Synthesis, characterization, and properties of new conducting polyaniline/copper sulfide nanocomposites.
For a polymer to become electrically conductive, it must imitate a metal which means that electrons in polymers must be free to move. The electrical property of polymeric materials has become an increasingly interesting area of research because these materials possess a great potential for solid-state devices (1-3). Most polymeric materials are poor conductors of electricity because of the large number of free electrons that are not available to participate in the conduction process (4). Polymers are usually poly-conjugated structures with poor electrical conductivity, but when treated with oxidizing or reducing agent, they can be converted into polymers with reasonable conductivity. Therefore, a polymer might become electrically conductive by means of doping. This class of material provides a tremendous scope for tuning their electrical conductivity from semiconducting to metallic regime by way of doping.
The electrical properties of polymers can be modified by the addition of fillers such as carbon or metal to impart high conductivity. These composites are used for interconnectors, printed circuit boards, encapsulation die attach, conductive adhesive, electromagnetic interface shield, aerospace engineering, etc (5-7). Unfortunately, these conductive fillers will impart high weight, poor surface finish, poor mechanical properties, and easy oxidative degradation to the end products. Nanoscale particles are more attractive due to intriguing properties arising from the nanoscale and large surface area. The nanoscale fillers may better improve the electrical and dielectric properties of host polymers than conventional metal fillers. Nano-structuration of conducting polymers and their composite emerged as a new field of research and development, directed to creation of new materials for use in modern and future technologies. The properties of nanocomposites are quite different from the constituent material due to interfacial interaction between nanostructure material and polymers (8). The properties of these materials can easily be tuned to yield the desired application through the variation of particle size, shape, and distribution of nanopar-ticles (9-11). To make these polymers technologically viable, the processability and thermal stability have to be improved. Incorporation of conducting polymers into composite has been extensively used to develop materials that combine the good mechanical properties, thermal stability, and processability of the matrix with the electrical properties of the conducting component (12-15).
Metal nanoparticles with different sizes and shapes can be mixed with conducting polymer to form composite materials, which represent a new class of materials that may combine desirable physical characteristics of both organic and metallic component with single composition. The metallic portion offers the electrical properties, whereas the polymer part can provide thermal stability and mechanical properties to the fabricated polymer (8), (16-22). Fabrication of polyaniline (PANI) composite with various materials has received great attention because of their unique properties and applications in various electrical devices (23). However, the main problem associated with the effective utilization of all the intrinsically conducting polymers, including PANI is inherent in their lower level of conductivity compared to metal, poor proc-essability, and infusibility. Hence the goal of this research is to produce hybrid materials where the inorganic components are truly dispersed at a molecular level in the organic matrix. Since copper is one of the metals with highest electrical and thermal conductivity, so the combination of PANI with copper sulfide (CuS) can yield functional materials having enhanced electrical and thermal properties. This study is focused on the synthesis and characterization of CuS nanoparticle inside a network of conducting polymer. The conductivity and thermal behavior of the system is also discussed.
The monomer aniline AR grade was purchased from Merck, India and double distilled before use. LR grade HCI, ammonium persulfate, copper acetate monohydrate, thioacetamide, cetyltrimethyl ammonium bromide (CTAB), acetone, and ethanol were obtained from SD Fine Chemicals, India and were used as received. De-ionized water was used as solvent in all the syntheses.
Synthesis of Polyaniline
PANI was synthesized by polymerization of aniline in the presence of hydrochloric acid as a catalyst and ammonium peroxidisulfate (APS) as an oxidant by chemical oxidative polymerization. The required amount of aniline monomer was taken in 500 ml RB flask containing 200 ml I M HCI solution and vigorously stirred at 45[degrees]C in an inert atmosphere. Calculated amount of APS solution was dissolved in 1 M HCI and kept at 0[degrees]C for an hour. It was then added drop wise to the above solution with continuous stirring at 0[degrees]C for 12 hr. The resulting precipitate was washed with acetone, ethanol, and de-ionized water to remove monomer, oligomer, and excess oxidant. The solid mass was dried under vacuum for 48 hr at 50[degrees]C and stored in a zipper bag.
Synthesis of Nanocrystalline Copper Sulfide
CuS nanoparticles (25, 50, 100, and 150 mmol)) were prepared by a simple and environmentally benign technique involving the reaction of copper acetate with thioa-cetamide [241. In a typical procedure, copper acetate monohydrate (appropriate amount) was dissolved in deionized water (10 ml). In another container, thioaceta-mide (appropriate amount) and CTAB (0.2 g) were dissolved in deionized water (10 m1).The copper acetate monohydrate solution was added to the thioacetamide solution dropwise at room temperature [(25 [+ or -] 2)[degrees]C] with continuous stirring over a period of 20 min. The solution turned golden brown as soon as the first drop of copper acetate monohydrate solution was added and the color became intense with further addition. The brown solution turned green over a period of 12 hr, which was identified as CTAB trapped nanocrystals (18).
