Materials Letters 71 (2012) 117–119

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Materials Letters

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Characterization of in situ synthesized hydroxyapatite/polyetheretherketonecomposite materials

Rui Ma a, Luqian Weng a,⁎, Xujin Bao b, Zhuo Ni c, Shenhua Song a, Weiquan Cai a

a Department of Materials Science and Engineering, Shenzhen Graduate School, Harbin Institute of Technology, University Town, Xili, Shenzhen 518055, PR Chinab Department of Materials, Loughborough University, Loughborough, Leicestershire LE11 3TU, UKc College of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen 518060, PR China

⁎ Corresponding author. Tel.: +86 755 26033464; faxE-mail address: luqianw@hotmail.com (L. Weng).

0167-577X/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.matlet.2011.12.007

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 September 2011Accepted 2 December 2011Available online 8 December 2011

Keywords:HydroxyapatitePolyetheretherketoneBiomaterialsComposite materialsIn-situ synthesis

Hydroxyapatite/polyetheretherketone (HA/PEEK) composite materials have potentials for use as load-bearing orthopedic materials due to PEEK outstanding properties, which could overcome the disadvantagesof currently used metallic and ceramic materials. In the study HA/PEEK composite materials were successfullyprepared via an in situ process, for the first time, in order to improve the mechanical performance of theexisting HA/PEEK composites. The structure of the lab-synthesized materials was characterized by infraredspectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM), and the mechanicalperformance was investigated by a tensile strength test. The results exhibit the strong bonding between hy-droxyapatite fillers and PEEK matrix and good mechanical properties of the composites, suggesting that thein situ synthesized HA/PEEK composite materials are promising for orthopedic applications.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Considerable research interests have been recently directed to-ward developing polymeric composite biomaterials for use in a hardtissue load-bearing implant environment [1]. Among them HA/PEEKis one of the most suitable candidates due to its combinational prop-erties of PEEK outstanding chemical stability and excellent mechani-cal properties and hydroxyapatite good bioactivity [2]. So far HA/PEEK composite materials have been commonly prepared by a com-pounding and melt blending process [3–5]. However, the mechanicalstrength and ductility of the materials decreased substantially withHA content increase [5]. Furthermore, the severe debonding betweenhydroxyapatite particles and PEEK matrix has been observed [3,5],which results in the deteriorating anti-fatigue properties of HA/PEEK composites [6]. Considering that the anti-fatigue property ofbiomaterials is most critical property for long term biological applica-tions, such as implants, the further research is much needed to im-prove the reliability by enhancing the bonding between HA fillersand PEEK matrix. The weak interaction between PEEK and HA fillersmay be attributed to the poor wetting property between them causedby their large polarity difference, the high melt point of PEEK and thehigh viscosity of PEEK and HAmixtures at processing temperatures inthe compounding and melt blending process. The process of in situsynthesizing composite materials can be adopted to solve these prob-lems since the pre-polymerized liquid has much lower viscosity than

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PEEKmelt, which could significantly improve the interaction betweenHA fillers and liquid phase, consequently HA fillers are able to befirmly embedded in PEEK after the completion of polymerization.The previous work [7] on the woven glass fabric reinforced in situ po-lymerized poly(butylene terephthalate) composites showed that thelow viscosity of the pre-polymerized precursors facilitated not onlythe dispersion of the fabric but also the wetting of the glass fabric.The present study aims to improve the interfacial bonding betweenPEEK and HA in the composite materials via an in situ synthesis pro-cess. The structure and mechanical properties of the composite sam-ples are characterized.

2. Materials and methods

Analytical grade di-terbutyl peroxide, p-dihydroxybenzene, sulfo-benzide, K2CO3 and Na2CO3 were used for PEEK synthesis. Commer-cial HA powders (99.5% purity) are agglomerates with a primaryparticle size about 200 nm long and 50 nm wide.

