Thermally Rearranged Polymer Membranes

As shown in Fig. 14, PIM-1 and PIM-7 have been found to exhibit substantially higher O /Nj selectivities (a(O2/N2)>3.0) than other polymers of similar permeability [41]. Other thermally rearranged [78] polyimides show excellent CO /CH separation selectivities. These materials were also shown to function as fuel cell membranes when doped with H3PO4 and proton conductivities of 0.15 S cm" were observed at 130°C [78] that is, higher than polybenzimidazole membranes. 

Figure 15.1 Comparing the CO2/CH4 Robeson upper bound for dense and thermally rearranged (TR) polymer membranes to the carbon membranes/ and the region for industrial applicability was suggested by Hillock et alf (Data for CMS membranes and industrial applicability region added to the original Robeson plot.)...

The thermal rearrangement of a-hydroxyl-PI membranes improves the gas permselectivity properties in comparison to a neat PI. By intro-dueing segments within the polymer that do not undergo thermal rearrangement, the gas separation properties of the thermally rearranged membrane can be modified. PI copolymers based on 4,4 -hexafluoroisopropylidene diphthalic anhydride and diamines 3,3 -dihydroxy-4,4 -diamino-biphenyI with 2,3,5,6-tetramethyI-l,4-phenyIenediamine or 9,9 -bis(4-aminophenyl)fluorene, thermally rearranged into poly(benzoxazole-co-imide), were tested [107]. 

In the above work, they followed the same thermal treatment protocols for all cases. However, the thermal treatment protocols also played an important role in the final microporous structure formation and size distribution of TR-PBO. The incorporation of the flexible efher linkages also affected the thermal rearrangement procedure of the hydroxyl-containing polyimides and transport properties of the resultant TR polymer membranes (Figure 5.50) [68]. 

Poly(substituted acetylene)s such as PTMSP and PMP, amorphous fluoro-polymers like Teflon AF and Hyflon AD, polymers with intrinsic microporosity, and thermally rearranged (TR) polymers are the candidate polymers for highly permeable glassy polymer membranes. The high free volume in glassy polymers contributes to enhanced diffusion and permeation of small gas molecules. The gas permeation performances of these highly permeable polymers even surpass upper bounds for CO2/N2, CO2/CH4 and H2/CO2 separations. 

In one recent report, aromatic polyimide polymer membranes cross-linked at elevated temperatures (up to 450 °C) showed resistance to plasticization to at least up to 3000 kPa (450 psia) CO2. Achieving uniform high temperature cross-linking of polymers in commercial membrane manufacturing has its own challenges. Solution-based chemistry to cross- link membrane may be more commercially amenable than the high temperature thermal rearrangements. 

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