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Preparation and luminescence studies of thermosensitive PAN luminous fiber based on the heat sensitive rose red TF-R1 thermochromic pigment
Dyes and Pigments 139 (2017) 693e700
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Preparation and luminescence studies of thermosensitive PAN luminous fiber based on the heat sensitive rose red TF-R1 thermochromic pigment Yang Jin a, Chen Shi a, Xiaoqiang Li a, Yiwen Wang a, Fangfang Wang b, Mingqiao Ge a, * a
School of Textile and Clothing, Key Laboratory of Eco-textiles, Ministry of Education, Jiangnan University, Jiangnan University, 1800 Lihu Avenue, Wuxi JiangSu Province, 214122, PR China College of Textile & Clothing, Nantong University, No.9 Seyuan Road, Nantong, Jiangsu Province, 226019, PR China
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a r t i c l e i n f o
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Article history: Received 27 October 2016 Received in revised form 24 December 2016 Accepted 3 January 2017 Available online 4 January 2017
The aim of this study was to prepare and demonstrate a new kind of thermosensitive luminous fiber based on optical interference of thermochromic pigments. The surface morphology of a thermosensitive luminous fiber, containing Sr2ZnSi2O7: Eu2þ, Dy3þ; Y2O2S: Eu3þ, Mg2þ, Ti4þ; and heat-sensitive rose red TF-R1 thermochromic pigment, was analyzed by scanning electron microscope (SEM). The X-Ray Diffraction (XRD) results revealed the crystal structure of the fiber samples and the synthesized rareearth luminescence materials. There was no degradation of the crystalline phases in the blend when blending and spinning. The thermodynamic stability and dynamic phase structure were analyzed by Thermal Gravity Analysis (TGA) and Differential Scanning Calorimetry (DSC). This thermosensitive polyacrylonitrile (PAN) luminous fiber was stable below 200 C. The fluorescence spectra, reflectivity spectra were measured to reveal the phosphorescence and thermochromism, and visual testing was also performed. The phosphorescence colors of the thermosensitive luminous fiber were directly related to the temperature. The prepared fiber samples had a red emission at room temperature, and because of the inner phase transition of the heat-sensitive rose red TF-R1 thermochromic pigment on heating, the fiber samples became colorless with blue light emission. This novel thermosensitive luminous fiber has many potential applications in optical and thermal sensors. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Thermosensitive luminous fiber Rare earth Thermochromic Sr2ZnSi2O7: Eu2þ, Dy3þ Y2O2S: Eu3þ, Mg2þ, Ti4þ Fluorescence
1. Introduction Luminous fiber is a very promising new functional material because of its high brightness, long afterglow time, and stability. In the decade since luminous fiber was first introduced to the public, it has attracted a large amount of research interest, and the technology has progressed substantially. Luminous fibers with SrAl2O4: Eu2þ, Dy3þ, Sr2ZnSi2O7: Eu2þ, Dy3þ and Y2O2S: Eu3þ, Mg2þ, Ti4þ blending have been successfully fabricated in polyethylene terephthalate, polypropylene, and polyamide to prepare luminous fiber with various light-emitting spectral characteristics [1e4]. For example, SrAl2O4: Eu2þ, Dy3þ luminous fiber has been developed for commercial purposes. Attempts have been made to blend binary or ternary long-lasting luminous materials with fiber-forming
* Corresponding author. Tel./fax: þ86 0510 85912329. E-mail address: [email protected] (M. Ge). http://dx.doi.org/10.1016/j.dyepig.2017.01.003 0143-7208/© 2017 Elsevier Ltd. All rights reserved.
polymers to analyze the possibilities of polychromatic lightemitting luminous fiber [5,6]. Polychromatic luminous fiber is the significant precursor that laid the foundation for the invention of thermosensitive luminous fiber. Polyacrylonitrile (PAN) is a widely used fiber-forming polymer in the textile and carbon-fiber industries. At present, it is possible to prepare commercial PAN fiber via wet spinning or gel spinning because of the structural characteristics of PAN [7]. Some organic solvents such as dimethylsulfoxide (DMSO), dimethylformamide(DMF), etc. have been used to prepare spinning solutions [8]. Based on the additive three primaries, Sr2MgSi2O7: Eu2þ, Dy3þ with blue light emission and Y2O2S: Eu3þ, Mg2þ, Ti4þ with red light emission, binary-blended PAN luminous fiber has polychromatic light-emitting behavior. Depending on the ratio of Sr2MgSi2O7: Eu2þ, Dy3þ to Y2O2S: Eu3þ, Mg2þ, Ti4þ, binary-blended PAN luminous fiber could emit blue, red, purple, or magenta light [9]. The fluorane derivative, consisting of spiro-isobenzofuran and a chromophore, is an important leuco dye. The heat-sensitive rose
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red TF-R1 is a typical fluorane derivative that has lignocaine as the chromophore with a red color. Moreover, because of the benzofluorane, which extends the conjugated system, the substituent with large conjugation will lead to a red shift [10e14]. Because of the above properties, the heat-sensitive rose red TF-R1 has been widely used in reversible thermosensitive materials. Based on the formation or destruction of the ternary compound among the leuco dye, developer, and co-solvent driven by interactions in the solvent, the temperature of the thermochromic behavior is influenced by the melting temperature of the co-solvent [15e19]. The thermochromic equation can be seen in Fig. 1 (the co-solvent is the tetradecanol). In this study, the PAN was employed as the fiber-forming matrix. Sr2ZnSi2O7: Eu2þ, Dy3þ; Y2O2S: Eu3þ, Mg2þ, Ti4þ; and a thermochromic pigment comprising the heat-sensitive rose red TF-R1, bisphenol A, and tetradecanol were blended in the PAN spinning solution via solution mixing. The spinning solution was well dispersed, and then spun into thermosensitive PAN luminous fibers [20e25]. Fig. 2 shows the process of light color change. A red filter membrane would be formed in the interior of binary-blended luminous fiber by blending in thermochromic pigment. On the macroscopic level, the thermosensitive luminous fiber presents red and has red light-emitting behavior. Rising temperatures encourage co-solvent fusion, and the heat-sensitive rose red TF-R1 will convert to a closed-ring structure and become colorless [26e29]. Thereupon, the barrier that blocks blue light disappears. The blue light can be easily seen with the naked eye when the fiber becomes colorless.
