Mechanical and thermal properties of graphene oxide/ultrahigh molecular weight polyethylene nanocomposites

Abstract

Graphene oxide (GO) was prepared according to a modified Hummers method, and a range of GO/ultrahigh molecular weight polyethylene (UHMWPE) composites were fabricated using liquid-phase ultrasonication mixing followed by hot-pressing. The thermal performances of the GO/UHMWPE composites were characterized by TGA and DSC. The dispersion of GO in GO/UHMWPE composites was investigated by FTIR and XRD. Moreover, the mechanical properties, including micro-hardness, tensile properties, and impact strength of GO/UHMWPE composites were also studied and the fractured surfaces were observed under SEM. The results show that the melting temperature of these composites was about 135 °C and the crystallinity was improved with the addition of GO. Moreover, the initial decomposition temperature was about 472 °C and the addition of GO improved the thermal performance of GO/UHMWPE. Furthermore, not only was the impact strength increased substantially with the addition of GO, but the micro-hardness was also improved gradually and the tensile properties were improved with the addition of GO. The thermal and mechanical performances of the GO/UHMWPE composites are influenced by the free-space effect and interaction-force effect.

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1. Introduction

Ultrahigh molecular weight polyethylene (UHMWPE) is widely used in medical care, pipelines and bearings as an excellent engineering plastic with low friction, outstanding chemical stability, low water absorption and high wear resistance.^1,2 In the past few years, UHMWPE has been widely used for artificial joints because of its outstanding mechanical properties and stability.³ However, the application of UHMWPE has been limited because of its low surface hardness and anti-fatigue capacity.^4,5 Therefore, considerable efforts have been made to improve the mechanical properties and tribological properties. Recently, researchers have made significant progress in overcoming these difficulties by carrying out various studies, involving crosslinking,⁶ the use of inorganic fillers^7,8 and carbon nanometer materials.^9,10 Unfortunately, the high additive amounts, high cost and unsatisfactory performance of these composites limit their potential applications.¹¹ Therefore, it is an urgent and significant task to develop new UHMWPE composites having low cost and satisfactory performance, which poses a great challenge for further research.

Graphene, first separated in 2004 with the help of a scotch tape, has excellent mechanical properties. Hence, researchers excitedly turn their attention to this material to discover its potential applications in the wide fields of science and engineering.¹² Because of its high specific surface area, extraordinary mechanical properties and low cost compared with carbon nanotubes, graphene has attracted significant attention as an effective reinforcement for high performance composites.^13–17 However, in the practical application of reinforced polymer, graphene requires functional activation treatment to obtain oxygen containing functional groups, which would make it more easily dispersible and soluble in polar solvents and form intercalated composites through the strong interactions with polymer molecules.^18,19 The high specific surface area, two-dimensional geometric structure and good interfacial adhesion of GO are beneficial to the stress transfer between GO and the polymer matrix. Based on previous investigation, the addition of graphene oxide or graphene nanofillers into a series of polymers has caused advantageous enhancements in mechanical and tribological properties. Shen et al.²⁰ and Shin et al.²¹ investigated the wear behaviors of polymers filled with GO in dry friction, and the results showed that the deformation and fracture of composites were reduced, whereas wear resistance was improved with the addition of GO. Chen et al.²² demonstrated that the addition of GO to UHMWPE not only improves the hardness and yield strength, but also shows good biocompatibility. Tai et al.²³ found that the hardness and wear resistance of GO/UHMWPE composites are both significantly improved. These have driven us to explore the feasibility of GO reinforced UHMWPE composites with enhanced mechanical properties as a new type of seawater self-lubricating bearing material. In addition, fundamental research on the thermal performance and impact strength of GO/UHMWPE composites are rarely reported, so this was also investigated.

In this study, a range of GO/UHMWPE composites were successfully manufactured. The melting points and the initial decomposition temperatures of GO/UHMWPE composites changed slightly compared with pure UHMWPE; therefore, the original production process could still be used in the industrial production of these composites without excess cost. The prepared GO/UHMWPE composites were studied and showed not only improved hardness²⁴ and tensile properties but also significant enhanced impact strength, and an empirical formula for impact strength was found for engineering predictions. A model of factors affecting the properties of the composites was also established.

