We performed a series of molecular dynamics simulations investigating the static and dynamic properties of polymer melts confined between planar solid surfaces. The solid–melt interface was found to be very narrow (approximately two segment diameters) and independent of chain length. Inside the interface the segment density profile was oscillatory, the bond orientation altered between directions parallel and normal to the solid surface, and the chain ends accumulated very close to the wall (in the absence of strong wall–segment attraction). The oscillations of the segment density profile were weaker and were dampened faster than those of a simple fluid density profile next to the same solid surface. This reflected the reduced ability of sequences of connected segments (chains) to layer themselves against a solid surface because of restrictions on their configurations imposed by the chain connectivity requirement. This effect made the solid–melt interface even narrower than that of a simple fluid. Only the chain portions lying inside the interface had their shape affected by the wall. Chain statistical segments inside the interface assumed orientations parallel to the wall. In the absence of wall–segment attraction, the size of the statistical segments inside the interface was unaffected. This situation resulted in an apparent decrease of the radius of gyration normal to the wall an apparent increase of the radius of gyration parallel to the wall and spatial independence of the total radius of gyration. The wall effect was gradually diminished and chains assumed their bulk dimensions when their center-of-mass was so far from the solid surface that no portions of the chain could reach the interface (i.e., at a distance comparable to the bulk radius of gyration). The microscopic dynamics of chain portions inside the interface were strongly anisotropic. The mobility increased in the direction parallel to the wall and decreased normal to the wall. This fact was caused by the angular asymmetry of the segment–segment collisions inside the interface, i.e., by the same mechanism that induces the segment layering. The total mobility inside the neutral wall–melt interface was identical with that in the bulk reflecting the fact that the average segment density inside the interface had essentially the bulk value. The presence of strong wall–segment attraction increased the average interfacial density above the bulk value and lowered the mobility of the interfacial chain portions in all directions. The mean-square displacement of the chain center-of-mass during a certain time interval was affected by the solid only if the chain had a portion of itself inside the interface for a fraction of this time interval. The longest relaxation time of the chains, a property that cannnot be localized properly on a length scale smaller than the interfacial width, exhibited a weak and strongly diminishing with chain length spatial dependence.

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A donor-acceptor, rod-coil diblock copolymer has been synthesized with the objective of enhancing the photovoltaic efficiency of the PPV-C 60 (PPV) poly(p-phenylenevinylene)) system by the incorporation of both components in a molecular architecture that is self-structuring through microphase separation. Diblock copolymers were obtained by using an end-functionalized rigid-rod block of poly(2,5-dioctyloxy-1,4phenylenevinylene) as a macroinitiator for the nitroxide-mediated controlled radical polymerization of a flexible poly(styrene-stat-chloromethylstyrene) block. The latter block was subsequently functionalized with C 60 through atom-transfer radical addition. In a spin-cast film of the final diblock copolymer, the luminescence from PPV is strongly quenched, indicating efficient electron transfer to C 60. Under suitable conditions, solution-cast films of these diblock copolymers exhibit micrometer-scale, honeycomb-like patterns of holes.

We have directly measured the entropic elasticity due to the uncoiling of individual polymer chains of poly(methacrylic acid) (PMAA) using the atomic force microscope (AFM). Covalent attachment of one chain end to a substrate and sufficiently low chain grafting densities were achieved by using a mixed monolayer technique that involved the co-chemisorption of (mono)thiol-functionalized PMAA and self-assembling alkanethiols on gold. Single molecule force spectroscopy experiments were carried out in good solvent conditions where the chains were tethered to a Si3N4 probe tip via nonspecific physisorption interactions. Upon retraction of the probe tip from the surface, single, continuous, attractive peaks in the force versus distance profiles were frequently observed. These peaks could be fit, for all chain bridging lengths, to entropic-based, statistical mechanical, random-walk formulations, i.e., the free ly j ointed chain (FJC) model and wormlike chain (WLC) model. The fits to both models yielded a statistical segment length or persistence length of ≈0.3 nm (approximately the length of a single PMAA monomer unit), thus suggesting that locally the chains are quite flexible. In addition to measuring entropic elasticity, we have also shown that single molecule force spectroscopy experiments are able to provide quantitative information on the statistical nature of adsorption of single polymer chains.

The equilibrium segregation of deuterated polystyrene-poly(2-vinylpyridine) diblock copolymers to interfaces between high molecular weight polystyrene and poly(2-vinylpyridine) homopolymers was measured by forward recoil spectrometry. The dependence of the integrated segregation on the equilibrium copolymer concentration in the PS phase is compared to predictions from a mean-field theory in which the copolymer chemical potential is the relevant parameter. Predictions from the theory are quantitatively accurate for values of the copolymer chemical potential, which are below a certain limiting value associated with the formation of block copolymer micelles. The segregation behavior in the regime where micelles are present is complicated by a strong tendency for micelles to segregate to the free polystyrene surface and by a weaker tendency for micelles to segregate to the interfacial region. Values of the copolymer chemical potential at the micelle transition are obtained from a careful analysis of the data and are in reasonable agreement with predictions from a simplified theory of micelle formation.

No abstract

Hydrophobins are small fungal proteins that self-assemble at hydrophilic/hydrophobic interfaces into amphipathic membranes that, in the case of Class I hydrophobins, can be disassembled only by treatment with agents like pure trifluoroacetic acid. Here we characterize, by spectroscopic techniques, the structural changes that occur upon assembly at an air/water interface and upon assembly on a hydrophobic solid surface, and the influence of deglycosylation on these events. We determined that the hydrophobin SC3 from Schizophyllum commune contains 16-22 O-linked mannose residues, probably attached to the N-terminal part of the peptide chain. Scanning force microscopy revealed that SC3 adsorbs specifically to a hydrophobic surface and cannot be removed by heating at 100 degrees C in 2% sodium dodecyl sulfate. Attenuated total reflection Fourier transform infrared spectroscopy and circular dichroism spectroscopy revealed that the monomeric, water-soluble form of the protein is rich in beta-sheet structure and that the amount of beta-sheet is increased after self-assembly on a water-air interface. Alpha-helix is induced specifically upon assembly of the protein on a hydrophobic solid. We propose a model for the formation of rodlets, which may be induced by dehydration and a conformational change of the glycosylated part of the protein, resulting in the formation of an amphipathic alpha-helix that forms an anchor for binding to a substrate. The assembly in the beta-sheet form seems to be involved in lowering of the surface tension, a potential function of hydrophobins.

A method for chemically modifying a surface with grafted monolayers of initiator groups, which can be used for a "living" free radical photopolymerization, is described. By using "living" free radical polymerizations, we were able to control the length of the grafted polymer chains and therefore the layer thickness up to ∼100 nm. Also, single-layer grafted block copolymers were obtained by subsequent polymerizations of styrene and methyl methacrylate monomers. The surface-grafted polymer and block copolymer layers were evidenced by direct imaging methods (transmission and scanning electron microscopy) and by indirect surface characterization methods (contact angle measurements, SFM, XPS, and IR). The ability to control the thickness of the grafted polymer as well as the synthesis of a grafted block copolymer layer in a well-defined manner affirms the "living" character of the surface-initiated free radical photopolymerization.