Thin Cellulose Nanofiber from Corncob Cellulose and Its Performance

Research Article pubs.acs.org/journal/ascecg

Thin Cellulose Nanofiber from Corncob Cellulose and Its Performance in Transparent Nanopaper Xingya Kang,†,‡ Peipei Sun,§ Shigenori Kuga,† Chao Wang,† Yang Zhao,† Min Wu,*,† and Yong Huang*,† †

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Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 29 Zhongguancun East Road, Haidian District, Beijing 100190, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China § School of Polymer Science and Engineer, Qingdao University of Science & Technology, Shandong, People’s Republic of China ABSTRACT: Cellulose separated from corncob was used as a new cellulose resource to produce esterified cellulose nanofiber (E-CNF) with hexanoyl chloride through one-step mechanochemical esterification by ball milling. The result showed that corncob cellulose was easily disintegrated and esterified to achevie a high DS, and then, thin nanofiber was compared to the common pulp cellulose resource. The DS of E-CNF was as high as 0.95, and the diameter was about 1.5−2.8 nm. Then, E-CNF was formed to nanopaper by vacuum filtration showing high optical transparency up to 89% at 550 nm. The transparent nanopaper had a Young’s modulus of 5.5 GPa and tensile strength of 110−125 MPa. Due to the introduction of alkyl chain, the wetting property of the nanopaper was changed from hydrophilicity to hydrophobicty. So, it may still work well in a humid environment. KEYWORDS: Corncob cellulose, Cellulose nanofiber (CNF), Transparent, Nanopaper, Hydrophobic

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INTRODUCTION Cellulose nanofiber (CNF) obtained by disintegration of native cellulose is gaining more and more interest as a component of advanced materials due to its sustainability, biodegradability, and unique physicochemical properties.1−3 While the common sources of cellulose are cotton, linen, and wood, cellulose is present in nearly all plant tissues at varying contents. Therefore, possible utilization of agricultural waste as a cellulose source is attracting attention of late. One example is corncob, which is wasted in large amounts in corn starch production. Thus, we are interested in the use of corncob as the source of CNF and its application. The application of CNF ranges from composite components4−6 and films7 to hydrogels8 and aerogels.9 One remarkable feature of CNF is formation of a transparent sheet when it is neat-compacted, unlike ordinary fibrous cellulose forming paper or nonwoven sheets. Thus, the CNF film may also be called nanopaper. Due to its transparency, flexibility, low thermal expansion, and high mechanical strength, CNF film is expected to be useful in electronic devices, such as flat-panel displays,10 solar cells,11 lithium ion battery cells,12 and also as electrodes when it is made conductive13,14 and electrically triggered shape-memory nanocomposites.15 There are also many works about CNF films with barrier properties toward water,16 oxygen,17 oil,18 and grease,19 being possibly useful in the packaging industry. © 2017 American Chemical Society

One shortcoming of cellulose nanopaper is the lack of water resistance arising from the hydrophilicity of cellulose, limiting its use in many fields. It is possible, however, to reduce the hygroscopicity of cellulose by introducing nonpolar groups to the surface of CNF. As such, there are reports of esterification,20 fluorination,21 silylation,22 and grafting.23 In this paper, to expand the source of cellulose, we prepared esterified CNF (E-CNF) from corncob cellulose through onestep mechanochemical esterification by ball milling based on our previous work.24 The efficiency of esterification−disintegration was assessed in terms of changes in degree of substitution (DS) and morphology of the products. Then, ECNF was formed to nanopaper for which optical, surface, and mechanical properties were studied for practical utility.

