Robust Alternating Multilayered Cellulose Nanofiber Composite Membranes Fabricated by High-Efficiency Ca 2+ -Precomplexed Vacuum-Assisted Self- Assembly towards Electromagnetic Interference Shielding

doi:10.21203/rs.3.rs-3249220/v1

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Research Article

Robust Alternating Multilayered Cellulose Nanofiber Composite Membranes Fabricated by High-Efficiency Ca 2+ -Precomplexed Vacuum-Assisted Self- Assembly towards Electromagnetic Interference Shielding

https://doi.org/10.21203/rs.3.rs-3249220/v1

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published 23 Jan, 2024

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One-dimensional cellulose nanofiber (CNF) is prone to constructing connected nanofiber networks in vacuum-assisted self-assembly due to intense hydrogen bonds, which exhibits great superiority in fabricating electromagnetic interference (EMI) shielding composite membranes. However, time-consuming vacuum-assisted assembly process creates vast bottlenecks for spreading EMI shielding CNF composites. Herein, a Ca²⁺-precomplexed vacuum-assisted self-assembly strategy is first proposed to high-efficiently assemble robust CNF&Carbon nanotube/Polyethylene oxide (CNF&CNT/PEO) composite membranes with alternating multilayer architectures. The introduction of Ca²⁺ pre-complexation can not only largely improve the fabrication efficiency but also immensely enhance the mechanical properties of alternating multilayered CNF&CNT/PEO membranes. The self-assembly time of CNF-5&CNT/PEO-4 is greatly decreased to 756 min at 0.45 mmol/L Ca²⁺, in comparison with the common vacuum-assisted filtration (i.e., without Ca²⁺) of 1192 min, owing to the hydrogen bonds between CNF and water broken down by Ca²⁺ complexation. With the Ca²⁺ concentration increasing from 0 to 0.23 further to 0.45 mmol/L, the tensile strength of CNF-3&CNT/PEO-2 is gradually reinforced from 40.2 to 43.2 and to 47.6 MPa, as a consequence of the excellent Ca²⁺ crosslinking and hierarchical “Zigzag” crack paths. The favorable electrical conductivity and unique alternating multilayered structures endow CNF-5&CNT/PEO-4 membrane with the maximal EMI shielding effectiveness (SE) of 43.3 dB. The Ca²⁺-precomplexed strategy sheds new light on high-efficient self-assembly of multilayered CNF composite membranes with pretty EMI shielding performances, which has prospective application in advanced electronics and microelectronic packaging.

Electromagnetic Interference Shielding

Alternating Multilayer

Ca2+-Precomplexed Vacuum-Assisted Self-Assembly

Mechanical Property

Fabrication Efficiency

Following the fast development of advanced electronic technology, electromagnetic radiation pollution has become a serious issue [1–4]. The need for high-efficient electromagnetic interference (EMI) shield is becoming more urgent to prevent undesirable electromagnetic radiation from disturbing the normal operation of electronic equipment and endangering human health [5–8]. To reduce electromagnetic pollution, traditional metals with superb electrical conductivity and shielding effectiveness (SE) are commonly used as electromagnetic shielding materials, but their application in the new generation of miniaturized and integrated electronic products is largely limited, due to the deficiencies of metal materials such as easy corrosion, high density, and non-flexibility [9, 10]. Nowadays, conducting polymeric composites (CPCs) have sparked vast attention from researchers, which is attributed to their lightness, anti-corrosion, low cost, and stability [11].

As most polymers are insulators in general, it is essential to incorporate conductive fillers into polymer matrix and construct an excellent conductive network to attenuate electromagnetic waves and elevate EMI SE [12]. Therefore, a variety of conductive fillers (e.g., graphene [13, 14], carbon nanotubes (CNT) [15, 16], reduced graphene oxide (rGO) [17, 18], metal nanowires [19], and MXenes [20–22]) are applied to prepare the EMI shielding materials. For instance, Wang et al. [23] fabricated PDA@CNTs/EVA composite membranes with high flexibility and mechanical fatigue resistance, by coating CNTs with Polydopamine (PDA) and then mixing them with ethylene-vinyl acetate (EVA) copolymer. The EMI SE of PDA@CNTs/EVA membrane reached 32.4 dB at 70 wt% filler content, whereas the overall performance of composite membrane was greatly dominated by PDA coating effect. Qian et al. [24] combined MXene with hexagonal boron nitride (h-BN) by ultrasonic blending to prepare MXene/h-BN hybrid membranes. The composite membrane with a thickness of 47.60 microns had a conductivity of 57.67 S/cm and a maximal EMI SE of 37.29 dB at the mass ratio (MXene:h-BN) of 7:3. In order to achieve excellent EMI SE, a large quantity of conductive fillers are usually added into the polymer matrix, which inevitably results in an increase of the composite thickness and a deterioration of the mechanical properties and processibility [25, 26].

