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X-ray Reflectivity Study of Ultrathin Liquid Films of Diphenylsiloxane-Dimethylsiloxane Copolymers Guennadi Evmenenko,* Haiding Mo, Sumit Kewalramani, and Pulak Dutta Department of Physics and Astronomy, Northwestern UniVersity, EVanston, Illinois 60208-3112 ReceiVed February 23, 2006. In Final Form: May 5, 2006 Using X-ray reflectivity, we observe drastic differences in the interfacial structure and molecular ordering of diphenylsiloxane-dimethylsiloxane copolymer thin films deposited on hydroxylated versus H-terminated (etched) silicon wafers. We find that substrate type and comonomer ratio determine the conformational arrangements in these liquid films. High-energy bonding between the substrate and the molecules and an increase in rigidity of the molecules due to replacement of methyl groups by phenyl groups leads to a specific molecular ordering at the liquid/solid interface and pronounced density oscillations in this region. The observed structural reorganizations are explained by the interplay and the established balance between the chain flexibility and the polymer-substrate interactions.
Introduction There is an increasing interest in understanding the properties and structural design of highly responsive smart material interfaces. In contrast to covalent chemistries, stimulated bulk restructuring and interfacial reorganization in materials through precisely tuned physical bonding (due to weak, noncovalent interactions: van der Waals, hydrophobic, and electrostatic) is still an unexplored field. Rational interface design and the subsequent performance of thin films implicitly require a precise understanding of the interfacial phenomena. Polymeric systems present a great chemical variety. However, the behavior of polymer films at interfaces is complicated by their chain architecture and the complexity of the segmentsurface contacts,1 nonequilibrium effects,2 dynamic interfacial response,3 etc. The critical role that such interfaces play in numerous physical systems has stimulated significant efforts to understand the interfacial behavior of polymeric films.4 Previous studies of polymeric thin films have highlighted the fact that an intricate balance between a number of competing factors dictates the structural arrangement of molecules at interfaces. For example, vibrationally resonant sum-frequency generation (VR-SFG) experiments show that the pendant phenyl groups of polystyrene thin films on surface-modified hydrophilic glass substrates lie in the surface plane.5 The improved adhesion on modified substrates is attributed to the development of attractive interactions between the surface hydroxyl groups and the π electron cloud of the phenyl ring. However, no such ordering was found for the related polymer poly-4-hydroxystyrene, on either hydrophobic or hydrophilic substrates, due to polymer self-interactions. This fact underlines the complex behavior of thin films of phenyl-containing polymers. The X-ray reflectivity (XRR) technique is an important method of structural analysis of buried interfaces. Our earlier XRR studies of a liquid of nonentangling molecules, tetrakis(2-ethylhexoxy)* To whom correspondence should be addressed. Phone: (847) 4913477. Fax: (847) 491-9982. E-mail:
[email protected]. (1) Netz, R. R.; Andelman, D. Phys. Rep. 2003, 380, 1. (2) Sommer, J.-U. Eur. Phys. J. E 2002, 9, 417. (3) Santore, M. M. Curr. Opin. Colloid Interface Sci. 2005, 10, 176. (4) Bucknall, D. G. Prog. Mater. Sci. 2005, 49, 713. (5) Wilson, P. T.; Richter, L. J.; Wallace, W. E.; Briggman, K. A.; Stephenson, J. C. Chem. Phys. Lett. 2002, 363, 161.
