polyethyleneimine film with enhanced hydrogen barrier properties by reactive layer-by-layer self-assembly

Accepted Manuscript Preparation of modified graphene oxide/polyethyleneimine film with enhanced hydrogen barrier properties by reactive layer-by-layer self-assembly Peng Li, Kuo Chen, Lili Zhao, Hongyu Zhang, Haixiang Sun, Xiujie Yang, Nam Hoon Kim, Joong Hee Lee, Q. Jason Niu PII:

S1359-8368(18)30919-3

DOI:

https://doi.org/10.1016/j.compositesb.2019.02.058

Reference:

JCOMB 6644

To appear in:

Composites Part B

Received Date: 23 March 2018 Revised Date:

11 January 2019

Accepted Date: 23 February 2019

Please cite this article as: Li P, Chen K, Zhao L, Zhang H, Sun H, Yang X, Kim NH, Lee JH, Niu QJ, Preparation of modified graphene oxide/polyethyleneimine film with enhanced hydrogen barrier properties by reactive layer-by-layer self-assembly, Composites Part B (2019), doi: https:// doi.org/10.1016/j.compositesb.2019.02.058. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Preparation of modified graphene oxide/polyethyleneimine

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film with enhanced hydrogen barrier properties by reactive

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layer-by-layer self-assembly

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Peng Lia,1, Kuo Chena,1, Lili Zhao b, Hongyu Zhanga, Haixiang Suna, Xiujie

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Yanga, Nam Hoon Kimc, Joong Hee Leec, Q. Jason Niua, *

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State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering,

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China University of Petroleum, Huadong, Qingdao 266580, China b

State Key Laboratory of Bioactive Seaweed substances, Qingdao Brightmoon

Seaweed Group, Qingdao 266580, China c

Advanced Materials Research Institute for BIN Convergence Technology &

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Department of BIN Convergence Technology, Chonbuk National University, Jeonju,

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Jeonbuk 561-756, South Korea

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*Correspondence authors: Q. Jason Niu ([email protected])

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These authors equally contributed to this work.

Abstract

Hydrogen barrier properties are characteristic of polymeric materials prepared with

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graphene; thus, they can be considered as a good substitute for the metal body of the

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traditional

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self-assembling film based on noncovalent force shows good hydrogen gas barrier

hydrogen

storage

tank.

Graphene

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oxide/polymer

layer-by-layer

ACCEPTED MANUSCRIPT properties. However, the dense film structure can be broken when the film is placed in

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water environment, especially acidic or alkaline environment, which induces to the leak

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of the hydrogen gas. Herein, a modified graphene oxide/polyethyleneimine reactive

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layer-by-layer self-assembled film for the hydrogen barrier was fabricated by the

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covalent bond self-assembled technology. Graphene oxide was modified with ethylene

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glycol diglycidyl ether to introduce epoxy groups that can react with polyethyleneimine

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to form covalent bonds. The modification time, modification pH value, and the feed ratio

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of graphene oxide/ethylene glycol diglycidyl ether were investigated in detail. Results

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indicate that the self-assembled films were prepared by covalent bonds between

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polyethyleneimine and modified graphene oxide. When the modification time was 6h,

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modification pH value was 2, and the feed ratio of graphene oxide/ethylene glycol

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diglycidyl ether was 0.05/0.23, the hydrogen transmission rate of 10-bilayer modified

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graphene oxide/polyethyleneimine self-assembled films was 289 cm3/m2·24h·0.1MPa,

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which was decreased by 78.8% compared to that of the polyethylene terephthalate

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substrate films (1365 cm3/m2·24h·0.1MPa). Furthermore, the modified graphene

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oxide/polyethyleneimine

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acid-resistance, alkali-resistance, salt-resistance and thermal-resistance properties.

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Keywords: A. Layered structures A. Thin films E. Assembly

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reactive

layer-by-layer

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self-assembled

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exhibit

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1.

Introduction

Hydrogen, as an important industrial raw material, is widely used in petroleum

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recovery and refining, fuel cells, electronics, food and chemical industries, and paint

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industry due to its advantages of environment friendly, light weight, high calorific value

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and wide availability [1-4]. However, currently limited technologies on hydrogen storage

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and transportation prevent its large-scale applications [5]. Presently, bulky steel tanks

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under low temperature and high pressure are commonly used to store and transport

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hydrogen. Nevertheless, high compression pressure is infeasible in the process of storage,

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because small hydrogen molecules can cause hydrogen corrosion and easily escape

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through the storage containers, which may lead to an explosion. Therefore, it is necessary

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to develop a new material with excellent hydrogen barrier properties to replace the metal

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body of traditional hydrogen storage tanks.

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Graphene has been widely applied in many fields due to its unique properties, such

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as excellent mechanical properties, electrical conductivity, and impermeability to

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gas/liquid [6-8]. Gopalsamy et al. reported the preparation of NiPd nanoalloy/graphene

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bifunctional nanocomposite for fuel cells [9]. In 2008, Bunch et al. [10] reported that

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graphene possessed gas barrier properties due to its special layered structure.

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Additionally, the polymeric materials prepared with graphene have good mechanical,

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chemical and thermal stabilities, and more importantly, hydrogen barrier properties.

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Therefore, it is considered as a good substitute for the metal body of the traditional

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hydrogen storage tank. Many technologies have been used to fabricate high quality

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ACCEPTED MANUSCRIPT graphene, for instance, micromechanical exfoliation [11], epitaxial growth [12],

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chemical vapor deposition [13, 14] and the reduction of graphene oxide (GO) [15].

