Synthesis, rheology and constitutive modelling of complex polymer model systems

Jun.-Prof. Dr. Valerian Hirschberg, Clausthal University of Technology

Understanding the effect of branching on the melt rheological properties in shear and elongation is of fundamental interest since the dawn of polymer science and allows to engineer material properties, with the classical example for Polyethylene of HDPE and LDPE. Besides the number and the molecular weight of the arms, the exact location of the branching points, i.e., the exact architecture of the polymer (its topology) is key to control shear and extensional rheological properties.

Continuing the pom-pom story from a purely theoretical approach, about twenty low-disperse, polystyrene samples with a pom-pom topology were synthesized with optimized synthetic routes via a combination of anionic polymerization and grafting onto, in the scale of up to 300 g per model system. The molecular parameters of the pom-poms such as molecular weight of the backbone (M_W,bb = 100 – 400 kg/mol), of the arms (M_W,a = 2.5 – 300 kg/mol) and the arm number at each end (q = 5-30) are systematically varied. The shear and elongational behaviour are investigated experimentally and modelled with the pom-pom and the Hierarchical Multi-mode Molecular Stress Function (HMMSF) model.

Shear thinning is found in small amplitude oscillatory shear (SAOS) measurements, depending on M_W,a, the backbone volume fraction and backbone self-entanglement. The zero-shear viscosity and the diluted modulus as a function of the backbone volume fraction are analysed. Additionally, criteria to predict backbone self-entanglement based on the effective backbone entanglement are investigated and compared with double reputation theory. The results of the pom-poms are compared with literature data of combs. In elongational flow, very high strain hardening factors > 100 are found. Following the pom-pom model and the Considère criterium, the SHF can be predicted based exclusively on the arm number q if the backbone is self-entangled by a factor of . For combs and pom-poms with similar molecular properties, similar SHF are obtained in elongational flow.