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Macromolecular Research, Vol. 21, No. 4, pp 414-418 (2013) DOI 10.1007/s13233-013-1019-4

www.springer.com/13233 pISSN 1598-5032 eISSN 2092-7673

Fabrication of Honeycomb-Structured Porous Film from Polystyrene via Polymeric Particle-Assisted Breath Figures Method Wei Sun*, Yuchen Zhou, and Zhongren Chen Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, P. R. China Received March 8, 2012; Revised May 5, 2012; Accepted May 8, 2012 Abstract: Polymeric particles were utilized as a second component to assist polymer patterning via the breath figures (BF) method. Polymeric constituents lessen the incompatibility between particles and polymers, which brings new characteristics in both pattern fabrication and particle allocation. The influence of the experimental parameters for two kinds of polymeric particles on the pattern morphology of the obtained film was investigated. Different assembling characteristics of particles under different circumstances are also discussed. Polymeric particles are proven to be able to function as BF array stabilizers and assist in pattern formation. Also, polymeric particles enriching the patterned pores present unique assembling characteristics which are much different from previously reported inorganic particles. The possibilities for the functionalization of BF arrays utilizing functional microgels were also tested. These results demonstrate a study in the polymeric particle-assisted BF method, which may provide new insight into novel functional material preparation methods via self-assembly. Keywords: breath figures method, pickering emulsions, polymeric particle, functional material.

Introduction

tion on the polymer surface. Besides polymers, nanomaterials are also becoming the candidates to perform BF method. Due to its unique “size effect” and superior physical/chemical performance, nanomaterial shows great potential with patterned structure. Wu and his co-workers have reported that a series of nanomaterials including polyoxometalates, DNA-surfactant complexes and single-molecule magnets, can be applied in the BF method and certain features of the nanomaterials were preserved within porous films.30-32 Nanoparticles, being the most representative form of nanomaterials, were also utilized to prepare BF patterns. Fluorocarbon-stabilized silver nanoparticles were firstly reported in preparation of porous films.33 Nanocrystals were also fabricated into ordered macroporous thin films via self-organization.34,35 The nanocrystals were found to be mechanically stable enough to withstand a focused ion beam etching without compromising the structure.35 Furthermore, Ji et al. used colloidal particles to assist polymers in forming patterned surfaces via BF method.36,37 A so-called particle-assisted BF method was developed. Particles, instead of amphiphilic surfactants, were successfully used as stabilizers for water droplets in BF method. That is just like the case in which solid particles were commonly used as emulsion stabilizers, the so-called Pickering emulsions.38 Solid particles are able to spontaneously assemble into fluid interface and function as excellent emulsion and foam stabilizers. It is accepted that the particles can act as steric barriers against coalescence of emul-

Breath figures (BF) method is emerging as an representive bottom-up surface-patterning technique to construct ordered arrays on the surface of polymeric films.1-5 Such highly ordered microporous films could have potential applications as separation membranes,6 photonic crystals,7 catalysts,8 electronics,9 immobilization of biomolecules,10 cell culture scalfolds,11 and superhydrophobic surfaces.12 The BF method represents a single-step technique for fabricating films with ordered microscale holes utilizing foggy arrays of water droplets as templates, which evaporate after they leave ordered imprints on the film surfaces.5 The crucial point for the success of carrying out the BF method has always been seen as the stabilization of the templating water droplets. In order to achieve preservation of the ordered arrays of water droplets during the evaporation of solvent, certain materials, mostly polymers, must be used to prevent the coalescence of water droplets. Since the establishment of the BF method by François et al. in 1994,1 a variety of polymers including linear polymer,13-17 amphiphilic copolymer,18-21 liquid-crystalline copolymer,22 rod-coil copolymer,23 star-shaped polymer,24-28 comb polymer,29 has been applied in this method. All of these polymers are capable of effectively stabilizing the templating water droplets so that fine water droplets arrays can lead to regular pattern forma*Corresponding Author. E-mail: [email protected] The Polymer Society of Korea

