Glass transition temperature many-molecule dynamics

However, in many cases the problem dictates which of the two approaches is more appropriate. Generally, covalently bound labels are suited for characterizing structure and dynamics of individual macromolecules. Examples are dynamics of a polymer chain end or the distribution of end-to-end distances of a polymer chain. Spin probes are suited for characterizing the collective behavior of molecules or, to use a more fashionable term, supramolecular behavior. For example, in a pure polymer, spin probes reside in the voids that make up the free volume. The characteristic tan-perature of nitroxide mobility, q (see Chapter 1) for spin probes thus depeuds ou the size of the probe and the size distribution of free volume voids. As this size distribution in turn depends on the structure of the polymer, the relation of Tsog to the glass transition temperature, T, of the polymer is not linear, even when comparing the same spin probe in different polymers. However, if the same probe is used, q does exhibit some correlation with T. Therefore, it is often (somewhat incorrectly) being referred to as an ESR T. ... 

While all relaxation times depend on temperature and pressure, only the global motions (viscosity, terminal relaxation time, steady state recoverable compliance) are functions of Mw (and to a lesser extent MWD). An example of the various dynamics of 1,4-polyisoprene are illustrated in Fig. 10. At frequencies beyond the local segmental relaxation, or at temperatures below Tg, secondary relaxation processes can be observed, especially in dielectric spectra. In polymers, many of these secondary processes involve motion of pendant groups. However, the slowest secondary relaxation, referred to as the Johari-Goldstein process, involves all atoms in the repeat unit (or the entire molecule for low M materials). This Johari-Goldstein relaxation serves as the precursor to the prominent glass transition. 

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