Surface charge on chitosan/cellulose nanowhiskers composite via functionalized and untreated carbon nanotube

2.1 Raw materials

Chitosan (CS), powder of low molecular weight (50,000–190,000 Da) exhibiting sufficient potential and more solubility, microcrystalline cellulose (MCC), acetic acid 98%, lithium chloride (LiCl), N, N dimethyl acetamide solution (DMAc) and sodium hydroxide (NaOH) purchased from Sigma-Aldrich were used as received without further purification. Un-MWCNTs with a purity of 98% and diameter range between 16 and 23 nm were synthesized according to Mubarak, Sahu (Mubarak et al., 2014) and f-MWCNTs from Jun, Mubarak (Jun et al., 2018).

2.2 Preparation of cellulose nanowhiskers (CNWs)

CNWs were prepared with MCC via chemical swelling technique and then followed by ultrasonic treatment Oksman, Mathew (Oksman et al., 2006). DMAc and 0.5% LiCl solution was used as a swelling agent. DMAc/LiCl-0.5% mixture was kept on stirring by using magnetic stirrer for 15 min. The concentration of MCC in DMAc/LiCl mixture was maintained as 10 wt%. A weighed amount of MCC powder was gently poured into DMAc/LiCl mixture while continuously agitated for 12 hrs. at 70 °C using magnetic stirrer. Afterwards, slightly swelled particles were transferred to a centrifuge bottle and centrifugation was repeated three times with the speed of 3000 RCF for 3 min. After every centrifuge cycle, supernatant was removed and replaced with distilled water. Subsequently, the suspensions were ultrasonicated for 10 min to obtain the product CNWs. Repeatedly, gel-like suspension was washed with distilled water and stored in an airtight container and refrigerated.

2.3 Preparation of CS/CNWs/MWCNTs nanocomposites

The CS/CNWs/MWCNTs composite was synthesized via solution casting processing method. The 0.5 wt% MWCNTs (both functionalized and treated MWCNTs in separate reactions) were added into 1% of the acetic acid solution and undergo dispersion by ultra-sonication bath for 1 h. After that, 2 wt% of chitosan powder was added into the solution. The mixture was stirred for 90 min at 50 °C to dissolve the chitosan powder and to form a homogeneous mixture. 5 wt% of CNWs were poured into the mixture and then stirred for 12 hrs. or more until a completely homogeneous mixture was formed. The mixture was then left at room temperature for 30 min until the air bubbles disappeared followed by ultra-sonication for 20 min to deeply remove the remaining air bubbles. The homogeneous mixture was casted into a petri dish and get evaporated at room temperature for 48 hrs. or more until a dry nanocomposite film was obtained. In order to speed up the evaporation process, the samples were placed in the fume hood. After the sample got completely dry, film was immersed in a 2% (w/v) sodium hydroxide for 20 min and then neutralized with the acetic acid and then washed thrice with distilled water. Nanocomposite film was easily removed from petri dish with the aid of NaOH. The prepared nanocomposites films were then dried at room temperature to remove the residues of water. All the samples were kept in a zip bag to avoid contact with air moisture [Fig. 1].

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Fig. 1

Schematic Illustration of Preparation of CS/CNWs/MWCNTs Nanocomposites.

