Dielectric characterization of B3 and B4 phases

Dielectric measurements on two samples formed from banana-shaped molecules with terminal alkyl or alkyloxy groups were carried out in a frequency range between 10^-3 Hz and 10 MHz. Both samples exhibit B₂ and B₃ phases; one of them has a B₄ state as well. As usual, two ranges of relaxation were detected in the B₂ phase, the fast reorientation about the long axes of the molecules and a slow collective process. Only one dielectrically active low frequency process could be separated in the B₃ and B₄ phases; this is probably related to the dynamics of superstructures. The high frequency limit of the dielectric constants gives a hint that neither phase is a classical solid. A quite strong increase in the conductivity at the transition into the B₃ phase is critically discussed.     

AFM investigations of a sample showing the B2 phase

Atomic force microscopy (AFM) images reveal that the liquid crystalline B₂ phase has no helical superstructure, in contrast to the B₇ phase. This is demonstrated on comparing the AFM images of focal-conic domains in the smectic A, cholesteric, B₂ and B₇ phases. The B₂ and B₇ phases are phases formed by banana-shaped molecules. The results point to the existence of ferroelectric clusters with a random size distribution which also cause the low frequency dielectric relaxation.     

Characterization of the B6 phase by dielectric measurements

Dielectric measurements on a sample from banana-shaped molecules were carried out between 1Hz and 10MHz. The sample exhibits at higher temperatures the B₆ phase and at lower temperatures the B₁ phase. No difference in the dynamical behaviour between these two phases with respect to the relaxation times, dielectric increments and distribution parameters could be detected. Both phases show a negative dipole correlation for the reorientation of the dipoles in the stiff middle part about the long axes.     

Enhanced conductivity and dielectric absorption in discotic liquid crystalline columnar phases of a vanadyl complex

A liquid crystalline vanadyl complex has been studied by DSC, polarizing optical microscopy, the reversal current technique, X-ray diffraction and frequency domain dielectric spectroscopy. The compound exhibits three columnar phases: rectangular ordered (Col_ro), rectangular disordered (Col_rd), and hexagonal disordered (Col_hd), all of which show a dielectric relaxation process at low frequencies. In the Col_ro low temperature phase this process seems to be connected with a slow relaxation of polarized polymeric chains inside the columns (mHz frequency range). However, in the Col_hd high temperature disordered phase this relaxation is faster (Hz range). It is interesting that the liquid crystalline phases studied show enhanced conductivity which changes by four orders of magnitude from 10^?9 S m^?1 in the orientationally disordered crystal (an ODIC phase) to 10^?5 S m^?1 in the Col_hd high temperature phase. Such a value of the conductivity is typical for semiconducting materials.     

From Sponges to Nanotubes: A Change of Nanocrystal Morphology for Acute‐Angle Bent‐Core Molecules

The crystalline (B₄) phase made of acute‐angle bent‐core molecules (1,7‐naphthalene derivatives), which exhibits an unusual, highly porous sponge‐like morphology, is presented. However, if grown in the presence of low‐weight mesogenic molecules, the same crystal forms nanotubes with a very high aspect ratio. The nanotubes become unstable upon increasing the amount of dopant molecules, and the sponge‐like morphology reappears. The phase is optically active, and the optical activity is an order of magnitude smaller than in the B₄ phase made of conventional bent‐core molecules. The optical activity is related to the spatial inhomogeneity of the layered structure and is reduced due to the low apex angle and low tilt of the molecules. The arrangement of molecules within the layers was deduced from the bathochromic absorption shift in the B₄ phase.     

Columnar phases of achiral vanadyl liquid crystalline complexes

Two liquid crystalline vanadyl complexes have been studied by frequency domain dielectric spectroscopy over the range 10 mHz to 13 MHz. The materials exhibit two or three columnar phases denoted Col_ro, Col_rd, and Col_hd that were identified by X-ray diffraction. In the higher temperature Col_rd phase, a relaxation process in the kHz range is observed that is attributed to the reorientation about the molecular short axis. A pronounced dielectric relaxation process shows up in the low temperature Col_ro phase at hertz and sub-hertz frequencies. This slow relaxation is assigned to reorientation of the molecular dipoles within the polar linear chains, which are aligned along the column's axis. Triangular wave switching studies at low frequency reveal processes inside the Col_ro phase which are most probably due to ionic/charges relaxations but a ferroelectric switching for an achiral discotic system cannot be ruled out completely. Below the Col_ro phase there is an orientationally disordered crystalline Cr_x phase with disordered side chain dipoles. A dielectric relaxation process connected with the intramolecular relaxation of the alkoxy side chains, similar to the beta-process of polymers, has been found in the lower temperature Cr_x phase.     

