The viscoelastic properties of dendrimers of generation 1-4 are studied using nonequilibrium molecular dynamics. Flow properties of dendrimer melts under shear are compared to systems composed of linear chain polymers of the same molecular weight, and the influence of molecular architecture is discussed. Rheological material properties, such as the shear viscosity and normal stress coefficients, are calculated and compared for both systems. We also calculate and compare the microscopic properties of both linear chain and dendrimer molecules, such as their molecular alignment, order parameters and rotational velocities. We find that the highly symmetric shape of dendrimers and their highly constrained geometry allows for substantial differences in their material properties compared to traditional linear polymers of equivalent molecular weight.
The molecular structure of fluids composed of dendrimers of different generations is studied using nonequilibrium molecular dynamics (NEMD). NEMD results for dendrimer melts undergoing planar Couette flow are reported and analyzed with particular attention paid to the shear-induced changes in the internal structure of dendrimers. The radii of gyration, pair distribution functions and the fractal dimensionality of the dendrimers are determined at different strain rates. The location of the terminal groups is analyzed and found to be uniformly distributed throughout the space occupied by the molecules. The fractal dimension as a function of strain rate displays crossover behavior analogous to the Newtonian/non-Newtonian transition of shear viscosity.
Nonequilibrium molecular-dynamics simulations are used to investigate the molecular shape of dendrimers and linear polymers in a melt and under shear. Molecules are modeled at the coarse-grained level using a finitely extensible nonlinear elastic bead-spring model. The shape of dendrimers and linear polymers at equilibrium and undergoing planar Couette flow is analyzed quantitatively and it is related to the shear viscosity. The shape of dendrimers responds differently to the influence of shear compared with linear polymers of equivalent molecular mass. However, in both cases the transition from Newtonian to non-Newtonian viscosity behavior corresponds to significant changes in molecular symmetry. This suggests that a shape analysis could be used to estimate the onset of shear thinning in polymers.
Structure and transport properties of dendrimers in dilute solution are studied with the aid of Brownian dynamics simulations. To investigate the effect of molecular topology on the properties, linear chain, star, and dendrimer molecules of comparable molecular weights are studied. A bead-spring chain model with finitely extensible springs and fluctuating hydrodynamic interactions is used to represent polymer molecules under Theta conditions. Structural properties as well as the diffusivity and zero-shear-rate intrinsic viscosity of polymers with varied degrees of branching are analyzed. Results for the free-draining case are compared to and found in very good agreement with the Rouse model predictions. Translational diffusivity is evaluated and the difference between the short-time and long-time behavior due to dynamic correlations is observed. Incorporation of hydrodynamic interactions is found to be sufficient to reproduce the maximum in the intrinsic viscosity versus molecular weight observed experimentally for dendrimers. Results of the nonequilibrium Brownian dynamics simulations of dendrimers and linear chain polymers subjected to a planar shear flow in a wide range of strain rates are also reported. The flow-induced molecular deformation of molecules is found to decrease hydrodynamic interactions and lead to the appearance of shear thickening. Further, branching is found to suppress flow-induced molecular alignment and deformation.
Flow properties of dendrimers are studied with the aid of nonequilibrium molecular dynamics techniques. Simulations are performed in the NpT ensemble using the NpT-SLLOD algorithm [P. J. Davis and D. J. Evans, J. Chem. Phys. 100, 541 (1994)] and are compared to the results from simulations performed in the NVT ensemble reported earlier [J. T. Bosko, B. D. Todd, and R. J. Sadus, Chem. Phys. 121, 12050 (2004)]. Shear thickening observed at high strain rates vanishes in systems kept under constant pressure. Also the exponents in the power-law dependencies of the viscosity and the normal stress coefficients change. The variations are significant only at high strain rates and do not affect largely microscopic properties such as shape, alignment, or rotation of molecules. The NpT-SLLOD algorithm has been applied to study various systems including dendrimers in solution and their blends with linear chain molecules of the same molecular mass, and some results for these systems are presented.
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