Mechanical properties of graphene oxide-based composite layered-materials

Materials Chemistry and Physics 234 (2019) 81–89

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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Mechanical properties of graphene oxide-based composite layered-materials Mina Sabzevari a, Duncan E. Cree a, *, Lee D. Wilson b a b

Department of Mechanical Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, S7N 5A9, Canada Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan, S7N 5C9, Canada

H I G H L I G H T S

� GO-based composites were successfully prepared using a green chemical approach. � The GO-based composites showed improved mechanical properties over pure GO. � Swelling and thermal properties were enhanced in GO-based composite materials. A R T I C L E I N F O

A B S T R A C T

Keywords: Graphene oxide Composite layered structure Microstructure Mechanical properties

In this study, two graphene oxide (GO) based composite systems were prepared by a facile cross-linking method. GO was cross-linked with chitosan (GO-CTS) or with Al3þ ions (GO-Al) to yield unique composite layeredmaterials with versatile properties. The microstructure and mechanical properties were evaluated across a range of cross-linker contents from 0.3 to 0.6 w/v%. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) results revealed greater layer thickness and interlayer distance upon cross-linking of GO. The formation of ionic bonds between the active functional groups of GO and the cross-linkers were supported by Fourier transform infrared (FTIR) spectroscopy, while thermal gravimetric analysis (TGA) and solvent swelling results provided support for the greater structural stability of GO-based composites. The energy-dispersive X-ray (EDX) spectroscopy was used to obtain the elemental analysis of the samples before and after cross-linking. When the CTS and Al3þ ions were added at 0.6 w/v% to GO, the tensile strengths improved by 146% and 83%, respec­ tively; while the tensile modulus increased by 64% and 41%, respectively, relative to pristine GO. The com­ posites reported herein have potential utility for diverse applications in tissue engineering, membrane-based pollutant removal and filtration for chemical separations.

1. Introduction Removal of organic contaminants and dyes from effluents that originate from the paper, food, and pharmaceutical industries are known to employ polymer membrane technology due to their facile operational requirements [1,2]. However, they have been reported to possess low mechanical performance under high pressure, with a ten­ dency to foul and a reduced chemical stability in the presence of acid/alkaline environments [1]. An alternative is to use graphene oxide (GO) and GO-based composite materials since they are known to have greater mechanical strength and resistance to strong acids/alkaline media when compared against conventional membrane materials [3,4]. GO refers to a single-layer of carbon atoms in a two-dimensional (2-D)

network structure [5]. When assembled into a composite, GO often forms a three-dimensional (3-D) layered sheet, where such materials have been developed using various manufacturing techniques such as filtration-assisted, casting/coating-assisted, layer-by-layer assembly, etc. [6,7]. GO-based materials are effective for a wide range of appli­ cations that include molecular separations, greenhouse gas capture and separations (e.g., CO2) [8,9], water decontamination and desalination [10–12]. GO sheets possess various polar functional groups (e.g., car­ boxylic acids, epoxides and hydroxyl) making GO an ideal candidate for the design of self-assembled membranes. Unique mechanical properties and structural stability of GO can be fine-tuned by enhancing the in­ teractions between adjacent GO sheets through the formation of non-covalent and/or covalent bonding [13]. The functional groups of

* Corresponding author. E-mail address: [email protected] (D.E. Cree). https://doi.org/10.1016/j.matchemphys.2019.05.091 Received 4 December 2018; Received in revised form 24 May 2019; Accepted 27 May 2019 Available online 28 May 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.

