Beschreibung:
<jats:title>Abstract</jats:title><jats:p>Dynamic mechanical properties (sound velocity and damping factor Q<jats:sup>−1</jats:sup>) have been determined in some amorphous ethylene‐propylene copolymers from −170 to −15°C. by an electrostatic method at a frequency of the order of 10<jats:sup>4</jats:sup> Hz.</jats:p><jats:p>Two abrupt changes of slope are observed in the v—T curves, one at T'<jats:sub>g</jats:sub> (−120°C.) and one at T<jats:sub>g</jats:sub> (−30 to −55°C.) and two regions of high dissipation are noticed in the Q<jats:sup>−1</jats:sup>‐T curves, one at temperatures between T'<jats:sub>g</jats:sub> and T<jats:sub>g</jats:sub> and one at temperatures above T<jats:sub>g</jats:sub>.</jats:p><jats:p>The transition at T'<jats:sub>g</jats:sub> and the lower temperature relaxation phenomenon are both attributed to molecular motions of CH<jats:sub>2</jats:sub> groups in the main chain and of a limited number of chain elements containing CH<jats:sub>3</jats:sub> branching. The transition at T<jats:sub>g</jats:sub> and the higher temperature relaxation phenomenon are connected with motions of large sections of the chain.</jats:p><jats:p>The experimental results are discussed on the basis of the structure of ethylene‐propylene copolymers and suggestions are made concerning the use of dynamic mechanical data for the characterization of these materials.</jats:p>