When assessing the quality of archwires, account should be taken of the surface quality, as it is this that determines corrosion resistance, biocompatibility and friction characteristics. The mechanical properties depend on the initial state; moderate plateau forces and plateau moments can only be achieved with martensitic archwires. In contrast to conventional steel alloys, the strength characteristics are heavily dependent on temperature and need to be known if NiTi archwires are to be used to optimal effect. In addition, the superelastic plateau is used only partially, if at all, when minimum leveling is required.

Hydrothermally stable particles of the metal-organic framework MIL-101(Cr) were incorporated into a polysulfone (PSF) matrix to produce mixed-matrix or composite membranes with excellent dispersion of MIL-101 particles and good adhesion within the polymer matrix. Pure gas (O2, N2, CO2 and CH4) permeation tests showed a significant increase of gas permeabilities of the mixed-matrix membranes without any loss in selectivity. Positron annihilation lifetime spectroscopy (PALS) indicated that the increased gas permeability is due to the free volume in the PSF polymer and the added large free volume inside the MIL-101 particles. The trend of the gas transport properties of the composite membranes could be reproduced by a Maxwell model.

Positron annihilation lifetime spectroscopy (PALS) and pressure-volume-temperature (PVT) experiments were performed to characterize the temperature dependent microstructure of the hole free volume in the low molecular weight glass-former phenyl salicylate (salol). The PALS spectra were analyzed with the new routine LT9.0 and the volume distribution of subnanometer size holes characterized by its mean and standard deviation sigma(h) was calculated. Crystallization of the amorphous sample was observed in the temperature range above 250 K, which leads to a vanishing of the positronium formation. The positronium signal recovered after melting at 303 K. A combination of PALS with PVT data enabled us to calculate the specific density N(h)('), the specific volume V(f), and the fraction of holes f(h) in the amorphous state. From comparison with dielectric measurements in the temperature range above T(B)=265 K, it was found that the primary structural relaxation slows down with temperature, faster than the shrinkage of the hole free volume V(f) would predict, on the basis of the Cohen-Turnbull (CT) free volume theory. CT plots can be linearized by replacing V(f) of the CT theory by (V(f)-DeltaV), where DeltaV is a volume correction term. This was interpreted as indication that the lower wing of the hole size distribution contains holes too small to show a liquidlike behavior in their surroundings. Peculiarities of the relaxation behavior below T(B)=265 K and the possible validity of the Cohen-Grest free volume model are discussed.

Atomistic molecular dynamics simulations of a chemically realistic model of atactic short-chain polystyrene between gold surfaces (111) and positron annihilation lifetime spectroscopy experiments on similar polystyrene thin films on gold were performed. Results from both approaches show that the free volume voids in the film have a slightly smaller average size than in bulk polystyrene. In agreement to that the existence of an interphase of higher density at the polymer−solid substrate interface is shown both by the simulation as well as in the experiment. The average shape of the voids is similar in the bulk and the film. ■ INTRODUCTIONPolymer−solid interfaces are at the center of intensive research due to their importance in polymer coatings, hybrid materials, lubrication, adhesion, etc. The polymer properties close to this interface are of paramount importance for the performance of these composite systems. From the viewpoint of fundamental science, the confinement imposed by a solid substrate on the polymer should affect the polymer's glass transition temperature, and this has led to a number of studies.Various properties of polymers confined in thin films, be it supported or free-standing, differ from their bulk properties. In the case of supported thin films, an interphase between the substrate and the bulk phase of the polymer is postulated, and the width of this interphase layer has been the focus of many studies. Confinement effects on polystyrene thin films and their implications for the polymer glassy dynamics were studied by molecular dynamics (MD) simulation. 1 For freestanding polymer films, the glass transition temperature in the boundary layer near the gas phase was found to be lower than in the bulk. 2 On a completely smooth, structureless, solid substrate modeled by a truncated 9−3 Lennard-Jones potential three layers with different density and glass transition behavior substrate, middle, surfacehave been shown in MD simulations. 3 MD simulations of polystyrene on gold, 4,5 polybutadiene on graphite, 6 and polyethylene on graphite 7 have shown the existence of a higher-density interface layer of a width of about 2 nm. The dynamical properties of polymers near an interface have also been probed using MD simulations. 4,6,8 Experimental work about polymers at the interface claim the existence of e.g. a "dead layer" or "Guiselin brushes" 9 of a few nanometers at the interface as permanently attached polymer chains where even washing with a good solvent cannot remove this layer again. Using gold nanoparticles as markers in a X-ray photon correlation (XPCS) experiment, 10 an irreversibly adsorbed layer with a surface reduced viscosity (compared to bulk viscosity) layer altering the dynamics up to a distance of approximately 20 nm from the substrate was postulated as interpretation of their results. With the recently developed technique resonance enhanced dynamic light scattering (REDLS), experiments on polybutadiene on gold 11 showed a slowing-down in dynamics, and hence an increase in viscosity goi...

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