Bioinspired Design of Strong, Tough, and Highly Conductive Polyol

Letter www.acsami.org

Bioinspired Design of Strong, Tough, and Highly Conductive PolyolPolypyrrole Composites for Flexible Electronics Fengxian Gao,†,§ Ning Zhang,*,§ Xiaodong Fang,*,† and Mingming Ma*,§ †

Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China CAS Key Laboratory of Soft Matter Chemistry, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Department of Chemistry, University of Science and Technology of China, Hefei 230026, China

§

S Supporting Information *

ABSTRACT: Inspired by the dynamic network structure of animal dermis, we have designed and synthesized a series of polyol-polypyrrole (polyol-PPy) composites. Polyols and polypyrrole are cross-linked by hydrogen bonding and electrostatic interactions to form a dynamic network, which helps to dissipate destructive energy. We have found a clear correlation between the mechanical properties of polyol-PPy composites and the polyols structure. Particularly, the PEE-PPy film shows both high strength and flexibility, leading to a remarkable tensile toughness comparable to cocoon silk. The combination of outstanding strength, ductility, and conductivity enables polyol-PPy composites (especially PEEPPy) as potential electronic materials for making flexible electronics.

KEYWORDS: polypyrrole, conducting polymers, flexible electronics, bioinspired materials, tough polymers

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other polymers,19,20 or by postsynthesis processing such as electrospinning.21 Despite the tremendous efforts, the flexibility of theses enhanced PPy films is still low (the elongation at break is typically less than 10%), which needs further improvement to meet the requirements of flexible electronics.1 Therefore, a general method to prepare strong, tough, and highly conductive PPy is desired, but remains a challenge. It is well-known that animal dermis possesses a network structure consisted by rigid collagen fibers and soft elastin fibers (Figure 1a). The two types of fibers interconnect with each other through supramolecular interactions to form a dynamic network, making animal dermis a sturdy and flexible material. Inspired by the dynamic network structure of animal dermis, we demonstrated that a dynamic network formed by supramolecular interconnection between a soft polymer and the rigid polypyrrole would yield a strong and tough conductive polymer material.22 The dynamic network structure formed by noncovalent cross-linking between polymer chains has also been demonstrated as an effective way to achieve tough polymers.23,24 Herein, we report the design of a series of polyolPPy composites, in which polyols and polypyrrole are crosslinked by hydrogen bonding and electrostatic interactions to form a dynamic network (Figure 1b). The dynamic network based on supramolecular interactions can dissipate mechanical energy efficiently through noncovalent bond breaking and

lexible electronics is a technology for assembling electronic circuits on flexible plastic substrates, which can be folded and twisted with negligible effect on its electronic function.1 Current flexible electronics is typically composed of a thin passive plastic substrate topped with a second layer of active electronic components such as inorganic conductors and semiconductors, which have strict limits in the extent of deformation due to the intrinsic rigidity of inorganic electronic materials.2 On the other hand, organic electronic materials such as semiconducting small molecules and conducting polymers can provide good mechanical flexibility, tunable electronic properties, convenience, and low cost of manufacture, which enable organic electronic materials as promising alternatives to inorganic electronic materials for making flexible electronics.3−6 Among organic electronic materials, polypyrrole (PPy) as one type of conducting polymer shows high conductivity, good environmental stability, low toxicity, and high biocompatibility. Therefore, PPy has been widely studied for applications as organic electronic components such as sensors,7,8 electrodes,9,10 and actuators.11,12 PPy can be prepared by electrochemical polymerization to form a self-standing polymer film. Because of its conjugated chain structure, conventional PPy films are brittle, which limits its application for flexible electronics. Many efforts have been devoted to improving the strength and conductivity of PPy film by optimizing the electro-polymerization conditions such as electrolyte anions,13 electrolyte solvents,14 pH,15 polymerization temperature,16 current density/potential,16 and polymer additives.14,17,18 The properties of PPy film can also be enhanced by forming composite with © XXXX American Chemical Society

Received: January 15, 2017 Accepted: February 7, 2017 Published: February 7, 2017 A

DOI: 10.1021/acsami.7b00717 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Structure design of polyol-PPy composites. (a) Dynamic network in animal dermis is formed by the interconnection between collagen fibers and elastin fibers. (b) Dynamic network structure formed between polyols and polypyrrole through hydrogen bonding and electrostatic interactions. (c) Photograph of a PEE-PPy film.

