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Square Grains in Asymmetric Rod-Coil Block Copolymers Bradley D. Olsen,† Michael F. Toney,‡ and Rachel A. Segalman†,* Department of Chemical Engineering, UniVersity of California, Berkeley, California 94720 and Materials Science DiVision, Lawrence Berkeley Laboratory, Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Menlo Park, California 94025 ReceiVed October 12, 2007 Unlike the rounded grains that are well known to form in most soft materials, square grains of microphase-separated lamellae are observed in thin films of a rod-coil block copolymer because of hierarchical structuring originating from the molecular packing of the rods. The square grains are oriented with lamellar layers parallel to the film interface and result from growth along orthogonal low-surface-energy directions as a result of the effects of the tetragonal crystalline lattice that forms within the rod-rich lamellar nanodomains of poly(2,5-di(2′-ethylhexyloxy)-1,4-phenylene vinylene)-b-polyisoprene (PPV-b-PI). These grain shapes form only for a narrow range of coil volume fractions around 72% as a result of kinetic barriers at lower coil fractions and disordering of the lattice at higher coil fractions, and the polydisperse grain size suggests that growth is nucleation-limited. The grains form in both weakly and moderately segregated polymers at all annealing temperatures below the order-disorder transition, and they are observed for all thicknesses at which parallel-oriented grains are grown.
Introduction Self-assembly of functional block copolymers on the 10 nm length scale1,2 provides an elegant path for fabricating and controlling nanoscale structures for organic electronics and solar cells3-5 or biotechnology.6 Optimization of the materials structure for these applications relies not only on nanoscale self-assembly but also on the details of domain orientation and grain size and shape in thin films. The incorporation of functional blocks such as semiconducting polymers or helical polypeptides into block copolymers fundamentally changes their physical behavior because of changes in the polymer topology and liquid crystalline interactions between the rodlike chains.7 The coexistence of microphase separation between the rod and coil blocks and liquid crystallinity causes nanoscale materials that self-assemble on dual length scales, resulting in non-classical block copolymer behavior in both bulk systems7-12 and thin films.13-18 * Corresponding author. E-mail
[email protected]. † University of California Berkeley and Lawrence Berkeley Laboratory. ‡ Stanford Linear Accelerator Center. (1) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: New York, 1998. (2) Segalman, R. A. Mater. Sci. Eng. R 2005, 48, 191-226. (3) de Boer, B.; Stalmach, U.; van Hutten, P. F.; Melzer, C.; Krasnikov, V. V.; Hadziioannou, G. Polymer 2001, 42, 9097-9109. (4) Sivula, K.; Ball, Z. T.; Watanabe, N.; Frechet, J. M. J. AdV. Mater. 2006, 18, 206-210. (5) Hulvat, J. F.; Sofos, M.; Tajima, K.; Stupp, S. I. J. Am. Chem. Soc. 2005, 127, 366-372. (6) Minich, E. A.; Nowak, A. P.; Deming, T. J.; Pochan, D. J. Polymer 2004, 45, 1951-1957. (7) Olsen, B. D.; Segalman, R. A. Macromolecules 2005, 38, 10127-10137. (8) Chen, J. T.; Thomas, E. L.; Ober, C. K.; Mao, G. P. Science 1996, 273, 343-346. (9) Radzilowski, L. H.; Carragher, B. O.; Stupp, S. I. Macromolecules 1997, 30, 2110-2119. (10) 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, 15481-15490. (11) Olsen, B. D.; Segalman, R. A. Macromolecules 2006, 39, 7078-7083. (12) Olsen, B. D.; Segalman, R. A. Macromolecules 2007. (13) Olsen, B. D.; Li, X.; Wang, J.; Segalman, R. A. Macromolecules 2007, 40, 3287-3295. (14) Heiser, T.; Adamopoulos, G.; Brinkmann, M.; Giovanella, U.; OuldSaad, S.; Brochon, C.; van de Wetering, K.; Hadziioannou, G. Thin Solid Films 2006, 511, 219-223. (15) Li, H. B.; Liu, Q. T.; Qin, L. D.; Xu, M.; Lin, X. K.; Yin, S. Y.; Wu, L. X.; Su, Z. M.; Shen, J. C. J. Colloid Interface Sci. 2005, 289, 488-497. (16) Park, J. W.; Thomas, E. L. AdV. Mater. 2003, 15, 585-588. (17) Park, J. W.; Thomas, E. L. Macromolecules 2006, 39, 4650-4653.
