Morphological examination
The cryo-fracture surface morphologies of HA nanoparticles and HDPE/HA (10%, 30%) are shown in Figure
1a, b and c respectively. A good distribution of HA nano particles, appear as bright spots in the dark HDPE matrix, was achieved. In spite of the HA particles were in the nano-range according to the manufacturer specifications, the cryo-fracture surface morphologies shows remarkable agglomeration of HA nano particles. This agglomeration could be attributed to the tendency of HA nano particles to decrease their contact surfaces with HDPE matrix.
Melt flow index (MFI)
The effects of HA and Gamma irradiation on the HDPE/HA MFI is shown in Figure
2. As can be observed, the MFI of HDPE/HA strongly decreases with gamma irradiation and HA nanoparticles content. This reduction in the HDPE nanocomposite MFI due to irradiation can be attributed to the chain crosslinking. On the other hand when HA is added to the polymer, the material viscosity increases and hence resistance to flow increases. Similar results have been also obtained where the MFI decreases due to irradiation and presence of HA for HDPE/HA (20:80)
[29].
Calorimetric measurements
The thermal properties for the melting and crystallization of irradiated (70 kGy) HDPE/HA (30%) nanocomposite is shown in Figure
3. This figure shows the overlay DSC curves as a function of temperature and time. The degree of crystallinity and melting temperature of the first heat for irradiated and non-irradiated neat HDPE and HDPE/HA nanocomposites are shown in Figures
4 and
5 respectively. The results indicated that the degree of crystallinity (Xc) decreased with increasing the HA nanoparticle ratio. Figure
4 indicated that the Xc of neat HDPE decreased by 19% due to the addition of 30% HA to the polymeric matrix. This figure also indicated that the Xc of neat HDPE and its nanocomposites increased due to gamma irradiation. The crystallinity of HDPE/HA (30%) increased by 20% due to irradiation with 70 kGy. The reduction on the HDPE/HA nanocomposite crystallinity can be attributed to the formation of HA nanoparticles conglomerates in the HDPE matrix. At higher ratios of fillers these conglomerates exceed the critical size and lose their nucleating capacity. Also the presence of HA nanoparticles restricted the mobility of the molecules and hence the crystallinity decreased. At high ratio of HA nanoparticles, the degree of crystallinity decreased not only due to conglomeration of fillers but also due to confinement and entanglement effects. Moreover, the effect of nanoparticles on polymer crystallization is strongly affected by the shape and ratio of nanoparticles
[17–19, 30]. On the other hand, the HA filler has higher specific heat capacity that make it a better heat conductor resulting in faster cooling of HDPE/HA nano-composite. This high rate of cooling resulted in thin lamellar formation rather than thick crystal growth leading to lower degree of crystallinity
[30, 31]. This decrease in the crystallinity has negative effects on the polymer mechanical properties where the higher degree of crystallinity leads to stiffer and stronger composite. However, for the HDPE/HA nanocomposite, these negative effects can be negated by the presence of HA nanoparticles which stiffen the polymer. The DSC results also indicated that the crystallinity of neat HDPE and HDPE/HA nano-composites increased due to irradiation. Such increase of crystallinity can be attributed to the crosslinking of polymer chains due to gamma irradiation. The DSC results also indicated that the melting temperature is slightly changed due to the addition of HA nanoparticles and gamma radiation as indicated in Figure
5.
Thermogravimetric analysis (TGA)
Figure
6a graphically displays the thermogravimetric analysis of neat HDPE and its nanocomposite. The weight loss vs. temperature profile of neat HDPE and its nanocomposites showed that the weight of these materials remains unchanged until the temperature reached 400°C. These results showed that the weight loss mainly occurred in the range of 420-480°C, 435-520°C and 450-540°C for neat HDPE, 10% HA and 30% HA, respectively, with negligible change at temperature higher than 550°C. This weight loss indicated that the decomposition temperature of HDPE/HA nanocomposites is higher than that of the neat material. These results suggested that the HA nanoparticles confers thermal stability to the HDPE matrix and this can be attributed to good dispersion of the HA nanoparticles. This phenomenon resulting in a delay in the polymer thermo-degradation process. Similar results have been obtained by Bikiaris B.
[32] who studied the effects of inorganic nanofillers on the thermal stability of polymers. Bikiaris attributed the increase of polymer thermal stability to many reasons such as shielding effects, gas impermeability of inorganic nanofillers that inhabit the formation and escape of volatile by-products during the degradation, reducing polymer chain mobility and then delaying the composite degradation. The irradiated specimen showed higher thermal stability than that of non-irradiated specimen, as shown in Figure
6b. It can be seen that the weight loss of irradiated specimens remained unchanged until the temperature reaches 433°C. This weight loss mainly occurred in the range of 433-495°C with negligible change at temperature higher than 505°C. Also, the improvements in the nanocomposite thermal stability due to irradiation can be attributed to the reduction of chain mobility which occurred as a positive effect due to gamma irradiation
[29].
