Wenchao Pang*a,
Jialiang Wua,
Qifan Zhangb and
Guangfei Lib
aSchool of Mechatronic Engineering, Qilu Institute of Technology, Jinan 250200, Shandong, China. E-mail: pangwenchaogg@sina.com
bSchool of Mechanical Engineering, Jiangnan University, Wuxi 214122, Jiangsu, China
First published on 7th December 2017
A method for enhancing the micro-hardness and tensile properties of cross-linked ultrahigh molecular weight polyethylene (UHMWPE) for total joint implants by radiation cross-linking after adding vitamin-E (VE) and graphene oxide (GO) was reported in this paper. Vitamin E was blended with UHMWPE powder at a concentration of 0.1 wt%, followed by adding GO at a concentration of 0.5 wt% and subsequently irradiated with 60Co gamma-rays at 100 kGy. The GO/VE/UHMWPE composite showed improved micro-hardness (∼8%), Young's modulus (∼28%), yield stress (∼33%) and fracture stress (∼37%) in comparison with UHMWPE. The OI of irradiated UHMWPE decreased from 1.9 to 0.7 after blending with VE and the OI of irradiated VE/UHMWPE increased from 0.7 to 1.3 after filling with GO. In addition, the addition of GO further reduced the gel content.
In order to solve those problems, the researchers have devoted a lot of efforts to balance the wear resistance, oxidation resistance and mechanical properties. Oral et al.10 limit distribution of crosslinking density in the implant though controlling the concentration of vitamin-E, so that high crosslink density on the surface is remained and low crosslink density on the bulk is controlled. The study has shown that this method improved the fatigue strength and wear resistance. Fu et al.11 study the effect of high temperature melting on the irradiated VE/UHMWPE blend. It is demonstrated that this method improves its wear and impact strength, but there is no effect on tensile properties of the polymer. Although the above techniques enhance the wear resistance and impact strength to some extent, they have some disadvantages such as the expensive cost and complication of process. Therefore, there is a crying need to find an advanced technology to fabricate new UHMWPE composite with excellent mechanical properties, well wear resistance and oxidation resistance.
Graphene oxide (GO), a functionalization of graphene, with excellent mechanical,12 thermal properties,13 and biocompatibility,14 has been widely used to enforce polymer properties.15–19 In our previous study,16 graphene oxide (GO) is used as reinforcement to enhance the mechanical properties of UHMWPE. It is demonstrated that GO could enhance its micro-hardness and impact properties with low concentration of GO. Chen et al.,17 Tai et al.18 and Suñer et al.19 also demonstrate that GO could improve the tensile properties, micro-hardness and resistance to wear of UHMWPE. Although researchers spend a lot of energy in GO/UHMWPE composites, the study of GO enhanced vitamin E-stabilized irradiated UHMWPE composite is rarely reported. Different from our previous study which just focuses on the effect of adding GO into UHMWPE,16 the GO/UHMWPE composite is exposed to irradiation treatment and VE is added to improve the oxidation resistance of irradiated composite in this paper.
Therefore, we propose to fill 0.5 wt% GO into 0.1 wt% vitamin E-stabilized UHMWPE blend prior to radiation cross-linking to further enhance its mechanical properties without sacrificing their resistance to oxidation. In order to confirm whether the mechanical properties of irradiated GO/VE/UHMWPE composite can be enhanced or not, tensile and micro-hardness test are performed. Since mechanical strength and oxidation are two critical factors to joint implant material, we also develop standard and aggressive accelerated aging test to study the molecular structure and oxidation of the irradiated GO/VE/UHMWPE composite. In addition, we also study the dispersion of GO, the degree of crystallinity and gel content, which are the three critical factors to the tensile performance and micro-hardness of the polymer composites.
Graphene oxide (GO) was prepared through the modified Hummer's methods.20 The mixture of 1 g graphite, 2 g NaNO3 and 70 ml concentrated H2SO4 was stirred for 1 h in ice bath. Then 6 g KMnO4 was slowly added to the mixture and the mixture was kept stirring for 2 h in ice bath, and then kept stirring for 1 h at 35 °C. 100 ml deionized water was slowly added into the mixture, and then there was red violet gas produced. After stirring for 0.5 h, the mixture was added into 10 ml H2O2 and then cooled to room temperature. The suspension was removed from the prepared thickened liquid by centrifuge and then it was washed with 10% HCl solution for three times. And it was washed with deionized water several times until pH was close to neutral. The resulting viscous object was dried at 60 °C for about 24 h under vacuum environment and the required GO was obtained.
