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Transparent and High Gas Barrier Films of Cellulose Nanofibers Prepared by TEMPO-Mediated Oxidation Hayaka Fukuzumi,† Tsuguyuki Saito,† Tadahisa Iwata,† Yoshiaki Kumamoto,‡ and Akira Isogai*,† Department of Biomaterials Sciences, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan, and Global R&D-Processing Development, Kao Corporation, 2606 Akabane, Ichikai-Machi, Haga-Gun, Tochigi 321-3497, Japan Received September 22, 2008; Revised Manuscript Received October 23, 2008
Softwood and hardwood celluloses were oxidized by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)mediated oxidation. The TEMPO-oxidized cellulose fibers were converted to transparent dispersions in water, which consisted of individual nanofibers 3-4 nm in width. Films were then prepared from the TEMPO-oxidized cellulose nanofibers (TOCN) and characterized from various aspects. AFM images showed that the TOCN film surface consisted of randomly assembled cellulose nanofibers. The TOCN films prepared from softwood cellulose were transparent and flexible and had extremely low coefficients of thermal expansion caused by high crystallinity of TOCN. Moreover, oxygen permeability of a polylactic acid (PLA) film drastically decreased to about 1/750 by forming a thin TOCN layer on the PLA film. Hydrophobization of the originally hydrophilic TOCN films was achieved by treatment with alkylketene dimer. These unique characteristics of the TOCN films are promising for potential applications in some high-tech materials.
Introduction In nature, plant celluloses form hierarchical structures consisting of highly crystalline cellulose microfibrils, each of which has 30-150 cellulose chains and 3-6 nm in a cross-sectional width, depending on the plant origins.1-4 Thus, bionanofibers having unique properties and sizes different from synthetic nanofibers are abundantly present in natural plant bodies, which have potential to be used as transparent and extremely strong films in high-tech fields. However, because of the presence of numerous hydrogen bonds between cellulose microfibrils in plant cell walls, it has been impossible to convert native cellulose fibers into aqueous dispersions of individual cellulose microfibrils without significant decreases in microfibril length. Mechanical agitation of cellulose fiber/water slurries using a high-pressure homogenizer, a double-disk refiner, or a grinder under harsh conditions yield nanosized cellulose fibrils, even though these are comprised of bundles of cellulose microfibrils.5-7 Acid hydrolysis with, for example, 50% sulfuric acid at 40 °C for 8 h, can convert native cellulose fibers to individual cellulose whiskers or microcrystals dispersed in water, but their lengths are 80%. The approximately 0.1% (w/v) TOCN/water dispersion was converted to a film with about 20 µm thickness on a surfacehydrophilized polytetrafluoroethylene (PTFE) membrane with 0.1 µm pore size (Advantec Toyo, Japan) by suction filtration. The TOCN films obtained were dried in a ventilated oven at 50 °C overnight without forced air-flow. Some of the films were then soaked in a 0.05% AKD dispersion for 10 s followed by rinsing thoroughly with water. The wet films were dried in the same manner as the original films, then heated at 100 °C for 10 min. Analyses. Surface images of the TOCN films were obtained using an AFM apparatus in tapping mode (SPI SPI 3800N and SI-DF20 Si cantilever, Seiko Instruments Inc., Japan). The film surfaces were observed using a field-emission-type SEM (Hitachi S-4000) after platinum sputtering at 20 mA for 120 s. The thermal decomposition temperatures were recorded in a nitrogen atmosphere using a thermogravimetric analyzer (Ulvac, TGD-9600, Japan) at 10 °C min-1. Thermal expansivity of films that were predried at 120 °C for 10 min were determined at 0.03 N load in a nitrogen atmosphere from 28 to 102 at 5 °C min-1 using a Shimadzu TMA-60 instrument. Light transmittance of the films was measured from 200 to 1000 nm using a Shimadzu UV-1700 UV-vis spectrometer, and was correlated based on the film thicknesses using the Lambert-Beer’s law. Contact angles of 2 µL water droplets on the films were measured at 23 °C and 50% relative humidity using a FAMAS DM500 apparatus (Kyowa Interface Science Co. Ltd., Japan). Moisture contents were calculated from weight decreases of the films after heating at 105 °C for 3 h. Tensile strengths and Young’s moduli of films 3 mm in wide and at least 20 mm in length were measured at 1.0 mm min-1 and 10 mm span length using a Shimadzu EZ-TEST instrument equipped with a 500 N load cell. The TOCN/water dispersion was cast on a plasma-treated PLA film with 25 µm thickness to make a TOCN coated layer 0.4 µm in thickness on the PLA film The TOCN/PLA film was subjected to oxygen permeability determination using a MOCON ML & SL (Modern Controls Inc., U.S.A.) instrument under dry conditions according to the standard method (ASTM 3985).16
Results and Discussion Optical Properties. Figure 1 shows the UV-vis transmittance of approximately 20 µm thick TOCN films prepared from TEMPO-oxidized softwood and hardwood celluloses. The transmittance at 600 nm was about 90% for the film prepared from softwood cellulose, while it was about 78% for the film prepared from hardwood cellulose. The hardwood cellulose contained xylan as the predominant hemicellulose, which has no C6 primary hydroxyls. The presence of the hydrophobic xylan molecules in the TEMPO-oxidized hardwood cellulose fibers probably interfered in part with complete dispersion of the TOCNs in water, resulting in the lower light transmittance. On the other hand, the softwood cellulose contained glucomannan, whose C6 primary hydroxyls can be converted to
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Figure 1. UV-vis transmittance of nanofiber films prepared from TEMPO-oxidized softwood and hardwood celluloses. The photograph shows light transmittance behavior of the nanofiber film prepared from TEMPO-oxidized softwood cellulose.
