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Enhanced photo-stability polyphenylene sulfide fiber via incorporation of multi-walled carbon nanotubes using exciton quenching
Composites Part A 129 (2020) 105716
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Enhanced photo-stability polyphenylene sulfide fiber via incorporation of multi-walled carbon nanotubes using exciton quenching ⁎
Zexu Hu, Kai Hou, Jialin Gao, Genming Zhu, Zhe Zhou, Hengxue Xiang , Tian Qiu, Meifang Zhu
T ⁎
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, International Joint Laboratory for Advanced Fiber and Low-dimension Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Polyphenylene sulfide fiber MWCNTs Photo-stability Exciton quenching
Polyphenylene sulfide (PPS) fiber is widely used in flame-retardant fabrics, high-temperature filter bags, among other applications. It has excellent heat and corrosion resistance. However, the fibers were characterized by weak photo-stability. Herein, an unique strategy based on second component induced shielding effect for constructing organic-inorganic hybrid nanocomposites were developed by using multi-walled carbon nanotubes (MWCNTs) as effective stabilizers. The photo-stability of PPS/MWCNTs composite fibers were investigated by Xenon-lamp weather resistance test chamber. Moreover, the mechanism for photo-stability enhancement was analyzed by Fluorescence spectrometer, differential scanning calorimeter and thermogravimetric analysis. Compared with neat PPS fiber, it was showed significant improvement in photo-stability of PPS/MWCNTs composite fibers resulting to increased strength retention ratio from 57.8% for neat PPS to 77.3% for PPS/ MWCNTs-1.0 fiber. It was finally concluded that the mechanism for enhanced photo-stability of PPS fiber was based on efficient quenching of optically generated excitons by MWCNTs resulted from steady-state fluorescence spectroscopy.
1. Introduction Polyphenylene sulfide (PPS) fiber is among those high performance fibers with inherent flame resistance, outstanding heat resistance [1], excellent chemical resistance [2] and electro-insulating [3] properties which is widely used as protective textiles [4]. However, the application of neat PPS fiber is restricted by its weak photo-stability. The presence of ultraviolet (UV) light in the environment degrades PPS to a darker color and lowers its mechanical strength. According to the literature reported [5,6], photo-degradation of PPS is mainly induced by morphology, oxygen and spectral region of excitation. In order to improve the photo-stability of PPS, numerous organic UV-absorbers have been investigated to shield light in the near-ultraviolet region [7]. However, the plasticization effect arising from the excessive use of organic UV-absorbers reduces the melt-spinnability and mechanical properties of PPS fibers. Thus, inorganic shielding agents with broadspectrum UV-shield and mechanical enhancement performance, such as nano-TiO2 and ZnO, were preferred materials for modified PPS fiber [810]. However, The defects of shielding nanoparticles are the low permissible contents (< 2%) in fibers and their inevitable agglomeration which could reduce the shielding efficiency due to the incomplete and incompact distribution.
⁎
Compared with its applicability in UV shielding, the method of fluorescence and exciton quenching has in the recent years shown better advantages for improving the photo-stability of polymer materials and organic molecules. Moreover, carbonaceous materials, such as CNTs, graphene, fullerene and carbon dots, have excellent electron acceptability from the excited state of organics which enhances their quick excitons quenching effect [11–14]. In this regard, the optically generated excitons of the fluorescent molecule are rapidly transferred to the surface of the carbon material through the interface [15–17]. This whole process suppresses the degradation reaction by cutting off the formation channel of the superoxide anion O2%. On the other hand, the process of electron transfer through carbonaceous materials is reversible. When the irradiated fluorochrome/carbon nanotubes composites are transferred to the dark environment, excitons in the carbon nanotubes are transferred back to fluorochrome [18]. Thirdly carbonaceous materials with a π-π conjugated structure have a strong electron adsorption capacity. They capture radicals from molecular light/ thermal degradation thereby improving the stability of the molecular chain [19–21]. Therefore, carbonaceous materials are simultaneously regarded as excellent excitons quenchers and outstanding light/thermal stabilizers for polymers [22,23]. Moreover, these carbonaceous materials (CNTs, graphene, fullerene) not only exhibit excellent excitons
Corresponding authors. E-mail addresses: [email protected] (H. Xiang), [email protected] (M. Zhu).
https://doi.org/10.1016/j.compositesa.2019.105716 Received 2 April 2019; Received in revised form 24 November 2019; Accepted 25 November 2019 Available online 26 November 2019 1359-835X/ © 2019 Elsevier Ltd. All rights reserved.
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the test environment was set to harsh outdoor weathering conditions. The irradiance time, temperature as well as environment humidity were 20 h/d, 63 ± 3 °C, and 60 ± 5% respectively.
quenching [24] and radicals scavenging properties [25,26], but also effectively enhance the mechanical behavior and electrical properties [27–29] of polymer fibers. It was showed in some report that the π-π conjugation structure between PPS and CNT can effectively increase the dispersion of the inorganic powders, resulting in the improvement of conductivity [30] and mechanical properties of the PPS [31]. In view of the fact that most research related to PPS/MWCNTs nanocomposites have mainly focused on the mechanical properties rather than photo-stability [32,33], we report the effect of the dosage and dispersibility of MWCNTs on the spinnability and photo-stability of PPS nanocomposite fibers in detail herein. Briefly, the PPS/MWCNTs nanocomposite fibers with good photo-stability were obtained by facile melting-spinning process. Afterward, the UV-light stability of PPS/ MWCNTs composite fibers were investigated by Xenon-lamp weather resistance test chamber. The enhancement mechanism of MWCNTs for PPS/MWCNTs fiber was first analyzed by combining the fluorescence behavior and thermal degradation of PPS molecular. On the basis of the steady-state PL spectrum and TGA results obtained in this work, it can be concluded that MWCNT can significantly improve the photo-stability of PPS fiber.
2.4. Characterization 2.4.1. Dispersion of MWCNTs in PPS matrix Dispersion of MWNCTs in PPS matrix was observed by a Field emission scanning microscopy (FESEM, S-4800, Hitachi) operating at 10 kV. 2.4.2. Mechanical properties of PPS/MWCNTs fibers Uniaxial tensile mechanical properties of PPS/MWCNTs fibers were tested using a single fiber tensile strength tester (XQ-1A, Shanghai New Fiber Instrument Co. Ltd., China) with a testing speed of 10 mm/min and gauge length of 10 mm. The test ambient temperature was 20 ± 3 °C and the relative humidity was 65 ± 5%. The mechanical properties of the samples were the average of 50 samples. 2.4.3. The thermal properties of PPS/MWCNTs fibers Differential scanning calorimeter (DSC, 204F1, NETZSCH Group) was used to determine the melting behaviors of PPS/MWCNTs nanocomposite fibers in the range of 20–320 °C at a heating/cooling rate of 10 °C/min.
