Melting and Crystallization Processes of EBBA III. Microscopic Observation

Melting and crystallization processes of EBBA (N-p-ethoxybenzylidene-p'-butyl aniline) have been studied by the method of microscopic observation. A solid sample obtained by rapid cooling (cooling rate: 40 K/min) from nematic to 5°C (sample A) shows granular texture. A band like crystal grows in sample A at room temperature. A solid sample obtained by quenching (cooling rate: 13 K/s) from nematic to about - 100°C (sample B) shows cloudy texture. In the heating process sample B transforms to a more stable crystalline form which shows mosaic texture. A needle crystal grows slowly in a supercooled nematic state, then smaller needle crystals grow rapidly around the needle crystal. The formation and transformation processes of solid modifications are explained on the basis of the idea of instability of them, and schematic phase diagram of those solid forms is presented.

Keywords: EBBA, liquid crystals, nematic, solids, crystal growth, microscopic observation     

Stabilization of polystyrene by styrylquinoline containing methacrylates

Abstract

The thermostability of polysterene with/without new styrylquinoline containing methacrylic fragments has been described in the present work. Polystyrene and copolymers based on styrene and new methacrylic styrylquinoline containing monomers were synthesized by free radical thermoinitiated polymerization in the presence of 2,2′-azobisisobutyronitrile (1?wt%) as initiator at inert atmosphere. The impact of 2-[2-(4-methoxyphenyl)ethenyl]quinolin-8-yl 2-methylpropyl-2-enoate (М1), 2-(2-phenylethenyl)quinolin-8-yl 2-methylpropyl-2-enoate (М2), 2-[2-(4-nitrophenyl)ethenyl]quinolin-8-yl 2-methylpropyl-2-enoate (М3) on thermal stability of polystyrene was investigated. The thermostability of polystyrene and copolymers ST:M1, ST:M2, ST:M3 were studied by dynamic thermograviametric analysis. It was shown that destruction of polystyrene with corresponding units M1-M2 starts at 41–42°С higher, than reference polystyrene.     

Low-temperature luminescence and phase structure of 5CB liquid crystal under different cooling regimes

Abstract

The effects of the 5CB liquid crystal (LC) cooling regimes on its luminescence spectra at the low temperature (T?=?4.2 K) have been found out and investigated. Experiments have been carried out at two cooling rates: 2 and 150?deg/min. Differential scanning calorimetry (DSC) data allow us to establish that the 5CB glass-like state (GLS) is formed during the rapid cooling of the sample. Peculiarities of the LC dimer formation under the different cooling conditions are discussed.     

The Smetic Polymorphism and the Phase Transitions in the Liquid Crystal 4, n-Hexyloxyphenyl-4, n′-Decyloxybenzoate

Three different methods — microscopical, depolarized light scattering and conoscopic have been used for the investigation of the polymorphism of 4, n-hexyloxyphenyl-4, n′-decyloxybenzoate (HOPDOB) at homeotropic orientation. A new textural transition is registered during cooling in smectic C phase connected with a change in the tilt (ω) and azimuthal (φ) angles of the molecular director. At cooling we found smectic E phase and by the conoscopic method we determined this phase as tilted smectic type (E_t) with 24° tilt angle. It is also found that B phase is a normal type smectic liquid crystal — B_N.     

Photoreaction of Cholesteryl trans-Cinnamate in Mesomorphic States

Abstract

The photoreaction of cholesteryl trans-cinnamate in K Br matrix was studied in the solid, mesomorphic and isotropic liquid states at 25?225°C, and compared with the solution reaction in n-hexane. Cholesteryl trans-cinnamate was converted into a dimer on irradiation at wave-length longer than 300 nm at these phases. The photoirradiation in the solution led to a cis-isomer at 25?30°C. The initial rate of the dimerization at the mesophase was found to be higher than that at the isotropic liquid phase. The photoreaction in the solid took place at a much slower rate than in the other states. These suggest that some kinds of ordering besides the mobility of the molecules of the cinnamate enhance the rate and affect the course of the reaction on irradiation in comparison with the reaction in the isotropic solution.     

