10898
Langmuir 2006, 22, 10898-10903
Surface-Induced Morphologies in Thin Films of a Rod-Coil Diblock Copolymer Ji-Woong Park* and Yo-Han Cho Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, Buk-gu, Oryong-dong 1, Gwangju 500-712, Korea ReceiVed August 8, 2006. In Final Form: October 29, 2006 A block copolymer containing a rodlike block is studied for its adsorption and formation of nanostructured thin films on the substrate surface. The block copolymer is poly(styrene-b-3-triethoxysilylpropylisocyanate) (PS-b-PIC) of which the PIC chain consists of repeating amide units with triethoxysilyl side groups. As the copolymer chains are adsorbed onto silica surfaces, the PIC blocks pack laterally on the plane in a smectic manner, and the PS chains segregate along the ordered PIC chains, resulting in stripe patterns. The width of the stripes formed on the silica surface appeared to be much larger that on the carbon surface. This was accounted for by the bilayered smectic packing of the rod blocks that is induced by rod-surface attractive interaction. The periodicity of the stripe pattern on the carbon surface indicates that interdigitated packing is preferred by the copolymers on the hydrophobic surface in a manner similar to those in the bulk state of rod-coils. Excess rod-coils on the bilayered smectic layer resulted in a terraced morphology due to large difference in the periodicity between the bilayered smectic layer at the substrate surface and the interdigitated smectic layer in the bulk.
Block copolymers consist of two or more chemically distinct macromolecules that are connected covalently. The morphological structure of the block copolymer thin films have been the focus of recent research activity because of their potential use in a variety of applications.1-4 Block copolymers with a surfaceattractive block self-assemble into nanostructured monolayers as the block is selectively adsorbed or bonded to the surface while being microphase separated from the blocks not adsorbed.5,6 The microstructure of the block copolymer monolayer depends on the conformational properties of constituting macromolecular blocks. In flexible coil-coil block copolymers, the polymer chains that are attracted to the surface are anchored onto the surface, whereas the blocks with weaker surface affinity segregate into circular or cylindrical microdomains.7 The surface-attractive blocks are usually stretched over the substrate surface because the adsorption interaction prevails over the entropy loss incurred by extending macromolecular chains. Rodlike polymers have extended conformations in solution, which may allow them to be readily adsorbed onto the surface without paying an entropic penalty. Lateral packing of the rods, together with their adsorption onto the surface, can result in monolayered rod crystals with the thickness of a rod diameter. A recent study on the self-assembly of oligomeric peptides on a mica surface shows the facile formation of nanoscale tapes composed of laterally packed rods with a thickness identical to the diameter of an extended oligopeptide chain.8 * Corresponding author. E-mail:
[email protected]. (1) Fasolka, M. J.; Mayes, A. M. Annu. ReV. Mater. Res. 2001, 31, 323. (2) Park, C.; Yoon, J.; Thomas, E. L. Polymer 2003, 44, 6725. (3) Senaratne, W.; Andruzzi, L.; Ober, C. K. Biomacromolecules 2005, 6, 2427. (4) Luzinov, I.; Minko, S.; Tsukruk, V. V. Prog. Polym. Sci. 2004, 29, 635. (5) Russell, T. P. Curr. Opin. Colloid Interface Sci. 1996, 107. (6) Spatz, J. P.; Moeller, M.; Noeske, M.; Behm, R. J.; Pietralla, M. Macromolecules 1997, 30, 3874. (7) Potemkin, I. I.; Kramarenko, E. Y.; Khokhlov, A. R.; Winkler, R. G.; Reineker, P.; Eibeck, P.; Spatz, J. P.; Moeller, M. Langmuir 1999, 15, 7290. (8) Whitehouse, C.; Fang, J. Y.; Aggeli, A.; Bell, M.; Brydson, R.; Fishwick, C. W. G.; Henderson, J. R.; Knobler, C. M.; Owens, R. W.; Thomson, N. H.; Smith, D. A.; Boden, N. Angew. Chem. Int. Ed. 2005, 44, 1965.
