Cross-stacking aligned non-woven fabrics with automatic self-healing properties for electromagnetic interference shielding

Journal Pre-proof Cross-stacking aligned non-woven fabrics with automatic self-healing properties for electromagnetic interference shielding Li Chen, Kun Guo, Shi-Lin Zeng, Long Xu, Cheng-Yuan Xing, Sheng Zhang, BangJing Li PII:

S0008-6223(20)30180-9

DOI:

https://doi.org/10.1016/j.carbon.2020.02.034

Reference:

CARBON 15084

To appear in:

Carbon

Received Date: 12 November 2019 Revised Date:

7 February 2020

Accepted Date: 14 February 2020

Please cite this article as: L. Chen, K. Guo, S.-L. Zeng, L. Xu, C.-Y. Xing, S. Zhang, B.-J. Li, Crossstacking aligned non-woven fabrics with automatic self-healing properties for electromagnetic interference shielding, Carbon (2020), doi: https://doi.org/10.1016/j.carbon.2020.02.034. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Credit author statement Li Chen: Conceptualization, Methodology, Software, Formal analysis, Investigation, Data Curation, Writing - Original Draft, Visualization Kun Guo: Methodology, Software, Visualization, Writing - Review & Editing Shi-Lin Zeng: Methodology Long Xu: Software, Writing - Review & Editing Cheng-Yuan Xing: Software, Writing - Review & Editing Sheng Zhang: Conceptualization, Methodology, Validation, Resources, Writing Review & Editing, Supervision, Project administration, Funding acquisition Bang-Jing Li: Validation, Resources, Supervision, Project administration, Funding acquisition, Writing - Review & Editing

Cross-stacking aligned non-woven fabrics with automatic self-healing properties for electromagnetic interference shielding Li Chen,a Kun Guo,c Shi-Lin Zeng,a Long Xu,b Cheng-Yuan Xing,b Sheng Zhanga,* and Bang-Jing Lib,*

a.

State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute

of Sichuan University, Chengdu,610065, Sichuan b.

Key Laboratory of Mountain Ecological Restoration and Bioresource Utilization,

Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu,610041, Sichuan c.

College of Pharmacy, Southwest Minzu University, Chengdu,610041, Sichuan

*Corresponding author. Tel: 86-28-82890646. E-mail address: [email protected] (Sheng Zhang); [email protected] (Bang-Jing Li).

Cross-stacking aligned non-woven fabrics with automatic self-healing properties for electromagnetic interference shielding Li Chen, a Kun Guo, c Shi-Lin Zeng, a Long Xu, b Cheng-Yuan Xing, b Sheng Zhang a,* and Bang-Jing Lib,* a.

State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute

of Sichuan University, Chengdu,610065, Sichuan b.

Key Laboratory of Mountain Ecological Restoration and Bioresource Utilization,

Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu,610041, Sichuan c.

College of Pharmacy, Southwest Minzu University, Chengdu,610041, Sichuan

Efficient, flexible and self-healing electromagnetic interference (EMI) shielding materials are highly desirable and valuable for electromagnetic shielding industry. Furthermore, in order to achieve a commercial-grade EMI shielding effectiveness, a large amount of filler is required. Here, a kind of self-healing EMI shielding fabric was fabricated by preparing aligned carbon nanotube-poly (2-hydroxyethyl methacrylate) (CNT-PHEMA) non-woven fabric through magnetic-field-assisted electrospinning and cross-stacking aligned fabric layers. The unique aligned CNT stacking and porous structure enable that the materials can effectively absorbing EMI even at very low CNT loading amount (0.17wt%). The host-guest interaction between β-cyclodextrin (β-CD) and adamantane (Ad) moieties not only made the fabric can adhere together automatically but also self-heal the scratch under 100% humidity environment. After the healing, the EMI shielding property can be restored

1

90.86±3.90%.

