Proton conducting grafted/crosslinked membranes prepared from poly(vinylidene fluoride-co-chlorotrifluoroethylene) copolymer

Poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF-co-CTFE)) backbone was grafted with crosslinkable chains of poly(hydroxyl ethyl acrylate) (PHEA) and proton conducting chains of poly(styrene sulfonic acid) (PSSA) to produce amphiphilic P(VDF-co-CTFE)-g-P(HEA-co-SSA) graft copolymer via atom transfer radical polymerization (ATRP). Successful synthesis and microphase-separated structure of the copolymer were confirmed by ¹H NMR, FT-IR spectroscopy, and TEM analysis. Furthermore, this graft copolymer was thermally crosslinked with sulfosuccinic acid (SA) to produce grafted/crosslinked membranes. Ion exchange capacity (IEC) increased continuously with increasing SA contents but the water uptake increased up to 6wt% of SA concentration, above which it decreased monotonically. The membrane also exhibited a maximum proton conductivity of 0.062S/cm at 6wt% of SA concentration, resulting from competitive effect between the increase of ionic groups and the degree of crosslinking. XRD patterns also revealed that the crystalline structures of P(VDF-co-CTFE) disrupted upon graft polymerization and crosslinking. These membranes exhibited good thermal stability at least up to 250°C, as revealed by TGA.