Furthermore, EMI SE is also significantly impacted by the structural layout of electromagnetic shielding materials [12]. Many types of structures, including sandwich [27, 28], isolation [29, 30], gradient [31, 32], and foam structures [33, 34] have recently been used in EMI shielding. The investigation results of Cheng et al. [35] showed that a lightweight porous carbon aerogel with a density of only 11.9 mg cm^− 3 was obtained by freeze-drying and heat treatment of a chitosan (CS)/MXene hydrogel. the EMI SE of 61.4 dB was observed for the porous carbon aerogel at 0.13 vol% MXene content and a thickness of 3 mm. However, the large volume of foam-structured shielding materials makes it very difficult to apply them in tiny electronic devices [36–38]. In an effort to fabricate thinner materials with excellent EMI SE, researchers have focused on composite materials with layer by layer construction [39, 40]. For example, Yang et al. [41] prepared composite membranes with alternating multilayer structures of individual aramid/polypyrrole nanofibers (AFPy) and MXene nanosheets/water-soluble polyurethane (WPU) layers by using alternating vacuum self-assembly and then thermocompression. The best-performing sample had an EMI SE of 40.9 dB, which was far higher than that of the directly mixing sample at the same thickness. Such results demonstrate that the alternating conductive layer structure can enhance the multiple reflections of EMWs, lengthen their propagation paths, and promote the dissipation of EMWs energy into thermal energy [42, 43].

Apart from EMI shielding performances, the mechanical property is also one of the most significant factors for the actual utilization of EMI shields. Cellulose nanofibers (CNF), which are environmentally friendly, biodegradable, high-crystallized, and one-dimensional, are derived from biomass cellulose and extensively employed as polymer matrix to fabricated EMI shielding composites [44, 45]. Yuan et al. [46] constructed CPCs with a double-layered structure by vacuum self-assembly and hot-compression, in which one layer is the derivative porous carbon (C-ZIF67)/cellulose nanofiber (CNF) mixture and the other is graphene nanoplate (GNP)/CNF layer. The results showed that the EMI SE of double-layer structured composite membrane at 0.1 mm thickness achieved to 50.5 dB in X-band frequency range, and the tensile strength was as high as 46.33 MPa, which was attributed to the construction of double-layer networks and the high performance of CNF. Evidently, the presence of CNF network provides a strong reliance for conductive fillers as a benefit for the reinforcement of mechanical properties. Moreover, CNF has an intrinsic trend to assemble connected networks by vacuum filtration, which is prone to the improvement of electrical conductivity and EMI SE.

To enable more convenient application of CNF as a matrix for CPCs, vacuum-assisted filtration has become one of the most commonly employed methodologies [47]. Nonetheless, the strong hydrogen bonds between CNF and water, due to the abundant hydrophilic groups on CNF chains such as -OH and -COOH, largely augment the difficulty of separating CNF from aqueous solution, which eventually causes low preparing efficiency and/or unsatisfactory structures of CPCs. Therefore, how to enormously enhance the fabricating efficiency of vacuum-assisted self-assembly on the promise of maintaining excellent mechanical properties and multi-functions of CNF composites is still a great challenging task, which is rarely mentioned by researchers at present.

In this investigation, the robust multilayer membranes composed of alternating CNF layers as supporting layers and CNT/PEO layers as conductive layers were successfully fabricated via a novel high-efficiency Ca²⁺-precomplexed vacuum-assisted self-assembly approach. The introduction of Ca²⁺ pre-complexation can not only largely improve the fabrication efficiency in vacuum-assisted self-assembly of CNF composites but also immensely enhance the mechanical performances of alternating multilayer CNF&CNT/PEO membranes. Moreover, CNF&CNT/PEO membranes with alternating multilayer constructions exhibit the definite advantages in EMI shielding, in contrast to CNF/CNT/PEO membranes with homogeneous structures, due to the “Zigzag” multi-reflection principle and the eminent electrical conductivity of CNT/PEO layers. The Ca²⁺-precomplexed vacuum-assisted infiltration strategy makes a great progress toward high-efficient fabrication and construction of robust multilayered EMI shielding CNF composite membranes, which could be envisioned to display immense implications for flexible smart electronics and next-generation intelligent devices.

