Iso-Frictional Mass Dependence of Diffusion of Polymer Melts Revealed by 1H NMR Relaxometry

We extract the translational diffusion coefficient D(T,M) from field cycling (FC) H-1 NMR relaxometry which provides the relaxation dispersion of poly(dimethylsiloxane), 1,4-poly(butadiene), poly(styrene), 1,4-poly(isoprene), and poly(propylene glycol) with various molecular masses M. Oligomers with very low M, nonentangled (M < M-e), and entangled (M > M-e) polymers are included. The low-frequency H-1 NMR relaxation dispersion is dominated by translational dynamics and allows extracting D via benefiting from an universal dispersion power-law characteristic of free diffusion. In order to correct for the additional mass dependence of the monomeric friction coefficient observed at low M and controlled by the M dependence of the glass transition, the segmental correlation time tau(s)(T,M) is taken from previous analyses of the FC susceptibility master curves. Consequently, we present the temperature independent, iso-frictional quantity D tau(s) proportional to F(M), which reveals the M-dependence of the pure collective polymer dynamics. While at the lowest M the quantity D tau(s) displays a trend to become M independent typical of simple liquids, it crosses over to a behavior characteristic of Rouse dynamics. In most systems, however, this crossover manifests itself in a rather narrow M interval as entanglement dynamics takes over at M > M-e. Thus, pure Rouse behavior is difficult to identify, yet the approach allows one to decide when a molecule becomes a polymer, in terms of the (smallest) Rouse unit.