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Energy Related Research  |  Biomedical Polymers  |  Photonic Materials  |

Aerospace Materials  |  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.

 1. 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 electrolytes.


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 electrolytes.

2. Polymer Electrolyte Fuel Cells

 Proton-conductive 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 electrodes.  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

 The 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)

 2. 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 group.

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.    

 Hydrolysis prod

            (b) For controlled drug or vaccine delivery.  Bioerodible 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 chemotherapeutic drugs.

             (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 possible.

 3. Biostable Polymers

             (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 specific compounds.  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.



1. Background

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

 Organic 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 Photochromic Materials

 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

 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.




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.  Although 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.

 2. Polyphosphazenes in Fire-resistant Composite Materials and Foams

 Considerable 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 aircraft construction.

 Lightweight 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.

 Fire-resistant polymers are needed for advanced aerospace, automotive, and housing applications.  The 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

 A 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 section).

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.


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Copyright 2006 H. R. Allcock Research Group
Last modified: 10/15/2017