Interplay of Template Constraints and Microphase Separation in

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Interplay of Template Constraints and Microphase Separation in Polymeric Nanoobjects Replicated from Novel Modulated and Interconnected Nanoporous Anodic Alumina Haider Bayat, Chia-Hua Lin, Ming-Hsiang Cheng, Marc Steuber, Jiun-Tai Chen, and Holger Schönherr ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00108 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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ACS Applied Nano Materials

Interplay of Template Constraints and Microphase Separation in Polymeric Nanoobjects Replicated from

Novel

Modulated

and

Interconnected

Nanoporous Anodic Alumina Haider Bayat,†a Chia-Hua Lin,†b Ming-Hsiang Cheng,b Marc Steuber,a Jiun-Tai Chen,*b and Holger Schönherr*a

Author ADDRESSES: aPhysical Chemistry I, Department of Chemistry and Biology & Research Center of Micro and Nanochemistry and Engineering (Cµ), University of Siegen, AdolfReichwein-Strasse 2, 57076 Siegen, Germany b

†

Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30010 These authors contributed equally in this work.

KEYWORDS: anodic aluminium oxide, pore diameter modulation, replication of phase separated polymeric nanoobjects

ABSTRACT: We report on a versatile strategy to fabricate shape-controlled polymeric nanoobjects with an internal compartmentalized structure by replication from pore-diameter modulated and interconnected nanoporous anodized aluminum oxide (AAO). The AAO with modulated pore diameters is synthesized in a refined Temperature Modulation Hard Anodization

ACS Paragon Plus Environment

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(TMHA) approach, by alternating the flow rate of an air stream cooling the electrolyte. By pore widening of the templates with modulated pore diameters, a stable interconnected 3D network with transversal ellipsoidal holes is fabricated as a second template structure. From these unprecedented template structures exhibiting modulated pores and an intricate 3D network structure, shape-controlled polymeric nanoobjects are replicated using the melt wetting method with polystyrene homopolymer (PS) and cylinder-forming block copolymer polystyrene-blockpolydimethylsiloxane (PS298-b-PDMS195). The replicated nanostructures are separated into individual anisotropically shaped nanocapsules by passing the replicas through a polycarbonate membrane followed by gentle sonication. Inside the nanoobjects, the microphase separation of PS298-b-PDMS195 affords parallel aligned nanophases, in which both domain sizes and morphologies are different compared to the unconfined bulk according to transmission electron microscopy analyses. These advanced AAO platforms and the nanoscale polymer replicas offer new routes for compartmentalized nanoobjects with potential future relevance for applications ranging from drug nanocarriers to biosensors.

1. Introduction Compartmentalized polymeric nanostructures, which are also highly interesting from a fundamental point of view, have garnered widespread attention as nanomaterials for many potential application areas. Prime examples are various types of micelles, vesicles or multicompartment compound micelles formed by the self-assembly of amphiphilic bock copolymers, which can be loaded with active compounds in preselected compartments.1 Such assemblies may either serve as model systems for cascade reactions in nanoreactors,2,3 and for the selective administration of drugs4 or signalling agents in various applications.5 By controlling


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the size, shape and arrangement of compartments of different polymeric building blocks or that of polymeric and other constituents in hybrid materials, important properties can be modulated, such as cell uptake.6 In the bulk, one characteristic length scale of compartmentalization may be defined by microphase separation of block copolymers, which is very well established and an interesting approach to obtain 3D nanometer-scale morphologies.1,7 These are important in fields such as nanolithography, optoelectronics, drug delivery, and organic solar cells.8–11 Recently, the morphologies of block copolymers in confined geometries have been investigated in order to extend the potential applications of block copolymers. One of the most commonly used confined geometries is the cylindrical confinement of nanopores e.g. in anodic aluminum oxide (AAO) templates.12–14 By controlling the relationship (commensurability) between pore diameter of the nanopores and the repeating period of the block copolymers, frustration of the polymer chains can be induced and novel morphologies which cannot be observed in the bulk can be achieved.15– 26

