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Synthesis and characterisation of nanocellulose-based polyaniline conducting films
Accepted Manuscript Synthesis and characterization of nanocellulose-based polyaniline conducting films D.Y. Liu, G.X. Sui, D. Bhattacharyya PII: DOI: Reference:
S0266-3538(14)00140-7 http://dx.doi.org/10.1016/j.compscitech.2014.05.001 CSTE 5801
To appear in:
Composites Science and Technology
Received Date: Revised Date: Accepted Date:
27 December 2013 21 April 2014 1 May 2014
Please cite this article as: Liu, D.Y., Sui, G.X., Bhattacharyya, D., Synthesis and characterization of nanocellulosebased polyaniline conducting films, Composites Science and Technology (2014), doi: http://dx.doi.org/10.1016/ j.compscitech.2014.05.001
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.
Synthesis and characterization of nanocellulose-based polyaniline conducting films D. Y. Liu*1, G. X. Sui1, D. Bhattacharyya2 1
2
Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China
Centre for Advanced Composite Materials, Department of Mechanical Engineering, The University of Auckland, Auckland, New Zealand, Private Bag 92019 *Email: [email protected] *Phone number: 0086 24 83970093
Abstract In this paper, a relatively new concept of using nanocellulose as matrix material in a composite system has been explored. The functionality of the composite has been enhanced by using polyaniline (PANI) as a functional component. These tunable electrically conducting biocomposites have potential applications in anti-static, electromagnetic interference shielding, sensors, electrodes, and storage devices. Nanocellulose was extracted by hydrolysing bleached flax yarn with sulphuric acid (60 wt. %) at 55 ˚C for 1 h under vigorous stirring. Thin composite films of nanocellulose with PANI inclusions at different loadings were manufactured using in-situ polymerisation where aniline-HCl was polymerised with ammonium peroxydisulfate (APS) as oxidant in aqueous nanocellulose suspension. Thin composite films showed improved combination of flexibility and conductivity. These films could be bent by 180º without breaking. The dependence of electrical conductivity on the concentration of polyaniline (0, 10, 20, 30 wt. %), was investigated. It was found that the conductivity of a film increased significantly with the increase in PANI content from 10 to 30%. The conductivity of the nanocomposite containing 30 wt % reached 1.9 × 10 -2 S/cm, which
1
shows promise in the application of paper-based sensors, flexible electrode and conducting adhesives. The composite film showed improved thermal stability above 300 ºC by 15 % less weight loss at 500 ºC compared to pure nanocellulose films. The morphologies, microstructures, thermal stability properties of the nanocomposite films were also investigated. Keywords: A. Nano composites, A. Polymers, B. Electrical properties, B. Flexible composites, B. Thermal properties 1 Introduction There is a growing interest in developing bio-based products derived from renewable sources and innovative processing technologies that can reduce the dependence on fossil fuels and encourage movement towards a sustainable material basis [1-2]. Cellulose is the most abundant natural bioresource in the world, and the annual biomass production is about 1 trillion tons [3]. Cellulose fibres have been widely used due to their sustainability and good mechanical properties. Nanocellulose fibrils has recently gained attention from researchers and industry because it has high tensile modulus (138 GPa), which is higher than that of the S-glass (86-90 GPa) and comparable to Kevlar (131 GPa), rendering them good reinforcement for natural and synthetic polymer matrices [46]. Cellulose nanowhiskers, with size ranging from a few to tens of nanometres in one dimension, have some unique properties, including renewable resource, excellent mechanical properties, high specific surface area, biodegradability, and biocompatibility [7]. Moreover, cellulose, rich in hydroxyl group, has good affinity with a variety of polymers, including conducting polymers [8-11]. Cellulose nanowhiskers can be prepared from a variety of sources, such as wood pulp, plant fibres (e.g. hemp, sisal, flax, ramie, jute, algae)[12], microbial (Acetobacter Xylinum)[13], sea creatures
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(tunicate)[14], fruits (banana and grape skin)[15], and even agricultural products (e.g. cornhusk, wheat straw)[16], which makes them more attractive and applicable. There are mainly three methods for producing nanocellulose, namely, chemical acid hydrolysis, chemical treatment in combination with mechanical refining and enzymatic method. Polyaniline, as one of intrinsically conducting polymers, is a very promising material because of its ease of synthesis, low cost monomer, tunable properties, and high environmental stability. It has potential applications in anti-static and electromagnetic interference shielding, sensors, electrodes, and batteries fields. However, it is very difficult to produce the neat polyaniline