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A facile route to transparent, strong, and thermally stable nanocellulose/ polymer nanocomposites from an aqueous Pickering emulsion Shuji Fujisawa, Eiji Togawa, and Katsushi Kuroda Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.6b01615 • Publication Date (Web): 13 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016
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A facile route to transparent, strong, and thermally
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stable nanocellulose/polymer nanocomposites from
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an aqueous Pickering emulsion
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Shuji Fujisawa,* Eiji Togawa, and Katsushi Kuroda
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Forestry and Forest Products Research Institute, Tsukuba 305-8687, Japan
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KEYWORDS Pickering emulsion, cellulose nanofibril, TEMPO-mediated oxidation, Polymer
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composite
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ABSTRACT Cellulose nanofibril (CNF) is a promising nanofiller for polymer nanocomposite
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materials, and a critical challenge in designing these materials is organization of the
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nanostructure using a facile process. Here, we report a facile aqueous preparation process for
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nanostructured polystyrene (PS)/CNF composites, via the formation of a CNF-stabilized
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Pickering emulsion. PS nanoparticles, with a narrow size distribution, were synthesized by free
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radical polymerization in water, using CNF as a stabilizer. The nanoparticles were easily
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collected by filtration and the resulting material had a composite structure of PS nanoparticles
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embedded in a CNF framework. The PS/CNF nanocomposite showed high optical transparency,
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strength, and thermal dimensional stability. Thus, this technique provides a simple and
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environmentally-friendly method for the preparation of novel CNF/polymer nanocomposite
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materials.
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INTRODUCTION
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A key element in the design of high-performance polymer nanocomposites is organization of
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the nanostructure to overcome the high surface energy of the nanofillers. Several strategies for
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engineering composite structures using a variety of nanofillers have been reported, such as
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efficient mechanical exfoliation, layer-by-layer deposition, and self-assembly templating.1-8
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Some of the most-studied nanofillers are carbon nanotubes,9-11 graphenes,12-14 and nanoclays.15-17
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A well-organized nanoscale structure could tailor macroscale material properties, such as the
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optical, mechanical, and thermal properties.
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Cellulose nanofibril (CNF) is a promising building block for nanocomposite materials.18 CNF,
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which provides structural support to plant bodies in nature, possesses high strength (2‒6
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GPa),19,20 high elastic moduli (130‒150 GPa),21-23 and low thermal expansion coefficients (4‒6
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ppm K‒1).24,25 Taking advantage of these unique properties, great effort has been made
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developing the use of CNF as a reinforcing filler in polymer nanocomposite materials.26,27
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However, during the development of these composites, incompatibility with hydrophobic
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polymers often becomes a critical issue. Generally, to achieve homogeneous distribution of CNF
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in hydrophobic polymer matrices, melt compounding at high temperatures or solution casting
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using organic solvents is necessary. In other words, to overcome the high interfacial energies that
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exist, nanocomposite preparation requires a lot of energy and careful processing, which hinders
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its commercial viability.
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Here, we report on a facile and scalable method for the preparation of a nanostructured
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PS/CNF composite, using an aqueous polymerization process. Surface-carboxylated CNF, with a
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width of ~3 nm, was prepared using TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-mediated
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oxidation of wood cellulose.28 The PS/CNF composite was successfully prepared by
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polymerizing a styrene monomer via a formation of an aqueous Pickering emulsion, using CNF
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as a stabilizer. This approach has recently appeared elsewhere using partially delignified wood as
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a stabilizer for PS, and the resulting composite showed improved mechanical properties.29 In this
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study, we demonstrate that ultrafine CNF prepared by TEMPO-mediated oxidation can construct
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an organized PS nanoparticle/CNF structure, and acts as a reinforcing nanofiller in the resulting
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composite material.
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EXPERIMENTAL SECTION
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Materials. A softwood bleached kraft pulp was kindly provided by Nippon Paper Industries
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Co. Ltd. in a never-dried state. TEMPO, a 2M sodium hypochlorite solution, sodium bromide,
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styrene, 2,2'- azobis(2,4-dimethylvaleronitrile) (ADVN), tetrahydrofuran (THF), and other
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chemicals were obtained from Wako Chemical Co. Ltd. and used as received. PS with a Mw of
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324 000 was purchased from Sigma Aldrich for use as a reference.
