Article pubs.acs.org/Langmuir
Intelligent Reversible Nanoporous Antireflection Film by SolventStimuli-Responsive Phase Transformation of Amphiphilic Block Copolymer Xiao Li, Xinhong Yu, and Yanchun Han* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, People's Republic of China ABSTRACT: An erasure-reconstruction porous structure with reversible antireflection (AR) property at near-infrared region (NIR) was prepared for the first time based on solvent-stimuli-responsive phase transformation of polystyrene-block-poly(4-vinypyridine) (PS-b-P4VP). The inhomogeneous porous structure with a dense skin and porous underneath, which was obtained by the nonsolvent-induced phase separation of PS-b-P4VP film from micelle solution with mixed solvents (tetrahydrofuran and dimethylformamide), was used as starting porous film. Then, the film was annealed by PS-selective solvent to erase the nanopores because the PS block was swollen effectively by its selective solvent. Afterward, the nonporous film was immersed in linear aliphatic acid to reconstruct the nanoporous structure (loosely packed micelles) by the combination of the hydrogen bond interaction and the positively charge-induced repulsion between each chain. Thus, an intelligent reversible AR property in the NIR region between a high-transmittance porous state (∼99.0%) and a lowtransmittance nonporous state (∼90.0%) was realized by alternate treatments of PS-selective solvent and linear aliphatic acids. This reversible erasure-reconstruction porous structure for switching between AR (98.0%) and non-AR (90.0%) properties could be recycled by at least four times.
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INTRODUCTION Antireflection (AR) film, inspired from moth eye, is necessary for photovoltaic, flat-panel display, solar cells, and all kinds of optical lenses because of its ability to improve transmittance property, reduce glare, and eliminate ghost images for light contrast.1−5 In recent years, based on the AR principle of destructive interference of reflected light from interfaces between the air−film and film−substrate, much efforts have been attempted to obtain low-refractive-index materials or structures to satisfy the following basic conditions: a λ/4 thick coating, where λ is the wavelength of the incident light; and nc = (nans)1/2, where nc, na, and ns are refractive indices of the coating, air, and substrate, respectively.6,7 Polymer AR coatings are significant in the development of the AR films because of their adherence to the flexible substrate and ease in processing large areas. Various methods that relied on polymer blends, block copolymer, polymer micelles, and polymer particles were performed to obtain proper AR structures (homogeneous, inhomogeneous, and gradient-refractive-index structures) that could satisfy different AR requirements, such as single wavelength, broadband wavelength, reversible transmittance, and multifunctional ARs.8−16 To date, intelligent reversible AR coatings have attracted considerable attention because of their potential application on architectural or vehicle windows, skylights, and internal partitions.17−19 Stimuli-responsive polymers are very suitable for this system because of their special response mechanism. Stimuli-responsive polymers could undergo relatively obvious © 2012 American Chemical Society
physical or chemical changes in response to small external changes under different environmental conditions.20 These stimuli could be divided into two categories, namely, chemical and physical. Chemical stimuli, such as pH, ionic factors, and chemical agents, change the interactions between polymer chains at the molecular level. Rubner et al.19 reported a phaseseparated polyelectrolyte multilayer film with a nanoporous structure, which could be erased and reconstructed by a reversible pH-induced swelling transition for the first time, based on the chemical stimuli. Such films have good reversible AR properties between 500 and 900 nm with reversible transmittance scope at 91.0 to 99.0% and could be easily patterned by inkjet printing technique with the potential for pH responsive biomaterial and membrane applications. Physical stimuli, such