Clay-Layered

Article pubs.acs.org/Biomac

Ultrastrong and High Gas-Barrier Nanocellulose/Clay-Layered Composites Chun-Nan Wu, Tsuguyuki Saito, Shuji Fujisawa, Hayaka Fukuzumi, and Akira Isogai* Department of Biomaterials Sciences, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan S Supporting Information *

ABSTRACT: Nanocellulose/montmorillonite (MTM) composite films were prepared from 2,2,6,6-tetramethylpiperidine1-oxyl radical (TEMPO)-oxidized cellulose nanofibrils (TOCNs) with an aspect ratio of >200 dispersed in water with MTM nanoplatelets. The composite films were transparent and flexible and showed ultrahigh mechanical and oxygen barrier properties through the nanolayered structures, which were formed by compositing the anionic MTM nanoplatelet filler in anionic and highly crystalline TOCN matrix. A composite film with 5% MTM content had Young’s modulus 18 GPa, tensile strength 509 MPa, work of fracture of 25.6 MJ m−3, and oxygen permeability 0.006 mL μm m−2 day−1 kPa−1 at 0% relative humidity, respectively, despite having a low density of 1.99 g cm−3. As the MTM content in the TOCN/ MTM composites was increased to 50%, light transmittance, tensile strength, and elongation at break decreased, while Young’s modulus was almost unchanged and oxygen barrier property was further improved to 0.0008 mL μm m−2 day−1 kPa−1.

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INTRODUCTION Innovative development of biobased materials with high mechanical strengths and gas-barrier properties produced by environmentally friendly processes is highly desirable. Such materials are indispensable in, for example, lightweight transportation bodies, high-performance packages, and electronic devices in place of heavy steel or fiber-reinforced/ petroleum-based materials in the building of a sustainable society. Organic/inorganic nanocomposites are promising materials that meet the above needs. Natural nacre, consisting of a hierarchical structure of chitin nanofibrils and hard calcium carbonate, is an example of a lightweight and layered nanocomposite with good mechanical properties.1 In the case of artificial organic/inorganic hybrid materials, it has been shown that the mechanical and oxygen-barrier properties of polymeric materials can be improved by adding 200.13 Moreover, because TOCNs have abundant anionically charged sodium C6-carboxyl groups on the nanofibril surfaces at a high density of ∼1.7 sodium carboxylate groups nm−2,14 electrostatic repulsions may effectively work not only between TOCNs but also between anionically charged MTM and TOCN in water. Zeta potentials of TOCNs and MTM were reported to be approximately −80 and −35 mV, respectively, in water at pH 7.15,16 These electrostatic effects are expected to lead to sufficient nanodispersibility of anionic MTM nanoplatelets in the anionic TOCN matrix. When MTM is applied as a nanocomposite filler, highly nanodispersed states or exfoliated structures of MTM in polymer matrices are required Received: March 23, 2012 Revised: May 5, 2012 Published: May 8, 2012 1927

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Figure 1. Light transmittances and photographs of the TOCN/MTM composite films with different MTM contents. (a) Light transmittances from 300 to 800 nm wavelength. (b) Light transmittances at 600 nm. (c−e) The half-folded TOCN, TM05, and TM50 films. Thicknesses of the films were about 5−7.7 μm. (f) Neat MTM film with ∼35 μm in thickness.

to achieve an efficient reinforcement effect.7 In this study, MTM nanoplatelets dispersed in water were prepared by simple mechanical agitation without the use of surfactant.

