Parallel micro- and mesophase transitions of thermoplastic polyurethane induced by thermal annealing

Abstract

We report on the first rheology–morphology evidence revealing that, under short-term thermal annealing well below the melting temperature, thermoplastic polyurethanes (TPUs) can undergo parallel micro- and mesophase transitions. While thermal annealing is shown to promote the well-known microphase transition, characterized by promoted hard segment (HS) associations, we have identified another mesophase transition, characterized by an enhanced aggregation of micron-sized ellipsoidal clusters that harbor the microphase above. Upon increasing system temperature beyond the melting temperature, while the microphase transition eventually diminishes due to its enthalpic nature, the corresponding mesophase transition persists and, thereby, notably modifies the rheological features of the annealed TPUs. The last feature is proposed to arise from the thermal-induced soft segment (SS) entanglement between two proximate TPU clusters during the HS associations, while the inter-cluster SS entanglement remains intact beyond the melting temperature due to its entropic nature. The present findings point to the potential to manipulate the multiscale phase behaviors of TPUs and similar materials for various end applications using easily implementable physical treatments such as thermal annealing.