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Flow Behaviors of Polymer Colloids and Curing Resins Affect Pore Diameters and Heights of Periodic Porous Polymer Films to Direct Their Surface and Optical Characteristics Jong Seong Park, Beu Lee, Ji Hoon Park, Yeon Jae Choi, Ji Eun Song, Min Gyu Kim, Ju A La, Seung Beom Pyun, and Eun Chul Cho Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03906 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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Revised MS, la-2018-03906h.R1

Flow Behaviors of Polymer Colloids and Curing Resins Affect Pore Diameters and Heights of Periodic Porous Polymer Films to Direct Their Surface and Optical Characteristics

†

†

Jong Seong Park, Beu Lee, Ji Hoon Park, Yeon Jae Choi, Ji Eun Song, Min Gyu Kim, Ju A La, Seung Beom Pyun, and Eun Chul Cho*

Department of Chemical Engineering, Hanyang University, Seoul, 04763, South Korea. *E-mail: †:

[email protected]

These two persons contributed equally to this work.

ACS Paragon Plus Environment

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ABSTRACT: Manipulation of both pore diameters and heights of two-dimensional periodic porous polymer films is important to extensively control their characteristics. However, except for using different sized colloid templates in replication methods, effective method that tunes these factors has rarely been reported. We find both parameters are controllable by adjusting the flow behaviors of polystyrene colloids and curing resin precursors while preparing phenolic resin and polydimethylsiloxane periodic porous films by embedding their precursors into colloidal crystal monolayers. We adjust the flow behaviors by either varying film preparation temperatures (≥ glass transition temperature of polystyrene) or using the precursors mixed with different amounts of a solvent that renders the colloids viscous. Consequently, the pore diameters and film heights change by 36–56% and 56–84%, respectively. Such modulation results in the change in height to dimeter ratios and the areal fractions of resins at air-film interfaces, thereby significantly changing the water contact angles on these surfaces and their photonic characteristics. This straightforward method does not require additional steps, differently sized colloids, or different amounts of precursors for these parameter controls. KEYWORDS: Periodic porous polymer films, flow behaviors, curing resins, polymer colloids, simultaneous height and pore controls, replication method.


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INTRODUCTION Two-dimensional periodic porous films have ordered arrays of uniform-sized pores with monolayer levels. They not only provide high surface areas, but also create various properties.1 For this reason, these films have been used as super-hydrophobic substrates,2–4 size-selective membranes,5–7 templates for catalytic reactions8 or cell growth,9–11 photonic crystal films,12–14 photodetectors,15 photovoltaic devices,16 and optical sensors.17–21 To date, the breath figure and replication methods are suggested to prepare the porous films. Breath figure methods produce pores by condensing water on the substrate surface, where solvents in the solvent–polymer mixtures are evaporated.3,5,7,9–11,22–31 Replication methods embed replicating components into either colloidal crystal monolayers or matster templates having a certain geometries, and the porous structures are obtainable after removing the colloids.1,2,4,8,12–16,19–21,32–41 Regardless of the preparation method, pore diameter is one of the important factors that determine the characteristics of the films. The breath figure method controls the pores by introducing a humidity condition and using different volumes of casting hydrophobic polymers.22–25 While the replication methods mostly modulate the pore diameters by using colloids of different sizes, other methods have also been suggested. For example, pore diameters of polydimethylsiloxane (PDMS) films were tuned by deforming polymer colloid crystal monolayer templates, prior to the embedment of a PDMS precursor.33 In addition, the pores of the polyurethane films were also controlled by curing a polyurethane precursor at different configurations.34 In some cases, the manipulation of both pores and heights is also necessary to vary the functions of films extensively. For examples, it was reported that the pore structures and the surface characteristics of porous films for membrane applications can greatly affect the viscosity and slip characteristics of fluids42–44 and thus affect the fluids flow.45,46 In replication method, the film height and size of the pores could be simultaneously tunable by using colloids of different sizes because the changes in the diameters mostly accompany the height changes of the porous films. In addition, from


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either electrochemical techniques35–38 or electrodeless metallic depositions,39 the pore diameters and film heights could be also controllable by adjusting the deposition times of reductive/electroactive replicating components. However, for polymer films, it does not appear to be easy to control over these parameters by adjusting amount and concentration of replicating components. Alternatively, it was reported that the pores and heights of fluorinated porous films were modulated by removing the top layer with an adhesive tape.3 However, the structures might be altered depending on the peeling speeds, and thus, fine-tuning of the pores and heights might not be reproducible. Therefore, it is necessary to develop a robust method effectively regulating the pore diameters and heights of periodic porous films without additional steps, without using differently sized colloids, and without using different amounts of precursors.

