Applications


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.

 

Polyphosphazenes in Biomedical Research

A number of polyphosphazenes produced in our program have proved to have properties that are particularly important in medical research.  These include solid polymers that bioerode to harmless products and which have many advantages as scaffolds for tissue regeneration, macromolecules that form responsive hydrogels and membranes, polymers that serve as microsphere delivery vehicles for drugs and vaccines, and block copolymers for micell formation.  The following are illustrative examples.


           Polyphosphazene Scaffolds for Bone Regeneration. We have been part of a long-standing collaboration with the groups of Dr. Cato Laurencin at the University of Virginia and Dr. Paul Brown at Penn State to develop polyphosphazenes that can serve as resilient but bioerodible supports for in vivo bone regeneration.  The strongest emphasis has been placed on polymers that have carboxylic acid function groups that can form ionic crosslinks with calcium ions and polyphosphazenes with amino acid ethyl ester side groups that bioerode to phosphate, ammonia, amino acid, and ethanol.  The phosphate/ammonia combination is a pH-buffered system which overcomes many of the disadvantages of the polyesters that have been investigated intensively for this application.  The idea is this: fiber matrices seeded with the patient's own osteoblast cells will serve as a support for broken or diseased bones.  The polymer erodes hydrolytically as the osteoblast cells multiply and fill the space between the fibers.  Eventually the polymer will disappear and be replaced completely by living bone.  Our challenge has been to design polyphosphazenes that hydrolyze at a specific rate and maintain their strength as the erosion process proceeds.  To this end we have synthesized a broad portfolio of polymers with different amino acid ester side groups either alone or in combination with hydrophobic cosubstituents.  The most promising materials have been fabricated by electrospinning into micro- and nano-fibers, as shown in the following scanning electron microscope photograph.

            Membranes and Hydrogels.  Polyphosphazenes with alkyl ether side groups, such as -OCH2CH2OCH2CH2OCH3, are soluble in and stable to water.  When lightly crosslinked by gamma radiation they absorb water and swell to hydrogels, but they do not dissolve.  The hydrogels show a lower critical solution temperature (LCST) which means that below a certain temperature they are expanded.  Above that temperature they contract and expel the water.  The LCST depends on the length, branching, and end groups of the side chains.  The LCST for several of these hydrogels is near human body temperature which means that, in the form of membranes, they may be used for the stepped delivery of drugs or to control the activity of trapped enzymes or cells.  In the expanded state molecules in solution can reach or leave the enzymes.  In the contracted state of the membrane the activity of the enzyme is curtailed.  The behavior of the membrane can be tuned further by the incorporation of aryloxy cosubstituent group that bear carboxylic acid functional units.  These hydrogels expand and contract in response to changes in pH, cation charge, or the ionic strength of the medium.  In collaboration with Dr. Fahran Gandhi at Penn State we are also investigating the possibility that phosphazene hydrogels can be developed as artificial muscles.


            Microspheres.  Polyphosphazenes with carboxylic acid functional groups such as compound 1, are soluble in water as their sodium salts but insoluble as calcium salts because the divalent cations form ionic crosslinks between the chains. 

Thus, treatment of solutions of the sodium salt with soluble calcium salts can bring about precipitation of the polymer in the form of microspheres.  Biological agents present as co-solutes in the polymer solution will be trapped in the microspheres and will be released in the presence of sodium ions, which de-crosslink the chains.  This phenomenon was first developed through a collaboration with the group of Dr. Robert Langer at M.I.T. for mammalian cell encapsulation, but it has been utilized specifically for the oral delivery of vaccines by Parallel Solutions Inc. in Cambridge, Massachusetts, and has progressed through human clinical trials.


            Micelles.  Phosphazene di-block copolymers formed via the living cationic polymerization technique (see Fundamental Science section) can be designed to show amphiphilic character, with one block hydrophilic and the other hydrophobic.  Such polymers form nanometer-size micelles in aqueous media, with the hydrophilic block on the outside and the hydrophobic block in the interior.

This work, in collaboration with the group of Dr. Chulhee Kim at Inha University in Korea, is aimed at the controlled delivery of hydrophobic drugs via the circulatory system.  Block copolymers that have been investigated include species with two different phosphazene blocks, or a phosphazene block and organic polymer or silicone block.  The phosphazene block can be hydrophilic or hydrophobic, and may also be bioerodible, for example, with amino acid ester side groups.

