Structure and Dewetting Behavior of Polyhedral Oligomeric

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Langmuir 2007, 23, 902-907

Structure and Dewetting Behavior of Polyhedral Oligomeric Silsesquioxane-Filled Polystyrene Thin Films Nao Hosaka,† Naoya Torikai,‡ Hideyuki Otsuka,†,§ and Atsushi Takahara*,†,§ Graduate School of Engineering and Institute for Materials Chemistry and Engineering, Kyushu UniVersity, Hakozaki, Higashi-ku, Fukuoka 812-8581, and Neutron Science Laboratory, Institute of Materials Structure Science, High Energy Accelerator Research Organization, Oho, Tsukuba, Ibaraki 305-0801, Japan ReceiVed July 31, 2006. In Final Form: September 30, 2006 Polyhedral oligomeric silsesquioxane (POSS) meets increasing interest as a building unit for inorganic-organic hybrid materials. The incorporation of cyclopentyl-substituted POSS (CpPOSS) into polystyrene (PS) thin films led to an inhibition of dewetting. In this paper, the dispersion state of CpPOSS in the CpPOSS/PS hybrid films and, furthermore, the relationships between the structure and dewetting inhibition effect are discussed. Structural analysis of the hybrid films revealed that CpPOSS segregated to the film surface and crystallized. The segregation of CpPOSS to the surface changes the surface free energy and spreading coefficient of the film. Interfacial structure was also roughened by the segregation of CpPOSS, which can contribute to the inhibition of dewetting by pinning the contact line of the PS film with the substrate. The inhibition of dewetting can be attributed to the modification of the film surface and interface by the segregation of CpPOSS.

Introduction Miniature devices and various applications such as coatings, lubrication, or electronic packaging demand progressively thinner polymer films. However, producing stable and defect-free films is particularly problematic in very thin films which tend to break up and dewet from the substrates.1 Since dewetting can often compromise the effectiveness of the coatings, this fundamental problem motivates the development of strategies for stabilizing polymer thin films against dewetting.2-5 Previously, Barnes et al. discovered a novel stabilizing method through addition of nanoparticles to polymer thin films.6 The addition of a small amount of fullerene nanoparticles to a spincoating polymer solution led to an inhibition of dewetting in the films. In that case, fullerene nanoparticles were found to segregate to the polymer-substrate interface, which modified the interface and caused the inhibition effect. Since then, several nanofillers such as dendrimers7 and polymer nanoparticles8 have been reported to bring retardation of the dewetting of polymer films on an inorganic substrate. The addition of nanoparticles was also effective for polymer films on an organic substrate, such as a polymer bilayer, and colloidal silica9 and nanogel particles10 were reported to inhibit the dewetting of the upper layer of polymer * To whom correspondence should be addressed. Phone: +81-92-6422721. Fax: +81-92-642-2715. E-mail: [email protected]. † Graduate School of Engineering, Kyushu University. ‡ High Energy Accelerator Research Organization. § Institute for Materials Chemistry and Engineering, Kyushu University. (1) Reiter, G. Phys. ReV. Lett. 1992, 68, 75-78. (2) Yerushalmi-Rozen, R.; Klein, J.; Fetters, L. J. Science 1994, 263, 793795. (3) Henn, G.; Bucknall, D. G.; Stamm, M.; Vanhoorne, P.; Je´roˆme, R. Macromelecules 1996, 29, 4305-4313. (4) Netz, R. R.; Andelman, D. Phys. ReV. E 1997, 55, 687-700. (5) Yuan, C.; Ouyang, M.; Koberstein, J. T. Macromolecules 1999, 32, 23292333. (6) Barnes, K. A.; Karim, A.; Douglas, J. F.; Nakatani, A. I.; Gruell, H.; Aims, E. J. Macromolecules 2000, 33, 4177-4185. (7) Mackay, M. E.; Hong, Y.; Jeong, M.; Hong, S.; Russell, T. P.; Hawker, C. J.; Vestberg, R.; Douglas, J. F. Langmuir 2002, 18, 1877-1882. (8) Krishnan, R. S.; Mackay, M. E.; Hawker, C. J.; Van Horn, B. Langmuir 2005, 21, 5770-5776. (9) Sharma, S.; Rafailovich, M. H.; Peiffer, D.; Sokolov, J. Nano Lett. 2001, 1, 511-514.

