Effect of long‐chain branching on the relation between steady‐flow and dynamic viscosity of polyethylene melts

The steady‐state viscosity η, the dynamic viscosity η′, and the storage modulus G′ of several high‐density and low‐density polyethylene melts were investigated by using the Instron rheometer and the Weissenberg rheogoniometer. The theoretical relation between the two viscosities as proposed earlier is:{article}{empty}{document}η ( γ ) = ∫ H( ln τ ) h( θ )g( θ )^0.7ex3\!/ 3 2.-\!0.7ex2τ d ln τ{document} , where {article}{empty}{document}θ = γ τ / γ τ 2 . - 2{document} ; {article}{empty}{document}γ{document} is the shear rate, H is the relaxation spectrum, τ is the relaxation time, {article}{empty}{document}$ g( θ ){ } = { }( {{2 {/ { {2 π }} . -} π }} )[ { - 1 θ { } + { }{θ {/ { {θ {( {1 + θ ^2 } )}}} . -} {( {1 + θ ^2 } )}}} ] document , and articleemptydocument h( θ ){ } = { }( {{2 {/ { {2 π }} . -} π }} )[ { - 1 θ { } + { }{{θ ( {1{ } - { }θ ^2 } )} {/ { {{θ ( {1{ } - { }θ ^2 } )} {( {1{ } + { }θ ^2 } )^2 }}} . -} {( {1{ } + { }θ ^2 } )^2 }}} ] ${document} . Good agreement between the experimental and calculated values was obtained, without any coordinate shift, for high‐density polyethylenes as well as for a low density sample with low nw, the weight‐average number of branch points per molecule. The correlation, however, was poor with low‐density samples with large values of the long‐chain branching index nw. This lack of coordination can be related to nw. The empirical relation of Cox and Merz failed in a similar way.