Crossover in dynamics of polymeric liquids: back to T_II?

Light scattering spectra of two polymers, polyisobutylene (PIB) and polystyrene (PS), were analyzed in the broad frequency range at temperatures above the glass transition (T_g). At high temperatures, the spectra followed the qualitative scenario suggested by mode-coupling theory (MCT) of the glass transition. The crossover temperature (T_c) was defined to be approximately 1.35 T_g in PIB and approximately 1.15 T_g in PS. At lower temperatures (T < T_c), the light scattering spectra deviated strongly from the idealized MCT scenario. Different signs of the dynamic transition around T_c 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.