2. Experimental 2.1. Materials Polyphenylene sulfide (PPS, Fortron 0309P4, 1.35 g/cm3, Tg ≈ 89 °C, Tm ≈ 280 °C) was purchased from Celanese Corporation. Multi-walled carbon nanotubes (MWCNTs, outer diameter: 10–20 nm; length: 0.5–2 μm; purity: 95%) were supplied from Chengdu Organic Chemicals Co., LTD.
2.4.4. Fluorescence quenching of MWCNTs in PPS/MWCNTs fibers The fluorescence quenching behavior of MWCNTs in PPS/MWCNTs nanocomposite fibers was determined by a steady-state fluorescence measurement at room temperature using a commercial fluorimeter (QM/TM, Photon technology international). The samples were tightly wrapped in parallel on the cardboard to make the test surface uniform. The fluorescence emission intensity traces of samples were obtained by using a 330 nm laser as the excitation source. And the fluorescence emission behavior of samples was monitored at 390 nm to detect the fluorescence excited intensity.
2.2. Fabrication of PPS/MWCNTs nanocomposite fibers PPS/MWCNTs nanocomposite fibers were prepared in two stages. First, PPS nanocomposite resins with different MWCNTs contents (0, 0.1, 0.2, 0.5, 1.0, 1.5 and 2.0 wt%) were produced by twin screw extruder with a processing temperature of 290 °C. Prior to this procedure, PPS chips and MWCNTs were dried in a vacuum drum dryer and made sure the moisture was below 50 ppm. Second, the PPS/MWCNTs nanocomposite fibers were fabricated by melting spinning. The small aperture micropore spinnerets (< 300 μm) were used to prepare smalldiameter fibers (< 20 μm) which are suitable for clothing. The parameters and the spinning properties of PPS-MWCNTs fibers are listed in Table 1. The resultant nanocomposite fibers are abbreviated to PPS/ MWCNTs-x, where x represent the mass percentage of MWCNTs.
2.4.5. Thermal degradation behavior of PPS/MWCNTs Thermal gravimetric analyzer (TGA, 209F1, NETZSCH Group) was used to determine the degradation behavior of PPS/MWCNTs nanocomposite fibers in air atmosphere at a heating rate of 10 °C/min. 3. Results and discussion 3.1. Dispersion of MWCNTs in PPS matrix Dispersion of inorganic powders plays an important role in nanocomposite fiber spinnability and properties, which depends on the compatibility of inorganic powder with polymer matrix [34-37]. SEM micrographs of PPS/MWCNTs nanocomposite resins are presented in Fig. 1. From Fig. 1(a)–(e), it was clear that when the content of MWCNTs was less than 1.0 wt%, MWCNTs had good dispersibility in PPS matrix without formation of agglomerates or entanglements.
2.3. Photo-aging treatment of PPS/MWCNTs fibers Photo-stability of PPS/MWCNTs composite fibers were investigated by Xenon-lamp weather resistance test chamber (TXH-T, Shanghai Changguan Electronic Technology Co. Ltd., China) with a 1800 W xenon arc lamp. The samples were placed on a 10 mm * 20 mm bracket and
Table 1 Melting-spinning parameters and spinnability of PPS/MWCNTs nanocomposite fibers. Samples
Spinning temperature (°C)
Take-up speed (m/min)
Drawing/ annealing temperature (°C)
Drawn ratio
Spinnability
PPS PPS/MWCNTs-0.1 PPS/MWCNTs-0.2 PPS/MWCNTs-0.5 PPS/MWCNTs-1.0 PPS/MWCNTs-1.5 PPS/MWCNTs-2.0
318 318 318 318 318 318 318
600 600 600 600 600 600 600
90/180 90/180 90/180 90/180 90/180 90/180 90/180
4.5 4.5 4.5 4.5 4.5 4.3 –
+ ++a + ++ + ++ + ++ + ++ + −b
Note: + means good spinnability (No damage of yarn during spinning and drawing). − means poor spinnability (Intermittent drawing process). 2
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Fig. 1. SEM micrographs of PPS/MWCNTs nanocomposite resins with different MWCNTs content: (a) 0 wt%, (b) 0.1 wt%, (c) 0.2 wt%, (d) 0.5 wt%, (e) 1.0 wt%, (f) 1.5 wt%, (g) 2.0 wt%.
3.2. Mechanical properties of PPS/MWCNTs nanocomposite fibers
Moreover, it was worth noting that most of the MWCNTs were still embedded in the PPS matrix. This is attributed to the strong interfacial adhesion between PPS and MWCNTs. It was seen from Fig. S1 (Supporting information) that the benzene ring structure of PPS molecular and the surface of the MWCNTs formed a strong π-π interaction, which caused the D-band of MWCNTs to red-shift from 1338 cm−1 to 1350 cm−1 [31]. From Fig. 1(f) and (g), when the content of MWCNTs was higher than 1.5 wt%, the MWCNTs in PPS matrix exhibited significant entanglements and aggregation. This is due to the excessive volume of MWCNTs powder. MWCNTs and PPS powder were difficult to mix evenly before melt compounding. Some MWCNTs entangled and agglomerated at the beginning of the melting blending process. The surface of agglomerated MWCNTs was not infiltrated enough by the PPS melt and difficult to be re-dispersed, which led to the decrease of spinnability (Table 1). Therefore, under the premise of ensuring continuous industrial melting spinning of PPS fiber, 0–1.0 wt% MWCNTs content was used to discuss their effect on the light aging resistance of PPS. Moreover, with the addition of MWCNTs, the hydrophilicity of PPS/MWCNTs nanocomposites increased. The contact angles of PPS/ MWCNTs nanocomposite film were decreased from 92.7° to 82.1° (Fig. S5, Supporting information) [38–40].
