Polyacrylamide Nanoweb: Highly Reliable Soft

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Organic Electronic Devices

PEDOT:PSS/Polyacrylamide Nanoweb – Highly Reliable Soft Conductors with Swelling-Resistance Gwang Mook Choi, Seung-Min Lim, Yoo-Yong Lee, SeolMin Yi, Young-Joo Lee, Jeong-Yun Sun, and Young-Chang Joo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00314 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 24, 2019

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ACS Applied Materials & Interfaces

PEDOT:PSS/Polyacrylamide Nanoweb – Highly Reliable Soft Conductors with Swelling-Resistance Gwang Mook Choi†, Seung-Min Lim†, Yoo-Yong Lee†, Seol-Min Yi†, Young-Joo Lee†, Jeong-Yun Sun†,‡,*, and Young-Chang Joo†,‡,*

† Department of Materials Science; Engineering, Seoul National University, 1 Gwanakro, Gwanak-gu, 151-744 Seoul, Republic of Korea

‡ Research Institute of Advanced Materials (RIAM), Seoul National University, 1 Gwanak-ro, Gwanak-gu, 151-742 Seoul, Republic of Korea

ACS Paragon Plus Environment

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KEYWORDS: PEDOT:PSS, polyacrylamide, nanoweb structure, stretchable conductor, mechanoelectrical stability, electrical strain insensitivity, swelling-resistance

ABSTRACT

According to the recent growth in interest of human-friendly devices, soft conductors, which are conductive materials with an inherent compliance, must have a low electrical strain sensitivity under large deformation conditions, environmental stability in water, and reliability even for complex and repeated deformation, as well as non-toxic characteristics. In this study, we fabricated a PEDOT:PSS/polyacrylamide nanoweb that satisfies all of the above requirements through a web microstructure with entangled conductive nanofibers. Since the web structure can be deformed through structural alignment, the conductive path is stably maintained during deformation, which makes it highly conductive, electrically stable, and electrically strain insensitive. The tangled nanofibers are composed of PEDOT:PSS as the conductive component and polyacrylamide as a binding material, so it is non-toxic and has the soft properties of the material itself, which can withstand large deformations. Additionally, the material has a good electrical stability against repeated deformation, so that the resistance increased only by 13% after a 50% strain was repeated 1000 times. Notably, electrical instabilities such as noise and hysteresis were not evident during the repeated deformations. Finally, the nanoweb has excellent swelling resistance and maintains its mechanical and electrical characteristics in water.


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Introduction

Soft electronic devices have the advantage of being mechanically stretchable, so that they can be conformably adhered to a complicated surface and can operate even when deformed. Due to these merits, recently, soft electronics have been developed in various applications such as stretchable displays,1, 2 stretchable sensors,3, 4 and solar cells5 for wearable or attachable devices. In particular, as the field of real-time health care grows, research on human-friendly soft electronic devices that are capable of being attached to the skin or even inside the human body is actively proceeding, in order to diagnose the health status of a human in real time and respond appropriately and quickly.6-10 To fabricate a human-friendly electronic device, it is necessary to develop a soft conductor with no toxicity and similar mechanical properties to the human body. In addition, the device should have an appropriate electrical conductivity according to the purpose of the application. Electrical characteristics, such as resistance, should also be stable during complex and repeated deformation and not change significantly, even during a large strain. Finally, as the human body is full of water, swelling should not occur under a water environment.


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The most commonly developed soft conductor is a metal-based conductor that realizes stretchability by designing a wavy11,

12

or a buckled shape,13 nanowires,14 and

composites.15 However, there are mechanical limits due to the inherent rigidity of metals. Since there are differences in the elastic modulus and Poisson’s ratio values between metal and stretchable substrates, a lateral stress occurs when stretched, and wrinkles and delamination can easily occur when attached on skin or a surface inside the body. A conductive polymer gel, which consists of conductive polymers as the conductive part and a hydrated matrix polymer as the container,16, 17 is one good alternative for a soft conductor due to its inherent softness. However, the conductivity is low, and the current fluctuates during deformation since the conduction path is unstable. Moreover, the conductive gel is rapidly swollen in the solvent, which degrades the electrical and mechanical properties. These limits are critical barriers to practical device applications, especially in skin-attachable or body-implantable devices for medical devices.