Scheme 1. Schematic illustrating the strategies to simultaneously control the pore diameter and height of phenolic resin and PDMS periodic porous films. Symbols and Abbreviations: Tg, glass transition temperature of PS colloids; Tcure, curing temperatures of resin precursors; THF, tetrahydrofuran.


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Here, we suggest a novel approach that simultaneously modulates the pore size and height of periodic polymeric porous films, replicated from colloidal crystal monolayers, by controlling the flow behaviors of polymer colloids and curing resin precursors (Scheme 1). We synthesize monodisperse linear polystyrene (PS) colloids to prepare the colloidal crystal monolayers. Either a phenolic resin or a PDMS precursor is embedded into the monolayers to produce periodic porous films. As for the phenolic-resin-based film, we introduce different curing temperatures, which are equal to or above the glass transition temperature (Tg) of PS colloids,31,47 to control the flow behaviors of the PS colloids and curing resin precursors. While the film curing temperatures are increasing from 100 °C (close to Tg) to 160 °C (far above Tg), the pore diameters and film heights significantly decrease. Importantly, the different film structures significantly alter the water contact angles, and also change the light reflectance of the films that are attached on a silicon wafer. As for the PDMS periodic porous films, their pore sizes and heights are tunable by adding different amount of tetrahydrofuran (THF) to the PDMS precursor, not only to render the colloids viscous or plasticized but also to decrease the viscosity of the PDMS precursor. We find that the transmittances of PDMS films are greatly tunable at specific optical frequencies, along with tunable water contact angles. Here we discuss the mechanism underlying the structural changes of the films in connection with the flow behaviors of PS colloids and curing resin precursors in monolayer-resin composites.

EXPERIMENTAL SECTION Materials: We used ethanol (99.9%) from Fisher Scientific (USA). We purchased dioctyl sulfosuccinate sodium salt from Wako Chemical, Ltd. (Japan). We purchased polyvinylpyrrolidone (Mw ~ 55,000), styrene (≥99%), 1-butanol (99.9%), sodium 4-vinylbenzenesulfonate (≥90%), 2methoxyethanol (≥99.9%), and hydroxypropyl cellulose (99%, Mw ~ 100,000) from Sigma–Aldrich (Yongin, Korea). We purchased benzoyl peroxide (74%) from Daejung (Korea). We purchased polydimethylsiloxane

precursor

(Sylgard®

184),

α,α′-azobis(isobutyronitrile)


(98.0%),

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tetrahydrofuran (HPLC), acetone (99.5%), and phenolic resin precursor (CB-8057) from Dow Corning (USA), Junsei Chemical (Japan), J. T. Baker (USA), Samchun chemicals (Korea), and Kangnam Chemical Co. (Korea), respectively. Single-side polished silicon wafers (P type, 525 ± 25 μm thickness) and cover glass (22 mm × 22 mm) were purchased from Namkang Hi-Tech (Korea) and Paul Marienfeld GmbH & Co. (Germany), respectively. Synthesis of PS Colloids: We referred literatures48–50 for the synthesis of PS colloids. Briefly, for the synthesis of 0.9 m PS colloids, 1.8 g of poly(vinylpyrrolidone) and 0.05 g of sodium 4vinylbenzenesulfonate were dissolved in a mixture of 15 g deionized water (18.2 MΩ⸱cm) and 70.4 g ethanol, and a reactor containing the mixture was heated to equilibrate at 70 °C. In the meantime, 0.125 g α,α′-Azobis(isobutyronitrile) was dissolved in 12.5 g styrene. Upon the thermal equilibrium of the reactor, a styrene–AIBN mixture was added to the reactor under stirring and N2 pursing. The polymerization was conducted for 18 h. For the synthesis of 2.3 m PS spheres, 1.8 g of poly(vinylpyrrolidone) and 0.2 g of dioctyl sulfosuccinate sodium salt were dissolved in 85.4 g ethanol, and a reactor containing the mixture was heated to equilibrate at 70 °C. In the meantime, 0.125 g of α,α′-Azobis(isobutyronitrile) was dissolved in 12.5 g of styrene. Upon the thermal equilibrium of the reactor, styrene–AIBN mixture was added to the reactor under stirring and N2 pursing. The polymerization was conducted for 18 h. For the synthesis of 3.2 m PS spheres, 1.8 g of poly(vinylpyrrolidone) and 0.2 g of dioctyl sulfosuccinate sodium salt were dissolved in 85.4 g ethanol, and a reactor containing the mixture was heated to equilibrium at 70 °C. Meanwhile, 0.25 g of α,α′-Azobis(isobutyronitrile) was dissolved in 25 g of styrene. Upon the thermal equilibrium of the reactor, styrene–AIBN mixture was added to the reactor under stirring and N2 pursing. The polymerization was conducted for 18 h. For the synthesis of 6.1 m PS spheres, 1.5 g of hydroxypropyl cellulose was dissolved in 42.5 mL ethanol and 2-methoxyethanol, and a reactor containing the mixture was heated to equilibrate at 65 °C under stirring and N2 pursing. After 30 min,