1. Introduction Recent advances in electronic, such as personal computers and portable electronics have brought huge conveniences to humanity, but the use of these equipment has resulted in severe electromagnetic interference (EMI) pollution, and the effects of EM wave radiation to human body have emerged as more important problems. Electromagnetic wave not only cause various disease to people [1-3], but also induce micro-devices to malfunction [4-6]. Preventing electromagnetic wave pollution, in addition to reducing the electromagnetic wave radiation of the electronic products themselves, the most effective way is to develop and use EMI shielding materials. Among them, fabrics with electromagnetic protective are a kind of very important and widely used EMI shielding material type because of their flexibility, light weight, low cost, and facile preparation. In general, the fabric products made of polymer materials with poor EMI shielding properties. The electromagnetic protective capability of fabrics was usually endowed by two methods: 1) surface treatments: coating conductive layer on the surface of fabric; 2) hybrid fabrics: blending shielding or adsorbing fillers, with polymeric fibers to form fabrics. The first coating method always suffers from poor durability. Therefore, the hybrid fabric with intrinsic electromagnetic protective properties attract great attention recently.

2

Reflection,

absorption,

and

multi-reflection

are

three

dominated

mechanisms for electromagnetic protective fabrics. Metal-based EMI shielding fabrics show high EMI shielding effectiveness due to the high reflection of metals on electromagnetic wave [7, 8]. However, the reflected waves may cause other problems, such as the electric noise. Moreover, metal fillers are heavy, rigid, corrosive and inconvenient to process [9-12]. Therefore, high multi-reflecting and absorbing materials are desirable. Many Carbon-based materials, e.g. carbon filaments, carbon fibres, carbon nanofibers, carbon nanotubes (CNTs) and graphene have been used as fillers to fabricate EMI shielding materials [13-17]. CNTs are one of the most studied materials due to the combined light weight, excellent mechanical and electronic properties, and larger aspect ratio. Lots of research have been carried out to prepare CNT filled polymer for EMI shielding materials and to understand their shielding

mechanism.

Generally,

the

EMI

shielding

effectiveness

of

CNTs-based materials depend on filler loading content. However, high filler loading requirement to achieve acceptable EMI shielding performance causes agglomeration tendency of nanofillers and nonuniform dispersion, and then resulting in poor mechanical properties. In order to improve the EMI shielding effectiveness of CNTs-based materials at low CNT loading content, porous structure has been introduced to the structure of EMI adsorbing composites. Multi-reflection usually occurs when there are numerous surface areas or interfacial areas in materials, since the electromagnetic wave reflecting and

3

scattering at various inhomogeneous interfaces. The synergistic effect of adsorption and multi-reflection make the EMI shielding materials showing good EMI shielding effectiveness even at low filler concentration [18, 19]. For example, Gupta and co-worker designed a polystyrene/nanotube foam by foaming method. Its EMI shielding effectiveness was about 20 dB at 7 wt% CNT loading [5]. Nasouri et al. reported a poly (vinyl alcohol)/CNT composite nanofibers

through

electrospinning

processing.

The

response

surface

methodology model predicted that this material showed 31.6 dB absorption and 8.5 dB reflection at conditions of 7.7wt% CNT, and 3 mm of the sample thickness [20]. Xu and Zhong group developed cellulose/nanotube porous composites using ice-temple freeze-drying method, which showed a shielding effectiveness of 40 dB with merely 0.51wt% CNT under a thickness of 2.5 mm owing to the preferential distribution of CNT on the cellulose cell walls [13]. Although

various

porous

materials

with

sufficiently

EMI

shielding

effectiveness have been achieved, in practical applications, they may encounter severe degradation of EMI shielding performance after mechanical damage. Recently, various self-healing functional materials, which were able to restore their original functionality, have attracted burgeoning interests, but up to now, porous EMI shielding materials with self-healing properties are seldom reported. In this paper, we designed a kind of self-healing nonwoven fabric composed of poly (2-hydroxyethyl methacrylate) (PHEMA) and single-walled

4

CNTs by electrospinning technology for EMI shielding. These materials have two special features. Firstly, CNT-PHEMA nanofibers were uniaxially aligned in the non-woven fabric by introducing magnetic field and the CNT aligned along fibres. Secondly, CNTs were connected to PHEMA with β-CD and Ad host-guest interactions. The aligned fabrics are easy to be stacked together and show autonomic self-healing ability after being damaged due to host-guest interaction. Peng and co-workers have demonstrated that aligned CNT films could act as building block to prepare microwave absorption materials, and the absorption properties could be controlled by stacking them with different intersectional angels [21]. In this study, aligned CNT-PHEMA fabric also could be used as light-weight, and adjustable EMI shielding. Furthermore, the CNT alignment and porosity of non-woven fabric enable the material to show good EMI shielding effectiveness (20.42 dB) at 11.3 GHz at low CNTs content (0.17wt%), reaching the commercial grade and can shield 99.99% EM. Crack can lead to a large decline of EMI shielding ability. However, after self-healing process, the maximum

can recover 90.86±3.90%.

