Study of the Transport Properties of Membranes Modified with Polyaniline Electrosynthesized at Different Temperatures
Authors: Alvarez Cerda Luis Manuel , Montes Rojas Antonio , Torrez Rodriguez Luz María 
Published: May 2020

Even though the ion exchange membranes (IEM) have been a commercial success, the necessity to improve the selectivity properties is constant. One way to reach this purpose is to modify the surface of the commercial membrane using electronic conducting polymers like polyaniline (PAni) The emeraldine salt form has a conductivity similar to a metal and has a particular hydrophilic, so that the integration does not have any substantial contributions on electric resistance or hydrophilic characteristics, but it influences transport properties. In literature recommends taking care of the synthesis temperature, as a control parameter, to obtain the best characteristics possible in hydrophilic and conductivity. Therefore, with the decrease of the temperature, the reaction rate is slow down and the amount of the material deposited is less to obtain an orderly and better arrangement on the surface of the membrane. Among the results obtained in our laboratory highlights that, if the polymerization is carry on a rank between 20 and 40 °C, the crystalline properties of the deposit are better at 20°C. Nevertheless, in literature, there is no evidence if the crystallinity is better as we decrease the temperature under 20°C.

Therefore, we studied the effect of the voltamperometric synthesis temperature of the PAni upon an IEM in a rage of 10 to 20 °C, setting the sweep cycles constant. We modified in a cell with three electrodes and fix the number of cycles at 30. The sweep rate was 100 mV/s and the working temperatures were 10, 15 y 20 °C. The work solution had a concentration of 0.1M of aniline and 1.0M of H2SO4. The deposit is only on one face of the membrane. We made the characterization of the transport properties of the membranes by chronopotentiometry to obtain the transport number (tNO3-), and by UV-VIS spectroscopy, we obtained the ion exchange capacity (IEC). Finally, we determinate the contact angle and the dragging static energy (DSE), as well as the infrared spectra (IR) of each modified membrane.

One first result was the obtainment of the typical voltamperometric response of the polymer during the polymerization on their different oxidation states. In all cases, the current magnitude of the peaks increased with the progression of the cycles applied, which confirms the growing of the polymeric deposit on the membrane surface. Whereas, in an aniline free solution, the voltamperograms of the deposits present a magnitude proportional to the synthesis temperature. In the evaluation of the oxidation peak located at 273 mV, we can see the proportionality between the current magnitude and the temperature (0.05, 0.366 and 0.780 mA at 10, 15 and 20 °C respectively); and the current is related with the quantity of the electroactive material deposited. At this point, when the temperature increases the arrangement becomes more disordered. We observed this tendency in the values of the DSE (4.69 and 4.99 to 13.91 at the temperature 10, 15 and 20 °C respectively).

Additionally, the IR spectra of the modified membranes confirm the presence of conductive PAni in our deposits, observing the characteristic bands at 1150 and 1240 cm-1, which are distinctive of the conductivity and presence of polaron respectively, Figure 1A. On the other hand, in the case of the transport number obtained, this decreases as the temperature increases. We must consider that the tNO3- has a relation with the amount of material deposited on the membrane during electrosynthesis. This leads to the channels in the deposit narrowing as more polyaniline is present and therefore it is more difficult for an ion to pass through. Another effect that may contribute to a decrease tNO3- is hydrophilic of the modified membrane surface. Indeed, as the temperature increases, the values obtained for the contact angle increase, in other words, the membrane becomes more hydrophobic, because of this, it is more difficult for a hydrated ion to pass through an increasingly hydrophobic environment. Finally, concerning the IEC, this increases as the temperature increases, because the polymer has fixed charges (Emeraldine salt form). Thus, as the number of materials increases with temperature, as mentioned above, then the deposit brings more and more fixed charges to the membrane.

In conclusion, as the synthesis temperature decreases, the amount of material deposited decreases because the polymerization reaction slows down, while as the temperature increases, the amount of material deposited on the membrane increases and the arrangement becomes more disordered. The tNO3- decreases due to increased hydrophobicity and reduced space in the membrane channels. Finally, the fixed charges of the PAni increase the IIC. Results summary in Figure 1B.

Figure 1