reforming,25 which enables these polyol-PPy composites to show superior mechanical properties to conventional PPy. We have found a clear correlation between the mechanical properties of the polyol-PPy composites and the molecular structure of polyols. Among these polyols, the tetra-armed polyethylene glycol (pentaerythritol ethoxylate, PEE) provides the best performance. The PEE-PPy film as shown in Figure 1c provides both high tensile strength (125 MPa) and high flexibility (elongation-at-break 75%), leading to remarkable tensile toughness (73 MJ/m3) that is comparable to the values of cocoon silk (70 MJ/m3). Besides the outstanding mechanical properties, the PEE-PPy film also shows a high conductivity (115 S/cm). The combination of outstanding strength, ductility, and conductivity enables polyol-PPy composites (especially PEE-PPy) as a potential electronic material for fabricating flexible electronics. The polyol-ppy composite films were synthesized by electropolymerization of pyrrole in organic electrolyte solution at the presence of polyol-borate complex. When polyols are mixed with boron trifluoride diethyl etherate (BFEE), the terminal alcohol groups of polyols can react with boron trifluoride to form borate ester bond, which connects polyols into polymers with negative charges on tetra-coordinated boron atoms. The anionic polyol-borate polymers can be electrically attracted to the anode and trapped in the growing PPy matrix as macromolecular counterions.22 In the formed polyol-PPy composite films, polyols and PPy are interconnected through hydrogen bonding and electrostatic interaction to form a dynamic network structure (Figure 1b).

To systematically investigate the effect of the molecular structure of polyols on the properties of polyol-PPy films, we selected and tested ten polyols. Six polyols are linear polyethylene glycol with different molecular weight (MW), which are referred as PEG 200, PEG 400, PEG 600, PEG 800, PEG 1000, PEG 2000 (the number indicating the numberaverage molecular weight Mn). Two polyols are linear polypropylene glycol (PPG 400, PPG 2000). Two tetraarmed polyols are pentaerythritol ethoxylate (Mn ∼ 800) referred as PEE, and pentaerythritol propoxylate (Mn ∼ 630) referred as PEP 600 (Figure S1). To focus on the “molecular structure−property” relationship of polyol-PPy films, we kept other factors of the electro-polymerization condition the same for all the polyols, including electrolyte solvents, temperature, current density and working electrodes. The volume percentage of polyols in the electrolyte solution was also fixed at 5%. For comparison, we also synthesized PPy film in the same organic electrolyte solution without adding any polyols. Taking PEEPPy as an example (Figure 1c), the obtained polyol-PPy films are generally black-colored compact films with a mirrorlike surface and good flexibility. The ATR-IR spectra of polyol-PPy composite films and PPy without polyols are shown in Figure S2a, which are similar to each other. The bands at 1509, 1410, 1265, and 1113 cm−1 are assigned as pyrrole N−H in-plane bending, pyrrole C−N ring stretching, B−O stretching, and C−O stretching. Raman interrogation of these polyol-PPy films also showed similar spectra to each other (Figure S2b), which indicates similar PPy skeleton structure through all these polyol-PPy composites. B


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Figure 2. Mechanical properties of polyol-PPy composites. (a) Content of polyols in the polyol-PPy composites. (b−e) Mechanical properties of polyol-PPy films plot to the MW of polyols: (b) elongation at break, (c) Young’s modulus, (d) tensile strength, and (e) tensile toughness. (f) Three typical polyol-PPy films’ stress−strain curves.

correlates with its outstanding mechanical properties as discussed later. With the same amount of charge passing through the working electrode, the obtained weight of polyol-PPy composite films (mpolyol+ppy) was always higher than the PPy film (mppy). The difference in weight would be due to the “doping” of polyols into the polyol-PPy composite films, referred as (mpolyol). When plotting mpolyol/mpolyol+ppy over the MW of polyols (Figure 2a), we found two clear patterns relating to the molecular structure of polyols: (1) Polyethylene glycol (PEG) series tends to dope more than the polypropylene glycol (PPG) series. The low doping of PPG can be attributed to the irregular orientation of methyl groups along the PPG chain, which would hinder the compact interaction between PPG and PPy.

The similarity of IR and Raman spectra of different polyol-PPy composites infers that the variation in the molecular structure of polyols has little effect on the electro-polymerization process of pyrrole and the skeleton structure of formed PPy. On the other hand, the physical properties of polyol-PPy films are clearly dependent on the molecular structure of polyols. The PPy film synthesized without polyols shows the lowest density (∼1.5 g/cm3, Table S1), whereas the addition of polyols significantly improved the density of obtained polyolPPy films to 1.65−1.9 g/cm3. The density of components in the electrolyte solution: isopropanol (0.79 g/cm3), polyethylene glycols (1.1−1.2 g/cm3), polypropylene glycols (∼1.0 g/cm3), and boron trifluoride diethyl etherate (1.13 g/cm3), is lower than PPy itself. Therefore, the enhanced density of polyol-PPy films can only be attributed to the more compact packing between polyols and PPy than PPy itself. Particularly, the PEEPPy film provides the highest density (∼1.91 g/cm3), which C


