Letter www.acsami.org
Combined High Stretchability and Gas Barrier in Hydrogen-Bonded Multilayer Nanobrick Wall Thin Films Shuang Qin,† Yixuan Song,† Michael E. Floto,‡ and Jaime C. Grunlan*,†,‡,§ †
Department of Materials Science and Engineering, ‡Department of Chemistry, and §Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843, United States S Supporting Information *
ABSTRACT: Hydrogen-bonded multilayer thin films are very stretchable, but their gas barrier properties are modest compared to more traditional ionically bonded assemblies. In an effort to improve the gas barrier of poly(ethylene oxide) (PEO)− poly(acrylic acid) (PAA) multilayer films without sacrificing stretchability, montmorillonite (MMT) clay platelets were combined with PAA and alternately deposited with PEO. A ten-bilayer PEO/PAA+MMT film (432 nm thick), deposited on a 1 mm PU substrate, resulted in a 54× reduction in oxygen transmission rate after exposure to a 20% strain. This system is the best combination of stretchability and gas barrier ever reported. KEYWORDS: layer-by-layer assembly, hydrogen bonding, nanobrick wall structure, oxygen barrier, clay nanoplatelets ransparent, ultrathin gas barrier films are important in food, pharmaceutical, and electronic packaging.1−3 With nanometer-scale control over film structure, layer-by-layer (LbL) assembly has proven to be an efficient method for preparing thin film gas barrier coatings.3−5 LbL-deposited nanocoatings containing clay, with a nanobrick wall structure, exhibit especially low gas permeability.6−8An assembly prepared from montmorillonite clay (MMT) and polyethylenimine (PEI), with a thickness below 120 nm, was shown to reduce the oxygen transmission rate (OTR) of 179 um thick polyethylene terephthalate (PET) by 3 orders of magnitude.9 The strong ionic bonding between anionic MMT and cationic PEI facilitates a high level of exfoliation and orientation of clay platelets, resulting in an extremely tortuous brick wall structure. This structure is critical for oxygen barrier because it significantly lengthens the pathway for diffusing oxygen molecules. Despite their super gas barrier, these nanobrick wall assemblies are typically stiff and cracks develop upon stretching. Unlike stretchable conductors, where surface defects are tolerable because conductivity remains if the network structure survives the strain,10 stretchable gas barrier films must be crackfree because the cracks function as oxygen transmission highways.11 For example, the cracks that appear after 10%
T
© XXXX American Chemical Society
strain of a PEI/MMT nanocoating leads to an orders of magnitude increase in gas permeability.12,13 Hydrogen-bonded multilayer thin films are generally less stiff than their ionically bonded counterparts. The weaker hydrogen bonding creates a looser film structure with lighter cross-link density, enabling greater strain without damage.14,15 A hydrogen-bonded assembly of poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA) was recently shown to remain crack free even at 100% strain and PEO/tannic acid maintained its barrier after repeated exposure to 100% strain.13,16 Unfortunately, stretchability typically improves at the cost of gas permeability, because the barrier of LbL-deposited films relies upon density, which correlates to stiffness.17 One strategy to improve this situation is to modify hydrogen-bonded systems to improve barrier while retaining their stretchability. An early attempt to accomplish this involved adding a hydrogen-bonded polyglycidol (PGD) layer to ionically bonded PEI/MMT, which withstood a 10% strain and maintained a 4× OTR reduction on a thick PET substrate.12 Combining hydrogen-bonded PEO/PAA layers with ionically bonded Received: January 17, 2017 Accepted: February 23, 2017 Published: February 23, 2017 A
DOI: 10.1021/acsami.7b00844 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 1. (a) Schematic of the PEO/PAA+MMT layer-by-layer process. (b) Thickness as a function of bilayers deposited for films with varying MMT concentration and (c) FTIR spectra of MMT, PAA, and a 50:50 PAA+MMT mixture.
