Deformation and reinforcement of thin-film composite (TFC) polyamide-imide (PAI) membranes for osmotic power generation

To understand the deformation of thin-film composite (TFC) membranes in high-pressure pressure retarded osmosis (PRO) processes, TFC membranes have been fabricated by interfacial polymerization upon Torlon^® polyamide-imide (PAI) porous substrates. Experimental results show that reinforcements can be achieved by controlling the chemistry and morphology of the substrate and TFC polyamide layer. The PAI substrate exhibits reinforced properties and performance by pre-compression and polydopamine (PDA) cross-linking modification, while the interfacial properties between the substrate and TFC layer are improved by the adhesive PDA cushion layer. The responses of the modified PAI substrate under high pressures have been examined in terms of structural compaction and permeation decline. When subjected to a hydraulic pressure of 14bar for periods up to 1600min, the PDA modified substrate exhibits a better mechanical stability and less deformation. The PDA modified substrate shows a steady-state water permeability that doubles the steady-state value of the unmodified substrate. In addition, casting membranes on polyester fabrics and post-treatments by chloride and ethanol show positive effects on membrane performance in PRO processes. As a result, the modified TFC membrane achieves a 3-fold increment in water flux and a 16-fold rise in power density (2.84W/m² vs. 0.16W/m²) as compared with the unmodified TFC membrane. Positron annihilation spectroscopy (PAS) has been employed to investigate membrane morphological changes after high pressure PRO processes. The high hydraulic pressure mainly deforms the porous substrate but has little effect on the TFC dense layer. These results may provide important insights into the hydraulic characteristics of TFC membranes and are essential to the design of PRO membranes for osmotic power generation.