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Hongbin Zhang, a,b Rui Hao,a John K. Jackson,c Mu Chiao*b and Haifeng Yu*a Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/
Ultrathin free-standing Janus films were fabricated at airwater interfaces using azopyridine derivatives and poly(acrylic acid) via multi-level self-assembly on molecular and microscopic scales, which showed distinct asymmetric water wetting abilities on different surfaces. Janus materials which possess asymmetric architecture based on two incompatible sides of different chemistry or morphology, have emerged as a new division of functional materials over past few years1 and attracted significant attention rapidly due to their fascinating properties and diverse potential applications in the fields such as interface stabilizers,2 biological sensors,3 optical probes,4 antireflection coatings5 and drug delivery systems.6 Such anisotropic materials can exist in different forms, including Janus particles7 (spheres, cylinders and discs), fibers8 and films.9 In recent years, major progress has been achieved in the preparation of nano/micro-sized Janus materials/particles by self-assembly, modification with mask protection and phase separation.10 However, producing Janus materials with larger scale two-dimensional (2D) structure, especially Janus ultrathin films, remains a challenge and has not been developed sufficiently. Asymmetric surface modification on a 2D platform can be a practicable way to obtain Janus films. Inspired by the Janus feature of lotus leaves, Cheng et al. fabricated a Janus film with poly(dimethylsiloxane) and epoxy resin using fresh natural lotus leaves as bio-template.11 Janus films manufactured using such methods are usually thick and may need a couple of steps. On the other hand, being different from nano-Janus particles and Janus films with large thickness, Janus ultrathin films could be directly produced at interfaces between two distinct phases, for instance, at liquidliquid interfaces. Kulkarni et al. grew Janus porous silica films by hydrolysis and condensation reactions between methyltrimethoxysilane and water at heptane-water interfaces.12 The films showed anisotropic water wetting ability on their two surfaces. In a similar study by Rao’s group, various nanocrystalline Janus films of metal oxides, metal chalcogenides and gold were successfully prepared at toluene-water interfaces.13 Through electropolymerization at the interface of dichloromethane and water, Song et al. fabricated a Janus polypyrrole film with totally different morphologies on the surfaces facing different media, which presents
This journal is © The Royal Society of Chemistry 2012
isotropic and anisotropic underwater superoleophobicity ability.14 Unlike the liquid-liquid interface approach, only a few reports have focused on air-liquid interface fabrication of Janus films. Fujii et al. proposed a synthetic method with potential to produce bulk quantities of films with softness, asymmetry and regularity.15 They coated two-dimensional polystyrene particle arrays at the air-water interface by polymerizing pyrrole from the aqueous phase to form the soft Janus colloidal crystal film. Furthermore, self-assembly is an elegant and powerful way to fabricate materials with novel structure and properties, which is also an essential feature of many strategies to generate Janus structures on the nano/micro scale. Using prepared Janus nanoparticles, Sashuk et al. constructed Janus monolayers at two-phase interfaces including air-water interfaces17 and liquid-liquid interfaces.18 However, the only report on macroscopic Janus film from self-assembled nonJanus materials reported to date (to our knowledge) is from Ou and coworkers.16 They reported a silver reduced graphene oxide Janus film formed through an evaporation-induced self-assembly process in a Teflon dish, during which the silver (air side) and graphene oxide (solution side) were separated from their aqueous solution. Herein, we present a manufacturing process for macroscopic freestanding Janus ultrathin films obtained at air-water interfaces. The film formation involves a multi-level self-assembly process, including molecular self-assembly and layer-layer self-assembly. This is coupled with selective exposure of different exterior compositions under different environments, resulting in unique surface morphologies of the films and distinct water wetting ability on their two surfaces. The film was composed of two kinds of molecular building blocks. One is a synthetic azopyridine derivative, p-(dodecyloxy)pridylazobenzene (DPAB), which contains one linear 12 carbon alkyl chain. Detailed information of the synthesis and characterization of DPAB is described in the ESI.† Azopyridine derivatives have been recently explored for the fabrication of novel functional materials through self-assembly arising from their π–π stacking interactions among the azopyridine chromophores and the potential ability to conjugate with other materials via hydrogen bonding (H-bond) or halogen bonding.19,20 The other material is poly(acrylic acid) (PAA) with molecular weight of 2000, which may combine with azopyridines by forming hydrogen bonds between pyridyl groups and carboxylic acid groups.21 We anticipated that the amphiphilic complex of DPAB-PAA might exhibit different
J. Name., 2012, 00, 1-‐3 | 1
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Cite this: DOI: 10.1039/x0xx00000x
Janus Ultrathin Film from Multi-‐Level Self-‐Assembly at Air-‐ Water Interfaces
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assembly behaviour at the interface of water and air and probably form an asymmetric film.
