bk-2010-1051.ch015

Chapter 15

Downloaded by PENNSYLVANIA STATE UNIV on June 25, 2012 | http://pubs.acs.org Publication Date (Web): November 9, 2010 | doi: 10.1021/bk-2010-1051.ch015

Ion Effects on Trisilanolphenyl-POSS as an Adhesion Promoter Sarah M. Huffer, Ufuk Karabiyik, Joshua R. Uzarski, and Alan R. Esker* Department of Chemistry and the Macromolecules and Interfaces Institute, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 *[email protected]

Polyhedral oligomeric silsesquioxanes (POSS) have been an area of intense interest during the past two decades. Their unique properties may facilitate aerospace applications as space-survivable coatings and ablative insulation. Recent studies showed that trisilanol-POSS derivatives form self-assembled monolayers at the air/water (A/W) interface. One particular derivative, trisilanolphenyl-POSS (TPP), even forms Langmuir-Blodgett (LB) multilayer films by Y-type deposition on hydrophobic surfaces. In addition, TPP can complex metal ions, such as Al3+. The purpose of this study was to improve adhesion between ceramics and metals and metals and polymers by preparing multilayer films with and without metal ions using TPP as an adhesion promoter. These multilayer systems were prepared by spincoating to make the layers of polystyrene, the LB-technique to create the TPP layers, and physical vapor deposition (PVD) to produce alumina layers. The resulting films were characterized for quality and stability using atomic force microscopy (AFM), optical microscopy (OM), X-ray photoelectron spectroscopy (XPS), and dewetting experiments. Results for alumina PVD show that films on TPP coated Si wafers are blistered and cracked in contrast to films on Si wafers coated with TPP plus aluminum ions. Similarly, polystyrene dewetting on TPP layers was suppressed when the TPP layers contained Al3+.

© 2010 American Chemical Society In Advances in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Introduction Polyhedral oligomeric silsesquioxanes (POSS) have been studied with great interest for more than two decades (1–3). These organic/inorganic hybrid molecules have potential applications as low k dielectrics (4, 5), high temperature nanocomposites (6–8), and space-survivable coatings (9–12). Additionally, trisilanol-POSS derivatives have been found to self-assemble as monolayers at the air/water (A/W) interface (13–16). Figure 1 shows a generic trisilanol-POSS. The OH groups attached to the cage are hydrophilic and can lie on the subphase while the R groups (phenyl for this study) are hydrophobic and take up a position away from the water surface. For the case of trisilanolphenyl-POSS (TPP), this amphiphilic character also allows the formation of Langmuir-Blodgett (LB) films. In general, amphiphilic materials can be studied at the A/W interface with techniques such as florescence microscopy (17, 18) and Brewster angle microscopy (BAM) (19, 20). Moreover, amphiphilic species that can be transferred to solid substrates avail themselves to high resolution techniques, such as atomic force microscopy (21, 22) and X-ray diffraction (23, 24). In this study, the ability to prepare and characterize thin POSS films will be used to study POSS-metal interactions. Extensive research has focused on siloxane/metal complexes (25–33). Metallasilsesquioxanes have been used as representative surfaces for zeolites, clays, or silicates that are instrumental in catalysis. For example, titanasilsesquioxanes are active catalysts in the oxidation of olefins. The incompletely condensed silsesquioxanes, such as trisilanol-POSS, are the most useful in creating model surface silanol sites found in zeolites and amorphous silicas (31).

Figure 1. Structure of a trisilanol-POSS derivative (R = phenyl for this study).

Figure 2. Configurations for the multilayer films utilized in this study. 184 In Advances in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