2. Experimental 2.1 Material 2-Hydroxyethyl methacrylate (HEMA) was purchased from J&K Chemical Technology (Beijing, China). β-cyclodextrin (β-CD) was purchased from Chron Chemical Reagent Factory (Chengdu, China) were applied directly without further treatment. p-Toluene sulfonyl chloride (p-TsCl) was purchased from Adamas Reagent Co., Ltd. (Shanghai, China). Single-walled carbon 5

nanotubes

(SWCNTs),

1-pyrenebutyric

acid,

2,2-azobis

(AIBN),

1-adamantanecarboxylic acid (Ada-COOH), dicyclohexylcarbodiimide (DCC), 6-chloro-1-hydroxibenzotriazol (HOBT) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). Thionyl chloride was purchased from Changlian Reagent Co., Ltd. (Chengdu, China). Triethylamine was purchased from Zhiyuan Reagent Factory (Tianjin, China). 2.2 The process of sample synthesis 2.2.1 Preparation of per-6-iodo-β-cyclodextrin (6-OTs-β-CD) 6-OTs-β-CD was prepared using the way according to Quan et al [22]. 20 g β-CD was dispersed into 400 mL of deionized water under continuous stirring, and then, 5.2 g p-TsCl was added in slowly to ensure that the substitution occurred at the C6 position. After vigorous agitation 10 h at ambient temperature, 80 mL of 2.5 M sodium hydroxide solution was added dropwise slowly. The obtained suspension was filtered to remove unreacted p-TsCl, and then, 24 g ammonium chloride was added into the filtrate solution to adjust the solution pH value to 8.0, some white precipitate appeared. The resultant solution was placed in the refrigerator approximately 4 °C and kept overnight. After vacuum filtration, the precipitation was collected and washed by acetone three times to remove unreacted p-TsCl and β-CDs. The white precipitation was recrystallized at 90 °C at least three times. Finally, after being dried at 50 °C for 2 days under vacuum drying, the 6-OTs-β-CD yield was 4.42 g. The corresponding characterization is shown in Fig. S1. 2.2.2 Preparation of per-6-deoxy-6-amino-β-cyclodextrin (6-NH2-β-CD)

6

6-OTs-β-CD (4 g) was dissolved in 87 mL of ammonia. After stirring for 4 h at 75 °C, the obtained solution was cooled and added into appropriate amount of acetone solution, then masses of white precipitation precipitated immediately. The white precipitation obtained by suction filtration. Dissolved the white precipitation into mixed solution of H2O/CH3OH (v/v=3:1), and then acetone was added for precipitation for removing the unreacted 6-OTs-β-CD and ammonia. This operation was repeated several times. The white precipitations were dried at 50 ℃ in vacuum for one days[23]. The corresponding characterization is shown in Fig. S1. 2.2.3 Preparation (Py-β-CD)

of

per-6-(1-pyrenebutylamino)-6-deoxy-β-cyclodextrin

6-NH2-β-CD (1000 mg, 0.88 mmol) and 1-pyrenebutylic acid (508 mg, 2.18 mol), DCC (400 mg, 1.94 mmol) and HOBT (200 mg, 1.48 mmol) were added at 0 ℃ under nitrogen atmosphere to DMF solution (10 mL), then the reaction mixture was stirred two days at room temperature. After filtering to removal of insoluble salts, and the filtrate was reprecipitated with acetone. The precipitate was obtained by filtration washed with water several times to remove 6-NH2-β-CD. After drying under vacuum at 50 ℃, the yellow solid Py-β-CD was obtained, and the yield was 618 mg [24]. The corresponding characterization is shown in Fig. S2. 2.2.4 Self-assembly of Py-β-CD and SWCNT The hybrids were prepared using the way according to the literature previously reported [24]. Py-β-CD (100 mg, 0.07 mmol) was dissolved in 0.1

