Unusual elastic behavior of ultrathin polymer films: Confinement-induced/ molecular stiffening and surface tension effects

Prior experiments of membrane inflation (biaxial extension) on ultrathin polymer films have evidenced a dramatic stiffening in the rubbery regime whose origins remain unclear. Here, we describe results from bubble inflation measurements on ultrathin films of a previously unexamined material [poly (n -butyl methacrylate) (PBMA)] and show that the observed rubbery stiffening behavior can be attributed to a mixture of "molecular or confinement-induced stiffening" and surface tension effects. The results suggest that the molecular stiffening contribution is significant, with the macroscopic surface tension being unchanged even for films as thin as 20 nm. It is also found that PBMA exhibits a significantly different behavior from both poly(vinyl acetate) and polystyrene, illustrating that the magnitude of the molecular or confinement-induced stiffening in ultrathin films depends on the polymer repeat unit structure.