Improvement of the resistance performance of carbon/cyanate ester composites during vacuum electron radiation by reduced graphene oxide modified TiO₂

Abstract

Electron irradiation in outer space causes severe damage to the polymer materials of spacecrafts. An effective approach to prevent such damage is to incorporate nanoparticles into the polymeric materials. Herein, we fabricated modified cyanate ester (CE) and carbon/CE composites by the incorporation of reduced graphene oxide–TiO₂ (rGO–TiO₂) nanoparticles and studied their resistance performance to electronic radiation. Compared with the carbon/TiO₂/CE composite, the interlayer shear strength of the resulting carbon/rGO–TiO₂/CE composite increased by 10.4% and its mass loss reduced by 16.5%. Scanning electron microcopy (SEM) images showed that there are more cracks at the fiber and resin interfaces of carbon/CE than at the interfaces of carbon/rGO–TiO₂/CE after irradiation. X-ray photoelectron spectroscopy (XPS) investigation showed that irradiation with 160 keV electrons could break the chemical bonds at the surface layer of the pristine CE resin, which is effectively prevented by the incorporation of rGO–TiO₂ nanoparticles.

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1. Introduction

Polymer materials have been applied widely in spacecrafts because of the advantages of high strength-to-weight ratios, near-zero coefficients of thermal expansion, chemical inertness, and thermo-optical characteristics.^1,2 However, polymeric materials are usually damaged by many space environmental threats like UV radiation, atomic oxygen (AO), thermal cycling (TC) and charged particle radiations such as electrons and protons.^3–10 Therefore, it is important to foresee these damages and protect polymeric matrix from severe working conditions.

Electron irradiation is one of the severe and inevitable space environmental factors, which is harmful to the durability of polymeric materials of spacecraft. The effects of electron irradiation on the polymer matrix composites have been reported. Gao et al.¹¹ investigated the effect of irradiation with <200 keV electrons on AG-80 epoxy resin. It was found that electron irradiation could lead to the accumulation of electric charges. In the case where the charges are accumulated to a given extent, electric discharging occurs, which could result in ablation on the resin surface. Yu et al.¹² studied the effects of electron irradiation on the surface of carbon/bismaleimide. The results indicated that electron irradiation in ultrahigh vacuum environment could change the surface molecular structure and chemical composition of the carbon/bismaleimide, resulting in damage effects such as mass loss and outgassing of the material skin, this may eventually result in the degradation of the material. Fig. 1(a) shows the interactions between the electron and the materials. The electron radiation is bremsstrahlung, in which the electrons interact with the spacecraft materials. Because the mass of electrons is very light, electrons are prone to deviate after colliding with atoms and their propagation path in the material is bent. Three pathways are possible: electrons are absorbed by the material, penetrated into the material, or are scattered backwards. Electrons interact with the spacecraft materials leading to ionization energy loss and radiation energy loss. As the bond energies of polymer molecules are considerably smaller than those of electrons, ionization and excitation of many molecules will occur to influence the performance of the materials.

Fig. 1 A schematic showing the electron irradiation of CE (a) and rGO–TiO₂/CE (b).

Carbon/cyanate ester (CE) composites have become a complementary option to traditional carbon/epoxy composites in structural aerospace applications because of the high performance of CE resins such as high temperature-resistance, excellent dielectric properties, low water absorption rate, and good compatibility with carbon fibers.^13,14 Nevertheless, radiation damage of CE is a major restriction for their practical applications. A promising approach is to incorporate nanoparticles into the polymeric matrix. Jiang et al.¹⁵ studied the resistance to charged particles for TiO₂/epoxy nanocomposites. Experimental results show that under electron irradiation, the incorporation of TiO₂ nanoparticles into the M40/EP648 reduced the mass loss and increased the interlayer shear strength. We recently reported that TiO₂ was incorporated into cyanate ester resin to prepare TiO₂/CE nanocomposites by casting and curing. Due to the shielding function of the TiO₂ modifier, it effectively reduces irradiation damage of the TiO₂/CE nanocomposites.¹⁶ The electrons incident to the internal of the materials will mostly reflect off the surfaces due to the blockage of TiO₂ nanoparticles. Furthermore, some electrons incident to the material will lose their energy via absorption by TiO₂ nanoparticles, thereby the degree of damage to the polymeric matrix by electron irradiation is reduced. In addition, recent studies showed that reduced graphene oxide (rGO) can be a reinforcing material in blends with polymers to enhance their electrical conductivities.^17,18 Yang et al.¹⁹ added rGO to polyvinyl alcohol (PVA) and effectively improved the electrical conductivity of rGO/PVA composites. Therefore, reduced graphene oxide may be an alternative material to prevent the accumulation of electric charges in cyanate resin matrix. The electron irradiation resistance of reduced graphene oxide and TiO₂ is shown in Fig. 1(b).

