Morphology and wettability control of honeycomb porous films of amphiphilic fluorinated pentablock copolymers via breath figure method

Abstract

Amphiphilic pentablock copolymers of poly (trifluoroethyl methacrylate)-b-poly (methyl methacrylate)-b-poly (ethylene glycol)-b-poly (methyl methacrylate)-b-poly (trifluoroethyl methacrylate) (PTFEMA-b-PMMA-b-PEG-b-PMMA-b-PTFEMA) with three different block ratios as well as molecular weights were used to fabricate honeycomb structured porous films through the breath figure technique. Several critical influencing factors such as macromolecular structures, solvent properties, copolymer concentrations and the substrates were investigated to control the morphology of the pores. The results showed that chloroform utilized as a solvent with an appropriate concentration of 45 mg mL⁻¹ on the substrate of silicon wafer offered the optimum condition. It could be evidenced that the pore sizes of the honeycomb films were increased by enhancing the molecular weight of the copolymer. In addition, an increase in the copolymer concentration leads to a decrease in the pore size, and even causes a few porous structures to disappear. The viscosity of the liquid substrates also affects the pore size, and it was found that a larger pore size was formed with an increase in the viscosity. The porous films possessed better hydrophobic and oleophobic properties than the flat films. In addition, the pincushion structure films exhibited the best hydrophobic and oleophobic properties and an excellent water-adhesion ability in the reverse state. This work may facilitate our understanding of the breath figure process and assist us in preparing films under different conditions, which show different perspectives as micro-suckers, hydrophobic and oleophobic membranes.

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1. Introduction

Microporous films with a regular and ordered patterned structure demonstrate an attractive topic of research, especially for their potential applications such as in superhydrophobic polymer films, filter membranes, catalysts, cell culture substrates and optical materials.^1–7 The breath figure (BF) method has been regarded as a powerful tool for the fabrication of strictly ordered and closely packed 2D and 3D nano- and microstructures^8,9 due to its facile, economical, nontemplated and versatile nature^10,11 to prepare films with ordered honeycomb structures, in which the template consists of an ordered array of water droplets that may be removed by simple evaporation. Generally, the BF process involves three steps: the evaporation of a volatile solvent from a polymer solution in the presence of a humid atmosphere, the accumulation of water vapor molecules as well as the condensation of water droplets, and finally the sacrifice of the water droplets template to form the porous films.

The driving forces of the accumulation of water vapor molecules and the condensation of water droplets are considered to be the capillary force and the surface convection resulting from the temperature gradient generated between the humid atmosphere and the casting solution, which is caused by the solvent evaporation of the casting solution. The final water droplets with a tunable diameter in the range from 0.2 to 10 μm¹² serve as sacrificial templates prior to the complete solidification of the polymer solution, which leads to a highly ordered hexagonal array honeycomb patterned film after the evaporation of both the solvent and water due to Marangoni convection.^12,13 The key points in this procedure are to prevent the coalescence of water droplets and precipitation of the polymer on the solution surface, which could be affected by the nature of the solvent, macromolecular structures, relative humidity of ambience, the polymer concentrations and substrates.^1,14–20

