Richard P. Wool, Department of Chemical Engineering and Center for Composite Materials, University of Delaware, 150 Academy St, Newark, DE 19716
The percolation model of entanglements (Wool 1993) makes unique predictions regarding the dynamics of polymer chains in the terminal relaxation zone of reptating linear polymer melts of molecular weight M. We show that the percolation entanglement model and the packing model are related through the dependence of the characteristic ratio C„V on the monomer molecular weight Mj as C„V ~ Mj1/2. When percolation occurs during relaxation of entangled chains in the terminal zone, we observe some unusual results. These include: (a) for homopolymers of molecular weight M >> Mc, reptating chains appear to be non-reptating as their ends and centers relax at the same rate during percolation. (b) During stress relaxation, the random coil dimensions Rg(//) and Rg(ƒÎ) are predictably not fully relaxed when the stress and birefringence relax to zero. (c) The matrix molecular weight P effects on relaxation time ƒä of the probe chain M >> Mc are as follows: Rouse-like dynamics is observed for reptating chains with ƒä ~ PM2 when M>>P and (d) the relaxation time ƒä ~ PoM3 when P>>M, in accord with reptation. These unusual results predicted by entanglement percolation are supported by significant experimental data from selectively deuterated polystyrene chains HDH, DHD and DPS. Entanglement Percolation complements Constraint release and dominates the Chain End Fluctuation mechanisms in the understanding of the dynamics of polymers in the melt and concentrated solutions.