Research of Materials Science March 2014, Volume 3, Issue 1, PP.1-9
Synthesis, Structure and Properties of Bacterial Cellulose/polyaniline/manganese Dioxide Nanocomposites Via Layer-by-layer Interfacial Polymerization Hubin Lin1,2, Chongming Du1,2, Zhidan Lin1# 1 Jinan University, Guangzhou 510632, PR China 2 Huizhou Changyi New Materials Company, Huizhou, 516005, PR China #
Email: linzd@jnu.edu.cn
Abstract A new nanocomposite based on bacterial cellulose (BC), polyaniline (PANI), and manganese dioxide (MnO2) was synthesized and characterized. Initially, the bacterial cellulose absorbed KMnO4 oxidant acidic aqueous solution, then contacted and reacted with toluene/aniline solution on one side of BC. After several times of absorption and reaction processes alternately, layer-by-layer PANI and MnO2 in situ on the surface of BC were formed. Because BC with acidic aqueous solution of oxidant restricts the penetration of toluene/aniline solution, it can control the oxidative polymerization at one side of BC. The BC/PANI/MnO2 composites with single conductive surface were successfully produced with the surface resistivity of 55.5 Ω cm. The BC component in the new BC/PANI/MnO2 composite can absorb the electrolyte solution and change to multilayer composite of electrode/electrolyte septum material. The multilayer nanocomposites of electrode/electrolyte septum materials can be used as ideal electrode materials in supercapacitors. Keywords: Bacterial Cellulose; Polyaniline; Manganese Dioxide; Conductive Material; Interfacial Polymerization; Nanocomposites
1 INTRODUCTION Electronic conducting polymers have been a new class of materials in the recent years and have received extensive attention in academic and industrial research activities because of their great potential applications in electrical, optical, biological, and other sectors. Polyaniline (PANI) and its derivatives make an important class of conducting materials which provide good stability, and processability at relatively low production cost. Polyaniline/cellulose nanocomposites have been attractive as electric conductive and flexible electrode materials in electrical devices applications [1–3]. Cellulose pulp, cellulose derivatives, cotton cellulose, microcrystalline cellulose, and bacterial cellulose (BC) membrane have been used for preparation of these conducting nanocomposites [4–8]. Bacterial cellulose is a special kind of cellulose that is produced through fermentation of bacteria in static or agitated culture [9]. Having similar molecular structure as natural cellulose, BC provides specific ultrafine network structure and particular properties, such as sufficient porosity, large purity and crystallinity, good mechanical properties, great water holding capability, excellent biodegradability, and biocompatibility [10,11]. Recently, the preparation of BC/PANI conducting nanocomposites via in situ polymerization of aniline nanoparticles onto the BC membrane was explored [12–20]. Despite these advantages, there are some concerns in these fabrication methods. The utilization of PANI is small in the reaction system [15]. Only few of PANI macromolecules are loaded on BC macromolecular chains, most of which are discarded as waste with other reaction materials, resulting in huge amounts of waste materials [17]. The -1http://www.ivypub.org/rms
double sides or bulk of the composite membranes are conductive that make them useful as flexible electrode materials through pasting on the insulating membranes, such as sulfuric acid paper [21]. In application of these composites, bonding with the substrate has been a challenging issue. Moreover, it is difficult to compound the energy storage materials, such as manganese dioxide (MnO2), with the membrane, which has special significance to the new batteries [22–26]. To overcome the above problems, we used the permselecivity with materials of the nanofiber network in wet BC and designed a new synthetic route of layer-by-layer interfacial polymerization in which polyaniline and MnO2 could be formed in situ on the BC surface. Toluene disperses aniline and controls its migration rate towards aqueous oxidant acidic solution through BC, which could limit the oxidative polymerization at the single side of the aqueous oxidant acidic solution with BC. Thus, the nanocomposite membrane of BC/PANI/MnO2 can be used as flexible electrode without pasting on the sulfuric acid paper. In this paper, using various concentrations of KMnO4 as oxidant, the single side conductive BC/PANI/MnO2 nanocomposite membranes were produced. The FTIR, SEM, TGA, and surface resistance measurement analytical methods were applied to study the molecular structure, microstructure and electrical properties of composite membranes.
2 Experimental 2.1 Materials The oxidation agent of potassium permanganate (KMnO4) was purchased from Tianjin Damao Chemical Reagent Company, China. Hydrochloric acid (HCl) and toluene were supplied by Shantou Guanghua Chemical Reagent Company, China. Methylphenol was purchased from Sigma-Aldrich Company, USA. All reagents were analytical grade. Bacterial cellulose film (dry thickness 0.014 mm) was kindly provided by Hainan Yeguo Foods Company, China.
