The
fractional free volume Fv can then be estimated as Fv = C Vf I3. Here
C = 0.0018 Å3 is used as suggested in the literatureNote that, in a polymer, the free volume sizes distribute over a wide
range, and hence Vf calculated from the average radius R is not
strictly the average free volume size in the real sense; it needs to
be evaluated statistically from the hole volume distribution. However,
literature very often reports this usage and in the present
work, we follow this common practice of calling Vf as the average
free volume cell size.