Simple strain measure for entangled polymers

In the classical theory of Doi and Edwards for entangled polymers the strai n measure Q arises from the assumption of affine deformation of the tube se gments. On the other hand, we have recently argued [G. Marrucci, F. Greco, and G. Ianniruberto, "Possible role of force balance on entanglements," Mac romol. Symp. (in press)] that force balance on the nodes of the entangled n etwork generates departures from affinity. An ad hoc "lattice" model of the network was then proposed to fulfill force balance automatically, at least for single-step deformations. The corresponding strain measure is linked t o the square root of the Finger tensor, and is therefore easy to use. Predi ctions obtained with the new strain measure are here compared with a variet y of step strain data. (C) 2000 The Society of Rheology. [S0148-6055(00)001 04-8].