The effect of annealing on the behavior of polyetheretherketone composites compared to pure titanium

Polyetheretherketone (PEEK) has become increasingly popular in biomedical applications due to its favorable biocompatibility, biostability, mechanical strength, and elastic modulus, all of which are similar to those of natural bones. This paper investigates the effects of annealing on the behavior of PEEK ternary composites. PEEK samples were annealed and characterized by mechanical tests, Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR), Scanning Electron Microscopy (SEM), Energy Dispersive x-ray Spectroscopy (EDS) and physical property testing. Results showed that annealing has an appositive effect on the properties of PEEK. The properties of the ternary composites were also compared with those of pure PEEK and Ti as a control.


Introduction
Fixture dental implant is a device surgically inserted into the jaw bone to support a prosthodontic or orthodontic implant [1,2]. Fixtures implant, composed of Ti, are Known to show elastic modulus (E) mismatches when associated with bone tissue [3][4][5]. For instance, the elastic modulus of Ti (110-150 GPa) may be up to 10 times higher than that of the cortical bone, which resulting in the concentration of stresses at the bone-implant mastication interface loading. Therefore, materials with an elastic modulus more similar to that the surrounding bone could induce a stress distribution through the structural materials at low intensity to the supporting bone tissue, thereby reducing the stress shielding at the interface [5,6]. Polyetheretherketone reinforced by carbon fibre is gradually becoming a primary candidate to replacement for metallic implants because its adjustable mechanical properties are similar to those of human cortical bone; as such, its use can mitigate concerns over the risk of bone resorption caused by stress shielding due to elasticity mismatches between implants and human bones [7,8]. Osseointegration, in which implants are integrated with the surrounding bone, is critical for successful bone regeneration and healing in dental/orthopedic applications. Although significantly improved biological properties have been attained with the use of PEEK-based TiO 2 [9], Ti [10] and calcium silicate [11] and nano-hydroxyapatite (n-HAp) [12][13][14], binary composites through blending modification, these simple reinforcements usually impair the mechanical properties (i.e. tensile strength and work-to-failure limit) of PEEK, effecting adjacent tissues [14,15], and hindering the clinical applications of the material. A recent study in this field highlighted the importance of ternary composites in compensating for the shortcomings of conventional binary composites; ternary composites promote the advanced mechanical properties and biocompatibility of materials to mimic the constituents and structure of natural bone [16].
Feng et al investigated the effect of incorporated hydroxyapatite and carbon fibre into polyetheretherketone (PEEK/n-HAp/CF) and the results show that the composite strength and modulus of this material are significantly improved by the inclusion of carbon fiber [17].
Deng et al improved the biocompatibility and mechanical properties of PEEK by adding 25 wt% n-HAp and 20 wt% CF into the polymer. It was found that the improved of mechanical properties (the E and tensile strength) with biological assays revealed high bioactivity for composites than those recorded for pure PEEK [16].
Anxiu et al developed a novel biocomposite based PEEK matrix reinforced with n-HA and CF for osteogensis enhancement. The results show that the surface hydrophilicity of the ternary biocomposite improved by using oxygen plasma combined with sandblasting. Also, the surface treatment led to promote MG-63 cells proliferation and differentiation in vitro [18].
The objective of the present study is to prepare and characterize a novel polymeric composite reinforced with nano bioceramic and carbon fiber in order to enhance the properties. Also estimate the effects of annealing on the mechanical, physical and surface properties of these composites. Afterward, comparsion has been conducted for the properties of resultant polymer samples before and after annealing with properties of pure titanium for use as implants in the human body.

Nanoindentation curve
In a typical nanoindentation experiment, the indenter is placed in contact with the material surface and then penetrated to a certain depth or load. A nanoindentation curve the load as a function of the displacement of the indenter is then plotted, and the loading and unloading patterns could be determined. Figure 1 shows the unloading process and parameters associated with the contact geometry. The depth of penetration is the total displacement into the sample, and the hardness (H) and modulus values are determined [19].

Hardness and Young's modulus calculation
The load and displacement curve can be used to determine the H and E of the material [21]. After each complete cycle consisting of the loading and unloading of the sample, the results are plotted as agraph load of the indenter as a function of penetration depth displacement -as in figure 1.
where: P -load, hc -penetration depth, hf-residual depth, hc -contact depth, hmax-maximal depth, S-contact stiffness. The analysis of the results indentation experiment was based on Oliver and Pharr method, as explained in the following relationships [21,22].: reduced modulus: Where: E -modulus of specimen, E′ -modulus of intender ν -Poisson's ratio.

