Preparation, characterization and methanol permeability of proton conducting membranes based on sulfonated ethylene-vinyl alcohol copolymer

Sulfopropylated ethylene-vinyl alcohol copolymers (s-EVOHs) have been synthesized via reacting 1,3-propanesultone with EVOH. Solution-cast membranes from the s-EVOH and its blend with EVOH of ethylene segment ratio of 44 mol% (44E) were prepared. The results of FT-IR, wide angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) showed that pendant sulfonic acid groups diminished the crystallinity of pristine EVOH. It was observed that these blend membranes showed lower swellability and methanol permeability with the increasing blending ratio of 44E. Small angle X-ray scattering (SAXS) further showed no nano-phase separation of ionic domain existed in the matrix, and solid-state 23Na NMR displayed the aggregates of ionic functionalities in the prepared membranes were smaller than those in Nafion. The highest proton conductivity of these membranes is 0.05 S/cm at 30 °C. The methanol permeability of the membranes ranges from 10−8 to 10−6 cm2/s and is highly correlated with the bound water degree and the corresponding amount of hydroxyl groups in the matrix. An optimum amount of vinyl alcohol segments in ion channels will act as an effective methanol barrier without causing greater swelling ratio.