Differently-charged graphene-based multilayer films by a layer-by-layer approach for oxygen gas barrier application

Accepted Manuscript Differently-charged graphene-based multilayer films by a layer-by-layer approach for oxygen gas barrier application Hongyu Liu, Jianping Wu, Cuiyun Liu, Bingli Pan, Nam Hoon Kim, Joong Hee Lee PII:

S1359-8368(18)32196-6

DOI:

10.1016/j.compositesb.2018.08.137

Reference:

JCOMB 5967

To appear in:

Composites Part B

Received Date: 14 July 2018 Accepted Date: 29 August 2018

Please cite this article as: Liu H, Wu J, Liu C, Pan B, Kim NH, Lee JH, Differently-charged graphenebased multilayer films by a layer-by-layer approach for oxygen gas barrier application, Composites Part B (2018), doi: 10.1016/j.compositesb.2018.08.137. 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.

ACCEPTED MANUSCRIPT

Differently-charged graphene-based multilayer films by a layer-

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by-layer approach for oxygen gas barrier application

Hongyu Liu1, Jianping Wu1, Cuiyun Liu1, Bingli Pan1, Nam Hoon Kim2*, Joong Hee Lee2*

Department of Chemical Engineering & Pharmaceutics, Henan University of Science & Technology, Luoyang, China,

Department of BIN Fusion Technology, Chonbuk National University, Jeonju, Jeonbuk, 561-756,

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Republic of Korea

--------------------------------------------------------------------------------------------------------------*Corresponding author: Fax: +82 63 270 2341; Tel: +82 63 270 2342 Email address: [email protected] (Prof. J. H. Lee) and [email protected] (Prof N. H. Kim) ----------------------------------------------------------------------------------------------------------------

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ACCEPTED MANUSCRIPT Abstract Polyelectrolyte-decorated graphenes with positive and negative charges were prepared using graphene

oxide

as

a

precursor

along

with

polyallylamine

hydrochloride

and

poly(styrenesulfonate sodium) as surface modifiers, respectively. Driven by an electrostatic

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interaction, the as-obtained polyelectrolyte-decorated graphene sheets were self-assembled via a layer-by-layer method, producing graphene based composite multilayers. Field emission

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scanning electron microscopy (FE-SEM) images showed that the surface of composite film exhibited a mass of characteristic wrinkles of graphene. Interestingly, the cross-sectional FE-

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SEM of the film exhibited an unstratified structure, indicating a good dispersion of graphene sheets in the composite film. The results of oxygen gas permeability testing demonstrated that the resulting composite films had excellent oxygen gas barrier properties. The oxygen gas transmission rate (OTR) of the composite film with 24 bilayers was as low as 1.2 cm3·m-2·d·atm-1, exhibiting potential for use in packaging applications.

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Keywords: A. Layered structures; A. Polymer-matrix composites; B. Physical propertie; E.

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Surface treatments; D. Electron microscopy

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Introduction

Materials with high gas barrier properties are of great importance in food and drug packaging applications for protection from the adverse effects of oxygen and water vapor. However, the

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widely-used packaging materials made of synthetic plastic are permeable to most of small molecules, such as gases and vapors. Thus, a great deal of attention has been paid so far to improving the gas barrier properties of polymeric material [1-3]. One of the more effective

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routes for enhancing gas barrier properties is to introduce barrier coatings onto the polymer

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matrix. For instance, assembling two-dimensional inorganic materials such as surfacemodified graphene or clay sheets onto the polymer matrix via a facile layer-by-layer (LbL) method has attracted tremendous attention [4,5]. LbL assembly utilizes the alternating deposition of electrolytes onto a pretreated substrate on the basis of electrostatic interaction,

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hydrogen bonding, or covalent bonding. Furthermore, LbL films are extremely easy to realize by performing facile coating processes, like dip-coating, spray-coating, spin-coating, or other processes.

