Effect of Alkyl Chain Length on the Phase Situation of Glass-Forming Liquid Crystals

Abstract

The phase behaviour of the latest synthesised compound belonging to a family of (S)-4′-(1-methyloctyloxycarbonyl) biphenyl-4-yl 4-[‘m’-(2,2,3,3,4,4,4-heptafluorobutoxy) ‘m’alkoxy]-benzoates (where ‘m’ means 3, 5 or 7 methylene groups) is described by polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and Fourier-transform infrared absorption spectroscopy. It has been shown that as the length of the alkyl chain increases, a given liquid crystal possesses a greater number of mesophases and at a higher temperature it goes into the isotropic liquid phase. All examined compounds form a chiral smectic phase with antiferroelectric properties (SmCA* phase), in which the temperature range of occurrence increases with the length of the molecule. The number of methylene groups also affects the glass transition. The compound with the shortest alkyl chain (‘m’ = 3) is vitrified from the conformationally disordered crystal phase. For the compound with five -CH2- groups (‘m’ = 5), a glass transition from the monotropic high-order hexatic smectic SmXA* phase is observed. In the case of the liquid crystal with the longest carbon chain (‘m’ = 7), the vitrification from the less ordered SmCA* phase is visible. Differences in the crystallization kinetics, e.g., the nucleation-controlled mechanism for the compound with the shortest carbon chain vs. the complex phenomenon for its longer homologs, are discussed.