The GO/VE/UHMWPE composite was prepared by liquid ball milling mixing and compression molding process, followed by irradiation in vacuum environment. Based on our previous study16 and other literatures,17,19 the overall performance of GO/UHMWPE composites could achieve the optimal effect with the incorporation of 0.5 wt% GO. The details of sample preparation were as follows: firstly, 0.5 g GO powder and 0.1 g VE were dispersed in 200 ml alcohol solution followed by ultrasonic for 0.5 h. Then, 99.4 g UHMWPE powders were filled into the dispersed solution and then the solution was treated by ultrasonic for 0.5 h. Subsequently, the mixture solution was mixed by ball milling at the speed of 400 rpm for 2 h and then kept in a water bath at 60 °C until the solvent was completely dry. Next, the dried mixture powders were further ball-mixed for 2 h, and prepressed at the pressure of 10 MPa for 0.25 h followed by heating for 2 h at 180 °C, and then cooled down to room temperature under 15 MPa for about 3 h in air. Many studies report that vacuum packaging is effective at minimizing polymer damage due to oxidation.21–23 So, the prepared UHMWPE, VE/UHMWPE and GO/VE/UHMWPE composite were packed using vacuum foil packing bags and then irradiated by 60Co gamma-rays (WuXi EL PONT Radiation Technology Co., Ltd) with a dose of 100 ± 8 kGy at room temperature.
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The cryogenic fracture morphology of the composite was characterized by Scanning Electron Microscopy. 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. The crystallinity of the irradiated and non-irradiated GO/VE/UHMWPE composite was studied by TA Q2000 (TA Instruments Corporation, USA) under nitrogen atmosphere. The crystallinity of samples was computed on basis of the following equation:
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Fig. 1 Image of (a) tensile specimens of composite: white materials are UHMWPE and black materials are 0.5% GO/UHMWPE composites and (b) diagram of indentation hardness test. |
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Fig. 2 FT-IR spectrums of (a) vitamin-E; (b) irradiated and non-irradiated GO; (c) irradiated GO/VE/UHMWPE composite. |
As shown in Fig. 2c, the location of the primary absorption peak of irradiated GO/VE/UHMWPE composite has no significant change compared to that of irradiated VE/UHMWPE composite. Absorption peak of C–H stretching appear at 2840 cm−1 and 2918 cm−1, the bending peak of C–H bonds appears at 1462 cm−1, the CC stretching bonds appear at around 1650 cm−1, the CH2 peak appear at 720 cm−1, which indicated that the filler of GO led no pronounced effect to the molecular structure of irradiated VE/UHMWPE blend. However, the intensities of the major absorption peak are increased dramatically, attribute to the conjugation effect of C–C bonds and polar functional groups on the surface of GO. Moreover, there is a weak peak appears around 1715 cm−1 (C
O) of irradiated GO/VE/UHMWPE compare to irradiated VE/UHMWPE blend, indicating that GO is filled in the composite successfully.
It is known that irradiation could lead to oxidation on the surface of UHMWPE, accompany by decreasing of the mechanical properties and wear resistance.24 Although vitamin-E could hinder the oxidation of UHMPWE, the resistance to oxidation of irradiated GO/VE/UHMWPE is not clearly yet. So, an accelerated aging test is performed to investigate the oxidation resistance of the samples. The oxidation index of both aged and unaged samples is also measured on basis of FT-IR and the results are plotted in Fig. 3. The OI of irradiated but non-aged UHMWPE is relatively high. It may be caused by the oxidation in air during the process of hot pressing. The OI of irradiated UHMWPE before and after aging are 1.17 ± 0.05 and 1.90 ± 0.02 respectively for almost increasing by 62.39% due to aging, which shows irradiated UHMWPE has poor resistance to oxidation. The OI of irradiated VE/UHMWPE composite is 0.57 ± 0.032 and run up to 0.63 ± 0.03 after aging for increasing by 10.5% which is significantly lower than that of the virgin UHMWPE. When GO is filled into irradiated VE/UHMWPE blend, the OI of irradiated GO/VE/UHMWPE before and after aging are 1.23 ± 0.041 and 1.38 ± 0.05 respectively for increasing by 12.19% due to aging. It is similar to irradiated VE/UHMWPE, which indicates that the incorporation of GO has no dramatically effect to the resistance to oxidation. In other words, the irradiated GO/VE/UHMWPE composite has well resistance to oxidation due to the existence of vitamin-E. However, the OI of irradiated GO/VE/UHMWPE composite before and after aging are higher than that of aged and irradiated VE/UHMWPE blend, which attribute to the abundant carbonyl groups on the surface of GO as discussed above.
