Biaxially oriented lamellar morphology formed by the confined crystallization of poly(1,4-butylene succinate) in the oriented blend with poly(vinylidene fluoride)

The oriented crystallization of poly(1,4-butylene succinate) (PBSU) in the oriented blend with poly(vinylidene fluoride) (PVDF) was investigated at various temperatures. Films of the miscible blend with a composition of PBSU/PVDF = 30/70 (wt %) were prepared by solution casting and were subsequently quenched in ice water from the molten state. Oriented films of the blend were prepared by uniaxially stretching the melt-quenched films. The drawn films were heat-treated with the length of the sample fixed at 130 degreesC for 5 min to melt the PBSU component, followed by quenching in ice water or isothermal crystallization at various temperatures. The crystal orientation and the lamellar textures of the obtained samples were studied using wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM). The orientation of PBSU crystals was found to be significantly dependent on the crystallization temperature when PBSU was crystallized in the oriented matrix of PVDF. When the sample was crystallized at a temperature below 30 degreesC, the c axis of PBSU crystals was parallel to the stretching direction. In contrast, the crystal b axis of PBSU was found to orient parallel to the stretching direction at a crystallization temperature higher than 60 degreesC, and thus the samples that crystallized at higher temperatures were biaxially oriented. The stretched samples displayed very strong equatorial scatterings with a long period of about 50 nm. The equatorial scattering was attributed to the periodic arrangement of PBSU-rich domains in the PVDF-rich matrix. It is considered that the morphology of the miscible blend is segregated into the fibrillar morphologies of PBSU and PVDF, forming a periodic structure with a long period of 50 nm perpendicular to the stretching direction. The TEM studies revealed that PBSU-rich domains dispersed in the PVDF-rich matrix were elongated into ribbon like domains with a width of 20-50 nm, which was consistent with the SAXS and WAXD results. On the basis of the morphologies of the blend, it was deduced that the confinement of the crystallization of PBSU within the ribbonlike domains results in the unique biaxial orientation textures. The effect of these unique orientation textures on the mechanical properties was also studied. It was found that the tensile strength of the biaxially oriented samples was higher than that of the as-drawn sample in the perpendicular direction.