sodium carboxylate groups by the TEMPO-mediated oxidation. The TOCN film obtained was transparent and flexible (photograph, Figure 1). The absorption band shoulder at about 250 nm in Figure 1 is caused by C6 aldehyde groups formed as intermediate structures during TEMPO-mediated oxidation of the C6 primary hydroxyls. Aldehyde contents of TEMPOoxidized wood celluloses are generally lower than 0.3 mmol/g.14,15 The absorption at 250 nm disappeared after reduction of the TEMPO-oxidized celluloses with NaBH4 or oxidation with NaClO2. Surface Observations. The surface of TOCN films was observed using atomic force microscopy (AFM), which showed that the film surface consisted of randomly assembled nanofibers (Figure 2a). Their widths were estimated to be 3-4 nm from the height of AFM images, in good agreement with TEM measurements.12,13 When the film surface was observed using scanning electron microscopy (SEM, Figure 2b) after platinum sputtering, fibrous structures 20-30 nm in width were observed. It is probable that the nanofibers on the film surface were covered with relatively thick platinum layers by the sputtering process, which is necessary to be able to observe such nanosized organic materials by SEM. Mechanical Properties. The densities, moisture contents, and tensile properties of the TOCN films prepared from the softwood and hardwood celluloses are listed in Table 1. Because the density of cellulose Iβ crystals is 1.63 g cm-3,17 the measured values of 1.45-1.46 g cm-3 indicate the presence of some disordered regions in the TOCN films. The densities and moisture contents of the TOCN films were very similar to those of commercial cellophane film. However, the tensile strengths and Young’s moduli of the TOCN films were more than 200% and about 100%, respectively, larger than for cellophane film. The high crystallinity (ca. 75%) and the high aspect ratios of the nanofibers likely resulted in such high and characteristic strengths of the TOCN films. Interestingly, the tensile strengths of TOCN films prepared from the softwood and hardwood celluloses were not significantly different. Thermal Properties. Thermal degradation of the TOCN films started to occur at about 200 °C in a nitrogen atmosphere, while for the original celluloses, degradation began at 300 °C. Thus, the formation of sodium carboxylate groups from the C6 primary hydroxyls of cellulose microfibril surfaces by the TEMPOmediated oxidation leads to a significant decrease in the thermal degradation point (Figure 3), which is detrimental. The initial decreases in weight observed at about 60-100 °C for both of the two samples are probably the result of the loss of residual
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Figure 4. Thermal expansion behavior of nanofiber film prepared from TEMPO-oxidized softwood cellulose obtained in a dry nitrogen atmosphere after drying at 120 °C for 30 min. Four different specimens were measured, and the data are shown as different colors. Table 2. Oxygen Permeability of the Original 25 µm Thick PLA Film and the Film Coated with 0.4 µm Thick TEMPO-Oxidized Cellulose Nanofibers O2 permeability (mL m-2 day-1 Pa1-) polylactic acid (PLA) film PLA film coated with TOCN
Figure 2. AFM (a) and SEM (b) images of the surface of a nanofiber film prepared from TEMPO-oxidized softwood cellulose. Table 1. Tensile Properties of Nanofiber Films Prepared from TEMPO-Oxidized Softwood and Hardwood Celluloses tensile Young’s moisture density strength modulus elongation content 3 [GPa] [%] [%] [g/cm ] [MPa] TEMPO-oxidized softwood cellulose hardwood cellulose
1.46 1.45
233 ( 44 6.9 ( 1.4 7.6 ( 0.2 222 ( 11 6.2 ( 1.6 7.0 ( 2.4
13 15
moisture from the samples, because the degrees of weight loss for both the TOCN film and the original cellulose roughly corresponded to their moisture contents. The thermal expansivity of TOCN films was measured in a nitrogen atmosphere, after heating the film at 120 °C for 10
Figure 3. Thermogravimetric curves of the original and TEMPOoxidized celluloses as measured in a nitrogen atmosphere.
