Infrared Invisibility Cloak Based on Polyurethane–Tin Oxide

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Infrared Invisibility Cloak Based on Polyurethane−Tin Oxide Composite Microtubes Jihun Ahn,†,§ Taekyung Lim,‡,§ Chang Su Yeo,† Taekuk Hong,‡ Sang-Mi Jeong,‡ Sang Yoon Park,*,† and Sanghyun Ju*,‡ †

Advanced Institutes of Convergence Technology, Seoul National University, Suwon-si, Gyeonggi-do 16229, Republic of Korea Department of Physics, Kyonggi University, Suwon, Gyeonggi-do 16227, Republic of Korea

‡

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ABSTRACT: An invisibility cloak based on visible rays with a refractive index similar to that of air can effectively conceal people or objects from human eyes. However, even if an invisibility cloak based on visible rays is used, an infrared (IR) thermography camera can detect the heat (thermal radiation) emitted from different types of objects including living things. Therefore, both visible and IR rays should be shielded using an invisibility cloak produced by an appropriate technology. Herein, we developed a textile cloak that can almost completely conceal people or objects from IR vision. If a person or object is covered with an IR- and thermal-radiation-shielding textile woven with polyurethane (PU)−tin oxide (SnO2) composite microtubes, serving as an IR invisibility cloak, IR and thermal radiation emitted from the person or object can be simultaneously blocked. Furthermore, the IR- and thermal-radiation-shielding characteristics could be improved further by filling the core of the PU−SnO2 composite microtubes with heat-absorbing materials such as water and paraffin oil in place of air. In addition, the external surface of the IR- and thermal-radiation-shielding textile serving as an IR-reflecting cloak can be waterproofed to enable certain IR- and thermal-radiation-shielding functions under various environmental conditions. KEYWORDS: infrared-reflecting cloak, polyurethane−tin oxide, composite microtube, infrared shielding, hydrophobic

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INTRODUCTION Infrared (IR) is a light in the wavelength range of 0.78−1000 μm that is invisible to the human eye, accounting for 45% of the total solar energy. Thermal radiation is thermal energy in the wavelength range of 0.1−100 μm that is generated by the vibrations and collision energy of electrons, atoms, and molecules in matter. All objects with a surface temperature above absolute zero (−273.15 °C) emit an electromagnetic radiation, which is proportional to their intrinsic temperature. In general, people and objects at room temperature emit infrared wavelengths of 8−25 μm, which can be blocked with the IR-reflective/absorbing material. However, since heat is generated in the object along with IR, heat is propagated to the IR-reflective/absorbing material covering the object for IR shielding. As the temperature of the IR-reflective/absorbing material rises, IR is emitted from the IR-reflective/absorbing material. Thus, to develop an invisibility cloak in the IR range, it is also necessary to develop a material capable of simultaneously (i) reducing IR transmission through IR shielding and (ii) reducing heat transfer through thermal radiation shielding. IR- and thermal-radiation-shielding materials are expected to have a large market in the future technological industry because they are applicable in a variety of fields including automobiles, aerospace, energy devices, construction materials, semiconductors, and displays. The © XXXX American Chemical Society

excellent air permeability of IR rays allows the detection and tracking of targets using IR rays of a specific frequency. Thus, the development of IR stealth technology relies on preventing the leakage and distortion of unnecessary or critical information. Particularly, for military operations, the development of broad-band IR-shielding materials is essential for concealing military materials, equipment, and facilities from being observed by night vision and thermal vision cameras. For civilian purposes, it is required to develop efficient IR- and thermal-radiation-shielding materials to suppress the increase in the temperature of buildings, automobiles, and crop cultivation facilities. Methods to suppress IR and thermal radiation include shielding, dispersion, and cooling. IR- and thermal-radiationshielding technologies based on these include (i) an optically thin-film structure for preventing the incidence of IR rays from an external source on the target using a reflector, (ii) a multilayer thin-film structure capable of scattering or reflecting IR rays owing to the multilayered feature and fine patterning, and (iii) a metastructure capable of controlling the permeation, absorption, and reflection of electromagnetic waves by Received: December 26, 2018 Accepted: March 13, 2019

A

DOI: 10.1021/acsami.8b22535 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. Optical and structural properties of seven types of PU−SnO2 composite microtubes. (A) Optical images of PU−SnO2 composite microtubes densely wound around a glass plate. (B) FE-SEM images of a single strand of PU−SnO2 composite microtubes. (C) Cross-sectional FE-SEM images of the PU−SnO2 composite microtubes showing the internal hollow structure. (D) EDS elemental maps showing the distributions of carbon, tin, and oxygen in the PU−SnO2 composite microtubes.

