In Situ Reduction of Graphene Oxide Nanosheets in Poly(vinyl alcohol

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In Situ Reduction of Graphene Oxide Nanosheets in Poly(vinyl alcohol) Hydrogel by #-Ray Irradiation and Its Influence on Mechanical and Tribological Properties Yan Shi, Dangsheng Xiong, Jianliang Li, and Nan Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05948 • Publication Date (Web): 05 Aug 2016 Downloaded from http://pubs.acs.org on August 8, 2016

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The Journal of Physical Chemistry

In Situ Reduction of Graphene Oxide Nanosheets in Poly(vinyl alcohol) Hydrogel by γ-Ray Irradiation and Its Influence on Mechanical and Tribological Properties Yan Shi,† Dangsheng Xiong,*,†,‡ Jianliang Li,† Nan Wang§ †

School of Materials Science and Engineering, Nanjing University of Science and

Technology, Nanjing 210094, Jiangsu, P. R. China ‡

Jiangsu Key Laboratory of Advanced Micro/nano Materials and Technology,

Nanjing, PR China §

Synergetic Research Center on Advanced Materials (SRCAM), Nanjing, PR China

ABSTRACT: Graphene oxide containing poly (vinyl alcohol) (PVA/GO) composites prepared by freezing-thawing method were irradiated by γ-ray with the doses of 50, 100, 150 and 200 kGy in order to improve their strength and wear resistance. The effects of irradiation dose on the mechanical, thermal and tribological properties were evaluated. Microstructure and composition of PVA/GO hydrogels before and after irradiation were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The results revealed that the irradiation can in situ reduce the GO sheets dispersed in the PVA matrix, and the reduced graphene oxide acts as the crosslinking point of the dangling bond between molecular chains and formed covalent bonding after γ-ray irradiation, which endows them high strength and the improved thermal stability. Compared to the non-irradiated PVA/GO hydrogels, a 270% enhancement in compressive strength was obtained when the applied irradiation dose was 150 kGy. The friction coefficient of PVA/GO hydrogels increased with the increasing irradiation dose due to the loss of hydrophilicity. However, the wear resistance significantly improved via irradiation treatment.

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INTRODUCTION Hydrogels, which are hydrophilic polymer swollen with water, 1 have widely investigated for orthopedic and biomedical applications, such as cartilage replacement, 2 drug delivery systems, 3 , 4 biosensors, 5 wound dressing. 6 They are generally formed by crosslinking of polymer chains by covalent bonds, van der Waals interactions, or physical entanglements.7 Poly(vinyl alcohol) (PVA) hydrogel is an intriguing biomaterial that has been recognized as a very promising substitute for repairing diseased or damaged tissue such as cartilage, nucleus pulposus, and human lens due to its excellent biocompatibility, non-toxicity, water absorption ability and low friction coefficient. 8 – 11 Unfortunately, the poor mechanical strength, wear resistance and water-retention properties of pure PVA hydrogel has limited its development and applications. Incorporation of nanofillers such as carbon nanotubes (CNT), clay and silica into hydrogels has been considered to be an effective way to enhance the mechanical strength and toughness of hydrogels,12–15 but the costly and complicated methods for synthesizing and purifying CNT hinder their production on an industrial scale.16 Recently, the development of graphene-based nanocomposites has attracted tremendous attention. Graphene, a unique allotrope of carbon, is a one-atom planar planar sheet composed of sp2-hybridized carbon atoms arranged in a honeycomb lattice.17,18 Since it was first discovered by Geim in 2004,19 it has attracted tremendous interest due to its remarkable physiochemical properties arising from its extraordinary mechanical strength,20 large specific surface area,21 high intrinsic mobility,22 excellent thermal and electrical conductivities.23–25 These unique features endow great promise for potential applications in the area of chemistry, physics, and nanocomposites.20,26–282930 Similar to CNT, the processing and difficulty in dispersion of graphene in the polymer matrix constitutes the major challenge to utilize it in preparing nanocomposites.16,31 Chemical reduction of graphene oxide (GO), the oxygenated counterpart of graphene, has been considered as a most economical and efficient approach that can improve the dispersion of graphene but also realize mass production of graphene from cost-efficient natural graphite. 32 Up to now, GO has been reduced by various techniques, such as using reduced agents (including hydrazine,16 hydroquinone,33 sodium borohydride,34 and L -ascorbic35,36) and solvothermal reduction methods.37,38 Salavagione16 reduced GO in PVA/GO solutions by the addition of hydrazine, and the resulting composites experienced a 35° decrease in melting temperature (Tm) and a 2


