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halloysite composite membranes
Polymer Testing 32 (2013) 265–271
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Material properties
Physico-chemical characterisation of chitosan/halloysite composite membranes R.T. De Silva a, Pooria Pasbakhsh a, *, K.L. Goh b, Siang-Piao Chai c, H. Ismail d a
Mechanical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon Selatan, Bandar Sunway, 46150 Selangor Darul Ehsan, Malaysia b School of Mechanical and Systems Engineering, Newcastle University, NE1 7RU, United Kingdom c Chemical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon Selatan, Bandar Sunway, 46150 Selangor Darul Ehsan, Malaysia d School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 11800 Pulau Penang, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 October 2012 Accepted 12 November 2012
Chitosan membranes reinforced by halloysite nanotubes (HNTs) at concentrations from 2 to 15 (w/w%) have been prepared by solution casting to investigate the optimal physicochemical properties for biomedical applications. Tensile test data revealed that the membranes reinforced with 5 (w/w%) HNTs yielded the highest Young’s modulus (0.52 0.01 GPa) and strength (81.6 4.4 MPa). Electron micrographs of the fractured surfaces implicated the interplay between individual HNTs and agglomerates of HNTs in the stress transfer mechanism. Infrared spectra revealed interaction between the HNT siloxane and chitosan functional groups. Thermogravimetric results demonstrated that the thermal stability of the membranes increased with HNT concentration. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Halloysite nanotubes Chitosan biocomposite membranes Microtensile testing Scanning electron microscope Thermal properties
1. Introduction Chitosan, ‘poly-b (1,4)-2-amino-2-deoxy-D-glucose’, is a deacetylated product of chitin which may be derived from crab and shrimp shells and fungal mycelia [1,2]. Owing to its non-toxic nature, antimicrobial activity and good compatibility with living tissues, chitosan has been identified during the last 20 years as a good candidate for use as a biomaterial [3]. Additionally, it possesses good adhesion and film forming capability, as well as high hydrophilicity, biocompatibility and biodegradability [4]. Accordingly, chitosan can be produced in various forms such as membranes [5], films [6], fibrous mats [5] and sponges [7]. From a structurefunction point of view, of the many varied forms, chitosan membrane is the most versatile, particularly for biomedical applications such as tissue engineering, bone implantation and artificial skin [5,8], as well as for filtration applications such as waste water filtration and reverse osmosis [9]. * Corresponding author. Tel: þ60 3 55146211; fax: þ60 3 55146207. E-mail address: [email protected] (P. Pasbakhsh). 0142-9418/$ – see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymertesting.2012.11.006
However, chitosan has low mechanical strength, thermal stability and water and gas barrier properties, which has restricted the opportunity for wider applicability [3,10]. Different types of material as micro and nanofillers, such as silica, hydroxyapatite, calcium phosphate, carbon nanotubes (CNTs) [1] and organo modified montmorillonite (OMMT), have been studied for reinforcing chitosan [11]. Of these, CNTs and OMMT have shown potential for reinforcing and enhancing the thermal stability of chitosan membranes [1,2,12,13]. Nevertheless, it is well known that the synthesis and functionalization processes of CNTs are complicated and costly [14]. As for OMMT, the standard organic treatment is not sufficient to generate well-intercalated and/or exfoliated morphologies in polymer matrix [15]. Halloysite nanotubes (HNTs), first reported by Berthier in 1826, are natural aluminosilicates (Al2Si2O5(OH)4.nH2O) with nano-tubular structures [14]. Typically, the length of HNTs varies from 50-5000 nm, with an external diameter of 20-200 nm and internal diameter of 10-70 nm [16]. HNTs possess several remarkable properties such as biodegradability, large aspect ratio and high surface area
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to volume ratio. Consequently, HNTs have found applications in polymer reinforcement [17] and drug delivery [18]. In the former application, HNTs have been used as nanofillers to reinforce polymers such as epoxy [19], polypropylene [20], polyamide [21], styrene rubber [22] and ethylene propylene diene monomer [23,24]. Very recently, mechanical properties of HNT incorporated chitosan films have been investigated and it has been observed that adding 7.5 (w/w%) of HNTs increased the tensile stress and Young’s modulus of chitosan/HNT composite by 134% and 65%, respectively [25]. Zheng et al. [26] have pointed out the feasibility of synthesizing chitosan/HNT hydrogels as an absorbent for removing ammonium ions from synthetic water. Findings from the study of the bioelectrochemical properties of chitosan/ HNT composite films demonstrate the feasibility of using the films for immobilizing horseradish peroxidase [27]. Studies on the physical properties of chitosan containing chrysotile and HNTs have demonstrated the feasibility of fabricating fibrous chitosan-based structures reinforced by HNTs as well as chrysotile [28]. Among all the studies on mechanical properties of polymer/HNT nanocomposites, the effect of Young’s modulus of single HNTs has been rarely studied. Recently Lu et al. [29] measured the Young’s modulus of single HNTs by transmission electron microscopy on a bending stage. In an earlier study, Guimarães et al. [30] used a “self-consistent charged densityfunctional tight-binding” method to predict the Young’s modulus of single wall HNTs without any interpretation with any polymer matrix. Here, we report on a feasibility study to establish a simple solution-casting approach for fabricating chitosan-based composite membranes reinforced by HNTs. Physico-chemical characterisation was carried out to measure the tensile, thermal and chemical properties of the membranes at different HNT concentrations. Microstructural characterisation was carried out to identify the mechanism of reinforcement and failure in the membrane. Also, for the first time, the Young’s modulus of a single HNTs was estimated by applying the rule of mixtures for these composites.
