Dense membranes were prepared from three different rigid block copolyaramides, one block bearing two bulky hexafluoro (-CF3) groups and a lateral tert-butyl group (-C-(CH3)(3)), and the second block without the lateral tert-butyl group. The effect of block length, at constant comonomer concentration, on thermal properties as well as gas permeability coefficients and separation factors is analyzed. The results indicate that block copolyaramide membranes present a density that is quite similar but slightly lower, as the length of the blocks that form the copolymer increase, that falls in between the density of the homopolyamides. The fractional free volume, FFV, increases in the block copolymers as the block length increases. This result is attributed to an inefficient packing of the copolymer molecules as the block length gets larger. As a result, the permeability and diffusion coefficients in the block copolymers are larger than those in the parent homopolymers. The gas separation factors remain with a minimum change even though there is a gain in gas permeability; therefore, block copolymerization using highly rigid blocks, due to differences in packing, presents the advantage of a higher gas permeability coefficient with a minimum loss in selectivity. The rigidity of these copolymers presents advantages for high temperature applications. (C) 2013 Elsevier B.V. All rights reserved.