Article pubs.acs.org/Langmuir
Hierarchically Structured Multifunctional Porous Interfaces through Water Templated Self-Assembly of Ternary Systems Alberto S. de León,† Adolfo del Campo,‡ Marta Fernández-García,† Juan Rodríguez-Hernández,*,† and Alexandra Muñoz-Bonilla*,† †
Instituto de Ciencia y Tecnología de Polímeros (ICTP-CSIC), C/Juan de la Cierva 3, 28006 Madrid, Spain Instituto de Cerámica y Vidrio (ICV-CSIC), C/Kelsen 5, 28049 Madrid, Spain
‡
ABSTRACT: Herein, a facile water-assisted templating approach, the so-called breath figures method, has been employed to prepare multifunctional and hierarchically structured porous patterned films with order at different length scales (nano- and micrometer). Tetrahydrofuran solutions of ternary blends consisting on high molecular weight polystyrene, an amphiphilic block copolymer, polystyrene-b-poly[poly(ethylene glycol) methyl ether methacrylate] (PS40-b-P(PEGMA300)48), and a fluorinated copolymer, polystyrene-b-poly(2,3,4,5,6-pentafluorostyrene) (P5FS21-bPS31), have been used to obtain films varying the proportion of the three components. Confocal micro-Raman spectroscopy and atomic force microscopy demonstrated the preferential location of the different functionalities in the films. Because of the breath figures mechanism, the amphiphilic copolymer yield pores enriched in hydrophilic functionality while the fluorinated copolymer remained mixed with the PS matrix and eventually also forming self-assembled nanostructures at the surface. As a consequence, two levels of order can be observed, i.e., micrometer size pores with nanostructured domains due to the block copolymer self-assembly. In addition, the distribution of the amphiphilic copolymer within the holes is not regular being located principally on the edges of the cavities. This can be attributed to the coffee stain phenomenon occurring in the water droplets as a consequence of the segregation of the block copolymers to the droplets and their self-assembly.
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evaporation for fabricating porous films.12,13 The characteristics of the holes such as pore size, distribution or the formation of mono- and multilayer pores can be easily modulated by the processing conditions. Furthermore, taking into account the formation mechanism of the pore the chemical functionality of the cavities can be controlled simultaneously with the topography. This is due to the formation mechanism of the breath figures, where the precipitation of the polymers around the water droplets implies the orientation of the hydrophilic groups preferably to the wall of the holes. The location of the polar and functional groups mainly inside the pores opens even more the field of applications and the potential of these materials as micrometer-sized reactors, sensors, filters, or catalytic sites. Besides the formation of pores and the modification of their chemical composition, hierarchical surface patterns have been prepared, for instance, developing the breath figures process onto structured substrates such as TEM grids or in combination with photolithography.14−16 Other examples described in the literature concern the creation of hierarchical
INTRODUCTION The surface of a material dictates many of their properties and, consequently, their applicability. For instance, it is well-known that the control of both the chemical composition and the topology of the surface is crucial to enhance the wetting properties of the surface, which recently have allowed the preparation of superhydrophobic surfaces for self-cleaning applications.1,2 Moreover, functional micropatterned surfaces with periodic order have been used for optical applications such as photonic crystals3 or even for cell culturing purposes.4,5 With the aim of designing functional and/or patterned polymeric surfaces two different strategies have been proposed, i.e., top-down approaches based mainly on the use of lithographic techniques and the bottom-up methods based on self-assembly of singular elements. However, the preparation of functional and hierarchically structured interfaces can be only carried out by multistep complex procedures that combine selfassembly processes with micrometer length scale templating methods.6−10 In this sense, the breath figures method11 has been used in the past years as alternative to these approaches to functionalize and for structuring polymeric materials at microand nanometer length scales. Breath figures is a simple technique which involves the condensation of water droplets on the surface of a polymer solution during the solvent © 2012 American Chemical Society
Received: March 30, 2012 Revised: May 22, 2012 Published: May 23, 2012 9778
