Nonequilibrium Nature of Flow-Induced Nucleation in Isotactic Polypropylene

Abstract

Based on classical nucleation theory, the current entropic reduction model (ERM) of flow-induced crystallization (FIC) treats external work as perturbation on the framework of equilibrium thermodynamics, which, however, obscures the nonequilibrium nature of FIC. In this work, in situ investigation on FIC under strong flow by combining a unique homemade extensional rheometer and ultrafast X-ray scattering reveals a constant critical strain or time for nucleation in isotactic polypropylene melt in a wide temperature range from 130 to 170 °C. Our discovery contradicts the strain–temperature equivalence predicted by ERM but unveils the nonequilibrium nature of FIC. To account for the temperature independence of flow-induced nucleation, a tentative kinetic pathway of nucleation describing stretch-induced hierarchical structural transitions is proposed through which the capability of flow as driving force is justified.