films because of its infusibility, poor mechanical properties, and poor solubility in all available solvents except doping with a suitable dopant or modifying the monomer [17]. Cellulose has been recognised as good matrix/substrate for biodegradable batteries, sensors, and actuators [11, 18, 19]. The polymer composites containing polyaniline are mostly investigated for blending with commercial polymers in order to obtain improved processability and fairly good mechanical properties together with good conductivity for practical applications [2022]. However, the electrical conductivity of the composite was not improved effectively; especially the composites became more brittle due to addition of polyaniline. Cellulose fibers have been used to reinforce brittle conducting polymers, such us polypyrrole (PPy), polyaniline (PANI) and polythiophene (PTP) for energy storage applications [23]. Cellulose and PPy all polymer composite battery with high reported charge capacities and charging rates [11]. Recently, Polyaniline-based aqueous suspensions containing polyaniline (PANi) contents ranging between 5 and 80 wt.% have been successfully developed, the composite films showed high mechanical strength of 178 MPa, and a
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percolation threshold of electrical conductivity of 4.57 vol.% of PANi content[24]. Compared with PPy and PTP, PANI has relatively high theoretical specific capacity [23]. Combining cellulose nanowhiskers and polyaniline is promising for developing green functional polymer nanocomposites. Recently, polyaniline modified and cellulose nanowhisker reinforced smart composites have been reported [25]. In this research, nanocellulose was used as the matrix and polyaniline was added as conducting component to produce the nanocellulose-based flexible and electrically conducting composite films. Aqueous nanocellulose suspension has good film formability because of strong hydrogen bond between the whiskers, which facilitates the film forming ability of the composites. The combination of nanocellulose and polyaniline gives good conductivity and excellent mechanical properties to the nanocomposites. The composites, combining good mechanical properties of cellulose nanowhiskers and conductivity of polyaniline, have great potential in anti-corrosion coatings, conducting adhesives, anti-static and electromagnetic interference shielding materials, biodegradable smart sensors/actuators, and batteries. 2 Materials and testing methods 2.1Materials Bleached flax yarns were purchased from Jayashree Textiles, Kolkata, India. Sulphuric acid with concentration of 95-97 %, was supplied by Merck KGaA, Darmstadt, Germany. Sodium hydroxide was purchased from Ajax Finechem Pty Ltd., Taren Point, Australia. Aniline hydrochloride and Ammonium persulfate (APS) were bought from Sigma-Aldrich, Inc, St Louis, USA. The chemicals were used as received without further purification. 2.2 Preparation of cellulose nanowhiskers
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Cellulose nanowhiskers were prepared by acid hydrolysis, following a procedure reported earlier [5]. The oven dried flax yarns were hydrolysed in sulphuric acid solution (60 wt.%) at 55°C for 1 h. Then the suspension was centrifuged and diluted with D. I. water, which was followed by neutralisation with NaOH solution (10 wt.%) to remove free acid. The suspension was freeze-dried prior to being redispersed in D. I. water. 2.3 Preparation of nanocellulose/polyaniline composite films 0.2g aniline HCl powder and 0.44 g APS were dissolved in distilled water. Aniline HCl solution was added into cellulose nanowhiskers aqueous suspension (0.5 wt%) followed by dropwise addition of APS. The weight ratio of aniline HCl monomer and nanocellulose is 1:9, 2:8, and 3:7. The mixture was kept stirred for 24 hr at room temperature and then was diluted and washed with D. I. water under centrifugation until the supernatant was clear. The mixture was cast directly onto petri dishes. The composite film was peeled off after water fully evaporated at room temperature. 2.4 Testing methods The morphology of the nanocomposites was studied using field emission scanning electron microscopy (FE-SEM, FEI XL30s) with an accelerating voltage of 5 kV. Fourier transform infrared – attenuated total reflectance (FTIR, ATR-FTIR, Nicolet 8700, USA) spectroscopy was used to analyse the chemical structures of the specimens. All spectra were collected with 4 cm-1 wave number resolution after 64 continuous scans at a wavelength range of 4000 to 600 cm-1. UV-Vis absorption analysis was recorded with UV-Vis scanning spectrophotometer (UV-2101 PC, Shimadzu, Japan) on PANI solution by using N, N-dimethylformamide (DMF) as solvent. Thermal stability analysis was carried out with thermogravimetric analysis (TGA, Q5000, USA). Samples
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were heated in open platinum pan from room temperature to 600 ºC, under a nitrogen atmosphere in order to avoid thermoxidative degradation due to oxygen, at a heating rate of 10 ºC/min.
