New Method to Probe the Surface Properties of Polymer Thin Films by Two-Dimensional (2D) Inverse Gas Chromatography (iGC).

Polymer-based functional surface coatings are extensively used in advanced technologies, including optics, energy, and environmental applications. Surface thermodynamic properties profoundly impact the molecular interactions that control interfacial behaviors, such as adhesion and wettability, which in turn dictate coating processes and performance. Conventionally, contact angle measurements are used to assess the surface energy of polymer films and coatings, where the wettability of a surface is assessed using probe fluids (liquid drops). However, contact angle measurement oftentimes can be nontrivial due to the roughness or chemical heterogeneity of the solid surface, as well as the potential for the liquid drop to swell or even dissolve the material being measured. Alternatively, inverse gas chromatography (iGC) is a versatile technique to measure surface thermodynamics and Lewis acid-base properties while also providing environmental control such as temperature and humidity. Despite these benefits, the application of iGC has been limited to powders or fibers, while the direct measurement of supported thin films or coatings is still a nascent area of research. This creates a challenge when using iGC as a comprehensive platform for measuring the physicochemical properties of solid surfaces. Here, we demonstrate how to effectively use iGC to characterize the surface energy of supported polymer thin films by using a two-dimensional (2D) film holder and modifying operational controls, such as the concentration range of the injected gas probe molecules. This enables the precise control of surface coverage required for analyzing samples having minimal surface area, such as thin films. Poly(methyl methacrylate) (PMMA) was employed as a benchmark to determine suitable iGC parameters and to validate our approach on polymer thin films. The seminal work presented here expands the capability of state-of-the-art iGC to embrace supported thin films (2D iGC) that could either be smooth or display texture/roughness (patterned films) as well as coatings with heterogeneous chemical/structural composition.

The Role of Side Chains and Hydration on Mixed Charge Transport in n-Type Polymer Films.

Introducing ethylene glycol (EG) side chains to a conjugated polymer backbone is a well-established synthetic strategy for designing organic mixed ion-electron conductors (OMIECs). However, the impact that film swelling has on mixed conduction properties has yet to be scoped, particularly for electron-transporting (n-type) OMIECs. Here, the authors investigate the effect of the length of branched EG chains on mixed charge transport of n-type OMIECs based on a naphthalene-1,4,5,8-tetracarboxylic-diimide-bithiophene backbone. Atomic force microscopy (AFM), grazing-incidence wide-angle X-ray scattering (GIWAXS), and scanning tunneling microscopy (STM) are used to establish the similarities between the common-backbone films in dry conditions. Electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) and in situ GIWAXS measurements reveal stark changes in film swelling properties and microstructure during electrochemical doping, depending on the side chain length. It is found that even in the loss of the crystallite content upon contact with the aqueous electrolyte, the films can effectively transport charges and that it is rather the high water content that harms the electronic interconnectivity within the OMIEC films. These results highlight the importance of controlling water uptake in the films to impede charge transport in n-type electrochemical devices.

Salts as Additives: A Route to Improve Performance and Stability of n-Type Organic Electrochemical Transistors.

Organic electrochemical transistors (OECTs) are becoming increasingly ubiquitous in various applications at the interface with biological systems. However, their widespread use is hampered by the scarcity of electron-conducting (n-type) backbones and the poor performance and stability of the existing n-OECTs. Here, we introduce organic salts as a solution additive to improve the transduction capability, shelf life, and operational stability of n-OECTs. We demonstrate that the salt-cast devices present a 10-fold increase in transconductance and achieve at least one year-long stability, while the pristine devices degrade within four months of storage. The salt-added films show improved backbone planarity and greater charge delocalization, leading to higher electronic charge carrier mobility. These films show a distinctly porous morphology where the interconnectivity is affected by the salt type, responsible for OECT speed. The salt-based films display limited changes in morphology and show lower water uptake upon electrochemical doping, a possible reason for the improved device cycling stability. Our work provides a new and easy route to improve n-type OECT performance and stability, which can be adapted for other electrochemical devices with n-type films operating at the aqueous electrolyte interface.

Trace Water Effects on Crystalline 1-Ethyl-3-methylimidazolium Acetate.

