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17-10-2007
Book Chapter
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4. TITLE AND SUBTITLE
5a. CONTRACT NUMBER
Hydrophobic Silsesquioxane Nanoparticles and Nanocomposite Surfaces (Preprint)
5b. GRANT NUMBER
5c. PROGRAM ELEMENT NUMBER
6. AUTHOR(S)
5d. PROJECT NUMBER
Joseph M. Mabry, Ashwani Vij, Brent D. Viers, Wade W. Grabow, Darrell Marchant, &
Scott Iacono (AFRL/RZSM); Patrick N Ruth & Isha Vij (ERC)
5e. TASK NUMBER
5f. WORK UNIT NUMBER
23030521
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AFRL/RZSM
9 Antares Road
Edwards AFB CA 93524-7401
AFRL-RZ-ED-BK-2007-473
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Air Force Research Laboratory (AFMC)
AFRL/RZS
5 Pollux Drive
Edwards AFB CA 93524-7048
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NUMBER(S)
AFRL-RZ-ED-BK-2007-473
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Approved for public release; distribution unlimited (PA #08375A)
13. SUPPLEMENTARY NOTES
For publication in the American Chemical Society Series Book Chapter, “Silicones and Silicone-Modified Materials”
14. ABSTRACT
Fluorinated Polyhedral Oligomeric Silsesquioxanes are hydrophobic nanoparticles. One compound, FD8T8, is ultrahydrophobic,
possessing a water contact angle of 154°. This is believed to be the most hydrophobic and lowest surface tension crystalline
substance known. Analysis of the x-ray crystal structure indicates a large number of Si…F contacts may lead to
ultrahydrophobicity.
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Dr. Joseph M. Mabry
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RESERVE THIS SPACE
Hydrophobic Silsesquioxane Nanoparticles and
Nanocomposite Surfaces (Preprint)
An Overview of the Synthesis and Properties of Fluorinated
Polyhedral Oligomeric SilSesquioxanes (POSS) and
Fluorinated POSS Nanocomposites
Joseph M. Mabry*, Ashwani Vij*, Brent D. Viers, Wade W.
Grabow, Darrell Marchant, Scott T. Iacono, Patrick N. Ruth#, and
Isha Vij#
Air Force Research Laboratory
#
ERC Incorporated
Materials Applications Branch
Edwards Air Force Base, CA 93524
e-mail: joseph.mabry@edwards.af.mil, ashwani.vij@edwards.af.mil
Fluorinated Polyhedral Oligomeric Silsesquioxanes are
hydrophobic nanoparticles.
One compound, FD8T8, is
ultrahydrophobic, possessing a water contact angle of 154°.
This is believed to be the most hydrophobic and lowest surface
tension crystalline substance known. Analysis of the x-ray
crystal structure indicates a large number of Si…F contacts
may lead to ultrahydrophobicity.
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Introduction
Polyhedral oligomeric silsesquioxanes (POSS) continue to be explored for use in
many new applications.1 Applications include space-survivable coatings,2-4 and
ablative and fire-resistant materials.5-7 POSS compounds have a rigid, inorganic
core and have been produced with a wide range of organic functionality. Due to
their physical size, POSS incorporation in polymers generally serves to reduce
chain mobility, which often results in the improvement of both thermal and
mechanical properties.
The addition of fillers to polymeric matrices is of major technological
importance. However, the effects of this process are still not fully understood.
Filler addition can impart enhanced scratch resistance, increase thermal or
mechanical properties, and improve processing parameters. There has been
much effort to optimize the factors in the addition of filler. One factor is filler
chemistry. Silicate and carbon black based fillers are quite common. They are
often inexpensive and their incorporation into many polymer systems is fairly
straightforward. When miscibility is a problem, surface modification of the
fillers to further enhance their compatibility is widespread. The silylation of
surface silanol groups on silica fillers is a good example. Processing is another
factor that has been optimized. The use of high shear to break up large
agglomerates or aggregates of nanoscopic particles is common. These
approaches yield nanoscopic species with large surface areas, which should
favor physisorption and/or chemisorption between the polymer chain and the
filler.
A number of reports have detailed that POSS materials can act as reinforcing
fillers (or reinforcing comonomers) in a number of nanocomposite systems. 8-10
The results reported herein are somewhat different in that the monodisperse
POSS building blocks seem to be rather non-interacting. Specifically, the
organic functionality surrounding the silsesquioxane core is composed of
fluoroalkyl moieties. Fluoroalkyl compounds are known to be basically inert.
