Article pubs.acs.org/Biomac
Superior Reinforcement Effect of TEMPO-Oxidized Cellulose Nanofibrils in Polystyrene Matrix: Optical, Thermal, and Mechanical Studies Shuji Fujisawa, Tomoyasu Ikeuchi, Miyuki Takeuchi, Tsuguyuki Saito, 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: Polystyrene (PS) composites reinforced with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibrils (TOCNs) with various weight ratios were fabricated by casting and vacuum-drying mixtures of PS/N,Ndimethylformamide (DMF) solution and TOCN/DMF dispersion. TOCNs of 3 to 4 nm width were dispersed homogeneously at the individual nanofibril level in the PS matrix, such that the TOCN/PS nanocomposite films exhibited high optical transparencies and their tensile strengths, elastic moduli, and thermal dimensional stabilities increased with increasing TOCN content. Dynamic mechanical analysis showed that the storage modulus of the TOCN/ PS films increased significantly with TOCN content above the glass-transition temperature of PS by the formation of an interfibrillar network structure of TOCNs in the PS matrix, based on percolation theory. The outstanding and effective polymer reinforcement by TOCNs results from their high aspect ratio, high crystallinity, and nanodispersibility in the PS matrix.
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nanowhiskers have relatively low aspect ratios (∼50),20,21 filler additions of >10 wt % are required for significant reinforcement of polymer matrices. Cellulose nanofibrils with high aspect ratios can be isolated from wood celluloses by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation and successive gentle mechanical treatment in water.22−26 The obtained TEMPO-oxidized cellulose nanofibrils (TOCNs) have uniform widths of 3 to 4 nm, high aspect ratios greater than 100,22−25 and high elastic moduli of 145 GPa.27 Therefore, TOCNs would have potential application in achieving more effective reinforcement in polymer matrices than conventional cellulosic nanomaterials. Even though some TOCN/polymer composites have been reported in previous papers,28−32 the efficient reinforcement effects of TOCNs as nanofillers dispersed at individual nanofibril level in the matrices have not been achieved yet. In this study, we demonstrate the outstanding nanoreinforcement effect of TOCNs in a polystyrene (PS) matrix. The TOCN/PS composite films were prepared by mixing PS solution in N,N-dimethylformamide (DMF) and TOCN with free carboxyl groups (TOCN-COOH)/DMF dispersion with various weight ratios and subsequent casting and vacuumdrying of the mixtures. Two TOCNs with different aspect ratios
INTRODUCTION Fiber-reinforced composite materials have attracted considerable interest over the decades. With only a small amount of fiber addition, mechanical properties of plastics, such as strength, elastic modulus, and thermal deformation, can be improved substantially in some cases. Many kinds of fiberreinforced composites have been reported thus far generally using glass fibers,1 carbon fibers,2−4 and carbon nanotubes.5−7 However, the main drawbacks of these fillers are high production cost, high energy consumption during production and difficulty in recycling. Therefore, the possibility of using renewable and environmentally friendly reinforcement materials has generated renewed interest in recent years. Cellulose is a promising candidate as a sustainable polymer reinforcement material. This material is the most abundant and renewable carbon resource on earth and exists in nature in a fibrous crystal state called cellulose microfibril.8 Plant cellulose microfibrils have high aspect ratios of ∼4 nm width, lengths greater than 1 μm,9 and high elastic moduli (∼140 GPa).10 These unique properties provide potential applications in designing composite materials with high transparency and high mechanical properties. The reinforcement of conventional polymers with plant cellulose nanofibrils such as microfibrillated celluloses (MFCs)11−14 and sulfuric acid-treated cellulose nanowhiskers (or nanocrystalline celluloses)15−19 has been explored intensively in recent years. However, because MFCs consist of bundles of cellulose nanofibrils and cellulose © 2012 American Chemical Society
Received: April 19, 2012 Revised: May 24, 2012 Published: May 29, 2012 2188
dx.doi.org/10.1021/bm300609c | Biomacromolecules 2012, 13, 2188−2194
Biomacromolecules
Article
