Preparation and Luminescence Properties of Nylon 6 Composite Nanofibers

ZHANG ManXI Peng23WANG YanSHU Dengkun

1 School of Materials Science and Engineering,Tiangong University,Tianjin 300387,China 2 State Key Laborary of Separation Membranes &Membrane Process,Tianjin 300387,China 3 Tianjin Key Laboratory of Advanced Fibers and Energy Storage,Tianjin 300387,China

Abstract:The luminescent fibers have a good application prospect. The feature of this paper is that efficient luminescent nylon 6 (PA6) composite nanofibers are successfully prepared by electrospinning. The luminescent PA6 composite nanofibers composed of PA6,Eu(BA)3Phen and Tb(BAO)3Phen (BA= P-methylbenzoic acid,BAO=P-methoxybenzoic acid,and Phen=1,10-phenanthroline). The structure and properties were characterized by scanning electron microscopy (SEM),Fourier transform infrared spectroscopy (FT-IR),fluorescence spectroscopy,and thermogravimetriy (TG) analysis. The correspondence between polymer matrix and as-prepared composite nanofibers properties has also been studied in detail. Through using hexafluoroisopropanol(HFIP) as solvent,Eu(BA)3Phen/PA6 and Tb(BAO)3Phen/PA6 composite nanofibers exhibit good luminescence properties. It is noted that only 5% rare earth luminescent materials are added to Tb(BAO)3Phen/PA6 composite nanofibers,and the luminescence intensity of the as-prepared nanofibers reaches half that of the pure rare earth luminescent materials. Furthermore,uniform dispersion of pure rare earth luminescent materials in the as-prepared nanofibers gives the composite nanofibers good mechanical properties and thermal stability. These results provide an important basis for the preparation and wide application of new PA6 luminescent fibers.

Key words:luminescence;nanofibers;rare earth complex;nylon 6 (PA6)

Introduction

Recently,distinctive luminescent properties of trivalent lanthanide ions have absorbed much attentions due to their extensive applications in many fields,such as white light-emitting diodes[1],flexible display[2],luminescent sensors[3]and medical diagnostics[4]originated from their 4f electron transition.Among rare earth ions,Eu3+can emit brilliant red light based on their energy level transitions of5D0→7F2[4].Tb3+mainly shows green light through their transitions of5D4→7F5[5].Dy3+is able to show emission in the blue region via transitions of4F9/2→6H13/2[6].White light and full-colors lights can be obtained by adjusting the ratio of different rare earth ions.However,pure rare earth ions show the low luminescent intensity because their f-f transition is forbidden[7].

Among the luminescent materials with different lanthanide ions,rare earth complexes show better luminescent properties than pure rare earth ions due to the antenna effect of ligands and the f-f electron transition of rare earth ions[8].In particular,benzoic acid rare earth complexes exhibit excellent luminescence properties,such as high luminescence intensity,long luminescence lifetimes,and good color purity.According to these advantages,benzoic acid rare earth complexes have been used in lasers[9],phosphors[10],chemosensors[11],bioimaging probes[12],and flexible display[13].However,the thermal stability and processing properties of rare earth complexes are so poor that they cannot meet the needs of the wide applications[14].To overcome these shortcomings,the rare earth complexes usually were incorporated into inorganic,organic,and polymer matrices[15].

Among many polymer materials,nylon 6 (PA6) is a better fiber-forming polymer.The PA6 nanofibers prepared by electrostatic spinning technology have smooth surface,narrow diameter distribution and good mechanical properties,and have a good application prospect[25].However,PA6 luminescent fibers with rare earth complexes have not been reported.In previous studies,conventional PA6 electrospinning solvent systems[26],such as formic acid or acetic acid,were used.But,it was difficult to prepare PA6 luminescent fibers with good luminescence performance.These results indicate that solvent selection is very important to prepare luminescent fibers by electrospinning.In this study,hexafluoroisopropanol(HFIP) was selected as the spinning solvent,and Eu(BA)3Phen and Tb(BAO)3Phen were used as the luminescent materials to prepare PA6 luminescent fibers with red and green lights by electrospinning.The dispersion state of rare earth complexes in nanofibers was characterized by scanning electron microscopy (SEM) and energy dispersive spectra (EDS).The optical mechanism of rare earth ions in PA6 matrix was analyzed through the Judd-Ofelt theory.

