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Langmuir 1998, 14, 4860-4864
Phase Separation of Polystyrene and Bromo-Polystyrene Mixtures in Equilibrium Structures in Thin Films D. Slep,†,‡ J. Asselta,‡ M. H. Rafailovich,*,‡ J. Sokolov,*,‡ D. A. Winesett,§ A. P. Smith,§ H. Ade,§ Y. Strzhemechny,| S. A. Schwarz,| and B. B. Sauer⊥ Hilord Chemical Corp., Hauppauge, New York 11788, Department of Material Science, State University of New York at Stony Brook, Stony Brook, New York 11974, Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, Physics Department, Queens College, Flushing, New York 11367, and E. I. Dupont de Nemours and Company Inc., Experimental Station, Wilmington, DE 19880 Received April 10, 1998. In Final Form: June 17, 1998 Scanning transmission X-ray microscopy (STXM), atomic force microscopy (AFM), and dynamic secondary ion mass spectrometry (DSIMS) were used to obtain the true three-dimensional concentration profiles of polystyrene (PS) and bromo-polystyrene (PBrx)0.79S where x ) fraction of monomers brominated) blend films as a function of PBrx)0.79S concentration. Upon annealing, it is found that the PBrS becomes encapsulated by the PS. The encapsulation provides for a continuous PS phase for all blend compositions and explains the observed structures that are formed for different PBrS volume fractions. The encapsulation allows us to estimate the dispersive contribution of the PBrS surface energy to be less than 20.6 dyn/cm.
1. Introduction Polystyrene PS and poly(p-bromo)styrene (PBrxS) phase separation has been studied previously by Strobl et al.1 who demonstrated that for high degrees of bromination the two polymers are completely immiscible. Reich et al.2 have shown that the deformable nature of the vacuum interface permits the formation of distinct three-dimensional topographies driven by phase separation in binary polymer blend films. The spreading condition for a polymer blend is much more complex than that of a simple homopolymer film. Although the two components separately wet the surface, their interfacial tension can destabilize the film. There are two situations where this can occur. The first is where the difference in surface tensions between the two immiscible blend components is small. The second is where the difference in surface tension is large but the same component segregates at both the vacuum and substrate interfaces. Karim et al.3 demonstrated that, in the second situation, the phase separation between two polymers manifests itself in surface roughening. The morphology of PS/PBrx)1S thin films spun on Si wafers that occurs immediately after spinning in a cosolvent of the PS and PBrS has been studied by Affrossman et al.4 The spinning is a dynamic process where nonequilibrium structures become cast as the solvent evaporates. The rate of solvent evaporation can play a role in topographic modulation. In this paper, we concentrated on the late * To whom correspondence should be addressed. † Hilord Chemical Corp. ‡ State University of New York at Stony Brook. § North Carolina State University. | Queens College. ⊥ E. I. Dupont de Nemours and Co. Inc. (1) Strobl, G. R.; Bendler, J. T.; Kambour, R. P.; Shultz, A. R. Macromolecules 1986, 19, 2683. (2) Reich, S.; Cohen, Y. J. Polym. Sci., Polym. Phys. Ed. 1981, 19, 1225. (3) Karim, A.; Slawecki, T. M.; Kumar, S. K.; Douglas, J. K.; Satija, S. K.; Han, C. C.; Russell, T. P.; Lui, Y.; Overney, R.; Sokolov, J.; Rafailovich, M. H. Macromolecules 1998, 31, 857. (4) Affrossman, S.; Henn, G.; O’Neill, S. A.; Pethrick, R. A.; Stamm, M. Macromolecules 1996, 29, 5010.
