GAO Xingcao()， MA Mengjiao()， LI Congcong()， DING Liushan()， YAN Xuhuan ()， XI Peng( )，

1 Tianjin Polytechnic University， Tianjin 300387， China2 Tianjin Key Laboratory of Advanced Fibers and Energy Storage, Tianjin 300387， China

Abstract： A new multi-branched benzoic acid rare earth complex (MBBAL-Eu(III) complex) was prepared. The advanced tested technologies were employed to characterize the composition and structure of as-prepared complex. Scanning electron microscopy (SEM) image shows that as-prepared complex has a layer structure. Transmission electron microscopy (TEM) image presents that the shape of MBBAL-Eu(III) complex is similar to oval; the size is in the range of 20-50 nm. The thermogravimetric analysis (TGA) curves of MBBAL-Eu(III) complex reveal that as-prepared complex has good thermal stability. The PET luminescence fibers with MBBAL-Eu(III) complex were prepared through melt-spinning and electrospun methods. The results prove that as-prepared fibers with MBBAL-Eu(III) complex have good luminescent properties and show bright red light.

Key words： luminescence fiber; rare earth complex; thermal stability; solvent resistance; electrospun

Introduction

Environmental pollution and the increase of greenhouse gas emissions have made the novel revolution of dyeing and printing become a crucial topic. In recent years， there is growing interest in exploring luminescence fibers and fabrics， since they can exhibit specific and fascinating colors without dyeing and printing[1-3]. In these luminescent products， the rare earth luminescent materials are playing an important role due to their excellent luminescent properties， such as extremely sharp emission bands， long lifetime， and potential high internal quantum efficiency[4-6]. A few luminescence fibers and fabrics had been prepared through melt-spinning， wet-spinning and composite spinning technologies[7-9]. However， the luminescent materials are mostly inorganic materials in as-prepared luminescence fibers and fabrics. The compatibility of these inorganic materials and fiber-forming polymers is poor. After a period of time， these luminescent materials will be precipitated from fiber-forming polymers， which make as-prepared luminescence fibers and fabrics lose luminescent properties.

Compared with inorganic luminescent materials， the organic rare earth complexes have good compatibility with the fiber-forming polymers. The luminescence fibers and fabrics with the organic rare earth complexes present better luminescent properties than those with inorganic luminescent materials[10-13]. However， pure organic rare earth complexes usually have poor thermal stability and processing properties， which restrict their applications. So, the design and synthesis of the organic rare earth complex with good thermal stability and processing properties have become a much imminent problem[14-15].

In this paper， we designed and synthesized a new multi-branched benzoic acid ligand(MBBAL). The organic rare earth complex was prepared through coordination reaction of the MBBAL, 1，10-phenenthroline and europium ions. The Fourier transform infrared (FT-IR)，1H nuclear magnetic resonance (1H NMR)， elemental analysis (EA)， scanning electron microscopy(SEM)， transmission electron microscopy(TEM) and thermogravimetric analysis(TGA) were used to characterize the structure and properties of the organic rare earth complex. The good thermal stability and solvent resistance of the as-prepared organic rare earth complex endow the complex with wide applications on luminescent products.

1 Experimental

1.1 Reagents and materials

Methyl 4-(chloromethyl) benzoate was purchased from Tokyo Chemical Industry Co.， Ltd.， Japan. Glycerol was from Tianjin Fengchuan Chemical Reagent Technology Co.， Ltd.， China. Eu2O3(99.99%)was purchased from Shanghai Yuelong New Materials Co.， Ltd.， China. NaH and 1，10-phenanthroline were purchased from Aladdin Bio-Chemical Technology Co.， Ltd.， China. Poly(ethylene terephthalate) (PET) was purchased from Tianjin Huaxinying Polyester Material Technology Co.， Ltd.， China. Other chemicals were purchased from Tianjin Kemiou Chemical Reagent Co.， Ltd.， China. All materials were used without further purification.

1.2 Synthesis of the multi-branched benzoic acid ligand

The reaction was performed in a three-necks round bottomed flask fitted with an overhead stirred， nitrogen inlet and an addition funnel[16-17]. First， 45 mL N，N-Dimethylformamide (DMF) and 1.60 g NaH were added into the flask and stirred at 0 ℃ for 20 min. And then， 0.70 mL glycerol was dropped into the mixture and stirred at 0 ℃ within 40 minutes. Meanwhile， methyl 4-(chloromethyl) benzoate (5.54 g， 0.03 mol) was dissolved in DMF (15 mL). The clear solution was dropped into above reaction solution at 0 ℃ within 20 min. The reaction was continued under nitrogen at 80 ℃. Completion of the reaction was indicated by the disappearance of absorbed peak of C—Cl in methyl 4-(chloromethyl) benzoate molecule using FT-IR spectroscopy. At last， the reaction solution was filtered and evaporated， and crude product was prepared. The final MBBAL was got through recrystallization.

