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Surface versus Confinement Induced Morphology Transition in Triblock Copolymer Films: A Grazing Incidence Small Angle Neutron Scattering Investigation P. Mu¨ller-Buschbaum,*,† E. Maurer,† E. Bauer,† and R. Cubitt‡ Physik-Department, TU Mu¨nchen, LS E13, James-Franck-Strasse 1, 85747 Garching, Germany, and Institut Laue-LangeVin, 6 rue Jules Horowitz, b.p. 156, 38042 Grenoble, France ReceiVed May 23, 2006. In Final Form: August 16, 2006 The internal nanostructure resulting from microphase separation in triblock copolymer films of polyparamethylstyreneblock-polystyrene-block-polyparamethylstyrene, P(pMS-b-Sd8-b-pMS), has been investigated with grazing incidence small angle neutron scattering (GISANS). X-ray reflectivity, grazing incidence small-angle X-ray scattering (GISAXS), optical microscopy and atomic force microscopy (AFM) complement the investigation. The influence of two limiting interfaces present in confinement is compared to the presence of only one surface. GISANS allows for the detection of structures in the very limited sample volume of confined films as well as for a depth sensitivity to probe the near free surface part of bulk films. With respect to the surface a perpendicular oriented lamella is observed. In contrast to the shrinkage of the characteristic lamellar spacing in confinement at the free surface, a slight increase is determined.
Introduction Block copolymers built from immiscible monomer units (A and B) has attracted long standing attention due to new possibilities emerging from the microphase separation process.1 Due to the chemical bond between the immiscible parts, block copolymers self-assemble into well-controlled nanoscale structures instead of commonly observed micron-sized structures. The morphology is selected by minimizing the free energy for the buildup of internal interfaces compared to the conformational entropy to adopt this morphology.2-5 In the bulk important parameters are (a) the product of the Flory-Huggins interaction parameter χ with the total degree of polymerization N, (b) the volume fraction f, (c) the copolymer architecture,6-8 (d) the conformational asymmetry,9 and (e) the fluctuation effects.10 In A-B diblock copolymers a rich variety of different structures ranging from spheres to cylinders to lamellae and to a more complex bicontinuous cubic phase have been reported. Increasing complexity by adding a third monomer unit C to an A-B-C triblock copolymer significantly enriches the number of possible structures and rather colorful phase diagrams result.11-14 In the case of the triblock copolymer being of the type A-B-A, the situation simplifies. In contrast to A-B diblock copolymers, the mean-field phase diagrams of A-B-A triblock copolymers are † ‡
TU Mu¨nchen. Institut Laue-Langevin.
(1) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: Oxford, U.K., 1998. (2) Fredrickson, G. H. Macromolecules 1987, 20, 2535. (3) Turner, M. S. Phys. ReV. Lett. 1992, 69, 1788. (4) Fischer, M. E.; Nakanishi, H. J. Chem. Phys. 1981, 75, 5857. (5) Tang, W. H. Macromolecules 2000, 33, 1370. (6) Leibler, L. Macromolecules 1980, 13, 1602. (7) Hodrokoukes, P.; Floudas, G.; Pispas, S.; Hadjichristidis, N. Macromolecules 2001, 34, 650. (8) Pochan, D. J.; Gido, S. P.; Pispas, S.; Mayes, J. W. Macromolecules 1996, 29, 5099. (9) Matsen, M. W.; Bates, F. S. J. Polym. Sci., Polym. Phys. 1997, 35, 945. (10) Fredrickson, G. H.; Helfand, E. J. Chem. Phys. 1987, 87, 697. (11) Alexandridis, P.; Spontak, R. J. Curr. Opin. Colloid Interface Sci. 1999, 4, 130. (12) Schmalz, H.; van Guldener, V.; Gabrielse, W.; Lange, R.; Abetz, V. Macromolecules 2002, 35, 5491. (13) Abetz, V.; Markgraf, K.; Rebizant, V. Macromol. Symp. 2002, 177, 139. (14) Bohbot-Raviv, Y.; Wang, Z. G. Phys. ReV. Lett. 2000, 85, 3428.
highly asymmetric as a result of the higher entropic penalty in deforming the central B blocks so as to accommodate the two outer blocks into the A domains. The phase diagram is dominated by the lamellar phase. For symmetric triblock copolymers with a high degree of polymerization (of the order of N ) 106), fluctuation corrections, using the angle-dependent Hartree approximation,15 do not give rise to an additional bcc phase of spheres. The corrected phase diagram allows only for direct transitions from the disordered to the lamellar and hexagonal phases. In the strong segregation regime no order-to-order transitions are possible. The presence of external surfaces and of confinement effects in ultrathin films may alter the morphology of materials strongly. In contrast to bulk samples, in a thin film geometry, the interactions with both boundaries influence morphology and orientation. In the case of complete wetting of each interface by one component of the triblock copolymer, a lamellar order parallel to these interfaces is forced,16-18 as observed in diblock copolymer films.19-23 If one interface is free, like a film surface against vacuum, the film thickness is quantisized in units of the lamellar bulk period Lo. If the initial film thickness is not commensurate with this constraint, in many systems an incomplete top layer is formed.24 This commensurability effect basically affects the morphology of the surface and islands or holes are created.25,26 (15) Mayes, A. M.; de la Cruz, M. O. J. Chem. Phys. 1991, 95, 4670. (16) Sakurai, S.; Aida, S.; Okamoto, S.; Ono, T.; Imaizumi, K.; Nomura, S. Macromolecules 2001, 34, 3672. (17) Finne, A.; Andronova, N.; Albertsson, A. C. Biomacromolecules 2003, 4, 1451. (18) Shin, D.; Shin, K.; Aamer, K. A.; Tew, G. N.; Russell, T. P.; Lee, J. H.; Jho, J. Y. Macromolecules 2005, 38, 104. (19) Anastasiadis, S. H.; Russell, T. P.; Satija, S. K.; Majkrzak, C. F. Phys. ReV. Lett. 1989, 62, 1852. (20) Russell, T. P.; Coulon, G.; Deline, V. R.; Miller, D. C. Macromolecules 1989, 22, 4600. (21) Stamm, M.; Go¨tzelmann, A.; Giessler, K. H.; Rauch, F. Prog. Colloid Polym. Sci. 1993, 91, 101. (22) Mansky, P.; Russell, T. P.; Hawker, C. J.; Mays, C.; Cook, D. C.; Satija, S. K. Phys. ReV. Lett. 1997, 79, 237. (23) Torikai, N.; Noda, I.; Karim, A.; Satija, S. K.; Han, C. C.; Matsushita, Y.; Kawakatsu, T. Macromolecules 1997, 30, 2907. (24) Mansky, P.; Russell, T. P.; Hawker, C. J.; Pitsikalis, M.; Mays, J. Macromolecules 1997, 30, 6810. (25) Stocker, W.; Beckmann, J.; Stadler, R.; Rabe, J. P. Macromolecules 1996, 29, 7502.