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EXPERIMENTAL SECTION

Materials. Corncob cellulose was kindly provided by Ji’nan Shengquan Group Co., Ltd. The material is the residue of corn saccharification, purified by alkali, acid, and bleaching treatments to a cellulose content of 90.4%. The dry powder was used without further purification. The esterification agent was hexanoyl chloride (HC, Alfa Aesar, U.S.A.). N,N-Dimethylformamide (DMF) and t-butyl alcohol Received: November 28, 2016 Revised: January 20, 2017 Published: January 31, 2017 2529

DOI: 10.1021/acssuschemeng.6b02867 ACS Sustainable Chem. Eng. 2017, 5, 2529−2534

Research Article

ACS Sustainable Chemistry & Engineering

was set as 25 °C, and the relative humidity (RH) was 0%, 50%, and 90% to obtain the equilibrium adsorption capacity. The water contact angle of CNF nanopaper was tested by a DataPhysics OCA-20 using static sessile drop method. Tensile mechanical properties of CNF nanopaper was performed by a MTS Sintech-1 (MTS Systems) with a strain rate of 4 mm/min. Specimen strips were 5 mm wide and 30 mm long, with gauge length of 20 mm. The thickness of samples were measured separately before the measurements. The samples were conditioned in a measuring environment for 48 h before testing. At least five replicates were measured for each sample.

were provided by Beijing Chemical Reagent Co. All the reagents were of analytical grade and used without further purification. Preparation of CNF. E-CNF was produced in the same way as Huang et al.24 Typically, 0.5 g of corncob cellulose, 23 mL of DMF, and 2.0 mL of HC were added to a 45 mL zirconia pot containing seven 10 mm zirconia balls. The molar amount of HC was 1.5 times that of the hydroxy group of cellulose. Ball milling was carried out under programmed punctuated operation (working time 20 min and interval of 2 min) with a rotation of 200 rpm at room temperature. The milling time was varied from 1 to 24 h. The milled sample was collected by rinsing with DMF and washed with DMF by centrifugation for 5 min at 12,000 rpm at least three times to remove unreacted HC. The dry sample was obtained by freeze-drying of the wet sample exchanged from DMF to t-butyl alcohol by centrifugation. The samples were denoted as E***-CNF; E means “esterified”, and *** indicates the ball milling time. For example, E4-CNF means a milling time of 4 h. For comparison, the corncob cellulose was ball milled with DMF for 6 h without HC and treated in the same way as E-CNF to get the unmodified CNF(Un-CNF). In this paper, without special note, CNF is referred to as unmodified CNF and esterified CNF collectively. Preparation of CNF Nanopaper. E-CNF or Un-CNF suspension in DMF was diluted to 0.1% (w/w) and sonicated by a tip-type generator (JY99-IIDN, Ningbo Scientz Biotechnology, China) to get a uniform suspension. The suspension was vacuum-filtrated through a Nylon-6 membrane filter with a pore size of 0.65 μm. The wet film was peeled from the membrane and dried in an oven at 60 °C overnight. The thickness of the nanopaper was controlled by the amount of suspension used. Characterization. The original corncob cellulose and freeze-dried CNF were ground with KBr in a weight ratio of 1:100. The spectra were obtained in transmission mode in the range of 400−4000 cm−1 with a resolution of 2 cm−1 and accumulation of 32 scans by FTIR spectrometer (Varian 3100). The DS was calculated by weight gain by eq 1. Here, m1 is the dry weight of the starting cellulose; m2 is the dry weight of E-CNF; 162 is the molar mass (g/mol) of anhydroglucose unit; 98 is the molar mass (g/mol) increase by esterification with a single HC molecule.

DS =

162 × (m2 − m1) 98 × m1

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RESULTS AND DISCUSSION Evolution of Esterification. Mechanochemical esterification made cellulose undergo defibrillation and esterification simultaneously (Figure 1), which was confirmed by an FTIR

Figure 1. Esterification of cellulose nanofiber with HC by ball milling.

(1)

The original corncob cellulose was compressed into pellet. The XRD patterns of the pellet and CNF nanopaper were recorded by an X’Pert PRO X-ray diffractometer (Bruker AXS GmbH) with Cu radiation (λ = 0.154184 nm) from 5° to 60°. The increment step was 0.02°, and the scan speed was 0.05 s/step. The crystallinity degree was calculated by eq 2, where I002 is the intensity of the crystalline region of cellulose (2θ = 22.2°) and Iam is the intensity of the amorphous phase (2θ = 18.6°).

