Precisely Tuning the Clay Spacing in Nanobrick Wall Gas Barrier Thin

Apr 18, 2013 - ... Jose A. Plaza Hernández , Antonio Bódalo Santoyo , Elisa Gómez ... Morgan A. Priolo , Kevin M. Holder , Tyler Guin , Jaime C. Gr...
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Precisely Tuning the Clay Spacing in Nanobrick Wall Gas Barrier Thin Films Morgan A. Priolo, Kevin M. Holder, Stephen M. Greenlee, Bart E. Stevens, and Jaime C. Grunlan* Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843-3123, United States S Supporting Information *

ABSTRACT: The influence of clay-layer spacing on gas barrier thin films of sodium montmorillonite clay and polyelectrolytes, created via layer-by-layer assembly, is investigated. The alternate deposition of polymers and clay leads to the assembly of a nanobrick wall structure that is highly impermeable to gases. In an effort to tailor the thickness (or spacing) between clay layers, films with differing numbers of polymer layers between clay depositions were examined. Films analyzed for their thickness, clay concentration, transparency, nanostructure, and oxygen barrier as a function of layers (or spacing) between clay depositions reveal linear growth, optical clarity, and low OTR at 100 nm thick and containing only four clay layers. An optimal thickness between clay layers appears to exist for achieving the highest oxygen barrier LbL films (PO2 < 1 × 10−21 cc(STP)·cm/(cm2·s·Pa)). This knowledge can ultimately minimize deposition steps and lead to decreased thin film fabrication times. KEYWORDS: multilayers, clays, nanocomposites, thin films, membranes

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particles.34−41 Because of the large choice of materials and ease of production, LbL assembly has been widely studied as a relatively inexpensive technique for creating thin films that exhibit antimicrobial,42−44 electrically conductive,45−47 superhydrophobic,48−50 and drug delivery51−53 properties. In addition to imparting steel-like strength,54 nanoplatelet clays have been used in LbL assemblies to deposit flame retardant coatings on cotton55 and foam56 and oxygen barrier coatings on plastic substrates that rival ceramic and metallic thin films.16−20 In previous reports, it was shown that clay-layer spacing has a dramatic effect on the barrier of polymer−clay films fabricated using LbL.17,19 The present work examines the growth of assemblies made with cationic poly(allyl amine) (PAAm), anionic poly(acrylic acid) (PAA), and sodium montmorillonite (MMT) clay, in an effort to understand the influence that claylayer spacing has on gas barrier behavior. By depositing a precise number of BLs of PAAm/PAA between each clay deposition, the thickness between each clay layer (CL) can be finely tuned. The deposition of 1.5 or 18.5 PAAm/PAA BLs varies the thickness between CLs from 4 to 21 nm, respectively. When four clay layers, with a spacing of 4 nm per CL, are deposited with PAAm/PAA in a film that is 100 nm thick, a 42% reduction in oxygen permeability of the neat PAAm/PAA coating is observed. A 100 nm film with 6 nm of PAAm/PAA per CL, still containing four total clay layers, lowers the permeability by 66%, yielding an oxygen transmission rate (OTR) more than 2 orders of magnitude lower than the bare poly(ethylene terephthalate) (PET) substrate.