silane,6 and thin films of poly(dimethylsiloxane)s (PDMS) and poly(methylhydrodimethyl)siloxanes7-10 reveal that a balance between confinement-restricted segmental ordering and noncovalent interactions between the substrate surface determine the organization of the molecules in the vicinity of the substrate. These effects are of course, dependent on the nature of the substrate and the molecular architecture of the polymers. We obtained evidence for molecular layering induced by geometrical confinement for thin films of PDMS of a low molecular weight (up to 2000 g/mol). PDMS molecules have pronounced inherent conformational flexibility of main-chain backbones and high mobility of segments and entire molecules. An increase in chain length and/or interaction between the molecules and the substrate surface leads to a suppression of density oscillations and a change in the density profile in the vicinity of the substrate.8,9 Recently, electronic structure and molecular dynamics studies also confirmed the importance of adhesive interactions between PDMS molecules and hydroxylated silicon substrate for the ordering of polymer segments near the liquid/solid interface.11 Thus, the two principal factors that govern the structural organization of polymers near interfaces are the flexibility of the molecular chains and the nature of interactions at the interface. A comprehensive understanding of the role of each of the factors is still being sought. In the present work, we have used the XRR technique to study the structural behavior of thin films of phenyl-containing siloxanes. These materials can form thermotropic mesophases without any mesogens (Tgs ∼ 134-164 K).12 The phenyl groups increase the rigidity in the silicone chain12-14 and simultaneously enhance the adhesive interactions with the substrate. Interplay between these two competing influences can produce specific (6) Yu, C.-J.; Richter, A. G.; Datta, A.; Durbin, M. K.; Dutta, P. Phys. ReV. Lett. 1999, 82, 2326. (7) Evmenenko, G.; Dugan, S. W.; Kmetko, J.; Dutta, P. Langmuir 2001, 17, 4021. (8) Evmenenko, G.; Yu, C.-J.; Kmetko, J.; Dutta, P. Langmuir 2002, 18, 5468. (9) Evmenenko, G.; van der Boom, M. E.; Yu, C.-J.; Kmetko, J.; Dutta, P. Polymer 2003, 44, 1051. (10) Evmenenko, G.; Mo, H.; Kewalramani, S.; Dutta, P. Polymer 2006, 47, 878. (11) Tsige, M.; Soddemann, T.; Rempe, S. B.; Grest, G. S.; Kress, J. D.; Robbins, M. O.; Sides, S. W.; Stevens, M. J.; Webb, E. J. Chem. Phys. 2003, 118, 5132. (12) Godovsky, Y. K.; Papkov, V. S. AdV. Polym. Sci. 1989, 88, 129. (13) Lee, M. K.; Meier, D. J. Polymer 1993, 34, 4883. (14) Grigoras, S.; Qian, C.; Crowder, C.; Harkness, B.; Mita, I. Macromolecules 1995, 28, 7370.
10.1021/la060522d CCC: $33.50 © 2006 American Chemical Society Published on Web 06/09/2006
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Figure 1. Schematic representation of the chemical structure of the P(DPS-DMS) molecule.
structural rearrangements. Moreover, our recent studies of crystal growth of tetraphenylsilane from toluene solutions on silicon oxide and etched silicon surfaces revealed a high degree of alignment of crystals perpendicular to the solid surface (unpublished data). The preferential alignment in tetraphenylsilane crystals was determined by the first layer of molecules, which has a specific ordering at the substrate surface due to the attractive interactions between phenyl polarizable groups of solute molecules and OH- or H-atoms of the substrate. The first layer subsequently acts as a template for layer-by-layer growth in the vertical direction. From this point of view, tetraphenylsilane as well as other molecules containing symmetric tetraaryl groups are very interesting systems, which can produce different structural rearrangements at the molecular level. These results have stimulated our current studies of the structural behavior of phenylbased siloxanes. Experimental Procedures The silicone fluids, PDMS of molecular weight 950 g/mol and structurally near random liquid diphenylsiloxane-dimethylsiloxane copolymers P(DPS-DMS)15 with different fractions of the DPS group, are products of Gelest Co. Inc. and were used as received. The chemical structure of the P(DPS-DMS) molecule is presented in Figure 1. The average molecular Mw and the mole percent of DPS groups, respectively, are: 800 g/mol and 5% (sample PDM-0421), 1000 g/mol and 8.5% (sample PDM-0821), and 2000 g/mol and 20% (sample PDM-1922).16 The substrates (3 in. × 1 in. × 0.1 in.), silicon (100), were purchased from Umicore USA, Inc., Semiconductor Processing Division. They were cleaned in a strong oxidizer, a mixture of 70% sulfuric acid and 30% hydrogen peroxide (7:3 v/v), for 1 h at 90 °C, rinsed with copious amounts of distilled water (18 MΩ cm), and stored under water. Prior to preparing the films, the wafers were removed from water and blown dry under a stream of nitrogen. For removing the native oxide layer from the silicon substrates, we used a wet chemical treatment method with HF:17-20 silicon wafers were (1) cleaned by immersion in a freshly prepared piranha solution (concentrated H2SO4/H2O2 (30%) ) 7:3 v/v) at 80 °C for at least 45 min; (2) rinsed with copious amounts of deionized water; and (3) immersed in 5% aqueous HF for 30 s followed by etching in 1% HF solution for 3 min. Then the wafers were rinsed with deionized water and blown dry under a stream of nitrogen. All these steps were carried out just prior to film preparation. We spread uniform thin films by dipping the substrates in dilute solutions of silicone fluids in hexane and withdrawing the wafers (15) Itoh, T.; Yang, M. H.; Chou, C. J. Chem. Soc., Faraday Trans. 1996, 92, 3593. (16) PDM-0421, PDM-0821, and PDM-1922 are abbreviations used by the manufacturing company, Gelest Co., Inc. The mole percent of DPS groups is an average value, and the real values can deviate by as much as 20% of this value. (17) Grundner, M.; Jacob, H. Appl. Phys. A 1986, 39, 73. (18) Cerofolini, G. F.; Meda, L. Appl. Surf. Sci. 1995, 89, 351. (19) Palermo, V.; Jones, D. Mater. Sci. Semicond. Process. 2001, 4, 437. (20) Endo, K.; Arima, K.; Hirose, K.; Kataoka, T.; Mori, Y. J. Appl. Phys. 2002, 91, 4065.
