Freestanding
polystyrene thin films were found to reach elongations 2 orders of
magnitude larger than what is found in bulk tests. We performed planar
tensile testing at multiple strain rates using a push-to-pull based
technique to obtain a quantitative stress–strain response of
microtomed thin films with thicknesses ranging from 200 to 500 nm. In situ optical microscopy combined with postmortem transmission electron microscopy experiments were able to reveal
the progression and microstructure of the deformation. It was found
that the films undergo shear yielding and that crazing only occurs
if the films are thermally annealed prior to testing. Regardless of
thermal pretreatment, the microtomed thin films exhibited extreme
ductility, which is at odds with previous reports on the mechanical
properties of polystyrene thin films. The strain-softening amplitude
was also found to directly depend upon the film thickness for unannealed
thin films. Strain softening was not measured in thin films thermally
annealed before quantitative testing. Comparisons to other relevant
phenomena and current theoretical models are discussed in light of
the extreme ductility that was found in what is a nominally brittle
glassy polymer.