Here, a new soft conductor with a web microstructure that can maintain a stable conduction path while still demonstrating elasticity was developed. This conductor was


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fabricated by inserting the conductive element crystallized PEDOT:PSS (poly(3,4ethylenedioxythiophene):polystyrene sulfonate) sheets inside the polyacrylamide nanofibers attached to each other to form a nanoweb structure. PEDOT:PSS was chosen as a conductive component due to high electrical conductivity up to 4600 S cm-1,18 environmental stability, mechanical flexibility and easy processibility in addition to good biocompatibility. Until recently, PEDOT:PSS has been produced for functional devices, such as interconnects, supercapacitors, bioelectrodes, and sensors.19-26 The new soft conductor was named PEDOT:PSS/polyacrylamide nanoweb in consideration of these structures and constituents.

The web microstructure has the following advantages. First, the web structure consists of thin yarns clinging to each other, so that it is possible to expand and contract through structural alignment while the material is not significantly damaged or deformed. Therefore, if a conductive component is inserted into the fibers that form the web structure, then the conductive path is also not damaged greatly even when deformed, so that the current can flow stably. Second, if a conductive component is inserted into the


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fiber, it is protected from the external environment such as polar solvents, and thus, excellent environmental stability can also be achieved. The advantages of this web microstructure can be a breakthrough solution to solve the electrical and environmental instabilities in conventional soft conductors.

The PEDOT:PSS/polyacrylamide nanoweb is so highly stretchable that it can be stretched over 500%, and the average elastic modulus is 25 kPa, which can be adjusted to skin. The large deformable region is due to the combination of the alignment of the web structure and the elasticity of the material itself. Furthermore, the nanoweb not only has high electrical mobility and conductivity but also higher stability and reliability since PEDOT: PSS inside the nanoweb is densely packed, and polyacrylamide acts as a binder to help secure the conduction path. In addition, because it is possible to deform during alignment, the change in the resistance to deformation is very insensitive. Finally, since polyacrylamide is strongly bound, it has a swelling resistance even in a solvent environment such as water and maintains electrical and mechanical stability in a solvent. Therefore, a highly conductive and stable soft conductor was fabricated successfully.


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2. Experimental

Fabrication process. Forty microliters of 0.2 M ammonium persulfate (APS) and 3.4 μL of N, N, N ', N'-tetramethylethylenediamine (TEMED) were added as an initiator and accelerator to 9 mL of a 1 M acrylamide solution for the polymerization of polyacrylamide. The solution was continually stirred to prevent aggregation during polymerization. Next, 0.16 g of the PEDOT: PSS solute, which was sublimed and dried for 4 days in a highvacuum state using a freeze dryer (FD5508, Ilshin Biobase) after freezing the PEDOT: PSS solution (Clevios PH 1000, Heraeus) at -20 °C, was dissolved in 10.8 mL of distilled water, and the PEDOT:PSS solution was added to the polyacrylamide solution. After sufficient stirring, electrospinning was carried out with the PEDOT:PSS/polyacrylamide solution. Under electrospinning conditions, the voltage was 20 kV, the injection rate was 0.3 mL h-1, the distance between the needle and the collector was 18 cm, the temperature was 25 °C, and the humidity was 30%. Then, a heat treatment was performed at 120 °C


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for one day to improve the bonding force in the nanofiber, and the thermally treated nanofibers were wetted with DMSO, an organic solvent for secondary doping, to form a conduction path among the PEDOT: PSS nanofibers. The DMSO-treated nanofibers were again dipped in distilled water, and the solvent was replaced with water. The final obtained stretchable conductors is referred to as the PEDOT:PSS/polyacrylamide nanoweb due to its material and structure. X-ray diffraction. XRD data were obtained from an XRD analytical instrument (D8 Advance, Bruker) using the parallel beam mode that is recommended by the instrument manufacturer to characterize thin-film samples. The PEDOT:PSS/polyacrylamide nanofibers before and after DMSO wetting were used as analytical samples for comparison.

