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.