Fig. 1 Chemical structures of materials and fabrication process of the Janus film (The scale bars represent 5 mm).
Fig. 2 (a) The Janus film on air-water interface bearing water droplets on its air-side surface. (b) Water contact angles on the Janus film surfaces of both sides and their average values. The error bars represent mean ± standard deviation (n = 5). Figure 1 shows the fabrication process for the ultrathin Janus film (the thickness is about 12 to 25 µm). Briefly, DPAB (144.0 mg) and PAA (14.4 mg) were dissolved in tetrahydrofuran (THF, 500 µL) with a pyridyl to carboxylic acid ratio of approximately 2∶1. After mixing thoroughly, 10 µL solution was pipetted and dropped onto the surface of deionized water. When the solvent was gone by evaporation and diffusion into water, the ultrathin Janus films with thicknesses of approximately 5 to 8 µm were obtained quickly on the air-water interface. Solutions composed of mixing ratios of about 1∶1 and 1∶2, and pure DPAB solution were also applied for comparison, but the film quality was not good and sometimes the films would not even form. These observations implied that the component ratio of DPAB and PAA which decides the solution viscosity and finally the movability of molecules for self-assembly plays an important role in the film formation. The unique asymmetric wetting property of the composite Janus film on air-side and water-side surfaces is shown in Fig. 2. Fig. 2a shows the water droplets supported by the Janus film formed at the air-water interface. The average values of water contact angles on the Janus film surfaces of the air side and water side were measured to be 143° and 42°, respectively (Fig. 2b). These large differences between hydrophobic and hydrophilic asymmetric surfaces may arise from the combined effects of chemical composition and surface morphology of the Janus films.
Journal Name The surface morphology of the resulting film was initially observed under an optical microscope. The obtained images ViewS3 Article Online including polarizing optical images are shown in Fig. (ESI †). DOI: 10.1039/C4CC06798C Basically, the surface of the film was uniform but not smooth on microscale (Fig. S3 a&c, ESI†), and there were tiny birefringent domains homo-dispersed in the film (Fig. S3 b&d, ESI†), suggesting a part of the DPAB molecules or the complex were self-assembled into small ordered structures during the film formation. A submicrostructure of the film surface could be distinguished at higher magnification. Fig. 3 shows the images taken by scanning electron microscope (SEM). Images (a) to (c) are from the surface of the air side and (d) to (f) are from that of the water side. Interestingly, both surfaces consisted of interlaced flakes with lamellar structures. However, the flakes on air side were relatively distorted and smaller than those on water air, and partly embedded in a transition layer which was mostly composed of excess DPAB in the film. The different morphologies may be ascribed to the different environmental conditions that the surface of film faced during manufacture and the different mechanisms of solvent loss (evaporation to air or diffusion to water). Regardless of the small morphological differences, the flakes of lamellar structure indicated that a similar multi-level self-assembly occurred with no dependence on growing environment (air or water). Section view SEM images of the Janus film were also obtained (Fig. S4, ESI†), showing that two rough surfaces are linked by a relatively compact transition layer with thickness of 10-15 µm. To further examine the self-assembled lamellar structure, X-ray diffraction (XRD) measurements were performed on the film. Fig. 4 shows the XRD profiles of DPAB and the composite film. In the wide-angle region (Fig. 4a), the film shows similar diffraction patterns (with smaller intensity peaks) to DPAB, indicating that any excess free DPAB which did not form H-bonds with PAA precipitated in the film partially in the crystalline form similar to the starting DPAB material.18 The sharp peaks at 2θ ≈ 1.09° (d = 8.09 nm) and wide peaks 2θ ≈ 0.46° (d = 19.11 nm) observed in the small-angle region (Fig. 4b) imply that the composite film contained the lamellar structure formed by DPAB itself. However, a broad diffraction at 2θ of 0.55°-0.62° appeared in the composite film after deduction of the effect of the excess DPAB (insert of Fig. 4b). The presence of this peak may indicate that, as well as the lamella of DPAB with low spacing, another lamellar structure with a wider spacing existed in the film. The widest spacing of the lamellar structure in the film is about 62.21 nm which is probably from the flakes observed by SEM in Fig. 3.
