Morphologies and Properties of PET Nano Porous Luminescence Fiber

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Morphologies and Properties of PET Nano Porous Luminescence Fiber: Oil Absorption and Fluorescence-indicating Functions Dengkun Shu, Peng Xi, Shuwang Li, Congcong Li, Xiaoqing Wang, and Bowen Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16655 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 6, 2018

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ACS Applied Materials & Interfaces

Morphologies and Properties of PET Nano Porous Luminescence Fiber: Oil Absorption and Fluorescence-indicating Functions Dengkun Shua, Peng Xi a, b*, Shuwang Lia, Congcong Lia, Xiaoqing Wanga, Bowen Chenga* a

Tianjin Key Laboratory of Advanced Fibers and Energy Storage, Tianjin Polytechnic University, 399 Bin Shui West Road, 300387, Tianjin, PR China

b

State Key laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, 100080, Beijing, PR China

ABSTRACT: A polyethylene terephthalate (PET) nano porous luminescence fiber (PNPLF) was prepared through electrospun technology. The SEM and TEM images show that the surfaces of the fibers are covered with pores. The diameter of the fiber is 250-500 nm and diameter of the pores is 20-180 nm. The water and oil contact angles of PNPLF are 135° and 27°，respectively. The oil absorption value of the as-prepared PNPLF achieves 135 g/g and has a good oil absorption function. The as-prepared PNPLF has good luminescence properties and fluorescent-indicating function. Even trace amounts of oil can also cause obvious change of fluorescence intensity of PNPLF which has a good stability from 20°C to 70 °C. The breaking stress of yarn of PNPLF reaches 117cN. Furthermore, the good mechanical properties and thermal properties of PNPLF provide important basic conditions for their wide applications. Keywords: oil absorption; fluorescent-indicating; porous fiber; nano fiber; PET INTRODUCTION In the process of exploitation, transportation, storage and using of petroleum, leakage and emission always cause serious pollution to the environment, especially to water environment.1-3 Oil pollution can spread to the survival environment of people, such as atmosphere; soil; the circulation of nature and human activities. So oil pollution treatment has become an important topic. With the development of oil pollution treatment technology, some methods have been applied. In these methods, oil absorption materials with different oil absorption mechanisms have been widely studied. Oil absorption mechanism can be classified into chemical, physical and biological mechanism. Physical oil absorption is a more environmental friendly method because the oil absorption materials can be reused after removing the adsorbed oil.4-6 Owing to the extremely high specific surface area, nano fiber has been widely used in absorption, filtering and separation. As a continuous and efficient method of preparing nano fibers, electrospun technology has been widely studied and applied. The application of the electrospun nanofibers in oil absorption has received extensive attention due to its excellent characteristics. Wang and coauthors prepared superhydrophobic thermoplastic polyurethanes (TPU) film by electrospun for oil-water separation.7 Wu and coauthors prepared porous polystyrene (PS) nanofibers by electrospun with high oil adsorbing capacity.8 Zhu and coauthors used the polyvinyl chloride (PVC)/polystyrene (PS) fibers to adsorb oil.9 Li et al. used poly(lactic acid) ultrafine fibers for the removal of oil from water.10 However, these electrospun nanofbers have poor mechanical properties, which limits the wide application of these nanofibers. As a common fiber-forming polymer, PET has many good mechanical properties and can be used to electrospin. N. Strain et al. prepared PET nanofiber (PNF) by electrospun which was used for smoke filtration.11 Beatriz Veleipinho et al. explored the effects of solvent and concentration on the properties

