Peculiarities of self-diffusion studies on polymer systems by the NMR pulsed field gradient technique

Abstract

Abstract The measurement of polymer self-diffusion coefficients in solution and melt with the NMR pulsed field gradient technique demands special care because of the slow and generally complicated diffusion process. The echo attenuations are derived and discussed for the following special cases: (i) the macromolecules have a distribution of molar masses, (ii) the species undergo diffusion with two mechanisms, (iii) the diffusion is hindered. The effects of chemical exchange and inner gradients are also discussed. In all these cases deviations from an exponential echo attenuation with a time-dependent self-diffusion coefficient are to be expected, leading to a complication of the interpretation of the experimental data. Based on the mathematical expressions thus obtained and on measurements with a highly sophisticated NMR pulsed field gradient spectrometer, it is shown that the NMR self-diffusion data of polymers in solution and melt so far reported are most likely the effect of mere center-of-mass motions of the macromolecules.