2.2. Preparation of rare earth luminescent materials Sr1.95ZnSi2O7: Eu2þ0. 02, Dy3þ0. 03 and Y2O2S: Eu3þ0.04, Mg2þ0.05, Ti4þ0.05 were synthesized using high temperature solid state method. After preliminary milling, these raw materials were respectively dissolved in appropriate amounts of absolute ethanol, followed by ultrasonic dispersion for 30min and mechanical mixing for 30min in order to get the homogeneous mixture. The samples were heated by adding flux (the ratio of H3BO3 to Sr1.95ZnSi2O7: Eu2þ0. 02, Dy3þ0. 03 is 10 mol.%; the ratio of Na2CO3 to Y2O2S: Eu3þ0.04, Mg2þ0.05, Ti4þ0.05 is 20mol.%) to a high temperature of 1300 C for 3 h in a reducing atmosphere. The sintered products were re-milled in ball mill and sieved with 600 mesh to get the desired size. 2.3. Preparation of PAN thermosensitive luminous fiber
2. Experimental details
The rare earth luminous materials and the thermochromic pigment were added in homogeneous PAN/DMSO solution as the spinning dope(PAN/DMSO is 20% at mass-volume concentration). The thermochromic pigment was added in the PAN/DMSO spinning dope by the mass ratios of 10% and 15% to PAN. The prepared Sr2ZnSi2O7: Eu2þ, Dy3þ (the mass ratio was 4% to PAN)and Y2O2S: Eu3þ, Mg2þ, Ti4þ (the mass ratio was 6% to PAN)were uniformly mixed in spinning dope. Wet spinning was carried out using the syringe and a boost device. The preparatory dope solution was spun into coagulation bath (deionized water) at room temperature(25 C) with the push speed of 6 mL/min. The preparation process of thermosensitive luminous fiber process was given in Fig. 3.
2.1. The raw materials
2.4. Instrumental measurements
SrCO3, ZnO, SiO2, Eu2O3, Dy2O3, H3BO3, Y2O3, S, TiO2, 4MgCO3$Mg(OH)2$6H2O, Na2CO3 of analytical reagent grade as the starting materials were purchased from Sinopharm Chemical Reagent Co., Ltd., China. The polyacrylonitrile(PAN) powder were produced by the Shaoxing Gimel Advanced Materials Technology Co., Ltd. The dimethyl sulfoxide(DMSO) of analytical reagent grade as the spinning solvent was supplied by Sinopharm Chemical Reagent Co., Ltd., China. The commercial heat sensitive rose red TF-R1 pigment (the theoretical thermochromism temperature is 38 C) was purchased from Shenzhen Bianse Science and Technology Co., Ltd.
The surface morphologies of luminous fiber were characterized on Hitachi TM3030 scanning electron microscopy. The conventional fluorescence spectra and the luminescence color of thermosensitive luminous fibers were obtained at 25 C, 35 C and 45 C with an excitation wavelength of 360 nm using the fluorescence spectrophotometer (FS5 Fluorescence spectrometer produced by Edinburgh Instruments) with the Xe flash lamp as an excitation source; the slit was 2 nm in width; the excitation wavelength was from 400 to 700 nm and the dwell time was 0.2s; chromaticity coordinates and reflectivity were tested using Macbeth color-Eye 7000A color measuring and matching
Fig. 1. The thermochromic process of reversible color change compound.
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Fig. 2. The temperature sense lending to light color change process of thermosensitive PAN luminous fiber.
Fig. 3. The preparation process of thermosensitive luminous fiber.
instrument made in the USA. The X-ray diffraction was measured using Bruker D8 Advance made in Germany at room temperature; the scanning range of samples was from 10 to 80 and the speed of scanning was 4 /min using Cu radiation. The Differential Scanning Calorimetry (DSC) was measured by Q200 produced by TA Instruments; the measured temperature was 30 C350 C and the heating rate was 10 C/min. The thermogravimetry(TGA and DTG) was characterized by Q500 produced by TA Instruments; the measured temperature was 50 C600 C and the heating rate was 10 C/min. The image of thermochromic and thermosensitive luminescent phenomena were taken by Canon G15 camera; the
thermosensitive luminescent phenomena was taken by high dynamic range (HDR) mode. Before all the optical tests, samples had been kept in darkness for 20 h to rule out the error. 3. Result and discussion 3.1. Microstructure analysis Fig. 4(a) shows the surface features of thermosensitive PAN luminous fiber (15%). The average diameter of the fiber sample is about 350e400 mm. As we can see on the left side of Fig. 4(a), the
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Fig. 4. The surface feature of thermosensitive PAN luminous fiber(a) and (b) with 15% thermochromic pigment; (c) Sr2MgSi2O7: Eu2þ, Dy3þ, (d) Dy3þ;Y2O2S: Eu3þ, Mg2þ, Ti4; the inset picture of (b) shows the image of thermochromic pigment.
prepared fiber samples are flexible so that they can be stretched and twisted. Most of the particles of rare-earth luminescent material were embedded in the PAN, and only a portion of each particle was exposed on the surface. It is easy to see that there are many bright particles on the otherwise homogeneous fiber surface. By examining these sporadic particles at higher magnification, as in Fig. 4(b), it can be verified that the particles with irregular shape consist of Sr2MgSi2O7: Eu2þ, Dy3þ [Fig. 4(c)]; and the others with a regular hexahedral shape consist of Y2O2S: Eu3þ, Mg2þ, Ti4þ [Fig. 4(d)]. The particle size of Sr2MgSi2O7: Eu2þ, Dy3þ is in the range from 10 mm to 30 mm. From Fig. 4(d), it can be seen that the particle size of Y2O2S: Eu3þ, Mg2þ, Ti4þ is more uniform, and is in the range from 5 mm to 10 mm. It is easy to see that these two kinds of luminescent materials have some differences in morphology and particle size. This result could be ascribed to the different lattice parameters that affect the habit of the crystal and its physical properties. By carefully observing Fig. 4(b), it can be seen that there are numerous shallow ditches and small nodes on the fiber surface. Due to the blend of inorganic particles, the swelling behavior of the polymer leads to the bumpy surface. From the inset picture in Fig. 4, it can be seen that the small nodes are thermochromic pigment with regular spherical shapes in the range of 2e4 mm in diameter.
