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chitosan composite foam with enhanced elastic property
Accepted Manuscript Title: Preparation and characterization of carbon nanotubes/chitosan composite foam with enhanced elastic property Author: Jia Yan Tianhao Wu Zezun Ding Xiaokang Li PII: DOI: Reference:
S0144-8617(15)01029-2 http://dx.doi.org/doi:10.1016/j.carbpol.2015.10.049 CARP 10466
To appear in: Received date: Revised date: Accepted date:
23-2-2015 29-9-2015 14-10-2015
Please cite this article as:http://dx.doi.org/10.1016/j.carbpol.2015.10.049 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Preparation and characterization of carbon nanotubes/chitosan composite foam with enhanced elastic property
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Abstract: Carbon nanotubes/chitosan (CNTs/CHI) composite foams with ordered lamellar structure were prepared by unidirectionally freezing a dispersion of CNTs in chitosan aqueous solution and subsequent freeze drying. The structure and thermal stability of the composite foams have been characterized by wide-angle X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and thermogravimetry analysis. And their elastic behaviors were investigated by cyclic compression tests. The produced CNTs/CHI composite foams have better recoverability and improved elastic properties compared with the pure chitosan foams. Freezing rate, fraction of CNTs and density are the important factors affecting on the micro morphology, elasticity and mechanical strength of CNTs/CHI composite foams. Due to less ice dendrites and thicker lamellas being formed under low freezing rate, the CNTs/CHI composite foams prepared under low freezing rate (6 mm min-1) possesses better mechanical properties than those prepared under high freezing rate (10 cm min-1). With the increasing CNTs fraction, the recovery ability of CNTs/CHI composite increases and achieves the maximum at a critical point, and then decreases dramatically due to the inadequate chitosan matrix and aggregation of CNTs. The critical point herein appears at the CNTs fraction ≥0.5 and ≥0.3 respectively, for the samples with density of 0.02 and 0.01 g/cm3. The CNTs/CHI composite foams with high density (0.02 g/cm3) possess better elasticity and mechanical strength than the ones with low density (0.01 g/cm3).
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Jia Yan*, Tianhao Wu, Zezun Ding, Xiaokang Li State Key Laboratory of Structural Analysis for Industrial Equipment, Dalian University of Technology, 2 Linggong Road, Ganjingzi District, Dalian 116024, P. R. China
Keywords: Chitosan; Carbon nanotubes; Foams; Elastic properties; Nanocomposites * Corresponding author. Tel: 86-411-84706692. E-mail: [email protected] (Jia Yan) 1. Introduction Chitosan is a linear polysaccharide derived from partial deacetylation of chitin. It can be commonly found in shells of marine crustaceans and therefore is considered as a natural, nontoxic, cheap and commercially produced material. In the recent decades, it has been reported to be applied in orthopedics [Martino, Sittinger, & Risbud, 2005], sensors [Nasution, Nainggolan, Hutagalung, Ahmad, & Ahmad, 2013], nanocarriers [Tang, Song, Chen, Wang, & Wang, 2013] and wastewater purification [Cui et al., 2013], due to its excellent biodegradability, biocompatibility, suitability for cell ingrowth, electrochemical properties and adsorptivity for metal ions. Usually, chitosan exists as powders, films or fibers with low porosity. However, for some applications such as bone and soft tissue engineering, high porosity and good mechanical properties are demanded. The three-dimensional (3D) porous structure can promote the ingrowth of tissue cells, such as rapid penetration of cells, multicellular spheroid, nerve and blood capillary into the materials with maintenance of their biological 1
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function, resulting in rapid tissue regeneration. Therefore, it is important for the development of cell carriers and foams for tissue engineering. In addition, the foam, whether in the process of handling or in vivo, is often exposed to mechanical compression. Thus, it must be designed to have recoverability against compressions while maintaining the sufficient porosity. Therefore developing porous chitosan-based materials system with excellent mechanical properties especially elasticity and anti-fatigue performance becomes mandatory. It has been found the porous carbon nanotubes/chitosan (CNTs/CHI) composite system have excellent properties for bone tissue engineering [Abarrategia et al., 2008; Venkatesan, Qian, Ryu, Kumar, & Kim, 2011], but mechanical properties of the prepared composites were not investigated, and the porous structure was disordered. And it has been reported that CNTs can be utilized to reinforce chitosan film [Azeez, Rhee, Park, Kim, & Jung, 2013], but at present there is rare research being reported about the effect of CNTs on mechanical properties of porous chitosan materials. As the most simple and environment-friendly preparation method, ice templating has been widely used to prepare porous or fabric polymers, ceramics and assemblies of nanomaterials [Gutiérrez, Ferrer, & Monte, 2008; Yan, Chen, Jiang, Tan, & Zeng, 2009; Zhang et al., 2005] in the recent decades, for various applications in areas such as tissue engineering [Abarrategia et al., 2008; Venkatesan, Qian, Ryu, Kumar, & Kim, 2011], wastewater treatment [Zhang, Qiu, Si, Wang, & Gao, 2011], catalyst carries [Nakagawa, Yasumura, Thongprachan, & Sano, 2011; Nishihara, Mukai, Shichi, & Tamon, 2010] and drug delivery [Gutiérrez et al., 2007]. In this method, directionally growing ice crystals are utilized as templates to prepare porous materials with ordered micro structure, inspired by a natural principle that when water is frozen, impurities originally present in water will be expelled from the forming ice and entrapped within channels between the ice crystals [Deville, Saiz, Nalla, & Tomsia, 2006]. In this paper we prepare CNTs/CHI composite foams with aligned lamellar structure using ice-templating under different freezing rates and CNTs/chitosan concentrations, investigate their mechanical behaviors, and discuss the possible mechanism combining the macro mechanical properties to their micro structures. It is found CNTs can obviously improve the elasticity of chitosan foams, and the freezing rate, density and CNTs fraction are important factors effecting on their morphologies and mechanical properties. This is important to the material design for further practical application of CNTs/CHI composites in fields of tissue engineering, orthopedics and nanocarriers. 2. Experimental 2.1. Chemicals Chitosan (degree of deacetylation: 75~85%; molecular weight: 190,000-310,000 Da) and acetic acid were purchased from Sigma-Aldrich (Shanghai) Trading Co. Ltd.; CNTs (Carboxylic multi-walled carbon nanotubes) with diameter of 40~60 nm and length of 0.5~2 µm were purchased from Shenzhen Nanotech Port Co. Ltd. All of the chemicals were used as received. 2.2. Preparation of samples The CNTs/CHI composite foams were produced by unidirectionally freezing a
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mixture of CNTs and chitosan aqueous solution and subsequent freeze drying. Typically, chitosan aqueous solution (1 wt.%) was prepared by dissolving chitosan (0.5 g) in 50 ml of aqueous solution of acetic acid (0.1 M, pH 5), and then 0.5 g of CNTs were dispersed in the chitosan solution under stirring and subsequent sonication to obtain CNTs suspension with a CNTs concentration of 1 wt.%. For unidirectional freezing, the resulting suspension was poured into a round polypropylene tube (inner diameter ~ 1.3 cm) and vertically immersed into liquid nitrogen (-196 ºC) at a desired velocity controlled by a motor until all of the liquid was totally frozen, as shown in Fig. 1a. The immersion rate of 6 mm min-1 and 10 cm min-1 were used to perform low and high freezing rate respectively. Then, the frozen samples were freeze-dried for 48 hours to remove the ice. Finally the CNTs/CHI composite foam with a CNTs fraction of 0.5 and a density of ~0.02g/cm3 was obtained. To prepare samples with the density of 0.01 g/cm3, all steps were the same except for the halved concentration of CNTs and chitosan. The obtained CNTs/CHI composites under the low and high immersion rates were denoted as CNTs/CHI-L-x and CNTs/CHI-H-x respectively, where x was the fraction of CNTs in the CNTs/CHI composite foams. To prepare the pure chitosan foams with densities of 0.01g/cm3 and 0.02g/cm3, the chitosan aqueous solution with concentration of 1 wt. % and 2 wt. % were used respectively for unidirectional freezing, and the other steps were similar to those for preparation of CNTs/CHI composite foams. 2.3. Characterization of samples The products were investigated by wide-angle X-ray diffraction (XRD) patterns (Siemens D5000, Cu Kα, radiation wavelength λ= 0.154 nm). The Fourier transform infrared spectroscopy (FTIR) measurements were performed in attenuated total reflectance (ATR) mode. The micro-structures of produced samples were characterized using a scanning electron microscope (SEM, S-4800, Hitachi). The thermal behavior of the products was investigated by thermo-gravimetric analysis (TGA) in air atmosphere on TA Instrument 2950. The temperature range is employed from 200 to 750 with a heating rate of 10 ºC min-1. 2.4. Mechanical measurements The cyclic compression test was carried out by cyclically compressing a cylindrical sample (diameter ~1.1 cm, height ~1 cm) to a constant strain of 50% on the Instron3345 (loading capacity 0.001 ~ 10 N) at a constant loading and unloading speed of 2 mm min-1. When we prepared samples for cyclic compression test, the top and bottom of the freeze-dried samples were cut off due to their curved surfaces and irregular micro-structures derived from the instable freezing in these parts, and the middle parts in shape of cylinder were chosen as specimens.
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ip t cr us an M Fig. 1 The experimental setup (a) and processing principle (b and c) for the preparation of CNTs/CHI composite foams.