the polymer solution, the nanoparticles are not uniformly dispersed in the polymer solution, resulting in poor interfacial interaction between the nanoparticles and the polymer, thereby decreasing the flexibility and elasticity of the fibers. To solve these problems, a composite fiber with an IRreflective/absorbing material uniformly embedded in the polymer matrix was produced by mixing the IR-reflective/ absorbing material with a hydrophilic polymer solution in a sol−gel form with excellent dispersion. Furthermore, the IRreflective/absorbing material in the sol−gel form was hydrophobically treated to prevent the embedded IR-reflective/ absorbing material from dissolving the polymer matrix in the aqueous solution. The IR-shielding property can be maintained even if the inside is exposed due to external friction. It also maintains excellent flexibility and elasticity properties of the fiber. In this study, we investigated whether IR- and thermalradiation-shielding textile woven with polyurethane (PU)−tin oxide (SnO2) composite microtubes can effectively shield IR and thermal radiations from objects and evaluated the IR- and thermal-radiation-shielding performance of the developed microtube after filling its interior with various heat-absorbing materials. During wet-spinning, the hollow structure generated through the solvent−nonsolvent extraction of a polyurethane (PU)−SnO2 composite solution was used as a container and filled with heat-absorbing materials using a syringe. In addition, the IR- and thermal-radiation-shielding textile woven with PU−SnO2 composite microtubes were rendered waterproof to verify whether they show certain IR- and thermal-radiationshielding performance when they come in contact with water.

artificially controlling the permittivity and permeability of objects.1−6 The multilayer thin-film structure has been used in parts of satellites or spaceships that need IR and thermal radiation shielding, and it provides IR-shielding feature for windows of automobiles and buildings. Currently, spaceships and satellites use multilayer thin-film structures based on polyimide−polyester.7−10 Furthermore, studies are underway to reinforce aerogels with polymers and apply them to spaceships and space bases.11,12 For military purposes, active/passive thermal-radiation-shielding films based on polymeric organic and inorganic composite materials containing inorganic nanoparticles are being developed to prevent IR detection.13−15 In particular, technologies to shield IR and thermal radiation by coating films or glass substrates with organic, inorganic, or organic−inorganic hybrids have been already commercialized for energy saving.16−18 However, these thin-film-based materials have limitations in applying to objects having various curved large-area surfaces or humans taking various actions. Thus, there is an urgent need for the development of broad-band IR-shielding and far-infrared thermal image shielding flexible material technology to keep up with the recent developments in wearable electronic devices based on flexible materials with freeform surfaces. Representative studies that are currently underway in the development of flexible materials for IR and thermal radiation shielding include the formation of various porous structures in the form of fibers to use the insulating property of air.19,20 Furthermore, composite fibers mixed with metal and metal oxide materials capable of reflecting IR to minimize IR absorption and transmission are being investigated.21,22 The representative materials capable of reflecting or absorbing IR are magnesium oxide (MgO), silicon oxide (SiO2), zirconium oxide (ZrO2), antimony−tin oxide (ATO), indium tin oxide (ITO), and Sb2O3−ZnO (Sb−Zn).23,24 However, when the IR-reflective/absorbing materials are coated on the flexible fabric material, the IR-shielding performance may be deteriorated because of the fiber surface coating being peeled off or buried due to the external surface friction. In addition, when the IR-reflective/absorbing nanoparticles are mixed with

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RESULTS AND DISCUSSION The PU−SnO2 composite microtubes serving as the IR- and thermal-radiation-shielding materials were fabricated by wetspinning a PU−SnO2 solution. PU, which is a representative elastomer, can be uniformly mixed with a heterogeneous material through a hydrophilic urethane bond. Moreover, a hollow structure can be fabricated by a simple manufacturing method called wet-spinning, which is widely used in the B

DOI: 10.1021/acsami.8b22535 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 2. Physical, mechanical, and optical properties of PU−SnO2 composite microtubes. (A) TGA thermograms and (B) Raman spectra of the PU microtube and seven types of PU−SnO2 composite microtubes. (C) Representative strength and strain at break values of various PU−SnO2 composite microtubes. (D) Water contact angles of 10 types of composite microtubes (Microtube 1, Microtube 2, Microtube 3, Microtube 4, Microtube 4-1, Microtube 4-2, Microtube 4-3, Microtube 5, Microtube 6, and Microtube 7). The inset shows the optical image of water droplets placed on a textile produced with Microtube 5. IR spectra of the PU microtube and various composite microtubes (Microtube 1, Microtube 2, Microtube 3, Microtube 4, Microtube 5, Microtube 6, and Microtube 7) in the wavenumber range of (E) 1630−1740 cm−1 and (F) 2−14 μm.

