Interface influenced phase transitions of alkylcyanobiphenyl liquid crystals A calorimetric study

Abstract

Using an AC-calorimetry technique, we report measurements of the heat capacity and the phase shift between the applied heating power and the resulting thermal oscillations on the thermotropic liquid crystal series of alkylcyanobiphenyl (nCB) above the tricritical point. Specifically, we studied the first order phase transition smectic A to isotropic as a function of substrate, under atmospheric pressure. Different combinations of sapphire, Kapton type 200H polyimide film and air, are used to vary systematically the interfaces encountered by a liquid crystal. The calorimeter uses a sapphire disk, 10 mm in diameter and 0·13 mm thick, above which the samples are placed. A second sapphire disk, or a disk-shaped Kapton film, are used to sandwich the liquid crystal. Air interface samples are droplets allowed to spread naturally on the chosen substrate. The chosen geometry is such that interface effects appear to be maximized. Striking features found in the heat capacity and the phase shift studies with 10CB and 12CB will be presented for the different interfaces.