Recent Progress on Self-Healable Conducting Polymers.

Materials able to regenerate after damage have been the object of investigation since the ancient times. For instance, self-healing concretes, able to resist earthquakes, aging, weather, and seawater have been known since the times of ancient Rome and are still the object of research. During the last decade, there has been an increasing interest in self-healing electronic materials, for applications in electronic skin (E-skin) for health monitoring, wearable and stretchable sensors, actuators, transistors, energy harvesting, and storage devices. Self-healing materials based on conducting polymers are particularly attractive due to their tunable high conductivity, good stability, intrinsic flexibility, excellent processability and biocompatibility. Here recent developments are reviewed in the field of self-healing electronic materials based on conducting polymers, such as poly 3,4-ethylenedioxythiophene (PEDOT), polypyrrole (PPy), and polyaniline (PANI). The different types of healing, the strategies adopted to optimize electrical and mechanical properties, and the various possible healing mechanisms are introduced. Finally, the main challenges and perspectives in the field are discussed.

Nanostructuring mechanical cracks in a flexible conducting polymer thin film for ultra-sensitive vapor sensing.

The swelling of electrically conducting polymer films upon absorption of vapors like alcohol or moisture is widely known. However, this swelling leads to feeble changes in charge transport characteristics. We demonstrate a colossal enhancement (from ∼6% to 108%) in the vapor-induced resistance change for a representative system, poly(3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT:PSS). This is achieved when the films are nanostructured by strain-induced quasi-periodic parallel cracks, which is then followed by crack engineering. The cracks are nanostructured such that the charge carrier percolation pathways are nearly turned off in the absence of alcohol vapor or at low humidity. These percolation pathways are restored upon alcohol vapor or humidity exposure. When used as an alcohol sensor, this system shows ultra-high sensitivity of 106 for methanol vapors, when compared to ethanol vapors (2 × 102). When used as a humidity sensor in the range 60-100% RH, a resistance ratio of 1.5 × 102 is realized. The different extent of response to alcohol vapors and humidity is attributed to the dominance of the surface ionic conduction process in the former. These sensing characteristics are achieved with short response and recovery time (<5 s). The developed sensing platform outperforms commercial portable breath analyzers. While cracks have been utilized for developing ingenious strain sensors in the literature, here we demonstrate an approach based on the same that substantially amplifies vapor response.

Swelling kinetics and electrical charge transport in PEDOT:PSS thin films exposed to water vapor.

We report the swelling kinetics and evolution of the electrical charge transport in poly(3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT:PSS) thin films subjected to water vapor. Polymer films swell by the diffusion of water vapor and are found to undergo structural relaxations. Upon exposure to water vapor, primarily the hygroscopic PSS shell, which surrounds the conducting PEDOT-rich cores, takes up water vapor and subsequently swells. We found that the degree of swelling largely depends on the PEDOT to PSS ratio. Swelling driven microscopic rearrangement of the conducting PEDOT-rich cores in the PSS matrix strongly influences the electrical charge transport of the polymer film. Swelling induced increase as well as decrease of electrical resistance are observed in polymer films having different PEDOT to PSS ratio. This anomalous charge transport behavior in PEDOT:PSS films is reconciled by taking into account the contrasting swelling behavior of the PSS and the conducting PEDOT-rich cores leading to spatial segregation of PSS in films with PSS as a minority phase and by a net increase in mean separation between conducting PEDOT-rich cores for films having abundance of PSS.

Wrinkle and crack-dependent charge transport in a uniaxially strained conducting polymer film on a flexible substrate.

We investigate charge transport in poly(3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT:PSS) films on functionalized polydimethylsiloxane (PDMS) substrates under varying uniaxial strain up to 16%. Strong anisotropy in transport is observed at a large applied strain (ε > 4%), which is understood in terms of an extrinsic process, involving a change in density of cracks from a few cracks per mm at ε = 4% to >100 cracks per mm at ε = 16%. The quasi-periodic cracks are aligned perpendicular to the stretching direction. A strain-history dependent response of the resistance of PEDOT:PSS films cycled through a uniaxial strain up to 4% is also observed, for current paths which are both parallel and perpendicular to the direction of stretching. The resistance-strain plots of strained PEDOT:PSS films for the second and subsequent few strain cycles follow the reverse path of the previous strain cycle. This unique strain-history dependence of resistance helps to identify the source of resistance changes at a low strain (ε < 4%). We demonstrate that the out-of-plane uniaxial wrinkle arrays that appear in a direction parallel to stretching have the same hysteresis response as the resistance, and therefore wrinkle formation governs the low-strain resistance changes. These phenomena are extensively investigated with dc-conductivity and frequency-dependent-ac-conductivity measurements, and surface morphological studies of the films under various applied strains. Our work quantitatively identifies the contributions of wrinkles and cracks to the change in resistance of PEDOT:PSS under an applied strain.

Anomalous charge transport in reduced graphene oxide films on a uniaxially strained elastic substrate.

We investigate temperature-dependent charge transport in reduced graphene oxide (rGO) films coated on flexible polydimethylsiloxane (PDMS) substrates which are subject to uniaxial strain. Variable strain, up to 10%, results in an anisotropic morphology comprising of quasi-periodic linear array of deformations which are oriented perpendicular to the direction of strain. The anisotropy is reflected in the charge transport measurements, when conduction in the direction parallel and perpendicular to the applied strain are compared. Temperature dependence of resistance is measured for different values of strain in the temperature interval 80-300 K. While the resistance increases significantly upon application of strain, the temperature-dependent response shows anomalous decrease in resistance ratio R 80 K/R 300 K upon application of strain. This observation of favorable conduction processes under strain is further corroborated by reduced activation energy analysis of the temperature-dependent transport data. These anomalous transport features can be reconciled based on mutually competing effects of two processes: (i) thinning of graphene at the sites of periodic deformations, which tends to enhance the overall resistance by a purely geometrical effect, and (ii) locally enhanced inter-flake coupling in these same regions which contributes to improved temperature-dependent conduction.