Ultralight cellulose porous composites with manipulated porous

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Ultralight cellulose porous composites with manipulated porous structure and carbon nanotube distribution for promising electromagnetic interference shielding Liang-Qing Zhang, Shu-Gui Yang, Lei Li, Biao Yang, Hua-Dong Huang, Ding-Xiang Yan, Gan-Ji Zhong, Ling Xu, and Zhong-Ming Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14738 • Publication Date (Web): 01 Nov 2018 Downloaded from http://pubs.acs.org on November 2, 2018

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ACS Applied Materials & Interfaces

Ultralight Cellulose Porous Composites with Manipulated Porous Structure and Carbon Nanotube Distribution for Promising Electromagnetic Interference Shielding

Liang-Qing Zhang, Shu-Gui Yang, Lei Li, Biao Yang, Hua-Dong Huang, Ding-Xiang Yan, Gan-Ji Zhong*, Ling Xu* and Zhong-Ming Li

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, China

*Corresponding Authors: *Phone: +86-28-8540-0211. Fax: +86-28-8540-6866. E-mail: [email protected]. (G.-J.Z.) [email protected] (L. X.)

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ABSTRACT Lightweight conductive polymer composites (CPCs) based on biomass could be a promising candidate for electromagnetic interference (EMI) shielding application. Herein, tailoring porous microstructure and regulating the distribution of carbon nanotubes (CNT) in cellulose composites are attempt to achieve highly efficient EMI shielding properties accompanying desired mechanical property and low density. Specifically, aligned porous structure is fabricated by ice-template freeze-drying method, meanwhile, CNT is regulated to decorate inside the cellulose matrix (CNT-matrix/cellulose porous composites) or to directly bind over the cellulose cell walls (CNT-interface/cellulose porous composites). It is found that, owing to the preferentially distribution of CNT on the cell walls, the CNT-interface/cellulose porous composites posse a very high electrical conductivity of 38.9 S/m with an extremely

low

percolation

threshold

of

0.0083

vol%

with

regard

to

CNT-matrix/cellulose porous composites. Therefore, a shielding effectiveness of 40 dB with merely 0.51 vol% CNT under a thickness of 2.5 mm is achieved in CNT-interface/cellulose porous composites, which is attributed to efficient multiple reflections and the accompanying absorption with promoted conductivity and better-defined porous structure. More laudably, the CNT-interface/cellulose porous composites reveal a superior mechanical property with a specific modulus of 279 MPa g-1 cm3. The value behind the current work is to pave an effective way to fabricate environmentally benign, high-performance EMI shielding materials to practically boost numerous advanced applications of cellulose. 2


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KEYWORDS: cellulose porous composite, carbon nanotube, electromagnetic interference shielding, ice-template freezing, structure manipulation

■ INTRODUCTION Owning to increasing demands in electronic reliability, healthcare, and national defense security, electromagnetic interference (EMI) shielding materials are wildly developed. The traditional metal-based shielding materials usually have the problem of high mass density, poor flexibility, undesirable corrosion susceptibility, and limited tuning of shielding effectiveness (SE). Recently, conductive polymer composites (CPCs) with porous structure have stirred up burgeoning interest for applications to dissipate and attenuate electromagnetic radiation, owning to the advantages in lightweight property, chemical stability, shaping ability, material saving and design flexibility.1-4 The lightweight porous structure, typically incorporated with carbon fillers, is responsible for the forming of three-dimensional (3D) conductive networks, which are crucial for the attenuation of the electrical current.5 Especially, the incorporation of voids in CPCs can efficaciously mitigate the impedances mismatch for microwave propagation between the materials and voids, which leads to effective electromagnetic energy attenuation ability to eradicate the secondary pollution caused by the microwave reflection.6-8 Several methods are used to fabricate porous EMI shielding materials, such as, foaming method7,9-11, salt-leaching12, phase separation13, template-directed chemical vapor deposition (CVD) method14, etc. Using foaming method, CPC porous materials incorporated with carbon nanofiber (CNF) and CNT were developed by Gupta and his 3