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2.4 Characterizations

The samples with different concentrations of f-MWCNTs or untreated MWCNTs on Cs/CNWs were prior prepared for testing. The characterization studies would analyze their surface morphology, chemical composition, mechanical properties, and temperature decomposition, with Energy-dispersion X-ray Spectroscopy (EDX), Field Emission Scanning Electron Microscope (FESEM), Fourier-Transform Infrared Spectroscopy (FTIR), Mechanical Properties testing and Thermogravimetric Analysis (TGA). The morphology of the samples was examined by using a Field Emission Scanning Electron Microscope (FESEM, FEI Quanta 400) and Energy-dispersion X-ray Spectroscopy (EDX). For FESEM examination, samples were prepared by placing small pieces of sample films on the sample holder i.e. SEM’s stage using a conductive carbon-tape. Each small piece of sample film was coated with an ultra-thin gold (Au) layer for a minimum of 100 s using an ion-sputtering machine 108 Auto Sputter Coater). Furthermore, the chemical composition was analyzed with Energy-dispersion X-ray Spectroscopy (EDX). The Fourier-transform infrared (FTIR) transmission spectra were performed with Perkin Elmer 1600 Series in ATR mode to characterize and evaluate the surface structure of nanocomposite samples. The samples were placed on an optic window with a diamond crystal. For better contact between the crystal and sample, the pressure of the applicator was maintained with the help of a pressure control spot. After each measurement, both the anvil of the pressure applicator and the crystal plate was cleaned thoroughly. The tensile strength properties were analyzed based on ASTM standard (D-3822) using an INSTRON Universal Testing Machine and testing was performed at room temperature. The initial gauge length L₀ and thickness of 10 mm and 6 mm, respectively, were prepared for testing. The testing was performed with a speed of 5 mm/min with a load cell of 500 N. The thermal stability of the nanocomposites sample was analyzed with Mettler Toledo Thermogravimetric Analyzer, Perkin Elmer STA 6000. The TGA analysis was performed with a heating rate of 20 °C/min from 30 to 900 °C by using a nitrogen gas flow of 50 cm/min. Each testing was repeated three times and the obtained values were noted for further analysis and discussion.

3.1 Scanning electron microscopy

The scanning electron microscope analysis explored the core-shell structure of CS/CNWs/f-MWCNTs and CS/CNWs/Un-MWCNTs. As a neat chitosan and CS/CNWs-5% had been analyzed to compare the structure with modified nanocomposites. The SEM image in Fig. 1 (i) shows a smooth surface with some scratch look indicated a good dissolution of chitosan in acetic aqueous solution and formed a chitosan nanocomposite film (Qian and Yang, 2006; Jung et al., 2007). In short, Fig. 1 (ii) show a disorganized and some fractured look surface structure which indicates the presence of CNWs in chitosan matrix (Tanjung et al., 2017). The high aspect ratio and strong intermolecular hydrogen bonding of CNWs formed the aggregation with chitosan matrix (Akhlaghi et al., 2013). There were no large or unequal aggregates observed in the chitosan matrix which indicates that CNWs are well dispersed in this matrix. Meanwhile, upon fracturing, most of the CNWs got broken and then embedded instead of separated out of the matrix. It is most likely with the results of the strong interfacial adhesion between the nanofiller and matrix. Consequently, the properties and performance of chitosan nanocomposite would directly be affected by the uniform dispersion of nanofiller in the chitosan matrix. Therefore, it correlates with the mechanical properties analysis which means that the addition of 5 wt% CNWs on the chitosan would improve or alter the tensile strength and Young’s Modulus (Tanjung et al., 2017).

As studied in literature review, the chitosan matrix was highly insoluble in alkaline and natural organic solvents while well dissolved in acid aqueous solvent solution (pH < 6). The amino group of chitosan would be protonated during the dissolution with the acetic acid aqueous solution and resulting as a polycation. On the other hand, the acid oxidation functionalized MWCNTs had the carboxylic acid groups at the sidewalls and end of the structure which present the negative charge. The preparation of chitosan, CNWs and MWCNTs were well mixed by ultra-sonication and magnetic stirrer where the chitosan matrix would absorb the CNWs and MWCNTs on the surface at certain molecular level by electrostatic interaction which resisted the MWCNTs and CNWs aggregation and formed a dark grey solution (Qian and Yang, 2006). From FESEM micrographs, it is easy to distinguish between MWCNTs and CNWs due to both differences in fibrous shape. The CNWs are present as disorganized fracture look while the MWCNTs were in network or threadlike (Liu et al., 2005). The MWCNTs were easy to aggregate during the evaporation process through the π bond interaction and high surface specific area. In the meantime, the evaporation of water solvent could strengthen the agglomeration of nanowhiskers (Liu et al., 2016). The SEM images obviously shows that the homogeneous dispersion of MWCNTs in CS/CNWs-5% is significantly improved probably by the Van der Waals forces between CS/CNWs and surface of MWCNTs (Xue et al., 2017). This appearance shows a strong interfacial adhesion between CS/CNWs and MWCNTs. This strong bonding between CS/CNWs and MWCNTs help to improve the mechanical properties of nanocomposites. With the presence of a functional group of chitosan, CNWs and MWCNTs would form a laminated film with sufficient tensile strength but ductile nanocomposite films (Tanjung et al., 2017). In both SEM images for CS/CNWs/f-MWCNTs in Fig. 1 (iv) and CS/CNWs/Un-MWCNTs in Fig. 1 (iv) clearly show a quite similar micrograph which indicates Un-MWCNTs also capable to well disperse in CS/CNW-5% and formed a nanocomposites film where no agglomerations were observed. Therefore, FESEM shown that CS/CNWs/f-MWCNTs and CS/CNWs/Un-MWCNTs nanocomposite films were successfully synthesized with the presence of each chitosan, CNWs and MWCNTs. In the overall observation, the presence of CNWs and MWCNTs clearly alternate the surface morphology while the Un-MWCNTs and f-MWCNTs leading to form a slightly different morphology, corresponding with different mechanical properties and degree of crystallinity (Tanjung et al., 2017).