10.1021/la061455q CCC: $33.50 © 2006 American Chemical Society Published on Web 09/21/2006
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With decreasing film thickness the confinement gives rise to an interplay between the intrinsic length scale of the bulk structure and the geometry of the film.27-30 This leads to transitions between phases of identical symmetries but different orientations with respect to the confining walls. As an example, lamellar domains reorientate from a parallel to a perpendicular arrangement.31 Several different morphologies, deviating from the bulk structure near the surface and due to confinement, were found on silicon surfaces in the case of a polystyrene-b-polybutadieneb-polystyrene triblock copolymer, which had a cylindrical microdomain structure in the bulk.32 The near-surface morphology was found to deviate from the bulk lamellar morphology depending on the film thickness in the system of the lamellarforming triblock copolymer polystyrene-b-polybutadiene-b-poly(methyl methacrylate).33 In contrast, the strong polymer-polymer interactions of a poly(n-octadecyl methacrylate)-b-poly(tert-butyl acrylate)-b-poly(n-octadecyl methacrylate) triblock copolymer accounted for the good agreement between bulk and surface structures.34 Within the present investigation, the effect of the free surface and of confinement is revisited with advanced scattering experiments making use of grazing incidence small angle neutron scattering (GISANS).35-38 Whereas our previous investigations35-38 focused on structure creation introduced by the dewetting of a confined thin film, in the present investigation no dewetting is involved. The examined model system consists of the A-B-A triblock copolymer poly(p-methylstyrene)-bpolystyrene-b-poly(p-methylstyrene) with a fully deuterated central polystyrene block P(pMS-b-Sd8-b-pMS) and both components being nearly symmetrical in the number of monomer units. Due to the high molecular weight, the strong segregation regime is addressed. In contrast to other triblock copolymers, in the case of P(pMS-b-Sd8-b-pMS) from the AFM topography no information about the internal distribution of the blocks can be obtained. Because both monomer units, deuterated styrene (Sd8) and p-methylstyrene (pMS), differ only by one methyl group, despite the deuteration, their chemical and mechanical properties are very similar. Therefore surface characterization methods such as friction and stiffness measurements39 yield not enough contrast to distinguish between the components and a selective dissolution40 of one component is not possible. However, in the case of X-rays, contrast even for significantly different polymers is not large, whereas with neutrons strong contrast between two components can be generated by deuteration. For X-rays the scattering contrast between the two monomer units δ(Sd8)/ (26) Huang, E.; Mansky, P.; Russell, T. P.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Hawker, C. J.; Mays. J. Macromolecules 2000, 33, 80. (27) Turner, M. S. Phys. ReV. Lett. 1992, 69, 1788. (28) Geisinger, T.; Mu¨ller, M.; Binder, K. J. Chem. Phys. 1999, 111, 5241. and Geisinger, T.; Mu¨ller, M.; Binder, K. J. Chem. Phys. 1999, 111, 5251. (29) Koneripalli, N.; Levicky, R.; Bates, F. S.; Ankner, J.; Kaiser, H.; Satija, S. K. Langmuir 1996, 12, 6681. (30) Tang, W. H. Macromolecules 2000, 33, 1370. (31) Morkved, T. L.; Jaeger, H. M. Europhys. Lett. 1997, 40, 643. (32) Rehse, N.; Knoll, A.; Konrad, M.; Magerle, R.; Krausch, G. Phys. ReV. Lett. 2001, 87, 035505. (33) Krausch, G.; Magerle, R. AdV. Mater. 2002, 14, 1579. (34) Wu, W.; Huang, J.; Jia, S.; Kowalewski, T.; Matyjaszewski, K.; Pakula, T.; Gitsas, A.; Floudas, G. Langmuir 2005, 21, 9721. (35) Mu¨ller-Buschbaum, P.; Gutmann, J. S.; Stamm, M.; Phys. Chem. Chem. Phys. 1999, 1, 3857. (36) Mu¨ller-Buschbaum, P.; Gutmann, J. S.; Stamm, M.; Cubitt, R.; Cunis, S.; von Krosigk, G.; Gehrke, R.; Petry, W. Physica B 2000, 283, 53. (37) Mu¨ller-Buschbaum, P.; Wolkenhauer, M.; Wunnicke, O.; Cubitt, R.; Stamm, M.; Petry, W. Langmuir 2001, 17, 5567. (38) Mu¨ller-Buschbaum, P.; Cubitt, R.; Petry, W. Langmuir 2003, 19, 7778. (39) Krausch, G.; Hipp, M.; Bo¨ltau, M.; Mlynek, J. Macromolecules 1995, 28, 260. (40) Walheim, S.; Bo¨ltau, M.; Mlynek, J.; Krausch, G.; Steiner, U. Macromolecules 1997, 30, 4995.
