The Influence of Thermoreversible Diels–Alder Bonds on the Structure, Morphology, and Rheology of Recyclable Thermoplastic Polyurethanes

Conventional thermoplastic polyurethanes (PU) are difficult to recycle due to their high melt viscosity. In this work, a semi-crystalline polyurethane comprising thermoreversible Diels–Alder (DA) cycloadducts in the backbone was prepared and compared to an analogue containing conventional butanediol (BDO) chain extender. The DA-PU was shown to successfully dissociate at high temperatures and reach a very low melt viscosity of 4 Pa.s. Isothermal crystallization experiments showed that the crystallization kinetics of DA-PU were strongly dependent on the melting temperature because of its dynamic molecular weight. SAXS and rheological measurements showed that DA-PU exhibited a more phase-mixed melt morphology than its BDO-PU analogue, which displayed distinct hard-soft segment phase separation. These morphological differences were reflected in the rheological measurements. Frequency sweep measurements showed that master curves could be constructed for DA-PU by time-temperature or time-molecular weight superposition. Kinetic and thermodynamic parameters Ea, ΔH⦵ and ΔS⦵ for the DA reaction were obtained from rheology and FTIR. The data shows that incorporating DA cycloadducts in thermoplastic polyurethanes provides a fundamental change to thermal properties, morphology, and rheology, suggesting a different paradigm for processability and properties.