On the role of pore formation in oscillatory neck propagation in poly(ethylene terephthalate)

Two regimes of oscillatory neck propagation in amorphous poly(ethylene terephthalate) are known. The first is related to the formation of pores, which presumably reduce heat conductivity and facilitate a fast temperature rise in the region of the transition of the unoriented polymer to a neck. The pores are formed if extension and compression of a polymer sample are accompanied by the development of shear bands. The second oscillation mode is observed in the absence of pores. In this case, the heating of the polymer and the stress release are much slower and, in order to develop self-excited oscillations, it is necessary to load a polymer specimen by a series-connected spring (at a conventional length of test samples). The annealing of PET in water suppresses the development of shear bands, thus altering the oscillation regime.