The ability of the surface-attractive rods to form nanostructured monolayers can be explored further by attaching a coil-like block to the end of the rodlike blocks. The resultant copolymer has a structure of rod-coil block copolymers that may give arrays of nanometer-thick organic structure at the surface via rod-coil microphase separation and rod-rod packing occurring in two dimensions. Linear or dendritic molecules containing oligomeric rods have been reported to exhibit unique organized monolayers on solid or liquid surfaces.9-12 The axes of the rodlike units in most of the oligomeric monolayers align perpendicular to the surface because of weak interactions between the surface and the rod employed in the studies. The rod-coil block copolymer containing a sufficiently long, surface-attractive rod would give different results as the rod-surface interactions become comparable to rod-rod or rod-coil packing interactions. We reported recently that a rod-coil block copolymer consisting of a surface-attractive rodlike block (PIC) and a hydrophobic coil-like block (PS) readily formed a nanopatterned monolayer on the silica surface when the substrate had been immersed in the copolymer solution.13 The adsorbed PIC blocks could be covalently bonded in-plane onto the surface using the reaction between their alkoxy silyl side groups and the hydroxyl groups of the silica surface. The PS blocks were aggregated to form nanoscale mounds over the PIC layer. The arrangement of the PS mounds on the surface was irregular or had only shortrange order, indicating the lack of rod-rod interchain packing interaction during the adsorption of the copolymer chains onto the substrate. In the immersion coating method used in the previous study, the adsorption and chemical bonding of copolymer chains to the surface occurred in the isotropic solution state, which probably caused the disordered surface pattern to yield. (9) Genson, K. L.; Holzmueller, J.; Ornatska, M.; Yoo, Y. S.; Par, M. H.; Lee, M.; Tsukruk, V. V. Nano Lett. 2006, 6, 435. (10) Holzmueller, J.; Genson, K. L.; Park, Y.; Yoo, Y. S.; Park, M. H.; Lee, M.; Tsukruk, V. Langmuir 2005, 21, 6392. (11) Li, H. B.; Liu, Q. T.; Xu, M.; Bu, W. F.; Lin, X. K.; Wu, L. X.; Shen, J. C. J. Phys. Chem. B 2005, 109, 2855. (12) Tsukruk, V. V.; Genson, K.; Peleshanko, S.; Markutsya, S.; Lee, M.; Yoo, Y. S. Langmuir 2003, 19, 495. (13) Park, J.-W.; Thomas, E. L. J. Am. Chem. Soc. 2002, 124, 514.
10.1021/la062352a CCC: $33.50 © 2006 American Chemical Society Published on Web 11/17/2006
Letters
Langmuir, Vol. 22, No. 26, 2006 10899
Figure 1. In-plane smectic ordering of the rod-coil block copolymers on different substrate surfaces. (a and b) TEM images of films on a silicon nitride membrane grid treated with oxygen plasma and on a carbon support surface, respectively. The scale bars indicate 500 nm. (c and d) Schematic drawings of the bilayer and the interdigitated in-plane smectic packing.