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Fig. 3 OI of irradiated and irradiated + aged UHMWPE composite. Error bars represent standard deviation. Different letters denote significant difference (P < 0.05). |
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Fig. 4 SEM images of the cryogenic fractured surfaces: flat surface of pure UHMWPE (a), fibrotic surface of VE/UHMWPE blend (b) and coarse surface of GO/VE/UHMWPE nanocomposite (c). |
Material | Tm (°C) ± SD | Crystallinity (%) ± SD |
---|---|---|
a SD: standard deviation. Means in the same column with different small superscript letters indicate significant difference at P < 0.05. | ||
UHMWPE | 133.9 ± 0.1e | 50.6 ± 1.2e |
VE/UHMWPE | 134.3 ± 0.2d | 50.9 ± 1.1d,e |
GO/VE/UHMWPE | 133.1 ± 0.2f | 54.2 ± 1.0b,c |
Irradiated UHMWPE | 135.0 ± 0.2c | 52.7 ± 1.3c,d,e |
Irradiated VE/UHMWPE | 134.9 ± 0.3c | 53.1 ± 1.4c,d |
Irradiated GO/VE/UHMWPE | 135.4 ± 0.1b | 56.3 ± 1.4b |
The results show that the filler of both GO and VE have no significant effect on the melting temperature of non-irradiated GO/VE/UHMWPE composite. Pang et al.16 and Chen et al.17 also reports that GO has slightly impact on melting temperature of GO/UHMWPE composites. After irradiation, the melting temperature of all the irradiated samples is increased slightly. GO, with larger specific surface area, is grafted to the molecular chain of VE/UHMWPE composite in irradiation treatment process and enhance the interaction force between GO and VE/UHMWPE matrix, which could increase the melting temperature of samples. Moreover, Premnath et al.25 also demonstrates that the crosslink could hinder the melting process, especially with high dose irradiation.
In addition, the incorporation of GO dramatically increases the crystallinity of GO/VE/UHMWPE nanocomposite by 6.45%, compared to VE/UHMWPE blend. This can attribute to the huge specific surface area of GO, which can acts as nucleation sites for crystallization. Therefore, many nucleation can growth together and form a larger scale of crystallite so rapidly that lead to a higher degree of crystallinity. In addition, the irradiated GO/VE/UHMWPE composite has higher degree of crystallinity because the recrystallization of the molecular chain scission generated in the irradiation process. Meanwhile, the two-dimensional wrinkled GO might hinder the mobility of chain scission, which lead to the chain scission assemble around GO surface and improve the probability of recrystallization.
In the thermal formation of the composites during industrial manufacture process, their properties may be affected due to the fracture of the molecular chain, which may be caused by plasticization molding under certain temperature conditions. Thermogravimetric analysis (TGA) is used to study the thermal stability of the composites, which is very necessary for polymer materials. The sample of TGA decomposition curves of pure UHMWPE showing demarcated temperatures (T1 and T2) is shown in Fig. 5a. The initial decomposition temperature, T1 (Fig. 5b), is 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) increases slightly with the addition of VE, which may be because the addition of VE increases the liquidity of UHMWPE molecular chain. The addition of GO slightly decreases the T1 of VE/UHMWPE. The addition of VE increases the end of the linear weight loss temperature (T2) of UHMWPE and the addition of GO also increases the T2 of VE/UHMWPE blend. The incorporation of VE and GO could slightly increase the thermal stability of UHMWPE. After irradiation treatment, thermal stability of all composites decreases due to the molecular chain of composites is severed and the length of molecular chain is decreased during irradiation treatment process. The results are in accordance with the study reported by Wang et al.26 which found that gamma ray and ion beam irradiation slightly decreased the thermal properties of UHMWPE. However, the influence of addition of VE/GO on the thermal performance is not significant (P > 0.05).
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Fig. 6 Gel content of the irradiated samples. Error bars represent standard deviation. Different letters denote significant difference (P < 0.05). |
The gel content of irradiated UHMWPE is 90.25% ± 1.5 and decreases to 88.72% ± 1.2 after filling vitamin-E into UHMWPE matrix. It may be due to the property of stabilizing free radicals of vitamin-E, and it is similar to previous research.27 The gel content of irradiated GO/VE/UHMWPE composite is further reduced with the incorporation of GO. This may be that the mobility of chain scission is hindered by the strong interface binding force between GO and VE/UHMWPE matrix. In addition, Gonçalves et al.28 and Kolanthai et al.29 demonstrate that GO/graphene has ability to scavenge free radicals to some extent. As a result, the incorporation of GO further reduces the crosslink density of GO/VE/UHMWPE nanocomposite.
