Molecular Weight Dependence of Glassy Dynamics in Linear Polymers Revisited

Dielectric spectra of a series of polydimethylsiloxanes and polystyrenes were measured covering a wide range of molecular weights M including the monomeric limit. From spectral analysis the time constants tau(alpha)(T) of the segmental dynamics are extracted, allowing determination of the glass-transition temperatures T-g(M) and tau(alpha)(M) as well as the parameters D(M) and T-0(M), respectively, Delta(M) = T-g - T-0, of the Vogel-Fulcher-Tammann equation. In addition, we analyzed fragility m(M) including here also data on polybutadiene. It turns out that all the quantities show particularities as a function of M which are not explained by common wisdom. T-g(M) as well as T-0(M) data reveal a noncontinuous M dependence with three distinct regimes which may become plausible when comparing our results with rheological and NMR data. We attribute the three regimes to simple liquid, Rouse, and entanglement dynamics, i.e., T-g(M) and T-0(M) traces show kinks at the Rouse unit M-R and entanglement molecular weight M-e. Fragility m(M) shows only one kink at M-e. This fact can be explained by the behavior of Delta(M), which increases with M in the simple liquid regime but saturates already at MR; a similar behavior is observed for the exponential prefactor tau(0)(M). We conclude that polymer dynamics specifically modify glassy dynamics.