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of electrospun PNF.12 However, detailed studies on oil absorption properties of PNF and microfiber are still rarely reported at present. In our researches, we select PET as fiber-forming polymer and successfully prepared PET nano porous luminescence fiber through electrospun. The surfaces of the fibers are covered with nano pores. The optimum spinning parameters, pore-forming mechanism and oil absorption properties of PNPLF were discussed. The mechanical and thermal properties of PNPLF were also characterized. The luminescent material was also introduced to the PET nano porous fiber to explore the oil absorption process. The results prove that the as-prepared PNPLF can maintain good mechanical properties in relatively wide temperature range. The oil absorption value of the as-prepared PNPLF achieves 135 g/g, and the value is more than many results reported by other literatures.4,13,14 The fluorescence intensity of the PNPLF has a significant change with the increase of oil absorption value. Through the change of fluorescence intensity, the oil absorption properties of PNPLF can be quickly evaluated. The luminescence property can be recovered after deoiling to the fiber. These properties lay the foundation for application of PNPLF in oil absorption fields. EXPERIMENTAL SECTION Materials. PET, Hexafluoroisopropanol (HFIP) and dichloromethane (DCM) were purchased from Shanghai Aladdin biological technology co., Ltd.. P-methylbenzoic acid terbium complex (PATC) was synthesized as reported literature.15 Other chemicals were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. and used as received. Preparation of Electrospinning Solution. The spinning solutions used to fabricate PNPLF were prepared as follows: Firstly, 0.2 g PET and 0.002 g PATC were added in mixed solvent (VHFIP : VDCM= 1:3), and the mixed solution was stirred by magnetic stirring equipment for 10 h at room temperature. The concentration of the electrospinning solution was 7% -10% (w/v). Preparation of PNPLF. The as-prepared spinning solutions were placed in 10 mL plastic syringes, which had 22G stainless-steel needles attached to them (inner diameter of 0.41 mm). During electrospinning, the solution flow rate was maintained at 1 mL/h by using an injection pump. The voltage was maintained at 15 kV. The distance between the tip of the needle and the collecting aluminum plate was 20 cm. The spinning time was 1.5 h. The spinning temperature was 25 °C and the humidity was adjusted from 30% to 80 %. The PNPLF was formed on the collecting aluminum plate and roll. The samples were removed from collection devices for the further characterization. Oil Absorption and Deoiling Treatment of PNPLF. The experimental methods and process of oil absorption were as follows: First, the mass of the dried PNPLF sample was weighed. And then, the sample was added to a beaker which was filled with engine oil. After five minutes, the PNPLF sample was taken out from oil and statically placed until the oil of fiber surface completely removed. The as-prepared PNPLF sample, which absorbed oil, was weighed. The oil absorption capacity of PNPLF was calculated through the mass difference of samples before and after absorbing oil. In this work, n-hexane was selected to remove the absorbed oil. When the oil absorption samples were dipped into the n-hexane, the absorbed oil can be removed through stirring continuously. At last, the deoiling PNPLF sample was dried in vacuum oven at 60°C. The different oil absorption and deoiling cycle samples were prepared based on above process. Preparation of PNPLF Samples with Different Oil Absorption Value. The PNPLF sample was dipped into mixed solution with different mass percentage of engine oil and n-hexane for five minutes. These samples were taken out from mixed solution, and the as-prepared samples were heated in

