The fabrication and neutron shielding property of polyphenylene sulfide containing salicylic acid/Gd 2 O 3 composites

Abstract

A series of neutron absorbing materials with good neutron absorbing capacity, high strength and good thermal property were designed and prepared in this work. First of all, polyphenylene sulfide containing different mole content of salicylic acid (SAPPS) in the main chain was synthesized by nucleophilic substitution polymerization under high pressure. Then the composites with different content of nano Gd₂O₃ and modified PPS were prepared by melt blending method. The testing results indicated that the copolymers SAPPS were synthesized successfully, there was an interface interaction between nano Gd₂O₃ and the matrix without the need for surfactants or coupling agents. Additionally, the content of nano Gd₂O₃ had no obvious influence on the thermal property of the composites. While following with the increase of the content of nano Gd₂O₃, the tensile strength of the composites increased firstly and then decreased, when the content of nano Gd₂O₃ was 10 wt%, the tensile strength of 10%Gd₂O₃/5%SAPPS reached the maximum value of 74.9 MPa. The results of neutron shielding testing indicated that the content of nano-particles had a large effect on the neutron shielding rate of composites. The neutron shielding rate of 50%Gd₂O₃/5%SAPPS composite was up to 83%. All of these results indicated that the Gd₂O₃/5%SAPPS had potential to be applied to the high-temperature resistance and thermal shielding materials in nuclear and aerospace applications.

Keywords

PPS copolymers high-temperature resistance neutron shielding

Introduction

With the increasing maturity of nuclear technology, nuclear energy has been used in medical, industrial, military and other fields, and it has gradually become an indispensable energy source.^1,2 Generally, nuclear energy needs to be protected or shielded to avoid nuclear leakage due to its radiation damage in the application of nuclear energy. At the present stage, nuclear shielding materials mainly include traditional lead products and concrete, and present-day polyethylene, boron-polyethylene and so on. However, traditional shielding materials, such as lead products and concrete, are difficult to be used widely in nuclear radiation situations with special requirements on weight, volume and mobility due to their single function, high density and difficulty in moving.^3–7 Although the densities of polyethylene and boron-polyethylene are much less than lead and concrete, also it can resolve the problems of heavy weight and difficulty in moving of traditional shielding materials, their application is somewhat limited in nuclear and aerospace applications because of their relatively lower shielding effect, mechanical properties, heat resistance and anti-irradiation properties.^8–12 Thus, it is critical to design and develop the novel materials with good comprehensive properties among thermal and mechanical performance, heat resistance and anti-irradiation.

Poly(1,4-pheynylene sulfide) (PPS), which are composed of para-substituted phenylene rings and sulfur atoms, is a semi-crystalline engineering thermoplastic. Because of its exceptional physical and chemical properties, such as good radiation, excellent chemical resistance, heat resistance, good electronic properties, PPS is widely used in aerospace industry, electronics, motor industry and so on.^13,14 While PPS has some disadvantages of relatively low thermal and mechanical properties, difficulty in coloring, insolubility and poor interfacial interaction, so the chemical structure and physical blend modification of PPS is thought to be a necessary way to improve it properties. In the chemical structure modification, incorporating chromophore group in the main chain of PPS can endow the modified PPS with brilliant color, and the prepared copolymers show different colors in the concentrated sulfuric acid.¹⁵ The hydrophilicity of polymers can be improved well by introducing active groups such as carboxyl group, amino group and so on.^16,17 Additionally, by replacing dichlorobenzene with dichlorobenzophenone¹⁸ or dichlorodiphenyl sulphone,¹⁹ the obtained poly(phenylene sulfide ketone)(PPSK) is a crystalline polymer with a melting point of 360°C, which was much higher than that of PPS. While poly(phenylene sulfide sulfone) (PPSS) PPSS is an amorphous polymer with a glass transformation temperature of about 220°C. Its solubility is greatly improved due to the addition of sulfone group, and it can be dissolved in many solvents, such as NMP, DMI, DMAC and so on. In the physical blending modification, glass fiber,^20,21 carbon fiber²² and particles (CaCO₃, SiO₂, Fe₃O₄, carbon nanotubes, graphene, etc.)^23–27 are usually added into the matrix of PPS to improve its tensile strength. The toughness of PPS can be improved by adding other polymers such as PA66, rubber and other elastomers into the PPS matrix.^28–32 It can be found that, as a nuclear shielding material, there are relatively few studies on the nuclear shielding performance of PPS. Yamaoka and Miyata studied the stability of the PPS film in the low-temperature irradiation environment, and the results showed that the mechanical and electrical properties of the PPS film did not change significantly after irradiation.³³ Kaçal also demonstrated that PPS has good gamma ray and neutron absorption capacity.³⁴ The results show that PPS has excellent neutron shielding performance, but it needs to be further improved.

