Secondary glass relaxation

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. 

Most crystalline polymers with metylenic groups in their structure and with a degree of crystallinity below 50% present a sub-glass relaxation whose intensity and location scarcely differ from those observed for the amorphous polymer in the glassy state. The temperature dependence of this relaxation follows Arrhenius behavior, and its activation energy is of the same order as that found for secondary processes in amorphous polymers. 

The molecular motions of the PNF (i.e., primary and secondary molecular relaxations) were documented by dielectric and dynamic (Rheovibron) mechanical measurements (14) (Figure 5). The -160°C relaxation has been ascribed to the combined onset of the trifluoroethoxy, (3, and fluoroalkoxy, 3", side chain motion (15,16). The — 50°C relaxation has been ascribed (15) to the glass transition (segmental backbone) of the PNF. 

Figure 8, Transition map for PIB. The hypersonic result corir siderahly extends the available frequency range. The primary (P) glass-rubber relaxation line and secondary (S) relaxation line are from Ref. 22.

Figures 1 and 2 show the storage modulus and tan 6 for HX-205 and F-185 neat resins as a function of temperature. E for HX-205 decreases gradually with an Increase In temperature and does not show any transition Indicative of secondary molecular relaxation until the temperature reaches the glass transition temperature, which Is approximately 60 C. E for F-185, on the other hand, shows a transition starting at about -50 C accompanied by an Increase In tan 6. The tan 6 continues to Increase until the temperature reaches the glass transition temperature at which point the tan 6 Increases drastically.

Another DMA analysis is shown in Fig. 4.170 for poly(vinyl chloride), [-CHCl-CHjlx- The data for G, G", and tan 6 are given as a function of temperature for one frequency. The glass transition occurs at about 300 K, as indicated by the drop in G and the peaks in G" and tan 6. In addition, there is a broad peak in G" and in tan 6, indicating a secondary, local relaxation in the glassy state. Semicrystalline... 

Many polymeric liquids display a maximum in G" at higher frequencies than those associated with the primary glass relaxation. The secondary maximum can have a relaxation strength (as measured by the value of the distribution of relaxation times) that exceeds the primary glass relaxation strength. The frequencies of maximum loss often obey tiie relation ... 

This is consistent with the local intramolecular character of many of these relaxations. It is also observed that the breadth of the distribution of relaxation times associated with tiie secondary relaxations is often much larger than that associated with the primary glass relaxation. 

The dynamic mechanical thermal properties of PHCT copolymers in the amorphous state showed that all the samples exhibited two main relaxation processes the primary a relaxation associated with the glass transition temperature and the secondary P relaxation, attributed to the local segmental motions of the cyclohexylene rings, as well as the cooperative motions of the methylene, carbonyl and phenylene groups (51). Both, a and P-relaxations temperatures increased with the content of CHDM units in the copolymer. 

Strain-rate Dependence. In the dispersion region of primary or secondary glass transitions, there is a viscoelastic strain-rate dependence of mechanical parameters its magnitude is related to the relaxation strength. Viscoelastic relaxation processes are no longer effective, at least below 30 K. 

At low temperature the material is in the glassy state and only small ampU-tude motions hke vibrations, short range rotations or secondary relaxations are possible. Below the glass transition temperature Tg the secondary /J-re-laxation as observed by dielectric spectroscopy and the methyl group rotations maybe observed. In addition, at high frequencies the vibrational dynamics, in particular the so called Boson peak, characterizes the dynamic behaviour of amorphous polyisoprene. The secondary relaxations cause the first small step in the dynamic modulus of such a polymer system. 

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. 

Fig. 6. Transition map for PPG showing the primary glass-rubber relaxation (a process) and the secondary relaxation (J3 process)...

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