Vinyl benzoxazine: a novel heterobifunctional monomer that can undergo both free radical polymerization and cationic ring-opening polymerization

Abstract

A novel heterobifunctional monomer, namely (6-ethenyl-3-phenyl-3,4-dihydro-2H-1,3-benzoxazine (VBOZO)), which contains both a vinyl group and benzoxazine group, was successfully prepared using the normal Wittig method from an aldehyde-containing benzoxazine precursor of 3-phenyl-3,4-dihydro-2H-1,3-benzoxazine-6-carbaldehyde. VBOZO underwent free radical polymerization initiated by azodiisobutyronitrile (AIBN) to form a homopolymer P(VBOZO) with a number-average molecular weight of about 5050 g mol⁻¹. Meanwhile the benzoxazine group remains unchanged in the copolymer and can undergo cationic polymerization at high temperature to form crosslinked polymers. Benzoxazine-containing linear P(VBOZO) has a T_g of about 125 °C and shows much better thermal stability than polystyrene. The peak temperature assigned to the cationic ring-opening polymerization of P(VBOZO) was centered at 263.0 °C. The VBOZO also copolymerized with styrene under free radical conditions to get the copolymer P(VBOZO-co-St). The polymerization character of VBOZO is very similar to styrene and the copolymer composition is very close to the feed ratio of monomers. All copolymers show high thermal stability and the char yields at 800 °C are over 20%.

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Introduction

Polybenzoxazine has attracted much attention due to its excellent mechanical and thermal properties with good handling capability for material processing and composite manufacturing.^1–10 Benzoxazine monomers can be obtained in large batches using a simple Mannich reaction and then polymerized through thermal cationic ring-opening of the oxazine ring without any initiator or catalyst. Their immensely rich molecular design flexibility allows for the design and preparation of various molecular structures with desired properties. Moreover, they release no reaction byproducts during the ring-opening polymerization reactions. Furthermore, no volatiles are released upon polymerization, and nearly zero shrinkage is achieved due to their ring-opening polymerization nature. These fascinating characteristics make polybenzoxazine a promising candidate for various applications such as electronics, aerospace, composites, coatings, and adhesives.^11–15

From a practical standpoint, polybenzoxazine still suffers from some disadvantages and these limitations will restrict its wide applications in high-tech fields. Usually benzoxazine monomers are powder-like organics and their film-forming ability cannot meet the requirements for most applications. Additionally, polybenzoxazines are brittle as a consequence of their rigid molecular structure and the high crosslink density of polybenzoxazine. So, several strategies have been developed to overcome the associated shortcomings of benzoxazine resins, such as (i) the preparation of modified monomers with an additional polymerizable group,^12,16 (ii) the synthesis of novel polymeric benzoxazine precursors,¹¹ and (iii) blending with high performance polymers or fillers and fibers.^13,15 The most direct route to increase the molecular weight and impact resistance of polybenzoxazine is the synthesis of well-defined high molecular weight polymeric precursors by incorporating benzoxazine units either as a side chain or as an chain-end or in the main chain.^11,12

To obtain a well-defined polymer, it is very important to choose both the right polymerization technology and right monomer. Chain growth polymerization, including free radical, ionic, ring-opening and coordination reactions, is the most powerful method to obtain commercial polymers. How to get a well-defined polymeric benzoxazine precursor with a high molecular weight depends mainly on the right choice of monomer.

The term bifunctional monomer refers to a monomer that possesses two polymerizable groups.^17–28 If the two polymerizable groups have identical reactivity (a homobifunctional monomer), they are used mainly to prepare crosslinked polymers in one step due to a lack of group selectivity. On the contrary, as the monomer has two kinds of polymerizable group (a heterobifunctional monomer) which are a low copolymerizing combination and have large differences in reactivity, a noncrosslinked soluble polymer whose main chain is formed by the more reactive functional group will be obtained with suspended pendants of the remaining less reactive group. Furthermore, the remaining functional group on the side chain in every repeating unit does not lose reactivity and will be allowed to undergo polymerization under more fitting conditions. These functional polymers can be used as self-crosslinkable materials.

