Rheology and Phase Change of Polymers and Vesicles

Abstract

This thesis investigated interactions between polymers and vesicles, focusing on the electrostatic interactions and hydrophobic interactions. Systems examined experimentally included a polyelectrolyte with oppositely charged surfactant vesicles, and a hydrophobically modified polymer with the vesicles. Polymers employed were poly (acrylamide-co-diallyldimethylammonium chloride) (PADA), and 2-hydroxyethyl cellulose hydrophobically modified with hexadecyl groups (hmHEC) respectively. The vesicles were composed of an anionic surfactant sodium dodecyl benzenesulfonate (SDBS) and a zwitterionic surfactant lauryl sulfonate betaine (LSB). The experimental methodology was rheometry. For pure PADA solutions, they showed a behavior of Newtonian fluid at low concentrations, while shear thinning took place at high concentrations. Intermolecular hydrogen bonds were the driving force for entanglements and network. Based on the distinct concentration dependence of zero-shear viscosity, three concentration regimes were identified: the dilute regime C < C*(ca. 1wt %), the semidilute regime C* < C < C** (ca. 3wt %), and the concentrated regime C >C**. For PADA-SDBS/LSB vesicle mixture solutions, the rheological properties exhibited nonmonotonic functions of vesicle concentration. At low vesicle concentrations, zero-shear viscosity decreased with concentration, while it beccame increased at higher concentrations. According to the oscillatory shear results, both crossover modulus and apparent relaxation time decreased with the vesicle concentration at low vesicle concentrations. However, at higher vesicle concentrations, they increased. In addition, salt effect on viscosity was also investigated. The effect was pronounced for PADA-vesicle mixture solutions, but not significant for pure PADA solutions. For pure hmHEC solutions, shear thickening behavior was observed at intermediate concentrations and shear rates. Four concentration regimes were identified: the dilute regime (C<0.15wt %), the semidilute unentangled regime (0.15wt %< C<0.25wt %), the semidilute entangled regime (0.25wt %< C<0.4wt %), and the concentrated regime (C >0.4wt %). The four regimes showed distinct concentration dependences of zero-shear viscosity. For hmHEC-vesicle mixture solutions, the variation of rheological properties with vesicle concentration was not monotonic. The zero-shear viscosity was initially increased with vesicle concentration and reached the maximum at a certain concentration. Beyond the concentration, the viscosity was decreased with concentration. For the crossover modulus and apparent relaxation time, their trends were similar to that of the zero-shear viscosity. Strong gelation can take place for certain compositions.