Tensile test: The PEDOT:PSS/polyacrylamide nanoweb was cut into a size of 10 mm × 40 mm × 1 mm (w × 1 × t), and a clamp made of an acrylic plate was attached to both ends with an adhesive, which prevents slippage during the tensile test Therefore, accurate strain values can be obtained. The tensile test was measured with a tensile machine (Instron 3343). The experiment was conducted in two modes. First, the sample


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was pulled until the breakage occurred. Second, the strain was repeated 10 times from 0% strain to 100% strain and again to 0% strain. All sample deformation rates were kept constant at a rate of 5 mm min-1. Additionally, the electrical resistance of the PEDOT:PSS/polyacrylamide nanoweb was observed during mechanical deformation. The electrical resistance was measured using a resistance measuring device (Agilent 34410A multimeter). Hall

effect

measurement.

The

Hall

effect

measurement

of

the

PEDOT:PSS/polyacrylamide nanoweb was performed using an HL5500PC (Bio-Rad) instrument. The samples were fabricated with a 10 mm × 10 mm × 0.3 mm (w × 1 × t) sample, and silver paste was applied to the four corners, which were used as electrodes for the current flow and the voltage application. The intensity of the applied electric field was 0.51 T, and a voltage of 20 mV was applied in the direction of the current flow.

3.

Results and Discussion

3.1.

Fabrication process of PEDOT:PSS/polyacrylamide nanoweb


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Figure 1 shows the fabrication process of the PEDOT:PSS/polyacrylamide nanoweb. The fabrication process can be divided into the solution preparation and nanoweb fabrication. Figure 1A shows the solution preparation. Polyacrylamide was chosen as a soft binder since it is highly water absorbent and forms a soft gel when hydrated. Acrylamides, as monomers, were dissolved in deionized water and polymerized in 70℃ for 2 h by ammonium persulfate (APS) and N, N, N’, N’-tetramethylethylenediamine (TEMED) as the initiator and accelerator, respectively. The amount of initiator and the synthesis temperature were optimized to the extent that polyacrylamide had a sufficiently long molecular length and solution was not excessively viscous. The longer a polymer chain is, the more tangled up it can get, which improves the binding force and consequently the mechanical stability. However, if the polymer chain is too long, the viscosity of the solution becomes too large, and it is difficult to produce nanofibers through electrospinning. Figure S4 (Supporting information) shows the gel permeation chromatography (GPC) analysis for the directly synthesized polyacrylamide. It was


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confirmed that synthesized polyacrylamide exhibits a sufficiently long chain polymer at 2,250 kDa, which is a suitable polymer weight.

The freeze-dried PEDOT:PSS, which was frozen and sublimated in vacuum, was used as the PEDOT:PSS solute to obtain the high crystallinity of PEDOT:PSS. Because the crystallinity of PEDOT is related to the charge carrier mobility, increasing the crystallinity is very significant for a high conductivity. Freeze-dried PEDOT:PSS was redissolved in another deionized water solution, and both the polyacrylamide solution and freeze-dried PEDOT:PSS solutions were mixed. Then, the PEDOT:PSS/polyacrylamide nanofibers were fabricated by electrospinning the mixed solution.


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Figure 1. Fabrication process of the PEDOT:PSS/polyacrylamide nanoweb. (A) PEDOT:PSS/polyacrylamide solution preparation for electrospinning. Polyacrylamide with optimized polymer chains was synthesized to a viscosity capable of electrospinning and performed the role of a binder for holding PEDOT:PSS. (B) Formation of PEDOT:PSS/polyacrylamide nanoweb. The electrospun nanofiber was thermally treated to be densified for mechanical and chemical stability. The heat-treated nanofibers were immersed in DMSO and DI water to obtain the crystallization of PEDOT and softness, respectively.

The conditions of electrospinning were as follows: the applied voltage was 20 kV, the flow rate was 0.3 mL h-1, and the distance between needle and collector was 18 cm. The temperature was 25 °C, and humidity was approximately 30%.