2. Experimental

2.1 Materials

UHMWPE was supplied by the Shanghai Research Institute of Chemical Industry, China. High-purity graphite powder (99.9%, 325 mesh) was purchased from Qingdao Jinrilai graphite Co. Ltd., China. Other reagents were of analytical grade and were commercially available.

2.2 Preparation of the GO/UHMWPE composites

Graphene oxide samples were prepared from natural graphite powders according to a modified Hummers method.²⁵ A range of GO/UHMWPE nanocomposites with different GO content were synthesized, with UHMWPE being intercalated into the layer of GO. The prepared GO samples were stirred in alcohol for 30 min and then ultrasonicated for 30 min to form a well-dispersed suspension. Furthermore, the suspensions were added into the UHMWPE powders and ultrasonicated for 1 h. Finally, alcohol was removed at 60 °C in a water bath and the solid products were ground with the aid of a planetary ball mill to obtain the nanocomposite powders. The powders were molded by hot-pressing, which involved pre-pressing under a pressure of 5 MPa and then heating at 200 °C for 2 h followed by pressing under 10 MPa until the temperature dropped to room temperature. In order to investigate the effects of GO on the properties of the composite materials, composites with GO contents of virgin, 0.1, 0.3, 0.5, 0.7 and 1.0 wt% GO were prepared.

2.3 Characterization and thermal property measurements of the composites

Thermal weight loss was studied by means of a thermogravimetric analyzer (TGA, TGA/1, Switzerland) with temperature ranging from 50 to 800 °C at a rate of 20 °C min⁻¹ under a nitrogen atmosphere. Crystallinity and melting temperature of the composites were determined by differential scanning calorimetry (DSC, TA, Q200) at a scanning rate of 10 °C min⁻¹ from 20 to 200 °C under a nitrogen atmosphere. Fourier transform infrared spectrometry (FTIR, Nicolet iS10, USA) was employed to investigate the characteristics of the composites. X-Ray diffractometry (XRD, D8, Germany) was performed using Cu-Kα radiation and scanned from 10° to 90° at the rate of 6° min⁻¹.

2.4 Mechanical property measurements of the composites

Micro-hardness was measured by a micro-hardness tester with a load of 10 g, and the reported results were the average of twenty times each content. The tensile properties were measured by a universal testing machine. Tensile sample sizes were 33 mm in effective length, 6 mm in width and 3.2 mm in thickness (Fig. 1a). The tensile tests were performed at a rate of 20 mm min⁻¹. Dynamic mechanical thermal analysis (DMTA) was conducted for UHMWPE and composites by an instrument (TA, DMA Q800), using the dual cantilever configuration. The size of the specimens for DMTA testing was 35 × 12 × 4 mm³. The complex modulus of each sample was determined at a constant frequency of 1 Hz, raising the temperature from 40 to 180 °C at a heating rate of 3 °C min⁻¹. The impact strength was measured by a cantilever beam impact tester with the sample size of 80 mm in length, 10 mm in width and 4 mm in thickness (Fig. 1b) and the fractured surfaces were observed under a field emission scanning electron microscope (SEM, HITACHI, S-4800). An empirical formula for impact strength of composites was found through fitting by origin. Impact tests were performed six times for each content.

Fig. 1 Images of (a) tensile specimens of GO/UHMWPE composites and (b) impact specimens of GO/UHMWPE composites.

3. Results and discussion

3.1 Structure of the prepared materials

The FTIR spectra of the GO/UHMWPE composites are shown in Fig. 2. It can be seen that the position of the infra-red absorption peak of the GO/UHMWPE composites does not change with increasing amounts of GO, illustrating that the addition of GO does not cause a great influence on the molecular structure of UHMWPE, and the degradation of UHMWPE in the ultrasonic dispersion and hot pressing process does not occur. This is because ultrasonication can accelerate the diffusion process through the effects of cavitation, micro-agitation and micro-streaming,²⁶ so GO could be well-distributed when doped into the UHMWPE matrix. The obvious peaks in the image were explained as follows: 2924 cm⁻¹ (the asymmetric stretching vibration peak of C–H), 2851 cm⁻¹ (the symmetric stretching vibration peak of C–H), 1469 cm⁻¹ (the in-plane bending vibration peak of C–H), and 722 cm⁻¹ (the rocking vibration peak due to the high degree of polymerization and long molecular chain of UHMWPE). With the addition of GO, other carbon bonds appeared when the GO content was 0.1 wt%, 0.3 wt%, 0.7 wt% and 1 wt% at 1742 cm⁻¹ (C=O), 1724 cm⁻¹ (C=C), 1053 cm⁻¹ (C–O–C), respectively. The existence of these peaks indicate that GO has been intercalated homogeneously into the UHMWPE matrix, which is of great importance to the performance of the composites.

