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The primary focus of this work was to evaluate the relationship between free volume distribution, chain motion, crystallinity, and the gas properties of PET upon strain-induced crystallization (SIC) at different stretch ratios. The formation of a three-phase structure containing rigid amorphous phase, mobile amorphous phase, and crystalline phase upon SIC was confirmed by differential scanning calorimetry and positron annihilation lifetime spectroscopy (PALS). Dynamic mechanical analysis and PALS indicated that there was a significant reduction in the fractional free volume upon orientation at an extension ratio of 3 × 3. Sub-T g relaxation studies indicated that the activation energy of mechanical relaxation decreased with increasing the stretch ratio. Gas transport studies revealed that the reduction in permeability coefficient was mainly due to reduction in diffusivity. Permeation studies using gas molecules with different sizes revealed that strain-induced crystallization affects the free volume distribution as well as reducing the average fractional free volume.

SYNOPSISThe effect of modification of the central moiety of the dianhydride residue and isomerism on the gas transport and physical properties were compared for six polyimides containing the hexafluoroisopropylidene group in the diamine residue. Substitution of bulkier groups within the dianhydride residue resulted in disruption of chain packing and slight increases in resistance to chain motions which led to an increase in permeability with little loss in selectivity. The permeabilities and diffusivities in the meta connected polyimide isomers were considerably lower than in the para connected polyimide isomers. Similarly, the permselectivities in the meta connected isomers were consistently higher than in the para connected isomers. These lower permeabilities and higher permselectivities were a result of the more dense packing and a significant suppression of small scale motions in the meta connected isomers. The suppression of segmental mobility in the meta connected isomers was indicated by an increase in the sub Tg transition temperatures in these materials relative to the para connected isomers. The differences in transport properties for these polyimides were attributed to contributions by several factors, including: ( 1) total free volume ( 2 ) distribution of free volume ( 3 ) intersegmental resistance to chain motions, and ( 4 ) intrasegmental resistance to chain motions.

This paper discusses the effect of penetrant-induced conditioning on the gas transport properties of four fluorine- containing polyimide isomers. The effects of substitution of bulky groups within the dianhydride residue of the polyimide and isomerism of the diamine residue on the penetrant-induced hysteresis of CO2 permeability were investigated. The CO2 permeability in the polyimide films that had been conditioned with carbon dioxide at 60 atm are compared to those in the untreated films. Exposure of the polyimides to CO2 at 60 atm resulted in 4.5- to 10-fold increases in the permeability of CO2 relative to unconditioned films. The permeability enhancement was a result of increases in both the solubility and diffusivity following conditioning. There was also a surprising maximum in the permeability at intermediate pressures in conditioned samples of two of the polyimides. The permeability of CO2 in the conditioned polymers initially increased with decreasing pressure, exhibited a maximum value at approximately 30 atm, and decreased in permeability at low pressures.

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