Wavelength Selective Antireflective Coatings on Plastics with

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Wavelength Selective Antireflective Coatings on Plastics with Hydrophobic Surface Sucheta De, Debrina Jana, Samar Kumar Medda, and Goutam De Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie400395c • Publication Date (Web): 19 May 2013 Downloaded from http://pubs.acs.org on May 21, 2013

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Industrial & Engineering Chemistry Research

Wavelength Selective Antireflective Coatings on Plastics with Hydrophobic Surface Sucheta De,† Debrina Jana, Samar Kumar Medda and Goutam De* Nano-Structured Materials Division, CSIR-Central Glass & Ceramic Research Institute, 196, Raja S. C. Mullick Road, Kolkata-700 032 (India). †Present address: Institute of Nano Science and Technology, Habitat Centre, Sector-64, Phase-X, Mohali-160062 (India)

KEYWORDS. Antireflective coatings, hydrophobic surface, tunable reflection wavelength, twolayer optical design, inorganic-organic hybrid.

*Corresponding Author. Email: [email protected]

1 ACS Paragon Plus Environment


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ABSTRACT: Wavelength selective antireflective (AR) coatings have been deposited on hardcoated CR-39® ophthalmic lenses and polycarbonate (PC) sheets following a two-layer (high and low index) quarter wavelength (λ⁄4) optical design. Covalently bonded SiO2–polyethyleneoxide (PEO)‒TiO2 and SiO2‒PEO inorganic-organic composite based coatings were used as high and low index coatings, respectively. 99% transmissions (reflection loss minimized to ~1%) have been achieved in the visible wavelength regions after deposition of such AR coatings on hardcoated CR-39®. The pre-determined reflection minima/transmission maxima can be tuned over the entire visible wavelength region by changing simply the physical/optical thickness of the two layers (λ⁄4 design) thereby generating different reflection colors. The AR coated CR-39® lenses showed a surface hardness of 3H (ASTM D3363). Applying a thin (≤50 nm) hydrophobic layer on top, the surface hardness value can be increased to 5H due to decrease of frictional coefficient. Deposition of such thin hydrophobic layer (λ⁄4 in the UV region) does not alter any AR property in the visible region. In case of PC the surface hardness value can be close to 4H after application of such hydrophobic coating. Coated CR-39® and PC substrates have passed all the standard tests related to the adhesion, abrasion, surface hardness and chemical endurance. These AR coatings with hard and hydrophobic surface (static water contact angle 102±3°) can find wide range of applications starting from visual comfort in optical related appliances to antireflective covers on solar cell with a modest self-cleanable property.


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1. INTRODUCTION Transparent optical grade plastics such as CR-39® (allyl diglycol carbonate; trademark of PPG, USA) and polycarbonate (PC) are replacing glasses in many sectors mainly because of their good optical quality, light weight and high impact resistance than glass. While CR-39® (thermoset polymer) possesses a relatively hard surface (pencil hardness ~3H; ASTM D 3363) but not unbreakable, the surface of PC (thermoplastic in nature) is extremely soft (pencil hardness ~HB) but it is unbreakable. So two different approaches were established to increase the surface hardness and durability of these scratch prone plastics for their practical uses.1-5 In case of CR39®, a SiO2–PEO (PEO stands for polyethylene oxide) based formulation with coating thickness ~2 µm was found to be suitable to achieve a surface hardness of 6H or higher.1,4 As PC is very soft, a relatively thicker (~5 µm) inorganic-organic (SiO2–PEO–PMMA) based hard-coatings containing boehmite nanoparticles embedded and bonded with the organic polymers were needed to obtain a surface hardness of ∼4H.5 It is noteworthy here that for better viewing purposes, development of antireflective (AR) coatings on these hard-coated surfaces are essential. AR coatings can effectively enhance the transmittance of light through an optical surface to obtain clearer and brighter views of images and to avoid the formation of ‘ghost pictures’ originating from reflection. Applications include plastic eyeglass lenses as well as optical lenses for sensors inside cars, in solar cells, photographic equipment of mobile phones and an increasing number of shields and covers to protect screens and displays. The formation of AR coatings on plastic substrates faces many problems. One challenge to develop AR coatings on organic substrates like plastic is that the coating must be developed at temperatures that will not melt or deform the structure. Current literature encompasses a wide array of pertinent studies for reducing the reflection losses of various substrates including plastics.6-32 The available global technologies in



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this area based on CVD,6 PECVD,7,8 plasma polymerization,9 PVD,10 sputtering,8,11,12 and Ebeam evaporation13 are not only expensive but also the coatings are not durable and scratch resistance enough. The poor adhesion between coating material and plastics is the main constraint of these technologies. Extensive work has also been done to develop AR coatings on glass, plastic and other related substrates using different chemical routes 14-18 including sol-gel.1932

Some of the reported sol-gel based19,25,27-32 processes can be applicable for the deposition of

AR coatings on plastic substrates. Among these, the molecular level inorganic-organic hybrid based technologies28-32 are found to be most suitable because the inorganic component provides hardness and the organic counterpart helps in adhering the plastics. However, issues like stability and reproducibility of coating sols, poor abrasion and adhesion resistant properties of the AR coatings and curing conditions (temperature/UV) suitable for plastics are still very much in concern.37 A very few low temperature sol-gel based techniques have been reported for plastic substrates at low cost.25,27,31 In this work AR coatings have been developed on the hard-coated plastics1-5 using a two-layer quarter wavelength (λ/4) design. Layer thicknesses are chosen to produce destructive interference in the light beams reflected from the interfaces, and for such destructive interference, layer thickness should be one-quarter of the wavelength of the light (λ/4). Here, we describe a simple sol-gel derived inorganic-organic hybrid nanocomposite approach for developing AR coatings on hard-coated CR-39® and PC substrates by dip-coating method. Our earlier report showed that the hard-coated CR-39® and PC have about 88−92% transmission (reflection loss ∼12−8%) in the visible wavelength region.1,4,5 So AR coatings have been developed to reduce the reflection loss. The two layers used for this purpose are made of high refractive index (HRI) and low refractive index (LRI) matrices based on silica-polyethyleneoxide-titania (SiO2−PEO−TiO2) and


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silica−polyethyleneoxide (SiO2−PEO) polymeric organic-inorganic networks, respectively. The combined effects of these high and low index coatings generate the AR property. Using this AR coating the reflection loss could be minimized to ~0.5% and 1.5−2% per surface for CR-39® ophthalmic lenses and PC sheets, respectively in the pre-determined wavelength region. The predetermined minimum reflection wavelength can be tuned by changing the optical thickness of layers using the same pair of HRI and LRI sols. Further, application of a very thin hydrophobic layer increases surface hardness (4-5H), durability with modest self-cleaning property without affecting the AR design.

