Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 1271−1278
pubs.acs.org/journal/ascecg
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* 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 S Supporting Information *
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 environmentally friendly and one-pot fabrication of regenerated cellulose nanofibers (CNFs)/graphene FTCFs directly from raw materials of cellulose and graphite based on ionic liquid. Prepared FTCFs exhibit simultaneous and extraordinary improvement of tensile strength (135.4%) and toughness (459.1%) with graphene loading of only 0.1 wt %. In addition to the contribution of graphene sheets as reinforced filler, the morphology analysis reveals that the diameter size of regenerated CNF plays the key role in 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 self-assembly of cellulose molecules in the 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 a simple and green strategy have potential in large-scale industrial applications. KEYWORDS: Nanocomposite, Cellulose nanofiber, Graphene sheets, Toughening, Reinforcement, Transparent and conductive film
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environmentally 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 promising advanced materials. Nevertheless, the enhancements of tensile strength and toughness are generally conflicted.24−26 Inspired by the “brick-and-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 to think that the mechanical
INTRODUCTION Flexible transparent conductive films (FTCFs), composed of electrical conductor and plastic substrates, have been widely applied in optoelectronic devices because of their unique properties.1−5 Many conductive nanoparticles have arisen as conductors of FTCFs. Graphene, because of 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 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 substrates 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. For realization of 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 byproduct, always involved in the fabrication of CNF and graphene themselves, are not © 2017 American Chemical Society
Received: October 6, 2017 Revised: November 6, 2017 Published: November 19, 2017 1271
DOI: 10.1021/acssuschemeng.7b03608 ACS Sustainable Chem. Eng. 2018, 6, 1271−1278
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Figure 1. (a) Schematic illustration of the in situ regeneration procedure of CNF/graphene nanocomposite 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) WAXD profiles. (d) Optical transparency and electrical conductivity of as-prepared films with various graphene contents. Academy of Sciences). 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, and 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 10 000 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. Preparation of Regenerated CNF/Graphene Nanocomposite 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 asprepared 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 were washed with deionized water repeatedly to guarantee the complete removal of AmimCl. Finally, the regenerated cellulose/graphene nanocomposite hydrogels were transferred onto the poly(methyl
properties of CNF/graphene nanopapers may be tailored by controlling the size of CNF diameter. 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 in 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.
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EXPERIMENTAL SECTION
Materials. The ionic liquid, AmimCl, was kindly provided by Prof. Zhang (Key Laboratory of Engineering Plastics, Joint Laboratory of Polymer Science and Materials, Institute of Chemistry, Chinese 1272
DOI: 10.1021/acssuschemeng.7b03608 ACS Sustainable Chem. Eng. 2018, 6, 1271−1278
Research Article
ACS Sustainable Chemistry & Engineering methacrylate) (PMMA) plates and fixed with 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 2θ = 5−40° with a step interval of 0.05° and scanning rate of 0.02° min−1. The hybrid nanopapers were operated using the 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 twoprobe method. I−V curves were recorded with a CHI 660D electrochemical 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. An 18 N load cell was used with a normal strain rate of 0.5 mm min−1 at ambient conditions. At least four specimens were measured from each sample. For a study of 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.
image shows that the 3D structure of regenerated CNF in composite film has compressed from a loose to dense state through the drying process. For evaluation of 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 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 film surface because of the twodimensional character and high aspect ratio of the graphene sheets.31,32 Here, this isotropic distribution may be due to the relatively small aspect ratio of graphene sheets. In addition, the simultaneous and rapid regeneration of CNF and graphene sheets may also disturb the preferential orientation of graphene sheets.19 On the other hand, a micro-Raman image (Figure S3) is also collected to 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 26.4° (d-spacing = 0.334 nm) occurs. This 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 to that of the regenerated pure cellulose sample, there are broad amorphous peak located at 20.2° for all the regenerated CNF/ graphene nanocomposite films while the crystalline diffraction peaks of cellulose could be hardly identified. This 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 nanocomposite films is depicted in Figure 1d. It can be seen that the transmittance of the 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 literature.31,33 Such a high transparency of our specimens may be attributed to the nanosize effect of regenerated CNF and the dense 3D network structure of the resulted nanocomposite films as shown in Figure 1a.24,34 In Figure 1d, the conductivity of the films reaches 2.5 S m−1 with only 0.3 wt % 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 nanocomposite films exhibit excellent flexibility as demonstrated in Figure S5.
