Fabrication of Chemically Tunable, Hierarchically Branched Polymeric

May 31, 2016 - ... and Macromolecular Chemistry, University of Hamburg, Bundesstraße 45, ... Gecko-mimicking,1−3 dry-adhesive,4−6 and superhydropho-...
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Fabrication of Chemically Tunable, Hierarchically Branched Polymeric Nanostructures by Multi-branched Anodic Aluminum Oxide Templates Hanju Jo,† Niko Haberkorn,‡ Jia-Ahn Pan,† Mohammad Vakili,† Kornelius Nielsch,§ and Patrick Theato*,† †

Institute for Technical and Macromolecular Chemistry, University of Hamburg, Bundesstraße 45, 20146 Hamburg, Germany Institute of Organic Chemistry, University of Mainz, Duesbergweg 10-14, 55099 Mainz, Germany § Institute of Applied Physics, University of Hamburg, Jungiusstraße 11, 20355 Hamburg, Germany ‡

S Supporting Information *

ABSTRACT: In this paper, a template-assisted replication method is demonstrated for the fabrication of hierarchically branched polymeric nanostructures composed of post-modifiable poly(pentafluorophenyl acrylate). Anodic aluminum oxide templates with various shapes of hierarchically branched pores are fabricated by an asymmetric two-step anodization process. The hierarchical polymeric nanostructures are obtained by infiltration of pentafluorophenyl acrylate with a cross-linker and photoinitiator, followed by polymerization and selective removal of the template. Furthermore, the nanostructures containing reactive pentafluorophenyl ester are modified with spiropyran amine via post-polymerization modification to fabricate ultravioletresponsive nanostructures. This method can be readily extended to other amines and offers a generalized strategy for controlling functionality and wettability of surfaces.

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INTRODUCTION Over the past decade, the fabrication of one-dimensional anisotropic nanostructures, such as asymmetric polymeric nanopillars, has attracted considerable attention as a result of their potential to control the wettability and adhesive property of surfaces. In particular, the biomimetic surfaces, such as Gecko-mimicking,1−3 dry-adhesive,4−6 and superhydrophobic7−9 surfaces, have been studied comprehensively. Arrays of nanopillars have been generally fabricated by a templateassisted replication method, which is a cost-effective fabrication method with a high throughput. Numerous studies have applied a “template synthesis” method, which uses the infiltration of a polymer melt or solution into the nanocavities of anodic aluminum oxide (AAO) by external forces or wetting phenomena.10−18 This method enables production of wellordered and highly monodispersed nanostructures with tailored dimensions and high aspect ratios on a large area. AAO membranes feature hexagonally packed, highly ordered pores with a narrow size distribution and have been intensively used to produce various polymeric structures.19−26 AAO can be fabricated by a well-developed two-step anodization process, and the pore parameters can be easily tuned by varying the experimental parameters during the anodization step, resulting in adjustable pore lengths and pore diameters.27−29 AAO allows for the fabrication of various shapes of nanopillars by replication of their shapes, in particular high aspect ratio and hierarchical nanopillars, which can play an important role in © XXXX American Chemical Society

controlling the wettability and adhesive property. The complexity of structures provides different wetting and adhesive properties of materials comprising identical chemical composition. In particular, hydrophobicity of a surface can be altered on the basis of the roughness caused by nanostructuring. A number of studies have shown that the superhydrophobic surfaces can be obtained by applying the replication of complex and hierarchical AAO membranes.30,31 In addition, hierarchically branched AAO membranes have been exploited to fabricate the highly complex nanostructures, which are required for multifunctional nanodevices. Since Li et al. introduced AAO membranes composed of Y-shaped pores fabricated by a reduction of the applied voltage, various researchers developed the method to produce the multi-level, multi-branched nanostructures with various materials, and these approaches advanced the new fields of nanoscale applications.32−36 Several methods have already been introduced for the fabrication of hierarchical nanopillars; however, these approaches focused mostly on mimicking of the topography of biological surfaces or the changes of the physical structures of the nanopillars only.37−41 To overcome the limitation of variation of the physical structures and to allow for a versatile adjustment of the surface chemistry, a post-polymerization modification method Received: January 17, 2016 Revised: May 28, 2016