INTRODUCTION Flexible, transparent layers with a high barrier to atmospheric gases (e.g., oxygen and moisture vapor) are a key component for flexible electronics, food, and pharmaceutical packaging.1−3 Monolithic, inorganic oxide layers, such as SiOx or AlxOy, have been extensively investigated for their high barrier performance and optical clarity at thicknesses typically below 100 nm,4−6 but these films are prone to defect formation during deposition, poor substrate adhesion, and failure upon flexure at strains less than 2%.7−9 Additionally, the best inorganic oxides generally require extreme processing conditions (e.g., plasma-enhanced chemical vapor deposition) and suffer from low productivity and high costs when layered with polymers.1,10 Clay-filled polymer composites have been shown to improve barrier over neat polymer films,11−13 but exhibit lower transparency and oxygen barrier relative to the aforementioned metal-oxide layers.14,15 A relatively new technique, layer-by-layer (LbL) assembly, has been shown to generate polymer−clay thin films that exhibit extremely low oxygen transmission rates, high transparency in the visible light spectrum, and tunable gas barrier behavior.16−21 Layer-by-layer assembly is a thin film fabrication technique used to build nanoscale coatings by exposing a substrate to oppositely charged, aqueous mixtures and generally follows the scheme in Figure 1a.22−26 Each complementary pair of layers, as shown in Figure 1b, is known as a bilayer (BL) and can range in thickness from a few angstroms to tens or hundreds of nanometers. The adsorption thickness and properties of these deposited layers can be tailored by altering individual mixture pH,17,27−29 species molecular weight,30,31 and deposition temperature.32,33 In many cases, one or more of these individual layers contain charged or functionalized nano© 2013 American Chemical Society

Received: January 4, 2013 Revised: April 17, 2013 Published: April 18, 2013 1649

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microbalance (QCM) were deposited onto polished Ti/Au crystals, purchased from Maxtek, Inc. (Cypress, CA), with a resonance frequency of 5 MHz. Films characterized by UV−vis spectrometry were deposited onto 1 mm thick fused quartz glass slides, purchased from Structure Probe, Inc. (West Chester, PA). Film Preparation. All film deposition mixtures were prepared using 18.2 MΩ·cm deionized water and rolled for at least 24 h to achieve homogeneity. Prior to deposition, the pH of 0.2 wt % aqueous PAAm and PAA solutions, and 2.0 wt % MMT mixtures, were altered to 7.5 using 1.0 M HCl or 1.0 M NaOH. A 30 min piranha treatment was used to clean silicon wafers and fused quartz glass slides.62 Caution! Piranha solution reacts violently with organic materials and should be handled with extreme caution. Prior to deposition, silicon wafers and glass slides were rinsed sequentially with water, acetone, and water again, and finally dried with filtered air. PET films were initially cleaned by rinsing with water, methanol, and water again before being dried with filtered air. Prior to deposition, cleaned PET substrates were corona treated to create a negative surface charge using a BD-20C Corona Treater from Electro-Technic Products, Inc. (Chicago, IL).63,64 A 5 min plasma cleaning treatment, using a PDC32G plasma cleaner from Harrick Plasma (Ithaca, NY), was performed on QCM crystals prior to deposition. Each appropriately treated substrate was initially dipped into the PAAm solution for 5 min, rinsed with deionized water, and dried with filtered air. The same procedure was followed when the substrate was next dipped into the PAA solution. Once this initial bilayer was deposited, the above procedure was repeated for subsequent layers using 5 s PAAm and PAA dip times and 1 min MMT dip times, until the desired number of layers was achieved. All films were prepared using home-built robotic dipping systems.65,66 Film Characterization. Film thickness was measured after each CL was deposited using an α-SE spectroscopic ellipsometer (J. A. Woollam, Lincoln, NE). Thin film mass deposited was measured with a Research Quartz Crystal Microbalance (QCM) (Maxtek Inc., Cypress, CA). Thin film absorbance was measured using a U SB2000-UV−vis spectrometer (Ocean Optics, Dunedin, FL). Carbon stabilized, Formvar-coated 150 mesh nickel grids (Electron Microscopy Sciences, Hatfield, PA) were used to pick up thin cross sections (∼100 nm in thickness), which were imaged using a JEOL 1200 EX (Parbody, MA) TEM, at an accelerating voltage of 100 kV and calibrated magnifications. Thin films, deposited on PET, were coated with carbon, embedded in epoxy, and sectioned onto water prior to imaging. Scanning electron micrographs were obtained using a JEOL JSM-7500F scanning electron microscope (JEOL Ltd., Tokyo, Japan) equipped with gentle beam capabilities, allowing the specimen stage to have a bias voltage for mitigating sample charging. In accordance with ASTM D-3985, OTR testing was performed by MOCON (Minneapolis, MN) using an Oxtran 2/21 ML instrument at 23 °C and 0% RH. Prior to overnight desiccation, films deposited on PET for OTR testing were placed in an oven at 70 °C for 15 min, immediately following film growth, to remove excess moisture in the film.