2.1 Materials

Cellulose nanofibers (CNF) dispersion with a certain concentration (~ 3.2 wt%) was obtained from Zhongshan NanoFC Bio-material Film (China). Carbon nanotube (CNT) with the purity > 90% was purchased from Organic Chemicals Institution (Chengdu, China). CNT indicates the length of ~ 30 µm and a mean diameter of 15–25 nm. Polyethylene oxide (PEO, $\stackrel{-}{{M}_{\nu }}$=1.0 ×10⁶ g/mol) and anhydrous calcium chloride (CaCl₂) were supplied by Macklin Biochemical Merchant (Shanghai, China). Distilled water used in this study was obtained in the laboratory. All chemicals were used without further purification.

2.2 Preparation of CNF&CNT/PEO membranes

The alternating multilayer CNF&CNT/PEO membranes were obtained by a multilayer self-assembly route with the aid of Ca²⁺ pre-complexation under alternating vacuum filtration conditions, and the preparation process is shown in Fig. 1. A certain amount of CNF dispersion was added to the configured CaCl₂ solution (0.45 mmol/L) and mechanically stirred for 24 h to obtain a homogeneous state with a CNF concentration of 1 mg/mL. In addition, CNT/PEO dispersion was prepared by dissolving 10 mg PEO into 100 ml CaCl₂ solution (0.45 mmol/L) with continuous stirring for 30 min and subsequently dispersing 100 mg CNT under ultrasonication (800 W) for 10 min. Then, the separate CNF dispersion (20 mL) and CNT/PEO dispersion (20 mL) were alternately filtered in a fixed vacuum (-0.08 MPa). The cellulose acetate filtering paper has a 47 mm diameter and 0.22 µm pore size. Here, the CNF is settled as exterior layers to secure interior CNT/PEO layers from abrasion. For the sake of simplicity, the prepared multilayer-structured composite membranes are named as CNF-n + 1&CNT/PEO-n, where n + 1 and n denote the layer number of CNF and CNT/PEO in the membranes, respectively. For comparison, pure CNF membranes and homogeneous composite membranes were also obtained by the vacuum filtration of the mixing CNF and CNT/PEO dispersions, where Homogeneity-3, Homogeneity-5, Homogeneity-7 and Homogeneity-9 correspond to 3, 5, 7, and 9-layer CNF&CNT/PEO, respectively.

In order to investigate the role of Ca²⁺ in the composite membranes, the selected CNF-3&CNT/PEO-2 and Homogeneity-5 with different CaCl₂ concentrations (0, 0.23, 0.45, and 0.68 mmol/L) were prepared via the similar process. All the samples were stored in a vacuum dryer at 35°C for 24 hours, after which they were peeled off from the filter papers.

2.3 Characterizations

The morphology of CNF&CNT/PEO membranes was viewed on Scanning electron microscope (SEM, ZEISS gemini360, Germany) accompanied by energy dispersive spectrometer (EDS). The lattice structures of CNF&CNT/PEO membranes were detected by X-ray diffractometer (XRD, Rigaku Smartlab, Japan). The electrical conductivity of the membranes was measured by a four-pin probe (Signatone, USA). A vector network analyzer (VNA, E5071C, Agilent Technologies, USA) was used to investigate EMI SE of the samples by waveguide method at the frequency between 8.2 and 12.4 GHz (X-band). Prior to testing, the samples were cut into a rectangular shape (22.86 mm × 10.16 mm). Based on the scattering factors (S₂₁ and S₁₁) obtained by VAN, the power coefficients of reflection (R), transmission (T), and absorption (A), the total EMI SE (SE_T), reflective EMI SE (SE_R), absorptive EMI SE (SE_A), and multi- reflections (SE_M) of the samples can be derived from the following equations:

$$R={\left|{S}_{11}\right|}^{2}and T={\left|{S}_{21}\right|}^{2}$$

$$A=1-T-R$$

$${SE}_{R}=10\text{log}\left(\frac{1}{1-R}\right)$$

$${SE}_{A}=10\text{log}\left(\frac{1-R}{T}\right)$$

$${SE}_{T}=10\text{log}\left(\frac{1}{T}\right)={SE}_{A}+{SE}_{R}+{SE}_{M}$$

where SE_M is usually negligible at SE_T≥15 dB. The tensile performances of CNF&CNT/PEO membranes were characterized on a universal tester (M1176, Instron, America) with a tensile speed of 1 mm/min. Each sample was tested at least 5 times for the average value.