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Nevertheless, stable single-layer graphene sheets cannot be obtained by these

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technologies due to the strong van der Waals attraction between different graphene layers.

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At the same time, graphene is difficult to be modified, which limits its large-scale

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production and application [16]. However, the chemical oxidation process of graphene is

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easy to operate on a large scale, and its oxidation products can participate in various

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chemical reaction processes.

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Compared with graphene, GO is a single-atomic-layer sheet, and GO sheet is easy to

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exfoliate and disperse in water because of its oxygen-containing groups. Therefore, GO

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is a promising material for preparing gas barrier composite films. For the past few years,

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reports on the fabrication of gas-barrier films via self-assembly of GO and polymers have

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been accessible. The regenerated cellulose/graphene oxide composite films prepared by

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Huang Huadong et al. [17] had excellent oxygen barrier properties. The O2 permeability

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coefficient was reduced by about 1000 times relative to the pure regenerated cellulose

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film at a low graphene oxide nanosheets loading of 1.64 vol%. Layek et al. prepared a

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layer-structured graphene oxide/polyvinyl alcohol (PVA) nanocomposite coated films

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that can significantly improve the hydrogen barrier properties [1]. The hydrogen gas

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transmission rates (GTRs) decreased significantly in GO/PVA nanocomposite coated

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films (GTRs = 5 cm3/m2·24h·0.1MPa) compared to uncoated polyethylene terephthalate

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(PET) films (GTRs = 122 cm3/m2·24h·0.1MPa). Bandyopadhyay et al. obtained

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nylon films [18]. The coated films exhibited noticeable reduction in the hydrogen gas

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transmission rate (H2GTR), and triethylenetetramine - modified graphene oxide (TET -

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mGO)/polyurethane (PU) with 22 wt% TET-mGO exhibited 93% decrease in H2GTR

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than the bare nylon film. High molecular weight polyethyleneimine (HPEI) modified

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graphene oxide coated PET films capable of significantly improving hydrogen barrier

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properties were fabricated by Park et al. [19]. The reduced graphene oxide (rGO)/HPEI

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film with a coating thickness of 9.5 µm showed a hydrogen gas transmission rate (GTR)

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value of 8 cm3/m2·24h·0.1MPa, representing a nearly 95.3% decrease in the GTR value

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compared to the uncoated PET film (GTR = 169.5 cm3/m2·24h·0.1MPa). Moreover, in

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addition to being widely used in the field of gas separation, GO can also be applied to

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areas such as water treatment [20-24].

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Layer-by-layer (LBL) self-assembly technology, as an effective method, has been

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broadly used to fabricate composite films [25, 26]. This technology was firstly reported

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by Decher et al. in 1991 [27], who used mutually adsorbed polyelectrolyte molecules with

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opposite electric charges to construct composite films via the LBL deposition method.

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Because the LBL self-assembly technology can effectively control the thickness and

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properties of films, it has been employed to fabricate nanoscale ultrathin composite films

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in the past two decades. Different driving forces are utilized in the LBL self-assembly

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process, such as electrostatic force [28], hydrophobic interactions [29], hydrogen

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bonding [30], and covalent bonding [31]. Recent studies on the LBL self-assembly

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strength [32], concentration [33], pH [34], and molecular weight [35] of the polymer

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solution. Due to the advantages of simple operation, being flexible, and precise control,

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the LBL self-assembly technology has also extended to prepare multifunctional thin

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films applied in perm-selective and gas-barrier membranes [36, 37]. Kim et al. obtained a

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high performance oxygen barrier film by alternately stacking negatively charged GO and

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positively charged amino-ethyl-functionalized GO (AEGO) on PET substrates by this

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technology [38]. Liu Hongyu et al. reported a facile approach for the fabrication of

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chemically-modified reduced graphene oxide based multilayer films for hydrogen barrier

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applications also by the LBL self-assembly technology [39]. Zhao Lili et al. reported the

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layer-by-layer self-assembly technology in the electric field to improve hydrogen barrier

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properties of GO/ polyethyleneimine (PEI) LBL self-assembled films [40, 41].

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In recent years, based on the electrostatic layer-by-layer self-assembly technology,

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the reactive layer-by-layer self-assembly technology has been developed by alternative

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coatings of two kinds of organic functional groups that can react to each other to form

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covalent bonds. Compared with the electrostatic LBL self-assembled film, the acid, alkali

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and salt resistance of the reactive LBL self-assembled film is improved, which widens its

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application scope. Nanoscale polyurethane films, polyimide films and linear polyamide

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films have been successfully prepared through the method [42]. Based on conventional

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condensation

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self-assembled films were obtained by controlling the concentration of two alternately

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polymerization

reaction,

the

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mGO/PEI

reactive

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polymerization, but the film thickness is much easier to control than that produced

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through interfacial polymerization, and the surface is smooth. Studies on the preparation

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of composite films have concentrated on the reaction between two kinds of monomers.

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There have been a few reports on the fabrication of polymer/nanomaterial composite

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films by the formation of covalent bonds. Jia et al. [43] reported that carboxylic acid and

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dihydric alcohol as cross-linking agents were used to esterify hydroxyl and carboxyl

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groups on the surface of GO to prepare covalently crosslinked GO membranes. Satti et al.

