Search results for "INTERFACE"
showing 10 items of 2139 documents
Two-Dimensional Assembly Formation of Hydrophobic Helical Peptides at the Air/Water Interface: Fluorescence Microscopic Study
1995
Monolayer formation of hydrophobic α-helical peptides, X-(Ala-Aib) 8 -Y (X=Boc-, HOOCCH 2 CH 2 CO-, biotinyl, biotinyl-(Sar) 3 -; Y=OMe, OBzl, OH), at the air/water interface was studied by the fluorescence microscopic method. Some peptides showed a mound in the π-A isotherm. When the monolayer containing a small amount of FITC-labeled peptide was held at the surface pressure corresponding to the top of the mound, bright and dark domains were observed by fluorescence microscopy. Domain formation was also observed by the addition of a cationic dye (DiIC 1 ) into the subphase underneath the peptide monolayer. The mound in the π-A isotherm is, therefore, ascribed to the phase transition from a…
Surface plasmon near-field imaging of very thin microstructured polymer layers.
2004
We report on the near-field imaging of microstructured polymer layers deposited on an homogeneous metal thin film on which a surface plasmon mode is excited. The microstructures in the polymer layers are designed by electron beam lithography, and the near-field imaging is performed with a photon scanning tunneling microscope (PSTM). We show that, despite their very small height, the microstructures can be conveniently imaged with a PSTM thanks to the field enhancement at the surface of the metal thin film supporting the surface plasmon. The influence of the illumination conditions on the contrast of the PSTM images is discussed. In particular, we show that both the field enhancement and the…
Static and dynamic properties of supercooled thin polymer films
2004
The dynamic and static properties of a supercooled (non-entangled) polymer melt are investigated via molecular-dynamics (MD) simulations. The system is confined between two completely smooth and purely repulsive walls. The wall-to-wall separation (film thickness), D, is varied from about 3 to about 14 times the bulk radius of gyration. Despite the geometric confinement, the supercooled films exhibit many qualitative features which were also observed in the bulk and could be analyzed in terms of mode-coupling theory (MCT). Examples are the two-step relaxation of the incoherent intermediate scattering function, the time-temperature superposition property of the late time alpha-process and the…
Glass transition of polymer melts: Test of theoretical concepts by computer simulation.
2003
Abstract Polymers are good glass formers and allow for the study of melts near the glass transition in (meta-)stable equilibrium. Theories of the glass transition imply such an equilibrium and can, hence, be tested by the study of polymer melts. After a brief summary of the basic experimental facts about the glass transition in polymers, the main theoretical concepts are reviewed: mode coupling theory (MCT), entropy theory, free-volume theory, the idea of a growing length describing the size of cooperative regions, etc. Then, two basic coarse-grained models of polymers are described, which have been developed aiming at a test of these concepts. The first model is the bond-fluctuation model …
Anisotropic Stick-slip friction of highly oriented thin films of poly(tetrafluoroethylene) at the molecular level
1996
Lateral force microscopy (LFM) studies of poly(tetrafluoroethylene) (PTFE) films with molecular resolution are reported. Thin PTFE layers with a high degree of orientation were obtained by pressing and sliding a block of polymer on a clean, heated muscovite mica substrate. LFM nanographs obtained on these films by scanning at directions between ca. 40 and 90° with respect to the film orientation direction, i.e. with respect to the direction of the polymer chains, showed a “stick-slip” type frictional motion of the LFM probe tip at the molecular level. The friction force observed at constant load decreased with decreasing scan angles. Chain-chain packing distances obtained by LFM and contact…
Scaling concepts for polymer brushes and their test with computer simulation
2004
After a brief review of the scaling concepts for static and dynamic properties of polymer brushes in good solvents and Theta solvents, the Monte Carlo evidence is discussed. It is shown that under typical conditions the diameter of the last blob is of the order of 10-20% of the brush height, and therefore pronounced deviations from the self-consistent field predictions occur. In bad solvents, lateral microphase separation occurs leading to an irregular pattern of "dimples". Particularly interesting is the response of brushes to shear deformation, and the interaction between two interpenetrating brushes. Recent attempts to understand the resulting shear forces via molecular-dynamics simulati…
Molecular modeling studies of interactions between sodium polyacrylate polymer and calcite surface
2013
Abstract The interactions between calcite pigment and sodium polyacrylate dispersing agent, widely used in papermaking as paper coating components, were investigated using classical force field and quantum chemical approaches. The objective was to understand interactions between the calcite surface and sodium polyacrylate polymer at 300 K using molecular dynamics simulations. A quantum mechanical ab initio Hartree–Fock method was also used to obtain detailed information about the sodium polyacrylate polymer structure. The effect of water molecules (moisture) on the interactions was also examined. Calculations showed that molecular weight, branching and the orientation of sodium polyacrylate…
Frictional Drag Mechanisms between Polymer-Bearing Surfaces
2001
The fundamental features of friction between two polymer-bearing surfaces in relative sliding motion are investigated by molecular dynamics simulation. Adsorbed and grafted polymers are considered in good and bad solutions. The solvent is not treated explicitly but indirectly in terms of a Langevin thermostat. In both systems, we observe shear thinning that is attributed to an orientation of the radius of gyration along the sliding direction. This effect is particularly strong for surfaces bearing polymer brushes. In this case, the shear stresses are mainly determined by the degree of the interpenetration of brushes.
Photoinduced electron transfer in molecular organizates at the gas-water interface
1994
Abstract The influence of the molecular geometry is of fundamental importance for a better understanding of the photoinduced electron transfer mechanism. Because of their typical molecular structures, cyclophane rings have proved to be suitable for this purpose as electron acceptor molecules adsorbed under an amphiphile monolayer. We used a pyrene-labelled phospholipid derivative both as molecular anchor for the cyclophane ring and electron donor molecule. The co-spreading technique was used to prepare the complex monolayers. Surface pressure and surface potential measurements have indicated similar monolayer behaviour as with dimyristoylphosphatidic acid as anchor molecule, leading to the …
On the tribology and rheology of polymer brushes in good solvent conditions: a molecular dynamics study
2003
Tribological and rheological properties of two polymer brushes in relative sliding motion and good solvent conditions are investigated by means of molecular dynamics (MD) simulations. The lateral forces between the brushes are found to decrease logarithmically with increasing relative sliding velocity v0 over a range of more than one decade in v0. We also observe an almost logarithmic relaxation of the end-to-end distance vector that occurs after sliding is stopped. The coincidence of these logarithmic dependencies support the picture that friction between polymer brushes is small due to the retraction of the polymers from the interpenetration zone. The shear stress relaxes almost instantly…