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δ(pMS) ) 3.62/4.18 ) 0.9 is small, which makes them hardly distinguishable whereas a strong contrast results for neutrons with δ(Sd8)/δ(pMS) ) 20.14/4.91 ) 4.1. Consequently with X-ray scattering the triblock copolymer P(pMS-b-Sd8-pMS) basically can be regarded as a homopolymer without internal contrast whereas with neutron scattering the main scattering results from the deuterated component (Sd8) only. For the molecular weight of the investigated samples, Rg is of the order of 15 nm. Therefore confined triblock copolymer films have to be very thin in film thickness and, along this line, have an extremely small scattering volume. In addition, they have to be prepared on top of solid substrates to achieve mechanical stability. Both factors make the common transmission geometry disadvantageous. Even the use of a sample stack consisting of several equally prepared samples41 only gives rise to a scattering signal that contains a small contribution of the polymer film. One way to overcome this problem is the use of the reflection geometry as demonstrated.35-38 In this article, the influence of confinement versus surface effects is addressed. For this purpose GISANS measurements (at the ILL at beamline D22) at films of different thickness are compared with measurements of various penetration depths of the neutron beam. With decreasing incident angle the penetration depth of the neutron beam is reduced. As a consequence, the scattering depth (the probe thickness of the top part of the bulky film) is reduced as well. This allows for probing the free-surface induced morphology transition in triblock copolymer films. Not addressed is the regime of film thicknesses characterized by the presence of a lamellar ordering parallel to the surface. Experimental Section Sample Preparation. The triblock copolymer poly(p-methylstyrene)-block-polystyrene-block-poly(p-methylstyrene), denoted P(pMS-b-Sd8-b-pMS) was prepared anionically (Polymer Standard Service, Mainz). The nearly symmetric P(pMS-b-Sd8-b-pMS) had a fully deuterated polystyrene block and two equal sized protonated polyparamethylstyrene blocks. The degree of polymerization of the PSd block compared to the total chain was fPSd ) NPSd/N ) 0.51. From the molecular mass Mw ) 280 000 g/mol (narrow molecular weight distribution Mw/Mn ) 1.1) and the polymer-polymer interaction parameter of PSd and PpMS χ ) A + B/T with A ) -0.011 ( 0.002 and B ) 6.8 ( 1 K (42) a value of χN ∼ 29 can be calculated. Thus the investigated system belongs to the strong segregation regime. The radius of gyration of the unperturbed molecule in the bulk is Rg ) 15 nm and the bulk lamellar spacing is Lbulk ) 54 nm. To ensure reproducible preparation conditions of the thin triblock copolymer films as well as to enable the preparation of highly confined films, the native oxide covered Si(100) surfaces (MEMC Electronic Materials Inc., St. Peters, MO) were cleaned prior to the spin-coating. The cleaning includes 15 min at 80 °C in an acid bath consisting of 100 mL of 80% H2SO4, 35 mL of H2O2, and 15 mL of deionized water, rinsing in deionized water and drying with compressed oilfree nitrogen immediately before spin-coating (1950 rpm for 30 s). After the spin-coating homogeneous P(pMS-b-Sd8-b-pMS) films result. When the concentration of the polymer-toluene solution was varied, the film thickness varied between 0.7 and 231 nm as measured by X-ray reflectivity. The homogeneity of the films was checked with atomic force microscopy and from the small value of the surface roughness (of the order of 0.5 nm) obtained from the X-ray data. Instead of annealing of the prepared films, storage under toluene vapor atmosphere (pressure p ) 0.8po, temperature 296 K) was applied to allow the structure of the triblock copolymer to come into equilibrium. After 14 h storage the samples were quenched in ambient (41) Kumar, S. Private communication.
Morphology Transition in Triblock Copolymer Films air and examined as described below. Reproducibility was checked by the preparation of several identical samples. Optical Investigation. The sample surfaces were observed with optical microscopy using a Zeiss Axiotech 25H optical microscope with a magnification between 4 and 100. A Hitachi KP-D50 CCD camera recorded the micrographs. Grazing Incidence Small Angle Neutron Scattering. GISANS measurements were performed at the D22 beamline at the neutron reactor ILL (Grenoble). For comparison, in the common transmission geometry SANS experiments were performed in addition to the GISANS measurements, which make use of the reflection geometry. For GISANS the sample was placed vertically on a two-circle goniometer with a z-translation table.36 Extremely narrow crossslits with typical openings of mm and a large collimation distance of 17.6 m were used. The background was minimized due to the completely evacuated pathway except a small region of (10 mm in front and behind the sample. Experiments were carried out at a wavelength of 0.6 nm (wavelength selector, ∆λ/λ ) 10%). Details concerning the beamline are reported elsewhere.43 The scattered intensity at one fixed angle of incidence Ri is detected with a twodimensional detector (128 × 128 pixel array). Two large sampledetector distances were operated. At 17.6 m a resolution, in terms of smallest accessible wavevector component, of 4.45 × 10-3 nm-1 and at 14.4 m 5.44 × 10-3 nm-1 was achieved. The GISANS information is extracted from horizontal slices (with respect to the sample surface) taken at the critical angle of PpMS. Statistics of these slices are improved by integrating the intensity over the two neighboring detector lines corresponding to ∆qz ) (1.91 × 10-2 nm-1 and ∆qz ) (2.33 × 10-2 nm-1 for the two sample -detector distances, respectively. Typical counting times were of the order of hours to obtain reasonable statistics. Grazing Incidence Small-Angle X-ray Scattering. Complementary GISAXS experiments were performed at the BW4 beamline at the synchrotron HASYLAB (DESY, Hamburg). The selected wavelength was λ ) 1.38 Å. For further details concerning the beamline, see ref 44. A setup of high quality entrance slits and a completely evacuated pathway was used. The two entrance crossslits defined the beam divergence in and out of the plane of reflection to match the detector resolution. A sample-detector distance of 12.7 m allowed for a comparable high-resolution experiment (2.85 × 10-3 nm-1) to the GISANS measurements. A beam-stop in front of the detector was installed at the position of the specular peak to shield the detector. At one fixed angle of incidence Ri ) 0.397° the two-dimensional intensity distribution was recorded with a twodimensional detector consisting of a 512 × 512 pixel array. A detailed description of GISAXS as an advanced scattering technique is given in ref 45. X-ray Specular Reflectivity. With a laboratory X-ray source (Θ-Θ reflectometer Seifert XRD 3003TT) reflectivity measurements of the samples were performed. A Ge(110) channel cut crystal is used to monochromasize the beam (λ ) 1.54 Å). The sample was placed on a specially designed vacuum chuck and was measured in air. Reflectivity curves of the as prepared films exhibited wellpronounced fringes due to the low surface roughness of typically 5 Å. From a fit to the reflectivity data, the film thicknesses of the as prepared samples l were obtained.46-48 The density was close to the mean density of both components PS and PpMS. Due to the weak scattering contrast a resolution of the internal order was poorly achieved.49 (42) Schnell, R.; Stamm, M. Physica B 1997, 234-236, 247. (43) Bu¨ttner, H. G.; Lelievre-Berna, E.; Pinet, F. Guide to Neutron Research Facilities at the ILL; ILL: Grenoble, France, 1997; p 32. (44) Gehrke, R. ReV. Sci. Instrum. 1992, 63, 455. (45) Mu¨ller-Buschbaum, P. Anal. Bioanal. Chem. 2003, 376, 3. (46) Parratt, L. G. Phys. ReV. 1954, 95, 359. (47) Born, M.; Wolf, E. Principles of Optics, 2nd ed.; Pergamon Press: Oxford, U.K., 1964. (48) James, R. W. In The Optical Principles of the Diffraction of X-rays; OxBow Press: Woodbridge, CT, 1962. (49) Stamm, M.; Schubert, D. W. Annu. ReV. Mater. Sci. 1995, 25, 325.