Unique Ordered TiO2 Superstructures with Tunable Morphology and Crystalline Phase for Improved Lithium Storage Properties

Unique ordered TiO₂ superstructures with tunable morphology and crystalline phase were successfully prepared by the use of different counterions. Dumbbell‐shaped rutile TiO₂ and nanorod‐like rutile mesocrystals constructed from ultrathin nanowires, and quasi‐octahedral anatase TiO₂ mesocrystals built from tiny nanoparticle subunits were achieved. Interestingly, the obtained anatase mesocrystals have a fine microporous structure and a large surface area. The influence of the counterions in the reaction system is discussed and possible mechanisms responsible for the formation of the unique ordered TiO₂ superstructures with different morphologies and crystalline phases are also proposed based on a series of experimental results. The obtained TiO₂ superstructures were used as anode materials in lithium ion batteries, and exhibited higher capacity and improved rate performance; this is attributed to the intrinsic characteristics of the mesoscopic TiO₂ superstructures, which have a single‐crystal‐like and porous nature.     

Influence of halogen substituent on the mesomorphic properties of five-ring banana-shaped molecules with azobenzene wings

Three new series of bent-shaped molecules with 4-chlororesorcinol, 4-bromoresorcinol or 4-fluororesorcinol as the central unit, and azobenzene with different alkoxy chain length as side arms were synthesised. The mesophase behaviour was investigated by polarising optical microscopy, and differential scanning calorimetry. A representative example has also been characterised by X-ray diffraction (XRD) studies. It is found that almost all of the materials prepared are monotropic liquid crystalline. Depending on the substituent at the central unit and on the chain length nematic phases, B₆ phases, a B₄-like dark conglomerate phase and a modulated/undulated anticlinic SmC phase were found. As a unique feature, upon reducing the chain length a transition from nematic to B₆-type smectic phases was observed, which is reverse to usually observed phase sequences. The UV–vis absorption spectroscopy was also performed to study the effect of light-induced trans–cis-isomerisation on the prepared compounds.     

Dielectric behaviour of the B1 phase

Dielectric measurements on two different samples formed by banana-shaped molecules and showing a B₁ phase were carried out. Three relaxation ranges were detected: a very fast intramolecular motion, the fast reorientation about the long axes and a slow collective process. The data are discussed in relation to known X-ray investigations and dielectric results for homologous samples showing the B2 state. The main difference between B₁ and B₂ is that in B₁ no evidence for ferroelectric order in the short range was detected.     

Phase behaviour of thermotropic banana-shaped compounds under pressure

The phase behaviour of two achiral bent core banana-shaped compounds, the hexyloxy (compound I) and decyloxy (compound II) members of the 1,3-phenylene bis[N-(2-hydroxy-4-n-alkoxybenzylidene)-4′-aminobenzoate] series was investigated under hydrostatic pressures up to 300?MPa using high pressure differential thermal analysis and light transmission methods. The reversible transition sequence crystal (Cr₁)–B₁ phase–isotropic liquid (I), observed at room pressure for compound I, remains in the pressure region up to c 70?MPa. At higher pressures a pressure-induced crystalline phase (Cr_i) appears between the Cr₁ and B₁ phases, its temperature region becoming wider with increasing pressure. The temperature vs. pressure phase diagram shows a triple point of 72.9?MPa and 160.3°C for the Cr₁, Cr_i and B₁ phases, indicating the lower limit of pressure for the Cr_i phase. In compound II the reversible transition sequence crystal (Cr₁)–B₂ phase–I is seen over the whole pressure region, and the temperature range of the B₂ phase remains unaltered. It is concluded that both the B₁ and B₂ banana phases are stable over the whole pressure region studied.     

Dielectric relaxation of a short molecule in a liquid-crystalline solvent

Abstract

Dielectric relaxation measurements of 5 mole % 4-n-hexyloxycyanobenzene (I) dissolved in 4-n-pentyloxyphenyl-trans-4-n-octylcyclohexylcarboxylate (II) were carried out from 1 kHz to 10 MHz in the nematic, smectic A and smectic B phases. The relaxation frequency of I parallel to the director is about 05 MHz in the S_Bphase and increases rapidly at the transition from S_B to S_A.     

