MESOSCOPIC AND SUBMICROMETRIC ORDERING IN EVAPORATED POLYMER FILMS: INFLUENCE OF THE SURFACE DEFECTS

Ed. Bormashenko1*, R. Pogreb1, O. Stanevsky1, Ye. Bormashenko1, T. Stein1, N. Litvak1, A. Shulzinger1, O. Gendelman2

1The College of Judea and Samaria, The Laboratory of Polymer Materials, Ariel, 44837, Israel, E-Mail:

2Faculty of Mechanical Engineering, Technion, Technion City 32000, Haifa, Israel

Abstract: The influence of surface defects on the patterning of evaporated polymer films, deposited on solid substrates by a dip-coating process was investigated. It was shown that surface defects favor formation of mesoscopically ordered patterns. The characteristic dimension of the mesoscopic cell is about 50 μm. The characteristic scale of the mesoscopically patterned area is about an order of magnitude more than the size of the defect (about 100 μm) itself. The structure of mesoscopically scaled cells was studied by means of optical and SEM microscopy. A semi-quantitative model explaining the phenomenon of mesoscopic cell formation is proposed.

INTRODUCTION

Evaporation-induced submicrometric and mesoscopic ordering in organic and non-organic liquids was under intensive investigation recently [1–11]. Nanoscaled self-assembling, allowing formation of strictly ordered honeycomb structures comprised of air holes dispersed in the solid matrix was explained satisfactorily in the framework of approach, developed by Kralchevsky and Nagayama [1, 12]. However evaporation-induced mesoscopic patterns formation remains mysterious, in spite of much theoretical effort devoted to understanding of the mechanisms responsible for mesoscopic structuring. Two main approaches could be recognized: the first treats mesoscopic structuring as a process of cracking under solvent evaporation [6, 13–15]; and the second relates mesoscopic self-assembling to various kinds of surface instabilities, such as Marangoni surface tension instability [8–9, 16–22].It has to be emphasized that describing physical processes occurring in evaporating films turns out to be very challenging, since couplings between the microstructure and hydrodynamic flow are highly non-linear [18, 23–25].

We have shown already that evaporation of the same polymer solutions could bring into existence patterns ordered on mesoscopic and submicrometric scales, depending on the drying conditions [11, 26]. This work is devoted to one more factor, which (as it will be demonstrated) has a decisive impact on the patterning: namely, the impact of the surface defects.

EXPERIMENTAL

Polystyrene (PS) 143 E supplied by BASF, was dissolved in a mixture of dichloromethane CH2Cl2 (90% wt.) and chloroform CHCl3 (10% wt.). Solvents were supplied by Karlo Erba Reagenti, the concentration of PS in the solutions was 5% wt. Polypropylene substrates were coated under conditions of a fast dip-coating process. Substrates were cleaned thoroughly with acetone and ethyl alcohol and rinsed with a large amount of distilled water. Substrates were perforated in a way described in Figure 1. Diameter of the holes was 100 μm, separations between holes were: L = 10 mm, H = 20mm (see Figure 1). Large distances between holes avoided their reciprocal interference during the pattern formation.

Fig.1. Perforated polypropylene substrate used for dip-coating process.

Fig. 2. Scheme of dip-coating process.

When the substrate is dip-coated, the liquid film runs out from the polymer solution, adheres to the substrate surface and solidifies during the evaporation of the solvent (see Figure 2). Dip-coating was carried out with a high pulling speed V = 47 cm/min. Pulling direction was vertical. Films were immediately dried with a hot air current (v = 5 m/s), under isothermal conditions. Gas current velocity was measured with anemometer PROVA AVM 03. Gas stream was stabilized with 0.5 m length tube. The drying temperature was td = 65ºC. We have demonstrated already that the above-noted mixture of chlorinated solvents components, when dried under the mentioned conditions, promotes formation of submicrometric honeycomb structures [27]. The structure of the dry film was studied by means of optical and scanning electron microscopy. Images were processed with SIAMS 600 software.

RESULTSANDDISCUSSION

We established that surface defects exert a decisive impact on the patterns formation. “Brain-like” mesoscopic patterns were formed in the vicinity of surface defects (see Figure 3). In specific cases polymer film covered the hole (Figure 3b-c), and in some cases the hole remained unfilled (Figure 3a), however in all situations the hole promoted formation of mesoscopic patterns. The characteristic dimension of the mesoscopic cell was ~50 μm. The size of the mesoscopically patterned area could be estimated as 800–900 μm. Areas which were away from the surface defects were coated with the submicrometric structure, as reported previously by the authors [11] and depicted in Figure 4a. We relate formation of these pores to the solvent evaporation under drying. It has to be emphasized that drying procedure was performed under the temperature (65°C), which is higher than the solvents boiling points (60°C for chloroform and 39.5°C for dichloromethane).