MWCNTs are effective reinforcement agents for polymer fibers. To quantitatively illustrate the effect of MWCNTs on PPS fibers, the mechanical properties of PPS/MWCNTs nanocomposite fibers are presented in Fig. 2. From Fig. 2(a) and (b), it was clear that with the increase of MWCNTs dosage, the strength of composite fibers first increased and then decreased, showing a maximum strength 4.41cN/ dtex when the dosage of MWCNTs was about 0.1 wt%. This result was directly related to the effect of MWCNTs on the crystallization properties and fiber orientation of PPS/MWCNTs fibers. On the one hand, the WAXD curves of PPS/MWCNTs nanocomposite fibers also illustrated that the degree of crystallinity of PPS/MWCNTs nanocomposite fibers showed the same trend to breaking strength which was attributed to MWCNTs as effective heterogeneous nucleating agents (Fig. S2 Supporting information and Table S1) [41,42]. When the amount of MWCNTs was less than 1.0 wt%, the nucleation effect of MWCNTs dominated the total crystallization ability. However, excessive usage of MWCNTs restricted the movement of molecular chains, leading to a decrease in PPS crystallinity (Fig. S3 and Table S2, Supporting information). Moreover, the strong π-π interaction between PPS and MWCNTs prohibit free movement of PPS molecular. Therefore, under the same process, the degree of orientation of the PPS fibers decreased (Fig. S4 and Table S3, Supporting information). However, when the 3
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Fig. 2. Mechanical properties of PPS/MWCNTs nanocomposite fibers: (a) uniaxial tensile mechanical properties of PPS/MWCNTs fibers, (b) stress-strain curves of PPS/MWCNTs fibers, (c) stress-strain curves of PPS/MWCNTs fibers after 8d under the photo-aging treatment, (d) breaking strength retention of PPS/MWCNTs fibers after 8d under the photo-aging treatment. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
entanglement of PPS. However, with the addition of MWCNTs, the melting-point gradient of PPS composite fibers showed a decreasing trend from 6.7 to 3.1 °C. This result directly shows that the molecular chain scission behavior of PPS fiber was suppressed by the MWCNTs (see Table 2).
content of MWCNTs was 1.0 wt%, the strength of PPS/MWCNTs nanocomposite fiber was over 4.0 cN/dtex, which was still suitable for textile and clothing. The fundamental purpose of this paper is to study the effect of adding MWCNTs on the photo-stability of PPS fibers which is directly linked to the fibers’ mechanical properties. PPS molecular chain is prone to fracture under the UV-light by series of photo-chemistry reactions [5–7], which leads to a decrease in molecular weight and reduces the mechanical properties of fibers. In addition, the breaking of molecular chains will destroy the entanglements between molecules leading to low mechanical properties of fibers. As shown in Fig. 2(c) and (d), the strength of neat PPS fiber decreased sharply and the strength retention was only 57.85%. It was worth noting that with the addition of MWCNTs, the mechanical strength retention rate of the PPS/MWCNTs fibers increased and reached 77%. It is inferred that MWCNTs can reduce the photochemical reaction efficiency of PPS molecules initiated by photoelectrons and oxygen molecules thereby improving the photo-stability of PPS fibers.
3.4. Photo-stability enhancement mechanism of PPS/MWCNTs nanocomposite fibers Based on the above analysis, it can be concluded that the MWCNTs were able to reduce loss of PPS fiber’s mechanical properties in the illumination environment by inhibiting degradation of molecular chains. Moreover, it is well known that the MWCNTs can be used as fluorescence quenchers which absorb the photo-exciton and improve the light aging resistance of organic molecules in turn [17,18,25]. In order to study the mechanism of photo-stability enhancement of MWCNTs, the steady-state fluorescence emission spectrum of PPS/ MWCNTs nanocomposite fibers was characterized under 330 nm excitation light. As was observed in Fig. 4, the fluorescent emission peaks of benzene ring and benzene-thioether conjugate structural units in PPS molecular were showed in 355 nm and 381 nm. As the dosage of MWCNTs increased, the intensity of fluorescence emission peak of PPS reduced sharply and the position of peaks blue shifted. When the MWCNTs content was 0.03 wt%, the fluorescence emission intensity at peak of PPS fibers decreased to 1/2 compared to neat PPS fiber. This is linked to the fact that the electron adsorption of MWCNTs and its π-π conjugated structure with PPS can rapidly absorb electrons and reduce their activities [43–45]. This absorption behavior not only decreased the activity of photo-induced exciton but also reduced the rate at which the O2 molecule can be combined with the excitons to form singlet oxygen, thereby inhibiting the photochemical reaction of the PPS
3.3. Thermal properties of PPS/MWCNTs nanocomposite fibers In order to further reveal the effect of photo-aging on PPS fibers and photo-stability enhancement of MWCNTs, Fig. 3 shows the melting behavior of PPS/MWCNTs fibers at different aging time. As shown in Fig. 3 the initial melting and melting peak temperatures of PPS/ MWCNTs nanocomposite fibers gradually moved towards a lower temperature as a result of increasing the aging time. The melting point of PPS fiber after 8d aging reduced from 281 °C to 274.3 °C, and the melting-point gradient (ΔT) was 6.7 °C. This is mainly attributed to the fracture of the PPS molecular chain after aging under the xenon lamp, resulting into reduced relative molecular mass and molecular chain 4
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Fig. 3. Melting behaviors of PPS/MWCNTs nanocomposite fibers aged in 0–8 d. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
and dynamic Stern- Volmer constants respectively. As can be seen from Fig. 4(b), the ratio of (F0/F)/[Q] to [Q] is different from that of Ref. [25]. This was due to the fact that the dynamic quenching efficiency of solid-state PPS/MWCNTs at room temperature was low which only appeared at the phase interface between PPS and MWCNTs. Therefore, MWCNTs achieved exciton quenching mainly through the static quenching method such as energy transfer quenching and formation of non-fluorescent compounds, such that the value of KS tended to 0. When the content of MWCNTs was less than 0.2 wt%, the relative fluorescence intensity of PPS varied linearly with the content of MWCNTs. It can be obtained from the Stern-Volmer plot fitting that the value of KD is 1.717, inferring that the quenching behavior of low content MWCNTs in PPS matrix is mainly static quenching via energy transduction or non-fluorescent complex formation. When the content of MWCNTs was higher than 0.5 wt%, the relative PL intensity ratio of PPS was nonlinearly correlated to the content of MWCNTs. This observation is attributed to the fluorescence emission peak of benzenethioether covalent bond in PPS which blue shifted and coincide with the fluorescence emission peak of the benzene ring. Although the fluorescence intensity is affected by benzene ring fluorescence exciton quenching efficiency was in principle gradually enhanced.
