Morphological Study of the Organization Behavior of Rod−Coil

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Morphological Study of the Organization Behavior of Rod-Coil Copolymers and Their Blends in Thin Solid Films Nikos P. Tzanetos,† Vassilis Dracopoulos,‡ Joannis K. Kallitsis,†,‡ and Valadoula A. Deimede*,†,‡ Department of Chemistry, University of Patras, G.R.-26500 Patras, Greece, and Foundation of Research and Technology Hellas, Institute of Chemical Engineering and High-Temperature Chemical Processes, P.O. Box 1414, G.R.-26500 Patras, Greece Received April 1, 2005. In Final Form: June 3, 2005 A detailed study of the self-assembly ability of triblock coil-rod-coil copolymers containing a rigid di(styryl)-anthracene segment covalently linked to oxadiazole-based blocks and their binary blends with oxadiazole-based homopolymers is presented here. The self-organized microdomains seem to pack into a fascinating ordered hexagonal structure obtained at a critical concentration without any significant influence of the sample preparation method, based on evidence obtained by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and fluorescence microscopy studies. The compatibilization efficiency of these coil-rod-coil copolymers in polymer blends composed of an electron-accepting polyoxadiazole and a luminescent polyanthracene-based pair was studied by atomic force microscopy (AFM). The common feature of all observed morphologies is the compatibilizing function of the rod-coil molecule, which intercalates between the incompatible domains to prevent the formation of well-defined phase separated nanostructured surfaces.

Introduction One of the attractive interests in materials science and molecular electronics is related with the design of ordered nanostructures with well-defined size and shapes.1-4 Selforganization of molecules through noncovalent forces has the great potential for creating such supramolecular architectures.5,6 An example of the self-assembling systems is provided by rod-coil block copolymers that are a fascinating class of materials both in terms of fundamental interest and practical applications. Moreover, rod-coil block copolymers have a strong tendency to self-organize into a variety of supramolecular structures in nanoscale dimensions.7-9 This supramolecular organization arises not only from the general thermodynamic incompatibility of the blocks but also by the tendency of the rod blocks to order into anisotropic structures. * Corresponding author. Phone: 302610-965248. Fax: 302610997122. E-mail: [email protected]. † University of Patras. ‡ Institute of Chemical Engineering and High-Temperature Chemical Processes. (1) Lehn, J. M. Supramolecular Chemistry; VCH: Weiheim, Germany, 1995. (2) Muthukumar, M.; Ober, C. K.; Thamas, E L. Science 1997, 277, 1225. (3) Sleytr, U. B.; Messner, P.; Pum, D.; Sara, M. Angew. Chem., Int. Ed. 1999, 38, 1034. (4) Oesterbacka, R.; An, C. P.; Jiang, X. M.; Vardeny, Z. V. Science 2000, 287, 839. (5) Whitesides, G. M.; Mathias, J. P.; Seto, C. P. Science 1991, 254, 1312. (6) Percec, V.; Ahn, C.-H.; Ungar, G.; Yeardley, D. J. P.; Mo¨ller, M.; Sheiko, S. S. Nature 1998, 391, 161. (7) (a) Lee, M.; Cho, B.-K.; Ihn, K. J.; Lee, W.-K.; Oh, N.-K.; Zin, W.-C. J. Am. Chem. Soc. 2001, 123, 4647. (b) Lee, M.; Cho, B.-K.; Zin, W.-C. Chem. Rev. 2001, 101, 3869. (8) (a) Stupp, S. I.; LeBonheur, V.; Walker, K.; Li, L. S.; Huggins, K. E.; Kesser, M.; Amstutz, A. Science 1997, 276, 384. (b) Stupp, S. I. Curr. Opin. Colloid Interface Sci. 1998, 3, 20. (c) Pralle, M. U.; Whitaker, C. M.; Braun, P. V.; Stupp, S. I. Macromolecules 2000, 33, 3550. (9) Lecle`re, P.; Calderone, A.; Marsitzky, D.; Francke, V.; Geerts, Y.; Mu¨llen, K.; Bre´das, J.-L.; Lazzaroni, R. Adv. Mater. 2000, 12, 1042.