240 g di-terbutyl peroxide, p-dihydroxybenzene and sulfobenzidewith a mass ratio 1:2:5 were charged in a 500 ml four-necked flaskequipped with a reflux condenser. The mixture was first stirred for0.5 h at ambient temperature, subsequently heated to 180 °C at5 °C/min under constantly stirring in pure nitrogen atmosphere. At180 °C, 10 g K2CO3 and 5 g Na2CO3 were added, and the mixturewas further heated to 320 °C at a heating rate of 5 °C/min with stir-ring. Subsequently, HA powder with different contents was intro-duced to the mixtures respectively, and the mixtures were held for3 h at 320 °C before cooling. The composite materials were harvested

118 R. Ma et al. / Materials Letters 71 (2012) 117–119

by washing the mixtures with deionized water and acetone for fivetimes respectively, and dried in vacuum at 140 °C for 24 h.

The HA content in the final composite products was 2.6, 5.6, 8.7,13.4 and 23.9 vol.% respectively, measured by the weight loss of thesamples burned at 700 °C in air for 2 h. The chemical and crystallinestructures of lab-prepared materials were examined by Fourier infra-red spectrometer (FTIR) (Thermo Nicolet-380 FT-IR spectrophotome-ter) and X-ray diffraction (XRD) (Rigaku D/Max-2500PC), using CuKα radiation (λ=0.1542 nm) operated at 40 kV and 200 mA, respec-tively. The microstructure of the composite materials was observedby scanning electron microscopy (SEM). The mechanical propertiesof the dumbbell-shaped composite specimens were measured, in ac-cordance to ASTM D638 for tensile testing, by an Instron IX MaterialTesting System at room temperature.

Fig. 2. XRD patterns of HA powder (a), lab synthesized PEEK (b) and HA/PEEK compositesamples (5.6vol.%HA/PEEK (c), 13.4vol.%HA/PEEK (d) and 23.9vol.%HA/PEEK (e)).

3. Results and discussion

The FTIR spectra of commercial HA powder, lab-synthesizedPEEK and HA/PEEK composite materials with various HA contentare shown in Fig. 1. The PEEK spectrum (Fig. 1b) is in accordancewith that of PEEK reported in the previous study [8]. FTIR resultsof HA/PEEK composites (Fig. 1c–e) show that the spectra are theoverlapped ones of PEEK and HA, and the intensity of HA absorptionbands in the composites is gradually increased with HA content.Neither the bands shift nor the new absorption bands are identified,suggesting that the lab-synthesized HA/PEEK composites are themixture of these two compounds without forming identifiable newchemical bonds.

XRD measurements were carried out to HA powder, lab-synthesized PEEK and HA/PEEK composites (Fig. 2). The XRD patternof the lab-synthesized PEEK (Fig. 2b) matches that of the PEEK used inthe previous studies [8]. The result, together with the FTIR resultshown in Fig. 1b, indicates that pure PEEK has been successfully syn-thesized. Comparing with those of pure PEEK (Fig. 2b) and pure HApowders (Fig. 2a), the patterns of the HA/PEEK composites showthat no extra peaks were created or lost with the addition of HA pow-der, indicating that no appreciable new interfacial crystalline phaseswere formed. Furthermore, no apparent shift of each reflection peakis observed to occur, as compared to pure PEEK. The results is in dif-ference with the previous studies on AlN/PEEK [9] and nano-SiO2/PEEK composites [10], where 2θ shift toward higher angles of PEEKpeaks occurred, as compared to pure PEEK, suggesting that the inter-action between PEEK molecules and HA powders may be chemicallyweaker than that between PEEK and micro-sized AlN powder orSiO2 nanoparticles, as also supported by the FTIR results (Fig. 1).

Fig. 1. FTIR spectra of HA powder (a), lab synthesized PEEK (b) and HA/PEEK compos-ite samples (5.6vol.%HA/PEEK (c), 13.4vol.%HA/PEEK (d) and 23.9vol.%HA/PEEK (e)).