nonsolvent water. Figure 1C shows the cross-sectional SEM images of the PU−SnO2 composite microtubes with seven different compositions (Microtubes 1−7). Every PU−SnO2 composite microtube has a hollow structure with a pore size of ∼110 μm. To confirm the distribution of PU and SnO2 in the produced PU−SnO2 composite microtube, energy-dispersive X-ray spectroscopic (EDS) elemental mapping of the cross section of Microtube 5, as a representative sample, was carried out (Figure 1D). The distribution of C, Sn, and O in the EDS maps reveals uniform distribution of PU and SnO2 within the PU−SnO2 composite microtube. Figure 2A shows the thermograms obtained by the thermogravimetric analysis (TGA) of the PU and seven types of PU−SnO2 composite microtubes. The pyrolysis of the material started at ∼200 °C, and weight loss occurred continuously until ∼450 °C. PU polymers are generally composed of both hard and soft segments. When PU−SnO2 composite microtubes are heated to a high temperature, a loss of mass in two stages occurs due to the thermal decomposition of the hard and soft segments of PU.26 At temperatures above 450 °C, a small quantity of SnO2 residues dispersed within the polymer matrix is left during the degradation of the polymer chains. An increased amount of SnO2 uniformly anchored to the PU matrix enhances the thermal stability of the latter and consequently improves the thermodynamic properties of the PU−SnO2 composite microtubes. Thus, at 700 °C, the amount of the residue of the PU−SnO2 composite microtubes is approximately proportional to the weight of SnO2 added during the fabrication of the microtubes. The TGA data up to 700 °C shows that as the content of SnO2 in the composite increases, the ratio of the remaining mass increases from 7.6 for the PU microtube to 13.3, 14.7, 18.4, 22.3, 24.0, 25.2, and 25.6% for Microtube 1, Microtube 2, Microtube 3, Microtube 4, Microtube 5, Microtube 6, and Microtube 7, respectively. Figure 2B shows the Raman spectra of the PU and PU− SnO2 composite microtubes. No peak is detected in the Raman spectrum of the pure PU microtube, whereas four peaks related to SnO2 appeared in the Raman spectra of PU−SnO2

industries. This study fabricated PU−SnO2 composite microtubes, which are homogeneously mixed with a PU matrix, maintaining the hollow structure during wet-spinning, using SnO2 sol−gel, which is a well-known metal oxide material for IR reflection. PU−SnO2 composite microtubes (referred to as Microtube 1, 2, 3, 4, 5, 6, and 7) were fabricated using PU− SnO2 solutions obtained by dissolving different quantities of a tin oxide precursor, tin(II) chloride dihydrate (0.5, 0.7, 0.8, 0.9, 1.0, 1.1, and 1.2 g, respectively), in the same amount of a 12 wt % PU solution. Figure 1A shows the photographs of the PU−SnO2 composite microtubes prepared with seven different ratios of PU/SnO2 densely wound around a glass slide. As shown in the field-emission scanning electron microscopy (FESEM) images in Figure 1B, the diameters of the PU−SnO2 composite microtubes are almost identical at ∼200 μm, regardless of the concentration of the SnO2 precursor. The structure of the polymer microtubes produced by the wetspinning process is determined by phase inversion process parameters such as the viscosity, flow rate, and solubility (solvent/nonsolvent interaction) of the polymer solution and the coagulation rate. During the fabrication of a PU microtube using wet-spinning, when the PU solution extracted through a needle is injected into the nonsolvent water in the coagulation bath, the PU polymer of the PU solution is coagulated to form a microtube. Because the PU polymer has hydrophilic urethane bonding, the solvents (tetrahydrofuran (THF) and dimethylformamide (DMF)) present in the PU solution can be easily exchanged with water, which is a nonsolvent in the bath, before the PU microtube is completely coagulated. The phase separation of the PU polymer occurred by water impregnated inside the PU microtube, forming a hollow structure inside the microtube (Figure S1).25 Since both the PU matrix and the embedded SnO2 are hydrophilic even during the wet-spinning of the PU−SnO2 composite microtube, the exchange of DMF and THF solvents in the composite microtube and the nonsolvent water is relatively easy as in the fabrication of a PU microtube. Subsequently, a PU−SnO2 composite microtube was formed by phase separation of composite polymers in C

