简介概要

Electrospun and in situ self-polymerization of polyacrylonitrile containing gadolinium nanofibers for thermal neutron protection

来源期刊：Rare Metals2019年第3期

论文作者：Chun-Hong Wang Li-Min Hu Zhi-Feng Wang Ming Zhang

文章页码：252 - 258

摘要：In this work, the polyacrylonitrile containing gadolinium nanofibers for thermal neutron protection were successfully fabricated by electrospunning and followed by in situ self-polymerization. Scanning electron microscopy（SEM） and energy-dispersive spectroscopy（EDS） results show that there are no beads on the smooth surface of the nanofibers and gadolinium elements are uniformly dispersed in the matrix. The thermal analysis and FTIR results prove that gadolinium methacrylate is induced in situ selfpolymerization during the heat treatment. The leaching rate of Gd³⁺ decreases from 79.97% to 10.74% tested by lowfield nuclear magnetic resonance（LF-NMR） method after the self-polymerization of gadolinium methacrylate in the matrix when the nanofibers were immersed in water for7 days. The thermal neutron shielding analysis calculated by MCNP program shows that above 99% thermal neutrons are absorbed when traveling through the 2-mm-thick polyacrylonitrile containing gadolinium nanofibers.

详情信息展示

稀有金属(英文版) 2019,38(03),252-258

Electrospun and in situ self-polymerization of polyacrylonitrile containing gadolinium nanofibers for thermal neutron protection

Chun-Hong Wang Li-Min Hu Zhi-Feng Wang Ming Zhang

School of Chemistry and Chemical Engineering, Yangzhou University

Testing Center,Yangzhou University

作者简介：*Ming Zhang,e-mail:lxyzhangm@yzu.edu.cn;

收稿日期：25 July 2017

基金：financially supported by Postgraduate Research & Practice Innovation Program of Jiangsu Province (No. KYLX_1337);the Excellent Doctorial Dissertations Fund of Yangzhou University;

Electrospun and in situ self-polymerization of polyacrylonitrile containing gadolinium nanofibers for thermal neutron protection

Chun-Hong Wang Li-Min Hu Zhi-Feng Wang Ming Zhang

School of Chemistry and Chemical Engineering, Yangzhou University

Testing Center,Yangzhou University

Abstract：

In this work, the polyacrylonitrile containing gadolinium nanofibers for thermal neutron protection were successfully fabricated by electrospunning and followed by in situ self-polymerization. Scanning electron microscopy(SEM) and energy-dispersive spectroscopy(EDS) results show that there are no beads on the smooth surface of the nanofibers and gadolinium elements are uniformly dispersed in the matrix. The thermal analysis and FTIR results prove that gadolinium methacrylate is induced in situ selfpolymerization during the heat treatment. The leaching rate of Gd³⁺ decreases from 79.97% to 10.74% tested by lowfield nuclear magnetic resonance(LF-NMR) method after the self-polymerization of gadolinium methacrylate in the matrix when the nanofibers were immersed in water for7 days. The thermal neutron shielding analysis calculated by MCNP program shows that above 99% thermal neutrons are absorbed when traveling through the 2-mm-thick polyacrylonitrile containing gadolinium nanofibers.

Keyword：

Gadolinium; Polyacrylonitrile; Electrospinning; In situ self-polymerization; Thermal neutron shielding;

Received： 25 July 2017

1 Introduction

With the fast development of nuclear science and technology,neutron irradiation has been beneficiary to many fields,such as medicine,industrial production,agriculture,research and so on [ 1, 2, 3, 4, 5] .However,due to the strong penetration,capability neutron can easily cause damage to human as well as the environment.Protection from neutron radiation is important to keep people and environments safe.In this regard,the demand for neutron radiation shielding material has been getting stronger during the last decade [ 6, 7, 8] .Traditionally,the elements of boron,lithium,cadmium and RE (Sm,Gd,Eu,etc.) are utilized to absorb thermal neutrons [ 9, 10, 11, 12] .Among them,gadolinium has the highest thermal neutron capture cross section [ 13] .In addition,hydrogenous materials are quite effective to slow down the fast and intermediate neutrons to thermal neutrons by elastic scattering [ 14] .As a result,the polymer containing gadolinium is a type of ideal neutron shielding material.However,this kind of functional materials is nearly unreported until now.

