Morphology, thermal and mechanical properties of poly (ε-caprolactone) biocomposites reinforced with nano-hydroxyapatite decorated graphene

Journal of Colloid and Interface Science 496 (2017) 334–342

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Regular Article

Morphology, thermal and mechanical properties of poly (e-caprolactone) biocomposites reinforced with nano-hydroxyapatite decorated graphene Keqing Zhou a,⇑, Rui Gao a, Saihua Jiang b a b

Faculty of Engineering, China University of Geosciences (Wuhan), 388 Lumo Road, Wuhan, Hubei 430074, PR China School of Mechanical and Automotive Engineering, South China University of Technology, Wushan Road 381, Guangzhou 510641, PR China

g r a p h i c a l a b s t r a c t Hydroxyapatite-graphene hybrids were synthesized by a hydrothermal method and incorporated into poly (e-caprolactone) by a solvent blending method. The mechanical properties and biocompatibility of PCL nanocomposites were improved obviously.

a r t i c l e

i n f o

Article history: Received 19 January 2017 Revised 13 February 2017 Accepted 15 February 2017 Available online 16 February 2017 Keywords: A. Graphene A. Hybrid A. Polymer-matrix composites (PMCs) B. Thermal properties B. Mechanical properties

⇑ Corresponding author. E-mail address: [email protected] (K. Zhou). http://dx.doi.org/10.1016/j.jcis.2017.02.038 0021-9797/Ó 2017 Elsevier Inc. All rights reserved.

a b s t r a c t In this work, hydroxyapatite (HAP) nanorods decorated on graphene nanosheets (HAP-Gs) was synthesized by a hydrothermal method. The structure, elemental composition and morphology of the HAP-Gs hybrids were characterized by X-ray diffraction, Fourier transform infrared and Transmission electron microscopy. Subsequently, the hybrids were incorporated into poly (e-caprolactone) (PCL) via a solution blending method. Optical images and scanning electron microscopy observation revealed not only a well dispersion of HAP-Gs hybrids but also a strong interfacial interaction between hybrids and PCL matrix. The influence of HAP-Gs hybrids on the crystallization behavior, crystal structure, thermal stability, mechanical properties and biocompatibility of the PCL nanocomposites was investigated in detail. The results showed that the crystallization temperature of PCL was enhanced obviously, but the crystal structure was not affected by the incorporation of HAP-Gs hybrids. The mechanical properties of PCL bionanocomposites were improved obviously. Ó 2017 Elsevier Inc. All rights reserved.

K. Zhou et al. / Journal of Colloid and Interface Science 496 (2017) 334–342

1. Introduction In the past few decades, the development of biodegradable polymers has attracted extensive attention owing to the environmental problems induced by the accumulation of plastic waste. As one kind of biodegradable and biocompatible aliphatic polyesters, PCL has provided a number of potential applications from agricultural usage to engineering and biomedical devices [1]. However, the further practical applications for PCL are limited by some drawbacks such as slow crystallization rate and poor mechanical properties. To overcome these difficulties, one promising alternative is to prepare nanocomposites by the combination of various types of fillers. It has been reported the incorporation of nanofillers, such as carbon nanotubes, layered silicate, polyhedral oligomeric silsesquioxanes and graphite oxide can greatly improved the physical properties of PCL with low loading [2–5]. The presence of well-dispersed nanoparticles can not only act as the anisotropic reinforcements, but also as the nucleating agents, both contributing to the improvement of mechanical and thermal properties of PCL. Moreover, filled with nanostructured particles is expected to produce new PCL based biodegradable and biocompatible composites with even unexpected properties [6]. As an emerging 2D material, graphene and its derivatives have attracted considerable attention in numerous fields due to its high surface area and chemical stability, unique electronic and outstanding mechanical properties [7–9]. The addition of graphene with very low loadings into a polymeric matrix can significantly improve the properties of polymer matrix, such as mechanical, thermal, electrical, flame retardant and gas barrier properties [10–13]. Therefore, more attention has been drawn to graphene as a potential reinforcement material in the polymer nanocomposite [14]. It has been reported that incorporation of graphene into PCL can improve the crystallization and mechanical properties [15–17]. However, it is worth noting that the homogeneous dispersion and reasonable interfacial interaction between the graphene nanosheets and polymer matrix play vital roles in the final properties of the polymer nanocomposites. But actually, polymer nanocomposites with well dispersed graphene nanosheets are hard to achieve because pristine graphene has a pronounced tendency to agglomerate and even restack in polymer matrices due to the strong van der Waals force and p-p interactions which limits its dispersion in polymer matrices [18]. Meanwhile, the formation of strong interface between graphene nanosheets and the polymer matrix is still an urgent issue that needs to be solved. Therefore, to improve and stabilize the dispersion state of graphene nanosheets in host polymers constitutes the greatest challenge in graphene based polymer nanocomposites. HAP is well known for its unique properties such as excellent bioactivity, biocompatibility, osteoinductivity and osteoconductivity, and has been widely used as clinically available bone substitutes and implantation material [19]. However, the intrinsic brittleness, low mechanical strength and fracture toughness of pure HAP limits its further application under load-bearing conditions [20]. Therefore, the main focus of HAP research has been to improve its mechanical performance by combining it with various reinforcements such as polymers, metals and carbon nanomaterials [21–23]. However, an ideal reinforcement material would impart mechanical integrity to the composite without diminishing its bioactivity. Among these composites, PCL/ HAP composites are a strong candidate for applications as hard tissue regeneratives, in which the bioactive HAP component provides a favorable environment for cells to attach, proliferate, and differentiate, while the biodegradable PCL polymer matrix provides the required flexibility and moldability [24]. Therefore, if HAP is used as the filler for PCL, the strength of PCL might be improved to meet the clinical requirements. In return, PCL might avoid the brittleness problem of HAP.

