Letter www.acsami.org
Regioselective Synthesis of Eight-Armed Cyclosiloxane Amphiphile for Functional 2D and 3D Assembly Motifs Huie Zhu,*,† Buket Akkus,† Yu Gao,† Yida Liu,† Shunsuke Yamamoto,† Jun Matsui,‡ Tokuji Miyashita,† and Masaya Mitsuishi*,† †
Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ‡ Department of Material and Biological Chemistry, Yamagata University, 1-4-12 Kojirakawamachi, Yamagata 990-8560, Japan S Supporting Information *
ABSTRACT: A crystalline tetramethylcyclotetrasiloxane (TMCS)-derived amphiphile was regioselectively synthesized with eight peripheral hydrophilic amide groups and hydrophobic dodecyl chains by Pt(0)-catalyzed hydrosilylation and amidation reactions. The as-synthesized materials showed ordered lamellar structure formation in the powder form. It also exhibits superior two-dimensional (2D) monolayer formation properties at the air−water interface with unexpectedly high collapse surface pressure and elastic modulus. The monolayers act as two-dimensional building blocks with finely controllable thickness on a several nanometer scale irrespective of the substrate type and properties. The amphiphile forms nanofibers spontaneously by good−poor solvent strategies, which contributes to porous three-dimensional (3D) structures possessing superhydrophobic surface wettability. KEYWORDS: amphiphile, hydrosilylation, LB film, superhydrophobic, self-assembly yclic siloxanes, e.g., trimer and tetramer [−(SiRR′O)n−], where R and R′ can be hydrogen, halogen, alkyl- or -aryl, and where n = 3 or 4, are generally used to develop inorganic− organic hybrid siloxane oligomers and polymers through a ringopening polymerization for various applications such as antifoaming agents, lubricants, cosmetics, silica precursor, and coatings.1−3 The siloxane hybrid materials generally exhibit unique flexibility, low surface energy, hydrophobic features, and low toxicity. Of those cyclic siloxanes, 1,3,5,7-tetramethylcyclotetrasiloxane (TMCS, [CH3SiHO]4) bearing four functional Si−H groups, offers diverse opportunities to develop widely various inorganic−organic hybrid materials through efficient hydrosilylation reaction at moderate temperature between − Si−H and olefins catalyzed by Karstedt’s catalyst, [Pt(0)-1,3divinyl-1,1,3,3-tetramethyldisiloxane complex].4 The difficulty in controlling the regioselectivity of the hydrosilylation reaction, however, has significantly prevented its application in the development of fine chemicals with novel properties. The reaction system generally contains mixtures of α- and βadditions products, in which the ratio of α:β products can be significantly impacted by the substituents of the olefins because of its steric hindrance and/or electronic effect.5−7 In particular, large π-conjugated and fluoroalkyl substituents generated dominant β-products (95%) over styrene (11%) by steric hindrance.5 The electron-withdrawing effect from substituents such as anhydride, acrylonitrile and acrylate in olefins generally impedes the Pt(0) hydrosilylation. However, the anhydride, acrylonitrile, and acrylate can be further functionalized through
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chemical reaction that is necessary to further design and functionalization toward new materials. To address this problem, we proposed that a methylene group between vinyl and electron-withdrawing substituents would weaken the electronic effect but maintain the steric hindrance effect, thereby leading to high reaction efficiency accompanied by high regioselectivity. To date, most TMCS derivatives, even linear siloxane polymers of high molecular weight, are liquid, amorphous, and too flexible to be handled without irreversible cross-linking. Those limits impede the development of novel siloxane materials and entail numerous difficulties for practical applications. Surmounting these hurdles is important for crystallizing siloxanes, perhaps by introducing amide groups to the chemical structures, thereby leading to increased stiffness. Amphiphiles containing both hydrophobic and hydrophilic parts can assemble at interfaces to form various assemblies having special structures and functionalities.8 Such assemblies might include two-dimensional monolayer formation at the air−water interface and three-dimensional (3D) superhydrophobic surface at the air−solid interface. In consideration of the low surface energy of siloxane, rational design of amphiphilic siloxane-based molecules is promising for producing hybrid mesostructures with a long-range order through control of Received: May 24, 2017 Accepted: August 14, 2017 Published: August 18, 2017 28144
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Figure 1. (a) Synthetic schemes of TMCS-TSA and TMCS-DDA, with photographs of (b) TMCS, (c) TMCS-TSA, and (d) TMCS-DDA.