In fact, some modified benzoxazines which possess a second polymerizable group have the potential to be used as bifunctional monomers to prepare well-defined polymeric benzoxazine precursors. Among them double-bond-related benzoxazines are highly attractive to researchers, as this type of benzoxazine can first undergo chain polymerization to obtain a well-defined linear polymer with dense benzoxazine pendent groups. Although allyl-containing benzoxazine monomers^29–37 have been widely studied, almost no research has reported that a linear polymeric benzoxazine precursor was prepared by these monomers using chain growth polymerization technology. A maleimide group^38–40 was introduced into benzoxazine and served as another polymerizable group. It was found that the bifunctional monomer showed a two stage process of thermal polymerization. Fusible polymaleimides with a T_g of around 100 °C could be obtained by thermal polymerization at 130 °C. Further polymerization of the polymaleimides at 240 °C resulted in a completely cured resin showing a T_g of 204 °C. But the structural details of the fusible polymaleimides were not given in that paper. The free radical copolymerization character of this monomer with styrene was also studied.²⁵ The first well-defined linear polymeric benzoxazine precursor formed through free radical polymerization was reported by Yagci’s group.⁴¹ A heterobifunctional monomer (BEM) possessing both methacrylate and benzoxazine functionalities was synthesized and characterized. BEM was then copolymerized with styrene by free radical polymerization using AIBN as initiator. It was found that the free radical copolymerization of this monomer is a convenient pathway that allows for the easy synthesis of a polymeric precursor with side-chain benzoxazines which can be cured thermally. But they didn’t supply any details about a BEM homopolymer. To further extend the study, Yagci and Ishida⁴¹ developed a new methacryloyl-benzoxazine-containing monomer and studied its photopolymerization behavior as well as its thermal polymerization. As the free radical initiator, benzoyl peroxide (BPO), was added into the monomer at 70 °C and it was found that even under such mild conditions the decomposition of the monomer is still happening; it appears that the monomer is relatively sensitive to heat, and at least some part of the molecule will undergo decomposition before its polymerization. It is demonstrated that photopolymerization^42,43 is feasible for anchoring the acrylate in the monomer in spite of the difficulty experienced with free radical polymerization with and without an initiator. A norbornene functionalized benzoxazine was also reported for preparing highly crosslinked polybenzoxazines.⁴⁴

In this article, we propose a new heterobifunctional benzoxazine, namely vinyl benzoxazine, which can be used as a universal monomer to prepare a linear polymeric precursor with a side-chain benzoxazine group. The typical character of this styrene-like monomer is that the second polymerizable double bond directly connects to the benzoxazine structure without linking spacers. Both the homo- and copolymerization characters of this monomer were studied.

Experimental

Reagents and solvents

p-Hydroxybenzaldehyde (Alfa, 98%, recrystallized from water), aniline (Alfa, 99+%), azodiisobutyronitrile (Sinopharm, 95%), styrene (Alfa, 99%, purification according to a standard method), potassium tert-butoxide (Alfa, 97%), methyltriphenyl-phosphonium bromide (98+%), formaldehyde (Sinopharm, 37%), THF (Sinopharm, 99%), dichloromethane (Sinopharm, 99%), petroleum ether (Sinopharm, 60–90 °C), ethyl acetate (Sinopharm, 98%) and toluene (Sinopharm, 98%).

3-Phenyl-3,4-dihydro-2H-1,3-benzoxazine-6-carbaldehyde (ABOZO)

ABOZO was prepared according to a reported method with some modifications.^45,46 Into a 500 mL three-necked flask with a stirrer, aqueous formaldehyde solution (32 g, 0.4 mol) and toluene (60 mL) were added at room temperature. The mixture was cooled down to 5 °C using ice water under stirring and then a cooled mixture of aniline (18.6 g, 0.2 mol) and toluene (20 mL) was dropped into the flask under rapid stirring and the temperature was kept below 5 °C. After 30 min stirring at this temperature, p-hydroxybenzaldehyde (24.4 g, 0.2 mol) in toluene (60 mL) was added into the flask under stirring. The temperature was then brought back to ambient temperature automatically without heating and kept under room temperature for one hour with continuous stirring. Then heating was applied and the temperature was gradually raised to 95 °C and stirred at this temperature for 5 h. Part of the solvent was removed from the reaction mixture under reduced pressure and chloroform was added to dissolve the residues. The resulting solution was washed with 0.5 mol L⁻¹ NaOH aqueous solution and water three times to remove the impurities and unreacted reagents. After that, the solvent was evaporated using a rotary evaporator. Then, the crude yellow product was recrystallized from toluene three times to afford a white crystalline powder (ABOZO) (mp 98–99 °C, yield 68%). ¹H NMR (CDCl₃, TMS, ppm): 9.79 (1H, CHO), 7.60 (1H, Ar-H), 7.54 (1H, Ar-H), 7.25 (2H, Ar-H), 7.09 (2H, Ar-H), 6.94 (1H, Ar-H), 6.89 (1H, Ar-H), 5.41 (2H, CH₂), 4.64 (2H, CH₂).