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half reduction in crystallinity, and the composites with 1 wt% of RGO exhibited good electrical conductivity with a percolation threshold. Yang39 prepared PVA/graphene nanocomposites by reducing graphite oxide in the polymer matrix in a simple solution processing, and the results showed 16% and 32% higher of modulus and tensile stress for PVA/3.5 wt% graphene nanocomposite than those of pure PVA. Sheng 40 synthesized PMMA/graphene nanocomposites by in situ thermal reduction of GO in the PMMA matrix, and the mechanical properties of the nanocomposites were significantly improved. When the graphene nanosheets loading was 1.5 wt%, the storage modulus and the glass transition temperature increased by 45% and 7.5 ℃, respectively. However, most of reduced agents are poisonous and explosive, and are difficult to be removed. Solvothermal reduction methods are always needed high temperature (>100℃) and even high pressure.32 Therefore, the reduction of GO via γ-ray irradiation under room temperature is proposed to be an environmental-friendly and effective approach. Irradiation with γ-ray has been studied as a clean and facile method for the formation and modification of hydrogel materials. The radiation process exhibits various advantages, such as easy control of processing, environmentally friendly, an ability to be scaled up, has a great penetrating distance through material atoms and has the possibility of finishing the hydrogel formation and sterilization in one technological step. 41 , 42 Yang 43 investigated and compared the properties of PVA/ws-chitosan hydrogels prepared by repeated freeze-thawing and by irradiation, and found that hydrogels made by freeze-thawing followed by irradiation had a high mechanical strength. Shi44 studied the effect of γ irradiation dose on the material properties of PVA/PVP hydrogels, and found that hydrogels treated with irradiation dose of 100 kGy showed the best mechanical strength and the lowest friction coefficient, and the wear resistance of hydrogels was improved after irradiation. Recently, γ-ray irradiation has been applied in the reduction of GO. Importantly, besides the advantages mentioned above, reduction can be carried out without using excess reducing agent via irradiation, and the irradiation can also decrease the aggregations of graphene nanosheets due to its mild irradiation reduction rate.45 Zhang32 prepared graphene sheets by γ-ray induced reduction of a graphene oxide (GO) suspension in N,N-dimethyl formamide (DMF), and the reduced GO can be re-dispersed in many organic solvents. Li46 confirmed that GO was successfully reduced and modified by γ-irradiation in an ethylenediamine/water blend solution. 3



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Ansón-Casaos 47 studied the effect of γ-irradiation dose on the structure and composition of chemically synthesized few-layered graphene materials, and found that changes in Raman spectra and XPS spectra after γ-irradiation were greater than those induced by chemical reduction on GO, demonstrating that the graphene carbon lattice was strongly affected by γ-irradiation. He48 developed an irradiated-induced reduction and self-assembly method to fabricate graphene aerogel (GA) from GO sheets suspending in isopropanol/water solution, and the resulting GA showed honeycomb-like porous structure with high C/O ratio and thermal stability, indicating the effectiveness of γ-ray irradiation to reduce and self-assembly GO sheets. In this paper, we report a facile and eco-friendly method for the preparation of PVA/graphene nanocomposites with uniformly dispersed graphene nanosheets into the PVA matrix. First, PVA/GO hydrogels were formed by freezing and thawing method. Then, the GO was in situ reduced in the hydrogel composites by γ-ray irradiation. Unlike earlier reports, GO was reduced first and then incorporation with polymers, and to our knowledge, there are less reports about in situ reduction of GO within hydrogel matrix. In order to confirm the reduction of GO in PVA/GO composites after γ-ray irradiation, both non-irradiated and irradiated hydrogel samples were comprehensively characterized by using X-ray diffraction (XRD), Fourier transform infrared (FT-IR),

Raman spectroscopy and X-ray photoelectron

spectroscopy (XPS), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The effects of irradiation dose on microstructure, mechanical, thermal and tribological properties of PVA/GO hydrogel composites were also investigated. EXPERIMENTAL METHODS Materials. GO was purchased from Nanjing FAME Bearing Co. Ltd., China. PVA, saponiﬁed greater than 99% with a polymerization degree of 1700 was bought from Kuraray Co. Ltd., Japan. Preparation of PVA/GO Hydrogels. To prepare PVA/GO composite hydrogels, PVA powder was dissolved in deionized water at 95 ℃ to form an aqueous solution. Then, GO aqueous suspension was dripped into the PVA solution, and then the mixed solution was stirred at 95 ℃ for 10 h. After held at a higher temperature to remove air bubbles, the mixed solution was purred into molds and subjected to five cycles of freezing at -20 ℃ for 18 h and thawing at room temperature for 6 h. The amount of 4


