For 4-cyano-3-fluorophenyl 4-butylbenzoate (4CFPB), the process of the crystallization of the CrII phase was studied in microscopic (POM), calorimetric (DSC), and dielectric (BDS) nonisothermal experiments with various (0.5−50 K/min) heating of the metastable nematic phase obtained from its glass. Growth of areas of crystal CrII in the microscopic texture of nematic phase during heating allows estimation of degree of crystallinity D(T) vs temperature curves similar to these obtained basing on DSC heat flow curves and for slow heating with help of dielectric relaxation (BDS) method. Two types of CrII crystallization mechanisms seem to be identified: (1) strong ϕ dependence on temperature of full crystallization T c (ϕ) and half time of crystallization t 1/2 (ϕ) on slow heating up to 5 K/min points to diffusioncontrolled mechanism with the energy barrier 57 kJ/mol, and (2) small effect of faster heating on T c (ϕ) and t 1/2 (ϕ) seems to illustrate thermodynamic mechanism with energy barrier 180 kJ/mol. The scenario of two mechanisms of CrII crystallization is in agreement with the results of new method proposed by Mo et al., using combination of Avrami and Ozawa equations for description of nonisothermal crystallization. In addition to crystallization of CrII of 4CFPB, at higher temperature range CrII−CrI transformation to a stable CrI crystal was digitalized based on microscopic and DCS results for heating at 1 K/min. ■ INTRODUCTIONThe well-known crystallization phenomenon is still not clearly described as it depends on many factors like a type of nucleation of crystal grains, a nucleation rate, and a rate of growth of crystallites in the melt substance. 1,2 Calculations of absolute nucleation and growth rates are difficult, but each substance has its own temperature ranges where nucleation and growth are favorable. Usually, the rate I(T) of nucleation has its maximum at lower temperature than the maximum for the rate G(T) of crystal growth. 2 Moreover, both rates may be complicated functions of temperature and of details of the experiment used (e.g., cooling rate). The driving force of overall crystallization depends on viscosity to entropy relationships and their temperature changes. 3 Lower temperature parts of I(T) and G(T) curves reflect growing viscosity (transport parts), while the higher temperature parts are the results of larger diffusivity/molecular mobility (thermodynamic parts) in the substance under study. 2 If these curves have no temperature range in common, no crystallization is detected on cooling. Instead, vitrification is observed, no matter how slowly the temperature is decreased. Then, crystallization is expected on heating. In isothermal studies of crystallization kinetics, degree D(t) of crystallinity (or crystallization ratio) is described in terms of Avrami model 4,5Linear dependence of log{−ln[1 − D(t)]} vs log t is expected: slope n A describes the dimensionality of the process and the nucleation mode (instantaneous, prolonged in time, 1-, 2-or 3-dimensional etc.), and the k parameter d...

The results of dielectric relaxation spectroscopy and polarizing microscope observation of the 4'-butyl-4-(2-methylbutoxy)azoxybenzene (abbreviated as 4ABO5*) are presented. Numerical analysis of the dielectric spectra results points to complex dynamics of 4ABO5* molecules in isotropic, cholesteric, and crystalline phases. Two well-separated maxima on the imaginary part of dielectric permittivity and the third low frequency relaxation process, hidden in the conductivity region, were detected and described in cholesteric and crystalline phases. Temperature dependence of mean relaxation times characterizing flip-flop motions and rotation around long axes, observed in all phases, is of the Arrhenius type.

This paper presents studies of dielectric response of chosen Schiff bases, which have similar molecular structures with different isomerizations of an azomethine bridging group, alkyloxy chain length with n=5 or n=6 carbon atoms, and a bromine or chlorine halogen terminal atom. Significant differences in the values of the maximum of dielectric absorption related to flip-flop molecular jumps in hexagonal smectic-B(Cry) phases have been found despite small differences of molecular dipole moments in these substances. This phenomenon is discussed in relation to the possibilities of the creation of dimers and to steric factors favoring motions.

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Signicant dierences in phase sequence and dynamics between 4-bromobenzylidene-4-pentyloxyaniline (5BBA) and-bromobenzylidene-4-heksyloxyaniline (6BBA) substances were detected by broadband dielectric spectroscopy. In the smectic A (6BBA), the smectic B (found in both substances) and the smectic E (5BBA) phases the relaxation was ascribed to reorientations of the molecules around the short axes. Slow dynamics detected in the smectic E and in two crystalline phases of 5BBA was ascribed to conformational changes of molecular chains. Coexistence of Cr(I) and Cr(II) conformationally disordered crystal (CONDIS) phases was observed and vitrication of Cr(II) was identied with help of polarizing microscopy.