the effect of different stabilizers on the formation of...

The Effect of Different Stabilizers on the Formation of Self-Assembled

Porous Film Via the Breath-Figure Technique

Masoud Amirkhani, Nicola Berger, Mohamed Abdelmohsen, Frank Zocholl, Manuel Rodrigues

Goncalves, Othmar Marti

Institut fur Experimentelle Physik, Universitat Ulm, Ulm, Germany

Correspondence to: M. Amirkhani (E-mail: [email protected])

Received 7 June 2011; revised 6 July 2011; accepted 12 July 2011; published online 3 August 2011

DOI: 10.1002/polb.22325

ABSTRACT: The Breath-Figure technique was employed to

imprint honeycomb structures in the polymer films via the con-

densation of water vapor on the surface of an evaporating poly-

mer solution. Generally, the condensed water droplets can be

stabilized by an end-functional polymer or by particles added to

the polymer solution. In this study, we carried out a systematic

experiment on the effect of different stabilizers on the porous

honeycomb structure under identical physical conditions. The

end-functional polymer produced a large area of regular spheri-

cal bubbles, whereas adding particles to the polymer solution

leads to smaller arrays of the flattened bottom bubbles. The

separation length between pores was larger for polymer/particle

sample than that of the end-functional polymer films. In the reg-

ular area of polymer/particle film many bubbles were not deco-

rated by particles. VC 2011 Wiley Periodicals, Inc. J Polym Sci

Part B: Polym Phys 49: 1430–1436, 2011

KEYWORDS: breath-figure; composite polymer; functional poly-

mer; pickering emulsion; self-assembly

INTRODUCTION Porous polymer films have long been inves-tigated because of their possible use in micro-lenses,1 solarcells, catalysts, antireflection coatings,2 and membranes. Po-rous surfaces also have applications in superhydrophobicsurfaces, with non-wettable and self-cleaning properties.3

These surfaces have been successfully produced by peelingoff the top layer of polymer film, leaving a pincushion struc-ture with a contact angle of y ¼ 170.3 Furthermore, micro-structured surfaces in the field of tissue engineering serve asa cell growth material.4,5 Among many methods of fabrica-tion of porous polymer films, the Breath-Figure (BF)method6–8 has attracted much attention due to the simpleprocess of fabricating pores. This technique produces two-dimensional honeycomb networks that may be used as softlithography templates.9 The periodic spherical structuresformed via BF were used in the fabrication of two-dimen-sional photonic crystals.10

Breath-Figure employs solvent evaporative cooling on thesurface of a solution under humid conditions in which watervapor condenses and forms droplet at the interface betweenthe polymer solution and humid air. Then, because of con-vective current due to evaporation of solvent and airflowacross the surface, they pack a hexagonal array. In order toprevent coalescence of water droplets, one may use stabil-izers such as end-functional polymers11,12 and particles.13–15

After complete evaporation of the solvent and water, traces

of water droplets remain in the polymer film, and it becomesporous with a honeycomb structure.

In practice, the production of porous honeycomb films bythe BF method is simple, but control of all characteristics ofthe film is rather complex. Karthaus et al underlined the im-portance of solvent, polymer properties, and humidity of the

atmosphere.16 A different approach was followed by Limaye,

proposing a convective current induced by a large tempera-

ture gradient to be the dominant factor in the final pattern

formation.17 Recent research focused on the effect of solvent

choice and attempted a modeling of the evaporation process,

including thermocapillary convection and Benard-Marangoni

convection.18

The formation process consists of four steps, 1) condensa-tion of water droplets, 2) growth and hexagonal packing ofwater droplets, 3) interfacial interaction of polymer andwater, and 4) evaporation of solvent and water. However,these steps do not provide a strict chronological order. Theprocess starts with the condensation of water droplets(step 1) and finishes after evaporation of both solvent andwater (step 4) meanwhile condensation and evaporation areconstantly present. The growth and hexagonal packing of thewater droplets occur where the droplets either sink into thesolvent or form a monolayer.19,20

