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Ultrastrong, Chemically Resistant Reduced Graphene Oxide-based Multilayer Thin Films with Damage Detection Capability Tyler Guin,† Bart Stevens,† Michelle Krecker,† John D’Angelo,† Mohammad Humood,† Yixuan Song,‡ Ryan Smith,§ Andreas Polycarpou,† and Jaime C. Grunlan*,†,‡,§ †
Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843-3123, United States Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843-3003, United States § Department of Chemistry, Texas A&M University, College Station, Texas 77843-3012, United States ‡
S Supporting Information *
ABSTRACT: Multilayer thin films of graphene oxide (GO) and poly(vinylamine) (PVAm) were deposited via layer-by-layer assembly. Poly(vinylamine) pH was used to tailor film thickness and GO layer spacing. Graphene oxide concentration in the films was controlled through simple pH adjustment. Thermal reduction of the PVAm/GO multilayer thin films rendered them electrically conductive, which could be further tailored with PVAm pH. These reduced films also exhibited exceptionally high elastic modulus of 30 GPa and hardness of 1.8 GPa, which are among the highest of any graphene-filled polymer composite values ever reported. Cross-linking of these films with glutaraldehyde improved their chemical resistance, allowing them to survive strongly acidic or salty solutions. Additionally, scratches in the films can be instantaneously detected by a simple electrical resistance measurement. These films are promising for a variety of packaging and electronic applications. KEYWORDS: reduced graphene oxide, layer-by-layer assembly, nanoindentation, cross-linking, poly(vinylamine)
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INTRODUCTION Graphene displays extraordinary mechanical,1 thermal,2 and electrical properties3 due to its unique two-dimensional nature.4 This nanoplatelet is ideal for gas barrier thin films,5 as it is impermeable to gases and liquids.6 It is also electrically conductive and useful for supercapacitors.7−9 Unfortunately, graphene is difficult to solution-process due to its poor solubility.10,11 Graphene oxide (GO) is an oxidized version of graphene,4 which is readily dispersible in water and displays a strong anionic surface charge.12 It can be reduced through chemical, electrical, or thermal means to produce reduced graphene oxide (rGO),13 which is simply modestly imperfect graphene (i.e., some oxidation sites remain postreduction).4 Polymer nanocomposites utilizing rGO display high modulus,14 gas barrier,15 thermal stability,16 and electrical conductivity.17 The structure of these nanocomposites has a strong influence on their properties,18,19 but it is difficult to control the nanostructure.11 Layer-by-layer (LbL) assembly is a facile method to fabricate thin composite films from waterborne materials,20,21 such as GO, with excellent control of the nanostructure.22 LbL assembly most commonly deposits multilayer ultrathin films through alternate exposure of a substrate to aqueous solutions containing oppositely charged polyelectrolytes.21,23−25 GObased multilayer thin films display super gas barrier,24,26−30 gas separation,29 and mechanical properties30 due to their high packing density and highly oriented nanostructure.31 It was © XXXX American Chemical Society
recently shown that the surface platelet packing, and therefore gas barrier, of GO-based thin films can be modified and improved by adjusting the pH of the GO deposition solution.26 In a separate study, it was shown that thermal reduction produced electrically conductive rGO-based multilayer films, which also displayed improved resistance to moisture.27 Similar rGO-based multilayer films display high specific capacitance (>1550 F/cm3)32 and electrical conductivity33 due to their wellordered structure. Despite all these impressive properties, there has been little work detailing how to modify the multilayer structure directly, which would allow for improvements in these rGO-based nanocomposite films. In this work, poly(vinylamine) (PVAm) was alternately deposited with GO via LbL assembly. The thin-film nanostructure was controlled through simple pH adjustment. The thickness of the films was found to be directly correlated to the degree of protonation (DOP) of the PVAm solution, which allows for direct control of GO platelet spacing through pH adjustment. The concentration of GO in these films was monitored by UV−vis spectroscopy, and it was found that GO concentration is directly linked to solution pH. PVAm/GO films were thermally reduced to produce electrically conductive PVAm/rGO films, and it was found that the rGO spacing and Received: December 23, 2015 Accepted: February 17, 2016
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DOI: 10.1021/acsami.5b12596 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Schematic of layer-by-layer (LbL) assembly procedure and the chemical structures of PVAm and GO. (Kimberly Clark, Neenah, WI) on both sides after a brief rinse with DI water. Mechanical properties of PVAm/GO, PVAm/rGO, and xPVAm/rGO films were evaluated with a TriboScope TS-75 (Hysitron, Inc., Eden Prairie, MN), mounted on a multimode atomic force microscope (AFM) (Bruker, Billerica, MA). AFM images of films before indentation were taken in tapping mode. Indentation tests were carried out by use of a Berkovich tip, with a tip radius of 150 nm. Calibration was performed by the Oliver and Pharr method,34 on fused quartz (E = 69.6 GPa), from 15 to 50 nm. Indentation depth was
