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Green Fabrication of Regenerated Cellulose/Graphene Films with Simultaneous Improvement of Strength and Toughness by Tailoring the Nanofiber Diameter Tongping Zhang, Xiaofang Zhang, Yuwei Chen, Yongxin Duan, and Jianming Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03608 • Publication Date (Web): 19 Nov 2017 Downloaded from http://pubs.acs.org on November 25, 2017

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ACS Sustainable Chemistry & Engineering

Green Fabrication of Regenerated Cellulose/Graphene Films with Simultaneous Improvement of Strength and Toughness by Tailoring the Nanofiber Diameter

Tongping Zhang,† Xiaofang Zhang,† Yuwei Chen, Yongxin Duan*, Jianming Zhang* Key Laboratory of Rubber-Plastics, Ministry of Education/ Shandong Provincial Key Laboratory of Rubber-plastics, Qingdao University of Science & Technology, Qingdao City 266042, People’s Republic of China

* To whom all correspondence should be addressed. Mailing address: No. 51-1, Wuyang Road, Qingdao 266045, China. Fax: +86-532-84022791 E-mail: [email protected] (J. Z.), [email protected] (Y.D.).

ACS Paragon Plus Environment

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ABSTRACT: The development of green and facile synthesis techniques for flexible, transparent and conductive films (FTCFs) is in great demand with the rapid consumption of electronics. Herein, we report the environmental friendly and one-pot fabrication of regenerated cellulose nanofibers (CNF)/graphene FTCFs directly from raw materials of cellulose and graphite based on ionic liquid. As prepared FTCFs exhibit simultaneous and extraordinary improvement of tensile strength (135.4 %) and toughness (459.1%) with graphene loading of only 0.1 wt%. Besides the contribution of graphene sheets as reinforced filler, the morphology analysis reveals that the diameter size of regenerated CNF plays the key role on tailoring the mechanical properties of regenerated CNF/graphene film. Meanwhile, our results show that the diameter of regenerated CNF is dependent on the dispersion state of graphene sheets. The disruptive selfassembling of cellulose molecules in regeneration process induced by the hydrophobic interaction between graphene sheets and cellulose chains is proposed to explain the reduction of diameter size of regenerated CNF in the presence of graphene. The high performance FTCFs fabricated by such simple and green strategy have the potential in large-scale industrial applications. KEYWORDS:

Nanocomposite,

Cellulose

nanofiber,

Graphene

sheets,

Toughening,

Reinforcement, Transparent and conductive film INTRODUCTION Flexible Transparent Conductive Films (FTCFs), composed of electrical conductor and plastic substrates, have been widely applied in optoelectronic devices owing to their unique properties.15

Many conductive nanoparticles have been arising as conductor of FTCFs. Graphene, due to its

intrinsic physicochemical properties, is one of the most promising candidates.6-8 To further meet the requirements of future consumer electronics, cellulose nanofiber (CNF), derived from


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abundant natural cellulose with low thermal expansion (2.7×10-6/K), high optical transparency and outstanding elastic modulus (about 150 GPa), has been considered as one of most optimal flexible substrate for green, reproducible and low cost FTCFs.3,9-16 Currently, considerable attention has been devoted to fabricate FTCFs based on cellulose nanofiber (CNF) and graphene. To realize the homogeneous dispersion of graphene in cellulose matrix, Layer-by-layer (LBL) assembly17,18 and vacuum filtration19-22 are the most commonly used methods to fabricate CNF/graphene nanocomposites. However, both methods are sophisticated in procedure and not suitable for large-scale production. Moreover, chemical reactions and waste by-product, always involved in the fabrication of CNF and graphene themselves, are not environmental-friendly.18,22,23 Thus, it remains a significant challenge to obtain CNF/graphene nanopapers through a green and facile approach. On the other hand, the continuous improvement in mechanical properties is also highly desired for CNF/graphene nanopapers, as the promising advanced materials. Nevertheless, the enhancement of tensile strength and toughness are generally conflict.24-26 Inspired by the “brickand-mortar” microstructure of nacre, Xiong et al.18 successfully fabricated transparent conductive membranes with synergistic high strength and toughness by using reduced graphene oxide (RGO) nanosheets and modified cellulose nanocrystal (CNC) as the starting materials. It is believed that the existence of dense covalent and hydrogen bonding between CNC and RGO endowed those membranes with excellent mechanical properties. Recently, Zhu et al.24 reported that both the strength and toughness of cellulose nanopaper increase simultaneously as the diameter of the constituent CNF decreases (from a mean diameter of 27 µm to 11 nm). This anomalous but highly desirable scaling law inspired us that the mechanical properties of CNF/graphene nanopapers may be tailored by controlling the size of CNF diameter.


