Bulk viscosity and its unstable behavior upon storage in polyimide precursor solutions

Abstract

The bulk viscosity behavior of poly(amic acid) and its amine salt solutions, PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine), has been investigated. Both PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) solutions show strong concentration and molecular weight dependence on bulk viscosity, and display critical values on the concentration dependence of bulk viscosity because of increasing the molecular chain interactions and entanglements. PAA(ODPA/o-tolidine) possesses a higher bulk viscosity than PAS(ODPA/o-tolidine) at the same concentration, indicating a stronger resistance to shear flow. The temperature dependence of bulk viscosity follows the exponential Arrhenius type relation. The activation energies of the fluids depend on the nature, concentration, and molecular weight of the polymers used. It has been found that both PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) solutions inherently have poor bulk viscosity stability upon storage. The bulk viscosity of the polymer solutions decreases dramatically with time upon storage at room temperature while increasing during the storage at -18 °C. A molecular mechanism in terms of a "temporary junction" is suggested to explain the bulk viscosity behavior of PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) in concentrated solutions.