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Multifunctional Nanostructured Conductive Polymer Gels: Synthesis, Properties, and Applications Fei Zhao,†,§ Ye Shi,†,§ Lijia Pan,*,‡ and Guihua Yu*,† †
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Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States ‡ Collaborative Innovation Center of Advanced Microstructures, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China CONSPECTUS: Conductive polymers have attracted significant interest over the past few decades because they synergize the advantageous features of conventional polymeric materials and organic conductors. With rationally designed nanostructures, conductive polymers can further exhibit exceptional mechanical, electrical, and optical properties because of their confined dimensions at the nanoscale level. Among various nanostructured conductive polymers, conductive polymer gels (CPGs) with synthetically tunable hierarchical 3D network structures show great potential for a wide range of applications, such as bioelectronics, and energy storage/conversion devices owing to their structural features. CPGs retain the properties of nanosized conductive polymers during the assembly of the nanobuilding blocks into a monolithic macroscopic structure while generating structure-derived features from the highly crosslinked network. In this Account, we review our recent progress on the synthesis, properties, and novel applications of dopant cross-linked CPGs. We first describe the synthetic strategies, in which molecules with multiple functional groups are adopted as cross-linkers to cross-link conductive polymer chains into a 3D molecular network. These cross-linking molecules also act as dopants to improve the electrical conductivity of the gel network. The microstructure and physical/chemical properties of CPGs can be tuned by controlling the synthetic conditions such as species of monomers and cross-linkers, reaction temperature, and solvents. By incorporating other functional polymers or particles into the CPG matrix, hybrid gels have been synthesized with tailored structures. These hybrid gel materials retain the functionalities from each component, as well as enable synergic effects to improve mechanical and electrical properties of CPGs. We then introduce the unique structure-derived properties of the CPGs. The network facilitates both electronic and ionic transport owing to the continuous pathways for electrons and hierarchical pores for ion diffusion. CPGs also provide high surface area and solvent compatibility, similar to natural gels. With these improved properties, CPGs have been explored to enable novel conceptual devices in diverse applications from smart electronics and ultrasensitive biosensors, to energy storage and conversion devices. CPGs have also been adopted for developing hybrid materials with multifunctionalities, such as stimuli responsiveness, self-healing properties, and super-repellency to liquid. With synthetically tunable physical/chemical properties, CPGs emerge as a unique material platform to develop novel multifunctional materials that have the potential to impact electronics, energy, and environmental technologies. We hope that this Account promotes further efforts toward synthetic control, fundamental investigation, and application exploration of CPGs. and 2D nanosheets11 have been developed and applied in a range of technological areas, such as sensors, electronics, and energy storage and conversion devices. However, the electrical properties of these nanostructured conductive polymers could be weakened by structural defects induced by inhomogeneous aggregation, severe restacking, and poor contacts during processing and assembly.12 The development of nanostructured conductive polymers with tunable microstructures and controllable chemical/physical properties still remains a challenge.
1. INTRODUCTION Conductive polymers have been researched over the past few decades owing to their unique ability to provide tunable electrical conductivity and flexibility during processing.1,2 The conductivity of conductive polymers depends on the molecular structures of the constituent materials, the level of doping, and the ordering of the molecular packing.3 With the rapid emergence of nanoscience and nanotechnology, it is anticipated that conductive polymers with well-defined nanostructures can translate the properties of their bulk forms and exhibit unusual chemical/physical properties because of the confined dimensions of the nanomaterials.4−6 Conductive polymers with various nanostructures including 0D nanoparticles,7,8 1D nanofibers,9,10 © 2017 American Chemical Society