Figure 1. Schematic of layer-by-layer assembly with positively and negatively charged mixtures (a) and a cross-sectional illustration of the resultant thin film (b).

This four clay layer assembly exhibits an oxygen permeability of 9.8 × 10−22 cc(STP)·cm/(cm2·s·Pa), which is approximately 2 orders of magnitude lower than that of SiOx thin films,4,7,57 3 orders of magnitude lower than that of some of the best polymer/clay composites,14,15 and 4 orders of magnitude lower than that of EVOH copolymer barrier films.58 It is believed that this control of layer thickness can lead to tailored film architectures with precise, single-polymer deposition between clay layers, which will increase fabrication speeds, reduce overall manufacturing costs, and broaden the application space. Moreover, the use of PAAm, PAA, and MMT, which are deposited from water, represents a cost-effective recipe, especially when compared to the significant cost associated with capital equipment for vacuum deposition processes. Optimizing the behavior of transparent and flexible high gas barrier films, like those presented in this work, is a significant step toward their use for a variety of applications, such as flexible electronics and food packaging.1,59,60

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RESULTS AND DISCUSSION Influence of Polymer Bilayers on Polymer−Clay Film Growth. The number of BLs of PAAm/PAA between each clay layer was set to 1.5, 6.5, 11.5, 16.5, and 18.5 in an effort to understand the influence of CL spacing on the properties of gas barrier films. LbL films are denoted in the text and figures as [(PAAm/PAA)γ/MMT]β, where γ is the number of BLs of PAAm/PAA deposited and β is the number of CLs deposited (or total deposition cycles). The value of γ includes an extra half BL throughout this study because it is necessary for a single PAAm layer to be deposited immediately prior to an MMT deposition. Figure 2a reveals exponential film growth of PAAm/PAA as a function of BLs deposited through the first 31 BLs and linearly increases, with a growth rate of 5.4 nm per BL, beyond that. This film growth behavior is typical for weak polyelectrolytes, with film thickness increasing exponentially

EXPERIMENTAL SECTION

Materials. Natural sodium montmorillonite (MMT) (trade name Cloisite NA+) clay was supplied by Southern Clay Products, Inc. (Gonzalez, TX) and used as received. Individual MMT platelets have a negative surface charge in deionized water, a reported density of 2.86 g/cm3, a thickness of 1 nm, and a nominal aspect ratio (S /d) ≥ 200.61 Poly(allyl amine) (PAAm) (Mw = 15 000 g/mol, 15 wt % in water), purchased from Polysciences, Inc. (Warrington, PA), and poly(acrylic acid) (PAA) (Mw = 100 000 g/mol, 35 wt % in water), purchased from Sigma-Aldrich (Milwaukee, WI), were used as received. Films characterized by oxygen transmission rate (OTR) testing and transmission electron microscopy (TEM) were deposited onto 179 μm thick poly(ethylene terephthalate) (PET) film (trade name ST505, produced by DuPont-Teijin), purchased from Tekra (New Berlin, WI). Films characterized by ellipsometry were deposited onto 500 μm thick, single-side-polished, silicon wafers, purchased from University Wafer (South Boston, MA). Films characterized by a quartz crystal 1650

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by-layer assembly is capable of depositing clay nanoplatelets in a controlled, oriented manner, with their largest dimension parallel to the substrate.19,56,68 UV−vis spectroscopy data are shown in Figure 3a as the average visible light transmission as a