This is largely because they are non-polarizable and have low surface free
energies. Fluoroalkyl chains are often rigid, due to steric and electronic
repulsion. These POSS materials are monodisperse and crystalline. The melting
point of the POSS is lower than the processing conditions of the fluoropolymers,
so one can safely assume that hard filler effects should not be an issue. In this
regard, one may expect that these materials could exhibit small molecule,
solvent-like, characteristics. The POSS could be well dispersed and act as
molecular ball bearings. This paper will discuss parameters and surface
properties
of
simple
blends
of
FluoroPOSS
materials
in
poly(chlorotrifluoroethylene) matrices.
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Experimental
Materials
POSS compounds, (1H,1H,2H,2H-heptadecafluorodecyl)8Si8O12 (FD8T8) and
(1H,1H,2H,2H-tridecafluorooctyl)8Si8O12 (FO8T8) were prepared using
previously reported methods.11 Poly(chlorotrifluoroethylene) (PCTFE) was
obtained from Daikin.
Composite Preparation
Fluoropolymer composites have been prepared by melt blending five, ten, or
fifteen weight percent of FD8T8 or FO8T8 into PCTFE. All samples were
blended in a DACA Micro Compounder for 3 minutes at 100 rotations per
minute. The DACA Micro Compounder is a conical co-rotating twin-screw
extruder with a bypass allowing the material to circulate for specified times. The
capacity of the mixer is 4.5 cm3. The mean shear rate is approximately 100 s-1
and is reported based on a treatment given in literature.12 The blends were
compounded at 280 ºC. Samples for contact angle measurements were prepared
into thin films. The films were prepared by compression molding two grams of
the polymer-blend extrudate utilizing a Tetrahedron compression molder. The
polymer extrudate was placed between two sheets of thick aluminum foil at 10
ºC greater than the compounding temperatures for 10 minutes using one ton of
force. All films were less than 0.3 mm thick and about 80 mm in diameter and
appeared homogenous and similar to the respective unfilled fluoropolymer.
Contact Angle
Contact Angle analyses were performed on a First Ten Angtroms 110 series
system using a syringe metering pump. Deionized water was used as the
interrogating liquid. Small drops of water were accurately metered onto a flat
surface, and the full screen image of the drop was captured with the frame
grabbing software coupled to a CCD camera operating at the optimized zoom
and contrast. The contact angle was determined via the software suite or via
graphical fitting of the contact tangents in the captured image. Both approaches
2 degrees. Only the value of the
gave the same nominal value within
quasistatic advancing angle is reported.
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Results and Discussion
FluoroPOSS Synthesis
FluoroPOSS were produced by the base-catalyzed hydrolysis of trialkoxy
silanes. In small-scale syntheses, these compounds tend to condense into T8
cages (Figure 1), rather than cage mixtures, as has been previously observed in
the base-catalyzed synthesis. This is significant because the usual method to
produce T8 cages is the acid-catalyzed hydrolysis of trichlorosilanes, which
results in a much longer reaction time and the production of an undesirable
acidic byproduct.
R
R
RSiX3
KOH / H2O
solvent
Si
O O
Si O
O R
Si
O
Si
O
R
O
R
Si
O
Si
RO
O
Si
O
R
Si O
R
R8T8
Figure 1. Synthesis of Fluoroalkyl8T8.
A variety of FluoroPOSS compounds have been produced, including FD8T8 and
FO8T8. Synthesis is currently underway on a number of others. The yields for
these reactions are often nearly quantitative. The byproduct is a resinous
material that is formed when the condensation is less controlled. The resin is
typically removed by extraction.
An interesting occurrence has been observed during the scale-up of the
FluoroPOSS synthetic procedure. During large-scale syntheses FluoroPOSS
compounds, cage mixtures are often formed. A cage mixture is a combination of
cages with eight, ten, and twelve silicon atoms. However, during a purification
step involving extraction of the basic catalyst, the cage mixture is converted
exclusively to eight-member cages (Figure 2). Because this step involves the
dissolving of the compound into a fluorinated solvent, it is believed that the
presence of the catalyst allows conversion of the cage mixture to the most
thermodynamically stable product. Apparently, in the fluorinated solvent used,
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the eight-member cage is the favored product. This conversion can be followed
by 29Si NMR spectroscopy. Calculations are underway to confirm this
hypothesis.
