Nano-dynamic mechanical and thermal responses of single-walled carbon nanotubes reinforced polymer nanocomposite thinfilms

Accepted Manuscript Nano-dynamic mechanical and thermal responses of single-walled carbon nanotubes reinforced polymer nanocomposite thin-films Gunasekaran Venugopal, Jipsa Chelora Veetil, Nivea Raghavan, Varu Singh, Ashwini Kumar, Azhagurajan Mukkannan PII:

S0925-8388(16)32244-7

DOI:

10.1016/j.jallcom.2016.07.209

Reference:

JALCOM 38378

To appear in:

Journal of Alloys and Compounds

Received Date: 13 May 2016 Revised Date:

5 July 2016

Accepted Date: 19 July 2016

Please cite this article as: G. Venugopal, J.C. Veetil, N. Raghavan, V. Singh, A. Kumar, A. Mukkannan, Nano-dynamic mechanical and thermal responses of single-walled carbon nanotubes reinforced polymer nanocomposite thin-films, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.07.209. 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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Nano-dynamic mechanical and thermal responses of single-walled carbon nanotubes reinforced polymer nanocomposite thin-films *

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Gunasekaran Venugopal1,2 , Jipsa Chelora Veetil1,3, Nivea Raghavan1, Varu Singh4, Ashwini Kumar5, Azhagurajan Mukkannan6

Nanomaterials Research Lab (NmRL), Department of Nanosciences and Technology,

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1

Karunya University, Coimbatore - 641 114, Tamil Nadu, India. Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577, Japan. 3

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COSDAF, Department of Physics and Materials science, City University of Hong Kong, Hong Kong Special Administrative Region, PR China.

4

New Industry Creation Hatchery Center, Tohoku University, 6-6-10 Aramaki Aza Aoba, Aoba-ku, Sendai 980-8579 Japan.

Department of Mechanical and Aerospace Engineering, School of Engineering,

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Tohoku University, Sendai, Japan.

6

Frontier Research Institute for Interdisciplinary Sciences (FRIS), Tohoku University,

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6-3 Aoba, Sendai 980-8578, Japan.

Corresponding author:

E-mail address: [email protected] Tel- +91-98947 89648

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Abstract In recent years, nanocomposites are of great interest in various engineering applications such as lightweight, high-strength low-density, anti-static coatings and corrosion resistant

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coatings due to their excellent mechanical, electrical and chemical properties. In this study, we demonstrated the preparation of single-walled carbon nanotubes (SWCNT) reinforced chitosan polymer nanocomposite thinfilms via solution-casting method and examined their nano-dynamic

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mechanical (nano-DMA) and thermal behaviors. The surface morphology and crystalline nature of nanocomposite were investigated by using scanning electron microscopy, transmission

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electron microscopy and X-ray diffraction techniques. The presence of functional groups and molecular interactions between SWCNT and chitosan were further studied by using Fourier transform infrared spectroscopy. From the thermo-gravimetric study, it is observed that the SWCNT reinforced chitosan nanocomposite shows a superior heat-barrier property and an

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improved thermal stability. Nano-DMA results revealed that the SWCNT reinforced chitosan thinfilm has superior storage modulus of ~8 GPa indicating the storage capacity of elastic energy in deformation process. The observed loss modulus further confirms the viscous behavior of

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nanocomposite. Our experimental results suggested that SWCNT reinforced chitosan based nanocomposites could be a potential candidate for the various mechanical engineering and

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industrial thermal applications.

Keywords: nano-DMA; SWCNT reinforced chitosan nanocomposite thin-film, Storage modulus, Thermal property.

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1. Introduction Biodegradable polymers are of enormous interest due to its excellent properties and represent a potential alternative to non-biodegradable materials [1-3]. Applications of bio-

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degradable polymers have been limited to certain extent due to its disadvantages such as high hydrophobicity, poor mechanical and thermal properties etc., [4-7]. Modification is necessary to enhance the properties of the polymer and considerable efforts have been made to improve the

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properties of the polymer with the help of nanotechnology [8]. Nanocomposite formation is one of the effective methods to improve the performance of the polymer material by incorporation of

