Investigation of solid-state transitions in linear and crosslinked amorphous polymers

This paper deals with the determination of the main characteristics and the assignment of the molecular motions leading to the solid-state transitions observed in amorphous polymers at temperatures lower than the glass-rubber transition. First, the specific features of these secondary transitions (β, y, δ,...) are briefly described. Then, the behaviour of various polymer systems is analysed: poly(cycloalkyl methacrylates) and their intracycle motions, poly(ethylene tere-phthalate) and bisphenol A polycarbonate β transition and the effect of small molecule antiplasticisers, aryl-alkyl polyamide transitions, the β transition of poly(methyl methacrylate) and its maleimide as well as glutarimide random copolymers. Additionally linear polymers, aryl-aliphatic epoxy systems, with or without antiplasticisers, illustrate the case of crosslinked amorphous polymers. Whereas all the systems considered were investigated by dynamic mechanical measurements and 13 C solid-state nuclear magnetic resonance experiments, some systems were also studied by other techniques such as dielectric relaxation and molecular modelling. Poly(methyl methacrylate) and bisphenol A polycarbonate constitute the best examples of the level of description of the molecular motions involved in the β transition and, in particular, the nature and extent of the cooperativity which develops in the high-temperature part of the transition. This is achieved by combining all the experimental and modelling techniques presently available.