Figure 2. Thickness as a function of PAAm/PAA bilayers deposited (a) and clay layers deposited with varying numbers of PAAm/PAA bilayers, γ, between clay depositions (b). The inset in (b) reveals the agreement between average γ thickness in (b) and neat PAAm/PAA film growth from (a). Error in (b) is less than 1 nm.

until a saturation point is reached and relatively thick linear growth prevails.67 Figure 2b shows the linear growth and influence of PAAm/ PAA on the thickness of polymer−clay films created with varying γ values. Also, the inset in Figure 2a confirms the polymer-only film growth with the average polymer thickness between CLs observed in Figure 2b. These average polymer thickness values are the slopes of the individual trend lines from Figure 2b, with 1.5 nm subtracted from each slope value to account for the thickness of a deposited clay layer. The lack of exponential growth in films containing clay is due to the extent of clay depositing onto the film surface, disrupting the exponential growth of PAAm/PAA into the underlying subclay layers. With each deposition of clay, the exponential polymer buildup is restarted, as though each clay layer acts as a new substrate for film growth. This phenomenon was reported previously when branched polyethylenimine (PEI) and PAA were deposited onto a pre-existing PEI/PAA film that had been chemically cross-linked to promote film stability.28 This effect is desirable for this study because altering the γ value of these films provides the ability to tune the thickness between CLs. It is also the basis for using 2 wt % MMT suspensions for film deposition (see the Experimental Section). The inset in Figure 2b confirms this idea of exponential growth hindrance, where the average polymer thickness values between clay layers trends precisely with PAAm/PAA growth measurements. While these methods of measurement anecdotally suggest clay platelet alignment, UV−vis spectroscopy and cross-sectional transmission electron microscopy can verify that each clay-layer deposition results in platelet orientation parallel to the substrate surface. Optical Clarity and Nanobrick Wall Structure. Previously reported polymer−clay studies have shown that layer-

Figure 3. Average visible light transmission of films consisting of four clay layers as a function of PAAm/PAA bilayers (γ) deposited on fused quartz (a). Transmission electron microscope cross-sectional image of a four clay layer film, made with 18.5 PAAm/PAA bilayers between clay, deposited onto PET film (b). The white double arrow spans the LbL film thickness. Scanning electron microscope surface images of silicon wafers coated with a four clay layer film, made with 18.5 PAAm/PAA bilayers between clay (c) and a 38-bilayer PAAm/PAA film (d). Error in (a) is less than 0.07%.

function of γ equal to 1.5, 11.5, and 18.5 (the two extremes and the median examined here). These data indirectly confirm that clay platelets have successfully been deposited in a highly aligned, highly exfoliated state, as light transmission remains above 96% throughout the visible light spectrum. This level of control of the deposition state, where structures contain exfoliated, uniformly dispersed clay platelets in a polymeric matrix, has been regarded as the key to successful implementation of polymer−clay nanocomposites.69 In conjunction with the data in the inset of Figure 2a, ellipsometric thickness and optical clarity strongly suggest clay is mostly depositing as a single layer of clay, with minimal clay stacking, in an extremely dense, highly packed state. A transmission electron microscope (TEM) image of the cross-section of a clay-filled film, shown in Figure 3b, highlights the nanobrick wall structure that these films exhibit. Individual clay layers can be seen in this image as dark lines, oriented horizontally and parallel to the PET substrate surface. This film containing four CLs, made with γ equal to 18.5, was deposited onto 179 μm thick PET film, and was embedded in epoxy prior 1651

