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Article Curved-Mechanical Characteristic Measurements of Transparent Conductive Film-Coated Substrates Using Common-Path Optical Interferometry

Bor-Jiunn Wen * and Jui-Jen Hsu

Department of Mechanical and Mechatronic Engineering, National Taiwan Ocean University, Keelung 20224, Taiwan; [email protected] * Correspondence: [email protected]

Abstract: This study proposes a method for measuring curved-mechanical characteristics based on a whole-folding test for transparent conductive film-coated polymer substrates using common- path optical interferometry. Accordingly, 80-, 160-, and 230-nm indium tin oxide films coated on 40 × 40 mm 125-µm-thick polyethylene terephthalate (PET) substrates, and monolayer graphene films coated on 40 × 40 mm 250-µm-thick PET substrates are inspected and analyzed under the curving conditions of 50-, 30-, 20-, and 10-mm radii before and after an 11,000 whole-folding cycle test based on a 10-mm folding radius. This study utilizes the changes in the phase retardations of transparent conductive film-coated polymer substrates under different curving conditions before and after 11,000 whole-folding cycles to analyze the substrates’ residual stress characteristics that were the   direct result of manufacturing process parameters. The results from this study of curved-mechanical characteristic measurements of flexible transparent conductive substrates can provide designers Citation: Wen, B.-J.; Hsu, J.-J. Curved-Mechanical Characteristic with improved product development and can assist manufacturers in improving the manufacturing Measurements of Transparent design of enhanced coating processes. Conductive Film-Coated Polymer Substrates Using Common-Path Keywords: curved-mechanical characteristic measurements; whole-folding test; transparent Optical Interferometry. Coatings 2021, conductive film-coated polymer substrate; common-path optical interferometry 11, 766. https://doi.org/10.3390/ coatings11070766

Academic Editor: Roman A. 1. Introduction Surmenev Flexible substrates are critical components of flexible electronics. Because different flexible electronic product applications have different requirements for substrate materials, Received: 1 June 2021 research on substrate materials determines the developmental direction of flexible electron- Accepted: 24 June 2021 ics. For example, magnetic material films or nanoparticles are deposited on transparent Published: 25 June 2021 flexible substrates (e.g., polyethylene terephthalate (PET), polyimide, polystyrene, poly- methylmethacrylate, or cyclic olefin copolymer) for developments of magnetic sensors Publisher’s Note: MDPI stays neutral or absorbers [1–5]. Because the flexible transparent optical film itself is not conductive, with regard to jurisdictional claims in published maps and institutional affil- depositing a transparent conductive film on it as a conductive electrode (e.g., indium tin iations. oxide (ITO) or aluminum doped zinc oxide) is necessary. The current commonly-used transparent conductive material is an ITO film, which has the advantages of low resistivity (1–5 × 10−6 Ωm) and high light transmittance. In recent years, ITO conductive oxides have been deposited on PET plastic films. This represents one of the most popular research directions for flexible electronic materials [6–9]. However, after depositing the flexible trans- Copyright: © 2021 by the authors. parent optical or conductive film on the polymer substrate, it still exhibits poor water and Licensee MDPI, Basel, Switzerland. oxygen resistance, temperature, humidity, and mechanical properties of load conditions. This article is an open access article distributed under the terms and Accordingly, major domestic and foreign manufacturers and research institutes are seeking conditions of the Creative Commons alternatives. Therefore, in recent years, for the application of flexible substrates or sensors, Attribution (CC BY) license (https:// graphene materials have been added to improve the conductivity, mechanical proper- creativecommons.org/licenses/by/ ties, electromagnetic effects, heat conduction effects, and optical transmittance of flexible 4.0/). transparent optical films [10–14]. For quality mechanical measurements of graphene film

Coatings 2021, 11, 766. https://doi.org/10.3390/coatings11070766 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 766 2 of 13