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that the light absorbance of the MTM/water dispersion was significantly decreased by the mechanical pretreatment, showing that the dispersibility of MTM nanoplatelets in water could be enhanced. TOCN/MTM Composite Films. The 2% MTM nanoplatelet dispersion was added to the 0.1% TOCN dispersion (30 g) at various weight ratios under stirring. After stirring for 1 h, the TOCN/MTM dispersion was poured in a polystyrene Petri dish with 50 mm diameter and oven-dried at 40 °C for 3 days. The TOCN/MTM composite film formed on the dish was peeled off and stored at 25 °C and 50% RH. The obtained TOCN/MTM composite films with weight ratios TOCN:MTM of 99:1, 95:5, 90:10, 75:25, and 50:50 were coded as TM01, TM05, TM10, TM25, and TM50, respectively. Analyses. Moisture contents of the films were determined from weight losses at 150 °C by thermogravimetric analysis using a Rigaku Thermoplus TG-8120. Light transmittance spectra and thicknesses of the films were measured using a JASCO V-670 UV−vis spectrophotometer.21,22 The film thickness was measured from at least five reflectance spectra of different positions for each sample obtained by UV−vis spectrophotometry, based on the interference fringe technique,23,24 and shown as a mean value. Standard deviation of film thickness was within 5% of the mean value. The thickness values measured by the UV−vis spectrophotometry were consistent with those determined using a digimatic micrometer (Mitutoyo Series 227). Tensile tests of the films were performed using a Shimadzu EZ-TEST instrument equipped with a 500 N load cell at 25 °C and 50% RH. Rectangular strips 2 × 30 mm in size were cut from the films and tested with a span length of 10 mm at a rate of 1.0 mm min−1, and at least 10 measurements were carried out for each sample.25 SEM observation of the film cross sections was carried out with a Hitachi S4000 field-emission microscope at 1 kV. The samples subjected to the SEM observation were first coated with osmium using a Meiwafosis Neoc Osmium Coater at 10 mA for 5 s. X-ray diffraction (XRD) patterns of the films were acquired in reflection mode using a Rigaku RINT 2000 diffractometer with monochromator-filtered Cu Kα radiation (λ 0.15418 nm) at 40 kV and 40 mA. Fourier transform infrared (FT-IR) spectra of the films were recorded using a JASCO FT/IR-6100 spectrometer in transmission mode with a resolution of 4

EXPERIMENTAL SECTION

Materials. A softwood bleached kraft pulp was supplied by Nippon Paper Industries (Tokyo, Japan) in never-dried state with a water content of 80% and used as the wood cellulose for TEMPO-mediated oxidation. MTM was supplied by Kunimine Industries and used as the clay without further chemical modification. The cation-exchange capacity of the MTM was 1.15 meq g−1. Platelet thickness and aspect ratios of the MTM were approximately 1 nm and 300−600, respectively, according to the manufacturer. TEMPO, NaBr, a 2 M NaClO solution and other chemicals were of laboratory grade (Wako Pure Chemicals, Osaka, Japan) and used as received. TEMPO-Oxidized Cellulose Nanofibril Dispersion. A TEMPOoxidized cellulose was prepared from the wood cellulose by the TEMPO/NaBr/NaClO system in water at pH 10, according to previously reported methods.13,17−19 The TEMPO-oxidized cellulose thus obtained was further treated with 1% w/v NaClO2 (100 mL) at pH 4.8 for 2 days to oxidize the C6-aldehyde groups remaining in the cellulose to C6-carboxyls.13,20 The two-step-oxidized cellulose had a carboxylate content of 1.21 mmol g−1.13,20 The TEMPO-oxidized cellulose/water slurry at a 0.11% w/v consistency was mechanically treated with a double-cylinder type homogenizer for 1 min and subsequently with an ultrasonic homogenizer for 4 min.13,17,21 The unfibrillated fraction (200) and high crystallinity (∼75%) of TOCN.13,26 Mechanical properties of the TOCN film were significantly improved by combination with the MTM nanoplatelets. The TM05 film had a high Young’s modulus, an ultrahigh tensile strength, and a high elongation at break of 18 GPa, 509 MPa, and 7.6%, respectively, despite having a low density of 1.99 g cm−3. Although the tensile strength and elongation had their maximum values for the TM05 film, Young’s moduli had high levels >18 GPa for all composite films at 5−50% MTM content. The high tensile strength of the TM05 film is similar to those of stainless steels such as AISI 316 L and much higher than those of biobased organic/inorganic composite materials reported to date.8,28 The strength reinforcement efficiency by MTM nanoplatelets in the TM05 film far exceeded those in the MTM-reinforced composite materials previously reported.29−32 In addition, this high strength was combined with 7% elongation at break at only 3 to 4 vol % of inorganic MTM phase. The Young’s modulus of 18 GPa indicated that each MTM nanoplatelet was discrete and fully functional as reinforcement. This high strength is probably due to an efficient nanocomposite effect through uniform distribution of MTM nanoplatelets on the nanometer level in the composite films,7 forming numerous hydrogen bonds and ionic interactions at the interfaces between the TOCNs and MTM nanoplatelets. Of course, the mechanical properties of the composite films in Figure 2 should be changed when tested

