Facile synthesis of P(EDOT/Ani) : PSS with enhanced heat shielding efficiency via two-stage shot growth

Poly(3,4-ethylenedioxythiophene/aniline) : poly(styrene sulfonate), P(EDOT/Ani) : PSS, with enhanced absorption of near infrared light, was prepared by oxidative polymerization. We demonstrated that a two-stage shot growth process optimizes the absorption of the polymer in the near infrared region via a controlled monomer addition time. In other words, the optical properties of the polymer complex were improved by controlling the time intervals of aniline monomer addition. P(EDOT/Ani) : PSS was characterized by Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The shielding efficiency of the P(EDOT/Ani) : PSS films was calculated by using data from ultraviolet, visible, and near infrared (UV-vis-NIR) spectroscopy. The introduction of polyaniline to PEDOT increased the absorption in the near infrared area. In comparison with PEDOT : PSS film, the total shielding efficiency of the P(EDOT/Ani) : PSS film increased to 65.8% from 54.6% at 60% transmittance. The maximum NIR shielding efficiency (SENIR) of the film is 92.7% and the transmittance is 46.5%. Also, large-scale P(EDOT/Ani) : PSS film was fabricated using roll-to-roll slot-die equipment and a heat shielding test of the film was conducted by measuring the temperature variation, in order to prove the enhanced heat shielding effect. P(EDOT/Ani) : PSS prepared by a two-stage shot growth system showed excellent potential as a heat shielding material.


Introduction
Conjugated polymers have attracted intense interest for applications in photonics, sensing, light emitting diodes and energy harvesting. [1][2][3][4][5] In particular, poly (3,4-ethylenedioxythiophene) (PEDOT) has been actively studied due to its excellent electrical and optical properties and its environmental stability. PEDOT has very poor solubility in water, but when polystyrene sulfonate (PSS), a linear ionic polymer, is introduced, its solubility is improved. PSS also acts as a dopant and enhances the electrical conductivity of PEDOT. Therefore PEDOT : PSS has been used in various applications such as a hole transport layer for solar cells, thermoelectric devices, and transparent electrodes. [6][7][8][9] In addition, PEDOT : PSS can be used as a heat shielding lm material because of its high optical absorption in the near infrared (NIR) region. A low band gap for the conjugated polymers contributes to excellent absorption at longer wavelengths of light. 10 Recently, energy conservation has been an important issue and research has been conducted in many related elds. Ordinary glass typically has high transmittance and low reectance in the visible and near infrared regions. To prevent energy losses inside buildings and vehicles, it is necessary to block sunlight in the summer and to prevent loss of indoor heat in the winter. Therefore, many studies have been carried out to develop smart window coating materials with high absorption in the infrared region. Inorganic heat shielding materials such as interstitially doped tungsten oxide (M x WO 3 , M ¼ Li, Na, K, Rb, or Cs) or indium tin oxide (ITO) are the main types of materials that have been investigated. 11 Wang et al. fabricated NIR shielding lms using F-TiO 2 and K x WO 3 . The K x WO 3 was in the form of nanorods as the NIR shielding materials were synthesized with Na 2 WO 4 . 12 Zhang et al. prepared transparent heat insulation coatings using antimony doped tin oxide (ATO) dispersions. 13 These inorganic coating materials are promising alternatives for low-emissivity (low-E) glass for energy efficient windows. But they have the disadvantages of having a high cost and a difficult coating process due to the requirement for a high-temperature treatment process and many additives for coating stability. A heat shielding lm coated with organic materials is more cost-effective and simpler to manufacture. Chen et al. synthesized polypyrrole (PPy) nanoparticles and polyacrylic acid (PAA) resin for UV/NIR shielding lm. 14 In this study, we investigated the enhanced heat shielding effect of PEDOT and polyaniline (PAni) composites, synthesized by oxidative polymerization. 15 By introducing PAni to PEDOT : PSS, absorption in the NIR region signicantly improved with a minimal loss in transmittance. In addition, the P(EDOT/ Ani) : PSS lm have an excellent coating ability without the use of additives such as leveling agents and binders, and the lm is stable under high temperature (85 C) or high temperature/ humidity (85 C/85% R.H.) conditions. Finally, we fabricated a large area P(EDOT/Ani) : PSS lm (500 mm in width Â 150 m in length) with improved heat shielding efficiency using roll-toroll slot-die equipment.

Characterization
The FT-IR spectrum of P(EDOT/Ani) : PSS solution was measured by a FT-IR spectrometer in ATR mode (Bruker, model Vertex 70). The UV-vis-NIR absorbance of P(EDOT/Ani) : PSS lms was measured by a spectrophotometer (JASCO Corporation, model V-770). X-ray photoelectron spectroscopy was performed using an X-ray photoelectron spectrometer (K-alpha, Thermo VG) equipped with a 180 spherical sector analyzer and monochromated Al X-ray source (Al Ka line: 1486.6 eV). Infrared bulbs (250 W, PHILIPS) and thermometers were used to obtain a temperature variation curve for the heat shielding lms.