The incorporation of metals in silsesquioxanes also improves the properties of the molecule by making them more rigid (33) as well as rendering them catalytically active for Diels-Alder reactions, polymerization, and epoxidation (31). Metallasilsesquioxanes also have electron withdrawing sites for bonding, and selective hydroxyl groups allow certain reagents to react at the surface, as occurs during alkene metathesis (33). In addition to catalytic activity, POSS-metal complexes in composites with polymers are also being studied for enhancing mechanical and thermomechanical properties. Chian, et al., has shown that POSS fillers in an epoxy resin increase both the maximum shear strain and the maximal stress (34). Complexes of POSS with metals, such as aluminum, have been shown to increase the modulus of resins used in dental applications (35). Liu, et al., also discovered that POSS enhanced thermomechanical properties, such as higher glass transition and initial thermal decomposition temperatures, in epoxy networks (36). As already noted, POSS has long been studied for thermal protective systems (9–12). In this study, we will examine if the bonding properties of metallasilsesquioxanes are suitable for using these complexes as adhesion promotion layers in thermal protective systems. We recently discovered that metal ions, such as copper, silver, and iron, could be incorporated into LB-films of TPP. For the present study, the influence of Al3+ ions incorporated into LB-films of TPP on the bonding of aluminum oxide layers prepared through physical vapor deposition (PVD) to a silicon wafer through a POSS film (schematically depicted in Figure 2A), and on the bonding of polystyrene to an alumina surface through a POSS layer (schematically depicted in Figure 2B) will be examined by x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and optical microscopy.

Experimental Materials Trisilanolphenyl-POSS obtained from Hybrid Plastics, Inc., and used without further purification was dissolved in chloroform (0.05-0.5 mg·mL-1, HPLC grade) to prepare spreading solutions for LB-film deposition. Because of slow dissolution, the samples were prepared and stored for at least 24 h at room temperature in specially sealed vials to avoid the evaporation of chloroform. Aluminum nitrate was obtained from Sigma Aldrich, and 1 mM aluminum nitrate in Millipore water (18.2 MΩ, < 10 ppb organic) was used as a subphase for making the metal ion/TPP LB-films. Aluminum pellets (3-5 mesh) were also obtained from Sigma Aldrich with a purity of 99.999% for preparing alumina films through PVD. Polystyrene with a number average molar mass, Mn = 1460 g/mol, and a polydispersity index, Mw/Mn = 1.06, was used as received from Polymer Source, Inc. for dewetting studies. The polystyrene was dissolved in methyl ethyl ketone (MEK) as a 1% solution by weight. 4" (100) silicon wafers were obtained from Waferworld, Inc. 185 In Advances in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Substrate Preparation Silicon wafers were cut into roughly 25 x 40 mm2 pieces. The wafer pieces were cleaned in a boiling mixture of 28% NH4OH: 30% H2O2: Millipore water mixture in a 1:1:5 ratio by volume for 1.5 hours. Next, the wafers were placed in a concentrated H2SO4: 30% H2O2 mixture in a 7:3 ratio by volume. At this stage, the surface is a hydrophilic silica surface that can be used to create films depicted in Figure 2b after first rinsing the films with water and drying with nitrogen. In order to obtain the hydrophobic silicon surface depicted in Figure 2A, the hydrophilic silicon wafer was further treated with a buffered oxide etch (HF) solution (DOE and Ingalls, CMOS grade) for 5 minutes and exposed to 40% NH4F solution (DOE and Ingalls, CMOS grade). The wafers were subsequently rinsed with Millipore water and dried with nitrogen.

Film Preparation A standard KSV 2000 LB-trough (KSV Instruments, Inc.) was used to prepare LB-films of TPP and TPP with aluminum ions. The LB-trough is filled with Millipore water or a 1 mM aluminum nitrate solution at 22.5 °C. The barriers and suction are used to concentrate and remove surface active impurities from the trough until the surface tension of the subphase matches the literature value. Next, the TPP solution was spread between the barriers of the trough. After allowing ~20 min for the spreading solvent to evaporate, the TPP was compressed to a constant transfer surface pressure of 9.5 mN·m-1. The LB-films depicted in Figure 2a were created by dipping the hydrophobic silicon wafer through the TPP film at a rate of 10 mm·min-1, with waiting intervals of 15 s under the surface and 300 s above. Thirty layers of TPP were created by Y-type deposition with quantitative transfer ratios. For films depicted in Figure 2B, a single layer of TPP or TPP with aluminum ions was deposited on top of the hydrophilic alumina surface during the upstroke. Polystyrene was subsequently spincoated on top of the TPP monolayer using a solution of 1% PS by weight in MEK at a spinning rate of 4000 rpm. The PS film was annealed at 45 °C (glass transition temperature) for 2 hours to remove residual solvent. The PS films are ~ 28 nm (from ellipsometry).

