A new phenomenon in necking of some polymers, including poly(ethylene terephthalate) (PETP) was detected. It was found that extension of PETP films under certain conditions results in periodic stress oscillations and a periodic change in appearance of the sample. The conditions at which self‐oscillations appear have been determined, and the principal regularities of this regime of deformation are described. The following factors are critical for the appearance of self‐oscillation: speed of straining and compliance of the sample. The self‐oscillation of stress and formation of the periodic transverse bands is attributed to heat dissipation during necking corresponding to local temperature jumps and periodic strong variation of elasticity modulus due to poor heat conductivity of the polymer. Changing the external conditions of heat transfer influences the possibility and development of the effect. The phenomenon is common for various crystallizing polymers, being dependent on physical properties of the polymer and conditions of deformation.

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A new method for obtaining porous and porous fiber polymers is presented. This method is based on using gel‐type technology (without previously preparing polymer solutions) for crystallizable polymers, preparing polyethylenes, and including polyethylenes of very high molecular mass and isotactic polypropylene. The method consists in swelling crystalline polymer films at elevated temperatures in a proper solvent with subsequent precipitation with a non‐solvent at different conditions. In this case, simultaneous or consecutive processes of phase separation of amorphous or/and crystalline type occurs; stretching the sample can also accompany this process. Complete phase diagrams of two‐ and three‐component systems (polymer‐solvent and polymer‐solvent precipitator) were constructed. Temperature‐concentration boundaries of amorphous separartion (binodal) and crystallization (liquidus) are reported for the system polyethyleneo‐xylenedimethyl formamide. Phase transitions of both types influence characteristics of the resultant porous structure. They were prepared by simultaneous (precipitation of a gel by dimethyl formamide at 25°C) or consecutive (precipitation with a hot non‐solvent at 138°C and following cooling) phase separation. Studied were the effect of experimental conditions (temperature, times for solvation and precipitation, polymer molecular mass, the thermodynamic quality of solvents and parameters of film stretching) on peculiarities of the structure and quantitative characteristics of final porous and fiber‐porous polyolefins. It has been demonstrated that the method proposed allows us to obtain a crystalline and highly porous polymer with open poros, a bimodal size distribution and with a highly developed inner surface. Further high strength and small shrinkage are characteristic of the fiber‐porous materials. The method under discussion appears to be universal, it does not require a preliminary preparation of polymer solutions and can be realized within a general technology of polymer films and sheet processing. Highly porous polymers obtained by this technology, primarily based on polyethylenes of very high molecular mass, can be used as neutral supports for multi‐functional membranes, polymeric covers, frame systems for implants and other applications.

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The pore structure of heat-treated nonwoven materials is determined by the conditions of heating and cooling them. The effect of the shrinkage properties of a bicomponent fibre on the pore structure of the materials is manifested at a treatment temperature above the melting point of polypropylene.The effectiveness of using nonwoven materials for filtration and as fibrous sorbents is basically determined by their pore structure. At the same time, the mechanical properties of these materials should ensure resistance to development of deformation. The comprehensive requirements imposed on filtering materials and fibrous sorbents have made it necessary to combine mutually exclusive characteristics such as low density and high mechanical strength.One possible method of obtaining highly porous and strong materials involves to addition of shrinkable fibres to the nonwovens and conducting heat treatment in the final stage of production. The proposed methods of formula and process modification of nonwovens can affect their pore structure. In studying the formation of the pore structure in nonwovens containing polypropylene fibres in [1], it was previously shown that this process takes place in conditions of significant shrinkage over the area of the material and changes in the initial structure formed in the needle-punching stage. The change in the structure of the materials causes the formation of closed pores unavailable for transfer of gases and liquids in the bulk of the materials.We investigated pore structure formation in nonwovens containing bicomponent fibres treated with heat at different temperatures. Nonwovens made of a blend of polyester fibres with a linear density of 0.33 tex (TU 6-13-0204077-95-91) and 0.44 tex bicomponent fibre with an outer polypropylene shell and a poly(ethylene terephthalate) core (South Korea) were investigated. The materials contained 20 and 40 wt. % bicomponent fibres. The samples of the nonwoven material were made by the mechanical method of spinning the fibre web followed by needle-punching at a punching density of 160 p./cm 2 .The real (V r ) and apparent (V a ) pore volumes of samples with an area of 100 cm 2 , determined with the following equations [2], were used as the pore structure characteristics of the initial and thermostated materials:where V 1 is the volume of the sample, cm 3 (according to GOST 3811-72); m 1 is the weight of the sample, g; ρ is the density of the fibre blend determined pyknometrically and equal to 1.21 g/cm 3 ; m 2 is the weight of the sample after holding in water, g; ρ w is the density of the liquid (water), g/cm 3 .To determine the apparent pore volume, the disk-shaped samples were placed in distilled water for two days. After removal from the water, the samples were placed on a screen and left there until all of the liquid had drained. The samples were weighed in a plastic container of known weight, which excluded losses of water in handling the samples during the experiment.The dependence of the real and apparent pore volumes of the materials on t...