Short-Term Morphology Relaxation of Thermoplastic Polyurethane Elastomers after Fast Strain Steps

Abstract

Strain steps are applied to elastomers in a pneumatic relaxometer and monitored by small-angle X-ray scattering (SAXS). The relaxometer provides a rise time of 13 ms for strain pulses of step height ?e = ±1 in strain. The basic character of the 2D SAXS frames is examined and corresponding invariants Q(t) are analyzed. Three thermoplastic polyurethanes (TPU) of hardness 85 Shore A with different soft segments are studied both unannealed and annealed. The first response of all materials is a fast morphology conversion which finishes within tmc =250 ms. Because it has been untraceable, it is characterized by a settling stroke Q(tmc) - Q(0). The second response is a slow morphology adjustment process which complies with logarithmic relaxation. It is characterized by a relaxation rate DQ = Q(10 t)/Q(t) - 1. Comparison indicates that the nanoscopic morphology relaxation processes appear to have little direct relation to the macroscopic stress relaxation curves. The materials differ with respect to hard-domain morphology stability and morphology recovery. Most unstable is the morphology of the annealed polyether-based material. It forms nanofibrillary entities when strained. © 2020 The Authors. Published by Wiley-VCH GmbH