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However, HAP has a strong tendency to agglomerate in PCL matrix even at low loadings and the interfacial adhesion between HAP and the polymer matrix is poor, which will adversely affects the mechanical properties of the nanocomposites. Accordingly, it is very necessary to modify the surfaces of HAP nanoparticles as the fillers, weak agglomeration and improve the interface structure between the filler and matrix of the composite. Recently, polymer nanocomposites based on hybrid nanofillers which comprise two or more heterogeneous nano elementary units with different properties, have been demonstrated to be a more attractive strategy with effective enhancement effects on mechanical, thermal stability, flame retardancy and thermal conductivity properties of the composites [25–31]. Moreover, it can be expected that the agglomeration of the nanoparticles and weak interfacial interaction problems in polymer nanocomposites could be solved simultaneously by using such functional hybrid nanofillers [32]. Therefore, the combination of graphene nanosheets with secondary nanomaterials integrates unique characters and functions of the two components which are not found in either of their individual components [33]. Inspired by this, it may be an efficient method to improve the crystallization, mechanical and biocompatibility properties of the PCL nanocomposites by the combining graphene and commonly used bone substitutes and implantation material (HAP). As an exfoliated two-dimensional material, graphene can be used as a perfect supporter to ameliorate the dispersion of HAP nanoparticles in the polymer matrix. In addition, the restacking of graphene nanosheets can be effectively prevented by the tightly decorated nanoparticles, which is beneficial for promoting the dispersibility of graphene in polymer matrices [28]. Therefore, integrating graphene nanosheets and HAP nanoparticles will result in the development of new composites with well dispersion, good mechanical property and excellent biocompatibility. Hydroxyapatite/graphene nanohybrids have been explored since HAP is the inorganic component of bone and offers a favorable environment for bone regeneration [34]. Nevertheless, to the best of our knowledge, there are no reports of an evaluation of their reinforcement effect in polymer nanocomposites published to date. Therefore, the target of the present study was to investigate the influence of HAP-graphene hybrids on the thermal behavior, mechanical properties and biocompatibility of PCL nanocomposites. In present study, a hybrid graphene supported HAP (HAP-Gs) is synthesized by a facile hydrothermal method. The HAP-Gs nanohybrids are characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Transmission electron microscopy (TEM). Then different weight percentages of HAP-Gs hybrids are incorporated, to obtain PCL composites via a solvent blending method and studied the influence of HAP-Gs on the morphology, thermal behavior, mechanical properties and biocompatibility of PCL nanocomposites. The research reported herein is expected to be of great interest for a better understanding of the relationship between structure and properties of polymer nanocomposites.