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to sectioning in preparation for imaging (see the Experimental Section). The sectioning of the film is the source of “waviness” observed in this image, with ultramicrotomy facilitating stress relaxation in the film. This TEM image demonstrates the precise control over film thickness and nanoparticle orientation that LbL assembly can uniquely provide. It also confirms the growth behavior observed by ellipsometry, shown in Figure 2b, where polymer deposition between each clay layer is consistent throughout the entire film’s thickness. Scanning electron microscope images of the surface of silicon wafers coated with four clay layers, deposited with 18.5 PAAm/PAA bilayers between clay, and 38 bilayers of PAAm/PAA, can be seen in Figure 3c,d. Surface images confirm that clay layer deposition occurs uniformly with each platelet oriented parallel to the substrate’s surface, with minimal surface features seen in Figure 3c. The extremely smooth, defect-free surface seen in Figure 3d is expected for highly charged, thinly growing polymer multilayers.28 With a highly aligned, densely packed nanostructure of clay nanobricks in a mortar-like polymer thin film, these nanobrick wall assemblies are expected to dramatically improve the oxygen barrier with relatively few clay layers. Clay Spacing Influence on Oxygen Barrier and Film Composition. Filling polymer matrixes with clay platelets (or inorganic disk-shaped nanoparticles) is one of the most common methods of lowering the gas permeability of a polymer.3,11,12,15,70,71 Unique to the layer-by-layer assembly technique is its control over platelet orientation, ability to deposit clay from exfoliated solution in an exfoliated state within the film, and tailorability to include clay anywhere within the thin film assembly. The deposition of clay parallel to the substrate is the key to the success of clay-filled thin films, where this orientation forces the platelets’ largest dimension to be maximized for gas barrier improvement. The aspect ratio (S /d) for montmorillonite clay ranges from 100 to 1000 and is maximized in the film when horizontally oriented (i.e., parallel to the substrate surface), creating an extremely tortuous pathway for permeating molecules. This increased tortuosity of LbL films has been shown to produce super gas barrier assemblies less than 500 nm thick.16,18 Improvements to these films have been observed when clay-layer spacing has been increased,17 clay concentration has been increased,20 or when exponentially growing polymer assemblies are deposited with clay.19 Films made from renewable resources and food contact approved materials have also been shown to benefit from increased thickness between clay layers.56 Figure 4 shows the mass deposited, oxygen transmission rate (OTR) data and clay spacing for thin films with increasing thickness between clay layers. The thickness of each film was fixed at approximately 100 nm (the thickness of ((PAAm/ PAA)18.5/MMT)4) in an effort to decouple the effects of increasing clay spacing on barrier from increasing film thickness. To maintain constant film thickness while varying the spacing between CLs, films created with γ equal to 1.5, 6.5, 11.5, and 16.5 were deposited with 36.5, 35.5, 32.5, and 29.5 PAAm/PAA BLs, respectively, prior to the first clay-layer deposition. After this initial CL was deposited, three more CLs with the corresponding γ values were deposited for an ultimate thickness approximately equal to the film thickness of γ equal to 18.5 at four CLs. The additional polymer necessary to maintain constant thickness was deposited in accordance to the thickness measurements via ellipsometry of PAAm/PAA (see Figure 2a) and preceded clay deposition to ensure that each clay-based film’s terminal layer was MMT.

Figure 4. Mass deposited as a function of clay layers for films deposited onto QCM crystals (a), where filled points denote polymer deposition and unfilled denote clay deposition. Oxygen transmission rate (filled) and clay spacing (unfilled) as a function of PAAm/PAA bilayers (γ) deposited onto 179 μm PET film (b). The dotted lines are provided as guides for the eye and are not trend lines for the plotted data. Error in (a) is less than 1%.