CI (%) =

I002 − Iam × 100 I002

Figure 2. FTIR of original corncob cellulose, Un-CNF, and E-CNF.

spectrum (Figure 2). The new band at 1744 cm−1 was the C O stretching of the ester group, and 2903 cm−1 was the C−H stretching of the alkyl chain, together showing a successful introduction of the ester group. The intensity of CO stretching increased with ball milling time, indicating progressive esterification. The DS grew rapidly in the first 12 h and gradually leveled off, reaching 0.96 at 24 h of milling (Figure 3). This level was significantly higher than that of the bleached kraft pulp cellulose, ∼0.58.24 The time course of DS was plotted in Figure 3 as compared to the pulp. The figure shows the remarkable readiness of corncob cellulose to disintegrate and undergo surface esterification. The X-ray diffraction patterns of the nanopaper (Figure 4) show near preservation of the original crystallinity. This behavior indicated that microfibrillar morphology was preserved, and therefore, the esterification had taken place only on the surface of microfibrils. E-CNF nanopaper preserved the same crystal structure as the original cellulose. According to eq 2, the crystallinity degree for Un-CNF nanopaper was 68%, while the crystallinity degrees for E4-CNF, E6-CNF, E8-CNF, and E12-CNF nanopapers were 59%, 54%, 50%, and 44%, respectively. Compared to Un-CNF nanopaper, the crystallinity

(2)

The morphology of the original corncob cellulose powder and freeze-dried CNF from t-butyl alcohol were observed by SEM (JEOL JSM-4800) at an acceleration voltage of 10 kV. The specimens were coated with gold by ion sputtering. The morphology of CNF was observed by TEM (JEOL E-2100) at 200 kV. A drop of suspension of CNF was deposited on a carbon filmcoated copper grid. The surface morphologies of E-CNF and E-CNF nanopapers were examined using a Bruker Multimode 8 in scanasyst mode. The E-CNF suspension was casted on a freshly cleaved mica surface and air-dried. E-CNF nanopaper was pasted on the mica surface. Light transmittance of E-CNF nanopaper was measured from 300 to 800 nm using a UV−vis-NIR spectrometer (Varian Cary 5000) containing an integrating sphere taking air as the reference. Water vapor sorption of CNF nanopaper was measured by a Bei Shide 3H-2000PW dynamic vapor sorption analyzer. The temperature 2530


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Figure 3. Variation of DS of E-CNF from corncob cellulose and pulp cellulose. (The plot of pulp cellulose was replotted from ref 24).

Figure 5. Precipitation of Un-CNF in DMF (a). Flow birefringence of E6-CNF in DMF (b). (Solid contentwas 0.1 wt % for both.).

Figure 6. SEM of original corncob cellulose (a), Un-CNF (milled in DMF for 6 h) (b) and E6-CNF (c).

Figure 4. XRD spectra of original corncob cellulose, Un-CNF, and ECNF nanopapers.

degree of E-CNF nanopaper decreased with the increasing of ball milling time or DS. Due to esterification, the amount of hydroxyl group was reduced, and the hydrogen bond interaction between cellulose molecules chains was weakened, resulting in disruption of the regular arrangement of cellulose molecules and then reduced crystallinity. Dispersibility and Morphology of CNF. Ball milling (6 h) of the corncob cellulose in DMF with no esterifying agent gave a partially dispersed suspension. The fibrous solid, however, tended to aggregate and precipitate soon as shown in Figure 5a. In contrast, the esterified materials were dispersed as nanofibers, which were stably suspended on standing with flow birefringence as shown in Figure 5b. Figure 6 shows the SEM of the original cellulose, Un-CNF (milled in DMF for 6 h), and E6-CNF. Typical fibril widths in Figure 6b and c were 0.24 and 0.12 μm, respectively, based on the scale bar, both much wider than the expected value of several nanometers. This discrepancy obviously resulted from lateral coagulation by dry sample preparation for SEM. In fact, TEM images in Figure 7 show typical widths of 10−100 nm for Un-CNF and several 10 nm for E-CNF. In the latter, the individualizing effect of prolonged reactive ball milling was evident. More accurate estimation of the fibril width was possible by AFM (Figure 8). For E6-CNF, the typical width of well-dispersed fibrils were in the range of 1.5−2.8 nm, while some fibrils were as thin as 1.0 nm. In addition to high efficiency in esterification, corncob cellulose gave higher ultimate DS than wood pulp (Figure 3). This difference was likely a result from the difference in the

