Glass transition of liquid-crystalline 4-alkoxyphenyl and 4-cyanophenyl 4-(2,4-dialkoxybenzoyloxy) benzoates

Abstract

Some 4-alkoxyphenyl and 4-cyanophenyl 4-(2,4-dialkoxy) benzoates show a nematic phase and a glassy phase at low temperatures. The glass-transition phenomena were examined as a function of the sweep rate in both heating and cooling processes. The derivatives intrinsically experience two kinds of transition process in the nematic phase. Two transitions were observed when the sweep rate was < 1 K min^–1, and these overlapped when the sweep rate was > 1 K min^–1. The transition temperatures show an interesting dependence on the chain length of three alkoxy groups; i.e. lengthening the lateral alkoxy group at position 2 lowers the glass-transition temperature (T_g) while lengthening both alkoxy groups at the terminal positions increases T_g. The thermodynamic parameters are discussed in terms of the McMillan theory of the glass transition.