Table 2 Melting points of PPS/MWCNTs nanocomposite fibers aged in 0–8d. Samples
PPS PPS/MWCNTs-0.1 PPS/MWCNTs-0.2 PPS/MWCNTs-0.5 PPS/MWCNTs-1.0
ΔT (°C)
Melting Point (°C) 0d
2d
5d
8d
281.0 280.0 280.5 282.9 283.2
278.0 277.0 276.6 281.8 282.3
275.1 276.8 276.8 280.9 281.6
274.3 274.8 275.6 279.5 280.1
6.7 5.2 4.9 3.4 3.1
molecule to improve the overall light stability of the PPS fiber. This exciton quenching process is also similar to graphene and carbon quantum dots [46,47]. In order to effectively evaluate the exciton quenching of MWCNTs, the Stern-Volmer nonlinear equation is used to calculate the quenching efficiency [48,49].
F0/ F = 1 + (KD + K S )[Q] + KD K S [Q]2 where [Q] is the concentration of the MWCNTs, and KS and KD are static
Fig. 4. (a) Steady-state PL spectrum of PPS/MWCNTs nanocomposite fibers and (b) Stern-volmer plot fitted from Steady-state PL spectrum. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 5
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Fig. 5. Fluorescence excitation spectrum and the peak position of PPS-MWCNTs nanocomposite fibers with different contents of MWCNTs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
In order to further analyze the effect of higher content (> 0.5 wt%) of MWCNTs on the electronic excitation and fluorescence behavior of PPS matrix, the fluorescence excitation spectra of PPS composite fibers were characterized by fluorescence absorption peaks at 385 nm. It can be seen from Fig. 5 that as the content of MWCNTs increased, the fluorescence excitation peak of PPS fiber shifted from 370 nm to 357 nm. This is owing to the band gap of the luminescent groups in the PPS molecular chain which was enhanced by MWCNTs with strong absorption electron effect resulting to a blue shift of the excitation and emission peaks. When the content was 1.5 wt%, MWCNTs presented agglomerates, the relative specific surface area in the matrix increased slowly, but the fluorescence excitation wavelength of the PPS/ MWCNTs-1.5 nanocomposite fiber did not decrease. Finally, due to the blue shift in the peak of the fluorescence excitation light, the light intensity of sunlight or xenon lamp decreased at the peak of the corresponding excitation light, which was beneficial to reduce the efficiency of exciton formation/photo-degradation and further improve the photostability of the PPS fiber. It is well known that MWCNTs are both electron donors and electron acceptors, and that they are free radical scavengers due to their strong adsorption of free radicals (such as OHs, OOHs, CH3O) [50,51]. In order to further confirm the free radical scavenging properties of MWCNTs, Fig. 6 gives the thermal degradation behavior of PPS/MWCNTs fibers. The thermogravimetric parameters were tabulated in Table3. Compared with the thermogravimetric curves between PPS and PPS/ MWCNTs-1.0, the increment of thermal decomposition temperature of nanocomposite fiber was insignificant in the first stage of thermal-degradation. It can be inferred that MWCNTs are difficult to react rapidly with free radicals generated from the fractured PPS molecular chain due to their low migration rate and barrier property. Therefore, the free radical quenching performance of MWCNTs cannot account for
Table 3 Thermogravimetric parameters of PPS/MWCNTs nanocomposite fibers. Samples
Tonset/°C
Td/°C
T2%/°C
T5%/°C
T10%/°C
T50%/°C
PPS PPS/MWCNTs-1.0
480.5 483.4
499.5 501.7
485.1 494.1
499.2 504.5
509.5 513.5
623.7 656.8
enhancement of light aging resistance in PPS nanocomposite fibers. Based on the above analysis, the photo-stability enhancement mechanism of MWCNTs in PPS nanocomposite fibers is illustrated in Fig. 7. The dynamic quenching behavior of excitons were weak owing to the barely diffusion behavior of MWCNTs and macromolecules in the solid system [48]. Besides, the radical scavenging of MWCNTs with low loading is barely useful for the photo-stability of PPS fiber. It is probable that the photo-stability of PPS/MWCNTs nanocomposite fiber is improved via static-state quenching of excitons by MWCNTs through the formation of non-fluorescent complex. When such a complex absorbs UV light, the excitons are quickly adsorbed onto the surface of MWCNTs and return to the ground state without emission of a photon and photochemical reaction, thus suppressing singlet oxygen formation and PPS degradation [15,17,18]. Moreover, the strong interaction between the PPS molecular chain and MWCNTs causes the maximum fluorescence excitation to shift blue [17,52], thus decreasing the photochemical reaction efficiency under the same xenon source while broadening the energy gap of the chromogenic group. Therefore, the synergistic effect of static quenching behavior and blue shift of excitation light effectively reduces the yield of exciton of the PPS molecular which inhibits a series of photochemical reactions and improves the photo-stability of PPS fibers.
Fig. 6. TGA (a) and DTG (b) curves of PPS nanocomposite fibers (in air atmosphere). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 6
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Fig. 7. Schematic of possible exciton quenching mechanism of MWCNTs in PPS fiber [15,17,18,48,52] (Where S0, S1, S2 are ground state, the first-excited singlet state and the second- excited singlet state, respectively, vr, ic, ec are vibrational relaxation, intersystem crossing and external conversion, respectively, abs and f1 mean absorb excite light and fluorescence, Eg mean energy gap of PPS molecular.) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
4. Conclusions
China Postdoctoral Science Foundation (2018M631980).
PPS/MWCNTs nanocomposite fibers were fabricated by meltingspinning method. MWCNTs were used as effective photo-stabilizers, and had a strong π-π interaction with the PPS molecular. This molecular interaction caused the blue-shift of fluorescence excitation light, resulting in decreased photochemical reaction efficiency but amplified the energy gap of the chromogenic group. Moreover, due to the high electron adsorption and electron mobility of MWCNTs, the excitons were quickly adsorbed onto the surface of MWCNTs and returned to the ground state without photochemical reaction, thus suppressing singlet oxygen formation and PPS degradation. It's worth noting that when the amount of MWCNTs was less than 1.0 wt%, they could uniformly and stably be dispersed in the PPS matrix. The mechanical strength of PPS fibers was increased from 4.15 cN/dtex to 4.41 cN/dtex due to the heterogeneous nucleation of MWCNTs. Moreover, the inhibiting effect of MWCNTs increased the breaking strength retention rate of fiber from 57.8% to 77.3%. Therefore, PPS fibers with good photo-stability are suable for making protective fabrics.