bilayer films. However, if unfavorable interaction existed between the particle and the matrix polymer, dewetting of the films was observed even in nanoparticle-filled polymer thin films.9 These experiments showed the potential of using nanoparticles to prevent the dewetting of the polymer thin films; however, there could be a situation in which the nanoparticles can cause inhibition of dewetting. A recent molecular dynamics simulation11 proposed that the ideal situation for the inhibition of dewetting was that in which the nanoparticles were confined to the polymersubstrate interface and pinned the contact line of the dewetting front. In that simulation, the inhibition of dewetting not only was a result of a pinning effect but also was affected by the mobility of the nanoparticles, the interaction between the particle and the polymer, and the size of the nanofiller. The authors previously reported that the addition of polyhedral oligomeric silsesquioxane (POSS) into polystyrene (PS) thin films led to an inhibition of dewetting.12 POSS, the most common octameric structure of which is characterized by the formula R8Si8O12, is a nanosized material with a silica core and organic groups on the surface providing a high compatibility with organic polymers. POSS nanostructured chemicals, with sizes of from 1 to 3 nm in diameter, can be thought of as the smallest possible particles of silica. This nanosized material has the ability to functionalize the silicon corners with a variety of organic substituents and can be incorporated into polymer systems without any decrease in optical transparency.13 These salient features of POSS are an advantage over current filler nanotechnologies, so POSS derivatives meet increasing interest as building units for inorganic-organic hybrid materials.14,15 (10) Wei, B.; Gurr, P. A.; Genzer, J.; Qiao, G. G.; Solomon, D. H.; Spontak, R. J. Macromolecules 2004, 37, 7857-7860. (11) Luo, H.; Gersappe, D. Macromolecules 2004, 37, 5792-5799. (12) Hosaka, N.; Tanaka, K.; Otsuka, H.; Takahara, A. Compos. Interfaces 2004, 11, 297-306. (13) Kim, K. M.; Keum, D. K.; Chujo, Y. Macromolecules 2003, 36, 867875. (14) Marcolli, C.; Calzaferrit, G. Appl. Organomet. Chem. 1999, 13, 213226. (15) Li, G.; Wang, L.; Ni, H.; Pittman, C. U., Jr. J. Inorg. Organomet. Polym. 2001, 11, 123-154.

10.1021/la062255h CCC: $37.00 © 2007 American Chemical Society Published on Web 11/15/2006

Structure and Dewetting of POSS-Filled PS Films

Figure 1. Chemical structure of CpPOSS.

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incorporation of CpPOSS into PS thin films resulted in the enhancement of the thermal stability of the films. The present work is focused on the dispersion state of CpPOSS in the CpPOSS/ PS hybrid films, which is expected to affect the dewetting behavior of the hybrid films. In this study, the details of the dewetting behavior and the dispersion state of CpPOSS in the CpPOSS/PS hybrid film, especially at the surface and interface of the films, were investigated. Furthermore, the relationship between the dewetting inhibition and structure of the hybrid thin film is discussed. Experimental Section

Figure 2. Optical micrographs of (a) a 53 nm thick pure PS2k film and (b) a 56 nm thick CpPOSS/PS2k (15/85, w/w) hybrid film annealed at 373 K for 180 min. The length of the bars is 300 µm.