materials, most studies have used non-contact Raman [15–19]; others have used atomic microscopy [20–22]. These mechanical characteristic measurements require precision and expensive equipment, such as Raman and atomic force microscopes, and the measurement range can be as high as hundreds of microns. For large deformations of flexible electronics, the bending radius of curvature is approximately millimeters to centimeters. For large areas, fast confirmation and further analysis of the interface stress of flexible graphene substrates are difficult, and the cost of high detection equipment is considerable for general manufacturers. In addition, the major features of flexible electronics are the bending storage and operation of curving conditions, such as the display or transmission of electrical signals during curving [23–25]. Therefore, flexible electronic stress analysis under curving conditions enables researchers to obtain an im- proved design for enhanced coating processes. Some studies [26–28] have utilized optical phase retardation measurement technology to measure the stress states of transparent materials. Of these technologies, single-point [29–31], area-based [32–35], and area-based optical phase retardation measurement methods with scanning platforms [36–38] have been used. In addition, some literatures utilize an X-ray reflectometer to analyze the stresses of the films [39–41]. However, to measure and analyze stress states, measured states must be under the plane. Therefore, this study proposes a method for measur- ing the curved-mechanical characteristics of transparent conductive film-coated polymer substrates using common-path optical interferometry. To analyze the mechanical charac- teristics of transparent conductive film-coated PET substrates in curving, an automatic sliding-folding testing platform (ASTP) [42] is used to conduct a whole-folding test in this study. Accordingly, 80-, 160-, and 230-nm ITO films coated on 40 × 40 mm 125-µm-thick PET substrates and monolayer graphene films coated on 40 × 40 mm 250-µm-thick PET substrates were inspected and analyzed under the curving conditions of 50-, 30-, 20-, and 10-mm radii before and after an 11,000 whole-folding cycle test based on a 10-mm fold- ing radius was conducted. The curved-mechanical characteristic measurement results of flexible transparent conductive substrates can provide designers with improved product development or assist manufacturers in improving the manufacturing design of enhanced coating processes.

2. Methods To measure and analyze the curved-mechanical characteristics of transparent conduc- tive film-coated polymer substrates, this study develops a common-path optical interfero- metric technique to measure the phase retardations of a sample on a curved clamper, as shown in Figure1. The light from a flat white light source passes through a color filter and a PR polarizer and is thus transformed into a linearly polarized light at 45◦ to the y axis. The light next passes through a liquid crystal modulation and a tested sample on a curved clamper with an r radius of curvature. The light then passes through an AR analyzer with a transmission axis at −45◦ to the y axis, and finally through an imaging lens and camera to obtain the phase retardations of the sample during curving. Because the collimated light transmitted to the transparent curved test sample causes refraction and phase retardation, this study derives the phase retardation influence factor in curving. The phase retardation under the curving of the test sample is next measured using common-path optical interfer- ometry. Finally, the phase retardation influence factor in curving is subtracted from it, and the phase retardation of the sample in curving is obtained. Because the thickness of the transparent conductive film of the test sample is much lower than that of the polymer substrate, this study considers only the refraction effect of the polymer substrate under curving, as shown in Figure2. Figure2a,b shows the optical paths of the test sample of the collimated light source transmission under outer and inner curves. Coatings 2021, 11, x FOR PEER REVIEW 3 of 13

Sample PR y Curved y AR ° 45° clamper -45 CoatingsCoatings 20212021,, 1111,, x 766 FOR PEER REVIEW x x 3 3of of 13 13 r Flat white Liquid crystal Sample on Color filter Polarizer Analyzer Lens Camera light source modulation curved clamper Sample PR y Curved y AR Figure 1. Schematic of phase retardation measurements under the curving° of a test sample by 45° clamper -45 common-path optical interferometry. x x r Because the thickness of the transparent conductive film of the test sample is much Flat white Liquid crystal Sample on Color filter Polarizer Analyzer Lens Camera lowerlight source than that of the polymermodulation substrate, thiscurved study clamper considers only the refraction effect of the polymer substrate under curving, as shown in Figure 2. Figure 2a,b shows the optical FigureFigurepaths 1.of 1. Schematic theSchematic test sample of ofphase phase of retardation the retardation collimated measurements measurements light source under undertransmission the curving the curving ofunder a test of a outersample test sampleand by inner by common-pathcommon-pathcurves. optical optical interferometry. interferometry.

Because the thickness of the transparent conductive film of the test sample is much lowerConductive than that offilm the polymer substrate,Conductive this study filmconsiders only the refraction effect of the polymer substrate under curving, as shown in Figure 2. Figure 2a,b shows the optical Polymer substratepaths of the test sample of the collimated light sourcePolymer transmission substrate under outer and inner curves. D A θ ConductiveE film Conductive film F 1 lo t θ1 θ do A 2 Polymer substrateθ2 θ d t Polymer1 isubstrate F θ E 1 li D D A θ y E r F 1 lo t θ1 r yθ2 do A θ2 θ d t 1 i F θ E 1 li D B θ1 θ1 B O O y (a)r (b) r y Figure 2. Optical paths of the test sample of the collimated light source transmission under (a) outer and (b) inner curves, whereFigure the 2. Optical vertical, paths horizontal, of the test and sample perpendicular of the collimated directions light are the source y, z, andtransmission x axes, respectively. under (a) outer and (b) inner curves, where the vertical, horizontal, and perpendicular directions are the y, z, and x axes, respectively. B θ1 θ1 B In Figure2, the curvingO radiusO r is based on the curving center O. According to Snell’s law [27In ],Figure the following 2, the curving equation radius is obtained: r is based on the curving center O. According to Snell’s (a)law [27], the following equation is obtained: (b) n sinθ = n sinθ (1) 1𝑛 𝑠𝑖𝑛𝜃1 = 2𝑛 𝑠𝑖𝑛𝜃2 (1) Figure 2. Optical paths of the test sample of the collimated light source transmission under (a) outer and (b) inner curves,