stage morphology of thin films of a similar blend, PS and PBrx)0.79S, after annealing above the glass-transition temperature of both components. The resulting surface of the film of the two polymers was imaged with an atomic force microscope (AFM). Since the frictional and mechanical properties of PS and PBrS are similar, lateral force microscopies cannot be used to distinguish between chemical differences.5 We therefore employed scanning transmission X-ray microscopy (STXM),6,7 a technique that can discern lateral chemical differences with a spatial resolution of 400 Å. The concentration of the component normal to the substrate surface was profiled with dynamic secondary ion mass spectrometry (DSIMS), and a comprehensive three-dimensional profile of the composition was obtained. 2. Experimental Section A monodisperse (Mw/Mn ) 1.04), 96K molecular-weight standard of polystyrene was obtained from Polymer Laboratory. The PBrx)0.79S samples were made from this batch of PS by the bromination procedure used by Kambour et al.8 The bromine is in the C4 position in the aromatic ring, making the compound poly(p-bromo)styrene. The same batch of polystyrene was used for all samples that were brominated, in the thin films and in the samples for bulk analysis. The bulk polystyrene and bromo-polystyrene samples were characterized with mass analysis and differential scanning calorimetry (DSC). Bulk samples from the same batch used for the films were sent to Dessert Analytical for a mass analysis of the bromine content in the PBrxS and was found to have a degree of bromination of approximately 79% (PBrx)0.79S). The glass-transition temperatures (Tg) were measured on the PS and PBrx)0.79S using a Mettler T11 DSC. The values were very close to accepted values.9 (5) Krausch, G.; Hipp, M.; Boltau, M.; Marti, O.; Mlynek, J. Macromolecules 1995, 28, 260. (6) Ade, H.; Zhang, X.; Cameron, S.; Costello, C.; Kirz, J.; Williams, S. Science 1992, 258, 5084, 972. (7) Zhang, X.; Ade, H.; Jacobsen, C.; Kirz, J.; Lindaas, S.; Williams, S.; Wirick, S. Nucl. Instrum. Methods Phys. Res. 1994, A347, 431. (8) Kambour, R. P.; Bendler, J. T.; Bopp, R. C. Macromolecules 1983, 16, 753. (9) Bicerano, J. Predict. Polym. Properties 1993, 154.
S0743-7463(98)00413-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/25/1998
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Figure 1. STXM reference spectra of PS and PBrS. Sample
Accepted Tg values
Measured Tg
PS standard 96K PBrx)0.79S
109 °C 142 °C (for x ) 1)
109.5 °C 139.1 °C
To remove the oxide layer, the silicon wafer substrates were first cleaned by heating above 80 °C in a solution of deionized water, hydrogen peroxide, and ammonium hydroxide. The silicon wafers were dipped in a water and hydrofluoric acid solution, until it was hydrophilic.10 The films were spun cast immediately after this step in order to reduce the amount of oxidation on the substrate. Solutions were made of PS and PBrx)0.79S dissolved in chlorobenzene, at a solid-to-solvent ratio of 25 mg/mL. The PS and PBrx)0.79S solutions were then mixed at different ratios prior to spinning onto the silicon wafers. Note that the solutions made were allowed to sit overnight before being spun cast onto the silicon in order to reach equilibrium. Films were spun at a rate of 2500 rpm for 60 s. The thickness of the films were measured with an ellipsometer model Rudolph “auto EL” and found to range from 400 to 700 Å. The films were then examined with an optical microscope, and it was noted that, as previously reported,4 separation was obvious prior to annealing. The films were annealed at 180 °C in a vacuum of 10-4 Torr for time varying from 0.5 to 7 days and then quenched with air to atmospheric pressure. The topography of the samples were analyzed with a DI 3000 atomic force microscope (AFM) in the contact mode using a SiN2 tip. No change in the morphology was observed between the samples annealed 7 days and 24 h. Therefore, it was assumed that 24 h of annealing was sufficient time for the polymer blend to achieve a state of equilibrium. The volume fraction of the PBrS in the blend was profiled using dynamic secondary ion mass spectroscopy (DSIMS). The samples were prepared by the same annealing procedure described above. A 200-Å sacrificial layer of PS was floated on the sample surface, to allow the sputter to equilibrate. Further details on the sample preparation are found in a paper by Shwarz et al.11 A 2-keV rastered Ar+ ion beam was used to sputter the sample at a rate of approximately 400 Å/h negative ions of O, Si, and Br were detected. Small corners of the samples were floated onto carbon-coated grids by dipping them into KOH solution at 80 °C, which the silicon oxide layer then attaches. The samples were subsequently studied with the scanning transmission X-ray Microscopy on beamline ×1A (X1A-STXM) at the National Synchrotron Light Source at Brookhaven National Laboratory. The ×1A- STXM (10) Higashi, G. S.; Chabal, Y. J.; Trucks G. W.; Raghavachari, K. Appl. Phys. Lett. 1990, 56, 656. (11) Shwarz, S. A.; Willkens, B. J.; Pudensi, M. A. A.; Rafailovich, M. H.; Sokolov, J.; Zhao, X.; Zhao, W.; Zheng, X.; Russell, T. P.; Jones, R. A. L. Mol. Phys. 1992, 76, 937.