of Y2O2S: Eu3þ, Mg2þ, Ti4þ (YOS) could be indexed as hexagonal with the space group of P-3m1(164). The crystal size is a ¼ b ¼ 3.784 nm, c ¼ 6.589 nm. The results suggest that the
3.2. XRD analysis The XRD patterns are shown in Fig. 5. The diffraction peaks of rare-earth luminescent materials, which were prepared by a high temperature solid state method, are presented in accordance with the standard Joint Committee on Powder Diffraction Standards Cards No. 39-0235 and No. 24-1424, respectively. The most basic form of Sr2MgSi2O7: Eu2þ, Dy3þ (SZSO) is tetragonal, and belongs to the space group of P-421m (113). The crystal size is a ¼ b ¼ 8.002 nm, c ¼ 5.1706 nm. In addition, the diffraction peaks
Fig. 5. The x-ray diffraction pattern of thermosensitive luminous fiber with 15% thermochromic pigment, Sr2ZnSi2O7: Eu2þ, Dy3þ, Y2O2S: Eu3þ, Mg2þ, Ti4þ and thermochromic pigment.
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dopants occupy the chemical substitutional sites. However, the occupying behavior does not destroy the crystal structure of the luminescent materials. There are two diffraction peaks on the curve of the PAN fiber at 17 and 29 . However, the diffraction peak at 29 is very weak, because the PAN fiber was prepared without further drawing. The amorphous region accounts for a large proportion of the as-formed PAN fiber. The crystallinity and orientation are much lower than they would be in a fully drawn yarn. From the diffraction patterns of the thermosensitive luminous fiber, it can be inferred that the characteristic peaks of the thermochromic pigments, Sr2MgSi2O7: Eu2þ, Dy3þ and Y2O2S: Eu3þ, Mg2þ, Ti4þ are superimposed over the curve of the PAN itself. This result indicates that there were no interactions between the thermochromic pigments, Sr2MgSi2O7: Eu2þ, Dy3þ and Y2O2S: Eu3þ, Mg2þ, Ti4þ. Moreover, the phase structure of the PAN was not destroyed by wet spinning.
Fig. 7 shows the TGA and DTG curves that reveal the thermal stabilities of the PAN fiber, thermochromic pigment, and thermosensitive luminous fiber. The highest weight-loss peak of the PAN fiber appears at 313 C. It is evidence of rapid breaking of the cyano group. On the DTG curve of the thermosensitive luminous fiber, a shoulder peak can be clearly observed at 313 C, and its weight loss peak appears at 324 C. This may be the reason that the interaction between the PAN matrix and thermochromic pigment makes the peak temperature move toward the average temperature. In addition, the weight-loss peaks of those three samples range from 250 C to 300 C; this indicates that the PAN matrix, thermochromic pigment, and thermosensitive luminous fiber are stable at 200 C.
3.3. Thermal stability analysis
The reflectance curves, which reveal the color of the thermosensitive luminous fiber under different temperature conditions, are shown in Fig. 8. The reflectance curves of pure PAN fiber are also shown for comparison. The pure PAN fiber has only a weak absorption behavior at 400e450 nm in the visible light band (400e700 nm). The chromaticity coordinate of pure PAN fiber was (0.3198, 0.3386), as shown in Fig. 9. The reflectance curves and the CIE 1931 color chromaticity coordinate reveal that the pure PAN fiber is white and should reflect most visible light. It can be seen from Fig. 8 that the fiber samples reflect most of the wave range from 600 nm to 700 nm regardless of the temperature. This indicates that the fiber samples with the heat-sensitive rose red TFR1 blending do not absorb the red waveband no matter the fiber sample was decoloration or not. However, the blue and green wavebands are almost absorbed under low temperature conditions (25 C and 35 C) because of the open chaining of phthalimide. The interaction between heat-sensitive rose red TF-R1 and bisphenol A forms cationic dyes with a conjugated structure between that of spiro-benzene and dimethylamino. This large conjugated structure could absorb visible light at 400e600 nm. Once the temperature had risen to 45 C, the central carbon atom from the SP2 hybridized orbital would become an SP3 hybridized orbital, and the heatsensitive rose red TF-R1 would recover its original structure with decoloration. The conspicuous rise of reflectance in the blue and green bands indicates that the decoloring phenomenon could clearly be seen at 45 C. In addition, the reflectance curves of the fiber samples at the same temperature have a similar form. However, the curves of the samples with a lower content of
Fig. 6 shows the DSC curve of thermosensitive luminous fiber with 15% thermochromic pigment blended in. The curves of the PAN fiber and the thermochromic pigment are presented for comparison. It is obvious that there are three crystallization peaks in the curve of the thermosensitive luminous fiber. The endothermic peaks at 44 C corresponding to the thermochromic pigment reveal the complete range of melting temperatures of tetradecanol, which is blended in the thermochromic pigment as the co-solvent. The melting temperature of the co-solvent directly determines the decolorization temperature of the thermochromic pigment. The curve of the thermochromic pigment shows that tetradecanol begins melting at 35 C and is completely melted at 44 C. There is another endothermic peak that belongs to the melting behavior of bisphenol A at 145 C in both the thermosensitive luminous fiber and the thermochromic pigment. An exothermic peak can be observed at 301 C on the curve of the thermosensitive luminous fiber. Because the decomposition temperature of PAN is lower than its melting temperature, the heat released by breaking of chemical bonds results in the exothermic peak. The exothermic peak of the thermosensitive luminous fiber appears at 301 C, which is lower than that of the pure PAN fiber. Although the blending particles do not react with the PAN matrix, the blending behavior could still reduce the crystallinity of the PAN matrix.
Fig. 6. The DSC cures of PAN fiber, thermochromic pigment and thermosensitive fiber.
