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 V_f/V_sp, where V_f (cm³/g) is the free volume andV_sp (cm³/g) is the specific volume of the polymer, the latter parameter being the reciprocal density ρ. The value of V_f can be calculated using Bondi (1968) formula V_f = V_sp – V_oc, where V_oc is the occupied volume, and it can be estimated as V_oc = 1.3 V_w, where V_w 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 V_ocis estimated from packing...