A. I. Kulebyakina

Polymer Micelles with Hydrophobic Core and Ionic Amphiphilic Corona. 1. Statistical Distribution of Charged and Nonpolar Units in Corona

Polymer micelles with hydrophobic polystyrene (PS) core and ionic amphiphilic corona from charged N-ethyl-4-vinylpyridinium bromide (EVP) and uncharged 4-vinylpyridine (4VP) units spontaneously self-assembled from PS-block-poly(4VP-stat-EVP) macromolecules in mixed dimethylformamide/methanol/water solvent. The fraction of statistically distributed EVP units in corona-forming block is β = [EVP]/([EVP]+[4VP]) = 0.3-1. Micelles were transferred into water via dialysis technique, and pH was adjusted to 9, where 4VP is insoluble. Structural characteristics of micelles were investigated both experimentally and theoretically as a function of corona composition β. Methods of dynamic and static light scattering, electrophoretic mobility measurements, sedimentation velocity, transmission electron microscopy, and UV spectrophotometry were applied. All micelles possessed spherical morphology. The aggregation number, structure, and electrophoretic mobility of micelles changed in a jumplike manner near β ~ 0.6-0.75. Below and above this region, micelle characteristics were constant or insignificantly changed upon β. Theoretical dependencies for micelle aggregation number, corona dimensions, and fraction of small counterions outside corona versus β were derived via minimization the micelle free energy, taking into account surface, volume, electrostatic, and elastic contributions of chain units and translational entropy of mobile counterions. Theoretical estimations also point onto a sharp structural transition at a certain corona composition. The abrupt reorganization of micelle structure at β ~ 0.6-0.75 entails dramatic changes in micelle dispersion stability in the presence of NaCl or in the presence of oppositely charged polymeric (sodium polymethacrylate) or amphiphilic (sodium dodecyl sulfate) complexing agents.

Biodegradable multi-liposomal containers

The method of preparing biodegradable multi-liposomal containers via modification of anionic liposomes with a cationic polymer and subsequent adsorption of the obtained liposome-polymer cationic complex on the surfaces of negatively charged polylactide micelles with grafted polyoxyethylene chains is described. Liposomes preserve their integrity in a ternary micelle-polycation-liposome complex, a circumstance that allows the complex to be used as a multi-liposomal container for encapsulation of bioactive compounds.

Visualization of structural rearrangements responsible for temperature-induced shrinkage of amorphous polycarbonate after its deformation at different conditions

Structural rearrangements during the temperature-induced shrinkage of amorphous polycarbonate after its tensile drawing below and above the glass transition temperature, rolling at room temperature, and solvent crazing have been studied with the use of the direct microscopic procedure. This evidence demonstrates that the character of structural rearrangements during the temperature-induced shrinkage of the oriented amorphous polymer is primarily controlled by the temperature and mode of deformation. In the case of the polymer sample stretched above the glass transition temperature, the subsequent temperature-induced shrinkage is shown to be homogeneous and proceeds via the simultaneous diffusion of polymer chains within the whole volume of the polymer sample. When polymer deformation is carried out at temperatures below the glass transition temperature, the subsequent temperature-induced shrinkage within the volume of the polymer sample is inhomogeneous and proceeds via the movement of rather large polymer blocks that are separated by the regions of inelastically deformed polymer (shear bands or crazes).

Structural approach to the study of deformation mechanism of amorphous polymers

A new microscopic procedure for the visualization of structural rearrangements in amorphous polymers during their deformation to high strains is described. This approach involves the deposition of thin (several nanometers) metallic coatings onto the surface of the deformed polymer. Subsequent deformation entails the formation of a relief in the deposited coating that can be studied by direct microscopic methods. The above phenomenon of relief formation provides information concerning the deformation mechanism of the polymer support. Experimental data obtained with the use of this procedure are reported, and this evidence allows analysis of the specific features of structural rearrangements during deformation of the amorphous polymer at temperatures above and below its glass transition temperature under the conditions of plane compression and stretching, uniaxial tensile drawing and shrinkage, rolling, and environmental crazing. This direct structural approach originally justified in the works by Academician V.A. Kargin appears to be highly efficient for the study of amorphous polymer systems.

Visualization of Structural Rearrangements during Annealing of Solvent-Crazed Poly(ethylene terephthalate)

A direct microscopic procedure is used for studying structural rearrangements during the annealing of PET samples after solvent crazing. Even at room temperature, solvent-crazed PET samples experience shrinkage which is provided by processes taking place in crazes. This shrinkage is observed at temperatures up to the glass transition temperature of PET and proceeds via drawing together of crack walls. Once the glass transition temperature is attained during annealing, the spontaneous self-elongation of the polymer sample occurs. The mechanism of this phenomenon is proposed. The low-temperature shrinkage of the polymer sample is related to the entropy contraction of highly dispersed material in crazes that has a lower glass transition temperature than that of the bulk polymer. This shrinkage cannot be complete, owing to crystallization of the oriented polymer in the volume of the crazes. As a result of crystallization, the oriented and crystallized polymer in the crazes coexists with the regions of the unoriented initial PET. As the annealing temperature approaches the glass transition temperature of the bulk PET, its strain-induced crystallization takes place. As a result, the regions of the unoriented polymer between crazes are elongated along the direction of tensile drawing and the sample experiences contraction in the normal direction.

