Effect of the applied magnetic field on formation of complex polyaniline films
© Dimitriev et al. 2008
Received: 22 October 2007
Accepted: 21 October 2008
Published: 21 October 2008
Formation of complex polyaniline (PANI) films with diamagnetic TCNQ and paramagnetic metal ion impurities cast under applied magnetic field was studied. It has been found that the applied magnetic field affects interaction of PANI chains with the impurities and induces formation of magnetically ordered regions in the complex film doped by paramagnetic metal ions in contrast to the reference film of the same composition but prepared under ambient conditions. The magnetically ordered regions have been observed directly by scanning magnetic force microscopy. It was found a correlation in distribution of the magnetically ordered regions and peculiarities of the surface relief of a film. Electronic absorption spectra and conductivity measurements showed that an applied stationary magnetic field can suppress the interaction of PANI chains and paramagnetic metal ions and lowers conductivity of the resulting complex film up to one order of magnitude as compared with the reference film. An alternating magnetic field was found to improve interaction of PANI and diamagnetic TCNQ molecules. The mechanisms of the magnetic field influence on the complex film formation are discussed.
PACS: 75.70.-i; 72.80. Le; 78.40. Me
Effect of an applied magnetic field on chemical reactions, growth of organic films and functional state of organic materials is a subject of continuing interest of scientists in different areas of science at both fundamental and practical levels [1–7]. Despite many controversial results in this field because of usually small effects the magnetic field renders on growth and properties of functional films, some progress in understanding of how an applied magnetic field can control physical and chemical properties of organic materials has been achieved. It is known that the magnetic field can influence the orientation of a molecule through rotation of molecular fragments possessing a magnetic moment to adjust it to the applied field direction or renders the effect of translational motion of a molecule bearing a magnetic moment along the gradient of the applied field . Since many organic functional materials have benzene and/or heterocyclic rings the applied magnetic field will induce an electric current inside the ring with the induced magnetic moment antiparallel to the applied field vector; thus the applied field will interact with the aromatic rings which act as diamagnetic dipoles. The above effects, however, can be noticeable when the magnetic forces overcome those of intermolecular interactions. So, it is expected that the effects can be observed either under a strong magnetic field or when the film is formed from a liquid or gaseous phase. Particularly, it was found that the magnetic field can induce an orientational effect on some organic polymers and biological molecules , as well as it can enhance mass transfer and morphological changes in organic films via magnetomechanical effect [10, 11].
The effect of strong and stationary applied magnetic fields on formation and properties of polyaniline (PANI) has also been studied in the literature [12–22]. In the majority of these works two aspects of the applied magnetic field are considered, i.e., how strong magnetic fields of the order of 0.4–10 T affect chemical polymerization or electropolymerization of aniline, and how strong magnetic fields affect properties of PANI molecules with liquid crystalline groups or rare-earth dopants. Particularly, it was found that the applied magnetic field effects on aniline polymerization [15, 16, 19] and on specific morphology of the resulting PANI particles [12, 14], that PANI molecules can be oriented in a solution , and that PANI films polymerized under a strong magnetic field applied along the film surface become anisotropic and show improved electroactivity and conductivity [18, 20, 21].
We have recently developed novel conducting complexes based on emeraldine base (EB) of PANI and tetracyanoquinodimethane (TCNQ) [23, 24] as well as PANI and transition metal salts [25–27]. Particularly, it was shown that coordination of metal ions to the polymer chain serves two purposes. The first one is doping of the polymer chains, i.e., conversion of the insulating polymer form to the conductive form. The second purpose is directed to induce a specific morphology of the film. Since many transition metal ions possess paramagnetic properties, the applied magnetic field can better control formation of PANI-transition metal complexes, acting on both magnetic moments of metal ions and diamagnetic benzenoid or quinoid groups of PANI molecules. It is suggested that more weak applied magnetic fields are sufficient to affect the resulting film properties formed from the complex molecules bearing spins. This work is a systematic study of the effect of modest magnetic fields, not more than 0.1 T, both stationary and alternating, on morphology and properties of complex PANI films cast from solutions. This study aims to reach a better understanding of mechanisms of the applied magnetic field on organization of complex polymer molecules upon their condensation from a solution to a film.