2. Materials and methods 2.1. Fabrication of chitosan/HNT composite membranes Chitosan flakes (low molecular weight w50,000 daltons, based on viscosity; 85% of deacetylation) was purchased from Sigma Aldrich. HNTs were provided by Imerys Tableware New Zealand. One gram of chitosan was mixed in 100 ml of 1% acetic acid solution and stirred for 20 h at 500 rpm. Next, 9 ml of distilled water was mixed with HNTs at 2, 5, 10 and 15 (w/w%) with respect to 1 g of chitosan. Following this, the mixtures were placed in an ultrasonic bath (Thermo D-60) for 30 min at a temperature of 50 C. Thereafter, the mixture was stirred for 20 h at 350 rpm. Subsequently, the solutions of chitosan and HNTs were mixed together and stirred for 8 h at 1000 rpm. Next, 15 g from the resultant respective solutions was poured into plastic petri dishes (diameter, 8.5 cm) and the dishes were placed in an oven for approximately 20 h at 40 C. After membrane formation, each dish was filled with sodium hydroxide (30 ml, 0.1 M) and left for 30 min to neutralize any left-over acetic acid. Following this, the membranes were rinsed with distilled water and submerged in distilled water for 30 min to remove the sodium hydroxide. Finally, the membranes were peeled off from the plastic dishes (Fig. 1(a)). The solution casting method yielded circular, translucent membranes. A similar procedure was employed to prepare membranes containing only chitosan. 2.2. Physico-chemical characterisation Strips of membranes, 3 mm by 15 mm (Fig. 1(b)) were tested for mechanical properties by stretching to rupture at a pre-determined displacement rate of 0.06 mm/s, using a high-throughput small-scale horizontal tensile test rig (Fig. 1(c)). The stress-strain curve of the respective specimens were derived from the load-displacement data; here stress was found by dividing the load by the cross-sectional area of the specimen and strain was equal to the ratio of the displacement to the initial grip-to-grip length of the taut
Fig. 1. (a) A typical image of a chitosan-based membrane reinforced by HNTs, (b) Schematic of membrane mounted on a paper template prior to tensile testing, (c) Stretching a chitosan-based membrane specimen using the microtensile test rig.
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specimen. The cross section of the membranes was measured under an Olympus BU41 optical microscope. The thickness of the bulk (around the central region) of the membrane ranged from 17 to 23 micrometer. Tensile properties of the membranes such as Young’s modulus (E), strength (s) and strain at rupture (ε) were obtained from the stress-strain curve. A total of ten specimens were tested for each HNT concentration. The fractured surfaces of tensile samples with 2, 5 and 15 (w/w%) of HNTs were investigated by using a Supra-35VP SEM. To prevent electrostatic charging during observation, the samples were coated with a thin layer of platinum. Fourier transform infrared (FTIR) spectroscopy was carried out to analyse the interaction between chitosan and HNTs by comparing the chemical structure of the chitosan/ HNTs composite membranes (at different HNT concentrations) with respect to those from pure chitosan flakes and HNTs. All samples were examined using a Thermo Scientific IS10 with a wavenumber range of 400-4000 cm1 with 0.2 cm1 resolution. Thermogravimetric analysis (TGA) was carried out using a TGA Q50 to investigate the thermal stability of the chitosan/HNT composite membranes over a temperature range of 25 C to 600 C at a rate of 10 C/min under nitrogen atmosphere. One-way analysis of variance (ANOVA) was implemented by “Tukey’s Post Hoc test”, using commercial software (SPSS, version 18) to investigate the significance differences between the means of the E, s and ε with
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respect to HNT concentration. In all the cases, P < 0.05 was considered as statistically significant. Linear regression analysis was used to determine the significance of the linear relationship between E and the volume fraction of HNTs (corresponding to the HNT concentration). 3. Results and discussion 3.1. Tensile properties Fig. 2(a) shows that chitosan membranes with 5 (w/w %) of HNTs has the highest strength (s) compared to the other concentrations. The ANOVA test revealed that not all of the means of s, among the different HNT concentrations, are equal (P < 0.05); Tukey’s Post Hoc test revealed significant difference between the means for (i) 0 (w/w%) versus 2 to 15 (w/w%), (ii) 2 (w/w%) versus 5 (w/ w%) and (iii) 5 (w/w%) versus 10 and 15 (w/w%). Correspondingly, the plot of s versus HNT concentration describes an increasing trend from 0 to 5 (w/w%), a decreasing trend from 5 to 10 (w/w%) and fluctuations of s with no appreciable trend from 10 to 15 (w/w%). Of note is that the chitosan membrane with 0 (w/w%) HNT concentration yielded s ¼ 60.8 2.8 MPa; this value falls within the range of values, from 4 MPa to 70 MPa, reported for the tensile strength of chitosan in the literature [31–35]. The wide range of values may be due to the chitosan molecular weight (30 kDa to 80 kDa), the percentage deacetylation of chitosan, and the composite’s
Fig. 2. Graphs of (a) tensile strength, s (MPa), (b) Young’s modulus, E (GPa), and (c) strain at break, ε (%), of chitosan-based membranes reinforced by HNTs, versus HNT concentration. Data are plotted as means SEM (SEM: standard error of mean); vertical bars represent SEM.