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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, and (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 the 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 facilitates the exfoliation of graphite in AmimCl (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 it is shown that there are single- or multilayers as confirmed by the corresponding selected area electron diffraction. Therefore, the subsequent graphene loading denoted in this work represents the content of graphene sheets containing both single-layer and multilayer graphene. 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). Afterward, the mixed solution is cast on a glass plate and then immersed into deionized water to form a composite gel film via a 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 (CNFs) which are woven into the 3D network structure during the regeneration process. Finally, the wet gel is dried to form a paperlike regenerated CNF/graphene film with outstanding optical transparency. The corresponding cross-sectional SEM 1273
DOI: 10.1021/acssuschemeng.7b03608 ACS Sustainable Chem. Eng. 2018, 6, 1271−1278
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Figure 2. (a) Stress−strain curves, (b) tensile strength, (c) toughness and (d) strain at break of the regenerated CNF/graphene nanocomposite 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.
Thus, the CNF/graphene nanocomposite films with flexibility, transparency, and electrical conductivity have been successfully fabricated by the green and facile method proposed here. Mechanical Properties of the Regenerated CNF/ Graphene Nanocomposite Films. As mentioned in the Introduction section, to develop FTCFs with simultaneous high strength and high toughness is still a great challenge. We therefore pay attention to the investigation of 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, and 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 compositions. 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 m−3, about a 135.4% and 459.1% increase compared to those 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 literature 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 for 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. This should result from the gradual aggregation of graphene sheets with the increase of graphene loading as confirmed previously. The origin of the 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 blocks38−41 [for example, shin et al.38 found that there is an extraordinary improvement in toughness and strength when GO/SWNT hybrid nanofillers were used to reinforce the poly(vinyl alcohol) (PVA)], and (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,c, the fine dispersion of graphene sheets in FTCFs with 0.1 wt % graphene content may have some contribution to the synergetic improved stiffness and ductility. However, the synergetic effect of hybrid nanofillers or alteration 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 %). For further understanding of the origin of the simultaneous improvement in stiffness and toughness, cross-sectional SEM images of regenerated CNF/graphene nanocomposite films with various graphene contents and the corresponding analysis 1274
DOI: 10.1021/acssuschemeng.7b03608 ACS Sustainable Chem. Eng. 2018, 6, 1271−1278
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ACS Sustainable Chemistry & Engineering of size distribution are displayed in Figure 3. As shown in Figure 3a, many salient points could be observed. Those points
Figure 4. Changes of mean diameter sizes of CNF, the tensile strength, and toughness of as prepared CNF/graphene nanocomposite films as a function of graphene content.
trend. However, their values show the opposite trend as compared 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 to 20.8 nm), which is in agreement with the anomalous scaling law: the smaller, the stronger and the tougher, reported by Zhu et al.24 In particular, the relationship between the 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 will the introduction of graphene sheets reduce the average diameter of regenerated CNF in the nanocomposite? We envision the following mechanism to understand 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, because of the preference of water molecules 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 self-assembly 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 nanocomposite 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 to 1.0 wt %. On the basis of 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
Figure 3. (a) Cross-sectional SEM images of regenerated CNF/ graphene nanocomposite films with various graphene contents and (b) corresponding statistical distribution of mean diameters.
should be the cross sections of regenerated CNF,15,44−46 which are uniformly distributed and parallel to the surface of the film. 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 summarizes 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 changes of tensile strength and toughness of FTCFs as a function of graphene content present a similar 1275
DOI: 10.1021/acssuschemeng.7b03608 ACS Sustainable Chem. Eng. 2018, 6, 1271−1278
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Figure 5. Schematic illustration of the structural evolution of regenerated CNF of (a) cellulose/AmimCl and (b) cellulose/graphene/AmimCl in water.
size of regenerated CNF. It is worthwhile to mention that the diameter of regenerated CNF should be related to the dispersion state of graphene sheets in the mixed solution of cellulose/graphene/ILs.
Author Contributions
CONCLUSIONS In conclusion, the CNF/graphene FTCFs have been successfully fabricated using ionic liquid AmimCl as the cosolvent of exfoliated graphene sheets and cellulose via a simple and green strategy. The as-prepared nanocomposite films possess synergistically improved strength and 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 results 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 into the design of novel multifunctional nanocomposites based on regenerated cellulose.
ACKNOWLEDGMENTS The authors acknowledge the financial support from National Natural Science Foundation of China (51603112 and 51573082), Taishan Mountain Scholar Foundation (TS20081120 and tshw20110510), Key Laboratory of Rubber-Plastics, Ministry of Education/Shandong Provincial Key Laboratory of Rubber-Plastics, and Qingdao University of Science & Technology (KF212-02005403).
†
T.Z. and X.Z. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03608. Fabrication procedures, cross-sectional TEM images, Raman image, WAXD patterns, photos, and Young’s modulus (PDF)
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REFERENCES
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AUTHOR INFORMATION
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Jianming Zhang: 0000-0002-0252-4516 1276
DOI: 10.1021/acssuschemeng.7b03608 ACS Sustainable Chem. Eng. 2018, 6, 1271−1278
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