V.G. Baranov

Thermodynamics of crystallization of macromolecules of various degrees of coiling

Abstract The crystallization of flexible-chain molecules under conditions of molecular extension is considered. The molecular orientation was characterized by a parameter β that is the ratio of end-to-end distance to the contour length of macromolecule. Calculations were performed for two types of crystals: folded-chain crystals and extended-chain crystals. Thermodynamical analysis shows that the decrease of conformational entropy owing to the drawing of the molecule into a crystal makes an essential contribution to the free-energy change. This value is of great importance in the definition of the equilibrium state of a system and so it is the reason why it is impossible to get the degree of crystallinity equal to 1. The values of molecular orientation at which each of two mechanisms of crystallization takes place are evaluated. It is shown that there is a critical value β* as a boundary between two ranges of extensions: at β < β*, the crystallization with the formation of folded-chain crystals is thermod...

Thermo-mechanical study of relaxation effects in polymers☆

Abstract It was shown that thermo-mechanical spectrometry can only be used in thermo-mechanical studies if temperature shows a linear variation over a period of time. A principle was proposed and a description given of the structural features of a linear temperature programmer. A study was made of the thermo-mechanical behaviour of twelve polymer fibres of different chemical structures. It was found possible to examine the energetics of corresponding processes as a result of the displacement of the transition temperature of relaxation due to a variation of the rate of heating and the sample used in thermo-mechanical tests.

Features of the crystallization of polychloroprene during annealing

Abstract The dependence of the time and heat of additional isothermal crystallization on the temperature of the main process of isothermal crystallization has been established during the annealing of polychloroprene. Crystallization during annealing leads to a redistribution of the intensities and temperatures of mechanical loss peaks. According to the proposed model crystallization results in additional crystallite formation on the main crystallite surface within intercrystallite layer limits. The redistribution of various types of mobility of portions of macromolecules of the intercrystallite layer is regarded as being a liquid-liquid type phase transition which accompanies crystallization and annealing of amorphous-crystalline elastomers.

A possibility of observing the structure forming process in polypropylene crystallization

The existence of the specific metastable phase made it possible to carry out multistage electron microscopic investigation of the supermolecular structure formation in the crystallization of polypropylene in the oriented state. Comparison with the data of electron diffraction yielded a direct evidence of the fact that the formation of structural units of the supermolecular organization of crystalline PP, i.e. fibrils and lamellae, occurs in the stage of orientation, and crystallization itself only fixes these structures. Die Existenz einer spezifischen metastabilen Phase erlaubt es, mehrstufige elektronenmikroskopische Untersuchungen der Bildung der ubermolekularen Struktur bei der Kristallisation von Polypropylen im orientierten Zustand durch zufuhren. Der Vergleich mit Elektronenbeugungsaufnahmen liefert einen direkten Beweis dafur, das die Bildung von Struktureinheiten der ubermolekularen Ordnung im kristallinen Polypropylen, d. h. der Fibrillen und Lamellen, wahrend der Orientierung erfolgt und die Kristallisation diese Strukturen nur fixiert.

The structure and properties of polyethylene films obtained during orientational crystallization

Highly oriented films possessing increased tensile strengths (based on initial cross section) and elastic moduli are produced directly in the extrusion process. The dependence of the structure and properties of these films on hardening temperature is studied. Assumptions are made concerning the formation and crystallization mechanisms of films produced with higher molecular orientation.

Mono- and multimolecular nucleation in polymer crystallization

General relations for the nucleation rates are obtained with allowance for the contribution of the change of free energy in nucleation by two mechanisms — monomolecular, with the formation of folded nuclei, and multimolecular, i.e., nucleation with straightened polymer chains. It is shown that for flexible coiled molecules the formation of folded nuclei is more probable. However, as the degree of coiling decreases, whatever the reasons for the uncoiling of the macromolecules, the relative probability of formation of multimolecular nuclei increases, and at a certain critical value of the chain-folding parameter β=h/L the situation is reversed.

Experimental verification of main ratios using the method of small angle polarized light scattering

Abstract Calculation of the distribution of scattering intensity of polarized light by large anisotropic particles in polymers is purely a “model” calculation and is based on the appropriate selection of parameters of the scattering element immersed in an isotropic matrix and subsequent analysis of interaction with electromagnetic radiation. This analysis normally enables ratios to be derived which relate the dimensions and form of element with the pattern of scattering. However, the approach itself and the approximation used (Rayleigh-Hans) require an experimental verification of these ratios. This verification was made precisely for two main ratios which relate parameters of spherulitic supermolecular organization of polymers with the pattern of small angle polarized light scattering. It was shown that these ratios are valid for systems containing spherulites with dimensions of one to ten microns.

Structure formation in a crosslinked polyethylene melt crystallizing under uniaxial tension

The formation of supermolecular structure has been investigated in the case of medium-pressure polyethylene film subjected to a dose of 30 Mrad of n, γ radiation. The melt, prestretched at 160°C to various degrees of elongation α, was crystallized at constant temperature. The results are interpreted on the basis of an analysis of the effect of the mechanical field on the growth kinetics and the orientation relative to the direction of extension of the individual crystallites; the first factor determines the shape and the second the sign of the spherulite.

Study of the supermolecular transitions associated with the reorientation of high-pressure polyethylene film

Small-angle optical and X-ray diffraction techniques have been used in a comparative study of the processes of orientation and reorientation of high-pressure polyethylene film. It is shown that at large stretch ratios both processes are governed by the same laws. In these supermolecular transitions it is possible to observe spherulite-macrofibril conversions during orientation and macrofibril-macrofibril conversions during reorientation, a direct genetic relationship being preserved between the dimensions of the starting spherulites or macrofibrils and the dimensions of the macrofibrils formed by stretching. However, the major period of the oriented film, the basic element of the microfibrils forming the macrofibril, is independent of the major period of the starting film and depends only on the temperature at which stretching is carried out.

Effect of formation conditions on the supermolecular organization and mechanical properties of polyethylene film

Films formed by extruding medium-pressure polvethylene through a flat-slot have a spherulitic supermolecular organization whose principal parameters (size and shape of the spherulites) are determined by the extrusion conditions. Thus, as the draw-down ratio increases, the radius of the spherulites decreases, and their degree of flattening relative to the direction of extension increases. Stretching these films leads to a transition from a spherulitic to an orientational supermolecular order, whose period is genetically related to the diameter of the starting spherulites. Films containing flattened spherulites have a yield point anisotropy opposite in sign to the degree of flattening. The mechanical anisotropy, like the degree of flattening, increases with increase in the draw-down ratio. The probable cause of flattened spherulite formation is the draw-down process, whose mechanical field may retard the radial growth of the spherulites in the take-off direction.