The goal of stretchable electronics is to accelerate the expansion of the Internet of Things by establishing reliable electrical conductive sub-circuits and components to adapt to a large number of deformed scenes, including sensors, displays, actuators, energy storage devices, and flexible robots.
In order to minimize material waste and production costs, and to manufacture large-area electronic products, coating or printing a stretchable conductor on an insulating substrate is the most promising for the development of complex materials that combine the superior performance of the substrate and the conductor.
One of the methods.
Compared with low-strain traditional conductors that can cause serious delamination problems, the performance of com" href="">paintable and stretchable conductors when the substrate is deformed to maintain an increasing strain is crucial .
For example, next-generation robots and wearable devices can withstand 30% of the pulling force on the skin and 100% of the pulling force at the joints.
Some commercial sportswear are designed to withstand 400% of the pulling force.
However, coating stretchable conductors on various substrates has proven to be a very challenging task, and requires comprehensive consideration of the transparency, adhesion, mechanical compliance, shape adaptability and processing properties of the coating.
【Introduction to Achievements】
Recently, East China University professor Wupei Yi , professor Sun Sheng Tong jointly reported only one kind prepared by the concentration of induced spontaneous ion ring-opening polymerization of a natural small molecule gel coat lipoic acid α- (the TA) .
A related paper entitled " Adaptive Ionogel Paint from Room-Temperature Autonomous Polymerization of α-Thioctic Acid for Stretchable and Healable Electronics " was published on " Adv.
[Highlights of this article]
(1) The ionic liquid prevents the depolymerization of polyTA by forming COOH···O=S hydrogen bonds, thereby forming a super-stretched ionic gel with widely adjustable mechanical and electrical conductivity, self-healing properties, and tissue-like strain adaptability .
(2) Ion gel coatings have universal adhesion and adjustable rheology, and can be directly coated on various substrates with any shape (including porous materials, three-dimensional printing frames and elastic threads) to make them have Ionic conductivity.
(3) The ionic gel coating has high sensitivity and high durability similar to the skin, indicating the great potential of the ionic gel coating in the emerging soft and scalable electronic products.
Point 1 Design and Preparation of PolyTA Ion Gel Coating
As shown in Figure 1a, the author found for the first time that TA was dissolved in a relatively high concentration of ethanol.
Within 12 hours, as the viscosity increased, polyTA was slowly and autonomously formed.
Adding a certain amount of [EMI][ES] (TA:IL molar ratio 10:1), even if ethanol is removed, the formed polyTA can be completely stable, forming an amorphous three-dimensional network polyTA ion gel (Figure 1b) .
When the TA:IL molar ratio is 5:1, after mixing TA powder, ethanol and [EMI][ES], the liquid precursor is quickly transformed into a gel.
The presence of IL significantly promotes the polymerization of TA, making the gelation time within a few minutes, although the complete monomer conversion will be completed in a longer time, which is determined by the time-dependent rheological test and the degree of reaction NMR monitoring (Figure 1d).
After drying, the ion gel showed a light yellow inherited TA color, but the light transmittance was as high as 95% in the visible light range of 500-800 nm (Figure 1e).
The SEM image of the ion gel) shows a compact and uniform surface morphology, which is also supported by the surface morphology of the nano-scale AFM (Figure 1f).
The author also constructed a unit containing polyTA and [EMI][ES] through molecular dynamics simulation.
PolyTA is represented as a fully flexible chain with a typical random coil structure (Figure 1g).
In addition, the prepared polyTA ion gels with different TA:IL molar ratios all have higher ionic conductivity.
Due to the increase in the number of carriers, with the increase of [EMI][ES] content, the conductivity is 1.
05~ Between 20.
3 mS m -1 (Figure 1h).
Figure 1 a) Schematic diagram of ring-opening polymerization (ROP) induced by the concentration of TA in ethanol at room temperature.
b) Schematic diagram of the internal structure of polyTA ion gel.
c) Quick ROP photo of TA mixed with ethanol and [EMI][ES].
The resulting gel can be diluted to any concentration and used as an ionic gel coating for various coatings.
d) Rheological monitoring of gel process when TA concentration is 1 g ml -1 .
e, f) Photographs and AFM images of transparent polyTA ion gel.
g) A simulated polyTA chain containing 20 repeating units in the ionic gel.
h) The electrical conductivity of polyTA ion gel with different [EMI][ES] content.
Point 2 The stabilization mechanism and internal interaction of PolyTA ionic gel
In order to verify the existence of hydrogen bonds in the polyTA ion gel (Figure 2a), the author compared the different components with ATR-FTIR and 1 H-NMR spectra.
As shown in Figure 2b, a new v(COOH) shoulder peak appeared in the polyTA ion gel near 1734 cm -1 , which corresponds to the COOH that forms a weak hydrogen bond with [ES], and was observed at 1703 cm -1 There are mainly strong dimerization hydrogen bonds in the obtained pure polyTA.
