Kinetically Controlled Supramolecular Block Copolymer Self-Assembly: Multicolor Photonic Crystal Patterns from a Single Formulation

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES27 Dec 2022Kinetically Controlled Supramolecular Block Copolymer Self-Assembly: Multicolor Photonic Crystal Patterns from a Single Formulation Shuai Deng, Renhua Deng, Xi Mao, Bijin Xiong, Mian Wang, Senbin Chen, Wolfgang H. Binder, Jintao Zhu and Zhenzhong Yang Shuai Deng State Key Laboratory of Materials Processing and Die & Mould Technology, Key Laboratory of Materials Chemistry for Energy Conversion and Storage (Ministry of Education), School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 , Renhua Deng *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Materials Processing and Die & Mould Technology, Key Laboratory of Materials Chemistry for Energy Conversion and Storage (Ministry of Education), School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 , Xi Mao State Key Laboratory of Materials Processing and Die & Mould Technology, Key Laboratory of Materials Chemistry for Energy Conversion and Storage (Ministry of Education), School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 , Bijin Xiong State Key Laboratory of Materials Processing and Die & Mould Technology, Key Laboratory of Materials Chemistry for Energy Conversion and Storage (Ministry of Education), School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 , Mian Wang State Key Laboratory of Materials Processing and Die & Mould Technology, Key Laboratory of Materials Chemistry for Energy Conversion and Storage (Ministry of Education), School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 , Senbin Chen State Key Laboratory of Materials Processing and Die & Mould Technology, Key Laboratory of Materials Chemistry for Energy Conversion and Storage (Ministry of Education), School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 , Wolfgang H. Binder Chair of Macromolecular Chemistry, Faculty of Natural Science II (Chemistry, Physics, and Mathematics), Martin Luther University Halle-Wittenberg, Halle (Saale) , Jintao Zhu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Materials Processing and Die & Mould Technology, Key Laboratory of Materials Chemistry for Energy Conversion and Storage (Ministry of Education), School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 and Zhenzhong Yang Department of Chemical Engineering, Tsinghua University, Beijing 100084 https://doi.org/10.31635/ccschem.022.202202530 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Photonic crystal (PC) patterns with tunable and changeable nonvolatile structural colors printed from a single ink are of great interest for optical products but have rarely been reported because most inks can only output one respective structural color. Herein, we propose a facile yet effective kinetically controlled self-assembly strategy to address this challenge. An ink formulation containing supramolecular block copolymers (SBCPs) is developed. SBCP patterns were printed by direct-ink-writing followed by solvent annealing to generate different structural colors by simply controlling the annealing time. The self-assembly kinetic regime suggests that different colors result from various kinetically trapped metastable states. In turn, the variation in structural color enables "visualization" of the self-assembly dynamics. Furthermore, we demonstrate that these kinetically trapped structures exhibit different responsive color-change behaviors. In addition, this kinetic control strategy can be synergistic with thermodynamic control to extend the color range. This study provides a facile yet effective solution for well-designed PC patterns with tunable, responsive, and unfading colors printed from the simplest single-nozzle printer with a single colorless ink, presenting great potential in broad applications, including information storage, encryption, and anti-fake. Download figure Download PowerPoint Introduction Photonic crystal (PC) materials are promising candidates for the new generation of optical products, including printing, decoration, anti-fake, sensors, and photonic devices, because of their unfading features.1–8 Among various PCs, those composed of block copolymers (BCPs) that are solution processible, flexible, and stimuli responsive have attracted substantial research interest.9–12 Since 2000, significant progress has been achieved in BCP PCs in the form of films,13–15 microspheres,16–22 or three-dimensional printed parts from linear BCPs (LBCPs),23–31 brush BCPs (BBCPs),32–36 or linear-brush supramolecular BCPs (SBCPs) with pendant side groups.37–39 Currently, the methodology for patterning BCP PCs with tunable structural colors is one of the focuses of this research field.40–42 Various applications demand patterned PC materials with tunable color. Although great progress has been achieved in BCP PC films, well-designed patterns of BCP PC have rarely been reported. One way to obtain BCP PC patterns is selective "color developing" or etching of precoated films.43–45 For example, a writable and