Molecular dynamics simulations were performed for a series of AB 2 dendrimer models, in explicit-solvent solutions where the ratio R g /L ͑R g is the radius of gyration and L the size of the simulation box͒ is kept between 0.15рR g /Lр0.2. Results on static properties ͑size, shape, density profiles͒ are in good agreement with recent theoretical and experimental studies. Dynamic properties are systematically investigated on the local and entire molecule length scale. The dynamic characteristics of the examined models capture the qualitative behavior observed experimentally in dendrimer molecules. The systematic and comparative nature of this study affords detailed insight into the origin and the relative contribution of different relaxational mechanisms in the observed dynamic spectra.

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Brownian dynamics simulations of perfect dendrimers up to the sixth generation have been performed under the influence of simple shear flow. Hydrodynamic and excluded volume interactions have been taken into account explicitly. The onset of shear-thinning is observed to occur at lower shear rates for larger dendrimers (i.e. more generations). As the generation increases, the zero shear rate intrinsic viscosity reaches a maximum and begins to fall. The radius of gyration, the hydrodynamic radius, the translational mobility, and radial density profiles including the location of the terminal groups are also reported.

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Brownian dynamics simulations of hyperbranched polymers with different degrees of branching have been performed under the influence of simple shear flow. Hydrodynamic and excludedvolume interactions have been taken into account explicitly. Shear-thinning effects have been observed for all simulated degrees of branching. As the molecular weight of highly branched structures increases, the zero shear rate intrinsic viscosity reaches a maximum and begins to fall similar to the intrinsic viscosity behavior of perfectly branched dendrimers. In the absence of shear, static structure factors, S(k), for hyperbranched polymers with the smallest number of monomers studied resemble those of a three-arm star. As the number of monomers increases and as the degree of branching increases, the S(k) curves for the hyperbranched polymers begin to illustrate features associated with S(k) curves for hard spheres. Further insight into the shape and interior density of these structures is obtained through the ratio of the radius of gyration, Rg, to the hydrodynamic radius, Rh. The ratio Rg/Rh is observed to approach unity as the number of monomers and the degree of branching increase.

Brownian dynamics simulations of a polymer chain described by three different models under the influence of a shear flow have been performed. Model A is a freely jointed Kramers chain consisting of beads connected by rigid rods. Model B is a freely jointed chain consisting of finitely extensible nonlinear elastic (FENE) springs. Excluded volume and hydrodynamic interactions are not taken into account in either of these two models. Model C is a chain with rigid bonds, valence, and torsional angle potentials, excluded volume and hydrodynamic interactions. Asymptotic dependencies [η]∼γ̇−1/3 and [η]∼γ̇−2/3 for the intrinsic viscosity [η] at large shear rates γ̇ for models A and B, correspondingly, have been obtained. Asymptotic dependencies for the first normal stress coefficient Ψ1∼γ̇−4/3 do not depend on the particular choice of model. At intermediate shear rates [η]∼γ̇−1/2 is followed for all models. Scaling dependencies of rheological properties on molecular weight have been studied. Results of the simulations show that chains are not fully stretched even at extremely high shear rates but form rather compact anisotropic objects. Correlation functions of the chain end-to-end vector relax quicker with increasing shear rate and reveal evidence of the end-to-end vector flipping between orientations parallel and antiparallel to the flow direction.

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