on the topic below to go to the corresponding section.
Energy Related Research
| Surface Science
The development of new polymers for energy-related research
has been a cornerstone of our research for a number of years (see publication
list). Specifically, we design and
find ways to synthesize ion-conductive polymers for use as electrolytes in
rechargeable lithium batteries, polymer electrolyte fuel cells, and dye-based
solar cells. Work on
supercapacitors is just beginning.
Secondary Lithium Batteries
A severe need exists for new lithium battery energy storage
science especially in automotive technology.
The development of new polymer electrolyte systems to replace the
flammable organic electrolytes currently used in lithium batteries has become a
major challenge. Numerous
ion-conductive organic polymers have been investigated for energy storage, but
many of these are limited by their low ionic conduction - often in the range of
only 10-5 to 10-7 S/cm.
We have focused on a range of polyphosphazenes which have linear or
branched oligo-ethyleneoxy side units such as O(CH2CH2O)nCH3.
These gum-like polymers are capable of solubilizing lithium salts and
facilitating ion-pair separation by coordination to the cations.
In addition, low volatility small molecule counterparts such as
2, have also been studied as
Recent work using electrolytes
based on composites of the high polymers with the cyclic oligomers in the
presence of small amounts of propylene carbonate has produced fire-resistant ion
conductive systems that have conductivities in the target 10-3 S/cm
range. Of special interest is the
mechanism of ion conduction in these systems.
Recent work has provided evidence that, contrary to previous
interpretations, both anion and cation transport is important in these
2. Polymer Electrolyte Fuel Cells
polymers based on the polyphosphazene platform are being synthesized and studied
for uses in polymer electrolyte fuel cells.
The phosphazene polymers have aryloxy side groups functionalized with
sulfonic acid, phosphonic acid, or sulfonimide groups.
The advantages of the polyphosphazenes are the ease of property tuning
via cosubstituent groups, access to phase-separated hydrophilic/hydrophobic
composite materials, and the electrochemical stability of the polymers.
These polymers appear to be especially useful for incorporation into
direct methanol or ethanol fuel cells because they are resistant to alcohol
crossover- which is a serious problem with classical polymers like Nafion.
We are also studying the mechanism of proton transport in these membranes
using solid state NMR spectroscopy and are finding evidence for the existence of
organized, ice-like water associated with the acidic side groups as well as the
normal liquid water in the hydrophilic domains.
3. Dye-based Solar Cells
The use of volatile organic
solvents in the electrolytes of dye-based solar cells is a factor that lowers
the lifetime of these devices. We
are investigating polymeric and oligomeric phosphazenes, similar to those used
in the lithium battery work, as electrolytes in Gratzel cells.
A major challenge is to facilitate penetration of the electrolyte into
the nanostructured TiO2
Advances have been made using TiO2 nanosphere-type electrodes
and those with nanowires or nano-columns as part of the surface structure.
This program is part of a collaboration with Professors Thomas Mallouk,
Craig Grimes, and Mark Horn at Penn State.
1. General Principles
ability to tune the properties of polyphosphazenes by the use of different
skeletal architectures and by the introduction of different side groups makes
them particularly attractive materials for uses in biomedicine.
Three types of biological properties have been the focus of our program -
polymers that bioerode (hydrolyze) to harmless small molecules, others that are
biostable and suitable for uses in cardiovascular devices or sensors, and still
others that change shape or permeability in response to external stimuli
(so-called 'intelligent" or "life-like" polymers)
Polyphosphazenes as Bioerodible Materials
(a) For tissue engineering.
One of the most important uses for hydrolytically sensitive polymers is as
scaffolds for the regeneration of mammalian tissues or organs from a patient's
own cells. As part of a long-term
NIH-supported collaborative program with the group of Dr. Cato Laurencin, now at
the University of Connecticut Medical Center, we have developed more than 20
different polymers that are platforms for colonization by osteoblasts for the
regeneration of bone. The most
useful materials are polyphosphazenes with amino acid esters or dipeptide esters
as side groups linked to the main chain through the amino terminus of the side
Three methods are being explored to optimize this system.