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pattern using the self-assembly of block copolymers17−19 or the use of polymer blends with incompatible phases.20−22 In particular, our group previously reported the formation of nanostructuration in the interior of microcavities by combination of breath figures and self-assembly of block copolymers, using a polymeric blend based on an amphiphilic ABC triblock copolymer and a high molecular weight polystyrene matrix.23,24 The triblock copolymer, poly(2,3,4,5,6-pentafluorostyrene)-bpolystyrene-b-poly[poly(ethylene glycol) methyl ether methacrylate] (P5FS21-b-PS31-b-P(PEGMA300)38) was designed to include a hydrophilic segment of P(PEGMA300), a hydrophobic block of P5FS, and a central segment of polystyrene to compatibilize with the polystyrene matrix. In this case, the enrichment of the amphiphilic triblock copolymer within the holes was directed by the hydrophilic segment of the copolymer. In addition, micellar nanostructuration was generated on the pore surface caused by the self-assembly of the block copolymer. The structuration observed within the holes could even be tuned by soft annealing, varying from micellar structures to a lamellar phase. In this previous contribution the structuration and functionalization are limited to the inner part of the pore where the block copolymer is concentrated during the evaporation process. In order to enhance both the structuration process and to extend the surface chemical composition to other surface areas in the present article, we investigated further the hierarchical structuration obtained from polymer blends of diblock copolymers and high molecular weight homopolymer matrix. PS matrix will be mixed with two block copolymers, P5FS21-b-PS31 and PS40-b-P(PEGMA300)48, instead of the triblock copolymer, P5FS21-b-PS31-b-P(PEGMA300)38. Thus, the complexity of the system significantly augments by the combination of the breath figures process with the phase separation and self-assembly of two different block copolymers. As will be described, the incorporation of two different copolymers allowed the preparation in one single step of multifunctional hierarchically structured surfaces with controlled topography by combination of different effects acting simultaneously at different length scales. In addition to the above-mentioned breath figures approach, the coffee-ring25 effect acting during the evaporation of the condensed water droplets will be responsible for both the microscale topography and the original chemical functional group distribution within the pores. Finally, the self-assembly of block copolymers allowed the formation of structures at the nanometer scale. The characterization of these systems, due to their complexity, requires powerful characterization techniques. Most of the techniques found in the literature to analyze the chemical composition of the surface and the functionality inside the pores are methods based on fluorescence or luminescence such as confocal fluorescence microscopy or cathodoluminescence.26 However, the use of these methods is limited to specific components containing, for instance, fluorescent substances or the use of indirect method as the posterior attachment of fluorescent indicators. In this sense, confocal micro-Raman microscopy is postulated as an excellent and powerful candidate to study the breath figures mechanism and all the process associated, such as self-assembly of block copolymers, phase separation of incompatible components, or decoration of certain hydrophilic structures of the internal cavity of the walls.
Article
EXPERIMENTAL SECTION
Materials. The amphiphilic block copolymer polystyrene-b-poly[poly(ethylene glycol) methyl ether methacrylate] (PS40-b-P(PEGMA300)48) (copolymers are labeled with the degree of polymerization of each block) and the fluorinated block copolymer polystyrene-b-poly(2,3,4,5,6-pentafluorostyrene) (P5FS21-b-PS31) were synthesized via atom transfer radical polymerization (ATRP) as previously reported.23,24,27,28 Polystyrene (PS) (Aldrich, Mw = 2.50 × 105 g mol−1) was used as polymeric matrix. Tetrahydrofuran (THF) was purchased from Scharlau. Round glass coverslips of 12 mm diameter were supplied from Ted Pella Inc. Film Preparation. Polymer blend solutions were prepared by dissolving both PS matrix and the block copolymers in THF. The PS concentration was 50 or 80 wt %, and the copolymers proportion was
Table 1. Formulation of the Binary and Ternary Blends Used To Prepare the Microstructured Films via Breath Figures and Their Average Pore Size pore size (μm) PS matrix sample (wt %) S1 S2 S3 S4 S5 S6 S7 S8 S9 S10
80 80 80 80 80 50 50 50 50 50
P5FS21b-PS31 (wt %)
PS40-bP(PEGMA300)48 (wt %)
20 15 10 5 0 50 37.5 25 12.5 0
0 5 10 15 20 0 12.5 25 37.5 50
RH50% 1.5 0.8 1.4 2.7 2.7
± ± ± ± ±
0.2 0.2 0.5 0.4 0.4
RH70% 2.0 1.0 2.0 3.2 3.7 2.3 2.9 2.7 2.3 400
± ± ± ± ± ± ± ± ± ±
0.6 0.3 0.4 0.5 0.6 0.1 0.2 0.4 0.4 100
