Uniaxial Extension of Ultrathin Freestanding Polymer Films

The mechanical properties of ultrathin polystyrene (PS) films have been shown to change as the thickness approaches the average size of a polymer molecule. Previous measurements of the uniaxial stress-strain relationship for ultrathin polymer films have required the use of liquid-support layers. However, the influence of the liquid support layer, specifically water, on the mechanical properties of PS films has remained an open question. Here, we introduce a method for directly measuring the complete stress-strain response of ultrathin freestanding polymer films. For freestanding PS thin films, we observe a constant elastic modulus and maximum stress with decreasing thickness for film thicknesses as thin as 30 nm, consistent with the liquid supported measurement. From the freestanding measurements, we identify that the liquid supporting layer leads to enhanced craze stability for ultrathin PS films. We compare these results to the previous liquid-supported measurements and provide insights into how the liquid surface interactions can alter polymer behavior in thin polymer films.