Materials. Two kinds of monodispersed PS films with reported average molecular masses and polydispersity values of Mn ) 2100 and Mw/Mn ) 1.06, respectively (PS2k), and Mn ) 44000 and Mw/ Mn ) 1.04 (PS44k) and deuterated PS with Mn ) 38500 and Mw/Mn ) 1.07 (d-PS38k) were purchased from Polymer Source, Inc. 1,3,5,7,9,11,13,15-Octacyclopentylpentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane (CpPOSS; Figure 1) and cyclopentyltrimethoxysilane were purchased from Aldrich Chemical Co. Sample Preparation. Mixtures of CpPOSS and PS with specific compositions were dissolved in toluene. The CpPOSS mass fractions relative to PS were in the range of 0-15 wt %. The solutions were filtered using 0.2 µm poly(tetrafluoroethylene) filters and spin-coated onto Si substrates. Si wafer substrates were acid-cleaned in a 70/30 volume ratio solution of concentrated H2SO4 (97%) and H2O2 (34.5%) at 353 K for 60 min, rinsed in deionized water, and dried under vacuum before spin-coating. The thicknesses of the films were estimated by ellipsometry using an imaging ellipsometer (Nippon Laser & Electronics Laboratory Inc.) with the optical constants (refractive index, n, and extinction coefficient, k) of the films and the substrate being (1.58, 0.000) and (4.14, 0.045), respectively. As a model surface of CpPOSS, a cyclopentyltrimethoxysilane monolayer was prepared on a Si substrate by the chemical vapor adsorption method16 at 373 K for 180 min. The acid-cleaned Si substrate was photochemically cleaned by vacuum ultraviolet (λ ) 172 nm) ray/ozone treatment before preparation of the monolayer to completely decompose the contaminant on the substrate. Observation of Dewetting Behavior. PS2k films and CpPOSS/ PS2k hybrid films were annealed at various temperatures above the Tg of pure PS2k (331 K, evaluated from differential scanning calorimetry (DSC)) with a hot stage directly under an optical microscope, OLYMPAS BX51 (Olympas Optical Co., Ltd.), to observe the dewetting process in real time under reflection mode. The hot stage was purged by a N2 atmosphere. X-ray Photoelectron Spectroscopy (XPS). The Ar ion sputtering XPS measurement of the CpPOSS/PS44k hybrid film was conducted with a PHI Quantera SXM (ULVAC-PHI, Inc.) with an Al KR X-ray source at 15 kV and 25 W. The Ar ion gun was accelerated at 500 V corresponding to a sputter rate of 1.0 nm min-1 for a SiO2 standard. The emission angle of the photoelectrons, θ, was kept constant at 45°. The surface chemical compositions of the CpPOSS/PS44k hybrid films were also evaluated using a PHI 5800H (Physical Electronics Co., Ltd.) with an Al KR X-ray source at 14 kV and 350 W. The measurements were carried out at several θ values from 15° to 90°. The analytical depth, d, from the outermost surface is dependent on θ and is given by d ) 3λ sin θ

Figure 3. Dewetting area fraction versus CpPOSS concentration of a ca. 60 nm thick CpPOSS/PS2k hybrid film annealed at 373 K for 60 min.

In the previous work, cyclopentyl-substituted POSS (CpPOSS; Figure 1) was introduced into PS thin films as a nanofiller. The

(1)

where λ is the inelastic mean-free path of photoelectrons in the solids.17 Neutron Reflectivity (NR). The NR measurement was performed on a CpPOSS/d-PS38k hybrid film using an advanced reflectometer (16) Hozumi, A.; Ushiyama, K.; Sugimura, H.; Takai, O. Langmuir 1999, 15, 7600-7604. (17) Stuart, B. H. Polymer Analysis; Wiley: West Sussex, 2002; Chapter 6.

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Figure 4. Dewetting area fraction versus annealing time of a 57 nm thick PS2k film and a 56 nm thick CpPOSS/PS2k (10/90, w/w) hybrid film annealed at 393 K.

Figure 5. Optical micrograph of a 56 nm thick CpPOSS/PS2k (10/90, w/w) hybrid film annealed at 393 K for 60 min. The length of the bar is 300 µm. for interface and surface analysis (ARISA) on the H5 beamline of the Neutron Science Laboratory, High Energy Accelerator Research Organization.18 The NR data were analyzed with Parratt 32 software from HMI Berlin. The NR data were plotted versus the scattering vector, q, which was defined by q ) (4π sin θ)/λ

(2)

where λ and θ are the wavelength and incident angle of the neutron, respectively. X-ray Diffraction (XRD). XRD measurements of CpPOSS/PS44k hybrid films were carried out in symmetrical reflection geometry on a Rigaku RINT 2500V (Rigaku Denki Co., Ltd.) with a Cu KR X-ray source at 40 kV and 400 mA. The data collection time was 3 s per step at 0.05° intervals. Atomic Force Microscopy (AFM). AFM observation was performed with an SPA-400 (Seiko Instruments Inc.). AFM images were obtained under constant force mode under weak repulsive force using a 20 µm × 20 µm scanner and a rectangular Si3N4 tip with a spring constant of 0.05 N m-1. Contact Angle Measurements. The static contact angles of 2 µL droplets of deionized water and diiodomethane on PS44k films, CpPOSS/PS44k hybrid films, and a cyclopentyltrimethoxysilane monolayer were measured using a DSA-10 (Kru¨ss Co., Ltd.). A liquid droplet was introduced onto the solid surface through the microsyringe, and the needle was moved away from the droplet. The droplet was equilibrated for a few seconds, and it was confirmed (18) Torikai, N.; Furusaka, M.; Matsuoka, H.; Matsushita, Y.; Shibayama, M.; Takahara, A.; Takeda, M.; Tasaki, S.; Yamaoka, H. Appl. Phys. A: Mater. Sci. Process. 2002, 74 (Suppl.), S264-S266.