A four-probe conductivity apparatus (Keithley 6517, USA) was
used for electrical conductivity test. Storage modulus was tested on dynamic thermal mechanical analysis (DMTA, Q800, USA) at a heating rate of 3 ºC / min. The dynamic storage was determined at a constant frequency of 1 Hz as a function of temperature from 20 to 150 ºC. 3 Results and discussions 3.1Morphology Flexible nanocellulose-based polyaniline electrically conductive films are obtained by the in-situ polymerisation of aniline HCl onto cellulose nanowhiskers. The composite has very good film formability by easily casting the resulted aqueous mixtures onto petri dish, which is not the case for pure polyaniline powder. This method is also environment friendly without any chemical solvent being involved. The resulting composite film appears in dark green colour and has high flexibility so that it can be bent by 180 º without breaking, Figure 1. Figure 2 shows the typical morphology of nanocellulose film and nanocellulose/PANI composite films. The nanocellulose film is composed of randomly oriented cellulose nanowhiskers with diameters varying from a few to around 20 nm. The composites have the similar morphology; however, the surface is rougher and the diameters of the whiskers are larger with an increase in PANI content due to the polymerisation of polyaniline on the whisker surface. The whiskers are well bonded with each other, which indicate the existence of strong hydrogen bonding in spite of surface coating of PANI powder. No detectable polyaniline precipitation is observed from the top surface
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of the films with 10 and 20 wt.% PANI, indicating that tiny polyaniline has been preferably polymerised on the surfaces of cellulose nanowhiskers. Individual cellulose nanowhiskers could not be distinguished and aggregates of PANI powder can be seen on the surface of the composite film with 30 wt. % PANI. Fracture surfaces of pure nanocellulose and nanocellulose/PANI composite films are shown in Figure 3. Nanocellulose film shows layered structure due to the strong hydrogen bonding among the whiskers. The composite film shows a similar layered structure (contributing to good flexibility) and white dots are clearly seen at the fractured ends of the whiskers, indicating good bonding between cellulose nanowhiskers and PANI powder. 3.2 Chemical structure The FTIR spectra of nanocellulose, PANI powders, and nanocellulose/PANI composite films are shown in Figure 4. The characteristic peaks of PANI are at1582, 1492, 1304, 1152, and 818 cm-1 are corresponding to C=C stretching peak of quinone ring, benzene ring, and bending vibration of C-H (N=quinoid=N), and C-H( 1, 4 substitute, out of plane), respectively [26, 27]. The peak at 1582 cm-1 confirms the presence of a pronated imine function. For nanocellulose film, the bands at 3334, 2897 cm-1 are related to stretching of hydroxyl groups and C-H bond. The band at 1642 results from the H-O-H bending of the absorbed water. The peak at 1160 cm-1 corresponds to the C-O stretching [28]. For the spectra of composite film, three peaks are appearently different from those of the components. Additional peaks at 1575 and 1495cm-1 belong to PANI, denoted by line 1 and 2, besides the characteristic peaks from cellulose. The peak intensity at 818 cm-1 increases with an increase in PANI content, proving the presence of PANI on surfaces of cellulose nanowhiskers.
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UV-vis absorption spectra for the composite show two absorption bands at 280-400 nm and 500-700 nm. These peaks represent benzenoid segment π- π* electron transition and quinoid segment π - π* electron transition, respectively [29]. The absorption bands of PANI depended on the pH of the solution. The first absorption bands are flat and distorted due to the overlapping of the two peaks for the HCl-doped PANI solution. 3.3 Physical structure X-ray diffraction patterns of nanocellulose based composite films are shown in Figure 5. The peaks are typical for cellulose I and show the highest intensity at 2 theta = 22.5˚, accompanied by two close weak peak in the region 2 theta = 14.6 and 16.4˚ . However, no characteristic peaks of PANI at 2 theta =25˚ is seen due to its low crystalline level and content. This result is different from the distinct crystalline structure in polyaniline/iron oxide and polyaniline/carbon nanotube nanocomposites [30,31]. The reason for the low crystallinity of PANI formed on nanocellulose is not understood yet. There is not much difference for different content of PANI except that the peak intensity of nanocellulose at 2 theta = 22.5˚ decrease with an increase in PANI concentration from 10% to 30%. 3.4 Electrical conductivity The electrical conductivity of polyaniline relies on the degree of doping, oxidation state, particle morphology, crystallinity, interior intrachain interactions, molecular weight, etc [32, 33]. The electrical conductivities of the composite films are 2.8×10-6S/cm, 1.3×10-2 S/cm and 1.9×10-2 S/cm for the composite containing 10, 20, and 30 wt.% PANI, respectively. The conductivity has increased significantly compared to pure cellulose
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nanowhisker film (practically an insulator). The significant increase in conductivity with the increase in PANI content from 10 to 20 wt.% PANI probably results from the dense coverage of tiny PANI particles on the surface of cellulose nanowhsikers and the formation of a connected network. The significantly high conductivity of the composite attributes to the homogenous formation of PANI layer on the surfaces of cellulose nanowhiskers. However, the conductive properties can be improved by the efficient doping and crystallinity enhancement of polyaniline. 3.5 Thermal stability The thermogravimetric curves of pure nanocellulose film, pure PANI powder, and nanocellulose /PANI composite films with PANI contents of 0 to 30 wt. % are illustrated in Figure 6. The slight weight loss of all materials results from the water evaporation at around 100 °C. As seen from the curve, pure cellulose nanowhisker film appears to be decomposed after 300 °C, which corresponds to pyrolysis of cellulose [34]. For the PANI powder, there are three major stages for weight loss, around 100, 240-350, and 420–550°C, which can be attributed to the removal of moisture, HCl dopant, and the degradation of the PANI molecules, respectively. This result is similar to that of the HCl-doped Emerald Salt(ES)-form PANI powder reported by Wei et al. [35]. The composite film with 30 wt.% PANI shows higher thermal stability at temperature above 500 °C by 15% weight loss, compared to pure cellulose nanowhiskers. The DSC curves of the composite films are shown in Figure 7. Pure nanocellulose film exhibits a sharp melting peak at 211 °C, indicating the crystalline structure
