Methylnadimide End-Capped Polyimides for Use in High Temperature Composites—A New Approach to Their Synthesis

Abstract

Two methylnadimide end-capped polyimides have been prepared by a new water synthesis procedure and their properties compared to similar polyimides synthesized in the conventional solvents, cresol and NMP. The water method has been shown to be a practical alternative for the synthesis of a wide range of polyimides with no evidence of contaminating polyamic acids. The present results showed that with appropriate adjustment of the synthesis conditions very useful methylnadimide end-capped polyimides could be obtained. It was found that the amount of methylnadimide end-cap was reduced under 180°C synthesis conditions, and even at lower temperatures with prolonged heating, and that the water reaction is best carried out at 165°C for these polyimides. One of the uncured resins, Polyimide 1 was quite soluble in common solvents allowing complete characterization. It was found that the amount of endcap introduced by either conventional or water synthesis procedures was less than the theoretical maximum. Examination of the cured neat resin from polyimide 1 showed it to consist of a portion of insoluble, possibly cross-linked, material and a considerable amount of soluble polymer of wide molecular weight distribution. The potential usefulness of these polymers as high-temperature composite matrices was indicated by the excellent tensile properties of polyimide 2B, which on curing produced a tough voidless polymer with a tensile strength of 120 MPa, a modulus of 3.9 GPa and a strain at break of 9.7%.