We show that hydrophobic flexible polyelectrolyte molecules of poly(2-vinylpyridine) and poly(methacryloyloxyethyl dimethylbenzylammonium chloride) are trapped and frozen due to adsorption on the mica surface, and the observed AFM single molecule structures reflect the molecular conformation in solution. An increase of the ionic strength of the solution induces the cascade of abrupt conformational transitions due to the intrachain segregation from elongated coil to compact globule conformations through intermediate pearl necklace-globule conformations with different amounts of beads per chain. The length of the necklaces and the number of beads decrease, while the diameter of beads increases with the increase of ionic strength. Coexistence at the same time of extended coils, necklaces with different amounts of beads, and compact globules indicates the cascade of the first-order-type phase transitions.
[Show abstract][Hide abstract] ABSTRACT: Visualization and study of reconformation of polyelectrolytes (PEs) of different architecture is of great fundamental and practical interest. Verification of theoretical predictions with experiment is of essential importance. On the other hand, a wide range of bottom-up techniques based on patterning of matter on the length scale of a few nanometers have been recently developed. Particularly interesting is the possibility of using self-assembled single molecule structures as templates for the deposition of inorganic matter, in particular metals. Synthetic "normal-sized" polymers of various architecture, like poly-2-vinylpyridine (P2VP) or polystyrene-poly(2-vynil pyridine) P2VP7-PS7 star-like block copolymer, adsorbed on solid substrates have been visualized for the first time with molecular resolution by AFM in different conformation. This finding allowed us to study largely discussed problem, a coil-to-globule transition of PEs. It was found that PE molecules undergo conformational transitions from stretched worm-like coil to compact globule via set of necklace-like globules, as the fraction of charged monomers decreases with an increase of pH and ionic strength. These results are in good agreement with recently developed DRO theory for weakly charged flexible PEs in poor solvent. The size of the deposited single molecules correlates very well with molecular dimensions in solution obtained in light scattering experiments. PE single molecules of various architectures was mineralized in different conformations that constitutes the route to nanoparticles with desired shape (including wire-shape and star-shaped), size, and composition (including metallic, magnetic and semiconductive nanoparticles). It was shown that molecular details of the adsorbed linear flexible PE molecules determine the dimensions of the nanostructures after metallization and that observed sizes are consistent with the decoration of single molecules with nanoclusters. Thus those metallized nanoparticles (cluster assembles) reflect the conformation of original adsorbed PE molecules. The dimensions of the obtained nanowires are significantly smaller than those previously reported. All of these features are of the potential benefit in applications for nanodevices. Metallization of the PS7-P2VP7 improves AFM resolution due to the selective deposition of Pd clusters along the P2VP chains. For the first time, the number of the P2VP second generation arms of the heteroarm block-copolymer was directly counted in the single molecule AFM experiment. Simple contrasting procedure was developed to improve AFM visualization of positively charged polymer chains deposited on the substrates of relatively high roughness. This method allows increasing the thickness of the resulting structures up to 10 nm, and, consequently, provide visualization of polymer chains on rough surfaces. This innovation is important for the development of single molecule experiments with polymer chains. The reaction of HCF-anion could be used for recognition of polycation molecules, when polycations, polyanions and neutral molecules coexist on the surface. Recently, the study was strongly restricted to atomically smooth surfaces. The contrasting procedure extends the range of substrates (Si-wafers, chemically modified or patterned Si-wafers, polished glasses, polymer films, etc) appropriate for the experiments. Thus, polymer single molecules can be considered not only as representative of the ensemble molecules, but also as individual nanoscale objects which can be used for future nanotechnology for the fabrication of single molecule electronic devices. Also these findings are important from fundamental point of view, since developed approach can be successfully applied for investigation of various "classical" problems in polymer science, such as polymer reconformation, interpolyelectrolyte complex formation, polymer diffusion, adsorption, etc.
[Show abstract][Hide abstract] ABSTRACT: Polymers exhibit extended structures at high temperatures or in good solvents and collapsed configurations at low temperatures or in poor solvents. This fundamental property is crucial to the design of materials, and indeed has been extensively studied in recent years. In this paper, the collapse of polyethylene rings on an attractive surface was investigated by using molecular dynamics simulations. It is found that the collapse of ring chains on the attractive surface is of distinct difference from their free counterparts, where the collapse becomes more continuous and a one-stage instead of two-stage collapse can be identified by the specific heat. Some hairpin-like crystal structures are found at low temperatures, which are induced by the adsorption interaction of polymer-surface. For a given chain length, the results were further compared with those of the adsorbed linear chains. Due to the topological constraint of ring chains, the number of hairpin structures is clearly less than that of the linear chains. These numerical simulations may provide some new insights into the folding of ring polymers under adsorption interactions.
Chinese Journal of Polymer Science 08/2013; 31(8). DOI:10.1007/s10118-013-1307-7 · 1.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Unimolecular micelles constituted from the star-shaped PS7−P2VP7 polysterene/poly(2-vinylpyridine) block copolymer single molecules were studied with molecular resolution using AFM. In selective solvent the unimers show core−shell structures with very pronounced segregation into the collapsed core and the extended shell formed by stretched polymer arms of P2VP and PS in water and toluene, respectively. Metallization of P2VP arms of the copolymer leads to the organic−inorganic nanocomposite with 1−3 nm in diameter palladium clusters localized in the outer shell of unimers, forming starlike structures with metallized arms and improving the contrast to resolve the fine molecular morphology.
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