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3. Results and discussion As can be seen from Fig. 2a, both the pure chitosan foam and CNTs/CHI composite foams prepared by ice-templating were foam-like and self-standing. In addition, it could be easily seen with naked eyes that there were many microgrooves on the lateral side of these prepared samples. The low-magnified SEM images (Fig. 2b) further confirmed these unidirectional micro-channels going throughout the CNTs/CHI composite foam from the bottom to top. In the images of cross sections parallel (Fig. 2c) and perpendicular (Fig. 2d) to the direction of ice crystals growth (i.e. the direction of immersion into liquid nitrogen), it was observed that the foams are composed of a great number of lamellas or sheets which extended unidirectionally. In the micro scale, these lamellas seemed to align parallel with interspace of 10~50µm, and their thickness was uniform (Fig. 2e). Furthermore, in a zoomed SEM image (Fig. 2f) of an individual CNTs/CHI lamella, it was observed that CNTs were well dispersed in and mixed with the polymer matrix. As shown in Fig. 1b, while the CNTs/CHI mixture solution was freezing, the growing ice crystals expelled the impurities (CNTs and chitosan in this case) originally present in water, and some of the expelled CNTs and chitosan were entrapped within the inter space between ice crystals, creating a lamellar microstructure oriented in a direction parallel to the movement of the freezing front. After the ice crystals were removed by
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freezing-drying, a reverse duplicate of ice crystals array could be obtained. It should be noted that while the ice platelets grew vertically, in the three-dimensional space, these platelets simultaneously grew horizontally from the mold wall to the axis driven by the radial gradient of temperature. As a result, the lamellas in the final CNTs/CHI foam also centered around the axis of the cylindrical sample block, as shown in Fig. 2c. In addition, the lamellas in CNTs/CHI composite foams were not smooth, and some antenna-like structures were observed on their surfaces (Fig. 2e and g). These antennas are probably resulted from the entrapping of chitosan and CNTs between the side-branch crystals which are formed when impurities freeze-concentrate around the primary ice cells, causing suppercooling and secondary instability formation perpendicular to the freezing (immersion) direction [Zhang et al., 2005], as illustrated in Fig. 1c. It was found that the CNTs/CHI-H-0.5 (Fig. 2g) possessed denser and longer antennas then CNTs/CHI-L-0.5 (Fig. 2e). This is because high emersion rate results in higher supercooling, forming more side-branch crystals. When the polypropylene tube containing the solution is immersed into liquid nitrogen at a low rate such as 6 mm/min, ice crystal growing rate is actually faster than the immersion rate, and the growing front of ice crystals is much higher than the surface of liquid nitrogen. At this condition, the supercooling around the primary ice cells is slighter, leading to lesser antennas on lamellas. While at the condition of high immersion rate such as 10 cm/min, as the solution is fast immerged by liquid nitrogen, the vertical temperature gradient at growing ice fronts is fast increased, obtaining a higher freezing rate, simultaneously the supercooling becomes seriously due to the dramatically increased horizontal temperature gradient, leading to a great number of side-branch crystals growing between the vertically growing prime ice cells. And as a result, larger amount of long antennas on lamellas could be obtained. For the pure chitosan foam, similar lamellar structure but few antennas could be observed (Fig. 2h and Fig. 3a, d). This is because the high heat conductivity of CNTs can promote the formation of suppercooling, whereas the freeze-concentrate of pure chitosan caused much slighter supercooling around the primary ice cells. Therefore, the surfaces of pure chitosan lamellas were smooth, and few antennas could be observed on them.
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Fig. 2 Digital photographs of the pure chitosan foam (left in a) and CNTs/CHI composite foam (right in a), and SEM images of CNTs/CHI-L-0.5 (b-f), CNTs/CHI-H-0.5 (g) and the pure chitosan foam prepared at low immersion rate (h). (b) Lateral surface of the sample block, (c-h) cross sections parallel (c and e-h) and perpendicular (d) to the direction of ice crystals growth (i.e. the direction of immersion into liquid nitrogen). The images in e-h were taken after cyclic
compression tests. Fig. 3 shows the SEM images of pure chitosan foams and CNTs/CHI composite 6
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foams with density of 0.01 g/cm3 and 0.02 g/cm3 respectively. It was observed that the samples with both densities exhibited ordered and unidirectional lamellar micro-structures. But the lamellas in less dense samples were wavy (Fig. 3a and b), wheras the ones in higher dense samples were flat (Fig. 3d-f). The lamellas in the former were thiner, and there were more defects in them than those in the latter, thus they were easier to deform under external force and their own gravity, resulting in wavy layers. It was found that even under the same density, the samples’ morphology changed at different CNTs fractions. In the SEM image of CNTs/CHI-L-0.66 composite foam with the density of 0.02 g/cm3 (Fig. 3g), discontinuous CNTs/CHI lamellas incompletely enwrapped by chitosan matrix and some small CNTs aggregates between lamellas were observed, which was very different from the images of CNTs/CHI-L-0.3 and CNTs/CHI-L-0.5 showed in Fig. 3e and f respectively. And in the case of 0.01 g/cm3, the similar phenomenon happened in the samples with the CNTs fraction of 0.5 (Fig. 3c). It should be noted that for the samples with the same density, larger CNTs content, smaller chitosan content. When the content of CNTs is increased to a critical point, the decreasing chitosan is not enough to form continuous matrix and totally enwrap CNTs, leading to the discontinuous lamellas and CNTs aggregates. It is indicated that for the samples with density of 0.02 and 0.01 g/cm3, the critical point herein appears at the CNTs fraction ≥0.5 and ≥0.3 respectively. When the fraction of CNTs was further increased, as showed in Fig. 3h, the lamellas in CNTs/CHI-L-0.8 foam cracked badly and CNTs were exposed outside of the lamellas (Fig. 3i), indicating that the content of polymer matrix is too small to remain the entire lamellar structure and enwrap CNTs. For the samples with density of 0.01 g/cm3, no self-standing sample could be obtained when the fraction of CNTs was more than 0.5.
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ip t cr us an M Fig. 3 SEM images of pure chitosan foams and CNTs/CHI composite foams with density of 0.01 g/cm3 (a-c) and 0.02 g/cm3 (d-i). The CNTs fraction is 0 (a and d), 0.3 (b and e), 0.5 (c and f), 0.66 (g) and 0.8 (h and i) respectively.
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XRD patterns of carboxyl modified CNTs, raw chitosan powders, chitosan foams, and CNTs/CHI composite foams are shown in Fig. 4. In the pattern of CNTs, the diffraction peak at 26.0° was observed, which is the characteristic peak of (002) plane of the hexagonal graphite structure with interlayer spacing of 0.34 nm in CNTs [Li et al., 2015]. The pattern of raw chitosan powders showed two main peaks at 10.5° and 20.0°. The diffraction peak around 10° is considered to be the (020) reflection of hydrated crystals of α-chitin chains remaining in the raw chitosan, and the peak at 20° corresponds to the (040) plane of chitosan [Choi, Kim, Pak, Yoo, & Chung, 2007; Ogawa, 1991]. While in the pattern of chitosan foams, only a broad diffraction peak at 20.0° was observed, suggesting an amorphous state of chitosan in the sample [Han, Yan, Chen, & Li, 2011; Kim, & Lee, 2011]. The disappearance of the peak at 10.5° suggests the hydrated crystals in chitosan were totally destroyed in the preparation process of chitosan foams. Similar phenomena were also reported by previous literatures. Kam et al. reported the addition of acetate ions would disrupt the chain alignment in the partially deacetylated chitosan films [Kam, Khor, & Lim, 1999]. Ogawa reported the “as received” chitosan which was produced by solid-state deacetylation of chitin was highly crystallized, whereas the crystallinity of “regenerated” chitosan via dissolving in water and drying would decrease a lot unless
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it was heated. In our experiments, the chitosan foam totally lost its crystallinity. We attribute this to the fast freezing process and subsequent freeze-drying. When the raw chitosan powders were dissolved in the aqueous acetic acid solution, the acetate ions and hydrogen bonding between water and chitosan destroyed the long-range order of molecular chains of chitosan. And then, in the process of freezing, the chitosan was quickly expelled from and compacted between the growing ice crystals, without chance to adjust the molecular chains to recover their order, leading to the amorphous chitosan after freeze-drying. For CNTs/CHI composite, the main characteristic peaks of chitosan and CNTs were observed at 20.0° and 26.0° respectively without shifting and any other new diffraction peak, indicating a physical interaction rather than chemical reaction between chitosan and CNTs.
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Fig. 4 XRD patterns of (a) carboxyl modified CNTs, (b) raw chitosan powders, (c) chitosan foams
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and (d) CNTs/CHI composite foams.
Fig. 5 shows FTIR spectra of chitosan foam, carboxyl modified CNTs and CNTs/CHI composite foam. For chitosan foam, the broad absorption band near wave number 3445 cm−1 correspond to O-H and N-H stretching of alcohol and amine groups. The weak absorption bands at wave numbers 2917 and 2880 cm−1 correspond to C-H stretching of hydrocarbon in the chitosan. The characteristic absorption bands at wave numbers 1650 and 1374 cm−1 correspond to the C=O and C-O stretching of amide group, respectively. In addition, the absorption band at wave number 1593 cm−1 corresponds to N-H deformation of amino groups, and bands at 1155, 1078 and 1030 cm−1 correspond to the symmetric stretching of the C-O-C and involved skeletal vibration of the C-O stretching, respectively [Brugnerotto et al., 2001]. As carboxylic multi-walled carbon nanotubes (MWCNTs-COOH) were used in our experiments for better dispersion in chitosan aqueous solution, the absorption bands of carboxylic stretching were observed in FTIR spectrum of pure MWCNTs-COOH foam. The absorption bands at wave numbers 1717 and 1213 cm−1 correspond to the C=O and C-O stretching of carboxylic group grafted on CNTs, respectively [Ntim, Sae-Khow, Witzmann, & Mitra, 2011]. And the peak around 1588 cm-1 is assigned to the C=C 9
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stretching in CNTs [Eklund, Holden, & Jishi, 1995; Li et al., 2015]. However, these characteristic absorption bands of CNTs were not observed in the FTIR spectrum of CNTs/CHI composite foam, resulting in a similar curve to that of chitosan. It is probably because absorption peaks of CNTs were overlapped with or submerged by those of chitosan due to their weak signals or small amount exposed on the lamellar surfaces in composites, since ATR mode used in FTIR measurements herein characterizes the component on the surface of sample. The FTIR results further indicate there are mainly physical interaction rather than chemical reaction between chitosan and CNTs in the composite foams.