746 1
min to remove the residual moisture (Figure 4). The coefficient of thermal expansion (CTE) of TOCN film was about 2.7 ppm K-1, which is much lower than that of glass (about 9 ppm K-1). The extremely low CTE is caused by the high crystallinity of the nanofibers comprising the TOCN film. This characteristic, as well as the high transparency without the presence of any coexisting polymers, is highly advantageous for flexible display panels and electronic devices. Oxygen-Barrier Properties. A polylactic acid (PLA) film surface-coated with TOCN was subjected to oxygen permeability determination under dry conditions. The unmodified PLA film had an oxygen permeability of about 746 mL m-2 day-1 Pa1-. Surprisingly, the oxygen permeability was decreased to 1 mL m-2 day-1 Pa1- (Table 2) by the thin TOCN layer (see Experimental Section). The oxygen permeability of the TOCNcoated PLA film was close to the value for typical synthetic polymer films such as poly(vinylidene chloride) and polyethylene-poly(vinyl alcohol) copolymers that have high oxygen barrier functionality. Thus, transparent and high oxygen-barrier films can be obtained by casting TOCN dispersion on PLA films, which are available for packaging as wholly biomass-sourced films for foods and medicines. The biodegradability of TEMPOoxidized cellulose has been already reported in detail.18,19 Such high oxygen-barrier properties are also required for films in display panels and other electronic device applications. Hydrophobization of TOCN Film Surface. The high carboxylate content of TEMPO-oxidized wood celluloses (1.3-1.6 mmol g-1), is necessary to convert wood cellulose fibers to TOCN films having the aforementioned characteristics, as well as to make aqueous dispersions of completely separated nanofibers.12,13 However, the highly hydrophilic nature of the TOCN films, because of their high carboxylate content, may lead in turn to low resistance to water. The change in the contact angle with time of a water droplet placed on TOCN film was measured (Figure 5). Predictably, the original TOCN film was hydrophilic, and the initial water-contact angle was as low as 47°. The contact angle decreased with time as a result of partial penetration of water into the film. For comparison, the initial
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the TEMPO-mediated oxidation. Although the original TOCN films are hydrophilic, hydrophobization is achieved by a simple soaking treatment in the AKD dispersion. These unique characteristics of the TOCN films are promising for potential applications in flexible display panels, various electronic devices, and transparent and biodegradable packaging films with high oxygen-barrier properties. Acknowledgment. This research was supported by the Japan Society for the Promotion of Science (JSPS, Grant No. 18380102) and by New Energy and Industrial Technology Development Organization (NEDO, Grant No. 1023001) from 2007. Figure 5. Change in the contact angle with time of a water droplet on nanofiber film prepared from TEMPO-oxidized softwood cellulose before and after treatment with a 0.05% alkyl ketene dimer dispersion.
contact angles of cellophane film and a glass plate were 33 and 48°, respectively, when measured under the same conditions. Alkylketene dimer (AKD) is a typical hydrophobizing chemical used for paper. When an AKD dispersion is added to pulp fiber slurries, cationic AKD particles are efficiently adsorbed through electrostatic interactions on anionic sites or dissociated carboxyl groups originally present in wood cellulose fibers, whose contents are 0.02-0.08 mmol g-1.20,21 TOCN film treated with a 0.05% AKD dispersion had a water-contact angle 94°, and this value was maintained for 10 s. Transparency was maintained for the AKD-treated films. Thus, the hydrophilic TOCN films can be made hydrophobic using a simple soaking method with the cationic AKD dispersion. The abundant carboxylate groups in TOCN film are likely to behave as adsorption sites for the cationic AKD dispersed particles. Consequently, the presence of significant amounts of carboxylate groups in the TOCN films is advantageous for versatile and efficient modification of the original properties of the films.
Conclusion New TEMPO-oxidized cellulose nanofiber (TOCN) films prepared from softwood cellulose are transparent and flexible and have high tensile strengths, substantially high oxygen barrier properties, and extremely low coefficient of thermal expansion caused by high crystallinity of native cellulose. The thermal degradation point of cellulose, however, decreased from 300 to 200 °C by the introduction of carboxylate groups through
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(11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21)
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