molecular interaction. However, the TMCS-derived amphiphiles reported to date have important limitations as nanostructure building blocks. For instance, they have liquid states and weak molecular interaction. Monolayers of TMCSderived amphiphiles at the air−water interface generally show low collapse surface pressure of less than 20 mN m−1 and inhomogeneous morphologies.2,9 Moreover, multilayer transfer onto substrates is hindered by weak molecular interaction. Our previous studies of poly(N-dodecylacrylamide) (pDDA) conducted over several decades have examined amphiphilic polymers intensively because of their excellent monolayer formation properties that derive from the hydrogen bonding network among amide groups.10 Abundant functional copolymers created by incorporating DDA units through radical polymerization exhibited amphiphilicity and monolayer stability on a water surface and precise multilayer structures with 1.72 nm per layer.10,11 Consequently, combining a TMCS core with peripheral alkyl-amide side chains is expected to give rise to unprecedented physical properties because of the synergetic effects of the hydrophobic TMCS cores, the strong amide
hydrogen bonding network and the hydrophobic interactions of alkyl chains. No report of the relevant literature describes TMCS amphiphiles with peripheral amide groups and alkyl chains. For this study, an eight-armed cyclic TMCS-derived amphiphile TMCS-DDA was demonstrated with a lowsurface-energy cyclosiloxane core surrounded by eight hydrophilic amide groups for hydrogen bonding sites and equivalent hydrophobic dodecyl chains. The TMCS-DDA amphiphile was synthesized by a two-step process including regioselective hydrosilylation reaction and amidation reaction starting from a commercially available 1, 3, 5, 7-tetramethylcyclotetrasiloxane (TMCS) reactant. By virtue of the special molecular structure and configuration of TMCS-DDA, the hydrophobic TMCS cores were anchored firmly at the air−water interface because of the synergetic effects of the hydrogen bonding network among amide groups and hydrophobic interactions of vertically aligned dodecyl chains, capable of forming stable monolayer assemblies as two-dimensional (2D) nanobuilding blocks. Not limited to monolayer assemblies at the air−water interface, the 28145
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ACS Applied Materials & Interfaces cyclic amphiphile was also capable of self-assembly through a simple good−poor solvent strategy into three-dimensional (3D) porous structures possessing superhydrophobic properties. Through two-step reactions, eight-armed dodecylacrylamidetetramethylcyclotetrasiloxane (TMCS-DDA) was synthesized from TMCS with a yield of 64% (Figure 1a). Figure 1b−d depicts photographs of the reactant TMCS, the intermediate product TMCS-TSA, and the final product TMCS-DDA, respectively, with accompanying physical state and viscosity changes attributable to the reaction. The starting chemical TMCS is colorless and transparent liquid. After the first step reaction, the obtained TMCS-TSA becomes a honeylike viscous liquid. The final product becomes a lily white solid after the amidation reaction. This solid product engenders easy operation and handling in comparison with other viscous cyclosiloxane compounds and polymers.2,3 Moreover, it has good solubility in chloroform and toluene. Details related to synthesis are presented in the Supporting Information. The nuclear magnetic resonance (1H NMR) spectra of TMCS, TMCS-TSA, and TMCS-DDA are shown in Figure 2, which presents their chemical structures through the exact assignment