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GO powder was 0.1 wt% of PVA powder, and the polymer concentration was kept 15 w/w%. After that, the as-prepared hydrogels immersed in the oxygen-free deionized water were then exposed to 60Co γ-ray irradiation at a dose rate of 0.89 kGy/h at room temperature across a series of total doses comprised 50 kGy, 100 kGy, 150 kGy and 200 kGy. Non-irradiated PVA/GO hydrogels were also prepared as controls to evaluate the impact of irradiation dose on the material properties in terms of microstructure, mechanical and friction properties. Characterization Techniques. Scanning electron microscopy (SEM, FEI Quanta 250FEG, German) was used to evaluate the morphology of GO powder and the cross-section of PVA/GO hydrogels. The hydrogel samples were placed in freezer dryer (FD-1A-50, China) for at least 2 days to remove all water, and then the samples were sputter-coated with a layer of gold for SEM observations. Field emission transmission electron microscopy (FETEM) image of GO were recorded by using Tecnai G2 20 LaB6 (FEI, USA). A multimode 8 Atomic force microscopy (AFM, Bruker, Gremany) was used to determine the thickness of GO sheets. Fourier transform infrared ray (FT-IR, Nicolet MAGNA-IR 750, USA) spectroscopy of GO and both non-irradiated and irradiated PVA/GO composites was performed in the range of 4000–550 cm−1with the resolution of 4 cm−1. X-ray diffraction (XRD) patterns were obtained with a Bruker-AXS D8 Advance X-ray diffractometer (Bruker, Gremany) with Cu Kα radiation (λ= 0.1541 nm) from 5° to 60° (by step of 0.02°). The tube voltage and tube current were kept at 40 kV and 40 mA, respectively. Raman spectra were recorded on a Raman microscope system (Aramis, France) using green (532 nm) laser excitation. X-ray photoelectron spectroscopy (XPS) analysis was taken by a PHI QUANTERA II photoelectron spectrometer (ULVAC PHI, Japan). Survey spectra were obtained at 100 eV pass energy, whereas high resolution peak scans were performed at 20 eV pass energy. Water Content. For all the hydrogel samples, the water content, W, was determined according to the following equation, W=

୑౩ ି୑ౚ ୑౩

(1)

where Ms and Md represent the fully swollen and completely dried sample, 5



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respectively. Five independent samples were tested for each set of hydrogels (n=5). Mechanical Properties Test. Unconfined compression tests were carried on Instron 5943 under a rate of 4 mm/min until the compression strain reached 65% since the PVA/GO hydrogels are soft material. The sample size was approximately 12 mm in diameter and 4 mm in height. The compressive strength of hydrogels was obtained. Three independent samples were tested for each set of hydrogels (n=3). Thermal properties and crystallinity. Differential scanning calorimetry (DSC, Mettler-toledo, Switzerland, Model: DSC823e) were carried out from 50 ℃ to 300 ℃ at a heating rate of 10 ℃/min, under N2 atmosphere. The speed of nitrogen gas flow was 30 ml/min. The crystallinity degree was calculated using the heat of fusion of a perfect crystal of PVA sample (∆Hf* = 138.6 J/g):49 ୼ୌ

Crystallinity = ୼ୌ ౜ × 100 ౜∗

(2)

Thermogravimetric analysis (TGA) was carried out using a TGA/SDTA851E thermal analyzer instrument from 50 ℃ to 600 ℃ at a heating rate of 10 ℃/min, under N2 atmosphere. Tribological Properties Evaluation. The friction properties were evaluated by sliding against CoCrMo ball with a diameter of 8 mm on a UMT-Ⅱ multi-functional micro-friction test machine at room temperature. The countersurface of CoCrMo ball was polished to the roughness Ra ≤ 0.02 µm. The hydrogel samples were manufactured as a shape of disk with 45 mm in diameter and 4 mm in thickness. The deionized water served as the lubricant. Before the friction tests, the hydrogel sample and friction pair were kept contact at applied load for 30 s. The sliding speed and applied normal load were kept at 0.08 m/s and 5 N. The test duration was typically 30 min, and the friction coefficient evolution was continuously monitored during the test. The test conditions were summarized in Table 1. Each of test was repeated three times (n=3). In order to appraise the influence of irradiation dose on wear resistance of PVA/GO hydrogels quickly, the hydrogels were sliding against CoCrMo ball with a sliding speed of 0.08 m/s under 10 N for 45 min. After that the surface morphplogy the tested hydrogels was measured by using a white-light interferometer (ContourGT-K 3D Optical Microscope, Germany).

