SMART transfer method to directly compare the mechanical response of water-supported and free-standing ultrathin polymeric films

Abstract

Intrinsic mechanical properties of sub-100 nm thin films are markedly difficult to obtain, yet an ever-growing necessity for emerging fields such as soft organic electronics. To complicate matters, the interfacial contribution plays a major role in such thin films and is often unexplored despite supporting substrates being a main component in current metrologies. Here we present the shear motion assisted robust transfer technique for fabricating free-standing sub-100 nm films and measuring their inherent structural–mechanical properties. We compare these results to water-supported measurements, exploring two phenomena: 1) The influence of confinement on mechanics and 2) the role of water on the mechanical properties of hydrophobic films. Upon confinement, polystyrene films exhibit increased strain at failure, and reduced yield stress, while modulus is reduced only for the thinnest 19 nm film. Water measurements demonstrate subtle differences in mechanics which we elucidate using quartz crystal microbalance and neutron reflectometry.

Water‐assisted mechanical testing of polymeric thin‐films

“…Upon confinement near the end‐to‐end distance of the polymer chain, the interchain entanglements are reduced resulting in a decay in the yield stress. This trend has been observed in both wrinkling metrologies and uniaxial tensile testing (film‐on‐water and free‐standing film) 19,21,23,24,39,56 . The influence of confinement on fracture strain is more complicated, as there exists a delicate balance between interchain entanglement density, thin‐film T g , chain orientation, and defect density.…”

“…Most recently, our group has developed a more robust transfer method and successfully released a 20 nm thick PS film from the water surface through shearing motion assisted robust transfer (SMART). Instead of directly lifting the film out of the water, which can cause film rupture due to the interfacial tension, our group used shear motion to allow the water meniscus to gently retreat from the film surface 56 …”

“…This trend has been observed in both wrinkling metrologies and uniaxial tensile testing (film-on-water and free-standing film). 19,21,23,24,39,56 The influence of confinement on fracture strain is more complicated, as there exists a delicate balance between interchain entanglement density, thinfilm T g , chain orientation, and defect density. Lee et al observed the onset fracture strain to increase for confined 654 kDa PS supported on PDMS, attributed to the free surface mobile layer.…”

“…(D) schematic of shear motion assisted robust transfer (SMART) process and the exerted force on the thin‐film at different steps. (reproduced with permission 56 . 2021, springer nature [Color figure can be viewed at wileyonlinelibrary.com]…”

“…Instead of directly lifting the film out of the water, which can cause film rupture due to the interfacial tension, our group used shear motion to allow the water meniscus to gently retreat from the film surface. 56 We will discuss the techniques individually in this section.…”