of
nanocellulose. However,This temperature is much lower than that of microcrystalline cellulose which situated at 350°C[36]. The reason is propbably due to smaller size,
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lower crystallinity, lower polymerization degree of nanocellulose. PANI showed two peaks during the DSC measurement. The first exothermic peak between 160 and 240°C is possibly generated by the crosslinking reaction resulting from a coupling of two neighbouring –N=Q=N- groups (where Q represents the quinoid ring ) to give two – NH-B-NH- groups (where B represents the benzenoid ring ) through a line of the N with its neighbouring quinoid ring[37]. The endothermic peak at 240 - 350 °C could be caused by the evaporation of HCl dopant, which is in aggreement of TGA analysis. A significant difference is seen from pure nanocellulose film and nanocellulose containing PANI composite films. The absence of sharp melting peak of nanocellulose can be seen with the introduction of PANI coating on nanocellulose surface. 3.6 Thermomechanical properties The composite film shows good flexibility and high rigidity due to the existence nanocellulose. Cellulose nanopaper shows high tensile strength and modulus of over 200 MPa and 13 GPa, respectively [38]. Storage modulus of nanocellulose film exhibited slow decrease with an increase in temperature. The storage modulus at low temperature has a value over 11GPa. The modulus of the composite are 7.8, 6.1, and 5.4 GPa for the film containing 10, 20, and 30 wt.% PANI, respectively, which is much higher than that of pure PANI (0.9±0.2 GPa) and recently reported value at the same composition ratio [24,39]. The modulus of the composites decreased with an increase in PANI content. Thus the composites containing more than 30% PANI are not useful for conducting films due to their brittleness. The mechanical properties are mainly contributed by interconnected network of nanocellulose whiskers. The existence of PANI particles weakened the hydrogen bond among cellulose nanowhiskers. 4 Conclusions
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Flexible and electrically conductive nanocellulose-based polyaniline(PANI) ecofriendly composites have been fabricated by in-situ oxidative polymerization of aniline hydrochloride in stable aqueous nanocellulose suspension by using ammonium peroxydisulfate (APS) as oxidant. PANI particles are formed on the surfaces of cellulose nanowhiskers and under SEM observation, no free polyaniline particles are observed on the top surfaces of the composite films with 10 and 20 wt.% PANI. However, aggregates of PANI particles are seen on the surface of the composite film with 30wt.% PANI. The thin composite film shows good flexibility and the film with thickness of 50 μm can be bent by 180 without breaking. The electrical conductivity of as prepared composite films significantly increases from 10-6 to 10-2 S/cm when the content of PANI increases from 10 to 20 wt. %, and retains the same order of magnitude when the content of PANI increases from 20 to 30 wt.%. The composite film exhibits improved thermal stability above 300 ºC the film containing 30 wt.% PANI shows only 15 % less weight loss at 500 ºC. The flexible and conductive biodegradable nanocellulose-based composites are promising for using in sensors, batteries and conductive adhesives. Acknowledgements The authors would like to thank the Ministry of Research and Innovation NZ for financial support, late A/Prof. A. J. Easteal for the experimental discussion, and Mr. S. Crump and Mr. S. Cawley for their assistance in chemical and electrical measurements. References [1] Pandey JK, Kumar AP, Misra M, Mohanty AK, Drzal LT, Singh R P. Recent advances in biodegradable nanocomposites. J Nanosci Nanotechn. 2005;5(4): 497526.
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Figure captions: Figure 1. Optical images of nanocellulose based composite film containing 20 wt.% PANI after bending by 180º, with thickness of 50μm Figure 2. SEM images of nanocellulose based composite films containing (a)0, (b)10, (c)20, and 30 wt.% PANI Figure 3 Fracture surface of nanocellulose-based composite films containing (a)0, (b)10, (c)20, and (d) 30 wt.% PANI Figure 4. FTIR spectra of pure nanocellulose film and nanocellulose based composites fillms containing (a)0, (b)10, (c)20 and (d) 30% PANI, and (e) 100% PANI powder Figure 5. UV-Vis spectra of nanocellulose based composites fillms containing 10, 20 and 30% PANI Figure 6. XRD patterns of nanocellulose based composite films with 10, 20, 30% PANI Figure 7. TGA curves of pure nanocellulose fillm and nanocellulose based composite films with 10, 20, 30% PANI , 100% PANI shown for comparision Figure8. DSC curves of pure nanocellulose fillm and nanocellulose based composite films with 10, 20, 30% PANI , 100% PANI shown for comparision Figure 9. DMA curves of pure nanocellulose fillm and nanocellulose based composite films 10, 20, and 30% PANI
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S0266-3538(14)00140-7 http://dx.doi.org/10.1016/j.compscitech.2014.05.001 CSTE 5801
To appear in:
Composites Science and Technology
Received Date: Revised Date: Accepted Date:
27 December 2013 21 April 2014 1 May 2014
Please cite this article as: Liu, D.Y., Sui, G.X., Bhattacharyya, D., Synthesis and characterization of nanocellulosebased polyaniline conducting films, Composites Science and Technology (2014), doi: http://dx.doi.org/10.1016/ j.compscitech.2014.05.001
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.
