Among other materials, polymers tend to be cheap, flexible, and low-density. However, they often lack thermal stability and mechanical robustness. This is why additives are frequently used to modify a polymer's properties for the desired application. In this study, we use molecular dynamics simulations to measure the effect of monomer-scale additives on composite dynamics near the glass transition temperature. We first examine the short and long-timescale dynamics, characterized by the Debye-Waller factor and relaxation time respectively. For each additive size, we then use the Localization Model (LM) to find a relation between the Debye-Waller factor and the relaxation time across temperatures. We find that the transition in size from molecular to NP-scale additives is associated with an independence of LM parameters for NP-scale additives, which we term the ``nanoparticle plateau''. Furthermore, we find that LM parameters are independent of the interaction strength of additives with the polymer matrix. Near the glass transition, polymer dynamics are known to become heterogeneous, meaning that certain regions have higher mobility than others. We thus use the Adam-Gibbs (AG) model to relate the size of these high-mobility regions to the relaxation time, showing that this size is an equivalent predictor to the Debye-Waller factor for the relaxation time. We perform this analysis using two geometries: A ``bulk'' system that simulates a macroscopic sample of composite far from any surface and a ``film'' system that simulates an ultrathin film (just several nm thick) lying on a rough, attractive substrate. Because the film's substrate is known to decrease mobility and the film's free interface is known to increase mobility in pure (no additive) systems the film possesses an inherent gradient in dynamics that could be modified by additives. Thus, in the film system, the above analysis is performed in smaller regions at different distances from each interface. We find that additives tend to suppress the mobility gradient at the substrate and enhance the mobility gradient at the free interface, producing faster averaged dynamics compared to the analogous bulk systems.
"},{"label":"Degree","value":["Bachelor of Arts with Departmental Honors"]},{"label":"Author-supplied Keywords","value":["Polymer composites","Molecular dynamics","Glass transition"]},{"label":"Administrative Note","value":"Drupal Node ID: 24224