6-Ethenyl-3-phenyl-3,4-dihydro-2H-1,3-benzoxazine (VBOZO)

The standard Wittig method was adopted to prepare VBOZO. Under an argon atmosphere, methyltriphenylphosphonium bromide (10.072 g) was added into a 100 mL two-necked flask containing 40 mL of dry THF, then potassium tert-butoxide (5.62 g) was added into the flask under stirring. Rapidly the colorless solution turned bright yellow and the stirring was continued for 1 h. ABOZO (4.78 g, 20 mmol) in THF (20 mL) was dropped into the flask under stirring and the temperature was kept below 25 °C. After finishing the addition, the reaction proceeded at room temperature for 5.5 h. Then water was added to stop the reaction. The THF and water were removed using a rotary evaporator and the residue was extracted with CH₂Cl₂ (70 mL × 2). The organic phase was washed with water thoroughly and dried with anhydrous Na₂SO₄. After evaporation of the solvent, a viscous liquid was left. VBOZO was obtained as a white powder using chromatography (eluent: petroleum ether/ethyl acetate, 6/1). Yield 33%, mp 42–44 °C. Elemental analysis: found C, 81.43; H, 6.49; N, 5.77, calculated based on C₁₆H₁₅NO: C, 80.98; H, 6.37; N, 5.90.

Polymerization of VBOZO

VBOZO (710 mg, 3 mmol), AIBN (14.2 mg) and toluene (5 mL) were mixed in a polymerization tube. The reaction mixture was treated with three freeze–thaw cycles and the tube was sealed under argon. The tube was then heated up and stirred at 60 °C for 24 h under an argon atmosphere. After the reaction mixture was cooled to room temperature, the reaction solution was dropped into a large amount of methanol and the resulting precipitate was filtered off and collected to afford the crude product with a light yellow color. Purification was done using a dissolving-precipitation method three times in a toluene (or DMF)–methanol mixture. Finally 0.20 g of a white powder was obtained and the yield was 28%.

For copolymerization with styrene, the same procedure was used except a different amount of styrene was added and the molar ratio of VBOZO to styrene was varied.

Characterization

The structures of the compounds were verified using proton (¹H) and carbon (¹³C) nuclear magnetic resonance spectroscopy (NMR) using a Bruker AV400 NMR spectrometer at a proton frequency of 400 MHz at room temperature. Fourier transform infrared (FTIR) spectra were recorded with a Bruker Vertex 70 FTIR spectrometer. Solid samples were prepared as KBr pellets and liquid samples were prepared by casting onto KBr windows, and the spectra were recorded at room temperature in the region of 4000–400 cm⁻¹ with a resolution of 4 cm⁻¹. The thermal stability was investigated using thermogravimetric analysis (TGA) performed on a TG/DTA6300 thermogravimetric analyzer. Nitrogen was used as a purge gas for all testing. A heating rate of 20 °C min⁻¹ with a flow rate of 100 mL min⁻¹ was used for all tests. Differential scanning calorimetry (DSC) was undertaken using TA Instruments Q20 running TA Q Series Advantage software on samples placed in hermetically sealed aluminum pans. Experiments were conducted at a heating rate of 5 °C min⁻¹ from −20 to 300 °C under flowing nitrogen (20 mL min⁻¹). Elemental analysis was performed on an Elementar Vario Micro cube elemental analyzer. The molecular weight of the polymer was determined using gel permeation chromatography (GPC) in THF using polystyrene as standards. The flow rate of THF was maintained as 1 mL min⁻¹. The chromatograms were recorded using a Waters 510 pump and Waters 410 differential RI detector. Mass spectrometry (MS) was undertaken using Bruker Daltonics microTOF II electrospray ionization high resolution mass spectrometry, the quality range was 50–20 000 of m/z.

