Gecko-like Branched Polymeric Nanostructures from Nanoporous

Aug 30, 2018 - Here, we report a simple method to produce hierarchically shaped polymeric one-dimensional nanostructures. More specifically, dual-size...
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Gecko-like Branched Polymeric Nanostructures from Nanoporous Templates Iwona Blaszczyk-Lezak, Diana Juanes, Jaime Martin, and Carmen Mijangos Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01923 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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Gecko-like Branched Polymeric Nanostructures from Nanoporous Templates Iwona Blaszczyk-Lezaka, Diana Juanesa, Jaime Martínb,c, Carmen Mijangos*,a,d,e

a) Instituto de Ciencia y Tecnología de Polímeros, Consejo Superior de Investigaciones Científicas (CSIC), Juan de la Cierva 3, 28006 Madrid, Spain b) POLYMAT and Polymer Science and Technology Department, Faculty of Chemistry, University of the Basque Country UPV/EHU, Manuel de Lardizabal 3, 20018 Donostia-San Sebastián, Spain c) Ikerbasque, Basque Foundation for Science, E-48011, Bilbao, Spain d) Donostia International Physics Center (DIPC), Paseo Manuel de Lardizábal 2, 20018 Donostia-San Sebastián, Spain e) Materials Physics Center (CFM), CSIC-UPV/EHU, Paseo Manuel de Lardizábal 5, 20018 Donostia-San Sebastián, Spain Email: [email protected]

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ABSTRACT Here, we report a simple method to produce hierarchically shaped polymeric 1Dnanostructures. More specifically, dual-sized polymer nanowires are fabricated employing multibranched anodic aluminum oxide (AAO) templates. By fine selection of the anodization conditions we achieve branched nanopores having a first segment of 400 nm in diameter from which seven further 55-nm-in-diameter pores arise. Wetting of such nanopores with polymer melts – e.g. poly (ε-caprolactone) (PCL) and polystyrene (PS) – allows for the nanomolding of their respective inverse nanostructures, i.e. dual-sized multibranched polymer nanowires that, when supported on a flat surface, strongly resemble the spatulae of geckos´ toes. The structural features of the dual-sized polymer nanostructures, namely crystalline phase, crystallinity, texture, etc., are furthermore characterized and interpreted within the context of polymer phase transitions in confined media. Our work presents a readily applicable approach to produce soft nanomaterials of high morphological complexity, thereby with promising implications in the nanotechnology area, for example, in biomimetic solid adhesion.

Keywords template-based strategies, multibranched anodic aluminum oxide (AAO) templates, complex nanoscale morphologies, dual-sized polymer nanostructures, hierarchically branched polymer nanoarchitectures, crystallinity, crystal orientation, confined media

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INTRODUCTION Most recent challenges in the field of nanotechnology are demanding the combination of complex nanoscale morphologies with specific chemical functionalities and the easy, highthroughput manufacturing of nanoscale elements over large areas. An illustrative example of the above is dry adhesion. Enormous scientific efforts are being devoted towards developing dry adhesives via biomimetic approaches. Many of these approaches are inspired by the toes of geckos, which thanks to a dense array of flexible hairs, from which nanoscale branchednanowire-like structures (spatulae) protrude, are perfectly able to adhere themselves to any surface1 . Because of their intrinsic elasticity as well as the easy tunability of their properties via chemistry – during the synthesis or in an ulterior chemical modification – polymers are, with no doubt, the preferred choice to mimic the hairy toe pad of geckos; thereby to produce dry adhesives.

Large arrays of densely packed polymer nanowires can be easily produced by nanomolding strategies using the nanopores of anodic aluminum oxide (AAO) matrices as nanomolds

2-5

.

By suitable selection of the AAO template as well as the infiltration method / polymerization conditions, highly uniform nanowires and nanotubes can be achieved, with large size flexibility and high throughput

2, 4-9

. Subsequent developments have allowed, moreover, to

increase the complexity level of the achieved polymer nanostructures, e.g. to produce polymer-polymer core-shell structures

10-13

. In this line, a major milestone has recently been

set with the establishment of guidelines to grow AAO templates having hierarchically branched nanopores in a controlled manner

