Water self-diffusion in aqueous associative polymer solutions

Water self-diffusion in aqueous model associative polymer (AP) solutions, hydrophobically end-capped poly(ethylene oxide), C(12)EO(200)C(12) (AP9,) and C(12)EO(90)C(12) (AP4), has been studied with the NMR-PGSE method and compared to the diffusion in nonmodified poly(ethylene oxide) PEO. It was found that it decreases monotonically with increasing polymer concentration, giving D-i/D-0 approximate to 0.2 at 50 wt % (D-0 being the water self-diffusion coefficient in the absence of polymer), independently of polymer molecular weight and modification. In further evaluation of the data, the cell-diffusion model was used. Such an analysis suggests that up to a polymer content of about 2 wt % AP9, water diffusion is not significantly affected by the polymer. Above this concentration, up to about 10 water molecules per EO group are affected in AP9 and AP4 solutions. On increasing the temperature, water self-diffusion increases, following an Arrhenius-like equation, with E(a) equal to that of pure water at low polymer content (10 wt %). The activation energy increases with polymer content, and at 50 wt %, E(a) is about 30 kJ/mol, independently of polymer type. A minor difference in E(a) between AP4 and AP9 solutions at intermediate polymer content is likely to originate from the ability of AP4 to form well-developed cubic phase structures. An increase in temperature was found to lead to a slight dehydration of the associative polymer EO monomers closest to the hydrophobic core.