erasable "whiteboard" of BCP PC film has been reported, which has potential in encryption applications.46 In addition, inkjet printing,41,46–50 a localized material deposition technique, can obtain well-designed patterns directly with minimum usage of raw materials but has rarely been used to print BCP PC patterns. A possible reason is that, unlike color printing with traditional inks containing dyes, the structural colors of PC patterns can hardly be modulated by the simple overlap of pixels with different inks jetted independently from different nozzles. Although BCP PC films with various structural colors across the entire visible spectral range have been realized by changing the molecular weight of BCP or the content of additives,51 the high-throughput fabrication of nonvolatile colorful BCP PC patterns is still an enormous challenge. One of the major problems is that various formulations are required for different colors, which means complex custom material synthesis, expensive cost, and complicated processing procedures. Thus, the generation of different structural colors for PC patterns printed with a single formulation of BCP ink is of great interest.40 Studies of the stimuli response of BCP PC films or microspheres showed that adjusting the swelling degree of the material changes the structural color.52,53 These swollen PCs have considerable applications in some cases.14,18 However, these structural colors usually cannot be retained after deswelling, which takes place after solvent evaporation. Nevertheless, tuning the structural color via control of the swelling degree has shown great potential in detection and anti-fake. In contrast, regulation of the structural colors of nonvolatile PCs from the same BCP formulation is highly expected but has rarely been reported.40,53 A recent study realized BBCP PC patterns with tunable colors by varying the printing speed and drying temperature through direct-ink-writing (DIW) technology,40 which is impressive progress in tuning nonvolatile structural colors from the same formulation. However, it remains difficult to obtain PC patterns with good monochromaticity and varying structural color involving changes in pattern morphology (e.g., width and thickness). Herein, we developed a facile approach to obtain well-designed SBCP PC patterns with tunable and changeable structural colors printed from a single ink, whose structural colors are nonvolatile and have good monochromaticity. Briefly, patterns were printed by DIW with an ink containing blends of SBCP and a homopolymer, followed by postannealing to promote disorder-to-order transformations to generate structural colors (Figure 1a–c).54,55 More importantly, we propose a kinetically controlled supramolecular self-assembly strategy for tuning structural colors. We reveal that the different kinetically trapped metastable states can generate different structural colors or exhibit different responsive color-change behaviors, which can be well tuned simply by controlling the annealing time until they finally reach their thermodynamically stable equilibrium states. In addition, we demonstrate that these SBCP PCs have promising potential in broad applications, including information storage, encryption, and anti-fake. Figure 1 | Schematic illustrations: (a) multicolor PC patterns printed by using a single SBCP ink, (b) chemical structural formula of SBCP, and (c) color modulation by kinetic control. (λ and D represent the reflectance spectrum wavelength and periodic distance, respectively; PS-b-P2VP is poly(styrene-block-2-vinylpyridine), PDP is 3-n-pentadecylphenol, DBSA is dodecylbenzene sulfonic acid, and hPS is the homopolymer polystyrene.) Download figure Download PowerPoint Experimental Methods Materials PS2048-b-P2VP2048 (the subscripts are the degrees of polymerization, and the molecular weight distribution is Mw/Mn = 1.29) and homopolymer polystyrene (hPS; PS19, Mw/Mn = 1.06) were purchased from Polymer Source, Inc. (Montreal, Quebec, Canada). 3-n-Pentadecylphenol (PDP) and the solvent propylene glycol monomethyl ether acetate (PGMEA) were purchased from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Dodecylbenzene sulfonic acid (DBSA; purity >96%) was purchased from Bide Pharmatech Co., Ltd. (Shanghai, China). (3-Iodopropyl)trimethoxysilane (C6H15IO3Si, 95%) was purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Chloroform (CHCl3, purity ≥99.0%) and ethanol (EtOH, purity ≥99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and Shanghai Titan Scientific Co., Ltd. (Shanghai, China), respectively. All the materials were used as received without further purification. Preparation of inks Poly(styrene-block-2-vinylpyridine) (PS-b-P2VP) was dissolved in PGMEA at a concentration of 20.0 mg/mL. For the hPS/PS-b-P2VP(PDP)1.0 ink, equivalent masses of PDP and hPS were dissolved in the above polymer solution, wherein the molar ratio of PDP to the pyridine group of P2VP was x/n = 1.0. The ink has a low viscosity of 3.9 mPa·s. To prepare inks with different x/n values, the above ink was mixed with an ink of neat PS-b-P2VP with various