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[44] reported that a carbodiimide coupling agent was used to crosslink GO sheets and

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polyallylamine hydrochloride (PAH) to prepare composite films with tensile strengths up

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to 146 MPa. Ruoff et al. [45] reported that the chemically crosslinked graphene oxide

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sheet material can be produced through the reaction between amino-groups on

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polyallylamine (PAA) chains and epoxy groups on GO surface. Liu Hongyu et al. [46]

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prepared a PEI modified graphene oxide (PEI-mGO)/polyvinyl alcohol (PVA) composite

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films to enhance the hydrogen barrier properties of the film. The PEI-mGO/PVA film

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with 3.0 wt% of PEI-mGO content exhibited almost 95% decrease in the GTR value

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compared to PVA films.

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At present, the hydrogen barrier properties of the LBL self-assembled film prepared

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by the non-covalent bond, such as hydrogen bond or electrostatic force is easy to

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deteriorate in the acid, alkali or salt environment, which is disadvantage to hydrogen

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storage and transportation. Therefore, it is necessary to use covalent bonds instead of

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hydrogen barrier properties in different environments. In the present study, epoxy groups

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were introduced to the surface of GO to form covalent bonds with PEI to obtain

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mGO/PEI reactive layer-by-layer self-assembled films, which possessed acid, alkali, salt

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and thermal resistance. GO is a nanomaterial with a large size, and the reactivity of the

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surface hydroxyl group is lowered due to the limitation of its mobility. Therefore, it is

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necessary to introduce more reactive epoxy groups as much as possible by changing the

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reaction conditions to provide more covalent bonds for film self-assembly, which makes

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the film have more stable hydrogen barrier properties in different environments. So GO

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was modified with EDGE to introduce epoxy groups on the surface of GO. Then with the

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hydrogen barrier properties as the main index, the single factor control variable method

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was adopted to explore the recommended modification conditions, such as modification

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times, modification pH values and feed ratios. In addition, the effects of covalent bonds

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self-assembly process on the thickness, nanostructure and micromorphology of the film

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were analyzed. The hydrogen transmission rate (H2TR), ultraviolet (UV) absorbance,

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infrared (IR) absorbance and film thickness were measured. The surface topography of

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the films was observed by an atomic force microscopy (AFM).

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2. Experiment

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2.1. Materials

GO (oxidation degree > 95%) in the powder form was purchased from the Suzhou

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Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, China. Pure

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PEI (MW = 10,000 g.mol-1) was purchased from Aladdin Co., Ltd. (Shanghai, China).

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The concentration was 0.5 wt% for PEI water solution, 0.5 wt% for PEI ethanol solution,

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and 0.05 wt% for GO suspension. Pure ethylene glycol diglycidyl ether (EDGE) (MW =

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170 g.mol-1) was purchased from SA Chemical Technology Co., Ltd. (Shanghai, China).

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Pure ethylene glycol diglycidyl ether (EDGE) (MW = 400 g.mol-1) was purchased from

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Aike Reagent Co. Ltd. (Sichuan, China). Pure ethylene glycol diglycidyl ether (EDGE)

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(MW = 600 g.mol-1) was purchased from Hua Na Reagent Co. Ltd. (Sichuan, China). Pure

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industrial polyester film, PET (type 6020; thickness = 160 µm), was purchased from

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Yuxiang Electronic Material Co., Ltd. (Shanghai, China). The PET film, which was used

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as the deposition substrate for the gas barrier measurements, was cleansed with ultrapure

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water and ethanol, and then pretreated with an alkali–amine solution, which was prepared

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by mixing sodium hydroxide (8.4%) and ethylenediamine (0.6%). The PET film was

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dipped into the alkali–amine solution for 1 h at 70℃, then was rinsed with ultrapure water

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and dried before the LBL self-assembly.

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2.2. Modification and characterization of GO

The reaction between GO suspension (100 mL, 0.05 wt %) and EDGE (0.65g) was

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performed at 190 rpm for 6hours at 45℃. The reaction was accomplished based on the

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following steps: Step 1: The product was filtered by PTFE membrane with a pore size of

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0.45µm. Step 2: The product was rinsed with ultrapure water and ethanol. Step 3: The

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product was dispersed in ethanol (100mL) by ultrasonication (Autotune 500w,

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Shimadzu, Japan) for 30 min with a high-intensity sonicator to obtain a concentration of

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0.05 wt% mGO ethanol dispersion suspension. Step 4: The product was dried for 40 min.

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Step 5: Fourier transform infrared spectrometer (Vertex 7.0, Bruker Corporation,

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Germany), Raman spectra, and X-ray photoelectron spectroscopy (Thermo escalab

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250Xi, Thermo electron, USA) were used to characterize the chemical structure of

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mGO.

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2.3. Preparation of self-assembled films

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The preparation process of mGO/PEI films consists of the following steps: (1) Si

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wafers were pretreated with a piranha solution (70% H2SO4 and 30% H2O2) for 1 h

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under 90℃. (2) Preparation of the PEI solution at a concentration of 0.5 wt% and GO

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suspension at a concentration of 0.05 wt%. (3) The Si wafers and PET film were dipped

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into the PEI solution for 20 min. (4) The Si wafers and PET film were rinsed with

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ultrapure water and dried on a spin coater. (5) The Si wafers and PET film were

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immersed in mGO suspension for 20 min, then rinsed and dried again. This process

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bilayer. This entire process was repeated to prepare mGO/PEI films with different

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numbers of bilayers.

