Secondary transition relaxation process

The temperature position of the secondary fi relaxation (about 290 K 1 Hz), generally attributed to partial rotations of the side chains COOR, is only slightly affected by the polarity and volume of the substituent R but decreases markedly (by 120 K) on removal of the a-methyl group on the main chain. The experimental data obtained contradict the assumption that there is a certain relationship between this temperature and the glass transition temperature. Nevertheless, we can infer that the pertinent molecular mechanism in polymethacrylates differs from that in polyacrylates, probably due to the different participation of the main chains. The values of the individual contributions to the activation energy were estimated by employing a procedure similar to that used in the y relaxation process, and their sum was found to agree approximately with the experimental values. 

In order to compare primary dynamics with secondary relaxation steps, we depict on the left-hand side of Fig. 15 the anisotropic spectra (a-c), which consist mainly of spectral components with the same linear polarization as directly induced by the pump pulse. On the right-hand side of the figure the corresponding isotropic spectra (d-f) are shown. In the latter spectral components can notably contribute that result from a relaxation process, where the initially orientation of the OH transition dipole is (partially) lost. 

PS, in either its atactic or syndiotactic form, is a polymer which shows no segmental mobility of chain segments below its glass transition temperature. Secondary relaxation processes which can be attributed to mobility in the main chain are missing. Therefore, these materials do not exhibit long-range energy... 

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... 

Solution The mechanical properties of polymers depend on time and temperature. The time dependence is usually expressed as a frequency dependence, which to a first approximation is related to time by 2jt 1) = 1/t where X> is the frequency. The combined dependence of molecular processes of viscoelastic materials on frequency and temperature can be described by an activation energy E, . Ej, is about 100 kcal/mol and 10 kcal/mol for primary and secondary transitions, respectively. This implies that the relaxation processes associated with the molecular motions shift to higher temperatures at higher frequencies however, the secondary transition shifts more than the primary transition. Therefore, if tests are conducted at high frequencies, the resolution between the energy absorption peaks for primary and secondary transitions that are close to each other is poor. Thus, in this case, the P and a peaks, which are relatively distinct at 0.1 Hz, merge at 50 Hz, and there is a shift in the peak to higher temperatures. 

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].

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. 

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