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Materials Chemistry and Physics 234 (2019) 81–89

GO can be further modified by surface functionalization and chemical cross-linking to form versatile composite materials [12]. Chemical cross-linking of GO represents a facile method for developing tailored GO-based composite materials at relatively low cost due to the minimal energy input requirements [2]. GO sheets cross-linked with Mg2þ, Ca2þ, Mn2þ, and Al3þ ions have been reported to significantly improve me­ chanical stiffness (10–200%), fracture strength (~50%) and thermal stability (flammability), as compared to pristine GO [13,14]. Chitosan (CTS) is a renewable and relatively inexpensive biocompatible polymer with abundant amine groups [15]. Recent studies have shown that cross-linking GO sheets with CTS led to improvements in tensile strength, stiffness and desalination performance of the resulting GO-based composites [16–21]. The formation of cross-links between GO sheets yield stronger materials, where the metal ions or CTS polymer chains tend to resist deformation between GO sheets within the same plane (cf. Fig. 1). In a previous report, it was demonstrated that GO-based composite materials that contain CTS have potential utility as efficient adsorbents for waterborne contaminant removal applications [22]. GO-based composites of this type can serve as materials for membrane separa­ tion technology under medium to high stress levels. To the best of our knowledge, no similar comparative studies have been performed on materials derived from a green synthesis of GO-based composites that contain an organic biopolymer (CTS) and an inorganic cross-linker (Al3þ). The physical (swelling and thermal), mechanical and texture properties of the 2-D layered GO sheets were tailored by the incremental addition of CTS or trivalent metal (Al3þ) ions using a facile and sus­ tainable chemical approach to yield versatile 3-D GO composite framework materials. The structure and properties of GO-based com­ posites were systematically characterized and discussed. The results reported herein demonstrate that the strength of both GO-based com­ posites can be significantly enhanced over pristine GO. Overall, this study not only affords a facile route for the preparation of GO-based systems (especially for GO-metal ion composites), but also provides new insights on the preparation of GO-based materials. Therefore, the aim of this study was to prompt new ideas for the next generation of GO-based composite membranes. The results obtained herein are likely to contribute to prospective applications of GO-based materials in tissue engineering (e.g., bone substitutes), water treatment membranes (e.g., molecular separation), food packaging industry and drug delivery. It should be noted that for practical use of these materials, sufficient me­ chanical strength is required to endure the physiochemical stress of the

media without mechanical breakdown. Additionally, adequate me­ chanical strength and physical robustness is necessary for handling and transportation purposes. 2. Materials and methods 2.1. Materials For the preparation of GO powders, graphite flakes 325 mesh, 99.8% were obtained from Alfa Aesar. Potassium permanganate (KMnO4), so­ dium nitrite (NaNO3), sulfuric acid (98%) and hydrogen peroxide (30 v/ v%) were purchased from Sigma-Aldrich. To produce GO-based com­ posites, low molecular weight CTS (Mw ¼ 95,0000 Da, ~75%–85% deacetylation) and aluminum nitrate nonahydrate (Al (NO3)3⋅9H2O) ACS reagents were used. 2.2. Fabrication of graphene oxide-based composite materials As detailed in our previous work [22], GO was synthesized using graphite flakes as the starting material by means of the modified Hummer’s method [23]. The resulting GO was washed and centrifuged several times with millipore water, followed by washing with 30% HCl aqueous solution and ethanol until the solution reached neutral pH~7. Then, the product was dried in a vacuum oven at 40 � C to obtain the final GO powder. A facile green chemical approach was used to chemically cross-link GO with CTS or aluminum nitrate. Solutions of GO, low mo­ lecular weight CTS, and aluminum nitrate were prepared individually. Appropriate amounts of CTS and Al (III) salts were dissolved in 100 mL of 1 v/v% glacial acetic acid and millipore water, respectively. The respective cross-linker solutions were added dropwise to the GO solution and stirred continuously for ca. 6 h, followed by several washings with millipore water until the solution reached pH 7. The prepared GO-based composites are listed in Table 1. Each of the prepared solutions were poured onto a polystyrene substrate and vacuum dried at 40 � C for 48 h. A block diagram of the synthetic procedure is illustrated in Fig. 2. 2.3. Characterization of graphene oxide-based composite materials The cross-sectional morphology of GO and its composites (GO-CTS and GO-Al) were studied by use of scanning electron microscopy (SEM) (Hitachi Model SU8010) with a 20 kV accelerating voltage. Fourier transform infrared (FTIR) spectra was used to determine the presence of chemical functional groups in samples using a Bio-RAD FTS-40 IR spectrophotometer in reflectance mode with a resolution of 4 cm 1 over Table 1 GO and cross-linker solution conditions for preparation of GO-based composites. Sample ID

