(Invited) Charge Transport and Structural Dynamics in Bulk and Ultrathin Films of Polymerized Ionic Liquids
Authors: Sangoro Joshua R 
Published: February 2020

Polymerized ionic liquids (PolyILs) are a novel class of functional polymers that combine the unique physicochemical properties of molecular ionic liquids with the outstanding mechanical characteristics of polymers [1,2]. This special mix of features might help to circumvent the key limitations of low molecular weight ionic liquids, namely, leakage and poor mechanical properties while utilizing their outstanding characteristics such as low vapor pressures, wide liquidus ranges, high thermal stability, high ionic conductivity, and wide electrochemical windows. PolyILs have shown remarkable advantages when employed in electrochemical devices such as dye-sensitized solar cells, lithium batteries, actuators, field-effect transistors, light emitting electrochemical cells, and electrochromic devices, among others. Despite their promising prospects as ideal polymer electrolytes, the role of molecular structure, morphology, and polymer dynamics on charge transport in PolyILs remains poorly understood. According to classical theories, the self-diffusion and ion transport in electrolytes are controlled by structural relaxation. These approaches predict similar temperature dependence for the dc conductivity and structural dynamics. Although this prediction has been shown to hold reasonably well for low molecular weight aprotic ionic liquids, it fails for PolyILs [2,3]. In addition, the impact of morphology charge transport is not considered within the framework of these theories.

            The first part of this talk will focus on new insights obtained from experimental studies employing broadband dielectric spectroscopy, temperature-modulated differential scanning calorimetry, rheology, and scattering techniques to elucidate charge transport and structural dynamics in a systematic series of bulk polymerized ammonium- and imidazolium- based ionic liquids. Detailed analyses reveal strong decoupling of these processes in the PolyILs, implying the limitation of the classical theories in describing charge transport and molecular dynamics in these materials, in contrast to low molecular weight systems (4,5). In addition, the strong correlation observed between ionic conductivity from dielectric experiments and morphologies from scattering studies will be reviewed.

In the second part of this talk, our recent results from a new approach to dielectric experiments yielding details about ion dynamics in ultrathin films of polymerized ionic liquids down to sub-5nm film thicknesses will be discussed. While the characteristic charge transport rates remain bulk-like down to about 5 nm film thicknesses, ionic-liquid/substrate interactions play an increasingly significant role in determining ion dynamics in confined polymerized ionic liquids (5). These results will be considered within the context of recent models of charge transport and polymer dynamics under confinement.

References

1)      Mecerreyes, D. Progress in Polymer Science 2011, 36, 1629.

2)      Sangoro, J. R. and Kremer, F. Accounts of Chemical Research 2012, 45, 525

3)      Sangoro, J. R. et al., Soft Matter 2014, 10, 3536.

4)      Wojnarowska, Z. et al., Macromolecules 2015, 48 (23), 8660-8666.

5)      Heres, M. F. et al., Submitted.