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vacuum oven to remove n-hexane at 60°C. At last, the PNPLF samples with different oil absorption value were prepared. In the mixed solution of engine oil and n-hexane, the mass percentage of engine oil were 0%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 50%, 60%, 70%, 80%, 90% and 100%. Measurements and Characterization of the Samples. The morphologies of samples were observed by a Hitachi S4800 (Hitachi Ltd., Japan) field-emission scanning electron microscope (FE-SEM) and a Hitachi H7650 (Hitachi Ltd., Japan) transmission electron microscope (TEM). The surface area and pore size distributions of PNPLF were analyzed by Brunauer–Emmett–Teller (BET) method (Autosorb 1C system, Quantachrome, USA). The Fourier transform infrared (FTIR) spectra of samples were collected with a Nicollet NEXUS-670 FTIR spectrometer through KBr disks for wavenumbers of 4000–500 cm-1.The water/oil contact angle of PNPLF was measured by a SL200C (KNIO) contact angle apparatus. The mechanical performance of the PNPLF was measured by electronic single fiber strength meter. The nano fiber was prepared into strip sample, and the diameter was 0.15 mm. The thermal property of the samples was determined by a Netzsch STA 449F3 thermogravimetric analysis (TGA) system under oxygen atmosphere. The heating rate was 10 °C/min, and the investigated temperatures zone were 50–800 °C. Steady-state luminescence spectra were collected using a HORIBA FluoroLog-3-UltraFast spectrophotometer. The excitation wavelength range was 200-400 nm, and the emission wavelength range was 400-750 nm; the slit width was 0.2nm. The luminescent quantum yield (Φ) was measured through Edinburgh FLS 920 fluorescence spectrophotometers. The luminescence lifetime was performed using the same equipment with an MCS mode, for which the R928P PMT was used; the light source was a xenon flash lamp, and the duration was 2-3 µs. The fitting software used was DAS6. To determine the fluorescence property of PNPLF under different temperature, the fluorescence intensity of fiber was measured from 20 °C to 100 °C, and the temperature interval was 10 °C. RESULTS AND DISCUSSION Morphologies of PET Nano Luminescence Fiber (PNLF). Humidity is an important parameter to prepare porous fibers through electrospinning.16 Figure 1 shows the morphologies of the as-prepared PNLF via electrospun technology under different relative humidity conditions. The result shows that the surfaces of the fibers are smooth when the relative humidity was among 30%-50%; the diameters of

Figure 1. SEM images of the electrospun fibers under different relative humidity. a: 30%; b:40% c:50% d: 60% e: 70% f: 80%. (The spinning voltage was 15 kV; the deposition distance was 20 cm; the propulsion rate was 1mL/h; the spinning solution concentration was 7 % (w/v); the spinning temperature was 25 °C; the addition amount of PATC was 1 % )

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the fibers lie in 200-500 nm. When the relative humidity reached 60 %, the surface of the fiber began to form pores. However, the coverage rate of the microporous is low. The diameter of the fiber is 200-400 nm, and diameter of the pores is 50-100 nm. The quantity of the pores increased when the relative humidity added up to 70%, and the coverage rate of the pores increased evidently. The diameter of the fiber was 200-480 nm, and diameter of the microporous was 50-150 nm. When the relative humidity was up to 80%, the surface of the fiber was covered with pores. The diameter of the fiber was 180-450 nm. Furthermore, from Figure 2a-d, the fiber sample with same average diameter and pore size can be attained through the adjustment of the electrostatic spinning parameters. The BET data of the as-prepared PNLF under 80% relative humidity were tested to characterize specific surface area, pore sizes and pore size distributions of the samples. From the Figure 2e, it can be found that the specific surface area of as-prepared sample is 157 m2/g. The pore size is mainly distributing in 20-50 nm.

Figure 2. a-c: SEM images of PNPLF in different locations; d: electrostatic spinning nonwovens; e: typical N2 adsorption-desorption isotherms and pore size distribution plots of PNPLF (inset). Figure 3a and b show the TEM images of the as-prepared PNLF under 30% and 80% relative humidity, respectively. The as-prepared PNLF under 30% relative humidity has a smooth surface. The TEM image of the PNLF under 80% relative humidity indicates that the fiber has a rough surface due to the porous structure.17

Figure 3. TEM images of the PNLF under 30% (a) and (b) 80% relative humidity. (The spinning voltage was 15 kV; the deposition distance was 20 cm; the propulsion rate was 1mL/h; the spinning solution concentration was 7 % (w/v); the spinning temperature was 25 °C; the addition amount of PATC was 1 % ) Forming Pore Mechanism of PNPLF. Pore-forming mechanism of PNPLF on the electrospinning process is shown in Figure 4. The PET electrospinning process was conducted in a hermetical 4


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environment, and the external factors of electrospinning including temperature and humidity which could be artificially controlled. PET spinning solution was stretched into nano fibers from liquid drop under the force of electric power. The solvent volatilized rapidly in the process of the fiber being stretched, and the process was conducted in an external environment with 60% to 80% relative humidity. The temperature of the surface of the fiber was rapidly decreased when the solvent volatilized. The water vapor was concentrated into tiny water drops and distributed in the surface of the fiber.18 Owing to the nano scale of fiber, the water drops were also in nano scale. The fibers have not solidified completely when the tiny water drops were forming, so the surface of the fiber was depressed into pores in the region where the water vapor concentrated.19,20 Humidity is directly relevant to the mass percentage of water vapor in the spinning external environment. The larger the mass percentage of water vapor is, the higher the humidity is. The water vapor is easier to form liquid droplets on the surface of the nanofiber when it cool down, and the surface of the nanofiber is easier to form a porous morphology.21