DOI: 10.1021/acsami.8b22535 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

composition between those of Microtube 4 and Microtube 5, increased to 127.0, 128.3, and 137.1 nm, respectively. This confirms that the major factor affecting the WCA of PU−SnO2 composite microtubes is their surface roughness. Figure 2E shows the IR spectra of the PU microtube and different PU−SnO2 composite microtubes (Microtube 1−7) determined by Fourier transform infrared (FT-IR) spectroscopy. The PU polymer can form various kinds of hydrogen bonds owing to the urethane moieties with N−H and CO groups in its structure. As shown in Figure 2E, the band at 1691−1716 cm−1 corresponds to the CO stretching of the urethane bonds; the hydrogen-bonded CO stretching peaks (1691 cm−1) appear at a lower wavenumber than the stretching peak (1716 cm−1) of the “free” (non-hydrogenbonded) C  O groups.31 Since the diameter of the PU− SnO2 composite microtubes remained constant regardless of the amount of the SnO2 precursor in the composite microtubes, the area occupied by the SnO2 clusters on the microtube surface increases with increasing SnO2 content and the intensity of the CO stretching peak of the PU polymer gradually decreases. Figure 2F shows the IR transmissions of the PU microtube and different PU−SnO2 composite microtubes (Microtube 1 to Microtube 7) in the attenuated total reflection (ATR) mode. Note that the peak at 4.2 μm corresponds to CO2. In the ATR mode, the variation in the FT-IR peak intensity occurring in the internally reflected beam is determined by irradiating the sample with an infrared beam. The IR reflection effect by SnO2 and the air trap effect by the hollow structure were simultaneously determined in the ATR mode while maintaining the shape of the hollow microtubes without squeezing or damaging the sample. The results confirm that the PU−SnO2 composite microtubes can decrease the external IR radiation as well as have thermal insulating properties. The reflectance of a material is expressed by the Fresnel equation (eq 1), and the reflectance of the material increases as the refractive index n value increases.

composite microtubes. SnO2 has a crystalline tetragonal rutile structure and shows three typical Raman-active modes: ∼475 cm−1 (Eg), ∼630 cm−1 (A1g), and ∼775 cm−1 (B2g). The mode at 475 cm−1 (Eg) is the translational mode of the oxide, that at 630 cm −1 (A 1g) corresponds to symmetric O−Sn−O stretching, and that at 755 cm−1 (B2g) corresponds to asymmetric O−Sn−O stretching.27 Furthermore, the peak at ∼355 cm−1 indicates some defects in the surface sites, such as oxygen vacancies and lattice disorders, that are generated during the hydrolysis of the SnCl2 solution.28 Figure 2C shows the mechanical properties, the strength and strain, of the produced microtubes that should always be verified during textile production. Through tensile testing, the tensile strength (maximum load) and the strain (elongation) at which the PU− SnO2 composite microtube breaks were determined. As the SnO2 content in the microtube increased, the strength and strain values of the PU−SnO2 composite microtube decreased gradually (strengths of 4.13, 3.00, 2.89, 2.93, 2.54, 2.21, and 1.85 mN tex−1, respectively, and strains of 466.19, 356.63, 351.39, 350.57, 285.67, 211.05, and 157.92%, respectively, were determined for Microtube 1, Microtube 2, Microtube 3, Microtube 4, Microtube 5, Microtube 6, and Microtube 7). This is because as the amount of SnO2 cluster increases, it interferes with the interaction of the PU polymer chains and the orientation of the PU polymer in the matrix, thus weakening the tensile strength and strain of the PU−SnO2 composite microtube.29 To render the PU−SnO2 composite microtubes hydrophobic, a self-assembled monolayer (SAM) of (1H,1H,2H,2Hheptadecafluorodec-1-yl)phosphonic acid (HDF-PA), which contains a phosphoric acid anchor group, was formed on them. HDF-PA molecules bind strongly to the microtubes through interfacial bonding based on a condensation reaction between their phosphonate groups and SnO2 on the microtube surface. X-ray photoelectron spectroscopic (XPS) analysis revealed the formation of the HDF-PA SAM with the manifestation of a fluorine peak (Figure S2). Figure 2D shows the water contact angles (WCAs) of the seven types of microtubes. The WCA showed a gradual tendency to increase starting from Microtube 1 (112.3 ± 0.8°) to Microtube 7 (142.9 ± 1.0°). In particular, the WCA increased steeply from 126.2 ± 0.5° for Microtube 4 to 140.5 ± 0.5° for Microtube 5. Therefore, additional microtubes were produced with 0.925, 0.950, and 0.975 g of the SnO2 precursor (referred to as Microtube 4-1, Microtube 4-2, and Microtube 4-3, respectively), which showed WCAs of 130.2 ± 0.4°, 132.8 ± 0.5°, and 135.5 ± 0.4°, respectively. A typical strategy for forming a hydrophobic surface is to lower the surface energy while simultaneously increasing the surface roughness. Fluorine is known as a representative element that decreases the surface free energy by forming stable covalent bonds with carbon.30 Thus, upon increasing the content of the SnO2 precursors during the production of the PU−SnO2 composite microtubes, the quantity of SnO2 in the composite increased, which in turn increased the bonding of HDF-PA, and as a result, the WCA showed an increasing trend. Additionally, the atomic force microscopy (AFM) images of each PU−SnO2 composite microtube revealed that the surface roughness gradually increases from 82.7 for Microtube 1 to 97.8, 107.3, 110.9, 159.4, 180.8, and 184.6 nm for Microtube 2, Microtube 3, Microtube 4, Microtube 5, Microtube 6, and Microtube 7, respectively (Figure S3). Furthermore, similar to the WCA results, the roughnesses of Microtube 4-1, Microtube 4-2, and Microtube 4-3, which have an intermediate

