Humidity-Responsive Gas Barrier of Hydrogen-Bonded Polymer–Clay

Aug 27, 2012 - It is well-known that gas barrier behavior in most polymer and composite materials degrades at elevated humidity. In an effort to reduc...
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Humidity-Responsive Gas Barrier of Hydrogen-Bonded Polymer− Clay Multilayer Thin Films Kevin M. Holder,† Morgan A. Priolo,‡,§ Kimberly E. Secrist,∥ Stephen M. Greenlee,§ Adam J. Nolte,*,○ and Jaime C. Grunlan*,‡,§,⊥ †

Department of Chemistry, ‡Materials Science & Engineering Program, §Department of Mechanical Engineering, ⊥Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States ∥ Department of Chemistry, ○Department of Chemical Engineering, Rose-Hulman Institute of Technology, Terre Haute, Indiana 47803, United States ABSTRACT: It is well-known that gas barrier behavior in most polymer and composite materials degrades at elevated humidity. In an effort to reduce this trend, the influence of relative humidity (RH) on the gas barrier of thin films comprising montmorillonite clay and polyvinylpyrrolidone, created via layerby-layer assembly, was investigated. These hydrogen-bonded thin films approximately doubled in thickness when RH was increased to 100% but returned to within 1% of the original thickness when RH was decreased to 0%, with minimal swelling/deswelling hysteresis. Transmission electron microscopy reveals a highly aligned nanobrick wall structure, which has a clay concentration of 74 wt % and greater than 95% visible light transmission. The oxygen transmission rate (OTR) through these films, deposited on 179 μm poly(ethylene terephthalate) film, remarkably decreases as a function of RH. A 40-BL film has an OTR of 3.9 (cc/(m2·day·atm)) at 0% RH, while exposure to 100% RH decreased this value by 11%. In this case, greater spacing between clay layers and maintenance of tight packing within the layers (due to relatively weak H-bonding between polymer and clay) combine to create a more tortuous path at high humidity. This study marks the first polymer−clay assembly that exhibits improved gas barrier at high humidity, which is important for various packaging applications (e.g., food and flexible electronics). gas barrier,11−15 electrically conductive,23−25 flame retardant,26−28 and sensing29−31 properties by simply exposing a substrate to oppositely charged mixtures in an alternating fashion.32 Film thickness of LbL assemblies can be adjusted by altering the deposition species’ molecular weight33,34 or solution pH,35,36 or by altering environmental factors such as temperature or relative humidity.37,38 Because these films are prepared with hydrophilic ingredients, they are prone to swelling in humid environments.37 Clay-based assemblies often show a reduction in gas barrier (i.e., increase in gas transmission rate) at high humidity as a result of this swelling.11,13 In an effort to prepare clay-based assemblies that maintain low oxygen permeability at high humidity, this swelling phenomenon is exploited here. Thicker polymer between clay layers has previously been shown to dramatically improve gas barrier,12,13 while polyvinylpyrrolidone (PVP) is known to swell within all-polymer LbL assemblies when exposed to humidity.39,40 More importantly, PVP has been shown to grow successfully, and swell more dramatically at high humidity, when deposited with montmorillonite (MMT) clay.41 In this

1. INTRODUCTION Flexible, transparent gas barrier layers are necessary for the advancement of flexible displays, along with improving food and pharmaceutical packaging.1−3 Inorganic metal-oxide layers, such as SiOx, exhibit the lowest gas transmission rates and highest transparency among commercially available barriers. Despite having excellent as-deposited barriers, these thin films are prone to cracking when flexed and require complex fabrication when layered with polymers to improve barrier and flexibility.2,4−8 Flexible polymer−clay composites have been shown to modestly improve the barrier over the neat polymer matrix but exhibit low optical clarity and relatively high gas transmission rates.1,9,10 By using layer-by-layer assembly, polymer−clay thin films have been shown to exhibit high gas barrier, rivaling that of thin metal oxides, while maintaining the mechanical properties of a traditional polymer composite.11−15 The best of these films is 50 nm thick and exhibits an oxygen transmission rate of 0.005 cc/m2·day due to high clay concentration (∼37 wt %) and near complete exfoliation of platelets oriented parallel to the substrate’s surface.13 This unique structure is known as a ‘nanobrick wall’. Layer-by-layer (LbL) assembly has become a popular method for fabricating multifunctional thin films, typically less than 1 μm thick.16−18 These films, deposited from water under ambient conditions, can be made to display antimicrobial,19−22 © 2012 American Chemical Society

