Interfacial Polymerisation of Pyrrole: Formation of Thin Films and Noble Metal Nanocomposites
PANI is a polymer with a slightly hydrophilic character. It can be solubilised in water upon high dilution and sonication in order to stop unwanted clustering. In contrast with PANI, polypyrrole (PPy) is a totally insoluble polymer that cannot be solubilised in any polar or non-polar solvent (organic or water). Based on this unique lack of solubility for PPy black, polymerisation in a liquid-liquid interface should be expected to lead to formation of a suspended thin film at the boundary of the two phases, which can be removed from the interface and purified with reaction solvents readily. The lack of solubility of black PPy powder, in contrast with its oligomers, in any solvent has been assigned to extensive crosslinking between polymer chains [13a]. For a detailed analysis of the mechanism of this interfacial polymerisation, see the review by Dallas and Georgakilas [13b].
Figure 4.4 shows the mechanism for oxidative formation of PPy through a redox reaction with metal cations. It involves oxidation of the monomer with metal cations, which could be ferric ions derived from iron nitrate or silver cations derived from silver nitrate (for Fe3+; Eox = 0.77 V and for Ag+; Eox = 0.80 V), both of which are dissolved in aqueous solutions. The first report of pyrrole polymerisation was as an oxidative polymerisation occurring in aqueous solutions, where addition of iron nitrate led to spontaneous formation of black PPy powder. Usually, the reaction takes place in aqueous solutions in the presence of an oxidiser. The pyrrole monomer has solubility in water of 7.5 wt% [14]. The most used oxidative agents are ferric ions, which oxidise the monomer by attacking the а-position of the pyrrole ring, and the reaction propagates through subsequent addition of pyrrole molecules. This black PPy precipitate is a charged polycation with crosslinked chains, and with nitrate as a counterbalancing anion.
Cyclodextrins (CD) are highly soluble macromolecules in water thanks to their decoration with hydroxyl groups. Their remarkable solubility in water has made them appealing candidates for the synthesis of inclusion complexes and for biomedical applications. They have also been applied in conductive polymers, and have been used for the synthesis of a 3D network from branched PPy nanofibres through interfacial polymerisation. The interfacial reaction takes place in the presence of the sulfonated P-cyclodextrins (SP-CD) which, due to the sulfonate ions, act as a doping agent. The effect of the concentration of SP-CD in polymerisation solvents, the type of oxidant used to initiate the reaction, and the temperature on the electrical properties of PPy-SP-CD was investigated [15]. Similarly, interfacial polymerisation in the presence of SP-CD has been reported for the aniline monomer [16], and led to the formation of chiral PANI nanobundles. The chiral nanostructures stem from the intertwining of PANI and CD sulfate chains during polymer growth and they were isolated exclusively from aqueous solution irrespective of the organic solvent used [16].
Figure 4.4 The interfacial oxidative polymerisation of pyrrole towards the synthesis of carbon nanotubes (CNT)-PPy nanocomposites. Reproduced with permission from P. Dallas and V. Georgakilas, Advances in Colloid and Interface Science, 2015, 224, 46. ©2015, Elsevier [13b]