as temperature, electric or magnetic fields, and mechanical stress, can influence the level of various energy sources and change molecular interactions at critical onset points.20 Cho et al.17 studied hierarchical structures with a surface morphology consisting of a nanopillar array on a wrinkled poly(dimethylsioxane) film based on physical stimuli. The film could be reversibly switched between a transparentstretched state (∼90.0%) and an opaque-released state (∼25.0%) at 400 to 800 nm light region by controlling the microscale roughness through the exploitation of the Received: April 29, 2012 Revised: June 20, 2012 Published: June 21, 2012 10584
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Table 1. Solvent Characteristics Employed in This Study boiling point (°C) polarity δsolvent (J/cm3)1/2 χPS/solvent χP4VP/solvent
THF
HCCl3
TOL
CS2
AA
PA
DMF
IPA
67.0 4.2 18.6 0.35 0.60
61.7 4.4 19.0 0.35 1.62
110.6 2.4 18.2 0.35 2.0
46.3 2.6 20.5 0.44 0.88
117.9 6.2 20.7 0.45 0.81
141.0 3.1 20.3 0.44 1.07
152.8 6.4 24.8 1.65 0.34
82.4 4.3 23.5 1.74 0.42
Figure 1. (a) SEM topographic image of original porous PS-b-P4VP films; AFM images of (a) after annealed by different solvents for 2 h: (b1) THF (the insets were the SEM top-view and cross-sectional images of the film, respectively); (c) HCCl3; (d) TOL; (e) CS2. AFM images of (a) after annealed in THF for different times: (b1) 2 h, (b2) 6 h; (b3) 8 h; (b4) 18 h; (b5) 24 h. The scale bar of each AFM images is 4 μm × 4 μm.
fabricated by nonsolvent-induced phase separation of PS-bP4VP micelles. Then, the nanoporous film with AR property was annealed by PS-selective solvent to obtain effective swollen volume of large PS blocks for the erasure of nanopores. The nanoporous structure was constructed by immersing the film into linear aliphatic acid to obtain loosely packed micelles, based on the combination of the hydrogen bond interaction and the positively charge-induced repulsion between each chain. The AR property of PS-b-P4VP films could be recycled by an alternate process of PS-selective solvents and linear aliphatic acids. This reversible erasure-reconstruction porous structure for switching between AR (98.0%) and non-AR (90.0%) properties could be recycled by at least four times.
mechanically sensitive reversible wrinkling patterns (external strain). However, studies on the intelligent reversible light transmittance AR films are rare, and the previous studies focused on the reversible AR property in the visible light region. The pHinduced reversible porosity was due to changes in the molecular chain level, which limited the pore size changes only in the tens of nanometers proper for visible light AR. By contrast, when the aforementioned hierarchical structures film went through mechanical stress to modulate the structure on the micrometer level, the highest reversible transmittance was only achieved at 90%. Therefore, an intelligent reversible light transmittance AR film at the near-infrared region (NIR) is worthy of further investigation. AR coatings in the NIR are particularly very useful for output couplers, dichroic mirrors, lenses, and so on.3,21 In the present work, an erasable nanoporous polystyreneblock-poly(4-vinypyridine) (PS-b-P4VP) film with an intelligent reversible AR property between high-transmittance porous and low-transmittance nonporous states in the NIR is presented for the first time. The precursor porous film was
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EXPERIMENTAL SECTION
Materials. The diblock copolymer of PS-b-P4VP
(
MPS = 130 000; MP4VP = 75 000; polydispersity
Mw/Mn = 1.25) was purchased from Polymer Source Inc. The solvents 10585