Synthesis and characterization of nanocellulose-based polyaniline conducting films D. Y. Liu*1, G. X. Sui1, D. Bhattacharyya2 1
2
Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China
Centre for Advanced Composite Materials, Department of Mechanical Engineering, The University of Auckland, Auckland, New Zealand, Private Bag 92019 *Email: [email protected] *Phone number: 0086 24 83970093
Abstract In this paper, a relatively new concept of using nanocellulose as matrix material in a composite system has been explored. The functionality of the composite has been enhanced by using polyaniline (PANI) as a functional component. These tunable electrically conducting biocomposites have potential applications in anti-static, electromagnetic interference shielding, sensors, electrodes, and storage devices. Nanocellulose was extracted by hydrolysing bleached flax yarn with sulphuric acid (60 wt. %) at 55 ˚C for 1 h under vigorous stirring. Thin composite films of nanocellulose with PANI inclusions at different loadings were manufactured using in-situ polymerisation where aniline-HCl was polymerised with ammonium peroxydisulfate (APS) as oxidant in aqueous nanocellulose suspension. Thin composite films showed improved combination of flexibility and conductivity. These films could be bent by 180º without breaking. The dependence of electrical conductivity on the concentration of polyaniline (0, 10, 20, 30 wt. %), was investigated. It was found that the conductivity of a film increased significantly with the increase in PANI content from 10 to 30%. The conductivity of the nanocomposite containing 30 wt % reached 1.9 × 10 -2 S/cm, which
1
shows promise in the application of paper-based sensors, flexible electrode and conducting adhesives. The composite film showed improved thermal stability above 300 ºC by 15 % less weight loss at 500 ºC compared to pure nanocellulose films. The morphologies, microstructures, thermal stability properties of the nanocomposite films were also investigated. Keywords: A. Nano composites, A. Polymers, B. Electrical properties, B. Flexible composites, B. Thermal properties 1 Introduction There is a growing interest in developing bio-based products derived from renewable sources and innovative processing technologies that can reduce the dependence on fossil fuels and encourage movement towards a sustainable material basis [1-2]. Cellulose is the most abundant natural bioresource in the world, and the annual biomass production is about 1 trillion tons [3]. Cellulose fibres have been widely used due to their sustainability and good mechanical properties. Nanocellulose fibrils has recently gained attention from researchers and industry because it has high tensile modulus (138 GPa), which is higher than that of the S-glass (86-90 GPa) and comparable to Kevlar (131 GPa), rendering them good reinforcement for natural and synthetic polymer matrices [46]. Cellulose nanowhiskers, with size ranging from a few to tens of nanometres in one dimension, have some unique properties, including renewable resource, excellent mechanical properties, high specific surface area, biodegradability, and biocompatibility [7]. Moreover, cellulose, rich in hydroxyl group, has good affinity with a variety of polymers, including conducting polymers [8-11]. Cellulose nanowhiskers can be prepared from a variety of sources, such as wood pulp, plant fibres (e.g. hemp, sisal, flax, ramie, jute, algae)[12], microbial (Acetobacter Xylinum)[13], sea creatures
2
(tunicate)[14], fruits (banana and grape skin)[15], and even agricultural products (e.g. cornhusk, wheat straw)[16], which makes them more attractive and applicable. There are mainly three methods for producing nanocellulose, namely, chemical acid hydrolysis, chemical treatment in combination with mechanical refining and enzymatic method. Polyaniline, as one of intrinsically conducting polymers, is a very promising material because of its ease of synthesis, low cost monomer, tunable properties, and high environmental stability. It has potential applications in anti-static and electromagnetic interference shielding, sensors, electrodes, and batteries fields. However, it is very difficult to produce the neat polyaniline films because of its infusibility, poor mechanical properties, and poor solubility in all available solvents except doping with a suitable dopant or modifying the monomer [17]. Cellulose has been recognised as good matrix/substrate for biodegradable batteries, sensors, and actuators [11, 18, 19]. The polymer composites containing polyaniline are mostly investigated for blending with commercial polymers in order to obtain improved processability and fairly good mechanical properties together with good conductivity for practical applications [2022]. However, the electrical conductivity of the composite was not improved effectively; especially the composites became more brittle due to addition of polyaniline. Cellulose fibers have been used to reinforce brittle conducting polymers, such us polypyrrole (PPy), polyaniline (PANI) and polythiophene (PTP) for energy storage applications [23]. Cellulose and PPy all polymer composite battery with high reported charge capacities and charging rates [11]. Recently, Polyaniline-based aqueous suspensions containing polyaniline (PANi) contents ranging between 5 and 80 wt.