Figure 7. TEM of Un-CNF (a) and E-CNF of different ball milling time (b).

width of microfibril. Huang et al.24 proposed a scheme explaining the relation between DS and fibril width based on the number of hydroxyl groups exposed on the fibril surface. Their model was based on the traditional square-shaped cross section of microfibril. Modification of the cross-section shape was proposed by Ding and Himmel,25,26 which is being gradually adopted by researchers.27−29 On the basis of this hexagonal model, the number of surface hydroxyls and DS can be formulated as in Figure 9. On the basis of this scheme and observed DS values in surface esterification, the chain number of nanofiber was estimated as m = 36 or more for wood pulp cellulose and m = 28−18 for corncob cellulose. On the other hand, the thinnest fibrils observed by AFM were 1.0−1.5 nm, corresponding to m = 10 or m = 7 with expected DS of 1.20 or 1.29 (Figure 9). It has been reported that many soft tissues of higher plants such as fruit flesh contain ultrathin cellulose fibrils of this order,30 and corncob cellulose apparently belongs to this category. Optical Property of CNF Nanopaper. The nanopaper was formed from the CNF suspensions by vacuum filtration. The Un-CNF nanopaper (left) was translucent, while the E62531


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CNF nanopaper (right) was transparent (Figure 10a). This difference apparently came from the difference in the level of nanoscale dispersion. Because the diameter of E6-CNF was less than 3 nm as described above, the fibers and the voids between them were much smaller than the wavelength of visible light, making the nanopaper transparent. The E6-CNF nanopaper was not only transparent but also foldable (Figure 10b), being beneficial in optical display applications. Figure 10c shows development in the transparency of the nanopaper by prolonged ball milling. It was 83% at 550 nm for E4-CNF nanopaper and increased to 89% for E12-CNF nanopaper. Optical transparency of E-CNF nanopaper increased with ball milling time, resulting from better dispersion and smaller diameter of E-CNF. The improved dispersion of E-CNF implied high levels of individualized E-CNF, which ensured elimination of large aggregates during the filtration. The AFM image (Figure 10d) visualized the surface of E12-CNF nanopaper formed by densely packed nanofibers without obvious aggregates, further showing that the light scattering on the interface between air and nanofiber was negligible, leading to transparent nanopaper. Wetting Properties. The hygroscopicity of cellulose, which was adverse in materials applications, arises from the abundant hydroxyl groups. E-CNF, in contrast, showed significantly reduced hydrophilicity as a result of surface esterification of fibrils. The hydrophobic property of E-CNF nanopaper was strongly related to the DS value, i.e., to the ball milling time. The hygroscopic property of the nanopaper was measured by water vapor sorption of the nanopaper under 0%, 50%, and 90% RH at 25 °C (Figure 11). The hygroscopic property increased with the increase in RH for all nanopapers. At 90% RH, the water vapor sorption sharply decreased from 16.57% of Un-CNF nanopaper to 4.07% of E12-CNF nanopaper. The water vapor sorption of E-CNF nanopaper decreased with the DS value or ball milling time, and the similar trend was demonstrated for 50% RH. In addition, the water contact angle of the nanopaper was measured to show the change from hydrophilicity to hydrophobicity (Figure 12). The contact angle of the nanopaper increased with the DS value or ball milling time. It increased from 48 °C of the unmodified nanopaper to 92 °C of the E12-CNF nanopaper. This level of

Figure 8. AFM images of E6-CNF. Dispersed nanofiber with height profile on silicon substrate (a) and dispersion (b).

