ARTICLE pubs.acs.org/Biomac
Toward “Strong” Green Nanocomposites: Polyvinyl Alcohol Reinforced with Extremely Oriented Cellulose Whiskers Ahmed Jalal Uddin,† Jun Araki,‡ and Yasuo Gotoh*,† † ‡
Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan International Young Researcher Empowerment Center, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan ABSTRACT: To exploit the maximum potential of cellulose whiskers (CWs), we report here for the first time the successful fabrication of nanocomposites reinforced with highly oriented CWs in a polymer matrix. The nanocomposites were prepared using polyvinyl alcohol (PVA) and a colloidal suspension of cotton-derived CWs. The macroscopically homogeneous PVA-CW suspensions were extruded into cold methanol to form gel fibers followed by a hot drawing. Compared to the neat PVA fiber, the as-spun fiber containing a small amount of CWs (5 wt % of solid PVA) showed higher drawability, leading to an extremely high orientation of CWs with the matrix PVA. The stress-transfer mechanism, a prime determining factor for high mechanical properties of nanocomposites, was studied by X-ray diffraction. The stress on the incorporated CWs was monitored by applying an in situ nondestructive load to the composite fibers. The applied stress to the whole sample was found to be effectively transferred to the CWs inside the composites, suggesting strong interfacial bonding between the filler and the matrix. Effective stress transfer to the oriented whiskers resulted in outstanding enhancement in mechanical properties of the nanocomposites.
’ INTRODUCTION Cellulose fibers are bundles of microfibrils consisting of monocrystalline cellulose domains linked by amorphous domains. The diameter of the fibrils ranges from 5 to 20 nm and their lengths can reach several tens micrometers depending on the source of the cellulose.1 Upon hydrolysis with strong sulfuric or hydrochloric acid, the nearly endless microfibrils undergo longitudinal cleavage and release stiff rod-like nanoparticles of cellulose whiskers (CWs) that have a high aspect ratio in the range of 20-100. In suspension, they form chiral nematic phases resembling the highly textured organizations of microfibrils found in natural biological systems.2,3 The highly crystalline CWs are devoid of chain folding, with excellent physical properties approaching those of perfect crystals. Their modulus and strength were demonstrated to be as high as 140-150 and 7 GPa, respectively.4-7 The outstanding mechanical properties, renewability and biodegradability of the CWs make them promising candidates to create nanocomposites with superior performance and extensive future applications.8 Favier et al.9 first reported the use of CWs as a reinforcing phase. Subsequently, the performance of CWs incorporated into different natural and synthetic polymer matrices has been studied. These polymers include poly(β-hydroxyoctanoate),10 starch,11 polylactic acid,12,13 silk fibroin,14 cellulose acetate butyrate,15 poly(styrene-co-butyl acrylate),16,17 polyvinyl chloride,18 waterborne epoxy,19 atactic polypropylene,20 poly(oxyethylene),21 and polyvinyl alcohol (PVA).22,23 Most of these studies focused on composite films or electrospun nanofibers where the CWs are dispersed randomly or are poorly oriented in the polymer matrix. However, all of these studies have reported improvements in mechanical and thermal properties of the r 2011 American Chemical Society
respective composites. Similar with the polymer molecules, the excellent mechanical properties of CWs in longitudinal direction can only be fully exploited if uniaxial orientation is achieved. In this context, the fabrication of composite fibers is the best approach to obtain highly oriented whiskers in a composite that coincidently occurs during uniaxial drawing of a fiber. Good interfacial interaction and stress transfer between CWs and polymer matrices is essential for good mechanical properties of composites. Hence, the choice of the substrate is an important factor. The hydrophilic nature of CWs combined with hydrophobic polymers results in reduced composite properties due to difficulties in compounding8 and low adhesion force between the two components. CWs have a high affinity to water, and thus, water is a preferred medium for processing and dispersing CWs in composites.24 From this perspective, water-soluble polyvinyl alcohol (PVA) is a well-suited matrix to