Phase Equilibrium of the Ga–Al–Sb system

Employing the method of liquid-phase epitaxy (LPE) solid-solutions of Ga_1–xAl_xAl_xSb (0 ≦ x ≦ 0.8) have been obtained. The dependence of Sb solubility on Al concentration in the liquid phase at 403°C, 452°C, 500°C has been established. The dependence of AlSb concentration in the solid phase on the composition of the liquid phase has been investigated at 452°C. Using the chemical constants equilibrium method, the phase equilibrium of the Ga–Al–Sb system in the region of liquid phase composition near the Ga-rich corner of the phase diagram has been calculated. The comparison of experimental and calculated data for the liquid and solid phases shows their agreement within the limits of experimental error.     

New Effects in the Poly-N-Vinylcaprolactam/Titanium(IV) Oxides Nanocomposite System and Their Nature

Nanocomposites based on poly-N-vinylcaprolactam (PVC), characterized by different drying temperatures of aqueous PVC solution at 25°С (PVC25) and 40°С (PVC40), with titanium(IV) oxide nanoparticles (TONs) (η-phase (TP: TiO_{2 – x} · mH₂O) with a superstructure to the anatase structure and ordered solid solution with partial replacement of oxygen atoms in the η-phase with peroxo group O ₂ ^2– (PP: TiO _x (O₂)_{2 – x} · nH₂O)) have been obtained for the first time and characterized by X-ray diffraction and IR spectroscopy. It is found that a common feature of TP/PVC and PP/PVC obtained by dry mixing and grinding is the decrease in the number of water molecules for TP and PP in the interlayer space of the structure and their attachment to the PVC amide groups; the differences are related to the specificity of PVC behavior in these systems: mechanodestruction of PVC in PP/PVC40 and different numbers of water molecules in PVC25 and PVC40.     

High performance and low level of image sticking fringe field switching mode liquid crystal display with photo-alignment layers

Abstract

A novel photo-alignment fringe field switching mode liquid crystal display (PA-FFS-LCD) with a wide nematic temperature range from ?40?°C to 95?°C was developed. The new developed PA-FFS-LCD shows fast gray-to-gray response time (<20?ms at 25?°C and < 300?ms at ?30?°C), wide viewing angle, and high contrast ratio about 1600 owing to use an isomerization-type photo-alignment material. Image sticking was not recognized owing to a suitable arrangement of pixel and common electrodes.     

Mechanochromic Photoluminescent Liquid Crystals Containing 5,5′-Bis(2-phenylethynyl)-2,2′-bithiophene

5,5′-Bis(2-phenylethynyl)-2,2′-bithiophene derivative 1 containing dendritic moieties linked through amide groups was synthesized. Compound 1 exhibits a rectangular columnar phase on slow cooling from the isotropic liquid. In contrast, a cubic phase is formed as a metastable liquid-crystalline phase by rapid cooling of the isotropic melt. Mechanical shearing induces a cubic–columnar phase transition, accompanied by the change of a photoluminescent color from yellow-green to green. The infrared spectra of 1 suggest that the mechanical stimuli can induce the partial dissociation of the hydrogen bond. This may lead to more disordering of the π-stacks of chromophores.     

New Mesogenic Compounds Containing a Terminal-Substituted Benzoxazole Unit

A series of novel mesogenic 2-(4-alkoxyphenyl-1-yl)-benzoxazole derivatives bearing different substituents (H, NO₂, CH₃, Cl, coded as nPB-H, nPB-N, nPB-M, and nPB-C, respectively) at the 5-position were prepared and characterized. nPB-N, nPB-M, and nPB-C exhibited enantiotropic smectic mesophases with the mesophase ranges 3 °C–32 °C and 3 °C–82 °C on heating and cooling processes, whereas nPB-H showed no mesophases. The substituents with the stronger electron withdrawing effect let to the wider mesomorphic temperature domain. The nPB-M, nPB-C, and nPB-H displayed intense emission in CH₂Cl₂ solutions with λ_max peaks of the photoluminescence spectra at 350–355 nm when excited at their absorption maxima.     

