Phase behavior and nucleation kinetics of Octaphenylcyclotetrasiloxane

Abstract

Phase behaviour and certain aspects of phase-transformation kinetics for Octaphenylcyclotetrasiloxane (OPCTS) have been determined by means of differential scanning calorimetry and optical microscopy. Three solid phases have been observed: 3 (stable at room temperature), 2 (stable above 76.6°C), and 1 (stable above 189.5°C). The 3-and 2-phases are optically anisotropic and mechanically brittle; the 1-phase is optically isotropic and soft or plastic. Although the 3-phase is thermodynamically unstable above 76.6°C, its transformation to the 2-phase is sufficiently slow that the 3 → 1 transition can be studied. The transition temperatures (and corresponding latent heats) for the 3 → 2, 3 → 1, 2 → 1, and melting transitions are: 76.6°C (∼2.9 KJ/mol), 186.0°C (47.3 KJ/mol), 189.6°C (43.76 KJ/mol), and 204.9°C (1.95 KJ/mol). The entropy of melting (ΔS_m = 0.491 R) is one of the lowest measured for any solid. The low-melting entropy, isotropy, and plasticity of the 1-phase lead to the conclusion that it is an orientationally-disordered crystal mesophase (plastic crystal).

The kinetic behavior of the 3 → 2 phase transformation, as determined by DSC, supports a theoretical model taking homogenous nucleation as the rate-determining process.