Generally,most inorganic rare earth compounds have poor compatibility with polymer which will cause uneven dispersion and poor mechanical properties.Reducing the size of rare earth particles modified with coupling agent is a usual way to improve the compatibility with polymer [ 15, 16, 17, 18, 19] .But it normally has a complex procedure.Using the organic rare earth compounds to prepare polymer composites can achieve a certain degree of improvement in dispersion and compatibility.Zhang et al. [ 20, 21] had prepared Eu(BA)3(TPPO)2/polyvinyl pyrrolidone (PVP)and Eu(TTA)3(TPPO)2/PVP composite fibers through electrospinning,and the organic rare earth compounds showed good compatibility with PVP.Applying an unsaturated rare earth monomer followed by polymerization is possibly a new effective approach to solve the above problems [ 22, 23] .Liu et al. [ 24] prepared a series of Gd(AA)3/NR composites by an in situ reaction method.They found that occurrence of the in situ polymerization of Gd(AA)3 in composites greatly improves the dispersion.They mainly focused on the fabrication of rare earth/rubber composite materials.The nanofibers containing rare earth fabricated by electrospinning and in situ self-polymerization have rarely been reported until now.

In this paper,polyacrylonitrile containing gadolinium nanofibers were firstly prepared by electrospinning and followed by in situ polymerization of gadolinium methacrylate in the nanofibers.The thermal neutron shielding property was also calculated by MCNP program [ 25] .We hope polyacrylonitrile containing gadolinium nanofibers can be used as thermal neutron radiation protective clothing in the nuclear industry in the coming future.

2 Experimental

2.1 Materials

All the raw materials were purchased and used without further purification including gadolinium oxide (Gd₂O₃,Shanghai Yuelong Chemical Factory),methacrylic acid(MAA,Sinpharm Chemical Reagent Co.,Ltd.),dicumyl peroxide (DCP,Sinpharm Chemical Reagent Co.,Ltd.),N,N-dimethylformamide (DMF,Sinpharm Chemical Reagent Co.,Ltd.) and polyacrylonitrile with average molecular weight of 15,000 (PAN,Jinshan Petrochemistry Co.,Ltd.).

2.2 Preparation of gadolinium methacrylate(Gd(MAA)3)

Gd(MAA)₃ was prepared by adding Gd₂O₃,MAA and H₂O with the molar ratio of 1:10:20 into a 500-ml three-necked flask.The reaction was carried out at 90℃for 1 h until all the powders were dissolved and the solution became transparent.The solution was filtered while still hot,and the filtrate was concentrated by distilling water for about2 h.A white precipitate was obtained by adding ethyl alcohol in excess to the filtrate.The solid was filtered off,washed with ethyl alcohol and dried under vacuum.The yield ratio of the final product was about 80%yield.

2.3 Preparation of PAN containing Gd nanofibers

As shown in Fig.1,the Gd(MAA)3(1.29 g),DCP(0.18 g)and PAN(3.00 g) powders were dissolved in DMF(20.00 g) under room temperature and stirred for 24 h until the solution became transparent.The polymer solution was put into a syringe/capillary tube connected to a high voltage source.The voltage between the electrode and the counter electrode could be controlled by the high voltagepower supply which here was set at 12 kV.The solution flow rate was 1.0 ml·h^-1 controlled by the syringe pump.The as-spun nanofibers were collected on the grounded conductive target with a tip-to-collector distance of 15 cm.Then,the as-spun nanofibers were heat-treated in air condition at 160℃for 3 h.

Fig.1 Preparation of PAN containing Gd nanofibers

2.4 Characterizations

The morphologies of Gd(MAA)3 powders and nanofibers were observed by field-emission scanning electron microscope (FESEM,Hitachi,S-4800).The energy-dispersive X-ray (EDX) mapping was done using field-emission transmission electron microscope (FETEM,FEI,Tecnai G2 F30 S-TWIN).The thermal characteristics of Gd(MAA)3 powders and nanofibers were obtained by thermogravimetric analyzer (TGA,PE,Pyris 1) and differential scanning calorimetry (DSC,PE,DSC 8500).The heating rate was 10℃·min^-1 for TGA and 5℃.min^-1 for DSC,the temperature range was from 30 to 800℃for TGA and from 30 to 350℃for DSC.The Fourier transform infrared (FTIR) spectra of the samples were recorded at a resolution of 0.1 cm^-1 on a Varian Cary 610 spectrometer.The low-field nuclear magnetic resonance (LF-NMR,Niumag Corporation,MicroMR) was used to measure the concentration of Gd³⁺in the aqueous solution(details were shown in supporting information (Fig.S1)).The thermal neutron (0.025 eV) shielding property of PAN containing Gd nanofibers was calculated by Monte Carlo N particle transport code (MCNP) program (details were shown in supporting information (Fig.S2)).