Fig. 4 XRD spectra of DPAB and the composite Janus film: (a) spectra in the wide-angle region, (b) spectra in the small-angle region.
Fig. 3 SEM images of the Janus film surfaces: (a-c) the surface on air-side, (d-f) the surface on water-side.
2 | J. Name., 2012, 00, 1-‐3
We considered that the flakes on the Janus film surfaces were formed by the self-assembly of H-bonded complex DPAB-PAA. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) was employed to detect the chemical composition on surfaces of the Janus film (see Fig. S5, ESI†). Both sides of the Janus film showed similar FTIR spectra with the characteristic absorbed peaks of DPAB and PAA, indicating PAA was involved in This journal is © The Royal Society of Chemistry 2012
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Journal Name the construction of flakes on both surfaces of the Janus films. Moreover, the peak of PAA centred at 2944 cm-1 which represents the -OH of the carboxylic acid disappeared in the film and a new peak appeared at 1943 cm-1 on both surfaces. These variations and a noted shift of the C=O peak of PAA from 1697 cm-1 to 1715 cm-1 after the film was formed imply the formation of H bonds (-O-H…N) between the carboxylic acid of PAA and the pyridyl group of DPAB.21,22
COMMUNICATION tends to be exposed on aqueous side of the film while DPAB on airside, which finally determines the unique asymmetric wetting Article Online property of the Janus film. The fabrication method byView self-assembly 10.1039/C4CC06798C at air-water interface is probably scalable toDOI: produce other ultrathin free-standing Janus films on macroscopic scale. The result also reveals a promising way to fabricate multifunctional materials with simple building blocks under a pre-designed environment.
Notes and references a
Department of Materials Science and Engineering, College of Peking
University,
P.
R.
China.
Email:
[email protected] b
Department of Mechanical Engineering, University of British Columbia,
Canada. Email: [email protected] c
Department of Pharmaceutical Sciences, University of British Columbia,
Canada † Electronic Supplementary Information (ESI) available: The synthesis and characterization of DPAB, the polarizing optical images and ATRFTIR spectra of the Janus film. See DOI: 10.1039/c000000x/ 1
(a) J. Du and R. K. O'Reilly, Chem. Soc. Rev., 2011, 40, 2402; (b) S. Jiang, Q. Chen, M. Tripathy, E. Luijten, K. S. Schweizer and S. Granick, Adv. Mater., 2010, 22, 1060; (c) F. Wurm and A. F. M. Kilbinger, Angew. Chem. Int. Ed., 2009, 48, 8412; (d) C.