Powder of emeraldine base (EB) of polyaniline (PANI) was dissolved in N-methylpyrrolidinone (NMP) to prepare a stock solution with concentration of 1 wt.%, followed by treatment of the solution in the ultrasonic bath and filtering of the solution to remove undissolved particles. Salts of CoCl2, Eu(NO3)3, as well as tetracyanoquinodimethane (TCNQ), were dissolved in NMP to prepare solutions with desirable concentrations. Mixtures of EB and CoCl2, EB and Eu(NO3)3, and EB and TCNQ were prepared by addition of the salt or TCNQ solutions to the EB solution with certain proportion of EB (calculated per tetramer units) to metal ions or TCNQ. The mixture usually consisted of 40 ml of the EB stock solution and 10 ml of the salt or TCNQ solution of concentration of 10-1 M to provide an approximate ratio 1:2 of a tetramer unit of the EB to metal cations or TCNQ.
The solutions were used for preparation of films by drop-casting onto glass plates. One series of samples was prepared under ambient conditions, and the other series of samples was prepared under the same temperature and atmospheric conditions but using an applied magnetic field. In the case when a stationary magnetic field was used, the sample on a thin glass substrate was placed in between two stationary magnets, with the opposite poles separated by about 0.5 cm and creating a magnetic field perpendicular to the sample surface with the strength of about ~60 mT in the area of the sample. The surface area of the magnetic poles exceeded that of the sample and the linear size of the poles was much larger the separation distance between the poles, so that the field between the poles was considered to be homogeneous.
In the case of applying an alternating magnetic field, a substrate with the sample was placed onto the solid surface under which a piece of a stationary magnet has been installed. The magnet had a magnetic axis oriented parallel to the substrate surface and a strength measured at the substrate surface of about ~15 mT. During evaporation of the solvent from the drop-cast film, the magnet was rotating around the axis which was perpendicular to the substrate surface with a frequency of several Hertz, so that the magnetic field vector laid mainly at the substrate surface but changed its direction continuously.
Magnetic force microscopy (MFM) measurements were performed by Dimension 3000 NanoScope IIIa scanning probe microscope. Magnetic force gradients were measured using frequency modulation at LiftMode operation (i.e., topography was scanned at the first pass in the tapping mode and magnetic field gradient was scanned at the second one using oscillation frequency shift of the magnetic probe moving at a constant height over surface). The value of lift scan height was about 100 nm; this value was chosen to provide fine resolution of MFM image and not to influence sharp surface features. The Veeco magnetic force etched silicon probes (MESP) with coercivity of ~400 Oe and magnetic moment of 1·10-13 Electro-Magnetic Unit (EMU) were used. Nominal tip radius was 25 nm.
Optical microscopy studies of the film morphology have been performed using a microscope "Axiostar plus" (Carl Zeiss) equipped with a photo-camera and a computer. Electronic absorption spectra were recorded using a dual-beam spectrophotometer "Specord M40" in the transmission mode, bare glass plate served as a reference. IR absorption spectra of the samples prepared on the germanium wafers were recorded using a dual-beam spectrophotometer "Specord M80".
The electrical in-plane conductivity of the films was measured using a standard four-probe technique. During the measurement, an appropriate constant current, I, in the range 0.1 – 10 μA was maintained on two outer probes, and the voltage drop, V, was measured across two inner probes, using a UNI-T M890C+ electrometer. The resulting conductivity, σ, was found according to the expression for the thin-film approximation , σ = ln2 (I/π d V), where d is the film thickness.