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0.6 g benzoyl peroxide dissolved in 15 mL styrene was added. After 2 h, the reaction temperature was raised to 75 °C. The polymerization was conducted for 24 h. Fabrication of Colloidal Crystal Monolayers by Using PS Colloids: The as-synthesized PS colloids were purified by centrifugation, removal of aqueous solution, and re-dispersion of the colloids by adding ethanol. The purification process was repeated two more times. The PS colloids were re-dispersed in 1-butanol for the preparation of colloidal crystal monolayers. Specifically, the butanol-PS dispersion was dropped on the surface of deionized water in a petri dish, and the PS colloidal crystal monolayer was transferred onto a silicon wafer. The as-synthesized PS colloids were purified by first centrifugation, removal of supernatant, and re-dispersion in ethanol. After purification of the ethanol–PS colloid dispersion, the PS colloids were once more re-dispersed in 1butanol. The butanol–PS dispersion was dropped on the surface of deionized water filled in a petri dish, and the PS colloidal crystal monolayer was transferred onto a silicon wafer (1.3 cm × 1.3 cm). The PS colloidal crystal monolayer was dried at room temperature. Fabrication of Phenolic-resin-based Periodic Porous Films: First, a phenolic resin precursor mixture was dissolved in ethanol. The concentration of the precursor in ethanol varied depending on the diameters of PS colloids assembled in the monolayers. We used 2 wt % for the 0.9 m colloid, 3.5 wt% for the colloids of 2.3 and 3.2 m, and 4 wt% for the 6.1 m colloid. Next, we dropped the ethanol solution on the surface of the PS colloidal crystal monolayers with the different amount depending on the diameters of PS colloids assembled in the monolayers. While 17 L of the solution was dropped on the monolayers with PS colloids of 0.9 and 2.3 m, the 17 L of the ethanol solution was dropped twice on the monolayers with PS colloids of 3.2 and 6.1 m. After drying the sample overnight at room temperature to evaporate the ethanol, the phenolic resin precursors were cured at 100–160 °C for 6 h under vacuum. Then, all the PS colloids were removed from the sample by immersing it in tetrahydrofuran for 2 h. The periodic concave microstructure was washed with tetrahydrofuran and de-ionized water several times and dried at room temperature.


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Fabrication of PDMS Periodic Porous Films: Sylgard® 184 was used with a mixing ratio of siloxane oligomer to siloxane crosslinker = 10:1. First, the precursor was mixed with THF. The content of THF was changed from 0 wt% to 20 wt%. The precursor–THF mixtures were poured into the PS colloidal crystal monolayer bearing the diameter of 0.9 m. After curing for 24 h at 25 °C, the PS was removed by immersing the samples into acetone at 55 °C for 2 h. Then, the PDMS films were separated from the substrate and the film was further cleaned by immersing it into pure acetone at 55 °C for 2 h. Characterization: Optical microscopy was used (BX 51, Olympus, Japan). Scanning electron microscopy (SEM, S-4800, HITACHI, Germany) was used to investigate the morphologies of the colloidal crystal monolayer films and the colloidal crystal patterns. The transmission and reflectance characteristics of the colloidal crystal patterns were obtained using a UV-Vis spectrophotometer (V670, JASCO, Japan) equipped with a 60 mm integrating sphere (ISN-723). A Spectralon white reflectance standard (Labsphere, USA) was used as a reference. The contact angle of the DI water was measured by using a goniometer (PHOENIX-150, Surface Electro Optics, South Korea).

