Phase structure of poly(styrene-co-acrylic acid)/poly(ethylene oxide) blends by spin-label EPR and DSC

Blends of poly(styrene-co-acrylic acid) \P(S-AA)\ random copolymers with poly(ethylene oxide) (PEO) have been studied by the electron paramagnetic resonance (EPR) spin-label method and by differential scanning calorimetry (DSC). The copolymers contained up to 25 mol% acrylic acid monomers and the blends contained up to 60 wt.% PEO. The spin label is attached to some acrylic acid units and provides information on phases containing the copolymer. The presence of crystalline PEO in the blends is detected by DSC and verified by wide-angel X-ray scattering (WAXS). EPR and DSC data indicate that pure polystyrene and PEO are immiscible in the temperature range 300-420K. The presence of two amorphous phases is detected by EPR for some blends based on a copolymer containing 5 mol% acrylic acid monomer. The miscibility of the two components in the blend increases with increasing acrylic acid content of the copolymer. The melting points of crystalline PEO have been measured by DSC in untreated blends and in blends annealed by heating above the melting point of pure PEO and crystallization at ambient temperature. The presence of two melting transitions in the annealed samples can be explained by a phase diagram of the system consisting of two amorphous phase swith a miscibility gap and a crystalline phase. It is suggested that the blend obtained from a solution of the two components is not in thermodynamic equilibrium, and the importance of annealing, especially in systems containing a crystallizable component such as PEO, is emphasized.