Article pubs.acs.org/JPCC
Graphene-Oxide-Sheet-Induced Gelation of Cellulose and Promoted Mechanical Properties of Composite Aerogels Jing Zhang, Yewen Cao, Jiachun Feng,* and Peiyi Wu State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science and Laboratory of Advanced Materials, Fudan University, Shanghai 200433, P. R. China
J. Phys. Chem. C 2012.116:8063-8068. Downloaded from pubs.acs.org by OPEN UNIV OF HONG KONG on 01/23/19. For personal use only.
S Supporting Information *
ABSTRACT: By taking advantage of cellulose, graphene oxide sheets (GOSs), and the process of freeze-drying, we propose a simple and effective method to prepare green cellulose aerogels with significant mechanical improvements. The addition of GOSs could accelerate the gelation of cellulose solution, which was confirmed by differential scanning calorimetry and rheology. Detailed investigations including dynamic light scattering and ultraviolet spectroscopy revealed the existence of interaction between GOSs and cellulose chains, which might be responsible for the promotion of the gelation process. With the incorporation of only 0.1 wt % GOSs, the compression strength and Young’s modulus of the composite aerogels were dramatically improved by about 30 and 90% compared to with those of pristine cellulose aerogels, respectively. This method is believed to provide possibilities to combine the extraordinary performances of GOSs with the multifunctional properties of environmentally friendly cellulose-based aerogels, thus holding great potential for biological applications in the future.
1. INTRODUCTION Aerogels, which are produced by the replacement of solvent with air, have received considerable attention because of their fascinating performances such as ultralow density, low heat conductivity, and large inner surface area and their various practical applications such as kinetic energy absorbers, thermal and acoustic insulating materials, reinforcing platforms, and scaffolds.1,2 Compared with inorganic aerogels, organic aerogels are gaining more interest because of their better toughness.3 Among organic aerogels, cellulose aerogels show great prospect in future application owing to their biodegradation, environment friendliness, and reproducibility.4,5 Following Kistler,6 who first created aerogels composed of cellulose, numerous attempts to prepare cellulose aerogels have been energetically pursued.4,7−11 More recently, the photoswitchable superabsorbent,12 superhydrophobic, superoleophobic,13 or magnetic cellulose aerogels14 have also been prepared by using various functionalized celluloses. However, for their better utilization, disadvantages related to the inferior mechanical properties of cellulose aerogels, which resulted from the defects such as dangling ends and loops in their structures, are the main challenges that have to be overcome.15 A typical technique for preparing cellulose-based materials comprises the following steps: (i) dissolving of cellulose in proper solvents, (ii) casting, (iii) extracting the solidified castings to initiate cellulose coagulation, and (iv) drying of the obtained cellulose gel to obtain the cellulose-based materials.16 In the past decades, the development of green solvents of cellulose makes it easy to prepare cellulose-based material.15,17,18 In this context, it is worth mentioning the new © 2012 American Chemical Society
mixture solvent of NaOH/thiourea aqueous systems (usually made of ∼9.5 wt % NaOH mixed with 4.5 wt % thiourea solution) invented by Zhang et al.19 This solvent has been demonstrated to be an environmentally friendly system and in which the “dissolution” of cellulose could be achieved at low temperature (−8 to −5 °C) within short time periods. By using this solvent, a series of novel regenerated cellulose-based materials with homogeneous structure and excellent transparency have been successfully prepared.20,21 However, Liang et al.22 found that the casting and coagulation steps usually lead to the lack of mechanical performance due to the weak hydrogen bonding interaction and the quick rearrangement of cellulose chains. Furthermore, they also found that if the pregelation process was introduced into the preparation, then the above drawbacks would be partially overcome. Presently, the pregelation process, based on the thermal irreversible gelation properties of cellulose solution, has been frequently employed to prepared cellulose-based materials, especially cellulose aerogels, because the slow sol−gel transition favors a homogeneous and dense structure.18,22 For instance, Liang et al.23 fabricated cellulose aerogels with improved strength on the basis of the solution pregelation technique. Nevertheless, the pregelation process usually takes a long time to obtain the cellulose physical gel, and the improvement in mechanical properties is also rather limited. Received: November 14, 2011 Revised: February 6, 2012 Published: March 20, 2012 8063
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have been reported on the performance improvement of cellulose aerogels using GOSs. Taking these considerations into account, in this study, we showed a simple, fast, and environmental friendly preparation of cellulose-based aerogels by incorporating GOSs into the cellulose matrix using NaOH/thiourea aqueous solution as the processing solvent. Herein, we focused on the dispersion of GOSs in cellulose matrix and the promotion of the gelation behavior of cellulose solution. Afterward, compared with neat cellulose aerogel, the compression stress and Young’s modulus aerogel have been significantly raised, respectively. Thermal stability of composite aerogel had also been improved.
