Investigation of chiral smectic phases and conformationally disordered crystal phases of the liquid crystalline 3F5FPhH6 compound partially fluorinated at the terminal chain and rigid core

Complementary methods are applied to investigate the phase transitions and crystallization kinetics of the liquid crystalline compound denoted as 3F5FPhH6. Two crystal phases are confirmed, and one of them is the conformationally disordered (CONDIS) phase. Complexity of the melt crystallization process is revealed by the analysis with Friedman’s isoconversional method. The melt crystallization of 3F5FPhH6 shows different mechanisms depending on temperature, which is explained by the relation between the thermodynamic driving force and the thermal energy of translational degrees of freedom. The studied compound crystallizes even during fast cooling (30 K/min), unlike similar compounds with different fluorosubstitutions of the benzene ring, which form the smectic glass for moderate cooling rates. The tendency to vitrification of the smectic phase decreases apparently with the decreasing stability width of the $SmC_{A}$* phase and the increasing relaxation time of the collective relaxation process in this phase, at least for homologues differing from 3F5FPhH6 only by the type of fluorosubstitution

Distinguishing the Focal-Conic Fan Texture of Smectic A from the Focal-Conic Fan Texture of Smectic B

This publication presents methods of distinguishing the focal texture of the conical smectic phase A (SmA) and the crystalline smectic B phase (CrB). Most often, characteristic transition bars are observed in polarized light at the temperature point of the SmA–CrB phase transition. TOApy software transforms each image from a series of images recorded during POM observation to a function of light intensity versus temperature. Thermo-optical analysis is a powerful quantitative tool to notice this phase transition, but it has some limitations. The other applied method, the local binary pattern (LBP) algorithm, with high probability, detects differences between the textures of the conical focal fan of the SmA and CrB phases. The LBP algorithm is an efficient tool for texture classification

Kinetics of Non-Isothermal and Isothermal Crystallization in a Liquid Crystal with Highly Ordered Smectic Phase as Reflected by Differential Scanning Calorimetry, Polarized Optical Microscopy and Broadband Dielectric Spectroscopy

The kinetics of the non-isothermal and isothermal crystallization of the crystalline smectic B phase (soft crystal B, SmBcr) in 4-n-butyloxybenzylidene-4′-n′-octylaniline (BBOA) was studied by a combination of differential scanning calorimetry (DSC), broadband dielectric spectroscopy (BDS) and polarized optical microscopy (POM). On cooling, part of the SmBcr phase undergoes conversion to a crystalline phase and the remainder forms a glassy state; after the glass softens, crystallization is completed during subsequent heating. By analyzing the area of the crystal growing in the texture of SmBcr as a function of time, the evolution of degree of crystallinity, D(t), was estimated. It was demonstrated that upon heating, D(t) follows the same Avrami curve as the crystallization during cooling. Non-isothermal crystallization observed during slow cooling rates (3K/min ≤ ϕ ≤ 5K/min) is a thermodynamically-controlled process with the energy barrier Ea ≈ 175 kJ/mol; however, the crystallization occurring during fast cooling (5 K/min > ϕ ≥ 30K/min) is driven by a diffusion mechanism, and is characterized by Ea ≈ 305 kJ/mol. The isothermal crystallization taking place in the temperature range 274 K and 281 K is determined by nucleus formation

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

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

Mesomorphic and physicochemical properties of liquid crystal mixture composed of chiral molecules with perfluorinated terminal chains

New binary mixture of thermotropic liquid crystalline compounds, possessing both ferro– and antiferroelectric smectic $C^{\ast}$ phases, has been studied by complementary methods: calorimetric, X–ray powder diffractometry and electro-optic. Additionally, quantum–chemical model based on the density functional theory was also applied. It was found that mixture studied did not exhibit antiferroelectric phase at any rate of heating or cooling in contradiction to its ingredients. On the other hand it has a wide temperature range of ferroelectric smectic $C^{\ast}$ phase, which is invariant with respect to different heating and cooling rates within the range of thermal hysteresis. Furthermore, temperature dependence of switching time and spontaneous polarization were measured

Vibrational Dynamics of a Chiral Smectic Liquid Crystal Undergoing Vitrification and Cold Crystallization

Vibrational dynamics in the glass transition and the cold crystallization process of (S)-4′-(1-methyloctyloxycarbonyl) biphenyl-4-yl 4-[7-(2,2,3,3,4,4,4-heptafluorobutoxy) heptyl-1-oxy]-benzoate (3F7HPhH7) was studied by Fourier transform infrared spectroscopy (FTIR) during cooling/heating experimental runs. The measured spectra processing was supported by quantum chemical density functional theory (DFT) calculations (frequency assignments). The perturbation-correlation moving window two-dimensional analysis (PCMW2D) was performed to examine how the height of individual absorption bands change under with temperature. Two-dimensional correlation analysis (2D-COS) was used to detect freezing-in or activation of the stochastic movements during the vitrification and the cold crystallization processes. Upon cooling, the vitrification process involves freezing-in of the stochastic movements of ester groups. Upon heating, as the cold crystallization process begins, the first to respond are the vibrations of the C–O–C and C=O groups in the rigid core

Dynamics of two glass forming monohydroxy alcohols by field cycling 1H NMR relaxometry

The dynamics of 2,2-dimethylbutan-1-ol (2,2-DM-1-B) and 3,3-dimethylbutan-1-ol (3,3-DM-1-B), two glass forming monohydroxy alcohols, was investigated by field cycling1H NMR relaxometry in their liquid phase, including the supercooled regime. Nuclear Magnetic Relaxation Dispersion (NMRD) curves (i.e. longitudinal relaxation rate R1vs1H Larmor frequency), acquired for the two alcohols at different temperatures in the 0.01-35 MHz frequency range, were analyzed in terms of suitable models for internal motions, overall molecular reorientations, and molecular self-diffusion, and the corresponding correlation times were quantitatively determined. In addition, trends of1H R1as a function of the frequency square root at low frequencies, where the contribution of translational motions dominates, were exploited to achieve an independent determination of the self-diffusion coefficients (D), which does not require the separation of different motional contributions to relaxation. Good agreement was found between D values determined by the two methods, thus corroborating the model used for the description of the NMRD curves. Self-diffusion was found to be slower and more strongly temperature dependent for 2,2-DM-1-B with respect to 3,3-DM-1-B, whereas molecular reorientations were quite similar for the two isomeric alcohols. Correlation times for molecular reorientations were found to be at least one order of magnitude shorter than those reported in the literature for the Debye-like relaxation observed by dielectric spectroscopy in the liquid phase