Phase transformations in the La1−xNdxNi3.5Al1.5–H2 (x = 0.1 and 0.2) system

Interactions in the La_1?xNd_xNi_3.5Al_1.5-Н₂ (x = 0.1 and 0.2) system was studied from room temperature up to 950 °C at the initial hydrogen pressure of 5 MPa through differential thermal (DTA) and X-ray phase analyses. Heating two-phase alloys (x = 0.1 and 0.2) in hydrogen results in their disproportionation (at 530 and 560 °С, respectively) and the formation of NiAl and unidentified amorphous products. The single-phase La_0.9Nd_0.1Ni_3.5Al_1.5 alloy decomposes in hydrogen at 900 °С into a hydride of rare-earth metals and an Ni₃Al intermetallic; traces of NiAl and hydride of a phase of the CaCu₅-type structure have also been observed. Heating the disproportionated samples in vacuum to 520–550 °С leads to their recombination into a homogenized phase with a CaCu₅-type structure. In other words, the increase of neodymium content shifts the reaction equilibrium of La_1?xNd_xNi_3.5Al_1.5 alloys with hydrogen towards recombination.     

Structural evolution in the solidification process of Cu–Sn alloys

The liquid structures in the solidification process of the Cu_100−xSn_x (x = 10, 20, 30, 40) alloys were investigated by X-ray diffraction method. The results show that the Cu₃Sn and quasi Cu₃Sn structures exist in the Cu–Sn liquids. Which arouses an obvious structure change indicated by the correlation radius r_c at about 900 °C. In addition, the structure change induced by the liquid–solid transition at about 30–40 °C below the liquidus is also observed.     

Evolution of the vanadium pentoxide V<Subscript>2</Subscript>O<Subscript>5</Subscript> crystal surface after vacuum annealing

A complex study of the (001) cleavage surface of a V₂O₅ single crystal annealed in vacuum at 450 and 550°C has been performed. Tunnel microscopy of the sample surface annealed in vacuum at 550°C showed the formation of a plane with a corundum structure, reconstructed according to the V₂O₃ (0001)-(1/√3 × 1/√3)R30° type, on the surface. X-ray photoelectron spectroscopy revealed a significant modification of interatomic bonds in the surface layers of V₂O₅ single crystal after vacuum annealing at 550°C, which is related to the partial reduction of V⁵⁺ ions and formation of lower vanadium oxides. These modifications lead to a decrease in the electrical resistivity of V₂O₅ and the occurrence of bending at a temperature of 61.5°С in the temperature dependence of the sample resistance, which is indicative of phase transition.     

The transition layers in AlGaAs/GaAs and InGaAsP/GaAs heterostructures grown by LPE

AlGaAs/GaAs and InGaAsP/GaAs heterostructues were grown by liquid phase epitaxy (T_o = 800–770 °C), the transition layers (TLs) were measured with Auger profiling. The Auger profiling of the InGaAsP/GaAs heterostructures after a long heating (1–6 hours, 770 °C) has been made too. The analysis of the experimental data led to the conclusion that the fundamental values of the TL in above mentioned systems are about 10 and 1 to 2 nm, correspondingly. This difference between the minimum widths of the TL in AlGaAs/GaAs and InGaAsP/GaAs system is connected with the different speed of solid state diffusional processes which occur at the initial stpeps of liquid – solid contact.     

Supramolecular assembly in the chiral N-carbamoyl compound 2-ureido-pentanedioic acid

The compound 2-ureido-pentanedioic acid was synthesized and characterized by FT-IR and NMR spectra. Its molecular structure was solved by single crystal X-ray diffraction. In the title compound, C₆H₁₀N₂O₅, the chiral structure is stabilized by intermolecular O–H···O and N–H···O hydrogen bonds in a supramolecular assembly formed by infinite chains parallel to the bc plane with graph set C(4) connected by amide-acid dimers R²₂(8) into a three-dimensional network.     

Molecular and crystal structures of the product of the reaction between tetrafluoroborate tris[triphenylphosphinegold(I)]oxonium and calix[4]arene

The molecular and crystal structures of a complex salt in which the cation is tris[triphenylphosphinegold(I)]oxonium and the anion is deprotonated calix[4]arene are determined. The tris[triphenylphosphine-gold(I)]oxonium cation forms a centrosymmetric, doubly charged, hexanuclear dimer. The oxonium oxygen atoms are characterized by a nonplanar bond configuration: the Au-O-Au bond angles vary in the range 92.2(2)°–101.8(2)° due to aurophilic interactions (Au?Au) occurring at distances in the range 2.9650(4)–3.1836(4) Å. The monomers are joined into dimers via pairs of Au?Au interactions at a distance of 3.1880(5) Å. The anion adopts a cone conformation. The deprotonated oxygen atom of the anion is involved in the formation of two hydrogen bonds as a proton acceptor. In the crystal, two anions and one cation form a cluster in which the large-sized dimer cation is capped by the two calix[4]arene anions on opposite sides: { A-K-A}. In turn, these clusters form zigzag chains due to stacking interactions involving one of the benzene rings.     

