Polymerized Ionic Liquid Triblock Terpolymers: Synthesis and Characterization

Polymerized ionic liquid (PIL) block copolymers are a distinct set of block copolymers that incorporate unique physiochemical properties of PILs (e.g., high solid-state ionic conductivity, high chemical, thermal and electrochemical stability, and widely tunable physical properties) into block copolymer architecture, which allows for self-assembly into a range of nanostructures, where morphology type and domain size are tunable. To date, PIL block copolymers have been synthesized by a number of research groups and have shown distinct conductivity-morphology relationships as it relates to various applications. Several block chemistries have been explored, yet almost all reports are exclusive to (AB) diblock copolymers. Although AB diblock copolymers can provide orthogonal properties of high ion conduction and high mechanical strength in a solid-state material, there are limitations to AB diblock copolymers, such as limited set of morphologies, domain sizes, and a lack of means to achieve a combination of more than two properties simultaneously (e.g., conductivity, strength, flexibility). Moreover, spatially connected 3D network morphologies, which often result in the highest ion conductivities, occur only over a small compositional range in AB diblock copolymers.

In this study, a PIL (ABC) triblock terpolymer was synthesized to explore a PIL-containing polymer with a richer compliment of properties, a wider diversity of cation/backbone chemistries, and a broader compositional range to obtain a variety of continuous 3D network morphologies. Specifically, poly(MMA-b-MEBIm-Br-b-HMA) was synthesized via sequential reversible addition-fragmentation chain-transfer (RAFT) polymerization and subsequently quaternized, where A block = methyl methacrylate (MMA), B block or PIL block = 1-[(2-methacryloyloxy)ethyl]-3-butylimidazolium bromide] (MEBIm-Br), and C block = hexyl methacrylate (HMA). Reaction kinetics of this PIL ABC triblock terpolymer were investigated and these results were utilized in scaling up the polymerization reaction to > 2 g quantities with narrow polydispersities and controllable block compositions. Chemical, thermal, morphological, and ion conductive properties were characterized using ¹H nuclear magnetic resonance (NMR) spectroscopy, elemental analysis (EA), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), small angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and electrical impedance spectroscopy (EIS).