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A simple strategy for high stretchable, flexible and conductive polymer films based on PEDOT:PSS-PDMS blends
Organic Electronics 76 (2020) 105451
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A simple strategy for high stretchable, flexible and conductive polymer films based on PEDOT:PSS-PDMS blends
T
Rubai Luoa,b, Haibin Lia, Bin Dua,b,∗, Shisheng Zhoua,b, Yuxiang Zhua a b
Faculty of Printing, Packaging Engineering and Digital Media Technology, Xi'an University of Technology, Xi'an, 710048, China Shaanxi Collaborative Innovation Center of Green Intelligent Printing and Packaging, Xi'an University of Technology, Xi'an, 710048, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Flexible electronics PEDOT:PSS PDMS Doping Conductive polymer films Integrated circuits
Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and polydimethylsiloxane (PDMS) are typical conductive polymers and transparent elastomers, respectively, which are widely used in the field of stretchable electronics. This paper described a simple, cost-effective route for the preparation of PEDOT:PSSPDMS conductive polymer films through direct manufacturing after miscibility. The surfactant, P-t octylophenol (Triton X-100), significantly improved the miscibility of PEDOT:PSS and PDMS. The tensile modulus, elongation at break, sheet resistance, normalized resistance and surface morphology, internal PEDOT:PSS conformation and long-term stability of the films (nine samples total) prepared by polymer blends were studied by varying the concentration groups of ethylene glycol (EG) and Triton X-100 in the mixture. By adjustment, the conductive polymer films exhibit a sheet resistance of 20 Ω sq−1 and an elongation at break of about 82%, and have excellent piezoresistive effects and long-term stability. Moreover, we also demonstrated the feasibility of stretchable conductors by fabrication of stretchable LED circuits. On account of the excellent stretchability and conductivity, together with the facile fabrication process, we believe that our novel strategy could be directly integrated to high performance stretchable electronics.
1. Introduction Stretchable (flexible) electronics manufacturing is an emerging electronic technology that combines organic/inorganic conductive materials with polymers or metal substrates. Stretchable electronics have received increasing attention with their unique flexibility/ductility and efficient, low-cost manufacturing processes, and were believed to gradually have broad application prospects in displays, sensors, RFID tags, and wearable devices [1]. Stretchable electronics should follow the mechanical movement of the active materials and maintain high conductivity at large strain. Although a variety of metal/non-mental conductors and conducting polymers, such as silver nanowire (AgNW) [2,3], carbon nanotubes (CNT) [4,5], graphene (GR) [6–9], polyaniline (PANI) [10], polypyrrole (PPY) [11,12] and poly(3,4-ethylene dioxythiophene) (PEDOT) [13–15] have been developed for diverse applications, their good electrical conductivity is limited by poor mechanical properties. However, traditional conductors are not stretchable, while traditional elastomers like polyurethane (PU), natural rubber (NR), butadiene rubber (SBR) and poly(dimethylsiloxane) (PDMS) are not conductive. Two methods have been used in implementing stretchable interconnects: formation of wavy or mesh configuration using
∗
conductive materials, and converting global tensile strain into local bending strain to improve the tensile adaptability to elastic materials [16]. Another method is to directly fill the conductive material into the elastomer to adapt to the strainability by the inherent stretchability of the filling structure [17,18]. However, these methods have some problems, such as the percolation dependent conductivity is highly strainsensitive and the combination of two materials is often not satisfactory in both conductivity and stretchability. The stretchable interconnect by modifying elastomer or conductive materials have been enhanced by several research groups. At present, the main methods are to adjust the external shape and internal structure of the elastic matrix and improving the surface properties of the elastomer. Ge et al. reported a fabrication method for binary-network structured polyurethane sponge–Ag nanowire–poly(dimethylsiloxane) (PUS-AgNW-PDMS) stretchable conductors with high performance, and confirmed high conductivity of 19.2 S/cm and a resistance change (R/ R0) of 160% at a 100% strain [19]. The key to achieve impressive combinations of elasticity and conductivity lies in the choice of commercial polyurethane (PU) sponge with an interconnected and junctionfree macroporous structure as the skeleton to support the 2D AgNW networks. As one of the most commonly used elastomeric substrates,
Corresponding author. Faculty of Printing, Packaging Engineering and Digital Media Technology, Xi'an University of Technology, Xi'an, 710048, China. E-mail address: [email protected] (B. Du).
https://doi.org/10.1016/j.orgel.2019.105451 Received 13 April 2019; Received in revised form 11 August 2019; Accepted 11 September 2019 Available online 14 September 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.
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PEDOT:PSS, 100% PDMS, 30% PDMS-b-PEO). The introduction of such block copolymers opens up new avenues for the direct preparation of stretchable conductive composite films. P-t-octylophenol (Triton X-100, Fig. 1) is non-ionic surfactant additive which has amphiphilic properties (hydrophilic “head” and hydrophobic “tail”) [38]. When added to the PEDOT:PSS, Triton X-100 was utilized to increase PEDOT:PSS films conductivity by stabilizing PEDOT nanosized particles [39]. It is widely used to regulate hydrophile-lipophile balance (HLB) and critical micelle concentration (CMC) [40]. In previous studies, it has been confirmed that such surfactants can further improve the performance of the PEDOT:PSS on the basis of secondary doping—i.e., as a “tertiary dopant [41].” In addition to improved wetting behavior and conductivity, Triton X-100 can reside the dried PEDOT:PSS film acts as a plasticizer [42] and inhibit PDMS crosslinking reaction [43]. Although there have been many studies on the secondary and tertiary doping of PEDOT:PSS, the integrated preparation of high performance PEDOT:PSS/PDMS conductive polymer films by two kinds of doping has rarely been reported. In this work, we reported a simple, controllable, and low cost effective route for the fabrication of stretchable conductive polymers through combining PEDOT:PSS with PDMS. The composite conductor produced can achieve excellent electrical conductivity and stretchability simultaneously by simple doping as compared with a composite conductor manufactured by a conventional method. The PDMS is not mixed with most aqueous solvents due to its high hydrophobicity, however. The key to the construction of the miscible structures lies in the choice of Triton X-100 to reduce the hydrophobicity of PDMS and blend it with PEDOT:PSS. Moreover, the miscibility, surface morphology, conductivity, mechanical property and long-term stability of the films prepared from the blends were investigated by changing the relative compositions of EG and Triton X-100. Finally, the possibility of their use for stretchable interconnects is examined.
Fig. 1. Chemical structures of PEDOT: PSS, PDMS, EG, and Triton X-100.
