Novel imidazolium-based poly(ionic liquid)s with different counterions for self-healing

Abstract

Novel imidazolium-based norbornene polymerized ionic liquids (PILs) with different counterions (CH₃SO₃⁻, CF₃SO₃⁻, CF₃(CF₂)₃SO₃⁻, FSI⁻, Tf₂N⁻) for self-healing are synthesized via ring-opening metathesis polymerization (ROMP) utilizing the Grubbs third generation catalyst (G3). As proved by the changes in the chemical shift of the imidazolium proton in ¹H NMR spectra in CDCl₃, variation of the counterion brings different interaction intensities between imidazolium and counterions. The glass transition temperature (T_g) was mainly influenced by the size of the counterion. For large counterions such as FSI⁻ and Tf₂N⁻, they not only formed loose ionic aggregates leading to weak physical cross-links, but also acted as plasticizers in the polymer matrix, resulting in a decrease of T_g. X-ray scattering tests also proved that small counterions were prone to forming packed ion clusters leading to intensive ion aggregation. The ionic associations in the polymer matrix serve as physical cross-links, restricting the mobility of the surrounding polymer chain and improving the mechanical strength. However, sufficient chain mobility was in favor of the dynamic rearrangement of ion aggregation over the fracture surface that imparted the self-healing behavior. To balance the mechanical properties and healing performance, PIL blends with appropriate T_g were prepared by blending a high T_g PIL (such as CH₃SO₃⁻, CF₃SO₃⁻ or CF₃(CF₂)₃SO₃⁻) with varying amounts of a low T_g PIL (i.e. Tf₂N⁻). Via this approach, the T_g of the resulting blends could be systemically tuned by adjusting the blending ratio and counterion. As expected, both excellent mechanical performance and self-healing capability of the well-designed PILs were achieved simultaneously.