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Fig. 5 FTIR spectra of carboxyl modified CNTs, CNTs/CHI composite foams and chitosan powders.
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Fig. 6 shows the TGA and DTG curves of chitosan, CNTs/CHI-0.1, CNTs/CHI-0.66 and CNTs examined in air atmosphere. For chitosan, in the low temperature range between 40~120 °C, a mass loss of ~10% was observed. This step can be attributed to the evolution of adsorbed water in chitosan foams. Subsequently, with the increase of temperature, the pure chitosan foam decomposed in two thermo-oxidative degradation stages in air atmosphere. The first thermo-oxidative degradation process occurred in the temperature range 200~380 ºC, and reached the maximum rate at 290 ºC. The significant weight loss of ~60% in this stage is attributed to the main decomposition (oxidation and rupture of polysaccharide chains) of chitosan. The second thermo-oxidative degradation process occurred in the temperature range 400-620 ºC, with a maximum rate at 520 ºC. The weight loss in this stage is attributed to the further decomposition and oxidation of residual char formed in the first thermo-oxidative degradation process, which can result in the release of CO and CO2. For CNTs, a weak thermo-oxidative degradation occurred in the temperature range 200~400 ºC, due to the decomposition of grafted organic groups. Subsequently the major thermo-oxidative degradation occurred at 400~670 °C, and the maximum 10
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degradation rate occurred at 610 °C. The weight went to ~3 wt. % in this degradation process due to the complete loss of the carbon by oxidation, and the residue was probably the residual metal catalysts and their oxides which remained in the raw material [Bom et al., 2002; Stefov, Najdoski, Bogoeva-Gaceva & Buzarovskaba, 2014]. For CNTs/CHI-0.1, the shape of TGA curve was similar to that of chitosan foam, and it had lower weight loss than chitosan at the same temperature. It was shown in DTG curves that CNTs/CHI-0.1 had three thermo-oxidative degradation stages. The first two stages were similar to that of pure chitosan foam with a little bit shifts to low temperature, and the third one occurred at the same temperature range where CNTs were oxidized. This indicates that before ~500 ºC, the weight loss of the composite foam is mainly derived from decomposition of chitosan, whereas after 500 ºC it is derived from oxidization of CNTs. With the increase of CNTs fraction, as shown in the TGA and DTG curves of CNTs/CHI-0.66, the main thermo-oxidative degradation occurred at the same temperature ranges, but the main mass loss moved to high temperature (>500 ºC), suggesting a further improved thermo-oxidative stability. The above results suggest that the thermo-oxidative stability of the composite foam can be increased by incorporation of CNTs into chitosan walls in the porous structure.
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Fig. 6 (a) TGA and (b) DTG curves of the chitosan foam, CNTs/CHI-L-0.1, CNTs/CHI-L-0.66 and CNTs.
It is found that the prepared CNTs/CHI composite foams possess enhanced elasticity compared to chitosan foams. Fig. 7 shows the digital photographs of pure chitosan and CNTs/CHI composite foams before and after they were compressed to 50% of original length. When the compressive stress was unloaded, the resilience of chitosan foam was not obvious (Fig. 7b), whereas the CNTs/CHI composite foam almost recovered to its original shape (Fig. 7d) without obvious deformation. We further demonstrated this improvement by the accurate cyclic compression test.
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and after (b, d) they were compressed to 50% of original length.
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Fig. 8a shows the stress-strain (σ-ε) curves of cyclic compressions on the CNTs/CHI-L-0.5 composite foam. In the first compression cycle, the stress-strain curve showed two distinct stages during the loading process. The stress increased linearly at ε ≤ 13% which belonged to a linear-elastic region, recording the elastic deformation of lamellas in the CNTs/CHI composite foam. Subsequently, stress increased slowly, indicating the reach of a plateau region, which recorded a non-linear deformation of sample. The unloading curve was totally different from loading one, generating a hysteresis loop, which indicates that a large portion of mechanical energy was absorbed during compression. In the second cycle, a positive stress response appeared at a strain of 6%, indicating the sample recovered to 94% of the original length after the first cycle. The unrecovered part is due to the plastic deformation happening partially in the CNTs/CHI composite foam during the compression process. In all of the subsequent cycles, hysteresis loops could be observed and obvious linear-elastic and plateau regions appeared on loading curves. However, in these cycles, recovery ratio of sample, maximum compressive stress and area of hysteresis loop became smaller and tended towards stability after 10 cycles. The observations of the hysteresis and stress level softening in the CNTs/CHI-L-0.5 composite foam suggest it exhibits viscoelastic behavior under compression [Suhr et al., 2007]. Eventually, even being compressed for 50 cycles，the CNTs/CHI-L-0.5 composite foam still could recover to over 80% of its original length and keep its original macroscopic shape without bend or crack. Such excellent compressibility, similar viscoelastic property to soft-tissue materials such as tendon and muscle [Fung, 1993; Sanjeevi, 1982] and good biocompatibility make the CNTs/CHI composite foam an ideal material for application in tissue engineering. By contrast, the pure chitosan
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Fig. 7 Digital photographs of pure chitosan and CNTs/CHI-L-0.5 composite foams before (a, c)
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foam exhibited a poor compressibility. As shown in Fig. 8b, the sample only recovered to 62% of its original length after 5 compression cycles, and since the second cycle the loading curves rapidly reached densification region almost without elastic and plateau regions. Therefore, we deem that addition of CNTs can obviously improve the elasticity of chitosan foams. The micro morphologies of samples can help to reveal the mechanism of the enhanced elasticity in CNTs/CHI composite foams. In SEM images of the samples after cyclic compression tests, it was observed the lamellas in CNTs/CHI-L-0.5 were orderly aligned and flat (Fig. 2e), whereas the ones in the pure chitosan foam were wavy, somewhere twisting together (Fig. 2h). This well interprets their different mechanical behaviors demonstrated in cyclic compression tests. The lamellas in the pure chitosan foam deform under compressive force and cannot recover to their original shape well after unloading, leading to an obvious plastic deformation in the macro scale. While lamellas in CNTs/CHI-L-0.5 composite foam can survive in the compression cycles without obvious buckling and shifting. We deem the well dispersed CNTs (Fig. 2f) endow their intrinsic elasticity [Falvo et al., 1997] to the lamellas in composite foams, leading to a good elasticity.
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Fig. 8 Stress-strain (σ-ε) curves of cyclic compressions on (a) CNTs/CHI-L-0.5, (b) pure chitosan foam and (c) CNTs/CHI-H-0.5 with the density of 0.02 g/cm3 at the maximum strain of 50%. (d) The recoverable compressive strain and maximum compressive stress response at a constant strain amplitude of 50% for different cycles derived from (a), (b) and (c).
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It was also found that the CNTs/CHI composite foams had very different compressive properties when they were prepared under low and high freezing rates respectively. As 14
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shown in Fig.8c and d, after 5 compression cycles, the CNTs/CHI-H-0.5 only could recover to 68% of original length, which was better than that of pure chitosan foams (62%) but much worse than that of CNTs/CHI-L-0.5 (87%). And simultaneously, the max compressive stress of CNTs/CHI-H-0.5 at 50% strain was also much smaller than that of CNTs/CHI-L-0.5. These results indicate CNTs/CHI-L-0.5 has better elasticity and strength than CNTs/CHI-H-0.5. This is probably attributed to the formation of massive antenna-like structures and thinner lamellar thickness at high immersion (freezing) rate. It should be noted that the forming of more antennas means less matter being used to form the aligned lamellas that are the major force-bearing structures in the CNTs/CHI composite foams. On the other hand, it has been known that the lamella thickness decreases with the increase of freezing rate [Gutiérrez et al., 2007; Zhang et al., 2005]. For these two reasons, the lamella thickness (~ 200 nm) in CNTs/CHI-H-0.5 (Fig. 2g) was lower than that (~ 500 nm) in CNTs/CHI-L-0.5 (Fig. 2e and 3f). And this well interprets the lower max compressive stress and worse recovery capability in CNTs/CHI-H-0.5 than those in CNTs/CHI-L-0.5 (Fig. 8d). Besides freezing rate, density and CNTs fraction were another two important factors effecting no the mechanical properties of CNTs/CHI composite foams. The stress-strain curves of cyclic compressions on the pure chitosan foam and CNTs/CHI composite foams with the density of 0.01 g/cm3 were showed in Fig. 9. As shown in Fig. 9a, the pure chitosan foam recovered to 68% of its original length after 5 cycles, but the elastic modulus and maximum compressive stress were obviously decreased compared to the one with a density of 0.02 g/cm3 (Fig.8b). This agrees with the observation in SEM images (Fig. 3a and d) and the general knowledge for foams that smaller density, smaller strength when the materials system is the same. The introduction of CNTs improved the elasticity, but the strength was decreased (Fig. 9b). This is similar to the above observation on the foams with the density of 0.02 g/cm3. Fig. 9c shows the recovery percentage of original volume at the strain of 50% after 5 compression cycles as a function of CNTs content in the CNTs/CHI composite foams with the two densities. For the foams with density of 0.02 g/cm3, the recovery ability increased with the increase of CNTs fraction, achieving the maximum (87% of original length) at the CNTs fraction of 0.5, and then decreased dramatically. In the case of 0.01 g/cm3 density, similar trend with the maximum at the CNTs fraction of 0.3 was observed, and no compressive test could be applied on the samples with the fraction of CNTs more than 0.66, because they were too weak to shape and self-stand. It is mentioned above that when the CNTs fraction is more than 0.5 and 0.3 respectively, the CNTs/CHI composite lamellas in samples with the two densities become discontinuous and CNTs aggregated seriously (Fig. 3c, g and h). It is this change in micro structure leads to the poor elasticity and mechanical strength.