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The structure and morphology of the produced film dependon the condensation, growth, and hexagonal packing of waterdroplets, interfacial interaction of polymer and water, andevaporation of solvent and water. Indeed, each single stagecan be significantly affected by multiple critical factors suchas temperature and relative humidity as well as concentra-tion, viscosity, density, surface free energy, and solubility pa-rameters of the casting solution.18–23

The mixing of polymers and nanoparticles is opening path-ways for engineering flexible composites that exhibit advan-tageous electrical, optical, or mechanical properties.21 Recentadvances reveal routes to exploit both enthalpic and entropicinteractions22 so as to direct the spatial distribution of nano-particles and thereby control the macroscopic performanceof the material.23 The mixture of particles in a water/oil sys-tem results in self-assembly of particles at the polymer-water interface20,24 and it produces a polymer composite af-ter evaporation of all liquids. Breath-Figure is a techniquethat potentially can be used to produce polymer composites.At the interface of polymer solution and water, the particlestend to form a closed-packed array, lowering the systemenergy. Therefore, adding an adequate amount of particlesto the polymer solution can stabilize water droplets andproduce hexagonal arrangement of droplets by BFtechnique.13,14

This work was carried out with the intention of deepeningthe understanding of the effect of different stabilizers onthe formation of BF patterns. Nevertheless, different poly-mer11 and polymer/particles13–15 systems have been usedto produce BF patterns but as the structure and morphol-ogy of the polymer porous film depends strongly on thephysical condition, it is very difficult to elucidate the effectof different stabilizers on the porous film structure. Thus,we utilized two kinds of stabilizers in the same physicalcondition and applied the BF method to compare the sta-bilizers effect on the pattern formation. In addition, differ-ent solvents were employed to examine the pattern forma-tion in PS-mCOOH and PS films. Film morphology wascharacterized by optical microscopy and scanning electronmicroscopy (SEM). After reliable production of poroushoneycomb polymer films, subsequent experimentsfocused on different stabilization mechanisms were done.The effect of particle size and concentration on the honey-comb structure of PS-mCOOH and PS films, and the self-as-sembly of particles at the polymer-water interface wasinvestigated.

EXPERIMENTAL SECTION

MaterialsBy carefully fine-tuning all experimental conditions BF arrayswere produced from PS-mCOOH, whereas for other polymers(block copolymers, star-shaped polymers and linear conju-gated polymers) a wider range of casting conditions is toler-ated.23 In our case, the choice of polymer was kept simple inorder to compare the results obtained with PS and particle-stabilized patterns.

Both PS-mCOOH (Mw ¼ 50K g/mol) and PS (Mw ¼ 45K g/mol) were purchased from Sigma-Aldrich. Three differentsolvents (toluene, chloroform and carbon disulfide) withevaporation rate of 2.24, 11.6, and 22.4 were used for thiswork. Evaporation rates are obtained from material safetydata sheets (MSDS) in reference to butyl acetate.26 The con-centration of polymer (2 wt %), relative humidity (80%),temperature (room temperature) and flow speed (around1 m/s) were kept constant for all experiments. Aqueous so-lution of non-functionalised silica microspheres of differentsizes (0.5 lm, 0.7 lm, 1.0 lm, 1.5 lm) were purchased fromThermo Scientific.

Sample PreparationFlow of humid air was regulated by a valve to control thespeed of nitrogen flow that passes through a water bubbler.Moist air is guided over the substrate, which is situated in aclosed box to regulate humidity, temperature, and velocity ofairflow. A moisture meter and an anemometer record rela-tive humidity and speed of gas through an opening in thebox. Each sample was prepared by casting 25 ll of finalsolution on the substrate.

Silica microspheres were washed in ethanol and then sepa-rated from the solution by centrifugation. This step wasrepeated several times. To remove all remaining liquid themicrospheres were dried in an oven at 70 �C for 24 h. Thenthe polymer solution was added to the silica and, to preventparticles from aggregation and obtain a uniform dispersion,the solution was put in an ultrasonic bath for 1–2 h, depend-ing on particle size and concentration.

