| Proton exchange membrane fuel cell(PEMFC) is a kind of novel clean and efficient energy source. Its research and development application has obtained rapid developement in recent years. As the key component of PEMFCs, polymer proton exchange membrane(PEM)serves as an electrolyte for transporting protons from anode to cathode and a separator for preventing mixing of the fuel(i.e. hydrogen or methanol) and the oxidant. To date, the commercial PEM is DuPont’s Nafion membrane(a perfluorosulfonate PEM). However, some shortcomings such as high cost, high methanol permeation(in direct methanol fuel cell) and dehydration above 80℃ limit the further application of Nafion membrane. So there is an urgent need to research and develop non-fluorinated PEMs which have high performance and low cost under the condition of high temperature and low humidity. Aromatic polymer is an important skeleton material to prepare low cost and high performance PEM due to its excellent comprehensive performance. Most of aromatic polymer PEMs are main chain type sulfonated PEMs in which the sulfonic acid groups are directly incorporated onto the backbones of aromatic polymers, and they suffer from poor dimensional stability and a loss in mechanical performance due to its high water swelling ratio under the condition of high humidity. However, by the idea of molecular design, the side chain sulfonated aromatic polymer PEMs can overcome these drawbacks because they have the characteristics of micro-phase separation structures between hydrophilic and hydrophobic domains by locating the sulfonic acid groups far away from the polymer main chains. This is one of the important direction to prepare high performance PEM. In this study, bisphenol A type polysulfone was used as the matrix, through elaborate molecular design and effective macromolecular reaction,several kinds of side chain sulfonated aromatic polymer PEMs with the micro phase separation structure were smoothly prepared. The relationship between the structures and properties of the PEMs were mainly investigated. A strong regularity structure-activity relationship was obtained. Obviously, the results of this paper have obvious innovation in the design of high performance aromatic polymer PEMs and have an important scientificsignificance and application value on promoting the development of fuel cell.First, based on the Chloromethylated reaction of PSF, the benzene sulfonic acid type side chain sulfonated PSF PEMs were successfully prepared.(1) Chloromethylated polysulfone(CMPSF) was first prepared. On this basis, two side chain type sulfonated polysulfones with micro-phase separation structure, PSF-BSS and PSF-BDSS were obtained via nucleophilic substitution reactions with sodium 4-hydroxybenzenesulfonate and disodium1,2-dihydroxybenzene-3,5-disufonate as nucleophilic reagents, respectively. The structures of the sulfonated PSFs were fully characterized by infrared spectroscopy(FTIR) and NMR-H spectrum(1H-NMR). The effects of main factors on nucleophilic substitution were examined and the reaction conditions were optimized. The experimental results show that the nucleophilic substitution reaction of CMPSF is SN1 reaction. Dimethyl sulfoxide(DMSO)with strong polarity is the appropriate solvent because the polarity of the solvent benefits the SN1 reaction. The reaction was allowed to be carried out at 100 ℃ for 40 h, the bonding amount of sulfonic acid group of PSF-BSS and PSF-BDSS could reach up to 1.54 mmol/g and 1.58 mmol/g, respectively.(2) With PSF-BSS and PSF-BDSS as membrane materials, the corresponding PEMs were prepared by solution casting method. The basic properties of the PEMs were determined and the effects of the two factors, the aromaticity of the main chain and the degree of micro-phase separation between hydrophilic and hydrophobic domains on the properties of the PEMs were mainly studied. The research results show that the two PEMs,PSF-BDSS and PSF-BSS PEMs have high proton conductivity, but PSF-BDSS have better performance than PSF-BSS. For the PSF-BDSS PEM, the proton conductivities at room temperature and at 80 ℃ are 4.7×10-2 S/cm and 8.1×10-2 S/cm, respectively. The PEMs have excellent dimensional stability(the swelling ratios of PSF-BDSS are 8.6% at 25℃ and 30% at80℃, respectively). They have fine thermal stability(they are stable up to 270℃) and oxidative stability(they have better oxidative stability than Nafion 115).Subsequently, two kinds of benzene sulfonic acid type side chain sulfonated PSF PEMs with different length of side chains were obtained based on the chloroacylation reaction of PSF. Chloroacetylchloride and chlorobutyryl chloride were used as electrophilic