Langmuir 1996, 12, 4317-4320
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Microdomain Morphology Analysis of Block Copolymers by Atomic Force Microscopy with Phase Detection Imaging Ph. Lecle`re, R. Lazzaroni, and J. L. Bre´das* Service de Chimie des Mate´ riaux Nouveaux, Centre de Recherche en Electronique et Photonique Mole´ culaires, Universite´ de Mons-Hainaut, Place du Parc, 20, B-7000 Mons, Belgium
J. M. Yu, Ph. Dubois, and R. Je´roˆme Centre d’Etude et de Recherche sur les Macromole´ cules, Universite´ de Lie` ge, Institut de Chimie, B6, B-4000 Sart-Tilman (Lie` ge), Belgium Received January 30, 1996. In Final Form: May 14, 1996X We use atomic force microscopy (AFM) with phase detection imaging (PDI) in order to study the surface microdomain morphology of thick (i.e., ca. 2 mm) films of triblock copolymers. We present here the results obtained on a poly(methyl methacrylate)-block-polybutadiene-block-poly(methyl methacrylate) (PMMAb-PBD-b-PMMA) copolymer prepared by using a 1,3-diisopropenylbenzene (DIB)-based difunctional anionic initiator. Our data illustrate the interest of PDI for the elucidation of surface phase separation in block copolymers. We show that the surface of thick films studied by this new technique exhibits a two-phase structure corresponding to the two types of components.
I. Introduction The increasing importance of block copolymers arises mainly from their unique properties in solution and in the solid state, which are a consequence of their molecular structure.1 In particular, sequences of different chemical composition are usually incompatible and therefore have a tendency to phase segregate. The microdomain formation process in the solid state is directly related to the specific molecular architecture, which can be designed by using existing monomers or polymers. Since their discovery, block copolymers have been considered for numerous applications in domains ranging from impact resistance2 to surface modification of textiles, surfactants in cosmetics,3 and biomedical and pharmaceutical products.4 In this context, the combination of appropriate sequences, for instance of polystyrene and polybutadiene, led to the appearance of a new class of materials, called “thermoplastic elastomers”. At temperatures below the melting point or glass transition temperature of the thermoplastics blocks, those domains effectively act as cross-links for the elastomeric matrix. The materials then behave as a vulcanized rubber in many aspects, while at high temperature, they can be processed with standard thermoplastic processing techniques such as extrusion and injection molding. Structural studies of block copolymers have been based mostly on electron microscopy and X-ray scattering; these techniques showed that segregated microphases can be spheres, cylinders, or lamellae. The latter tend to form a regularly-repeating lamellar order while the cylinders arrange in a bidimensional hexagonal lattice, and the spheres give rise to cubic lattices.5 These mesomorphic structures have been extensively studied by Hashimoto X
Abstract published in Advance ACS Abstracts, August 15, 1996.
(1) Riess, G.; Bahadur, P. In Encyclopedia of Polymer Science and Engineering; Mark, H. F., Bikales, N. M., Overberger, C. G., Menges, G., Eds.; Wiley: New York, 1989; p 324. (2) Angelo, R. J.; Ikeda, R. M.; Wallach, M. L. Prepr.sAm. Chem. Soc., Div. Org. Coat. Plast. Chem. 1974, 34, 103. (3) Helfand, E. In Recent Advances in Polymer Blends, Grafts and Blocks; Sperling, L., Ed.; Plenum Publishing Corp.: New York, 1974; p 117. (4) Lundsted, L. G.; Schmolka, I. R. In Block and Graft Copolymerization; Ceresa, R. J., Ed.; Wiley: New York, 1976; Vol. 2, p 37.