In the studies by Ma et al. PS-b-PDMS nanorod cores embedded in PMAA matrix shells were

fabricated via a coaxial two-fluid electrospinning technique and the commensurability was investigated by TEM characterization and simulations.27,28 Cheng et al. introduced lamellaeforming PS-b-PDMS block copolymers into AAO nanopores via a solvent vapor annealing process and the microphase-separated morphology could be tuned by varying the composition of the mixed solvents.29

Since the structure and design of the templates are of crucial importance in these approaches, the fabrication of AAO with defined pore geometries is discussed here in some detail. Masuda et al.30 reported, following their pioneering work, a two-step anodization technique to fabricate


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well-ordered AAO templates. This method was subsequently developed and refined to tune interpore distances, pore diameters and pore lengths by changing the anodization parameters and type of anodization, the so-called mild anodization (MA) and hard anodization (HA).31–33 The self-ordering of the pore structure in conventional AAO was attributed by Nielsch et al. due to the mechanical stresses and volume expansion involved.34 Beyond the formation of straight cylindrical pores, several other diverse structures based on imprinted AAO templates with different geometrical options35 and in particular with modulated pore diameters were synthesized to date, which are useful means to manipulate e.g. light. Among others, Rugate filters36,37, distributed Bragg-reflector (DBR)38–40, waveguides41 and 3Dinterconnected networks in AAO have been reported.42,43 The anodization approaches utilized are mainly pulse anodization and cyclic anodization, which in conjunction with wet chemical widening procedures afford versatile modulated

pores with different diameters.44 Similar

structures were obtained, albeit with much more effort, by Lee and co-workers, who fabricated modulated pores along the pore length by combining the conventional MA and HA steps.45 To overcome this issue, the mentioned pulse anodization46 and cyclic anodization47 were introduced to perform a continuous modulation without any need for an exchange of the electrolyte. Modulated pores were also synthesized by periodically changing the temperature at the Al/Al2O3 interface via changing the flow rate of the cooling electrolyte during hard anodization. In this process, the current density and hence the pore diameters were varied by temperature modulation and therefore this process is known as temperature modulated hard anodization (TMHA). It was also shown that such complicated AAO templates can be successfully replicated by the layer-by-layer deposition of polyelectrolytes to afford free standing structures after template removal.48


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Although the AAO membranes with modulated pores have been widely used investigated in recent years, reports of block copolymer microphase separation inside such pore-diameter modulated AAO are scarce. One main question is how the morphologies of diblock copolymer, such as PS298-b-PDMS195 that was studied here, transform inside confined modulated nanopores in comparison to the bulk and thin film state. To address this question, we introduce here first of all a versatile air flow-assisted hard anodization approach to vary in a controllable and adjustable manner the range of straight AAO pores as well as modulated and 3D-AAO networks with one type of electrolyte based on a new variant of TMHA. The designed nanostructures obtained were subsequently widened to different extents and replicated by filling with homopolymer PS and diblock copolymer PS298-b-PDMS195. The analysis of the filling of the modulated AAO pores and the 3D-AAO network was complemented by the analysis of the frustrated microphase separated nanostructures obtained (Figure 1).

Figure 1. Schematics of the fabrication of modulated AAO pores and 3D-AAO networks in three steps (I, II, and III) as well as subsequent replication by the polymer melt wetting method. The three steps (I, II, and III): fabrication of modulated pores via air flow TMHA, removal of the aluminum in CuCl2 solution and of the barrier layer to afford free standing membranes after the


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treatment in 0.5 M H3PO4, respectively. Approach A (a, b, and c): filling with PS298-b-PDMS195, etching of alumina in 5 wt % H3PO4, and formation of nanocapsules by processing through a polycarbonate membrane followed by gentle sonication, respectively. Approach B (d, e, and f): formation of 3D-AAO network after widening (III) the templates in 0.5 M H3PO4, filling with PS and etching of alumina in 5 wt % H3PO4 to yield 3D-polymeric nanoobjects, respectively.

