Structural changes of segmented copolyetheresteramides with uniform aramid units induced by melting and deformation

The structural changes induced by melting and deformation of segmented copolymers with uniform bisesteraramid units and poly(tetramethyleneoxide) (PTMO) based soft segments are investigated. With DSC, WAXS and IR it was shown that the uniform bisesteraramid units self assemble in the melt through the formation of hydrogen bonds. This leads to fast and nearly complete crystallization into lamellae with a thickness of only one bisesteraramid repeat unit (1.8 nm). IR-dichroism measurements showed that during the first 300% strain the bisesteraramid units preferentially orient transverse to the polymer chain direction, after which they become oriented in the polymer chain direction. This is attributed to the high aspect ratio of the thin aramid crystalline lamellae. The following model for the structural changes during deformation was proposed: first the bisesteraramid crystalline network is disrupted and the lamellae are, due to their high aspect ratio, preferentially oriented perpendicular to the polymer chain direction. At the same time, the bisesteraramid lamellae are broken up in their lateral dimension into eventually ‘square’ crystallites. These small crystallites are oriented parallel to the polymer chain direction. The orientation of the polymers was also studied with synchrotron WAXS measurements. After approximately 300% strain, the polyether phase strain crystallizes. The bisesteraramid orientation is irreversible while the PTMO orientation disappears upon strain release. PTMO1000 based soft segments and PTMOm soft segments strain crystallize reversibly and PTMO segments longer than 1000 g/mol strain crystallize irreversibly.