T. E. Grokhovskaya

Stabilization of scleral collagen by glycerol aldehyde cross-linking

The paper aims at the evaluation of prospects for using glyceraldehyde as a cross-linking agent for the scleral tissue. Stability parameters (denaturation temperature, Youngs modulus, ultimate tensile stress, proteolytic resistance) and analytical parameter (fluorescence intensity) were determined during the glycation process of isolated rabbit sclera. The analysis of fluorescence spectral characteristic provided information about some glycation products. The glyceraldehyde treatment was resulted in a significant increase in thermal stability, proteolytic resistance and improvement of biomechanical characteristics (Youngs modulus, ultimate tensile stress). Unique properties of the reaction between scleral collagen and glyceraldehyde are observed at short cross-linking times. The appearance of intermediate collagen fraction with lowest thermal and proteolytic stability was detected.

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.

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.

Thermal stability of collagen II in cartilage.

Collagen is the main component of connective tissue. It was shown that collagens fell into several types: collagen I (collagen found in skin, derma, ligaments, and tendons), collagen II (collagen of cartilage tissue), etc. Thermal stability of collagen I was thoroughly studied in [1, 2]. The temperature of denaturation of collagen I in fibrils is 60–70°ë and its specific enthalpy of denaturation is ∆ H d 60 ± 10 J/g [2]. Much less is known about thermal behavior of collagen II [3]. Cartilage tissue is a three-dimensional network of collagen II fibers embedded in gel produced by a network of proteoglycan (PG) fibers composed mainly of glycosaminoglycans (GAG). Development of new medical methodological approaches (e.g., changes in cartilage tissue shape induced by nondestructive laser heating) makes it important to study the problems of stability of cartilage tissue and thermal stability of collagen II [4]. The goal of this work was to study thermal and thermomechanical behavior of cartilage tissue and to determine the conditions of complete denaturation of collagen II. Cartilage tissue of calf nasal septum was obtained by dissection post mortem and used within 12 h. Incubation buffer (1000 ml) contained 0.1 M Tris–HCl (Amerco, the United States), 25 mM EDTA (Quality Biological, the United States), 5000 IU of penicillin, and 5 mg of streptomycin. Samples of cartilage tissue used in experiments were of three series: A, B, and C. The control samples (series A) were exposed to incubation buffer. Samples of series B were exposed to 0.1% trypsin (Sigma, United States) in incubation buffer. Samples of series C were exposed to 0.1% α -chymotrypsin (Samson-Med, Russia) in incubation buffer. Incubation time was 20 h; temperature, 37°ë . Thermal behavior of samples was studied by the method of differential scanning calorimetry using a Metter TA 4000 standard calorimeter. Analysis was performed in air-tight boxes at a heating rate of 10° C/min.

Sclera of the glaucomatous eye: Physicochemical analysis

It has been shown that the collagen content and denaturation temperature in scleral tissue increase during the development of glaucoma whereas the fluorescence intensity does not change. These effects may be related to the disturbance of collagen catabolism.

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.

Structural Aspects of the Deposition of Metal Coatings on Polymer Films

An electron-microscopic study of the formation of thin metal coatings (gold and aluminum) on a PET film is performed. During the deposition of gold coatings, a well-pronounced interfacial polymer/metal layer is formed, while, in the case of aluminum coatings, the polymer/metal interface is well-defined. The assumption is made that this effect is caused by different chemical activities of metals deposited on the polymer.

Effect of supramolecular organization of a cartilaginous tissue on thermal stability of collagen II

The thermal stability of collagen II in various cartilaginous tissues was studied. It was found that heating a tissue of nucleus pulposus results in collagen II melting within a temperature range of 60–70°C; an intact tissue of hyaline cartilage (of nasal septum and cartilage endplates) is a thermally stable system, where collagen II is not denatured completely up to 100°C. It was found that partial destruction of glycosaminoglycans in hyaline cartilage leads to an increase in the degree of denaturation of collagen II upon heating, although a significant fraction remains unchanged. It was shown that electrostatic interactions of proteoglycans and collagen only slightly affect the thermal stability of collagen II in the tissues. Evidently, proteoglycan aggregates play a key role: they create topological hindrances for moving polypeptide chains, thereby reducing the configurational entropy of collagen macromolecules in the state of a random coil.