volume ratios. Preparation of PC patterns The inks were printed on substrates (glass slides treated with (3-iodopropyl) trimethoxysilane or polyimide films) using a DIW printer (Sonoplot Microplotter, SonoPlot, Inc., Middleton, Wisconsin, United States) with a 50 μm (orifice size) nozzle. The jetting voltage was 3.5 V, and the nozzle moving speed was 1000 μm/s. Each pattern contained 20 printed layers. Then, each sample was placed in a 25 mL glass vial, to which 45 μL chloroform was added. The glass vial was sealed and placed in an oven at 60 °C to allow annealing for the desired time. Preparation of samples for grazing-incidence small-angle x-ray scattering and Fourier transform infrared spectroscopy To obtain SBCP films, the samples were prepared by drop casting followed by solvent annealing as described above. Characterization Scanning electron microscopy (SEM) images were obtained by a Hitachi SU8010 SEM (Hitachi, Ltd., Tokyo, Japan) operated at an acceleration voltage of 3 kV. Cross-sectional SEM samples were prepared by fracturing the samples (with Si substrates) after freezing in liquid nitrogen. To identify inner structures, P2VP microdomains were selectively stained with iodine vapor for 1 h at room temperature. Grazing-incidence small-angle X-ray scattering (GISAXS) experiments were performed on a Xeuss system of Xenocs France, equipped with a semiconductor detector (Pilatus 3R, 1 M) attached to an X-ray source (MetalJet-D2), with a wavelength λ = 0.134 nm. The sample-to-detector distance was set at 6402 mm. Fourier transform infrared (FT-IR) spectroscopy was performed on a Bruker INVENIO-V70 spectrometer (Bruker, Billerica, Massachusetts, United States) using attenuated total reflection mode. Differential scanning calorimetry (DSC) experiments were performed on a DSC (Q2000, TA Instruments, Newcastle, Delaware, United States). Samples of 3–5 mg were heated from 30 to 180 °C at a heating rate of 10 °C/min. Rheology experiments were performed on a modular compact rheometer (MCR 102, Anton Paar, Graz, Steiermark, Austria) at 25 °C. The shear strain of the polymer solution was tested as a function of shear rate (variation range: 0.1–100) by using the cone-plate mode. A fiber-optic spectrometer (USB4000, Ocean Optics Inc., Dunedin, Florida, United States) was used to measure the reflection spectra of the PC patterns. Photographs of the PC patterns were taken with a mobile phone camera or under a dermoscope. Results and Discussion Printed PC patterns with good monochromaticity An approach combining DIW with postannealing was established to prepare SBCP PC patterns. We developed an ink containing a blend of PS-b-P2VP, PDP, and hPS. PDP and hPS were added to enlarge the P2VP and PS domains of the PS-b-P2VP assemblies, respectively. PDP can graft to P2VP via hydrogen bonding (Figure 2a),38,56,57 whereas hPS can embed in PS domains via the "wet-brush" effect.58 Equal amounts of PDP and hPS were added to maintain the volume ratio (VPS+hPS:VP2VP+PDP) at approximately 1:1 to target a lamellar morphology. A green solvent, PGMEA, was applied as the solvent for the ink. An as-formed ink (the relative molar ratio of PDP with respect to pyridine ring x/n = 1.0) was printed through a DIW printer onto a substrate. Ink was extruded in the form of a continuous fluid from the nozzle, and a pattern was obtained after drying. Figure 2 | (a) Schematic illustration of the formation of SBCP, (b) reflectance spectra and inset photographs of SBCP (x/n = 1.0) patterns with different substrates before and after annealing, (c) SEM images of the cross-sections from patterns before (0 h) and after annealing (12 h). Download figure Download PowerPoint The as-printed pattern initially did not exhibit structural color (Figure 2b, bottom inset). An SEM image of the cross-section of the pattern at 0 h (Figure 2c, bottom) revealed that its nanostructure was disordered, where the bright microdomains belong to P2VP due to higher conductivity (because of quaternization) after staining by iodine vapor, while the dark regions are therefore composed of PS. The disordered structures were kinetically trapped metastable states, which were formed due to lack of time for ordered self-assembly of SBCP during the fast solvent evaporation process (∼15 s). Therefore, annealing was applied to allow a disorder-to-order transformation through chain rearrangement driven by microphase separation. Fortunately, a bright green color appeared after annealing for 12 h (Figure 2b, middle right inset, and Supporting Information Figure S1a) without the need for further crosslinking and swelling processes. An ordered, alternating, in-plane lamellar nanostructure (parallel orientation with respect to the substrate) was confirmed by SEM (Figure 2c). The maximum reflection wavelength (λmax) was 564 nm with a half-peak width of only 69 nm (Figure 2b), indicating good monochromaticity of the PC pattern, which is better than that of BBCP prepared by DIW.40 This approach also enabled the printing of PC patterns onto flexible substrates. Polyimide films are widely used for photonic and electric