Cross-link density of a dispersed rubber measured by 129Xe chemical shift

Xe-129 NMR chemical shifts of adsorbed xenon (natural isotopic abundance) were used to probe the free volume in a cross-linked rubber, polybutadiene (PB), alone and occluded within high impact polystyrene (HIPS). Shift dependency on cross-link density, or N-m (average number of monomer units between chemical cross-links) determined by the classical solvent swelling method, was detailed for high-cis-1,4-PB. A decrease in N-m from 262 to 4 caused the shift to increase 4 ppm. A curve inflection, evident at 218 ppm (and N-m = 30), corresponded to a free volume with an average spherical diameter of 0.49 nm using the empirical (shift-pore size) relation known for zeolites. This diameter is essentially equal to that of the PB chain (0.48 nm) at its T-g (glass transition temperature). T-g plotted against N-m showed the same inflection (at N-m = 25), corroborating a regime with more severe interchain contacts when the free volume diameter is inferior to the chain diameter. The empirical relation between Xe-129 shift and N-m observed for pure PB was used to estimate the cross-link density of PB dispersed in HIPS as a function of cure time. Cross-linkage was greatly slowed after 8 h of curing, when N-m estimated by NMR spectroscopy was 10.