| The fabrication of structurally regulated organic ultrathin films by simple and convenient methods has gained a lot of attention in the fields of materials science and interface science, and ultrathin film through layer-by-layer (LbL) assembly became one of the most active fields because of its strong potential in buildup of new materials. Adsorption of polymer is the basis of problem of LbL assembly, although it is a problem that is far from full understood, because of the complexity of both polymer itself and the influent factor, and because of the limit of traditional research methods. Problems such as the adsorption structure of polymer on substrate, the conformation of special polymer in solution, the driving force of adsorption of polymer are not clearly enough to direct the design and the control of LbL assembly. It is expected that there will be more powerful methods to investigate the adsorption of polymer on molecular level.Atomic force microscopy (AFM) based single molecule force spectroscopy (SMFS) with high spatial resolution and extreme force sensitivity, was specially built up for the study of nano-mechanics. It is powerful for the characterization of elasticity of polymer strands, inter-chain or intra-chain secondary structure of polymer. Furthermore, it is an ideal and efficient method to study the adsorption of polymers on molecular level. This paper planned to investigate the adsorption of two kinds of polymer used in LbL assembly, aiming for a better understanding of conformation of polymer in solution, as well as structure and driven force of adsorption of polymers.Poly(allylamine hydrochloride) (PAH), which is frequently used in fabricating polyelectrolyte multilayer films, was studied in the works of the second chapter. Plenty of force-extension curves with a long plateau were obtained in water, indicating that train-like structure was predominant when PAH was adsorbed on the substrate. The average desorption force in water was found to be equal to 109±7 pN. It was found that the peak-type force-extension curves of PAH in water were not able to be fitted by the modified freely-jointed chain model. Additionally, there was a flat region in the derivative of force-extension curves. Thus, it was inferred that PAH chain in water was in a special conformation and underwent a"conformational transition"under the stretching of an external force. This phenomenon did not appear in the SMFS experiment in 1 mol·L-1 urea solution, which indicated that urea was able to break the special conformation. According to the discussion based on literatures, it is speculated that hydrophobic interaction could play an important role in the formation of the special conformation.In the works of the third chapter, the adsorption of polymer driven by charge-transfer (CT) interaction was studied. The average desorption force of PVK from the substrate modified with groups of 3,5-dinitrobenzamido was found to be 29 pN by analyzing the height of the plateau on the force-extension curves. The average bonding energy was calculated from the desorption force to be - 4.4 kJ·mol-1. It is in agreement in the order of magnitude withΔrG?m, which was calculated from UV-vis spectra using Benesi-Hildebrand equation. The average desorption force was found to be strongly dependent on the electron deficiency of the group modified on the substrate. This result further confirmed the speculation that the plateau can be attributed to the adsorption of PVK driven by CT interaction. So far as we know, it's the first report of the direct detection of the adsorption force of polymer driven by CT interaction using single molecule force spectroscopy. This work is also helpful for understanding the driven force for layer-by-layer assembly based on CT interaction.
|
PDF Full Text Request