Effect of cellulose solubility on the thermal and mechanical properties of regenerated cellulose/graphene nanocomposites based on ionic liquid 1-allyl-3-methylimidazoliun chloride

Abstract

Regenerated cellulose/ionic liquids (ILs)–reduced graphene oxide (IRGO) nanocomposites have been prepared via a simple and green method based on 1-allyl-3-methylimidazoliun chloride (AmimCl). To one's surprise, it is found that the presence of a small amount of IRGO sheets can largely decelerate the dissolution of cellulose in AmimCl, and thus the thermal and mechanical properties of resultant cellulose/IRGO nanocomposites could be controlled simply by changing the dissolution time. Our results suggest that the residual cellulose fibrils with micrometre size are beneficial to the thermal stabilities of regenerated cellulose/IRGO films when short dissolution time is used. However, for obtaining cellulose/IRGO nanocomposites with improved mechanical properties, full dissolution and mixture of cellulose in IRGO/AmimCl solution with prolonged time are required. Compared with the regenerated pure cellulose film, our cellulose/IRGO nanocomposite prepared with dissolution time of 4 h exhibits about 23 °C increase in decomposition temperature, and 10.7% increase in tensile strength and remarkably 387.5% increase in strain at break, respectively.

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1. Introduction

Graphene has attracted tremendous attention due to its unique one-atom-thick two-dimensional structure and exceptional thermal, mechanical and electrical properties.^1–3 Incorporating graphene sheets into polymer composite materials is a practical route to demonstrate the great potential of graphene in practical applications.^4–7 In recent years, graphene sheets and its derivatives have been used to fabricate nanocomposites materials with numerous pertroleum-based polymers.^3–8 However, considering the ever-increasing demand on environmental protection and sustainable development, natural biopolymers⁹ are attracting more and more attention in preparing graphene-based nanocomposites.

Cellulose, the most abundant natural polymer in the world, is widely utilized in bioengineering as alternatives to non-biodegradable petroleum-based materials.¹⁰ A great deal of studies are focused on cellulose owing to its low cost, environmental friendliness, renewable, biodegradable and biocompatible. Nevertheless, there are difficulties in processing the cellulose in solution or as a melt due to its strong inter- and intra-molecular hydrogen bonds,¹¹ which largely limit the application of cellulose. Recently, Rogers et al.¹² and Zhang et al.¹³ has reported that some room-temperature ionic liquids (ILs) can be used as effective and green solvents for cellulose. Meanwhile, it is reported that the regeneration of cellulose in ILs can be achieved with green solvents, such as water^12,14 and even compressed CO₂.^15–17 Hence, the ILs could be easily recovered from the remaining solution for cycling utilization. Besides the outstanding dissolving capability and recyclability, the ILs are attractive due to their negligible vapor pressure, variable structures, thermal and chemical stability.^12–18 Therefore, the utilization of ILs opens a promising way to prepare the cellulose/graphene composites in a green routine.

In previous studies, graphene oxide (GO)^4,7,19–21 is widely used as the precursor for the preparation of the cellulose/graphene nanocomposite based on ILs. For instance, Zhang et al.²² successfully fabricated cellulose/reduced graphene oxide (RGO) film based on 1-allyl-3-methylimidazoliun chloride (AmimCl), in which the RGO was chemically reduced GO by hydrazine monohydrate, exhibiting enhanced stiffness. Peng et al.²³ demonstrated that GO can be simultaneously reduced and surface functionalized by natural cellulose in 1-butyl-3-methylimidazoliun chloride (BmimCl) with the addition of ammonia solution to accelerate the reduction reaction and stabilize the RGO sheets, the as-prepared cellulose/RGO paper showed robust mechanically flexibility. However, the preparation methods of graphene/ILs solution are complicated and non-environmental friendly, which are carried out by solvent exchange using rotary evaporator,^22,23 following the chemical reduction with reducing agents, such as toxic hydrazine, hydriodic acid and so on.^24–26 On the other hand, due to the strong π–π stacking interaction between graphene sheets, surfactants or stabilizers^{23,24,27–30} are always employed to prevent the re-stacking of graphene sheets in ILs and the matrix. Therefore, it remains a critical challenge to obtain uniformly dispersed graphene sheets in ILs without the assistance of any additives.