3 Results and discussion

The quality of nanofibers fabricated by electrospinning is strongly influenced by the physical properties of the solution,such as the viscosity,surface tension and conductivity [ 26, 27, 28] .The average diameter of the PAN nanofibers could increase from about 165 to 860 nm by varying the concentration from 10 wt%to 20 wt%,while there are beads in the nanofibers when the concentration is10 wt%and the surface of the nanofibers becomes rough when the concentration is 20 wt%(Fig.S3).The average diameter of the PAN nanofibers could decrease from about460 to 380 nm by varying the distance between needle and grounded plate from 10 to 20 cm,and all the surfaces of the nanofibers are smooth (Fig.S4).Figure 2a shows SEM image of pure PAN nanofibers of which the PAN concentration is 15 wt%.It is found that the surface of the nanofibers is smooth without any beads in the nanofibers,and the average diameter of the nanofibers is (470±43)nm.Figure 2b-d is SEM images of Gd(MAA)3 powders and Gd(MAA)3/PAN nanofibers before and after heat treatment.It could be seen that Gd(MAA)3 is a micronsized rod-like crystal.But in the as-spun nanofibers,the rod-like crystal Gd(MAA)3 powders could not be found,meaning that the Gd(MAA)3 is well dispersed in the fibers.The as-spun nanofibers also have a smooth surface and the average diameter of the nanofibers increases from 470 to578 nm.The morphology of nanofibers does have obvious change after heat treatment,as shown in Fig.2d.

Figure 3 is EDS mapping of as-spun Gd(MAA)3/PAN nanofibers and it reveals that gadolinium elements are well dispersed in the nanofibers.This may be because that Gd(MAA)3 can dissolve in DMF solution at molecule level although Gd(MAA)3 is a micron-sized crystal powder.During the electrospinning procedure,DMF would be volatilized in a short period while PAN would be solidified as fiber.Gd(MAA)3 does not have enough time to gather as micron-size crystal,as a result,Gd(MAA)3 can still be dispersed in PAN nanofibers matrix at molecule level.

Figure 4 shows TGA curves of the PAN nanofibers and as-spun Gd(MAA)3/PAN nanofibers.According to TGA curves,the 5%weight loss temperatures of the PAN nanofibers and as-spun Gd(MAA)₃/PAN nanofibers are 311and 295℃in the nitrogen atmosphere,respectively.The presence of Gd(MAA)3 could separate the adjacent C≡N groups of PAN macromolecular chain and reduce the hydrogen bond formation of macromolecular chain that might be due to the decrease in thermal stability of the nanofibers composite.When the atmosphere changes to oxygen and the temperature increases to 800℃,the PAN nanofibers lost almost 100%weight while the as-spun Gd(MAA)₃/PAN nanofibers have 13.97 wt%residual and the residual is Gd₂O₃,meaning that the content of Gd in the fibers is 12.11 wt%,almost the same as the formula.

Then,LF-NMR technology was used to study the leaching rate of Gd³⁺from the nanofibers when the nanofibers were immersed in the water last for 7 days.As shown in Fig.5,79.97%Gd³⁺is leached from the as-spun nanofibers while only 10.74%Gd³⁺is leached from the heat-treated nanofibers.This result shows that although Gd(MAA)₃ could be uniformly dispersed in the as-spun nanofibers matrix,there are no chemical interactions between each other.When the as-spun nanofibers are immersed in the good solvent of Gd(MAA)3 such as water,Gd³⁺would easily leach from the matrix into the solvent.After heat treatment,Gd(MAA)3 might be induced in situ self-polymerization by DCP in the nanofibers and poly(-Gd(MAA)₃) could hardly leach from the nanofibers into water.