1 Experiments

1.1 Materials

PA6 (powder,with a weight-average molecular weight of 1.2×105) was purchased from Tianjin Haijing Xinli Co.Ltd.,China.HFIP was purchased from Tianjin Kemiou Chemical Reagent Co.Ltd.,China.P-methoxybenzoic acid terbium complex (Tb(BAO)3Phen) and p-methylbenzoic acid europium complex (Eu(BA)3Phen) were synthesized as reported in Ref.[26].Their particle size ranges from 60 nm to 80 nm.

1.2 Preparation of electrospinning solution

The spinning solutions used to fabricate PA6 composite nanofibers were prepared as follows.Firstly,1 g PA6 was dissolved in 5.25 mL HFIP.Secondly,the appropriate amount of rare earth complexes was added.Finally,the mixed solution was stirred for 10 h at room temperature until transparent.

1.3 Preparation of PA6 composite nanofibers with luminescent performance

As shown in Fig.1,the as-prepared spinning solutions were placed in a 10 mL plastic syringe,which had a 22G stainless-steel needle attached (an inner diameter of 0.41 mm).During the electrospinning,the solution flow rate was maintained at 0.8 mL/h by using a microinjection pump.The spinning voltage was 18 kV.The distance between the tip of the needle and the collecting aluminum plate was 15 cm.The spinning time was 2 h.The spinning temperature was 25 ℃ and the humidity was adjusted to 60%.The PA6 composite nanofibers were formed on the collecting aluminum plate and removed for further characterization.

Fig.1 Preparation process of PA6 composite nanofibers with luminescent function

1.4 Measurements and characterization of the samples

The morphologies of samples were observed by a Hitachi S4800 (Hitachi Ltd.,Japan) field-emission SEM (FE-SEM).EDS of samples were collected by Edax Apollo XL (Edax Co.Ltd.,America).Steady-state luminescence spectra of samples were collected by an F-380 fluorescence spectrophotometer (Gangdong Co.Ltd.,China).The excitation wavelength range was 200-400 nm,the emission wavelength range was 400-760 nm,and the slit width was 2 nm.The Fourier transform infrared (FT-IR) spectra of samples were collected with a Nicollet NEXUS-670 FT-IR spectrometer through KBr disks for wavenumbers of 4 000-500 cm-1.Photoluminescence lifetimes and photoluminescence quantum yields of samples were obtained by an Edinburgh FLS1000 spectrophotometer.The thermal decomposition performances of samples were tested by a 209F3 TARSUS thermogravimetric analyzer.The mechanical performance of the PA6 composite nanofibers was measured by the YG026 (Jigao Co.Ltd.,China) electronic single fiber strength meter.The nanofibers were prepared into a yarn-like sample,the diameter was 0.15 mm,and the stretching rate was 20 mm/min.

2 Results and Discussion

2.1 Morphology and structure of PA6 composite nanofibers

Morphologies and elemental compositions of Eu(BA)3Phen/PA6 and Tb(BAO)3Phen/PA6 composite nanofibers were characterized through FE-SEM and EDS.From Figs.2(a) and (b),it can be found that Eu(BA)3Phen/PA6 and Tb(BAO)3Phen/PA6 composite nanofibers have uniform diameter distribution.The average diameters of Eu(BA)3Phen/PA6 and Tb(BAO)3Phen/PA6 composite nanofibers are 668 nm and 665 nm,respectively.Figures 2(c) and (d) show the EDS of Eu(BA)3Phen/PA6 and Tb(BAO)3Phen/PA6 composite nanofibers.The results reveal that Eu(BA)3Phen/PA6 and Tb(BAO)3Phen/PA6 composite nanofibers consist of C,O,N,Eu and Tb elements;the Eu and Tb elements come from Eu(BA)3Phen and Tb(BAO)3Phen,respectively.As shown in Figs.2(e) and (f),as-prepared Tb(BAO)3Phen/PA6 and Eu(BA)3Phen/PA6 composite nanofibers present bright green and red colors,respectively.At the same time,the PA composite nanofiber film has a good mechanical flexibility.The film can be bended 180oby hand.The results provide an important basis for the applications of Eu(BA)3Phen/PA6 and Tb(BAO)3Phen/PA6 composite nanofibers with luminescent function.