We recently successfully produced thermal reduced GO in AmimCl through a simple mixing of aqueous GO with AmimCl at high temperature.³¹ Such ILs-reduced GO (IRGO) can be stably dispersed in AmimCl up to several months without the help of any additional stabilizers. Therefore, thus prepared IRGO/AmimCl solution could be directly used to fabricate the cellulose/graphene nanocomposites in a simple and green way. Unexpectedly, the presence of IRGO significantly slow down the dissolution rate of cellulose in AmimCl, and it is also found that the thermal and mechanical properties of the resulting materials are strongly depending on the dissolution time of cellulose in IRGO/AmimCl. In present study, the relationship between the dissolution time, dispersion of IRGO sheets and the properties of the regenerated cellulose/IRGO nanocomposites has been studied systematically combining with various techniques.

2. Experimental section

2.1 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 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. IRGO was prepared as described in our previous work.³¹ In brief, well-dispersed RGO in ILs (IRGO/ILs: 2 mg ml⁻¹) was obtained by mixing GO aqueous solution with AmimCl at 180 °C for 2 h to reduce GO and evaporate water synchronously, as illustrated in the first step of Scheme 1.

Scheme 1 Illustration of the preparation of cellulose/IRGO nanocomposites materials based on ionic liquid AmimCl.

2.2 Preparation of cellulose/IRGO/AmimCl solutions with different dissolution time

The key steps of the presented synthesis process are illustrated in Scheme 1. In a typical procedure, following the preparation of IRGO/AmimCl solution, 200 mg cellulose powder, as well as 1 ml well-dispersed IRGO/AmimCl (2 mg ml⁻¹), were added in sequence into 5 g AmimCl in sealed reaction vessels to prepare IRGO/cellulose nanocomposites with the loading of 1 wt%. Then, the mixed solution was heated with a stirrer at 90 °C for dissolving the cellulose and getting the homogeneous solution. In order to examine the effect of dissolution time on the properties of resulted films, four bottles of cellulose/IRGO/AmimCl solutions with the dissolution time of 1 h, 2 h, 3 h and 4 h are respectively prepared. At the same time, pure cellulose/AmimCl solution was obtained as a control by dissolving cellulose in AmimCl for 1 h at the same condition (cl-1 h).

2.3 Preparation of regenerated cellulose/IRGO nanocomposites films with different dissolution time

The as-prepared cellulose/IRGO/AmimCl solutions were cast onto glass plates to give a thickness of 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 in a mass of deionized water to regenerate cellulose/IRGO nanocomposites and washed with deionized water repeatedly to guarantee the removal of AmimCl. Finally, the regenerated cellulose/IRGO nanocompositers hydrogels were transferred onto the poly(methylmethacrylate) (PMMA) plates and fixed with a tape, then regenerated cellulose/IRGO nanocomposites films, as well as the pure regenerated cellulose film, were obtained by drying the fixed hydrogels at room temperature for 12 h. The pure regenerated cellulose film was prepared by the same procedure. On the regeneration method, it should be mentioned that Mu et al.³² recently employed compressed CO₂ as a good anti-solvent for regeneration of biomass composite materials from ionic liquids. It was found that the dispersion of nanoparticles on biomass material could be tuned by changing the pressure of CO₂. However, for getting large-size composite film with regular shape, special device maybe required with this method. Therefore, we choose water rather than compressed CO₂ as the anti-solvent in the present study.

2.4 Characterization

Polarized optical micrographs (POM) were obtained using a BX51 Olympus polarized optical microscope (Olympus, Japan) equipped with an Olympus DP72 CCD camera. The cellulose solution with different dissolution times were sandwiched between two pieces of cover glasses and monitored by a digital camera.

Scanning electron microscopy (SEM) images of the samples were obtained on a JEOL SEM 6700 operating at 5 kV. For observing the dispersion of graphene in cellulose, the regenerated cellulose/IRGO materials and pure cellulose materials were freeze-dried for SEM characterization. The samples were sputtered with gold for better observation.

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 cellulose/IRGO nanocomposites and pure cellulose films were cut into 10 × 15 mm to be recorded in the range of 2θ = 5–40° with a step interval of 0.05° and scanning rate of 0.02° min⁻¹.

Transmission electron microscopy (TEM) analysis of cellulose/IRGO/ILs solution was conducted on a JEOL JEM-2200 FS instrument with an accelerating voltage of 200 kV, a drop of the sample was placed on parafilm and then touched the drop with one face of the carbon-coated TEM grid, which was stored under vacuum overnight prior to characterization by TEM to remove any absorbed water.

Raman microscopy images and the corresponding spectra were recorded using a Laser micro-Raman spectrometer (Thermo Fisher) with an excitation wavelength of 532 nm at room conditions. For comparison purposes, the GO and graphite powders are also tested.

Thermal analysis of the samples was performed using a Perkin Elmer TGA 6 (Perkin Elmer Instruments, USA). The temperature ranged from 30 to 700 °C with a heating rate of 10 °C min⁻¹ under nitrogen.