varied (Table 1). The total polymer concentration was 30 mg/mL in all the solutions. Films were prepared from these solutions by casting onto glass wafers under controlled humidity inside of a closed chamber. Measurements. Scanning electron microscopy (SEM) micrographs were taken using a Philips XL30 with an acceleration voltage of 25 kV. The samples were coated with gold−palladium (80/20) prior to scanning. The analysis of the pore size (average diameter), pore size distribution, and Voronoi tessellation graph were performed using the image analysis software (ImageJ). Atomic force microscopy (AFM) measurements were conducted on a Multimode Nanoscope IVa, Digital Instrument/Veeco operated in tapping mode under ambient conditions. Small-angle X-ray diffraction measurements with X-ray synchrotron radiation were performed in the soft-condensed matter beamline A2 at Hasylab (Hamburg, Germany), working at a wavelength of 1.50 Å. The experimental setup included a specimen holder and a MARCCD detector for acquiring two-dimensional SAXS patterns (sample-to-detector distance being 260 cm). The different orders of the long spacing of rat-tail cornea (L = 65 nm) were utilized for calibration of the SAXS detector. The 2D X-ray diffractograms were processed using the A2tool program developed to support beamline A2 data processing. The profiles were normalized to the primary beam intensity, and the background from an empty sample was subtracted. Contact angles were measured with deionized water on a goniometer KSV Theta (KSV Instrument, Ltd., Finland) at room temperature. Water droplets of 3.0 μL were placed on the films, and a charge coupled device camera was used to capture the images of the water droplets for the determination of the contact angles. The chemical composition and the distribution of the different components on the polymeric films were determined using confocal Raman microspectroscopy integrated with AFM on a CRM-Alpha 300 RA microscope (WITec, Ulm, Germany) equipped with Nd:YAG dye laser (maximum power output of 50 mW power at 532 nm). 9779
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Figure 1. SEM images of samples (a) S1, (b) S2, (c) S3, (d) S4, and (e) S5 prepared at 70% RH and (f) S1, (g) S2, (h) S3, (i) S4, and (j) S5 prepared with at 50% RH.
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RESULTS AND DISCUSSION Binary and ternary blends were prepared by mixing the block copolymers, P5FS21-b-PS31 and PS40-b-P(PEGMA300)48, with the PS matrix in different proportions (Table 1) and dissolved in THF with a total concentration of 30 mg/mL. As the typical breath figures method, porous films were prepared from these polymer solutions by casting in a moist atmosphere with 50 and 70% of relative humidity (RH). Initially, the films containing 80 wt % of PS matrix and only 20 wt % of copolymers, samples S1−S5, were studied to investigate the influence on the pores array formation by mixing an amphiphilic and a hydrophobic copolymer with the PS matrix. As shown in Figure 1a−e, the preparation of the samples at 70% RH implies the presence of microsized holes on the surface of the films, with slightly irregular arrays. Concerning the films of the binary blend PS/P5FS21-b-PS31, sample S1 (Figure 1a), SEM image reveals that is composed of a large area of relatively ordered pattern with cavities of 2.0 ± 0.6 μm diameter. The incorporation of the amphiphilic copolymer affords the formation of ternary blends PS/ P5FS21-b-PS31/PS40-b-P(PEGMA300)48 (Figure 1b−d), where the amounts of both copolymers were varied, remaining constant the total percentage of block copolymers relative to the PS matrix. From the SEM micrographs it can be seen that the presence of the amphiphilic copolymer causes loss of order and irregular arrays of holes are observed. Regarding the size of the pores, the diameter increases as the percentage of PS40-bP(PEGMA300)48 augments in the blend, presenting sizes of 1.0 ± 0.3, 2.0 ± 0.4, 3.2 ± 0.5, and 3.7 ± 0.6 μm for samples S2− S4 and the binary blend S5, respectively. In the case of the binary blend the PS/PS40-b-P(PEGMA300)48 (Figure 1e) the holes are arranged in more regular pattern as happened with the other binary blend PS/P5FS21-b-PS31. A similar trend was observed for the films prepared from the same polymeric solution, samples S1−S5, at lower RH, 50% (Figure 1f−j). Disordered porous structures were obtained for the ternary blend, while much more regular arrays were formed for the binary blends (Figure 1f,j). Therefore, and as expected, the sizes of the holes are smaller for all the compositions in comparison with the films prepared under higher RH, 70%. In this case, at 50% RH (Figure 1f−j), the average pore sizes are 1.5 ± 0.3, 0.8 ± 0.2, 1.4 ± 0.5, 2.7 ± 0.4, and 2.7 ± 0.4 μm for the samples S1, S2, S3, S4, and S5, respectively. It is well-known that characteristics of the honeycomb patterned films such as the pore diameter are strongly dependent on the experimental
