Preparation and characterization of proton-conducting crosslinked diblock copolymer membranes

The synthesis and properties of crosslinked diblock copolymers for use as proton-conducting membranes are presented. A polystyrene-b-poly(hydroxyl ethyl methacrylate) diblock copolymer at 56: 44 wt % was sequentially synthesized via atom transfer radical polymerization. The poly(hydroxyl ethyl methacrylate) (PHEMA) block was thermally crosslinked by sulfosuccinic acid (SA) via the esterification reaction between -OH of PHEMA and -COOH of SA. Proton nuclear magnetic resonance and Fourier transfer infrared spectra revealed the successful synthesis of the diblock copolymer and the crosslinking reaction under the thermal condition of 120°C for 1 h. The ion-exchange capacity continuously increased from 0.25 to 0.98 mequiv/g with increasing SA concentration because of the increasing number of charged groups in the membrane. However, the water uptake increased up to an SAof 7.6 wt %, above which it decreased monotonically (maximum water uptake ∼ 27.6%). The membrane also exhibited a maximum proton conductivity of 0.045 S/cm at an SA concentration of 15.2 wt %. The maximum behavior of the water uptake and proton conductivity with respect to the SA concentration was considered to be due to a competitive effect between the increase of ionic sites and the crosslinking reaction according to the SA concentration. All the membranes were thermally quite stable at least up to 250°C, presumably because of the block-copolymer-based, crosslinked structure of the membranes.