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Figure 1. Schematic picture of the experimental setup used in the GISANS measurements. The incident angle with respect to the sample horizon is denoted Ri, the exit angle Rf, and the out-of-plane angle ψ. A two-dimensional detector is used to measure one complete set of diffuse scattering. As sketched, the diffusely scattered intensity in the scattering plane, the detector scan, exhibits a Yoneda peak50 and a specular peak as common features. Atomic Force Microscopy. Using a PARK Autoprobe CP atomicforce microscope the sample surface was probed. All measurements were performed in air at room temperature. The amplitude of the oscillation of the tip was calibrated with respect to the vertical position of the piezoelectric scanner. The distance calibration of the piezocontroller was performed with a standard gold grating. Each scanned micrograph consisted of 265 lines, scanned with 0.25 Hz up to 1.0 Hz. Several images were measured for each sample. Micrographs were recorded at different sample positions. Avoiding contact with the sample minimized the tip-induced sample degradation. The silicon gold coated conical cantilevers had resonant frequencies at approximately f ) 60 kHz and a spring constant of approximately 2.1 N m-1. At each individual sample position scans with different ranges from 0.5 µm × 0.5 µm to 10 µm × 10 µm were performed. From the raw data the background due to the scanner tube movement was fully subtracted to determine the values of the rms-roughness over the complete scan area. In addition to the topography data the error signal was recorded to probe possible surface islands.
Results and Discussion In contrast to the standard transmission geometry, in GISANS35-38 the probed surface is illuminated under a very shallow angle of incidence Ri. Figure 1 shows a typical sketch of the experimental setup. Note that in the sketch the complete setup is shown rotated by 90° to allow for a simple use of the terms horizontal and vertical (defined with respect to the sample surface). To define a coordinate system, the incident beam was directed along the x-axis and the sample surface is defined as the (x, y)-plane. Thus the (x, z)-plane denotes the plane of incidence and reflection and the condition for specular scattering is given by qx ) qy ) 0 and qz > 0, with the scattering vector b q ) (qx, qy, qz). The specular peak fulfils the specular condition (Ri ) Rf). Diffusely scattered intensity is observed for qx * 0 or qy * 0. Consequently, the two-dimensional detector (see Figure 1) basically contains diffusely scattered information. Vertical lines mainly carry a qz dependent information and GISANS is observed along the out-of-plane direction and satisfies the condition qy * 0.45 Whereas neutron reflectivity measurements have become routine, the number of GISANS experiments is still rather limited. In neutron reflectivity only the specular information is used, if there is no multidetector, and the intensity is measured as a function of the wave vector component qz perpendicular to the
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Figure 2. (a) SANS data measured in the common transmission geometry at a bulk-like P(pMS-b-Sd8-b-pMS) film. The direct beam is blocked by a square beam top. (b) GISANS data measured at the same sample by applying reflection geometry. The chosen incident angle is Ri ) 0.70°.
surface. As a consequence, the density profile perpendicular to the surface is probed and no lateral information is accessible. With a multidetector off-specular reflectivity is accessible, which is, however, limited to micron-sized structures. Thus neutron specular reflectivity allows one to detect a surface parallel orientation of a lamellar structure but not to probe an in-plane structure. In contrast, GISANS yields a lateral information due to the probed qy dependence. Therefore in GISANS a surface perpendicular orientation is detectable.35-38 Moreover, in the geometry shown in Figure 1, a surface parallel orientation of a lamella will cause the appearance of Bragg peaks along the vertical 2d detector direction. So, in summary, GISANS experiments are advantageous as compared to neutron reflectivity in case small lateral structures have to be probed. In the case the of small roughnesses (with ||qz|2 C(R)| , 1), which is fulfilled for the investigated triblock copolymer films (as revealed by X-ray reflectivity), a good approximation is that the GISANS signal is directly proportional to the diffuse scattering factor
S(q) )
∆F2 -[q2z +(q/z )2]σ2/2 2 e |qz| |qz|2
∫∫C(R) exp(-ıq|R) dR
(1)
with the density contrast ∆F and the rms-surface roughness σ. Thus the GISANS signal is directly proportional to the Fourier transform of the height-height correlation function C(R) of the effective surface.51 It is observable in y-cuts (horizontal slice) of the 2D intensity distribution (see Figure 1). A. Bulk Structure. To probe a bulk-like structure, very thick P(pMS-b-Sd8-b-pMS) films with thicknesses dsum ) 231 ( 1 nm are investigated. The given error bar accounts for the deviation of the film thickness between different samples as well as for the film thickness uncertainty of the fit to the X-ray reflectivity data. Films are directly investigated after preparation via spincoating by SANS and GISANS to demonstrate the differences between a transmission and a reflection geometry. Due to the large film thickness, in contrast to thinner films, in transmission geometry sufficient intensity is present. Figure 2 shows the twodimensional intensity as measured with (a) SANS and (b) GISANS. The SANS data exhibit a strong circular ring-shaped intensity maximum that is centered at the direct beam (blocked by the square beam stop in Figure 2a). This maximum results from the presence of a well-defined structure factor given by the bulk lamellar spacing Lbulk. The intensity around the ring is uniform. (50) Yoneda, Y. Phys. ReV. 1963, 131, 2010. (51) Salditt, T.; Metzger, T. H.; Brandt, Ch.; Klemradt, U.; Peisl, J. Phys. ReV. B 1995, 51, 5617.
