Raisa V. Talroze

Liquid-crystalline polymer composites with CdS nanorods: structure and optical properties.

We report on the structure, uniaxial orientation, and photoluminescent properties of CdS nanorods that form stable nanocomposites with smectic C hydrogen-bonded polymers from the family of poly(4-(n-acryloyloxyalkoxy)benzoic acids. TEM analysis of microtomed films of nanocomposites reveals that CdS nanorods form small domains that are homogeneously distributed in the LC polymer matrix. They undergo long-range orientation with the formation of one-dimensional aggregates of rods when the composite films are uniaxially deformed. The Stokes photoluminescence was observed from CdS NRs/LC polymer composites with emission peak located almost at the same wavelength as that of NRs solution in heptane. An anti-Stokes photoluminescence (ASPL) in polymer nanocomposites was found under the excitation below the nanoparticles ground state. The mechanism of ASPL was interpreted in terms of thermally populated states that are involved in the excitation process. These nanocomposites represent an unusual material in which the optical properties of anisotropic semiconductor nanostructures can be controlled by mechanical deformation of liquid-crystalline matrix.

Phase separation effects and the nematic–isotropic transition in polymer and low molecular weight liquid crystals doped with nanoparticles

Properties of the nematic–isotropic phase transition in polymer and low molecular weight liquid crystals doped with nanoparticles have been studied both experimentally and theoretically in terms of molecular mean-field theory. The variation of the transition temperature and the transition heat with the increasing volume fraction of CdSe quantum dot nanoparticles in copolymer and low molecular weight nematics has been investigated experimentally and the data are interpreted using the results of the molecular theory which accounts for a possibility of phase separation when the system undergoes the nematic–isotropic transition. The theory predicts that the nematic and isotropic phases with different concentrations of nanoparticles may coexist over a broad temperature range, but only if the nanoparticle volume fraction exceeds a certain threshold value which depends on the material parameters. Such unusual phase separation effects are determined by the strong interaction between nanoparticles and mesogenic groups and between nanoparticles themselves.

Antiferroelectric alignment and mechanical director rotation in a hydrogen-bonded chiral SmC* A elastomer

A new type of chiral smectic elastomer based on poly[4-(6-acryloyloxyhexyloxy)benzoic acid] is discussed. The layer structure and the molecular tilt stabilized by hydrogen bonding between side groups are identified by X-ray measurements. Well aligned and optically clear monodomain samples with smectic layers in the film plane are obtained by uniaxial stretching and then frozen-in by additional gamma-radiation crosslinking. In this monodomain state, two opposite orientations of director tilt are distributed through the sample thickness and alternate between neighbouring layers in a zigzag fashion. This structure of the stress-aligned chiral smectic C elastomer is similar to that of antiferroelectric liquid crystals of the smectic C* A type. Further mechanical stretching in the layer plane induces a gradual c-director reorientation along the new stress axis, when a threshold deformation ~ 20% is exceeded. The (reversible) transition proceeds as a director azimuthal rotation around the smectic C cone, with the layers essentially undistorted and the tilt angle of the side mesogenic groups preserved.

Mesophase polymers in the coming decade: problems and trends

Polymers containing mesogenic groups in side and main chains of macromolecules represent liquid crystalline (LC) polymers as a special class of polymeric materials. Peculiarities of electrooptical behaviour of side-chain polymers as ferroelectric materials and optically- recording media as well as the future of LC engineering plastics based on main-chain polymers are discussed.

Matrices based on acrylic liquid-crystalline copolymers for the design of composites with quantum dots

Acrylic ternary cholesteric copolymers that selectively reflect light in a wide temperature range and carry functional carboxyl groups are synthesized and characterized. As shown by X-ray diffraction analysis, differential scanning calorimetry, optical microscopy, spectroscopy, and transmission electron microscopy, the cholesteric structure is formed in the copolymers. The optical properties of the copolymers are investigated. It is shown that homogeneous nanocomposites, in which the liquid-crystalline order is preserved, may be created.

Role of the Polymer Matrix on the Photoluminescence of Embedded CdSe Quantum Dots

The photoluminescence (PL) of CdSe quantum dots (QDs) that form stable nanocomposites with polymer liquid crystals (LCs) as smectic C hydrogen-bonded homopolymers from a family of poly[4-(n-acryloyloxyalkyloxy)benzoic acids] is reported. The matrix that results from the combination of these units with methoxyphenyl benzoate and cholesterol-containing units has a cholesteric structure. The exciton PL band of QDs in the smectic matrix is redshifted with respect to QDs in solution, whereas a blueshift is observed with the cholesteric matrix. The PL lifetimes and quantum yield in cholesteric nanocomposites are higher than those in smectic ones. This is interpreted in terms of a higher order of the smectic matrix in comparison to the cholesteric one. CdSe QDs in the ordered smectic matrix demonstrate a splitting of the exciton PL band and an enhancement of the photoinduced differential transmission. These results reveal the effects of the structure of polymer LC matrices on the optical properties of embedded QDs, which offer new possibilities for photonic applications of QD-LC polymer nanocomposites.