Incorporating with reinforcement additives that have strong interaction with the matrix is a widely used technique to improve the mechanical properties of polymer materials, and some attempts in preparation of cellulose-based materials have been reported.24,25 Cellulose whisker is the most common filler used in reinforcing cellulose composites because of its high aspect ratio and high mechanical performance.26 Qi21 found that when the cellulose whisker content increased from 0 to 10 wt %, the tensile strength of cellulose film increased from 87 to 124 MPa because a rigid filler network formed through inducing a mechanical percolation phenomenon. In fact, some materials that have strong interaction with cellulose matrix could also have a reinforcement effect. Different molecular weight PEGs with ether oxygen atoms, which have been reported to form hydrogen bonding interaction with cellulose chains,27,28 were adopted to increase the tensile strength of cellulose gel.23 Notwithstanding the considerable efforts in the preparation of cellulose-based composites with improved properties made by addition reinforcement agents, the questions are open in the preparation of cellulose aerogels. Considering the inferior mechanical properties of cellulose aerogels and the time-consuming pregelation process, the explorations on more efficient reinforcing additives for cellulose aerogels as well as its effects on the preparation, are urgently required. As the precursor of graphene, graphene oxides sheets (GOSs) have been studied as the reinforcing fillers to enhance polymer matrix29,30 because of their high aspect ratio, excellent modulus, and intrinsic strength. Moreover, containing numerous oxygen functional groups on their surfaces,31 GOSs could have strong interaction with polar polymers such as poly(vinyl acetate) (PVA),32−36 poly(N-isopropylacrylamide) (PNIPAM),37−39 chitosan,40 and deoxyribonucleic acid (DNA).41 The strong interactions not only lead to a prominent reinforcing effect of GOSs to polymer matrix but also facilitate the fabrication of GOS/polymer composite hydrogels. Among widely investigated GOS-modified hydrogels, PVA/GO and PNIPAM/GO hydrogels have been extensively investigated because of their attractive environmentally friendly and sensitive characteristics.32,33,39 GO sheets would form pHsensitive hydrogels with PVA, which relied on the assembly of GOSs and the cross-linking effect of PVA chains.32 Zhang et al.33 prepared the PVA/GO composite hydrogels by a freeze/ thaw process, and a 132% increase in tensile strength and a 36% improvement of compressive strength were obtained. Such remarkable improvements in mechanical properties are suggested to be due to not only the extraordinary properties of GOSs but also the strong hydrogen bonding between GOSs and PVA. Recently, Alzari et al.39 successfully synthesized PNIPAM nanocomposite hydrogels with reduced graphene oxides by using frontal polymerization and found that G′ modulus and complex viscosity increased by increasing RGO content. In fact, there are already some pioneer works devoted to the composite of cellulose and GOSs.42,43 Zhang et al. prepared a cellulose composite film with improved tensile strength by the addition of GOSs using ionic liquid as solvents.42 More recently, Seppala et al. successfully synthesized composite paper using amine-modified nanofibrillated cellulose and GOSs. The composite paper showed a significantly enhanced tensile strength of 273 MPa that is 1.4 and 2.8 times higher than that of the cellulose papers and graphene oxide paper, respectively.43 However, to the best of our knowledge, there is no attempt, and systemic investigations
