September 4th, 2020
June 26th, 2020
June 8th, 2020
March 27th, 2020
Ion Dynamics of Monomeric Ionic Liquids Polymerized In Situ within Silica Nanopores
ACS Applied Materials & Interfaces, 2020, 12(39), 44325-44334.
Polymerized ionic liquids are a promising class of versatile solid-state electrolytes for applications ranging from electrochemical energy storage to flexible smart materials that remain limited by their relatively low ionic conductivities compared to conventional electrolytes. Here, we show that the in situ polymerization of the vinyl cationic monomer, 1-ethyl-3-vinylimidazolium with the bis(trifluoromethanesulfonyl)imide counteranion, under nanoconfinement within 7.5 ± 1.0 nm diameter nanopores results in a nearly 1000-fold enhancement in the ionic conductivity compared to the material polymerized in bulk. Using insights from broadband dielectric and Raman spectroscopic techniques, we attribute these results to the role of confinement on molecular conformations, ion coordination, and subsequently the ionic conductivity in the polymerized ionic liquid. These results contribute to the understanding of the dynamics of nanoconfined molecules and show that in situ polymerization under nanoscale geometric confinement is a promising path toward enhancing ion conductivity in polymer electrolytes.
Wetting and Chain Packing across Interfacial Zones Affect Distribution of Relaxations in Polymer and Polymer-Grafted Nanocomposites
Macromolecules, 2020, 53(13), 5315-5325.
Polymers exhibit deviations from their bulk physical properties in the vicinity of solid interfaces due to changes in configurations, entanglements, and relaxation dynamics at the interfacial regions. By comparing grafted and nongrafted polymer nanocomposite systems based on poly(methyl methacrylate) and silica, we show that the distribution of relaxation times exhibits both commonly reported slower mobility and faster modes that depend on the nature of the interfacial zone, matrix molecular weight, and loading level of nanomaterials. These findings are derived from studies using broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC) to probe molecular and interfacial dynamics. By systematically examining nanocomposites based on nonfunctionalized "bare" Si nanoparticles (NPs) dispersed in PMMA matrices and on PMMA-grafted Si NPs (PMMA-g-NPs) in PMMA matrices, we probe the effects of interfacial interactions and confinement in each of these cases on the glass transition temperature, Tg, the mean time scales, and spectral shapes of the dielectric relaxation. The faster relaxation modes are attributed to the increasing importance of chain wetting and packing in the interfacial zones around nanofillers, especially in the polymer-grafted system. These insights are used to generate a unifying molecular framework that explains the enhancement in numerous macroscopic physical properties of polymer and polymer-grafted nanocomposites, which suits them for myriad applications.
Surface-Induced Ordering Depresses Through-Film Ionic Conductivity in Lamellar Block Copolymer Electrolytes
ACS Macro Letters, 2020, 9(4), 565-570.
Lamellar block copolymers based on polymeric ionic liquids (PILs) show promise as electrolytes in electrochemical devices. However, these systems often display structural anisotropy that depresses the through-film ionic conductivity. This work hypothesizes that structural anisotropy is a consequence of surface-induced ordering, where preferential adsorption of one block at the electrode drives a short-range stacking of the lamellae. This point was examined with lamellar diblock copolymers of polystyrene (PS) and poly(1-(2-acryloyloxyethyl)-3-butylimidazolium bis(trifluoromethanesulfonyl)imide) (PIL). The bulk PS-PIL structure was comprised of randomly oriented lamellar grains. However, in thin PS-PIL films (100-400 nm), the lamellae were stacked normal to the plane of the film, and islands/holes were observed when the as-prepared film thickness was incommensurate with the natural lamellar periodicity. Both of these attributes are well-known consequences of preferential wetting at surfaces. The ionic conductivity of thick PS-PIL films (50-100 ?m) was approximately 20x higher in the in-plane direction than in the through-plane direction, consistent with a mixed structure comprised of randomly oriented lamellae throughout the interior of the film and highly oriented lamellae at the electrode surface. Therefore, to fully optimize the performance of a block copolymer electrolyte, it is important to consider the effects of surface interactions on the ordering of domains.