Effect of hydration on the mechanical properties and ion conduction in a polyethylene-b-poly(vinylbenzyl trimethylammonium) anion exchange membrane

Abstract

a b s t r a c tAnion exchange membranes (AEM) are promising solid polymer electrolytes utilized in alkali fuel cells and electrochemical energy conversion devices. AEMs must efficiently conduct ions while maintaining chemical and mechanical stability under a range of operating conditions. The ionic nature of AEMs leads to stiff and brittle membranes under dry conditions while at higher hydrations, water sorption causes significant softening and weakening of the membrane. In this work, a new polyethylene-b-poly(vinylbenzyl trimethylammonium) polymer (70 kg/mol) was cast into large (300 cm 2 ), thin (12 7 3 μm) membranes. These membranes exhibited improved elasticity over previously tested AEMs, minimal dimensional swelling, and moderate ionic conductivity (5 7 2 mS/cm at 50°C, 95% RH in the bromide form). Extensional testing indicated a 95% reduction in Young's modulus between dry and hydrated states. Further investigation of the complex modulus as a function of hydration, by dynamic mechanical analysis, revealed a sharp decrease in modulus between dry and hydrated states. Mechanical softening was reversible, but the location of the transition displayed hysteresis between humidification and dehumidification. Conductivity increased after membrane softening; suggesting bulk mechanical properties can identify the hydration level required for improved ion transport. Understanding the relationship between ion conduction and mechanical properties will help guide AEM development and identify operating conditions for sustained performance.

Novel Processing of a Poly(phenyleneoxide) −b–Poly(vinylbenzyltrimethylammonium) Copolymer Anion Exchange Membrane; The Effect On Mechanical And Transport Properties

“…The membrane stiffness was reduced drastically, almost by ~ 80% from initial values at dry conditions when humidity of the samples was increased to 95 %RH. Vandiver et al have described this effect as the plasticizing effect due to water softening the membranes [44]. Similar behavior has been observed in other AEMs under similar experimental conditions [32,43].…”

“…These SCMP membranes showed linear trends for chloride and bicarbonate conductivity at 95 %RH, implying the ions were fully dissociated, in contrast to the observation for the same anions with the MP polymer that showed curved behavior, as these anions in that materials where not fully dissociated and showed VTF behavior [8]. Presumably improved morphologies of the SCMP membranes led to the observed higher ionic conductivity in all anionic forms ( Figure 5) The conductivity values for the membranes were higher than the reported values of conductivity for these anions under similar operating conditions to other AEMs [12,16,[44][45][46].…”

“…The amount of water in the membrane affects the mechanical strength of the membranes [44]. At dry conditions, the membrane stiffness increased with the melt pressing time, and for all membranes the Young's modulus was > 400 MPa (Figure 9).…”

Water Uptake Study of Anion Exchange Membranes

“…AEMs. 42,66,[69][70][71][72]80,81 Equilibrium water uptake in such membranes was previously modeled by the Park model [77][78][79] and by the New Dual Mode Sorption (NDMS) model 82 was previously attributed to the flexibility of the membrane matrix and its affinity for water. 83 The membranes investigated herein are built from different polymeric backbones, with different quaternary ammonium FGs.…”

Anion exchange membranes with eight flexible side-chain cations for improved conductivity and alkaline stability

“…On the other hand, a block copolymer AEM architecture is conducive to the formation of more defined hydrophilic/hydrophobic microphase separated morphology, thus greatly improving ion conduction [37]. However, block copolymer-type AEMs typically have a large ion channel size accompanied by high dimensional water swelling ratio, which leads to inferior mechanical properties and even a loss of mechanical strength under high temperature conditions [38]. Furthermore, the alkaline stability is generally not significantly improved compared with main chain-type AEMs.…”