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

Abstract

Flexible electronic devices composed of soft materials, such as polymers and elastomers, require high mechanical durability to maintain their performance during cyclic bending, where large bending can lead to fracture. To design the appropriate structure for such devices, it is essential to utilize a neutral mechanical plane (NMP) in which the strain becomes zero inside bending materials. Despite the importance of identifying the NMP position for this utilization, the NMP position in soft materials has rarely been studied experimentally because of the difficulty in measuring internal strain in largely bending materials. Herein, the NMP position of bending polydimethylsiloxane (PDMS) film, which is a common soft material used in flexible electronic devices, is experimentally quantified via internal strain measurement with a cholesteric liquid crystal sensor. This is the first reported identification of the NMP position, revealing that bending of the PDMS film reversibly shifts the NMP position within ≈11% of the film thickness toward the inner bending surface. Considering this large NMP shift, a flexible electronic device with high mechanical durability is fabricated. The direct identification of the NMP position, which is enabled by the internal strain measurement, facilitates the development of device designs for flexible electronic devices.

Significant anisotropic deformation and optical shifts in stretched cholesteric liquid crystal elastomers

“…5,[15][16][17][18] The mechanochromic response also allows for their use in tensile and pressure sensors with high spatial resolution. 13,14,[19][20][21][22] These opto-mechanical features make CLCEs particularly interesting as tunable photonic soft materials.…”

Microwave Bonding of Thin Polymer Substrates Using Carbon Nanotubes for Flexible Interconnections

“…Flexibility at the assembly joint should be guaranteed to maintain the high mechanical and electrical performance of a flexible device without causing any physical damage. Several studies have highlighted the importance of the mechanical reliability of electronic packages against dynamic bending, twisting, and folding, which can induce the formation of cracks inside the joint area. − Conventional bonding methods for flexible packaging include organic adhesives such as epoxies due to their strong adhesion force, lightweight, and low cost. − Recently, anisotropic conductive film (ACF) technologies have been actively developed for flexible applications. The ACFs consist of insulating epoxy resin and various conductive particles and have a high potential to realize flexible devices such as chip-on-flex (COF). − However, organic-adhesive-based bonding techniques have inevitable limitations, including bending stiffness and mechanical reliability.…”

Bending Fatigue Analysis of Various Polymer Films by Real-Time Monitoring of the Radius of Curvature and Heat Generation

“…Polymer films within electronic devices have gained much attention in displays, soft robots, and mobilities due to their flexibility and durability. − These devices need to work under cyclic stretching, compressing, and bending without fatigue failure of a polymer substrate and the degradation of device performance. Fatigue is one of the most common failure mechanisms in polymeric materials subjected to cyclic deformation.…”