PACS. 62.40+i -Anelasticity, internal friction, stress relaxation, and mechanical resonances. PACS. 64.70Kb -Solid-solid transitions. PACS. 66.30−h -Diffusion in solids.Abstract. -YBa2Cu3O x exhibits a second-order tetragonal-to-orthorhombic transformation, at least in wide temperature ranges. The order parameter is proportional to the orthorhombic strain or to the O population difference between ( 1 2 0 0) and (0 1 2 0) sites in the Cu(1) plane. Quasi-static anelasticity experiments were performed at 100 and 120 • C on YBa2Cu3O x samples with O concentrations x between 6.29 and 6.9. The experiments show a drastic increase of both relaxation strength and relaxation time near the tetragonal-to-orthorhombic transformation at x 6.4, thus demonstrating i) large amplitudes and ii) a (critical) slowing-down of the order parameter fluctuations.

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Polymer multicomponent materials containing conjugated fragments (such as azo- or azomethine groups) have attracted significant attention due to their possible use in the efficient reversible stimuli-responsive, light-responsive devices. Light-responsive polyazomethines with azo groups in the backbone (Azo-Pams) are synthesized by polycondensation of tetrafluoro­benzene- or octafluoro­biphenylene-based bis-hydroxy­benzaldehydes with hexamethylene­diamine. The polymers feature the simultaneous combination of azo and azomethine groups inside one conjugated block. The synthesized polymers can be solution cast into flexible solid films with tensile strength of about 24 MPa. We found that the obtained Azo-Pam polymers have an amorphous structure (T g ∼ 134 °C) and high thermal stability (up to 300 °C). Their optical spectra depend on the solvent type and its pH which influence the dynamical equilibrium between the tautomeric forms. It also determines the balance between the main (azo form) and the ionized form in the solvent and allows to purposefully regulate the absorption maxima. We demonstrate the ability to record optical information using Azo-Pam films in the form of polarization gratings (holograms) with a regulated diffraction efficiency. A uniform orientation of liquid crystal is achieved by a polarized UV irradiation of glass substrates covered with the polymer Azo-Pam-II. The obtained data show that the synthesis strategy of the Azo-Pam polymers is an effective instrument for designing a wide range of stimuli-responsive and optical active materials.

A method to determine the surface azimuthal anchoring energy of a nematic liquid crystal is proposed. The technique implies the measurement of the director deviation on the cell substrate as a fbnction of strength and direction of the applied magnetic field. As an example, the dependence of the azimuthal anchoring coefficient on the exposure time is measured at the interface between the nematic K15 and polyvinylcinnamate film exposed by UV light. The analogous measurements performed in a wedge cell show that the method with magnetic field is more precise.