R
R
R
Si
O O
Si
O
O
R
Si
R O
Si
O
Si O
O
Si
R
R Si
O
O
R
Si
O
O
Si
R
R
Si R
O
Si
O
R
Si
O
O
R
O
O
O
Si
O
R
O
Si
R
Si
O
Si
R
O
O
Si O
R
O
Si
O
R
R
R
R
O
R
O
O
Si
O
O
R
Si
Si
R
O
Si
Si
O
R
R
Si
O
Si
O Si
R
R
R
O
Si
O O
Si
O
O
Si
O
Si
R O
O
Si
O
Si O
R
R
O
R
Si
O
O
R
Si
O
O
Si
OR
O Si
O
Si
R
O O
O
R
Si
R
Figure 2. Redistribution of cage mixture to T8 cages.
FluoroPOSS Properties
The properties of the FluoroPOSS compounds are quite interesting. They tend
to volatilize at approximately 300 C, rather than decompose, as is observed with
many POSS compounds. The FluoroPOSS are the highest molecular weight
POSS yet produced. The FD8T8 has a molecular weight of 3993.54 g/mol. The
density of these materials is also very high, with crystals of the FD8T8 having a
density of 2.067 g/mL.
Various surface properties of the FluoroPOSS compounds have been examined.
Water contact angles are a measure of the surface free energy of a surface. As
the surface energy decreases, the contact angle increases to a maximum of 180 .
The trend observed in the FluoroPOSS compounds is that the surface energy
decreases as the length of the fluoroalkyl chain increases. While this may not be
surprising, the observed contact angles are unexpectedly high. The FD8T8 has a
water contact angle of 154 (Figure 3), which is approximately 40 higher than
the water contact angle of polytetrafluoroethylene (PTFE). The correlation
between the chain length and the contact angle is not linear. The contact angle
appears to be increasing as the chain length increases.
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Drop of
H2O
FD8T8
Coated
Surface
Figure 3. Water drop on surface of FD8T8 with a contact angle of 154°.
Solid State Structures of FluoroPOSS
Due to the highly crystalline nature of FluoroPOSS compounds, single crystals
were grown from fluorinated solvents. Although these crystals exhibit different
morphologies, both materials investigated under this study belong to the triclinic
crystal system. Single crystal x-ray diffraction analysis of FD8T8 and FO8T8 at
room temperature did not allow for any reasonable structure solution as there is a
large amount of disorder due to the movement of fluoroalkyl chains, which is
consistent with observation of large diffused scattering. Our attempts to cool
crystals to low temperatures resulted in crystal shattering, probably due to a
rapid phase transition. This challenge was overcome by very slow cooling rates.
However, at 103 K, the quasi powder-like pattern of diffused scattering
disappears, resulting in sharper spots indicating significant ordering due to
reduced entropy. The ultrahydrophobic FD8T8 exhibits a structure with two
molecules within a large asymmetric unit and the fluoroalkyl chains propagating
in a zig-zag manner as seen in Figure 4.
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Figure 4. X-ray crystal structure of FD8T8.
The fluoroalkyl chains adopt both gauche and eclipsed conformations as a result
of close intra- and intermolecular Si…F contacts (Figure 5). In addition to these
contacts, intermolecular H…F contacts are also observed, which result from
lower disorder at 103K. The Si…F contacts are in the range of 3.0-3.5 Å, which
is below the sum of van der Waals radii of silicon and fluorine. These contacts
cause the non-fluorinated methylene groups to lie flat, along the axis of the rigid
fluoroalkyl chains, and increase the packing efficiency of the crystals with an
almost parallel arrangement of alkyl chains. This results in the formation of a
surprisingly unique structure due to self-assembly.
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Figure 5. Cage region of FD8T8 structures showing Si-F and H-F contacts.
POSS Fluoropolymers
The FluoroPOSS compounds mentioned above were blended into
poly(chlorotrifluoroethylene). For the purposes of this paper, FD8T8 and FO8T8
blends in PCTFE will be used to describe processing, thermo-mechanical, and
surface properties.
Processability
The processability of the samples were compared using torque and load, a
measure of the pressure generated in the compounder. The pressure is generated
in the mixer due to its conical design. With a constant volume of material
compounded and a fixed screw speed, the pressure generated is proportional to
the viscosity of the material. The lower the pressure, the lower the viscosity, and
the easier the material is to process. The second measure of processability is the
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torque output by the motor. This gives an indication of the mechanical energy
put into the system and is proportional to the current used by the motor. A
similar measure was utilized to characterize the processability of polyethylene
and hyperbranched polymer blends.13 The lower the torque output the more
processable the polymer for any given screw speed.