devices, such as flexible printed circuit boards, due to their excellent physics and chemical stabilities. The resistance of the polyimide film to high temperature and chemical solvents makes it suitable for the annealing process. As a proof of concept, a green hPS/PS-b-P2VP(PDP)1.0 PC pattern on a polyimide film was prepared (Figure 2b, center inset). The reflectance spectrum of the PC pattern on the polyimide film was consistent with that on the rigid glass substrate (Figure 2b). Color modulation of PC patterns printed from the same SBCP ink by kinetic control Interestingly, PC patterns with tunable, nonvolatile structural colors from the same ink were realized by kinetic control during solvent annealing. Although annealing has been widely used to drive disorder-to-order transformations, less attention has been given to their transition states for BCP PCs. We found that these transition states provide a facile solution for color modulation by simply controlling the annealing time. For example, a light cyan color appeared after annealing a hPS/PS-b-P2VP(PDP)1.0 PC pattern for 3 h (Figure 3a), which was confirmed by the reflectance spectrum peak at λmax = 497 nm (Figure 3b). The structural color evolution of patterns printed from the same hPS/PS-b-P2VP(PDP)1.0 ink was produced by prolonging the annealing time (Figure 3a). The light cyan color changed to medium aquamarine and then to green for patterns that were annealed for 6 and 9 h, respectively. An SEM image indicates local order structures formed for PCs annealed for 6 h ( Supporting Information Figure S2). Their reflection peak wavelengths redshifted accordingly (Figure 3b), which agreed well with the structural colors. Almost no further obvious color change occurred when prolonging the annealing time to 12 h or even 24 h, which suggests that the system was approaching its equilibrium state. We note that the half-peak width further decreased from 9 to 24 h, which indicates that the degree of long-range order of the full pattern was slightly improved. Although the structural colors were derived from the kinetically trapped metastable states, they showed good stability ( Supporting Information Figures S3 and S4). In contrast, these structural colors obtained by using the traditional swelling method showed poor monochromaticity and could not be kept in a dry state ( Supporting Information Figures S5 and S6). Figure 3 | (a) Photographs and (b) reflectance spectra of patterns printed from the same ink (x/n = 1.0) and annealed for various times. (c) 1D GISAXS curves of films coated from the same ink (x/n = 1.0) and annealed for various times. (d) Schematic illustration of the possible thermodynamic and kinetic regimes of a self-assembled SBCP system. Download figure Download PowerPoint The internal structure evolution of hPS/PS-b-P2VP(PDP)1.0 assemblies during annealing was further explored by GISAXS. The as-coated colorless film showed no obvious characteristic peak from the one-dimensional (1D) GISAXS curve (Figure 3c), which confirms that the initial state had disordered internal structures. Interestingly, for the film annealed for 3 h, where an intermediate state is formed according to Figure 3a, multiple integer order peaks were observed, indicating the presence of ordered lamellar structures.11 The estimated periodic distance determined from the equation D = 2π/q1 is 170.7 nm, where q1 belongs to the position of the 1st-order peak at ∼0.037 nm−1. With the increase in annealing time to 12 h, where a quasi-equilibrium state is formed according to Figure 2, q1 shifts to a lower region (∼0.033 nm−1), indicating that the periodic distance increases to 190.3 nm. In addition, the relative intensities of the second- and third-scattering peaks significantly increased, which indicates that the degree of order was also improved. By further prolonging the annealing time to 24 h, no obvious shift of the first-order peak but a slight shift of the second- and third-scattering peaks was observed from the 1D curves (Figure 3c), which were integrated over all directions from corresponding 2D GISAXS patterns ( Supporting Information Figure S7). Clearly, the GISAXS results regarding morphological transformation were consistent with those observed by SEM and detected by the reflectance spectra. The lamella spacing calculated via GISAXS (∼190 nm) was larger than that measured via the SEM image (∼160 nm), yet it is not hard to understand that the region of PC pattern at the break section underwent a stretch thinning process during the preparation of the SEM sample by fracture, thereby resulting in a decrease in lamella spacing. Similar results have been reported in the literature.40 Therefore, we suggest that SEM images can show the lamellar morphology directly, whereas the lamella spacings from GISAXS should be more reliable. The possible mechanism of structural color modulation was by combining the above with thermodynamic and kinetic The self-assembly process is by and different kinetically trapped metastable states in a local minimum of the can for each Similar to each of the metastable states of the