Solid state phase transitions in pentaerythritol and related polyhydric alcohols

Abstract

The solid state phase transformations in some hydrocarbon molecular crystals reversibly absorb large amounts of heat. Some of these materials have potential for use in composite building materials as heat storage media. In particular, the homologous series of polyhydric alcohols related to pentaerythritol can be used for heat storage over a range of temperatures suitable for solar heated buildings. The thermodynamic properties and mechanisms of solid state transformations have been studied for pentaerythritol (C(CH 2OH) 4); trimethylol ethane [(CH 3)C(CH 2OH) 3], neopentyl glycol ((CH 3) 2C(CH 2OH) 2) and their mixtures. Differential scanning calorimetry was used to characterize the temperatures and enthalpies of transformations and to measure the heat capacities over a range of temperature from ambient to above the melting temperatures. Infrared spectroscopy was performed on the pure alcohols over a range of temperatures including the solid state transformation. Partially deuterated samples of pentaerythritol were also studied. Results of these measurements support a mechanism for the solid state transformation which involves reversible breaking of nearest-neighbor hydrogen resonance bonds in the molecular crystals at the transformation temperatures. The binary mixtures of these three alcohols also exhibit solid state transformations which appear to occur by the same mechanism.