Polymer/magnetite nanocomposites with electrical and magnetic conductivity
Electrically conductive polymers play a significant role in the electrical and electronics industries. However, most conducting polymers suffer from low processability, which limits the effective utilization of these materials in commercial applications.1, 2
Several researchers have reported on the poor solubility and processability of polyaniline (PANI).3, 4 The properties of PANI can be improved by attaching more polar functional groups (which improve the intermolecular interactions), but this requires a tedious process that can reduce the conducting properties of PANI.5 Poly(anthranilic acid), PANA, can be used as a powerful substitute for PANI because it contains a carboxylic acid group in the main chain. This group can effectively react with polar monomers or polar filler particles through strong interactions. Although the conductivity of PANA is poor compared with PANI, its conductivity can be enhanced by adding conductive polar metal-oxide nanoparticles (NPs). In previous work, we used magnetite (Fe3O4) NPs—which are highly compatible with the PANA chain, through the interfacial interaction between active sites of both polymer and magnetite NPs—to achieve PANA composites with improved functionalities.6
More recently, we have carried out oxidative in situ polymerization of anthranilic acid with various concentrations of magnetite NPs in a one-step process.7 We then analyzed high-resolution transmission electron microscopy images of the nanocomposites and found that, in the case of those with 10wt% loading, NPs are uniformly distributed in the PANA—see Figure 1(a)—while an aggregation or cluster morphology is observed in those with a concentration of 20wt%: see Figure 1(b).
Using x-ray diffraction analysis (see Figure 2), we were able to observe a strong interfacial interaction between PANA and the magnetite NPs. We found that the addition of magnetite imparts a crystalline or regular-chain arrangement in the composite materials. We also noted that the intensity and broadness of the amorphous peak of PANA in the composite is reduced by the introduction of the NPs. This result proves that the magnetite NPs have undergone a uniform dispersion in the polymer chain, leading to a regular arrangement of the macromolecular chain in the polymer matrix.8
To determine the magnetic properties of our samples, we obtained magnetic hysteresis loops of PANA with varying magnetite-NP contents using a vibrational-sample magnetometer. The magnetization of the composite materials (shown in Figure 3) exhibits a typical hysteretic behavior, thus proving that they show ferromagnetic/superparamagnetic properties.9 This enhanced magnetic behavior arises mainly as a result of the strong interaction between magnetite NPs and the carboxyl group of PANA, which leads to a uniform distribution of NPs. With an increase in NP concentration, we found that the saturation of magnetization and the remanence values of the composites are increased, whereas the coercive force decreases.
We also investigated the temperature-dependent AC conductivity of our PANA/magnetite composites with respect to the frequency and concentration of the metal-oxide NPs. The plots, shown in Figure 4, indicate that the conductivity of the composites increases with the volume fraction of NPs, up to a certain concentration (i.e., the electrical percolation threshold). We found that the percolation threshold appears at a concentration of 15wt% magnetite as a result of the structure, morphology, interfacial interaction, and processing method used. The AC conductivity is shown to increase with frequency. Additionally, these graphs are almost linear, which is reflective of the hopping of electrons (charge conduction) from one zone to another—i.e., between Fe2+ and Fe3+ ions—taking place in a linear fashion.10We found the PANA composite with 15wt% magnetite to achieve the maximum AC electrical conductivity (−0.598S/cm at 106Hz).
We also analyzed the variation of current–voltage (I–V) characteristics in the pure PANA and the PANA/magnetite composites at room temperature using a Keithly 2400. The linear I–V dependence of the nanocomposite (see Figure 5) is linear in nature, suggesting systematic and ohmic behavior.11 The conductivity of the nanocomposites is much higher than the parent polymer, and increases significantly with an increased concentration of magnetite NPs. This is as a result of the continuous conductive network that is formed within the composite. Additionally, the nanofillers undergo interfacial interaction with the interaction zone of the macromolecular chain. Interfacial contact between the magnetite NPs and the PANA chain thus enhances the charge injection at the interface, leading to an increase in current flow in the resultant polymer matrix.12
In summary, we have developed PANA/magnetite nanocomposites via a one-step process. Our analysis suggests that interfacial interactions between the NPs and the polymer play an important role in the conductive network that forms within these novel nanocomposites. Additionally, we found that their DC and AC electrical conductivities were much higher than in the neat-polymer case, and that the conductivity increases with NP concentration and temperature. The PANA/magnetite composites that we have synthesized show a wide range of electrical properties at various temperatures, in addition to their magnetic behavior, and would thus be suitable for use in electromagnetic shielding applications within electronic devices. In our future studies, we will investigate the feasibility of using this polymeric composite in low-cost efficient sensors.
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