of each resonance for the protons. We tracked the resonance evolution of −Si−H and −Si−CH3 with reaction time in the proton NMR of the reaction mixture for TMCS-TSA (Figure S1a, b). Results show that the resonance for −Si−H at 4.7 ppm reduced gradually with reaction time and eventually disappeared, which is indicative of the successful and efficient substitution (100%) of reactive anhydride groups onto TMCS cores (Figure 2b). The simultaneous interplay of the throughbond shielding effect ruled by the characteristics of the peripheral chains on each silicon atom and the through-space shielding effect ruled by the spatial orientation of neighbors governs the methyl proton shift.12 The resonance for −Si−CH3 turned from a multiplet into a singlet at 0.096 ppm during reaction, which is also indicative of the successful substitution of −Si−H with the vinyl group, giving rise to a change in the chemical environment for the through-bond shielding effect in TMCS-TSA. The most well-known reaction mechanism of platinum(0)-catalyzed hydrosilylation is the Chalk−Harrod mechanism,13,14 which includes three steps: (1) Si−H oxidative addition to Pt, (2) alkene insertion into the Pt−hydride bond, and (3) Si−C reductive elimination. It is well-known that the hydrosilylation reaction consists of α- and β-additions during the alkene insertion step 2 in the Chalk−Harrod mechanism, thereby leading to formation of regioisomers.5,15 When vinyl groups having an electron-withdrawing group are used, silicon hydrides prefer α addition to match the polarization of the Siδ+Hδ‑ bond with the olefins.7 It is noteworthy that the TMCSTSA is a regioselective product possessing 98.2% β-structure determined by the integral ratio of methylene resonance (b, βstructure) to methyl resonance (g′, α-structure) at around 1.0 ppm in the TMCS-TSA 1H NMR spectra (Figures 2b).3 These results indicate that the electron-withdrawing effect of anhydride groups was shielded during hydrosilylation to restrict the α addition, while its steric hindrance effect was maintained to generate β addition from the terminal of vinyl group. The chemical structures of the intermediate TMCS-TSA and the final product TMCS-DDA were also verified using 13C NMR, 29 Si NMR, matrix-assisted laser desorption ionization/time-offlight mass spectroscopy (MALDI-TOF/MS), elemental analysis, and Fourier transform infrared spectroscopy (FT-IR) (Figures S1c−f, S2a, b, and S3 and Table S1). The X-ray diffraction (XRD) patterns in Figure 3 of TMCS-DDA powder and thin films were applied for structure confirmation. Intense
Figure 2. 1H NMR spectra of (a) TMCS, (b) TMCS-TSA, and (c) TMCS-DDA. The pink numerical digits under the spectra are the integrals of each resonance.
Figure 3. XRD patterns of TMCS-DDA (a) powder, (b) casting film from CHCl3 solution, and (c) self-assembly film from mixing solvents; and (d) schematic illustration of TMCS-DDA lamellar structure. 28146
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Figure 4. (a) π−A isotherms (top) and the corresponding ε−A isotherms (bottom) of TMCS-DDA monolayers at different temperatures. (b) BAM images of TMCS-DDA monolayer at 20 °C at different surface pressures: 10, 30, and 50 mN m−1. (c) AFM images of TMCS-DDA monolayer, 20layer, and the casting films on silicon substrates. (d) Schematic illustration of molecular packing in TMCS-DDA LB film and a photograph of a 180layer TMCS-DDA LB film on a silicon substrate (lower left).
