The thermodynamics of polyether sulfone-poly(ethylene oxide) mixtures

Mixtures of poly(ethylene oxide) and polyether sulfone have previously been shown to be miscible and undergo phase separation on heating, the minimum in the cloud point occurring at about 80°C and at 10:90 w/w PES:PEO composition. This paper describes an attempt to understand the thermodynamics of this system and to simulate the spinodal curves of the phase diagram using a modified form of the equation‐of‐state theory of Flory et al. Two methods were used to gain information about the interactional energy of the polymers. Heat of mixing measurements at 90°C on low molecular weight analogues were used to obtain a value of X12 of −40 J. cm−3. A modified form of the equations for the melting point depression of PEO in the blend was used to obtain values of X̄12 in the range of −35 to −50 J. cm−3 at 52°C, the melting point of PEO. In order to adjust the location of the simulated spinodal, using a value of X12 = −40 J. cm−3, it was necessary to assume an unfavourable non‐combinatorial entropy contribution of Q12 = −0,048 J. cm−3 · K−1. If the spinodals were simulated assuming Q12 = 0 and hence X12 ≡ X̄12 then a value of X12 = −18 J · cm−3 must be used. This is not inconsistent with the melting point depression result considering the large uncertainties in this method and the fact that the results are obtained at a lower temperature. The simulated spinodals are not very molar mass dependent which corresponds to the similar findings of the cloud point. The shape of the spinodal is dominated by the value of the surface area to unit volume ratio S2/S1 through its effect on the interactional terms in the spinodal equation.