The combination of interactions present in rod-coil block copolymers also results in novel structures on macroscopic length scales. Like most soft materials, grains of coil-coil block copolymers grow into shapes that minimize surface energy, resulting in curved interfaces and elliptical or flattened disk-like grains. In lamellar block copolymers, grains grow into elliptical shapes in thin films or ellipsoids of revolution in the bulk.19-21 However, in a model rod-coil block copolymer system the liquid crystalline aligning interactions between rods produce nanodomains with a very high bending modulus, resulting in straight interfaces that bound the nanoscale self-assembled lamellar grain structures.13 In these previously reported polymers with nearly symmetric block compositions (∼50 vol % coil), the grains grew into irregular polygon shapes with irregular numbers of sides, angles between sides, and lengths of sides. When the number of edges is relatively large, the grains approximate the classically rounded shape that minimizes interfacial energy. Using the same model rod-coil system poly(alkoxyphenylenevinylene-b-isoprene) (PPV-b-PI), we show that tuning the coil volume fraction of this rod-coil block copolymer allows the crystalline packing of the rod block to drive the formation of large, regular square grains of nanoscale lamellar structures that are extremely unusual for soft materials. Experimental Section Poly(2,5-di(2′-ethylhexyloxy)-1,4-phenylenevinylene)-b-polyisoprene (PPV-b-PI) block copolymers were synthesized as described previously.7 The polymers reported here have total molecular weights and coil volume fractions of 11,600 g/mol and 72% (PPV-b-PI-72) and 19 300 g/mol and 73% (PPV-b-PI-73). The polydispersity of the PPV blocks ranged between 1.05 and 1.17, and the polydispersities of all the PI blocks were less than 1.05. Both of these polymers form lamellar structures in bulk, as reported elsewhere.7,12 Samples were prepared for scanning force microscopy (SFM) and grazing-incidence X-ray diffraction (GIXD) by spin casting polymers from toluene solution onto (100) silicon wafers with a (18) Stupp, S. I.; LeBonheur, V.; Walker, K.; Li, L. S.; Huggins, K. E.; Keser, M.; Amstutz, A. Science 1997, 276, 384-389. (19) Balsara, N. P.; Marques, C. M.; Garetz, B. A.; Newstein, M. C.; Gido, S. P. Phys. ReV. E 2002, 66, 052802. (20) Huang, E.; Mansky, P.; Russell, T. P.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Hawker, C. J.; Mays, J. Macromolecules 2000, 33, 80-88. (21) Fasolka, M. J.; Mayes, A. M. Annu. ReV. Mater. Res. 2001, 31, 323-355.
10.1021/la703174e CCC: $40.75 © 2008 American Chemical Society Published on Web 01/26/2008
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native oxide surface (University Wafer, Boston, MA). Concentrations from 5 to 40 mg/mL and spin speeds between 900 and 6000 rpm were used to achieve film thicknesses ranging from 7 to 350 nm. Prior to annealing, the films are structureless, and samples were annealed under vacuum for 24 h to induce ordering. Sample thicknesses were measured prior to thermal annealing using a Sentech SE400 ellipsometer with a 632.8 Å laser. SFM samples were analyzed on a Digital Instruments MultiMode AFM operating in tapping mode. GIXD experiments were performed at beamline 11-3 of the Stanford Synchrotron Radiation Laboratory. Experiments were performed using a 0.9736 Å X-ray beam with a width of 150 µm and a height of 50 µm. Data were recorded on a 2D MAR345 image plate detector (pixel size 0.15 mm) with a sample to detector distance of 433 mm. Data were acquired for 20 min per frame at an incident angle of 0.08° (above the critical angle for these films but below the critical angle for Si such that the X-rays penetrated throughout the depth of the polymer film but did not penetrate into the Si substrate). Scattering angles were converted into q space accounting for the planarity of the detector and using the direct beam as the reference beam.