R=

(n − 1)2 i 2 yz zz = jjj1 − 2 n + 1{ (n + 1) k

2

(1)

SnO2, an n-type semiconductor material with a wide band gap of 3.6−3.8 eV, has a refractive index of ∼1.90 and is a relatively high-reflectance material.32 Therefore, the refractive index of the PU−SnO2 composite microtube is highly dependent on the content of SnO2. As the refractive index increases, the reflectance increases. As a result, it can be seen that the IR reflectance increases as the content of SnO2 increases. In addition, SnO2 is known to transmit radiation in the visible region, whereas it absorbs in the UV region and reflects radiations above the IR region.33,34 In general, a conductor such as metals can be regarded as a kind of solid-state plasma in which the nucleus constituting most of the atomic mass is located at the center and the free electrons move around it. Since the distribution of the density of free electrons is nonuniform, an electric field is generated. The generated electric field causes a kind of vibration phenomenon owing to the inertia generated by the electron motion, which is referred to as the plasma vibration. The IR absorption band of plasma vibration is proportional to the electron plasma frequency, ωp, according to eq 2. D

DOI: 10.1021/acsami.8b22535 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 3. Verification of the IR- and thermal-radiation-shielding properties of a textile produced with PU−SnO2 composite microtubes. (A) Schematic illustrating a textile with the IR- and thermal-radiation-shielding and water-repellent properties. The central panel shows an optical image of the IR- and thermal-radiation-shielding textile produced with PU−SnO2 composite microtubes (Microtube 5). The right panel shows the crosssectional FE-SEM image of Microtube 5 with a hollow structure. The optical image and the IR thermal image of a tank model, respectively, (B) and (E) before applying the IR-absorption textile, (C) and (F) after applying a commonly used textile (conventional polyester), and (D) and (G) after applying the IR-absorption textile (Microtube 5) fabricated in this study.

ωsp2 =

ωp2 1 + εm

the thermal IR reflectivity of SnO2, which lowers the IR transmission. Figure 3A shows a schematic of the IR- and thermalradiation-shielding textile woven with PU−SnO2 composite microtubes that are water repellent and do not transmit IR rays and heat. Based on the prior analysis of the mechanical and wettability properties, textiles were produced with Microtube 5, which showed a high WCA of ∼140° or greater and high strength and strain at break values determined through tensile testing. The center and right panels in Figure 3A show a photograph of the produced textile and the cross-sectional SEM image of Microtube 5 used in the textile, respectively. The PU−SnO2 composite microtube has both IR-shielding and thermal insulating properties owing to SnO2 clusters in its structure and the hollow structure of the composite microtube. To confirm this process, the IR thermography images of tanks heated with a uniform temperature were obtained under the three conditions: a tank without a textile cover, a tank covered with a nonporous polyester textile, and a tank covered with a PU−SnO2 composite microtube (Fiber 5)-based textile. In this case, IR camera images were obtained in a setting where a tank model was heated for 30 min or more to achieve thermal equilibrium under constant heating condition applying a constant current to the tank model at room temperature (23 °C). Figure 3B−G demonstrates the IR-shielding effect of the textile when it is applied as the IR- and thermal-radiationshielding material to an object that actually emits heat. In this experiment, a tank model composed of metals was produced

(2)

where ωsp is the surface plasma absorption frequency and εm is the dielectric constant of the surrounding medium. The electron plasma frequency (ωp) is proportional to the square root of the free electron density (N) (eq 3). ωp =

