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 013 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 lOCB and 12CB will be presented for the different interfaces.

High-resolution calorimetric investigation of the existence of a nematic phase for the dodecylcyanobiphenyl liquid crystal

“…Concerning the N phase reported for the 12CB trapped in a glass matrix (10), this might have been induced by the confinement. However, no indication for a nematic phase in 12CB was found by Iannacchione et al in very thin ac calorimetric cells with different surface coatings or in anopores with different surface agents (11,12). On the other hand, it has been shown recently that in case of 8CB confined to controlled pore glasses the nematic phase becomes increasingly stabilised with decreasing matrix pore diameter, i.e.…”

“…very small anomaly is related to surface effects similar to those reported in (11). The present measurements were performed in thicker samples (0.7-0.8 mm for our ACC runs compared with 0.13 mm samples in (11)), that were placed in cells with non-treated surfaces.…”

“…For 12CB, in particular, the influence of surfaces on the growth of smectic ordering was reported almost 15 years ago by Iannacchione et al (11,12). It was systematically explored for thin 12CB samples placed in cells with different surface coating (11), as well as for samples confined in anopore membranes with surface agents of various chain lengths (12).…”

High Resolution Calorimetric Studies at Phase Transitions of Alkylcyanobiphenyl Liquid Crystals Confined to Submicron Size Cylindrical Cavities

“…3D Since our "bulk" measurements consist of a drop of liquid crystal bound by the sapphire disk on one side and air at atmospheric pressure at the other, and based on a series of studies,' 14 these phenomena have been interpreted in terms of a temperature dependent anchoring energy and a reflection of how smectic layers need to accommodate themselves at the different interfaces. 44 In fact, no such features were observed in the identically performed studies with nematic nCB (n < 10) liquid crystals. The confined AI transition peaks are more rounded than bulk and is broader for the radial configuration; the bulk features completely disappeared upon confinement.…”

Surface-Induced Discrete Smectic Order in the Isotropic Phase of 12CB in Cylindrical Pores

“…Two types of scans are performed; qadi scans, which probe the longitudinal smectic correlations and cu scans to determine the sample mosaicity. [24], and is partly due to the smectic layering transitions occurring near TAi.…”