Impact of Proton Irradiation on Medium Density Polyethylene/Carbon Nanocomposites for Space Shielding Applications

The development of novel materials with improved radiation shielding capability is a fundamental step towards the optimization of passive radiation countermeasures. Polyethylene (PE) nanocomposites filled with carbon nanotubes (CNT) or graphene nanoplatelets (GNP) can be a good compromise for maintaining the radiation shielding properties of the hydrogen-rich polymer while endowing the material with multifunctional properties. In this work, nanocomposite materials based on medium-density polyethylene (MDPE) loaded with different amounts of multi-walled carbon nanotubes (MWCNT), GNPs, and hybrid MWCNT/GNP nanofillers were fabricated, and their properties were examined before and after proton exposure. The effects of irradiation were evaluated in terms of modifications in the chemical and physical structure, wettability, and surface morphology of the nanocomposites. The aim of this work was to define and compare the MDPE-based nanocomposite behavior under proton irradiation in order to establish the best system for applications as space shielding materials.


Introduction
Polyethylene (PE) is a widely researched material that finds application in different fields due to its low weight, low cost, and easy processability. Its high biocompatibility, good mechanical properties, and chemical resistance have made PE the material of choice for the commercial production of orthopedic prostheses and packaging [1,2]. In the space sector, PE and PE-based composites are used as a barrier against the hazardous space radiation environment [3,4]. In fact, it is widely recognized that materials composed of low atomic number atoms offer protection against radiation [5]. PE is composed of the ethylene monomer -[CH 2 -CH 2 ]-with high content of H atoms and is, therefore, the solid material with the most efficient radiation shielding properties [6]. However, PE is a dielectric polymer and does not possess enough strength and thermal stability to be considered as a structural material [5]. For this reason, carbon nanoparticles are widely investigated for the fabrication of novel PE nanocomposite materials with potentially enhanced mechanical and functional properties: high electrical conductivity for static charge dissipation, high thermal conductivity, radiation hardness, and mechanical integrity [5][6][7][8][9][10]. Many investigations have been reported on the multifunctional properties of nanocomposites based on Based on the electrical measurements, samples (5 cm × 5 cm × 0.5 cm) of MDPE/GNP at 5 wt%, 10 wt%, and 15 wt%, of MDPE/MWCNT at 5 wt%, and of MDPE/GNP3MWCNT1 were fabricated and subjected for an average time of 294 s to proton irradiation, at an energy of 64 MeV, current 1 nA, for a total dose of 50 Gy. The US Center for Diseases Control and Prevention on Acute Radiation Syndrome reports 50 Gy as the dose causing the fatal collapse of human cardiovascular and central nervous systems. The dose of 50 Gy is higher than the acceptable astronauts' exposure limits [39], and it was selected in this work to assess the shielding robustness of MDPE nanocomposites. The tests ( Figure  2) were conducted at the Crocker Nuclear Laboratory of the University of California, Davis (CA, USA). Based on the electrical measurements, samples (5 cm × 5 cm × 0.5 cm) of MDPE/GNP at 5 wt%, 10 wt%, and 15 wt%, of MDPE/MWCNT at 5 wt%, and of MDPE/GNP 3 MWCNT 1 were fabricated and subjected for an average time of 294 s to proton irradiation, at an energy of 64 MeV, current 1 nA, for a total dose of 50 Gy. The US Center for Diseases Control and Prevention on Acute Radiation Syndrome reports 50 Gy as the dose causing the fatal collapse of human cardiovascular and central nervous systems. The dose of 50 Gy is higher than the acceptable astronauts' exposure limits [39], and it was selected in this work to assess the shielding robustness of MDPE nanocomposites. The tests ( Figure 2) were conducted at the Crocker Nuclear Laboratory of the University of California (Davis, CA, USA). The Fourier transform infrared (FTIR) spectra of the nanocomposites before and after proton irradiation were studied using a Thermo-Scientific Nicolet Summit spectrometer equipped with an attenuated total reflection (ATR) accessory (ZnSe crystal). The ATR-FTIR spectra were acquired in the wavenumber range from 400 cm −1 to 4000 cm −1 , at a  The Fourier transform infrared (FTIR) spectra of the nanocomposites before and after proton irradiation were studied using a Thermo-Scientific Nicolet Summit spectrometer equipped with an attenuated total reflection (ATR) accessory (ZnSe crystal). The ATR-FTIR spectra were acquired in the wavenumber range from 400 cm −1 to 4000 cm −1 , at a resolution of 4 cm −1 and auto scanning speed of 2 mm/s, keeping the air as a reference. The equation proposed by Zerbi et al. [40] is used to determine the degree of crystallinity (X c ) from FTIR spectra: where I a and I b are the intensities of the bands at 730 cm −1 and 720 cm −1 , respectively. The constant 1.2333 corresponds to the relation of the intensities of these bands in the fully crystalline polyethylene spectrum [41].
