SCATTERING-STUDIES OF THE GELATION PROCESS IN AN AQUEOUS SYSTEM OF A NONIONIC POLYMER AND A CATIONIC SURFACTANT

Static and dynamic light scattering studies are presented on the temperature induced sol-gel transition of an aqueous system of the non-ionic cellulose derivative ethyl hydroxyethyl cellulose in the presence of a cationic surfactant. In dilute solution coil expansion is observed with increasing temperature. In the semidilute range structural changes of the network are found when approaching the sol-gel transition. These effects are explained by an enhanced binding of surfactant to the polymer. The relaxation of the correlation function, in the semidilute regime, is shifted towards longer times as the gelation threshold is approached. These results are interpreted in the framework of a model that has been constructed with the aid of the theoretical approach of Semenov. In this model the correlation function initially decays by a fast cooperative mode (single exponential). followed by a non-exponential decay (reptation-like mode) at longer times. The cooperative relaxation time is only slightly influenced by temperature, whereas the reptation-like relaxation time exhibits a drastic increase during the gelation process. This effect is attributed to a slowing down of the motion of the individual chains in the gelation zone. Within the framework of the model the determination of the gel point seems possible.