3.2 Fourier-transform Infrared Spectroscopy

The FTIR analysis is performed in the range of 400–4000 cm⁻¹ to determine the presence of functional groups in the nanocomposites films. As the neat CS and CS/CNW-5% has been analyzed to determine the possible presence of functional groups before going through the analysis for characteristic peak for CS/CNWs/f-MWCNTs and CS/CNWs/Un-MWCNTs. The neat chitosan in Fig. 2 (i) shows the —OH peak which the stretching vibration presence from 3200 to 3600 cm⁻¹ and the intramolecular hydrogen bonds, peak at 2880 cm⁻¹ indicate as CH. The presence of N-acetyl groups confirmed at the band around 1657 cm⁻¹, C—N stretching amide at around 1365 cm⁻¹ and the peak at 1594 cm⁻¹ due to the bending vibration of N—H, respectively. The stretching vibration band around 1058 cm⁻¹ represent as C—O—C bridge [β (1, 4) glyosidic bonds]. This analysis has an agreement with the several studies (Fernandes Queiroz et al., 2014; Abo Elseoud et al., 2018; Venugopal et al., 2016; Marques Neto et al., 2019). On the other hand, Fig. 2 (ii) shows the modified chitosan with CNWs. Chitosan and CNWs consists of almost similar spectrum characteristic peaks. In short, the CS/CNWs nanocomposite film show the characteristic peak as C⚌O stretching (amide I bands) would present at 1500–1750 cm⁻¹, the 1500–1600 cm⁻¹ would attribute as C—N stretching vibration (amide II bands) and N—H bending as the characteristic peaks for chitosan composites. The peak at 1161 and 1026 cm⁻¹ indicate CS/CNWs nanocomposite as saccharide structure and represent as C—O—C bridge [β (1, 4) glyosidic bonds]. The presence of stretching vibration at 1200–1700 cm⁻¹ indicates that the strong hydrogen bonds interaction via between functional groups of CNWs and chitosan which lead to a formation of the transparent nanocomposites film surface. The peak at 3200–3600 cm⁻¹ had become deeper which suggested the presences of same functional groups and strong interaction of sulphate functional groups of CNWs surface with —NH₂ functional groups of chitosan. This analysis indicated that CNWs were physically modified into the chitosan matrix by solution casting method. These characteristic peak results show agreement with several studies (Tanjung et al., 2017; Abo Elseoud et al., 2018; Geng et al., 2015). Fig. 2 (iii) shows the comparison of FTIR characteristic peaks of CS/CNWs/f-MWCNTs and CS/CNWs/Un-MWCNTs which displayed slightly different characteristic peaks. The characteristic peaks for 3200–3630 cm⁻¹ attributed to the stretching vibration of —OH overlapped with —NH stretching due to the intermolecular hydrogen bond for the polysaccharide of chitosan matrix. A peak at 2908 cm⁻¹ appeared due to the presence of CH₂ stretch of the CNWs composites. A peak observed at 1661 cm⁻¹ for both CS/CNWs/f-MWCNTs and CS/CNWs/Un-MWCNTs due to the presence of C⚌C stretching of MWCNTs, the peak at 1598 cm⁻¹ was due the C⚌O stretching amide I of chitosan, the band stretching vibration around 1038 cm⁻¹ represent the C—O—C bridge [β (1, 4) glyosidic bonds]. Comparative to neat chitosan, the C⚌O functional group is slightly shifted from 1653 cm⁻¹ to 1598 cm⁻¹ due to the network of intermolecular hydrogen bonds between MWCNTs with CS/CNWs (Choi et al., 2019). As clearly observed that the peak vibration for the CS/CNWs/Un-MWCNTs nanocomposite film is smaller than CS/CNWs/f-MWCNTs. It is most likely due to the lack of functional group in the Un-MWCNTs. There is a deformation of primary amine N—H stretching observed compared to neat chitosan which was shifted from 1594 cm⁻¹ to 1440 cm⁻¹ due to the interaction between the amine group of chitosan and functional groups of carbon nanotubes (Carson et al., 2009). The overall band at 1400–1650 cm⁻¹ for CS/CNWs/f-MWCNTs indicated the presence of carboxylic group of v(COOH) of functionalized MWCNTs which reacts with the amino functional group of chitosan during modification and converted into amide (—CONH—) group (Venkatesan et al., 2012). However, in CS/CNWs/Un-MWCNTs there is hard to analyze the primary amine N—H stretching and the section on 1600 cm⁻¹ till 400 cm⁻¹. It is most likely due to lack of functional group in Un-MWCNTs and had a defect to the structure of CS/CNWs during physical modification. Overall results indicated that the CNWs and f-MWCNTs are well dispersed and integrated into chitosan matrix during solution casting technique. However, the Un-MWCNTs also consider well dispersed while there is lack of intermolecular interaction with the CS/CNWs which clearly observed in FTIR analysis.

