In-situ photopolymerization and gelation of ionic liquids

Ionic liquids pose a novel and tunable medium for bulk polymerization of vinyl monomers. 1-vinylimidazole (VIm) displays intriguing viscosity growth trends when mixed with lithium bistriflimide (LiTf₂N) to form an ionic liquid (IL) and then photopolymerized via a free-radical mechanism. Previous work has shown that increasing [LiTf₂N] results in faster chemical conversion. We used rheology to monitor the in-situ synthesis of poly(VIm) from ILs containing varying ratios of VIm:LiTf₂N. Three distinct regimes were observed: (1) at low [LiTf₂N], samples increased in viscosity faster with [LiTf₂N] and behaved as solutions; (2) at intermediate [LiTf₂N], viscosity growth was maximized and samples underwent sol-to-gel transitions; (3) at high [LiTf₂N], viscosity growth slowed with [LiTf₂N] and samples exhibited viscoelastic behavior. We attribute gelation to Li+ complexing with imidazole pendant groups to form physical crosslinks. Our samples also exhibited "dark curing", the extent of which increased with [LiTf₂N]. EPR spectroscopy demonstrates that Li+ can stabilize propagating radicals, leading to enhanced curing following UV exposure. Taken together, these results demonstrate that the rheological behavior of photopolymerized ILs can be tuned by varying the composition and UV dosage.