7

M NaOH. Then to the mixture, purified SWCNTs (50 mg) was added and sonicated in low-energy ultrasonic bath (Bransonic 2510), followed by centrifugation. The supernatant was dialyzed against 0.1 M NaOH for a week to remove excess free Py-β-CD from the solution. Then the supernatant was dialyzed against deionized water for 48h to remove NaOH. The product was denoted as Py-β-CD-CNT. 2.2.5 Preparation of Ada-modified 2-hydroxyethyl methacrylate (HEMA-AD) According to the literature [25], 1-adamantanil chloride was obtained by converting 1-adamantanil chloride into acid chloride after being treated with thionyl chloride at 80 ℃. 1 mL HEMA and 1.6 mL triethylamine were dissolved in 100 ml dichloromethane with stirring at 0 ℃. 1.85 g of 1-adamantanoyl

chloride

was

dispersed

in

30

ml

of

anhydrous

dichloromethane, and added to the reaction solution slowly. The final yellow solution was extracted with HCl (1 M), NaHCO3 (1 M) and deionized water after stirring the mixture for 5 h in ice bath. Finally, anhydrous sodium sulfite was used to dehydrate the solution for 4 h, and HEMA-AD was obtained after solvent removal. The corresponding characterization is shown in Fig. S3. 2.2.6 Preparation of inclusion complexes between Py-β-CD-CNT and HEMA-AD The inclusion complex was formed by mixing Py-β-CD-CNT and HEMA-AD (with n(CD)/n(Ad)=1:2) in 4 mL DMSO/H2O (V/V=9:1). Then removing most solvent after vigorous stirring for two days at room temperature

8

and washing it using CH2Cl2. Finally, inclusion complexes were obtained at 50 °C for two days under vacuum drying. 2.2.7 Copolymerization of HEMA and inclusion complexes HEMA (8 mL) and 100 mg inclusion complexes were mixed in 25 mL absolute ethyl alcohol, by ultrasonic for 5 min. After addition of AIBN (20 mg), the mixture solution was treated again with the ultrasonic for 3 min. Next, the polymerization was carried out at 52 °C for 5 h under nitrogen after the reaction solution was degassed under nitrogen flow. Finally, the black viscous liquid was obtained. 2.2.8 Preparation of electrospinning fabric The final polymerization liquid was diluted with DMF to obtain electrospinning solution. The electrospinning process was carried out using electrospinning machine (YFSP-T(B), Tianjin). The prepared solution was added to a 5 mL plastic syringe with a needle (22G) and connected to the power supply, which can generate DC voltages. The needle was connected to a positive high-voltage power supply. We used a high voltage of 15 kV between the needle and the collector. The collector was two ferromagnets with distance of 3.5 cm for the collection of aligned electrospinning fabrics. The feed rate was 0.25 mL·h-1, after 4 h, the electrospinning fabric was collected about 200 µm and then dried in oven at 50 ℃ for 12 h. A 2mm control film was obtained by casting polymer solution into mould. 2.3 Characterizations The 1H NMR,

13

C NMR, and 2D 1H NOESY spectra were recorded on a

9

Bruker Avance-III400 MHz spectrometer. The UV/Vis and fluorescence spectra results were detected using Varioskan Flash (Thermo, USA). The thermogravimetric analysis (TGA) was characterized using Thermal Analyzer EXSTAR 6000 (Seiko Instruments Inc, Japan). Samples were heated from room temperature to 800 ℃ with a heating rate of 10 ℃/min under nitrogen atmosphere. The samples for SEM analysis were performed using JSM-7500F microscope after they were cut into small rectangle sputter-coated with a thin layer of gold (about 5 nm) (JSM-7500F, JEOL, Tokyo, Japan). The self-healing properties of fabrics were detected by a laser scanning confocal microscope (LSCM) (Zeiss, LSM 800, Germany) at room temperature. EMI shielding measurements were performed an Agilent N5230 vector network analyzer. The thickness values of CNT-PHEMA fabric with 8 layers was less than 2 mm. The thickness of fabric with 12,16 layers were equal to 2 mm. FT-IR spectra of non-woven fabrics and HEMA were recorded on a Varian Scimitar100 Fourier transform IR in ATR method. The Raman spectra were performed using Thermo Fisher DXRxi. To examine the nanofiber alignments in the fabrics statistically, 2D-SAXS measurement was performed in the Xeucs 2.0 system (GeniX3D Cu ULD, Xenocs SA, France). The dispersion of SWCNT in the nanofiber matrix was evaluated using FEI Tecnai F20 transmission electron microscopy (TEM) at an accelerating voltage of 120 kV. Mechanical properties were tested by Electronic Universal Test Machine-INSTRON 5567 (Simadzu, Japan) at a tensile speed of 50mm/min at room temperature (25 °C). Samples

