Differential scanning calorimetry of 11 solution blends of polystyrenes of molecular masses from 3 X 103 to 1 X 107 daltons with poly(a-methylstyrenes) of molecular masses from 1.9 X 104 to 5.1 X 105 daltons was carried out between 300 and 500 K. The various blends covered the ranges from miscibility to immiscibility. The glass transition temperatures, their ranges, and the change in heat capacity with composition in the transition ranges have been measured. Long before phase separation is macroscopically noticeable, it is possible to detect phase instability from a broadened glass transition region. This broadening is discussed in terms of concentration fluctuations.

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