2. EXPERIMENTAL SECTION 2.1. Materials. Graphite oxide solids used in our experiments were synthesized and purified from expandable graphite (Yingtai, China) by a modified Hummers method.44 Cellulose powder was supplied by Aladin Reagent Company (Shanghai, China). It was vacuum-dried at 55 °C for 24 h to remove any moisture before use. Unless otherwise stated, all other reagents and solvents were purchased from commercial suppliers and used as received. 2.2. Preparation of Cellulose and Cellulose/GOSs Aerogels. NaOH/thiourea/H2O mixed solution with a weight ratio of 9.5:4.5:86.0 was used as the cellulose solvent (CS). After storing the CS in −10 °C for 8 h, cellulose in the desired amount was immediately added to the CS with strong stirring at 3000 rpm for 5 min at the ambient temperature, and a transparent cellulose solution was obtained. To prepare cellulose/GOS dispersions, GOSs were added to the cellulose solution with the same stirring speed. The samples are denoted as CxGy, where x indicates the weight percent of cellulose in the solution, whereas y indicates the weight percent fraction of GOSs relative to the weight of cellulose used. Among these samples, only C10G0 and C10G0.1 were utilized to prepare aerogels. To observe the dispersion ability of GOSs in cellulose solution and in CS alone, we also prepared the samples C5G0, C5G0.1, and C0G0.1. For the preparation of aerogels, these two samples were transformed into a tube with 10 mm in diameter and stored at ambient temperature for 30 min to allow gelation. Then, the gelated samples were placed in deionized water several times for complete solvent exchange and freeze-dried using a FD-1A-50 lyophilizer (Boyikang, China) with a condenser temperature of −50 °C and an inside pressure of 20 Pa. After 24 h of freeze-drying, aerogels with cylindrical shape were finally obtained. 2.3. Measurements. Differential scanning calorimetry (DSC) measurements of cellulose solution and cellulose/ GOS dispersion were performed by Mettler-Toledo DSC with a heating/cooling rate of 15 °C/min at the temperature range of 25 to 120 °C. The dynamic rheology experiments were carried out on a Bohlin GeminiHRnano rheometer (Malvin Instruments, U.K.). Double coaxial cylinder geometry with a gap of 2 mm was used to measure dynamic viscoelastic parameters such as the storage modulus and loss modulus (G′ and G″) as functions of time (t). The value of the stain amplitude was set at 10%, which is within a linear viscoelastic regime. The sweep of frequency was set at 1 Hz. Ultraviolet spectra (UV−vis) of GOS solution, cellulose, and cellulose/ GOS dispersion in aqueous were recorded on a Shimadzu model 3150 spectrophotometer. The compression strength of the aerogels was measured on a SANS CMT-6503 universal testing machine (Shenzhen, China) according to the ISO6048064
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2002 standard. Before testing, the aerogels were tailored to the same size of 10 mm in length, and the strain rate of 5 mm/min was applied. The measurements were carried out seven times for each sample.
To clarify the gelation process of cellulose solution with the addition of GOSs, we have chosen DSC and rheological measurement to describe the acceleration effect. Figure 2 shows
3. RESULTS AND DISCUSSION 3.1. Gelation Behavior of Cellulose Solution and Cellulose/GOS Dispersion. The gelation process of cellulose solution has relation to the ambient temperature, concentration of cellulose solution, molecular weight of cellulose, and so on. According to Zhang et al.,45 for the same concentration of cellulose solution, the gelation temperature decreased from 60.3 to 30.5 °C, with the increase in Mη from 4.5 × 104 to 11.4 × 104. Considering that the molecular weight of cellulose used is relatively low in our work, we prepared a high concentration cellulose solution (10 wt %) to observe the gelation process at ambient temperature in a short time. Figure 1 shows the Figure 2. DSC curves for the heating process of cellulose solution (C10G0) and cellulose/GOS dispersion (C10G0.1); the heating rate is 15 °C/min.
