Polymorphic Self-Poisoning in the Isothermal Crystallization of Thermoplastic Polyurethanes

Thermoplastic polyurethanes (TPUs) are multiblock copolymers whose properties are strongly influenced by the crystallization of the hard segments (HS). Crystallized HSs based on 4,4'-methylenediphenyl diisocyanate/1,4-butanediol can develop two distinct polymorphs: the thermodynamically stable triclinic Form II or the kinetically favored paracrystalline Form I, each associated with different mechanical responses. While the effect of cooling rate on polymorphic crystallization has been studied, the isothermal crystallization kinetics of TPUs with varying HS content are less explored. Here, we investigate the isothermal crystallization of TPUs containing 29-80 wt % HS using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), polarized light optical microscopy (PLOM), and fast scanning calorimetry (FSC). TPUs with low HS content (29 and 33 wt %) crystallize exclusively in Form I, and the overall crystallization rate decreases monotonically with increasing temperature at low supercooling. In contrast, TPUs with ≥50 wt % HS display a nonmonotonic temperature dependence: the overall crystallization rate first increases with supercooling, then passes through a relative minimum, and rises again at larger supercooling. Structural analyses confirm that this inversion of the temperature coefficient of the crystallization rate originates from the competition between the formation of the two polymorphs. In agreement with previous literature, the rate minimum is tentatively attributed to polymorphic self-poisoning, in which Form I temporarily hinders the crystallization of Form II. These findings establish a direct link between polymorphic competition and crystallization kinetics in TPUs, providing new insights into structure formation and strategies for tailoring their properties.