3.1. Microstructures and morphology

As shown in Fig. 2a, CNT/PEO is stably dispersed in the (0.45 mmol/L) CaCl₂ solution after sonication. The SEM image of the monolayer CNT/PEO membrane (Fig. S1a) reveals that CNTs are tightly entangled together, because of the robust hydrogen bonds and orientation forces between PEO and CNT. In Fig. 2b, it is clearly shown that CNF is homogeneously dispersed in the CaCl₂ solution (0.45 mmol/L), exhibiting a typical Tyndall effect. In the SEM image of pure CNF (Fig. S1b), CNF is compactly stacked together to form a uniform and smooth membrane structure.

Figure 2c and 2d show the SEM images of CNF-2&CNT/PEO-1 without and with the addition of Ca²⁺ (0.45 mmol/L), respectively. The CNF-2&CNT/PEO-1 membrane without Ca²⁺ displays severe defect and fragmentation, making it very difficult to continue increasing the number of layers in the composite membrane. Nevertheless, the introduced Ca²⁺ can act as a bridge to enhance the interaction between CNT and CNF, and thus gives a more complete membrane morphology.

The alternating multilayer microstructures of CNF&CNT/PEO membranes at Ca²⁺ concentration of 0.45 mmol/L are depicted in Fig. 2e-h, in which the different morphologies of the two adjacent layers of CNF and CNT/PEO can be clearly identified owing to the diverse intrinsic properties. Moreover, both CNF and CNT/PEO layers show tightly packed lamellar arrangement, as a result of the strong interactions in the layers such as hydrogen bonds and “Ca-O” complexation bonds [48]. CNT forms a continuous conductive network under the protection of the supporting CNF layer, which facilitates the attenuation of electromagnetic waves. The EDS mappings shown in Fig. 2i-k illustrate the regular distribution of C, Ca, and O elements in the multilayers, further validating the assembly of perfect alternating multilayers and compactly stacked structures in the composite membranes.

The crystal structures of CNF&CNT/PEO membranes and its ingredients were explored by XRD analysis, as shown in Fig. 3a. Two prominent peaks located at 22.6° and 14.8° for the pristine CNF membrane are attributed to (020) and (110) cellulose crystal planes, respectively. CNT shows the sharp diffraction peaks at 26.1° and 43.2°, corresponding to the (002) crystal plane and (100) crystal plane, respectively. For CNT/PEO membrane, new weak peaks appear at 16.5° and 22.5°, which are attributed to (120) and (112) planes of PEO, respectively. the overall featured peaks can also be traced in XRD profiles of the CNF&CNT/PEO membranes (Fig. 3b), which elucidates the crystallographic structures of CNF, CNT, and PEO are pretty preserved in the Ca²⁺-precomplexed vacuum-assisted filtration process.

3.2. EMI shielding properties

It is well known that the EMI shielding properties of electrically conductive materials are highly correlated with their electrical conduction [49]. Naturally, the monolayer CNF membrane is non-conductive with a high resistance value of 7.9×10⁸ Ω·m, which is consistent with the previous report [50]. As Fig. 4a shows, the conductivity of composite membranes is significantly improved with the incorporation of CNT/PEO layer. As an example, the conductivity of CNF-2&CNT/PEO-1 reaches up to 961 S/m, which is 900 times more than the actual EMI shielding material requirement (1 S/m) in practical applications [51]. The tightly in-plane arranged CNT/PEO layer provides an interlinking conductive network for CNF&CNT/PEO membranes, which plays a decisive role in improving the electrical conductivity. The conductivity of CNF-5&CNT/PEO-4 membrane is gradually enhanced to 1513 S/m with the growing number of CNT/PEO layers, which is mainly attributed to the rising proportion of CNT in the whole membrane and the more compact lamellar structure.