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Figure 2. SEM topographic images of nonporous PS-b-P4VP film of Figure 1b1 after immersed by different solvents for 30 s. (a1) Acetic acid (the inset was the SEM cross-sectional images of the film); (b) propionic acid; (c) isopropanol. AFM topographic images of films of Figure 1(b1−e) after immersed in acetic acid for 30 s: (a1) Figure 1b1; (d) Figure 1c; (e) Figure 1d; (f) Figure 1e. AFM topographic images of the film of Figure 1b1 immersed in acetic acid for different times: (a2) 10 s; (a3) 20 s; (a4) 50 s; (a5) 120 s. The scale bar of each AFM figure is 4 μm × 4 μm. tetrahydrofuran (THF), chloroform (HCCl3), toluene (TOL), carbon disulfide (CS2) acetic acid (AA, pKa = 4.75), propionic acid (PA, pKa = 4.87), dimethylformamide (DMF), and isopropanol (IPA) were purchased from Beijing Chemical (China). Deionized water was used. The glass substrates were boiled in a piranha solution [7/3 (v/v) of 98% H2SO4/30% H2O2] for 30 min to remove the stains on the surface, then washed with deionized water, and dried under nitrogen (N2) flow. Sample preparation. The diblock copolymer was dissolved in a mixed solvent of THF and DMF (3 wt %) with concentrations of 2.0 wt %. First, the solutions were spin coated onto the glass substrates at 2500 rpm for 20 s. The sample was immersed in a nonsolvent bath (IPA) at room temperature for 12 h. Then, the sample was washed several times with deionized water, dried under N2 flow, and placed in a vacuum oven at room temperature for more than 24 h to remove the remaining solvent molecules in the sample. Then, the porous sample was exposed to the saturated PS-selective solvent vapor (THF, HCCl3, TOL, and CS2) in a closed vessel at −7 °C for 2 h. Finally, the sample was immersed into P4VP-selective solvents (DMF, AA, PA, and IPA) for different periods (10, 20, 30, 50, and 120 s as well as 1 h), washed several times with deionized water, and dried under N2 flow. Characterization. The surface topography of the spin-coated films was studied by atomic force microscopy (AFM). Images were obtained using a SPI3800N atomic force microscope (Seiko Instruments Inc., Japan) with a Si tip with a spring constant of 2 N/m. The cantilevers were operated slightly below their resonance frequency of approximately 72 kHz. Image acquisition was performed at ambient conditions. AFM was conducted in tapping mode to reduce tipinduced surface degradation and sample damages. Imaging was conducted in height mode. The morphology and cross-sectional images of the porous film were investigated by field emission scanning electron microscopy (Micro FEI PHILIPS XL-30-ESEMFEG). X-ray photoelectron spectra (XPS) were measured with VG ESCALAB MK (VG Company, U.K.) at room temperature using an Mg Kα X-ray source (hν = 1253.6 eV) at 14 kV and 20 mA. The
sample analysis chamber of the XPS instrument was maintained at a pressure of 1 × 10−7 Pa. Transmittance measurements at a spectral range of 800 to 2000 nm were performed using a Shimadzu UV-3600 spectrophotometer. The thickness of the nanoporous film on the silicon wafer and the refractive index (n) were measured by spectroscopic ellipsometry over a wavelength range of 300 to 800 nm at a fixed incident angle of 70° using a UVISEL spectroscopic ellipsometer (Jobin Yvon, France).
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RESULTS AND DISCUSSION The Erasure of Nanopores from Porous PS-b-P4VP Films by PS-Selective Solvents. As previously reported by the authors,22 an inhomogeneous porous polymer film with a dense skin and porous structure underneath was obtained by spin coating a micelle solution of PS-b-P4VP with THF and 3
Figure 3. XPS spectra of (a) the original porous PS-b-P4VP film; (b) the film treated in THF vapor for 2 h; (c) the THF-annealed film (b) immersed in acetic acid for 30 s. 10586
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Scheme 1. Schematic Representation of the Reversible Formation Process between Nonporous and Porous Structurea
The initial porous film was annealed by PS-selective solvent to erase the nanopores, because PS block was swollen effectively by its selective solvent. After that, the nonporous film was immersed in linear aliphatic acid to reconstruct nanoporous structure (loose packed micelles) by the combination of the hydrogen bonding interaction and the repulsion between each chain induced by positively charge. a
Table 2. Mean Transmittance Values of PS-b-P4VPs through Different Solvents Post-Processinga mean T%
THF
HCCl3
TOL
CS2
solvent annealing AA immersion
89.7 (±1.3)
87.3 (±1.8)
89.9 (±0.8)
86.7 (±2.2)
98.1 (±0.7)
88.1 (±2.0)
90.5 (±0.9)
88.2 (±1.9)
a
The wavelength coverage of mean transmittance is between 800 and 2000 nm.