The use of casting or dip-coating methods involving rapid solvent evaporation may allow liquid crystalline, crystalline, and/or microphase-separated regimes14 to be accessed by the copolymer chains before they are immobilized by chemical grafting onto the surface. Here we study the microstructure of the rod-coil copolymer thin films on solid surfaces with different surface energies. The study demonstrates that the rod-coil block copolymer, which self-assembles in the bulk state via two competing processes, liquid crystalline ordering and microphase phase separation, can self-assemble on solid surfaces into monolayers with different nanostructures depending on the affinity of its rodlike block with the substrate surface. The copolymer investigated here is poly(styrene-b-3-(triethoxysilyl)propylisocyanate)(PS-b-PIC) with molecular weights of the PS block of 39 000 g/mol and the PIC block of 23 000 g/mol with polydisperisity of the copolymer of 1.25. The synthesis of the copolymer is described elsewhere.13,14 Polyisocyanates have been known as helical rodlike polymers with persistence lengths of above 40 nm, and they show liquid crystalline behavior in solution.15,16 The amide carbonyl groups along the polyiso-
cyanate backbone facilitate the adsorption of the chains to hydrophilic surfaces,17,18 and the chemical reactivity of alkoxy silyl side groups to silanol allows covalent bonding of the PIC chains to the silica surface. First, we investigated the morphologies of thin regions near the edge of the drop-cast films on two different types of substrate surfaces using transmission electron microscopy (TEM, JEOL 2000FX or 200CX). Silicon nitride membrane window TEM grids (SPI supplies) were treated with oxygen plasma to generate hydroxyl groups so as to be available for covalent bonding with alkoxysilyl groups of the PIC chain. Carbon films deposited on mica surfaces, which can be floated off on water to be picked up with TEM grids, were used as a substrate with weaker chemical affinity to the rod block. About 10-200 µL droplets of the copolymer solution at a concentration of about 0.02% in toluene were cast on the substrates and were allowed to evaporate to dryness. In the case of the silicon nitride membrane window TEM grid, its surface was spotted with the copolymer solution using a micropipette. After drying for 30 min under atmospheric conditions, the grid was exposed to ruthenium oxide vapor for 5-10 min to stain the PS region. For the silicon nitride membrane
(14) Park, J. W.; Thomas, E. L. AdV. Mater. 2003, 15, 585. (15) Green, M. M.; Peterson, N. C.; Sato, T.; Teramoto, A.; Cook, R.; Lifson, S. Science 1995, 268, 1860. (16) Bur, A. J.; Fetters, L. J. Chem. ReV. 1976, 76, 727.
(17) Kawaguchi, M.; Ishikawa, R.; Yamamoto, M.; Kuki, T.; Kato, T. Langmuir 2001, 17, 384. (18) Ohkita, M.; Higuchi, M.; Kawaguchi, M. J. Colloid Interface Sci. 2005, 292, 300.
10900 Langmuir, Vol. 22, No. 26, 2006
Letters
Figure 2. Tapping mode AFM images of a dip-coated PS-b-PIC film. (a) Height-contrast image, (b) phase-contrast image, (c) typical height profile in the AFM scanning direction, and (d) schematic models for the zigzag stripes. Insets are Fourier power spectra. The solvent evaporation direction of the sample is parallel to the vertical direction of the image.
grid, about 100-Å-thick carbon was evaporated onto the copolymer film to minimize electron beam damage. The structures of thin regions near the edge of the films were examined by TEM. In Figure 1, we compare the TEM images of the films formed on the surfaces. Both images show stripe patterns consisting of alternating dark PS and bright PIC regions. It is noteworthy that the average periodicity of the stripe pattern was about 84 nm on the silicon nitride surface, varying in the range of 70 to 90 nm (Figure 1a), whereas it was only about 50 nm on the carbon support substrate (Figure 1b). The width of unstained PIC regions on the silicon nitride surface was larger than twice the length (∼20 nm) of a fully extended PIC block, which was estimated by multiplying the degree of polymerization of the PIC block (∼100) by the repeating unit length (∼2 Å). In contrast, on the carbon surface, the width of the unstatined PIC region was similar to the length of PIC block. The data are well explained by the two smectic structures, the bilayered (Figure 1c) packing on the silicon nitride surface and the interdigitated (Figure 1d) packing on the carbon surface.19-21 Although the rod-coils are known to favor interdigitated smectic packing, it appeared that the bilayered smectic packing (Figure 1a,c) was facilitated on the plasma-treated silicon nitride surface possessing higher affinity for the rod blocks. The area percentages of the dark and bright regions in the stripe pattern in Figure 1a, PS (25%) and PIC (75%), are different (19) Raphael, E.; deGennes, P. G. Physica A 1991, 177, 294. (20) Matsen, M. W.; Barrett, C. J. Chem. Phys. 1998, 109, 4108. (21) Halperin, A. Macromolecules 1990, 23, 2724.
