Johari-Goldstein process

At frequencies faster than for segmental relaxation, or at temperatures below Tg, secondary relaxation process 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 (Ngai and Paluch, 2004), involves all atoms in the repeat unit (or the entire molecule for low M-u, materials). This process is referred to as the Johari-Goldstein relaxation, and it serves as the precursor to the prominent glass transition. 

In order to identify the E-process of PB, the characteristic times are plotted in a relaxation time map (Fig. 22) [ 130]. As mentioned in Sect. 4, the three processes - the a-process, the Johari-Goldstein process, and the fast process - are commonly observed in most glass-forming materials including polymers and small molecules. Therefore, these three processes should not be considered as special features of polymers but as common features of glass-forming materials. On the other hand, the E-process is not observed in the relaxation time map of OTP [84], suggesting that the E-process is characteristic of polymers. 

Below Tg, in the glassy state the main dynamic process is the secondary relaxation or the )0-process, also called Johari-Goldstein relaxation [116]. Again, this process has been well known for many years in polymer physics [111], and its features have been estabhshed from studies using relaxation techniques. This relaxation occurs independently of the existence of side groups in the polymer. It has traditionally been attributed to local relaxation of flexible parts (e.g. side groups) and, in main chain polymers, to twisting or crankshaft motion in the main chain [116]. Two well-estabhshed features characterize the secondary relaxation. 

In [189] a simple two state model for the dynamic structure factor corresponding to the Johari-Goldstein jS-process was proposed. In this model the jS-relaxation is considered as a hopping process between two adjacent sites. For such a process the self-correlation function is given by a sum of two contributions ... 

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. 

In our discussion to this point we have considered that any higher frequency processes for T > Tg would be the 3 process (equation (3)) Johari, Goldstein and Smyth had shown that the 3 process (in the glass or just above Tg) was small and broad, just as in amorphous solid polymers. We have no experimental Information on the form of the high frequency process for the solute/o-terphenyl solutions of Table 2. We would suggest that for cases where AcQ /Ae >0.8 the 3 process would be very similar to that observed by Johari just above Tg (figure 4 above). 

It has been demonstrated that simple small-molecule glass-forming systems exhibit dielectric a- and /3-processes. It is clear from the work of Johari, Goldstein and Smyth that the interpretation of the behaviour for polymers may not require theories of chain dynamics. It appears that the behaviour of the two classes of sterns are accommodated by the general approach given above [see Eq. (22)] where for small mdecules only the autocorrelation functions are important while for polymers both auto- and crosscorrelation functions are involved. Williams and co-workers (Beevers and co-workers, 1976 Beevers and co-workers, 1977a, b Crosdey and Williams,... 

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