Creep behavior of ultra-thin polymer films

A novel microbubble inflation method has been used to determine the creep compliance of poly(vinyl acetate) and polystyrene ultra-thin films (13-300 nm thick) at temperatures from below to above the glass temperature. We present results that suggest that time-temperature and time-thickness superposition hold in the glassy relaxation regime. Although time-temperature superposition is found for the entire response curve for each thickness, we also find that time-thickness superposition fails as the long-time compliance is approached. This effect occurs because of a strong stiffening as the film thickness decreases. We also show first evidence of stiffening in the glassy regime of free standing films of polystyrene.