Density Measurements of Thin Polymeric Films using Magnetic Levitation

When a polymeric film—supported by a sacrificial release layer—is placed in a paramagnetic solution between two magnets, it lifts off and levitates to a height that is linearly dependent upon the mass density. Using the principle of magnetic levitation, we construct a simple metrology for directly measuring the mass density of polymeric films down to 3 nm thick--an important quantity for understanding the glassy dynamics of such films. We validate the technique with several polymer films with a range of densities (1-1.4 g cm^-3) and by measuring density variations as a function of thickness in polystyrene and polymethylmethacrylate films. Our results are in agreement with existing data obtained by reflectometry experiments and confirm the trends previously observed in PS and PMMA, with important implications for understanding the dynamics of such glassy systems. Moreover, we demonstrate the potential utility of our technique for studying crystallization in ultrathin films of polymers using polyethylene as a model system. MagLev of thin polymeric films provides a simple, accessible metrology that will be useful for polymer scientists and engineers to study and optimize the properties of thin films for electronics, barriers, and separation membranes.