Molecular relaxations in a glass of cholesteric liquid crystal

Abstract

Glasses, in which the orientations of molecules have a long-range order, can be obtained by supercooling the mesophases of liquid crystals. Such mesophases may be: (1) nematic, when the centres of their rod-, or lath-like molecules do not lie on a regular lattice, but there is a long-range order in the alignment of their long axes, called the director; (2) cholesteric (or twisted nematic) when the alignment (due to steric asymmetry of the individual molecules) of the nematic type is such that the local director rotates at a steady rate as one moves normal to it; and (3) smectic, when there is also a partial ordering in the positions of the molecules in planes related to the local direction of orientational order. The situation in the nematic and cholesteric phases is clearly akin to magnetic ordering and the glassy states of these phases are useful, as our models of disorder, in understanding the behaviour of structurally isotropic glasses. A study of molecular motions in the glassy cholesteric phase of cholesteryl hydrogen phthalate, given here, shows features which have a striking resemblance to those observed in the (positionally ordered) glass-like state of plastic crystals^1,2 and the (grossly disordered) molecular and polymeric glasses. This suggests that the characteristic features of the molecular motions in glasses must be explained in terms of the non-equivalence of the molecular environment, rather than in terms of the complexity of the molecule, its internal degrees of freedom, or the state of aggregation. This has implications for our concept of the microstructure of a glass.