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.