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Fig. 9 Stress-strain (σ-ε) curves of cyclic compressions on (a) pure chitosan foam and (b)
CNTs/CHI-L-0.1 composite foam with the density of 0.01 g/cm3 at the maximum strain of 50%. (c) Recovery percentage of original volume after 5 compression cycles as a function of CNTs content in the foams with densities of 0.01 g/cm3 and 0.02 g/cm3.
4. Conclusions CNTs/chitosan composite foams constructed by orderly aligned lamellas with uniform interspace and layer thickness at micron and submicron scale respectively were prepared by ice-templating. The characterized and test results reveal the prepared CNTs/CHI composite foams have better thermal stability and higher elasticity than the pure chitosan foams prepared by the same method, due to the introduction of CNTs into chitosan matrix. The mechanical properties of CNTs/CHI composite foams are mainly affected by immersion (freezing) rate, density and CNTs fraction. The samples prepared under low immersion rate (6 mm min-1) exhibit enhanced elastic 16
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properties than those prepared under high immersion rate (10 cm min-1). The samples with high density (0.02 g/cm3) possess better elasticity and mechanical strength than the ones with low density (0.01 g/cm3). For the foams with the same density but different CNTs fractions, with the increasing CNTs fraction the recovery ability increases and achieves the maximum at a critical point, and then decreases dramatically due to the inadequate chitosan matrix and aggregation of CNTs. The critical point herein appears at the CNTs fraction ≥0.5 and ≥0.3 respectively, for the samples with density of 0.02 and 0.01 g/cm3. It is anticipated that this CNTs/CHI composite foam with enhanced elastic property can be applied in fields of tissue engineering, orthopedics and nanocarriers. Acknowledgment The work is financially supported by the National Natural Science Foundation of China (No. 50902015), Fundamental Research Funds for the Central Universities (No. DUT12LK40) and Specialized Research Fund for the Doctoral Program of Higher Education (No. 20090041120036). References Abarrategia, A., Gutierrez, M. C., Moreno-Vicentea, C., Hortiguela, M. J., Ramos, V., Lopez-Lacomba, J. L., et al. (2008). Monte, Multiwall carbon nanotube foams for tissue engineering purposes. Biomaterials, 29, 94-102. Azeez, A. A., Rhee, K. Y., Park, S. J., Kim, H. J., & Jung, D. H. (2013). Application of cryomilling to enhance material properties of carbon nanotube reinforced chitosan nanocomposites. Composites Part B, 50, 127-134. Bom D., Andrews R., Jacques D., Anthony J., Chen B., Meier M. S., & Selegue J. P. (2002). Thermogravimetric Analysis of the Oxidation of Multiwalled Carbon Nanotubes: Evidence for the Role of Defect Sites in Carbon Nanotube Chemistry. Nano Letters, 2 (6), 615-619. Brugnerotto, J., Lizardi, J., Goycoolea, F. M., Arguelles-Monal, W., Desbrieres, J., & Rinaudo, M. (2001). An infrared investigation in relation with chitin and chitosan characterization. Polymer, 42 (8), 3569-3580. Choi, C. Y., Kim, S. B., Pak, P. K., Yoo, D. I., & Chung, Y. S. (2007). Effect of N-acylation on structure and properties of chitosan fibers. Carbohydrate Polymers, 68, 122–127. Cui, H., Chen, J., Yang, H., Wang, W., Liu, Y., Zou, D., et al. (2013). Preparation and application of Aliquat 336 functionalized chitosan adsorbent for the removal of Pb(II). Chemical Engineering Journal, 232, 372–379. Deville, S., Saiz, E., Nalla, R. K., & Tomsia, A. P. (2006). Freezing as a Path to Build Complex Composites. Science, 311, 515-518. Eklund, P. C., Holden, J. M., & Jishi, R. A. (1995). Vibrational modes of carbon nanotubes, spectroscopy and theory. Carbon, 33 (7), 959-972. Falvo, M. R., Clary, G. J., Taylor, R. M., Chi, V., Brooks, F. P., Washburn, S., et al. (1997). Bending and buckling of carbon nanotubes under large strain. Nature, 389, 582-584. Fung, Y. C. (1993). Biomechanics: Mechanical Properties of Living Tissue. Springer-Verlag, New York.
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Gutiérrez, M. C., Ferrer, M. L., & Monte, F. (2008). Ice-Templated Materials: Sophisticated Structures Exhibiting Enhanced Functionalities Obtained after Unidirectional Freezing and Ice-Segregation-Induced Self-Assembly. Chemistry of Materials, 20 (3), 634-648. Gutiérrez, M. C., García-Carvajal, Z. Y., Jobbágy, M., Rubio, F., Yuste, L., Rojo, F., et al. (2007). Poly(vinyl alcohol) Foams with Tailored Morphologies for Drug Delivery and Controlled Release, Advanced Functional Materials, 17, 3505–3513. Han, D. L., Yan, L. F., Chen, W. F., & Li, W. (2011). Preparation of chitosan/graphene oxide composite film with enhanced mechanical strength in the wet state. Carbohydrate Polymers, 83, 653-658. Kam, H. M., Khor, E., & Lim, L. Y. (1999). Storage of Partially Deacetylated Chitosan Films. Journal of Biomedical Materials Research, 48, 881-888. Kim, M. Y. & Lee, J. H. (2011). Chitosan fibrous 3D networks prepared by freeze drying. Carbohydrate Polymers, 84, 1329-1336. Li, S. Z., Gong, Y. B., Yang, Y. C., He, H., Hu, L. L., Zhu, L. F., et al. (2015). Recyclable CNTs/Fe3O4 magnetic nanocomposites as adsorbents to remove bisphenol A from water and their regeneration. Chemical Engineering Journal, 260, 231–239. Martino, A. D., Sittinger, M., & Risbud, Makarand V. (2005). Chitosan: A versatile biopolymer for orthopaedic tissue-engineering. Biomaterials, 26, 5983–5990. Nakagawa, K., Yasumura, Y., Thongprachan, N., & Sano, N. (2011). Freeze-dried solid foams prepared from carbon nanotube aqueous suspension: Application to gas diffusion layers of a proton exchange membrane fuel cell, Chemical Engineering and Processing, 50, 22–30. Nasution, T. I., Nainggolan, I., Hutagalung, S. D., Ahmad, K. R., & Ahmad, Z. A. (2013). The sensing mechanism and detection of low concentration acetone using chitosan-based sensors. Sensors and Actuators B, 177, 522– 528. Nishihara, H., Mukai, S. R., Shichi, S., & Tamon, H. (2010). Preparation of titania–silica cryogels with controlled shapes and photocatalysis through unidirectional freezing. Materials Letters, 64, 959–961. Ntim, A. S., Sae-Khow, O., Witzmann, F. A., & Mitra, S. (2011). Effects of polymer wrapping and covalent functionalization on the stability of MWCNT in aqueous dispersions. Journal of Colloid and Interface Science, 355, 383–388. Ogawa K. (1991). Effect of heating an aqueous suspension of chitosan on the crystallinity and polymorphs. Agricultural and Biological Chemistry, 55, 2375–2379. Sanjeevi, R. (1982). Aviscoelastic model for the mechanical properties of biological materials. Journal of Biomechnics, 15, 107-109. Stefov V., Najdoski M, Bogoeva-Gaceva G., & Buzarovskaba A. (2014), Properties assessment of multiwalled carbon nanotubes:A comparative study. Synthetic Metals, 197, 159–167. Suhr, J., Victor, P., Ci, L., Sreekala, S., Zhang, X., Nalamasu, O., et al. (2007). Fatigue resistance of aligned carbon nanotube arrays under cyclic compression. Nature Nanotechnology, 2, 417-421. Tang, D. L., Song, F., Chen, C., Wang, X. L., & Wang, Y. Z. (2013). A pH-responsive
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chitosan-b-poly(p-dioxanone) nanocarrier: formation and efficient antitumor drug delivery. Nanotechnology, 24, 145101. Venkatesan, J., Qian, Z. J., Ryu, B., Kumar, N. A., & Kim, S. K. (2011). Preparation and characterization of carbon nanotube-grafted-chitosan-Natural hydroxyapatite composite for bone tissue engineering. Carbohydrate Polymers, 83, 569-577. Yan, J., Chen, Z., Jiang, J., Tan, L., & Zeng, X. C. (2009). Free-Standing All-Nanoparticle Thin Fibers: A Novel Nanostructure Bridging Zero- and One-Dimensional Nanoscale Features. Advanced Materials, 21, 314-319. Zhang, H. F., Hussain, I., Brust, M., Butler, M. F., Rannard, S. P., & Cooper, A. I. (2005). Aligned two- and three-dimensional structures by directional freezing of polymers and nanoparticles. Nature Materials, 4, 787-793. Zhang, N. N., Qiu, H. X., Si, Y. M., Wang, W., & Gao, J. P. (2011). Fabrication of highly porous biodegradable monoliths strengthened by graphene oxide and their adsorption of metal ions. Carbon, 49, 827-837.
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CNTs/chitosan foams with lamellar structure were prepared by ice-templating.