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nanomaterial into the polymer matrix [8, 9]. Polymer nanocomposite contains both organic polymer matrix and inorganic nano-fillers like clay, metal nanoparticles, CNTs (carbon nano tubes) etc. Better morphologies and properties are expected from the polymer nanocomposite than the bare polymers due to the excellent properties of nano-fillers used [10, 11]. Proper

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dispersion, good alignment of the filler, large aspect-ratio and solvent selection are of prime importance while improving properties of nanocomposites [12, 13]. Chitosan is one of the abundant bio-degradable polymers available in nature which is

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obtained by deacetylation of chitin. Chitosan is chemically known as poly-b(1, 4)-2-aminodeoxy-D-glucose and it consists of amine and hydroxyl groups. Hydrophilic groups (–NH2 and –

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OH) of chitosan allow it to dissolve in an acetic acid aqueous solution [14-19]. Chitosan has wide applications due to its unique properties such as non-toxicity, biocompatibility, biodegradability, antimicrobial ability, film forming ability etc. In addition, chitosan has shown its potential in the field of biotechnology, biomedicine, agriculture, packaging and cosmetics. Low-thermal and mechanical stability of chitosan polymer is a major drawback and it may be improved by the addition of fillers like carbon nanotubes [20-24].

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Carbon nanotubes are known as ideal nano-fillers for polymer composites due to their excellent mechanical, thermal and electrical properties [25-27]. Carbon nanotubes can be of two types; single-walled carbon nanotube (SWCNT) and multi-walled carbon nanotube (MWCNT).

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A single-walled carbon nanotube has tensile strength of 45 GPa which is very much higher than a steel alloy. Another interesting fact is that the chitosan is an excellent dispersing agent of single-walled

carbon

nanotube

[28].

Covalent

functionalization

and

non-covalent

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functionalization of CNTs are common for better solubilization or to achieve better compatibility with polymers [25]. However, the covalent functionalization treatments may create defects in

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the surface and it affects the structure of the material. Non-covalent functionalization does not affect the structure of the material and it enhances interfacial properties of nanotubes [29-31]. Dynamic-mechanical analysis can be used to determine mechanical properties of bulk materials. However, in case of nano/micron-sized materials, a nanoscale dynamic mechanical analysis

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(nano-DMA) can be used to investigate their mechanical properties (like storage modulus, loss modulus) in smaller region very accurately. Recently, very few studies have been reported on MWCNT-polymer composite, CNT-filled polycarbonate composite materials and their

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mechanical properties [32]. However, there is no report yet about nano-DMA studies on SWCNT-polymer composites.

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Hence, in this present work, we used SWCNT as filler in chitosan polymer matrix to

form chitosan-SWCNT composite thinfilm and studied their nano-dynamic mechanical and thermal responses. Significant improvements in thermal and nano-dynamic mechanical properties were observed in comparison with bare polymer and SWCNT. These results may open up a new research direction to engineer the SWCNT based polymer nanocomposites with better thermal and mechanical properties.

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2. Experimental 2.1 Materials and methods Chitosan (91.3% de-acetylated) was purchased from HI-Media Laboratories and SWCNT

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was purchased from sigma Aldrich. Diameter of single-walled carbon nanotube was in the range of 1.2 to 1.5 nm. Chitosan was dissolved in distilled water with the help of acetic acid. Fourier Transform Infrared Spectroscopy (FT-IR) was used to find the changes in the structure and

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surface chemical bonding/molecular interactions. The surface morphologies of samples were analyzed by using a field emission scanning electron microscope (FESEM, Quanta. FEG 250).

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Elemental composition of the samples was examined by using an energy dispersive X-ray analyzer (EDAX) which was coupled to scanning electron microscope. The thermal stability and purity of the SWCNT samples were assessed by using TGA and DTA techniques (Model: Shimadzu DTG-60H). The specimens were scanned under continuous air flow and temperature

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was maintained from 25 to 900 0C at a ramp rate of 10 0C/min. Mechanical analysis was carried out using nano-indenter facility at Indian Institute of Technology-Bombay, India. 2.2. Preparation of Chitosan-SWCNT nanocomposite thin-film

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Chitosan-SWCNT was prepared by solvent casting method. Briefly, the polymer “chitosan” was dissolved in distilled water with help of acetic acid (1 wt. %) and the solution