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.compositesa.2019.105716. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Declaration of Competing Interest
[12] [13]
The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
[14] [15] [16] [17] [18] [19]
Acknowledgements This work was partially supported by the project was funded by Special Funds Project for Transformation of scientific and technological achievements in Jiangsu Province (BA2016108), the Program for National Key Research and Development Program of China (2016YFA0201702/2016YFA0201700), the Fundamental Research Funds for the Central Universities (2232018A3-01, 2232018D3-03) and
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Enhanced photo-stability polyphenylene sulfide fiber via incorporation of multi-walled carbon nanotubes using exciton quenching ⁎
Zexu Hu, Kai Hou, Jialin Gao, Genming Zhu, Zhe Zhou, Hengxue Xiang , Tian Qiu, Meifang Zhu
T ⁎
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, International Joint Laboratory for Advanced Fiber and Low-dimension Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Polyphenylene sulfide fiber MWCNTs Photo-stability Exciton quenching
Polyphenylene sulfide (PPS) fiber is widely used in flame-retardant fabrics, high-temperature filter bags, among other applications. It has excellent heat and corrosion resistance. However, the fibers were characterized by weak photo-stability. Herein, an unique strategy based on second component induced shielding effect for constructing organic-inorganic hybrid nanocomposites were developed by using multi-walled carbon nanotubes (MWCNTs) as effective stabilizers. The photo-stability of PPS/MWCNTs composite fibers were investigated by Xenon-lamp weather resistance test chamber. Moreover, the mechanism for photo-stability enhancement was analyzed by Fluorescence spectrometer, differential scanning calorimeter and thermogravimetric analysis. Compared with neat PPS fiber, it was showed significant improvement in photo-stability of PPS/MWCNTs composite fibers resulting to increased strength retention ratio from 57.8% for neat PPS to 77.3% for PPS/ MWCNTs-1.0 fiber. It was finally concluded that the mechanism for enhanced photo-stability of PPS fiber was based on efficient quenching of optically generated excitons by MWCNTs resulted from steady-state fluorescence spectroscopy.
1. Introduction Polyphenylene sulfide (PPS) fiber is among those high performance fibers with inherent flame resistance, outstanding heat resistance [1], excellent chemical resistance [2] and electro-insulating [3] properties which is widely used as protective textiles [4]. However, the application of neat PPS fiber is restricted by its weak photo-stability. The presence of ultraviolet (UV) light in the environment degrades PPS to a darker color and lowers its mechanical strength. According to the literature reported [5,6], photo-degradation of PPS is mainly induced by morphology, oxygen and spectral region of excitation. In order to improve the photo-stability of PPS, numerous organic UV-absorbers have been investigated to shield light in the near-ultraviolet region [7]. However, the plasticization effect arising from the excessive use of organic UV-absorbers reduces the melt-spinnability and mechanical properties of PPS fibers. Thus, inorganic shielding agents with broadspectrum UV-shield and mechanical enhancement performance, such as nano-TiO2 and ZnO, were preferred materials for modified PPS fiber [810]. However, The defects of shielding nanoparticles are the low permissible contents (< 2%) in fibers and their inevitable agglomeration which could reduce the shielding efficiency due to the incomplete and incompact distribution.
⁎
Compared with its applicability in UV shielding, the method of fluorescence and exciton quenching has in the recent years shown better advantages for improving the photo-stability of polymer materials and organic molecules. Moreover, carbonaceous materials, such as CNTs, graphene, fullerene and carbon dots, have excellent electron acceptability from the excited state of organics which enhances their quick excitons quenching effect [11–14]. In this regard, the optically generated excitons of the fluorescent molecule are rapidly transferred to the surface of the carbon material through the interface [15–17]. This whole process suppresses the degradation reaction by cutting off the formation channel of the superoxide anion O2%. On the other hand, the process of electron transfer through carbonaceous materials is reversible. When the irradiated fluorochrome/carbon nanotubes composites are transferred to the dark environment, excitons in the carbon nanotubes are transferred back to fluorochrome [18]. Thirdly carbonaceous materials with a π-π conjugated structure have a strong electron adsorption capacity. They capture radicals from molecular light/ thermal degradation thereby improving the stability of the molecular chain [19–21]. Therefore, carbonaceous materials are simultaneously regarded as excellent excitons quenchers and outstanding light/thermal stabilizers for polymers [22,23]. Moreover, these carbonaceous materials (CNTs, graphene, fullerene) not only exhibit excellent excitons
Corresponding authors. E-mail addresses: [email protected] (H. Xiang), [email protected] (M. Zhu).
https://doi.org/10.1016/j.compositesa.2019.105716 Received 2 April 2019; Received in revised form 24 November 2019; Accepted 25 November 2019 Available online 26 November 2019 1359-835X/ © 2019 Elsevier Ltd. All rights reserved.
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the test environment was set to harsh outdoor weathering conditions. The irradiance time, temperature as well as environment humidity were 20 h/d, 63 ± 3 °C, and 60 ± 5% respectively.
quenching [24] and radicals scavenging properties [25,26], but also effectively enhance the mechanical behavior and electrical properties [27–29] of polymer fibers. It was showed in some report that the π-π conjugation structure between PPS and CNT can effectively increase the dispersion of the inorganic powders, resulting in the improvement of conductivity [30] and mechanical properties of the PPS [31]. In view of the fact that most research related to PPS/MWCNTs nanocomposites have mainly focused on the mechanical properties rather than photo-stability [32,33], we report the effect of the dosage and dispersibility of MWCNTs on the spinnability and photo-stability of PPS nanocomposite fibers in detail herein. Briefly, the PPS/MWCNTs nanocomposite fibers with good photo-stability were obtained by facile melting-spinning process. Afterward, the UV-light stability of PPS/ MWCNTs composite fibers were investigated by Xenon-lamp weather resistance test chamber. The enhancement mechanism of MWCNTs for PPS/MWCNTs fiber was first analyzed by combining the fluorescence behavior and thermal degradation of PPS molecular. On the basis of the steady-state PL spectrum and TGA results obtained in this work, it can be concluded that MWCNT can significantly improve the photo-stability of PPS fiber.
2.4. Characterization 2.4.1. Dispersion of MWCNTs in PPS matrix Dispersion of MWNCTs in PPS matrix was observed by a Field emission scanning microscopy (FESEM, S-4800, Hitachi) operating at 10 kV. 2.4.2. Mechanical properties of PPS/MWCNTs fibers Uniaxial tensile mechanical properties of PPS/MWCNTs fibers were tested using a single fiber tensile strength tester (XQ-1A, Shanghai New Fiber Instrument Co. Ltd., China) with a testing speed of 10 mm/min and gauge length of 10 mm. The test ambient temperature was 20 ± 3 °C and the relative humidity was 65 ± 5%. The mechanical properties of the samples were the average of 50 samples. 2.4.3. The thermal properties of PPS/MWCNTs fibers Differential scanning calorimeter (DSC, 204F1, NETZSCH Group) was used to determine the melting behaviors of PPS/MWCNTs nanocomposite fibers in the range of 20–320 °C at a heating/cooling rate of 10 °C/min.
