On the dynamics of the highly anisotropic glass transition in a columnar discotic liquid crystal

Abstract

Abstract Recent experimental findings show that the dynamics and thermodynamics of glass transition in a columnar discotic liquid crystal are highly anisotropic. This anisotropy is discussed in the context of the coupling theory. The difference of almost thirty degrees Kelvin between the glass transition temperature of axial rotation around the columnar axis and that of the intracolumnar thermal expansion coefficient is rationalized by the disparity of intermolecular constraints (coupling parameter, n) for these two types of motion. Axial rotation of the disc-like molecule about the columnar axis is least constrained by intermolecular interactions, having a small coupling parameter and a weak Tg-scaled temperature dependence of its relaxation time. Dielectric and NMR data which probe the axial rotational motion are consistent with this expected behavior. Comparison with wholly amorphous glass-forming small molecule liquids show that the Tg-scaled temperature dependence of the α-relaxation of the columnar discotic liquid crystal is the weakest known. This unusual property is due to the α-relaxation associated with the axial motion, involving only a single degree of freedom. However, I expect that motion along the columnar axis will be more intermolecularly constrained and its relaxation time to have a much stronger Tg-scaled temperature dependence. This prediction cannot be fully verified until measurement of the dynamics of motion along the columnar axis becomes available.