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 (<i>Domain I</i>), crystallization initially occurs as a single low-temperature exothermic peak associated with the formation of metastable <i>Form I</i>. However, when the self-nucleation temperature (<i>T</i> _s) is within <i>Domain II</i> (the self-nucleation <i>Domain</i>), a second, higher-temperature crystallization exotherm emerges and progressively dominates as <i>T</i> _s decreases, indicating a change in polymorphic crystallization to the more ordered <i>Form II</i>. Therefore, self-nucleation not only accelerates crystallization kinetics but also alters the polymorphic outcome, favoring <i>Form II</i> over <i>Form I</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 <i>Form II</i> with decreasing <i>T</i> _s, as evidenced by its characteristic diffraction patterns and by the growing presence of <i>Form II</i> birefringent spherulites, particularly in high-HS-content TPUs. Notably, even TPUs with low HS content (29-33%), which are typically incapable of crystallizing in <i>Form II</i> under nonisothermal conditions, develop this polymorph induced by the thermal treatment applied by self-nucleation. The reason behind the formation of <i>Form II</i> by self-nucleation is the persistence of interurethane hydrogen bonds in the melt, which may favor the crystallization of <i>Form II</i> due to its higher content of bonded carbonyl and N-H groups with respect to <i>Form I</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.