Synthesis and characterization of achiral banana‐shaped liquid crystalline molecules containing bisnaphthyl moieties

Three analogous series of symmetric banana‐shaped liquid crystalline molecules containing bisnaphthyl units were synthesized and characterized. The effects of linking groups (on the side wings) and lateral meta‐fluoro substitutions (on the middle outer rings) on the mesogenic properties were examined. The type of mesophase depends on the lengths of terminal alkoxy chains. Thus, achiral molecules with shorter flexible chains (n = 8) exhibit a rectangular columnar (B₁) phase, while analogous derivatives with longer flexible chains (n = 12) display the B₂ phase. All lateral meta‐fluoro substituted analogues (series II) possess the lowest isotropization temperatures and the narrowest mesophasic ranges of the B₁ and B₂ phases. The B₁ and B₂ phases were confirmed by X‐ray diffraction, polarizing optical microscopy (POM) and electro‐optical (EO) switching experiments. An electric field‐induced transition from an antiferroelectric (tristable) state to a ferroelectric (bistable) state was observed in the EO measurements. Spontaneous polarization (by switching current response), tilt angle of chiral domains (by POM), and transmittance–voltage measurements of the B₂ phase in related compounds have been surveyed in this study.     

An isotopic effect in the dielectric spectra of Ark. 9/water systems

Dielectric spectra of H₂O and D₂O molecules in the L_α liquid crystalline phase of nonylphenoxy-poly(ethylenoxy)ethanol(Ark. 9)/water lyotropic systems have been investigated by dielectric time domain spectroscopy in the frequency range from 10 MHz to 10 GHz. By fitting the Cole-Cole formula to the dielectric spectra, obtained at different temperatures the dielectric increments, the relaxation times and the distribution parameters have been calculated. A strong retardation of water molecules has been found for the lamellar phase with low water content, i.e. 10 water molecules (H₂O or D₂O) per one Ark. 9 molecule. The relaxation times obtained at room temperature for the light and heavy water are 63 and 93 ps, respectively. It means that the retardation factor for D₂O molecules in the L_α phase is close to 1.5 and higher than that found for pure heavy water (1.25). Any decomposition of the dielectric spectra obtained seems to be unsubstantiated. The temperature dependences of the relaxation times acquired for both kinds of water obey the Arrhenius behaviour.     

Hindered reorientation of chiral molecules around their long axes in the S*C phase

Abstract

In this work, the high frequency dielectric relaxation process has been studied for the N*, S*_A and S*_C phases of the mixture ZLI 3654. To this end, time domain spectroscopy (TDS) was used to cover the frequency range from 10⁷ to 10¹⁰ Hz. The dielectric parameters obtained show that the high frequency spectrum is mostly influenced by the reorientation of molecules around their long axes. In addition, the latter process seems to be strongly hindered in the S*_C phase, because the necessary relaxation time is of the order of 1 ns, whereas for the N* and S*_A phases, the relaxation time is remarkably shorter and equal to 0.3 and 0.4 ns, respectively.     

Phase ordering of the liquid crystalline 'B7' phase by analysis of fractal growth patterns

The phase ordering process of bent-core mesogenic molecules is qualitatively different from that observed for conventional rod-like (calamitic) mesogens. Fractal dimensional analysis can be employed to characterize the phase ordering process of these unconventional mesogens. The phase transition between the isotropic melt and the liquid crystalline 'B₇' phase of a 'banana-molecule' material has been investigated in terms of fractal growth patterns, with respect to sample thickness and rate of temperature change. It is shown, that there is a crossover from confinement dominated phase growth to volume behaviour, while the cooling rate has no influence on the dimension of the fractal patterns observed during the phase ordering process.     

Thermal evolution of ferromagnetic metallic glasses

A traditional TG apparatus was modified by placing two permanent magnets producing a controlled magnetic field (TG(M): Magneto Thermogravimetry). This technique proved to be useful to study both structural relaxation and crystallisation of ferromagnetic metallic glasses. Results obtained for the amorphous alloys Fe₄₀Ni₄₀P₁₄B₆ and Fe_62.5Co₆Ni_7.5Zr₆Nb₂Cu₁B₁₅, are reported in this paper. Structural relaxation can be evaluated by measuring changes in Curie temperature induced by thermal treatments. Crystallisation in TG(M) is detected through a change in the measured apparent mass (difference between the sample mass and magnetic force driving it upward). These results were confirmed by DSC analysis. Whether the obtained crystalline phase is ferromagnetic, it can be identified through its Curie temperature, measured by TG(M). In fact the value of 770°C measured as Curie temperature of crystallised Fe_62.5Co₆Ni_7.5Zr₆Nb₂Cu₁B₁₅led to conclude that the only ferromagnetic crystalline phase is a-Fe. These hypothesis was confirmed by XRD analysis, showing that the first crystallisation yields to a-Fe nanocrystals. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dielectric relaxation studies of 6OCB/8OCB mixtures with the nematic-smectic A-nematic re-entrant phase sequence