Fig. 5 Schematic illustration of the multi-level self-assembly for the Janus film formation. Based on these results, we propose a possible mechanism (Fig. 5) of the self-assembly Janus films described in this study. The film formation may include two kinds of self-assembly processes. One is the self-assembly at the molecular level (molecular self-assembly) in which DPAB and DPAB-PAA complexes assemble separately or together through π–π stacking interactions into nano-layers. The molecular spacing of the layer is relatively small, corresponding to the peaks of larger angles in XRD spectra (Fig. 4b). The other is layer-layer self-assembly at the micro-level where the formed layers further stack together into the lamellar structure (the flakes seen in the SEM images) with large layer spacing which correspond to the broad peaks observed in the low angle X-ray determinations. Importantly, the asymmetric wetting property of the layers and lamellar structure of the Janus film result from the environment of self-assembly. On the side of film contacting air, the hydrophobic alkyl chains of DPAB tend to be exposed externally. While on the side contacting water, the exposed part is hydrophilic PAA. These lamella structural flakes may be fixed on the film surfaces by interlacing together and being partly embedded in the transition layer which may be composed of excess DPAB. The surface roughness enhanced by the interlaced flakes further amplifies the distinction in water wetting ability between two surfaces of the film.
Conclusions In summary, an ultrathin free-standing Janus film was successfully fabricated with hydrophobic DPAB and hydrophilic PAA at air-water interfaces by a unique multi-level self-assembly process. The resulting film shows distinct hydrophilic (water-side) and hydrophobic (air-side) surfaces on opposite sides of the films which are composed of interlaced layer-layer self-assembled flakes. Interestingly, the layers for flakes may be formed by molecular selfassembly and the exterior compositions of the flakes depend on the property of the two environments the surfaces were exposed to. PAA
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Kaewsaneha, P. Tangboriboonrat, D. Polpanich, M. Eissa and A. Elaissari, ACS Appl. Mater. Interfaces, 2013, 5, 1857. 2
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Engineering,
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J. Song, H. Liu, M. Wan, Y. Zhu and L. Jiang, J. Mater. Chem. A, 2013, 1, 1740.
15 S. Fujii, M. Kappl, H. J. Butt, T. Sugimoto and Y. Nakamura, Angew. Chem. Int. Ed., 2012, 124, 9947. 16 E. Ou, X. Zhang, Z. Chen, Y. Zhan, Y. Du, G. Zhang, Y. Xiang, Y. Xiong and W. Xu, Chem. Eur. J., 2011, 17, 8789. 17 V. Sashuka, R. Hołysta, T. Wojciechowskib and M. Fiałkowski, J.
Fiałkowski, Chem. Eur. J., 2012, 18, 2235; (b) V. Sashuka, K. Winter, A. Żywociński, T. Wojciechowski, E. Górecka and M
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Colloid Interf. Sci., 2012, 375, 180. 18 (a) V. Sashuka, R. Hołysta, T. Wojciechowskib, E. Górecka and M.
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4 | J. Name., 2012, 00, 1-‐3
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ChemComm Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: H. Zhang, R. Hao, J. K. Jackson, M. Chiao and H. Yu, Chem. Commun., 2014, DOI: 10.1039/C4CC06798C.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Hongbin Zhang, a,b Rui Hao,a John K. Jackson,c Mu Chiao*b and Haifeng Yu*a Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/
Ultrathin free-standing Janus films were fabricated at airwater interfaces using azopyridine derivatives and poly(acrylic acid) via multi-level self-assembly on molecular and microscopic scales, which showed distinct asymmetric water wetting abilities on different surfaces. Janus materials which possess asymmetric architecture based on two incompatible sides of different chemistry or morphology, have emerged as a new division of functional materials over past few years1 and attracted significant attention rapidly due to their fascinating properties and diverse potential applications in the fields such as interface stabilizers,2 biological sensors,3 optical probes,4 antireflection coatings5 and drug delivery systems.6 Such anisotropic materials can exist in different forms, including Janus particles7 (spheres, cylinders and discs), fibers8 and films.9 In recent years, major progress has been achieved in the preparation of nano/micro-sized Janus materials/particles by self-assembly, modification with mask protection and phase separation.10 However, producing Janus materials with larger scale