Pure GO-L Pure GO-H GOCTS-L GOCTSH GO-Al-L GO-AlH

Fig. 1. Schematic model proposed for the reaction of graphene oxide (GO) with two types of cross-linkers (Al3þ ions and CTS) to form GO-based compos­ ite materials.

GO concentration (mg/mL)

Cross-linker concentration (w/v%)

GO/ crosslinker ratio

Avg. thickness of composite membranes (μm)

1.0

0

0

19.6 � 1.5

3.0

0

0

22.1 � 1.2

1.0

0.3

1:0.3

29.6 � 1.6

1.0

0.6

1:0.6

33.7 � 1.2

1.0 1.0

0.3 0.6

1:0.3 1:0.6

26.1 � 1.4 31.2 � 1.2

Note: L and –H denote low and high levels of cross-linkers, respectively. In the case of pure GO-L and –H, this refers to dilute and moderate concentrations of GO. 82

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Materials Chemistry and Physics 234 (2019) 81–89

Fig. 2. Experimental procedure for GO-based composite preparation.

a spectral range of 500–4000 cm 1. The changes of ID/IG for GO-based composites and carbonaceous precursors (graphite and GO) were ob­ tained with an inVia Reflex Renishaw Raman spectrophotometer system 2000 operating with a resolution (¼ λ/2) of 0.257 μm and an excitation source at 514.5 nm. The degree of structural order-disorder was ob­ tained using a PANalytical Empyrean instrument equipped with a Co source and a linear X’Celerator detector. The X-ray diffraction (XRD) patterns were obtained in continuous mode in a range of 5–15� (2theta), with a scan speed of 3� /min. Thermal gravimetric analysis (TGA) was performed on a TA Instrument Q50 TGA system from 23 � C to 500 � C with a 5 � C/min heating rate under a nitrogen purge atmosphere. Elemental analysis of materials was obtained using a transmission electron microscope (TEM) Hitachi HT7700 instrument at 100 kV equipped with energy-dispersive X-ray (EDX) spectroscopy. The spectra were collected by a Bruker XFlash 6T/60 X-ray detector. The swelling behavior of GO and its composites were determined by submerging fixed amounts of membrane in vials containing 12 mL of millipore water whilst equilibrating on a horizontal shaker for ~24 h at 23 � C. The water swelling ratio (Sw) was estimated using Eq. (1) where Ws is the swollen sample weight and Wd is the dried sample weight after drying samples in an oven at 60 � C for 12 h to constant weight.

Sw ð%Þ ¼

Ws Wd �100% Wd

(1)

2.4. Mechanical properties of GO-based composites Tensile tests of GO and GO-based composites were carried out using a Dynamic Mechanical Analyzer (DMA) (TA instrument, Q800, USA). The DMA had a preload of 0.01 N, displacement ramp of 50 μm/N, clamp compliance of 0.178 μm/N and cross-head speed of 50 μ/min. The specimens had a gauge length of 16–20 mm (strain rate 10 5 s 1) and tested at 22 � C with a relative humidity (RH) of 40%. Three samples were used to characterize the triplicate average value of each membrane material. Rectangular samples with dimensions of 3 mm � 16 mm (width � length) were prepared by compression cutting with a sharp razor blade. The membrane thickness was by analysis of the SEM im­ ages. The width-to-thickness ratio was controlled to achieve a uniform deformation of the samples.