2. Experimental 2.1. Materials Polyphenylene sulfide (PPS, Fortron 0309P4, 1.35 g/cm3, Tg ≈ 89 °C, Tm ≈ 280 °C) was purchased from Celanese Corporation. Multi-walled carbon nanotubes (MWCNTs, outer diameter: 10–20 nm; length: 0.5–2 μm; purity: 95%) were supplied from Chengdu Organic Chemicals Co., LTD.
2.4.4. Fluorescence quenching of MWCNTs in PPS/MWCNTs fibers The fluorescence quenching behavior of MWCNTs in PPS/MWCNTs nanocomposite fibers was determined by a steady-state fluorescence measurement at room temperature using a commercial fluorimeter (QM/TM, Photon technology international). The samples were tightly wrapped in parallel on the cardboard to make the test surface uniform. The fluorescence emission intensity traces of samples were obtained by using a 330 nm laser as the excitation source. And the fluorescence emission behavior of samples was monitored at 390 nm to detect the fluorescence excited intensity.
2.2. Fabrication of PPS/MWCNTs nanocomposite fibers PPS/MWCNTs nanocomposite fibers were prepared in two stages. First, PPS nanocomposite resins with different MWCNTs contents (0, 0.1, 0.2, 0.5, 1.0, 1.5 and 2.0 wt%) were produced by twin screw extruder with a processing temperature of 290 °C. Prior to this procedure, PPS chips and MWCNTs were dried in a vacuum drum dryer and made sure the moisture was below 50 ppm. Second, the PPS/MWCNTs nanocomposite fibers were fabricated by melting spinning. The small aperture micropore spinnerets (< 300 μm) were used to prepare smalldiameter fibers (< 20 μm) which are suitable for clothing. The parameters and the spinning properties of PPS-MWCNTs fibers are listed in Table 1. The resultant nanocomposite fibers are abbreviated to PPS/ MWCNTs-x, where x represent the mass percentage of MWCNTs.
2.4.5. Thermal degradation behavior of PPS/MWCNTs Thermal gravimetric analyzer (TGA, 209F1, NETZSCH Group) was used to determine the degradation behavior of PPS/MWCNTs nanocomposite fibers in air atmosphere at a heating rate of 10 °C/min. 3. Results and discussion 3.1. Dispersion of MWCNTs in PPS matrix Dispersion of inorganic powders plays an important role in nanocomposite fiber spinnability and properties, which depends on the compatibility of inorganic powder with polymer matrix [34-37]. SEM micrographs of PPS/MWCNTs nanocomposite resins are presented in Fig. 1. From Fig. 1(a)–(e), it was clear that when the content of MWCNTs was less than 1.0 wt%, MWCNTs had good dispersibility in PPS matrix without formation of agglomerates or entanglements.
2.3. Photo-aging treatment of PPS/MWCNTs fibers Photo-stability of PPS/MWCNTs composite fibers were investigated by Xenon-lamp weather resistance test chamber (TXH-T, Shanghai Changguan Electronic Technology Co. Ltd., China) with a 1800 W xenon arc lamp. The samples were placed on a 10 mm * 20 mm bracket and
Table 1 Melting-spinning parameters and spinnability of PPS/MWCNTs nanocomposite fibers. Samples
Spinning temperature (°C)
Take-up speed (m/min)
Drawing/ annealing temperature (°C)
Drawn ratio
Spinnability
PPS PPS/MWCNTs-0.1 PPS/MWCNTs-0.2 PPS/MWCNTs-0.5 PPS/MWCNTs-1.0 PPS/MWCNTs-1.5 PPS/MWCNTs-2.0
318 318 318 318 318 318 318
600 600 600 600 600 600 600
90/180 90/180 90/180 90/180 90/180 90/180 90/180
4.5 4.5 4.5 4.5 4.5 4.3 –
+ ++a + ++ + ++ + ++ + ++ + −b
Note: + means good spinnability (No damage of yarn during spinning and drawing). − means poor spinnability (Intermittent drawing process). 2
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Fig. 1. SEM micrographs of PPS/MWCNTs nanocomposite resins with different MWCNTs content: (a) 0 wt%, (b) 0.1 wt%, (c) 0.2 wt%, (d) 0.5 wt%, (e) 1.0 wt%, (f) 1.5 wt%, (g) 2.0 wt%.
3.2. Mechanical properties of PPS/MWCNTs nanocomposite fibers
Moreover, it was worth noting that most of the MWCNTs were still embedded in the PPS matrix. This is attributed to the strong interfacial adhesion between PPS and MWCNTs. It was seen from Fig. S1 (Supporting information) that the benzene ring structure of PPS molecular and the surface of the MWCNTs formed a strong π-π interaction, which caused the D-band of MWCNTs to red-shift from 1338 cm−1 to 1350 cm−1 [31]. From Fig. 1(f) and (g), when the content of MWCNTs was higher than 1.5 wt%, the MWCNTs in PPS matrix exhibited significant entanglements and aggregation. This is due to the excessive volume of MWCNTs powder. MWCNTs and PPS powder were difficult to mix evenly before melt compounding. Some MWCNTs entangled and agglomerated at the beginning of the melting blending process. The surface of agglomerated MWCNTs was not infiltrated enough by the PPS melt and difficult to be re-dispersed, which led to the decrease of spinnability (Table 1). Therefore, under the premise of ensuring continuous industrial melting spinning of PPS fiber, 0–1.0 wt% MWCNTs content was used to discuss their effect on the light aging resistance of PPS. Moreover, with the addition of MWCNTs, the hydrophilicity of PPS/MWCNTs nanocomposites increased. The contact angles of PPS/ MWCNTs nanocomposite film were decreased from 92.7° to 82.1° (Fig. S5, Supporting information) [38–40].