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RESULTS AND DISCUSSION Synthesis and Morphologies of GO and Irradiated PVA/GO Composites. The PVA/GO hydrogels were synthesized by a freezing-thawing method. GO sheets act as physical cross-linkers in this case (Figure 1a and b), besides GO, nanoparticles such as clay and multiwalled carbon nanotubes have also been proposed as physical cross-linkers previously.50–52 Liang53 has found the efficient load transfer between GO and PVA matrix due to the molecule-level dispersion of GO in PVA matrix and strong interfacial adhesion attributed to H-bonding between them. Irradiation leads to either crosslinking or random chain scission of polymer. On the other hand, in the process of γ-ray irradiation, high-energy γ-ray irradiation interacts with PVA molecules and produce excided molecules, ions, electrons, and radicals. The resulting electrons are solvated with PVA solvent molecules, and the oxygen-containing functional groups can be reduced by the solvated electrons, and the GO nanosheets are thought to be reduced in the PVA matrix (Figure 1c), which would lead to significant changes in microstructure, mechanical and tribological properties of the hydrogel composites, and then various characterization techniques were used to confirm these.

(a)

(c)

(b) γ-ray irradiation

Figure 1. Proposed structure of PVA/GO hydrogels.

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SEM and TEM measurements were used to visualize the morphology of GO sheets, and the results are shown in Figure 2a and b. It is apparent that the average size of GO nanosheets is approximately 5 µm, and some crumple are observed due to the very thin thickness. The SEM image also indicates a curled morphology and wavy structure (Figure 2a), which are part of the intrinsic nature of GO sheets own.54 Based on the TEM image of GO nanosheets, the sheets show a restacked few layers structure about 4 layers (Figure 2b). Figure 2c exhibits a typical AFM image of GO deposited onto a cleaved mica substrate from an aqueous dispersion and the corresponding height profiles of the GO nanosheets. The average thickness of GO nanosheets is about 0.96 nm, as shown in Figure 2c. The SEM image in Figure 3a exhibits an internal three-dimensional porous network structure of PVA/GO hydrogel composite, and the structure of the composite becomes more homogeneous and denser when compared to the neat PVA hydrogel (Figure S1). After γ-ray irradiation, the structure of the composite becomes denser and compact, and the pores get smaller with the increasing irradiation dose, as shown in Figure 3b–e, which is mainly attributed to the great crosslinking induced by γ-ray irradiation.

Figure 2. (a) SEM, (b) TEM and (c) AFM image of graphene oxide.

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Figure 3. SEM images of PVA/GO hydrogel composites treated with the irradiation dose of (a) 0, (b) 50, (c) 100, (d) 150 and (e) 200 kGy. XRD patterns of GO, neat PVA and PVA/GO hydrogels treated with different irradiation dose are plotted in Figure 4. The typical diffraction peak of GO is observed at about 2θ = 10.3°, indicating that the distance between layers is about 0.84 nm,35 and the diffraction peak of neat PVA is located at 2θ = 19.3°. For PVA/GO composite, the XRD pattern only shows the characteristic diffraction peak from PVA, while the characteristic peak of GO does not appear, which suggests that GO has been fully exfoliated into individual sheets in the polymer matrix so that the regular and periodic structure of GO disappears.55,56 When the hydrogel composite was subjected γ-ray irradiation, the position of diffraction peak is just slightly shifted (Table 1), but the peak intensity significantly decreases compared with the non-irradiated one, which suggests that irradiation process significantly decreases degree of crystallinity of PVA/GO composite hydrogels. But the γ-ray total dose seems to have little effect on the XRD pattern of PVA/GO hydrogels, as clearly shown in Figure 4.

