Largely Improved Tensile Properties of Chitosan Film via Unique

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ARTICLES Largely Improved Tensile Properties of Chitosan Film via Unique Synergistic Reinforcing Effect of Carbon Nanotube and Clay Changyu Tang, Lixue Xiang, Juanxia Su, Ke Wang, Changyue Yang, Qin Zhang, and Qiang Fu* Department of Polymer Science and Materials, Sichuan UniVersity, State Key Laboratory of Polymer Materials Engineering, Chengdu 610065, China ReceiVed: October 13, 2007; In Final Form: January 9, 2008

In this work, a great synergistic effect of 2D clay platelets and 1D carbon nanotubes (CNTs) on reinforcing chitosan matrix has been observed for the first time. With incorporation of 3 wt % clay and 0.4 wt % CNTs, the tensile strength and Young’s modulus of the nanocomposites are significantly improved by about 171 and 124%, respectively, compared with neat chitosan. This could be understood as due to the formation of much jammed fillers network with 1D CNTs and 2D clay platelets combined together, as indicated by rheological measurement. Our work demonstrates a good example for the preparation of high performance polymer nanocomposites by using nanofillers of different dimension together.

Introduction In recent years, much attention has been paid to natural polymers due to the energy crisis and environmental problem.1 Chitosan, poly-β(1,4)-2-amino-2-deoxy-D-glusose, is the deacetylated products of chitin, the second abundant natural polymers on earth.2 Chitosan has been widely studied for biosensors, tissue engineering, separation membrane, water treatment and so on because of its good biocompatibility, biodegradability, and multiple functional groups.3 However, low mechanical properties and thermal stability of chitosan restrict its use in a wide-range application. Nanocomposite technology using nanofillers such as carbon nanotubes, clay, and silica has already proved to be an effective way to improve the mechanical, electrical, and thermal properties of polymers.4 Carbon nanotubes (CNTs) as 1D nanomaterials are considered to be an ideal reinforcing agent for polymer matrix5-9 because of their unique mechanical strength, nanometer scale diameter, and high aspect ratio. For example, with incorporation of only 0.8% CNTs into chitosan, the mechanical properties are greatly improved, but the tensile strength is difficult to be improved further by increasing the CNT content.10 On the other hand, clay platelets as 2D nanofillers for incorporation into polymeric matrices have been intensively studied in the past few years.11 Chitosan/clay composites were described in some literatures.12-15 Because of the polycationic nature of the chitosan in acid media, this biopolymer can intercalate into the pristine clay by means of cationic exchange process. Intercalated and exfoliated nanostructures are formed in this system, which can provide strong interactions between the clay and chitosan. The enhanced mechanical and thermal stability properties of chitosan were reported,12 and the enhancement was limited as the clay content was above 3 wt %. * To whom correspondence should be addressed. Phone: +86-2885460953. E-mail: [email protected].

It is crucial to have uniform filler dispersion within the polymer matrix and good interfacial adhesion between nanofillers and polymer matrix. The enhanced properties are usually “saturated” due to the filler aggregation, particularly at higher filler content. Thus it is difficult to further improve the mechanical properties of a polymer by using only CNTs or clay alone. Here we report our effort to improve the tensile properties of chitosan by using both CNTs and clay simultaneously. We demonstrate a remarkable synergistic effect of the chitosan/ CNT-clay hybrids prepared by just simple solution mixing. On one hand, the acidified CNTs attached hydrophilic groups such as -COOH and -OH, can be well dispersed in water. On the other hand, chitosan, a hydrophilic biopolymer, has -NH2 and -OH in each unit. The amino group in chitosan can be protonated to NH3+ in acid media, which is favor of the intercalation of polymer chain into clay by means of a cationic exchange process. Thus a good dispersion of clay and CNTs in chitosan solution is expected. Experimental Section Materials. Chitosan powder from crab shell, with a deacetylation degree of 86.98% and viscosity-average molecular weight of 186 000 g/mol, was bought from Zhejiang Golden-Shell Biochemical Co. Ltd. (Yuhuan, China). The unmodified pristine Na+-montmorillonite (MMT), with a cationic exchange capacity (CEC) of 96 mequiv/100 g, was purchased from Southern Clay Products, Inc., TX. Multiwall carbon nanotubes (MWNTs, purity > 95%, diameter 10-30 nm, length 5-15 µm), manufactured by CVD were purchased from Shenzhen Nanotech Port Co. (China). Glacial acetic acid, sulfuric acid (98%), and nitric acid (65%) were purchased from Kelong Chemical reagent plant (Chengdu, China). The raw MWNTs were treated in a mixture of concentrated sulfuric acid (98%)/nitric acid (3:1) at 50 °C for 24 h to increase the mounts of carboxylic and hydroxyl groups.

