Morphological investigation of AFR-PEPA-N imide oligomers and their cured polyimides and the remodification of AFR-PEPA-N to achieve liquid-crystalline behavior

The morphological investigation of AFR-PEPA-N and the development of a new polyimide have been established herein. AFR-PEPA-N is an imide oligomer that was created out of the need to attain a high temperature polyimide that is also resistant to hygrothermal and thermooxidative degradation. Previously, AFR700B was implemented in aerospace applications, but it was found to be hygrothermally unstable. It experienced a severe drop in its glass transition temperature and composite blistering. AFR700B was improved upon, by altering the chemical structure of the polyimide. The nadic end-cap was removed and replaced by a more hydrolytically stable end-cap. However this phenylethynyl-terminated end-group could possibly create semi-crystallinity or liquid-crystalline characteristics within the polymer.

Previous research suggests further study of the relationships between AFR-PEPA-N's oligomer crystallinity and the properties of phenylethynyl-terminated polyimides. This understanding is valuable in processing AFR-PEPA-N by resin transfer molding (RTM) to obtain its optimum properties. The investigation included the identification of a processing window, temperature overlap between the melting of residual crystals and crosslinking reactions, and liquid crystallinity behavior. These reactions were investigated primarily through birefringence.

The residual crystals were found to be innate in the oligomer powder and not created by preliminary thermal processing. Therefore a reasonable processing window was found based upon the reduction of crystal size by appropriate dissolution techniques. Possible nematic liquid-crystalline characteristics were found to be present at 360oC.

A new imide oligomer, which was based upon AFR-PEPA-N's original structure, was synthesized. The non-linear, flourinated backbone of AFR-PEPA-N was replaced with a co-linear backbone, pyromellitic dianhydride (PMDA). These modifications were made in hopes to improve upon the network structure by it becoming more regular and resistance to nano-sized defects in the final crosslinked structure. The initial characterization found that the new polyimide, AFR-P3, displayed a cure temperature at 350oC. The degree of cure reaches about 80 to 90 percent complete based upon the consumption of the carbon-triple bond. AFR-P3 did not show signs of liquid-crystalline behavior. However, there will be future work in creating a more rigid-rod, self-assembling oligomer that can attain optimum thermal and mechanical properties.