Influence of free volume change on the relative permittivity and refractive index in fluoropolyimides

The often observed decrease in relative permittivity (ε') and refractive index (n) that accompanies fluorine substitution is the result of several effects. Fluorine replacement for hydrogen is known to decrease local electronic polarization and is thought to frequently increase fractional free volume. Both of these effects can independently decrease ε' and n, and this paper attempts to assign the observed decreases between these mechanigms for a series of hydrogen/fluorine polyimide analogs.