Fig. 2 FTIR spectra of GO/UHMWPE composites.

Further evidence that proves GO was well dispersed in the UHMWPE matrix is the XRD patterns, which provide an important tool for determining the exfoliation of GO in the composites.^27,28 The XRD patterns of GO, pure UHMWPE and the 1.0% GO/UHMWPE composite are shown in Fig. 3. As can be seen, an apparent peak appeared at 2θ = 10°, which is the characteristic peak of GO while the characteristic peak of graphite appeared at 2θ = 26°, which means GO was obtained. The introduction of oxygen functional groups, such as hydroxyl and carboxyl between the layers of graphite typically increased the distances of the interlayers, which led to a shift of the diffraction peaks to lower angle values. Moreover, the location of the diffraction peaks of GO/UHMWPE are the same as virgin UHMWPE and have no peak at 2θ = 10° with addition of GO. This clearly demonstrates that GO was fully exfoliated in the UHMWPE matrix and UHMWPE does not degrade to a new substance. Composites with other GO content gave the same XRD results. Moreover, the peak strength of UHMWPE increased with the addition of GO, showing that the crystallinity of GO/UHMWPE is higher than that of pure UHMWPE, which is verified below.

Fig. 3 XRD patterns of GO, pure UHMWPE and 1.0% GO/UHMWPE composite.

3.2 The thermal performance of GO/UHMWPE

In the thermal formation of the GO/UHMWPE composites during industrial manufacture, their properties may be affected by the fracture of the molecular chain, which may be caused by plasticization molding under certain temperature conditions. TGA was used to study the thermal stability of the composites, which is very necessary for polymer materials. A sample of TGA decomposition curves of pure UHMWPE showing demarcated temperatures (T₁, T₂, T₃) is shown in Fig. 4a. As can be seen, the filling with GO produced a notable influence on the thermal properties of UHMWPE. The initial decomposition temperature, T₁ (Fig. 4b), was characterized by the corresponding temperature of the weight loss of 5 wt% in the thermogravimetric analysis. The temperature of weight loss of 5 wt% (T₁) increased gradually with the increase of GO content, which may be because the addition of GO was beneficial to the thermal stability of UHMWPE. The addition of GO increased the end of the linear weight loss temperature, T₂ (Fig. 4c), at the contents of 0.3–0.7%. The temperature of complete volatilization, T₃ (Fig. 4d), increased notably due to the addition of GO and was higher than that of pure UHMPE at all contents. This can be explained by as follows: on the one hand, the addition of GO could reinforce the interactions between GO and UHMWPE through van der Waals' forces and chemical bonds, therefore the motion of the UHMWPE molecular chain is limited; on the other hand, the slice layer structure of GO produces a larger free volume for the molecular chain motion of UHMWPE, the two aspects cancel each other out and the effect of the interaction forces is greater than the effect of free space, so the thermal performance is improved. Interestingly, Xu et al.²⁹ reported that increasing the GO content can improve the thermal stability of PVA/GO composites, which supports our conclusion. However, it can be noted that Xu's research was targeted toward fire retardants and was launched based on a large GO content, which reached 20%. In our study, such large additive amounts of GO are not necessary, although the thermal performance may be improved significantly with adding a large amount of GO. This is because the mechanical properties can be significantly enhanced with low additive amounts of nano-fillers, which is one of the major advantages of nanocomposites. Therefore, we consider that the temperature is satisfactory for meeting the manufacturing and working requirements.

Fig. 4 Typical decomposition curve (a) of TGA and thermal stability parameters (b, c and d) for GO/UHMWPE composites.