RESULTS AND DISCUSSION Simultaneous Control of Pore Diameters and Heights of Periodic Porous Films. In fact, we first expected that the film height and size of the pores would be tunable by embedding different amounts of replicating components into colloidal crystal monolayers. From the introduction of electrochemical35–38 or electrodeless depositions,39 the dimensions of the heights and pores of metallic/polypyrrole porous films were mostly controlled by adjusting the deposition times (=amount) of replicating components on substrates where the colloidal crystal monolayers were coated. However, with the phenolic resin precursor, the film height was not controlled effectively and reproducibly, and the porous structures were not well developed with a low amount of polymers (see Figure S1, Supporting Information). When introducing the electrodeposition method, the replicating


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components might be deposited from the bottom of the substrates to the top of the colloidal crystal, thereby enabling the modulation of the structures provided that the deposition conditions are properly established. Meanwhile, given that the polymer resins or precursors were mostly deposited from the top of the colloidal crystal to the bottom of the substrate, it was suspected that the uniform structures would be fairly affected by various factors, such as the flow (or viscous) behavior of the precursors and their wettabilities to polymer colloids or substrates. In the meantime, Li and Zhang induced deformation of the PS colloids arrayed in colloidal crystal monolayers by annealing at below their Tg (70–90 °C):33 with increasing annealing temperature, the spherical colloids were turning into dome-like shape ones. When the PDMS precursor was embedded into these colloidal monolayers and the PS colloids were removed, the PDMS porous films increased the pore sizes with increasing annealing temperatures. Later, Cheng et al. reported that the curing of a polyurethane precursor embedded in a colloidal crystal monolayer with “upside-down” configuration made the precursor flow toward the top of the colloidal monolayers until the precursors reached gelation.34 Therefore, the pore sizes were tunable. Those two studies inspired us to explore an effective way to extensively control the height and pore sizes of porous films without introducing differently sized colloids and without adjusting the amounts of curing resins. As illustrated in Scheme 1, we expected that the morphologies of PS colloids in the monolayer could significantly alter when they were exposed to temperatures higher than Tg of PS (100 °C) or exposed to solvents that can plasticize the PS colloids. If the resin precursors, which are embedded into the colloidal crystal monolayers, have low viscosity at these conditions, the precursors could also flow according to the morphology changes of the PS colloids. Meanwhile, the gelation of precursors should be also considered as this could hamper their deformation. Putting together, before the experiment, we hypothesized that the structure of the porous films (heights and pore diameters) could be significantly changed if the deformation rate of the PS colloids is faster than the gelation time of the curing resin precursor. On the contrary, the


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structural change might not be significant if the deformation rate of the colloids is slower than the gelation time or if the precursors are highly viscous at a certain condition.

Figure 1. Optical microscopy (OM) images showing the effect of temperatures on the time-course morphological changes in the colloidal crystal monolayers. PS colloids of diameter 0.9, 2.3, 3.2, and 6.1 m were used to produce four types of colloidal crystal monolayers.

To demonstrate the scenarios, we first made periodic porous films by using phenolic resin precursors and PS colloidal crystal monolayers and from the variation of preparation temperatures. Preliminarily, we observed the time-course changes in the morphologies of the colloidal crystal monolayers when they were exposed to different temperatures (Figure 1). For the fabrication of colloidal crystal monolayers, we synthesized four different linear PS colloids having diameters of 0.9 ± 0.05, 2.3 ± 0.12, 3.2 ± 0.16, and 6.1 ± 0.31 m (see Experimental Section). Note that no crosslinker was used to render PS colloids viscous at T ≥ Tg, 100 °C.31,47 The colloidal crystal monolayers were prepared according to the method described in the Experimental Section. As shown in the Figure 1, the resulting monolayer is well packed with a hexagonal symmetry. When these


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monolayers were exposed to 100 °C, none of the colloidal crystal monolayers showed significant changes in their morphologies until 6 h. Meanwhile, the monolayers were transformed into continuous films within 6 h at 120 °C. We observed that the transformations were faster and more complete when the monolayers were exposed to 160 °C.