Self-Assembly of an Amphiphilic Diblock Copolymer in Aqueous Solutions: Effect of Linear Charge Density of an Ionogenic Block

The effect of linear charge density of the ionogenic block on the selfassembly of the amphiphilic diblock copolymer based on polystyrene and poly(4�vin ylpyridine) partially alkylated by ethyl bromide in aqueous solutions at pH 9 is studied by UV spectrophotometry and dynamic and static light scattering. Dur� ing dispersion in water, the diblock copolymer forms micelles composed of the hydrophobic polystyrene core and the amphiphilic lyophilizing corona consisting of Nethyl�4�vinylpyridinium bromide ionic units and uncharged units of 4�vinylpyridine. It is shown that with a change in the fraction of charged units in the lyo� philizing block from 30 to 90 mol %, the thermodynamic quality of the solvent with respect to micelles is slightly monotonically improved and the hydrodynamic sizes of micelles in 0.05 M NaCl are increased. At the same time, such properties as the weightaverage deg ree of aggregation of macromolecules in a micelle, the dimensions of the corona in 0.05 M NaCl, and the dispersion stability of micelles in aqueous-saline solution abruptly change when the content of charged units in the lyophilizing block is 60-70 mol %. After addition of oppositely charged polyelectrolytes or lowmolecu larmass surfactants into the micellar solution, poly� electrolyte complexes form and the solubility of these complexes in an excess oppositely charged component likewise changes abruptly with variation in the fraction of charged units in the range from 60 to 70 mol %. A qualitative description is advanced to explain the effect of linear charged density of the lyophilizing block on the selfassembly and complexing behavior of diblock copolymers in aqueous solutions.

Visualization of structural rearrangements during annealing of solvent-crazed isotactic polypropylene

Structural rearrangements taking place upon the annealing of solvent-crazed isotactic PP are studied by the direct microscopic method. Independently of the type of its crystalline structure, solvent-crazed PP undergoes shrinkage in a wide temperature interval, starting even from room temperature and up to its melting temperature. This shrinkage is a result of the structural processes in crazes and proceeds via shutting down of the walls of individual crazes. This low-temperature shrinkage of solvent-crazed PP is assumed to have an entropy nature. This process involves the contraction of extended polymer chains and their transition into thermodynamically favorable conformations. This contraction is allowed because, upon annealing, the entropy contracting force increases. As a result, the crystalline framework of oriented PP melts down (amorphization), extended chains appear contracted, stored stresses relax, and subsequent recrystallization in the unstressed state takes place.

Synthesis and properties of conjugates involving liposomes, a linear polymer, and the micelle of a polylactide-poly(ethylene glycol) block copolymer

Multiliposome biodegradable conjugates have been obtained via electrostatic binding of a liposome-polylysine complex with micelles formed from a polylactide-poly(ethylene glycol) di- or triblock copolymer. It has been shown that the integrity of liposomes in conjugates is retained. Of special interest is a conjugate based on a polylactide-poly(ethylene glycol) diblock copolymer that exhibits stability in a physiological solution with 0.15 mol/L NaCl, but degrades in the presence of proteolytic enzymes. This fact allows the suggestion that the conjugate will be eliminated from the body after its transport function is completed. The results may be used to create biodegradable liposome containers for drug encapsulation and delivery.

Visualization of strain-induced structural rearrangements in amorphous poly(ethylene terephthalate)

A direct microscopic observation procedure is applied to study the deformation of amorphous PET decorated with a thin metal layer when stretching is performed at different draw rates and at temperatures below and above the glass transition temperature Tg. Analysis of the formed microrelief allows stress fields responsible for the deformation of the polymer to be visualized and characterized. When tensile drawing is performed at temperatures above Tg, inhomogeneity of stress fields increases with the increasing draw rate; at high draw rates, the stress-induced crystallization of PET takes place. In the case of drawing the polymer at temperatures below Tg, direct microscopic observations make it possible to visualize the development of shear bands that appear in the unoriented part of the polymer specimen adjacent to the neck. The shear bands are oriented at an angle of about 45° with respect to the draw direction. When necking involves the unoriented part of the polymer, shear bands abruptly change their orientation and become aligned practically parallel to the draw axis.

Fabrication and mechanical properties of composite based on β-chitin and polyacrylic acid

Squid β-chitin has been exfoliated in aqueous acrylic acid (AA), after which a composite film of chitin microfibrils in polyacrylic acid (PAA) has been prepared by in situ polymerization of the AA. The segregated chitin fibrils in the composite are 4-6nm in diameter, with an aspect ratio >250. After drying cast films of the composites containing 1, 2 and 3% (w/w) chitin at 140°C for four hours, there was a dramatic resistance to swelling in water, in that the dried films showed only small changes in shape and properties after four hours immersed in water.The most profound impact of the reinforcement on the mechanical properties is observed at high relative humidity (RH), when the PAA is in the rubbery state. At 97.5% RH and room temperature, the elastic moduli of the composites with 1, 2 and 3% (w/w) chitin were 150, 230 and 2100MPa respectively, compared to 65MPa for pure PAA. The main contribution to the filler-reinforcing effect is the high aspect ratio of fibrils and non-covalent interactions, but the stability in water suggests the presence of chemical bonding between the PAA and chitin.