the high surface tension of PDMS is typically addressed by UV/O3 treatment to facilitate uniform coating of a variety of conductive materials [20,21]. Although the combination of a conductive material and an elastic substrate can be solved by a number of methods, these methods often have disadvantages such as high processing cost, complicated process, and low yield. To overcome this drawback, a wave of searching for new composite materials that can afford both high electrical conductivity and also good mechanical elasticity has surged. Conductive polymers are good candidates due to the self-regulating nature of the molecular structure and the flexibility of electro-mechanical properties [22]. The PEDOT:PSS solution (Fig. 1) is one of major conducting polymers that is already commercially applied to stretchable electronics and organic light-emitting diodes (OLEDs) [23]. PEDOT:PSS solution has good film forming properties, high transparency, excellent conductance controllability and thermal stability [24]. PEDOT has low solubility in organic solvents or water, which can be tightly bounded to PSS chains by the electrostatic interaction, making it water-dispersible with high stability [25]. The conductivity of PEDOT:PSS film is about 1 S/cm−1, which makes it very limited in the application process. Fortunately, secondary doping can effectively improve the conductivity of PEDOT:PSS. High boiling polar organic compounds can be considered as effective “secondary dopants”, such as dimethyl sulfoxide (DMSO) [26,27], ethylene glycol (EG) [28–31], tetrahydrofuran (THF) [26], and glycerol [32,33]. The effect of these secondary dopants is attributed to the enlargement and coalescence of conductive grains of PEDOT in the solid film upon drying at elevated temperature, which produces large regions of uninterrupted pathways for charge carriers to traverse the film and increases carrier mobility and concentration [34]. This chemical treatment discussed in detail in the review by Po et al. The conductivity of PEDOT:PSS were increased by three orders of magnitude (1000 S/cm−1) [35]. However, hydrophilic PEDOT:PSS and hydrophobic PDMS are difficult to directly compound to prepare a stretchable conductor. To overcome this drawback, Teng et al. realized the synthesis of PEDOT/PDMS composite conductors by gelling PEDOT:PSS. The as-prepared composite conductors remain electrically stability under strain stress (ε = 43%) [36]. For this method, its complex manufacturing process and unsatisfactory strain capability limit the application prospects of the conductor. Therefore, the block copolymer, PDMS-b-PEO, was proposed as a mediator to prepare polymer blends of PEDOT:PSS and PDMS which are naturally insoluble with each other [37]. The miscibility of PDMS and PEDOT was significantly improved and the cured composite film had a tensile elongation at break of 75% at an optimal combination (200%
2. Experimental details 2.1. Material PEDOT:PSS aqueous solution (Clevios PH1000) was purchased from Heraeus. Its concentration was 1–1.3 wt% and had a ratio of PEDOT to PSS of 1: 2.5 by weight. PDMS (Sylgard 184) was received from Dow Corning in combination with a curing agent. Ethylene glycol (EG, 99 wt %) was supplied by the Macklin. Triton X-100 (laboratory grade) was purchased from Sigma Aldrich. All the chemicals were used as received without further purification. PET film was provided by the Continental Lab. 2.2. Preparation of composite polymer films The preparation process of the conductive polymer film is depicted in Fig. 2. First of all, to prepare polymer blends, PEDOT:PSS solution was mixed with PDMS, three constituents (curing agent, Triton X-100, EG) were dropped into a beaker in a stepwise manner with tight weight control. On the basis of the PEDOT:PSS weight, PDMS for 200 wt%, the relative weights of Triton X-100 and EG were varied: 0.1, 1, 10 wt% for Triton X-100 and 0, 7, 14 wt% for EG. The amount of the curing agent was controlled at 10% of the weight of PDMS. The mixtures were vigorously stirred at 300 rpm for 1 h to obtain homogeneous dispersions. Polymer blends stirred and degassed in a planetary mixer (Thinky ARV310, Japan) for 10 min at 2000 rpm. In the next step, the conductive polymer films were prepared by uniformly spread the dispersion of a polymer blend on mold and then dried at 100 °C for 1 h in an oven. Finally, after cooling down to room temperature, the conductive polymer films were carefully peeled off from mold and cut into rectangle strips of 50 × 15 mm2 for mechanical and electrical tests. The thickness of the films prepared in this way ranged from 300 to 500 μm. 2
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Fig. 2. Schematic illustration of fabrication sequence for conductive polymer film. Step 1: the polymer blend was prepared after secondary doping and tertiary doping. Step 2: selective masking of the PET film using Kapton tape and the defoaming polymer blend was dropped onto the unmasked areas. Step 3: thermal curing the polymer blend. Step 4: conductive polymer film was peeled-off for further characterization.
lower critical micelle concentration. According to previous reports, Triton X-100 can weakens the ionic interaction between PEDOT and PSS and expedite phase separation into separate phases, leading to enhanced crystallization of PEDOT segments and resulting in increased electrical conductivity [39].
2.3. Characterization The morphologies of the conductive polymer films were analyzed using scanning electron microscopy (SEM Hitachi S4700, Japan). The acceleration potential was 10–15 kV. The conductivities and square resistances of conductive polymer films were measured by the double electric test four-probe resistance tester (RTS-9, China). Chemical bonding states were investigated by X-ray photoelectron (XPS AXISULTRA, Kratos, UK). XPS spectra were obtained with monochromatic Al Kα (1486.71eV) line at a power of 100 W (10 mA, 10 kV). The Raman scattering spectra were measured by a Raman spectrometer (LabRam HR Evolution, France) with a 532 nm laser source. The conductive polymer films were fixed on a customized stage, the current (I)–voltage (V) characteristics and the resistances change upon deformation were measured using source meter (Keithley Model: 2461, United States). The tensile modulus was evaluated using a universal testing machine (Instron 5500, United States) with a 10 N load cell at a strain rate of 5 mm/min−1.
3.2. Mechanical properties of conductive polymer films Figs. 4 and 5 show the results of the mechanical measurements. Triton X-100 has a dramatic plasticizing effect on conductive polymer films, as seen in the decrease in tensile modulus and increase in elongation at break. This is because Triton X-100 is difficult to volatilize, which remains in the films to increase the free volume in the membrane and weaken the intermolecular forces between the polymer chains [43].It is to be noted that the absence of data (Fig. 4) from the sample containing 0 wt% EG and 10 wt% Triton X-100, which was too soft to measure. In each triad on the plots in which the concentration of EG in the conductor was kept constant at 0, 7, or 14 wt%, the concentration of Triton X-100 dramatically decrease the tensile modulus of the conductor, as manifested by easier to stretch. For samples with 7 wt% and 14 wt% EG, increasing the Triton X-100 from 0.1 wt% to 1 wt% results in a decrease in tensile modulus by more than two times, and a more pronounced decrease in modulus for samples with 10 wt% Triton X100. For samples with 0 wt% and 7 wt% EG, Increasing Triton X-100 from 0.1 wt% to 10 wt% resulted in an increase in elongation at break (Fig. 5). The maximum effective strain can reach about 95% which far exceeds the maximum stretch (ε ≈ 43%) of the PEDOT:PSS/PDMS composite gel [36]. We noticed that the increase in Triton X-100 from 1 wt% to 10 wt% produced a decrease in the elongation at break for samples with 14 wt% EG. This may be because excessive secondary doping can cause the film to become more fragile, affecting the plasticization of Triton X-100. The data shown in Figs. 4 and 5 also reveal a dependence of the mechanical properties of conductive polymer films on the amount of EG present. It can be seen from Fig. 4 that the film containing 7 wt% EG is the least stretchable, but the difference in elastic modulus is not as obvious as Triton X-100. This may be because during the preparation process, a large amount of EG volatilized at 100 °C, and thus has a small effect on the elastic modulus of the film. Meanwhile, the increase in EG concentration reduced the elongation at break of the film while the Triton X-100 concentration maintained constant (Fig. 5). These results indicate that the increase of EG concentration will reduce the relative
3. Results and discussion 3.1. Principle of tertiary dopant In order to understand the role of Triton X-100 in conductive polymer films in affecting the surface properties of the PDMS, we revealed the mechanism of action of Triton X-100 (Fig. 3). PEDOT:PSS is a polyelectrolyte complex prepared by the oxidative polymerization of PEDOT in the presence of PSS. In solution, it exists as relatively highMW chains of PSS decorated by relatively shorter oligomers of PEDOT [44]. The PEDOT with better conductivity is attached to the hydrophilic PSS backbone by polymerization (Fig. 3a). Fig. 3b shows the natural immiscibility of PDMS with PEDOT:PSS, which is completely coated by PDMS. Triton X-100 is a nonionic surfactant with a hydrophilic end and a hydrophobic end. The initial PEDOT:PSS drops form a circular island on the inside of PDMS liquid. The introduction of Triton X-100 turned out to relieve the interfacial tension between the two phases, making PDMS and PEDOT:PSS fully miscible (Fig. 3c). This is because the hydrophilic and hydrophobic ends of Triton X-100 are linked to PEDOT:PSS aqueous solution and PDMS, respectively, allowing PEDOT:PSS to be uniformly dispersed in PDMS, and no significant phase separation was observed. PEDOT:PSS can be dispersed in PDMS only at the assistance of Triton X-100, which results in homogenous mixing with a 3
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Fig. 3. Schematic illustration of the formation mechanism of polymer blends by Triton X-100. (a) PDMS polymer and PEDOT:PSS aqueous solution, (b) natural immiscibility of PDMS with PEDOT:PSS and Triton X-100, (c) PDMS and PEDOT:PSS are completely miscible after adding Triton X-100.
