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Formation of Hierarchical Porous Structure via Breath Figure Method
The porous structure films of PS containing silica nanoparticles of different hydrophobicity were obtained from breath figure method. For the porous films, the pores at the peripheral region and the center region were compared. It was found that the pores at the peripheral region appeared more uniform in size and showed higher degree of ordering. In addition, the patterns of the porous film became more disordered with the enhancing concentration of nanoparticles, and the pore size of the pore was increased. With the increase of the content of SiOH, that is, decrease in particle hydrophobicity, the Si elements were aggregated at the surface of the film and the interior of the pores. A hierarchical porous structure had been obtained at appropriate humidity and particle concentration. Finally, a possible mechanism of the hierarchical structure was proposed.
Breath figure (BF) has attracted great attention to prepare patterned ordered porous polymer films because of its facile, economical, time saving, and convenient implementation [1–3]. The prepared porous films have large surface area and ordered porosity, which have been proved to be promising in a variety of applications in the fields of superhydrophobic surfaces, micropatterned templates, catalysis, resolution separation, and responsive surfaces [4–12].
The BF method is an easy process to fabricate honeycomb patterned porous films because the water droplets are applied as templates and easy removed by simple evaporation. In this process, polymers dissolved in a volatile and water-immiscible solvent are cast under high humidity. The rapid evaporation of volatile solvent can decrease the solution surface temperature, initiating the nucleation of water droplets onto the surface of the solution . At the same time, the polymer aggregates at the interface between solution and water to stabilize the water droplets. Through thermocapillary effect and Marangoni convection, water droplets self-organize into close packed hexagonal arrays before their coagulation with each other [14, 15]. Eventually, after complete evaporation of the solvent and water droplets, hexagon-arranged honeycomb pores templating from water droplets are left on the polymer film surface. The surface morphologies of the porous films mainly depend on the polymer structure, humidity, concentration of solution, solvent, and the temperature of the solution surface [16–18]. A wide variety of polymers are devoted to obtain honeycomb structures. However, it is difficult to be applied if the polymer matrix requires complicated synthetic methods and high price. Polystyrene is a commercially available polymer, which is able to form porous structures by this process [19, 20].
Nanoparticles can be adsorbed at the fluid-fluid interfaces and significantly change the properties of these interfaces [21–23]. Introducing nanoparticles into the polymer matrix leads to the preparation of new honeycomb patterned composite and excellent property. Sun et al. [24, 25] utilized polymer and silica particles to assist in preparing honeycomb porous films through BF method. Particles are proven to be effective in serving as stabilizers for making fine patterned porous arrays. However, the silica particles with varied surface hydrophobicity have not been investigated systematically yet.
The wettability of ordered porous surfaces plays an important role in the practical applications. It is well known that the roughness of a surface can enhance the hydrophobic or hydrophilic properties. Ke et al.  prepared positively charged honeycomb films from PS-based block copolymer and silica particles by the breath figure method. The film surface is at a Cassie state where the negatively charged silica nanoparticles can selectively assemble on the external surface of films; however, silica nanoparticles assemble on both external and internal surfaces at a Wenzel state after prewetting. Consequently, by controlling the wetting states, nanoparticles selectively assemble inside or outside the pores influencing the wettability.
In addition to surface roughness, the investigations of surface chemistry of nanoparticles on the wettability are also important. In this paper, the porous structure films of the PS and silica nanoparticles of different surface hydrophobicity are fabricated from breath figure method. The surface morphology and the aggregation of Si elements of the porous structure films were investigated. The influences of important experimental factors, such as humidity and particle hydrophobicity, on the final morphology are discussed.
Polystyrene (PS) ( g/mol) was purchased from Aldrich. The silica particles used were fumed silica nanoparticles (Wacker Chemie), with primary diameter of 20–30 nm. The hydrophobicity of the particles is characterized by the relative SiOH content on their surface. The hydrophobicity increases with a decrease of the content of SiOH. In this study, the relative silanol content was 25%, 75%, and 100%, respectively. The solvent of chloroform (CHCl3) was of analytical grade and used without further purification.
2.2. Preparation of Honeycomb Films
The honeycomb film was fabricated via BF technology [18, 27]. The concentrations of 30 mg/mL of the PS and SiO2 particles (1 mg/mL, 2 mg/mL, and 5 mg/mL) with different contents of -OH (25%, 75%, and 100%) were dissolved in chloroform solution and then cast onto a silicon wafer under a humid airflow. After the complete evaporation of solvent and water droplets, porous film was obtained.
Surface morphologies of the films were characterized by optical microscopy (XJX-2, Nanjing, China) and a scanning electron microscope (SEM) carried on VEGA 3 LMH (esko TESCAN) with 20 kV accelerating voltage. The quantitative analysis was using energy dispersive spectrometry (EDS) (Oxford INCA X-ACT). The water contact angle was tested using the sessile drop method (JC2000D4 Powereach Tensiometer).
3. Results and Discussion
3.1. Spatial Variation of Porous Structures
Before the BF process, the PS solution drop was deposited on the glass slide. The morphology of the drop is shown in Figure 1(a). The contact angle of the drop was measured to be ~15.5°. The peripheral area was marked as region B and the center area was region C. The SEM images of region B and region C are shown in Figures 1(b) and 1(c), respectively.
For the hole structure of the porous films, the morphology at the peripheral area is noticeably different from that at the center area. At the peripheral area, honeycomb like ordered pores with uniform sizes are observed. In contrast, the pores formed at the center of the films are evidently less ordered and the size distribution is much broader than that at the peripheral area. The hole is distributed in a monolayer at the periphery of the drop, whereas at the center part, multilayers of holes are examined. Moreover, the size of holes becomes multidispersed.
