Effect of confinement on stiffness and fracture of thin amorphous polymer films

The elastic modulus, fracture strength, and onset fracture strain of supported glassy polystyrene films with a thickness ranging from 250 to 9 nm were quantitatively determined by a recently developed wrinkling-cracking method. Films with a thickness below about 40 nm showed a decrease in both elastic modulus and fracture strength with decreasing film thickness, whereas the onset fracture strain was shown to increase. The observed variations in mechanical properties with respect to the bulk counterparts support the notion that a mechanically soft thin layer having a loosely entangled chain network exists in the near-surface region of polymeric materials, whose contribution becomes more pronounced in thinner films.