Fig. 4. Tensile modulus of conductive polymer films as a function of the content of EG and Triton X-100.
Fig. 5. Elongation at break of conductive polymer films as a function of the content of EG and Triton X-100.
stiffness of the films. The effects produced by EG are attributed to changes in the microstructure of the films.
conductive polymer films. The lowest sheet resistance of 20 Ω sq−1 was achieved using 7 wt% EG and 10 wt% Triton X-100. Compared with the composite film which only added PDMS-b-PEO, the film prepared by our method has a 20-fold increase in the square resistance after doping in the case where the PEDOT:PSS concentration is halved. At the same time, the tensile properties of the film are also improved [37]. We also attempted to correlate the mechanical data sheet resistance. In each triad on the plots in which the concentration of EG in the conductor was kept constant at 0, 7, or 14 wt%, the concentration of Triton X-100 simultaneously increased the electrical and tensile properties. This may be related to the distribution of conductive particles inside the film.
3.3. Electric properties of conductive polymer films We studied the electrical properties of the conductive polymer films as a function of both the concentrations of EG and Triton X-100 present in films (Fig. 6). The doping of EG can greatly reduce the sheet resistance (increased the conductivity) of the film, as we expected due to precedent in the literature. Similar values of sheet resistance were obtained for films containing both 7 wt% and 14 wt% EG. For samples with 0 wt% and 7 wt% EG, increasing the Triton X-100 concentration could reduce the sheet resistance of the film (The sheet resistance of the sample without EG was greatly reduced). This is because the added nonionic surfactant weakens the ionic interaction between PEDOT and PSS, but EG has a priority for improving the electrical properties of the film. However, concentration of Triton X-100 in excess of 1 wt% increased the sheet resistance of the samples containing 14 wt% EG. It is indicated that excessive doping reduces the electrical properties of the
3.4. XPS analysis of conductive polymer films To further understand the conformation of PEDOT:PSS in conductive polymer films, XPS analysis was performed on the different doped films. As is shown in Fig. 7, All samples exhibit three peaks in the XPS spectra of S2p where the bimodal peak at 163–165.5 eV is derived from sulfur atom in the thiophene ring of PEDOT and the peak at 4
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Fig. 8. Raman Spectra of conductive polymer films as a function of the content of EG and Triton X-100.
Fig. 6. Sheet resistance of conductive polymer films as a function of the content of EG and Triton X-100.
Fig. 9. SEM images of conductive polymer films as a function of the content of EG and Triton X-100.
3.5. Raman spectroscopy of conductive polymer films Fig. 8 shows the Raman spectra of conductive polymer films with and without EG and Triton X-100. For the film with almost no doping, the vibrational modes of PSS are located at 987 cm−1 and 1108 cm−1. The vibrational modes of PEDOT are Ca-Ca’ inter-ring stretching at 1273 cm−1, Cb-Cb stretching at 1389 cm−1 and Ca=Cb symmetrical vibration at 1426 cm−1 [48]. It is observed that the presence or absence of the addition of Triton X-100 significantly affected the absorption intensity of the Raman spectrum of the film, which may be related to the miscibility of PDMS and PEDOT:PSS [39]. Further, compared with the film without secondary doping, the addition of EG causes a red shift of the symmetrical vibration of Ca = Cb, a disappearance of the shoulder at 1389 cm−1, and a decrease in the intensity of the Raman peak at 1273 cm−1, which is in good agreement with literature [49,50]. It is means that the secondary doping promotes the conformational transition of PEDOT from the benzene to the quinoid structure. The quinoid structure among PEDOT chain results in high charge carrier mobility. Remarkable, for three films with different degrees of doping (7 wt% EG, 0.1 wt% Triton X-100, 0 wt% EG, 10 wt% Triton X-100 and 7 wt% EG, 10 wt% Triton X-100), show the red shift from 1426 to 1422 cm−1, from 1426 to 1424 cm−1 and from 1426 to 1419 cm−1, respectively. The degree of increase Raman shift is consistent with the trend of decreasing sheet resistance.
Fig. 7. XPS (S2p) spectra of conductive polymer films as a function of the content of EG and Triton X-100.
166–170 eV is derived from sulfur atom in the sulfonate of PSS [45]. When the Triton X-100 concentration increases from 0.1 to 10 wt%, the peak intensity of films decreases in the S2p spectrum. It is indicated that the introduction of Triton X-100 promotes the uniform dispersion of PEDOT:PSS in PDMS. The peak area ratio of PEDOT to PSS rich regions can be used to understand the relative content of PEDOT and PSS on the surface. Sample containing only 0.1 wt% of Triton X-100 has a PEDOT/ PSS ratio of 0.41 which is consistent with previous studies [46]. When 10 wt% Triton X-100 and 7 wt% EG are added, respectively, the PEDOT/PSS ratio is 0.45 and 0.51. This is because the surfactant accelerates the phase separation of PEODT:PSS to form part of the TXPEDOT and TX-PSS complexes and EG promotes the enlargement and coalescence of PEDOT particles [47]. At the EG loading of 7 wt% and Triton X-100 loading of 10 wt%, the PEDOT/PSS ratio is increased to 0.55. The trend of increasing PEDOT/PSS ratio is consistent with the decrease in the square resistance of the conductive polymer film in Fig. 6. In addition, the XPS spectrum of S2p also verified that EG plays a major role in improving the electrical properties of the film. 5
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Fig. 11. (a) I-V measurement of conductive polymer films integrated with an LED at 0%, 30%, 50% and 70% strain. (b) optical photograph of the LED-integrated circuits with the conductive polymer films as electrical wires with bending and twisting test.
and Triton X-100 (Fig. 9). Samples containing 0.1 wt% of Triton X-100 show that PEDOT:PSS conductive particles are clearly deposited on the surface of the film, and significant phase separation is observed for PDSM and PEDOT:PSS. Increasing the concentration of Triton X-100 further to 10 wt%, it can be seen that the conductive particles are uniformly encapsulated in the PDMS, and no obvious phase segregation can be observed on the polymer films (which is particularly clear in the transition from 0.1 wt% Triton X-100 to 1 wt% Triton X-100). As we expected in Fig. 2, the good miscibility of the two polymers can be attributed to the increase in the concentration of the tertiary dopant. The SEM images of conductive polymer films with Triton X-100 loading of 0.1 wt% revealed that the doping of EG into PEDOT:PSS leads to the enlargement and coalescence of conductive grains of PEDOT in the solid film. However, when the EG concentration is increased to 14 wt%, the PEDOT conductive grains will shrink by a certain amount. It is demonstrated that the change of the square resistance of the sample containing 0.1 wt% of Triton X-100 in Fig. 6. In addition, sample containing 7 wt% EG and 10 wt% Triton X-100 has a smoother surface compared to other samples. The PEDOT:PSS became highly cross-linked with the matrix, thus leading to the high efficient transfer and more continuous surface, which produce large regions of uninterrupted pathways for charge carriers to traverse the film. This result agreed well with the sheet resistance measurement result.