3.4. Optical performance
Fig. 7. The TGA and DTG cures of PAN fiber, thermochromic pigment and thermosensitive luminous fiber.
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Fig. 8. The reflectance curves of thermosensitive luminous fiber with different ratios of themochromic pigment under different temperature conditions.
thermochromic pigment are upon the samples which contain more thermochromic pigments blended in. This demonstrates that the content of the thermochromic pigment could influence the color lightness of the thermosensitive luminous fiber. A more visual color expression of the thermosensitive luminous fiber is given in Fig. 9. The colors of thermosensitive luminous fiber under different temperature conditions are marked on the CIE 1931 color chromaticity diagram. These marked points reflect the changed trend of color with the temperature increases. It is obvious that the fiber samples become white and continuously approach the white point as the temperature goes up. The fluorescence spectra are shown in Fig. 10. When the fiber samples are at room temperature (25 C), the emission peaks at 616 nm and 626 nm can be seen at both 10% and 15%. These two emission peaks originate from the transitions of 5D0e7F2 of Eu3þ;
Fig. 9. The color of thermosensitive luminous fiber with different ratios of themochromic pigment under different temperature conditions on the CIE 1931 color chromaticity diagram.
another emission peak also emitted by this electron transition of Eu3þ is very weak at 596 nm. Except for the emission peaks at 596, 616, and 626 nm, no other emission could be seen in the red band; this indicates that the Eu3þ is the only luminescence center in Y2O2S: Eu3þ, Mg2þ, Ti4þ. Mg2þ and Ti4þ are just co-doped ions which enhance the depth and density of traps. The emission peak at 468 nm originating from the Eu2þ in Sr2MgSi2O7: Eu2þ, Dy3þ, which has transitions between the 8S7/2 ground state and the crystal field components of 4f6 5d1 excited state configuration, is difficult to see at room temperature. This may be because the red filter formed by the thermochromic pigment absorbs the band of 400e600 nm and intervenes in the fluorescence at 600e700 nm. This phenomenon confirms the intervention behavior of thermochromic pigment in Sr2MgSi2O7: Eu2þ, Dy3þ, Y2O2S: Eu3þ, Mg2þ, Ti4þ binary luminous fiber. When the temperature rises to 45 C, the emission in the blue band increases dramatically. The thermochromic pigment will gradually discolor at 35 C and will completely discolor at 45 C. The decolorization of thermochromic pigment is the primary cause of the change of fluorescence spectra. The discolored thermochromic pigment has little absorptive and interferential effects in the blue and green bands. As a result, the fluorescence of Sr2MgSi2O7: Eu2þ, Dy3þ can be seen. The trends of fluorescence color can be seen on the CIE 1931 color chromaticity diagram in Fig. 11. The fluorescence of Sr2MgSi2O7: Eu2þ, Dy3þ dominates the color of the light as the temperature goes up. As the temperature increases, the fluorescence of the thermosensitive luminous fiber changes its color from red to blue. The coordinates of Sr2MgSi2O7: Eu2þ, Dy3þ; Y2O2S: Eu3þ, Mg2þ, Ti4þ; and the fiber sample with 15% thermochromic pigment scatter around a straight line. It echoes with the additive three primaries. 3.5. Heat sensing luminescence Fig. 12 shows a visual test that could be observed with the naked eye. Half of a thermosensitive luminous fiber bundle was dipped into 45 C deionized water and the other half was kept in air. The room temperature was controlled by air-conditioning at 25 C. Under a standard light source, it could be seen that the fiber bundles were decolorized in the 45 C water bath, while the fiber
Fig. 10. The fluorescence spectra of thermosensitive luminous fiber with different ratios of thermochromic pigment under 25 C and 45 C; the fluorescence spectra of Sr2MgSi2O7: Eu2þ, Dy3þ(SZSO) and Y2O2S: Eu3þ, Mg2þ, Ti4þ (YOS) at room temperature.
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bundles dipped at 45 C to have stronger emission. Moreover, the emission bands or trap levels of Sr2MgSi2O7: Eu2þ, Dy3þ are deeper than those of Y2O2S: Eu3þ, Mg2þ, Ti4þ. This results in the thermosensitive luminous fiber having blue emission under high temperature conditions. 4. Conclusions
Fig. 11. The fluorescent color of thermosensitive luminous fiber with 15% themochromic pigment under different temperature on the CIE 1931 color chromaticity diagram.
A thermosensitive luminous fiber that can automatically sense heat and then change its color as well as its fluorescence emission has been successfully prepared for the first time via conventional wet spinning. The XRD results showed that the doping rare-earth ions simply replaced the chemical substitutional sites, but the crystal structures of the phosphors were not destroyed. The structure phases of blends and PAN remained unchanged after stirring and spinning. The TGA, DTG, and DSC results indicated that the pyrolysis temperature of the thermosensitive luminous fiber ranged from 250 C to 300 C, and it remain stable at 200 C. From the reflectivity curves, it can be seen that the fiber samples absorb the visible wavelengths of 400e600 nm, appearing red at room temperature. When the temperature was increased, the absorptivity of the thermosensitive luminous fiber at 400e600 nm was reduced, resulting in a white appearance. The fluorescence spectra and chromaticity coordinates showed that both 10% and 15% blends of the thermosensitive luminous fiber had a red emission behavior at room temperature and emitted blue fluorescence at 45 C. This functional fiber should have many applications in thermal and optical sensors. Acknowledgement This study was financially supported by the Fundamental Research Funds for the Central Universities (NO. JUSRP51505, JUSRP116020). Jiangsu province ordinary university academic degree graduate student scientific research innovation projects (NO. KYLX16-0791). A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (NO.1065211532080020). References
Fig. 12. The visual image of thermochromic and thermosensitive luminescent phenomena.
