Flexible multilayered aramid nanofiber/silver nanowire films with outstanding thermal durability for electromagnetic interference shielding
Introduction
With the continuous development of electronic information technology and the popularity of personal electronic equipment, electromagnetic radiation pollution has become a serious problem, which not only endangers the operation of equipment, but also causes information leakage and does harm to human health [1], [2], [3], [4]. Especially in the fields of aerospace, military and wearable electronics, there is an urgent need for high performance electromagnetic interference (EMI) shielding materials combined with high mechanical strength, flexibility and thermal durability [5], [6], [7]. Traditional metal-based electromagnetic shielding materials cannot be widely used in many circumstances because of their high density, poor flexibility [8], [9]. Recently, conductive polymer composite (CPC) composed of polymer substrates and conductive fillers have been extensively studied because of their light weight and flexibility [10], [11], [12], [13]. The commonly applied conductive nanofillers include grapheme [14], [15], [16], carbon nanotube (CNT) [17], MXene [18], [19], [20], metal nanoparticle (NP) [21], metal nanowire (NW) [19], [22], etc. Among these fillers, silver nanowire (AgNW) is widely used in the construction of CPC with electromagnetic shielding due to their ultra-high aspect ratio, outstanding electrical conductivity and excellent mechanical flexibility [23]. The incorporating of AgNW into various polymer substrates, including cellulose nanofibers (CNF) [24], [25], polyethylene terephthalate (PET) [23], [26], and polyurethane (PU) [23]. For example, a flexible paper consisted of cellulose and AgNW using a simple “blending-filtration-peeling” method, had an EMI shielding performance of 39 dB with 14.2 vol% AgNW content at X-band [27]. However, for polymer matrix, the thermal durability of the EMI shielding materials should be considered, especially in some high temperature application fields such as aerospace and military.
Poly (p-phenylene terephthalamide), also known as aramid and Kevlar fiber, possesses outstanding mechanical properties, good electrical insulation ability, high chemical resistance, prominent thermostability, and flame retardancy [28]. Aramid nanofiber (ANF), which can be synthesized by deprotonation of macroscopic aramid fibers in potassium hydroxide/dimethyl sulfoxide (KOH/DMSO) system, not only maintains the advantages of aramid fibers, but also shows nanoscale effects such as high aspect ratio and high specific surface area, becoming a potential reinforcing phase and polymer substrate [29], [30]. Therefore, the ANF can be utilized as the soft fibrous matrix to host a vast various of multifunctional nano-fillers, such as CNT [31], graphene [32], MXene [33], metal NP [34], boron nitride nanosheets [35] and other components, for the application in EMI shielding, energy storage, thermal management. Chen et al. [17] constructed multifunctional aerogel film using ANF, carbon nanotubes, showing excellent EMI shielding effectiveness (SE) (54 dB), self-cleaning performance and Joule heating property. Weng et al. [36] used a simple vacuum-assisted filtration method to fabricate ANF and MXene nanocomposite film and the ANF/MXene (20/80) film exhibited a high electrical conductivity of 879.0 S/cm and EMI SE of 40.6 dB.
Among the various systems of CPC, the main strategy is to disperse the nanofillers uniformly in the polymer matrix to form a continuously conductive network [37], [38], [39]. The composite exhibits sufficient EMI shielding performance when the content of nanofillers is higher than the percolation thresholds [40]. However, the insulating polymer substrates inevitably hinders the contact of the nanofillers, resulting in a decrease in electrical conductivity, and ultimately the nanofillers are unable to give play to their full potential for EMI shielding [41]. In addition, nanofillers often aggregate in the polymer substrate, resulting in a greatly decrease in the mechanical properties and EMI shielding properties of CPC. In order to solve these problems, segregated conductive structure with the conducting fillers wrapped by a polymer matrix has been successfully designed [42], [43], [44], [45], [46]. At the same time, a large number of studies demonstrated that the layered structure can effectively improve the EMI SE through multiple reflections at the interfaces [47], [48], [49], [50], [51], [52]. Fang et al. [53] prepared independent AgNW/polyaniline films with layered structure and embedded structure, respectively, and the layered structure displayed higher EMI shielding performance. Yuen et al. [54] also developed the composites by stacking ten layers of 0.1 mm MWCNT/PMMA, and the EMI SE is higher than that of a single piece of bulk MWCNT/PMMA with 1 mm thickness. Xi et al. [55] reported that multilayered structure with artificial insulating layers exhibited a higher EMI SE than those with no insulating layers.
Inspired by the above reports, multilayered ANF/AgNW nanocomposite films with high thermal durability and EMI shielding properties were successfully fabricated by a multiple filtration layer-by-layer process. The insulating ANF substrates alternated with AgNW layers and simultaneously served as the outer protective layers, while AgNW was tightly stacked together to assemble highly efficient conductive layers and also combined with ANF layers to form extensive hydrogen bonding at the interfaces. Therefore, the multilayered ANF/AgNW nanocomposite films exhibited efficient EMI shielding performance, excellent mechanical flexibility and thermal durability. With the increase of the number of layers, the EMI shielding and mechanical properties of the multilayered films are gradually improved.
Section snippets
Materials
Poly (p-phenylene terephthamide) (PPTA) pulp (Kevlar 49) was brought from Zhongli New Material Technology Co., Ltd (Cangzhou, China). Silver nanowire solution (1 wt%) was purchased from Tenna New Material Co., Ltd (Nanjing, China). Dimethyl sulfoxide (DMSO, AR) and Potassium hydroxide (KOH, AR) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
Preparation of ANF
ANF solution was derived from PPTA pulp according to the previously reported method [29]. PPTA pulp (20 g), KOH (20 g) and DI
Morphologies of ANF and AgNW
The fabrication process for the multilayered ANF/AgNW films are displayed in Fig. 1a. The dewatering processes were carried out by alternate filtration of ANF and AgNW dispersion, respectively. Fig. 1b shows ANF aqueous dispersion which were prepared by deprotonation and electrostatic repulsion of PPTA fibers. The KOH/DMSO system extracts the mobile hydrogen protons from the amide group of PPTA chain to produce the negatively charged PPTA chain. As the hydrogen protons on the amide groups in
Conclusions
In summary, flexible multilayered ANF/AgNW nanocomposite films with excellent thermal durability for high performance EMI shielding were successfully fabricated via solution-based multiple filtration layer by layer process. Results indicated that both EMI shielding and mechanical properties of the multilayered film were significantly improved compared with the directly mixed AFN/AgNW film under the same mass condition. Also, EMI shielding and mechanical properties of the multilayered films
CRediT authorship contribution statement
Shuang Li: Conceptualization, Methodology, Software, Validation, Visualization, Data curation, Formal analysis, Writing – original draft. Kunpeng Qian: Software, Visualization, Data curation. Sineenat Thaiboonrod: Writing – review & editing. Hongmin Wu: Writing – review & editing. Shaomei Cao: Investigation. Miao Miao: Investigation. Liyi Shi: Investigation. Xin Feng: Writing – review & editing, Supervision, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was financially sponsored by the Science and Technology Commission of Shanghai Municipality (20230742300 and 18595800700) and the project of “joint assignment” in Shanghai University led by Prof. Tongyue Gao from School of Mechatronic Engineering and Automation. We are grateful to Instrumental Analysis & Research Center of Shanghai University.
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