Rare earth elements (Sm), europium(Eu) and gadolinium(Gd), as a kind of luminescent elements and neutron shielding elements, are widely used in nuclear protection materials, among which Gd is the most widely used due to the larger neutron absorption cross section.³⁵ Besides, the gadolinium elements present itself as self-gamma (γ) absorbers because of relatively high atomic numbers (Z = 64).³⁶ On account of gadolinium metal is easily to be oxidized, gadolinium element is usually present in the form of gadolinium oxide (Gd₂O₃), so Gd₂O₃ is always blended with polymer matrix to improve the neutron shielding capability. The composites, such as Gd₂O₃/poly(vinyl alcohol)(PVA),³⁵ Gd₂O₃/epoxy composites³⁷ and h-BN/Gd₂O₃/high-density polyethylene(HDPE)³⁸ were found to have good neutron absorption capability, while their application was somewhat limited in nuclear and aerospace applications due to their poor thermal property and corrosion resistance.

Thus, in this work, in order to improve the disadvantages of traditional and current nuclear shielding materials, considering the modified PPS with polar groups in the side chain could improve its hydrophilicity and its good thermal and mechanical property, the modified PPS (SAPPS) with reactive group (salicylic acid) in the main chain was synthesized and used to be as polymer matrix to prepare nano Gd₂O₃/SAPPS composites. And the chemical structure, thermal properties and mechanical properties and neutron shielding performance of these obtained copolymers and their composites, were also characterized in detail.

Experimental

Materials

3,5-dichlorosalicylic acid (DSA) (AR, Aladdin reagent) and nano Gd₂O₃ (purity of 99.98%, average particle sizes is 42.3 nm) were Aladdin reagents. The 1,4-dichlorobenzene (DCB) (99.8%), sodium sulfide (Na₂S·xH₂O, Na₂S% = 45.5%) and N-methyl-2-pyrrolidone (NMP) were industrial products from Quzhou, Rainful Chemical Industry Company, Nanfine Chemical Industry Group Co., Ltd and Jiangsu Nanjing Jinlong Chemical Industry Company, respectively.

Synthesis of the polyphenylene sulfide containing salicylic acid (SAPPS)

A typical polymerization was performed as shown in Figure 1. Firstly, the Na₂S·xH₂O, NMP and catalyst³⁹ were added into a 1000 mL high pressure reactor. Then, the mixture was heated at 180°C for 1 h, protected by nitrogen, for removing water during the period. Secondly, DCB and DSA were added when the temperature drops below 120°C, then the temperature was raised to 220°C and kept for 3 h, then the mixture is incubated at 260°C for a further 3 h. After reaction, the product was washed with homemade deionized water and ethanol three times. After drying under vacuum at 80°C for 12 h, the modified PPS resins was obtained.

Here, the copolymers SAPPS(5%), SAPPS(10%) and SAPPS(15%) represent the copolymer with different DSA molar fractions (5%, 10% and 15%, respectively) in DCB/DSA. The ratio between them in different copolymers was shown in Table 1.

Table 1.

The molar ratio of DCB/DCA and the mechanical properties, thermal properties of copolymers.