Figure 2. (A) Side-view SEM images showing time-course changes in the morphologies of composite films composed of the colloidal crystal monolayer and the phenolic resin, when prepared at various preparation temperatures. PS colloids of 6.1 m diameter were used to prepare the colloidal crystal monolayer and the phenolic resin precursors were embedded into the monolayer. (B) Plots showing the average heights of phenolic resins as a function of times for various preparation temperatures. The data were obtained based on the analysis of the SEM images shown in (A). The dashed lines were shown to guide the changes of height at each temperature.

We next observed the time-course changes in the morphologies of composite films made of the colloidal crystal monolayer and a phenolic resin precursor, as a function of preparation temperatures.


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An amount of 17 L of an ethanol mixture containing the phenolic resin precursor was dropped on the surface of the colloidal crystal monolayer. After evaporation of the ethanol at 25 °C, the phenolic resin precursors were cured at 100 – 160 °C for 6 h under vacuum. As shown in Figure 2A, we observed side-view temporal changes in the morphologies of 6.1 m PS colloids in the colloidal crystal monolayer where the phenolic resin precursor was embedded. To understand the results, we also estimated the gelation times of the phenolic resin films at each curing temperature from the determination of the hardening time, with a glass probe, of precursor films coated on a glass slide. At 100 – 160 °C, the precursor film initially became viscous, and next turned sticky, and eventually turned into a soft gel. At 100 °C (close to Tg), there was no significnat change in the morphology of PS colloids and phenolic resin precursors for 6 h. The phenolic resin precursor stayed viscous and became gradually sticky for approximately 12 min after which the precursor became a soft gel. Since there was no significant change in the morphologies of PS colloids for 6 h (Figures 1 and 2A), it could be said that the time for the phenolic resin to reach gelation is much shorter than the time for the PS colloids to start flowing for the deformation of the phenolic resin (tcure, R < tflow, PS). As such case, the precursor became solidified by preserving the initial inverted structure. At 120 °C (slightly higher than Tg), the PS colloids exhibited deformation in 2 h when the colloidal crystal monolayer was not embedded with the phenolic resin precursor (Figure 1). In fact, the PS colloids in the monolayer were about to be coalesced around 10 min (see Figure S2, Supporting Information). Nevertheless, from Figure 2A, the PS colloids did not look significantly deformed for the first 2 h when the monolayer was embedded with the precursor. At 120 °C, the gelation time of phenolic resin precursor films was approximately 10 min, similar to the softening time of PS colloids. Therefore, it was speculated that the phenolic resin inverted structure influenced the flow behavior of PS colloids. Meanwhile, from the analysis of the SEM images (Figure 2B), the average height of the phenolic resin was decreased from 3.8 m before cure to 2.8 ± 0.4 m (n = 4)


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at 2 h. The results also suggested that the phenolic resin inverted structure was also influenced by the deformation of PS. At 6 h, the shape of PS colloids were collapsed. The height of phenolic resin at some regions was quite decreased (white arrow), resulting in a further decreased in the average height to 2.3 ± 0.9 m (n = 4). At 130 °C, the PS colloids rapidly changed their morphologies within 1 h, and the height of the phenolic resin was reduced to a considerable extent. The gelation time was approximately 10 min at 130 °C. When the colloidal crystal monolayer was not embedded with the phenolic resin precursor, the colloidal crystal monolayer turned into a film from 7 min (see Figure S3, Supporting Information). At 160 °C, there were significant changes in PS morphology, and the phenolic resins looked more collapsed. At 2 h and 6 h, it was difficult to clearly identify the phase boundaries between the PS and phenolic resin. While the gelation time was approximately 5 min at 160 °C, the PS colloidal crystal monolayers turned into a continuous film within 5 min (see Figure S4, Supporting Information). Therefore, at 130 °C and 160 °C, the time for PS colloids to flow and deform the phenolic resin precursor might be shorter than the time for the phenolic resin precursor to reach gelation (tcure, R > tflow, PS). Figure 2B summarizes the analysis of the SEM images. After curing the resin at 100 °C for 6 h, a post-cure was conducted at 160 °C for 0.5 h for complete curing of the phenolic resin. However, the resulting morphologies of the porous films after the removal of PS colloids were very different from the morphologies of the film prepared only at the cure temperature of 160 °C (Figure 3A), implying that the post-cure process did not significantly affect the structure of the film.


