Stretching and compression of a macromolecule under different modes of mechanical manupulations

This review is concerned with the response of an isolated polymer chain subjected to the action of the two different modes of the mechanical impact on the chain ends. In one mode, the end-to-end distance is changed in a controlled fashion and the fluctuating response force is measured; in the second case, an external stress field is applied to the chain end, and the measured response of the system is the fluctuating end-to-end distance. The main attention is focused on the results of the computer-aided simulation experiments and theoretical results. Upon stretching of an ideal chain, a real chain in a good solvent, or a globule, the resultant strain-force and force-strain dependences are shown to be different for chains with finite length L; however, this difference diminishes with an increase in the length of a molecule. When the anchored Gaussian chain is separated from the adsorbing surface, this difference disappears in the limit of high L; however, in the neighborhood of the phase transition, some characteristics (fluctuations, distribution functions) appear to be critically different under different impact modes even in the thermodynamic limit. The example of an abnormal system is discussed: The behavior of a polymer chain compressed by a small piston is different in the conjugated ensembles, and, as the system increases in size, this difference becomes even more pronounced.     

The rule parameters in liquid adsorption chromatography of polymer homologous series and their determination

The separation of polymer homologous series is governed by two rule parameters: the interaction parameter c of the repeat unit and the sorbent surface S (relation of pore volume to pore diameter). The interaction parameter is independent on column dimensions and pore diameter. In a plot of elution volumes Vn versus the difference DeltaV =Vn -Vn-1 in elution volumes of consecutive oligomers, straight lines are obtained, the intercept of which yields the accessible volume. Using the accessible volume as the hold-up volume in the calculation of the retention factor k, a strictly linear dependence of ln k versus the number of the repeat units n is observed (Martin's plot). The interaction parameter c can be determined experimentally by three different approaches: from the slope in a plot of V versus DeltaV, from the slope in Martin's plot (ln k versus n), and from the intercept in a plot of ln(DeltaV) versus n. The pore surface is obtained from the intercept in Martin's plot (ln k versus n) or the intercept in a plot of ln(DeltaV) versus n. The results obtained by the different approaches agree very well. The theoretical predictions are verified by comparison with the experimental data obtained on columns with different pore diameter and pore volume and thus, different pore surface at conditions corresponding to the same interaction parameter.     

Infrared and ESR spectroscopy of KTaO3 : Er

Infrared reflection and ESR spectra of KTaO₃ : Er³⁺ single crystals have been investigated. The frequency of the lowest optical phonon mode TO₁ has been found to increase as compared to undoped KTaO₃ crystals, which indicates the suppression of the ferroelectric instability of the system. The ESR spectra have revealed the presence of two Er³⁺ centers of different (cubic and non-cubic) symmetries in KTaO₃ : Er³⁺.     

The effect of a weak magnetic field on the mobility of dislocations in silicon

The magnetoplastic effect in dislocation silicon is discovered. It is shown that in the presence of tensile stresses (up to 20 MPa), the mechanically activated path of surface dislocation half-loops is limited mainly by the dynamics of defects in various slip systems relative to the applied load. The activation barriers for the motion of dislocations controlled by various conditions in the temperature range T=850–950 K are E _aF=2.1±0.1 eV and E _aS=1.8±0.1 eV. An increase in the path of surface dislocation half-loops and a change in the activation barriers are detected (E _aF=1.4±0.1 eV and E _aS=1.6±0.1 eV) after subjecting silicon to a magnetic field (B=0.7 T) for 30 min. Possible reasons behind the observed effects are discussed.     

On the Escape Transition of a Tethered Gaussian Chain; Exact Results in Two Conjugate Ensembles

Summary: Upon compression between two pistons an end-tethered polymer chain undergoes an abrupt transition from a confined coil state to an inhomogeneous flower-like conformation that is partially escaped from the gap. In the thermodynamic limit the system demonstrates a first-order phase transition. A rigorous analytical theory of this phenomenon for a Gaussian chain is presented in two ensembles: a) the H-ensemble, in which the distance H between pistons plays the role of the control parameter, and b) the conjugate f-ensemble in which the external compression force f is the independent parameter. A loop region for 〈f(H)〉 with negative compressibility exists in the H-ensemble, while in the f-ensemble 〈H(f)〉 is strictly monotonic. The average lateral forces taken as functions of H (or 〈H〉, respectively) have distinctly different behavior in the two ensembles. This result is a clear counterexample of the main principles of statistical mechanics stating that all ensembles are equivalent in the thermodynamic limit. Another theorem states that the thermodynamic potential as a function of volume must be concave everywhere. We demonstrated that the exact free energy in the H-ensemble contradicts this statement. Inapplicability of these fundamental theorems to a macromolecule undergoing the escape transition is clearly related to the fact that phase coexistence in the present system is strictly impossible. This is a direct consequence of the tethering and the absence of global translational degrees of freedom of the polymer chain.     

Crossovers between superconducting symmetry classes

We study the average density of states in a small metallic grain coupled to two superconductors with the phase difference π, in a magnetic field. The spectrum of the low-energy excitations in the grain is described by the random matrix theory whose symmetry depends on the magnetic field strength and coupling to the superconductors. In the limiting cases, a pure superconducting symmetry class is realized. For intermediate magnetic fields or couplings to the superconductors, the system experiences a crossover between different symmetry classes. With the help of the supersymmetric σ-model we derive the exact expressions for the average density of states in the crossovers between the symmetry classes A-C and CI-C.     

Molecular Structure and Spectral Characteristics of the Complex [Ni(Bu3P)2(NCS)2], and Its Catalytic Properties in Hydrosilylation Reaction

According to X-ray diffraction data, the nickel complex [Ni(Bu₃P)₂(NCS)₂] has a square-planar structure with the two phosphine ligands trans to each other and the thiocyanate ligand coordinated by the nitrogen atom. The IR and ¹H, ^{1 3}C, and ^{3 1}P NMR data show that the same structure is preserved in solutions. The catalytic activity of the complex in hydrosilylation of 1-hexene with chlorohydrosilanes HSiMe_{3 - n} Cl _n varies in the series HSiCl₃ > HSiMeCl₂ > HSiMe₂Cl. The reactions of the complex with hydrosilanes involve substitution of the anionic ligand and formation of silyl complexes.     

Second-order doppler-free spectroscopy

The observation of second-order Doppler-free optical resonances with a width of 50 Hz are reported for the first time. It was achieved due to the use of optical selection of cold particles from an absorbing gas. The experiments have been carried out by using a new laser spectrometer, supposed to obtain the saturated absorption resonances with a relative width 10^–13–10^–14. The results of experimental and theoretical studies of second-order Doppler-free effect influence on the shape of nonlinear optical resonances in transit-time conditions are considered.