Microstructural Aspects of Particle-Assisted Breath Figures in

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Microstructural Aspects of Particle-Assisted Breath Figures in Polystyrene-Alumina Hybrid Free Standing Film Lakshmi Vijaya, Ramya Rajan, Annu Raju, Thazhavilai Ponnu Devaraj Rajan, and Chorappan Pavithran J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02941 • Publication Date (Web): 26 Jun 2017 Downloaded from http://pubs.acs.org on June 26, 2017

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The Journal of Physical Chemistry

Microstructural Aspects of Particle-Assisted Breath Figures in Polystyrene-Alumina Hybrid Free Standing Film Lakshmi Vijaya, Ramya Rajan, Annu Raju, *Thazhavilai Ponnu Devaraj Rajan, and Chorappan Pavithran Materials Science and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology (CSIR-NIIST), Council of Scientific and Industrial Research (CSIR), Thiruvananthapuram 695019, India. Email: *[email protected], [email protected], Tel: 0091-471-2515327, Fax: 0091-471-2491712

Abstract Polystyrene-alumina hybrid free-standing films with amino functionalized breath- figure (BF) cavities were fabricated from a suspension of the amino-functionalized amphiphilic-modified alumina particles in polystyrene/chloroform solution by employing a one-step casting process. The role of the film thickness and the particle concentration on the BF microstructural aspects such as the pore size, pore density, amino group surface density etc. was reported. The dry film thickness increased from 0.91µm to 5.6 µm on increasing the wet film thickness from 0.2mm to 1mm. The pore size increased with the film thickness while the pore density and the porosity decreased. Relatively uniform pore size was observed for a ~3µm thick film in which the moderately ordered BF pattern was more or less maintained up to a particle concentration of 5 wt.% of the polystyrene content. While the 0.91 µm thick film exhibited through-pore structure, concavities were formed in thicker films. The particleassisted BF formation favoured the formation of amino-functionalized cavities. The surface density of the amino-functionality reduced on increasing the film thickness, whereas the density increased with the particle concentration. The implications were discussed. INTRODUCTION During the past decades, the fascinating structure of 2D-micropatterned polymer and polymer-inorganic hybrid films have gained importance due to their captivating properties

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suitable for optical, electrical, thermal and catalytic applications1-4. The Breath-figure (BF) method provides a relatively simple bottom-up approach to build this type of structures5-10. The self-assembly of nanoparticles at the BF interface has emerged as a useful tool to build functionalized micropatterned polymer-inorganic-hybrid films11-18 for various industrial applications such as catalytic support, tissue engineering, and photonic band gap materials etc19-21. In particular, the free-standing micropatterned films (FSM) are well-suited for many chemical and biological applications due to their flexibility and ease of post-modification to induce various functionalities. Functionalized FSM have added advantages because their topography and controlled surface properties facilitates tissue engineering applications. Moreover, the hybrid films exhibit enhanced mechanical strength, improved thermal and optical properties 22, 23. The BF method is based on the evaporative cooling of the polymer solution and the following development of hexagonal arrays of water droplets on the polymer solution surface. Drying of the film leaves the imprint of the water droplets. Several parameters like polymer solution concentration, humidity, evaporation rate, temperature etc. influence the BF pattern and the pore morphology7, 8. All these parameters are more or less influenced by the processing conditions and were well investigated and effectively correlated to the morphology. However, very few works were concerned with the thickness of the drop-cast film or the FSM. The thickness of the drop-cast film depends on the concentration and spreading ability of the polymer solution on a substrate material and the formation of the films having uniform thickness without any sophisticated technique is rather difficult. On the other hand, the FSM films were generally fabricated by spreading the polymer solution on water surface and control of the film thickness was possible by adjusting the temperature of the substrate surface22,

23

. However, the process demands BF compatible polymers like

amphiphilic co-polymers and surface active compounds for stabilizing the water surface.


