Article pubs.acs.org/Biomac
Hydrophobic, Ductile, and Transparent Nanocellulose Films with Quaternary Alkylammonium Carboxylates on Nanofibril Surfaces Michiko Shimizu,† Tsuguyuki Saito,† Hayaka Fukuzumi,‡ and Akira Isogai*,† †
Department of Biomaterials Science, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan ‡ Department of Chemistry and Material Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, Tokyo 152-8550, Japan S Supporting Information *
ABSTRACT: Hydrophobic, ductile, and transparent nanocellulose films were prepared by casting and drying aqueous dispersions of 2,2,6,6-tetramethylpiperidine-1-oxyl-oxidized cellulose nanofibrils (TOCNs) with quaternary alkylammoniums (QAs) as counterions for the surface carboxylate groups. TOCN films with tetramethylammonium and tetraethylammonium carboxylates showed high optical transparencies, strain-to-failure values (14−22%), and work-of-fracture values (20−27 MJ m−3). The ductility of these films was likely caused by the alkyl chains of the QA groups densely covering the TOCN surfaces and being present at the interfaces between the TOCN elements in the films. The water contact angle of the TOCN-QA films increased to ∼100° by introducing tetra(n-butyl)ammonium groups as counterions. Thus, TOCN film properties can be controlled by changing the chemical structure of the counterions from Na to QAs. The hydrophilic TOCN surfaces can be changed to hydrophobic simply and efficiently by the conversion from TOCN-Na to TOCN-QA, when TOCNs are used as nanofillers in hydrophobic polymer matrices.
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INTRODUCTION
When native celluloses are oxidized with catalytic amounts of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and NaBr in combination with NaClO used as a primary oxidant under aqueous conditions at pH 10 and room temperature, sodium carboxylate groups are formed position-selectively, densely, and regularly on the cellulose microfibril surfaces.26,27 The TEMPO-oxidized wood celluloses with carboxylate contents of 1.0−1.7 mmol g−1 obtained can be converted to TEMPOoxidized cellulose nanofibrils (TOCNs) dispersed in water as individualized nanoelements with homogeneous ∼3 nm widths by gentle mechanical disintegration treatment.28,29 In a previous study, we reported that TOCNs with bulky quaternary alkylammonium (QA) carboxylates (TOCN-QAs) can be dispersed in various solvents, including water, alcohol, and acetone.30 TOCN-QAs are prepared by exchanging the counterions of the Na carboxylate groups of TEMPO-oxidized wood celluloses (TOCs) with QAs such as tetramethyl, tetraethyl, tetra(n-propyl), and tetra(n-butyl) ammonium groups. The nanodispersibility of the TOCN-QAs can be explained in terms of the high degree of dissociation of bulky QA groups in organic solvents as well as water. In this study, self-standing TOCN-QA films were prepared, and their structures and optical, mechanical, hydrophobic, and gaspermeability properties were characterized and related to the chemical structures of the QAs.
