Graphene oxide/cellulose composite films with enhanced UV-shielding and mechanical properties prepared in NaOH/urea aqueous solution

Abstract

Based on a NaOH/urea aqueous system, a facile and convenient method was proposed to prepare graphene oxide/cellulose composite films with superior mechanical performances and excellent ultraviolet-shielding properties. Unexpectedly, it is found that more uniform dispersion of graphene oxide (GO) in a cellulose/NaOH/urea aqueous solution could be realized by mixing them at low temperature rather than room temperature. Epichlorohydrin (ECH) was introduced as a cross-linking agent to enhance the performance of GO/cellulose composite films. With the synergistic effect of chemical cross-linking and the strong hydrogen bonding interaction between GO and the cellulose matrix, the tensile strength, elongation at break and fracture energy of the thus prepared composite film with 0.5 wt% GO loading were significantly improved by 78%, 172% and 397%, respectively, compared with the pure cellulose film. Moreover, it is found that the cross-linked GO/cellulose composite film also possessed excellent ultraviolet-shielding properties. These enhanced properties indicate that the cellulose based composite material has great potential for use as a high performance bioplastic film.

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1. Introduction

Polymer films have been widely used in pharmaceuticals, food and electronic devices as packaging and protective materials, due to their desired properties, such as light weight, low cost, versatility and ease of processing. However, most of the commonly used polymer films are produced based on fossil fuels and are difficult to recover and reuse. With rapidly growing awareness of the protection of the environment, scientists are shifting focus from traditional petrochemical-based polymeric materials to environmentally friendly alternatives.^1,2 Cellulose, as the most abundant and renewable natural biomass resource, has received intense interest in fundamental research and applications as a promising substitute for petroleum-based materials because of its excellent biocompatibility and biodegradability.^3,4 Nevertheless, pure cellulose film usually presents unbalanced mechanical performances as well as limited functionality, impeding its development and application.

Incorporation of nanofiller has been thought to be an effective strategy to prepare composite films with high-performance and multi-functionality in recent years. Graphene, with excellent mechanical, thermal, electronic, and even biological properties,^5–8 has recently attracted great interest in the fabrication of carbon-based composites. As the precursor of graphene, graphene oxide (GO) have often been used as the reinforcing fillers to enhance properties of polymer. Because abundant oxygen containing groups on the surface of GO⁹ can form strong interactions with polar polymers, not only inducing a significant reinforcing effect but also facilitating the dispersion of GO in the polymer matrix.^10–12

In contrast to the excellent solution processability of GO, cellulose is hardly dissolved in common solvent due to the existence of strong inter- and intra-molecular hydrogen bonding. This issue largely limits the processing and application of cellulose. Over the past years, two kinds of green solvents have been developed for the dissolution of cellulose. One is room temperature ionic liquids,¹³ and the other is the NaOH/urea aqueous system.¹⁴ Benefit from these two great discoveries, GO/cellulose composites have been fabricated in recent years.^15–18 Compared with ionic liquids, NaOH/urea aqueous system has the advantage of low cost and thus are more suitable for large-scale production of cellulose based composites. Zhang et al.¹² reported that GO could be dispersed stably in cellulose/NaOH/urea solution instead of in pure NaOH/urea solvent. They proposed that high ionic strength of strong basic solution destroys electrostatic repulsion balance between GO sheets resulting in their aggregation. But with the presence of cellulose, the intermolecular hydrogen bonds and special steric effect between cellulose and GO restrain the agglomeration of GO so that stable dispersion of GO could be observed in cellulose/NaOH/urea solution. Similarly, Han et al.¹⁷ also found this phenomenon and fabricated homogeneous GO/cellulose composite films by adding a certain amount of microcrystalline cellulose into pre-cooled GO/NaOH/urea suspensions. In general, the tensile strength value of pure cellulose film regenerated from NaOH/urea system is about 50–70 MPa and the elongation at break is in the range of 3–8%. It had been expected that the addition of GO could enhance the mechanical properties of regenerated cellulose film. However, we noticed that the GO/cellulose films reported in literature only showed a very limited enhancement in tensile strength (less than 85 MPa) and elongation at break (less than 9%) with the incorporation of GO.^16–18 Besides, even by utilizing the hot pressing technology to fabricate the GO/cellulose composite film with highly oriented GO distribution in cellulose matrix, it was found that the expected reinforcing effect in mechanical properties had not been realized yet although as-prepared composite films possessed ultra-low O₂ permeability with the incorporation of 1.64 vol% GO.¹⁸

