Structure and Dynamics of a Polybutadiene Melt Confined between Alumina Substrates

Quantitative studies concerning the structure, the conformations, and the associated dynamics of confined polymers, at the molecular level, are of utmost importance toward a fundamental understanding of the behavior of polymer chains under confinement. Here, we examine the properties of cis-1,4 polybutadiene (cPB) melt, well above Tg, confined between alumina (001) substrates, via detailed atomistic molecular dynamics simulation. The cPB/alumina interaction is described via a recently developed machine-learned atomistic force-field, based on extensive ab initio density functional theory calculations, that results in an accurate atomistic description of the polymer structure at the vicinity of the interface. Structural, conformational, and dynamical heterogeneities of the polymer chains in the vicinity of the alumina substrate are observed. These include modification of chain configurations in the vicinity of the alumina substrate; desorption kinetics that are at least 3 orders of magnitude slower than the maximum bulk relaxation time; exclusive relaxation of the adsorbed segments through the desorption mechanism; slower (with respect to the bulk) relaxation dynamics of tail and loop segments dependent on the proximity to the substrate; slower dihedral transitions of adsorbed segments; and more than 3 orders of magnitude slower diffusion of adsorbed segments parrallel to the substrate. Additionally, the system exhibits long-time reorganization effects at the interface. Our findings are related to experimental observables by computing the complex dielectric permittivity of the segmental and chain dipole moment dynamics. These results shed light on recent experimental findings on confined polymers.