Thermal treatment of dense polyimide membranes

A study has been conducted to clarify the relationship between polymer structure, annealing temperature, and the extent of plasticization by high-pressure CO2 for two typical polyimide membranes; BTDA-DAPI (poly(3,3'-4,4'-benzophenone tetracarboxylic-dianhydride diaminophenylindane) and 6FDA-TMPDA (poly(2,2'bis-(3,4'-dicasrboxyphenyl) hexafluoropropane dianhydride-2,3,5,6-tetramethyl-1,4-phenylenediamine). Both membrane materials are exposed to varying levels of thermal annealing at 200 and 250 degrees C. The effect of this heat treatment on free volume is examined using positron annihilation lifetime spectroscopy (PALS), whereas fluorescence spectroscopy is used to monitor changes in electronic structure. Results show that thermal annealing causes a reduction in both the size and number of free volume elements. A strong relationship is found between the fluorescence peak intensity for 6FDA-TMPDA and both the membrane gas permeability and plasticization pressure. This correlation is most likely the result of the formation of charge transfer complexes, particularly at 250 degrees C. However, the formation of covalent crosslinks at these temperatures cannot be discounted. No fluorescence is observed for BTDI-DAPI. Although thermal annealing has a significant effect on the extent of plasticization in both polymers, it is found that the rate of plasticization is unaffected by the annealing temperature. (C) 2008 Wiley Periodicals, Inc.