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 (T c ), Form-II and Form-I develop at T c ≥ 130 °C and T c ≤ 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.

Quantifying the cooperative evolution of microphase segregation and nanostructural order in annealed polyurethanes of MDI‐BDO‐PTHF

“…A lower-temperature endotherm is consistently observed roughly 35 C above T a , in agreement with the expected range of roughly 20-40 C. 78 This peak is nearly independent of composition except at very low and very high T a . This has been assigned to the melting of the initially formed ordered structure, 50,79 in this case the increasing fraction of ordered paracrystalline domains due to the annealing procedure, or to relaxation of interfacial structures. 80 The monotonic behavior with T a had earlier been attributed to ordering of progressively longer hard segment sequences with increasing T a .…”

Polyurethane hard domain composed of mixed hard segments

“…So far, researchers mainly focused on single component TPU systems, and the study on TPU blends was still rare. 35,36 Xu et al investigated the effect of water absorption on the T g and the degree of phase separation of thermoplastic polyurethane blends by dynamic mechanical analysis, FTIR spectroscopy, and SAXS. 37 It was concluded that hydrogen-bound water rather than free water weakened the interfacial state between the soft and hard domains, thus lowering the T g of the samples.…”