Understanding the curvature of a bent polymer film is important for the research and development (R&D) of flexible electronic devices. Herein, we report that the curvature of a polymer film is successfully predicted by a stress–strain curve (S–S curve) of the polymer. The curvature of a bent polymer film depends on film thickness and chemical structure, which is experimentally confirmed. This dependence is demonstrated well by our theoretical model (the modified Elastica theory) based on a S–S curve. This method provides effective guideline for the R&D of flexible electronic devices.
be circumvented by exploiting an elementary mechanics that surface bending strain decreases linearly with a thickness of a substrate. For example, flexible substrates with a thicknesses of 10 µm experience peak surface strain of only 0.1% upon bending to the radius of curvature of 5 mm, and this strain remains well below the fracture limits of semiconductors (≈1%), metals (1-2%), and hard coatings (1-3%); indeed, the use of a substrate within a range below tens of micrometers enables comparable growth, resulting in high-performance durable flexible devices for epidermal, implantable, and wearable applications (Figure 1b). [10-20] On the other hand, the development of foldable electronics devices that have thicknesses of hundreds of micrometers is an emerging challenge. This highlights the ever-growing importance of the substrates strategically designed to reduce surface strain without thinning (Figure 1a right and 1b). However, it is not yet possible to measure nanoscale surface bending strain in real time: none of the currently available advanced methods fulfill all of the necessary spatial-temporal resolution, accuracy, precision, and a wide range of measurable materials. Electrical strain sensors including strain gauges exemplify the most common strain analytical method. [21-23] Although they provide surface bending strains of materials targeted in real time, their With the rapid development of flexible electronics and soft robotics, there is an emerging topic of preventing fracture in materials and devices integrated on largely bending film substrates of >100 µm thickness. The high demand for strategically reducing strain in bending materials requires a facile method that enables one to accurately and precisely analyze the surface bending strain in a wide variety of materials. This study proposes the surface-labeled grating method that is the fundamental and efficient technique for measuring surface bending strains merely by labeling a thin, soft grating onto various film substrates composed of flexible polymeric and rigid inorganic materials. The surface strain with a single-nanoscale (<1.0 nm) can be quantified in real time with no need of material information such as Poisson's ratio, Young's modulus, and film thickness. The fracture limit of a hard coating overlying flexible substrates is successfully determined by the accurate and precise quantification of surface bending strains. Furthermore, a multilayer film substrate with surface bending strain reduced by 50% prevents fractures of hard coatings and organic thin film transistors (OTFTs) since the strains remain below the fracture limit under large bending.
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.
Fluorescent polycarbonates were synthesized by embedding AIE-active diol monomers with simple structures in the polymer chain.
The measurement of bending curvature of polymer films characterised the bending hysteresis as a precursor phenomenon of fracture and fatigue. The measurement also enables us to predict the occurrence of bending hysteresis.
In order to clarify the mechanics of human swimming, a full-body swimming humanoid robot called "SWUMANOID" was developed as an experimental platform for research about human swimming. SWUMANOID had a detailed human body shape, created using three-dimensional scanning and printing equipment, and was developed as an experimental model substituting for human subjects. Not only the appearance but also the methodology to realize various swimming strokes was considered. In order to reproduce complicated swimming motions with high fidelity, 20 waterproof actuators were installed. The free swimming of the crawl stroke at a velocity of 0.24 m/s was realized in the previous study. However, it could not perform the breaststroke due to mechanical limitations. The objectives of this study were to realize the breaststroke for SWUMANOID by improving its lower limbs, and to investigate the swimming performance of the breaststroke experimentally. The lower body of SWUMANOID was fully redesigned, built, and connected to the upper body. The swimming motion of the breaststroke was created based on that of an actual swimmer. A free swimming experiment was conducted in a 25 m outdoor swimming pool. In addition, in order to discuss the experimental results in detail, the experiment was reproduced by the simulation. From the experiment, it was found that SWUMANOID could perform the breaststroke successfully. The swimming speed for the stroke cycle of 2.3 s was found to be 0.12 m/s. Since this swimming speed was considered low compared to that of the actual swimmer, the reason for the discrepancy was examined by simulation. From the simulation, it was found that one main reason for the low swimming speed was insufficient output power of the motors, especially for the knee and shoulder joints.
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.
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.
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