2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals were used as received. Tetraethyl orthosilicate (TEOS) and (3glycidoxypropyl)trimethoxysilane (GLYMO), titanium tetraisopropoxide (TTIP), were supplied by Sigma-Aldrich. Acetylacetone (acac), n-butanol, methanol and HNO3 were obtained from s.d. fine−Chem Ltd. Aluminum acetylacetonate (Al(acac)3) was supplied by Lancaster. HCl was obtained from Rankem fine chemicals Ltd. Mili-Q (Millipore) water (18.2 MΩ) was used throughout the study. 2.2. Preparation of SiO2–PEO based LRI sol. The LRI sol was prepared following the procedure reported by us with some modifications,33 and this LRI sol is found to be suitable for both the hard-coated CR-39® and PC (Lexan® grade) substrates. Briefly, the sol was prepared using TEOS, GLYMO, n-butanol, water, methanol, HNO3, and Al(acac)3. The last two chemicals were used in catalytic amounts for alkoxide hydrolysis and epoxy polymerization initiator,1,34 respectively. The ratio of mole fraction of alkoxides TEOS and GLYMO was 7:3 and sol had 15.1 wt % equivalent SiO2. The sol was diluted with n-butanol−acac mixture (0.33 mol acac/mol



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of Si) to 1.7 wt% equivalent SiO2. This diluted sol was filtered through Millipore 0.5 µm filter paper and kept in refrigerator (∼4 °C) for 24 h for aging. The sol provides LRI coatings of SiO2PEO composites after curing with the composition of LRI film SiO2:PEO = 76.2:23.8. Viscosity of the as prepared LRI sol was 3.0±1 cps. The sol remains stable for at least 12 months at 4±1° C. 2.3. Preparation of SiO2–TiO2–PEO based HRI sol. The HRI sol was prepared using TEOS, GLYMO, TTIP (high index component), acac, n-butanol, water, HNO3 and Al(acac)3. Catalytic amounts of last two chemicals were used for the same reason as in case of LRI sol, and acetylacetone (acac) was used to reduce the hydrolysis rate of Ti-alkoxide. The ratio of mole fractions of alkoxides TEOS, GLYMO and TTIP is 1:3:6. To prepare this sol, first, 9.22 mmol of TEOS, 55.3 mmol of TTIP and 210 mmol of n-butanol were mixed with stirring and stirring was continued for 15 min. This alkoxide mixture was then refluxed for ~2 h and 45 min and then the solution was cooled at room temperature. To this, a mixture of acac (0.5 mol per mol of Tialkoxide) and n-butanol was added and stirred for 1 h and the solution was kept overnight at room temperature. Then 27.7 mmol of GLYMO was added to the above solution and stirred for 2 h. The hydrolysis/condensation reactions of this mixed alkoxide solution was then carried out with acid (5 x 10-4 mol per mol of alkoxy group), water (0.5 mol per mol of alkoxy group) and nbutanol and stirred for 2 h. Thereafter the epoxy polymerizing agent Al (acac)3 (0.05 mol per mol of GLYMO) was added at this stage with stirring until it dissolved. The equivalent oxide (SiO2+TiO2) wt% in the sol was about 8%. This concentrated sol was then diluted with n-butanol in such a way that the equivalent oxide wt% became ~2%. Sol so obtained was filtered through Millipore filter paper (0.5 µm). Finally the filtered sol was left 24 h in refrigerator (∼4 °C) for aging. This aged sol could be stored at ambient condition or refrigerator. The equivalent mol% of SiO2, TiO2 and PEO in HRI film is 28.57: 47.62: 23.81. It is to be noted here that the amount of


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TTIP can be increased to get further high index coatings. However, good quality coating was obtained using up to the above combinations. Viscosity of the as prepared HRI sol was 3.0±1 cps. The sol remains stable for at least 12 months at 4±1° C. 2.4. Preparation of hydrophobic coating sol. The hydrophobic sol was prepared by mixing about 5 wt% solution of dichlorodimethyl silane (CDMS) and chlorotrimethyl silane (CTMS) (weight ratio of CDMS : CTMS=3:1) in n-hexane (solvent). As for example, 6.14 mmol of CDMS and 2.44 mmol of CTMS were mixed with 232 mmol of n-hexane and stirred for 1 h and the solution so obtained was ready for coating deposition on the AR coated surface. 2.5. Preparation of AR coatings. We have used hard-coated CR-39® and PC substrates for the deposition of AR coatings. SiO2–PEO and SiO2–PEO–PMMA–boehmite based hard-coatings were diposited on CR-39® and PC substrates, respectively. The details procedures for such inorganic-organic nanocomposite based hard-coating deposition techniques are reported in references 1 and 5, respectively. Thickness of such hard-coatings employed for CR-39® and PC substrates were ~2.0 µm and ~5.0 µm, respectively. These hard-coated lenses and sheets can be stored in ambient conditions. Prior to AR coating deposition, hard-coated CR-39® lenses and PC substrates (8×5 cm2)1,5 were cleaned with dust-free tissue paper soaked with isopropanol to remove the dust particles if any. AR coating was made by depositing successive HRI and LRI layers by dip-coating procedure (Dip-master 200, Chemat Corporation) following a two-layer λ/4 optical design on hard-coated CR-39® and PC substrates. In this procedure both side of the substrate can be coated. A schematic diagram showing the layers of AR coating assembly is shown in Figure 1. Hard-coated and cleaned CR-39® lenses/PC sheets were first immersed into the HRI sol and withdrawn maintaining velocities in the range of 2–22 cm min–1 from the sol perpendicular to the solution surface to obtain coatings of desired thickness values (after curing) required for various pre-determined wavelengths where minimum reflection has to be given (see 7 ACS Paragon Plus Environment