S0743-7463(96)00096-0 CCC: $12.00
et al.,6 particularly for styrene-isoprene block copolymers; these authors also studied the relationships between the size and characteristics of microdomain structure as a function of the molecular parameters of the constituent polymers. More complex structures such as a bicontinuous double-diamond arrangement have also been observed experimentally7 and rationalized theoretically.8 Progress in the understanding of surface morphology has lagged behind knowledge about the bulk, partly due to a paucity of suitable analysis techniques. Recently, two novel types of microscopies have become available which promise to lead to significant progress in this domain: low-voltage high-resolution scanning electron microscopy (LVHRSEM)9 and atomic force microscopy (AFM).10,11 While both techniques have a lateral resolution as good as 5 nm, only AFM is able to afford precise and accurate information in the direction perpendicular to the surface. Furthermore, compared to LVHRSEM, AFM has the advantages of avoiding staining with contrast agents and coating of the surface with an extra conductive layer. However, in AFM, the probe tip usually remains in permanent contact with the sample during the analysis, which can lead to sample distortion in the case of soft materials, such as elastomers. In order to prevent this unwanted effect, the so-called tapping mode was developed and used successfully.12 (5) Gallot, B. R. M. Adv. Polym. Sci. 1978, 29, 85. Gallot, B. In Liquid Crystalline Order in Polymers; Blumstein, A., Ed.; Academic: New York, 1978; p 223. (6) Hashimoto, T.; Nagatoshi, K.; Todo, A.; Hasegawa, H.; Kawai, H. Macromolecules 1974, 7, 364. Hashimoto, T.; Todo, A.; Itoi, H.; Kawai, H. Macromolecules 1977, 10, 377. Todo, A.; Kiuno, H.; Miyoshi, K.; Hashimoto, T.; Kawai, H. Polym. Eng. Sci. 1977, 17, 587. Hashimoto, T.; Shibayama, M.; Kawai, H. Macromolecules 1980, 13, 1237. Hashimoto, T.; Fujimura, M.; Kawai, H. Macromolecules 1980, 13, 1660. (7) Thomas, E. L.; Anderson, D. M.; Henkee, C. S.; Hoffman, D. Nature 1988, 334, 598. (8) Bates, F. S.; Fredrickson, G. H. Annu. Rev. Phys. Chem. 1990, 41, 525. (9) Schwack, D. W.; Vezie, D. L.; Reffner, J. R.; Thomas, E. L.; Annis, B. K. J. Mater. Sci. Lett. 1992, 11, 352. (10) Han, C. D.; Kim, J.; Kim, J. K. Macromolecules 1989, 22, 383. Collin, B.; Chatenay, D.; Coulon, G.; Ausserre, D.; Galot, Y. Macromolecules 1992, 25, 1621. Annis, B. K.; Schwark, D. W.; Reffner, J. R.; Thomas, E. L.; Wunderlich, B. Makromol. Chem. 1992, 193, 2589. (11) Wiesendanger, R. Scanning Probe Microscopy and Spectroscopy: Methods and Applications; Cambridge University Press: Cambridge, 1994.
© 1996 American Chemical Society
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Letters Scheme 1
In tapping mode atomic force microscopy (TMAFM), the cantilever is excited into resonance with a piezo driver and it only touches the surface very lightly and very briefly. The interactions between the tip and the sample lead to a modulation of the absolute value of both the resonance frequency and its amplitude, and the surface morphology is monitored by keeping this amplitude or frequency shift at a constant value. Using TMAFM, Van den Berg et al.12 have studied the phase separation process in thin films of commercial styrene-butadiene-styrene (SBS) triblock copolymers annealed in high vacuum. They have shown that the surface orientation of polystyrene cylinders depends on the film thickness in the 30-150 nm range. In such thin films, the observed surface microdomain morphology is directly related to the sample thickness. Therefore, we decided to study the surface morphology of thicker samples of radial triblock copolymers which are closer to the application purposes of such materials. In such systems, the surface phase separation depends on the chemical composition, sample preparation procedure, and bulk morphology. Here, we apply a recently-developed imaging technique, based on phase detection during tapping mode operation, to the surface characterization of a poly(methyl methacrylate)-block-polybutadiene-blockpoly(methyl methacrylate) triblock. In phase detection imaging (PDI), the phase lag of the cantilever oscillation, relative to the signal sent to the cantilever piezo driver, is monitored simultaneously to the topographic response. This approach provides both the amplitude and the phase difference, and the mapping of that phase during the analysis allows one to go beyond simple topographical imaging and to detect variations in composition. Compared to force modulation microscopy (FMM), which can also be used to study viscoelastic properties of polymer materials,13,14 PDI provides enhanced resolution (better than 10 nm vs 100 nm in FMM) and is less likely to induce damage on soft surfaces. (12) Van den Berg, R.; De Groot, H.; Van Dijk, M. A.; Denley, D. R. Polymer 1994, 35, 5778. Van Dijk, M. A.; Van den Berg, R. Macromolecules 1995, 28, 6773. (13) Maivald, P.; Butt, H. J.; Gould, S. A. C.; Prater, C. B.; Drake, B.; Gurley, J. A.; Elings, V. B.; Hansma, P. K. Nanotechnology 1991, 2, 103. (14) Nysten, B.; Legras, R.; Costa, J. L. J. Appl. Phys. 1995, 78, 5953.