Figure 4a reveals the mass increase as a function of CLs deposited for these thickness-normalized, 4-CL gas barrier assemblies. Films composed of γ equal to 1.5 and 6.5 were initially coated with the appropriate number PAAm/PAA BLs to achieve the thickness of a film with γ equal to 18.5 (i.e., 100 nm total film thickness). The significant initial shift in growth curves shows the excess mass deposited in the initial PAAm/ PAA layers, with coatings of γ equal to 1.5 and 6.5 achieving almost identical ultimate mass values. The volume fraction clay comprising these films ranges from 0.14 to 0.22 for films created with γ values equal to 1.5−18.5, respectively (see the Supporting Information). This modest increase in clay content is due to the growth mechanism of PAAm/PAA, shown in Figure 2b, with larger polymer mass deposition at larger numbers of PAAm/PAA BLs needed to normalize the ultimate thickness. That is, films deposited with γ equal to 18.5 restrict polymer growth to the relatively thin, exponentially growing regime, resulting in less mass deposition, whereas films requiring more than 30 initial PAAm/PAA BLs deposit more overall mass due to the relatively thick and linear growth regime. It can be seen in Figure 4b that the addition of four CLs, at a spacing of 4.2 nm, to PAAm/PAA lowers the OTR by 44%, from 0.052 cc/(m2·day·atm) to 0.29 cc/(m2·day·atm). The film with a clay spacing of zero in Figure 4b contains only polymer, consisting of 39.5 BLs of PAAm/PAA, and serves as a control film with equivalent thickness as the clay-based assemblies. This rapid decrease in OTR with only 14 vol % clay is believed to be due to the nanobrick wall structure shown in Figure 3b, with highly aligned clay platelets creating a highly tortuous pathway for permeating oxygen gas. This OTR of PAAm/PAA coated PET is decreased even further by the inclusion of four clay 1652

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Figure 4b above. This is due to their equal thickness values, which is the normalizing factor between OTR and O2 permeability. Because there are multiple regions within each film, the different regions of these films’ thicknesses (i.e., the PAAm/PAA initial layers and the 4 CL region) can be considered different parts of a laminated stack. Employing the rule of resistors in parallel (an extension of eq 3 above) to multiple coating stacks

layers that have a thickness between each layer of 6.1 nm, corresponding to γ equal to 6.5. The OTR increases only slightly as clay spacing increases, reaching a maximum of 0.28 cc/(m2·day·atm), revealing that the extra polymer required to achieve similar thicknesses effectively normalizes oxygen barrier data when the number of CLs is held constant. These films also have a relatively similar clay concentration, with a range of only 8 vol %. These data suggest that the amount of clay spacing deposited strongly influences the oxygen permeability of the clay-filled region of these films. Tuning Oxygen Permeability by Altering Clay Spacing. Most LbL-based nanocomposites are too thin (≪250 nm) to be removed from the substrate and tested as free-standing films. In this case, the use of ideal laminate theory is employed to calculate the permeability of ultrathin barrier coatings from the measured OTR values.4 The definition of ideal laminate theory permeability (PITL) is PITL

−1 ⎛ ϕP ϕLbL ⎞ =⎜ + ⎟ PLbL ⎠ ⎝ PP

d d ϕP = P , ϕLbL = LbL d d

−1 ⎛ 1 1 1 ⎞ ⎟⎟ OTR = ⎜⎜ + + OTR i OTR γ ⎠ ⎝ OTR PET

where i denotes the initial PAAm/PAA bilayer buildup’s contribution to the overall OTR, and γ denotes the CL region’s contribution; the O2 permeability of the CLs can be decoupled from that of the entire film. Using the same mathematics and assumptions from eqs 1−3, the O2 permeability of the clay layer region of these films, Pγ, is −1 ⎛ 1 2hi , γ ⎞ 1 Pγ = 2hγ ⎜ − − ⎟ OTR PET Pi ⎠ ⎝ OTR

(1)

