Ewa Juszyńska-Gałązka

Phase polymorphism and thermal stability of new cholesterol thioesters derivatives

We report the fundamental properties of a series of new mesogens with cholesteric esters, cholesteryl 4-(4-alkoxybenzoylothio)benzoates CnH2n+1OC6H4COSC6H4COOch, where ch = cholesteryl, for n = 7–10, nOSBch in short. They share a common feature, which is a high temperature cholesteric mesophase. In addition, the n = 10 compound has a monotropic ferroelectric smectic C* phase. The chemical structure and purity of nOSBch were confirmed by standard methods: 1H nuclear magnetic resonance (NMR), 13C NMR, Fourier transform infrared spectroscopy and elemental analysis. Differential scanning calorimetry (DSC), polarising optical microscopy and transmitted light intensity were used to study the polymorphism of these mesogens. The substances were proved stable up to 240–270°C (thermal stability deduced from DSC scans). The crystal structures were determined from X-ray diffraction data. Possible conformational disorder as well as nearest-neighbours intermolecular interactions were inferred from quantum chemical calculations.

Molecular dynamics of 4-cyano-3-fluorophenyl 4-butylbenzoate (4CFPB) glass-forming liquid crystal in unidirectional silicon nanopores

Complex dynamics of 4-cyano-3-fluorophenyl 4-butylbenzoate (4CFPB) in cylindrical non-intersecting nanopores, mean diameters 6 nm and 8 nm was studied by broadband dielectric spectroscopy. For relaxation processes ascribed to the flip-flop, reptation-like and collective motions, the short- and long-range time correlations were estimated and discussed. Size effect and differences in ordering of molecules in pores during cooling and heating runs were found. On cooling, vitrification occurred in pores, similarly as in bulk, while on heating two-step ordering of molecules along pore walls was evidenced by subsequent diminishing of ε″max(T). Explanation based on data concerning solid state polymorphism of 4CFPB in bulk is given.

Universal scaling of dielectric response of various liquid crystals and glass-forming liquids

Abstract We present a new generalized scaling relationship accounting both for the real and imaginary parts of the complex permittivity data. The generalized scaling procedure has been successfully used for various relaxation processes in liquid crystals (4-bromobenzylidene-4′-pentyloxyaniline, 4-bromobenzylidene-4′-hexyloxyaniline, 4′-butyl-4-(2-methylbutoxy)-azoxybenzene, 4-ethyl-4′-octylazoxybenzene), and in glass-forming liquids (glycerol, propylene carbonate, salol, cresolphthalein-dimethylether). As it is shown, one obtains common master-curve for liquid-like phases (isotropic liquid, cholesteric, nematic, smectic A), solid-like phases (smectic B, conformationally disorder crystal) and supercooled liquid phase.

Vibrational dynamics of glass forming: 2-phenylbutan-1-ol (BEP), 2-(trifluoromethyl)phenethyl alcohol (2TFMP) and 4-(trifluoromethyl)phenethyl alcohol (4TFMP) in their thermodynamic phases

ABSTRACT The complex polymorphism and vibrational dynamics of three glass-forming single-phenyl-ring alcohols (with and without fluorine atoms) have been studied by complementary methods. Glass of isotropic liquid phase and cold crystallization of metastable supercooled liquid state were detected. Temperature investigations of vibrational motions show important role of hydrogen bonds in interactions between molecules. Theoretical calculations for isolated molecule, as well as dimer- and tetramer-type aggregates of non-covalently bound molecules, allow for a good description of experimental spectra. Intermolecular interactions of molecules with ortho and para positions of CF3 group in phenyl ring have a similar influence on the spectra observed.

Dynamics of Dimethylbutanols in Plastic Crystalline Phases by Field Cycling 1H NMR Relaxometry

2,2-Dimethylbutan-1-ol (2,2-DM-1-B), 3,3-dimethylbutan-1-ol (3,3-DM-1-B), and 3,3-dimethylbutan-2-ol (3,3-DM-2-B) show a rich solid-state polymorphism, which includes one or more plastic crystalline phases (also referred to as orientationally disordered crystalline (ODIC) phases) and glass of the liquid or ODIC phases. In this work, the dynamics of the three isomeric alcohols was investigated in the liquid and plastic crystalline phases by fast field cycling 1H NMR relaxometry in the temperature range between 213 and 303 K. The analysis of the nuclear magnetic relaxation dispersion curves (i.e., longitudinal relaxation rate R1 vs 1H Larmor frequency) acquired for the different alcohols at different temperatures gave quantitative information on internal motions, overall molecular reorientations, and molecular self-diffusion. Self-diffusion coefficients were also determined in the liquid phase and in some ODIC phases of the samples from the trends of 1H R1 as a function of the frequency square root at low frequencies. Remarkable changes in the temperature trends of correlation times and self-diffusion coefficients were found at the transition between the liquid and the ODIC phase for 2,2-DM-1-B and 3,3-DM-1-B, and between ODIC phases for 3,3-DM-2-B, the latter sample showing a markedly different dynamic and phase behavior.

Positron annihilation in liquid crystals

Abstract Positron annihilation studies of liquid crystals are reviewed with particular reference to thermotropic liquid crystals with rod-like molecules. The studies of compounds exhibiting smectic A or smectic E phases indicate that local arrangement of dipole molecules play an important role because high electron density at the end group of molecules can influence substantially formation and annihilation of positronium. The obtained ortho-positronium lifetimes in these phases can be explained by antiparallel pairing of molecules in case of the smectic A phase or a structure with a nanosegregation of alkyl chains and others parts of molecules into sublayers and liquid-like state of alkyl chains in case of the smectic E phase.