The properties of solvent, such as insolubility in water and volatility determine whether the BF films can be formed perfectly. The most common solvents used for the BF technique include carbon disulfide and chloroform due to their proper volatility as well as insolubility.^13,21 On the other hand, the water-miscible solvents such as THF are not suitable for the fabrication of ordered porous films by the BF method because of their affinity with water. However, Li et al.²² and Hu's group¹⁴ utilized poly(L-lactide) and PS-b-PAA as copolymer matrix, respectively, and THF as the solvent can also fabricate honeycomb porous films. Therefore, the effect of solvent on the formation of honeycomb porous films is also related to the chemical structures. As a result, the macromolecular structure is a crucial element to determine the final morphology of the films. During the past two decades, it has been shown that various types of polymers with different macromolecular structures, such as rod–coil copolymers, star shaped polymers, comb copolymers, graft polymers, supramolecular polymers and dendritic polymers can be fabricated into honeycomb patterned films with a controlled pore size.^17,23–29 The polar groups, degree of branching and end-groups of the polymers can affect the precipitation and stabilization of the water droplets, and then influence the pore size. Furthermore, relative humidity of ambience is another factor which influences the water condensation on the solution surface, and consequently determines the pore size and regularity of the pore arrangement. Environments with a relative humidity of 50% or higher are necessary to promote favorable condensation.^15,30 In particular, a trend is observed in which the size of the pores in the films increases almost in a linear fashion with a higher humidity (>60%) as Hu et al. proposed.¹⁴ Moreover, the pore size is also considerably influenced by the polymer concentrations, which stabilize water droplets and determine the total contact area between the water phase and polymer solution phase.³¹ In addition, Stenzel³² observed a strong relationship (R = K/c, K is a constant dependent on the polymer material used) between the pore size (R) and the concentration (c) in solution while using amphiphiles, whereas less influence was observed while using various concentrations of star polymers.³³ Besides, various studies have shown that the substrate has an effect on the regularity and final quality of the pore array in the films. Billon and co-workers³⁴ fabricated a more regular organization in mica using poly(butyl acrylate)-b-polystyrene (PBA-b-PS) as a polymer, which is due to the electrostatic interactions between the cationic ionomer ends and the oxyanions of the mica surface. Xi et al.³¹ also found that mica was the best substrate for the casting of dendronized block copolymers in the BF method, but honeycomb films could not be successfully prepared on silicon wafer. However, honeycomb porous thin films of PS-b-PAA were obtained on silicon wafer by Hu's group.¹⁴ These phenomena can be ascribed to the different affinity of water and polymer solutions with the substrates.

Based on the above mentioned descriptions, we can observe that various explanations for the controlled arrangement of ordered patterns by the BF technology have been proposed.^35–39 However, the possible interplay of general mechanisms for the formation of all BF systems is complex, since each casting system is unique, and in some cases, no definite conclusions on the effect of each variable can be drawn. The complexity of the process with its manifold influences does not readily allow an absolute prediction, and even the developed empirical relationships merely work well in certain systems.⁴⁰ Furthermore, the identification of the crucial factors directing the formation is still a matter of debate, and subtle changes in the casting conditions can change the outcome of the film preparation significantly because of the complicated character of BF formation.⁴¹

The above controversial results confirm that it is of great significance to explore the key factors for the formation of BF through a variety of polymers as film-forming materials. In the present work, amphiphilic pentablock copolymers with PMMA and PTFEMA as hydrophobic segments and PEG as hydrophilic segment were utilized to prepare the porous films and to explore the mechanism of the BF process. As versatile building blocks, PTFEMA is a kind of special polymer with unique properties including thermal and chemical resistance and a lower surface energy, and thus have been extensively applied in many fields.^42,43 However, the low solubility, difficult processability and the high costs will certainly limit its applications. Thus, the introduction of PMMA and PEG to the PTFEMA can help reduce costs and improve the solubility and processability. In addition, PEG has attracted considerable attention because of its hydrophilicity and functional flexibility. Based on the pentablock copolymers, we expected to obtain the honeycomb porous films through the BF method with various excellent properties such as thermal and chemical resistance, hydrophobicity and oleophobicity.

