AbstractThe β‐relaxation atTgof the terephthalate and isophthalate series is due to molecular motions of the backbone chains in the amorphous region, but differs for the isophthalate series in that thep‐phenylene group does not exhibit the free rotation possible for them‐phenylene group. Consequently, the relaxation times of the terephthalate series are longer than for the isophthalate series. The γ‐relaxation (Tγ) for the higher homologues of the terephthalate series cannot be explained in terms of the poly(ethylene terephthalate) analogy. For poly(ethylene terephthalate) and poly(tetramethylene terephthalate), an induced cooperative type of motion of all the moieties is possible, whereby overlapping processes caused by “rocking vibrations” are observed as one γ‐peak. The resolution of the γ‐loss peaks for the above‐mentioned polyesters into components is not possible at the experimental frequency of 110 Hz. For poly(hexamethylene terephthalate) and poly(decamethylene terephthalate), the “rocking vibrations” between the moieties of the skeletal chain are reduced so that even at a test frequency of 110 Hz, the γ‐loss peaks could be resolved into two or three components. In the case of poly(decamethylene terephthalate), three components are resolved; the lowest temperature peak γ1is attributed to hindered motions of the methylene portions, the γ2peak is attributed to motions of the carbonyl group in the gauche conformation, and the γ3peak is attributed to the carbonyl group in the trans conformation of the skeletal chain in the amorphous region. The general observations obtained by other techniques were confirmed by the forced vibration analyses. As the length of the methylene chain increased,Tgdecreased. As crystallinity increased, the β‐relaxation moved to higher temperatures and the damping peak was smaller and broader. The damping peak moved to lower temperatures and increased in size as the length of the methylene chain increased. The damping peak was larger for the isophthalate homologue than for the co
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