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Figure 3. SEM images of PPy films. (a) Surface of PPy without polyols shows broccoli-like particles and defects. The arrows indicate defects. (b−d) SEM images of polyol-PPy films with (b) PEE 800, (c) PEG 800, and (d) PEP 600. Scale bar: 10 μm.

strength comparing to PPy without polyols (Figure 2d). This effect can be attributed to hydrogen bonding and electrostatic interactions between polyols and PPy, which are much stronger than the van der Waals’ force between typical plasticizer and plastics. From this point of view, polyols behaved as a dynamic cross-linking agent. Therefore, the addition of polyols into PPy greatly enhances the flexibility of PPy film with no compromise of the film’s strength. Tensile toughness is the energy consumed during the process of the sample being snapped, which can be measured by calculating the area value under the stress−strain curve.26 All the polyol-PPy films show enhanced tensile toughness comparing with PPy film without polyols (Figure 2e). The toughness of polyol-PPy films increases with the increases of MW of polyols, reaches a maximum when the MW of polyols reaches ∼600, and decreases for polyols with higher MW. In general, the PEG-PPy films show better tensile toughness than PPG−PPy films, which is due to the better incorporation of PEG with PPy chains. The toughening effect of polyols can be attributed to the dynamic network structure based on noncovalent cross-linking between polymer chains.22 The interaction between polyols and PPy are mainly hydrogen bonding and electrostatic interactions, which can be easily broken and reformed upon mechanical stress and help to dissipate the destructive energy.23,24,27,28 Among all the polyol-PPy films, PEE-PPy provides the highest tensile toughness (∼73 MJ/m3), which is 3.6 times higher than the PPy film without polyols (∼20 MJ/m3), and comparable to the tensile toughness of cocoon silk (70 MJ/ m3).29 This effect can be explained by the network structure formed by PEE (Figure 1b), which is more resistant to damage than the linear structure formed by linear PEG.23,24 In fact, the ductility of PEE-PPy is significant better than PEG1000-PPy and similar to PEG2000-PPy (Figure 2b), although the doping content of PEE in PEE-PPy film is significantly lower than PEG 1000 and PEG 2000 in corresponding films. At the meantime, the tensile strength of PEE-PPy film is highest among all the polyol-PPy films (Figure 2d). The combination of high tensile strength and high ductility enables a remarkable tensile

(2) Within the same series, higher MW polyols tend to dope more into the polyol-PPy films than lower MW polyols. A higher MW polyol would have more ethylene glycol or propylene glycol units in each polyol molecule, and generate stronger interaction with PPy chains. On the other hand, even only a few ethylene glycol or propylene glycol units in a polyol chain interact with PPy, the entire polyol chain would be incorporated into the film. Therefore, the doping extent is roughly linear to the MW of polyols. One interesting finding is that the doping extent for PEG 800 and PEE 800 are almost the same, despite that PEG 800 is linear, while PEE 800 is tetra-armed. This result is easy to explain, if considering that PEG 800 and PEE 800 have almost the same number of ethylene glycol units in their molecular structures. As shown in Figure 2b, all the polyol-PPy composites show enhanced ductility comparing to PPy without polyols. For polyol-PPy composites containing linear PEG or linear PPG, the elongation at break increases as the increasing of the MW of PEG or PPG. This effect reaches a maximum when the MW of polyols reaches ∼600, and levels off for polyols with higher MW. The addition of polyols between PPy chains can help to enlarge the distance between rigid PPy chains, which could enhance the motion of PPy chain segment upon mechanical stress. From this point of view, polyols serve as a plasticizer for PPy.22 Therefore, the Young’s modulus of polyol-PPy films should decrease as the increase of MW of polyols (due to the increased content of polyol in the polyol-PPy films), which is exactly the case shown in Figure 2c. The Young’s modulus of PPy film without polyol is ∼2.2 GPa, which decreases almost linearly as the increase of PEG’s MW, to ∼0.6 GPa for PEG2000PPy. In addition, the PPG−PPy films show lower flexibility (Figure 2b) and higher Young’s modulus than PEG−PPy films (Figure 2c), which can be attributed to the lower polyol doping content in PPG-PPy films than that in PEG−PPy films. However, different from typical plasticizer that usually leads to decreased tensile strength, all the polyol-PPy composites except for PEG2000-PPy show similar or enhanced tensile D


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Figure 4. Wide-angle X-ray diffraction of PEE-PPy film (a) before and (b) after being stretched to 40% elongation and released.

