The water-locking and cross-linking effects of graphene oxide on the load-bearing capacity of poly(vinyl alcohol) hydrogel

Abstract

The poor load-bearing capacity of the poly(vinyl alcohol) (PVA) hydrogel has limited its application in biomedical and orthopedic fields. In the present study, PVA/GO composite hydrogels were fabricated by the freeze–thaw method, and their mechanical and tribological properties as a function of GO content were evaluated. The results demonstrated that the GO sheets exhibited excellent interfacial interactions with the PVA hydrogel matrix. The tensile and compressive strength were improved by about 116% and 161% for the PVA/GO hydrogels with the optimized GO contents of 0.10–0.15 wt% compared to neat PVA. The GO sheets worked as the cross-linking points between the PVA molecular chains, and the individually dispersed GO sheets in the PVA matrix could maximize the filler's effect on restricting the movement of polymer chains, which was positive to the enhancement in the mechanical properties. Moreover, the water block effect of GO was thought to impede the water infiltration between the PVA molecules, which could enlarge the instantaneous pressurization under compressive load and further improve the load-bearing capacity of PVA. This was also beneficial to improving the tribological properties of PVA hydrogels.

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1. Introduction

Hydrogels, which are hydrophilic polymers swollen with water,¹ have been widely investigated as potential biomaterials for orthopedic and biomedical applications, such as replacements for cartilage and nucleus pulposus, etc.^2,3 They are generally formed by cross-linking of polymer chains through covalent bonds, physical entanglements, hydrogen bonding or van der Waals interactions.⁴ At present, the possibilities of hydrogels being used as permanent implants to replace damaged tissue have been investigated.⁵ Within the range of polymers capable of forming hydrogels, poly(vinyl alcohol) (PVA) hydrogel has attracted great interest due to its excellent biocompatibility, low toxicity, high water content. However, the neat PVA hydrogel suffers from the poor mechanical properties, water-locking ability, and inferior wear resistance, which limits its development. Integrate the hydrogel with other materials to form hydrogel composites is considered as one of the effective ways to improve the mechanical and tribological performances.⁶

Recently, nanomaterials have attracted great attentions for a variety of applications due to their structural features and special properties on a nanometer scale. Carbon nanostructures and clay minerals have been proposed as the effective filler materials. Coleman et al.⁷ found that the addition of multiwalled carbon nanotubes (MWNTs) in PVA and poly(9-vinyl carbazole) (PVK) increased both Young's modulus and hardness by factors of 1.8 and 1.6 at 1 wt% in PVA and 2.8 and 2.0 at 8 wt% in PVK. Haraguchi et al.⁸ first prepared nanocomposite hydrogels with a unique organic (polymer)/inorganic (clay) network structure, and these resulting hydrogels were highly stable, structurally homogeneous and had superior elongation with near-complete recovery. Chen et al.⁹ synthesized a novel layered double hydroxide (LDH)/polyacrylamide (PAM) nanocomposites by means of a convenient in situ polymerization method, and the swollen nanocomposites showed a greatly increased tensibility of >6236% at low inorganic content (≤2.3 wt%). However, the costly and multi-step methods for synthesizing and purifying carbon nanotubes hinder their production on an industrial scale.¹⁰ Whereas the clay is stiff inorganic platelet filler, the flexibility and biodegradability of these nanocomposites can be compromised.¹¹

Graphene oxide (GO), an important precursor for graphene, possesses high specific surface, abundant functional groups such as hydroxyl, epoxy, carbonyl, and carboxyl group, and extraordinary modulus and strength.^12,13 More important, GO is hydrophilic and can be easily dispersed in water as individual sheet.^12–15 It has been demonstrated that GO is efficient filler for the enhancement in mechanical properties and other properties of composite materials.^16,17 Li¹⁸ found that the introduction of amphiphilic GO enhanced the swelling ratio of PVA/GO hydrogels. Zhang⁶ prepared GO/PVA hydrogels with a 132% increase in tensile strength and a 36% improvement in compressive strength at a loading of 0.8 wt% GO. Liu¹⁹ fabricated PVA/chitosan (CS)/GO hydrogel nanofibers by electrospinning, and the tensile strength of the composites was 25% higher than that of the neat PVA/CS nanofibers. These results revealed that strong hydrogen bonding interactions between the oxygen-containing functional groups of GO sheets and the hydroxyl groups on PVA polymer chains led to the cross-linking of PVA molecules, which subsequently improved the load-bearing capacity of PVA.