To our best knowledge, the preliminary results presented in this work are the first example of TMAFM-PDI applied to the investigation of the morphology of block copolymers. With this technique, we have observed a dramatic improvement in the resolution of the microdomain segregation, compared to “classical” TMAFM images obtained on the same samples. II. Experimental Section The triblock copolymers were synthesized by classical anionic polymerization using a difunctional anionic initiator (DIBLi2) (see Scheme 1) prepared by addition of tert-butyllithium (t-BuLi) to 1,3-diisopropenylbenzene (DIB), as described elsewhere.15 Briefly, polymerization was carried out under a prepurified nitrogen atmosphere in a 2-L round-bottomed flask equipped with a rubber septum. Dried solvent and monomers were added into the reactor by using a stainless steel capillary or a syringe. Butadiene (BD) was polymerized overnight in cyclohexane (Chx)/ diethyl ether (DEE) (95/5 v/v) as a solvent at 20 °C. To the reaction vessel was added tetrahydrofuran (THF) containing a five-fold molar excess of diphenylethylene (DPE) compared to the anionic species. The polymerization solvent was then a 50/50 (v/v) THF/ cyclohexane mixture, which was cooled down to -78 °C for the polymerization of methyl methacrylate (MMA) and kept at this temperature for 1 h. A few drops of oxygen-free methanol was finally added to the reaction mixture, and the mixture was poured into a large volume of methanol. The precipitated copolymer was recovered by filtration and redissolved in Irganox 1010 (0.5 wt % copolymer) containing THF. THF was finally evaporated and the polymer dried under vacuum at 40 °C for 1 day. A P(MMA-b-BD-b-MMA) triblock with 16 000-b-65 000-b16 000 as the number average molecular weight and a final molecular weight dispersity of 1.1 was cast into thick films by pouring toluene/THF (40/60) solutions onto leveled glass plates which were kept away from dust as the solvent evaporated (the composition of the solvent mixture was selected on the basis of a systematic study to optimize the evaporation rate). The films were further dried under high vacuum until the samples reached a constant weight. The final film thickness was about 2 mm. As toluene and THF are good solvents for both PMMA and PBD, solvent effects are not considered to be responsible for the structure of the formed films. This means that the observed (15) Yu, J. S.; Dubois, Ph.; Je´roˆme, R.; Teyssie´, Ph. Macromolecules, in press. Yu, J. S.; Dubois, Ph.; Je´roˆme, R.; Teyssie´, Ph. J. Polym. Sci., Polym. Chem., in press.
Letters
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a
a
b
b
Figure 1. Tapping mode AFM image of a PMMA-b-PBD-bPMMA thick film (1.0 × 1.0 µm2 area): (a) Height image. The gray scale corresponds to 100 nm. (b) Phase image. The gray scale corresponds to 180°. O indicates the center of the zone around which the parallel structure is developed. I and II indicate the two distinguishable types of areas (see text for details).
