Effect of temperature on NMR self-diffusion in aqueous associative polymer solutions

The temperature dependence of polymer self-diffusion rates in aqueous solutions of C(12)EO(200)C(12) (AP9, M(w) = 9300) and C(12)EO(90)C(12) (AP4, M(w) = 4600) and nonmodified analogues PEO10 and PEO4 (M(w) = 10 000 and 3400) has been studied. The effect of the hydrophobic end-groups on the self-diffusion of the associative polymer (AP) was found to be proportional to the polymer content and inversely related to the temperature. The variation of the AP self-diffusion coefficient follows an Arrhenius behavior. The resulting apparent activation energies, E(a), increase with polymer content from 15 to 55 kJ/mol in the range 0.5-50 wt % for AP9, whereas the parent PEO10 shows an almost constant E(a) of about 25 kT/mol in the same concentration range. Activation energies derived from self-diffusion and low shear viscosity measurements were found to be quite similar. The distribution of self-diffusion coefficients often observed in AP systems is discussed in terms of distribution of aggregate sizes at low AP content and homogeneity of the network at higher contents. The residence time of an AP monomer in a hydrophobic domain was estimated as 0.1 ms at 25 degrees C and decreases with temperature. Also included are turbidity measurements on the AP systems.