MISCIBILITY IN PLASTIC PHASES - BINARY-SYSTEM NPG (NEOPENTYLGLYCOL) AMP (2-AMINO,2-METHYL-1,3-PROPANEDIOL)

The phase diagram of the plastic crystals neopentylglycol (NPG) and 2-amino,2-methyl-1,3-propanediol (AMP) has been determined. The phase diagram shows an extensive diversity of two-phase regions, due to the polymorphism of the pure substances. The binary system is characterized by two eutectoid invariants corresponding to the solid to plastic transitions and by one eutectic invariant corresponding to the melting process. The solubility boundaries, determined by means of X-ray powder diffraction, strongly depend on which phases are involved. The demixing region in the plastic state shows a very narrow two-phase domain (approximately 0.05 in mole fraction). The study of the mole-fraction dependence of the cell dimensions of the molecular alloys in the plastic phase is explained by means of the existence of different intermolecular interactions and geometrical effects.