Kinetics of Adsorption of Poly(o-methoxyaniline) Self-Assembled Films

Abstract

The kinetics of adsorption of poly(o-methoxyaniline) (POMA) via self-assembly has been investigated using UV−vis spectroscopy. Though experimental conditions, including type of substrate, affect the amount of polymer adsorbed, adsorption of the first POMA layer on glass substrates was shown to occur in two stages, reaching saturation within a few minutes. The first stage is represented by a first-order kinetics with characteristic times of 5−10 s. The second stage also has an exponential behavior with t 1.5 and characteristic times of hundreds of seconds, which was valid for all substrates when the shift in the polaronic band in the beginning of adsorption was taken into account. The t 1.5 within the exponential is believed to indicate a diffusion-controlled growth of small domains formed during the first stage of adsorption. The appearance of small domains and their subsequent growth are consistent with scanning electron micrographs obtained at distinct time periods of adsorption. When a POMA layer was made to adsorb on an already formed POMA/poly(ethenesulfonic acid) self-assembly multilayer, the adsorption process became faster as the number of bilayers already deposited increased.

Adsorption mechanisms in layer-by-layer films

“…The experimental procedures adopted for producing such SA lms have been described in ref. 18 , and therefore only a short description is given here. POMA was synthesized with ammonium peroxydisulfate in a 1.0 M HCl aqueous solution at 0 C. The monomer to oxidant ratio was 4:1.…”

Influence of the pH and substrate immersion time on the adsorption of poly(o-ethoxyaniline) in self-assembled films

“…Also, the shorter immersion time helps to prevent the competitive adsorption of water molecules, which in longer times would have more chance to attach to the substrate surface, reducing the number of sites for POEA adsorption. The water adsorption is very significant in these systems and according to the work done by Raposo et al 13 an energy is spent to remove water molecules from the substrate surface and to replace them by polymer chains. Besides, during the washing process both POEA and water molecules which are not very well adsorbed, are removed releasing sites for other molecules in a next immersion.…”

“…[11][12][13][14][15] The predominant mechanism will depend on the pH of polymeric solution. In our previous work we have observed that in pH 3 the adsorption of poly(o-ethoxyaniline) on quartz is driven by electrostatic attraction due to the protonated nitrogen atoms; however, if the solution pH is increased to 5 hydrogen bonding becomes more important.…”

“…At high pHs the POEA chains not significantly protonated will adsorb mainly through hydrogen bonding, via amine and imine sites, in agreement with the literature. [13][14][15]20 Hydrogen bonding allows the polymer to adsorb over itself without saturation under the conditions studied in Figure 2b and c. At low pHs although the hydrogen bonding might be also acting on the adsorption, the large number of protonated nitrogen atoms from POEA chains already attached would repel upcoming chains inducing an earlier interruption of the adsorption process.AFM images of POEA films obtained in two different time scales of substrate immersion, at pH 5, are presented in Figure 3. The AFM image of a clean quartz substrate without any film deposited (Figure 3a) reveals a globular morphology with very low roughness.…”

Filmes poliméricos ultrafinos produzidos pela técnica de automontagem: preparação, propriedades e aplicações

“…Na literatura são encontradas dezenas de trabalhos que tratam do tema [18][19][20][21][22][23] , revelando que mecanismos genéricos ainda não podem ser propostos, uma vez que sistemas muito diferentes têm sido investigados. Em filmes formados por camadas de polieletrólitos, o processo de automontagem é governado por interações eletrostáticas entre seus grupos iônicos, levando à formação de pares iônicos 18 .…”