Nanofibrillated and nanocrystalline celluloses, or “nanocelluloses”, have attracted increasing attention as high-performance and sustainable biobased nanomaterials. In particular, individualized cellulose nanofibrils have high aspect ratios (>300), high moduli (30−140 GPa), and high tensile strengths (2−3 GPa).1−4 Nanocelluloses with partial junctions between the cellulose microfibrils or their bundles can form various types of structured materials and composites.5−8 For example, selfstanding nanocellulose films exhibit high stiffness values, high tensile strengths, low oxygen permeability values, and low coefficients of thermal expansion and are, therefore, promising as packaging films and substrates for flexible electronic devices.9−13 However, most nanocellulose films and nanocellulose-containing composites have a hydrophilic and brittle nature, which often impairs their excellent properties. In this context, various approaches have been investigated to give hydrophobicity and ductility to nanocelluloses. Introduction of alkyl chains onto nanofibril surfaces via ionic or covalent bonds is often performed for this purpose.14−20 However, in most cases, the self-standing films and composite materials consisting of alkyl chain-grafted nanocelluloses are still brittle, and their stiffness and strength values are significantly low. Nanocellulose-based porous films containing ductile polymer layers on the nanofibril surfaces show high ductility, but they are still hydrophilic.21−25 Thus, the preparation of nanocelluloses with both sufficient hydrophobicity and ductility is still challenging in nanocellulose research. © 2014 American Chemical Society
Received: September 6, 2014 Revised: October 10, 2014 Published: October 13, 2014 4320
dx.doi.org/10.1021/bm501329v | Biomacromolecules 2014, 15, 4320−4325
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for each sample. The oxygen permeability (P−O2) of the TOCN films was determined at 23 °C and at 0, 35, and 50% RH, using a MOCON OX-TRAN 2/21 module (Modern Control Inc., DE; ASTM 3985). The water vapor permeability (P−H2O) of the TOCN films was determined using a MOCON PERMATRAN-W 1/50 water vapor transmission rate analyzer (Modern Control Inc.; ASTM 398). The moisture content of the upstream flow of the film was fixed to 10% RH and that of the downstream flow was set at 40 and 60% RH. The P− H2O was measured for >1 h to obtain a stable value.
EXPERIMENTAL SECTION
Materials. A never-dried, softwood breached kraft pulp with an αcellulose content of 90% and a viscosity-average degree of polymerization (DPv) of 1270 was provided by Nippon Paper Industries, Tokyo, Japan.30 The pulp was demineralized using 0.1 M hydrochloric acid.30 Tetramethylammonium [N(Me)4] hydroxide pentahydrate, aqueous solutions of 10% tetraethylammonium [N(Et)4] hydroxide, 10% tetra(n-propyl)ammonium [N(n-Pr)4] hydroxide, 0.5 M tetra(nbutyl)ammonium [N(n-Bu)4] hydroxide, and other chemicals were of laboratory grade (Wako Pure Chemicals, Osaka, Japan) and used without further purification. TEMPO/NaBr/NaClO oxidation in water at pH 10 and then postoxidation with NaClO2 in water at pH 4.8 were used to prepare a sodium salt of TEMPO-oxidized wood cellulose (TOC-Na) with a carboxylate content of 1.7 mmol g−1.30 The fibrous TOC-Na thus obtained was converted to TOCs with quaternary alkylammonium carboxylates (TOC-QAs) through TOC with protonated carboxyl groups (TOC-H).30 The TOC-QAs were stored at 4 °C without drying before use. TOCN-QA Films. The aqueous 0.1 w/v % TOC-QA slurry was converted to a TOCN-QA dispersion using double cylinder-type and ultrasonic homogenizers, according to our previous report.30 The DPv value of the TOCNs used in the present study was 450. The DPv of the original wood cellulose decreased from 1270 to 450 by TEMPOmediated oxidation, postoxidation with NaClO2, and successive mechanical disintegration treatments in water. The TOCN-QA dispersions thus obtained were poured into polystyrene Petri dishes and dried at 40 °C for 3 days. The resulting TOCN-QA films were conditioned at 23 °C and 50% relative humidity (RH) for at least 2 days before analysis. Another two TOCN films with sodium (TOCNNa) and ammonium (TOCN-NH4) carboxylates were prepared according to our previous report and used as references.31 Analysis. The light transmittance spectra of the TOCN-QA films were measured using a JASCO UV−vis−NIR spectrometer (Tokyo, Japan). The thicknesses of the self-standing TOCN-QA films were calculated from the interference patterns in the transmittance spectra.30,32 The refractive index of the TOCN-QA films, which was used in the calculation of film thickness, was determined at 23 °C and 50% RH using an Atago Abbe refractometer (Tokyo, Japan). The refractive indices were 1.55 for the TOCN-N(Me)4 and TOCNN(Et)4 films and 1.54 for the TOCN-N(n-Pr)4 and TOCN-N(n-Bu)4 films. Atomic force microscopy (AFM) images were obtained using a Veeco Nanoscope III (Digital Instruments, NY) operating in tapping mode. The weight ratio of the counterion (Na, NH4, or QA) in the films, excluding the moisture content, is given by
counterion(w/ w%) =
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RESULTS AND DISCUSSION Structures of TOCN-QA Films. The counterions of the carboxylate groups on the TOCN surfaces were completely ion exchanged with QAs, which was confirmed from the FTIR spectra and nitrogen contents of the films.30 Figure 1 shows the
Figure 1. Light transmittance spectra of the TOCN-QA films. The inset shows an AFM image of the TOCN-N(n-Bu)4 film surface.
