Uniaxial Elongational Behavior of Unentangled Polymer Melts

Uniaxial extensional behavior was examined for unentangled Polystyrene (PS; M = 27k) and Poly (tert-butyl styrene) (PtBS; M = 53k) in melt state with a commercially available filament stretching rheometer, VADER 1000. At sufficiently high stretching rates, strain hardening was observed in viscosity growth function of both PS and PtBS, in particular for PtBS. Stress relaxation just after stretch to Hencky strain of 2.5 was also examined. Relaxation time was insensitive to stretching rate when the Weissenberg number Wi defined with respect to the terminal relaxation rate in the linear regime is smaller than unity, whereas the initial relaxation (just after cessation of stretching) was considerably accelerated when Wi > 1. This result is consistent with previously reported results for entangled PS chains, where the initial Rouse relaxation was accelerated under fast stretch. Thus, the reduction of monomeric friction under stretching at high Wi plays a significant role in uniaxial extensional behavior of polymer melts irrespective of the entanglement.