Figure 2. Tapping mode AFM image of a PMMA-b-PBD-bPMMA thick film (300 × 300 nm2 area): (a) Height image. The gray scale corresponds to 50 nm. (b) Phase image. The gray scale corresponds to 180°. I and II indicate the two distinguishable types of areas (see text for details).
microdomain structure can be considered as a thermodynamic equilibrium morphology. All AFM images were recorded with a Nanoscope III microscope from Digital Instruments Inc. operated at room temperature in the tapping mode in air, using the microfabricated cantilevers provided by the manufacturer (spring constant of 30 N m-1). The system is equipped with the Extender electronics module to provide height and phase cartography. Images of each sample were taken at several locations, and the time for scanning was about 5 min. All images were made with the maximum available number of pixels (512) in each direction. The images presented here were culled from several recorded files. For image analysis, the Nanoscope image processing software was used. The images presented here were not filtered and are shown as captured. Repeated scans indicated that the observed structures were stable.
III. Results and Discussion Figure 1a shows a typical TMAFM height image of the surface of a PMMA-PBD-PMMA sample characterized by a 16 000-b-65 000-b-16 000 molecular weight. Extracting information on the microdomain morphology from this picture is not straightforward because the origin of the apparent height is not obvious in TMAFM. If the various surface elements are characterized by identical interactions with the cantilever tip, the measured surface would correspond to the actual topography. However, if
Figure 3. Schematic illustration of the surface microdomain structure of a PMMA-b-PBD-b-PMMA triblock copolymer film.
the interactions are different, as can be the case for polymers with different elastic moduli, the relation between the voltage detected by the photodetector and the physical height may not be direct. From the analysis of different height images, it appears that they are composed of elongated structures characterized by a lateral dimension of 30-40 nm. The limits of these domains are not well defined, but they can be seen almost everywhere on the height images (Figures 1a and 2a). From the same type of height images, Van den Berg et al.12 suggested that the softer phase is able to relax by protruding out of the surface of the film, as sketched in Figure 3. On the same basis, we propose that the brighter areas (marked I on Figures 1a and 2a) observed on our samples are related to domains of the soft phase, here PBD, while the darker areas (marked II) correspond to the hard phase, PMMA. By appropriate image processing, one can enhance the contrast between the two phases.12
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In Figure 1b, we present a typical result obtained when we mapped the phase difference between the piezo signal driving the oscillating tip and the response of the photodetector. The darker areas (II on Figures 1b and 2b) correspond to zerophase shift (the tip interacting with the surface remains in-phase with the piezo driver signal) while the white areas (I on Figures 1b and 2b) represent a 180° phase difference; that is, interaction with the surface makes the tip in phase opposition with the piezo signal. It must be noted that these images were captured simultaneously with the height images (Figures 1a and 2a). The contrast is significantly increased, and the surface morphology appears more clearly. Note also that the limits between the two phases are now much better defined in comparison to the height images (compare Figure 2a and b), as also observed with the same technique in the case of dust particles on silicon wafers.16 It is thus an important aspect of TMAFM-PDI that it is able to provide in real time the block copolymer morphology with a good contrast between the different phases. The phase image is composed of elongated bright structures with a width around 30 nm, separated by darker areas. These structures appear to be arranged concentrically around a given point marked O (Figure 1b) in the top-right part of the picture while the left part of the image seems to be less ordered. Image analysis indicates that bright areas typically represent approximately two-thirds of the surface while the darker zones occupy the remaining one-third. On the basis of the chemical composition of the studied samples (70% PBD and 30% PMMA), the white areas can be attributed to PBD while the darker ones are assigned to PMMA, as proposed from the height images. If we consider that the origin of the phase lag is mainly due to the interaction between the sample and the tip, the brighter the areas, the more important the interaction. In our view, the differences in viscoelasticity are likely responsible for the observed phase contrast, which therefore reflects the microdomain morphology. Due to the softness of PBD, the tip tends to remain longer in contact with that material and this leads to a delay in tip motion relative to the piezo signal. In contrast, the interaction with a harder surface, here PMMA, is expected to be shorter and the phase lag should remain close to zero. On this basis, the PBD phase will give rise to the brighter areas in the PDI-TMAFM images while the PMMA domains will appear darker. This is consistent with the interpretation deduced from the height data and the chemical composition. While it is well-known that various types of tip-sample interactions can affect the AFM signal in general,11 the phase contrast in PDI-TMAFM is thought to be mainly governed by viscoelastic properties.16 Note that a similar PDI-TMAFM image obtained on a pure homogeneous polymer surface shows no phase contrast. Figure 3 schematically illustrates the microdomain structure thought to be present at the sample surface. The fact that the rubbery phase (PDB) can expand out of the surface and partially cover the PMMA phase can explain why the limits of domains appear to be sharper on the phase image. As indicated in Figure 2a and b, the typical microdomain size assigned to PBD is larger in the height image than in the phase image (here 36 ( 2 nm vs 30 ( 2 nm). This is possibly due to the fact that, at the interface between two different domains (position C on Figure 3), the height is still larger than that above PMMA (positions B) but the elasticity, hence the phase lag, may be significantly reduced by the presence of hard PMMA beneath a thin PBD protrusion. Therefore, over positions (16) Chernoff, D. A. Unpublished results.