Preparation of samples
Biopolymer composites, containing 1.5% nano powders and 5% chopped CF, were fabricated via compounding and hot press. The mixture was obtained as follows. First, granule PEEK was prepared and dried in an oven at 80°C for 1 h prior to compounding and hot pressing. Next, chopped CFs with nanopowder were dispersed in ethanol by using an ultrasonic mixer to obtain a homogeneous mixture, and then added to the PEEK granules. Compounding was performed in an internal mixer (Haake) type (HBISYSTEM 90, AHAAKE BUCHLER PRODUCT, USA) at a temperature of 360°C and mixing speed of 90 rpm. Press was performed by using a hot press (TOYOSEIKI, Japan). The temperature of the hot press was adjusted to 360°C and 15 MPa of pressure was applied for (15) min. The mold was then removed from the hydraulic press and placed in a cooling system with water jets (5 l min −1 ) at room temperature. The testing samples were obtained by cutting the prepared plates according to the relevant ASTM standard for each test. Three specimens were tested at various filler ratios to obtain the best results. Titanium samples were cut by using wire cutting machine according to the relevant ASTM standard for each test and also three specimens were tests according to each test and grinding on SiC abrasive papers down to 4000, followed by polishing with 1 μm diamond paste. All properties for all samples types were measured at room temperature. Figure 2 illustrates the experimental setup diagram.

Annealing of the manufacture samples
After manufacture of the polymer, samples were annealed in an oven (ATRA-type PC-15, Iran), they were heated at 245°C (∼10°C min −1 ) for approximately 2 h. After heat treatment, the temperature was reduced at a rate of 10°C h −1 . The specimens were left inside the oven and allowed to cool naturally to room temperature.

Mechanical properties
The nanoindentation experiments were performed using a nanoindentation system (Triboscope nanomechanical test instrument, HYSITRON, Inc.) with the ISO 14577 method was used to measure the modulus and hardness of different materials. The experimental tensile test for polymer composites was performed on the a tensile test machine (SANTAM, type STM-50, Iran) with an ASTMD638 type IV standard While the tensile test for titanium was performed on tensile test machine (Instron-8502/model S/N, England) with an ASTM E8-04. For flexural test polymer samples inspection was done by using the same tensile test instrument to standard measurements ASTM D790-03. On the other hand, the flexural test for titanium was performed on (Computer Controlled electronic Universal, China). Samples prepared based on standard measurements ASTM E 290-97a. For compression test polymer samples were prepared according to standard ASTM D695-2002a was done by using a universal testing machine type ((LARYEE, China) while titanium samples prepared according to standard ASTM E9-89a was performed on the same bending instrument custom metal inspection.

Surface characterization
The prepared samples (i.e., PEEK, PEEK-1.5%n-HAp-5%CF and PEEK-1.5%n-TiO 2 -5%CF) were characterized via attenuated total reflection FTIR spectroscopy (BRUKER I11316, Germany) with an FTIR spectrophotometer according to ASTM E 1252-98. The sample morphology was determined by scanning electron microscopy SEM (TESCAN-type MIRA3, Czech) in low-vacuum mode with primary electron energy of 15 kV. The elemental composition of the samples was analyzed by energy dispersive x-ray (EDS) by using the same SEM device. Surface roughness was determined by using (TR200-type (TA620 Stand and Column, Time group Inc.)).

Thermal analysis and density
The thermal behavior of the prepared polymer samples was studied by using DSC (METTLER TOLEDO (Switzerland)) and thermal conductivity analysis. Approximately 10-15 mg was sealed in Al crucible under N 2 atmosphere. Thermal conductivity was also measured by using a device (FARNELL INSTRUMENTS LTD, Britain). Density was measured by using Archimedes' principle and weighting the specimens with an electronic balance under different conditions by using a device ((RADWAG, Poland (EU)).