Of particular interest to us are rod-coil block copolymers with luminescent rod blocks. These may offer a way to a rational design of materials with optoelectronic properties and the additional features of polymers such as easy processing, mechanical integrity, and absence of crystallization on devices. Several examples of such copolymers with for example oligo(p-phenylenevinylenes),10,11 oligo(phenylquinolines),12 oligo(p-phenylenes),13 and oligo(fluorenes)14 as the rod block have been reported. As expected, the optoelectronic properties vary with the supramolecular morphology. Thus, a critical issue for these copolymers is the understanding of how their surface morphology influences their optoelectronic properties,15 since spherical or cylindrical objects which are favorable for light-emitting applications and bicontinuous interpenetrating phases which are desirable in photovoltaic applications can be obtained from phase-separated organic semiconducting materials. It has been suggested that control of the phase separation in polymer blends could lead to efficient, large-area (10) (a) Tew, G. N.; Pralle, M. U.; Stupp, S. I. J. Am. Chem. Soc. 1999, 121, 9852. (b) Tew, G. N.; Li, L.; Stupp, S. I. J. Am. Chem. Soc. 1998, 120, 5601. (11) (a) de Boer, B.; Stalmach, U.; Nijland, H.; Hadziioannou G. Adv. Mater. 2000, 12, 1581. (b) Stalmach, U.; de Boer, B.; Videlot C.; van Hutten, P. F.; Hadziioannou G. J. Am. Chem. Soc. 2000, 122, 5464. (c) de Boer, B.; Stalmach, U.; van Hutten, P. F.; Melzer, C.; Krasnikov, V. V.; Hadziioannou G. Polymer 2001, 42, 9097. (12) (a) Jenekhe, S. A.; Chen, X. L. Science 1998, 279, 1903. (b) Chen, X. L.; Jenekhe, S. A. Langmuir 1999, 15, 8007. (c) Jenekhe, S. A.; Chen, X. L. Science 1999, 283, 372. (13) Tsolakis, P. K.; Koulouri, E. G.; Kallitsis, J. K. Macromolecules 1999, 32, 9054. (14) (a) Tsolakis, P. K.; Kallitsis, J. K. Chem. Eur. J. 2003, 9, 936. (b) Chochos, C. L.; Tsolakis, P. K.; Gregoriou, V. G.; Kallitsis, J. K. Macromolecules 2004, 37, 2502. (15) (a) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (b) Kato, T. Science 2002, 295, 2414. (c) Ikalla, O.; ten Brinke, G. Science 2002, 295, 2407. (d) Percec, V.; Glodde, M.; Bera, T. K.; Miura, Y.; Shiyanovskaya, I.; Singer, K. D.; Balagurusamy, V. S. K.; Heiney, P. A.; Schnell, I.; Rapp, A.; Spiess, H.-W.; Hudson, S. D.; Duan, H. Nature (London) 2002, 419, 384.

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Scheme 1. Chemical Structures of Side-chain Oxadiazole Homopolymer 1, Anthracene Homopolymer 2, Coil-Rod-Coil Block Copolymers 3 and 4 Used in This Study

photodiodes and full color displays.16-18 Thus, LEDs in which the emission color varies can be made by blending polymers with different emission and charge transport characteristics.16 Also, efficient photodiodes have been fabricated from mixtures of electron-accepting and holeaccepting polymers.17,18 The performance of this type of device except on the composition of the blend is very sensitive to the morphology of the film. The present work investigates the morphology of rodcoil copolymers containing a luminenscent di(styryl)anthracene unit as the rigid block and bearing one or two pendant oxadiazole units as the coil blocks and their blends in solid films, applying the complementary imaging techniques, like atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). We also investigate the effects of molecular composition, solution concentration, and sample preparation method on the obtained structures. In particular, the self-structuring behavior of the triblock as a function of solution concentration has been studied by using SEM and fluorescence microscopy techniques. The influence of sample preparation method on the obtained morphology of a binary blend of a triblock copolymer and oxadiazole homopolymer has been examined by TEM and SEM. Finally, AFM measurements have been carried out in order to investigate the phase morphology and topography of (a) binary blends of an oxadiazole-based homopolymer with electron transport properties and an anthracene-based homopolymer with luminescent func-