In this study, rGO–TiO₂ nanoparticles were fabricated, and the resistance to the electron irradiation of the corresponding prepared carbon/rGO–TiO₂/CE composites was investigated. To clarify the damaging effects of the electrons on the materials, the correlation of the mechanical properties and mass loss with the micro structures was analyzed before and after the electron irradiation. Our findings reveal the possibility of developing novel composites with superior radiation resistance to electrons for application in spacecrafts.

2. Experimental

Graphene oxide (GO) was prepared according to the modified Hummers method as described elsewhere.^20,21 rGO–TiO₂ was synthesized using a procedure described by Shen et al.²² 50 mg of GO was added to 30 mL water. The mixture was sonicated for 1 h to obtain a clear brown dispersion of graphene oxide. Subsequently, 50 mg of glucose and 1 mL of ammonium hydroxide were added to the GO solution to obtain part A. 50 mg of tetrabutyl titanate was added to 3 mL of ethanol. The above mentioned mixture was slowly dropped into a mixture of 2 mg ammonium chloride and 2.5 mL of water to obtain part B. Subsequently, part A and part B were mixed. The mixture was placed into an autoclave and heated at 160 °C for 4 h. When the reduction reaction was completed, the as-synthesized product (rGO–TiO₂) was isolated by centrifugation, washed with pure water and ethanol several times, and dried at 90 °C for 12 h.

The powder sample of reduced graphene oxide-loaded TiO₂ was characterized by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) and X-ray photoelectron spectrometry (XPS) analysis. The crystal phases of the sample were evaluated using a D/max-rB type X-ray diffractometer (XRD) with Cu Kα X-rays in the 2θ range from 5° to 70°. Fourier-transform infrared (FT-IR) spectra of the compounds were recorded on a VECTOR22 type device using the attenuated total reflectance method. The measurements were conducted in the wavenumber range of 4000–500 cm⁻¹, with a resolution of 4 cm⁻¹. Transmission electron microscopy (TEM) images were obtained using an FEI Tecnai G2S-Twin microscope at an accelerating voltage of 200 kV.

A commercial bisphenol A dicyanate ester (BADCy) was used as the matrix material, which was supplied by the Shanghai Institute of Synthetic Resins in China. T700 carbon fibers were chosen as the reinforcement for the composites, and were supplied by TORAY Company in Japan. The prepreg tape for the T700/cyanate composite specimens was prepared to impregnate the carbon fiber directly into the cyanate matrix. Unidirectional T700/CE composites were manufactured using 16 prepreg tapes stacked in an orientation, laid onto the mold and placed in an autoclave for curing. The curing cycles were as follows: heating to 130 °C from room temperature at a heating rate of 1.5–2.0 °C min⁻¹, holding for 40–60 min at 130 °C under a pressure of 0.6–0.7 MPa and then heating to 180 °C for 2 h for curing under a pressure of 0.6–0.7 MPa. Finally, the finished prepreg tape was also postcured for 3 h at 220 °C, and the temperature was allowed to decrease to room temperature thereafter. The volume fraction of T700 fibers in the manufactured composites was approximately 60%. The rGO–TiO₂/CE (Fig. S1†) and T700/rGO–TiO₂/CE composites were also prepared by the same procedure.

