Synthesis And Characterization Of Novel Modified PBI Proton Exchange Membranes

Phosphoric acid-doped PBI membranes have high proton conductivity at high temperature (>100℃) and exhibit good properties, such as low fuel permeability, good chemical resistance, etc. However, this kind of membranes still have some major problems, such as poor solubility, low PA doping level, acid leaching, etc. Two kinds of modification methods have been proposed to improve these drawbacks in this study. On the one hand, the twisted, non-coplanar and asymmetrical phthalazinone moiety was introduced into the polymer main chain, in order to reduce the regularity of the main chain, resulting in the improved solubility. Besides, we also introduced the triazine ring into the backbone of PBI in order to increase the PA doping level by hydrogen bond or acid-base interations. On the other hand, we investigated the influence of modification of morphology on membrane properties, namely introducing the pore structure and cross-linking structure into membranes.A novel diacid monomer containing triazine ring was prepared by coupling reaction and oxidation reaction successfully. Then PBIs containing phthalazinone structure and trizine ring (TPPBIs) were synthesized from DHPZ-DA, BAPT and 3.3’-diaminobenzidine (DAB) by solution polycondensation in polyphosphoric acid (PPA). Furthermore, the model compound of TPPBI was synthesized and tested by FT-IR, TOF-MS and NMR. The TPPBI polymers have excellent thermal properties, with the decomposition temperature for 5% and 10% weight loss (Td5% and Td10%) under nitrogen in the range of 529-561℃ and 598-623 ℃, respectively. The polymers maintain 72.6-80.8% of their original weight at 800℃.With the increasing content of trizine ring, the PA doping level has increased. The proton conductivity of acid doped TPPBI membranes with the highest doping level (15.4 mol H3PO4) reaches 11 mS·cm-1.Porous PPBI proton exchange membranes were prepared by mixing a low-molecular-weight compound (porogen) with polymer solution to increase the phosphoric acid doping level and proton conductivity. SEM images of the porous polymer membranes show the pore size of 2-4μm and the pore size became bigger as the porosity increased. The introduction of pore structure improved the phosphoric acid doping level and proton conductivity. The proton conductivity of pPPBI-40 membrane could reach 15.72 mS·cm-1 at 120℃ under anhydrous conditions. The mechanical strength to some extent decreased with the increasing doping level, but can still up to 9.5-13.7 MPa. The elongation at break increased to 72.8-203.9%.Silane-cross-linked porous PPBI membranes were successfully prepared by using a silane monomer, g-(2,3-epoxypropoxy)propyltrimethoxysilane (KH-560), as the cross-linker, and DBP as the porogen. FT-IR and solubility tests were used to characterize and confirm the cross-linked porous structure in the membranes. SEM images show the cross-section morphology of the cross-linked porous membranes and the pore size of 1-3μm. The proton conductivity was enhanced because the silane-cross-linked structure in the membranes could absorb more phosphoric acid by electrostatic interactions. However, with the KH-560 ratio increasing, the doping levels of the membranes first increased and then decreased, and the turning point appeared at the membrane with a KH-560 content of 4wt%, whose doping level was 21.48 mol H3PO4. Moreover, the values of all the cross-linked porous membranes are higher than the pristine PBI. The proton conductivity of cpPPBI-4 membrane could reach 12.89 mS·cm-1 at 120℃ under anhydrous conditions. The tensile strength of undoped membranes is 72.3-87.4 MPa while the elongation at break is 15.1-17.8%. Because of the plastification of phosphoric acid, the mechanical strength decreased with the increasing doping level. The tensile strength of doped membranes is 3.3-7.1 MPa while the elongation at break is 145.3-210.7%.