% have been successfully developed, the composite films showed high mechanical strength of 178 MPa, and a
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percolation threshold of electrical conductivity of 4.57 vol.% of PANi content[24]. Compared with PPy and PTP, PANI has relatively high theoretical specific capacity [23]. Combining cellulose nanowhiskers and polyaniline is promising for developing green functional polymer nanocomposites. Recently, polyaniline modified and cellulose nanowhisker reinforced smart composites have been reported [25]. In this research, nanocellulose was used as the matrix and polyaniline was added as conducting component to produce the nanocellulose-based flexible and electrically conducting composite films. Aqueous nanocellulose suspension has good film formability because of strong hydrogen bond between the whiskers, which facilitates the film forming ability of the composites. The combination of nanocellulose and polyaniline gives good conductivity and excellent mechanical properties to the nanocomposites. The composites, combining good mechanical properties of cellulose nanowhiskers and conductivity of polyaniline, have great potential in anti-corrosion coatings, conducting adhesives, anti-static and electromagnetic interference shielding materials, biodegradable smart sensors/actuators, and batteries. 2 Materials and testing methods 2.1Materials Bleached flax yarns were purchased from Jayashree Textiles, Kolkata, India. Sulphuric acid with concentration of 95-97 %, was supplied by Merck KGaA, Darmstadt, Germany. Sodium hydroxide was purchased from Ajax Finechem Pty Ltd., Taren Point, Australia. Aniline hydrochloride and Ammonium persulfate (APS) were bought from Sigma-Aldrich, Inc, St Louis, USA. The chemicals were used as received without further purification. 2.2 Preparation of cellulose nanowhiskers
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Cellulose nanowhiskers were prepared by acid hydrolysis, following a procedure reported earlier [5]. The oven dried flax yarns were hydrolysed in sulphuric acid solution (60 wt.%) at 55°C for 1 h. Then the suspension was centrifuged and diluted with D. I. water, which was followed by neutralisation with NaOH solution (10 wt.%) to remove free acid. The suspension was freeze-dried prior to being redispersed in D. I. water. 2.3 Preparation of nanocellulose/polyaniline composite films 0.2g aniline HCl powder and 0.44 g APS were dissolved in distilled water. Aniline HCl solution was added into cellulose nanowhiskers aqueous suspension (0.5 wt%) followed by dropwise addition of APS. The weight ratio of aniline HCl monomer and nanocellulose is 1:9, 2:8, and 3:7. The mixture was kept stirred for 24 hr at room temperature and then was diluted and washed with D. I. water under centrifugation until the supernatant was clear. The mixture was cast directly onto petri dishes. The composite film was peeled off after water fully evaporated at room temperature. 2.4 Testing methods The morphology of the nanocomposites was studied using field emission scanning electron microscopy (FE-SEM, FEI XL30s) with an accelerating voltage of 5 kV. Fourier transform infrared – attenuated total reflectance (FTIR, ATR-FTIR, Nicolet 8700, USA) spectroscopy was used to analyse the chemical structures of the specimens. All spectra were collected with 4 cm-1 wave number resolution after 64 continuous scans at a wavelength range of 4000 to 600 cm-1. UV-Vis absorption analysis was recorded with UV-Vis scanning spectrophotometer (UV-2101 PC, Shimadzu, Japan) on PANI solution by using N, N-dimethylformamide (DMF) as solvent. Thermal stability analysis was carried out with thermogravimetric analysis (TGA, Q5000, USA). Samples
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were heated in open platinum pan from room temperature to 600 ºC, under a nitrogen atmosphere in order to avoid thermoxidative degradation due to oxygen, at a heating rate of 10 ºC/min.
