Polyphosphazenes properties

Properties. One of the characteristic properties of the polyphosphazene backbone is high chain dexibility which allows mobility of the chains even at quite low temperatures. Glass-transition temperatures down to —105° C are known with some alkoxy substituents. Symmetrically substituted alkoxy and aryloxy polymers often exhibit melting transitions if the substituents allow packing of the chains, but mixed-substituent polymers are amorphous. Thus the mixed substitution pattern is deUberately used for the synthesis of various phosphazene elastomers. On the other hand, as with many other flexible-chain polymers, glass-transition temperatures above 100°C can be obtained with bulky substituents on the phosphazene backbone. 

Phosphazene polymers are inherently good electrical insulators unless side-group stmctures allow ionic conduction in the presence of salts. This insulating property forms the basis for appHcations as wire and cable jackets and coatings. Polyphosphazenes also exhibit excellent visible and uv radiation transparency when chromophoric substituents are absent. 

Applications. Many applications have been proposed for polyphosphazenes, particularly the non-cyclic polymers of high molecular weight, but those with the most desirable properties are extremely expensive and costs will have to drop considerably before they gain widespread use (cf. silicones, p. 365). The cheapest compounds are the chloro series... 

The physical properties of polyphosphazene depend on the nature and the number of substitutes. However, the flexibility of the P-N backbone is the property in common. Because of the weakness of the rotation energy around the N-P bond (3.38 and 21.8 kJ/mol, respectively for... 

Potin P and Jaeger RD. Polyphosphazenes Synthesis, structures, properties, applications. Eur Polym J, 1991, 415, 341-348. 

Anurima S, Nicholas RK, Swaminathan S, Lakshmi SN, Jacqueline LS, Paul WB, Cato TL, and Harry RA. Effect of side group chemistry on the properties of biodegradable L-alanine cosubstituted polyphosphazenes. Biomacromolecules, 2006, 7, 914-918. 

This account will summarise results in the development of n-conjugated materials incorporating phosphorus moieties with emphasis on the conceptual design and specific properties that result directly from the presence of the P-atom. Polyphosphazenes, which are the most familiar synthetic polymers incorporating phosphorus [8], will not be included in this review since they do not display the type of n-conjugation as sought in systems (A)-(D). 

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-... 

Of course, not all the phosphazene polymers that have been synthesized are equally important. Many of them, in fact, have a mere academic or speculative interest, and will not be described in this article. A few other classes of POPs, however, do occupy an important place in phosphazene history, and have been seriously considered for industrial development and commercialization. These polymers are basically those in which the properties of the inorganic -P=N- skeleton overlap to the highest extent those of the phosphorus side substituents. In the successive sections of this article we will describe in some detail the most important classes of polyphosphazenes that fulfil this condition. 

The general structure of polyphosphazenes substituted with fluorinated alcohols is described by the Formula below while the basic structure-property relationships for these substrates are collected in Table 9. 

A new series of properties are expected for polyphosphazenes when the percentage of inorganic elements inherently present in the -P=N- skeleton is artificially enhanced by introducing fluorinated alcohols as side phosphorus substituents. This facilitates their application in different fields. 

In conclusion, polyphosphazenes containing fluoroalkoxy groups as side phosphorus substituents constitute one of the most relevant class of macromolecules of this family and have attracted remarkable interest in the past because of their outstanding properties and wide range of applicability, especially in low and high temperature domains, and have received renewed interest in more recent times [399,457]. 

Polyphosphazenes are intrinsically fire-resistant materials because of the presence of phosphorus and nitrogen in the polymeric chain. A low flammability is thus one of the most important properties of polyphosphazenes, particularly of the polyaryloxyphosphazenes I, in which R may be H, halogens, and alkyl or alkoxy groups. 

The development relative to the flame-retardant properties of polyphosphazenes has been principally centred around the aryloxyphosphazenes copolymers II. 

Aryloxyphosphazene copolymers can also confer fireproof properties to flammable materials when blended. Dieck [591] have used the copolymers III, and IV containing small amounts of reactive unsaturated groups to prepare blends with compatible organic polymers crosslinkable by the same mechanism which crosslinks the polyphosphazene, e.g. ethylene-propylene and butadiene-acrylonitrile copolymers, poly(vinyl chloride), unsaturated urethane rubber. These blends were used to prepare foams exhibiting excellent fire retardance and producing low smoke levels or no smoke when heated in an open flame. Oxygen index values of 27-56 were obtained. 

