Nuclear magnetic resonance relaxation studies of polyacrylamide solution

The cohesive interaction among polymer chains in a polyacrylamide (PAAm)-D2O solution has been studied by NMR relaxation. The NMR relaxation times of PAAm in the good solvent D2O were measured at different temperatures. The results show that the solution system has a high local viscosity and that its relaxation characteristic is soft-solid-like. The temperature dependence of the relaxation behavior of the solution is obviously different from that of ordinary polymer solutions. The difference lies in the relaxation behavior of the methylene protons in the main chain of PAAm, as shown by analyzing the relaxation process with single exponential and biexponential decays. As the temperature increases, the solvation is weakened, leading polymer chains to form curling coils, thus hindering the movement of the methylene protons among the main chains. It can be expected from the existence of 80% fast-relaxing protons that there are a thigh number of entanglements among the polymer chains in PAAm solution. The information about entanglements among the polymer chains can be deduced from the biexponential dependence of the spin-spin relaxation on the concentration of the polymer solutions.