Spontaneous room-temperature crystallization of 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]) was observed upon removal of trace water. Sample purity was confirmed using analytical nuclear magnetic resonance spectroscopy to ensure that trace water or other contaminants did not produce this observation. Raman spectroscopy and simultaneous quartz crystal microbalance/infrared spectroscopy measurements were used to study molecular reorganization during crystallization and decrystallization using trace water in the form of atmospheric moisture. These experimental results were supplemented with density functional theory calculations that indicate imidazolium cation ring stacking and side chain clustering with an exclusive arrangement of the acetate anion in the cation ring plane upon water removal. Crystal structure formation was confirmed using two-dimensional wide-angle X-ray scattering. This natural crystallization is attributed to the removal of trace water over extended periods of time and calls attention to the molecular-level role of water in the structure of hygroscopic ionic liquid systems.

Hydration of a Side-Chain-Free n-Type Semiconducting Ladder Polymer Driven by Electrochemical Doping.

We study the organic electrochemical transistor (OECT) performance of the ladder polymer poly(benzimidazobenzophenanthroline) (BBL) in an attempt to better understand how an apparently hydrophobic side-chain-free polymer is able to operate as an OECT with favorable redox kinetics in an aqueous environment. We examine two BBLs of different molecular masses from different sources. Regardless of molecular mass, both BBLs show significant film swelling during the initial reduction step. By combining electrochemical quartz crystal microbalance gravimetry, in-operando atomic force microscopy, and both ex-situ and in-operando grazing incidence wide-angle X-ray scattering (GIWAXS), we provide a detailed structural picture of the electrochemical charge injection process in BBL in the absence of any hydrophilic side-chains. Compared with ex-situ measurements, in-operando GIWAXS shows both more swelling upon electrochemical doping than has previously been recognized and less contraction upon dedoping. The data show that BBL films undergo an irreversible hydration driven by the initial electrochemical doping cycle with significant water retention and lamellar expansion that persists across subsequent oxidation/reduction cycles. This swelling creates a hydrophilic environment that facilitates the subsequent fast hydrated ion transport in the absence of the hydrophilic side-chains used in many other polymer systems. Due to its rigid ladder backbone and absence of hydrophilic side-chains, the primary BBL water uptake does not significantly degrade the crystalline order, and the original dehydrated, unswelled state can be recovered after drying. The combination of doping induced hydrophilicity and robust crystalline order leads to efficient ionic transport and good stability.

In Situ Studies of the Swelling by an Electrolyte in Electrochemical Doping of Ethylene Glycol-Substituted Polythiophene.

Organic mixed ionic electronic conductors (OMIECs) have the potential to enable diverse new technologies, ranging from biosensors to flexible energy storage devices and neuromorphic computing platforms. However, a study of these materials in their operating state, which convolves both passive and potential-driven solvent, cation, and anion ingress, is extremely difficult, inhibiting rational material design. In this report, we present a novel approach to the in situ studies of the electrochemical switching of a prototypical OMIEC based on oligoethylene glycol (oEG) substitution of semicrystalline regioregular polythiophene via grazing-incidence X-ray scattering. By studying the crystal lattice both dry and in contact with the electrolyte while maintaining potential control, we can directly observe the evolution of the crystalline domains and their relationship to film performance in an electrochemically gated transistor. Despite the oEG side-chain enabling bulk electrolyte uptake, we find that the crystalline regions are relatively hydrophobic, exhibiting little (less than one water per thiophene) swelling of the undoped polymer, suggesting that the amorphous regions dominate the reported passive swelling behavior. With applied potential, we observe that the π-π separation in the crystals contracts while the lamella spacing increases in a balanced fashion, resulting in a negligible change in the crystal volume. The potential-induced changes in the crystal structure do not clearly correlate to the electrical performance of the film as an organic electrochemical transistor, suggesting that the transistor performance is strongly influenced by the amorphous regions of the film.

Reversible Electrochemical Charging of n-Type Conjugated Polymer Electrodes in Aqueous Electrolytes.