Materials. Scheme 1 shows the chemical structures of the polymers and copolymers investigated in this study. The oxadiazole homopolymer 119,20 and the anthracene homopolymer 221 were synthesized through free radical polymerization and a polycondensation process, respectively, as has been described previously. Moreover, two kind of copolymers containing a luminescent di(styryl)-anthracene unit as the rigid block and bearing one or two pendant oxadiazole units as the coil blocks were reported.19 These copolymers had been synthesized by atom transfer radical polymerization (ATRP) of the desired oxadiazole monomers using a properly modified conjugated segment as initiator, which resulted in triblock copolymers of the structures 3 and 4. The triblock copolymers show a narrow molecular weight distribution with polydispersity index in the range of 1.17-1.47 as determined from gel permeation chromatography (GPC) relative to polystyrene standards. The molecular weights of the polymers and the copolymers are given in Table 1. Preparation of Polymer Solutions. Triblock copolymers solutions were prepared by dissolving the different copolymers 3-4 (Table 1) in chloroform at room temperature and at various concentrations (0.03, 0.06, 0.09, 0.12, and 0.25 wt %). Polymer binary or ternary blends were prepared by mixing the constituent polymers at various compositions (70, 63, 30, and 27 wt % of polymer 2). The above blends were dissolved in chloroform, such that the final concentration of the prepared solution was 0.03 or 0.09 wt %. All solutions were then filtered through a Millipore

(16) Berggren, M.; Ingana¨s, O.; Gustafsson, G.; Rasmusson, J.; Anderson, M. R.; Hjertberg, T.; Wennerstro¨m, O. Nature 1994, 372, 444. (17) (a) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498. (b) Arias, A. C.; MacKenzie, J. D.; Stevenson, R.; Halls, J. J. M.; Inbasekaran, M.; Woo, E. P.; Richards, D.; Friend, R. H. Macromolecules 2001, 34, 6005. (c) Snaith, H. J.; Arias, A. C.; Morteani, A. C.; Silva, C.; Friend, R. H. Nano Lett. 2002, 2, 1353. (d) Kim, J.-S.; Ho, P. K. H.; Murphy, C. E.; Friend, R. H. Macromolecules 2004, 37, 2861. (18) Yu, G.; Heeger, A. J. J. Appl. Phys. 1995, 78, 4510.

(19) Tzanetos, N. P.; Kallitsis, J. K. Chem. Mater. 2004, 16, 2648. (20) (a) Jiang, X. Z.; Register, R. A.; Killeen, K. A.; Thomson, M. E.; Pschenitzka, F.; Sturm, J. C. Chem. Mater. 2000, 12, 2542. (b) Jiang, X. Z.; Register, R. A.; Killeen, K. A.; Thomson, M. E.; Pschenitzka, F.; Hebner, T. R.; Sturm, J. C. J. Appl. Phys. 2002, 91, 6717. (c) Jiang, X. Z.; Jen, A. K.-Y.; Phelan, G. D.; Huang, D.; Londergan, T. M.; Dalton, L. R.; Register, R. A. Thin Solid Films 2002, 416, 212. (d) Jiang, X. Z.; Philan, G.; Carlson, B.; Liu, S.; Dalton, L.; Jen, A. K.-Y. Macromol. Symp. 2002, 186, 171. (21) Konstandakopoulou, F. D.; Kallitsis, J. K. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 3826.

tionality at various compositions and (b) ternary blends in which a coil-rod-coil copolymer has been used as “compatibilizer” for the two incompatible homopolymers. Experimental Section

Behavior of Rod-Coil Copolymers

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Table 1. Molecular Weight Characteristics of the Polymersa polymer

Mn

Mw

PDI

wt % of the rigid part

1 2 3i 3ii 4

42 870 10 365 5240 8190 5590

99 860 21 180 6140 11 620 8240

2.33 2.04 1.17 1.42 1.47

11.3 7.3 10.6

a Molecular weights and polydispersity indices were determined with size exclusion chromatography (CHCl3, room temperature) using polystyrene standards.

filter 0.25 µm. More details about the blends composition are given in Table 2. Atomic Force Microscopy. AFM measurements were carried out with an Explorer SPM microscope (Veeco Inc.) equipped with a scanner of maximum ranges of 100 and 10 µm in xy and z directions, respectively. All imaging was conducted in tapping mode. Samples were prepared by depositing one drop of the filtered solution (0.03 wt %) onto a mica substrate and then spincoated. Transmission Electron Microscopy. TEM images were recorded with a Zeiss EM902 transmission electron microscope operated at 80K V. Electron micrographs were recorded on Ilford Pan film. Samples were prepared using the following procedure. A drop of the diluted polymer solution of a concentration 0.09 wt % was deposited directly on a thin carbon support film across 400 mesh copper grids help by a pair fine forceps. Most of the solution was removed by touching the grid edge to a filter paper wedge. The sample was allowed to air-dry at room temperature. Scanning Electron Microscopy. SEM measurements were performed using a Zeiss Supra 35VP microscope. Two different sample preparation methods were followed (a) a drop of dilute polymer solution (0.03-0.09 wt %) was deposited onto a holey Formvar carbon support grid (SPI Supplies) where most of it was blotted with a filter paper and sample dried in air and (b) sample droplets (0.09 wt %) were placed onto mica, spin coated, and coated with gold. In the case of using holey carbon support films, a successful spreading of the sample across the holes can be achieved, with the sample being supported only by the surrounding thin Formvar film, thus avoiding any interaction with the surface of the carbon support film. Fluorescence Microscopy. Fluorescence microscopy experiments were carried out with an Inverted Epifluorescence Microscope (Nikon ECLIPSE TE2000-U), equipped with a camera system (Nikon Digital Sight DS-5M-L1), using a super highpressure mercury lamp (100 W) as source and a YFP F41-028 filter (Chroma Technology Corp.). The samples were prepared by depositing one drop of the dilute polymer solution onto a holey Formvar carbon support grid.

