Branching of the Perfluorinated Chain Influences the Liquid-Crystalline Properties of Semifluorinated Alkanes: Perfluorooctyl- and Perfluoroisononyl-n-Alkanes—a Comparative Study

Abstract

Two series of semifluorinated-n-alkanes (SFAs) of comparable perfluorinated chain length but different terminal group constitution, namely perfluorooctyl-n-alkanes [F–(CF₂)₈–(CH₂)_n–H, in short F8Hn] and perfluoroisononyl-n-alkanes [(CF₃)₂CF(CF₂)₆–(CH₂)_n–H, in short iF9Hn] were synthesized. Their thermal properties were analyzed by means of differential scanning calorimetry (DSC), on both heating and cooling of the samples. The textures of the synthesized semifluorinated alkanes were investigated with the microscopy in polarized light. Similar to previously investigated perfluorodecyl-n-alkanes, perfluorooctyl-n-alkanes containing fewer than 12 methylene groups form smectic B phase. F8H8, F8H9, and F8H10 show a tendency to supercool and preserve smectic B ordering even below the transition temperature. For F8H11, the phase transition from smectic B to 3D crystal structure, manifesting itself as a rapid recrystallization, has been observed. Perfluorooctyl-n-alkanes of 12 or more methylene groups do not form liquid-crystalline phases. In contrast to perfluorooctyl-n-alkanes, perfluoroisononyl-n-alkanes are not capable of liquid-crystalline phase formation, regardless of the hydrogenated moiety length and the experimental conditions.

Keywords:

• differential scanning calorimetry
• liquid-crystalline properties
• semifluorinated alkanes

ACKNOWLEDGMENT

The State Committee for Scientific Research (KBN, Poland) is gratefully acknowledged for financial support (Grant No. 3 T09A 10527).

Notes

^a T_m, transition temp. (cryst. ⟷ isotropic or smectic).

^b T_pt, transition temp. (smectic ⟷ isotropic).

^c ΔH_m, transition enthalpy corresponding to T_m.

^d ΔH_pt, transition enthalpy corresponding to T_pt.

*substance may contain certain amounts of “supercooled smectic phase” at temperatures less than 15°C.

^a T_m, transition temperature (crystalline ⟷ isotropic).

^b ΔH_m, transition enthalpy corresponding to T_m