Extreme Ductility in Freestanding Polystyrene Thin Films

Abstract

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.

Highly Deformable, Ultrathin Large-Area Poly(methyl methacrylate) Films

“…In general, the preparation process is known to play a significant role on the mechanical properties of thin-film materials by promoting, for example, specific chain conformation or solvent residuals. 9 In our specimens, we can exclude the presence of solvent residuals as thermal annealing was performed before the transfer of the PMMA films on our testing platform. However, it is interesting to notice that our investigated PMMA films reproduced the morphology of the copper foil that we used in the first fabrication steps as a sacrificial substrate.…”

“…In the past, the occurrence of crazing was observed in polymeric thin films provided with both brittle and ductile behavior. In the case of PMMA, non-fibrillated crazes were previously reported in macroscopic specimens under tensile loading, although not associated with the high deformation capability that we found in our experiments .…”

“…Crazes usually nucleate at defects or impurities at the surface or within the material, which are likely to be present in our specimens as a consequence of the preparation method (commonly implemented during graphene production and manipulation). In general, the preparation process is known to play a significant role on the mechanical properties of thin-film materials by promoting, for example, specific chain conformation or solvent residuals . In our specimens, we can exclude the presence of solvent residuals as thermal annealing was performed before the transfer of the PMMA films on our testing platform.…”

“…In contrast, polymers were reported to show either an improvement or an even significant reduction of their mechanical performances (such as Young's modulus) with decreasing thickness. 6−8 Thus, the existence of conflicting reports in the literature 9 requires a more extensive investigation of the mechanical behavior of thin and ultrathin polymeric films.…”

“…As expected from the reduced probability of finding larger defects in smaller volumes, many inorganic materials or metals showed an improvement of their mechanical strength at the nanoscale. In contrast, polymers were reported to show either an improvement or an even significant reduction of their mechanical performances (such as Young’s modulus) with decreasing thickness. − Thus, the existence of conflicting reports in the literature requires a more extensive investigation of the mechanical behavior of thin and ultrathin polymeric films.…”