Fig. 3. a-b: typical brain-like mesoscopic structures observed in the vicinity of the surface defects, c: magnified optical microscopy image of the mesoscopic structure (area II in Fig. 3 b).

Interesting results were obtained when images, depicting submicrometric structures were processed with SIAMS 600 software, intended for processing of speckled images. Elements of broken hexagonal symmetry could be recognized (see Figure 4b, which displays segments connecting geometric centers of pores). The average distance between pores was established with SIAMS 600 as 960 nm; the average diameter of the pore was 360 nm.

Fig. 4. a: SEM image of the submicrometric scaled porous area (area I in Figure 3a), solvent mixture of dichloromethane (90%) and chloroform (10%), drying temperature – 65°C, b: the same image processed with SIAMS 600 software.

A closer inspection of the mesoscopic cells, displayed in Figure 3, was carried out with a scanning electron microscope. SEM study revealed that mesoscopic cells are separated with porous boundaries presented in Figures 3c, 5–6. Asymmetry in the shape of the patterns near the pore may be attributed to the overall flow of the polymer solution while drying takes place. Diameters of pores, forming the boundary, were estimated as 1-2 μm. The internal area of the mesoscopic cell contains very small pores of 100–300 nm in diameter presented in Figure 7. However, the surface density of these pores is less, comparatively to those of pores distant from the mesoscopically patterned area (such as depicted in Figure 4); the average distance between pores was estimated as 1800 nm.

Now let us discuss mechanisms responsible for mesoscopic patterning. It should be noted that classical Bénard or Bénard-Marangoni instabilities driven by the buoyancy or the surface tension hardly may be responsible for the pattern formation in our case [16–22]. The reason is that polymer film is very thin (~ 1-3 μm) and the characteristic time of the thermal conduction was estimated already as 10-2 s [26].

Fig. 5. SEM image of mesoscopically patterned structure.

Fig. 6. SEM image of typical boundaries separating mesoscopic cells formed in the mesoscopic “brain-like” structure.

Fig. 7. SEM image of the internal area of the mesoscopic cell.

Thus the whole process may be considered as isothermal, and Bénard or Bénard-Marangoni instabilities could not be invoked for the explanation of the phenomenon.

We already proposed another kind of mass transport instability, which is perhaps responsible for the mesoscopic cells formation [26]. De Gennes supposed that due to fast external drying, all solvent which leaves the solution is immediately removed from the system. Then, at an early stage of the evaporation process, a polymer-rich layer is formed [14]. According to earlier results, the characteristic thickness of such a layer may be on the order of 50-70 nm, about 20 times less than the characteristic thickness of the film by the end of drying. This boundary layer has an essentially lower diffusion coefficient than the bulk solution; therefore, the evaporation of the solvent is primarily governed by diffusion through this layer.

Now let us discuss the physical mechanism of diffusion instability: it is based on the observation that the local trough from outside the boundary layer has a tendency to grow as the reduced thickness facilitates the diffusion of the solvent in this place. Consequently, a local crest outside the boundary layer tends to grow as it suppresses the diffusion. Small-wavelength perturbations of this sort are suppressed by surface tension, and therefore there exists a critical scale for development of the layer instabilities. The critical wavenumber for stability was thus estimated as [26]:

,

where D is the characteristic diffusion coefficient, Δc – is the dimensionless concentration jump at the boundary layer, σ and ρ – surfacetension anddensity of the solution respectively, s – the boundary layer thickness. For the sake of numeric evaluation we take s~70 nm (this estimation for the thickness of the boundary layer has been obtained by De Gennes in paper [14]) and the physical parameters of the solvents established experimentally [28–30]. Our estimation results on kcrit~6·105m-1. This estimation corresponds to the typical scale for the loss of stability equal to 2π/kcrit~10 μm. This figure at least qualitatively coincides with experimental results presented in Figures [3, 5]. Actually, mesoscopic cells characteristic dimensions demonstrated a considerable dispersion: 10–100 μm, and it has to be stressed that mentioned above theoretical estimation is very rough and complete understanding of the mesoscopic patterning is not achieved still. However, it seems to be reasonable to conclude that surface defects promote the progress of the diffusion instability.

The results presented demonstrate that mesoscopic structural surface defects strongly affect the pattern formation process. Somewhat surprisingly, the characteristic scale of this influence is about an order of magnitude more than the size of the defect itself. Such long-range influence may be explained as related to the relatively long time necessary for full drying and complete development of the structure. In other terms, the surface defect (pore) influences formation of a relatively large number of cells in the course of the solution flow.