coworkers, demonstrating a new stage to fabricate light weight EMI shielding materials.9,10 Combining salt-leaching method and high-pressure compression molding, graphene/polystyrene porous composites with excellent EMI shielding performance were fabricated in our previous work.12 Zheng and his coworkers used a phase separation process to prepare polyetherimide (PEI)/graphene composite foam with micropores, indicating lightweight property and superior specific EMI SE.2 Chen’s group developed a graphene/PDMS composite foam by CVD approach, showing enhanced shielding performance with an extremely high specific SE of 500 dB g-1 cm3.14 Although multifarious porous materials with sufficient EMI SE have been achieved on the basis of these methods, porous CPCs with lower density are still required to be further extended to incorporate the superiority of lightweight feature; Furthermore, the present porous EMI shielding materials are basically fabricated based on petrochemical-derived polymers which would cause the diminishment of nonrenewable petroleum and serious environmental pollution, thereby limiting the widespread application of CPC porous materials on EMI shielding. Lightweight and biodegradable properties are crucial to the practical EMI shielding applications. Given the advantages such as biocompatibility, biodegradability, and excellent properties of mechanical, thermal and chemical stability, cellulose, an almost inexhaustible biopolymer resources, has grabbed considerable attention for the fabrication of porous EMI shielding materials in substitute of petroleum-based polymer.15,16 In our previous work, we have successfully prepared lightweight cellulose porous materials with ultra-low density and controllable microstructure 4


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using an environmentally friendly method,17 which helps build solid technological foundation to construct lightweight cellulose EMI shielding materials. After incorporation of CNT, which is known to be one of the most promising nanofillers with excellent electrical property,18 an ultra-low density of 0.095 g cm-3 as well as an EMI SE of 20 dB in the X-band frequency range have been achieved.16 However, a further attempt to increase the SE of cellulose porous composites is still a long-standing challenge, especially when the low density is required. Conventionally, increasing the loading of electric fillers used to be the most versatile strategy to initiate significant enhancement in shielding performance. Nonetheless, the increased filler content will inevitably engender the deteriorated mechanical property of cellulose composites. Therefore, a high EMI SE cellulose-based porous material with lightweight and superior comprehensive performance remains a daunting issue. In pursuit of high performance EMI shielding materials with highly efficient shielding ability as well as low material consumption, and simultaneously maintaining excellent mechanical properties, structure optimization is considered to be the most crucial issue. As typical examples, Zeng and his coworkers have launched a series of pioneering works to explore the anisotropic microstructure of lightweight high performance EMI shielding materials.19,20 Anisotropic microstructure was developed by directional freezing of an aqueous suspension of CNT and waterborne polyurethane (WPU), revealing a high EMI SE of about 50 dB and an ultrahigh Specific SE of 1148 dB cm3g−1 at 7.2 vol% CNT content.19 Significant finding reveals that the aligned porous composites exhibit substantially higher electrical conductivity, 5



mechanical strength and EMI SE than the isotropic micropore composites, along with ultra-lightweight and flexible properties.20 Anisotropic porous WPU/silver nanowire nanocomposites were also prepared, which have achieved a SE of 20 and 64 dB when the densities are merely 8 and 45 mg/cm3, respectively.2,21 In view of previous works, it is conceivable that the alignment of cell wall interfaces within the porous composites benefits the multiple reflections on the interfaces by taking full use of the aligned cell wall surface. Unfortunately, random and disordered microstructure is almost always the case in reported porous cellulose samples. Relative investigations addressing the structural regulation of the porous cellulose EMI shielding composites, including the corresponding preparation strategies, are still not sufficient due to the limitation in cellulose processing. Besides the amelioration of porous structure, enhancement of the microwave absorption in the CNT/cellulose cell walls, hereinafter referred to as the frame structure, is significant for low material consumption. As regards the distribution of conductive filler in the frame structure, it has been well acknowledged that the establishment of electrical conductive network is critical to the conductive performance and thus be estimated to have a conspicuous influence on the EMI shielding property. Herein, instead of simply increasing the mass ratio of the nanofillers, the regulation of conducive filler distribution in the frame structure is creatively proposed to achieve full utilization of the inherent shielding capability. Taking the aforementioned issues into consideration, we proposed, for the first time, to construct highly efficient EMI shielding material by mutually ameliorating the porous and frame structure. 6


