Methylnadimide end-capped polyimides for use in high temperature composites - a new approach to their synthesis

Two methylnadimide end-capped polyimides have been prepared by a new water synthesis procedure and their properties compared to similar polyimides syn thesized 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 presen t results showed that with appropriate adjustment of the synthesis conditio ns very useful methylnadimide end-capped polyimides could be obtained. It w as found that the amount of methylnadimide end-cap was reduced under 180 de grees C synthesis conditions, and even at lower temperatures with prolonged heating, and that the water reaction is best carried out at 165 degrees C for these polyimides. One of the uncured resins, Polyimide 1 was quite solu ble in common solvents allowing complete characterization. It was found tha t 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 poly mer of wide molecular weight distribution. The potential usefulness of thes e polymers as high-temperature composite matrices was indicated by the exce llent 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 a nd a strain at break of 9.7%.