Torsional mobility

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). 

Analysis of the conformational flexibility of the unique components of the investigated DBA systems in terms of torsional mobility and its effect on the 7t-conjugation. 

For symmetrical polymers, the chemical nature of the backbone chain is the important factor determining the chain flexibility and hence Tg. Chains made up of bond sequences which are able to rotate easily are flexible, and hence polymers containing -(-CH2-CH2-)-, -(-CH2-0- H2-)-, or -(-Si-O-Si-)- links will have correspondingly low values of Tg. For example, poly(dimethyl siloxane) has one of the lowest Tg values known (-123°C) presumably because the Si-0 bonds have considerable torsional mobility. 

One of the unusual properties of (PNF2) is its low-tanperature elasticity, which reflects a relatively high degree of torsional mobility of the chain. Of the three polymeric phosphazene halides, the fluoride has the lowest glass transition temperature, which is in accord with it having least interchain interaction and highest torsional freedom at low tanperatures (Table 12.37). 

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. 

A polymer with a low Tg has considerable torsional mobility. For example, poly(dimethylsiloxane) [Si(CH3)20] has a Tg of-127 °C. On the other hand high Tg s mean a stiff and rigid polymer. For example, poly(methyl-methacrylate) has a Tg of about 120 °C [1]. 

Oxygen atoms in a hydrocarbon chain give torsional mobility and materials flexibility. Nearby skeletal units have to be taken into consideration as ether linkages are generally stable to hydrolysis and to thermooxidation while polyaldehydes depolymerise readily at moderate temperatures. 

The silicone-oxygen bond has one of the highest torsional mobilities in any polymer backbone, e.g., Tg of polyfmethyl siloxane) is -130 °C. Polydimethylsiloxane (-[Si(CH3)20] -) is more stable to thermooxidative attack than is the aliphatic C-O bond but is sensitive to certain reagents such as acids and bases. 

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