A study of photooptical processes in photosensitive cholesteric azobenzene-containing polymer mixture under an action of the polarized and nonpolarized light

New liquid crystalline cholesteric mixture consisting of azobenzene-containing side-chain copolymer and the chiral low-molar-mass dopant (5%) was prepared and its properties were studied. It was shown that freshly prepared spin-coated thin films are optically isotropic. However, the films possess a noticeable circular dichroism with a maxima corresponding to the absorbance of the nonchiral azobenzene fragments. This phenomenon is an evidence for the existence of the helical supramolecular structure elements in the mixture films. An annealing of the films at temperatures above the glass transition leads to the strong increase in the circular dichroism due to formation of the perfect cholesteric phase. UV light irradiation results in the E-Z isomerization of azobenzene groups and the significant drop in circular dichroism values (almost to zero) both for the fresh and the annealed films that is associated with the transition from cholesteric to the fully isotropic state. An action of polarized UV and visible light leads to the photoorientation of azobenzene groups perpendicular to the electric vector of the polarized light. Kinetic features of the photoorientation processes in cholesteric phase in comparison with nematic systems were studied. It has been experimentally found that the helical order to some extent prevents the photoorientation of the side groups. Upon UV irradiation of the thick, planarly-oriented films of the mixture at temperatures higher than glass transition, the selective light reflection maximum is shifted to the long-wavelength spectral region. All photoinduced phenomena occurring in the mixture are thermally reversible and annealing of the films at temperatures above glass transition results in the back recovery to the cholesteric nonoriented state and to the initial helix pitch value.     

Some aspects of simulating structural transformation in alloys based on immiscible components

The question has been studied of using the reversal monotectic reaction as technological mean for obtaining alloys based on immiscible components (IC).……     

Universal relation for polymer crystallization rates from the melt

Analysis of spherulitic growth rate data for a number of linear polymers has shown that the temperature at maximum growth rate, $\text{T}{}^1$, is related to the glass transition temperature, T_g, through the empirical equation, $\text{T}{}^1\text{= 1.26 T}{}_{\mathrm{g}}$. The universal master curve for the temperature dependence of the growth rate of crystals from the melt in reduced Gandica—Magill coordinates, $\text{ln}\text{(}\text{G}\text{G}{}^1\text{) =}\text{f(T ? T}{}_{\mathrm{\infty }}\text{)}\text{(T}{}_{\mathrm{m}}\text{? T}{}_{\mathrm{\infty }}\text{)}$, is possible only on the condition that the following empirical equation holds: $\text{0.26 =}\text{T}{}_{\mathrm{\infty }}\text{T}{}_{\mathrm{g}}\text{?}\text{T}{}_{\mathrm{\infty }}\text{T}{}_{\mathrm{m}}$. Finally, limits of variation of the ‘conformational’ contribution to the excess entropy, and of the free volume fraction at $\text{T}{}^1$ were evaluated for some polymers.     

An easy way to Pd–Zn nanoalloy with defined composition from a heterobimetallic Pd(μ–OOCMe)4Zn(OH2) complex as evidenced by XAFS and XRD

The heterobimetallic lantern complex Pd(μ-OOCMe)₄Zn(OH₂) was found to be readily reduced with H₂ under fairly mild conditions (150–250 °C, 5–10% H₂/He) both in the carbon-supported and crystalline states to afford a Pd–Zn nanoalloy as evidenced by the ICP elemental analysis, EXAFS, XANES and XRD data.      

Two-Stage Ozonation – Adsorption Purification of Ground Water from Trichloroethylene and Tetrachloroethylene with Application of Commercial Carbon Adsorbents

ABSTRACT

The aim of the research is to choose the most efficient adsorbent for two-stage ozone-sorption purification of groundwater containing both trichloroethylene (TCE) and tetrachloroethylene (PCE) between three carbon sorbents produced in Russia (AUT-M, CAUSORB-221, and AG-3). Sorption isotherms of TCE and PCE on AUT-M and CAUSORB-221 at 296 K were fitted by the Freundlich equation. The better TCE and PCE sorption ability of AUT-M in comparison with CAUSORB – 221 and AG-3 was demonstrated.

The optimum parameters for ozonation and sorption stages of groundwater purification from TCE and PCE are elucidated using laboratory and pilot-plant scales. Prolonged test of this technology for purification of ground demonstrated that the higher achievable efficiency of destruction with ozone is 94% for TCE and 38% for PCE. Ozonation-sorption treatment of groundwater allows one to achieve TCE and PCE removal efficiency of 96-97% and 92-94% correspondingly. The most efficient carbon sorbent is microporous carbon fiber AUT-M. Using this sorbent, TCE and PCE concentrations in treated water decrease below the MPC level (5 μg/L) adopted in Russia. It is concluded that the combination of ozonation with sorption of residual contaminants by carbon sorbents is a promising way for the purification of waters containing chlorinated contaminants.     