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Figure 4. XRD patterns of (a) GO, (b) neat PVA, and PVA/GO hydrogels treated with (c) 0, (d) 50, (e) 100, (f) 150, and (g) 200 kGy of irradiation dose. Changes in Microstructure of PVA/GO Composites after Irradiation. The reduction of GO in PVA matrix via γ-ray irradiation was firstly investigated by FT-IR spectra, as shown in Figure 5. The characteristic peaks of GO at 1716, 1621, and 1050 cm–1 correspond to C=O and C=C, and C–O–C stretching vibrations, respectively. The absorption peak at 3000–3700 cm–1 in the spectrum of PVA and PVA/GO hydrogels is due to the symmetrical stretching vibration of hydroxyl groups, demonstrating the strong intermolecular and intramolecular hydrogen bonding. 57 Compared with neat PVA, the –OH stretching peak of PVA/GO hydrogel shifts to a higher wavenumber (~3293 cm–1), and the C=O and C–O–C stretching peak shifts to 1723 cm–1 and 1094 cm–1, indicating the presence of hydrogen bonding interactions between the hydroxyl groups of the PVA chains and the oxygen-containing functional groups of GO sheets.58 This interfacial interaction is helpful for the external force to be effectively transferred to GO to enhance the load-carrying capacity of hydrogels. After irradiation, the –OH stretching peak shifts to a lower number (~3280 cm–1), the characteristic peak corresponding to C=O almost disappears, and the relative intensity weakening of C–O–C stretching vibration at 1094 cm–1 is found. These phenomenon could be explained by the remove of oxygen-containing functional groups from GO, suggesting that γ-ray irradiation results in the reduction of GO. The reduction mechanism is likely to be because the produced H radicals attack the oxygen-containing functional groups on the surface of GO, leading to the formation of water molecules.59 PVA/GO-200kGy 1649 PVA/GO-0kGy Absorbance

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3280 PVA

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Figure 5. FTIR spectra of GO, PVA, the unirradiated and irradiated PVA/GO 10


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hydrogels. Raman spectroscopy is a fast and non-destructive method for the further characterization of structural changes in carbon materials. Figure 6 presents the effect of irradiation dose on Raman spectra of PVA/GO composites. Besides the characteristic peaks of PVA, spectral features include D band at 1345–1368 cm–1, the G band at 1580–1607 cm–1 and the second order 2D band at around 2649–2704 cm–1 of PVA/GO hydrogel composites treated with various irradiation doses, as summarized in Table S1. It is known that the D band that originates from zone-boundary phonons is ascribed to defects or a breakdown of translational symmetry, while the G band is attributed to the doubly degenerate zone center E2g mode related to phonon vibrations in sp2 carbon domains.60,61 It is clear that G bands of all PVA/GO hydrogels are un-shifted after irradiation when compared with the non-irradiated PVA/GO hydrogel. Hole doping from O2 with an assist from water molecules may account for this.62,63 In our particular system under γ-ray irradiation, oxygen atoms is thought to be exchanged between the carbon lattice and the water environment, and the resulting substitution of carbon by oxygen in the graphene lattice accounts for the up-shift of G band position.47,62

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Figure 6. Effects of irradiation dose on the Raman spectra of PVA/GO hydrogels. The intensity ratio of the D and G bands is widely used for characterizing the defect concentration in graphitic material. This ratio provides an indication of the degree of disorder in the graphite structure, and it is inversely proportional to the average size of the sp2 C=C cluster.64,65 Moon66 found an increasing D/G intensity ratio of reduced 11



graphene oxide treated with hydroiodic acid and acetic acid, suggesting the reduction process altered the structure of GO with a high quantity of structural defects. An increase in D/G intensity ratio reveals in-plane defects such as interstitials and vacancies and their clusters, leading to bond formation, disordering and turbulence of the basal planes eventually.65 As shown in Figure 7, the intensity ratio of these two bands significantly decreases after γ-ray irradiation (1.30 for the non-irradiated PVA/GO hydrogel), and the value gradually deceases from 1.15 to 0.93 when the total irradiation dose increases from 50 kGy to 200 kGy. It is indicated the repair of defects by recovery of the aromatic structure,67 that is, the reduction in defect concentration. It is also inferred that γ-ray irradiation mainly eliminates the oxygen-containing functional groups present in GO, as hydroxyl groups on GO sheets facilitate interaction with intercalated PVA molecules, which increases the dimensions of in-plane sp2 domains, followed by the decreasing disorder.59 1.4 1.2 1.0

