Glass-transition temperature bulk polymer motions below

A close connection exists between the presence of a flexible polymer skeleton and the flexibility of the bulk material. Macromolecular flexibility is often defined in terms of the glass-transition temperature, Tg. Below this temperature, the polymer is a glass, and the backbone bonds have insufficient thermal energy to undergo significant torsional motions. As the temperature is raised above the Y g, an onset of torsional motion occurs, such that individual molecules can now twist and yield to stress and strain. In this state the polymer is a quasi-liquid (an elastomer) unless the bulk material is stiffened by microcrystalfite formation. Thus, a polymer with a high Tt is believed to have a backbone that offers more resistance to bond torsion than a polymer with a low 7 g. 

It is important to mention at this point that, in the solid state, some of the above spectral parameters are not only determined by molecular motions, but also by the dynamics of the spin system. As a consequence, in bulk polymers at temperatures below the glass-transition temperature, the competition between molecular dynamics and spin dynamics may introduce some difficulties in the data interpretation. On the other hand, the spin dynamics phenomenon itself can be a most useful tool for studying the solid-state organization of heterogeneous materials. 

In this chapter, we consider the sensitivity of the different NMR parameters to both molecular motions and spin dynamics. The first two parts illustrate the capability of NMR to study the local dynamics of bulk polymers at temperatures above and below the glass-transition temperature, Tg, respectively. The third part is devoted to the NMR investigation of the solid-state organization of heterogeneous polymer systems. In this last section, examples are taken mainly from the field of polymer blends. 

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