Figure 4. Schematic diagram of forming pore mechanism for PNPLF. The porous morphology was formed when the water and solvent volatilized completely.22,23 Water was selected as the media of pore-forming in this pore-forming mechanism because water is the non-solvent of PET, therefore the nano fiber can solidify and be prepared. Other composite solvent of hexafluoroisopropanol /dichloromethane was adopted in this research. Hexafluoroisopropanol was used as the good solvent in this composite solvent system, and PET can be completely dissolved in hexafluoroisopropanol. This is a necessary condition that PNPLF can be prepared by electrospinning.24 In this composite solvent system, dichloromethane did not play the role to dissolve PET. However, dichloromethane can make the solvent volatilize rapidly and promote the process of phase separation which can promote the formation of micropores on surface of the fibers. Mechanical Properties of PNPLF. Mechanical strength of fiber is an important basic condition that the fiber can be applied.25 The electronic single fiber strength tester is shown in the illustration of Figure 5, and it shows the breaking strength and elongation of the as-prepared yarn of PNPLF under different relative humidity conditions.26,27 The breaking stress of yarn of PNLF is 105 cN, 115 cN, 104 cN, 105 cN,117 cN, 117 cN respectively. And the corresponding elongation is 27 mm, 22 mm, 27 mm, 25 mm, 25.5 mm and 25 mm respectively. The results illustrate that the PNLF has better mechanical properties than that of reported literatures.28

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Figure 5. Strength of yarns of the prepared PNLFs under different relative humidity conditions. Thermal Properties and FT-IR Spectrum of PNPLF. Figure 6a shows the optical microscope photographs of the PNPLF at different temperature. When the temperature is below 250°C, the PNPLF retains the fibrous morphology. The fiber begin to soften when the temperature reaches to 260°C; the fiber melts when the temperature reaches to 270°C; the fiber becomes to PET droplet when the temperature achieves 280°C, which is the melting point of PNPLF.29, 30 The morphology of fiber is an important condition of maintaining the mechanical properties of the fiber. Therefore, the PNPLF can be used to any field which the temperature is less than 250°C. In order to study the thermal stability of PNPLF, TGA tests were performed,31 and the results were shown in Figure 6b. The decomposition temperatures of PNF, PNLF and PNPLF are 406.30 °C, 408.69 °C and 404.66 °C, respectively. The results demonstrate that the addition of PATC does not affect the thermal stability of the PNPLF.

Figure 6. (a) Optical microscope photographs of the PNPLF at different temperatures. (b) TGA curves of PATC, PNF, PNLF and PNPLF. Figure 7a gives the FT-IR spectra of PNF, PATC and PNPLF, respectively. In the FT-IR spectra of PNPLF, the characterization peak of –C=O for net PET is shifted to 1716 cm-1. The result verifies that the Tb3+ ions of PATC have coordinated with ester groups of PET molecules (Figure 7b). 32-34 This kind of interaction makes the structure of the PATC more stable, and the thermal properties are significantly improved, the result has been proved by TGA curves of PNF, PNLF and PNPLF. Meanwhile, it can also be found that Figure 3, the PATC (1%) uniformly distribute in the PNLF, and there was no agglomeration of PATC owing to this interaction.