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Herein, we have demonstrated that the transparent conductive CNF/graphene films with both high strength and toughness can be fabricated directly from graphite and cellulose in one-pot with the aid of ionic liquid, 1- allyl-3-methylimidazolium chloride (AmimCl), a green solvent with high solubility to cellulose.27,28 Interestingly, it is found that the diameter of in situ regenerated CNF from cellulose/exfoliated graphene/AmimCl solution strongly depends on the content of graphene. The CNF/graphene nanopaper with 0.1 wt% graphene content has the smallest mean diameter (ca.20.8 nm) of regenerated CNF whereas it shows the largest improvement on mechanical properties, 135.4% increase in strength and 459.1% increase in toughness. This study revealed that not only the dispersion state of graphene but also the size of regenerated CNF determines the mechanical properties of CNF/graphene nanopapers. EXPERIMENTAL SECTION Materials. The ionic liquids, AmimCl was kindly provided by Prof. Zhang (Key Laboratory of Engineering Plastics, Joint Laboratory of Polymer Science and Materials, Institute of Chemistry, Chinese Academy of Sciences, China). α-Cellulose, with a degree of polymerization (DP) of 650, was purchased from Sigma Aldrich. The cellulose powder was dried at 70 °C in a vacuum oven for 12 h prior to use. Natural flake graphite was purchased from Qingdao JingRiLai graphite co., Ltd. Graphene sheets were prepared as follows: natural flake graphite and 0.02 wt% α-cellulose powder were dispersed into AmimCl, then the mixture was subjected to ultrasonication (100 W, 20 Hz) in the water bath for 6 h at 80 °C. Then, the mixture was centrifugated at 10000 rpm for 30 min to remove the bulk graphite. The supernatant solution with a graphene concentration of 1.35 mg mL-1 was collected to use in the next step, as shown in Figure S1.


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Preparation of regenerated CNF/graphene nanocomposites films with different graphene contents. The key steps of the presented synthesis process are illustrated in Figure 1. In a typical procedure, cellulose powder was mixed with different volumes of Graphene/AmimCl solution. Then, the mixtures were heated with a stirrer at 90 °C for 2 h and the resulted homogeneous solutions with various contents of graphene sheets were obtained. Later, the as-prepared solutions were cast onto glass plates to give a thickness of about 2.5 mm with a rectangle form and kept under reduced pressure to get rid of air bubbles. Then the degassed gels were immediately coagulated into a mass of deionized water to regenerate and washed with deionized water repeatedly to guarantee the completely removal of AmimCl. Finally, the regenerated cellulose/graphene nanocomposite hydrogels were transferred onto the poly(methylmethacrylate) (PMMA) plates and fixed with a tape. The regenerated CNF/graphene nanopapers were obtained by drying the hydrogels at room temperature and 30% humidity for 24 h. The pure regenerated cellulose film was also prepared as a reference by the same procedure. Characterization. Wide angle X-ray diffraction (WAXD) patterns were collected on a Bruker D8 Advance diffractometer with an incident wavelength of 0.154 nm (Cu Kα radiation), the regenerated CNF/graphene nanopapers and pure cellulose films were recorded in the range of 2 θ = 5 ~ 40o with a step interval of 0.05o and scanning rate of 0.02o min-1. The hybrid nanopapers were operated using frozen section for transmittance electronic microscopy (TEM) with a JEOL JEM-2200 FS electron microscope at 200 kV. The optical transmittance was measured using a UV-Vis spectrophotometer (UV-2550, Shimadzu). Confocal Raman microscopy with a 532 nm laser (Thermo Fisher) was employed to observe the dispersibility of graphene sheets in the plane of nanocomposite film. The electrical conductivities of the CNF/graphene samples were measured by a two-probe method. I-V curves were recorded with a CHI 660D electrochemical