2.2 Synthesis of Nanocomposite Films
FIG.1 SCHEMATIC DIAGRAM OF THE SYNTHESIS OF BC/PANI/MnO2 NANOCOMPOSITE FILMS
The solutions of KMnO4 were prepared in 1 mol/L HCl solution as aqueous acidic solvent at various concentrations -2http://www.ivypub.org/rms
of KMnO4 of 0.01, 0.02, and 0.03 mol/L. The aniline solution in toluene was prepared at concentration of 0.1 mol/L. Bacterial cellulose wet film was washed with deionized water and wrapped on a plastic pipe to make a cup. The bacterial cellulose wet film was squeezed using a vacuum filtration device. The acidic solution of KMnO4 was poured into the cup which was aborted by the squeezed BC film. After 20 min soaking, the acidic solution of KMnO4 was removed from the cup, and the aniline solution in toluene was poured on the cup. After being kept at 0°C for 1 h, PANI and MnO2 were formed at the bottom of the cup. The aniline solution in toluene was removed from the cup and the film was squeezed with a vacuum filtration device. The alternate soaking processes of the acidic solution of KMnO4 and the aniline solution in toluene were repeated for 20 times. Afterwards, the cup was placed in a vacuum filtration device to remove the remaining reagents and then washed with acetone and deionized water successively. Next, the cup was dried in a vacuum oven at 60°C for 24 h. In the final stage, the plane dry BC/PANI/MnO2 nanocomposite film was carefully removed from the cup. Various BC/PANI/MnO2 nanocomposite films were prepared using different concentrations of KMnO4 solutions. The films were named as samples 1, 2, and 3 for concentrations of 0.01, 0.02, and 0.03 mol/L, respectively. Reaction schematic diagram is shown in Fig. 1.
2.3 Analytical Methods and Procedures The two sides of BC/PANI/MnO2 nanocomposite film were examined using EQUINOX55 (BURKER) Fourier transform infrared (FTIR) spectroscopy technique. The FTIR spectra were recorded using an attenuated total reflectance accessory (ATR). The specimens were analyzed over the range of 4000 to 400 cm–1 with a spectrum resolution of 4 cm–1. All spectra were averaged over 30 scans. The various BC/PANI/MnO2 nanocomposite films prepared at different concentrations of KMnO4 were sputter-coated with gold before conducting scanning electron microscopy (SEM) examinations. The surface morphologies of BC/PANI/MnO2 nanocomposite films were observed on a Philips XL-30 environmental scanning electron microscope (ESEM) equipped with an EDS JED 2300 at an acceleration voltage of 15 kV. Energy-dispersive X-ray spectroscopy (EDS) spectra directly revealed the presence of the atomic elements in the samples. The BC/PANI/MnO2 nanocomposite film was immersed in methylphenol for 24 h; the BC film was removed and the residue in methylphenol was filtrated and dried to constant weight. The PANI and MnO2 contents in the BC/PANI/MnO2 nanocomposite films were calculated using the following formulas: MnO2% content = (weight of residue in methylphenol/weight of dried nanocomposites) × 100 PANI% content = (1– weight of dried BC/weight of dried nanocomposite – MnO2 percentage content) × 100 All data were averaged over the results obtained from five different samples. The samples of BC and BC/PANI/MnO2 were vacuum-dried for 24 h at 60°C before measurements. The surface resistivities of the two sides of the BC/PANI/MnO2 nanocomposite films were measured with a Bakon485 (HAKKO, China) surface resistance tester. Five different samples were examined and the averages were presented as results. The thicknesses of the BC/PANI/MnO2 nanocomposite films were measured using an instrument with the precision of 0.001 mm. The thermal decomposition behavior of the BC/PANI/MnO2 nanocomposite films was carried out with the temperature of 700°C using a thermogravimetric analyzer(TGA Q500, TA Instruments) in nitrogen atmosphere at the heating rate of 10°C/min.
3 RESULTS and DISCUSSIONS 3.1 Appearance of Nanocomposites The photographs in Fig.2 show the appearances of original BC membrane and BC/PANI/MnO2 composite membrane containing 58.2 wt % PANI. The original BC membrane appeared as white semitransparent with smooth surface. One side of the composite membrane revealed identical character as that of the original BC and the other side an obvious dark green color, indicating the successful merging of PANI only on one side of the BC membrane. Both -3http://www.ivypub.org/rms
original BC and BC/PANI/MnO2 nanocomposite membranes were rather flexible and curled.
