Design of Mixed Electron- and Ion-Conducting Radical Polymer-Based Blends

A nonconjugated radical polymer enables bimodal memory and in-sensor computing operation

This study reports intrinsic multimodal memristivity of a nonconjugated radical polymer with ambient stability. Organic memristive devices represent powerful candidates for biorealistic data storage and processing. However, there exists a substantial knowledge gap in realizing the synthetic biorealistic systems capable of effectively emulating the cooperative and multimodal activation processes in biological systems. In addition, conventional organic memristive materials are centered on conjugated small and macromolecules, making them synthetically challenging or difficult to process. In this work, we first describe the intrinsic resistive switching of the radical polymer that resulted in an exceptional state retention of >105 s and on/off ratio of >106. Next, we demonstrate its bimodal cooperative switching, in response to the proton accumulation as a biological input. Last, we expand our system toward an advanced in-sensor computing system. Our research demonstrates a nonconjugated radical polymer with intrinsic memristivity, which is directly applicable to future electronics including data storage, neuromorphics, and in-sensor computing.

Conductive Glassy Nonconjugated Open-Shell Radical Polymer with Organosulfur Backbone for Macroscopic Conductivity

Nonconjugated organic radicals with an open-shell radical active group exhibit unique functionality due to their radical pendant site. Compared with the previously studied doped conjugated polymers, radical polymers reveal superior processability, stability, and optical properties. Despite the success of organic radical polymer conductors based on the TEMPO radicals, it still requires potential design substitutions to meet the fundamental limits of charge transport in the radical polymer. To do so, we demonstrate that the amorphous, nonconjugated radical polymer with backbone-pendant group interaction and low glass transition temperature enables the macromolecules to have rapid charge transport in the solid state, resulting in conductivity higher than 32 S m-1. This charge transport is due to the formation of the local ordered regime with an energetically favored orientation caused by the strong coupling between the backbone and pendant group, which can significantly modulate the polymer packing with active electronic communications. The nonconjugate nature of the radical polymer maintains an optical transparency up to 98% at a 1.5 μm thick film. Thus, this effort will be a dramatically advanced model in the organic radical community for the creation of next-generation polymer conductors.

Graphene-Enhanced Ion Transport in Dual-Conducting Composite Films of Polyacetylene and an Imidazolium Iodide Ionic Liquid

Dual-conducting polymer films were synthesized by dispersing graphene in an aqueous solution of poly(vinyl alcohol) and 1-propyl-3-methylimidazolium iodide ([C₃mim]I) ionic liquid and thermally converting the poly(vinyl alcohol) to polyene in the presence of hydroiodic acid catalyst. The electrical and mechanical properties of the resulting free-standing films of the nanocomposite, containing different concentrations of graphene, were analyzed using electrochemical impedance spectroscopy (EIS) and dynamic mechanical analysis (DMA), respectively. Nyquist plots (imaginary vs real components of the frequency-dependent impedance) showed two characteristic arcs representing the composite's electronic and ionic conduction pathways. The conductivity values corresponding to both charge transport mechanisms increased with temperature and the graphene concentration. The enhancement in electronic conductivity is expected because of graphene's high electron mobility. Interestingly, ionic conductivity also showed a significant increase with graphene concentration, approximately triple the extent of the rise in the electronic conductivity, even though the loss and storage moduli of the films increased. (Generally, a higher modulus results in lower ionic conductivities in ionic gels.) Molecular dynamics simulations of the three-component system provided some insights into this unusual behavior. Mean square displacement data showed that the diffusion of the iodide anions was relatively isotropic. The iodide diffusion coefficient was higher in a blend with 5 vol % graphene than in blends with 3 vol % graphene or no graphene. The improvement is attributed to the interfacial effects of the graphene on the free volume of the blend. Furthermore, an exclusion of the iodide ions from the vicinity of graphene was observed in the radial distribution function analysis. The increase in the effective concentration of iodide due to this exclusion and the increase in its diffusion coefficient because of the excess free volume are the primary reasons for the observed enhancement in ionic conductivity by adding graphene.

Radical Polymer-Based Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) are an emerging platform for bioelectronic applications. Significant effort has been placed in designing advanced polymers that simultaneously transport both charge and ions (i.e., macromolecules that are mixed conductors). However, the considerations for mixed organic conductors are often different from the established principles that are well-known in the solid-state organic electronics field; thus, the discovery of new OECT macromolecular systems is highly desired. Here, we demonstrate a new materials system by blending a radical polymer (i.e., a macromolecule with a nonconjugated backbone and with stable open-shell sites at its pendant group) with a frequently used conjugated polymer. Specifically, poly(4-glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl) (PTEO) was blended with poly(3-hexylthiophene) (P3HT) to create thin films with distinct closed-shell and open-shell domains. Importantly, the sharp and unique oxidation-reduction (redox) potential associated with the radical moieties of the PTEO chain provided a distinct actuation feature to the blended films that modulated the ionic transport of the OECT devices. In turn, this led to controlled regulation of the doping of the P3HT phase in the composite film. By decoupling the ionic and electronic transport into two distinct phases and by using an ion transport phase with well-controlled redox activity, never-before-seen performance for a P3HT-based OECT was observed. That is, at loadings as low as 5% PTEO (by weight) OECTs achieved figure-of-merit (i.e., μC*) values >150 F V^-1 cm^-1 s^-1, which place the performance on the same order as state-of-the-art conjugated polymers despite the relatively common conjugated macromolecular moiety implemented. As such, this effort presents a design platform by which to readily create a tailored OECT response through strategic macromolecular selection and polymer processing.

Electronic and Magnetic Properties of a Three-Arm Nonconjugated Open-Shell Macromolecule

Nonconjugated radical polymers (i.e., macromolecules with aliphatic backbones that have stable open-shell sites along their pendant groups) have arisen as an intriguing complement to π-conjugated polymers in organic electronic devices and may prove to have superior properties in magneto-responsive applications. To date, however, the design of nonconjugated radical polymers has primarily focused on linear homopolymer, copolymer, and block polymer motifs even though conjugated dendritic macromolecules (i.e., polyradicals) have shown significant promise in terms of their response under applied magnetic fields. Here, we address this gap in creating a nonconjugated, three-arm radical macromolecule with nitroxide open-shell sites using a straightforward, single-step reaction, and we evaluated the electronic and magnetic properties of this material using a combined computational and experimental approach. The synthetic approach employed resulted in a high-purity macromolecule with a well-defined molecular weight and narrow molecular weight distribution. Moreover, epoxide-based units were implemented in the three-arm radical macromolecule design, and this resulted in a nonlinear radical macromolecule with a low (i.e., below room temperature) glass transition temperature and one that was an amorphous material in the solid state. These properties allowed thin films of the three-arm radical macromolecule to have electrical conductivity values on par with many linear radical polymers previously reported, and our computational efforts suggest the potential of higher generation open-shell dendrimers to achieve advanced electronic and magnetic properties. Importantly, the three-arm radical macromolecule also demonstrated antiferromagnetic exchange coupling between spins at temperatures < 10 K. In this way, this effort puts forward key structure-property relationships in nonlinear radical macromolecules and presents a clear path for the creation of next-generation macromolecules of this type.