Mechanical properties of regenerated cellulose/IRGO nanocomposites films 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, respectively. A 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.

3. Results and discussion

3.1 Dissolution of cellulose in AmimCl with the existence of IRGO

According to Zhang et al.,¹³ cellulose without any pretreatment or activation could be dissolved in ionic liquid AmimCl rapidly at temperature above 60 °C. They reported that cellulose with a degree of polymerization as high as 650 can be dissolved in AmimCl at 80 °C within 30 min. Therefore, we preconceive that one hour is enough to ensure the solubility of cellulose in IRGO/AmimCl solution at 90 °C. As shown in Fig. 1a, the cellulose/IRGO/AmimCl solution dissolved for 1 h shows uniformly black colour with the addition of 1 wt% IRGO sheets. However, once the solution is cooled to room temperature, the cellulose/IRGO/AmimCl solution behaves gel by the inversion (Fig. 1b). Nevertheless, followed by the inversion once the solutions with different dissolving time have been cooled to room temperature, it is extremely surprised to find that the cellulose/IRGO/AmimCl solutions dissolved for 1 h and 2 h appear gelled state, while those with longer dissolution time (3 h, 4 h), as well as the pure cellulose/AmimCl solution dissolved for 1 h (cl-1 h), can flow slowly, as shown in Fig. 1b.

Fig. 1 The photos of pure cellulose dissolved at 90 °C for 1 h and cellulose/IRGO nanocomposites solutions dissolved in AmimCl at 90 °C for different hours (a) and their inversions in the first 10 minutes (b).

Given the same preparation condition of the pure cellulose and cellulose/IRGO nanocomposites dissolved in AmimCl for 1 h, the gels might be due to the cross-linked networks originated from the existence of well-dispersed IRGO sheets. Shi et al.³⁰ and Zhang et al.³³ have already proposed that the presence of oxygen-functional groups together with the π-stacking of RGO sheets can result in the successful construction of hydrogel. As attested in our previous study,³¹ the IRGO sheets possess residual oxygenated functional groups, which could act as cross-linked hydrogel points. However, on the other hand, for the nanocomposites solutions with the same loading IRGO sheets, there still display fluidity discrepancy dissolved for 1 h and 4 h, respectively. In other words, the distinct gel behavior of the nanocomposites solutions should not be due to the existence of IRGO sheets. According to Duchemin et al.³⁴ and Zhang et al.,¹³ the dissolution of cellulose has related to the temperature, time, concentration, molecular weight, and so on. Hence, the coagulations formation might result from some insolvable cellulose in the cellulose/IRGO/AmimCl nanocomposites solutions with shorter dissolving time.

In order to explore the origin of the gel behavior, dissolving process of cellulose in cellulose/IRGO/AmimCl solutions at 90 °C, as well as cellulose/AmimCl solution, is monitored by POM and TEM as shown in Fig. 2. Fig. 2a reveals that pure cellulose can be dissolved in AmimCl at 90 °C within 0.5 h, being in agreement with the results reported by Zhang et al.¹³ However, it is clearly observed from Fig. 2b that the cellulose in IRGO/AmimCl solution is dissolved gradually from 1 h to 4 h. That is to say, the existence of IRGO sheets retards the dissolution of cellulose and it is the insolvable cellulose induces the coagulation formation dissolved at 90 °C for shorter time. The TEM images (Fig. 2c) show that there are still some residual nanometre-size rod-like cellulose crystals in cellulose/IRGO/AmimCl solution dissolved for 4 h, the width is about 5–10 nm. Zhang et al.³⁵ had also reported that there remain some rod-like fibrils in fresh 0.1 wt% cellulose/AmimCl solution dissolved at 60 °C for 1 h. Considering the structure and property of IRGO sheets reported in our previous work,³¹ it is speculated that parts of the effective ions that dissolve the cellulose are occupied by IRGO sheets, thus inhibiting the dissolution of cellulose in AmimCl.

Fig. 2 The POM images of pure cellulose (a), cellulose/IRGO nanocomposites (b) dissolved in AmimCl at 90 °C for different hours and the TEM images (c) of cellulose/IRGO nanocomposite dissolved in AmimCl at 90 °C for 4 h.