MWCNTs are effective reinforcement agents for polymer fibers. To quantitatively illustrate the effect of MWCNTs on PPS fibers, the mechanical properties of PPS/MWCNTs nanocomposite fibers are presented in Fig. 2. From Fig. 2(a) and (b), it was clear that with the increase of MWCNTs dosage, the strength of composite fibers first increased and then decreased, showing a maximum strength 4.41cN/ dtex when the dosage of MWCNTs was about 0.1 wt%. This result was directly related to the effect of MWCNTs on the crystallization properties and fiber orientation of PPS/MWCNTs fibers. On the one hand, the WAXD curves of PPS/MWCNTs nanocomposite fibers also illustrated that the degree of crystallinity of PPS/MWCNTs nanocomposite fibers showed the same trend to breaking strength which was attributed to MWCNTs as effective heterogeneous nucleating agents (Fig. S2 Supporting information and Table S1) [41,42]. When the amount of MWCNTs was less than 1.0 wt%, the nucleation effect of MWCNTs dominated the total crystallization ability. However, excessive usage of MWCNTs restricted the movement of molecular chains, leading to a decrease in PPS crystallinity (Fig. S3 and Table S2, Supporting information). Moreover, the strong π-π interaction between PPS and MWCNTs prohibit free movement of PPS molecular. Therefore, under the same process, the degree of orientation of the PPS fibers decreased (Fig. S4 and Table S3, Supporting information). However, when the 3
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Fig. 2. Mechanical properties of PPS/MWCNTs nanocomposite fibers: (a) uniaxial tensile mechanical properties of PPS/MWCNTs fibers, (b) stress-strain curves of PPS/MWCNTs fibers, (c) stress-strain curves of PPS/MWCNTs fibers after 8d under the photo-aging treatment, (d) breaking strength retention of PPS/MWCNTs fibers after 8d under the photo-aging treatment. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
entanglement of PPS. However, with the addition of MWCNTs, the melting-point gradient of PPS composite fibers showed a decreasing trend from 6.7 to 3.1 °C. This result directly shows that the molecular chain scission behavior of PPS fiber was suppressed by the MWCNTs (see Table 2).
content of MWCNTs was 1.0 wt%, the strength of PPS/MWCNTs nanocomposite fiber was over 4.0 cN/dtex, which was still suitable for textile and clothing. The fundamental purpose of this paper is to study the effect of adding MWCNTs on the photo-stability of PPS fibers which is directly linked to the fibers’ mechanical properties. PPS molecular chain is prone to fracture under the UV-light by series of photo-chemistry reactions [5–7], which leads to a decrease in molecular weight and reduces the mechanical properties of fibers. In addition, the breaking of molecular chains will destroy the entanglements between molecules leading to low mechanical properties of fibers. As shown in Fig. 2(c) and (d), the strength of neat PPS fiber decreased sharply and the strength retention was only 57.85%. It was worth noting that with the addition of MWCNTs, the mechanical strength retention rate of the PPS/MWCNTs fibers increased and reached 77%. It is inferred that MWCNTs can reduce the photochemical reaction efficiency of PPS molecules initiated by photoelectrons and oxygen molecules thereby improving the photo-stability of PPS fibers.
3.4. Photo-stability enhancement mechanism of PPS/MWCNTs nanocomposite fibers Based on the above analysis, it can be concluded that the MWCNTs were able to reduce loss of PPS fiber’s mechanical properties in the illumination environment by inhibiting degradation of molecular chains. Moreover, it is well known that the MWCNTs can be used as fluorescence quenchers which absorb the photo-exciton and improve the light aging resistance of organic molecules in turn [17,18,25]. In order to study the mechanism of photo-stability enhancement of MWCNTs, the steady-state fluorescence emission spectrum of PPS/ MWCNTs nanocomposite fibers was characterized under 330 nm excitation light. As was observed in Fig. 4, the fluorescent emission peaks of benzene ring and benzene-thioether conjugate structural units in PPS molecular were showed in 355 nm and 381 nm. As the dosage of MWCNTs increased, the intensity of fluorescence emission peak of PPS reduced sharply and the position of peaks blue shifted. When the MWCNTs content was 0.03 wt%, the fluorescence emission intensity at peak of PPS fibers decreased to 1/2 compared to neat PPS fiber. This is linked to the fact that the electron adsorption of MWCNTs and its π-π conjugated structure with PPS can rapidly absorb electrons and reduce their activities [43–45]. This absorption behavior not only decreased the activity of photo-induced exciton but also reduced the rate at which the O2 molecule can be combined with the excitons to form singlet oxygen, thereby inhibiting the photochemical reaction of the PPS
3.3. Thermal properties of PPS/MWCNTs nanocomposite fibers In order to further reveal the effect of photo-aging on PPS fibers and photo-stability enhancement of MWCNTs, Fig. 3 shows the melting behavior of PPS/MWCNTs fibers at different aging time. As shown in Fig. 3 the initial melting and melting peak temperatures of PPS/ MWCNTs nanocomposite fibers gradually moved towards a lower temperature as a result of increasing the aging time. The melting point of PPS fiber after 8d aging reduced from 281 °C to 274.3 °C, and the melting-point gradient (ΔT) was 6.7 °C. This is mainly attributed to the fracture of the PPS molecular chain after aging under the xenon lamp, resulting into reduced relative molecular mass and molecular chain 4
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Fig. 3. Melting behaviors of PPS/MWCNTs nanocomposite fibers aged in 0–8 d. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
and dynamic Stern- Volmer constants respectively. As can be seen from Fig. 4(b), the ratio of (F0/F)/[Q] to [Q] is different from that of Ref. [25]. This was due to the fact that the dynamic quenching efficiency of solid-state PPS/MWCNTs at room temperature was low which only appeared at the phase interface between PPS and MWCNTs. Therefore, MWCNTs achieved exciton quenching mainly through the static quenching method such as energy transfer quenching and formation of non-fluorescent compounds, such that the value of KS tended to 0. When the content of MWCNTs was less than 0.2 wt%, the relative fluorescence intensity of PPS varied linearly with the content of MWCNTs. It can be obtained from the Stern-Volmer plot fitting that the value of KD is 1.717, inferring that the quenching behavior of low content MWCNTs in PPS matrix is mainly static quenching via energy transduction or non-fluorescent complex formation. When the content of MWCNTs was higher than 0.5 wt%, the relative PL intensity ratio of PPS was nonlinearly correlated to the content of MWCNTs. This observation is attributed to the fluorescence emission peak of benzenethioether covalent bond in PPS which blue shifted and coincide with the fluorescence emission peak of the benzene ring. Although the fluorescence intensity is affected by benzene ring fluorescence exciton quenching efficiency was in principle gradually enhanced.
Table 2 Melting points of PPS/MWCNTs nanocomposite fibers aged in 0–8d. Samples
PPS PPS/MWCNTs-0.1 PPS/MWCNTs-0.2 PPS/MWCNTs-0.5 PPS/MWCNTs-1.0
ΔT (°C)
Melting Point (°C) 0d
2d
5d
8d
281.0 280.0 280.5 282.9 283.2
278.0 277.0 276.6 281.8 282.3
275.1 276.8 276.8 280.9 281.6
274.3 274.8 275.6 279.5 280.1
6.7 5.2 4.9 3.4 3.1
molecule to improve the overall light stability of the PPS fiber. This exciton quenching process is also similar to graphene and carbon quantum dots [46,47]. In order to effectively evaluate the exciton quenching of MWCNTs, the Stern-Volmer nonlinear equation is used to calculate the quenching efficiency [48,49].