As seen in Fig. 4b, pure CNF membrane has almost no shielding effect on electromagnetic waves in X-band, and the EMI SE is only 0.1 dB, which is in correlation with its inherently electrically insulating property. With the amalgamation of highly conductive CNT/PEO layers, CNF&CNT/PEO membranes illustrate excellent EMI SE (> 20 dB), satisfying the requirement of industrial EMI shielding commodity [52]. For instance, the EMI SE of CNF-2&CNT/PEO-1 in X-band is almost independent from frequency with the maximal value of 32.6 dB. The maximum EMI SE of CNF-5&CNT/PEO-4 is largely augmented to 43.3 dB following the increase of layers, which is parallel to the variation of electrical conductivity. CNF-5&CNT/PEO-4 membrane exhibits an optimal shielding efficiency of 99.995%, which means that only 0.005% of EMWs can be transmitted through CNF-5&CNT/PEO-4 membrane with 99.995% attenuation.

In view of a more profound explanation of the shielding mechanism of CNF&CNT/PEO membranes, the average total EMI SE (SE_T), absorptive SE (SE_A), and reflective SE (SE_R) are figured out and depicted in Fig. 4c. It is easy to find that both SE_A and SE_R show a significant increase with the incremental total number of layers, which is due to the polarization loss caused by the increase of the polarization interface and the “zigzag” multi-reflection theory resulting from the alternating multilayer structure [53]. The SE_A and SE_R of CNF-5&CNT/PEO-4 membrane are 26.4 and 14.3 dB, respectively, showing that SE_A accounts for 64.9% of SE_T. Natheless, the power coefficients (Fig. 4d) display that R coefficient (> 0.9) is far larger than A (< 0.1) for all the alternating multilayer membranes, which indicates that the EMI shielding theory of CNF&CNT/PEO membranes is a reflection-dominated loss theory. This phenomenon is mainly on account that most of the EMWs are directly reflected back when they touch the surface of the shielding material, whereas only a small fraction of EMWs can traverse into the interior of the material [54]. Additionally, it is apparently discovered that A coefficient tends to drop slightly as CNT content increases, which is attributed to the enhanced impedance mismatch.

To investigate whether the introduction of Ca²⁺ has an influence on the shielding effect of CNF&CNT/PEO membranes, CNF-3&CNT/PEO-2 membranes with different Ca²⁺ concentrations are also fabricated, and the EMI shielding properties are shown in Fig. 5. It is obviously seen that the electrical conductivity of CNF-3&CNT/PEO-2 membranes changes little in the range of 1030–1100 S/m with the increasing Ca²⁺ concentration (Fig. 5a), indicating the Ca²⁺ pre-complexation has little influence on the construction of electrical conduction pathway. The EMI shielding properties of CNF-3&CNT/PEO-2 membranes, including SE_T, SE_A, SE_R, A and R, aren’t nearly affected by the Ca²⁺ concentration (Fig. 5b-d), which is in great consistence with the electrical conductivity. Moreover, the thickness of CNF-3&CNT/PEO-2 membranes is slightly reduced from 99.6 µm to 87.4 µm with the incremental Ca²⁺ concentration (Fig. S4b), owing to the enhanced multilayer interaction and compact layered structures. For further investigation, the EMI shielding performances of pure CNF and alternating multilayered membranes without Ca²⁺ pre-complexation are also exhibited in Fig. S2. The conductivity and EMI shielding performances of alternating multilayer CNF&CNT/PEO membranes with the same layer number exhibit little difference at the Ca²⁺ concentration of 0 and 0.45 mmol/L, in good agreement with the analysis of Fig. 5. For instance, SE_T of CNF-5&CNT/PEO-4 membrane is 40.7 and 38.5 dB at the Ca²⁺ concentration of 0.45 and 0 mmol/L, respectively.

To demonstrate the superiority of alternating multilayer membranes in EMI shielding, the CNF/CNT/PEO composite membranes with a homogeneous structure were obtained as a control pair. In Fig. S3a, it is obvious that the electrical conductivity of homogeneous membranes also shows the enhancing tendency but is slightly lower than that of multilayered membranes at the same component contents. For example, Homogeneity-7 exhibits the electrical conductivity of 1207 S/m, which is lower than that of the corresponding CNF-4&CNT/PEO-3 (1513 S/m). Similarly, SE_T of Homogeneity-7 (31.1 dB) is explicitly smaller than that of NF-4&CNT/PEO-3 (38.6 dB), on account that EMWs are reflected many times during the transmission in multilayer membranes and the propagation path is prolonged (Fig. S3b-d). Taking the effect of the thickness on EMI shielding into consideration, the thickness of multilayer and homogeneous membranes is also tested and presented in Fig. S4a. Although the thickness of multilayered membranes is found to be moderately thicker than that of homogeneous membranes with the same ingredients, specific SE, SE/t (where t denotes the thickness), of multilayered membranes is obviously greater than that of homogeneous membranes, illuminating the advantage of alternating multilayered architectures to the homogeneous structures in EMI shielding performances.