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Figure 3. (A) SEM images showing the four types of colloidal crystal monolayers and the corresponding top-and side-view SEM images of the phenolic-resin periodic porous films prepared at different preparation temperatures, 100 °C and 160 °C. (B, C) Average pore diameters (B) and heights (C) of the porous films prepared at different curing temperatures by using four types of colloidal crystal monolayers as templates. The error bars are the standard deviations relative to the average values (n=50).

After the cure process, the PS colloids were removed by immersing the sample in THF for 2 h. Figure 3A shows the SEM images. Regardless of the colloidal sizes we used, the pore diameters of the porous films prepared at 160 °C appeared smaller than the films prepared at 100 °C. We compared the pore sizes of the films, based on the analysis of SEM images (Figure 3B). With the monolayer using 0.9, 2.3, 3.2, and 6.1 m PS colloids, the pore diameters were reduced by 36 % (from 0.83 ± 0.03 m at 100 °C to 0.53 ± 0.03 m at 160 °C), 52 % (2.13 ± 0.05 m to 1.03 ± 0.06 m), 56 % (3.08 ± 0.08 m to 1.36 ± 0.08 m), and 54 % (6.03 ± 0.15 m to 2.80 ± 0.13 m), respectively. In addition, the heights were more significantly changed as shown in Figure 3A and


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Figure 3C. With the monolayer using 0.9, 2.3, 3.2, and 6.1 m PS colloids, the film heights were reduced by 72 % (from 0.50 ± 0.02 m at 100 °C to 0.14 ± 0.01 m at 160 °C), 74 % (1.08 ± 0.05 m to 0.28 ± 0.02 m), 81 % (1.81 ± 0.09 m to 0.35 ± 0.06 m), and 84 % (3.41 ± 0.18 m to 0.56 ± 0.09 m), respectively.

Figure 4. Top view (A, C, E) and side view (B, D, F) SEM images of the PDMS periodic porous films prepared by casting PDMS precursors–THF mixtures onto the PS colloidal crystal monolayers. The THF contents in the mixture were (A, B) 0 wt%, (C, D) 10 wt%, and (E, F) 20 wt%.

We further extended the concept to the preparation of PDMS periodic porous films replicated from the PS colloidal crystal monolayer (with 0.9 m PS colloids). Sylgard® 184 was used with a mixing ratio of siloxane oligomer to siloxane crosslinker = 10:1. In fact, it was even more difficult to adjust the amount of the precursor embedded into the monolayer than in the case of the phenolic resin precursors, and thus, the precursor was poured into the colloidal crystal monolayer to form a thick


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film (thickness of 400 m) covering the top of the PS colloids. Moreover, we found that the structures of the periodic porous films did not show dependence on the preparation temperatures (Figure S5, Supporting Information), which was probably due to the high viscosity of the PDMS precursor and fast gelation at high temperatures. Alternatively, we mixed the precursor with THF to lower the viscosity of the precursor. In addition, as THF can dissolve the PS, it would smear into the PS colloidal crystal monolayer when the precursor–THF mixture was cast on the monolayer, thereby enabling the PS colloids to flow. For this reason, we expected that THF would not only make the PS colloids flow but also decrease the viscosity of precursors to adopt the deformation of colloids. In this case, the degree of structural changes could depend on the amount of THF. We prepared the PDMS periodic porous films by adjusting the amount of THF by 5, 10, and 20 wt% in the precursor–THF mixture. The curing was conducted at 25 °C for 24 h to sufficiently retard the gelation of PDMS. After the process, the PS was removed by immersing the samples into acetone at 55 °C for 2 h. And then, the PDMS films were easily separated from the substrate. Lastly, the surface of PDMS film was further cleaned by immersing the film in pure acetone at 55 °C for 2 h. Note that the use of acetone, instead of THF, did not significantly affect the structure of the PDMS film. Figure 4 presents the top- and side-view SEM images of the films prepared by using the precursor–THF mixture containing THF of 0, 10, and 20 wt%. Figure S6 in the Supporting Information showed OM images of all the PDMS films we prepared. The pore diameters were reduced by 45 % from 0.83 ± 0.03 m to 0.72 ± 0.03 m, and further to 0.46 ± 0.04 m when the THF content in the precursor mixtures changed from 0 wt% to 10 wt% and to 20 wt%. The heights were also decreased by 55 % from 0.40 ± 0.01 m to 0.30 ± 0.01 m and further to 0.18 ± 0.01 m. Surface and Photonic Characteristics of Periodic Porous Films. The two examples clearly demonstrate that the control of the flow behaviors of colloidal crystal monolayers and curing resin precursors are of great importance to manipulate the pore size and height of the periodic porous films. In addition, the method is straightforward because the geometries of the films are easily controllable


