Experimental and theoretical analyses of curvature and surface strain in bent polymer films

Kuwahara, Kohei; Taguchi, Ryo; Kishino, Masayuki; Akamatsu, Norihisa; Tokumitsu, Kayoko; Shishido, Atsushi

doi:10.35848/1882-0786/ab8346

Cited by 16 publications

(16 citation statements)

References 33 publications

(44 reference statements)

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“…All the theoretical surface bending strains in the polymer films were entirely consistent with those obtained experimentally (Figure 3a,b). The surface bending strain in the thin glass obtained from the experiment was also consistent with that calculated from the Elastica theory [33,34] (Figure 3b and Figure S7, Supporting Information). These consistencies clearly indicate that the surfacelabeled grating method yields accurate strains, suggesting that this method has a considerably wide range of measurable materials compared with the conventional optical methods.…”

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confidence: 82%

“…To prove the accuracy of this method, we used the modified Elastica theory which is an analytical model of curvature for a single-layer bent film that we recently reported. [33] As shown in Figure 3e, let s be the distance along the axis of the bent film from origin O; the expression for the angle between the line tangent to the bent film and the x-axis is θ (s). The angle θ (0) is also expressed by β, dividing the bending of a film into two groups: 0 ≤ β < π/2 (Figure 3e top) and β = π/2 (Figure 3e bottom).…”

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confidence: 99%

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Nanoscale Analysis of Surface Bending Strain in Film Substrates for Preventing Fracture in Flexible Electronic Devices

Taguchi

Akamatsu

Kuwahara

et al. 2021

Adv Materials Inter

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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.

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confidence: 99%

Nanoscale Analysis of Surface Bending Strain in Film Substrates for Preventing Fracture in Flexible Electronic Devices

Taguchi

Akamatsu

Kuwahara

et al. 2021

Adv Materials Inter

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“…The PDMS 100 film turned blue (Figure 2b and Movie S1, Supporting Information), whereas the PDMS 1400 film turned red (Figure 2c and Movie S2, Supporting Information). Their color changes were quantified via the reflectance spectrum analysis (Figure 2d); the incident angle was fixed at 0° toward the top of the bending film using a precisely bendable machine [ 40 ] to avoid the reflection wavelength shift caused by an oblique angle. [ 41,42 ] The reflection wavelength of the PDMS 100 film blueshifted by 103 nm (Figure 2e), whereas that of the PDMS 1400 film redshifted by 99 nm (Figure 2f).…”

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

et al. 2021

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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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“…The curvature (κ) was the most appropriate criterion to evaluate the bending characteristics, and thus, in the present study, the bending performance was evaluated by measuring the electrical resistance between the electrodes under various curvatures. 35 Figure 3c shows the I−V curves of the sensor in bending with various curvatures such as 0, 0.10, 0.15, and 0.20 mm −1 . An excellent linear relationship between the voltages and currents is observed in all of the I−V curves, indicating the Ohmic behavior and constant resistance of the sensor in the static state.…”

Section: Resultsmentioning

confidence: 99%

Novel Bending Sensor Based on a Solution-Processed Cu₂O Film with High Resolution Covering a Wide Curvature Range

et al. 2021

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A Cu 2 O film is prepared on a flexible polyethylene terephthalate substrate for a bending sensor using the spin−spray method, a facile and low-environmental-load solution process. The Cu 2 O bending sensor shows high sensitivity and high resolution not only over a wide range of curvatures (0 < κ < 0.21 mm −1 ) but also for very small curvature changes (Δκ = ∼ 0.03 mm −1 ). The bending response of the sensor exhibited a curvature change of high linearity with a good gauge factor (18.2) owing to the grainboundary resistance and piezoresistive effects of the fabricated Cu 2 O film. In addition, the sensor possesses good repeatability, stability, and long-term (>30 days) and mechanical fatigue durability (1000 bending−release cycles). The sensor is capable of detailed monitoring of large-and small-scale human motions, such as finger bending, wrist bending, nodding, mouth opening/ closing, and swallowing. In addition, excellent stability and repeatability of the monitoring performance is observed over a wide range of motion angles and speeds. All of these results demonstrate the potential of the flexible bending sensor based on the Cu 2 O film as a candidate for healthcare monitoring and wearable electronics.

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Experimental and theoretical analyses of curvature and surface strain in bent polymer films

Cited by 16 publications

References 33 publications

Nanoscale Analysis of Surface Bending Strain in Film Substrates for Preventing Fracture in Flexible Electronic Devices

Nanoscale Analysis of Surface Bending Strain in Film Substrates for Preventing Fracture in Flexible Electronic Devices

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

Novel Bending Sensor Based on a Solution-Processed Cu₂O Film with High Resolution Covering a Wide Curvature Range

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Experimental and theoretical analyses of curvature and surface strain in bent polymer films

Cited by 16 publications

References 33 publications

Nanoscale Analysis of Surface Bending Strain in Film Substrates for Preventing Fracture in Flexible Electronic Devices

Nanoscale Analysis of Surface Bending Strain in Film Substrates for Preventing Fracture in Flexible Electronic Devices

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

Novel Bending Sensor Based on a Solution-Processed Cu2O Film with High Resolution Covering a Wide Curvature Range

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Novel Bending Sensor Based on a Solution-Processed Cu₂O Film with High Resolution Covering a Wide Curvature Range