Ordering phenomena in composite monolayers and Langmuir–Blodgett films

Abstract The phenomenon of monolayer and Langmuir–Blodgett (LB) film stabilization via interaction with water soluble substrates like polyacrylic acid is studied. Hydrogen bonds (H-bonds) are shown to be responsible for the formation of composite monolayers based on tertiary amines and polyacid. An important influence of the contact time (before the compression) and amine amount spread on the subphase on the monolayer density is shown to be related to the diffusion of the polymer molecules up to the subphase surface. The structure of monolayers is found to depend on the amount of polyacid dissolved in water as well as on the order of introduction of the polymer into the water subphase (before or after compression). Composite monolayers based on the preprepared amine monolayer show the most dense and well aligned structure. This results in the formation of double layered LB films with highly periodic structures where polyacid amine residues are located between thin layers of H-bonded polyacid and aligned normally to the film plane. This binding of monolayers with water soluble polymers seems to be very important for the deposition of monolayers on the solid substrates and LB film preparation particularly when monolayers based on individual compounds cannot be transferred.

Formation of electrochromic systems based on noncovalently associated polymer-viologen complexes

New composite electrochromic systems combining the advantages of low-molecular-mass organic electrochroms—high molar-absorption coefficients in the colored state and high switching rates—with good properties of polymers—such as mechanical strength and film-formation ability—are prepared through the incorporation of low-molecular-mass viologens into a carboxyl-containing polymer matrix via noncovalent binding. The formation of noncovalently bound polymer-viologen complexes is confirmed by IR and UV spectroscopy. It is found that the degree of substitution in a viologen molecule strongly affects the character of interaction with the polymer matrix and the electrochromic properties of the composite.

Eutectic mesophase transitions and induced crystalline phase in mixtures of hexagonal columnar liquid crystal and mesogenic diacrylate.

Eutectic behavior and the induced crystalline phase in mixtures of hexagonal columnar liquid crystal, 2, 3, 6, 7, 10, 11-hexakis-(pentyloxy) triphenylene (HPTP)/reactive mesogenic diacrylate monomer, 4-(3-acryloyloxypropyloxy)-benzoic acid 2-methyl-1, 4-phenylene ester (RM257) have been investigated both experimentally and theoretically. To determine the theoretical phase boundaries, we combined the free energy of Flory-Huggins free energy for liquid-liquid demixing, Maier-Saupe free energy of nematic ordering, and Chandrasekhar-Clark free energy of hexagonal ordering. The calculated phase diagram of the HPTP/RM257 blend is essentially of a eutectic type that consists of isotropic (I), nematic (N), and hexagonal columnar (Col(h)) regions, and nematic + isotropic (N+I), hexagonal columnar + isotropic (Col(h)+I), and hexagonal columnar + nematic (Col(h)+N) coexistence regions, bound by the liquidus and solidus lines. Of particular interest is the formation of an induced crystalline phase (Cr(in)) in the intermediate compositions of the HPTP/RM257 mixtures, exhibiting Cr(2) (RM257) + Cr(in) in the RM257-rich and Cr(in) + Cr(1) (HPTP) in the HPTP-rich regions.

Microphase separation in block cooligomers based on a discotic liquid-crystalline block

Block cooligomers of 2,6,7,10,11-pentapentyloxy-3-(3-acryloylpropyloxy)triphenylene and poly(tert-butyl acrylate) have been synthesized by the atom-transfer living radical polymerization. The preformed 3,6,7,10,11-pentapentyloxy-3-(3-acryloylpropyloxy)triphenylene block comprising eight monomer units was used as a macroinitiator with a fixed length, whereas the length of the poly(tert-butyl acrylate) block was varied through a change in the monomer-to-macroinitiator ratio. The microphase separation phenomenon has been established with DSC and X-ray diffraction. However, the sizes of discrete structures observed via transmission electron microscopy and atomic-force microscopy lie in the micron-size length (0.1–3.0 μm). This fact is inconsistent with the molecular characteristics of individual blocks. It is proposed that the micron-sized structural elements are not true domain structures belonging to each phase but reflect the occurrence of aggregation in solution preceding formation of thin films.