where the vertical, horizontal,wherewhere and perpendicular n𝑛1and andn 2𝑛are aredi therections the refractive refractive are theindices y, indices z, and of xof theaxes, the air airespectively.r and and polymerpolymer substrate,substrate, respectively, θ𝜃1 is is the the incident incident angle angle of pointof point A, andA, andθ2 is 𝜃 the is refraction the refraction angle. angle. According According to the ∆ toABO, the under∆ABO,In theFigure under outer 2, the the curve, outer curving the curve, following radius the followingr is equation based onequation is the obtained: curving is obtained: center O. According to Snell’s law [27], the following equation is obtained: y 𝑦 sin𝑠𝑖𝑛𝜃θ1 = = (2)(2) 𝑛𝑠𝑖𝑛𝜃= r𝑛+𝑟+𝑡𝑠𝑖𝑛𝜃t (1) and under the inner curve, the following equation is obtained: whereand under 𝑛 and the inner𝑛 are curve, the refractive the following indices equation of the ai isr and obtained: polymer substrate, respectively, 𝜃 is the incident angle of point A, and 𝜃 is the𝑦 refraction angle. According to the 𝑠𝑖𝑛𝜃 =y (3) ∆ABO, under the outer curve, the followingsinθ equation= 𝑟 is obtained: (3) 1 r where 𝑦 is the distance between point A and the𝑦 incident light beam on the sample and 𝑠𝑖𝑛𝜃 = (2) wherethe extensiony is the distancepoint B from between the pointcenter A of and the the curve,𝑟+𝑡 incident and light𝑡 is beamthe thickness on the sample of the andsample the extension point B from the center of the curve, and t is the thickness of the sample based and under the inner curve, the following equation is obtained: on the extremely thin transparent conductive film. In the ∆AFD under the outer and inner 𝑦 curves, the approximate relationship can𝑠𝑖𝑛𝜃 be expressed= as [43] (3) 𝑟 t where 𝑦 is the distance between point Ad ≈and the incident light beam on the sample and(4) the extension point B from the center of the coscurve,θ2 and 𝑡 is the thickness of the sample

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where d is the path of the light passing through the sample. Through the trigonometric function relationship of cosθ2, under the outer curve, do can be rewritten as

n2t do = q (5) 2 2 y 2 n2 − n1( r+t )

and under the inner curve, di can be rewritten as

n2t di = q (6) 2 2 y 2 n2 − n1( r )

From the geometric relationship of ∆AFE and the angle formula of the trigonometric function, under the outer curving, lo is derived as q ( + )2 − 2 2 t r t y n1ty lo = docos(θ1 − θ2) = + q (7) r + t 2 2 2 y 2 (r + t) n2 − n1 r+t

and under the inner curving, li is derived as

p 2 2 2 t r − y n1ty li = dicos(θ1 − θ2) = + q (8) r 2 2 2 y 2 r n2 − n1 r

Figure2 shows only the upper half of the y axis, and because the lower half is symmetrical with the upper half, the phase retardation of the lower half is added to the upper half. Therefore, according to the definition of optical phase retardation [43], the phase retardation ∅o(x, y) of the flexible transparent conductive film-coated polymer substrate with or without curving under outer curving is q 2 n t (r + t)2 − y2 2 2 n2t 1 n1ty ∅o(x, y) = 2[n2do − n1lo] = 2[ q − − q ] (9) 2 2 y 2 r + t 2 2 2 y 2 n2 − n1 r+t (r + t) n2 − n1 r+t

and the phase retardation ∅i(x, y) of the flexible transparent conductive film-coated poly- mer substrate with or without curving under inner curving is q 2 n t (r)2 − y2 2 2 n2t 1 n1ty ∅i(x, y) = 2[n2di − n1li] = 2[ q − − q ] (10) 2 2 y 2 r 2 2 2 y 2 n2 − n1 r (r) n2 − n1 r

Therefore, ∅o(x, y) and ∅i(x, y) are the phase-retardation influence factors in the outer and inner curving, respectively. According to the literature [27,44], the light amplitude E in Figure1 using common-path optical interferometry is derived as    ◦   ◦  1 E = AR −45 ·S·M·PR 45 · (11) 0

where a polarizer and analyzer with 45◦ and 135◦ linearly polarized directions PR and AR     ◦ 1 1 ◦ 1 −1 to the y axis, respectively, are PR45  = 1 and AR−45  = 1 , 2 1 1 2 −1 1  iδ/2  e 0 T respectively. S = and 1 0  are the Jones matrix of the tested sample 0 e−iδ/2 and Jones vector of the light at the x axis [27], respectively, where δ is the total optical retardation of the multilayer-film substrate. In addition, M is the Jones matrix of n-layer Coatings 2021, 11, 766 5 of 13

liquid crystal modulation based on a twisted nematic liquid crystal with a maximum twist angle θ of 90◦, which is written as

