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Past differential scanning calorimetry and dielectric relaxation measurements have established that polystyrene (PS)‐poly(vinyl methyl ether) (PVME) mixtures exhibit a degree of compatibility when cast from toluene, whereas they are incompatible when cast from chloroform or trichloroethylene. The present study reports that toluene‐cast mixtures can be phase‐separated by thermal treatment at temperatures exceeding 125°C. This is true for samples containing 20–80 wt‐% PS. The temperature of phase separation varies with heating rate; isothermal heating times needed to cause phase separation increase rapidly as the temperature approaches 125°C. Reversibility of the phase separation process depends upon such factors as cooling rate, annealing time, treatment temperature, and thermal history. By annealing and/or slow cooling, all thermally phase‐separated mixtures have been brought back to their original state of compatibility. That is, there is no evidence for true irreversiblity of phase separation in thermally treated samples. Quench‐cooled samples remain phase‐separated indefinitely at room temperature, but this is attributed to rapid cooling below the glass transition of the PS. Chloroform‐cast and trichloroethylene‐cast mixtures have not been brought to a compatible state by thermal treatment, even after lengthy annealing and slow cooling steps.

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