Copolymer	DCB	DSA	T_d (°C)	T_m (°C)	Tensile strength(Mpa)	$η_{s p}$ (dLg⁻¹)
PPS	100%	0	498.8	283.0	79.5 ± 1.1	0.333
5%SAPPS	95.0%	5.0%	502.9	284.0	66.6 ± 3.5	0.293
10%SAPPS	90.0%	10.0%	490.8	284.2	60.5 ± 1.3	0.252
15%SAPPS	85.0%	15.0%	492.9	283.7	52.2 ± 1.8	0.209

Preparation of composites

In this study, the Gd₂O₃/SAPPS composites were blended using a HAAKE Minilab extruder at 300°C with a screw speed of 150 rpm. Here, the copolymer 5%SAPPS was selected as a matrix for the development of the nano-composites due to its optimal balance between intrinsic viscosity and thermal properties (From Table 1). The 5%SAPPS with 10, 20, 30, 40 and 50 wt% nano Gd₂O₃ were named 10%Gd₂O₃/5%SAPPS, 20%Gd₂O₃/5%SAPPS, 30%Gd₂O₃/5%SAPPS, 40%Gd₂O₃/5%SAPPS and 50%Gd₂O₃/5%SAPPS, respectively. After blending, the pellets were injection molded through an HAAKE injection-molding machine at the melt temperatures of 310°C and mold temperature of 60°C.

Characterization

Intrinsic viscosity

The intrinsic viscosity of the samples was obtained by dissolving 0.400 g of polymer in 100 mL 1-chloronaphthalene at 208°C ± 0.1°C and using a Cannon−Ubbelodhe viscometer. The intrinsic viscosity values were obtained by a one-point method (the Solomon–Ciuta equation) as follows:

η_{i n t} = \frac{\sqrt{2 (η_{s p} - l n η_{r})}}{C}

where $η_{r} = \frac{η}{η_{0}}$ , $η_{s p} = η_{r} - 1$ .

Fourier transform infrared analysis (FT-IR)

FT-IR spectra were studied by a Nexus 670 FT-IR spectrometer (Nicolet Instrument Co., USA). The spectral resolution was 4 cm⁻¹, ranging from 4000 to 500 cm⁻¹.

X-ray diffractograms (XRD)

The XRD patterns were recorded at Rigaku Smartlab diffractometer (Cu Kα, λ = 1.54056 Å, Rigaku, Japan).

X-ray photoelectron spectroscopy

The molecular interaction between SAPPS and nano Gd₂O₃ was measured using X-ray photoelectron spectroscopy (XPS, XSAM800, Kratos Company, England). The scan spectrum was obtained over a range of 0–1100 eV.

Thermal properties

Differential scanning calorimetry (DSC) was performed on a NETZSCH DSC 200 PC thermal analysis instrument (NETZSCH Company, Germany). The samples were heated to 300°C with the rate of 50°C/min and maintained this temperature for 2 min to eliminate their thermal history. Then, the temperature was cooled to 25°C and then reheated to 300°C, the last two steps was carried at a rate of 10°C/min. A TGA Q500 V6.4 Build 193 thermal analysis instrument (TA company, America) was used to examine the thermal stability at a heating rate of 10°C /min under a nitrogen atmosphere.

Mechanical testing

Tensile properties of SAPPS and Gd₂O₃/SAPPS composites were examined using Universal Testing Machine (Instron 5567, USA) at room temperature, the size of dumbbell samples was 63.5 mm (length)* 3.18 mm (width)* 3.18 mm (thickness).

Scanning electron microscopy

The morphology of the composites was investigated by scanning electron microscopy (SEM; JSM-5900LV, JEOL, Japan) at an acceleration voltage of 20 kV. All specimens were sputtered with 10 nm layer of gold prior to SEM observations.