Figure 2. Normalized specular XRR data for different silicone fluids deposited on the oxidized surface of the silicon substrate: (1) PDMS and (2-4) P(DPS-DMS) copolymers with different fractions of diphenylsiloxane, 5% (sample PDM-0421), 8.5% (PDM0821), and 20% (PDM-1922), respectively. The corresponding Patterson functions from the observed XRR data are presented in the inset (shifted vertically for clarity). at a constant speed of 1.5 mm/s.21 Solution concentrations were chosen to obtain films of thickness ∼80 Å. XRR studies were performed at beamline X23B of the National Synchrotron Light Source using a four-circle diffractometer in the specular reflection mode (i.e., the incident angle θ was equal to the exit angle). The reflected intensity was measured as a function of the vertical momentum transfer component qz ) (4π/λ)sin θ. X-rays of energy E ) 10.0 keV (λ ) 1.24 Å) were used for all measurements. The beam size was 0.4 mm (vertical) and 1.0 mm (horizontal). The samples were kept under a slight overpressure of helium during the measurements to reduce the background scattering from ambient gas and radiation damage. All experiments were performed at room temperature. Details of data acquisition and analysis procedures are described in our previous papers.6-8,22
Results and Discussion Thin films of diphenylsiloxane-dimethylsiloxane copolymers with different comonomer ratios were deposited on hydroxylated or H-terminated silicon wafers. Distinctive structural changes occur in these films when the rigidity of the polymer chains is enhanced due to steric interaction of their side groups by replacing the methyl groups with phenyl components and with the modification of the substrate surface. We have used a PDMS film as a control sample for comparison purposes (curve 1, Figure 2). Thin films of PDMS of different molecular weights up to 2000 g/mol and varying composition were comprehensively studied by our group recently.7-9 Normalized specular XRR patterns and corresponding Patterson functions for different silicone fluids deposited on oxidized surface of silicon substrate are presented in Figure 2. All Patterson functions show the large primary maxima, the position of which indicates the overall film thickness ∼70-80 Å. As is seen from curve 1 in the inset of Figure 2, the Patterson function for the PDMS film shows a few secondary maxima, which is characteristic of interfacial ordering (21) The interaction energy between substrate and copolymers is stronger than between substrate and solvent. This allows us to obtain a uniform film thickness over the footprint of the X-ray beam. This may not generally be the case. For example, if toluene is the solvent, films formed are nonuniform, with only a very thin uniform region adjusted to the substrate. (22) Evmenenko, G.; van der Boom, M. E.; Kmetko, J.; Dugan, S. W.; Marks, T. J.; Dutta, P. J. Chem. Phys. 2001, 115, 6722.
Diphenylsiloxane-Dimethylsiloxane Copolymers
Figure 3. Normalized specular XRR data for P(DPS-DMS) copolymer films (sample PDM-1922) deposited on oxidized (curve 1) and H-terminated (curve 2) surfaces of silicon substrates. (Bottom inset) Patterson function for the second XRR pattern. (Top inset) Electron density profiles for both films obtained by the modelindependent fitting: solid line, oxidized substrate surface and dashed line, substrate surface with silicon oxide removed.