  " iτ #  cos(−θ) sin(−θ) e 2n 0 cos(θ/n) sin(θ/n) n M = lim ( −iτ ) (12) −sin(−θ) cos(−θ) n→∞ 0 e 2n −sin(θ/n) cos(θ/n)

According to [44], the 2D phase-retardation distribution by common-path optical interferometry is derived as

2π∅tan(∅/2) ∅(x, y) = q (13) 2 4[(π2 + ∅2)tan2(∅/2)] − [2∅tan(∅/2)]

Because the collimated light transmitted to the transparent curved test sample causes refraction and phase retardation, this study utilizes the phase-retardation influence factors of (9) and (10) subtracted from (13) based on the method of 2D phase unwrapping [45] to obtain the correct phase retardation of the sample during curving. Finally, the correct phase retardation of the sample in the outer curving is derived as

∅co(x, y) = ∅(x, y) − ∅o(x, y) q 2 n t (r + t)2 − y2 2 2 πC n2t 1 n1ty = √ − 2[ q − − q (14) 4B − C2 2 2 y 2 r + t 2 2 2 y 2 n2 − n1 r+t (r + t) n2 − n1 r+t and the correct phase retardation of the sample in inner curving is derived as

∅ci(x, y) = ∅(x, y) − ∅i(x, y) q 2 n t (r)2 − y2 2 2 πC n2t 1 n1ty = √ − 2[ q − − q ] (15) 4B − C2 2 2 y 2 r 2 2 2 y 2 n2 − n1 r (r) n2 − n1 r

3. Experiment Figure3 depicts the instrument configuration of the phase retardation measurements of the test sample under a curving with a 30-mm radius by common-path optical inter- ferometry. To avoid light interference, this study measured the phase retardation of the sample in a dark room. In this study, a flat white light source with a color filter, PR, and liquid crystal modulation was used as a linearly polarized white light with a 535.38-nm central wavelength; this was a liquid crystal flat panel display. This study also utilized a 1388 × 1038-pixel camera (Basler/A631fc) with an AR analyzer in front to capture phase retardation images. Finally, liquid crystal modulation was controlled to perform four-step phase shifting [46] for 0, 1/6, 1/3, and 1/2 maximum intensities, image processing, and correction calculations, which was necessary to measure the optical retardation images under the curving of the test sample using LabVIEW software. Each phase retardation measurement was the average value of 15 measurement images. In this study, four curved clampers with radii of 10, 20, 30, and 50 mm were used to provide four corresponding curving conditions of the test sample, as shown in Figure4. In addition, each curved clamper had a measurement area of 10 × 10 mm or greater. Figure5 depicts a 125- µm-thick PET substrate and 80-, 160-, and 230-nm ITO films coated on 40 × 40 mm 125-µm-thick PET substrates (VisionTek Systems Ltd., Chester, UK). Monolayer graphene films coated on 40 × 40 mm 250-µm-thick PET substrates (Graphenea Inc., Cambridge, MA, USA) were utilized for curved-mechanical characteristic measurements in this study. Coatings 2021, 11, xCoatings FOR PEER 2021 REVIEW, 11, x FOR PEER REVIEW 6 of 13 6 of 13

optical retardationoptical ∅ retardation based on common-path∅ based on common-path optical interferometry optical interferometry could be obtained could be obtained Coatings 2021, 11, 766 using a least-squareusing afitting least-square algorithm fitting [47] algorithm as [47] as 6 of 13

∅ = 2.64𝑀𝑇∅ = − 2.64𝑀𝑇 19.08 − 19.08 (16) (16)

Figure 3. InstrumentFigure 3. Instrument configuration configuration of the phase of retardation the phase retardation measurements measurements of the test ofsample the test under sample a curving under awith a 50- Figure 3. Instrument configuration of the phase retardation measurements of the test sample under a curving with a 50- mm radius by common-path optical interferometry. mm radius by common-pathcurving optical with interferometry. a 50-mm radius by common-path optical interferometry.

Figure 4. Curved clampers of 10-, 20-, 30-, and 50-mm radii based on a measurement area of 10 × 10 mm or greater. Coatings 2021, 11, x FOR PEER REVIEWFigureFigure 4.4. CurvedCurved clampers clampers of of 10-, 10-, 20-, 20-, 30-, 30-, and and 50-mm 50-mm radii radii based based on a onmeasurement a measurement area of area 710 of × of 13

1010× mm10 mmor greater. or greater.

