Surface phenomena in discotic liquid crystals investigated using polarized FTIR transmission spectroscopy

Influence of uncoated and spin-coated surfaces of four different substrates on the bulk alignment of a discotic liquid crystal 2,3,6,7,10,11-hexapentyloxytriphenylene (HPT) was investigated using polarized FTIR transmission spectroscopy. The substrates investigated are: Si, ZnS, ZnSe and CaF2. A planar-heterogeneous alignment of HPT has been found on the uncoated surfaces of Si and CaF2 substrates whereas HPT is planar-homogeneously aligned on similar surfaces of ZnS and ZnSe. In contrast, a homeotropic alignment of HPT is observed on the surfaces of substrates of Si, CaF2 spin-coated by nylon 6/6. Under certain experimental conditions of temperature and time variations, the anchoring transition from planar to homeotropic is observed for HPT. The observed alignments of LC and the anchoring transition are explained in terms of the topology and the interactions of the LC molecule with the surfaces of the substrate and/or the polymer with which the substrate is spin-coated.