Tiago Outerelo Corvoa,b,d , Eric Drockenmullerc, Frédéric Restagnoa, Alexis Chennevièreb
a Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, 91405, Orsay, France
b Université Paris Saclay, Laboratoire Léon Brillouin, UMR 12 CNRS-CEA, CEA-Saclay, 91191, Gif-sur-Yvette, France
c Université Claude Bernard Lyon 1, CNRS, Ingénierie des Matériaux Polymères, UMR 5223, 69622, Villeurbanne, France
d Current address Dept. of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, United States
Lien vers la publication : Structure of Poly(ionic liquid)s in Bulk and Solutions by Small-Angle Neutron Scattering
Poly(ionic liquid)s (PILs) 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 solid-state viscoelastic electrolytes. They inherit their original nanostructure from ILs, arising from the segregation of the alkyl moiety leading to either a globular or a bicontinuous sponge-like structure depending on the side chain length [1,2]. Although the local structure is similar to that of ILs [3,4], its impact in a polymeric point of view, such as in terms of the chain conformation, remained unclear.
The present work focuses on a series of model imidazolium-based PILs (PCnVImTFSI) synthesized such as the side-chain length n is the only varying parameter with fixed degree of polymerization and dispersity, thereby tailoring the local interactions. Small angle neutron scattering was performed on a mixture of hydrogenated and deuterated PILs, yielding the scattering patterns in Figure 1a. The bulk structure of PIL melts is thus probed at length scales ranging from the polymer coil all the way down to the chain diameter and the distance between neighbouring chains. At the local scale in Figure 1b and with increasing n, the diameter of chains grows larger than the distance between backbones, hence providing direct evidence of interdigitation of long side chains. The most surprising result lies nonetheless at the scale of the polymer coil in Figure 1c, with a radius of gyration displaying a non-monotonic evolution with n, which only happens in melt and not in solutions. This suggests the flexibility of the main chain in the melt varies with n, with a potential contribution of modulated electrostatic repulsions before steric effects take over for longer side chains.
These SANS results highlight the importance of such a systematic approach using a model PIL series with varying side-chain length, bringing to light some remarkable properties of these materials. It paved the way to a broader investigation of the influence of local interactions. The distinct behaviours between short and long side chain PILs are somehow connected to other aspects of the materials. In terms of viscoelastic properties, long side chain PILs seem to behave similarly to other ion-containing polymers, such as ionomers, but the short side-chain ones exhibit contrasting properties. In thin films, the interfacial structure of these materials is again highly dependent on the side chain length. These aspects will soon be published in other contributions.
Figure 1: (a) SANS data of 1:1 mixtures of hydrogenated and deuterated PCnVImTFSI analogues of varying alkyl side chain length in the melt. (b) Chain diameter , and distance between neighbouring chains as functions of side chain length. (c) Radius of gyration as a function of side chain length. Inner sketch in (b) represents the interdigitated side chain situation.
References
[1] Canongia Lopes et al. “Nanostructural organization in ionic liquids.” The Journal of Physical Chemistry B 110.7 (2006): 3330-3335.
[2] Triolo A., et al. “Nanoscale segregation in room temperature ionic liquids.” The Journal of Physical Chemistry B 111.18 (2007): 4641-4644.
[3] H. Liu, and S. J. Paddison. “Alkyl chain length dependence of backbone-to-backbone distance in polymerized ionic liquids: An atomistic simulation perspective on scattering.” Macromolecules 50.7 (2017): 2889-2895.
[4] C. Iacob, et al. “Polymerized ionic liquids: Correlation of ionic conductivity with nanoscale morphology and counterion volume.” ACS Macro Letters 6.9 (2017): 941-946