Thermal analysis was performed using a double-furnace differential scanning calorimeter (DSC 8500, PerkinElmer, Waltham, MA, USA). Samples were sealed in aluminum pans with lids and measured in the temperature range from −45 • C to 150 • C with heating and cooling rates of 10 • C/min under nitrogen flow (20 cc/min). The degree of crystallinity was evaluated from the melting enthalpies (∆H m ) determined as the area under the melting peak in the DSC thermograms, using the following equation: where ∆H f is the latent heat of fusion of 100% crystalline polyethylene (288 J/g) [42] and w f is the weight fraction of the nanoparticles. The surface wettability of the nanocomposites was characterized by static contact angles (CA) measurements at the top surface using a DataPhysics OCA15Pro analyzer (DataPhysics Instruments, Filderstadt, Germany). The measurements were performed with the sessile drop method using degassed ultrapure water and diiodomethane as testing liquids. The determination of the contact angle values was performed according to the Young-Laplace fitting method using the DataPhysics SCA20 image analysis software. The values of the surface free energy (SFE) were determined with the Owens-Wendt method [43]: where γ s is the SFE of the solid that is analyzed, γ l is the SFE of the measuring liquid, the apexes d and p indicate the dispersive and polar components, respectively, and θ is the contact angle between the solid and the testing liquid. The reported values of the CA and SFE are the average of ten measurements. The morphology of the nanocomposites before and after proton irradiation was investigated using a VEGA II LSH scanning electron microscope (TESCAN, Brno, Czech Republic) with an accelerating voltage of 5 kV and a magnification of 500×. SEM images were acquired before and after proton exposure. The MountainsMap 7 software (Digital Surf, Besançon, France) was used to perform a 3D reconstruction of the specimen surface and the evaluation of surface roughness (R a ) from images acquired at different tilt angles (0 • and 1 • ) [44]. The R a was averaged over values determined on profiles extracted every 0.1 mm across the reconstructed 3D surface, typically 30 profiles for a surface area of 300 µm × 300 µm.

Electrical Properties
The volumetric electrical conductivity of MDPE/GNP at 2.5 wt%, 5 wt%, 10 wt%, and 15 wt%, of MDPE/MWCNT at 2.5 wt% and 5 wt%, and of MDPE/GNP 3 MWCNT 1 Nanomaterials 2023, 13,1288 5 of 17 at 20 wt% is given in Table 1. First, a significant enhancement of the electrical properties was observed, with the loading fraction increasing from 2.5 wt% to 5 wt% for both the MDPE/GNP and MDPE/MWCNT systems. Overall, the MDPE/MWCNT system shows higher electrical conductivities than the MDPE/GNP system at all mass concentrations due to the presence of the MWCNTs with their high aspect ratio. In addition, the difference between the electrical conductivity of the MDPE/GNP at 5 wt%, 10 wt%, and 15 wt% is negligible, meaning that further increasing the nanofiller concentration has a minimal effect. As regards the MDPE/GNP 3 MWCNT 1 20 wt% nanocomposite, it shows the highest electrical conductivity among the investigated systems due to the synergistic effect of the GNPs and CNTs nanoparticles: the high aspect ratio of MWCNTs is responsible for the high electrical conductivity and for preventing the restacking of the GNPs, while the GNPs inhibit the aggregation of the MWCNTs, creating an interconnected hybrid architecture [45]. In order to explore the potential applications of carbon-based multifunctional nanocomposites with high electrical conductivity in space radiation environments and the effects of different nanofiller loadings on the radiation sensitivity of these materials, samples (5 cm × 5 cm × 5 cm) of MDPE/GNP at 5 wt%, 10 wt%, and 15 wt%, of MDPE/MWCNT at 5 wt%, and of MDPE/GNP 3 MWCNT 1 20 wt% were subjected to proton radiation and their properties were investigated in terms of chemical structure, thermal behavior, wettability, and morphology.

