ADHESIVE PROPERTIES OF TRISILANOLPHENYL-POSS
Sarah M. Huffer and Alan R. Esker
Department of Chemistry (0212)
Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
shuffer@vt.edu and aesker@vt.edu
Abstract
Polyhedral oligomeric silsesquioxanes (POSS)
have been an innovative area of research for
the past twenty years. Their unique properties
allow for use in aerospace applications as
space-survivable coatings and insulation.
Recent studies showed that trisilanol-POSS
derivatives form self-assembled monolayers at
the air/water (A/W) interface. The purpose of
this study was to improve adhesion between
ceramics and metals (Configuration 1) and
metals and polymers (Configuration 2) by
preparing multilayer films at various pH
values and metal ion concentrations using
trisilanolphenyl-POSS (TPP).
These
multilayer systems were prepared by
spincoating to make the polymer layer of
polystyrene, the Langmuir-Blodgett (LB)
technique to create the TPP layer, and
physical vapor deposition (PVD) to produce
the aluminum layer. The resulting films were
characterized for quality and stability using
atomic force microscopy (AFM), optical
microscopy (OM), X-ray photoelectric
spectroscopy
(XPS),
and
dewetting
experiments.
Initial experiments on
Configuration 1 demonstrated that TPPaluminum ion complexes created a smooth
aluminum film on silica while TPP alone
caused a blistered aluminum surface.
Dewetting experiments on Configuration 2
showed that the polystyrene layer completely
dewet on TPP, but the TPP-aluminum ion
complexes suppressed dewetting.
Introduction
Polyhedral oligomeric silsesquioxanes
(POSS) have been studied with great interest
for over two decades.1-3 These inorganicorganic hybrid molecules have potential
applications as low k dielectrics,4,5 hightemperature nanocomposites,6-8 and spacesurvivable coatings.9-12 Recently trisilanolPOSS derivatives have been found to selfassemble as monolayers at the air/water
(A/W) interface.13-16 As can be seen in Figure
1, the OH groups attached to the cage are
hydrophilic and can rest on the subphase while
the R groups are hydrophobic and take up a
position away from the water surface. This
amphiphilic character also allows the
formation of Langmuir-Blodgett (LB) thin
films. Trisilanol-POSS derivatives can be
studied at the A/W interface with useful
techniques such as Brewster angle microscopy
(BAM)17,18 and florescence microscopy.19,20
Atomic force microscopy (AFM)21,22 and Xray diffraction23,24 also provide higher
resolution information about LB-films.
R
O Si OH
O
O
O
Si OH
R
R
OH
O Si
Si
O
O
Si R
R
O
Si O
R Si
R
Figure 1. Structure of a trisilanol-POSS
derivative where R is usually an organic
substituent.
Extensive research has focused on
siloxane/metal
complexes.25-33
Metallasilsesquioxanes have been used as
representative surfaces in zeolites, clays, or
silicates that are instrumental in catalysis. For
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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
The
incorporation of metals in silsesquioxanes also
improves the properties of the molecule by
making them more rigid33 as well as useful
catalysts for Diels Alder reactions,
polymerization, and epoxidation.31 These
metallasilsesquioxanes also have electron
withdrawing sites for bonding, and selective
hydroxyl groups allow certain reagents to
react at the surface.33
POSS has long been studied for
thermal protective systems.9-12 Using the
bonding properties of metallasilsesquioxanes,
it is possible to incorporate these complexes
into adhesives for aerospace applications. We
recently discovered that metal ions, such as
copper, iron, and silver, could be incorporated
in LB-films of TPP. This study investigates
how the incorporation of Al+3 ions in LB-films
of TPP alter adhesion between aluminum
oxide and silicon and a polymer and
aluminum oxide as depicted schematically in
Figure 2.
A)
Metal
B)
Polymer
POSS + Metal Ion
POSS + Metal Ion
Metal
Solid
Solid
Figure 2. Configuration of the multilayer
films prepared in this study.
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).
As a result 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 pellets (3-5 mesh) were obtained
from Sigma Aldrich with a purity of 99.999%.
Aluminum nitrate was also obtained from
Sigma Aldrich, and 1 mM aluminum nitrate in
Millipore water was used as a subphase for
making the metal ion/TPP LB films.