conditions (humidity, solvent, or type of polymer). From these results, it is clear that the combination of two block copolymers with very different character on a PS matrix affects the formation and the quality of the porous microstructures. In the case of the binary blend containing only the PS matrix and the fluorinated copolymer, the polymers precipitated around the water droplets during the process, preventing their coalescence and, consequently, forming a porous patterned film. Because of the formation mechanism of the breath figures, the amphiphilic structures should arrange around the water droplets, involving the orientation of hydrophilic groups preferably inside the holes once the solvent is evaporated and the porous arrays formed. For this reason, the increase of the PS40-b-P(PEGMA300)48 on blends containing this copolymer, S2−S5, entails the increase of the pore size because the amphiphilic structures can stabilize the formed water droplets reducing the surface tension and permitting their growth. Consequently, in the present study using ternary blends, the amphiphilic copolymer PS40-bP(PEGMA300)48 should be mainly located in the wall of the holes, while the hydrophobic copolymer P5FS21-b-PS31 is supposed to be mixed in the PS matrix between holes. This phase distribution differs from our previous contribution where both the fluorinated and the hydrophilic segments remained within the cavities due to the covalent attachment of the three blocks.8a Herein, we have employed confocal micro-Raman microscopy, for the first time to the best of our knowledge, to analyze the chemical composition of micropatterned surfaces formed by breath figures and phase separation. First, polymer blend films containing 80 wt % of PS matrix and 20 wt % of PS40-b-P(PEGMA300)48/P5FS21-b-PS31 block copolymers (S1−S5) were analyzed. Figure 3 shows the XY and the XZ micro-Raman mapping of the diverse binary and ternary polymer blends. The intensity of the signals associated with P5FS21-b-PS31 in the mixtures are rather low (as will be described below the fluorinated component is homogeneously distributed in the PS phase). Thus, the micro-Raman images were performed only using the signals corresponding to the ring breathing mode of the polystyrene (Raman shift at 1012 cm−1) and the strong band at 1735 cm−1 attributed to the carbonyl groups of the P(PEGMA300) segment and taken point by point with a step of 100 nm. The average Raman spectra of the pure PS, P(PEGMA300), and P5FS homopolymers are displayed in Figure 2. In the Raman micrographs of the samples that contain the amphiphilic copolymers it can be clearly appreciated that the 9780
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Besides, the amphiphilic copolymer tends to self-assembly into micelles in the water droplets. As a consequence, the so-called “coffee stain” phenomenon29−32 could occur in the water droplet, which involves the deposition of the block copolymer in ringlike fashion. This coffee-ring effect takes place when the contact line of a drying droplet of a polymer solution is pinned. Thus, the solvent evaporation rate is higher at the edge which is replenished by the liquid from the interior. The outward flow carries the solutes to the edge of the drops where they are dispersed and deposited forming the solid ring. In this particular case, the contact line of the droplets of the polymer solution is fixed during the precipitation of the PS matrix around the water droplets in the breath figures process. This coffee-ring effect could be the reason for the preferential enrichment of the edges in the copolymer after the complete evaporation of the water droplets. Previously, other watersoluble components, for instance polyvinylpyrrolidone (PVP), were mixed with a polystyrene matrix and used in the breath figure technique.21 PVP is dissolved in the condensed water droplets, and after the complete evaporation they remain located in the internal wall, principally at the bottom, forming protrusions. Herein, although in some holes of the sample S4 can be appreciated the amphiphilic copolymer on the bottom surface, as shown in Figures 1d and 3d, most of the amphiphilic copolymer is located on the edge of the holes. The drying process of the polymer solution drops is a complex system, dictated by the outward capillary flow along with the Marangoni effect. Therefore, the final shape of the polymer deposits (rim, dotlike, or uniform deposits) depends on factors that can modify the process such as the addition of surfactant,33,34 the shape of the suspended particles,35 concentration,36 or molecular weight.37 The amphiphilic block copolymer used in this study presents very different properties comparing to the hydrophilic PVP, as its capacity to form micellar aggregates, which could determined the final shape of the deposits. In contrast, water-insoluble amphiphilic structures are only oriented through the water droplets but not dissolved in them. Consequently, once the water is evaporated, the amphiphilic structures are distributed homogeneously within the cavities. Whereas the coffee stain phenomenon has been previously reported in planar surfaces, in breath figures arrays this effect has only been observed in nanoparticle-
Figure 2. Average Raman spectra and chemical structure of the pure P5FS, P(PEGMA300), and PS homopolymers.
