Effect of supercritical CO2 on phase structure of PEO/PVAc blends evaluated from SAXS absolute intensity measurement

Abstract

The effect of supercritical CO2 on the morphological structure of crystalline/amorphous PEO/PVAc blends was investigated by means of SAXS with the measurement of absolute scattering intensity. The morphological structure of PEO/PVAc exhibited a considerable change upon CO2 treatment as demonstrated by the drastic increase of scattering intensity, or the enhancement of electron density contrast between the crystalline and amorphous layers in the lamellar stacks, resulting from the swelling of amorphous PEO via the incorporation of CO2 into the interlamellar (IL) regions and/or the expulsion of PVAc from the IL regions. Upon CO2 treatment, the crystal and amorphous layer thickness (lc and la, respectively) were both increased. Compared with the increase of la, the increase of lc was relatively significant and was attributed to the occurrence of melting and recrystallization during CO2 treatment leading to thicker PEO crystals via a depression of equilibrium melting temperature and/or an increase of crystal fold surface free energy. The measured electron density contrast revealed that the distance of segregation in PEO/PVAc blends involved the extralamellar segregation before CO2 treatments and the swelling of interlamellar region dominated the drastic increase of scattering intensity after CO2 treatments. The finding of extralamellar morphology was consistent with the magnitude of volume fraction of lamellar stacks in the blends. The lamellar size distribution appeared to be broader and the lamellar stacks more disorganized for the blends after CO2 treatments according to SAXS one-dimensional correlation function profiles.