Description and molecular interpretations of anomalous compositional dependences of the glass transition temperatures in binary organic mixtures

Abstract The present study explores the applicability of a three-parameter equation: T g  =  φ 1 T g,1  + (1 −  φ 1 ) T g,2  +  φ 1 (1 −  φ 1 )[ a 0  +  a 1 (2 φ 1 –1) +  a 2 (2 φ 1 –1) 2 ] (where φ i and T g, i are, respectively, the weight fraction and the glass transition temperature of the i th neat component), recently proposed for describing the T g vs. composition dependencies in miscible binary polymer blends and copolymers (Brostow et al. Mater. Lett. 62 (2008) 3152 [16] ). Its efficacy is postulated here also for mixtures of polymers with low molecular mass organics (solvent, plasticizer or semicrystalline drug molecule phases) and very strongly/weakly associating polymer blends, including interpolymer complexes. Binary systems where entropic factors overcome the enthalpic ones were also considered. For several complicated (asymmetric or sigmoid) dependencies a description with better accuracy was achieved, compared to the common theoretical or semi-empirical functional forms, some of which require parameters that are not always readily available to the experimentalist or contain a superfluous number of fitting parameters. First- (linear) or second-order (parabolic) polynomial dependencies are established among its prime parameter a 0 and “interaction terms” of common T g ( φ ) functions, which are used as semi-quantitative measures of the strength of intermolecular interactions (e.g., parameter k GT of Gordon–Taylor, b of Jenckel–Heusch, and q of Kwei). Changes in the shape of the T g ( φ ) plots, and the corresponding a i fitting parameter estimates, are discussed in relation to important physicochemical phenomena and properties of the mixtures, such as, the strength of the hetero-contact forces and their composition dependence, irregular excess free-volume effects, as well as nanoscale effects arising from variation of components molecular mass, chains’ branching or organization in crystalline phases.