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Fig. 2

FESEM micrographs of CS (i) CS/CNW (ii) CS/CNW/Un-MWCNTs (iii) and CS/CNW/f-MWCNTs (iv) nanocomposites at 20 µm.

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3.3 Energy-dispersive X-ray Spectroscopy (EDS)

The elemental weight % of the chemical composites were analyzed with EDS analysis and all the samples i.e. neat chitosan, CS/CNWs-5%, CS/CNWs/f-MWCNTs and CS/CNWs/Un-MWCNTs did not presented any impurity as assumed from Fig. 3 (i-iv). Based on the results, the CS/CNWs/f-MWCNTs had higher oxygen content as compare to CS/CNWs/Un-MWCNTs. It could be explained that the acid functionalized MWCNTs consist of oxygenated functional group, whereby verified that the untreated MWCNTs lack of this functional group (Jun et al., 2018). Besides, CS/CNWs/f-MWCNTs and CS/CNWs/Un-MWCNTs had a higher carbon content compare to neat chitosan and CS/CNWs. The increase of carbon contents could state that both f-MWCNTs and Un-MWCNTs was well-dispersed in the CS/CNWs and directly increase on carbon contents in the nanocomposites.

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Fig. 3

FTIR spectra of Neat Chitosan (i) FTIR spectra of Chitosan/CNWs-5% (ii) and FTIR spectra of CS/CNWs/Un-MWCNTs (blue) and CS/CNWs/f-MWCNTs (red) (iii).

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On the other hand, nitrogen was only detected in CS/CNWs/f-MWCNTs nanocomposite due to the reaction between dialdehyde and amino group during solution casting modification (Xu et al., 2019). Furthermore, the oxygen content of the CS/CNWs, CS/CNWs/f-MWCNTs and CS/CNWs/Un-MWCNTs was decreased compared to neat chitosan. The estimated results are also enlisted in Table 1. So, the supporting information provided from EDS analysis shows that both f-MWCNTs and Un-MWCNTs is well attached and well dispersed with CS/CNWs-5%.

