The effects of polydispersity, entanglements, and crosslinks on pulsed‐gradient NMR diffusion experiments in polymer melts

We have used the pulsed‐gradient spin‐echo NMR method to measure diffusion in a highly polydisperse polymer, a hydroxyl‐terminated polybutadiene. We have modelled the expected spin‐echo attenuation assuming that the low‐molecular‐weight fractions are unentangled and constitute a Rouse‐like diluent to the entangled larger molecules. Fitting this simulation to our data is consistent with the model and permits the extraction of the range of diffusivities and a semiquantitative value for the molecular weight above which entanglements inhibit diffusion. The model is extended to include a monodisperse diluent present additionally, e.g., a curing agent. This extension is tentatively applied to polybutadiene resins undergoing curing and shows that the diffusivity of the remaining unreacted curing agent is little affected by network formation, whereas the retardation of the unreacted polymer molecules is clearly evident.