(27c) Functionalization of Polybenzimidazole Membranes for Pore Size Reduction, Increased Hydrophilicity and Surface Charge

Forward osmosis involves the movement of water across a selectively permeable membrane.  Unlike pressure driven processes, such as reverse osmosis (RO), the driving force for water permeation through the membrane in FO is the difference in osmotic pressure between the feed and draw solutions.  Recently, a hollow fiber nanofiltration (NF) forward osmosis membrane made of polybenzimidazole (PBI) was developed by researchers in Singapore.  PBI is a material with outstanding chemical resistance and thermal and mechanical stability.  PBI is a promising material for FO membrane development due to its ability to generate high water flux in FO processes.  The drawbacks with PBI include low salt rejection in the virgin state and hydrophobicity. 

This study focused on functionalization of PBI membrane surfaces using several different modifying agents, with the goal of reducing pore size, increasing hydrophilicity and imparting a surface charge.  The surface charge was expected to yield an increased rejection of ions and charged particles in the feed solution.  The modifying agents chosen for functionalization included: taurine, ethylene diamine, para-phenylene diamine and poly(acrylamide-co-acrylic acid).  These modifying agents were selected for their potential to increase hydrophilicity and impart a charge on the PBI surface, when in near neutral pH environments. 

Membranes were characterized using several analytical techniques to verify that modification was successful.   Fourier transform infrared spectroscopy in attenuated reflectance mode (FTIR-ATR) was used to detect changes associated with each of the modifying agents.  Zeta potential measurements were taken to detect changes in the membranes surface charge.  Environmental scanning electron microscopy (ESEM) was used to detect any changes in morphology of the membranes during and after modification.  Contact angle measurements were taken to determine changes in hydrophilicity.  Lastly, chemical force microscopy (CFM) analysis was used to compare changes in hydrophilicity and charge characteristics.  Functionalization, surface charge, and increased hydrophilicity were all verified.  In addition, transport characteristics were evaluated for all membrane modifications and virgin samples.