2. Experimental Section 2.1 Materials The

block

copolymer,

polystyrene-block-polydimethylsiloxane

(PS298-b-PDMS195)

was

purchased from Polymer Source Inc.. The subscripts denote the corresponding degrees of polymerization. The total number-average molecular mass (Mn) and polydispersity index (Đ) were 45.5 kg/mol and 1.15, respectively. Milli-Q water with a resistivity of 18.0 MΩ·cm was drawn from a Millipore Direct Q8 system (Millipore, Schwalbach, with Millimark Express 40 filter, Merck, Germany). Aluminum (Al, 99.9999%, 0.5 mm thick plate, Chempure), phosphoric acid (H3PO4, 85 %, Chemische Fabrik Budenheim, Germany), oxalic acid (H2C2O4, C.N. 615356-6, Merck), copper chloride dihydrate (CuCl2·2H2O, 99.999 %, Sigma Aldrich), perchloric acid (HClO4, 60− 62 %, J.T. Baker), chloroform (CHCl3, ≥ 99 %, Roth, Germany), potassium hydroxide (KOH, C.N. 1310-58-3, ROTH, Germany), hydrochloric acid (HCl, 37 %, VWR, Germany), silicon wafers ((100), Guv Team International Co. Ltd.), polycarbonate filters (VCTP with pore sizes of ~0.1 µm, Millipore Inc.), toluene (99.5 %, Echo Chemical), ethanol (99.9 %, Merck, Germany) and polystyrene (PS, 35 kg/mol, Sigma Aldrich and Đ = 2.0) were used as received.


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2.2 Anodization setup The anodization processes were carried out using a home-built setup, comprising a power supply (EA-PSI 8160-04 T, Electro-Automatic GmbH, Viersen, Germany) and a cooling system (LAUDA PK20, -40 to +150°C). The high purity Al sample mounted on a copper served as the anode and an Al sheet (2.5 cm × 9.0 cm, Bikar, Bad Berleburg, Germany) as the cathode. For applied potentials higher than 165 V, the power supply was coupled with another power suppler (power supply SMX 7220-D, Delta Elektronika). The anodization was conducted by supplying air from the cathode via a pipe made of PVC (inner radius of 0.4 cm) against the anode (compare schematic in Figure S1). The distance between the electrodes was 2.0 cm and the anodized area of the Al substrates was 0.5 cm2. A rubber O-ring (Wilhelm Jung GmbH) was used to seal the compartments.

2.3 Preparation of AAO Templates Prior to anodization, the Al plate was degreased in chloroform for 3 min, immersed in 0.3 M KOH for 5 min, rinsed with ethanol, and Milli-Q water and then followed by blow-drying in a nitrogen stream. The preparation of the AAO templates followed several steps: Electropolishing The cleaned Al substrate was electro-polished at room temperature in a 1:3 (V:V) mixture of HClO4 and ethanol at a constant voltage of 20 V for 4 min. Initially, the current was gradually increased starting from zero to reach a final current of 80 mA. This step facilitated to reduce the surface roughness and afforded a mirror-finished surface. Eventually, the substrate was thoroughly washed with Milli-Q water and dried in a nitrogen stream.