prepared PEO/PAA+MMT films are denoted as PPM [X%], where PPM represents PEO, PAA, and MMT. [X%] is the weight percent solids of MMT in the 1 wt % PAA+MMT deposition solution (Table S1). Films containing clay, poly(ethylene oxide) and poly(acrylic acid) were assembled on silicon wafers to monitor film growth as a function of the number of bilayers deposited. Figure 1b shows exponential growth for PPM [25%] and PPM [50%], whereas the PPM [100%] system grows linearly due to the absence of PAA. Similar growth patterns are observed with a quartz crystal microbalance (QCM), which measures the weight of deposited layers (Figure S1). The “in-and-out” interlayer diffusion of PAA within these films contributes to the observed exponential growth.19,20 When the chain mobility is insufficient for interlayer diffusion during the deposition, films grow linearly.21 This explains the changes in growth pattern as a function of clay concentration, with increasing clay concentration limiting the mobility of PAA. When MMT is dispersed in pH 2.5 water it aggregates and ultimately settles out of solution because of the house-of-cards structure it adopts.22 When dispersed in PAA solution, a stable light brown suspension is formed due to the interaction between polymer and clay, which is observed in FTIR spectra (Figure 1c). The spectrum of MMT alone exhibits Si−OH stretching vibrations at 3626 cm−1, Si−O stretching vibrations at 997 cm−1, and H−O−H bending vibrations at 1642 cm−1. The sharp peak at 1700 cm−1 for PAA containing samples is attributed to the CO stretching vibration of − COOH groups. In the spectrum of PAA+MMT, there is a shift and intensity reduction of the Si−O peak (from 973 to 1016 cm−1)
PEI/PAA layers yielded a 10× OTR reduction for a 1 mm thick polyurethane substrate exposed to 20% strain.18 PEO/PAA alone produces only a 5× reduction with the same strain on a much lower barrier 1.6 mm natural rubber substrate.13 In the present work, poly(acrylic acid) and clay were combined in a single deposition solution and alternately assembled with PEO in an effort to further improve gas barrier at high strain. A nanobrick wall structure with homogeneous coverage, high level of alignment, and exfoliation of clay platelets was confirmed with energy-dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM), and crosssectional transmission electron microscopy (TEM). Without PAA in the clay solution (i.e., a PEO/MMT assembly), clay misalignment was observed. A 10 BL PEO/PAA+MMT film (432 nm thick) reduced the OTR of a 1 mm polyurethane rubber substrate more than 2 orders of magnitude. Unlike ionically bonded nanobrick wall films, the hydrogen-bonded films are surprisingly stretchable and maintain high barrier at high strain. The 10 BL film reduced the OTR of the PU substrate by 54× after being subjected to a 20% strain. This system has the lowest OTR and permeability ever reported at such a high strain. This incredible stretchability is believed to be due to the hydrogen bonding between the PEO/PAA and PAA/MMT interfaces. Barrier films were deposited layer-by-layer onto the polyurethane with 0.1 wt % PEO and 1 wt % PAA+MMT solutions adjusted to pH 2.5 (Figure 1a). The reason for choosing pH 2.5 for assembling is that it yields the thickest PEO/PAA film.13 Varying the MMT:PAA ratio in the aqueous deposition solution was evaluated (keeping total solids at 1 wt %). The B
DOI: 10.1021/acsami.7b00844 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces and reduced intensity of the Si−OH peaks at 3626 cm−1. This interaction is believed to be the hydrogen bonding between PAA and MMT.23,24 The interaction between PAA and MMT has been well-studied.23−28 It was observed that the clay adsorbs PAA from aqueous solution and forms intercalated PAA/MMT that facilitates clay dispersion and exfoliation.28 As a result, MMT platelets remain stably suspended in PAA solution at pH 2.5. During layer-by-layer deposition, hydrogen bonds are formed between PAA and PEO. With the help of adsorbed PAA, the MMT nanoplatelets are homogeneously incorporated into the multilayer films. Uniform clay dispersion can be seen in EDS images (Figure S3). AFM images show varying topography with and without PAA. As shown in Figure 2a, the film with
Figure 3. Schematics of PEO/MMT films assembled (a) with and (b) without PAA present, and (c) hypothesized behavior of the thin film assembled with PAA when applying external strain.
