Relaxation molecular mobility

Heat content and heat flow Heat content and heat flow Dielectric properties Mechanical relaxation Volume expansion Electrical properties Molecular relaxations Molecular mobility... 

Holzmiiller, W. Molecular Mobility, Deformation and Relaxation Processes in Polymers. Vol. 26, pp. 1 —62. 

It is an unfortunate fact that several preexisting theories have tried to explain complicated mechanical phenomena of CB-reinforced rubbery materials but they have not been so successful." " However, a recent report might have a capability of explaining them collectively," when the author accepted the existence of the component whose molecular mobility is different from that of matrix mbber component in addition to the existence of well-known bound rubber component. The report described that this new component might be the most important factor to determine the reinforcement. These mbber components have been verified by spin-spin relaxation time 2 by pulsed nuclear magnetic resonance (NMR) technique, ° while the information obtained by NMR is qualitative and averaged over the sample and, therefore, lacking in the spatial... 

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. 

Molecular mobility (dynamics) 0.5-500 nm Relaxation times and line-shapes multidimensional exchange experiments... 

The transition strongly affects the molecular mobility, which leads to large changes in rheology. For a direct observation of the relaxation pattern, one may, for instance, impose a small step shear strain y0 on samples near LST while measuring the shear stress response T12(t) as a function of time. The result is the shear stress relaxation function G(t) = T12(t)/ < >, also called relaxation modulus. Since the concept of a relaxation modulus applies to liquids as well as to solids, it is well suited for describing the LST. 

LIG. 22 A schematic illustration of the dependence of NMR relaxation times T and T2 on the molecular correlation time, xc, characterizing molecular mobility in a singlecomponent system. Both slow and fast motions are effective for T2 relaxation, but only fast motions near w0 are effective in Tx relaxation. 

Roudaut et al. (1999a) used low-frequency pulsed-proton NMR and dielectric dynamic mechanical spectroscopies to study molecular mobility in glassy bread (<9%) as a function of temperature. Based on NMR results, they reported that some (if not all) of the water molecules were much more mobile than the polymer matrix whose relaxation time could not be measured within the 20-p,s dead time of the RF probe. 

Champion, D., Le Meste, M., and Simatos, D. 2000. Towards an improved understanding of glass transition and relaxations in foods Molecular mobility in the glass transition range. Trends Food Sci. Technol. 11, 41—55. 

The mode coupling theory [11] has emerged from the hydrodynamics of liquids. This theory is able to explain the splitting of molecular mobility into relaxation modes that are frozen at the glass transition and molecular motion that is still possible below Tg. 

Relaxation is another important aspect of NMR spectroscopy, since it is unique in providing information about molecular mobility. There are two independent relaxation mechanisms spin-lattice and spin-spin relaxation. 

Spin-spin (transversal) relaxation (relaxation time T2) is the second mechanism which is related to molecular mobility. It influences the half-height line widths w [tv = lj(n T2) in the frequency-domain NMR spectrum and is of some relevance in the NMR spectroscopy of quadrupolar nuclei such as, 2H, 14N, 170, 33S and others. In the context of this section, T2 is of very limited relevance. 

The dynamic mechanical thermal analyzer (DMTA) is an important tool for studying the structure-property relationships in polymer nanocomposites. DMTA essentially probes the relaxations in polymers, thereby providing a method to understand the mechanical behavior and the molecular structure of these materials under various conditions of stress and temperature. The dynamics of polymer chain relaxation or molecular mobility of polymer main chains and side chains is one of the factors that determine the viscoelastic properties of polymeric macromolecules. The temperature dependence of molecular mobility is characterized by different transitions in which a certain mode of chain motion occurs. A reduction of the tan 8 peak height, a shift of the peak position to higher temperatures, an extra hump or peak in the tan 8 curve above the glass transition temperature (Tg), and a relatively high value of the storage modulus often are reported in support of the dispersion process of the layered silicate. 

The above models describe a simplified situation of stationary fixed chain ends. On the other hand, the characteristic rearrangement times of the chain carrying functional groups are smaller than the duration of the chemical reaction. Actually, in the rubbery state the network sites are characterized by a low but finite molecular mobility, i.e. R in Eq. (20) and, hence, the effective bimolecular rate constant is a function of the relaxation time of the network sites. On the other hand, the movement of the free chain end is limited and depends on the crosslinking density 82 84). An approach to the solution of this problem has been outlined elsewhere by use of computer-assisted modelling 851 Analytical estimation of the diffusion factor contribution to the reaction rate constant of the functional groups indicates that K 1/x, where t is the characteristic diffusion time of the terminal functional groups 86. ... 

Often, however, there is insufficient molecular mobility in the solid state for asymmetric fluorine placement to significantly increase the dielectric constant. Below the glass transition temperature, only restrained local motions are possible, and below subglass relaxations such as the P relaxation in polyimides, even these limited motions are virtually eliminated, rendering orientation polarization negligible. 

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