FTIR Analysis
Samples (5 cm × 5 cm × 5 cm) of MDPE/GNP at 5 wt%, 10 wt%, and 15 wt%, of MDPE/MWCNT at 5 wt%, and of MDPE/GNP 3 MWCNT 1 at 20 wt% were subjected to proton radiation and the chemical changes induced by the exposure were identified by ATR-FTIR, obtaining the spectra (raw data) reported in Figures S1 and S2. The main absorption peaks of polyethylene are given in Table 2, in agreement with the literature [46]. The addition of GNP and MWCNT nanofillers did not significantly modify the shape of the vibrational spectra of the MDPE matrix. However, two main effects induced by irradiation in atmosphere are observed: oxidative degradation and crystallinity changes. Figures 3 and 4 show ATR-FTIR spectra of the nanocomposites before and after proton irradiation in the 750-700 cm −1 region, where the doublet at 718-729 cm −1 was analyzed. Table 3 illustrates the X c of the nanocomposites before and after proton irradiation, cal-culated using the equation proposed by Zerbi et al. [40]. The intensities of the bands at 729 cm −1 and 718 cm −1 and their ratios are reported in Table S1.   First, a slight decrease in Xc is observed as the GNP content increases, which can be attributed to the tendency of GNP to hinder the molecular mobility of the polymer matrix at relatively high concentrations (above 3-5 wt%) [47,48], thus limiting the growth of polyethylene crystallites [48]. The MDPE/MWCNT 5 wt% samples show slightly higher Xc values than the MDPE/GNP 5 wt% nanocomposites since the GNP filler imposes more constraints around the polymer chains, inducing a greater fraction of polymer chains to be trapped in the graphene network [49]. All nanocomposites show a decrease in Xc after exposure to proton radiation; however, this effect is negligible for the MDPE/MWCNT 5 wt% sample. Furthermore, as the filler content increases, the relative percentage variations in Xc (indicated as ΔXc/Xc) decrease, confirming an increasing shielding effect with increasing filler content. Overall, the decrease of Xc after irradiation can be explained by branching and cross-linking phenomena, in accordance with the current literature [50][51][52][53]. In the case of irradiation of polymeric materials, macro radicals will be generated both in the amorphous and crystalline phases. These radicals can then react with molecular or atomic oxygen leading to the formation of ketones, aldehydes, alcohols, and carboxylic acids. The selected GNP nanoparticles have nitrogen and oxygen atoms attached to the graphene sheets that are likely cleaved when irradiated [54,55]. The generated free radicals can react with polyethylene, leading to the formation of cross-linked bonds in the side chains of the polymer matrix. In this way, the increase of short-chain branching density decreases the lamellar thickness of the crystal structure, consequently reducing the Xc of irradiated samples [53,56].  First, a slight decrease in X c is observed as the GNP content increases, which can be attributed to the tendency of GNP to hinder the molecular mobility of the polymer matrix at relatively high concentrations (above 3-5 wt%) [47,48], thus limiting the growth of polyethylene crystallites [48]. The MDPE/MWCNT 5 wt% samples show slightly higher X c values than the MDPE/GNP 5 wt% nanocomposites since the GNP filler imposes more constraints around the polymer chains, inducing a greater fraction of polymer chains to be trapped in the graphene network [49]. All nanocomposites show a decrease in X c after exposure to proton radiation; however, this effect is negligible for the MDPE/MWCNT Nanomaterials 2023, 13, 1288 8 of 17 5 wt% sample. Furthermore, as the filler content increases, the relative percentage variations in X c (indicated as ∆X c /X c ) decrease, confirming an increasing shielding effect with increasing filler content. Overall, the decrease of X c after irradiation can be explained by branching and cross-linking phenomena, in accordance with the current literature [50][51][52][53]. In the case of irradiation of polymeric materials, macro radicals will be generated both in the amorphous and crystalline phases. These radicals can then react with molecular or atomic oxygen leading to the formation of ketones, aldehydes, alcohols, and carboxylic acids. The selected GNP nanoparticles have nitrogen and oxygen atoms attached to the graphene sheets that are likely cleaved when irradiated [54,55]. The generated free radicals can react with polyethylene, leading to the formation of cross-linked bonds in the side chains of the polymer matrix. In this way, the increase of short-chain branching density decreases the lamellar thickness of the crystal structure, consequently reducing the X c of irradiated samples [53,56].

