Crazing and yielding in glassy polymers of high molecular weight

This feature article presents many experimental results in order to explain and advance our understanding on crazing during tensile deformation of glassy polymers. Ample data are presented to show that crazing is an activated process and therefore takes time to emerge. For example, ductile polymers can also show crazing prior to yielding as demonstrated with bisphenol A polycarbonate and PMMA at elevated temperatures. As an integral part of the investigation, we show that crazing can emerge in absence of any ongoing deformation, i.e., taking place not during extension but after termination of the extension. More remarkably, glassy polymers such as polystyrene and amorphous poly(lactic acid) (PLA) can undergo macroscopic fracture without any ongoing extension after a rapid step extension that first results in massive crazing. Moreover, we investigate several ways to either suppress or promote craze formation. Specifically, it is shown that a craze-free ductile PLA turns brittle and develops crazing upon physical aging or the incorporation of solvent. Conversely, adequate pre-melt-stretching is found to prevent physical aging from causing crazing and brittle fracture. Crazing perhaps reveals the existence of structural and dynamic heterogeneity that can be more severe in one glassy polymer than another. For example, PMMA hardly shows crazing before brittle fracture at room temperature in contrast to PS. At the present, no theoretical description is available to predict which polymer is more prone to crazing since all glassy polymers of linear flexible chains show comparable characteristics in terms of the underlying chain network structure.