The mechanism of constraint release for an entangled linear chain of polystyrene diffusing by reptation is emphasized, dissolving large N chains in a matrix of shorter N, chains of the same species.Dynamic shear measurements allow us to define an average relaxation time f(NJVt) for the relaxation of the large chains, from which we derive another average relaxation time for the tube modification induced by constraint release. We find that, according to the Klein and Graessley theories, the tube could be considered as a Rouse chain (rmod(1V^VS) AT9*0-1) but constraint release is not directly connected to the reptation of the N, chains (rmod(A''ArB)2'3±0'1)-The process of constraint release included in the diffusion of chains in a binary blend at various component concentrations allows us to specify the idea of "cell" in the Graessley theory and to describe reasonably the variations of the limiting parameters and Je°w ith blend composition.

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Considering that the terminal relaxation of every chain in a linear melt is dominated by reptational and constraint release motions, we propose an expression for the relaxation modulus G(t) of entangled polymers as a function of the molecular weight distribution (MWD). The expressions of zero-shear viscosity rj0 and steady-state compliance =/e°d erived from G(t) describe correctly the experimental behavior: 0 scales with average molecular weight as 0 Mw3•4"3•8 for a constant polydispersity P; Je°i ncreases strongly with P and varies widely with the shape of MWD. We also predict a decrease of viscosity with polydispersity; this is a possible explanation of some experimental variations of viscosity, which do not follow the classical power law with Mw. The behavior expected for strictly monodisperse samples shows that it is necessary to consider a relaxation times distribution in the terminal region.

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