Secondary relaxation processes, glass transition and

FIGURE 10 Schematic indicating range of motions and corresponding frequencies for 1,4-polyisoprene (M = 500,000 g/mol) at -40°C. 

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. 

The Tg of an elastomer is below room temperature, typically lower than -30°C. Otherwise, at ambient temperature, the material would lack the flexibility associated with rubbery behavior. The glass transition is a second-order 

FIGURE 12 Specific volume of polystyrene (M = 110,000 g/mol) as a function of temperature at various pressures. The inset shows the variation of Tg with pressure. (From Roland and Casalini [186].) 

Modulated (or alternating) DSC (MDSC) [187-189] is a technique in which a modulated heating (or cooling) rate is superimposed on the usual constant heating (or cooling) rate. The measured heat flow is separated into the 

---

One of the most common methods utilized to characterize the phase behavior of polymer blends employs low amplitude cyclic deformation studies to obtain the elastic and viscoelastic properties. This method, termed dynamic mechanical characterization, yields high resolution of polymer transitions including secondary relaxation processes, crystalline melting transitions and of primary importance, the glass transition. This method maps the data over a broad temperature range to ascertain the phase behavior. 

Chapter 4 deals with the local dynamics of polymer melts and the glass transition. NSE results on the self- and the pair correlation function relating to the primary and secondary relaxation will be discussed. We will show that the macroscopic flow manifests itself on the nearest neighbour scale and relate the secondary relaxations to intrachain dynamics. The question of the spatial heterogeneity of the a-process will be another important issue. NSE observations demonstrate a subhnear diffusion regime underlying the atomic motions during the structural a-relaxation. 

In addition to primary a-relaxation, there are secondary relaxation processes that have transpired at earlier times. Most theories including those cited in the NY Times article have focused their attention on the primary a-relaxation and do not consider any secondary relaxation to be important for glass transition. It turns out secondary relaxation belonging to a special class has various properties indicating that it bears strong connection to the a-relaxation. Moreover, secondary relaxation of this special class is... 

In Chapter 7, Mano and Dionisio describe how electrical methods, and particularly dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarisation current (TSDS) techniques, play a major role as tools for e2q)loring molecular mobility. DRS enables molecular relaxational processes (both slow and fast) to be studied. For example, the localized motions of glass formers in the glassy state give rise to local fluctuations of the dipole vector that are the origin of the secondary relaxation processes detected by dielectric relaxation spectroscopy, while above, but near, the glass transition, cooperative motions result in a distinguishably different relaxation process (the a-relaxation). 

Figure 8. Dielectric loss spectra of tetra-ethyleneglycol dimethacrylate between -114 and -86 °C, in steps of 2 °C, showing two secondary relaxation processes the high loss values on the low frequency side for the highest temperatures is due to the incoming of the main relaxation process associated with the glass transition (Tg= -83 C). Detailed dielectric characterization is given in [47].

---