Molecular engineering of the glass transition: Glass-forming ability across a homologous series of cyclic stilbenes

We report on the glass-forming abilities of the homologous series 1,2-diphenylcyclo-butene, pentene, hexene and heptene-a series that retains the cis-phenyl configuration characteristic of the well-studied glass former, o-terphenyl. We find that the glass-forming ability shows a sharp maximum for the six-membered ring and demonstrate that this trend in glass-forming ability is a consequence of a maximum, for the 1,2-diphenylcyclohexene, of the reduced glass transition temperature T_g/T_m. Since the nonmonotonic trend in T_g/T_m is entirely due to variations in T _m, we conclude that the design target for maximizing the glass-forming ability across an homologous series should focus on the crystal stability and the factors that determine it.