To further interpret the EMI shielding theory, the transmission process of EMWs across the alternating multilayer membranes is intuitively modalized in Fig. 6. When EMWs reach to the outer CNF layer, only a small portion of EMWs are reflected due to the electrical insulation of CNF layer. Then, a large part of the transferred EMWs are reflected back at the CNF-CNT/PEO interface, because there are significant impedance mismatches between CNF and CNT/PEO layers [55, 56]. A handful of remaining EMWs entering the CNT/PEO layer are continuously consumed by immense polarization loss and conduction loss, due to the high-density electron carriers in the highly conductive networks. Afterwards, the EMWs are reflected and scattered several times between adjacent conducting CNT/PEO layers, by virtue of its unique alternating multilayer structure. Such multiple reflection can extend the transmission path and promotes the dissipation of the residual EMWs in the form of thermal energy. As a consequence, one can conclude that the eminent EMI shielding property of CNF&CNT/PEO membranes is mainly derivative from the fascinating alternating multilayer structures.

3.3. Fabrication efficiency

The fabrication time of multilayered hybrid membranes which is an important factor in the layer-by-layer self-assembly process can be significantly shortened by Ca²⁺ pre-complexation. The fabrication time of alternating multilayered membranes with and without the addition of Ca²⁺ (0.45 mmol/L) is shown in Fig. 7a. It can be obviously seen that the required time for preparing multilayer membranes is immensely lengthened with the increasing layer number. The preparation time of multilayered CNF-2&CNT/PEO-1 without Ca²⁺ is 204 min, while CNF-5&CNT/PEO-4 membrane without Ca²⁺ pre-complexation exhibits the largely increased preparation time of 1192 min with the growing layer number. In addition, the consumed time of alternating multilayered CNF&CNT/PEO is sharply reduced with the introduction of 0.45 mmol/L Ca²⁺. For instance, the filtration time of the Ca²⁺ pre-complexed CNF-5&CNT/PEO-4 is greatly decreased to 756 min, indicating the saving time of 436 min and 36.6% enhancement in fabrication efficiency. The most probable reason is that a large number of polar groups including hydroxyl, carboxyl, and ester in CNF (CNT) chains can form strong interactions with H₂O such as hydrogen bond and Van der Waals’ force, which retards the separation of CNF (CNT) from water to some extent. Under Ca²⁺ pre-complexation, the new ionic bond and complex bond will be constructed between CNF (CNT) and Ca²⁺, which will break the hydrogen bonds and largely weaken the intermolecular interaction between CNF (CNT) and H₂O (Fig. 7c). Thus, CNF and CNT are faster filtrated under vacuum with Ca²⁺ pre-complexation.

The Ca²⁺ concentration can largely influence the fabrication efficiency of vacuum-assisted filtration of CNF composite membranes. According to Fig. 7b, the fabrication efficiency of CNF-3&CNF/PEO-2 is largely improved with the increasing Ca²⁺ concentration. The filtration time of CNF-3&CNF/PEO-2 is reduced from 439 min without Ca²⁺ pre-complexation to 249 min at 0.68 mmol/L Ca²⁺. Ca²⁺ could complex some CNF chains and break some hydrogen bonds between CNF and water, enabling CNF to separate easily from water. In addition, the filtration time of homogeneous CNF/CNT/PEO membranes is also immensely decreased with the introduction of Ca²⁺ (Fig. S5). Homogeneity-9 exhibits the fabrication time of 376 min at 0.45 mmol/L Ca²⁺ but 607 min without Ca²⁺, indicating the 38.1% enhancement of preparation efficiency. What’s more, the preparation time of homogeneous membranes is observably smaller than that of multilayered membranes, due to the complicated and tedious procedures. Overall, the introduction of Ca²⁺ pre-complexation could greatly elevate the fabrication efficiency in vacuum-assisted self-assembly process.

