NMR self-diffusion of associative polymers in aqueous solution: The influence of the hydrocarbon end-chain length on the polymer transport dynamics in single- and two-component mixtures

This paper presents diffusion studies of two ''model'' associative polymers (hydrophobically end-capped poly(ethylene oxide))(AP) differing in end-group length, a factor of great significance in the transport rate of APs in aqueous solution. An increase in end-group length was found to reduce the self-diffusion coefficient. As expected, it also greatly influences the onset of aggregation, as monitored by static fluorescence, and the phase behavior. The longer the hydrocarbon end group, the lower the critical aggregation concentration and the lower the cloud point of the polymer solution; i.e., the lower critical solution temperature decreases. In line with oscillatory relaxation results obtained by T. Annable et al. (J. Rheol. (N.Y.) 37, 695 (1993)), the data of the present paper show that AP transport in equimolar mixtures of the two APs is characterized by two-component behavior over a wide range of semidilute polymer concentrations. The diffusion of the polymer is therefore judged to be molecularly determined in this concentration region; i.e., the polymers diffuse independently within a network structure. The dynamic hydrophobic domains, keeping the network together, are considered to be mixed, containing end groups of both lengths. We also find that the cloud point temperature of these mixed solutions are situated between those of the single component solutions following a two-state relation. (C) 1997 Academic Press.