PLbL

(5)

where hγ is the CL thickness, hi,γ is the initial PAAm/PAA bilayer buildup thickness based on that film’s γ value, and Pi is the calculated O2 permeability of PAAm/PAA from eq 3. The oxygen permeability of the clay layer region of these films can be seen as the unfilled data points in Figure 5. The O2 permeability of clay layers initially decreases by 11% when spacing increases by 45%, from 4.2 to 6.1 nm, confirming the trend of previously reported data, where increasing the thickness between CLs decreased the film’s permeability.17,19 However, beyond 6.1 nm between clay layers, the permeability begins to increase as a function of clay layer spacing, with a 90% increase in film permeability resulting from a 74% increase in spacing. By simply increasing the number of polymer bilayers between each CL from 6.5 to 18.5, the increase in polymer thickness increases the permeability by 340%, which suggests that 6.1 nm between CLs is an optimal thickness for low permeability [(PAAm/PAA)γ/MMT] films. These data reveal that there are diminishing returns for increasing the thickness between clay layers for LbL assemblies designed to decrease film permeability. The increased permeability as a function of polymer thickness is believed to be due to the decreased confinement effects for thicker films that exist mostly at the polymer−clay interfaces. That is, the polymer matrix exhibits a gradient of chain mobility, with increasing mobility as its proximity to clay decreases. These areas of greater chain mobility can offer a more permeable area for an O2 molecule to diffuse through. This hypothesis is supported by the exponentially increasing permeability as a function of clay spacing, which is similar to the exponentially increasing polymer thickness, shown in Figure 2a. It is clear that the LbL technique is able to fine-tune the permeability of clayfilled polymer nanocomposites by adjusting polymer deposition thickness.

(2)

where ϕP, PP, and dP are the volume fraction, permeability, and thickness of the PET substrate, while ϕLbL, PLbL, and dLbL are the volume fraction, permeability, and thickness of the LbL assembly deposited onto PET (d is the total thickness of the coated substrate). Because the permeability of a material is equal to its OTR multiplied by its thickness, the permeability of LbL coatings can be derived from eqs 1 and 2 as ⎛ 1 ⎞−1 1 = 2h⎜ − ⎟ OTR PET ⎠ ⎝ OTR

(4)

(3)

where 2h is the measured film thickness (h) multiplied by 2 to account for both sides being coated during the deposition process, OTR is the measured value of the coated PET, and OTRPET is the OTR of the PET used here (measured previously to be 8.559 cc/m2·day·atm). From eq 3, the oxygen permeability of the PAAm/PAA film deposited onto PET is calculated to be 11.8 × 10 −21 cc(STP)·cm/(cm 2 ·s·Pa). Figure 5 reveals that the O 2 permeabilities of the 4-CL, 100 nm thick films on PET (filled data points) trend similarly to their OTR values shown in

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CONCLUSIONS The influence of clay-layer spacing on gas barrier thin films, containing highly oriented montmorillonite clay “nanobricks”, was investigated. By altering the numbers of polymer layers between deposited clay layers, the thickness (or spacing) between clay layers was tuned from 4.2 to 20.9 nm. In addition to thickness, clay concentration and oxygen barrier were

Figure 5. Oxygen permeability as a function of clay layer spacing of films deposited onto PET with an overall thickness of 100 nm and a total of four clay layers. Filled data points denote permeabilities calculated using eq 3, and unfilled points denote permeabilities of the clay layer region using eq 5. 1653

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strongly influenced by simply altering the deposited clay layer spacing. On the basis of the calculated oxygen permeability, the [(PAAm/PAA)γ/MMT] recipe exhibits an optimal CL spacing of roughly 6 nm, corresponding to γ equal to 6.5. Combining very high transparency, flexibility, and microwavability, these thin films exhibit an OTR necessary for a variety of packaging applications. With optimized process parameters, such as clay spacing and deposition speeds, gas barrier assemblies can be achieved relatively quickly and more economically, making this a commercially friendly encapsulation technology.

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ASSOCIATED CONTENT

S Supporting Information *

Additional QCM data analysis and light transmission data (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 979-845-3027. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Texas Engineering Experiment Station (TEES) and The Dow Chemical Company for financial support of this research. Use of the Texas A&M University Microscopy and Imaging Center is acknowledged. Use of the Texas A&M University Materials Characterization Facility is acknowledged.

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REFERENCES

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