Photorheology and Gelation during Polymerization of Coordinated Ionic Liquids

Ionic liquids (ILs) containing reactive groups provide a tunable medium for bulk polymerization and network formation with potential applications as 3D-printable materials. In this study, dynamic rheology and real-time Fourier transform infrared spectroscopy are used to monitor the in situ photopolymerization and gelation of coordinated ILs containing varying molar ratios of 1-vinylimidazole (Vim) to lithium bistriflimide (LiTf2N). Three distinct regimes are observed: (1) at low [LiTf2N], samples increase in complex shear modulus (G*) and conversion faster with increasing [LiTf2N] and behave as solutions; (2) at intermediate [LiTf2N], G* growth and conversion achieve local maxima, and samples undergo sol-to-gel transitions during polymerization; (3) at high [LiTf2N], G* growth and conversion slow with [LiTf2N], and samples exhibit viscoelastic material behavior. Gelation is attributed to Li+ coordination with imidazole pendant groups to form physical cross-links between polymer chains, while the three regimes reflect the interplay of competing effects of increased polymer content and coordination-induced cross-linking. Rheological dark curing is also observed at high [LiTf2N] due to continued physical cross-linking by Li+ after cessation of UV light.