Figure 5. Flexibility and conductivity of PEE-PPy film. A long PEE-PPy stripe was (a) twisted into a spiral shape or (b) tied into a knot, and was stretched and released repeatedly. (c) Long PEE-PPy stripe can be used as a conductor in a circuit for lighting a blue LED lamp. The red dotted area indicates a copper wire, and the yellow dotted area is the PEE-PPy stripe under various deformation.

structure of polyol-PPy films. Figure 3 presents the SEM photographs of four PPy films with and without polyols. There are broccoli-like structure and many defects on the surface of PPy film without polyols (Figure 3a). The boundary of these large particles are defects, where cracks would form upon mechanical stress. As shown in Figure 3b−d, the particle size decreases with the addition of polyols into PPy films, and the packaging of these smaller particles are also greatly enhanced. Therefore, the polyol-PPy films are much smoother and denser than PPy film without polyols, which implies that polyols in the electrolyte solution could serves as surfactant to minimize particle size18 and also serve as glue to join adjacent particles

toughness of PEE-PPy (Figure 2f). Considering the strength and flexibility of materials (Figure S3), PEE-PPy film (indicated by the red triangle) is clearly superior to common polymers, and also better than other organic materials such as rubbers, woods and foams. As an organic materials, the mechanical performance of PEE-PPy film enter the range of metals and alloys. As a simple demo in Figure S4 and Movie S1, a small piece of PEE-PPy film (width of 7 mm and thickness of 30 μm) can be used as a rope to hold a 200-g weight. To understand the mechanism of polyols on enhancing mechanical properties of PPy films, we used scanning electron microscopy (SEM) and X-ray diffraction (XRD) to probe the E


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ACS Applied Materials & Interfaces together, leading to enhanced mechanical properties. Again, the PEE-PPy film shows the smallest particle size and the best packaging between particles (Figure 3b), which accounts for its outstanding mechanical properties. Wide-angle XRD was used to evaluate the microstructure of PEE-PPy film before and after stretching (Figure 4). Based on Hunter’s work on the PPy’s microstructure,30 three peaks with corresponding d spacing at 0.719, 0.502, and 0.382 nm were found for the PEE-PPy film. The d = 0.502 nm peak is attributed to solvents or counterion scattering; and the d = 0.719 nm peak is due to the repeated structure along the PPy polymer chain.30 These two peaks showed no much change after the PEE-PPy film was stretched to 40% elongation and then released. The d = 0.382 nm peak corresponds to the face to face distance between pyrrole rings on adjacent stacked planar chains.30 After being stretched and released, this d spacing decreased from 0.382 to 0.364 nm, while the peak became stronger and narrower. This change indicates that the interlaminar π−π stacking became more orderly and compact in the stretched PEE-PPy film. Besides the superior mechanical performance, the polyol-PPy films also shows good conductivity in the range of 85−115 S/ cm (Table S1), which is higher than most of previously reported PPy films.13−21 Again, PEE-PPy provides the highest conductivity (115 S/cm) among all the polyol-PPy films. The combination of high strength, ductility and conductivity enables polyol-PPy composites (especially PEE-PPy) as a potential electronic material for making flexible electronics. As a brief demo, the PEE-PPy film can be easily made into various shapes such as spiral stripe and knots, which can be stretched repeatedly and return to its initial state without any cracks (Figure 5a, b and Movie S2). When a long PEE-PPy stripe was used as a flexible conductor in a circuit, a blue LED in this circuit can be lighted with a 3 V battery (Figure 5c and Movie S3). The PEE-PPy stripe can be bent and twisted without affecting its conductivity, which demonstrates its potential as a flexible conductor for flexible electronics. In summary, we have demonstrated that the combination of a rigid polymer (polypyrrole) and a soft polymer (polyols) through hydrogen bonds and charge−charge interactions can yield a strong and ductile polyol-PPy composite. We have found a clear correlation between the mechanical properties of these polyol-PPy composite films and the molecular structure features of these polyols. Polyols serve as a dynamic cross-linker between PPy chains, whereas the dynamic network helps to dissipate destructive energy, leading to strong and ductile polyol-PPy composite films. Besides the outstanding mechanical properties, the polyol-PPy films also show high conductivity. Among all the polyol-PPy films, PEE-PPy film provides the best overall performance. The combination of high strength, ductility, and conductivity enables polyol-PPy composites (especially PEE-PPy) as a potential electronic material for making flexible electronics.

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Movie S3 (AVI)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (N.Z.). *E-mail: [email protected] (X.F.). *E-mail: [email protected] (M.M.). ORCID

Mingming Ma: 0000-0002-7967-8927 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Natural Science Foundation of China (81401531, 21474094), the Natural Science Foundation of Anhui Province (1508085QH154). The authors thank Prof. Liangbin Li and Prof. Ningdong Huang for their help with the measurement of XRD of PEE-PPy films.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00717. Preparation, characterization, and demo of polyol-pyrrole composites; Figures S1−S4; Table S1 (PDF) Movie S1 (AVI) Movie S2 (AVI) F


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Bioinspired Design of Strong, Tough, and Highly Conductive Polyol

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