The ultimate tensile strength of the lab-synthesized PEEK and thecomposite materials is given in Fig. 3. The tensile strength of HA/PEEKcomposites with the addition of 2.6 vol.% HA reaches to the value ashigh as 106 MPa, which is about 30% higher than that of lab-synthesized pure PEEK and 14% higher than the maximum tensilestrength of the reported composites with 10 vol.% HA content pro-duced by melt blending process [3]. According to the previous studies[11], the increase in tensile strength of the composites may be due toHA particles' intimate contact with PEEK matrix, consequently thelocal stress in matrix material under load being easily transferredinto rigid HA particles. Moreover, HA fillers in composites can inter-rupt and delay the propagation of micro-cracks or even stop theirgrowth via stress distribution, which is also attributed to improvetensile strength. However, when HA content in the composites con-tinue to increase, the severer HA agglomeration in polymer matrix re-sults in more defects and stress concentration, consequentlydecreasing the tensile strength.

The typical SEM micrographs of the fracture surface of the tensiletest samples with various HA content are shown in Fig. 4. The samplewith 2.6 vol.% HA (Fig. 4a) apparently exhibits the plastic deformationnature of the composites. Many stripes with a dimension of tens ofmicrons are formed due to drawing-out effect. HA fillers (seearrow) are firmly embedded in PEEK matrix without debonding dur-ing the plastic mode failure, indicating that the bonding between HAand PEEK sustains a very high tensile strength (over 100 MPa), whichis a remarkable improvement compared with the previous studies[3,5]. The HA fillers give a flattened broken texture, indicating thatthe cracks propagated through the inner part of fillers in the processof breaking, instead of through the interface between filler andmatrixas reported in the previous study [3]. With HA content increasing to

Fig. 3. Relationship between HA content and tensile strength of HA/PEEK composite.

HA

A B

C D

HA

HA

Fig. 4. Fractographs of HA/PEEK composites with different HA contents: (A) 2.6 vol.%; (B) 5.6 vol.%; (C) 8.7 vol.%; (D) 13.4 vol.%.

119R. Ma et al. / Materials Letters 71 (2012) 117–119

5.6 vol.%, the fractograph (Fig. 4b) shows that the composite mainlypossesses a drawn-out fibration feature, and HA fillers are firmly em-bedded in matrix with a flat surface. With HA content increasing to8.7 vol.%, the fractograph of the composite samples (Fig. 4c) shows arelatively smooth fracture surface, suggesting that the failure naturechanges to brittle mode. The interface between HA fillers and the ma-trix appears mostly seamless due to strong bonding. When the HAcontent is above 13.4 vol.%, the fractograph of the composites(Fig. 4d) gives flake crazes, a typical brittle failure nature. Differentfrom the samples with lower HA content (Fig. 4a–c), the HA agglom-erates in 13.4 vol.% HA/PEEK samples, not clearly exposed to SEM ob-servation, were covered by PEEK, suggesting that the crackspropagated through the PEEK matrix instead of through the interfacebetween HA agglomerates and PEEK matrix during failure probablybecause the binding force between HA particles and PEEK molecularchains was stronger than the failure strength of the samples.

The strong bonding between PEEK and HA observed in the studycan be attributed to in situ synthesis process, in which HA particleswere mixed into PEEK oligomers with short molecule chains andlow viscosity, and good wetting and contacting between HA fillersand PEEK oligomers were achieved. With continuing polymerization,PEEK oligomers on HA surfaces become polymeric molecules withlong molecular chain that firmly wrapped on HA surface, similar tothat of in situ preparation of CaCO3/polystyrene nanocomposites[12]. However, the strong bonding probably is due to physical factors,such as molecules chain wrapping and the interlock effect of PEEKmolecules on HA surface [13], because the FTIR and XRD results(Figs. 1 and 2) suggest that the chemical bonding between HA fillersand PEEK is weak.

4. Conclusions

Polyetheretherketone composite materials containing a variousamount of hydroxyapatite have been successfully synthesized via insitu synthesis process. The tensile strength of 2.6 vol.% HA/PEEK com-posites reaches as high as 106 MPa, which is attributed to the stronginterfacial bonding between PEEK and HA. This enhanced bondingmay be mainly due to the physical factors since the results suggest achemically weak interaction between PEEK and HA. Comparing toHA/PEEK composites previously prepared by other processes, the insitu synthesized composite materials in the study exhibit great im-provement on mechanical property and bonding state betweenPEEK and HA.

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