Polymer disentanglement in steady-shear flow

A procedure is presented for investigating entanglement loss in polymer liq uids during steady-shear flow. The method combines steady shearing with sma ll-amplitude step strain measurements to determine the elastic modulus G(e) of an entangled polymer network under steady-state flow conditions. In thi s study, superimposed step/steady-shear measurements are used to investigat e entanglement loss in narrow molecular weight distribution polystyrene/die thyl phthalate solutions with variable entanglement density (9 < N/N-e < 58 ). For all materials studied, G(e) decreases with increasing shear rate (ga mma) over dot over a wide range of rates. At high shear rates, an approxima te 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; fo r this material, a related scaling form G(e)((gamma) over dot) similar to ( gamma) over dot(-1) correctly describes the experimental results. We also f ind that the ratio of limiting shear modulus G(e)(0) to modulus at finite r ate 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), wh ere p takes on values of 1 and 1/2, depending on whether contour length str etching is taken to be affine p = 1, or nonaffine p = 1/2. For the lowest m olecular weight polymer investigated, the affine stretch result G(e)(0)/G(e )((gamma) over dot) approximate to [\E . u\] fairly describes the experimen tal results over the entire range of shear rate investigated. Other materia ls manifest a transition from an initially affine to a square-root nonaffin e response G(e)(0)/G(e)(gamma) approximate to [\E . u\](1/2), as the rate i s increased. Implications of these results on polymer contour length dynami cs are discussed. (C) 1999 The Society of Rheology. [S0148-6055(99)02206-3] .