Ne 2 meε∞ε0

(3)

where e is the electron charge, me is the effective mass of an electron, ε0 is the vacuum permittivity, and ε∞ is the optical dielectric constant.35 According to the equation above, a radiation with a frequency smaller than the electron plasma frequency (ωp) cannot pass through the material and is totally reflected. The electron plasma frequency (ωp) of ordinary metals lies in the range of purple wavelength (380−450 nm). In the case of a metal oxide such as SnO2 having a smaller free electron density (N) than metals, the electron plasma frequency (ωp) and the wavelength are smaller and longer, respectively, than those of the metals. Therefore, IR rays more than ∼2.5 μm cannot be transmitted and are reflected by the plasma vibration of SnO2 that shows metallic optical properties.24,36−38 Thus, as the content of SnO2 in the composite microtube increases, the exposed surface of the SnO2 cluster increases and the IR rays are reflected owing to E

DOI: 10.1021/acsami.8b22535 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 4. Verification of the IR- and thermal-radiation-shielding properties of textiles woven with hydrophobic PU−SnO2 composite microtubes filled with air, paraffin oil, or water in the hollow structure. (A) Filling the PU−SnO2 composite microtube with paraffin oil or water using a syringe. (B) Composite microtubes filled with paraffin oil or water. (C) Optical image of the IR- and thermal-radiation-shielding textile produced with hydrophobic PU−SnO2 composite microtubes filled with air, paraffin oil, or water and the cross-sectional image of the microtube. (D) Optical images and (E) IR thermal images of the IR- and thermal-radiation-shielding textiles based on the conventional polyester, PU textile, and PU− SnO2 composite microtubes filled with air, paraffin oil, or water placed on a hot plate at 40 °C. (F) Temperature changes of the IR- and thermalradiation-shielding textiles based on the conventional polyester, PU textile, and PU−SnO2 composite microtubes filled with air, paraffin oil, or water placed on a hot plate at 40 °C over time.

Figure 4 compares the IR- and thermal-radiation-shielding characteristics of the PU−SnO2 composite microtube before and after filling its hollow core with paraffin oil or water. To fill the hollow structure with these liquids, a PU−SnO2 composite microtube with a diameter of ∼600 μm was produced using an 18 gauge-syringe needle. Then, the hollow core of the produced PU−SnO2 composite microtube was filled with paraffin oil or water. Water is known to absorb IR. Paraffin oil has a low melting point of ∼40 °C and absorbs thermal energy depending on the phase change when it is externally heated. Note that a blue oil-soluble dye (Oil Blue N, dye content 96%, Sigma-Aldrich) and an orange water-soluble dye (fluorescein disodium salt, Samchun Pure Chemicals Co.) were used to distinguish the respective solutions filled into the hollow structure. As the solution entered the hollow structure, the white PU−SnO2 composite microtube was dyed according to the liquid injected (Figure 4A,B). Figure 4C shows the cross sections of the IR- and thermal-radiation-shielding textile and the PU−SnO2 composite microtubes filled with air, paraffin oil, or water within the hollow core. It can be observed that the microtube developed the dye color as the solution filled the hollow structure. Furthermore, all of the three textiles maintained their hydrophobic properties (Figure S7). Figure 4D,E confirms the IR- and thermal-radiation-shielding characteristics of each textile comprising microtubes filled with air, paraffin oil, or water. It is worth noting that the dye was not applied for each textile comprising microtubes filled with air, paraffin oil, or water. With the hot-plate temperature set to 40 °C, the three types of textiles were placed on the hot plate and observed with an IR thermography camera at room temperature (23 °C). The temperature of the hot plate observed using IR thermography camera was 36 °C. The common textile produced with a conventional polyester and the PU textile showed low IR- and thermal-radiation-shielding effects, and their temperatures were observed to be 35 °C