10

used in tensile strength measurements were rectangular shapes with a size of about 20*9*1.6±0.2 mm (8-layer non-woven fabrics). For each sample, each test was repeated three times. The ultimate strength and strain at break of the samples were obtained from the tensile tests and recorded as an average value. 3. Results and discussion 3.1 Preparations of aligned CNT-PHEMA non-woven fabrics Single-walled CNTs are selected as EMI shielding material fillers. However, usually, CNTs are easily agglomerated during the experiment, and it is difficult to disperse well in polymer solution. The previous article of our group has introduced Py-β-CD to CNTs for their better dispersion and more important, endowing self-healing properties to follow-up materials [26]. In this study, Py-β-CDs were coated on the surface of CNTs through π-π stacking using the similar method. Fig. S4 shows characterization of Py-β-CD-CNT. The resulted Py-β-CD-CNT could be dispersed in aqueous solution, forming a stable suspension, however, the unmodified CNTs precipitated in water. TGA data of SWCNT and Py-β-CD-CNT showed that the weight percentage of β-CD in Py-β-CD-CNT was about 61.22wt% (Fig. S7). Fig. S5a, b and Fig. 1a show preparation

procedure

of

CNT-PHEMA

electrospinning

solution.

The

CNT-PHEMA electrospinning solution was prepared in a two-step process: 1) self-assembly

of

Py-β-CD-CNT

and

HEMA-Ad;

2)

copolymerization

of

Py-β-CD-CNT/HEMA-Ad inclusion complexes and HEMA monomers using AIBN as initiator. It is well known that β-CD could interact with adamantane and its derivatives to form stable inclusion complexes [27, 28]. In this paper, Py-β-CD-CNT 11

could connect with HEMA-Ad through interaction between Ad and β-CD moieties. 2D 1H NOESY NMR spectrum of Py-β-CD-CNT/HEMA-Ad complexes showed an obvious correlation between signals of β-CD and Ad groups (Fig. S6a), suggesting that Ad groups captured in the cavity of β-CD moieties to form inclusion complexes. TGA results confirmed that the HEMA-Ad percent in inclusion complexes was 81.85wt%. (Fig. S7). These inclusion complexes contained many double bonds, so they could act as macro-monomers to copolymerize with HEMA monomers. Figure 1b is the IR spectrum of CNT-PHEMA copolymer. It can be seen the C=C vibration peak at 1603 cm-1 disappeared after copolymerization, indicating that the successful polymerization. In order to make sure that Py-β-CD-CNT and HEMA-Ad could still form inclusion complexes after polymerization with HEMA, 2D 1H NOESY NMR was measured (Fig. S6b), the resonances of Ad were obviously correlated with peaks at 3.5-4.0 ppm attributed to protons to Py-β-CD. Considering that no interaction exists between PHEMA backbone and Ad, these correlation peaks indicated that Ad still captured to the cavity of Py-β-CD to form inclusion complexes even after polymerization. The concentration of CNT was controlled as 0.17wt% to avoid agglomerating. The resulted CNT-PHEMA copolymer could dissolve in DMF very well as homogenous black viscous liquid and act as electrospinning solution for the electrospinning. Electrospinning technology is one of the most convenient way to produce nanofibers with increased surface area and controllable morphology. It has been reported that uniaxially aligned fibres

12

could easily be fabricated by using two strips of ferrite magnets as collector. In this work, the setup of collector consisting of magnets separated by a gap was shown in Fig. 1c. The width of the gap was 3.5 cm [29].Through the magnetic-field-assisted electrospinning technology, nanofibers will be aligned under the orientation of the magnetic field, and finally a non-woven fabric with a certain directionality was obtained. This fabric was grey (Fig. 1d), and porous mat with 200 µm thickness after electrospinning 4h.