Aside from the cross-linking of PVA molecules, the fluid infiltration is also responsible for the load-bearing capacity of PVA hydrogel. Like the natural cartilage, PVA hydrogel exhibits porous structure with a large amount of water, which endows them similar biphasic character.²⁰ Therefore, the water content is also essential to the mechanical and tribological properties of PVA hydrogel. As GO can play the role of fastening the water molecules due to its hydrophily, its incorporation endows PVA with high water contents and, on the other hand, impedes the water infiltration between the PVA molecules, which is also called water-locking effect. Thus, higher interstitial fluid pressurization would occur under load for the composites to support a considerable portion of load, which enhances the load-bearing capacity of hydrogels and reduces the solid-to-solid contact area. Moreover, the GO exhibits excellent lubrication property.²¹ If it transfers to the counterface, it will lead to the reduction of friction. To date, there are few reports on the tribological properties of PVA/GO composite hydrogels.

In the present study, PVA/GO hydrogel composites with GO as the reinforcement were prepared by the repeated freeze–thaw method. The composites were characterized by Scanning electronmicroscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, Raman spectroscopy, X-ray photoelectron spectrum (XPS), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The effects of GO content on mechanical and tribological properties of the hydrogel composites were investigated. The reinforcement and lubrication mechanisms of GO in PVA/GO hydrogel composites were discussed.

2. Experimental

2.1. GO synthesis

GO was prepared from natural graphite through a modified Hummers method.²² The as-prepared GO was purified by repeatedly washing and centrifugalizing with deionized water until pH of the solution was close to 7. Finally, the GO powder was dried at 60 °C under vacuum for at least 12 h to remove all the water inside. To prepare GO solution, a certain amount of GO powder was dispersed in deionized water by ultrasonic agitation for 20 min for later application.

2.2. Preparation of PVA/GO hydrogels

To prepare PVA/GO composite hydrogels, PVA, >99% saponified with a polymerization degree of 1700 (Kuraray Co. Ltd, Japan), was dissolved in deionized water at 95 °C to form an aqueous solution. Then, GO aqueous suspension was dripped into the PVA solution, and the mixed solution was stirred at 95 °C for 10 h. After being held at a higher temperature to remove air bubbles, the mixed solution was poured into molds and subjected to five cycles of freezing at −20 °C for 18 h and thawing at room temperature for 6 h. The amount of GO powder was 0, 0.05, 0.10, 0.15, and 0.20 wt% of the PVA powder, and the polymer concentration was kept at 15% (w/w).

2.3. Characterization

Scanning electron microscopy (SEM; FEI Quanta 250FEG, USA) was used to evaluate the surface morphology of the GO powder and the cross sections of the PVA/GO hydrogels. The accelerating voltage, and the spot size was kept at 20 kV and 3.5, respectively. The hydrogel samples were placed in a freeze 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) of GO were recorded using a Tecnai G2 20 LaB6 instrument (FEI, USA). 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° (in steps of 0.02°). The tube voltage and tube current were kept at 40 kV and 40 mA, respectively. The relative crystallinity degree was calculated as the ratio of the PVA crystalline area to the entire area of the spectrum.²³ Fourier transform infrared (FT-IR) spectroscopy (Nicolet MAGNA-IR 750, USA) was performed in the range of 4000–550 cm⁻¹ with a resolution of 4 cm⁻¹. Raman spectra were recorded on a LabRAM ARAMIS Raman microscope system using green (532 nm) laser excitation. X-ray photoelectron spectroscopy (XPS) analysis was performed with a PHI QUANTERA II photoelectron spectrometer (ULVAC PHI, Japan). Survey spectra were obtained at a 100 eV pass energy, whereas high-resolution peak scans were performed at a 20 eV pass energy. The deconvolution analysis of C 1s peaks was carried out using XPSPEAK software 4.1 (RCSMS Lab).