(JASCO V-670). TOCN morphologies were observed by transmission electron microscopy (TEM) using a JEOL JEM-2000EX microscope at an accelerating voltage of 200 kV according to a previously reported method.33,37 Any kinks present in individual nanofibrils were ignored for the length calculation; each length was measured as one connected TOCN, even though it had some kinks. The number of the measured fibrils was ∼150 for each sample. For TEM observations of film cross sections, the film specimen was embedded in epoxy resin (Epon 812), which was then cured at 60 °C. The embedded specimen was sectioned at 90° relative to the film surface using a Leica Ultracut-E microtome equipped with a diamond knife. Approximately 100 nm thick sections were stained with 1% uranyl acetate and 2% lead citrate, adjusted to pH 12, on a carbon support grid, and then observed by TEM under the same conditions described above. Differential scanning calorimetry (DSC) measurements were carried out using a Perkin-Elmer DSC 8500 instrument at a heating rate of 10 °C min−1. Tensile tests of films ∼150 μm thick were carried out using a Shimadzu EZ-TEST tensile tester equipped with a 500 N load cell. Specimens 2 × 30 mm in size were cut from the composite films using a cutter blade and measured at 0.5 mm min−1 and a 10 mm span length. At least five specimens were measured for each sample. Thermomechanical analysis was carried out using a 0.03 N load cell in a nitrogen atmosphere from 30 to 120 at 5 °C min−1 using a Shimadzu thermomechanical analyzer (Shimadzu, Japan, TMA-60). The change in specimen length with temperature was recorded and the linear coefficient of thermal expansion (CTE) was calculated. Dynamic mechanical analysis (DMA) was carried out on an RSA-G2 (TA Instruments). The samples were heated under a nitrogen atmosphere from room temperature to 230 °C at a heating rate of 5 °C min−1 and a frequency of 1 Hz.
were prepared and used to investigate the effect of TOCNs on nanostructures and the physical properties of the TOCN/PS composite films.
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EXPERIMENTAL SECTION
Materials. A never-dried softwood bleached kraft pulp (Nippon Paper, Japan), which contained ∼90% cellulose and 10% hemicelluloses, was used as the original wood cellulose sample. The wood cellulose was stirred in diluted HCl solution at a pH of ∼2.5 for 2 h at 1% consistency for demineralization and then washed repeatedly with water by filtration. TEMPO, sodium bromide, 13% sodium hypochlorite solution, DMF, and other chemicals and solvents were commercial products of laboratory grade (Wako Pure Chemicals, Japan) and used as received. An atactic PS with number- and weightaverage molecular weights of 170 000 and 350 000 g mol−1, respectively, was supplied by Aldrich and used as received. TEMPO-Mediated Oxidation. TEMPO-mediated oxidation was applied to 1 w/v % cellulose fiber/water slurry at pH 10 and room temperature, with NaClO and catalytic amounts of TEMPO and NaBr, using a previously reported method.22,23 The cellulose (1 g) was suspended in water (100 mL) containing TEMPO (0.016 g, 0.1 mmol) and sodium bromide (0.1 g, 1 mmol). The NaClO solution (3.8 or 10 mmol) was added to the slurry, and the mixture was stirred at room temperature and pH 10.33 Most of hemicelluloses present in the original softwood bleached kraft pulp are likely to be removed as water-soluble fractions during the washing process after TEMPOmediated oxidation.34 The TEMPO-oxidized cellulose (1 g) was treated further with sodium chlorite (1.8 g) in 0.5 M acetate buffer (90 mL) at pH 4 to 5 and room temperature for 2 days to convert residual C6-aldelhydes to C6-carboxylates. The TEMPO-oxidized celluloses thus prepared with NaClO of 3.8 and 10 mmol g−1 cellulose had carboxyl contents of 1.30 and 1.72 mmol g−1, respectively. The carboxyl contents were determined by the conductivity titration method.35 Preparation of TOCN/DMF Dispersions. The TEMPO-oxidized cellulose was suspended in water (150 mL) at a 0.1 w/v % solid content, and the slurry was homogenized at 7500 rpm for 1 min at room temperature using a double-cylinder-type homogenizer (Physcotron, Microtec Nition, Japan). The gel thus obtained was then sonicated for 4 min using an ultrasonic homogenizer with a 26-mmdiameter probe tip at 19.5 kHz and 300 W output power (US-300T, Nissei, Japan) and converted to a transparent aqueous dispersion of TOCNs with sodium carboxylate groups (TOCN-COONa). The TOCN-COONa/water dispersion was centrifuged to remove the small amount of partially fibrillated or unfibrillated fraction (
