Effect of branch point fluctuations on dynamics of entangled star polymers: A slip-spring simulation study

We examined the dynamical properties of entangled star-branched polymers using a discrete slip-spring model where motional constraints due to entanglements are imposed through slip-springs. Using a single set of slip-spring parameters, we first attempted to reproduce linear viscoelastic and dielectric experiments for both linear and star cis-polyisoprene (PI) melts. Since PI has a dipole parallel to the chain backbone, the dielectric data reflects the relaxation of the end-to-end vector of the entire chain. We found that the slip-spring model can predict well the viscoelastic and the end-to-end relaxations of linear PIs. For star PIs, although the viscoelastic properties are reasonably reproduced, the slip-spring model predicts end-to-end relaxations roughly 70% slower than those of the experiments when the branch point (BP) is modeled to be immobile as in the conventional slip-spring model. Allowing BP to fluctuate depending on spring and Brownian forces, we observed that this deviation is almost removed in some cases especially for relatively large molecular weights and cut roughly in half in others. Furthermore, using the slip-spring model with BP fluctuations, we examined the mean squared displacement and the intermediate scattering function to test the slip-spring model rigorously.