Direct surface modification of poly(VDF-co-TrFE) films by surface-initiated ATRP without pretreatment

Abstract

The direct surface modification of poly(vinylidene fluoride-co-trifluoroethylene) (VDF-co-TrFE) copolymer films via surface-initiated atom transfer radical polymerization (ATRP) is presented. The surface-initiated ATRP of tert-butyl acrylate (tBA) and styrene was carried out on poly(VDF-co-TrFE) films containing 75 mol% VDF at 383 K. Such a reaction was monitored by ¹H and ¹⁹F NMR, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and by water contact angle measurements. First, the ¹⁹F NMR spectra of poly(VDF-co-TrFE)-g-poly(tBA) graft copolymers revealed a reduction in the signal intensity at −123 ppm compared with that of the poly(VDF-co-TrFE) copolymer, indicating that the polymerization of tBA occurred exclusively by fluorine abstraction from the TrFE units. ATR-FTIR spectra of the resulting poly(VDF-co-TrFE)-g-poly(tBA) and poly(VDF-co-TrFE)-g-polystyrene (PS) films evidenced the characteristic absorption frequencies assigned to carbonyl and aromatic C–H stretching. The atomic ratio on the surface of polymer-grafted poly(VDF-co-TrFE) film observed by XPS well agreed with theoretical value of poly(tBA) and PS hompolymers. These results indicated that poly(tBA) and PS were successfully grafted onto the poly(VDF-co-TrFE) film surfaces forming grafting layers, the thickness of which was over 10 nm. The poly(VDF-co-TrFE)-g-poly(tBA) film was subsequently treated with p-toluenesulfonic acid and sodium hydrogen carbonate to modify the grafted chains into poly(acrylic acid sodium salt). Surface-grafted polystyrene was also converted to poly(styrene sulfonic acid sodium salt) by treatment with sulfonic acid. Each reaction step was characterized by ATR-IR and XPS. The static water contact angle of poly(VDF-co-TrFE) copolymer was remarkably reduced from 91 to 15° by hydrolysis of the tBA chains, sulfonation of the styrene chains and their neutralization. A hydrophilic surface was successfully achieved on the poly(VDF-co-TrFE) film by direct surface-initiated polymerization from the outermost surface of the TrFE unit without any change in the bulk physical properties.