Thermally rearranged 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. 

Choi Jl, Jung CH, Han SH, Park HB, Lee YM. Thermally rearranged (TR) poly(benzoxazole-co-pyrrolone) membranes tuned for high gas permeability and selectivity. J Membr Sci 2010 349(l-2) 358-368. 

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]. 

Scholes CA, Ribeiro CP, Kentish SE, Freeman BD. Thermal rearranged poly(benzoxazole-co-imide) membranes for C02 separation. J Membr Sci 2014 450 72-80. 

H. Wang, T.-S. Chung, D.R. Paul, Physical aging and plasticization of thick and thin films of the thermally rearranged ortho-functional polyimide 6FDA-HAB, Journal of Membrane Science 458 (2014) 27-35. 

Lee et al. first developed a new synthesis route to prepare thermally rearranged microporous [40] PBI (TR-PBI) membranes by alkaline treatment of... 

The above thermal rearrangement reaction was also evident from the weight change of the precursor membranes in the region of 300-450 °C during... 

PBO membranes prepared by the thermal rearrangement of the hydroxyl-containing polyimides or polyamides at elevated temperature were used for membrane-based gas separation. Lee s group has done considerable work on the thermally rearranged (TR) PBO membrane (TR-PBO) (Figure 5.38) for gas separation [67,80]. They had studied the effect of imidization methods on the properties of TR-PBO membranes. The final properties of the TR-PBO membranes depended on the synthetic methods to prepare polyimide precursors. There are three different routes for the synthesis of the polyimides (1) thermal, (2) chemical, and (3) solution thermal imidization using an azeotrope. They demonstrated the effect of these routes on the final properties of the PBOs. The precursor ort/jo-functional polyimides were synthesized from 4,4 -hexafiuoroisopropyli-dene diphthalic anhydrides and 2,2 -bis(3-amino-4-hydroxyphenyl)hexafiuoropropane. 

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]. 

Han et al. investigated the gas separation behavior of the PBOs (Figure 5.57) prepared from thermal rearrangement of the fluorinated o-HPAs [80]. The thermal rearrangement occurred at a comparatively low temperature (350 °C) than the precursor poly-imides. The cavity sizes and distribution of FFV elements were tuned to obtain a higher combination of permeability (Phj = 206 Barrer) and selectivity by changing the precursor HPA structure and thermal treatment. The reduction of CO2 solubility for PBO in comparison to the precursor HPAs improved the H2/CO2 selectivity (a = 6.2 at 210 °C, in which Ph2 > 200 Barrer) and moved the membrane performance to polymeric upper bound (Robeson upper bound). 

M. Calle, C.M. Doherty, A.J. Hill, Y.M. Lee, Cross-linked thermally rearranged poly (benzoxazole-co-imide) membranes for gas separation. Macromolecules 46 (20) (2013) 8179-8189. 

R. Swaidan, X. Ma, E. Litwiller, I. Pinnau, High pressure pure and mixed-gas separation of CO2/CH4 by thermally-rearranged and carbon molecular sieve membranes derived from a polyimide of intrinsic microporosity, J. Membr. Sci. 447 (2013) 387-394. 

B.C. Gandara, M. Cafle, H.J. Jo, A. Hernandez, J.G. Campa, J. Abajo, A.E. Lozano, Y.M. Lee, Thermally rearranged polybenzoxazoles membranes with biphenyl moieties monomer isomeric effect, J. Membr. Sci. 450 (2014) 369-379. 

Y.K. Ong, H. Wang, T.S. Chung, A prospective study on the appUcation of thermally rearranged acetate-containing polyimide membranes in dehydration of biofuels viaper-vaporation, Chem. Eng. Sci. 79 (2012) 41-53. 

Copolymerization is a method to control the gas transport performances of copolymer membranes as well as to confirm the effect of thermal rearrangement. Poly(benzoxazole-co-imide) membranes were obtained from the thermal rearrangement of poly(hydroxyl imide-c -imide)." Size and distribution of free volume cavities created during thermal conversion could be easily controlled by varying HPI composition in the copolymer. CO2 permeability of copolymer TR... 

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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