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Synthesis and Properties of Sulfonated Poly(Arylene Ether Ketone) Proton Exchange Membrane Mateirials

Author: ZhangHaiBo
Tutor: JiangZhenHua
School: Jilin University
Course: Polymer Chemistry and Physics
Keywords: Proton exchange
Type: PhD thesis
Year: 2005
Downloads: 302
Quote: 8
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The proton exchange membrane is a key component in proton exchange membrane fuel cell (PEMFC), which acts as a separator to prevent mixing of reactant gas and provides ionic pathways for protons transport. The most commonly used proton exchange membranes are perfluorinated copolymers with pendant sulfonic acid group, such as Nafion (Dupont), Dow membrane (DOW), which combine high conductivity and excellent chemical stability. However, the high price and limited operating temperature have prevented the perfluorinated sulfonated membranes applications in large scale. In addition, the high methanol permeation restrains their use in DMFC. Many efforts have been spent on searching for new alternative materials during past years. Among these materials, sulfonation of aromatic polymers is one of the main approaches due to their excellent thermal and mechanical properties as well as their resistance to oxidation and stability under acidic conditions. In addition, the organic-inorganic composite membranes represent one way to improve the PEM properties. Heteropolyacids are one of the most attractive inorganic modifiers because of their high proton conductivity and thermal stability. In our work, we prepared some novel sulfonated poly(arylene ether ketone) copolymer materials by molecular design, and investigated the relationship between structure and property in the view of the introduction of heteropolyacid, the effect of bulkyside groups containing fluorine and the sulfonate position of copolymers. Firstly, poly(arylene ether nitriles), as one class of engineering thermoplastics, possess excellent heat and chemical resistance. The cyano groups on the aromatic rings may interact with other functional groups through polar interaction,which facilitates them composite with some inorganic particles and blended with other functional polymer materials. Here, we present the synthesis and characterization of a series of high molecular weight, sulfonated poly(ether ether ketone) containing aromatic nitriles (SPAENK) with different sulfonation degree by direct copolymerization and prepared the HPA (phosphotungstic acid)/SPAENK composite membranes. All the coplolymers exhibited high intrinsic viscosities and well membrane formation, which indicated obtained copolymers with high molecular weight. The copolymer structure and sulfonate degree were determined by 1HNMR and FTIR. IR spectrum also indicated that the HPA particles interacted primarily with the sulfonic acid groups. The HPA/SPAENK40 composite membranes were tough and transparent, and the SEM image indicated that the HPA particles homogenously distributed within the polymer matrix. Though the introduction of HPA to sulfonate polymer reduced the stabilities of sulfonate groups, which may be due to the high acidity of HPA, the composite membranes were stable below 200℃satisfying the requirement of Fuel Cell working conditions. The water uptake and proton conductivity of SPAENKs increased with the rise of DS, temperature and relative humidity, which were consistence with the morphology analysis of TEM on the membranes. The membranes of highly sulfonated SPAENKs (SPAENK50 andSPAENK60) showed good proton conductivity, but their excessive swellings at high temperature limited their practical use. The proton conductivities of the composite membranes were higher than that of SPAENK and comparable with Nafion 117. Furthermore, the composite membranes showed lower conductivity decrease with temperture than pure SPAENK and Nafion117 when temperature above 100℃. However, the extraction of HPA particles from polymer matrix was a fatal problem for the practical use of HPA/sulfonated polymer composite membranes. Secondly, from the analysis of polymer molecular design, the introduction of bulky side groups will interrupt the close ‘pack’of the polymer chains and increase the space between them, which will favor the water retention and increase the connections of the ionic domains in the sulfonated copolymers. Here, we introduced fluorinated and nonfluorinated bulky pendants into sulfonated poly(ether ether ketone) by direct copolymerization of bisphenol monomers containing bulky side groups with different bifluoro-monomers and three series of sulfonated poly(ether ether ketone)s with different sulfonation levels were obtained and their properties were also explored. Three novel series of sulfonated poly(ether ether ketone)s containing bulky side groups with different sulfonate degree (DS) were synthesized directly by aromatic nucleophilic polycondensation reactions. The DS of the copolymers was controlled by varying the feed ratio of sulfonated monomer to unsulfonated monomer. These copolymers had high intrinsic viscosities and could be cast into tough and flexible films, which indicated the high molecular weight of the obtained copolymers. The structure and DS of the sulfonated copolymers were determined using FTIR and 1HNMR spectroscopy. The consistence of the experimentally measured DS values and the theoretical values indicated that the DSof copolymers were well controlled. All the copolymers exhibited good thermal stabilities (about 300℃in air). The water uptakes and proton conductivities of these sulfonated copolymers increased with the rise of DS and temperature. The membranes of highly sulfonated copolymers such as SFPEEK60 showed good proton conductivity (0.13S/cm, 80℃), especially at high temperature, which were comparable with that of Nafion117(0.13S/cm, 80℃), but their excessive swellings at high temperature restrained their practical use. Combined with the proton conductivity, water uptake and mechanical strength, the SFFPEEK40 and SFPEEK40 were more preferable for use as PEMs. The introduction of bulky side groups into sulfonated copolymers had great effects on their water uptakes and proton conductivities, which favored the copolymers to exhibit high water uptakes and proton conductivities, especially at high temperature. Though the incorporation of fluorine to pendant groups of sulfonated copolymers resulted in relative lower water uptake and conductivity, which could be explained in the view of the morphology of membranes by TP-AFM, this also decreased the water swelling and restrained the loss of mechanical strength due to excessive water sorption. These novel materials showed potential used as the proton-exchange membranes for fuel cells and further research on the performance of these films in a fuel cell is also ongoing in our group. The introduction of sulfonate groups on the pendants of polymer can decrease the restriction of the motion of the sulfonate acid, which will facilitate the formation of ‘ionic channels’in the polymer membranes. So, the study of sulfonated copolymers with sulfonate groups on the polymer pendants has absorbed much attention. However, due to the difficulty of synthesis, there have been relative fewer reports on

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