Bragg peaks near 2.8 nm−1 indicate a material with long-rangeorder lamellar structure, especially for the thin film (Figure 3d), which corresponds to a d-spacing value of 2.2 nm.16 This value shows close agreement with the summed chain length of DDA (1.72 nm)10 and n-propyl (0.597 nm).17 Peaks at 5.74 and 8.60 nm−1 might be associated with second-order and third-order lamellar structures. Other peaks might originate from in-plane ordering.16 Thermal gravimetric analysis (TGA) (Figure S4a) results demonstrate that the TMCS-DDA molecules start decomposing at 230 °C. The constant residual weight (7.60%) after 630 °C in the TGA curve that is almost identical to the calculated value (7.97%) for SiO contents based on the molecular structure. Differential scanning calorimetry (DSC) (Figures S4b, c) spectra demonstrate with a pair of sharp peaks corresponding to the melting point (Tm = 85.5 °C) and crystallization point (Tc = 79.2 °C). The result verifies the crystalline characteristics of TMCS-DDA. Surface pressure (π)−area (A) isotherms from 10 to 30 °C are shown in Figure 4a, from which a series of sharply rising curves manifest the formation of stable and compact Langmuir monolayers at the air−water interface.10 The collapse surface pressures exceeding 50 mN m−1 are superior to those of all reported cyclosiloxane-based amphiphiles monolayers and are equal to that of long alkyl acid, e.g., stearic acid: 55 mN m−1.2,18 Compression elasticity (ε)−A isotherms in Figure 4a were derived from π−A isotherm data based on equation S1 to obtain the monolayer elastic behavior.19,20 In the ε − A curves, each Langmuir monolayer shows a peak value of ε*, as presented in Table S2. A monolayer with a high ε* value is rigid and difficult to deform.20 The ε* value of TMCS-DDA exceeds 120.0 mN m−1, which was ascribed to the excellent twodimensional hydrogen bonding network formed in its monolayer at the air−water interface.20 In situ investigation of the monolayer at 20 °C by Brewster angle microscopy (BAM) is presented in Figure 4b at different compression states. The homogeneous surface is indicative of its good film stability, even near the collapsed state, 50 mN m−1. From top to bottom, panels of Figure 4c portrays atomic force microscopy (AFM) images of TMCS-DDA monolayer, multilayers, and casting film from chloroform solution. Multilayer formation was
achieved by transferring the monolayer to substrates such as silicon, glass, and PET. Quartz crystal microbalance (QCM) measurements showed a normalized frequency change for each deposition cycle indicative of regular monolayer transfer (Figure S5a). The bilayer thickness was ascertained using Bragg’s law as 3.90 nm according to the XRD pattern (Figure 4d and Figure S6). On the basis of these results, we inferred that the TMCS-DDA molecules at the air−water interface take conformation with the amide groups, strongly anchoring at the water surface, with the eight surrounding alkyl chains standing perpendicularly to the water surface because of the hydrophobic effect, and the hydrophobic TMCS cores being lifted up from the water surface without effects on the monolayer thickness (Figure S5b). Therefore, the monolayer thickness is equal to DDA chain length as 1.72 nm at the water surface.10 However, after transfer of monolayer to the substrates, slight relaxation from the propyl linkers gave rise to increased monolayer thickness because of the high steric hindranceinduced space reorientation of the alkyl chains. In view of the discussion presented above, we calculated the density as 1.11 g cm−3 for films on substrates using the occupied unit area of each TMCS-DDA molecule at the air−water interface (1.7 nm2 at 30 mN m−1 for transfer, Figure 4a), the monolayer thickness (1.95 nm), the molecular weight (2211.84 g mol−1), and Avogadro’s constant (6.02 × 1023 mol−1). Results show that the TMCS-DDA LB film density is even more dense than those of most organosilicon polymers and crystalline polymers, e.g., polydimethylsiloxane (PDMS, 0.965 g cm−3),21 polyethylene (0.93 g cm−3), polypropylene (0.91 g cm−3), and comparable to those of crystalline polyamides such as nylon-6,6 (1.14 g cm−3).22 Results suggest that the TMCS-DDA monolayers are densely packed because of multiple hydrogen bonded sites originating from the chemical structures and the external compression benefited by its higher collapse surface pressure. The monolayer shows a root-mean surface roughness value 2.1 nm (a measured area of 10 × 10 μm2) and a quite smooth surface (Figure 4c). With the layer number increase, it is noteworthy that the surface roughness remained constant as the monolayer surface, which also demonstrated the highly regular transfer of the monolayer onto substrates. A 180-layer film on a 28147
