Abstract
The exploration of flexible, transparent, anti-corrosion and thermally stable electromagnetic interference (EMI) shielding materials is still a huge challenge and urgently demanded in the civil and military fields. Herein, two novel high-performance poly (amide-imide)s (PAIs) were designed and used to fabricate sandwich structure EMI shielding films through simple layer-by-layer casting method. Combining advantages of PAI and silver nanowires (Ag NWs), PAI/Ag NWs composite films showed a shielding effectiveness (SE) of 35 dB and simultaneously possessed an optical transmittance of beyond 80%. More encouragingly, the composite films remained more than 92% EMI SE with random deformations (>5000 times). Furthermore, benefitting from the excellent properties of PAI matrix and sandwich structure of EMI shielding films, the EMI SE was almost unchanged even when they were exposed to air for 30 min at 160°C. On the other hand, the PAI/Ag NWs composite films still exhibited EMI SE of more than 30 dB when they were dipped into aqueous solution for 1 h by varying pH values from 2 to 13. These results demonstrated that PAI and Ag NWs was a good combination to prepare EMI shielding materials with excellent comprehensive performances.
Introduction
In the past decades, the rapid widespread of various electronic devices have made the space full of electromagnetic (EM) waves, which is not only bringing about chaos among different communication channels, but also causing harm to human health. In order to clear the effect of this unwanted EM radiations, developing of electromagnetic interference (EMI) shielding materials is inevitable.1–5 Traditional EMI shielding materials, such as metal (copper,6–8 aluminum,9–11 and nickel12–14) or metal oxide (Fe3O4,15–17 ZnO,18–20 TiO221–23), possess good conductivity and high permeability. However, they are not the optimal choice owing to their high density, poor mechanical flexibility as well as prone to corrosion. Especially, modern display devices and visual windows require that the EMI shielding materials must be transparent and flexible. To meet such harsh requirements, one of the most important approaches is to develop conductive polymer composites (CPCs) with good optical transmittance, high EMI shielding effectiveness (EMI SE) and excellent shielding reliability.5,24–25 Indium tin oxide (ITO) exhibits both high visible transmittance and good conductivity, and thus has been widely used as EMI shielding materials. For example, Xu et al. fabricated ITO films with an EMI SE of 30 dB and transmittance of 80%. 26 However, ITO suffers from inherent brittleness, which restricts its application in flexible devices. Other transparent CPCs have also been proposed, such as graphene,27–29 conducting polymers30–32 and MXene.33–35 Unfortunately, these CPCs possess either unsatisfactory EMI SE or low optical transmittance.
Recently, silver nanowires (Ag NWs) have been considered as a promising candidate to fabricate transparent and reliable EMI shielding materials due to their full spectrum transparency, high inherent conductivity and good mechanical flexibility.36–39 For example, Jia prepared an EMI shielding film based on Ag NWs through a simple Mayer-rod coating approach. 37 The Ag NWs composite films exhibited a high EMI SE of 31.3 dB and a good transmittance of 81%. Furthermore, these transparent films still exhibited a highly reliable shielding ability in a complex service environment, with 98% and 96% EMI SE retentions even after 30 min of ultrasound treatment and 5000 bending cycles, respectively. But it should be reminded that these metal nanowire networks are susceptible to oxygen and tend to degrade rapidly in harsh ambient conditions, such as damp heat and high temperature, which limits the long-term practical application.
It is well known that in addition to conductive fillers, structural arrangement of CPCs is also very important for EMI shielding films. In particular, layer-by-layer sandwich structure has been proven to be one of the effective models in combining the individual advantages of polymer matrix and conductive filler.40–43 For example, Hu et al. fabricated flexible transparent poly (ethersulfone)/silver nanowires/poly (ethylene terephthalate) (PES/Ag NWs/PET) sandwich-structured film for EMI shielding.
40
The PES/Ag NWs/PET showed a EMI SE of more than 31.3 dB and a good transmittance of 81%. However, like some other traditional transparent polymer films, such as poly (methyl methacrylate),44–46 poly (carbonate),
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Nylon,48,49 PET,40,50 also suffer from some inherent disadvantages of low glass transition temperatures (
Herein, we report a very simple method to fabricate highly efficient and reliable transparent EMI shielding films. A series of PAIs were designed and selected as polymer matrix due to their preferable properties, such as excellent thermal resistance, anti-corrosion and high mechanical properties. As we know, this is the first time that PAI is used to manufacture EMI shielding devices. Ag NWs was selected as conductive filler. Combining the advantages of PAI and Ag NWs, PAI/Ag NWs composites gave a high EMI SE of 35 dB and a good transmittance of beyond 80%. This could satisfy the industry-accepted SE requirement, such as modern display devices and visual windows. Furthermore, the sandwich structure of PAI/Ag NWs EMI shielding materials enabled the film a highly reliable shielding ability in complex service environment, with more than 90% EMI SE retentions even after acid-base solution treatment, thermal anneal in air condition and random bending cycles. The sandwich structured, flexible and transparent PAI/Ag NWs composites with excellent EMI shielding performance and good anti-corrosion and thermal stable properties has a bright potential in electronic and communication applications in high-tech fields, such as flexible electronics, artificial intelligence, and optoelectronic devices.
