Temperature-induced Evolution of Micro-structure and Chain Dynamics in Thermoplastic Polyurethane with Low Hard Segment Content as Studied by In Situ Synchrotron SAXS and Time-Domain NMR Experiments

The microstructural evolution of a thermoplastic polyurethane (TPU) with low hard segment content has been monitored utilizing in situ real-time synchrotron small angle X-ray scattering (SAXS) and time-domain nuclear magnetic resonance (NMR) measurements. The TPU is composed of 23 wt% of [4,4-methylenediphenyl diisocyanate (MDI)]-[1,4-butanediol (BD)] chain segments, which form hard domains, as [polytetrahydrofuran (PTHF)] forming soft domains. The number and distribution of monomer units in hard blocks is determined by the successive self-nucleation and annealing thermal fractionation technique. In situ SAXS method reveals heating-induced increase in the spacing of hard and soft domains, while time-domain H-1-NMR characterizes the changes in the phase composition and chain dynamics in these domains. A glassy fraction of short MDI-BD chain segments in hard domains passes through T-g above ambient temperature. At higher temperatures, MDI-BD nanocrystals start to melt. Sequence length distribution of MDI-BD chain segments causes a distribution in crystal sizes and wide melting temperature range. The melting is accompanied by the mixing of MDI-BD with PTHF segments in soft domains, and by increase in segmental mobility in these domains. Above 180 ?, the TPU melt is homogeneous on the scale above nanometers according to SAXS data.