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In the present work, aligned porous CNT/cellulose composites are fabricated by the unidirectional freeze-drying method22-24 (Figure 1). Previous works have already witnessed the capability of CNF forming an aligned porous structure. For example, Wicklein and coworkers25 constructed a micro honeycomb porous material based on CNF and graphene oxide. A CNF hydrogel with aligned nanofibers was also reported.26 However, the fabrication of CNF is complex, ineffective and high energy consumption.27 Hence, the green NaOH/urea aqueous solution28 is used to prepare cellulose solution for the construction of aligned porous structure. In addition, two distinct CNT distribution states in the porous material are achieved by adjusting the mixing process. As shown in Figure 1, CNT is preliminarily mixed with cellulose molecules in the green CNT/cellulose/ NaOH/urea aqueous solution in route 1. After unidirectional freeze drying process, CNT is homogenously dispersed in the matrix of frame structure (recorded as CNT-matrix/cellulose porous composite for brevity). In the other case (route 2), a reliable dip-coating approach is adopted to achieve CNT thin coatings on the porous cellulose surfaces by dealing with a concentrated CNT dispersion and porous cellulose material as the host substrate (recorded as CNT-interfaces/cellulose porous composite). The realization of CNT meticulous incorporating in cellulose matrix (CNT-matrix) or firmly attached to the interface of cell walls (CNT-interface) relies on the strong hydrogen bonding between CNT and cellulose, which have been extensively confirmed. As a noteworthy example, it was reported in our previous work that with the incorporation of CNT, cellulose film was simultaneously reinforced and toughed due to the formation of fortified interfacial 7



hydrogen bonding between CNT and cellulose chains.29 In addition, an efficient coating process has been adopted to fabricate cellulose fiber based nanocomposite paper, and the cellulose fibers were found to be homogeneously coated with interconnected CNT.30 Ultimately, following the principle of synergistically optimization of porous and frame structure, aligned porous cellulose composites with regulated CNT distribution were constructed to promote the multiple reflection and absorption, leading to highly efficient EMI shielding feature. EMI shielding mechanism was elucidated considering the aligned porous structure as well as selective distribution of the CNT conductive networks. This simple and feasible strategy of morphological amelioration and regulation proposed here would definitely present its capability of fabricating highly efficient green EMI shielding products.

Figure 1. Schematic illustration for the preparation process of CNT-matrix/cellulose porous composite (Rout 1) and CNT-interface/cellulose porous composite (Rout 2).

■ EXPERIMENTAL SECTION Materials. Cotton linters was used as the cellulose sample, which was obtained from Hubei Jinhuan Ltd (Xiangfan, China) with DP of 500 ± 50. The CNT (NC7000) was kindly provided by Nanocyl S.A., Belgium, possessing an average diameter and 8


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length of 9.5 nm 1.5 mm. NaOH, urea and cetyltrimethylammonium bromide (CTAB), both were analytical grade and purchased from Kelong Chemical Reagent Factory (Chengdu, China), were applied directly without further treatment. Preparation of CNT-matrix/cellulose porous composites. The fabrication procedure of CNT-matrix/cellulose porous composites was schematically shown in Figure 1 (Rout 1), which involved the dissolution process, unidirectional freezing, freeze-drying and etching process. Firstly, CNT/cellulose solution was prepared. A calculated amount of CNT with surfactant CTAB (weight ratio: 1.0:0.8) was first uniformly dispersed into 81g of deionized water with the help of ultrasonic dispersion for 15 min (an ultrasonic cell disruptor under a constant output power of 325 W). Following the dispersion of CNT, 12 g of urea and 7 g of NaOH were directly dissolved into the CNT suspension, and the resulted complex solution was put in the low temperature chamber to be pre-cooled to -12.5 oC. Subsequently, 5.0 g cellulose was quickly put into the pre-cooled solution under sufficiently stirring of 3000 rpm for 5 min at ambient temperature to obtain a homogeneous CNT/cellulose solution. With the dissolution of cellulose, CNT was homogenously mixed with cellulose at molecular level (as shown in Figure 1); then the CNT/cellulose solution was casted in a cylindrical polytetrafluoroethylene mold with stainless steel bottom. The solution was frozen from the bottom side by immersing the mold into liquid nitrogen. The sample chamber was sealed to reduce environmental disturbances. During the unidirectional freezing process, the ice crystals grew from bottom to top and acted as template for the phase separation (as shown in Figure 1). Afterward, the frozen bulk 9



