REVERSIBLE MORPHOLOGICAL-CHANGES IN POLY(ETHER ESTER) THERMOPLASTIC ELASTOMERS DURING DEFORMATION AS REVEALED BY SMALL-ANGLE X-RAY-SCATTERING

The scattering behavior of undrawn and drawn annealed bristles of thermoplastic elastomers with conventional and higher molecular weight based on poly(butylene terephthalate) as hard segments and poly(ethylene glycol) as soft segments in a ratio of 49/51 wt.% is studied. Small-angle x-ray scattering (SAXS) measurements with an area detector are carried out on single bristles under or without applied stress and with deformations up to 300%. At low macrodeformations (epsilon less-than-or-equal-to 30-40%) the morphology of the predrawn samples represents assemblies of parallel crystalline lamellae positioned perpendicular to the stretching direction. These morphological characteristics remain unchanged within the entire deformation range (up to epsilon = 300%) for the predrawn samples of lower molecular weight. For the initially undrawn sample of larger molecular weight reversible orientation and disorientation of the crystallites (microdomains) is established in the same deformation range. Common morphological features are found for the predrawn and undrawn samples with increased molecular weight at medium (epsilon = 50-150%) and high (epsilon = 150-300%) deformation ranges. For both samples in an unloaded relaxed state the x-ray patterns can be explained by a zig-zag arrangement of crystalline lamellae, i.e., the microdomains are inclined to the stretching direction. After loading, the microdomains transform to a position perpendicular to the stretching direction. This observed morphological transition is found to be reversible and becomes more pronounced with progressing deformation. It is suspected to contribute to reversible macrodeformations of thermoplastic elastomers in many cases and may be related to the large amount of tie-molecules created during solid-state reactions in those materials.