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the thickness versus withdrawal velocity calibration procedure given in Supporting Information with calibration curves Figure S1a,b). The coatings were dried at 60 °C for 60 min and finally cured at 85 °C for 1 h in an air oven. The HRI coated CR-39® lenses/PC sheets were then immersed into the LRI coating sol and withdrawn in a similar way by maintaining the withdrawal speeds as per the respective calibration curve (Figure S1b; Supporting Information). The coating assembly was dried at 60 °C for 60 min and followed by cured at 85 °C for 30 min in an air oven. Finally the lenses/sheets were UV cured (both surface) using a conveyorized UV curing machine. The conveyor speed was controlled so that the each coated surface receives energy corresponding to UV-A (2.7 J cm−2) and UV-C (0.30 J cm−2). Similar type of coatings on intrinsic (both side polished and also one side polished) silicon wafers were deposited for the IR spectral studies and RI/thickness evaluation by ellipsometry, respectively. 2.6. Preparation of hydrophobic coating. For hydrophobic coating deposition, the cured AR coated plastic lenses and sheets were dipped into the hydrophobic sol and kept inside the sol for about 60 s and then lifted out of sol with a withdrawal speed of 15 cm min−1. After deposition of hydrophobic coatings the samples were cured at 60 °C for 30 min followed by curing at 80 °C for 30 min. Thickness of the hydrophobic layer so obtained was ∼50 nm. 2.7. Characterization of the films. RI (n) and thickness of the HRI and LRI films deposited on Si-wafer were measured using spectroscopic ellipsometer, J. A. Woollam Co., Inc., USA. As the films are dielectric and transparent in the spectral region of interest the Cauchy dispersion model was chosen for the evaluation of n values. UV curing of the high and low index coating assemblies was done using a conveyorised UV curing machine fitted with a high power (5000 W) medium pressure Hg vapor lamp emitting a range of wavelength 200−400 nm with maximum intensity peaks in the UV-A region (300−380 nm). The UV-vis spectra of the AR coated films at


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different steps deposited on CR-39® and PC sheets were obtained using a Cary 50 scan spectrophotometer. Infrared absorption spectra of the film deposited on both side polished Siwafers were recorded by FTIR spectrometry (Nicolet, 380) with a resolution of 4 cm−1 and 200 scans. Transmission SAXS of sols were measured using a Rigaku SmatLab X-ray diffractometer operating at 9 kW with Cu Kα radiation and the results were analyzed using Rigaku’s Nanosolver software. For SAXS measurements sols were taken in a sealed silica glass capillary of diameter 1 mm and analyzed. Transmission electron microscopic (TEM) measurements were carried out with a JEOL 2010 transmission electron microscope. During the UV curing process the UV energy was monitored using a UV intensity meter (UV Power Puck, EIT), passing through the conveyor along with the sample. To evaluate the mechanical strength and chemical resistant of the coatings different tests were carried out, viz., cross cut and adhesive tape test following DIN 53151 or ASTM D 3359, abrasion test using a lens coating pencil hardness tester following ASTM D 3363, thermal test, boiling salt water test, 50 h in physiological NaCl solution, haze and clarity test. Haze and clarity (following ASTM 1003 specifications) of the uncoated and AR coated PC and CR-39 lenses were measured before and after abrasion (following the specification MIL-F-12397B). Static and dynamic water contact angles were measured using a KRUSS GmbH instrument (easy drop DSA20E model) in ambient condition. Static contact angle was measured using 6 µL water drop whereas advancing and receding contact angles were studied by increasing and withdrawing the volume of water from 1.5 to 11 µL and vice versa.



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3. RESULTS AND DISCUSSION 3.1. Principle for reducing the reflection using quarter wavelength optical design. The most important parameter in AR coating design is the RI of the film, which is related to the physical thickness. In the case of a one-layer coating, two criteria should be met: a film thickness of λ/4 and a RI intermediate between those of air and the substrate. For complete zero-reflection, the RI of the AR coating layer should have the geometric mean value (nons)1/2 of the refractive indices of air (n0) and the substrate (ns).35 Typical glass and most of the optical plastic substrates have an index of refraction mostly in between 1.45 and 1.65 in visible spectral region, which implies that the RI of the AR interference film must be between 1.20 and 1.25.36 Such a low index requirement makes it practically impossible to design a dense single-layer AR film. Further, the low RI is difficult to attain with any known low index coating material (the lowest index optical material is MgF2 with n = 1.38 at 600 nm).37 In this work, AR coating has been developed on hard-coated plastic (CR-39® and PC) substrates using a two-layer (high and low index) λ/4 optical design according to the Fresnel equation.38 This can be noted here that we are using the hard-coated CR-39® and PC for the deposition of AR-coatings. As these coatings are >2 µm in thickness, their RI values (1.489 and 1.522 in cases of CR-39® and PC, respectively) are considered as the RI of the substrates. If λ is the chosen wavelength, where maximum transmission or minimum reflection is required, then λ/4 (quarter wavelength) would be the optical thickness; and physical thickness of the coating can be obtained by dividing optical thickness by the RI (n) of the coating materials (to be deposited using high and low index sols). According to the Fresnel equation,38 If, n1= refractive index of the low index layer (top layer) n2 = refractive index of the high index layer (bottom layer)


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ns = refractive index of the substrate n0 = refractive index of the medium i.e. air (=1) Then, the reflection at quarter wavelength (λ/4) is given by: 2

Rλ/4 =

n12 ns - n 22 n0 n1 2 ns+ n2 2 n0

…….. (1)