Figure 2. Component thickness maps of a nominally 1000-Å thick, unannealed PBrS/PS blend with φPBrS ) 0.5 derived from several STXM images. Image area is 10 × 10 µm (240 × 240 pixels), Calculated average total thickness of this area is 865 Å. (a) Total thickness (sum of PS and PBrS maps); (b) PS map; (c) PBrS map; (d) profile of the row indicated by a line in Figure 2a-c. Note: Lower display limit of images are set to black for zero thickness and upper limit is scaled for maximum contrast.
Figure 3. AFM (10 × 10 µm) image of φPBrS ) 0.5 PS/PBrS sample where the shading corresponds to the z-axis with a maximum scale of 20 nm. (a) As cast sample prior to annealing. (b) Sample annealed at 180 °C for 7 days. utilizes zone plate optics to produce a micro probe, which the size of about 400 Å determines the spatial resolution. The samples are imaged in transmission mode in air. Details about the instrument are described by Jacobsen et al.12 To be able to perform quantitative compositional mapping, near edge X-ray absorption fine structure spectroscopy (NEXAFS) reference spectra were acquired of PS and PBrx)0.79S thin films with the STXM (Figure 1). The pre-carbon edge (280 eV) and post-carbon edge atomic continuum (317 eV) values from these spectra were scaled to the atomic X-ray absorption coefficients from the database of Henke et al.,13 assuming a uniform density of 1.07 g/cm3 for PS and 1.53 g/cm3 for PBrx)0.79S.14 Because of the complex fine structure exhibited by polymers right near the carbon absorption edge, the values of Henke only reflect the absorption coefficient accurately at energies well-above and -below the absorption edge. (12) Jacobsen, C.; Williams, S.; Anderson, E.; Brown, M. T.; Buckley, C. J.; Kern, D.; Kirz, J.; Rivers, M.; Zhang, X. Opt. Commun. 1991, 86, 351. (13) Henke, B. L.; Gullikson, E. M.; Davis, J. C. At. Data Nucl. Data Tables 1993, 54, 2, 181. (14) Zhang, X.; Balhorn, R.; Mazrimas, J.; Kirz, J. J. Struct. Biol. 1996, 116, 335.
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Figure 4. AFM scans (10 × 10 µm) of annealed PBrS/PS blends of volume fractions: (a) φPBrS ) 0.1; (b) φPBrS ) 0.2; (c) φPBrS ) 0.3; (d) φPBrS ) 0.4; (e) φPBrS ) 0.5; (f) φPBrS ) 0.6; (g) φPBrS ) 0.7; (h) φPBrS ) 0.8; (i) φPBrS ) 0.9.
Figure 5. Component thickness maps of a nominally 1500-Å thick, annealed PBrS/PS blend with φPBrS ) 0.5 derived from several STXM images. Image area is 29 × 29 µm (240 × 240 pixels). Measured average thickness is 1380-Å. (a) Total thickness (sum of PS and PBrS maps); (b) PS map; (c) PBrS map; (d) profiles of the row indicated by the line in Figure 5ac. Note: Lower display limit of images is set to black for zero thickness and upper limit is scaled for maximum contrast. Utilizing these normalized absorption coefficients and several characteristic X-ray micrographs of the sample under investigation, we can extract quantitative equivalent thickness maps for the two components with a spatial resolution of 400 Å by employing a singular value decomposition procedure.14
3. Results and Discussion The STXM images of the unannealed sample of a NPBrS ) 0.5 are shown in Figure 2 where the lightness is proportional to the thickness of what is scanned. Figure
Slep et al.
Figure 6. Component thickness maps of a nominally 1000-Å thick, annealed PBrS/PS blend with φPBrS ) 0.7 derived from several STXM images. Image area is 19 × 19 µm (240 × 240 pixels). Measured average thickness is 842 Å. (a) Total thickness (sum of PS and PBrS maps); (b) PS map; (c) PBrS map; (d) profile of the row indicated by the line in Figure 6a-c. Note: Lower display limit of images is set to black for zero thickness and upper limit scaled for maximum contrast.
Figure 7. DSIMS scan of a PBrS/PS blend with φPBrS ) 0.4. The normalized concentration profiles of PBrS is shown as a function of distance from the vacuum interface. The concentration of O and Si (arbitrary units) are also plotted to mark the vacuum and Si interfaces.
