Polymer disentanglement in steady-shear flow

A procedure is presented for investigating entanglement loss in polymer liquids during steady-shear flow. The method combines steady shearing with small-amplitude step strain measurements to determine the elastic modulus G(e) of an entangled polymer network under steady-state flow conditions. In this study, superimposed step/steady-shear measurements are used to investigate entanglement loss in narrow molecular weight distribution polystyrene/diethyl phthalate solutions with variable entanglement density (9 < N/N-e < 58). For all materials studied, G(e) decreases with increasing shear rate (gamma) over dot over a wide range of rates. At high shear rates, an approximate scaling relation G(e)((gamma) over dot) similar to (gamma) over dot(-1/2) can be defined for all but the most weakly entangled polymer solution; for this material, a related scaling form G(e)((gamma) over dot) similar to (gamma) over dot(-1) correctly describes the experimental results. We also find that the ratio of limiting shear modulus G(e)(0) to modulus at finite rate G(e)((gamma) over dot) is related to a molecular stretching functional [\E . u\] by G(e)(0)/G(e)((gamma) over dot) approximate to [\E . u\](P), where p takes on values of 1 and 1/2, depending on whether contour length stretching is taken to be affine p = 1, or nonaffine p = 1/2. For the lowest molecular weight polymer investigated, the affine stretch result G(e)(0)/G(e)((gamma) over dot) approximate to [\E . u\] fairly describes the experimental results over the entire range of shear rate investigated. Other materials manifest a transition from an initially affine to a square-root nonaffine response G(e)(0)/G(e)(gamma) approximate to [\E . u\](1/2), as the rate is increased. Implications of these results on polymer contour length dynamics are discussed. (C) 1999 The Society of Rheology. [S0148-6055(99)02206-3].