Cooling rate-dependent polymorphism in thermoplastic polyurethanes: effect of hard segments content

Thermoplastic polyurethanes (TPUs) are multi-block copolymers consisting of hard (HS) and soft segments (SS). The hard segment, based on 4,4′-methylenediphenyl diisocyanate and 1,4-butanediol (MDI/BD), crystallizes into two forms (Form I and Form II) depending on cooling conditions. While these polymorphs exhibit distinct mechanical properties, a detailed understanding of their formation conditions is lacking. This study explores how HS content and the cooling rate of the melt influence TPU polymorphism. Using conventional and fast scanning calorimetry, along with in-situ and ex-situ structural characterization, we developed a “polymorph map” correlating cooling conditions and HS content with final structures. For HS content above 50 wt%, both polymorphs coexist at cooling rates of 10–30 K/min, with Form I dominating as the cooling rate increases. Fully amorphous TPUs form at cooling rates >100–1000 K/min. At HS lower than 50 wt%, only Form I crystallizes. Pure Form II cannot form under non-isothermal conditions due to thermal degradation at rates below 1–3 K/min. Polarized light microscopy distinguishes the polymorphs: Form II displays birefringent spherulites. Quenched samples reveal a glass transition temperature linearly dependent on HS content, suggesting partial miscibility between HS and SS. These findings provide a framework for designing TPUs with tailored crystalline structures through precise control of HS content and processing conditions.