Letters
C, the phase signal would be typical of PMMA while the height would be closer to that of PBD (positions A). As mentioned before, the morphology of block copolymers essentially depends on their composition. Increasing the rubbery phase content (PBD in our case) of di- or triblock copolymers, one can observe successively a cubic structure of PBD spheres (up to 16% PBD), an orthorhombic structure formed by short rods of PBD (16-18%), a hexagonal structure formed by cylinders of PBD (1836%), a lamellar structure formed by alternating layers of PBD and PMMA (36-60%), an inverse hexagonal structure formed by cylinders of PMMA in a PBD matrix (60-80%), and finally an inverse cubic structure of PMMA spheres (over 80%).1 In the systems considered in this study, the composition (70% PDB) should correspond to a structure made of cylinders of PMMA in a PBD matrix; the dark elongated features appearing on Figures 1b and 2b thus correspond to cylinder-like PMMA domains. Those cylinders are mainly oriented parallel to the sample surface. The average repeat distance between the cylinders was determined from the 2D power spectrum (derived from the Fourier transform) of the image to be about 30 nm. This distance is in agreement with the value deduced from electron microscopy and small angle X-ray scattering (SAXS) measurements. These SAXS results will be presented in detail elsewhere.17 Note also that an average repeat distance of ca. 31 nm was reported by Van Dijk and Van den Berg12 for commercial SBS triblock copolymer (Kraton D-1102C, Shell Chemicals Europe) containing about 71% of PBD. It is however worth pointing out that the reported value has been determined from “flattened” and Fourier-filtered AFM pictures of thin films prepared by spin coating (thickness ranging from 40 to 150 nm). To summarize, we have shown that the use of phase detection imaging in tapping mode AFM of block copolymers significantly improves the quality of the data; this leads to a much better characterization of the surface morphology of thick layers of these compounds. By mapping the phase difference of the cantilever oscillation during the tapping mode scan, we obtained information concerning the viscoelastic properties of those block copolymers. These preliminary PDI-TMAFM data indicate that the hard segments of PMMA form cylindrical microdomains, with a typical diameter around 30 nm, that are dispersed in the elastomeric matrix of PBD. Acknowledgment. The authors are grateful to Emmanuel Lepleux (Instrumat, France) for technical assistance and helpful discussions. The research in Mons is supported by the Belgian Federal Government Office of Science Policy (SSTC) “Poˆle d’Attraction Interuniversitaire en Chimie Supramole´culaire et Catalyse”, the Belgian National Fund for Scientific Research FNRS/ FRFC, and an IBM Academic Joint Study. The research in Lie`ge is supported by the Belgian Federal Government Office of Science Policy (SSTC) “Poˆle d’Attraction Interuniversitaire: Polyme`res”. The collaboration between Mons and Lie`ge is partially supported by the European Commission (Human Capital and Mobility Network: Functionalized Materials Organized at Supramolecular Level). R.L. and P.D. are chercheurs qualifie´s du Fonds National de la Recherche Scientifique (FNRSsBelgium). LA9600964 (17) Sobry, R.; Van den Bossche, G.; Dubois, Ph.; Je´roˆme, R. To be published.
