R. V. Tal'roze

Monodomain liquid crystalline networks: reorientation mechanism from uniform to stripe domains

Monodomain acrylate-based networks have been synthesized by a two-step cross-linking procedure using gamma-irradiation. Strain-induced reorientation of the nematic director is studied by X-ray diffraction measurements. The geometrical shape (aspect ratio) of the monodomain films is found to affect drastically the dynamics and mechanics of the reorientation transition. Uniform continuous rotation of the director occurs in narrow samples with an aspect ratio AR=12 when the external mechanical field is applied perpendicular to the initial orientation. Under the same conditions, films having a lower aspect ratio (AR=2.5) demonstrate the formation of stripe domains with an alternating sense of director rotation (clockwise and counter-clockwise towards the stress axis). Deformation of a square film (AR=1) generates stripe domains in the geometrical centre of the sample, whereas a uniform continuous rotation is observed in other regions of the film. Finally, a comparison of experimental data and theoretical pred...

Liquid crystal acrylate-based networks : polymer backbone-LC order interaction

Abstract New synthetic pathways to poly- and monodomain acrylate-based networks via gamma-irradiation and chemical crosslinking are described. Strain-induced orientation transitions and phase behavior are studied by X-ray scattering, differential scanning calorimetry and optical microscopy. Analysis of the observed phenomena is given in terms of crosslinking conditions, network topology, structure and spatial distribution of crosslinks. The pretransitional behavior and the observed shifts in clearing point Tn–i are in good agreement with theoretical predictions. The reorientation process in monodomain networks is regulated via strain gradients generated by external mechanical field. The variation in geometrical shape of the samples gives an opportunity to control and realize two different reorientation mechanisms: uniform director rotation and reorientation via stripe domains. Gamma-irradiation is found to be a powerful tool for fixation of any current distribution of poly- and monodomain regions within the LC film. ‘Macroscopic memory’ of the information recorded on the films is observed and discussed.

Structure of β-N-dimethylamino-4-dodecyloxypropiophenone complexes with di- and polycarboxylic acids

Abstract The type of bonds responsible for the complexation of di- and polyacids with the tertiary amine β-N-dimethylamino-4-dodecyloxypropiophenone is studied by means of FTIR spectroscopy. The complexes are shown to be stable due to strong H-bonding with partial charge transfer. The characteristic composition for complexes of polyacrylic, polymethacrylic and malonic acids is calculated as 2:1 (number of carboxylic groups per number of amine molecules) whereas glutaric acid forms complexes of different composition including 1:1. The characteristic composition results from the structure of the initial acid. The structures of both the characteristic complex and “excess” acid are also discussed.

Sandwich structure containing liquid crystal polymer grafted on polymer support

Graft post-polymerization of mesogenic monomers onto fluorine-containing polymer support was initiated by the simultaneous action of vacuum ultraviolet radiation and atomic oxygen. The resultant two-layer structure possesses the combined physical–mechanical properties of the polymer-supporting film and the optical characteristics of the anisotropic liquid crystal layer.

Some unusual structures in thermotropic mesophase polymers

Random copolymers of polyester type have been shown to exhibit simultaneous crystallization into two different crystalline forms which are transformed into different hexagonal mesophase structures on heating. Some of these polymers have been found to form LC aperiodic smectic structures. Liquid Crystal elastomers, H-bonded networks and grafted LC materials are discussed. Mechanical field creates the director reorientation in monodomain elastomers and H-bond networks which is discussed in terms of soft elasticity. Grafting of particular LC polymer on fluorine containing polymer support results in the formation of composite sandwich-type material. Mechanical deformation leads to the macroscopic alignment of the LC grafted layer. There is a pronounced memory effect, which is manifested in the macroscopic alignment of the grafted layer under cooling from an isotropic to a LC state. The phenomenon observed results from the strong influence of the polymer support on the LC director orientation. The optical and mechanical properties of sandwiched samples can be regulated in wide limits.

Pretransitional phenomena in acrylate-based liquid crystal networks

The static Kerr effect is used to study the pretransitional phenomena in polymer networks even in the presence of inherent optical anisotropy of crosslinked materials. The pretransitional phenomena in a liquid crystal polymer and its networks are sensitive to the structure of the crosslinks and their distribution within the network. The bulky network junctions distributed more or less homogeneously within the network serve as defects which decrease the hypothetical second order phase transition temperature T* and suppress the nematic fluctuations in the polymer network above the clearing point.