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Dual Counterion Systems of Carboxylated Nanocellulose Films with Tunable Mechanical, Hydrophilic, and Gas-Barrier Properties Ryuji Kubo, Tsuguyuki Saito, and Akira Isogai Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.9b00057 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019
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Biomacromolecules
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Dual Counterion Systems of Carboxylated
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Nanocellulose Films with Tunable Mechanical,
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Hydrophilic, and Gas-Barrier Properties
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Ryuji Kubo, Tsuguyuki Saito, and Akira Isogai*
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Department of Biomaterials Science, Graduate School of Agricultural and Life Sciences, The
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University of Tokyo, Tokyo 113-8657, Japan
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ABSTRACT: Aqueous dispersions of C6-carboxylated cellulose nanofibrils with sodium
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counterions (CNF-COONa) and CNFs with tetraethylammonium counterions (CNF-COONEt4)
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were mixed at various weight ratios. Transparent, flexible CNF-COONa/NEt4 films were
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prepared by casting and drying aqueous mixtures with various Na/NEt4 molar ratios as dual
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counterion systems. The film density, Young’s modulus, and tensile strength decreased linearly
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with increasing molar ratio of the bulky NEt4 counterion. The film hydrophilicity was
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controlled by varying the Na/NEt4 molar ratio. The oxygen and water vapor permeabilities also
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increased with increasing molar ratio of bulky NEt4 counterions. The mechanical, hydrophilic,
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and gas-barrier properties were tuned by controlling the Na/NEt4 molar ratios of CNF films
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containing dual counterions. The results of model experiments using tetra(n-butyl)ammonium
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hydroxide indicated that the Na and quaternary alkylammonium counterions were
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homogeneously distributed among the carboxylated CNF elements in both dispersions and
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cast-dried films of the dual counterion systems. 1
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KEYWORDS: Cellulose nanofibril, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), Dual
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counterion system, Gas barrier, Ion exchange
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INTRODUCTION
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The preparation and characterization of various nanocelluloses have been extensively studied
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in the past two decades. This is because nanocelluloses are prepared from abundant and
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renewable biomass recourses and have unique functions and properties as bulk nanocellulose
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materials and nanocellulose-containing composites.1‒5 Nanocelluloses are primarily prepared
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from plant celluloses, with or without pretreatment, by mechanical disintegration in water.
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2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation is one of the methods used
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for chemical pretreatment of plant celluloses and it is used to prepare completely
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individualized TEMPO-oxidized cellulose nanofibril (TCNF) dispersions in water.6‒26 TCNFs
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prepared from wood cellulose under appropriate conditions have homogeneous widths of ~3
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nm and high aspect ratios of >100. Sodium carboxylate groups are densely, position-selectively,
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and regularly placed on the crystalline cellulose microfibril surfaces.9,24‒26 TEMPO-oxidized
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cellulose nanocrystals with homogeneous widths of ~3 nm and low aspect ratios of ~50 are
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prepared from wood cellulose by TEMPO-mediated oxidation and subsequent sonication of the
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TEMPO-oxidized cellulose in water for a long period.27,28
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TCNFs have abundant sodium carboxylate groups, i.e., 1‒1.7 mmol/g, on their surfaces, and
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these groups are exchangeable with other metal and alkylammonium counterions under
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aqueous conditions.24‒26 TCNF films with different metal and ammonium counterions show
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various levels of biodegradability in soil embedding tests.29 Protonated TCNF films 2
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(TCNF-COOH) have mechanical and gas-barrier properties that differ from those of
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TCNF-COONa films.30 When alkylammonium counterions are introduced into the carboxylate
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groups of TCNF, the corresponding TCNF-COONH2R and TCNF-COONR4 films become
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hydrophobic, depending on the chemical structure of the alkylammonium group.24‒26
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When fibrous TEMPO-oxidized celluloses with sodium carboxylate groups (TOCs-COONa)
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are soaked in dilute HCl solution and washed with water, fibrous TOCs-COOH are obtained.
