Ultra-High Molecular Weight Polyethylene Modifications Produced by Carbon Nanotubes and Fe2O3 Nanoparticles

Thin sheets of ultra-high molecular weight polyethylene (UHMWPE), both in pristine form and containing carbon nanotubes (CNTs) or Fe2O3 nanoparticles (NPs) at different concentrations, were prepared. The CNT and Fe2O3 NP weight percentages used ranged from 0.01% to 1%. The presence of CNTs and Fe2O3 NPs in UHMWPE was confirmed by transmission and scanning electron microscopy and by energy dispersive X-ray spectroscopy analysis (EDS). The effects of the embedded nanostructures on the UHMWPE samples were studied using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and UV–Vis absorption spectroscopy. The ATR-FTIR spectra show the characteristic features of the UHMWPE, CNTs, and Fe2O3. Concerning the optical properties, regardless of the type of embedded nanostructures, an increase in the optical absorption was observed. The allowed direct optical energy gap value was determined from the optical absorption spectra: in both cases, it decreases with increasing CNT or Fe2O3 NP concentrations. The obtained results will be presented and discussed.


Introduction
Ultra-high molecular weight polyethylene (UHMWPE) is a biocompatible polymer which, thanks to its interesting properties, can be employed as a high-performance material in several fields, such as biomedicine [1], engineering [2], microelectronics [3], chemistry, and physics [4][5][6][7]. In fact, it has good chemical and physical stability, mechanical resistance, a low friction coefficient, low wear volume, high crystallinity, high ultimate tensile strength, high biocompatibility [8], and high thermal conductivity [9]. Polymers such as UHMWPE have applications in microelectronics, e.g., as insulators in many electronics components, and in biomedicine, e.g., to realize organ-on-chip microsystems, where cells growing over polymeric membranes are used in simulations of biological barriers and transport processes [10], or as prostheses, e.g., as the femoral head and its acetabular cup [11]. With UHMWPE, it is also possible to create fixed and mobile joints for bones, substrates for cell cultures [12], thin foils to reduce wear during surface friction [13], materials resistant to water and saline solutions [14], lightweight high-strength biocompatible and water equivalent materials useful for radiotherapy research [15], or, by incorporating high hydrogen and deuterium content, it can be used to prepare peculiar samples useful for nuclear research [16]. Such applications can be improved by enhancing the properties of UHMWPE, for example by increasing its chemical and physical stability, its mechanical resistance, its wear resistance, and its biocompatibility. This because the properties of UHMWPE, such as

Materials and Methods
UHMWPE resin Ticona-GUR 1020 (ρ = 0.93 g/cm 3 , Mw ≈ 3 × 10 6 g/mol) was employed as a polymer. The matrix was filled with the powder of multi-walled carbon nanotubes (from Sigma Aldrich, Darmstadt, DE, Europe [38]), having diameters of 6-13 nm and lengths of 2.5-20 µm, or with iron oxide nanoparticles (from Sigma Aldrich, Darmstad, DE, Europe), having sizes in the order of~50 nm [39], both with a purity level of 99.999%. Nanocomposites were made by mixing the UHMWPE with different weight percentages (from 0.01% to 1%) of the CNTs (named, for simplicity, as "UHMWPE & CNT"), employing 99.8% pure ethanol (from Fluka Chemical Corporation, Ronkonkoma, NY, USA) as a dispersing means. More details about the preparation can be found in [23,31]. The UHMWPE & Fe 2 O 3 nanocomposites were also prepared by mixing the UHMWPE with different weight percentages of Fe 2 O 3 NPs ranging from 0.01 to 1 weight %, with final density