PS40-b-P(PEGMA300)48 is located into the holes (Figure 3b− e). The preferential localization of this amphiphilic copolymer makes possible the clear observation of the corresponding Raman peaks, contrary to the fluorinated block copolymer. The image of the ternary blend PS/PS40-b-P(PEGMA300)48/ P5FS21-b-PS31 containing only 5 wt % of the amphiphilic copolymer (Figure 3b) shows holes filled with it. Nevertheless, in this case, the holes are very small, and the spatial resolution may not be enough to distinguish between the center and the periphery of the cavities. In films prepared from the polymer blends containing higher amount of amphiphilic copolymer, 10, 15, and 20 wt % (Figure 3c−e), the hydrophilic P(PEGMA300) was observed on the edges of the holes. Equally, the XZ mappings depicted in Figure 3 reveal that the walls of the holes are enriched in hydrophilic blocks, in general accumulated mainly on the edge of the pores rather than at the bottom. In these cases, the use of the confocal microRaman spectroscopy for the characterization of the breath figures patterns allowed us to detect this “a priori” unexpected localization of the amphiphilic copolymer on the top of the cavities edges which has not been previously reported. In the present system, the amphiphilic copolymer has a large hydrophilic segment and can be dissolved and incorporated in the condensed water droplets during the film formation.
Figure 3. Raman micrographs of the polymer blend films formed with at 70% RH: (a) S1, (b) S2, (c) S3, (d) S4, and (e) S5. Red areas correspond to high intensity of the 1012 cm−1, thus evidencing the presence of PS-rich spaces, whereas the blue areas are related to a high intensity of the 1735 cm−1 signal that belongs to the P(PEGMA300) block. 9781
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Figure 4. Average Raman spectra for the interpore domains of the blend films S1, S2, S3, S4, and S5 formed at 70% RH. In these regions we observed an increase of the signal intensity at 1663 cm−1 related with an increase of the amount of fluorinated copolymer in the mixture (a), whereas the peak at 1735 cm−1 was not detected (b). As a consequence, the interpore domains are constituted by a mixture of PS and fluorinated copolymer.
Figure 5. AFM phase images of (a) the binary blend S1 and ternary blends (b) S2, (c) S3, and (d) S4, all prepared at 70% RH.
decorated pores.38 Nevertheless, the preferential enrichment of the pore edges in functional polymer chains in a ringlike fashion has not been described before. Previously, it was observed by cathodoluminescence and fluorescence microscopy the artificially enhancing of the intensity around the edges of the cavities in films prepared with the breath figures technique where the walls were decorated with CdSe nanoparticles26 or with hydrophilic π-conjugated segments of block copolymers.26,39 This was explained considering that the signal from a monolayer of material is too weak to be detected and the emission is only perceived from the rim of the holes where the amount of materials would seem to be larger due to a projection of the spherical shape of the cavity. In those articles the preferential location of the emitted material covering the entire wall of the holes was demonstrated by other techniques. In the present system the detection of rims around of the cavities composed on the amphiphilic copolymer cannot be an artificial effect because the XZ image also shows that the copolymer is mainly at the edge of the holes but not at the bottom. Therefore, the use of micro-Raman spectroscopy permits the detection of this effect on microstructured and multifunctional films created from polymer blends via combination with breath figures. The high resolution of this technique which permits the XY and XZ imaging allows us to observe the size and spatial distribution of the different phases or domains. However, the hydrophobic block copolymer, P5FS21-b-PS31, could not be easily distinguishable by micro-
Raman spectroscopy. The peaks characteristic of the pentafluorostyrene were only detected in PS matrix region (red area). The intensity of the bands at 1663 cm−1 associated with the aromatic −CFCF− stretching of the pentafluorostyrene was very small and difficult to detect as shown in the average spectra of the matrix region (Figure 4a). Besides, the intensity of the bands becomes lower as the amount of fluorinated copolymer decreases in the blend. These results suggest that the P5FS21-b-PS31 is homogeneously distributed on the film, which makes difficult the visualization of this component on the film. This fluorinated copolymer is supposed to be located far away from the water droplets and, consequently, outside of the pores, mixed in the PS matrix or along the surface of the film. Since fluorinated blocks present low surface energy, they are expected to be mainly on the surface of the films, forming a monolayer.40 This hypothesis was demonstrated by AFM, as can be seen in Figure 5. The phase image of the microstructured pores films of the binary blend PS/P5FS21-b-PS31 displays a lamellar morphology at the surface of the film between the pores, attributed to P5FS21-bPS31 block copolymer. However, a loss of order is exhibited as the proportion of the fluorinated copolymer decreases in the blend from S1 (Figure 5a) to S4 (Figure 5d), where smooth surface and only one phase are finally observed. Similar behavior in terms of the distribution of the components in the polymeric films was observed by micro9782
