An Experimental and Modeling Comparison of the Dynamics of Capped and Freestanding Films

We compare the influence of two different types of interfaces on the segmental dynamics of poly(2-chlorostyrene) (P2ClS): the simple free surface of freestanding films vs. the solid adsorbing surface of Aluminum (Al) capped films. Data on both have been measured via dielectric spectroscopy, and to understand and quantify the change in free space at these two types of interfaces we use the Cooperative Free Volume (CFV) rate model. Though adsorbing interfaces are often considered to be a source of slower dynamics, meta-stable packing imperfections at the interface can lead to more free space (lower density) near the interface and enhanced dynamics. CFV calculations show that the Al-capped interface does provide an excess of free space relative to bulk, and this excess is almost half as large as that at the P2ClS free surface. These calculations also show that "capping" an 18 nm thick freestanding film has an equivalent effect on the dynamics as applying a 19 MPa pressure increase. We also discuss the importance of accounting for the volume sensitivity of the bulk material, and, whether this can be sufficient to predict the magnitude of the confinement effect.