Azo initiator selection to control the curing order in dimethacrylate/epoxy interpenetrating polymer networks

Differential Scanning calorimetry (DSC) and Fourier-transform infrared (FT-IR) spectroscopic studies have been undertaken of the cure of interpenetrating polymer networks (IPNs) formed with imidazole-cured diglycidyl ether bisphenol-A (DGEBA) and with either diethoxylated bisphenol-A dimethacrylate (DEBPADM) or bisphenol-A diglycidyl dimethacrylate (bisGMA), polymerized by a range of azo initiators (AIBN64, VAZ088, VR110 and AZO168). Due to the differing decomposition rates of the azo initiators, the neat dimethacrylate resin either cured faster than (with AIBN64 and VAZ088), or similar to (VR110), or slower than (AZO168), the neat epoxy resin. In the neat DGEBA/1-methyl imidazole (1-MeI), DEBPADM/AIBN64, DEBPADM/VAZO88 and DEBPADM/VR110 resins, close to fun cure was achieved. For the neat, high-temperature DEBPADM/AZO168 resin, full cure was not attained, possibly due to the compromise between using a high enough temperature for azo decomposition while avoiding depolymerization or decomposition of the methacrylate polymer. IPN cure studies showed that, by appropriate initiator selection, it was possible to interchange the order of cure of the components within the IPN so that either the dimethacrylate or epoxy cured first. In the isothermal cure of the 50:50 DEBPADMIAIBN64:DGEBA/1-Mel IPN system, the cure rate of both species was less than in the parent resins, due to a dilution effect. For this system, the dimethacrylate cured first and to high conversion, due to plasticization by the unreacted epoxy, but the subsequent cure of the more slowly polymerizing epoxy component was restricted by the high crosslink density developed in the IPN. After post-curing, however, high conversion of both reactive groups was observed and the fully cured IPN exhibited a single high-temperature T-g, close to the T-g values of the parent resins. In the higher-temperature, isothermal cure of the 50:50 DEBPADMIVR110:DGEBA/1-Me1 IPN system, the reactive groups cured at a similar rate and so the final conversions of both groups were restricted, while in the 50:50 DEBPADM/AZO168:DGEBA/1-Mel system it was the epoxy which cured first. Both of these higher-temperature azo-initiated IPN systems exhibited single T(g)s, indicating a single-phase structure; however, the T(g)s are significantly lower than expected, due to plasticization by residual methacrylate monomer and/or degradation products resulting from the high cure temperature. (C) 2004 Society of Chemical Industry.