of molecules in thin PDMS films deposited on polished silicon wafers.7 Increasing the DPS fraction in the main molecule chain produces two distinctive changes in the XRR patterns (curves 2-4, Figure 2): (1) the number of observed Kiessig fringes decreases; this is indicative of an enhanced roughness of the film surface and (2) a drastic change for the film with the highest DPS content (curve 4). The corresponding Patterson function (curve 4, the inset) shows a pronounced maximum around 21 Å. This peak corresponds to a substantial structural reorganization near the oxidized silicon surface for the copolymer films with a high rigidity of molecular chains. Suppressed mobility of polymer segments near the solid interface is the main reason for such an arrangement. However, the presence of specific noncovalent interactions between the liquid and the surface molecules also have a significant effect on the interfacial molecular behavior and contribute significantly to the resulting density profile of the liquid film. To better understand the role of the substrate surface on the observed structural changes, we have removed the native silicon oxide layer using the wet chemical etching process described previously. XRR data for the sample PDM-1922 on native silicon oxide and H-terminated silicon surfaces are presented in Figure 3. On etched silicon surfaces, the reflectivity pattern looks similar to that observed for PDMS films (curve 1, Figure 2). However, the corresponding Patterson function (bottom inset, Figure 3) indicates more pronounced layering near the film-air interface. To compute the electron density profiles (top inset, Figure 3) for sample PDM-1922 on different substrates, we have followed the model-independent analysis of Sanyal et al.23 using a variable electron density within the film. A detailed description of the method can be found in ref 6. The density oscillations are found to be strongest at the substrate-film interface (on silicon with a native oxide layer) and at the air-film interface (on etched silicon), with a spacing of ∼15-17 Å. The observed spacing is consistent with the parameters obtained independently from molecular dynamics calculations (CS Chem 3D Pro, Cambridge(23) Sanyal, M. K.; Basu, J. K.; Datta, A.; Banerjee, S. Europhys. Lett. 1996, 36, 265.
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Soft Corp., Cambridge, MA). The PDM-1922 molecules can be approximated by an ellipsoid with axes of 15.2, 16.3, and 17.5 Å. It should be mentioned here that the confinement of molecules between two interfaces (solid substrate and free surface) may restrict the free arrangements of the polymer chains in thin films. Deviations from the unperturbed coil conformation occur in the vicinity of the confining surfaces on distances of ∼2-3Rg of the unperturbed chain dimensions.24,25 When the film thickness approaches twice the size of this confinement effect scale, the possible interference rearrangements of molecules may happen. In our case, the radius of gyration for the highest molecular weight sample, PDM-1922, is Rg ) ((15.22 + 16.32 + 17.52)/ 5)1/2 ∼ 12.7 Å and is ∼1/6 of the film thickness. However, principal structural features, which were observed for the thinnest films, are unchanged for film thicknesses up to 1600 Å. Therefore, we conclude that the interfacial structure and molecular ordering at the liquid-solid interface remains intact. Consider the case of thin P(DPS-DMS) films on H-terminated silicon first. The Patterson function and electron density profile indicate that the molecular layering goes through the entire film with a slightly pronounced amplitude at the air-film interface. Other silicone fluids, for example, PDMS, show such layering only near the substrate-film interface.7 The type of ordering observed for P(DPS-DMS) thin films is, however, not unique and has previously also been observed for non-phenyl-based polymers. Spin-coated PMDA-ODA polymer films of thicknesses around 400 Å (the same trend was also observed for thinner films ∼100 Å) were more ordered near the air-film interface than in the bulk,26 and no crystalline-like ordering was found near the etched silicon surface. Differences in mobility near the substrate and the air interfaces and strong interactions of the polymer molecules with the substrate were cited as the primary reasons for a lack of ordering near the substrate surface.26 In the case of phenyl-based P(DPS-DMS) polymers, the interaction between the polymer molecules and the etched silicon surface is expected to be negligible. Thus, we believe that the observed structural patterns for this film arise from a balance between the mobility and the rigidity of the copolymer molecules and the intermolecular (between different polymer layers) interactions. Strong interlayer interactions are known to cause the formation of ordered phases even when the rigidity of molecules is quite high. For example, in the case of polysiloxanes bearing aryl groups where the polymer chain flexibility can vary to a large extent, the macromolecules are still able to form ordered phases.27 Such polysiloxanes with symmetric side substituents on the silicon atom show a tendency to form thermotropic mesophases with columnar architecture. A disordered crystalline mesophase was found even for an oligomeric species of poly(diphenylsiloxane) with an average molecular weight greater than the critical value of ∼2400 g/mol.28 In the case of P(DPS-DMS) liquid films on a silicon surface with native oxide layers, adhesion is enhanced due to attractive interactions between the phenyl polarizable groups and the OHgroups of the substrate. The electron density profile changes drastically near the solid substrate as compared to the situation discussed above (Figure 3, top inset). The density fluctuations near the substrate are as high as 25% of the average bulk density, as compared to the density fluctuations observed for PDMS films (24) Kraus, J.; Mu¨ller-Buschbaum, P.; Kuhlmann, T.; Schubert, D. W.; Stamm, M. Europhys. Lett. 2000, 49, 210. (25) Mischler, C.; Baschnagel, J.; Dasgupta, S.; Binder, K. Polymer 2002, 43, 467. (26) Factor, B. J.; Russell, T. P.; Toney, M. F. Macromolecules 1993, 26, 2847. (27) Ganiecz, T.; Stan´czyk, W. Prog. Polym. Sci. 2003, 28, 303. (28) Harkness, B. R.; Tachikawa, M.; Mita, I. Macromolecules 1995, 28, 8136.