Results and Discussion Thin films of rod-coil block copolymers show bimodal microdomain orientation where lamellar grains oriented parallel to the substrate are bounded by defect regions composed of perpendicularly oriented lamellae. For asymmetric polymers with coil fractions near 72%, unusual square grains are observed by SFM phase imaging, as shown in Figure 1. Each grain has exactly four sides, and all the sides of the square are approximately equal in length. Contrast between the PPV-rich (light) and PI-rich (dark) nanodomains originates from the difference in mechanical properties, and the parallel oriented grains appear dark because of segregation of the PI block to the vacuum interface.13 In regions where the square grains are densely packed, the grains align with one another. This grain alignment has previously been observed in coil-coil block copolymers where the kinetics of grain growth result in correlations between the microdomain orientation of neighboring grains, all of which are elliptical.22,23 The unusual square grain shapes in rod-coil block copolymers form independent of molecular weight and strength of segregation, as illustrated by their growth in both moderately segregated (higher molecular weight) and weakly segregated (lower molecular weight) polymers in Figure 1. Square grains form at all annealing temperatures below the order-disorder transitions of the block copolymers, although qualitatively larger grains are formed by annealing at higher temperatures, as illustrated by a comparison between Figure 1c and d. These square grain shapes indicate a broken symmetry within the lamellar structure that results in certain preferred directions for grain growth. The broken symmetry derives from the crystallinity of the rod block and drives the formation of these square shapes. The PPV rod blocks within these block copolymer films pack onto a tetragonal unit cell with 4-fold symmetry in the plane of the lamellae, as shown by the grazing-incidence X-ray diffraction pattern in Figure 2. Because the [110] direction of the rod lattice orients perpendicular to interfaces,24 it appears that this is the lowest energy interface of the crystal. Therefore, the lattice orientation within the squares also likely has the 110 directions oriented perpendicular to the sides of the squares. Unlike the grains oriented parallel to the film surface, the perpendicularly oreiented nanodomains do not intersect at regular (22) Garetz, B. A.; Balsara, N. P.; Dai, H. J.; Wang, Z.; Newstein, M. C.; Majumdar, B. Macromolecules 1996, 29, 4675-4679. (23) Newstein, M. C.; Garetz, B. A.; Balsara, N. P.; Chang, M. Y.; Dai, H. J. Macromolecules 1998, 31, 64-76. (24) Olsen, B. D.; Alcazar, D.; Krikorian, V.; Toney, M. F.; Thomas, E. L.; Segalman, R. A. Macromolecules 2008, 41, 58-66.
Figure 1. Square grains in PPV-b-PI rod-coil block copolymers with approximately 72 vol % coil block. Scanning force microscopy phase imaging reveals square grains oriented parallel to the film surface separated by regions of perpendicular lamellae. Contrast is due to differences in mechanical properties between areas covered by the hard PPV blocks (light) and soft PI blocks (dark). Parallel grains appear dark because of segregation of the sticky PI block to the vacuum interface. (a) Close-up image of square grains of PPVb-PI-72 (90 nm thick film, annealed at 80 °C) showing the structure of individual perpendicular rod nanodomains bounding the grain. (b) Larger image of grains in PPV-b-PI-72 (78 nm thick film, annealed at 80 °C) shows that squares are the dominant shape observed for parallel oriented grains in this material. The polydispersity of grain size suggests a nucleation-limited growth mechanism. (c) Square grains are also formed in a higher molecular weight, moderately segregated polymer PPV-b-PI-73 (139 nm thick film, annealed at 120 °C). (d) Increasing the annealing temperature to 160 °C for a 178 nm film results in the formation of much larger grains that appear to overlap one another. (e) Schematic of film cross-section showing parallel and perpendicular orientation of lamellar nanodomains. (f) Schemiatic of film cross-section showing slightly tilted lamellae that produce overlapping and partially buried square grains. The z scale is 30° for all images.
right angles, indicating that the preferred grain growth directions are not a result of low energy angles for lamellar intersections. When a large number of grains are examined, polydispersity in grain size and several characteristic types of defects in shape are observed. The fact that polydispersity in grain size is observed for both molecular weights and all annealing temperatures suggests that each grain has grown for a different amount of time, implying that grain growth is limited by nucleation. Decreasing annealing time from 24 to 12 h results in fewer parallel grains per unit area, consistent with nucleation-limited growth. In some grains one of the two low-energy directions grows faster than the other, resulting in extended rectangular grains, as shown in Figures 1b,d. The difference in grain growth rate may be due to the presence of other grains that restrict growth; however, the low-energy directions of the PPV crystal structure consistently result in grains with right angles and four sides. The intersection of two grains results in the truncation of the corner of a square,
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Figure 2. Grazing-incidence X-ray diffraction of square-forming PPV-b-PI block copolymer. The formation of square grains in PPVb-PI block copolymers is driven by the tetragonal lattice of the PPV rod block crystals. Grazing incidence X-ray diffraction of PPV-bPI-72 demonstrates crystal formation in these films, and the peak indexing is consistent with that of the alkoxy-PPV homopolymer. This 2D diffraction pattern was taken at an incident angle of 0.08° from a 117 nm film that displayed mixed parallel and perpendicular orientation of the lamellar nanodomains.
