Effects of a Nanophase-Separated Structure on Mechanical Properties and Proton Conductivity of Acid-Infiltrated Block...

Acid-infiltrated block polymer electrolyte membranes adopting a spherical or lamellar nanophase-separated structure were prepared by infiltrating sulfuric acid (H₂SO₄) into polystyrene-b-poly(4-vinylpyridine)-b-polystyrene (S-P-S) triblock copolymers to investigate the effects of its nanophase-separated structure on mechanical properties and proton conductivities under non-humidification. Lamellae-forming S-P-S/H₂SO₄ membranes with a continuous hard phase generally exhibited higher tensile strength than sphere-forming S-P-S/H₂SO₄ membranes with a discontinuous hard phase even if the same amount of Sa was infiltrated into each neat S-P-S film. Meanwhile, the conductivities of lamellae-forming S-P-S/H₂SO₄ membranes under non-humidification were comparable or superior to those of sphere-forming S-P-S/H₂SO₄ membranes, even though they were infiltrated by the same weight fraction of H₂SO₄. This result is attributed to the conductivities of S-P-S/H₂SO₄ membranes being greatly influenced by the acid/base stoichiometry associated with acid-base complex formation rather than the nanophase-separated structure adopted in the membranes. Namely, there are more free H₂SO₄ moieties that can release free protons contributing to the conductivity in lamellae-forming S-P-S/H₂SO₄ membranes than sphere-forming S-P-S/H₂SO₄, even when the same amount of H₂SO₄ was infiltrated into the S-P-S.