Critical behavior of a smectic-Ato nematic phase transition imbedded in a random network of voids

Abstract

Department of Physics, Kent State University, Kent, Ohio 44240~Received 13 August 1997!We present the results of a systematic ac calorimetric study on the smectic-A to nematic ~Sm-A–N! phasetransition for the alkylcyanobiphenyl liquid crystals, 8CB and 9CB, imbedded in the randomly interconnectedvoids of Millipore cellulose membranes. The confining void size ranged from 5 to 0.025mm, which, althoughlarge, introduces substantial changes compared to the bulk behavior. The heat capacity peak at the Sm-A–Ntransition temperature changes from sharp and prominent to broad and small with increased confinement.Concomitant with the heat capacity peak shape change, critical behavior analysis reveals that the randomconfinement drives the heat capacity critical exponent from its bulk value towards zero, approaching thethree-dimensionalXY universality class value prediction of 20.007. This change in the critical exponent isliquid-crystal material and voids size dependent, and is equivalent to what occurs with increasing nematicrange that weakens the coupling between smectic and nematic order parameters. The Millipore confinementdoes not alter the nematic range; it is the randomness it introduces that changes the nature of the coupling. Inaddition, but in a manner not fully consistent with finite size effects, transition temperatures are shifted downfrom bulk and the transition enthalpy is suppressed. Supporting results are found in a study of binary mixturesof 9CB and 10CB. @S1063-651X~98!02504-5#PACS number~s!: 64.70.Md, 61.30.2v, 65.20.1wINTRODUCTION