Permanent and reversible

Thermoplastic Elastomers. These represent a whole class of synthetic elastomers, developed siace the 1960s, that ate permanently and reversibly thermoplastic, but behave as cross-linked networks at ambient temperature. One of the first was the triblock copolymer of the polystyrene—polybutadiene—polystyrene type (SheU s Kraton) prepared by anionic polymerization with organoHthium initiator. The stmcture and morphology is shown schematically in Figure 3. The incompatibiHty of the polystyrene and polybutadiene blocks leads to a dispersion of the spherical polystyrene domains (ca 20—30 nm) in the mbbery matrix of polybutadiene. Since each polybutadiene chain is anchored at both ends to a polystyrene domain, a network results. However, at elevated temperatures where the polystyrene softens, the elastomer can be molded like any thermoplastic, yet behaves much like a vulcanized mbber on cooling (see Elastomers, synthetic-thermoplastic elastomers). 

Peters 0, Back T, Lindauer U, Busch C, Megow D, Dreier J, Dirnagl U (1998) Increased formation of reactive oxygen species following permanent and reversible middle cerebral artery occlusion in the rat. J Cereb Blood Flow Metab 18 196-205... 

Not all contaminants or chemicals are created equal in their capacity to cause adi ersc effects. Thus, cleanup standards or action levels are based in part on the compounds toxicological properties. Toxicity data are derived largely from animal experiments in which llie aninuils (primarily mice mid rats) are exposed to increasingly liighcr concentrations or doses. Responses or effects can vary widely from no obscn ablc effect to temporary and reversible effects, to permanent injury to organs, to chronic functional impairment to ultimately, death. 

Neurotoxicity. Clinical signs indicative of disturbances of the nervous system in exposed humans have been well documented in short-term studies at high doses and appear to be reversible. These effects are characteristic of cyanide toxicity. Animal studies confirm findings in humans. In longer-term studies, effects on the nervous system have also been reported, but it is not certain if these effects are permanent or reversible following termination of acrylonitrile exposure. 

Li et al.22 investigated the effect of water for a platinum-promoted Co/y-Al203 catalyst during Fischer-Tropsch synthesis in a CSTR-type reactor. The catalyst lost activity in the presence of water, and it was found that small quantities of water (3-25 vol%) led to mild and reversible deactivation, whereas large amounts of water (>28 vol%) deactivated the catalyst more severely and permanently. The deactivation was attributed to the formation of cobalt oxide or cobalt aluminate. 

Topical minoxidil (Rogaine) is effective in reversing the progressive miniaturization of terminal scalp hairs associated with androgenic alopecia. Vertex balding is more responsive to therapy than frontal balding. The mechanism of action of minoxidil on hair follicles is unknown. Chronic dosing studies have demonstrated that the effect of minoxidil is not permanent, and cessation of treatment will lead to hair loss in 4-6 months. Percutaneous absorption of minoxidil in normal scalp is minimal, but possible systemic effects on blood pressure (see Chapter 11) should be monitored in patients with cardiac disease. 

The physical adsorption is characterized by weak intermolecular forces of the van der Waals type. The adsorbed particle must get close to the solid surface, since the van der Waals energy is proportional to the sixth power of reciprocal distance. The main feature of this interaction is its non-specificity, a considerable velocity and reversibility. An example of the physical adsorption is the adsorption of apolar molecules on an apolar surface resulting form disperse forces. Beside these forces the dipol-dipol interactions occur when molecules of the adsorbent or adsorbate can form permanent or induced dipoles (adsorption of gases or dipol liquids on apolar surfaces). 

Enzyme inhibitors are divided into two classes, irreversible and reversible. Irreversible inhibition implies destruction or permanent modification of chemical groups in the enzyme. In contrast, reversible inhibitors form a complex with the enzyme that can dissociate and release the active enzyme. An enzyme E can bind either to substrate S, to form an ES complex (which can go on to products) or to inhibitor I, to form the complex EL... 

Container molecules in general show an increasing number of applications and so do the container molecules based on imine type ligands. Many different shapes of open or nearly closed ones could already be synthesised. Those cages are known to encapsulate different types of guest molecules. This encapsulation can be selective and permanent or reversible. The container molecules described are also used for stabilisation of different compounds such as the allotrope P4. They can be used as gas or optical sensors. One of the described cages can also be opened and closed selectively. 

The hydrogenation of THEAQ is a zero-order reaction with respect to hydrogen and a first order reaction with respect anthraquinone [2]. The kinetics of the catalyst deactivation has been studied in a laboratory continuous reactor. Two deactivation mechanisms have been recognized, a reversible and fast one due to the presence of water a d a permanent and slow one probably due to the formation of... 

Poisoning by water vapor is a reversible effect and can be overcome by redrying the catalyst. Alkali poisoning, on the other hand, is permanent and may involve the formation of a salt, such as a manganite or cobaltite in the surface layer. In such cases, the manganese or cobalt atom is more completely coordinated and the reactivity of the surface considerably lessened thereby. Carbon monoxide is therefore oxidized only stoichiometrically by poisoned Mn02. 

Deformation may be of one or both of two types, irreversible deformation, called flow, and reversible deformation, called elasticity. The energy used in irreversible deformation is dissipated as heat, and the body is permanently deformed. The energy used in reversible deformation is recovered upon release of... 

Brightness reversion is one of the serious problems for paper industries, restorers, paper conservators, and librarians, because the brightness of neither unbleached nor bleached pulp is permanent, and the causes and prevention of reversion still are not known entirely. 

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