Self-Nucleation Enables Polymorphic Selection in Thermoplastic Polyurethanes

Abstract

This work investigates the self-nucleation behavior of thermoplastic polyurethanes (TPUs) with hard segment (HS) contents ranging from 29 to 80 wt %. Differential scanning calorimetry (DSC) reveals that upon cooling from the isotropic melt (Domain I), crystallization initially occurs as a single low-temperature exothermic peak associated with the formation of metastable Form I. However, when the self-nucleation temperature (T_s) is within Domain II (the self-nucleation Domain), a second, higher-temperature crystallization exotherm emerges and progressively dominates as T_s decreases, indicating a change in polymorphic crystallization to the more ordered Form II. Therefore, self-nucleation not only accelerates crystallization kinetics but also alters the polymorphic outcome, favoring Form II over Form I. This interpretation is further supported by ex situ Wide-Angle X-ray Diffraction (WAXD) and polarized light optical microscopy (PLOM) measurements, which confirm the increasing presence of Form II with decreasing T_s, as evidenced by its characteristic diffraction patterns and by the growing presence of Form II birefringent spherulites, particularly in high-HS-content TPUs. Notably, even TPUs with low HS content (29–33%), which are typically incapable of crystallizing in Form II under nonisothermal conditions, develop this polymorph induced by the thermal treatment applied by self-nucleation. The reason behind the formation of Form II by self-nucleation is the persistence of interurethane hydrogen bonds in the melt, which may favor the crystallization of Form II due to its higher content of bonded carbonyl and N–H groups with respect to Form I. These findings demonstrate that self-nucleation enables precise control over polymorphic selection in TPUs across a wide compositional range, offering a versatile strategy for tailoring material properties through thermal processing.