Synthesis of PolyanilinelCopper Sulfide Nanocomposite
PANI/CuS nanocomposite was synthesized via in situ polymerization using ammonium peroxidisulfate as oxidizing agent. CTAB (0.4 mmol) was dissolved in de-ionized water (40 ml) in a 500 ml RB flask to get a homogeneous solution followed by adding aniline monomer and stirred the mixture for 15 min. CuS nanoparticle solutions (25, 50, 100, and 150 mmol) were dispersed with the above solution and ultrasonicated over a period of 30 min. The polymerization was done in hydrochloric acid medium. The APS solution was added drop wise into the nanoparticle dispersed aniline solution at 0[degrees]C for an hour, it turned to characteristic black color, indicating the polymerization reaction. The polymerization was completed at 0[degrees]C for 12 h with constant mechanical stirring. The precipitated PANI/CuS nanocomposite was filtered and repeated washing with de-ionized water and ethanol. The fabricated polymer nanocomposite was dried under vacuum at 50[degrees]C for 48 hr.
The surface structure of the composite was investigated by using field emission scanning electron microscopy (FESCA)--Hitachi, SU 6600 FESEM). The ultraviolet-visible (UV-vis) absorption spectra of the polymer composite in dimethyl sulfoxide (DMSO) were recorded on a Hitachi U-3000 spectrophotometer. The IR spectra of the composites were recorded on a JASCO (model 4100) Fourier transform infrared (FT'IR) spectrophotometer in the region 500-4000 [cm.sup.-1]. X-ray diffraction (XRD) pattern of the sample was recorded on Philips X-ray diffrac-tometer using CuK[alpha] radiation ([alpha] = 1.5406 [A.sup.[degrees]). The diffractogram was recorded in terms of 20 in the range 20[degrees]-80[degrees]. Differential scanning calorimetry (DSC) studies were carried out by V2 6D TA instrument model DSC 2010. Initial scan was taking from 50 to 100[degrees]C to remove the thermal history effects and then cooled to room temperature. The samples were heated at a rate of 10 [degrees]C/min (atmosphere [N.sub.2]; flow 40 ml/min) in wide temperature range suitable for given sample. Thermal stability of the resulting composites was investigated by a Perkin Elmer thermo gravimetric analyzer with pure nitrogen gas at a heating rate of 20 [degrees]C/min. Alternating current (AC) resistivity of the samples was measured by Hewlett--Pack-ard LCR Meter, fully automatic system in a frequency range [10.sup.2]-[10.sup.6] Hz at room temperature. Direct current (DC) conductivities at room temperature were measured by using a standard four-probe method with a Keithley 2400 system digital electrometer. The samples for electrical conductivity measurements were prepared using a pressed pellet method.
RESULTS AND DISCUSSION
Scanning Electron Microscopy Analysis
The size and morphology of PANI and its nanocompo-site with different molar concentration of CuS nanopar-ticles are examined using scanning electron microscopy (SEM) (Fig. 1). Compared with that of PAN! in Fig. I a, SEM images of composite show a direct evidence for the formation of PANI layers on the surface of CuS nanopar-ticles. It is clear from the figure that the CuS nanopar-ticles are uniformly dispersed in the polymer matrix with spherical shape (100 mmol CuS embedded PANI). Uniform morphology of the nanocomposite is due to the coordination interaction between vacant orbital available in the valence shell of Cu in CuS and the lone-pair electrons of nitrogen of PAN! molecules. This type of interaction between the ligand and metal is responsible for the formation of spherical morphology 1251. As the concentration of nanoparticle in PANI increases, the size of the particle shows a slight variation in their dimension. The lowest diameter of CuS nanoparticle incorporated polymer matrix recorded is 139 nm (Fig. lc) and it can be observed that the size of spherical particles increased to 350 nm (Fig. Id). This type of variation in the dimension of nanocomposite has strong influence on various electrical properties of composite. Since the PANI/CuS nano-composite is spherical in shape with very good uniformity and adhesiveness, the observed porosity in these composite is less than other PAN! composites. So it can be easily concluded that the CuS nanoparticles are not simply mixed up or blended with the polymer, rather, they are strongly entrapped inside the PAN! chains.