Fig.2 PA6 composite nanofibers and nanofibrous film with 8% Eu(BA)3 Phen and Tb(BAO)3 Phen:(a)-(b) SEM images;(c)-(d) EDS;(e) luminescent photographs;(f) bending photograph

2.2 Luminescent properties of PA6 composite nanofibers

Figures 3(a) and (b) give luminescent spectra of Eu(BA)3Phen/PA6 composite nanofibers.In the excitation spectrum of Eu(BA)3Phen/PA6 composite nanofibers,there is a broad absorption band in the range of 200-400 nm.The highest absorbed peak is at 297 nm.The result indicates that Eu(BA)3Phen/PA6 composite nanofibers can absorb the most energy and exhibit the best luminescent performance under 297 nm ultraviolet light (UV) irradiation.The emission spectrum of Eu(BA)3Phen/PA6 composite nanofibers is shown in Fig.3(b).In Fig.3(b),three obvious fluorescent emission peaks were observed at 597,616 and 700 nm,respectively.These fluorescent emission peaks correspond to the energy level transitions of5D0→7F1,5D0→7F2and5D0→7F4of Eu3+ions[27].The highest emission peak appears at 616 nm.So the Eu(BA)3Phen/PA6 composite nanofibers show red color under 297 nm UV irradiation.

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(d)Fig.3 Luminescent spectra of (a)-(b) Eu(BA)3 Phen/PA6;(c)-(d) Tb(BAO)3 Phen/PA6 composite nanofibers including 8% rare earth complexes

Figures 3(c) and (d) present luminescent spectra of Tb(BAO)3Phen/PA6 composite nanofibers.The excitation spectrum of Tb(BAO)3Phen/PA6 composite nanofibers is similar to that of Eu(BA)3Phen/PA6 composite nanofibers.The strongest excitation peak is at 298 nm.There are four obvious peaks at 490,546,580 and 618 nm in the emission spectrum of Tb(BAO)3Phen/PA6 composite nanofibers.These peaks are attributed to the energy level transitions of5D4→7F6,5D4→7F5,5D4→7F4，and5D4→7F3of Tb3+ions[28].The highest peak is at 546 nm.However,the second peak (at 490 nm) is also significantly higher than the other two absorption peaks (at 580 nm and 618 nm),and its intensity is half of the peak at 546 nm.So the Tb(BAO)3Phen/PA6 composite nanofibers show yellow-green color under 298 nm UV irradiation.

2.3 Effect of rare earth complex contents on the luminescence properties of PA6 composite nanofibers

To study the effect of different amounts of rare earth complexes on the luminescence properties of PA6 composite nanofibers,the emission spectra of Eu(BA)3Phen/PA6 and Tb(BAO)3Phen/PA6 composite nanofibers with 1%,2%,3%,4%,5%,6% and 7% rare earth complexes were tested (shown in Fig.4) and the results were summarized in Table 1.From Fig.4 and Table 1,fluorescence intensities of composite nanofibers increased along with adding more Eu(BA)3Phen and Tb(BAO)3Phen complexes.However,when the content of rare earth complexes is more than 4%,the increased fluorescence intensity of the PA6 composite nanofibers is no longer obvious.Taking Tb(BAO)3Phen/PA6 composite nanofibers as an example,the content of Tb(BAO)3Phen increased from 4% to 7%,and the fluorescence intensity of composite nanofibers increased only by 2.5%.The results indicate that with the increase of the content of rare earth complexes,some rare earth complexes in the PA6 composite nanofibers begin to aggregate and could not be uniformly dispersed.Therefore,the fluorescence intensity of PA6 composite nanofibers was not changed significantly with the increase of rare earth complex contents.The result has been confirmed by the rare earth elemental mapping of Eu(BA)3Phen/PA6 and Tb(BAO)3Phen/PA6 composite nanofibers.

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(b)Fig.4 Emission spectra of PA6 composite nanofibers containing different mass percentages of (a) Eu(BA)3 Phen and (b) Tb(BAO)3 Phen

Table 1 Luminescence intensities of PA6 composite nanofibers with different rare earth complex contents

2.4 Effect of polymer matrix on the luminescence properties of PA6 composite nanofibers

Figures 5(a) and (b) show the emission spectra of Eu(BA)3Phen and Tb(BAO)3Phen complexes.As shown in Fig.5,Eu(BA)3Phen and Tb(BAO)3Phen exhibit high luminescent intensities,and their biggest values are 12 055 (a.u.) and 12 890 (a.u.),respectively.The two values are close.However,when the above two kinds of rare earth complexes were added to the PA6 polymer and prepared into nanofibers by electrospinning technology,their fluorescence intensities were significantly different.Figure 6 gives the Eu and Tb elemental mapping of Eu(BA)3Phen/PA6 and Tb(BAO)3Phen/PA6 composite nanofibers.In the two kinds of composite nanofibers,Tb3+ions were evenly distributed in polymer matrix and Eu3+ions showed certain aggregation phenomenon in composite nanofibers.This is an important reason that results in a decrease in the fluorescence intensity of Eu(BA)3Phen/PA6.

