Low relative permittivity polymers

The development of polymers with low relative permittivity provides an informative and instructive exercise in polymer design using the principles 

For advanced electronics applications polyimides have been the materials of choice for high-performance insulation. The simplest member of the polyimide series has the structural formula  

An additional benefit of fluorination is an increase in hydrophobicity through the effect of the highly polar C-F groups. This means that the level of absorbed water in the polymer at typical ambient humidity is much reduced. Water has a very high relative permittivity, so that its presence can significantly increase the relative permittivity of a material, and the level will also be liable to vary with ambient conditions, bringing uncertainty into design considerations for electronic systems. 

Adopting the fluorination approach in polyimides, relative permittivities can be reduced from 3.4 to 2.8, a common structure incorporating the hexafluoroLsopropylydene moiety (6F)  

An increase in fractional free volume will reduce the number of polarisable groups per unit volume, and thereby reduce the relative permittivity of the polymer. Quantitatively, the effect may be estimated by means of the Clausius-Mossotti/Lorenz-Lorentz model for dielectric mixing (Bottcher, 1978)  

---