14-16

, which has opened up a plethora of new

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possibilities for this nanofabrication approach . Moreover, in a very recent review17, Q Xu et al highlights the advances in the porous AAO-assisted rational synthesis of 1D hybrid and hierarchically branched nanoarchictectures. In addition, they also discussed the outstanding application of these complex nanoarchitectures, in combination with metals such as Au, Si, Co, Ag or CNT, may have as potential building blocks for various nanodevices. Nevertheless, although polymers offer the advantage of easy design and reproducible strategies for assembling and integrating 1D complex nanostructures into hierarchically branched nanodevices and systems, authors do no mentioned any example of polymer made. So, the possibility to fabricate complex polymer nanoarchitectures is open and seems necessary Herein, we show that rationally designed, branched AAO templates can be used to fabricate large area arrays of hierarchically shaped 1D-polymer nanostructures that strongly resemble 3 ACS Paragon Plus Environment

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the spatulae in gecko´s toes. More specifically, dual-sized nanowires consisting of 7 average narrow nanowire segments (around 55 nm in diameter) that protrude from a wider nanowire segment (400 nm in diameter) are produced. Furthermore, we discuss how this complex morphology impacts the structural features of polymers, namely, crystalline phase, crystallinity, texture, etc. RESULTS AND DISCUSSION Branched AAO templates are obtained following a three-step anodization process. Firstly, anodized aluminum oxide templates of 140 nm pore diameter and 1 and 10 µm of pore length were synthesized by a two-step electrochemical anodization process of ultrapure aluminum foils (99.999%) following the procedure of Masuda el al. Finally, a third anodization in 0.15 M oxalic acid solution was performed at the potential of 80 V at 2 - 3 ºC following the method described by Xu et al

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during 1 hour and 7 hours. As a result, small pores were

formed from the bottom layer of large pores, as later shown. See supporting information (SI) for detailed information concerning AAO fabrication. The morphology of different AAO templates was investigated by scanning electron microscopy (SEM) employing a FESEM Hitachi model SU8000 microscope. Figure 1 shows the surface (a) and cross section (b, c) of a branched AAO template produced as described above. In figure 1a, large pores with a diameter of 400 nm are observed, which corresponds to the morphology resulting from the anodization conditions during the 1st and the 2nd anodization steps, followed by a subsequent pore widening process. The smaller pores that can be also observed in Figure 1a are, however, created during the third anodization process. Figure 1b shows the cross section of a broken AAO template. Two clear pore regions of different diameters and lengths can be distinguished in this image. The upper part of the pores, which extends over a length of around 1,5 µm has 400 nm in diameter. At the bottom of this region, a number of narrower pores show up. The size of the narrow pores is 55 nm in diameter and 10 µm in length, although no shown here. We note that because of the manipulation of the samples for the SEM investigation, some of the pores appear broken in the picture; however, the high uniformity of the pores can be clearly appreciated. These results are in agreement with the morphology of the bottom side of alumina template (shown in Figure S1) which becomes visible after the elimination of aluminum part. The red circle drawn in the Figure S1a defines an area of equal size as the red circle included in Figure 1a. As can be observed, average 7 alumina protuberances are contained within the red circle 4 ACS Paragon Plus Environment

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which corresponds to external part of each of 7 small pores (Figure S1b). See Figure S1 in SI for better explanation.

Figure 1. SEM micrographs of branched AAO template: (a) surface view, (b) cross section and (c) detailed view of the bottom of wide pores, where the narrow pores start.

The main morphological features of these templates can be summarized as follows: i) wider pores are arranged in orderly fashion according to a honey-comb morphology. This periodicity is responsible for the uniformity of the pore diameters observed. ii) Narrow pores also exhibit a uniform pore size (55 nm) over their entire length. iii) From most of the bottom parts of the wide pores (red circle in Figure 1a), 7 average narrow pores grow separated approx. 150 nm one from each other. iv) Any intermediate size between 1 and 10 microns can be achieved, since the pore length is directly proportional to the anodization time. Our multibranched AAO templates can be readily employed to produce complex polymer nanostructures. For the fabrication of branched Polystyrene (PS) nanowires, a 1-mm-thick PS film was placed on the surface of the AAO templates. Then the infiltration of the branched AAO nanopores with PS (number-average molecular weight, Mn: 199.000, supplied by Sigma-Aldrich) melts was conducted by annealing the system at 200 ºC for 3 hours under vacuum (a weak mechanical force was applied during this step). Finally, the solidification of the polymer material was achieved by cooling the system to room temperature. Branched PS nanotubes (hollow branched PS) were obtained following a similar wetting procedure and shorter infiltration times (e.g. 2 h). Although in both cases, polymer infiltration process is under complete wetting regime, in the last case, polymer melt coats only the pore walls and not the center of pore as a result of the smaller PS amount and not mechanical pressing applied3 (see reference 3 for difference between nanotubes and nanofibers). PCL nanostructures were