In this section we will describe the general principles that determined the biological applications of polyphosphazenes in different domains, putting an effort into establishing their specific utilization on the basis of structure-property relationships. This argument has been covered by several different review articles in the past [400-406,626] and has been recently highlighted by H. R. Allcock [627] and E. Schacht [407]. 

The third topic in polyphosphazene biomaterials that will be described in this article concerns surface implications. One of the major problems in the utilization of polyphosphazenes in solid state is their exploitation in the construction of implantable devices, for which good physical properties, minimum biological response, and good resistance to fungal or bacterial colonization may be required. 

The modification of the surface properties of polyphosphazene films could be achieved in several different ways, and the most important types of modifications carried out over the years are reported in Table 23. 

Furthermore, polyphosphazene features are interpreted as the resultant combination of two basic contributions one coming from the properties inherently due to the polyphosphazene inorganic backbone (-P=N-), the other being due to the characteristics possessed by the exploited nucleophiUc substituents. 

Understanding the relationship between molecular structure and materials piroperties or biological activity is one of the most important facets of biomaterials synthesis and new-drug design. This is especially true for polyphosphazenes, where the molecular structure and properties can be varied so widely by small modifications to the substitutive method of synthesis. 

Table 1 is a summary of current knowledge of the relationship between side group structure in polyphosphazenes and biomedically important properties. Within rather broad limits two or more of these properties can be incorporated into the same polymer by a combination of different side groups attached to the same macromolecular chain. 

TABLE 1 Summary of Side Group Structure-Property Relationships in Polyphosphazenes... 

Allcock, H. R., and Kwon, S., Glyceryl polyphosphazenes Synthesis, properties, and hydrolysis. Macromolecules. 21. 

Unique combinations of properties continue to be discovered in inorganic and organometallic macromolecules and serve to continue a high level of interest with regard to potential applications. Thus, Allcock describes his collaborative work with Shriver (p. 250) that led to ionically conducting polyphosphazene/salt complexes with the highest ambient temperature ionic conductivities known for polymer/salt electrolytes. Electronic conductivity is found via the partial oxidation of unusual phthalocyanine siloxanes (Marks, p. 224) which contain six-coordinate rather than the usual four-coordinate Si. 

Moreover, the molecular structural, synthetic, and property nuances of these polymers illustrate many of the attributes, problems, and peculiarities of other inorganic macromolecular systems. Thus, they provide a "case study" for an understanding of what may lie ahead for other systems now being probed at the exploratory level. In short, an understanding of polyphosphazene chemistry forms the basis for an appreciation of a wide variety of related, inorganic-based macromolecular systems and of the relationship between inorganic polymer chemistry and the related fields of organic polymers, ceramic science, and metals. 

Such hybrid molecules and supramolecular solids offer the promise of systems with the flexibility, strength, toughness, and ease of fabrication of polymers, with the high temperature oxidative stability of ceramics, and the electrical or catalytic properties of metals. Polyphosphazene chemistry provides an illustration of what is possible in one representative hybrid system. 

TCNQ-Polyphosphazene Systems. Tetracyanoquinodimethane (XX) salts crystallize in the form of stacked arrays that allow electrical semiconductivity (42). Although this phenomenon has been studied in many laboratories, it has not been possible to fabricate conductive films or wires from these substances because of the brittleness that is characteristic of organic single crystals. However, it seemed possible that, if the flexibility and ease of fabrication of many polyphosphazenes could be combined with the electrical properties of TCNQ, conducting polymers might be accessible. 

The development of synthetic routes to new polyphosphazene structures began in the mid 1960 s (2-4). The initial exploratory development of this field has now been followed by a rapid expansion of synthesis research, characterization, and applications-oriented work. The information shown in Figure 3 illustrates the sequence of development of synthetic pathways to polyphosphazenes. It seems clear that this field has grown into a major area of polymer chemistry and that polyphosphazenes, as well as other inorganic macromolecules, will be used increasingly in practical applications where their unique properties allow the solution of difficult engineering and biomedical problems. 

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