The present invention relates to composite filtration membranes, methods of their preparation, and more specifically to crosslinked poly(meth)acrylates for selective layers of composite filtration membranes used in water purification.
Fluid filtration membranes include microfiltration (MF), nanofiltration (NF), reverse osmosis (RO), and ultrafiltration (UF) membranes. Nanofiltration membranes are mainly used for water softening and removal of organic contaminants. Nanofiltration is also used in the food industry for concentration and demineralization of whey, as well as for the concentration of sugar and juice. Nanofiltration membranes are usually fabricated by interfacial polymerization between i) a piperazine or an amine substituted piperidine or cyclohexane and ii) a polyfunctional acyl halide (Dow FILMTEC). Nanofiltration membranes are also made by coating ultrafiltration membranes with different polymer solutions. Nitto Denko has a commercial nanofiltration membrane containing a thin layer of polyvinyl alcohol coated on a porous polysulfone support. Nanofiltration membranes are also obtained by modifying reverse osmosis membranes (RO). In this process, a strong mineral acid such as phosphoric acid is contacted with a crosslinked polyamide layer at 100° C. to 150° C., which is followed by a treatment with a “rejection-enhancing agent” such as tannic acid to selectively plug microscopic leaks and defects.
Although many advances have been made in recent years for making commercial filtration membranes, improved solvent resistance, adhesion, and/or fouling resistance to organic foulants and biofoulants remain significant challenges. To overcome the fouling problem in membranes, several research groups synthesized new poly(ethylene glycol) (PEG) based copolymers, which were shown to be useful as high-flux nanofiltration membranes exhibiting low biofouling. The PEG graft copolymers are not crosslinked and hence adhesion to the support membrane is an issue when used for longer timescales.
Other thin film composite (TFC) nanofiltration (NF) membranes based on poly(vinylidene fluoride)-graft-poly(oxyethylene)methacrylate (PVDF-g-PEOM) have been used as a selective layer for water purification. The poly(oxyethylene) grafts were grown from the poly(vinylidene fluoride) backbone, using fluorine as an initiator for atom transfer radical polymerization (ATRP). Although these materials act as efficient selective layers for thin film composite NF membranes, they are expensive to manufacture. ATRP utilizes a copper-based catalyst, which is removed using several purification steps in the purification of the graft/comb copolymer.
Other polymer systems based on poly(acrylonitrile-graft-ethyleneoxide) PAN-g-PEO have also been used as selective layers for thin film composite NF membranes, but these membranes have to be cast on the UF support membrane by a phase-inversion method. Moreover, the selective layer is a thermoplastic polymer that offers no means of crosslinking to improve the toughness, adhesion and solvent resistance of the separation membranes.
PEO based graft copolymers have also been synthesized by ring opening metathesis polymerization (ROMP) of cyclooctene based monomers. These polymers have phenylazide based moieties that undergo crosslinking in the presence of UV light. A drawback of this system is the synthetic challenge of designing new exotic PEO and phenylazide based cyclooctene based monomers. Another drawback is the explosive nature of the azide functional group, rendering the compounds unsuitable for large scale manufacturing.
Consequently, a need continues for filtration membranes having improved mechanical and/or performance properties, which can be prepared using materials and methods suitable for large scale manufacturing.