reagents,CAPSF and CBPSF were obtained by introducing the flexible side chains with different numbers of carbon atom into the main chains of PSFs via Friedel-Crafts acylation reaction.Then the two kinds of benzene sulfonic acid type side chain sulfonated PSFs, 2PSF-BSS(sidechain containing two carbon atoms) and 4PSF-BSS(side chain containing four carbon atoms)were synthesized via nucleophilic substitution with sodium 4-hydroxybenzenesulfonate(HBSS) as reagent. The infrared spectroscopy(FTIR) and NMR-H spectrum(1H-NMR) were used to characterize the chemical structures of the sulfonated PSFs. The effects of main factors on Friedel-Crafts acylation reaction were examined and the reaction conditions were optimized. The experiment results show that the appropriate catalyst is AlCl3. The polarity of solvent has a great effect on Friedel-Crafts acylation reaction. CH2Cl2 with middle polarity is the appropriate solvent among several kinds of chlorinated hydrocarbons. Due to the Friedel-Crafts acylation reaction is an exothermic reaction, the appropriate reaction temperature is 50℃ considering the effects of chemical kinetics and thermodynamic factors.Two kinds of PEMs with different length of side chains were fabricated by solution casting method with 2PSF-BSS and 4PSF-BSS as membrane materials. Then the three kinds of benzene sulfoic acid type sulfonated polysulfones PEMs, 1PSF-BSS, 2PSF-BSS and4PSF-BSS were obtained combined with PSF-BSS(1PSF-BSS) reported in our previous study, and they have different length of side chains( the carbon atoms number of side chains are 1, 2 and 4, respectively). On this basis, the proton conductivities and dimensional stability of three PEMs were measured at 25℃ to 80℃ temperature range. The relationship between the side chain structures of three PEMs and properties were systematic studied, and especially the effects of the length of the side chains on the properties of the PEMs, i.e. the effects of degree of micro-phase separation on the performance of PEMs. The experimental results show that the three kinds of the PEMs, 1PSF-BSS, 2PSF-BSS and 4PSF-BSS have the characteristics of micro-phase separation structure between hydrophilic and hydrophobic domains. Compared with the main chain type sulfonated PSF PEMs, they have better dimensional stability and oxidative stability, as well as these properties are closed to that of Nafion series PEMs. For the three PEMs with the same degree of sulfonation, the length of the side chains have a positive effect on their main properties. Along with the increase of the length of the side chains, the flexibility of the side chains strengthens and the degree of micro-phase separation increases. It leads to the increase of the proton conductivities(the proton conductivity at 80℃: 1PSF-BSS, 7.1×10-2 S/cm; 2PSF-BSS, 7.8×10-2 S/cm;4PSF-BSS, 8.6×10-2 S/cm), the decrease of the swelling ratio(the swelling ratio at 80℃:1PSF-BSS, 29.6%; 2PSF-BSS, 26.8%; 4PSF-BSS, 22.1%) and the strengthening of theoxidative stability of the PEMs.Finally, CMPSF and CAPSF were used as the precursors, two kinds of aliphatic sulfonic acid type side chain sulfonated PSFs, 3PSF-SS(side chain containing three carbon atoms) and4PSF-SS(side chain containing four carbon atoms) were obtained via nucleophilic substitution with hydroxyehyl sulfonate sodium as nucleophilic reagent. On the basis of fully characterize their chemical structure by infrared spectroscopy(FTIR) and NMR-H spectrum(1H-NMR). For comparison, benzene sulfonic acid type side chain sulfonated polysulfones,1PSF-BSS was prepared according to the previous study route of ours. The relationship between the structures and performance of three kinds of side chain sulfonated PSF PEMs,1PSF-BSS, 2PSF-BSS and 4PSF-SS were mainly investigated. The preliminary results indicate that the aliphatic sulfonic acid type side chain sulfonated PSFs have better flexibility of the side chains compared with benzene sulfonic acid type side chain sulfonated PSFs, the movement of sulfonic acid at the end of side chain and the degree of micro-phase separation strengthens. It leads to the two PEMs, 3PSF-SS and 4PSF-SS have higher proton conductivities and water uptaking than 1PSF-BSS. For the two aliphatic sulfonic acid type side chain sulfonated PSF PEMs, 3PSF-SS and 4PSF-SS with the same degree of sulfonation.The flexibility of the side chains increases with increasing of the length of the side chains, the4PSF-SS PEM has higher proton conductivity and better dimensional stability than 3PSF-SS.The relationship between the structures and properties of PEMs need to be further deeply investigated. |
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