Macromolecules 2003, 36, 3307-3314
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Surface Morphology of Annealed Polystyrene and Poly(methyl methacrylate) Thin Film Blends and Bilayers Mark Harris, Guenter Appel, and Harald Ade* Department of Physics, North Carolina State University, Raleigh, North Carolina 27695 Received October 1, 2002 ABSTRACT: Thin films of polystyrene (PS) and poly(methyl methacrylate) (PMMA) were spun-cast onto silicon substrates, annealed, and analyzed by atomic force microscopy (AFM), total electron yield (TEY), and partial electron yield (PEY) near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in order to resolve conflicting prior literature regarding the tendency of PS to form a wetting layer or overlayer on top of PMMA. From the comparison of the three methods of analysis and on the basis of the extraordinary surface sensitivity of PEY NEXAFS, we conclude that PS does not form an overlayer in samples with morphologies near thermodynamic equilibrium. The PS forms droplets of a large size range on top of a PMMA layer that wets the hydrophilic SiOx substrate. From our results, the maximum thickness of a continuous PS wetting layer would be about 0.25 nm. This is in contrast to recent experiments that imply an equivalent PS wetting layer of about 5-10 nm is forming during annealing.
Introduction Polystyrene (PS) and poly(methyl methacrylate) (PMMA) are frequently used as binary model systems to study thin film polymer structure formation, polymerpolymer and polymer-substrate interactions, and pattern formation and phase separation dynamics in polymer thin films.1-7 In most cases, the results observed in annealed PS/PMMA systems are consistent with or where interpreted as phase-separated films without any wetting layer at the air interface, i.e., a situation in which neither polymer wets the other. Walheim et al. reported droplets of PS after annealing a 50/50 w/w sample for 12 h at 190 °C.3 Ade et al. have also found surface phase-separated PS and PMMA domains with PS droplets after annealing at 180 °C.2 If other morphologies were found for the same weight ratio, either the samples were not annealed1 or a different substrate was used.3 In contrast, results with PS/PVME8 and PS/PBrS9-11 systems clearly indicated wetting layers of PS over PVME and PBrS, respectively, due to different interfacial and surface energies in these systems. In contrast to the interpretation in prior investigations of PS/PMMA systems, recent investigations reported the formation of a PS overlayer during annealing of a PS/PMMA thin film polymer blend at 142 °C. On the basis of angle-resolved X-ray photoelectron spectroscopy (XPS) and friction force microscopy, Ton-That et al. concluded to find a PS overlayer,12 implying that this is the thermodynamically favored morphology. The measured PMMA concentration of only 9% at an electron takeoff angle of 60°, corresponding to the most surface-sensitive measurement with a depth sensitivity of 4.5 nm, would imply an equivalent PS wetting layer of about 10 nm. On the basis of thermodynamic equilibrium considerations, one would expect annealed PS and PMMA blends to be phase-separated in the bulk, but also right to the surface. For example, the surface tensions of 3K PMMA and 44K PS are γPMMA ) 41.1 mJ/m2 and γPS ) * Corresponding author: e-mail
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40.7 mJ/m2 at 20 °C, while γPMMA ) 32.0 mJ/m2 and γPS ) 32.1 mJ/m2 at 140 °C. The interfacial tension between these two materials is 3.2 mJ/m2 at 20 °C and 1.7 mJ/m2 at 140 °C.13 Since the surface tensions, γ, of pure PS and pure PMMA are very similar at typical annealing temperatures, the interfacial energy γPS/PMMA is the driving force in determining the surface morphology. For typical annealing temperatures, the interfacial tension γPS/PMMA is larger than the absolute value of the surface tension difference, so the spreading parameters for PS spreading on PMMA, SPS ) γPMMA - γPS γPS/PMMA ) -1.8 mJ/m2, and for PMMA spreading on PS, SPMMA ) γPS - γPMMA - γPS/PMMA ) -1.6 mJ/m2, are both negative at 140 °C. The spreading parameters decrease for higher temperatures, but both remain negative for temperatures
