Crossover in dynamics of polymeric liquids: Back toTll?

Light scattering spectra of two polymers, polyisobutylene (PIB) and polystyrene (PS), were analyzed in the broad frequency range at temperatures above the glass transition (Tg ). At high temperatures, the spectra followed the qualitative scenario suggested by mode-coupling theory (MCT) of the glass transition. The crossover temperature (Tc ) was defined to be approximately 1.35 Tg in PIB and approximately 1.15 Tg in PS. At lower temperatures (T < Tc ), the light scattering spectra deviated strongly from the idealized MCT scenario. Different signs of the dynamic transition around Tc are discussed. The difference between the suggested interpretation and an old idea of the liquid–liquid transition in polymeric liquids is stressed: we describe the transition as purely dynamic in nature.