TechniquesFor analysis by scanning electron microscopy (SEM) thepolymer samples were made electrically conductive by physi-cal vapor deposition (PVD) with gold-palladium at a filmthickness of 8 to 15 nm. We also used optical microscope tocheck each sample after preparation and all image analysiswas performed using ImageJ.27

RESULTS AND DISCUSSION

SolventPS-mCOOH and PS were dissolved in three different solvents(toluene, chloroform, and carbon disulfide) and used to pro-duce a regular pattern by BF method. In the PS-mCOOH solu-tion, the polar group (COOH) at the end of the PS chain ori-ented itself by interacting with the condensed water dropletsand promoting some interaction between the solution andthe water droplets. This interaction allows water droplets toget close to one another other without coalescing and, there-fore, producing regular pattern formations. However, Connalet al showed that for star polymers increasing the number ofpolar groups over a certain degree causes a decrease in reg-ularity of honeycomb structure.28 So it seems a small differ-ence of polarity between the terminal groups and the rest ofthe polymer chain is sufficient for the formation of regularstructure via the BF technique.19 The importance of solventchoice and viscosity has been underlined by Sharma et al, as

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well as a first attempt to model mass loss and diffusion of asolvent during evaporation.18

Under physical conditions of the present work, the dried-upfilm in toluene is rougher over a large area which indicatesthe existence of more convective currents and heat/masstransfer in comparison to two other solvents. Depending onthe density, interfacial tension, and kinetics of film drying,29

the porous film can be a three dimensional or two dimen-sional structure. In this study, toluene produced a 3-D (Figs. 1and 2) honeycomb structure,19 whereas chloroform andcarbon disulfide formed a single layer. Carbon disulfide didgenerate slightly lager regular patterns compared with chloro-form (Fig. 3) but its regular area was much bigger than tolu-ene. The size of the regular pattern can be attributed to theevaporation rate; for a more volatile solvent (Carbon disul-fide) the evaporation rate18,26 is high, so water dropletsaggregate less and large order areas can be produced.

The coefficient of variation (CV ¼ standard deviation/mean)is an indication of the variation in size within one area aswell as the degree of order of the hexagonally packed drop-lets. The results of CV (Table 1) for all solvents showed thatwithin an ordered area the regularity was almost the same,whereas the pore size changed over different order areas.Within hexagonally packed droplets, each bubble is sepa-rated by a thin wall of smaller than 0.14 lm and does notchange by changing the solvent.

Apparently, solvents had no big effect on the size of holes.However, there is a tendency for solvents with high evapora-tion rates to result in smaller hole size (Table 1), whichmeans water droplets had less time to grow.

Figure 4 shows a typical pattern of PS in chloroform, whichmeans linear PS (MW ¼ 50 Kg/mol) was not suitable toobtain a regular BF pattern by given experimental setup andcasting conditions. However, Peng et al successfully obtainedordered patterns with high molecular weight linear PS (Mw ¼223.2 Kg/mol), stating that proper viscosity of the solution isan important factor.30

Particle Assist Pattern Formation in PS SolutionTo produce a regular BF pattern, the emulsion of linear poly-styrene, solvent (continuous phase), and water droplets (dis-persed phase) has to be stabilized until evaporation of allliquids. Two different sizes of silica particles 0.5 and 1.5 lmwere dispersed in the polymer solution (chloroform wasused as the solvent) to stabilize the condensed water drop-lets (Figs. 5 and 6). For Pickering emulsions,31 a minimumconcentration of particles was required to obtain a pattern.Particle concentration for larger particles varied from 0.7 �107 to 1.4 � 107 particles/ll and for smaller particles variedfrom 3.6 � 107 to 7.2 � 107 particles/ll. The minimum con-centration of particles was required to obtain ordered pat-terns (for linear PS), and the maximum concentration corre-sponds to the upper limit of homogeneous distribution ofsilica particles in the solution without disturbing the patternformation. As a result we could elucidate that bigger par-ticles produce a regular pattern in lower concentration.Moreover, it was observed that higher concentration leads toa larger well-ordered area compared with the lower concen-tration. According to E ¼ pr2 c(1�|cos(how)|)

2 (where r, how.c are radius of particle, contact angle, and interfacial tension)the adsorption energy at the interface is proportional to r2.Hence, for larger particles the stabilization energy is higherthan smaller ones, which explains the difference in particleconcentration.