bundles that were kept in the air maintained their red coloring. Before moving the fiber bundle to darkness, these were excited by 365 nm UV for 5 min. Under the dark condition, a striking color difference could be seen by the naked eye. The fiber bundles that had been kept in air had a dim red emission while those that had been dipped into the hot water bath emitted conspicuous blue light. The intensity difference of phosphorescence can be ascribed to thermoluminescence theory. The electrons in the fiber bundles at the higher temperature have higher energy; therefore, the bound electrons in traps have higher escape rates. This causes the fiber
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Preparation and luminescence studies of thermosensitive PAN luminous fiber based on the heat sensitive rose red TF-R1 thermochromic pigment Yang Jin a, Chen Shi a, Xiaoqiang Li a, Yiwen Wang a, Fangfang Wang b, Mingqiao Ge a, * a
School of Textile and Clothing, Key Laboratory of Eco-textiles, Ministry of Education, Jiangnan University, Jiangnan University, 1800 Lihu Avenue, Wuxi JiangSu Province, 214122, PR China College of Textile & Clothing, Nantong University, No.9 Seyuan Road, Nantong, Jiangsu Province, 226019, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 October 2016 Received in revised form 24 December 2016 Accepted 3 January 2017 Available online 4 January 2017
The aim of this study was to prepare and demonstrate a new kind of thermosensitive luminous fiber based on optical interference of thermochromic pigments. The surface morphology of a thermosensitive luminous fiber, containing Sr2ZnSi2O7: Eu2þ, Dy3þ; Y2O2S: Eu3þ, Mg2þ, Ti4þ; and heat-sensitive rose red TF-R1 thermochromic pigment, was analyzed by scanning electron microscope (SEM). The X-Ray Diffraction (XRD) results revealed the crystal structure of the fiber samples and the synthesized rareearth luminescence materials. There was no degradation of the crystalline phases in the blend when blending and spinning. The thermodynamic stability and dynamic phase structure were analyzed by Thermal Gravity Analysis (TGA) and Differential Scanning Calorimetry (DSC). This thermosensitive polyacrylonitrile (PAN) luminous fiber was stable below 200 C. The fluorescence spectra, reflectivity spectra were measured to reveal the phosphorescence and thermochromism, and visual testing was also performed. The phosphorescence colors of the thermosensitive luminous fiber were directly related to the temperature. The prepared fiber samples had a red emission at room temperature, and because of the inner phase transition of the heat-sensitive rose red TF-R1 thermochromic pigment on heating, the fiber samples became colorless with blue light emission. This novel thermosensitive luminous fiber has many potential applications in optical and thermal sensors. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Thermosensitive luminous fiber Rare earth Thermochromic Sr2ZnSi2O7: Eu2þ, Dy3þ Y2O2S: Eu3þ, Mg2þ, Ti4þ Fluorescence
1. Introduction Luminous fiber is a very promising new functional material because of its high brightness, long afterglow time, and stability. In the decade since luminous fiber was first introduced to the public, it has attracted a large amount of research interest, and the technology has progressed substantially. Luminous fibers with SrAl2O4: Eu2þ, Dy3þ, Sr2ZnSi2O7: Eu2þ, Dy3þ and Y2O2S: Eu3þ, Mg2þ, Ti4þ blending have been successfully fabricated in polyethylene terephthalate, polypropylene, and polyamide to prepare luminous fiber with various light-emitting spectral characteristics [1e4]. For example, SrAl2O4: Eu2þ, Dy3þ luminous fiber has been developed for commercial purposes. Attempts have been made to blend binary or ternary long-lasting luminous materials with fiber-forming
* Corresponding author. Tel./fax: þ86 0510 85912329. E-mail address: [email protected] (M. Ge). http://dx.doi.org/10.1016/j.dyepig.2017.01.003 0143-7208/© 2017 Elsevier Ltd. All rights reserved.
polymers to analyze the possibilities of polychromatic lightemitting luminous fiber [5,6]. Polychromatic luminous fiber is the significant precursor that laid the foundation for the invention of thermosensitive luminous fiber. Polyacrylonitrile (PAN) is a widely used fiber-forming polymer in the textile and carbon-fiber industries. At present, it is possible to prepare commercial PAN fiber via wet spinning or gel spinning because of the structural characteristics of PAN [7]. Some organic solvents such as dimethylsulfoxide (DMSO), dimethylformamide(DMF), etc. have been used to prepare spinning solutions [8]. Based on the additive three primaries, Sr2MgSi2O7: Eu2þ, Dy3þ with blue light emission and Y2O2S: Eu3þ, Mg2þ, Ti4þ with red light emission, binary-blended PAN luminous fiber has polychromatic light-emitting behavior. Depending on the ratio of Sr2MgSi2O7: Eu2þ, Dy3þ to Y2O2S: Eu3þ, Mg2þ, Ti4þ, binary-blended PAN luminous fiber could emit blue, red, purple, or magenta light [9]. The fluorane derivative, consisting of spiro-isobenzofuran and a chromophore, is an important leuco dye. The heat-sensitive rose
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red TF-R1 is a typical fluorane derivative that has lignocaine as the chromophore with a red color. Moreover, because of the benzofluorane, which extends the conjugated system, the substituent with large conjugation will lead to a red shift [10e14]. Because of the above properties, the heat-sensitive rose red TF-R1 has been widely used in reversible thermosensitive materials. Based on the formation or destruction of the ternary compound among the leuco dye, developer, and co-solvent driven by interactions in the solvent, the temperature of the thermochromic behavior is influenced by the melting temperature of the co-solvent [15e19]. The thermochromic equation can be seen in Fig. 1 (the co-solvent is the tetradecanol). In this study, the PAN was employed as the fiber-forming matrix. Sr2ZnSi2O7: Eu2þ, Dy3þ; Y2O2S: Eu3þ, Mg2þ, Ti4þ; and a thermochromic pigment comprising the heat-sensitive rose red TF-R1, bisphenol A, and tetradecanol were blended in the PAN spinning solution via solution mixing. The spinning solution was well dispersed, and then spun into thermosensitive PAN luminous fibers [20e25]. Fig. 2 shows the process of light color change. A red filter membrane would be formed in the interior of binary-blended luminous fiber by blending in thermochromic pigment. On the macroscopic level, the thermosensitive luminous fiber presents red and has red light-emitting behavior. Rising temperatures encourage co-solvent fusion, and the heat-sensitive rose red TF-R1 will convert to a closed-ring structure and become colorless [26e29]. Thereupon, the barrier that blocks blue light disappears. The blue light can be easily seen with the naked eye when the fiber becomes colorless.
