Crossover in dynamics of polymeric liquids: Back to T-parallel to?

Light scattering spectra of two polymers, polyisobutylene (PIB) and polysty rene (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 disc ussed. The difference between the suggested interpretation and an old idea of the liquid-liquid transition in polymeric liquids is stressed: we descri be the transition as purely dynamic in nature. (C) 2000 John Wiley & Sons, Inc.