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The as-spun nanofibers were heated at 120 °C for 24 h to densify the polyacrylamide, which not only enhanced the mechanical strength and conduction path but also made it resistant to a polar solvent, i.e.; it did not dissolve. This resistance to a polar solvent made it possible to process using a polar solvent post treatment, such as DMSO and water. To verify the thermal treatment effect on polyacrylamide, pure polyacrylamide nanofibers without PEDOT:PSS were fabricated by electrospinning. Then, the change in the densification was checked from two polyacrylamide nanofiber samples with and without thermal treatment. The densification of polyacrylamide was ascertained by small-angle X-ray scattering (SAXS). As shown in Figure S7 (Supporting information), the scattering peak of the heated nanofibers was higher than that of unheated nanofibers. Therefore, the thermal treatment was confirmed for polyacrylamide to be densified and strongly bound to one another. After thermal treatment, however, the PEDOT:PSS/polyacrylamide nanofibers were not conductive. These unhydrated nanofibers were insulating because there is no percolation path for PEDOT:PSS inside the nanofiber. Polyacrylamide could act as a conduction barrier. Therefore, the nanofibers were soaked in DMSO, which can form a highly


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crystalline and continuous connection of PEDOT:PSS. Solvent post-treatment for conductivity enhancement of PEDOT:PSS is a well-known process. By adding a polar organic solvent, such as ethylene glycol (EG), glycerol, dimethyl sulfoxide(DMSO), and sorbitol, the conductivity of PEDOT:PSS can be improved by up to 3 orders of magnitude.27-29 Since the crystallinity of PEDOT was improved, the region of PEDOT-rich cores were transformed from small and curved domains to a long stretched network after solvent post-treatment. Thus, the connected areas among the better-oriented PEDOT rich grains were expanded, resulting in a well-aligned conduction path and high conductivity.30, 31 Among the solvents, DMSO was chosen due to its non-toxic feature and crystallization effect. By soaking the nanofibers in DMSO, the high crystallinity and percolated conduction path of PEDOT:PSS was confirmed, which can be proven with XRD analysis. After DMSO treatment, the nanofibers were soaked in deionized water again to hydrate the polyacrylamide. Unhydrated polyacrylamide is not soft, so it is easily ripped out during tension. However, after hydration, polyacrylamide becomes soft and elastically stretchable. In addition, water facilitated the surface of the nanofibers to slightly dissolve,


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the nanofibers were attached to one another during coagulation, which improved the mechanical strength and formed a multiconduction path. By this method, a highly conductive and stretchable conductor can be made.

3.2. Web-like microstructure for stable conduction paths As shown in Figure 2A, the PEDOT:PSS/polyacrylamide nanoweb was successfully fabricated. The material looks like typical PEDOT:PSS gel to the eye, which is a dark blue gel that is mechanically soft. However, the PEDOT:PSS/polyacrylamide nanoweb has a different microstructure from the PEDOT:PSS gel. Figure 2B shows the microstructural image of the freeze-dried PEDOT:PSS/polyacrylamide nanoweb observed by field emission scanning electron microscopy (FESEM). To prevent microstructural damage, the nanoweb was frozen at a temperature of -20 °C for 2 days and dried by sublimation in a high vacuum; this process is called freeze-drying. While typical PEDOT:PSS hydrogel has

evenly

distributed

pores

with

an

average

size

of

20

μm,16

the

PEDOT:PSS/polyacrylamide nanoweb has web structure, which was formed from nanofibers that are entangled and attached to one another. The web structure is dense


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because the average distance between the nanofibers is approximately 4 μm. Additionally, while the pores in hydrogel are relatively closed by polyacrylamide membranes, the pores in nanoweb are open and well connected with the other pores. Because the porous structure of the nanoweb demonstrated a better mechanical stretchability than the bulk, the nanoweb structure enhances the stretchability without a breakage of the conduction path.

Figure 2. The microstructure of the PEDOT:PSS/polyacrylamide nanoweb. (A) PEDOT:PSS/polyacrylamide nanoweb is a dark-blue-colored soft conductor. (B) SEM image of the freeze-dried PEDOT:PSS/polyacrylamide nanoweb. Nanofibers were well linked and formed a porous structure like a sponge. (C) XRD analysis results before and


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after DMSO treatment to identify the crystallized PEDOT. The intensity at 20° increased, indicating that the crystallinity of PEDOT inside the nanoweb was increased by DMSO. (D) A schematic to depict the microstructure of the PEDOT:PSS/polyacrylamide nanoweb. The nanofibers are entangled and attached to form a nanoweb, and a crystallized PEDOT forms a conductive path in the nanoweb.

