Ultrasonic relaxation of rotational-isomeric equilibria in polymer solutions

The sound absorption per wavelength µ in solutions of polystyrene in benzene and carbon tetrachloride deviates from proportionality with frequency in the low MHz range. This could be accounted for with good agreement by an additional, Debye-shaped excess absorption. Various models for the relevant relaxation process are discussed; the most probable one is the thermal relaxation of a rotational-isomeric equilibrium within the polymer chain, consisting of a rotation of a single monomer unit. Under that assumption, from the temperature dependence of position and amount of the absorption maximum the following data of the rotational potential were calculated: separation of the equilibrium energies ΔH= 0.9 kcal/mol, activation energy ΔH12= 7.5 kcal/mol, frequency factor ν= 1.4 × 1012 s–1.