values within the (0.935-1.018) g/cm 3 range. Both of the mixtures were kept in an ultrasound bath at room temperature (25 • C/2 h). The solvent was then separated under stirring in a heated plate and molded in a hot press at 200 • C for 20 min (at a pressure of about 20 MPa), obtaining sheets of about 50 mm × 50 mm in surface and 100 µm in thickness by releasing Teflon substrates. The pristine UHMWPE sheets had a semitransparent appearance, the UHMWPE & CNT sheets were black, while the UHMWPE & Fe 2 O 3 sheets were red, as is shown in Figure 1, in which a set of photos of some of the prepared UHMWPE samples with different CNT (a) and Fe 2 O 3 NP (b) concentrations is shown. The thicknesses of the prepared samples were measured using a micrometer. Their structural properties were evaluated using a Fourier transform infrared (FTIR) spectrometer (Jasco Mod. 4600, Jasco-Europe, Lecco, IT, Europe) equipped with an ATR (attenuated total reflectance) accessory in the (400-4000) cm −1 wavenumber range. Their optical properties were also investigated using a double-beam UV-Vis-nIR spectrophotometer (Jasco Mod. V-750, Jasco-Europe, Lecco, IT, Europe) in the (300-900) nm wavelength range. The obtained characterizations were compared with those of the pure UHMWPE. New York, USA) as a dispersing means. More details about the preparation can be found in [23,31] Figure 1, in which a set of photos of some of the prepared UHMWPE samples with different CNT (a) and Fe2O3 NP (b) concentrations is shown. The thicknesses of the prepared samples were measured using a micrometer. Their structural properties were evaluated using a Fourier transform infrared (FTIR) spectrometer (Jasco Mod. 4600, Jasco-Europe, Lecco, IT, Europe) equipped with an ATR (attenuated total reflectance) accessory in the (400-4000) cm −1 wavenumber range. Their optical properties were also investigated using a double-beam UV-Vis-nIR spectrophotometer (Jasco Mod. V-750, Jasco-Europe, Lecco, IT, Europe) in the (300-900) nm wavelength range. The obtained characterizations were compared with those of the pure UHMWPE. The surface morphology, size, shape, and dispersion level of the nanostructures embedded in the UHMWPE were microscopically investigated. A scanning electron microscope (SEM, Abingdon, Oxfordshire, UK) by Oxford instruments equipped with a TVIPS camera, operating in the secondary-electron mode at an acceleration voltage of 5 kV on areas of 25 μm × 25 μm, was employed. To obtain a clearer image of the structures The surface morphology, size, shape, and dispersion level of the nanostructures embedded in the UHMWPE were microscopically investigated. A scanning electron microscope (SEM, Abingdon, Oxfordshire, UK) by Oxford instruments equipped with a TVIPS camera, operating in the secondary-electron mode at an acceleration voltage of 5 kV on areas of 25 µm × 25 µm, was employed. To obtain a clearer image of the structures embedded in the UHMWPE matrix, further transmission electron microscopy (TEM) and scanning TEM-coupled energy dispersive X-ray spectroscopy (STEM-EDS) analyses were carried out. Specifically, TEM micrographs of the samples were obtained using a Jeol JEM-2200FS (Jeol, Akishima, Japan) field emission electron microscope equipped with an in-column energy filter. To enhance the image contrast and assist in the localization of elements of interest in the sample, the three windows method at a 250 eV, 270 eV, 294 eV energy shift and a 20 eV energy slit was adopted [40]. For the STEM-EDS analysis, a small volume of the active material suspended in ethanol was drop-cast onto a lacey carbon Cu grid. The Cu signal obtained in the EDS spectra due to the X-rays emitted by the Cu grid hit by backscattered electrons was not reported.

