Synthesis Of Proton Conducting Electrolytes Based On Fluoropolymers And Polynorbornene Derivatives Via Atom Transfer Radical Polymerization

Polymer electrolyte membrane fuel cells(PEMFCs) and direct methanol fuel cell (DMFC) have been identified as the promising sources for transport,stationary,and portable applications due to their low emissions and high conversion efficiency. Polymer electrolyte membrane(PEM) is one of the most critical components for PEMFC and DMFC systems.PEM widely used in PEMFC are perfluorosulfonic acid PEMs,such as DuPont's Nafion.Nation has high proton conductivity,excellent chemical and mechanical stability.However,high cost,poor barrier to methanol crossover and performance loss at high temperature or low humidity has limited its industrial applications.These limitations have stimulated worldwide research activities for developing new materials with high performance as proton exchange membrane.Fluoropolymers and Polynorbornene derivatives via addition polymerization have received much attention because of its excellent physical and chemical properties,such as high-temperature stability,excellent chemical resistance,were chosen to prepare the proton conducting polymer electrolyte membranes.Poly(vinylidene fluoride-co-hexafluoropropylene)(PVDF-HFP) grafted poly(styrene sulfonic acid)(PVDF-HFP-g-PSSA) and poly(sulfopropyl methacrylate) (PVDF-HFP-g-PSPMA) copolymers as proton conducting electrolytes by atom transfer radical polymerization(ATRP) of 4-styrene sulfonic acid(SSA) and sulfopropyl methacrylate(SPMA) at the secondary halogenated sites of PVDF-HFP is demonstrated.The structures of the PVDF-HFP-g-PSSA and PVDF-HFP-g-PSPMA copolymers were verified by Fourier transform infrared spectroscopy(FTIR) and proton nuclear magnetic resonance spectra(¹HNMR).The PVDF-HFP-g-PSSA copolymer membranes showed the IEC values ranging from 0.045 to 0.272 meq/g and the proton conductivities varying from 1.85×10(-4) to 9.8×10^-4 S/cm,all of which could be modulated by control of the polymerization time.The PVDF-HFP-g-PSPMA membranes showed the IEC values ranging from 0.05 to 0.59 meq/g and the proton conductivities varying from 2.58×10^-4 to 3.09×10^-3 S/cm,all of which could be modulated by control of the polymerization time.Polynorbornene derivatives(Poly(Norbornene-co-5-Norbornene-2-Methanol)) (PNB-OH) was prepared by the addition polymerization of norbornene(NBE) and 5-norbornene-2-methanol(NBE-CH₂OH) using ternary catalyst system of bis-(β-ketoamino) nickel(â…¡) in combination with co-catalyst B(C₆F₅)₃ and AlEt₃.The sulfonated polynorbornenes with sulfonic acid group in side chain were prepared by atom transfer radical polymerization(ATRP) of 4-styrenesulfonic acid sodium salt (SSA) or 3-sulfopropyl methacrylate(SPMA) from bromoisobutyrate polynorbornene (Poly[Norbornene-co-2-Norbornene-5-(2-bromine-2-methyl methacrylate)])(Br-PNB) which was synthesized by the reaction of PNB-OH with 2-bromoisbutyryl bromide. The structures of the Polynorbornene derivatives copolymers were verified by Fourier transform infrared spectroscopy(FTIR) and proton nuclear magnetic resonance spectra(¹HNMR).The PNB-g-PSSA and PNB-g-PSPMA showed the IEC values of 1.05 and 0.787meq/g,respectively.Consequently,the graft polymers based on polynorbornene proved to be promising as proton exchange membranes.