Validation of theoretical analysis of surface bending strain in polymer films by surface-labeled grating method

Abstract

Quantitative analysis of tension and compression imposed on surfaces of bending polymer films plays a key role in the design of flexible electronic devices. For over a decade, the analysis has relied on the classical beam theory that mainly deals with metals, glass, and cement; however, the applicable limit of the theory to largely bending polymer films has never been validated. We present that the classical beam theory accurately analyzes surface bending strains in single-layer and double-layer polymer films through measuring the strains by a surface-labeled grating method. The experimental analysis reveals that the bending strains on the outer and inner surfaces of the single-layer film are symmetrical, whereas those of the double-layer film are asymmetrical. These results are well explained by the classical beam theory considering stress–strain curves of polymer films. This approach will further advance the strain design of polymer films, which aids in the development of mechanically durable devices.

Bending creep behaviour of various polymer films analysed by surface strain measurement

“…To overcome the issue, we previously developed the surfacelabeled grating method utilizing optical diffraction to quantify surface strain in bending polymer films. [28][29][30] In this method, a soft thin-film grating label is attached to a polymer film to directly measure the surface strain in real-time without affecting the material's bending behaviour. Utilizing a laser beam with a diameter of mere 100-200 mm, we achieved highly localized strain measurements.…”

Interfacial Strain Analysis of Bending Bilayer Silicone Elastomer Films Using a Cholesteric Liquid Crystal Sensor

“…Various methods for strain analysis include the use of strain gauges, − digital image correlation, − optical diffraction, − and mechanochromism. − However, these methods are limited to measuring the surface and cross-sectional strain in bending polymer films owing to experimental constraints such as light interference by bending, difficulty in creating a speckle pattern, multi-diffraction, and low resolution. In our previous study, the internal strain distribution of a bending monolayer poly­(dimethylsiloxane) (PDMS) film, a common polymer used as a substrate for flexible electronic devices, was successfully measured employing a cholesteric liquid crystal elastomer (CLCE) sensor. , CLCEs have a cross-linked helical molecular structure, resulting in flexibility, selective reflection, and distinctive mechanical and optical characteristics.…”