Comparison of positional disorder in the liquid and glassy states of hydrocarbons: Dependence of disorder on molecular shape

Monte Carlo simulations of liquid and supercooled liquid states of neohexane, n-hexane, n-pentane, isopentane, neopentane and a model linear molecule are reported. A quantitative measure of the degree of disorder associated with the molecular centre of mass has been obtained from the minimal spanning tree method. The results suggest a strong dependence of the degree of disorder of the centre of mass on the molecular shape. The changes in the degree of disorder on cooling also depend on molecular geometry. There appears to be little difference in the structures as well as the magnitude of positional disorder of the liquid and the glassy states of linear molecules in contrast to those of globular molecules. Based on the results obtained from the simulations, regions have been identified in the m-σ plane, where one may expect plastic crystalline and liquid crystalline phases. This yields fresh insight into the nature of the structural phase diagram for polyatomic systems.