two-dimensional (2D) structure, especially Janus ultrathin films, remains a challenge and has not been developed sufficiently. Asymmetric surface modification on a 2D platform can be a practicable way to obtain Janus films. Inspired by the Janus feature of lotus leaves, Cheng et al. fabricated a Janus film with poly(dimethylsiloxane) and epoxy resin using fresh natural lotus leaves as bio-template.11 Janus films manufactured using such methods are usually thick and may need a couple of steps. On the other hand, being different from nano-Janus particles and Janus films with large thickness, Janus ultrathin films could be directly produced at interfaces between two distinct phases, for instance, at liquidliquid interfaces. Kulkarni et al. grew Janus porous silica films by hydrolysis and condensation reactions between methyltrimethoxysilane and water at heptane-water interfaces.12 The films showed anisotropic water wetting ability on their two surfaces. In a similar study by Rao’s group, various nanocrystalline Janus films of metal oxides, metal chalcogenides and gold were successfully prepared at toluene-water interfaces.13 Through electropolymerization at the interface of dichloromethane and water, Song et al. fabricated a Janus polypyrrole film with totally different morphologies on the surfaces facing different media, which presents
This journal is © The Royal Society of Chemistry 2012
isotropic and anisotropic underwater superoleophobicity ability.14 Unlike the liquid-liquid interface approach, only a few reports have focused on air-liquid interface fabrication of Janus films. Fujii et al. proposed a synthetic method with potential to produce bulk quantities of films with softness, asymmetry and regularity.15 They coated two-dimensional polystyrene particle arrays at the air-water interface by polymerizing pyrrole from the aqueous phase to form the soft Janus colloidal crystal film. Furthermore, self-assembly is an elegant and powerful way to fabricate materials with novel structure and properties, which is also an essential feature of many strategies to generate Janus structures on the nano/micro scale. Using prepared Janus nanoparticles, Sashuk et al. constructed Janus monolayers at two-phase interfaces including air-water interfaces17 and liquid-liquid interfaces.18 However, the only report on macroscopic Janus film from self-assembled nonJanus materials reported to date (to our knowledge) is from Ou and coworkers.16 They reported a silver reduced graphene oxide Janus film formed through an evaporation-induced self-assembly process in a Teflon dish, during which the silver (air side) and graphene oxide (solution side) were separated from their aqueous solution. Herein, we present a manufacturing process for macroscopic freestanding Janus ultrathin films obtained at air-water interfaces. The film formation involves a multi-level self-assembly process, including molecular self-assembly and layer-layer self-assembly. This is coupled with selective exposure of different exterior compositions under different environments, resulting in unique surface morphologies of the films and distinct water wetting ability on their two surfaces. The film was composed of two kinds of molecular building blocks. One is a synthetic azopyridine derivative, p-(dodecyloxy)pridylazobenzene (DPAB), which contains one linear 12 carbon alkyl chain. Detailed information of the synthesis and characterization of DPAB is described in the ESI.† Azopyridine derivatives have been recently explored for the fabrication of novel functional materials through self-assembly arising from their π–π stacking interactions among the azopyridine chromophores and the potential ability to conjugate with other materials via hydrogen bonding (H-bond) or halogen bonding.19,20 The other material is poly(acrylic acid) (PAA) with molecular weight of 2000, which may combine with azopyridines by forming hydrogen bonds between pyridyl groups and carboxylic acid groups.21 We anticipated that the amphiphilic complex of DPAB-PAA might exhibit different
J. Name., 2012, 00, 1-‐3 | 1
ChemComm Accepted Manuscript
Published on 01 October 2014. Downloaded by The University of Melbourne Libraries on 11/10/2014 09:57:28.
Cite this: DOI: 10.1039/x0xx00000x
Janus Ultrathin Film from Multi-‐Level Self-‐Assembly at Air-‐ Water Interfaces
ChemComm
Published on 01 October 2014. Downloaded by The University of Melbourne Libraries on 11/10/2014 09:57:28.
assembly behaviour at the interface of water and air and probably form an asymmetric film.
Fig. 1 Chemical structures of materials and fabrication process of the Janus film (The scale bars represent 5 mm).
