Neutral Mechanical Plane Shifting in Bending Elastomer Film Revealed by Quantification of Internal Strain

Abstract

Neutral mechanical plane (NMP) position of a bending elastomer film is experimentally identified by internal strain analysis utilizing reflection of a cholesteric liquid crystal elastomer sensor as described in article number http://doi.wiley.com/10.1002/adem.202101041 by Atsushi Shishido, Norihisa Akamatsu and co‐workers. The internal strain analysis revealed NMP shifting during elastic bending. Quantification of the NMP shifting in the bending elastomer film enables us to design a flexible electronic device that exhibits high mechanical durability.

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

“…These blue shifts were due to the decrease in the helical pitch of the CLCE sensors, indicating that out-of-plane compression was induced in the bent bilayer PDMS films, as reported previously (see insets). 27,28,30,31 The bent bilayer PDMS films containing the CLCE sensor were confirmed to be tightly bonded, even at 50% applied strain. Figure 5c,d shows the reflection spectra of the S 500 H 1000 and H 500 S 1000 films, respectively.…”

“…A blue shift was observed in the reflection spectra of the S 750 H 750 film, whereas a slight red shift and subsequent blue shift were observed in the reflection spectra of the H 750 S 750 film. The red shift represents the increase in the helical pitch of the CLCE sensors induced by the out-of-plane tension (see insets) . Through the reflection spectra measurement with the CLCE sensor, we found that, even at the same interface positions, the reflection wavelength shifts, acting as an indicator of the interfacial bending strain, were different.…”

“…This stress gap causes an increase in the in-plane tensile area (out-of-plane compression) and a decrease in the in-plane compressive area (out-of-plane tension) to balance these stresses in the bending bilayer PDMS films, resulting in the NMP shifting toward the inner bending surface (Figure ). ,− The NMP shifting reduces z of the H 750 S 750 film and the interface bending strain. This study demonstrates that estimating the initial NMP position and taking the NMP shifting into account are vital for understanding the interface strain in bending polymer films.…”

“…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. Because the reflection wavelength (λ) is proportional to the helical pitch ( P ) and refractive index ( n ), expressed as λ = nP , the bending strain imposed on the CLCEs changes the helical pitch and reflection wavelength.…”

“…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. 27,28 CLCEs have a cross-linked helical molecular structure, resulting in flexibility, selective reflection, and distinctive mechanical and optical characteristics. Because the reflection wavelength (λ) is proportional to the helical pitch (P) and refractive index (n), expressed as λ = nP, 29 the bending strain imposed on the CLCEs changes the helical pitch and reflection wavelength.…”