Fractional Free Volume (FFV)
Fractional free volume (FFV) is an empirical dimensionless parameter that characterizes free volume in polymers. FFV can be defined as the ratio Vf/Vsp, where Vf (cm3/g) is the free volume andVsp (cm3/g) is the specific volume of the polymer, the latter parameter being the reciprocal density ρ. The value of Vf can be calculated using Bondi (1968) formula Vf = Vsp – Voc, where Voc is the occupied volume, and it can be estimated as Voc = 1.3 Vw, where Vw is van der Waals volume of the repeat unit of the polymer. Such an estimate gives the FFV values of most polymers in the range of 10–25 %. However there are examples when FFV is as large as 35 % (polytrimethylsilyl propyne). In spite of its wide usefulness, such definition of FFV has many disadvantages and is being often criticized. In the case of glassy polymers, the density may depend on prior history of the sample and, to some extent, on the method of determination. The coefficient 1.3 in calculating Vocis estimated from packing...
- Bondi A (1968) Physical properties of molecular crystals, liquids, and gases. Wiley, New YorkGoogle Scholar