Synthesis And Properties Of Sulfonated Polybenzimidazoles (Polybenzothiazoles) As Proton Exchange Membranes

Proton exchange membranes fuel cells (PEMFCs) are one of the most attractivedevices as the clean power sources. Proton exchange membranes (PEMs) are one ofthe key components of PEMFCs. At present, Nafion, which is extensively used asPEMs has some disadvantages, such as the serious loss of proton conductivity underlow humidity or high temperature, high methanol permeability, and high cost. Toovercome those drawbacks, the main focus is on developing aromatic PEMs.H3PO4-doped polybenzimidazole membranes exhibited wonderful properties suchas an electro-osmotic drag number nearly approaching zero, high proton conductivityat elevated temperatures, and low methanol permeability. Thepolybenzimidazole-based PEMs have attracted particular attention. Sulfonatedpolybenzimidazole (sPBI) membranes have been developed after that, in order toattach the ionic groups to PBI backbone by chemical linkage, the grafting,post-sulfonation, and direct-polycondensation methods have been developed toprepare sPBI membranes. The direct-polycondensation method shows the advantagesby combination of various functional groups, resulting in excellent comprehensiveproperties of the products. On the other hand, the rigid-rod backbone andintermolecular acid-base interactions of sulfonated polybenzimidazoles often makethem poorly soluble and difficult to form membranes used as PEMs. Recently, theflexible moieties or bulky pendant groups were incorporated into the backbone ofsPBI in order to enhance the flexibility or disrupt the regular packing of polymerchains and thus to improve the solubility by our group. We have synthesized a seriesof sulfonated polybenzimidazoles. In this paper, the asymmetric1,4-naphthalenegroup was incorporated into the backbone of sPBI in order to disrupt the regularpacking of polymer chains and thus to prepare soluble sPBI. Moreover, bothbenzenesulfonic acid bulky pendant groups and phosphine oxide groups wereincorporated into the backbone of sPBI in order to increase the distance betweenpolymer chains and disrupt the regular packing of polymer chains, and then to improve the solubility of sPBI.Sulfonated polybenzoxazoles are a kind of high performance matrix materials ofPEMs. They were initially designed to be used as structural materials, which cannotbe used as PEM because of their low molecular weight or poor solubility. Three seriesof sulfonated polybenzothiazoles were prepared by polycondensation ofbis(3-sulfonate-4-carboxyphenyl) sulfone by our group. We have also developedhighly soluble sPBT as PEMs with high molecular weight for the first time. As shownin a comparison research, the incorporation of the flexibile hexafluoroisopropylideneunits and sulfophenyisulfonyl pendant groups could obviously enhance the solubilityof sulfonated polybenzoxazoles. In this paper, three series of sulfonatedpolybenzothiazoles were prepared by polycondensation of3,3’-disulfonate-4,4’-dicarboxylbiphenyl(SCBP),6-sulfonate-1,4-naphthalene dicarboxylic acid(SNAA), and bis(2-sulfonate-4-carboxyphenyl) ether, respectively. The flexiblehexafluoroisopropylidene units and diphenyl ether groups were incorporated into thebackbone of sPBT in order to enhance the flexibility or disrupt the regular packing ofpolymer chains.In Chapter2, soluble sulfonated polybenzimidazoles (sPBI) as proton exchangemembranes using asymmetric dicarboxylic acid monomers was synthesized. Firstly,6-sulfonate-1,4-naphthalene dicarboxylic acid (SNAA) and6,8-disulfonate-1,4-naphthalene dicarboxylic acid (DSNAA), were designed andsynthesized. Subsequently, mono-sulfonated polybenzimidazoles (sPBI-N100) weresynthesized by polycondensation of SNAA. Other four series of sulfonatedpolybenzimidazoles were also prepared by copolymerization of DSNAA. Theincorporation of the asymmetric1,4-naphthalene group enhanced the asymmetry ofpolymer chains and disrupted their regular packing. These sPBI ionomers thusexhibited excellent solubility in NMP, DMF and DMSO, so they could be cast to themembranes as PEMs. Those two series of sPBI showed high thermal stability, sPBIdisplayed a Td5above410oC and no glass transition in their DSC thermograms at thetemperature of90–350oC. Moreover, sPBI membranes showed excellent oxidativestability. The water uptake of those sPBI membranes is in the range of29.932.9%at 80oC, very close to that (30%) of Nafion117; their swelling ranges from8.8%to9.3%at80oC, which is less than half that (20%) of Nafion117. However, they displayedlow proton conductivity like other sPBI because of the acid-base interactions betweenthe sulfonic acid and benzimidazole moieties. As shown in AFM images, theconnectivity and width of hydrophilic domains of dual sPBI membranes increase withincreasing sulfonation degree. At the same time, the water uptake, swelling, andproton conductivity are also increased with increasing sulfonation degree. In addition,sPBI-N100with the lowest IEC showed the highest proton conductivity among all thesPBI. This is because it displayed more connected hydrophilic domains than any othersPBI.In Chapter3, soluble sulfonated polybenzimidazoles (sPBI-P) as proton exchangemembranes with phosphine oxide was synthesized. Sulfonate bis(4-methyl benzoate)phenyl phosphine oxide (sMBPO) was synthesized. Soluble sulfonatedpolybenzimidazoles phosphine oxide (sPBI-P) was prepared using sMBPO. Bothbenzenesulfonic