Design and development of new polymer membranes for water filtration applications

The overall objective of this thesis research is to design and develop new polymer membranes that overcome several limitations that exist in conventional water filtration membranes. Two separate research thrusts were explored. In the first study, new polymer coatings for water ultrafiltration (UF) membranes were developed to reduce protein adsorption and fouling. Fifteen different functional monomers were synthesized, coated, and polymerized onto UF supports. Their resistance to protein adsorption and membrane protein fouling were then examined. For the first time, certain simple quaternary phosphonium- and ammonium-based polymer coatings were shown to be effective at resisting protein adsorption and membrane fouling. The second research objective is to design and develop new nanoporous polymer membranes with uniform, sub-1-nanometer pores for water purification via a size-exclusion mechanism. Cross-linkable lyotropic liquid crystals (LLCs) were examined due to their ability to self-organize into regular, porous nanostructures when mixed with water. Photo-radical cross-linking of these LLC assemblies resulted in robust polymer membranes with uniform nanopores. A 1st-generation LLC membrane containing a type I bicontinuous cubic (QI) phase structure and 3D-interconnected nanopores was made using a gemini phosphonium monomer. Comprehensive water filtration experiments and the use of a modified Donnan-steric pore model (DSPM) showed that this membrane has an effective pore diameter of 0.90 nm with a monodisperse pore size. It can remove small organic and inorganic solutes better than a nanofiltration (NF) membrane and almost as well as a reverse osmosis (RO) membrane. It also resisted chlorine degradation and protein adsorption. However, this monomer is difficult and expensive to synthesis and process. A 2 nd-generation QI-phase gemini ammonium-based monomer was developed to overcome these issues. Two homologues were found to form Q I-phase. Water transport studies using supported QI-phase membranes of the 2nd-generation monomer demonstrated their ability to rejected solutes at a similar level to the 1st-generation system. Additionally, they are significantly easier and less expensive to process and synthesize. Potentially, these nanoporous polymer membranes have applications in water desalination and NF. Future work will examine thin-film processing and controlling the pore diameter of these LLC membranes in the 0.5--5 nm range.