Self-Nucleation Enables Polymorphic Selection in Thermoplastic Polyurethanes

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.