FIG.2 PHOTOGRAPHS OF (a) PURE BC FILM AND (b) BC/PANI/MnO 2 NANOCOMPOSITE FILM CONTAINING 58.2% PANI
3.2 Infrared spectroscopy results
FIG.3 FTIR-ATR SPECTRA OF TWO SIDES OF BC/PANI/MnO2 NANOCOMPOSITE FILM CONTAINING 58.2WT% PANI
Infrared spectra of vacuum-dried samples of the two sides of the PANI/BC/MnO2 composites containing 58.2 wt % PANI are shown in Fig. 3. All the characteristic peaks of BC were observed in the spectra of white smooth side. Absorption band at 3347 cm–1 is assigned to hydroxyl groups of cellulose and water. Absorption bands at 2899 and 1428 cm–1 are assigned to C–H stretching of CH2 and CH2 symmetric bending, respectively. Absorption bands at 1363 and 1161 cm–1 were assigned to stretching and bending modes of hydrocarbons in cellulose backbone and to asymmetric bridge C–O stretching, respectively. Absorption peaks at 1109 and 1055 cm–1 were assigned to skeletal vibrations involving C–O stretching [27]. The FTIR spectra indicated that after the reaction, one side of the composite film still kept the molecular structure of the original BC on which interfacial polymerization reaction did not take place. The hydroxyl peak at 3347 cm–1 of BC membrane disappeared and the characteristic peaks of PANI were instead observed in the spectra of the dark green color side of the composite film [28] Absorption peak at 3227 cm–1 was assigned to N–H stretching. Absorption peaks at 1558 and 1491 cm–1 were assigned to stretching of -4http://www.ivypub.org/rms
quinone ring and benzene rings in PANI, respectively[29]. Absorption band at 1291 cm–1 was assigned to the stretching of C–N band of benzene ring [16]. Absorption band at 819 cm–1 was assigned to out-of-plane bending vibration of C–H band of para-substituted benzene ring. The sharp peaks appearing at 581 cm–1 belong to the Mn–O vibrations [30]. This observation indicated that after the reaction, PANI and MnO2 were successfully produced on one side of the film at which the interfacial polymerization reaction took place. Comparing with the spectra of the two sides of the film, the characteristic peaks were not coincide, indicating that the PANI and MnO2 produced via interfacial polymerization reaction could not pass through BC membrane and the polymerization reaction was restricted only to one side of the BC film.
3.3 Morphology Studies
FIG.4 SEM MICROGRAPHS OF PURE BC FILM AND BC/PANI/MnO2 NANOCOMPOSITE FILM
Morphologies of the nanocomposites were examined using SEM techniques. The microstructures of the two sides of the BC/PANI/MnO2 nanocomposite membrane were directly observed. Figure 4 presents the SEM micrographs of the BC/PANI/MnO2 nanocomposite membranes containing various oxidant concentrations. In the SEM image of the original BC membrane (Fig. 4(a)), the nanofibers within 100 nm in diameter overlap with each other. With the addition of 0.01 mol/L KMnO4, the polymerization side of BC membrane (Fig. 4(b)) shows the overlapping fibers between 300 and 400 nm, indicating the successful polymerization reaction of PANI particles on the surface of the nanofibers of BC membrane with an interconnected electrically conductive network. When 0.02 mol/L KMnO4 was used, the SEM micrograph of the polymerized side of the BC membrane (Fig. 4(c)) appeared with a large number of particles uniformly dispersing on the whole surface of the BC film; no fiber was observed. This observation may be attributed to the increasing oxidant concentration, the mounts of PANI and MnO2 formed instantly on the interface between BC membrane and toluene, which led to the formation of larger particles on the fibers surfaces. When 0.03 mol/L of KMnO4 was used (Fig. 4(d)), the particle size on the interface reached 2 μm, and no fiber was observable. -5http://www.ivypub.org/rms
This may be due to the sizes of PANI and MnO2 particles that were too large to immerse into the nano-network BC membrane, but suspended on the BC surface. Formation of large PANI and MnO2 particles suspended on the BC membrane prevented production of a continuous membrane structure. Huge cracks of the membrane could be observed even at small magnifications (Fig. 4(e)). The obtained EDS spectra suggested that the main ingredient of the large particles in Fig. 4(d) is Mn, meaning that MnO2 is formed in situ on the BC surface via the method mentioned in this paper.