ATR-FTIR Spectroscopy
The attenuated total reflectance (ATR) accessory coupled to the FTIR spectrometer (Jasco-Europe, Lecco, IT, Europe), under normal atmospheric conditions, without vacuum, facilitated the evaluation of the UHMWPE IR transmittance vs. the wavenumber, as is shown in the spectrum of Figure 2, which is characterized by two sharp absorption peaks at about 2908 cm −1 and 2846 cm −1 due, respectively, to the C-H asymmetric and symmetric stretching vibrations in the -CH 2 -group. The spectrum also displays two smaller absorption peaks at 1454 cm −1 and 718 cm −1 . The peak at 1454 cm −1 can be attributed, in agreement with the literature [41], to the C-H deformation vibrations in -(CH 2 ) n -, while the peak at 718 cm −1 is due to the C-C rocking vibrations in -(CH 2 ) n -. Figure 2 also shows that a weak broad band appears between 3600 cm −1 and 3250 cm −1 ; this structure can be attributed to the O-H stretching vibrations. embedded in the UHMWPE matrix, further transmission electron microscopy (TEM) and scanning TEM-coupled energy dispersive X-ray spectroscopy (STEM-EDS) analyses were carried out. Specifically, TEM micrographs of the samples were obtained using a Jeol JEM-2200FS (Jeol, Akishima, Japan) field emission electron microscope equipped with an in-column energy filter. To enhance the image contrast and assist in the localization of elements of interest in the sample, the three windows method at a 250 eV, 270 eV, 294 eV energy shift and a 20 eV energy slit was adopted [40]. For the STEM-EDS analysis, a small volume of the active material suspended in ethanol was drop-cast onto a lacey carbon Cu grid. The Cu signal obtained in the EDS spectra due to the X-rays emitted by the Cu grid hit by backscattered electrons was not reported.

ATR-FTIR Spectroscopy
The attenuated total reflectance (ATR) accessory coupled to the FTIR spectrometer (Jasco-Europe, Lecco, IT, Europe), under normal atmospheric conditions, without vacuum, facilitated the evaluation of the UHMWPE IR transmittance vs. the wavenumber, as is shown in the spectrum of Figure 2, which is characterized by two sharp absorption peaks at about 2908 cm −1 and 2846 cm −1 due, respectively, to the C-H asymmetric and symmetric stretching vibrations in the -CH2-group. The spectrum also displays two smaller absorption peaks at 1454 cm −1 and 718 cm −1 . The peak at 1454 cm −1 can be attributed, in agreement with the literature [41], to the C-H deformation vibrations in -(CH2)n-, while the peak at 718 cm −1 is due to the C-C rocking vibrations in -(CH2)n-.   Figure 3 shows the ATR-FTIR spectra comparison between the pristine UHMWPE and the UHMWPE containing CNTs at weight concentrations of 0.01%, 0.1%, and 1.0%. Small differences in the spectra exist due to the different concentrations of embedded CNTs, but the characteristic vibrational peaks of the UHMWPE remain fairly constant. At higher wavenumbers, it is possible to observe that the transmittance increases with the CNT concentration (see up arrow). For instance, at 3500 cm −1 , the transmittance is 85%, 90%, 91%, and 92.5% for CNT concentrations of 0%, 0.01%, 0.1%, and 1%, respectively.  Figure 3 shows the ATR-FTIR spectra comparison between the pristine UHMWPE and the UHMWPE containing CNTs at weight concentrations of 0.01%, 0.1%, and 1.0%. Small differences in the spectra exist due to the different concentrations of embedded CNTs, but the characteristic vibrational peaks of the UHMWPE remain fairly constant. At higher wavenumbers, it is possible to observe that the transmittance increases with the CNT concentration (see up arrow). For instance, at 3500 cm −1 , the transmittance is 85%, 90%, 91%, and 92.5% for CNT concentrations of 0%, 0.01%, 0.1%, and 1%, respectively. Moreover, a detailed analysis of the main characteristic peaks of the UHMWPE shows that the transmittance peak-to-background ratio increases with the CNT concentration. In fact, the plot of the main peak-to-background at 2913 cm −1 vs. the CNT concentration increases, as is shown in Figure 4a. Thus, the embedded CNTs do not substantially modify the main UHMWPE vibrational peaks and make the polymer more transparent in the IR region. The band between 3600 cm −1 and 3250 cm −1 , attributed to the O-H vibrations, decreases in intensity with the CNT concentration, indicating