Liquid and Gas Permeation Studies on the Structure and Properties of Polyamide Thin-Film Composite Membranes

Abstract

This research was undertaken to improve the understanding of structure-property-performance relationships in crosslinked polyamide (PA) thin-film composite (TFC) membranes as characterized by liquid and gas permeation studies. The ultrathin PA selective layer formed by interfacial polymerization between meta-phenylene diamine and trimesoyl chloride was confirmed to contain dense polymer matrix regions and defective regions in both dry and hydrated states. The first part of this research studied the effect of non-selective convection through defective regions on water flux and solute flux in pressure-assisted forward osmosis (PAFO). Through systematic comparison with cellulose triacetate (CTA) and PEBAX-coated PA-TFC membranes, the existence of defects in pristine, hydrated PA-TFC membranes was verified, and their effects were quantified by experimental and modeling methods. In the membrane orientation of selective layer facing the draw solution, water flux increases of up to 10-fold were observed to result from application of low hydraulic pressure (1.25 bar). Convective water flux through the defects was low (< 1% of total water flux for PA-TFC membranes) and of little consequence in practical FO or reverse osmosis (RO) applications. However, it effectively mitigated the concentration polarization in PAFO and therefore greatly increased the diffusive flux through the dense regions. The second part of this research characterized the structures of the PA material and the PA selective layer by gas adsorption and gas permeation measurements. Gas adsorption isotherms (N2 at 77K, CO2 at 273K) confirmed the microporous nature of PA in comparison with dense CTA and polysulfone materials. Gas permeation through the commercial PA-TFC membranes tested occurred primarily in the defective regions, resulting in Knudsen gas selectivity for various gas pairs. Applying a Nafion coating layer effectively plugged the defects and allowed gas permeation through the dense PA regions, which significantly decreased gas permeance and increased gas selectivity. Specifically, high He and H2 selectivity against CO2 suggests the potential applications of this membrane in He recovery and CO2 capture in pre-combustion. Finally, the dense PA matrix was modified with two types of novel nanofiller to improve desalination performance in RO. A series of dense, nano-sized (1-3 nm) polyhedral oligomeric silsesquioxanes (POSS) with different functional groups were systematically incorporated into the PA matrix by physical blending or chemical fixation. The free volume of the PA matrix increased with addition of POSS, leading to water flux increases of up to 67 %, while maintaining high NaCl rejections. The effects of adding microporous, hydrophobic zeolitic imidazolate framework-8 (ZIF-8) nanoparticles into PA are presented in the last chapter. A 162 % water flux increase was achieved without decreasing NaCl rejection. This interesting result can be attributed to a less crosslinked PA structure and to the intrinsic desalination properties of ZIF-8.