Periodic Polyethylene Sulfonates from Polyesterification : Bulk and Nanoparticle Morphologies and Ionic Conductivities

Abstract

A series of polyethylene-like ionomers with periodic sulfonate groups were synthesized by polyesterification of octatetracontane-1,48-diol and tetrabutylammonium dimethyl sulfosuccinate. Optional cation exchange in aqueous dispersions replaced the NBu₄⁺ counterions for Na⁺ or Cs⁺. The defined periodic microstructure of the polymers leads to the formation of layered structures in bulk and to platelet-like self-stabilized nanoparticles in aqueous dispersion, as observed by X-ray scattering and transmission electron microscopy. The bulk polymers exhibit a semicrystalline structure with multiple stacks of ionic layers embedded in the crystallites with a layer-to-layer distance of 50–60 Å. Upon melting, the ionic layers in the NBu₄⁺-neutralized polymer transitioned to disordered ionic aggregates, while the layered ionic aggregates in the Cs⁺- and Na⁺-neutralized polymers transformed into ionic aggregates with hexagonal symmetry, which is an unprecedented order-to-order transition in microphase-separated ionomers and a direct result of the molecular periodicity in these polymers. The ionic conductivities depend on the ionic aggregate morphologies. When ionomers are semicrystalline and have layered aggregates, the ionic conductivity is decoupled from the polymer segmental motion and increases with the cation size (Na⁺ < Cs⁺ < NBu₄⁺). These new periodic polyethylene sulfonates produce a variety of ionic aggregate morphologies with good connectivity that can be further manipulated to improve ion dissociation and increase ionic conductivity.