Results and Discussion Rod-coil copolymers 3i and 3ii were initially examined by SEM in order to investigate their solid-state structure. SEM images of thin film on holey carbon coated grids obtained by solution casting of 3ii from chloroform at concentration 0.09 wt % are given in Figure 1. The copolymer self-assembled into a fascinating two-dimensional ordered hexagonally structure with micrometer dimensions. The domains of the sample are 1-2 µm in diameter with an interdomain spacing of approximately

Figure 1. SEM image of ordered hexagonal-structured film made by drop casting of copolymer 3ii from chloroform (0.09 wt %) onto a holey carbon support film. The inset shows the same structure spread across the holes, supported only by the thin Formvar film.

250 nm. The same structure was observed when the sample was spread across the holes and was supported only by the surrounding thin film (thus avoiding any interaction with the carbon support film), as shown in the inset of Figure 1. A concentration-dependent morphological transition from irregular circular to hexagonal-like structures was also observed for the same copolymer, as shown in Figure 2. SEM images of thin films on holey carbon coated grids obtained by solution casting of 3ii from chloroform at various concentrations were studied. More specifically, only circular structures were obtained from dilute solutions at a concentration of 0.03 wt % (Figure 2a). At higher concentration (0.06 wt %), hexagonal-like structures became evident and their number density is sufficiently high for the onset of 2D hexagonal ordering to be visible in regions of a monolayer film (Figure 2b, arrow), whereas the two-dimensional ordered hexagonal-like structure was observed at concentration as high as 0.09 wt % (Figure 2c). Moreover, on close inspection of Figure 2, panels b and c, it was observed that the hexagonal-like structures, shown in Figure 2b, have diameters which are approximately half that of the corresponding structures in Figure 2c. Above 0.09 wt %, the hexagonal-like structure was not further observed (not shown here). These results indicate that there is a critical concentration for this ordered self-assembly. To elucidate the influence of the oxadiazole block length on the obtained morphology, thin films of copolymer 3i with rod volume fraction (φrod ) 11.5%) obtained from different solution concentrations were examined. As the concentration is increasing from 0.03 wt % (Figure 2d) to 0.06 wt % (Figure 2e), a structure evolution was observed. Comparison of the copolymers 3ii and 3i at the same concentration 0.09 wt % (Figure 2, panels c and f), revealed a structural differentiation from hexagonal to circular-

Table 2. Composition of the Blends Prepared in This Study

blend

total polymer concentration (wt %)

anthracene homopolymer 2 (wt %)

oxadiazole homopolymer 1 (wt %)

1 2 3 4 5 6

0.03 0.03 0.03 0.03 0.03 0.09

70 63 63 30 27

30 27 27 70 63 30

triblock copolymer 3i (wt %)

triblock copolymer 4 (wt %)

triblock copolymer 3ii (wt %)

10 10 10 70

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Figure 2. SEM micrographs (a) 0.03 wt %, (b) 0.06 wt %, and (c) 0.09 wt % of the triblock copolymer 3ii film and (d) 0.03 wt %, (e) 0.06 wt %, and (f) 0.09 wt %, of 3i film as obtained by solution casting onto a holey carbon support film.