2.4. Water content

For all of the hydrogel samples, the water content, W, was determined according to the equationwhere M_S and M_d represent the fully swollen and completely dried samples, respectively. Five independent samples were tested for each set of hydrogels (n = 5).

2.5. Thermal properties and crystallinity

Thermogravimetric analysis (TGA) was carried out using a TGA/SDTA851E thermal analyzer from 50 to 600 °C at a heating rate of 10 °C min⁻¹, under a N₂ atmosphere.

Differential scanning calorimetry (DSC, Mettler Toledo, Switzerland, Model: DSC823e) was carried out from 50 to 300 °C at a heating rate of 10 °C min⁻¹, under a N₂ atmosphere. The speed of the nitrogen gas flow was 30 mL min⁻¹. The crystallinity degree was calculated using the heat of fusion of a perfect crystal of PVA (ΔH_f* = 138.6 J g⁻¹)²⁴

2.6. Mechanical properties test

The tensile stress–strain curves were obtained on Instron 3367 at a rate of 10 mm min⁻¹ until breaking occurred. Hydrogels used for tensile test were processed into a dumbbell shape. Unconfined compression tests were carried out in deionized water on an Instron 5943 instrument at a rate of 4 mm min⁻¹ at room temperature at until the compression ratio reached to 60%. Three independent samples were tested for each set of hydrogels (n = 3).

2.7. Friction tests

The friction properties of the hydrogels were evaluated by sliding them against a CoCrMo ball with a diameter of 8 mm on a UMT-II multifunctional microfriction test machine at room temperature. The countersurface of the CoCrMo ball was polished to a roughness of R_a ≤ 0.02 μm. The hydrogel samples were manufactured in the shape of disk with a 45 mm diameter and a 4 mm thickness. Deionized water served as the lubricant. Before the friction tests, the hydrogel sample and friction pair were kept contact at the applied load for 30 s. The sliding speed was 0.08 m s⁻¹, and applied normal load was in the range of 1–7 N. The test duration was typically 30 min, and the evolution of the friction coefficient was continuously monitored during the tests. Each test was repeated three times (n = 3). To evaluate the influence of the addition of GO on the wear resistance of the PVA/GO hydrogels, the PVA/0.1 wt% GO hydrogels were slid against the CoCrMo ball with a sliding speed of 0.08 m s⁻¹ under 5 N for 90 min. After that the three-dimentional wear morphology and roughness measurements of the tested hydrogels were obtained using a true color confocal microscope system (Axio CSM 700).

3. Results and discussion

3.1. Morphology and microstructure of GO and PVA/GO hydrogel composites

SEM and TEM measurements were used to visualize the morphology of the GO sheets, and the results are shown in Fig. 1a and b. It is clear that the average size of the GO nanosheets is approximately 50–150 μm, and some crumpling is observed. The SEM image also indicates a curled morphology and a wavy structure (Fig. 1a), which are part of the intrinsic nature of GO sheets.²⁵ Based on the TEM image of the GO nanosheets, the sheets show a restacked few-layer structure (Fig. 1b). The surface chemical composition of GO was evaluated by XPS. Survey spectrum (Fig. 1c) clearly shows the elements of carbon and oxygen. The C/O atomic ratio of GO is 2.1 : 1, which confirms the successful oxidization from pristine graphite. The asymmetric shape of the peak in C 1s spectrum (Fig. 1d) presents the expected presence of an oxygen contribution. It is clear that three different peaks centered at 283.9, 285.5, and 287.8 eV are observed, corresponding to C=C/C–C in aromatic rings, C–O (epoxy and hydroxyl groups), C=O (carbonyl and carboxyl), respectively.²⁶ It suggests the presence of abundant oxygen-containing functional groups on the GO sheets.

Fig. 1 (a) SEM, (b) TEM, (c) XPS survey spectrum, and (d) C 1s region of GO.