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RESULTS AND DISCUSSION Optical and Mechanical Properties. The TOCN/water and MTM/water dispersions were prepared separately, followed by mixing at various ratios, and TOCN/MTM composite films were prepared by casting the mixed dispersions and drying to mimic natural nacre. Figure 1a shows the light transmittance of the TOCN/MTM composite films. Thicknesses and densities of the films were 5−7.7 μm and 1.48 to 2.45 g cm−3, respectively, and both of these values increased with MTM content. Moisture contents of the TOCN/MTM films at 23 °C and 50% relative humidity (RH) were 5−9%. The Fabry−Perot fringes observed in Figure 1a indicate that the films had highly transparent and smooth surfaces.21,27 The light transmittance of the TOCN/MTM films at 600 nm linearly decreased with increasing MTM content due to lightscattering by the MTM nanoplatelets (Figure 1b). Figure 1c−f shows photographs of the half-folded TOCN, TM05, TM50 films, and the neat MTM film. Therefore, the TOCN/MTM films at 1−10% MTM contents are sufficiently flexible and transparent and their surfaces are smooth and glossy. Stress−strain curves, Young’s moduli, tensile strengths, and elongations at break of the films are shown in Figure 2, and detailed data are listed in Table S1 of the Supporting Information. The tensile test instrument used (Shimadzu EZTEST) was suitable for the thin TOCN/MTM composite films 1929

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under different RH conditions because the above interactions between TOCNs and MTM nanoplatelets are sensitive to moisture. When the neat MTM film was subjected to tensile testing, no data could be obtained because the film was too brittle. The most characteristic point concerning the mechanical properties of the TOCN/MTM composite films is the remarkable improvement of work of fracture for the TM05 film, which had an average work of fracture 25.6 MJ m−3 (Figure 3). This value was approximately six times as much as

Although highly self-aligned TOCN elements in the TOCN film provide high oxygen-barrier properties at 0% RH,25,26 oxygen molecules could gradually pass through nanosized pores in the TOCN film.26 The added MTM nanoplatelets, present as homogeneously distributed states in the TOCN film, may form multiwalls, efficiently hindering oxygen permeation through the TOCN/MTM films. However, the oxygen permeability of the TOCN film at 0% RH increased to 0.2 mL μm m−2 day−1 kPa−1 at 50% RH (Figure 4). This value clearly decreased with increasing MTM content in the composite films, and thus the MTM nanoplatelets contributed to improvement of oxygen barrier properties even at 50% RH. Nevertheless, clear differences in oxygen permeability were still observed for the TOCN/MTM composite films between 0 and 50% RH, probably due to the hydrophilic nature of TOCNs. Nano-Layered Structures. Figure 5 shows SEM images of cross sections of the TOCN, neat MTM, TM05, and TM50

Figure 3. Works of fracture of TOCN/MTM composite films.

that of the TOCN film and higher than those of spring steel (1 MJ m−3) and NFC paper (15.1 MJ m−3).33 Therefore, the TM05 film is highly tough and ductile probably because of a sufficiently nanodispersed state of MTM platelets in the TOCN matrix at the 5% MTM content. Oxygen Barrier Properties. The oxygen permeability of the TOCN film at 0% RH markedly decreased from 0.03 to 0.007 mL μm m−2 day−1 kPa−1 with only 1% MTM addition (Figure 4). The TM50 film had an even lower value of 0.0008 mL μm m−2 day−1 kPa−1. Therefore, the TOCN/MTM composite films showed excellent oxygen-barrier properties under dry conditions. The oxygen permeability of the TM50 film was markedly lower than those of commercial ethylene− vinylalcohol copolymer films (0.001 to 0.01 mL μm m−2 day−1 kPa−1), which are commonly used as oxygen-barrier films.34