Figure 3. Fluorescence micrograph of hexagonal-like structures in solution cast films on holey carbon coated grids of copolymer 3ii at concentration 0.09 wt %.

like morphologies, as the rod volume fraction is increased from 7.4% to 11.5% respectively. Further evidence about the shape and size of the copolymer structures was provided by fluorescence microscopy. Fluorescence micrograph of solution-cast film on holey carbon coated grids of copolymer 3ii at concentration 0.09 wt % (Figure 3) confirmed the existence of hexagonal-like structures which are homogeneously dis-

tributed in the solid film. These structures showed a strong yellow-green luminescence. To elucidate whether it is possible for the morphology formed by the copolymer to be influenced by blending with the respective coil-block homopolymer, blends of copolymer 3ii with oxadiazole in 70/30 composition were examined. TEM characterization of this blend (blend 6), in which the rod units content was around 5% in the total polymer weight, revealed the same ordered hexagonal structures with average diameters in the same range (Figure 4a) compared with the results of SEM observations of copolymer 3ii. For TEM measurements the sample cast from dilute chloroform solution (at concentration 0.09 wt %) onto a carbon supported film and prepared without any special stains. Therefore, the contrast observed is due to diffraction and phase contrast in the sample itself. We also investigated the influence of the sample preparation method on the obtained morphology by using different substrates and casting methods. SEM image of the former blend (blend 6) obtained by spin coating onto a mica substrate is given in Figure 4b. In the micrograph is clearly displayed the open surface structure of the blend film with hole diameters of 100-500 nm and the presence of an internal ordered hexagonal structure. The structures revealed by the two imaging techniques (SEM and TEM) are morphologically similar and thus result from the

Behavior of Rod-Coil Copolymers

Figure 4. (a) TEM micrograph of solution-cast thin film (0.09 wt %) onto a carbon support film of blend 6 and (b) SEM micrograph of the former blend obtained by spin coating onto a mica substrate, revealing the formation of ordered hexagonal structures.

polymer blend rather than the sample preparation method. A structure of this type can be utilized to trap solar light into the polymer layer by diffraction and guide the incident light into the polymer film, thus increasing the conversion efficiency of photovoltaic cells.22 Other groups have also reported similar hexagonal patterns. Hadziioannou et al.11a,11b reported that rod-coil block copolymers poly(p-phenylene vinylene)-b-polystyrene cast from CS2 (selective solvent for polystyrene) solution form highly ordered microporous honeycomblike aggregation structure with a characteristic scale length, an idea which was originally presented by Francois and co-workers.23 Jenehke et al.12 also reported on the selfassembling behavior of rod-coil diblock copolymers consisting of poly(phenylquinoline) as the rod block and polystyrene as the coil block. They observed that these rod-coil copolymers in a selective solvent for the coil segment self-assemble into hollow spherical micelles with diameters of a few micrometers, which subsequently selforganize into a two-dimensional hexagonal superlattice. Solution-cast micellar films were found to consist of (22) Stolz Roman, L.; Ingana¨s, O.; Granlund, T.; Nyberg, T.; Svensson, M.; Andersson, M. R.; Hummelen, J. C. Adv. Mater. 2000, 12, 189. (23) (a) Widawski, G.; Rawiso, M.; Franc¸ ois, B. Nature 1994, 369, 387. (b) Franc¸ ois, B.; Pitois, O.; Franc¸ ois, J. Adv. Mater. 1995, 7, 1041. (c) Pitois, O.; Franc¸ ois, B. Eur. Phys. J. B 1999, 8, 225.