Until now, more fluorinated copolymers are chosen for hydrophobic applications, but the hydrophobic and oleophobic performances still have much scope for improvement. The introduction of surface morphologies of honeycomb porous films, which enhanced the surface roughness, will be concerned to solve this problem. Chen et al.²⁷ fabricated highly ordered porous films from crystalline conjugated rod–coil diblock copolymers of PF-b-PSA (poly[2,7-(9,9-dihexylfluorene)]-b-poly(stearyl acrylate)) through the BF process, and the water contact angle was over 160° when the top layer was peeled off. Similarly, Yabu et al.⁶ prepared a superhydrophobic and oleophobic surface from fluorinated polymers by the BF method. It was found that the contact angles of porous and pin-cushion films were 145° and 170°, respectively, which are much higher than the contact angle (117°) of the relative flat film. This surface can be widely applied in self-cleaning surfaces, biological fields, and so forth. To further expand the application of the honeycomb porous films of the pentablock copolymers, the wettability, hydrophobic and oleophobic properties are investigated in this paper.

2. Experimental

2.1. Materials

The pentablock copolymers of PTFEMA-b-PMMA-b-PEG-b-PMMA-b-PTFEMA with three different molecular weights were synthesized through a one pot method via atom transfer radical polymerization. The synthesis processes is shown in Fig. S1 and S2 in ESI.† The pentablock copolymers were analyzed via FTIR and NMR, as shown in Fig. S3 and S4 in ESI.† The GPC analysis results are listed in Table 1. Water used in all of the experiments was de-ionized and ultrafiltrated to 18.2 MΩ with an ELGA Lab water system. All other reagents (benzene (C₆H₆), chloroform (CHCl₃), dichloromethane (CH₂Cl₂), tetrahydrofuran (THF) and n-dodecane) were analytically pure and used without further purification.

Table 1 GPC analysis results of the pentablock copolymers

Serial number	Block ratio PEG : PMMA : PTFEMA	Mn (g mol^−1)	PDI
Copolymer-1	9 : 21 : 15	26 790	1.49
Copolymer-2	9 : 21 : 11	23 340	1.58
Copolymer-3	9 : 11 : 7	17 200	1.57

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2.2. Preparation of the copolymer films

To prepare honeycomb porous films, the pentablock copolymers were dissolved in CHCl₃, CH₂Cl₂, C₆H₆ and THF, and then cast onto silicon wafer, ITO glass or stainless steel under a flow of moist air. The airflow was vertically blown over the solution surface. The solvent started to evaporate and water vapor condensed onto the solution surface simultaneously. After completely evaporating the solvent and water, the porous films were prepared. For comparison, nonporous films were also cast from CHCl₃ solution of the pentablock copolymer and dried under normal conditions.

2.3. Characterization

Scanning Electron Microscopy (SEM) images were used to observe apparent morphologies of porous films, which were carried out on VEGA 3 LMH (Česko TESCAN) with an accelerating voltage of 10 kV. Contact angles were measured using ultra-pure water or copolymer solution by the pendent drop method with a JC2000D4 Powereach Tensionmeter made by Shanghai Zhongchen Company. The copolymer solutions (45 mg mL⁻¹ in CHCl₃) were casted on glass slides (43 × 17 × 16 mm³), and the wetting liquid used was water. A microsyringe was used to deliver water to the film surface. For each angle reported, at least ten sample readings from different surface locations were averaged.

3. Results and discussion

3.1. Factors influencing the honeycomb porous films

It is noteworthy that the morphologies, pore size and regularity of the pores prepared from the BF method can be tailored by changing some conditions that affect the fabrication of the honeycomb porous films.^{17–19,44,45} In this section, some critical factors such as macromolecular structures, solvent properties, copolymer concentrations and substrates on the morphology of porous films are investigated.

3.1.1. The influence of macromolecular structures. The copolymers with different chemical structures and molecular weights used as film-forming materials have an effect on the pore size and regularity of the porous films in the BF technique, which has been reported in recent years.^13,17,18 The chemical structure and components of the copolymer usually determine whether the porous film can be formed, and the molecular weight of the copolymer may change the pore sizes and the arrangements of the pores.