FigureFigure 5. 5.Photos Photos of of 4 4× × 4 cm PET substrate and transparenttransparent conductiveconductive filmsfilms coatedcoated onon PETs.PETs.

To analyze the mechanical characteristics of the transparent conductive film-coated PET substrates under curving, an ASTP [42] was used to conduct a whole-folding test. The ASTP controlled the screw mechanism to enable the stage to move back and forth during the whole-folding test of the entire sample up to a 5-mm folding radius, as shown in Fig- ure 6. In addition, a whole-folding test based on a conducting-film layer outward in ten- sion was used to conduct the whole-folding test. Figure 6 depicts a whole-folding test cycle based on a 5-mm folding radius using ASTP.

Figure 6. Schematic of (a–c), (a,b) (in that order) within one whole-folding cycle based on a 5-mm folding radius by ASTP.

4. Experimental Results and Discussion Figure 7 and Table 1 show the phase retardation measurement results of a 125-μm- thick PET substrate under the curving conditions of 50-, 30-, 20-, and 10-mm radii using

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This study did not use optical-retardation, quarter-wave, or half-ware plates to cal- ibrate the optical retardation measurements using common-path optical interferometry. The average measurement value in a 10 × 10 mm area was calculated based on 10 mea- surements. The relationship equation between values of measurement MT and calibrated optical retardation ∅c based on common-path optical interferometry could be obtained using a least-square fitting algorithm [47] as

Figure 5. Photos of 4 × 4 cm PET substrate∅c = and2.64 tranMTsparent− 19.08 conductive films coated on PETs. (16)

ToTo analyzeanalyze thethe mechanicalmechanical characteristicscharacteristics ofof thethe transparenttransparent conductiveconductive film-coatedfilm-coated PETPET substrates under under curving, curving, an an ASTP ASTP [42] [42 was] was used used to conduct to conduct a whole-folding a whole-folding test. test.The TheASTP ASTP controlled controlled the screw the screw mechanism mechanism to enable to enable the stage the stageto move to moveback and back forth and during forth duringthe whole-folding the whole-folding test of the test entire of the sample entire sample up to a up 5-mm to a folding 5-mm folding radius, radius, as shown as shown in Fig- inureFigure 6. In 6addition,. In addition, a whole-folding a whole-folding test based test based on a onconducting-film a conducting-film layer layer outward outward in ten- in tensionsion was was used used to toconduct conduct the the whole-folding whole-folding test. test. Figure Figure 6 depictsdepicts aa whole-foldingwhole-folding test test cyclecycle basedbased onon aa 5-mm5-mm foldingfolding radiusradius usingusing ASTP.ASTP.

Figure 6. Schematic of (a–c), (a,b) (in that order) within one whole-folding cycle based on a 5-mm Figure 6. Schematic of (a–c), (a,b) (in that order) within one whole-folding cycle based on a 5-mm folding radius by ASTP. folding radius by ASTP. 4. Experimental Results and Discussion 4. Experimental Results and Discussion Figure7 and Table1 show the phase retardation measurement results of a 125- µm- thickFigure PET substrate 7 and Table under 1 show the curving the phase conditions retardation of 50-, measurement 30-, 20-, and results 10-mm of radii a 125- usingμm- common-paththick PET substrate optical under interferometry. the curving Theconditions negative of value50-, 30-, of 20-, phase and retardation 10-mm radii was using the result of compressive stress. As Table1 shows, as the radius of curvature decreased, the phase retardation decreased due to greater compressive stress. This study also measured the phase retardations of 80-, 160-, and 230-nm ITO films coated on 40 × 40 mm 125-µm- thick PET substrates under outer and inner curving conditions of 50-, 30-, 20-, and 10-mm radii before and after 11,000 whole-folding cycles based on a 10-mm folding radius using common-path optical interferometry, as shown in Table2. As Tables1 and2 show, after ITO was coated on PET, its phase retardation was greater than that of uncoated PET under the same curving conditions. This was particularly the case with 80- and 160-nm ITO- coated PETs. It can be inferred that because the Young’s coefficient (116 GPa [48]) of ITO is much greater than that of PET (4.1 GPa [49]), this caused the ITO film to bend, thereby reducing the bending compressive stress [50]. For the same reason, the phase retardations of ITO-coated PETs under ITO-film inner curving conditions were greater than those under outer curving conditions. However, compared with the other two films, because of the Coatings 2021, 11, 766 8 of 13