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preparation process. Considering the range of HNT concentrations used in this study, the magnitude of s in chitosan/HNT composite is higher than that of the membrane containing only chitosan. The increase in s is attributed to the possible interactions between the HNTs and chitosan. The structure of chitosan consists of two hydroxyl groups and an amino group in one glucosamine ring. The OH groups on the edge and surface of HNTs can interact with chitosan via hydrogen bonding; the interaction could be important for facilitating the transfer of stress (generated in the chitosan matrix) to the HNTs which functioned as a reinforcing phase. Statistical analysis of the variation in E with respect to HNT concentration revealed that the chitosan-based membranes reinforced by 2 (w/w%) of HNTs yielded the highest E (P < 0.05, ANOVA). In particular, Tukey’s Post Hoc test revealed significant difference between the means for 0 (w/w%) versus 2 to 15 (w/w%) of HNT concentration. Correspondingly, the plot of E versus HNT concentration (Fig. 3(b)) describes an increasing trend from 0 to 5 (w/w%) and fluctuations with no appreciable change from 5 to 15 (w/w%). The stiffening of chitosan membranes blended with HNTs is attributed to a corresponding increase in the structural rigidity of the chitosan polymer chains [10] arising from the interaction between adjacent HNTs and chitosan. To the best of our knowledge, there has been no report on the prediction of Young’s modulus of halloysite (EH) using experimental data derived from tensile testing of
polymer-based composites reinforced by HNTs. This paragraph explains an attempt to provide simple order-ofmagnitude estimate of EH. Application of the rule of mixtures for modeling the HNT reinforcement of chitosan membrane leads to an expression relating E to EH and the Young’s modulus of chitosan (EC), weighted by the respective volume fractions of VH and chitosan (VC), i.e. E ¼ KEHVH þ ECVC, where VH þ VC ¼ 1 and K is the filler reinforcement efficiency. Here, K is set to 0.2 [36] to model an array of randomly oriented HNTs in a chitosan matrix material. Given that the densities of HNT and chitosan are 2.53 g/cm3 and 0.3 g/cm3, respectively, it is easy to see that 2 and 5 (w/w%) HNT concentration corresponded to VH ¼ 0.0024 and 0.0060, respectively. By considering VH from 0 to 0.006, linear regression analysis of E versus VH revealed that E increased at a rate of 14 7.3 GPa for every unit increase in VH (F ¼ 64.17, P < 0.05, R2 ¼ 78.6 %). Consequently, this model predicted that EH ¼ 72.165 GPa. This value agrees with the E of larger HNTs (60 to 71 GPa for tubes with inner diameter >70 nm and outer diameter >158 nm) reported by Lu et al. [29]. According to their report, E of a single HNT could vary from 60 to 133 GPa depending on its’ inner and outer diameters. However, the actual E of a single HNT could be higher than these reported values and the lower E of HNT found in our study might be due to the agglomeration of tubes in some regions, which can reduce the effective modulus of dispersed HNTs in chitosan. Further increase in HNT concentration (to 10 and 15 (w/w%)) could dramatically increase the density of HNT
Fig. 3. Scanning electron micrographs of typical fractured morphology of chitosan-based membranes reinforced by (a) 2 (w/w%), (b) 5 (w/w%) and (c) 15 (w/w%) of HNTs.
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aggregates. These aggregates could no longer be able to provide reinforcement for the chitosan matrix due to the reduction in HNT-chitosan interfacial regions. Henceforth, the magnitude of E decreased from 5 to 10 (w/w%) HNTs. So the critical volume fraction of HNTs is estimated about 0.006, which corresponds to 5 (w/w%) HNT concentration. Fig. 2(c) shows a graph of ε versus HNT concentration. The ANOVA test revealed that not all of the means of ε among the different HNT concentrations are equal (P < 0.05); Tukey’s Post Hoc test revealed significant differences between the means of ε for (i) 0 (w/w%) versus 2 to 15 (w/w%), and (ii) 2 (w/w%) versus 5 and 10 (w/w%) and (iii) 5 (w/w%) versus 15 (w/w%). The overall profile of the plot of ε versus HNT concentration features a rapid drop in the magnitude of ε at small HNT concentration, i.e. 2 (w/ w%); a small appreciable increase in the magnitude of ε occurs at a slightly higher HNT concentration but further increase in HNT concentration yields a decreasing trend thereafter. Compared to the pure chitosan membranes, the smaller ε in the chitosan-HNT blends is consistent with the previous argument in stiffening of the membranes with the addition of HNTs. The overall reduced extensibility of the chitosan-membrane may be attributed to the increase in the structural rigidity of the chitosan polymer chain [10]. Similar trends have been observed in chitosan-based membranes reinforced by, e.g. OMMT [3]. 3.2. Morphology From Fig. 3, the light circular dots shown in the micrographs of the tensile fractured surfaces of the composite membranes are believed to be the end of the tubes. Hence, it can be seen that chitosan membranes with 2 and 5 (w/w %) of HNTs have better dispersion of HNTs than the
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membranes with 15 (w/w%). Additionally, embedded HNTs in chitosan matrix can be seen in the micrographs. These embedded HNTs confirmed a good interaction between HNTs and chitosan. Furthermore, micro-voids and pores are evident in the fractured surfaces (marked by arrows in Fig. 3) due to the pulled out nanotubes and dislodged agglomerated HNTs during the tensile testing. The presence of agglomerated HNTs was also observed, (circled in Fig. 3(c)). This could be due to stacking of two or more HNTs together, and referred to as HNT aggregated areas. At 15 (w/ w%) HNTs, only few HNT-HNT interactions were spotted due to the covering of HNT aggregated areas with chitosan. Nevertheless, the chitosan-HNT interaction at low HNT concentrations (2 and 5 (w/w%)) was dominant due to the presence of less micro-voids and pores compared to higher HNT concentration. Even though the morphology and dispersion for the membranes with 2 and 5 (w/w%) HNTs were very similar, the membranes with 5 (w/w%) HNTs has more HNTs which allow for better stress transformation than the 2 (w/w%) HNTs membranes. This is why membranes with 5 (w/w%) of HNTs have higher mechanical properties than the other compositions. According to the tensile test results, mechanical properties such as E and s of 15 (w/w%) membranes were less than the 5 (w/w%) HNTs membranes and were also slightly inferior to the membranes with 2 (w/w%) of HNTs due to (i) poor matrix-filler interaction and (ii) the presence of HNT aggregation, even though they have more HNTs to provide more opportunities for stress transformation. 3.3. FTIR analysis From Fig. 4, the presence of functional groups of chitosan and HNTs can be identified in the composite
Fig. 4. Fourier transform infrared spectra of (a) pure chitosan and (b) HNT. The remaining spectra are derived from chitosan-based membranes reinforced by (c) 0 (w/w%), (d) 5 (w/w%) and (e) 15 (w/w%) of HNTs.
