Kinetics of support-free interfacial polymerization polyamide films by in-situ absorbance spectroscopy

Understanding the kinetics of the interfacial polymerization (IP) reaction is one of the crucial factors in tailoring the characteristics and performance of reverse osmosis (RO) membranes. In this novel study, we report a kinetic investigation of polyamide (PA) film formation and growth by IP reaction between M-phenylenediamine (MPD) and trimesoyl chloride (TMC), in real-time, using the absorbance spectroscopy (AS) method. We investigated the effects of monomer concentrations, as well as that of IP reaction time for support free films, on kinetic behavior and film characteristics. The real-time results showed a significant effect of monomer concentration on the kinetics of reaction especially during the first few seconds of interfacial polymerization. We showed that the increase in thickness of film measured ex situ by transmission electron microscopy (TEM), follows similar trends to the in-situ absorbance with polymerization time, for this formulation. Furthermore, a quasi-linear trend between absorbance and film thickness, for formulations of TMC of 0.2 wt/wt% and MPD concentrations at or above 0.255 wt/wt%, was found, which was not apparent for lower MPD concentrations. This suggests that there may be a critical MPD concentration for a given TMC value that gives a ‘defect-free’ film; below this concentration a continuous (fully densified and defect free) film cannot form. This study confirms that the AS method, provides an opportunity to investigate the kinetics, predict the characteristics (structure) and optimize the IP conditions (monomer concentration ratio) to achieve RO and nanofiltration (NF) membranes with tailored performance.