Computational simulation of hyperbranched polymer melts under shear

In this work, hyperbranched polymers of different molecular weights and different molecular architectures have been simulated using a coarse-grained model and nonequilibrium molecular dynamics techniques. A number of structural parameters and the rheology of hyperbranched polymer melts under shear were analysed to explain the effect of the molecular structure and molecular weight on microscopic as well as macroscopic properties. In order to determine the shear-induced changes in the structural properties of hyperbranched polymers, various parameters were calculated at different strain rates. The radii of gyration which characterize the size of the polymer were evaluated. The relationship between the zero shear rate mean squared radius of gyration and the molecular weight as well as the Wiener index was established. The tensor of gyration was analysed and results indicate that hyperbranched polymer molecules have a prolate ellipsoid shape under shear. As hyperbranched polymers have compact, highly branched architecture and layers of beads have increasing densities which might lead to an unusual distribution of mass, the distribution of beads was also studied. The distribution of terminal beads was investigated to understand the spatial arrangement of these groups which is very important for hyperbranched polymer applications, especially in drug delivery. Flow birefringence was characterized by taking into account both form and intrinsic birefringences which result from molecular and bond alignment respectively. The melt rheology of hyperbranched polymer structures with different molecular weights and different number of spacers was also studied. Systems were simulated over a wide range of strain rates to capture the crossover behaviour from Newtonian to non-Newtonian regimes. Rheological properties including the shear viscosity and first and second normal stress coefficients were computed and the transition to shear thinning was observed at different strain rates for hyperbranched polymers of different sizes and topologies. The results were consistent with findings from NEMD simulations of linear and dendritic polymers. The stress-optical rule was tested and shown to be valid only in the Newtonian regime and violated in the strong flow regime where the rule does not take into account flow-induced changes of the micro structure. The stress-optical coefficient was found to be independent of the molecular weight and topology of polymers. Blends of hyperbranched polymers and linear polymers were also simulated and their rheological properties were investigated. Results show that even a small proportion of hyperbranched polymer in a melt of linear chains can reduce the shear viscosity of the whole system. This feature makes hyperbranched polymers a potential candidate as rheology modifiers. However there is no observed limit in the proportion of hyperbranched polymers in the samples above which the viscosity is stabilized. The viscosity drops continuously, correlating with the amount of hyperbranched polymers present.