Article pubs.acs.org/Langmuir
Polymer−Graphene Oxide Quadlayer Thin-Film Assemblies with Improved Gas Barrier Ping Tzeng,† Bart Stevens,‡ Ian Devlaming,‡ and Jaime C. Grunlan*,‡ †
Department of Chemical Engineering, Texas A&M University (TAMU) 3122 TAMU, College Station, Texas 77843-3122, United States ‡ Department of Mechanical Engineering, Texas A&M University (TAMU) 3123 TAMU, College Station, Texas 77843-3123, United States S Supporting Information *
ABSTRACT: Layer-by-layer assembly was used to create quadlayers (QLs) of chitosan (CH), poly(acrylic acid) (PAA), CH, and graphene oxide (GO). Electron microscopy confirmed GO coverage over the film and a highly ordered nanobrick wall structure. By varying pH deviation between CH and PAA, a thick and interdiffused polymer matrix was created because of the altered chain conformation. A 5 CH (pH 5.5)/ PAA (pH 3)/CH (pH 5.5)/GO QL assembly (48 nm) exhibits very low oxygen permeability (3.9 × 10−20 cm3 cm cm−2 Pa−1 s−1) that matches SiOx barrier coatings. In an effort to maintain barrier performance under high humidity, GO was thermally reduced to increase hydrophobicity of the film. This reduction step increased H2/CO2 selectivity of a 5 QL film from 5 to 215, exceeding Robeson’s upper bound limit. This unique water-based multilayer nanocoating is very promising for a variety of gas purification and packaging applications.
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INTRODUCTION Improving the gas barrier in packaging films is an ongoing endeavor. For the past 2 decades, technologies such as SiOx, composite films, and metallized plastic films have been successfully used to protect various items (food, electronics, etc.) from oxygen and moisture.1−3 In many cases, these barrier solutions require complex processing conditions,4 exhibit poor optical and/or mechanical behavior,5 and/or suffer from pinholes.6 Additionally, metal applied by vapor deposition can also pollute the environment and negatively impact human health.5 More recently, super gas barrier thin films have been deposited from water using layer-by-layer (LbL) assembly.7,8 These environmentally friendly nanocoatings overcome many of the limitations described for more traditional gas barriers. Having become popularized more than 20 years ago,9 LbL assembly is a simple processing technique that can be tailored via concentration,10 temperature,11 molecular weight,12 pH,13 and deposition time.14 By alternative exposure of a substrate to oppositely charged polyelectrolyte solutions (or nanoparticle suspensions), electrostatic attractions between the charged ingredients result in the buildup of anion/cation multilayer assemblies.15,16 LbL-deposited films have been used to deliver drugs,17 purify hydrogen gas,18 prevent reflection,19 tailor wettability,20 and protect polymeric substrates from fire.21 A high gas barrier is most often realized using clay as one of the ingredients.22,23 The electrostatic interaction between polyelectrolyte and clay forces the platelets to deposit in a highly © 2015 American Chemical Society
oriented fashion, creating a long tortuous pathway that reduces gas permeability.24 Nanobrick wall multilayer assemblies have shown an excellent barrier toward a variety of gases,7,25 but the presence of hydrophilic polymers causes plasticization in the presence of moisture, resulting in an increase in gas permeability.26 More hydrophobic platelets, such as graphene, have been examined to reduce this moisture sensitivity.27,28 Two-dimensional (2D) graphene (sp 2 carbon atom) possesses excellent electrical,29 mechanical,30 and thermal properties,31 making it an interesting platelet for LbL assembly. Considering the difficulty to exfoliate neutral, unmodified graphene, because of its strong van der Waals interactions, graphene oxide (GO) is a more appealing starting material.32 The hydroxyl and carboxyl groups on the GO surface enable aqueous deposition through electrostatic or hydrogen bonding (see structure in Figure 1b).33 In most cases, GO is paired with a cationic material to create a “bilayer” (BL) assembly.34,35 These multilayer assemblies exhibit low gas permeability but contain a high GO concentration (∼90 wt %),36 which is costly and produces strongly colored films. In the present study, chitosan (CH) and poly(acrylic acid) (PAA) were used to fabricate a “quadlayer” (QL) assembly with GO. Discrete, equally spaced layers of polymer and GO were Received: February 25, 2015 Revised: May 11, 2015 Published: May 13, 2015 5919
DOI: 10.1021/acs.langmuir.5b00717 Langmuir 2015, 31, 5919−5927
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Figure 1. Illustration of (a) LbL process, (b) materials used, and (c) nanobrick wall structure built from CH/PAA/CH/GO QLs.
Figure 2. Thickness of CH/PAA/CH/GO as a function of QLs deposited with varying (a) pH of CH and (b) pH of PAA. (c) Film growth of CH5.5/PAA3/CH5.5/GO is compared to a TEM cross-section of the 5 QL film. (d) Electron signal profile, scanned through the white dotted section in panel c. Valley points indicate the position of GO layers.
was discovered that this reduced GO assembly could also separate gas molecules with different sizes. With H2/CO2 selectivity of 215, this thin film membrane outperforms most known separation membranes. The advantages of this QL assembly, including its relatively low GO concentration (