The SEM images of porous films of PEG macroinitiator, copolymer-1, copolymer-2 and copolymer-3 with different block ratios and molecular weights prepared with the concentration of 45 mg mL⁻¹ in CHCl₃ are illustrated in Fig. 1. It is noticeably found that the SEM image of PEG macroinitiator film is a rod-like structure instead of a porous formation in Fig. 1a due to its hydrophilicity and low molecular weight.²⁷ On the contrary, the PMMA and PTFEMA blocks on copolymer with a long alkyl side chain could facilitate the stabilization of the water droplets and result in an ordered porous structure as evidenced in Fig. 1b–d.²⁷ Moreover, the increase in molecular weight of the pentablock copolymer seems to result in the formation of larger pores. The reason is that the higher molecular weight indicates a lower mole fraction and a faster solvent evaporation, which is beneficial for the growth of the condensed water droplets.⁴⁶ The lower mole fraction copolymer solution may lead to a delayed precipitation and provide enough time for the condensed water droplets to grow and finally form a larger pore size after sacrificing the water droplets.^17,18,22

Fig. 1 SEM images of films prepared from (a) PEG macroinitiator, (b) copolymer-1, (c) copolymer-2 and (d) copolymer-3. The concentration is 45 mg mL⁻¹, solvent is CHCl₃. The scale bars are 50 μm in (a) and 5 μm in (b)–(d).

3.1.2. The influence of solvent properties. Solvent property is an important factor in the formation of the ordered porous structures.^47,48 In the present work, the films were fabricated by casting solutions of copolymer-1 with a typical concentration of 45 mg mL⁻¹ in solvents of CH₂Cl₂, CHCl₃, THF and C₆H₆ to analyze how the solvent affects the porous structure, and the SEM images of the films are depicted in Fig. 2. It is shown that the attempts to prepare an ordered porous morphology from THF (Fig. 2c) and C₆H₆ (Fig. 2d) under identical conditions are unsuccessful, but regular patterns can be prepared from the CHCl₃ solution, in which most of the pores are surrounded by highly ordered hexagonal arrays (which are shown by white lines) as exhibited in Fig. 2b; the pore arrays obtained from CH₂Cl₂ are less ordered than those of CHCl₃, as shown in Fig. 2a.

Fig. 2 SEM images of copolymer-1 films dissolved in different solvents with the concentration of polymer at 45 mg mL⁻¹, (a)–(d) are CH₂Cl₂, CHCl₃, THF and C₆H₆, respectively.

The major differences between these solvents are their evaporation rate and the interactions between the solvent and the water droplets. The fast evaporation rate of the solvent corresponds to the rapid cooling rate of the copolymer solution surface and favors condensation of water vapor. For CH₂Cl₂ and CHCl₃, the solvent of CH₂Cl₂ may completely disappear from the system before water droplets form a regular packing. On the other hand, the lower volatility of CHCl₃ delays the evaporation of the solvent completely. Consequently, the water droplets have more time to grow and sink, resulting in the larger and regular pores in the films¹⁷ (Fig. 2a and b).

In addition, water and THF are miscible with each other. When water droplets condense onto the copolymer/THF solution, the copolymer cannot effectively stabilize the droplets, and the droplets diffuse in the solution because of its affinity with THF. However, the viscosity of the solution increases with the evaporation of THF; some droplets are formed without coalescence. Finally, the disordered morphology is formed (Fig. 2c). The C₆H₆ with low volatility evaporates too slowly to produce enough temperature gradient to allow sufficient surface cooling to condense water on the surface of the solution. As a result, the water droplets can't easily sink into the solution. Consequently, it does not allow the formation of obvious porous structures as illustrated in Fig. 2d. The above results indicate that the solvent is one of the key factors for the ordered arrangement in BF method, which would determine whether the ordered porous films can be formed.

