Block length distribution, morphology, and property of thermoplastic polyurethanes affected by production method: A polymer-by-process investigation

Two thermoplastic polyurethanes (TPUs) based on the same monomers and composition were produced by two different production methods – hand mix batch process and continuous band casting. The soft segment was poly(hydrofuran) (PTHF) with a molar mass of 1000 and the hard segment was made of 1,4-butanediol (BD) and 4,4′-methylene diphenyl diisocyanate (MDI). The hard segment content amounted to 42%. The distinctions in crystallization and thermal behavior and mechanical properties of the two TPUs were ascribed to differences in the hard and soft block length distribution caused by the different production methods. Using thermal fractionation – a series of successive self-nucleation and annealing steps – the minor differences in hard block length distribution could be shown and quantified. The length distribution of the hard and soft blocks of the machine-produced sample was narrower than that of the hand-cast sample. The former with the narrow block distribution showed thicker and mechanically and thermally more stable hard domains. The more uniform block length distribution facilitated crystallization and resulted in improved tensile recovery behavior and elasticity. The second with the broader distribution, however, showed the highest tensile strength at break, which was ascribed to an improved strain-induced hardening of the soft phase.