How Entangled Polymer Chains Relax

It will be shown through a series of experiments with selectively deuterated model polymers that stress relaxation occurs through a mechanical percolation process which permits large clusters of entangled polymers to stress relax before their conformations are fully relaxed. We find that: (a) Reptating homopolymer chains with molecular weight M $>>$ M$_{c}$ appear to be non-Reptating as their ends and centers relax at the same rate in a Rouse-like manner during percolation. (b) The mechanical relaxation time .$\tau $(M) is related to the Reptation time T$_{r}\sim $ M$^{3}$ by .$\tau $(M) = T$_{r}$[(1-M$_{c}$/M) M$_{e}$/M$_{c}$]$^{2}$, which is the origin of the viscosity behaving as .$\eta \sim $M$^{3.4}$ (c) During stress relaxation, the random coil dimensions R$_{g}$(//) and R$_{g}$(.$\bot )$ are significantly not relaxed when the stress and birefringence relax to zero. (d) Matrix molecular weight P effects on relaxation time .$\tau $(M) of the probe chain M are as follows: When the probe chain M$>>$P, the matrix P-chains percolate and Rouse-like dynamics is observed for the M-Reptating chains with .$\tau $(M) $\sim $ P$^{1}$M$^{2}$. (e) When the matrix P$>>$M, percolation does not occur for the M-chain and the relaxation time of the probe chain .$\tau $(M) $\sim $ P$^{o}$M$^{3}$ is in accord with DeGennes Reptation theory. These results clearly suggest that current notions of polymer rheology involving chain end fluctuation and constraint release need to be reconsidered. .