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Figure 7. (a) FTIR spectra of PNF, PATC and PNPLF. (b) Schematic diagram of coordination of PET and PATC. Oil Absorption Property of PNLF. Figure 8a shows the oil absorption property of the prepared PNLF under different relative humidity. The results showed that the oil absorption values of the prepared PNLF under 30%, 40% and 50% relative humidity are 53g/g, 57g/g and 60g/g, respectively. The oil absorption values of the prepared PNPLF samples under 60%, 70% and 80% relative humidity are 89g/g, 127g/g and 135 g/g, respectively. The porous fiber had a larger specific surface area due to the existence of the pores on the surface of the fiber. The tested results of SEM showed that the quantity of the pores increased with the growing of relative humidity. Therefore, the oil absorption value of as-prepared PNLF increased with the increase of electrospinning relative humidity.35,36 To investigate the actual application properties of the PNPLF, the oil absorption values of different cycle times PNPLF samples were shown in Figure 8b. The results show that PNPLF has maintained the good oil absorption property after 20 times oil absorption and deoiling cycles. The oil absorption value reaches 101.5 g/g.

Figure 8. Oil absorption value of the as-prepared PNLF under different relative humidity (a); oil absorption value of the as-prepared PNLF under 80% relative humidity after different absorption and deoiling cycle times (b). Figure 9a gives the result of the water contact angle of PNPLF. Figure 9b shows the change process of oil contact angle of PNPLF. The water contact angle of PNPLF is 135°. The oil contact angle which the oil drop came into contact with the PNPLF is 92°, and the oil contact angle became to 27° after 3 seconds. PET is a hydrophobic material and the surface roughness of PNPLF is relatively large. Therefore, the PNPLF has a big water contact angle of 135°, and the oil contact angle is 27°. These

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results further verify the as-prepared PNPLF has good oil absorption property.37,38

Figure 9. (a) Water contact angle and (b) Oil contact angles of PNPLF. Fluorescence-indicating Functions to Oil Absorption of PNPLF. The fluorescence spectrum is an important method to evaluate luminescence properties of luminescent materials. In the excitation spectrum of PNPLF (Figure 10a), there is an obvious broad peak from 250 nm to 325 nm. The strongest peak lies at 303 nm. The results reveal that the as-prepared PNPLF can be effectively excited through 250-325 nm ultraviolet light.39,40 The best excitation wavelength is 303nm. Figure 10b shows the fluorescence emission spectra of PNPLF. The four characteristic peaks appear at 493 nm, 546 nm, 589 nm and 623 nm, respectively. These peaks correspond to the energy level transition of 5D4 → 7F6， 5

D4 → 7F5, 5D4 → 7F4 and 5D4 → 7F3 for Tb3+ ions.41,42 The highest peak appears at 546 nm. The results

verify that as-prepared PNPLF can show bright green light.

Figure 10. Fluorescence spectra of PNPLF before and after oil absorption. a: excitation spectrum; b: emission spectrum. Figure 10 presents the fluorescence spectra of PNPLF after adsorbing oil and deoiling. The fluorescence spectra show that the fluorescence intensity of the PNPLF after absorbing oil drops to 424 (a.u.), and the fluorescence intensity significantly reduced. However, the best excitation and emission wavelengths remain unchanged, which are 303 nm and 546 nm, respectively. The reason of the reduction of fluorescence intensity is that the oil absorbed the energy of the excitation light, and then the energy transport to the PNPLF reduced. When the oil is removed, the fluorescence intensity of PNPLF reverts to 1504 (a.u.), which is 76.23% of the initial fluorescence intensity. The result shows that a little oil still remain in the PNPLF. The fluorescence intensity of PNPLF is sensitive to trace amount of oil. To verify the fluorescence-indicating functions of PNPLF with different oil adsorbing values, the PNPLF samples with different oil adsorbing values were prepared.43-45 The fluorescence emission 8


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spectra, fluorescence lifetimes and quantum efficiencies were tested. Figure 11 and 12 give the fluorescence emission spectra and decay curves of the PNPLF samples with different oil adsorbing values. The results were summarized in Table 1. Based on Table 1, it can be found that the fluorescence intensity of the PNPLF sample gradually decreased with the increase of oil absorption value. There is corresponding relation between the fluorescence intensity and oil absorption value of PNPLF sample. These results indicate that fluorescence intensity of PNPLF has a good fluorescence-indicating function to oil absorption value.46,47 As shown in Table 1, the change of fluorescence lifetimes and quantum efficiencies of the PNPLF sample are not obvious until the oil absorption value reaches 35%. The result shows that the fluorescence lifetime and quantum efficiency of the PNPLF sample is not sensitive to low oil absorption value.

