Novel high performance thermosetting polyimides

Abstract

Soluble imide thermosetting (reactive) oligomers of controlled moleular weights were synthesized utilizing 3-phenylethynylmdine, 4-phenylethynylphthalic anhydride or phenylmaleic anhydride as the reactive endcapping agents. The phenylethynyl endcapping agents were synthesized by a ppalium-catalyzed coupling reaction in yields greater than 80% after purification. The polyitmide oligomers were synthesized via solution imidization techniques, using the ester-acid derivatives of various dianhydrides and various aromatic diamines. A "one pot" procedure utilizing NMP as the solvent and o-dichlorobenzene as the azeotroping agent reproducibly exhibit fully imidized soluble polyimides with <Mw>/<Mn> values of ~2.0 determined by gel permeation chromatography (Universal Calibration). Successful molecular weight control was achieved using the reactive endgroups with imide oligomers synthesized with <Mp> values of 2,000 to 15,000 g/mole. The phenylethyny] and phenylmaleic endgroups permitted sufficient flow and low melt viscosities (< 10 PaS) prior to curing of the reactive endgroups at temperatures of 350°C or higher. Matrix composite and structural adhesives were also synthesized by the incorporation of triarylphosphine oxide moieties into the polyimide backbone, which generated char yields as high as 80% in air by TGA analysis.

Thermally cured samples displayed excellent solvent resistance with gel contents of 95% or higher in most cases. Glass transition temperatures comparable to high molecular weight linear analogs were also obtained upon thermal curing of the imide oligomers. Several systems exhibited excellent thermal stability at temperatures up to 700 °F (371°C) aged in air determined by dynamic and isothermal thermal gravimetric analysis. Graphite fiber composite specimens prepared from these reactive imide systems by conventional and small scale "powder prepregging" techniques displayed moduli values equal to or greater than specimens prepared from commercial materials.

Model studies were also conducted utilizing imide compounds analyzed by several spectroscopic techniques. These results indicate that crosslinking (possibly including cyclotrimerization) and chain-extension/branching both occur during the thermal curing of phenylethynyl groups. Phenylmaleic model imide studies indicate that Michael addition mechanisms at relatively high solution imidization temperatures may be a significant factor in limiting successful molecular weight control in phenylmaleic anhydride endcapped imide oligomers. However, two-stage reaction/curing permitted well defined networks to be generated.