129Xe NMR Investigation of the Free Volume in Dendritic and Cross-Linked Polymers

129 Xe NMR spectroscopy was used to study the free-volume changes associated with cross-linking of linear polymers and extensive chain branching in dendritic tree molecules. Qualitative changes in the free volume in model networks of poly(oxypropylene) and in starburst dendrimers of poly(amidoamine) were studied as a function of the cross-link density and the generation, respectively. Consistent with the present understanding of these end-linked networks, the change in chemical shift of the xenon was found to correlate well with the reciprocal of the average molecular weight between cross-links of the end-linked networks. With dendrimers, the chemical shift of xenon varied linearly with the generation number. A modified form of the chain-end free volume theory applicable to dendritic molecules was used to interpret the data.