Figure 5. (a) The cyclic transmission spectra by THF annealing and acetic acid immersion. (b) The mean transmittance value at 800−200 nm as a function of cycle number N according to the transmission data of (a). (c) The cyclic transmission spectra by THF annealing and propionic acid immersion.
wt % DMF. The film was then subjected to nonsolvent-induced phase separation before it reached a dry state. The dense skin surface morphology was fixed because of the rapid evaporation of THF during the spin coating. In the subsequent nonsolvent immersion, an inner nanoporous structure was formed because the DMF was replaced by the nonsolvent (with similar solubility to DMF). The inhomogeneous porous polymer film showed a tunable wavelength AR between visible and NIR light wavelengths because of the regulation of film thickness and immersion time of the nonsolvent. In the current paper, an intelligent AR film with switchable light transmittance properties in the NIR by selective solvents alternatively postprocessing the inhomogeneous porous PS-b-P4VP film is presented.
Figure 4. Transmission spectra of (a) original continuous PS-b-P4VP film and porous PS-b-P4VP films after annealed by THF for different times; (b) the films annealed by THF for 2 h, then immersed into acetic acid for different times. Note that both sides of the glass substrates were coated with the PS-b-P4VP films and the incident light was normal.
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obviously facilitated the fusion of the PS block, resulting in a considerably longer worm-shaped pattern (Figure 1b3). Further solvent annealing resulted in the fusion of the longer wormlike PS-block together with P4VP cylindrical microdomains oriented parallel or normal to the film surface (Figure 1b4,b5).25 In the present work, exposure to the selective solvent vapors was not the thrust to obtain different film morphologies, rather, the use of the solvent annealing made the effective swollen volume fraction of larger PS to erase the nanopores. The prolonged annealing time provided the sufficient mobility of polymer chains to obtain long-range structural order. Therefore, 2 h exposure to selective solvent was sufficient to erase film nanopores. The Reconstruction of Porous PS-b-P4VP Films by Linear Aliphatic Acids. The subsequent reconstruction of PSb-P4VP nanoporous film was realized by selective solvents for P4VP-blocks. The THF-annealed film with relatively compact and dense surface morphology with only several pores in the range of 30 to 80 nm in diameter (Figure 1b1) was exposed to P4VP selective solvents, such as linear aliphatic acid (AA and PA), IPA, and DMF for 30 s. After acid immersion process, the film that was immersed in AA generated a porous structure (Figure 2a1) with loosely packed nanoparticles with ∼50 nm diameter. The cross-sectional images (Figure 1b1 (inset images) and Figure 2a1 (inset image)) also showed that the relatively compact structure was changed into loosely packed nanoparticles with many nanopores after immersed into acetic acid. Another linear aliphatic acid, PA, could also regenerate the similar structure similar to AA, showing relatively densely packed nanoparticles of ∼50 nm diameter with only several nanopores imbedded in the film (Figure 2b), which would decrease the porosity ratio of the film. In terms of IPA and P4VP good solvent DMF, the film with nonporous structure would keep the same state even if the immersion time was prolonged to more than 1 h by IPA (Figure 2c) or completely dissolved by DMF once the film was immersed into this solvent. Thus, the most appropriate solvent to reconstruct nanoporous structure was acetic acid. Then, the films, annealed by THF, HCCl3, TOL and CS2, were all immersed into AA for 30 s (Figure 2). The initial spinodal-like structure by THF and HCCl3 annealing were transformed into numerous nanosized spherical particles with lots of gaps among these particles for THF (Figure 2a1) and also numerous nanosized spherical particles fused together with only a small quantity of gaps for HCCl3 (Figure 2d). The corresponding bigger size phase separation morphologies of the TOL- and CS2-annealed films after AA treatment appeared as spherical particles when particles with approximately 250 nm diameter fused together with a small quantity of large blank areas for both of the TOL and CS2 (Figure 2e,f). Finally, the optimal time of AA treatment was selected by immersing the THF-annealed films into AA for different periods from 10 to 150 s. The porous morphology showed a dependence on the duration of the AA treatment. A spinodallike pattern surface with many nanopores (Figure 2a2,a3) appeared under a short immersion time (10 and 20 s). The surface exhibited numerous spherical particles with ∼50 nm diameter when the immersion time was increased to 30 s. There were many nanogaps among these particles (Figure 2a1). Further increase in the immersion time to 50 s resulted in the covering of various spherical particles on the surface, but the substrate began to be exposed at several small-sized areas (Figure 2a4). When the immersion period was prolonged to