and heat was generated by passing electricity through it. The IR thermal images of the tank model (Figure 3B,E), which can be viewed with naked eyes, indicate emission of different thermal IR rays from different parts of the tank. The highest heat was generated from the engine and wheels of the tank. The temperatures of the engine and wheels of the tank model were measured to be approximately 33 and 40 °C, respectively. Figure 3C shows a photograph of the heat-emitting tank model covered with a common textile made of polyester, and Figure 3F shows an IR emission photograph taken with an IR thermography camera. Figure 3F clearly shows thermal IR radiations from the top part directly contacting the tank model and the engine and wheels that generate different thermal IR rays. In this configuration, the temperatures of the engine and wheels were measured to be approximately 28 and 31 °C, respectively. In contrast, the thermal IR radiation of the tank model covered with the IR- and thermal-radiation-shielding textile is not well observed by thermal imaging and the model appears blue similar to the surrounding background. Note that, as shown in Figure 3D,G, the temperature of every part was found to be approximately 26 °C. Furthermore, as shown in Figure S5, in the case of a tank covered with the IR- and thermal-radiation-shielding textile, even when the tank was heated for up to 120 min, the textile remained at the initial temperature of 26 °C. This experiment demonstrates that the PU−SnO2 composite microtube has the capability to suppress the transmission of IR as well as has thermal insulating properties. The air trapped in the hollow core of the PU−SnO2 composite microtubes lowers the heat transfer and the IR rays are scattered and reflected at the interfaces with different refractive indices, thus leading to a reduction in their transmittance. Moreover, the IR transmission is also reduced due to the reflection of the IR rays by the plasma vibrations of the SnO2 clusters on the surface of the PU−SnO2 composite microtube. F

DOI: 10.1021/acsami.8b22535 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces (polyester) and 33 °C (PU), respectively, after 60 min. In contrast, the temperature of the textile produced with the PU− SnO2 composite microtube was measured to be 30 °C when it was filled with air, 28 °C when filled with paraffin oil, and 27 °C when filled with water. Figure 4F shows the observation results of five textiles with 2 mm spacing on a hot plate at 40 °C for up to 120 min. The temperatures of the conventional polyester and PU textile increased to 35 and 33 °C, respectively, within 10 min, and the elevated temperatures were maintained because almost all of the heat energy applied from the hot plate was delivered. On the other hand, the temperatures of PU−SnO2 composite microtubes filled with air, paraffin oil, and water increased to 28 °C (air), 26 °C (paraffin oil), and 25 °C (water) after 30 min. After saturation at 60 min, the temperature was slightly increased to 31 °C (air), 28 °C (paraffin oil), and 27 °C (water) after 120 min. Although a slight temperature rise in PU−SnO2 composite microtubes filled with air, paraffin oil, and water was observed over time, the temperature increase ranged from 2 to 3 °C, which was much lower than that of the conventional polyester and PU textile. When the IR transmission was measured via FT-IR spectroscopy, the IR transmission of the composite microtubes filled with air, paraffin oil, or water decreased in the same order as that of the conventional polyester used as the control (Figure S9). A dye was mixed with water and paraffin oil to ensure the filling of the microtube with the liquid. To eliminate the effect of the IR peak resulting from the dye during the FTIR measurements in the ATR mode, microtubes filled with pure water or paraffin oil were used. In the spectra (Figure S9), the broad peak at 2.9 μm is caused by water and the peak at 3.4 μm is due to paraffin oil. Note that the peak at 4.2 μm is related to CO2. In general, IR radiation has a strong thermal effect because the wavelength of the IR rays is almost the same as the intrinsic wavelength of the molecule that constitutes the material; thus, as the material is exposed to IR, the electromagnetic resonance phenomenon occurs and the material smoothly absorbs the IR wavelength.39 The higher density and specific heat of water (998 kg m−3 and 4182 J kg−1 K−1, respectively) and paraffin oil (880 kg m−3 and 2180 J kg−1 K−1, respectively) present within the microtubes compared to those of air (1.205 kg m−3 and 1005 J kg−1 K−1, respectively) increase the IR absorption of the composite microtubes, thereby decreasing the IR transmittance. Specifically, water with the highest density and specific heat has the highest IRabsorption capacity and the composite microtubes filled with water showed a higher IR-shielding effect.

with paraffin oil or water, better thermal-radiation-shielding characteristics were observed than when it was filled with air. Furthermore, imparting hydrophobicity to the IR- and thermal-radiation-shielding textile woven with PU−SnO2 composite microtubes through HDF-PA processing prevented the distortion of the IR- and thermal-radiation-shielding characteristics owing to the swelling of the microtube due to wetting by rain or water. The IR- and thermal-radiationshielding textile developed as an IR-reflecting cloak composite microtube is expected to be applicable in the wearable IR stealth technology in the future toward concealing people and objects from IR thermography cameras.