Neutron shielding properties

The neutron shielding properties of all the Gd₂O₃/5%SAPPS composites containing different content of nano Gd₂O₃ were investigated by determining their corresponding neutron shielding ratio. The tests were carried out by at the Institute of Nuclear Science and Technology using the experimental setup shown in Figure 2. In this work, a ²⁴¹Am/Be neutron source was sealed in a thick PE barrel. Meanwhile, a stack of 4 cm paraffin wax was used as a neutron moderator, which was effective to thermalize the emitted fast neutrons from ²⁴¹Am/Be. The neutron detector was a BF₃ neutron detector. Each composite plate with a diameter of 5.5 cm and thicknesses of 2 cm. The same thickness with different gadolinium oxide content were placed between the neutron detector and the paraffin sheets. The test was divided into two steps: Firstly, record the total number of neutrons (I₀) without composites for 600 s, Secondly, measure the counts of neutrons (I) passing through the composites for 600 s. Therefore, the thermal neutron shielding ratio can be expressed as I/I₀.

Results and discussion

FT-IR analysis

The FT-IR spectra of pure PPS, 5%SAPPS, 10%SAPPS, 15%SAPPS and Gd₂O₃/5%SAPPS composites were shown in Figure 3. From Figure 3(a) and (b), SAPPS (5%, 10%, 15%) shows a broad peak centered at 3400 cm⁻¹ assigned to the −OH groups, which is coincident with that of water. Benzene ring stretching modes were found at 1647 cm⁻¹, 1573 cm⁻¹ and 1471 cm⁻¹, and the new unsharp peaks of copolymers at about 1690 cm⁻¹ could be attributed to the stretching vibration of −C=O. Above all, the spectra of FT-IR evidence that the polymerization of DCB and DSA with sodium sulfide occurs during this reaction. From Figure 3(c), two peaks presented at about 560 cm⁻¹ and 470 cm⁻¹ were assigned to the Gd–O stretching frequencies.⁴⁰ The characteristic peaks of gadolinium oxide were not seen in the FT-IR spectrum of the composites because they were masked by the peaks of the copolymers.

XRD characterization

The XRD patterns of pure PPS, the modified polymer and nano-composites were shown in Figure 4. The pure PPS showed the most intense diffraction peak that appeared at 20.6°, arising from the overlap of (111) and (200). Other peaks were found at 18.8°, 25.5° and 27.4°, assigned respectively to the (110), (112) and (211) crystal planes.⁴¹ The diffraction pattern of the derivatives is almost indistinguishable except intensity from that of pure PPS. In the Figure 4(b), nano Gd₂O₃ at 2θ = 22.3°, 27.6°, 31.7°, 39.5° and 47.4° were corresponding to (211), (222), (400), (332) and (440) planes of nano Gd₂O₃ in the composites, respectively. These peaks were not detected in neat PPS, which strongly pointed that some nano Gd₂O₃ had deeply embedded into polymer matrix.

Intrinsic viscosity

The intrinsic viscosities of the pure PPS, 5%SAPPS, 10%SAPPS, 15%SAPPS were 0.333, 0.293, 0.252 and 0.209, respectively. Following with the increase of the mole content of salicylic acid in the main chain, the intrinsic viscosity decreased. The results showed that the molecular weight of copolymers might be lower than that of the obtained copolymers.

Corrosion resistance

As shown in Table 2, SAPPS were insoluble in the selected solvent at room temperature, indicating that all copolymers maintained the good insolubility of PPS.

Table 2.

Corrosion resistance of copolymer.

Copolymer	PPS	5%SAPPS	10%SAPPS	15%SAPPS
98%H₂SO₄	—	—	—	—
HCl	—	—	—	—
CH₃COOH	—	—	—	—
NMP	—	—	—	—
DMF	—	—	—	—
DMAc	—	—	—	—
DMSO	—	—	—	—
NaOH	—	—	—	—
DCM	—	—	—	—
C₆H₅CH₃	—	—	—	—

—: insoluble at room temperature; DCM: Dichloromethance; DMAc: N,N-dimethyl acetamide; DMF: N,N-dimethyl formamide; DMSO: dimethyl sulfoxide; NMP: N-methyl pyrrolidone.