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2.4. Modification of GO

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During the reaction of EDGE modified GO, the epoxy groups on EDGE reacts

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with the hydroxyl groups on the surface of GO to form ether bonds, so the epoxy groups

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at the other end of EDGE were grafted on GO. But the reaction produced new hydroxyl

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groups, and the resulting hydroxyl groups could interact with new epoxy groups to form

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a crosslinked structure until the epoxy groups were exhausted. Therefore, the

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modification time, the modification pH and the feed ratio of GO/EDGE affect the

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composition of functional groups on the surface of mGO, which further affects the

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hydrogen barrier properties of mGO/PEI self-assembled films. In order to obtain

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reactive self-assembled films with good hydrogen barrier properties, the hydrogen

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transmission rate of reactive self-assembled films prepared with modified GO under

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different conditions were measured to explore suitable modification conditions.

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GO modified under different conditions was investigated: (1) The other

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modification conditions were kept consistent, and GO was modified with EDGE at

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different modification time, pH values and feed ratios to prepare mGO. (2)

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Characterized the surface functional group composition before and after GO

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modification by Fourier Transform Infrared Spectrometer (FTIR) and X-ray

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of GO. (3) The 10-bilayer mGO/PEI reactive layer-by-layer self-assembled films were

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fabricated and the H2TRs of the films were measured to investigate the hydrogen barrier

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properties. (4) With the hydrogen barrier properties as the main index, the mGO/PEI

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self-assembled films prepared at different modification times (pH values, feed ratios)

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were compared to determine the suitable modification conditions.

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2.5. Test and characterization of films

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Some indexes are used to analyze the results, including absorbance, thickness,

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morphologies, surface roughness, and hydrogen barrier properties. The absorbance of

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the film was measured by a UV–visible light (UV–Vis) spectroscopy with the

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wavelength range of 190–600 nm. The films were cut into discs with a diameter of 97

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mm and then inserted into a standard quartz cell (thickness = 1 mm). After taking a

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baseline calibration with a blank sample, the UV–Vis spectra were obtained for the

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films with different numbers of bilayers. The thickness of mGO/PEI films were

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measured using an Alpha-SE Ellipsometer (EC-400 and M-2000V, J. A. Woollam Co.

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Inc., Lincoln, NE, USA). The surface morphologies and the surface roughness were

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imaged with a multimode scanning probe microscope (MultiMode® 8, Bruker Corp.,

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Billerica, MA, USA), operated in tapping mode. The hydrogen barrier properties were

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measured with a pressure permeation instrument (Labthink Instruments Co., Ltd.,

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MPa was maintained throughout the measurements of the gas transmission rate.

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3. Results and discussion

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In this section, the structure of mGO were characterized by FTIR, Raman spectra

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and XPS firstly. In order to obtain reactive self-assembled films with good hydrogen

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barrier properties, GO was modified with EDGE under different condition, and the

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modified GO were studied by XPS in this section, and FTIR in Supplementary Data

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which support the XPS data further. Then, the characterization and hydrogen gas barrier

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properties of self-assembled films were discussed in detail. The photos of prepared GO,

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mGO as well as self-assembled films were shown in the Supplementary Data.

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3.1. Characterization of GO and mGO

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The introduction of more epoxy groups onto GO surfaces is crucial for the reactive

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layer-by-layer self-assembly process. Fig. 1 shows the scheme of GO modification by

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EDGE. Both ends of the EDGE molecular chain are epoxy groups, which can react with

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the hydroxyl groups on the surface of GO. The reaction of EDGE with GO further

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increases the epoxy group content of mGO surface, thereby promoting its covalent bond

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self-assembly with PEI. After modification, the dispersion stability of mGO in ethanol

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is improved as shown in Fig. S1 of Supplementary Data.

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Fig. 2a shows the FTIR of GO and mGO. The absorption peaks at 1720 cm-1 and

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1050 cm-1 were assigned to the stretching vibration of the carboxyl groups and bending 13

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GO. Simultaneously, the peak at 1050 cm-1 moves to 1080 cm-1, which indicates the C-O

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band absorption peak of hydroxyl groups on the surface of mGO moves to the position of

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the ether C-O bond absorption peak. Therefore, the content of epoxy groups increases.

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Fig. 2b shows the Raman spectra of GO and mGO. The characteristic D and G peaks of

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GO and mGO are presented. The D and G bands were fitted with Gaussian-shaped

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functions to estimate the ID/IG ratio between the areas of the peaks, which is an indication

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of the degree of medium range disorder. The positions of G peak and D peak have no

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change, while the ID/IG ratio (ID/IG = 1.09) that represents the disorder degree of the mGO

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lamellar structure increases compared to that of GO (ID/IG = 0.91). The results indicate

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that the degree of the disorder of the GO sheets increases after changing the structure of

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GO surface functional groups. The aforementioned changes confirm that the epoxy group

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content of mGO is increased, and EDGE was successfully grafted on the surface of GO.

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The modification of GO by EDGE was also validated by XPS. Fig. 3 shows the XPS

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C1s spectra of GO and mGO. The C1s high resolution spectra is presented to estimate the

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change of functional groups on the GO and mGO surfaces. The curves with binding

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energies at 288 eV, 286.4 eV, and 287.1 eV are corresponding to the C=O bond, C-OH

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bond and C-O-C band, respectively. The peak intensity of the C-O-C band of mGO

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increases significantly compared to that of GO, the peak intensity of the C=O band

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decreases slightly, and the peak intensity of the C-OH band changes little. These

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with EDGE, resulting in a significant increase in the epoxy group content of the mGO.

