Glass transition and ultrasonic relaxation in polystyrene

The authors present measurements of the glass transition and the ultrasonic relaxation modulus in a series of monodisperse polystyrenes. The temperature dependence of the modulus was analyzed using Havriliak-Negami relaxation model (HN) and Vogel-Tammann-Fulcher equation (VTF) for the relaxation time. The results allowed us to determine the fragility index, m, which decreases with increasing molecular weight, M{sub n}. Furthermore, the relaxation time was found to saturate at high molecular weights and varies as M{sub n}{sup p}, in the low molecular weight region. The exponent is p {approx} 2 at high temperatures and p {approx} 7 at low temperatures close to T{sub g}.