The electron irradiation test was performed in a simulator within the energy range of 30–200 keV. Specimens were placed in the vacuum chamber at a pressure of 1.2 × 10⁻⁵ Pa. The electron beam was perpendicular to the specimen surface. The electron energy was chosen as 160 keV and the flux as 5.0 × 10¹¹ e s⁻¹ cm⁻². The fluence was altered within the range of 1.0 × 10¹⁵ to 2.0 × 10¹⁶ e cm⁻². A liquid-nitrogen screen was placed inside the test chamber, maintaining the environment temperature at 173–193 K.

The interlayer shear strength was measured on an MTS-810 type tester. Interlaminar shear strength tests were conducted in accordance with the GB3357-82 standard, and each of the resulting data point is an average of five separate measurements.

The mass loss of the specimens before and after the irradiation was measured using a high precision microbalance with a sensitivity of 10⁻⁶ g. The mass loss ratio was calculated using the following equation:

where m₀ is the mass of non-irradiated specimens and m_t is the mass of the specimens exposed to electron irradiation with different fluence.

The change in the surface morphology of the irradiated specimens was examined using a Phenom G2 pro type optical microscope. The interface analysis was performed with an SU8000 type scanning electron microscope. XPS analyses were performed using a Perkin-Elmer PHI-5700 ESCA system with Al(Kα) X-ray irradiation (energy = 1486.6 eV). A small amount of standard graphite was coated on the surface center of the specimens to calibrate the negative charge effect.

3. Results and discussion

3.1 Characterization of reduced graphene oxide–TiO₂ (rGO–TiO₂)

FTIR spectroscopy is commonly used to identify interactions between graphene and TiO₂ nanoparticles. Fig. 2 shows FTIR transmittance spectra of GO and rGO–TiO₂. As shown in the FTIR spectrum of GO, all the representative absorption peaks, including those at 3402 cm⁻¹ (O–H stretching vibration), 1618 cm⁻¹ (C=C stretching of aromatic), 1728 cm⁻¹ (C=O stretching of COOH groups), 1225 cm⁻¹ (C–O stretching of epoxide groups) and 1050 cm⁻¹ (C–O stretching vibration of alkoxy/COOH), were clearly observed.^23–25 As can be seen in the FTIR spectrum of rGO–TiO₂, the oxygen-containing functional groups (C=O and C–O) of GO decreased dramatically, while the skeletal vibration of the graphene sheets corresponding to the absorption peak at 1553 cm⁻¹ appeared, indicating that the GO sheets were deoxidized to graphene. In addition, a typical peak of Ti–O–C appeared around 800 cm⁻¹,²⁶ indicating that chemical bonds are formed between graphene and TiO₂ nanoparticles.

Fig. 2 FTIR transmittance spectra of GO and rGO–TiO₂.

The formation of GO and rGO–TiO₂ was further characterized by XPS. Fig. 3(a) shows the XPS spectrum of GO with the symmetric C 1s peak at 284.4 eV for C–C bonds, while the oxygen-containing carbonaceous bands (C–O) appeared at higher binding energies. For rGO–TiO₂ (Fig. 3(b)), the intensities of all C 1s peaks for carbon bound to oxygen decreased dramatically or disappeared, which indicates that GO was efficiently reduced into graphene during the hydrothermal reaction process. In addition, there is a relatively weak peak located at 288.9 eV, attributed to carboxyl carbon (O=C–O). This surface functional group indicated that the –OH groups on TiO₂ possibly react with the –COOH groups on the GO surface through esterification to form O=C–O–Ti bonds,²⁷ which is in accordance with the FT-IR observation discussed above. Fig. 3 (c) and (d) show the XPS of TiO₂ and rGO–TiO₂ nanocomposites in the Ti 2p3/2, Ti 2p1/2, and O 1s binding energy regions. For pure TiO₂, the determined binding energies of Ti 2p3/2 and Ti 2p1/2 are 458.6 and 464.3 eV, respectively, and the O 1s peak at 529.9 eV for TiO₂ is mainly attributed to the oxygen in the TiO₂ crystal lattice, which is in good agreement with a previously reported data.²⁸ After the formation of rGO–TiO₂ nanocomposites, a shift in the Ti 2p and O 1s binding energies was observed in the XPS spectrum. Normally, this type of shift is induced by a change of the chemical state or coordination environment.

