Simulating Local T g Reporting Layers in Glassy Thin Films

• Published in: Macromolecules 2016-03, Vol.49 (5), p.1822-1833
• Main Authors: DeFelice, Jeffrey, Milner, Scott T, Lipson, Jane E. G
• Format: Article
• Language: English
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/acs.macromol.6b00090

One of the most notable deviations from bulk fluid properties is the onset of a thickness-dependent glass transition temperature (T g) for nanometrically thin polymer films. Experimental and theoretical observations suggest that this behavior is a response to the interfaces, which perturb the local properties of a film and play an increasingly important role in influencing the global properties of a film as its thickness decreases. In this work, we probe the global and local properties of free-standing films using our limited mobility (LM) model, which is a simple kinetic lattice model that simulates free volume and mobility in a fluid. We provide insight about the role of mobility in affecting the thickness-dependent film-average T g of free-standing polymer films by characterizing the depth to which mobility propagates from a free surface, i.e., the “mobile layer depth”. We also consider the effect of “stacking” free-standing polymer films, where confinement by interfaces composed of the same material yields T g suppression intermediate to that of substrate supported and free-standing films. In order to characterize the local properties of a film, we utilize “reporting layers” located near the free surface and film interior, from which we compute local glass transition temperatures and make connections with experimental results reported for real polymer films.