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Graphene is a two-dimensional carbonic material with single-atomic thickness that exhibits

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outstanding gas barrier properties. Great efforts have been made toward preparing graphenebased polymeric composites for enhanced gas barrier application [6-13]. However, it is very difficult to incorporate virgin graphene sheets into a polymer matrix due to the inert nature of the graphene sheets. Therefore, surface modification of graphene is essential when employing graphene sheets as nanofillers to enhance the gas barrier, thermal, and mechanical properties as well as the electrical conductivities of polymer matrices [14-18]. On the other hand, the derivative of graphene, graphene oxide (GO) has diverse oxygen-containing groups, such as 3

ACCEPTED MANUSCRIPT hydroxyl, carbonyl, carboxyl and epoxide groups decorated either in GO sheets or on the edges of GO sheets [19]. These oxygen-containing groups facilitate the reactions of GO sheets with other functional molecules and polymers in order to produce surface-modified

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graphene sheets with miscellaneous applications. Therefore, GO is a versatile platform for producing surface-modified graphene. Furthermore, GO can be separated inexpensively from graphite oxide, which can be obtained from naturally abundant graphite via Hummers method.

graphene,

respectively.

Then,

graphene-based

composited

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polyelectrolyte-decorated

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In this paper, GO was utilized as a versatile precursor for the preparation of two kinds of

multilayers were prepared via a facile LbL method for yielding a highly-ordered dispersion of graphene sheets. Due to the unique distribution of graphene sheets in the composite film, a slightly improved oxygen gas barrier property was achieved. Polyallylamine hydrochloride

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(PAH) and poly(styrenesulfonate sodium) (PSS) were used as surface modifiers of GO in order to yield surface-modified graphene sheets with opposite charges. Accordingly, the oppositely-charged PAH-modified reduced graphene oxide (PAH-RGO) and PSS-modified

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reduced graphene oxide (PSS-RGO) were assembled, with this assembly driven by an

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electrostatic interaction. A schematic diagram of the LbL assembly process is shown in Figure 1. The substrate polyethylene terephthalate (PET) with a thickness of 117 µm was first activated with a NaOH solution (1M) so as to create negative charges on the surface, followed by rinsing with an enormous amount of deionized water. Then, the activated PET substrate was subjected to alternate dipping/rinsing in PAH-RGO and PSS-RGO solutions in order to construct LbL films. 2. Experimental 4

ACCEPTED MANUSCRIPT 2.1 Materials Natural flake graphite was obtained from Najing Pionerr Nano Co. Ltd., China. Concentrated sulfuric acid (95%), hydrochloric acid, hydrogen peroxide, and potassium permanganate were

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purchased from Shanghai Chemical Reagent Co. Ltd., China. Polyallylamine hydrochloride (PAH, Mw~17 000) and poly(styrenesulfonate sodium) (PSS, Mw~70 000) were purchased

2.2 Preparation of surface modified graphene

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from Sigma-Aldrich. The materials were directly used without further purification.

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The oppositely charged PAH-RGO and PSS-RGO were prepared as follows: Graphite oxide was fabricated from expanded graphite by a modified Hummers method as described in our previous study [6]. Following sonication for 15 minutes, graphite oxide dispersion was converted into GO solution. By reacting GO with PSS and PAH in the presence of hydrazine,

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GO was reduced and converted to PSS-RGO and PAH-RGO, respectively [20, 21]. After being processed as Fig. 1, PAH-RGO and PSS-RGO were successfully assembled into LbL films with n bilayers by performing a continuous alternate dipping process. The as-obtained

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LbL films were assigned as (PAH-RGO/PSS-RGO)n.

3. Results and discussion 3.1. FTIR Analysis

The successful conversion of GO to PSS-RGO and PAH-RGO was confirmed by Fourier transform infrared (FTIR) spectroscopy. As shown in Fig. 2, GO exhibited the peaks of carbonyl groups at 1726 cm-1 and epoxide groups at 1247 cm-1, respectively. The sharp peak at 1652 cm-1 corresponds to the C=C stretching vibration. These results were all in line with 5

ACCEPTED MANUSCRIPT those of a previous report [22]. However, the peaks of carbonyl and epoxide groups disappeared in PSS-RGO and PAH-RGO, indicating the efficient reduction of GO to RGO. Furthermore, the presences of the C-N vibration peak at 1497 cm-1 in PAH-RGO and the S=O

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vibration peak at 1000 cm-1 in PSS-RGO demonstrated the successful introductions of PAH and PSS onto the surfaces of graphene sheets, forming polyelectrolytes modified PAH-RGO and PSS-RGO, respectively [20,21]. The successful introductions of PAH and PSS modifiers

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endowed opposite surface charges onto the graphene sheets and facilitated the next LbL

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deposition. 3.2 XRD analysis

The XRD patterns, shown in Fig. 3, also disclosed the transformation of GO into PAH-RGO and PSS-RGO. As could be seen, graphite oxide exhibited a sharp diffraction peak at 2θ =

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10.5˚, which corresponded to an interlayer spacing of 0.84 nm which was attributed to the decoration of oxygen containing groups onto the carbon layers [23]. Moreover, a small peak presented at 2θ = 20.9˚ was related to the un-oxidized graphite region in graphite oxide [24].