a higher transmittance in the that the transmittance peak-to-background ratio increases with the CNT concentration. In fact, the plot of the main peak-to-background at 2913 cm −1 vs. the CNT concentration increases, as is shown in Figure 4a. Thus, the embedded CNTs do not substantially modify the main UHMWPE vibrational peaks and make the polymer more transparent in the IR region. The band between 3600 cm −1 and 3250 cm −1 , attributed to the O-H vibrations, decreases in intensity with the CNT concentration, indicating a higher transmittance in the polymer containing the nanotubes, according to the plot shown in Figure 4b, relative to the transmittance values at 3380 cm −1 .   [42], are indistinguishable from the background in our spectra. Thus, the transmission peaks caused by the C-H stretching fact, the plot of the main peak-to-background at 2913 cm −1 vs. the CNT concentration increases, as is shown in Figure 4a. Thus, the embedded CNTs do not substantially modify the main UHMWPE vibrational peaks and make the polymer more transparent in the IR region. The band between 3600 cm −1 and 3250 cm −1 , attributed to the O-H vibrations, decreases in intensity with the CNT concentration, indicating a higher transmittance in the polymer containing the nanotubes, according to the plot shown in Figure 4b, relative to the transmittance values at 3380 cm −1 .   [42], are indistinguishable from the background in our spectra. Thus, the transmission peaks caused by the C-H stretching  [42], are indistinguishable from the background in our spectra. Thus, the transmission peaks caused by the C-H stretching vibrations in the CH 2 group remain in their wavenumber positions, but their peak-to-background intensity increases with the concentration of CNTs embedded in the UHMWPE. Moreover, even if we do not reveal any of the CNTs' characteristic IR features, probably due to their diameters being too large or their concentrations being too low, as indicated in the literature [43,44], we observe a decrease in IR absorption in the polymer nanocomposites at these wave numbers as the CNT concentration increases. Figure 5 shows the ATR-FTIR spectra comparison between the pristine UHMWPE and the UHMWPE containing Fe 2 O 3 at weight concentrations of 0.01%, 0.1%, and 1.0%. Besides the characteristic peaks of the UHMWPE, one can observe below 700 cm −1 (in the so-called fingerprint region) some features around 600 cm −1 that can be attributed, in agreement polymer nanocomposites at these wave numbers as the CNT concentration increases. Figure 5 shows the ATR-FTIR spectra comparison between the pristine UHMWPE and the UHMWPE containing Fe2O3 at weight concentrations of 0.01%, 0.1%, and 1.0%. Besides the characteristic peaks of the UHMWPE, one can observe below 700 cm −1 (in the so-called fingerprint region) some features around 600 cm −1 that can be attributed, in agreement with the literature [45], to the Fe-O stretching mode. These features become more and more pronounced as the Fe2O3 nanoparticle concentration increases (down arrow on the right). The IR transmission is strongly reduced with the presence of embedded Fe2O3 nanoparticles. The band associated with the O-H vibration around 3430 cm −1 increases in intensity with the concentration of embedded NPs (down arrow on the left).  Figure  6a shows the transmittance value of the large band at about 3430 cm −1 as a function of the Fe2O3 weight concentration, indicating a significant transmission decrease with the Fe2O3 NP concentration. Figure 6b shows the decrease in the transmittance at the large peak located at about 610 cm −1 with the Fe2O3 NP weight concentration. Both plots indicate that the IR absorption increases with the presence of the Fe2O3 NPs in the UHMWPE.  Figure 7 shows the UV-Vis absorption spectra of the UHMWPE with embedded CNTs at different weight concentrations. Unlike the IR radiation, the absorption in the near UV and visible regions increases with the CNT concentration, a finding which is in agreement with the literature, and with the color change in the samples, which become more and more black and absorbent [46]. The absorbance increment depends on the CNT  Figure 7 shows the UV-Vis absorption spectra of the UHMWPE with embedded CNTs at different weight concentrations. Unlike the IR radiation, the absorption in the near UV and visible regions increases with the CNT concentration, a finding which is in agreement with the literature, and with the color change in the samples, which become more and more black and absorbent [46]. The absorbance increment depends on the CNT concentration and light wavelength. A comparison of the absorbance at 400 nm and 700 nm vs. CNT concentration is presented in Figure 8.  