Beschreibung:
<jats:title>Abstract</jats:title><jats:p>Dynamic mechanical properties (sound velocity and damping factor) have been determined in poly‐α‐methylstyrene in the temperature range from 60 to 470°K., at frequencies in the range 10<jats:sup>3</jats:sup> ÷ 10<jats:sup>6</jats:sup> Hz. Two high dissipation regions are found in the damping‐temperature curves: one, at temperatures below <jats:italic>T</jats:italic><jats:sub>g</jats:sub>, characterized by a loss peak at about 140°K., and another, at temperatures above <jats:italic>T</jats:italic><jats:sub>g</jats:sub> (405°K.), characterized by a very rapid increase of losses with increasing temperatures. The loss peak at low temperature (δ) is probably due to rotational motions of the phenyl rings in the side chain; the corresponding activation energy is about 4000 cal.‐mole<jats:sup>−1</jats:sup>, much higher then that of polystyrene, because of the sterical hindrance of CH<jats:sub>3</jats:sub> in the <jats:italic>α</jats:italic> position. A <jats:italic>δ</jats:italic> transition is also found in other vinyl polymers containing aromatic groups as side chains: by plotting the logarithm of the activation energy as a function of the reciprocal maximum damping temperature, determined in different polymers, points nearly lying on a straight line are obtained. The main transition is associated with cooperative motions of chain segments. Practically no difference is found between the dynamic mechanical properties of atactic and syndiotactic samples of poly‐<jats:italic>α</jats:italic>‐methylstyrene.</jats:p>