For zero reflection condition, the value of Rλ/4 = 0 and from there we have, n2 = n1√ns. In our case, ns = 1.489 or 1.522 (refractive index of the hard coating material on CR-39® and PC substrate); n1 = 1.489 = refractive index of the low index layer. Hence, the calculated value of n2 = 1.816 and 1.837 for CR-39 and PC substrates, respectively. It is to be noted here that all the RI values have been reported at 500 nm. 3.2. Concepts of Sols and Films. Two different sols namely LRI and HRI sols were used to develop such AR coatings. The LRI sol is the same sol which was used to prepare hard-coating on CR-39 lenses.1,4 We have diluted this sol to obtain the proper thickness (see section 2.2). This LRI sol was prepared by hydrolyzing GLYMO-TEOS mixture in acidic condition to facilitate generation of silica network and in situ SiO2 nanoparticles in the sol.1 These in situ generated SiO2 nanoparticles remained covalently attached with the silica network. In this sol catalytic amount of Al(acac)3 was added and coatings were prepared. Al(acac)3 facilitated the polymerization of GLYMO originated epoxy groups to form an inorganic-organic cross-linked hybrid nanocomposites. This sol after diluted with n-butanol–acetylacetone mixture was used to prepare PEO–SiO2 based LRI coatings with a RI value of ~1.489 after curing. The transmission SAXS study (see experimental for details; section 2.7) of the LRI sol shows existence of SiO2 nanoparticles of average size 4.6 nm with a dispersion of 41% (Figure S2; Supporting Information). This value is quite close to the size of SiO2 nanoparticles estimated by reflection



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SAXS studies of hard-coatings deposited on CR-39.1 In the HRI sol we have introduced the high index component TiO2 to increase the RI. The preparation procedures are almost similar to that of LRI sols (see section 2.3). As expected, using this HRI sol a SiO2–TiO2–PEO based coatings with relatively high RI (1.763) value can be prepared after curing. The formation of such networks has been discussed in the next section. 3.3. FTIR studies. A detailed FTIR study has been undertaken of both the HRI and LRI coated samples. Figure 3a shows the FTIR spectra of LRI coatings at different stages, viz. as-prepared, cured at 60 and 85 °C and UV-cured. As-prepared LRI film shows peaks corresponding to Si−C covalent linkage at ~1200 cm−1, Si−O−Si (asymmetric stretch) overlapping with Si−O−C at around 1090 cm−1,2,3 Si−OH at 944 cm−1,2,3 epoxide ring vibrations at 910 and 855 cm−1,39,40 Si−O−Si (symmetric stretch) at 795 and 444 cm−1.2,3 In case of dried (60 and 85 °C) and UVtreated film, all the bands remain unaffected except the epoxide bands which gradually weakened and almost disappeared after curing at 85 °C due to polymerization of epoxy groups to poly(ethylene oxide). This indicates that SiO2−PEO network formation occurs during the thermal curing steps and no further densification of the coating was observed after UV curing. As a result, the RI value does not increase after UV curing. There is a basic difference between HRI and LRI coating matrices from the view point of RI values before and after UV curing. Unlike LRI, the HRI coating shows a substantial increase of RI value and decrease of film thickness to ~30−45%. This can be explained from the FTIR spectral evolution. Figure 3b shows the FTIR spectra of HRI coating at different processing stages, viz. as-prepared, cured at 60 and 85 °C, and UV-cured. The as-prepared HRI film showed a sharp pair of peaks at 1587 and 1528 cm−1 due to C−C+C−O stretching arising from Ti-acac chelate (marked as 1 and 2 in Figure 3b).41,42 The presence of bands in the 1370−1280 cm−1


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region (marked by 3 and 4)43 are attributed to the Ti−O−C vibrations and these vibrations also support that acac is bonded to Ti as chelating ligand.41-43 All these bands gradually weakened during thermal curing (up to 85 °C) and almost disappeared after UV treatment (as shown in Figure 3b). This suggests that the thickness reduction of the film was the result of elimination of the chelating ligands (acac) leading to further densification of the film matrix. As a result, RI value of the coating was increased. The Si−C near 1200 cm−1 (marked by 5), Si−O−Si (asymmetric stretch) overlapping with Si−O−C at 1093 and 1036 cm−1 (marked by 6 and 7), Si−OH along with Si−O−Ti vibrations near 932 cm−1 (marked as 8),44 epoxide ring vibrations at 904 and 855 cm−1 (shoulder) (marked by 9 and 10),39,40 and Si−O−Si vibration at 795 and 455 cm−1 peaks2,3(marked by 11 and 12) were also observed up to the drying stage (60 °C). The epoxide bands were however disappeared after thermal (85° C)/UV treatment due to polymerization of epoxy groups to PEO. The gradual weakening of Si−O−Si band (overlapping with Si−O−C) and strengthening of Si−O−Ti (marked by 8) after thermal treatment followed by UV curing at 85 °C followed by UV-treatment is expected (Figure 3b) due to the formation of more Si−O−Ti linkages. 3.4. Refractive index and Thickness calibration of the LRI and HRI films. The RI value of LRI coating remains same i.e. 1.489 before and after UV-treatment whereas RI value of HRI coating changes from ~1.63 to ~1.763 (reported at 500 nm) after UV curing. Figure 2 shows the RI values of the UV cured LRI and HRI coatings in the visible wavelength region. As we have mentioned accurate measurement of physical thickness (henceforth described as “thickness”) value of each layer is very important for fabricating AR coating, the optical and physical thickness values for obtaining minimum reflections at various pre-determined wavelengths (viz. 425, 500, 550, 600, 650, 700 and 750 nm) were calculated and given in Table 1. To obtain the



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desired physical thickness, during the dip-coating of the substrates from the LRI and HRI sols, withdrawal speeds were controlled. The procedure for estimating the required withdrawal speed from the calibration curves has been discussed in the Supporting Information and Figure S1. We found a maximum variation of coating thickness of the order of ±3 nm prepared from different batches of sols; however, as expected the RI of the coatings remains almost unaltered. 3.5. Fabrication of AR coatings and their optical properties. The AR coatings have been deposited on previously hard-coated CR-39 and PC substrates. Figures 4 and 5 show the representative optical transmission spectra of the hard-coated CR-39 and PC substrates, respectively, at different stages of AR coating deposition. The % transmission value for uncoated CR-39 lens is ~92%. As the RI of hard coating (RI=1.489) is slightly less than that of uncoated CR-39 lens (RI=1.498) a very slight increase in transmission has been observed after deposition of hard-coatings. Deposition of HRI layer on hard-coating decreases the transmission value to ~4−6% due to reflection loss. When LRI coating is deposited on to the HRI layer, the optical transmission increases up to ~5−7% (i.e. transmission increases to 97−99%) compared to the uncoated lens. It may be noted here that the slight ups and downs in the spectra have been observed due to the interference arising from the thick hard-coatings and difference in refractive index values of the deposited layers. The % transmission value for uncoated PC sheet is ~88%. Hard-coated PC shows ~2% more transmittance than uncoated PC substrate (RI=1.589) because of lower RI (1.522 at 500 nm)5,37 value of hard-coatings. Deposition of HRI layer on hard-coated PC decreases the transmission value ~4−6% (Figure 5). After deposition of LRI layer onto the HRI (i.e. fabrication of AR coating), the % transmission of PC substrate increases to ~8% compared to the uncoated PC. The optical interference is supposed to be more in case of PC substrates due to the larger thickness of