2. Experimental 2.1. Materials Natural flake graphite (EG) with an average particle size of 325 mesh was supplied by Qingdao Tianhe Graphite Co., Ltd. (China). Potassium permanganate (KMnO4), sodiumnitrate (NaNO3), sulfuric acid (H2SO4, 98%), hydrogenperoxide (H2O2, 30% aq), hydrochloric acid (HCl, 36% aq), calcium nitrate tetrahydrate (Ca (NO3)2
4H2O), di-ammonium hydrogen phosphate ((NH4)2HPO4) were purchased from Sinopharm Chemical Reagent Co. All of the chemicals were of analytical grade and used as received without

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further purification. PCL granules were kindly provided by Hefei Keyan Chemical Materials Company (Hefei, China). 2.2. Synthesis of HAP-Gs hybrids Graphite oxide was synthesized by using a modified Hummers method [35]. The HAP-Gs hybrids were synthesized by a facile hydrothermal method [21]. In a typical procedure, 30 mg of GO was added to 30 mL of distilled water with the assistance of sonication for 2 h to form a homogeneous suspension. Then, 0.5 mM Ca (NO3)2
4H2O was dissolved into the abovementioned homogeneous GO suspension. Meanwhile, 0.3 mM of (NH4)2HPO4 was dissolved in 30 mL deionized water and gradually dripped to the abovementioned solution. The pH value of the solution was adjusted to 10.5 by adding ammonium hydroxide solution. The mixture was then stirred for 30 min and transferred to a Telfonlined autoclave, which was tightly sealed and placed into a hot air oven at 180 °C for 12 h. Subsequently, the autoclave was gradually cooled to room temperature, the precipitates were collected by centrifugation, washed with deionized water and ethanol for several times. The obtained final product was dried in a vacuum oven at 80 °C for overnight before further characterization. The pure Gs samples were obtained under same synthesis conditions without the addition of Ca(NO3)2
4H2O and (NH4)2HPO4.

isothermally at 100 °C for 3 min to eliminate the thermal history. The samples were then cooled to 0 °C at a cooling rate of 10 °C/ min. The DSC curves were recorded and analyzed. The degree of crystallization can be calculated by this equation: Xc (%) = DHm/ DH0  100%, where DHm is the apparent value of enthalpy of melting, DH0 is the extrapolated value of the enthalpy, corresponding to the melting of a 100% crystalline PCL (136.1 J g 1) [36]. Thermogravimetric analysis (TGA) was carried out using a Q5000 thermoanalyzer instrument (TA Instruments Inc., New Castle, DE) under air flow of 25 mL min 1. In each case, the samples were heated from room temperature to 800 °C at a linear heating rate 20 °C min 1. The tensile strength and elongation at breaking of PCL nanocomposites were measured according to the Chinese standard of GB 13022–91 on an MTS CMT6104 universal testing machine (MTS Systems Co. Ltd, P.R. China). The stretching rate was 50 mm min 1. Five parallel runs were done in the case of each sample to get the average. Ultraviolet–visible diffuse eactance spectroscopy and ultraviolet–visible absorption spectra were recorded on a Perkin-Elmer Lambda 950 UV/Vis-NIR spectrophotometer (Perkin-Elmer).

3. Results and discussion 3.1. Characterization of GO, Gs and HAP-Gs hybrids

2.3. Preparation of PCL/HAP-Gs nanocomposites PCL nanocomposites with 0.2%, 0.5%, 1% and 2% HAP-Gs hybrids were prepared by a solvent blending method. In a typical experiment, 0.1 g HAP-Gs hybrid was dispersed in DMF with several hours of ultrasonication and strong mechanical stirring to obtain homogeneous suspension. Then 10.0 g PCL granules were incorporated into the above homogeneous suspension. After 2 h of ultrasonication and strong mechanical stirring, the black slurry obtained was flocculated in hexane, and further dried at 50 °C for 12 h. Then the obtained samples were hot pressed at 65 °C to form specimens for test. PCL nanocomposites with 1% Gs were prepared by a similar procedure. 2.4. In vitro protein adsorption Fibrinogen and lysozyme were chosen as a model protein for our investigation. The protein-adsorption experiments of PCL nanocomposites were performed as follows: PCL nanocomposites (5 cm  5 cm) were immersed in aqueous solutions that contained different protein (500 lg/mL). Each solution was shaken at a constant rate for 60 min at 37 °C. Then, the solution was centrifuged and the amount of protein in the supernatant was measured by UV/Vis absorption. 2.5. Characterization XRD patterns were taken on a Japan Rigaku D/Max-Ra rotating anode X-ray diffractometer equipped with a Cu-Ka tube and Ni filter (k = 0.1542 nm). FTIR spectra were obtained with a Nicolet 6700 spectrometer (Nicolet Instrument Corporation, Madison, WI). TEM (JEM-2100F, Japan Electron Optics Laboratory Co., Ltd.) was employed to investigate the morphology of GO, Gs, HAP-Gs hybrids. The distribution of Gs and HAP-Gs hybrids in PCL matrix was evaluated by an optical microscope (OM, Leika DMLP, Germany) at room temperature. SEM (JSM-6800F, JEOL) was used to observe the fracture surface structure of PCL and its nanocomposites. Thermal analysis of samples was conducted by differential scanning calorimetry (DSC) (Perkin-Elmer Diamond DSC). The melting and crystallization studies were performed in the temperature range of 0–100 °C at a heating rate of 10 °C/min, and hold

