Prediction of bulk density and molecular packing in model dendrimers with different chain stiffness

By means of molecular dynamics simulations of model dendrimers, we analyze the dependence of the bulk density and molecular packing on the dendrimer molecular weight and intrinsic stiffness. We find that the density is consistently higher in flexible dendrimers than in the rigid ones with a large bending angle. The density values change slightly within the first two generations to reach a plateau. We interpret these results in terms of free volume, showing that the enhanced accessible free volume that characterizes the end-dendron monomers is counterbalanced by the higher number of internal monomers, leading to a constant bulk density for generations larger than three. The added stiffness affects the geometrical properties and the molecular rearrangement of the bulk, reducing the short-range local order and the packing efficiency favoring the dendrimer interpenetration. Our prediction for the bulk density matches and rationalizes experimental and previous all-atom simulation results. © 2010 American Chemical Society.