2.2. Preparation of rare earth luminescent materials Sr1.95ZnSi2O7: Eu2þ0. 02, Dy3þ0. 03 and Y2O2S: Eu3þ0.04, Mg2þ0.05, Ti4þ0.05 were synthesized using high temperature solid state method. After preliminary milling, these raw materials were respectively dissolved in appropriate amounts of absolute ethanol, followed by ultrasonic dispersion for 30min and mechanical mixing for 30min in order to get the homogeneous mixture. The samples were heated by adding flux (the ratio of H3BO3 to Sr1.95ZnSi2O7: Eu2þ0. 02, Dy3þ0. 03 is 10 mol.%; the ratio of Na2CO3 to Y2O2S: Eu3þ0.04, Mg2þ0.05, Ti4þ0.05 is 20mol.%) to a high temperature of 1300 C for 3 h in a reducing atmosphere. The sintered products were re-milled in ball mill and sieved with 600 mesh to get the desired size. 2.3. Preparation of PAN thermosensitive luminous fiber
2. Experimental details
The rare earth luminous materials and the thermochromic pigment were added in homogeneous PAN/DMSO solution as the spinning dope(PAN/DMSO is 20% at mass-volume concentration). The thermochromic pigment was added in the PAN/DMSO spinning dope by the mass ratios of 10% and 15% to PAN. The prepared Sr2ZnSi2O7: Eu2þ, Dy3þ (the mass ratio was 4% to PAN)and Y2O2S: Eu3þ, Mg2þ, Ti4þ (the mass ratio was 6% to PAN)were uniformly mixed in spinning dope. Wet spinning was carried out using the syringe and a boost device. The preparatory dope solution was spun into coagulation bath (deionized water) at room temperature(25 C) with the push speed of 6 mL/min. The preparation process of thermosensitive luminous fiber process was given in Fig. 3.
2.1. The raw materials
2.4. Instrumental measurements
SrCO3, ZnO, SiO2, Eu2O3, Dy2O3, H3BO3, Y2O3, S, TiO2, 4MgCO3$Mg(OH)2$6H2O, Na2CO3 of analytical reagent grade as the starting materials were purchased from Sinopharm Chemical Reagent Co., Ltd., China. The polyacrylonitrile(PAN) powder were produced by the Shaoxing Gimel Advanced Materials Technology Co., Ltd. The dimethyl sulfoxide(DMSO) of analytical reagent grade as the spinning solvent was supplied by Sinopharm Chemical Reagent Co., Ltd., China. The commercial heat sensitive rose red TF-R1 pigment (the theoretical thermochromism temperature is 38 C) was purchased from Shenzhen Bianse Science and Technology Co., Ltd.
The surface morphologies of luminous fiber were characterized on Hitachi TM3030 scanning electron microscopy. The conventional fluorescence spectra and the luminescence color of thermosensitive luminous fibers were obtained at 25 C, 35 C and 45 C with an excitation wavelength of 360 nm using the fluorescence spectrophotometer (FS5 Fluorescence spectrometer produced by Edinburgh Instruments) with the Xe flash lamp as an excitation source; the slit was 2 nm in width; the excitation wavelength was from 400 to 700 nm and the dwell time was 0.2s; chromaticity coordinates and reflectivity were tested using Macbeth color-Eye 7000A color measuring and matching
Fig. 1. The thermochromic process of reversible color change compound.
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Fig. 2. The temperature sense lending to light color change process of thermosensitive PAN luminous fiber.
Fig. 3. The preparation process of thermosensitive luminous fiber.
instrument made in the USA. The X-ray diffraction was measured using Bruker D8 Advance made in Germany at room temperature; the scanning range of samples was from 10 to 80 and the speed of scanning was 4 /min using Cu radiation. The Differential Scanning Calorimetry (DSC) was measured by Q200 produced by TA Instruments; the measured temperature was 30 C350 C and the heating rate was 10 C/min. The thermogravimetry(TGA and DTG) was characterized by Q500 produced by TA Instruments; the measured temperature was 50 C600 C and the heating rate was 10 C/min. The image of thermochromic and thermosensitive luminescent phenomena were taken by Canon G15 camera; the
thermosensitive luminescent phenomena was taken by high dynamic range (HDR) mode. Before all the optical tests, samples had been kept in darkness for 20 h to rule out the error. 3. Result and discussion 3.1. Microstructure analysis Fig. 4(a) shows the surface features of thermosensitive PAN luminous fiber (15%). The average diameter of the fiber sample is about 350e400 mm. As we can see on the left side of Fig. 4(a), the
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Fig. 4. The surface feature of thermosensitive PAN luminous fiber(a) and (b) with 15% thermochromic pigment; (c) Sr2MgSi2O7: Eu2þ, Dy3þ, (d) Dy3þ;Y2O2S: Eu3þ, Mg2þ, Ti4; the inset picture of (b) shows the image of thermochromic pigment.
prepared fiber samples are flexible so that they can be stretched and twisted. Most of the particles of rare-earth luminescent material were embedded in the PAN, and only a portion of each particle was exposed on the surface. It is easy to see that there are many bright particles on the otherwise homogeneous fiber surface. By examining these sporadic particles at higher magnification, as in Fig. 4(b), it can be verified that the particles with irregular shape consist of Sr2MgSi2O7: Eu2þ, Dy3þ [Fig. 4(c)]; and the others with a regular hexahedral shape consist of Y2O2S: Eu3þ, Mg2þ, Ti4þ [Fig. 4(d)]. The particle size of Sr2MgSi2O7: Eu2þ, Dy3þ is in the range from 10 mm to 30 mm. From Fig. 4(d), it can be seen that the particle size of Y2O2S: Eu3þ, Mg2þ, Ti4þ is more uniform, and is in the range from 5 mm to 10 mm. It is easy to see that these two kinds of luminescent materials have some differences in morphology and particle size. This result could be ascribed to the different lattice parameters that affect the habit of the crystal and its physical properties. By carefully observing Fig. 4(b), it can be seen that there are numerous shallow ditches and small nodes on the fiber surface. Due to the blend of inorganic particles, the swelling behavior of the polymer leads to the bumpy surface. From the inset picture in Fig. 4, it can be seen that the small nodes are thermochromic pigment with regular spherical shapes in the range of 2e4 mm in diameter.
