(464d) Effect of Confinement on the Relaxation Dynamics in an Antiplasticized Polymer Melt
Recently we have developed a coarse grained model which exhibits antiplasticization in a polymer melt. Antiplasticization is the behavior exhibited when certain low molecular weight solvents are added to a polymer melt and cause an increase in the density and elastic constants, while a reduction in the glass transition temperature, Tg. This differs from the more widely studied plasticizing solvents, where the density, elastic constants, and Tg are all decreased upon addition of the solvent. Using molecular dynamics simulations, we have characterized our model and investigated the effect of antiplasticization on the relaxation dynamics of the polymer in both bulk and free-standing thin film geometries. In the bulk, we show that antiplasticization makes the system a stronger glass-former by reducing the size of the cooperatively rearranging regions (string-like motions) and showing a weaker temperature dependence of the relaxation times of the system. In addition, we characterize the fragility in terms of the ratios of the characteristic temperatures to show that the pure polymer is a more fragile glass former. Next, we investigated the effect of confinement to ultra-thin freestanding films on the glass-forming properties for both the pure polymer and the antiplasticized system. Upon confinement, we find that the pure polymer becomes a stronger glass-former, and the cooperative rearrangements in the pure polymer film are heterogeneous and are more cooperative near the free surfaces. In contrast, the antiplasticized melt shows little change in its fragility. The antiplasticized film is also has a more homogeneous distribution of cooperative rearrangements, and the free surface effects do not extend throughout the thin film.