role this anionic polymer plays in building the hydrogenbonded nanobrick wall structure. PAA acts to first stabilize and then separate the MMT platelets in the pH 2.5 solution. The platelets are then incorporated in an aligned structure in the PEO/PAA multilayer film. The hydrogen-bonded brick wall nanostructure in these films serves to reduce the oxygen transmission rate (OTR) by creating an extremely torturous pathway for permeating molecules. Figure 4a shows the OTR of the PEO/PAA +MMT multilayers, each with a thickness of ∼500 nm. The transmission rate is nearly an order of magnitude lower than a 20 BL PEO/PAA film that obtained 10× OTR reduction on natural rubber.13 A 10 BL PPM [25%] film reduces the OTR of a 1 mm thick PU rubber substrate by a factor of 80. PPM [50%] reduces OTR by more than 100×. This improved oxygen barrier is likely due to higher MMT concentration, based upon EDS measurement (Figure S2). As expected based on structural analysis, PPM [100%] has the worst barrier among the three films evaluated, despite having the highest MMT concentration. Figure 3b highlights the large gaps between MMT platelets that are caused by clay misalignment and act as oxygen transmission highways that increase permeability. Scanning electron microscopy (SEM) was used to image each film’s surface before and after applying strain (Figure 4b). The texture observed at 0% strain (for all samples) comes from the polyurethane substrate. For PPM [25%], no cracks are observed up to 20% strain. When stretched 20%, cracks perpendicular to the stretch direction appear in PPM [50%] and PPM [100%], which makes PPM [25%] the best candidate for a truly stretchable gas barrier. A 10 BL, 432 nm thick PPM [25%] film exhibits 54 and 46× OTR reduction after 10 and 20% strain, respectively, relative to an uncoated 1 mm PU substrate (Figure 4a). The permeability of PPM [25%] remains almost 5 orders of magnitude lower at both 10 and 20% strain. This appears to be the lowest OTR and permeability even reported for a gas barrier thin film after 20% strain. The previous best system maintained a 10× reduction on the same substrate.18 This superior oxygen barrier after high strain is believed to be due to the hydrogen-bonded nanobrick wall structure. When exposed to external strain, the bond-slip and reorientation along the PEO/PAA and PAA/MMT interfaces reduce the strain felt by the thin film (Figure 3c).29 When there are more clay platelets present, they constrain polymer chain motion and reduce the stretchability of the
Figure 2. Atomic force microscope phase images of 10 BL (a) PPM [25%] and (b) PPM [100%] deposited on silicon. TEM crosssectional images of 10 BL (c) PPM [25%] and (d) PPM [100%] deposited on PET.
PAA (i.e., PPM [25%]) is relatively featureless, which suggests the clay platelets are fully embedded in the polymer matrix. The topography is very similar for the PPM [50%] film. In Figure 2b, clay platelets are easily recognized, with similar topography to ionically bonded PEI/MMT films.9 Clay misalignment can also be observed in this PPM [100%] film (Figure 2d), which leads to a rougher surface. Height images of PPM [25%], PPM [50%], and PPM [100%] on silicon wafers have root-meansquare (RMS) surface roughness of 47.2, 54.9, and 70.2 nm, respectively. The decreasing surface roughness with increasing PAA content suggests better alignment of clay platelets in the film. Clay alignment in these multilayer films is more clearly observed using cross-sectional TEM images of 10 BL films deposited on PET. As shown in Figure 2c, individual MMT platelets are the dark parallel lines in the PPM [25%] film, revealing a well-ordered nanobrick wall structure that is typical for ionically bonded polymer/clay assemblies.6,9 Platelet stacking and misalignment is observed in the PPM [100%] film (Figure 2d), which is the likely cause of its greater thickness. Figure 3 shows schematics of these two different film structures. High levels of clay orientation are only observed in films containing PAA (Figure 3a), demonstrating the critical C
DOI: 10.1021/acsami.7b00844 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 4. (a) Oxygen transmission rate of 10 BL PPM [25%], deposited on polyurethane rubber, after exposure to varying strain. (b) Scanning electron microscope surface images of PPM [25%], PPM [50%], and PPM [100%] after exposure to varying levels of strain.