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achieved by wetting branched AAO templates with PCL (Mn: 81.000) melt at 80 ºC for 3 h (in vacuum). The residual polymer material remaining on the surface of the templates after infiltration was removed using sharp blades in all the samples. The morphology of different polymer nanostructures was investigated by scanning electron microscopy (SEM). In order to extract the polymer nanostructures from the templates, the aluminum substrate was first dissolved by treatment with a mixture of HCl, CuCl2 and H2O and then, the alumina was dissolved in 10 % H3PO4 solution. In order to stand the branched pillars, a polymer was placed over the surface of the template prior to its elimination. Shown in Figure 2a is the cross section of a branched AAO template whose pores have been filled with polystyrene (PS, artificially colored in purple). The molten PS is able to fill completely both the wide and the narrow regions of the nanopores, which gives rise, after solidification, to dual-sized PS nanostructures that are perfect inverse replicas of the original pores. Thus, the diameter of the wider segment of the PS nanowires is 400 nm while that of the narrower segments amounts to 55 nm. Interestingly, this is a morphology that strongly resembles the spatulae of geckos´ toes, as depicted in Figure 2b. Hence, in order to fully mimic the natural nanomorphology of gecko´s feet 19, we set on to liberate our hiperbranched polymer nanostructures from the templates. Figure 2c shows a SEM image of the array of branched PS nanowires supported on a polymer substrate. It is clear from the picture that the array of free-standing wide PS segments maintains the hexagonal symmetry inherited from original nanopores, while the narrower nanowires protruding from them collapse due to their high aspect ratio.

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Figure 2. SEM micrographs of the branched PS nanostructure: (a) cross section of a branched AAO template whose pores have been filled with PS constituting nanowires. PS is artificially colored in purple for clarity. (b) The similar morphology of our PS nanostructure and spatula in gecko’s toes is demonstrated in (b), (c) surface view of PS nanostructures released from the AAO template, (d) cross section of a branched AAO template whose pores have been filled with PS constituting nanotubes.

Having demonstrated the suitability of our methodology to prepare dual-sized polymer nanowires we further scrutinize our approach, this time producing polymer hierarchical polymer nanotubes. This can be easily produced by changing the infiltration conditions of the polymer melt2-3. In Figure 2d are shown the nanotubes thus produced. These consist of a hollow segment (400 nm in diameter), from which 55-nm-in-diameter nanowires protrude. Nanotubes are broken as a consequence of the mechanical treatment to obtain the SEM images. We note that the empty space in the nanotubes can be readily filled with another polymer 11, which results in nanostructures of even more elevated complexity.

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In order to investigate the effect of this hierarchical nanoscale morphology on the properties of polymers, specifically to investigate whether this feature impacts on their structural behavior, dual-sized PCL nanowires (with diameters of 400 nm and 55 nm) were fabricated and their phase behavior and microstructural features studied. PCL nanostructures were achieved by wetting branched AAO templates with PCL melt (PCL was provided by Polymer Source) at 80 ºC for 3 h (in vacuum). The residual polymer material remaining on the surface of the templates after infiltration was removed using sharp blades in all the samples. The Figure S3 shows a SEM picture of these PCL nanowires. Grazing-Incidence wide angle X-ray scattering (GIWAXS) was employed for the structural characterization of the branched PCL nanostructures (a schematic of a GIWAXS experiment geometry is depicted in Fig. 3a. Wide-angle X-ray scattering measurements were performed at the SAXS line of Elettra Sincrotrone (Italy). A wide band-pass (1.47%) X-ray beam with a wavelength of 1.54 Å was shone on the samples with incidence angles between 0.5° and 1°. A Pilatus 1M detector with a pixel size of 172µm was placed at a distance of 17.0 cm from the samples. The exposure time for room temperature measurements was 10s. The nanowire arrays were placed in the setup so that the PCL nanowires were aligned with the z-direction (Figures 3a). For the temperature resolved WAXS measurements, an Anton Paar hot stage was employed and heating and cooling rates of 10 °C/min were used. We note that when “large” incident angles, α, are used (e.g. α > 0.5) this technique allows to probe the whole sample thickness, allowing us to investigate the entire nanowires. Fig. 3b shows the 2D-GIWAXS pattern acquired for the PCL nanostructures crystallized at -10 ºC/min. The pattern exhibits clear ring-like Bragg reflections, which evidence the semicrystalline nature of the confined PCL. The azimuthal integration of the 2D pattern at each scattering angle, θ, give rise to the 1D-GIWAXS pattern shown in Fig. 3c. The 1DGIWAXS pattern is characterized by the 3 main reflections of the orthorhombic unit cell of PCL, corresponding to the stack of (110), (111) and (200) lattice planes (in ascending order)20. This results agree with those obtained by Suzuki et al., who also reported the formation of orthorhombic PCL crystals inside AAO nanopores