acid bulky pendant groups and phosphine oxide groups wereincorporated into the backbone of sPBI in order to disrupt the regular packing ofpolymer chains and then improve the solubility of sPBI. As sPBI-P is soluble inDMSO, they could be cast into homogeneous membrane as PEMs. Incorporation ofthe phosphine oxide groups to the backbone enhanced the water-keeping ability ofsPBI-P. sPBI-P membranes with the same IEC showed39%more water uptake thanthat of other sPBI. They also showed high thermal, oxidative stability and excellentmechanical properties. However, they displayed low proton conductivity like othersPBI because of the acid-base interactions between the sulfonic acid andbenzimidazole moieties. But their proton conductivity could be improved by thedoping or blending method.In Chapter4, soluble sulfonated polybenzothiazoles as proton exchange membraneswith biphenyl group was synthesized. Two series of sulfonated polybenzothiazoles bypolycondensation of2,5-diamino-1,4-benzenedithiol dihydrochloride (DABDT) and3,3’-disulfonate-4,4’-dicarboxylbiphenyl (SCBP) with4,4’-dicarboxylbiphenyl(DCBP) or2,2-bis(4-carboxyphenyl) hexafluoropropane (6FA), respectively. The first series was expressed as sPBT-DP, while the other series was denoted as sPBT-BP.Contrast research shows that the flexible hexafluoroisopropylidene moieties wereincorporated into the backbone of sPBT in order to disrupt the regular packing andenhance the solubility. The sPBT-BP series showed high thermal and oxidativestability as well as excellent mechanical properties. Moreover, the sPBT-BPmembranes exhibited high proton conductivity and outstanding dimensional stability.For example, sPBT-BP57.5displayed a proton conductivity of0.094S/cm, anin-plane swelling of9.7%and a through-plane swelling of35%. Additionally, thewater uptake, in-plane swelling, and through-plane swelling of sPBT-BP membranesincreased only slightly with increasing temperature because of strong intermolecularinteractions.In Chapter5, soluble sulfonated polybenzothiazoles as proton exchange membraneswith1,4-naphthalene group was developed to synthesized. Two series of sulfonatedpolybenzothiazoles (sPBT) were synthesized by polycondensation of6-sulfonate-1,4-naphthalene dicarboxylic acid (SNAA),2,5-diamino-1,4-benzenedithiol dihydrochloride (DABDT) and1,4-naphthalenedicarboxylic acid (NA) or2,2-bis(4-carboxyphenyl) hexafluoropropane (6FA),respectively. The first series was expressed as sPBT-NA, while the other series wasdenoted as sPBT-6F. The sPBT-NA series are insoluble in common solvents, sPBT-6Fseries are soluble in DMSO due to the incorporation of the flexiblehexafluoroisopropylidene moieties onto polymer chains. sPBT-6F showed highthermal and oxidative stability as well as excellent mechanical properties. Moreover,they showed appropriate water uptake and swelling in plan and thickness. Forexample, sPBT-6F70displayed a water uptake of34%, an in-plane swelling of4.4%and a through-plane swelling of44.5%at80oC, this is less than half that (20%) ofNafion117. Especially, the water uptake and swelling of sPBT-6F70almost remainsunchanged in the range of25–90oC. The storage modulus of sPBT-6F membranesalmost exhibited an increasing trend up to180oC. sPBT-6F membranes showed asecondary relaxation at around80oC. It could cause the membrane to change therelative place of the sulfonic acid and the basic benzothiazole group as well as thedistance between them, thus forming stronger intermolecular acid-base interactions and facilitating the increase of storage modulus. These strong intermolecularinteractions could facilitate the sPBT-6F membranes to resist the swelling. ThesesPBT-6F membranes also show high proton conductivity, the proton conductivity ofsPBT-6F90showed almost twice that of Nafion117, and the proton conductivity ofsPBT-6F80, sPBT-6F85and sPBT-6F95is also as high as that of Nafion117.In Chapter6, soluble sulfonated polybenzothiazoles as proton exchange membraneswere synthesized by incorporation of flexible diphenyl ether groups to the backbone.2,5-Diamino-1,4-benzenedithiol dihydrochloride, bis(2-sulfonate-4-carboxyphenyl)ether, and bis(4-carboxyphenyl) ether or2,2-bis(4-carboxyphenyl) hexafluoropropanewere polycondensated to synthesize sPBTs (sPBT-PE and sPBT-6F). As shown in thecomparison study, the incorporation of flexible diphenyl ether groups into thebackbone of sPBT could disrupt the regular packing and enhance the solubility; theflexible hexafluoroisopropylidene moieties were incorporated into the backbone ofsPBT, which could enhance the flexibility of the polymer chains and then highlyincrease the solubility. Both series sPBTs show high thermal and oxidative stability aswell as excellent mechanical properties. Furthermore, both of them exhibited highproton conductivity and outstanding dimensional stability. For example, theydisplayed a proton conductivity of0.11–0.13S/cm at80oC, but only exhibited anin-plane swelling of14–15%and a through-plane swelling of7.1–30%even at98oC.Equally importantly, their in-plane and through-plane swelling almost showed nochange with increasing temperature because of strong intermolecular interactions. Thebenzothiazole units thus could be regarded as a new structural unit to fabricate theswelling of PEMs, facilitating to design low-swelling PEMs. Compared with thesPBT-6F series, the sPBT-PE series with an equal IEC displayed higher protonconductivity but lower solubility and elongation at break.