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the hard-coating (~5 µm) and relatively larger difference between the RI values of PC substrate and other layers. It can be noted here that the calculated HRI values for the high index layer (n2) are 1.816 and 1.837 for the CR-39 and PC substrates, respectively for obtaining 0 reflections at 500 nm (see section 3.1). However, experimentally we could achieve a maximum n2 value of 1.763 at 500 nm. For this reason, it is not possible to achieve ~100% transmission or ~0% reflection. 3.6. Wavelength selective AR coatings. The transmittance maxima or the reflection minima of the AR coatings can be tuned over the entire range of visible region. Figure 6a shows the photos of AR coated CR-39 lenses with maximum transmittance or minimum reflectance at around 425, 500, 550, 600, 650, 700 and 750 nm, respectively. The AR coatings were fabricated using HRI/LRI layers of physical thickness values required for minimum reflection at these desired wavelengths (see Table 1). The photos of CR-39® lenses were taken keeping the lenses under the fluorescent tube light and the reflecting images of tube light are observed to be different in color (viz. orange-yellow, magenta, violet, bluish violet, blue, bluish green and greenish). Figures 6b and c show the corresponding optical reflectance and transmittance spectra of the lenses, respectively as mentioned above. Figure 6b shows that a major amount of reflection has been minimized to ~0.5% per surface (i.e. ~1%; considering both the surfaces) at the desired wavelength by the deposition of such AR coatings on hard-coated CR-39® lenses. Furthermore, the reflection minima can be tuned at different wavelengths, and as a consequence different reflecting colors can be generated. Because of the applications of AR coatings in pre-determined minimum reflection wavelengths, the combined effect of reflected lights of other wavelengths makes such a beautiful systematic color variations. As for example, when a minimum reflection wavelength is ‘500 nm’, the reflection light from this region (see Figure 6b) will be minimum,



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but reflected lights from other wavelengths would cause a coloring effect of magenta. AR coated PC sheets also showed similar (Figure S3, Supporting Information) optical features like CR-39 lenses. Now considering at a specific wavelength 550 nm, if we solve the eq (1) using the experimental ns (1.483 for CR-39®), n1 (1.483) and n2 (1.751) values (these RI values are taken from Figure 2), it gives a minimum reflection (Rλ/4 at 550 nm) per surface ~0.1% which is quite close to the experimentally observed reflection value (~0.5%; Figure 6b) in case of CR-39® lens. However, the same calculation considering the ns value (1.519) of PC at 550 nm gives minimum reflection value per surface close to ~0.45% which is however deviates from the experimental value (~1.5–2%; see Figure S3d; Supporting Information). This could be associated with the absorption of the hard-coating material in the ~400–550 nm regions (see the %T difference between the uncoated and hard-coated PC in Figure 5). It can also be pointed out here that although the entire process is reproducible, the calculated wavelengths for which coatings were deposited, and the experimentally obtained wavelengths (at which minimum reflections were obtained) are differed by ±20 nm in both the cases i.e. CR-39® and PC. This difference is expected to be due to the variation of experimentally given HRI/LRI thicknesses compared to the calculated values. From Figure S1 (calibration curves) it could be observed that the thickness data points have variations in the range of ±2–3 nm. Calculation shows a thickness variation of film 1 nm can shifts the AR position ∼7 nm. It may also be noted here that we have used Si wafer to measure the thickness values to obtain the lifting speed/thickness calibration curves which have been used for the hard-coating surfaces. As Si wafer has high RI, it helps in measuring the RI values of the deposited HRI/LRI layers very accurately. If glass or plastics are used, RI of such HRI/LRI layers could not be measured accurately because of comparable RI values of the substrates and deposited layers. However, difference in surface characteristics of Si wafer and the


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inorganic-organic hard-coatings (where AR layers were deposited), some variation of coating thickness is also expected. All these limitations are responsible for difference (±20 nm) in the experimental and theoretical (pre-determined) minimum reflection values. The overall hardness of these AR coating surfaces were measured following the ASTM D 3363 specifications and found to be of the order of 3H. 3.7. TEM study. To characterize the HRI and LRI coating materials, TEM studies were performed. Bright field TEM images of the HRI and LRI layers with two different magnifications are shown in Figure S4 (Supporting Information) along with the corresponding electron diffraction patterns. From TEM and electron diffraction studies it was observed that both the HRI and LRI coatings are amorphous in nature. 3.8. RI and thickness values along the depth of AR coating assembly. To understand the RI profile of the layers, a similar type of AR coating assembly (i.e. hard coating used for CR-39®1 followed by HRI/LRI layers) have been deposited on one side polished silicon wafer and analyzed using a spectroscopic ellipsometer. Figure 7 shows the change of refractive index values (RI) along the thickness (t) of AR coating designed for 550 nm. In this study the ellipsometric data was recorded at the wavelength of 500 nm. It clearly shows that RI remains constant at 1.489 up to the thickness of hard coating (~1.94 µm), then sharply increases to 1.75 for HRI layer (thickness 81 nm) and then decreases down to 1.489 due to the LRI layer (thickness 88 nm). This depth profile data clearly shows the exact AR geometry. The thickness values obtained from this depth profile are close to the physical thickness values (see Table 1, LRI=92.5 nm and HRI=78 nm) given for fabricating AR effect at 550 nm. The study indicates that practically there is no mixing between two layers at the interface. 3.9. Properties of AR coated CR-39® lenses and PC sheets. For practical applications, these AR coatings on plastics must meet some important criteria such as optical transparency, good 17 ACS Paragon Plus Environment


