Hydroxyl terminated PEEK-toughened epoxy–amino novolac phthalonitrile blends – Synthesis, cure studies and adhesive properties

Self cure promoting, amine-containing novolac–phthalonitrile (APN) resins of varying compositions were synthesized and characterized. APN possessing amine functionalities reduced the cure initiation temperature from 310 °C (typical of pure phthalonitrile systems) to 180 °C. It showed excellent thermal stability up to 420 °C and high char residue of 77–79 %. Co-reaction of APN with diglicydyl ether of bisphenol A (DGEBA) led to a decrement in their thermal stability though improved their adhesive properties. Evidences were obtained for epoxy–amine, epoxy–phthalonitrile and amine–phthalonitrile reactions. The latter reactions led to formation of oxazoline, triazine and phthalocyanine groups in the network. These were rationalized by density functional theory studies on model compounds. The extents of epoxy–amine and epoxy–phthaonitrile reactions were quantified. Introduction of hydroxyl terminated poly ether ether ketone (PEEK) reduced the brittleness of the blends and improved their lap shear strength. Toughening of epoxy–amino novolac phthalonitrile networks occurred through phase separation of PEEK segments in cured matrix.