Fig. 3. SEM micrographs of GO and the GO-based composites: (a) GO (b) GO-CTS-L and (c) GO-Al-L. 83

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glucopyranose ring. The carbonyl band for GO at 1730 cm 1 is present due to –COOH and is absent in the GO-CTS spectra. A greater intensity of the amide II band at 1595 cm 1 was observed for GO composites, where the IR band was slightly red-shifted to a lower wave number value. The broad peak in the 3000 cm 1 to 3600 cm 1 range was attributed to the –OH groups of GO and amine stretching bands from the GO-CTS com­ posites. The formation of cross-links between negatively charged car­ boxylic and epoxy groups on the surface of GO with the amine groups of CTS is supported, which is in agreement with other studies [25,26]. The cross-linking of GO sheets with Al3þ ions was further supported by various spectral signatures. A noteworthy observation for the GO-Al composite revealed a much lower C¼O band intensity related to C¼O groups of GO at 1730 cm 1 and epoxy groups at ca.1100 cm 1, when compared to the IR bands of GO described above. This suggests the functional groups (carboxyl and epoxy) of GO have reacted with Al3þ ions through a ring-opening reaction. The result is a decrease in IR in­ tensity of the vibrational band in the spectra of GO-Al [16]. The low intensity band at ca. 1380-1410 cm 1 was related to a nitrate ion (NO3 ) vibrational contribution from the presence of residual aluminum nitrate species, further indicating that excess Al3þ ions was present to fully coordinate the COO groups of GO. The presence of the nitrate group in the spectra of GO-Al composite showed that the Al3þ species were incorporated into the GO composite structure. It should be noted that these results were identical for both low (-L) and high (-H) concentra­ tions of cross-linkers.

3. Results and discussion 3.1. Morphology of GO-based composites, SEM analysis SEM was used to determine changes in surface morphology, porosity and texture of GO and its composites. SEM micrographs of the crosssectional images of GO and GO-based composites are shown in Fig. 3. The obtained GO composite membranes had thickness values in the range of 19.6 � 1.5 μm to 33.7 � 1.2 μm. The cross-sectional view of the prepared GO material (Fig. 3 (a)) revealed multiple and dense layers of GO sheets stacked consecutively. The cross-sectional view of the GObased composites (GO-CTS in Fig. 3b and GO-Al in Fig. 3c) resembled the cross-sectional morphology of GO material with some dimensional variability. The GO-based composites showed an interconnected layered-assembly with wrinkled edges while some layers had irregular shapes. This effect relates to the presence of either CTS or Al (III) species in the 2-D plane of the GO sheets. As given in Table 1, the overall GO sheet thickness and interlayer GO distance increased. In the case of GOCTS composites, a slightly greater sheet thickness was noted when compared to GO-Al-based composites. The SEM results suggest that the GO sheets displayed greater adhesive interactions with CTS upon crosslinking in comparison with Al3þ ions [24]. This observation parallels the molecular weight differences for each cross-linker and the role of ag­ gregation of CTS in the presence of GO. The variable morphology and textural changes observed in the GO-based composites revealed that the type of cross-linker (CTS or Al3þ ions) modified the structure of GO sheets.

3.3. Degree of order-disorder of GO-based composites. Raman spectroscopy

3.2. Chemical characterization of GO-based composites. FTIR spectroscopy

The Raman spectrum of GO exhibited D- and G-band signatures at 1350 cm 1 and 1580 cm 1, respectively (Fig. 4 (b)). While the G-band relates to the sp2-hybridized carbon network of graphene, the D-band was associated with structural imperfections on the graphene basal plane and partially disordered structure of GO [27]. The intensity ratios of the D- and G-bands (D/G) represent the degree of disorder in the graphene-based network structure [28]. Upon cross-linking of GO with CTS and Al3þ ions, the intensity of D-band increased and became broader. Furthermore, cross-linking of GO led to changes in the D/G intensity ratio, from 0.98 in GO to 1.16 in GO-based composites with Al3þ ions and 1.24 in GO-based composites with CTS. This variation in the D/G signal intensity ratio corresponded to defects of the regular GO layered structure upon cross-linking. The observed effect was noted to be greater when GO was cross-linked with CTS relative to cross-linking with Al3þ species. In addition, the greater D/G intensity ratio can be due to the alteration of the functional groups of the GO structure and

