(217j) Non-Equilibrium Molecular Dynamics Studies of Miscibility and Rheological Behaviour of Blends of Dendrimer/Linear and Hyperbranched/Linear Polymers Under Planar Extensional Flow

Authors: 
Todd, B. D., Swinburne University of Technology
Daivis, P. J., RMIT University



We present nonequilibrium molecular dynamics simulations (NEMD) of the miscibility, rheology, and structural properties of polymeric blends based on linear polymer chains (187 beads per chain) with dendrimers and hyperbranched molecules of generations 1-4 at different concentrations ($4\%, 8\%, \text{and} 12\%$) with respect to the linear matrix undergoing planar elongational flow (PEF). Melt rheological properties including the first and second extensional viscosities obtained from constant pressure simulations were found to fall into the range between those of pure dendrimer and hyperbranched polymers and pure linear polymer melts.  A small amount of dendrimer or hyperbranched polymers added in the melt of the linear chains can significantly reduce the extensional viscosity of the blend system compared to the pure high molecular weight linear melt. However, this drop in the viscosity is correlated with the proportion of dendrimer and hyperbranched polymers in the blends and also with the geometry and generation of the dendritic molecules. The inter-penetration of linear molecules into the dendrimers and hyperbranched molecules is measured by the mean squared displacement of the molecular centre of mass of linear molecules toward the dendritic molecules. Results reveal that linear polymers can be found inside the layers of the dendrimers and hyperbranched polymers, indicating interpenetration of these molecules into each other. In support of this, we also calculate the averaged gyration radius of the linear chains and also dendrimer and hyperbranched molecules and compare them to their values of the pure melt of individual molecules. Linear chains showed significant decrease in size for all the prepared blends and dendrimers and hyperbranched polymers showed increased gyration radius. These results demonstrate that linear chains have reduced their size in order to penetrate into the cavities of the dendritic molecules while dendritic molecules have been expanded to host the linear molecules. This tendency of these molecules toward each other is a sign of miscibility of these two species. To help demonstrate this further we calculate ratios of the eigenvalues of the gyration tensor to study shape of the molecules in the blend. The distribution of the dendritic molecules throughout the linear matrix and also miscibility of these two species was studied using configurational snapshots after reaching a steady state between two states that are far from each other in time. The combination of structural analysis suggests that the systems of blends are miscible.

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