Figure 6. Depth profile of a 129 nm thick CpPOSS/PS44k (15/85, w/w) hybrid film evaluated from Ar ion sputtering XPS measurement.

Figure 7. Concentration of Si for a ca. 130 nm thick CpPOSS/ PS44k (10/90, w/w) hybrid film before and after annealing at 413 K for 90 min. that the shape of the droplet did not change anymore before the contact angles were measured. The surface free energies of the samples were calculated from the static contact angles using Owens and Wendt’s method.19

Results and Discussion Low molecular weight PS2k was used as a matrix polymer for the investigation of the dewetting behavior to observe the film dewetting in the experimental time scale. CpPOSS/PS2k hybrid films were prepared to be approximately 60 nm in thickness. Optical microscopy indicated that the films were smooth and uniform when inspected immediately after spincoating. Figure 2 shows the optical micrographs of a 53 nm thick pure PS2k film and a 56 nm thick CpPOSS/PS2k (15/85, w/w) hybrid film annealed at 373 K for 180 min. The PS2k film broke up by the creation and growth of holes, which finally contacted each other, creating cellular structures. The rims decayed into droplets at the final stage of the dewetting as shown in Figure 2a. On the other hand, no appreciable dewetting was observed by optical microscopy in the CpPOSS/PS2k hybrid film even after the annealing treatment, which resulted in the pure PS2k films completely dewetting. This result showed that the incorporation of CpPOSS into PS2k films eliminated the dewetting of the films. Figure 3 shows the dependence of the dewetting inhibition on the CpPOSS concentration. The dewetting process was observed in real time by reflective optical microscopy, and the dewetting area was calculated from the optical micro(19) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 1741-1747.

Structure and Dewetting of POSS-Filled PS Films

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Table 1. Static Contact Angles of Water and Diiodomethane and Surface and Interfacial Free Energies and Spreading Coefficient of the PS44k Film, CpPOSS/PS44k Hybrid Films, and the CpPOSS Model Surface sample

θH2O/ deg

θCH2I2/ deg

γd/ mJ m-2

γh/ mJ m-2

γ/ mJ m-2

S/ mJ m-2

Si substrate PS44k film CpPOSS/PS44k (10/90, w/w) hybrid film CpPOSS/PS44k (15/85, w/w) hybrid film CpPOSS model surface