the DSC traces during the gelation process for C10G0 and C10G0.1. The traces for neat cellulose solution presented two exothermal peaks: one located at 56 °C and the other at 105 °C, which could be attributed to the sol−gel transition of cellulose solution and the evaporation of water, respectively. As for C10G0.1 solution, the DSC trace was slightly different: the sol−gel transition temperature shifted almost 20 °C lower in comparison with that of cellulose, whereas the evaporation temperature of water had almost no obvious change. Besides, the incorporation of GOSs in cellulose also broadens the exothermal peak of sol−gel transition. Therefore, the results of DSC suggest that the sol−gel transition could take place in lower temperature and in a wider temperature range after incorporation of GOSs in cellulose solution. In other words, the gelation rate of cellulose solution is prominently accelerated. Figure 3 shows the results of rheological measurement for the gelation process in C10G0 and C10G0.1 at a frequency value of 1 Hz. Referred to the above DSC results, 50 °C was chosen as the test temperature to observe the sol−gel transition in a relatively short time. In general, the crossover of the storage moduli G′ and the loss moduli G″ curves is regarded as the determination of the gel point, which was taken as Ggel. The crossover of G′ and G″ indicates that gelation could take place
Figure 1. Digital pictures of gelation process of cellulose/GOS dispersion (C10G0.1) (black) and cellulose solution (C10G0) (yellow). After standing at room temperature for 20 min (a) and for 1 h (b).
gelation process of C10G0 and C10G0.1. After standing for 1 h at room temperature, they gradually transformed into the solidlike state, which could not flow any longer (Figure 1b). It indicates that the cellulose physical cross-linked network formed with time. The color of pure cellulose solution and gel is a little yellow because of the thiourea used in CS, whereas the color is black after the addition of GOSs. Besides, it should be noted that the gelation time of C10G0.1 is shorter than that of C10G0. With the addition of GOSs, the composite dispersion could not flow after ∼20 min, whereas the cellulose solution still could flow, although the viscosity is higher (Figure 1a). That is to say, the cross-linked network forms much faster in the composite dispersion compared with that in the cellulose solution. This indicated that there should be certain roles of GOSs in rigidifying the assembly, which might originate from the interactions between GOSs and the cellulose molecules. A similar situation has been reported by Shi and coworkers,41 who found that the addition of GOSs could accelerate DNA to form multifunctional hydrogels. They proposed that the strong binding of DNA chains to GOSs through π−π stacking and hydrophobic interactions as well as hydrogen bonding interaction was the reason for the promotion in forming DNA hydrogels. Because none of the above two interactions exists between GOS and cellulose, the hydrogen bonding interaction should be the most possible interaction responsible for the acceleration of gelation behavior.
Figure 3. Rheological results of cellulose solution (C10G0) and cellulose/GOS dispersion (C10G0.1). 8065
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cellulose-containing solution for a long time. Interestingly, the colors of both C5G0.1 and C0G0.1 become more and more black with prolonging the placement, which is similar to the phenomenon of deoxygenation of GOSs under strong basic conditions described by Fan et al.48 Hence, we suppose that the reduction of GOSs probably took place during this process. (See Figures S1 and S2 of the Supporting Information.) In fact, GNS-based composites have already been prepared through the reduction of GOSs with alkali dioxides,49,50 and systemic research of this aspect is under study in our laboratory. Figure 5 demonstrates the UV spectra of GOS solution, cellulose/GOS, and aqueous cellulose dispersion. The curve of
in both C10G0 and C10G0.1 samples. However, the gelation time is 853 s for C10G0 and 377 s for C10G0.1, which indicates that the sol−gel transition of C10G0.1 took place earlier than that of C10G0. It is also notable that the values of G′ and G″ for C10G0.1 are significantly larger than those for C10G0, which suggests that much stronger intermolecular association has occurred when mixed with GOSs and more entanglement development of cellulose chains has occurred with the addition of GOSs, leading to the formation of elastic gels. The results of both DSC and rheology verified the acceleration of gelation that was observed in forenamed section. 3.2. Dispersion Ability of GOSs in CS with and without Cellulose. Figure 4 shows the digital picture of dispersion of
Figure 4. Digital picture of dispersion of cellulose and GOSs in CS. From left to right: C5G0 (a), C5G0.1 (b), and C0G0.1 (c). (d) and (e) were obtained after standing (b) and (c) for 6 h at room temperature.
