Fragility and glass transition of 1,2-propanediamine and 1,2-propanediol binary mixtures

The glass transition behaviors have been investigated for binary mixtures in addition to pure substances using differential scanning calorimetry (DSC). Although interesting mixing effects have been observed at the glass transition temperature (T_g), findings on the heat capacity jump at T_g (ΔC_p) and the kinetic fragility index (m) have been less discussed so far. In this study, we investigated the glass transition and fragility of the 1,2-propanediamine (12PDA) and 1,2-propanediol (12PDO) binary mixture integrally over the entire miscible composition range using DSC, while measuring the composition dependences of T_g, ΔC_p, and m. The DSC study revealed that these composition dependences were not the same and deviated from the ideal mixing prediction. More specifically, T_g exhibited a distinct positive peak, ΔC_p varied in the form of a sigmoidal curve, and m exhibited a complex negative deviation from the ideal mixing prediction. These results suggested that the intrinsic hydrogen bonds between each pure substance were dramatically changed upon mixing, involving the formation of extra hydrogen-bonding structures between the two components at intermediate compositions. Considering the difference in the composition dependences of T_g, ΔC_p, and m, these three parameters were found to reflect different aspects of mixing effects on glass transition and solvation structure. The melting behavior, but also the unique changes in T_g and m upon mixing, suggested the existence of a stable intermolecular compound, namely (12PDA)_0.50(12PDO)_0.50.