Naphthalene-based Side-chain-type Sulfonated Poly(Arylene Ether Ketone) Copolymers For Proton Exchange Membranes: Synthesis And Performance Studies

Fuel cells are electrochemical devices that directly convert the chemical energy of a fuel into electrical and thermal energy. Due to the high efficiency, low emission, and environmentally friendly, it has achieved more and more attention. Direct methanol fuel cell (DMFC) is a kind of proton exchange membrane fuel cell which use methanol as fuel. Proton exchange membranes are the key components of DMFC systems. They act on conducting the protons and separating the fuel from oxidants so they must possess high stabilities, good mechanical properties, high conductivity, and low methanol permeability as well. Up to date, the commercial available PEMs are Nafion membrane, which have excellent chemical stability and high proton conductivity. However the poor thermal stability, high cost, especially the high methanol permeability limited their industrial application.Sulfonated aromatic polymers have been studied as one of the promising candidates for PEMs because of their excellent thermal and mechanical stability, relative high proton conductivity, low methanol permeability and low cost. Now the most of the sulfonated aromatic polymer contain sulfonic acid group randomly distributed along polymer main chain. It has been reported that microstructure this kind of polymer shows less pronounced hydrophilic / hydrophobic separation which lead to unconspicuous phase separation. At the low sulfonated degree the channels were highly branched and with many dead-end which could not used in fuel cell. But when the sulfonated degree became higher, the excess water uptake would influent the dimension stability even dissolve in water at high temperature. Above all, researchers want to prepare the sulfonated aromatic polymers both possess high proton conductivities and good dimension stability by designing the molecule structures and the structure of the polymers.One promising way to enhance properties of sulfonated aromatic polymers is to distinctly separate the hydrophilic sulfonic acid group regions from the hydrophobic polymer main chain by locating the sulfonic acid group on the flexible pendant side chains. Since the flexibility, the side chain type sulfonic acid groups are prone to aggregated in to hydrophilic domain, which could help to hydrophilic / hydrophobic separation. Synthesize of the side chain type sulfonic aromatic polymer was not easy to perform no matter the post-sulfonated method or synthesize a new monomer. Besides, there are few reports on the effect of the structure of main chain on the properties of the side-chain-type sulfoanted aromatic polymer. So in this study we aim to investigate an easy way to synthesize side-chain-type sulfonated aromatic polymers and by changing the main chain to figure out the influence of the main chain on the properties of the polymers. We also use the cross-linking method to improve the high Ds membranes'properties for use in fuel cell.Firstly, we investigated a novel side chain type sulfonic aromatic polymer. The monomer 1,5-bis(4-fluorobenzoyl)-2,6-dimethoxynaphthalene (DMNF) was prepared via Friedel-Crafts acylation. The naphthalene-based Poly (arylene ether ketone) (DMNF) copolymers containing methoxy groups with different mole ratio of DMNF were synthesized by polycondensation. And then by the demethylation which convert MNPAEKs copolymer to HNPAEKs. The structure of DMNF, MNPAEKs and HNPAEKs were all confirmed by 1H NMR. The results meet our previous expectation. The thermal and mechanical properties of MNPAEKs and HNPAEKs indicated that they are both novel plastic with high performance. The sulfobutyl groups were grafted on HNPAEKs by the nucleophilic ring opening reaction with the 1, 4-butane sultone. As expected, the 1H NMR showed the successfully introduce sulfuric groups in to the aromatic polymer. All the SNPAEKs were characterized by TGA and mechanical properties, meanwhile, the membrane properties for fuel cells application such as water uptake, proton conductivity and methanol transport had been studied. The result showed that the naphthalene-based poly (arylene ether ketone) containing sulfobutyl pendant groups not only possessed excellent dimensional stability, but also with high proton conductivity and low methanol permeability.In chapter 4, the fluorin moiety containing naphthalene-based poly(arylene ether keton) with methoxy groups was synthesized by polycondensation using DMNF, 3,4-Difluorobenzophenone (DFB) and hexafluorobisphenol A. Then by the demethylation and post sulfonic method we get the side chain type sulfonic aromatic polymer with fluorin in main chain (6F-SNPAEKs). The 1H NMR confirmed that reaction proceeded smoothly. 6F-SNPAEKs exhibit good thermal and mechanical properties. Meanwhile since the introduce fluorin into the main chain, it enhance the hydrophobic of the main chain, which help to maintain the dimensional stability of the polymer. So the 6F-SNPAEKs with high Ds also possessed low water uptake and low methanol permeability. Due to the high Ds it could get, the proton conductivity of 6F-SN-90 reached 0.081 Scm-1 at room temperature.In chapter 5, to take advantage of both block polymer and side chain type, a serious of blocked side chain type sulfonic aromatic polymer was synthesized. Make use of hydroquinone and DMNF to synthesize different length (3, 9, 15) oligomer containing methoxy groups. Then different length of block polymer was synthesized by polycondensation using the oligomer, DFB and BPA. By demethylation and post sulfonic method, side chain type sulfonated aromatic polymer in different length block (Block-x) was synthesized. The 1H NMR confirmed that we got the polymer as expected. TEM was used to investigate the microstructure of the membranes. The results showed that with the increasing of the length of the block, the hydrophilic / hydrophobic became more obvious. The longer of the block the more flexible of the polymer and the water uptake became bigger also. It found that the side chain type block sulfonated aromatic polymer showed higher proton conductivity.In chapter 6, the cross linked membranes composited with high Ds SNPAEKs and epoxy TMBP with different curing agent DDS (STD-x) and BDSA (STB-x) were prepared. SEM was used to investigate the morphology of the cross section of the composite membranes. It found that STB-x composite membranes showed good compatibility, but the STD-x membranes'compatibility degrade with the increase of ratio of the TMBP. Introduce cross linking method enhance the mechanical property, but the degradation of compatibility could weaken the mechanical property also. After comparing the water uptake, swelling ratio, proton conductivity and methanol permeability of the composite membranes, it found that the introduction of epoxy cross linking method could improve the comprehensive performance of the PEMs and the curing agent with sulfonic agent could improve the compatibility.