19.4 98.5 99.6 100.2 82.9

40.7 20.5 29.2 40.5 49.4

28.1 49.4 46.1 40.3 31.7

41.4 0.1 0.0 0.0 4.2

69.5 49.5 46.1 40.3 35.9

-21.1 -17.8 -12.7

graphs as a percentage of the bare substrate in the micrographs. The addition of CpPOSS into the PS2k film had a tendency to inhibit the film from dewetting; however, the degree of inhibition depended on the CpPOSS concentration. Holes were formed on the hybrid films with CpPOSS concentrations lower than 15 wt %. To observe the dewetting behavior of the hybrid films, the CpPOSS/PS2k (10/90, w/w) hybrid film was annealed at 393 K. The dewetting behavior of the hybrid film was completely different from that of the pure PS2k film. Figure 4 shows the dewetting behavior of the pure PS2k film and the CpPOSS/PS2k hybrid film annealed at 393 K. The pure PS2k film was completely dewetted after 20 min of annealing; however, the growth of the holes formed on the hybrid film stopped before the final stage of dewetting was reached. Figure 5 shows the optical micrograph of the CpPOSS/PS2k hybrid film annealed at 393 K for 60 min. This behavior implied that the initial state of the hybrid film was changed through the annealing process into a favorable state for dewetting inhibition. The spin-coated films were not in the equilibrium state; therefore, CpPOSS in the hybrid films would try to move to the equilibrium position during the annealing treatment. In addition, the CpPOSS concentration per unit area at the rim of the holes was expected to increase in the dewetting process, and the dewetting behavior of the hybrid film might suggest that the CpPOSS concentration per unit area played an important role in the dewetting inhibition. To understand the origin of this dewetting inhibition, the dispersion state of CpPOSS in CpPOSS/PS hybrid films was investigated. CpPOSS/PS hybrid films for structural analysis (CpPOSS/PS44k and CpPOSS/d-PS38k hybrid films) were prepared to be 110-130 nm in thickness. The dispersion state of CpPOSS in a 129 nm thick CpPOSS/ PS44k (15/85, w/w) hybrid film was investigated by Ar ion sputtering XPS measurement. A CpPOSS concentration of 15 wt % showed the elimination of dewetting in CpPOSS/PS2k hybrid films. Figure 6 shows the depth profile of the CpPOSS/ PS44k hybrid film evaluated from Ar ion sputtering XPS measurement. The XPS result shows the enrichment of Si and O on the surface of the film, indicating the segregation of CpPOSS. The Si and O concentrations on the surface of the film were about 6 times higher than the theoretical values of the mixture of CpPOSS/PS44k. The Si concentration on the surface of the film was 10.5%, which is close to the Si concentration in the CpPOSS molecule, 13.3%, suggesting considerable coverage of the surface by CpPOSS. In the middle region of the film, the concentrations of Si and O were evaluated as 80% of the theoretical value of the mixture. Oxygen was also detected at the interface between the Si substrate and the CpPOSS/PS44k layer, but it is not clear whether this means the segregation of CpPOSS to the interface because of the existence of a silicon oxide layer on the Si substrate. In the observation of dewetting behavior in CpPOSS/PS2k hybrid films, a CpPOSS concentration of 10 wt % suggested that the change of the dispersion state of CpPOSS in the hybrid films contributed to the dewetting inhibition. Therefore, the surface

composition of the CpPOSS/PS44k (10/90, w/w) hybrid films before and after annealing treatment was investigated by XPS measurements to estimate the structural changes through the annealing process. The films were annealed at 413 K, which is above the Tg of pure PS44k (361 K, evaluated from DSC), for 90 min under vacuum. No appreciable dewetting was observed by optical microscopy even in the pure PS44k films after this annealing treatment because the dewetting rate of the PS44k films was much slower than that of the PS2k films. The XPS results are shown in Figure 7. The horizontal axis shows the sine of the emission angle of the photoelectrons, θ, corresponding to the depth near the surface region. The Si concentration increased with a decrease of sin θ, and segregation of CpPOSS to the film surface was also indicated. The Si concentration at the surface of the CpPOSS/PS44k (10/90, w/w) hybrid film was lower than that of the CpPOSS/PS44k (15/85, w/w) film; however, segregation of CpPOSS to the surface was enhanced by the annealing process as shown in Figure 7. The surface segregation of CpPOSS can change the surface energetics of the film. Surface energetics is an important factor causing film rupture. The key to whether a film is stable or not on a certain substrate is given by a spreading coefficient, S:20

S ) γB - (γA + γAB)

(3)

where γA and γB are the surface free energies of the polymer film and substrate, respectively, and γAB is the free energy of the film-substrate interface. If S is negative, then the film is unstable. To understand the dewetting of PS films, preliminary measurements of the surface energy of the acid-cleaned Si substrates, PS44k films, and CpPOSS/PS44k hybrid films were performed by the static contact angle measurement of deionized water and diiodomethane. PS44k films and the hybrid films were annealed at 413 K for 90 min before the measurements. As a model surface of CpPOSS, the contact angle of the cyclopentyltrimethoxysilane monolayer prepared on the Si substrate was also evaluated. The surface free energy, γ, and its dispersive and hydrogen-bonding components, γd and γh, respectively, were calculated from the static contact angles of the samples using Owens and Wendt’s equation.19 The interfacial free energy of the film and acidcleaned Si substrate, γAB, was evaluated using the extended Fowkes equation19

γAB ) γA + γB - 2(γdAγdB)1/2 - 2(γhAγhB)1/2

(4)

The values of the contact angles, θ, and free energies are shown in Table 1. S of the films on the substrate was evaluated from the surface energies shown in Table 1. As shown in Table 1, γ of the CpPOSS model surface, 35.9 mJ m-2, was lower than that of PS44k, 49.5 mJ m-2. This result suggested that the surface segregation of CpPOSS was obtained by the lower γ of CpPOSS than that of the PS matrix. The segregation of the low γ component to the surface affected the (20) Adamson, A. W. Physical Chemistry of Surface, 5th ed.; Wiley: New York, 1990; Chapter 10.