The synthesis route of the ternary rare earth complex with MBBAL and 1，10-phenanthroline according to the following process. MBBAL (0.000 5 mol， 0.247 g) was dissolved with stirring in ethanol (30 mL) at 50 ℃ to obtain an MBBAL-ethanol solution (solution 1). A solution of EuCl3(0.000 5 mol， 0.130 g) in ethanol was prepared and added to solution 1; the pH value of the mixed solution was adjusted by NH3·H2O. After the mixed solution was stirred at 50 ℃ for 2 h， it was cooled to 25 ℃ and maintained for 24 h at this temperature. Finally， the precipitate was quickly filtered; the as-prepared product was washed five times with ethanol (10 mL each time) and deionized water (10 mL each time),respectively. The samples were kept in a vacuum oven at 80 ℃ for 24 h to obtain the final product (MBBAL-Eu(III) complex)[18-20].

1.3 Preparation of PET luminescence fibers with MBBAL-Eu(III) complex

The PET luminescence fibers with MBBAL rare earth complex via melt-spinning technology were prepared according to the following process[21]. PET pellets were first dried in a vacuum oven at 120 ℃ for 12 h. PET luminescence pellets were prepared through melt-blended method using twin screw extruder. The barrel temperature was set 230 - 280 ℃ from the feed zone over six zones; the rotation speed was 80 r/min. The content of MBBAL-Eu(III) complex was 1% in as-prepared PET luminescence pellets.

PET luminescence fibers were prepared using a single screw extruder connected to a syringe pump， the screw diameter was 20 mm. The polymer was extruded through a spinneret with twelve orifices. The total extrusion rate was 30 g/min，i.e. 2.5 g/min per orifice. The temperature of the spinneret head and the orifice was 260 ℃. The temperature of the screw was 245 ℃ (zone 1)， 260 ℃ (zone 2)， 260 ℃ (zone 3)， 260 ℃ (zone 4)， 260 ℃ (zone 5) and 260 ℃ (zone 6). The spinning chamber and the syringe pump had a temperature of 280 ℃. The extruded fibers were wound up on a godet roll with a diameter of 190 mm after passing through two ceramic guides.

The nano-micron meters PET luminescence fibers were prepared via electrospun technology. Firstly， PET (1 g) was dissolved in a component solvent of DMF and ethyl acetate; the mass percent of PET in the solvent was 17%. Then， MBBAL-Eu(III) complex was added to this solution in different amounts. The as-prepared solutions were stirred continuously for 24 h to obtain uniform electrospinning solutions. Finally， a series of nano-micron meters fibers were obtained with electrospinning， the electrospinning voltage was 15 kV， the advancing speed was 0.6 mL/h， and the receiving distance was 20 cm.

1.4 Measurements and characterization of the samples

FT-IR spectra of samples were collected with a Nicollet NEXUS-670 FT-IR spectrometer through KBr disks for wavenumbers of 400-4 000 cm-1.1H NMR spectrum of the ligand was collected using a JEOL EX-400 NMR spectrometer at room temperature with dimethyl sulfoxide as the solvent. EA was performed using an Elementary Vario EL analyzer. SEM image of the MBBAL-Eu(III) complex was recorded using a Hitachi S4800 (Hitachi Ltd.， Japan) field-emission scanning electron microscope operated at an acceleration voltage of 10 kV; the sample along with the conductive adhesive was fixed on a copper target and sputter coated with gold. TEM image of the MBBAL-Eu(III) complex was recorded using a Hitachi H7650 (Hitachi Ltd.， Japan) transmission electron microscope. The thermal stabilities of the samples were investigated with a Netzsch STA 449F3 TGA system. The heating rate was 10 ℃/min， and the temperatures investigated were 0-1 000 ℃. Steady-state luminescence spectra were collected using a HORIBA FluoroLog-3-Ultra Fast spectrophotometer.

2 Results and Discussion

2.1 Structure and morphology of MBBAL-Eu(III)complex

Figure 1 shows the1H NMR spectrum of the MBBAL. As shown in Fig. 1， the chemical shift at 12.91 ppm (a) corresponds to the proton peak of COOH. The chemical shifts at 7.50-8.00 ppm (b) belong to the proton peak of benzene ring. The chemical shift at 4.86 ppm (c) is the proton peak of CH2. The chemical shift at 3.65 (d) ppm corresponds to the proton peak of CH2—O[22-23]. These results prove that the new ligand has been prepared successfully.

Table 1 Content of each element in the complex

Fig. 1 1H NMR spectrum of the MBBAL complex

Fig. 2 FT-IR spectra of(a) MBBAL; (b) 1，10-phenanthroline; (c) MBBAL-Eu(III) complex

Fig. 3 Structure schematic of MBBAL-EU(III)complex

Fig. 4 Images of the MBBAL-Eu(III)complex: (a) SEM and(b) TEM

2.2 Thermal properties of MBBAL-Eu(III) complex

The thermogravimetric analysis of MBBAL-Eu(III) complex was carried out in nitrogen and air atmosphere. In Fig. 5， the MBBAL-Eu(III) complex has good thermal stability before the temperature reaches 305 ℃ in nitrogen atmosphere. In air atmosphere， the TG curve of MBBAL-Eu(III) complex is similar to that in nitrogen atmosphere until the temperature reaches 485 ℃. The result indicates that the thermal degradation temperaure of the complex is higher than the melt extrusion temperature (280 ℃) of the PET. So, the PET luminescence fibers were able to be produced through melt-spinning method.

