Viscosity glass phase transition

Because of their novel topologies, polyrotaxanes have properties different from those of conventional polymers. Solubility, intrinsic viscosity, melt viscosity, glass transition, melting temperature and phase behavior can be altered by the formation of polyrotaxanes. The detailed changes are related both to the properties of the threaded cyclics and to the backbone and the threading efficiency. 

As opposed to the liquid-crystal transformation, the liquid-glass transformation is not a phase transition and therefore it can not be characterized by a certain transition temperature. Nevertheless, the term "the vitrification temperature , Tv, is widely used. It has the following physical meaning. As opposed to crystallization, vitrification occurs when the temperature changes continuously, i.e. over some temperature interval, rather than jump-wise. Inside this interval, the sample behaves as a liquid relative to some of the processes occurring in it, and as a solid relative to other processes occurring in it. The character of this behaviour is determined by the ratio between the characteristic time of the process, t, and the characteristic relaxation time of the matrix, x = t//G, where tj is the macroscopic viscosity and G is the matrix elasticity module. If t x, then the matrix should be considered as a solid relative to the process, and if t > x it should be considered as a liquid. The relation tjx = 1 can be considered as the condition of the matrix transition from the liquid to the solid (vitreous) state, and the temperature Tv at which this condition is realized as the temperature of vitrification. Evidently, Tv determined by such means will be somewhat different for the processes with different characteristic times t. However, due to the rapid (exponential) dependence of the viscosity rj on T, the dependence of Tw on t (i.e. on the kind of process) will be comparatively weak (logarith-... 

The increase in viscosity associated with the formation of either an amorphous solid at the glass transition temperature or a crystalline lattice at the phase transition temperature will significantly reduce the mobility of most radicals. An increase in viscosity in rubbery liquids, due to decreases in temperature down towards the glass transition, will also reduce mobility. Consequently, recombination of primary radicals at the site of formation would be favored, lowering their yield. Other reaction pathways not involving diffusion and migration might also be favored. These effects are discernible and sometimes can be pronounced. 

Classical methods are designed to obtain thermodynamic and transport information, for example molar volume, density, viscosity, and surface tension. The effects of pressure and temperature on these properties can also be evaluated, and thus phase transition information such as melting points and glass transition temperatures. If molecular dynamics (in contrast to Monte Carlo) is used, data relating to reorientation of molecules, self-diffusion and residence times are all available. Information can also be obtained from the simulation equations on the contribution made by kinetic, coulombic, intramolecular and dispersion energies to the total potential energy. However, because the charges are fixed and there is no explicit wavefunction included in the classical methods, no electronic information can be obtained. 

Molecular dynamics has proved to be a powerful method for simulating and/or predicting several features of polymer systems. Properties on either side of the glass transition temperature (see Section 1.5) have been successfully simulated, as has the solid-to-liquid transition, and provided descriptions of the dynamics (segmental motions, chain diffusion, conformational transitions, etc.) that are in accord with relaxation measurements and such bulk properties as shear viscosities and elastic moduli. The method may also provide a good description of the variation in heat capacity and other thermodynamic fimctions across a phase transition. Several collections of these investigations have recently been published. ... 

The properties of conventional standard materials cannot be significantly altered, e.g., if oil is heated, it will become thinner, whereas smart materials can exhibit volume, shape, and size changes or phase transitions in response to environmental conditions, such as temperature [203], pH [204], pressure [205], electric or magnetic fields [206], light [207], or moisture. A smart material with variable viscosity may turn from a nonviscous fluid to a solid. Smart materials recently have been used in a wide range of applications, such as coffee pots, cars, eye glasses, etc. 

The release and the oxidation processes of the encapsulated D-limonene are closely related to the structural changes in the capsule matrices. Physico-chemical changes caused by the phase transition of carbohydrate from amorphous glass to rubbery are commonly expressed with the temperature difference between the storage temperature, T, and the glass transition temperature, Tg, of the carrier matrices, T — Tg. The idea is based on the fact that the viscosity (or relaxation time) of the carrier matrices follows the Wflliams-Landel-Ferry (WLF) equation expressed as a function of T — Tg (Williams et al., 1955). Therefore, the release rate constants k and the oxidation rate... 

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