Physical Vapor Deposition (PVD) Figure 3 shows a schematic of the apparatus that held the silicon wafers for PVD. The wafers were inverted to face the aluminum pellet, which sat in a boron nitride crucible (Kurt J. Lesker Co.) wrapped in a tungsten filament. A diffusion pump was used to reduce the pressure to 5x10-6 mmHg. The filament connected to the power source heated the crucible and its contents. The temperature was increased slowly to around 1000 °C, and the aluminum was vaporized in this manner for 5 minutes to ensure that a layer of aluminum had been deposited. The aluminum rapidly oxidizes to form an aluminum oxide surface. 186 In Advances in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 3. Schematic depiction of the interior of the PVD apparatus Dewetting Experiments Dewetting was studied using an optical microscope operating in the reflection mode (Axiotech Vario 100 HD, Carl Zeiss Inc.). The films were annealed on a temperature-controlled heating stage (Linkam) from 25 to 200 °C at 1 °C·min-1 and images of dewetting as a function of temperature were captured and viewed using Scion Image software. X-ray Photoelectorn Spectroscopy (XPS) XPS measurements were made using a PHI 5400 (Perkin- Elmer) with Mg-Kα radiation at an angle of 45°. Atomic Force Microscopy (AFM) AFM images were obtained in the Tapping Mode™ with a Digital Instruments Dimension 3000 Scope with a Nanoscope IIIa controller using etched single crystal silicon tips. 5 µm × 5 µm images were captured at a set-point ratio of ca. 0.6.

Results Adhesion between Silica or Silicon and Alumina Figure 4 shows a height and phase image for an aluminum layer placed onto a hydrophilic silica coated wafer by physical vapor deposition. X-ray photoelectron spectroscopy (XPS) confirmed that aluminum was present in high percentages on the silica surface with prominent aluminum 2s and 2p peaks. A tall oxygen peak was present at approximately 520 eV, indicating that the aluminum rapidly oxidizes to aluminum oxide. AFM images of the resulting film in Figure 4 reveal the root mean square (RMS) roughness is ~0.3 nm. Physical vapor deposition can also be used to place a uniform alumina layer onto a hydrophobic silicon wafer (HF etched) with properties comparable to the alumina layer on the silica surface. 187 In Advances in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 4. Height (left) and phase (right) AFM images of the alumina coating obtained from PVD onto a hydrophilic silica surface. The 5 x 5 µm2 images have z-scales of 5 nm and 5 degrees, respectively.

Figure 5. Representative optical microscopy images of alumina coatings for films depicted in Figure 2A on A) 30 layers of TPP, B) 30 layers of TPP, and C) 30 layers of TPP with aluminum ions. All images are 2.28 x 3.04 mm2. 188 In Advances in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Adhesion between TPP and Alumina Having, established that a uniform alumina layer can be deposited onto silica and silicon surfaces, TPP was LB-deposited onto HF etched silicon with and without Al3+. Subsequently, an alumina layer was added by physical vapor deposition to obtain the films depicted in Figure 2A. The TPP-coated silicon without aluminum ions developed surfaces with gross defects. Both blisters and cracks (Figure 5A and 5B) were observed. Independent of the type of defect observed, the portion of the hydrophobic Si wafer not coated with TPP exhibited a smooth alumina surface. This observation indicates that the blistered morphology was not an artifact of the deposition process, but rather a consequence of different interactions between TPP and alumina vs. alumina and the HF etched Si. In contrast, when Al3+ was co-transferred with the TPP film, a homogeneous film with no visible blistering formed (Figure 5C) indicating that the presence of the aluminum ions inhibited dewetting of the aluminum/alumina surface during PVD and the subsequent oxidation of the film. Figure 6 shows representative height and phase AFM images of the same alumina-coated sample in Figure 5B. The images were taken from smooth areas of the alumina film. The RMS roughness of the alumina film on 30 layers of TPP was 23 nm (vs. < 1 nm on silica). Figure 7 shows a representative height and phase image for the same film presented in Figure 5C (30 layers of TPP co-transferred with aluminum ions). The most striking difference is the three-fold increase in RMS roughness to ~69 nm when Al3+ is present in the TPP layer. Hence, two factors could be contributing to the improvement of aluminum/alumina wetting on the TPP/aluminum ion layer. One possibility is a more favorable surface energy, while the other is enhanced wetting due to the increased roughness. Nonetheless, the rougher TPP/aluminum ion films clearly inhibit gross failures like blistering and cracking that occur without the aluminum ions.

