Relations between steady flow and oscillatory shear measurements

Abstract

Results are given of a comparison between dynamic oscillatory and steady shear flow measurements with some polymer melts. Comparison of the steady shear flow viscosity,η, with the absolute value of the dynamic viscosity, ¦η¦, at equal values of the shear rate,q, and the circular frequency,ω, has shown the relation thatCox andHerz had found empirically to be substantially correct. Further, the coefficients of the normal stress differences obtained by streaming birefringence techniques have been compared with 2G′ (ω) ·ω − 2 in the same range of shear rates as covered by the viscosity measurements (G′ is the real part of the dynamic shear modulus). Two polystyrenes with narrow molecular weight distribution showed the same shift factor along theω orq axis for the normal stress coefficients with respect to 2G′ (ω) · ω − 2 and the steady shear flow viscosities with respect to the real part of the dynamic viscosity,η′. For two polyethylenes the results are not so conclusive owing to the smallness of the shift factor found. An empirical equation is proposed predicting the main normal stress difference from dynamic measurements only.