Film Preparation. The Langmuir-Blodgett
technique
was
utilized
to
create
TPP/aluminum ion films. A standard KSV
2000 LB trough from KSV Instruments, Inc.,
was used to prepare the LB films. First the
TPP was spread between the barriers on the
trough, which is filled with Millipore water or
a 1 mM aluminum ion solution. 4" 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,
and Millipore water in a 1:1:5 ratio by volume
for 1 1/2 hours. Next the wafers were placed
in a mixture of concentrated H2SO4 and 30%
H2O2 in a 70:30 ratio by volume. To create a
hydrophobic surface for Figure 2A, the silicon
wafer was treated with HF (DOE and Ingalls,
CMOS grade) for 5 minutes and exposed to
40% NH4F solution. The wafers were then
rinsed with Millipore water and dried with
nitrogen. The TPP was compressed to a
constant pressure of 9.5 mN•m-1. The LB
films were created by dipping the hydrophobic
wafer into 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. For Figure 2B, however, a
single layer was made on top of the
hydrophilic aluminum surface. Polystyrene
(PS) was spin coated on top of the TPP
monolayer using a solution of 1 wt% PS in
methyl ethyl ketone (MEK).
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
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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.
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 has turned to aluminum oxide.
Figure 4 shows an AFM image of the resulting
film. The roughness is 0.275 nm.
Silicon Wafer
Figure 4. Height and phase AFM images of
the alumina coating from PVD on the
hydrophilic silicon surface. Scan size is 5 x 5
x 5 microns, and the z-range for the height and
phase images are 5 nm and 5 degrees,
respectively.
Tungsten Filament
Crucible
Figure 3. Schematic of the inside of physical
vapor deposition apparatus.
XPS. XPS measurements were made using a
PHI 5400 (Perkin-Elmer) with Mg-K
radiation.
AFM. AFM images were obtained in the
Tapping ModeTM with a Digital Instruments
Dimension 3000 Scope with a Nanoscope IIIa
controller using etched single crystal silicon
tips. 5 m  5 m images were captured at a
set-point ratio of ca. 0.6.
Results
Adhesion between silica and aluminum. The
first objective was to coat bare silica with
aluminum to see how it initially adheres to the
surface. In order to get aluminum to stick, a
hydrophilic SiO2 surface (no HF etching) was
left on the silicon wafer. X-ray photoelectric
spectroscopy (XPS) confirmed that aluminum
Adhesion between hydrophobic Si and Al2O3
for the configuration in Figure 2A. TPP was
then LB-deposited on HF etched silicon with
and without Al+3 and covered with Al2O3 to
understand how TPP affected the adhesion
between the silicon and the Al2O3 film. The
TPP-coated silicon without aluminum ions
developed a blistered surface. The portion of
the wafer not coated with TPP exhibited a
smooth alumina surface. When Al3+ was cotransferred with the TPP film, a homogeneous
film with no visible blistering formed
indicating that the aluminum ions converted
TPP into a favorable surface for PVD of
alumina coatings. Future work will focus on
coating polymer surfaces with alumina with
and without TPP adhesion promotion layers.
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studies for the polystyrene film are currently
underway.
A
Figure 5. Height and phase AFM images of
the alumina coating from PVD on thirty layers
of TPP. Scan size is 20 x 20 x 20 microns,
and the z-range for the height and phase
images are 200 nm and 50 degrees,
respectively.
Figure 6. Height and phase AFM images of
the alumina coating from PVD on thirty layers
of TPP and aluminum ions. Scan size is 20 x
20 x 20 microns, and the z-range for the
height and phase images are 500 nm and 30
degrees, respectively.
Adhesion between aluminum and polystyrene
for the configuration depicted in Figure 2B.
As discussed above, it was possible to coat
silica with aluminum without an adhesion
promotion layer. Since the aluminum oxide
layer is hydrophilic, only a single monolayer
of TPP can be LB-deposited on alumina
during the upstroke as subsequent layers were
removed
(downstroke)
and
replaced
(upstroke) during Y-type deposition to yield a
net monolayer. A layer of TPP complexed
with Al3+ was also transferred to the
aluminum-coated silica. MEK provides an
appropriate solvent to spincoat polystyrene
onto the TPP treated surfaces. In initial spin
coating tests, XPS showed MEK did not
remove TPP from the surface. Dewetting
B
C
Figure 7. Optical microscopy images at 20x
magnification of a single layer of TPP coated
with polystyrene (Mn = 1460 g/mol) at A) 110
°C B) 140 °C and C) 160 °C. Image size is
0.76 x 0.57 mm2
A
B
C
Figure 8. Optical microscopy images at 20x
magnification of a single layer of TPP and
aluminum ions coated with polystyrene (Mn =
1460 g/mol) at A) 110 °C B) 140 °C and C)
160 °C. Image size is 0.76 x 0.57 mm2
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Conclusions
Aluminum ions influenced the
adhesion between TPP LB-films and alumina
coatings prepared by PVD. These preliminary
results suggest that TPP films containing
aluminum ions may be suitable for promoting
adhesion between alumina and polymers and
is the subject of ongoing research.
Future Work
Acknowledgements
The authors would like to thank the
Virginia Space Grant Consortium (2005-06)
for funding, the POSS group at Edwards Air
Force Base for materials, and members of Dr.
John Morris’ group at Virginia Tech for help
with PVD.
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