Porous materials from crystallizable polyolefins produced by gel technology

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 degrees C) or consecutive (precipitation with a hot non-solvent at 138 degrees 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.