Integrating Superwettability within Covalent Organic Frameworks for Functional Coating

Abstract

•Imparting superwettability on COFs via pore-surface engineering•Retained crystallinity and porosity after conferring superwettability on the COF•Improved stability of the pristine COF after being imparted with superhydrophibicity•Integration of the superhydrophobic COF with substrates to expand its application Wetting is a ubiquitous phenomenon that can be observed anywhere from high tides on the beach to ion channels in cell membranes. Nature has adapted over millennia to process special wettability with unique functions. Inspired by nature, artificial superhydrophobic materials have garnered widespread application. Existing coating systems for preparing superhydrophobic surfaces are predominantly confined to nonporous materials. Covalent organic frameworks (COFs), an emerging class of crystalline, porous organic materials, have rapidly grown into a major area of chemical research. Herein, we successfully impart superhydrophobicity on a COF via pore-surface engineering and a series of substrates that, after being coated with this COF, show superhydrophobicity, which greatly expands its possibilities for numerous applications. By virtue of the intrinsic properties of COFs, the composites show great promise in tackling challenges associated with energy and the environment. Despite the availability of a variety of skeletons for covalent organic frameworks (COFs), control of pore-surface wettability remains undeveloped, which could immensely expand their overall versatility. Herein, we contribute an effective strategy for imparting superwettability on COFs by chemically coating the pore surface with perfluoroalkyl groups to confer them with superhydrophobicity. Taking advantage of controllable modification, the resultant COFs retain the porosity and crystallinity of the pristine COFs. Benefiting from the bulk superhydrophobicity of the COF crystals and the feasibility of COF synthesis, they can be used as a coat or in situ integrated within various substrates; once applied, this renders them superhydrophobic. Given the modular nature, this protocol is compatible with the development of various specific wettabilities in COFs, which thereby constitutes a step for expanding the scope of COF applications and provides many opportunities for the processing of advanced materials and devices. Despite the availability of a variety of skeletons for covalent organic frameworks (COFs), control of pore-surface wettability remains undeveloped, which could immensely expand their overall versatility. Herein, we contribute an effective strategy for imparting superwettability on COFs by chemically coating the pore surface with perfluoroalkyl groups to confer them with superhydrophobicity. Taking advantage of controllable modification, the resultant COFs retain the porosity and crystallinity of the pristine COFs. Benefiting from the bulk superhydrophobicity of the COF crystals and the feasibility of COF synthesis, they can be used as a coat or in situ integrated within various substrates; once applied, this renders them superhydrophobic. Given the modular nature, this protocol is compatible with the development of various specific wettabilities in COFs, which thereby constitutes a step for expanding the scope of COF applications and provides many opportunities for the processing of advanced materials and devices. Organic porous materials have technological importance and a myriad of functions and applications.1Slater A.G. Cooper A.I. Function-led design of new porous materials.Science. 2015; 348: aaa8075Crossref PubMed Scopus (891) Google Scholar, 2Das S. Heasman P. Ben T. Qiu S. Porous organic materials: strategic design and structure-function correlation.Chem. 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Drug delivery in covalent organic nanosheets (CONs) via sequential postsynthetic modification.J. Am. Chem. Soc. 2017; 139: 4513-4520Crossref PubMed Scopus (374) Google Scholar It can be envisioned that imparting different superwettabilities onto COFs could allow the generation and integration of novel interfacial functional systems into devices to expand the realm of possibilities for such materials to be used in tackling current and future challenges involving energy, environment, and health. Given the increasing demand of water-repellent materials,51Levkin P.A. Svec F. Fréchet M.J. Porous polymer coatings: a versatile approach to superhydrophobic surfaces.Adv. Funct. Mater. 2009; 19: 1993-1998Crossref PubMed Scopus (282) Google Scholar, 52Darmanin T. Guittard F. Recent advances in the potential applications of bioinspired superhydrophobic materials.J. Mater. Chem. A. 2014; 2: 16319-16359Crossref Google Scholar, 53Hayase G. Kanamori K. Hasegawa G. Maeno A. Kaji H. Nakanishi K. 