Glass transition of binary systems: peculiarities at low temperature

The present paper refers to the well known fact that the glass transition-composition curves of miscible binary systems (e.g. polymer--diluent and polymer-polymer mixtures) often exhibit a singular point when the Tg values of the two components are far apart. Two different equations apparently apply above and below the temperature of this singularity. Starting from Couchman's entropic treatment of the glass transition of mixtures, and from Angell's suggestion for the calculation of To ('ideal glass transition temperature' of the component with higher Tg), it is shown that below To an equation should be applicable to binary mixtures which differs considerably from the equations normally used in the literature. The new equation gives a satisfactory description of selected experimental data in the proximity of the component with lower Tgs.