UV--vis spectroscopy is a powerful technique to characterize the interfacial interaction between PANI and Cus nanoparticles. The UV visible absorption spectra of pure PANI and PANI/CuS composites are shown in Fig. 2. PANI depicts a shoulder at ~350 nm and a sharp intense peak at ~456 nm. The shoulder at ~350 nm corresponds to the [pi]-[pi] * transition of the benzenoid ring, while the sharp peak at ~445 nm is attributed to the transition from the valance band to the antibonding polaron state . However, in the spectrum of PANI/CuS composites, the characteristic peak assigned to polaron-[pi] transition is slightly shifted to lower wavelength with increasing concentration of nanopar-ticles. It indicates that the insertion of nanoparticle affects the interaction between PANI and metal sulfide nanopar-ticles. This is in good agreement with the SEM studies.
The molecular structure of PANI and PANI/CuS nano-composite has been identified through FTIR spectra, as shown in Fig. 3. The characteristic vibrations of PANI are observed at 1585, 1519, 1289, 1234, 1119, and 800 [cm.sup.-1] (Fig. 3a). The peaks at 1585 and 1519 [cm.sup.-1] are assigned to C--C ring stretching vibrations. The characteristic peaks appearing at 1289 and 1119 [cm.sup.-1] corresponded to C--N stretching (--N--benzenoid--N--) and C=N stretching (--N=quinoid=N--), respectively. The band appearing at 1234 and 800 [cm.sup.-1] are attributed to the C--N stretching vibration and the C--H out-of plane bending vibration, respectively(27). The band at 3319 [cm.sup.-1] could be attributed to NH stretching mode of PANI. Figure 3b shows the FTIR spectra of PANI/CuS nano-composite. The prominent sharp and intense spectral band at 616 [cm.sup.-1] is assigned to typical absorption of CuS (16). It is also evident from the figure that the incorporation of CuS nanoparticle to PANI causes some observable changes in the spectrum of composites. The band located at 1289 and 1234 [cm.sup.-1] are merged and shifted to broad band at 1212 [cm.sup.-1], indicate the intermolecular interaction between CuS nanoparticle and the electronegative nitrogen of PANI. Furthermore, the observed spectral features also reflect physical ordering and aligned conformation of the nanostructured polymer matrix (28), which is in correlation with the SEM and UV studies. It is also evident from the figure that the characteristics absorption of the NH band of nanocomposite shifted to higher frequency (i.e., 3319-3339 [cm.sup.-1] region with a sharp peak. The ordering of nanostructured molecular chains results in an ordered regularity in polymer composite, that leads to shifting and sharpening of spectral bands (29).
X-Ray Diffraction Analysis
XRD pattern of CuS, PANI, and their nanocomposites are shown in Fig. 4. The main peak of synthesized CuS (Fig. 4d) appeared at 20 = 27.9[degrees], 29.3[degrees], 32.3[degrees], 32.9[degrees], and 48.2[degrees], indicating that the CuS particles are crystalline in nature (24). It is observed that pure PANI has a broad peak at about 2[theta]-25.7[degrees], indicating its amorphous nature (Fig. 4a) (30). However, the XRD patterns of PANI/CuS composites (Fig. 4b and c) exhibit the characteristic peak of PANI along with the crystalline peaks of CuS, owing to the systematic alignment of polymer chain (31). The increase in ordering of polymer composite with the addition of CuS nanoparticles indicates that the structure of PANI is strongly influenced by the molar concentration of nanoparticles. The increase in degree of regularity in arrangement or ordering of polymer chain is due to the strong interfacial interaction between the polymer and nanoparticles. Hence, the orientation of conducting polymer nanocomposites is of much interest, because more highly ordered polymer matrix could display a conductive property such as metallic state.
Average particle size can be determined by Scherrer's formula, D = K[lambda]/[beta]cos[theta], where K is the particle shape factor generally taken as 0.9, [lambda] is the wavelength of Cu K[alpha] radiation ([lambda] = 1.54 A[degrees]), 0 is the diffraction angle of the most intense peak, and [beta] is the half height of diffraction angles in radians. When the reflecting peaks at 2[theta] =28.07[degrees], 29.3[degrees], 32.2[degrees], and 48.5[degrees] are chosen to calculate the average diameter, the average size of CuS nanopar-tide is about 43 nm.