X-ray photoelectron spectroscopy analysis

Figure 5 showed the core level high-resolution XPS spectrum of S (2p), C (1s), O (1s), Gd (3d) energy levels of 40%Gd₂O₃/5%SAPPS composite and 40%Gd₂O₃/PPS composite. S (2p) spectra showed two peaks at 164.6 eV and 163.5 eV which thanked to a spin-orbital splitting of 3p1/2 and 3p3/2 energy levels of S element, respectively. C (1s) spectra displayed a main peak at 284.8 eV that corresponded to the C–C/C–H. O (1s) spectra exhibited one major peak at 532.3 eV. The slight shift of S (2p) spectra may be attributed to the addition of DSA. It could be observed that the same shifts in the spectra of C (1 s), O (1 s) and Gd (3d). This may be owed to the hydrogen bonding with the –OH groups of Gd₂O₃ nanoparticle.⁴² By the way, Gd (3d) spectra demonstrated two major peaks at 148.5 eV and 143.1 eV, which caused by a spin-orbital splitting of 3d3/2 and 3d5/2 energy levels of Gd, respectively.

Thermal properties

In Figure 6, it showed the thermal properties of the copolymers and its composites. From Figure 6(a) and Table 1, It could be found all the polymer have obvious melting point, indicating they were crystalline polymers, which agreed with the result of XRD. Following with the increase of the content of salicylic acid, the melting point of the polymer appeared to increase first and then decrease, and the values were from 283.7 to 284.2°C. The reason might be as follows: on the one hand, copolymers’ hydrogen bonding between molecules increased the intermolecular interaction because of the introduction of salicylic acid, which caused an increase in the melting point of the copolymer. However, with the further increase of the content of salicylic acid in the main chain of PPS, the regularity of polymers’ chain decreased, and its effect on the reduction of melting point may be higher than that of hydrogen bond on the increase of melting point, so the melting point of copolymers firstly increased and then decreased. From Figure 6(b), it could be seen that the melting point of the composite decreased slightly with the increase of nano Gd₂O₃ content. Generally speaking, inorganic particles as nucleating agents can promote the crystallization of polymer, which was beneficial for the crystallizations of polymers. While the interaction between inorganic particles and polymer limited the intermolecular movement of polymer, resulting in the decline of its crystallinity. The decrease of melting point of the composites may be determined by both of these factors. Although the melting point of the composites decreased slightly, the values were also munch higher than that of HDPE (T_m = 145°C, T_d = 300°C) and PVA (T_m = 230°C, T_d = 220°C).^35,38 As shown in Figure 7(a), the initial degradation temperatures (T_d) of the copolymers 5%SAPPS, 10%SAPPS and 15%SAPPS were 502.9°C, 490.8°C and 492.9°C, respectively. And there were no significant differences from that of pure PPS. From these, it can be concluded that the obtained copolymers and its composites have good thermal performance.

Table 3.

Thermal parameters obtained from DSC and TGA analysis and Mechanical properties for composites.

Materials	T_d (°C)	T_m (°C)	Tensile strength (MPa)
5%SAPPS	502.9	284.0	66.6 ± 3.5
10%Gd₂O₃/5%SAPPS	501.3	283.2	74.9 ± 2.0
20%Gd₂O₃/5%SAPPS	500.4	283.2	62.3 ± 8.7
30%Gd₂O₃/5%SAPPS	502.1	283.8	64.8 ± 2.0
40%Gd₂O₃/5%SAPPS	500.8	283.6	60.4 ± 3.5
50%Gd₂O₃/5%SAPPS	500.5	281.1	48.6 ± 2.3

T_d:Initial thermal decomposition temperature; T_m:Melting point temperature.

