Polyimide membranes and their preparation

A method for preparing a polyimide membrane comprising:

• providing a solution containing a solvent and grafted polyimides, the grafted polyimides having molecular weights of 10,000-300,000 and containing monomers A and, optionally, also containing monomers B;
• each of the monomers A, which are the same or different, having the following formula:
• and each of the monomers B, which are the same or different, having the following formula:
• in which
• each of the Ar1 moieties, independently, is
• each of the Z moieties, independently, being C(CF3)2, C(═O), O, S, S(O2), C(CH3)2, CH2, or O(-Ph-)O;
• each of the Ar2 moieties, independently, is
• each of the X, X1, X2, and X3, independently, being H, C1-C5 alkyl, C1-05 alkoxyl, phenyl, and phenyoxyl;
• and each of the Y moieties, independently, being C(CF3)2, C(═O), O, S, S(O2), C(CH3)2, CH2, or O(-Ph-)O;
• each of the Ar3 moieties, independently, is
• each of the R moieties, independently, being C(CF3)2, C(═O), O, S, S(O2), C(CH3)2, CH2, or O(-Ph-)O;
• each of the R1 moieties, independently, being H, C1-C5 alkyl, C1-C5 alkoxyl, phenyl, and phenyoxyl;
• each of the n values, independently, being 0, 1, 2, or 3;
• and
• each of the T moieties, independently, is a thermal labile side chain containing a —O— group linked to the carbonyl group in the Ar3 moiety in a monomer A, and also containing at least one hydroxyl group;
• evaporating the solvent to form a membrane;
• cross-linking Ar3 moieties via T moieties by covalent bonding therebetween to form a cross-linked membrane, the covalent bonding resulting from reaction between the hydroxyl group in a T moiety and the carbonyl group in an Ar3 moiety;
• and
• heating the cross-linked membrane so as to both decompose the T moieties and form covalent bonds between aryl rings in the Ar1, Ar2, and Ar3 moieties,
• wherein, if the grafted polyimides also contain the monomers B, the molar ratio of the monomers B to the monomers A is 0.11 to 99, and a monomer A and a monomer B are linked as follows: