Characteristic Rheological Behaviors in Startup Shear of Entangled Polymer Melts

Abstract

The present work explores the characteristic rheological behaviors of polystyrene (PS) and poly(methyl methacrylate) (PMMA) melts in the startup shear. Through a detailed analysis of the stress overshoot, unexpected non-universal features are observed as follows. For all the examined melts, with increasing shear rate, the peak shear stress increases and the peak time decreases. The peak stress and the peak time exhibit a power-law relation, for which the scaling exponent is −1/2 for PS when the applied shear rate is larger than the reciprocal of the terminal time. Meanwhile, for PMMA melts, three scaling regimes with the exponent of −1, −1/4, and −1/2 are evident with the increase of the applied shear rate. Consequently, the relation between the peak shear stress and the peak time is not universal and chemistry dependent, even if the stress and the time are normalized by the plateau modulus and the Rouse relaxation time. This non-universality can be found in the literature data for the other entangled polymers.