Structure and Dynamics of Star Polymer Films from Coarse-Grained Molecular Simulations

We simulate the structure and dynamics of star polymer films of varying arm mass M-a and number of star arms f on a supporting solid substrate with an attractive interaction and compare to the corresponding properties of thin films of linear polymers. While the spatial variation of segmental density profile is only weakly dependent on star polymer topology, the polymer topology significantly affects both the average rate of relaxation and the spatial variation of the average segmental relaxation time tau(alpha). as a function of depth from the film substrate, z. In particular, we observe a general slowing down in the rate of relaxation of the entire film with an increasing functionality f, a general trend attributed mainly to an alteration of the effective cohesive interaction strength when f is increased. The mobility gradients in the film, quantified by the relaxation time obtained from the intermediate scattering function on a layer-by-layer basis, are also significantly altered by f In particular, the width of the substrate interfacial layer grows with increasing f saturating in value around f = 12, while the width of the interfacial zone near the "free" boundary, where the polymer dynamics is greatly accelerated compared to the film interior, is less influenced by changes in polymer topology. These observations are qualitatively consistent with the interpretation of recent X-ray photon correlation spectroscopy on supported star polymer films.