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workstation, (Chenhua, Shanghai, China) a 100 kΩ standard resistor was employed to validate the two-probe method for acquiring the bulk resistivity of sample slices. Mechanical properties of regenerated CNF/graphene nanopapers and pure cellulose film were evaluated using dynamic mechanical analyses (DMA Q800 TA). The specimen sizes used were typically in the range of 50 mm×4 mm×25 ~ 35 µm, length, width and thickness, respectively. A 18 N load cell was used with a normal strain rate of 0.5 mm/min at ambient conditions. At least four specimens were measured from each sample. To study the fracturing of the nanopapers, the specimens were fractured using a tensile tester and the exposed cross-sections were sputtered with a thin layer of Aurum to promote conductivity before SEM (JEOL SEM 6700) observation. RESULTS AND DISCUSSION Preparation of FTCFs based on regenerated CNF and directly exfoliated graphene The solution-processed procedure of FTCFs is shown in Figure 1a, including three steps: (1) the preparation of mixed solution, (2) regeneration process, (3) drying process. In the first step, the green solvent AmimCl, which can effectively and rapidly dissolve cellulose, was used to prepare exfoliated graphene directly from graphite powder by ultrasonic treatment according to previous literature.29,30 The modified procedure for preparing exfoliated graphene with ionic liquid is presented in Figure S1. Notably, we discovered that the existence of a small amount of cellulose effectively facilitate the exfoliation of graphite in AmimCl (the detailed information will be reported in a separate paper). The inset in Figure S1 presents the TEM images of thus exfoliated graphene sheets, in which show that there are single or multi-layers as confirmed by the corresponding selected area electron diffraction. Therefore, the subsequently graphene loading denoted in this work represents the content of graphene sheets containing both singlelayer and multilayer graphene.


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Figure 1. (a) The schematic illustration of the in-situ regeneration procedure of CNF/graphene nanocomposites films based on AmimCl. The enlarged figures from left to right respectively show the microstructure of cellulose/graphene/AmimCl solution, a typical SEM image of freeze-dried CNF/graphene hydrogel and a typical cross-sectional SEM image of dried CNF/graphene film with 0.1 wt% graphene sheets. (b) Cross-sectional TEM images (c) the WAXD profiles and (d) the optical transparency and electrical conductivity of as prepared films with various graphene contents.


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Subsequently, the as prepared graphene/AmimCl solution is mixed with cellulose/AmimCl solution to obtain the homogeneous and stable cellulose/graphene/AmimCl solution (as illustrated by the carton in Figure 1a). Afterwards, the mixed solution is casted on a glass plate and then immersed into deionized water to form a composite gel film via regeneration process as shown in Figure 1a.27,28 As seen from the SEM image of freeze-dried cellulose/graphene hydrogel, the cellulose chains are self-assembled into cellulose nanofibers (CNF) which is woven into the 3D network structure during the regeneration process. Finally, the wet gel is dried to form a paper-like regenerated CNF/graphene film with outstanding optical transparency. The corresponding cross-sectional SEM image shows that the 3D structure of regenerated CNF in composite film has compressed from loose to dense state through the drying process. To evaluate the dispersion of graphene sheets in nanopapers, the ultrathin frozen slices of the regenerated CNF/graphene nanopapers with various graphene loading are produced along the direction perpendicular to the film plane. Figure 1b reveals that the average size of graphene sheets is around 200 nm, and the graphene sheets turn to aggregate more heavily with increasing the graphene content from 0.1 wt% to 1.0 wt%. Unexpectedly, it is clearly seen that the graphene sheets are uniformly distributed throughout the matrix without preferred orientation (Figure 1b, Figure S2), that is, the graphene sheets with both parallel and perpendicular orientation can be observed. Generally, the compressive forces and gravitational forces will cause the graphene sheets to align parallel with the composite films surface due to the two-dimensional character and high aspect ratio of the graphene sheets.31,32 Here, this isotropic distribution may be owing to the relatively small aspect ratio of graphene sheets. Besides, the simultaneous and rapid regeneration of CNF and graphene sheets may also disturb the preferential orientation of graphene sheets.19 On the other hand, Micro-Raman image (Figure S3) is also collected to