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was freeze-dried at 20 Pa and -45 oC (FD-1A-50, Boyikang Instrument Co., Ltd., China). A slow sublimation rate of ice under low temperature and pressure was crucial for the scaffold drying process to avoid remelting of the aligned ice template, shrinkage and crack formation of the frame structure. Then a stepwise washing approach was adopted on the cellulose scaffold, which was necessary to preserve the scaffold structure by avoiding swelling and partially dissolve of the cellulose structure. The dried sample was immersed in 25 ml 2-propanol. The washing liquid was diluted step by step: adding 15 ml water after 2 h, 25 ml after 4 h, 25 ml after 6 h and 35 ml after 8 h. On the second day the immersing liquid was replaced with pure water and a final washing step was adopted on the third day to etch away all left NaOH and urea from the cellulose scaffold. Finally, the washed porous scaffold was freeze-dried again to obtain the aligned porous CNT-matrix/cellulose porous composite. CNT-matrix/cellulose porous composites with a series of CNT contents of 1, 5, 10, 15, 30 and 50 wt% were prepared by controlling the weight content of CNT in the aqueous suspension. The volume contents of CNT (VCNT) were calculated from the weight content (WCNT) using the density of CNT (ρ = 2.1 g cm-3) and porous composites (ρ*) by VCNT = (ρ*×WCNT)/ρ×100%16, which were estimated to be 0.04, 0.17, 0.22, 0.37, 0.61 and 0.83 vol%, as shown in Table S1. The pure cellulose aligned porous material was prepared according to the same process. Preparation

of

CNT-interface/cellulose

porous

composites.

CNT-interface/cellulose porous composite was fabricated by a simple dip-coating method (Figure 1. Route 2). The NaOH/urea/cellulose/water solution with the weight 10


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ratio of 7:12:5:81 was prepared for the fabrication of pure cellulose aligned porous material. After the unidirectional freezing, drying and etching processing, the pure cellulose porous material with aligned pore structure was achieved. With the aid of an estimated amount of CTAB, CNT of different concentrations was dispersed in deionized water (weight ratio of CNT and CTAB is 1:0.4) by ultra-sonication for 30 min under a constant output power of 325 W. Then, the prepared pure porous cellulose materials, acting as a 3D scaffold, were directly immersed into the CNT suspensions for 30 min followed by drying in an air-circulating oven at 70 oC for 12 h, and the porous CNT-interface/cellulose porous composites were obtained. The CNT content was adjusted by controlling the concentration of CNT suspension as well as the repeated time of dipping. Similarly, the weight contents of CNT in CNT-interface/cellulose obtained by weighing the samples before and after dip-coating process were converted into volume contents, which were estimated to be 0.01, 0.09, 0.34 and 0.51 vol% (see Table S2, the dip-coating parameter is recorded as a*b form, where ‘a’ represents the CNT concentration of CNT suspension and ‘b’ represents repeated times). Characterization. The samples were polished into regular cuboids-shape and the volume and weight were measured respectively to determine the density of CNT/cellulose porous composites. The morphology of aligned porous CNT/cellulose composites was observed by Field emission scanning electron microscope (SEM, FEI Inspect-F, Finland). The SEM samples were firstly immersed in liquid nitrogen for 20 min and quickly impact fractured through the orientation direction. The fractured 11



surfaces were sprayed with gold prior to observation. The accelerated voltage was maintained at 5 kV. Based on SEM observation, energy-dispersive X-ray spectroscopy (EDS) was also conducted. EDS was an eﬀective characterization technique to study the solid surface chemistry. Transmission electron microscopy (TEM; FEI Tecnai F20) was employed to observe the dispersion morphologies of the CNT in CNT-matrix/cellulose porous composite and CNT-interface/cellulose porous composite at an accelerating voltage of 200 kV. A piece of porous composite was embedded in epoxy and a thin section of the specimen for TEM imaging was obtained using a microtome equipped with a diamond knife. Two-line method was used for the measurement of electrical conductivity of the CNT/cellulose composites which were polished into cuboids-shaped samples. And in order to guarantee close contact between the samples and electrodes the opposite profiles of the samples along the conductive direction were coated with silver paste. Keithley 4200SCS (USA) was employed to measure the electrical resistance of the samples. The electrical conductivity (σ) was calculated by the equation σ = l / (R·A), where A and l are the effective area and length of the measuring electrode, respectively. At least three samples for each component are prepared for the measurement of electrical conductivity. The EMI SE characteristic in the frequency range of 8.2–12.4 GHz (X-band) for these CNT/cellulose samples (diameter and thickness of 13 and 2.5 mm, respectively) were measured using Agilent N5230 vector network analyzer which is connected with a coaxial test cell (APC-7 connector), according to ASTM ES7-83 and ASTM D4935-99 (specific description on the measurement setup was shown in our 12


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previous works).15,31 The APC-7 connector was a precision coaxial connector that was used on laboratory microwave test equipment and could be applied at frequencies up to 18 GHz. The compression test along orientation direction on the aligned porous samples was characterized by a universal electronic compression machine (Model 5576, Instron Instrument, USA). The compressing speed and span length were set to be 3 mm/min and 10 mm, respectively. Prior to testing, the CNT/cellulose porous composites were trimmed into cuboid shape with 10 mm in height and 5 mm in width and length.

