The dependency of diamine diffusivity and diamine-acyl chloride reactivity on film formation has limited to the available monomers and crosslinkers to a small group of molecules. While polyesters, polyimides, polyureas and few other polymers can be synthesized via interfacial polymerization; these monomers have not historically produced high performance water treatment membranes. And while many different monomers were considered, virtually all polyamide water treatment membranes are made by a reaction between m-phenylene diamine (MPD) and trimesoyl chloride (TMC) for RO membranes or piperazine and TMC for nanofiltration membranes.
This limitation results in a performance gap in membrane applications. Current commercial membranes are divided into a group of very high sodium chloride (NaCl) rejecting RO membranes and low to medium NaCl rejecting NF membranes while a performance gap exists especially between mild to high NaCl rejection . This in turn narrows down the growth opportunities for emerging membrane applications where these mid to high NaCl rejection levels are needed.
This deficit of available monomers is tied to the manufacturing process which also limits the control over thickness due to its rapid polyamide formation. For the last decade or so, few new manufacturing approaches have been suggested to overcome one or more of these issues. Support free interfacial polymerization, molecular layer-by-layer formation, spin coating and molecular layer deposition has been some of those. EAIP) is another one of those emerging methods.
This study focuses on using electrospray assisted interfacial polymerization (EAIP) as a means of expanding the monomers available to use for TFC fabrication. Monomers reported in this study are ethylenediamine (EDA) -an aliphatic diamine-, o-phenylene diamine (OPD) and p-phenylenediamine (PPD) -structural isomers of MPD-, and bulky fluorinated monomers like 5-trifluoromethyl-1,3,phenylenediamine and 1,2-diamino, 4-5-difluorobenzene. These fluorinated diamines have never been used before for membrane fabrication and are shown to be able to form polyamide by molecular dynamics simulations which are later verified experimentally.
EAIP atomizes the solvents into small droplets and help distribute the monomers homogenously and enables them to react on much smaller interfaces. So far, it has been shown to successfully fabricate thin film composite materials with controlled thickness. In addition to controlling the membrane thickness by adjusting the amount of deposited material, it can also allow membrane chemistry adjustments by enabling more monomers to be used. As it is decoupling the interfacial polymerization from diffusion limitations, low diffusivity monomers or low reactivity monomers become available to use.
In this study, we explore a wide variety of monomers that are not commonly used in interfacial polymerization, including o-phenylene diamine (OPD), p-phenylene diamine (PPD), aliphatic diamines like ethylene diamine (EDA), or even bulky diamines like 5-trifluoromethyl-1,3,phenylenediamine and 1,2-diamino, 4-5-difluorobenzene.
Through using these monomers or various blends of monomers as diamine alternatives, we show that EAIP can be used to fabricate membranes to fill performance gaps that exist in current conventional membranes offerings.