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investigate the dispersion of graphene sheets in the plane of nanocomposite film. As shown in Figure S3, the graphene sheets (the green region) are homogeneously dispersed throughout the cellulose matrix (the blue region). Wide-angle X-ray diffraction (WAXD) is employed to further study the dispersion of graphene sheets and its effect on the condensed structure of regenerated cellulose.17,33 As shown in Figure 1c, with the graphene content increasing up to 0.3 wt%, the characteristic diffraction peak of graphite located at 26.4o (d-spacing =0.334 nm) occurs. It indicates that the aggregation of graphene sheets in the cellulose matrix appears with increasing the graphene content. This is consistent with the previous TEM observation (Figure 1b). In addition, similar with that of regenerated pure cellulose sample, there are broad amorphous peak located at 20.2° for all the regenerated CNF/graphene nanocomposites films while the crystalline diffraction peaks of cellulose could be hardly identified. It suggests that the crystallinity of regenerated cellulose samples is very low, closing to amorphous state (Figure S4). This result also reveals that the incorporation of graphene sheets does not influence the phase structure of cellulose. The effect of graphene content on the transparency and electrical conductivity of as prepared nanocomposites films is depicted in Figure 1d. It can be seen that the transmittance of regenerated pure cellulose sample with the thickness of about 25 µm is 97.7% at 550 nm wavelength, and the addition of graphene sheets reduces its transparency to 90.4% with 1.0 wt% graphene loading. However, the transmittance of all the nanocomposites is still relatively higher than that of the other CNF/graphene nanopapers reported in the literatures.31,33 Such high transparency of our specimens may be attributed to the nanosize effect of regenerated CNF and the densely 3D network structure of the resulted nanocomposites films as shown in Figure 1a.24,34 In Figure 1d, the conductivity of the films reaches to 2.5 S m-1 with only 0.3 wt%


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graphene loading. The electrical conductivity is higher than that of the amine-modified CNF/RGO composite paper with 3 wt% RGO (1.1 S m-1),35 which may be contributed to the homogeneous dispersion of graphene sheets. Moreover, the as prepared nanocomposites films exhibit excellent flexible as demonstrated in Figure S5. Thus, the CNF/graphene nanocomposites films with flexibility, transparency and electrical conductivity have been successfully fabricated by the green and facile method proposed here. The mechanical properties of the regenerated CNF/graphene nanocomposites films

Figure 2. (a) Stress-strain curves, (b) tensile strength, (c) toughness and (d) strain at break of the regenerated CNF/graphene nanocomposites films containing different amounts of graphene sheets and pure cellulose film as reference. The presented error bars were calculated from the standard deviation of parallel measurements.


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As mentioned in introduction section, to develop FTCFs with simultaneous high-strength and high-toughness is still a great challenge. We therefore pay our attention to investigate the mechanical properties of as prepared FTCFs. Figure 2a shows the typical tensile stress-strain curves of the regenerated CNF/graphene nanopapers with various graphene loadings. The corresponding tensile strength, toughness, elongation at break are depicted in Figure 2, and the Young's modulus is presented in Figure S6. Compared to regenerated pure CNF nanopaper, the data in Figure 2 clearly shows that addition of graphene sheets to the CNF matrix results in a significant increase of comprehensive mechanical properties for all composition. In addition, the mechanical properties go through a maximum at 0.1 wt% graphene loading. The optimum tensile strength and toughness for the CNF/graphene nanocomposite film with 0.1 wt% graphene loading reach 197.3 MPa and 14.2 MJ/m3, about 135.4 % and 459.1 % increase compared to that of the pure CNF film, respectively. These results suggest that transparent conductive CNF/graphene films with simultaneous improvement in strength and toughness have been achieved successfully. The increased toughness is a result of increased ultimate tensile strength and failure strain.24 Usually, the elongation at break of CNF/graphene nanopapers reported in the literatures is lower than that of neat sample or remains unchanged.17,19 Herein, the elongation at break of the as prepared nanopapers has been improved remarkably. For example, it is 8.6 % for the nanopaper with 0.1 wt% graphene, around 138.9 % higher than that of the regenerated pure CNF film whose elongation at break is 3.6 %. The failure strain value decreases gradually with increasing the content of graphene sheets, it is 4.8 % when the graphene content increases to 1.0 wt%, but still higher than that of neat film. It should be resulted from the gradual aggregation of graphene sheets with the increase of graphene loading as confirmed previously. The origin of the


