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 Bu₃N, Hex₃N, Oct₃N, and pyridine, were insoluble in H₂O, only the salt with Et₃N (PAS(Et₃N)) 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(Et₃N) 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(Et₃N) in H₂O showed larger activation energy of viscosity than that in NMP. However, PAA and PAS(Et₃N) 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.