Article pubs.acs.org/Langmuir
Honeycomb Films with Core−Shell Dispersed Phases Prepared by the Combination of Breath Figures and Phase Separation Process of Ternary Blends A. del Campo,† A. S. de León,‡ J. Rodríguez-Hernández,*,§ and A. Muñoz-Bonilla*,§ †
Instituto de Cerámica y Vidrio (ICV), Consejo Superior de Investigaciones Científicas (CSIC), C/Kelsen 5, 28049 Madrid, Spain Mechano(Bio)chemistry, Max Planck Institute of Colloids and Interfaces, Science Park Potsdam-Golm, 14424 Potsdam, Germany § Instituto de Ciencia y Tecnología de Polímeros (ICTP), Consejo Superior de Investigaciones Científicas (CSIC), C/Juan de la Cierva 3, 28006 Madrid, Spain ‡
ABSTRACT: Herein, we propose a strategy to fabricate core−shell microstructures ordered in hexagonal arrays by combining the breath figures approach and phase separation of immiscible ternary blends. This simple strategy to fabricate these structures involves only the solvent casting of a ternary polymer blend under moist atmosphere, which provides a facile and low-cost fabrication method to obtain the porous structures with a core−shell morphology. For this purpose, blends consisting of polystyrene (PS) as a major component and PS40-b-P(PEGMA300)48 amphiphilic copolymer and polydimethylsiloxane (PDMS) as minor components were dissolved in tetrahydrofuran and cast onto glass wafers under humid conditions, 70% of relative humidity. The resulting porous morphologies were characterized by optical and confocal Raman microscopy. In particular, confocal Raman results demonstrated the formation of core−shell morphologies into the ordered pores, in which the PS forms the continuous matrix, whereas the other two phases are located into the cavities (PDMS is the core while the amphiphilic copolymer is the shell). Besides, by controlling the weight ratio of the polymer blends, the structural parameters of the porous structure such as pore diameter and the size of the core can be effectively tuned.
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INTRODUCTION During the last decade, there has been a considerable interest in designing novel materials by blending polymers, as an effective approach to develop materials with improved and/or unprecedented properties.1−3 The properties of polymeric blends are largely affected by the phase morphology. Therefore, the control of this phase morphology is a key factor to achieve the best combination of the desired properties. The understanding and control of the morphology in binary blends have been extensively explored, and most of the results indicated that the size of the minor phase would depend principally on the viscosity and the interfacial tension. However, in the case of multicomponent blends, although they are very attractive due to the wide variety of microstructured morphologies that can be formed, the complexity of these systems significantly augments and requires a more extensive investigation. In recent years, many researchers have focused on studying ternary polymer blends, that present a wide variety of microstructured morphologies with interesting properties4−8 including ultralow conductive percolation,7 low dielectric loss,6 or a drop in resistivity.8 A particularly interesting core−shell morphology has been observed when the minor component forms a shell around the small domains of the second minor component, which is itself dispersed in a third polymer, the matrix or major component. It has been observed that the final © XXXX American Chemical Society
blend morphology depends, among others, on the nature of the components, blend composition, or the molecular weight of each component.9−11 Furthermore, the morphology can be affected strongly by the interactions at the interfaces between polymer pairs and can be predicted by the following equation12,13 λ31 = γ12 − γ31 − γ23
where γ12, γ32, γ32 are the interfacial tensions for each component pair, and λ31 is the spreading coefficient for the shell formed by component 3, core by component 1 while index 2 refers to the matrix. The coefficient λ31 must be positive to form the core−shell structure, a complete wetting behavior. However, the surface chemical composition of the polymer blends may largely differ from the bulk composition because of the asymmetrical environment at the air−polymer interface, thus affecting many of the surface properties such as wettability. For instance, it was reported that the surface morphology in polymer blends can be controlled by the selective migration of desired components to the surface using blends of highmolecular-weight polymers and surface-active polymers.14 In Received: January 24, 2017 Revised: February 28, 2017 Published: March 1, 2017 A
