Cross-linked polymers secondary relaxation

The activation energy of secondary relaxation in polymers can be measured by means of dynamic mechanical or dielectric relaxation spectroscopy. Dynamic mechanical or dielectric relaxation spectra of polymers can be obtained as a function of temperature at different frequencies. As an example, the dielectric relaxation behavior of the secondary relaxation in some AB cross-linked polymers is shown in Figure 4.63. 

Figure 4.63 Dielectric loss spectra for the secondary relaxation regions in an AB cross-linked polymer [85],...

The one general class of polymers that fall outside this concept is the thermoplastic elastomers, one example of which was discussed previously. These materials can be processed (and reprocessed) at high temperature, yet they maintain properties of cured rubber at use temperatures. This system functions by the formation of either hard plastic, crystalline, or ionic domains that, at use temperature, act as cross-link sites because multiple chains are involved in the domains. Upon heating, the integrity of these domains breaks down, and the polymer chains can easily flow past one another. It should be noted that at use temperatures these systems have a three-dimensional network. Such systems tend to show more creep and stress relaxation than cured systems, as the network is formed via weaker secondary effects rather than primary chemical bonds. These problems become more severe as the use temperature is increased because ultimately the network cannot remain intact at processing temperatures. For any network, its structure is important in defining the performance of the... 

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