CONCLUSIONS

Processes of self-organization in evaporated polymer films were investigated. Polystyrene films were deposited on specially prepared polypropylene substrates that included surface defects. It was demonstrated that surface defects exert decisive influence on the patterns formation. Mesoscopically scaled patterns were observed in the vicinity of the surface defects. The characteristic scale of the mesoscopically patterned area is about an order of magnitude more than the size of the defect. SEM microscopy revealed that mesoscopic cells are separated with porous boundaries. Diameters of pores, forming the boundary, were estimated as 1-2 μm. The internal area of the mesoscopic cell is comprised of very small pores of 100–300 nm in diameter. The mechanism of mesoscopic patterning is discussed.

ACKNOWLEDGEMENTS

The work has been supported by the Israel Ministry of Absorption. The authors are grateful to Professor M. Zinigrad, for his continuous support of our research activity. Oleg Gendelman is grateful to the Taub and Shalom Foundations for financial support. We thank Mrs. Albina Musin for her help in preparing the paper.

REFERENCES

[1] Srinivasarao M., Collings D., Philips A., Patel S., Science 292 (2001) 79.

[2] Yabu H., Tanaka M., Ijiro K., Shimomura M., Langmuir 19 (15) (2003) 6297.

[3] Karthaus O., Maruyama N., Cieren X., Shimomura M., Hasegawa H., Hashimoto T., Langmuir 25 (16) (2000) 6071.

[4] Jenekhe S. A., Chen X. L., Science 283 (1999) 372.

[5] Grosso D., Cagnol F., de A. A. Soler Illia G. J., Crepaldi E. L., Amenitsch H., Brunet-Bruneau A., Bourgeois A., Sanchez C., Adv. Funct. Mater. 14 (4) (2004) 309.

[6] Ma X., Xia Y., Chen E-Q., Mi Y., Wang X., Shi A-C., Langmuir 20 (22) (2004) 9520.

[7] Mitov Z., Kumacheva E., Phys. Rev. Letters 81 (16) (1998) 3427.

[8] Weh L., Ventur A., Macromol. Mater. Eng. 289 (2004) 227.

[9] Weh L., J. Colloid and Interface Science 235 (2001) 210.

[10] Karthaus O., Grasio L., Maruyama N., Shimomura M., Thin Solid Films 327-329 (1998) 829.

[11] Bormashenko E., Pogreb R., Stanevsky O., Biton Y., Bormashenko Y., J. Materials Science 39 (2004) 639.

[12] Kralchevsky P. A., Nagayama K., Adv. in Colloid and Interface Science, 85, 145.

[13] De Gennes P. G., Eur. Phys. J. E 6 (2001) 421.

[14] De Gennes P. G., Eur. Phys. J. E 7 (2002) 31.

[15] Lee W. P., Routh A. F., Langmuir 20 (23) (2004) 9885.

[16] Reichenbach J., Linde H., J. Colloid Interface Sci. 84 (1981) 433.

[17] Golovin A. A., Nepomnyashchy A. A., Pismen L. M., Physica D 81 (1995) 117.

[18] Oron A., Nepomnyashchy A. A., Phys. Rev. E 69 (2004) 016313.

[19] Merkt D., Physica D 185 (2003) 196.

[20] Margerit J., Collinet P., Lebon G., Iorio C. S., Legros J. S., Phys. Rev. E 68 (2003) 041601.

[21] Linde H., Velarde M. G., Wierschem A., Waldhelm W., Loeschke K., Rednikov A. Y., J. Colloid Interface Sci. 188 (1997) 16.

[22] Moussy C., Lebon G., Margerit J., Eur. Phys. J. B 40 (2004) 327.

[23] Panizza P., Courbin L., Cristobal G., Rouch J., Narayanan T., Physica A 38 (2003) 322.

[24] Brunisma R., Rabin Y., Phys. Rev. E 49 (1) (1994) 554.

[25] Frank A., European J. of Mechanics Fluids B 22 (2003) 445.

[26] Bormashenko E., Pogreb R., Stanevsky O., Biton Y., Bormashenko Y., Stein T., Gaisin V. Z., Cohen R., Macromolecular Materials and Engineering 290 (2005) 114.

[27] Bormashenko Ed., Pogreb R., Stanevsky O., Bormashenko Ye., Stein T., Cohen R., Nunberg M., Gaisin V. Z., Gorelik M., Gendelman O., Mesoscopic and submicroscopic patterning in thin polymer films: impact of the solvent, submitted to Materials Letters.

[28] Prielmeier F. X., Lüdemann H.-D., Molecular Physics 58 (3) (1986) 593.

[29] Harris K. R., Lam H. N., Raedt E., Easteal A. J., Price W. E., Woolf L. A., Molecular Physics 71 (6) (1990) 1205.

[30] Waggoner R. A., Blum F. D., Macelroy J. M. D., Macromolecules 26 (1993) 6841.

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