2b corresponds to the PS equivalent thickness map while Figure 2c corresponds to the PBrS map. A total thickness map is obtained by summing the PS and PBrS images and is shown in Figure 2a. Corresponding profiles of the row indicated by the lines in the three images are displayed in Figure 2d. The topographical image (Figure 2a) is seen to be in agreement with the AFM data obtained by Affrossman et al.4 From the Figure 2a,c, it is clear that the PBrx)0.79S rich regions correspond to the raised sections seen in the AFM scans as proposed by Affrossman et al. From the cross-sectional scans, one can determine the actual percentage of the different polymers in the structures. From Figure 2d it is clear that the two phases are almost completely separated immediately after spinning. The topographical features show that the initial threedimensional morphology need not be the equilibrium ones. We also demonstrate this with AFM scans in Figure 3,
Phase Separation of PS and PBrS Mixtures
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Figure 8. The three-dimensional AFM view of an annealed φPBrS ) 0.3 sample. The shaded areas are the postulated locations of the PBrS phase, as deduced from STXM and DSIMS. Inset: The surface topography, as seen in a plan view.
where we show an unannealed versus an annealed NPBrS ) 0.5 image. In Figure 4, AFM images are shown of annealed samples with different PBrx)0.79S volume fractions. The STXM images corresponding to annealed samples of φPBrS ) 0.5 and φPBrS ) 0.7 are shown in Figures 5a-d and 6a-d, respectively. From the figures, it can be seen that the STXM topographical images are in qualitative agreement with the AFM images. From the AFM images in Figure 4, it is seen that for φPBrS < 0.5, the PBrx)0.79S phase forms isolated droplets; then size and density increase with concentration. At φPBrS ) 0.5, the droplets begin to coalesce and the PBrS phase begins to form continuous structures. These structures are quite different from the discontinuous droplets seen in the unannealed samples. This indicates that considerable flow can occur between the two completely immiscible fluids above Tg. The solvents drying during the spin-casting process essentially freezes the phases at nonequilibrium, although already-separated state. The difference of unannealed and annealed phase separation of a PS/PBrS blend at φPBrS ) 0.5 has been previously observed also with a PBrS of x of less than 0.12.15,16 The rapid approach to equilibrium of the system is facilitated by the continuity of the PS phase and subsequent flow of matter, as discussed by Krausch et al.17 As the PBrx)0.79S concentration continues to increase, or when PBrx)0.79S becomes the majority phase, the morphology changes from droplets to surface holes in a continuous PBrx)0.79S layer. As the concentration of PBrx)0.79S increases, the diameter of the holes shrinks. From the STXM data in Figure 6 for the φPBrS ) 0.5 film we see that the holes are only in the PBrx)0.79S layer. The holes do not go to the silicon substrate; rather, they go (15) Kraush, G.; Mlynek J.; Straub W.; Brenn R.; Marko J. F. Europhys. Lett. 1994, 28 (5), 323. (16) Bruder, F.; Brenn, R. Phys. Rev. Lett. 1992, 69 (4), 624. (17) Kraush, G.; Kramer, E. J.; Rafailovich, M. H.; Sokolov, J. Appl. Phys. Lett. 1994, 64 (20), 2655.
down into a PS layer approximately 200-Å thick. Hence, a continuous PS layer extends below the PBrx)0.79S regions. The STXM data were taken in the transmission mode; therefore, it was not possible to determine whether the PS encapsulated the PBrS by segregating to the vacuum surface as well. The volume fraction of PBrS in a φPBrS ) 0.4 annealed sample, as determined with dynamic secondary ion mass spectroscopy (DSIMS), is shown in Figure 7. Bromine, silicon, and oxygen ion concentrations are plotted as a function of distance from the vacuum surface. The PBrS concentration was obtained from the Br- fraction according to the normalization procedure outlined in ref 13. We assume an incompressible blend where φPS ) 1 - φPBrS. From the figure, we can see that the PBrS concentration drops off at the vacuum and the silicon interface, where the position of the silicon and vacuum interfaces is marked by the O2 peaks. The surface oxygen is due to slight oxidation of the PS film and is always visible in SIMS spectra. From the figure, we can see that the PBrS is encapsulated in a PS layer. A cutaway drawing superimposed on the three-dimensional AFM view of the measured annealed morphologies at φPBrS ) 0.3 and φPBrS ) 0.7 is shown in Figures 8 and 9, respectively. From the figure it is clearly seen that the morphologies formed always maintain the continuity of the PS phase. As a result, the observed surface morphologies are not symmetric in PS and PBrx)0.79S content. Hence, although the interfacial energy would have been minimized if the PS formed spheres in the PBrx)0.79S rich phases, the continuity constraint at the interfaces requires the formation of the hole morphologies. The segregation of PS to the vacuum is expected since the surface tension of PS (γPS) is less than that of PBrS (γPBrS) where γPS ) 30.5 dyn/cm and γPBrS ) 33.7 dyn/cm. The spreading coefficient is18 (18) Hobbs, S. Y.; Dekkers, M. E. J.; Watkins, V. H. Polymer 1988, 29, 1598.