of Y2O2S: Eu3þ, Mg2þ, Ti4þ (YOS) could be indexed as hexagonal with the space group of P-3m1(164). The crystal size is a ¼ b ¼ 3.784 nm, c ¼ 6.589 nm. The results suggest that the
3.2. XRD analysis The XRD patterns are shown in Fig. 5. The diffraction peaks of rare-earth luminescent materials, which were prepared by a high temperature solid state method, are presented in accordance with the standard Joint Committee on Powder Diffraction Standards Cards No. 39-0235 and No. 24-1424, respectively. The most basic form of Sr2MgSi2O7: Eu2þ, Dy3þ (SZSO) is tetragonal, and belongs to the space group of P-421m (113). The crystal size is a ¼ b ¼ 8.002 nm, c ¼ 5.1706 nm. In addition, the diffraction peaks
Fig. 5. The x-ray diffraction pattern of thermosensitive luminous fiber with 15% thermochromic pigment, Sr2ZnSi2O7: Eu2þ, Dy3þ, Y2O2S: Eu3þ, Mg2þ, Ti4þ and thermochromic pigment.
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dopants occupy the chemical substitutional sites. However, the occupying behavior does not destroy the crystal structure of the luminescent materials. There are two diffraction peaks on the curve of the PAN fiber at 17 and 29 . However, the diffraction peak at 29 is very weak, because the PAN fiber was prepared without further drawing. The amorphous region accounts for a large proportion of the as-formed PAN fiber. The crystallinity and orientation are much lower than they would be in a fully drawn yarn. From the diffraction patterns of the thermosensitive luminous fiber, it can be inferred that the characteristic peaks of the thermochromic pigments, Sr2MgSi2O7: Eu2þ, Dy3þ and Y2O2S: Eu3þ, Mg2þ, Ti4þ are superimposed over the curve of the PAN itself. This result indicates that there were no interactions between the thermochromic pigments, Sr2MgSi2O7: Eu2þ, Dy3þ and Y2O2S: Eu3þ, Mg2þ, Ti4þ. Moreover, the phase structure of the PAN was not destroyed by wet spinning.
Fig. 7 shows the TGA and DTG curves that reveal the thermal stabilities of the PAN fiber, thermochromic pigment, and thermosensitive luminous fiber. The highest weight-loss peak of the PAN fiber appears at 313 C. It is evidence of rapid breaking of the cyano group. On the DTG curve of the thermosensitive luminous fiber, a shoulder peak can be clearly observed at 313 C, and its weight loss peak appears at 324 C. This may be the reason that the interaction between the PAN matrix and thermochromic pigment makes the peak temperature move toward the average temperature. In addition, the weight-loss peaks of those three samples range from 250 C to 300 C; this indicates that the PAN matrix, thermochromic pigment, and thermosensitive luminous fiber are stable at 200 C.
3.3. Thermal stability analysis
The reflectance curves, which reveal the color of the thermosensitive luminous fiber under different temperature conditions, are shown in Fig. 8. The reflectance curves of pure PAN fiber are also shown for comparison. The pure PAN fiber has only a weak absorption behavior at 400e450 nm in the visible light band (400e700 nm). The chromaticity coordinate of pure PAN fiber was (0.3198, 0.3386), as shown in Fig. 9. The reflectance curves and the CIE 1931 color chromaticity coordinate reveal that the pure PAN fiber is white and should reflect most visible light. It can be seen from Fig. 8 that the fiber samples reflect most of the wave range from 600 nm to 700 nm regardless of the temperature. This indicates that the fiber samples with the heat-sensitive rose red TFR1 blending do not absorb the red waveband no matter the fiber sample was decoloration or not. However, the blue and green wavebands are almost absorbed under low temperature conditions (25 C and 35 C) because of the open chaining of phthalimide. The interaction between heat-sensitive rose red TF-R1 and bisphenol A forms cationic dyes with a conjugated structure between that of spiro-benzene and dimethylamino. This large conjugated structure could absorb visible light at 400e600 nm. Once the temperature had risen to 45 C, the central carbon atom from the SP2 hybridized orbital would become an SP3 hybridized orbital, and the heatsensitive rose red TF-R1 would recover its original structure with decoloration. The conspicuous rise of reflectance in the blue and green bands indicates that the decoloring phenomenon could clearly be seen at 45 C. In addition, the reflectance curves of the fiber samples at the same temperature have a similar form. However, the curves of the samples with a lower content of
Fig. 6 shows the DSC curve of thermosensitive luminous fiber with 15% thermochromic pigment blended in. The curves of the PAN fiber and the thermochromic pigment are presented for comparison. It is obvious that there are three crystallization peaks in the curve of the thermosensitive luminous fiber. The endothermic peaks at 44 C corresponding to the thermochromic pigment reveal the complete range of melting temperatures of tetradecanol, which is blended in the thermochromic pigment as the co-solvent. The melting temperature of the co-solvent directly determines the decolorization temperature of the thermochromic pigment. The curve of the thermochromic pigment shows that tetradecanol begins melting at 35 C and is completely melted at 44 C. There is another endothermic peak that belongs to the melting behavior of bisphenol A at 145 C in both the thermosensitive luminous fiber and the thermochromic pigment. An exothermic peak can be observed at 301 C on the curve of the thermosensitive luminous fiber. Because the decomposition temperature of PAN is lower than its melting temperature, the heat released by breaking of chemical bonds results in the exothermic peak. The exothermic peak of the thermosensitive luminous fiber appears at 301 C, which is lower than that of the pure PAN fiber. Although the blending particles do not react with the PAN matrix, the blending behavior could still reduce the crystallinity of the PAN matrix.
Fig. 6. The DSC cures of PAN fiber, thermochromic pigment and thermosensitive fiber.
3.4. Optical performance
Fig. 7. The TGA and DTG cures of PAN fiber, thermochromic pigment and thermosensitive luminous fiber.
