Unveiling the gradient crystalline structure in the poly(ether-b-amide) foams

Abstract

The microscopic crystalline structures of unfoamed bead (UFB), foamed bead (FB), foamed bead-welded part (FBWP), extrudate (EXT) and foamed extrudate (FEXT) of poly(ether-b-amide) were investigated by micro-wide-angle X-ray diffraction/small angle X-ray scattering (μWAXD/SAXS). The crystallinity of the polyamide hard segment was improved after super-critical foaming (SCF), and double-melting peaks were incurred. A gradient crystalline structure with a core-skin difference was found in the FB, EXT, and FEXT, where higher crystallinity and orientation were found in the skin parts, i.e., the crystallinity in the skin could be 1 % higher than that in the core. It is not beneficial for a good steam-welding. The failure mode during tear process of FBWP is majorly adhesive failure, secondly cohesive failure of the skin detachment from the core, confirmed by digital image correlation (DIC) of the tear propagation and scanning electron microscope (SEM) of the fractured surface The tensile strength of FEXT and FBWP was 1/4 and 1/16 of that of hot-pressed film (HPF) respectively. The evolution of the microscopic structures of HPF, FEXT and FBP during tensile elongation were probed by in situ WAXD/SAXS. The long periods were increased remarkably after SCF, due to the increased crystallinity and improved phase separation. The lattice orientation of FEXT increased rapidly upon stretching, during which the strain-induced crystallization occurred. However, that of FBWP was not so high, since the inadequate inter-bead adhesion led the FBWP to break prematurely.