Tensile storage and loss modulus E’, E”
The variations of tensile dynamic mechanical properties (storage modulus E’ and loss modulus E”) of neat HDPE and its HA nanocomposites with frequency are presented in Figure
7. The measurements were carried out at frequencies ranging from 0.01 to 100 Hz at room temperature (25°C). The results showed a strong dependence of the tested material’s viscoelastic behavior on the testing frequency (loading rate), where the storage and loss moduli increased significantly with increasing the testing frequency. These results confirmed that the viscoelastic behavior of HDPE is strain rate dependent. It is noticed that E‘ is more than E” for all types of HDPE specimens which indicated that the elastic behavior of the material is dominant over the viscous one. Additionally, the storage and loss moduli (E’, E”) of all nano-composites increased with increasing the HA nanoparticles. For example, at 1 Hz, the storage modulus increased from 1.16 to 1.55, 1.69 and 1.79 GPa due to the addition of 10, 20 and 30% HA nanoparticles compared to neat HDPE. For loss modulus, its value increased from 275 to 320, 332 and 403 MPa due to the addition of 10, 20 and 30% HA nanoparticles compared to neat HDPE at the same testing conditions. This increase in the moduli of nanocomposites can be attributed to the increase in the stiffness of polymeric matrix as a result of the decrease of free volume and the mobility restriction due to the presence of HA nanoparticles
[17–19].
The effects of gamma irradiation dose on the storage and loss moduli of neat HDPE and its nanocomposite as a function of testing frequency are shown in Figure
8. The results indicated that the storage and loss modulus significantly increased with increasing the irradiation dose. For example, at 1 Hz, the storage modulus increased from 1.16 to 1.31 and 1.36 GPa due to the irradiation with 35 and 70 kGy compared to non-irradiated neat HDPE. For loss modulus, its value increased from 275 to 300 and 350 MPa due to irradiation with 35 and 70 kGy compared to non-irradiated neat HDPE at the same testing conditions. The improvement in the viscoelastic behavior due to gamma irradiation can be attributed to the crosslinking of HDPE chains that occurred in the amorphous region resulting in creation of rigid regions HDPE matrix
[31].
Creep-recovery behavior
Figure
9 shows the creep-recovery response of neat HDPE and its nanocomposites at different loading conditions. The tests were carried out at stress levels of 2, 5, 10 MPa at a temperature of 25°C. The results showed an initial elastic deformation and subsequently, the strain slowly increased as long as the stress is maintained. After stress removal (at 8 hrs of loading), the elastic strain is quickly recovered, and the portion of the creep strain is recovered slowly in the course of time. The creep test results indicated that the creep strain for neat HDPE and its nanocomposite increases (or the creep resistance decreases) with increasing the applied stress as expected. Moreover, the creep resistance increased significantly with the addition of HA nanoparticles. The addition of 10%, 20% and 30% HA increased the creep resistance of HDPE nanocomposites by 25%, 41% and 67% respectively comparing to the neat HDPE. Conversely, the remaining residual strain decreased by 42%, 57% and 57% respectively. The improvement of creep resistance due to the addition of HA nanoparticles to HDPE matrix can be attributed to the improvement of polymeric matrix stiffness due to the reduction of free volume and the chains mobility restriction
[17–19]. Figure
10 shows the effects of gamma irradiation dose on the creep-recovery resistance of HDPE/HA (30%) as a function of time. The results indicated that the creep resistance increases due to irradiation and the remaining residual strain decreased. Irradiation improved the creep behavior of HDPE/HA nanocomposites due to chain crosslinking that occur in the amorphous region resulting in rigid regions in the polymer matrix.
Relaxation behavior
The relaxation test for neat HDPE and its nanocomposite specimens were carried out at predetermined strain level of 3%. The applied strain was held constant while the decaying in the stress with time was recorded for a period of 3 hrs. The tests were conducted under a constant temperature of 25°C. Figure
11 shows the stress relaxation graphs of the irradiated and non-irradiated neat HDPE and its 30% HA nanocomposite. The plots showed that, the relaxation stress levels of HDPE/HA nanocomposites and irradiated specimens are higher than that for neat HDPE specimens, over the whole test duration. Furthermore, the initial and remaining relaxation stresses also have higher values compared to non-irradiated neat HDPE specimens. The increase of the initial stress can be attributed to the changes in nanocomposite stiffness due to the presence of HA nanoparticles in the polymer matrix. With reference to Figure
11, it is clear that the relaxation stress value of irradiated HDPE/HA (30%) specimens after 3 hrs reduced to 37% of its initial value through the relaxation test, while it dropped to 24% of initial value for neat HDPE specimens. The value of stress reduction occurred in HDPE/HA specimens is higher than that on of neat HDPE, indicating that in the biomedical applications (bone substitutes and plates, hip joint cup etc), HDPE/HA nanocomposite would transfer a large amount of stress to the surrounding bone (reduce stress shielding). This phenomenon will lead to an improvement in the artificial joint performance
[33].