Morphological Changes under Strain for Different Thermoplastic Polyurethanes Monitored by SAXS Related to Strain at Break

In tensile tests of six different hand-cast thermoplastic polyurethanes (TPU) with a hard-segment content = 0.51, the volume fraction of hard domains (v(h)) has been tracked. The materials exhibit a linear increase up to a strain of approximate to 0.5 (strain-induced phase separation). Thereafter a linear decrease of v(h) is indicated. When stretched by 100% about 25% of the initially present hard domains are destroyed. The extrapolation to v(h) = 0 is close to the elongation at break. Characteristic materials parameters are suggested by the found relation. The tests (maximum strain: 3) are monitored by small-angle X-ray scattering. The morphology is closer to particle scattering than that of machine-processed material. Therefore the longitudinal scattering appears fittable by a 1D statistical model that separates the weak discrete scattering from the height distribution of hard domains, whose weight returns the variation of v(h) with the strain. All TPUs are based on 4,4'-methylene diphenyl diisocyanate (MDI). The other raw components are systematically varied. Two MDI units determine the height of the average hard domain. The variation of v(h) with strain is described by a tensile augmentation (in analogy to the tensile strength) which is realized at an end-of-growth strain. The following hard domain destruction is governed by a consumption factor. Deviations between extrapolated and measured strain at break may be related to the TPU composition.