Controlled Swelling of Monolithic Films as a Facile Approach to the Synthesis of UHMWPE Membranes

A new method of fabricating porous membranes based on ultra-high molecular weight polyethylene (UHMWPE) by controlled swelling of the dense film was proposed and successfully utilized. The principle of this method is based on the swelling of non-porous UHMWPE film in organic solvent at elevated temperatures, followed by its cooling and further extraction of organic solvent, resulting in the formation of the porous membrane. In this work, we used commercial UHMWPE film (thickness 155 μm) and o-xylene as a solvent. Either homogeneous mixtures of the polymer melt and solvent or thermoreversible gels with crystallites acting as crosslinks of the inter-macromolecular network (swollen semicrystalline polymer) can be obtained at different soaking times. It was shown that the porous structure and filtration performance of the membranes depended on the swelling degree of the polymer, which can be controlled by the time of polymer soaking in organic solvent at elevated temperature (106 °C was found to be the optimal temperature for UHMWPE). In the case of homogeneous mixtures, the resulting membranes possessed both large and small pores. They were characterized by quite high porosity (45-65% vol.), liquid permeance of 46-134 L m^-2 h^-1 bar^-1, a mean flow pore size of 30-75 nm, and a very high crystallinity degree of 86-89% at a decent tensile strength of 3-9 MPa. For these membranes, rejection of blue dextran dye with a molecular weight of 70 kg/mol was 22-76%. In the case of thermoreversible gels, the resulting membranes had only small pores located in the interlamellar spaces. They were characterized by a lower crystallinity degree of 70-74%, a moderate porosity of 12-28%, liquid permeability of up to 12-26 L m^-2 h^-1 bar^-1, a mean flow pore size of up to 12-17 nm, and a higher tensile strength of 11-20 MPa. These membranes demonstrated blue dextran retention of nearly 100%.