A metallurgical approach to the pre‐yield and yield behavior of glassy polymers

This paper discusses some new mechanical and small angle neutron scattering (SANS) data on glassy polymers, both thermoplastics and thermoset resins, from the point of view of dislocation‐like defects introduced in the molecular chain arrangement by deformation. In the pre‐yield stage, a new parameter, the work‐hardening rate K is introduced and its measurement is defined. Experiments are reported which show that K can be used as a very sensitive probe for microstructural changes during physical aging or curing. In one hand, the theory of yielding is revisited to make clear how dislocations and their propagation in polymers depend on specific features like entanglements and chain stiffness. On this basis, experimental internal stresses and activation volumes at yield (i.e., the temperature slope of yield stress) are accounted for. On the other hand, SANS data provide us with experimental evidence at the scale of 10 to 20 Å of the dislocation nature of the molecular “shear defects” introduced in the polymer by deformation. Finally, temperature is known to have a pronounced influence on yield processes. It is shown that two distinct deformation modes exist below and above a critical temperature T c . Above T c , a dislocation climb, which probably involves β‐processes, gives rise to a “diffusional” deformation mode where chains within a (diffuse) shear band are no longer oriented. A tentative formalization of this behavior, and its relation to the small strain creep of polymers, are then presented.