Recently, it has been shown by our group that the composition dependence of the glass transition temperature (T,) of polymer-diluent (and polymer-polymer) systems has to be described, in some occasions, by two different analytical expressions, one applicable above and one below a critical temperature l s 2 ) To2. At this characteristic temperature, which corresponds to the 'ideal glass transition' 3,4) of the component with higher Tg, a cusp can sometimes be observed in experimental T,-composition curves 5 ) .
Starting from the Couchman entropic and enthalpic treatments6-'), which assume entropy and enthalpy continuity at the glass transition temperature of the mixtures, and taking care of limitations related to the existence of Toz, two different sets of T,composition relationships have been derived, which are used to fit experimental data in the compositional ranges close to pure components 1 and 2, respectively2). To the best of our knowledge, no attention has been payed up to now to another quantity obtainable from calorimetric measurements: the specific heat increment at constant pressure AC, of the mixture at its glass transition temperature.
From the very few Acp data reported in the literature for binary s y ~t e m s ~-' ~) , it is possible to observe that the Ac,-composition curve appears to be composed of two different parts that approximately meet at the composition at which a cusp is observed in the correspondent T,-composition curves. In the present paper a tentative interpretation of the phenomenon, based on the recently published dual treatments of the T, of binary systems 'sZ), will be presented.
From the basic equations of Couchman's entropic and enthalpic treatments6-') it follows 0 1994, Hiithig & Wepf Verlag, Zug