Free Volume Estimation
Originally, free volume was interpreted (H. Fujita, M.H. Cohen and D. Turnbull, J.S. Vrentas and J.L. Duda) as an abstract parameter that fits the equation of free volume model. The methods for quantitative determination of free volume were proposed by S. Sugden and later by A. Bondi (1968). According to the latter, free volume Vf (cc/g) can be estimated as the difference: Vf = Vsp – Vocc, where the specific volume is defined as Vsp = 1/ρ (ρ is the polymer density), while occupied volume Vocc can be found as 1.3 Vw, where Vw is the van der Waals volume of the repeat unit of the polymer. The Vw value can be estimated from the increments tabulated for various groups (Van Krevelen and te Nijenhuis 2009). The factor 1.3 is taken from the packing density of molecular crystal at 0 K (accounting for inaccessible volume). Bondi’s formula was criticized: the factor 1.3 might not be a universal constant, applicable in all cases; Vwof repeat units cannot be considered as the sum of increments...
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