The 1 H NMR comparison of ion gel, polyTA and [EMI] [ES] further proved the existence of COOH···[ES] hydrogen bonds (Figure 2c).
As shown in Figure 2d, the interaction energy between polyTA and [ES] is much greater than the interaction energy between polyTA and [EMI] and the interaction energy between [EMI] and [ES], which indicates that the polyTA+[ In the case of EMI][ES], the COOH····[ES]H bond is thermodynamically stable, and indeed reduces the potential energy of polyTA from 544.
2 to 777.
1 kcal mo l-1 .
Similar to other reported polyTA elastomers, when polyTA ion gel is heated at 119°C, due to thermally unstable H bonds and dynamic disulfide bonds, it exhibits a reversible gel-sol transition, such as a temperature-dependent rheological curve 2e) as shown.
The author further used variable temperature FTIR spectroscopy to study the changes of v(COOH) and v(S=O) bands with increasing temperature.
The results show that the heating process is driven by the change of COOH··[ES] H bond, which has the highest heat sensitivity, which reemphasizes its leading role in the formation of stable polyionic gels.
Figure 2 a) Schematic diagram of COOH···[ES]H bond (atomic tag is used for IR and NMR analysis); b, c) ion gel (TA: IL=5:1), [EMI][ES] and Comparison of ATR-FTIR and 1 H NMR spectra of poly TA ; d) The interaction energy of four pairs of components was calculated by molecular dynamics simulation; e) Storage and loss of poly TA ion gel in a heating-cooling cycle Modulus changes with temperature.
f) The variable temperature FTIR spectrum of the ionic gel in the CO stretching zone of 25～151℃ (interval 3℃).
gi) Synchronous and asynchronous spectra of PCMW and 2DCOS produced by (f).
In (gi), warm colors (red and yellow) represent positive spectral intensity, while cool colors (blue) represent negative spectral intensity.
Point 3 Strain adaptation and healing properties of PolyTA ionic gel
As a coating for stretchable conductors, polyTA ion gel coatings have very important tensile properties.
As shown in Figure 3a, all polyTA ion gels have particularly high tensile properties.
As long as the amount of IL is simply changed, the mechanical properties such as elongation, modulus and tensile strength can be widely adjusted to meet Different application requirements.
Figure 3 a) Tensile stress-strain curves of ionic gels with different TA:IL molar ratios.
b) Tensile stress-strain curve of ionic gel (TA:IL=5:1) at different strain rates.
c) The relationship between G and G'and strain amplitude (γ) in the strain sweep experiment.
d) The shear stress differential modulus (K)σ of ionic gels with different IL content.
e) Classical time-temperature superimposed displacement, the reference temperature is 25℃.
The illustration shows the time-temperature horizontal displacement factor (aT) drawn according to the Arrhenius equation.
f) The optical micrograph records the healing process of the ion gel at 55°C, and the tensile stress-strain curves of the original and self-healing ion gel at different healing times.
Point 4 Strain adaptation and healing properties of PolyTA ionic gel
5 wt%5 wt%，polyTA，(4a)。4b，HPC，300%，（）-，26%99%（=10）。polyTApolyTA（4e-4i）。
polyTA。5aTA0%200%，，。(5b)，polyTA。，，，(5c)。，，/(5d)。polyTA，200%2500，(5e)，polyTA。，polyTA3D(5 g-5 h)。（5i）。，（5 j）。，polyTA，。
In summary, the author reports a polyTA ion gel coating with a simple preparation method, which can be sprayed or penetrated adaptively to various insulations with arbitrary shapes (planar, non-planar, porous, 3D printing, textiles, etc.
) In the substrate, so that it has stable ionic conductivity in the air.
Different from the traditional complex ion conductor coatings, poly-TA ion gel coatings can be quickly obtained by simply mixing TA, IL and ethanol at room temperature by the rapid and autonomous polymerization of poly-TA ion gel coatings under high concentrations of TA.
The formation of stable poly-TA ion gel coating is due to the key stabilizing effect of the hydrogen bond between [ES] and the carboxylic acid of TA.
They coexist with the strong hydrogen bond of the side chain of dimer TA to make the synthesized ion gel It has a wide range of adjustable mechanical properties.
For example, depending on the content of IL and/or HPC additives, the polyTA ion gel can be highly stretchable, elastic and recoverable, strain adaptable, and self-curing.
Because polyTA ion gel has high ionic conductivity and good interfacial adhesion, all ion gel coating substrates are transformed into skin-like sensitive and reliable strain sensors, with electrical response for real-time monitoring of deformation .
This study not only fundamentally clarified the hindered and stable mechanism of ion-assisted TA self-polymerization, but also opened up a new way for the application of stretchable ion gel coatings in the field of integrated electronics.