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Preparation of CNF aqueous dispersion. CNF was prepared from softwood bleached kraft
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pulp by TEMPO-mediated oxidation.28 The pulp (1 g) was oxidized with TEMPO (0.1 mmol),
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sodium bromide (1 mmol), and sodium hypochlorite (10 mmol) in water (100 mL) at pH 10. The
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oxidized pulp was washed thoroughly with distilled water by filtration, then suspended in water
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at a concentration of 0.1 % w/v. CNFs were dispersed in water using a double-cylinder-type
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homogenizer (Physcotron, Microtec Nition, Japan) at 7,500 rpm for 1 min and an ultrasonic
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homogenizer (VP-300N, TAITEC) for 4 min. The unfibrillated fraction (˂ 5%) was then
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removed by centrifugation at 10,000g for 30 min. The carboxylate content of CNF was
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determined to be 1.7 mmol g−1 by electric conductivity titration. The introduction of the carboxyl
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groups was also confirmed by evaluating the chemical structure of the CNF using solid-state 13C
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cross-polarization/ magic-angle spinning (CP/MAS) nuclear magnetic resonance (NMR). The
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CNF with a uniform width of 3 nm and average length of ~400 nm was successfully prepared,
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and formed transparent dispersion in water (Figure S1 in the Supporting Information). The
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concentration of the CNF dispersion was adjusted to 1.0 % w/w using an evaporator at 40 ºC
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under reduced pressure.
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Preparation of CNF-stabilized Pickering emulsion. Styrene (1 mL) was added to the CNF
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aqueous dispersion (9 mL) and the styrene/water emulsion, stabilized by CNFs, was formed by
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ultrasonication (60 W, 20 KHz) in an ice bath.
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Preparation of PS/CNF composite. The PS/CNF composite was prepared by polymerizing
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the styrene monomer inside the emulsion. ADVN was used as an initiator at a concentration of
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0.01 mol L‒1 (0.3 % w/w) based on styrene monomer. Styrene containing the initiator (1 mL)
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was added to the CNF dispersion (9 mL) and emulsified by ultrasonication as described above.
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In order to polymerize the styrene, the suspension was gently stirred at 200 rpm in a water bath
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kept at 70 ºC. After the polymerization reaction, methanol (20 mL) and 0.1 M HCl (0.1 mL) was
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added to the system and white the precipitate formed was thoroughly washed with methanol by
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filtration through a 0.1-µm pore size PTFE membrane. The obtained precipitate was vacuum
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dried at 40 ºC for more than a week. Transparent PS/CNF composite films, ~200 µm thick, were
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formed by pressing the composite samples at 160 ºC and 3 MPa for 30 sec.
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Size distribution measurement of Pickering emulsion. The emulsion size distribution was
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determined using a laser diffraction particle size analyzer (λ = 405 nm; SALD-7500nano,
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Shimadzu). The emulsion was dispersed in water and analyzed using a flow cell unit.
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C NMR analysis. Solid-state
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C CP/MAS NMR analysis was carried out using an NMR
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spectrometer (CMX Infinity 300, Chemagnetics). The spectra were obtained using a 1.2-ms
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contact time and 3.0 s repetition time. The samples were loaded into a 4-mm zirconia rotor and
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spun at 10,000 Hz during the measurements.
