Shear and elongational rheology of model polystyrene pom-poms.

Understanding the effect of branching on the melt rheological properties in shear and elongational flow is of fundamental interest to polymer science. Besides the number and the molecular weight of the arms as well as of the backbone, the exact location of the branching points, thus the exact topology is key to control shear and extensional rheological properties. Fourteen low-disperse, polystyrene pom-pom samples were synthesized with two optimized synthetic routes via a combination of anionic polymerization and grafting onto, in a scale of up to 300 g per model system. The molecular parameters of the pom-poms such as molecular weight of the backbone (MW,b=100 - 400 kg.mol−1), of the arms (MW,a=7 - 300 kg.mol−1) and the arm number at each end (q=5 - 30) are systematically varied. The shear and elongational behavior are investigated experimentally and analyzed within the framework of the pom- pom model. High shear thinning is found in small amplitude oscillatory shear (SAOS) measurements. The results in shear revealed up to five times lower zero-shear viscosities for pom-poms compared to combs with similar MW,b, MW,a and total arm number. For the pom-poms, maximum strain hardening factors beyond 100 are found in elongation, depending for the pom-poms exclusively on the total arm number, as the elongational viscosity is by a factor of [q²/ln(√3×q)] above the linear-viscoelastic envelope (LVE) in the backbone rubber plateau, as predicted by the pom- pom model and the Considère criterion. [ABSTRACT FROM AUTHOR]