The influence of fluorine terminal group and flexible spacers on properties of the liquid crystal dimers

ABSTRACT

Two series of symmetric LC dimers with the same rigid group and different fluorine terminal group (F and F₃C) have been synthesised by varying the length of flexible spacers, termed 1 F series (1 F-4, 1 F-6, 1 F-8) and 3 F series (3 F-4, 3 F-6, 3 F-8). Chemical structures and LC properties of the six symmetric LC dimers were characterised by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance hydrogen spectrometer (¹H NMR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and polarised optical microscopy (POM). The 1 F series dimers only show nematic phase. While for 3 F series, smectic phase appeared, and with the increase of flexible spacers, the smectic phase is dominant. For the formation of smectic phase of 3 F series, the larger intermolecular transverse force coming from CF₃ may be the driving force, and the long flexible spacer makes it possible. The melting temperatures of the 1 F series and 3 F series gradually decrease as the length of the flexible spacer increases, respectively, indicating that molecular rigidity has a greater influence on the melting temperatures. The melting temperatures of the 3 F series dimers decrease faster, which may attribute to the influence of CF₃ on the conformation of the dimers.

Graphic abstract

KEYWORDS:

• Liquid crystal
• fluorinated liquid crystal
• flexible spacer
• symmetric liquid crystal dimers

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was financially supported by the Fundamental Research Funds for the Central Universities: [Grant Number N180705004].