Rheological properties of branched polystyrenes: nonlinear shear and extensional behavior
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Nonlinear shear and uniaxial extensional measurements on a series of graft-polystyrenes with varying average numbers and molar masses of grafted side chains are presented. Step-strain measurements are performed to evaluate the damping functions of the melts in shear. The damping functions show a decreasing dependence on strain with an increase in mass fraction of grafted side chains. Extensional viscosities of the melts of graft-polystyrenes exhibit a growing strain hardening with increasing average number of grafted side chains as long as the side branches have a sufficient molar mass to be entangled. Graft-polystyrenes with side arms below the critical molar mass Mc for entanglements of linear polystyrene but above the entanglement molar mass Me of linear polystyrene (Me < Mw,br < Mc) still show a distinct strain hardening. With decreasing molar mass of the grafted side chains (Mw,br < Me) the nonlinear-viscoelastic properties of the graft-polystyrene melts approach the behavior for a linear polystyrene with comparable polydispersity.