Iso-Frictional Mass Dependence of Diffusion of Polymer Melts Revealed by1H NMR Relaxometry

Abstract

We extract the translational diffusion coefficient D(T,M) from field cycling (FC) 1H 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 < Me), and entangled (M > Me) polymers are included. The low-frequency 1H 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 τs(T,M) is taken from previous analyses of the FC susceptibility master curves. Consequently, we present the temperature independent, iso-frictional quantity Dτs ∝ F(M), which reveals the M-dependence of the pure collective polymer dynamics. While at the lowest M the quantity Dτs 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 > Me. 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.