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was stirred for 24 hrs at room temperature. SWCNTs were added into the chitosan solution and the mixture was sonicated for 2 hrs. The mixture was then poured into a glass dish and dried in a vacuum oven (at 70 0C) to obtain a uniform film [33]. 2.3. Instrumentation: nano-DMA analysis For the chitosan film and nanocomposite film, the dynamic-mechanical analysis was performed using nano-indenter. Nano-DMA is a powerful method which can be used to measure mechanical properties of the sample in nanoscale. It functions with CMX control algorithms and 5

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provides a continuous measurement of mechanical properties (such as hardness, storage modulus, loss modulus, complex modulus) as a function of depth into a sample’s surface [34]. Nano-indentation system is capable of in-situ SPM imaging and it can use the same probe to scan

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the sample surface immediately before and/or after the experiment permits for precise placement of the test as well as near-instant observation of events or sample recovery. It is also equipped with a standard maximum force upto 10 µN and a noise floor of less than 30 µN and it can cover

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a large range of sample testing possibilities. 3. Results and Discussion

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The surface morphology has been investigated by using FE-SEM, EDAX and TEM analyses which are shown in Fig. 1. The FE-SEM images [Fig. 1 (a, b)] show a clear polymer aggregation of chitosan and CNT reinforcement into polymer matrix. TEM image [Fig. 1(d)] reveals a clear image of SWCNT reinforcement in polymer matrix. However, in order to study

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the composition of composite, an energy dispersive x-ray (EDAX) analysis was carried out. EDAX is an analytical technique which is normally used for the elemental analysis of the sample. EDAX spectra of CNT reinforced chitosan composite is shown in Fig. 1(c). The

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observation of a sharp carbon peak further confirms the existence of CNT in the composite. The structural characterization of chitosan, SWCNT and composite was carried out by

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using x-ray diffraction technique and their results are presented in Fig. 2. We observed the characteristic peaks at 25.2o for SWCNT and at 12.8°, 22.4° for chitosan. However, in case of CNT reinforced polymer, the characteristic peak was shifted to 20.9°. This peak shift and the reduction in x-ray intensity are further confirming the formation of nanocomposite [35]. The nanocomposite thinfilm used in nano-indentation experiment is shown in Fig. 3. The existence of functional groups in chitosan, CNT and chitosan-CNT composite was further identified by using FT-IR spectroscopy and their results are shown in Fig. 4. FT-IR 6

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spectra collect the spectral data in a wide spectral range. In Figure 4(a), for the case of chitosan, we observed an absorption band at 1157 cm-1 due to asymmetric stretching of the C-O-C bridge [β (1, 4) glyosidic bonds]. The peak at 1563 cm-1 is attributed to N-H bending of NH2 group and

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the peak at 2874 cm-1is due to C-H stretching and bending of CH3. The existence of -C=O groups were observed at 1680 cm-1 and CH3 symmetrical angular deformation results in the peak at 1376 cm-1 [17, 36, 37]. The characteristic peak of SWCNT at 1450 cm-1 was observed which

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was attributed due to aromatic C=C stretching. In figure 4(b), the spectra of SWCNT reinforced chitosan is shown where it can be seen that the C-O stretching and N-H bending bands are

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shifted to lower frequency. This shift is mainly due to the strong interaction between the hydrophobic group of chitosan (–COCH3) and the surface of CNT [23, 31] which further confirms the formation of SWCNT-chitosan nanocomposite.

The thermo-gravimetric (TGA) profiles of chitosan and SWCNT-chitosan nanocomposite

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are presented in Fig. 5(a). The thermograms were recorded from 25 to 800 ºC in nitrogen atmosphere. We observed a non-oxidative degradation which was occurred in two steps and the range was below 200 ºC and between 200-400 ºC. The first step weight loss was started at 100

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ºC which is due to the absorbed water content in the chitosan and second step weight loss was occurred at 600 ºC which is attributed due to the degradation and deacetylation of chitosan.

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Addition of SWCNT to the chitosan matrix has improved the thermal properties of the composite. Hence, in case of composite, the weight loss was started at 100 ºC due to the elimination of water. The overall weight loss was 85.3 % for pure chitosan and 34.1 % for nanocomposite respectively up to 800 ºC. So, it can be concluded that an excellent heat barrier property of SWCNT resulting an improved thermal stability of the nanocomposite [36, 38]. The differential thermal analysis (DTA) curve [Figure 5(b)] of nanocomposite shows the existence of

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a well resolved exothermic peak at 476 ºC which is related to the single decomposition reaction indicating the low weight loss and high stability behavior of CNT reinforced chitosan nanocomposite.

