The segmental responses to orientation and relaxation of thermoplastic poly(ether-ester) elastomer during cyclic deformation: An in-situ WAXD/SAXS study

Abstract

Poly(ether-ester) (PEE) consists of crystalline poly(butylene terephthalate) (PBT) hard segments and amorphous poly(tetramethylene oxide) (PTMO) soft segments, thus endowing the material with superior elasticity and high tensile strength during application. The segmental responses to orientation and relaxation during cyclic deformation of PEE films were probed by a combination of in situ wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) in order to elucidate the contributions to mechanical properties. Reversible strain-induced crystallization (SIC) of PTMO was confirmed within the scope of the strain. The initial α phase of PBT hard segment did not transform to β phase upon stretching. SAXS showed progressive orientation of PBT ribbon-like lamellae at lower stresses, followed by formation of oriented nano-fibrils at higher stresses. Only when the film was unloaded after low stresses was the long period in the strain direction mostly reversible. For relaxing from medium and high stresses, tilting of PBT lamellae domains was found in the relaxed states, leading the elastic performance to decrease. Under high loading stresses, SAXS scattering in the strain direction is due to correlations between hard and soft blocks along the length of well oriented “load bearing” nano-fibrils which also comprise PTMO SIC. High speed melt-spun PEE fibers were also investigated by SAXS to understand the effect of super-fast extension induced orientation on the morphology and mechanical properties.