Fig. 1 a) Schematic illustration about preparation process of CNT-PHEMA containing host-guest groups; b) FT-IR spectrum of HEMA and non-woven fabric; c) Schematic illustration of the preparation of multilayer overlap non-woven fabrics and d) digital photograph of flexible with 16-layer overlapping non-woven fabrics. (size:2cm*1cm*2mm)

13

Fig. 2a and b show the low and high magnification SEM images of aligned CNT-PHEMA non-woven fabric. It can be seen CNT-PHEMA fibres with diameter of 1.64±0.36 µm are arranged roughly in one direction, while nanofibers randomly aligned without the assistance of magnetic field (Fig. 2c). The mechanism could be described as follows: electrospinning jet just like an electrifying solenoid during the electrospinning process, it would generate a magnetic field in its every turn. When electrospinning fibre attracted to the collector surface based on two ferrite magnets, interaction between generated magnetic field would force fibres to align along the collection magnetic field [29, 38-40]. Finally, the fibres land on the magnets and across the gap. Since fibres were arranged in a directional manner under the assisted of magnetic field, SWCNTs were distributed along the direction of fibres under the action of fibres [30-33]. TEM characterization shows that single-walled carbon nanotubes are arranged along the direction of nanofiber, and single-walled carbon nanotubes are not bundled but separated from each other, indicating that Py-β-CD has a good dispersion effect on SWCNTs (Fig. 2d).

14

Fig. 2 SEM images of the CNT-PHEMA nanofibers with low-magnification a) (scale bar of 500µm) and high-magnification b) (scale bar of 50µm) with magnetic-field-assisted electrospinning technology; c) SEM image of the CNT-PHEMA nanofibers without the assistance of magnetic field and d) TEM images of the single CNT-PHEMA nanofiber.

3.2 Cross-stacking aligned CNT-PHEMA of non-woven fabrics Peng s’ group has demonstrated that the microwave absorption performance could be tuned effectively by cross-stacking aligned CNT films. The reflection loss value was maximal when the intersectional angle between neighbour layers was 45° and this value increased with the increase of the layer number [21]. This discovery represents a new strategy to tune and enhance the EMI shielding ability of materials. However, the aligned CNT films were pasted together by commercial glue in that work, resulting in uncontrollable factors and inaccurate. In this study, the non-woven fabrics could be adhered together by themselves without any glue under the environment of 100% humidity with 12 h. Fig. S5c shows overlapping process with non-woven

15

fabrics with an intersectional angle of 45°between two neighbouring ones. As shown in Fig. S8, non-woven fabric composed of overlapping electrospinning fabrics was still flexible, which can be bent easily. Fig. S9 shows the stress−strain curves of 8-layer non-woven fabric, it can be seen mechanical strength of non-woven fabric was 7.77±0.87 MPa. The self-adhesion ability of CNT-PHEMA fabric is due to the host-guest interaction between β-CD and Ad. As Harada et al. and our group have reported before, the host and guest gel could stick together since the host and guest groups formed inclusion

complexes

[34-36].

In

this

study,

two

layers

overlapping

electrospinning fabric could be stuck together since there still existed a lot of free β-CD and Ad groups in fabric.

3.3 EMI shielding performance of multilayer overlapping non-woven fabrics The aligned CNT-PHEMA fabric shows EMI shielding effect. The EMI shielding performance of the material obtained by the S-parameters ( and

) in the X band (8.2-12.4 GHz) was determined by the vector network

analysis, and using the follow equations, we can calculate the reflected power (R), transmitted power (T), absorbed power (A), EMI SE (SE

), microwave

reflection (SE ) and microwave absorption (SE ): R=|

|

(1)

T=|

|

(2)

16

A=1−R−T = −10 lg!1 − "#

(4)

= −10 lg $

(5)

= SE

(3)

%

+

& +

(6)

is waves multiple reflection. Firstly, it can be seen cross-stacking electrospinning CNT-PHEMA fabrics

(16 layers) with or without aligned showed higher EMI shielding values than the direct casting film from 8 to 12.4 GHz, although they had same thickness (2 mm). These results suggested that the porous structure of non-woven fabric was beneficial for EMI shielding. In general, the EMI mechanism of materials including absorption, reflection, and multi-reflection. Fig. 3 shows the total EMI SE ( SE

), the absorption of electromagnetic waves ( SE ), and

reflection of electromagnetic radiation (SE ) for different samples. It can be seen the electrospinning fabrics showed obviously higher SE lower SE

values but

values than casting film. For the CNT-PHEMA fabric with 8, 12

layers, although the SETotal values were lower than casting film within the frequency range of 9.0 to 10.0, they still showed higher SEA value.

