Synthesis and Characterization of Novel Polybenzimidazoles Containing Sulfonated Biphenyl Unit for Proton Exchange Membranes
|School||Shanghai Jiaotong University|
|Course||Polymer Chemistry and Physics|
|Keywords||sulfonated polybenzimidazoles biphenyl hexafluoroisopropyl phosphine oxide proton exchange membrane|
At present, non-fluorinated proton exchange membrane (PEM) attracts much attention in the field of PEM. As a kind of high performance polymers, polybenzimidazoles (PBI) are regarded as an excellent matrix of PEM materials. Since the phosphoric acid doped polybenzimidazole PEM was reported, polybenzimidazoles have been of interest as PEM. A series of Polybenzimidazole based PEM was prepared by grafting, post-sulfonation, direct polycondensation, and blending. The direct polycondensation method shows the advantage such as the designable structure of molecules, thus sulfonated polybenzimidazoles with desired structure could be obtained by polycondensation of the designated monomers and the properties of the resulting membrane could be controlled. Up to now, only a few sulfonated polybenzimidazole-containing PEM have been prepared by direct polycondensation.In Chapter 2, 3,3’-disulfonate-4,4’-dimethyl biphenyldicarboxylate and 3,3’-disulfonate-4,4’-dicarboxylbiphenyl were synthesized, which could be polymerized with 3,3’-diaminobenzidine to attach the sulfonic aicd onto the backbone of the resulting polymer. In addition, bis(4-methyl benzoate)phenyl phosphine oxide was also synthesized and thus the triphenyl phosphine oxide moieties could be incorporate into the product by polycondensation. The product therefore shows loose arrangement of main chains and high solubility due to the phenyl side group. At the same time, it displays high water retention and adhesive ability with inorganic compounds arising from phosphine oxide mioeties, suitable for preparing high temperature PEM or inorganic compounds doped PEM.In Chapter 3, the hexafluoroisopropyl unit was introduced to the product by polycondensation of 3,3’-disulfonate-4,4’-dicarboxybiphenyl and 2,2-bis(4-carboxyphenyl) hexafluoropropane with 3,3’-diaminobenzidine in order to prepare soluble sulfonated polybenzimidazoles (sPBI). sPBI polymers exhibit high thermal and oxidative stability, the IEC of which is in the range of 1.12~2.59 meq/g. sPBI-70 membrane displays a water uptake of 19.5% and a swelling of 10.0% at 80 oC, lower than the water uptake and swelling of Nafion 117, respectively. This illustrated that sPBI-70 membrane possesses low water uptake and excellent dimensional stability. sPBI polymers show a trend that the water uptake and swelling decrease with increasing temperatures, which is anormal in comparison with the reported phenomena. This was explained by dynamic mechanical analysis. The investigation demonstrated that sulfonic acid groups show secondary relaxation at about 60 oC and thus their position changes, leading to the result that the interaction between the sulfonic acid and imidazole moiety enhances and that the resistance to deformation increases. sPBI membranes only show a proton conductivity in the order of magnitude of 10-3 S/cm, owing to the interaction between the sulfonic acid and imidazole moieties. TEM images of sPBI membranes denote an obvious hydrophilic/hydrophobic microphase separation structure. The hydrophilic domains aggregate to form ionic channels, the connectivity of which increases with the increase of sulfonation degree. This is favorable for sPBI membranes to absorb water and conduct protons, resulting that the water uptake and swelling rise with increasing temperatures.In Chapter 4, the triphenyl phosphine oxide moiety was incorporated to the product by polycondensation of 3,3’-disulfonate-4,4’-dicarboxybiphenyl and bis(4-methyl benzoate)phenyl phosphine oxide with 3,3’-diaminobenzidine in order to prepare soluble sulfonated polybenzimidazoles (sPBI-PO). The phosphine oxide moiety could improve the water uptake and proton conductivity of the product. sPBI-PO polymers exhibit high thermal and oxidative stability. sPBI-PO-70 membrane exhibits a water uptake of 26.46%,a swelling of 7.08 %, and a proton conductivity of 4.00×10-3 S/cm at 80 oC, its water uptake and proton conductivity are higher than those of sPBI-70 membrane, respectively, due to the water retention of phosphine oxide groups. sPBI-PO membranes also show a anormal tendency that the water uptake and swelling decrease with increasing temperatures, arising from the above-mentioned reasons. The AFM images demonstrated that sPBI-PO have a hydrophilic/hydrophobic microphase separation structure. Contrary to most of random sulfonated polymers, sPBI-PO polymers show a trend that the connectivity of ionic channels increases but the width of them decreases as the sulfonation degree rises. The increase of the connectivity of ionic channels improves the proton conductivity, while the binding of the surrounding hydrophobic domains gains with the decrease of the width of ionic channels, favorable for the decrease of swelling and the increase of dimensional stability.