Amorphous reversible process

Because of the kinetic energy present in the molecule, amorphous flexible polymer chains are usually in constant motion at ordinary temperatures. The extent of this wiggling-like segmental motion decreases as the temperature is lowered in this reversible process. The temperature at which this segmental or micro-Brownian motion of amorphous polymers becomes significant as the temperature is increased is called the glass transition temperature, Te The term free volume is used to describe the total vplume occupied by the holes. 

Most elastomers are amorphous, but those with regular structures can crystallize when cooled to extremely low temperatures. Vulcanized soft rubber, which has a low cross-link density, when stretched crystallizes in a reversible process, and the oriented polymer has a high modulus (high stress for small strains, i.e., stiffness) and high tensile strength. 

Since at temperatures below the Tg the chains of an amorphous polymer are randomly distributed and immobile, the polymers are typically transparent. These glassy polymers behave like a spring and when subjected to stress, can store energy in a reversible process. However, when the polymers are at temperatures slightly above the Tg, i.e., in the leathery region, unless crosslinks are present, stress produces an irreversible deformation. 

Thermo-reversible has the following meaning if an amorphous polymer is heated to above Tg, it readily reaches thermodynamic equilibrium by definition the sample has then "forgotten" its history, any previous ageing it may have undergone below Tg having been erased. In other words it is completely rejuvenated. Ageing therefore is a thermo-reversible process to which one and the same sample can be subjected an arbitrary number of times. It has just to be retreated each time to the same temperature above Tg. 

On this conceptual premise, uptake and release of solvent can be paralleled to a solid-gas reaction, whereby the reactants are the molecules in the crystalline solid and the molecules in the gas phase and the product is the solvated crystal. Clearly, the same reasoning applies to the reverse process, i.e. generation of a new crystalline form by means of gas release. In gas-solid reactions, gases are reacted directly with crystals or amorphous phases to give complete conversion and usually quantitative yields. What would then be the difference between a solvation reaction and a reaction leading to new molecular/ionic species if not the energetic scale of the processes and the fact that in solvation processes molecules retain their chemical identity ... 

Maciejewski and Reller confirm [6] that (as yet) there is no experimental evidence for the existence of an amorphous or metastable intermediate in the dissociation of calcite. They show that the CaO formed by CaCOj dissociation under high vacuum reacts relatively rapidly at low temperatures (below 320 K) in the reverse process on CO2 readmission. Carefully controlled conditions are required to isolate and to study any highly active phases that may be formed during solid state decompositions. 

The effect does not seem to be related to thermodynamic barriers since the electrochemical insertion of lithium into common intercalation compounds, such as LiVaOs or VeOn, is an easily reversible process. In fact, electrochemical and spectroscopical studies carried out in liquid electrolytes have clearly demonstrated that these intercalation compounds can easily and repeatedly accept Li" ions without undergoing significant structure alterations. It has to be pointed out, however, that these studies have been carried out at low temperatures. Recent investigations performed at elevated temperatures (i.e. around 120°C) have indicated a crystalline to amorphous... 

The symbol Lg derives from latent heat, ie, the recoverable heat in a reversible process. Thermolytic cleavage of primary chemical bonds in the polymer backbone to produce volatile fuel and char is obviously not a reversible process, but the symbol Lg will be used throughout to conform with the literature in the fire sciences. Table 2 illustrates the magnitude of these enthalpic terms for amorphous PMMA, polystyrene (PS), and semicrystalline polyethylene (PE). The stored heat Ahs was obtained by numerical integration of heat capacity versus temperature from ambient to the dissociation temperature as per equation 24. The dissociation... 

It is shown in fig. 129 that b,f of the amorphous Laj5Al25Ni2o ribbon remains unchanged in the T, range below 406 K, decreases from 1.0 to 0.04 in the narrow Tg range between 406 and 434 K and shows a nearly constant value of about 0.03 in the Tg range above 434 K. The change in b,f with 7), indicates that the amorphous ribbon becomes brittle at 416 K in the case of an annealing time of 1.8 ks. The exothermic reaction for the as-quenched sample as well as the endothermic reaction for the annealed sample is a reversible process for the La-Al-Ni amorphous alloy with a wide supercooled liquid... 

Changes inoi by filling control the density of the amorphous phase. The experimental data show that in the melts of oligoesters, for the whole range of interlayer thicknesses there are loosely packed regions. The dependence of the reduced specific volume, Va, on is nonlinear and characterized by the alternation of more dense and less dense regions typical for dissipative structures, formed as a result of non-reversible processes under non-equilibrimn conditions. ... 

The industrial catalyst for n-butane oxidation to maleic anhydride (MA) is a vanadium/phosphoras mixed oxide, in which bulk vanadyl pyrophosphate (VPP) (VO)2P207 is the main component. The nature of the active surface in VPP has been studied by several authors, often with the use of in situ techniques (1-3). While in all cases bulk VPP is assumed to constitute the core of the active phase, the different hypotheses concern the nature of the first atomic layers that are in direct contact with the gas phase. Either the development of surface amorphous layers, which play a direct role in the reaction, is invoked (4), or the participation of specific planes contributing to the reaction pattern is assumed (2,5), the redox process occurring reversibly between VPP and VOPO4. 

The cyclotrisilazane (R = Me) produced in reaction (14) is recycled at 650°C [by reaction with MeNHo) the reverse of reaction (14)] to increase the yield of processible polymer. Physicochemical characterization of this material shows it to have a softening point at 190°C and a C Si ratio of 1 1.18. Filaments 5-18 pm in diameter can be spun at 315°C. The precursor fiber is then rendered infusible by exposure to air and transformed into a ceramic fiber by heating to 1200°C under N2- The ceramic yield is on the order of 54% although, the composition of the resulting amorphous product is not reported. The approach used by Verbeek is quite similar to that employed by Yajima et al. (13) in the pyrolytic preparation of polycarbosilane and its transformation into SiC fibers. 

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