Detection of Heterogeneities in Dry and Swollen Polymer Networks by Proton Low-Field NMR Spectroscopy

We report on the implementation of modern proton multiple-quantum NMR methods for the characterization of molecular order and dynamics in polymer networks and melts on cost-efficient low-resolution low-field NMR instrumentation. The method permits the extraction of chain order parameter distributions, and is therefore sensitive to a heterogeneity of cross-links and other topological constraints. Data from samples with bimodal network chain length distributions acquired at 20 MHz (0.47 T) are in quantitative agreement with results obtained at high field (500 MHz). It is shown for the first time that the chain order distribution is broadened upon swelling, providing evidence for the nontrivial nature of the swelling process.