Synthesis and Morphology of Semifluorinated Polymeric Ionic Liquids

Abstract

Polymeric ionic liquids (POILs) are important materials in the field of ionic liquid gating, requiring the precise synthesis of new POILs with tailored structural variability and defined nanoscaled structure. In the current contribution, using reversible addition–fragmentation chain-transfer polymerization (RAFT) technique, the homopolymerization of three imidazolium-based acrylates with different counterions is reported, namely 1-[2-acryloylethyl]-3-methylimidazolium bis(trifluoromethane)sulfonamide (APMIN(Tf)2), 1-[2-acryloylethyl]-3-methylimidazolium hexafluorophosphate (APMIPF6), and 1-[2-acryloylethyl]-3-methylimidazolium tetrafluoroborate (APMIBF4), to afford the respective poly(ionic liquid)s (POILs). All polymerizations display pseudo-first-order kinetics and a rapid growth of P(APMIN(Tf)2), P(APMIPF6), and P(APMIBF4), yielding homopolymers with controlled molar mass, revealing a strong influence of the counterions on the polymerization rate, increasing in the order of BF4Θ < PF6Θ < N(Tf)2Θ. As a direct determination of molecular weights via size exclusion chromatography (SEC) of the POIL homopolymers from RI and UV detectors was not successful, we developed an alternative strategy to generate accurate, “normal” SEC peaks of POILs using RAFT copolymerization technique: APMIN(Tf)2 was copolymerized with a semifluorinated monomer 2,2,2-trifluoroethyl acrylate (TFEA), allowing to study the influence of the comonomer feeding ratio on the resulting SEC signals of P(APMIN(Tf)2-co-TFEA) copolymers. We found that when the feeding molar ratio of TFEA is adjusted to 0.77, symmetric SEC peaks from the resulting P(APMIN(Tf)2-co-TFEA) copolymers are obtained. Furthermore, copolymerizations of TFEA with the other two IL monomers, APMIPF6 and APMIBF4, are also performed to afford P(APMIPF6-co-TFEA) and P(APMIBF4-co-TFEA) copolymers. Moreover, the propensity of the so-obtained POIL random copolymers P(APMIN(Tf)2-co-TFEA), P(APMIPF6-co-TFEA), and P(APMIBF4-co-TFEA) to grow a new block (polypentafluorostyrene, PPFS) is explored, intending to generate the fluorinated POIL triblock copolymers P(APMIN(Tf)2-co-TFEA)-b-PPFS-b-P(APMIN(Tf)2-co-TFEA), P(APMIPF6-co-TFEA)-b-PPFS-b-P(APMIPF6-co-TFEA), and P(APMIBF4-co-TFEA)-b-PPFS-b-P(APMIBF4-co-TFEA), respectively. The morphology and size of such semifluorinated POILs are investigated using transition electron microscopy (TEM), atomic force microscopy (AFM), and dynamic light scattering (DLS), revealing the aggregated nanoparticles from P(APMIN(Tf)2-co-TFEA) due to the mesoscale organization of the ionic “multiplets”. Significantly larger and crowded globular objects/aggregates are formed from the chain-extended POIL triblock P(APMIN(Tf)2-co-TFEA)-b-PPFS-b-P(APMIN(Tf)2-co-TFEA) copolymers under the same conditions.