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predominantly segregate to the Si surface, the following must be true in order to achieve the lowest possible free energy state.21
γPS/Si + γPBrS < γPBrS/Si + γPS
(2)
Hydrogen passivation of the silicon surface produces a slightly hydrophobic nonpolar surface with a surface energy of γso = γsod ) 44.7 dyn/cm,19 where γsod is the dispersive component of the HF-etched bare silicon surface. The dispersive component of the polymer/solid interfacial energy, γsld, a measure of the nonpolar interactions, can be obtained if the dispersive fraction of the surface energy is known:22
γsld ) [(γsod)1/2 - (γ)1/2]2
Figure 9. The three-dimensional AFM view of an annealed φPBrS ) 0.7 sample. The shaded areas are the postulated locations of the PBrS phase, as deduced from STXM and DSIMS. Inset: The surface topography, as seen in a plan view.
S ) (γPBrS - γPS) - γPS/PBrS
(1)
where γPS/PBrS is the interfacial tension and is approximately 1 dyn/cm.19 At equilibrium, this involves obtaining the proper balance between the three competing terms, minimizing the interfacial energies between the components while satisfying the requirements of eq 1. The spreading coefficient is calculated to be S ) 2.2 dyn/ cm or S > 0; therefore, PS spreads on PBrS. The enrichment of PS at the vacuum interface has also been noted by Bruder et al.10 and Krausch11 et al. with a PS/PBrS blend at φPBrS ) 0.5 with partially brominated PBrS and by Ade et al.,20 using photoemission electron microscopy (PEEM) on the same samples as our own. The segregation of PS to the silicon surface is somewhat more surprising since both Bruder and Krausch do not observe preferential wetting. On the other hand, the wetting layer we measure is rather narrow, of the order of 100 Å, which is smaller than the resolution of profiling techniques used in these references. Furthermore, the wetting layer is also dependent on the degree of H passivation or stripping of the oxide layer. For PS to (19) Zhao, W.; Zhao, X.; Rafailovich, M. H.; Sokolov, J.; Mansfield, T.; Stien, R. S.; Composto, R. C.; Kramer, E. J.; Jones, R. A. L.; Sansone, M.; Nelson, M. Physica B 1991, 173, 43-46. (20) Ade, H.; Winesett, D. A.; Smith, A. P.; Anders, S.; Stammler, T.; Heske C.; Slep, D.; Rafailovich, M. H.; Sokolov, J.; Stohr, J., to be submitted to Appl. Phys. Lett. 1998.
(3)
Assuming that the PS surface tension is nonpolar,19 γPS = γPSd ) γ, we find for PS from eq 3, γPS/Sid ) 1.35dyn/cm at the passivied silicon interface. From eq 2, this implies that γPBrS/Si J 4.6 dyn/cm. Substituting this value into eq 3 and solving for γPBrS/Sid, we get γPBrS/Sid < 20.6 dyn/cm. This indicates that the surface tension for PBrS is approximately 39% polar. In conclusion, we have found that the cast structures of PS/PBrS blends are not the equilibrium ones. Upon annealing, it is found that the PBrS becomes encapsulated by the PS. The encapsulation provides for a continuous PS phase for all blend compositions and explains the observed structures which are formed for different PBrS fractions. The encapsulation allows us to estimate the dispersive contribution of the PBrS surface energy, which is found to be less than 20.6 dyn/cm. Acknowledgment. M. H. Rafailovich and J. Sokolov are supported by NSF DMR-9732230 (MRSEC Program) and DOE-SG02-93-ER45481. H. Ade, D. A. Winesett, and A. P. Smith are supported by an NSF Young Investigator Award DMR-9458060. STXM data was acquired with the X-1A STXM developed by the group of Janos Kirz and Chris Jacobsen at SUNY Stony Brook, with support from the Office of Biological and Environmental Research, U.S. DOE under contract DE-FG02-89ER60858, and NSF under Grant DBI-9605045. The zone plates were developed by Steve Spector and Jacobsen of Stony Brook and Don Tennant of Lucent Technologies Bell Labs, with support from the NSF under Grant ECS-9510499. The NSLS and ALS are supported by the Office of Basic Energy Sciences, Energy Research, Department of Energy. LA9804132 (21) Aussere, D.; Raghunathan, V. A.; Maaloum, M. J. Phys. II 1993, 3, 1485. (22) de Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827.