10.1021/jp709977m CCC: $40.75 © 2008 American Chemical Society Published on Web 03/12/2008

Largely Improved Tensile Properties of Chitosan Film Preparation of the Nanocomposite Films. Chitosan solution of 2 wt % was prepared by dissolving chitosan (CS) in a 2% (v/v) aqueous acetic acid solution using a laboratory magnetic stirrer at 200 rpm for 1 h and filtered through the filter paper to remove the impurity under vacuum. The clay was first swelled in 40 mL of distilled water and ultrasonicated for 15 min. Then, the clay suspension was added into the chitosan solution with clay content ranging from 1 to 5 wt % (with respect to Chitosan), followed by stirring at 60 °C for 5 h. Subsequently, desired amount of CNTs suspension was added into the mixture solution with stirring for 1 h. Upon cooling, the resulting solution was degassed for 3 h under vacuum. After that, chitosan/clay/CNT solutions were poured into a hydrophobic glass plate and heated at 50 °C to remove the solvents. Films were cut into test samples (60 mm × 10 mm) using a razor blade and were kept in an oven at 50 °C for 5 h to remove the remaining water before mechanical test. Mean thickness of the films was about 0.1 mm. It should be noted that the temperature of forming film, gas bubble in cast solution, and dispersion of nanofiller would greatly affect the properties of film because: (i) The higher temperature of forming film allows fast evaporation of solvent (over 60 °C) in film, which could result in defects (microvoid) in film. However, too low temperature (for example, room temperature) could lead to aggregation of CNTs during the slow process of forming film. (ii) The cast solution should be completely degassed; otherwise, gas bubbles in film would result in a large decrease of tensile strength of film as a defect. (iii) CNTs should be ultrasonicated well in water before mixing with chitosan solution. Enough stirring time was necessary for good dispersion of CNTs in chitosan solution. A typical procedure of chitosan nanocomposite film is available in Supporting Information. Measurements. Fourier transform infrared (FTIR) spectra were measured on a nicolet560 spectrophotometer in a transmission mode at 2-cm-1 intervals and 16× scanning. CNTs powder samples were mixed with KBr, and the resulting mixture was pressed into disks (about 0.3 mm in thickness) for FTIR test. A background spectrum containing no sample was subtracted from the spectra. Wide-angle X-ray diffraction (XRD) pattern of the samples were obtained by X’Pert Pro X-ray diffactometer with Cu KR radiation (λ ) 0.15418 nm) under a voltage of 40 kV and a current of 40 mA. Samples were scanned over the range of diffraction angle 2θ ) 1-30°, with a scan speed of 0.5°/min at room temperature. To measure the relative crystallinity (Xc) of film, the amorphous areas and crystalline peak areas were measured, and Xc was calculated from diffracted intensity data (2θ ) 6-30°) with the following relationship21