PVA is a physical hydrogel cross-linked by the intermolecular and intramolecular hydrogen bonds. The GO sheets are expected to be well-dispersed in PVA matrix, and the oxygen-containing functional groups on their surfaces can form strong interfacial interactions with PVA by hydrogen bonding (Fig. 2a), which would provide effective load transfer between the GO and PVA matrix.¹⁶ The SEM images in Fig. 2 exhibit an internal three-dimensional porous network structure of the hydrogels. With the incorporation of GO into PVA, the structure of the composites becomes homogeneous and dense (Fig. 2c) when compared to that of the neat PVA hydrogel (Fig. 2b). The porous structure of the PVA/GO composites becomes denser with increasing GO content (Fig. 2d). It is because that the GO sheets can act as physical cross-linking points to promote cross-linking.

Fig. 2 (a) Proposed structure of PVA/GO hydrogel, and SEM observation of (b) neat PVA, (c) PVA/0.10 wt% GO, and (d) PVA/0.20 wt% GO hydrogels.

Fig. 3 presents the XRD patterns of GO, neat PVA, and PVA hydrogel composites with different amount of GO. The characteristic diffraction peak of GO is observed at about 2θ = 10.3°, which indicates that the distance between layers is about 0.84 nm,²⁷ and the diffraction peak of neat PVA is located at 2θ = 19.3°. Interestingly, after the GO is incorporated into the PVA matrix, the XRD pattern of PVA/GO hydrogel only shows the characteristic peak from PVA, while the characteristic peak of GO disappears. This is because of the disorder and loss of structural regularity of GO in the PVA/GO composites, therefore, the GO was fully exfoliated and dispersed at the molecular level in the polymer matrix.^28–30 The result is consistent with other reports.^6,16 The intensity of the characteristic peak of the PVA/GO composites is significantly higher than that of the neat PVA, which indicates the increasing relatively crystallinity with the incorporation of GO into PVA. With increasing GO content, the intensity of the characteristic peak of the composites increases first and then decreases. The relative crystallinity degree calculated from XRD patterns is 38.9, 41.8, 47.6 and 44.5% for PVA/GO composites prepared with 0.05, 0.10, 0.15 and 0.20 wt% of GO, respectively, which is higher than that of the neat PVA (36.9%).

Fig. 3 XRD patterns of GO, neat PVA and PVA/GO hydrogel composites.

Fig. 4 shows the FT-IR spectra of GO, neat PVA and PVA/GO hydrogel composites. In the spectrum of GO, a broad and strong absorption at 3353 cm⁻¹ is due to O–H stretching vibration of carboxyl groups and the absorbed water. The characteristic peaks at 1719 and 1624 cm⁻¹ correspond to C=O stretching vibration and C=C skeletal stretching vibration, respectively. The characteristic peaks for the skeletal vibrations of C–OH and C–O–C appear at 1378 and 1226 cm⁻¹. The absorption peak at 3000–3700 cm⁻¹ in the spectrum of PVA and PVA/GO composites is due to the symmetrical stretching vibration of hydroxyl groups, demonstrating strong intermolecular and intramolecular hydrogen bonding.³¹ With the addition of GO, the –OH stretching peak is shifted to a smaller wavenumber, and the C=O stretching peak shifts to 1734 cm⁻¹. A decrease in –OH stretching of the PVA/GO composite is found with increasing GO content, indicating the reduction of hydrogen bonding between PVA molecules. This behavior is because that the incorporation of GO cuts off the hydrogen bond among the hydroxyl groups in the PVA and that the hydroxyl groups from PVA undergo hydrogen bonding with oxygen-containing functional groups on the surface of GO sheets.³¹ With the increase of GO content, hydrogen bonding between PVA molecules is further inhibited, and the interfacial interactions between GO and PVA enhanced. The interfacial interactions between the GO and PVA matrix are helpful for the external force to be effectively transferred to GO to enhance the load-bearing capacity of hydrogels.^16,32.

Fig. 4 FT-IR spectra of GO, neat PVA and PVA/GO hydrogel composites.