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Fig. 8. The reflectance curves of thermosensitive luminous fiber with different ratios of themochromic pigment under different temperature conditions.
thermochromic pigment are upon the samples which contain more thermochromic pigments blended in. This demonstrates that the content of the thermochromic pigment could influence the color lightness of the thermosensitive luminous fiber. A more visual color expression of the thermosensitive luminous fiber is given in Fig. 9. The colors of thermosensitive luminous fiber under different temperature conditions are marked on the CIE 1931 color chromaticity diagram. These marked points reflect the changed trend of color with the temperature increases. It is obvious that the fiber samples become white and continuously approach the white point as the temperature goes up. The fluorescence spectra are shown in Fig. 10. When the fiber samples are at room temperature (25 C), the emission peaks at 616 nm and 626 nm can be seen at both 10% and 15%. These two emission peaks originate from the transitions of 5D0e7F2 of Eu3þ;
Fig. 9. The color of thermosensitive luminous fiber with different ratios of themochromic pigment under different temperature conditions on the CIE 1931 color chromaticity diagram.
another emission peak also emitted by this electron transition of Eu3þ is very weak at 596 nm. Except for the emission peaks at 596, 616, and 626 nm, no other emission could be seen in the red band; this indicates that the Eu3þ is the only luminescence center in Y2O2S: Eu3þ, Mg2þ, Ti4þ. Mg2þ and Ti4þ are just co-doped ions which enhance the depth and density of traps. The emission peak at 468 nm originating from the Eu2þ in Sr2MgSi2O7: Eu2þ, Dy3þ, which has transitions between the 8S7/2 ground state and the crystal field components of 4f6 5d1 excited state configuration, is difficult to see at room temperature. This may be because the red filter formed by the thermochromic pigment absorbs the band of 400e600 nm and intervenes in the fluorescence at 600e700 nm. This phenomenon confirms the intervention behavior of thermochromic pigment in Sr2MgSi2O7: Eu2þ, Dy3þ, Y2O2S: Eu3þ, Mg2þ, Ti4þ binary luminous fiber. When the temperature rises to 45 C, the emission in the blue band increases dramatically. The thermochromic pigment will gradually discolor at 35 C and will completely discolor at 45 C. The decolorization of thermochromic pigment is the primary cause of the change of fluorescence spectra. The discolored thermochromic pigment has little absorptive and interferential effects in the blue and green bands. As a result, the fluorescence of Sr2MgSi2O7: Eu2þ, Dy3þ can be seen. The trends of fluorescence color can be seen on the CIE 1931 color chromaticity diagram in Fig. 11. The fluorescence of Sr2MgSi2O7: Eu2þ, Dy3þ dominates the color of the light as the temperature goes up. As the temperature increases, the fluorescence of the thermosensitive luminous fiber changes its color from red to blue. The coordinates of Sr2MgSi2O7: Eu2þ, Dy3þ; Y2O2S: Eu3þ, Mg2þ, Ti4þ; and the fiber sample with 15% thermochromic pigment scatter around a straight line. It echoes with the additive three primaries. 3.5. Heat sensing luminescence Fig. 12 shows a visual test that could be observed with the naked eye. Half of a thermosensitive luminous fiber bundle was dipped into 45 C deionized water and the other half was kept in air. The room temperature was controlled by air-conditioning at 25 C. Under a standard light source, it could be seen that the fiber bundles were decolorized in the 45 C water bath, while the fiber
Fig. 10. The fluorescence spectra of thermosensitive luminous fiber with different ratios of thermochromic pigment under 25 C and 45 C; the fluorescence spectra of Sr2MgSi2O7: Eu2þ, Dy3þ(SZSO) and Y2O2S: Eu3þ, Mg2þ, Ti4þ (YOS) at room temperature.
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bundles dipped at 45 C to have stronger emission. Moreover, the emission bands or trap levels of Sr2MgSi2O7: Eu2þ, Dy3þ are deeper than those of Y2O2S: Eu3þ, Mg2þ, Ti4þ. This results in the thermosensitive luminous fiber having blue emission under high temperature conditions. 4. Conclusions
Fig. 11. The fluorescent color of thermosensitive luminous fiber with 15% themochromic pigment under different temperature on the CIE 1931 color chromaticity diagram.
A thermosensitive luminous fiber that can automatically sense heat and then change its color as well as its fluorescence emission has been successfully prepared for the first time via conventional wet spinning. The XRD results showed that the doping rare-earth ions simply replaced the chemical substitutional sites, but the crystal structures of the phosphors were not destroyed. The structure phases of blends and PAN remained unchanged after stirring and spinning. The TGA, DTG, and DSC results indicated that the pyrolysis temperature of the thermosensitive luminous fiber ranged from 250 C to 300 C, and it remain stable at 200 C. From the reflectivity curves, it can be seen that the fiber samples absorb the visible wavelengths of 400e600 nm, appearing red at room temperature. When the temperature was increased, the absorptivity of the thermosensitive luminous fiber at 400e600 nm was reduced, resulting in a white appearance. The fluorescence spectra and chromaticity coordinates showed that both 10% and 15% blends of the thermosensitive luminous fiber had a red emission behavior at room temperature and emitted blue fluorescence at 45 C. This functional fiber should have many applications in thermal and optical sensors. Acknowledgement This study was financially supported by the Fundamental Research Funds for the Central Universities (NO. JUSRP51505, JUSRP116020). Jiangsu province ordinary university academic degree graduate student scientific research innovation projects (NO. KYLX16-0791). A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (NO.1065211532080020). References
Fig. 12. The visual image of thermochromic and thermosensitive luminescent phenomena.
bundles that were kept in the air maintained their red coloring. Before moving the fiber bundle to darkness, these were excited by 365 nm UV for 5 min. Under the dark condition, a striking color difference could be seen by the naked eye. The fiber bundles that had been kept in air had a dim red emission while those that had been dipped into the hot water bath emitted conspicuous blue light. The intensity difference of phosphorescence can be ascribed to thermoluminescence theory. The electrons in the fiber bundles at the higher temperature have higher energy; therefore, the bound electrons in traps have higher escape rates. This causes the fiber
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