3.4 Mechanical properties

The mechanical properties were studied on the tensile strength (TS), Young’s modulus (YM) and elongation break (EB%) of nanocomposites at room temperature. The mechanical properties of a neat chitosan matrix were poor with TS 21.6 ± MPa which similar to reported one (Corsello et al., 2017; Rong et al., 2017). The data in Table 2 confers that with 5% concentration of CNWs, the EB (%) only had a slight change from 13.5% to 12.5%. However, tensile strength and modulus were increases to 22.5 MPa and 402.5 MPa compared to neat CS. It is because CS is inherently hydrophilic in nature and a very polar matrix which can easily aggregate with CNWs (Corsello et al., 2017; Rong et al., 2017; Li et al., 2009). Moreover, CNWs could homogeneously have dispersed in the chitosan polymer matrix. However, it was difficult to have a well-dispersed CNWs with a concentration higher than 5% due to the high weight percentage of nanofiller suspension that can deteriorate the homogeneity of Chitosan/acetic acid/CNWs system (Geng et al., 2015). Consequently, 5 wt% of CNWs was chosen as the best ratio for CS/CNWs nanocomposite and used for further study.

Fig. 4 (i) showed the effect of different concentration of Un-MWCNTs and f-MWCNTs on the tensile strength of CS/CNWs nanocomposites films. A comparative study had been performed by using the Un-MWCNTs and f-MWCNTs as an additional nanofiller. The effect of Young's Modulus and Elongation Break vs concentration is shown in Fig. 4 (ii) and (iii) respectively. The results shown in Table 2 show that with the presence of additional nanofiller the Tensile strength (TS) and Young’s Modulus (YM) got significant improvement but Elongation break (EB) was decreased. Decrease in EB% can be supported by the various study as with the increasing of nanofiller would impart the stiffness effect to the nanocomposites which decrease the deformability of CS/CNWs nanocomposites films (Li et al., 2009; Yao Rong et al., 2017). Moreover, with the increase in loading of Un-MWCNTs and f-MWCNTs, the TS and YM of CS/CNWs film were increased.

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Fig. 4

EDS spectra of CS (i), CS/CNWs (ii), CS/CNWs/f-MWCNTs (iii) and CS/CNWs/Un-MWCNT (iv) composite films.

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By adding 0.5% f-MWCNTs to CS/CNWs the TS and YM were increased by about 70% (from 22.5 MPa to 38.25 MPa) and 60% (402.5 MPa to 644 MPa) respectively, but decrease the EB by 44.08% (12.5–6.99%). However, an addition of 0.5% Un-MWCNTs, the TS and YM gave rise for about 39% (22.5 MPa to 31.25 MPa) and 21% (402.5 MPa to 535.76 MPa) respectively and decrease the EB approximately 36.2% (12.5–7.98%). As the overall significant perceived from the results is that the addition of 0.1% of MWCNTs had drastically enhanced the mechanical properties of CS/CNWs film. However, with the further increase for nanofiller aggregation at higher concentration of MWCNTs, the mechanical properties observed in Table 2 only increased slightly. With a higher concentration additional nanofillers, it was hard to completely well disperse in CS/CNWs solution due to the strong internal Van der Waal force and the high viscosity of chitosan solution. Other than that, a low weight percent of chitosan matrix which dissolved in acetic acid unable to contribute incorporation for a high amount of nanofiller reinforcement.

Significantly, it is perceived that the addition of Un-MWCNTs to CS/CNWs experienced lower mechanical properties as compared to the f-MWCNTs. Nevertheless, both f-MWCNTs and Un-MWCNTs are capable to be dispersed in a 1% acetic acid aqueous solution. An acid functionalized MWCNTs would present with a number of the hydrophilic group such as —OH and —COOH group along their backbone and lead to overcome the strong internal Van der Waal force interaction (Rong et al., 2017; Shang et al., 2013). The presence with poor or unequal dispersion and solubility of nanofiller in the CS/CNWs matrix would lead to inequivalent enhancement for the properties of nanocomposites. A f-MWCNTs with no sign of structure devastation as a nanofiller well-dispersed with CS/CNWs nanocomposites film might obtain with a slightly different mechanical properties results (Shang et al., 2013; Wang et al., 2005; Sun et al., 2011). Other than that, an improvement of mechanical properties for a CS/CNWs with Un-MWCNTs as nanofiller had the agreement with the study reported by (Aryaei et al., 2014). The observation of mechanical properties could be supported by previous researches which performed chitosan matrix modified with MWCNTs (Wang et al., 2005; Aryaei et al., 2014). Chitosan is a very polar and inherent hydrophilic matrix which could be modified with two nanofillers and enhanced its mechanical properties. Therefore, it can be explained that the addition of two nanofillers had extended to a formation of percolate network within the chitosan matrix (Amri et al., 2013). As this distinctly results with forming a stronger intra and inter hydrogen bonding between nanofillers and chitosan matrix which facilitated the formation of a rigid interface (Lewandowska, 2015). The overall increment for mechanical properties can be attributed to the interaction of both f-MWCNTs and Un-MWCNTs as extra nanofiller with CS/CNWs. Suggestively, the result represents a strong interaction of CS/CNWs with the Un-MWCNTs and f-MWCNTs, the YM and TS would increase as the extra energy was needed to overcome the intra and inter molecular bonding energy.