21

. The diffraction rings

observed in Fig. 3b suggest that crystals are isotropically oriented within the dual-sized nanostructures. According to this and previous results 21-22, we conclude that PCL crystals do not seem to grow with a clear preferred orientation within AAO nanopores, like other polymer crystals typically do

23-25

. We hipothesize that this is related to the fact that PCL 8

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crystals nucleate by heterogeneous, instantaneous nucleation at the AAO nanopore wall. Thus many lamellar crystallites develop simuntaneously in different directions, emanating from the surface of the heterogeneity, which results in randomly oriented crystals.

Figure 3. (a) Grazing-Incidence wide angle X-ray scattering (GIWAXS) geometry employed for the structural characterization of dual-sized PCL nanostructures. 2D (b) and 1D (c) GIWAXS patterns for PCL nanostructures. The 1D pattern results from azimuthal integrations of the 2D pattern DSC heating (d) and cooling (e) traces for PCL nanostructures and bulk. Azimuthally integrated GIWAXS intensity measured upon heating (f) and cooling (g) the PCL nanostructures at 10 °C min−1.

Because thermically activated processes leading to structural transitions in soft matter, such as crystal nucleation and melting, are profoundly influenced by spatial confinement 24-28 we went on to study the thermal behavior of dual-sized PCL nanostructures. Differential scanning calorimeter (DSC, Perkin Elmer D7) was used for the thermal characterization of the nanostructures. Heating and cooling runs at 20 ºC/min were carried out under a constant flow of nitrogen. For the DSC study, the aluminum substrates attached to the AAO templates were selectively etched as described above. Bulk PCL samples were also examined for comparison. Fig. 3d shows the DSC heating trace for the PCL nanostructure and bulk PCL (for comparison). Both heating traces exhibit a single, intense endothermic peak at T∼58 ºC. Moreover, the GIWAXS intensity plot acquired upon heating the PCL nanostructures (Fig. 3f) 9 ACS Paragon Plus Environment

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shows that Bragg reflections disappear at that temperature, which, all together, indicates that orthorhombic PCL crystals melt out at T∼58 ºC. Unlike the heating trace, the cooling trace of dual-sized PCL nanowires shows notable differences with respect to that of bulk PCL (Fig. 3e). More specifically, while bulk PCL presents a single crystallization event (at T∼27 ºC), 3 exothermic peaks are found for PCL nanostructures, centered at T ∼ 30 ºC, T ∼ 19 ºC and T ∼ -7 ºC. We must note that, although this phenomenon has been previously reported for nanoconfined

21-22

its origin seems to be

still controversial. Suzuki et al. interpreted that each exothermic processes account for a crystal population that crystallizes via different nucleation mechanisms, as each nucleation mechanism require a different energetic barrier to be overcome, which is further transduced into different crystallization temperatures – the higher the energetic barrier for the nucleation, the lower the crystallization temperature

21

. In our case, this effect may be further enhanced

because of dual-sized nature of our nanopores, which may impose two levels of confinement to the PCL in a single sample. Hence, we cannot exclude the possibility that PCL crystals nucleate via different mechanism within 400 nm pore regions and within 55 nm pore regions, which would result in multiple exothermic events in the DSC cooling trace. In line with the latter argument, Shi et al. have recently claimed that the multiple exotherms found in nanoconfined PCL result from the presence of distinct polymer fractions that crystallize under different confinement scenarios. These polymer fractions can be constituted, for instance, by the polymer confined inside the pores and by the residual polymer material that remains on the template surface 22. Thus, the different confinement scenarios that each polymer fraction is subjected to would determine the nucleation mechanism that is active. It also worth noting that the first exothermic peak in the DSC cooling trace of PCL nanostructures shows up at higher temperature than that of the bulk, suggesting a higher nucleation rate of crystals in the branched nanostructures. This is further confirmed by temperature-resolved GIWAXS experiments shown in Fig. 3g, where the appearance of the Bragg reflections can be appreciated at T ∼ 40 ºC.

