Secondary relaxations in the glassy regime

In this section we present only several secondary relaxations occurring in glassy PMMA, to complement the discussions in earlier sections of the a-relaxations in PMMA. 

McCnun et al. (1967) have listed the various secondary relaxations observed in PMMA. The one that has been most widely studied is the p-relaxation of the COOCH3 ester side group. It has been studied extensively both by dielectric relaxation and by mechanical relaxation. We consider here only the latter mechanical relaxations, which are summarized in Fig. 5.10, where the relaxation information is plotted as either the temperature of the loss peak in dynamic experiments at constant frequency or as the frequency of the loss peak in experiments conducted at constant temperature. All results are plotted along the slanted P-line. The figure also shows on the left side a steep line that represents the a-relaxation. This should not have appeared on this kinetic plot, and its slope is too steep to show its characteristic curvature representing the WLF form of relaxation. It is included only to show the relative positions of the specific secondary relaxations against the a-relaxation. 

Another prominent secondary relaxation in PMMA is associated with the hydrophilic character of the ester side groups that can accommodate 1 %-2% of water in normal practice unless the PMMA is thoroughly dried. Such presence of water in PMMA results in a prominent and relatively broad dip in the modulus. Its relaxation property is summarized also in Fig. 5.10. 

Finally, also a low-temperature relaxation peak associated with the a-methyl group in the main chain is included in Fig. 5.10. 

In addition to these secondary relaxations exhibited by atactic PMMA there are other prominent relaxations in stereoregular PMMA as well as isotactic and syndiotactic PMMA that have been discussed and summarized by McCrum et al. (1967). 

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