Fig.2 SEM images of a pure PAN nanofibers,b Gd(MAA)3,c as-spun Gd(MAA)₃/PAN nanofibers and d heat-treated Gd(MAA)3/PAN nanofibers

Fig.3 a SEM image and EDS mappings (b C,c N,d O and e Gd) of as-spun Gd(MAA)₃/PAN nanofibers

Fig.4 TGA curves of a PAN nanofibers and b as-spun Gd(MAA)₃/PAN nanofibers

Fig.5 Leaching rate of Gd³⁺tested by LF-NMR technology

DSC curve of Gd(MAA)₃ shown in Fig.6a has a significant exothermic peak at 275-325℃while TGA curve in Fig.6b exhibits almost no mass loss from 100 to 350℃,which both prove that Gd(MAA)3 could be induced to selfpolymerization by thermal in solid state [ 29] .When DCP and Gd(MAA)3 were mixed together,the significant exothermic peak comes up between 150 and 180℃instead of the one comes up between 275 and 325℃,as shown in Fig.6a.That means Gd(MAA)3 could be induced self-polymerization at low temperature by free radical coming from the decomposition of DCP.In addition,the disappearance of C=C in Gd(MAA)3 and as-spun nanofibers after heat treatment is the direct evidence of the in situ self-polymerization of Gd(MAA)3 in the solid state and in the nanofibers matrix,as shown in Fig.7.

In Fig.8,the probable mechanism of the in situ selfpolymerization of Gd(MAA)₃ in the PAN nanofibers might be expressed as follows:In the as-spun Gd(MAA)3/PAN nanofibers,part of DCP could contact with Gd(MAA)3 and the others are isolated dispersed in the matrix.During the heat treatment,the former decomposed and generated free radical that would cause Gd(MAA)3 to contact with DCP in situ self-polymerization.The latter decomposed and generated a large of heat energy which would also cause Gd(MAA)3 self-polymerization.Because of the large atomic radius of gadolinium,methacrylate ions are coordinated by the gadolinium atoms and have different structural functions:One of the acid residues behaves as a bidentate bridging ligand,while the other two residues function as tridentate chelate-bridging ligands [ 30] .So Gd(MAA)3 was dispersed in the nanofibers matrix as the form of coordination polymer.Although Gd(MAA)3 could hardly move to touch with other monomer in the solid state,it would still be induced in situ self-polymerization by free radical and heat energy as the structure of coordination polymer.Besides,the average number of methacrylate ions coordinated by Gd atoms is three,the poly(Gd(MAA)₃) is a cross-linked polymer that could hardly dissolve in water.

Fig.6 a DSC and b TGA curves of Gd(MAA)₃

Fig.7 FTIR spectra of a Gd(MAA)₃,Gd(MAA)₃ and DCP after heat treatment and b as-spun Gd(MAA)₃/PAN nanofibers and heat-treated Gd(MAA)₃/PAN nanofibers

Fig.8 Schematic illustration of in situ self-polymerization of Gd(MAA)₃ in PAN nanofibers

Fig.9 Thermal neutron shielding property of pure PAN and PAN containing Gd nanofibers

As shown in Fig.9,the pure PAN nanofibers barely have thermal neutron shielding ability.Above 90%thermal neutron could penetrate the 2-mm-thick pure PAN nanofibers.While the PAN containing Gd nanofibers show strong thermal neutron shielding ability because of the high Gd content (theoretical Gd content is 11.43 wt%and actual Gd content is 12.11 wt%).Above 99%thermal neutron is absorbed when traveling through the 2-mm-thick PAN containing Gd nanofibers.The result reveals that the PAN containing Gd nanofibers can be used as thermal neutron radiation protective material in the nuclear industry.

4 Conclusion

In summary,the PAN containing Gd nanofibers were successfully fabricated by electronspinning method and followed by the in situ polymerization of gadolinium methacrylate in the nanofibers.Using Gd(MAA)3 as gadolinium source can offer two obvious advantages.On the one hand,gadolinium can be uniformly dispersed in PAN matrix at molecule level.On the other hand,the in situ self-polymerization of Gd(MAA)₃ makes Gd element exist stably in the nanofibers even if the nanofibers are immersed in the good solvent of Gd(MAA)₃.These advantages can improve the reliability of PAN containing Gd nanofibers used as thermal neutron radiation protective material.In addition,firstly compounding organic rare earth compounds containing unsaturated bonds with polymer and then inducing in situ self-polymerization may be an effective method to produce highly content and uniformly dispersion of polymer containing rare earth functional composites.