as shown in Figures 1a,b. In addition, overlapping square shapes and square grains partially covered by thin layers of perpendicular nanostructures are observed in the higher molecular weight polymer annealed at 160 °C, as shown in Figure 1d and schematically illustrated in Figure 1f. These large, overalapping grains in thin films demonstrate that the square shapes can be templated by just a single lamellar rod nanodomain, indicating that the grain shape is a property of the individual rod sheets and not the overall lamellar microphase.
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The observation of square grains only for coil fractions around 72 vol % coil block is due to the interplay between kinetic and thermodynamic effects. Polymers with a coil volume fraction of 67% or less form irregular polygon shaped grains with a variable number of straight sides and unequal side lengths, as previously reported for symmetric block copolymers.13 Polymers with coil volume fractions of 77% or more form highly disordered lamellar structures. The increased mobility in increasingly asymmetric polymers is due to the higher isoprene content. This allows the bounding lamellar interfaces to form into square geometries. However, too high of a PI content results in disordering of the PPV crystalline lattice,24 disrupting the broken symmetry that drives the formation of the square grains. The narrow region around 72-73% coil block represents a compromise between these kinetic and thermodynamic effects where the crystalline structure may effectively template grain growth. Square grain formation is also independent of film thickness in these asymmetric lamellar films. Like more symmetric polymers, increasing thickness of the films results in a reorientation of the structure at the vacuum interface from predominantly parallel lamellae to predominantly perpendicular grains, as shown in Figure 3. However, the increase in coil fraction results in a decrease in the number of lamellar layers required for reorientation to five. Square grains are observed for all thicknesses below the reorientation thickness. Because of symmetric wetting of the polyisoprene block at both the Si and vacuum interfaces,13 these films also form islands or holes for thicknesses not commensurate with integer multiples of the natural lamellar domain spacing. Square grains form both smooth films and films with islands or holes. As shown in Figure 3, the boundaries of island or hole regions are templated by the boundaries between parallel and perpendicular grains. In films less than a single monolayer, the
Figure 3. Scanning force microscopy images of PPV-b-PI-72 show a reorientation of the lamellar microstructures from parallel to perpendicular orientation with increasing film thickness. In films thicker than five monolayers, the entire vacuum interface is coated in perpendicular lamellae. Height images (a-c) have a 50 nm z scale, and phase images (d-f) have a 30° z scale. Reorientation is driven by the decaying surface orientation effect of the Si substrate with increasing film thickness. Square grains are observed for all thicknesses greater than a single monolayer and less than the reorientation thickness, regardless of the presence of islands or holes. Film thicknesses are 24.8 nm (a, d), 92.8 nm (b, e), and 130 nm (c, f).
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island or hole interfaces cause corners of the grains to become rounded and the grains to be distorted into irregular shapes with relatively few right angle corners and irregular length sides.
demonstrate multidimensional templated order ranging from the crystallinity of the rods (∼1 nm) to the rod-coil block copolymer lamellar self-assembly (∼10 nm) to the templated organization of the grains (∼1 µm).
Conclusions Regular square grain structures can be self-assembled by exploiting the hierarchical structure of rod-coil block copolymers, templating the structure of the rod crystalline lattice onto the larger length scale microphase separated structure. These square structures are formed reliably for a wide range of film thicknesses, annealing temperatures, and molecular weights, but they are only realized across a narrow range of coil fractions because of the interplay of thermodynamic and kinetic effects. These structures
Acknowledgment. We gratefully acknowledge support from a NSF-Career grant. X-ray experiments were performed at the Stanford Synchrotron Radiation Lab, operated by Stanford University and supported by the Department of Energy Office of Basic Sciences. B.D. Olsen gratefully acknowledges the Fannie and John Hertz Foundation for a graduate fellowship. LA703174E
