Preparation And Characterization Of Polymer-based Homogeneous Ion Exchange Membrane As Well As Ion Transport Therein

Ever since last century,ion exchange membranes and related technologies have found many applications in water treatment,environmental protection,fuel cells,and cleaner production.To meet the requirements for application in rigorous environment,ion exchange membranes with desired properties,such as high permselectivity,low electrical resistance,good mechanical and chemical stability, need exploring.With regard to anion exchange membranes,the traditional preparation method involves a much complicated process—chloromethylation,in which the commonly used chloromethyl methyl ether(CME) and bis-chloromethylether(BCME) are now considered carcinogens.Therefore,it is necessary to develop new routes to prepare anion exchange membranes without such chloromethylation process.As for cation exchange membranes, mostly-perfluorinated sulfonic acid ionomers,such as Nafion^? series,have been wildly used due to their high proton conductivity and substantial durability. Nonetheless,the extremely high manufacturing cost,large fuel permeability,and poor proton conductivity at elevated temperature restricts perfluorinated membranes from wide industrialization.Therefore,the development of alternatives from inexpensive materials is urgent,and these cation exchange membranes should resist high temperature,oxidants,solvents,and degradation.Apart from the manufacture cost,fundamental research on the construction of ion exchange membranes should not be ignored.Only upon a better understanding of the relationships between membrane structure and properties,membranes with better performances can be developed.In view of those considerations above,a high-quality and low-cost polymer material was prepared through Friedel-Crafts reaction,and a route without traditional chloromethylation process was developed for preparation of anion exchange membrane from such polymer material.The superior performance of such membrane indicates its potential industrial applications.When this polymer material was blended with another functional material,a new membrane with special inter-molecular crosslinking structure was obtained.During the membrane preparation,no additional crosslinking agent was needed but self crosslinking occurred.Such special crosslinked structure imparted the final membrane excellent physical and chemical stability and led to an orderly arrangement of functional groups for OH^- transfer.Taking use of this inter-molecular crosslinking,a new cation exchange membrane was successfully prepared from a semi-crystalline perfluorosulfonic polymer and amorphous poly(ether-ketone).This membrane has a multi-phase structure and combines the advantages of perfluorosulfonic polymers and aromatic ion-exchange membranes;moreover,this membrane exhibited physicochemical stability comparable to that of Nafion^?-112 membranes and higher proton conductivity than the latter.Upon this excellent performance,a unique transfer mechanism was proposed,which can provide theoretical guidance for development of ion exchange membranes with high proton conductivity at low water contents.The main conclusions based on this research are as follows:(1) Friedel-Crafts chloroacetylation,instead of chloromethylation,can be used to prepare anion exchange membranes from a linear engineering plastics polymer, Poly(2,6-dimethyl-1,4-phenylene oxide)(PPO).According to the membrane forming of PPO,the chloroacetylation degree should be control around 50%.To achieve the best electrochemical properties,the optimum reaction conditions are as follows: amination time=48-50 hours,amination concentration=0.91mol/l,and amination temperature=35-45℃.Under these conditions,the membrane exhibited an ion exchange capacity[IEC]of 1.15mmol/g dried membrane,water content of 0.4-0.6 g/g wet membrane,and area resistance up to 0.2Ωcm².(2) An anion exchange membrane with partially inter-crosslinked network was prepared from the blend of chloroacetylated poly(2, 6-dimethyl-1,4-phenyleneoxide)(CPPO)/bromomethylated poly (2,6-dimethyl-1,4-phenylene oxide)(BPPO) by a Friedel-Crafts reaction without addition of any crosslinking reagent or catalyst.The inter-crosslinking structure is different from conventional 3-dimension interpenetrating network,and the crosslinkages are immobilized and dispersed uniformly inside membrane. Consequently,it could enhance the membrane's flexibility and stability significantly; moreover,the membrane exhibited excellent hydroxyl conductivity:0.032 S cm^-1 at 25℃.Hence,the membrane is qualified for application in low temperature direct methanol alkaline fuel cells(DMAFCs)application.(3) A novel cation exchange membrane was prepared from the blend of sulfonated poly(phthalazinone ether sulfone kentone)(SPPESK) and perfluorosulfonic polymer(FSP) via acid-base interactions.This membrane exhibited physicochemical stability comparable to that of Nafion^?-112 membranes and higher proton conductivity than the latter.(4) Inside the FSP/SPPESK blend membrane,there exists a multi-phase structure:hydrophobic electrostatically-interacted acid-base pairs constitute the interlayers between two hydrophilic phases(FSP and SPPESK).The multi-phase membrane,due to these acid-base pairs,exhibited good phase compatibility,and excellent thermal and dimensional stability.Notably,a channel is constructed throughout the interlayer,and protons transport directly through these hydrophobic zones by hydrogen bonds forming and breaking between acid-base pairs and water molecules.(5) The morphology of different phases in the membrane changed significantly after annealing treatment.In FSP phase,crystalline degree increased significantly after annealing treatment;meanwhile,acid-base reaction occurred in the SPPESK phase.The recrystalization and acid base reaction induced by annealing are both dependent on temperature.Therefore,it is feasible to prepare desired membranes by controlling the annealing conditions.