Xc ) [Ac/(Ac + Aa)] × 100% Where Ac and Aa are the areas of the crystalline and amorphous regions, respectively. Ultrathin films with thicknesses of 50-70 nm for transmission electron microscopy (TEM) observation were prepared via cutting from the epoxy block with embedded nanocomposite films at room temperature using a Leica ultramicrotome with diamonds knife. The ultrathin films were collected in a trough filled with water and placed on the copper grids and then were observed using TEM (JEM-2010) under an accelerated voltage of 200 kV. The failure surface of the chitosan/clay/CNTs nanocmoposite film (after tensile tests) was observed via scanning electron microscopy (SEM, JSM-5900LV). The fractured surfaces were

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Figure 1. FTIR spectra of (a) raw MWNTs and (b) acidified MWNTs.

coated with gold and investigated in an SEM instrument under an acceleration voltage of 20 kV and a working distance of 10 mm. The tensile strength, elongation, and Young’s modulus of the samples were measured on an Instron universal Testing Machine 4302 (with a 500 N load cell and pneumatic side-action grips) at room temperature with gauge length of 20 mm and a tensile rate of 5 mm/min according to ASTM D882. The specimen dimension was 60.00 mm in length, 10.00 mm in width, and 0.1 mm in thickness. Four parallel measurements were carried out for each sample. Oscillatory shear measurements were performed on a controlled stress RH7D rheometer (Bohlin Instruments Ltd) using a 40-mm diameter cone and plate geometry with a cone angle of 4°. The rheometer is equipped with a thermostatic bath at a temperature of 25.0 ( 0.1 °C. The instrument was calibrated, and rotational mapping was performed to ensure the accuracy of the measurements before measurement. The gap between the cone and the plate is 0.15 mm during measurement. The sample was allowed to equilibrate for at least 5 min at the desired temperature before oscillatory shear measurements. Frequency sweep with an angular velocity between 0.01 and 100 rad/s were performed in the linear viscoelastic regime at a low stress of 0.2. Results and Discussion The FTIR spectrum of raw CNTs in Figure 1 shows some peaks with very low intensity located at 3448 and 1723 cm-1, corresponding to OH and CdO stretching, respectively. However, the intensities of these characteristic bands are increased significantly after treatment in sulfuric acid (98%)/nitric acid, indicating that a large amount of carboxylic and hydroxyl groups has been generated on the surface of CNTs. Figure 2 shows the XRD patterns of pure clay (sodium montmorillonite), neat chitosan, chitosan/clay, chitosan/CNTs, and chitosan/CNT-clay composite with various amounts of clay and CNTs (here MWNTs are used). The neat chitosan film shows weak crystalline peaks around 2θ ) 11.3, 18.2, and 23° in the XRD spectrum. These crystalline peaks become stronger and sharper, accompanying a new peak at about 8.5°, after incorporating desired amounts of CNTs or clay into chitosan. The result suggests an enhanced crystallinity or denser packing in the main chain in comparison with the neat chitosan.13 The basal spacing (d001) of pure clay has been calculated to be 1.2 nm from a diffraction peak at 2θ ) 7.2° using Bragg function. The characteristic peak of the clay disappears in the XRD spectrum for both chitosan/clay and chitosan/CNT-clay composite when the clay content is less than 5 wt %, which means the formation of exfoliated structure of clay. In the case of

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Figure 2. XRD pattern of neat chitosan, clay, and nanocomposites with different contents of CNTs and clay: (a) pure clay; (b) neat chitosan; (c) chitosan/0.4% CNTs; (d) chitosan/1% clay; (e) chitosan/ 2% clay; (f) chitosan/2% clay/0.4% CNTs; (g) chitosan/3% clay; (h) chitosan/3% clay/0.4% CNTs; (i) chitosan/5% clay/0.4% CNTs.