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transport and a hierarchically porous structure for ion diffusion (Figure 2B). The chemical and physical properties of the dopant molecule cross-linked CPGs can be tuned by changing the cross-linker used in the synthesis. Figure 2C shows that a polypyrrole (PPy) gel with an interconnected fiber structure is synthesized using CuPcTs as the cross-linker owing to the supramolecular effect. However, PPy gels with a “necklace-like” 1D nanostructure and a granular nanostructure are obtained when indigo carmine and indigo carmine dehydrate are used (Figure 2D).21 These varied microstructures play important roles in electrical and electrochemical properties of the resulting CPGs. In addition, the structure and properties of the CPGs can further be tuned by controlling the synthetic conditions, such as solvents, precursor ratio, temperature, etc. Figure 2E and F shows that the interfacial synthesis generates an interconnected hollow-sphere microstructure of PPy gel.23 Using different organic solvents and varying the ratio of the polymer monomers to the cross-linkers, the microstructural parameters of PPy gels can be adjusted.24
Inspired by the chemical/structural features and synthetic approaches of natural gels,13,14 conductive polymer gels (CPGs) with 3D networked structures were recently developed by crosslinking the conjugated polymer chains using molecules with multiple functional groups.15,16 CPGs show the unique features of gel materials: they are dilute cross-linked systems and exhibit no flow when in the steady-state. This monolithic structure inherits the conductive properties of the conjugated polymeric chains and generates highly tunable chemical/physical properties derived from its cross-linked network,2,17 including flexibility, stretchability, ionic conductivity, electrochemical activity, and so forth. Meanwhile, CPGs have emerged as a unique material platform to develop functional materials by building interpenetrating structures with a second polymeric network, loading specific nanoparticles, or serving as a precursor for graphitic carbon frameworks.18 With the above-mentioned advantageous features, CPGs have been widely studied and applied in diverse applications, including energy storage and conversion, catalysis, responsive devices, sensors, drug delivery, and super-repellent surfaces (Figure 1).18
2.2. Synthesis of Interpenetrating Double Network Structured Gels Based on CPGs
The design and synthesis of interpenetrating double network structured gels has enabled multifunctional gels because the hybrid gel may inherit the advantages of each component and offer exciting new features because of the synergic effects between the two polymeric networks (Figure 3A).25−27 CPGs can be an ideal matrix to introduce a second gel network to build an interpenetrating double networked hybrid gel owing to their hierarchically porous structure. The interpenetrating double network structured hybrid gels based on CPGs can be synthesized in two ways. The first method is a two-step polymerization in which one of the gel networks is first constructed and acts as the supporting matrix for in situ polymerization of the second polymeric network. poly(Nisopropylacrylamide) (PNIPAM)/polyaniline (PANI) and PNIPAM/PPy hybrid gels have been successfully synthesized using this method (Figure 3B).28 The second method is the onestep method in which two polymers are polymerized and crosslinked within one precursor solution.29
Figure 1. Conductive polymer gels combine the appealing features of organic conductors and polymeric gels and are thus being used in a wide range of applications.
In this Account, we present the synthetic approaches for crosslinking molecule-enabled CPGs and their hybrid materials, introduce the structure-derived features of CPGs, and explore their various technological applications.
2.3. Synthesis of Hybrid Gels with Functional Particles Based on CPGs
2. SYNTHETIC STRATEGIES
CPGs can act as a supporting matrix to immobilize functional particles that present unconventional features, including high surface area, tunable conductivity, and excellent biocompatibility.30,31As shown in Figure 4A and B, the functional particles, including inorganic particles and biomolecules, can be loaded onto the surface of a preformed CPGs network in a simple solution-based deposition to form the first type of hybrid gel.32 Cross-linking agents, such as glutaraldehyde, can be further introduced to create the chemical bonding between the CPG network and the functional particles to improve the linking strength. In this hybrid gel, the CPGs act as a supporting matrix to immobilize these functional particles within an electrochemical electrolyte or bioenvironments as well as transport mediates between functional particles and substrates. In another type of hybrid gels shown in Figure 4C and D, the functional particles are conformably coated by the conductive polymer layer and interconnected with each other through the CPG network. In situ polymerization is essential for the preparation of this type of hybrid gel.