FTIR spectra were obtained to distinguish the presence of unique chemical bonds and functional groups in GO before and after crosslinking with CTS and Al3þ ions. In Fig. 4 (a), the FTIR spectra of GO confirmed successful oxidative conversion of graphite due to charac­ teristic IR bands ~3200 3600 cm 1, ca. 1680-1730 cm 1 ca. 15501650 cm 1, and ca. 1050-1100 cm 1. These IR signatures correspond to stretching bands of –OH, carbonyl groups (C¼O), C¼C vibration of sp2 carbon skeleton, and C–O–C bonds of the epoxy groups. The presence of amide I (NHCO) and amide II (N–H) bands in the CTS membrane are supported by two bands at 1645 cm 1 and 1580 cm 1, respectively. The FTIR spectra of the GO-based composites that contain CTS (GO-CTS) have similar spectral features of both precursors (GO and CTS). How­ ever, the glucopyranose signature for CTS at 895 cm 1, along with the signatures near 1018 cm 1 and 1154 cm 1 correspond to the

Fig. 4. (a) FTIR and (b) Raman spectra of GO and GO-based composites containing Al3þ ions and CTS. 84

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introduction of defects according to the type of cross-linking and CTS self-assembly of GO composites, as reported elsewhere [29].

cellulose biopolymers [33]. The moderate thermal stability of the GO-Al composite reported herein (cf. Fig. 5 (b)) parallel the increased thermal stability of cross-linked GO/Al3þ sheets reported elsewhere which yielded composite materials with fire retardant properties [16].

3.4. Structural order-disorder of GO-based composites. XRD analysis XRD results of GO and its composites were used to assess the struc­ ture and interlayer spacing of the GO sheets upon cross-linking as shown in Fig. 5 (a). The XRD data of GO showed a sharp characteristic reflec­ tion at 2θ ¼ 13.4� , along with oxidation of graphite via introduction of oxygen functional groups. The interlayer distance (d) between adjacent GO sheets can be estimated according to Bragg’s law as 0.77 nm [30]. Compared to the GO pattern, GO-based composite materials exhibited a peak shift in the XRD pattern at 2θ ¼ 10.8� and 2θ ¼ 10.4� for GO-Al and GO-CTS composites, respectively. Notable shifts in the XRD pattern of GO-based composites suggest an increase in the d-spacing. For example, the interlayer distance between adjacent GO sheets increased from 0.77 nm in GO to 0.95–0.98 nm in the GO-composites. XRD results suggested that the d-spacing of the GO sheets increased upon cross-linking by the presence of a “sandwiched” CTS chain and/or metal ions intercalated between the basal planes of GO. This trend in interlayer spacing provides additional support that intercalation of cross-linkers occur between the GO sheets upon composite formation.

3.6. Elemental analysis of GO-based composites. EDX analysis

3.5. Thermal stability of GO-based composites. TGA investigation

Equilibrium swelling and water uptake properties of GO-based composite materials were compared in Fig. 7. The degree of solvent swelling of a material can be related to the extent of solvent infiltration into the material matrix. The results for GO-based composites (GO-CTS and GO-Al) displayed reduced SD in water relative to pristine GO, in accordance with the greater accessibility of the polar groups of pristine GO. The SD was greater for GO-CTS composites as listed in Table 3. In Fig. 6, the GO-based composite membranes were allowed to undergo solvent swelling in water, where the membranes did not dissolve after 48 h under quiescent conditions. The reduced SD of GO-CTS composites can be related to the lesser number of polar functional groups (-COOH) on the GO surface due to cross-linking, in agreement with the IR spectral results. Similarly, for GO-Al composites, the SD decreased due to coor­ dination with the GO sheets. The different SD values in GO-based composite materials was related to cross-linking effects that altered the hydrophile-lipophile balance (HLB) of GO-Al and GO-CTS compos­ ites versus GO. As well, variable pillaring effects occurred due to the smaller size of Al (III) versus the larger CTS biopolymer that is known to impart variable pore volume and composite rigidity [33]. The trend in SD results concurred with the denser arrangement of GO-based com­ posites according to the SEM results (cf. Fig. 3 (a)). The reduced SD for GO-based materials may be due to the partial loss of structural order in

