Alkyldimethylammoniumethyl Methacrylate Ionic Liquid Monomers and Their Poly(Ionic Liquid)s: Chain Length Dependent Crystallinity, Thermoresponsiveness, and Self‐Assembly

The studies of structure-property relationship in ionic liquid monomers (ILMs) and their corresponding poly(ionic liquid)s (PILs) are required to gain deeper understanding of their tunable physical, chemical, and mechanical properties. In this work, a series of 2-(alkyldimethylammonium)ethyl methacrylate bromide ([R-DMAEMA][Br]) ILMs are synthesized followed by free-radical polymerization to obtain the analogous PILs with alkyl chains of varying lengths. The introduction of long alkyl chain induces crystallinity to these ILMs, whose melting temperature, enthalpy of fusion, and crystallinity increase with increasing the length of alkyl-chain. Exchange of Br^- with bulkier NTf₂ ^- leads to a substantial change in the crystalline behavior of ILMs. The bromide-based PILs show detectable semicrystallinity due to the presence of crystallizable pendent long alkyl chains. Both bromide- and NTf₂ ^--based ILMs show strongly birefringent crystals of various morphologies but, the Br^--based PILs give fibrillar morphology of very weak birefringence. No such fibrils are observed for NTf₂ ^--based PILs. Except ILMs, the Br^--based PILs exhibit upper critical solution temperature (UCST)-type phase behavior in CHCl₃, THF, CHCl₃/ⁱPrOH, and THF/ⁱPrOH mixed solvents. The NTf₂ ^--based PILs exhibit such thermoresponsiveness only in THF. The Br^--based PILs self-assemble into vesicular nanostructures in CHCl₃ and THF capable of encapsulating model Eosin B dye.