Melting Kinetics of Superheated Polymer Crystals Examined by Isothermal and Nonisothermal Fast Scanning Calorimetry

Abstract

Quantitative analyses of the melting kinetics of superheated polymer crystals, including the isothermal analysis of the time dependence of melting and nonisothermal analysis of the heating-rate dependence of melting, were carried out by fast scanning calorimetry on linear polyethylene that has a narrow melting temperature region. The time evolution of the decrease in the total crystallinity during melting was modeled using a first-order kinetic equation with a rate coefficient, which defines a characteristic melting time under isothermal conditions. A superheating-dependent rate coefficient characterizes the heating-rate dependence of the nonisothermal melting. The obtained results of both analyses consistently suggest the presence of a superheating-dependent melting kinetics unique to initially metastable long-chain polymer crystals with the melting rate nonlinearly dependent on superheating. The dependence is exponential at least in a certain range of superheating. Examinations of the dependence on crystallization temperature, which controls the thickness of lamellar crystals, suggest an activated process of detaching a whole crystalline stem. A possible mechanism of the activation barrier is discussed in connection with distinct types of chain folding.