Deformation, Yield and Fracture of Amorphous Polymers: Relation to the Secondary Transitions

Abstract

This paper deals with the mechanical properties (plastic deformation, micromechanism of deformation, fracture) of various amorphous polymers: poly(methyl methacrylate) and its maleimide and glutarimide copolymers, bisphenol A (and/or tetramethyl bisphenol A) polycarbonate, and aryl-aliphatic copolyamides. First, the required background on molecular characteristics, micromechanisms of deformation and fracture characterisation is recalled. Then, the results are discussed for each polymer series, considering information obtained on the motions involved in secondary transitions (mostly β transitions) analysed in a previous paper. The importance of the cooperative motions occurring in the high temperature part of the transition is unambiguously pointed out. Furthermore, in glutarimide copolymers, as well as in aryl-aliphatic copolyamides, it is shown that the dependence of toughness on the entanglement density fails and only the consideration of cooperative β transition motions can consistently account for the results. Finally, concerning the change of strain softening with increasing temperature, two opposite behaviours are observed for polymers in which β transitions result, on the one hand from side-group motions (strain softening decreases) and on the other hand from certain main-chain units (strain softening increases). Two different mechanisms have been proposed, based on a softening of the polymer medium by β transition motions associated with either an intramolecular cooperativity (in the first case), or an intermolecular cooperativity (in the second case) of these motions. Thus, combination of the results of the analysis of the relation of chemical structure to β transition motions with the present conclusions yields a molecular description of the whole set of behaviours involved in the mechanical properties of amorphous polymers, till fracture.