Mechanisms of structure and performance controlled thin film composite membrane formation via interfacial polymerization process

Kinetics and mechanisms involved in the formation of crosslinked polyamide (PA) membrane from p-phenylenediamine (PPD) and trimesoyl chloride (TMC) grown within a porous polysulfone substrate via interfacial polymerization (IP) process were studied through weight gain, permeability and selectivity of the membrane. A modified diffusion- and reaction-controlled model was derived, in which membrane structural parameters including apparent wet membrane density (ρ), dry polymer density (ρp), the number of functional groups on polymer chains (NCOCl) and polymer molecular weight (Mw), were correlated with the kinetics of thin film composite (TFC) membrane formation. According to the change of the main inhibition reaction rate constant as function of these four variables, a multi-stage membrane formation mechanism was developed. It elucidated many unique phenomena induced by this IP process, such as the primary membrane formation caused by polymer precipitation, the membrane perpendicular growth along water–oil interface, the crosslinking reaction, the “self-limiting” tendency and the limited membrane thickness. Effects of these phenomena and post-treatment on membrane structure and performance were discussed. It was shown that the permeability and selectivity of PA membrane can be effectively controlled by reaction time and the TMC and PPD concentrations. This study extended existing theories and techniques regarding IP process and structure–performance control of TFC membrane.