Solution properties of polyamic acids and their amine salts

Solution properties such as viscosity, critical concentration, radius of gyration, and activation energy in aqueous solutions are described for [BPDA/PDA]polyamic acid (PAA) and their salts with various amines (PAS). Although PAA and their salts with Bu(3)N, Hex(3)N, Oct(3)N, and pyridine, were insoluble in H2O, only the salt with Et(3)N (PAS(Et(3)N)) was soluble in H2O. The different solubility of PAS suggests that higher base strength as well as shorter alkyl length of amines enhance water solubility of PAS. PAS(Et(3)N) showed critical concentrations at 18, 14, and 7.8 wt% in NMP, NMP/H2O (1/1), and H2O respectively. The radius of gyration, which is calculated from the critical concentration, in H2O is 1.3 times larger than that in NMP; that is, the polymer chain expands in H2O rather than in NMP. PAS(Et(3)N) in H2O showed larger activation energy of viscosity than that in NMP However, PAA and PAS(Et(3)N) showed similar activation energies in the NMP solution. Therefore, it is concluded that the amine salts of the polyamic acids are hydrated by several water molecules in H2O, resulting in the larger radius of gyration and lager activation energies.