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3.2. Investigation of GO modification conditions

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Fig. 4a to 4d displays the XPS C1s spectra of mGOs modified at different time, and

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demonstrates the functional group composition before and after GO modification. The

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peak intensity of the C-O-C band of mGO increases compared to that of GO, the peak

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intensity of the C=O band decreases slightly, and the peak intensity of the C-OH band

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exhibits little change after 2h, 4h, and 6h modifications. However, the peak intensity of

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the C-O-C band of mGO decreases and the peak intensity of the C-OH band increases

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compared to those of GO after 24h modification. These trends indicate that the content of

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epoxy groups on the surface of mGO (modified for 2h, 4h，6h) increases, the content of

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carboxyl groups decreases, and the content of hydroxyl groups changes little. However,

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the content of the epoxy groups on the surface of mGO modified for 24h relatively

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reduces, but the content of the hydroxyl groups increases. The reason may be that long

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reaction time leads to the reaction of epoxy groups and water in the weak acid

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environment to form glycol structures. mGO modified under different times were also

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characterized by FTIR which were shown in Supplementary Data (Fig. S2).

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Fig. 5a to 5d displays the XPS C1s spectra of mGOs modified under different pH

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values. The peak intensity of the C-O-C band of mGOs increases significantly under

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acidic conditions (pH=2 and pH=4.5) compared to that of GO, and reaches the maximum

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Additionally, under alkaline conditions, the peak intensity of C-O-C and C=O bands

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decreases, while the peak intensity of the C-OH band increases. This meant that under

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alkaline conditions, the content of epoxy groups and carboxyl groups of mGOs decreases,

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while the content of the hydroxyl groups increases. The possible reason is that epoxy

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groups open loops to react with water to form the glycol structure and GO is easy to be

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reduced under alkaline conditions. Fig. S3 shows the FTIR of mGOs modified under

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different pH values, which support the results of XPS above.

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Fig. 6a to 6d shows the XPS C1s spectra of mGOs under different ratios of GO to

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EDGE. With the increase of the amount of EDGE added, the peak intensity of the C-O-C

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band increases, which means the content increase of epoxy groups. However, when the

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feed ratio of GO/EDGE increases to 0.05:1.3, the C-O-C band peak intensity slightly

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decreases while the C-OH band peak intensity increases, which indicates the slight

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reduction of the epoxy groups content and the increase of the hydroxyl groups content.

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Similar results can be obtained from FTIR spectra shown in Fig. S4. The reason for this

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phenomenon may be attributed to the strong hydrophobicity of EDGE. Moreover, with

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the feed ratio of GO/EDGE increases to 0.05:1.3, EDGE cannot fully mix with the

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hydrophilic GO due to the internal system established by itself. During the reaction,

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EDGE first reacts with water to form an ethylene glycol structure before reacting with

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GO.

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ACCEPTED MANUSCRIPT Fig. 7a shows the H2TRs of 10-bilayer mGO/PEI self-assembled ((mGO/PEI)10)

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films with mGOs modified under different times. As shown in the figure, the H2TRs of

319

mGO/PEI films are significantly reduced compared to PET films, which means that the

320

hydrogen barrier properties of PET films are improved effectively through mGO/PEI

321

reactive layer-by-layer self-assembly process. Although the mGO modified for 2 h

322

contained more epoxy groups, the hydrogen barrier properties of mGO/PEI reactive

323

layer-by-layer self-assembled films (H2TR = 298 cm3/m2·24h·0.1MPa) are not as good as

324

those prepared by the modification time for 6h (H2TR = 232 cm3/m2·24h·0.1MPa).

325

Therefore, 6 h period is recommended for the modification of GO in this study.

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Fig. 7b shows the H2TRs of (mGO/PEI)10 films prepared with mGOs modified

327

under different pH values. The H2TRs of mGO/PEI films (pH = 2, H2TR = 251

328

cm3/m2·24h·0.1MPa; pH = 4.5, H2TR = 232 cm3/m2·24h·0.1MPa) is significantly

329

reduced compared to PET films (H2TR = 1356 cm3/m2·24h·0.1MPa). This indicates that

330

mGO (modified at pH = 2 and pH = 4.5)/PEI self-assembled films possessed better

331

hydrogen barrier properties compared to PET films. But the H2TR of mGO/PEI films (pH

332

= 9.5) was 848 cm3/m2·24h·0.1MPa, which shows that the hydrogen barrier properties of

333

self-assembled films were deteriorated. The reason may be related to the higher degree of

334

reduction of mGO under alkaline conditions. Additionally, mGO is easily dispersed in

335

ethanol under acidic conditions, but mGO modified under alkaline conditions tends to

336

agglomerate and has poor dispersion stability. Therefore, mGO (modified under alkaline

337

conditions) is disadvantageous for the self-assembly adsorption process, and the

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self-assembled film prepared therefrom possesses a non-uniform structure and relatively

339

poor hydrogen gas barrier properties. Therefore, the modification pH of pH = 2 was

340

recommended for GO modification in this study. Fig. 7c shows the H2TRs of (mGO /PEI)10 films prepared with mGOs modified

342

under different GO/EDGE ratios. When GO/EDGE = 0.05:0.23, mGO/PEI

343

self-assembled films possessed good hydrogen barrier properties (H2TR = 232

344

cm3/m2·24h·0.1MPa) compared to PET films (H2TR = 1356 cm3/m2·24h·0.1MPa). But

345

when the GO/EDGE ratio increased to 0.05:1.3, the hydrogen barrier properties of

346

mGO/PEI self-assembled films began to deteriorate (H2TR = 316 cm3/m2·24h·0.1MPa).