17

Fig. 3 Comparison of EMI shielding performance. Variations in a) , b) SE , and c) SE as a function of frequency for aligned multilayer overlapping non-woven fabrics, non-aligned non-woven fabrics and the direct casting film; d) The SE histogram comparison of electrospinning non-woven fabrics and the direct casting film with different frequencies.

Fig. 4 schematically illustrates the mechanism of electromagnetic waves transfer across electrospinning fabric. The incident electromagnetic waves can easily enter the inside of fabric with less reflectivity due to their high porosities. The entered waves are attenuated by multi-reflecting and scattering between the fibres and CNTs [11, 43-45]. As consequence, waves are difficult to escape from the fabric until being absorbed. In contrast, casting films shielded microwaves by both absorption and reflection. The absorption-dominant EMI shielding performance is superiority in practical EMI applications since this

18

kind of materials would not cause the secondary microwave pollution to the surrounding environment by the reflection.

Fig. 4 Schematic diagram of electromagnetic interference shielding on porous non-woven fabric.

Fig. 3 shows EMI shielding effectiveness of multilayer overlapping non-woven fabrics with different layers. It can be seen from Figure 3c and d that EMI shielding effectiveness of 16-layer aligned non-woven fabrics are better than that of 16-layer non-aligned electrospinning fabrics obviously at 11.3 and 12.4 GHz with same thickness. It has been demonstrated that aligned porous structure of the CNT-PHEMA non-woven fabric provides more efficient interfaces coming from CNTs orientation, which are perpendicular to the propagation direction of electromagnetic waves [31].

19

As shown in Fig. 4, electromagnetic waves propagated perpendicular to fabrics, scatter or absorption will be more effectively owning to fibres form a porous structure to promote waves absorption as well as a scatter network of CNTs. The overlapped aligned CNT-PHEMA fabrics composed of CNTs aligned at different direction. When the incident electromagnetic waves entered fabric, their propagation got more interference at layers in which aligned CNTs are perpendicular to the direction of waves, but because of the arbitrary distribution of CNTs within non-aligned fabrics, there were not obvious interface distinction, so EMI shielding of 16-layer non-aligned fabrics is not well to 12 and 16-layer aligned fabrics with same thickness. For conventional EMI shielding materials, the absorption properties can be only tuned by varying the thickness. But for aligned nanomaterial films, the absorption properties could also be controlled by other parameters, such as the intersectional angles and stacked number. It has been reported that aligned CNT can enhance the microwave absorption intensity effectively and stacking aligned CNT films showed best absorption performance when the intersectional angle between two neighborhood films was 45°, since enhancement of polarization and balance between reflection and conversion of the electromagnetic wave at 45° [21, 37]. In this study, we found that the orientational feature of stacked fabric also affected their absorption properties. The stacked aligned multi-layers fabric with isotropic feature showed better absorption ability than the anisotropy ones. Figure S10 showed the EMI shielding performance of fabric with intersectional angles of 45, 60,

20

and 90° with 6 layers. It was found that the one with intersectional angle of 60° but not 45° showed the best absorption properties. These results may since that fabric with intersectional angle of 60° showed dominant isotropic feature after overlapping 6 layers. As shown in Fig. S11, the SAXS image of 6 layers fabric with intersectional angle of 60° was circle. This isotropic feature has been proved to be beneficial for the microwave absorption from a viewpoint of polarization[21, 41, 42]. When the intersectional angle was 45°, the fabric was isotropic after overlapping 4 or multiple of 4 layers. As shown in Fig. 5 the SAXS image of the aligned CNT-PHEMA fabric showed a circle after overlapping four layers with 45°, implying its isotropy. Considering that the optimal intersectional angle was 45°, the layer numbers of CNT-PHEMA fabric were multiples of 4 for the EMI shielding measurement. It also can be seen from Fig. 3c, d, EMI shielding performance increased as the number of aligned non-woven fabrics layer increasing. These results may due to the fact more layers fabric involved more CNTs in preventing the invasion of wave radiation. The unique porous cross-stacking aligned structure endowed the CNT-PHEMA good EMI shielding property even at very low CNT loading amount (0.17wt%). The sample with 16 layers exhibited 20.42 dB EMI effectiveness at 11.3 GHz, which can already satisfy the use of commercial materials.

