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 e xperimental data combined with recoverable creep compliance data taken previously on the same sample provides information on the local segme ntal motion in a wide time window of more than II decades. Significant reduction of the ''fragility'' (i.e., a scaled temperature dependence of the retardation time) of the low molecular weight polystyrene comp ared with high molecular weight polystyrene, seen first by Santangelo and Roland by shear modulus measurement, is confirmed. The broader tim e window available in the present work shows for the fu st tithe that, unlike high molecular weight polystyrene, the dispersion of the low m olecular weight sample is strongly temperature dependent and narrows w ith increasing temperature. By considering the low molecular weight po lymer as a blend of chain ends and inner repeat units, the results are interpreted by the coupling model.