Received: June 18, 2012 Revised: August 15, 2012 Published: August 27, 2012 19851

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Figure 1. (A) Thickness and (B) mass as a function of bilayers deposited for (PVP/MMT)x films.

methanol, and water again, dried with filtered air, heated at 70 °C for 15 min, and finally corona treated with a BD-20C corona treater from Electro-Technic Products, Inc. (Chicago, IL), to create a negative surface charge prior to deposition.45,46 Each appropriately treated substrate was then dipped into the PVP solution for 5 min, rinsed with deionized water at an unaltered pH ≈ 5.25, and dried with filtered air. The same procedure was followed with MMT. After this initial bilayer (BL), the above procedure was repeated for subsequent layers using 5-s PVP dip times and 1-min MMT dip times until the desired numbers of bilayers were deposited. Deposition times were chosen on the basis of previous work showing that polymers can be deposited more quickly than nanoparticles and yield films that exhibit equivalent barrier behavior.47 All films were prepared using home-built robotic dipping systems.48,49 2.3. Film Characterization. Film thickness was measured every five bilayers (on silicon wafers) using an F20 thin film reflectometer (Filmetrics, San Diego, CA) under both ambient and humidity-controlled conditions. A refractive index of 1.43 was assumed for these measurements, with thickness data reported only for an average goodness of fit value no more than 3% from unity. OTR testing was performed by MOCON (Minneapolis, MN) using an Oxtran 2/21 ML instrument at 0% and 100% RH. In order to remove residual moisture, films deposited on PET were placed in an oven at 70 °C for 15 min immediately following deposition. A quartz crystal microbalance (QCM) (Maxtek Inc., Cypress, CA) was used to measure mass deposited per layer (on Ti/Au crystals). A USB2000 UV−vis spectrometer (Ocean Optics, Dunedin, FL) was used to measure thin film absorbance at wavelengths between 190 and 900 nm. In order to obtain the absorbance of the coating, the absorbance of a bare quartz substrate was subtracted from the absorbance spectra of the coated quartz slide. Thin film cross sections were imaged using a JEOL 1200 EX TEM (Parbody, MA) at calibrated magnifications and an accelerating voltage of 100 kV. Before imaging, these films were coated with carbon, embedded in epoxy, and sectioned onto water. Thin sections, approximately 100 nm thick, were picked up onto carbon-stabilized, Formvar-coated 150 mesh nickel grids (Electron Microscopy Sciences, Hatfield, PA). A humidity-controlled glovebox (Electro-Tech Systems, Inc., Glenside, PA) was used to monitor film thickness as a function of relative humidity, and data are reported as relative film

study, LbL assemblies of PVP and MMT clay were used to investigate the influence of PVP swelling on oxygen transmission rate (OTR) tested at 0% and 100% RH. The OTR of a 40-BL PVP/MMT film (∼378 nm thick) is 3.92 cc/m2·day under dry conditions, but improved by 11% when relative humidity was increased to 100%. It is believed that the relatively weak hydrogen bonding between the hydroxyl groups of MMT and the carbonyl groups of PVP allows swelling of the film to occur without significantly altering platelet packing parallel to the substrate. This gas barrier improvement after exposure to humidity is the first such demonstration for LbL assemblies and marks an important step in the evolution of this unique gas barrier technology.

2. EXPERIMENTAL SECTION 2.1. Materials. Polyvinylpyrrolidone (PVP) (Mw = 360,000 g/mol), with a density of 1.2 g/cm3, was purchased from Sigma-Aldrich (Milwaukee, WI) and used as received. Natural sodium montmorillonite (MMT) clay was provided by Southern Clay Products, Inc. (Gonzales, TX) and used as received. MMT platelets have a negative surface charge in deionized water, a reported density of 2.86 g/cm3, thickness of 1 nm, and an aspect ratio (l/d) of approximately 100− 1000.42,43 Poly(ethylene terephthalate) (PET) film (trade name ST505 by DuPont-Teijin Films), with a thickness of 179 μm, was purchased from Tekra (New Berlin, WI) and used as the substrate for OTR testing and transmission electron microscopy (TEM). Single-side-polished, 500-μm-thick silicon wafers were purchased from University Wafer (South Boston, MA) and used for film growth and swelling characterization by reflectometry. 2.2. Film Preparation. Solutions were prepared using 18.2 MΩ deionized water and rolling for 24 h to ensure homogeneity. Prior to deposition, the pH of each 0.5 wt % aqueous solution of PVP was altered to 4 using 1.0 M HCl. MMT suspensions were prepared in deionized water at a concentration of 2.0 wt % and were used at their natural, unaltered pH of approximately 10. Silicon wafers and fused quartz glass slides were treated with piranha solution for 30 min prior to rinsing with water, acetone, and water again, and finally dried with filtered air before deposition.44 Caution! Piranha solution reacts violently with organic materials and should be handled with extreme care. PET films were rinsed with water, 19852