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adhesion and abrasion resistivity, hardness and resistance against chemical attack. AR coated lenses and sheets are totally crack-free and homogenous when observed against reflection. Several standard tests1,4 were performed on such AR coatings and these tests gave satisfactory results. The AR coated CR-39® lenses were subjected to the adhesion test following ASTM and DIN procedures (cross-cut adhesive tape test), and after the test the coated surface was examined under optical microscope using 100× magnification. It showed no peeling off of the coatings from the substrate and can be classified as ASTM 5B (highest standard). These lenses could resist more than 20 abrasion cycles following the U.S. federal specifications without any noticeable damage. No crazing or damage of the coating was observed after 2 times boiling in salt solution test, indicating its compatibility with the substrate and chemical inertness as well as thermal stability. The coatings remained unaffected after being immersed in physiological NaCl solution for 50 h at room temperature. The coatings were kept at 70 °C for 6 h in an air oven and no crazing or damage was observed. Haze and clarity of the uncoated and AR coated PC sheets and CR-39® lenses were measured before and after abrasion (following specification MIL-F12397B). It has been observed that the haze and clarity values of the AR coated substrates are excellent and remain low even after abrasions (Table 2). 3.10. Deposition of hydrophobic layer to improve the surface hardness. As mentioned previously the overall hardness of AR coating is of the order of 3H; however, after deposition of a thin hydrophobic layer (thickness ~50 nm; refractive index ~1.475) using the hydrophobic sol followed by thermal curing (see section 2.6) the overall hardness value increases to 5H (in case of CR-39® lenses) with hydrophobic surface (static water contact angle, θ varied in the range of 99−105°). Similar results were also obtained in case of PC substrates; however in this case the hardness value could be improved to ~4H. Considering low thickness (~50 nm) and refractive index (~1.475) values of such hydrophobic layer optical effect is expected in the UV region (near 18 ACS Paragon Plus Environment

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295 nm). However, there is no possibility of distortion of AR quality in the visible wavelength region. For this reason the reflection colors and optical spectra of the AR coating assemblies remain similar as shown in Figure 6. Increase of surface hardness after deposition of hydrophobic top layer is due to the decrease of frictional coefficient which in turn increases the hardness of the film.45 To understand the hydrophobic characteristics of the AR and hydrophobic coated CR39® lenses the water contact angle hysteresis was measured.46,47 For this purpose the advancing and receding water contact angles were measured by increasing the volume of water from 1.5 to 11 µL and followed by withdrawing the volume of water in a dynamic mode. The rate of increasing or decreasing of water volumes was maintained at 0.08 µL s−1 As shown in Figure S5, we observe that the advancing water contact angle (θA) varies in the range of 106–103° whereas at the time of receding the contact angle (θR) decreases to 90–92°. So the result shows an overall water contact angle hysteresis (θA–θR) in the range of ~11–15°. As θR remains always >90° we can say the surface is hydrophobic with modest self-cleaning characteristics.

4. CONCLUSIONS Tunable AR coatings on previously hard-coated plastic ophthalmic lenses and PC sheets have been developed following a two-layer (high and low index) quarter wavelength coating design. High (SiO2-TiO2−PEO) and low (SiO2−PEO) index inorganic-organic hybrid sols have been used for the deposition of such AR coatings followed by a combined thermal (85 °C) and UV curing. The % reflection at a desired wavelength can be minimized to ~1% (~99% transmission) in case of CR-39® lens. In case of PC ~96–97% transmission can be achieved at desired wavelengths. By changing the two-layer optical AR design the reflection minima or transmission maxima can be tuned to the entire visible wavelength of light. The tunable reflection colors were originated from



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the umbrella shaped reflection curves with pre-determined optical design. The surface hardness of the AR coated CR-39® lens and PC sheets is found to be ∼3H; however, deposition of a thin hydrophobic coating as a top layer further increases the overall surface hardness value to ∼5H (CR-39®) and ~4H (PC) with hydrophobic surface without affecting the optical property of AR coating. This approach provides a productive route for the fabrication of durable wavelength selective antireflective coatings on plastic substrates with hydrophobic surface. These AR coatings with hydrophobic surface can find wide range of optical applications as well as a hard and self-cleanable antireflective covers on solar cells because more sun light can be entered through selective wavelength of light and the surface will remain clean from the dust.

ASSOCIATED CONTENT Supporting Information. Thickness versus withdrawal speed calibration procedure with figure (Figure S1), SAXS of LRI sol (Figure S2), Reflection and Transmission spectra (%) of AR coated PC sheets (Figure S3), TEM images of HRI and LRI coatings (Figure S4) and advancing and receding contact angle studies (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Tel: +91 33 23223403. Fax: 91 33 24730957; E-mail: [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS DST (Govt. of India) is thankfully acknowledged for providing funds. SD and DJ thank CSIR for providing fellowship.

REFERENCES (1) Medda, S. K.; De, G. Inorganic-Organic Nanocomposite Based Hard-Coatings on Plastics using in-situ Generated Nano SiO2 Bonded with ≡Si-O-Si−PEO Hybrid Network Ind. Eng. Chem. Res. 2009, 48, 4326; 6906. (2) Medda, S. K.; Kundu, D.; De, G. Inorganic-Organic Hybrid Coatings on Polycarbonate. Spectroscopic Studies on the Simultaneous Polymerization of Methacrylate and Silica Networks. J. Non-Cryst. Solids 2003, 318, 149. (3) De, G.; Kundu, D. Gold-Nanocluster-Doped Inorganic-Organic Hybrid coatings on Polycarbonate and Isolation of Shaped Gold Microcrystals from the Coating Sol. Chem. Mater. 2001, 13, 4239. (4) De, G.; Medda, S. K. A Process of Making Thermally Curable Inorganic-Organic Hybrid Coating Sol Providing Anti-Scratch Coatings on Plastics. Indian Patent 196846, 2003. (5) De, G.; Kundu, D.; Medda, S. K. A Process of Manufacturing Inorganic-Organic Hybrid Sol and Useful as Scratch Resistant Polycarbonate Sheets and Lenses and other Related Plastics. Indian Patent 228274, 2003. (6) Kavakli, Y. G.; Kantarli, K. Single and Double-Layer Antireflection Coatings on Silicon. Turk. J. Phys. 2002, 26, 349. (7) Wang, Y.; Cheng, X.; Lin, Z.; Zhang, C.; Zhang, F. Optimization of PECVD Silicon Oxynitride Films for Anti-Reflection Coating. Vacuum 2004, 72, 345.



