Thermomechanical properties of polytetrafluoroethylene in different zones of impact of continuous CO2 laser radiation

The pseudo-network structure of the amorphous block of polytetrafluoroethylene is formed by branching points, the crystallites of low- and high-melting crystalline polymorphs and the cluster segments of macromolecules. The polymer treated by laser irradiation is amorphized, with the degree of amorphization and other changes depending on the radiation fluence. A depression of the molecular flow onset and initial melting temperatures of the crystalline polymorphs, a decrease in the molecular mass, and the disappearance of the crystalline branching points characteristic of the original polymer have been observed in the irradiated polymer. Areas with different degrees of modification of the molecular structure appear in the dynamic mode with moving boundaries during continuous CO₂ laser irradiation. The difference in absorbance of the crystalline and amorphous portions of the polymer at the laser emission wavelength and a relatively high transmittance make laser-induced degradation differ from thermolysis in contact with a hot surface.     

Accurate distance determination of nucleic acids via Förster resonance energy transfer: implications of dye linker length and rigidity

In Fo?rster resonance energy transfer (FRET) experiments, the donor (D) and acceptor (A) fluorophores are usually attached to the macromolecule of interest via long flexible linkers of up to 15 ? in length. This causes significant uncertainties in quantitative distance measurements and prevents experiments with short distances between the attachment points of the dyes due to possible dye-dye interactions. We present two approaches to overcome the above problems as demonstrated by FRET measurements for a series of dsDNA and dsRNA internally labeled with Alexa488 and Cy5 as D and A dye, respectively. First, we characterize the influence of linker length and flexibility on FRET for different dye linker types (long, intermediate, short) by analyzing fluorescence lifetime and anisotropy decays. For long linkers, we describe a straightforward procedure that allows for very high accuracy of FRET-based structure determination through proper consideration of the position distribution of the dye and of linker dynamics. The position distribution can be quickly calculated with geometric accessible volume (AV) simulations, provided that the local structure of RNA or DNA in the proximity of the dye is known and that the dye diffuses freely in the sterically allowed space. The AV approach provides results similar to molecular dynamics simulations (MD) and is fully consistent with experimental FRET data. In a benchmark study for ds A-RNA, an rmsd value of 1.3 ? is achieved. Considering the case of undefined dye environments or very short DA distances, we introduce short linkers with a propargyl or alkenyl unit for internal labeling of nucleic acids to minimize position uncertainties. Studies by ensemble time correlated single photon counting and single-molecule detection show that the nature of the linker strongly affects the radius of the dye's accessible volume (6-16 ?). For short propargyl linkers, heterogeneous dye environments are observed on the millisecond time scale. A detailed analysis of possible orientation effects (κ(2) problem) indicates that, for short linkers and unknown local environments, additional κ(2)-related uncertainties are clearly outweighed by better defined dye positions.     

Pyrrolo[1,2-<Emphasis Type="Italic">a</Emphasis>]quinoxalines based on pyrroles (Review)

Published data on methods for the synthesis of pyrrolo[1,2-a]quinoxalines, based on derivatives of pyrroles and also on compounds not originally derivatives of quinoxalines or pyrroles, are reviewed and classified.     

2,2'-Bipyridinyl carboranes as B,N,N-ligands in cyclometallated complexes of platinum(II)

Novel B,N,N-cyclometallated Pt(II) complexes of 2,2'-bipyridin-6-yl carboranes exhibit absorption and emission similar to relative Pt(II) complexes of aromatic C,N,N-ligands: the same transitions but lower intensities. DFT calculations suggest the former emits from the (3)MLCT state while for the latter the mixed (3)ICT-MLCT transitions should be considered.     

The effect of the sputtering process ambient on the magnetic state and phase composition of the film nanocomposites (Fe0.45Co0.45Zr0.10) x (Al2O3)1−x

The effect of oxygen-containing ambient arising at sputtering of granular nanocomposites (Fe_0.45Co_0.45Zr_0.10) _x (Al₂O₃)_1−x (30 at.%≤x≤65 at.%) on their magnetic state and phase composition has been investigated. It was shown that the presence of oxygen resulted in the formation of oxide shells preventing the ferromagnetic interaction between Co_0.45Fe_0.45Zr_0.10 nanoparticles and also the formation of metallic percolative net beyond the percolation threshold (as opposed to the films prepared in pure argon atmosphere).     

Radiation from relativistic electrons in homogeneous condensed matter at large angles (≫γ−1)

The radiation spectra from 900 and 400 MeV electrons in thin Ta, Cu, and Sn foils are measured at an angle of 19° with respect to the direction of motion of the beam. The radiation yield and its dependence on the electron energy agree satisfactorily with the theory of polarization bremsstrahlung. This result represents the first direct observation of polarization bremsstrahlung from ultrarelativistic electrons in homogeneous condensed matter. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 3, 145–149 (10 February 1996)     

Supramolecular Crystallization of Monodispersed Silica Spherical Particles (MSSP) in Concentrated Suspensions

The supramolecular crystallization of MSSP concentrated suspensions in alcohol and ethyl ether with various ammonia concentrations is found to occur as a phase transition, with appearence of a pre-transitional state. The temperature dependence of the time, it takes for this state to be established, evidences for the presence of processes of nucleation and activation character.