First, the rates of hydrolysis are controlled by the types of side groups
on the polyphosphazene, and by their disposition along the chains.
Second, the sensitivity to hydrolysis is being modified via composites
produced between bioerodible polyphosphazenes and poly(lactic-glycolic acid)
(PLGA) a long-existing commercial bioerodible polymer that suffers from some
limitations. The phosphazene
modifies the physical properties of the PLGA and reduces the acidity of the
hydrolysis medium. Third, the
physical form of the polymer affects the biomedical behavior - whether the
polymer is in the form of films, porous constructs, or micro- or nano-fiber
mats. Fifth, the eroding polymer
systems must also provide a means for the controlled release of growth factors
to encourage osteoblast replication.
Understanding how to balance these factors and enhance the compatibility
with osteoblasts and their rate of colonization is the major challenge that we
are attempting to solve. In general
terms, our group at Penn State focuses on the polymer design, synthesis, and
characterization aspects, while our colleagues at the University of Connecticut
concentrate on the materials science and biological evaluations.
(b) For controlled drug or vaccine delivery.
polyphosphazenes are useful not only
for growth factor release but also
for the controlled release of vaccines and drugs.
One aspect of controlled release that has received considerable attention
is the use of polyphosphazene microspheres for the oral delivery of vaccines.
This system, invented through a joint program between Penn State, MIT,
and VRI/Parallel Solutions utilizes a polyphosphazene polyelectrolyte which is
crosslinked by calcium ions and decrosslinked by monovalent cations.
The decrosslinking step in the small intestine releases the trapped
vaccine molecules and immunizes the patient.
Currently our group is also exploring the use of micelles derived from
amphiphilic polyphosphazene block copolymers for the delivery of
(c) Shape-memory polymers.
We have recently shown that specific bioerodible polyphosphazenes
have shape-memory characteristics.
The materials can be physically programmed to return to a predetermined
shape when warmed to a certain temperature (see photograph).
Potential uses as stents or other tissue reinforcement materials may be
(a) Possible cardiovascular materials.
A number of investigators worldwide have examined the use of
fluoroalkoxyphosphazene polymers as
replacements for polyurethanes or silicone rubber in cardiovascular devices.
The phosphazenes have surfaces that are less prone to induce thrombus
formation than most polymers. This
is an application that holds strong potential for future developments, and some
of our surface science work (see later) is targeted to further developments in
this area. Other elastomers that
are emerging from our program, such as hybrids of phosphazenes and silicones,
are also of interest for this application.
(b) Membranes and bioresponsive
materials. The use of biostable
membranes for the controlled release of drugs or for hemodialysis is of ongoing
interest in the biomedical community.
Our recent research has focused on the use of polyphosphazene hydrogels
and nanoporous films as responsive membranes for biomedical uses.
Hydrogels are derived from water-soluble, water-stable polyphosphazenes
that have been lightly crosslinked.
Specific gel membranes have been produced that open or close to the passage of
drug molecules following changes in pH, ion strength, or the replacement of
monovalent by multivalent cations.
In addition, hydrogels produced from polyphosphazene polyelectrolytes
crosslinked by di- or trivalent cations can be induced to expand or contract by
the application of an electric current that electrochemically reduces or
oxidizes the cation. This is the basis
of variable permeability membranes and simple muscle-like devices that bend and
unbend when an electric current is applied.
(c) Detection devices.
Recent work in our laboratories has
examined the use of polyphosphazene hydrogels
as surfaces for the cultivation of nerve cells.
Devices constructed by our collaborators can detect and amplify the weak
electrical signals generated by the cells when they are exposed to the vapor of
The advantage of the polyphosphazene system is that its molecular
structure can be fine-tuned to favor the growth of specific cell lines (for the
detection of different compounds) in different regions of the device.
Collaborative work on the use of polyphosphazenes for stem cell
cultivation is just beginning.
Phosphazene rings and polymers
provide an almost unique platform for the development of optical materials.