In the present study, we report that the dispersion of GO in cellulose/NaOH/urea aqueous solution is dependent on the mixing temperature. That is, the mixture of GO suspensions with the cellulose solution at temperature lower than room temperature is more effective to obtain uniformly dispersed GO/cellulose/NaOH/urea aqueous solution. Based on this finding, together with the introducing of chemical crosslinking agent, we successfully prepare graphene oxide/cellulose composite films with superior mechanical performances. Moreover, it is found that thus prepared GO/cellulose composite films also possessed excellent ultraviolet-shielding (UV-shielding) properties. These enhanced properties are attributed to the uniform dispersion of the GO, as well as the strong chemical and physical interaction between the GO and the cellulose matrix.

2. Experimental section

2.1 Materials

Cellulose raw material (cotton liners, with a degree of polymerization of about 600) was supplied by Hubei Chemical Fiber Co., Ltd. (Xiangfan, China). It was vacuum-dried at 60 °C for 24 h to remove any moisture prior to use. Flaky graphite was purchased from Jinrilai Graphite Co., Ltd. (Qingdao, China). ECH, NaOH, and urea were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Unless otherwise stated, all other reagents were of analytical reagent grade and were used as received without further purification.

2.2 Preparation of GO/cellulose composite films

The GO was obtained using the modified Hummer's method.¹⁹ A suspension of 4.6 mg ml⁻¹ GO was obtained for further experiments. As confirmed by AFM data in ESI (Fig. S1†), as-prepared GO show an average thickness of ca. 1 nm, which indicates the successfully exfoliation into individual sheets in water. To prepare a series of regenerated cellulose composite materials containing various GO loadings, NaOH/urea/H₂O solvent with a weight ratio of 14 : 24 : 162 was firstly precooled to −12 °C, and then 6 g cotton linter pulp was added immediately into the solvent with vigorous stirring for 5 min to obtain a homogeneous cellulose solution (150 ml). The viscous solution was centrifuged at 6000 rpm for 10 min at 5 °C to exclude the few remaining undissolved part and air bubbles. Subsequently, 0.78–5.22 ml GO suspension and 120 mg ECH (about 0.2 ml) were mixed into the 30 ml cellulose solutions at 5 °C and stirred vigorously until obtaining homogeneous GO/cellulose solutions with GO loading of 0, 0.3, 0.5, 1.0 and 2.0 wt%, respectively. After that, the as-prepared composite solutions were immediately cast on a glass plate to give a thickness of 2.5 mm and then placed in the oven at 80 °C for 1 h to obtain gelatinous films, which were subsequently immersed in deionized water for 2 days to eliminate NaOH and urea. Finally, the hydrogel films were transferred into a poly(methylmethacrylate) (PMMA) plate and fixed with tapes. After drying the fixed hydrogels at room temperature for 12 h, regenerated GO/cellulose composites films were obtained. The pure regenerated cellulose film was prepared by the same procedure. The average thickness of all thus prepared composite films is ca. 26 μm.

2.3 Characterization

Atomic force microscope (AFM) images were recorded on a Multimode 8 Bruker microscope with a silicon cantilever using the scanasyst mode. AFM samples were prepared by depositing a dilute aqueous GO suspension on a freshly cleaved mica surface.

To study the dispersion of GO in cellulose solution, the GO/cellulose solution were sandwiched between two pieces of cover glasses and monitored by a BX51 Olympus polarized optical microscope equipped with a DP72 CCD camera.

