Ion Specific, Thin Film Confinement Effects on Conductivity in Polymerized Ionic Liquids

Abstract

Acrylate-based polymerized ionic liquids (PILs) with ammonium (Am) or imidazolium (Im) cations and tetrafluoroborate (BF 4 ) or bis(trifluoromethanesulfonyl)imide (TFSI) anions were synthesized and spin coated onto gold interdigitated electrodes on silica to investigate nanoconfinement effects on ion transport. The film thickness ranged from 23 to 313 nm. A significant reduction of the inplane conductivity was observed in some PIL thin films with thickness below 100 nm. Specifically, Am BF 4 PIL showed the largest conductivity drop (ca. 50% difference between a 22 nm and a 261 nm film) while Im TFSI PIL showed almost no change under confinement. The difference in conductivity drop is discussed in terms of (i) differences in interfacial layer thickness by fitting a two-layer conductivity model and (ii) potential changes in glass transition temperature (T g ) under confinement, which were estimated using the Vogel−Fulcher−Tammann fits of bulk conductivities. Decreasing film thickness also caused the dielectric loss peaks to shift to lower frequency, indicating that the ion diffusion process slowed under confinement.

Ordered Nanostructures in Thin Films of Precise Ion-Containing Multiblock Copolymers

“…Understanding the structural characteristics of polymer thin films is critical to numerous emerging technologies. , For example, self-assembly of block copolymer ( e.g. , layer and cylinder) in thin films is essential to using these materials for nanolithography because the phase behavior can be significantly perturbed relative to the bulk behavior due to interfacial and confinement effects. − Also, conjugated polymers are widely studied in thin-film geometries because the alkyl chain crystallinity and π–π stacked assemblies impact the electron transport properties, , which are valuable for transistors, light-emitting diodes, and photovoltaics. − Similarly, structure–property relationships of ion-containing polymer thin films have received significant interest for their ion transport abilities. − The perfluorinated sulfonic acid polymers ( e.g. , Nafion) are among the most extensively studied ionomers for their applications in the catalyst layer of fuel cells and solar-fuel generators. , Specifically, the studies of Nafion have indicated that producing aligned morphologies of phase-separated sulfonic acid aggregates in a thin film can improve the proton transport properties or catalytic performance. ,− …”

“… 8 − 11 Similarly, structure–property relationships of ion-containing polymer thin films have received significant interest for their ion transport abilities. 12 − 15 The perfluorinated sulfonic acid polymers ( e.g. , Nafion) are among the most extensively studied ionomers for their applications in the catalyst layer of fuel cells and solar-fuel generators.…”

Eliminating the Tg-Confinement Effect in Polystyrene Films: Extraordinary Impact of a 2 mol % 2-Ethylhexyl Acrylate Comonomer

“…The glass transition temperature, T g , is a key characteristic of polymer materials because many physical and mechanical properties change markedly when a polymer undergoes a glass transition from a rubbery state to a glassy state or vice versa. , For polymer films confined at the nanoscale, perturbations originating at the polymer/free surface and the polymer/solid interfaces often result in deviations of the measured T g from the bulk T g . − Such nanoconfinement behavior is important because it can affect the commercial application of ultrathin polymer films and may aid in understanding the underlying mechanisms of glass transition behavior, which has been a long-standing, unresolved challenge. − Indeed, the T g -confinement behavior of polymers has been the subject of intense research since 1994 when Keddie et al , first reported that, relative to bulk T g , the T g s of polystyrene (PS) and poly­(methyl methacrylate) ultrathin films supported on silicon decrease and increase, respectively, with decreasing nanoscale thickness.…”

Structure and Dynamics of Star-Polymer/Polymer Nanocomposites

“…Conductivity in specific thin-film systems is influenced by solvation-segmental dynamics of ethylene-oxide side chains [23]. The same study that drew connections between substrate properties also concluded that conductivity is affected by confinement [17].…”

“…In some cases, the manufacturing of film substrates requires glass transition temperatures greater than the annealing temperature ≈ 150 • C [12]. Moreover, properties of the thin-film substrate, such as the conductivity, are also dominated by dynamics [17].…”