Results and discussion
Synthesis of P(EDOT/Ani) : PSS via two-stage shot growth P(EDOT/Ani) : PSS was synthesized by oxidative polymerization of EDOT and aniline using sodium persulfate (NaS 2 O 8 ) in an aqueous medium. In this work, we introduce a two-stage shot growth process, which is a two-step monomer addition system, for the preparation of a conjugated polymer with enhanced absorbance in the near infrared region. First, the EDOT monomer was polymerized, then aniline was added as a second monomer at certain time intervals. Aer initiating the EDOT polymer, the aniline addition time was varied in order to achieve a polymer with improved NIR absorbance. Fig. 1 shows the synthesis process for P(EDOT/Ani) : PSS along with the absorbance spectra in the 200-2500 nm range for PEDOT : PSS and PAni : PSS. Typically, PSS displays a strong absorption peak at 230 nm due to p-p* transition of the benzene rings in the PSS system. 16 The absorption spectrum of PEDOT shows a broad band in the visible and near infrared regions. The characteristic absorbance is associated with polaron or bipolaron states of PEDOT. 17 In addition, the reectance of PEDOT gradually increases to 30% in the near-infrared region (1100-2500 nm), as shown in Fig. 1(b). However, a relatively weak absorption of PEDOT at 700-1000 nm wavelength is to be improved for efficient heat shielding effect. A hybridization with PEDOT and PAni is appropriate for increasing absorption in the NIR area. Because, PAni shows strong absorption in the visible region and 780-900 nm wavelength although has a weak absorption at longer wavelengths (see Fig. 1(c) and S1 †). 18 To minimized the absorption offset at wavelengths longer than 1000 nm of PEDOT by PAni, we also optimised polymerization system of P(EDOT/ Ani) : PSS using the two-stage shot growth. In terms of the reaction kinetics, the rate constant for the chain propagation of PAni is greater than that for PEDOT. The rate constants of PAni are rst order, and the formation of the polymer results in expedited oxidative polymerization. 19 The reaction rate of PEDOT is sensitive to the reactant concentration because the rate constant is third order. In other words, the oxidative reaction rate of EDOT is very slow. 20 Hence, addition of aniline as the second monomer can inhibit the chain propagation of PEDOT. As shown in Fig. S2, † FT-IR spectra of P(EDOT/Ani) : PSS with several time intervals of aniline monomer addition reveal a difference in chain growth of PEDOT and PAni. There is no characteristic absorbance peak of polymer in the 1300-4000 cm À1 region because the water baseline peak is removed (see Fig. S2 in the ESI †). In contrast, the bands at 602-1030 cm À1 , assigned to C-O-C deformation and asymmetric and symmetric C-S-C deformation of PEDOT, show that the absorbance increases with increasing interval times of aniline addition. 21,22 Also, interference with PEDOT growth is clearly conrmed by the optical properties of P(EDOT/Ani) : PSS lms. We obtained transmittance spectra of P(EDOT/Ani) : PSS lms for different addition times of aniline monomer by using UVvis-NIR spectroscope. We observed that the transmittance of the lms at 1000-2700 nm, a characteristic band for PEDOT, decreased with increasing growth of PEDOT chains. As the addition time of the aniline monomer was delayed, the polymerization of PEDOT proceeded well (see Fig. 2(a)). When the polymerization of aniline was initiated and then EDOT was added to the solution as the second monomer, the growth of PEDOT chains was interrupted (see Fig. 2(b)). Evidence of this interruption was also observed by X-ray photoelectron spectroscopic analysis. A quantitative analysis of the XPS data indirectly demonstrates a difference in the rate of polymerization of EDOT and aniline in heterogeneous systems. The XPS S2p (sulfur) data of P(EDOT/Ani) : PSS and P(Ani/EDOT) : PSS with different addition times for the second monomer are shown in Fig. 3  In general, conducting polymers absorb visible light well because they have a low band gap. The absorption spectrum of PEDOT is related to the PEDOT oxidation state (polaron or bipolaron state). PEDOT is capable of absorbing light with longer wavelengths than those of visible light. Moreover, the introduction of polyaniline into PEDOT results in an excellent shielding effect in the NIR region. The heat shielding efficiency (SE heat ) of the P(EDOT/Ani) : PSS lms is dened by the following equation: The transmitted efficiency (TE) is obtained by the following equation: The transmitted intensity (I T ) corresponds to the sum of spectral irradiance for each wavelength: Fig. 4(a) shows the solar spectral irradiance and the transmitted solar energy spectrum of the P(EDOT/Ani) : PSS lm. The solar power density was calculated by integrating the spectral irradiance at each wavelength, and was plotted for each region. The ultraviolet (UV) region corresponds to a wavelength from 280 to 400 nm, visible light is 400-780 nm, and near infrared light is 780-2700 nm. As shown in Fig. 4(a), we can predict a reduction of solar energy by P(EDOT/Ani) : PSS lm. Also, the shielding effect of NIR light is excellent, but the transmittance in the visible light region is low. These properties result in an enhanced heat shielding effect. The heat shielding efficiency of the 60% transmittance P(EDOT/Ani) : PSS lm is 65.8%, which is more than 11.2% higher than the 60% transmittance pristine PEDOT : PSS lm. The heat shielding effect of P(EDOT/ Ani) : PSS lm is dependent on lm thickness. Fig. 4(b) shows the transmittance spectra of lms at various thicknesses. The transmittance at 550 nm decreased from 77.7% to 46.5% as the thickness increased, but the heat shielding effect increased at the same time. The shielding efficiency in the UV, NIR regions  This journal is © The Royal Society of Chemistry 2018 was signicantly improved. This result is equivalent to an increase in absorbance as the concentration of the lm increases. The calculated shielding efficiency for the P(EDOT/ Ani) : PSS lms with different thickness is summarized in Table 1. For the 47% transmittance lm, SE UV , SE NIR , and SE total were calculated to be 86.9%, 92.7%, and 79.0%, respectively.