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EXPERIMENTAL SECTION

Fabrication of PU−SnO2 Composite Microtubes. A PU solution (12 wt %) was prepared by dissolving PU powder (0.58 g) in dimethylformamide (2.62 g; Sigma-Aldrich) and tetrahydrofuran (1.64 g; Sigma-Aldrich) under stirring for ∼6 h. Thereafter, the SnO2 precursor, tin(II) chloride dihydrate (SnCl2·2H2O, Sigma-Aldrich), was added to the PU solution (4.84 g) in a beaker and stirred for 12 h. During this time, SnCl2 hydrolyzed and condensed to form a SnO2 sol−gel mixture. Using this method, we prepared several PU−SnO2 solutions with different concentrations of the SnO2 precursor [0.5 (9.4), 0.6 (11.0), 0.7 (12.6), 0.8 (14.2), 0.9 (15.7), 1.0 (17.1), 1.1 (18.5), and 1.2 (19.9) g (wt %)]. A disposal syringe containing PU− SnO2 solutions (10 mL) was attached to the syringe pump. Then, the PU−SnO2 composite microtubes were drawn out using a co-flow wetspinning machine (Invisible co.) and injected into a coagulation bath, which was rotating at 10 rpm, containing deionized water at the rate of 70 mL min−1 through a stainless steel 18 gauge-syringe needle and then allowed to stand for 20 min. The extracted PU−SnO2 composite microtubes were dried at room temperature for 12 h. The PU−SnO2 composite microtubes were then rendered hydrophobic by reacting with a 5 mM solution of HDF-PA (Apollo Scientific) in isopropyl alcohol (JT Baker) for 30 min. The PU−SnO2 composite microtubes with HDP-PA were dried at room temperature for 6 h. Analysis of Optical, Electrical, and Mechanical Properties of the PU−SnO2 Composite Microtubes. The surface morphology and the diameter of the PU−SnO2 composite microtube were observed using an FE-SEM microscope (FE-SEM, S-4800, Hitachi), and chemical mapping was carried out using an energy-dispersive Xray spectrometer (EDS, 7593-H, Horiba). The thermal stability of the PU−SnO2 composite microtubes was determined via TGA (Mettler Toledo, TGA/DSC 1) in the range of 100−700 °C at the rate of 10 °C min−1 under a nitrogen flow. Raman spectra of the PU−SnO2 composite microtube were recorded using a Raman spectrometer (Acron, UniNanotech). Tensile strength evaluation and mechanical elongation of ∼50 mm long PU−SnO2 composite microtubes were carried out using a thermal mechanical analyzer (TMA, Hitachi, TMA7100) at room temperature. To identify the self-assembled monolayer after HDF-PA treatment on the PU−SnO2 composite microtubes, elemental composition was observed via XPS (K-Alpha plus, Thermo Scientific). The wettability of the PU−SnO2 composite microtube was evaluated using a contact angle analyzer (Phoenix 300, SEO Co.). Surface roughness was characterized via AFM (XE150, PSIA). IR transmission was measured using an FT-IR spectrometer (Tensor 27 Spectrometer, Bruker), and IR images were captured using an IR thermography camera (T-420, FLIR) capable of measuring the wavelength of 7.5−13 μm. In the experiment for identifying the IR- and thermal-radiation-shielding characteristics, the images on the textiles with 2 mm spacing on a hot plate at 40 °C were obtained after 60 min. In the experiment for identifying the IRshielding and thermal insulating properties using a metal tank model, a constant current was applied to the tank model at room temperature (23 °C) and the textile was covered under a constant heating condition. After 30 min, the IR thermography images were obtained under the thermal equilibrium condition.

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CONCLUSIONS This study demonstrated an IR- and thermal-radiationshielding textile as an IR-reflecting cloak that can conceal people or objects so that they are not detected by an IR thermography camera. In general, people and objects emit thermal radiation. Consequently, to shield IR rays and heat generated by them, they must be completely shielded. Furthermore, textile-type, flexible IR-shielding materials are required for wearable devices. The IR- and thermal-radiationshielding textile woven with PU−SnO2 composite microtubes proposed in this study showed flexibility owing to their textile form and had excellent thermal-radiation-shielding capability owing to the pores inside the microtube filled with air and the SnO2 in the microtube imparted IR-shielding properties. In particular, when the hollow core of the microtube was filled G

DOI: 10.1021/acsami.8b22535 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b22535.

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FE-SEM image of a PU microtube; XPS, AFM, ATRmode FT-IR images, IR- and thermal-radiation-shielding properties, and wettability of the PU−SnO2 composite microtube; simulation and IR transmission characteristics of the PU−SnO2 microtubes filled with air, paraffin oil, or water (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.Y.P.). *E-mail: [email protected] (S.J.). ORCID

Sanghyun Ju: 0000-0002-3620-1600 Author Contributions §

J.A. and T.L. contributed equally to this work.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (MSIP) (2017M3C1A9069593, 2017M3A7B4025166, 2017R1D1A1B04030415, 2018M3A7B4070987, and 2019R1A2C2010614).

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DOI: 10.1021/acsami.8b22535 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX