Thermal study of octylcyanobiphenyl liquid crystal confined to controlled-pore glass

We study the phase behavior of octylcyanobiphenyl (8CB) liquid crystal confined to control porous glass (CPG) using high-resolution calorimetry. We used silanized and nonsilanized CPG matrices with characteristic void diameters ranging from 400 to 20 nm. With decreasing diameter the first-order isotropic to nematic (I-N) and second-order nematic to smectic A (N-SmA) phase transition temperatures monotonously decrease. The weakly first-order features of the I-N transition approaches the smeared tricritical behavior. The effective N-SmA critical exponent apparently decreases toward the 3D XY value for diameters above 100 nm, however, the N-SmA transition progressively broadens and becomes strongly suppressed for diameters below 100 nm. A simple phenomenological model is used for a qualitative explanation of results. (C) 2004 Elsevier B.V. All rights reserved.