from the composition of PS (63%) and PIC (37%) in the block copolymer. This resulted from the PIC chains being extended over the surface and the PS blocks forming elevated domains via aggregation similarly to those in the immersion-coated monolayer.13 The TEM image of the copolymer layer in Figure 1a also contains dark cloudlike regions covering the stripe pattern. This can be accounted for by the repulsive interaction of the PS chains at the PS-PIC interface. When PIC blocks order in the bilayer smectic manner on the surface, all of the PS chains come out of the PS-PIC interface where the PS-PIC junctions are localized. Because a stable conformation of PS is the random coil, the interfacial area per chain of the PS block is larger than that of the PIC. Smectic packing of the rods results in crowded coils at the interface. Therefore, as more copolymer chains pack closely onto the surface, the PS blocks extend over the bottom PIC stripe layer to decrease the repulsive energy, forming a continuous second layer that appears as dark cloudlike regions in the TEM after staining with ruthenium oxide. In contrast, the interdigitation of PICs reduces the density of PSs at the interface to half of that of bilayered PIC packing because the PS-PIC interface of the interdigitated smectc structure consists of both the PIC chain ends and the PIC-PS junctions. The interdigitated smectic packing scheme also works well for the thicker films because the interdigitation of the PIC rods causes little crowding of PS blocks, resulting in an edge-on lamella of an interdigitated smectic over a large area of the carbon surface. The brightness of the TEM image in Figure 1b changes gradually over different regions without discrete domains, indicating that the same edge-on
Letters
Langmuir, Vol. 22, No. 26, 2006 10901
Figure 3. ΑFM images of a self-assembled monolayer of the PS-b-PIC block copolymer formed by annealing the dip-coated films on the Si wafer surface under a toluene-saturated atmosphere. (a and b) Height- and phase-contrast images over an area of 2 × 2 µm2, (c) phase contrast image over an area of 10 × 10 µm2, and (d) cross-sectional view of a schematic model for the bilayer smectic stripe covered with top PS brushes.
lamellar structure forms in different regions of the film regardless of its thickness. The extended conformation of PS blocks on the bilayer smecticpacked PICs facilitated the formation of a monomolecular layer of the block copolymer. The film with the bilayered smectic stripe pattern could be coated on a large area of the substrate surface by employing the dip-coating method in which the liquidsubstrate boundary moves continuously while depositing nearly a monolayer-thick film with a dipping motion. A silicon wafer was immersed in a THF solution of PIC-b-PS in a concentration of about 0.2-0.5 w/v% for about 10 s. Then the substrate was vertically removed from the solution by an approximate speed of 0.5 cm/s. The substrate was held at the position right above the solution level until the solution flew down the wafer surface. Horizontal interference fringes appeared along the border of the liquid film moving downward. The thickness of the films obtained in this way was approximately 5 nm as measured by AFM for the height difference between a coated and an uncoated region. In Figure 2 are shown the tapping mode AFM images of the dip-coated PIC-b-PS films on the silicon wafer surface. No characteristic pattern was observed in the height-contrast AFM
image (Figure 2a) whereas wavy stripe patterns appeared clearly in the phase-contrast image that was obtained by the hard-tapping method using large drive amplitudes or low set-point values. The water contact angle of the dip-coated film was about 85°, which is close to the value known for PS. These data suggest that the stripe pattern is covered with PS blocks as discussed above. The Fourier transform image shows long-range orientational order in the direction of solvent evaporation. The periodicities varied in the range of 55-90 nm (73 nm is the peak value in the Fourier transform image), depending on the tilt of the stripes relative to their average direction. The zigzag undulation of the stripes can be accounted for by the evaporationinduced strain causing smectic C-like tilting of the rods relative to the rod-coil interface. A similar pattern has been observed in thin films of poly(styrene-b-n-hexylisocyanate) rod-coil block copolymers.22 Although Figure 2 exhibits a long-range orientational order that originates from liquid crystalline ordering, the images shown (22) Chen, J. T.; Thomas, E. L.; Ober, C. K.; Mao, G.-p. Science 1996, 273, 343.