thicker ITO film, the phase retardation of the 230-nm ITO-coated PET was slightly less than that of the uncoated PET. Table2 shows that the phase retardations of the 160- and 230-nm ITO-coated PETs were the largest and smallest of the three films under different curving conditions, respectively, prior to the 11,000 whole-folding-cycle test. This was because the Coatings 2021, 11, x FOR PEER REVIEW 9 of 13 manufacturing process parameters caused residual stress in the ITO film-coated PET and produced a large or small value of phase retardation in the test sample [50]. In addition, compared to the results before and after the 11,000 whole-folding-cycle test, the phase retardationscompressive were stress not in significantlygraphene/PET different 1 that wa unders the the direct outer result and of inner the curvingmanufacturing conditions pro- ofcess 50-, parameters. 30-, and 20-mm For monolayer radii. Nevertheless, graphene the on phase the second retardations sample of (graphene/PET 80-, 160-, and 230-nm 2), the ITOphase films retardations coated on 125-wereµ m-thicknot significantly PET substrates different under under outer the and same inner conditions curving conditionsbefore and ofafter a 10-mm the 11,000 radius whole-folding-cycle after an 11,000 whole-folding-cycle test. Nevertheless, test the were phase smaller retardations than those of prior gra- tophene/PET the test, as 2 shown under inouter Table and2. Figures inner 8curving and9 show conditions the phase of retardationthe 10-mm measurementradius after the images11,000 ofwhole-folding-cycle 80-, 160-, and 230-nm test ITOwere films smaller coated than on those 125-µ beforem-thick by PET approximately substrates under 1.5 rad. a 10-mmThe results radius suggest condition that before a compressive and after theresidual 11,000 stress whole-folding-cycle in the graphene/PET test using 2 was common- induced pathunder optical the same interferometry. conditions of Table the 10-mm2 shows radius that thebefore changes the 11,000 in phase whole-folding-cycle retardation of the test. 160-After and the 230-nm 11,000 ITO-coated whole-folding-cycle PETs before test, and a after compressive the test were residual the largest stress and in smallest,the gra- respectively,phene/PET 2 of was the threereleased. films The under results a 10-mm suggest radius that condition. the curved-mechanical The results suggest characteristics that the curved-mechanicalof the graphene/PET characteristics 2 were better of than the graphe 160- andne/PET 230-nm 1, due ITO-coated to the manufacturing PET were the process worst andparameters best of the causing three ITOless residual film-coated stress PETs in the due monolayer to the manufacturing graphene film-coated process parameters PET. caused residual stress in the ITO film-coated PET.

Figure 7. Phase retardation measurement images of a PET substrate under the curving conditions of (aFigure) 50-, ( b7.) Phase 30-, (c )retardation 20-, and (d )measurement 10-mm radii usingimages common-path of a PET substrate optical under interferometry. the curving conditions of (a) 50-, (b) 30-, (c) 20-, and (d) 10-mm radii using common-path optical interferometry.

Table 1. Phase retardation measurement results of a 125-µm-thick PET substrate under the curving Table 2. Phase retardation measurements of 80-, 160-, and 230-nm ITO films coated on 40 × 40 mm 125-μm-thick PET substrates under the curvingconditions conditions of of 50-, 50-, 30-, 30-, 20-, 20-, and and 10-mm 10-mm radii radii using before common-path and after 11,000 optical whole-folding interferometry. cycles based on a 10-mm folding radius using common-path optical interferometry. 50 mm 30 mm 20 mm 10 mm Before 11,000−3.19 Whole-Folding rad Cycles−3.31 rad After− 3.4311,000 rad Whole-Folding−3.47 Cycles rad Radius Folding 80-nm ITO 160-nm ITO 230-nm ITO 80-nm ITO 160-nm ITO 230-nm ITO (mm) Condition (rad) (Rad) (Rad) (Rad) (Rad) (Rad) Outer −2.15 −1.23 −3.75 −2.09 −1.27 −3.81 50 Inner −1.63 −0.62 −3.62 −1.76 −0.38 −3.65 Outer −2.00 −0.47 −3.78 −1.88 −0.68 −3.77 30 Inner −1.32 −0.35 −3.79 −1.59 −0.62 −3.78 Outer −2.18 −0.87 −3.75 −1.97 −0.50 −3.71 20 Inner −2.04 −0.56 −3.74 −2.22 −1.05 −3.69

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Table 2. Phase retardation measurements of 80-, 160-, and 230-nm ITO films coated on 40 × 40 mm 125-µm-thick PET substrates under the curving conditions of 50-, 30-, 20-, and 10-mm radii before and after 11,000 whole-folding cycles based on a 10-mm folding radius using common-path optical interferometry.

