| A series of linear and H-shaped polybutadienes (PBDs) were prepared by living anionic polymerization. These polymers were prepared in a novel way using a DPE derivative instead of a difunctional initiator to prepare the cross-bar. This new synthesis strategy was designed to reduce the presence of intermediate species and undesired by-products in the final samples. Details of the structures were revealed using temperature gradient interaction chromatography (TGIC) and the results were compared with those indicated by size exclusion chromatography (SEC) data from reputable researchers at three laboratories. Discrepancies in molecular weights and molecular weight distributions were observed amongst the three sets of SEC data. Furthermore, TGIC, which is believed to have a higher resolution than SEC, revealed that the H-shaped PBDs were actually mixtures of components having several molecular structures. Rheological characterizations were carried out over a broad frequency range by combining small amplitude oscillatory shear (SAOS) and creep (and recovery in some cases) data. Time-temperature superposition was found to work well at temperatures ranging from -75°C to 25°C, and dynamic moduli inferred from creep (or creep/recovery) data agreed well with the SAOS data at the reference temperature Tref = 25°C. Material constants including the zero shear viscosity eta0, plateau modulus G0N and steady state compliance J0S were extracted from experimental data. The effects of long-chain branching and polydispersity of arms and cross-bars on rheological behavior were studied. Two tube-based models: The Hierarchical-3.0 model developed by Larson et al. and the branch-on-branch (BoB) model of Das et al. were evaluated and compared quantitatively. The evaluation consisted of a summary of peer reviews, sensitivity and uncertainty analyses and comparison of simulations with experimental data. Hierarchical-3.0 gave the most accurate predictions for the H-shaped polymers when polydispersity in the structure was taken into account. |
