Toughening of alicyclic epoxy resin by in situ polymerization of modifier vinyl monomers using alkylborane as a radical initiator

Abstract In this study, we attempted to improve the toughness of alicyclic epoxy cured resin using a vinyl polymer as a modifier which was generated with alkylborane as a radical initiator in parallel with the curing of the resin. Alkylborane has the ability to initiate radical polymerization around room temperature in the presence of oxygen, and the use of diethylmethoxyborane (DEMB) instead of a conventional peroxide radical initiator (DBPC) was found to allow copolymerization of styrene (St) and N ‐phenylmaleimide (NPMI) in an alicyclic epoxy resin in the early stage of the multistep curing. As a result, only when DEMB was used as an initiator, a cured product with a phase‐separated structure at which ca. 50–100 nm St/NPMI copolymer phases were dispersed was obtained. The fracture toughness ( K IC ) of the modified cured resin with DEMB was 0.77 MN/m 3/2 , which was 30% higher than that of the modified resin with DBPC. Furthermore, the glass transition temperature of the modified cured resin with DEMB reached 247°C, which was 87 and 63°C higher than the unmodified rein and the modified resin with DBPC, respectively, since DEMB acted as a Lewis acid to promote cationic polymerization of the alicyclic epoxy resin.