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blends containing 20 wt % of copolymers, with sizes ranging between 1 and 3 μm, while the density and regularity of the holes seems to be higher, as expected. However, in the binary blend S10 the large amount of hydrophilic P(PEGMA300) produces extensive coagulation of the water droplets, and large and heterogeneous pores were obtained. Samples S7 and S8 presented the highest regularity compared with the rest of the films (Figures 1 and 6). Therefore, and with the purpose of obtaining hierarchical structures, the packing of pores in sample S8 was quantitatively analyzed by Voronoi polygon construction.41 Low-magnification SEM image of S8, to envisage a large area of film, was processed by software ImageJ obtaining the Voronoi tessellation of the image (Figure 7). The probabilities of the pores with 5, 6, 7, and 8 nearest neighbors (P5, P6, P7, and P8) are 0.15, 0.63, 0.21, and 0.01, respectively, which gives the conformational entropy, S = −∑Pnln Pn, of 0.96. This value is significantly less than 1.71 reported for random packing of pores.41 Although much less entropy values have been previously found for breath figures patterns,42−44 this result points out that the obtained patterns are moderately ordered taking into account the complexity of the system and the use of water-miscible solvent as THF. Micro-Raman spectroscopic mapping shows that in sample S9 containing a high amount of hydrophilic component the hole sizes on the surface are smaller than that of the internal diameter as can be appreciated in the XZ mapping (Figure 8c). Concerning the polymer distribution on the films, similar results were found as compared with samples S2−S5 (80 wt % of PS matrix). Figures 8b and 8c display XY and XZ mappings of the ternary blend S8 and S9 at 70% RH, respectively. In both cases the amphiphilic copolymer PS40-b-P(PEGMA300)48 does not cover all the inner part of the pore. Instead, only the edge is concentrated in PS40 -b-P(PEGMA300)48 which can be associated with the coffee stain phenomenon. Regarding the binary blend composed of PS matrix and PS 40 -b-P(PEGMA300)48, a larger amount of hydrophilic component in the initial blend enhanced the water condensation, and instead of homogeneous pores, the coagulation of the water droplets gives heterogeneous and macrophase-separated structures, as illustrated in Figure 8d. Concerning the fluorinated copolymer, the peaks associated with the polypentafluorostyrene segments are significantly more intense compared to samples S1−S4 with P5FS21-bPS31 up to 20 wt %, as it can be appreciated in the average Raman spectrum of the matrix, corresponding to the region between holes (Figure 9a). XZ mapping of sample S6 contains 50 wt % of PS matrix, and 50 wt % of P5FS21-b-PS31 was also constructed using the band at 1663 cm−1 (green) associated with P5FS regions (Figure 9b) where it can be clearly observed that the P5FS21-b-PS31 is homogeneously distributed along the whole film. In the case of the ternary blends, the fluorinated
Raman spectroscopy and AFM on the samples prepared at lower humidity, 50% RH. Scheme 1 illustrates the formation mechanism of the films. In a first step, the evaporation of the organic solvent carried out in Scheme 1. Schematic Representation of the Methodology Employed To Prepare Structured Interfaces
humid atmosphere provokes the water condensation. The presence of three components with different affinity for water and air induces the rearrangement of the components at the surface. As a consequence, the amphiphilic block copolymer PS40-b-P(PEGMA300)48, soluble in water is concentrated in those areas where the water droplets were condensed (ii and iii). Similarly, the high affinity of the fluorinated moieties for the polymer−air interface induces the rearrangement of the P5FS21-b-PS31 toward the areas comprised between two water droplets. When the organic solvent has been completely removed, the condensed water droplets begin to evaporate (iv). The evaporation occurs from the edge of the droplet, i.e., the periphery of the pore, and induces the diffusion of the dissolved block copolymer toward the external part of the pore (v). Upon complete evaporation of the water contained in the pore the block copolymer is located, forming a ringlike structure on the edge of the pore (vi). Next, the proportion of block copolymers in the blend with respect to the PS matrix was increased up to 50 wt %. In principle, it is expected that the use of higher content of block copolymers favors the visualization of the different polymer phases on the films. Again, several samples were prepared including two binary blends (S6 and S10) and three ternary blends (S7, S8, and S9). The SEM images of the films prepared at 70% RH did not reveal significant differences in pore size as the proportion of the copolymers varied in the blends (Figure 6a−d). In general, the cavities are apparently smaller than the
Figure 6. SEM images of samples (a) S6, (b) S7, (c) S8, (d) S9, and (e) S10 prepared at 70% RH. 9783
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Figure 7. SEM image of S8 film and its corresponding Voronoi diagram.