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Modulated Pores via TMHA After the electropolishing treatment, the Al substrate was anodized by applying an initial lower voltage (130 V) for 10 min as a protective layer followed by a gradual increase (10 V/min) to reach the final voltage (230 V). Afterwards, the anodization was conducted for 1 hour. The entire experiment was performed in 0.075 M H2C2O4 containing 10 vol % of ethanol at -4°C at an air flow rate of 40 cm3/s. Ethanol was used as an anti-freezing agent and contributed to dissipate the heat generated during the anodization process. Subsequent to this, pores were modulated by temperature modulation by changing the flow rate of air against the anode between 20 cm3/s and 40 cm3/s (see Figure S1). The length of the segment with constant pore diameter could be accurately controlled by manually changing in the duration of the constant flow rate intervals of the air. 3D-AAO Networks Modulated pores with underlying Al were converted into 3D-AAO networks, as follows: Aluminum Removal Prior to the acidic dissolution of the barrier layer, the underlying supporting Al was removed from the backside by exposing it to a solution containing CuCl2·2H2O (3.5 g), HCl (100 mL), and Milli-Q water (100 mL). The solution was continuously replaced with fresh solution and the process was carried out until a dark brown membrane appeared, showing complete removal of the metallic Al. Barrier Layer Removal and Pore Widening The pre-fabricated modulated AAO templates were chemically treated by immersing in H3PO4 (0.5 M) at 39°C for 135 min in total, which resulted to remove the barrier layer (65 min) and widened pores with the formations of holes (70 min) in the transversal section (3D-AAO


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networks). An acidic treatment with different times yielded wide ranges of modulated pore radii in the longitudinal section.

2.4 Polymer Filling via Melt Wetting Method To fill the pores (modulated and 3D-AAO networks) with PS homopolymer and PS298-bPDMS195block copolymer, 5wt % solution of PS and PS298-b-PDMS195 in toluene were prepared. The polymer solution (50 µL) was then dropped onto a silicon wafer. After solvent evaporation, the polymer film on the silicon wafer was further dried using a vacuum pump. Subsequently, an AAO template was placed on top of the PS and PS298-b-PDMS195 film. The melt wetting process was carried out at 170°C for 24 h. After the melt wetting process, the AAO template was dissolved selectively by immersing it into a 5 wt % H3PO4 solution for 72 h, followed by filtration through a polycarbonate membrane and gentle sonication (Supporting Information, Figure S2). Then the samples were washed with deionized water for several times and the PS and PS298-b-PDMS195 nanostructures were subsequently obtained.

2.5 Field Emission Scanning Electron Microscopy (FESEM) Measurements The FESEM data were acquired on a Zeiss Ultra 55cv field emission scanning electron microscope (Zeiss, Oberkochen, Germany). All measurements were performed with an operation voltage of 10 kV with the Inlens secondary electron detector. Prior to the measurements, samples were coated with 5 nm gold (Edwards Sputter Coater S150B at 15-20×10-1 mmHg) to make the surface conductive. For the analysis of the FESEM micrographs, ImageJ software (scanning probe image processor, Version 5.0.7) was used.


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2.6 Transmission Electron Microscopy (TEM) Measurements A JEOL JEM-2100 transmission electron microscope with an acceleration voltage of 200 kV was used to determine the internal structure of the microphase separated block copolymers nanoobjects. Before the TEM measurements, the samples were placed onto TEM copper grids coated with Formvar. For the analysis of the TEM micrographs, ImageJ software (scanning probe image processor, Version 5.0.7) was used.

2.7 Small Angle X-Ray Scattering (SAXS) SAXS measurement was performed at the NSRRC beamline BL23A1 using a 0.5 mm photon beam with a wavelength of 1.24 Å. For the sample preparation, first, 60 mg of PS298-b-PDMS195 block copolymers was dissolved in 1 mL of toluene and stirred for 24 h to achieve complete dissolution. Then the sample bottle was placed in hood to slowly evaporate toluene at room temperature. The residual solvent was then completely eliminated by placing the sample bottle in an oven at 60°C for 72 h. At last, thermal annealing treatment was conducted by heating the block copolymer samples at 170°C for 12 h to increase the long range ordering of block copolymers. The block copolymer samples were taken out, sealed between two thin Kapton windows, and measured at an ambient temperature.

