Diffusion in Entangled Polymer Solutions: Constraint Release

We have used the proton pulsed-gradient spin-echo NMR method to measure the self-diffusion D of entangled polybutadienes (PBD) in tetrachloromethane at 70$^{\circ}$C, in order to elucidate the origins of the departure of the molecular-weight (M) exponent of D from the reptational -2.0, and to characterize any differences between melt and solution behavior. M(PBD) ranged from 12 kDa to 100 kDa; PBD concentration (volume fraction ) v was 0.350, 0.422, and 0.514. Entanglement ratios R were calculated as M/M$_{e}$(v), where for PBD M$_{e}$(1) = 4.5 kDa and M$_{e}$(v) = M$_{e}$(1)/v$^{1.2}$, yielding a range of R between 2 and 25. Preliminary results, plotted in the form log (DM$^{2}$v$^{1.2})$ \textit{vs.} log R, adhere to a master curve with slope near -1, suggesting at most modest differences from the known behavior of melts. The relatively rapid approach to the melt asymptote, near R = 20, supports our earlier contention that constraint release rather than contour length fluctuation is the cause of the observed M-exponent of D. Further work, using solutions of binary blends based on high-M PBD, is in progress.