An elemental analysis of GO-based composites was carried out that neglected the H content. The GO-CTS composites had variable carbon (73.0%), nitrogen (11.4%) and oxygen (15.7%) contents, as shown in Fig. 6 (a), while the GO-AL had carbon (68.6%), nitrogen (8.6%), oxygen (14.4%) and aluminum (8.4%) contents as shown in Fig. 6 (b) and given in Table 2. The nitrogen content in GO is attributed to the presence of nitrate ions in GO arising from synthesis process using Hummer’s method. The greater nitrogen content in GO-CTS composites compared to GO is in agreement with the presence of CTS in the composite. Also, the unique presence of 8.4 wt %, aluminum in GO-Al composites observed in Fig. 6 (b) is related to existence of Al ions originating from aluminum nitrate in the GO-Al composite. 3.7. Swelling degree (SD) of GO-based composites. Water stability analysis

The TGA results for GO and its composites are compared in Fig. 5 (b), where a weight loss event between 80 and 100 � C occurred for GO which relates to the loss of volatiles and/or adsorbed water. The higher tem­ perature event can be related to the breakdown of the GO framework. By contrast, the GO-CTS composite showed two thermal events at ca. 140–155 � C and 250–270 � C. The first event can be attributed to the desorption of bound water and the loss of functional groups on the GO surface. The second thermal event (ca. 250–270 � C) relates to the onset of pyrolytic decomposition of GO framework and cross-linker units (CTS), as described in detail elsewhere [22]. Furthermore, the GO-Al composite revealed two minor weight losses at 180–225 � C due to either the desorption of coordinated species (nitrate) from Al (III) sites or disproportionation of GO, as reported previously [31,32]. The weight losses at 340 � C occurred due to the decomposition of Al sites along with the GO functional groups facilitate degradation of the GO-Al network. The TGA results confirmed that the GO-based composites (GO-CTS and GO-Al) displayed a gradual loss profile that started at higher tempera­ ture in accordance with the variable thermal stability due to cross-linking effects. By contrast, GO had lower thermal stability in accordance with the role of pillaring effects reported for cross-linked

Fig. 5. (a) XRD patterns and (b) TGA curves of GO and its composite materials. 85

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Fig. 6. Elemental analysis spectra results of GO and the GO-based composites: (a) GO-CTS-L and (b) GO-Al-L.

loading conditions and geometry of the samples may contribute to substantial variations in the measured mechanical properties [34]. In this study, the mechanical properties of GO membranes cross-linked with CTS and Al3þ ions were evaluated and compared. Stress-strain behavior of pristine precursors (GO, CTS) and GO-based composites (GO-CTS and GO-Al) with variable cross-linker content (w/v

Table 2 Relative elemental analysis result of GO and GO-based composites. Sample ID

GO

GO-CTS

GO-Al

Element Carbon Nitrogen Oxygen Aluminum

wt. % 66.8 8.8 24.4 0

wt. % 73.0 11.4 15.7 0

wt. % 68.6 8.6 14.4 8.4

Table 3 Water swelling properties of GO and its composite forms.

Note: H content was neglected for this relative analysis.

Sample

both types of composites (as observed in the XRD results of Fig. 5 (a)). The greater solvent swelling of composite materials with a lower content of cross-linker (0.3% w/w over 0.6% w/w) relates to the moderate accessibility of polar functional groups and hydrophile character of the surface sites of the GO sheets (e.g., –OH, –COOH, etc.). In particular, the SD was reduced for GO-CTS composites and gradually leveled off when the cross-linker contents rose above 0.3% w/w. Therefore, greater cross-linking led to reduced swelling [22].