Fig. 5 shows the DSC curves of the GO/UHMWPE composites. As can be seen from the Fig. 5a, even though the weight fraction of GO was different, the peak melting temperature appeared at about 135 °C, illustrating that the addition of GO has little effect on the melting point of the composites. The initial decomposition temperature and the melting temperature are much higher than the working temperature of these composites as potential materials of seawater self-lubricating bearings. Fig. 5b shows that the crystallization temperature had little variation, first decreasing slightly and then increasing with the adding of GO. The enthalpies of melting are 109.8 J g⁻¹, 129.9 J g⁻¹, 128.2 J g⁻¹, 133.7 J g⁻¹, 134.1 J g⁻¹ and 120.8 J g⁻¹ for virgin UHMWPE and 0.1%, 0.3%, 0.5%, 0.7%, 1% of GO, respectively. The crystallinity of GO/UHMWPE composites can be calculated in terms of the following formula (eqn (1)):

where ϕ is the weight fraction of GO in the composites, ΔH is the melting enthalpy of composites, which were actually measured from the DSC curves, and ΔH⁰ is the melting enthalpy of 100% crystalline PE taken as 293 J g⁻¹.³⁰ As shown in Fig. 6, the crystallinity of the composites obtained from the formula (1) increased substantially from 37.5% to 44.4% when the addition of GO reached 0.1 wt% and fluctuated mildly with the continued addition of GO, then decreased with the addition of 1 wt% GO. Even so, the crystallinity was still higher than that of pure UHMWPE. It can be explained that because of the larger specific surface area of GO, the polymer is adsorbed onto the surface of GO, which can be used as a heterogeneous nucleating site for crystallization, thus improving the crystallinity of UHMWPE. However, with the continued increase in the GO content, a large amount of GO was dispersed into the UHMWPE matrix. The strong interaction forces between GO and the matrix limit the crystal growth, leading to the crystallinity being stable within a range.

Fig. 5 DSC curves of GO/UHMWPE composites: (a) heating curve, (b) cooling curve.

Fig. 6 Crystallinity curves of GO/UHMWPE composites.

3.3 Micro-hardness

The micro-hardness values of virgin UHMPWE and GO/UHMWPE composites are detailed in Fig. 7. As can be seen, the addition of GO notably increases the micro-hardness of UMWPE. With the increased content of GO, the micro-hardness increased correspondingly. A small addition of GO can enhance the micro-hardness of UHMWPE significantly. This is mainly because of the two-dimensional structure of GO and its remarkable mechanical properties. The two-dimensional structure could bear load and transfer it to a large two-dimensional lamellar structure.

Fig. 7 The micro-hardness of GO/UHMWPE composites with different contents of GO.

3.4 Tensile properties

As can been seen in Table 1, although the contents of GO in all nanocomposites were low, the enhancement of Young's modulus and yield strength were realized. Unfortunately, it should be noted that the ultimate tensile strength decreased once GO was added into the UHMWPE matrix at a low concentration of 0.1 wt%, but then the values reached maximum at the concentration of 0.5 wt%. The Young's modulus and yield strength were also increased and then decreased as the GO content increased. At the GO content of 0.5 wt%, these values reached the maximum. This is due to the strong interaction-forces between GO and the UHMWPE matrix. The layered structure of GO plays a role in load transfer to a larger area and the relatively loosened structure could absorb some energy due to the movement of molecular chains. A similar phenomenon was validated from other graphene/GO reinforced polymer composites. Wang et al.³¹ reported that the tensile strength of GO/polybenzimidazole had an optimal value at the GO content of 0.3 wt% and decreased the properties of composites with too high or too low GO amounts.

Table 1 Tensile properties of GO/UHMWPE nanocomposites and virgin UHMWPE^a

Material	Young's modulus (MPa) ± SD	Yield strength (MPa) ± SD	Ultimate tensile strength (MPa) ±SD
a SD : standard deviation.
UHMWPE	601.32 ± 27.81	23.45 ± 1.21	32.77 ± 3.53
GO/UHMWPE 0.1 wt%	634.75 ± 30.59	23.86 ± 0.92	33.51 ± 4.05
GO/UHMWPE 0.3 wt%	641.88 ± 31.29	24.05 ± 0.85	35.29 ± 4.21
GO/UHMWPE 0.5 wt%	664.38 ± 28.32	24.57 ± 1.19	36.91 ± 3.98
GO/UHMWPE 0.7 wt%	645.14 ± 30.83	24.03 ± 1.05	34.59 ± 5.38
GO/UHMWPE 1.0 wt%	644.26 ± 29.40	23.97 ± 1.13	33.12 ± 4.09

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3.5 DMTA results of GO/UHMWPE composites