The high electron density in the skeleton and the broad window of
transparency from the near ultraviolet to the near infrared provides an
excellent starting point for the design and synthesis of a variety of optical
and photonic materials. Basically,
beyond the properties provided by the backbone, optical properties can be
generated and tuned by the choice of different organic side groups or different
skeletal architectures, as illustrated by the following examples.
2. Controlled Refractive Index Materials
side groups with high electron densities, such as aryl, halogenoaryl, or
sulfur-containing groups allow the refractive index values of phosphazene
oligomers and polymers to be as high as 1.75.
Low refractive index values can be generated via the use of fluorinated
organic side groups. Thus,
combinations of different side groups allow fine-tuning of the refractive index.
The polymers may be in the form of transparent glasses, or ring systems
can be incorporated into cyclolinear or clear cyclomatrix resins.
Coupled with refractive index tuning is the need to minimize chromatic
dispersion, which typically rises with high refractive index materials that have
absorptions in the near ultraviolet.
Recent progress toward low chromatic dispersion materials has been made
with the use of sulfur-containing side groups.
3. Liquid Crystalline, NLO, and
Side groups such as aromatic azo
or cinnamyl units employed as side groups along a polyphosphazene chain give
rise to polymer-effect liquid crystalline and/or non-linear-optical phenomena.
The spiropyran/merocyanine photochemical
rearrangement is the basis of many photochromic devices including variable
density eyeglasses. Linkage of
spiropyrans to a polyphosphazene chain has been accomplished as a means to
control the response time.
4. Electroluminescent Polymers
that emit light of specific wavelengths of light when stimulated with an
electric current are the basis of a growing number of devices, including flat
panel computer and television screens.
We have synthesized the first phosphazene-based polymers that are
electroluminescent. These have a
cyclolinear architecture (see example structure below), in which conjugated
unsaturated organic oligomeric chains link cyclotriphosphazene units.
The cyclophosphazene units interrupt the electron delocalization in the
organic regions and define the emission wavelength.
Other organic side groups on the phosphazene rings provide solubility in
organic solvents, a prime requirement for the fabrication of flat panel devices.
HIGH PERFORMANCE ELASTOMERS
Polyphosphazenes have several attributes that make them
valuable for aerospace, arctic, energy, or oil drilling applications.
These properties include low temperature flexibility and elasticity;
resistance to hydrocarbon fuels, oils, and hydraulic fluids; fire resistance,
radiation resistance, and ultraviolet stability.
The specific applications fall into four main categories based on (1)
elastomeric properties, and (2) fire-resistant composite materials and foams.
1. Phosphazene Elastomers
This is one of the most established uses for
polyphosphazenes are known with glass transition temperatures that range from
-100oC to more than +200oC, species with fluoroalkoxy
and/or organosilicon side groups with glass transitions in the -60oC
range are of special value in aerospace, automotive, arctic, and oil drilling
engineering. They are used to
fabricate O-rings, seals, and shock-absorbing devices for low temperature uses.
The presence of two or more different types of side groups in these
polymers ensures a lack of crystallinity and provides the molecular free volume
needed for elasticity. The
fluoroalkoxy derivatives are also noted for their ability to absorb and dampen
impact energy, a valuable property for certain applications.
Illustrations of components manufactured from phosphazene elastomers are
shown in the following photographs
This aspect of the field evolved in industry directly from
our earliest fundamental discoveries.
Our current work is designed to investigate different side groups and
skeletal architectures that give rise to elastomeric character, and to cooperate
with engineering specialists to identify advanced uses.
Polyphosphazenes in Fire-resistant Composite Materials and Foams
work has been done in our group and elsewhere to examine the chemistry that
underlies the formation of composites formed between polyphosphazenes and
organic polymers, or between polyphosphazenes and sol-gel silicates.
This is also related to the formation of block copolymers derived from
polyphosphazenes and organic polymers or silicones.
Compatibility between two polymers usually depends on the existence of
weak attractive forces such as hydrogen bonding.
Thus, polyesters and several other organic macromolecules form compatible
blends with phosphazenes that bear etheric, acidic, or amino side groups.
The main technological interest in such composites is based on the
ability of the phosphazene to serve as a non-volatile fire retardant or impact
absorber for the organic polymer.
