3480
Langmuir 2005, 21, 3480-3485
Self-Organization of Polystyrenes into Ordered Microstructured Films and Their Replication by Soft Lithography A. Bolognesi,* C. Mercogliano, and S. Yunus Istituto per lo Studio delle Macromolecole, CNR, Via E. Bassini 15, 20133 Milano, Italy
M. Civardi, D. Comoretto, and A. Turturro Dipartimento Chimica e Chimica Industriale, Universita` degli Studi di Genova, Via Dodecaneso 31, 16146 Genova, Italy Received October 19, 2004. In Final Form: January 24, 2005 We report on the formation of ordered arrays of micrometric holes on the surface of polystyrene (PS) films cast from volatile solvents in the presence of humidity at different temperatures. The formation mechanism is investigated for PS having different molecular weights, polydispersities, and carboxylic terminations. Among the chosen materials, a highly regular honeycomb microstructured morphology is obtained on the surface of films prepared with dicarboxy-terminated PS with 〈molecular weight〉 ) 100 000. Experiments and observations on film formation indicate that polar groups are playing a fundamental role in this process. Tuning the surface tension by means of polar terminations allows the film morphology to be modified and in particular the preparation of two- or three-dimensional microstructured films. Finally, we show how these structures can be replicated by soft lithography and then used in the fields of photonic crystals and organic electronics.
1. Introduction The preparation of ordered polymeric films by the selforganization of matter is an active field of research due to its potential applications in the area of nano- and microtechnology. Many strategies are currently investigated to control the structure, the dimensions, and the regularity of the patterns that can be obtained. As an example, the ordered nanophase segregation occurring in block copolymers or the formation of microstructured polymer films, presenting an ordered array of holes, is currently exploited in optoelectronics, photonics, membranes, catalysis, microlens technology, and pico-beaker fabrication.1-3 While phase separation in block copolymers is a well-established phenomenon,4 the formation of arrays of holes, ordered in a honeycomb structure, has not been fully elucidated yet even though several papers have been published on this subject.5-10 Holes result from the condensation of water microdroplets on the evaporative cooling surface of the polymer solution. In accordance with Srinivasarao et al.,9 Govor et al.,11 and Franc¸ ois and Pitois,12 we believe that the overall phenomenon is driven by surface tension effects. (1) De Rosa, C.; Park, C.; Thomas, E. L.; Lotz, B. Nature 2000, 405, 433. (2) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (3) Erdogan, B.; Song, L.; Wilson, J. N.; Park, J. O.; Srinivasarao, M.; Bunz, U. H. F. J. Am. Chem. Soc. 2004, 126, 3678. (4) Krausch, G.; Magerle, R. Adv. Mater. 2002, 14, 1579. (5) Widawski, G.; Rawiso, M.; Franc¸ ois, B. Nature 1994, 369, 387. (6) Franc¸ ois, B.; Widawski, G.; Rawiso, M.; Cesar, B. Synth. Met. 1995, 69, 463. (7) Jenekhe, S. A.; Chen, X. L. Science 1999, 283, 372. (8) De Boer, B.; Stalmach, U.; Nijland, H.; Hadziioannou, G. Adv. Mater. 2000, 12, 1581. (9) Srinivasarao, M.; Collings, D.; Philips, A.; Patel, S. Science 2001, 292, 79. (10) Maruyama, N.; Koito, T.; Nishida, J.; Sawadaishi, T.; Cieren, X.; Ijiro, K.; Karthaus, O.; Shimomura, M. Thin Solid Films 1998, 327, 854.