Conjugated polymers achieve redox activity in electrochemical devices by combining redox-active, electronically conducting backbones with ion-transporting side chains that can be tuned for different electrolytes. In aqueous electrolytes, redox activity can be accomplished by attaching hydrophilic side chains to the polymer backbone, which enables ionic transport and allows volumetric charging of polymer electrodes. While this approach has been beneficial for achieving fast electrochemical charging in aqueous solutions, little is known about the relationship between water uptake by the polymers during electrochemical charging and the stability and redox potentials of the electrodes, particularly for electron-transporting conjugated polymers. We find that excessive water uptake during the electrochemical charging of polymer electrodes harms the reversibility of electrochemical processes and results in irreversible swelling of the polymer. We show that small changes of the side chain composition can significantly increase the reversibility of the redox behavior of the materials in aqueous electrolytes, improving the capacity of the polymer by more than one order of magnitude. Finally, we show that tuning the local environment of the redox-active polymer by attaching hydrophilic side chains can help to reach high fractions of the theoretical capacity for single-phase electrodes in aqueous electrolytes. Our work shows the importance of chemical design strategies for achieving high electrochemical stability for conjugated polymers in aqueous electrolytes.

Generalizable Framework for Algorithmic Interpretation of Thin Film Morphologies in Scanning Probe Images.

We describe an open-source and widely adaptable Python library that recognizes morphological features and domains in images collected via scanning probe microscopy. π-Conjugated polymers (CPs) are ideal for evaluating the Materials Morphology Python (m2py) library because of their wide range of morphologies and feature sizes. Using thin films of nanostructured CPs, we demonstrate the functionality of a general m2py workflow. We apply numerical methods to enhance the signals collected by the scanning probe, followed by Principal Component Analysis (PCA) to reduce the dimensionality of the data. Then, a Gaussian Mixture Model segments every pixel in the image into phases, which have similar material-property signals. Finally, the phase-labeled pixels are grouped and labeled as morphological domains using either connected components labeling or persistence watershed segmentation. These tools are adaptable to any scanning probe measurement, so the labels that m2py generates will allow researchers to individually address and analyze the identified domains in the image. This level of control, allows one to describe the morphology of the system using quantitative and statistical descriptors such as the size, distribution, and shape of the domains. Such descriptors will enable researchers to quantitatively track and compare differences within and between samples.

Ion Exchange Gels Allow Organic Electrochemical Transistor Operation with Hydrophobic Polymers in Aqueous Solution.

Conjugated-polymer-based organic electrochemical transistors (OECTs) are being studied for applications ranging from biochemical sensing to neural interfaces. While new polymers that interface digital electronics with the aqueous chemistry of life are being developed, the majority of high-performance organic transistor materials are poor at transporting biologically relevant ions. Here, the operating mode of an organic transistor is changed from that of an electrolyte-gated organic field-effect transistor (EGOFET) to that of an OECT by incorporating an ion exchange gel between the active layer and the aqueous electrolyte. This device works by taking up biologically relevant ions from solution and injecting more hydrophobic ions into the active layer. Using poly[2,5-bis(3-tetradecylthiophen-2-yl) thieno[3,2-b]thiophene] as the active layer and a blend of an ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, and poly(vinylidene fluoride-co-hexafluoropropylene) as the ion exchange gel, four orders of magnitude improvement in device transconductance and a 100-fold increase in kinetics are demonstrated. The ability of the ion-exchange-gel OECT to record biological signals by measuring the action potentials of a Venus flytrap is demonstrated. These results show the possibility of using interface engineering to open up a wider palette of organic semiconductors as OECTs that can be gated by aqueous solutions.

A Reversible Structural Phase Transition by Electrochemically-Driven Ion Injection into a Conjugated Polymer.