Figure 9. Model cross section of cellulose elementary fibril. Black dots on horizontal edges indicate hydroxyl groups on surface; m is the number of molecular chain; DS is the highest value according to the model.

Figure 10. Photographs of Un-CNF and E6-CNF nanopapers (a). Photograph of foldable E6-CNF nanopaper (b). Light transmittance of E-CNF nanopaper (c). AFM image of the surface of E12-CNF nanopaper (d). 2532


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This difference could be ascribed to the introduction of a hexanoyl group to the nanofiber surface. The unmodified or TEMPO-oxidized CNFs had hydrophilic groups on the surface, which can form mutual hydrogen bonding after nanopaper formation. Together with the internal crystalline structure, such CNFs can form largely homogeneous hydrogen bonding networks throughout the bulk nanopaper, leading to mechanical characteristics similar to bulk crystalline cellulose. In contrast, the surface-esterified CNF nanopaper was a kind of nanocomposite in which hydrogen bonding was disrupted by the short alkyl chain on the surface of every nanofiber. These surface domains would have a plasticizing effect on the bulk sheet, reducing both tensile strength and Young’s modulus. Similar behavior had been observed for other surface-esterified CNFs.20,35,36

Figure 11. Water vapor sorption of Un-CNF and E-CNF nanopapers under different RH.

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CONCLUSIONS Corncob cellulose was found to be a suitable source of nanofiber for one-step mechanochemical esterification−dispersion. The diameter of the obtained E-CNF ranged 1.5−2.8 nm, typical of higher plants’ soft tissue cellulose. Reflecting on this thinness, a rise in DS was faster than for wood pulp, and ultimate DS was as high as 0.95, substantially higher than 0.58 for wood pulp. Thus, the corncob cellulose is a favorable material for surface-esterified cellulose nanofibers. The nanopaper made by vacuum filtration showed high optical transparency up to 89% at 550 nm as well as good hydrophobicity. The nanopaper made of E-CNF by 6−12 h ball milling showed a Young’s modulus of 5.5 GPa and tensile strength of 110−125 MPa. These characteristics may find utility as advanced materials in electronics and other applications.

Figure 12. Water contact angle of Un-CNF and E-CNF nanopapers.

contact angle is similar to those of common polyolefins, making the nanopaper a possible commodity material. Mechanical Property. Figure 13 showed the influence of reactive ball milling on the mechanical properties of the

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AUTHOR INFORMATION

Corresponding Authors

*Min Wu. E-mail: [email protected]. Tel: +86-1082543500. *Yong Huang. E-mail: [email protected]. Tel: +86-1082543478. ORCID

Min Wu: 0000-0003-0542-4235

Figure 13. Change in tensile strength and elastic modulus of nanopaper by ball milling time.

Notes

The authors declare no competing financial interest.

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nanopaper. The introduction of a surface ester group caused a large increase in strength and modulus from the unmodified nanopaper at 4 and 6 h ball milling. This behavior could be ascribed to disintegration of cellulose into CNFs and coverage of them by hexanoyl groups, both would contribute to tighter packing of fibers in the nanopaper. While esterification− dispersion proceeded with further ball milling of more than 6 h, mechanical performances reached a maximum of 6 h of ball milling (Figure 3). This behavior was likely to be the influence of mechanical damages to the nanofibers; though keeping continuity and crystallinity, the nanofibers subjected to continued ball impacts would get more and more breaks and kinks, which would have adverse effects on mechanical performances of the nanopaper. Overall, the esterified corncob nanopaper had representative values of tensile strength of 120 MPa and Young’s modulus of 5 GPa. Typical cellulose nanopaper was reported to give a tensile strength of 130−230 MPa and Young’s modulus of 10−15 GPa, both 2−3 times that of the present material.19,23,31−34

ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (Nos. 51373191 and 51472253), training fund of Shandong Province (ZR2015PE021), and Chinese Academy of Sciences Visiting Professorships.

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Thin Cellulose Nanofiber from Corncob Cellulose and Its Performance

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