Polymer and Materials Science
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
Mechanical damping measurements were carried out in the range of 103-105 cps and between 60°K. and the softening point on some substituted Polystyrenes and poly(vinyl benzoates) containing different substituents (methyl groups, methoxy groups, and halogen atoms) either in the ring or in the main chain. The ortho and meta ring-substituted polystyrenes do not show any secondary mechanical relaxation in the glassy state, although all the other substituted polystyrenes, exhibit a low-temperature damping peak (δ process) (which is in some way connected with ring motions) whose height and temperature location depend on nature, position, and number of substituents. Substituents in the para position of the ring or in the α position in the backbone chain shift the δ peak of the unsubstituted polystyrene towards higher temperatures; this shift is accompanied by an increase of the apparent activation energy E*. Substitution in the β position, on the contrary, does not affect the δ peak. Analogous results are obtained for substituted poly(vinyl benzoates), which exhibit, in addition, a β relaxation effect, associated with carboxyl group motions. A very good correlation is found between the values of E* and the limiting relaxation time τ for the δ relaxation of polystyrenes and poly(vinyl benzoates), similarly substituted in the ring, indicating that the δ relaxation leads to absorption curves in the mechanical relaxation spectrum which are characteristic of the structure of the aromatic side chain.
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