For the first time, we report the C4s' transport properties: solubility, diffusivity, permeability in 6FDA-DAM polymer, one of most permeable glassy polymers with significant nC4 vs. iC4 selectivity. An nC4 permeability of 3.7 Barter, and nC4/iC4 ideal selectivity of 21 was found in pure 6FDA-DAM polymer membrane. Mixed matrix films were successfully fabricated using a 6FDA-DAM as the matrix, with up to 35 wt% loading of MFI, modified by a two-step Grignard treatment (GT) that produced Mg(OH)(2) whiskers on the surface of the MFI particles. The permeability of nC4 more than doubled; however, the selectivity for the C4s remained the same. Permeation of mixed matrix films with impermeable GT-uncalcined-MFI agree with Maxwell modeling of films with an impermeable solid loading, thereby supporting the existence of a defect-free interface between MFI particles and the polymer matrix. This indicates that the MFI is too permeable to optimally match the properties of the 6FDA-DAM, which is one of the most permeable selective matrix polymers available. It appears unlikely that any currently known, adequately selective glassy polymer can match the high permeability of nC4 in MFI to enable development of promising composite membrane for the C4s separation based on MFI. Therefore a smaller pore size zeolite is required for a better match. (C) 2009 Elsevier B.V. All rights reserved.