The phase formation and crystallinity of GO, Gs and HAP-Gs hybrids are investigated by XRD (Fig. 1a). GO exhibits a typical diffraction peak at 2h = 11.0°, which is ascribed to the diffraction of (0 0 2) plane of GO. For the Gs, a weak and broad diffraction peak appearing at 2h = 20–30° corresponds to (0 0 2) plane, which is mainly attributed to the oxygen-containing groups in the structure of GO removed during the hydrothermal process [37]. As for the HAP-Gs hybrids, it shows noticeable peaks at 25.9° and 32.0°, corresponding to the (0 0 2) and (2 1 1) planes, respectively. In addition, characteristic peaks were observed at 32.3°, 33.1°, 35.6°, 39.9°, and 49.6°, corresponding to the (1 1 2), (3 0 0), (2 0 2), (3 1 0), and (2 1 3) planes, respectively. These diffraction peaks are well in accordance with the standard pattern of HAP (JCPDS card No. 09–0432), suggesting that the synthesized HAP has a hexagonal crystalline structure with high purity [38]. The phases of GO, Gs and HAP-Gs hybrids are further confirmed by FTIR spectra (Fig. 1b). The FTIR spectra of GO, Gs, and HAP-Gs show broad bands ranging from 3400–3500 cm 1, corresponding to the OAH stretching vibration of adsorbed water molecules. In addition, it can be observed that the FTIR spectrum of GO shows the characteristic absorption peaks of oxygen-containing functional groups. The absorption bands at 1723, 1389, 1220 and 1068 cm 1 are ascribed to the C@O stretching vibration, OAH deformation vibration, stretching vibrations of CAO (epoxy) and CAO (alkoxy), respectively [39]. The strong peak at 1620 cm 1 may be attributed to the C@C stretching vibration of the unoxidized graphitic domains or the vibration of the absorbed water molecules [40]. In comparison with that of GO, the bands associated with oxygen-containing functional groups of the Gs decreases significantly, and some of the bands disappear. The disappearance of the C@O peak at 1723 cm 1 compared to GO provides a solid indication of GO reduction. For the HAP-Gs, the intense peaks at 567 and 604 cm 1 are attributed to the bending vibrations of the OAPAO in the PO34 groups. The asymmetric stretching vibrations of the PAO in the PO34 groups are located at 1096 and 1033 cm 1, which indicate the formation of HAP succesfully. Moreover, it is clear to observe that there are no characteristic bands for oxygen-containing groups between 1300 and 1600 cm 1 and that the band at 1723 cm 1 are disappeared, which is confirmed that the oxygen-containing groups are removed to a high extent based

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Fig. 1. XRD patterns (a) and FTIR spectra (b) of GO, Gs and HAP-Gs hybrids.