Because the optical absorption analysis gives information about the band structure of a given material, it is possible to calculate the UHMWPE optical energy gap (Eg) from its optical absorption spectra using the following Mott and Davis relation [47]:

UV-Vis Spectroscopy
where α and hν are the absorption coefficient and incident photon energy, respectively, B is a constant, Eg is the value of the optical energy gap between the valence band and the conduction band, and the exponent n is an empirical index that characterizes the involved transition type. In fact, the n exponent takes the value of 1/2, 2, 3/2, and 3 for al- Because the optical absorption analysis gives information about the band structure of a given material, it is possible to calculate the UHMWPE optical energy gap (Eg) from its optical absorption spectra using the following Mott and Davis relation [47]: where α and hν are the absorption coefficient and incident photon energy, respectively, B is a constant, Eg is the value of the optical energy gap between the valence band and the conduction band, and the exponent n is an empirical index that characterizes the involved transition type. In fact, the n exponent takes the value of 1/2, 2, 3/2, and 3 for allowed direct, allowed indirect, forbidden direct, and forbidden indirect transitions, respectively. For the determination of the direct optical Eg, (αE) 2 has been plotted as a function of photon energy E. The plots for the pristine UHMWPE sample and for those containing CNTs at different concentrations are shown in Figure 9a-d respectively. The α coefficient has been calculated in terms of the Beer-Lambert law as follows: where d is the thickness of the sample and A is its optical absorbance.  to about 2.85 eV (for the sample containing 1 wt% CNTs). This decrease in the optical energy gap values may be attributed to the presence of new electronic levels due to the embedded nanostructures within the energy band gap. In fact, it is well known that carbon atoms, carbon aggregates, and carbon clusters are supposed to be rich with charge carriers that enhance electrical conductivity and, consequently, also influence the optical properties of such materials [49]. The decrease in the optical energy gap implies an increase in the electrical conductivity of the UHMWPE polymer containing the CNTs. The value of the optical energy gap Eg has been calculated taking into account the linear portion of the fundamental absorption edge of the UV-Vis spectra plotted in Figure 9, extrapolating it and finally determining the intersection of the extrapolated line with the photon energy axis. The obtained optical energy gap Eg values are indicated in Figure 9.
As is shown in Figure 9a, the pristine UHMWPE exhibits an optical band gap of about 3.30 eV, which is in agreement with the literature [48], while the UHMWPEs containing CNTs at different concentrations have optical energy gaps that decrease with the increasing CNT concentrations, ranging from 3.19 eV for 0.01% to 2.85 eV for 1%. The four plots of Figure 9 indicate the R 2 coefficient values relative to the best fit of the linear portion of the (αE) 2 vs. E curve used for the optical energy gap evaluation. Figure 10 depicts the deduced decreasing trend of the optical energy gap Eg vs. the CNT concentration in the UHMWPE. It decreases from 3.30 eV (for the pristine sample) to about 2.85 eV (for the sample containing 1 wt% CNTs). This decrease in the optical energy gap values may be attributed to the presence of new electronic levels due to the embedded nanostructures within the energy band gap. In fact, it is well known that carbon atoms, carbon aggregates, and carbon clusters are supposed to be rich with charge carriers that enhance electrical conductivity and, consequently, also influence the optical properties of such materials [49]. The decrease in the optical energy gap implies an increase in the electrical