As shown in Figure 2C, after DMSO post-treatment, a broad peak at approximately 20° appeared, which corresponds to the XRD pattern of crystallized PEDOT.32 This result means that PEDOT:PSS was densely packed and crystallized inside the polyacrylamide due to post-treatment with DMSO, which made the nanoweb exhibit a higher carrier mobility and conductivity. Based on these data, the microstructure of the PEDOT:PSS/polyacrylamide nanoweb is drawn in Figure 2D. PEDOT:PSS is evenly dispersed inside the polyacrylamide nanofiber forming web structure. The crystallized PEDOT-rich grains (red strap) are connected and form a long-stretched network by DMSO, as proven in previous studies,30,

31

and they are wrapped by densely packed

polyacrylamide (blue-curved cylinder). This structure can explain the improved mechanical and electrical properties compared with the conventional PEDOT:PSS gel. In the PEDOT:PSS gel, PEDOT:PSS is not wrapped with a protective binder, so the current


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is unstable during deformation. On the other hand, the nanoweb exhibits stable conduction, since it is protected by a polyacrylamide binder. Additionally, the web structure can be deformed through alignment, so that the conduction path can be stably maintained. Therefore, the current can be more stable and reliable in the nanoweb rather than in gel, even during deformation.

3.3.

Measurement of electrical-mechanical performance

Electrical, mechanical, and mechanoelectrical analyses were carried out to confirm whether the PEDOT:PSS/polyacrylamide nanoweb actually demonstrated improved performance compared to that of conventional conductors. First, we conducted Hall effect measurements to identify the electrical factors of the nanoweb. PEDOT:PSS/polyacrylamide hydrogel, which was fabricated by acrylamide and N,N,N’,N’-tetramethylethylenediamine (MBAAm),33 was also analyzed for comparison with the nanoweb. Since the Hall measurement was a DC system, and the voltage was 20 mV, no chemical reaction such as water decomposition occurred during the analysis, and only the electric current due to holes existing in PEDOT exists. The results of Hall effect measurement are shown in Figure 3A. The charge density of the PEDOT:PSS/polyacrylamide nanoweb containing 12 wt% PEDOT:PSS is 0.45×1020 cm-3, which is slightly lower compared with that of the 12 wt% PEDOT:PSS gel. In contrast, the carrier mobility of the nanoweb is 0.042 cm2 V-1 s-1, which is approximately 100 times larger than that of the gel. Consequently, the conductivity of the nanoweb is 0.304 S cm-1, which is approximately 20 times larger than the gel. For the 20 wt%


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PEDOT:PSS/polyacrylamide nanoweb, the carrier mobility is 0.273 cm2 V-1 s-1, and the conductivity is 3.11 S cm-1, which is approximately 100 times larger than that of the 12 wt% gel. In the case of the nanoweb, because PEDOT:PSS can be isolated in the polyacrylamide nanofibers, it can cause a decrease in the charge density. However, since PEDOT:PSS was condensed inside the nanofibers without being dispersed in the solvent, the conduction paths of crystallized PEDOT were effectively formed by DMSO, which improved the charge carrier mobility. As a result, as the growth of the carrier mobility is enough to complement a decrease in the charge density, the PEDOT:PSS/polyacrylamide nanoweb shows a much higher conductivity than the PEDOT:PSS gel. Next, tensile testing was carried out to confirm the mechanical properties of the nanoweb. As shown in Figure 3B, the PEDOT:PSS/polyacrylamide nanoweb was glued to two acryl plates to eliminate the compression effect by the jigs and to mount on the tensile machine. Then, the sample was stretched until mechanical rupture occurred. A uniaxial extension was conducted with a constant rate of 5 mm min−1. The stress-strain curve of the nanoweb is displayed in Figure 3C. The average elastic modulus is 25 kPa, which is mechanically compatible to human skin.34, 35 Additionally, the nanoweb can be stretched over 500%, and the maximum strain endured was 660%. Notably, the nanoweb was shown to tolerate deformations similar to or even greater than hydrogels without cross-linkers. Polyacrylamide acts as a conjugate and enhances the mechanical strength,


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and moreover, the web structure is a structure where the force is easily dispersed through alignment, so that it can endure a large deformation. Therefore, polyacrylamide can be used in an environment such as in human bodies where complicated and large deformations occur.