The storage modulus (E′) plots for the virgin UHMWPE and the GO/UHMWPE composites are presented in Fig. 8. It shows that the rubbery plateau level is higher for the GO/UHMWPE composites than for the virgin UHMWPE. Moreover, the rubbery modulus significantly increases with filling of GO. The inset map in Fig. 8 shows that the E′ of virgin UHMWPE is about 850 MPa and the E′ of 0.5 wt% GO/UHMWPE composites is about 1060 MPa, i.e. an evident increase of about 210 MPa. However, the E′ begins to decrease as the GO content increases over 0.5 wt%. A possible reason for the reduction of E′ with increasing GO content could be related to the aggregation of GO nanosheets. The micro-sized cracks might have initiated at the GO aggregate sites and propagated throughout the samples.^32,33 As a result, E′ would be lowered by the addition of GO at high GO concentrations.

Fig. 8 Storage modulus (E′) by the DMTA study of virgin UHMWPE and GO/UHMWPE composites with various GO contents.

3.6 Impact strength

Fig. 9a shows the variation of impact strength values with the increase of GO content in UHMWPE. It can be seen that the impact strength of GO/UHMWPE composites increased remarkably with the addition of GO. When GO addition reached 1.0 wt%, the impact strength increased from 17.27 to 90.96 compared with pure UHMWPE, corresponding to a growing amount of more than 500%. It was noted that just a little addition of GO could remarkably enhance the impact strength of UHMWPE. This is because the relatively loosened layer structure of composites, caused by the slice layer structure of GO, produced a larger free volume for the molecular chain motion of UHMWPE. In addition, the interaction forces between the GO layers and UHMWPE matrix were increased with increasing amounts of polar groups on GO. Moreover, the larger interface of GO is due to its bi-dimensional structure, resulting in more effective load-transfer. The external energy generated by the impact was dispersed into a larger space and converted into heat energy through the crank motion of the main chain link in the flexible molecular chains, so the impact strength was increased substantially. Therefore, we believe that there is a bright future for the application of GO/UHMWPE composites due to their excellent impact strength.

Fig. 9 Impact strength curves (a) and fitted curves (b) of GO/UHMWPE composites.

As shown in Fig. 9b, the data were fitted to get an empirical formula of impact strength of GO/UHMWPE composites for the purpose of obtaining the impact strength of composites with different GO content directly through the empirical formula calculation. The smaller filling content was one of the advantages of nano-filled composites, and therefore the experiment was carried out only with the addition of 1 wt% of GO. Therefore, the range of GO content in the empirical formula was from 0 wt% to 1 wt%. The empirical formula was calculated from the equation below (eqn (2)):

y = 61.74x² + 11.64x + 17.73 (2)

where y is the impact strength of GO/UHMWPE composites and x is the content of GO. The adjusted coefficient of determination was up to 0.995, so the impact strength of the composites with GO content from 0 wt% to 1 wt% could be obtained directly from the empirical formula as a means of engineering prediction. In an effort to validate the correctness of eqn (2), the impact strength of 0.7 wt% composites was tested. There is little deviation between the value obtained from eqn (2), which is 56.13, and the value obtained from the actual experiment, which is 53.92^+13.34_−12.12, illustrating that eqn (2) holds up.

As can be seen in Fig. 10, the fractured surface of pure UHMWPE was relatively flat and had obvious gaps. This is because the structure of UHMWPE is a hybrid system, which is composed of a crystalline phase and a non-crystalline phase, and the crystalline phase is too small to form a completed spherulite.³⁴ This structure of UHMWPE is beneficial to the movement of molecular chains, therefore UHMWPE has good impact properties. The fractured surface significantly changed with the GO addition. When the amount of GO added was 0.1 wt%, the fractured surface became a little uneven. Because of its smaller dosage, GO was sparsely dispersed in the UHMWPE matrix, but the layered structure of GO also can be seen from the magnified image. When the GO content increased to 0.5 wt%, more GO was uniformly dispersed in the UHMWPE matrix. The fractured surface changed again to become flatter than that of 0.1 wt%, and there emerged some filamentous even mesh structure, which is due to the strong interaction between GO and UHMWPE matrix. When the GO content increased to 1 wt%, GO layers were uniformly dispersed in the UHMWPE matrix, and the GO that embedded into the molecular chain of UHMWPE together with the polymer were tightly pressed into a flake structure. It can be seen from Fig. 9 that the lamellar structure became more and more continuous, uniform and compact with the increase in GO content. The interaction between GO and UHMWPE became stronger with the addition of GO, and the lamellar structure is advantageous to the load transfer, therefore the addition of GO could significantly improve the impact strength of UHMWPE.

