Nanofiltration membranes based on poly(vinylidene fluoride-co-chlorotrifluoroethylene)-graft-poly(styrene sulfonic acid)

Graft copolymers comprising poly(vinylidene fluoride-co-chlorotrifluoroethylene) backbone and poly(styrene sulfonic acid) side chains, i.e. P(VDF-co-CTFE)-g-PSSA were synthesized using atom transfer radical polymerization (ATRP) for composite nanofiltration (NF) membranes. Direct initiation of the secondary chlorinated site of CTFE units facilitates grafting of PSSA, as revealed by FT-IR spectroscopy. The successful "grafting from" method and the microphase-separated structure of the graft copolymer were confirmed by transmission electron microscopy (TEM). Wide angle X-ray scattering (WAXS) also showed the decrease in the crystallinity of P(VDF-co-CTFE) upon graft copolymerization. Composite NF membranes were prepared from P(VDF-co-CTFE)-g-PSSA as a top layer coated onto P(VDF-co-CTFE) ultrafiltration support membrane. Both the rejections and the flux of composite membranes increased with increasing PSSA concentration due to the increase in SO₃H groups and membrane hydrophilicity, as supported by contact angle measurement. The rejections of NF membranes containing 47wt% of PSSA were 83% for Na₂SO₄ and 28% for NaCl, and the solution flux were 18 and 32 L/m² hr, respectively, at 0.3 MPa pressure.