Novel glass-forming liquid crystals .4. Effects of central core and pendant group on vitrification and morphological stability

To unravel the effects of the volume-excluding central core and the mesogenic pendant group on both the glass-forming ability and morphological stability of the thermally quenched glass, nine model compounds were synthesized that contain various nematogenic and cholesteryl pendant groups. The glass-forming ability of the melt and morphological stability of the thermally quenched glass were assessed using the DSC, XRD, and hot-stage POM techniques. With cyanobiphenyl as the pendant group, the following descending order in morphological stability against thermally activated recrystallization was established: trans-cyclohexane > allexo-bicyclo[2.2.2]oct-7-ene > cubane > cis-cyclohexane > benzene. While the cyclohexane compound containing three cyanoterphenyl groups showed a strong tendency to crystallize upon quenching, the chiral nematic system in which one of the cyanoterphenyl groups is substituted by a cholesteryl group showed superior glass-forming ability and morphological stability. Additionally, with cis-cyclohexane as the central core the angular 6-(4-cyanophenyl)naphthyl group, a stronger nematogen, showed a comparable glass-forming ability but a superior morphological ability in comparison to the cyanobiphenyl group. However, with allexo-bicyclo[2.2.2]oct-7-ene as the central core, the angular 1-phenyl-2-(6-cyanonaphth-2-yl)ethyne, also a stronger nematogen, turned out to be inferior to the cyanobiphenyl group with respect to morphological stability. It appears that the glass-forming ability and morphological stability of the hybrid system are determined by the characters of both the volume-excluding core and the pendant group, a delicate structural balance between the two constituents, and the stereochemistry of the hybrid system.