347

Moreover, excessive EDGE (GO/EDGE = 0.05:1.3) made mGO easy to aggregate, and

348

the dispersion stability was deteriorated, which was disadvantageous for preparing the

349

hydrogen barrier film with a uniform structure. In addition, the H2TR of the mGO/PEI

350

self-assembled film (GO/EDGE = 0.05:0.65) was 232 cm3/m2·24h·0.1MPa, which is

351

close to the H2TR of the mGO/PEI self-assembled film (GO/EDGE = 0.05:0.23).

352

Therefore, the feed ratio of GO/EDGE=0.05:0.23 is recommended for the modification

353

of GO.

354

3.3. Growth and hydrogen barrier properties of mGO/PEI films

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Different bilayers of mGO/PEI reactive layer-by-layer self-assembled films were

356

prepared under the recommended modification conditions (modification time = 6h,

357

modification pH = 2 and GO/EDGE = 0.05/0.23). Fig. S5 shows the photos of mGO/PEI

18

ACCEPTED MANUSCRIPT

358

self-assembled films with different bilayers. As the number of self-assembled bilayers of

359

mGO/PEI films increases, the transparency of the film decreases gradually，which

360

indicates that the adsorption amount of mGO is gradually increased. Fig. 8 shows the absorbance and thickness of mGO/PEI and mGO/PEI

362

self-assembled films with different bilayers. The absorbance and thickness of the

363

self-assembled films increase linearly with the number of bilayers, and exhibit the same

364

growth trend. In addition, since there was no electrostatic repulsion during the

365

self-assembly process, there was more mGO deposited on the surface of PEI. The

366

absorbance (1.470) of the 10-bilayer mGO/PEI reactive self-assembled film is about 3

367

times relative to that of the 10-bilayer GO/PEI electrostatic self-assembled film (0.46).

368

Simultaneously, the EDGE introduced onto the surface of GO increases the interlayer

369

spacing of the mGO adsorption layer of the reactive self-assembled film, thereby the

370

thickness of the composite film obtained by reactive self-assembly technology increases.

371

The thickness (62 nm) of the 10-bilayer mGO/PEI reactive self-assembled film is larger

372

than that of the 10-bilayer GO/PEI electrostatic self-assembled film (47 nm).

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Fig. 9 shows the H2TR of GO/PEI and mGO/PEI self-assembled films with different

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bilayers. The change trend of H2TR of the mGO/PEI reactive self-assembled films is

375

basically the same as that of the GO/PEI electrostatic self-assembled films. As the

376

number of self-assembled bilayers increases, the H2TR of the multilayer composite films

377

gradually decreases. The hydrogen barrier properties of mGO/PEI reactive

378

self-assembled films are slightly worse than GO/PEI electrostatic self-assembled films,

19

ACCEPTED MANUSCRIPT and the H2TR of the 10-bilayer GO/PEI electrostatic self-assembled films (289

380

cm3/m2·24h·0.1MPa) reduces by 18.7% compared to that of mGO/PEI reactive

381

self-assembled films (235 cm3/m2·24h·0.1MPa). Moreover, the H2TR of the 10-bilayer

382

mGO/PEI reactive self-assembled films (289 cm3/m2·24h·0.1MPa) reduces by 78.8%

383

compared to that of PET substrate films (1365 cm3/m2·24h·0.1MPa).

384

3.4. FTIR characterization of mGO/PEI films

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Fig. 10 shows the FTIR of a 30-bilayer GO/PEI electrostatic self-assembled film

386

and a 30-bilayer mGO/PEI reactive self-assembled film. In the IR spectra of the

387

mGO/PEI film, the peak intensity of the methyl group at 2920 cm-1 and the peak intensity

388

of the methylene group at 2820 cm-1 increase compared to the GO/PEI electrostatic

389

self-assembled film, which indicates the content increase of methyl groups and

390

methylene groups of the mGO/PEI film. It is also can be seen that the C=O band

391

stretching vibration peak intensity of the carboxyl group at 1720 cm-1 is decreased,

392

indicating the content reduction of the carboxyl groups. The results are consistent with

393

the reduction in the carboxyl groups content and the increase in methyl/methylene groups

394

content after the modification of GO to mGO. Moreover, compared to that of the GO/PEI

395

self-assembled film, the N-H in-plane bending vibration peak of the mGO/PEI

396

self-assembled film secondary amine groups at 1640 cm-1 moves to 1550 cm-1. This

397

indicates that some primary amine groups were converted to secondary amine groups.

398

Simultaneously, a broad peak of the out-of-plane bending vibration of the secondary

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ACCEPTED MANUSCRIPT amine groups appear at 818 cm-1. These alterations indicate that the content of secondary

400

amino groups in the mGO/PEI self-assembled film was increased. Based on the analysis,

401

the primary amine groups in PEI reacted with the epoxy groups in mGO to form covalent

402

bonds, and the primary amine groups were converted into the secondary amine groups.

403

Therefore, the content of secondary amino groups on the mGO/PEI self-assembled films

404

increased. It is also confirmed that the mGO/PEI self-assembled film was prepared by

405

forming covalent bonds between mGO and PEI.

406

3.5. Comparison of the GO/PEI film and mGO/PEI film properties

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10-bilayer GO/PEI electrostatic self-assembled films and a 10-bilayer mGO/PEI

408

reactive self-assembled films prepared under recommended conditions were soaked in

409

the HCl solution (pH = 2), NaOH solution (pH = 12), NaCl solution (1M) and

410

heat-treated at 80℃for different durations. Then the acid-resistance, alkali-resistance,

411

salt-resistance and heat-resistance properties of the self-assembled films were analyzed

412

by comparing the H2TR of multilayer films under different treatment conditions, as

413

shown in Fig. 11. With the impregnation time of 10-bilayer GO/PEI films in HCl (pH =

414

2), NaOH (pH = 12) and NaCl (1M) increased from 0 h to 5 h, the hydrogen transmission

415

rate of GO/PEI films increased by 159.15%, 292.34% and 174.89%, respectively.

