Amorphous material/polymers/regions relaxations

The data shown in fig. 7.18 for polytetrafluoroethylene (PTFE) provide a good example of how the variation of shear modulus and tanS with temperature can depend strongly on the degree of crystallinity of the sample. Because tan 3 for the y relaxation increases as the crystallinity decreases and tan 3 for the p relaxation decreases, the former is associated with the amorphous material and the latter with the crystalline material. The behaviour of tan 3 for the a relaxation strongly suggests that, in this polymer, it is associated with the amorphous regions. 

Bulk polymers are composed of amorphous and crystalline regions which have distinct domain structures and chain dynamics within the domains. In this article, some examples illustrating the use of solid-state NMR methods to probe the structure and dynamics of different polymer domains in multiphase polymer systems are reviewed. Observed chemical shifts are influenced by the population of conformers present in the different phases. Also, different components in relaxation time decay curves can be observed as a result of heterogeneity. By means of solid-state NMR techniques, unique information on the structural details of the domains in different dynamic states can be derived. Furthermore, phase separation can be detected in the order of nanometer scale. Such information is useful in designing the morphology and nanostructure of advanced polymer systems, to engineer desired physical and chemical properties into the materials. 

In addition to the crystalline (a ) relaxation, other (broader) relaxations are observed in partially crystalline polymers arising from the amorphous regions of the material. These relaxations are related to similar broad absorptions found in amorphous polymers except they are perturbed due to the proximity of amorphous and crystalline regions (see ref. 10 for the example of crystalline poly(ethylene terephthalate) and further discussion below). 

The DSC is widely used to measure the glass-rubber transition temperature (Tg-value), which is an important parameter for polymer characterisation. The Tg-value represents the temperature region at which the (amorphous phase) of a polymer is transformed from a brittle, glassy material into a tough rubberlike liquid. This effect is accompanied by a step-wise increase of the DSC heat flow/temperature or specific heat/ temperature curve. Enthalpy relaxation effects can hamper the... 

In polymers and ceramics, NMR can be used to determine quantitatively the amounts of amorphous and crystalline material in a sample. Molecules in amorphous regions move more than molecules in crystalline regions, so the relaxation times of molecules in these different environments is different. NMR can measure the difference in relaxation times and relate that to the percent crystallinity of the sample. Whether a sample is amorphous, crystalline, or a combination of both directly impacts the material s physical properties and behavior. It is an important piece of information for materials scientists, polymer chemists, and engineers to know. 

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