Concentration-dependent proton magnetic cross relaxation in aqueous polyoxyethylene solutions

Longitudinal proton magnetic relaxation rates were measured in aqueous solutions of polyoxyethylene (POE) 400 as a function of the polymer concentration. At all concentrations the plot of reduced magnetization vs. pulse separation in the 180°-t-90° sequence yields a nonlinear line which is easily separated into two relaxation processes. Both relaxation rates increase faster with the concentration above 50% POE than below. Above this concentration, the fractions of 2 magnetization ascribed to each of the two relaxation phases deviate from the values expected from the chemical composition of the solution. This demonstrates the transfer of magnetization between these phases, i.e., the cross-relaxation phenomenon. The relaxation rates of POE and water protons in the absence of cross relaxation, the cross-relaxation rates, and the normalized intensities of the relaxation components for each of the relaxation phases were computed according to Edzes and Samulski after modifications required by the specificities of the POE-water system. According to the present data, along with those from literature, it is concluded that polymer ordering within the aqueous solution takes place above 50% POE, providing thus a structural basis for the apparent cross-relaxation phenomenon.