Analysis of Manufacturing Methods of Ion Exchange Membranes for Desalination

An emerging approach to membrane based desalination is to remove ions from water via electrodialysis. Membranes for this application need to be highly selective, and thin membranes can offer advantages by reducing mass transfer resistance. A recently reported additive manufacturing approach can enable access to ultrathin and highly selective polyamide-based membranes and could be adapted to prepare conductive ultrathin ion exchange membranes (IEMs) for electrodialysis. These materials could allow for more efficient ion transport and increased membrane selectivity. It is theorized that these polyamide-based IEMs can be manufactured and optimized using an electrospray technology to deposit monomers directly onto a supporting membrane. Once such experimental membranes are created, ion transport properties must be characterized.

Molecular additive manufacturing may enable the production of highly tunable, selective, low resistance ion exchange materials, and linear polyamide IEM analogs can be used as control materials to inform transport property characterization and analysis. Linear polyamide membranes composed of m-phenylenediamine (MPD), isophthaloyl chloride (IPC), and terephthaloyl chloride (TPC) can be produced via stirred interfacial polymerization. The addition of a sulfonated diamine comonomer is necessary because strong ion exchange groups are required to facilitate charge transport in electrodialysis applications. Sulfonated copolymers have been studied for their potential use in electric field-driven membrane processes, but the polymerization reaction used to prepare sulfonated polyamide copolymers is complicated by competition between the charged (sulfonated) and uncharged diamine comonomers. Yield calculations and FTIR analysis suggest that incorporation of sulfonate comonomers is indeed challenged by these factors. Ionic conductivity (determined using electrochemical impedance spectroscopy, EIS, and a direct current method) was measured to characterize the materials considered in this study and to provide insight into the properties, stability, and structure of the electrospray membranes. By analyzing the ionic conductivity properties of membranes produced by both linear polymerization and additive manufacturing, this presentation considers the viability of electrospray technology as a means to make ultrathin polyamide-based IEMs.