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Figure 2. (A) Visible light transmission as a function of wavelength for (PVP/MMT)x films. Absorbance at 530 nm as a function of bilayers (inset). (B) TEM cross-sectional image of a 40-BL film deposited on PET (double arrow spans this film’s compressed thickness).

decreases only slightly with increasing bilayers, from an average of 97.7% for a 10-BL film to 95.8% for a 40-BL film. This modest decrease in transmission is directly linked to increasing film thickness, as expected based on the Beer−Lambert Law (i.e., absorbance is proportional to thickness, assuming uniformity). Absorbance as a function of bilayers deposited produces a straight line (inset of Figure 2A) that further confirms linear growth and suggests these films are very uniform. The high transparency observed for these PVP/MMT thin films is consistent with previously reported clay-filled LbL films,12−14 and is attributed to nearly perfect platelet exfoliation and alignment, with the largest dimension of each clay nanoplatelet lying parallel to the substrate. A transmission electron micrograph, shown in Figure 2B, reveals the nanobrick wall structure of these multilayered films. The double arrow spans the thickness of the film, and it can be seen that the cross section is smaller than the reflectometry measurements (see Figure 1A). This disparity is primarily due to the preparation of thin film sections by ultramicrotomy, where sections are embedded in epoxy that is cured at 50 °C and cut at an angle that yields images with the illusion of an altered film thickness. This sectioning technique also causes compression of the film that reduces the observed thickness relative to what would be expected from an untouched film. Highly aligned clay platelets, deposition characteristic of polymer−clay LbL assembly, can be seen in the image as dark, horizontal lines, oriented parallel to the PET substrate surface. This nanobrick wall structure of polymeric mortar and clay nanobricks is the key to a good gas barrier LbL film. The swelling properties of PVP, coupled with the hydrogen-bonding nature of these assemblies, produce greater clay spacing perpendicular to the substrate at high humidity that maintains this nanobrick wall structure and provides further improvement in the gas barrier. 3.2. Influence of Humidity on Film Thickness. One of the most common methods used to improve polymer gas barrier performance is to add clay.1,10−15,51,52 The high level of clay concentration and orientation afforded by LbL deposition produces a better gas barrier than conventional clay-filled composites.12−15 Exceptionally low oxygen permeability has been achieved with electrostatically grown polymer−clay thin films under dry conditions (PO2 ≤ 5 × 10−22 cm3(STP)·cm/

thickness normalized to the thickness at 5% RH. The relative swelling measurement technique was performed as previously described.38 The average temperature during investigation was 26.1 ± 2.6 °C. Systematic uncertainties in relative humidity and temperature readings were 0.25% RH and 1 °C, respectively. Films were equilibrated at 5% RH for 10 min, and a measurement was recorded upon stabilization of the thickness reading. The RH was then increased to approximately 10%, and a thickness measurement was recorded after another 5-min equilibration. Thickness was subsequently measured as the RH was increased (absorption) by 10% increments, until 90% RH, and then decreased (desorption) by 10% decrements to 10% RH. This cycle was repeated a second time, swelling up to 99.7% RH and deswelling to 10% RH, using the same 10% steps. All film thicknesses were normalized to the initial 5% RH thickness values.