The low frequency relaxation process was studied for 6OCB/8OCB mixtures with three concentrations (27.0, 27.3 and 27.5 wt %) exhibiting the isotropic-nematic-smectic A-nematic re-entrant-crystalline phase sequence and four mixtures (28.5, 30.0, 35.0 and 40.0 wt %) with the isotropic-nematic-crystalline phase sequence. In the liquid crystalline phases, all dielectric spectra could be excellently described by the Debye equation. The relaxation time tau passes smoothly through the phase transitions separating the liquid crystalline phases. The activation barriers hindering the molecular rotations around the short axes are practically the same in the nematic and smectic phases and become larger in the re-entrant nematic phase. Smaller values of the barrier in the nematic phase of mixtures in comparison with those obtained recently for pure 6OCB and 8OCB are explained as an effect of weakening of the molecular interactions caused by increased dipole-dipole associations between molecules in mixtures in relation to pure substances. The slightly larger activation barrier in the nematic re-entrant phase indicates stronger molecular associations in this phase. d     

Dielectric relaxation studies of 6OCB/8OCB mixtures with the nematic-smectic A-nematic re-entrant phase sequence

The low frequency relaxation process was studied for 6OCB/8OCB mixtures with three concentrations (27.0, 27.3 and 27.5 wt %) exhibiting the isotropic-nematic-smectic A-nematic re-entrant-crystalline phase sequence and four mixtures (28.5, 30.0, 35.0 and 40.0 wt %) with the isotropic-nematic-crystalline phase sequence. In the liquid crystalline phases, all dielectric spectra could be excellently described by the Debye equation. The relaxation time tau passes smoothly through the phase transitions separating the liquid crystalline phases. The activation barriers hindering the molecular rotations around the short axes are practically the same in the nematic and smectic phases and become larger in the re-entrant nematic phase. Smaller values of the barrier in the nematic phase of mixtures in comparison with those obtained recently for pure 6OCB and 8OCB are explained as an effect of weakening of the molecular interactions caused by increased dipole-dipole associations between molecules in mixtures in relation to pure substances. The slightly larger activation barrier in the nematic re-entrant phase indicates stronger molecular associations in this phase. d     

Structural and thermal interpretation of the synergy and interactions between the fire retardants magnesium hydroxide and zinc borate

The mechanism of cooperative action of commercial fire retardants is interpreted as resulting from specific chemical reaction and phase changes. This investigation focuses on the thermally initiated interactions between two forms of commercially available fire retardant compounds. The fire performance of a polyolefin with a metal hydroxide fire retardant, magnesium hydroxide, can significantly reduce the heat release rate through absorption of heat during conversion to its metal oxide. Formation of water, followed by vaporisation, decreases heat and dilutes volatiles from polymer degradation. The second form of fire retardant compounds are zinc borates (2ZnO·3B₂O₃·3H₂O and 4ZnO·B₂O₃·H₂O), that undergo dehydration with increasing temperature. Differential thermal analysis and wide-angle X-ray spectroscopy indicated that various structural changes occurred during heating. Endothermic transitions were observed for all components, while zinc borate (2ZnO·3B₂O₃·3H₂O) showed an exothermic crystallisation transition at relatively high temperature. The exotherm was modified by the development of a new crystalline phase, magnesium orthoborate (3MgO·B₂O₃) that formed on reaction with magnesium oxide (MgO) at temperatures greater than 500 °C. Formation of crystalline zinc oxide (ZnO) was also detected. From zinc borate (4ZnO·B₂O₃·H₂O), ZnO was primarily formed. No new crystalline phases were observed in the presence of MgO over the temperature range investigated.     

Synthesis and mesomorphic properties of four-ring liquid crystals containing 2,3-difluorophenyl and 1,3-dioxane units

Two series of novel liquid crystalline materials were synthesized: 2-(2,3-difluoro-4-alkoxyphenyl)-5-[4-(4-alkylcyclohexyl)phenyl]-1,3-dioxanes (A _m-n and 2-(2,3-difluoro-4-alkylphenyl)-5-[4-(4-alkylcyclohexyl)phenyl]-1,3-dioxanes (B _m-n ). Their mesmorphic properties were characterized by polarizing optical microscopy and differential scanning calorimetry. All these compounds exhibit broad mesomorphic phases, and most of them show wide smectic C phases and the phase sequence Cr-SmC-SmA-N-I. The relationship between properties and chemical structures is discussed in detail.