Melting behavior of polymorphic MDI/BD-block TPU investigated by using in-situ SAXS/WAXS and FTIR techniques. Hydrogen bonding formation causing the inhomogeneous melt

Abstract

MDI/BD hard segment based TPUs exhibit two distinct polymorphs, form I and form II, when it is solution-cast at 60 and 145 °C. Form I shows paracrystalline features, whereas form II typically shows semi-crystalline spherulitic structures with birefringence. The melting behavior of these two forms is investigated using in-situ FTIR and simultaneous in-situ SAXS and WAXS techniques. The long spacing of form I is smaller than that of form II with increasing the temperature up to 200 °C, as the thermal expansion increases the size of the hard and soft domains and therefore also the length of the long spacing. Partial melting of the hard domains above 200 °C increases the long spacing while the hard domains that survive in the melt reorganize into larger domains. As the size increment above 200 °C of form I is larger than that of form II, the long spacing of form I becomes larger than that of form II. The presence of significant H-bonding between the urethane carbonyl CO and the amide N–H and the diffuse scattering confirm the inhomogeneous melt containing tiny MDI/BD hard block aggregates. The presence of these aggregates in the melt may influence the crystallization behavior of the MDI/BD hard blocks during cooling.