Figure 6. 20 x 20 µm2 height (left) and phase (right) AFM images of the alumina coating from PVD on 30 layers of TPP (film configuration depicted in Figure 2A). The images have z-scales of 200 nm and 50 degrees, respectively.

189 In Advances in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 7. 20 x 20 µm2 height (left) and phase (right) AFM images of the alumina coating from PVD on 30 layers of TPP plus Al3+ (film configuration depicted in Figure 2A). The images have z-scales of 500 nm and 30 degrees, respectively.

Figure 8. 0.57 x 0.76 mm2 optical microscopy images of polystyrene (Mn = 1460 g/mol) on alumina coated with a single layer of TPP as a function of temperature: A) 110 °C, B) 140 °C, and C) 160 °C. Adhesion between Polystyrene and Alumina with TPP Adhesion Promotion Layers As discussed above, it was possible to coat silica with aluminum/alumina without an adhesion promotion layer. Since the resulting aluminum oxide layer is hydrophilic, only a net monolayer of TPP can be LB-deposited onto alumina as TPP only adds during an upstroke and is removed during a downstroke. This behavior is the same for TPP complexed with Al3+. In a control experiment, 190 In Advances in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


XPS showed that TPP is not removed from the alumina surface by MEK during spincoating (solvent only). Hence, MEK provides an appropriate solvent for spincoating polystyrene directly onto the TPP treated surfaces to obtain the films schematically depicted in Figure 2B. Temperature ramp experiments from 25 °C to 200 °C at 1 °C·min-1 were conducted to study the dewetting properties and to test the adhesion of a polystyrene film to the modified alumina. Figure 8 shows the dewetting of the polystyrene layer on top of a single layer of TPP. At temperatures above 110 °C (Figure 8A), dewetting began around nucleation sites, such as impurities and defects. As the temperature increased from 110 °C (Figure 8A) to 140 °C (Figure 8B), the polystyrene film developed holes (white circles) around these nucleation sites. At 160 °C (Figure 8C), the polystyrene film was completely dewet, and droplets of polystyrene (irregular features on the lighter alumina background) remained on the surface. In contrast, Figure 9 shows how Al3+ in the TPP monolayer suppresses dewetting of the polystyrene layer at the same temperatures shown in Figure 8. This suppression of dewetting relative to a TPP adhesion promotion layer may reflect subtle changes in surface energy attributed to the presence of Al3+ ions. The exact role of the aluminum ions within the TPP film is not yet completely understood, and additional experiments are necessary to ultimately understand how the ions inhibit the dewetting process.

Figure 9. 0.57 x 0.76 mm2 optical microscopy images of polystyrene (Mn = 1460 g/mol) on alumina coated with a single layer of TPP with Al3+ as a function of temperature: A) 110 °C, B) 140 °C, and C) 160 °C.


Conclusions


Aluminum ions influenced the adhesion between TPP LB-films and alumina coatings prepared by PVD. AFM results for the configuration schematically depicted in Figure 2A revealed that the roughness of the alumina film increased with the addition of Al3+ to the TPP layers. Nonetheless, OM images gave visual confirmation that the alumina layer on the TPP with Al3+ film did not exhibit the cracking and blistering that occurred on TPP coatings without Al3+. For the configuration schematically depicted in Figure 2B, the addition of Al3+ suppressed dewetting of the polystyrene layer up to 200 °C. Without Al3+, the polystyrene layer began to dewet at 110 °C and had completely dewet around 160 °C.

Acknowledgments The authors would like to thank the Virginia Space Grant Consortium (200506) and the National Science Foundation (CHE-0239633 and DMR-0244141) for funding, the POSS group at Edwards Air Force Base for materials, and Prof. John Morris at Virginia Tech for the use of his PVD apparatus.

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