Rheological Response of Ultrathin Polymer Films

A novel microbubble inflation experiment has been developed for the purpose of measuring the biaxial creep compliance response of ultrathin polymer films. Here we have performed experiments on two polymeric films, poly(vinyl acetate) (PVAC) and polystyrene (PS) having thicknesses ranging from 25 to 100 nm. Three findings come from these studies. 1) PVAc films show a segmental dynamics that is quantitatively the same as in the bulk with no reduction in the glass temperature T$_{g}$ for even the thinnest films; 2) early results suggest that for the thinnest PS films the T$_{g}$ decreases by up to 30 $^{o}$C which demonstrates dramatically and quantitatively that the segmental dynamics of nanometer thick polymer films do not follow a universal behavior, i.e., the results seem to depend on chemical structure; 3) for both polymers we find that the creep compliance increases from the glassy value to a rubbery plateau that is greatly reduced from the bulk behavior, i.e., the stiffness is increased. This result suggests that entanglements in ultra thin films are much more effective than they are in the bulk---perhaps because of the two-dimensional aspect of the film.