The stiffening of ultrathin polymer films in the rubbery regime: The relative contributions of membrane stress and surface tension

We have previously reported on a new nanobubble inflation technique we have developed to measure the absolute creep compliance of polymer films at thicknesses down to ∼10 nm (1-5). One of the more surprising results to come out of these studies was the observation that as the film thickness decreased the rubbery plateau compliance was seen to decrease dramatically. At the thinnest film thickness (∼10 nm) the compliance decreased (or conversely the stiffness increased) by some 3 orders of magnitude from the bulk value. It is known that, at the nanometer scale, surface tension effects can become significant. We examine here the contribution of surface tension to the observed stiffening and show that, while the contribution is important, it does not account for the degree of stiffening observed. This suggests that molecular or confinement induced stiffening is the major factor.