The focused helium ion beam is ideally suited to precision milling of thin films avoiding implantation artifacts.

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.

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A method for small-scale testing and imaging of freestanding, microtomed polymer films using a push-to-pull device is presented. Central to this method was the development of a sample preparation technique which utilized solvents at cryogenic temperatures to transfer and deposit delicate thin films onto the microfabricated push-to-pull devices. The preparation of focused ion beam (FIB)-milled tensile specimens enabled quantitative in situ TEM tensile testing, but artifacts associated with ion and electron beam irradiation motivated the development of a FIB-free specimen preparation method. The FIB-free method was enabled by the design and fabrication of oversized strain-locking push-to-pull devices. An adaptation for push-to-pull devices to be compatible with an instrumented nanoindenter expanded the testing capabilities to include in situ heating. These innovations provided quantitative mechanical testing, postmortem TEM imaging, and the ability to measure the glass transition temperature, via dynamic mechanical analysis, of freestanding polymer films. Results for each of these mentioned characterization methods are presented and discussed in terms of polymer nanomechanics. Graphic Abstract