Secondary relaxation processes amorphous polymers

The effect of diluents on the viscoelastic behavior of amorphous polymers is more complex at temperatures below T, i.e., in the range of secondary relaxation processes. Mechanical, dielectric and NMR measurements have been performed to study the molecular mobility of polymer-diluent systems in this temperature range (see e.g. From extensive studies on polymers such as polycarbonate, polysulfone and polyvinylchloride, it is well known that diluents may suppress secondary relaxation processes. Because of the resulting increase in stiffness, these diluents are called antiplasticizers . Jackson and Caldwell have discussed characteristic properties... 

Conventional models or theories consider only the segmental relaxation of amorphous polymers and the primary relaxation of nonpolymeric glass-formers in the change of molecular mobility with temperature and pressure (and concomitant changes in free volume and/or configurational entropy) leading to vitrification. Here we wish to recognise two different kinds of secondary relaxation processes. There... 

Secondary relaxation processes have been correlated with a number of physical and mechanical properties. For example, there is a good correlation between impact strength and the occurrence of main-chain secondary loss processes [24-26], especially for amorphous polymers [21]. There is also reasonable correlation between secondary loss processes and gas permeability [27-29]. 

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. 

Stress induced thermal events have been produced in all of the amorphous polymeric glasses examined to date. Surprisingly, these processes are always observable in a narrow temperature range above the ambient temperature, independent of the chemical structure of the polymer. Therefore, these effects are not, as might be expected, associated with the materials sub Tg secondary relaxations. The evidence for this assertion is presented in Figure 7 which shows the response of a variety of similarly stressed polymers approximately one hundred days after compaction. From top to bottom, in order of their respective glass transition temperatures are polystyrene (MW=37,000) ... 

For constant-frequency (sometimes referred to as isochronal) experiments on amorphous polymers, the highest temperature relaxation is the glass transition, also referred to as a, while secondary p and y relaxations are commonly observed. There also may be a 8 process. In this case all the relaxations are associated with the amorphous phase. The y and 8 relaxations (and in certain cases also the p relaxation) are below -100 °C. These relaxations would be missed if the only cooUng device available were a mechanical chiller that would cool to only -100 °C. Figure 5.18 illustrates the various loss peaks associated with the transitions in amorphous and crystalUne polymers. 

It is well established that all solid amorphous polymers exhibit a principal (or a) and a secondary (or process (McCrum et al., 1967 Ishida, 1969 Hedvig, 1977 Baird, 1973 Wada, 1977). Many polymers exhibit further relaxations e.g., poly-(methyl methacrylate) exhibits five processes (see McCrum et al., 1967, p. 250). Not all relaxation processes are observed in a dielectrics experiment. A process is dielectrically active only if it involves the reorientation of the dipole-moment vector. ... 

Colombini and co-workers [42] used DMTA and DETA (Chapter 12) to explore the relaxation processes occurring in amorphous and semi-crystalline polyethylene naphthalene-2,6,-dicarboxylate. The two secondary relaxations P and P, the main a-relaxation and the p-relaxation processes were revealed by both mechanical and electro viscoelastic responses of the polymer. The DMTA results clearly identified the T(a) loss factor peak. 

Experimentally it is found that AFis positive for relaxations in polymers so equation (37) shows that Qv < Qp if temperature of a material is raised at constant pressure, part of the decrease in is due to the increase in volume accompanying the increase in temperature. The effect of pressure can be very large for the primary (a) relaxation in amorphous polymers with (31n /5P)t lying in the range (1-5) x 10 atom" or can be extremely small, as found for the secondary () ) relaxation in amorphous polymers e.g. for poly(ethylene terephthalate) and poly(vinyl chloride)" ]. Use of data provides a test of theories for relaxation in the glass transition region (a process)." " ... 

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