416

However, the hydrogen transmission rate of mGO/PEI films treated under the same

417

conditions above increased by 54.36%, 193.96% and 88.26%, respectively. It

418

demonstrates that with the increasing processing time, the hydrogen barrier properties of

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ACCEPTED MANUSCRIPT the GO/PEI and mGO/PEI self-assembled films soaked in acid, alkali and salt solutions

420

deteriorate. Moreover, after immersion in HCl (pH = 2), NaOH (pH = 12) and NaCl (1M)

421

solution for 5 h, the hydrogen transmission rate of mGO/PEI films were 24.47%, 4.99%

422

and 13.16% lower than that of GO/PEI films, respectively. It demonstrates that mGO/PEI

423

reactive self-assembled films have better acid, alkali, and salt resistance compared to

424

GO/PEI electrostatic self-assembled films. In addition, with the processing time of

425

10-bilayer GO/PEI films and mGO/PEI films at 80℃ increased from 0 h to 5 h, the

426

hydrogen transmission rate of the GO/PEI and mGO/PEI films increased by 32.34% and

427

7.27%, respectively. Hence, mGO/PEI reactive self-assembled films have better heat

428

resistance than GO/PEI electrostatic self-assembled films. In summary, mGO/PEI

429

reactive self-assembled films possess better acid, alkali, salt and heat resistance compared

430

to GO/PEI electrostatic self-assembled films.

431

4. Conclusions

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Hydrogen barrier properties are characteristic of polymeric materials prepared with

433

graphene; thus, they can be considered as a good substitute for the metal body of the

434

traditional hydrogen storage tank. In this study, ethylene glycol diglycidyl ether (EDGE)

435

was used to modify graphene oxide (GO) by grafting epoxy groups on GO.

436

Subsequently covalent bonds between modified graphene oxide (mGO) and

437

polyethyleneimine (PEI) were formed to fabricate the multilayer composite films with

438

stable hydrogen barrier properties in different environments. Results demonstrate that

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ACCEPTED MANUSCRIPT the epoxy groups of EDGE were successfully grafted onto the GO surface and the

440

dispersion stability of GO in the solvent was meliorated. Furthermore, the reaction

441

degree of EDGE modified GO was controlled by changing the reaction time, pH value

442

and feed ratio of GO/EDGE. The parametric analysis has revealed that the following

443

conditions secure the best resistance of the film to hydrogen gas transmission: the

444

modification reaction time = 6 h, pH = 2, and GO/EDGE = 0.05/0.23. The mGO/PEI

445

self-assembled films prepared under these conditions showed a linear increase in

446

absorbance and thickness as the number of bilayers increased, and the hydrogen barrier

447

properties were also gradually improved. The hydrogen transmission rate (289

448

cm3/m2·24h·0.1MPa) of 10-bilayer mGO/PEI reactive self-assembled film reduced by

449

78.8% compared to that of polyethylene terephthalate substrate films (1365

450

cm3/m2·24h·0.1MPa). In addition, with the impregnation time of 10-bilayer GO/PEI and

451

mGO/PEI films in HCl (pH = 2), NaOH (pH = 12) and NaCl (1M) increased from 0 h to 5

452

h, the hydrogen transmission rate of mGO/PEI reactive self-assembled films were

453

24.47%, 4.99% and 13.16% lower than that of GO/PEI electrostatic self-assembled films,

454

respectively. The reactive self-assembled film has better acid, alkali, and salt resistance

455

compared to the electrostatic self-assembled film.

456

Acknowledgements

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The authors gratefully acknowledge the financial support from the Fundamental

458

Research Funds for the Central Universities (No. 15CX02015A ,16CX05009A,

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ACCEPTED MANUSCRIPT 18CX05006A 24720164002A), the National Natural Science Foundation of China (grant

460

no. 21502227) , the Province Key Research and Development Program of Shandong (No.

461

2016GSF115032), the Postdoctoral application Program of Qingdao (No. T1604013), the

462

State Key Laboratory of Separation Membranes and Membrane Processes (Tianjin

463

Polytechnic University, NO. M1-201601),State Key Laboratory of Heavy Oil Processing

464

SLKZZ-2017009, and Qingdao Original innovation plan (applied research project - youth

465

project (grant No. 17-1-1-64-jch) ).

466

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Figure Captions

589

Fig. 1 Scheme of GO modification with EDGE.

590

Fig. 2 The spectra of GO and mGO: (a) FTIR; (b) Raman spectra.

591

Fig. 3 XPS C1s spectra of: (a) GO; (b) mGO.

592

Fig. 4 XPS C1s spectra of mGOs modified under different times.

593

Fig. 5 XPS C1s spectra of mGOs modified under different pH values.

594

Fig. 6 XPS C1s spectra of mGOs modified under different feed ratios.

595

Fig. 7 H2TR of 10-bilayer mGO/PEI films with mGOs modified under different

596

conditions: (a) different times; (b) different pH values; and (c) different feed ratios.

597

Fig. 8 Absorbance and thicknesses of GO/PEI and mGO/PEI films with different

598

bilayers.

599

Fig. 9 H2TR of GO/PEI films and mGO/PEI films with different bilayers.

600

Fig. 10 FTIR of 30-bilayer GO/PEI and mGO/PEI self-assembled films.

