Free volume is a key property regulating penetrant (gas phase) or solute (solution phase) permeability in polymeric membranes. Thus, we investigated the origins of free volume in two types of Teflon AF 2400 nanocomposites. In nanocomposites from glassy Teflon AF and fluorophilic, solid silica nanospheres, the soft matrix shows exceptionally high apparent fractional free volume (aFFV(P))-reaching over 50% at high particle loading. Adding the higher density FNPs to polymer films actually decreases film density. Nanocomposites from plasticized Teflon AF show similarly high aFFV at high particle loading. However, the nano-scale morphologies of the films are very different. The nanocomposites with glassy polymer exist as a polymer matrix with entrained particles. At higher FNP loadings, where the characteristic distance between particles is smaller than the radius of gyration of the polymer, the polymer exists as a vine-like network entraining the particles. Nanocomposite films with plasticized polymer have a packed, core-shell particle morphology with FNPs as the core and plasticized Teflon AF as the eight-nm shell. While the aFFV(P)s of the two types of films at high FNP loading are similar, systematic studies reveal different mechanisms for the formation of the apparent free volume elements. Distinctive free volume properties lead to remarkably different transport behavior. (C) 2013 Elsevier B.V. All rights reserved.