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Fabrication of Honeycomb-Structured Porous Film from Polystyrene via Polymeric Particle-Assisted Breath Figures Method

sion droplets by forming coherent particle layers around the droplets.39 In the case of BF method, it has been demonstrated that particles can assist better stabilization of templating water droplets and at the same time achieve directed assembly of the particles into the interior walls of the patterned holes.36,37 The employment of particles in the BF method provides a novel way to perform BF method, and also establish a new concept of how the particles can participate in a dynamic self-assembly process. In the particle-assisted BF method, inorganic colloidal particles such as silica particles were commonly used,36,37 producing hybrid films with inorganic particles decorating polymeric matrix. Up to now, no researchers has conducted a systematic study on the behavior of “soft” particles in BF method. Polymeric constituents lessen the incompatibility between the particles and polymers, which may bring changes in both pattern fabrication and particle allocation. In the current work, we carefully examined the influence of the experimental parameters of two kinds of polymeric particles on the pattern morphology of the obtained film. Different assembling characteristics of particles under different circumstances are also discussed. We tend to explore the new topic of fabricating polymer/polymeric particle composite patterning in this study.

Experimental Materials. Polystyrene (PS) (Mw=1.92×105) was purchased from Sigma-Aldrich. Monodispersed PS particles with ~200 nm diameters were purchased from Yiyi Nano (China). The poly(dimethylsiloxane) (PDMS) prepolymer and curing agent were purchased from Dow Corning (Sylgard®184). Silica particles (with mean diameters of 200 nm) were prepared by hydrolysis of tetraethoxysilane in an alcohol medium in the presence of water and ammonia by the procedure originally described by Stöber et al..40 The PS particles were cross-linked to be redispersed in chloroform. The PS particles were washed thoroughly with ethanol and deionized water by repeated centrifugation, and were collected by drying at 70 oC for 12 h before use. Synthesis of Poly(N-isopropylacrylamide)-co-Acrylic Acid Microgels. Poly(N-isopropylacrylamide) (PNIPAm)co-acrylic acid (AA) microgels were prepared by precipitation polymerization.41 NIPAm monomer (0.475 g), AA (0.024 g), and N,N-methylenebisacrylamide (0.052 g) dissolved in water (98 g) at room temperature were tirred at 400 rpm under N2 for 30 min and then heated to 60 oC. After stabilizing the system at 60 oC for 15 min, polymerization was initiated by addition of potassium persulfate (0.060 g)/ water (2 g) solution. The reaction was conducted under stirring at 60 oC for 4 h. The resultant microgel particles were dialyzed for 1 week against deionized water (twice daily changes of water) to remove surfactant and unreacted molecules. After dialysis, PNIPAm-co-AA microgels were conMacromol. Res., Vol. 21, No. 4, 2013

centrated by ultracentrifugation at 10,000 rpm for 1 h and redispersed in deionized water. The micogel particles were then collected by freeze dehydration for further redispersion in ethanol. Film Preparation. Before film preparation, casting solution was prepared by mixing PS chloroform solution with different particle suspensions. Linear PS was weighed in a sample vial and dissolved in chloroform to prepare polymer solutions with concentration of 10 g L-1. Particles were dispersed either in ethanol or chloroform depending on their surface wettability properties for better mixing in the casting solution. Specifically, as-prepared PNIPAm-co-AA microgel particles were dispersed in ethanol. PS particles were dispersed in chloroform. After adding certain particle suspension into polymer solutions, the casting solution was mixed ultrasonically. Quickly after the mixing, the casting solution was transferred onto a clean glass substrate dropwise. At the same time, a humidified flow of air was directed onto the liquid films on the substrate. After solidification, the obtained film was dried at room temperature. Preparation of the Patterned PDMS Substrate. Fabrication of PDMS negative replicas from polymer film templates was achieved by means of soft lithography. Specifically, PDMS prepolymer was mixed with a curing agent at a 10:1 weight ratio, and was carefully poured onto the BF film. After curing at 130 oC for 2 h and removal of the PS mold by dissolution in chloroform, PDMS negative replica were obtained. Characterizations. The surfaces of the microstructured films were characterized with a field-emission scanning electron microscope (FE-SEM; FEI, SiRion100), operating at a 25 kV accelerating voltage. Samples for SEM were coated with gold prior to observation.

