(16d) Ionic Liquids in Polyurethane Ionomers
Single-ion conductors have potential utility for electronic devices like battery membranes, actuators and other applications. For such applications, both high ionic conductivities and high modulus are desirable. To achieve these two goals, polyurethane ionomers can be suitable candidates: the hydrogen bonded hard phase provides good modulus while the soft phase (PEO in our system) can still facilitate ion transportation. However, the presence of ions such as sodium ions usually elaborates soft phase Tg and resulting in low ionic conductivity.
In our study, para-phenylene diisocyanate (pPDI) was chosen as the hard segment since its symmetry and tendency to microphase separate from soft phase which is based on polyethylene glycol (PEG) due to its good solvation ability to ions. Anionic groups are either attached in the hard segment or soft segment with mobile cations range from small single-atom sodium ions to multi-atom ionic liquid type cations including ammonium, imidazolium, and phosphonium.
The thermal and counterion dynamic properties of the ionomers were characterized by differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DRS). Introducing ionic liquid type cations reduces the soft phase Tg because of much weaker Coulombic interaction between cation and anion. By replacing Na+ with large ether-oxygen containing ammonium, Tg can be reduced by 60K and ionic conductivity was significantly improved up to 5 orders of magnitude. The conductivity was found strongly correlated to polymer chain relaxation as prediction of Debye-Stokes-Einstein (DSE) equation suggesting that ion transport is based on cation diffusion along polymer chain. To further understand ion transport under electric field, we apply an electrode polarization (EP) model to separate the contribution of number density of charge carriers and their mobility. The conducting ion content was found to follow Arrhenius equation and the cation mobility has Vogel-Fulcher-Tammann (VFT) temperature dependence.