Thermal Analysis by Differential Scanning Calorimetry
The thermal behavior of the nanocomposites was analyzed by DSC before and after proton irradiation. Thermograms of heating and cooling of the samples are reported in Figures 5 and 6, respectively. Results from thermal analysis are summarized in Tables 4 and 5. As the samples were irradiated in a solid state, the first heating cycle was used to investigate changes in crystallinity induced by radiation, and the results were compared with those obtained by FTIR.

Thermal Analysis by Differential Scanning Calorimetry
The thermal behavior of the nanocomposites was analyzed by DSC before and after proton irradiation. Thermograms of heating and cooling of the samples are reported in Figures 5 and 6, respectively. Results from thermal analysis are summarized in Tables 4  and 5. As the samples were irradiated in a solid state, the first heating cycle was used to investigate changes in crystallinity induced by radiation, and the results were compared with those obtained by FTIR.     As shown in Table 4, only small differences in the values of the melting (T m ) and crystallization (T c ) temperatures are observed among the different nanocomposites and after the radiation process. Results regarding the degree of crystallinity calculated by DSC (Table 5) confirm the same trend showed by ATR-FTIR analysis. In fact, also, in this case, comparing the X c values before and after proton exposure, a decrease in crystallinity caused by the irradiation can be observed, and this is more evident in the presence of the GNP filler. Further, the presence of a shoulder in the nanocomposites containing GNP was observed ( Figure 5), and it can be ascribed to the melting of imperfect crystals. This phenomenon can be related to the more constraints imposed by the GNP filler on the polymer matrix [49], favoring the formation of imperfect crystalline lamellae. For the samples containing GNPs, the decrease of ∆H c upon irradiation confirms that the formation of cross-links and chain branches hinders the polymer chain's mobility and chain reorganization during the crystallization process, leading to the formation of imperfect and thinner lamellae [57]. As anticipated in the ATR-FTIR analysis, the values of X c and ∆H c of the MDPE/MWCNT 5 wt% nanocomposite are less affected by proton irradiation. Although the trend in the degree of crystallinity is the same, X c values determined by FTIR are higher for all samples. This has already been reported in the literature and can be mainly ascribed to some limitations of the DSC technique. In fact, the crystallinity of the polymer is temperature dependent, and the estimated crystallinity determined by DSC at the melting temperature will differ from the value at ambient temperature [48,58]. Moreover, the differential nature and overlap of multiple thermal events (chain relaxation, melting of different crystal forms) can affect the values of X c determined by DSC. Nevertheless, the DSC technique allows for detecting bulk features, whereas ATR-FTIR mainly reveals the surface characteristics of the material. Overall, these two techniques should be considered complementary to better understand the behavior of the nanocomposites. In fact, it is possible that irradiation induced chain-branching and cross-linking on top of the irradiated surface, while chain scission is predominant in the bulk of the sample.

