Phase structure of poly(3-hydroxy butyrate)/poly(vinyl acetate) blends probed by small-angle X-ray scattering

The semicrystalline morphology of poly(3-hydroxy butyrate) (PHB)/poly(vinyl acetate) (PVAc) blends at four crystallization temperatures (T_c = 70, 90, 110, and 125°C) was probed by small-angle X-ray scattering (SAXS). Morphological parameters including the crystal thickness (l_c) and the amorphous layer thickness (l_a) were deduced from the interphase distribution function. Blending with PVAc thickened the PHB crystals. The amorphous layers were also swollen upon blending, but l_a displayed a maximum in its variation with PVAc composition. A zero-angle scattering was observed in the Lorentz-corrected SAXS profiles of PHB/PVAc 60/40 blend crystallized at 110 and 125°C and 50/50 composition crystallized at 90-125°C. Such a zero-angle scattering was attributed to the presences of amorphous domains between the lamellar stacks so as to generate gaps in the sequential stacking of lamellae. These amorphous domains were generated by the interfibrillar segregation of PVAc, where PVAc was expelled beyond several layers of lamellae and subsequently resided in the regions between the lamellar stacks. On the basis of the composition variation of l_a, the observation of zero-angle scattering, and the magnitude of the volume fraction of lamellar stacks, it was concluded that when PVAc composition was ≤20 wt%, the morphology was predominantly characterized by the interlamellar segregation; above this composition, interlamellar and interfibrillar segregation coexisted, and the extent of interfibrillar segregation increased with the increases of PVAc composition and crystallization temperature.