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Figure 1. Schematic illustration of the fabrication of hierarchically branched polymeric nanostructures. phosphoric acid (6.0 wt %) at 45 °C for 48 h. For route A (see Figure 1), the second anodization was initially performed in an aqueous solution of phosphoric acid (1.0 wt %) at 195 V for 12 min and the barrier layer was subsequently thinned out by consecutive reduction of the applied voltage to 80 V (−0.02 V/s). After exchange of the electrolyte against oxalic acid (0.3 M), the reduction of the applied voltage was continued to 0 V (exponential decay of the applied voltage). A further thinning process of the barrier layer of the template was conducted in phosphoric acid (10 wt %) for 1 h at room temperature. Subsequent anodization resulted in the formation of branched pores, which were further processed in oxalic acid (0.3 M) at 40 V for 3−7 min. Pore widening was performed in aqueous phosphoric acid (10 wt %) at room temperature for 15 min. To follow route B (see Figure 1), the second anodization step was performed at 40 V for 3−7 min in 0.3 M oxalic acid solution at 1−8 °C. Pore widening was performed in aqueous phosphoric acid (5 wt %) solution at room temperature. Fabrication of Polymeric Nanopillars. Pentafluorophenyl acrylate (PFPA) was synthesized following a procedure described in section S1 of the Supporting Information. A solution of PFPA with 1,6-hexanediol diacrylate (10 wt %) and diphenyl(2,4,6trimethylbenzoyl)phosphine oxide (TPO) (5 wt %) was drop-cast onto an AAO template, and vacuum was applied for 5 min at room temperature to assist the infiltration. Afterward, a polyethylene terephthalate (PET) film, which was modified with vinyltrimethoxysilane, hence, exposing the vinyl group on the surface to enhance adhesion, was covered on top of AAO as a supporting substrate and irradiated with UV light (365 nm) for 30 min to polymerize PFPA. Sacrificial AAO was removed in subsequent steps. First, aluminum was removed by a CuCl2·HCl (0.02 M) solution with an ice bath, and then aluminum oxide was removed by 10 wt % phosphoric acid solution at 45 °C for 1 h. Subsequently, the sample was washed with sufficient deionized water. Before the water evaporated, the sample was frozen by dipping it into liquid nitrogen and placed in a freeze-drying station for several days. Surface Modification of Polymeric Nanopillars. Spiropyran amine was synthesized following a procedure described in section S2 of the Supporting Information. Spiropyran amine (5 wt %) was dissolved in methanol and stirred for several days until a homogeneous solution was obtained. The free-standing nanopillars were immersed in

that is able to alter the physicochemical properties of surfaces is applied on the surfaces of the nanopillars, where various functional groups can be attached by mild reaction.42 Herein, we present a simple method to fabricate chemically modifiable hierarchical poly(pentafluorophenyl acrylate) [poly(PFPA)] nanostructures by the template-assisted method with multi-level AAO templates, followed by an exemplarily postpolymerization modification. On the basis of the structures of the AAO templates, various corresponding poly(PFPA) nanostructures, such as raspberry shape, flower shape, or branch shape, are prepared. Individual hierarchical poly(PFPA) nanostructures show the significantly different wettability properties, and it is proven by the dynamic contact angle (CA) measurements. Poly(PFPA), a well-established active ester-based polyacrylate, is used as the infiltrated material because it allows for the post-polymerization modification with various amines under mild conditions with high yields, and the surfaces of poly(PFPA) nanostructures are eventually modified with photoresponsive spiropyran amine, which is able to photoisomerize upon ultraviolet (UV) irradiation via postpolymerization modification. The chemically modified surfaces show reversible changes of wettability properties upon UV irradiation.

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EXPERIMENTAL SECTION

Fabrication of Hierarchically Branched AAO Templates. High-purity aluminum chips (Goodfellow, 99.999% Al) were placed in an anodization cell and electropolished in a vigorously stirred mixture of ethanol and perchloric acid (3:1, v/v) with an applied voltage of 20 V for 5 min at 5 °C. Subsequently, the cells were washed with deionized water and isopropanol. For the fabrication of the hierarchically branched AAO templates, an asymmetric two-step anodization was applied. In the first step, electropolished aluminum was anodized in an aqueous solution of phosphoric acid (1.0 wt %) at 175 V for 3 h, and sequentially the voltage was increased to 195 V and applied for 21 h. After the first anodization, the aluminum oxide layer was removed in aqueous solution of chromic acid (1.8 wt %) and B

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Figure 2. SEM images of standard and hierarchically branched AAO templates and the corresponding polymeric nanostructures: (a) standard AAO template, (b) free-standing regular poly(PFPA) nanopillars, (c) branched AAO template fabricated via route A, (d) flower shape of poly(PFPA) nanopillars, (e) two-tiered branched AAO template fabricated via route A, (f) branch shape of poly(PFPA) nanopillars, (g) hierarchically patterned AAO template fabricated via route B, and (h) raspberry-like poly(PFPA) replication (scale bar of 200 nm).