3.1.3. The influence of copolymer concentrations. Generally, copolymer concentration is directly related to the mass of the solvent, which would determine the duration of solvent volatilization of the copolymer solution and finally influence the morphologies of the copolymer solidification. Moreover, copolymer concentration can affect the density and viscosity of the solution, and further influence the height of water droplets on the surface of the copolymer solution, which would affect the pore size of honeycomb porous films. Fig. 3 depicts the SEM images of copolymer-1 with the increasing concentration from 5 to 75 mg mL⁻¹. It is obvious that the highly ordered films are fabricated with the copolymer with concentrations ranging from 15 to 60 mg mL⁻¹. When the concentrations are 5 and 75 mg mL⁻¹, respectively, disordered patterns are observed.

Fig. 3 SEM images of copolymer-1 films, the concentrations of (a)–(f) are 5, 15, 30, 45, 60, 75 mg mL⁻¹.

It is noteworthy that the average pore sizes of the obtained films decrease from 1.59 ± 0.15 μm (CV = 9.4%), 1.39 ± 0.10 μm (CV = 7.2%), 0.94 ± 0.03 μm (CV = 3.2%) to 0.80 ± 0.09 μm (CV = 11.3%) when the copolymer concentrations increase from 15 to 60 mg mL⁻¹, as illustrated in Fig. 4. The correlation between the pore size and the solution concentration is almost a linear variation. The CV is an indication of the variation as well as the degree of order in the pore size. The relatively small coefficient of variation suggests the regularity and the similar distance of any two neighbouring pores. Moreover, the most ordered porous film is obtained in a solution with a concentration of 45 mg mL⁻¹, and the pores are in a regular hexagonal arrangement to reduce free energy.⁴⁰

Fig. 4 Average pore size as a function of concentration of the porous films prepared from copolymer-1/CHCl₃ solutions. Coefficient of variation (CV)²⁷ is a normalized dispersion of a probability distribution.

It is known that the pore size is determined by the size of the condensed water droplet. The pore sizes of water droplets relate to the growth rate and growth time. With the increase in the concentration, the process of phase-inversion from liquid to solid is accelerated, which then shortens the growth time of the droplets.²¹ On the other hand, the growth rate of the droplets is proportional to the surface temperature gradient (ΔT) between the temperature of solution surface and atmosphere, which is described as⁴⁹

dR/dt ∼ ΔT^0.8 (1)

It is addressed by Henry's Law that the more concentrated solution corresponds to the lower vapor pressure and the slower evaporation rate of the solvent,^21,46 which leads to the smaller decrease in ΔT. Therefore, the growing rate and growing time of the water droplets on the solution surface with a higher copolymer concentration are both slower, and the obtained water droplets have a smaller size as a result, which leads to a smaller pore size. In addition, the precipitation rate is the key factor in the formation of the ordered porous film. Generally, a higher copolymer concentration leads to a faster precipitation rate by which the water droplets could be encapsulated and solidified immediately, and the smaller pore size could be formed. Moreover, a higher concentration corresponds to a higher viscosity, which would lead to an increased efficiency of the encapsulation of condensed water droplets. Consequently, the droplets do not have enough time to grow larger,^15,22,46 and small pores are formed. The solution with an excessively low concentration (5 mg mL⁻¹) with a viscosity too low to encapsulate the droplets or prevent their coalescence²⁷ resulted in the formation of a disordered and irregular arrangement (Fig. 3a). With the increase in copolymer concentration, the water droplets can be more easily stabilized by the solution, which is in favor of arraying the droplets in an orderly manner. The precipitation rate would be enhanced with the increase in the copolymer concentration,⁵⁰ which can stabilize water droplets and inhibit them from aggregating to form an ordered porous structure (Fig. 3b–e). In addition, no apparent holes resulted from the high copolymer concentration (75 mg mL⁻¹),^19,51 as exhibited by the red arrows in Fig. 3f. The reason is that the highest viscosity of the copolymer solution makes it more difficult for the water droplets to sink into the copolymer solution, consequently resulting in the slower growth of droplets and smaller pores. These results suggest that a suitable copolymer concentration is needed for the formation of the ordered honeycomb structures.