■ RESULTS AND DISCUSSION Oriented microstructure of CNT/cellulose porous composites.

Figure 2. Morphology of CNT-matrix/cellulose porous composites. SEM images of the aligned porous composites with various CNT contents: (a) 0.04 vol%, (b) 0.17 vol%, (c) 0.22 vol%, (d) 0.37 vol%, (e) 0.61 vol%, and (f) 0.83 vol% in longitudinal section.

13



Figure 3. SEM images for porous cellulose (a) and CNT-interface/cellulose porous materials (b) in longitudinal cross section and the cell wall surface morphology before (c) and after coated with 0.01vol% (d), 0.09 vol% (e), 0.34 vol% (f) and 0.51 vol% (g) CNT.

The morphology and structure of the porous CNT/cellulose composites are characterized by SEM, as illustrated in Figure 2 and 3. Figure 2 presents the porous structure of CNT-matrix/cellulose composites. The longitudinal section images intuitively reveal an aligned layered structure of micro-ribbon array, which is commonly obtained from unidirectional freezing process.22,32-34 The distance between adjacent layers ranges from 20 to 80 μm, and thus the pore sizes are distributed in micrometer-scale (Figure S2). The pore size is a little larger than that of previously reported porous materials which are unidirectionally freezed from water suspension system.22,25,26,35-38 The main reason lies in the relatively lower undercooling of the cellulose solutions. During the unidirectional freezing process, ice-crystal nuclei is rapidly formed, followed by the unidirectional growth from the bottom of the mold to the upper side. Meanwhile, the growing ice crystals exclude CNT and cellulose molecules from the freezing front, leading to aligned structures in the longitudinal 14


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direction. After the sublimation of ice, the corresponding pore is formed. In the cellulose/NaOH/urea aqueous solution, the networks constructed by hydrogen bonding between NaOH “hydrates” and cellulose chains are stable at low temperature,39 and the crystallization temperature of the NaOH/urea solvent is as low as -12.6 oC,40 which is much lower than that of water, accordingly, the undercooling for the cellulose solution is relatively low. Consequently, the arising ice-crystal nuclei density is lower than that in water solution under the same freezing temperature, which directly contributes to a larger pore size. Indeed, the undercooling plays a pivotal role on the phase assembly of cellulose,17 thus alter the domain size in the freezing process. After the sublimation of ice template, a well-shaped monolith with anisotropic porous structure is retained (further confirmed by Figure S4, Figure F5 and Figure S6, Supporting Information). It is noteworthy that a larger size of ice crystals results in a thicker cell wall, which in turn gives a desirable strength of the frame structure to efficaciously avoid undesirable shrinkage and collapse during drying process, thus the pore size is enlarged. The loading of CNT plays a crucial role on the pore size and ultimate cell wall morphology. It is impressive that the pore size dramatically decreases (Figure S2) with the increasing of CNT content, and an enormous morphological evolution takes place from the dense laminate structure (insets in Figure 2a and b) to the loose-knit network (insets in Figure 2e and f). Take the 0.61 vol% CNT composites as an example, it can be seen that the aligned cell walls are slackly flocculated from small clumps. The loose-knit structure possibly stems from the decreased solid 15