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SEC-MALLS analysis. The molecular weight of PS was determined by size-exclusion
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chromatography using a multi-angle laser light scattering (SEC-MALLS) detector (DAWN
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HELEOS II, Wyatt Technology) with THF as an eluent. The SEC-MALLS system consisted of a
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degasser (DGU-20A, Shimadzu), a high-pressure pump (LC-20AD, Shimadzu), a stainless steel
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inline filter with a 0.1 µm poly(tetrafluoroethylene) (PTFE) membrane (Millipore), an auto-
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sampler (SIL-20A HT, Shimadzu), a column oven (CTO-10AS VP, Shimadzu), a SEC column
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(KF-806M, Shodex), a MALLS detector, and a refractive index detector (Optilab rEX, Wyatt
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Technology). The SEC conditions were as follows: flow rate 0.5 mL min−1, injection volume 50
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µL, sample concentration 0.3 % w/v, and column temperature 40 ºC. A value of 0.180 mLg−1
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was used for the refractive index increment value (dn/dc) of PS. A PS standard (Mw 30,000,
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Mw/Mn 1.06; Pressure Chemical Co.) was used for the MALLS calibration.
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SEM analysis. A field-emission scanning electron microscope (FE-SEM) (S-4800, Hitachi)
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was used to analyze the surface structure of PS/CNF composite. Secondary electron images were
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obtained at an acceleration voltage of 1 kV with about 5 nm osmium coating by an osmium
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coater (Neo Osmium Coater, Meiwafosis) at 5 mA for 5 s.
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UV-vis spectroscopy. Transmittance spectra of the composite films were monitored using an ultraviolet–visible light spectrophotometer (UV-2400PC, Shimadzu).
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Tensile testing. Tensile properties of the PS/CNF composite film were investigated using a
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tensile tester (EZ Graph, Shimadzu) equipped with a 500-N load cell. The specimens were tested
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with a span of 10 mm at a crosshead speed of 1 mm min‒1.
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Thermomechanical analysis (TMA). Thermal dimensional stability of the films was
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evaluated using a thermomechanical analyzer (TMA-60, Shimadzu). The linear coefficient of
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thermal expansion (CTE) of the film was determined using a 0.03 N load cell under a nitrogen
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atmosphere from 30 to 150 ºC at 5 ºC min‒1.
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RESULTS AND DISCUSSION
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CNF-stabilized Pickering emulsion. The CNF-stabilized Pickering emulsion was successfully
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formed by ultrasonication of the styrene/CNF aqueous dispersion mixture, and the emulsions
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were individually dispersed in water (Figure 1). In the absence of CNF, a phase separation
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occurred spontaneously and reduced the interfacial energy of the styrene/water system (Figure 1a
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left). Therefore, CNFs stabilized the oil in water (o/w) emulsion by adsorption at the
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styrene/water interface, as previously reported CNF-stabilized Pickering emulsions.30-34
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Cellulose has natural amphiphilic properties and it is likely that the hydrophobic surface of the
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CNFs preferentially attracted the styrene phase.31,35,36 The repulsive forces between the
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emulsions arise primarily from osmotic pressure caused by the carboxyl groups introduced by
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the TEMPO-mediated oxidation.32,37 The density of the surface carboxylate groups on the CNFs
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was calculated to be as high as 1.7 groups nm−2.38 Therefore, the Pickering emulsion showed
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remarkable coalescence stability by exposing the carboxyl groups on the surfaces.
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Figure 1. CNF-stabilized Pickering emulsion formed at a 10/90 ratio (o/w). (a) Photograph of
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styrene/water mixture with (right) and without (left) CNF, (b) optical microphotographs, (c) size
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distribution.
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The Pickering emulsion showed a monomodal size distribution (Figure 1b and c). The larger
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emulsion particles became smaller with increasing sonication time and when sonicated for longer
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than 1 min the size leveled off at a median diameter of ~9.0 µm (Figure S2). The emulsion
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showed good stability. The size distribution of the emulsion remained unchanged for over a
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week (Figure S3), and no appreciable creaming was observed. In some previous studies, o/w
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emulsions stabilized by cellulose nanocrystals rapidly form a creaming layer, due to the density
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difference between the oil and water phases.30-32 The long-term stability in this study is likely
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due to both the stronger repulsive forces between the emulsions and the increased viscosity of
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the aqueous phase, caused by the ultrafine CNFs.39 The increased viscosity could cause
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decreased velocity of creaming (or sedimentation). In this way, hydrophobic styrene was
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successfully encapsulated in the CNF and homogeneously dispersed in the aqueous phase.
