Direct NMR evidence for a local resriction in the segmental chain mobility of a model ionomer

To probe the local mobility of the chain segments directly attached to ionic multiplets, two model ionomers have been synthesized. The first one is a carboxylato-telechelic deuterated polystyrene in which the chain segments bearing the sodium carboxylate end groups have been selectively protonated; the second one has a protonated polystyrene backbone terminated at both ends with a deuterated polystyrene segment bearing the sodium carboxylate groups. Both polymers have been studied by high-resolution solid-state C-13 NMR. Line-width measurements of the protonated units have clearly shown that the dipolar interactions of the sodium carboxylate end groups are responsible for a mobility restriction of the chain end segments. This experiment is thought to be the first direct evidence for the model recently proposed by Eisenberg et al. for ionomer morphology according to which the ionic multiplets are surrounded with a shell of constrained polymer segments.