Polymer Nanocomposite Materials. Группа авторов

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СКАЧАТЬ Reproduced with permission. [111] Copyright 2018, American Chemical Society. (f) Cross sectional SEM image and (g) TEM image of the TPU/CNTs/PDMS nanofiber composite. SEM images of the nanofiber composite at different strain during stretching, Source: (f)–(g) Reproduced with permission. [112] Copyright 2019, Elsevier B.V. (h) ε = 10%, (i) ε = 70%. Source: (h)–(i) Reproduced with permission. [113] Copyright 2019, Elsevier B.V.

In addition to the aforementioned interfacial interaction, ultrasonication induced nanofiller anchored onto the nanofiber surface or the skeleton of the foam is wildly used to prepare conductive polymer sponge [106, 115] or nanofiber [116] composite. Its working mechanism is as follows: under ultrasonication, a large number of cavitation bubbles will be generated in the solution, and these cavitation bubbles will be blasted generating micro-jets [117], which will break up the nanofiller aggregates in the solution, and at the same time, the dispersed nanofillers can rush to polymer scaffold at high speed. The viscoelastic elastomer at the interface between fiber and CNTs would be soften or even partially melt under the instantaneous high temperature and high pressure. At this point, the well dispersed nanofillers under ultrasonication would be anchored in situ on the surface of elastomer surface. Gao et al. [118] first reported this technique to prepare the CNTs decorated TPU nanofiber composite. Recently, they [112] prepared multifunctional nanofiber composite by dip coating one PDMS layer onto the CNTs/TPU surface. The fluffy CNTs forms conductive shell on the fiber surface as shown in Figure 2.5f,g. The PDMS layer acts not only as adhesion promoter enhancing the interfacial interaction but also as low surface energy material endowing the CPCs surface with low surface energy. CPCs with dual conductive networks are also reported by Gao and his coworkers by combination of ultrasonication and dip-coating [113]. The acidified carbon nanotubes (ACNTs) decorated nanofiber composite was immersed in Ag precursor solution, and then AgNPs were generated after the precursor was reduced. PDMS was also used to improve the interfacial adhesion between the AgNPs. The dual conductive structures on the fiber surface can be observed in Figure 2.5h. The outer surface of the CPC is surrounded by silver nanoparticles, and ACNTs in the interlayer could be observed after stretching the fiber composite as shown in Figure 2.5i.

2.4 Application in Sensors

CPCs have been widely used for different sensors, due to its light weight, portability, flexibility, and low cost [90, 119]. The sensing mechanism of CPC sensors is based on the resistance variation of CPCs when experiencing external stimulus like mechanical deformation [120, 121], temperature variation [122, 123], and adsorption of vapors or organic solvent [124, 125]. The resistance of CPCs is affected by the evolution of conductive network under external stimulus [5]. These transient resistance change can be detected as a signal for sensing purpose.

      2.4.1 Strain Sensor

      As a device for detecting object deformation, CPC based strain sensors have been widely used for health monitoring, electronic skin, wearable electronics, etc. For the purpose of monitoring, the resistance (conductive networks) should response upon external strain/stress [119]. Thus, CPCs should possess excellent resilience such that the material could be elongated under external strain (destruction of conductive networks) and recovered immediately after the removal of the stress (reconstruction of conductive networks).

Conductive natural rubber/elastomer composites decorated with different conductive nanofillers are explored as promising candidates of wearable strain sensors [110, 126, 127]. The sensors show broad workable sensing range, high sensitivity, and excellent recoverability. Among these materials, conductive fabric composite strain sensors have shown promising applications in wearable electronics, thanks to their breathability, skin affinity, and so on [128, 129]. For example, they can be incorporated into clothing or gloves to detect human body motions [111, 114] and detect the motions of human body as shown in Figure 2.6. Figure 2.6a–d demonstrate that the fiber strain sensor could be used for human–machine interfaces by simply incorporating the fiber composites onto a glove, demonstrating the potential application in remote control of hand robot. Figure 2.6e–j show sensing performance of the smart integrated glove in monitoring motions like head-forward, shoulder imbalance, kyphosis, etc. In order to improve the environmental suitability of the sensors, self-protective CPCs with superhydrophobicity have attracted great interests [128, 130] from the academia. This superhydrophobicity endows the sensor with self-cleaning behavior and can prevent the water or even corrosive solution diffusion into the materials, which can extend their applications in some harsh conditions. To reveal the strain sensing mechanism, the evolution of conductive network of the CPC under different strains is observed by using the SEM [126].

      Hydrogel as a new type of conductive polymer composite demonstrating preferable bio-compatibility, self-healing, and self-adhesiveness is an ideal material for strain sensing usage [131, 132]. Zhang et al. [133] developed a MXene (Ti₃C₂T_x)/PVA hydrogel sensor with outstanding sensing performance by mixing MXene nanosheet with PVA hydrogel. The hydrogel composite shows outstanding stretchability, self-healing property, and strong adhesiveness to skin, which can adhere to human skin without the assistance of bonding materials to detect subtle motions including facial expression, vocal signals, handwriting, and finger bending, demonstrating high accuracy and sensitivity.

      2.4.2 Piezoresistive Sensor

In recent years, CPC based pressure sensors as a branch of smart material, which could respond to compressive deformation and transform mechanical forces into electrical signals, have been extensively developed [134]. To meet the demands of different applications, innovated material structures and fabrication strategies are developed to prepare flexible and wearable pressure sensors with excellent sensing performance. CPCs containing an elastic matrix and conductive nanofillers are the most popular candidates of the piezoresistive sensor [135]. Textiles with fiber networks are often used as flexible scaffolds of piezoresistive sensors by carbonization or coating conductive fillers onto the fiber surface [136, 137]. Li et al. [137] fabricated flexible and electrically conductive carbon cotton (CC)/PDMS composites by infiltrating PDMS glue into the CC scaffold. The CC/PDMS demonstrates a sensitivity of 6.04 kPa⁻¹, a working pressure of 700 kPa and durability over 1000 cycles. CPCs with a porous structure (foam, aerogels) are another ideal active materials of piezoresistive sensors because of their highly reversibility and hence reusability after large-deformation cycles [138, 139]. Zhong and coworkers [140] successfully fabricated lightweight MXene (Ti₃C₂) aerogel with ultra-stable lamellar structure by freeze drying the mixture of bacterial cellulose fiber and MXene nanosheets. The carbon aerogel used for pressure sensor demonstrates ultrahigh compressibility and superelasticity, a wide linear range and low detection limits.

Figure 2.6 (a) Photograph showing the smart glove integrated with the fiber СКАЧАТЬ