Fig. 2 (a) The Janus film on air-water interface bearing water droplets on its air-side surface. (b) Water contact angles on the Janus film surfaces of both sides and their average values. The error bars represent mean ± standard deviation (n = 5). Figure 1 shows the fabrication process for the ultrathin Janus film (the thickness is about 12 to 25 µm). Briefly, DPAB (144.0 mg) and PAA (14.4 mg) were dissolved in tetrahydrofuran (THF, 500 µL) with a pyridyl to carboxylic acid ratio of approximately 2∶1. After mixing thoroughly, 10 µL solution was pipetted and dropped onto the surface of deionized water. When the solvent was gone by evaporation and diffusion into water, the ultrathin Janus films with thicknesses of approximately 5 to 8 µm were obtained quickly on the air-water interface. Solutions composed of mixing ratios of about 1∶1 and 1∶2, and pure DPAB solution were also applied for comparison, but the film quality was not good and sometimes the films would not even form. These observations implied that the component ratio of DPAB and PAA which decides the solution viscosity and finally the movability of molecules for self-assembly plays an important role in the film formation. The unique asymmetric wetting property of the composite Janus film on air-side and water-side surfaces is shown in Fig. 2. Fig. 2a shows the water droplets supported by the Janus film formed at the air-water interface. The average values of water contact angles on the Janus film surfaces of the air side and water side were measured to be 143° and 42°, respectively (Fig. 2b). These large differences between hydrophobic and hydrophilic asymmetric surfaces may arise from the combined effects of chemical composition and surface morphology of the Janus films.
Journal Name The surface morphology of the resulting film was initially observed under an optical microscope. The obtained images ViewS3 Article Online including polarizing optical images are shown in Fig. (ESI †). DOI: 10.1039/C4CC06798C Basically, the surface of the film was uniform but not smooth on microscale (Fig. S3 a&c, ESI†), and there were tiny birefringent domains homo-dispersed in the film (Fig. S3 b&d, ESI†), suggesting a part of the DPAB molecules or the complex were self-assembled into small ordered structures during the film formation. A submicrostructure of the film surface could be distinguished at higher magnification. Fig. 3 shows the images taken by scanning electron microscope (SEM). Images (a) to (c) are from the surface of the air side and (d) to (f) are from that of the water side. Interestingly, both surfaces consisted of interlaced flakes with lamellar structures. However, the flakes on air side were relatively distorted and smaller than those on water air, and partly embedded in a transition layer which was mostly composed of excess DPAB in the film. The different morphologies may be ascribed to the different environmental conditions that the surface of film faced during manufacture and the different mechanisms of solvent loss (evaporation to air or diffusion to water). Regardless of the small morphological differences, the flakes of lamellar structure indicated that a similar multi-level self-assembly occurred with no dependence on growing environment (air or water). Section view SEM images of the Janus film were also obtained (Fig. S4, ESI†), showing that two rough surfaces are linked by a relatively compact transition layer with thickness of 10-15 µm. To further examine the self-assembled lamellar structure, X-ray diffraction (XRD) measurements were performed on the film. Fig. 4 shows the XRD profiles of DPAB and the composite film. In the wide-angle region (Fig. 4a), the film shows similar diffraction patterns (with smaller intensity peaks) to DPAB, indicating that any excess free DPAB which did not form H-bonds with PAA precipitated in the film partially in the crystalline form similar to the starting DPAB material.18 The sharp peaks at 2θ ≈ 1.09° (d = 8.09 nm) and wide peaks 2θ ≈ 0.46° (d = 19.11 nm) observed in the small-angle region (Fig. 4b) imply that the composite film contained the lamellar structure formed by DPAB itself. However, a broad diffraction at 2θ of 0.55°-0.62° appeared in the composite film after deduction of the effect of the excess DPAB (insert of Fig. 4b). The presence of this peak may indicate that, as well as the lamella of DPAB with low spacing, another lamellar structure with a wider spacing existed in the film. The widest spacing of the lamellar structure in the film is about 62.21 nm which is probably from the flakes observed by SEM in Fig. 3.
Fig. 4 XRD spectra of DPAB and the composite Janus film: (a) spectra in the wide-angle region, (b) spectra in the small-angle region.
Fig. 3 SEM images of the Janus film surfaces: (a-c) the surface on air-side, (d-f) the surface on water-side.