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CNTs can improve the elastic property and thermal stability of chitosan scaffolds.
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S0144-8617(15)01029-2 http://dx.doi.org/doi:10.1016/j.carbpol.2015.10.049 CARP 10466
To appear in: Received date: Revised date: Accepted date:
23-2-2015 29-9-2015 14-10-2015
Please cite this article as:
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Preparation and characterization of carbon nanotubes/chitosan composite foam with enhanced elastic property
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Abstract: Carbon nanotubes/chitosan (CNTs/CHI) composite foams with ordered lamellar structure were prepared by unidirectionally freezing a dispersion of CNTs in chitosan aqueous solution and subsequent freeze drying. The structure and thermal stability of the composite foams have been characterized by wide-angle X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and thermogravimetry analysis. And their elastic behaviors were investigated by cyclic compression tests. The produced CNTs/CHI composite foams have better recoverability and improved elastic properties compared with the pure chitosan foams. Freezing rate, fraction of CNTs and density are the important factors affecting on the micro morphology, elasticity and mechanical strength of CNTs/CHI composite foams. Due to less ice dendrites and thicker lamellas being formed under low freezing rate, the CNTs/CHI composite foams prepared under low freezing rate (6 mm min-1) possesses better mechanical properties than those prepared under high freezing rate (10 cm min-1). With the increasing CNTs fraction, the recovery ability of CNTs/CHI composite increases and achieves the maximum at a critical point, and then decreases dramatically due to the inadequate chitosan matrix and aggregation of CNTs. The critical point herein appears at the CNTs fraction ≥0.5 and ≥0.3 respectively, for the samples with density of 0.02 and 0.01 g/cm3. The CNTs/CHI composite foams with high density (0.02 g/cm3) possess better elasticity and mechanical strength than the ones with low density (0.01 g/cm3).
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Jia Yan*, Tianhao Wu, Zezun Ding, Xiaokang Li State Key Laboratory of Structural Analysis for Industrial Equipment, Dalian University of Technology, 2 Linggong Road, Ganjingzi District, Dalian 116024, P. R. China
Keywords: Chitosan; Carbon nanotubes; Foams; Elastic properties; Nanocomposites * Corresponding author. Tel: 86-411-84706692. E-mail: [email protected] (Jia Yan) 1. Introduction Chitosan is a linear polysaccharide derived from partial deacetylation of chitin. It can be commonly found in shells of marine crustaceans and therefore is considered as a natural, nontoxic, cheap and commercially produced material. In the recent decades, it has been reported to be applied in orthopedics [Martino, Sittinger, & Risbud, 2005], sensors [Nasution, Nainggolan, Hutagalung, Ahmad, & Ahmad, 2013], nanocarriers [Tang, Song, Chen, Wang, & Wang, 2013] and wastewater purification [Cui et al., 2013], due to its excellent biodegradability, biocompatibility, suitability for cell ingrowth, electrochemical properties and adsorptivity for metal ions. Usually, chitosan exists as powders, films or fibers with low porosity. However, for some applications such as bone and soft tissue engineering, high porosity and good mechanical properties are demanded. The three-dimensional (3D) porous structure can promote the ingrowth of tissue cells, such as rapid penetration of cells, multicellular spheroid, nerve and blood capillary into the materials with maintenance of their biological 1
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function, resulting in rapid tissue regeneration. Therefore, it is important for the development of cell carriers and foams for tissue engineering. In addition, the foam, whether in the process of handling or in vivo, is often exposed to mechanical compression. Thus, it must be designed to have recoverability against compressions while maintaining the sufficient porosity. Therefore developing porous chitosan-based materials system with excellent mechanical properties especially elasticity and anti-fatigue performance becomes mandatory. It has been found the porous carbon nanotubes/chitosan (CNTs/CHI) composite system have excellent properties for bone tissue engineering [Abarrategia et al., 2008; Venkatesan, Qian, Ryu, Kumar, & Kim, 2011], but mechanical properties of the prepared composites were not investigated, and the porous structure was disordered. And it has been reported that CNTs can be utilized to reinforce chitosan film [Azeez, Rhee, Park, Kim, & Jung, 2013], but at present there is rare research being reported about the effect of CNTs on mechanical properties of porous chitosan materials. As the most simple and environment-friendly preparation method, ice templating has been widely used to prepare porous or fabric polymers, ceramics and assemblies of nanomaterials [Gutiérrez, Ferrer, & Monte, 2008; Yan, Chen, Jiang, Tan, & Zeng, 2009; Zhang et al., 2005] in the recent decades, for various applications in areas such as tissue engineering [Abarrategia et al., 2008; Venkatesan, Qian, Ryu, Kumar, & Kim, 2011], wastewater treatment [Zhang, Qiu, Si, Wang, & Gao, 2011], catalyst carries [Nakagawa, Yasumura, Thongprachan, & Sano, 2011; Nishihara, Mukai, Shichi, & Tamon, 2010] and drug delivery [Gutiérrez et al., 2007]. In this method, directionally growing ice crystals are utilized as templates to prepare porous materials with ordered micro structure, inspired by a natural principle that when water is frozen, impurities originally present in water will be expelled from the forming ice and entrapped within channels between the ice crystals [Deville, Saiz, Nalla, & Tomsia, 2006]. In this paper we prepare CNTs/CHI composite foams with aligned lamellar structure using ice-templating under different freezing rates and CNTs/chitosan concentrations, investigate their mechanical behaviors, and discuss the possible mechanism combining the macro mechanical properties to their micro structures. It is found CNTs can obviously improve the elasticity of chitosan foams, and the freezing rate, density and CNTs fraction are important factors effecting on their morphologies and mechanical properties. This is important to the material design for further practical application of CNTs/CHI composites in fields of tissue engineering, orthopedics and nanocarriers. 2. Experimental 2.1. Chemicals Chitosan (degree of deacetylation: 75~85%; molecular weight: 190,000-310,000 Da) and acetic acid were purchased from Sigma-Aldrich (Shanghai) Trading Co. Ltd.; CNTs (Carboxylic multi-walled carbon nanotubes) with diameter of 40~60 nm and length of 0.5~2 µm were purchased from Shenzhen Nanotech Port Co. Ltd. All of the chemicals were used as received. 2.2. Preparation of samples The CNTs/CHI composite foams were produced by unidirectionally freezing a
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mixture of CNTs and chitosan aqueous solution and subsequent freeze drying. Typically, chitosan aqueous solution (1 wt.%) was prepared by dissolving chitosan (0.5 g) in 50 ml of aqueous solution of acetic acid (0.1 M, pH 5), and then 0.5 g of CNTs were dispersed in the chitosan solution under stirring and subsequent sonication to obtain CNTs suspension with a CNTs concentration of 1 wt.%. For unidirectional freezing, the resulting suspension was poured into a round polypropylene tube (inner diameter ~ 1.3 cm) and vertically immersed into liquid nitrogen (-196 ºC) at a desired velocity controlled by a motor until all of the liquid was totally frozen, as shown in Fig. 1a. The immersion rate of 6 mm min-1 and 10 cm min-1 were used to perform low and high freezing rate respectively. Then, the frozen samples were freeze-dried for 48 hours to remove the ice. Finally the CNTs/CHI composite foam with a CNTs fraction of 0.5 and a density of ~0.02g/cm3 was obtained. To prepare samples with the density of 0.01 g/cm3, all steps were the same except for the halved concentration of CNTs and chitosan. The obtained CNTs/CHI composites under the low and high immersion rates were denoted as CNTs/CHI-L-x and CNTs/CHI-H-x respectively, where x was the fraction of CNTs in the CNTs/CHI composite foams. To prepare the pure chitosan foams with densities of 0.01g/cm3 and 0.02g/cm3, the chitosan aqueous solution with concentration of 1 wt. % and 2 wt. % were used respectively for unidirectional freezing, and the other steps were similar to those for preparation of CNTs/CHI composite foams. 2.3. Characterization of samples The products were investigated by wide-angle X-ray diffraction (XRD) patterns (Siemens D5000, Cu Kα, radiation wavelength λ= 0.154 nm). The Fourier transform infrared spectroscopy (FTIR) measurements were performed in attenuated total reflectance (ATR) mode. The micro-structures of produced samples were characterized using a scanning electron microscope (SEM, S-4800, Hitachi). The thermal behavior of the products was investigated by thermo-gravimetric analysis (TGA) in air atmosphere on TA Instrument 2950. The temperature range is employed from 200 to 750 with a heating rate of 10 ºC min-1. 2.4. Mechanical measurements The cyclic compression test was carried out by cyclically compressing a cylindrical sample (diameter ~1.1 cm, height ~1 cm) to a constant strain of 50% on the Instron3345 (loading capacity 0.001 ~ 10 N) at a constant loading and unloading speed of 2 mm min-1. When we prepared samples for cyclic compression test, the top and bottom of the freeze-dried samples were cut off due to their curved surfaces and irregular micro-structures derived from the instable freezing in these parts, and the middle parts in shape of cylinder were chosen as specimens.
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ip t cr us an M Fig. 1 The experimental setup (a) and processing principle (b and c) for the preparation of CNTs/CHI composite foams.