Before 11,000 Whole-Folding Cycles After 11,000 Whole-Folding Cycles Folding Radius (mm) Condition 80-nm ITO 160-nm ITO 230-nm ITO 80-nm ITO 160-nm ITO 230-nm ITO (rad) (Rad) (Rad) (Rad) (Rad) (Rad) Outer −2.15 −1.23 −3.75 −2.09 −1.27 −3.81 50 Inner −1.63 −0.62 −3.62 −1.76 −0.38 −3.65

Coatings 2021, 11, x FOROuter PEER REVIEW −2.00 −0.47 −3.78 −1.88 −0.68 −3.7710 of 13 30 Coatings 2021, 11, x FORInner PEER REVIEW −1.32 −0.35 −3.79 −1.59 −0.62 −3.7810 of 13

Outer −2.18 −0.87 −3.75 −1.97 −0.50 −3.71 20 Inner −2.04 −0.56 −3.74 −2.22 −1.05 −3.69 Outer −1.38 −0.37 −3.79 −2.46 −3.15 −3.89 10 Outer −1.38 −0.37 −3.79 −2.46 −3.15 −3.89 10 OuterInner −1.382.28 −0.260.37 −3.793.78 −2.462.97 −3.154.08 −3.894.04 10 Inner −2.28 0.26 −3.78 −2.97 −4.08 −4.04 Inner −2.28 0.26 −3.78 −2.97 −4.08 −4.04

Figure 8. Phase retardation measurement images of (a,d) 80-, (b,e) 160-, and (c,f) 230-nm ITO films Figure 8. Phase retardation measurement images of (a,d) 80-, (b,e) 160-, and (c,f) 230-nm ITO films coatedFigure on8. Phase 125-μ m-thickretardation PET measurement substrates under images (a– cof) outer (a,d) 80-,and ((bd,–ef)) 160-, inner and curving (c,f) 230-nm conditions ITO of films a µ coatedcoated10-mm on onradius 125- 125- μbeforem-thickm-thick the PET PET 11,000 substratessubstrates whole-folding-cycl underunder (aa––cc))e outer test using and ( (dcommon-path–ff)) inner inner curving curving optical conditions conditions interferome- of of a a 10-mm10-mmtry. radius radius before before the the 11,000 11,000 whole-folding-cycle whole-folding-cycl teste test using using common-path common-path optical optical interferometry. interferome- try.

FigureFigure 9. 9.Phase Phase retardation retardation measurement measurement images images of of ( a(,ad,d)) 80-, 80-, ( b(b,e,)e) 160-, 160-, and and ( c(,cf,)f) 230-nm 230-nm ITO ITO films films µ coatedcoatedFigure on on9. 125-Phase 125-μm-thick m-thickretardation PET PET measurement substratessubstrates underunder images (aa––c cof)) outer(a,d) 80-,and (( (bd,–eff)) )160-, inner inner and curving curving (c,f) 230-nm conditions conditions ITO of films of a a 10-mmcoated10-mm radius onradius 125- after μafterm-thick the the 11,000 11,000 PET whole-folding-cyclesubstrates whole-folding-cycle under (a– testc) test outer using using and common-path common-path (d–f) inner curving optical optical interferometry.conditions interferometry. of a 10-mm radius after the 11,000 whole-folding-cycle test using common-path optical interferometry. Table 3. Phase retardation measurements of monolayer graphene 1 and 2 films coated on 40 × 40 mm 250-μm-thick PET Tablesubstrates 3. Phase under retardation curving conditions measurements of 50-, of 30-, monolayer 20-, and graphene10-mm radii 1 and before 2 films and coated after 11,000 on 40 ×whole-folding 40 mm 250-μ m-thickcycles based PET substrateson a 10-mm under folding curving radius conditions using common-path of 50-, 30-, optical20-, and interferometry. 10-mm radii before and after 11,000 whole-folding cycles based on a 10-mm folding radius using common-path optical interferometry. Before 11,000 Whole-Folding Cycles After 11,000 Whole-Folding Cycles Radius Folding Graphene/PETBefore 11,000 1 Whole-Folding Graphene/PET Cycles 2 Graphene/PET After 11,000 Whole-Folding1 Graphene/PET Cycles 2 Radius(mm) ConditionFolding Graphene/PET(Rad) 1 Graphene/PET(Rad) 2 Graphene/PET(Rad) 1 Graphene/PET(Rad) 2 (mm) Condition Outer (Rad)−4.02 (Rad)−4.51 (Rad)−3.81 (Rad)−4.41 50 OuterInner −1.974.02 −4.124.51 −2.953.81 −4.284.41 50 OuterInner −1.973.58 −4.124.48 −2.953.75 −4.284.34 30 OuterInner −2.283.58 −4.374.48 −3.413.75 −4.304.34 30 20 OuterInner −3.752.28 −4.504.37 −3.743.41 −4.434.30 20 Outer −3.75 −4.50 −3.74 −4.43