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Significantly, because of the extreme water-repellent properties of superhydrophobic surfaces, virtually all aqueous liquids, including inorganic acidic and basic solutions, are prevented from permeating the material, thereby greatly improving its tolerance against variable pH environments. To show the applicability of the superhydrophobic COF, experiments were designed to integrate them with a series of substrates, including melamine foam, paper, and magnetic liquid, to confer them with superhydrophobicity. By combining the intrinsic properties of COFs and the bulk superhydrophobicity, such functional nanocoatings hold great promise to be applied as “suction skimmers” in oil-spill recovery, self-cleaning surfaces, coats for a magnetic liquid marble in microfluidic applications, and protectors for the next generation of microelectronic devices. This work therefore expands the possibilities of COFs for a plethora of target-specific applications via rational modification of pore-surface properties. To test the feasibility of pore channel engineering for controlling the wettability of COFs, we selected a COF bearing the vinyl functionality synthesized from the condensation between 1,3,5-tris(4-aminophenyl)-benzene and 2,5-divinylterephthalaldehyde, which was developed by our group, for proof of principle because of its excellent chemical stability, large pore size, and abundant high reactivity vinyl groups for potential chemical transformations (Figure 1).23Sun Q. Aguila B. Perman J. Earl L. Abney C. Cheng Y. Wei H. Nguyen N. Wojtas L. Ma S. Postsynthetically modified covalent organic frameworks for efficient and effective mercury removal.J. Am. Chem. Soc. 2017; 139: 2786-2793Crossref PubMed Scopus (652) Google Scholar Given the low surface free energy of fluorinated compounds,53Hayase G. Kanamori K. Hasegawa G. Maeno A. Kaji H. Nakanishi K. A superamphiphobic macroporous silicone monolith with marshmallow-like flexibility.Angew. Chem. Int. Ed. 2013; 52: 10788-10791Crossref PubMed Scopus (115) Google Scholar especially for the long chain ones, in conjugation with the facility and controllability of the thiol-ene click reaction,52Darmanin T. Guittard F. Recent advances in the potential applications of bioinspired superhydrophobic materials.J. Mater. Chem. A. 2014; 2: 16319-16359Crossref Google Scholar 1H,1H,2H,2H-perfluorodecanethiol was chosen to modify the pore surface of COF-V to manipulate the wettability. Because the enhancement of hydrophobicity, by increasing the grafting degree of fluorinated compounds, is at the expense of both porosity and crystallinity of the material, reaction conditions were screened to achieve the trade-off between hydrophobicity and the retention of intrinsic properties of the COF. Under optimal synthetic conditions, reacting COF-V with a 10% (v/v) 1H,1H,2H,2H-perfluorodecanethiol trifluorotoluene solution in the presence of a catalytic amount of azobisisobutyronitrile (AIBN) for 2 hr, resulted in the desired material (COF-VF; Figures 1 and S1). To explain the effect of the degree of modification and the type of perfluoroalkyl group on the superhydrophobic properties of the resultant materials, we provide detailed structure-property relationships in Figures S2–S4. Figures 2A and 2B show scanning electron microscopy (SEM) images of the COF materials before and after pore-surface modification and reveal that no noticeable morphological changes occurred and that both of them showed a large quantity of uniform nanofibers with diameters of about 80 nm. To examine the change of surface functionalities after chemical modification, we performed Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (TEM) energy dispersive X-ray spectroscopy (EDX) mapping, and solid-state NMR spectroscopy. The appearance of new peaks at 1,241 and 1,212 cm−1, which were assigned to the C–F stretching vibration53Hayase G. Kanamori K. Hasegawa G. Maeno A. Kaji H. Nakanishi K. A superamphiphobic macroporous silicone monolith with marshmallow-like flexibility.Angew. Chem. Int. Ed. 2013; 52: 10788-10791Crossref PubMed Scopus (115) Google Scholar together with the presence of the C–F (292.2 eV), as well as elements F (F1s at 689.6 eV) and S (S2p at 163.4 eV) signals in the FTIR (Figure S5) and XPS spectra of COF-VF (Figure S6), respectively, indicate the successful incorporation of perfluoroalkyl groups onto COF-V. The EDX mapping via TEM verifies the homogeneously distributed F, N, and S elements throughout COF-VF (Figure S7). To provide additional proof, we employed solid-state NMR analyses. As shown in Figure S8, the 19F magic-angle spinning (MAS) NMR spectrum of COF-VF gave clear F signals with the same chemical shifts as those of 1H,1H,2H,2H-perfluorodecanethiol. In addition, the appearance of a noticeable peak at 26.3 ppm attributed to the alkyl carbon species from the reacted vinyl groups confirms the covalent bond formation between vinyl groups on COF-V and 1H,1H,2H,2H-perfluorodecanethiol. However, the relative intensity of the peak ascribed to the vinyl group did not change obviously in comparison with that in COF-V, suggesting that only a small part of the vinyl groups participated in the reaction (Figure S9). To quantify the degree of post-synthetic modification, we evaluated the content of F species in COF-VF by elemental analysis. The results showed that the weight percentage of F species in COF-VF was 5.2 wt %, which means that about 