D/G intensity ratio

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0.8 0.6 0.4 0.2 0.0

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Figure 7. The D/G Raman intensity ratio as function of irradiation dose The 2D band originates from the splitting in the π-electron dispersion and is due to the interactions between the basal planes of graphite.61 The 2D band is very sensitive to the stacking order of the graphene sheets along the c-axis as well as to the number of layers, and shows greater structure (often a doublet) with increasing number of graphene layers.68 After γ-ray irradiation, there are some variations in the position of 2D band (Table S1), which is due to the structural changes of GO sheets during γ-ray irradiation. The relative increase in intensity and sharpening of the 2D band for the irradiated PVA/GO with the increasing irradiation dose is attributed to a decrease in the number of graphene layers and a smaller starking distance across the interlayer spacing. It indicates the successfully reduction of GO within PVA/GO composites via 12


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γ-ray irradiation, and a higher reduction degree of GO can be obtained under larger irradiation dose treatment. O1s

C1s

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Figure 8. XPS survey scans of the non-irradiated and irradiated PVA/GO hydrogels. More evidence of the irradiation reduction mechanism of GO in PVA matrix is demonstrated by XPS analysis. Highlights on the C 1s and O 1s peaks are shown in the low resolution XPS spectra of both non-irradiated and irradiated PVA/GO hydrogels (Figure 8). For PVA/GO hydrogels, the overall intensity of peak associated with O 1s decreases after irradiation while that of the C 1s peak inversely increases. The weight percentage of the main elements are listed in Table S2, also the elemental ratio of C/O calculated from those data. The C/O ratio of PVA/GO–200 kGy is 2.53, which is 27% higher than that of the non-irradiated PVA/GO hydrogel at 1.99. This change again suggests the remove of oxygen-containing functional groups from GO in the process of γ-ray irradiation, which is also consistent with the C 1s spectra (Figure 9a and c). Similar to the XPS spectra of GO (Figure S2), the C 1s XPS spectrum of PVA/GO composite clearly indicates three components attributable to carbon atoms involved in different functional groups: the aromatic C–C (sp2-hybridized carbon), C–O (hydroxyl and epoxy), and C=O (carbonyl and carboxyl) groups, centered at 283.1, 284.3 and 287.3 eV, respectively. While after γ-ray irradiation treatment, the peak intensity of C–O significantly decreases (Figure 9c), about half of the peak intensity for non-irradiated PVA/GO hydrogel, which is a signature that GO is reduced within PVA matrix through γ-ray irradiation. In the O 1s spectra of PVA/GO composite (Figure 9b), the two deconvoluted components are observed at 530.6 and 531.3 eV, which are assigned to C–O (oxygen singly bonded to 13



aliphatic carbon) and C=O (oxygen doubly bonded to aromatic carbon) groups, respectively.61,69 Decreases in magnitude of both C–O and C=O peaks occur in the irradiated PVA/GO hydrogel (Figure 9d), and the elimination of oxygen-functional groups via γ-ray irradiation further confirms the successful reduction of GO within PVA matrix. 9000 8000

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Figure 9. (a,c) C 1s and (b,d) O 1s regions of PVA/GO-0kGy (a,b) and PVA/GO-200kGy (c,d) hydrogel The influence of irradiation dose on the water content of PVA/GO hydrogels is shown in Figure 10. It is clear that the water content of PVA/GO hydrogel significantly decreases with the increasing irradiation dose. The losses of oxygen-containing functional groups of GO for the irradiated PVA/GO hydrogels diminish their own hydrophily, leading to the decreased water absorbing capacity. On the other hand, the γ-ray irradiation promotes crosslinking of polymer chains, and then the network structure of hydrogel composites becomes denser, which leaves less space to accommodate water molecules.42 The water content of PVA/GO hydrogel is higher than that of pure PVA hydrogel (~81.3%), which has been reported in our previously study.70 It is interesting that after irradiation with the same dose of 200 kGy, the water content of the irradiated PVA/GO composite is lower than that of the 14


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irradiated pure PVA hydrogel,71 the hydrophobicity of the reduced GO can account for this. 90