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Fig. 7 The micro-hardness of the composites. Error bars represent standard deviation. Different letters denote significant difference (P < 0.05). |
As shown in Fig. 8, the tensile properties (Young modulus, yield stress, fracture stress) show little significant change with the incorporation of vitamin-E, compare to that of virgin UHMWPE before and after irradiation. These results are consistent with previous studies.6,31 Fortunately, the mechanical properties are dramatically improved with the incorporation of GO. The Young modulus is increased by 19.93%, from 357.41 Mpa to 428.67 Mpa, compares to that of VE/UHMWPE blend (Fig. 8a). As discussed above, GO possesses huge specific surface area and abundant polarity oxygen containing functional groups on the surface and margin, which could result in powerful interface interactions between GO and polymer matrix. The powerful interface interactions cooperate with the inherent high hardness of GO lead to an increase in Young modulus. In addition, the increased crystallinity of GO/VE/UHMWPE also promotes the increase in Young modulus.
In general, yield stress appears to be associated with crystallinity of composites.32 In present study, the yield stress is improved obviously with the filler of GO into VE/UHMWPE composite (Fig. 8b). The obtained results are consistent with the variation trend of crystallinity of GO/VE/UHMWPE composite. Moreover, GO could transfer load from polymer matrix to GO. Hence, the increased yield stress can be associated to the enhancement effect of GO. On the other hand, the incorporation of GO also has a significant effect on the fracture stress which is increased by 16.96% (Fig. 8c). This result also confirm the powerful interface interactions between GO and VE/UHMWPE matrix. As shown in Fig. 8d, the addition of VE has little effect on the maximum strain of UHMWPE. And there is a decrease in maximum strain of samples with irradiated treatment. However, it should be mentioned that the maximum strain decreased once GO is added into the UHMWPE/VE blend.
In this work, we also characterize the influence of irradiation on mechanical properties of all the studied specimens. The results show that, the young modulus, yield stress and fracture stress of samples are further increased to some extent after irradiation. On one hand, this is because of the strong interfacial interactions improved by irradiation crosslinking. Irradiation could result in cleavage of C–C and C–H bond, and generates free radicals which can recombine with each other to form molecular bonds and increases the interface interactions. Moreover, Kolanthai et al.29 investigates the effect of graphene on the mechanical properties of UHMWPE/LLDPE (linear low-density polyethylene)/graphene. The results show that both irradiation and the filler of graphene could enhance the interfacial interactions between graphene and UHMWPE/LLDPE matrix. On the other hand, further increased crystallinity of all studied samples is observed after irradiation, which is a crucial factor to mechanical properties. Thus, the irradiation result in improved mechanical properties compared to non-irradiated UHMWPE based composites.
Generally speaking, surface hardness is also a critical factor to wear resistance. Ge et al.33 show that the plastic contact areas between polymer matrix and metal friction pair would be decreased with increasing hardness. The hardness of non-irradiated VE/UHMWPE blend is reduced slightly compared to virgin UHMWPE. Fortunately, the hardness is improved with the incorporation of GO. This mainly due to the transferring load performance of GO. In addition, the increased crystallinity of GO/VE/UHMWPE composite also plays an important role in improved hardness. According to the prediction of Buchanan,31 the hardness increases with the increasing crystallinity. After irradiation, the hardness of UHMWPE is enhanced dramatically, because of the increase of crystallinity and crosslinking density. Although, the crosslinking density is decreased slightly with the incorporation of GO, the hardness of irradiated GO/VE/UHMWPE composite increases all the same compares to that of irradiated VE/UHMWPE blend. This indicates that the performance of GO and crystallinity play dominant roles.
Table 2 summarizes the percentage variation of mechanical properties of similar irradiated and non-irradiated UHMWPE-based composites with respect to the virgin UHMWPE. Comparing the results of mechanical properties presented here with those for other irradiated and non-irradiated filler/UHMWPE composites in the literatures,19,34,35 it becomes evident that only a very small amount of GO can significantly improve the mechanical properties of UHMWPE. In the Table 2, the contents of fillers are optimal for effective enhancement of mechanical properties. For common fillers such as CNT and HAP, a relatively high content of filler is needed to effectively improve the mechanical properties.34,35 In contrast, the mechanical properties can be significantly improved by the incorporation of GO with a much lower content.19 As shown in Table 2, irradiation further improved the mechanical properties of both CNT/UHMWPE and GO/UHMWPE. Compares with irradiated CNT/UHMWPE, the irradiated GO/VE/UHMWPE possesses the similar mechanical properties with much low fillers amount.