conductivity of the UHMWPE polymer containing the CNTs. Figure 10 depicts the deduced decreasing trend of the optical energy gap Eg vs. the CNT concentration in the UHMWPE. It decreases from 3.30 eV (for the pristine sample) to about 2.85 eV (for the sample containing 1 wt% CNTs). This decrease in the optical energy gap values may be attributed to the presence of new electronic levels due to the embedded nanostructures within the energy band gap. In fact, it is well known that carbon atoms, carbon aggregates, and carbon clusters are supposed to be rich with charge carriers that enhance electrical conductivity and, consequently, also influence the optical properties of such materials [49]. The decrease in the optical energy gap implies an increase in the electrical conductivity of the UHMWPE polymer containing the CNTs.  Figure 11 shows the UV-Vis optical absorption spectra of the UHMWPE with embedded Fe 2 O 3 NPs at different concentrations. In this case, the optical absorbance increases with the Fe 2 O 3 NP concentration. As with the UHMWPE reinforced with CNTs, the direct optical energy gap Eg value for the UHMWPE containing Fe 2 O 3 NPs at different concentrations has been deduced from the (αE) 2 vs. E curves ( Figure 12).    Even the UHMWPE containing Fe 2 O 3 NPs at different concentrations has an optical energy gap that decreases with increasing Fe 2 O 3 NP concentrations ranging from 3.25 eV for 0.01% to 2.70 eV for 1.0%. The weak broadband absorption that already appears at a concentration of 0.1% in the region between 2.0 eV and 2.5 eV (indicated with an arrow in Figure 12c,d) can be assigned, according to the literature [50], to quasi-spherical and/or polyhedral-shaped Fe 2 O 3 nanoparticles with an average diameter of 75 nm, and could arise from the Laporte forbidden d-d transitions of Fe 3+ cation in the octahedral coordination site [50]. The four plots of Figure 12 indicate the R 2 coefficient values relative to the best fit of the linear portion of the (αE) 2 vs. E curve used for the optical energy gap evaluation.
The morphological examination of the CNT and Fe 2 O 3 structures embedded in the UHMWPE polymeric matrix was performed using SEM. Images of the UHMWPE/CNT and UHMWPE/Fe 2 O 3 nanocomposites with magnifications of 11.1 kX and with a view field of 25 µm are shown in Figure 13a,b, respectively. In both cases, the images reveal small aggregates of nanoparticles on their surfaces whose average size is about 100 nm or less.
In the TEM micrograph (Figure 14a), several CNTs tangled in the UHMWPE polymeric matrix have been indicated by red arrows, and a more evident big agglomerate of CNTs with a diameter of about 80 nm has been marked with a dashed red circle. The appearance of the tangled CNTs suggests their sub-optimal dispersion. Typically, a state of complete dispersion is achieved by optimizing the dispersion protocol, especially during the breakdown step, which is necessary to avoid the entangling of CNTs.

uation.
The morphological examination of the CNT and Fe2O3 structures embedded in the UHMWPE polymeric matrix was performed using SEM. Images of the UHMWPE/CNT and UHMWPE/Fe2O3 nanocomposites with magnifications of 11.1 kX and with a view field of 25 μm are shown in Figure 13a,b, respectively. In both cases, the images reveal small aggregates of nanoparticles on their surfaces whose average size is about 100 nm or less. In the TEM micrograph (Figure 14a), several CNTs tangled in the UHMWPE polymeric matrix have been indicated by red arrows, and a more evident big agglomerate of CNTs with a diameter of about 80 nm has been marked with a dashed red circle. The appearance of the tangled CNTs suggests their sub-optimal dispersion. Typically, a state of complete dispersion is achieved by optimizing the dispersion protocol, especially during the breakdown step, which is necessary to avoid the entangling of CNTs. Differential scanning calorimetry (DSC) analysis of the pristine UHMWPE and the polymer containing 1 wt% CNTs and Fe2O3 NPs are in progress. The preliminary results confirm the data in the literature, i.e., that the UHMWPE melting temperature of about 138 °C increases to 139 °C and 140 °C with the inclusion of CNTs and Fe2O3 NPs, respectively [23,[51][52], indicating a slightly higher thermal stability in the obtained nanocomposites.

