Novel ultraporous and microporous membranes from immiscible polymer blends were produced via melt processing and post-extrusion treatments. Polystyrene (PS)/polypropylene (PP) and poly(ethylene terephthalate) (PET)/polypropylene (PP) blend systems with different rheological properties were studied. The blends were first compounded in an intermeshing co-rotating twin-screw extruder and subsequently extruded through a sheet die to obtain the precursor films. These were post treated by uniaxial or biaxial deformation (100-500%) with respect to original dimensions to induce a microporous structure. The porosity is induced by drawing the precursor film at a temperature below the glass transition temperature (Tg) of the minor phase. Crazing structure is initiated during the post-step treatments. The dimensions of the crazes are then enlarged by a series of stretching processes that comprise consecutive steps of cold stretching followed by hot stretching. Rates of craze initiation and growth depend strongly upon applied stress conditions and deformation temperature. Microscopy and finite element stress analysis suggest that microporous structures are formed by a crazing mechanism. Shear yielding also occurs along with the crazing. The films were then subjected to heat setting at elevated temperatures to stabilize the porous structure which consisted of three-dimensional uniform microcracks in the order of a few hundred nanometers across the thickness of the membranes. The effects of phase morphology, degree of dispersion, interfacial adhesion of the membrane components as well as processing and post-treatment conditions were studied to relate processing and blend morphology with membrane structure.
In the case of membrane precursors from binary uncompatibilized PP/PS blend systems, the domain size increases with increasing dispersed phase concentration due to increased coalescence. The domain size distribution also broadens as the minor phase concentration increases. The limiting domain size of 0.37 µm was obtained at 1 wt% dispersed phase concentration. For ternary blend systems containing a block copolymer, it was found that the block copolymer had profound effects on blend morphology by decreasing interfacial tension as well as suppressing coalescence. The dispersed phase domain size could be reduced by as much as 40% in comparison to the uncompatibilized system. Moreover, it is shown that the mixing protocol plays a critical role in morphology development in the compatibilized blend system affecting the ability of the copolymer to migrate to the interface between minor and major blend components.
Results of the present study have lead to a discovery of a unique group of microporous films. Mesoporous, membranes with pore size ranging from 2 to 25 nm can be produced via melt processing and post-extrusion treatments in the absence of solvents. The membrane structures obtained by this process are expected to be extremely useful for applications such as ultrafiltration and battery separators. The films have transport and mechanical properties that are suitable for membrane processes that operate at 2 to 10 bars and are expected to be used in relatively high temperature environments for liquid and gas separations.
The fabrication process developed here is shown to be a promising technique for producing mesoporous and microporous membranes. The process has several potential advantages over other membrane fabrication processes, i.e.: no solvents are required; high production rate resulting in lower production costs; inexpensive polymers can be used as starting materials.
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