Fig. 10 SEM images of the fractured surfaces of the GO/UHMWPE composites with different amounts of GO: (a and b) virgin; (c and d) 0.1 wt%; (e and f) 0.5 wt%; (g and h) 1 wt%.

Table 2 summarizes the percentage variation of mechanical properties of similar composites with respect to the virgin materials. Comparing the results of mechanical properties presented here with those for other filler/UHMWPE composites in the literature,^35–37 it becomes evident that only a very small amount of GO can significantly improve the mechanical properties of UHMWPE. In Table 2, the contents of other fillers are optimal for effective enhancement of mechanical properties. For common fillers such as HA, CNTs and zeolites, a relatively high content of fillers is needed to effectively improve the mechanical properties.^35–37 In contrast, the mechanical properties can be significantly improved by the incorporation of graphene or GO with a much lower content.^28,38 Obviously, the GO nanosheets showed a superiority in enhancing the mechanical properties of UHMWPE over other fillers, including HA, CNTs and zeolite. It shows that our composites are overall improved in their mechanical properties, especially the impact property is significantly improved.

Table 2 Percentage variation of mechanical properties of similar composites with respect to the virgin materials

Composites	Filler fraction (wt%)	Tensile modules	Max tensile strength	Impact energy	Hardness	Ref.
HA/UHMWPE	22.8	+788%	−13%			35
CNTs/UHMWPE	5	+82%	−13%			34
Zeolite/UHMWPE	10	+31%	−11%	+24.86%		37
Graphene/chitosan	2.3	+112%			+60%	38
GO/polybenzimidazole	1.0	+23%	+17%			28
GO/UHMWPE	0.5	+11%	+12%	+134%	+8%	This work
GO/UHMWPE	1.0	+7%	+1%	+426%	+19%	This work

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3.7 Space and force model (free-space effect and interaction-force effect)

What exactly influences the thermal and mechanical properties of GO/UHMWPE composites? Based on the abovementioned analysis, a model of the free-space effect and interaction-force effect of GO/UHMWPE nanocomposites was established. The two aspects mainly affecting the performance of GO/UHMWPE nanocomposites, such as the thermal properties and impact strength, are free-space effect and interaction force effect, which have been verified above. The free-space effect refers to the relatively loosened layered structure caused by the slice layer structure of GO, which produces larger free space for the molecular chain motion of UHMWPE. The interaction-force effect refers to the interaction force between GO layers and UHMWPE and the force will increase with increasing the polar groups of GO. Therefore, it can be concluded that with the amounts of GO layers increasing in GO/UHMWPE nanocomposites, the free-space effect is obvious initially and then tends to be a constant while the interaction-force effect always increases. In particular, excessive addition of GO will cause an uneven dispersion phenomenon, leading to agglomeration to affect the performance of the GO/UHMWPE composites. In other words, the interaction force effect will increase and play a dominant role along with the increase in GO content.

4. Conclusions

Graphene oxide was produced using a modified Hummers method and GO reinforced UHMWPE composites were prepared successfully using liquid-phase ultrasonication mixing and hot pressing. GO sheets were dispersed uniformly into the polymer matrices. GO could improve the mechanical and thermal properties of UHMWPE with small loading compared with other fillers reported in the literature such as CNTs. Crystallinity was improved with the increase of the GO content compared with pure UHMWPE. The addition of GO improved the thermal performance of GO/UHMWPE. The tensile strength had an optimal value at the GO content of 0.5 wt%. Moreover, small amounts of GO could obviously increase the micro-hardness and the impact strength. There was strong interaction between GO and the UHMWPE matrix, and the lamellar structure of GO was advantageous to the load transfer. The empirical formula was found to be directly related to the impact strength of GO/UHMWPE composites in the range of virgin to 1 wt% for the purpose of engineering prediction. Moreover, the performances of GO/UHMWPE composites were influenced by two aspects: the free-space effect and the interaction-force effect. For thermal properties, these two aspects were mutually offset. While for mechanical properties, these two aspects were mutually reinforced. With the increasing addition of GO, the interaction-force effect plays a leading role.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (grant no. 51305166) and the Natural Science Foundation of Jiangsu Province, China (BK20130143).

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