Processing

1
Characterization of Variable Platform for Robust
Sensor and Separation Nanocomposite Membranes
J. M. W. Olson1, Y. Blum2, W. R. Chung1
1Chemical Materials Engineering, San José
State University, 2SRI International
Objectives
Results
Materials
The goal of this research project is to create
and characterize nanocomposite membranes using
nanoparticles andsiloxane-based. Siloxane-based
polymers have already been already reported as
selective membranes for CO2. The chosen
nanoparticles are nano graphene platelets that
were recently discovered and are considered a low
cost replacement of carbon nanotubes for
enhancing mechanical properties of nanocomposite
materials as well as electrical conductivity.
Such composite membranes hold potential to
produce a viable gas/electrical sensing membrane.
Hence the creation and characterization of such
materials is of interest.
Scanning Electron Microscopy (SEM) was preformed
on the PRMS/graphene nanocomposites, the
pre-coated filters, and the raw filters. The
Image at right was taken of a raw glass filter.
The filter is clearly highly porous, and the
permeability of the composites can not be
determined by this mechanical support. The image
of the polymer-coated filter, shown below,
indicates uniform coating throughout.
The siloxanes selected for this study are
polydimethylsiloxane (PDMS), a polymer with
numerous commercial applications, and modified
polyhydromethylsiloxane (polyhydridomethylsiloxane
, PHMS), based on a polymer platform developed by
SRI International with great chemical
diversity. Both polymers possess high thermal
resistance and high CO2 selectivity. As a result
of their molecular composition, they differ in
their polarity and cross-linking characteristics.
The conventional PDMS is non-polar, lightly
crosslinked, and elastomeric in nature. Replacing
one methyl group with a different functional or
crosslinking group (e.g. modified PHMS, PRMS) can
change the affinity and degree of crosslinking.
Theory
The process of producing nano-scale graphene
plates begins with a polymeric precursor. This
precursor is then carbonized (fully or
partially). This results in an intercalated
graphite flake that looks like that pictured top
left. This single flake is made up of many, many
layers of graphene (middle left). Next,
chemicals and heat are added to exfoliate the
layers. The flake will expand from a thickness
of 80-100 µm to a thickness of up to 10 mm or
more in three seconds. Fast digital photography
by Lee et al. captured this rapid transformation
(bottom left). The exfoliated graphite is then
ball-milled (or otherwise crushed) until
nano-sized graphene, as shown bottom far left,
results. The graphene flakes will consist of, as
in the transmission electron microscope image at
right, a small number of layers with total
thickness on the nano-scale. The graphene
platelets used in this project have an average
diameter of 35 µm, with a maximum of 100 µm. In
thickness, at least 80 are less than 100 nm.
There are several mechanisms proposed to explain
why the addition of a dispersed phase within a
polymer matrix would enhance the gas
permeability, selectivity, and sensing abilities
of the material ? Nanogaps the dispersed phase
is not perfectly adhered to the matrix, resulting
in nanogaps along the interface through which gas
might travel ? Polymer chain packing disruption -
increases the free volume through which gases as
passed ? Chemical modification chemical
interactions between the dispersed phase and the
diffusing material Transport of large molecules
is diffusion limited, while transport of small
molecules is absorption limited. Therefore,
mechanisms which increase diffusion, while not
affecting absorption, may increase selectivity
and sensing of larger molecules.
SEM on the composites was performed on the
surface and on a cross-section of a specimen cut
with scissors. The composite appeared very rough,
as shown below right. The rough nature of the
composite increases the chance that there are
straight paths through the composite for the gas.
Also, literature evidence suggests that platelets
laying perpendicular to the gas flow will be most
effective. Future processing will focus on
reducing this roughness.
Adapted from B.Z. Jang and W.C. Huang,
Nano-scaled graphene plates, US Patent 7071258
(October 21, 2002).
The structure of graphene, above, is rather like
an unrolled carbon nanotube. Graphene shares many
of the properties of carbon nanotubes like high
thermal and electrical conductivity and strength.
In fact, its shape may make it superior to
nanotubes for the studied application.
In order to create the most efficient sensitivity
with the least amount of material, the path the
gas molecules must travel should be as long as
possible. This will also increase the
selectivity. To achieve this, a tortuous path is
created, as in the figure below, left. The best
possible shape for the dispersed phase is thus a
flake, oriented perpendicular to the net path of
gas flow. To this end, pre-exfoliated graphene
has been selected as the dispersed phase to be
placed in various crosslinked siloxane matrices.
Lee, D. Cho, and L.T. Drzal, Real-time
observation of the expansion behavior of
intercalated graphite flake, J. Mater. Sci., 40,
231-234 (2005).
Coated glass filter
It is important that the dispersed phase be
exfoliated, as illustrated at right. This will
result in maximization of the surface area for
possible chemical reaction.
Nanocomposite membrane
K. Kalaitzidou, H. Fukushima, and L.T. Drzal,
Mechanical properties and morphological
characterization of exfoliated graphite-polypropyl
ene nanocomposites, Compos. Part A, 38,
1675-1682 (2007).
K. Kalaitzidou, H. Fukushima, and L.T. Drzal,
Mechanical properties and morphological
characterization of exfoliated graphite-polypropyl
ene nanocomposites, Compos. Part A, 38,
1675-1682 (2007).
Hay, J.N. and S.J. Shaw (2000). Clay-Based
Composites Online. Available at
http//www.azom.com/details.asp?ArticleID936
(accessed September 21, 2007). WWW Article.
Lee, D. Cho, and L.T. Drzal, Real-time
observation of the expansion behavior of
intercalated graphite flake, J. Mater. Sci., 40,
231-234 (2005).
Processing
Cross-sectional views of a specimen (above)
showed clear delineation between the composite
and the supporting substrate. The composite forms
a nice, thick coating. Individual graphene
flakes could be observed, as in right. It appears
to be the expected radius. These flakes are so
thin that structure beneath may be seen in the
image.
Gas Measurements
Future Work
Acknowledgments J. Olson is supported by Defense
Microelectronics Activity Cooperative Agreement
H94003-07-2-0705. Scanning electron microscope
images were possible thanks to National Science
Foundation grant 0421562 for Major Research
Instrumentation. Graphene donated by Angston
Materials, LLC. Thanks to Prof. E. Allen, Prof.
M. McNeil, D. Hui, A. Micheals, T. Olson , N.
Peters and D. Verbosky.
The next step is to perform gas transport
measurements and compare the performance of the
polymer-only filters with the composite
membranes. This will reveal if the roughness
indicated by the SEM is providing fast paths
through the membranes. These gas measurements
will first be done with nitrogen gas and argon
gas. Later carbon dioxide will be used, and
possibly other gases. At right is the test
chamber that has been designed and constructed
for this purpose with automated computer data
acquisition.
? Additional membranes will be created using
smaller amounts of graphene by volume. ?
Additional membranes will also be made with
modified PHMS and PDMS. ? Process refinements
will focus on creating flatter surfaces with
flakes aligned parallel to the substrate.