2.8 Atomic Force Microscopy (AFM) Atomic force microscopy (AFM, Bruker Innova) under soft tapping mode conditions was applied to determine the surface morphologies of the block copolymers thin films on Si. The sample was prepared by spin coating a 1 wt % PS298-b- PDMS195 solution in toluene onto a silicon wafer at 2000 rpm for 40 s. The sample was imaged with AFM after being dried using a


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vacuum pump. The cantilevers had nominal values of thickness, width, length, force constant and resonance frequency of 2.8 µm, 27 µm, 150 µm, 7.4 N/m and 160 kHz, respectively.

3. Results and Discussion 3.1 Modulated Pores and 3D-AAO Networks The fabrication of AAO with modulated pores and 3D-AAO networks (interconnected pores) is based on TMHA afforded by modulating the air flow, which was carried out at a final constant potential of 230 V (0.075 M H2C2O4 containing 10 vol % ethanol at -4°C). Initially, the anodization was started at a lower voltage (130 V), which was followed by a gradual increase with a rate of 10 V/min to avoid electric breakdown of the sample. Conducting one hour of preanodization at 230 V (resulting in straight pores) led to a stable current density of 15 mA/cm2 under a flow of air at a constant rate. By decreasing the flow rate of air from 40 cm3/s to 20 cm3/s, the convection of the cold electrolyte decreased, which resulted in an increase in the temperature of the barrier layer, which in turn caused a change in current density from 15 mA/cm2 to 700 mA/cm2. The temporal variation of voltage and current density is shown in Figure 2a. This process generates diameter-modulated pores. As shown in Figure 2a, after the formation of the initial cylindrical straight pores in the pre-anodization step, 6 cycles to fabricate modulated pores by varied flow rate intervals of the electrolyte occurred. During this stage, the current density was gradually increased from 15 mA/cm2 to 700 mA/cm2 within 10 s. Then it decreased back to the steady state for the same period. During operation, however, a transient that lasted ~2s was observed, which is caused by turning from higher to lower flow rates of air and vice versa. The duration of anodization at different current densities determined the lengths of the modulated pores and narrow segments.


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Figure 2. (a) Plot of voltage and current density vs. time during the initial stage of modulation during HA (230 V) using 0.075 M H2C2O4 containing ethanol (10 vol %) at -4°C. This process yielded pores with an interpore distance of 510 ± 10 nm. (b) Schematic illustration of modulated pores by modulating the air flow rates.

During anodization, the periodic changes of the flow rate between 20 cm3/s and 40 cm3/s afforded periodic changes in the pore diameter. The length of the segment with constant pore diameter could be accurately controlled by changing the duration of the constant flow rate intervals of the air. Consequently, the length of the wide segments (490 ± 20 nm) and narrow segments (119 ± 10 nm) corresponded to intervals of 4 s and 2s, respectively. Pore diameters in the wide and narrow segments were 130 ± 15 nm and 47 ± 11 nm, respectively (Figure 3a). Modulated pores were then widened to enlarge the dimensions (Figure 3b).


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Figure 3. Cross-sectional FESEM micrographs: (a) Modulated pores by applying air-assisted TMHA using a final voltage of 230 V in H2C2O4 (0.075 M) containing ethanol (10 vol %), which acts as anti-freezing agent at -4°C. Pore diameters are 130 ± 15 nm and 47 ± 11 nm in the wide (yellow arrows) and narrow segment (red arrows), respectively. The length of the modulated pores (4 s interval at constant flow rate) and interpore distances are 490 ± 20 nm and 510 ± 10 nm, respectively. (b) High magnification section of panel (a) after 17 min widening. Pore diameters are 195 ± 30 nm and 183 ± 18 nm in the wide and narrow segment, respectively. (c) Schematic illustrating the flow rates. Higher and lower flow rates correspond to the narrow and wide segments, respectively.

