The effects of supercritical carbon dioxide (SC CO,) fluids on the morphology and/or conformation of poly(ethylene oxide) (PEO) in PEO/poly(methyl methacrylate) (PMMA) blends were investigated by means of differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and Fourier transform infrared (FTIR). According to DSC data for a given blend, the melting enthalpy and, therefore, degree of crystallinity of PEO were increased, whereas the melting temperature of PEO was decreased, with SC CO, treatment. The enhancement of PEO crystallization with SC CO, treatment, as demonstrated by DSC data, was supported by WAXD data. According to FTIR quantitative analyses, before SC CO2 treatments, the conformation of PEO was transformed from helix to trans planar zigzag via blending with PMMA. This helix-to-trans transformation of PEO increased proportionally with increasing PMMA content, with around 0.7% helix-to-trans transformation per 1% PMMA incorporation into the blend. For a given blend upon SC CO, treatments, the conformation of PEO was transformed from trans to helix. This trans-to-helix transformation of PEO decreased with increasing PMMA contents in the blends because of the presence of interactions between the two polymers. (C) 2004 Wiley Periodicals, Inc.