Experimental section
Materials
4,4’-Diaminodiphenylmethane (DCP) and 2,2-bis(4-aminophenyl)hexafluoropropane (DFP) were purchased from Tokyo Chemical Industry (TCI) (Shanghai) Chemical Industry Co., Ltd. 1-Methyl-2-pyrrolidinone (NMP), pyridine (Py), triphenyl phosphite (TPP), isopropanol, γ-aminobutyric acid (GABA), silver nitrate (AgNO3), poly (vinyl pyrrolidone) (PVP), FeCl3·6H2O, anhydrous calcium chloride (CaCl2) and anhydrous calcium chloride (LiCl), acetone, ethanol, ethylene glycol and trifluoroacetic acid-d (isotopic) (TFA-d) were purchased from Aladdin Co., Ltd. 4,4'-Oxydiphthalic anhydride (ODPA) were purchased from Changzhou Sunlight Medical Co., Ltd. All chemicals were used without any purification.
Synthesis
Synthesis of polymer matrixes
Synthesis of diimide-diacid (DIDA) monomer. As shown in the Scheme 1, the DIDA monomer was synthesized by the condensation of ODPA with GABA through dehydration cyclization as our previous work.51–54 Firstly, ODPA (18.6 Synthetic route of diimide-diacid monomer and poly (amide-imide)s.
Synthesis of PAIs. PAI-DCP and PAI-DFP were synthesized with varying aromatic diamine (ADA) monomers (DCP and DFP), respectively. All polymers were synthesized by the similar method. The synthetic detail is described as below taking PAI-DCP as an example. Diimide-diacid (2.40
PAI-DCP: FTIR (ATR, v, cm−1): 3327 (amide N-H), 1770/1707 (imide C=O), 1675 (amide C=O), 1394 (C-N), 1274/1232 (aromatic ether C-O-C). 1H NMR (TFA-d, 500 MHz, ppm): 7.87–7.89 (dd, 2H), 7.49 (t, 2H), 7.40 (d, 2H), 7.38 (d, 2H), 7.25 (s, 2H), 7.23–7.25 (d, 2H), 7.21 (s, 2H), 7.14–7.16 (d, 2H), 7.11 (s, 2H), 4.04 (s, 2H), 3.85 (t, 2H), 3.77–3.80 (t, 2H), 2.95–2.97 (m, 2H), 2.67 (t, 2H), 2.18 (t, 2H), 2.05–2.07 (t, 2H).
PAI-DFP: FTIR (ATR, v, cm−1): 3308 (amide N-H), 1770/1708 (imide C=O), 1674 (amide C=O), 1397 (C-N), 1256/1239 (aromatic ether C-O-C), 1202/1173 (C-F). 1H NMR (TFA-d, 500 MHz, ppm): 7.87 (d, 2H), 7.84 (dd, 2H), 7.52 (d, 2H), 7.45–7.50 (m, 2H), 7.42 (d, 2H), 7.38 (d, 2H), 7.34 (s, 2H), 7.25–7.32 (m, 2H), 3.94 (s, 2H), 3.8–3.86 (m, 2H), 3.0–3.05 (m, 2H), 2.62 (t, 2H), 2.17 (t, 2H), 2.08 (t, 2H).
Synthesis of silver nanowires
Ag NWs was prepared by a modified polyol method with some modifications.
