Start-up and relaxation of well-characterized comb polymers in simple shear

We report on the shear flow start-up and the relaxation upon flow cessation of anionically synthesized comb polymers of different chemistries. The experimental data, obtained with a cone partitioned-plate geometry in order to avoid artifacts, showed that the start-up shear flow of combs exhibits systematic dependencies on the branching structure. They were interpreted by invoking dynamic dilution and hierarchical relaxation, which are known to control the linear viscoelastic response. For all combs studied here, the backbones remained entangled after dynamic dilution due to branch relaxation. We combined the important molecular parameters (i.e., the number and molar mass of the branches) into a single parameter, the number of entanglements of the dynamically diluted backbone, ZBB DIL., which we found to be the main scaling parameter for the observed nonlinear flow behavior. The steady viscosities as function of Weissenberg number were less shear-thinning compared to linear analogues, and the higher the amount of branching, the less thinning they became, reflecting a broader relaxation spectrum, and being consistent with the behavior of commercial branched polymers. The strain at maximum viscosity was higher for combs in comparison to linear polymers, a finding attributed to nonlinear hierarchical relaxation. The maximum in viscosity (scaled with steady viscosity) became lower with increased degree of branching due to the action of dynamic dilution. The viscosity peaks became broader for combs with an increased degree of branching, which is again a reflection of a broader relaxation spectrum. The initial relaxation rate upon cessation of steady shear increased with shear rate and seemed to reflect the loss of entanglements of combs in steady shear due to the action of convective constraint release. The relaxation was found to be independent of branching structure, suggesting that for the time ranges considered here, the loss of orientation of the backbones scales with the longest relaxation time, and is hence an effect of linear relaxation mechanisms (i.e., mainly reptation of the backbone). (C) 2013 The Society of Rheology.