2.1. Synthesis of Dopant Molecule Cross-Linked CPGs
Conventional methods to build up a 3D nanostructured network of conductive polymers mainly include template-guided methods using either a hard or soft template.19,20 However, these methods usually result in gel composites that consist of both conductive and nonconductive components, thus deteriorating the conductivity of the resulting gels. We recently found that molecules with multiple functional groups, such as phytic acid and copper phthalocyanine-3,4′,4″,4‴-tetrasulfonic acid tetrasodium salt (CuPcTs), can cross-link the conductive polymer chains, leading to CPGs with 3D networked structures free of insulating components (Figure 2A).15,21,22 The mechanism is that each of these molecules can react with more than one conductive polymer chain by protonating the nitrogen groups and through electrostatic interactions, thus acting as cross-linkers and dopant molecules. These dopant-molecule cross-linked CPGs exhibit high electrical and ionic conductivities because they construct a heavily doped and interconnected polymer network for electron 1735
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Figure 2. (A) Schematic and photograph of the 3D network structured PANI gel using phytic acid as the dopant and cross-linker. (B) SEM image showing the interconnected network of dendritic PANI nanofibers. Reprinted with permission from ref 15. Copyright 2012 PNAS. (C) Illustration of the supramolecular self-sorting mechanism that aligns the PPy chains to form the 1D nanostructure. (D) SEM images of different nanostructured PPy gels with varied dopants, scale bar: 1 μm. Reprinted with permission from ref 21. Copyright 2015 American Chemical Society. (E) Schematic of interfacial synthesis of the hollow-sphere-structured PPy. Reprinted with permission from ref 23. Copyright 2014 Nature Publishing Group. (F) SEM and TEM image of the PPy showing its interconnected hollow-sphere structure, scale bar: 1 μm. Reprinted with permission from ref 24. Copyright 2014 Royal Society of Chemistry.
Figure 3. (A) Schematic of an interpenetrating double network structured hybrid gel. (B) Schematic of the synthesis process of a hybrid hydrogel composed of PNIPAM and conductive polymers. Reprinted with permission from ref 28. Copyright 2015 WILEY-VCH Verlag GmbH & Co.
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polymers show poor elasticity owing to their rigid backbones caused by a highly conjugated structure. By forming an interconnected hollow sphere geometry, CPGs can exhibit an effective elastic modulus that is capable of withstanding large effective strains and stresses (Figure 5B). For example, a PPy gel foam with a hollow-sphere structure shows a high modulus and reversibility in cycles of compression tests, demonstrating its structure-derived mechanical elasticity. The CPGs also exhibit interesting surface properties.15 Owing to their swelling nature, CPGs are highly hydrophilic (Figure 5C), thus providing the ability to maintain a high water content and prevent the leakage and inactivation of the surface loaded species within water contained environments. In addition, because the organic composition of the CPGs is similar to the extracellular matrix, they exhibit excellent biocompatibility. The surface properties of CPGs can be further modified by introducing functional groups and other polymeric chains.
Figure 4. (A,B) Schematic and TEM image of the hybrid gel synthesized by loading catalytic and bioparticles onto the surface of preformed CPGs. Reprinted with permission from ref 32. Copyright 2013 American Chemical Society. (C,D) Schematic illustration and TEM image of hybrid gel synthesized by in situ polymerization of CPGs with electrochemically active particles. Reprinted with permission from ref 40. Copyright 2013 Nature Publishing Group.
4. APPLICATIONS 4.1. Energy Storage Devices
Supercapacitors and lithium ion batteries are key members of electrochemical energy storage systems.33 Supercapacitors utilize the near-surface charge storage, showing high power density and exceptional cyclic performance.34 In comparison, lithium ion batteries show limited charge/discharge rates and cycle life because their charge/discharge processes rely on moving lithium ions between the negative and positive electrode as well as the chemical reactions between lithium ions and the cathode/anode materials.35 Owing to the chemical, structural, and electrical features of CPGs, they have been explored in supercapacitor and lithium ion battery devices as an active electrode material and bifunctional binder, respectively. Conductive polymers have been studied for supercapacitors as pseudocapacitive materials, which take advantage of fast and reversible exchange reactions at or near the electrode surface.36 Although they show high pseudocapacitance and mechanical flexibility, these conjugated polymers suffer from poor cycle lifetime and rapidly decaying capacitance in electrochemical processes, possibly because of the high volume change and the change of doping states during electrochemical reactions. The adoption of CPGs as a supercapacitor electrode material has
3. STRUCTURE-DERIVED PROPERTIES Owing to the polymeric nature and the high synthetic tunability, CPGs synergize the advantageous features of conventional gel materials and inorganic conductors while showing unique properties. First, the conductivity and electrochemical properties of CPGs can be tuned over a wide range by changing the crosslinkers, which also act as the dopants. The conductivity of the polymer depends highly on the dopants used and the level of doping. As mentioned above, different types of cross-linkers at different concentrations can be adopted in the synthesis of CPGs, which can lead to a controlled conductivity of the resulting CPGs (Figure 5A).21 PANI and PPy gels show typical conductivities from 0.1 to 1 and from 0.1 to 10 S cm−1, respectively. This tunable conductivity endows CPGs with the ability to act as insulators, semiconductors or conductors for a range of applications. The mechanical properties of CPGs can be improved by controlling their microstructures.24 Traditional conductive