From the absence of intensity maxima, the absence of a preferential orientation of the lamellae in the bulk film is concluded. Thus the lamella has a powder-like orientation. Similarly to the observations in diblock copolymer films of PSd8 and PpMS, immediately after spin-coating, a microphase separation structure is present.52,53 A higher order maximum is missing, emphasizing the absence of long ranged correlations at this stage of the self-organization process. The GISANS data show a more complicated intensity distribution. Figure 2b pictures the example of the twodimensional intensity distributions recorded at D22 at an angle of incidence above the critical angle of the triblock copolymer film and of the silicon substrate (Ri > Rc). In the case of rotationally isotropic samples, the scattering data are symmetric with respect to the qy ) 0 position (vertical center of Figure 2b). The characteristics of the scattered intensity are well separated: Specular peak (peak in the center of detector) and Yoneda peak (peak below the specular peak). Moreover, due to the large film thickness, even under this small angle of incidence a refracted transmission signal is detected. This transmission signal gives rise to a ring-shaped intensity maximum, very similar to the SANS experiment, which is elliptically deformed due to refraction effects. As a consequence, the two intensity rods, symmetrically placed with respect to the direct beam are caused by the bulk scattering, which is refracted through the silicon substrate and the half-ellipse of intensity is the remaining intensity part of the originally circular intensity maximum, which is refracted through the polymer surface. Due to the scaling by the Fresnel transmission functions, the corresponding intensities are enhanced at the positions of the critical angles. Due to the differences in the scattering contrast at the two interfaces (polymer film toward silicon substrate and polymer film toward vacuum) two different critical angles are observed. To summarize, in the reflection geometry the two-dimensional detector collects the ordinary GISANS signal, due to a surface scattering, together with a refracted bulk scattering, which is close to a SANS experiment with a 90° rotated sample orientation (with respect to the standard transmission geometry). For a deeper analysis selected line cuts are compared in Figure 3. In Figure 3a a line cut calculated from the radial averaged SANS data is displayed together with an out-of-plane cut from the GISANS data. The out-of-plane cut was performed at an exit angle Rf ) Rc(pMS) < Rc(Si) < Rc(Sd8). The ring-shaped intensity maximum causes the strong peak in the intensity of the SANS cut (marked with “A” in Figure 3a) (52) Giessler, K.-H.; Endisch, D.; Rauch, F.; Stamm, M. Fresenius J. Anal. Chem. 1993, 346, 151. (53) Mu¨ller-Buschbaum, P.; Gutmann, J. S.; Lorenz-Haas, C.; Mahltig, B.; Stamm, M.; Petry, W. Macromolecules 2001, 34, 7463.
Morphology Transition in Triblock Copolymer Films
Figure 3. (a) log-log plot of line cuts from SANS (triangles) and GISANS (spheres) data from bulk-like P(pMS-b-Sd8-b-pMS) films immediately after preparation. The intensity is displayed as a function of the wave vector component qy in the plane of the sample and perpendicular to the neutron beam. (b) log-log plot of out-of-plane cuts from GISAXS (triangles) and GISANS (spheres) data of bulklike films after toluene vapor exposure. The solid lines are model fits as explained in the text. The positions of the most prominent in-plane lengths are marked with arrows and denoted with A, B. The dashed lines indicate the different resolution limits in terms of smallest accessible wave vector component. The curves are shifted along the y-axis for clarity.
at a position close to the bulk lamellar spacing L/bulk. From a standard SANS fit to the data (solid line) a value of L/bulk ) 47 ( 3 nm is determined. In the GISANS out-of-plane cut an intensity maximum (marked with “A” in Figure 3a) originating from the same effect is also visible. The solid line is a fit in the framework of the distorted wave Born approximation (DWBA) assuming a random lamellar structure, a form factor with a Gaussian type of distribution and the resolution function of the experimental setup. Superimposed is the refraction corrected transmission fit yielding the strong maximum corresponding to L/bulk ) 47 ( 3 nm. As compared to the bulk value of Lbulk ) 54 nm, immediately after preparation, the microphase separation structure is squeezed matching the observation from diblock copolymer films.52,53 The rapid spin-coating procedure does not allow for the bulk equilibrium structure; however, due to the strong segregation regime addressed by the chosen copolymer units, self-organization starts already during spin-coating. Due to the increased resolution in a reflection geometry (GISANS) as compared to a transmission geometry (SANS)54 a second structural feature is observed in the out-of-plane cut. A weak intensity maximum occurs at smaller qy values (marked with “B” in Figure 3a). We attribute the extracted in-plane length L/B ) 300 ( 20 nm to a superstructure, perhaps indicating the domain size. Within the domains the lamellae are well oriented, whereas the orientation is random between different domains. From optical microscopy and AFM the presence of islands at the surface can be excluded. Figure 4 shows typical AFM and optical microscopy data. No dominant surface feature with an in-plane length on the order of L/B is visible. It should be noted that the actual number characterizing the improvement of the experimental resolution by changing from the transmission to the reflection geometry will depend on the detailed experimental settings. In the presented example the number was 6.5. No attempt was undertaken to independently improve the individual setups with respect to resolution, to demonstrate the general fact: In transmission the very small (54) Mu¨ller-Buschbaum, Casagrande, M.; Gutmann, J. S.; Kuhlmann, T.; Stamm, M.; Cunis, S.; von Krosigk, G.; Lode, U.; Gehrke, R. Europhys. Lett. 1998, 42, 517.
