Formation of ordered mesoscopic patterns in polymer cast films by dewetting

O. Karthaus, L. Gra˚sjo, N. Maruyama, M. Shimomura*

Research Institute for Electronic Science, Hokkaido University, Kita 12, Nishi 6, Kita-ku, N12W6 Sapporo, Japan

Abstract

Ordered arrays of polymer aggregates can be prepared by a simple process: rinsing of smooth hydrophilic surfaces with a dilute solutionof a hydrophobic polymer. The patterns consist of submicron-size polymer aggregates which can form superlattices over several tens ofmicrometers, thus self-assembling several hundred aggregates in an orderly fashion. Atomic force microscopy shows that the structuresconsist of isolated ‘polymer islands’, separated by the bare substrate. The ordering of the aggregates can be explained by the formation ofregular, dissipative structures caused by convection during the evaporation of the solvent, followed by dewetting of the hydrophobicpolymer solution from the smooth hydrophilic surface. 1998 Elsevier Science S.A. All rights reserved

Keywords:Dissipative structure; Dewetting; Hydrophobic polymer; Mesoscopic pattern; Nanosize dot

1. Introduction

Dewetting is a spatiotemporal process in which a thinliquid film contracts and forms droplets on a surface.Mostly, dewetting is an unwanted process. For coatings,lubrication or adhesion, thin homogeneous films are desir-able, thus a lot of attention is focused on the wetting anddewetting properties of polymer thin films. Dewetting hasbeen intensively studied [1–4] and described theoretically[5,6] in the case of polymer melts on flat substrates. In nocase has the formation of a regular pattern in dewetted filmsbeen observed. Recently we found the formation of anordered polymer patterns upon rinsing a mica surface witha dilute solution of polystyrene, but we were unable to pro-vide a conclusive evidence for the formation mechanism [7].

In this paper we show that the pattern formation is notlimited to polystyrene on mica, but can be extended to otherhydrophobic polymers and to other hydrophilic substrates.The size of the aggregates can be controlled by using twodifferent preparation techniques, rinsing and casting,respectively.

2. Experimental

Polystyrene (Mw 9800 g/mol; Mw/Mn , 1.05, andMw

45 000 g/mol; Mw/Mn undetermined), poly(vinylstearate)(Mw 90 000 g/mol), and poly(hexylthiophene) (Mw 87 000g/mol) were purchased from Aldrich; benzene was UvasolGrade from Merck, Germany. All glassware was thoroughlycleaned with detergent solution (dcn 90, Decon Labora-tories, UK) followed by immersion in alkaline water/etha-nol (1:1) with sonication. Mica was cleaved immediatelybefore use. Silicon wafers (Si(111) or (100)) (Nilaco,Japan) and glass substrates (Matsunami, Japan) were usedas purchased. Samples were prepared by two methods. (1)Rinsing a freshly cleaved mica surface with a dilute benzenesolution of polystyrene. The mica was held at an angle of60–70° during the rinsing process. (2) Evaporation of asolution droplet on a horizontal substrate, e.g. silicon orslide glass. Optical micrographs were taken in the reflectionmode (Olympus BH-2) and recorded with a CCD camera(Sony DXC 755). Atomic force microscopy (AFM) imageswere taken in the AC mode, in which a vibrating tip isscanned over the surface (Olympus NV 2500 aqac). NIHImage, Version 1.61, was used as the software for fast Four-ier transformation (FFT).

Thin Solid Films 327–329 (1998) 829–832

0040-6090/98/$ - see front matter 1998 Elsevier Science S.A. All rights reservedPII S0040-6090(98)00771-8

* Corresponding author. Tel. +81 11 7063666; fax: +81 11 7064974;e-mail: okart@imdes.hokudai.ac.jp

3. Results and discussion

3.1. Optical microscopy

All three investigated hydrophobic polymers (polystyr-ene, poly(vinylstearate) and poly(hexylthiophene)) formordered patterns on hydrophilic surfaces upon the evapora-tion of their benzene solutions. Here, we will discuss theresults obtained from polystyrene and poly(hexylthio-phene), a functional polymer for optical and electricalapplications. Fig. 1a shows an optical micrograph of a poly-styrene pattern on Si obtained by the horizontal evaporationtechnique of a 100 mg l−1 benzene solution. The aggregatesize and the inter-aggregate spacing is fairly homogeneousover a large area. The inset in Fig. 1a is an FFT of Fig. 1a.The hexagonal order is clearly seen. No dependence on themolecular weight or the polydispersity of the used polystyr-ene was observed. The size and the inter-aggregate spacingis concentration-dependent, as can be seen in Fig. 1b. A 10mg l−1 benzene solution of poly(hexylthiophene) formssmaller aggregates that are closer together. Also in thissample, a hexagonal order of the aggregates can be seenfrom the FFT in the inset of the figure.

The dewetting of the polystyrene solution from the sili-con substrate was followed in situ by optical microscopy. Itwas found that the dewetting occurs at the 3-phase-line(air–solution–substrate) of the droplet. Fig. 2 shows avideo sequence of the dewetting. Upon the receding of thesolvent front, finger-like extensions of the 3-phase-line canbe seen that lead to the formation of solution droplets (‘bud-ding’) upon further receding. Both the spacing of the fingersat the 3-phase-line as well as the ‘budding’ frequency isregular, thus an ordered pattern of polymer aggregates isformed. Fingering instabilities at 3-phase-lines are well-known and described theoretically [8] as well as experimen-tally [9]. The origin of the instability is differences of sur-face tension, the Marangoni effect [10], along the 3-phase-line. These differences can be caused by a thermal gradient[8] perpendicular to the 3-phase-line. In our case of a poly-mer in solution, local inhomogeneities in concentration dur-ing the evaporation of the solvent are probably the reasonfor fingering. With increasing length of the finger, Rayleighinstabilities lead to a bottle-neck formation with consecu-tive ‘budding’ of solution droplets.

