RHEOLOGICAL AND DYNAMIC LIGHT-SCATTERING-STUDIES ON AQUEOUS-SOLUTIONSOF A HYDROPHOBICALLY-MODIFIED NONIONIC CELLULOSE ETHER AND ITS UNMODIFIED ANALOG

Dynamic light-scattering (DLS) and oscillatory shear experiments have been carried out on aqueous solutions of ethyl(hydroxyethyl)cellulose (EHEC) and of a hydrophobically modified analogue (HM-EHEC) in the pre sence of various amounts of sodium dodecyl sulfate (SDS). Both the the ological and the DLS measurements revealed significant polymer-surfact ant interaction for the EHEC/SDS and HM-EHEC/SDS systems at surfactant concentrations of 8-10 and 4-5 mm, respectively. The observed theolog ical and hydrodynamic effects for both systems were found to be strong ly dependent upon the level of surfactant addition, with at first an i ncrease and then a decrease in the values of parameters such as the dy namic viscosity, the storage and loss moduli, the slow relaxation time from DLS, and the wave vector dependence of the slow inverse relaxati on time. The strength of interaction was considerably stronger in solu tions of the hydrophobically modified polymer at moderate surfactant c oncentrations due to enhanced hydrophobic associations. At higher surf actant concentrations, a breakdown of chain associations occurred. The theological properties of the hydrophobically modified polymer at mod erate surfactant concentrations suggest that the elastic response domi nates at higher frequencies. The time correlation data obtained from t he DLS experiments revealed, for both systems at all surfactant concen trations, the existence of two relaxation modes, one single exponentia l at short times followed by a stretched exponential at longer times. The values of the slow relaxation time were constantly found to be sev eral orders of magnitude higher than those of the fast relaxation time , with a maximum value observed for the HM-EHEC/SDS system at a surfac tant concentration of about 4 mm. The fast mode was always diffusive, while the slow mode exhibited a stronger and more complex wave vector dependence. Under certain polymer-surfactant conditions, the DLS resul ts at low scattering angles indicated the formation of large clusters.