3.5 Thermogravimetric analysis

Thermogravimetric analysis was performed to analyze the thermal behavior of the modified nanocomposites. As for the comparative study, the chitosan/CNWs-5% had been modified with f-MWCNTs and Un-MWCNTs. The thermal analysis in Fig. 5 (I and ii) has shown three distinct weight loss for CS/CNWs/Un-MWCNTs and four for CS/CNWs/f-MWCNTs respectively. The first stage showed a minor weight loss of about 4% at 50.6 °C for CS/CNWs/Un-MWCNTs and 64.5 °C for CS/CNWs/f-MWCNTs which was signified to the evaporation of water vapors (Venugopal et al., 2016; Yu et al., 2009). While nanocomposite films sample undergo preparation, the samples had been left in fume hood for 72 hrs. with the purpose of removing the water vapors and solvent, then kept in a close zip bag. However, there might be some water vapors physically absorbed to the surface of nanocomposites during testing preparation. In the second stage, a long range of decomposition for weight loss occur at 380 °C about 7.8% for CS/CNWs/f-MWCNTs and around 5.6% for CS/CNWs/Un-MWCNTs which started at 475 °C and most likely due to the chitosan matrix involved a minor depolymerization or decomposition of glucosamine, amino side and N-acetyl side groups (Shawky et al., 2012). The third stage undergone minor weight loss of about 0.8% for the CS/CNWs/f-MWCNTs occurred at a temperature range of 536–751.4 °C. It was most likely attributed to the —COOH group of f-MWCNTs which had damaged the surface of the chitosan matrix or due to the defect on f-MWCNTs structure during acid oxidation and affect the nanocomposite (Jun et al., 2018; Carson et al., 2009). Lastly, a sharp weight loss region from 751 °C to 838 °C for CS/CNWs/f-MWCNTs and 638 °C to 837 °C for CS/CNWs/Un-MWCNTs was observed which might be ascribed for the thermal degradation of chitosan polymer. Compared to the study performed by (Rong et al., 2017), the neat chitosan and CS/CNW-5% would have a sharp weight lost at 250 °C and 350 °C, respectively (see Fig. 6).

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Fig. 5

Dependence of TS (i), YM (ii) and EB (%) (iii) upon concentration of MWCNTs on Chitosan/CNWs Nanocomposite Films.

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Fig. 6

TGA analysis of CS/CNWs/Un-MWCNTs (i) and CS/CNWs/f-MWCNTs (ii) composite films.

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However, this research indicated that the addition of f-MWCNTs and Un-MWCNTs would prevent or delay the degradation of CNWs remarkably and the addition of CNWs together with MWCNTs had a more strong interaction with chitosan matrix leading to more effective and enhanced thermal stability of chitosan matrix (Liu et al., 2016; Carson et al., 2009; Celebi and Kurt, 2015). The overall degradation of CS/CNWs/Un-MWCNTs nanocomposites film showed almost similar behavior to CS/CNWs/f-MWCNTs which indicates that the Un-MWCNTs also enable to be well dispersed in chitosan matrix (Liu et al., 2016). It is most likely due to the received Un-MWCNTs was in high purity by little or absence of any sites defect (Carson et al., 2009). Therefore, the extended results showed that the f-MWCNTs had a higher increment on the thermal stability of CS/CNWs as compare to Un-MWCNTs.