We find that conjugated polymers can undergo reversible structural phase transitions during electrochemical oxidation and ion injection. We study poly[2,5-bis(thiophenyl)-1,4-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)benzene] (PB2T-TEG), a conjugated polymer with glycolated side chains. Using grazing incidence wide-angle X-ray scattering (GIWAXS), we show that, in contrast to previously known polymers, this polymer switches between two structurally distinct crystalline phases associated with electrochemical oxidation/reduction in an aqueous electrolyte. Importantly, we show that this unique phase change behavior has important physical consequences for ion-polaron pair transport. Notably, using moving front experiments visualized by both optical microscopy and super-resolution photoinduced force microscopy (PiFM), we show that a laterally propagating ion-polaron pair front in PB2T-TEG exhibits non-Fickian transport, retaining a sharp step-edge profile, in stark contrast to the Fickian diffusion more commonly observed in polymers like P3MEEMT. This structural phase transition is reminiscent of those accompanying ion uptake in inorganic materials like LiFePO4. We propose that the engineering of similar properties in future conjugated polymers may enable the realization of new materials with superior performance in electrochemical energy storage or neuromorphic memory applications.

Fullerene Active Layers for n-Type Organic Electrochemical Transistors.

Organic electrochemical transistors (OECTs) are currently being developed for applications ranging from bioelectronics to neuromorphic computing. We show that fullerene derivatives with glycolated side chains can serve as n-type active layers for OECTs with figures of merit exceeding the best reported conjugated-polymer-based n-type OECTs. By comparing two different fullerene derivatives, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and 2-(2,3,4-tris(methoxtriglycol) phenyl) [60]fulleropyrrolidine (C60-TEG), we find that the hydrophilic glycolated side chains in C60-TEG enable volumetric doping of C60-TEG films. In contrast, the hydrophobic nature of PCBM prevents ions from penetrating into the material. Our results demonstrate that small-molecule semiconductors follow many of the same design principles established for conjugated polymers and can function as high-performing mixed electronic/ionic conductors for efficient, fast OECTs.

Polymer Crystallinity Controls Water Uptake in Glycol Side-Chain Polymer Organic Electrochemical Transistors.

We study poly(3-{[2-(2-methoxyethoxy)ethoxy]methyl}thiophene-2,5-diyl) (P3MEEMT), a new polythiophene derivative with ethylene glycol-based side chains, as a promising semiconducting polymer for accumulation-mode organic electrochemical transistors (OECTs) with figures of merit comparable to those of state-of-the-art materials. By characterizing the OECT performance of P3MEEMT transistors as a function of the anion, we find that large hydrophobic anions lower the threshold voltage. We find that, compared to poly(3-hexylthiophene-2,5-diyl) (P3HT), P3MEEMT has faster anion injection rates, which we attribute to the hydration of the P3MEEMT crystal lattice. We study P3MEEMT-based OECT and organic field-effect transistor (OFET) performance as a function of film crystallinity and show that changing the crystallinity of the polymer by thermal annealing increases the OFET mobility yet decreases the OECT mobility. We attribute this difference to the fact that, unlike OFETs, OECTs operate in aqueous environments. To probe how hydration affects the operation of OECTs, we investigate the role of water in electrochemical doping using electrochemical quartz microbalance (EQCM) gravimetry. We find that steady-state hydration and hydration dynamics under electrochemical bias differ dramatically between the crystalline and amorphous P3MEEMT films. These results suggest that the presence of water reduces the electronic connectivity between the crystalline regions of P3MEEMT, thus lowering the mobility in solution. Overall, our study highlights the importance of the role of polymer hydration and nanoscale morphology in elucidating design principles for OECT operation.

Incorporating spike-rate adaptation into a rate code in mathematical and biological neurons.

For a slowly varying stimulus, the simplest relationship between a neuron's input and output is a rate code, in which the spike rate is a unique function of the stimulus at that instant. In the case of spike-rate adaptation, there is no unique relationship between input and output, because the spike rate at any time depends both on the instantaneous stimulus and on prior spiking (the "history"). To improve the decoding of spike trains produced by neurons that show spike-rate adaptation, we developed a simple scheme that incorporates "history" into a rate code. We utilized this rate-history code successfully to decode spike trains produced by 1) mathematical models of a neuron in which the mechanism for adaptation (IAHP) is specified, and 2) the gastropyloric receptor (GPR2), a stretch-sensitive neuron in the stomatogastric nervous system of the crab Cancer borealis, that exhibits long-lasting adaptation of unknown origin. Moreover, when we modified the spike rate either mathematically in a model system or by applying neuromodulatory agents to the experimental system, we found that changes in the rate-history code could be related to the biophysical mechanisms responsible for altering the spiking.