3.1.4. The influence of substrates. Mica, silicon wafer and glass are usually used as the substrates to fabricate different pore patterns of porous films by the BF method, despite how the substrates affect the pattern formation is still inconclusive.^30,44,52,53 To further investigate the mechanism of the substrates on the formation of porous films, the copolymer-1 solutions spread on different solid substrates of ITO glass, stainless steel, silicon wafer are studied, and the SEM images of the honeycomb films are illustrated in Fig. 5.

Fig. 5 SEM images of copolymer-1 films dissolved in CHCl₃ on different solid substrates: (a) ITO glass, (b) stainless steel, (c) silicon wafer.

It is shown that the pore structures exhibit regular hexagonal arrays on the substrate of silicon wafer with uniform pore sizes of 0.94 ± 0.03 μm (CV = 3.2%) (Fig. 5c), while the pore patterns formed on the ITO glass and the stainless steel are less ordered, with non-uniform pore sizes of 1.72 ± 0.31 μm (CV = 18.0%) and 1.62 ± 0.17 μm (CV = 10.5%) (Fig. 5a and b). The smallest CV value reveals that the substrate of silicon wafer results in an ordered microporous structure. This could be ascribed to the different affinity of water and copolymer solutions with different substrates, which are demonstrated by their contact angles listed in Table 2.

Table 2 Contact angles (CA) of water and copolymer solution on different substrates

Solid substrates	ITO glass	Stainless steel	Silicon wafer
CA of water/°	74.3 ± 2.4	67.2 ± 2.1	40.5 ± 2.2
CA of copolymer solution/°	14.2 ± 0.4	12.9 ± 0.3	9.5 ± 0.3

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It is evident that it is easy for the water droplets to be adsorbed onto the silicon wafer as a steady template for the formation of porous films. Moreover, it is also facile for the copolymer solution to be spread onto the silicon wafer. The water droplets are easily adsorbed onto the hydrophilic substrates, which causes the template's stability to increase, such that the formation of highly ordered porous structure is feasible. In addition, it is well known that the wetting of solid substrates with copolymer solution is beneficial to the periodicity and regularity of pores.^14,31 These processes lead to ordered porous films fabricated on the silicon wafer. On the other hand, the ITO glass with the highest contact angle of the copolymer solution may restrain the spreading behavior of copolymer solutions on the substrates by slowing the nucleation of water droplets at the water/CHCl₃ interface, which accordingly yields the largest pore sizes. As a result, average pore size decreases as hydrophilicity of the copolymer solution increases on the substrate.

Compared with the solid surface, the liquid substrate with a large surface tension (e.g., water) would be beneficial to the spreading of the copolymer solution by a simple qualitative analysis. Wan et al.¹ prepared honeycomb films on the surface of various organic solvents, and found that the surface tension of the liquids was crucial to the formation of through-pores. However, there is no research on the viscosity of liquid substrates, which influences the spread and fluidity. In this work, dimethyl silicone with four different viscosities are selected as substrates and investigated to analyze the morphology, and the results are depicted in Fig. 6.

Fig. 6 SEM images of the pentablock copolymer film prepared at the dimethyl silicone surfaces. The viscosity is (a) 10, (b) 100, (c) 1000 and (d) 60 000.

It is noteworthy that with the increase in the viscosity of dimethyl silicone, the average pore size increased. Specifically, a hexagonal pore morphology is observed (Fig. 6d) in the film prepared from the dimethyl silicone with a viscosity of 60 000, which is different from the circular pores in the films prepared from the dimethyl silicone with viscosities of 10, 100 and 1000. The copolymer solution spread well at the interface of dimethyl silicone with a lower viscosity because of its better fluidity. The water droplets are beneficial to precipitate at the copolymer surface due to the short evaporation time of the solvent, consequently resulting in the slower growth of water droplets and the formation of smaller pores and vice versa.