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membranes. The main functional groups that appeared in the chitosan spectrum were vibration of OH groups at 3353 cm1 and 3297 cm1, C–H bond (–CH3) at 2873 cm1, carbonyl bonds (C]O) of amide groups C]O–NHR at 16801480 cm1 can also be seen in composite membranes. The bending vibration of methylene groups at 1375 cm1 and asymmetric vibration of CO at 1150 cm1 [11], also appeared in the chitosan membranes, with 0, 5 and 15 (w/w%) of HNTs. This indicated that functional groups of chitosan were also repeated in the membranes. For the pure HNTs, the main bands appearing in the spectrum included; O–H deformation of inner hydroxyl groups at 911 cm1, in-plane Si–O stretching at 1031 cm1, O–H stretching of inner hydroxyl groups at 3621 cm1, O–H stretching of inner – surface hydroxyl groups at 3695 cm1 [37], which also appeared in the chitosan membranes with 5 and 15 (w/w%) of HNTs. The peak at 1118 cm1 in the pure HNT spectrum corresponded to perpendicular Si–O stretching appears in the 15 (w/w%) HNTs membrane, but not in the other membranes. This finding implied that at high concentration, HNTs were not uniformly interacted with chitosan due to the formation of HNT aggregates, which was confirmed by SEM results earlier. The intensity of the band at 1031 cm1 (external surface siloxane groups) for 5 and 15 (w/w%) HNTs were decreased to the same amount. This consistency implied that the external surface siloxane groups (Si–O) (v ¼ 1031 cm1) of HNTs had chemically interacted with the chitosan up to the maximum extent at both concentrations. The functional groups (COH, COC and CH2OH) corresponding to the peak at 1063 cm1 in pure chitosan can be seen in 0 (w/w%) HNTs membrane, but disappeared after adding HNTs. This suggested that these functional groups were reacted with some other functional groups in HNTs. Moreover, the amide I bands for chitosan membranes for 5 and 15 (w/w%) HNTs were slightly shifted towards low wavelengths. The amide I band for pure chitosan and chitosan membranes was at 1653 cm1 for the 0 (w/w%) HNTs which was shifted to 1647 cm1 and 1650 cm1 for the 5 (w/w%) and 15 (w/w%) HNTs, respectively. Theoretically, amide I bands shifting to low wavelengths indicate the occurrence of hydrogen-bonding between the chitosan and fillers [2]. More shifting of amide I band at 5 (w/w%) HNTs indicated stronger hydrogen-bonding between chitosan and HNTs than in compositions; this was earlier confirmed by tensile strength results. 3.4. Thermal stability analysis As far as 50% remaining weight was concerned, the corresponding temperatures of chitosan membranes with 2, 10 and 15 (w/w%) of HNTs have drastically increased (Table 1). Enhancement of thermal stability could occur due to two main reasons. Char residue contributed to enhance the thermal stability of a composite. Char formation occurred at the expense of volatiles and the char itself acted as a barrier and hindered diffusion of volatile loss [38]. Hence, char residue of chitosan/HNT composites may act as a barrier and hinder the volatile escape while helping to improve the thermal stability. As mentioned by Du et al.
Table 1 Summary of the TGA results for CS and its composites. HNT Temperature Temperature Remaining Temperature weight (%) at maximum composition at 5% weight at 50% weight (loss weight loss ((w/w%)) loss ( C) rate/ C) ( C) 0 2 5 10 15
45 50 45 50 45
370 375 380 396 455
35 38 37 39 44
295 302 300 301 297
[15], the lumens of HNTs contributed to enhance the thermal stability of the polymer/HNT nanocomposite. In general, the length of HNTs varies from 50 to 5000 nm, with an external diameter of 20-200 nm and internal diameter of 10-70 nm [16]. Therefore, the HNTs’ lumen may have a high volume percentage (between 10 to 30%) [16]. During the initial degradation stage of the membranes, considerable amounts of degraded products of chitosan may be trapped within the lumens of HNTs. Hence, it may have led to effective delay in mass transport and, therefore, thermal stability significantly increased at high concentrations, especially by adding 15 (w/w%) HNTs into the chitosan. 4. Conclusions Chitosan/HNTs membranes were prepared using solution casting by adding 2, 5, 10 and 15 (w/w%) of natural halloysite nanotubes, and it was found that membranes with 5 (w/w%) of HNTs had the highest Young’s modulus and tensile strength. By incorporation of 5 (w/w%) of HNTs, Young’s modulus and tensile strength improved by 21% and 34%, respectively, due to uniform dispersion of HNTs, higher Young’s modulus of HNTs (w72 GPa) compared to chitosan, and interactions between chitosan’s hydroxyl and CO groups with external surface siloxane groups of the HNTs. At high concentrations, mechanical properties were slightly inferior to the membranes with 5 (w/w%) HNTs due to HNT agglomeration and less HNT-chitosan interaction. SEM micrographs showed that the chitosan/HNT (15 (w/w %)) had more micro-voids, pores and embossed HNTs, which confirmed the lower filler-matrix interaction compared to the other membranes. The addition of HNTs into the chitosan matrix in this composite system facilitated the formation of hydrogen bonds. According to the FTIR spectra, chitosan membranes with 5 (w/w%) HNTs showed stronger chemical interaction between chitosan and HNTs compared to the other compositions. TGA results revealed that the thermal stability has increased, since the HNTs contained lumen which delayed the mass transport by entrapping the degraded products. It is found that the addition of natural nanotubes of halloysite as an environmentally friendly nanofiller to chitosan biopolymer can increase the possible applications of this biopolymer, especially when the mechanical properties are concerned. Acknowledgements This project was funded by a grant (FRGS/2/2010/SG/ MUSM/02/6) from the Ministry of Higher Education,
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Material properties
Physico-chemical characterisation of chitosan/halloysite composite membranes R.T. De Silva a, Pooria Pasbakhsh a, *, K.L. Goh b, Siang-Piao Chai c, H. Ismail d a