Fig. 2. TEM images of the GO (a), Gs (b) and HAP-Gs hybrids (c, d).

on hydrothermal reaction. The emergence of the characteristic absorption peaks of HAP and Gs in the spectra of HAP-Gs suggested the successful conjugation of HAP onto the surface of Gs. The FTIR results further confirm the successful formation of the HAP-Gs hybrids which are good in agreement with the XRD results. The surface morphology and microstructure of the GO, Gs and HAP-Gs hybrids are observed by TEM which are shown in Fig. 2. Fig. 2a shows the exfoliated GO consists of large thin sheets with diameters up to several micrometers. After reduction, the structure of Gs changes obviously, some thin sheets closely fold up with each other in the structure (Fig. 2b–d). present the TEM images of the as-prepared HAP-Gs hybrids, from which it can be seen that numerous rod-like particles are distinguished from the flake background. The nanorods are randomly scattered throughout the surface and are densely attached on the surface of graphene nanosheets. Another interesting phenomenon is that, even after a long time of sonication during the preparation of the TEM specimen, the HAP nanorods remain strongly anchored on the surface

of graphene nanosheets with a high density, as shown in Fig. 2c and d, suggesting the strong interaction between HAP nanorods and graphene nanosheets. Such interaction combined with good mechanical flexibility of graphene sheets prevents the agglomeration of HAP nanorods to form large particles. Moreover, the HAP nanorods on the surface of graphene nanosheets can act as spacers to efficiently prevent the closely restacking of graphene nanosheets, which is benificial for the dispersion of graphene nanosheets in the polymer matrices. 3.2. Dispersion and interface interaction As is well known, the dispersion and interfacial interaction between nanofillers and polymer matrix are vital to final performances of polymer nanocomposites [41]. The optical images are used to evaluate the overall dispersion state of HAP-Gs and Gs in the PCL nanocomposites, as shown in Fig. 3. It can be seen that HAP-Gs hybrids show homogeneous distribution throughout

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Fig. 3. Optical images of PCL/1% HAP-Gs (a) and PCL/1% Gs nanocomposites (b).

Fig. 4. SEM images of the fractured surfaces for neat PCL (a and b), PCL/1% HAP-Gs (c and d) and PCL/1% Gs nanocomposites (e and f).

matrix, while Gs has a poor distribution state with obvious aggregate structure in the PCL matrix. Therefore, it is clear that the HAP nanorods decorated on the surface of graphene nanosheets can inhibit the restacking of the graphene nanosheets, which is conducive to improve the dispersion of graphene nanosheets. In addition, SEM is further performed to observe the fracture morphologies of the composites and reveal the interfacial interaction

between the nanofillers and polymer matrix. Fig. 4 shows the SEM images of the fractured surfaces for pure PCL, PCL/1% HAPGs and PCL/1% Gs nanocomposites. The morphological difference between neat PCL and its composites is clearly visible. For pure PCL, it has a uniform surface morphology revealing a rather smooth surface (Fig. 4a and b). Compared to neat PCL, the fractured surface of PCL nanocomposites with HAP-Gs or Gs shows considerably

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different fractographic features, as shown in Fig. 4c–f. After incorporating HAP-Gs hybrids, the PCL nanocomposite shows a relative rough surface. No obvious agglomerates and pull-out of graphene nanosheets are observed in the SEM images of the fractured surface of PCL/HAP-Gs nanocomposites, demonstrating that the graphene nanosheets exhibit excellent dispersion and are embedded and tightly held in the PCL matrix with strong interfacial adhesion. The strong interfacial interaction can support a significant stress transfer and thus substantially reinforce the properties of polymer nanocomposites. However, the bare graphene nanosheets agglomerated obviously in the PCL matrix (Fig. 4e and f). The homogeneous dispersion of the HAP-Gs hybrids and strong interfacial interaction between HAP-Gs hybrids and PCL matrix are mainly attributed to the following two reasons: (1) the HAP nanorods decorated on the surface of graphene nanosheets can inhibit the restacking of graphene nanosheets, (2) PCL may form hydrogen bonds with the hydroxy groups on the HAP surface [41]. The good dispersion and strong interfacial interaction are conducive to the property improvements listed below. 3.3. Thermal behavior Much more attention should be paid to the crystallization study because it affects not only the crystalline structure and morphology of semi-crystalline polymers but also the final physical properties and biodegradability of biodegradable polymers [42]. DSC tests are carried out to study the effect of HAP-Gs hybrids and Gs on the crystallization behavior of PCL and the DSC crystallization exotherms for pure PCL, PCL/1% HAP-Gs and PCL/1% Gs nanocomposites are shown in Fig. 5a, and the crystallization properties determined from DSC runs are summarized in Table 1. It is obvious