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PS/CNF composite. The PS/CNF composite was prepared by polymerizing the encapsulated
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styrene monomer at 70 ºC using ADVN as an initiator. Figure 2a and b compare the dispersion
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before and after polymerization. The size of the emulsion particles decreased significantly during
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the polymerization process and a stable dispersion of smaller particles was obtained. We initially
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Figure 2. Photographs and optical micrographs of CNF-stabilized emulsion containing an
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initiator (a) before and (b) after polymerization for 8 h.
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assumed that microparticles, reflecting the size of the Pickering emulsion (~10 µm), would result
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from the polymerization. However, it was found that the polymerization produced
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monodispersed nanoparticles of ~150 nm in diameter (Figure 3). In Figure S4, it can be seen that
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the PS nanoparticles were squeezed out of the CNF-stabilized emulsion, reducing the emulsion
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diameter. In general, Pickering emulsions could be used as templates for preparation of
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microparticles.40,41 However, the CNF-stabilized Pickering emulsion in this study is not so robust
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for the polymerization that the PS nanoparticles came out of the emulsion. This behavior seems
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complex, but can be understood as a dispersion polymerization-like process. Dispersion
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polymerization is a useful technique for synthesizing monodispersed polymer particles in the
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region of about 0.1‒10 µm, by conducting the reaction in a poor solvent for the polymer.42-44 The
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polymerization proceeds in the solvent and/or monomer and as a result, the polymer particles are
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precipitated in the medium. In this study, PS nanoparticles were precipitated in the aqueous
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medium in the same way and each individual particle was stabilized by CNF networks.
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Therefore, a unique film structure, where the PS nanoparticles were confined by the CNF
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framework, was easily obtained by filtration and subsequent drying.
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Figure 3. (a) Photograph, (b‒e) SEM images, and (f) Schematic illustration of PS/CNF
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composite with 12 % w/w CNF before melt pressing. (b,c) film surface, (d) edge, and (e) side
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surface.
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The composite was composed purely of PS and CNF, demonstrated by solid-state
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C
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CP/MAS NMR analysis (Figure S5). The CNF content in the composite film was calculated to
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be 12 % w/w based on the conversion of styrene (83 %), which was also confirmed by
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comparing the peak areas of PS (~146 ppm ) and CNF (~105 ppm) in solid-state
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NMR spectra. Mw of PS was as high as 1 560 000, with a polydispersity (Mw/Mn) of 1.59 (Figure
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4). The yields of the PS were more than 80 % and the PS showed high Mw values when the
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initiator amount was 0.01−0.05 mmol/mL against styrene monomer (Figure S6). The Mw value is
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one order higher than that of commercially used PS which is synthesized by bulk polymerization
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(typical Mw: ~150 000‒400 000,45 see Figure S7). Interestingly, when this polymerization was
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conducted without forming the micron-sized Pickering emulsions by ultrasonication, the
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obtained PS showed lower M w and higher M w/Mn values, likely due to a heterogeneous
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polymerization (Figure S8). Moreover, the Mw is about 10 times higher than that of partially
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d e l i g n i f i e d
w o o d - s t a b i l i z e d
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C CP/MAS
e m u l s i o n
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Figure 4. Molecular mass distribution of PS. (a) Elution pattern and Mw plots, and (b) double
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logarithmic plots of radius of gyration (Rg, z) vs Mw for PS.
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polymerization (Mw : 124 000),29 likely due to the formation of uniform Pickering emulsion in
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this study. Therefore, the Pickering emulsion acted as a microreactor and enhanced heat transfer
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due to the higher volume-to-surface ratio, which could contribute to the efficient polymerization
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of encapsulated styrene.46 The slope of the conformation plot for PS was 0.59 (Figure 4b), which
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corresponds to the conformation of linear flexible polymers in good solvents.47 There was no
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convincing evidence of covalent bond formation between PS and CNFs in the NMR spectrum
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and the SEC-MALLS result showed that only pure PS chains were detected at high yield
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(~100 %). Therefore, pure PS without branching was synthesized through the polymerization
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process.