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(b)Fig.5 Emission spectra of (a) Eu(BA)3 Phen and (b) Tb(BAO)3 Phen

Fig.6 Rare earth elemental mapping of Eu(BA)3 Phen/PA6 and Tb(BAO)3 Phen/PA6 composite nanofibers

The lifetime of5D0(τobs) is another important parameter to evaluate the fluorescence properties of sample,which is dominated by the total radiative transition rate of5D0→7FJ= 1,2,3,4,ARAD,and the non-radiative transition rateANR,which can be written asτobs-1=ARAD+ANR.Figure 8 gives the luminescent life-time curves of Eu(BA)3Phen/PA6 and Tb(BAO)3Phen/PA6 composite nanofibers.As shown in Fig.8,the luminescent lifetime value of Eu(BA)3Phen/PA6 is lower than that of Tb(BAO)3Phen/PA6.The tested results of fluorescence lifetime for as-prepared composite nanofibers are consistent with the results of fluorescence intensity.According to the correlated Ref.[32],the magnetic dipole allowed5D0→7F1transition of Eu(BA)3Phen/PA6 was taken as a reference for the whole spectrum andARAD(5D0→7F1) ≈50 s-1.The calculated values ofARAD,ANRandτobsof samples were listed in Table 2.From Table 2,it is noted that theANRvalue of Eu(BA)3Phen/PA6 is higher than that of pure Eu(BA)3Phen.The result indicates that the non-radiative energy transfer between the Eu(BA)3Phen and PA6 molecules may also occur owing to the aggregation of Eu-complexes in the fibers.

Table 2 Interaction parameters of ligand fields and luminescent lifetimes

Fig.7 FT-IR spectra of Eu(BA)3Phen/PA6 composite nanofibers

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2.5 Thermal and mechanical properties of composite nanofibers

Thermogravimetry (TG) analysis is an important method to evaluate thermal stability of the composite nanofibers.Figure 9 shows TG curves of Eu(BA)3Phen/PA6 and Tb(BAO)3Phen/PA6 composite nanofibers.As a comparison,TG curves of Eu(BA)3Phen and Tb(BAO)3Phen were also given.The initial decomposition temperatures of Eu(BA)3Phen/PA6 and Tb(BAO)3Phen/PA6 are 380 ℃ and 386 ℃，respectively.The initial decomposition temperatures of pure Eu(BA)3Phen and Tb(BAO)3Phen are 202 ℃ and 177 ℃.It can be seen that the thermal stabilities of the as-prepared composites nanofibers are better than those of the corresponding pure rare earth complexes.These results indicate that the PA6 matrix can make the rare earth complex more stable.The good thermal stabilities of composite nanofibers provide an important path for their wide application.

Tensile load-elongation curves of composite nanofibers were displayed in Fig.10.As shown in Fig.10(a),the breaking elongation of Tb(BAO)3Phen/PA6 is larger than that of Eu(BA)3Phen/PA6.It benefits from evenly dispersed of Tb(BAO)3Phen in composite nanofibers.At the same time,it can be found that the mechanical properties of nanofibers spun with HFIP as the solvent are better than those spun with formic acid as the solvent.From Fig.10(b),it can also be seen that the fluorescent intensities of the composite nanofibers fiber spun with HFIP as the solvent are better than those of the composite nanofibers spun with formic acid as the solvent.These results verify that using HFIP as the solvent,composite nanofibers with good properties can be prepared by electrospinning.

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Fig.10 Composite nanofibers with 8% Eu(BA)3Phen and Tb(BAO)3Phen in different solvent systems of (a) tensile load-elongation curves and (b) fluorescence intensity

3 Conclusions

In this paper,Eu(BA)3Phen/PA6 and Tb(BAO)3Phen/PA6 composite nanofibers were successfully prepared.As-prepared PA6 composite nanofibers exhibit characteristic energy level transitions of Eu3+and Tb3+ions.When the content of rare earth complexes ranges from 4% to 8%,the Eu(BA)3Phen/PA6 and Tb(BAO)3Phen/PA6 composite nanofibers have good luminescent properties,showing bright red and green lights.Judd-Ofelt theory reveals that the uniform dispersion of rare earth luminescent materials in as-prepared composite nanofibers is an important factor affecting the luminescence intensity of composite nanofibers.The coating action of polymer matrix to the rare earth complexes endows the composite nanofibers with good thermal stability and mechanical properties.