Improving Fabrication and Analyzing Gas Separation Behaviors of Polyamide Membranes Made Using Interfacial Polymerization of M-Phenylenediamine and P-Phenylenediamine.

Polyamide (PA) membranes made from the interfacial polymerization (IP) of m-phenylenediamine (MPD) with trimesoyl chloride (TMC) are popular for applications such as nanofiltration and water desalination, with some research in CO₂ capture. Despite the conventional fabrication of those membranes involving MPD, little research has gone into the fabrication and testing of one of its isomers: P-phenylenediamine (PPD). PPD has exhibited similar performance when used instead of MPD, typically having slightly lower rejection and higher permeation in both reverse osmosis (RO) and gas separations. Due to the para-orientation, PPD has higher reactivity than MPD during polymerization. This increase in reactivity usually does not affect the fabrication process; however, in studies where larger acyl chlorides were used to enhance flexibility and improve flux, PPD-fabricated membranes contained fewer defects. This resulted in a significantly better reported rejection than in MPD-fabricated membranes. During fabrication, the PPD solutions turned orange as time progressed, indicating the PPD was decomposing in the light. After about a day, the PPD-fabricated membranes and their soaking solution change color to a dark purple/black color, indicating the oxidation of the PPD monomer. The membranes themselves also changed color from blue, to purple, indicating this affected the PA as well. Due to its noticeable reaction in light and oxygen, PPD solutions and membranes were then stored in dark, air-tight environments. Because the PPD monomer can react with the environment and contaminate the membrane, future PPD-fabricated membranes were stirred in a vial of DI water and a magnetic stir bar on high intensity for 5 minutes before storing to encourage the leaching and separation of excess monomer from the membrane after fabrication. The permeance of CO2 in the PPD-membranes proved to be close to MPD-fabricated membranes. Since the MPD-fabricated membranes exhibited no color change while in solution, the procedure of the MPD-fabricated membranes did not change; however, excess MPD on fabricated membranes and reactions with the surrounding environment that do not change the color of the solution could still affect the performance of the membranes.