Secondary relaxation transitions

Experimental data collected so far show that most secondary relaxations (transitions, dispersions) in glassy polymers are a consequence of the conformational isomerization of short sections of main or side chains and that their kinetics may be satisfactorily described by means of the site model in which stable conformations are separated by a potential... 

The basic methods of detecting the secondary relaxation transitions are dynamic mechanical spectroscopy (DMS) and differential thermal analysis in some cases information can be obtained by evaluation of the dielectric characteristics, by NMR, and by DSC. 

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)...

A typical loss maximum of this type was observed for poly(methyl methacrylate) containing caprolactam or derivatives of cyclohexane12,13. It is noteworthy70 that in the latter case the relaxation induced by the cyclohexyl group present in the incorporated plasticizer and the secondary relaxation of poly(cyclohexyl methacrylate) or poly(cyclohexyl acrylate) are characterized by an identical temperature position, 190 K (1 Hz), and activation energy, 47.9 kJ/mol (AU = 47.7 kJ/mol is reported for the chair-chair transition of cydohexanol). Hence, it can be seen that the cyclohexyl ring inversion, which represents a specific molecular motion, is remarkably insensitive to the surrounding molecules. 

As mentioned earlier, we usually encounter two characteristic secondary relaxations in polymethacrylates and polyacrylates (below the glass transition temperature) which are assigned to side-chain motions1,12,13,15 The p relaxation due to partial rotation of... 

It has also been established for the copolymers HEMA-AAm (Fig. 7) that the temperature and intensity of the secondary dispersion do not change systematically with the copolymer composition. Thus, molecular motions underlying the ji or /S dispersions in polymethacrylates or polyacrylates differ from each other. This may be ascribed to a different cooperation of the backbone, but the results obtained so far do not suffice for a more precise interpretation. Likewise, it is difficult to explain156) that PAAc, in contrast to PMAAc, does not exhibit any secondary relaxation above the liquid nitrogen temperature. It is to be noted that the effect of stereoregularity and diluents168) on the /8 relaxation is not easy to estimate because the concomitant decrease in T accounts for the overlapping of the /3 and a transitions. 

The co transition is a weak and wide transition centred around 60 °C, in the temperature range of the co secondary relaxation observed by dielectric relaxation. In both cases, the position and width of the co peak are independent of the copolyamide composition. 

DSC and DTA. They can be used to confirm suspicious glass transitions revealed by DSC and most important, they can further quantify molecular mobility associated with sub-glass transitions. For example, DSC analysis of poly (ethylene 2,6-naphthalene dicarboxylate) (PEN) only revealed the presence of a glass transition around 112 °C (Hardy et al., 2001). DMA analysis of the same sample, however, revealed two secondary relaxations below this glass transition (Hardy et al., 2001). In the case of humic materials, it is not uncommon for DSC to fail to detect clear thermal transitions due to their heterogeneous nature, which contributes to overlap/ broadening or washout of thermal transitions. As such,TMA and DMA represent powerful, complementary tools to DSC. 

Although molecular mobility is severely restricted below the glass transition temperature, the dynamic glass transition temperature (main transition or, conventionally -relaxation) in polymers as it have been described above, is usually accompanied by subglass secondary relaxations labeled as p, y, S, relaxations. The glass transition at low temperatures is assumed to be caused by the cooperative motion of many particles, while the secondary relaxations have a more localized molecular... 

The P zone extend over a large temperature range. This is a characteristic of a secondary process which involve local motions of the lateral groups [155], They are more diversified movements with a large spectrum of relaxation times. Therefore, thermal cleaning of the t.s.c. global spectra is used to study the broad relaxation peaks of the low temperature secondary relaxation [42], This is effective because it allows one to excite only the specific transition of interest [155],... 

The variation of fanS with temperature at 1 kHz for the six poly(thiocarbonate)s is represented in Fig. 2.86. In all cases a prominent relaxation associated to the glass transition temperature labelled as a -relaxation is observed in Figure PT-1. A secondary relaxation which covers a range of about hundred degrees and which by comparison with the results reported for PCs is labeled as y relaxation. Between 80°C and 100°C a slightly dielectric activity is observed (f) zone) and at — 120°C another relaxation labelled as 5 relaxation for polymers 4,5 and 6. 

Analysis of XH T2 relaxation as a function of temperature yields information on molecular motions. Side-groups and local chain motions (secondary relaxations) cause a change in T2 below the Tg. The value of T2 increases with the amplitude and the frequency of molecular motions. The glass transition that occurs in the time scale in the NMR T2 relaxation... 

The mechanical dispersion peaks in low-Tg epoxies such as Bisphenol-A based resin (Epon 828, products from Shell Development Company) have been the subject of numerous studies 143,145148,152 "155, l59>. The alpha-dispersion peak related to the glass transition can undoubtly be attributed to the large-scale cooperative segmental motion of the macromolecules. The eta-relaxation near —55 °C, however, has been the subject of much controversy 146,153). One postulated origin of the dispersion peak is the crankshaft mechanism at the junction point of the network epoxies (Fig. 17). The crankshaft motion for linear macromolecules was first propos-ed 163 166> as the molecular origin for secondary relaxations which involved restricted motion of the main chain requiring at least 5 and as many as 7 bonds 167>. This kind of... 

Extensive dynamic mechanical property studies have been carried out on hydrogen-bonded (81) and nonhydrogen-bonded (60,82) polyurethanes. Several secondary relaxations were found in addition to the major hard- and soft-segment transitions. Molecular mechanisms could... 

There have been proposed several types of short-range molecular motions to account for the various transitions. Following Heijboer (1977), Fig. 13.30 illustrates schematically possible modes of molecular motion in secondary relaxation in glassy polymers. The classification is outlined below. 

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. 

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