Figure 8. Raman micrograph of the films: (a) S6, (b) S8, (c) S9, and (d) S10 prepared at 70% RH. Red areas correspond to high intensity of the 1012 cm−1, thus evidencing the presence of PS-rich spaces, whereas the blue areas are related to a high intensity of the 1735 cm−1 signal that belongs to the P(PEGMA300) block.
Figure 9. (a) Average Raman spectra for the matrix domains of the blend films S1 and S6 formed at 70% RH. (b) Raman micrograph of the polymer blend S6 film formed at 70% RH constructed from the peak at 1663 cm−1 associated with the aromatic −CFCF− stretching.
copolymer is also homogeneously located within the PS matrix; however, to simplify the figure, the Raman images were performed with the PS signals, and red color distribution is observed in Figure 8b,c. The distribution of the block copolymers with an enrichment of the PS40-b-P(PEGMA300)48 within the pores and the P5FS21-b-PS31 outside evidenced by micro-Raman clearly indicates that the chemical distribution can be controlled by the combination of breath figures and coffee-ring methodologies. As exposed before, functional and hierarchical microstructured patterns were obtained using relative low
amount of block copolymers, only 20 wt %. Nevertheless, the augment of the block copolymers proportion in the blend up to 50 wt % favors the visualization of the different polymer microphases on the films. This increment of the amount of the block copolymer should allow a further nanoscale order by selfassembly of block copolymers. As was investigated by AFM and depicted in Figure 10, two different areas containing either a lamellar phase or cylinder structure with an “edge-on arrangement” can be distinguished. Near the pore, the microphase is oriented parallel to the pore surface. According to micro-Raman, this region contains PS40-b9784
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Figure 10. AFM phase and height images of the pore edge in a film prepared from a blend (50 wt % PS, 25 wt % P5FS21-b-PS31, and 25 wt % PS40-bP(PEGMA300)48) by casting at RH 70%. In the region close to the pore a parallel oriented lamellar phase could be detected as a result of the selfassembly of the PS40-b-P(PEGMA300)48 block copolymer. Outside of this region, in the interpore area the P5FS21-b-PS31 forms equally nanostructured lamellar domains.