The pre-fabricated modulated template was immersed for in H3PO4 (0.5 M) at 39°C for 135 min, after etching the underlying Al, where 65 min were needed initially to remove the barrier layer. This acidic treatment yielded pore radii of 217 ± 17 nm (Figure 4a) and the formation of holes in the transversal sections (Figure 4b). Hence, 3D networks with interconnected pores in the transversal sections were achieved. The different rates of oxide dissolution in acidic medium are attributed to the different chemical composition associated with ion incorporation during fluctuation of current densities.31,51,52 The holes in the transversal section were created as a result of spatially confined dissolution of the oxide layer between the modulated pores in the


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longitudinal section.42,50 In this work, holes which are ellipsoidal in shape are smaller in size by a factor of 10 compared to the longitudinal pores (Figure 4c and 4d).

Figure 4. Formation of 3D-AAO networks. (a) Side-view FESEM micrograph of 3D-AAO networks having radii, wall thickness and interpore distances of 217 ± 17 nm, 77 ± 15 nm and 510 ± 10 nm, respectively. The red rectangle indicates a pore with an array of holes located at each plane of the hexagonal cell. (b) Cross-sectional FESEM micrograph of 3D-AAO network. The pre-fabricated modulated template was immersed in H3PO4 (0.5 M) at 39°C for 135 min, after etching the underlying Al. The holes are ellipsoidal in structure, and the sizes are 115 ± 20 nm and 230 ± 25 nm along the semi-minor axis and the semi-major axis, respectively. Panels (c) and (d) show histograms of the pore and ellipsoid areas, respectively.


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Additionally, a multilayered template was fabricated, which consisted of alternative stratified layers of straight pores and 3D networks (Figure 5). This approach may be used to adjust desired internal porosity affording high surface area.

Figure 5. Cross-sectional FESEM micrographs of multilayered 3D-AAO networks and straight pores. The template was synthesized initially by the performance of the modulation at an applied potential of 230 V (0.075 M H2C2O4) containing ethanol (10 vol %), which acts as anti-freezing agent at -4°C. Subsequently, the template was treated in 0.5 M H3PO4 at 39°C for 135 min.

3.2 Polymeric Nanorods and Nanocapsules by Replication of AAO Templates The AAO templates fabricated according to the two methods discussed above were replicated to yield polymeric nanorods and nanocapsules via the melt wetting method with PS and PS298-bPDMS195. Prior to replication, the morphology of PS298-b-PDMS195 was studied in the bulk and on a flat silicon substrate. To this end, SAXS measurements were applied to characterize the microphase separated morphology and self-assembly of PS298-b-PDMS195 in the bulk. The SAXS measurement is presented in q-I curve (Figure 6a), where I represents the scattered intensity and q represents the scattering vector. The ratio of the relative q values of PS298-b-PDMS195 is close


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to 1: √3: √4: √7, which is characteristic for the morphologies of hexagonally packed cylinders of PS298-b-PDMS195 in the bulk state. The value of q1 (0.01494 Å-1) was evaluated to determine the domain spacing (length of one repeating period of block copolymers) of PS298-b-PDMS195 and was found to be 42 nm. In addition, the domain spacing of spin-coated block copolymers on a flat Si substrate (Figure 6b) and replicated from modulated pores obtained by method A (Figure 7c, TEM) were on average 37 ± 4 nm and 35 ± 5 nm, respectively. The difference of domain spacing between the bulk and thin film states can be attributed to several factors, such as the difference in post-treatments, the interfaces, and the measuring techniques. Additional TEM images of the PS-b-PDMS block copolymer nanostructures using the pore-size modulated AAO membranes are also provided in the Supporting Information (Figure S3).

Figure 6.(a) SAXS data of PS298-b-PDMS195 block copolymer acquired for the bulk state. (b) Soft tapping mode AFM height image of cylindrical PS298-b-PDMS195 block copolymer coated on a flat silicon substrate.