The dynamic rheology experiments were carried out on an ARES G2 dynamic rheometer (TA, USA). Two parallel plates with a gap of 2 mm were used to measure dynamic viscoelastic parameters such as the storage modulus (G′) as functions of angular frequency (ω). The value of the strain amplitude was fixed at 10%, which is within the linear viscoelastic region, and the sweep of frequency was set at 1 Hz.

Scanning electron microscopy (SEM) images of the samples were obtained on a JEOL SEM 6700 operating at 3 kV. For observing the dispersion of GO in the cellulose matrix, freeze-dried GO/cellulose hydrogels were used for SEM characterization. The samples were sputtered with gold for better observation.

Transmission electron microscopy (TEM) analysis of GO/cellulose films was conducted on a JEOL JEM-2200 FS instrument with an accelerating voltage of 200 kV, and the sample was prepared by using a microtome equipped with a diamond knife.

Wide-angle X-ray diffraction (WAXD) patterns were collected on a Rigaku Ultima IV diffractometer with an incident wavelength of 0.154 nm (Cu Kα radiation). The crystallinity index (CI) of the regenerated pure cellulose and composite films was calculated as the following equation:

CI = 100 × (I₀₂₀ − I_am)/I₀₂₀ (1)

where I₀₂₀ is the maximum intensity of the principal (020) diffraction peak (2θ = 21.9 for cellulose II) and I_am is the intensity of diffraction attributed to amorphous cellulose (2θ = 16 for cellulose II).²⁰

Ultraviolet-visible (UV-vis) spectroscopy was recorded on a UV-2550 (SHIMADZU) UV-Vis spectrophotometer.

Mechanical properties of GO/cellulose composite films and pure cellulose film were evaluated using dynamic mechanical analyses (DMA Q800 TA) with a span length of 10 mm at a testing speed of 0.5 mm min⁻¹. Before testing, the films were tailored into rectangular strips with the width of 4 mm and the thickness were measured separately for each sample. The average values of the mechanical properties were evaluated for three samples, respectively.

3. Results and discussion

Fig. 1 schematically illustrates the preparation process of GO/cellulose composite films. In the first step, GO aqueous suspensions were mixed with cellulose/NaOH/urea solution at low temperature (5 °C) rather than at room temperature to obtain homogeneous GO/cellulose solution. At the beginning, GO was mixed with cellulose/NaOH/urea solution at room temperature following the literature.¹² Occasionally, we noted that GO was more easily dispersed in cellulose/NaOH/urea solution in winter than that operated in summer. This phenomenon lets us consider that mixing temperature may affect the dispersion of GO in cellulose solution, which will be discussed in detail in the later chapter. After the mixture of GO and cellulose solution, ECH as the cross-linking agent²¹ was added into the GO/cellulose solution for improving the interface bonding force of GO sheets and cellulose molecular chains. Subsequently, the GO/cellulose/ECH solution was cast on a glass plate and then placed in the oven at 80 °C for 1 h. During the annealing process, the solution turn to gel, finally, the gelatinous film were washed and dried to form the regenerated GO/cellulose films. The cartoon in Fig. 1 displays the predictable microstructure formation in the sampling process of GO/cellulose composite films. Initially, cellulose is naturally dissolved into freedom molecular chains in the NaOH/urea solution due to the broken of hydrogen bonding between cellulose chains.¹⁴ After the incorporating of GO, hydrogen bonds could be formed between oxygen-containing groups on the GO surface and hydroxyl groups of cellulose molecular chains¹⁵ Subsequently, the addition of ECH renders chemical cross-linking among GO and cellulose chains. As shown in the digital photo of Fig. 1, thus prepared regenerated pure cellulose film is of great transparency and flexible, and its surface is fairly smooth. In contrast, the composite films show uniform color that gradually get darker with increasing the GO loadings, indicating the homogeneous dispersion of GO in the cellulose matrix.

Fig. 1 Schematic illustration of the fabrication process of GO/cellulose composite materials based on NaOH/urea aqueous solution. The digital photo shows the optical transparency of the GO/cellulose films with various GO loadings.