SBCP system in this study can different structural colors no structural color it is which are different from those at the equilibrium state the minimum of the of the metastable states are possible by annealing because polymer are mobile when swollen by solvent and can rearrangement to decrease the of the system and be trapped in a new state with the of annealing solvent until its equilibrium Therefore, the to control transformations provides a for color In the a disordered metastable state initially formed due to fast annealing, the disordered structures ordered structures through chain rearrangement because a lower was derived from a higher degree of As the transformations are only the new state a lower than the the degree of order of the SBCP patterns until their equilibrium state (Figure In addition, control experiments were to the control mechanism ( Supporting Information Figure In the of solvent annealing at room temperature a pattern with a spectral peak at λmax nm was obtained after annealing for 24 h, which suggests that it is an metastable state that is still from the equilibrium state. the annealing was at 60 °C but in the of the solvent no structural color appeared even after These results that solvent a major in chain and the self-assembly is also temperature kinetic and thermodynamic control of structural color This kinetic control strategy can be synergistic with thermodynamic control to extend the color range. is well that the structural color of the equilibrium state is on the molecular To in the of BCP with various degrees of polymerization, we the SBCP strategy to the molecular weight by simply varying the content of PDP By adjusting PC patterns with tunable structural colors, from to were obtained after annealing for 24 h ( Supporting Information Figure Their λmax from to nm as a function of x/n ( Supporting Information Figure which has a to that of with various degrees of The spectra confirmed that the relative content of P2VP ( Supporting Information Figure modulation of structural color was by combining the SBCP strategy with A of PC patterns were prepared by varying the PDP content (x/n = 1.0) and annealing time from 3 to 24 h (Figure Thus, the of λmax can be well tuned by x/n or (Figure More we found that less time is for containing more PDP to reach their respective quasi-equilibrium states. As shown in Figure the SBCP pattern with x/n = h to approach whereas that with x/n = only This is because the PDP side can stretch the P2VP of the P2VP This can also can directly form structural color during a dry while molecular weight need a annealing time to a in some to generate structural color. The SBCP strategy in this study the annealing while the crosslinking and swelling method usually takes h of annealing for the disorder-to-order transformation of Figure | (a) spectra of PC patterns with various PDP content and annealing and (b) their λmax evolution as a function of annealing time (c) spectra of patterns annealed for various times. Download figure Download PowerPoint This strategy an obvious in the structural color of BCP PCs a single color to a range of the formulation is less to generate or even colors even x/n = 1.0. the degree of of PS-b-P2VP should address this but it great in the of PS-b-P2VP with a higher degree of We note that with the SBCP the color range can be by changing the pendant side As a proof of concept, PDP is by which can to P2VP by and hydrogen With a formulation of PC patterns with and colors were with spectra the full visible light range (Figure We note that even the molecular weight of DBSA is to that of PDP, the by DBSA are for the of the layers. BCP PCs potential applications The of printing, the SBCP and self-assembly can be applied for the fabrication of PCs with broad including information storage, encryption, and anti-fake. The colors by control provide an excellent solution for anti-fake, which can hardly be For example, PC patterns of with or colors were by annealing time (Figure which has potential in the of In addition, control can also be applied for information and As shown in Figure a pattern of was in which the were annealed for 24 h while were not of and were all after swelling the pattern with ethanol colorful was selectively 2 while colorless because of disordered internal structures. Furthermore, DIW printing the of complex such as 10 which enables the of information and encryption (Figure Figure | of SBCP (a) with (b) information encryption with a pattern, and (c) information and encryption with a hPS/PS-b-P2VP(PDP)1.0 Download figure Download PowerPoint In we have a facile printing for the patterning of nonvolatile BCP PCs more importantly, an kinetically controlled self-assembly strategy to tunable and changeable nonvolatile structural colors with good monochromaticity from a single colorless ink. We reveal that the different kinetically trapped metastable states can generate different structural colors or exhibit different responsive color-change behaviors, where their metastable states can be well tuned simply

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