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the solution for 18 h under ambient conditions and then washed with deionized water and methanol. The freeze-drying technique was applied to prevent the collapse of the nanopillars. For the photoisomerization of spiropyran, the sample was irradiated with UV light (254 nm) for 20 min. Measurement. The topographies of the fabricated hierarchically branched AAO templates and their poly(PFPA) replicas were investigated by scanning electron microscopy (SEM, Sigma-Zeiss) after gold sputtering (Cressington sputter coater) onto the polymeric nanopillars with a thickness of less than 3 nm. To measure the wetting properties of the nanostructures, CA measurements were conducted using a DataPhysics OCA 20. In general, a minimum of five data points on each sample were measured. The post-modified nanopillars were investigated by a Nicolet iS10 Fourier transform infrared (FTIR) spectrometer.

RESULTS AND DISCUSSION Modifiable hierarchical poly(PFPA) nanostructures were fabricated by a template-assisted method with multi-branched AAO templates. Two routes yielded different shapes of templates, which ultimately determined the structures of poly(PFPA). The multi-branched AAO templates were prepared by the fabrication procedure shown in Figure 1. Briefly, a first anodization was conducted in a phosphoric acid electrolyte under constant voltage, and the pores grown during the anodization indented aluminum and caused the hemispherical concave texture on the substrate. The aluminum oxide layer formed by the first anodization was removed to obtain the textured aluminum substrate, and the substrate was used for the C

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Figure 3. Static, advancing, and receding CAs for the poly(PFPA) thin film and the poly(PFPA) nanostructures shown in panels b, d, f, and h of Figure 2, including the photographs of a water droplet on the respective surfaces.

hemispherical concaves (diameter of ∼500 nm) via routes A and B, respectively. The great advantage of the present method is that the pore length and diameter of both hierarchical structures can be diversified by varying the conditions of anodization. Figure 2 shows the various shapes of nanostructures that were prepared from the different shapes of AAOs. As a reference structure, free-standing nanopillars were prepared with AAO fabricated by the conventional two-step anodization without additional anodization (diameter of ∼180 nm and length of 1−2 μm) (panels a and b of Figure 2). For the fabrication of the free-standing nanorods, the freeze-drying technique, which sublimes the water among the nanopillars, had to be applied to prevent the capillary forces, which act on the collapse of nanopillars. AAOs fabricated via route A resulted in a flower shape (Figure 2c) and a branched shape (Figure 2e) of AAO templates by employing the third anodization at 40 V and 1−8 °C for a shorter time of 3 and 7 min for the former and later, respectively, and a raspberry shape of the AAO template was produced via route B (Figure 2 g), for which the second anodization was conducted for 5 min at 40 V. It is worthy to note that different nanostructures are fabricated depending upon the length of the second anodization time, as shown in section S4 of the Supporting Information. Using this approach, various shapes of AAO templates can be easily fabricated by controlling the anodization time and type of electrolytes. Furthermore, the corresponding shapes of poly(PFPA) nanostructures were prepared by the infiltration of monomer solution into these hierarchically branched AAO templates (panels d, f, and h of Figure 2), followed by in situ polymerization. Previously, Salsamendi et al. studied the radical polymerization of a fluorinated acrylic monomer (1H,1H,2H,2H-perfluorodecyl acrylate) in the nanocavities of AAO templates and the effect of the dimension of AAO on the hydrophobicity of the perfluorodecyl acrylate surfaces.43 The