concentration of the CNT/cellulose rich phase. During unidirectional freezing process, the ice crystals grow from the bottom nuclei to the upper side, continuously repelling the CNT and cellulose molecules to constitute into the CNT/cellulose rich phase, until a critical particle concentration is reached, which is closely related to the random close packing of CNT and cellulose molecules. When the CNT content increases, a lower critical concentration for the ultimate CNT/cellulose rich phase is achieved owing to the rigid peculiarity of CNT that hindered the bonding and rearrangement of cellulose molecules, and ultimately a flabbily stacked frame structure is obtained after the sublimating of the ice in freeze-drying process. Moreover, the existence of CNT can also induce the splitting of the ice crystals owing to its lower mobility, leading to the nanoscale pores generated within the aligned cell walls. Table S1 gives the density of CNT-matrix/cellulose porous composites. The density of pure cellulose porous material is 0.057 g/cm3. Upon the incorporation of CNT, it increases to 0.073 g/cm3 at 0.04 vol% of CNT. Further, increase of the CNT content to 0.17 vol% leads to the increased density of 0.091 g/cm3. But beyond 0.22 vol% of CNT content, inversely, the density readily decreases to 36 mg/cm3 for 0.83 vol% CNT content, and the corresponding porosity is as high as 98.03%. When the CNT content is relatively low, CNT are closely embedded in cellulose matrix, forming dense and thick cell walls, as detected in the insets of Figure 2a and b. This comes from the adequate hydrogen bonding interactions between CNT and cellulose molecules, which stimulate the evolution of CNT/cellulose-rich phase. However, the existence of enormous amount of rigid CNT would inevitably smash the arrangement of cellulose molecules during 16


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phase separation process. Consequently, the morphology of the cell walls varies from dense lamellar structure to interconnected CNT and cellulose strand network (Figure 2c ~ f). Furthermore, the strongly built CNT networks can improve the geometrical stability of the frame structure, therefore the undesirable shrinkage of the scaffold during drying process is effectively suppressed, which reflects the superiority of CNT in constructing ultra-lightweight foams. It is noteworthy that the density of the CNT-matrix/cellulose foams is considerably lower than the reported values of porous EMI shielding materials expanded by melt foaming, isotropic freeze drying and chemical vapor deposition. For example, Kuang and coworkers reported a density of 0.30 g/cm3 for 10 wt% CNT/PLA foam by melt foaming method.41 CNT/cellulose aerogel composite with a density of 0.095 g/cm3 was constructed by isotropic freeze drying method.14 Using the template directed CVD method, Chen et al. fabricated a 3D interconnected network structure for graphene, which shows an ultra-low density of 0.06 g/cm3.14 In comparison with the conventional porous material preparation methods reported in the open literature, the strategies utilized herein also own the flexibility of regulating the selective distribution of CNT. The CNT-interface/cellulose porous composites with various CNT contents are effortlessly prepared with the stable skeleton of aligned porous cellulose. After CNT coating, the well-defined structure inherits the 3D scaffold structure of the neat cellulose porous material (Figure 3a, b), and the average distance between adjacent layers changes little (Figure S3). The surfaces of cell walls are uniformly coated with interconnected CNT, and wider and 17



thicker cell walls are observed ascribed to the encapsulation of CNT on the cellulose surface. When increasing the CNT loading, the density of CNT-interface/cellulose porous composites tardily increases from 0.063 g/cm3 at 0.01 vol% CNT to 0.077 g/cm3 at 0.51 vol% CNT, comprehensively resulting from the mass addition of CNT and CTAB as well as the slight volume shrinkage during dip coating process. Furthermore, the distribution morphology of CNT on the whole cellulose cell wall surface is also detected (Figure 3c-f). It is obvious that the CNT are intimately coated on the surface of cellulose ligaments with uniform dispersion state, forming interconnected conductive networks. The pure cellulose foam possesses a smooth and clean surface (Figure 3c). With only 0.01 vol% CNT content, the surface is found to be entirely coated with interconnected CNT and the entanglement density of CNT keep rising monotonously with increasing CNT content while no aggregation can be observed. It is worthy to mention that the aligned porous cellulose material serves as an ideal skeleton for the dip coating method. First of all, the cellulose cell walls are stable in water which comes from the adequate hydrogen bonding between cellulose molecules. The frame structure will not be dilapidated in the immersing solution, thus the well-defined structure can be maintained. Secondly, the cellulose matrix is hydrophilic. During the immersing process, cellulose molecules absorb water to swell to a favorable extent for the anchoring of CNT with the cell wall surface. Moreover, the CNT surfaces are decorated with some carboxylic and hydroxyl functions which enable the forming of hydrogen bonding between CNT and cellulose hydroxyl groups. 18


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FTIR results can further verify the consistent conclusion (Figure S1, Supporting Information). Therefore, taking the above points into consideration, the conclusion is easily achieved that the fabrication of CNT-interface/cellulose porous composite could obtain highly stable CNT networks firmly anchoring on the cellulose walls.

Figure

4.

