Phase diagram and intermolecular compounds in diamine–dihydric alcohol binary systems

To investigate the equilibrium behavior underlying the supercooled state of liquid mixture in polyamine-polyhydric alcohol binary systems, we have studied the phase diagrams of ethylene diamine–ethylene glycol (EDA-EG), 1,2-propanediamine–1,2-propanediol (12PDA-12PDO), and 1,3-propanediamine-1,3-propanediol (13PDA-13PDO) systems using differential scanning calorimetry. Our findings revealed that the (EDA)_x(EG)_1−x system showed two types of intermolecular compounds of the EDA concentration x = 0.25 and x = 0.50. The former compound showed incongruent melting, whereas the latter one fused in congruent melting as the peak in the liquidus line. When it comes to the (12PDA)_x(12PDO)_1−x system, an intermolecular compound was found at the 12PDA concentration x = 0.5 exhibiting congruent melting property. It is also found the increase of the liquidus line in the (13PDA)_x(13PDO)_1−x system at the 13PDA concentration x = 0.5 which is likely to form an intermolecular compound. However, the phase diagram in the 12PDO-rich side of (12PDA)_x(12PDA)_1−x and in the 13PDO-rich side of (13PDA)_x(13PDO)_1−x could not be fully determined due to crystallization difficulties. It was suspected that the nucleation rate was insufficient in these concentration ranges. In pure EDA, a phase transition in the crystal was not observed in the previously reported temperature range in the adiabatic method, possibly because the transition was extraordinarily supercooled and superheated during the cooling and heating processes, respectively. Considering the small values of enthalpy and entropy of transition, it can be inferred that the transition between phases Iα and II is of the displacive type.