Stress relaxation of polymer melts in biaxial and planar elongations

Abstract

We measured biaxial and planar stress relaxation moduli of polystyrene (PS) and low-density-polyethylene (LDPE) melts. For both PS and LDPE, and for both biaxial and planar elongations, the relaxation moduli were factored into strain and time dependent factors. The strain dependent factor, i.e., the damping function was larger for LDPE than for PS. The damping function for the uniaxial elongation was estimated from the uniaxial elongational viscosity data. The damping functions for uniaxial (hE), biaxial (hB) and planar (hP) elongations were found to be in order of HB < hP < hE, if compared at the same strain. The damping function for shear deformation (hγ) was also measured. It was found that hP was smaller than hγ even if plotted against the strain invariant, although the critical strain, at which the damping functions started to decrease, was almost the same for hP and hγ.