Mechanical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon Selatan, Bandar Sunway, 46150 Selangor Darul Ehsan, Malaysia b School of Mechanical and Systems Engineering, Newcastle University, NE1 7RU, United Kingdom c Chemical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon Selatan, Bandar Sunway, 46150 Selangor Darul Ehsan, Malaysia d School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 11800 Pulau Penang, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 October 2012 Accepted 12 November 2012
Chitosan membranes reinforced by halloysite nanotubes (HNTs) at concentrations from 2 to 15 (w/w%) have been prepared by solution casting to investigate the optimal physicochemical properties for biomedical applications. Tensile test data revealed that the membranes reinforced with 5 (w/w%) HNTs yielded the highest Young’s modulus (0.52 0.01 GPa) and strength (81.6 4.4 MPa). Electron micrographs of the fractured surfaces implicated the interplay between individual HNTs and agglomerates of HNTs in the stress transfer mechanism. Infrared spectra revealed interaction between the HNT siloxane and chitosan functional groups. Thermogravimetric results demonstrated that the thermal stability of the membranes increased with HNT concentration. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Halloysite nanotubes Chitosan biocomposite membranes Microtensile testing Scanning electron microscope Thermal properties
1. Introduction Chitosan, ‘poly-b (1,4)-2-amino-2-deoxy-D-glucose’, is a deacetylated product of chitin which may be derived from crab and shrimp shells and fungal mycelia [1,2]. Owing to its non-toxic nature, antimicrobial activity and good compatibility with living tissues, chitosan has been identified during the last 20 years as a good candidate for use as a biomaterial [3]. Additionally, it possesses good adhesion and film forming capability, as well as high hydrophilicity, biocompatibility and biodegradability [4]. Accordingly, chitosan can be produced in various forms such as membranes [5], films [6], fibrous mats [5] and sponges [7]. From a structurefunction point of view, of the many varied forms, chitosan membrane is the most versatile, particularly for biomedical applications such as tissue engineering, bone implantation and artificial skin [5,8], as well as for filtration applications such as waste water filtration and reverse osmosis [9]. * Corresponding author. Tel: þ60 3 55146211; fax: þ60 3 55146207. E-mail address: [email protected] (P. Pasbakhsh). 0142-9418/$ – see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymertesting.2012.11.006
However, chitosan has low mechanical strength, thermal stability and water and gas barrier properties, which has restricted the opportunity for wider applicability [3,10]. Different types of material as micro and nanofillers, such as silica, hydroxyapatite, calcium phosphate, carbon nanotubes (CNTs) [1] and organo modified montmorillonite (OMMT), have been studied for reinforcing chitosan [11]. Of these, CNTs and OMMT have shown potential for reinforcing and enhancing the thermal stability of chitosan membranes [1,2,12,13]. Nevertheless, it is well known that the synthesis and functionalization processes of CNTs are complicated and costly [14]. As for OMMT, the standard organic treatment is not sufficient to generate well-intercalated and/or exfoliated morphologies in polymer matrix [15]. Halloysite nanotubes (HNTs), first reported by Berthier in 1826, are natural aluminosilicates (Al2Si2O5(OH)4.nH2O) with nano-tubular structures [14]. Typically, the length of HNTs varies from 50-5000 nm, with an external diameter of 20-200 nm and internal diameter of 10-70 nm [16]. HNTs possess several remarkable properties such as biodegradability, large aspect ratio and high surface area
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to volume ratio. Consequently, HNTs have found applications in polymer reinforcement [17] and drug delivery [18]. In the former application, HNTs have been used as nanofillers to reinforce polymers such as epoxy [19], polypropylene [20], polyamide [21], styrene rubber [22] and ethylene propylene diene monomer [23,24]. Very recently, mechanical properties of HNT incorporated chitosan films have been investigated and it has been observed that adding 7.5 (w/w%) of HNTs increased the tensile stress and Young’s modulus of chitosan/HNT composite by 134% and 65%, respectively [25]. Zheng et al. [26] have pointed out the feasibility of synthesizing chitosan/HNT hydrogels as an absorbent for removing ammonium ions from synthetic water. Findings from the study of the bioelectrochemical properties of chitosan/ HNT composite films demonstrate the feasibility of using the films for immobilizing horseradish peroxidase [27]. Studies on the physical properties of chitosan containing chrysotile and HNTs have demonstrated the feasibility of fabricating fibrous chitosan-based structures reinforced by HNTs as well as chrysotile [28]. Among all the studies on mechanical properties of polymer/HNT nanocomposites, the effect of Young’s modulus of single HNTs has been rarely studied. Recently Lu et al. [29] measured the Young’s modulus of single HNTs by transmission electron microscopy on a bending stage. In an earlier study, Guimarães et al. [30] used a “self-consistent charged densityfunctional tight-binding” method to predict the Young’s modulus of single wall HNTs without any interpretation with any polymer matrix. Here, we report on a feasibility study to establish a simple solution-casting approach for fabricating chitosan-based composite membranes reinforced by HNTs. Physico-chemical characterisation was carried out to measure the tensile, thermal and chemical properties of the membranes at different HNT concentrations. Microstructural characterisation was carried out to identify the mechanism of reinforcement and failure in the membrane. Also, for the first time, the Young’s modulus of a single HNTs was estimated by applying the rule of mixtures for these composites.