Results and Discussion The Application of PS Particles in the Formation of Ordered Microporous Films. PS polymer was used to be the choice of main material for film preparation. PS has been widely used as a model polymer to carry out methodology study on BF method.42,43 Linear PS commonly can’t be directly prepared into honeycomb patterned films. It may be explained that the linear PS without any polar substituents cannot effectively envelop the water droplets so that aggregation of droplets happens easily. So it makes PS become ideal candidate for research of how the second component assist the patterning process via BF method. And we chose PS particles which have the same composition with the polymer so the advantage of polymeric particles on the compatibility with polymer matrix over inorganic counterparts can be insured. We used both silica and PS particles to assist the pattern preparation of PS, making comparisons between inorganic particles with polymeric ones in detailed characteristics of 415

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assembling morphology. The size of both silica and PS particles were set to be around 200 nm. That is based on two reasons. Firstly, particles with such size were proved to be able to assist the BF pattern formation in previous study.36 Secondly, with pore size of the patterned surfaces as much as several micrometers, 200 nm-sized particles not only can be conveniently characterized using SEM, but also can be located through fairly large surface area of the interior walls of the pores. Figure 1(a), (b) shows typical assembling morphology of silica particles. One can easily recognize all the silica particles from the patterned structure. The particles decorate the upper rims of the patterned holes in ordered arrays, which is in good accordance with previous report.36 It has been demonstrated that the preferred arrangement of the particles on the pore rims rather than the bottoms is due to the capillary flow induced by the evaporation of the solvent.36 While in the case of PS particles, the polymer/particle hybrid structure seems much more merged, showing indistinct particle-decorating surface topography. To be specific, silica particles only partially locat into the polymer matrix, hanging along the pore rims. While PS particles merge themselves deeply into the inside walls of the pores, only show small portions of the particles. That fact reflects the positions the particles take during the process of water templating. Particles would spontaneously adsorb into water/oil interfaces which was the surfaces of water droplets in the case of BF method. Hydrophobic PS particles reside most of their parts into polymer solution (oil). After the complete evaporation of solvent and water, the polymeric particles are deeply trapped into polymer matrix. And the silica particles operate the opposite way around due to their hydrophilic properties. Besides, PS particles achieve better dispersion within PS solution than silica particles since their good compatibility with polymer matrix. So the structural integrity for PS particles can be optimal.

Figure 1. SEM images of film fabricated from solutions prepared by adding 15 µL of silica particle (a,b) and PS particle (c,d) alcoholic suspension (10 mg/mL) into 1 mL PS chloroform solution with concentration of 10 g L-1. The scale bars are 2 µm. 416

The Influence of the Experimental Parameters of Polymeric Particles on the Formation of Ordered Microporous Films. Different application quantities of PS particles were used in two different ways to explore the influence of the experimental parameters of the polymeric particle on the formation of ordered microporous films. Figure 2 shows different patterns using different application quantities of the PS particle alcoholic suspension with same concentrations. It is obvious that the patterns of the film surface become increasingly disordered with larger application quantities. The ordered hexagonal arrangement of the pores has been compromised with increasing particle application quantities. And the pore size increases with larger application quantity of the particle alcoholic suspension. It can be explained that with more particles applied, surfaces of the templating water droplets are adsorbed by more particles, which slows the evaporation of the water droplets and gives them more time to grow. The longer growth time may both result in larger pore size and poor pattern regularity because water droplets tend to coalesce with each other. SEM images with larger magnification of pattern morphologies with different particle application quantities are shown in Figure 3. With low application quantity (Figure 3(a)), it is harder to locate the particles on the upper surface of the film. That is because the pore openings are relatively small, and the particles are covered underneath the “roofs” of the pores. To prove this, we made a reverse replica of the porous film by molding with PDMS (see Experimental section). Figure 3(b) shows the PDMS protuberance arrays replicated from the structure of Figure 3(a). The PS particles are closely packed around the outer rims of the PDMS protuberances, which proved the existence of the particles residing in the inner corner of the original pores. Figure 3(c) shows almost full particle coverage on the inner walls of the open pores. Closer look reveals a perfect hexagonal particle

Figure 2. SEM images of film fabricated from solutions prepared by adding 10 µL (a), 30 µL (b), 40 µL (c), 50 µL, and (d) of PS particle alcoholic suspension (10 mg/mL) into 1 mL PS chloroform solution with concentration of 10 g L-1. The scale bars are 50 µm (a,b,d) and 10 µm (c). Macromol. Res., Vol. 21, No. 4, 2013

Fabrication of Honeycomb-Structured Porous Film from Polystyrene via Polymeric Particle-Assisted Breath Figures Method

Figure 3. SEM images with larger magnification of film fabricated from solutions prepared by adding 10 µL (a), 40 µL (c), 50 µL (d) of PS particle alcoholic suspension (10 mg/mL) into 1 mL PS chloroform solution with concentration of 10 g L-1. (c) The PDMS replica molded from the structure shown in Figure 2(a). The scale bars are 2 µm (a,b,c) and 5 µm (d).

