(552h) Density Measurements of Thin Films of Polymers with Magnetic Levitation

This manuscript describes the use of magnetic levitation (MagLev) to measure the density of thin films of hydrophobic polymers ( > 3 nm in thickness). We validate the performance of the technique by (i) measuring the mass densities of a range of hydrophobic polymers, and (ii) measuring variations in density as a function of the thickness of two model polymeric films: poly(styrene) (PS) and poly(methyl methacrylate) (PMMA). Our results show that, as the thickness decreases to below 100 nm, the films of PS increase in density while the films of PMMA decrease in density. These results agree with trends in the dependency of density on thickness of polymeric films observed previously using indirect measurements, such as reflectometry; MagLev, however, provided a direct source of evidence for this phenomenon. MagLev is also compatible with non-transparent, uneven, and/or ultra-thin (down to 3 nm) polymeric films difficult to measure the densities otherwise. We also demonstrated the use of MagLev to study the crystallization-induced changes in density in thin films (200 nm) of poly(-caprolactone), a model semi-crystalline polymer. MagLev provides a simple, accessible metrology to measure the density of thin polymeric films; it will be useful for polymer scientists, materials scientists and engineers to study and optimize the properties of thin films designed for uses in electronics, barriers, and membrane-based separation.