Mechanical properties

The average tensile strength of the copolymers 5%SAPPS, 10%SAPPS, 15%SAPPS and Gd₂O₃/5%SAPPS composites were shown in Figure 8. As shown in the figure, with the increase of mole content of salicylic acid in the main chain of PPS, the tensile strength of copolymer decreased. When the mole content of salicylic acid was 5%, the tensile strength of copolymer reached the maximum value of 66.6 MPa. Additionally, following with the increase of the content of nano Gd₂O₃ in the composite, the tensile strength of the composite materials increased initially and then decreased. And the maximum tensile strength reached up to 74.9 MPa when the content of nano Gd₂O₃ was 10 wt%. The increase of tensile strength might be due to the effect of the content of filling particle and the interfacial interaction between a matrix and filling particle. Additionally, the mechanical property of 50%Gd₂O₃/5%SAPPS composite decreased obviously because the nano-particles were agglomerating seriously. By the way, the overall tensile strengths of the composites were above 60 MPa except for 50%Gd₂O₃/5%SAPPS composite, indicating that they had good mechanical property.

Morphological analysis

Figure 9 showed SEM images of the fractured surfaces of nano Gd₂O₃, PPS and Gd₂O₃/5%SAPPS composites. It was seen from Figure 9(a) of Gd₂O₃ nanoparticles were observed to have average particle of 42.3 nm. And it could be observed that the fractured surface of 5%SAPPS was smooth. Nevertheless, as shown in Figure 9(c) to (h), the fractured surfaces of the composites materials were relatively rough, and the nanoparticles were dispersed in the composite substrate without cavity phenomenon, indicating that the interface of nano Gd₂O₃ and copolymer was well bonded. When the content of nano Gd₂O₃ in the composite was over 10 wt%, the aggregation of nano Gd₂O₃ formed. Following with the increase of content of nano Gd₂O₃ in the composite, the agglomeration became more and more obvious. The aggregation of nanoparticles can decrease the mechanical properties of composites, which is consistent with the results of mechanical research.

Neutron shielding properties

The neutron shielding properties of Gd₂O₃/5%SAPPS composites with different contents of fillers was shown in Figure 10. From Figure 10, the results revealed that the neutron shielding rate of the composite was increased with the increase of the content of nano Gd₂O₃. The neutron shielding rate of pure copolymers 5% SAPPS was only 50%, while the shielding rate of 50%Gd₂O₃/5%SAPPS was up to 83%. This enhancement might be mainly based on the combined effects of two aspects. On the one hand, hydrogen atoms in the pure modified polymer have a good shielding effect on neutron.³⁴ On the other hand, gadolinium atoms also have a good shielding effect on neutron.⁴³ Meanwhile, it should be pointed out that the neutron shielding property of pure 5%SAPPS exhibited better neutron shielding property than other polymers, such as PVA, HDPE, PEEK and so on.^35,38,44 Additionally, when the content of nano Gd₂O₃ in the composite was 10 wt%, the shielding rate was about 80%, which increased by 30% compared with the pure modified 5%SAPPS. With the further increase of nano Gd₂O₃, the shielding growth rate of neutron decreases, indicating that thermal and low-energy neutrons might have been completely absorbed so that high content of nano Gd₂O₃ has no obvious effect on the shielding performance of composites.

Conclusions

The novel Gd₂O₃/SAPPS composites were prepared in the work. There was a good interface between SAPPS and nano Gd₂O₃. And the obtained composites had good thermal properties. The content of nano-sized Gd₂O₃ had little effect on the thermal properties of the composites, but great effect on their mechanical properties. With the increase of nano Gd₂O₃, the mechanical properties of nano-particles increased firstly and then decreased. In addition, the content of nano Gd₂O₃ had large influence on the neutron shielding performance of composites, when the content of gadolinium oxide was 50 wt%, the composite material showed the highest neutron shielding ratio, and the value was up to 83%. These results indicated that as a high-temperature resistant neutron shielding material, the obtained composite may have potential applications in nuclear reactors and aerospace.

Footnotes

Acknowledgements

The authors acknowledge the support by National Natural Science Foundation of China (No. 51773123) and the Sichuan Science and Technology Program (Grant No.: 2020YFG0093).

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Yumin He

Xi Zhang

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