Simultaneously Enhancing the Mechanical Strength and Ionic Conductivity of Stretchable Ionogels Enabled by Polymerization-Induced Phase Separation

Abstract

Stretchable ionogels have been considered as ideal materials for constructing flexible electronics. However, current ionogels suffer from the well-known trade-off between mechanical strength and conductivity. Here, we develop a simple strategy based on polymerization-induced phase separation to simultaneously enhance the mechanical performance and conductivity of the ionogels by randomly copolymerizing a hydrophobic and a hydrophilic monomer in a hydrophobic ionic liquid (IL). The polymerization process induced the formation of a bicontinuous network containing a polymer-rich phase and a solvent-rich phase. The polymer-rich domains with hydrogen bonds can bear loading, greatly improving the mechanical strength; meanwhile, the solvent-rich domains form conductive nanochannels to enhance the conductivity. The resulting copolymer ionogel is highly stretchable (500% strain), and the optimal fracture stress and conductivity are 0.29 MPa and 3.4 mS/cm, achieving 7.8- and 2.3-fold enhancements compared with that of the prepared homogeneous (pure PMEA) ionogel at the same IL content, respectively. Moreover, the ionogels also exhibit anti-swelling properties in various liquids and self-adhesiveness. Potential applications of this ionogel as a wearable sensor in a complex environment are further demonstrated.

Study on fabricating transparent, stretchable, and self-healing ionic conductive elastomers from biomass molecules through solvent-free synthesis

“…S16†). Unfortunately, the transparencies of these THICEs were relatively low (at room temperature) due to microphase separation 28,64–66 (Fig. S17†).…”

Stretchable, Low-Hysteresis, and Recyclable Ionogel by Ionic Liquid Catalyst and Mixed Ionic Liquid-Induced Phase Separation

“…Recently, by tailoring the cross-linking network structure with phase separation, highly stretchable and tough ionogels are achieved. – The random copolymers and block copolymers can be dispersed in IL to generate two different phases (hard phase and soft phase) . The hard phase consists of IL-phobic regions (poor compatibility with IL or IL insoluble) as physical cross-linking, while the soft phase consists of IL-philic regions (good compatibility with IL) to enable large deformation . Nevertheless, the copolymer content is too high (>30 wt %), and these ionogels show high hysteresis and low resilience due to the high energy dissipation for the release of chain entanglements and relaxation of dangling chains under large and repetitive actions, which result in unstable signals with hysteresis and fluctuation and limit their practical applications. – Until now, almost all of the stretchable ionogels based on phase separation are fabricated by swelling the random copolymers or block copolymers in one IL. – Importantly, the mixed solvent-induced phase separation strategy has been widely used for toughening hydrogels with a low polymer content (<10%). , It is widely known that ILs as special solvents show good diversity and designability, and about 10 18 ILs with different physicochemical properties (polarity, hydrophobicity, and solvent miscibility) can be formed by combining different cations and anions. , Nevertheless, utilizing the mixed IL-induced phase separation strategy to fabricate a stretchable and tough ionogel with low hysteresis has not yet been studied.…”

“…20 The hard phase consists of IL-phobic regions (poor compatibility with IL or IL insoluble) as physical cross-linking, while the soft phase consists of IL-philic regions (good compatibility with IL) to enable large deformation. 21 Nevertheless, the copolymer content is too high (>30 wt %), and these ionogels show high hysteresis and low resilience due to the high energy dissipation for the release of chain entanglements and relaxation of dangling chains under large and repetitive actions, which result in unstable signals with hysteresis and fluctuation and limit their practical applications. 22−24 Until now, almost all of the stretchable ionogels based on phase separation are fabricated by swelling the random copolymers or block copolymers in one IL.…”

Viscoelastic Fluorescent π-Conjugated Polymer Liquids toward Direct Visualization of Microphase Separation

“…26,27 The remarkable enhancement in mechanical strength and ionic conductivity for the flexible and stretchable electronics was achieved through precise control of the polymerization-induced phase separation. 28,29 In addition, the phase separation resulting from polymer blending has been utilized for 3D printing of porous materials to achieve precise control of micro-and macromorphology. 30 These examples all are based on immiscible polymer blends.…”

“…Viscoelastic conjugated polymers (VCPs) have attracted much attention in the past decade due to their excellent optoelectronic functions, good deformability, low weight, and stimuli responsiveness. − VCPs have been widely recognized and used as promising materials for active and smart materials in soft robots, emitters, − sensors, − and actuators . Many researchers recently reported several strategies to create room-temperature VCPs through the covalent functionalization of a π-conjugated backbone with bulky and flexible alkyl side chains that act as “internal plasticizers” to soften the rigidity of polymers. − The previous findings indicated that precise control of the phase statesfrom the “liquid” to “viscoelastic” to “solid” stateof CPs can be achieved by altering the ratio of alkyl side chains to the CP backbone. − Alternatively, polymer blending can provide excellent performance that can be easily achieved by combining multiple components of different polymers with distinctive properties into a single material. , Moreover, polymer blending is regarded as a powerful and versatile tool to control the morphologies and properties of these multicomponent polymeric systems, typically leading to various ranges of phase behaviors that directly influence the associated physical, viscoelastic, thermal, morphological, and rheological properties and play a crucial role in the practical applications. − Numerous studies and applications on polymer blending have been reported, such as all-polymer solar cells, where the active layer is usually modulated by polymer blending, while the phase separation morphology of its blended system is the key to the photovoltaic conversion efficiency of organic solar cells. , The remarkable enhancement in mechanical strength and ionic conductivity for the flexible and stretchable electronics was achieved through precise control of the polymerization-induced phase separation. , In addition, the phase separation resulting from polymer blending has been utilized for 3D printing of porous materials to achieve precise control of micro- and macromorphology . These examples all are based on immiscible polymer blends.…”