The cellulose solubility of regenerated cellulose/IRGO nanocomposites and pure cellulose films has further been confirmed using the WAXD data shown in Fig. 3. By comparison, the WAXD pattern of raw cellulose powder with form I crystal is also present. For raw cellulose powder, the WAXD patterns show three typical peaks at 2θ = 14.8, 16.8 and 22.7°, corresponding to the (11̄0), (110) and (002) planes of cellulose form I crystal, respectively.³⁶ The regenerated cellulose/IRGO nanocomposites film dissolved in AmimCl for 1 h shows similar but a little bit weak WAXD patterns as raw cellulose powder. With the dissolving time increasing, the diffraction intensities of the WAXD peaks obviously decrease, indicating the cellulose in the nanocomposites is dissolved gradually. Moreover, as dissolution time increased longer than 2 h, the amorphous scattering increased, emerging as two broad scattering located at 2θ = 12.0 and 20.2°, in line with that of regenerated pure cellulose film.^13,35–37 The above data proves that the cellulose is indeed progressively dissolved with time increasing from 1 h to 4 h. Furthermore, it is noted that the diffraction peak assigned to layer distance of graphene sheets doesn't appear. This suggests that the cellulose can effectively prevent the aggregation of IRGO sheets.

Fig. 3 The WAXD patterns of regenerated cellulose/IRGO nanocomposites films dissolved in AmimCl for different hours, as well as raw cellulose powder and regenerated pure cellulose film dissolved in AmimCl for 1 h.

3.2 Dispersion of IRGO in regenerated cellulose matrices

To investigate the dispersion of IRGO sheets in the cellulose matrices, freeze-dried technology that can maintain the original structure of the sample is employed. Fig. 4a shows the SEM images of freeze-dried regenerated cellulose/IRGO nanocomposites materials dissolved in ILs with different time. The nanocomposites materials present 3D fibrous web-like structure with the incorporation of IRGO sheets. Specially, different with the denser and compact network structure with thicker fibrils of regenerated pure cellulose materials shown in Fig. 4b, the IRGO sheets are embedded into the cellulose matrix, and the cellulose fibrils are loosely arranged. At the same time, the fibrils are thinner and compact with dissolving time increasing. As expected, the structure, morphology and dispersibility of IRGO sheets can be observed clearly. It is apparent that the IRGO sheets are uniformly dispersed in the nanocomposites with the size of 1 μm. With dissolving time increased from 1 h to 4 h, the IRGO sheets are changed from multilayered to nearly transparent plates shown in dashed ring in Fig. 4b, indicating the thickness of the IRGO sheets is decreased gradually.

Fig. 4 The SEM images of freeze-dried cellulose/IRGO nanocomposites (a) and pure cellulose materials (b) dissolved in AmimCl for different hours, the micro-Raman image (c) of the cellulose/IRGO nanocomposite, and the Raman spectra of graphite, GO and the cellulose/IRGO nanocomposite shown in (d).

The homogenous distribution of the IRGO flakes is further verified by mapping regenerated cellulose/IRGO nanocomposites surface with micro-Raman spectrometer. As shown in Fig. 4c, from the middle image, where the reddish orange areas indicate the IRGO sheets and the green regions mean regenerated cellulose matrix, the IRGO sheets is uniformly dispersed throughout the cellulose matrix. Besides, the Raman spectra of the IRGO flakes measured at the spot of the dashed ring shows the characteristic bands of graphene (Fig. 4d). The Raman spectra of GO sheets and graphite powder are also carried out for comparison, as shown in Fig. 4d. The typical features in the Raman spectra of graphite are two prominent peaks located at 1570 and 1342 cm⁻¹, which correspond to the well-documented G (the in-plane bond stretching vibration of sp² bonded carbon atoms) and D (amorphous carbon and defects) bands, respectively.^23,38 Notably, the G band is located at 1595 cm⁻¹ for GO, and it moves to 1589 cm⁻¹ for the IRGO sheets in the regenerated cellulose/IRGO nanocomposites film, which is close to the value of graphite and identifies the reduction of GO during the heating treatment based on AmimCl.³⁹ On the other hand, the D/G intensity ratio of IRGO is smaller than that of GO, the increased D/G ratio shows the successful reduction of GO to be RGO.^23,40–43