Then, the selective solvent annealing was employed to erase the nanopores in the film. According to the Flory−Huggins theory,23 the polymer and solvent are completely miscible over the entire composition range when χp‑s < 0.5, indicating that solvents always show different preferential affinity to one of the blocks and that the solvent vapors of varying selectivity for the two blocks could change the film surface morphology. Therefore, after the nonsolvent induction of the inhomogeneous nanoporous structure generation, the selective solvent annealing was chosen for the erasure of the nanopores. In the nonsolvent-induction process, the PS block shrunk, in contrast to the swollen P4VP-block because of the hydrogen bond interaction between 4VP units and IPA, and the nanopores came from the previously occupied positions by the nonsolvent. The selective solvent of PS was used for the swelling of the PS chains, which has volume fraction that was bigger than P4VP based on the intrinsic property of the block copolymer. Information on the selective solvents is presented in the Table 1. Compared with the spongelike porous structure of the original film (Figure 1a), the observed morphology was different after the film was exposed to solvent vapor for 2 h (Figure 1), although the solvents (THF, HCCl3, TOL, and CS2) all turned out to be beneficial for the PS block, relying on the χPS/solvent and χP4VP/solvent values. The vessels with solventsaturated vapor were placed in an environment of −7 °C to slow down the speed of the solvent annealing. Upon THF and HCCl3 vapor annealing, the films showed spinodal-like pattern (Figure 1b1,c), covered by the PS blocks to decrease the surface free energy. The size of the phase separation was small with PSblock (light areas) around 200 nm. Upon TOL and CS2 vapor annealing, the films also showed spinodal-like pattern (Figure 1d,e) with much bigger PS aggregation size of approximately 500 nm. The differences in surface morphologies, observed after THF, HCCl3, TOL, and CS2 vapor annealing, could be elucidated by considering the polymer−solvent interaction parameter (χp‑s ) values and solvent polarity. Previous description had shown that all these four solvents were PSselective solvents with χPS/solvent < 0.5 (Table 1), and the poor solvents for P4VP with χP4VP/solvent > 0.5, which made PS preferentially move to the free surface. However, considering that the compatibility of the block and solvents was also based on the polar interaction and that P4VP was a polar segment, THF and HCCl3, with higher polarities than TOL and CS2, may present a degree of compatibility with 4VP segments. The lower polarities of TOL and CS2 only strengthened the incompatibility between 4VP segments and solvents, that is, the strong incompatibility between the PS block and the P4VP block in TOL or CS2 solvent vapor pushed the PS chains away from the P4VP chains. Therefore, the copolymer chains were highly stretched, leading to an increase in the phase separation size and a decrease in the contact areas of the PS and P4VP blocks. The selective swelling of the PS block was the factor that led to an increase in the volume fraction of PS.24 Hence, the selective solvent annealing was a useful method for erasing the nanopores in the film. Furthermore, the effect of the selective solvent annealing time on erasing film nanopores was investigated. THF annealing was taken as the example. When the THF annealing time was extended, the simple spinodal-like pattern, instead of other PS-b-P4VP patterns, was evident (Figure 1). The isolated islands of PS aggregations tended to merge with adjacent ones after exposure to THF vapor for 6 h, leading to worm-shaped pattern (Figure 1b2). Prolonged solvent annealing for 8 h 10588