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However, the sharp peak in the XRD pattern of graphite oxide disappeared in both PAH-RGO

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and PSS-RGO XRD patterns, demonstrating the full exfoliation of graphite oxide into surface-modified graphene sheets during the sonication process. Furthermore, broad peaks at 2θ = 20.7˚ for PSS-RGO and 19.1˚ for PAH-RGO were observed, elaborating the partial agglomerates of graphene sheets during the reduction process of GO. These results were in line with those of previous reports [25,26]. 3.3 TGA study The incorporation of the polyelectrolytes PAH and PSS is of great importance for performing 6

ACCEPTED MANUSCRIPT sequential LbL deposition. Due to the chemical inertia of graphene sheets, it is very difficult to construct LbL films directly using original graphene sheets. However, the alternating deposition of graphene sheets can easily be realized through means of incorporation of

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versatile modifiers onto graphene sheets, such as PAH and PSS. The successful introduction of PAH and PSS onto the graphene sheets was further proven by the TGA study. As can be seen in Fig. 4, GO exhibits two weight losses occurring around 100 ℃ and 200 ℃,

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corresponding to the release of residual water and the pyrolysis of oxygen-containing groups

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out of GO sheets, respectively [27]. By comparison, there are no obvious weight losses around 100 ℃ for both PAH-RGO and PSS-RGO, demonstrating the efficient reduction of GO to RGO. Furthermore, the decomposition of the surface modifiers out of graphene sheets was observed to be 300-450 ℃ and 350-550 ℃ for PAH-RGO and PSS-RGO, respectively. On

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the contrary, the weight loss during the elevated temperatures in TGA results indicated the efficient functionalization of PAH and PSS onto RGO, respectively. 3.4 SEM study and water contact angle test

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The surface morphologies of NaOH-activated PET substrate and the substrate coated with

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four bilayers as well as the cross-sectional fracture of the LbL film with 24 bilayers were characterized through field emission scanning electron microscopy (FE-SEM); the results are reported in Fig. 5. As can be seen, following activation by NaOH, the surface of PET substrate was uneven, therefore producing a negative charge dominating the PET surface [25]. This surface variation was in line with corresponding water contact angles. Neat PET exhibited a water contact angle of about 70º. However, it was around 36.5º for NaOHactivated PET. This obvious decline in water contact angle was due to the setup of 7

ACCEPTED MANUSCRIPT hydrophilic surface of PET. After anchoring of four bilayers, the presence of the characteristic crinkles of flexible graphene sheets in Fig. 5(b) served as direct proof for the assembly of polyelectrolyte-decorated graphene onto the PET substrate [26]. Furthermore, the water

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contact angles of LbL films with four and 24 bilayers were ca. 33.6º and 31.8º, respectively. These values were lower than those of NaOH-activated PET. The hydrophilic surface modifiers of graphene sheets were responsible for this further decrease in water contact angle.

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Due to the fact that the PET film was very hard to fracture, a cross-sectional SEM image of

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LbL film was obtained by depositing LbL film onto a glass sheet with the same procedures as those for the PET substrate. As can be seen in Fig.5 (c), no aggregates of graphene sheets could be observed in the LbL film, demonstrating that the multilayers were properly constructed as expected.