Results and discussion

Synthesis and characterization of VBOZO

The new heterobifunctional monomer, VBOZO, was first prepared in our group and its synthetic routes are shown in Scheme 1. First ABOZO was obtained according to Gu’s method⁴⁶ in good yield. The aldehyde group was transformed into a double bond using the universal Wittig method. Though VBOZO has a low melting point (42–44 °C) and always appears as a viscous liquid, crystals can be obtained readily after storing for a short time. High purity VBOZO suitable for polymerization was obtained using chromatography and its chemical structure was characterized using different methods.

Scheme 1 Synthesis route for VBOZO and its free radical polymerization and curing reactions.

The IR spectrum of VBOZO is shown in Fig. 1. The peak at 1627 cm⁻¹ is attributed to the vinyl group connected to the benzene ring and it means that the aldehyde group has successfully changed into a double bond. The peak located at 1600 cm⁻¹ is a typical benzene ring absorption. The bands at 1489 cm⁻¹, 1235 cm⁻¹, 1028 cm⁻¹ and 944 cm⁻¹ are assigned to the characteristic modes related to the benzoxazine ring. These peaks confirm the presence of both a vinyl and benzoxazine unit.

Fig. 1 FT-IR spectra of VBOZO and P(VBOZ).

The ¹H NMR spectrum of VBOZO is shown in Fig. 2. A set of peaks appear in the range of 6.69–7.25 ppm, which all come from the benzene ring. The chemical shifts for double bond protons are located at 6.61 ppm (=CH–Ph), and 5.64 ppm and 5.11 ppm (CH₂=C–). The benzoxazine ring is characterized by resonance peaks at 4.66 ppm (Ar-CH₂–N) and 5.45 ppm (O–CH₂–N). Compared with reported results, these peaks shifted to higher ppm due to the enlarged conjugated structure. Meanwhile, the integrated peak area ratio of these protons is consistent with the theoretical value based on the chemical structure of VBOZO.

Fig. 2 ¹H NMR spectrum of VBOZO (solvent: DMSO-d₆).

The ¹³C NMR spectrum of VBOZO is shown in Fig. 3. The peaks assigned to the benzene ring and double bond appear at 154.12 ppm, 148.06 ppm, 136.05 ppm, 130.48 ppm, 129.28 ppm, 128.56 ppm, 127.12 ppm, 125.68 ppm, 124.44 ppm, 121.58 ppm, 120.61 ppm, 118.41 ppm, 116.76 ppm and 111.70 ppm. The typical benzoxazine ring protons appear at 79.62 ppm (–O–CH₂–N–) and 50.45 ppm (Ar-CH₂–N–).

Fig. 3 ¹³C NMR spectrum of VBOZO (solvent: CDCl₃).

The thermal properties of VBOZO were studied using DSC and the results are shown in Fig. 4. From the DSC curve, the sharp endothermic peak located at 42.7 °C is assigned to the melting point of VBOZO and the value agrees very well with that obtained from the melting point detector. The small exothermic process around 120–170 °C is due to the thermally initiated free radical homopolymerization of VBOZ. Most free radical monomers show thermal polymerization character at relatively high temperature. The ring-opening polymerization of benzoxazine occurs in the temperature range of 200–250 °C, with the exothermic peak located at 219.35 °C.

Fig. 4 DSC curve of VBOZO.

Polymerization of VBOZO initiated using AIBN and characterization of the corresponding polymer

As expected, the newly designed heterobifunctional monomer VBOZO can be initiated using AIBN, a typical free radical initiator under normal polymerization conditions. After polymerization and purification, a white powder was obtained and the sample was used for structural analysis. After polymerization, the typical double bond absorption (peak appearing at 1627 cm⁻¹) disappears completely (Fig. 1) and a set of new peaks located at 2920 cm⁻¹ and 2850 cm⁻¹ due to the polymer chain appears, which means that the monomers have been transformed into a polymer under the polymerization conditions. More importantly, the characteristic absorptions for a benzoxazine unit, such as peaks at 1230 cm⁻¹, 1030 cm⁻¹ and 948 cm⁻¹, still remain. This result shows that under free radical polymerization conditions, the benzoxazine unit is stable enough and doesn’t undergo thermally initiated ring-opening polymerization.