arrangement over the interior polymer walls. Such a fine particle array over large inner surface area corresponds to the similar particle arrangement which is commonly found in the case of particle-stabilized emulsions. It is believed that if particles were placed onto the liquid interfaces, interactive capillary forces, which are mediated by capillary bridges between two particles, will tend to organize the particles in a hexagonal lattice.39 Such particle patterns can be well preserved through the evaporation of the solvent because PS particles have good compatibility with the polymer matrix. If we further increase the quantity of particles, the inner surfaces fully covered by particles would show crest-like morphology (Figure 3(d)). It may be the result from the distorted interfaces with overly adsorbed particles. We also tried another way to access the influence of the experimental parameters of the polymeric particle. We used same application quantity of the PS particle alcoholic sus-

Figure 4. SEM images of film fabricated from solutions prepared by adding 10 µL of PS particle alcoholic suspension with concentration of 5 mg/mL (a), 10 mg/mL (b), and 15 mg/mL (c,d) into 1 mL PS chloroform solution with concentration of 10 g L-1. The scale bars are 2 µm (a,b,d) and 10 µm (c). Macromol. Res., Vol. 21, No. 4, 2013

pension with different particle concentration. As shown in Figure 4, particles show more subtle changes with increasing particle concentration comparing the case with different application quantities. Particle coverage of the inner surfaces shows a top-down spreading tendency. Solid particles have reported to have a preferential settlement onto threephase lines during the BF method process.36 Particles tend to accumulate from the top under the influence of the capillary flow. The pore size also increases with higher particle concentration due to the same reason with increasing application quantity. Application of Microgels in the Particle-Assisted BF Method and Its Functional Modification. Beside PS particles, PNIPAm-co-AA microgels were also applied in the BF method. It can be seen from Figure 5(a), (b) that microgels decorating honeycomb structure can be formed. Flattened circular PNIPAm-co-AA microgels are attached from the rim all the way to the bottom of the pores. PNIPAm-coAA microgel is a kind of hydrogels with high water-absorbable ability. We investigate the water adsorption with the microgel particle decorated PS films. For the microgel blended spin-coated PS sample, the initial contact angle is 96o (Figure 5(c)). The contact angle decreased for 20o over 10 min. While for the microgel-assisted PS patterned film with the same microgel content with the flat one, sample shows a significant water-absorbable property. With initial contact angle as high as 110o, one can observe a rapid sinking of the water droplet into the bulk of the sample. After 5 min, the original water droplet with contact angle of 110o disappeared from the surface (Figure 5(d)). The higher initial contact angle for patterned surface is due to the enhanced roughness. And the tremendous water-absorbable property

Figure 5. (a,b) SEM images of film fabricated from solutions prepared by adding 30 µL of PNIPAm-co-AA microgel alcoholic suspension with concentration of 10 mg/mL into 1 mL PS chloroform solution with concentration of 10 g L-1. The scale bars are 5 µm. (c,d) Images indicating the change of water contact angle of corresponding flat (c) and patterned (d) film surfaces. 417

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can be explained by the effective enrichment of the microgels in the patterned pores on the film surface. While for the spin-coated sample, the microgels just spread evenly through the bulk, cannot provide the sample with functional surface properties. Here by applying functional particle-assisted BF method, a novel, functional modification method of BF arrays with ease can be provided.

Conclusions This work has studied polymeric particle-assisted, bottomup surface patterning technique which is based on the combination of BF method and Pickering emulsions. PS particles and PNIPAm-co-AA microgels are both used for assisting patterning. The detailed patterning morphology as well as the assembling characteristics has been discussed. The influence of the experimental parameters of the polymeric particle on both BF array geometry and particle allocation is studied. Polymeric particles are proved to be able to function as BF array stabilizers and assist in patterning formation. And polymeric particles enriching the patterned pores present unique assembling characteristics which are much different from previously reported inorganic particles. The application quantity of the particles dramatically affects the pattern morphology. We also tested the possibilities of the functionalization of BF arrays utilizing functional microgels. These results demonstrate a study in the polymeric particleassisted BF method which may provide new insight into novel functional material preparation method via selfassembly. Acknowledgment. Financial support from the National Natural Science Foundation of China (No. 21104036 and No. 21274070), K. C. Wong Magna Fund in Ningbo University Scientific Innovation Platform Programme of Ningbo (No. 2011A31002) is gratefully acknowledged.

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