Swelling (%)

CTS Pure GO GO-CTS-L GO-CTS-H GO-Al-L GO-Al-H

Comments

356.5 683.3 394.0 279.2 165.4 143.2

Totally dissolved Partially stable Expanded gel-like Expanded gel-like Stable hydrated film Stable hydrated film

Table 4 Literature comparison of mechanical properties of GO produced in different forms.

3.8. Tensile strength and modulus of GO-based composites

Material form

Although mechanical properties of GO and GO-based composite membranes have been well-studied as summarized in Table 4, the literature values for GO sheets are wide-ranging for experimental and theoretical estimates. The variability relates to the various fabrication processes and the relative content of the oxygen-containing groups that yield complex microstructures during GO preparation. Additionally,

Single-layer GO sheet GO films GO papers

Thickness (μm)

Tensile modulus (GPa)

Tensile strength (MPa)

Ref.

0.0007

208

77

[35]

0.055 1–30

695–697 32

8–11 15–133

[36] [37]

Fig. 7. Swelling test of GO-based composite materials in water. (a) After 24 h and (b) 72 h. 86

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%) are shown in Fig. 8. As well, the corresponding properties of tensile strength and tensile modulus for these systems are shown in Fig. 9. In Fig. 8 (a), GO membranes with two thickness values (GO-L and GO-H) had tensile strength in the range of 14.76–15.93 MPa and a tensile modulus from 0.37 to 0.41 GPa. Additionally, pure CTS membrane had a tensile strength and modulus of 10.27 MPa and 0.36 GPa, respectively. In Fig. 8 (b), the GO-based composite membranes revealed higher values of tensile strength, tensile modulus, elongation at break (ductility) and fracture strength. For instance, the GO-based composites containing 0.3 w/v% CTS had an elongation at break that increased substantially relative to unmodified precursor (GO or CTS). The tensile strength, tensile modulus and elongation at break of the GO-CTS-L were 30.11 MPa, 0.67 GPa, and 1.68%, respectively. By comparison, the values of GO-Al-L composites are 26.18 MPa, 0.56 GPa, and 0.46%, respectively. Therefore, with addition of 0.3 w/v% CTS and Al3þ ions, the tensile strength of unmodified GO membranes increased by 101% and 74% over GO, respectively. By contrast, the tensile modulus of the composites improved by 61% and 33%, respectively, as compared to the pristine GO. Moreover, the addition of 0.3 w/v% CTS and Al3þ ions had a greater effect on the ductility (elongation % at break) of the GO membrane by 290% and 7%, respectively, as compared to pure GO. The GO membranes were brittle and tended to disintegrate in aqueous media (e.g., water). However, the GO-based composite membranes were more stable, where the flexibility and brittleness were tunable according to the relative composition of GO and cross-linker type. This indicates that formation of stable bonds occurred between individual GO sheets (via cross-linking) along with significant changes to the mechanical behavior of the GO-based composites at different stress levels. The membrane flexibility and tensile strength of GO were enhanced by the addition of small amounts of cross-linker species. Furthermore, the enhanced tensile strength and fracture strength of GO-based composite membranes was ascribed to the planar structure of graphene sheets which directs the rearrangement of either CTS and/or Al3þ ions during tensile deforma­ tion as illustrated by the structural model in Fig. 10. The enhancement in the properties of the composites was observed to depend on the relative cross-linker content. For instance, the intro­ duction of 0.6 w/v% of cross-linkers (CTS or Al3þ ions) with GO led to a tensile strength of 36.99 MPa for GO-CTS-H and 27.34 MPa for GO-Al-H. This represents an enhancement of 146% and 83%, respectively, while the modulus increased by 64% and 41%, respectively. The foregoing results showed that incremental addition of cross-linkers had positive effects on the mechanical tensile properties. The greater tensile strength of GO-CTS composite membranes relates to the uniform dispersion of CTS in the GO matrix and the strong bonding between components. The adhesion between GO and CTS resulted in a robust interfacial region that enhanced the tensile strength of the GO membranes. On the other hand,

Fig. 9. Tensile strength and tensile modulus for GO and GO-based composites.