Contact Angle Measurements
The wetting behavior before and after proton irradiation was investigated by static contact angle measurements in sessile drop configuration. The water contact angle (WCA) and SFE values are given in Table 6, showing an alteration of the WCA and SFE of irradiated samples compared with the non-irradiated ones. The analysis revealed the hydrophobicity of the non-irradiated nanocomposites, which are characterized by WCA values above 105 • . The WCA strongly increases with nanofiller content, reaching 115.7 • ± 2.5 • for the hybrid nanocomposite. After proton exposure, the WCA of all nanocomposites decreased, with the largest variation in surface wettability observed in the MDPE/MWCNT 5 wt% nanocomposite (11.1%). The surface hydrophobicity of the PE/GNP nanocomposites was reduced in proportion to the GNP content, with the smallest WCA decrease (3.2%) for the MDPE/GNP 5 wt% surface and the largest (8.4%) for the MDPE/GNP 15 wt% surface. It is known that the WCA of a polymer surface depends on chemical functional groups and asperities: it decreases with increasing surface energy and surface smoothness [59]. Hence, to further investigate the decrease of the WCA values occurring upon irradiation, a surfacefree energy analysis following the Owens-Wendt method [43] was carried out with two different testing liquids (water and diiodomethane). As shown in Table 6, the SFE decreases at increasing nanofiller content in non-irradiated samples. The dispersive component (γ d ), which is due to the dispersive interactions among non-polar molecules, is predominant over the polar one (γ p ) for all investigated samples. After proton exposure, the SFE and its dispersive component show a marked increase in the nanocomposite samples. On the contrary, the polar component decreases in GNP-loaded nanocomposites, and increases in the MDPE/MWCNT 5 wt% nanocomposite. The decrease of surface hydrophobicity upon irradiation is in agreement with reports in the literature [21,22,60]. It can be ascribed to an oxidation phenomenon in the MDPE/MWCNT 5 wt% nanocomposite (as revealed by the increase of γ p ) and to the formation of nonpolar chain branches in GNP-loaded nanocomposites (as revealed by the increase of γ d ). In fact, it was demonstrated that the oxygen content in proton-irradiated MWCNTs is higher than in non-irradiated ones [61]. This behavior has not been observed for the MDPE/GNP 3 MWCNT 1 20 wt% samples, and this can be explained by the predominant presence of GNP (GNP/MWCNT ratio of 3:1), which may plausibly mitigate this phenomenon. In addition, the morphological analysis of the nanocomposite surface reported below shows the improved surface smoothness of irradiated samples, which also contributes to the decrease of the WCA [59]. Table 6. Water contact angles (WCA) and surface free energies (SFE) with dispersive (γ d ) and polar (γ p ) components of MDPE/GNP 5 wt%, MDPE/GNP 10 wt%, MDPE/GNP 15 wt%, PE/MWCNT 5 wt% and MDPE/GNP 3 MWCNT 1 20 wt% before and after proton irradiation.

Morphological Analysis
The surface morphology of the nanocomposites before and after proton irradiation was investigated by SEM (Figures 7 and 8). The images reveal the erosive effect of radiation that is more evident at high nanofiller loadings, with the surfaces of the nanocomposites appearing smoother after exposure.
Due to the formation of entanglements at high nanofiller contents, we find an increasing number of surface asperities with increasing filler concentrations. In particular, we observe a high number of small peaks on the surface of MDPE/GNP specimens. By contrast, the surface morphology of the MDPE/MWCNT 5 wt% and MDPE/GNP 3 MWCNT 1 20 wt% samples is characterized by the presence of large peaks. The reason is attributed to the large aggregates formed due to the weak interaction of the pristine MWCNTs with the MDPE matrix and to the van der Waals forces acting among the MWCNT nanoparticles. A 3D reconstruction of the surface profiles was performed to quantify the effect of the radiation exposure at the top surface of the nanocomposite samples. 3D images were obtained with the Mountains Map software, starting from two SEM images acquired at two different tilt angles (0 • and 1 • ). Results of the 3D surface reconstruction are reported in Figures 9 and 10.