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Recently, we had reported a surfactant-free method to fabricate FSM hybrid film using a BF incompatible polymer and employing the ability of amphiphilic-modified inorganic particles to stabilize the breath-figures24. During the present study, we further investigated the possibility of BF microstructure tailoring for finding different end-use applications. The BF morphology could be successfully correlated to the film thickness/ particle concentration. Moreover, possibility exists for controlling the amino group surface density by tailoring the BF morphology. Post-modification of the film surfaces through the labile amino group by simple chemical reactions is another advantage. The present approach opens a modest route for the microstructure tailoring of the FSM with amino-functionalized cavities which also facilitate the site-specific post-modification with biomolecules or heavy metals25, 26. EXPERIMENTAL SECTION Materials: The materials used in the present study were amino-functionalized amphiphilic alumina particles (prepared from alumina powder of aps ~100 nm, Sumitomo corporation, Japan), Polystyrene (GPPS, Mw~3.6 KDa, PI=1.6), Chloroform, Fluorescamine (Synthetic reagents, Merck Specialties Pvt. Ltd., India). Methods: The amino-functionalised amphiphilic-alumina particles were prepared by silane modification using a mixture of aminopropyltriethoxysilane and vinyl triethoxysilane in alcohol-water mixture followed by in situ polymerization of styrene as reported in our previous publication24. The modified particles having hydrophobic/hydrophilic ratio of ‘4’ was used for the hybrid film preparation. The suspension for fabrication of the hybrid film was prepared by dispersing the modified particles in polystyrene (PS)/chloroform solution of concentration of 15 mg/mL. FSM was prepared by casting the suspension in a glass petri-dish of 3.5 cm diameter, swirling the dish for the uniform spreading of the suspension and then drying off the solvent under ambient temperature of ~30 °C and relative humidity of ~80 %.



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The film, which got detached from the glass surface on adding methanol, was removed from the dish and dried at 60 °C. Keeping the particle concentration constant at 3 wt. % of PS, volume of the casting was varied from 0.2 to 1.0 ml. The effect of particle concentration was studied by fixing the cast volume at 0.5 ml and varying the particle loading in the range of 3 to 6 wt.%. The hybrid films with plain surfaces were also prepared by drying the suspension under non-humid conditions. The thickness of the suspension film in the petri- dish, termed as the wet film thickness (WFT), was calculated from the suspension volume and the surface area of the petri-dish. The dry film thickness (DFT), which is the thickness of the film after drying, was measured from the magnified image of the cross-section of the sample held between two glass slides under a stereo microscope (Zeiss Discovery.V20) and using the ProgRes® CapturePro 2.8. JENOPTIK Optical System software. The porosity of the film was measured by solvent penetration method using 1-butanol as the solvent and the theoretical true density of the hybrid27. The surfaces of the films were characterized by SEM (JEOL JSM-5600LV) using samples which were gold-coated by employing JEOL JFC-1200 fine coater and AFM (Bruker multimode 8 AFM). The pore size, strut thickness and pore density were measured from the images using ‘imagej’ software. RESULTS AND DISCUSSION Earlier we had shown that the BF microstructure in PS/alumina hybrid drop-cast film was dependent on PS solution concentration, alumina content and its state of dispersion in the polymer solution24. The alumina was modified to have a hydrophobic/hydrophilic ratio of ‘4’ for its effective dispersion in PS/chloroform solution. The suspension from PS solution concentration of 15 mg/ml and alumina concentration of 3 wt. % with respect to PS was critical to form micropatterned film with the most closely packed arrays of the concavities of narrow size distribution. Hence, an identical composition was used for preparing FSM of


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different thickness. The minimum amount of the suspension for forming circular film in the petri-dish was 0.2 ml and the corresponding WFT was 0.207 mm. Figure 1 shows the digital photograph and the optical microscopic images (A-D) of a typical FSM of polystyrene/alumina hybrid. The BF microstructure was more or less uniform except towards the edges of the film where the pores of wide size range coexisted.

Figure 1. The photograph of a typical FSM and the optical microscopic images of the film from centre to edge.

