Studies Of Novel Sulfonated Block Copolymers For Proton Exchange Membranes

To protect the environment and reduce the consumption of traditional fossil fuels, many countries are dedicating studies to develop new energy conversion technologies. Due to high energy conversion efficiency and low pollution, polymer electrode membrane fuel cells (PEMFCs) become one of the most promising power sources, especially for vehicular transportation and other portable applications. As the key component of a PEMFC, the proton exchange membrane (PEM) has attracted widely investigations. Nowadays the most famous and commercially available PEM is Dupont's Nafion which was invented in the end of 1960's. It shows excellent oxidative and chemical stability, cross-linked ionic channels in structure as well as high proton conductivity when fully hydrated. But certain drawbacks such as high cost and possible pollution of fluorum containing materials have limited its potential applications. Thus, many efforts have been devoted to develop alternative proton conductive materials.Aromatic polymers are one kind of functional materials of high performance. They are widely used in many areas due to their versatile properties. Most of them are easily synthesized and exhibit good mechanical properties. Combining these aromatic chains and aliphatic macromolecules which are similar to Nafion by block copolymerization, the block copolymers possess the physicochemical properties of all individual blocks simultaneously, for example good mechanical properties, high phase separation and proton conductivity. The thesis is based on this strategy.In chapter 2, fluoro-terminated poly(ether ether ketone) (PEEK-F) of different molecular weights were synthesized from bisphenol A and 4,4'-diflurobenzophenone. Then the end groups was changed to amines by 4-aminophenol. Combining PEEK-NH2 and acyl chloride terminated polybutadiene (PB-COCl) which was modified from carboxyl-terminated polybutadiene and thionyl chloride, PEEK-b-PB was block copolymerized. After the selective post-sulfonation of PEEK-b-PB using acetyl sulfate as the sulfonating reagent which was synthesized from concentrated sulfuric acid (98%) and acetic anhydride, the sulfonic acid groups were only attached onto PB blocks and a series of PEEK-b-SPB were successfully prepared. Their molecular weights could be controlled by adjusting the length of PEEK chains. PEEK-b-SPB exhibited good film-forming ability and thermal stability. Their membranes showed enhanced tensile strength and Young's modulus when the sulfonation degree was increased, the maximums were 60 MPa and 2.38 GPa, respectively. With the increasing sulfonate groups and the promotion of PB soft chains, TEM clearly displayed the ionic domains grew to large ionic aggregates from small separated clusters. Proton conductivity was dependent on ion exchange capacity (IEC) and temperature. PEEK-b-SPB with shorter PEEK chains exhibited better conductivity. Most of them were higher than 0.01 S/cm, which was the lowest value of practical interest for use as PEM in fuel cells. At 20℃and 80℃, the best values of them in water were 0.023 S/cm and 0.037 S/cm, respectively.In order to increase the proton conductivity, based on PEEK-b-SPB, a series of all-block-sulfonated copolymers SPEEK-b-SPB were prepared. Partially sulfonated PEEK (SPEEK) was synthesized from bisphenol A, 4,4'-diflurobenzophenone and its sulfonated product 3,3'-disulfonate- 4,4'-difluorobenzophenone. Due to the sulfonation of PEEK, the proton conductivity of most membranes at 20℃was increased to 0.02 S/cm at least. The highest values at this temperature and 80℃were 0.033 S/cm and 0.064 S/cm, respectively. Because sulfonic groups were successfully grafted onto both PEEK and PB blocks, the mechanical properties of SPEEK-b-SPB were enhanced, too. The maximums of tensile strength and Young's modulus increased to 77.6 MPa and 2.71 GPa, respectively. Besides large ionic aggregates, spherical ionic domains with an average size of approximately 30-60 nm which were uniformly distributed in the PEEK phase could also be found in the micro structure of membranes. In addition, most SPEEK-b-SPB membranes showed enough mechanical strength and good dimensional stability at hydrated state. They could keep stable for 120 minutes at least in Fenton test (3% H2O2 and 3 ppm FeSO4, 80℃).In chapter 3, amino-terminated sulfonated polyimide (SPI-NH2) was synthesized from 1,4,5,8-Naphthalenetetracarboxylic dianhydride (NTDA) and 4,4'-diaminodiphenyl ether-2,2'-disulfonic acid (ODADS) which was sulfonated from 4,4'-diaminodiphenyl ether. Instead of PEEK-NH2, SPI-NH2 was used to prepare SPI-b-PB. Then the relevant post-sulfonated product SPI-b-SPB was successfully obtained, too. The employment of these wholly sulfonated aromatic blocks simplified the experimental steps. Meanwhile, the sulfonation degree was largely increased. Thus, the proton conductivity of SPI-b-SPB membranes enhanced a lot. All of them were higher than 0.04 S/cm at 20℃, the maximum was over 0.11 S/cm at 80℃. Mechanical properties were kept, at dry state the tensile strength and Young's modulus reached 76.5 MPa and 2.8 GPa, respectively. The strength of most membranes which were immersed in water for 240 h was even around 50 MPa. However, when sulfonation degree was too high, SPI-b-SPB became soluble in water, and the oxidative stability was worse.To develop the dimensional stability of SPI-b-SPB, a novel sulfonated diamine was synthesized instead of 4,4'-diaminodiphenyl ether-2,2'-disulfonic acid (ODADS). With this new monomer, a series of sulfonated block copolymers DSPI-b-SPB which contained benzenesulfonic acid terminated pendant groups were prepared. Compared with SPI-b-SPB, the IEC of these new membranes obviously decreased, but water uptake and proton conductivity increased because of the rigid side chains. The conductivity could reach 0.08 S/cm at 20℃and 0.16 S/cm at 80℃, which were even better than those values of Nafion-117 in the same condition. Due to the low IEC, dimensional and oxidative stability improved, as well. TEM clearly showed that two kinds of ionic distributions existed in micro structure, simultaneously.