XRD analysis
Figure 3. shows the X-ray diffraction spectra of MgAl2O4 and PLA/ MgAl2O4 materials. It is seen that combustion synthesized MgAl2O4 spectrum possesses broad and weak peaks while calcination treatment results in tremendous changes. Hereby, the peaks at 18.98, 24.31, 36.85, 44.77, 59.40, 62.20, and 77.29 ˚ are related to the (111), (220), (311), (400), (422), (511), (440), and (533) diffraction planes of MgAl2O4, respectively which is in a good agreement with JCPDS. 21-11052 [31] In addition, PLA/ MgAl2O4 composites include a peak at 2θ = 17˚[19] on a hill, which is related to semi-crystalline PLA.
Figures 4 shows a reasonable matching of the diffraction patterns with the standard spectra of PLA and MgAl2O4 spinel, which were extracted through the employment of MAUD software. According to the related calculations, the percentages of MgAl2O4 particles in PLA, PLA/4% MgAl2O4, and PLA/8% MgAl2O4 specimens were 0, 3.3, and 7.1, respectively. This result interestingly confirms the uniform distribution of reinforcement particles in the matrix of produced specimens.
To study about the structure of the synthesized materials, the molecular structure of PLA and MgAl2O4 were drawn via the employment of VESTA software. Figures 5 (a) and (b) show the schematic views of PLA and MgAl2O4 structures that have been made by the CIF No. 7242509 and 1010129, respectively. It is seen that PLA polymer possesses branched shape structure. By contrary, MgAl2O4 spinel has a crystal structure which is repeated in X, Y, and Z directions. This issue is in a good agreement with the described XRD data which the samples possessing PLA showed a wide peak while MgAl2O4 related spectra included sharp diffraction bands.
FESEM Analyses
Figure 6 shows the FESEM images of MgAl2O4 particles before and after calcination. It can be seen that even before calcination, the sub-micron particles have been formed without any agglomeration. Meanwhile, the calcination procedure results in slight particle growth. In addition, the EDX data show that before calcination the main elements are Mg, Al, and O, inducing that relatively low temperature of combustion synthesis has been adequate for good crystallization of pure MgAl2O4. Moreover, given that the calcination has occurred under normal atmosphere, one can expect that the percentage of oxygen increases within heat treatment procedure.
Figure 7 (a) shows the FESEM image of the unreinforced PLA produced by the explained slurry casting. According to Figs. 7 (b) and (c) which correspond to the composites possessing 4 wt% and 8 wt% MgAl2O4 reinforcement, it was found that the spinel particles have been uniformly distributed throughout the composites. This result agrees well with the Rietveld calculations which were carried out via MAUD software. Figure 7 (d) depicts the EDX results of PLA/8% MgAl2O4 composite. Accordingly, it is seen that high quantity of carbon element originates from PLA matrix. Additionally, no impurity can be found in the prepared composites.
The FESEM images of the PLA/4% MgAl2O4 and PLA/8% MgAl2O4 composites after being placed in SBF for 4 weeks are seen in Fig. 8. It was interestingly understood that after 4 weeks of immersion, remarkable parts of MgAl2O4 elements have been substituted by other elements which means enhanced bio-activity of the composite specimens[3]. Herein, the reduction of Mg and Al elements as well as the increase of O element can be considered as strong evidences for the deposition of hydroxyapatite on the surface of the slurry casted materials. This phenomenon is interestingly similar to our previous research which it was reported that the addition of MgAl2O4 particles to PLA polymer gives rise to the increase of wettability and enhanced degradation of PLA/MgAl2O4 composites[8].
FTIR Analyses
The FTIR results of MgAl2O4, PLA, and PLA/MgAl2O4 compounds have been presented in Fig. 9. In this spectra, the peaks at 500–700 cm− 1 are attributed to Mg-O bond stretching. In other words, similar to what was explained by XRD results, the FTIR characterization proves the successful formation of MgAl2O4 crystal structure. The peak at 870 cm− 1 originates from the C–C stretching of C–COO. In addition, the bands at 1000–1200 and 1375–1475 cm− 1 are because of C-O stretching and CH3 stretching, respectively. Moreover, the peaks at 1650–1750 cm− 1 are from C = O bond stretching while those in 2850–3000 cm− 1 originate from -CH stretching. The relatively weak peaks at 1250–1500 and 1600–1800 cm− 1 regions of the composite specimens originate from the vibrations of the carbonates as well as –OH bending components [22, 28]. As a matter of fact, it can be concluded that the addition of MgAl2O4 to PLA gives rise to the improvement of hydrophilic and biodegradability behaviors of composite specimens which will be discussed in details.
Degradation Analyses
The pH of SBF solution in the second and fourth weeks varied in the range from 7.6 to 7.8. Noteworthy, in the fourth week, the pH of PLA/ MgAl2O4 composites meaningfully increased. This phenomenon logically originates from the degradation of materials in the SBF solution. In other words, the addition of MgAl2O4 leads to the replacement of calcium and phosphorus by H+ ions, as a result, the pH of the SBF solution has increased.
Figure 10 demonstrates the variations of Ca and P elements of SBF in the second and fourth weeks in terms of (mg/L). The amounts of these elements interestingly decreased in the fourth week, which is due to the formation of hydroxyapatite on the surface of the degraded composites. This result confirms the EDX data of PLA/8% MgAl2O4 after immersion in SBF solution for 4 weeks which was already described.