Flexible electronic devices composed of polymers and elastomers require high mechanical durability to maintain their performance during cyclic bending. To design the appropriate structure for such devices, it is important to identify the position of a neutral mechanical plane (NMP) where there is no strain inside a bending material. In this study, the NMP position of bending polydimethylsiloxane (PDMS) film, which is a common soft material used in flexible electronic devices, is experimentally identified through internal strain measurement using a cholesteric liquid crystal sensor. Notably, the NMP of the bending PDMS film reversibly shifts toward the inner bending surface. Further, considering the NMP shifting enables us to fabricate a flexible electronic device with high mechanical durability. Quantifying the NMP position facilitates the development of device designs for flexible electronics.
Over the past decades, flexible electronics such as flexible liquid crystal devices composed of polymer film substrates have been dramatically growing. Understanding bending behavior of polymer films is the key to designing flexible electronic devices with high mechanical durability. Although various bending analysis methods have been proposed, they are still limited to macroscopic and qualitative analyses. Recently, we have newly proposed a method for analyzing the surface bending strain in flexible materials, termed surface labeled grating method. This method enables us to quantitatively evaluate the surface bending strain by monitoring the diffraction angle of a He-Ne laser beam that passes through a grating label attached on a sample. In this study, we measure the surface bending strain in polyethylene terephthalate films and reveal that tension and compression occurs in their outer and inner surfaces, respectively.
To develop flexible devices that have mechanical durability, understanding the bending behavior of soft material components is quite important. However, measuring bending strain in soft materials has been limited to their surface due to experimental constraints. In addition to the surface strain analysis, internal strain analysis will further clarify the bending behavior of soft materials. In this study, we quantified internal strain in bending polydimethylsiloxane (PDMS) films, which are a common soft material, through the selective reflection of a cholesteric liquid crystal elastomer (CLCE). The strain analysis with the CLCE revealed that internal strains depend on the position of the bending PDMS films. This internal strain quantification of soft materials leads to the development of flexible devices with high mechanical durability.
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