Miscibility behavior and reequilibration of binary poly(amic acid) mixtures

Abstract

Five poly (amic acid) solutions based on PMDA‐PDA, PMDA‐ODA, PMDA‐6F, ODPA‐ODA, and 6FDA‐ODA were prepared in N‐methylpyrrolidone at a polymer concentration of ca. 10 wt %. From these five poly (amic acid) solutions, six different binary blends were prepared: PMDA‐PDA/PMDA‐ODA, PMDA‐PDA/PMDA‐6F, PMDA‐ODA/6FDA‐ODA, PMDA‐ODA/ODPA‐ODA, PMDA‐PDA/ODPA‐ODA, and PMDA‐PDA/6FDA‐ODA. These blends were then characterized with respect to miscibility in the ternary state (polyamic acid‐1/polyamic acid‐2/N‐methylpyrrolidone), the condensed state (ca. 70 wt % polymer), and the fully imidized state. All blends except for PMDA‐PDA/PMDA‐6F yielded homogeneous mixtures in the ternary solution of 10 wt % polymer concentration. The PMDA‐PDA/PMDA‐6F mixture eventually became homogeneous after 10 days of mixing at room temperature. Upon solvent evaporation (condensed state) and full cure (imidized state) two blends incorporating ODPA‐ODA as one of the components exhibited apparent miscibility as evidenced by optical microscopy. The remaining blends exhibited large‐scale phase separation upon solvent evaporation with no significant differences in the overall morphology between the condensed and imidized state. However, as in the case of the PMDA‐PDA/PMDA‐6F ternary system, the morphology in the condensed and imidized state was strongly dependent on the mixing time of the precursor poly(amic acid) components; the phase‐separated domain size decreased with increasing mixing time, eventually leading to complete miscibility. These results are discussed with respect to the proposed “polymer‐monomer” reequilibration reaction as well as its implications with respect to the preparation of polyimide molecular composites. Copyright © 1991 John Wiley & Sons, Inc.