3.3 Effect of dissolution time on the thermal and mechanical properties of regenerated cellulose/IRGO composites

As is mentioned above, the incorporation of IRGO sheets can slow down the dissolution of cellulose in ILs, then how about the properties of the resulted cellulose/IRGO nanocomposites films with different cellulose solubility? The thermogravimetry analysis (TGA) is employed to examine the thermal properties of the nanocomposites films prepared for different dissolving time, the regenerated pure cellulose film is also tested. From Fig. 5a, the regenerated cellulose/IRGO nanocomposites films are more stable than regenerated pure cellulose film, most of which is composed of amorphous regions. As quantificationally expressed in Fig. 5b, the decomposition temperatures of the regenerated cellulose/IRGO nanocomposites films are decreasing gradually with dissolving time increasing. Herein, the higher stability of the samples with shorter dissolving time results from the remaining insolvable micrometre-size cellulose in the nanocomposites materials. On the other hand, the max decomposition temperature of regenerated pure cellulose locates at 294.7 °C, whereas the nanocomposites materials dissolved in ILs for 4 h with the same solubility it is 317.9 °C, about 23 °C higher than regenerated pure cellulose film. That is to say, the homogenous dispersion of IRGO sheets can extremely elevate the max decomposition temperature of cellulose matrix. According to Yao et al.,⁴⁴ this improvement is attributed to the suppression of the mobility of the cellulose segments at the interface between the IRGO sheets and cellulose matrix, thus induce the increased stability of the cellulose matrix.

Fig. 5 (a) TGA curves and (b) the max degradation peak position of regenerated cellulose/IRGO nanocomposites and pure cellulose films.

Fig. 6 shows the mechanical properties of regenerated cellulose/IRGO nanocomposites films and pure cellulose film. With similar dissolution degree of regenerated pure cellulose and cellulose/IRGO nanocomposites films dissolved for 4 h, compared to the former, the tensile strength and the strain at break of later are improved obviously. For example, the tensile strength and strain at break for the regenerated cellulose/IRGO film dissolved for 4 h are 102 MPa and 7.8%, respectively. 10.7% increase in tensile strength and remarkably 387.5% increase in strain at break compared with those of regenerated pure cellulose film. In particular, the strain at break improvement is extraordinary that is rarely reported in other observations.^22,44,45 The enhancements in the tensile strength and strain at break of the regenerated nanocomposites films should result from the homogeneous dispersion of IRGO sheets throughout the cellulose matrices, thus transfer the load from cellulose matrices to IRGO sheets. This consequences are consistent with previous studies about the incorporation of graphene and their derivatives into polymer matrices.^22,44,45 However, compared to the regenerated pure cellulose film, the decreased modulus of regenerated nanocomposite film dissolved for 4 h should be caused by the poor compatibility of cellulose and IRGO sheets as shown in Fig. 4a, where the IRGO sheets act as the stress concentration point.

Fig. 6 Stress–strain curves of regenerated pure cellulose and cellulose/IRGO nanocomposites films dissolved in AmimCl for different hours.

Besides the incorporated IRGO sheets, the cellulose solubility, that is, the dissolving time, is another effective factor to influence the mechanical properties of regenerated cellulose/IRGO nanocomposites films. As illustrated in Fig. 6, with the dissolving time increasing, the tensile strength, stress at break and the modulus of the regenerated nanocomposites films are elevated gradually. For instance, the tensile strength, stress at break and the modulus values of regenerated nanocomposites film dissolved for 1 h are 43.5 MPa, 2.3% and 3.4 GPa, respectively. Once the dissolving time increases to 4 h, those values tend to be 103 MPa, 7.8% and 4.7 GPa, respectively. Surprisingly, the mechanical properties of regenerated cellulose/IRGO nanocomposites films dissolved in AmimCl for longer time (12 h) is extremely enhanced, as revealed in Fig. S1,† and the specific causes are to be further studied in the future. Hence, the insolvable cellulose is obviously unfavourable factor to the mechanical properties of regenerated nanocomposites films, which is the result of weaker interface interaction between the well-dispersed IRGO sheets, dissolved cellulose matrices and insoluble cellulose fibrils.

4. Conclusions

In summary, the cellulose/IRGO nanocomposites with various cellulose solubilities have been successfully prepared via a simple and green method based on an ionic liquid AmimCl. Our results show that well dispersed IRGO sheets can obviously slow down the dissolution rate of cellulose in AmimCl and therefore the solubility of cellulose in IRGO/AmimCl suspension could be easily controlled by changing the dissolution time. Both the IRGO sheets and the insoluble cellulose can apparently increase the thermal stability of the regenerated nanocomposites films. Nevertheless, it is found that the mechanical properties could be largely improved only for the cellulose/IRGO nanocomposite films prepared with longer dissolving time. It is expect that this work may shed some light on the preparation of cellulose/graphene nanocomposites for various applications.

Acknowledgements

The authors acknowledge the financial support from Taishan Mountain Scholar Constructive Engineering Foundation (TS20081120 and tshw20110510), and Natural Science Fund for Distinguished Young Scholars of Shandong Province (JQ200905) is greatly appreciated.

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Footnote

† Electronic supplementary information (ESI) available: Stress–strain curves of regenerated pure cellulose and cellulose/IRGO nanocomposites films dissolved in ILs for 4 h and 12 h. See DOI: 10.1039/c5ra15160k

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