3.1 Dispersibility of GO in cellulose/NaOH/urea solution

Because of electrostatic repulsion and hydrophilicity, GO can be dispersed well in water to form a stable colloidal solution.²² However, the addition of electrolytes may destroy the dispersion equilibrium of GO sheets due to the change in ionic strength.²³ As shown in Fig. 2a, GO suspensions turn to be aggregated heavily when it is added into NaOH/urea solution with strong alkaline character. In contrast, GO seems to be dispersed stably in cellulose/NaOH/urea solution to form a homogeneous GO/cellulose solution at room temperature as reported in previous literature.^12,17,18 To examine the effect of the mixing temperature on the dispersibility of GO in cellulose/NaOH/urea solution, GO in cellulose solutions was investigated by POM. As shown in Fig. 2b, relatively uniform dispersion of GO in cellulose/NaOH/urea aqueous solution can be seen by mixing them at −10 °C and 5 °C. However, obvious aggregation of GO can be observed for the specimen obtained at room temperature (25 °C). This result indicates that the mixture temperature indeed influences the dispersion of GO in cellulose/NaOH/urea solution.

Fig. 2 (a) Digital photos for comparing the stability of GO suspension in NaOH/urea (sample A) and cellulose/NaOH/urea solution (sample C). The stability of pure cellulose/NaOH/urea solution without the addition of GO (sample B) is also provided for reference, (b) POM images of GO/cellulose solution with 0.3% GO loading mixed at different temperatures.

Rheological measurements are further performed to investigate the effect of mixing temperature on the dispersibility of GO in cellulose/NaOH/urea solution. The results from Fig. 3a show that when mixing at lower temperatures (−10 °C and 5 °C), storage modulus G′ curves of both are similar and almost linear with angular frequency (ω) increasing. As the mixing temperature increasing to 25 °C, the G′ value of the GO/cellulose solution has deviated from the linear type and higher than those of GO/cellulose solutions mixing at −10 °C and 5 °C. Moreover, Fig. 3b shows that viscosity of GO/cellulose solution mixed at 25 °C is higher than those obtained at −10 °C and 5 °C in the low-frequency region. Wang et al.²⁴ had found that the uniform dispersion of GO can decrease the viscosity of GO/cellulose solution. Therefore, these rheological data also suggest that GO is dispersed better in GO/cellulose solution mixed at a lower temperature than that at room temperature, which is in agreement with the observation of the POM.

Fig. 3 Storage modulus G′ (a) and viscosity η* (b) as a function of angular frequency for GO/cellulose solutions with 0.3% GO loading mixed at different temperatures.

According to the above characterization, it is verified that more uniform dispersion of GO in cellulose/NaOH/urea aqueous solution could be realized by mixing them at low temperature (≤5 °C). This observation should be related to the dissolution state of cellulose molecular chains in NaOH/urea solution. As reported,²⁵ cellulose/NaOH/urea solution obtained at low temperature (−12 °C) tends to gel with increasing temperature. It means the strong intermolecular association and more physical cross-linkings of cellulose molecular chains start to form at higher temperature. In other words, the interaction among cellulose molecular chains is relatively weak at the lower temperature so that the complexation of GO with cellulose molecular chain becomes easier at low temperature than that at high temperature. Thus, to achieve good dispersion of GO sheets in the cellulose matrix, the mixing of the cellulose solution and GO suspensions is preferably conducted at low temperature for preparing GO/cellulose composites. Considering that the dispersion of GO in cellulose solution at 5 °C is similar to that at lower temperature, such as −10 °C, as shown in Fig. 2b, we therefore conduct the mixture of the GO and cellulose solution at 5 °C for preparing composites in all experiments, which is easy to handle and energy-saving.

