Understanding the Influence of Different Interfacial Interactions on the Glass Transition Temperature and Self-Diffusion Coefficient in Unentangled Polymer Thin Films

In thin films, physical properties such as the glass transition temperature (T_g) and self-diffusion diffusion coefficient (D) are often different compared to those in thick films due to their higher interfacial area to volume ratio and other potential factors. These effects typically occur in submicron thick films and have been termed nanoconfinement effects. Understanding polymer dynamics under nanoconfinement is critical to advance nanotechnology; this knowledge could be exploited for tailoring material properties with applications ranging from nanocomposites to lithography, among others. However, there are several unsolved issues relating to nanoconfined mobility that are of primary interest such as the relationship between T_g and D. From a dynamic length scale point of view, T_g reflects the cooperative segmental mobility of 10-100 repeat units while D reflects the large-scale transport motion of the entire chain, and it is reasonable to expect they could be affected differently when polymer is nanoconfined. In this study, we employed fluorescence recovery after patterned photobleaching to evaluate the D of fluorescently labeled poly(isobutyl methacrylate) (PiBMA) as a model polymer. Films 16 - 300 nm in thickness were spin coated onto silica substrates possessing attractive polymer/substrate interactions and poly(cyclohexylethylene) (PCHE) substrates possessing weaker polymer/substrate interactions, then D and T_g were measured. In addition, the effects of free surfaces were investigated by capping the free surfaces atop PiBMA thin films with a PCHE layer. The effects of friction (e.g., between the substrate or capping layer and the confined polymer melt) on the T_g-D relationship will also be presented.