Figure 5. (A) The conductivity of the CPGs is highly tunable by changing the cross-linker. Reprinted with permission from ref 21. Copyright 2015 American Chemical Society. (B) CPG foam sustains a large weight and shows a high modulus and reversibility in cycling compression tests. Reprinted with permission from ref 23. Copyright 2014 Nature Publishing Group. (C) CPGs show highly hydrophilic properties. Reprinted with permission from ref 15. Copyright 2012 Proceedings of the National Academy of Sciences of the United States of America. 1737
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Figure 6. (A) SEM image of nanostructured PPy gel with CuPcTs as the dopant. (B) Specific capacitance vs current density for CuPcTs−PPy and pristine PPy. Reprinted with permission from ref 21. Copyright 2015 American Chemical Society. (C) CV curves of the PPy gel based supercapacitor under different bending conditions. Reprinted with permission from ref 24. Copyright 2014 Royal Society of Chemistry. (D) Schematic of Si nanoparticle/PANI hybrid gel composite electrodes. (E) Electrochemical cycling of in situ polymerized Si particle-PANI composite electrodes. Reprinted with permission from ref 40. Copyright 2013 Nature Publishing Group. (F) CPG framework constructs a 3D network for electron transport and a porous structure to facilitate the transport of ions through hybrid gel electrodes. (G) CPG coating is shown to enhance the dispersity of active particles. (H) Rate characteristics of Fe3O4 nanoparticles and PPy hybrid gels. Reprinted with permission from ref 41. Copyright 2017 WILEY-VCH.
capacity and excellent stability (Figure 6D and E).40 The PANI gels play multiple functions, including as a conductive framework (a supporting structure with mechanical robustness), adhesive coating, and surface modifier, which contribute to the exceptional electrochemical performance in lithium-ion batteries. Since the conductive additives are eliminated, the gravimetric energy density and power density are greatly enhanced. In addition, the porous gel matrix accommodates the large volume expansion of the Si particles during lithiation, thus improving the electrode stability. The properties of the CPG binder can also be tuned by carefully controlling the type and amount of cross-linker. Recently, 3D nanostructured gel frameworks consisting of Fe3O4 nanoparticles and PPy gels were prepared by our group and used as anode material for high-performance lithium ion batteries (Figure 6F). Both phytic acid and CuPcTs were adopted as cross-linkers, and the electrochemical tests showed that the CuPcTs cross-linked gel framework exhibited a significantly enhanced capacity, especially at high charge/ discharge rates. This can be attributed to the higher electrical conductivity of the PPy gel cross-linked by CuPcTs and the enhanced dispersity of the active particles (Figure 6G and H).41 We demonstrated that the CPG can form a uniform coating on each active particle, which can prevent the aggregation of active particles and eliminate the poor ionic transport within aggregates of particles. The CPGs based binder system has been further used for cathode materials42 and extended to ternary systems.43 Carbon nanomaterials such as carbon nanotubes can be incorporated to form 3D conductive ternary framework with high mechanical robustness.
been demonstrated to greatly improve the performance of supercapacitors.15 The 3D networked structure of CPGs establishes a continuous network to promote the transport of electrons, provides short ion diffusion pathways and a large surface area for redox reactions, and accommodates volume changes. For example, the specific capacitance of the CuPcTs doped PPy is much higher than that of pristine PPy (Figure 6A and B). The improvements can be attributed to the increase in their conductivities. In addition, with structure derived elasticity, CPGs have been used to fabricate highly flexible supercapacitor devices.24 This solid-state supercapacitor based on PPy gels has shown negligible capacitance change, even at a highly bent state (Figure 6C). In addition to their supercapacitors application, CPGs can be potentially used as an electrode material for lithium ion batteries.37 However, conductive polymers possess poor stability and low conductivity at reduced states. The unique properties of CPGs with synthetically controllable structures make them promising as a bifunctional binder in the battery electrode.38 In a traditional binder system, the conductive phases consisting of polymer and conductive additives are randomly distributed and may lead to bottlenecks and poor contacts.38,39 Distinct from the traditional binder system, the CPGs are intrinsically conductive, and their networks can serve as electrically conductive pathways to interconnect each active particle and the current collector. In addition, the porous structure can facilitate ion diffusion from the electrolyte to active particles. To serve as a binder material, CPGs are in situ polymerized with active materials to form hybrid gels. For instance, a Si and PANI hybrid gel electrode were developed that showed high 1738
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Figure 7. (A) SEM image of the CPG derived carbon framework. Reprinted with permission from ref 46. Copyright 2015 Nature Publishing Group. (B) Schematic of active sites on the edges and in the pores of the carbonized CPG framework. Reprinted with permission from ref 47. Copyright 2015 American Chemical Society. (C) RRDE measurements of the ORR at an N and P codoped carbon framework electrode with different catalyst loadings. (D) The performance of primary Zn-air batteries based on the PANI gel derived carbon framework. Reprinted with permission from ref 46. Copyright 2015 Nature Publishing Group.