Fig. 5 shows the Raman spectra of GO, neat PVA and PVA/0.10 wt% GO hydrogel composite. The characteristic peaks located at 1354 and 1578 cm⁻¹ correspond to D and G bands for GO, respectively. The D band corresponds to disorder resulting from structural defects, and the G band is attributed to the doubly degenerate zone center E_2g mode related to phonon vibrations in sp² carbon domains.^21,33 For the neat PVA, the most intense band centered at 2912 cm⁻¹ arises from the stretching vibrations of –CH₂, and another peak at 1430 cm⁻¹ is assigned to the stretching vibration of –CH in the PVA molecules.³⁴ In the Raman spectrum of PVA/GO hydrogel, both characteristic peaks of PVA and GO appear. The intensity ratio of the D and G bands, which is used to characterize the defect concentration in graphitic material,³⁵ is found to be 1.1 and 1.3 for GO and PVA/GO composite, respectively. The increase of D/G ratio suggests the generation of a large number of sp² carbon domains with a smaller average size in PVA/GO.³⁶

Fig. 5 Raman spectra of GO, neat PVA and PVA/0.10 wt% GO hydrogel composite.

Fig. 6 gives the water content of PVA/GO composite hydrogels as a function of GO content. It is obvious that the incorporation of GO leads to the remarkable increase in water content for the hydrogels. This may be because of the introduction of more hydrophilic groups from GO to the composites. However, PVA/GO composites prepared with different amount of GO show the similar values of water content.

Fig. 6 Effect of GO content on water content of PVA/GO hydrogels.

3.2. Thermal behavior and crystallinity of PVA/GO composites

The thermal stability of GO and PVA/GO hydrogel composites was evaluated by TGA. As shown in Fig. 7a, GO starts to lose weight below 60 °C due to the loss of absorbed moisture. The major mass loss occurs at 150–250 °C due to the pyrolysis of oxygenated functionalities, and the steady loss observed for temperatures above 250 °C is attributed to the release of more stable oxygen functionalities.³⁷ In the case of PVA/GO hydrogel composites, a visible delayed decomposition is observed compared to the neat PVA hydrogel. As shown in the DTG curves (Fig. 7b), it is obvious that all composites present the similar decomposition process. The peak temperature (T_p) of the DTG curve indicates the temperature corresponding to the maximum weight-loss rate. The T_p values of the PVA/GO composites gradually increase to 370.5 and 440.9 °C (PVA/0.2 wt% GO), which are increased by 10.9 °C and 4.5 °C, respectively, compared to those of the neat PVA hydrogel.

Fig. 7 (a) TGA and (b) DTG curves for neat PVA and PVA/GO hydrogel composites.

It can be found from the DSC curves that both melting temperature (T_m) (Fig. 8a) and crystallinity degree (Fig. 8b) of the PVA/GO hydrogel composites increase with increasing GO content. The increase in T_m is ascribed to the constrained polymer chains by the hydrogen bonding interactions between the GO and PVA matrix.³⁸ In addition, the individually dispersed GO sheets in PVA matrix can maximize the fillers' effect on restricting mobility of polymer chains,³⁹ which ensures the efficient load transfer at the interface and results in the increasing T_m. On the other hand, GO is thought to be able to constrain and order polymer chain arrangement. A “molecule movement restriction” effect³⁷ leads to the increased crystallinity for the composites. However, the crystallinity degree of the composites decreases when the GO loading is further increased to 0.20 wt% (Fig. 8b), because the great cross-linking between the GO and PVA matrix may break the symmetry and regularity of the polymer chains, which is not conducive to crystallization.⁴⁰ The DSC results are in good agreement with the XRD findings, whereas crystallinity degrees of all samples deduced from DSC are lower than those calculated from XRD, which is consistent with other reports.^41,42 The different conditions of the two instrumental analysis techniques can account for these differences. The crystallinity degree calculated from DSC is temperature dependent, based on enthalpy of crystallization and fusion.⁴³ These values of crystallinity degree are obtained at higher temperatures may cause their decreases when compared to the values measured from XRD at room temperature.

Fig. 8 (a) DSC curves for PVA/GO hydrogel composites, (b) effect of GO content on crystallinity of the composites.

