Thermodynamic modeling of the C(CH2OH)4–(NH2)(CH3)C(CH2OH)2 binary system

A new calculated phase diagram of pentaerythritol [PE: C(CH2OH)4]–2-amino-2-methyl-1,3-propanediol [AMPL: (NH2)(CH3)C(CH2OH)2] ‘plastic crystals’ is presented. The low temperature PE-rich α phase has a tetragonal structure whereas the AMPL-rich β phase has a monoclinic structure. Upon heating, these low temperature α or β phases transform to high temperature γ or γ′ plastic crystal phases. The PE-rich γ phase has an FCC structure and the AMPL-rich γ′ phase has a BCC structure. The system exhibits complex behavior with one low temperature peritectoid, a high temperature eutectoid, and a peritectic. The α and β phases are assumed to be regular solutions and the plastic crystal phases (γ and γ′) are described using sub-regular solution models. The optimization for excess Gibbs energy parameters was performed using Thermo-Calc (TCC) software. The magnitudes of the interaction parameters are relatively large compared to those for similar plastic crystal binary systems.