3.2 Morphology and microstructure of GO/cellulose composite films

Fig. 4a and b show the SEM images of freeze-dried regenerated GO/cellulose hydrogels with 0.5% and 2% GO loading, respectively. For both samples, there are typical three-dimensional fibrous networks formed by the gelation process of cellulose solution at high temperature.²⁶ Of note, in the GO/cellulose composite with a small amount of GO loading (Fig. 4a), GO sheets are nearly transparent and embedded into the cellulose matrix, as indicated by the red circle. However, with increasing the GO loading, the multilayered GO stacking could be clearly observed as shown in Fig. 4b. To further characterize the dispersion of GO in the cellulose matrixes, the cross-sectional TEM images of GO/cellulose composite films were provided in Fig. 4c and d. In accordance with the SEM observation, the image of composite film with a small amount of GO loading (0.5%) demonstrates better dispersion of GO than that of higher GO loading (2%). Interestingly, Fig. 4c and d suggest the GO sheets are apt to be parallel to the surface of the composite film. Huang et al. had speculated that gravitational forces induce the GO to align parallel with the composite films surface as a result of the high degree of anisotropy and high aspect ratio of the GO.¹⁸ They think that such parallel orientation of GO sheets could improve the mechanical performance of the cellulose film.

Fig. 4 Typical SEM images of freeze-dried GO/cellulose hydrogels (a and b) and cross-sectional TEM images of GO/cellulose composite films (c and d) with 0.5% and 2% GO loadings, respectively.

Fig. 5 shows the wide-angle X-ray diffraction (WAXD) profiles of the pristine, regenerated pure cellulose film and GO/cellulose composite film with 0.5% GO loading. For pristine cellulose material, the typical diffraction peaks are loaded at 2θ = 14.8°, 16.2° and 22.7°, corresponding to the (11̄0), (110) and (020) planes of the cellulose form I crystal.²⁷ For the regenerated cellulose film, the XRD pattern displays three typical diffractions located at 2θ = 12.1°, 19.7° and 21.9° which are assigned to the cellulose form II crystal, respectively.²⁸ Obviously, the diffraction peaks of the GO/cellulose composite film were similar to that of the regenerated cellulose film. It suggests that GO/cellulose composite film has form II crystal. However, the overlapped (110) and (020) diffraction peaks in the 2θ region of 18–24° for regenerated pure cellulose merged to a broad for GO/cellulose film. Meanwhile, the peak intensity of GO/cellulose film in the 2θ region of 18–24° decrease largely compared with that of regenerated cellulose film. The crystallinity index (CI) of regenerated pure cellulose film and GO/cellulose composite film with 0.5% GO loading were 54.05% and 38.95%, respectively. The obvious decrease in crystallinity may suggest that GO sheets and cellulose molecular chains are successfully combined by chemical cross-linking. Of note, it is found that the diffraction peak position (12.1°) of cellulose shifts to 11.9° after incorporation of GO. This suggests that the presence of GO sheets affect the regeneration process of cellulose to some extent, resulting in the change of interplanar crystal spacing of cellulose.

Fig. 5 WAXD profiles of pristine, regenerated cellulose and GO/cellulose composite film with 0.5% GO loading.

3.3 Optical properties of GO/cellulose composite films

The transparencies of cellulose and GO/cellulose films are monitored by UV-vis spectroscopy. As shown in Fig. 6a, the transparencies of GO/cellulose composite films decrease with increasing the GO loading in the wavelength range of 200–900 nm. For quantitative analysis, the effect of GO loading on the transmittance of composite films at specific wavelengths (300 nm and 600 nm) is displayed in Fig. 6b. The data shows that regenerated pure cellulose film has the highest transmittance for not only visible light (92.7% at 600 nm) but also UV light (67.4% at 300 nm). Interestingly, for GO/cellulose composite film with a small amount of GO loading (<1%), the UV light transparencies of as prepared GO/cellulose composite films are decreased sharply with the increase of GO loading, whereas there is only limited sacrifice in visible light transmittance. When GO loading reaches 2%, there is also a sharp decrease in visible light transmittance, which should be caused by the aggregation of GO sheets. Of note, the GO/cellulose film with only 0.3% GO loading can absorb 76.8% and the addition of 2% GO can absorb 99.9% UV light. It suggests that thus prepared GO/cellulose films have the potential application as UV-shielding materials. This character should be attributed to UV photons absorbance character of GO^29,30 and oriented distribution morphology of GO in the cellulose matrix as demonstrated in Fig. 4.

