Polyethylene-Based Block Copolymers for Anion Exchange Membranes

Block copolymer membranes with a semicrystalline polyethylene component were prepared by anionic polymerization and postpolymerization functionalization reactions. Polybutadiene-b-poly­(4-methylstyrene) (PB-b-P4MS) precursors with four different block compositions and 92–95% 1,4-content in the polybutadiene block were produced by living anionic polymerization in a nonpolar solvent. The polybutadiene block was subsequently hydrogenated to prepare a polyethylene block, and the hydrogenated block copolymers were then brominated at the arylmethyl group of the P4MS block. Subsequent quaternization reaction with trimethylamine led to the anion exchange membranes. The degree of crystallinity in the polyethylene block was determined by differential scanning calorimetry to be approximately 24–27%. The postpolymerization modification reactions were examined by 1H NMR and IR spectroscopy. The amount of quaternary ammonium groups was quantified by ion exchange capacity (IEC) measurements. Membranes with IEC’s ranging from 1.17 to 1.92 mmol/g were prepared. The IEC was varied by changing the relative amount of P4MS in the precursor block copolymer, and water uptake and ionic conductivity were found to increase with increasing IEC. Small-angle X-ray scattering (SAXS) experiments and transmission electron microscopy (TEM) showed phase-separated, bicontinuous structures at all compositions. Materials with higher IEC show improved ionic conductivity as well as lower activation energy of ion conduction compared to less functionalized membranes. The hydroxide conductivity of the block copolymer membrane with an IEC of 1.92 mmol/g reached 73 mS/cm at 60 °C in water. Tensile measurements indicated excellent mechanical properties of the semicrystalline membranes for potential use as alkaline fuel cell membrane materials.