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 Ge
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/Ne < 58).
For all materials studied, Ge
decreases with increasing shear rate
over a wide range of rates. At high shear rates, an approximate scaling relation Ge()∼−1/2
can be defined for all but the most weakly entangled polymer solution; for this material, a related scaling form Ge()∼−1
correctly describes the experimental results. We also find that the ratio of limiting shear modulus Ge(0)
to modulus at finite rate Ge()
is related to a molecular stretching functional 〈∣E⋅u∣〉
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 Ge(0)/Ge()≈〈∣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 Ge(0)/Ge(γ)≈〈∣E⋅u∣〉1/2,
as the rate is increased. Implications of these results on polymer contour length dynamics are discussed. © 1999 Society of Rheology.