The differences in the domain spacing are attributed to chain stretching and compression as a


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result of the confined geometry in order to be commensurate with the volume and diameter of the modulated pores, causing a frustration of the polymer chains and the generation of an altered morphology, which is different from those in the bulk state and thin film state.

Figure 7. FESEM and TEM micrographs of PS298-b-PDMS195nanorods: (a) FESEM micrograph of closely connected modulated nanorods having diameters of 196 ± 15 nm in the wide segment with lengths of 490 ± 20 nm (4 s interval at constant flow rate). (b) FESEM of nanorods having diameters of 196 ± 15 nm and 100 ± 10 nm in the wide and narrow segments, respectively. The lengths of modulated nanorods in the wide and narrow segment are 400 ± 15 nm (3 s interval at constant flow rate) nm and 90 ± 15 nm, respectively. (c) TEM micrograph of PS298-b-PDMS195 nanorods. The darker and lighter regions indicate the PDMS and the PS domains, respectively. The diameter and length of PS298-b-PDMS195 nanorods in the wide segment are 130 ± 15 nm and 325 ± 20 nm, respectively. The domain spacing of block copolymer nanorods is 35 ± 5 nm. In Figure, (a) and (b) were replicated from the templates initially widened for 18 min, while (c) was the original template without widening. The used templates were without transversal holes.

For the separation of the ellipsoidal structures into isolated polymeric nanocapsules, the modulated PS298-b-PDMS195 nanorods were filtered repeatedly through a polycarbonate


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membrane (pore sizes ~0.1 µm), sonicated gently in water and filtered again afterwards. From the FESEM data shown in Figure 8a, nanoparticles with diameters in the wide segment of 200 ± 20 nm were obtained. The domain spacing in the block copolymer was 35 ± 5 nm (Figure 8b). Parallel cylinder morphologies of the block copolymers were observed in the nanostructures due to the stronger interactions between PDMS and AAO walls than those between PS and AAO walls. Up to 3 cylinders, which are equally spaced in those nanoobjects, were observed. The possible arrangements to accommodate cylinders inside a modulated nanoobject are their location away from the alumina interface (Figure 8c), at the interface (Figure 8d) and cross-cut (Figure 8e) showing cylinders formed away from the interface. For the PS-b-PDMS block copolymers confined in AAO membranes with other single pore sizes (200 and 60 nm), similar parallel cylinder morphologies could be observed, as shown in Figure S4.

Figure 8. (a) FESEM micrograph of PS298-b-PDMS195 nanoobjects after filtration and gentle sonication with diameters of 130 ± 20 nm in the wide segment and 670 ± 30 nm in lengths.


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Nanocapsules were replicated from the templates without any initial widening step. (b) TEM micrograph of nanocapsules having diameters of 128 ± 17 nm in the wide segment and lengths of 660 ± 30 nm (6 s interval at constant flow rate). The domain sizes are 35 ± 5 nm. A barrier layer was completely removed in 65 min by treatment in H3PO4 (0.5 M) at 39°C and used without widening. The used templates were without transversal holes. (c, d, and e) Schematic of cylinders forming in a modulated pore away from the interface, at the interface and cross-cut away from the interface, respectively.

Finally, the 3D-AAO interconnected networks (method B) were also successfully replicated with PS homopolymer. The PS nanostructures had defined diameters of 451 ± 25 nm in the wide segments. In Figure 9a, nanostructures are indicated that are consistent with a complete replication, some parts were observed (Figure 9b), however, in which the replication was not fully achieved. This could be attributed to the breakdown of very thin lateral polymer structures in the elliptical holes between pores (Figure 9b and Figure S5, rectangles in red).