2. Materials and methods 2.1. Fabrication of chitosan/HNT composite membranes Chitosan flakes (low molecular weight w50,000 daltons, based on viscosity; 85% of deacetylation) was purchased from Sigma Aldrich. HNTs were provided by Imerys Tableware New Zealand. One gram of chitosan was mixed in 100 ml of 1% acetic acid solution and stirred for 20 h at 500 rpm. Next, 9 ml of distilled water was mixed with HNTs at 2, 5, 10 and 15 (w/w%) with respect to 1 g of chitosan. Following this, the mixtures were placed in an ultrasonic bath (Thermo D-60) for 30 min at a temperature of 50 C. Thereafter, the mixture was stirred for 20 h at 350 rpm. Subsequently, the solutions of chitosan and HNTs were mixed together and stirred for 8 h at 1000 rpm. Next, 15 g from the resultant respective solutions was poured into plastic petri dishes (diameter, 8.5 cm) and the dishes were placed in an oven for approximately 20 h at 40 C. After membrane formation, each dish was filled with sodium hydroxide (30 ml, 0.1 M) and left for 30 min to neutralize any left-over acetic acid. Following this, the membranes were rinsed with distilled water and submerged in distilled water for 30 min to remove the sodium hydroxide. Finally, the membranes were peeled off from the plastic dishes (Fig. 1(a)). The solution casting method yielded circular, translucent membranes. A similar procedure was employed to prepare membranes containing only chitosan. 2.2. Physico-chemical characterisation Strips of membranes, 3 mm by 15 mm (Fig. 1(b)) were tested for mechanical properties by stretching to rupture at a pre-determined displacement rate of 0.06 mm/s, using a high-throughput small-scale horizontal tensile test rig (Fig. 1(c)). The stress-strain curve of the respective specimens were derived from the load-displacement data; here stress was found by dividing the load by the cross-sectional area of the specimen and strain was equal to the ratio of the displacement to the initial grip-to-grip length of the taut
Fig. 1. (a) A typical image of a chitosan-based membrane reinforced by HNTs, (b) Schematic of membrane mounted on a paper template prior to tensile testing, (c) Stretching a chitosan-based membrane specimen using the microtensile test rig.
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specimen. The cross section of the membranes was measured under an Olympus BU41 optical microscope. The thickness of the bulk (around the central region) of the membrane ranged from 17 to 23 micrometer. Tensile properties of the membranes such as Young’s modulus (E), strength (s) and strain at rupture (ε) were obtained from the stress-strain curve. A total of ten specimens were tested for each HNT concentration. The fractured surfaces of tensile samples with 2, 5 and 15 (w/w%) of HNTs were investigated by using a Supra-35VP SEM. To prevent electrostatic charging during observation, the samples were coated with a thin layer of platinum. Fourier transform infrared (FTIR) spectroscopy was carried out to analyse the interaction between chitosan and HNTs by comparing the chemical structure of the chitosan/ HNTs composite membranes (at different HNT concentrations) with respect to those from pure chitosan flakes and HNTs. All samples were examined using a Thermo Scientific IS10 with a wavenumber range of 400-4000 cm1 with 0.2 cm1 resolution. Thermogravimetric analysis (TGA) was carried out using a TGA Q50 to investigate the thermal stability of the chitosan/HNT composite membranes over a temperature range of 25 C to 600 C at a rate of 10 C/min under nitrogen atmosphere. One-way analysis of variance (ANOVA) was implemented by “Tukey’s Post Hoc test”, using commercial software (SPSS, version 18) to investigate the significance differences between the means of the E, s and ε with
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respect to HNT concentration. In all the cases, P < 0.05 was considered as statistically significant. Linear regression analysis was used to determine the significance of the linear relationship between E and the volume fraction of HNTs (corresponding to the HNT concentration). 3. Results and discussion 3.1. Tensile properties Fig. 2(a) shows that chitosan membranes with 5 (w/w %) of HNTs has the highest strength (s) compared to the other concentrations. The ANOVA test revealed that not all of the means of s, among the different HNT concentrations, are equal (P < 0.05); Tukey’s Post Hoc test revealed significant difference between the means for (i) 0 (w/w%) versus 2 to 15 (w/w%), (ii) 2 (w/w%) versus 5 (w/ w%) and (iii) 5 (w/w%) versus 10 and 15 (w/w%). Correspondingly, the plot of s versus HNT concentration describes an increasing trend from 0 to 5 (w/w%), a decreasing trend from 5 to 10 (w/w%) and fluctuations of s with no appreciable trend from 10 to 15 (w/w%). Of note is that the chitosan membrane with 0 (w/w%) HNT concentration yielded s ¼ 60.8 2.8 MPa; this value falls within the range of values, from 4 MPa to 70 MPa, reported for the tensile strength of chitosan in the literature [31–35]. The wide range of values may be due to the chitosan molecular weight (30 kDa to 80 kDa), the percentage deacetylation of chitosan, and the composite’s
Fig. 2. Graphs of (a) tensile strength, s (MPa), (b) Young’s modulus, E (GPa), and (c) strain at break, ε (%), of chitosan-based membranes reinforced by HNTs, versus HNT concentration. Data are plotted as means SEM (SEM: standard error of mean); vertical bars represent SEM.