Figure

3.

Electrical,

mechanical

and

mechanoelectrical

properties

of

the

PEDOT:PSS/polyacrylamide nanoweb. (A) Hall analysis results to analyze the electrical characteristics. Since dense PEDOT: PSS conduction paths were formed inside the nanoweb, the carrier mobility and the conductivity showed were higher than those of the


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hydrogel. (B) The nanoweb specimens prepared for the tensile test and in the stretched state

from

λ=1

(left)

to

λ=5

(right).

(C)

Stress-strain

curve

for

the

PEDOT:PSS/polyacrylamide nanoweb. Average strain limit is over 500%, and the average elastic modulus is 25 kPa. (D) Cyclic test repeated 10 times at a 100% strain. The slope gradually increases since it was structurally aligned at the initial strain and materially stretched after the initial strain. (E) Resistance change as a function of the tensile strain. Nanoweb shows lower electrical strain sensitivity than elastic conductor such as ionic gel (R/R0=(L/L0)2). (F) Resistance change as a function of the fatigue cycle to the 10000th cycle with a 50% strain. (G) The resistance change as a function of strain. The representative values from 1st to 10000th cycles were indicated.

To confirm the elasticity of the nanoweb, the uniaxial deformation, which was stretched at a 100% strain and released again, was repeated 10 times. The stress and strain of each cycle were measured during each test. Repeated tensile deformation was conducted with a constant rate of 3 mm min-1. As shown in Figure 3D, from the second cycle, the stress-strain curve has a curved shape in which the slope gradually increases. This phenomenon may be due to energy dissipation mechanism, which are often observed from typical hydrogels under large deformation.36 Finally, to confirm the mechanoelectrical properties of the nanoweb, we observed changes in the electrical resistance during the tensile tests. Figure 3E shows the resistance change following the strain of the PEDOT:PSS/polyacrylamide nanoweb. The


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resistance change in the nanoweb is more strain insensitive compared with that of the other elastic conductors. Generally, the resistance (R) can be calculated by an inherent factor such as resistivity (ρ) and geometric factors such as the length of current direction (L) and area of the cross section perpendicular to the current direction (A), R=ρL/A. Assuming that the conductor is an elastomer, the volume (V) of elastomer should be conserved, and the product of area and length should also be constant, V0=A0L0=AL=V. Therefore, assuming that the inherent resistivity is constant, the ratio of the new resistance to the initial resistance (R/R0) is the same as the square of the ratio of the stretched length to the initial length (L/L0), which is R/R0=LA0/(L0A)=(L/L0)2. Typically, this equation of the resistance ratio can be applied to an elastic conductor including an ionic gel conductor.37 Comparatively, the PEDOT:PSS/polyacrylamide nanoweb shows low electrical strain sensitivity since the nanoweb can keep percolation paths safely in a large deformation due to the web structure and stretchable binder. As shown in Figure 3E, the resistances of the PEDOT:PSS/polyacrylamide nanoweb were almost unchanged until a 100% strain. This finding means that the electric power consumed by the nanoweb is almost constant, even during deformation, and a constant power can be supplied to the


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devices when a nanoweb is used as a circuit. Over a 100% strain, however, the resistance of the nanoweb gradually increased. This phenomenon resulted from a disconnection of the PEDOT:PSS conduction paths. The percolation of PEDOT:PSS inside the nanoweb can be damaged and disconnected at a large deformation, independent of polyacrylamide binders. Fortunately, the connection of PEDOT:PSS can be enhanced by increasing the amount of PEDOT:PSS. When the density of PEDOT:PSS increased, the percolation of PEDOT:PSS was compact. Therefore, as the concentration of PEDOT:PSS increases, the percolation path becomes more densely connected, so that the electrical strain insensitivity is improved, and the strain region where conduction occurs is widened. As shown in Figure 3E, At 250% strain, the resistance of the nanoweb with 20 wt% PEDOT: PSS increased 2.1 times while the resistance of the nanoweb with 12 wt% PEDOT: PSS increased 5.4 times. Additionally, the nanoweb with 20 wt% PEDOT:PSS can be operated over a 400% strain, while nanowebs with 12 wt% and 17 wt% PEDOT:PSS electrically failed at an approximately 260% strain. Repeated fatigue testing, stretching and releasing the samples was also carried out up to a 50% strain for 10000 cycles, in which the period of one cycle was 10 s. The


