Phase inversion with gel/sol/gel-treatment — a new technology for the formation of hollow-fiber membranes at a wet spinning process

The preparation of hollow-fiber membranes with high asymmetric morphology by a wet spinning process by phase inversion of polymer solutions with a nonsolvent medium is normally connected with the formation of a relatively dense unwelcome skin. One possibility to eliminate this disadvantage is a phase inversion process with gel/sol/gel-treatment, whose physicochemical peculiarities are descibed in this paper. In the gel/sol/gel-treatment an initially wet hollow-fiber membrane is contacted with a solvent and subsequently with a nonsolvent again. Such a technology causes a second phase inversion at the outer surface of the hollow-fiber membrane. Results about parameters influencing the gel/sol/gel-process are presented as an illustration for the treatment of polyacrylonitrile hollow-fiber membranes with a special structure. In conclusion, first ideas of the mechanism of this process are formulated.