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The structure of noncrystalline regions of HDPE oriented filaments prepared by two different techniques with various degrees of orientation X has been studied by density measurements, small-angle X-ray scattering and DTA. It is shown that at A > 10 the noncrystalline regions of the oriented structure prepared from a crystallized material have 80 nrn voids (pores) in addition to an amorphous phase. The volume fraction of pores grows with increasing A. If a sample is prepared by crystallization from apre-orientedmelt, it does not contain discontinuities, and its noncrystalline regions consist of the amorphous material alone. In this case the density of the amorphous phasedoes not depend on the degree of sample orientation and has a constant magnitude typical of highly oriented PE samples.

Hard elastic samples of linear polyethylene were prepared by melt extrusion at a high velocity of the melt flow and by subsequent annealing of crystallized samples. The deformation behavior of hard elastic samples obtained by annealing of as-spun samples at different temperatures has been analyzed at uniaxial extension resulting in formation of porous structure. Mechanical properties of microporous films in the longitudinal and transverse directions have been investigated. Composite systems consisting of a microporous polyethylene film and a thin layer of an electroconducting polymer have been prepared. Mechanical properties of composite systems, such as elastic modulus, tensile strength, and break elongation, have been compared with the properties of polyethylene substrates.

St. Petersburg 199004, Russia and V. A. MARIKHIN and L. P. MYASNIKOVA A. F. lo8e Physical Technical Institute Russian Academy of Sciences St. Petersburg 194021, RussiaThis paper reports on the theoretical and experimental studies of structure formation and strengthening (stiffening) of flexible-chain polymers. Two techniques of strengthening relying on the melt extrusion, i.e., orientational crystallization (crystallization initiated by melt extension) and drawing (uniaxial stretching of a crystallized polymer) are analyzed by theory. The experiments involved preparation and study of melt extruded films and film fibers of linear polyethylene formed by the two techniques mentioned above. The effect of the degree of orientation and other parameters of the formation processes on the mechanical characteristics and the factors limiting the ultimate values of these characteristics are discussed. It is shown that multistage drawing succeeds in achieving a higher tensile strength and elastic modulus (1.2 and 35 GPa, respectively) than the orientational crystallization, which gives 0.8 and 15 GPa. The strengthening by drawing is accompanied by microcrack formation. In contrast, no discontinuities are observed in orientationally crystallized samples up to their ultimate extension.