Orientational Mobility in Dendrimer Melts: Molecular Dynamics Simulations

We have simulated the melt of poly(carbosilane) dendrimers using atomistic models and have reproduced :the effect predicted by the analytical theory; i.e., orientational autocorrelation functions of a segment from the same layer (numbered from periphery) are practically identical and do not depend on dendrimer size. The frequency dependences of the dielectric and NMR relaxation were obtained and studied in detail. The main contribution to the maxima of these dependences is given by the pulsation, process. It leads to a shift of the maxima to low frequencies for the core segment in comparison with the maxima for peripheral segments. The contribution of local reorientation can also be significant, and in some cases this contribution manifests as an additional maximum. The nontrivial scaling laws in the frequency dependences of dielectric permittivity and NAIR relaxation rate averaged over all layers of a dendrimer macromolecule are found. A similar scaling law is observed in the experiments on NMR relaxation but is not described by the analytical theory.