Fig. 10. Variation of normalized resistance of conductive polymer films with different concentrations of EG and Triton X-100 when films were stretched. (a) samples with 0 wt% EG, increasing the Triton X-100 from 0.1 wt% to 10 wt%. (b) samples with 7 wt% EG, increasing the Triton X-100 from 0.1 wt% to 10 wt %. (c) samples with 14 wt% EG, increasing the Triton X-100 from 0.1 wt% to 10 wt%.
3.7. Electro-mechanical properties of conductive polymer films We studied the changes in the electrical resistance in the conductive polymer films during stretching up (Fig. 10). The resistance of the films was measured in situ upon stretching until electrical failure occurred. The normalized resistance (R/R0) of samples containing 0.1 wt% Triton X-100 are increased rapidly at relatively low strain compared to other samples (Fig. 10a). Fig. 10a and b shows that as the Triton X-100 content of the composite increases, the extent to which the sample electrically endures escalates: the electrodes with a high Triton X-100
3.6. Surface morphology of conductive polymer films To attempt to correlate the microstructure of the films as apparent on the surface to the sheet resistance and miscibility, we obtained SEM images of samples with all nine combinations of concentrations of EG 6
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Table 1 The change in sheet resistance and elongation at break of conductive polymer films after three months at room temperature. Sample
Sheet resistance
0 wt% EG, 0.1 wt% Triton X-100 7 wt% EG, 0.1 wt% Triton X-100 14 wt% EG, 0.1 wt% Triton X-100 0 wt% EG, 1 wt% Triton X-100 7 wt% EG, 1 wt% Triton X-100 14 wt% EG, 1 wt% Triton X-100 0 wt% EG, 10 wt% Triton X-100 7 wt% EG, 10 wt% Triton X-100 14 wt% EG, 10 wt% Triton X-100
1.5 0.055 0.062 0.87 0.039 0.023 0.096 0.02 0.027
initial
(kΩ sq−1)
Sheet resistance
three months
(kΩ sq−1)
electrical failure 0.7 0.251 1.96 0.059 0.03 0.335 0.031 0.029
Max stretch 65% 50% 35% 75% 73.2% 71.7% 95% 82% 70%
initial
Max stretch
three months
41.25% 31.5% 27.6% 70% 72.5% 70.9% 66.6% 73% 55%
deposited on the surface of the PDMS is sufficiently in contact with the air to absorb moisture, causing the film to swell. When the Triton X-100 concentration is increased to 1 wt%, the films have good mechanical stability, that is, the tensile strength of the sample showed a negligible change. However, when the concentration of Triton X-100 is further increased to 10 wt%, the film is seriously aged.
content remain electrically intact for a large strain. We attributed the initial increase in resistance to the generation of minor cracks on the films and the result of increase in the particle separation when the films are stretched. The large increase in resistance thereafter to major cracks that led to the catastrophic failure of the films. The doping of Triton X100 can effectively delay the generation of cracks. However, Fig. 10c shows a different trend. It is illustrate that excessive doping reduces the electrical stability dependence of the films. We observed a much smaller increase in the relative resistance for the film with 7 wt% EG and 10 wt% Triton X-100 for up to 78% strain. The higher stability probably is attributed to the synergy effects of surface structure by the strain and enhanced adhesion of PDMS by the embedding. We noted that a sample containing 14 wt% EG and 1 wt% Triton X-100 has a more stable resistance growth trend. Therefore, such sample shows promising for fabricating strain sensors based on conductive polymer films. To test the conductive polymer films as stretchable conductors, a LED was illuminated with the as-made elastomeric conductors (Sample with 7 wt% EG and 10 wt% Triton X-100) as electrical wires and the I–V curves were measured when the films was under tensile strains of 0%, 30%, 50% and 70%. Fig. 11a shows a typical I–V response at specific strains, which clearly exhibits the ohmic properties of devices consisting of LED lights integrated with the stretchable conductors. At all stages, the LED is turned on at 2.4 V. When the film was stretched from 0% to 70% strain, the current decreased as the tensile strength increased. Fig. 11a also shows that the brightness of the LED lights exhibited almost no obvious degradation as conductive polymer films was stretched to a strain of 70% by increasing the tensile strain under constant voltage. It is implied that our synthetic conductors could maintain constant resistance with different voltages applied while enduring 70% tensile strain. In addition to simple uniaxial stretching, the conductive polymer films we prepared can be bent and twisted at will while maintaining stable electrical conductivity, as shown in Fig. 11b. The above demonstrates that our scheme is a promising approach in high performance stretchable electronics. Table 1 shows the sheet resistance and elongation at break of the conductive polymer films after standing for three months at room temperature. It can be clearly seen that the conductivity of the films without secondary doping is seriously degraded, and even electrical failure occurs. However, the addition of EG stabilizes the square resistance of the film, and the square resistance remains within an acceptable range. This is because PEDOT:PSS is hygroscopic, resulting in a decrease in the ratio of electron-ion conductivity hence enhance ionSeebeck, while organic solvent treatment can effectively suppress hygroscopicity [51]. Remarkably, the sheet resistance of the samples containing 0.1 wt% Triton X-100 also become unstable after prolonged placement. When the Triton X-100 concentration is increased to 1 wt% and 10 wt%, the square resistances of the films are not significantly increased. This indicates that good miscibility of PDMS and PEDOT:PSS is beneficial to maintain long-term stability of the electrical properties of the film [52]. Furthermore, it can be observed from Table 1 that severe mechanical degradation occurred in samples containing 0.1 wt% Triton X-100. The reason for this phenomenon is the PEDOT:PSS
4. Conclusions This paper described a simple, cost-effective route for the preparation of PEDOT:PSS-PDMS conductive polymer films were explored through direct manufacturing. Polymer blends of PEDOT:PSS and PDMS that are naturally insoluble in each other were prepared using a surfactant, Triton X-100, as a mediator. We discovered that the addition of Triton X-100 significantly improved the mechanical compliance, while improving electrical performance to some extent. The SEM of films surface shows that PEDOT: PSS and PDMS were mixed well without noticeable phase separation at a 10 wt% Triton X-100 to PEDOT: PSS by weight. Under doping with 7 wt% EG, 10 wt% Triton X100, the conductive polymer film show the lowest sheet resistance (20 Ω sq−1) and its elongation at break reached about 82%. At the Triton X-100 loading of 1 wt%, stretchable conductors made of this blend exhibit excellent piezoresistive effects and long-term stability. Moreover, we have demonstrated the feasibility of stretchable conductors by fabrication of stretchable LED circuits. On account of the excellent stretchability and conductivity, together with the facile fabrication process, we believe that our novel strategy could be directly integrated to a variety of high performance stretchable electronics. Acknowledgement This work was supported in part by NSF of the Science and Technology Department of Shaanxi Province under Grant No. 2019JM122, NSF of the Science and Technology Department of Shaanxi Province under Grant No. 2018JQ5100, NSF of the Key Laboratory of Shaanxi Provincial Department of Education under Grant No. 15JS075, Doctoral Research Initiation Fund of Xi'an University of Technology under Grant No. 108-451119007, and Shaanxi Collaborative Innovation Center of Green Intelligent Printing and Packaging. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.orgel.2019.105451. References [1] J.-S. Noh, Conductive elastomers for stretchable electronics, sensors and energy jarvesters, Polymers 8 (4) (2016) 123 https://doi.org/10.3390/polym8040123. [2] J.Y. Woo, K.K. Kim, J. Lee, J.T. Kim, C.-S. Han, Highly conductive and stretchable Ag nanowire/carbon nanotube hybrid conductors, Nanotechnology 25 (28) (2014) 285203 https://doi.org/10.1088/0957-4484/25/28/285203. [3] S. Zhang, Y. Li, Q. Tian, L. Liu, W. Yao, C. Chi, P. Zeng, N. Zhang, W. Wu, Highly conductive, flexible and stretchable conductors based on fractal silver,
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A simple strategy for high stretchable, flexible and conductive polymer films based on PEDOT:PSS-PDMS blends
T
Rubai Luoa,b, Haibin Lia, Bin Dua,b,∗, Shisheng Zhoua,b, Yuxiang Zhua a b
Faculty of Printing, Packaging Engineering and Digital Media Technology, Xi'an University of Technology, Xi'an, 710048, China Shaanxi Collaborative Innovation Center of Green Intelligent Printing and Packaging, Xi'an University of Technology, Xi'an, 710048, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Flexible electronics PEDOT:PSS PDMS Doping Conductive polymer films Integrated circuits
Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and polydimethylsiloxane (PDMS) are typical conductive polymers and transparent elastomers, respectively, which are widely used in the field of stretchable electronics. This paper described a simple, cost-effective route for the preparation of PEDOT:PSSPDMS conductive polymer films through direct manufacturing after miscibility. The surfactant, P-t octylophenol (Triton X-100), significantly improved the miscibility of PEDOT:PSS and PDMS. The tensile modulus, elongation at break, sheet resistance, normalized resistance and surface morphology, internal PEDOT:PSS conformation and long-term stability of the films (nine samples total) prepared by polymer blends were studied by varying the concentration groups of ethylene glycol (EG) and Triton X-100 in the mixture. By adjustment, the conductive polymer films exhibit a sheet resistance of 20 Ω sq−1 and an elongation at break of about 82%, and have excellent piezoresistive effects and long-term stability. Moreover, we also demonstrated the feasibility of stretchable conductors by fabrication of stretchable LED circuits. On account of the excellent stretchability and conductivity, together with the facile fabrication process, we believe that our novel strategy could be directly integrated to high performance stretchable electronics.