40
PVP (0.2
Fabrication of PAI/Ag NWs composites
PAI (0.4
Materials characterization
Fourier transform infrared spectra were recorded by a Nicolt A vatar 370 FTIR spectrometer and measured by KBr pellet method for monomers and attenuated total reflection mode for PAI films. The solubility test was performed by dissolving the samples in different solvents at room temperature or 90°C for 3 h. The inherent viscosity was measured with an Ubbelohde viscometer at 30°C in NMP at a concentration of 0.01 g/mL. Differential scanning calorimetry (DSC) was carried out using a DSC8000 instrument from PE Instruments at a heating rate of 20 °C/min under nitrogen atmosphere from 30°C to 300°C. Thermogravimetric analysis (TGA) was carried out by TGA4000 instrument from PE Instruments at a heating rate of 10 °C/min under nitrogen atmosphere in the temperature range from 30°C to 750°C. Thermomechanical analysis (TMA) were carried out on a DMA8000 instrument adopting static stretching mode at a heating rate of 5°C/min with 0.005
Results and discussion
Design and synthesis of PAIs
In order to achieve such advanced EMI shielding materials, the polymer matrix should possess the following properties at the same time. First, the polymer matrix should be adequately soluble to form a uniform thin film by drop casting or other simple methods. Second, the polymer matrix film should be transparent. Third, the polymer matrix should have good thermal stability and high mechanical properties. Finally, the polymer matrix should be insensitive to acid-base solution and common organic solvents. Traditional transparent polymers such as PMMA, PET exhibit good solubility and processability, but their thermal stability and mechanical properties are poor. On the other hand, aromatic PI shows good thermal stability and mechanical properties, but its solubility and processability are not good. To avoid these drawbacks, PAI was selected as polymer matrix. The designed PAIs structure formed by ODPA, and ADA ((DCP), 2,2-bis(4-aminophenyl)hexa-fluoropropane (DFP)) can give good mechanical, thermal and anti-corrosion properties, the amide group and suitable aliphatic chain can produce good solubility and processability. As shown in the Figure 1, the non-coplanar structure of ODPA can increase the inter-chain distance, in addition, the flexible chain can increase the torsion angle between ODPA and ADA, which is be beneficial to reduce the crystallinity for achieving high optical transmittance. 3D structure of the repeating units of PAI-DCP (a) and PAI-DFP (b).
A diacid monomer bearing thermal stable aromatic benzene ring and good processability aliphatic alkyl chain was synthesized as shown in the Scheme 1. The dehydration cyclization of ODPA with GABA gave the corresponding DIDA monomer. The polymer matrixes (PAI-DCP, PAI-DFP) were respectively prepared by Yamazaki Higashi phosphorylation of DIDA with two different diamine monomers (DCP and DFP) in the presence of TPP and Py. In the FTIR spectra of ODPA and DIDA as shown in the Figure 2, the characteristic peak of dianhydride C=O at 1779 cm−1 disappeared, and in instead, the characteristic peaks of imide C-N at 1403 cm−1 and carboxyl O-H at 3440 cm−1 appeared, suggesting the successful formation of DIDA monomer. In the FTIR spectra of polymer matrixes, the characteristic peak of carboxyl O-H disappeared while the characteristic peak of amide N-H at 3310 cm−1 appeared as summarized in Table S1, furthermore, the trifluoromethyl C-F was observed in the FTIR spectrum of PAI-DFP, suggesting the successful synthesis of PAI-DCP and PAI-DFP. Moreover, these changes were also confirmed by nuclear magnetic resonance spectra as shown in the Figure S1. FTIR spectra of ODPA, DIDA, PAI-DCP and PAI-DFP.
Physical and chemical properties of PAIs
Unlike aromatic poly (imide), all PAIs exhibited good solubility in some common organic solvents, such as N-methyl-2-pyrrolidone (NMP), m-cresol. The inherent viscosities of PAIs were evaluated to be 0.94 dL/g for PAI-DCP and 0.64 dL/g for PAI-DFP, suggesting that these two polymers with high molecular weight were successfully prepared, although the two amine monomers possess different reactivity due to the electron-donating hydrogen atom and electron-withdrawing trifluoromethyl groups. This property allowed for processing polymer matrixes into films by casting the polymer solutions in NMP. A representative example of a PAI film is shown in Figure 3(a), indicating its high transparency. As shown in the Figure 3(b), the transmittances of all the PAI films at the 550 nm were higher than 88% measured by ultraviolet-visible (UV-vis) spectroscopy, and the cutoff wavelengths were estimated to be 360 nm for PAI-DCP and 350 nm for PAI-DFP as summarized in Table S2, suggesting that the PAI films only exhibited a subtle yellow hue. On the other hand, the higher optical transmittance PAI-DFP is ascribed to the presence of electron-withdrawing trifluoromethyl groups on the polymer backbone, which prevented the formation of CT complexes by sterically repelling adjacent chains, and weakening the electron density in the diamine unit.60,61 In addition, the hazes were investigated to be 1.8% for PAI-DCP and 0.6% for PAI-DFP, the better clarity of PAI-DFP is ascribed to its lower crystallinity as shown in the Figure S2. These results suggested that incorporation of fluorine containing functional groups into the polymer main chain was an effective strategy to improve the optical properties of polymer films. (a) Digital photograph of PAI film; (b) Optical transmittance of PAI pure films (black line: PAI-DCP; red line: PAI-DFP).
