Prevention of void formation in particulate-filled polymer composites: Effects of thermoplastic matrices and residual solvent

One of the key concerns in fabrication of particulate-filled polymer composites using polyimide (PI) as a matrix is formation of voids due to entrapment of water during thermal imidization of poly(amic acid) (PAA) precursor. Using hexagonal boron nitride (hBN) flakes as the filler material, this study investigated the effects of thermoplastic PI and fabrication conditions on morphological structures of hBN/PI composite films, in particular void fraction ϕv. A decrease in ϕv was observed in thermoplastic PI composites in comparison with its thermosetting counterpart. The morphological factors analyzed by SEM and WAXD showed a good correlation with the out-of-plane thermal diffusivity. A substantial decrease in ϕv was further achieved by using a solvent with higher boiling point and by shortening the drying time to increase the residual solvent in the hBN/PAA precursor films, from which the remaining solvent was eliminated during the final imidization step. Under optimized conditions, ϕv < 3% at 60 vol% hBN content was achieved through lowered glass transition temperatures of the precursor and the plasticizing effect of the solvent.