Results and discussion
Effect of magnetic field on formation of pristine PANI films
Thus, one can suggest that an applied magnetic field facilitates a proton exchange between chain fragments in highly entangled chains or between the polymer backbone and the residual solvent molecules in thin film with corresponding formation of polarons on the polymer chain. This process should be controlled by the N-H...N intermolecular hydrogen bonding whose energy is of the order of 0.1 eV . A crude estimation shows that to reach such an energy level the magnetic field should be several orders of magnitude higher than that used in our experiments because it creates the energy density of only ~10-6 eV per a hydrogen bond (assuming that the polymer has one hydrogen bond per a tetramer unit and that the polymer chains are closely packed). However, the weak external influence of the magnetic field can be quite effective when there is an additional factor affecting the hydrogen bonding, for example, thermal motion of the chains. That is why the above effects were observed only at elevated temperature and upon a slow formation of films from a solution. Moreover, the intramolecular proton migration which leads to interchange of the benzenoid and quiniod groups in the polymer backbone requires smaller energy contributions and therefore it is more affected by the external magnetic field.
Effect of magnetic field on formation of surface relief of the complex films
Effect of magnetic field on magnetic ordering in the complex films
Interaction of PANI molecules and paramagnetic metal ions in the presence of applied magnetic field
The changed ability of doping of PANI chains by the Co2+ and Eu3+ ions under applied magnetic field can be due to available magnetic moment of these ions. Doping of PANI usually results in appearance of radical nitrogen ions on the polymer chain which are paramagnetic. Interaction of paramagnetic metal ions and paramagnetic nitrogen ions is unfavorable in the presence of the applied magnetic field, because anti-parallel orientation of interacting spins will be suppressed. Thus, the applied magnetic field is unfavorable for doping of the EB chain by paramagnetic metal ions.
Effect of magnetic field on formation of PANI:TCNQ complex films
TCNQ is a diamagnetic molecule due to the negative magnetic susceptibility of its quinoid moieties . As has been shown previously, TCNQ can interact with the EB polymer via charge-transfer interaction, accepting an electron from the electron-rich nitrogen atom of the amine group of EB and leading to formation of anion-radical of TCNQ•- and cation-radical of EB, respectively . The resulting complex film has also the changed electrical and optical properties . Interaction of the EB and TCNQ can also change the diamagnetic status of the EB:TCNQ complex if spin unpairing of the resulting radicals takes place in the complex molecule.
A moderate magnetic field was found to influence formation of both pristine and complex PANI films, leading to the changed film morphology and physical properties. It has been found that the alternating magnetic field affects greatly the morphology of the complex films which was explained by magnetomechanical effect, i.e., a motion of magnetized particles or diamagnetic molecular fragments in the presence of the applied magnetic field. The electric component of the alternating electromagnetic field can probably contribute to this effect also. However, the observed correlation in distribution of topographical features and magnetically ordered regions on the film surface testifies to significant contribution of the magnetic field component to this effect. Therefore, one can conclude that the influence of the applied magnetic field on the film morphology can be enhanced via the presence of paramagnetic impurities in the film structure.
It has been found that the applied magnetic field tends to increase the amount of radicals on the polymer chain or in the PANI-TCNQ charge-transfer complex, i.e., to induce proton or electron exchange, respectively. We speculate that the origin of this effect consists in that the applied magnetic field supports the existence of radical molecular fragments formed spontaneously upon thermal motion and intermolecular interaction during film formation. Two different effects of the complex film formation owing to the suggested mechanism have been observed. The first one, when one of the components bears unpaired spin, is the suppressed interaction of EB and paramagnetic metal ions leading to the phase separation and decreased conductivity of the resulting complex films. The second effect, when both counterparts have paired spins, is the improved interaction of the EB and TCNQ leading to the electron exchange and formation of homogeneous complex films. However, further experiments to probe spin amount in the samples prepared under applied magnetic field are necessary to clarify the observed effects in more detail.
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