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We considered that the flakes on the Janus film surfaces were formed by the self-assembly of H-bonded complex DPAB-PAA. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) was employed to detect the chemical composition on surfaces of the Janus film (see Fig. S5, ESI†). Both sides of the Janus film showed similar FTIR spectra with the characteristic absorbed peaks of DPAB and PAA, indicating PAA was involved in This journal is © The Royal Society of Chemistry 2012
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Journal Name the construction of flakes on both surfaces of the Janus films. Moreover, the peak of PAA centred at 2944 cm-1 which represents the -OH of the carboxylic acid disappeared in the film and a new peak appeared at 1943 cm-1 on both surfaces. These variations and a noted shift of the C=O peak of PAA from 1697 cm-1 to 1715 cm-1 after the film was formed imply the formation of H bonds (-O-H…N) between the carboxylic acid of PAA and the pyridyl group of DPAB.21,22
COMMUNICATION tends to be exposed on aqueous side of the film while DPAB on airside, which finally determines the unique asymmetric wetting Article Online property of the Janus film. The fabrication method byView self-assembly 10.1039/C4CC06798C at air-water interface is probably scalable toDOI: produce other ultrathin free-standing Janus films on macroscopic scale. The result also reveals a promising way to fabricate multifunctional materials with simple building blocks under a pre-designed environment.
Notes and references a
Department of Materials Science and Engineering, College of Peking
University,
P.
R.
China.
Email:
[email protected] b
Department of Mechanical Engineering, University of British Columbia,
Canada. Email: [email protected] c
Department of Pharmaceutical Sciences, University of British Columbia,
Canada † Electronic Supplementary Information (ESI) available: The synthesis and characterization of DPAB, the polarizing optical images and ATRFTIR spectra of the Janus film. See DOI: 10.1039/c000000x/ 1
(a) J. Du and R. K. O'Reilly, Chem. Soc. Rev., 2011, 40, 2402; (b) S. Jiang, Q. Chen, M. Tripathy, E. Luijten, K. S. Schweizer and S. Granick, Adv. Mater., 2010, 22, 1060; (c) F. Wurm and A. F. M. Kilbinger, Angew. Chem. Int. Ed., 2009, 48, 8412; (d) C.
Fig. 5 Schematic illustration of the multi-level self-assembly for the Janus film formation. Based on these results, we propose a possible mechanism (Fig. 5) of the self-assembly Janus films described in this study. The film formation may include two kinds of self-assembly processes. One is the self-assembly at the molecular level (molecular self-assembly) in which DPAB and DPAB-PAA complexes assemble separately or together through π–π stacking interactions into nano-layers. The molecular spacing of the layer is relatively small, corresponding to the peaks of larger angles in XRD spectra (Fig. 4b). The other is layer-layer self-assembly at the micro-level where the formed layers further stack together into the lamellar structure (the flakes seen in the SEM images) with large layer spacing which correspond to the broad peaks observed in the low angle X-ray determinations. Importantly, the asymmetric wetting property of the layers and lamellar structure of the Janus film result from the environment of self-assembly. On the side of film contacting air, the hydrophobic alkyl chains of DPAB tend to be exposed externally. While on the side contacting water, the exposed part is hydrophilic PAA. These lamella structural flakes may be fixed on the film surfaces by interlacing together and being partly embedded in the transition layer which may be composed of excess DPAB. The surface roughness enhanced by the interlaced flakes further amplifies the distinction in water wetting ability between two surfaces of the film.
Conclusions In summary, an ultrathin free-standing Janus film was successfully fabricated with hydrophobic DPAB and hydrophilic PAA at air-water interfaces by a unique multi-level self-assembly process. The resulting film shows distinct hydrophilic (water-side) and hydrophobic (air-side) surfaces on opposite sides of the films which are composed of interlaced layer-layer self-assembled flakes. Interestingly, the layers for flakes may be formed by molecular selfassembly and the exterior compositions of the flakes depend on the property of the two environments the surfaces were exposed to. PAA
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