A four-probe conductivity apparatus (Keithley 6517, USA) was
used for electrical conductivity test. Storage modulus was tested on dynamic thermal mechanical analysis (DMTA, Q800, USA) at a heating rate of 3 ºC / min. The dynamic storage was determined at a constant frequency of 1 Hz as a function of temperature from 20 to 150 ºC. 3 Results and discussions 3.1Morphology Flexible nanocellulose-based polyaniline electrically conductive films are obtained by the in-situ polymerisation of aniline HCl onto cellulose nanowhiskers. The composite has very good film formability by easily casting the resulted aqueous mixtures onto petri dish, which is not the case for pure polyaniline powder. This method is also environment friendly without any chemical solvent being involved. The resulting composite film appears in dark green colour and has high flexibility so that it can be bent by 180 º without breaking, Figure 1. Figure 2 shows the typical morphology of nanocellulose film and nanocellulose/PANI composite films. The nanocellulose film is composed of randomly oriented cellulose nanowhiskers with diameters varying from a few to around 20 nm. The composites have the similar morphology; however, the surface is rougher and the diameters of the whiskers are larger with an increase in PANI content due to the polymerisation of polyaniline on the whisker surface. The whiskers are well bonded with each other, which indicate the existence of strong hydrogen bonding in spite of surface coating of PANI powder. No detectable polyaniline precipitation is observed from the top surface
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of the films with 10 and 20 wt.% PANI, indicating that tiny polyaniline has been preferably polymerised on the surfaces of cellulose nanowhiskers. Individual cellulose nanowhiskers could not be distinguished and aggregates of PANI powder can be seen on the surface of the composite film with 30 wt. % PANI. Fracture surfaces of pure nanocellulose and nanocellulose/PANI composite films are shown in Figure 3. Nanocellulose film shows layered structure due to the strong hydrogen bonding among the whiskers. The composite film shows a similar layered structure (contributing to good flexibility) and white dots are clearly seen at the fractured ends of the whiskers, indicating good bonding between cellulose nanowhiskers and PANI powder. 3.2 Chemical structure The FTIR spectra of nanocellulose, PANI powders, and nanocellulose/PANI composite films are shown in Figure 4. The characteristic peaks of PANI are at1582, 1492, 1304, 1152, and 818 cm-1 are corresponding to C=C stretching peak of quinone ring, benzene ring, and bending vibration of C-H (N=quinoid=N), and C-H( 1, 4 substitute, out of plane), respectively [26, 27]. The peak at 1582 cm-1 confirms the presence of a pronated imine function. For nanocellulose film, the bands at 3334, 2897 cm-1 are related to stretching of hydroxyl groups and C-H bond. The band at 1642 results from the H-O-H bending of the absorbed water. The peak at 1160 cm-1 corresponds to the C-O stretching [28]. For the spectra of composite film, three peaks are appearently different from those of the components. Additional peaks at 1575 and 1495cm-1 belong to PANI, denoted by line 1 and 2, besides the characteristic peaks from cellulose. The peak intensity at 818 cm-1 increases with an increase in PANI content, proving the presence of PANI on surfaces of cellulose nanowhiskers.
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UV-vis absorption spectra for the composite show two absorption bands at 280-400 nm and 500-700 nm. These peaks represent benzenoid segment π- π* electron transition and quinoid segment π - π* electron transition, respectively [29]. The absorption bands of PANI depended on the pH of the solution. The first absorption bands are flat and distorted due to the overlapping of the two peaks for the HCl-doped PANI solution. 3.3 Physical structure X-ray diffraction patterns of nanocellulose based composite films are shown in Figure 5. The peaks are typical for cellulose I and show the highest intensity at 2 theta = 22.5˚, accompanied by two close weak peak in the region 2 theta = 14.6 and 16.4˚ . However, no characteristic peaks of PANI at 2 theta =25˚ is seen due to its low crystalline level and content. This result is different from the distinct crystalline structure in polyaniline/iron oxide and polyaniline/carbon nanotube nanocomposites [30,31]. The reason for the low crystallinity of PANI formed on nanocellulose is not understood yet. There is not much difference for different content of PANI except that the peak intensity of nanocellulose at 2 theta = 22.5˚ decrease with an increase in PANI concentration from 10% to 30%. 3.4 Electrical conductivity The electrical conductivity of polyaniline relies on the degree of doping, oxidation state, particle morphology, crystallinity, interior intrachain interactions, molecular weight, etc [32, 33]. The electrical conductivities of the composite films are 2.8×10-6S/cm, 1.3×10-2 S/cm and 1.9×10-2 S/cm for the composite containing 10, 20, and 30 wt.% PANI, respectively. The conductivity has increased significantly compared to pure cellulose