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PEDOT:PSS/polyacrylamide nanoweb was passivated by ecoflex to prevent vaporization and other environmental effects during the test. The resistance was recorded during every fatigue cycle. Typically, PEDOT:PSS is a rigid polymer, which is easily broken during a large deformation38 or repeated deformation. Although PEDOT:PSS is contained in a gel, it is still unstable and degraded since there are differences of the mechanical characteristics between PEDOT:PSS and gel. It was confirmed that the current of PEDOT:PSS gel was unstable during repeated deformation. In one of the previous studies, the resistance of PEDOT:PSS gel increased 57% compared with the initial resistance after the 1000th cycle.16 On the other hand, in case of the nanoweb, as shown in Figure 3F, the resistance only increased to 13% compared with the initial resistance after the 1000th cycle and 3.2 times larger than the initial resistance after the 10000th cycle. This increase means that the stability of the nanoweb improved more than that of the PEDOT:PSS gel. As shown in Figure 3G, we plotted the results of the resistancestrain curves. The representative resistances from the 1st to 10000th cycle were indicated. Notably, the hysteresis or noise of resistance change was not observed at all during fatigue testing. It means that the paths of PEDOT:PSS were inherently stable. This


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stability suggests that PEDOT:PSS core-shell particles were evenly dispersed in polyacrylamide, so the particles did not form a large crystal, which makes PEDOT:PSS rigid. Instead, the paths of PEDOT:PSS could be formed by combining polyacrylamide and small parts of the crystallized PEDOT:PSS, which made the paths conductive and stretchable. Furthermore, polyacrylamide wrapped PEDOT:PSS and protected the conduction paths from damage by deformation, which improved the electrical stability during repeated deformation.

3.4.

Swelling-resistance in water

The unique property of the PEDOT:PSS/polyacrylamide nanoweb is the swelling resistance in the solvent. Figure 4 shows the swelling tests of the PEDOT:PSS/polyacrylamide nanoweb and PEDOT:PSS hydrogel. The nanoweb and hydrogel were submerged in water together for 30 min, 1 h, and 2 h, and the weight and resistivity changes were confirmed. The resistivity was measured by considering the geometric expansion and the resistance. Assuming that the volume expanded isotropically and was proportional to the weight of absorbed water, the resistivity can be obtained by the resistance divided by the -1/3 square of the weight.


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Figure 4. Swelling-resistance of PEDOT:PSS/polyacrylamide nanoweb in water. (A) The size change in the PEDOT:PSS/polyacrylamide nanoweb and PEDOT:PSS hydrogel with time in water. (B) The weight and C) resistivity change as a function of time in water. While the weight of PEDOT:PSS gel increased to 623% after 2 h, the weight of the nanoweb increased to 15% after 2 h. Additionally, the resistivity of PEDOT:PSS gel increased to 383% after 2 h, but the resistivity of nanoweb only increased to 27%. (D) Nanoweb conductor for the LED was produced and submerged in water for 24 h. In the case of PEDOT:PSS/polyacrylamide nanoweb, the conductivity and mechanical properties were maintained, while the PEDOT:PSS hydrogel deteriorated after being swollen.