1. Introduction Stretchable (flexible) electronics manufacturing is an emerging electronic technology that combines organic/inorganic conductive materials with polymers or metal substrates. Stretchable electronics have received increasing attention with their unique flexibility/ductility and efficient, low-cost manufacturing processes, and were believed to gradually have broad application prospects in displays, sensors, RFID tags, and wearable devices [1]. Stretchable electronics should follow the mechanical movement of the active materials and maintain high conductivity at large strain. Although a variety of metal/non-mental conductors and conducting polymers, such as silver nanowire (AgNW) [2,3], carbon nanotubes (CNT) [4,5], graphene (GR) [6–9], polyaniline (PANI) [10], polypyrrole (PPY) [11,12] and poly(3,4-ethylene dioxythiophene) (PEDOT) [13–15] have been developed for diverse applications, their good electrical conductivity is limited by poor mechanical properties. However, traditional conductors are not stretchable, while traditional elastomers like polyurethane (PU), natural rubber (NR), butadiene rubber (SBR) and poly(dimethylsiloxane) (PDMS) are not conductive. Two methods have been used in implementing stretchable interconnects: formation of wavy or mesh configuration using
∗
conductive materials, and converting global tensile strain into local bending strain to improve the tensile adaptability to elastic materials [16]. Another method is to directly fill the conductive material into the elastomer to adapt to the strainability by the inherent stretchability of the filling structure [17,18]. However, these methods have some problems, such as the percolation dependent conductivity is highly strainsensitive and the combination of two materials is often not satisfactory in both conductivity and stretchability. The stretchable interconnect by modifying elastomer or conductive materials have been enhanced by several research groups. At present, the main methods are to adjust the external shape and internal structure of the elastic matrix and improving the surface properties of the elastomer. Ge et al. reported a fabrication method for binary-network structured polyurethane sponge–Ag nanowire–poly(dimethylsiloxane) (PUS-AgNW-PDMS) stretchable conductors with high performance, and confirmed high conductivity of 19.2 S/cm and a resistance change (R/ R0) of 160% at a 100% strain [19]. The key to achieve impressive combinations of elasticity and conductivity lies in the choice of commercial polyurethane (PU) sponge with an interconnected and junctionfree macroporous structure as the skeleton to support the 2D AgNW networks. As one of the most commonly used elastomeric substrates,
Corresponding author. Faculty of Printing, Packaging Engineering and Digital Media Technology, Xi'an University of Technology, Xi'an, 710048, China. E-mail address: [email protected] (B. Du).
https://doi.org/10.1016/j.orgel.2019.105451 Received 13 April 2019; Received in revised form 11 August 2019; Accepted 11 September 2019 Available online 14 September 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.
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PEDOT:PSS, 100% PDMS, 30% PDMS-b-PEO). The introduction of such block copolymers opens up new avenues for the direct preparation of stretchable conductive composite films. P-t-octylophenol (Triton X-100, Fig. 1) is non-ionic surfactant additive which has amphiphilic properties (hydrophilic “head” and hydrophobic “tail”) [38]. When added to the PEDOT:PSS, Triton X-100 was utilized to increase PEDOT:PSS films conductivity by stabilizing PEDOT nanosized particles [39]. It is widely used to regulate hydrophile-lipophile balance (HLB) and critical micelle concentration (CMC) [40]. In previous studies, it has been confirmed that such surfactants can further improve the performance of the PEDOT:PSS on the basis of secondary doping—i.e., as a “tertiary dopant [41].” In addition to improved wetting behavior and conductivity, Triton X-100 can reside the dried PEDOT:PSS film acts as a plasticizer [42] and inhibit PDMS crosslinking reaction [43]. Although there have been many studies on the secondary and tertiary doping of PEDOT:PSS, the integrated preparation of high performance PEDOT:PSS/PDMS conductive polymer films by two kinds of doping has rarely been reported. In this work, we reported a simple, controllable, and low cost effective route for the fabrication of stretchable conductive polymers through combining PEDOT:PSS with PDMS. The composite conductor produced can achieve excellent electrical conductivity and stretchability simultaneously by simple doping as compared with a composite conductor manufactured by a conventional method. The PDMS is not mixed with most aqueous solvents due to its high hydrophobicity, however. The key to the construction of the miscible structures lies in the choice of Triton X-100 to reduce the hydrophobicity of PDMS and blend it with PEDOT:PSS. Moreover, the miscibility, surface morphology, conductivity, mechanical property and long-term stability of the films prepared from the blends were investigated by changing the relative compositions of EG and Triton X-100. Finally, the possibility of their use for stretchable interconnects is examined.
Fig. 1. Chemical structures of PEDOT: PSS, PDMS, EG, and Triton X-100.