Figure 5. SEM images of the cross sections of the (a) TOCN, (b) neat MTM, (c) TM05, and (d) TM50 films.

films. The nanofibril structures of TOCNs and sheet-like MTM nanoplatelets can be clearly observed in Figures 5a and 5b, respectively. The SEM image of the TM05 film (Figure 5c) reveals that thin, closely packed and nanolayered structures of MTM in the vertical direction were formed by compositing with TOCNs, which may have played a significant role in the strong binding behavior of MTM nanoplatelet planes. The TM50 film (Figure 5d) shows more sheet-like and highly aligned structures of MTM nanoplatelets.34−37 Therefore, the formation of nanolayered MTM structures mimicking natural nacre has been achieved by compositing with anionically charged and highly crystalline TOCNs as the matrix. Energydispersive X-ray (EDX) images (Figure S2 in Supporting Information) of cross sections of the TM50 film revealed that the TOCNs and MTM nanoplatelets were both homogeneously distributed in the film, without aggregation. FT-IR spectra of the TOCN/MTM composite films indicate that hydrogen bonds are the primary binding interactions between

Figure 4. Oxygen permeabilities of the TOCN/MTM films with 0− 50% MTM contents measured at 0 or 50% relative humidity. The thicknesses of the coated layers on poly(ethylene terephthalate) (PET) films were ∼1 μm. Left and right Y axes both show the same oxygen permeability but with different scales. 1930

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TOCN and MTM, resulting in ultrahigh mechanical and oxygen-barrier properties (Figure S3 in Supporting Information). XRD patterns show that d spacing of the MTM nanoplatelets in the TOCN matrices was substantially increased by compositing with TOCNs (Figure S4 in Supporting Information), although completely exfoliated structures with no MTM diffraction peak could not be formed in the TOCN/ MTM composite films. The crystal sizes of the MTM nanoplatelets in the composite films, calculated from the full widths at half-maximum of the MTM (001) peaks, decreased with decreasing MTM content. The anionically charged TOCNs may have contributed to the formation of partially exfoliated MTM nanoplatelet structures in the composite films.

CONCLUSIONS TOCN/MTM composite films with high transparency and flexibility can be prepared by a simple mixing and drying process, mimicking natural nacre, with excellent mechanical and oxygen-barrier properties, despite having low densities. In particular, the composite films with 5% MTM content had ultrahigh tensile strength (509 MPa), high Young’s modulus (18 GPa), and remarkably high work of fracture (25.6 MJ m−3), which shows an unprecedentedly high reinforcement efficiency of organic matrix by MTM. These characteristic properties are caused by nanolayered and homogeneously distributed structures of MTM nanoplatelets in the TOCN matrix, forming numerous hydrogen bonds and ionic interactions at the interfaces between MTM nanoplatelets and TOCNs. The primary driving force for forming the nanolayered structures of TOCN/MTM composite films arises from the highly anionically surface charged and crystalline TOCNs with high aspect ratios >200 used as the matrix nanofibers. Therefore, these TOCN/MTM composites, prepared by a biomimetic technique, have numerous potential applications as new biobased organic/inorganic nanocomposites that are lightweight and possess ultrahigh mechanical strengths and toughnesses. Energy efficiency in conversion processes from aqueous TOCN/MTM dispersions to dried nanocomposites should be improved in the future. ASSOCIATED CONTENT

S Supporting Information *

Light absorbances of the mechanically untreated and treated MTM/water dispersions, detailed mechanical properties, SEMEDX results of the TM50 film, FT-IR spectra of the TOCN/ MTM composite films, and XRD patterns of TOCN/MTM composite films. These materials are available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +81 3 5841 5538. Fax: +81 3 5842 5269. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was partially supported by Scientific Research S (21228007) and Encouragement of Young Scientists A (23688020) from the Japan Society for the Promotion of Science (JSPS). 1931

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