CONCLUSIONS Our results demonstrate that large-area arrays of branched polymer nanowires similar to those in gecko´s toe pads can be produced employing simple template-based strategies. To achieve so, we first developed nanoporous AAO templates having dual sized branched pores. More 10 ACS Paragon Plus Environment

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specifically, these branched pores consist of 7 average narrow nanopores (55 nm in diameter) arising from a wider pore (400 nm in diameter). The length of both the narrow and wide nanopore regions can be finely tuned as a function of time. Multibranched polymer nanowires and nanotubes can be subsequently produced by wetting the AAO nanopores above with polymer melts. For example, we are able to produce PS and PCL nanostructures. We demonstrate that dual sized PCL nanowires are semicrystalline at room temperature, with an orthorhombic unit cell, and exhibit no preferential orientation of crystals. Clearly, the benefits of the processing approaches presented in this paper are not limited to the materials above, thereby opening up a plethora of new possibilities for the processing of soft, morphologically complex nanostructures with potential applications in, for example, solid adhesion.

SUPPORTING INFORMATION Supporting information is available including 3 figures. ACKNOWLEDGEMENTS Financial support from the Spanish Ministerio de Ciencia, Innovación e Universidades under projects MAT2014-53437-C2 and MAT2017-83014-C2-2-P is acknowledged. J.M. thanks MEC for the Ramón y Cajal contract and Fundación Iberdrola (Ayudas a la Investigación en Energía y Medio Ambiente 2017) for financial support. The authors thank D. Gómez for SEM experiments and R.M. Michell for support in DSC measurement.

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18. Masuda, H.; Fukuda, K., Ordered metal nanohole arrays made by a two-step replication of honeycomb structures of anodic alumina. science 1995, 268 (5216), 1466. 19. Xue, L.; Sanz, B.; Luo, A.; Turner, K. T.; Wang, X.; Tan, D.; Zhang, R.; Du, H.; Steinhart, M.; Mijangos, C., Hybrid Surface Patterns Mimicking the Design of the Adhesive Toe Pad of Tree Frog. ACS nano 2017, 11 (10), 9711-9719. 20. Chatani, Y.; Okita, Y.; Tadokoro, H.; Yamashita, Y., Structural studies of polyesters. III. Crystal structure of poly-ε-caprolactone. Polymer Journal 1970, 1 (5), 555. 21. Suzuki, Y.; Duran, H.; Akram, W.; Steinhart, M.; Floudas, G.; Butt, H.-J., Multiple nucleation events and local dynamics of poly (ε-caprolactone)(PCL) confined to nanoporous alumina. Soft Matter 2013, 9 (38), 9189-9198. 22. Shi, G.; Liu, G.; Su, C.; Chen, H.; Chen, Y.; Su, Y.; Müller, A. J.; Wang, D., Reexamining the Crystallization of Poly (ε-caprolactone) and Isotactic Polypropylene under Hard Confinement: Nucleation and Orientation. Macromolecules 2017, 50 (22), 9015-9023. 23. Martin, J.; Scaccabarozzi, A. D.; Nogales, A.; Li, R.; Smilgies, D.-M.; Stingelin, N., Confinement effects on the crystalline features of poly (9, 9-dioctylfluorene). European Polymer Journal 2016, 81, 650-660. 24. Martín, J.; Iturrospe, A.; Cavallaro, A.; Arbe, A.; Stingelin, N.; Ezquerra, T. A.; Mijangos, C.; Nogales, A., Relaxations and Relaxor-Ferroelectric-Like Response of Nanotubularly Confined Poly (vinylidene fluoride). Chemistry of Materials 2017, 29 (8), 3515-3525. 25. Shingne, N.; Geuss, M.; Thurn-Albrecht, T.; Schmidt, H.-W.; Mijangos, C.; Steinhart, M.; Martín, J., Manipulating Semicrystalline Polymers in Confinement. The Journal of Physical Chemistry B 2017, 121 (32), 7723-7728. 26. Martín, J.; Dyson, M.; Reid, O. G.; Li, R.; Nogales, A.; Smilgies, D. M.; Silva, C.; Rumbles, G.; Amassian, A.; Stingelin, N., On the Effect of Confinement on the Structure and Properties of SmallMolecular Organic Semiconductors. Advanced Electronic Materials 2018, 4 (1). 27. Efremov, M. Y.; Olson, E. A.; Zhang, M.; Zhang, Z.; Allen, L. H., Glass transition in ultrathin polymer films: calorimetric study. Physical review letters 2003, 91 (8), 085703. 28. Michell, R. M.; Blaszczyk-Lezak, I.; Mijangos, C.; Mueller, A. J., Confinement effects on polymer crystallization: from droplets to alumina nanopores. Polymer 2013, 54 (16), 4059-4077.

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