Pressure and temperature dependence of molecular motion in organic plastic crystals

The proton NMR relaxation times T1 and T1ρ have been measured as a function of temperature and pressure in the plastic crystals cyclohexane, hexamethylethane (HME) and hexamethyldisilane (HMDS) for pressures up to 2.8 kbar. The T1 data provide information on the molecular reorientation. This motion is only weakly dependent on pressure. The reorientational activation volumes for the three materials studied were found to be temperature dependent and generally less than 10% of the molar volume Vm. The T1ρ data have been analysed in terms of the translational molecular self-diffusion, using a weak-collision theory. A value of 1.1Vm was obtained for the activation volume for self-diffusion in HME, independent of pressure and temperature over most of the range studied. Diffusional activation volumes for the other two substances were found to be dependent on both pressure and temperature, ranging from 0.9 to 0.3Vm in cyclohexane and from 0.55 to 0.2Vm in HMDS. The observed temperature and pressure dependences of the activation enthalpies and volumes are discussed in relation to the mechanisms of the molecular motion. The complexity of the diffusive motion appears to correlate with the entropy of fusion of the material. While the dominant defect responsible for self-diffusion is usually thought to be a relaxed vacancy, there is some evidence to suggest that, in parts of the plastic phase, translational motion may proceed by more than one mechanism.