Figure 2 (A-F) shows the SEM images of the films of different thickness. Evidently, the pore size increased with the film thickness. The wet film thickness and the corresponding dry film thickness, pore size, strut thickness, feature/pore density and porosity are summarized in table 1. 5 ACS Paragon Plus Environment


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Figure 2. SEM images of the FSM hybrid films having dry film thickness of (A) 0.91µm (B) 1.52µm (C) 3.01µm (D) 3.94µm (E) 4.83µm and (F) 5.65µm. Pore distribution graphs for A to C is shown below the respective SEM images. The films contained 3 wt. % alumina with respect to the PS content.

Table 1. The wet film thickness (WFT) and the corresponding dry film thickness (DFT), average pore size, average strut thickness, pore density and porosity of the patterned films WFT DFT (mm) (µm) 0.207 0.91 0.311 1.52 0.519 3.01 0.727 3.94 0.935 4.83 1.039 5.65

Pore size (µm) 0.87±0.90 1.10±0.08 2.10±0.04 4.50±0.17 3.80-5.00 4.20-7.00

Strut thickness ( µm) 0.96±0.27 1.19±0.22 1.60±0.16 4.40±0.48 3.00-7.00 7.00-9.00

Pore density (cavities/cm2) 3.1×108 2.2×108 1.5×108 9.3×106 9.0 ×106 4.0 ×106


Porosity (%) 94.3 89.3 84.1 80.7 67.8 41.2

I

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t can be seen from the table that the average cavity size gradually increased from 0.87 µm to 2.1 µm with DFT from 0.91µm to 3µm. Further increase of the film thickness resulted in the formation of relatively large size cavities (>4µm) and the pattern became progressively irregular. The strut thickness also exhibited the same trend. The increase of the cavity size and the strut thickness caused a decrease of the pore density. The 3µm thick film exhibited near to uniform BF microstructure and the 0.91µm thick film exhibited the pore density maximum of 3.1x108 cavities/cm2. The pore size distribution for the films having DFT of 0.91-3µm are shown below the respective SEM images. It is worth to note that the pattern regularity enhanced with the thickness. More precisely, the uniformity of the pattern can be expressed in terms of the conformational entropy (S), estimated using the equation S= ΣPnlnPn, where number ‘n’ is the co-ordination number and Pn is the fraction of the polygon having a particular coordination number of the Voronoi polygon28. The ‘S’ assumes a value of 1.71 or highly random distribution of BF patterns, and zero for a perfectly ordered pattern. The conformational entropies calculated for the films having DFT of 0.91, 1.52 and 3.01 µm were 0.68, 0.65 and 0.50 respectively, which confirmed that the cavities of narrow size distribution assumed near to hexagonally close packed structure in 3µm thick film. Evaporative cooling of the wet film surfaces causes the condensation and nucleation of water droplets on the film surfaces from the atmospheric moisture. The droplets grow with solvent evaporation time (t) and the droplet radius (R) is proportional to t1/3 in the beginning of the evaporation and R~t at the end28, 29. The difference at the end is due to coalescence of water droplets which occurs at the last stages of BF formation. The BF cavity size is related to the size of the stabilized water droplet. Delay in stabilization causes the growth/coalescence of the droplet and increase of the cavity size. In particle-assisted BF formation, the droplets are stabilized by the absorption of the particles at the droplet/solution



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interface. The particles from the suspension are carried to the surface layers under capillary flow induced by the fast evaporation of the solvent. These particles further migrate to the droplet/solution interface under hydrodynamic drag force11,

