(698f) When Does a Branched Polymer Becomes a Particle?

We investigate coarse-grained polymer melts of star polymers by molecular dynamics (MD) simulations to quantify the progressive transition between a diffuse and anisotropic state similar to linear random coil polymers to a relatively dense and symmetrical structural form more similar to colloidal particles as the number of star arms f  is increased. A recent study  [1] demonstrated that this transition is apparent qualitatively in the scaling of the monomer density with polymer molecular mass and in the present work we refine this type of analysis to show that this transition occurs in our model at a relatively small and sharply defined range of f, i.e., f = 5-6 arms. Since ring polymers are branched polymers in a general sense, we also compared our melt dynamics of star polymers with melt dynamic data for ring polymers. Surprisingly, the scaling of the monomer density of ring polymer melts is found to be remarkably similar to stars at their crossover functionality and the overall shape anisotropy of rings in their melt state is likewise similar to star melts at their crossover functionality. In other words, we find that ring polymers, a molecular structure with no free-ends, exhibit a similar conformational structure as star polymer having 5-6 free-ends so that both the rings and these few arm star polymers can be considered to be particle like in nature. These findings challenge simple models of the structure and dynamics of polymer melts that emphasize the central importance of chain free ends on chain packing and dynamics in the melt state. To better understand these trends, we performed a series of MD simulation to determine the size, shape, and isokintetic temperature for each molecular topology and these results are summarized from a general perspective.