Figure 8. (A) Schematic of the sensing mechanism of the CPG-based electrode platform. (B) Instant current−time response curves of the metabolites being successively added in the PBS solutions with the CPG based electrode. Reprinted with permission from ref 50. Copyright 2015 American Chemical Society. (C) Schematic of structural elasticity of the hollow-sphere-structured PPy. (D) Transient response to a 20 mg weight and a flower petal (8 mg) on the micropatterned EMCP device, and a chess board with pieces distributed on the pressure sensor array. Reprinted with permission from ref 23. Copyright 2014 Nature Publishing Group.
In recent works, Wei et al.47 synthesized a PANI gel derived carbon framework with large quantities of pores, from micro- to meso- and to macro-size. The resulting carbon framework exhibited high conductivity, ORR activity and good stability in acidic electrolytes and was used for high-performance proton exchange membrane fuel cells. In another study, a phytic acid cross-linked PANI gel was derived into N and P codoped carbon frameworks through carbonization.46 The resulting carbon framework showed remarkable bifunctional catalytic activities toward both ORR and OER reactions and can be used for Zn-air batteries (Figure 7C and D). The codoping generated synergistic effects to improve the electrocatalytic activities toward both the OER and ORR because the overpotential can be reduced.
4.2. Energy Conversion Applications
The reactions of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) play key roles in energy conversion applications that create electrochemical power, such as fuel cells, lithium−air batteries, and the generation of oxygen.44−46 In this regard, CPG derived carbon frameworks, which can be readily prepared by a thermal treatment, show various advantages. First, they have a hierarchically porous structure to accommodate the linkage of various speciestransport channels to their active sites (Figure 7A). Second, they can provide larger numbers of active sites as compared to conventional porous carbon materials (Figure 7B). In addition, the doping of N or other elements can be completed and tuned in situ during heat treatment because the cross-linker can provide the essential elemental atoms. Lastly, CPG derived carbon frameworks can show high catalytic activities for both ORR and OER.
4.3. Sensors
Sensing, an ability to respond to environmental changes, is an important functionality of next-generation “smart” materials and systems. Owing to the unique electrical, chemical, and 1739
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Figure 9. (A) Schematic of the working mechanism of the switch device based on thermal-responsive and conductive hydrogels. (B) Demo circuit controlled by the states of the switch and the cycling performance of the switching device. Reprinted with permission from ref 28. Copyright 2015 WILEY-VCH Verlag GmbH & Co. (C) Schematic of the water-retaining effects and interaction effects of the PNIPAM after incorporation of PEI. (D) Release curves of different drug models. Reprinted with permission from ref 29. Copyright 2015 WILEY-VCH.
Figure 10. (A) Water droplet deposited on a superhydrophobic 3D nanostructured gel surface. (B,C) Superhydrophobic coating enabled by the CPG exhibits excellent stretchability, and shows high efficiency for oil/water separation. Reprinted with permission from ref 55. Copyright 2014 American Chemical Society. (D) Schematic of the proposed mechanism of the supramolecular gels’ self-healing behavior. (E) PPy/supramolecular gel hybrids can electrically self-heal. (F) Photos showing that the PPy/supramolecular gel hybrid can self-heal cracks at room temperature. Reprinted with permission from ref 56. Copyright 2015 American Chemical Society.