The ¹H NMR spectrum of P(VBOZO) is shown in Fig. 5. Compared with the ¹H NMR spectrum of VBOZO (Fig. 2), the peaks assigned to the double bond (6.61 ppm, 5.64 ppm and 5.11 ppm) completely disappear, and new wide peaks located in the range of 0.75–2.25 ppm are found which are attributed to main chain polyethylene protons. This change indicates that the double bonds have undergone polymerization. The peaks between 6.0–7.5 ppm are due to benzene protons. After free radical polymerization, the typical benzoxazine peaks locating at 5.28 ppm (–O–CH₂–N–) and 4.31 ppm (Ar-CH₂–N–) still exist. It also certifies that the benzoxazine groups remain unchanged during free radical polymerization.

Fig. 5 ¹H NMR spectrum of P(VBOZO) (solvent: DMSO-d6).

The molecular weight of P(VBOZO) was measured using a GPC method, and the number-average molecular weight (M_n) and weight-average molecular weight (M_w) are 5050 g mol⁻¹ and 9360 g mol⁻¹, respectively, and the polydispersity index (PDI) is 1.85. The molecular weight of P(VBOZO) is relatively low at present, with the possible reason being the high steric hindrance of the benzoxazine unit in the monomer.

The thermal properties of P(VBOZO) were studied using DSC and TGA. From the DSC result (Fig. 6), we find the T_g of P(VBOZO) is about 125 °C, higher than that of polystyrene. The high T_g results in a large spatial volume for the benzoxazine group and it needs more free volume to rotate around the main chain. A sharp exothermic peak attributed to the ring-opening polymerization of benzoxazine appears in the temperature range of 230–290 °C; the peak temperature is nearly 263 °C and possesses a polymerization enthalpy of 198 J g⁻¹. Compared with the monomer, the peak temperature increased dramatically due to the rigid nature of the benzoxazine-containing polymer.

Fig. 6 DSC and TGA curves of P(VBOZO).

The thermal stability of P(VBOZO) is also shown in Fig. 6. No weight loss is observed below 250 °C. The 5% and 10% weight loss temperatures of P(VBOZO) are as high as 306 °C and 363 °C, respectively. High thermal stability is very important for a benzoxazine-containing thermoplastic resin. This type of polymer has the potential to be processed as a traditional thermoplastic resin and then a post-curing procedure is applied to obtain high performance crosslinked materials. The char yield (CR) of P(VBOZO) at 800 °C is as high as 35%, which is much bigger than that of polystyrene. It is well known that at such high temperatures the char yield of polystyrene is near zero and the normal limited oxygen index is about 18%.^47,48 The high thermal stability of P(VBOZO) comes from the fact that the benzoxazine unit in P(VBOZO) will undergo ring-opening polymerization at high temperatures and result in a crosslinked polymer (Scheme 1).^1,11–15 So this research provides a brand new route to prepare high thermal stability thermoplastic resin with a potential curing group in the main chain.

To better understand the curing processes of P(VBOZO), the curing kinetics of P(VBOZO) were investigated using non-isothermal differential scanning calorimetry (DSC) at different heating rates (Fig. 7).^49–52 It can be observed that the exothermic peak shifts to a higher temperature with a higher heating rate. Kissinger, Ozawa, and Flynn–Wall–Ozawa methods were used to determine the kinetics parameters and built the kinetics models. The activation energy of P(VBOZO) is 145.0 kJ mol⁻¹ (Kissinger method), 146.2 kJ mol⁻¹ (Ozawa method) and 144.5 kJ mol⁻¹ (Flynn–Wall–Ozawa method). Similar to a traditional bisphenol-A based benzoxazine resin, P(VBOZO) also shows self-catalysis character.

Fig. 7 DSC thermograms of P(VBOZO) at different heating rates.

