Development of novel proton conducting polymer membranes and investigation of their structure-property relationship

Polymer electrolyte membrane fuel cell (PEMFC) is considered as the most promising fuel cell techniques for automotive and portable power applications due to its simplicity in design and operational properties. PEM is the key element of PEMFC, as the performance of fuel cell is directly affected by membrane properties such as high proton conductivity and durability. Due to the high cost of production of NafionTM and environmental concerns resulting from its high fluorine content, it is of need to design novel membrane materials. In this study, multi-arm star block copolymer, star block and graft block ionomers were synthesized. The properties of ionomers as proton conducting membrane and their PEMFC performances were defined. Although free radical polymerization has been widely used in materials synthesis, it lacks effective control over the compositions and functionalities as do ionic polymerizations. Controlled/living radical polymerization (C/LRP) methods including atom transfer radical polymerization (ATRP), nitroxide mediated radical polymerization (NMP), and reversible addition-fragmentation chain transfer polymerization (RAFT) are versatile synthetic tools that enables the preparation of new (co)polymers with precisely controlled molecular weight, relatively low dispersities, different composition (block, graft, alternating gradient copolymers), and diverse functionalities. The term "click" refers to versatile, efficient, specific and energetically favored chemical reactions, which could become universal tools first in synthetic chemistry lately extremely popular in polymer and materials science. Click reactions, in particular, Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction and Diels-Alder reaction, have recently shown to be extremely powerful tools for advanced macromolecular design. In this thesis, novel proton conducting polymers with different well-defined macromolecular architectures were successfully synthesized using controlled/ "living" radical polymerization methods particularly ATRP, click chemistry and post-sulfonation reaction. The synthesized structures are multi-arm star and star block copolymer ionomers and graft block copolymers possessing non-fluorinated ionic and fluorinated blocks, which are all tailored to achieve nano- and micro-scopic phase separation of ionic and hydrophobic domains. In the first study, multiarm star polystyrene (PS) and multiarm star polystyrene-b-poly(2,2,3,3,3-pentafluoropropyl methacrylate) (PS-b-PFPMA) block copolymers with the average number of arms 15, 11 and 6 were successfully prepared via ATRP and Diels-Alder (DA) click chemistry based on arm first approach. Second, 3-arm star PS (PS)3, 3-arm star PFPMA (PFPMA)3 and 3-arm star (PFPMA-b-PS)3 block copolymers were synthesized by using ATRP method based on core first approach. The third step of this study was to the preparation of poly(2,2,3,3,3-pentafluoropropyl methacrylate)-b-poly(9-anthracenylmethyl methacrylate)-g-(polystyrene) (PFPMA-b-PAnMMA-g-PS) graft block copolymer by one pot CuAAC and Diels–Alder click reactions. All polymers were converted to their ionomers by sulfonation of phenyl ring of polystyrene block. The preparation of all proton conducting the membranes from sulfonated multiarm star block copolymer, 3-arm star block and graft block copolymer and its sulfonated poly(phenylene oxide) (SPPO) blends carried out using solvent casting method. Blend membranes were prepared in order to increase the flexibility of the membranes. Characterization of membranes were performed to investigate the effect of the arm number and sulfonic acid content in terms of ionic conductivity, water uptake, thermal and chemical stability. Moreover, morphology of star, star block ionomers and membranes were characterized by Atomic Force Microscope (AFM), Small Angle X-ray Scattering (SAXS) and Scanning Electron Microscopy (SEM) to examine the effect of different IECs, different number of arms, hydration of the membranes and the molecular weight of star block ionomers on the structure-property relationship. Finally, polymer electrolyte membranes with blend type of 3-arm star and graft block copolymer ionomer with SPPO having desired properties including more flexibility and higher proton conductivity were selected and characterized in their single-cell proton exchange membrane fuel cell (PEMFC) performances.