Phase transitions in “plastic crystals”

Abstract

The solid state phase transitions in certain organic molecular plastic crystals reversibly absorb large amounts of heat. These have potential applications for use in thermal energy storage. Pure plastic crystals undergo first order transformations but the binaries exhibit complex behavior. A phase diagram for two energetic materials, 2,2-dimethyl-1,3-propanediol (NPG) and 2-amino-2-methyl-1,3-propanediol (AMPL) plastic crystals is proposed. This diagram was determined by high temperature Guinier X-ray diffraction and differential scanning calorimetric methods. It is found that the AMPL rich binaries supercool and the high temperature phase can be triggered by straining the material to release an estimated amount of 50–53 cal g −1 at room temperature. The structure of the high temperature γ′ phase of AMPL is determined as body centered cubic with a lattice parameter 6.778 Å at 92 °C; which is much lower than that of 8.8381 Å at 46.5 °C for the face centered cubic γ phase of NPG. A eutectoid γ → α + β transformation occurs at 20 °C. In the AMPL rich mixtures heat is absorbed in the α + γ and α + γ′ regions over a wide temperature range. An interesting feature is that there are binary equilibria between the γ and γ′ phases, with molecular rotations. Some rather uncommon thermal contractions with abrupt changes of 28% are observed in γ and γ′ phases in heating AMPL-25 mol.% NPG binary mixture.