active catalysts for the electro-oxidation of the hydrogen peroxide. Hence, the glucose enzyme sensor based on the PtNP/PANI hybrid gel exhibited unprecedented sensitivity. To further explore the potential of CPGs, we modified the PtNP/ PANI hybrid gel with different enzymes to develop biosensing platforms that can detect diverse metabolites simultaneously with high sensitivity, a wide linear range and rapid response times.50 The sensing mechanism of the biosensor platform based on a CPG/PtNPs/enzyme hybrid electrode is illustrated in Figure 8A. The PANI gels provide hierarchical pores, nanostructured matrices, and particularly solvated surfaces resulting from the hydrophilic nature of the hydrogels. These features are favorable for enhancing molecule permeability. As a result, our CPG based biosensor platform presents excellent sensing
mechanical properties, CPGs have been studied as an effective platform or an active component to boost the performance of these sensing systems. CPGs may be a promising candidate for the interfacial material in biosensors.15,48,49 They exhibit some attractive features: first, they are good immobilization materials for biological recognition species, such as enzymes; second, they exhibit excellent biocompatibility, because their high water content and organic composition are similar to the extracellular matrix. Based on the considerations above, we demonstrated an ultrahigh-sensitivity glucose sensor based on hydrogel heterostructured electrodes constructed by polyaniline (PANI) with attached Pt nanoparticles (PtNPs).32 The monodisperse PtNPs, which were densely loaded onto the gel matrix, behaved as highly 1740
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Accounts of Chemical Research performance for uric acid, cholesterol, and triglyceride metabolites over a wide linear range with high sensitivities, low sensing limits and rapid response times (Figure 8B). The hierarchical 3D-structure of CPG also enables a tunable resistance regulated by external stimuli, and hence the CPG could be utilized in applications involving resistive sensors.51 For example, a new type of piezoresistive sensor with CPGs was designed where the active layer is both conductive and elastic and imparts ultrahigh sensitivity and reproducible sensing characteristics.23 The key innovation of the material design is the elastic microstructured CPG, consisting of interconnected hollowsphere structures of PPy. This hollow sphere structure enabled the PPy to elastically deform and recover upon the application and release of external pressure (as shown in the schematic illustration in Figure 8C), thereby promoting the contact stability of the pressure sensor and endowing the device with stable and reproducible sensing performance. The created pressure sensor can recognize pressures of less than 1 Pa with a short response time, illustrated through the practical application of a chessboard (Figure 8D).
and plastics, because the gel precursors have good wettability and a high affinity for different substrates. In addition, the superhydrophobic coating exhibits excellent mechanical properties, including flexibility, mechanical robustness and stretchability (Figure 10B). The novel multifunctional superhydrophobic coating is promising for various applications, including waterproof coatings, self-cleaning surfaces, antifouling surfaces, industrial oil recovery, and oil spill cleaning (Figure 10C). CPGs can also contribute to other types of multifunctional gel materials. For example, a self-healing supramolecular gel is introduced into the PPy conductive gel framework, thus forming an interpenetrating double networked structure.56 The hybrid gel shows a fast and efficient self-healing behavior that comes from the dynamic assembly or disassembly and association or dissociation behaviors of the supramolecular gel (Figure 10D). Figure 10E and F shows that the hybrid gels can electrically and physically self-heal. A self-healing electrical circuit could be fabricated using this hybrid gel to reveal its future applications in self-healing electronics.
4.4. Development of Responsive Gels
5. CONCLUSIONS As a unique class of nanoarchitectured polymeric materials, CPGs combine the advantages of organic conductors and polymeric materials, such as excellent electronic property, superhydrophilicity, and biocompatibility. More importantly, this unique material shares some of the benefits of gels, exhibiting structure-derived properties, including a large specific surface area, an accommodating framework, excellent flexibility, tunable mechanical strength, and electrochemical properties. With more dedicated research efforts devoted to this material, materials scientists and chemists may move from the rough construction of random structures and the simple chemical modification/ functionalization of CPG to the control/regulation of ordered microscopic structures and the exploration of synergetic multicomponent compositions for designated applications in different advanced systems with unprecedented functions and performances. In future studies, the nanostructure and the associated structure-derived properties of CPGs need to be further tailored at the molecular level. New cross-linkers/ dopants need to be exploited to pursue optimized electronic/ ionic conductivity. Meanwhile, the backbones of CPGs can be modified by introducing functional side chains or copolymerizing with other polymers to enable unexpected properties. Beyond synthetic modifications, advanced characterization tools need to be used to accurately depict CPG system, such as the aggregation states of polymeric chains, the interactions between doping molecules and polymer backbones, mapping of dopant dispersion, and surface states between gel matrix and incorporated functional materials. Along with the fundamental understanding of the double network of CPGs, the complex interpenetrating system based on different polymers and/or copolymers with designed nanostructures encompasses many research topics of fundamental and applied interest. For potential novel applications, CPGs and their hybrid materials can be applied for artificial skin, environmental temperature/humidity conditioner, and all-weather solar energy harvesting.