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scattering vectors are shadowed by the beam stop, due to the intrinsic need to protect the detector against the direct beam. In reflection, the out-of-plane cut is selected far away from the direct beam and thus the size of the beam stop and not the detector resolution itself determines the experimental resolution.45 In Figure 3b the effect of 14 h toluene vapor storage is shown. As compared to Figure 3a, the peak position in the GISANS data is shifted. During toluene vapor storage the polymer films are swollen by the toluene molecules and thereby plasticized. The structure created by spin-coating can relax toward an equilibrium structure, quite comparable to the situation of annealing above the glass transition temperature. In contrast to annealing, the vapor storage acts quicker. Due to the small differences in the solubilities of the different blocks of the block copolymer additional morphological transitions are not introduced. After the toluene vapor exposure a value of Lbulk ) 54 nm is calculated, which matches the bulk equilibrium value. However, the random orientation of the lamellar domains remains, which is concluded from the presence of the ellipsoidal shaped intensity maximum in the two-dimensional GISANS data (not displayed, very similar to Figure 2b). Moreover, in Figure 3b the different scattering contrast between X-rays and neutrons is visualized. GISANS data are compared to GISAXS data by two out-of-plane cuts performed at the same qz value. Whereas in the neutron data a strong peak (marked with “A” in Figure 3b) from the ordered bulk of the film is probed, in the X-ray data only a very broad and weak peak of ill defined position remains. Moreover, the second length scale, detectable in the GISANS data, is not present in the GISAXS data at all, although both experimental resolutions are very similar (the resolution in the GISAXS experiment was even better than in the GISANS experiment). B. Confined Thin Films. To address the effect of confinement, the P(pMS-b-Sd8-b-pMS) film thickness was varied between 0.7 and 231 nm. In Figure 5 X-ray reflectivity data of films with different thicknesses are displayed together with the model fit. A large qz range was covered with a high resolution ∆qz to account for the big thickness variation. As compared to the radius of gyration of the unperturbed molecule in the bulk (Rg ) 15 nm), a confinement of 0.7/15∼0.05 was achievable. Films of smaller thickness were no longer homogeneous and exhibited a typical dewetting pattern after the spin-coating already. Therefore 0.7 nm was the smallest film thickness addressed within this investigation. Different chemical surface treatments applied to the Si substrates might enable thinner films. As visible in Figure 5 within the full covered film thickness range, the reflectivity data exhibit well-pronounced fringes due to the small surface roughness. The critical angle of total external reflection as revealed by the X-ray reflectivity corresponds to the averaged electron density of Sd8 and pMS. Figure 4 shows typical optical microscopy and AFM data of the bulk-like films. The confined films exhibit very similar surface features. On the surface no islands are present, which is consistent with a surface roughness of the order of 0.5 nm. In contrast to our earlier work dealing with homopolymer, polymer blend and diblock copolymer films,35-38 in the presented film thickness regime no dewetting of the triblock copolymer films occurs. A stabilization against dewetting was found in the diblock copolymer P(Sdb-b-pMS) at larger film thickness only.37,38 Thus the increased rigidity of the triblock copolymer as compared to that of the diblock copolymer seems to shift the stability regime against dewetting toward thinner films. The absence of Bragg peaks in the X-ray reflectivity data is a further direct proof of a missing perpendicular order of the P(pMS-b-Sd8-b-pMS) film. Already the weak X-ray
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Figure 4. (a), (b) AFM topography, (c) AFM error signal, and (d) optical microscopy micrographs showing the representative surface of the thick triblock copolymer film before (a), (b) and after (c), (d) toluene vapor storage.
Figure 5. X-ray reflectivity data (dots) displayed together with model fits (solid lines). From the bottom to the top the film thickness increases: 0.7, 27.0, 71.8, and 230.8 nm. For clarity the data are successively shifted along the y-axis. The inset shows a small qz range to illustrate the high-frequency oscillation in the reflectivity data of the thickest P(pMS-b-Sd8-b-pMS) prepared.
scattering contrast between the monomer units Sd8 and pMS would at least cause broad and weak Bragg peaks. As shown by GISANS, the lamellae are randomly oriented inside the bulklike film. In the case of the confined films, the film thickness dsum < 2Lbulk does not permit a lamellar polymer stacking, which is oriented parallel to the substrate surface. In the case of confined thin films, in transmission geometry no scattering signal from the polymer structure was detected and only with GISANS was such a signal measured. In GISANS due to the small incident angle the probed sample volume is significantly larger than that in SANS. Figure 1 shows the example of the two-dimensional GISANS intensity distributions recorded at an incident angle Ri > Rc above the critical angle of the triblock copolymer film and of the silicon substrate. Data of the strongest confined film (film thickness 0.7 nm) are presented. Due to the isotropy (with respect to sample rotation) again the scattering data are symmetric with respect to the qy ) 0 position (horizontal
Figure 6. (a) log-log plot of horizontal cuts from GISANS data from samples stored for 14 h under toluene atmosphere. The intensity is displayed as a function of the wave vector component qy in the plane of the sample and perpendicular to the neutron beam. From the bottom to the top the prepared film thickness increases (dsum ) 0.7, 27.0, 71.8, and 230.8 nm). The solid lines are model fits within the DWBA as explained in the text. The dashed line indicates the resolution limit. All curves are shifted along the y-axis for clarity. (b) Lamellar spacing Lo normalized by the bulk value Lbulk as a function of the total triblock copolymer film thickness dsum. The solid line is a guide to the eye.
center of Figure 1). The characteristics of the scattered intensity are well separated: specular peak (peak in the center of detector) and Yoneda peak (central peak below the specular peak with two separated side maxima). The presence of well-pronounced intensity maxima in the Yoneda region, right and left to the central line of the two-dimensional detector, is a clear fingerprint of a well-ordered structure oriented in the plane of the sample (parallel to the substrate and film surface). In the case of thin copolymer films, with symmetric block ratio, this ordering is well-known to result from a so-called perpendicular lamella. Due to the very small film thickness, the copolymers cannot adopt a parallel (to the substrate surface) orientation of the lamella. In the bulk the lamellar spacing is Lbulk ) 54 nm and a squeezing of the lamellar thickness in only possible to a very limited extent.53,55 Instead of handling the full two-dimensional intensity distributions, we again restrict to selected cuts to display the impact of confinement on the thin film structure. In Figure 6a a log-log (55) Giessler, K. H.; Rauch, F.; Stamm, M. Europhys. Lett. 1994, 27, 605.
