Delay of re-entanglement kinetics by shear-induced nucleation precursors in isotactic polypropylene melt

Abstract

Rheological measurements were carried out to in-situ monitor the re-entanglement process of large amplitude oscillatory shear flow (LAOS) modified partially disentangled isotactic polypropylene (i-PP) melt. The characteristic entanglement recovery time (τent) was essentially longer than the average reptation time (τD) of fully entangled melt. At temperatures above 200 °C, the Arrhenius temperature dependence of τent and τD are the same, displaying a normal chain diffusion-controlled mechanism with flow activation energy of 41.4 kJ/mol. However, when the temperature is lower than 200 °C, τent is unexpectedly longer than expectation extrapolated from high temperatures, and a much higher activation energy of 98.6 kJ/mol is observed for the re-entanglement process. Based on our understanding, the re-entanglement process of shear-induced disentanglements in i-PP melt was postponed by the existence of quasi-ordered clusters created by shear at relatively low temperatures and their dissipation turns to be the rate-determining step for the whole recovery. Moreover, morphological study under polarized optical microscope showing much higher nucleation density which originates from the flow-induced precursors corroborates our hypothesis. Meanwhile, the surviving time of LAOS-generated nucleation precursors is comparable to the recovery time of disentangled chains, suggesting that the delay effect of nucleation precursors on chain diffusion may last over the entire re-entanglement process. Notwithstanding the obstruction of “extra” physical entanglements for chain diffusion, the molecular weight (Mw) dependence of entanglement recovery still follows the reptation model-based power law: τent=(a+b)Mw3.4, where a and b is the contribution of chain diffusion and dissipation of precursors, respectively.