asymmetric anodization via route A or B. To pursue route A, the second anodization was initially performed in 1 wt % phosphoric acid at 195 V for 12 min on the textured aluminum substrate, and subsequently, the barrier layer was thinned out by consecutive reduction of the applied voltage to conduct the additional anodization. Following the thinning process, the barrier layer was further thinned out by a wet chemical etching step in 10 wt % phosphoric acid for 1 h. Cross-sectional SEM images of the templates before and after the thinning out of the barrier layer are shown in section S3 of the Supporting Information. For the formation of secondary branched pores, a third anodization was carried out in 0.3 M oxalic acid at 40 V for 3−7 min, which resulted in the formation of the secondary pores (pore diameter of ∼40 nm). Subsequently, a PFPA monomer solution containing cross-linker (1,6-hexanediol diacrylate) and photoinitiator (TPO) was infiltrated into the templates and photopolymerized by irradiation with UV light after the supporting substrate was covered. The sacrificial AAO template was then removed, and the corresponding poly(PFPA) nanostructures were freeze-dried to prevent collapse and aggregation of the polymeric nanopillars. For route B, the second anodization was performed in 0.3 M oxalic acid at 40 V for 3−7 min on the textured aluminum substrate. The smaller secondary pores (pore diameter of ∼40 nm) were formed on hemispherical concaves (diameter of ∼500 nm), which originated from the first anodization and the oxide removal. The length of the secondary pores was controlled by the time of anodization from 1 to 8 min. To ensure that the monomer solution can infiltrate easily into the pores, pore widening was performed in 5 wt % phosphoric acid at room temperature prior to infiltration. The multi-branched AAO templates were obtained via either of the two different pathways. The secondary pores (pore diameter of ∼40 nm) were formed on either the thinned out nanopores (diameter of ∼300 nm and length of ∼1 μm) or the D

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Figure 4. Schematic illustration of post-polymerization modification with activated ester to spiropyran amine and the SEM images of nanostructures before and after modification.

than the regular nanopillars (104.9° ± 5°) and the flower shape of nanopillars (123.1° ± 10°) that have no secondary pillars on the top of nanopillars. The results clearly show that the surface becomes more hydrophobic with the advancement of the structural complexity on the contact area. Furthermore, the hierarchically branched poly(PFPA) nanostructures were chemically modified with a lightresponsive spiropyran photochromic dye via post-polymerization modification. Post-polymerization modification is one of facile methods to substitute various chemical groups into another under mild conditions for a short time. Following the established activated ester−amine chemistry, the pentafluorophenyl ester group was substituted with a spiropyran amine group as shown in Figure 4. To conduct post-polymerization modification, the polymeric nanostructures composed of poly(PFPA) are immersed in spiropyran amine solution (5 wt % in methanol for 18 h), resulting in the surface modification of the nanostructures with spiropyran. A SEM image of spiropyran-modified nanostructures showed that the nanopillars kept their initial structures, even after modification (see also section S6 of the Supporting Information). The conversion of the pentafluorophenyl ester moieties into the spiropyran-bound amide groups was proven by FTIR spectroscopy, and the resulting spectra showed the signals for N−H bending vibration and the CO amide stretching vibration at 3306 and 1650 cm−1, respectively (Figure 5). In addition, the change in the surface color, from yellow to violet, is observed after irradiation with UV light (254 nm) with bare eyes (see section S7 of the Supporting Information). Spiropyran is a well-known photochromic compound that can be reversibly isomerized by irradiation with light.46−49 Irradiation with UV light (e.g., 365 nm) results in photoisomerization into a merocyanine form, which is highly polar as a result of the zwitterionic resonance form, as shown in Figure 6. Consequently, the wetting properties of the surface can be significantly changed by surface-bound UV-responsive spiropyran. Upon UV irradiation, the static CAs of all spiropyranmodified nanostructures decreased as a result of the formation of the zwitterionic merocyanine form, which contains charges, and the decrease of static, advancing, and receding CAs was observed. As shown in Figure 6, the pristine static CAs of all structures decreased after post-polymerization modification because the hydrophobic pentafluorophenyl groups were substituted with the relatively more hydrophilic spiropyran amine groups. Furthermore, the CAs decreased further after UV irradiation, regardless of the type of structures, as a result of the photoisomerization of spiropyran. For example, the static CA of free-standing spiropyran-modified nanopillars decreased

