High-resolution calorimetric study of the nematic to smectic- A transition in aligned liquid crystal–aerosil gels

Abstract

High-resolution ac calorimetry has been used to study the nematic to smectic-A (N-SmA) phase transition in the liquid crystal octylcyanobiphenyl (8CB) confined in aligned colloidal aerosil gels. A stable and robust nematic alignment was achieved by repeated thermal cycling of the samples in the presence of a strong uniform magnetic field. In some ways (such as transition temperature and integrated enthalpy), the dependence of the specific heat peak associated with the N-SmA transition on the aerosil density for aligned gels is consistent with that observed in unaligned (random) gel samples. However, a power-law analysis reveals that the behavior of the critical exponent α is quite different. For random gels, α varies gradually with aerosil density, whereas we find that α for aligned gels shifts abruptly to an XY-like value for the lowest aerosil density studied and remains essentially constant as the sil density increases. This aerosil density independence of α is consistent with the critical behavior of the smectic correlation lengths obtained from an x-ray scattering study of 8CB in aligned aerosil gels. The combined calorimetric and x-ray results indicate that the role of quenched randomness in aligned gels of 8CB+sils differs significantly from that in random gels.