Thermal treatment of dense polyimide membranes

Abstract

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 °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 °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. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1879–1890, 2008

Pure- and mixed-gas permeation properties of highly selective and plasticization resistant hydroxyl-diamine-based 6FDA polyimides for CO2/CH4 separation

“…4 shows the results of the emission spectra for the 6FDA-mPDA, 6FDA-DAP, and 6FDA-DAR membranes. CTCs are typically formed between the nucleophilic fivemember rings and electrophilic six-member rings in polyimides [20,21,43]. The hydroxyl groups are likely to form hydrogen bonds, thereby tightening the polymer interchain structure to strengthen inter-chain CTCs, as evidenced by a previous study, which showed that hydroxyl moieties increased the intersegmental cohesive energy density (CED) in OH-functionalized polyimides [44].…”

“…However, many polyimides have similar limitations in their practical use as cellulose acetate because of CO 2 -induced plasticization, which often results in unacceptable loss in CO 2 /CH 4 selectivity [16][17][18][19]. However, recent studies demonstrated that plasticization in polyimides can be mitigated to a large extend by various methods, such as: (i) Thermal annealing [20][21][22][23], (ii) chemical cross-linking [22,[24][25][26][27][28][29][30] and (iii) formation of charge transfer complexes [31].…”

Effects of Thermal Treatment and Physical Aging on the Gas Transport Properties in Matrimid®

“…Therefore, the physical and chemical properties of glassy polymers are particularly influenced by thermal history [16][17][18][19][20][21][22][23][24][25] and physical aging [26][27][28][29][30][31][32], which can substantially change the material response to the external stresses and, what is of particular importance in the present discussion, can definitely affect its permeation properties.…”

“…Other works outline that different thermal treatments of the polymer matrix can influence significantly the transport rate of small size gases: the higher is the temperature of the pretreatment the lower is the final permeability, independently of the penetrant nature. The explanation proposed for this behavior is the reduction of the free volume available for the penetrant diffusion in the polymer matrix [17,[19][20][21].…”

“…The mass transport properties of Matrimid polyimide have been widely investigated [20,28,29,42,43] showing interesting values both of permeability and selectivity, which make it potentially suitable for CO 2 /CH 4 separation even if its tendency to plasticize in the presence of CO 2 [41] could inhibit its use in high pressure applications.…”