Effect of Mesogen Length on the Thermal Diffusivity of Polymethacrylate Smectic Liquid Crystals

Thermal diffusivity (α) was investigated for a series of side-chain liquid crystal (LC) PnPhEBp polymethacrylates composed of 4-biphenyl benzoate mesogenic units attached to the polymer backbones via n methylene spacers. PnPhEBp exhibits two distinct smectic (Sm) LC phases, SmA and SmB. α_|| measured parallel to the LC director orientation exhibits a positive correlation with the degree of LC orientation, quantified by the order parameter S. In contrast, α_⊥ measured parallel to the Sm layers (i.e., perpendicular to the director orientation) remains relatively constant and independent of both S and the nature of the Sm LC phase. This suggests that α_⊥ is not influenced by the short-range positional ordering of mesogens in the layers that distinguishes SmB from SmA. Notably, the α_|| values observed for PnPhEBp are comparable to those reported for other LC PnPhEPhOMe polymethacrylates containing shorter phenyl benzoate mesogens. A comparison of values at similar S indicates that PnPhEBp, which has a longer mesogen, exhibits a smaller α_||. This demonstrates that long mesogens enhance the S of LCs rather than α.