Composition–Structure–Property Mappings Defining Ionic Liquid Crystal Elastomers

Abstract

Incorporating ionic groups within liquid crystalline matrices expands the palette of functionalities beyond traditional thermomechanical responses. This study explored a range of ionic monomers, which were copolymerized with canonical liquid crystalline monomers to synthesize ionic liquid crystal elastomers (iLCE). Introducing ionic groups allows for modulation of the nematic order as a function of the structure, concentration, and polarity of the ionic moieties. Imidazolium-based monomers are distinguished by their ability to preserve the nematic order, in contrast to monomers characterized by anionic moieties as side groups. The structural attributes of the ionic groups also impact dielectric constants of these materials. Again, the divinyl imidazolium-based systems are distinguished by static dielectric constants of approximately 100. The dielectric constants correlate with the glass transition temperatures, and the glass transition temperatures correlate with the DC conductivity. The effect of ionic groups among the mesophases impacts a host of transport, electromechanical, and thermomechanical responses. The modulation of the mesophase behavior by substituting the diacrylate reactive mesogens with monoacrylate systems offers another route for controlling the properties of iLCE. Weakening the coupling between the mesophases by tailoring their core structure offers a direct route for tuning the nematic isotropic temperature of the iLCE and the resulting thermomechanical actuation profiles. The utilization of PEDOT:PSS electrodes allows for harnessing the ion transport to elicit athermal actuation. The temporal response of this actuation scales with the ionic conductivity characterizing the materials. The ability to tune the mechanical properties in iLCE as a function of their composition also allows for magnifying the adhesion properties. The systems characterized by polymerizable anionic species are present significantly stronger pressure-sensitive adhesion than the imidazolium-based systems. The broad trade space of properties presented by iLCE presents compelling directions for modulating composition–structure–property mappings in molecularly ordered polymers, where modalities for cross-coupling between stimuli become possible.