Pre- and post-functionalization of thermoresponsive cationic microgels with ionic liquid moieties carrying different counterions

Herein, we study the incorporation of hydrophilic-to-amphiphilic alkylated N-vinylimidazolium based ionic liquid monomers with different counter anions into N-vinylcaprolactam (VCL) based microgels via precipitation polymerization, and their resulting properties. Two strategies, following either a pre-functionalization of the comonomers by counterion exchange with subsequent polymerization, or post-functionalization via counterion exchange on pre-synthesized microgels, are applied and compared. The employed anions, i.e., halides (X-), methanesulfonate (MeS-), tetrafluoroborate (BF4-), and bistriflimide (NTf2-), were selected because of their kosmotropic-to-chaotropic solution properties. The resulting microgels with 10 mol% degree of functionalization were characterized by light scattering techniques, particularly with respect to their temperature-dependent swelling behavior and electrophoretic mobility. Differences in the counterion effect can be correlated to the solvation of the anions given by the Hofmeister series, cation-anion interaction strength, as well as the hydrophobicity of the vinylimidazolium moiety, i.e., the alkyl chain length. In this context, chaotropic anions will lead to a decrease in microgel size and electrophoretic mobility, while the volume phase transition temperature decreases as a function of the alkyl chain length. Moreover, the pre-functionalization approach can give access to different morphologies, such as anisotropic microgels, while generally restricting the microgel swelling, compared to the post-functionalization approach. Our results will allow for the controlled design of new functional microgels via simple precipitation polymerization with potential applications in drug delivery, biocatalysis and self-healing materials.