Effect of Two Crystalline Forms on the Multiple Melting and Crystallization Kinetics of Thermoplastic Polyurethane

Abstract

Exploring the origin of multiple melting of thermoplastic polyurethane (TPU) is of great significance for comprehending the mechanism for its phase transition. Herein, the effects of two crystal forms (Form-I and Form-II) and crystal form transition on the melting behaviors and crystallization kinetics of TPU are reported. As the function of the crystallization temperature (Tc), Form-II and Form-I develop at Tc ≥ 130 °C and Tc ≤ 130 °C, respectively, which leads to distinct differential scanning calorimetry (DSC) melting curves of TPU. It is found that the repeated melting–recrystallization occurs during the melting of Form-I, which includes the crystal form transition of Form-I to Form-II, resulting in four melting peaks in the DSC heating curves of Form-I. The results reveal that the mesophase (Form-I) could transform into the stable crystal form (Form-II) through melting–recrystallization upon heating in TPU, and TPU has the ability to continuously reorganize the ordered structure upon heating. Meanwhile, the intervention of Form-I significantly increases the overall isothermal crystallization rates, and the motion ability of TPU molecular chains is the crucial factor affecting the generation of Form-I.