Structure and optical characteristics of poly(p-phenylenevinylene) prepared by vapor deposition polymerization

Films based on poly(p-phenylenevinylene) are prepared by pyrolitic polymerization of α,α′-dichloro-p-xylene. During monomer precipitation, the temperature on a substrate is 25, 50, or ?196°C. Subsequent annealing of the precursor at 250°C yields the final product: the copolymer of p-phenylenevinylene and p-xylylene with an approximate composition of 4: 1. The surface morphology, structure, and optical characteristics of the polymer are studied. The mean-square surface roughness of the precursor is 5 nm. Thermal treatment increase the samples’ roughness up to 10 nm. When the precursor is transformed into poly(p-phenylenevinylene), the roughness coefficient decreases from 0.85 ± 0.05 to 0.74 ± 0.05 owing to the formation of a rougher surface. Characterization of the optical characteristics of the synthesized poly(p-phenylenevinylene) shows that the maximum effective conjugation chain length achieves 12 units in the copolymer prepared when the temperature on the substrate is ?196°C. As the temperature on the substrate increases, the conjugation length decreases to 8 units upon precipitation. Luminescence analysis reveals the effective excitation-energy transfer from short chain fragments of poly(p-phenylenevinylene) to long chain fragments. Electron parameters of the material are estimated: i.e., the band gap, the Huang-Rhys factor, the Stokes shift, and the oscillation energy of molecules.     

Polysaccharide-based natural and synthetic nanocomposites

Modern concepts on the supramolecular organization of natural nanocomposite materials based on structural polysaccharides are discussed, beginning from the stage of biosynthesis. The methods for obtaining nanocrystals and nanofibrils of cellulose and chitin, as well as their structure and properties, are discussed. Various methods for the synthesis of nanocomposites with the use of water-soluble, water-dispersed, and hydrophobic polymer matrixes are described. Special attention is given to surmounting the incompatibility of polymer and polysaccharide particles.     

Structure and characteristics of chitosan-based fibers containing chrysotile and halloysite

With the use of the methods of X-ray diffraction and electron microscopy, chitosan fibers prepared by coagulation into an alcohol-alkali mixture are shown to possess a two-phase structure containing C- and O-type crystallites. These fibers and composite fibers containing halloysite and Mg chrysotile nanotubes are characterized by anisotropic structure, i.e., by the orientation of both chitosan crystallites and Mg chrysotile particles along the fiber axis. A comparison of the rates of shear induced by passing of a polymer solution through a die and the data of rheological studies allows the conclusion that the structuring of chitosan solution under the applied field of shear stresses and the orientation of polymer macromolecules and filler nanotubes occur. An increase in the draw ratio during fiber spinning does not assist orientation of polymer crystallites but, in contrast, increases surface defectiveness and leads to the nucleation of longitudinal cracks; as a result, the strength of fibers decreases. The introduction of 5 wt % Mg chrysotile into the chitosan matrix markedly increases the mechanical characteristics of the composite fibers owing to the reinforcing action of oriented filler nanotubes.     

Kinetic investigation of the polymerization of <Emphasis Type="Italic">D</Emphasis>,<Emphasis Type="Italic">L</Emphasis>-lactide and glycolide via differential scanning calorimetry

The kinetics of polymerization of D,L-lactide and glycolide are studied via differential scanning calorimetry at different temperatures and concentrations of the catalyst tin octanoate. For the polymerization of D,L-lactide and glycolide, the enthalpies are determined to be ?17 ± 1.5 and 16.5 ± 1.5 kJ/mol, respectively. The time to attain reaction equilibration decreases from 300 to 100 min with an increase in temperature from 200 to 220°C. The time of reaction at 200°C decreases from 280 to 100 min as the concentration of the catalyst is increased from 500 to 830 ppm. When the polymerization of glycolide is conducted at temperatures below 200°C, the reaction is accompanied by crystallization of polyglycolide and an increase in the total enthalpy of the process.     

Creation of a microrelief by thermally stimulated shrinkage of metal-coated polypropylene films

A study was made of the conditions of producing micron-sized reliefs by thermally stimulated shrinkage of metallized commercial biaxially oriented polypropylene films. It was shown that, in annealing within a temperature range close to the polypropylene melting point, the film shrinks in two directions to form folded structures on the surface. The folds are about 2–8 μm in size. The shrinkage and the relief depth depend on the annealing temperature. The relief remains on the film surface after the removal of the aluminum layer.     

Nanocomposite nonwoven materials based on polyamide-6 and montmorillonite,prepared by electrospinning of the polymer melt

The effect of montmorillonite on the structure and properties of nonwoven microfibrous materials based on polyamide-6 and prepared by electrospinning of the polymer melt was studied. Addition of 3% montmorillonite into the melt increases its viscosity and electrical conductivity, with the mean diameter of the formed fibers increasing from 8 to 12 μm. As shown by X-ray diffraction, IR spectroscopy, and differential scanning calorimetry, pellets of the pristine polyamide are characterized by prevalence of crystals of the stable α-form, whereas in the composites and nonwoven materials the metastable γ-form prevails. Addition of montmorillonite only slightly influences the contact angles, and the resulting materials exhibit nearly superhydrophobic properties.     

Effect of crystallization conditions on the shape of polymer single crystals: Experimental and theoretical approaches

A modified system of Mansfield equations with different step propagation rates to the right and left was applied to describe the shape of solution-grown single crystals of long-chain alkanes with asymmetrically curved faces ?ub;110?ub;. Solution of this system of equations and simulation of the shape of single crystals of polyethylene, α-polyvinylidene fluoride, and polyethylene oxide shows good agreement with the experimental data. Comparison of the simulated shapes and experimental micrographs makes it possible to determine the ratio of the intensity of secondary nucleation to the average propagation rate of the layer as well as the ratio of the rates of lateral steps to the right and left and to calculate the absolute values of the main crystallization parameters when the growth rates of single crystals in certain directions are known. It is shown that the asymmetry of the step propagation rate to the right and left is significant only for crystallization at very small supercoolings.     

Driving Forces of the Self-Assembly of Supramolecular Systems: Partially Ordered Mesophases

The main aspects are considered of the self-organization of a new class of liquid crystalline compounds, rigid sector-shaped and cone-shaped dendrons. Theoretical approaches to the self-assembly of different amphiphilic compounds (lipids, bolaamphiphiles, block copolymers, and polyelectrolytes) are described. Particular attention is given to the mesophase structures that emerge during the self-organization of mesophases characterized by intermediate degrees of ordering, e.g., plastic crystals, the rotation-crystalline phase in polymers, ordered and disordered two-dimensional columnar phases, and bicontinuous cubic phases of different symmetry.     

Self-Assembly of Supramolecular Aggregates Based on Sector- and Cone-Shaped Dendrons and Bolaamphiphiles

Using a number of classes of such sector-shaped macromolecules as derivatives of 2,3,4- and 3,4,5- tri(dodecyloxy)benzenesulfonic acid and dendrimers based on gallic acid as an example, the main stages in the formation of supramolecular ensembles are considered: the formation of individual supramolecular aggregates due to the weak noncovalent interactions of mesogenic groups, and the subsequent ordering within these aggregates, which lowers the free energy of a system. Supramolecular aggregates are in turn organized into two- or three-dimensional supramolecular lattices. It is shown that the shape of the supramolecular aggregates and its change along with temperature are functions of the chemical structure of the mesogenic group (resulting in the controlled design of complex self-organizing systems with a given response to external stimuli).