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3. Results and discussion As can be seen from Fig. 2a, both the pure chitosan foam and CNTs/CHI composite foams prepared by ice-templating were foam-like and self-standing. In addition, it could be easily seen with naked eyes that there were many microgrooves on the lateral side of these prepared samples. The low-magnified SEM images (Fig. 2b) further confirmed these unidirectional micro-channels going throughout the CNTs/CHI composite foam from the bottom to top. In the images of cross sections parallel (Fig. 2c) and perpendicular (Fig. 2d) to the direction of ice crystals growth (i.e. the direction of immersion into liquid nitrogen), it was observed that the foams are composed of a great number of lamellas or sheets which extended unidirectionally. In the micro scale, these lamellas seemed to align parallel with interspace of 10~50µm, and their thickness was uniform (Fig. 2e). Furthermore, in a zoomed SEM image (Fig. 2f) of an individual CNTs/CHI lamella, it was observed that CNTs were well dispersed in and mixed with the polymer matrix. As shown in Fig. 1b, while the CNTs/CHI mixture solution was freezing, the growing ice crystals expelled the impurities (CNTs and chitosan in this case) originally present in water, and some of the expelled CNTs and chitosan were entrapped within the inter space between ice crystals, creating a lamellar microstructure oriented in a direction parallel to the movement of the freezing front. After the ice crystals were removed by
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freezing-drying, a reverse duplicate of ice crystals array could be obtained. It should be noted that while the ice platelets grew vertically, in the three-dimensional space, these platelets simultaneously grew horizontally from the mold wall to the axis driven by the radial gradient of temperature. As a result, the lamellas in the final CNTs/CHI foam also centered around the axis of the cylindrical sample block, as shown in Fig. 2c. In addition, the lamellas in CNTs/CHI composite foams were not smooth, and some antenna-like structures were observed on their surfaces (Fig. 2e and g). These antennas are probably resulted from the entrapping of chitosan and CNTs between the side-branch crystals which are formed when impurities freeze-concentrate around the primary ice cells, causing suppercooling and secondary instability formation perpendicular to the freezing (immersion) direction [Zhang et al., 2005], as illustrated in Fig. 1c. It was found that the CNTs/CHI-H-0.5 (Fig. 2g) possessed denser and longer antennas then CNTs/CHI-L-0.5 (Fig. 2e). This is because high emersion rate results in higher supercooling, forming more side-branch crystals. When the polypropylene tube containing the solution is immersed into liquid nitrogen at a low rate such as 6 mm/min, ice crystal growing rate is actually faster than the immersion rate, and the growing front of ice crystals is much higher than the surface of liquid nitrogen. At this condition, the supercooling around the primary ice cells is slighter, leading to lesser antennas on lamellas. While at the condition of high immersion rate such as 10 cm/min, as the solution is fast immerged by liquid nitrogen, the vertical temperature gradient at growing ice fronts is fast increased, obtaining a higher freezing rate, simultaneously the supercooling becomes seriously due to the dramatically increased horizontal temperature gradient, leading to a great number of side-branch crystals growing between the vertically growing prime ice cells. And as a result, larger amount of long antennas on lamellas could be obtained. For the pure chitosan foam, similar lamellar structure but few antennas could be observed (Fig. 2h and Fig. 3a, d). This is because the high heat conductivity of CNTs can promote the formation of suppercooling, whereas the freeze-concentrate of pure chitosan caused much slighter supercooling around the primary ice cells. Therefore, the surfaces of pure chitosan lamellas were smooth, and few antennas could be observed on them.
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Fig. 2 Digital photographs of the pure chitosan foam (left in a) and CNTs/CHI composite foam (right in a), and SEM images of CNTs/CHI-L-0.5 (b-f), CNTs/CHI-H-0.5 (g) and the pure chitosan foam prepared at low immersion rate (h). (b) Lateral surface of the sample block, (c-h) cross sections parallel (c and e-h) and perpendicular (d) to the direction of ice crystals growth (i.e. the direction of immersion into liquid nitrogen). The images in e-h were taken after cyclic
compression tests. Fig. 3 shows the SEM images of pure chitosan foams and CNTs/CHI composite 6
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foams with density of 0.01 g/cm3 and 0.02 g/cm3 respectively. It was observed that the samples with both densities exhibited ordered and unidirectional lamellar micro-structures. But the lamellas in less dense samples were wavy (Fig. 3a and b), wheras the ones in higher dense samples were flat (Fig. 3d-f). The lamellas in the former were thiner, and there were more defects in them than those in the latter, thus they were easier to deform under external force and their own gravity, resulting in wavy layers. It was found that even under the same density, the samples’ morphology changed at different CNTs fractions. In the SEM image of CNTs/CHI-L-0.66 composite foam with the density of 0.02 g/cm3 (Fig. 3g), discontinuous CNTs/CHI lamellas incompletely enwrapped by chitosan matrix and some small CNTs aggregates between lamellas were observed, which was very different from the images of CNTs/CHI-L-0.3 and CNTs/CHI-L-0.5 showed in Fig. 3e and f respectively. And in the case of 0.01 g/cm3, the similar phenomenon happened in the samples with the CNTs fraction of 0.5 (Fig. 3c). It should be noted that for the samples with the same density, larger CNTs content, smaller chitosan content. When the content of CNTs is increased to a critical point, the decreasing chitosan is not enough to form continuous matrix and totally enwrap CNTs, leading to the discontinuous lamellas and CNTs aggregates. It is indicated that for the samples with density of 0.02 and 0.01 g/cm3, the critical point herein appears at the CNTs fraction ≥0.5 and ≥0.3 respectively. When the fraction of CNTs was further increased, as showed in Fig. 3h, the lamellas in CNTs/CHI-L-0.8 foam cracked badly and CNTs were exposed outside of the lamellas (Fig. 3i), indicating that the content of polymer matrix is too small to remain the entire lamellar structure and enwrap CNTs. For the samples with density of 0.01 g/cm3, no self-standing sample could be obtained when the fraction of CNTs was more than 0.5.
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ip t cr us an M Fig. 3 SEM images of pure chitosan foams and CNTs/CHI composite foams with density of 0.01 g/cm3 (a-c) and 0.02 g/cm3 (d-i). The CNTs fraction is 0 (a and d), 0.3 (b and e), 0.5 (c and f), 0.66 (g) and 0.8 (h and i) respectively.
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XRD patterns of carboxyl modified CNTs, raw chitosan powders, chitosan foams, and CNTs/CHI composite foams are shown in Fig. 4. In the pattern of CNTs, the diffraction peak at 26.0° was observed, which is the characteristic peak of (002) plane of the hexagonal graphite structure with interlayer spacing of 0.34 nm in CNTs [Li et al., 2015]. The pattern of raw chitosan powders showed two main peaks at 10.5° and 20.0°. The diffraction peak around 10° is considered to be the (020) reflection of hydrated crystals of α-chitin chains remaining in the raw chitosan, and the peak at 20° corresponds to the (040) plane of chitosan [Choi, Kim, Pak, Yoo, & Chung, 2007; Ogawa, 1991]. While in the pattern of chitosan foams, only a broad diffraction peak at 20.0° was observed, suggesting an amorphous state of chitosan in the sample [Han, Yan, Chen, & Li, 2011; Kim, & Lee, 2011]. The disappearance of the peak at 10.5° suggests the hydrated crystals in chitosan were totally destroyed in the preparation process of chitosan foams. Similar phenomena were also reported by previous literatures. Kam et al. reported the addition of acetate ions would disrupt the chain alignment in the partially deacetylated chitosan films [Kam, Khor, & Lim, 1999]. Ogawa reported the “as received” chitosan which was produced by solid-state deacetylation of chitin was highly crystallized, whereas the crystallinity of “regenerated” chitosan via dissolving in water and drying would decrease a lot unless
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it was heated. In our experiments, the chitosan foam totally lost its crystallinity. We attribute this to the fast freezing process and subsequent freeze-drying. When the raw chitosan powders were dissolved in the aqueous acetic acid solution, the acetate ions and hydrogen bonding between water and chitosan destroyed the long-range order of molecular chains of chitosan. And then, in the process of freezing, the chitosan was quickly expelled from and compacted between the growing ice crystals, without chance to adjust the molecular chains to recover their order, leading to the amorphous chitosan after freeze-drying. For CNTs/CHI composite, the main characteristic peaks of chitosan and CNTs were observed at 20.0° and 26.0° respectively without shifting and any other new diffraction peak, indicating a physical interaction rather than chemical reaction between chitosan and CNTs.
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Fig. 4 XRD patterns of (a) carboxyl modified CNTs, (b) raw chitosan powders, (c) chitosan foams
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and (d) CNTs/CHI composite foams.
Fig. 5 shows FTIR spectra of chitosan foam, carboxyl modified CNTs and CNTs/CHI composite foam. For chitosan foam, the broad absorption band near wave number 3445 cm−1 correspond to O-H and N-H stretching of alcohol and amine groups. The weak absorption bands at wave numbers 2917 and 2880 cm−1 correspond to C-H stretching of hydrocarbon in the chitosan. The characteristic absorption bands at wave numbers 1650 and 1374 cm−1 correspond to the C=O and C-O stretching of amide group, respectively. In addition, the absorption band at wave number 1593 cm−1 corresponds to N-H deformation of amino groups, and bands at 1155, 1078 and 1030 cm−1 correspond to the symmetric stretching of the C-O-C and involved skeletal vibration of the C-O stretching, respectively [Brugnerotto et al., 2001]. As carboxylic multi-walled carbon nanotubes (MWCNTs-COOH) were used in our experiments for better dispersion in chitosan aqueous solution, the absorption bands of carboxylic stretching were observed in FTIR spectrum of pure MWCNTs-COOH foam. The absorption bands at wave numbers 1717 and 1213 cm−1 correspond to the C=O and C-O stretching of carboxylic group grafted on CNTs, respectively [Ntim, Sae-Khow, Witzmann, & Mitra, 2011]. And the peak around 1588 cm-1 is assigned to the C=C 9
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stretching in CNTs [Eklund, Holden, & Jishi, 1995; Li et al., 2015]. However, these characteristic absorption bands of CNTs were not observed in the FTIR spectrum of CNTs/CHI composite foam, resulting in a similar curve to that of chitosan. It is probably because absorption peaks of CNTs were overlapped with or submerged by those of chitosan due to their weak signals or small amount exposed on the lamellar surfaces in composites, since ATR mode used in FTIR measurements herein characterizes the component on the surface of sample. The FTIR results further indicate there are mainly physical interaction rather than chemical reaction between chitosan and CNTs in the composite foams.