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The carboxy groups of TOCs-COOH can be completely and stoichiometrically ion-exchanged
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to tetraalkylammonium carboxylate groups (TOCs-COONR4) in water through neutralization
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with aqueous R4N(OH) solutions.31‒34 The fibrous TOCs-COONR4 can then be converted to
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completely nanodispersed TCNF-COONR4 by mechanical disintegration in water. The
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TCNF-COONR4 films prepared from aqueous TCNF-COONR4/water dispersions by casting
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and drying have unique mechanical, hydrophobic, and oxygen-barrier properties, depending on
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the lengths and structures of the introduced alkyl chains.34
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TCNF-COONa is hydrophilic and nanodispersible in water, whereas TCNFs-COONR4 are
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hydrophobic and nanodispersible not only in water but also in some organic solvents such as
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methanol, N,N-dimethylacetamide, dimethylsulfoxide, and acetone.33 In this study, aqueous
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TCNF dispersions containing sodium and tetraethylammonium (NEt4) counterions with
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various Na/NEt4 molar ratios were prepared as dual counterion systems by mixing aqueous
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TCNF-COONa
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TCNF-COONa/NEt4 films with various Na/NEt4 molar ratios were prepared from these
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mixtures by casting and drying. The mechanical, hydrophilic, and gas-barrier properties of the
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films were investigated in terms of the Na/NEt4 molar ratios. The purpose of this study is to
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propose new methods to achieve tenability of mechanical, hydrophilic, and gas-barrier
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properties of TCNFs using the dual counterion systems. This is because the properties of CNF
and
TCNF-COONEt4
dispersions
at
various
weight
ratios.
3
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films required for applications or end products vary depending on the intended use.35‒46 The
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distributions of Na and NEt4 counterions in the TCNF elements and the possibility of
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ionexchange between Na and NEt4 ions in aqueous TCNF-COONa/NEt4 dispersions were
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investigated by performing model experiments in which tetra(n-butyl)ammonium ions were
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used instead of NEt4 ions.
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MATERIALS AND METHODS
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Materials. A commercial never-dried softwood bleached kraft pulp (SBKP) was
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demineralized by soaking in a dilute HCl solution at pH 2 and ambient temperature for 2 h and
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subsequently washing with water by filtration. The SBKP was stored in the never-dried state at
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4 °C before use. Aqueous 20% (w/w) tetraethylammonium hydroxide and 10% (w/w)
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tetra(n-butyl)ammonium hydroxide solutions, TEMPO, NaBr, 2 M NaClO solution, NaBH4,
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and other chemicals and solvents were purchased from the Fujifilm Wako Pure Chemical
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Corporation (Tokyo, Japan) and used as received.
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Preparation of Fibrous TEMPO-Oxidized Cellulose. SBKP (3 g based on dry weight) was
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suspended in water (300 mL) and oxidized with TEMPO (0.0456 g, 0.3 mmol), NaBr (0.3 g, 3
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mmol), and 2 M NaClO (15 mL, 30 mmol) at pH 10 and ambient temperature for 4 h.47 NaBH4
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(0.3 g) was added to the same container at pH 10 (to achieve one-pot reduction) for 3 h to
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reduce the small amounts of C6-aldehydes and C2/C3-ketones present in the oxidized cellulose
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to the corresponding alcoholic hydroxy groups.27,48 The TEMPO-oxidized and then
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NaBH4-reduced cellulose was washed with water by filtration. The sodium carboxylate content
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of the fibrous TEMPO-oxidized and NaBH4-reduced cellulose (TOC-COONa) was 1.43
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mmol/g, as determined by conductivity titration.49 This fibrous TOC-COONa was soaked in a 4
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dilute HCl solution at pH 2 and room temperature for 2 h and the washed with water by
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filtration to prepare a fibrous protonated TOC (TOC-COOH).33 The fibrous TOC-COOH was
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suspended in water, and aqueous tetraethylammonium hydroxide was added to the mixture.
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The amount of Et4N(OH) added was equimolar with respect to the protonated carboxy groups
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in TOC-COOH, and TOC-COONEt4 was obtained through neutralization.33
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Preparation
of
Aqueous
Dispersions
of
TCNFs
Containing
Na
and
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Tetraethylammonium Counterions. The fibrous TOC-COONa and TOC-COONEt4 were
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separately converted to ~0.1% (w/w) aqueous TCNF-COONa and TCNF-COONEt4
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dispersions,
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double-cylinder-type homogenizer (Physcotron NS-56, Microtec Co., Ltd., Japan) and an
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ultrasonic homogenizer (US-300T, Nihon Seiki, Japan) under previously reported
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conditions.30,33,47 The TCNF-COONa or TCNF-COONEt4 dispersion was centrifuged at 12 000
105
g for 10 min to remove the small amount of unfibrillated fraction (