Electrical

conductivity

of

CNT-matrix/cellulose

porous

composites

and

CNT-interface/cellulose porous composites with various CNT contents in longitudinal direction (a), and the insert presents a log–log plot of the conductivity as a function of φ-φc (exponent t =1.17 and critical volume content φc= 0.043 vol% for CNT-matrix/cellulose porous composites and t =1.56 and φc= 0.0083 vol% for CNT-interface/cellulose porous composites). TEM images of CNT dispersions in 0.17 vol% CNT-matrix/cellulose porous composite (b) and 0.09 vol% CNT-interface/cellulose porous composite (c).

Electrical conductivity and EMI shielding effectiveness. It is well acknowledged that the electrical conductivity presents an intrinsic capability of a material to absorb electromagnetic radiation, which plays a crucial role on EMI shielding performance. Figure 4a shows the electrical conductivity of the porous CNT/cellulose composites along longitudinal direction with various CNT 19



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loadings. With the increasing of CNT content, the electrical conductivity raises continuously

for

both

CNT-matrix/cellulose

porous

composites

and

CNT-interface/cellulose porous composites. The conductivity increases nearly 13 orders of magnitude for the CNT-matrix/cellulose porous composites when the CNT loading increases from 0 to 0.17 vol%, demonstrating a typical percolation behavior. According to classical percolation theory, the relationship between the electrical conductivity of porous CNT/cellulose composites and the CNT content can be described by the power-law equation σ = σ0(φ − φc)t, where σ represents the composite conductivity, σ0 is a constant related to the intrinsic conductivity of CNT, φ is the volume fraction of CNT, and φc is the percolation threshold. When the CNT concentration is above the percolation threshold, a sustained connecting CNT network will be established, which enables the unconstrained transportation of electrons throughout the whole material. t is a critical exponent depending only on the dimensionality of the system. The log σ versus log (φ−φc) plots is illustrated in the inset in Figure 4a. According to the best fitting result, a critical exponent of 1.17 for CNT-matrix/cellulose porous composites and 1.56 for CNT-interface/cellulose porous composites are achieved. It is a very low value for a porous structure which indeed represents two-dimensional (2D) conductive networks.42 This estimate corresponds well with the 2D distribution state of CNT in the CNT/cellulose aligned porous composites which is clearly presented in Figure 2 and 3. The CNT are located in the highly aligned cell walls for CNT-matrix/cellulose porous composites or on the surface for CNT-matrix/cellulose porous composites, and the low values of t imply 20


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that the charge must flow along the surface and interface of the aligned cell walls. The 2D distribution of CNT has also quite effectively reduced the percolation threshold. The insulator-to-conductor transition of the CNT-matrix/cellulose porous composite and CNT-interface/cellulose porous composite located at 0.043 and 0.0083vol % respectively, which is not only much lower than that of the other cellulose composites16,43-46 but also lower than most of the CPC foams.2,41,47 Notably, the percolation threshold for CNT-interface/cellulose porous composite is almost one magnitude lower than the values in CNT-matrix/cellulose porous composites, probably arising from the distribution state of CNT. The establishment of conductive networks in CNT-matrix/cellulose porous composites and CNT-interface/cellulose porous composites are further confirmed by TEM observation (Figure 4b and c). In CNT-matrix/cellulose porous composite, CNT is uniformly and separately dispersed in the cell wall (Figure 4b). While in CNT-interface/cellulose porous composite, the CNT is closely packed in a restricted space–the surface of cellulose cell wall (Figure 4c). In this case, more perfect and robust conductive networks can easily be achieved by CNT successively overlapping each other. EDS analysis is used to further reflect the distribution state of CNT (Figure S7). Despite of their ultra-high porosity, the composites possess high conductivity ascribed to the seamlessly interconnected CNT networks which would provide fast electron transport channels inside it. Specifically, CNT-matrix/cellulose porous composite own an electrical conductivity as high as 8.14 S/m with a CNT content of 0.84 vol%. What is more intriguing, the CNT-interface/cellulose porous composite already reaches an equivalent electrical 21



conductivity of 8.56 S/m with only 0.34 vol% CNT, and an outstanding electrical conductivity of 38.90 S/m is achieved by further increasing CNT content to 0.51 vol%. The superiority of electrical conductivity of CNT-interface/cellulose porous composite is also directly related to the perfect CNT networks situated on the aligned cell wall surface. It is well acknowledged that the electronic transportation directly decides the conductivity of the composite and there are two modes for electronic transport, i.e., the migrating and hopping electrons.48,49 The hopping electrons with full of energy, can jump over the energy barrier between CNT. Since cellulose matrix is insulator, the energy barrier between CNT in CNT-matrix/cellulose porous composite is higher than the CNT-interface/cellulose porous composite. Thus more electrons in CNT-interface/cellulose porous composite with lower energy state are able to jump across the interface between CNT, leading to the promoted electrical conductivity.

