Studies of Phase Diagram of a Liquid Crystal with 4-[2-(3-Fluorophenyl)ethyl]biphenyl Core of Molecules

Kołek, Łukasz; Massalska-Arodź, Maria; Majda, Dorota; Wantusiak, B.; Zalewski, Sławomir; Kula, Przemysław

doi:10.12693/aphyspola.122.370

Cited by 11 publications

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“…7−9 It is especially true for glass-formers, where the phenomenon of the so-called cold crystallization (cc) occurs on heating a substance. 10−14 For low molecular organic materials, the cc (a process typical for polymers and called recrystallization) has been identified on heating after softening of glasses of isotropic phases, 12−14 nematics, 7,15,16 crystal-like smectics, [7][8][9][10]17 conformationally disordered crystalline phases, 10,18 and ferroelectric phases 9,19 of liquid crystalline materials and also of plastic phases of alcohols. 20,21 In many cases, a metastable crystal is formed (one or more) which transforms on further heating to the stable crystalline phase.…”

Section: ■ Introductionmentioning

confidence: 99%

“…Studies of phase transitions in liquid crystalline materials revealed complex polymorphism of liquid-like and solid-like phases. − It is especially true for glass-formers, where the phenomenon of the so-called cold crystallization (cc) occurs on heating a substance. − For low molecular organic materials, the cc (a process typical for polymers and called recrystallization) has been identified on heating after softening of glasses of isotropic phases, − nematics, ,, crystal-like smectics, − , conformationally disordered crystalline phases, , and ferroelectric phases , of liquid crystalline materials and also of plastic phases of alcohols. , In many cases, a metastable crystal is formed (one or more) which transforms on further heating to the stable crystalline phase. Research on the kinetics of cold-crystallization in liquid crystalline glass-formers was conducted by polarizing microscopy (POM) along with calorimetric (adiabatic and/or DSC) methods ,, and broadband dielectric spectroscopy (BDS), also performed in real time of the experiment. ,,, …”

Section: Introductionmentioning

confidence: 99%

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Kinetics of Cold Crystallization of 4-Cyano-3-fluorophenyl 4-Butylbenzoate (4CFPB) Glass Forming Liquid Crystal. I. Nonisothermal Process As Studied by Microscopic, Calorimetric, and Dielectric Methods

Rozwadowski

Massalska-Arodź

Kołek

et al. 2015

Crystal Growth & Design

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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...

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