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nanowhisker film (practically an insulator). The significant increase in conductivity with the increase in PANI content from 10 to 20 wt.% PANI probably results from the dense coverage of tiny PANI particles on the surface of cellulose nanowhsikers and the formation of a connected network. The significantly high conductivity of the composite attributes to the homogenous formation of PANI layer on the surfaces of cellulose nanowhiskers. However, the conductive properties can be improved by the efficient doping and crystallinity enhancement of polyaniline. 3.5 Thermal stability The thermogravimetric curves of pure nanocellulose film, pure PANI powder, and nanocellulose /PANI composite films with PANI contents of 0 to 30 wt. % are illustrated in Figure 6. The slight weight loss of all materials results from the water evaporation at around 100 °C. As seen from the curve, pure cellulose nanowhisker film appears to be decomposed after 300 °C, which corresponds to pyrolysis of cellulose [34]. For the PANI powder, there are three major stages for weight loss, around 100, 240-350, and 420–550°C, which can be attributed to the removal of moisture, HCl dopant, and the degradation of the PANI molecules, respectively. This result is similar to that of the HCl-doped Emerald Salt(ES)-form PANI powder reported by Wei et al. [35]. The composite film with 30 wt.% PANI shows higher thermal stability at temperature above 500 °C by 15% weight loss, compared to pure cellulose nanowhiskers. The DSC curves of the composite films are shown in Figure 7. Pure nanocellulose film exhibits a sharp melting peak at 211 °C, indicating the crystalline structure
of
nanocellulose. However,This temperature is much lower than that of microcrystalline cellulose which situated at 350°C[36]. The reason is propbably due to smaller size,
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lower crystallinity, lower polymerization degree of nanocellulose. PANI showed two peaks during the DSC measurement. The first exothermic peak between 160 and 240°C is possibly generated by the crosslinking reaction resulting from a coupling of two neighbouring –N=Q=N- groups (where Q represents the quinoid ring ) to give two – NH-B-NH- groups (where B represents the benzenoid ring ) through a line of the N with its neighbouring quinoid ring[37]. The endothermic peak at 240 - 350 °C could be caused by the evaporation of HCl dopant, which is in aggreement of TGA analysis. A significant difference is seen from pure nanocellulose film and nanocellulose containing PANI composite films. The absence of sharp melting peak of nanocellulose can be seen with the introduction of PANI coating on nanocellulose surface. 3.6 Thermomechanical properties The composite film shows good flexibility and high rigidity due to the existence nanocellulose. Cellulose nanopaper shows high tensile strength and modulus of over 200 MPa and 13 GPa, respectively [38]. Storage modulus of nanocellulose film exhibited slow decrease with an increase in temperature. The storage modulus at low temperature has a value over 11GPa. The modulus of the composite are 7.8, 6.1, and 5.4 GPa for the film containing 10, 20, and 30 wt.% PANI, respectively, which is much higher than that of pure PANI (0.9±0.2 GPa) and recently reported value at the same composition ratio [24,39]. The modulus of the composites decreased with an increase in PANI content. Thus the composites containing more than 30% PANI are not useful for conducting films due to their brittleness. The mechanical properties are mainly contributed by interconnected network of nanocellulose whiskers. The existence of PANI particles weakened the hydrogen bond among cellulose nanowhiskers. 4 Conclusions
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Flexible and electrically conductive nanocellulose-based polyaniline(PANI) ecofriendly composites have been fabricated by in-situ oxidative polymerization of aniline hydrochloride in stable aqueous nanocellulose suspension by using ammonium peroxydisulfate (APS) as oxidant. PANI particles are formed on the surfaces of cellulose nanowhiskers and under SEM observation, no free polyaniline particles are observed on the top surfaces of the composite films with 10 and 20 wt.% PANI. However, aggregates of PANI particles are seen on the surface of the composite film with 30wt.% PANI. The thin composite film shows good flexibility and the film with thickness of 50 μm can be bent by 180 without breaking. The electrical conductivity of as prepared composite films significantly increases from 10-6 to 10-2 S/cm when the content of PANI increases from 10 to 20 wt. %, and retains the same order of magnitude when the content of PANI increases from 20 to 30 wt.%. The composite film exhibits improved thermal stability above 300 ºC the film containing 30 wt.% PANI shows only 15 % less weight loss at 500 ºC. The flexible and conductive biodegradable nanocellulose-based composites are promising for using in sensors, batteries and conductive adhesives. Acknowledgements The authors would like to thank the Ministry of Research and Innovation NZ for financial support, late A/Prof. A. J. Easteal for the experimental discussion, and Mr. S. Crump and Mr. S. Cawley for their assistance in chemical and electrical measurements. References [1] Pandey JK, Kumar AP, Misra M, Mohanty AK, Drzal LT, Singh R P. Recent advances in biodegradable nanocomposites. J Nanosci Nanotechn. 2005;5(4): 497526.
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Figure captions: Figure 1. Optical images of nanocellulose based composite film containing 20 wt.% PANI after bending by 180º, with thickness of 50μm Figure 2. SEM images of nanocellulose based composite films containing (a)0, (b)10, (c)20, and 30 wt.% PANI Figure 3 Fracture surface of nanocellulose-based composite films containing (a)0, (b)10, (c)20, and (d) 30 wt.% PANI Figure 4. FTIR spectra of pure nanocellulose film and nanocellulose based composites fillms containing (a)0, (b)10, (c)20 and (d) 30% PANI, and (e) 100% PANI powder Figure 5. UV-Vis spectra of nanocellulose based composites fillms containing 10, 20 and 30% PANI Figure 6. XRD patterns of nanocellulose based composite films with 10, 20, 30% PANI Figure 7. TGA curves of pure nanocellulose fillm and nanocellulose based composite films with 10, 20, 30% PANI , 100% PANI shown for comparision Figure8. DSC curves of pure nanocellulose fillm and nanocellulose based composite films with 10, 20, 30% PANI , 100% PANI shown for comparision Figure 9. DMA curves of pure nanocellulose fillm and nanocellulose based composite films 10, 20, and 30% PANI
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