3.4. Mechanical properties

In addition to the wonderful conductivity and EMI SE, prominent mechanical performances also have great significance for the promising prospect of EMI shielding membranes. Fig. S6a shows the representative stress-strain curves of pure CNF membrane, and the tensile strength and fractured strain of pure CNF membrane decreases slightly with the addition of Ca²⁺ (Fig. S6b), as a consequence of the stress concentration points produced by the agglomeration of CNF with Ca²⁺ pre-complexation. Besides, the mechanical properties of alternating multilayered CNF composite membranes at 0.45 mmol/L Ca²⁺ are shown in Fig. 8a and d. All the alternating multilayered membranes illustrate the brittle fracture without yielding and necking. The tensile strength and strain at break of layered CNF&CNF/PEO membranes at 0.45 mmol/L Ca²⁺ exhibit a slight decreasing trend with the rising layer number, because of the reducing CNF loading fraction and the increasing interfaces. For instance, the tensile strength and breaking elongation of CNF-5&CNT/PEO-4 are decreased to 34.8 MPa and 4.4%, respectively. Nevertheless, CNF-5&CNT/PEO-4 has the superb flexibility and can be folded without cracks and/or factures (Fig. S9), which is vital to the application in bendable electronic devices.

As a control, the mechanical properties of multilayered CNF&CNF/PEO membranes without Ca²⁺ were also detailedly characterized and shown in Fig. S7. A moderate declining tendency in the tensile strength and fracture strain with the increase of layer number are also observed for multilayered CNF&CNF/PEO membranes at 0 mmol/mL Ca²⁺. Moreover, the mechanical properties of multilayered CNF&CNF/PEO membranes at 0 mmol/mL Ca²⁺ are mildly worsened in comparison with the corresponding multilayered membranes at 0.45 mmol/L Ca²⁺. Exemplified by CNF-5&CNT/PEO-4 membrane, the tensile strength and fracture strain are reduced to 31.4 MPa and 3.3% without Ca²⁺ pre-complexation, respectively. This indicates that the introduction of Ca²⁺ can effectively reinforce the stretching properties of the multilayered CNF composite membranes, owing to the largely enhanced interfacial interaction via Ca²⁺ crosslinking.

Figure 8b and e describe the tensile performances of the homogeneous CNF/CNT/PEO membranes at Ca²⁺ concentration of 0.45 mmol/L. The homogeneous membranes are also brittlely fractured with quite finite strain. It is similar to alternating multilayered membranes that the tensile strength and breaking strain of homogeneous membranes are continuously reduced from Homogeneity-3 to Homogeneity-9. The minimal tensile strength of 42.2 MPa and fracture strain of 4.5% are achieved for Homogeneity-9 with 0.45 mmol/L Ca²⁺. Notably, the mechanical properties of homogeneous membranes are superior to the corresponding multilayered membranes, arising from the absence of weak CNF-CNT/PEO layer-to-layer interfaces. Likewise, the tensile properties of homogeneous membranes at 0.45 mmol/L Ca²⁺ are also higher than those without Ca²⁺ pre-complexation (Fig. S8). Homonegeity-9 at 0 mmol/L Ca²⁺ shows the lower tensile strength of 38.1 MPa and fracture strain of 6.2%.

For sake of further investigating the impact of Ca²⁺ on the tensile performances of alternating multilayered membranes, Fig. 8c and f depict the mechanical properties of CNF-3&CNT/PEO-2 with different Ca²⁺ concentration. It is obviously observed that the tensile strength of CNF-3&CNT/PEO-2 is gradually enhanced from 40.2 to 43.2 further to 47.6 MPa, as the Ca²⁺ concentration increases from 0 to 0.23 and to 0.45 mmol/L, on account of the excellent Ca²⁺ crosslinking. However, the tensile strength of CNF-3&CNT/PEO-2 at 0.68 mmol/L Ca²⁺ is largely reduced to 40.3 MPa, which can be explained by the fact that excessive Ca²⁺ pre-complexation may lead to the agglomeration of CNF and induce the stress concentration points.