The differences in the pore morphology between the peripheral and center regions resulted from the difference in evaporation rate at different regions.
During the fast evaporation of the CHCl3, Marangoni convection can actively transport most of the PS together with the water droplets from center to edge of the solution drop. Thus, the effective concentration of the PS is higher near the contact lines than that at the center of the solution drop . During the volatilization of solvent, the contact line of the solution is pinned, possibly due to both surface roughness of the substrate and the deposition of the PS . The pinning effect depends on the initial contact angle of the deposited solution on the silicon wafer substrate . The evaporation rate is higher at the periphery than at the center. Correspondingly, the faster vitrification at the peripheral region also helps to limit the growth and coalescence of the trapped water droplets . In comparison, the delayed gelation/vitrification at the center region results in longer growth and coalescence time for the water droplets, and thus the larger and polydisperse pores.
3.2. Influence of Nanoparticles
With the presence of particles, a particle layer could form at the water solution interface which possesses shear and dilational moduli . The mechanical properties of the particle layer can influence the growth of droplets. It is reasonable to expect smaller droplet size corresponding to stronger particle layer, which is indeed in line with the previous rheological study of the particle layer . Figure 2 shows different patterns using different concentrations of the silica nanoparticles with the same PS solution.
It is obvious that the patterns of the film surface become increasingly disordered with greater nanoparticle concentration. The ordered hexagonal arrangement of the pores has been compromised with increasing particle concentration, and the pore size is larger for higher application quantity of the particle. When the particle concentration exceeds a critical value above one concentration, the porous films are not formed. This is because the adsorption of too much nanoparticles at the water droplet surface may result in a jamming state to the interface, which hinders the droplet growth and leads to the formation of irregular shaped droplets instead of uniform droplet pattern .
At an appropriate particle concentration, ordered porous structure can be obtained. However, the average pore size depends on the hydrophobicity of the particles. The SEM images of the porous structure with particles of varied hydrophobicity are shown in Figure 3. The average pore size as a function of the hydrophobicity particles of the porous films are shown in Figure 4. It is noteworthy that the average pore sizes of the obtained films decrease from 0.83 μm, 1.09 μm to 1.29 μm when the SiOH contents increase from 25% to 100%. With the increase of -OH contents, the pore sizes are increased. It indicates that when the PS is fixed, the augment of units of -OH will enhance the capability of holding water during the water-organic interface, which enlarges the pore size. On the other hand, the existence of hydrophilic segment is beneficial to stabilize the water droplets at the interface, resulting in forming more ordered porous film. Hydrophilic segment is known to form inverse structures in organic solvent . When contacting with water, these inverse structures start interacting with water, leading to a rearrangement of PS around the water droplets. With the evaporation of the solvent and water, the pores enriched with hydrophilic functionality are formed.
In order to investigate where the particles were distributed, EDS was utilized and the Si elements were marked as red dots in Figure 3. For the most hydrophobic particle used in the present work (25% SiOH), most of the particle are distributed at the rim of the pores. With the increase of SiOH (100% SiOH), the particles also appear at the surface of the film and the interior of the pores. This is mainly attributed to the hydrophilic nature of these particles.
3.3. Formation of Hierarchical Porous Structure
When humidity is increased to 95%, the hierarchical porous structure has been obtained in Figure 5, which is characterized by large porous pores containing many small holes in their interiors with ( and are the diameter for small and large holes, resp.).
This hierarchical porous structure can only be formed at high relative humidity (RH = 95%). In this case, coalescence among water droplets could be enhanced, consequently leading to polydispersity in droplet sizes . The formation mechanism can be illustrated in Figure 6. The water droplets approach on the surface of the polymer solution (Figure 6(a)). The first unstable layer of water droplets was condensed at the solution surface as shown in Figure 6(b). Due to the high RH, the droplets close to the first layer condensation have coalesced to larger droplets. With the volatilization of solvent and water droplets, the smaller pores with metastable state are formed in Figure 6(c). The coalesced larger water droplets are dragged into the polymer solution through a thermocapillary effect, and then the large pores are formed as shown in Figure 6(d). The first unstable layer is squeezed to form a spherical shape because of the gravity of the larger water droplet. This process can repeat to form multilayer structure until the solvent and water droplet are thoroughly evaporated (Figure 6(e)).
The size of the periodic microstructures in honeycomb porous falls within the range of hundreds of nanometers to tens of micrometers, matching the requirements of templates, separation, cell culture, biosensing, optical and optoelectronic devices, and so on [1, 36–38].
In this paper, the porous films of PS and silica nanoparticles of different -OH are prepared through breath figure method. For all the porous films obtained, the pore morphology at the peripheral regions is found more uniform and ordered than that at the center region. Such a difference is attributed to the interplay between the contact line pinning and the evaporation rate. The patterns of the film become more disordered with the enhancing concentration of nanoparticles, and the pore size is increased. Moreover, the average pore size and the distribution of particles depend on the particle hydrophobicity. With the increase of SiOH content, the pore size tends to increase and the silica nanoparticle seems aggregate at the surface of the film and the interior of the pores. Interestingly, when the humidity is increasing to 95%, the hierarchical porous structure was observed, which is characterized by larger porous pores containing many small holes in the interior. This hierarchical porous structure is probably due to the humidity enhanced droplet coalescence and the thermocapillary effect caused by evaporation.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work was supported by the Foundation of Shaanxi Provincial Education Department, China (Grant no. 17JK1123), the Xi’an Science and Technology Project (Grant no. 2017CGWL15), and the Fundamental Research Funds for the Central Universities (Grant no. JUSRP11702). The authors thank Mr. H. B. Li and Mr. X. Lu for the help in experiments.
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