Fig. 6 (a) UV-vis spectra of neat cellulose and GO/cellulose films with different GO loadings, (b) effect of GO loading on the transparency of GO/cellulose films at specific light wavelengths (300 nm and 600 nm), respectively.

3.4 Mechanical properties of GO/cellulose composite films

Fig. 7a presents the typical tensile stress–strain curves of pure cellulose film and GO/cellulose composite films with different GO loadings. As expected, the incorporation of GO into the cellulose matrix does play a significant effect on the mechanical performance of composite films. The tensile strength, elongation at break and fracture energy of the composite films exhibit an apparently increasing trend as depicted in Fig. 7b–d. For example, the tensile strength, elongation at break and fracture energy of GO/cellulose film with 0.5% GO loading are 129.2 MPa, 15.45%, and 16.31 MJ m⁻³, respectively. Compared with those of neat cellulose films, there is prominently 78% enhancement in tensile strength, 172% enhancement in elongation at break and 397% enhancement in fracture energy. Nevertheless, with increasing the content of GO from 0.5% to 2%, the tensile strengths of the composite films decrease from 129.2 MPa to 82.3 MPa, and the elongation at break decrease from 15.45% to 3.11%, the fracture energies decrease from 16.31 MJ m⁻³ to 1.81 MJ m⁻³. These results should be caused by the inhomogeneous dispersion of GO sheets in the cellulose matrix with the increasing of GO loadings. As revealed in Fig. 4, there is aggregation of GO nanosheets with higher GO loadings, which thus results in the decrease of mechanical properties of composites films. Herein, the pronounced enhancement of mechanical properties for GO/cellulose composites with the addition of small amount of GO loading should come from two aspects. One is due to the effective dispersion of small amount of GO in cellulose matrix improved by the low-temperature mixture of GO and cellulose/NaOH/urea solution. The other reason is more likely to result from the cross-linking effect of ECH. When ECH/GO/cellulose solution turns into hydrogel by thermal treatment at 80 °C, regenerated fibrous network was formed via intermolecular interaction. Simultaneously, reactions among epoxy groups of ECH and hydroxyl groups of cellulose and GO occurred. Thus, both the physical cross-linking by cellulose itself and chemically cross-linking among the ECH, cellulose and GO will contribute to the sol–gel transition.^12,21 Enhanced interactions between them are also conducive to the improvement of mechanical properties for composites.

Fig. 7 (a) Typical stress–strain curves of cellulose and GO/cellulose films with different GO loadings. (b) Tensile strength, (c) elongation at break, (d) fracture energy of cellulose and GO/cellulose films with different GO loadings derived from (a).

4. Conclusions

In this study, GO-incorporated cellulose (GO/cellulose) composite films were fabricated using NaOH/urea aqueous solution as the solvent. POM images and the rheological behaviour indicated that GO dispersed better in cellulose/NaOH/urea solution by mixing them at low temperature rather than room temperature. Moreover, our results demonstrated that the addition of ECH as crosslinking agent into well-dispersed GO/cellulose composite would greatly improve the mechanical performances due to enhanced interactions between GO sheets and cellulose molecular chains. As compared to the results of previous studies that adopted GO as additives for preparing the cellulose composite, GO/cellulose composite reported here showed large improvements in tensile strength and toughness simultaneously. For instance, the mechanical strength, toughness and fracture energy of the composite film with 0.5% loading were drastically improved by 78%, 172%, and 397% compared with the pure cellulose film, respectively. The composite film also possesses excellent UV-shielding properties. Meanwhile, GO loading of 2% rewards the composite film nearly the complete ultraviolet shielding ability. Accordingly, this study presented here broadens the application of both cellulose and GO-based materials in the packaging and protective industry.

Acknowledgements

The authors acknowledge the financial support from National Natural Science Foundation of China (51573082) and Taishan Mountain Scholar Foundation (TS20081120 and tshw20110510).

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Footnote

† Electronic supplementary information (ESI) available: A typical AFM image of GO sheets deposited onto a mica substrate from an aqueous dispersion. See DOI: 10.1039/c6ra16535d

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