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(18) Huang, C. S.; Liu, H. Y. Method of Wet Coating for Applying Anti-Reflective Film to Substrate. U.S. Patent 7,507,437, 2009. (19) Schulz, U. Review of Modern Techniques to Generate Antireflective Properties on Thermoplastic Polymers. Appl. Opt. 2006, 45, 1608. (20) Langlet, M.; Burgos, M.; Coutier, C.; Jimenez, C.; Morant, C.; Manso, M. Low Temperature Preparation of High Refractive Index and Mechanically Resistant Sol-Gel TiO2 Films for Multilayer Antireflective Coating Applications. J. Sol-Gel Sci. Technol. 2001, 22, 139. (21) Vincent, A.; Babu, S.; Brinley, E.; Karakoti, A.; Deshpande, S.; Seal, S. Role of Catalyst on Refractive Index Tunability of Porous Silica Antireflective Coatings by Sol-Gel Technique. J. Phys. Chem. C 2007, 111, 8291. (22) Kim, S.; Cho, J.; Char, K. Thermally Stable Antireflective Coatings Based on Nanoporous Organosilicate Thin Films. Langmuir 2007, 23, 6737. (23) Zhang, X.T.; Sato, O.; Taguchi, M.; Einaga, Y.; Murakami, T.; Fujishima, A. Selfcleaning Particle Coating with Antireflection Properties. Chem. Mater. 2005, 17, 696. (24) Lee, D.; Rubner, M. F.; Cohen, R. E. All Nanoparticle Thin Film Coatings. Nano Lett. 2006, 6, 2305. (25) Chen, D.; Yan, Y.; Westenberg, E.; Niebauer, D.; Sakaitani, N.; Ray Chaudhuri, S.; Takamatsu, M. Development of Anti-Reflection (AR) Coating on Plastic Panels for Display Applications. J. Sol-Gel Sci. Technol. 2000, 19, 77. (26) Antonello, A., Brusatin, G.; Guglielmi, M.; Bello, V.; Mattei, G.; Zacco, G.; Martucci, A. Nanocomposites of Titania and Hybrid Matrix with High Refractive Index. J. Nanopart. Res. 2011, 13, 1697. (27) Li, X.; He, J. In situ Assembly of Raspberry and Mulberry-like Silica Nanospheres toward Antireflective and Antifogging Coatings. ACS Appl. Mat. Interfaces 2012, 4, 2204.



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(28) Zhang, R.; Wu, X.; Yang, J. M. Inorganic-Organic Hybrid Nanocomposite Antiglare and Antireflection Coatings. U.S. Patent 20090004462 A1, 2009. (29) Sager, B. M.; Sheats, J. R. Anti-reflective coatings. U.S. Patent. 20110019277 A1, 2011. (30) Li, H.; Wang, A. E.; Das, S. K. Coating Composition, Process for Producing Antireflective Coatings, and Coated Articles. U. S. Patent 5,580,819, 1996. (31) Park, S. S.; Zheng, H. Composition and Method for a Coating Providing Anti-Reflective and Anti-Static Properties. U. S. Patent 6,638,630, 2003. (32) Zhang, R.; Wu, X.; Yang, A. J. M. Inorganic-Organic Hybrid Nanocomposite Antiglare and Antireflection Coatings. Int. Pub. WO/2006/093748, 2006. (33) De, G.; De, S.; Medda, S. K. A Process of Making Inorganic-Organic Hybrid Sols for the Deposition of Antireflective (AR) Coatings on Plastic Substrates Indian Patent Application No. 1898/Del/2009, 2009. (34) Zhang, Z.; Sakka, S. Hydrolysis and Polymerization of Dimethyldiethoxysilane, Methyltrimethoxysilane and Tetramethoxysilane in Presence of Aluminum Acetylacetonate. A complex Catalyst for the Formation of Siloxanes. J. Sol-Gel Sci. Technol. 1999, 16, 209. (35) Macleod, H. A. Thin Film Optical Filters; Adam Hilger Ltd.: Bristol, U.K., 1986. (36) Yoldas, B. E.; Partlow, D. P. Wide Spectrum Antireflective Coating for Fused Silica and other Glasses. Appl. Opt. 1984, 23, 1418. (37) Thomas, I. M. Porous Fluoride Antireflective Coatings. Appl. Opt. 1988, 27, 3356. (38) Cox, J. T.; Hass, G. Physics of thin films; Hass, G.; Thun, R. E, Eds.; Academic press: New York, 1964; pp. 239-304. (39) Innocenzi, P.; Brusatin, G.; Guglielmi, M.; Bertani, R. New Synthetic Route to (3Glycidoxypropyl)trimethoxysilane-Based Hybrid Organic-Inorganic Materials Chem. Mater. 1999, 11, 1672.


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(40) Dean, K.; Cook, W. D.; Bruchill, P.; Zipper, M. Curing Behaviour of IPNs Formed from Model VERs and Epoxy Systems Part II. Imidazole-Cured Epoxy. Polymer 2001, 42, 3589. (41) Fang, Q.; Meier, M.; Yu, J. J.; Wang, Z. M.; Zhang, J. Y.; Wu, J. X.; Kenyon, A.; Hoffmann, P.; Boyd, I. W. FTIR and XPS Investigation of Er-Doped SiO2–TiO2 Films Mat. Sci. Engg. B 2003, 105, 209. (42) Medda, S. K.; De, S.; De, G. Synthesis of Au-Nanoparticle Doped SiO2–TiO2 Films: Tuning of Au-Surface Plasmon Band Position through Controlling the Refractive Index. J. Mater. Chem. 2005, 15, 3278. (43) Léaustic, A.; Babonneau, F.; Livage, J. Structural Investigation of the HydrolysisCondensation Process of Titanium Alkoxides Ti(OR)4 (OR=OPri, OEt) Modified by Acetylacetone. 1. Study of the Alkoxide Modification. Chem. Mater. 1989, 1, 240. (44) Que, W.; Zhou, Y.; Lam, Y. L.; Chan, Y. C.; Cheng, S. D.; Sun, Z.; Kam, C. H. Microstructural and Spectroscopic Studies of Sol–Gel Derived Silica–Titania Waveguides. J. Sol-Gel Sci. Technol. 2000, 18, 77. (45) Gao, F.; Kotvis, P. V.; Tysoe, W.T. The Frictional Properties of Thin Inorganic Halide Films on Iron Measured in Ultrahigh Vacuum. Tribol. Lett. 2003, 15, 327. (46) Gao, L.; McCarthy, T. J. Contact Angle Hysteresis Explained. Langmuir 2006, 22, 6234. (47) Choi, W.; Tuteja, A.; Mabry, J. M.; Cohen, R. E.; McKinley, G. H. A Modified Cassie– Baxter Relationship to Explain Contact Angle Hysteresis and Anisotropy on non-Wetting Textured Surfaces. J. Colloid Interface Sci. 2009, 339, 208.



