Dramatic stiffening of ultrathin polymer films in the rubbery regime

Recently, we (P.A. O'Connell, G.B. McKenna, Science 307, 1760 (2005)) introduced a novel nano-bubble inflation method to measure the absolute creep compliance of nanometer thick polymer films. In that work it was shown that even at film thicknesses as small as 27.5nm the glass temperature was unchanged for poly(vinyl acetate) (PVAc). Perhaps more importantly, and the subject of the present work, was the observation that these ultrathin films show a dramatic stiffening in the rubbery plateau regime, i.e., the compliance was reduced by over two orders of magnitude compared to the bulk material. In the present work we substantiate the previous results in a study of the thickness dependence of the rubbery compliance of PVAc and polystyrene (PS) films for thicknesses from 13nm to 276nm. We show the substantial stiffening of the plateau region for both materials. Furthermore, the rubbery compliance (inverse of stiffness) scales with approximately the second power (1.8 +/- 0.2) in the film thickness for both materials.