ИСТИНА

In this contribution the spin probe technique is applied to the study of rotation mobility and orientation alignment of photochemically active azobenzene-containing liquid-crystalline (LC) polymers. The structure of the considered materials incorporates mesogenic groups responsible for the formation of liquid-crystalline phases and azobenzene moieties capable of reversible trans-cis photoisomerization. Irradiation of these materials with light induces collective reorientation of both photoactive and mesogenic moieties in the direction perpendicular to the electric field vector of light. The molecular dynamics of the nitroxide spin probes was determined by numerical simulation of EPR spectra. It was found that in the studied polymers molecular mobility is described as follows: – In the vicinity of the glass transition temperature the probes motion is restricted to amplitude limited stochastic reorientations around the equilibrium position. These fast motions, known as quasi-librations [1], lead to the partial averaging of effective spin-hamiltonian parameters. – In the nematic liquid crystalline phase the investigated polymers become aligned by the magnetic field. The kinetics of EPR spectrum change during this alignment process was used to quantify the reorientation times of molecular fragments using the model [2]. The obtained values of transverse reorientation times lie in the range of 10–5÷10–7 s. – In the isotropic state of the polymer melt, the rotation correlation times can be determined directly from the shape of the EPR spectrum using the SLE approach [3]. The values 10  || 5 10 s    and 8  7 10 s     were obtained in the vicinity of isotropic–nematic phase transition. The numerical simulation of angular dependence of EPR spectrum of anisotropic samples was used to determine the orientation distribution function of the probe molecules, i. e. the number density of molecules oriented at each direction. The evolution of orientation distribution during the process of light induced alignment (photo-orientation) of the material was measured by means of recording EPR spectrum angular dependences at different times of light irradiation of the sample. This information, combined with the data obtained by optical techniques (polarized microscopy and polarized absorbance spectroscopy) was used for determining of the mechanism of photo-orientation process. It has been shown that photo-orientation proceeds via the rearrangement of microscopic domain structure upon light irradiation.