Imaging Gel Networks by In Situ Transmission Electron Microscopy

Visualizing polymer gel networks at the nanoscale remains an immense challenge. Transmission electron microscopy (TEM) can be performed on chemically and cryogenically stabilized gels, but either approach is prone to artefacts, and solvent immobilization puts dynamics of solutes, as well as of the network itself, out of reach. Here, TEM is explored for imaging gels solvated by ionic liquids, which due to nonvolatility, are stable in vacuum. Physical gels are created by frustrated crystallization of polymer solutions supersaturated by cooling. Beneficially, these gels are rigid, suppressing local structural fluctuations, and due to crystallinity, offer additional diffraction contrast. A model system is polyethylene glycol solvated by the ionic liquid ethylmethylimidazolium ethylsulfate [EMIM][EtSO4]; solutions of modest concentration gel across the openings of a TEM grid as temperature falls below the polymer's melting temperature. Imaging reveals a heterogeneous network with features from 5 to 200 nm, consistent with small angle x-ray data. The crystalline polymer forms high contrast linear struts that are interconnected by lower contrast amorphous polymer. A 3D representation of the network is gained by electron tomography.