DOI: 10.1021/acs.langmuir.7b00266 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
methanol and used as a polymeric matrix. THF was used without further purification as solvent for the preparation of the porous films. Round glass coverslips of 12 mm diameter were obtained from Ted Pella Inc. Film Preparation. Porous polymeric surfaces were prepared by the breath figures method from different ternary blends. Blends consisting of HPS as the matrix and PS40-b-P-(PEGMA300)48 and PDMS as minor components were dissolved in THF by varying the proportion of the components. The total polymer concentration in all solutions was 30 mg/mL. From these polymeric solutions, films were prepared by casting onto glass wafers at room temperature under controlled humidity, RH ≈ 70%, inside of a closed chamber. Characterization. The morphology, chemical composition, and distribution of the different components on the polymeric films were analyzed using confocal Raman microscopy integrated with atomic force microscopy (AFM) on a CRM-Alpha 300 RA microscope (WITec, Ulm, Germany) equipped with Nd:YAG dye later (maximum power output of 50 mW at 532 nm). Scanning electron microscopy (SEM) images were collected in a Philips XL30 microscope with an acceleration voltage of 25 kV. The samples were precoated with gold− palladium (80/20). The water contact angle measurements were performed in a KSV theta goniometer. The volume of the droplets was controlled to be 2.0 μL, and a charge-coupled device camera was used to capture the images of the water droplets for the determination of the contact angle values.
these cases, the obtained microstructures either at the surface or into the bulk are typically dispersed in the matrix in a random manner. Attempting to control the distribution of the components at the surface, this article involves the preparation of ordered microstructures combining two self-assembled processes, that is, the phase separation of incompatible ternary blends and the breath figures approach as a physical template method.15 The breath figures method is a simple and very efficient method for preparing films with pores ordered to form a honeycomb pattern.16−21 This technique consists of the evaporation of a polymer solution in a moist atmosphere. During the solvent evaporation, the temperature of the solution decreases, inducing the condensation of water droplets on top of the evaporating solution. The water droplets form a stabilized polymer/water interface and can self-organize into an ordered hexagonal array favored by the Bénard−Marangoni convection, and finally, a film with ordered porous structure is obtained after complete solvent and water evaporation. Besides, this breath figures formation process involves the preferential orientation of the hydrophilic segments of the polymers toward the interior of the pores once the solvent is evaporated and the porous arrays formed.22−24 Remarkably, when polymer binary blends composed of hydrophobic and hydrophilic polymers are used, a phase separation occurs simultaneously during the pore formation. As a result, the hydrophilic polymer component segregates inside of the cavities, whereas the rest of the surface is formed almost exclusively by the hydrophobic polymers.25−27 Taking advantage of these synchronized processes, our group previously prepared ordered microstructures by breath figures approach using ternary blends of incompatible polymers.15 In this previous work, ternary polymer blends based on the highmolecular-weight polystyrene (HPS) matrix, PS 40 -b-P(PEGMA300)48 amphiphilic copolymer, and a surface-active poly(2,3,4,5,6-pentafluorostyrene) homopolymer were used. Different microstructures produced due to the partial wetting were obtained depending on the blend composition, most of them ordered in hexagonal patterns because of the breath figures mechanism. On the basis of this idea, herein, we designed the ternary system to achieve complete-wetting microstructures ordered in honeycomb arrays. For this purpose, ternary blends consisting of HPS as a major component and PS40-b-P(PEGMA300)48 and polydimethylsiloxane (PDMS) as minor components were used in the breath figures process. In addition to the formation of self-assembled honeycomb films by breath figures, the phase separation of the incompatible polymers occurring in the honeycomb patterns will be described. According to our knowledge, there is no precedent in the fabrication of ordered hexagonal arrays with the core− shell morphology by the combination of these two processes, breath figures and phase separation.