Figure 5. (A) EMI shielding effectiveness of CNT-matrix/cellulose and CNT-interface/cellulose porous composites with various CNT contents in the X-band region, (B) Specific SE (SE divided by density) vs filler content, comparison of shielding performance of CNT/cellulose porous composites in this work and EMI shielding materials ever reported.

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EMI shielding has acquired importance and urgency in a wide range of areas (e.g. electronic newsletters, aerospace industry).50-53 The high electrical conductivity of CNT/cellulose porous composites is helpful to develop high performance EMI shielding materials particularly with ultralow CNT loading. The shielding performance is measured in the direction that is along the in-plane direction of the aligned porous CNT/cellulose composites. EMI SE of both CNT-matrix/cellulose and CNT-interface/cellulose porous composites is recorded in the X-band range (8.2 to 12.4 GHz). As shown in Figure 5A, EMI SE of both CNT-matrix/cellulose and CNT-interface/cellulose porous composites remains almost constant in the X-band frequency range, reflecting that the conductive networks in both porous CNT/cellulose

composites

are

homogenous

regular.54

and

The

SE

for

CNT-matrix/cellulose porous composite with 0.22 vol% CNT reaches 20 dB, which basically achieves the target for practical EMI shielding application. The SE continuously climbs up to 27, 30 and 35 dB as the CNT content increases to 0.37, 0.61 and 0.83 vol%, respectively, basically originated from the fact that more CNT is involved in preventing the invasion of EM radiation.54 Furthermore, the establishment of micro-current network increases the electrical conductivity of CNT/cellulose porous

composites,

and

thus,

enhances

the

EMI

SE.49

The

SE

of

CNT-interface/cellulose porous composite with only 0.34 vol% CNT readily reaches a comparable level with that of 0.83 vol% CNT-matrix/cellulose porous composites, and further increase to 40 dB with 0.51 vol% CNT, which distinctly bring the conclusion to light that a higher efficient EMI shielding material can be fabricated by 23



regulating the distribution state of CNT. The shielding performance ( ≈ 40 dB for CNT-interface/cellulose composites containing 0.51 vol% CNT at 2.5 mm thickness) is outstanding among most of the carbon-filled porous CPCs under analogous sample thickness with even much more CNT content. Such as 28 dB for 2.2 vol% MWCNT/WPU composites at 2.3 mm,19 24 dB for MWCNT/PP composites with MWCNT loading of 5 vol% of 2.8 mm,55 9–12.8 dB for graphene/PEI composites with filler loading of 5.87 vol% and at 2.3 mm thickness2 (Table S3, Supporting Information). Considering both EMI shielding and the density of the materials, the specific SE (SSE, EMI SE divided by the related material density) of the EMI shielding materials vs CNT volume content is summarized to manifest the EMI shielding performance (Figure 5B). The SSE of CNT/cellulose porous composites presents a considerably large value ranging from 425 to 944 dB cm3 g−1, which is superior to the level of previous reported porous systems with comparable CNT content (16.4 dB cm3 g−1 for 0.8 vol% graphene/PMMA foam,11 71.4 dB cm3 g−1 for 0.7 vol% MWCNT/PLA foam,41 219.0 dB cm3 g−1 for 0.45 vol% MWCNT/cellulose aerogel,16 18.8 dB cm3 g−1 for 0.75 vol% graphene/PEI foam,2 91.7 dB cm3 g−1 for 0.54 vol% AgNW/cellulose foam56). To our knowledge, the SSE of the aligned porous CNT/cellulose is comparable with the highest value reported in the pioneering work based on MWCNT/WPU (1050 dB cm3 g−1 for 1.1 vol%, 541 dB cm3 g−1 for 2.2 vol% and 401 dB cm3 g−1 for 7.2 vol% MWCNT/WPU foam)19. The SSE is closely related to the foam density, and hence the difficulties in achieving low density porous materials of the fabrication technology inevitably lead to low SSE (

Ultralight cellulose porous composites with manipulated porous

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