3.3. Mechanical properties of PVA/GO hydrogels

Fig. 9a and b shows the tensile and compressive behaviors of PVA/GO hydrogels with different GO content. The relevant results are plotted in Fig. 10. It is manifest that the incorporation of GO leads to the significant reinforcement in both tensile and compressive strength of the PVA/GO hydrogel composites compared to those of the neat PVA hydrogel. For example, a 116% increase in tensile strength (∼1.4 MPa) is gained with only 0.15 wt% GO loading. The elongation of the composites is also increased with the incorporation of GO (Fig. 10a). When the GO content is 0.10 wt%, a 161% improvement in compressive strength (∼0.947 MPa) is obtained (Fig. 10b). This significant enhancement can be explained by the large aspect ratio of the GO sheets and the strong interfacial interactions between the GO and PVA matrix (Fig. 2a), and importantly, the GO sheets are uniformly dispersed in the PVA matrix.¹⁶ In addition, the GO sheets are thought to be able to bound part of water molecules due to the abundant oxygen-containing functional groups on its surface and impede the water infiltration between the PVA molecules (Fig. 11), improving the water-locking ability of the composites. So under the compressive load, the interstitial liquid within the porous network becomes more difficult to be forced out with increasing GO content, enlarging the instantaneous pressurization and improving the ability to resist deformation. When the GO content is further increased, both tensile and compressive properties decrease. This may be attributed to the agglomeration of GO sheets at higher concentrations due to their large surface activation energy, which reduces their specific surface area and hence their interfacial bonding strength between GO and PVA.¹⁷ The results indicate that the addition of GO improves the water-locking ability of the composites, and the proper content of GO could toughen and strengthen the PVA hydrogel.

Fig. 9 Representative (a) tensile and (b) compressive stress–strain behaviors for PVA/GO hydrogels with different GO weight loadings.

Fig. 10 Effect of GO content on (a) tensile strength and elongation at break, and (b) compressive strength of PVA/GO hydrogels.

Fig. 11 The schematic diagram of the hydrogel under compression.

3.4. Tribological properties of PVA/GO hydrogels

General trends in the friction coefficient over time for PVA/GO hydrogels with different amount of GO are shown in Fig. 12. The results demonstrate that the friction coefficient of the neat PVA hydrogel remains stable at the first 500 s. With time prolonged, the friction coefficient increases significantly, and the sample is worn out soon. The friction coefficients of the PVA/GO hydrogel composites keep stable during the entire friction tests. PVA hydrogel has been considered as a biphasic material due to its porous structure with large amount of water. Its lubricating behaviors show the characters of biphasic and boundary lubrication.^20,44,45 Based on the biphasic theory, the incorporation of GO leads to the improvement in the hydrophily of the composites and a denser network structure, as confirmed by the increasing water content and the SEM images described above. This is positive to water-locking and fluid load supporting during sliding, and hence the low friction. Fig. 12 shows that the friction coefficient decreases with increasing GO content. The load-bearing capacity plays the dominant role for the frictional properties. The high load-bearing capacity diminishes the contact area during sliding, which is helpful for the reduction of friction.⁴⁶ Moreover, the excellent lubrication of GO transfer film is also responsible for the reduction of friction of the PVA/GO hydrogels.

Fig. 12 Effect of GO content on the friction coefficient of PVA/GO hydrogels (5 N, 80 mm s⁻¹, deionized water).

Fig. 13 gives a visual sight for the lubrication mechanism of PVA/GO hydrogel against CoCrMo ball under liquid lubrication. When the normal load is applied to the PVA/GO hydrogel (Fig. 13a), the interstitial water is extruded from the deformed polymer network to the frictional interface to provide lubrication. The GO sheets are uniformly dispersed in the PVA matrix, so the ball embedded in the hydrogel would contact with the sheets (Fig. 13b). As there are abundant oxygen-containing functional groups present on the surface of the GO sheets, the molecular chains existed in the lubricant are thought to attach to the GO surface easily, which is conducive to the formation of lubrication film on the interface of the ball (Fig. 13c). Specifically, as shown in Fig. 13d, during sliding, the exposed GO sheets on the surface of the hydrogel absorb water molecules to promote the formation of boundary film. Moreover, during continuously sliding, the GO sheets are transferred to the counterface and attached on the ball by van der Waals forces, which would also reduce the friction force.