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It was noted that the LbL film presented a homogeneously soft polymeric phase without the forming of hierarchical structures. This phenomena might be due to the successful introduction of polyelectrolytes onto the graphene sheets. The grafted polymers with opposite

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charges had strong interactions, therefore resulting in a molecular level blend between the

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soft polymeric chains. This entanglement of polyelectrolyte chains might sweep the graphene sheets under the entangled polymer chains. Similar results were reported by other groups [28,29]. The streaks presented in Fig. 5(c) might have resulted from the stress during the breakage. The presence of the streaks also demonstrated that the graphene sheets were successfully surface decorated by polymer electrolytes. The Fig. 5(d) with a higher magnification also exhibited that graphene sheets were uniformly dispersed throughout the LbL film. 8

ACCEPTED MANUSCRIPT 3.5 UV-vis analysis In order to investigate the assembly of graphene linearly or exponentially stacking onto PET surface, ultraviolet visible (UV-vis) spectroscopy was employed, with the results concluded

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in Fig. 6. The absorbance peaks for LbL films with different bilayers were located at 303 nm due to the n-π* transition of carbonyl groups in surface-modified graphene sheets. Moreover, the corresponding absorbencies increased uniformly against the number of bilayers,

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indicating the uniform assembly of PAH-RGO and PSS-RGO onto the PET substrate. The

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good linearity in the absorbance plot at 303 nm with assembly bilayer numbers demonstrated a uniform increase in film thickness [30,31]. This circumstance was quite different from the exponential increase in thickness observed in some LbL films constructed by pure polymers. The LbL assembly of pure polymer precursors was reported to exhibit an exponential growth

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in thickness initially, followed by a linear increase [32]. However, the assembly of polyelectrolyte-modified graphene sheets exhibited a linear increase in film thickness within the scope of our investigation. The exponential growth was due to the penetration of polymer

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chains into inner layers during the deposition process because of charge overcompensation.

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However, graphene sheets terminated the penetration of polymer chains into the inner layers in the presented LbL case, therefore leading to a completely linear increase in thickness [29]. 3.6 Mechanical performance The effect of multilayer film deposition on the mechanical property of the PET substrate was studied, and the results were reported in Fig. 7. The neat PET substrate exhibited a tensile strength of 39.8 MPa. With the deposition of LbL multilayers onto the PET substrate, the tensile strengths of the films increased dramatically. The deposition of four bilayers on the 9

ACCEPTED MANUSCRIPT PET substrate improved the tensile strength to 42.2 MPa. Further increasing the bilayer number resulted in a steady increase in tensile strength of the films [33]. Therefore, the maximum tensile strength of 66.1 MPa was achieved for LbL film with 24 bilayers

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in the scope of experimentation. In consideration of the neat PET value, it indicated that the deposited 24 bilayers contributed one half of the tensile strength, representing the fact that the coated 24 bilayers were comparable to those of the PET film. Beyond all question, the

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parallel-aligned and highly-ordered graphene sheets in the LbL multilayers contributed to the

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dramatic improvement in the mechanical property. A similar trend in tensile modulus was observed for the LBL assemblies. The maximum tensile modulus of 4.9 GPa was achieved with the coated 24 bilayers. 3.7 Oxygen gas barrier performance

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Graphene sheets have been reported as an effective filler for improving the gas barrier property of polymer matrix [11,33,34]. Herein, we investigated the oxygen gas transmission rate (OTR) of as-prepared LbL films. Figure 8 showed the OTR and permeability coefficients

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of LbL films as a function of bilayer numbers. As can be seen in Fig. 8, assembling surface-

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modified graphene onto PET substrate could greatly reduce the permeation of oxygen gas through the substrate due to the barrier effect of graphene sheets. The OTR value greatly decreased from 25.6 cm3·m-2·d-1·atm-1 for pristine PET substrate to 5.3 cm3·m-2·d-1·atm-1 for the LbL film with 12 bilayers. The corresponding permeability coefficients decreased from 2.99 cm3·mm·m-2·d-1·atm-1 for neat PET to 0.64 cm3·mm·m-2·d-1·atm-1 for the LbL film with 12 bilayers. Clearly, the highly-ordered arrangement of modified graphene sheets during the LbL process was responsible for the 79.3% decrease in OTR value. Due to the ordered 10

ACCEPTED MANUSCRIPT stacking of graphene sheets, oxygen molecules had to adopt a tortuous pathway in order to diffuse through the composite films, which maximized the diffusion length of oxygen molecules through the composite films [35]. Further, some reports concluded the layer-by-

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layer distribution of inorganic graphene sheets and organic polymers as a “brick and mortar” structure [36,37]. By virtue of highly-ordered embedded impermeable graphene sheets, it was reasonable to achieve the ideal oxygen gas barrier property for the LbL films. The OTR is

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only 1.2 cm3·m-2·d-1·atm-1 for the LbL film with 24 bilayers. Based on the prominent decrease

