Abstract
Nonlinear rheological behavior under uniaxial elongation was examined for unentangled melts of polystyrene (PS27; molecular weight M = 27k) and poly(p-tert-butylstyrene) (PtBS53; M = 53k) having nearly the same number of Kuhn segments per chain (nK = 30 and 35 for PS27 and PtBS53, respectively). For both materials, the steady state elongational viscosity, ηE, exhibited strain-rate-hardening and then strain-rate-softening on an increase of the Weissenberg number Wi ≥ 0.3 (Wi = eτ1eq, with τ1eq and e being the longest relaxation time in the linear viscoelastic regime and the Hencky strain rate, respectively). For the unentangled melts, the hardening and softening were free from any entanglement nonlinearity, so that the hardening was unequivocally related to the finite extensible nonlinear elasticity (FENE) of the chain, and the softening, to the suppression of the FENE effect due to reduction of the segment friction ζ occurring for the highly stretched and oriented chain. Thus, the ζ reduction, speculati...
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