Dispersion of nano-carbon filled polyimide composites using self-degradated low molecular poly(amic acid) as impurity-free dispersant

Polyimide (PI)-based composite films incorporated with carbon black (CB), carbon nanotube (CNT) and carbon nanofiber (CNF), respectively, were prepared using low-molecular-weight poly(amic acid) (PAA), a precursor of PI, as an impurity-free dispersant. High-energy ball mill was employed not only to downsize the nano-carbon agglomerates, but also to cut off the PAA chains for in-situ stabilizing the dispersion. Effect of the ball milling time, procedure, and filler species on the filler dispersion was investigated by means of electrical resistivity reproducibility, morphology observation, and mechanical testing. Comparing with direct dispersion of the nano-carbon in PAA, the composite films fabricated by a two-step approach, that is dispersion from the in-situ degradated low-molecular-weight PAA stabilized nano-carbon slurry, presented a uniform electrical conductivity with a lower percolation concentration and excellent reproducibility in the percolation region. A significant improvement in the Young's modulus for the CNT loaded PI film was achieved, which was much more effective than those filled with CB or CNF.