Early-Stage Phase Separation in Polyimide Precursor Blends: An Atomic Force Microscopy Study

The surface of polyimide precursor blends is studied using atomic force microscopy (AFM). The acid (PAA) and ester (PAETE) precursors in the blends are of the same polyimide. The topographical contrast is due to the complexation of solvent with the acid precursor, causing the latter to swell. Early-stage phase separation prior to impingement is studied for a blend with 20% weight fraction of PAA relative to PAETE. The growth rate of the domains is by nucleation and growth. The PAA-rich discontinuous phase grows at a rate of ∼t¹ rather than the t^1.5, which is usually obtained for surface-induced phase separation. The study also demonstrates the effect of moisture on the acid precursor. It is further shown that due to ∼ 15-fold faster kinetics of imidization of PAA relative to PAETE, the phase-separated topography is maintained even after both precursors convert to the same polyimide.