10902 Langmuir, Vol. 22, No. 26, 2006
Letters
Figure 4. Height-contrast AFM image of a terraced edge of a thicker island formed on the bilayer-smectic rod-coil monolayer. No stripe pattern is observed on the monolayer because of the reason discussed in the text. A postulated schematic model for packing rod-coils on the surface as viewed from the rod axis direction is shown at the bottom.
in Figures 1 and 2 were obtained from the films prepared by rapid evaporation of solvent; therefore, it is unlikely that the rod-coils had sufficient time to organize into their equilibrium structures. Because the reaction of alkoxy silyl groups with the silica surface proceeds slowly, the structure formed by dip coating may be further adjusted to a better-ordered structure by annealing under a solvent-saturated atmosphere. A dip-coated film prepared by the method described above was exposed to a toluene-saturated atmosphere for 15 h. Figure 3 shows that the copolymer chains reorganized into a well-ordered stripe pattern with a nearly constant periodicity of 70 nm, which is still about 20 nm larger than that (∼50 nm) of interdigitated smectic packing (Figure 1d), indicating that the rod blocks in the annealed film are more closely packed into the bilayer smectic. The thickness of the films was still about 5 nm. Uniform coverage over a large area with no bare substrate surface indicates that the copolymer layer had effectively adhered to the surface although the polymer chains were locally mobile under the annealing condition. The AFM image showed nearly featureless topography (Figure 3a) while giving a well-ordered stripe pattern in the phase-contrast image obtained by the hard-tapping method (Figure 3b,c), indicating that the smectic-ordered stripe pattern is covered with extended PS blocks that are similar to those in the dip-coated sample (Figure 2). A question remaining about the morphology of the rod-coils on the surface concerns the layering of more rod-coil molecules over the bilayer smectic monolayer. Solvent annealing of dipcoated films sometimes yielded thin islands on top of the stripe monolayer, probably via the aggregation of excess copolymer chains that are not directly adsorbed to the substrate surface but are trapped on the film surface. Interestingly, terraced morphology
was observed on the edge of the islands, as shown in Figure 4. The surface of the island region exhibited a holey texture and was elevated about 10 nm from the bottom monolayer, as measured by AFM. A similar holey image was obtained on the TEM for the region close to the stripe pattern in the drop-cast film (Supporting Information). This morphology resembles the image of the perforated lamella obtained from other types of rod-coil block copolymers.23 Considering that the length of the PIC chain (20 nm) is much larger than the height increment (10 nm) and that the majority of the block copolymer is the PS coil block, the rod blocks should be aligned horizontally as thin crystals, probably similar to the “fencelike” rod-coil assembly that was predicted long ago.19 On the edge region of the films, the smectic bottom layer and the holey top layer were connected by a ladderlike stripe layer located at an elevation of about 5 nm from the surface of the smectic bottom layer. The width of the ladder was similar to the periodicity of the smectic bottom layer. The thickness increment of about 5 nm suggests that layering of the rod-coil molecules occurs in a head-to-head fashion as shown by the schematic model in Figure 4. At longer distances from the substrate surface, discrete stripes form, and holey surface morphology is observed on the film. In conclusion, we show that rod-coil block copolymers with a surface-attractive rod order in the bilayered smectic pattern rather than in the interdigitated smectic packing that is known as a more stable microstructure form for the rod-coil system. Excess rod-coil molecules on the bilayer smectic rod-coil layer result in frustrated, terraced morphology because the copolymer chains at the surface and in the bulk prefer different packing (23) Tenneti, K. K.; Chen, X. F.; Li, C. Y.; Tu, Y. F.; Wan, X. H.; Zhou, Q. F.; Sics, I.; Hsiao, B. S. J. Am. Chem. Soc. 2005, 127, 15481.
Letters
structures. This study demonstrates that the rod-surface interaction is a key factor controlling the self-assembly of the copolymer containing a rodlike block at the surface. Acknowledgment. This work was supported by a Korean Research Foundation grant funded by the Korean government (KRF-2005-205-D00035) and the Program for Integrated
Langmuir, Vol. 22, No. 26, 2006 10903
Molecular Systems at Gwangju Institute of Science and Technology in Korea. Supporting Information Available: TEM image in a region thicker than the monolayer of a film on a silicon nitride membrane grid. This material is available free of charge via the Internet at http://pubs.acs.org. LA062352A
