Nanomechanical testing of freestanding polymer films: in situ tensile testing and Tg measurement

Abstract

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

Mechanical and Compositional Implications of Gallium Ion Milling on Epoxy Resin

“…Particularly, Wang et al [ 32 ], using diglycidyl ether of bisphenol A (DGEBA) and piperidine (cyclic amine), equivalent to the one studied here, proposed the core-shell model, stating that the epoxy specimens could be regarded as a ‘composite’ material, consisting of an epoxy core surrounded by an FIB peening-induced stiff layer. Similar stiffening effects of the surface was reported by Nathan for a PMMA film in tension [ 31 ] and by Schamel for epoxy resin pillars in compression [ 39 ]; the three studies used Ga + FIB milling with an acceleration voltage of 30 kV.…”

“…Only few studies have used FIB to fabricate small-scale polymeric specimens for mechanical testing, to study the effects of size on mechanical behavior [ 25 , 31 , 32 , 33 ]. Moon demonstrated patterns of wrinkled stiff skin on a polymeric substrate upon exposure to FIB [ 33 ].…”

“…Wang performed micro compression experiment on FIB-milled epoxy micropillars and concluded that a non-negligible stiff surface skin is created, and estimated a Young’s Modulus of 30 GPa and a thickness of 30 nm [ 32 ]. Nathan demonstrated a tensile test of polymethylmethacrylate (PMMA) under an optical microscope and in situ TEM [ 31 ]. They highlighted the sensitivity of polymeric materials to the TEM electron beam and to the radiation effects by the FIB.…”

Current trends in nanomechanical testing research

“…As evidenced by the high number of related contributions, the use of the FIB has become essential in the field of nanomechanical testing [17,18,[20][21][22]. This has inevitably raised concerns about possible measurement artifacts due to ion damage incurred during the specimen preparation [20,[23][24][25]. While detrimental effects can be ruled out for some applications [20], in other cases, workarounds are actively being developed to minimize [24] or to completely avoid [23] exposure of the samples to highly energetic ion beams.…”

“…Their development has been a dominant trend in nanomechanical research throughout the past decade, with their application seemingly only being limited by the high costs associated with the techniques. This trend is continued in the focus issue, which features applications both inside scanning electron microscopes (SEM) [17,18,21,22] and transmission electron microscopes (TEM) [23,24].…”