Probing the Effect of Prepolymerization on the Structure and Properties of Phthalonitrile Resin/Basalt Fiber Composites

Prepolymerization is a common procedure in the practical application of high-performance composites based on thermosetting resins, but very few reports studied the effects of prepolymerization on the curing reactivity, the chemical structure, and the properties of the final composites. Therefore, in this work, a phthalonitrile resin with a benzoxazine moiety was selected for prepolymerization and for preparing composites with basalt fiber cloth. It was revealed that the molecular weight increased slightly after prepolymerization, consuming a small number of functional groups and releasing groups with catalytic activity. Consequently, the curing reactivity of the prepolymers increased and then decreased; the toughness and strength increased, but the thermal properties increased and then decreased after prepolymerization. All of the variations resulted from the combined function of generated active groups and decreased molecular mobility; that is, more active groups such as phenolic hydroxyls were released after prepolymerization, but the molecular weight was increased, and functional groups were consumed, which led to difficulty in the motion of the resin melt and the crosslinking reaction.