Morphology Transition in Triblock Copolymer Films
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plot of the intensity distribution of out-of-plane scans measured at qz ) 1.73 nm-1 is shown. The presented cuts were performed at exit angles Rf ) Rc(pMS) < Rc(Si), which corresponds to exit angles below the critical angle of Sd8, too. The experimental resolution is marked with the dashed line. The Fresnel transmission functions act in this geometry only as overall scaling factors. The solid lines are DWBA based fits assuming structure and form factor as well as the resolution of the experimental setup. As visible in Figure 6a, independent of the film thickness, all out-of-plane cuts exhibit intensity maxima well inside the range of resolvable lateral lengths. Whereas the maximum is very intense in the case of the thick, bulk-like P(pMS-b-Sd8-b-pMS) film, its intensity decreases with decreasing film thickness. However, even in the case of only 0.7 nm film thickness a weak but visible peak is seen. It has to be noted that 8 h counting time was necessary even at the beamline D22, which provides a very high neutron flux. With decreasing value of the film thickness, the intensity maximum shifts in its position toward larger values of the wave vector component qy (in the plane of the sample and perpendicular to the beam). This shift resembles the shrinkage of the most prominent in-plane length of the ordered structure. Due to the missing real space information as well as the absence of higher order intensity maxima in the out-of-plane cuts, we interpret this shrinkage as a decrease of the lamellar spacing Lo, rather than a morphological transition due to confinement. Such a transition, e.g., from a lamellar to a cylindrical structure would introduce a change in the most prominent in-plane length as well. In the case of other triblock copolymer systems such transitions have been reported in the literature. For example, several different structures were found for a polystyrene-b-polybutadiene-bpolystyrene (SBS) triblock copolymer (with a cylindrical microdomain structure in the bulk) on a Si substrate. As a function of film thickness, the morphology of SBS changes from perforated layers to cylinders with parallel and perpendicular orientation followed by a disordered phase in the thinnest part of the film.56 In contrast to SBS, in the system P(pMS-b-Sd8-b-pMS) order is still present in very thin films. From the fit to the data the lamellar spacing Lo is extracted. To emphasize the confinement induced shrinkage, Lo is displayed normalized by the value of the bulk lamellar spacing Lbulk as a function of the total triblock copolymer film thickness dsum (see Figure 6b). The solid line is a guide to the eye. Obviously, by very strong confinement, a shrinkage of more than 30% results. The interaction between polymer molecules and an impenetrable surface can mediate strong deviation from the bulk behavior. The bulk conformational properties of these polymer chains are strongly modified in contact with the surface due to a subtle competition between the loss of entropy at the surface and the gain of internal energy. In a limiting case this might result even in a collapse of the copolymer to a compact phase.57,58 In general, contributions from wetting of an interface, a nematiclike ordering of stretched chains at a wall, and the enrichment of free end segments near a film interface need to be accounted for in the free energy density.59 However, a quantitative analysis is difficult due to the lack of knowledge about the parameters entering in the free energy density. In a qualitative analysis the degree of polymerization N allows for a simple distinction in a surface parallel lamellar orientation for low N and a surface
perpendicular lamellar orientation for large N.60 The observed perpendicular orientation in the case of confined thin films is in agreement with these considerations. In the bulk, block copolymer chains are stretched, which causes an entropic loss with respect to the Gaussian conformation. In contact with a hard wall, alignment along the wall can stabilize the stretched conformation by lowering the entropic loss. Thus a shrunken lamellar spacing of the perpendicularly aligned lamellae resembles a reduced stabilization by the substrate and a tendency toward a more Gaussian-like conformation. In contrast to the strong stretching, which was reported in the case of ultrathin polystyrene-block-poly(2-vinylpyridine) diblock copolymers,61 in the case of shrinkage, there is no gain in adsorption energy. Nevertheless, a quantitative understanding of the dependency shown in Figure 6b remains not possible and will require further experiments that address, for example, a variation of the substrate material. C. Surface Induced Structure. The strength of the surface field and the deformability of the bulk structure determine how the system rearranges in the vicinity of a surface. This causes either an orientation of the bulk structure or the formation of a surface reconstruction. The stability regions of the different phases are modulated by the film thickness via interference and confinement effects. The most intriguing effect of the presence of interfaces is a deviation from the bulk microdomain structure in the vicinity of one interface. This effect is best seen at large film thicknesses, and to work out the surface contribution part, we revisit the bulk-like P(pMS-b-Sd8-b-pMS) film with a surface sensitive scattering experiment. Upon variation of the incident angle in the GISANS experiment a limited surface sensitivity can be achieved. Well-known from X-ray experiments62 at incident angles smaller than the critical angle, a limited penetration of the beam inside the sample results. With
(56) Knoll, A.; Horvat, A.; Lyakhova, K. S.; Krausch, G.; Sevink, G. J. A.; Zvelindovsky, A. V.; Magerle, R. Phys. ReV. Lett. 2002, 89, 035501. (57) Orlandini, E.; Seno, F.; Stella, A. L. Phys. ReV. Lett. 2000, 84, 294. (58) Singh, Y.; Kumar, S.; Giri, D. J. Phys. A 1999, 32, L407. (59) Pickett, G. T.; Witten, T. A.; Nagel, S. R. Macromolecules 1993, 26, 3194.
(60) Busch, P.; Posselt, D.; Smilgies, D. M.; Rheinla¨nder, B.; Kremer, F.; Papadakis, C. M. Macromolecules 2003, 36, 8717. (61) Spatz, J. P.; Mo¨ller, M.; Noeske, M.; Behm, R. J.; Pietralla, M.; Macromolecules 1997, 30, 3874. (62) Dosch, H. Critical Phenomena at Surfaces and Interfaces; SpringerVerlag: Berlin, 1992.
D)
λ
x2π
∑ [x(Rj2 - Rc2)2 + 4β2 - (Rj2 - Rc2)2]-1/2
(2)
j)I,f
the scattering depth D of the neutrons is calculated using the bulk critical angle Rc ) x2δP(pMS-b-Sd8-b-pMS) of the mean refractive index of P(pMS-b-Sd8-b-pMS). Figure 7 shows this scattering depth normalized by the film thickness as a function of the incident angle. At the critical angle D increases very strongly and the full triblock copolymer film is probed. In a series of GISANS experiments, different incident angles were chosen: Ri ) 0.067, 0.162, 0.306, 0.417, 0.465 and 0.719° (marked with arrows in Figure 7). The first two angles are smaller than any critical angle of the materials under investigation, the third one is below the critical angles of the mean bulk P(pMSb-Sd8-b-pMS) and Sd8 but above the critical angles of pMS and Si, the fourth is only below the critical angle of the deuterated monomer Sd8 and the sixth and above all involve critical angles. As Figure 7 shows, the smallest probable surface near depth is 32 nm and a higher surface sensitivity is not achievable in the neutron scattering experiment. Upon increasing the incident angle the probed depth was increased to 41 nm (see inset of Figure 7) before the full film was illuminated. Again, instead of full two-dimensional intensities, we restrict to selected cuts. In addition to the out-of-plane cut, typical for
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Figure 7. Scattering depth D normalized by the thickness of the bulk-like triblock copolymer film dsum as a function of the incident angle Ri. Selected incident angles at which individual GISANS experiments were performed are marked with arrows. The inset shows a zoomed in part emphasizing on the region smaller than the bulk critical angle.
