Local Segmental Dynamics of Low Molecular Weight Polystyrene: New Results and Interpretation

The molecular dynamics of an 1100 low molecular weight polystyrene are reexamined by photon correlation spectroscopy measurements. The new experimental data combined with recoverable creep compliance data taken previously on the same sample provides information on the local segmental motion in a wide time window of more than 11 decades. Significant reduction of the “fragility” (i.e., a scaled temperature dependence of the retardation time) of the low molecular weight polystyrene compared with high molecular weight polystyrene, seen first by Santangelo and Roland by shear modulus measurement, is confirmed. The broader time window available in the present work shows for the first time that, unlike high molecular weight polystyrene, the dispersion of the low molecular weight sample is strongly temperature dependent and narrows with increasing temperature. By considering the low molecular weight polymer as a blend of chain ends and inner repeat units, the results are interpreted by the coupling model.