Formation of crystal-like structure and effective hard domain in a thermoplastic polyurethane

Traditionally, the hard domain of polyurethane was considered as the physical crosslinking point and has been studied for several decades. However, due to the hierarchical and metastable nature of the hard phase, its delicate structure hasn't been resolved thoroughly and its formation mechanism was unclear too. In this work, crystal-like structure in an amorphous polyurethane was discovered by conventional DSC experiments, and Flash DSC was used to study the thermodynamics and kinetics of such crystal-like structure by one-step and two-step temperature annealing experiment. It was found that the formation of crystal-like structure included nucleation and growth. However, its equilibrium melting point couldn't be obtained by Hoffman-Weeks' method and its structural regularity was weaker than the crystal. Moreover, the nucleus had more compact structure than its outer layer and owned higher melting point. With the help of time dependent FTIR experiment, we proved that the hard domain formed much earlier than the crystal-like structure. Such result implied that the crystal-like structure was birthed inside the hard domain and its core was the reorganized hard segments, which acted as nucleus. After that, the reorganization of outer layer began, and was the growth process of crystal-like structure. Furthermore, time dependent oscillatory rheological experiment proved that the physical network in polyurethane formed much later than the formation of crystal-like structure, which meant the growth of hard domain was slow. In conclusion, the formation of effective hard domain, which could act as physical crosslinking point, experiences hydrogen bonding association, crystal-like structure formation and size expansion processes.