3.2. Atomic force microscopy

Generally speaking, faster evaporation and lower concen-tration of the polymer gives smaller aggregates anddecreased inter-aggregate spacings. Rinsing a tilted sub-strate with a more dilute solution thus leads to smalleraggregates. In order to show the versatility of the methodtowards other substrates, mica was used as a substrate. Fig.3a shows a 30× 30mm2 scan of a mica surface rinsed with a20 mg l−1 benzene solution of polystyrene. The rinsingdirection is from left to right. Most of the 30× 30 mm2

area consists of an ordered array of submicron-size dots.Fig. 3b shows the height profile of a row of dots. Theyhave a regular height of 16 nm, a diameter of 300 nm atthe bottom and a spacing of ca. 1mm. The inset in the lowerright corner of Fig. 3a shows the Fourier transform of thepattern. A long-range order can be seen, although it is nothexagonal, but the dots are rather aligned along lines paral-lel to the rinsing direction, forming a rhombic superstruc-ture. Friction force measurements [11] and force mapping[12,13] can be used to investigate the chemical compositionof surfaces on a nanometer scale. The inset in the lower leftcorner of Fig. 3a shows the friction force image of thesample. The dots show a higher friction value than the sur-rounding area. Compared to crystalline mica, amorphouspolystyrene is a soft material, and control experimentswith freshly cleaved mica and a thick, homogeneous poly-styrene film show that mica has a lower friction value than

Fig. 1. (a) Optical micrograph of polystyrene aggregates on Si in thereflection mode. The sample was prepared by evaporation of a 100 mgl −1 benzene solution on a horizontal substrate. The dark dots represent thepolymer aggregates. The inset is the Fourier transform of the figure. (b)Optical micrograph of poly(hexylthiophene) aggregates on Si in the reflec-tion mode. The sample was prepared by evaporation of a 10 mg l−1

benzene solution on a horizontal substrate. The small dark dots representthe polymer aggregates. The large gray shadows are artifacts from themicroscope optics. The inset is the Fourier transform of the figure.

830 O. Karthaus et al. / Thin Solid Films 327–329 (1998) 829–832

polystyrene. Furthermore, the value of the friction force ofbare mica is comparable to that of the lower friction area in

Fig. 3a, whereas the higher friction value of polystyrenematches with the value of the higher friction area. Thisfinding indicates that there is an island structure of physi-cally separated polystyrene aggregates on mica.

4. Conclusion

We have described the formation of ordered polymerpatterns by a simple solution-casting of hydrophobic poly-mers onto hydrophilic substrates. Ordered arrays of polymeraggregates can be achieved. The size and spacing of theaggregates can be controlled by the concentration of thesolution and the casting technique (horizontal or tilted sub-strate). The smallest achieved polymer aggregates have adiameter of 300 nm and a height of 16 nm, forming arrayswith inter-aggregate spacings of approx. 1mm.

The origin of the pattern formation is dissipative struc-tures that form at the 3-phase-line and are induced by theMarangoni effect [10]. Because dissipative structures in

Fig. 2. Video image of the 3-phase-line of an evaporating benzene solutionof polystyrene. The frames were taken every 100 ms. The solution is in theupper part of each frame, the 3-phase-line moves from the lower leftcorner to the upper right corner. The bright dots with the dark halo arethe polystyrene aggregates.

Fig. 3. (a) 30× 30 mm2 AFM image of the topography of a mica surfacerinsed with a 20 mg l−1 benzene solution of polystyrene. Thez-scale of theimage (indicated by the color gradient) is 30 nm. The inset in the lower leftcorner is an enlarged friction force image. The inset in the lower rightcorner is the Fourier transform of (a). (b) Height profile of a row ofaggregates.

831O. Karthaus et al. / Thin Solid Films 327–329 (1998) 829–832

solutions are less dependent on molecular parameters,ordered mesoscopic structures could possibly be preparedfrom a wide variety of functional polymers. This hopefullywill open a new field in science, where the fixation of dis-sipative structures leads to new functional materials.

References

[1] G. Reiter, Langmuir 9 (1993) 1344.[2] A. Faldini, R.J. Composto, K.I. Winey, Langmuir 11 (1995) 4855.[3] G. Reiter, P. Auroy, L. Auvray, Macromolecules 29 (1996) 2150.[4] S. Sheiko, E. Larmann, M. Mo¨ller, Langmuir 12 (1996) 4015.

[5] P.-G. de Gennes, Rev. Mod. Phys. 57 (1985) 827.[6] F. Brochard-Wyart, P.-G. de Gennes, H. Hervert, C. Redon,

Langmuir 10 (1994) 1566.[7] O. Karthaus, K. Ijiro, M. Shimomura, Chem. Lett. (1996) 821.[8] S.M. Trojan, E. Herbolzheimer, S.A. Safran, J.F. Joanny, Europhys.

Lett. 10 (1989) 25.[9] A.M. Cazabat, F. Heslot, S.M. Trojan, P. Carles, Nature 346 (1990)

824.[10] L.E. Schriven, C.V. Sternling, Nature 187 (1960) 186.[11] R.M. Overney, E. Meyer, J. Frommer, et al., Nature 359 (1992) 133.[12] M. Rademacher, J.P. Cleveland, M. Fritz, H.G. Hansma, P.K.

Hansma, Biophys. J. 66 (1994) 2159.[13] L. Grasjo, O. Karthaus, N. Maruyama, M. Shimomura, Polymer

Prepr. Japan 46 (1997) E239.

832 O. Karthaus et al. / Thin Solid Films 327–329 (1998) 829–832