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by simply modulating the processing conditions. From a practical perspective, it is also important to understand the effect of changes in pore size and height on the characteristics and function of periodic porous films. From Figure 3, it was found that the height to diameter ratios of the films were 0.5 – 0.6 when the films were prepared at 100 °C, but the ratios were reduced to 0.20 – 0.27 at 160 °C. The results were attributable to the significant reduction in the heights of films at 160 °C. Since the decrease in the ratios could result in the change in the surface compositions (=areal fractions) of resin at air-film interfaces, we first investigated the water contact angles on the periodic porous phenolic resin films prepared at two curing temperatures (100 and 160 °C, Figure 5A, B). For the films made from 0.9, 2.3, 3.2, and 6.1 m PS colloids, the contact angles were substantially reduced by 41° (from 127° at 100 °C to 85° at 160 °C), 41° (128° to 87°), 48° (134° to 86°), and 49° (138° to 89°), respectively. The water contact angles on the porous film prepared at 160 °C were close to that of a non-porous smooth phenolic resin film (79°). We estimated the areal fraction of the phenolic resin exposed to air surface (Figure 5C). The fractions were increased from 0.18 – 0.22 when the film was prepared at 100 °C to 0.80 – 0.88 at 160 °C. The Cassie–Baxter model51 explains the changes in the contact angles of a porous surface with the equation cos = fR cosR − fair, where fR and fair are the areal fractions of the phenolic resin and air, respectively, and cosR is the contact angle of a flat phenolic resin film (79°). Based on the equation and areal fractions estimated from the SEM images, the contact angles should be 138 – 141° at 100 °C and 87 – 93° at 160 °C, which are approximate to those of the experimental results.


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Figure 5. (A) Water contact angles on the periodic porous phenolic resin films prepared at 100 °C and 160 °C by using four types of colloidal crystal monolayers as templates. (B) Average water contact angles, analyzed from images in (A), of the periodic porous phenolic resin films. The error bars are the standard deviations relative to average values (n=5). (C) Areal fraction of phenolic resins in the porous films exposed to air surface. The fractions were estimated from SEM images shown in Figure 3A. (D) Reflectance spectra of a bare silicon wafer and the periodic porous phenolic resin films prepared at 100, 130, and 160 °C on the silicon wafer by using the 0.9-m-colloidal crystal monolayer template. Flat film is the flat phenolic resin film (~10 m) cast on the Si wafer, and the data was included for comparison.

The present porous films replicated from the 0.9-m-PS colloidal crystal monolayers changed the pore sizes from 0.83 at 100 °C to 0.53 m at 160 °C, and the height from 0.50 at 100 °C to 0.14 m at 160 °C. As the sizes are equivalent to the wavelengths of the visible and near-infrared (VIS–NIR) lights, we further investigated the optical characteristics of the films, prepared at 100, 130, and 160 °C, by measuring the reflectance spectra of these porous films adhered on a silicon wafer (Figure 5D). The film prepared at 100 °C has a reflectance peak of approximately 723 nm, which is characteristic of photonic crystals.1,2,52 The peak was shifted to 640 and 612 nm when the film was


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prepared at 130 and 160 °C, respectively. The optical patterns are different from the optical spectra of a phenolic resin film (~10 m).