21

Fig. 5 Schematic diagram of overlap for magnetic-field-assisted aligned non-woven fabrics and corresponding SAXS characterization. (bar: 1cm)

3.4 Self-healing properties of multilayer overlapping non-woven fabrics Unlike other porous EMI shielding materials, aligned CNT-PHEMA fabrics developed by this paper showed self-healing properties due to the 22

host-guest interaction between β-CD and Ad. The scratches on the fabrics can be self-healed well under 100% humidity. LSCM was used to evaluate the self-healing capacity of fabric visually. As shown in Fig. 6a, the crack caused by scratching disappeared under 100% humidity in 24 h. The self-healing behaviour also was confirmed by SEM, as seen from Fig. S12. Fig. 6b showed the self-healing mechanism. Many β-CD/Ad links were broken at the fracture surface when the fabric got scribing. Under high humidity, fibres were expanded slightly, making both sides of scratches may contact each other, and then exposed β-CD and Ad groups on the two sides reformed inclusion complexes across interfaces. As a result, the scratches were repaired. The interface still existed to some extent (Figure S12), which may be due to the fact that not only reversible interactions were broken, and many covalent bonds were also damaged as the fabric was cut. In contrast, the non-woven fabric obtained by directly mixing PHEMA with SWCNTs cannot self-heal scratch in the humidity environment due to lack of host-guest interaction (Fig. S13). The EMI shielding materials are usually applied on the surface of the substrate, so they easily get damage in service. After being damaged, the electromagnetic waves could irradiate through the cracks of the fabric and form reflection. The restoration of fabric morphology may also lead to the recovery of EMI shielding performance.

23

Fig. 6 a) LSCM characterization of non-woven fabrics before and self-healing after scratching; b) Self-healing mechanism of non-woven fabric; Variations in c) of Original-16-layer ()* fabric, Cracked-16-layer fabric and Healed-16-layer fabric.(Based on triple test)

+,-./0

Table 1

1

with 16-layer non-woven fabric 16-layer fabric

+,-./0

1

Original

20.39±0.73

Cracked

11.63±2.31

Healed

18.53±1.08

Loss rate

42.66±13.57%

Self-healing efficiency

90.86±3.90%

Figure 6c and Table 1 showed the maximum EMI

values of

aligned CNT-PHEMA non-woven fabric with 16-layers before and after self-healing at 12.4 GHz. It can be seen the maximum

value decreased

42.66±13.57% owing to crack. However, the values recovered to 18.53±1.08 dB after self-healing process. The healing efficiency was 90.86±3.90%. These results indicate that the self-healing process not only repair the appearance of

24

fabric, but also recover their EMI effectiveness well. The EMI shielding performance did not recover completely may be due to the fact that the crack was not fully healed and the interface existed. 4. Conclusion In this paper, we developed a kind of flexible and porous EMI shielding fabric, which had combination of good EMI absorption properties and self-healing ability, by preparing aligned CNT-PHEMA non-woven fabric through magnetic-field-assisted electrospinning and cross-stacking aligned fabric. CNT alignment and porosity of fabric enable that the materials can effectively absorbing EMI even at very low CNT loading amount (0.17wt%). The EMI shielding effectiveness reached 20.42 dB at 11.3 GHz for sample with 16 stacking layers under thickness of 2 mm, which can shield 99.99% EM at high frequencies in X-band and satisfy commercial grade. The host-guest interaction between CD and Ad matrix not only made fabric can adhere together automatically but also self-heal the scratch under 100% humidity environment. After self-healing, the EMI shielding property can be restored 90.86±3.90%. This self-healing EMI shielding fabric shows great potential in military and civil applications.

Conflicts of interest The authors declare no competing financial interest.

25

Acknowledgements This work was funded by the National Natural Science Foundation of China (Grant No. 51573187).

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Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: