Polyphosphazenes glass transition

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. 

The final class of polymers containing carboranyl units to be mentioned here is the polyphosphazenes. These polymers comprise a backbone of alternating phosphorous and nitrogen atoms with a high degree of torsional mobility that accounts for their low glass-transition temperatures (-60°C to -80°C). The introduction of phenyl-carboranyl units into a polyphosphazene polymer results in a substantial improvement in their overall thermal stability. This is believed to be due to the steric hindrance offered by the phenyl-carborane functionality that inhibits coil formation, thereby retarding the preferred thermodynamic pathway of cyclic compound formation (see scheme 12). 

A theoretical analysis of the conformational energies of PDCP is presented. The results indicate that the bond pair P—N—P possesses a considerable conformational freedom, whereas the bond pair N-P-N is relatively rigid. This difference explains the low glass transition temperatures and large end-to-end distances measured for polyphosphazenes. All the calculated magnitudes are extremely sensitive to the energy Eler) that controls the statistical weight of the conformations tg, to, tg , gt, ct, and g t, relative to tt for the bond pair P-N-P. A qualitative explanation for this sensitivity is discussed. 

The versatility of water-soluble polyphosphazenes is in the variations in the structures that can be prepared. Structures with a low glass-transition temperature backbone can be modified with a variety of versatile side units. 

The versatility of water-soluble polyphosphazenes is in the variations in the structures that can be prepared. Structures with a low glass-transition temperature backbone can be modified with a variety of versatile side units. These may find use in solid polymeric ionic conductors, as a means to entrap and immobilize enzymes with retention of enzymic activity, and in biological functions as hydrogels with the capability of exhibiting biocompatibility and... 

Table 3.1 Physical Properties of Selected Polyphosphazenes Sequenced by Decreasing Glass Transition Temperature3...

As a coating offers increased anti-icing effectiveness and durability than fluorocarbon and silicone elastomers. These icephobic coats can reduce the accumulation of ice on products such as rooftops, aircraft, radomes, antennas, ships, and power-transmission lines. The weight of such accumulations of ice has led to aircraft crashes, fallen power lines, etc. The icephobic coats reduce the adhesive force between ice and a surface. Polyphosphazene elastomers possess these desired properties, in addition have low glass transition temperature (Tg), good environmental stability, curability, and moderate cost. 

The Backbone. The linear inorganic backbone imparts an unusual combination of properties. First, perhaps unexpectedly in view of the unsaturated structure, the skeletal bonds have a low barrier to torsion (perhaps as low as 0.1-0.5KcaF repeating unit), which becomes translated into one of the most flexible backbones known throughout polymer chemistry. This means that some polyphosphazenes have glass-transition temperatures (Tg) as low as -100 °C. It also means that, in the absence of microcrystallinity, numerous polymers of this type are rubbery elastomers. This is a key property for... 

Polyphosphazene synthesis provides additional possibilities for preparing liquid crystal polymers with different properties. As noted above, the substitution process (Figure 2) enables one to synthesize a wide variety of polymers. The phosphazene inorganic backbone Is a highly flexible polymer chain glass transition temperatures can... 

Recently, a mixed-substituent polyphosphazene (polymer V) was synthesized and the second-order NLO properties were investigated (17). The nitrostilbene/trifluoroethoxy ratio was approximately 36 64. Due to the low glass transition temperature of V (T - 25 C), the second harmonic signal decayed to zero within a few minutes. However, polymer V is a prototype which offers many opportunities for further tailoring the molecular structure of polyphosphazenes to generate an optimum combination of NLO and physical properties (17). 

Polyphosphazenes are suitable materials to be used as carriers for nonlinear optical (NLO) chromophores. Second order NLO properties have been studied for the polymer (128) and blends of (129) with the free chromophore (130) or the cyclophosphazene (131). All systems have glass transition temperatures higher than 135°C and a wide transparency window. The system (129)-(130) appears to exhibit the highest second-harmonic generation (SHG) response. For possible applications the SHG capability has to be enhanced. ... 

Polyphosphazenes with ferrocenyl substituents 35 have also been synthesized via the functionalization of poly-(methylphenylphosphazene) and related polymers by means of a deprotonation-electrophilic addition strategy (e.g., see Equation (10)). This versatile reaction sequence has yielded materials with, for example, degrees of substitution of 45% and 36% for polymers 35 (R = H and Me), respectively. The molecular weights of the polymers were M = Z.O x 10 and 1.5 x 10 for 35 (R = H and Me), respectively (with PDI values of 1.4-2.0). The glass transition temperatures increased in comparison with the unsubstituted polymer (Tg = 37°G) for 35 with values of 92 °C (R = H) and 87 °G (R = Me). 

Polyphosphazenes are among the most flexible polymers known. This is reflected in low glass transition temperatures (Tg). Only below the glass transition temperatures all conformational mobility is frozen and polymer becomes a glass. The skeletal flexibility of polyphosphazenes seems to arise from many factors. 

The glass transition temperatures for a few representative polyphosphazenes are given in Table 3.8. 

The parent polyphosphazenes [NPF2]n and [NPCyn, have extremely low glass transition temperatures of -95 °C and -66 °C respectively. This data suggests that these polymers have a lot of torsional freedom or skeletal flexibility. Only below the glass transition temperatures all the torsional motion of the polymers is arrested. 

Table 3.8. Glass-transition temperatures of various types of polyphosphazenes...

What is the relationship of the glass transition temperatures with the structures of polyphosphazenes Small-sized substituents on phosphorus (such as chlorine or fluorine) aid in low Tg s. Thus, poly(fluorophosphazene)s which contain small-sized fluorine substituents (and because fluorines have very poor van der Waals interactions) have very low Tg s. 

The trends in the glass transition temperatures are similar to what have been observed in polyphosphazenes. Thus, replacing the chlorines with aryloxy substituents increases the Tg. One of the highest Tg s is observed for the anilino derivative, [ NP(NHPh)2 CNHPh ] (+112 °C). 

As mentioned earlier the glass transition temperatures of poly(thionylphosphazene)s indicate that these are polymers with a flexible backbone. These polymers have five side-groups per repeat unit which is similar to poly(carbophosphazene)s and poly(thiophosphazene)s. On the other hand, a comparable polyphosphazene unit has six side-chains. This has interesting consequences on the glass transition temperatures of poly(thionylphosphazene)s (Table 5.3). 

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. 

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