Composition-dependent properties of segmented polyurethanes

We studied segmented polyurethanes (SPU) from poly(diethylene glycol adipate) with molar masses M_SFT = 780, 1365, 1780 and 3200 g/mol (soft fragments, SFT), 2,4-toluene diisocyanate and 2,4-toluene diamine (stiff fragments, STF) with essentially monodisperse molar mass distribution of STF. They were characterized by WAXS, SAXS, heat capacity measurements, mainly from 150–450 K, and enthalpies of solution in dimethyl formamide at room temperature, ΔH_sol. The experimental results may be summarized as follows.

• The glass transition temperatures of SFT decrease with an increase of M_SFT, but remain essentially composition-invariant at fixed M_SFT, whatever the STF volume fraction φ.
• The surface-to-volume ratio of STF domains exhibits sudden, jump-like transitions at volume fractions φ* = 0.23 and φ** = 0.40, respectively.
• Exothermic (i.e., negative) values of ΔH_SOl exhibit a jump-like decrease at φ* = 0.23.

It has been shown that the main cause of the latter effect is the transition of SFT from a somewhat extended to an essentially unperturbed conformation.     

Fiber Single-Crystal Growth from the Melt for Optical Applications

The micro-pulling-down technique has been applied to produce undoped and 0.1% Nd³⁺-doped Y₃Al₅O₁₂ garnet single-crystal fibers with controlled diameter. The hot zone construction and fiber-pulling conditions have been optimized. As a result, growth of transparent and homogeneous fiber crystals has been achieved. The crystals have been grown under 10⁵ Pa (1 atm) of argon atmosphere, with the pulling rate varied in the range of 0.3–2.0 mm/min. One hundred percent of the starting melt has been normally transformed into a crystalline material, with an empty crucible ready for the next growth procedure. The as-produced fibers demonstrate promising optical properties.     

Reconstruction of fluid Langmuir monolayers under shear forces

Scanning probe microscopy observations of monolayers of a classical boundary lubricant, stearic acid (STA), reveal long-range dynamics of wear and reconstruction of monomolecular films under the shear forces caused by the sliding tip. In the range of sliding velocities studied, the friction forces behave non-monotonically with a maximum value around 0.2 m/s. The STA monolayer in a fluid state displays a flow of material from the worn area and its redistribution to form local heterogeneous multilayers. The range of influence of the material removal on the monolayer structure around the worn area is up to 80 m, or more than an order of magnitude larger than the actual worn area. Surface diffusion of mobile organic material is responsible for the observed long-range spreading of local shear stresses produced in the area of wear. It demonstrates the non-local nature of the stress-induced reconstruction in the fluid monolayers, in contrast with solid monolayers, which show very localized wearing processes.     

Distinguishing between cis/trans isomers of monounsaturated fatty acids by FAB MS

Fast-atom bombardment (FAB) mass spectrometry in the negative ion mode can be used to unambiguously distinguish between cis and trans isomers of monounsaturated fatty acids by the relative signal strengths of an intense pair of ion signals. Under normal FAB ionization/desorption conditions, the deprotonated molecules (i.e., [M - H]-) of six fatty acids underwent charge remote fragmentation. A characteristic fragmentation pattern of two intense peak clusters of peaks with three weak intervening clusters of peaks are used in each case to identify the position of the double bond. The possibility of resonance electron capture occurring during the FAB process is discussed.     

Growth and characterization of InP nanowires with InAsP insertions

We report on the fabrication by Au-assisted molecular beam epitaxy of InP nanowires with embedded InAsP insertions. The growth temperature affects the nucleation on the nanowire lateral surface. It is therefore possible to grow the wires in two steps: to fabricate an axial heterostructure (at 420 degrees C), and then cover it by a shell (at 390 degrees C). The InAsP alloy composition could be varied between InAs0.35P0.65 and InAs0.5P0.5 by changing the As to P flux ratio. When a shell is present, the InAsP segments show strong room-temperature photoluminescence with a peak wavelength tunable from 1.2 to 1.55 mum by adjusting the As content. If the axial heterostructure has no shell, luminescence intensity is drastically reduced. Low-temperature microphotoluminescence performed on isolated single wires shows narrow peaks with a line width as small as 120 microeV.