4% of the vinyl groups were involved in the thiol-ene reaction. To characterize the crystalline structure of COF-VF, we carried out powder X-ray diffraction (PXRD) measurements. COF-VF exhibited an intense peak at 2.8° along with some relatively weak peaks at 4.9°, 5.9°, 7.5°, and 24.9°, which agree well with the pristine pattern of COF-V, thus revealing the retention of crystallinity and structural integrity after the introduction of perfluoroalkyl groups (Figure 2C). N2 sorption isotherms collected at 77 K showed that COF-V and COF-VF exhibited similar adsorption behavior (Figure 2D). The BET surface area of COF-VF was calculated to be as high as 938 m2 g−1, rivaling that of COF-V (1,152 m2 g−1), suggesting that the post-synthetic modification process has little effect on the pore structure of the pristine material and is thereby still accessible for guest molecules (see pore-size distributions of these samples in Figure S10). Thermogravimetric analysis (TGA; Figure S11) conducted in N2 atmosphere revealed that COF-V and COF-VF exhibited very similar curves with a decomposition temperature at around 400°C, indicative of their excellent thermal stability. This was further confirmed by variable temperature PXRD, and both of them retained their crystallinity up to 300°C with decreased crystallinities observed after that (Figures S12 and S13). To investigate the effect of perfluoroalkyl group incorporation on the wettability of the COF material, we measured the water contact angles (CA) of the surface. COF-VF exhibited a static water CA of about 167°, thus revealing a superhydrophobic surface (superhydrophobic materials have a contact angle exceeding 150° for a water droplet; Figure 2B, inset).52Darmanin T. Guittard F. Recent advances in the potential applications of bioinspired superhydrophobic materials.J. Mater. Chem. A. 2014; 2: 16319-16359Crossref Google Scholar By contrast, COF-V and COF-V modified with alkyl groups gave rise to CAs of only 113° and 122°, respectively (Figures 2A [inset] and S14, respectively). Therefore, the incorporation of perfluoroalkyl groups significantly increased the hydrophobicity. However, when oil dropped onto the surface of COF-VF, it was quickly absorbed, and COF-VF displayed a CA close to 0° (Figure S15). To further elucidate the superhydrophobicity and superoleophilicity of COF-VF, we performed vapor adsorption experiments. Water adsorption isotherms revealed that COF-VF was highly hydrophobic with a negligible water adsorption even at P/P0 up to 0.9 (<10 mg g−1), whereas COF-V exhibited a water adsorption capacity of 56 mg g−1 (Figure S16). In contrast, it had a toluene adsorption isotherm that showed a sharp uptake at very low pressure (P/P0 < 0.1) and attained a saturation capacity exceeding 680 mg g−1 at P/P0 = 0.88. To further investigate the impact by introducing fluorine species on the materials' affinity toward H2O and oil, we collected sorption isotherms of COF-V and COF-VF for H2O and toluene and compared them at 298 and 323 K (Figures S17–S19). COF-VF exhibited a higher affinity toward toluene and a lower affinity to H2O in relation to COF-V. These results indicate that the large channels in COF-VF are restricted to water yet permitted to toluene and exhibit superior hydrophobic and oleophilic behaviors, which offer exceptional abilities to overcome the problems associated with the adsorption of harmful volatile organic compounds in humid environments. Given the importance of chemical stability for practical applications, the tolerance of COF-VF under a wide range of conditions was tested. Notably, after 1 week of treatment in 12 M HCl and 14 M NaOH at room temperature, as well as boiling water, COF-VF still retained its crystallinity and porosity (Figures S20 and S21). To further evaluate the chemical shielding effect resulting from superhydrophobicity, we monitored the PXRD patterns of COF-VF exposed to 100% relative humidity under HCl or NH3 atmosphere at room temperature. COF-VF did not show noticeable change in the PXRD patterns, even after aging under the above conditions over 48 hr (Figure S20). In sharp contrast, COF-V could not survive in 2 M HCl or a humid HCl gas atmosphere after suspension and exposure for only 12 hr (Figure S22). These results indicate that hydrophobic modifications to the COF material can appreciably safeguard the crystallinity. We attribute the observed ultrastability of COF-VF toward acid and base to the extreme water-repellent properties of superhydrophobic surfaces, which serve as chemical shields to prevent the permeation of acidic and basic aqueous solutions. By embracing the features of superhydrophobicity and superoleophilicity together with high porosity and chemical stability, COF-VF could be beneficial in mitigating environmental problems caused by the release of harmful organic compounds. However, COF-VF was synthesized as a microcrystalline powder, and therefore its applications in real-world separation could be affected by poor processability and handling.55Kandambeth S. Biswal B.P. Chaudhari H.D. Rout K.C. Kunjattu H.S. Mitra S. Karak S. 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