Molecular motions in a series of linear aliphatic polyesters [poly(ethylene adipate), poly(ethylene sebacate), poly(hexamethylene sebacate), and poly(decamethylene 1,16‐hexadecanedicarboxylate)] were studied by dielectric measurements. Two loss maxima were observed for each polymer in the temperature range from −196 to about 60°C and in the frequency range from 110 to 105 Hz. The loss maxima of these polyesters, lying between −17 and −38°C at 110 Hz (β‐relaxation), are due to the micro‐Brownian motions of amorphous main chains. It was found that these β‐relaxations are well described by the WLF equation. The loss maxima in the range from −88 to −109°C at 110 Hz (γ‐relaxation), are attributed both to local mode motions of main chains in the amorphous region and to motions of the polar groups involved at the chain ends. For the β‐relaxation, no simple relation between the methylene sequence length and the loss peak temperature was found. Furthermore, as the methylene sequence length decreased, the effective dipole moment of the polyesters increased gradually. These facts were explained in terms of interchain dipole attraction.

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