chitosan/5% clay/0.4% CNTs composite, a new lower broadened peak around 2θ ) 4-6° appears due to the formation of intercalated structure with some exfoliation. This result is in good agreement with that reported in the literature,12 and our work further indicates that the presence of CNTs will not affect the exfoliation of clay in chitosan. Figure 3 shows the TEM micrograph of the cross section of chitosan/3%clay/0.4%CNTs composite. The TEM image shows the existence of individual nanotubes, and no aggregates of CNTs are observed. On the basis of the XRD results and TEM image, it can be concluded that the chitosan/CNT-clay nanocomposites with an exfoliated structure of clay and well-dispersed CNTs have been successfully prepared through the simple solution-intercalation/mixing method. A strong electrostatic interaction and hydrogen bond could be also formed between chitosan and these two nanofillers, which facilitate the dispersion and interfacial adhesion.10,13 CNTs can insert into a clay network, as indicated by red arrows in Figure 3a. In the Figure 3b (a high magnification TEM image of Figure 3a), a part of CNTs located in clay platelets regions, indicated by red arrows, is seen to be connected with the clay

Tang et al. platelets and form a new type of clay-CNTs network. In this unique fillers network, clay platelets probably impede the motion of CNTs in chitosan matrix during deformation, which could result in an enhanced interaction between chitosan chains and the fillers. The unique synergistic effect of CNT and clay on the reinforcing of chitosan is demonstrated in Figure 4. For both chitosan/clay and chitosan/CNTs binary nanocomposites, the tensile strength increases with the nanofiller content as shown in Figure 4a, b. The increase of tensile strength is saturated at 3 wt % clay for chitosan/clay and 0.4 wt % CNTs for chitosan/ CNTs, which may be caused by aggregation of the nanofillers at higher content. The results are basically consistent with the literature10,12 and suggest that it is difficult to further increase the tensile strength of chitosan by only using clay or CNTs alone. However, by using both clay and CNTs, a dramatic increase of tensile strength of chitosan is observed. Typical stress-strain curves are shown in Figure 4c. The mechanical properties of chitosan and its nanocomposites are summarized in Table 1. With incorporation of both clay (3 wt %) and CNTs (0.4 wt %), the tensile strength of the nanocomposite is increased by 171% from 42 to 114 MPa and Young’s modulus (calculated from the slope of the curves) is increased by 124% from 1400 to 3142 MPa, compared to neat chitosan. Both the tensile strength and Young’s modulus of chitosan/CNT-clay composites are much higher than those corresponding binary nanocomposites chitosan/clay (72 and 2753 MPa) or chitosan/CNTs (73 and 2682 MPa). However, the increase of mechanical properties of chitosan film is accompanied with an obvious decrease of elongation at break, from 12% for neat chitosan to 4-7% for the composites. This may be due to the enhanced rigidity of chitosan chain, resulting from the restriction effect of nanofillers. It is interesting to see that the chitosan/CNTclay ternary composites have not only higher strength but also a slightly larger elongation than those corresponding binary nanocomposites. In some case, the simultaneously improved strength and toughness of polymer nanocomposites have been reported, with incorporation of oriented or functionalized nanofillers.19,20

Figure 3. TEM images of chitosan/3% clay/0.4% CNTs composite (a) at low magnification and (b) at high magnification.

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Figure 4. (a) Tensile strength of chitosan/clay nanocomposite as a function of clay content; (b) tensile strength of chitosan/CNTs nanocomposite as a function of CNT content; (c) stress-strain behavior for neat chitosan, chitosan/0.4% CNTs, chitosan/3% clay, and chitosan/3% clay/0.4% CNTs composites; (d) tensile strength of chitosan/clay/CNTs nanocomposite as a function of clay content.

Figure 6. Frequency response of storage modulus (G′) for chitosan nanocomposite. The measurement was carried out at 25 °C. Figure 5. SEM image showing an overall morphology of a failure surface for chitosan/3% clay/0.4% CNTs nanocomposite.