15

. The continuous layer of

particles absorbed at the droplet/solution interfaces act as a mechanical barrier, preventing the droplets from coalescence. In other words, the amount of particles available at the wet film surface layers decides the droplet size on the film. In this investigation, the particle concentration in the suspension was fixed at 3 wt.% of PS. However, the settling tendency of the particles under gravity reduces the particle concentration in the surface layers and the effect become more and more prominent as the wet film thickness/height of the suspension increases. In order to confirm this, we prepared hybrid films with plain surfaces by drying of the suspensions in a non-humid atmosphere. These films were post-treated with ninhydrin, which upon reacting with the amino groups on the particles surface produces dark coloured spots30. Figure 3 shows the optical microscopic images of the films from suspensions of different WFT. It is clear from the images that the particle concentration at the film surfaces decreases as the WFT increased. The particle concentration sharply decreased on increasing the WFT above 0.5 mm. As said earlier, during BF formation, the reduced particle concentration in the surface layer results in the growth of the water droplets. The droplet continues to grow until the amount of the particles satisfied the requirement of forming a continuous layer at the water droplet/solution interface and stabilize the droplet. The cavity size is related to the size of the stabilized droplet. This may explain the observed increase of the cavity size with the WFT. The sharp increase of the cavity size from ~2.1 to ~4.5 µm in the films of DFT above 3 µm indicates that the unstabilized droplets grow by its coalescence at WFT above 0.5 mm.


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Figure 3. Optical images of ninhydrin-treated plain films with varying thickness (A) 0.91µm, (B) 1.5 µm (C) 3.0 µm (D) 3.9 µm (E) 4.8 µm

When compared to that was observed for the drop-cast film from 3µL suspension, the FSM from 0.5 ml suspension exhibited a more uniform microstructure (figure S1). The castings for the FSM in a confined area possessed a reduced surface area per unit volume of the suspension than that of the drop-cast suspension. The longer solvent evaporation time due to the reduced surface area favoured uniform growth of the water droplets and its selforganization in a more ordered manner while the film was sufficiently wet to allow the droplets to produce its impressions on it. Interestingly, the 0.91 µm thick dry film exhibited a through-pore structure which allowed water to penetrate through it. Through-pore was confirmed by AFM analysis of the film surfaces in contact mode. Figure 4 shows the AFM images and the height profile of the film surfaces. The pore size measured at the surfaces exposed to air was 870±90 nm, whereas the size of the pore openings at the surfaces in contact with the substrate appeared significantly lower than the former. Also, the openings were found at irregular intervals. Ma



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et al.23 prepared through-pore structured BF patterned film by spreading a polymer solution onto a surfactant monolayer modified water surface. They observed uniform distribution of through-pores with identical pore openings at both sides of the film. During BF formation, the liquid substrate facilitated the water droplets pass through the polymer solution to produce identical pore openings at both sides of the film. In contrast, the solid substrate in the present case, resisted the water droplets from passing through the film. It could be possible that the water droplets having the size identical or higher than that of the film thickness reached the substrate surface and ruptured the film to form through-pores. The spherical shape of the droplets caused the difference in size of the pore openings at opposite sides of the film.

Figure 4. AFM 2D image and height profile of 0.91µm thick film showing pore openings at the film surfaces (A) exposed towards air(corresponding 3D image is shown in inset) and (B) in contact with the substrate. The relatively low-size spots seen in figure B are assumed as due to surface roughness which might have caused by particle setting. Billon and his coworkers31 studied the effect of the wet film thickness of a polymer film on BF microstructure. For 250 µm thick film, they observed the formation of a mono-layer. Increase of the film thickness to 500 µm led to partial second structured layer. A complete double layer was observed with 1000 µm thick film. Such a trend was not observed in the present case where the WFT was varied in the same range. The PS used in this investigation 10 ACS Paragon Plus Environment

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was BF incompatible (Mw 3.6 KDa) due to its high molecular weight32 and the formation of BF pattern was due to the particles. In particle-assisted BF formation, the particles in excess of forming a monolayer at the droplet/solution interface are absorbed at the interface to form additional layers17.

Billon et al. used a BF compatible polymer in carbon disulphide to

produce the bi-layered BF structure. However, under the present experimental conditions, the particle-assisted stabilization mechanism does not favour the formation of a second structured layer of the water droplets.