Figure 11. The fluorescence emission spectra of the PNPLF samples with different oil adsorbing values.

Figure 12. The decay curves of the PNPLF samples with different oil adsorbing values. Table 1. The fluorescence intensities, fluorescence lifetimes and quantum efficiencies of PNPLF samples with different oil adsorbing values. Mass percentage of oil (%)

Luminescence properties of PNPLF Intensity(a.u.) τ (µs) Φoverall (%) (5D4 → 7F5)

0 5 10 15 20 25

1973 1809 1682 1523 1423 1322

1101 1055 1053 1023 1022 1022 9


28.26 28.14 28.26 28.21 28.31 29.04


30 35 40 50 60 70 80 90 100

1206 1047 891 770 562 555 172 169 143

1020 997 863 802 691 680 307 299 276

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27.08 27.27 27.52 27.32 26.92 26.76 21.95 20.37 20.46

Figure 13 presents the fluorescence intensities after the different cycle times for PNPLF samples. The results prove that when the cycle time reaches 20 times, the fluorescence intensity of PNPLF can still reaches 81.60 % of the initial fluorescence intensity; there is no significant change in the fluorescence lifetime of the PNPLF (Figure 14). These results indicate that the PNPLF has a good practical application value.

Figure 13. The fluorescence intensities after the different cycle times for PNPLF samples.

Figure 14. The fluorescence lifetimes of PNPLF samples after 20 time cycles. Figure 15 shows the change of fluorescence intensities for PNPLF under different temperatures.48-50 From the emission spectra, it can be found that the corresponding fluorescence intensities are 2429, 2400, 2333, 2192, 2020, 1996, 1399, 1089 and 638 (a.u.), when the environment temperatures are 20 °C, 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, 80 °C, 90 °C and 100 °C, respectively. The change of fluorescence intensity is in a small range when the temperature ranges from 20 to 70 °C. The temperature range can adapt to most of oil absorption processes.

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Figure 15. Fluorescence spectra of PNPLF under different temperature. CONCLUSIONS PNPLF has been prepared by electrospun, and the surface of the fiber is covered with pores. The diameter of the fiber is 180-450 nm and the diameter of the pores is 20-180 nm. The oil absorption properties of the PNPLF were investigated. The result shows that the oil absorption value of the fiber reaches 135g/g. The result of mechanical property indicates that the fiber has an excellent mechanical property, which guarantees that the fiber can be applied in many fields. The TGA curves show that the addition of PATC does not affect the thermal stability of the fiber. The fluorescence spectra show that the PNPLF has good fluorescence property, and the fluorescence intensity reaches 1973 (a.u.) when the addition of PATC is only 1%. And the tests of fluorescence properties show that the fluorescence intensity of PNPLF has an obvious reduction after absorbing oil. The fluorescence intensity could recover to 76.23% of the initial intensity after deoiling treatment. The fact proves that there is a little oil remains in the PNPLF after deoiling treatment. These results verify that the fluorescence intensity of PNPLF is sensitive to trace amount of oil and PNPLF has a good fluorescent-indicating function to the oil absorption process. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Xi P). *E-mail :[email protected] (Cheng BW). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors appreciate the financial support provided by the Natural Scientific Foundation of China (51373118) and the Application Fundamental and Advanced Technology Research Proposal Project of Tianjin, China (13JCYBJC17200). REFERENCES (1) Hackett, B.; Comerma, E.; Daniel, P.; Ichikawa, H. Marine Oil Pollution Prediction. Oceanography 2009, 22 (3), 168-175. (2) Atlas, R. M.; Bartha, R. Fate and effects of polluting petroleum in the marine environment. Residue reviews 1973, 49 (0), 49-85. 11



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Morphologies and Properties of PET Nano Porous Luminescence Fiber

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