Liquid Crystalline Properties and Molecular Packing of Semifluorinated <i>n</i>-Alkanes F(CF₂)₁₀(CH₂)<i>_m</i>H

Abstract

The phase transitions for a homologous series of semifluorinated n-alkanes, F(CF2)10(CH2)mH, with 2 ≤ m ≤ 19 (F10Hm), have been characterized using a polarizing microscope, DSC, X-ray diffraction, and dilatometry. Thermal analysis and optical textures indicate the presence of the smectic−smectic phase transition when the number of methylene units ranges from 6 to 12. The phase-transition temperature depends on the length (m) of the hydrocarbon segment. The density decreases with increases in the carbon number of the hydrogen segment. The data for both the thermal properties and the density suggest that the smectic−smectic phase transition is significantly first-order, which is primarily caused by a change in the molecular packing for the molecular rearrangement, while a transition for the clearing point is related to disordering of the entire molecule. The volume jump shows that F10Hm have the same molecular packing in the high-temperature mesophase and that their magnitude is greater than that of the conventional liquid crystalline compounds having an aromatic−mesogenic core. The effects of the fluorinated segment for the phase transitions are also discussed herein.