Glassy properties

Specific heat measurements40 indicate that the assumption of static frozen glass disorder at low temperatures may be too restrictive and that the intra-H-bond hydrogen motion may still persist in the form of quantum tunnelling. To check this hypothesis, 2D deuteron NMR and 87Rb and 2H SLR measurements were carried out at low temperatures. With site-specific NMR measurements, it was also hoped to identify the microscopic nature of the "two-level" states which determine the low T glassy properties of these systems. 

The present results constitute a step forward in our understanding of the microscopic details that may lead to disparate behaviour in macroscopic glassy properties such as the fragility and also come into line with results given in Ref. 22 on a study of the isomeric effect concerning another glassy material. 

On a broader level, the topology of the diagrams can be used to infer global characteristics of potential landscapes that control the dynamic relaxation of ensembles. For example, most speculations as to whether protein landscapes have funneling or glassy properties [49,50] have been based on computational studies performed on lattice models that attain simplicity at the expense of accuracy [51-53]. However, whereas these models may adequately account for important packing constraints... 

Atomistically detailed models account for all atoms. The force field contains additive contributions specified in tenns of bond lengtlis, bond angles, torsional angles and possible crosstenns. It also includes non-bonded contributions as tire sum of van der Waals interactions, often described by Lennard-Jones potentials, and Coulomb interactions. Atomistic simulations are successfully used to predict tire transport properties of small molecules in glassy polymers, to calculate elastic moduli and to study plastic defonnation and local motion in quasi-static simulations [fy7, ( ]. The atomistic models are also useful to interiDret scattering data [fyl] and NMR measurements [70] in tenns of local order. 

The polymers of the 2-cyanoacryhc esters, more commonly known as the alkyl 2-cyaiioacrylates, are hard glassy resins that exhibit excellent adhesion to a wide variety of materials. The polymers are spontaneously formed when their Hquid precursors or monomers are placed between two closely fitting surfaces. The spontaneous polymerisation of these very reactive Hquids and the excellent adhesion properties of the cured resins combine to make these compounds a unique class of single-component, ambient-temperature-curing adhesives of great versatiUty. The materials that can be bonded mn the gamut from metals, plastics, most elastomers, fabrics, and woods to many ceramics. 

Interest is maintained ia these materials because of the combination of mechanical, corrosion, electric, and magnetic properties. However, it is their ferromagnetic properties that lead to the principal appHcation of glassy metals. The soft magnetic properties and remarkably low coercivity offer tremendous opportunities for this appHcation (see Magnetic materials, bulk Magnetic materials, thin film). 

The selectivity of pervaporation membranes varies considerably and has a critical effect on the overall separation obtained. The range of results that can be obtained for the same solutions and different membranes is illustrated in Figure 41 for the separation of acetone from water using two types of membrane (89). The figure shows the concentration of acetone in the permeate as a function of the concentration in the feed. The two membranes shown have dramatically different properties. The siUcone mbber membrane removes acetone selectively, whereas the cross-linked poly(vinyl alcohol) (PVA) membrane removes water selectively. This difference occurs because siUcone mbber is hydrophobic and mbbery, thus permeates the acetone preferentially. PVA, on the other hand, is hydrophilic and glassy, thus permeates the small hydrophilic water molecules preferentially. 

Properties and Structure. Phosphoms(V) oxide, the extremely hygroscopic acid anhydride of the phosphoric acids, exists in several forms but is often referred to by its empirical formula, P2O3. Three crystalline polymorphs, two distinct Hquids, and several amorphous or glassy soHds are recogni2ed. Some properties of the various forms of phosphoric oxide are Hsted in Table 10. 

Although vitreous siUca is a simple, single-component glass, its properties can vary significantly, depending on thermal history, the type and concentration of defects, and impurities. Vitreous siUca can, however, be one of the purest commercially available glassy materials. In synthetic vitreous sihcas, for example, total metal contamination is typically measured ia the 50—100 ppb range. Even at such a low level of impurities, differences ia properties, such as uv-transmission, are observed for various siUcas. 

Silicon dioxide [7631-86-9] Si02, exists in both crystalline and glassy forms. In the former, the most common polymorph is a-quartz (low quartz). All commercial appHcations of crystalline quartz use a-quartz, which is stable only below ca 573°C at atmospheric pressure. Some of the properties of a-quartz are Hsted in Table 1. 

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