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Polymers
and Supercritical Fluid Applications
Interest in supercritical fluid processing of polymers has grown over
the last 15 years, and many purification, fractionation, and polymerization
applications have emerged. A significant motivation for applying this
technology to polymers is increased performance demands required of polymer
products coupled with the technical limitations of more traditional purification
and fractionation methods. With increasing scrutiny of industrial solvents,
supercritical fluid technology, especially using carbon dioxide, is receiving
widespread attention as an environmentally conscious method for replacing
various organic solvents used in industrial operations.
Phasex has been instrumental in developing many of the supercritical
fluid processes currently in production or advanced development: purification
of brain shunts, fractionation of medical polymers, devolitilization of
space grade and high vacuum adhesives, and production of narrow molecular
weight range high density disk lubricants.
A few examples are presented here to demonstrate the breadth of polymer
applications using supercritical fluids.
Polymer Extraction and Fractionation
It is possible to tailor the performance of polymers by modifying certain
properties such as molecular weight, polydispersity, or crystallinity
via supercritical fluid extraction or fractionation. For example, the
undesirably high viscosity of a polymer can be reduced by separating the
very high molecular weight species from the polymer via supercritical
fluid fractionation. Similarly, undesired low molecular weight or cyclic
species in a silicone polymer, which can migrate in some high temperature
application, can be removed by SCF extraction. Because their dissolving
power can be fine-tuned, often to high degree of selectivity, supercritical
fluids can separate polymers by molecular weight, which, as suggested
earlier, can enhance their performance.
Fractionation of a Perfluoroether
An example is presented to demonstrate the effectiveness of SCF fractionation
in preparing narrow polydispersity polymer fractions which are useful
in characterizing structure-property relations, elucidating reaction kinetics,
and even as calibration standards.
For the exact determination of molecular weight by SEC, narrow standards
of the polymer being analyzed must be used, but they are not generally
available, especially for polymers like the perfluoropolyethers (Krytox®,
Fomblin®), high molecular weight silicones, and polyethylene and its
copolymers.
The polydispersity of the parent polymer shown is 1.87. It has been reduced
to an average of 1.08 for the nine fractions. Molecular distillation cannot
carry out the fractionation of this polymer because its vapor pressure
is too low, but supercritical fluids have been effective in producing
narrow standards of the high molecular weight (>8,800) perfluoropolyether.
Fractionation of Polyolefins
The properties of supercritical fluids can also be manipulated so as
to fractionate polymers by, for example, crystallinity. Fractionation
of polyethylene by molecular weight and side chain branching is another
example presented here of the advantages offered by supercritical fluid
processing.
Narrow fractions of polyethylene and its copolymers are desired for
many reasons e.g., for GPC calibration standards, properties evaluation,
kinetics studies or catalyst performance analysis. Generally, the fractions
are not commercially available. For the specific case of GPC standards,
hydrogenated polybutadiene is sometimes used, but it is not a good model
for commercial polyethylene, such as LDPE, HDPE, LLDPE and certainly not
for experimental copolymers; there is, for example, no short and long
chain branching on hydrogenated polybutadiene, and although molecular
weight ranges of HDPE or LLDPE can be matched, the hydrodynamic volume
cannot be.
Quantities of very small (mg) size can be obtained by GPC fractionation
or by anti-solvent methods, and a laboratory process called TREF (Temperature
Rising Elution Fractionation) can produce small quantities of polyethylene
separated by crystallinity; however, producing preparative amounts requires
many liters of solvent to process even a 5g charge.
 At
a preparative bench scale, fractionation with supercritical fluids produces
large quantities of narrow MW fractions. Additionally, CITREF, Phasex
Corporation's supercritical variant of TREF, can separate ethylene polymers
and copolymers by side chain branching and chemical composition, again
producing large fractions. The HDPE described here was fractionated by
the process Phasex terms increasing pressure profiling.
Mn, Mw, and polydispersity
fraction table
Supercritical fluids can also fractionate polyethylene and its copolymers
(e.g., acrylate, methacrylate, acrylic acid, vinyl acetate) by crystallinity/side
chain branching/chemical composition. The process Phasex terms CITREF
(Critical Isobaric Temperature Rising Elution Fractionation) can, like
pressure profiling, produce large fractions for fundamental studies, polymer
properties determination, or catalyst performance evaluation. A commercial
LLDPE was fractionated by amount of side branching. CITREF has separated
the LLDPE, not by narrow MW, but by crystallinity (melting point).