The primary aim of using the amino-functionalised alumina was to introduce aminofunctionality to the BF cavities. The fluorescent adduct that formed between the amino group and non-fluorescent fluorescamine allowed the assessment of amino-functionality of the film surfaces33. The fluorescence microscopic images of fluorescamine (4 mM solution in 3:1 v/v methanol/water mixture) treated FSM (figure S2) indicated the elevated presence of amino groups inside the BF cavity. The particle-assisted BF stabilization mechanism favoured functionalization of the film with the amino groups at the cavity walls only. Figure 5 shows the fluorescence spectra (Spex-Fluorolog FL22 spectrofluorimeter) at 450-495 nm at which the amine-flourescamine adduct emit blue radiation. It can be seen from the spectra that the intensity of the fluorescence emission decreased from 4.05×106 µA to 1.8×106 µA with the increase of DFT from 0.91 µm to 3.9 µm, indicating that an analogous decrease in the surface density of the amino group occurs with increase of the film thickness. This was expected since increase of the DFT which caused a reduction in the total pore surface area.



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Figure 5. Variation of fluorescence intensity of the fluorescamine treated FSM with the film thickness. The inset shows the spectrofluorometric curves. The bathochromic shift of λmax is due to the requirement of more energy to excite the electron from the adduct on increasing the film thickness34

In drop-cast PS/alumina film, increase of the particle concentration above 3 wt.% resulted in irregular BF pattern24. In order to verify whether the same was true for the relatively thicker FSM film, the particle concentration was varied in the range of 3-6 wt. % in 3 µm thick dry film (WFT 0.5 mm). Figure 6 shows the SEM images of the films and the corresponding morphological features such as pore size, strut thickness, pore density are presented in Table 2. It can be seen that the increase of the particle loading from 3 wt.% resulted in the increase of the cavity size and strut thickness which decreased the pore density. The nano alumina particles tended to form agglomerates when the particle loading exceeded 3 wt. % in the polymer solution24. The faster settling of the agglomerates reduces the effective concentration of the particles in the surface layers of the suspension. The observed increase of the cavity size on exceeding the particle concentration above 3 wt.% indicated that the effective particle concentration in the surface layers of the castings reduced below 3 wt.%. However, in contrast to that was observed for the drop-cast films, the FSM with particle loading of 3 to 5 wt. % exhibited moderately ordered pattern (conformational entropy value within the range of 0.50-0.74. Possibly, the large casting area which probably increase the particle density at the solution surface and the longer solvent evaporation time, 12 ACS Paragon Plus Environment

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favoured uniform growth of the water droplets and its self-organization to achieve moderately ordered pattern.

Figure 6. SEM images of 3 µm thick hybrid film containing different particle concentration of (A) 3 wt. %, (B) 4 wt. % (C) 5 wt. % and (D) 6 wt. %. Table 2. The particle concentration and corresponding changes in cavity size, strut thickness and pore density Alumina (wt.% ) 3 4 5 6

cavity size (µm) 2.1±0.04 2.4±0.08 3.1±0.09 4.0±0.22

Strut thickness (µm) 1.6±0.17 1.6±0.16 3.0 ±0.52 3.7 ±0.57

Pore density (cavities/cm2) 1.50×108 0.74×108 0.35×108 1.5 × 107

The spectrofluorometric data (figure 7) showed that the fluorescence intensity increased with the particle concentration, although pore size minimum and pore density maximum (pore-wall area maximum) was observed for the film containing 3 wt. % alumina.



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Figure 7. Variation of fluorescence intensity of the fluorescamine treated FSM with particle concentration. The inset shows the spectrofluorometric curves

This could be due to the fact that the particle in excess of forming a monolayer at the droplet/solution interface formed additional layers in the dried film17. Consequently, the fluorescence activity tended to spread over the inter-cavity surfaces (strut) at particle concentration above 3 wt. %. However, the films exhibited improved hardness and thermal properties with the increase of the particle concentration (figure S3 and S4). Conclusions In conclusion, free-standing micropatterened polystyrene-alumina hybrid films with amino functionalized BF cavities were fabricated by casting the suspensions of the aminofunctionalised alumina particles in polystyrene/chloroform solution in a flat bottom glass dish. Tailoring of the BF microstructural aspects such as pore size and pore density without significant variation in the pattern uniformity was possible by varying the film thickness and particle concentration. The particle-assisted BF stabilization mechanism favored aminofunctionalisation of the film at the cavity walls. While the through-pore structured 0.91µm thick film of high porosity and having amino group surface concentration maximum could be useful for filtration and heavy metal separation, the thicker films with the functionalised