A decrease in the height of surface peaks in irradiated nanocomposites is clearly visible on the color-scaled SEM images, which corresponds to an increased surface smoothness.  Due to the formation of entanglements at high nanofiller contents, we find an increasing number of surface asperities with increasing filler concentrations. In particular, we observe a high number of small peaks on the surface of MDPE/GNP specimens. By contrast, the surface morphology of the MDPE/MWCNT 5 wt% and MDPE/GNP3MWCNT1 20 wt% samples is characterized by the presence of large peaks. The reason is attributed to the large aggregates formed due to the weak interaction of the     Table 7 presents the R a values of the nanocomposites before and after proton irradiation. Despite the different nanofiller concentrations, the MDPE/GNP 3 MWCNT 1 20 wt% and MDPE/GNP 15 wt% non-irradiated nanocomposites show similar values of the R a . This result is explained by the presence of both GNP and MWCNT nanofillers in the hybrid system, where the graphene nanoplatelets prevent the aggregation of nanotubes by physically hindering the process due to their large surface area [45]. All nanocomposites show a significant decrease of R a upon exposure to proton radiation, with the largest variation observed in the MDPE/GNP 15 wt% (76.5%). This result confirms the cleavage effect of irradiation on the edges of the GNPs and subsequent surface erosion.

Conclusions
In this work, the sensitivity to proton radiation of nanocomposites made of mediumdensity polyethylene (MDPE) loaded with different amounts of GNP and MWCNT nanofillers was investigated. Different techniques were used to study the nanocompositeʹs response in terms of chemical structure, thermal behavior, wettability, and morphology. Results regarding the degree of crystallinity evaluated by FTIR and DSC unveiled a higher decrease of crystallinity after proton irradiation in the nanocomposites filled with GNP due to the branching and cross-linking mechanisms induced by the radiation. The MDPE/MWCNT 5 wt% nanocomposite showed the highest degree of crystallinity, with unnoteworthy changes after irradiation. The DSC analysis showed the thermal stability of the investigated nanocomposites in response to radiation, with Tm and Tc that remained unaltered. Despite a decrease in the WCA values of all nanocomposites after proton exposure, all samples maintained a hydrophobic surface and, therefore, a low tendency to adsorb water vapor from the environment. Lastly, a 3D reconstruction of the surface profiles was performed to quantify the effect of the radiation exposure at the top surface of the nanocomposite samples, revealing an increased surface smoothness after irradiation. Nevertheless, this effect is less marked in the MDPE/GNP 5 wt% and MDPE/MWCNT 5 wt% nanocomposites.
Overall, the results of this work show that the MDPE/MWCNT 5 wt% material is the best MDPE/nanocarbon system for use in radiation shielding applications due to the negligible changes observed in the physico-chemical properties after proton irradiation.

Conclusions
In this work, the sensitivity to proton radiation of nanocomposites made of mediumdensity polyethylene (MDPE) loaded with different amounts of GNP and MWCNT nanofillers was investigated. Different techniques were used to study the nanocomposite's response in terms of chemical structure, thermal behavior, wettability, and morphology. Results regarding the degree of crystallinity evaluated by FTIR and DSC unveiled a higher decrease of crystallinity after proton irradiation in the nanocomposites filled with GNP due to the branching and cross-linking mechanisms induced by the radiation. The MDPE/MWCNT 5 wt% nanocomposite showed the highest degree of crystallinity, with unnoteworthy changes after irradiation. The DSC analysis showed the thermal stability of the investigated nanocomposites in response to radiation, with T m and T c that remained unaltered. Despite a decrease in the WCA values of all nanocomposites after proton exposure, all samples maintained a hydrophobic surface and, therefore, a low tendency to adsorb water vapor from the environment. Lastly, a 3D reconstruction of the surface profiles was performed to quantify the effect of the radiation exposure at the top surface of the nanocomposite samples, revealing an increased surface smoothness after irradiation. Nevertheless, this effect is less marked in the MDPE/GNP 5 wt% and MDPE/MWCNT 5 wt% nanocomposites.
Overall, the results of this work show that the MDPE/MWCNT 5 wt% material is the best MDPE/nanocarbon system for use in radiation shielding applications due to the negligible changes observed in the physico-chemical properties after proton irradiation.