Figure 5. UV−vis spectra of GOSs solution, cellulose/GOS, and cellulose dispersion in aqueous.
cellulose and GOSs in CS. C5G0 is a transparent solution (Figure 4a), which suggests that cellulose was well-dissolved in used solvent. With the addition of GOSs, the color of C5G0.1 turned brown and the solution was still homogeneous (Figure 4b), which indicated the good dispersion of GOSs in cellulose solution. Noticeably, the situation for sample C0G0.1 (Figure 4c), which contains the same amounts of GOSs alone compared with the sample C5G0.1, was quite different. GOSs aggregated immediately just after their addition, and the bad dispersion could not be slightly improved, even through about 1 h of ultrasound. This phenomenon reminded us of the discussion of the interaction between carbon materials and cellulose. Gokhale et al.46 once found that SWCNTs and cellulose could disperse in water without any assistance. They attributed this to the intermolecular hydrogen bonds between cellulose and nanotube that led to the good codispersion. Similarly, in our study, GOSs could disperse in cellulose containing CS instead of only in CS. We may also attribute this to the intermolecular hydrogen bonds between cellulose and GOSs, as discussed in detail in the follow section. Regarding the reason why strong basic solution (CS) does not have a positive effect on dispersion, it is likely to be the high ionic strength of CS. According to Wallace’s work,47 GOSs dissolve in water as colloids through strong electric repulsion, which would be destroyed by the electrolytes. With the addition of salts, the ionic strength of GOS solution would increase, which reduces the repulsion between GOSs and leads to their aggregation. In this work, the high ionic strength of solution caused by the high concentration of NaOH reduces the repulsion between GOSs and finally results in their aggregation. After 6 h of standing, sample C5G0.1 is still homogeneous (Figure 4d), whereas GOSs in C0G0.1 aggregate and precipitate (Figure 4e). This confirms that GOSs could remain stable in
GOSs exhibits a maximum at ∼230 nm, which is associated with a π→π* transition, and a shoulder at ∼300 nm, which is associated with a n→π transition.47 The absorption peak of cellulose/GOS is broader than that of GOS or cellulose, indicating the formation of the cellulose/GOS complex, which might be due to the interaction between cellulose and GO sheets.51 Moreover, there are two peaks on the DLS profile (Figure S4 of the Supporting Information): the lower peak is attributed to single cellulose molecular chain, and the higher peak is attributed to the cellulose inclusion complexes (ICs). Compared with pure cellulose solution, the addition of GOSs makes cellulose IC have a higher value, indicating that GOSs have been engaged in cellulose chains through the strong interaction between them,52 which agrees with the result of UV−vis spectra. It is worth noting that the absorption peak of π→π* transition located at ∼230 nm of GOS has a red shift to ∼280 nm for cellulose/GOS, showing the reduction of GOSs in CS,47 which is consistent with what we have observed. No obvious broad absorption of thiourea confirms that the thiourea has been completely eliminated,53 which is also corroborated by the DTG result of thiourea. (See Figure S3 of the Supporting Information.) According to the experiment results, we suppose a possible mechanism for the acceleration of the gelation behavior of cellulose solution with the addition of GOSs, as is shown in Figure 6. First, the rapid dissolution of cellulose at low temperature is a dynamic supramolecular assembly process mainly through hydrogen bonds. The small molecules of thiourea and NaOH form overcoats around cellulose chains by a hydrogen bond, which leads to the dissolution of cellulose. The association of hydrogen bonds is a reversible equilibrium reaction. At low temperature, the speed of the exchange of hydroxyl groups between cellulose molecular and NaOH is low; 8066
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Figure 6. Schematic gelation process of the cellulose solution and cellulose/GOS dispersion at ambient temperature.