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Fig. 5 FTIR spectra of carboxyl modified CNTs, CNTs/CHI composite foams and chitosan powders.
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Fig. 6 shows the TGA and DTG curves of chitosan, CNTs/CHI-0.1, CNTs/CHI-0.66 and CNTs examined in air atmosphere. For chitosan, in the low temperature range between 40~120 °C, a mass loss of ~10% was observed. This step can be attributed to the evolution of adsorbed water in chitosan foams. Subsequently, with the increase of temperature, the pure chitosan foam decomposed in two thermo-oxidative degradation stages in air atmosphere. The first thermo-oxidative degradation process occurred in the temperature range 200~380 ºC, and reached the maximum rate at 290 ºC. The significant weight loss of ~60% in this stage is attributed to the main decomposition (oxidation and rupture of polysaccharide chains) of chitosan. The second thermo-oxidative degradation process occurred in the temperature range 400-620 ºC, with a maximum rate at 520 ºC. The weight loss in this stage is attributed to the further decomposition and oxidation of residual char formed in the first thermo-oxidative degradation process, which can result in the release of CO and CO2. For CNTs, a weak thermo-oxidative degradation occurred in the temperature range 200~400 ºC, due to the decomposition of grafted organic groups. Subsequently the major thermo-oxidative degradation occurred at 400~670 °C, and the maximum 10
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degradation rate occurred at 610 °C. The weight went to ~3 wt. % in this degradation process due to the complete loss of the carbon by oxidation, and the residue was probably the residual metal catalysts and their oxides which remained in the raw material [Bom et al., 2002; Stefov, Najdoski, Bogoeva-Gaceva & Buzarovskaba, 2014]. For CNTs/CHI-0.1, the shape of TGA curve was similar to that of chitosan foam, and it had lower weight loss than chitosan at the same temperature. It was shown in DTG curves that CNTs/CHI-0.1 had three thermo-oxidative degradation stages. The first two stages were similar to that of pure chitosan foam with a little bit shifts to low temperature, and the third one occurred at the same temperature range where CNTs were oxidized. This indicates that before ~500 ºC, the weight loss of the composite foam is mainly derived from decomposition of chitosan, whereas after 500 ºC it is derived from oxidization of CNTs. With the increase of CNTs fraction, as shown in the TGA and DTG curves of CNTs/CHI-0.66, the main thermo-oxidative degradation occurred at the same temperature ranges, but the main mass loss moved to high temperature (>500 ºC), suggesting a further improved thermo-oxidative stability. The above results suggest that the thermo-oxidative stability of the composite foam can be increased by incorporation of CNTs into chitosan walls in the porous structure.
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Fig. 6 (a) TGA and (b) DTG curves of the chitosan foam, CNTs/CHI-L-0.1, CNTs/CHI-L-0.66 and CNTs.
It is found that the prepared CNTs/CHI composite foams possess enhanced elasticity compared to chitosan foams. Fig. 7 shows the digital photographs of pure chitosan and CNTs/CHI composite foams before and after they were compressed to 50% of original length. When the compressive stress was unloaded, the resilience of chitosan foam was not obvious (Fig. 7b), whereas the CNTs/CHI composite foam almost recovered to its original shape (Fig. 7d) without obvious deformation. We further demonstrated this improvement by the accurate cyclic compression test.
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and after (b, d) they were compressed to 50% of original length.
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Fig. 8a shows the stress-strain (σ-ε) curves of cyclic compressions on the CNTs/CHI-L-0.5 composite foam. In the first compression cycle, the stress-strain curve showed two distinct stages during the loading process. The stress increased linearly at ε ≤ 13% which belonged to a linear-elastic region, recording the elastic deformation of lamellas in the CNTs/CHI composite foam. Subsequently, stress increased slowly, indicating the reach of a plateau region, which recorded a non-linear deformation of sample. The unloading curve was totally different from loading one, generating a hysteresis loop, which indicates that a large portion of mechanical energy was absorbed during compression. In the second cycle, a positive stress response appeared at a strain of 6%, indicating the sample recovered to 94% of the original length after the first cycle. The unrecovered part is due to the plastic deformation happening partially in the CNTs/CHI composite foam during the compression process. In all of the subsequent cycles, hysteresis loops could be observed and obvious linear-elastic and plateau regions appeared on loading curves. However, in these cycles, recovery ratio of sample, maximum compressive stress and area of hysteresis loop became smaller and tended towards stability after 10 cycles. The observations of the hysteresis and stress level softening in the CNTs/CHI-L-0.5 composite foam suggest it exhibits viscoelastic behavior under compression [Suhr et al., 2007]. Eventually, even being compressed for 50 cycles，the CNTs/CHI-L-0.5 composite foam still could recover to over 80% of its original length and keep its original macroscopic shape without bend or crack. Such excellent compressibility, similar viscoelastic property to soft-tissue materials such as tendon and muscle [Fung, 1993; Sanjeevi, 1982] and good biocompatibility make the CNTs/CHI composite foam an ideal material for application in tissue engineering. By contrast, the pure chitosan
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Fig. 7 Digital photographs of pure chitosan and CNTs/CHI-L-0.5 composite foams before (a, c)
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foam exhibited a poor compressibility. As shown in Fig. 8b, the sample only recovered to 62% of its original length after 5 compression cycles, and since the second cycle the loading curves rapidly reached densification region almost without elastic and plateau regions. Therefore, we deem that addition of CNTs can obviously improve the elasticity of chitosan foams. The micro morphologies of samples can help to reveal the mechanism of the enhanced elasticity in CNTs/CHI composite foams. In SEM images of the samples after cyclic compression tests, it was observed the lamellas in CNTs/CHI-L-0.5 were orderly aligned and flat (Fig. 2e), whereas the ones in the pure chitosan foam were wavy, somewhere twisting together (Fig. 2h). This well interprets their different mechanical behaviors demonstrated in cyclic compression tests. The lamellas in the pure chitosan foam deform under compressive force and cannot recover to their original shape well after unloading, leading to an obvious plastic deformation in the macro scale. While lamellas in CNTs/CHI-L-0.5 composite foam can survive in the compression cycles without obvious buckling and shifting. We deem the well dispersed CNTs (Fig. 2f) endow their intrinsic elasticity [Falvo et al., 1997] to the lamellas in composite foams, leading to a good elasticity.
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Fig. 8 Stress-strain (σ-ε) curves of cyclic compressions on (a) CNTs/CHI-L-0.5, (b) pure chitosan foam and (c) CNTs/CHI-H-0.5 with the density of 0.02 g/cm3 at the maximum strain of 50%. (d) The recoverable compressive strain and maximum compressive stress response at a constant strain amplitude of 50% for different cycles derived from (a), (b) and (c).
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It was also found that the CNTs/CHI composite foams had very different compressive properties when they were prepared under low and high freezing rates respectively. As 14
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shown in Fig.8c and d, after 5 compression cycles, the CNTs/CHI-H-0.5 only could recover to 68% of original length, which was better than that of pure chitosan foams (62%) but much worse than that of CNTs/CHI-L-0.5 (87%). And simultaneously, the max compressive stress of CNTs/CHI-H-0.5 at 50% strain was also much smaller than that of CNTs/CHI-L-0.5. These results indicate CNTs/CHI-L-0.5 has better elasticity and strength than CNTs/CHI-H-0.5. This is probably attributed to the formation of massive antenna-like structures and thinner lamellar thickness at high immersion (freezing) rate. It should be noted that the forming of more antennas means less matter being used to form the aligned lamellas that are the major force-bearing structures in the CNTs/CHI composite foams. On the other hand, it has been known that the lamella thickness decreases with the increase of freezing rate [Gutiérrez et al., 2007; Zhang et al., 2005]. For these two reasons, the lamella thickness (~ 200 nm) in CNTs/CHI-H-0.5 (Fig. 2g) was lower than that (~ 500 nm) in CNTs/CHI-L-0.5 (Fig. 2e and 3f). And this well interprets the lower max compressive stress and worse recovery capability in CNTs/CHI-H-0.5 than those in CNTs/CHI-L-0.5 (Fig. 8d). Besides freezing rate, density and CNTs fraction were another two important factors effecting no the mechanical properties of CNTs/CHI composite foams. The stress-strain curves of cyclic compressions on the pure chitosan foam and CNTs/CHI composite foams with the density of 0.01 g/cm3 were showed in Fig. 9. As shown in Fig. 9a, the pure chitosan foam recovered to 68% of its original length after 5 cycles, but the elastic modulus and maximum compressive stress were obviously decreased compared to the one with a density of 0.02 g/cm3 (Fig.8b). This agrees with the observation in SEM images (Fig. 3a and d) and the general knowledge for foams that smaller density, smaller strength when the materials system is the same. The introduction of CNTs improved the elasticity, but the strength was decreased (Fig. 9b). This is similar to the above observation on the foams with the density of 0.02 g/cm3. Fig. 9c shows the recovery percentage of original volume at the strain of 50% after 5 compression cycles as a function of CNTs content in the CNTs/CHI composite foams with the two densities. For the foams with density of 0.02 g/cm3, the recovery ability increased with the increase of CNTs fraction, achieving the maximum (87% of original length) at the CNTs fraction of 0.5, and then decreased dramatically. In the case of 0.01 g/cm3 density, similar trend with the maximum at the CNTs fraction of 0.3 was observed, and no compressive test could be applied on the samples with the fraction of CNTs more than 0.66, because they were too weak to shape and self-stand. It is mentioned above that when the CNTs fraction is more than 0.5 and 0.3 respectively, the CNTs/CHI composite lamellas in samples with the two densities become discontinuous and CNTs aggregated seriously (Fig. 3c, g and h). It is this change in micro structure leads to the poor elasticity and mechanical strength.