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ACCEPTED MANUSCRIPT Fig. 11 H2TR of 10-bilayer GO/PEI and mGO/PEI films treated under different

602

conditions: (a) pH=2 HCl; (b) pH=12 NaOH; (c) 1M NaCl; and (d) 80℃.

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Wavenumber, cm-1

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1000

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400

GO/PEI mGO/PEI

200

600

400

200

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1

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3

4

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3

Time, h

4

5

ACCEPTED MANUSCRIPT Fig. 2

a

b ID/IG = 1.09

mGO

1080 cm-1

GO

ID/IG = 0.91

GO

1050 cm-1 4000

3500

3000

2500

2000

1500

1000

G band

D band

1720 cm-1

3350 cm-1

1588 cm-1

1352 cm-1

500

RI PT

Intensity

Transmittance

mGO

1000

1500

2000

SC M AN U TE D EP AC C

2500

Wavenumber , cm-1

Wavenumbers, cm-1

3000

ACCEPTED MANUSCRIPT Fig. 3

C-C/C=C

C-O-C

C=O

C=O C-OH

292

290

288

286

284

282

Binding Energy (eV)

292

290

RI PT

C-C/C=C

Intensity

Intensinty

C-O-C

288

SC M AN U

(b)

TE D EP

286

Binding Energy (eV)

(a)

AC C

C-OH

284

282

ACCEPTED MANUSCRIPT Fig. 4

C-O-C

C-C/C=C

C-O-C

C=O C-OH

292

290

288

286

C=O

284

282

292

290

C-O-C C-C/C=C

Intensity

C=O

C=O

TE D

C-OH

286

Binding Energy (eV)

EP

(c) 6h

AC C

282

SC M AN U

C-O-C

Intensinty

284

(b) 2h

C-C/C=C

288

286

Binding Energy (eV)

(a) GO

290

C-OH

288

Binding Energy (eV)

292

RI PT

Intensity

Intensinty

C-C/C=C

284

282

292

290

288

C-OH

286

Binding Energy (eV)

(d) 24h

284

282

ACCEPTED MANUSCRIPT Fig. 5

C-C/C=C

C-O-C

C-O-C

RI PT

Intensity

Intensinty

C-C/C=C

C=O

C=O C-OH

292

290

288

286

C-OH

284

282

292

290

Binding Energy (eV)

286

284

282

SC

Binding Energy (eV)

(b) pH=2.0

M AN U

(a) GO

C-C/C=C C-O-C

C-C/C=C

C-O-C

Intensity

Intensinty

288

C=O

292

290

288

C=O

TE D

C-OH

286

Binding Energy (eV)

AC C

EP

(c) pH=4.5

284

282

292

290

C-OH

288

286

Binding Energy (eV)

(d) pH=9.5

284

282

ACCEPTED MANUSCRIPT Fig. 6 C-O-C

C-C/C=C

C-O-C

C=O

C=O

C-OH

292

290

288

286

C-OH

284

282

292

290

288

Intensity

C-O-C

Intensinty

C=O

TE D

C-OH

286

284

Binding Energy (eV)

EP

(c) GO/EDGE=0.05/0.65

AC C

282

SC C-O-C

C-C/C=C

288

284

(b) GO/EDGE=0.05/0.23

M AN U

(a) GO

290

286

Binding Energy (eV)

Binding Energy (eV)

292

RI PT

Intensity

Intensinty

C-C/C=C

282

C-C/C=C

C-OH

C=O

292

290

288

286

284

Binding Energy (eV)

(d) GO/EDGE=0.05/1.30

282

ACCEPTED MANUSCRIPT Fig. 7 1-Uncoated 2-GO/PEI 3-(mGO)2h/PEI

1200

1400

5-(mGO)6h/PEI

800

6-(mGO)10h/PEI

1-Uncoated 2-GO/PEI 3-(mGO)pH=2.0/PEI

1200

4-(mGO)4h/PEI

1000

b

4-(mGO)pH=4.5/PEI 5-(mGO)pH=9.5/PEI

1000

7-(mGO)24h/PEI 600 400 200

800 600

RI PT

3 2 H2TR, cm /(m ⋅24h⋅0.1MPa)

1400

1600

a

H2TR, cm3/(m2⋅24h⋅0.1MPa)

1600

400 200

0

1

2

0

3

4

5

7

6

1

Numbers

H2TR, cm3/(m2⋅24h⋅0.1MPa)

1400

c

1-Uncoated 2-GO/PEI 3-(mGO)GO/EDGE=0.05/0.23/PEI

1200

4-(mGO)GO/EDGE=0.05/0.65/PEI 5-(mGO)GO/EDGE=0.05/1.30/PEI

800 600 400 200 0

1

M AN U

1000

2

3

EP

TE D

Numbers

AC C

3

Numbers

SC

1600

2

4

5

4

5

ACCEPTED MANUSCRIPT Fig. 8 2.5

150

Absorbance - GO/PEI Absorbance - mGO/PEI Thickness - GO/PEI Thickness - mGO/PEI

60

0.5

30

0.0

0 0

5

10

15

20

25

EP

TE D

M AN U

Bilayers

30

RI PT

1.0

SC

90

Thickness, nm

120

1.5

AC C

Absorbance

2.0

ACCEPTED MANUSCRIPT Fig. 9 1400

GO/PEI mGO/PEI

H2TR, cm3/(m2⋅24h⋅0.1MPa)

1200 1000

600 400 200 0 0

5

10

15

20

25

30

AC C

EP

TE D

M AN U

Bilayers

SC

RI PT

800