Investigations on the formation of porous membranes during non-solvent induced phase separation

Abstract

A common method for the production of membranes is immersion precipitation. For this, a polymer is dissolved in a suitable solvent and then precipitated by immersing it into a precipitation bath consisting of a proper non-solvent. Induced by an exchange between the solvent and the non-solvent phase separation occurs, which leads to the formation of a porous structure. The resulting structure allows the selective separation of a mixture of substances, which is based on a size exclusion mechanism. At the moment, the disadvantage of the commonly used solvents for membrane fabrication is their classification as being hazardous for humans and the environment. This is the reason why there is an increased interest in replacing the conventional solvents by less harmful alternatives. However, the challenge is that despite changing the solvent, the membrane properties must still be controllable. In order to ensure the required control of the resulting membrane properties by adjusting the influencing parameters, it is essential that the membrane fabrication process is well understood. In the first part of this work a method for the characterization of the thermodynamics of polymer solution phase equilibria was developed. In course of the validation of this method, it could be shown that the method provides reliable and reproducible data, which in comparison to the previously established method provides a higher information content. Furthermore, the method was applied for the characterization of a polymer solution system, which is commonly used for membrane preparation, and the results were compared to those of the previously used cloud point titration method. In the second part of this work a comparative study on the influences of polymeric additives on the membrane formation process and the resulting membrane properties was conducted. The focus was particularly laid on the comparison between conventional and alternative solvents in order to allow a substitution of hazardous solvents through less harmful alternatives. It could be shown that there are differences in the observed effects in dependence of the applied solvent. However, it could also be demonstrated that the gained mechanistic understanding can be used for adjusting the membrane preparation process, so that the properties of the fabricated membranes remain controllable. The third part of this work also focused on the influence of variations during the immersion precipitation process using different solvents. In order to obtain an overall picture of the influencing factors on membrane formation, the effects of non-solvent additives in the solution, of variations in the polymer concentration and of altered precipitation conditions were investigated. It could be shown that all three factors have an influence on the characteristics of the fabricated membranes in the investigated solvent systems. In addition, it was repeatedly demonstrated that the choice of the solvent is of high importance for the manifestation of the respective effects.