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rule that superhydrophobicity is dependent on both surface chemistry (equilibrium water contact angle of flat surface >90°) and surface topography (high surface roughness).26 The flat TMCS-DDA casting and LB films show respective water contact angles of 95.5 and 99.6°, each exceeding 90°, as shown in the inset images of Figure 4c, thereby satisfying the first prerequisite condition for a superhydrophobic surface. We then investigated self-assembly behaviors of amphiphilic TMCSDDA through a good−poor solvent strategy in a mixed organic solvent system of chloroform (good solvent)/THF (poor solvent)/AK-225 (poor solvent, a mixture of CF3CF2CHCl2 and CClF2CF2CHClF) following a procedure explained in the Supporting Information and Figure S9. It is particularly interesting that scanning electron microscopy (SEM) images of the TMCS-DDA self-assembly (SA) film show a threedimensional (3D) fiber network with considerable porosity (Figure 5a). The 100 ± 20 nm fibers are regular, mutually
silicon substrate is shown in Figure 4d, from which a beautiful and homogeneous interference color macroscopically verifies the uniform surface of the TMCS-DDA LB film. In contrast, rough surface morphologies were visible in the films cast on flat silicon substrates from either chloroform (Figure 4c bottom) or toluene (Figure S7). A flexible polyethylene terephthalate (PET) substrate was also suitable for TMCS-DDA monolayer deposition (Figure S5c). The critical surface energies (γc) of the TMCS-DDA casting and LB films were determined, respectively, as 23.3 and 20.8 mN m−1 using Zisman method23 with a homologous series of n-alkane liquids of hexane, octane, decane, dodecane, and tetradecane with respective surface tensions γl = 18.4, 21.8, 23.9, 25.4, and 26.7 mN m−1 (Figure S8a).24 The result shows more closely packed − CH3 groups in the LB film surface than in the casting film surface, as well as a standing-up alkane side chain. For solid films on substrates, the modes of amide I (C O stretching, νCO) and amide II in FT-IR spectra are useful to identify the hydrogen bonding interaction. Generally, the amide I peak appears in four regions at 1676, 1660, 1646, or 1638 cm−1. The 1676 cm−1 band is assigned to a free amide I mode. The 1660, 1646, and 1638 cm−1 bands are assigned to the hydrogen-bonded amide I modes.25 Therefore, hydrogen bonding interaction exists in both the casting film (1633 cm−1) and the LB film (1643 cm−1) (Figure S3). Particularly the respective differences Δν (amide I-amide II) of 87 and 92 cm−1 for casting and LB films show perfect agreement with the presence of H-bonded amide groups.16 However, peak red shifts of amide I and amide II in the LB film were detected in comparison with the casting film, indicating slightly weaker hydrogen bonding in the LB film and longer N−H to CO distance in the former than that of the latter.25 The result might originate from steric hindrance effects among stand-up alkyl chains at the air−water interface, which engenders a larger space between molecules. The respective vectors of incident light at the transmission and reflection−absorption (RAS) mode are parallel and vertical to the substrate surface. Extinction of the peak at 3293 cm−1 (νNH) for RAS mode (Figure S3a) shows the hydrogen bonding network taking a parallel orientation to the substrate surface. However, the casting film from chloroform and a self-assembly film from mixing solvents showed no difference for either mode (Figures S3b, c), which indicates random orientation of hydrogen bonding interactions. Because of its controllable thickness, regular layer structures, and smooth surface, the TMCS-DDA LB film will be studied in our future work for ultrathin dielectric layer formation.11,21 Superhydrophobicity is characterized by the repellence of water from a surface with contact angles of water (θ) greater than 150°.26,27 Superhydrophobic surfaces have received attention for many applications such as oil−water separation and self-cleaning. Simultaneous control of surface chemistry and topography can form extremely water-repellent surfaces. To date, most fabrications of superhydrophobic surface are based on fluorinated materials or complicated processes such as photolithography to produce microstructures and nanostructures. Amphiphiles are well-developed for unique selfassemblies through spontaneous formation of microstructures and nanostructures. The structuring can be driven by hydrogen bonding interactions, hydrophobic interactions, or other interactions. However, exploitations of fluorine-free cyclosiloxane amphiphiles are seldom reported for superhydrophobic applications. Apparently, most researchers are following the
Figure 5. (a) SEM image of TMCS-DDA SA film with an inset photograph of a water droplet on the surface and (b) schematic image of the interaction region of hydrophobic and hydrogen-bonding interaction in the TMCS-DDA molecule.
