High-resolution calorimetric study of the antiferroelectric liquid crystals methylheptyloxycarbonylphenyl octyloxybiphenyl carboxylate and its octylcarbonylbiphenyl analog

The antiferroelectric smectic-${\mathit{C}}^{\mathrm{*}}$ (Sm-${\mathit{C}}^{\mathrm{*}}$) phases of two closely related chiral compounds have been studied with high-resolution calorimetric techniques. Both methylheptyloxycarbonylphenyl octyloxybiphenyl carboxylate (MHPOBC) and its octylcarbonylbiphenyl analog (MHPOCBC) exhibit Sm-${\mathit{C}}_{\mathrm{\alpha }}^{\mathrm{*}}$ and Sm-${\mathit{C}}_{\mathit{A}}^{\mathrm{*}}$ antiferroelectric phases. The Sm-A–Sm-${\mathit{C}}_{\mathrm{\alpha }}^{\mathrm{*}}$ transitions are mean-field second order and well described by the usual extended Landau model. Three restructuring Sm-${\mathit{C}}^{\mathrm{*}}$ transitions in MHPOBC studied with a relaxation calorimeter are shown to be sharp first-order transitions with very small latent heats. Pretransitional heat capacity associated with the Sm-${\mathit{C}}_{\mathit{A}}^{\mathrm{*}}$–Sm-${\mathit{I}}_{\mathit{A}}^{\mathrm{*}}$ (hexatic smectic-${\mathit{I}}_{\mathit{A}}^{\mathrm{*}}$) transition and freezing of the monotropic Sm-${\mathit{C}}_{\mathit{A}}^{\mathrm{*}}$ phase was studied in MHPOCBC.