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2a1,b). Meanwhile, from the XPS spectra (Figure 3c) the presence of the nitrogen atom from P4VP blocks increased when the THF-annealed PS-b-P4VP film was immersed in the AA, which also proved the selective solvent AA was the reason for forming micellar particle with P4VP blocks as the shells. The protonation of the nitrogen atom of pyridine unit also induced positive charges at the P4VP chains. Therefore, the P4VP chains were stretched themselves and were mutually exclusive for adjacent chains. The cyclic change between the two structures was elucidated by illustrating the alternate process in Scheme 1. At higher solvent acidity, protonation was stronger to ensure that the repulsion ability was sufficient to generate considerably more gaps among these micelles. This phenomenon explains why the film immersed by PA has a relatively more compact micelle morphology and worse recovery of AR property than the AA process. When the linear aliphatic acids were replaced by P4VP good solvent DMF or organic alcohol with weaker protonation ability, the reconstruction of porous structure would not occur. Hence, linear aliphatic acid with strong ability of protonation was the key in the reproduction of the porous structure in this research system. Reversible AR Property through Erasable-Reconstruction Nanoporous Structure. The AR properties of the films on glass substrates were investigated by using the transmission spectra. Previously, the PS-b-P4VP film achieved an excellent transmittance of 99.2% at near-infrared light wavelength (800− 2000 nm) after nonsolvent-induced phase separation.22 First, the above-mentioned AR properties of the inhomogeneous porous PS-b-P4VP film annealed by the PS-selective solvent were examined. The mean transmittance values of the THF−, HCCl3−, TOL−, and CS2− annealed films decreased to below 90.0% in the NIR (800−2000 nm) (Table 2). Therefore, only THF solvent annealing was chosen for the study of the effect of the annealing times on the AR properties. The transmittances of the PS-b-P4VP porous THF-annealed films for different periods (Figure 4a) decreased to approximately 88.0%, based on the AR properties of these films. No further decrease in transmittance was observed even when the annealing time was prolonged. This phenomenon may be due to the fact that once the PS-b-P4VP film was exposed to the PS-selective solvent vapor, the increase in the volume of the PS block by the selective solvent swelling was limited to a certain value. Thus, the increased volume would occupy original pores volume.30,31 Only the mean transmittance of film annealed by THF returned to 98.1% after acetic acid treatment. The transmittance of films annealed by the other three solvents all showed almost similar mean transmittance values (∼88.9%) without remarkable additional increase before and after the AA treatment (Table 2). The transmittance spectra of THF-annealed film through different AA immersion periods are shown in Figure 4b. The spectrum of nonporous THF-annealed film is also shown for comparison. The transmittance increased significantly when the AA immersion time increased from 10 and 20 s to 30 s. On the basis of the 30 s AA immersion, the recovered transmittance spectrum achieved the best AR effect with the highest transmittance of 99.2% at 1273 nm and a mean transmittance value of 98.1% in the NIR (800 to 2000 nm). Extending immersion time to over 30 s resulted in a continuously decreasing transmittance. The initial increase and subsequent decrease trend in the transmittance also proved that the nanopores generated by AA increased with the immersion time initially and that a longer immersion period might have caused
120 s, the areas with exposed substrate and the size of the particles also increased (Figure 2a5). Finally, the film began to peel off when the immersion period was prolonged to over 3 min. Mechanism of the Erasure-Reconstruction Nanoporous Structure Process. The nanoporous structure of the PS-b-P4VP film could be recycled by an alternate process of PS-selective solvents (THF) and linear aliphatic acids (AA) treatments. First, the original prepared PS-b-P4VP porous film (Figure 1a) showed network-like porous inner structure.22 By contrast, the annealed film (Figure 1b1) showed relatively compact and dense surface morphology with only several pores in the range of 30 to 80 nm diameter. After AA immersion, the film generated a porous structure (Figure 2a1) with loosely packed nanoparticles with 50 nm diameter, in contrast to the original porous structure. The thickness of the PS-b-P4VP film also alternated from approximately 181.5 nm (n = 1.33) to approximately 258.9 nm (n = 1.197) because of the reversible porous and nonporous transition. The reversible erasurereconstruction porous structure could be recycled for at least four times. The cyclic process involving PS-selective solvents for erasing nanopores could be elucidated by the selective solvent annealing that induced the increase in the volume of the PS block. All PS-selective solvents (THF, HCCl3, TOL, and