Secondary relaxation pressure dependence

There are also glass-formers that have a resolved secondary relaxation that is not the JG relaxation according to the established criteria [38], but lack an apparent JG peak in their loss spectra at ambient pressure. These glass-formers include BMPC [75], dibutyl phthalate (DBP) [77], diethyl phthalate (DEP) [76], 2PG, 3PG [101,102], m-fluroaniline (m-FA) [44], and bis-5-hydroxypentylphthalate (BHPP) [228,229]. One criterion is the lack of a pressure dependence of their relaxation times, as shown for BMPC in Fig. 30. NMR measurements of molecular motion in BMPC had shown [230] that the... 

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. 

Pratt and Smith 2001)]. This substitution affects the packing of the chains and reduces the temperatures both of the a transition (Tg) and the secondary relaxations (Tp). In contrast to the segmental relaxation signal, the p relaxation, which involves localized motions, is only weakly dependent on changes in crystallinity and applied pressure (i.e., variations of free volume), although as was shown by Hakme et al. (2005), deformation increases the strength of its low-temperature (Pi) component. 

Abstract The present study demonstrates, by means of broadband dielectric measurements, that the primary a- and the secondary Johari-Goldstein (JG) /3-processes are strongly correlated, in contrast with the widespread opinion of statistical independence of these processes. This occurs for different glassforming systems, over a wide temperature and pressure range. In fact, we found that the ratio of the a- and P- relaxation times is invariant when calculated at different combinations of P and T that maintain either the primary or the JG relaxation times constant. The a-P interdependence is quantitatively confirmed by the clear dynamic scenario of two master curves (one for a-, one for P-relaxation) obtained when different isothermal and isobaric data are plotted together versus the reduced variable Tg(P)/T, where Tg is the glass transition temperature. Additionally, the a-P mutual dependence is confirmed by the overall superposition of spectra measured at different T-P combinations but with an invariant a-relaxation time. 

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