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Figure 8. NR profile and fitting curve of a 111 nm thick CpPOSS/ d-PS38k (10/90, w/w) hybrid film. The solid curve is the result of a model fit corresponding to the depth profile model of b/V shown in the inset.

surface energetics of the films. The value of γ of the CpPOSS/ PS44k (10/90, w/w) film was lower than that of pure PS44k films, indicating surface segregation of CpPOSS reduced the γ of the films. The CpPOSS/PS44k (15/85, w/w) hybrid film had an even lower γ than CpPOSS/PS44k (10/90, w/w) film, indicating the surface coverage of the film by CpPOSS was extended with an increase of the CpPOSS concentration. On the basis of the values of the surface and interfacial free energies, the dewetting of the PS film can be explained by the negative S, -21.1 mJ m-2. The change of the surface energies in the CpPOSS/PS44k hybrid film also brought an increase of S, which can reduce the dewetting of the films. However, S was still a negative value even in the CpPOSS/PS44k (15/85, w/w) hybrid film, -12.7 mJ m-2, so the change of S seems to retard the progress of the dewetting but not enough for elimination of the dewetting. The interfacial structure, which is also an important factor affecting the dewetting behavior of the films, was investigated by NR measurement. The NR profile for the CpPOSS/d-PS38k (10/90, w/w) hybrid film is shown in Figure 8. The solid curve in Figure 8 is the result of a model fit corresponding to the depth profile of the scattering length density (b/V) shown in the inset. b/V of d-PS38k was estimated as 6.05 × 1010 cm-2 from preliminary NR measurement of a pure d-PS38k thin film, and b/V of CpPOSS was evaluated as 0.82 × 1010 cm-2 from the reported density, 1.33 g cm-3,21 and calculated scattering length, 99.4 fm, of CpPOSS.22 Since b/V of the d-PS38k matrix is much larger than that of the CpPOSS nanofiller, the distribution of the nanofiller in the film can be studied by NR measurement. At the surface of the hybrid film, as shown in the inset of Figure 8, the boundary of the air and the film was broadened due to the segregation of CpPOSS to the film surface, indicated by XPS measurements, and the morphological roughness of the surface. The b/V of the hybrid film was lower than that of pure d-PS38k in the middle region of the film because of the existence of CpPOSS and extensively decreased near the film-substrate interface. At the interface, the roughness of the Si substrate can be ignored, so the result means the formation of a CpPOSS enrichment layer at the film-substrate interface like other nanofillers.6-8 The segregation of CpPOSS at the film-substrate (21) Bassindale, A. R.; Lui, Z.; MacKinnon, I. A.; Taylor, P. G.; Yang, Y.; Light, M. E.; Horton, P. N.; Hursthouse, M. B. Dalton Trans. 2003, 2945-2949. (22) Rauch, H.; Waschkowski, W. In Neutron Data Booklet, 2nd ed.; Dianoux, A. J., Lander, G., Eds.; OCP Science: Philadelphia, 2003; Chapter 1.

Hosaka et al.

Figure 9. XRD profiles of a ca. 130 nm thick CpPOSS/PS44k (10/90, w/w) hybrid film before and after annealing at 413 K for 90 min.

