Novel side-chain-type cardo poly(aryl ether sulfone) bearing pendant sulfoalkyl groups for proton exchange membranes

A series of cardo poly(aryl ether sulfone) copolymers bearing pendant hydroxyl ethyl groups (−CH2CH2OH) were synthesized from a new biphenol monomer 2-(2-hydroxyethyl)-3,3-bis(4-hydroxyphenyl)isoindolin-1-one (PPH–OH), 4,4′-biphenol(BP), and 4,4′-difluorodiphenylsulfone (DFDPS). Sulfonic acid groups were readily grafted to the copolymers by reaction of the hydroxyl group with 1,3-propanesultone. The resulting sulfonated copolymers SPES-C-X (where X refer to the content of PPH–OH in the bisphenol monomers) contain side chain sulfoalkyl groups (–CH2CH2–O–CH2CH2CH2SO3H). TEM photographs revealed that SPES-C-100 membrane exhibited characteristic hydrophilic/hydrophobic microphase separated morphology with small interconnected and hydrophilic clusters (1.5–2 nm). Such microphase separated morphology renders the membrane high proton conductivity of 0.056 S/cm (IEC = 1.41 mequiv./g), and low dimensional changes of SPES-C-100 at room temperature. SPES-C-X membranes exhibited the methanol permeability in the range of 13.8–5.3 × 10−8 cm2/s, which were much lower than those of SBPES (8.7 × 10−7 cm2/s) and Nafion 117 (2.4 × 10−7 cm2/s).

Research highlights▶ Novel side-chain-type sulfonated cardo poly(aryl ether sulfone)s contain side chain sulfoalkyl groups (–CH2CH2–O–CH2CH2CH2SO3H) have been synthesized and characterized. ▶ The side-chain-type sulfonated polymer showed higher proton conductivity than the main-chain-type ionomer with similar IEC value. ▶ The side-chain-type membrane exhibited characteristic hydrophilic/hydrophobic microphase separated morphology with small interconnected and hydrophilic clusters.