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

The effect of supercritical CO₂ on the morphological structure of crystalline/amorphous PEO/PV Ac blends was investigated by means of SAXS with the measurement of absolute scattering intensity. The morphological structure of PEO/PV Ac 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 PV Ac from the IL regions. Upon CO₂ treatment, the crystal and amorphous layer thickness (l_c and l_a, respectively) were both increased. Compared with the increase of l_a, the increase of l_c 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/PV Ac 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.