As shown in Figure 4A, hydrogel was rapidly swollen, and the volume of hydrogel was expanded, which reduced the concentration of cross-linked polymer in the unit volume and consequently weakened the mechanical strength of the gel. The swollen hydrogel was too feeble


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to support the structure; therefore, it was easily ripped in stretched state. On the other hand, the PEDOT:PSS/polyacrylamide nanoweb was mostly not swollen and maintained its shape. Figure 4B and C show the weight ratio and resistivity ratio following the swelling time. When the hydrogel was swollen, it was difficult for the charge carrier to hop to the other PEDOT, and the conductivity decreased since the PEDOT:PSS particles dispersed in gel were separated from each other by the solvent. Experimentally, the weight of the PEDOT:PSS hydrogel increased by 623%, and the resistivity increased 383% after a 2 h submersion in water. On the other hand, the PEDOT:PSS/polyacrylamide nanoweb showed that the weight increased only 15%, and the resistivity decreased approximately 2% under the same condition. It was also confirmed that the current was reduced by only approximately 17% when the current was measured 90 days later in water. (Figure S11, Supporting information) In contrast to the hydrogel, which maintains its structure with cross-linked polyacrylamide, the nanoweb maintained its structure by entangling densified polyacrylamide, which made it stronger and resisted swelling. The mechanical-electrical stability in a polar solvent is a very useful property for soft conductors in water-based systems, such as bid devices inside body or diving equipment. Figure 4D shows a stretchable LED circuit with swelling resistance composed of the PEDOT:PSS/polyacrylamide nanoweb and PEDOT:PSS hydrogel. The nanoweb-based LED circuit was immersed in water and operated for 24 h. As a result, it was confirmed that the circuit operates stably without any damage. This finding is because the nanoweb does not swell even after 24 h in water, and the PEDOT:PSS conduction path is stable. It was confirmed that not only the electrical characteristics but also the mechanical properties were not degraded. Alternatively, in the case of the hydrogel, it was confirmed that the LED circuit did not work due to a large increase in the resistance because of the swelling. Additionally, the microstructures of the nanoweb and hydrogel were confirmed. As shown on the


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right side of Figure 4D, it was confirmed that the swelling of the hydrogel caused a detachment of PEDOT:PSS, but the nanoweb showed a tight connection with PEDOT:PSS.

4. Conclusion

We successfully fabricated highly reliable and strain-insensitive soft conductor, PEDOT:PSS/Polyacrylamide nanoweb. The mixed solution of PEDOT:PSS and polyacrylamide was electrospun to produce PEDOT:PSS/Polyacrylamide nanofibers, which help to maintain the paths of PEDOT:PSS inside stably. Then, the paths of percolation between PEDOT:PSS sheets were strongly linked by DMSO that helps to improve the crystallinity and area of PEDOT. Finally, the nanofibers were soaked in deionized water to be hydrated, which made it soft. This soft and conductive materials are called PEDOT:PSS/Polyacrylamide nanoweb. Since nanoweb structure can mechanically expand and contract without deforming or damaging the material itself, the current can be stably supplied without changing the resistance even when deformed. Also, nanoweb has stable and well crystallized conduction paths, which improves charge


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carrier mobility and conductivity. As a result of checking the electrical characteristics of the nanoweb, 20wt% PEDOT:PSS contained nanoweb exhibited excellent electrical performance with a high charge carrier mobility of 0.27 cm2V-1 s-1 and a high conductivity of 3.11 S cm-1. Nanoweb also showed the remarkable strain-insensitivity with the resistance increasing 25% compared with initial resistance at 50% strain, which is much lower than ionic gel conductor. Furthermore, nanoweb has an excellent reliability as the resistance just increased 13% compared with initial resistance at 1000th cycle of repeated 50% strain without hysteresis or noise. Finally, the swelling resistance of nanoweb helps it retain the mechanical and electrical characteristics in polar solvent such as water. By these virtues, it is believed that PEDOT:PSS/Polyacrylamide nanoweb is an appropriate materials used as soft interconnects or electrodes in body-implantable devices or underwater-devices.39, 40


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ASSOCIATED CONTENT

Supporting Information.

The supporting information contains details for the structural concept of PEDOT:PSS/polyacrylamide nanoweb, GPC data of synthesized polyacrylamide and FT-IR data, X-ray diffraction, SAX profile, SEM images, long-term swelling-resistance test, impedance measurement of PEDOT:PSS/polyacrylamide nanoweb. Supporting Information is available free of charge from the ACS Applied Materials & Interfaces home page (https://pubs.acs.org/journal/aamick).

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

*E-mail: [email protected].


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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea(NRF) Grant funded by the Korean Government(MSIP)( NRF-2016R1A5A1938472)

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Polyacrylamide Nanoweb: Highly Reliable Soft

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