ИСТИНА

The spin probe technique has proven informative in the study of orientation alignment and orientation dynamics of liquid-crystalline and polymer materials. These systems gain much interest nowadays in view of their ability to change orientation upon the action of external forces, such as magnetic and electric fields and irradiation with light. However, the mechanism of such reorientation is not sufficiently well understood, partly because of the lack of reliable and detailed experimental data on the orientation alignment of these materials in the course of their reorientation. The present contribution summarizes a series of experimental and theoretical works, devoted to spin probe study of reorientation of liquid crystals and liquid-crystalline polymers. The following experimental systems are considered: (a) the reorientation of nematic and smectic liquid crystals in the magnetic field; (b) photo-induced ordering and re-ordering of azobenzene-containing liquid crystals and liquid-crystalline polymers To monitor the fast reorientation dynamics of low-molecular-weight liquid crystals, the kinetics of ESR signal change from the spin probe was recorded after an abrupt turn of magnetic field direction. The series of such kinetics recorded at a net of magnetic fields, was converted into the temporal evolution of the spin probe ESR spectrum, with the time resolution of 5 ms. For photoinduced orientation of liquid-crystalline polymers, the angular dependences of ESR spectra of the spin probes were directly measured at different irradiation times, which yield the time evolution of orientation distribution function of the polymer. The obtained data show that liquid crystal reorientation, both in the magnetic field, and upon light irradiation, does not proceed via mere rotation of the director, as it is often assumed in literature. The realistic mechanism of this process should take into account the microscopic inhomogeneity of liquid-crystalline materials. Magnetic field action or light irradiation induces the growth of domains or microscopic regions with favourable orientations, and diminishing of perpendicularly oriented regions. The authors acknowledge the financial support from RFBR (grant No. 16-33-60139 mol-dk).