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Figure 5. (a) Photograph of PS/CNF composite film with 12 % w/w CNF after melt pressing, (b)
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UV-Vis transmittance spectra, (c) stress-strain curves and (d) thermal expansion behavior of
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PS/CNF composite and PS films.
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Transparent PS/CNF composite film was simply formed by subsequent melt pressing (see
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Figure 5a and S8). The film showed high optical transparency with a transmittance of 88 % at
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600 nm, as high as that of PS films (90%) (Figure 5b). Moreover, the transmittance is
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comparable to that of a PS/CNF nanocomposite film prepared by solvent casting from N,N-
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dimethylformamide (89 %).48 Therefore, in this study, high optical transparency was achieved
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using the organic solvent-free process.
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Figure 5c shows the stress‒strain curves of the films. Incorporation of CNF improved the
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mechanical strength of PS. Young’s modulus, ultimate strength, and strain to failure of the
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composite film was 3.4 ± 0.1 GPa, 50.1 ± 2.6 MPa, and 1.5 ± 0.1 %, respectively (Table S1).
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These improvements are more marked than those of PS/CNF nanocomposite film prepared by
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solvent casting.48 Although the modulus and strength increased, the improvement is not so
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significant for the CNF content. Moreover, the nanocomposite became brittle, which is likely due
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to the insufficient attractive interfacial interaction between the hydrophilic CNF and hydrophobic
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PS, as discussed in our previous study.48 This problem could be solved by selective surface
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modification of the CNF.38 Ben Elmabrouk et al. have reported on preparation of poly(styrene-
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co-hexylacrylate)/cellulose whiskers nanocomposites via miniemulsion polymerization, and the
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obtained nanocomposite showed better mechanical properties even with less than 5 wt % of the
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whisker loadings.49 In the work, they added a low amount of reactive silane to stabilize the
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miniemulsion. In contrast, the PS/CNF composite in this study did not require any other
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stabilizers, and the obtained composite showed good mechanical properties due to the
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reinforcement by the CNF. Moreover, the composite showed better thermal dimensional stability
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(Figure 5d). The CTE of the composite was 56.7 ppm K‒1, below the glass transition region of
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PS (Tg: ~100 ºC), while that of PS was 143 ppm K‒1. The CTE above Tg significantly decreased
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from > 7000 to 151 ppm K‒1 with the addition of CNF, which is mainly due to the strong
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hydrogen bonding network between the nanofibrils.50 The Tg of the PS seems not to have been
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significantly changed in the presence of CNFs as previously reported.48 In this way, the
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mechanical properties and thermal dimensional stability of PS film were effectively improved
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due to the homogeneous distribution of CNF throughout the material.
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CONCLUSIONS
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A nanostructured PS nanoparticle/CNF composite was successfully prepared using a facile
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aqueous process. The process involved the encapsulation of a styrene monomer in a CNF-
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stabilized Pickering emulsion and subsequent polymerization. The PS/CNF composite was easily
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collected by filtration and subsequent melt pressing allowed the preparation of a transparent
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PS/CNF composite film. The film showed good mechanical properties and thermal dimensional
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stability due to the reinforcement by the homogeneously distributed CNFs. This material could
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be extended to nanotechnology application such as flexible electronic devices and solar cells
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combining transparency and thermostable properties. Moreover, this novel CNF/nanoparticle
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structure could be useful for optical applications or as a template for porous materials. Therefore,
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this technique paves the way toward novel green nanocomposite production using a facile and
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scalable process.
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ASSOCIATED CONTENT
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Supporting Information
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Photographs and size distributions of CNF aqueous dispersion and CNF-stabilized Pickering
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emulsions, SEM images of the emulsion, molecular mass distribution of PS, solid-state CP MAS
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charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
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Corresponding Author
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*Phone: +81 29 829 8270. Fax: +81 29 874 3720. E-mail:
[email protected] C NMR spectrum, and tensile properties of the composite. This material is available free of
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENT
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This research was supported by Grants-in-Aid for Scientific Research (Grant number
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JP15H06848) from the Japan Society for the Promotion of Science (JSPS).
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REFERENCES
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