Thermoplastic polyurethanes : Domain morphology evolution under mechanical and thermal load as a function of composition

Abstract

In this work three groups of thermoplastic polyurethane (TPU) materials have been studied. In the first group, Machine-cast TPUs are strained and monitored by small-angle X-ray scattering (SAXS). In the second study TPUs with varying diisocyanates and chain extenders are strained and monitored by (SAXS). In the third study TPUs in presence of nucleating agents are monitored by SAXS under thermal load. Upon stretching hard domains are destroyed. Most stable are the domains of materials with Hard Segment Content (HSC)=30. Domain stability decreases with increasing HSC. Chord distribution functions (CDF) exhibit the same sequence of static long-period bands. The band positions form a Fibonacci series, related to the underlying polyaddition process. This indicates a nearly quasicrystalline arrangement of stringed hard domains, identified as the strain probes of the discrete SAXS. In the second study, the model-free data inspection shows the difference in nanoscopic straining mechanisms. From these results a onedimensional morphological model for the analysis of the longitudinal SAXS is built. Its components are particle scattering and the two strongest scattering entities made of 2 hard domains with some soft phase in between (soft domain). Thus the model comprises 1 (solo) and 2 (duos). For all materials the average hard domains are about 6 nm high, and the domain heights vary by 38 to 48 percent.