Studies of phase diagram and glass transitions of a liquid crystal with ferro- and antiferroelectric phasesl

Based on the results of the differential scanning calorimetry, of transmitted light intensity measurements and of texture observations the phase diagram of 4-(6-hepta-fluoro-butano-iloxy-hexy-loxy)bi-phenyl-4'-carbo-xy-late(S)-4-(methylo-hepty-loxy-1-carbonyl)-phenyl (4H6) was obtained. The following phases were identified on cooling: isotropic, smectic A, smectic C*, smectic C*A phases and a highly ordered phase called SmX and its glass. During heating transformation from glass of SmX to SmX phase and then transition to a metastable Cr2 phase, evolving to the more stable Cr1 phase, were observed. On further heating SmC*A, SmC* and Sm phases were identified. When Cr2 was cooled, a glass transition was also observed

Studies of phase diagram of a liquid crystal with 4-[2-(3-fluorophenyl)ethyl]biphenyl core of molecules

Basing on the results of di erential scanning calorimetry and transmitted light intensity methods and texture observations in the temperature range from −50 ◦C to 90 ◦C phase diagram of 1-[3- uoro-4-(1-methylheptyloxycarbonyl)phenyl]-2-[4′ -(2,2,3,3,4,4,4-hepta uorobutoxybutoxy)biphenyl-4-yl]ethane was established. Monotropic system of thermodynamic phases was found. On cooling the isotropic phase transforms at 85 ◦C to ferroelectric SmC∗ phase and further two liquid crystalline phases, antiferroelectric SmC∗ A and SmI, and a metastable crystal Cr2 reveal. During heating exothermic anomaly in the wide temperature range was observed and ascribed to evolution of Cr2 to the stable crystal Cr1. On further heating both SmC∗ A and SmC∗ phases were observed. Thermodynamic parameters were calculated for all phase transitions. During rapid cooling (10 K/min) vitri cation of metastable Cr2 crystal at T_g = −115 ◦C was registered

Molecular dynamics and cold crystallization process in a liquid-crystalline substance with para-, ferro- and antiferro-electric phases as studied by dielectric spectroscopy and scanning calorimetry

In this article, molecular dynamics and the cold crystallization kinetics of 4-(6-heptafluorobutanoiloxyhexyloxy) biphenyl-4′-carboxylan(S)-4-(1-methyloheptyloxycarbonyl) phenyl (abbreviated as 3F6Bi and/or 4H6) are presented. Rich polymorphismof the liquid-crystalline (SmA*, SmC*, SmC*A and SmI*A) phases and partially disordered crystal CrI and glassy GCrI were observed upon cooling. Both, molecular and collective relaxation processes were observed in the para-, ferro- and antiferro-electric liquid-crystalline phases over the frequency range of 3 × 10−2 to 3 × 106 Hz. An additional bias field in the dielectric experiments was used to identify individual processes. The high heating rates (5–10 K/min) phase sequence is the same as in case of the cooling experiment. On slow heating (0.5–2 K/min), cold crystallization of SmI*A to the more stable crystal CrII phase was observed in the dielectric and calorimetric experiments. The crystallization kinetics was analyzed using the Mo equation, which is a combination of the Avrami and Ozawa models. The activation energy of crystallization was calculated to be 138 and 99 kJ/mol using the Kissinger and Augis-Bennett models, respectively

Reassessment of structure of smectic phases: Nano-segregation in smectic E phase in 4-n-alkyl-4′-isothiocyanato-1,1′-biphenyls

Based on new diffraction data from aligned samples of smectic E (SmE) phase of 4-n-alkyl-4′-isothiocyanato-1,1′-biphenyls, systematics against the alkyl chain length n is analyzed. In order to perform the analysis, the molecular form factor approximated by a box-shaped distribution is calculated while taking the rounding of the distribution at corners into account. The analysis clearly shows the nano-segregated layered structure, which does not fit to the traditional structural view of SmE phase but does fit to the model the present authors proposed recently. Some implications of this conclusion are discussed in relation to the importance of the molten state of alkyl chains in most of real mesogens revealed previously through thermodynamic analyses

Pressure-Temperature Phase Diagrams for four 4,4'-Dialkylbiphenyl Compounds

The high pressure phase behaviour of four 4,4'-dialkylbiphenyl compounds (Cm-Cn, = 5, 6,7, = 6, 7) has been studied with differential thermal analysis. The pressure dependence of the phase transitions has been determined up to 200 MPa. In one substance a pressure limited and in another a pressure induced phase was observed. Volume changes accompanying the transitions to the isotropic phase were calculated using the Clausius-Clapeyron equation and the enthalpy changes from DSC measurements at 1 atm. They are compared with the data for other two-ring compounds