Fig. 3 High-resolution XPS spectra of C 1s for GO (a) and rGO–TiO₂ (b). The Ti 2p XPS spectra of TiO₂ and rGO–TiO₂ (c). The O 1s XPS spectra of TiO₂ and rGO–TiO₂ (d).

Fig. 4 shows the XRD patterns of GO and rGO–TiO₂ nanocomposite, respectively. A characteristic (002) peak of GO appeared at a 2θ value of 10.5°. Compared to GO, the corresponding (002) peak of the rGO–TiO₂ nanocomposite disappeared, further confirming the conversion of GO to reduced graphene in the final composite (Fig. 4(b)). In addition, the peaks located at 25.4°, 37.8°, 48.0°, 53.9° and 62.7° were indexed to the (101), (004), (200), (105) and (204) crystal planes of anatase TiO₂. No typical diffraction peaks of graphene were observed for rGO–TiO₂ nanocomposites. This is because the main characteristic peak of graphene at 25° overlaps with the (101) peak of anatase TiO₂.²⁹ This finding indicates that graphene as a carrier does not affect the crystalline structure of TiO₂.

Fig. 4 XRD patterns of GO (a) and rGO–TiO₂ nanocomposite (b).

To explore the nature of TiO₂ on the graphene sheets, the microstructure of rGO–TiO₂ was investigated by TEM. Fig. 5 clearly shows that graphene is a solid support for the TiO₂ nanoparticles. Moreover, the TiO₂ nanoparticles prevent the graphene sheets from agglomeration during the reduction process. During the formation of the nanocomposite, due to the interactions between the hydrophilic functional groups (e.g., –OH, –COOH) of GO plane and the hydroxyl groups of TiO₂, TiO₂ nanoparticles could chemically adhere onto the surfaces of GO before its reduction to graphene.²⁷ Fig. 5 shows that the TiO₂ nanoparticles are well dispersed on the transparent graphene sheets. Based on the TEM images, the diameters of the TiO₂ nanoparticles were estimated to be in the range from 10 to 20 nm.

Fig. 5 TEM image of rGO–TiO₂.

3.2 Alteration of the interlayer shear strength

Fig. 6 shows the effects of the rGO–TiO₂ nanoparticles on the interlayer shear strength (ILSS) of the T700/rGO–TiO₂/CE composite after electron irradiation. As can be seen from Fig. 6, the ILSS of the T700/CE composite decreased with an increase of the irradiation fluence. This could be due to the fact that electron irradiation may trigger the degradation reactions of the polymer, resulting in decreased interfacial adhesion between the carbon fibers and resin matrix. The incorporation of the rGO–TiO₂ nanoparticles significantly enhanced the ILSS of the resulting T700/rGO–TiO₂/CE composite after electron irradiation. This could be due to the absorption and reflection of radiations by the nanoparticles. As a result, degradation of the composite is prevented. T700/rGO–TiO₂/CE with 3 wt% rGO–TiO₂ gave the maximum ILSS value even after intense irradiation. The reason is that the nanoparticles added may disperse the stresses around the cracks and thus prevent their further development. However, excessive nanoparticles can spontaneously agglomerate in the matrix, resulting in reduced mechanical strength of the composites. Compared with the 3% T700/TiO₂/CE, the ILSS of T700/rGO–TiO₂/CE increased by 10.4%, indicating that the reduced graphene oxide has strengthening effects on the T700/rGO–TiO₂/CE composite.

Fig. 6 Interlayer shear strength of T700/rGO–TiO₂/CE composites with different rGO–TiO₂ contents vs. irradiation fluence.