Responsive gel materials can actively respond to environmental stimuli and regulate their physical/chemical properties, thus becoming critically important for a range of technological applications.52,53 CPGs are found to be a promising “host” material for the development of responsive gels with excellent conductive properties. The interconnected network of CPGs provides hierarchical pores to accommodate another responsive gel framework, forming an interpenetrating double network structured gel. The rich interactions between the conductive polymer chains and the responsive gel networks help maintain the structural integrity during responsive changes and tune the responsive behavior. For instance, a highly thermoresponsive and conductive hybrid gel was synthesized by in situ formation of CPGs within a PNIPAM matrix by our group.28 This hybrid material successfully inherits the conductive and responsive properties of each component and maintains the high thermal sensitivity owing to high porosity. The thermoresponsive gel can be applied as a temperature sensitive switcher with “heating OFF” and “cooling ON” (Figure 9A). The “switching” behavior is highly efficient and reversible owing to the excellent responsive sensitivity and mechanical properties of hybrid gels (Figure 9B). Our studies further revealed that the responsive behavior of hybrid gels can be tuned by controlling the interactions between two polymeric networks.29 A polyelectrolyte, polyethylenimine (PEI), was incorporated into the PNIPAM matrix (Figure 9C). PEI was found to provide structural modification of the PNIPAM network and tune the water content in the PNIPAM hydrogel, modifying the interaction between the hydrogel matrix and the charged drugs. Therefore, the hybrid gel can be used as a general drug carrier for loading both positively and negatively charged drugs, as well as achieving controlled release rates (Figure 9D). 4.5. Other Multifunctional Gels
With a morphology that is similar to that of the lower surface of a lotus leaf, CPGs have been used for the development of superhydrophobic coatings with high stretchability.54,55 To achieve the superhydrophobicity, a silica layer is coated on the PANI gel, followed by the surface silanization. The modified gel framework shows the morphology with 3D interconnected nanofibers that can create superhydrophobic surfaces (Figure 10A). This superhydrophobic gel coating can be applied to virtually any substrate, including metals, cement, wood, fabrics,
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Accounts of Chemical Research ORCID
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Guihua Yu: 0000-0002-3253-0749 Author Contributions §
F.Z. and Y.S. contributed equally to this work.
Notes
The authors declare no competing financial interest. Biographies Fei Zhao is a postdoctoral researcher at University of Texas at Austin with Professor Yu. He received his B.S. in Material Science and Engineering and Ph.D. in Chemistry from Beijing Institute of Technology. Ye Shi is a Ph.D. student at University of Texas at Austin under the supervision of Professor Yu. He received his B.S. and M.S. degrees in Polymer Science and Engineering at Zhejiang University. Lijia Pan is a Professor in School of Electronic Engineering, Nanjing University, China. He received his B.S. degree in polymer science from South China University of Technology and Ph.D. in polymer physics from University of Science and Technology of China. Guihua Yu is an Assistant Professor of Materials Science and Engineering at University of Texas at Austin. He is an elected Fellow of Royal Society of Chemistry (FRSC), Sloan Research Fellow and Camille Dreyfus Teacher-Scholar. He received his B.S. degree (2003) from University of Science and Technology of China and Ph.D. (2009) in chemistry from Harvard University, followed by postdoctoral research (2012) at Stanford University.
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ACKNOWLEDGMENTS G.Y. acknowledges the Center for Mesoscale Transport Properties, an Energy Frontier Research Center supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award #DE-SC0012673 for financial support, National Science Foundation award NSF-CMMI-1537894, and Camille Dreyfus Teacher-Scholar Award. L.P. acknowledges the financial support from NSFC 61674078.
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