GO has hydrophilic groups (e.g., –COOH and –OH) on its surface imparting negative surface charges when dispersed in solvent [35] fa­ voring high dispersion of individual GO sheets in water. CTS is a hy­ drophilic biopolymer with abundant –OH and –NH groups that may undergo protonation at variable pH conditions to adopt a positive electrostatic potential. Cross-linking between GO and CTS yielded amide linkages which notably increased the mechanical properties, as illus­ trated by the proposed model (Fig. 10). In the case of GO-Al composites, the introduction of a metal cation species may result in covalent and/or ionic bonding between the COOH and –OH groups of the GO sheets. This resulted in a homogeneous codispersion of bonded GO-metal ions, as illustrated in Fig. 9. GO mem­ branes modified with CTS (GO-CTS) had higher tensile strengths and greater elongation at break compared to GO-Al composite membranes. The elongation at break for GO membranes with 0.6 w/v% CTS increased by about 200% in comparison with GO. By comparison, GO-Al composites containing 0.6 w/v% Al cross-linker increased by about 15% (Fig. 7). Moreover, the anionic nature of GO and polycationic nature of CTS resulted in favorable electrostatic attraction and reduced chain segmental mobility. This phenomenon led to enhanced thermal and tensile properties as shown by the TGA results in Fig. 5 (b), which are in agreement with other reports [24,38]. 4. Conclusion In this study, a first comparative report of GO-based composites with one of two cross-linker types (CTS or Al3þ ions) are described and characterized, where the role of cross-linker content (%) and type on the

Fig. 8. Tensile stress versus strain curves: (a) Unmodified synthetic precursors (GO and CTS) and (b) GO-based composite membranes. 87

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Fig. 10. A conceptual structural model for GO-based composites upon cross-linking to account for changes in tensile strength. CL indicates cross-linker (CTS or Al3þ ions).

composite stability and tensile strength were studied. Greater thickness and interlayer separation of GO cross-linked membranes were revealed by SEM results, where cross-linking of GO was supported by FTIR spectral results. In addition, a greater interlayer distance, D/G intensity ratio and thermal stability of GO-based composites were supported by XRD, Raman and TGA results. The elemental analysis of the samples revealed higher nitrogen and Al contents in GO-CTS and GO-Al com­ posites, respectively. Water swelling results for the GO-based composites revealed greater stability in water over the pristine GO materials. The tensile strength of the GO-CTS and GO-Al composites upon addition of 0.3 w/v% CTS and Al3þ ions exceeded the tensile strength of pristine GO by at least 64% and 33%, while the tensile modulus improved by 58% and 33%, respectively. The cross-linking method reported herein is a sustainable, facile, and low-cost method for potential large-scale pro­ duction of GO-based composites. GO-based composites have potential applications in advanced coatings, pollutant removal, and membrane filtration media for separation technology, where future work will be reported on membrane performance for long-term cross-flow filtration studies that employ GO-based materials.

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Acknowledgements The authors are grateful for the support of the Natural Sciences and Engineering Research Council of Canada Discovery Grant (RGPIN 2012418729 and RGPIN 2016-06197) for the financial support of this study. Support and expert technical assistance from Leila Dehabadi at the University of Saskatchewan for obtaining IR spectra and TGA profiles are greatly appreciated. The authors thank Jayaraman Raghavan and Jardel Nobrega Dos Santos at the University of Manitoba for performing DMA testing. Abbreviations and nomenclature GO CTS Al MW

graphene oxide chitosan Aluminum molecular weight

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