Ion Conducting Membranes

            Lithium Ion Conductors.  Most rechargeable lithium batteries contain a flammable organic liquid electrolyte.  Such batteries are prone to leakage, evaporation of the solvent, and the danger of fire.  It has been recognized for many years that lithium energy storage devices with a solid polymer membrane as the electrolyte would be safer, lighter, and more robust than existing systems.  Polyphosphazenes with alkyl ether side chains and with dissolved lithium salts are some of the most promising candidates for this application.  They are non-flammable, have low glass transition temperatures, and are among the few polymers that dissolve salts like lithium triflate or lithium trifluoromethylsulfonimide.  We began this program at the fundamental level working with the groups of Dr. Duward Shriver at Northwestern University and Dr. Roger Frech at the University of Oklahoma, and have continued in recent years at the device level working with Dr. Digby Macdonald at Penn State.  More than 30 different polyphosphazenes have been designed and synthesized in our program, and prototype batteries have been fabricated and evaluated.

We have also developed cyclic trimeric phosphazenes with branched alkyl ether side groups that serve to enhance lithium ion conduction in a number of polymeric membranes.

 

Proton Conducting Fuel Cell Membranes.  The incorporation of acidic functional units into the side groups of polyphosphazenes yields polymers that are good proton conductors.  They have advantages over other advanced fuel cell membranes because they are unaffected by oxidation or reduction reactions, are resistant to free radicals, resist methanol crossover, and can be fire retardant.  Typical polyphosphazenes synthesized and studied in our program have aryloxy side groups with sulfonic acid, phosphonic acid, or sulfonimide functional units.  The evaluation of membranes produced from these polymers has been carried out in the group of Dr. Serguei Lvov in the Energy Institute at Penn State.  The proton conductivities are similar to that of Nafion (an industry standard) but the phosphonic acid type membranes in particular have methanol diffusion coefficients one to two orders of magnitude less than Nafion membranes.  Thus, they are candidates for use in direct methanol fuel cells and possibly in small scale hydrogen-oxygen fuel cells.

Photonic Materials

            Traditional photonic materials (optical waveguides, non-linear-optical switches, etc.) have traditionally utilized totally inorganic materials.  The ease with which different side groups can be incorporated into polyphosphazenes has opened the possibility that these polymers may find uses in communications technology.  Recent work in our program has been focused on polymer glasses with high refractive indices, second-order non-linear optical materials, liquid crystalline polymers, photochromic, and electroluminescent materials.  For example, we have shown that polymers with aryloxy side groups have refractive index values in the range of 1.6-1.75.  Others with both aryloxy and halogenated alkoxy side groups are possible materials for thermo-optical switches.  Polymers with aromatic azo or biphenyl side groups undergo liquid crystalline transitions, or have non-linear optical characteristics.  Photochromic polymers with spiropyran side groups change color reversibly when exposed to ultraviolet light or when heated.

We have recently used phosphazene rings as "insulators" in the backbone of photoluminescent and electroluminescent poly(phenylenevinylenes) to control the conjugation length and hence control the color of the emitted light.  Red, blue, and green emitters have been produced.  This work is being carried out in collaboration with the group of Dr. Mary Galvin at the University of Delaware.  The photoluminescent behavior is illustrated in the photograph

 

High Performance Elastomers

 

            One of the major areas of application for polyphosphazenes is as high performance elastomers.  These are linear-type polyphosphazenes with fluorinated and/or aryloxy side groups.  Many of them have low glass transition temperatures (-60 oC is typical), are fire-resistant, hydrophobic, resistant to high energy radiation, and are thermally stable.  An important aspect of our program is to control the polymer structure at the molecular level to amplify specific properties such as adhesion, chemical and radiation resistance, and biocompatibility.  Hybrid polymers that contain both phosphazene and organosilicon components are of growing interest, as are species with functional side groups that promote coordination to metals or other materials.  The ability to control vapor or liquid permeability, and the generation of specific surface properties are other aspects.  Several of these objectives mesh with our interests in the development of new polymeric materials for uses in biomedicine, aerospace and automotive applications, fuel cells, advanced energy storage devices, and semiconductor technology. 

 


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Last modified: 04/21/12