Sustainable Polyelectrolyte Complex Membranes Produced via Aqueous Phase Separation

Polymeric membranes are typically prepared using a technique known as the non-solvent induced phase separation (NIPS). In this technique, a polymer is first dissolved in an organic solvent such as N-methyl pyrrolidone (NMP), and immersed in a non-solvent bath which typically comprise of water. Due to the insolubility of the polymer in water, it precipitates as a solid porous membrane. The polymer solution thermodynamics and the membrane precipitation kinetics can be controlled to obtain membranes with desired morphology and separation performance. For decades, NIPS has been the primary production method to produce commercial polymeric membranes. However, the largest downside of this technique is its reliance on organic solvents like NMP which are reprotoxic, harmful for the environment, and therefore, unsustainable. Owing to these facts, the European Union has restricted the use of NMP and there are increasing demands to utilize more sustainable alternatives for polymeric membrane production. In this thesis, we alleviate this problem by utilizing a safe and sustainable solvent, i.e. water. We demonstrate that water can be used as a solvent and a non-solvent for polyelectrolytes depending on its pH. When two oppositely charged polyelectrolytes are mixed, they form a polyelectrolyte complex (PEC), which is typically insoluble in water. A weak and a strong polyelectrolyte can be mixed to obtain a homogeneous solution at high pH, where the weak polyelectrolyte is uncharged. Immersing such a solution in low pH bath causes the weak polyelectrolyte to acquire charge and form a polyelectrolyte complex with the strong polyelectrolyte. The newly formed PEC is insoluble in water and precipitates as a solid porous membrane. In this thesis, several tuning parameters such as the molecular weight of the polyelectrolytes, the polymer solution concentration, the polyelectrolyte mixing ratio, and the precipitation bath conditions were investigated to obtain membranes with desired morphology and separation performance. Such parameters provide similar control over the membrane structure as in NIPS. Sustainable membranes for micro-, ultra-, and nanofiltration applications were successfully produced utilizing this new aqueous phase separation (APS) technique. Additionally, the versatility of this new technique was established by successfully producing sustainable hollow fiber membranes. The research work presented in this thesis opens up a new field of sustainable membranes and hopefully will be a stepping stone for further research on green fabrication technologies.