Polymer containing micelle

Scheme 3. Tiron.

Figure 12. Schematic illustrating micellar/microemulsion nanoparticle synthesis. Reprinted with permission from [4], G. G. Wallace and P. C. Innis, J. Nanosci. Nanotech. 2, 441 (2002). © 2002, American Scientific Publishers.

by extended polymer synthesis in the presence of self-assembled polystyrene cores that are selectively removed by toluene.

Selvan et al. [96, 97] utilized a block copolymer micelle of polystyrene-block-poly(2-vinylpyrridine) in toluene exposed to tetrachloroauric acid which was selectively adsorbed by the micelle structure. On exposure of this solution to pyrrole monomer, doped polypyrrole was observed to be synthesized concurrently with the formation of gold nanoparticles as a result of the reduction of the bound AuCl-. The nano-particles formed had a monodispersed (7-9 nm) gold core surrounded by a PPy shell after annealing at 130 °C for 1 hour. Depending on the postsynthesis treatment, different shaped nanoparticles such as spherical, cubic, tetrahe-dral, and octahedral shapes were formed due to micelle coagulation. Dendritic nanoaggregate structures were also reported to form via a vapor phase polymerisation of the pyrrole monomer onto cast films of the block copolymer. In related work by Zhou et al. [98, 99] metal ion reduction of Au, Pd, and Pt salts with a w-conjugated polymer, poly(dithiafulvene) (PDF), capable of electron donating to the metal salts thereby reducing them and itself being oxidized to provide steric and electrosteric stabilization of the nanoparticle. The resultant metal cores of the composite nanoparticles were 5-6 nm in size. Interestingly, the surface plasmon absorption band in the UV-vis spectrum for a metal nanoparticle (Pd, Pt) was significantly redshifted to 550 nm from an anticipated 510-525 nm. This shift was attributed to the electronic interaction of the oxidized PDF with the metallized cores.

Using a chemical approach involving inverse microemul-sions, it has been shown that small monodisperse polyaniline particles can be produced [100-102]. Inverse microemulsion polymerization of polyaniline by Chan et al. [101] was reported to give particles of 10-35 nm diameter that were spherical and could be synthesized using either the chemical or electrochemical route. The presence of stabilizing surfactant was found to be detrimental to the electrical conductivity of these particles when dried, due to formation of an insulating barrier around the nanoparticles in the bulk. After washing to remove the surfactant, chemically prepared materials exhibited conductivities of up to 10 S cm-1 while for electrochemically prepared materials conductivities as high as 200 S cm-1 were reported. Gan and co-workers have used [102] an inverse microemulsion approach that involves formation of barium sulfate nanoparticles, which are then coated by polyaniline. These composite nanoparticles had a reported conductivity from 0.017 to 5 S cm-1, with particles ranging in size from 10 to 20 nm. Xia and Wang [103] have used ultrasonication during inverse microemulsion polymerization of polyaniline to produce spherical nanoparticles with diameters of 10-50 nm and conductivities in the order of 10 S cm-1. The ultrasonication was found to increase the rate of polymerization of aniline which is typically slow when the microemulsion route is used. Rate increases are obtained by acceleration of the heterogeneous liquid-liquid chemical reactions in solution. A secondary advantage is the prevention of aggregation due to particle agitation induced by ultrasonication.

Polypyrrole nanotubes have also been synthesised via an inverse microemulsion self-assembly route by Jang and

Yoon [104]. Nanotubes were synthesised by using bis(2-ethylhexyl)sulfosuccinate (20.3 mmol) in hexane (40 mL) which form tube- or rodlike micelles, due to the presence of a double tail on a small hydrophilic head group, when aqueous FeCl3 (1 mL, 9.0 M) was added. The Fe3+ cations are effectively trapped in the center core of the micelle. Addition of pyrrole (7.5 mmol) to this solution then results in interfacial polymerization at the micelle surface, resulting in hollow nanotubes 95 nm in diameter and up to 5 /m in length. The electrical conductivity of these nanotubes was up to 30 S cm-1 and was observed to be a function of the FeCl3 oxidant concentration. More interestingly, the conductivity of these materials did not appear to be limited by the presence of the surfactant molecule, which in other emulsion polymerization routes tends to lead to a thick insulating coating that limits the observable conductivity.

2.3. Physical Templates

Conducting polymers have been grown within zeolites [105, 106]. With channels as small as 0.3 nm this allows assembly of single molecular wires. Conductivities of 10-9 S cm-1 for such wires contained within zeolites have been reported and this increases to 10-2 S cm-1 when the polymer is extracted from the host structure.

Martin and co-workers [107-109] have used controlled pore size membranes as templates to electrochemically grow fibrillar mats of ICPs. Similar structures have also been produced using nanoporous particle track-etched polycarbonate membranes with both polypyrrole [110-113] and polyani-line [114] via chemical and electrochemical techniques. The approach involves oxidation of the monomer within the pores of a template. This is achieved electrochemically as illustrated in Figure 13. The electrode substrate used can be either a conventional flat surface system such as solid gold, platinum, or glassy carbon, resulting in structures such as shown in Figure 14a. Alternatively, more novel electrode substrates such as platinized PVDF membranes with nominal pore size of 0.45 / m can be used (Fig. 14b).

Carbon nanotubes (CNTs) have been directly synthesized via a template carbonization of polypyrrole on an alumina template membrane (Whatman Anopore) with 0.2 / m diameter pores and 60 / m in total thickness [115]. Polypyr-role was chemically deposited on and through the membrane

0 0

Post a comment