the high surface tension of PDMS is typically addressed by UV/O3 treatment to facilitate uniform coating of a variety of conductive materials [20,21]. Although the combination of a conductive material and an elastic substrate can be solved by a number of methods, these methods often have disadvantages such as high processing cost, complicated process, and low yield. To overcome this drawback, a wave of searching for new composite materials that can afford both high electrical conductivity and also good mechanical elasticity has surged. Conductive polymers are good candidates due to the self-regulating nature of the molecular structure and the flexibility of electro-mechanical properties [22]. The PEDOT:PSS solution (Fig. 1) is one of major conducting polymers that is already commercially applied to stretchable electronics and organic light-emitting diodes (OLEDs) [23]. PEDOT:PSS solution has good film forming properties, high transparency, excellent conductance controllability and thermal stability [24]. PEDOT has low solubility in organic solvents or water, which can be tightly bounded to PSS chains by the electrostatic interaction, making it water-dispersible with high stability [25]. The conductivity of PEDOT:PSS film is about 1 S/cm−1, which makes it very limited in the application process. Fortunately, secondary doping can effectively improve the conductivity of PEDOT:PSS. High boiling polar organic compounds can be considered as effective “secondary dopants”, such as dimethyl sulfoxide (DMSO) [26,27], ethylene glycol (EG) [28–31], tetrahydrofuran (THF) [26], and glycerol [32,33]. The effect of these secondary dopants is attributed to the enlargement and coalescence of conductive grains of PEDOT in the solid film upon drying at elevated temperature, which produces large regions of uninterrupted pathways for charge carriers to traverse the film and increases carrier mobility and concentration [34]. This chemical treatment discussed in detail in the review by Po et al. The conductivity of PEDOT:PSS were increased by three orders of magnitude (1000 S/cm−1) [35]. However, hydrophilic PEDOT:PSS and hydrophobic PDMS are difficult to directly compound to prepare a stretchable conductor. To overcome this drawback, Teng et al. realized the synthesis of PEDOT/PDMS composite conductors by gelling PEDOT:PSS. The as-prepared composite conductors remain electrically stability under strain stress (ε = 43%) [36]. For this method, its complex manufacturing process and unsatisfactory strain capability limit the application prospects of the conductor. Therefore, the block copolymer, PDMS-b-PEO, was proposed as a mediator to prepare polymer blends of PEDOT:PSS and PDMS which are naturally insoluble with each other [37]. The miscibility of PDMS and PEDOT was significantly improved and the cured composite film had a tensile elongation at break of 75% at an optimal combination (200%
2. Experimental details 2.1. Material PEDOT:PSS aqueous solution (Clevios PH1000) was purchased from Heraeus. Its concentration was 1–1.3 wt% and had a ratio of PEDOT to PSS of 1: 2.5 by weight. PDMS (Sylgard 184) was received from Dow Corning in combination with a curing agent. Ethylene glycol (EG, 99 wt %) was supplied by the Macklin. Triton X-100 (laboratory grade) was purchased from Sigma Aldrich. All the chemicals were used as received without further purification. PET film was provided by the Continental Lab. 2.2. Preparation of composite polymer films The preparation process of the conductive polymer film is depicted in Fig. 2. First of all, to prepare polymer blends, PEDOT:PSS solution was mixed with PDMS, three constituents (curing agent, Triton X-100, EG) were dropped into a beaker in a stepwise manner with tight weight control. On the basis of the PEDOT:PSS weight, PDMS for 200 wt%, the relative weights of Triton X-100 and EG were varied: 0.1, 1, 10 wt% for Triton X-100 and 0, 7, 14 wt% for EG. The amount of the curing agent was controlled at 10% of the weight of PDMS. The mixtures were vigorously stirred at 300 rpm for 1 h to obtain homogeneous dispersions. Polymer blends stirred and degassed in a planetary mixer (Thinky ARV310, Japan) for 10 min at 2000 rpm. In the next step, the conductive polymer films were prepared by uniformly spread the dispersion of a polymer blend on mold and then dried at 100 °C for 1 h in an oven. Finally, after cooling down to room temperature, the conductive polymer films were carefully peeled off from mold and cut into rectangle strips of 50 × 15 mm2 for mechanical and electrical tests. The thickness of the films prepared in this way ranged from 300 to 500 μm. 2
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Fig. 2. Schematic illustration of fabrication sequence for conductive polymer film. Step 1: the polymer blend was prepared after secondary doping and tertiary doping. Step 2: selective masking of the PET film using Kapton tape and the defoaming polymer blend was dropped onto the unmasked areas. Step 3: thermal curing the polymer blend. Step 4: conductive polymer film was peeled-off for further characterization.
lower critical micelle concentration. According to previous reports, Triton X-100 can weakens the ionic interaction between PEDOT and PSS and expedite phase separation into separate phases, leading to enhanced crystallization of PEDOT segments and resulting in increased electrical conductivity [39].
2.3. Characterization The morphologies of the conductive polymer films were analyzed using scanning electron microscopy (SEM Hitachi S4700, Japan). The acceleration potential was 10–15 kV. The conductivities and square resistances of conductive polymer films were measured by the double electric test four-probe resistance tester (RTS-9, China). Chemical bonding states were investigated by X-ray photoelectron (XPS AXISULTRA, Kratos, UK). XPS spectra were obtained with monochromatic Al Kα (1486.71eV) line at a power of 100 W (10 mA, 10 kV). The Raman scattering spectra were measured by a Raman spectrometer (LabRam HR Evolution, France) with a 532 nm laser source. The conductive polymer films were fixed on a customized stage, the current (I)–voltage (V) characteristics and the resistances change upon deformation were measured using source meter (Keithley Model: 2461, United States). The tensile modulus was evaluated using a universal testing machine (Instron 5500, United States) with a 10 N load cell at a strain rate of 5 mm/min−1.
3.2. Mechanical properties of conductive polymer films Figs. 4 and 5 show the results of the mechanical measurements. Triton X-100 has a dramatic plasticizing effect on conductive polymer films, as seen in the decrease in tensile modulus and increase in elongation at break. This is because Triton X-100 is difficult to volatilize, which remains in the films to increase the free volume in the membrane and weaken the intermolecular forces between the polymer chains [43].It is to be noted that the absence of data (Fig. 4) from the sample containing 0 wt% EG and 10 wt% Triton X-100, which was too soft to measure. In each triad on the plots in which the concentration of EG in the conductor was kept constant at 0, 7, or 14 wt%, the concentration of Triton X-100 dramatically decrease the tensile modulus of the conductor, as manifested by easier to stretch. For samples with 7 wt% and 14 wt% EG, increasing the Triton X-100 from 0.1 wt% to 1 wt% results in a decrease in tensile modulus by more than two times, and a more pronounced decrease in modulus for samples with 10 wt% Triton X100. For samples with 0 wt% and 7 wt% EG, Increasing Triton X-100 from 0.1 wt% to 10 wt% resulted in an increase in elongation at break (Fig. 5). The maximum effective strain can reach about 95% which far exceeds the maximum stretch (ε ≈ 43%) of the PEDOT:PSS/PDMS composite gel [36]. We noticed that the increase in Triton X-100 from 1 wt% to 10 wt% produced a decrease in the elongation at break for samples with 14 wt% EG. This may be because excessive secondary doping can cause the film to become more fragile, affecting the plasticization of Triton X-100. The data shown in Figs. 4 and 5 also reveal a dependence of the mechanical properties of conductive polymer films on the amount of EG present. It can be seen from Fig. 4 that the film containing 7 wt% EG is the least stretchable, but the difference in elastic modulus is not as obvious as Triton X-100. This may be because during the preparation process, a large amount of EG volatilized at 100 °C, and thus has a small effect on the elastic modulus of the film. Meanwhile, the increase in EG concentration reduced the elongation at break of the film while the Triton X-100 concentration maintained constant (Fig. 5). These results indicate that the increase of EG concentration will reduce the relative
3. Results and discussion 3.1. Principle of tertiary dopant In order to understand the role of Triton X-100 in conductive polymer films in affecting the surface properties of the PDMS, we revealed the mechanism of action of Triton X-100 (Fig. 3). PEDOT:PSS is a polyelectrolyte complex prepared by the oxidative polymerization of PEDOT in the presence of PSS. In solution, it exists as relatively highMW chains of PSS decorated by relatively shorter oligomers of PEDOT [44]. The PEDOT with better conductivity is attached to the hydrophilic PSS backbone by polymerization (Fig. 3a). Fig. 3b shows the natural immiscibility of PDMS with PEDOT:PSS, which is completely coated by PDMS. Triton X-100 is a nonionic surfactant with a hydrophilic end and a hydrophobic end. The initial PEDOT:PSS drops form a circular island on the inside of PDMS liquid. The introduction of Triton X-100 turned out to relieve the interfacial tension between the two phases, making PDMS and PEDOT:PSS fully miscible (Fig. 3c). This is because the hydrophilic and hydrophobic ends of Triton X-100 are linked to PEDOT:PSS aqueous solution and PDMS, respectively, allowing PEDOT:PSS to be uniformly dispersed in PDMS, and no significant phase separation was observed. PEDOT:PSS can be dispersed in PDMS only at the assistance of Triton X-100, which results in homogenous mixing with a 3
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Fig. 3. Schematic illustration of the formation mechanism of polymer blends by Triton X-100. (a) PDMS polymer and PEDOT:PSS aqueous solution, (b) natural immiscibility of PDMS with PEDOT:PSS and Triton X-100, (c) PDMS and PEDOT:PSS are completely miscible after adding Triton X-100.