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preparation process. Considering the range of HNT concentrations used in this study, the magnitude of s in chitosan/HNT composite is higher than that of the membrane containing only chitosan. The increase in s is attributed to the possible interactions between the HNTs and chitosan. The structure of chitosan consists of two hydroxyl groups and an amino group in one glucosamine ring. The OH groups on the edge and surface of HNTs can interact with chitosan via hydrogen bonding; the interaction could be important for facilitating the transfer of stress (generated in the chitosan matrix) to the HNTs which functioned as a reinforcing phase. Statistical analysis of the variation in E with respect to HNT concentration revealed that the chitosan-based membranes reinforced by 2 (w/w%) of HNTs yielded the highest E (P < 0.05, ANOVA). In particular, Tukey’s Post Hoc test revealed significant difference between the means for 0 (w/w%) versus 2 to 15 (w/w%) of HNT concentration. Correspondingly, the plot of E versus HNT concentration (Fig. 3(b)) describes an increasing trend from 0 to 5 (w/w%) and fluctuations with no appreciable change from 5 to 15 (w/w%). The stiffening of chitosan membranes blended with HNTs is attributed to a corresponding increase in the structural rigidity of the chitosan polymer chains [10] arising from the interaction between adjacent HNTs and chitosan. To the best of our knowledge, there has been no report on the prediction of Young’s modulus of halloysite (EH) using experimental data derived from tensile testing of
polymer-based composites reinforced by HNTs. This paragraph explains an attempt to provide simple order-ofmagnitude estimate of EH. Application of the rule of mixtures for modeling the HNT reinforcement of chitosan membrane leads to an expression relating E to EH and the Young’s modulus of chitosan (EC), weighted by the respective volume fractions of VH and chitosan (VC), i.e. E ¼ KEHVH þ ECVC, where VH þ VC ¼ 1 and K is the filler reinforcement efficiency. Here, K is set to 0.2 [36] to model an array of randomly oriented HNTs in a chitosan matrix material. Given that the densities of HNT and chitosan are 2.53 g/cm3 and 0.3 g/cm3, respectively, it is easy to see that 2 and 5 (w/w%) HNT concentration corresponded to VH ¼ 0.0024 and 0.0060, respectively. By considering VH from 0 to 0.006, linear regression analysis of E versus VH revealed that E increased at a rate of 14 7.3 GPa for every unit increase in VH (F ¼ 64.17, P < 0.05, R2 ¼ 78.6 %). Consequently, this model predicted that EH ¼ 72.165 GPa. This value agrees with the E of larger HNTs (60 to 71 GPa for tubes with inner diameter >70 nm and outer diameter >158 nm) reported by Lu et al. [29]. According to their report, E of a single HNT could vary from 60 to 133 GPa depending on its’ inner and outer diameters. However, the actual E of a single HNT could be higher than these reported values and the lower E of HNT found in our study might be due to the agglomeration of tubes in some regions, which can reduce the effective modulus of dispersed HNTs in chitosan. Further increase in HNT concentration (to 10 and 15 (w/w%)) could dramatically increase the density of HNT
Fig. 3. Scanning electron micrographs of typical fractured morphology of chitosan-based membranes reinforced by (a) 2 (w/w%), (b) 5 (w/w%) and (c) 15 (w/w%) of HNTs.
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aggregates. These aggregates could no longer be able to provide reinforcement for the chitosan matrix due to the reduction in HNT-chitosan interfacial regions. Henceforth, the magnitude of E decreased from 5 to 10 (w/w%) HNTs. So the critical volume fraction of HNTs is estimated about 0.006, which corresponds to 5 (w/w%) HNT concentration. Fig. 2(c) shows a graph of ε versus HNT concentration. The ANOVA test revealed that not all of the means of ε among the different HNT concentrations are equal (P < 0.05); Tukey’s Post Hoc test revealed significant differences between the means of ε for (i) 0 (w/w%) versus 2 to 15 (w/w%), and (ii) 2 (w/w%) versus 5 and 10 (w/w%) and (iii) 5 (w/w%) versus 15 (w/w%). The overall profile of the plot of ε versus HNT concentration features a rapid drop in the magnitude of ε at small HNT concentration, i.e. 2 (w/ w%); a small appreciable increase in the magnitude of ε occurs at a slightly higher HNT concentration but further increase in HNT concentration yields a decreasing trend thereafter. Compared to the pure chitosan membranes, the smaller ε in the chitosan-HNT blends is consistent with the previous argument in stiffening of the membranes with the addition of HNTs. The overall reduced extensibility of the chitosan-membrane may be attributed to the increase in the structural rigidity of the chitosan polymer chain [10]. Similar trends have been observed in chitosan-based membranes reinforced by, e.g. OMMT [3]. 3.2. Morphology From Fig. 3, the light circular dots shown in the micrographs of the tensile fractured surfaces of the composite membranes are believed to be the end of the tubes. Hence, it can be seen that chitosan membranes with 2 and 5 (w/w %) of HNTs have better dispersion of HNTs than the
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membranes with 15 (w/w%). Additionally, embedded HNTs in chitosan matrix can be seen in the micrographs. These embedded HNTs confirmed a good interaction between HNTs and chitosan. Furthermore, micro-voids and pores are evident in the fractured surfaces (marked by arrows in Fig. 3) due to the pulled out nanotubes and dislodged agglomerated HNTs during the tensile testing. The presence of agglomerated HNTs was also observed, (circled in Fig. 3(c)). This could be due to stacking of two or more HNTs together, and referred to as HNT aggregated areas. At 15 (w/ w%) HNTs, only few HNT-HNT interactions were spotted due to the covering of HNT aggregated areas with chitosan. Nevertheless, the chitosan-HNT interaction at low HNT concentrations (2 and 5 (w/w%)) was dominant due to the presence of less micro-voids and pores compared to higher HNT concentration. Even though the morphology and dispersion for the membranes with 2 and 5 (w/w%) HNTs were very similar, the membranes with 5 (w/w%) HNTs has more HNTs which allow for better stress transformation than the 2 (w/w%) HNTs membranes. This is why membranes with 5 (w/w%) of HNTs have higher mechanical properties than the other compositions. According to the tensile test results, mechanical properties such as E and s of 15 (w/w%) membranes were less than the 5 (w/w%) HNTs membranes and were also slightly inferior to the membranes with 2 (w/w%) of HNTs due to (i) poor matrix-filler interaction and (ii) the presence of HNT aggregation, even though they have more HNTs to provide more opportunities for stress transformation. 3.3. FTIR analysis From Fig. 4, the presence of functional groups of chitosan and HNTs can be identified in the composite
Fig. 4. Fourier transform infrared spectra of (a) pure chitosan and (b) HNT. The remaining spectra are derived from chitosan-based membranes reinforced by (c) 0 (w/w%), (d) 5 (w/w%) and (e) 15 (w/w%) of HNTs.