Results and discussion
The properties of the composites were enhanced by adding nano ceramics and CFs. Table 1 illustrated the mechanical properties of the polymer composites, with neat PEEK and CP-Ti as control sample was compare  The results of the tensile, compression, bending modulus and bending strength testing of the composites were similar in nature to the nanoindentation results. The flexural strain at break showed the opposite behavior in the prepared polymer samples [23]. Figure 3 shows the surface roughness of the polymer composites and CP-Ti. The surface roughness of the ternary polymer composites was higher than of neat PEEK. The sample containing 1.5%TiO 2 +5%CF also showed higher roughness than values obtained for those of neat PEEK and CP-Ti [26]. Figure 4 illustrates the IR spectra of HAp, TiO 2 and cf. The TiO 2 spectrum exhibits an intense and wide band centered at∼3426.99 cm −1 , which could be attributed to e O−H stretching; peaks at 1654.66 and 1043.50 cm −1 arise from the bending vibrations of coordinated H 2 O and Ti−OH [27], the peak at ∼566.42 cm −1 is related to the Ti−O−Ti stretching, and the peak at 1465.75 cm −1 is assigned to TiO 2 lattice vibrations [28].
The IR curves of CF shows an absorption peak at 1433.61 cm −1 , which is attributed to the coupling vibrations of -COOH, and a peak at 1186.31 cm −1 , which refers to the deformation vibrations of O-H [29]. Broad peaks located at 3431.50 and 1658.93 cm −1 are assigned to the stretching vibrations of hydroxyl agroup, and a group of bands between 954.48 and 588.94 cm −1 are characteristic of the out-of-plane bending vibrations of C-H in aromatic rings [30]. The peak at 1628.73 cm −1 could be ascribed to the stretching vibrations bonds of C=C and C=N, wide peak at 3569.46 cm −1 is associated with the stretching vibrations O-H and the peak at 2923 cm −1 is related to C-H stretching vibration bonds [31].
In the HAp spectrum, the absorption peak at 3443.  [32]. Thus, the chemical structure of the PEEK substrate not has been altered by the addition of nanoparticles and chopped CF bio-composite samples. In addition, no new peaks and manifestly peaks shifts were observed.
Where ΔH: the apparent melting enthalpy was defined as the integral of the melting peak. ΔH°m: is the melting enthalpy of the 100% crystalline PEEK, i.e., 130 J g −1 . W PEEK : is the weight fraction of the PEEK matrix. T m and T g for all samples were relatively constant (∼339°C and ∼146°C respectively), thereby indicating that the addition of nanoparticles and CFs has no distinct effect on the polymer T m and T g . Thus, the crystal size of PEEK does not change after addition of the nanoparticles and fibres. The T c of the composites showed a decreasing trend due to the inhibitory effect of the reinforcement on the PEEK mobility [34,35]. Figure 6 shows the fracture morphologies with EDS of neat PEEK and ternary polymer composites observed via SEM at low and high magnification. The shape of CFs was clearly visible, and the black holes represent spaces from which the CFs were pulled when the composites were broken. The nano particles (white spots) were uniformly embedded in the composite, and randomly oriented CFs (i.e., a bright line like fibers) could be observed in the PEEK matrix. Debonding among CFs, nanoparticles and the PEEK matrix was not evident. This finding indicates good adhesion between the mixed components [24,36,37]. For EDS spectra of PEEK, n-HAp/ CF/PEEK and n-TiO 2 /CF/PEEK before and after annealing, here, the intensity of C peak decreased, whereas those of HAp, TiO 2 and CF peaks increased upon addition to the matrix. The EDS mapping images show that C, O and Ca, P and Ti are homogeneously distributed throughout the material.  Figure 7 compares the evolution of the thermal conductivities of neat PEEK, the ternary polymer composites and Cp-Ti. The conductivity of the ternary polymer appears to be higher than that of pure PEEK and lower than that of CP-Ti [38,39]. Figure 8 shows the relationship between the sample type and density; here, the density appears to increase after the incorporation of reinforcing material (with a density higher than that of the PEEK matrix) [40].

Annealing results
The effects of heat treatment were studied via mechanical, physical, chemical analyses and morphological properties. It is known that heat-treating PEEK between the Tg and melting temperature could increase the crystallinity of the material. Since the reinforcement was not thermally affected at a low annealing temperature of 245°C, these findings suggested that the PEEK matrix has undergone structural changes through crystallization [41]. After heat treatment of the samples was analyzed by DSC. Table 2 shows the major thermal characteristics and the calculated degree of crystallinity. It illustrated that heat treatment can promote crystallization temperature. The reason is that heat treatment can induce crystallization, and in turn, the induced crystals can accelerate the crystallization temperature [42]. It was worth noting that the average hardness results from nanoindentation measurements of annealed samples were depicted in table 1, and compared with the results from results of unannealed samples could be noted the heat treatment process seems to increase the average hardness of PEEK samples. The increment of the average hardness with annealing seems to be a result of crystallization in the samples. The obtained results stay in good agreement with the crystallinity level of the materials studied. In addition, even the amorphous regions can undergo a degree of reordering during annealing with a reduction in the free volume content. Generally, the average hardness increases with increasing degree of crystallinity of the polymer. A similar trend is observed for reducing the modulus of elasticity the results show that the modulus values for all samples increase with heat treatment due to increased crystallinity of the materials this result was in close agreement with [43]. For other mechanical properties it was found these properties changed significantly during annealing, in general, the heat treatment was seen to modify the modulus and Ultimate strength, flexural modulus, flexural strength, surface roughness, compression strength and compression modulus were generally higher than untreated samples because of the increased crystallinity and altered crystal structure, which resulted from annealing, affected characteristics of samples this result agrees with the reported values [44].