Synthesis,structure, and mesomorphism of novel liquid crystalline acrylate monomers and polymers

A series of novel liquid crystalline monomers (M₁?M₈) and side chain polymers base polyacrylate backbone were synthesized. The chemical structures were characterized by FT-IR and ¹H-NMR spectra. The mesomorphism and thermal behavior was investigated by polarizing optical microscopy, differential scanning calorimetry, and thermogravimetric analysis. The relationships of structure and mesomorphism are discussed in detail. The eight monomers and their corresponding polymers all show enantiotropic nematic phase. With increasing the spacer length or flexibility of the terminal group, the melting temperature (T_m) and isotropic temperature (T_i) of the corresponding monomers and polymers all decreased. However, with increasing the rigidity of the mesogenic core, T_m and T_i of the corresponding monomers and polymers all increased. TGA showed that all the polymers obtained in this study had excellent thermal stability.     

Gel-glass transformation in the SiO2Fe2O3 system

Gel-glass transformation has been studied by Mössbauer spectroscopy, DTA-TG analyses and X-ray diffractometry for four compositions in the SiO₂Fe₂O₃ system (A: 5 wt% Fe₂O₃, B: 10 wt% Fe₂O₃, C: 20 wt% Fe₂O₃, D: 40 wt% Fe₂O₃).The gels were prepared by the hydrolysis of silicon tetraethoxide and iron triethoxide and successively dried and heated in oxygen in the temperature range 40–1000°C.Samples A and B gave typical amorphous X-ray patterns up to 700°C; heating at higher temperature yielded the precipitation of quartz, cristobalite and hematite in sample A, cristobalite and hematite in sample B. Crystallization was also detected by DTA in sample A for which X-ray diffraction exhibited a larger effect.In samples C and D crystallization took place starting from 300°C with the precipitation of hematite, which remained the only crystalline phase up to 1000°C.The presence of hematite was confirmed by the obtained Mössbauer spectra which showed the characteristic sextet. The apportion of iron ions in the Fe³⁺ and Fe²⁺ oxidation states was also determined, together with the attribution of the probable coordination states for Fe³⁺ ions.Complex magnetic structure appeared in samples treated above 800°C.     

Amination of brominated nanoporous activated carbon beads for the preparation of CO2 adsorbents

Abstract

Nanoporous activated carbon (NAC) beads that prepared by carbonization of microporous polymer 2,6-di-tert-butyl pyridine beads were subjected to bromination followed by amination to synthesize the NAC derivatives carrying amino groups. Grafted bromine groups, 0.41?mmol of Br per 1?g of NAC, are hydrolytic stable at 100 °С and can be successfully substituted by amino groups. SEM, TEM, and texture studies were shown a nanoporosity decrease. However, the aminated NAC beads maintained a narrow nanopore size distribution and high specific surface area, retaining a spherical shape. These beads were evaluated for capturing CO₂ and showed good reversible adsorption capacity. The preparation method can easily be scaled up, and the NAC beads can be used for other adsorption applications where shaped carbon solids with developed nanoporosity are required.     

Graphene oxide modified with aminoazobenzol group stabilized blue phase liquid crystals

Liquid-crystalline blue phases are stable only for very narrow temperature range between the isotropic and the chiral nematic phase that severely hinders their applicability. Herein, aminoazobenzol group was chemically grafted onto graphene oxide (GO) via amidization reaction. Successful grafting of aminoazobenzol group was confirmed using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), UV-vis absorption spectra and Thermogravimetric analysis (TGA). The resultant aminoazobenzol group modified GO sheets which is reduced (GO-Az) were easily redispersed in common organic solvents. By doping different contents of GO-Az nanosheets could stabilize blue phase and increase the blue phase range. When doped with 0.5 wt% GO-Az, the mixtures show the wider range with 5.4°C than the range with 3.6°C of blue phase liquid crystals without GO-Az. Meanwhile, the phase sequence and the range of the aforementioned phases are reproducible upon heating and cooling, which shows that the blue phases doped with GO-Az nanosheets are thermodynamically stable.