Differential Scanning Calorimetry
DSC has been used to study the glass transition and melting behavior of chemically prepared PANI and its nanocomposite. DSC scan of PANI and PANI/CuS nano-composite are given in Fig. 5. It is observed from the figure that PANI exhibits one endothermic peak at 84[degrees]C. This endothermic peak is due to the vaporization of water combined with the glass transition behavior (30). However, in the case of nanocomposite the endothermic peak shifted to higher temperatures at 88[degrees]C for 50 mmol and 95[degrees]C for 150 mmol of CuS incorporated PAM. The significant enhancement of glass transition temperature by incorporating inorganic phase into the polymer matrix through in situ polymerization is attributed to the strong interfacial interaction between the polymer and nanopar-ticles, which ultimately leads to the restriction of polymer chain mobility. DSC curve of PANI/CuS nanocomposite exhibits a weak endothermic dip at 210[degrees]C, which may be due to the melting of PAM chain. It can be observed that PANI does not exhibit any melting peak as compared to the nanocomposite, indicating the less ordered structure of PAM molecule in the absence of metal sulfide. Hence it can be concluded that thermal parameters such as glass transition temperature depend on the concentrations of CuS in PANI.
The thermal degradation behavior of PANT and PANT! CuS composites are given in Fig. 6. The first weight loss of PANI and its nanocomposite below 250[degrees]C corresponds to the removal of physiosorbed water and dopant ions. The second weight loss occurring between 350 and 600[degrees]C attributed to degradation of polymer backbone. The thermal stability of the polymer and its nanocomposite is an important property for designing the materials for a particular use in a specific field. In such cases, the dispersion of nanoparticle in the polymer matrix has an important role in changing the thermal behavior. It can be observed from the figure that the thermal stability of the composite increases with the increase in concentration of CuS nano-particle. Another important point noted is the percentage of weight residue remaining at 600[degrees]C. Approximately, 8 wt% residue is obtained for PANI, while 29% weight residue is observed for 150 mmol CuS. This infers that PANI/CuS nanocomposite has better flame retardency than pure PANT. The increased thermal stability of the nanocomposite is due to the increased interfacial interaction between CuS and PANI, which is in good agreement with SEM, XRD, and DSC measurements.
AC Electrical Conductivity
Figure 7 shows the AC conductivity behavior of PANI and CuS incorporated PANI at different frequencies. It is evident from the figure that the AC electrical conductivity of the PANI/CuS nanocomposites is significantly higher than that of pure PANI. The comparatively lower conductivity of PANI is due to its amorphous nature. The improvement in AC conductivity of PANI/CuS nanocom-posites arises due to the uniform dispersion of CuS nano-particles in the PANI (which are supported from SEM). So the nanocrystalline nature of CuS particles may give rise to an increase in orderness of the composite (confirmed from XRD and DSC), which favors better electrical transport. It can be seen that the conductivity of PANT/ CuS nanocomposite increases not only with the frequency but also with the molar concentration of nano-particle. The frequency dependent AC conductivity is considered to be the result of an interface charge polarization (Maxwell--Wagner Siliar effect) and the intrinsic electric dipole polarization (33). Almost all the composites show similar behavior up to 104 Hz, i.e., there is not much variation in the conductivity with frequency during this range. Moreover, the conductivities of all the samples are higher at higher frequencies indicating the formation of excess charge carriers (polaron and bipolaron).
The variation of dielectric constant (er) with frequency at room temperature for PANT, and different concentration of CuS containing PANI samples is plotted in Fig. 8. It is evident that the dielectric constant initially decreases rapidly, and as frequency increases Er attains a constant value. The dielectric behavior of the sample in the present study can be explained by the mechanism of polarization process in composites. For a multicomponent system, the free charge carriers migrating through the material leads to a space charge build up at the interface of the constituents owing to mismatch of the conductivities. The electron exchange between the copper atom of CuS and the lone pair electrons of nitrogen atom of PANI molecules gives local displacement of electrons in the direction of applied electric field, inducing polarization in nanocomposites. For polar polymers, the alternating current frequency is an important factor because of the time taken to align the polar dipoles. At low frequency region, the dipoles have sufficient time to align with the field before they change the direction leading to high dielectric value. In an overlapped potential well, at least a few holes executing interwel hopping reverse the direction of motion when the electric field direction reverses. Thus interwel hopping contributes to dielectric relaxation at low frequencies. In normal dielectric behavior, the dielectric constant remains almost constant at high frequency because beyond a certain frequency of dielectric field, the interwel hopping becomes prominent, and the charge carriers do not get enough time for long range hopping before the field reversal (34). As a result of high frequency region only intrawel hopping exists, because the average hopping distance for intrawel hopping on lattice spacing while that for interwel hopping is of the order of few nanometers. So polarization decreases as the signal frequency increases.
