The effect of chain stiffness on the dynamics of polymer/star polymer composites

Nanoscale additives, such as nanoparticles (NP) or NP functionalized with tethered polymer chains, have been widely used to modify the properties of polymer materials. Star polymers are a limiting case of polymer tethered NP where the core NP size approaches the molecular scale. We use molecular simulations to consider a composite (or blend) of a modest fraction of star polymers embedded within a linear polymer matrix. We systematically study how the star architecture and internal star interactions impact the magnitude of changes to the composite properties. We find that stiffening star arms substantially increases the composite relaxation time and increases the glass transition temperature. To comprehend these observations, we consider the special case of a composite with stiff two-armed stars (worm-like chains) to eliminate the effects of star architecture, and also examine the properties of pure melts of stiffened stars. We show that the relaxation of the incoherent scattering function of the composite can be approximated as a simple linear combination of the relaxation of the pure polymer system and pure star system. We further show that this approximation fails at late time because it does not account for polymer-matrix interactions.