intersecting to form hierarchical nanostructures. The water contact angle portrayed in the inset image is 163°, which indicates superhydrophobicity. However, contact angles of nalkane solvents such as hexane and dodecane cannot be detected on the TMCS-DDA SA film surface because of the rapid adsorption. The critical surface tension of the porous surface was found to be 30.6 mN m−1 using Zisman method (Figure S8b).23 The results demonstrate that oil−water separation is achievable for poor solvents (Table S3) of TMCS-DDA with surface tension lower than 30 mN m−1 using the porous SA film. Figure 5b presents a schematic illustration of existing interactions in the TMCS-DDA materials. Strong 28148
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versity. We thank Prof. Takahiro Gunji, Tokyo University of Science, and Prof. Yoshiro Kaneko, Kagoshima University, for their fruitful discussion. Furthermore, we are grateful to Prof. Hidetoshi Oikawa and Assistant Prof. Tsunenobu Onodera, IMRAM, Tohoku University, for the use of an X-ray diffractometer.
hydrogen bonding interaction between amide groups bridges the hydrophobic siloxane core and the hydrophobic alkyl chains. The hydrogen bonded amide groups would pack in the center of the nanofibers leaving the alkyl chains exposed to the organic solvent environment, thereby leading to nanostructure formation.28 Because of the exposed alkyl chains and porous nanostructures, the water droplet on the TMCS-DDA superhydrophobic surface is in a Cassie−Baxter state,29 where it is suspended on top of the solid TMCS-DDA nanofibers and on air trapped inside the pores. Theoretically, we calculated the composition of solid fraction and air pocket fraction respectively as 4.9 and 95.1% using the Cassie−Baxter formula (see equation S2) in which the water contact angle values of the flat casting film (95.5°) and the SA porous film (163°) were used. The result verified the SA films possess highly porous structure caused by the networked nanofiber structures, which will be studied for oil−water separation and gas separation in our future work. In summary, we regioselectively synthesized a branched organic−inorganic hybrid cyclosiloxane amphiphile, which possesses a hydrophobic cyclosiloxane core, hydrophilic hydrogen bonding anchors, and hydrophobic peripheral alkyl chains. Because of this special chemical structure, the versatile amphiphilic molecules allowed us not only to achieve stable and flat monolayer building blocks on a nanometer scale, but also to manipulate the self-assembly porous nanostructures for a superhydrophobic surface. Consequently, results of this study are expected to shed light on additional design of cyclosiloxanebased amphiphiles toward unprecedented siloxane nanostructure formation and applications as ultrafine dielectric coating with sub-2 nm precision and oil−water separation applications.
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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.7b07331. Materials, experimental details, and supplemental figures (PDF)
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REFERENCES
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Masaya Mitsuishi: 0000-0002-7069-9860 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work was partially supported by Grants-in-Aid for Young Scientists (B) (16K17953) and Scientific Research (B) (16H04197) from the Japan Society for the Promotion of Science (JSPS) and for research in Innovative Areas (15H00719) from the Ministry of Education, Culture, Sports, Science, and Technology in Japan (MEXT). The work was also supported by the Cooperative Research Program “Network Joint Research Center for Materials and Devices: Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials (MEXT), Tohoku University Center for Gender Equality Promotion (TUMUG), and Polymer • Hybrid Materials Research (PHyM) Center Project, Tohoku Uni28149
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