Based on the analysis mentioned above, one can draw a conclusion that the formation of the honeycomb porous film of the pentablock copolymer would be affected by the molecular weight, solvent and copolymer concentration, and the regularity of the pores is influenced by the use of an appropriate solvent and substrate. In addition, the molecular weight and copolymer concentration have an effect on the pore size.

3.2. Wettability of the copolymer films

Copolymers composed of fluorinated acrylate have been broadly applied and proven to possess excellent surface performance, hydrophobic and oleophobic properties,⁴³ but how the wettability can be affected, and whether superhydrophobic films could be obtained are our concerns. To solve these problems, water and n-dodecane contact angles of flat films and related porous and peeled films are summarized in Fig. 7.

Fig. 7 Water and n-dodecane contact angles of different films: (a) copolymer-1, (b) copolymer-2, (c) copolymer-3 at flat, porous and peeled states. The concentration of copolymer solutions is 45 mg mL⁻¹.

It is clearly shown that the contact angles of the three copolymers increased to 95.98°, 89.13° and 79.17° when flat films are deposited (Fig. 7), which is higher than that of the glass (45.64°) due to the intrinsic hydrophobic properties of the copolymer. It is well known that the low surface energy of the surface could result in hydrophobic effects.

Surface energy can be calculated from contact angle measurements by the Owens and Wendt method. The equilibrium contact angle is well defined by Young's equation,⁵⁴

σ_s = σ_sl + σ_l cos θ (2)

where σ_s, σ_l, σ_sl are the surface energies at solid–vapor, liquid–vapor and solid–liquid interfaces, respectively. θ is the contact angle of liquid on a solid surface. The surface energies for σ_l and σ_s can be expressed as follows:

σ_l = σ^d_l + σ^p_l (3)

σ_s = σ^d_s + σ^p_s (4)

where σ^d_l and σ^p_l are the dispersive and polar contributions of the surface energy for the liquid, and σ^d_s and σ^p_s are for the solid, respectively. The interfacial energy (σ_sl) can be calculated from σ_s and σ_l based on the geometric mean method and the following equation:

substituting the σ_sl in eqn (3) with (6), we get

If the contact angles of two different liquids on the same polymer surface are known, and σ^d_s and σ^p_s can be obtained from eqn (6), the surface energy of the polymer film can be calculated using eqn (4). In this paper, ultra-pure water and n-dodecane are selected as the probe liquids to determine the surface energies of copolymer films. The values of σ^d_l (23.9 mN m⁻¹) and σ^p_l (48.8 mN m⁻¹) for water and σ^d_l (23.9 mN m⁻¹) and σ^p_l (0 mN m⁻¹) for n-dodecane are used in the calculation. The resultant surface energies (σ_S) of the flat films of the three copolymers are summarized in Table 3.

Table 3 Surface energies of the three pentablock copolymers

Solid substrates	σ^ds (mN m^−1)	σ^ps (mN m^−1)	σs (mN m^−1)
Copolymer-1	17.47	3.01	20.48
Copolymer-2	19.96	4.64	24.60
Copolymer-3	22.25	8.29	30.54

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It is obvious that the highest fluorinated segment of copolymer-1 results in the lowest surface energy. As a result, one can observe that the surface energy decreases with the increasing amount of fluorinated segments, which results in stronger hydrophobic properties.

