The effect of thermal hysteresis on the polymer chain packing and permeation properties of two 6FDA-based polyimide isomers was investigated. Thermal quenching resulted in a small increase in the fractional free volume of the polyimides with respect to the samples that had been annealed. Quenching from above the glass-transition temperature also resulted in larger increases in the permeabilities for both 6FDA–6FmDA and 6FDA–6FpDA with respect to annealed samples. Meta-connected 6FDA–6FmDA exhibited a larger increase in the permeability after quenching than the para-connected isomer, 6FDA–6FpDA. This larger increase in the permeability for 6FDA–6FmDA may have been due to differences in the effects of the increases in the free volume on the intersegmental resistance to chain motions. Although physical aging over a 3-month period resulted in a reduction in the permeability of quenched samples of 6FDA–6FpDA, the quenched samples maintained higher permeabilities than the annealed samples. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1174–1182, 2004
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