85

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Figure 10. Effect of irradiation dose on the water content of PVA/GO hydrogels Effect of Irradiation Dose on Mechanical and Thermal Properties of PVA/GO Composites. The application of γ-ray irradiation on PVA/GO hydrogel composites leads to the significant improvement of their compressive mechanical property due to the occurrence of crosslinking of the composites, as clearly shown in Figure 11. On the other hand, the increased surface activity of reduced GO sheets induced by irradiation makes the reduced GO more prone to be the crosslinking point, which is also responsible for the enhancement of mechanical property. The compressive strength of the composites increases first, while then decreases with the increase of irradiation dose. That is because irradiation process may result in either crosslinking or degradation of polymer, and the degradation of the polymer occurs under the overdose of irradiation. The compressive strength of the composites reaches a maximum value with the dose of 150 kGy, which is a 270% improvement compared with the non-irradiated hydrogel sample. It has demonstrated in the previously research that the pure PVA hydrogel gained the largest compressive strength at 100 kGy, which is about 65% higher than that of the non-irradiated pure PVA, and the compressive strength decreased with the increasing irradiation dose.71 It is indicated that the incorporation of GO could resist degradation for PVA. In another submitted manuscript, the prepared PVA/rGO composites with 0.1 wt% rGO shows a compressive strength of ~0.95 MPa, which is remarkably lower than that of the PVA/GO-150kGy hydrogel composite in the present study (~2.87 MPa), indicating 15



the efficiency of in situ reduction of GO. However, the influence of irradiation on the polymer itself or the reduction of GO dominates this significantly improvement is still unclear, and it would be investigated and discussed in the following research. 3.2 2.8

Compressive Strength/MPa

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2.4 2.0 1.6 1.2 0.8 0.4 0.0 0

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Figure 11. Effects of irradiation dose on the compressive strength of PVA/GO hydrogels. PVA is a semicrystalline polymer, and its degree of crystallinity is closely related to the mechanical properties. DSC curves were utilized to estimate the thermal behavior and the degree of crystallinity in the present study, and the influence of irradiation dose was explored (Figure 12). The melting temperature and the calculated crystallinity of the hydrogel composites are summarized in Table 1 indicating the continuous decreases with the increasing irradiation dose. However, these results are in contradiction with the observation made for GO/UHMWPE composites that the crystallinity of irradiated composites increases by 5.8% compared to non-irradiated samples due to the formation of perfect crystals induced by γ-ray irradiation.72 That is because the molecular chain scission caused by irradiation and the higher mobility of the new shorter chains lead to great cross-linking, which breaks the symmetry and regularity of the polymer chains, and hinders the crystal growth. On the other hand, the decreasing crystallinity as function of irradiation dose also demonstrates that the mechanical enhancement of irradiated PVA/GO composites is not ascribed to the change in crystallinity. In general, great crosslinking induced by irradiation and the successful reduction of GO contribute to the effective improvement of mechanical property.

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200kGy Heat flow/mW/g

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Figure 12. DSC curves for PVA/GO hydrogels irradiated with different dose. The thermal stability of PVA/GO hydrogels treated with various irradiation doses was also evaluated by TGA with a nitrogen atmosphere. Figure 13 shows the TGA and corresponding DTG results. Three degradation steps could be observed for both non-irradiated and irradiated PVA/GO composites from Figure 13a. The first weight loss process before 200℃ was associated with the loss of absorbed moisture, independent of the irradiation dose for all samples. In the second weight loss process at about 360℃ is associated to –OH collapse and partial main chain decomposition. The onset of the thermal degradation temperature of the composites is shifted to higher temperature after irradiation. The third weight loss process at approximately 440 ℃ is attributed to residual main chain decomposition. The peak temperature (Tp) of DTG curve represents the temperature corresponding to the maximum weight loss rate. As shown in Figure 13b and Table 1, the Tp of PVA/GO composite irradiated with a dose of 200 kGy appears at about 371.6 and 440.6℃, which is increased by 10.8℃ and 4.5℃, respectively, compared to the non-irradiated samples. The values of irradiated composites are higher than that of reported graphene sheets or reduced graphene oxide based polymer composites.40,73,74 The increasing thermal stability for the irradiated PVA/GO composites indicates the occurrence of crosslinking in the matrix due to radiation mechanisms.

17



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0 kGy 50 kGy 100 kGy 150 kGy 200 kGy 100

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Figure 13. (a) TGA, and (b) DTG curves for PVA/GO hydrogels with different irradiation dose.