interface can change the energetics at the interface and affect the dewetting behavior of the film. Furthermore, the CpPOSS enrichment layer roughened the interfacial structure and was expected to cause the pinning of the dewetting front. The driving force of the segregation of CpPOSS at the film-substrate interface has not been proved yet; nevertheless, it can be attributed to the entropic effect. Polymer chains existing in the interface should be in a confined state. This entropic penalty of the low molecular weight component, such as CpPOSS, is smaller than that of the polymers, so CpPOSS molecules segregate to the film interfaces. Less unfavorableness of the interaction between CpPOSS and the Si substrate than that between PS and the substrate is also expected to cause the segregation of CpPOSS to the interface. The dispersion state of CpPOSS in the hybrid films was also investigated by XRD. XRD measurements were performed on the CpPOSS/PS44k (10/90, w/w) hybrid film before and after annealing at 413 K for 90 min. The results are shown in Figure 9. In the hybrid film after annealing, a diffraction originating from a CpPOSS crystallite was observed at q ) 5.8 nm-1, which did not appear at the hybrid film before annealing. This implied that CpPOSS, which segregated to the surface and interface of the films, formed the crystallite. The crystallization of CpPOSS was expected to roughen the interfaces and, in addition, reduce the mobility of PS molecules at the surface. However, crystallization of CpPOSS in the middle region of the films made the aggregates and roughened the film surface. Figure 10 shows the AFM images along with the line profile of the CpPOSS/PS44k (10/90, w/w) hybrid film before and after annealing at 413 K for 90 min. The line profile revealed that the hybrid film was relatively smooth before annealing, several nanometers in roughness. The initially smooth CpPOSS/PS44k film was roughened through the annealing process as shown in Figure 10b, which implied aggregation of CpPOSS in the hybrid film. The roughness of the CpPOSS/PS44k hybrid film after annealing was tens of nanometers in height and hundreds of nanometers in width. In the XRD measurements, there was no evidence of penetration of the PS matrix into the CpPOSS crystallite, which might bring the shift of the diffraction peak, and the low affinity between PS and CpPOSS seemed to cause the aggregation of CpPOSS. Future work will be focused on the improvement of the affinity between CpPOSS and PS. The results mentioned above indicated that the inhibition of dewetting was closely related to the dispersion state of CpPOSS in the CpPOSS/PS hybrid films. Structural analysis revealed CpPOSS segregation to the film surface and interface. The segregation of CpPOSS in CpPOSS/PS44k (10/90, w/w) was enhanced through the annealing procedure. The annealing process

Structure and Dewetting of POSS-Filled PS Films

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Figure 10. AFM images and line profiles of a ca. 120 nm thick CpPOSS/PS44k (10/90, w/w) hybrid film (a) before and (b) after annealing at 413 K for 90 min.

also promoted the crystallization of CpPOSS in the films. These phenomena gave an explanation of the dewetting behavior observed in the CpPOSS/PS2k (10/90, w/w) hybrid film, that is, the arrest of the growth of the holes on the hybrid films before the final stage of the dewetting was reached. The CpPOSS/PS2k (15/85, w/w) hybrid film showed a strong inhibition effect of dewetting, and no holes were created in the films under the annealing process adopted in this study. This implied that sufficient coverage of the interfaces by CpPOSS was achieved even before annealing in that film. The CpPOSS enrichment layer changed the energetics of the surface and interface and retarded the dewetting. However, the change of the energetics did not seem to be enough for the entire inhibition of the dewetting. The rough interfaces brought by the segregation and crystallization of CpPOSS also played an important role in the dewetting inhibition through immobilizing the PS molecules and pinning the dewetting front.

Conclusion The present study demonstrated that the addition of CpPOSS to the PS thin films can actually stabilize the films against dewetting. The degree of inhibition depended on the CpPOSS

concentration, and strong inhibition was observed at the hybrid film containing 15 wt % CpPOSS. Structural analysis revealed that CpPOSS segregated to the film surface and film-substrate interface. Segregation and crystallization of CpPOSS were enhanced by the annealing process. The dewetting behavior of the hybrid film with 10 wt % CpPOSS suggested that the structural changes through the annealing process played an important role in dewetting inhibition. These results suggested that inhibition of dewetting can be attributed to the segregation and crystallization of CpPOSS at the film surface and film-substrate interface, accompanying modification of the energetics and morphologies of the interfaces. Acknowledgment. The present work was partially supported by a Grant-in-Aid for the 21st Century COE Program “Functional Innovation of Molecular Informatics” from the Ministry of Education, Culture, Science, Sports and Technology of Japan. N.H. acknowledges the financial support of a Grant-in-Aid for JSPS Fellows. Ar ion sputtering X-ray photoelectron spectroscopy was kindly conducted by ULVAC-PHI, Inc. LA062255H