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that, compared with pure PCL, PCL nanocomposites with HAP-Gs and Gs show far higher crystallization temperatures (Tc), especially for the PCL/1% HAP-Gs nanocomposites, indicating the presence of efficient nucleating effect of the HAP-Gs hybrids. The obvious nucleating effect can be attributed to the exfoliated structure of graphene nanosheets and HAP nanorods within the PCL matrix which was also suggested by previous reports [43,44]. Although the nucleation of the graphene oxide is disclosed, the melting point (Tm) of the composites is not increased significantly with respect to the pure PCL which is good agreement with previous research work [45]. The degree of crystallinity (Xc) of the PCL nanocomposites, which can be calculated by comparing melt enthalpy (DHm) with the theoretical enthalpy of 100% crystalline PCL, decreases relative to the neat PCL, which is in accordance with previous work [36]. The decrement of the Xc is mainly attributed to the decreased mobility of PCL chain which is caused by the anisotropic 2D sheet structure of graphene or by the increased system viscosity [46]. It is of interest to study the effect of HAP-Gs or Gs on the crystal structure of PCL in the PCL nanocomposites. Fig. 5b illustrates the WAXD patterns of neat PCL and its two nanocomposites. Neat PCL presents three main diffraction peaks at 2h = 21.3°, 21.9°, and 23.6°, corresponding to (1 1 0), (1 1 1), and (2 0 0) planes of the orthorhombic crystal form, respectively [16]. Moreover, as shown in Fig. 5b, compared with pure PCL, both the PCL nanocomposites with HAP-Gs and Gs show similar diffraction peaks, suggesting that the crystal structures of PCL remain unchanged despite the presence of HAP-Gs hybrids or Gs in the PCL nanocomposites. Thermal stability of PCL and its nanocomposites is further evaluated by TGA. Fig. 5c and d illustrates the TGA and DTG curves under air atmosphere for neat PCL and its nanocomposites with different loadings of HAP-Gs hybrids. It is obvious to note that

Fig. 5. DSC curves (a), XRD patterns (b), TG and DTG curves (c and d) of PCL and its nanocomposites.

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Table 1 Thermal properties of PCL and its nanocomposites. Sample

DHc (J/g)

DHm (J/g)

Tc (°C)

Tm (°C)

Xc (%)

PCL PCL/1.0% HAP-Gs PCL/1.0% Gs

51.2 48.4 49.6

75.0 74.4 74.3

28.7 33.7 30.7

65.4 66.3 66.1

55.1 54.7 54.6

the presence of higher content of HAP-Gs hybrids in the nanocomposites results in worse thermal stability than neat PCL. It is completely different with previous reports which demonstrate that the inclusion of reduced graphene nanosheets in PCL composites has a positive influence on its thermal property [47]. The possible explanation for reducing thermal degradation temperature might be caused by the following two reasons: (1) the HAP-Gs hybrids degrade at lower temperatures than neat PCL, (2) the surface alkalinities of the HAP can hydrolytically cleave the ester groups of PCL, which are the main reason for the decrease in thermal stability of PCL nanocomposites [48]. However, it can be obtained from DTG curves that the maximum mass loss rate of the PCL nanocomposites is lower than that of neat PCL, especially for the PCL/2.0% HAP-Gs, suggesting that graphene nanosheets function as an effective barrier to inhibit the mass loss during the thermal degradation process [28]. In addition, PCL nanocomposites with HAP-Gs hybrids have higher amount of residues than neat PCL, which provides a protective shield of mass and heat transfer. 3.4. Mechanical properties Typical stress–strain behaviors for the PCL and its nanocomposites with varying HAP-Gs hybrids loadings are shown in Fig. 6a, and the detailed data are summarized in Fig. 6b. It is evident from these curves that the overall deformation behaviors of the PCL nanocomposites are similar to those of the pure PCL, and the yields are distinct for all specimens. Fig. 6b clearly reveals that the tensile strength of the PCL/HAP-Gs nanocomposites increased with the increasing concentration of HAP-Gs up to 0.5 wt%, which was attributed to the complete dispersion of the HAP-Gs hybrids in the PCL polymer matrix. Unfortunately, when the concentration of HAP-Gs was further increased, the tensile strength of the PCL nanocomposites gradually decreased. However, it is still higher than that of the pure PCL. Simultaneously, it can be observed that the elongation at break of the nanocomposites increases with increasing HAP-Gs content level. The above results indicate that the interfacial stress can be well transferred from PCL matrix to graphene nanosheets. In order to further demonstrate the influence of the decorated HAP nanorods on the performances of the