Self-organization of holes may be due to thermocapillary convection as suggested by Be´nard-Marangoni13 and Be´nard-Rayleigh’s13 works. Indeed, Xu et al.14 have recently investigated the mechanism of surface texture formation in solid coatings and films. They have shown that, under particular conditions and in certain fluid layers, vertical temperature gradients induce various types of convection phenomena giving rise to ordered patterns. It is interesting to note that honeycomb nanostructured films have been obtained with several macromolecules having different structural complexities such as starshaped polystyrene (PS), poly(styrene)-block-poly(pphenylene) (PS-b-PPP) copolymers,5,6 poly(styrene)-blockpoly(2,5-dioctyloxy-p-phenylene vinylene),8 nitrocellulose,15 poly(3-octylthiophene), octylsulfoxy poly(p-phenylene-vinylene),11 atactic PS terminated by a carboxylic acid group,9 poly(methyl methacrylate) (PMMA), and crown ether-containing series such as PS-crown-PS, PMMA-crown-PMMA,16 and a semi-rod-coil block copolymer of styrene and isoprene with an oligothiophenemodified side chain.17 Carbon disulfide (CS2) is the solvent which is mainly used, but literature also reports the use of toluene,9 xylene,14 amyl acetate,15 and chloroform.18 However, due to the complexity of the phenomenon, its (11) Govor, V.; Bashmakov, I. A.; Kiebooms, R.; Dyakonov, V.; Parisi, J. Adv. Mater. 2001, 13, 588. (12) Pitois, O.; Franc¸ ois, B. Eur. Phys. J. B 1999, 8, 225. (13) For Be´nard-Marangoni and Be´nard-Rayleigh convection, see, for instance, Probstein, R. F. Physicochemical Hydrodynamics; Butterworth: Boston, 1989. (14) Xu, S.; Li, M.; Mitov, Z.; Kumacheva, E. Prog. Org. Coat. 2003, 48, 227. (15) Govor, V.; Bashmakov, I. A.; Kaputski, F. N.; Pientka, M.; Parisi, J. Macromol. Chem. Phys. 2000, 201, 2721. (16) Peng, J.; Han, Y.; Fu, J.; Yang, Y.; Li, B. Macromol. Chem. Phys. 2003, 204, 125-130. (17) Hayakawa, T.; Horiuchi, S. Angew. Chem., Int. Ed. 2003, 42, 2285. (18) Peng, J.; Han, Y.; Yang, Y.; Li, B. Polymer 2004, 45, 447.
10.1021/la047427u CCC: $30.25 © 2005 American Chemical Society Published on Web 03/05/2005
PS Self-Organization and Replication
Langmuir, Vol. 21, No. 8, 2005 3481
Table 1. Molecular Characteristics of Investigated PS’s PS
〈Mw〉 × 10-3
〈Mw〉/〈Mn〉
PS12K PS13K PS45K PS100K-2COOH PS200K-COOH PS200K PS227K
12 13.7 45 106 200 200 227
1.1 1.06 unknown 1.02 unknown bimodal 1.03
full understanding is not yet achieved. In particular, no definitive correlation between polymer properties and working conditions on the formation of microstructure was found. For instance, in all the papers so far reported, the presence of polar groups on the polymeric chains was never stressed. In our opinion this point could be important when evaporation takes place in a wet environment. To better understand the physicochemical mechanism underlying the formation of ordered arrays of holes in polymeric films, we focused our attention on PS. In this paper we have investigated the role played by polar substituents in PS chains on the microstructuring of films cast from their solutions. We choose PS and its -COOH-terminated derivatives because this model system is available with well-defined molecular weight and polar group concentration. We will also show how to control the preparation of two- and three-dimensional structures and how two-dimensional patterned PS films can be replicated on semiconducting polymers by means of soft lithography, thus, opening interesting perspectives in photonic crystal preparation and in organic electronics. 2. Materials and Methods Linear PS’s used in this work were purchased from Aldrich and used without further purification. Their molecular weights and polydispersities are summarized in Table 1 together with those of PS12K, which was synthesized by the “living” free radical polymerization method, using 2,2,6,6-tetramethylpiperidinylN-oxyl as an initiator. Macromolecular chains of PS100K2COOH and PS200K-COOH have two and one COOH terminal groups, respectively. Toluene and CS2 polymer solutions with concentrations ranging from 0.5 to 4 g/100 mL were prepared at room temperature. Solutions were spread on a glass slide over an area of about 1 cm2. Thicknesses up to several micrometers were reached. The evaporation process took place inside a chamber filled with moist air. Vapor was produced by heating at different controlled temperatures a water bath inside the chamber. Complete evaporation of the solvent occurred in 30-60 s. The surface and cross sections of the microstructured films were characterized by scanning electron microscopy (SEM) with a Leica-Stereoscan model 440, operating at a 20-kV accelerating voltage. Samples were made conductive by deposition of a gold layer (