LRI coating Two-layer HRI coating AR coatings Hard coating Substrate

Figure 1. Schematic representation of AR coating design on a hard-coated plastic substrate.

1.84 1.82 1.80 1.78

Refractive index

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 33

HRI coating

1.76 1.74 1.72 1.70 1.52 1.50

LRI coating

1.48 1.46 400

500

600

700

800

Wavelength (nm)

Figure 2. Refractive index values of HRI and LRI coatings with respect to wavelength of light after UV curing.


0.6

0.3

0.0 1800

1600

Si-OH

85 °C UV cured

1400

1200

1000

Si-O-Si

Si-C As-prepared 60 °C

0.9

Si-O-C, Si-O-Si

Absorbance

1.2

Si-O-Si

(a)

1.5

800

600

400

-1

Wavenumber (cm )

1.0 6 7

(b) 2 1 0.8 AsAs prepared

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


} Epoxide ring vibration

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12

89 10 11

3

5 4

0.6 60 °C 0.4 85 °C 0.2

UV cured

0.0 1800

1600

1400

1200

1000

800

600

400

-1

Wavenumber (cm )

Figure 3. FTIR spectra of (a) LRI and (b) HRI coatings in different processing steps as indicated in the figure. Y-axis was shifted for clarity.



% Transmission

100

Lens + hard-coat + HRI + LRI layers

95

Lens+hard-coat

Uncoated lens

90

85

Lens + hard coat + HRI layer

80 400

500

600

700

800

Wavelength (nm)

Figure 4. Optical transmission spectra at the different processing stages during fabrication of the AR coating on CR-39® ophthalmic lenses.

100

PC + hard-coat + HRI + LRI layers PC+hard coat

% Transmission

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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90 PC sheet 80 PC + hard coat + HRI layer 70

60

50 400

500

600

700

800

Wavelength (nm)

Figure 5. Optical transmission spectra at the different processing stages during the fabrication of AR coating on PC sheets.


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425 nm

500 nm

550 nm

600 nm

(a)

650 nm

750 nm

700 nm

100

(b)

7

98

6 5

Hard Coated CR-39 lens

4 3 2

500 nm 550 nm 600 nm

650 nm 700 nm

750 nm

94 92

Hard coated CR-39 lens

90 88 86

1 0

425 nm

96

% Transmission

% Reflection per surface

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


84 425 nm

400

500 nm

500

550 nm 600 nm 650 nm 700 nm

600

700

750 nm

800

82 400

(c) 500

Wavelength (nm)

600

700

800

Wavelength (nm)

Figure 6. (a) Photos of AR coated CR-39 lenses showing the reflection color over the entire visible range; (b) and (c) are their optical reflectance (per surface) and transmittance spectra.



1.80 1.75

Refractive index

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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HRI coating (RI = 1.75, t = 81 nm)

1.70 1.65 1.60 Hard coating (RI = 1.489, t = 1940 nm)

1.55 1.50

LRI coating (RI = 1.489, t = 88 nm)

1.45 0

500

1000

1500

2000

Distance from substrate (nm)

Figure 7. Ellipsometric depth profile showing the change of refractive index along the thickness of hard-coating and two-layer AR coating assemblies designed at 550 nm. The ellipsometric data was recorded at the wavelength of 500 nm.


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Table 1. Calculated optical and physical thickness values of LRI (SiO2–PEO) and HRI (SiO2– TiO2–PEO) coatings for obtaining minimum reflections at desired wavelengths. For the thickness calculation at a given wavelength, RI value (n) was taken at that particular wavelength as shown in Figure 2.

Minimum reflection designed at λ (nm) 425

Optical thickness (nm) = λ/4 LRI HRI

Physical thickness (nm) = λ/4n LRI HRI

106

106

70.5

59

500

125

125

84

71

550

137.5

137.5 92.5

78

600

150

150

101

86

650

162.5

162.5 110

94

700

175

175

119

101

750

187.5

187.5 128

109



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Table 2. Haze and clarity data of uncoated, hard-coated and AR coated CR-39 ophthalmic lenses and PC sheets (before and after abrasion).

Sample

Haze (ASTM 1003) 0.31

Clarity (ASTM 1003) 100

Haze after 10 Haze after 20 abrasion abrasion cycles cycles 11.1

Uncoated CR-39 Hard coated CR-39

0.33

99.9

-

0.57

Hard coated CR-39 0.34 with AR coating

99.8

-

1.20

PC uncoated

0.56

99.7

25.8

-

Hard coated PC

1.08

99.5

1.84

-

Hard coated PC with 0.30 AR coating

99.5

2.44

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List of figure captions.

Figure 1. Schematic representation of AR coating design on a hard-coated plastic substrate. Figure 2. Refractive index values of HRI and LRI coatings with respect to wavelength of light after UV curing. Figure 3. FTIR spectra of (a) LRI and (b) HRI coatings in different processing steps as indicated in the figure. Y-axis was shifted for clarity. Figure 4. Optical transmission spectra at the different processing stages during fabrication of the AR coating on CR-39® ophthalmic lenses. Figure 5. Optical transmission spectra at the different processing stages during the fabrication of AR coating on PC sheets. Figure 6. (a) Photos of AR coated CR-39 lenses showing the reflection color over the entire visible range; (b) and (c) are their optical reflectance and transmittance spectra. Figure 7. Ellipsometric depth profile showing the change of refractive index along the thickness of hard-coating and two-layer AR coating assemblies designed at 550 nm. The ellipsometric data was recorded at the wavelength of 500 nm.


Wavelength Selective Antireflective Coatings on Plastics with

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