Figures 5 and 6 illustrate that particles assembled in thewater side of the structure, which can be attributed to thehydrophilic properties of the particles. In Pickering emulsion,wettability of particles define the location of particle; if theparticle is hydrophilic it located itself in water, and if it ishydrophobic it located itself in polymer solution.32

In comparison with the highly-ordered BF pattern producedby PS-mCOOH, stabilization in PS/particle systems results inlower film quality and smaller regular patterns. The bubbleswere not spherical-shaped but flattened at the bottom andaround 6–9 lm wide (Fig. 5 and 6, side view). The flattenedbottom can be attributed to the contact angle of water on PS

FIGURE 1 PS-mCOOH in Toluene; SEM side view of multilay-

ered film.

FIGURE 2 PS-mCOOH in Toluene; optical image.



surface. As depicted in Figures 5 and 6, particles decoratedjust the rim of the droplets and the rest of the droplets arein direct contact with the solution of PS, so the water drop-lets should have almost the same contact angle on the PSfilms.33 Capillary flow carried the particles toward the upperpart of the bubble, and the silica particles assembled and

decorated the rim of the pore.34 Furthermore, the wall sepa-rating two bubbles from one another was much thicker com-pared to PS-mCOOH films (Fig. 3) and varied from t ¼ 0.5 tot ¼ 1.5 lm.

Figures 5 and 6 illustrate that not all bubbles in the orderedarea were decorated by particles. The optical pictures aswell as the SEM side view showed a regular pattern despitethe significant lack of particles (Fig. 5 and 6). A stabilizeddroplet as well as a nearby droplet can change thermocapil-lary convection and heat/mass flux around itself, and itseems this change in thermocapillary convection can stabi-lize other droplets to some extent. Hence, an inhomogeneousdistribution of particles throughout the bubbles suffices tostabilize most of the pattern. Thermocapillary convectionand heat/mass flux do change viscosity throughout the sam-ple so it seems the gradient of viscosity is one of the domi-nating factors in the pattern formation by BF technique.

Particles Embedded in Self-organizedPorous Polymer FilmsIn this section, the effect of particles on pattern formation ofPS-mCOOH and also assembly of particles in the interface ofwater and solution was investigated. Experiments were car-ried out using toluene (Fig. 7a, 7d) for 3-D structure andchloroform (Fig. 7e, 7f) for 2-D structure. Silica particleswith size of 0.5, 0.7, 1, 1.5 lm were used in concentrationsof 0.2 � 107, 0.5 � 107, 1.3 � 107, 1.8 � 107, 3.6 � 107 and7.2 � 107 particles/ll. The tendency to form ‘‘clusters,’’

FIGURE 3 PS-mCOOH in chloroform; (a) Surface of the poly-

mer film(optical image); (b) SEM top view, partly peeled; (c)

SEM side view.

TABLE 1 PS-mCOOH in Different Solvents: Bubble Diameter D

and the Coefficient of Variation CV

Solvent D (lm) CV

Toluene 5.78 6 0.83 4.6 6 1%

Choloform 5.46 6 0.28 2.2 6 0.6%

Carbon disulfide 5.13 6 0.85 4.3 6 0.6%

FIGURE 4 Linear polystyrene in chloroform. Only the formation

of isolated droplets can be visualized, scale bar is 15 lm.



which impede pattern formation, rises for increasing concen-tration and particle size.

