Rubber-elasticity and electrochemical activity of iron(ii) tris(bipyridine) crosslinked poly(dimethylsiloxane) networks

Abstract

2,2′-Bipyridine-terminated poly(dimethylsiloxane)s (bpyPDMS) with number average molecular weights, M_N, of 3300, 6100, 26 200, and 50 000 g mol⁻¹ were synthesized. When mixed with Fe(BF₄)₂ at low concentrations, red solutions formed with UV-vis spectra that match those of iron(II) tris(2,2′-bipyridine) (Fe(bpy)₃²⁺). Upon solvent evaporation, Fe(bpy)₃²⁺ crosslinked PDMS networks (bpyPDMS/Fe(II)) formed, and were studied using oscillating shear rheometry. The shear storage moduli (0.084 to 2.6 MPa) were found to be inversely proportional to the M_N of the PDMS, though the storage moduli at low molecular weights greatly exceeded the storage moduli of comparable covalently crosslinked PDMS networks. The shear storage moduli exhibited the characteristic rubbery plateau up to ∼135 °C. Films of bpyPDMS/Fe(II) coated onto electrodes were found to be electrochemically active, especially so when the PDMS M_N is low. The Fe(bpy)₃²⁺ crosslinks can be reversibly oxidized over ∼500 nm away from the electrode surface in the presence of a suitable electrolyte.