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

Syndiotactic polystyrene (sPS) is a chemically resistant, high-heat, semicrystalline polymer which is currently under development by The Dow Chemical Co. The research reported herein was undertaken to determine the critical fracture strength, i.e., the critical stress intensity factor, K 1C , and the fracture energy, G 1C , of sPS. The studies were aimed at developing a basic understanding of the failure mechanism and toughness of sPS. This work included investigations of the effect of molecular weight, as well as flow-induced anisotropy. Scanning electron microscopy (SEM) was used to aid in the determination of the failure mechanism. During failure testing, it was observed that sPS fails with a slow, controlled crack growth and ruptures with an almost nondetectable amount of yielding, as based on a tensile dilatometry investigation and a plane strain, biaxial yield experiment. The proposed failure mechanism, based on the scanning electron micrographs, is one of constrained crazing, followed by void coalescence with the spherulite nucleators acting as stress concentrators in the system. The damage appears to be greatly confined, with little initial cold-drawing of the spherulites. Addition of a nucleator reduces the K 1C values somewhat, as added nucleation sites proliferate the sites for stress concentration across the sample.

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The recent development of in situ Transmission Electron Microscopy (TEM) nanomechanical testing techniques has mostly benefitted our understanding of fundamental deformation mechanisms in hard materials such as metals and ceramics [1]. Here, we report on recent progress extending these techniques to study polymeric materials using an in situ TEM nanoindenter with a Push-to-Pull (PTP) device to perform quantitative tensile tests on thin polymer sheets.

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