Phenylphosphine Oxide Containing Polyimide Matrix Resins for Atomic Oxygen-resistant Fiber-reinforced Composites

Phenylethynyl-endcapped imide oligomers with different calculated molecular weights (calcd number average molecular weight (Mn)) and phosphorous (P) contents (P loading) were synthesized by the reaction of diethyl ester of 4,40-oxydiphthalic acid, monoethyl ester of 4-phenylethynyl phthalic acid, and the P-containing diamines including [2,5-bis(4-aminophenoxy)phenyl] diphenylphosphine oxide and di(3-aminophenyl)methylphosphine oxide via in situ polymerization of monomeric reactant method. The amide–ester solutions were impregnated with carbon fiber (Cf) and quartz fiber (Qf) cloth to give high quality prepregs that were then thermally cured to yield the Cf-reinforced polyimide (PI) composite (Cf/PI) and Qf-reinforced PI composite (Qf/PI), respectively. The Cf/PI composites showed high mechanical properties with a flexural strength of 1554 + 93 MPa, a flexural modulus of 112 + 3 GPa and an interlayer shear strength of 90 + 5 MPa, respectively. The atomic oxygen (AO) resistances of the neat resins and composites were evaluated on a ground-based simulation facility. Experimental results indicated that the P structures offer PIs with 55–75% lower weight losses and a self-healing property. The fiber-reinforced composites showed further lower erosion rates than neat resins. In addition, the effects of calcd Mn and P loadings on the melt processabilities of the B-staged imide oligomers and the AO resistance of the cured PIs were systematically investigated.