Polymerized ionic liquids (PIL) are macromolecules composed of covalently linked ionic liquid (IL) monomers. As such, they present a unique combination of properties from both polymers and ILs and are promising for potential applications as solid-state electrolytes. They inherit their particular nanostructure from ILs, arising from the segregation of the alkyl moiety leading, in imidazolium-based ILs, to either a globular or a bicontinuous sponge-like structure depending on the alkyl chain length. The addition of such interactions to macromolecules provides new opportunities to design polymer materials with targeted functionalities, highly related to both structural and dynamic properties of PILs.
The present work focuses on a series of imidazolium-based PILs (PCnVImTFSI) with varying side-chain length n, thereby finely tuning the local interactions. Using small angle neutron scattering and wide angle X-ray scattering, the bulk structure was probed at length scales ranging from the polymer coil to the chain diameter. The former showed a surprising non-monotonic evolution with n, while at the latter scale, interdigitation of was observed for long side-chains. This suggests the flexibility of the main chain varies with n, with a potential contribution of modulated electrostatic repulsions.
Although numerous applications rely on interfacial behaviours, and ILs themselves are unusual in that regard, arranging in a layered structure, very little is known about PILs at interfaces. To address this, we focused on the structure of spin-coated thin films of the same series of PILs using both specular X-ray reflectivity (XRR) and Grazing Incidence Wide Angle X-Ray Scattering (GIWAXS), so that all directions of space were explored. While short side-chain PCnVImTFSI form homogenous films, a lamellar structure develops in long sidechain ones. Both the interlayer spacing and the in-plane distance between neighbouring chains in the vicinity of the interface depend on the thickness of the film. We suggest a molecular picture of such confinement effect, involving a preferential orientation of pendant groups close to the interface in thinner films.
Besides deeply interesting structural considerations, these materials are also rare examples of polyelectrolytes presenting a melt state, despite their ionic nature, akin to ionomers. An increase in ionic content, however, leads to the loss of ionomers’ processability. PILs, on the other hand, have an ionic fraction of 100% without losing their viscoelastic properties. Changing the length of the alkyl side chain, as it was done in our structural study, is a way to vary the polarity of interactions at fixed ion fraction. Hence, the linear viscoelasticity of the PCnVImTFSI series with varying sidechain length n was probed. These results are discussed in connection with the structural features mentioned above and similarities with associating polymers such as ionomers.
This fundamental approach of the structure and dynamics of a model PIL series with varying side-chain length brought to light some remarkable properties of these materials. Hopefully this could ultimately be extended to other and more complex PIL chemical structures and provide better guidance to make use of the wide variety of IL ion pairs, polymeric architectures, temperature conditions or confinement design to reach precise targeting of applications.
LPS and LLB