Compared with the flat films, the contact angles of the honeycomb films from copolymer-1 to copolymer-3 are improved to 117.6°, 109.23° and 105.33° due to the presence of the highly ordered honeycomb pattern, which enhances the surface roughness and amplifies the hydrophobic properties.^23,34 In addition, no matter for flat films or porous films, copolymer-1 always possesses higher values of water contact angles than that of copolymer-2 and copolymer-3, due to the increased hydrophobicity and fluorinated segments.¹⁸

To further promote the roughness effect, the top layer of the honeycomb porous film is peeled off using an adhesive tape to obtain a pincushion structure (Fig. 8a). It is clearly shown that the pincushion structure from the three copolymers can enhance the hydrophobicity and raise the contact angles of water to 140.63°, 135.20° and 131.12°, respectively. The fluorine content of the copolymers is slightly low, which makes them unable to impart excellent water repellence. Consequently, the contact angles cannot reach superhydrophobic values. Compared with the honeycomb film, the increased contact angle is due to the increased surface roughness of the pincushion structure. Based on the analysis, we can derive that surface hydrophobicity is mainly caused by the chemical heterogeneity and geometrical roughness of the surface.^55,56 The relationship between the rough topography and the contact angle is illustrated by the Cassie–Baxter equation:^57–60

cos θ_r = f₁ cos θ − f₂ (7)

where f₁ and f₂ are the fractions of the solid and air on the surface (f₁ + f₂ = 1), respectively. θ_r and θ are the contact angles on the rough surface and flat surface, respectively. The fractions of the solid on the porous and peeled films are given in Fig. 9. Considering copolymer-1 as an example, the f_1porous of copolymer-1 is 0.599. In particular, the f_1peeled of copolymer-1 is dramatically decreased to 0.253, indicating that a much smaller portion of solid is in contact with water, and more hydrophobicity is found on the pincushion surface.

Fig. 8 (a) SEM image of the pincushion structure film of copolymer-1. (b) The water-adhesion ability for the peeled film. (c) The model of (b). The inset of (a) shows the water contact angle of the pincushion structure film.

Fig. 9 Fractions of the solids on the porous and peeled films of (a) copolymer-1, (b) copolymer-2 and (c) copolymer-3.

It is noteworthy that the pincushion structure also exhibited excellent water-adhesion ability. The water droplet inhibits rolling and sticks firmly on the pincushion structure film when the film is reversed, as presented in Fig. 8b, which is affected by the van der Waals force or hydrogen bonding between the water molecules and the C=O groups of the copolymer.^61–63 In fact, air in the pores is trapped by the copolymer walls and the bottom of the water droplet (green parts in Fig. 8c). This closed air volume in the pincushion configuration induces a capillary-like force to maintain the water in suspension, which acts as micro-suckers.³⁴ It is important to note that the chemical composition and pincushion structure play a critical role in exhibiting hydrophobicity and strong adhesion with water.

The honeycomb porous films possess an oleophobic property as shown in Fig. 7. The trend of the oleophobic property is similar to the hydrophobic property, which is related to the fluorinated segment and the surface structure. Similarly, the oleophobicity is affected by both the surface chemistry and the surface roughness, which originate from intrinsic oleophobic properties of the copolymer and microstructures of the films.

4. Conclusions

Highly ordered honeycomb films have been successfully prepared from the amphiphilic pentablock copolymers composed of PEG, PMMA and PTFEMA by the breath figure method. The pore sizes and regularity of the honeycomb porous films are influenced by macromolecular structures, solvent properties, copolymer concentrations and substrates. It is demonstrated that CHCl₃ utilized as a solvent at a suitable concentration of 45 mg mL⁻¹ on the substrate of silicon wafer offers the optimum condition. Moreover, the pore sizes of honeycomb films are decreased with the increasing concentration of the copolymer solution and the decreasing molecular weight. The viscosity of the liquid substrates also affects the pore size, and it was found that a larger pore size is formed with an increase in the viscosity. Furthermore, the increase in the amount of fluorinated segments and surface roughness results in stronger hydrophobic and oleophobic properties. In addition, the pincushion structure films have excellent water-adhesion ability.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant no. 51301139), Natural Science Foundation of Shaanxi Province (Grant no. 2012JQ1016), Innovation Project of Science and Technology of Shaanxi Province (Grant no. 2013KTCG01-14) and NPU Foundation for Fundamental Research (NPU-FFR-JCY20130147).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra08472a

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