Figure 7. Stress−strain curves (a) and Young’s modulus as well as compression strengths (b) of cellulose and cellulose/GOS aerogels.
briefly speaking, hydroxyl groups are fixed. With the temperature increased, the reaction speed also increases; if the cellulose chains have more exposed hydroxyl groups in a moment, then they have more opportunities of association that form a cross-link network structure. If GOSs have been added to the cellulose solution, then, in the same heating process, well-dispersed GOSs promote the speed of forming the physical cross-linking networks because GOSs have large amounts of oxygen-containing groups in the plane and these groups would have interaction with hydroxyl groups exposed in cellulose chain through hydrogen bonding. Therefore, GOSs would be regarded as cross-link agents in cellulose gelation. 3.3. Mechanical Properties of Cellulose/GOS Aerogel. We further explore the effect of GOSs in cellulose matrix. The representative stress−strain curves of cellulose and cellulose/ GOS aerogels are shown in Figure 7. The reinforcing effect of GOSs is rather significant: With the addition of 0.1 wt % GOSs in cellulose aerogel, the Young’s modulus and compression strength of the aerogel increased by 90% from 38.5 to 58.1 MPa and 30% from 0.87 to 1.13 MPa, respectively. The ideal reinforcing ability of GOSs is attributed not only to the strong interfacial adhesions with the matrix54 but also to the high moduli and aspect ratio, which play an important role in unique compression behavior.55 Compression stress−strain behavior of aerogel usually includes two regions. In low strain, the linear elastic in nature is expected to be primarily due to elastic cell wall bending. With the help of high moduli of GOSs, the elastic cell wall is facilitative to resist the pressure. In higher strain, the linear stress−strain behavior gradually changes to nonlinear stress-stain. The material would collapse in this region, which is related to the structure of cell wall, especially in polymer aerogels. The nanopaper-like structure maintains its loadbearing capacity better than a conventional polymer after “plastic yielding”. Hence, the high aspect ratio of GOS sheets contributes to enhance the aerogel in this region. Furthermore, the remarkable improvement in the mechanical properties should also be closely related to the good dispersion of GOSs in cellulose solution as well as in aerogel. In fact, besides the
mechanical performances, the thermal properties have also been improved with the corporation of GOSs. (See Figure S5 of the Supporting Information.) By optimizing the processing conditions and the ratios of GOSs in cellulose aerogels, it is possible to prepare cellulose aerogels with improved mechanical and thermal properties, which are of particular importance for many potential applications.
4. CONCLUSIONS A novel and easy method has been proposed to prepare cellulose/GOS aerogel with high mechanical strength and good thermal stability. This kind of composite aerogel represents the first example of aerogel prepared by combining the extraordinary performances of GOSs with environmentally friendly and reproducible cellulose via a convenient process. The good dispersion of GOSs in cellulose matrix and the acceleration of cellulose solution gelation with the addition of GOSs prove that GOSs have a strong interaction with the cellulose chain, which leads to the resultant cellulose/GOS composite aerogel exhibitsing a 30% increase in compression strength and a 90% increase in Young’s modulus compared with neat cellulose aerogel. Considering the high performance of the aerogel and the biocompatibility of GOSs and cellulose, our composite aerogel is attractive for a variety of biological and environmental applications such as tissue engineering, drug delivery, and high-performance composites. Furthermore, this work broadens the application scope of GOSs and promotes the development of both cellulose and GOS-based materials.
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ASSOCIATED CONTENT
S Supporting Information *
Additional TGA traces and XRD spectra for GOSs, DTG trace for thiourea, DLS for cellulose solution and cellulose/GOSs dispersion, and TGA and DTG traces for cellulose and cellulose/GOSs aerogels. This material is available free of charge via the Internet at http://pubs.acs.org. 8067
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AUTHOR INFORMATION
Corresponding Author
*Tel: 86 21 6564 3735. Fax: 86 21 6564 0293. E-mail: jcfeng@ fudan.edu.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of China (21174032, 20874017), National Basic Research Program of China (2011CB605704), PetroChina Company Limited, and PetroChina Innovation Foundation (2011D-5006-0504).
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