Abstract
Multi-block copolymers composed of poly (N-phenylmaleimide-alt-styrene) and flexible segments such as polydimethylsiloxane or polyoxyethylene were prepared by copolymerization of N-phenylmaleimide and styrene with macromolecular azo-initiators containing polydimethylsiloxane or polyoxyethylene segments, and used for the modification of high performance epoxy resin, which is a mixture of N,N, N', N'-tetraglycidyldiaminodiphenylmethane, dicyclopentadienyl-type epoxy resin, naphthalene-type epoxy resin and bisphenol-A diglycidyl ether, cured with 4, 4' -diaminodiphenyl sulfone. The fracture toughness (KIC) and flexural strength of the modified resin were strongly dependent on both the molecular weight (Mw) and structure (composition) of the modifier, while the modulus and glass transition temperature of the modified resin were maintained in this modification system. The addition of 5 phr of poly [poly (N-phenylmaleimide-alt-styrene) -block-polydimethylsiloxane] (PMSZ) having low Mw (157,000) and high dimethylsiloxane (VPS) content (40 mol%), or high Mw (664,000) and low VPS content (11 mol%) brought about increases (ca. 70%) in KIC of the modified resin without large loss of flexural strength, compared with the control resin. The addition of 12-15 phr of poly [poly (N-phenylmaleimide-alt-styrene) -block-polyoxyethylene] (PMSE : Mw, 300,000, oxyethylene content 35mol%) resulted in the largely increased KIC (210%) with rather large loss of flexural strength. PMSE was more effective for toughening of the epoxy matrix resin compared with corresponding PMSZ (Mw 371,000, VPS content 30 mol%) or poly (N-phenylmaleimide-alt-styrene) (PMS : Mw 397,000) without flexible segments. It was concluded that this is due to the high compatibility of polyoxyethylene segments in PMSE with the matrix resin. These modified resins with high KIC had the co-continuous-phase morphology. The toughening mechanism was discussed in terms of the morphological and dynamic viscoelastic behaviors of the modified epoxy resin system.