enriched in P5FS21-b-PS31. Thus, the breath figures and the coffee stain phenomenon induce the phase separation and the spatial distribution of the two block copolymers employed in the blends in different domains. The resulting micropatterned film shows two microphases at the surface corresponding to different block copolymer regions. SAXS experiments were performed in nonporous films of each block copolymer to analyze the possible effect of porous pattern. The characteristic distances in the corresponding morphologies of PS40-bP(PEGMA300)48 and P5FS21-b-PS31 block copolymers are 14.5 and 17 nm, respectively. These values are comparable to
P(PEGMA300)48 that self-assembled forming alternating layers of P(PEGMA300) and PS. The lamellar/cylinder spacing is ca. 17 nm. The rest of the surface has an additional lamellar phase with a lamellar spacing of ca. 19 nm. Taking into account the information obtained from the micro-Raman analysis in these areas only either P5FS21-b-PS31 or PS can be found. As a consequence, the nanostructures observed must have been caused by the microphase separation of the fluorinated diblock copolymer. Because of the low surface energy of the fluorinated moieties, the P5FS block preferably orients toward the air− polymer interface,40 and the surface between the pores is 9785
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those obtained by AFM measurement in the porous films (17 and 19 nm). Therefore, we can conclude that there is not effect of confinement. Afterward, the surface properties of these microstructured patterns containing amphiphilic and superhydrophobic block copolymers were investigated via contact angle measurements. For this purpose, the blends with higher content of block copolymers, 50 wt %, were employed to better analyze the influence of the chemical composition. Initially and for comparison, films were performed by spin-coating from the solutions S6−S10 under dry conditions (using nitrogen flow) in order to obtain flat and homogeneous films. The static contact angle, θS, of the films containing different proportion of the hydrophobic (PS31-b-P5FS21) and the amphiphilic (PS40-bP(PEGMA300)48) block copolymers are shown in Table 2. An
has proved to be an extremely useful technique to study the patterns produced, where the distribution of the different components of the films is dictated not only by the degree of compatibility between them but also by the water-assisted process itself. Since the pores are formed by the condensed water during the process, they are enriched in the amphiphilic PS40-b-P(PEGMA300)48, which tends to migrate toward the water droplets. On the other hand, the fluorinated P5FS21-bPS31 was homogeneously distributed in the PS matrix and on the outmost surface of the film. A remarkable result appreciated by confocal micro-Raman was the preferential location of the amphiphilic copolymer in the edges of the holes instead of covering the whole cavities. This selective functionalization has not been observed before and could be attributed to the water solubility and surfactant properties of the amphiphilic copolymer. Moreover, the self-assembly of block copolymers allowed the formation of structures at the nanometer scale. In summary, the mixing of a homopolymer with block copolymers exemplifies a versatile and potent possibility of making use of the combination of breath figures, phase separation, and selfassembly to fabricate multifunctional and hierarchically patterned films. Further works will be focused on ternary blends where the PS matrix is mixed with P5FS homopolymer and PS40-b-P(PEGMA)48 or with P(PEGMA300) homopolymer and P5FS21-b-PS31, in order to study the influence of the PS segment of the block copolymer to compatibilize the copolymer with the matrix.
Table 2. Static Contact Angles Measured on Flat Surfaces (Obtained under Dry Atmosphere, 16% RH via SpinCoating) and on Porous Films (Prepared at 70% RH by Casting) θS (deg) sample S6 S7 S8 S9 S10
flat surfaces 104 100 98 93 89
± ± ± ± ±
2 1 1 1 2
porous surfaces 111 110 113 111 103
± ± ± ± ±
2 1 1 2 2
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increase in PS40-b-P(PEGMA300)48 concentration accompanied of a decrease in P5FS21-b-PS31 leads to a further diminishment in contact angle. Despite the hydrophilicity of the P(PEGMA300) block, the reduction in contact angle caused by the increase of this segment amount from S6 to S10 is rather low. This is because of the high number of methyl terminal groups that are exposed to the surface.45 In contrast, contact angles on porous surfaces not only depend on the surface composition, and the values in this case are dictated by more complex rules. In general, structured porous films show a strong lotus effect, presenting high hydrophobicity and, consequently, higher contact angles.46 As shown in Table 2, the contact angles of the porous films are higher than those found on smooth surface films. Furthermore, the composition of the films seems to have no influence on the contact angle values. These results differ from the flat surfaces where the contact angles varied with the composition, meaning that the main parameter in porous surfaces is the roughness. Only a slightly decrease of contact angle from approximately 111° to 103° was observed on the binary blend S10, with 50 wt % of PS40-b-P(PEGMA300)48, where the fluorinated copolymer is absented in the blend and macrophase separation with huge holes were obtained.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (J.R.-H.);
[email protected] (A.M-B.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the MINECO (Projects MAT2010-17016, MAT2010-21088-C03-01, and COST Action MP0904 SIMUFER). A. Muñ o z-Bonilla gratefully acknowledges the MINECO for her Juan de la Cierva postdoctoral contract, and A. S. de León thanks the MECD for his FPU predoctoral fellowship. Authors thank Dr. M. Hoyos for his valuable discussions on the coffee stain effect. The synchrotron work leading to these results was performed at DESY and received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 312284. We are grateful for collaboration of the Hasylab personnel, especially Dr. S. S. Funari.
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