resulted n(B)PFA nanostructures showed the superhydrophobic properties, in which the CA switched from 114° to 159° with nanostructuring. Their results demonstrate that the nanostructured fluorinated polymers can be fabricated in the nanocavities of AAO templates by in situ polymerization and provide the superhydrophobic surface having a “lotus effect”. To study the wetting properties of hierarchically branched poly(PFPA) nanostructures, the surface wettability of the obtained branched nanostructures was investigated by CAs of water droplets on the nanostructures. It is well-known that the surface roughness and heterogeneity are assumed to cause a CA hysteresis because the rough structures have domains that hinder the motion of water droplets, thus causing an increase in the advancing CA and a decrease in the receding CA.44 Therefore, the measurement of the static CAs is not sufficient to characterize the wetting behavior of nanostructured surfaces. To define the dynamic wetting characteristics of the nanostructures shown in Figure 2, the advancing water CA θa and the receding water CA θr, which refer to the maximum expanding and contracting angles of the water droplets, were measured. Interestingly, the CAs of the nanostructures dramatically changed, depending upon the shapes of the nanostructures. The static, advancing, and receding CAs of a poly(PFPA) thin film and the poly(PFPA) nanostructures were compared, and a CA difference depending upon the morphology was found. The hierarchically branched nanostructures showed a prominent CA hysteresis, which represents the difference between the advancing angle and the receding angle, compared to the flat surface. The advanced roughness of surfaces hinders the movement of the water droplet and causes the considerable CA hysteresis. This can be explained by Wenzel’s hydrophobicity mode, which suggests that the CA hysteresis increases with the surface roughness because the contact interface area increases as a result of the increase in surface roughness.45 As shown in Figure 3, the raspberry shape of the nanostructure has the highest static CA (131.3 ± 3°) and the largest hysteresis (87.4 ± 7°) and the static CA is enhanced by 52.65% compared to the flat surface. In addition, it is observed that the CAs of nanopillars (Figure 3) increase with the development of roughness on the top surface of nanopillars. The top view SEM images of the surface of nanopillars show the development of roughness depending upon the structures of AAO templates used (see section S5 of the Supporting Information), and the images support the fact that the CA increases with the surface roughness. Following the trend of roughness, the branched shape of nanostructures containing secondary pillars on the head of nanopillars has a higher static CA (126.3° ± 2°) E

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smaller, all of hierarchically branched structures showed the decrease of CAs after UV irradiation. In addition, the CAs were reversibly switchable upon UV irradiation, followed by thermal relaxation (see section S8 of the Supporting Information). For this, the samples were kept in the dark for 1 day and the CA was remeasured. Under dark, spiropyran returned to the initial state and the CA increased repeatedly. The result showed that the wetting properties of nanopillars can be altered by simple modification, even on complex structures. This concept illustrates the potential applicability of our approach in constructing a wide range of stimuli-responsive materials by post-modification of the precursor nanostructures.

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CONCLUSION A facile approach for the fabrication of hierarchically branched polymeric nanostructures by using multi-branched AAO templates was presented. The modified two-step anodization process allowed for the independent variation of the pore length and diameter of both hierarchical substructures by controlling the anodization time and types of electrolyte, so that various structures of AAO templates can be prepared. By infiltration of the PFPA monomer into the templates and subsequent polymerization of the monomer, it was possible to replicate the shapes of AAO templates and fabricate hierarchically branched polymeric nanostructures. Furthermore, the hierarchically branched poly(PFPA) nanostructures were chemically modified with spiropyran amine via a postpolymerization modification method, resulting in lightresponsive nanostructures. The concept presented here is expected to be easily extended to the preparation of stimuliresponsive multi-structured surfaces by modification with

Figure 5. FTIR spectra of free-standing nanopillars before and after post-polymerization modification.

up to 15.92% after UV irradiation from 101.2° ± 10° to 87.3° ± 10° as a result of the formation of the zwitterionic merocyanine form. In comparison to previous studies on the spiropyranmodified thin films, the changes of CAs before and after UV irradiation are smaller but appear around the CA of 90°; hence, it is assumed that the roughness of structures, which generate the hydrophobic surface, offsets the effect of the photoisomerization of spiropyran.48 Even though the changes were

Figure 6. Static, advancing, and receding CAs of spiropyran-modified nanostructures before and after UV irradiation and the reduction percentage of static CAs before and after UV irradiation. The change of the chemical configuration of spiropyran upon UV irradiation is presented as an inset. F

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various functional amines via simple post-polymerization modification.

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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/acs.langmuir.6b00163. Syntheses of PFPA (S1) and spiropyran amine (S2), SEM images of AAO templates before and after the thinning procedure (S3), SEM images of poly(PFPA) nanostructures produced by AAO fabricated via route A with different anodization times (S4), SEM images of the surface of nanopillars (S5), SEM images of spiropyranmodified nanostructures (S6), image of the nanopillar array before and after modification with spiropyran (S7), and wettability switching cycles upon UV irradiation and the cessation of UV light (S8) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +49-40-42838-6009. Fax: +49-40-42838-6008. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Hanju Jo gratefully appreciates the support from the Evonik Foundation. REFERENCES

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DOI: 10.1021/acs.langmuir.6b00163 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.6b00163 Langmuir XXXX, XXX, XXX−XXX