Tetrasulfonated Poly(arylene biphenylsulfone ether) Block Copolymer and Its Composite Membrane Containing Highly Sulfonated Blocks

Abstract

Among different types of fuel cells, proton exchange membrane (PEM) fuel cells show the highest power density and have been actively investigated for automobile applications. Although Nafion has been typically considered as a PEM for practical applications, there are still some limitations such as the high cost and the degradation of proton conductivity at high temperature (> 80 C). Several approaches have been examined to improve proton conductivity. The strategies have included changing the acidity and position of sulfonic acid groups and the control of membrane morphologies. Recently, highly sulfonated moieties or sequenced hydrophilic and hydrophobic groups as block copolymers were claimed to improve proton conductivity under high temperature. However, none of these alternative membranes could compete with Nafion membranes. Related to these reports, we wish to study the thermal and electrochemical properties and morphological characteristic of a block membrane enhanceing the number of sulfonic acid groups in a repeat unit of a polymer. Here, we report on highly sulfonated poly(arylene biphenylsulfone ether) block copolymer (5) made using tetrasulfonated 4,4'-bis[(4chloro-3-sulfonatophenyl)sulfony]biphenyl-2,2'-disulfonate (TsBCSBPD, 2) and its composite membrane using phosphotunstic acid (PWA). In the previous paper, we described the synthesis and characterization of tetrasulfonated poly(arylene biphenylsulfone ether) random copolymers made using TsBCSBPD (2). The block copolymer 4 was prepared as follows: First, hydrophobic oligomer 1 was made from 6F-BPA (4,4'-hexafluoroisopropylidene diphenol), DCDPS (4,4'-dichlorodiphenyl sulfone), and K2CO3 in dimethyl acetamide (DMAc). 7 Second, hydrophilic oligomer 3 was prepared from TsBCSBPD (2) and 6F-BPA in the same manner. Finally, we synthesized a block copolymer 4 from the hydrophobic 1 (Mw 10 kDa) and hydrophilic oligomer 3 (Mw 13 kDa) using a nuclephilic substitution reaction. This was then followed by hydrolysis with sulfuric acid (Scheme 1). The polymer had a high molecular weight (Mw 57 kDa), which supported the formation of a block structure. The H NMR spectrum of B37 copolymer is shown in Figure 1. H NMR analysis confirmed 37% sulfonation (the degree of sulfonation on each phenylene ring in the repeating unit). The B37 membrane was prepared by solution casting the acid form of the B37 copolymer in DMAc (10% w/v) and the B37-P30 (B37:PWA = 10:3, wt/wt) composite membrane was prepared by casting the acid form of the B37 and a commercial PWA (H3PWO40, Fluka Chemicals) in DMAc (10% wt/v). These castings were carried out onto a clean glass plate and then dried by a heat lamp at 80 C for 24 h. A summary of the thermal properties of the fabricated membranes is listed in Table 1. The TGA (thermogravimetric analysis) of B37, B37-P30, and Nafion N117 membranes