F0/ F = 1 + (KD + K S )[Q] + KD K S [Q]2 where [Q] is the concentration of the MWCNTs, and KS and KD are static
Fig. 4. (a) Steady-state PL spectrum of PPS/MWCNTs nanocomposite fibers and (b) Stern-volmer plot fitted from Steady-state PL spectrum. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 5
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Fig. 5. Fluorescence excitation spectrum and the peak position of PPS-MWCNTs nanocomposite fibers with different contents of MWCNTs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
In order to further analyze the effect of higher content (> 0.5 wt%) of MWCNTs on the electronic excitation and fluorescence behavior of PPS matrix, the fluorescence excitation spectra of PPS composite fibers were characterized by fluorescence absorption peaks at 385 nm. It can be seen from Fig. 5 that as the content of MWCNTs increased, the fluorescence excitation peak of PPS fiber shifted from 370 nm to 357 nm. This is owing to the band gap of the luminescent groups in the PPS molecular chain which was enhanced by MWCNTs with strong absorption electron effect resulting to a blue shift of the excitation and emission peaks. When the content was 1.5 wt%, MWCNTs presented agglomerates, the relative specific surface area in the matrix increased slowly, but the fluorescence excitation wavelength of the PPS/ MWCNTs-1.5 nanocomposite fiber did not decrease. Finally, due to the blue shift in the peak of the fluorescence excitation light, the light intensity of sunlight or xenon lamp decreased at the peak of the corresponding excitation light, which was beneficial to reduce the efficiency of exciton formation/photo-degradation and further improve the photostability of the PPS fiber. It is well known that MWCNTs are both electron donors and electron acceptors, and that they are free radical scavengers due to their strong adsorption of free radicals (such as OHs, OOHs, CH3O) [50,51]. In order to further confirm the free radical scavenging properties of MWCNTs, Fig. 6 gives the thermal degradation behavior of PPS/MWCNTs fibers. The thermogravimetric parameters were tabulated in Table3. Compared with the thermogravimetric curves between PPS and PPS/ MWCNTs-1.0, the increment of thermal decomposition temperature of nanocomposite fiber was insignificant in the first stage of thermal-degradation. It can be inferred that MWCNTs are difficult to react rapidly with free radicals generated from the fractured PPS molecular chain due to their low migration rate and barrier property. Therefore, the free radical quenching performance of MWCNTs cannot account for
Table 3 Thermogravimetric parameters of PPS/MWCNTs nanocomposite fibers. Samples
Tonset/°C
Td/°C
T2%/°C
T5%/°C
T10%/°C
T50%/°C
PPS PPS/MWCNTs-1.0
480.5 483.4
499.5 501.7
485.1 494.1
499.2 504.5
509.5 513.5
623.7 656.8
enhancement of light aging resistance in PPS nanocomposite fibers. Based on the above analysis, the photo-stability enhancement mechanism of MWCNTs in PPS nanocomposite fibers is illustrated in Fig. 7. The dynamic quenching behavior of excitons were weak owing to the barely diffusion behavior of MWCNTs and macromolecules in the solid system [48]. Besides, the radical scavenging of MWCNTs with low loading is barely useful for the photo-stability of PPS fiber. It is probable that the photo-stability of PPS/MWCNTs nanocomposite fiber is improved via static-state quenching of excitons by MWCNTs through the formation of non-fluorescent complex. When such a complex absorbs UV light, the excitons are quickly adsorbed onto the surface of MWCNTs and return to the ground state without emission of a photon and photochemical reaction, thus suppressing singlet oxygen formation and PPS degradation [15,17,18]. Moreover, the strong interaction between the PPS molecular chain and MWCNTs causes the maximum fluorescence excitation to shift blue [17,52], thus decreasing the photochemical reaction efficiency under the same xenon source while broadening the energy gap of the chromogenic group. Therefore, the synergistic effect of static quenching behavior and blue shift of excitation light effectively reduces the yield of exciton of the PPS molecular which inhibits a series of photochemical reactions and improves the photo-stability of PPS fibers.
Fig. 6. TGA (a) and DTG (b) curves of PPS nanocomposite fibers (in air atmosphere). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 6
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Fig. 7. Schematic of possible exciton quenching mechanism of MWCNTs in PPS fiber [15,17,18,48,52] (Where S0, S1, S2 are ground state, the first-excited singlet state and the second- excited singlet state, respectively, vr, ic, ec are vibrational relaxation, intersystem crossing and external conversion, respectively, abs and f1 mean absorb excite light and fluorescence, Eg mean energy gap of PPS molecular.) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
4. Conclusions
China Postdoctoral Science Foundation (2018M631980).
PPS/MWCNTs nanocomposite fibers were fabricated by meltingspinning method. MWCNTs were used as effective photo-stabilizers, and had a strong π-π interaction with the PPS molecular. This molecular interaction caused the blue-shift of fluorescence excitation light, resulting in decreased photochemical reaction efficiency but amplified the energy gap of the chromogenic group. Moreover, due to the high electron adsorption and electron mobility of MWCNTs, the excitons were quickly adsorbed onto the surface of MWCNTs and returned to the ground state without photochemical reaction, thus suppressing singlet oxygen formation and PPS degradation. It's worth noting that when the amount of MWCNTs was less than 1.0 wt%, they could uniformly and stably be dispersed in the PPS matrix. The mechanical strength of PPS fibers was increased from 4.15 cN/dtex to 4.41 cN/dtex due to the heterogeneous nucleation of MWCNTs. Moreover, the inhibiting effect of MWCNTs increased the breaking strength retention rate of fiber from 57.8% to 77.3%. Therefore, PPS fibers with good photo-stability are suable for making protective fabrics.
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.compositesa.2019.105716. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Declaration of Competing Interest
[12] [13]
The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
[14] [15] [16] [17] [18] [19]
Acknowledgements This work was partially supported by the project was funded by Special Funds Project for Transformation of scientific and technological achievements in Jiangsu Province (BA2016108), the Program for National Key Research and Development Program of China (2016YFA0201702/2016YFA0201700), the Fundamental Research Funds for the Central Universities (2232018A3-01, 2232018D3-03) and
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