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RESULTS AND DISCUSSION Different ternary blends were prepared by mixing the amphiphilic block copolymer PS40-b-P(PEGMA300)48 and the PDMS with the HPS matrix and dissolved in THF to obtain a 30 mg/mL solution. Table 1 summarizes the weight ratio of the Table 1. Chemical Composition of the Different Blends Used to Prepare the Microstructured Films and the Pore Diameter (D) Obtained for Each Composition Determined in the Center of the Films HPS (wt %)
PS40-b-P(PEGMA300)48 (wt %)
PDMS (wt %)
87.5 75.0 62.5 50.0 86.5 50.0
12.0 24.5 37.0 49.5 12.0 48.5
0.5 0.5 0.5 0.5 1.5 1.5
D (μm) 1.8 2.4 3.2 8.7 2.3 8.5
± ± ± ± ± ±
0.7 0.2 0.2 0.7 0.4 0.9
HPS/PS-b-P(PEGMA300)/PDMS polymeric blends. Then, several films were made by casting of these ternary polymer blend solutions onto glass wafers under 70% of relative humidity. Figure 1 shows the optical images of the resultant films, which clearly indicate the formation of typical microporous structures obtained by the breath figures approach. Noticeably, the diameter of the pores decreases as the content of the amphiphilic copolymer (PS40-b-P(PEGMA300)48) diminishes in the blend, from 8.7 to 1.8 μm, as shown in Table 1. It is wellknown that the pore diameter of these honeycomb-patterned films depends on the experimental conditions such as humidity, temperature, solvent, or type of the polymer.29−31 The use of amphiphilic compounds also influences the dimension of the cavities because these amphiphilic structures are able to arrange around the condensed water droplets and stabilized them, reducing the surface tension and permitting their growth. Consequently, larger pore sizes are obtained when the content of amphiphilic polymers increases in the blend.26
EXPERIMENTAL SECTION
Materials. The amphiphilic block copolymer polystyrene-b-poly[poly(ethylene glycol) methyl ether methacrylate] [PS40-b-P(PEGMA300)48] (copolymer is labeled with the degree of polymerization of each block) was synthesized via atom transfer radical polymerization (ATRP), as previously reported.28 The precise composition of the block copolymer and the number average molecular weight were calculated by 1H NMR, Mn = 18 500 g/mol, whereas the polydispersity index determined by size exclusion chromatography (SEC) was PDI = 1.21. PDMS trimethylsiloxy terminated (Alfa Aesar, Mw = 770 g/mol) was used as received. Highmolecular-weight polystyrene (HPS) (Aldrich, Mw = 2.50 × 105 g/ mol) was precipitated from a tetrahydrofuran (THF) solution in B
DOI: 10.1021/acs.langmuir.7b00266 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 1. Optical images of the honeycomb porous films cast from THF solution of HPS/PS40-b-P(PEGMA300)48/PDMS ternary blends with a weight ratio of (a) 50.0/49.5/0.5, (b) 62.5/37.0/0.5, (c) 75.0/24.5/0.5, and (d) 87.5/12.0/0.5.
As aforementioned, the breath figures approach not only induces topographical changes due to the pore formation at the surface but also involves the preferential orientation of the hydrophilic segments inside of the cavities. This special chemical distribution obtained in porous films prepared by breath figure method has been previously evidenced by several techniques including AFM, 25 fluorescence microscopy (FM),24,32 cathodoluminescence (CL),33 time-of-flight secondary-ion mass spectrometry (ToF-SIMS),34 and confocal Raman microscopy (CRM).26 However, whilst AFM, FM, and CL techniques provide partial information and require components with special characteristics, labeling or other sample preparation techniques, both ToF-SIMS and CRM provide direct information about the chemical composition of the polymeric surfaces. However, confocal Raman microscopy offers another additional advantage. It is a powerful and nondestructive tool to study the chemical distribution of surfaces also generating depth profiles in contrast to ToF-SIMS that only provides information of the extreme surface (