Fig. 5 TG curves of MBBAL-Eu(III) complex in nitrogen (a) and air atmosphere (b)

2.3 Luminescent properties of MBBAL-Eu(III)complex

Figure 6 gives the fluorescence spectra of MBBAL-Eu(III) complex. In the excitation spectrum of the complex， the excitation wavelength range of 200-500 nm. There are three obvious peaks locating at 302 nm， 396 nm and 466 nm. The stronger broad band from 250 nm to 375 nm is attributed to the π→π* transition of MBBAL[26-28]. The weak peaks at 396 nm and 466 nm are assigned to the7F0→5L6and7F0→5D2transitions of the Eu(III) ions. The results prove that the MBBAL can absorb ultraviolet light from 275 nm to 345 nm. The absorbed UV lights are transmitted to Eu3+ions and sensitize Eu3+ions to give luminescence. The strongest absorbed peak is at 302 nm.

Fig. 6 Fluorescence spectra of MBBAL-Eu(III) complex

In the emission spectrum of the MBBAL-Eu(III) complex， the strongest emission peak is at 616 nm， which is assigned to the5D0→7F2transition. This leads MBBAL-Eu(III) complex to show bright red light. The half high width of the peak is 15 nm. The other weak emission peaks located in 593 nm， 653 nm and 702 nm are attributed to5D0→7F1，5D0→7F3and5D0→7F4transitions of Eu3+ions. The intensities of these peaks are only about 1/6 of5D0→7F2emission peak. The results indicate that MBBAL-Eu(III) complex has good color purity.

Figure 7 shows the emission spectra of the MBBAL-Eu(III) complex at different pH values. From Fig. 7， it can be found that the MBBAL-Eu(III) complex has good luminescent properties within the pH ranging from 7.5 to 9.0. Figure 8 gives the luminescent properties of the MBBAL-Eu(III) complex in different solvent. As shown in Fig. 8， the MBBAL-Eu(III) complex shows good red light in different solvents. The results prove that the MBBAL-Eu(III) complex has good solvent resistance. With the MBBAL-Eu(III) complex， the luminescence fibers can be prepared through wet-spinning.

Fig.7 Fluorescence spectra of the MBBAL-Eu(III) complex at different pH values

Fig.8 Luminescent properties of the MBBAL-Eu(III) complex in different solvents

2.4 Luminescent properties of PET luminescence fibers with MBBAL-Eu(III) complex

Figure 9 gives the photos of PET luminescence master-batch and fibers with MBBAL-Eu(III) complex through melt-extruded method. As presented in Fig. 9， the PET luminescence master-batch with MBBAL-Eu(III) complex is white， which indicates that MBBAL-Eu(III) complex won’t happen oxidative decomposition in melted-extruded process of the mixture of MBBAL-Eu(III) complex and PET. The melted-spinning PET luminescence fibers show bright red light.

Fig.9 Photos of PET luminescence master-batch and fibers with MBBAL-Eu(III) complex through melt-extruded method

As the PET could not be produced directly through wet-spinning， the electrospun technology was employed to investigate the processing properties of MBBAL-Eu(III) complex in polymer solution. Figure 10 (a) gives the SEM image of electrospun PET luminescence fibers. The diameter distribution of electrospun PET luminescence fibers is in the range of 60-140 nm. The emission spectra of electrospun PET luminescence fibers are presented in Fig.10(c). It can be found that the luminescence intensity of electrospun PET luminescence fibers including 3% MBBAL-Eu(III) complex reaches 1 400. The value is more than quarter that of net MBBAL-Eu(III) complex (as shown in Fig.6). These results prove that MBBAL-Eu(III) complex can be evenly distributed among the PET molecules. After electrospinning， the luminescent properties of MBBAL-Eu(III) complex were significantly enhanced. Figure 10(d) gives the photos of the nonwoven under sun and UV lights. The result proves that the nonwoven could show bright red light under UV lights.

Fig.10 SEM image and luminescence properties batch of PET electrospun luminescence fibers and nonwoven: (a) SEM image; (b)diameter distribution; (c) emission spectra; (d) the nonwoven under sun and UV lights

3 Conclusions

Based on luminescent mechanism of rare earth complex， a novel multi-branched benzoic acid complex was prepared. The PET luminescence fibers with MBBAL-Eu(III) complex were processed through melt-spinning and electrospun methods. The results indicate that the MBBAL-Eu(III) complex has good thermal stability and solvent resistance. The melt-spinning and electrospun PET luminescence fibers show bright red light. These results indicate that the potential applications of the MBBAL-Eu(III) complex on luminescent products.