3. RESULTS AND DISCUSSION 3.1. Film Growth, Optical Transmission, and Structure. Multilayer assemblies were deposited with PVP and MMT to investigate the swelling behavior of PVP under humid conditions and its influence on the oxygen barrier of these thin films. LbL films are represented in the text and figures as (PVP/ MMT)x, where x is the number of bilayers deposited. Figure 1 shows the linear growth and mass of films deposited. The thickness and mass deposited were measured under ambient conditions after five and two bilayers, respectively, in an attempt to eliminate the substrate’s influence on the data. The film is shown to grow linearly having a total thickness of 378 nm at 40-BL (Figure 1A). Quartz crystal microbalance (QCM) measurements confirmed this linear growth trend (Figure 1B). QCM data also reveal that this thin film contains 74.1 wt % clay, which is similar to that of other LbL films made with clay12−14 but unheard of for bulk composites. Difficulties related to aggregation make it very difficult to incorporate more than 10 wt % clay into bulk materials by using more traditional processing techniques.50 Even more impressive is the fact that this high clay concentration is accompanied by high visible light transmission (i.e., the nanobrick wall structure is completely transparent). Light transmission, as a function of wavelength (390−750 nm), through these PVP/MMT assemblies (Figure 2A) 19853

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Figure 3. Relative thickness as a function of RH for (A) (PVP/MMT)30 and (B) (PVP/MMT)40.

(cm2·s·Pa)); however, permeability increases significantly at high humidity.11,13 The swelling properties of PVP and its hydrogen-bonding nature with MMT, which is in contrast to the reported polymer−clay electrostatic interactions, is hypothesized to allow the film to maintain tighter lateral clay platelet packing and improve the barrier properties of the film under humid conditions.41 The strong interactions between the polymer and clay, when grown via electrostatic interactions in the case of previously studied PEI/MMT assemblies, most likely cause gaps to form between platelets during swelling, but weaker hydrogen bonds between clay and PVP allow for some slip that preserves the unswollen, lateral packing and platelet overlap. PVP/MMT assemblies are interesting because of the plasticization by water that occurs to deposited PVP under high RH conditions. This happens when the water molecules reach a certain concentration in the film and significantly lower the Tg of PVP, which then transitions from a glassy state to a less glassy, or rubbery, state.53−55 After this transition, PVP swells more quickly as the free volume within PVP decreases and it forms hydrogen bonding with water. As PVP swells and becomes denser, it is expected that the clay layers within the film mostly maintain their tight lateral packing between polymer layers instead of swelling with the polymer as is the case with electrostatically grown polymer−clay films. This is because the hydrogen bonds are weaker than electrostatic bonds, allowing them to break and reform as PVP expands, leaving the individual clay layers virtually untouched with regard to packing and overlap. It has also been shown previously that increasing the spacing between clay layers within a film can improve the gas barrier.12,13 In the case of PVP/MMT, the clay spacing changes with humidity, having greater spacing at higher humidity. With the maintenance of tightly packed clay layers afforded by hydrogen bonds, this allows for the improvement of oxygen barrier when exposed to high humidity. Relative thickness measurements of (PVP/MMT)30 and (PVP/MMT)40 are shown in Figure 3 as a function of RH. Relative humidity was cycled between 10 and 99.7% for each film. For the 30-BL film (Figure 3A), relative thickness during swelling ranged from approximately 1.00 at 10% RH up to 1.20 at 90% RH and then dramatically increased to 1.83 at 99.7% RH. A similar trend was observed for the deswelling process, where relative thickness was measured to be 1.24 at 90% RH

and 1.01 at 10% RH. A thickness disparity of less than 5% at both ends of the cycle, and a maximum thickness disparity of 5% at 70% RH, suggests that there is minimal alteration of the film structure. Similar trends are observed for the 40-BL film (Figure 3B), with relative thickness ranging from 1.00 at 10% RH to 1.25 at 90% RH and again dramatically swelling to 1.93 at 99.7% RH. Deswelling the film showed minimal hysteresis, much like the 30-BL film, with measured relative thickness values of 1.29 at 90% RH and 1.01 at 10% RH. Each film was cycled a second time to test the integrity of the film after being exposed to high humidity and very similar results were obtained, providing further evidence that these films maintain their highly oriented structure. The maintenance of an ordered structure with multiple swelling/deswelling cycles suggests the oxygen barrier could now be maintained or even improved at high humidity. 3.3. Influence of Humidity on Oxygen Barrier. The oxygen transmission rate of (PVP/MMT)30 and (PVP/ MMT)40 films, deposited on PET, was measured at 0 and 100% RH to determine how humidity-induced swelling influences the gas barrier. It was recently shown that increasing the spacing between clay layers increases the barrier of polymer−clay LbL films.12 This improved barrier is attributed to a longer tortuous pathway of oxygen molecules within the film. Subjecting PVP/MMT films to elevated RH causes their clay spacing and associated tortuous pathway to become greater, as the average thickness per bilayer at 0% RH (∼8.8 nm) doubles at 100% RH. This increase in tortuous pathway length decreases OTR, creating improved barrier with increased humidity, as shown in Figure 4. The OTR of the bare PET film used in this study is approximately 8.6 (cc/(m2·day·atm)) at 0% RH and 6.6 (cc/(m2·day·atm)) at 100% RH. When this PET film is paired with a (PVP/MMT)30 nanocoating, the OTR is decreased by more than a factor of 2 (∼4.2 [cc/(m2·day·atm)]) under dry conditions. More importantly, an OTR of 4.092 (cc/ (m2·day·atm)) is achieved at 100% RH. A 40-BL coating exhibited a larger improvement at high humidity, with an OTR of 3.922 (cc/(m2·day·atm)) under dry conditions and 3.488 (cc/(m2·day·atm)) at 100% RH, resulting in an overall decrease in OTR of 11.1%. This 40-BL PVP/MMT assembly lowers the OTR of PET by 47%, while only increasing the thickness by 0.8%. This positive correlation between humidity and barrier is unprecedented and marks a significant step in the development of flexible thin film gas barriers. 19854

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BL films deposited on 179 μm PET is shown to decrease with increasing relative humidity. A 40-BL film exhibited an 11.1% decrease in OTR at 100% RH, which is unprecedented for any polymer−clay composite. This study demonstrates, for the first time, the ability of a layer-by-layer assembled thin film of polymer and clay to exhibit greater gas barrier upon exposure to humidity. This humidity-responsive gas barrier behavior is an important step toward using these films for practical applications such as flexible electronics and food packaging.

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AUTHOR INFORMATION

Corresponding Author

*[email protected] (A.J.N.); [email protected] (J.C.G.) Notes

The authors declare no competing financial interest.

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Figure 4. Oxygen transmission rate as a function of RH for 30 and 40BL PVP/MMT assemblies on 179 μm PET film.

ACKNOWLEDGMENTS We acknowledge The Dow Chemical Company, the National Science Foundation Grant DBI-0116835, and the Texas Engineering Experiment Station for financial support of this work. A.J.N. and K.E.S. acknowledge support in part from the Lilly Endowment, Inc. under Grant No. 2004-1872-000, Faculty Success Grants to Attract and Retain Rose-Hulman Faculty.

Even the modest reduction in oxygen barrier of these PVP/ MMT coatings with increasing humidity is significant relative to previous studies that exhibited significant decreases in barrier with increasing RH.11,13 PVP/MMT coating permeability was determined via decoupling from the total permeability using a previously described method.56 Both 30-BL and 40-BL films displayed similar trends. At 40-BL, the permeability increased by a small factor of approximately 2 with values of 0.062 (10−16 cm3(STP)·cm/(cm2·s·Pa)) and 0.123 (10−16 cm3(STP)·cm/ (cm2·s·Pa)) at 0 and 100% RH, respectively. This is the same factor by which the thickness increased at 100% RH, suggesting that the dry and swollen film contributions to overall barrier performance were approximately the same. This was verified by calculating the barrier improvement factor (BIF), which can be represented as P BIF = s PT (1)

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REFERENCES

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where Ps is the permeability of the substrate (i.e., bare PET) and PT is the total permeability (i.e., PET and nanocoating). Bare PET has a permeability of 17.5 (10−16 cm3(STP)·cm/ (cm2·s·Pa)) at 0% RH and 13.5 (10−16 cm3(STP)·cm/ (cm2·s·Pa)) at 100% RH, while the total permeability of the 40-BL film was 8.1 (10−16 cm3(STP)·cm/(cm2·s·Pa)) and 7.2 (10−16 cm3(STP)·cm/(cm2·s·Pa)), respectively. These values translate to a barrier improvement factor of 2.2 at 0% RH and 1.9 at 100% RH. This maintenance of BIF at 100% RH demonstrates the unique humidity-responsive barrier properties of these films.

4. CONCLUSION The layer-by-layer assembly of hydrogen-bonding polyvinylpyrrolidone and montmorillonite clay platelets was used to demonstrate the ability to improve gas barrier with increasing relative humidity. These thin films grow linearly as a function of bilayers deposited and exhibit minimal swelling hysteresis when subjected to increasing and decreasing levels of relative humidity. Visible light transmission data revealed that these films were greater than 95% transparent, even with a clay concentration greater than 74 wt %. The orientation of clay platelets parallel to the substrate upon deposition was confirmed by TEM. Oxygen transmission rate of 30- and 4019855

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