PCL, the mechanical properties PCL nanocomposites with 1 wt% of Gs and HAP-Gs are compared, which are also shown in Fig. 6a and b. It is clearly observed that the tensile strength and elongation at break of the PCL nanocomposites with HAP-Gs hybrids is better than that of the PCL nanocomposites with bare Gs. The improvement in mechanical properties of nanocomposites could be attributed to the intrinsic properties of graphene nanosheets, strong interaction and adhesion between HAP-Gs hybrids and the PCL matrix as well as uniform dispersion [45]. 3.5. Protein adsorption PCL is a well-established biocompatible and biodegradable material having found use in many applications including drug delivery, sutures and as a scaffold for tissue engineering. When PCL is used as blood-contacting material, it has also been challenged like other polymers for cardiovascular devices by diverse and complex reactions of the blood components to the biomaterials, including plasma protein adsorption and thrombogenesis [49]. It has been well demonstrated that blood coagulation on biomaterial surfaces is related to protein adsorption that occurred within seconds following blood contact [50]. The types and amounts of adsorbed proteins are known to affect subsequent platelet adhesion and activation, which also plays a major role in surface thrombogenesis. Especially, fibrinogen is of particular importance due to its high concentration in blood and its high platelet affinity [51]. And the protein adsorption on the material surface depends on the surface chemistry, surface topography, surface energy (hy drophilicity/hydrophobicity) and charge, the mobility of the surface functional groups, and so forth [52]. Therefore, the proteinadsorption performance of the PCL nanocomposites is studied by using different protein as a model protein, as shown in Fig. 7. It can be shown that the resistance to the fibrinogen and lysozyme adsorption on the PCL nanocomposites is improved, especially for the PCL nanocomposites with 0.5% HAP-Gs hybrids, compared with that of the pure PCL and PCL nanocomposites with bare graphene nanosheets. It is reported that the platelet density and the amount of fibrinogen adsorbed on the material surface are positively correlated [53]. Moreover, previous study has proved

Fig. 6. Typical stress–strain curves of PCL and its nanocomposites (a) and mechanical properties of the PCL and its nanocomposites: tensile strength (left) and elongation at break (right) (b).

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Fig. 7. Quantity of the adsorbing proteins on PCL and PCL nanocomposites.

that lysozyme could activate platelet [54], which implies that the less lysozyme adsorption is beneficial for resisting platelet adhesion. Thus, the results of this study indicate that the PCL nanocomposites with HAP-Gs hybrids has a great effect on reducing the platelet adhesion and are promising for applications in biomedical fields. 4. Conclusion In this work, HAP-Gs hybrids were synthesized by a hydrothermal method. The composition and structure of hybrid materials were confirmed by XRD, FTIR and TEM. The morphological study exhibited the HAP nanorods were decorated tightly on the surface of graphene nanosheets. Biodegradable PCL/HAP-Gs nanocomposites were prepared via a solution mixing method. The dispersibility, thermal properties, mechanical properties and biocompatibility of the PCL/HAP-Gs nanocomposites were studied in detail. Optical images and SEM observation revealed not only a well dispersion of HAP-Gs hybrids but also a strong interfacial interaction between the hybrids and PCL matrix. The DSC results showed that the crystallization temperature of PCL nanocomposites enhanced obviously. However, the presence of HAP-Gs hybrids did not change the crystallization mechanism and crystal structure of PCL. The thermal degradation temperature of the PCL nanocomposite with HAP-Gs hybrids was lower than that of pure PCL which was mainly due to the negative catalytic degradation effect of HAP on the PCL resin. The tensile strength and elongation at break of PCL nanocomposites were improved simultaneously which were mainly attributed to the strong interfacial interactions between HAP-Gs hybrids and PCL matrix as well as good dispersion of HAP-Gs hybrids. The proteins adsorption experiments show that the as-prepared PCL/HAP-Gs nanocomposites had a better resistance to adsorb proteins than the pure PCL and PCL nanocomposites with bare graphene, suggesting that the PCL nanocomposites containing HAP-Gs hybrids had a great effect on reducing the platelet adhesion, which indicates that the PCL/HAP-Gs nanocomposites are promising for applications in biomedical fields. Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (CUG160607), Natural Science Foundation of Guangdong Province (No. 2014A030310122) and China Postdoctoral Science Foundation (No. 2015M572309).

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