Coatings 2021, 11, 766 10 of 13

This study also measured the phase retardation of two monolayer graphene films coated on 40 × 40 mm 250-µm-thick PET substrates under the curving conditions of 50-, 30-, 20-, and 10-mm radii before and after 11,000 whole-folding cycles based on a 10-mm folding radius using common-path optical interferometry, as shown in Table3. As Table3 shows, because the Young’s coefficient of monolayer graphene (1 TPa [51]) is much greater than that of PET, the phase retardations of monolayer-graphene-coated PETs under graphene-film inner curving conditions were greater than those under outer curving conditions, which caused the monolayer graphene film to bend, thereby reducing the bending compressive stress [50]. In addition, the phase retardations of monolayer graphene on the first PET sample (graphene/PET 1) under the inner curving conditions before 11,000 whole-folding cycles based on a 10-mm folding radius were obviously greater than those after the test by approximately 1 rad. The results suggest that there was a residual compressive stress in graphene/PET 1 that was the direct result of the manufacturing process parameters. For monolayer graphene on the second sample (graphene/PET 2), the phase retardations were not significantly different under the same conditions before and after the 11,000 whole-folding-cycle test. Nevertheless, the phase retardations of graphene/PET 2 under outer and inner curving conditions of the 10-mm radius after the 11,000 whole-folding- cycle test were smaller than those before by approximately 1.5 rad. The results suggest that a compressive residual stress in the graphene/PET 2 was induced under the same conditions of the 10-mm radius before the 11,000 whole-folding-cycle test. After the 11,000 whole-folding-cycle test, a compressive residual stress in the graphene/PET 2 was released. The results suggest that the curved-mechanical characteristics of the graphene/PET 2 were better than graphene/PET 1, due to the manufacturing process parameters causing less residual stress in the monolayer graphene film-coated PET.

Table 3. Phase retardation measurements of monolayer graphene 1 and 2 films coated on 40 × 40 mm 250-µm-thick PET substrates under curving conditions of 50-, 30-, 20-, and 10-mm radii before and after 11,000 whole-folding cycles based on a 10-mm folding radius using common-path optical interferometry.

Before 11,000 Whole-Folding Cycles After 11,000 Whole-Folding Cycles Radius (mm) Folding Condition Graphene/PET 1 Graphene/PET 2 Graphene/PET 1 Graphene/PET 2 (Rad) (Rad) (Rad) (Rad) Outer −4.02 −4.51 −3.81 −4.41 50 Inner −1.97 −4.12 −2.95 −4.28 Outer −3.58 −4.48 −3.75 −4.34 30 Inner −2.28 −4.37 −3.41 −4.30 Outer −3.75 −4.50 −3.74 −4.43 20 Inner −2.15 −4.38 −3.24 −4.41 Outer −3.84 −2.37 −3.96 −3.91 10 Inner −2.76 −2.25 −3.60 −3.90

5. Conclusions This study presented a method for measuring the curved-mechanical characteristics of transparent conductive film-coated polymer substrates using common-path optical interferometry. To analyze the mechanical characteristics of the transparent conductive film-coated PET substrates under curving, this study utilized an ASTP for a whole-folding test. The study then investigated and analyzed 80-, 160-, and 230-nm ITO films coated on 40 × 40 mm 125-µm-thick PET substrates and monolayer graphene films coated on 40 × 40 mm 250-µm-thick PET substrates under the curving conditions of 50-, 30-, 20-, and 10-mm radii before and after an 11,000 whole-folding cycle test based on a 10-mm folding radius. Experimental results revealed that the curved-mechanical characteristics of the 160- and 230-nm ITO-coated PET were the worst and best of the three ITO film-coated PETs, due to the manufacturing process parameters causing residual stress in the ITO film-coated Coatings 2021, 11, 766 11 of 13

PET. In addition, residual compressive stresses were observed in the graphene/PET 1 and PET 2 samples. The results suggest that the curved-mechanical characteristics of the graphene/PET 2 were better than graphene/PET 1, due to the manufacturing process parameters causing less residual stress in the monolayer graphene film-coated PET. The results of the curved-mechanical characteristic measurements of flexible transparent con- ductive substrates can provide designers with improved product development or assist manufacturers in improving the manufacturing design of enhanced coating processes.

Author Contributions: Methodology, B.-J.W.; software, B.-J.W. and J.-J.H.; validation, B.-J.W. and J.-J.H.; formal analysis, B.-J.W. and J.-J.H.; investigation, B.-J.W.; resources, B.-J.W.; data curation, B.-J.W.; writing—original draft preparation, B.-J.W.; writing—review and editing, B.-J.W.; visual- ization, B.-J.W.; supervision, B.-J.W. All authors have read and agreed to the published version of the manuscript. Funding: The authors wish to thank National Science Council of Taiwan for financial support (Grant No.: 108–2221-E−019–056). Conflicts of Interest: The authors declare no conflict of interest.

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