A Simple Access to the (Log/Normal) Molecular Weight Distribution Parameters of Polymers Using PGSTE NMR

A log/normal MWD is characterized by two parameters, its mean molecular weight M(0) and width sigma. It is demonstrated how these parameters can be obtained by model fitting a stretched exponential function (SEF), as characterized by two parameters beta and D(S), to the PGSTE response curve. Based on simulations, two general empirical equations relating beta and D(S) to M(0) and sigma are found. The model enables the MWD characteristics to be determined if the scaling law between diffusivity and molecular weight is known. The sensitivity and relative error of sigma and M(0) are discussed and the applicability of the model is illustrated by analyzing experimental NMR response curve of some PEO samples. The key advantages of this technique are its simplicity, numerical robustness, and reliability.