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concavities can find tissue engineering applications such as bio-film preparation, immobilization of biomolecules and hydrophilic bacteria etc. Supporting Information: Comparison of drop cast films and FSM, Fluorescence Microscopic image of fluorescamine treated FSM. Plot of hardness Vs particle loading and the corresponding optical images of the nano indentation residue, Thermogravimetric curves of the FSM with varying particle loading. Acknowledgment The authors thank CSIR, New Delhi for research fellowship and the Director and members of Materials Science and Technology Division, CSIR-NIIST. The authors are also thankful to Mr. M. R. Chandran. Mrs. Lucy Paul, Mrs. Sowmya, NIIST respectively for SEM analysis. References 1. Kurono N.; Shimada R.; Ishihara T.; Shimomura M. Fabrication and Optical Property of Self-Organized Honeycomb-Patterned Films. Mol. Cryst. Liq.Cryst. Sci. Technol.

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Jingcheng H. Fabrication of Freestanding

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25. Min, E.; Wong, K. H.; Stenzel, M. H. Microwells with Patterned Proteins by a SelfAssembly Process Using Honeycomb-Structured Porous Film, Adv. Mater. 2008, 20, 3550-3556. 26. Seeberger, P. H.; Werz, D. B. Synthesis and Medical Applications of Oligosaccharides, Nature 2007, 446, 1046-1051. 27. Anders L. Materials Characterization of Thin Film Electrodes for PEFC – Survey of Methods and an Example, J. of New Mater. Electrochem. Sys. 2004, 7, 21-28. 28. Steyer A.; Guenoun P.; Beysens D.; Knobler C. M. Two-Dimensional Ordering During Droplet Growth on a Liquid Surface, Phys. Rev. B 1990, 42, 1086. 29. Knobler C. M.; Beysens D. Growth of Breath Figures on Fluid Surfaces, Europhys.

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Properties

of

Thin

Films

of

Benzo-Annelated


Metal-Free

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Porphyrazines. J. Lumin. 2008, 128, 661–672.

TOC Graphic



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Figure 1. The photograph of a typical FSM and the optical microscopic images of the film from centre to edge. 327x505mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 2. SEM images of the FSM hybrid films having dry film thickness of (A) 0.91µm (B) 1.52µm (C) 3.01µm (D) 3.94µm (E) 4.83µm and (F) 5.65µm. Pore distribution graphs for A to C is shown below the respective SEM images. The films contained 3 wt. % alumina with respect to the PS content. 140x257mm (300 x 300 DPI)



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Figure 3. Optical images of ninhydrin-treated plain films with varying thickness (A) 0.91µm, (B) 1.5 µm (C) 3.0 µm (D) 3.9 µm (E) 4.8 µm 143x162mm (300 x 300 DPI)


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Figure 4. AFM 2D image and height profile of 0.91µm thick film showing pore openings at the film surfaces (A) exposed towards air(corresponding 3D image is shown in inset) and (B) in contact with the substrate. The relatively low-size spots seen in figure B are assumed as due to surface roughness which might have caused by particle setting. 103x86mm (300 x 300 DPI)



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Figure 5. Variation of fluorescence intensity of the fluorescamine treated FSM with the film thickness. The spectrofluorometric curves of different films are shown inset. 287x204mm (300 x 300 DPI)


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Figure 6. SEM images of 3 µm thick hybrid film containing different particle concentration of (A) 3 wt. %, (B) 4 wt. % (C) 5 wt. % and (D) 6 wt. %. 170x137mm (300 x 300 DPI)



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Figure 7. Variation of fluorescence intensity of the fluorescamine treated FSM with particle concentration. The spectrofluorometric curves of different films are shown inset.

81x62mm (300 x 300 DPI)


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TOC 225x136mm (96 x 96 DPI)


Microstructural Aspects of Particle-Assisted Breath Figures in

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