3.3 Alteration of the mass loss ratio

The effects of the nanoparticles on the mass loss of the composites under electron irradiation are shown in Table S1† and Fig. 7. The mass loss ratios of the three experimental composites obviously increased with increasing fluence up to 1.0 × 10¹⁶ e cm⁻² and then leveled off. Within the exposure range of 1.0 × 10¹⁵ to 2.0 × 10¹⁶ e cm⁻², T700/CE showed the largest mass loss, followed by T700/TiO₂/CE and T700/rGO–TiO₂/CE. The mass loss of T700/rGO–TiO₂/CE was about 16.5% less than that of T700/TiO₂/CE due to the addition of reduced graphene oxide.

Fig. 7 The mass loss of the composites as a function of the electron irradiation fluence: (a) T700/CE, (b) T700/TiO₂/CE, and (c) T700/rGO–TiO₂/CE.

At the early stage of electron irradiation with a fluence less than 1.0 × 10¹⁶ e cm⁻², the mass loss ratio was remarkably increased. This might be explained as follows: (i) volatilization of the absorbed water and residual organic species in CE under high vacuum during the curing process and (ii) outgassing of small volatile molecules in CE induced by electron irradiation. However, as the irradiation fluence is further increased, a carbon-rich layer is formed on the surface of the materials, preventing the ejection of gaseous molecules from the surface layer of the materials.

3.4 Alteration of the surface morphology

Spacecrafts are vulnerable to energetically charged particles in the working space environment. Because the polymer materials used for spacecrafts are insulators, electrons penetrating the thin skin can be trapped in the dielectric materials to cause accumulation of electric charges. When the charges are accumulated to a certain extent, electric discharging occurs, which results in ablation of the resin surface³⁰ and malfunction of electronic equipments inside the spacecraft.

Fig. 8 shows the surface discharge of pristine CE, TiO₂/CE and rGO–TiO₂/CE after electron irradiation with a fluence of 2.0 × 10¹⁶ e cm⁻². There are some distinct discharge ablation phenomena on the surface of pristine CE, as shown in Fig. 8(a). However, with the addition of TiO₂ nanoparticles, the amounts of discharge stripe were significantly reduced on the surface of TiO₂/CE. Because of the faster interfacial charge-transfer rate and larger surface area of graphene, rGO–TiO₂/CE exhibited the least discharge stripes on its surface relative to the other samples. This finding indicates that the rGO–TiO₂ nanoparticles added can effectively reduce charge accumulation on the resin surface of the spacecrafts, which is required for their safe use.

Fig. 8 Optical microscopy images of pure CE (a), TiO₂/CE (b) and rGO–TiO₂/CE (c) exposed to the electron irradiation with the fluence of 2.0 × 10¹⁶ e cm⁻².

3.5 Alteration of the interface morphology

It is well known that the interface plays a decisive role in the performance of the composites. Therefore, we studied the damaging effects of the electron irradiation on the interfaces of the T700/CE, T700/TiO₂/CE and T700/rGO–TiO₂/CE composites by SEM.

Fig. 9 shows the morphologies of the interfaces of the T700/TiO₂/CE and T700/rGO–TiO₂/CE composites before and after the electron irradiation. Fig. 9(a) and (b) present the SEM images of the T700/CE composite before and after the electron irradiation (30 000×). Compared with the original samples, obvious cracks appeared in the interfaces between the fibers and resin matrix after irradiation at the fluence of 2.0 × 10¹⁶ e cm⁻². This finding indicates that the bonding strength of the interface is reduced by the electron irradiation. The adhesion of the fiber and resin matrix after irradiation was improved with the addition of TiO₂ nanoparticles into the CE resin, as shown in Fig. 9(c) and (d). This shows that TiO₂ effectively prevents irradiation damage of the composites. Fig. 9(e) and (f) show that the fiber still closely bonds with the resin matrix before and after electron irradiation with the addition of rGO–TiO₂ nanoparticles. This could be due to the quick charge dispersion onto the surface of reduced graphene oxide, which effectively relieves and eliminates the electrons to prevent further damage to the composites.