Fig. 4. Tensile modulus of conductive polymer films as a function of the content of EG and Triton X-100.
Fig. 5. Elongation at break of conductive polymer films as a function of the content of EG and Triton X-100.
stiffness of the films. The effects produced by EG are attributed to changes in the microstructure of the films.
conductive polymer films. The lowest sheet resistance of 20 Ω sq−1 was achieved using 7 wt% EG and 10 wt% Triton X-100. Compared with the composite film which only added PDMS-b-PEO, the film prepared by our method has a 20-fold increase in the square resistance after doping in the case where the PEDOT:PSS concentration is halved. At the same time, the tensile properties of the film are also improved [37]. We also attempted to correlate the mechanical data sheet resistance. In each triad on the plots in which the concentration of EG in the conductor was kept constant at 0, 7, or 14 wt%, the concentration of Triton X-100 simultaneously increased the electrical and tensile properties. This may be related to the distribution of conductive particles inside the film.
3.3. Electric properties of conductive polymer films We studied the electrical properties of the conductive polymer films as a function of both the concentrations of EG and Triton X-100 present in films (Fig. 6). The doping of EG can greatly reduce the sheet resistance (increased the conductivity) of the film, as we expected due to precedent in the literature. Similar values of sheet resistance were obtained for films containing both 7 wt% and 14 wt% EG. For samples with 0 wt% and 7 wt% EG, increasing the Triton X-100 concentration could reduce the sheet resistance of the film (The sheet resistance of the sample without EG was greatly reduced). This is because the added nonionic surfactant weakens the ionic interaction between PEDOT and PSS, but EG has a priority for improving the electrical properties of the film. However, concentration of Triton X-100 in excess of 1 wt% increased the sheet resistance of the samples containing 14 wt% EG. It is indicated that excessive doping reduces the electrical properties of the
3.4. XPS analysis of conductive polymer films To further understand the conformation of PEDOT:PSS in conductive polymer films, XPS analysis was performed on the different doped films. As is shown in Fig. 7, All samples exhibit three peaks in the XPS spectra of S2p where the bimodal peak at 163–165.5 eV is derived from sulfur atom in the thiophene ring of PEDOT and the peak at 4
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Fig. 8. Raman Spectra of conductive polymer films as a function of the content of EG and Triton X-100.
Fig. 6. Sheet resistance of conductive polymer films as a function of the content of EG and Triton X-100.
Fig. 9. SEM images of conductive polymer films as a function of the content of EG and Triton X-100.
3.5. Raman spectroscopy of conductive polymer films Fig. 8 shows the Raman spectra of conductive polymer films with and without EG and Triton X-100. For the film with almost no doping, the vibrational modes of PSS are located at 987 cm−1 and 1108 cm−1. The vibrational modes of PEDOT are Ca-Ca’ inter-ring stretching at 1273 cm−1, Cb-Cb stretching at 1389 cm−1 and Ca=Cb symmetrical vibration at 1426 cm−1 [48]. It is observed that the presence or absence of the addition of Triton X-100 significantly affected the absorption intensity of the Raman spectrum of the film, which may be related to the miscibility of PDMS and PEDOT:PSS [39]. Further, compared with the film without secondary doping, the addition of EG causes a red shift of the symmetrical vibration of Ca = Cb, a disappearance of the shoulder at 1389 cm−1, and a decrease in the intensity of the Raman peak at 1273 cm−1, which is in good agreement with literature [49,50]. It is means that the secondary doping promotes the conformational transition of PEDOT from the benzene to the quinoid structure. The quinoid structure among PEDOT chain results in high charge carrier mobility. Remarkable, for three films with different degrees of doping (7 wt% EG, 0.1 wt% Triton X-100, 0 wt% EG, 10 wt% Triton X-100 and 7 wt% EG, 10 wt% Triton X-100), show the red shift from 1426 to 1422 cm−1, from 1426 to 1424 cm−1 and from 1426 to 1419 cm−1, respectively. The degree of increase Raman shift is consistent with the trend of decreasing sheet resistance.
Fig. 7. XPS (S2p) spectra of conductive polymer films as a function of the content of EG and Triton X-100.
166–170 eV is derived from sulfur atom in the sulfonate of PSS [45]. When the Triton X-100 concentration increases from 0.1 to 10 wt%, the peak intensity of films decreases in the S2p spectrum. It is indicated that the introduction of Triton X-100 promotes the uniform dispersion of PEDOT:PSS in PDMS. The peak area ratio of PEDOT to PSS rich regions can be used to understand the relative content of PEDOT and PSS on the surface. Sample containing only 0.1 wt% of Triton X-100 has a PEDOT/ PSS ratio of 0.41 which is consistent with previous studies [46]. When 10 wt% Triton X-100 and 7 wt% EG are added, respectively, the PEDOT/PSS ratio is 0.45 and 0.51. This is because the surfactant accelerates the phase separation of PEODT:PSS to form part of the TXPEDOT and TX-PSS complexes and EG promotes the enlargement and coalescence of PEDOT particles [47]. At the EG loading of 7 wt% and Triton X-100 loading of 10 wt%, the PEDOT/PSS ratio is increased to 0.55. The trend of increasing PEDOT/PSS ratio is consistent with the decrease in the square resistance of the conductive polymer film in Fig. 6. In addition, the XPS spectrum of S2p also verified that EG plays a major role in improving the electrical properties of the film. 5
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Fig. 11. (a) I-V measurement of conductive polymer films integrated with an LED at 0%, 30%, 50% and 70% strain. (b) optical photograph of the LED-integrated circuits with the conductive polymer films as electrical wires with bending and twisting test.
and Triton X-100 (Fig. 9). Samples containing 0.1 wt% of Triton X-100 show that PEDOT:PSS conductive particles are clearly deposited on the surface of the film, and significant phase separation is observed for PDSM and PEDOT:PSS. Increasing the concentration of Triton X-100 further to 10 wt%, it can be seen that the conductive particles are uniformly encapsulated in the PDMS, and no obvious phase segregation can be observed on the polymer films (which is particularly clear in the transition from 0.1 wt% Triton X-100 to 1 wt% Triton X-100). As we expected in Fig. 2, the good miscibility of the two polymers can be attributed to the increase in the concentration of the tertiary dopant. The SEM images of conductive polymer films with Triton X-100 loading of 0.1 wt% revealed that the doping of EG into PEDOT:PSS leads to the enlargement and coalescence of conductive grains of PEDOT in the solid film. However, when the EG concentration is increased to 14 wt%, the PEDOT conductive grains will shrink by a certain amount. It is demonstrated that the change of the square resistance of the sample containing 0.1 wt% of Triton X-100 in Fig. 6. In addition, sample containing 7 wt% EG and 10 wt% Triton X-100 has a smoother surface compared to other samples. The PEDOT:PSS became highly cross-linked with the matrix, thus leading to the high efficient transfer and more continuous surface, which produce large regions of uninterrupted pathways for charge carriers to traverse the film. This result agreed well with the sheet resistance measurement result.
