On the character of the glass transition in columnar discotics

The glass transition of highly ordered columnar discotics within the framework of the concept of fragility is discussed. The discussion is based on experimental findings from dielectric relaxation spectroscopy, calorimetric, and Xray scattering measurements. The findings are that discotics in their three-dimensionally ordered columnar plastic state are strong glass formers and those in the two-dimensionally ordered columnar hexagonal phase are fragile. We were unable to find correlations between the fragility and the degree of non-exponentiality of the relaxation function and correlations between fragility and the magnitudes of the stepwise change of the specific heat. The change of the thermal expansion of the lattice parameters at the glass transition, however, seems to depend on the phase type and is due to the anisotropy of the discotic structures. (C) 1998 Elsevier Science B.V. All rights reserved.