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synergetic improvement in stiffness and ductility will be discussed in the next section. Origin of the simultaneous improvement of strength and toughness for regenerated CNF/graphene films So far, several strategies have been reported to simultaneously improve the stiffness and toughness of polymer nanocomposites: (1) Improving the dispersion and interfacial interaction of nanofiller in matrix.10,19,36,37 (2) Utilizing the synergetic effect of hybrid nanofillers composed of 1D and 2D building blocks.38-41 For example, shin et al.38 found that there is an extraordinary improvement in toughness and strength when GO/SWNT hybrid naofillers was used to reinforce the polyvinylalcohol (PVA). (3) Tailoring the phase structure and morphology of the polymer matrix itself.34,37,42,43 For instance, Shah et al.42 obtained enhanced toughness and stiffness of polyvinylidene fluoride (PVDF) nanocomposites via nanoclay-directed crystal structure and morphology. As suggested by the cross sectional TEM images and WAXD data shown in Figure 1b and Figure 1c, the fine dispersion of graphene sheets in FTCFs with 0.1 wt% graphene content may do some contribution to the synergetic improved stiffness and ductility. However, the synergetic effect of hybrid nanofillers or altering in the phase structure should be precluded for expounding the simultaneous and extraordinary improvement in tensile strength and toughness at optimum graphene loading (0.1 wt%). To further understand the origin of the simultaneous improvement in stiffness and toughness, cross sectional SEM images of regenerated CNF/graphene nanocomposites films with various graphene contents and corresponding analysis of size-distribution are displayed in Figure 3. As shown in Figure 3a, many salient points could be observed. Those points should be the cross section of regenerated CNF,15,44-46 which are uniformly distributed and parallel to the surface of the film.


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Figure 3. (a) Cross-sectional SEM images of regenerated CNF/graphene nanocomposites films with various graphene contents and (b) the corresponding statistical distribution of mean diameters.


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From the corresponding statistical size-distribution of regenerated CNF (Figure 3b), it is clearly observed that the regenerated pure CNF film presents the biggest mean diameter (49 nm), and the nanocomposite film with 0.1 wt% graphene sheets has the smallest mean diameter (20.8 nm). The average diameters of regenerated CNF are upsized with increasing graphene content, while still smaller than that of the regenerated pure cellulose sample.

Figure 4. The changes of mean diameters size of CNF, the tensile strength and toughness of as prepared CNF/graphene nanocomposites films as a function of graphene content.

Figure 4 summarized the relationship between graphene contents and the diameter size of regenerated CNF, tensile strength and toughness of CNF/graphene films. From it, it is clear to find that the change of tensile strength and toughness of FTCFs as a function of graphene content presents the similar trend. However, their values show the opposite trend to the mean diameter size of regenerated CNF. That is, the tensile strength and toughness increase (135.4% and 459.1%, respectively) with the decrease of the diameter of the regenerated CNF (from a mean diameter of 49 nm to 20.8 nm), which is agreement with the anomalous scaling law: the smaller, the stronger and the tougher, reported by Zhu et al..24 Specially, the relationship between the