Copolymerization with styrene and properties of the copolymers

The copolymers of VBOZO with styrene were also prepared by free radical polymerization and the data is listed in Table 1. The molecular weight of the copolymer is a little higher than that of the P(VBOZO) homopolymer. The possible reason is that the steric hindrance resulting from VBOZO is reduced by styrene. From the IR spectra of these copolymers, the typical absorption peaks for benzoxazine appear at 1230 cm⁻¹, 1030 cm⁻¹ and 950 cm⁻¹. The composition of the copolymers was calculated from their ¹H NMR spectra and the results are collected in Table 1. The data shows that the composition of the copolymers is very close to the feed ratio of the monomers, which means VBOZO and styrene have a similar polymerization character.

Table 1 Free radical polymerization of VBOZ with St in toluene

Polymer/feed ratio	Conversion^a (%)	Composition (NMR)	Mn^b	Mw/Mn^b
a Determined from weight.b Determined using GPC measurements.
P(VBOZ)	28	100	5050	1.85
P(VBOZO/St) 2 : 1	49	69.4/30.6	8950	1.48
P(VBOZO/St) 1 : 1	42	52.2/47.8	6600	1.73
P(VBOZO/St) 1 : 2	37	35.2/64.8	7600	1.35

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The thermal ring-opening polymerization character of the copolymers was studied using DSC. From the DSC curves, we can figure out that the peak temperatures for the benzoxazine polymerization of the three copolymers are 263 °C, 254 °C and 272 °C (Fig. 8).

Fig. 8 DSC thermograms of P(VBOZO-co-St).

The thermal stability of the three copolymers was evaluated using TGA (Fig. 9) and the data are listed in Table 2. Both the 5% and 10% weight loss temperatures of the copolymers are higher than that of polystyrene. Most importantly, exceeding 400 °C, the thermal stability of the copolymers is much better than that of polystyrene. At 500 °C, polystyrene nearly decomposes completely and the char yield is 0.95%, while all copolymers show high thermal stability and the char yields at 800 °C are over 20%. So introducing benzoxazine groups into a thermoplastic polymer can effectively increase the thermal stability and fire retardancy. We can further imagine that if we pre-treat the copolymers within a suitable temperature range and make the benzoxazine group undergo ring-opening polymerization thoroughly, then the final crosslinked polymer will show much better thermal stability.

Fig. 9 TGA thermograms of P(VBOZO-co-St) and PS.

Table 2 Thermal properties of P(VBOZO-co-St) and PS

Polymer	T5%^a (°C)	T10%^b (°C)	Yc800^c (%)
a The 5% weight loss temperature under a nitrogen atmosphere.b The 10% weight loss temperature under a nitrogen atmosphere.c Char yield at 800 °C under a nitrogen atmosphere.
P(VBOZO-co-St) 1 : 1	349	382	30.7
P(VBOZO-co-St) 1 : 2	330	385	20.1
PS	329	353	0.95

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Conclusions

Heterobifunctional monomers combine different polymerization technologies together and can act as ideal starting materials for the preparation of well-defined polymer precursors. Benzoxazines possesses a high polymerization temperature during cationic ring-opening polymerization and shows excellent thermal stability under normal free radical polymerization conditions. Introducing a vinyl group into a benzoxazine structure is a direct route to prepare a novel benzoxazine-containing heterobifunctional monomer. In this research, a novel heterobifunctional monomer, VBOZO, which contains both a vinyl group and benzoxazine group, was successfully prepared using the Wittig reaction. VBOZO can be initiated using AIBN to obtain a well-defined homopolymer P(VBOZO) with pendent benzoxazine groups. The M_n of P(VBOZO) is about 5050 g mol⁻¹. The benzoxazine-containing linear P(VBOZO) has a T_g of about 125 °C and shows much better thermal stability than that of polystyrene. The peak temperature assigned to cationic ring-opening polymerization of P(VBOZO) is located at 263.0 °C. The VBOZO also copolymerized with styrene under free radical conditions to get copolymer P(VBOZO-co-St). The polymerization character of VBOZO is very similar to styrene and the copolymer composition is very close to the feed ratio of monomers. All copolymers show high thermal stability and the char yields at 800 °C are over 20%.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21274049), and the Natural Science Foundation of Hubei Province, China (Grant No. 2015CFB188), and Opening Project of Key Laboratory of Optoelectronic Chemical Materials and Devices of Ministry of Education, Jianghan University (No. JDGD-2013-06).

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