Temperature prole and stability of P(EDOT/Ani) : PSS lm
To conrm the degree of heat shielding by P(EDOT/Ani) : PSS lm, a simple test was conducted using black boxes and infrared lamps (details in the Experimental section). A box measuring 300 mm Â 210 mm Â 150 mm made of black acrylate was prepared. A thermometer measuring the temperature variation was placed in the black box and the P(EDOT/Ani) : PSS lm was attached to a glass window on the open side of the box. The distance between the black box and the infrared lamp was 500 mm. Fig. 5(b) shows the equipment for measurement of the temperature variation with either glass alone or with P(EDOT/Ani) : PSS lm attached. Aer turning on the IR lamps for 1800 s, the temperature with glass alone increased from 22 C to 51 C (DT ¼ 29 C), but with the 60% transmittance P(EDOT/Ani) : PSS lm, it only increased to 30 C (DT ¼ 10 C). The temperature variation of P(EDOT/ Ani) : PSS lm was also smaller than that of pristine PEDOT : PSS and PAni : PSS lms, whose temperature variation (DT) are 13 C, 12 C, respectively. (see Fig. 5(a)). These results effectively demonstrate that the P(EDOT/Ani) : PSS lm is applicable for energy saving eld. We also measured the stability of the lms under harsh conditions to test the lm durability. The UV light resistance of the lm was measured with 254 nm wavelength light for 250 h. As shown in Fig. 6, the rst column represents variation of the optical properties at 280-2700 nm wavelength (see UV-vis-NIR spectra of Fig. S3 in the ESI †). Aer UV irradiation for 250 h, the change in the heat shielding efficiency was 2.0% and the reduction of transmittance (at 550 nm) was 4.1%. These slight changes were caused by degradation of the polymer in UV light. The other columns show the stability test results under high    temperature (85 C) and high temperature/humidity (85 C/85%) conditions. Aer 250 h, the variations in heat shielding efficiency were 2.3% and 2.6%, and the transmittance was reduced by 4.7% and 5.3% respectively. These results are encouraging because there was little change in the transmittance under the harsh conditions. Finally, a P(EDOT/Ani) : PSS heat shielding lm of large surface area was fabricated by a roll-to-roll slot-die coating process. A dispersion of P(EDOT/Ani) : PSS was poured onto a PET substrate (500 mm in width Â 150 m in length) and then the coating took place. Residual solvent was removed using a convection oven at 120 C for 3 min (see Fig. 7).

Conclusions
In conclusion, we report the successful polymerization of 3,4ethylenedioxythiophene and aniline in aqueous medium by twostage shot growth. A conjugated polymer complex with enhanced absorption in the near infrared area was obtained by controlling time intervals for aniline monomer addition. P(EDOT/Ani) : PSS was also successfully applied as an efficient heat shielding material. Introduction of polyaniline to PEDOT : PSS signicantly improves the shielding efficiency in comparison with pristine PEDOT : PSS. The shielding efficiency of P(EDOT/ Ani) : PSS lm was calculated using UV-vis-NIR spectra. The total shielding efficiency increased by 11.2% at the same transmittance, which is 60% in 550 nm wavelength. We also fabricated a large-scale lm (500 nm in width Â 150 m in length) of P(EDOT/ Ani) : PSS and conducted a heat shielding test of the lm by measuring the temperature variation. Compared with the PEDOT : PSS and PAni lm, the P(EDOT/Ani) : PSS lm exhibited better the heat shielding effect. By employing the two-stage shot growth process, we easily synthesized the desired polymer composites. Our results indicate that P(EDOT/Ani) : PSS can facilitate commercial application of heat shielding lm and simplify the preparation of nanocomposites.

Conflicts of interest
There are no conicts to declare.