nanocoating. As a result, the PPM [50%] film exhibits greater barrier loss with strain, yielding only 16 and 17× OTR reduction at 10 and 20% strain, respectively. It is worth noting that the loss of barrier from 0 to 20% strain for PPM [25%] is similar to that of the PEO/PAA film,13 demonstrating that the incorporation of some amount of MMT clay platelets can improve the gas barrier without compromising stretchability. The barrier properties of these 10 BL films are summarized in Table 1.
requiring gas barrier for elastomeric substrates (e.g., balloons, tires, seals, etc.).
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00844. Experimental details, QCM curves of three samples, and EDS maps of atomic percentage and distribution (PDF)
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Table 1. Barrier Behavior of Ten Bilayer Thin Films Deposited on 1 mm Thick Polyurethane Rubber
system PU substrate PPM [25%] PPM [50%]
OTR (cc/m2 day atm)
0 10 20 0 10 20 0 10 20
138 138 138 1.74 2.54 3.04 1.29 8.11 8.43
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: 979-845-3027.
permeability (× 10−16 (cm3 cm)/ (cm2 Pa s)) strain (%)
ASSOCIATED CONTENT
S Supporting Information *
ORCID
Jaime C. Grunlan: 0000-0001-5241-9741
thickness (nm)
total
432 432 432 549 549 549
1200 1200 1200 15 22 26 11 71 73
coatinga
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Science Foundation (CBET 1403686) and infrastructural support from the Texas A&M Engineering Experiment Station (TEES), Microscopy & Imaging Center (MIC), and Materials Characterization Facility (MCF) at Texas A&M University.
0.02 0.03 0.03 0.02 0.11 0.11
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a Thin film permeability decoupled from the PU substrate was calculated using a previously described method.30
REFERENCES
(1) Dou, Y.; Pan, T.; Xu, S.; Yan, H.; Han, J.; Wei, M.; Evans, D. G.; Duan, X. Transparent, Ultrahigh-Gas-Barrier Films with a Brick− Mortar−Sand Structure. Angew. Chem., Int. Ed. 2015, 54, 9673−9678. (2) Robertson, G. L. Food Packaging: Principles and Practice, 3 rd ed.; CRC Press: Boca Raton, FL, 2016. (3) Priolo, M. A.; Holder, K. M.; Guin, T.; Grunlan, J. C. Recent Advances in Gas Barrier Thin Films via Layer-by-Layer Assembly of Polymers and Platelets. Macromol. Rapid Commun. 2015, 36, 866−879. (4) Richardson, J. J.; Cui, J.; Björnmalm, M.; Braunger, J. A.; Ejima, H.; Caruso, F. Innovation in Layer-by-Layer Assembly. Chem. Rev. 2016, 116, 14828−14867. (5) Yan, N.; Capezzuto, F.; Buonocore, G. G.; Lavorgna, M.; Xia, H.; Ambrosio, L. Gas-Barrier Hybrid Coatings by the Assembly of Novel Poly (vinyl alcohol) and Reduced Graphene Oxide Layers through
In conclusion, a highly stretchable gas barrier nanocoating was obtained by successfully incorporating highly aligned clay platelets in a hydrogen-bonded PEO/PAA multilayer assembly. A 432 nm thick, 10 BL PPM [25%] film deposited on 1 mm PU exhibits 80, 54, and 46× OTR reduction at 0, 10, and 20% strain, respectively, and retains a permeability 5 orders of magnitude lower than that of the rubber substrate. This is the best barrier ever reported for a transparent polymeric coating exposed to such a high strain. This incredible oxygen barrier at high strain comes from the unique hydrogen-bonded brick wall nanostructure, making these films well-suited for applications D
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ACS Applied Materials & Interfaces Cross-Linking with Zirconium Adducts. ACS Appl. Mater. Interfaces 2015, 7, 22678−22685. (6) Song, Y.; Hagen, D. A.; Qin, S.; Holder, K. M.; Falke, K.; Grunlan, J. C. Edge Charge Neutralization of Clay for Improved Oxygen Gas Barrier in Multilayer Nanobrick Wall Thin Films. ACS Appl. Mater. Interfaces 2016, 8, 34784−34790. (7) Soltani, I.; Smith, S. D.; Spontak, R. J. Effect of Polyelectrolyte on the Barrier Efficacy of Layer-by-Layer Nanoclay Coatings. J. Membr. Sci. 2017, 526, 172−180. (8) Song, Y.; Tzeng, P.; Grunlan, J. C. Super Oxygen and Improved Water Vapor Barrier of Polypropylene Film with Polyelectrolyte Multilayer Nanocoatings. Macromol. Rapid Commun. 2016, 37, 963− 968. (9) Priolo, M. A.; Gamboa, D.; Grunlan, J. C. Transparent Clay− Polymer Nano Brick Wall Assemblies with Tailorable Oxygen Barrier. ACS Appl. Mater. Interfaces 2010, 2, 312−320. (10) Cheng, T.; Zhang, Y.; Lai, W. Y.; Huang, W. Stretchable ThinFilm Electrodes for Flexible Electronics with High Deformability and Stretchability. Adv. Mater. 2015, 27, 3349−3376. (11) Yanaka, M.; Henry, B.; Roberts, A.; Grovenor, C.; Briggs, G.; Sutton, A.; Miyamoto, T.; Tsukahara, Y.; Takeda, N.; Chater, R. How Cracks in SiOx-coated Polyester Films Affect Gas Permeation. Thin Solid Films 2001, 397, 176−185. (12) Holder, K. M.; Spears, B. R.; Huff, M. E.; Priolo, M. A.; Harth, E.; Grunlan, J. C. Stretchable Gas Barrier Achieved with Partially Hydrogen-Bonded Multilayer Nanocoating. Macromol. Rapid Commun. 2014, 35, 960−964. (13) Xiang, F.; Ward, S. M.; Givens, T. M.; Grunlan, J. C. Super Stretchy Polymer Multilayer Thin Film with High Gas Barrier. ACS Macro Lett. 2014, 3, 1055−1058. (14) Kharlampieva, E.; Sukhishvili, S. A. Hydrogen-Bonded Layer-byLayer Polymer Films. J. Macromol. Sci., Polym. Rev. 2006, 46, 377−395. (15) Lutkenhaus, J. L.; Hrabak, K. D.; McEnnis, K.; Hammond, P. T. Elastomeric Flexible Free-Standing Hydrogen-Bonded Nanoscale Assemblies. J. Am. Chem. Soc. 2005, 127, 17228−17234. (16) Xiang, F.; Givens, T. M.; Ward, S. M.; Grunlan, J. C. Elastomeric Polymer Multilayer Thin Film with Sustainable Gas Barrier at High Strain. ACS Appl. Mater. Interfaces 2015, 7, 16148−16151. (17) Humood, M.; Chowdhury, S.; Song, Y.; Tzeng, P.; Grunlan, J. C.; Polycarpou, A. A. Nanomechanical Behavior of High Gas Barrier Multilayer Thin Films. ACS Appl. Mater. Interfaces 2016, 8, 11128− 11138. (18) Cho, C.; Xiang, F.; Wallace, K. L.; Grunlan, J. C. Combined Ionic and Hydrogen Bonding in Polymer Multilayer Thin Film for High Gas Barrier and Stretchiness. Macromolecules 2015, 48, 5723− 5729. (19) Picart, C.; Mutterer, J.; Richert, L.; Luo, Y.; Prestwich, G. D.; Schaaf, P.; Voegel, J.-C.; Lavalle, P. Molecular Basis for the Explanation of the Exponential Growth of Polyelectrolyte Multilayers. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 12531−12535. (20) Lavalle, P.; Gergely, C.; Cuisinier, F.; Decher, G.; Schaaf, P.; Voegel, J.; Picart, C. Comparison of the Structure of Polyelectrolyte Multilayer Films Exhibiting a Linear and an Exponential Growth Regime: an In Situ Atomic Force Microscopy Study. Macromolecules 2002, 35, 4458−4465. (21) Salehi, A.; Desai, P. S.; Li, J.; Steele, C. A.; Larson, R. G. Relationship between Polyelectrolyte Bulk Complexation and Kinetics of Their Layer-by-Layer Assembly. Macromolecules 2015, 48, 400−409. (22) Tombácz, E.; Szekeres, M. Colloidal Behavior of Aqueous Montmorillonite Suspensions: The Specific Role of pH in the Presence of Indifferent Electrolytes. Appl. Clay Sci. 2004, 27, 75−94. (23) Fareed, M. A.; Stamboulis, A. Nanoclays Reinforced Glass Ionomer Cements: Dispersion and Interaction of Polymer Grade (PG) Montmorillonite with Poly (acrylic acid). J. Mater. Sci.: Mater. Med. 2014, 25, 91−99. (24) Shi, Y.; Xue, Z.; Wang, X.; Wang, L.; Wang, A. Removal of Methylene Blue from Aqueous Solution by Sorption on Lignocellulose-g-Poly (acrylic acid)/Montmorillonite Three-Dimensional Cross-
Linked Polymeric Network Hydrogels. Polym. Bull. 2013, 70, 1163− 1179. (25) Tran, N. H.; Dennis, G. R.; Milev, A. S.; Kannangara, G. K.; Wilson, M. A.; Lamb, R. N. Interactions of Sodium Montmorillonite with Poly (acrylic acid). J. Colloid Interface Sci. 2005, 290, 392−396. (26) Haraguchi, K.; Takehisa, T. Nanocomposite Hydrogels: A Unique Organic-Inorganic Network Structure with Extraordinary Mechanical, Optical, and Swelling/De-Swelling Properties. Adv. Mater. 2002, 14, 1120. (27) Qiu, H.; Yu, J. Polyacrylate/(Carboxymethylcellulose Modified Montmorillonite) Superabsorbent Nanocomposite: Preparation and Water Absorbency. J. Appl. Polym. Sci. 2008, 107, 118−123. (28) Tran, N.; Wilson, M.; Milev, A.; Dennis, G.; Kannangara, G.; Lamb, R. Dispersion of Silicate Nano-Plates within Poly (acrylic acid) and Their Interfacial Interactions. Sci. Technol. Adv. Mater. 2006, 7, 786−791. (29) Erbaş, A.; Horinek, D.; Netz, R. R. Viscous Friction of Hydrogen-Bonded Matter. J. Am. Chem. Soc. 2012, 134, 623−630. (30) Roberts, A.; Henry, B.; Sutton, A.; Grovenor, C.; Briggs, G.; Miyamoto, T.; Kano, M.; Tsukahara, Y.; Yanaka, M. Gas Permeation in Silicon-Oxide/Polymer (SiOx/PET) Barrier Films: Role of the Oxide Lattice, Nano-Defects and Macro-Defects. J. Membr. Sci. 2002, 208, 75−88.
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