T.I. Volkov

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.