To visually elucidate the fracture mechanism of Ca²⁺ pre-complexation alternating multilayered membranes, the crack extension process under tensile action is vividly exhibited in Fig. 8g. When the multilayer composite membrane is subjected to tensile loading, the brittle CNT/PEO layer will rapture first owing to the finite strain and weak intermolecular interaction. Nevertheless, the neighboring strong protective CNF layers hinder the crack extension, which resists the tensile stress and protects the whole membrane from fracture. With the further increase of tensile stress, the cracks permeate into CNF layers in a nano-“Zigzag” path, triggering the long-chain CNF molecules to stretch in the tensile direction until fracture. Meanwhile, the cracks also slide along the CNF-CNT/PEO interlayer interfaces, inducing the micro-“Zigzag” crack path. Such a multilevel crack extension mechanism is able to dissipate a large amount of tensile energy during tension process, in aid of the substantial enhancement in the mechanical properties. Moreover, the Ca²⁺ cross-linking greatly improves the interfacial interaction between CNF and CNT/PEO and thus prompts the stress transfer and fracture energy absorption in the multilayer hybrid membranes.

In summary, the fabrication efficiency of vacuum-assisted self-assembly for CNF aqueous dispersion is highly improved for the first time with the introduction of Ca²⁺. Alternating multilayered CNF&CNT/PEO membranes were successfully assembled by a feasible and efficient Ca²⁺-precomplexed alternating vacuum-assisted self-assembly methodology. The introduction of Ca²⁺ pre-complexation greatly reduces the filtration time of CNF-5&CNT/PEO-4 to 756 min at 0.45 mmol/L Ca²⁺, revealing 36.6% time decrease in comparison with that of CNF-5&CNT/PEO-4 without Ca²⁺ pre-complexation. Besides, the tensile strength of CNF-3&CNT/PEO-2 is gradually enhanced from 40.2 to 43.2 further to 47.6 MPa, as the Ca²⁺ concentration increases from 0 to 0.23 and to 0.45 mmol/L, on account of the excellent Ca²⁺ crosslinking and hierarchical “Zigzag” crack paths. Due to the excellent conductivity of CNT/PEO layers and multilayered structures, CNF-5&CNT/PEO-4 exhibits the maximum EMI SE of 43.3 dB, indicating the effective 99.995% blocking of EMWs. Strikingly, the introduction of Ca²⁺ has little influence on the EMI shielding performances. This high-efficient fabrication strategy provides valuable insights into designing and fabricating robust multilayered CNF composite membranes for EMI shielding in wearable or portable electronic devices.

Supporting Information

SEM images of CNT/PEO and CNF membranes; EMI shielding performances of CNF&CNT/PEO membranes without Ca²⁺ and homogeneous CNF/CNT/PEO membranes with 0.45 mmol/L CaCl₂; Thickness of CNF&CNT/PEO and CNF/CNT/PEO membranes; Preparation time for homogeneous CNF/CNT/PEO membranes; Mechanical properties of pure CNF, CNF&CNT/PEO membranes, and CNF/CNT/PEO membranes without Ca²⁺; Digital photograph of bending CNF-5&CNT/PEO-4.

Author Contribution

Lingjun Zeng completed experimental performance and characterization, prepared Fig. 1-8, and Fig. S1-9, conducted data analysis, and wrote the main original manuscript text; Bai Xue designed the experiment, revised the grammar and expression of the manuscript, updated the formal analysis, finished the funding acquisition; Changmei Wu tested mechanical properties; Wenjing Qi prepared Fig. 7c; Peng Ai prepared Fig. 6; Lan Xie checked the manuscript, corrected the data analysis, and finished the funding acquisition; Qiang Zheng supervised the authenticity of this work. All authors reviewed the manuscript.

Funding

This work was supported by National Natural Science Foundation of China (No. 51963003 and 52263003); Guizhou Provincial Science and Technology Projects (No. ZK[2022]Maj019 and [2020]1Z044).

Data availability:

Data will be made available on request.

Conflicts of interest

There are no conflicts to declare.

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No competing interests reported.

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Robust Alternating Multilayered Cellulose Nanofiber Composite Membranes Fabricated by High-Efficiency Ca 2+ -Precomplexed Vacuum-Assisted Self- Assembly towards Electromagnetic Interference Shielding

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Abstract

Figures

1. Introduction

2. Experimental section

2.1 Materials

2.2 Preparation of CNF&CNT/PEO membranes

2.3 Characterizations

3. Results and discussion

3.1. Microstructures and morphology

3.2. EMI shielding properties

3.3. Fabrication efficiency

3.4. Mechanical properties

4. Conclusions

Declarations

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1