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Figure 4. Normalized specular XRR data for PDM-1922 liquid films of different thicknesses: around 70 Å (1), 60 Å (2), and 40 Å (3).
of similar thickness and molecular weight, which are only about 0.2%. The positions of two minima on the XRR pattern, at 0.2 and 0.5 Å-1, that give this characteristic size (2π/∆qz, where ∆qz is the spacing between the minima) are always present independent of the film thickness in the ultrathin film regime (Figure 4). We believe that an interplay between the higher energy bonding between the substrate and the molecules and an increase in the rigidity of molecules (due to replacement of methyl groups by phenyl groups) leads to a specific molecular (segmental) ordering at the liquid/solid interface and pronounced density oscillations in this region. The differences in molecular interactions at the hydroxylated and H-terminated silicon surfaces can be understood by observing the roughness of the formed polymer films. For PDM-1922 films deposited on a hydroxylated surface of silicon, the film roughness is around 10 Å and is independent of the film thickness (501600 Å). The roughness of these films on etched silicon surfaces (4-6 Å) is similar to that observed for PDMS films and is close to the expected film roughness of ∼4 Å due to capillary wave fluctuations at the interface (see, i.e., ref 29). High surface roughness in thin P(DPS-DMS) films can be explained by the destabilizing effect of the effective molecular interactions between
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the free liquid surface and the substrate. A change in the magnitude and character of the interactions at the near substrate region and their translation transfer through the entire film (with surfaceinduced modification of the average order) leads to different coupling between the interfaces in the case of P(DPS-DMS) copolymer films. It is a well-established fact that the inner structure of the liquid film, induced by the boundary conditions and coupling of the two interfaces, significantly affects the dewetting behavior of such films. The central hypothesis of the spinodal dewetting models is that strong long-range forces amplify the capillary waves.30 In regard to the roughness of polymer films, a mention of solvent-polymer interactions is also necessary. In our experiments, the choice of hexane, acetone, or tetrahydrofuran as the solvent, for films formed on silicon substrates with a native oxide layer, does not alter the near substrate structure of the films. At the same time, the use of tetrahydrofuran makes the film surface very rough (Kiessig fringes are less distinctive; data not shown). We believe that the relevant solubility parameter to determine such an effect is the solubility component due to dispersion forces. However, a clear understanding of the effect of solvents on the final film structure requires further systematic studies. Thus, we have shown that a balance between the factors, namely, the flexibility of molecular chains and attractive interactions between the liquid molecules and the substrate, is responsible for conformational rearrangements at the liquid/solid interface.31 Adequate mobility of the polymer segments near the solid interface is required for such an arrangement. This result again emphasizes the importance of a balance between entropic and energetic factors for a controlled modification of thin liquid films. Acknowledgment. This work was supported by the U.S. National Science Foundation under Grant DMR-0305494. XRR measurements were performed at beamline X23B of the National Synchrotron Light Source, which is supported by the U.S. Department of Energy. LA060522D (29) Ocko, B. M.; Wu, X. Z.; Sirota, E. B.; Sinha, S. K.; Deutsch, M. Phys. ReV. Lett. 1994, 72, 242. (30) Vandenbrouck, F.; Valignat, M. P.; Cazabat, A. M. Phys. ReV. Lett. 1999, 82, 2693. (31) McCarthy, D. W.; Mark, J. E.; Clarson, S. J.; Schaefer, D. W. J. Polym. Sci., Polym. Phys. Ed. 1998, 36, 1191 mention that the symmetry of repeat units of phenyl-based siloxanes impacts bulk crystallization properties of polymers. Symmetry considerations may also be relevant to interfacial characteristics of ultrathin polymer films and need further experimental investigation.
