Polyaniline membranes

The unique transport characteristics of conducting polymer membranes that we have demonstrated arise from their electrochemical properties. For the purpose of this discussion polypyrroles will be used, however, similar principles apply to transport across polyaniline membranes. 

Doping and dedoping of polyaniline membranes with aqueous HCl solution The emereddine base powder was dissolved in NMP (8 wt %). The emeraldine base solution in NMP was cast onto a glass plate and the solvent was removed under 120T for 3 hours. The as-cast membrane was, then, immersed into a 4M HCl solution for 24 hours to give a fully doped membrane. The fully doped membrane was completely dedoped by immersion into IM NH4OH solution for 48 hours. The dedoped membrane was subsequently redoped with 0.0150, 0.0175, 0.0200 ( 0.0002), and 1 M HCl solutions. Each membrane was dried under vacuum for 48 hours at room temperature. 

EPR experiment Narrow strips of polyaniline membrane (1mm x 5mm) were... 

Table I. Transport characteristics of various polyaniline membranes...

Park et al. [102] reported a noticeable reduction of the methanol uptake in Nafion/polypyrrole composite membranes, which is accompanied by a similar decrease in the water sorption. A Nafion/polyaniline membrane containing polyanilines with different oxidation states is reported [127] to reduce around 10 % the methanol uptake, particularly the oxidized state, along with a very small reduction of the water uptake. A similar study on Nafion/polyaniline membrane reported a larger reduction (40 %) of the methanol uptake, which reached 50 % when Si02 was added to the composite membrane, without any significant effect on the water uptake [125]. 

Flash Welding Route to Asymmetrical Polyaniline Membranes... 

Schmidt, V.M., D. Tegtmeyer, and J. Heitbaum. 1995. Transport of protons and water through polyaniline membranes studied with online mass-spectrometry. / Electroanal Chem 385 149. 

Fig. 33.8 Permeability (in barrens) versus size (in angstroms) for selected gases permeating as-cast polyaniline membranes.

Fig. 33.10 Effect of redoping, to different levels, on oxygen and nitrogen permeabilities of a polyaniline membrane. ( ) O2 ( ) N2 (O) a.

Fig. 33.11 The effects of doping, undoping, and partially redoping a polyaniline membrane are shown schematically in a hypothetical cross section of a polyaniline film (a) As-cast (b) doped (c) undoped (d) redoped.

Fig. 33.12 Permeability (in barrers) for gases permeating polyaniline membranes doped with the halogen acids HX(aq) (X = F, Cl, Br, and I) and undoped with NH4OH.

The use of polypyrrole and its derivatives was investigated soon after the reported discovery of polyaniline membranes. Liang and Martin [65] demonstrated that thin films of polypyrrole could be grown on alumina (An-opore) support membranes by using interfacial polymerization techniques. Doped polypyrrole films were found to be porous, showing Knudsen diffusion with an O2/N2 selection coefficient of 0.94. However, poly(A/-methyl-pyrrole) films were nonporous and showed good gas transport and selectivity. For example, a Ajxm poly(A/-methylpyrrole) film doped with NOs" ions had an oxy-... 

The transport of water through supported polyaniline membranes grown electrochemically was investigated by Schmidt et al. [71]. A 25-30% increase in water permeation was observed for sulfuric acid-doped polyaniline compared to the undoped polymer. The doped polymer was believed to have a more open structure, accounting for the enhanced water permeation. However, a simpler explanation is the increased hydrophil-icity of doped polyaniline. Analogous increases in methanol transport through both polyaniline and polypyrrole were found [72,73]. 

Since chemically synthesized free-standing polyaniline membranes are very robust and quite stable in aqueous solutions, it is interesting to explore their liquid permeation properties to elucidate the transport mechanism and identify potentially useful applications. 

A pervaporation setup is shown in Fig, 33.16. A freestanding polyaniline membrane (40-100 /tm thick) is placed on a porous stainless steel support and sealed with an 0-ring. About 200 mL of feed solution at room temperature is put in the feed reservoir, and the downstream side of the pervaporator is evacuated. The permeant collector is cooled to -65°C to maintain a low... 

Pervaporation was next carried out to test the selectivity of polyaniline membranes toward acetic acid-water mixtures. Different feed ratios of acetic acid and water were pervaporated through both undoped and doped polyaniline, as shown in Fig. 33.18, where the feed water content is plotted versus the permeant water content. For comparison the vapor-liquid equilibrium curve for acetic acid-water [78] is plotted just above the line of no separation. From Fig. 33.18 it is clear that undoped polyaniline has a small preference for permeating water over acetic acid at essentially any composition. However, this small preference at an average water permeability of about 0.5 g mm/(m--h) is too low to have any utility. More interesting is fully HCl-doped polyaniline, which permeates water over acetic acid in a much more selective fashion. In fact, even with a mixture of 859f acetic acid-15% water, at least 93 wt % of the permeant was water. It should be pointed out that when undoped membranes are used in the presence of acids, they will partially dope the polyaniline, the extent depending on the pH of the acid used. However, when a... 

Fig. 33.18 Pervaporation of acetic acid-water mixtures through doped and undoped polyaniline membranes. All polyaniline experiments were carried out at room temperature. The vapor-liquid equilibrium curve is plotted based on data points from Ref. 78 at 110-115.3°C and 760 mm Hg.

The permeability of the fully HCl-doped polyaniline membrane of Fig. 33.18 is shown in Fig. 33.19. Here the feed water content is plotted versus the permeability in gram-millimeters per square meter per hour fg mm/ (m h)]. The lower curve indicates the permeability of acetic acid, the middle curve represents the permeability of water, and the upper curve is the overall combined permeability. Clearly the acetic acid permeability of doped polyaniline is exceedingly low except when pure acetic acid is used. Water permeability increases from 0.23 g mm/(m h) at 15 wt % water content in the feed to >1 g mm/(m h) when pure water is the feed. The total permeability essentially parallels the water permeability. 

Fig. 33.19 Permeability of water-acetic acid mixtures through fully HCl-doped polyaniline membranes at room temperature.

Fig. 33.21 Pervaporation of a mixture of organic acids and water through a doped polyaniline membrane. The feed consists of approximately 25 wt % of each acid in water.

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