blend with CWs.25 The hydroxyl groups in PVA can interact with the hydrophilic surface of the CWs, leading to strong hydrogen bonding between the components. PVA is also a solution-spinnable polymer, and high molecular orientation can be achieved by hot-drawing of the unoriented PVA fiber, which concomitantly leads to high orientation of the CWs in the polymer matrix. PVA is also a biodegradable and biocompatible polymer,26 making it suitable in combination with cellulose materials to produce “green” nanocomposites. PVA has a broad range of industrial and technical applications. In a fiber composite form, it has the potential to be used in fiber-reinforced concrete (FRC) due to its high strength, Received: October 25, 2010 Revised: January 4, 2011 Published: February 04, 2011 617
dx.doi.org/10.1021/bm101280f | Biomacromolecules 2011, 12, 617–624
Biomacromolecules
ARTICLE
Table 1. Compositions of PVA Solution and PVA-CW Mixtures weight (g) sample
a
PVA solutiona
CW suspensionb
concentration (wt %) water 25.0
PVA
CW
CW/PVA ratio (%)
PVA
25
PVA-CW5%
25
7.5
17.5
15 15
0.75
5
PVA-CW10%
25
15.0
10.0
15
1.50
10
PVA-CW15%
25
22.5
2.5
15
2.25
15
PVA-CW20%
25
30.0
-5.0
15
3.00
20
PVA-CW30%
25
45.0
-20.0
15
4.50
30
30% PVA solution. b 5% aqueous whisker suspension.
high modulus, low density, good alkali resistance and, importantly, good interfacial hydrogen bond formation with the cement matrix. It has been shown that incorporating PVA into cementitious materials can drastically reduce the weight of the material, while simultaneously improving axial and bending strength, ductility and durability by resisting crack formation.27 As a low density and relatively inexpensive reinforcement for concrete, the demand for PVA is increasing, as an alternative to asbestos, steel, and glass fibers.28 The present work explores the preparation and characterization of PVA-CW composites, with the CWs embedded in a highly oriented form. CWs hydrolyzed from native cotton were used in this study. Homogeneous suspensions of PVA and PVACW solutions were gel spun, followed by subsequent hot drawing to the maximum extent. The morphology, structure, thermal, and mechanical properties of the fibers were studied. The reinforcing effect in composites is governed not only by the filler and matrix properties, but also to a significant extent by the interfacial interaction between the two. In the last part of this paper, the reinforcing performance of CWs in PVA composites is evaluated by studying the stress transfer mechanism from the matrix to the filler using an X-ray diffraction technique. Such observations have not been reported previously for CWreinforced nanocomposites.
to obtain a 30 wt % concentrated PVA solution. As summarized in Table 1, varying amounts of the as-prepared CWs suspension and water were added (or evaporated) to the as-prepared PVA solution to adjust the PVA concentration to 15% in the final solution. The mixtures were then homogenized in the same rotary mixer at 80 °C for 1 h. The ratio of the weight of the CWs to the weight of PVA ranged between 5 and 30%. The neat PVA and PVA-CW mixtures were deaerated in a vacuum oven at 80 °C for 4 h prior to spinning. A total of 15 wt % PVA-CW solutions with varying concentrations of CWs were prepared as the spinning dope. Gel Spinning. Gel spinning was carried out using a syringe pump and a syringe with a needle. A heater surrounding the syringe was set to 75 °C, and the spinning dopes were injected at 0.38 mL min-1 through a 0.80 mm diameter needle into cooled methanol maintained at a temperature between -15 and -20 °C. The spun gel fibers were kept immersed in the cooled methanol bath for 24 h and then wound into a bobbin. The spun fibers were then again kept in methanol at room temperature for 4 h. The fibers were subsequently dried in air for 24 h. Hot Drawing. The as-spun fibers were drawn in a hot oven with a hand-operated drawing apparatus. Special jaws were designed to provide proper gripping of the fibers and to prevent slippage during the drawing. Around 10 spun fibers of 5 mm length were clamped between two jaws. After placing the apparatus in a hot oven at 210 °C, the fibers were drawn to their maximal draw ratio (DRmax) at a rate of 2 mm s-1 by rotating a hand-driven knob outside the oven. Transmission Electron Microscopy (TEM). The CWs were imaged using a TEM. For the sample preparation, a few drops of aqueous CW suspension (