Challenges and Future Outlook
UHMWPEs modified by nanoparticles and nanostructures represent an important challenge for the realization of innovative polymeric materials with peculiar physical and chemical properties. UHMWPE is a biocompatible material with excellent insulating properties and chemical stability. It represents one of the lightest and most resistant materials on the market, with a high Young's modulus under different stresses, high wear resistance, and low permeability. It has high resistance to acids, alcohols, bases, esters, petrol, fats, and oils. UHMWPE is used in many fields, from biomedicine to microelec-

Challenges and Future Outlook
UHMWPEs modified by nanoparticles and nanostructures represent an important challenge for the realization of innovative polymeric materials with peculiar physical and chemical properties. UHMWPE is a biocompatible material with excellent insulating properties and chemical stability. It represents one of the lightest and most resistant materials on the market, with a high Young's modulus under different stresses, high wear resistance, and low permeability. It has high resistance to acids, alcohols, bases, esters, petrol, fats, and oils. UHMWPE is used in many fields, from biomedicine to microelectronics, and from engineering to chemistry.
The synthesis of nanocomposite materials using nanoparticles embedded into UHMWPE can enhance some of its properties and obtain not only more mechanically resistant, harder, and stronger polymers, but also polymers that absorb more light and ionizing radiations. The embedding of nanostructures in UHMWPE can improve the welding and the wetting properties of UHMWPE with other substances, modify its permeability, and modify the amount of carbon, oxygen, hydrogen, and other elements in its composition. Specifically, the UHMWPE+CNTs nanocomposite has been employed to improve the welding process with other polymers, while the UHMWPE+Fe 2 O 3 nanocomposite has been employed to enhance glass transition temperature, as has been reported in previous investigations.
In the future, it will be possible to modify only some parts of the polymer, realizing conductive tracks, printing electrical resistance and capacitance in the UHMWPE substrates. It will also be possible to embed microelectronic devices in UHMWPE and to make more efficient biomedical prostheses with greater durability and reliability, meeting the demand for prostheses to replace bone joints subjected to high pressure loading.

Conclusions
In this paper, we have studied the effects of reinforcing UHMWPE with CNTs and Fe 2 O 3 NPs on its structural and optical properties using ATR-FTIR and UV-vis spectroscopies. The morphological examinations of the CNTs and Fe 2 O 3 structures embedded in the UHMWPE using SEM revealed, in both cases, small aggregates of nanoparticles with average sizes of about 100 nm or less. The presence of the embedded nanostructures was also revealed through the ATR-FTIR spectra at the highest used concentration, and no significant vibrational change in the UHMWPE structure was observed. In the IR region, a decrease in the absorption was measured as the CNT concentration grew, while an increase in the absorption was measured using the Fe 2 O 3 NPs. For both embedded nanostructures, the allowed direct optical energy gap value Eg of the corresponding UHMWPE nanocomposite was calculated from the relative optical absorption spectrum.
Our study shows that, at the employed concentrations of CNTs or of Fe 2 O 3 NPs, and at temperatures during the nanocomposite preparation not higher than 200 • C, the embedded nanostructures do not significantly change the UHMWPE structure. Their main effect consists in introducing new electronic levels within the UHMWPE energy bandgap. This presence affects the UHMWPE's optical properties, reducing its optical energy gap as the CNT or of Fe 2 O 3 NP concentration increases.
The employment of UHMWPE as a composite polymer represents an open challenge due to the different problems that are to be overcome in its use. The dispersion of nanoparticle fillers represents a difficult challenge due to its high viscosity. By increasing the nanoparticle concentration, the surface/volume ratio can be increased, and the surface properties of the nanoparticles become a major factor in influencing its interfacial properties and its mechanical, optical, and electrical behavior. The results presented above can be used to employ the obtained UHMWPE nanocomposites in applications in which it is necessary to increase optical absorbance and electrical conductivity. In contrast, the possibility of varying the IR absorption by embedding CNTs or Fe 2 O 3 NPs may be useful in modifying the IR laser absorption in thin UHMWPE foils to control the laser melting effect and their welding to different interfaces. Finally, the reduction of the energy gap can be used to increase the electrical conductivity of the UHMWPE or to generate electron traps in the polymer band gap, thus modifying its insulator-like band structure.