Search results for "Diffusion"
showing 10 items of 1615 documents
Enlarged pore size in nanoparticulated bimodal porous silicas: Improving accessibility
2016
Abstract Mass-transfer kinetics seems to be highly favored in siliceous materials constructed from the aggregation of mesoporous nanoparticles. Besides intra-particle mesopores, over the course of the aggregation process an inter-particle (textural) large pore system is generated. Diffusion constrains through the resulting hierarchically structured pore systems mainly depend on the characteristics of the intra-particle mesopores. By using alkanes as swelling agents, we have been able to significantly increase the intra-particle mesopore size in previously well characterized UVM-7 materials. The Winsor-III-like behavior associated with the presence of alkanes in the hydro-alcoholic reaction …
Tracer diffusion in polymer and organic liquids close to the glass transition studied by forced rayleigh scattering
2007
Translational diffusion coefficients D of photochronic dye molecules have been measured by forced Rayleigh scattering in polymer diluent systems at compositions from pure polymer to pure diluent. D could be measured down to below the glass transition temperature T g. In the pure diluent, D(T g) was found typically about two decades below that in the pure polymer but still far above the Stokes-Einstein prediction extrapolated from high temperatures. Some experimental results are in contrast with predictions from free-volume theory.
Spatial scale-dependent tracer diffusion in bulk polycarbonate studied by holographic relaxation.
2005
The diffusion of a photochromic dye tracer in polycarbonate was studied by a holographic relaxation technique (forced Rayleigh scattering) at temperatures close to the glass transition temperature. By varying the holographic grating period the results could be interpreted via the spatial scale dependence of apparent diffusion coefficients within a two-state diffusion model. This indicates inhomogeneities on the scale of a few micrometers in the polymer glass.
Effects of pressure and of thermal history on the structural phase transition of KCN
1992
The cubic-noncubic structural phase transition of a KCN single crystal is studied as a function of thermal history and of hydrostatic pressure up to 2 kbar. The standard sequence of phases is cubic-monoclinic-orthorhombic on cooling and orthorhombic-cubic on heating. The monoclinic phase is also absent in first cooling runs at low pressures (p≦250 bar). The width of the monoclinic field inp, T phase diagram is increased in second cooling runs. The effects are discussed in terms of a martensitic transformation behavior and random strain fields.
Penetrant diffusion in frozen polymer matrices: A finite-size scaling study of free volume percolation
1996
The diffusion of penetrant particles in frozen polymer matrices is investigated by means of Monte Carlo simulations of the bond fluctuation model. By applying finite-size scaling to data obtained from very large systems it is demonstrated that the diffusion process takes place on a percolating free volume cluster describable by a correlated site percolation model which falls into the same universality class as random percolation. The diverging correlation length entails a pronounced dependence of the diffusion constant on the size of the simulated system. It is shown that this dependence is appreciable for a wide range of parameters around the transition. \textcopyright{} 1996 The American …
Diffusion of spherical micronetworks in polymer diluent systems and melts studied by dynamic light scattering techniques
1994
Spherical polystyrene (PS) micronetworks can be prepared in microemulsion with bulk radii of 5–60 nm and different cross-linking densities. The diffusion of these PS spheres has been studied in polymer diluent systems ranging from dilute solutions to plasticized melts by using forced Rayleigh scattering and photon correlation spectroscopy. On increasing the PS concentration, a colloid glass transition is observed at a volume fraction ΦC ≈ 0.64 of the swollen spheres. At higher concentration inside the “colloid glass” state the sphere diffusion is slowed down and becomes very complex but can be observed up to the limit of a melt of collapsed spheres.
Biological evaluation of PLLA membranes, with different pore diameters, to stimulate cell adhesion and growth in vitro
2015
Polymeric membranes prepared via DIPS (Diffusion Induced Phase Separation) are widely studied and utilized as scaffolds for the regeneration of tissue. In this work, poly (L)-lactide membrane are prepared through a DIPS protocol starting from a ternary solution made of polymer, dioxane (solvent) and water (non-solvent). A three-dimensional, porous and mechanically stable membrane is desirable for ingrowth of human bronchial epithelial cells. Polymeric membranes prepared via DIPS (Diffusion Induced Phase Separation) are widely studied and utilized as scaffolds for the regeneration of tissue. In this work, poly (L)-lactide membrane are prepared through a DIPS protocol starting from a ternary …
QENS from polymer aggregates in supercritical CO2
2000
Abstract We report QENS measurements from PS-b-PFOA aggregates in supercritical CO2. Line shapes are dominated by localized diffusive modes and segmental dynamics of the anchored, finite-length PFOA chains. For Q⩽0.6 A−1, we obtain effective diffusion coefficients of ≅0.8 10−6 cm2/s. At higher Q, a single component is not sufficient as shown by excess intensity on the flanks. For Q⩾1.5 A−1, the wings reflect contributions due to a distribution of faster, more localized chain modes.
Diffusional enhancement of holograms: phenanthrenequinone in polycarbonate
2002
As a further development of the manufacture of holographic gratings by the diffusion of phenanthrenequinone in polymer glass, the traditionally employed poly(methyl methacrylate) is replaced by poly(bisphenol-A-carbonate). The post-exposure growth and decay of the gratings in the two materials are compared at different spatial periods and temperatures. The medium with polycarbonate demonstrates a fortunate combination of significantly improved stability of the gratings and their faster development. Our results indicate that the polycarbonate materials promise to be suitable media for holographic optical elements and data storage.
Martensitic phase transformation cubic-orthorhombic of NaCN and (NaCN)0.98 (KCN)0.02
1990
The ferroelastic cubic-orthorhombic transition of single crystals NaCN and (NaCN)0.98 (KCN)0.02 has been studied by x-ray diffraction. The domain configuration obtained is consistent with the Wechsler, Lieberman, Read-model for martensitic transformations.