Network fraction and molecular motions in polymer composites: An NMR relaxation and self‐diffusion study

We have used NMR T2 relaxation and pulsed‐gradient spin‐echo diffusion techniques to study properties of Arco R45M hydroxyl‐terminated polybutadiene, either unfilled or containing 65 wt.% filler particles (SiO2, NaCl, Al) and cured with isophorone diisocyanate (IPDI), as functions of IPDI content. A short T2 relaxation component arises from the network (gel) whose amount is greatest (up to 92%) near NCO/OH stoichiometry. Gel fraction and stoichiometry concentration both are affected slightly by filler surface reactivity but principally by filler particle size. The diffusion rate of the nonnetwork (sol) molecules has a range of 1–2 orders of magnitude. This range is narrowest near stoichiometry for the smallest filler (SiO2), i.e., the situation in which the sol molecules are least mobile. Branching theory and the hypothesis of a layer of reduced mobility in a wide vicinity of the filler particles provides semiquantitative explanations of these observations.