Figure 8. (a) log-log plot of horizontal cuts from GISANS data at the central position of the beam stop. (b) log-log plot of outof-plane cuts from GISANS data at the critical angle. The samples were stored for 14 h under a toluene atmosphere. The intensity is displayed as a function of the wave vector component qy. From the bottom to the top the incident angle decreases (Ri ) 0.067, 0.162, 0.306, 0.417, 0.465, and 0.719°). The solid lines are model fits as explained in the text. The dashed line indicates the resolution limit. All curves are shifted along the y-axis for clarity.
GISANS experiments,35-38 a horizontal cut to the center of the beam stop is chosen. Such a cut shows the transmitted neutron intensity free of refraction effects because it obeys qz ) 0 and refraction only occurs in the qz-direction. In Figure 8a the corresponding cuts are shown. From the bottom to the top the incident angle decreases and the solid lines are fits with a standard SANS model of randomly oriented lamella. The probed intensity addresses the bulk structure, although the incident angle is small, as explained in the first part of this section. From the fit to the data the bulk lamellar spacing Lbulk is determined again. A penetration of the neutron beam through the bulk of the sample causes an intensity increase. This immediately explains the absence of any scattering signal for the two incident angles smaller than all critical angles of the related materials (pMS, Si, and Sd8). In the case of such a small incident angle, the neutron beam is not penetrating the triblock copolymer film. The out-of-plane cuts are collected in Figure 8b. As explained, in case the bulk film is penetrated, these cuts are composed of
Mu¨ller-Buschbaum et al.
surface scattering and a bulk scattering contribution, giving rise to strong intensity maxima. With a variation of the incident angle, the position of these maxima change due to the refraction effect and have not to be confused with a change in the lamella spacing. With decreasing incident angle the intensity of this maximum arising from bulk-like scattering increases, due to the projection of the neutron beam, an increasing film volume is probed. This tendency is stopped at the moment, at which the neutron beam can no longer penetrate the film due to total external reflection. Consequently, these out-of-plane cuts contain only structural information from the surface and no bulk scattering signal. The probed polymer volume significantly drops, which explains the reduced intensity. However, the presence of intensity maxima in these out-of-plane cuts can only be explained by the existence of a well-ordered surface lamellar structure. This structure is a perpendicularly oriented lamella (with respect to the substrate surface), which caps the powder-like oriented structure in the bulk at the free surface. Obviously, wetting of the free surface by one of the blocks, PSd8 or PpMS, is not introducing a parallel orientation. However, the difference in surface tensions γ(PpMS)/γ(PS) ) 0.8663 suggests that the surface is covered with PpMS and the perpendicular lamella is possibly buried under a PpMS layer. Although a lamellar orientation perpendicular to the surface was observed in the confined thin films as well as at the free surface, both lamellae differ in the lamellar spacing. The value of the lamellar spacing calculated from the DWBA based fit to the data in the case of the free surface is Lo ) 1.05Lbulk. Thus in the surface part of the bulk triblock copolymer film the ordered structure is slightly stretched as compared to that of the bulk. The percentage of expansion nevertheless is only 5%. It does not depend on the small changes in the probed scattering depth but is observed in all three experiments with Ri < Rc(P(pMS-bSd8-b-pMS)). The free surface acts as a wall, which orients the copolymer chains as well. However, its interaction differs from the hard wall interaction of the substrate. The penalty in conformational entropy due to the increased stretching is balanced by the gain in adsorption energy.
Summary In a thin film situation two constraints, the surface field and the film thickness, are simultaneously present. The surface field can either orient the bulk structure or stabilize deviations from bulk structures, such as wetting layers, and lamella, which are identified as surface reconstructions. Within a wide range, the film thickness is modulating the stability regions of the different phases via interference of surface fields. The presented work focuses on triblock copolymer films and compares the effect of the free surface with the effect introduced by confinement. Due to the special selected model system P(pMS-b-Sd8-bpMS) the powerful real space characterization techniques based on scanning probe ideas are no longer applicable and scattering turns out to be the probe of choice. The weak contrast between the monomers Sd8 and pMS in X-ray scattering, causes neutron scattering experiments to be superior. In addition, the extremely limited sample volume in confined films forces a reflection geometry (GISANS), whereas in the case of thick films standard transmission and reflection geometry experiments have been compared. The intrinsically higher resolution of the reflection geometry might be a strong argument for GISANS investigations (63) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; John Wiley & Sons: New York, 1989.
Morphology Transition in Triblock Copolymer Films
replacing SANS measurements for other thick film samples as well. Moreover, it should be pointed out that the reflection geometry not only is limited to solid support based films but also can be extended to free-standing films or foils as well.64 A lamellar stacking of the triblock copolymer oriented parallel to the substrate surface is not observed in all kind of samples. In bulk-like thick copolymer films the lamella are arranged in randomly oriented domains, which are capped by a surface perpendicular lamella. The value of the lamellar spacing in the capping layer is slightly larger than the bulk value. Therefore the adsorption energy at the free surface favors a slight stretching of the chains as compared to the bulk. In contrast, in the confined thin films a stronger deviation from the bulk structure is observed. In good agreement with a qualitative understanding based on the degree of polymerization, the lamella exhibits an orientation perpendicular to the substrate surface. However, the value of the (64) Schneider, G. J.; Mu¨ller-Buschbaum, P.; Po¨pperl, T.; Maurer, E.; Bauer, E.; Gehrke, R.; Go¨ritz, D. HASYLAB Annu. Rep. 2004, 383.
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lamellar spacing is significantly reduced and in the strongest confinement introduced in this investigation a shrinkage of more than 30% results. Consequently, the polymer chains deviate less from the Gaussian conformation than they do in the bulk. So, to compare, the free surface and confinement introduce two different situations, which affect the size of the lamella in opposite directions. Instead of a reduced lamellar spacing as observed in the confinement situation, the spacing is increased at the free surface. This might be explained by the absence of a second ordering interface in case the free surface is probed. Acknowledgment. We thank B. Toperverg and H. Lauter for stimulating discussion. S. Cunis helped by setting-up the BW4 beamline at the HASYLAB for GISAXS experiments and G. Schneider and T. Po¨pperl participated the GISAXS measurements. We obtained financial support by the DFG (Mu1487/4) and within the HASYLAB project II-03-025. LA061455Q