CS2) could erase the porous structure of the PS-b-P4VP film. However, only the nonporous THF-annealed film could reproduce the porous structure by the next AA treatment. The interaction between the acid and PS-b-P4VP was dependent on the hydrogen bond between the carboxyl group and the pyridine units. Therefore, only the film surface with pyridine unit residues after solvent annealing could induce the interaction and penetration of acid to generate the porous structure. According to the polymer−solvent interaction parameters, χPS/THF = 0.35 and χP4VP/THF = 0.60, THF was a good solvent for both PS and P4VP blocks, providing the mobility for both blocks during exposure to its vapor, but at the same time THF was slightly selective for PS block among the four selective solvents.25 The presence of the nitrogen atom from P4VP blocks at the polymer−air interface upon THF solvent annealing was evident in the XPS data (Figure 3), which can be used to calculate the molar ratio of nitrogen elements. By contrast, the weight percent of nitrogen atom of the PS-b-P4VP copolymer was 6.7% according to the theory calculation. The original porous PS-b-P4VP film, generated from IPA-induced phase separation process, was covered with 5.1% nitrogen atom (Figure 3a). After THF solvent annealing, the film surface showed 1.0% nitrogen atom (Figure 3b). This phenomenon implied that P4VP blocks rarely existed at the interface after THF annealing, ensuring the hydrogen bond interaction and penetration of acid to generate nanopores in the film. The other part of the cyclic process involving the linear aliphatic acid for reproducing porous structure could be explained as the combination of the hydrogen bond interaction and the positively charge-induced repulsion between each chain.26−29 The nitrogen atom of the pyridine unit was protonated in an acid medium that led the P4VP blocks to become increasingly hydrophilic and compatible with the acid solvent.27 However, the PS chains were in a poor solvent and shrunk to minimize their surface energy in the presence of acid. Thus, the micelles, with PS blocks as the cores and P4VP blocks as the shells, were observed in the acid medium (Figure 10589
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the porous structure to collapse and even exposed the substrate. Thus, 30 s immersion period is the optimal recovery time to obtain the highest transmittance value. Therefore, the PS-b-P4VP film no longer acted as an AR coating during the nonporous state, and the transmittance value decreased to almost the same value as that of a bare glass substrate, that is, lower than 90.0% (Figure 5a). When the nonporous film was exposed to AA, the transmittance increased effectively to as high as 98.0% (Figure 5a). Thus, when the alternate process of THF annealing and AA immersion was repeated, the film exhibited excellent reversible switching between AR and non-AR state for four cycles (Figure 5b). Two factors lead to the instability of the films after repeated solvent treatments. First, the pore size and the pore ratio were not constant after every time AA processing. Second, the interaction between the film and substrate was weak and some part of the film might be peeled off the substrate after repeated solvent treatments for many times. The recovery of the AR property by PA was not as beneficial as that of AA because of the low porosity of the film surface (Figure 5c). This phenomenon is caused by the longer hydrocarbon tails and the weaker ability of protonation by PA.25
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CONCLUSION An erasable nanoporous PS-b-P4VP film with reversible AR property in the NIR was demonstrated. The precursor porous film was fabricated by nonsolvent-induced phase separation of PS-b-P4VP micelles. The nanopores were erased by PSselective solvent to obtain a compact structure. Then, the nanoporous structure was constructed by linear aliphatic acid to obtain loosely packed micelles. This reversible erasurereconstruction porous structure for switching between AR and non-AR properties in the NIR (800−2000 nm) could be recycled four times by an alternate process of PS-selective solvents and linear aliphatic acids. Therefore, this work provided a novel approach to fabricate intelligent reversible AR films at NIR light region by directly selective-solvent inducing for the long-term optical application.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (20834005, 20923003, 21004064) and National Basic Research Program of China (973 Programs-2009CB930603).
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REFERENCES
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