Capillary levelling as a probe of thin film polymer rheology

While measuring the rheology of bulk polymer systems is routine, when the size of a system becomes comparable to molecular dimensions, properties of flow are hard to measure and poorly understood. We present the results of experiments that are easily performed and can probe the rheological properties of polymer films that are mere tens of nanometres in thickness. Glassy bilayer polymer films are prepared with height profiles well approximated by a step function. Upon annealing such stepped bilayer films above the glass transition, the height profiles broaden due to gradients in the Laplace pressure. By varying the molecular weights of the stepped films, we control the rate at which the broadening takes place. A scaling relation derived in the lubrication approximation is used to show that the rate of broadening is consistent with that expected from polymer rheology in the bulk, thus validating the technique as a uniquely versatile rheological probe for thin polymer films.