Table 1. The summary of the results from XRD, DSC and TGA for various hydrogel samples. XRD DSC TGA Samples 2θ/° Tm/℃ Crystallinity/% Tp-second/℃ Tp-third/℃ PVA/GO-0kGy 19.5 225.3 34.3% 360.8 436.1 PVA/GO-50kGy 19.4 221.1 33.6% 366.3 437.3 PVA/GO-100kGy 19.6 218.7 20.3% 365.8 438.4 PVA/GO-150kGy 19.5 216.4 18.7% 365.3 439.5 PVA/GO-200kGy 19.5 215.8 15.4% 371.6 440.6

18


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Effect of Irradiation Dose on Tribological Properties of PVA/GO Composites. General trends in the friction coefficient over time for PVA/GO hydrogels treated with different irradiation dose are presented in Figure S3, and their average values of friction coefficient as function of irradiation dose are plotted in Figure 14. The results demonstrate that the friction coefficient of both non-irradiated and irradiated hydrogels remain stable during the entire friction tests due to the biphasic lubrication mechanism. The solid matrix deforms and resists the flow of interstitial fluid under the applied normal load, and then interstitial fluid pressurization occurs to support considerable proportion of total load. Besides, the migrating contact area delivers re-swelling period to the hydrogel, which promotes the sustainability of the pressurization and consequent stable and low friction coefficient.2,75 However, the friction coefficient of hydrogels is observed to increase with the increasing irradiation dose. This may be attributed to the decreased water content after irradiation, which increases the solid to solid contact area between the hydrogel and the CoCrMo ball. On the other hand, as confirmed by FTIR, Raman and XPS analysis, the decrease of oxygen-containing functional groups after γ-ray irradiation treatment is thought to decrease the hydrophily of hydrogels, which is not conducive to lubrication. 0.12

0.11

Friction Coefficient

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Figure 14. Average value of friction coefficient of hydrogels as function of irradiation dose. The representative 3D surface morphologies of the non-irradiated and irradiated PVA/GO hydrogels after 45 min of friction under 10 N are given in Figure 15. After γ-ray irradiation, the wear tracks of hydrogels become significantly shallow and narrow, and the depth diminishes about 4 times for PVA/GO–100 kGy hydrogel 19



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(Figure 15b, ~ –30µm) compared with the non-irradiated samples (Figure 15a, ~ –150µm). The depth of the wear track further decreases when the irradiation dose continues to increase (Figure 15c). It is indicated that the wear resistance of PVA/GO hydrogel is significantly improved via γ-ray irradiation treatment. That is because, after irradiation, the high aspect ratio and surface activity of reduced GO offers a large surface area available for interaction with PVA molecules, which facilitates effective load transfer between GO and PVA. In addition, irradiation leads to a denser and tough structure for the composite, which is also conducive to the improvement of wear resistance.

Figure 15. 3D surface morphologies of (a) the non-irradiated PVA/GO hydrogels, and irradiated hydrogels with a dose of (b) 100 kGy and (c) 200 kGy after friction tests.

CONCLUSIONS γ-ray irradiation was found to be a facile and eco-friendly method for the reduction of GO within PVA matrix. In the present study, PVA/GO hydrogel composites were synthesized by a repeated freezing-thawing method, and then the GO sheets were in situ reduced by γ-ray irradiation. The effects of irradiation dose on the microstructure, 20


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composition, mechanical, thermal and tribological properties of the hydrogel composites were investigated. The results demonstrated the well dispersion of GO in the PVA matrix by XRD and SEM, and confirmed the successfully in situ reduction of GO within the hydrogel composites via FT-IR, Raman spectroscopy and XPS spectra. The G/D band intensity ratio from Raman spectra gradually decreased from 1.30 to 0.93 as the irradiation dose increased from 0 to 200 kGy, and the C/O ratio obtained from XPS analysis increased from 1.99 to 2.53. The losses of oxygen-containing functional groups of GO nanosheets diminished the hydrophily of the composites, resulting in the decreased water content and the increased friction coefficient with the increasing irradiation dose. After γ-ray irradiation, the thermal property of the composites improved, and a 270% enhancement in compressive strength was obtained with an irradiation dose of 150 kGy when compared to the non-irradiated PVA/GO hydrogels, and the wear resistance of PVA/GO hydrogel is significantly improved via γ-ray irradiation treatment. ASSOCIATED CONTENT Supporting Information SEM image of neat PVA hydrogel; XPS spectra of GO; Friction coefficient of PVA/GO hydrogels treated with various irradiation doses over time; the summary of the maximum intensity position of D, G, and 2D bands from Raman spectra; and the elemental analysis data from XPS spectra. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Phone: (+86)025-84315325. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This project is supported by the National Natural Science Foundation of China (Grant nos. 51575278 and 11172142), the Fundamental Research Funds for the Central Universities (No. 30910612203 and 30915014103), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). 21



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TOC Graphic

γ-ray irradiation

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In Situ Reduction of Graphene Oxide Nanosheets in Poly(vinyl alcohol

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