The variation of dielectric loss factor (imaginary part of the complex permittivity) with various frequency of PANI and their nanocomposite at room temperature is given in Fig. 9. From the plots it is observed that dielectric loss of nanocomposite is higher than that of PANI due to interfacial polarization. In dielectric nanostructured composite, interface with large volume fraction contains a large number of defects, such as dangling bonds, vacancies, vacancy cluster, and microporosities, which causes a space charge build-up at interfaces (35). This accumulation of space charge leads to field distortions, and dielectric loss depends upon the quantity of nanoparticle present in the composite as well as the geometrical shape of its dispersion (36). The dielectric loss factor decreases as the frequency increases for all the samples due to free charge motion within the material (37). Dielectric loss is a direct function of relaxation process and the origin of this relaxation is the local motion of polar groups. At high frequency dielectric loss becomes constant. This is due to the fact that at higher frequency, the exchange frequency of electric charge carriers cannot follow alternation of the AC electric field applied beyond a certain critical frequency.
DC Electrical Conductivity Studies
Figure 10 shows the room temperature current voltage (I--V) characteristics of PANI and its nanocomposite with various molar concentrations of metal sulfide nanopar-ticles. It is quite interesting to see that the conductivity of nanocomposite is higher than that of pure PANI and also the conductivity increased with the increase in content of CuS nanoparticles. Figure 10 shows a linear variation of electric current with applied voltage. The linear variation shows that PANI and the CuS nanoparticle incorporated polymer exhibit ohmic conduction. In an amorphous system like conducting polymers, the microscopic conductivity depends upon the chain length, conjugation, doping level etc., whereas macroscopic conductivity depends on the homogeneities, compactness of pellets, and also micro particle orientation (38). In the present composite, aniline monomer being uniformly polymerized on the surface of CuS nanoparticles leads to a homogeneous structure of polymer composite. Pure PANI is light weight polymer with poor compactness and the micro particles are randomly oriented. The linkage among the polymer particle through grain boundaries is very poor which result in relatively lower conductivity. In the case of nanocomposite, the growing polymer chain is uniformly adsorbed on the surface of CuS particle and thereby helps to acquire a spherical shape (confirmed from SEM), which leads to an improvement in the compactness of the polymer nano-composite. This change in compactness runs in parallel with the improvement in the internal ordering of the polymer planes as revealed by XRD analysis. At higher molar concentration of nanoparticles, the change in compactness becomes more significant due to the enhanced interaction among the polymer and nanoparticles. These interactions produce an ordered arrangement of PAN! chain (crystallinity) with an enhancement of the intermolecular component of bulk conductivity.
PANI/CuS nanocomposite has been synthesized successfully through a simple and environmentally benign technique by in situ polymerization. The SEM analysis revealed that the CuS nanoparticles are uniformly dispersed in the polymer matrix and the size distribution of nanoparticles are in the nanosize range. UV--vis spectroscopy supports the formation of nanocomposites by the change in wavelength as compared to that of pure PANI. The FTIR spectrum confirms the presence of sharp and resolved infrared bands reflecting upon the chemical interaction between the nanoparticles and PANI. The XRD patterns indicated that PANI/CuS nanocomposite show an ordered arrangement of polymer chain, whereas the PANI synthesized is amorphous in nature. Increase in glass transition temperature with increase in concentration of nano-particles shows the increased orderness in the polymer composite than the pure PANI. The thermal stability of the nanocomposite increasing with increase in concentration of nanoparticles is due to the improved interaction between the polymer chain and the nanoparticle. The higher AC conductivity of PANI/CuS composites than that of PANI may be due to the strong interaction between the metal particles and electronegative nitrogen of PANI. Dielectric constant and dissipation factor (tan 6) of nanocomposites are also increased with increase in CuS content. Due to the ordered structure of polymer composite, the DC conductivity is significantly higher than that of PANI and also the conductivity increased with increase in concentration of nanoparticle. From the above studies it can be concluded that due to high thermal stability, high conductivity and high dielectric constant along with high dissipation factor, the fabricated PANI/CuS nanocomposite can be used as multifunctional materials for different nanoelectronic devices.
Correspondence to: M.T. Ramesan; e-mail: in email@example.com DOI 10.1002/pen.23572 Published online in Wiley Online Library (wileyonlinelibrary.com). [c] 2013 Society of Plastics Engineers
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Department of Chemistry, University of Cal/cut, Kerala 673 635, India
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|Date:||Feb 1, 2014|
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