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ultimate tensile strength and diameter of CNF is well in line with the formulation proposed by Zhu et al.,24 that is, the ultimate tensile strength σ∝1/ D , with D being the mean CNF diameter. On the other hand, these results also indicate that the diameter size of regenerated CNF is heavily affected by the graphene contents. Why the introduction of graphene sheets will reduce the averaged diameter of regenerated CNF in nanocomposite? We envision the following mechanism understanding the effect of graphene contents on the mean diameter size of regenerated CNF. As illustrated in Figure 5a, when the cellulose/AmimCl solution is exposed to deionized water, the H-bonds between cellulose and AmimCl are weakened or even destroyed with addition of water, owing to water molecules prefer to form H-bonds with AmimCl. Then, the H-bonds between cellulose chains connect again to form regenerated CNF and precipitate out from the cellulose/AmimCl solution.47 When the graphene sheets are added, the hydrophobic interaction between cellulose chains and graphene sheets will induce the coassembly of cellulose chains and graphene sheets.19 In other words, the existence of graphene sheets disrupts the selfassembly of cellulose chains during the regeneration process of cellulose/AmimCl solution in water. Hence, as depicted in Figure 5b, the mean diameter size of CNF in the nanocomposites samples is smaller than that of pure regenerated cellulose. In addition, the dispersion of graphene sheets should also influence the assembly of cellulose chains. For the nanocomposite sample with 0.1 wt% graphene content, the well dispersed graphene sheets result in the thinnest diameter of regenerated CNF. With graphene content increasing, the contact areas between graphene sheets and cellulose chains are decreased because of the aggregation of graphene sheets. Thus, the assembled diameter size of regenerated CNF grows gradually with increasing the graphene content from 0.1 wt% to 1.0 wt%.


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Figure 5. Schematic illustration of the structural evolution of regenerated CNF of Cellulose/AmimCl (a) and Cellulose/Graphene/AmimCl (b) in water.

Based on the above results and analysis, we can conclude that the origin of the simultaneous improvement of strength and toughness for regenerated CNF/graphene film at the optimum graphene content (0.1 wt%) should be attributed to the nice dispersion of graphene and the smallest diameter size of regenerated CNF. It is worthwhile mentioning that the diameter of regenerated CNF should be related to the dispersion state of graphene sheets in the mixed solution of cellulose/graphene/ILs. CONCLUSIONS In conclusion, the CNF/graphene FTCFs have been successfully fabricated using ionic liquid AmimCl as the co-solvent of exfoliated graphene sheets and cellulose via a simple and green strategy. The as prepared nanocomposites films possess synergistic improved strength and


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toughness due to the nice dispersion of graphene and the smaller diameter size of regenerated CNF at optimum graphene loading. The most interesting finding is that the introduction of nanofiller like graphene will reduce the diameter size of regenerated CNF, which in turn resulting in the remarkable improvement in both strength and toughness. This phenomenon has been ignored in previous studies. We believe that this work could provide a new insight to design novel multifunctional nanocomposites based on regenerated cellulose.

ASSOCIATED CONTENT Supporting Information. The procedure of the fabrication of graphene solution in AmimCl with 0.02 wt% cellulose and its TEM images. Cross-sectional TEM images and Raman image of CNF/graphene nanocomposite film with 1.0 wt% graphene loading. WAXD patterns of regenerated pure cellulose film and native cellulose. The photos and Young`s modulus of as prepared nanocomposites films. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (J. Z.); Fax: +86 532 84022791; Tel: +86 532 84022604; Author Contributions Tongping Zhang and Xiaofang Zhang contributed equally to this work. All authors declare no conflict of interest. ACKNOWLEDGMENT


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The authors acknowledge the financial support from National Natural Science Foundation of China (51603112, 51573082 and 51503113) and Taishan Mountain Scholar Foundation (TS20081120 and tshw20110510). REFERENCES 1. Lai, Y. T.; Tai, N. H. One-step process for high-performance, adhesive, flexible transparent conductive

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Table of Contents Graphic

A green and one-pot preparation of regenerated CNF/graphene flexible transparent conductive films with simultaneous high strength and toughness.


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Graphene Films with

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