Polyphosphazenes Thermal properties

Polyphosphazenes offer unique thermal properties and have shown a number of uses in the field of electronics and medicine. 

Among inorganic heterocycles, perhalogenated cyclic phosphazenes occupy a very prominent place as the precursors for polyphosphazenes whose properties can he tuned by changing the substituents on the phosphorus sites (J). The heterocycles, as such, provide a robust framework for building a variety of novel molecules that have high thermal stability and flame retardancy, and have also been used recently to prepare a variety of dendrimers (2). The most widely studied among these are the perchlorinated cyclophosphazenes. Their syntheses reactions, properties, and application potential have been well documented a, 3-7). 

Polyphosphazenes have diverse thermal properties, and as one would anticipate these would also depend upon the nature of side-groups. Many... 

Thermal methods including differential thermal analysis (DTA), thermo-gravimetric analysis (TGA), differential scanning calorimetry (DSC) and thermogravimetric/dilferential thermal analyzer (TG-DTG) have been employed mainly for the analysis of polymers and particularly the characterization and determination of the thermal properties of cyclotripho-sphazenes " and polyphosphazenes for example. Obviously, full characterizations have been carried out using spectroscopic and structural studies. 

To conclude this synthetic section, it appears very clear that the experimental approaches for preparation of POPs are very numerous and give accessibility to phosphazene polymers and copolymers with different structures and properties. Moreover, it has been recently estimated [10,383] that the total number of polyphosphazenes reported up to now in the literature is about 700, and that these materials can find potential practical application as flame- and fire-resistant polymers [44,283, 384-388] and additives [389, 390] thermally stable macromolecules [391] chemically inert compounds [392] low temper-... 

For halogen or alkoxy-substituted polymers Tg values are low (between -60 and —100° C). Alkoxy poly phosphazene with Ci - C3 substituents are elastomers. Higher alkoxy- or aryloxy-substituted polyphosphazenes are thermoplastics. Fluoroalkoxypolyphosphazenes exhibit a good stability toward diluted acids and bases. Some of them have outstanding thermal stability and good flame-retardant properties. 

Figure 2 Properties in polyphosphazenes are determined hy (1) the backbone bonds that control the inherent flexibility of the polymer via their influence on bond torsional freedom, and also provide photo-and thermo-oxidative stahihty (2) the side groups control polymer solubility, reactivity, thermal stability, crystallinity, cross-linking, and (indirectly) polymer flexibility (3) free volume between the side groups affects polymer motion, solvent penetration, membrane behavior, and density (4) functional groups (usually introduced hy secondary reactions) affect soluhihty, biological behavior, proton conduction, cross-hnking, and many other properties...

This specification, prepared by the Navy, covers polyphosphazene elastomeric foam material for thermal insulation on piping, in either sheet or tubing form. Polyphosphazene foam has excellent fire-retardant properties and is suitable for use in the range -20 to 180°F (-29 to 82.TC) in tubular form (Form T). Form S covers sheet form. 

Sulfonated aromatic polymers have been widely studied as alternatives to Nafion due to potentially attractive mechanical properties, thermal and chemical stability, and commercial availability of the base aromatic polymers. Aromatic polymers studied in fuel cell apphcations include sulfonated poly(p-phenylene)s, sulfonated polysulfones, sulfonated poly(ether ether ke-tone)s (SPEEKs), sulfonated polyimides (SPIs), sulfonated polyphosphazenes, and sulfonated polybenzimidazoles. Representative chemical structures of sulfonated aromatic polymers are shown in Scheme 3. Aromatic polymers are readily sulfonated using concentrated sulfuric acid, fuming sulfuric acid, chlorosulfonic acid, or sulfur trioxide. Post-sulfonation reactions suffer from a lack of control over the degree and location of functionalization, and the... 

The presence of the phosphorus-nitrogen backbone confers to these kind of polymers thermo-oxidative stability, fire resistance, very high torsional mobility (low barrier to skeletal bond twisting), high refractive index, and hydrophilicity. On the other hand, the side groups in polyphosphazenes control other properties such as solubility, secondary reaction chemistry, thermal decomposition, and resistance to hydrolysis. The possibility of tuning the properties of polyphosphazenes thanks to their synthetic flexibility has led to enormous interest in their applications in several areas of research. 

In addition, the phosphorus-nitrogen backbone inherently possesses a unique range of unusual properties. For example, it is extremely flexible which in turn can give rise to low glass-transition temperatures, particularly in the case of poly(alkoxyphosphazenes) such as the n-butoxy derivative (Tg = -105°C) (1,4). Furthermore, the backbone is thermally and oxidatively stable, as well as optically transparent from 220 nm to the near infrared region, which makes it resistant to breakdown in many harsh environments, as evidenced by the flame-retardant properties of many polyphosphazenes. 

Polyphosphazenes, owing to their inorganic backbone, are characterized by properties that are not common to organic pol5miers, namely low temperature flexibility, nonflammability, good thermal stability, and biocompatibility. 

The derivatization reactions of the poly(alkyl/aryIphosphazenes) afford many new polyphosphazenes with a variety of functional groups and significantly extend the versatility of this class of polymers. Furthermore, the chemistry described above demonstrates that it will be possible to incorporate a number of side-groups into the polymer to enhance the thermal, mechanical, and electrical properties. A large number of graft copolymers of polyphosphazenes should also be accessible via initiation of anionic polymerizations by the polymer anions. 

Bose, S., Pramanik, N., Das, C.K., Ranjan, A., and Saxena, A.K. (2010) Synthesis and effect of polyphosphazenes on the thermal, mechanical and morphological properties of poly(etherimide)/ thermotropic liquid crystalline polymer blend. Mater. Des., 31,1148-1155. 

---