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.