Ion Gel Network Formation in an Ionic Liquid Studied by Time-Resolved Small-Angle Neutron Scattering.

We report the time-resolved small-angle neutron scattering (SANS) study of tetra-arm poly(ethylene glycol) (TetraPEG) polymer network formation in a typical ionic liquid (IL), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mim][TFSA]). To observe time-dependent SANS profiles, the reaction rate for the AB-type cross-end coupling reaction of TetraPEG macromers was controlled by adding an analogous protic IL, 1-ethylimidazolium TFSA ([C2imH][TFSA]). At polymer concentrations higher than the overlap concentration ( c*), the SANS profile remained unchanged during the gelation reaction, indicating that the network structure was independent of macromer connectivity in a semidiluted solution. On the other hand, at low polymer concentrations, an increase in the SANS profile intensity was clearly observed. The correlation length (ξ), estimated by a fitting analysis based on the Ornstein-Zernike function, increased as the reaction proceeded. This result indicated that the sparsely dispersed macromers formed clusters during the cross-linking process and polymer size growth followed thereafter. We found that the network formation process and homogeneity of the network structure were strongly dependent on the polymer concentration in IL solutions.