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 me sogenic pendant group on both the glass-forming ability and morphologi cal stability of the thermally quenched glass, nine model compounds we re synthesized that contain various nematogenic and cholesteryl pendan t groups. The glass-forming ability of the melt and morphological stab ility of the thermally quenched glass were assessed using the DSC, XRD , and hot-stage POM techniques. With cyanobiphenyl as the pendant grou p, the following descending order in morphological stability against t hermally activated recrystallization was established: trans-cyclohexan e > allexo-bicyclo[2.2.2]oct-7-ene > cubane > cis-cyclohexane > benzen e. While the cyclohexane compound containing three cyanoterphenyl grou ps showed a strong tendency to crystallize upon quenching, the chiral nematic system in which one of the cyanoterphenyl groups is substitute d by a cholesteryl group showed superior glass-forming ability and mor phological stability. Additionally, with cis-cyclohexane as the centra l core the angular 6-(4-cyanophenyl)naphthyl group, a stronger nematog en, showed a comparable glass-forming ability but a superior morpholog ical ability in comparison to the cyanobiphenyl group. However, with a llexo-bicyclo[2.2.2]oct-7-ene as the central core, the angular 1-pheny l-2-(6-cyanonaphth-2-yl)ethyne, also a stronger nematogen, turned out to be inferior to the cyanobiphenyl group with respect to morphologica l stability. It appears that the glass-forming ability and morphologic al stability of the hybrid system are determined by the characters of both the volume-excluding core and the pendant group, a delicate struc tural balance between the two constituents, and the stereochemistry of the hybrid system.