CRITICAL-BEHAVIOR OF A SMECTIC-A TO NEMATIC PHASE-TRANSITION IMBEDDEDIN A RANDOM NETWORK OF VOIDS

We present the results of a systematic ac calorimetric study on the sm ectic-A to nematic (Sm-A-N) phase transition for the alkylcyanobipheny l liquid crystals, 8CB and 9CB, imbedded in the randomly interconnecte d voids of Millipore cellulose membranes. The confining void size rang ed from 5 to 0.025 mu m, which, although large, introduces substantial changes compared to the bulk behavior. The heat capacity peak at the Sm-A-N transition temperature changes from sharp and prominent to broa d and small with;increased confinement. Concomitant with the heat capa city peak shape change, critical behavior analysis reveals that the ra ndom confinement drives the heat capacity critical exponent from its b ulk value towards zero, approaching the three-dimensional XY universal ity class value prediction of -0.007. This change in the critical expo nent is liquid-crystal material and voids size dependent, and is equiv alent to what occurs with increasing nematic range that weakens the co upling between smectic and nematic order parameters. The Millipore con finement does not alter the nematic range; it is the randomness it int roduces that changes the nature of the coupling. In addition, but in a manner not fully consistent with finite size effects, transition temp eratures are shifted down from bulk and the transition enthalpy is sup pressed. Supporting results are found in a study of binary mixtures of 9CB and 10CB.