These two new polymers are quite thermal stable even at high temperatures as shown in the Figure 4(a), both of PAI-DCP and PAI-DFP showed a 5% weight loss at above 400°C. As shown in the Figure S3, the glass transition temperatures ( Differential scanning calorimetry (a) and thermomechanical analysis (b) curves of poly (amide-imide)s (black line: PAI-DCP, red line: PAI-DFP).
Electromagnetic interference shielding performance of the sandwich structured PAI/Ag NWs films
To exploit the flexible, high transparency, excellent mechanical and thermal properties of the PAIs, as shown in the Figure 5(a), these two polymers were used as substrate to fabricate high-performance EMI shielding devices, in which silver nanowires (Ag NWs) was chosen as conductive filler. As shown in the Figure 5(b), the PAI/Ag NWs composites exhibited an EMI SE of above 35 dB in a wide range. More important, the optical transmittance was more than 80% at 500 nm as shown in the Figure S5. Schematic diagram (a) and EMI SE curve (b) of PAI/Ag NWs electromagnetic interference shielding films.
In terms of practical application, a reliable EMI SE is extremely important to EMI shielding devices. Firstly, the reliability of shielding performance under mechanical strain conditions was conducted, the PAI/Ag NWs sandwich films was folded by 5000 times with a bending angle of 180o. It is found that the EMI SE maintained a stable value close to the original value as shown in the Figure 6(a), suggesting the PAI/Ag NWs shielding films showed little sensitivity to mechanical deformation. In addition, there is no crease after this violent mechanical folding, suggesting the surface adhesion force is enough high to prevent the Ag NWs stripped from PAI substrate. Furthermore, it is worth emphasizing that the PAI/Ag NWs composite films still exhibited an EMI SE value of 32 dB, which exceeded the requirement of commercial applications. To describe the excellent flexibility of PAI/Ag NWs shielding films more precisely, a concept of EMI shielding durability (SD) was proposed, and the value can be calculated as the following equation. EMI SE variation of PAI/Ag NWs films with different treatments. (a) Bending cycles, (b) thermal annealing, (c) acid-base immersing, (d) organic solvents immersing. The variation is defined as the ratio of EMI SE after the treatments to the original EMI SE at 2.5 GHz.

Then the thermal reliability was investigated. As shown in the Figure 6(b), the PAI/Ag NWs shielding films with the new material still kept transparent even after in-air thermal annealing (160°C for 30 min), exhibiting 81% transmittance at 550 nm. This is a breakthrough compared to conventional PIs and PAIs, which typically exhibit significant yellowing ascribed from the formed CT complex, and further oxidation during the thermal annealing process. In addition, EMI SE only dropped slightly. The excellent thermal reliability of PAI/Ag NWs shielding films is mainly owing to the low CTE and high
Conclusion
Two novel PAIs with flexible, high transparency, excellent mechanical and thermal properties have been synthesized through careful molecular structure design. Combining the advantages of PAI and silver nanowires, the PAI/Ag NWs composite films showed EMI SE of 35 dB and possessed an optical transmittance of beyond 80% simultaneously. Furthermore, the PAI/Ag NWs composites films exhibited high reliability in acid-base, thermal, mechanical strain conditions. Even in these harsh environments, more than 90% EMI SE was kept due to the high-performance of PAI and sandwich structure of EMI shielding films. These results suggested that PAI materials can be used as versatile platform for organic devices that require multifunctional factors.
Supplemental Material
sj-pdf-1-ppc-10.1177_09673911221107292 – Supplemental Material for Design and synthesis of high-performance poly(amide-imide) for multifunctional electromagnetic interference shielding films
Supplemental Material, sj-pdf-1-ppc-10.1177_09673911221107292 for Design and synthesis of high-performance poly(amide-imide) for multifunctional electromagnetic interference shielding films by Changlong Zhuang, Yanbin Wang, Chengbi Chang, Zicheng Fan, Shuhan Hou, Chen Zhou, Hao Dong, Leier Xia, Zhonglin Luo and Biaobing Wang in Polymers and Polymer Composites
Footnotes
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Natural Science Foundation of Jiangsu Province (BK20160280), Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJCX20_0976).
References
Supplementary Material
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