Role of Water on Correlated Structural and Mechanical Response of Water-Supported and Free-Standing Ultrathin Films

Glass transition phenomena of confined polymer films have been heavily investigated for the last two decades, yet the mechanical properties of such sub-100 nm thin films have remained elusive until much more recently. The film on water (FOW) tensile test has enabled characterization of thin polymer films, however, the influence of water on the mechanics of confined films remains unclear. Here, we developed a free-standing (in air) tensile platform to study sub-100 nm thin films and compare the mechanical properties obtained to that of the FOW technique thereby elucidating the influence of the interface on the mechanics of these thin films. Characterization of stiff glassy polystyrene and soft semicrystalline poly(3-hexylthiophene) films has been achieved down to 15 and 80 nm respectively. We observed minimal difference in moduli obtained from FOW and in air measurements, while yield stress and crack-onset-strain were greater for FOW. The influence of water was further explored through quartz crystal microbalance and neutron reflectometry, which indicate a water content as high as 13.6 %. Overall, this work establishes a novel technique for measuring the mechanical properties of free-standing thin films and provides an in-depth analysis of the role of the polymer-water interface.