3.4 Properties of Nanocomposite Films Table 1 presents properties of the BC/PANI/MnO2 nanocomposite membrane with various concentrations of oxidant. As the results indicated, when the oxidant concentration increased, the thickness of the nanocomposite membrane and the PANI/MnO2 layer and the content of PANI and MnO2 increased as well. Increasing the concentration of oxidant increased the particle size and the contents of both PANI and MnO2, which led to the decrease of PANI/MnO2 coating strength (Fig. 4). When oxidant concentration reached 0.03 mol/L, PANI and MnO2 particles became too large to immerse into the network structure of BC that could even not bond together to make an integral film. This observation suggested that the nanofibers network structure of BC plays a great role in promoting the conductivity of PANI. The surface resistivity of the BC side of BC/PANI/MnO2 nanocomposite membrane was above 108 Ω cm and was in fully insulated state. With increasing oxidant concentration, the surface resistivity of the PANI/MnO2 side of BC/PANI/MnO2 membrane initially decreased and then increased. The surface resistant of samples 1 and 2 were 55.5 and 89.3 Ω cm, respectively, and both reached the conductivity value of 101 Ω cm. It was the increasing content of the non-conductive MnO2 that led to the increasing surface resistivity of the PANI/MnO2 side. Our observations showed that the single-side BC/PANI/MnO2 nanocomposite membrane with the ability to conduct electricity could be used as a perfect flexible electrode material. TABLE 1 PROPERTIES OF BC/PANI NANOCOMPOSITE FILMS Reagent Concentration
Film thickness (mm)
(mol/L)
Sample
Oxidan
Monome
t
r
—
—
0.014
1
0.01
0.1
2
0.02
0.1
3
0.03
0.1
BC
Total
Film composition
Surface resistivity (Ω cm)
(%)
PANI/
PANI/
Tensile Max stress(N)
PANI
MnO2
—
—
—
—
1.77×108
9.20
0.088
0.074
58.20
13.22
55.5
1.81×108
7.62
0.117
0.103
65.89
19.11
89.3
1.56×108
7.09
MnO2 layer
MnO2 side
BC side
Cannot form integral film
3.5 Thermal Stability Thermal gravity analysis technique provided good understanding of the thermal stability of materials. The TGA thermograms of pure BC and BC/PANI/MnO2 nanocomposite films are presented in Fig. 5. The TGA thermogram of pure BC showed a three-stage weight loss profile [16]: the initial weight loss at 40−100°C attributed to evaporation of the moisture inside BC; the second weight loss at 200−380°C due to removal of the molecular fragments, such as O−H and CH2−OH groups; and the last weight loss at 380−700°C that consists of decomposition of the cellulose backbone. The TGA thermograms of BC/PANI/MnO2 nanocomposite films exhibited a four-stage weight loss profile: initial weight loss at 40−100°C attributed to evaporation of the moisture inside BC; the second weight loss at 100−220°C due to the removal of HCl; the third weight loss at 220−380°C attributed to removal of the O−H and CH2−OH groups and the side chains of PANI; and eventually the forth weight loss at 380−700°C through the structural decomposition of PANI and cellulose backbones. In contrast to pure BC, BC/PANI/MnO2 nanocomposite films exhibited a delayed 200−400°C decomposition stage and produced a larger amount of residues due to the decomposition products of MnO2 and the PANI. All the obtained results demonstrated that the PANI and MnO2 -6http://www.ivypub.org/rms
coatings act as protective barriers on the surface of BC against thermal degradation and thermally stabilize the material.
FIG.5 TGA THERMOGRAMS OF PURE BC AND PURE BC FILM AND BC/PANI/MnO2 NANOCOMPOSITE FILM
4 CONCLUSION In this study, we successfully synthesized flexible BC/PANI/MnO2 nanocomposite membranes with one conductive side and one electrode resistant side via layer-by-layer interfacial polymerization. The experimental observations indicated that microstructures of the nanocomposite membranes are significantly affected by the concentration of oxidant. At small concentrations of oxidant (less than 0.02 mol/L KMnO4), the PANI and MnO2 particles could form films with good conductivity; reaching the surface resistant of the nanocomposite membrane to the magnitude of 101 Ω cm. With large concentrations of oxidant (larger than 0.03 molL KMnO4), the PANI and MnO2 particles were too large to bond together and develop an integral membrane, which resulted in a surface with large conductivity resistant. The nanofiber network structure of BC played a great role in promoting the conductivity of PANI. The BC component with BC/PANI/MnO2 composite material, formed through adsorption of electrolyte solution, could be used as layer composite of electrode/electrolyte septum material with potential applications in electric devices.
ACKNOWLEDGEMENTS This project was supported by Natural Science Foundation of China and Project of Science (Grants Nos. 21101076 and 21176100).
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AUTHORS 1
Hubin Lin (1963-), male, was born in Guangdong, China. He studied in Jinan University, China, for master degree. His major field of research is engineering plastics modification.
2
Chongming Du (1963-), was born in Guangdong, China. Engineer, and currently, he is doing research on engineering plastics modification. 3
Zhidan Lin, male, was born in Guangdong, China. Associate professor, master student supervisor, currently, he is doing research on polymer-based functional materials and engineering plastics modification.
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