Quantification Of Interactions At The Polymer-Substrate Interface: Implications For Nanoscale Behavior

The physical properties of polymer films of nanoscale thickness have been shown to exhibit film thickness-dependent behavior, largely associated with intermolecular interactions between the film and its external interfaces. These effects are typically manifested in the thickness dependencies of the glass transition temperatures (T(g)s), and segmental dynamics, over length scales ranging from a few nanometers to tens of nanometers in many polymer/substrate systems. Moreover, different experimental techniques, including incoherent neutron scattering, spectroscopic ellipsometry, and broadband dielectric spectroscopy, have yielded thickness-dependent T-g trends of the same systems that appear to be at odds. To this end, the first direct quantitative measurements of the length scales of interfacial interactions of strategically selected polymer/substrate systems, obtained using Kelvin force probe microscopy, are reported. These findings reveal pathways to understand and reconcile seemingly the contradictory experimental observations about T-g vs. film thickness trends, and dynamics, at the nanoscale. Our findings have broader implications for understanding the physical properties of polymer films in the nanoscale thickness regime.