Real-time monitoring of interfacial polymerization using fluorescent dyes

Polyamide (PA) thin film composite membranes are the commercial standard for nanofiltration and reverse osmosis. Establishing their synthesis-structure-performance relationships (SSPs) is imperative for rational membrane design. Complementary to detailed post-manufacturing characterization, real-time measurements during interfacial polymerization (IP) are key to understand the SSPs. A new method for in situ characterization of IP was developed by combining simple droplet-based experiments and fluorescence microscopy. Simultaneously, real-time data from droplet- and microfluidic-based experiments were demonstrated to agree. Eight commercial and tailormade dyes were tested and the associated pitfalls in the IP system were identified. Four main conclusions are drawn. First, pH measurements show a pH drop from ∼9 to a minimum of ∼5.3 within 70 ms after initiation of IP. Second, dye partitioning can be exploited to probe densification of the forming PA film. Third, within the first 70 ms of IP, a diffusion-limiting barrier for molecules >260 g mol−1 is formed. Yet, the film requires 30–40 s to acquire its full rejecting potential. Fourth, overlaying in situ and performance data supports the proposed SSP for varying TMC concentration, based on step-growth polymerization kinetics. Testing other fluorescent markers and synthesis conditions will expand the opportunities generated by in situ monitoring using fluorescence microscopy to establish SSPs.