The tensile strengths of chitosan/CNT-clay ternary composite as function of clay content are shown in Figure 4d. The clay content varies from 1 to 5 wt %, and the CNT content ranges from 0.2 to 0.6 wt %. At a fixed ratio of chitosan/CNTs, the tensile strength of the ternary composites is greatly enhanced with an increase of the clay content. With 3 wt % clay added, the tensile strength of the ternary composites reaches the

maximum value in different fraction of CNTs. The tensile strength of the nanocomposite is decreased as the clay content is up to 5 wt %, resulted from the aggregation of clay particles, as indicated by XRD results (from exfoliated structure to intercalated structure). Figure 5 shows a typical fracture surface of the composites (containing 3 wt % of clay and 0.4 wt % of CNTs) after tensile tests. A uniform distribution of clay and CNTs is observed, with the ends of the broken CNTs on the fracture surface. The observation that most CNTs are broken rather than pulled out from the matrix indicates a strong

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Figure 7. Schematic illustration of morphology of clay and CNTs in chitosan nanocomposites: (a) chitosan/0.4% CNTs; (b) chitosan/3% clay; (c) chitosan/3% clay/0.4% CNTs. The interaction and networks in the system could include: (1) clay-clay network; (2) clay-CNTs network; (3) CNTs-polymer-clay bridging; (4) polymer-polymer network.

TABLE 1: Mechanical Properties and Crystallinity of Neat Chitosan and Its Nanocomposites samples

tensile modulus, E (MPa)

tensile strength, σy (MPa)

elongation at break, b (%)

crystallinity, Xc (%)

neat chitosan chitosan/3% clay chitosan/0.4% CNTs chitosan/3% clay/0.4% CNTs

1400 ( 54 2753 ( 61 2682 ( 32 3142 ( 25

42.8 ( 1.6 72.4 ( 3.8 73.1 ( 2.4 114.2 ( 5.3

12.1 ( 3.3 4.7 ( 1.2 5.2 ( 1.4 6.5 ( 2.3

12.9 40.3 20.6 43.4

interfacial adhesion between the CNTs and chitosan matrix. Good dispersion and interfacial stress transfer are important factors for preparing reinforcing nanocomposites. This results in a more uniform stress distribution and minimizes the presence of stress concentration center.5 Additions of CNTs or clay improve greatly the crystallinity of chitosan, which is an important factor for the enhancement of tensile properties of chitosan. The mechanical properties and crystallinity of chitosan and its nanocomposites are listed in Table 1. With incorporation of clay or CNTs, obviously increased crystallinity of chitosan could partly contribute to the mechanical enhancement of chitosan matrix. However, chitosan has almost the same crystallinity (40-43%) in chitosan/clayCNTs ternary nanocomposites as in chitosan/clay nanocomposites, as shown in the XRD pattern and Table 1. Obviously here the crystallinity plays a less important role in the further increase of the mechanical properties for chitosan/clay-CNTs ternary nanocomposites compared to chitosan/clay nanocomposites. As so far, few literatures have been reported on the synergistic effect of CNTs and clay for improving the properties of polymer matrix. The thermal and flame retardant properties of the related EVA matrices were enhanced when organo-modified clays and carbon nanotubes were added simultaneously.16 Zhang et al. have successfully grown carbon nanotubes on clay platelets to form 3D nanostructure filler.17 The tensile modulus and the tensile strength of the nylon-6 composite were greatly improved, by about 290 and 150%, respectively, by the incorporation of only 1 wt % CNT-clay hybrid filler due to the synergistic effect of CNTs and clay platelets for their homogeneous dispersion and strong interaction with the polymer matrix. To understand the unique synergistic effect of CNT and clay on the reinforcing of chitosan, rheological measurement was carried on the 5 wt % concentrated solution of the composites (we failed to carry out the rheological measurement on chitosan melt due to its easy degradation). The storage modulus G′ at low-frequency oscillation is shown in Figure 6. Not much difference of the G′ is seen for chitosan/clay, chitosan/CNTs, and chitosan/clayCNTs nanocomposites in the high ω regime. However, the G′ in the low ω regime is significantly different among the composites. The G′ of chitosan/CNTs nanocomposites (CNTs content is 0.4 wt %) is slightly higher than that of basal polymer. An increased G′ is observed for chitosan/clay nanocomposites (clay content is 3 wt %). One observes a very narrow plateau