Figure 9. FESEM micrographs of replicated PS from 3D-AAO network: (a) Low magnification micrograph of replicated PS. (b) High magnification micrograph of replicated PS with junction points marked in red. The diameter in the wide segment and the length of the PS nanostructures


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(templates were synthesized during 8 s interval at constant flow rate) are 451 ± 25 nm and 997 ± 70 nm, respectively. The pre-fabricated modulated template was immersed in H3PO4 (0.5 M) at 39°C for 135 min to achieve interconnected pores.

4. Conclusions In the so called air-assisted Temperature Modulated Hard Anodization (TMHA) method modulated AAO nanopores were achieved with a high level of control. The modulated pores were separated by narrow segments at all nanoscales ranging from 119 ± 10 nanometers to 6 ± 0.1 micrometers. The diameters of modulated pore in the wide segment ranged from 130 ± 15 nm to 451 ± 25 nm. The lengths of modulated pores were adjusted from 325 ± 25 nm to 997 ± 70 nm, in the longitudinal section. Pre-designed modulated pores were transformed into 3D-AAO networks with ellipsoidal holes in the transversal section after widening process and afforded in increase in the internal porosity. The aforementioned defined geometries of modulated pores and 3D-AAO networks were replicated by PS298-b-PDMS195 block copolymer and PS homopolymer (35 kg/mol) using the melt wetting method. By filtrating PS298-b-PDMS195 into AAO with modulated pores, nanorods comprised of PS298-b-PDMS195 were obtained and self-assembled into parallel alignment. Furthermore, the domain spacing of PS298-b-PDMS195 in the modulated pores, in bulk state and in the thin film state was found to differ. These differences in the domain spacing are attributed to the confinement effect with least domain spaces in AAO pores. Similarly, a polymeric 3D-PS network was successfully obtained by filling a 3D-AAO network with PS homopolymer. The successfully replicated nanostructured polymeric nanorods and nanocapsules with controlled shapes provide new pathways to fabricate functional compartmentalized nanosystems for studies of drug delivery, solar, biomedicine and so forth.


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ASSOCIATED CONTENT Supporting Information. Scheme for anodization setup; Photograph of filtration through polycarbonate membrane; TEM micrographs of PS298-b-PDMS195 nanorods; PS298-b-PDMS195 block copolymers introduced into two discrete AAO membranes with different single pore sizes; FESEM micrograph of replicated PS from 3D-AAO network.

AUTHOR INFORMATION Corresponding Author *Holger Schönherr. E-mail: [email protected] * Jiun-Tai Chen. E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources This work was supported by the German Academic Exchange Service (Deutscher Akademischer Austauschdienst, DAAD), the European Research Council (ERC grant to H.S., no. 279202), the University of Siegen, and the Ministry of Science and Technology of the Republic of China.


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Notes The authors declare no competing financial interest.

Acknowledgement The authors gratefully acknowledge Prof. Dr. Xin Jian (Department of Mechanical Engineering, University of Siegen, Germany) for the access to the FESEM and Dipl.-Ing. Gregor Schulte for excellent technical support. We appreciate Dr. U-SerJeng and Dr. Chun-Jen Su of the National Synchrotron Radiation Research Center (NSRRC) for their help in the SAXS measurements.

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For Table of Contents use only

Interplay of Template Constraints and Microphase Separation in Polymeric Nanoobjects Replicated from

Novel

Modulated

and

Interconnected

Nanoporous Anodic Alumina Haider Bayat,†a Chia-Hua Lin,†b Ming-Hsiang Cheng,b Marc Steuber,a Jiun-Tai Chen,*band Holger Schönherr*a

An AAO scaffold with modulated pores is synthesized in a refined TMHA approach. A subsequent acidic treatment of modulated pores led to the formation of an interconnected 3D network. The templates are applied to replicate polymeric systems using the melt wetting method with PS homopolymer and cylinder-forming block copolymer PS298-b-PDMS195. Replicas are transformed into shaped nanocapsules and 3D-polymeric nanoobjects.


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Interplay of Template Constraints and Microphase Separation in

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