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in the OTR values of LbL films, it is considered to have potential for packaging applications. 4 Conclusions

In summary, two kinds of polyelectrolyte-modified graphene, PAH-RGO and PSS-RGO, were alternately deposited onto the PET substrate through an LbL method driven by

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electrostatic interaction. UV-vis spectroscopy results indicated that the LbL film grew linearly rather than exhibiting an exponential growth against the number of bilayers. The surface of the LbL film was rough and presented characteristic crinkles of graphene sheets. The OTR

·d-1·atm-1 for the LbL film with only 12 bilayers. This great decrease in OTR was considered

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value greatly decreased from 25.6 cm3·m-2·d-1·atm-1 for pristine PET substrate to 5.3 cm3·m-

to be the tortuous pathway for oxygen gas molecules to diffuse through the composite films by the highly-ordered arrangement of graphene sheets. Thus, it was believed that the asprepared LbL film had potential packaging applications.

Acknowledgements This study was supported by the Nano-Material Technology Development Program 11

ACCEPTED MANUSCRIPT (2016M3A7B4900117) and the X-mind Corps Program (2017H1D8A2030449) through the National Research Foundation (NRF) funded by the Ministry of Science and ICT of Republic of Korea. We also gratefully acknowledge the financial supports by research project of

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HAUST (No. 13480051), National Natural Science Foundation of China (51675162, 21373078).

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1 Schematic illustration for the preparation of LbL film. Fig. 2 FTIR spectra of PSS-RGO, PAH-RGO, and GO.

Fig.4 TGA curves of GO, PSS-RGO, and PAH-RGO.

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Fig. 3 XRD patterns of graphite oxide, PAH-RGO, and PSS-RGO.

Fig. 5 SEM images of (a) activated PET substrate and (b) LbL film of (PAH-RGO/PSS-

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RGO)24.

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Fig. 6 UV-vis spectra of (a) LbL films with different bilayers and (b) plot of absorbencies at 303 nm versus the number of bilayers.

Fig. 7 Mechanical properties of the LbL films with different bilayers. Fig. 8 Oxygen gas barrier properties of the LbL films with various bilayers.

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Table 1 Water contact angle results of the substrate and LbL films.

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Figures

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Fig. 1 Schematic illustration for the preparation of LbL film.

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PAH-RGO

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Transmittance (%)

PSS-RGO

4 000

3 500

3 000

GO

2 500

2 000

1 500

1 000

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Wavenumber (cm )

Fig. 2 FTIR spectra of PSS-RGO, PAH-RGO, and GO.

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PAH-RGO

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Intensity(a.u.)

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2θ (degree)

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Fig. 3 XRD patterns of graphite oxide, PAH-RGO, and PSS-RGO.

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PAH-RGO

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20 100

200

300

400

500

600

o

Temperature ( C)

Fig.4 TGA curves of GO, PSS-RGO, and PAH-RGO

19

700

TE D

M AN U

SC

RI PT

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Fig. 5 SEM images of (a) activated PET substrate, (b) LbL film of (PAH-RGO/PSS-RGO)24,

AC C

EP

(c) cross-sectional FE-SEM image of LBL film with 24 bilayers, and (d) enlarged view of (c).

20

SC

RI PT

ACCEPTED MANUSCRIPT

M AN U

Fig. 6 UV-vis spectra of (a) LbL films with different bilayers and (b) plot of absorbencies at 303 nm versus the number of bilayers.

Tensile modulus (GPa)

4

EP

50

40

5

TE D

60

AC C

Tensile strength (MPa)

70

3

30

0

4

8

12

16

20

24

Number of bilayers

Fig. 7 Mechanical properties of the LbL films with different bilayers

21

ACCEPTED MANUSCRIPT

4

RI PT

15

2

3

2

OTR (cm /m d atm)

3

20

Permeability coefficient(cm mm/m d atm)

25

10

0

4

8

M AN U

0

12

16

20

1

2

SC

3

5

0 24

number of layers (n)

TE D

Fig. 8 Oxygen gas barrier properties of the LbL films with various bilayers

EP

Table 1 Water contact angle results of the substrate and LbL films Water contact angle

Neat PET

70º

NaOH activated PET

36.5º

LbL film with 4 bilayers

33.6º

LbL film with 24 bilayers

31.8º

AC C

Sample

22