Controlled Postassembly Functionalization of Mesoporous Copolymer Membranes Informed by Fourier Transform Infrared Spectroscopy

The need for nanofiltration membranes with tailored selectivity and reduced fouling propensity has led to the development of a multitude of processes that incorporate targeted functionality on membrane surfaces. These processes are often time-consuming, and additional development is needed to realize functionalization protocols that are compatible with roll-to-roll membrane fabrication processes. This work investigated the use of copper-catalyzed azide–alkyne cycloaddition (CuAAC) click reactions whereby a reactive solution containing an alkynyl-terminated molecule is deposited on a copolymer membrane with azide-functionalized surfaces. The progress of the reaction was quantified by using Fourier transform infrared spectroscopy to monitor the decreasing intensity of the peak associated with the azide groups as a function of time. The rate of membrane functionalization was controlled through the concentrations of the copper catalyst and alkynyl-terminated molecule in the reactive solution. Reactive solutions containing 0.1 mol of CuSO4 per mole of alkynyl-terminated reactant (0.1 Cu:Alkyne) resulted in slow reaction rates whereby diffusion created a uniform concentration profile of the alkynyl-terminated molecule across the membrane thickness. These conditions resulted in full conversion of the azide groups in 10–15 μm thick membranes within 5–25 min. For reactive solutions with a value of 0.3 Cu:Alkyne, the nearly instantaneous reaction rate resulted in a reactive front that penetrated into the membrane to achieve full conversion in 60–180 s. Energy dispersive X-ray spectroscopy was used to visualize the progression of the reactive front. Moreover, a model that predicts the penetration of the reactive front, and hence the percent conversion of the azide moieties, as a function of the reactive solution formulation and processing time was developed and validated. These findings offer a controlled functionalization protocol with reactive front velocities that are compatible with roll-to-roll processing rates.