INVESTIGATION OF THE DIFFUSION PROCESS IN CROSS-LINKED POLYSTYRENES BY MEANS OF NMR IMAGING AND SOLID-STATE NMR-SPECTROSCOPY

The diffusion process of a solvent (dioxane) in samples of different c ross-linked polystyrenes can be monitored by combined NMR imaging and solid-state NMR techniques. NMR imaging data reveal a strong dependenc e of the diffusion behavior upon the cross-link density: in the case o f lowest cross-linking (1 %), a case II diffusion process takes place, whereas in the case of highest cross-linking (5 %), Fickian diffusion occurs. The 2.5 % cross-linked polystyrene shows intermediate behavio r. Solid-state NMR relaxation parameters indicate great differences be tween the unswollen and the swollen state. Whereas the relaxation data obtained in the unswollen state are almost independent of the degree of cross-linking, great differences are observed between the 1 % and 5 % cross-linked polymers in the swollen state, with both the highest o verall mobility and, in particular, a high mobility of the phenyl grou p in the case of the 1 % cross-linked material. This behavior can be d irectly related to the different types of diffusion, suggesting that t he strong interaction of the highly mobile phenyl groups with the diox ane in the low cross-linked material favors case II diffusion. In the 5 % cross-linked materials, with a lower overall mobility and no predo minant high phenyl group motion, Fickian diffusion occurs. Increased c ross-linking corresponds to increasing T1rhoC and T1C values, thus ind icating reduced mobility.