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Next, we present the results of nano-DMA analysis in figure 6 which was performed at room temperature (27 ºC). Figure 6(a) represents the frequency sweep test results of chitosan polymer film. Storage Modulus (E’) and Loss Modulus (E’’) were plotted against frequency.

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From the results, it is observed that the elastic property of chitosan polymer has an ability to revert back to original shapes as the storage modulus and loss modulus are approximately equal.

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During deformation, the stored elastic energy and loss energy released as heat which are approximately equal, confirming the perfect elastic behavior of chitosan polymer. Figure 6(b) shows the frequency sweep test results of SWCNT reinforced chitosan nanocomposite film. The results reveal a high storage modulus of ~8 GPa for composite film

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than bare chitosan film (1.8 GPa) and the value of loss modulus confirms the viscous behavior of nanocomposite film. Higher storage modulus further indicates that high energy can be stored elastically during the deformation. Higher storage and loss modulus are more pronounced at

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lower frequencies. This could be attributed due to the interaction between the CNT and polymer resulting the formation of network structure in the polymer nanocomposite which is already

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evidently seen through our FT-IR analysis. Our results are reasonably comparable to the previous reports on CNT-polymer nanocomposites [39-41].

4. Conclusion

In conclusion, we successfully investigated nano-DMA

and

thermal

responses

of

SWCNT reinforced chitosan nanocomposite thinfilm prepared using solvent casting method. The SEM, EDAX and HR-TEM analyses revealed the well-defined surface morphology of chitosan 8

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polymer and nanocomposite. The XRD and FT-IR analyses further confirmed the crystalline nature,

molecular

interactions

and

functional

groups

present

in

nanocomposite.

Thermogravimetric analyses have shown that SWCNT reinforced chitosan nanocomposite

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exhibits a superior hear barrier property than the pure chitosan film. In addition, nano-DMA studies further confirmed that the SWCNT reinforced chitosan thinfilm exhibits superior storage modulus of ~ 8 GPa indicating the storage capacity of elastic energy in deformation process. The

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observed loss modulus has confirmed the viscous behavior of nanocomposite. Our results will be

ACKNOWLEDGEMENTS

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a fortification for further research applications of CNT reinforced polymer nanocomposites.

The authors express their profound thanks to DST-Nanomission, New Delhi and Management of Karunya University for providing financial support and they extend their thanks

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to technical staffs at Center for Research in Nanotechnology (CRN), Karunya University for their help in sample characterization. The author (G.V) acknowledges a continuous encouragement and support from the Vice-chancellor and the Registrar of Karunya University,

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Coimbatore, India. The authors gratefully acknowledge the Nano-Indenter central facility at IIT-

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B, Mumbai, India, for taking nano-DMA test for our samples in time.

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Figure & caption

Figure 1. SEM image of (a) chitosan polymer (b) SWCNT reinforced chitosan composite and (c) EDAX spectra of SWCNT reinforced chitosan composite (d) TEM image of SWCNT

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reinforced chitosan composite.

Figure 2. XRD spectra of chitosan polymer, SWCNT and SWCNT reinforced chitosan

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composite.

Figure 3. Nanocomposite thinfilm used in nano-DMA experiment.

Figure 4. FT-IR spectra of (a) chitosan and SWCNT (b) SWCNT reinforced chitosan nanocomposite Figure 5. (a) TGA thermogram of chitosan and nanocomposite (b) DTA thermogram of nanocomposite.

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Figure 6. Nano-DMA analysis of (a) chitosan polymer film (b) SWCNT reinforced chitosan nanocomposite thinfilm.

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ACCEPTED MANUSCRIPT Highlights
SWCNT reinforced polymer nanocomposite thinfilms prepared via solution-casting method.
Surface morphology and crystalline nature of composite thin film studied using SEM, HR-TEM, XRD and FT-IR.

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Nano-dynamic mechanical properties of nanocomposite film were measured by nano-indentation.
Thermal properties of nanocomposite was analyzed using TG/DTA.
SWCNT-polymer nanocomposite exhibits superior heat barrier property and improved

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thermal stability.