Stribeck, A.; Pöselt, E.; Eling, B.; Jokari-Sheshdeh, F.; Hoell, A.: Thermoplastic polyurethanes with varying hard-segment components. Mechanical performance and a filler-crosslink conversion of hard domains as monitored by SAXS. European Polymer Journal 94 (2017), p. 340-353
10.1016/j.eurpolymj.2017.07.020

Abstract:
When monitoring tensile tests of thermoplastic polyurethanes (TPU) by small-angle X-ray scattering (SAXS) we find a filler-to-crosslink conversion of hard domain function. Its strength is related to the chemical composition and governs the mechanical performance of the TPUs. Acting as fillers, the domains provide a high modulus of elasticity. Once the domains take load, they lose their filler function and the material gains in extensibility. All the five machine-cast TPUs have soft segments from PTHF® 1000 and a hard segment content ≈ 45%. The hard segments are built from different diisocyanates (DI) and diols (chain extenders, CE). The base material has hard segments made from 1,4-butanediol (BD) and methylene diphenyl diisocyanate (MDI). Two other TPUs contain as DIs either the hydrogenated, isomeric MDI (H12MDI) or hexamethylene diisocyanate (HDI), respectively. In two other materials the CE is varied. Here the BD is replaced by either the shorter 1,3-propanediol (PD) or by the longer 1,6-hexanediol (HD). A morphological model is fitted to the SAXS data. It returns nanoscopic parameters, e.g. discriminating between the total (Vt) and the crosslinked (Vx) volume of hard domains. Fillers are Vf = Vt−Vx. Results show that Vf-domains can be converted into Vx-domains. Hydrogenation of the aromatic base DI does not change Vt, but Vx lags behind. Young’s modulus is higher (filler function, high Vf), but the material breaks earlier (low Vx). Generally, Vt increases for small strains (strain-induced domains, SIDs) and decreases for strain>1. SIDs start as fillers. When MDI is replaced by HDI the formation of SIDs is boosted leading to strain-induced hardening – only at low strain. At higher strain the modulus lowers (conversion Vf→ Vx). Only in this material are so many domains converted that Vx increases during stretching. The material breaks late. The long CE increases the average distance between crosslink domains and narrows the distribution of the distances. With the medium CE domains appear less stable at low strain.