We present a calorimetric study of the phase behavior of octylcyanobiphenyl (8CB) liquid crystal confined to a controlled-pore glass (CPG). We used CPG matrices with characteristic void diameters ranging from 400 to 20 nm. In bulk we obtain weakly first-order isotropic to nematic $(I-N)$ phase transition and nearly continuous character of the nematic to smectic-$A$ $(N-\mathrm{Sm}A)$ phase transition. In all CPG matrices the $I-N$ transition remains weakly first order, while the $N-\mathrm{Sm}A$ one becomes progressively suppressed with decreasing CPG pore radius. With decreased pore diameters both phase transition temperatures monotonously decrease following similar trends, but increasing the stability range of the N phase. The heat-capacity response at the weakly first order $I-N$ and continuous $N-\mathrm{Sm}A$ phase transitions gradually approaches the tricritical-like and three-dimensional $\mathrm{XY}$ behavior, respectively. The main observed features were explained using a bicomponent single pore type phenomenological model.

• Received 24 April 2003

DOI:https://doi.org/10.1103/PhysRevE.68.021705