Cation Transport in Ionic Layers of Sulfonated Crystalline Telechelic Polyethylenes

We present a set of sulfonated telechelic polyethylene ionomers that demonstrate ion transport of metal cations within layered ionic aggregates in a crystalline polymer matrix. These precise ionomers consist of 48 backbone carbons with sulfonated end groups that are fully neutralized by a counterion, C₄₈(SO₃X)₂ (X=Li+ or Na+). The morphology is characterized using X-ray scattering and ion conductivity is characterized using electrical impedance spectroscopy. These telechelic polyethylenes exhibit well defined ionic layers and a variety of crystalline backbone morphologies below T_m. The polyethylene backbone packs in a hexagonal crystal at high temperatures in both the Li+ and Na+-containing systems, with an Arrhenius activation energy (E_a) for ion transport of 120 and 53 kJ/mol, respectively, indicating decoupled ion transport through the layers. The E_a in the hexagonal regime is significantly lower than E_a in the room temperature backbone morphologies of these polymers, orthorhombic in C₄₈(SO₃Na)₂, and disordered crystals in C₄₈(SO₃Li)₂. The low E_a of 53 kJ/mol in the hexagonal regime of C₄₈(SO₃Na)₂ is a promising sign for the ability of ionic layers to facilitate ion transport in crystalline polymers, for potential application as solid polymer electrolytes.