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
c
m
1
c
1
Relative load (F /F )
Relative torque (M /M )
These two measures of the processability of the polymer blends were recorded at
30 second intervals during processing. It was found, within a 95% confidence
interval, that the load and torque values were constant for the duration of
processing excluding the first 30 sec. Therefore, an average value for both
torque and load is assigned to each processing run. In order to investigate the
effect of the addition of FluoroPOSS, relative torque and relative load values
were computed utilizing the average values in comparison to the average values
found for the unfilled resins. Figure 6 shows the relative torque and load values
with respect to the weight percentage of POSS added to PCTFE blends. The
solid symbols represent the relative torque values whereas the relative load
results are illustrated by the open symbols. The square symbols symbolize the
results of the FD8T8 blends and the circular symbols show the FO8T8 blend
results. One will notice that the processability of 10 weight percent of either
POSS in PCTFE is improved by greater than 30 percent. In addition, the FD8T8
exhibits a greater effect on the torque and load for the PCTFE blends.
M
0
0
2.5
5
7.5
0
10
Wt % FluoroPOSS
Figure 6. Effect of FluoroPOSS on torque and load for PCTFE blends.
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Surface Properties
While fluoropolymers are known for their hydrophobicity and low coefficients
of friction, incorporation of FluoroPOSS may help to improve these properties
even further. Water contact angles are a measure of surface hydrophobicity and
provide insight into the free energy of the surface. Contact angles have been
obtained on PCTFE nanocomposites containing FD8T8 and FO8T8.
Technologies that may benefit from the blending of FluoroPOSS into
fluoropolymers include abrasion resistance, lubricity, anti-icing, and non-wetting
applications. Figure 7 shows a drop of water on the surface of a PCTFE film.
The contact angle was measured at 88 . Figure 6 also shows a drop of water on
the surface of a PCTFE blend containing 10% FD8T8. The contact angle for this
film was measured at 128 . There is a 40 increase in contact angle with just
10% added FD8T8.
Figure 7. Water contact angles of 88 and 128° on PCTFE films.
It has also been observed, as one might expect, that the contact angle increases
with increasing weight percent POSS. Contact angles have been obtained on
other fluoropolymers as well. All show a similar trend. It should be noted that a
surface with a contact angle of 90 or higher is considered a “non-wetting”
surface, while a surface with a contact angle below 90 is considered “wetting.”
Unfilled PCTFE has a contact angle of 88 . Addition of FluoroPOSS produces a
“non wetting” surface.
Other Properties
In order to determine the effect of the FluoroPOSS on the mechanical properties
on PCTFE, dynamic mechanical analysis (DMTA) was performed. The
variation in moduli and glass transition temperatures seen with the addition of
FluoroPOSS is small enough to be statistically insignificant.
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The level of dispersion of POSS compounds into polymer systems is largely
dependent on surface chemistry. Atomic Force Microscopy (AFM) and
Scanning Electron Microscopy (SEM), along with the element mapping
capability of SEM, were used to determine nanoparticle dispersion. These
techniques indicate good to excellent particle dispersion in the polymer matrix.
Conclusions
Two fluorinated Polyhedral Oligomeric Silsesquioxanes (FluoroPOSS) have
been produced. The large-scale synthesis results in the production of cage
mixtures, which can then be converted to T 8 cages by a redistribution reaction.
The FluoroPOSS compounds are hydrophobic, with the FD8T8 possessing a
water contact angle of 154°, making it ultrahydrophobic. Analysis of single
crystal x-ray data indicates that molecular scale surface roughness may lead to
ultrahydrophobicity.
These compounds have been blended into poly(chlorotrifluoroethylene)
(PCTFE). These POSS fluoropolymers may be useful as low friction surfaces or
hydrophobic coatings. Contact angle measurements of the POSS fluoropolymers
show an improvement of water contact angles over the unfilled materials. The
FD8T8/PCTFE composite shows a contact angle improvement of 40 over the
unfilled material. The low surface energy POSS compounds also appear to act as
a processing aid during fluoropolymer processing, significantly reducing both
the torque and load measurements in the extruder. Thermal and mechanical
properties of the blended fluoropolymers do not differ significantly from those of
the unfilled polymers.
Acknowledgements
The authors would like to thank the Air Force Office of Scientific Research and
the Air Force Research Laboratory, Propulsion Directorate for funding.
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References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
POSS is a registered trademark of Hybrid Plastics Inc., Fountain Valley, CA
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