light transmittance spectra of the TOCN-QA films with thicknesses of ∼10 μm. All of the TOCN-QA films had high optical transparencies of ∼90% in the visible light region. The interference fringes of the spectra show that all of the TOCNQA films had highly smooth surfaces to reflect the visible light. The AFM image showed that the film surface consisted of randomly oriented and individualized TOCN elements (see the inset of Figure 1). The fundamental properties of the TOCN-QA films are listed in Table 1. The counterion weight ratio in the films
A × B × 100 A(B − 1) + 1000
where A is the carboxyl content of TOCN-H (mmol g−1) and B is the molecular weight of the counterion. The film density was calculated from the volume and weight of the films. The moisture content of the films was calculated from the film weights before and after heating at 105 °C for 3 h, and their standard deviations were within 1.3%. The pore diameters in the TOCN-QA films at average depths of 620 nm from the film surface were determined using a positron annihilation lifetime spectroscopy (PALS) facility in the National Institute of Advanced Industrial Science and Technology (Tsukuba, Japan). The experimental details of the PALS analysis are described elsewhere.33 Tensile tests of the films were carried out using a Shimadzu EZ-TEST tensile tester (Tokyo, Japan) equipped with a 500-N load cell at 23 °C and 50% RH. The film specimens had widths and lengths of 2 and 30 mm, respectively. At least five specimens were measured at a rate of 1.0 mm min−1 for each sample with a span length of 10 mm. The fractured surfaces of the films used in the tensile test were observed by scanning electron microscopy using a Hitachi field-emission microscope S-4800 (Tokyo, Japan) at 1.5 kV after coating with osmium using a Meiwafosis Neo osmium coater (Tokyo, Japan) at 10 mA for 5 s. The contact angle of a 2 μL water droplet on the film surfaces was measured at 23 °C and 50% RH using a Kyowa FAMAS DM500 apparatus (Saitama, Japan), and five measurements were carried out
Table 1. Fundamental Properties of the TOCN-QA Films TOCN-COOR Na NH4 QA
a
N(Me)4 N(Et)4 N(n-Pr)4 N(n-Bu)4
counterion content (w/w %)
densitya (g cm−3)
moisture contenta (%)
3.8 3.0 11.4 18.4 24.4 29.6
1.42 1.55 1.51 1.43 1.40 1.34
10.5 7.9 8.1 7.3 6.9 7.0
pore diameter (nm) 0.46 0.46 0.50 0.51 0.51
Measured after conditioning at 23 °C and 50% RH.
increased up to ∼30 w/w % with increasing alkyl chain length of the QAs. It should be noted that the counterion weight ratio of the TOCN-N(n-Bu)4 film was seven times greater than that of the TOCN-Na and TOCN-NH4 films. The density of the TOCN-QA films decreased with increasing alkyl chain length of the QAs, and the TOCN contents in the TOCN-QA films decreased correspondingly. The moisture content and pore 4321
dx.doi.org/10.1021/bm501329v | Biomacromolecules 2014, 15, 4320−4325
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diameter in the TOCN-QA films slightly decreased and increased, respectively, with increasing alkyl chain length of the QAs. Tensile Properties of TOCN-QA Films. The stress−strain curves and the tensile properties of the TOCN-QA films are shown in Figure 2. As the counterions of the carboxylates were
N(Et)4 films significantly increased up to ∼22% and ∼27 MJ m−3, respectively, which are advantageous compared with other nanocellulose films and sheets with strain-to-failure values of
