Structural relaxation time molecular mobility dependences

This asymmetry in the recovery means that the structural-relaxation time depends not only on the temperature but also on the instantaneous structure. In particular, when the approach toward equilibrium is from above, the initial structure of the material will have a higher molecular mobility than that of the equilibrium structure, although the difference decreases as the departure from equiUbrium decreases. On the other hand, the opposite is true when the approach toward equilibrium is from below. This feature of isothermal structural relaxation is sometimes referred to in the literature as the nonlinearity of the process, in the sense that the structural relaxation cannot be described by the linear differential equation... 

There is a second relaxation process, called spin-spin (or transverse) relaxation, at a rate controlled by the spin-spin relaxation time T2. It governs the evolution of the xy magnetisation toward its equilibrium value, which is zero. In the fluid state with fast motion and extreme narrowing 7) and T2 are equal in the solid state with slow motion and full line broadening T2 becomes much shorter than 7). The so-called 180° pulse which inverts the spin population present immediately prior to the pulse is important for the accurate determination of T and the true T2 value. The spin-spin relaxation time calculated from the experimental line widths is called T2 the ideal NMR line shape is Lorentzian and its FWHH is controlled by T2. Unlike chemical shifts and spin-spin coupling constants, relaxation times are not directly related to molecular structure, but depend on molecular mobility. 

Pulsed NMR measurements have been made as a function of temperature to study the pyrolysis behaviour of oil shales. The method is sometimes referred to as NMR Thermal Scanning. To overcome receiver dead time, a 9O-T-9O90 pulse sequence is used to form the solid echo . The echo signal is then decomposed into a rigid (short relaxation time) and a mobile (longer relaxation time) component. These data are then related to various properties of the system. Parameters that can be extracted from the NMR data relate to the hydrogen content, phase structure, molecular mobility, and free radical content. By measuring the temperature dependence of these parameters... 

Mao C, Chamarthy SP, Pinal R (2006a) Time-dependence of molecular mobility during structural relaxation and its impact on organic amorphous solids an investigation based on a calorimetric approach. Pharm Res 23(8) 1906-1917... 

Special attention is always paid to the questions of estimation of molecular mobility of polymer chains [54-56]. The reasons are obvious thermodynamically non-equilibrium solid-like media, particularly relaxation media, and their physical properties are defined by passing relaxation molecular processes in them, which in turn depend on features of the chemical constitution of the molecular chains and the structural organisation of the polymers [56]. As for parameters, there exist different points of view in describing these processes. So, for example, it is assumed that fast relaxations are defined by the mobility of free chains placed between densely packed domains, which are at the same time nodes of macromolecules physical entanglements network. Such treatment corresponds to the main postulates of the cluster model of the structure of polymers in the amorphous state [13], with the aid of which structure elements can be quantitatively described. 

When a solid undergoes the transition between its glassy and rubbery state, the key parameter that changes is the molecular relaxation time. The amount of molecular motion that can occur at any instant is determined by the mobility of the structure, which in turn depends upon the energy available to move the molecules. 

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