Conductivity, morphology, and dynamics of single-ion conducting random and block copolymers

Ionomers, where ionic groups are covalently bonded to the polymer backbone, have the potential application as both separator and electrolyte for cationic and anionic conducting rechargeable batteries. The chemical structure that provides the best ionic conductivities remains a challenging yet elusive goal, thus the structure-property relationship of single-ion conducting ionomers is thoroughly investigated in this study, using random copolymer ionomers with nonionic methacryl ethylene oxide as both polycation and polyanions, with the latter also made into block copolymers.;Random copolymer ionomers with ethylene oxide side chains are synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization, to systematically test effects of ion content. Dielectric relaxation spectroscopy (DRS) is used to measure the conductivity, dielectric constant and segmental relaxations in these ionomers and the electrode polarization at very low frequencies is used to assess the number density of simultaneously conducting ions and their mobility. Glass transition temperature (Tg) increases gradually as ions are incorporated at low ion content in polyethylene oxide-based styrene sulfonate ionomers then sharply as the ion content reaches 1:9 ion to ether oxygen (EO) ratio. The ionomer with 1:81 ion to EO ratio shows highest room temperature conductivity that results from the best combination of number density of simultaneously conducting ions and their mobility. Ion states are investigated by Fourier transform infrared spectroscopy and can be related to dielectric measurements. The microphase separation that is anticipated in the ionomers with higher ion contents is probed by X-ray scattering and linear viscoelastic (LVE) measurements.;Polycations with weak-binding aromatic phosphonium groups are also investigated with possible application for alkaline fuel cells and fluoride ion batteries. Tg broadens and increases with ion content (less than 1°C per mol% of ionic comonomer), and the increase in Tg per mol% ion ionic monomer is much smaller than that for styrene sulfonate ionomers (∼3 °C/mol%). The dynamics of these phosphonium ionomers are probed by LVE measurements, and two clear glassy relaxations can be seen in the LVE master curves though only one Tg is seen in differential scanning calorimetry that broadens with ion content.;In order to serve as both electrolyte and separator in rechargeable batteries, ionomers are required to exhibit both sufficient modulus and high ionic conductivity. However, good ionic conductivity is mostly correlated with low Tg materials where rapid segmental motion of the polymer aided ion transport, and decoupling of the electrical and mechanical properties of polymeric electrolytes is needed. By using poly(dimethylacrylamide) as the hard block with T g=120 °C of ∼50/50 diblock copolymer ionomers, the modulus is as high as 50 MPa at 60 °C for diblock copolymer ionomers with ∼100 k and ∼50 k molecular weights but decreases with ion content owing to more phase mixing. The ion containing poly(ethylene oxide)-based block, synthesized by RAFT polymerization, provides a low-Tg (∼ -50 °C) medium for ion conduction. The conductivity of ionomeric diblock copolymers at 60 °C is ∼2 x 10-8 S/cm at ∼10 mol% ionic monomer in the soft block (∼10-6 S/cm at 60 °C for PEO-based random copolymers with similar ion content) and increases with increasing ion content, and polymer chain dynamics in the soft block are studied by DRS under an applied ac field.;Small amplitude oscillatory shear is not only used to detect the mechanical response to assess the modulus of the glassy block, but the low-frequency response in the LVE master curves is combined with X-ray scattering and transmission electron microscopy to probe the diblock copolymer morphology. The ∼100 k molecular weight diblock copolymers exhibit lamellae morphology and longer range order is seen for the diblock copolymer with lowest ion content since the extent of phase mixing increases with ion content. No long range order can be observed in ∼50 k molecular weight diblock copolymers, and the lowest molecular weight (∼25 k) diblock copolymers are disordered at all temperatures studied.