Structure, solubility and solution rheology of poly(ionic liquids)*

We explore the solubility, scattering properties and rheological behaviour of a poly(ionic liquid) (PIL) in 24 solvents covering a broad range of dielectric constants. The PIL is found to be soluble in polar aprotic solvents, but not in non-polar or protic ones. For high dielectric constant media, (ε ≥ 30), the correlation length scales as ξ ∼ c^-1/2, as expected by the scaling theory. The local chain conformation is approximately independent of solvent permittivity if the Bjerrum length is ≤ 1 nm. As the Bjerrum length increases beyond 1 nm, chains become less stretched because more counterions condense onto the polymer backbone, and the electrostatic blob size increases.

For solvents with intermediate permitivitties (ε ≈17-30), the correlation length scales as ξ ∼ c^-1/3, suggesting a partially collapsed (pearl necklace) conformation. We use the Dobyrnin-Rubinstein model to estimate mass per bead and bead volume fraction, establishing correlations with the solvent’s properties. In tetrahydrofuran (ε ≈ 8), the scattering and rheological behaviour matches that of neutral polymers.

The scaling model expects semidilute non-entangled  solutions to follow Rouse dynamics. The specific viscosity (η_sp) is predicted to be equal to number of correlation blobs per chain (N/(cξ³)). We compute the ratio η_spcξ³ in several solvents and find that it does not take a constant value, in disagreement with the scaling model. Our results help establish a framework to understand the solution rheology of polymerised ionic liquids.