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multilayers of hexagonally ordered arrays of spherical holes whose diameter, periodicity, and wall thickness depend on copolymer molecular weight and block composition. Taking into account the above findings, solvent quality is a critical parameter to obtain highly ordered superstructures. By using a solvent that is selective for one of the blocks only, the rod-coil diblock copolymers can already be pre-assembled into a particular supramolecular structure prior to film casting and evaporation of the solvent. Recent studies on the formation mechanism of such honeycomb structures have revealed that these develop due to condensation of small water droplets on the surface of the polymer solution.24 Water condensation is caused by surface cooling as a result of rapid evaporation of the volatile organic solvent. The role of the polymer in this formation process seems to be twofold. First, the polymer prevents the coalescence of the water droplets via adsorption at the solvent-water interface. Second, upon evaporation of the organic solvent, the polymer can vitrify the pattern created by the condensation of the water droplets. This suggests that the morphologies previously reported are mostly due to the film formation process and are only little or not influenced by the rod-coil type molecular architecture. However, in our case, ordered hexagonal-like superstructures were obtained by solution casting from a volatile solvent, which is a good solvent for both blocks. Additionally, it should be mentioned that the formation of these reproducible hexagonal-like structures was confirmed by several microscopic techniques. Taking into consideration the above information, we studied the compatibilization efficiency of the rod-coil block copolymers on polymeric pairs composed of anthracene and oxadiazole-based homopolymers in compositions shown in Table 2. AFM phase and threedimensional topographic images of thin films as obtained by spin coating of the binary blends from dilute solutions (0.03 wt %) are shown in Figure 5. The solvent used was chloroform, a good solvent for the constituent polymers and the different blocks of the copolymers. Figure 5a displays a nonuniform phase separated 70:30 w/w blend (blend 1). This uneven surface domain structure was induced by incompatibility between the two homopolymers on the surface. A clear inversion in topographic contrast was observed when the composition is inverted (Figure 5b). The domains, which are made of the minority component, are lower than the matrix in the 70:30 system, whereas they appear higher in the 30:70 blend (blend 4). Therefore, the AFM images can be interpreted by considering that anthracene homopolymer-rich domains constitute the higher parts in topography (i.e., brighter areas) and the oxadiazole homopolymer-rich domains constitute the lower parts (darker areas). The surface morphology of 30:70 w/w blend had a pitted structure, with the pits about 15 nm in depth and 0.1-1 µm in width. To enhance the compatibility of 70:30 and 30:70 w/w homopolymer blends, triblock copolymers 3i and 4 were blended at 10 wt %. As mentioned above, these copolymers incorporate a di(styryl)-anthracene unit as the rigid block and bearing one or two pendant oxadiazole units as the coil blocks. A significant morphological change of the ternary copolymer 3i-based blend (blend 2) is shown in Figure 6a. This structure is reminiscent of the interconnected “wormlike” structures. Their height is 9.25 nm whereas their average width is in the range 50-150 nm. In Figure 6b, the copolymer 4-based blend (blend 3) (24) Klok, H.-A.; Lecommandoux, S. Adv. Mater. 2001, 13, 1217.

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Figure 5. AFM phase and topographical three-dimensional images of (a) the irregular morphology of 70:30 w/w blend (blend 1), (b) the pitted morphology of 30:70 w/w blend (blend 4) on mica substrates from chloroform solutions at 0.03 wt %. Each image is a 2.35 × 2.35 µm tapping-mode AFM scan.

Figure 6. AFM phase and topographical three-dimensional images of (a) the “wormlike” morphology of 63:27:10 w/w blend (blend 2), (b) the “wormlike” morphology of 63:27:10 w/w blend (blend 3), and (c) the pitted morphology of 27/63/10 w/w blend (blend 5) on mica substrates from chloroform solutions at 0.03 wt %. Each image is a 2.35 × 2.35 µm tapping-mode AFM scan.

Behavior of Rod-Coil Copolymers

revealed similar phase morphology compared to the copolymer 3i-based blend. Consequently, the change of the coil block structure did not influence the obtained blend morphology but only affected the size of the structure formed. Figure 6c illustrates the decrease in size of the dispersed phase for copolymer 3i-based blend (blend 5) compared to binary blend 4. Thus, the phase separation of the former blend is much finer than the phase separation of the binary blend 4. The average diameter of these pits is 50 nm, whereas their depth is 8.66 nm. On the basis of the above results, we can conclude that triblock copolymer 3i functions as “combatibilizer” for the two incompatible homopolymers, improving thus its surface morphology and resulting in the formation of welldefined phase separated nanostructured surfaces. Conclusions Morphological characterization of coil-rod-coil block copolymers containing di(styryl)-anthracene units as the rod block and oxadiazole homopolymers as the flexible blocks and their blends with oxadiazole homopolymers, is reported. Reproducible hexagonal symmetry microstructures that are regular in size and shape, applying scanning electron microscopy, transmission electron mi-

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croscopy and fluorescence microscopy, are obtained at a critical concentration without being influenced by the sample preparation method. We also investigated the phase morphology and topography of binary blends of an oxadiazole and an anthracene-based homopolymer at various compositions, and ternary blends in which the copolymer functions as “compatibilizer” for the two incompatible homopolymers, by atomic force microscopy. The morphology of the binary blends is improved by the addition of the triblock copolymer, resulting in the formation of well-defined phase separated nanostructured surfaces. Acknowledgment. The authors are indebted to assistant Prof. Z. Lygerou (Medicine School, University of Patras) for the fluorescence microscopy measurements. The TEM measurements have been performed at Max Planck Institute for Polymer Research (MPIP, Mainz, Germany). This work was partially supported by the Operational Program for Education and Initial Vocational Training on “Polymer Science and Technology” 3.2a, 33H6, administered through the Ministry of Education in Greece. LA050867G