Fig. 10. Variation of normalized resistance of conductive polymer films with different concentrations of EG and Triton X-100 when films were stretched. (a) samples with 0 wt% EG, increasing the Triton X-100 from 0.1 wt% to 10 wt%. (b) samples with 7 wt% EG, increasing the Triton X-100 from 0.1 wt% to 10 wt %. (c) samples with 14 wt% EG, increasing the Triton X-100 from 0.1 wt% to 10 wt%.
3.7. Electro-mechanical properties of conductive polymer films We studied the changes in the electrical resistance in the conductive polymer films during stretching up (Fig. 10). The resistance of the films was measured in situ upon stretching until electrical failure occurred. The normalized resistance (R/R0) of samples containing 0.1 wt% Triton X-100 are increased rapidly at relatively low strain compared to other samples (Fig. 10a). Fig. 10a and b shows that as the Triton X-100 content of the composite increases, the extent to which the sample electrically endures escalates: the electrodes with a high Triton X-100
3.6. Surface morphology of conductive polymer films To attempt to correlate the microstructure of the films as apparent on the surface to the sheet resistance and miscibility, we obtained SEM images of samples with all nine combinations of concentrations of EG 6
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Table 1 The change in sheet resistance and elongation at break of conductive polymer films after three months at room temperature. Sample
Sheet resistance
0 wt% EG, 0.1 wt% Triton X-100 7 wt% EG, 0.1 wt% Triton X-100 14 wt% EG, 0.1 wt% Triton X-100 0 wt% EG, 1 wt% Triton X-100 7 wt% EG, 1 wt% Triton X-100 14 wt% EG, 1 wt% Triton X-100 0 wt% EG, 10 wt% Triton X-100 7 wt% EG, 10 wt% Triton X-100 14 wt% EG, 10 wt% Triton X-100
1.5 0.055 0.062 0.87 0.039 0.023 0.096 0.02 0.027
initial
(kΩ sq−1)
Sheet resistance
three months
(kΩ sq−1)
electrical failure 0.7 0.251 1.96 0.059 0.03 0.335 0.031 0.029
Max stretch 65% 50% 35% 75% 73.2% 71.7% 95% 82% 70%
initial
Max stretch
three months
41.25% 31.5% 27.6% 70% 72.5% 70.9% 66.6% 73% 55%
deposited on the surface of the PDMS is sufficiently in contact with the air to absorb moisture, causing the film to swell. When the Triton X-100 concentration is increased to 1 wt%, the films have good mechanical stability, that is, the tensile strength of the sample showed a negligible change. However, when the concentration of Triton X-100 is further increased to 10 wt%, the film is seriously aged.
content remain electrically intact for a large strain. We attributed the initial increase in resistance to the generation of minor cracks on the films and the result of increase in the particle separation when the films are stretched. The large increase in resistance thereafter to major cracks that led to the catastrophic failure of the films. The doping of Triton X100 can effectively delay the generation of cracks. However, Fig. 10c shows a different trend. It is illustrate that excessive doping reduces the electrical stability dependence of the films. We observed a much smaller increase in the relative resistance for the film with 7 wt% EG and 10 wt% Triton X-100 for up to 78% strain. The higher stability probably is attributed to the synergy effects of surface structure by the strain and enhanced adhesion of PDMS by the embedding. We noted that a sample containing 14 wt% EG and 1 wt% Triton X-100 has a more stable resistance growth trend. Therefore, such sample shows promising for fabricating strain sensors based on conductive polymer films. To test the conductive polymer films as stretchable conductors, a LED was illuminated with the as-made elastomeric conductors (Sample with 7 wt% EG and 10 wt% Triton X-100) as electrical wires and the I–V curves were measured when the films was under tensile strains of 0%, 30%, 50% and 70%. Fig. 11a shows a typical I–V response at specific strains, which clearly exhibits the ohmic properties of devices consisting of LED lights integrated with the stretchable conductors. At all stages, the LED is turned on at 2.4 V. When the film was stretched from 0% to 70% strain, the current decreased as the tensile strength increased. Fig. 11a also shows that the brightness of the LED lights exhibited almost no obvious degradation as conductive polymer films was stretched to a strain of 70% by increasing the tensile strain under constant voltage. It is implied that our synthetic conductors could maintain constant resistance with different voltages applied while enduring 70% tensile strain. In addition to simple uniaxial stretching, the conductive polymer films we prepared can be bent and twisted at will while maintaining stable electrical conductivity, as shown in Fig. 11b. The above demonstrates that our scheme is a promising approach in high performance stretchable electronics. Table 1 shows the sheet resistance and elongation at break of the conductive polymer films after standing for three months at room temperature. It can be clearly seen that the conductivity of the films without secondary doping is seriously degraded, and even electrical failure occurs. However, the addition of EG stabilizes the square resistance of the film, and the square resistance remains within an acceptable range. This is because PEDOT:PSS is hygroscopic, resulting in a decrease in the ratio of electron-ion conductivity hence enhance ionSeebeck, while organic solvent treatment can effectively suppress hygroscopicity [51]. Remarkably, the sheet resistance of the samples containing 0.1 wt% Triton X-100 also become unstable after prolonged placement. When the Triton X-100 concentration is increased to 1 wt% and 10 wt%, the square resistances of the films are not significantly increased. This indicates that good miscibility of PDMS and PEDOT:PSS is beneficial to maintain long-term stability of the electrical properties of the film [52]. Furthermore, it can be observed from Table 1 that severe mechanical degradation occurred in samples containing 0.1 wt% Triton X-100. The reason for this phenomenon is the PEDOT:PSS
4. Conclusions This paper described a simple, cost-effective route for the preparation of PEDOT:PSS-PDMS conductive polymer films were explored through direct manufacturing. Polymer blends of PEDOT:PSS and PDMS that are naturally insoluble in each other were prepared using a surfactant, Triton X-100, as a mediator. We discovered that the addition of Triton X-100 significantly improved the mechanical compliance, while improving electrical performance to some extent. The SEM of films surface shows that PEDOT: PSS and PDMS were mixed well without noticeable phase separation at a 10 wt% Triton X-100 to PEDOT: PSS by weight. Under doping with 7 wt% EG, 10 wt% Triton X100, the conductive polymer film show the lowest sheet resistance (20 Ω sq−1) and its elongation at break reached about 82%. At the Triton X-100 loading of 1 wt%, stretchable conductors made of this blend exhibit excellent piezoresistive effects and long-term stability. Moreover, we have demonstrated the feasibility of stretchable conductors by fabrication of stretchable LED circuits. On account of the excellent stretchability and conductivity, together with the facile fabrication process, we believe that our novel strategy could be directly integrated to a variety of high performance stretchable electronics. Acknowledgement This work was supported in part by NSF of the Science and Technology Department of Shaanxi Province under Grant No. 2019JM122, NSF of the Science and Technology Department of Shaanxi Province under Grant No. 2018JQ5100, NSF of the Key Laboratory of Shaanxi Provincial Department of Education under Grant No. 15JS075, Doctoral Research Initiation Fund of Xi'an University of Technology under Grant No. 108-451119007, and Shaanxi Collaborative Innovation Center of Green Intelligent Printing and Packaging. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.orgel.2019.105451. References [1] J.-S. Noh, Conductive elastomers for stretchable electronics, sensors and energy jarvesters, Polymers 8 (4) (2016) 123 https://doi.org/10.3390/polym8040123. [2] J.Y. Woo, K.K. Kim, J. Lee, J.T. Kim, C.-S. Han, Highly conductive and stretchable Ag nanowire/carbon nanotube hybrid conductors, Nanotechnology 25 (28) (2014) 285203 https://doi.org/10.1088/0957-4484/25/28/285203. [3] S. Zhang, Y. Li, Q. Tian, L. Liu, W. Yao, C. Chi, P. Zeng, N. Zhang, W. Wu, Highly conductive, flexible and stretchable conductors based on fractal silver,
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