For example, polyphosphazenes with arylcarboxylic acid side groups react with
and are excellent fire retardants for polyurethanes, including those used in
polyphosphazene foams have been produced in industry as non-flammable thermal
and electrical insulation. Recently
we have shown that fluorinated polyphosphazenes are soluble in supercritical
carbon dioxide, and that expanded foams are formed when the pressure is
released. Non-flammable hydrophobic
foams produced from fluoroalkoxy phosphazenes have a density less than one, and
are thus of interest in flotation devices.
polymers are needed for advanced aerospace, automotive, and housing
phosphorus-nitrogen skeleton in polyphosphazenes is inherently fire resistant
and fire retardant, and this property, combined with elasticity, adhesion,
hydrophobicity or superhydrophobicity, is an attractive motivation for
engineering researchers to utilize these materials and for us to design and find
ways to synthesize new polymers based on this skeleton.
1. Importance of Surface Character
Many engineering and biomedical applications
depend on the surface character of a polymeric material as well as on the
internal characteristics. It
follows that the surface properties of phosphazene elastomers and thermoplastics
control several areas of application.
Properties such as hydrophobicity or hydrophilicity, adhesion, biological
compatibility, or corrosion protection are important.
In polyphosphazene science and technology the materials surface may
depend on the "native" surface character of the pristine material, which often
depends on the structure of the polymer side groups, or it may depend on
post-synthesis modification methods.
Treatment of a surface with chemical reagents or by means of an
atmospheric plasma are methods being used in our program to produce different
surfaces. Added to these factors is
the ability of a polymer surface to change in response to the medium with which
it is in contact. Contact with
water may cause hydrophilic units of the polymer to migrate to the surface,
while contact with a hydrophobic organic solvent will bring the hydrophobic
components to the interface. The
picture below illustrates the behavior of a droplet of water on a hydrophilic
and a hydrophobic or superhydrophobic phosphazene interface.
2. Chemical Surface Modification
In earlier work we showed that
certain side groups attached to a polyphosphazene chain can undergo facile
surface reactions when the solid polymer is treated with aqueous solutions of
various reagents. For example,
surface trifluoroethoxy side groups can be replaced by exposure to solutions of
a variety of sodium alkoxides including those that have functional groups at the
opposite end of the reagent from the alkoxide unit.
In this way pendent amino units at a surface are available for coupling
to biologically useful molecules.
Aryloxy side groups can be surface-sulfonated to convert a hydrophobic interface
to a hydrophilic one. The use of
these surface "wet chemistry" reactions is an ongoing part of both our
biomedical work and investigations of adhesive interfaces.
However, recently this interfacial chemistry has been supplemented by use
of environmental plasma techniques.
3. Plasma Modification
collaboration with the group of Professor Seong Kim at Penn State has allowed us
to make use of environmental (ie. atmospheric pressure) plasma equipment to
bring about rapid changes in the surface structure and properties of
polyphosphazenes. The simplest case
is the treatment of films of fluoroalkoxy-substituted polyphosphazenes to
produce hydrophilic or superhydrophobic interfaces using oxygen, nitrogen,
methane, or trifluoromethane in the plasma gas.
These reactions can take place with the use of shadow masks, a process
that allows the formation of patterned regions of hydrophilic or hydrophobic
character that are needed for lithography or soft contact printing as well as
selective immobilization of biological reagents.
We are also using this method to modify the surfaces of bioerodible
polyphosphazenes that are used in tissue engineering experiments (see Biomedical
The wide range of unique properties that can be generated
from polyphosphazenes has made these polymers the focus of interest for numerous
advanced applications, especially where no other polymers exist with the
required combinations of characteristics. These applications include several
challenging areas of biomedical research, uses as ion conductive membranes for
rechargeable polymer lithium batteries and fuel cell membranes, advanced
elastomers for applications in the aerospace engineering, photonic materials,
and fire-resistant polymers. Members of our group do the polymer design,
synthesis, and characterization studies, and we then collaborate with
specialists in each of the applications areas.