Figure 6. (A) Average water contact angles of the periodic porous PDMS films as a function of THF contents in the precursor–THF mixture used to cast onto the PS colloidal crystal monolayers. The error bars are the standard deviations relative to average values (n = 3). (B) Transmittance spectra of periodic porous PDMS films prepared by using the precursor–THF mixture bearing different amounts of THF. The transmittance of flat PDMS film (black line) is also included for comparison.

Figure 6 showed the water contact angles (Figure 6A) and light transmittances (Figure 6B) of PDMS periodic porous films whose dimensions were modulated by different amount of THF in the precursor–THF mixtures. From the analysis of Figure 4, the ratio of pore depth to diameter was 0.48 for the film made only with PDMS precursor (without THF), but the ratios were changed to 0.42 and 0.39 when the films were prepared using precursors mixed with 10 wt% and 20 wt% THF, respectively. Therefore, due to change in the areal fraction of resin at the air-film interfaces, water contact angles on the films were changeable. The porous PDMS film made only with PDMS


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precursor (without THF) had a contact angle of 132°, and the contact angles were decreased to 127°, 122 ± 2°, and 115 ± 2° with increasing content of THF in the precursor to 5, 10, and 20 wt% during the preparation of films. Note that the water contact angle of flat PDMS film was 110°. From the estimation of areal fractions using SEM images (Figure 4), the water contact angles were calculated to be 137° (no THF content), 128o (10 wt% THF), and 113° (20 wt% THF). In addition to the wettability changes, interesting photonic characteristics of the PDMS porous films were observed. The transmittances of flat PDMS films were measured to be approximately 92% with a reference of air transmittance (100%). The periodic porous PDMS films had characteristic peaks at visible frequencies. The maximum peak was 662 nm when the films were made with no THF, and the peak was shifted to 638 (5 wt% THF), 634 (10 wt% THF), and 632 nm (20 wt% THF) with increasing content of THF in the precursor–THF mixtures. Furthermore, the transmittances at the maximum peaks decreased from 103% to 93%, probably owing to the decrease in film height. The results suggest that these periodic porous patterns on the surface of PDMS films are very useful for modulating the light transmittances at specific visible frequency ranges.

CONCLUSIONS To date, the synthesis of two-dimensional periodic porous films has mainly focused on tuning the pore sizes and their periodicities. In the present research, we demonstrate that the pore diameters and heights of the periodic porous polymeric films can be extensively manipulated by tuning the flow behaviors of colloids and curing resin precursors. The key factor to tune the flow behaviors of colloids and curing resins are either the preparation temperatures or the content of solvent added to the precursors. Depending on the characteristics of curing resin precursors, it is desirable to develop different strategies to present the structural changes of the porous films. Importantly, for the pore and height controls, the method does not require any additional step, differently sized colloids, and different amounts of curing resins.


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We further demonstrate that the method is very effective in the extensive modulation of the surface and photonic characteristics of the films. Therefore, we anticipate the method could be useful in fabricating porous films with tunable surface, photonic, and membrane characteristics. We also anticipate this straightforward method can provide much benefit to those who want to fabricate various periodic porous films with one type of colloids, and thus, to exhibit various film characteristics.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI:. It includes SEM images of phenolic resin films prepared by embedding different amount of phenolic resin precursors into a PS colloidal crystal monolayer, OM images of PS colloidal crystal monolayers exposed at 120, 130, and 160 °C for some period of time, OM images of the periodic porous PDMS films both prepared at different curing temperatures and prepared by casting PDMS precursors-THF mixtures of different THF contents onto the PS colloidal crystal monolayers.

AUTHOR INFORMATION Corresponding Author *(E. C. Cho) E-mail: [email protected] ORCID Eun Chul Cho: 0000-0001-6408-3392 Author Contributions †J.

S. Park and B. Lee contributed equally to this work.

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT The authors very much thank prof. Kyung-Do Suh and his laboratory members at Hanyang University for the technical assistance of the work. The authors acknowledge the financial support from a research grant (NRF-2015R1A2A2A01007003) from the National Research Foundation of Korea (NRF).


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Flow Behaviors of Polymer Colloids and Curing ... - ACS Publications

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