We observed that the silica particles were more soluble inPS-mCOOH than PS solution, because the polar group of PS-mCOOH decorates the hydrophilic silica particles and pre-vents them from aggregating. Figure 7 shows examples ofparticle-assembly in a porous polymer film, and it can beobserved that silica particles only assemble at the polymer-water interface. Similar to the PS solution, silica particlestend to decorate the upper rim of the bubble and can rarelybe found in the bottom,34 also, the particles are less wettedby the water droplets in PS-mCOOH compared with PS. Thehydrophilic end of PS-mCOOH adsorbs on the surface of par-ticles and the hydrophobic end stretches out from particlesso the surface of silica particles may become more hydro-phobic.35 In contrast, PS does absorb very little to the sur-face of particles and the hydrophilicity of surface of particlesalmost remains unchanged, which results in a lower contactangle for PS-mCOOH. We also observed that most of thesilica particles for PS-mCOOH/toluene film, were in lowerpores (Fig. 7c). This is because water droplets adsorb most

of the particles and drag them to a lower layer while theysink. All of the analysis showed that particle size and con-centration did not have any readable effect on the assemblyprocess and pattern formation.

CONCLUSION

Breath-Figure (BF) technique produced highly ordered honey-comb structures by blowing an airflow parallel to the surfaceof polymer solution. The condensed water droplets are dis-persed in a polymer solution environment (similar to water inoil emulsion) and stabilized by the end-functional polystyrene(PS-mCOOH), or by silica particles (for polystyrene solution).The end-functional polystyrene produced a porous honey-comb structure for all solvents (toluene, chloroform and car-bon disulfide), whereas the non-functional polymer (PS) solu-tion did not form any regular pattern. The results show thatwithin an ordered area, regularity of the pattern for all sol-vents was similar, whereas the size of the regular area wassignificantly larger for chloroform and carbon disulfide. Silicaparticles in different sizes and concentrations were added toboth polymer solutions. For all samples, particles were carriedto a higher position by capillary flow and preferred to

FIGURE 5 SEM: Silica particles (0.5 lm particles) in PS-chloro-

form and their assembly at the water polymer interface. (a) Top

view, particles can be seen in the rim of pores; (b) Side view.

FIGURE 6 SEM: Silica particles (1.5 lm particles) in PS-chloro-

form and their assembly at the water polymer Interface. (a) Top

view, particles can be seen in the rim of pores; (b) Side view.



assemble near the opening of the holes. Bigger particlesproduced more regular bubbles with lower concentrationscompared with smaller particles in PS/particle films.

The size of the bubbles varied in different areas from 4.8 to7 lm for PS-mCOOH films and from 6 to 9 lm for PS/

particle films. The bubbles in the PS/particles samples were

flattened at the bottom and horizontally wider compared

with spherical bubbles obtained by PS-mCOOH, but the vol-

ume remains almost constant. The pattern formation of

PS-mCOOH/particles films was not affected by size and

FIGURE 7 Assembly of silica particles at the water-polymer interface. (a) PS-mCOOH-Toluene (0.5 lm particles); (b) PS-mCOOH-

Toluene (0.7 lm particles); (c) PS-mCOOH-Toluene particles were mostly in lower layers(1lm particles); (d) PS-mCOOH-Toluene

(1 lm particles); (e) PS-mCOOH-chloroform solution, preferred assembly near the upper opening (0.5 lm particles); (f) PS-mCOOH-

chloroform solution (1 lm particles).



concentration of particles, which means that the functional

part of polymer dominated particles in pattern formation.

The separation length of the pores was larger for PS/particle

samples than PS-mCOOH films. It is also found that in PS/

particle films just a few decorated droplets were needed to

produce a larger regular pattern, which leads to the conclu-

sion that the addition of particles is not solely responsible

for stabilizing the water droplets during pattern formation. It

seems in addition to the absolute value of viscosity sug-

gested by different authors, the gradient of viscosity and the

flow of the material throughout samples also plays an impor-

tant role in pattern formation. In summary, we can conclude

that stabilizers did change the shape of pores, regularity, and

separation length between the pores but did not affect their

volume. The investigation showed that end-functional poly-

mer is more suitable for pattern formation by BF technique,

and PS/particles cannot produce 3-D porous film.

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