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Fig. 9 Stress-strain (σ-ε) curves of cyclic compressions on (a) pure chitosan foam and (b)
CNTs/CHI-L-0.1 composite foam with the density of 0.01 g/cm3 at the maximum strain of 50%. (c) Recovery percentage of original volume after 5 compression cycles as a function of CNTs content in the foams with densities of 0.01 g/cm3 and 0.02 g/cm3.
4. Conclusions CNTs/chitosan composite foams constructed by orderly aligned lamellas with uniform interspace and layer thickness at micron and submicron scale respectively were prepared by ice-templating. The characterized and test results reveal the prepared CNTs/CHI composite foams have better thermal stability and higher elasticity than the pure chitosan foams prepared by the same method, due to the introduction of CNTs into chitosan matrix. The mechanical properties of CNTs/CHI composite foams are mainly affected by immersion (freezing) rate, density and CNTs fraction. The samples prepared under low immersion rate (6 mm min-1) exhibit enhanced elastic 16
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properties than those prepared under high immersion rate (10 cm min-1). The samples with high density (0.02 g/cm3) possess better elasticity and mechanical strength than the ones with low density (0.01 g/cm3). For the foams with the same density but different CNTs fractions, with the increasing CNTs fraction the recovery ability increases and achieves the maximum at a critical point, and then decreases dramatically due to the inadequate chitosan matrix and aggregation of CNTs. The critical point herein appears at the CNTs fraction ≥0.5 and ≥0.3 respectively, for the samples with density of 0.02 and 0.01 g/cm3. It is anticipated that this CNTs/CHI composite foam with enhanced elastic property can be applied in fields of tissue engineering, orthopedics and nanocarriers. Acknowledgment The work is financially supported by the National Natural Science Foundation of China (No. 50902015), Fundamental Research Funds for the Central Universities (No. DUT12LK40) and Specialized Research Fund for the Doctoral Program of Higher Education (No. 20090041120036). References Abarrategia, A., Gutierrez, M. C., Moreno-Vicentea, C., Hortiguela, M. J., Ramos, V., Lopez-Lacomba, J. L., et al. (2008). Monte, Multiwall carbon nanotube foams for tissue engineering purposes. Biomaterials, 29, 94-102. Azeez, A. A., Rhee, K. Y., Park, S. J., Kim, H. J., & Jung, D. H. (2013). Application of cryomilling to enhance material properties of carbon nanotube reinforced chitosan nanocomposites. Composites Part B, 50, 127-134. Bom D., Andrews R., Jacques D., Anthony J., Chen B., Meier M. S., & Selegue J. P. (2002). Thermogravimetric Analysis of the Oxidation of Multiwalled Carbon Nanotubes: Evidence for the Role of Defect Sites in Carbon Nanotube Chemistry. Nano Letters, 2 (6), 615-619. Brugnerotto, J., Lizardi, J., Goycoolea, F. M., Arguelles-Monal, W., Desbrieres, J., & Rinaudo, M. (2001). An infrared investigation in relation with chitin and chitosan characterization. Polymer, 42 (8), 3569-3580. Choi, C. Y., Kim, S. B., Pak, P. K., Yoo, D. I., & Chung, Y. S. (2007). Effect of N-acylation on structure and properties of chitosan fibers. Carbohydrate Polymers, 68, 122–127. Cui, H., Chen, J., Yang, H., Wang, W., Liu, Y., Zou, D., et al. (2013). Preparation and application of Aliquat 336 functionalized chitosan adsorbent for the removal of Pb(II). Chemical Engineering Journal, 232, 372–379. Deville, S., Saiz, E., Nalla, R. K., & Tomsia, A. P. (2006). Freezing as a Path to Build Complex Composites. Science, 311, 515-518. Eklund, P. C., Holden, J. M., & Jishi, R. A. (1995). Vibrational modes of carbon nanotubes, spectroscopy and theory. Carbon, 33 (7), 959-972. Falvo, M. R., Clary, G. J., Taylor, R. M., Chi, V., Brooks, F. P., Washburn, S., et al. (1997). Bending and buckling of carbon nanotubes under large strain. Nature, 389, 582-584. Fung, Y. C. (1993). Biomechanics: Mechanical Properties of Living Tissue. Springer-Verlag, New York.
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Gutiérrez, M. C., Ferrer, M. L., & Monte, F. (2008). Ice-Templated Materials: Sophisticated Structures Exhibiting Enhanced Functionalities Obtained after Unidirectional Freezing and Ice-Segregation-Induced Self-Assembly. Chemistry of Materials, 20 (3), 634-648. Gutiérrez, M. C., García-Carvajal, Z. Y., Jobbágy, M., Rubio, F., Yuste, L., Rojo, F., et al. (2007). Poly(vinyl alcohol) Foams with Tailored Morphologies for Drug Delivery and Controlled Release, Advanced Functional Materials, 17, 3505–3513. Han, D. L., Yan, L. F., Chen, W. F., & Li, W. (2011). Preparation of chitosan/graphene oxide composite film with enhanced mechanical strength in the wet state. Carbohydrate Polymers, 83, 653-658. Kam, H. M., Khor, E., & Lim, L. Y. (1999). Storage of Partially Deacetylated Chitosan Films. Journal of Biomedical Materials Research, 48, 881-888. Kim, M. Y. & Lee, J. H. (2011). Chitosan fibrous 3D networks prepared by freeze drying. Carbohydrate Polymers, 84, 1329-1336. Li, S. Z., Gong, Y. B., Yang, Y. C., He, H., Hu, L. L., Zhu, L. F., et al. (2015). Recyclable CNTs/Fe3O4 magnetic nanocomposites as adsorbents to remove bisphenol A from water and their regeneration. Chemical Engineering Journal, 260, 231–239. Martino, A. D., Sittinger, M., & Risbud, Makarand V. (2005). Chitosan: A versatile biopolymer for orthopaedic tissue-engineering. Biomaterials, 26, 5983–5990. Nakagawa, K., Yasumura, Y., Thongprachan, N., & Sano, N. (2011). Freeze-dried solid foams prepared from carbon nanotube aqueous suspension: Application to gas diffusion layers of a proton exchange membrane fuel cell, Chemical Engineering and Processing, 50, 22–30. Nasution, T. I., Nainggolan, I., Hutagalung, S. D., Ahmad, K. R., & Ahmad, Z. A. (2013). The sensing mechanism and detection of low concentration acetone using chitosan-based sensors. Sensors and Actuators B, 177, 522– 528. Nishihara, H., Mukai, S. R., Shichi, S., & Tamon, H. (2010). Preparation of titania–silica cryogels with controlled shapes and photocatalysis through unidirectional freezing. Materials Letters, 64, 959–961. Ntim, A. S., Sae-Khow, O., Witzmann, F. A., & Mitra, S. (2011). Effects of polymer wrapping and covalent functionalization on the stability of MWCNT in aqueous dispersions. Journal of Colloid and Interface Science, 355, 383–388. Ogawa K. (1991). Effect of heating an aqueous suspension of chitosan on the crystallinity and polymorphs. Agricultural and Biological Chemistry, 55, 2375–2379. Sanjeevi, R. (1982). Aviscoelastic model for the mechanical properties of biological materials. Journal of Biomechnics, 15, 107-109. Stefov V., Najdoski M, Bogoeva-Gaceva G., & Buzarovskaba A. (2014), Properties assessment of multiwalled carbon nanotubes:A comparative study. Synthetic Metals, 197, 159–167. Suhr, J., Victor, P., Ci, L., Sreekala, S., Zhang, X., Nalamasu, O., et al. (2007). Fatigue resistance of aligned carbon nanotube arrays under cyclic compression. Nature Nanotechnology, 2, 417-421. Tang, D. L., Song, F., Chen, C., Wang, X. L., & Wang, Y. Z. (2013). A pH-responsive
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chitosan-b-poly(p-dioxanone) nanocarrier: formation and efficient antitumor drug delivery. Nanotechnology, 24, 145101. Venkatesan, J., Qian, Z. J., Ryu, B., Kumar, N. A., & Kim, S. K. (2011). Preparation and characterization of carbon nanotube-grafted-chitosan-Natural hydroxyapatite composite for bone tissue engineering. Carbohydrate Polymers, 83, 569-577. Yan, J., Chen, Z., Jiang, J., Tan, L., & Zeng, X. C. (2009). Free-Standing All-Nanoparticle Thin Fibers: A Novel Nanostructure Bridging Zero- and One-Dimensional Nanoscale Features. Advanced Materials, 21, 314-319. Zhang, H. F., Hussain, I., Brust, M., Butler, M. F., Rannard, S. P., & Cooper, A. I. (2005). Aligned two- and three-dimensional structures by directional freezing of polymers and nanoparticles. Nature Materials, 4, 787-793. Zhang, N. N., Qiu, H. X., Si, Y. M., Wang, W., & Gao, J. P. (2011). Fabrication of highly porous biodegradable monoliths strengthened by graphene oxide and their adsorption of metal ions. Carbon, 49, 827-837.
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Highlights
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CNTs/chitosan foams with lamellar structure were prepared by ice-templating.
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CNTs can improve the elastic property and thermal stability of chitosan scaffolds.
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The CNTs/CHI foams exhibit a viscoelastic behavior similar to some soft tissues.
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The mechanical properties are affected by freezing rate, density and CNTs
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They have potential applications in fields of tissue engineering and nanocarriers.
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