Fig. 9 Interface morphologies of T700/CE, T700/TiO₂/CE and T700/rGO–TiO₂/CE composites before and after electron irradiation at the fluence of 2.0 × 10¹⁶ e cm⁻². Magnification is at 30 000×. (a) T700/CE composite before irradiation, (b) T700/CE composite after irradiation, (c) T700/TiO₂/CE composite before irradiation, (d) T700/TiO₂/CE composite after irradiation, (e) T700/rGO–TiO₂/CE composite before irradiation, and (f) T700/rGO–TiO₂/CE composite after irradiation.

3.6 Alteration of the surface chemical composition

Fig. 10 shows the XPS C 1s spectra of the specimens before and after the electron irradiation. Four peaks with binding energies of 284.60, 285.70, 286.62, and 288.22 eV were fitted and ascribed to –C–C–, –C–N–, –C–O– and –C–O–C–, respectively. Their concentrations were calculated based on the relevant peak areas and are presented in Table 1.

Fig. 10 XPS C 1s spectra of the composites before and after electron irradiation at the fluence of 2.0 × 10¹⁶ e cm⁻²: (a) T700/CE composite before irradiation, (b) T700/CE composite after irradiation, (c) T700/TiO₂/CE composite before irradiation, (d) T700/TiO₂/CE after irradiation, (e) T700/rGO–TiO₂/CE before irradiation, and (f) T700/rGO–TiO₂/CE after irradiation.

Table 1 Correlative functional groups of the composites before and after electron irradiation

Samples	Fluence (e cm^−2)	The concentration of correlative functional groups (%)
		–C–C–	–C–N–	–C–O–	–C–O–C–
T700/CE	0	75.0	18.4	5.1	1.5
	2 × 10^16	81.1	13.6	3.9	1.4
T700/TiO2/CE	0	73.2	13.6	8.9	4.3
	2 × 10^16	78.3	12.3	6.2	3.1
T700/rGO–TiO2/CE	0	77.4	16.9	4.1	1.5
	2 × 10^16	80.5	15.2	2.8	1.5

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Electron irradiation, especially at high irradiation fluence, induces breakdown of the chemical bonds of the polymer surface. The massive intensity of the C–C peak was related to the carbon on the surface layer of the specimens. Under exposure, C–C and C–H bonds are inclined to break to form volatile gaseous radiolytic products, as evidenced by the increased intensity of C–C peaks (Table 1). However, with the addition of reduced graphene oxide, the wrinkled exterior surface of reduced graphene oxide could provide nanoscale space limitations, which enhances the bonding between the polymer chains. Therefore, T700/rGO–TiO₂/CE exhibits better resistance under irradiation than other samples.

4. Conclusions

In this study, T700/rGO–TiO₂/CE composites were prepared by incorporating the as-synthesized rGO–TiO₂ nanoparticles into T700/CE composites. In this designed structure, the electrons could be absorbed and reflected by TiO₂ nanoparticles and transferred quickly in the interface by the reduced graphene oxide with larger surface areas. As a result, the small molecules generated from the broken –CH₂–CH₂– bonds in the surface of the cyanate ester resin during vacuum electron radiation were released. In other words, the damage to the composite caused by the electrons could be effectively relieved and eliminated by rGO–TiO₂. Under a fluence of 2.0 × 10¹⁶ e cm⁻², the mass loss caused by the electrons is reduced by 16.5% after rGO–TiO₂ modification. More importantly, the ILSS of the composites was also increased after modification. Our findings demonstrate that the fabricated T700/rGO–TiO₂/CE composite has promising radiation resistance properties, which are required for space applications.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (No. 51078101, 51173033), the Fundamental Research Funds for the Central Universities (NO. HIT. BRETIII. 201224 and 20132), and Shanghai key Laboratory of Deep Space Exploration Technology (No. 13dz2260100).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11113g

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