Fig. 13 Lubrication mechanism of PVA/GO hydrogels against CoCrMo ball under liquid lubrication.

The SEM, Raman spectra and XPS analysis results on the CoCrMo ball surface after wear tests are presented in Fig. 14. As shown in Fig. 14a, some dark grey and relatively transparent regions appear on the ball surface, which is supposed to be the peeled GO sheets. The similar Raman spectra feature of GO was obtained on the ball surface (Fig. 14b). The D and G bands are located at 1350 and 1597 cm⁻¹, respectively, and the intensity ratio of D/G is approximately 1.1, which indicates that partial GO is transferred to the surface of counterface. As shown in Fig. 15c, aside from some composition of the CoCrMo ball, the C 1s and O 1s peaks are also shown in the XPS spectra of the ball surface, which is derived from GO (Fig. 14c). For the C 1s spectrum (Fig. 14d), the relative intensity of C–O band becomes larger than that of C=C/C–C, which implies the increased degree of oxidation of GO arising from the interactions between the GO and water molecules in the lubricant. The results confirm our conjecture described above that the GO transfer film is presented on the surface of the couterface ball.

Fig. 14 (a) SEM image, (b) Raman spectra, and (c) XPS analysis recorded on the ball surface after wear test and (d) its C 1s.

Fig. 15 Effect of load on the friction coefficient of PVA/GO hydrogels (80 mm s⁻¹, deionized water).

Fig. 15 displays the friction coefficients of PVA/GO hydrogels varied with the applied normal load. The friction coefficient decreases first and then increases with increasing load, independent of the GO content. With the increase of load, more interstitial water is squeezed out from the porous network to provide lubrication, leading to the decreased friction coefficient. As the load further increases, the hydrogel deforms severely, and the contact area between the counter pairs enlarges, thus the friction coefficient increases. The amount of GO affects the water-locking ability and load-bearing capacity of the hydrogel composites, therefore, the critical load of the friction coefficient transition is different for hydrogels with various GO content.

Three-dimensional surface morphology of the PVA/GO composite before and after 90 min of friction is shown in Fig. 16. Before friction, the surface of the PVA/GO hydrogel is smooth with a surface roughness of 6.741 μm (Fig. 16a). After 90 min of friction, it can be seen a clear wear track (Fig. 16b) with about 70 μm in width and 70 μm in depth (Fig. 16c), and the roughness of the worn sample increases significantly, about two times larger than that of the initial hydrogel. For the neat PVA hydrogel, it is worn out in a few minutes, so its wear morphology is not given. The individually dispersed GO in PVA hydrogel offers a large surface area available for interactions with the polymer molecules due to its high aspect ratio, which facilitates effective load transfer across the GO–PVA interface. Moreover, the incorporation of GO endows the composite with a denser and tough structure and the enhanced load-bearing capacity, which are also helpful to resist wear. Thus, the incorporation of GO improves the wear resistance of PVA hydrogels.

Fig. 16 Wear morphology of PVA/0.10 wt% GO hydrogels before friction (a) and (b) after 90 min of friction. (c) Height profile taken along the white line in (b).

4. Conclusions

In this article, GO, an excellent filler, was incorporated into PVA to prepare PVA/GO hydrogel composites by a repeated freeze–thaw method. The effects of GO content on the microstructure, water content, and mechanical and friction properties of PVA/GO hydrogels were investigated. The results demonstrated that the incorporation of GO could significantly improve the load-bearing capacity of the hydrogels due to the excellent interfacial interactions between the GO and PVA matrix. When compared to the neat PVA hydrogel, a 116% improvement in tensile strength and a 161% enhancement in compressive strength were obtained with the addition of GO at low concentration. The friction coefficient of PVA/GO hydrogels decreased with increasing GO content due to the increased water-locking ability and the load-bearing capacity, as well as the formation of GO transfer film on the counterface. The addition of GO improved the wear resistance of PVA hydrogel composites.

Acknowledgements

This project was supported by the National Natural Science Foundation of China (Grants 51575278 and 11172142), the Fundamental Research Funds for the Central Universities (No. 30910612203 and 30915014103), the learning-research-production prospective study project of Jiangsu (No. BY2016004-08), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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