The effect of excess volume on molecular mobility and on the mode of failure of glassy poly(ethylene terephthalate)

To gain further understanding regarding the modes of molecular motion which contribute to the ductility and toughness of glassy polymers, and the factors that influence such molecular mobility, a study has been undertaken of the effect of the amount of excess volume in glassy poly(ethylene terephthalate) on the mode of failure observed in tensile tests and on the associated level of molecular motion. The results of tensile stress-strain studies on films annealed at temperatures below the glass transition temperature indicate that there is a relationship between the mode of failure and the level of excess volume trapped in the glassy polymer. From dynamic mechanical loss studies of the annealed films, however, it is evident that the modes of motion associated with the major subsidiary dynamic mechanical absorption, i.e., the γ-relaxation process, are relatively unaffected by the loss of free volume caused by the densification of the glassy polymer on annealing. Further, these studies suggest that the limited segmental mobility associated with the onset of the glass transition phenomenon may be significant with regard to the ductile behavior of the glassy polymer.