Nanomechanical measurements in ultrathin polymer films

Using a nano-bubble inflation technique developed in our lab we have measured the absolute creep compliance of polymer films down to a thickness of 9nm [1-3]. Materials studied were a poly (vinyl acetate) (PVAc), a polystyrene (PS) and a polycarbonate (PC). All three materials showed dramatic stiffening of the rubbery plateau compliance as the thickness was reduced below approximately 100nm, with the stiffness increasing by some three orders of magnitude for the thinnest films. In the region where the stiffness is thickness dependent, the stiffness of all three materials varies as approximately the inverse of the square of the thickness. Interestingly, while the change in stiffness was common to all materials, the reduction in the glass transition temperature which is commonly observed in thin free standing films was very different. The PS showed a decrease of approximately 50K at a thickness of 11nm which is comparable to literature data. The PVAc showed no change in T_g down to 23nm while the PC showed a large decrease of ∼90K at 9nm.