Synthesis and Characterization of Poly(vinylidene fluoride)-g-sulfonated Polystyrene Graft Copolymers for Proton Exchange Membrane

Abstract

A series of poly(vinylidene fluoride)-g-sulfonated polystyrene (PVDF-g-SPS) graft copolymers were systematically synthesized and examined with the focus of understanding how the polymer microstructure (backbone molecular weight, graft density, graft length, sulfonic acid concentration, ion exchange capacity, etc.) affects their morphology, water uptake, and proton conductivity under various environmental conditions (temperature and relative humidity). The synthesis of these relatively well-defined graft structures involves three reaction steps, including the preparation of PVDF copolymers containing a few mol% of chlorotrifluoroethylene (CTFE) units, the ATRP graft-from reaction to incorporate several polystyrene side chains, and the subsequent sulfonation reaction on the PS side chains. The PVDF-g-SPS graft copolymer with a combination of a high PVDF backbone molecular weight (Mn >300 000 g/mol), very low SPS graft density (0.3 mol%), and high graft length (SPS content >30 mol%) presents an interesting material for the proton exchange membrane (PEM). This graft copolymer self-assembles into a microphase-separated morphology with randomly oriented long-range lamella/cylinder ionic channels (with small widths) that are imbedded in the hydrophobic semicrystalline PVDF matrix. This graft copolymer morphology offers a high ion exchange capacity (IEC = 2.75 mmol/g) and resistance to excessive water swelling, which yields notably higher proton conductivity than Nafion under 30−120 °C and high humidity conditions.