Structure and swelling of polymer networks: insights from NMR

Abstract

This review addresses the relationship between the structure of elastomers and gels, i.e., polymer networks far above the glass transition, and their macroscopic properties. The network structure is characterized by the density of crosslinks, the chain length distribution, topological defects, and entanglement effects, and can be tuned by varying the preparation conditions. These quantities ultimately determine the functional (mostly mechanical) properties, and sensitively affect the swelling behavior. Here, we review current efforts using different NMR spectroscopy methods to unravel structural details of various networks made from poly(dimethylsiloxane), and correlate these with their swelling behavior in solvents of different quality. It was found that a specific NMR observable based upon segmental orientation correlations reflects not only the network chain structure, but also the thermodynamic state. This allowed us to draw relevant conclusions on the validity of the Flory–Rehner theory for network swelling, and on the applicability of simple models to describe network elasticity.