Mn and Mw for all fractions
 DSC
thermograms of the thirteen LLDPE fractions separated by side chain branching
are shown in the figure on the right and are compared with the DSC of
the parent polymer. The narrow transition range and the increasing transition
temperature of each fraction obtained by CITREF are readily seen. The
CITREF fractions of polyethylene are useful for the overall evaluation
of the polymerization process, the narrow fractions providing quantitative
information on kinetic profiles, and catalyst life and performance (mols
monomer/mol of active catalyst).
Other polyolefins and copolymers produced by virtually any catalyst
technology can be readily processed with supercritical fluids by pressure
profiling, for molecular weight distribution, and by CITREF, for crystal
unity and chemical composition distribution.
Polyolefins and copolymers fractionated by molecular weight, side chain
branching, and chemical composition-at the kilogram scale-can facilitate
your determination of polymer structure/property relationships and polymerization
catalyst performance. Let Phasex fractionate your polymers for kinetic
studies, catalyst life/performance evaluation, properties determination,
and new product development.
Extraction of Medical Polymers and Devices
Many medical devices that are in contact with the body or body fluids
or that are surgically implanted in the body are composed of silicone
polymers because of their biocompatibility. The silicone parts are lightly
cross linked to retain structure, but the cyclic byproducts present in
the silicone polymer are not incorporated into the matrix; thus, they
can migrate. Since the volatility of the cyclics is so low, high temperature
vacuum or nitrogen stripping is ineffective. Organic liquid extraction
can be effective in removing the interfering species, but the issue of
residual solvents in the devices then becomes a concern. Supercritical
fluid extraction of residual cyclics from medical devices is attractive
especially because the purification process cannot be reasonably carried
out by any other technique. Cyclics and low molecular weight oligomers
content can be as high as 4 wt%, and extraction with supercritical CO2
can reduce the level of these species to less than 10ppm. Several examples
of medical products that have been extracted using SCFs include aorta
and other arterial grafts, neuro-shunt lines, and catheters.
Supercritical fluids have also been used to extract other medical and
ocular materials, for example, to purify methacrylate functionality silicone
macromonomers that are used in the manufacture of soft contact lenses.
Because these macromonomers are very heat labile and have very low volatility
(and thus require high temperature to purify them even under high vacuum),
they are virtually impossible to purify by any traditional process.
 Purification
of Reactive Monomers
Reactive monomers are inherently difficult to process by traditional
methods. SCFs offer a useful alternative for extracting, for example,
odors from an acrylate monomer. The HPLC traces shown demonstrate that
even minute quantities of impurities can be readily extracted from a
temperature sensitive methacrylic monomer.
Impregnation of Porous and Polymeric Matrices
Extraction and fractionation are the most common operations using supercritical
fluids, but the process can be reversed to deposit materials, for example,
into a porous or polymeric substrate. In this application a supercritical
fluid is used to convey an organic compound into micropores of a substrate,
the pressure/temperature then reduced to bring about precipitation in
the micropores. With polymeric substrates, the SCF first swells the polymer
then conveys and deposits a compound in the matrix. Monomers and polymers
can also be impregnated into a porous substrate, but the procedure is
a bit more complex, viz., the pressure/temperature must be selected so
as to dissolve the entire polymer (not just the low molecular species).
Because the dissolving power of SCFs can be adjusted over wide ranges,
the conditions can be manipulated so as to dissolve and deposit the entire
polymer homogeneously.
Photoresists
With the rapid evolution of products in the microelectronics industry
there is an increasing need for higher purity materials and safer, more
efficient solvents, and supercritical fluids are proving effective in
satisfying these needs in diverse photoresist applications ranging from
polymer purification and fractionation to image developing. SCFs have
been applied to improving the performance of photoresist polymers by purification
and/or fractionation. Specialty polysiloxane and polysilane polymers that
are being developed as photoresists have broad molecular weight distribution
resulting in a variable sensitivity to radiation at 248-254 nm wavelength.
Sensitivity can be controlled by using near-monodisperse fractions of
these polymers in resist applications, but there is no traditional synthesis
method that can produce monodisperse polysilanes or polysiloxanes; SCF
fractionation overcomes this problem.
Supercritical fluids have also been applied as developers for photoresist
imaging; they eliminate problems associated with organic liquid developers
such as swelling and image distortion and they minimize solvent waste.
Furthermore, utilization of SCFs for photoresist imaging is applicable
to a number of new polymer systems under development as next-generation
resists. SCF imaging has been demonstrated with several polymer systems
including silanes and siloxanes, fluorinated methacrylates, and siloxane-modified
methacrylates for creating positive or negative tone images.
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