at a low-frequency regime. The frequency independence of G′ at a low-frequency regime is regarded as a pseudo-solid-like behavior and indicating a formation of percolating filler network. Within such locally correlated filler network, the relaxation of the chains upon the applied low-amplitude strain is incomplete at long time range, suggesting that the free movement of chains is restricted by the spatially confined geometry constituted by the anisotropic clay tactoids and nanoplatelets.18 It is interesting to note a much enhanced G′ and even broader plateau for chitosan/clay-CNTs nanocomposites (with 3 wt % of clay and 0.4 wt % of CNTs). This very strong pseudo-solid-like behavior suggests a much higher degree of restricted relaxation behavior. This result provides the evidence that the 2D clay platelets network could be very much enhanced by combination of 1D CNTs and result in a formation of 3D network in chitosan, as schematically shown in Figure 7. In chitosan/CNTs with 0.4 wt % of CNTs, the nanotubes can be well dispersed in chitosan, but no filler network could be formed due to its low concentration (Figure 7a). In chitosan/clay with 3 wt % of clay, formation of 2D clay platelets network is possible (Figure 7b), as indicated by rheological measurement. In chitosan/clay-CNTs ternary nanocomposites, the 1D CNTs set in 2D clay platelets network, or the 1D CNTs are confined in the 2D clay platelets network, resulting in a much jammed and conjugated 3D clay-CNTs network (Figure 7c). The interaction and networks in the system could include: (1) clay-clay network, (2) clay-CNTs network, (3) CNTs-polymer-clay bridging, and (4) polymer-polymer network. The formation of different networks and interactions could be the main reason for the observed synergistic effect of CNT and clay on the reinforcing of chitosan. Conclusion In summary, the high-performance chitosan/clay-CNTs ternary nanocomposites have been successfully prepared by a simple solution-intercalation/mixing method in acid media. A uniform distribution and fine dispersion for both CNTs and clay in chitosan have been evidenced. A great synergistic effect of 2D clay platelets and 1D CNTs on reinforcing chitosan matrix has been observed. With incorporation of 3 wt % clay and 0.4 wt % CNTs, the tensile strength and Young’s modulus are significantly improved by about 171 and 124%, respectively, compared with neat chitosan. This could be understood as due

Largely Improved Tensile Properties of Chitosan Film to a formation of 3D conjugated filler network with 1D CNTs inserted in the 2D clay platelets network. Details of the enhancement mechanism need further investigation. Our work demonstrates a good example for the preparation of high performance polymer nanocomposites by using nanofillers of different dimension together. And it can be expected that chitosan may play more important roles used as a biomaterial with largely improved tensile properties. Acknowledgment. We would like to express our sincere thanks to the National Natural Science Foundation of China for Financial Support (20404008, 50533050, and 20490220). This work is subsidized by the Special Funds for Major State Basic Research Projects of China (2003CB615600) and by the Ministry of Education of China (20050610030). The authors gratefully acknowledge Dr. Souke Yan of Chinese Academy of Science for TEM observation and Ms. Xin-Yuan Zhang of Analytical and Testing Center in Sichuan University for the SEM observation. Supporting Information Available: Modification of glass plate; acidification of raw MWNTs; a typical procedure of chitosan ternary nanocomposite; a Schematic illustration of chemical structure of chitosan in acid media; FTIR experimental results. The materials is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Kylma, J.; Seppala, J. V. Macromolecules 1997, 30, 2876. (b) Chabba, S.; Netravali, A. N. Green Chem. 2005, 7, 576. (c) Kobayashi, S.; Uyama, H. Macromol Chem Phys. 2003, 204, 235.

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