Investigating the electromechanical response mechanism of ionic block copolymers

One of the rapidly developing frontiers in research and engineering applications is focused on the rational design of materials that have targeted functionalities and tunable responses to external stimuli. Such materials stand to revolutionize how structural components are designed for applications including flexible electronics, biomimetic devices, nanofluidics, as well as separation chemistry. A chief obstacle to achieving this capability is the lack of a fundamental understanding of how chemical structure and morphology give rise to macromolecular properties. With this in mind, our research explores how polymer structure, counterion species, and film morphology affects the electromechanical response of materials when exposed to external electric fields. We report here the development of an AB ionic diblock copolymer Poly(lBDIMAPF₆-bstyrene) (hereafter, 1-BDIMA/PS) where an imidazolium ring paired with a PF₆ counter-anion in the ionic B block imparts electromechanical responsiveness, while polystyrene provides structural support as the A block. As a thin (∼30 nm) film on a Si substrate, 1-BDIMA/PS adopts a semi-lamellar structure as confirmed by neutron reflectometry (NR) and TOF-SIMS. Additionally, when 1-BDIMA/PS is prepared as a thick (∼100 μm) membrane sandwiched between two gold leaf electrodes, the film actuates under low applied potentials (< 1 V). Preliminary simulations on coarse-grained models of a similar system also confirm counter-ion mobility under applied electric fields. Collectively, these results indicate that 1-BDIMA/PS is not only an excellent candidate for future electromechanical responsiveness studies, but that the sensitivity of NR to scattering length density as well as subnanometer changes in thickness make it a promising tool for investigating polymer and counterion mobility under applied fields in the future.