Composites | Filler fraction (wt%) | Young's modulus (MPa) | Yield stress (MPa) | Fracture stress (MPa) | Ref. |
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a CNT: carbon nanotubes; HAP: hydroxyapatite. | |||||
CNT/UHMWPE | 3 | 37.9% | 4% | 7.9% | 34 |
CNT/UHMWPE-I (90 kGy) | 3 | 62.9% | 20.6% | 15.7% | 34 |
GO/UHMWPE | 0.5 | 12.4% | 11.9% | 24.4% | 19 |
GO/UHMWPE-I (75 kGy) | 0.5 | 18.2% | 17.1% | 1% | 19 |
HAP/UHMWPE | 1% | 5.3% | −43.4% | 35 | |
HAP/UHMWPE-I (100 kGy) | 1% | −32.8% | −52.4% | 35 | |
GO/VE/UHMWPE | 0.5/0.1 | 16.8% | 16.2% | 27.8% | This work |
GO/VE/UHMWPE-I (100 kGy) | 0.5/0.1 | 39% | 32.7% | 38.1% | This work |
In early studies, Shen et al.36 reports that irradiation produces crosslinking in amorphous regions and wear is observed to be much lower for chemically crosslinked cups. Perez and Vanderhart 37 also report that irradiation induced crosslink formation and produced Y-type branches and H-type branches. In addition, Kolanthai et al.29 reports that GO could graft on polymer chains during the irradiation process, and then the interactions at polymer–graphene interface are enhanced. Anson-Casaos et al.38 reports that graphene materials could role as radical scavengers and crosslinking generators. Based on the above reports, a postulated enhancement mechanism of irradiated GO/VE/UHMWPE composite is established, as shown in Fig. 9. Irradiation treatment results in the breaking of C–C bonds and C–H bonds of UHMWPE and then forms free radicals, as shown in Fig. 9a. The breaking of C–C bonds in irradiated materials is a random process, and the broken C–C bonds recombine to form new C–C bonds. The conjugated double bonds which consist in carbon nanostructures give carbon nanomaterials a great ability of electron donor–acceptor.29,39 Therefore, irradiation crosslinking treatment not only enables UHMWPE molecule to form reticular structure, but also causes the broken C–C bonds of GO to react with the broken C–C bonds of UHMWPE to form new C–C bonds, as shown in Fig. 9b. The C–C chemical bonds between GO filler and UHMWPE matrix produce by irradiation treatment are much stronger than van der Waals' force between GO filler and UHMWPE matrix,16 which lead to more excellent mechanical properties of irradiated composite.
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Fig. 9 Mechanism of radical formation during irradiation (a) and enhancement mechanism of composite with irradiation and without irradiation (b). |
Moreover, VE is added into UHMWPE to improve the oxidation resistance of blends after irradiation treatment. The oxidation process of irradiated UHMWPE can be explained by the formation of free radicals and their subsequent reactions with oxygen.40 The formation of hydrocarbon groups in UHMWPE (Fig. 9a) and their reactions with oxygen are key steps in a series of oxidation processes. The hydrocarbon radicals react with oxygen to form peroxy group. These peroxy groups absorb hydrogen atoms from neighboring UHMWPE molecules and form hydrogen peroxide and new hydrocarbon radicals. Fortunately, the cyclic reactions described above are terminated because hydrogen atoms in VE replace hydrogen atoms in UHMWPE to combine with highly reactive free radicals COO˙ to form stable compounds. Therefore, due to the absence of VE, the GO/VE/UHMWPE composite show well resistance to oxidation, which avoid the negative effects during irradiation process.
(1) The incorporation of GO increases the crystallinity of GO/VE/UHMWPE composite and the irradiation treatment further increases the crystallinity.
(2) Tensile properties and micro-hardness of GO/VE/UHMWPE composite are improved by adding of GO due to the excellent mechanical properties of GO. And the addition of VE improves the oxidation resistance of composite.
(3) Mechanical properties of GO/VE/UHMWPE composite are further improved after irradiation treatment because new C–C chemical bonds are generated between GO filler and UHMWPE matrix to improve combining power between filler and matrix. Meanwhile, the effects of oxidation resistance of VE and irradiation cross-linking of UHMWPE molecular chain are also essential. The GO/VE/UHMWPE composite may enable the use of cross-linked blends in total joint implants in more mechanically demanding applications.
However, there are some limitations in this study. For example, the fracture toughness, fatigue strength, resistance to delamination and wear resistance of the material are not reported, even though they are key mechanical properties for implant applications. And they should to be further studied in the future.
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