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membranes. The main functional groups that appeared in the chitosan spectrum were vibration of OH groups at 3353 cm1 and 3297 cm1, C–H bond (–CH3) at 2873 cm1, carbonyl bonds (C]O) of amide groups C]O–NHR at 16801480 cm1 can also be seen in composite membranes. The bending vibration of methylene groups at 1375 cm1 and asymmetric vibration of CO at 1150 cm1 [11], also appeared in the chitosan membranes, with 0, 5 and 15 (w/w%) of HNTs. This indicated that functional groups of chitosan were also repeated in the membranes. For the pure HNTs, the main bands appearing in the spectrum included; O–H deformation of inner hydroxyl groups at 911 cm1, in-plane Si–O stretching at 1031 cm1, O–H stretching of inner hydroxyl groups at 3621 cm1, O–H stretching of inner – surface hydroxyl groups at 3695 cm1 [37], which also appeared in the chitosan membranes with 5 and 15 (w/w%) of HNTs. The peak at 1118 cm1 in the pure HNT spectrum corresponded to perpendicular Si–O stretching appears in the 15 (w/w%) HNTs membrane, but not in the other membranes. This finding implied that at high concentration, HNTs were not uniformly interacted with chitosan due to the formation of HNT aggregates, which was confirmed by SEM results earlier. The intensity of the band at 1031 cm1 (external surface siloxane groups) for 5 and 15 (w/w%) HNTs were decreased to the same amount. This consistency implied that the external surface siloxane groups (Si–O) (v ¼ 1031 cm1) of HNTs had chemically interacted with the chitosan up to the maximum extent at both concentrations. The functional groups (COH, COC and CH2OH) corresponding to the peak at 1063 cm1 in pure chitosan can be seen in 0 (w/w%) HNTs membrane, but disappeared after adding HNTs. This suggested that these functional groups were reacted with some other functional groups in HNTs. Moreover, the amide I bands for chitosan membranes for 5 and 15 (w/w%) HNTs were slightly shifted towards low wavelengths. The amide I band for pure chitosan and chitosan membranes was at 1653 cm1 for the 0 (w/w%) HNTs which was shifted to 1647 cm1 and 1650 cm1 for the 5 (w/w%) and 15 (w/w%) HNTs, respectively. Theoretically, amide I bands shifting to low wavelengths indicate the occurrence of hydrogen-bonding between the chitosan and fillers [2]. More shifting of amide I band at 5 (w/w%) HNTs indicated stronger hydrogen-bonding between chitosan and HNTs than in compositions; this was earlier confirmed by tensile strength results. 3.4. Thermal stability analysis As far as 50% remaining weight was concerned, the corresponding temperatures of chitosan membranes with 2, 10 and 15 (w/w%) of HNTs have drastically increased (Table 1). Enhancement of thermal stability could occur due to two main reasons. Char residue contributed to enhance the thermal stability of a composite. Char formation occurred at the expense of volatiles and the char itself acted as a barrier and hindered diffusion of volatile loss [38]. Hence, char residue of chitosan/HNT composites may act as a barrier and hinder the volatile escape while helping to improve the thermal stability. As mentioned by Du et al.
Table 1 Summary of the TGA results for CS and its composites. HNT Temperature Temperature Remaining Temperature weight (%) at maximum composition at 5% weight at 50% weight (loss weight loss ((w/w%)) loss ( C) rate/ C) ( C) 0 2 5 10 15
45 50 45 50 45
370 375 380 396 455
35 38 37 39 44
295 302 300 301 297
[15], the lumens of HNTs contributed to enhance the thermal stability of the polymer/HNT nanocomposite. In general, the length of HNTs varies from 50 to 5000 nm, with an external diameter of 20-200 nm and internal diameter of 10-70 nm [16]. Therefore, the HNTs’ lumen may have a high volume percentage (between 10 to 30%) [16]. During the initial degradation stage of the membranes, considerable amounts of degraded products of chitosan may be trapped within the lumens of HNTs. Hence, it may have led to effective delay in mass transport and, therefore, thermal stability significantly increased at high concentrations, especially by adding 15 (w/w%) HNTs into the chitosan. 4. Conclusions Chitosan/HNTs membranes were prepared using solution casting by adding 2, 5, 10 and 15 (w/w%) of natural halloysite nanotubes, and it was found that membranes with 5 (w/w%) of HNTs had the highest Young’s modulus and tensile strength. By incorporation of 5 (w/w%) of HNTs, Young’s modulus and tensile strength improved by 21% and 34%, respectively, due to uniform dispersion of HNTs, higher Young’s modulus of HNTs (w72 GPa) compared to chitosan, and interactions between chitosan’s hydroxyl and CO groups with external surface siloxane groups of the HNTs. At high concentrations, mechanical properties were slightly inferior to the membranes with 5 (w/w%) HNTs due to HNT agglomeration and less HNT-chitosan interaction. SEM micrographs showed that the chitosan/HNT (15 (w/w %)) had more micro-voids, pores and embossed HNTs, which confirmed the lower filler-matrix interaction compared to the other membranes. The addition of HNTs into the chitosan matrix in this composite system facilitated the formation of hydrogen bonds. According to the FTIR spectra, chitosan membranes with 5 (w/w%) HNTs showed stronger chemical interaction between chitosan and HNTs compared to the other compositions. TGA results revealed that the thermal stability has increased, since the HNTs contained lumen which delayed the mass transport by entrapping the degraded products. It is found that the addition of natural nanotubes of halloysite as an environmentally friendly nanofiller to chitosan biopolymer can increase the possible applications of this biopolymer, especially when the mechanical properties are concerned. Acknowledgements This project was funded by a grant (FRGS/2/2010/SG/ MUSM/02/6) from the Ministry of Higher Education,
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