Determination of the Local Cross-Link Density in Polyisobutylene-Based Elastomers by NMR Imaging

Gravimetrical Flory−Rehner experiments are combined with MRI experiments to study the spatially dependent degree of cross-linking in unfilled, 1,6-hexamethylenediamine-cured poly(isobutylene−p-methylstyrene−p-bromomethylstyrene) terpolymers. MRI relaxometry reveals two proton T 2 relaxation decay times in CCl4 swollen specimens: a fast decaying component reflecting the constrained chain segments near cross-links and entanglements and a slow decaying component originating from less constrained, remote chains. On the basis of a linear relation between the bulk M n,eff (number-average molecular weight between effective cross-links) and the volume-averaged T 2 decay times, MRI allows the determination of the local cross-link density in inhomogeneously cross-linked specimens by means of the spatially dependent T 2 decay times. MRI solvent diffusion experiments reveal that the ingress kinetics of cyclohexane in PIB terpolymers is Fickian. It is shown that the diffusion coefficient varies linearly with the cure state and the volume-averaged T 2 decay times.