Near-infrared and rheological investigations of epoxy-vinyl ester interpenetrating polymer networks

Rheological and near-infrared (NIR) spectroscopic investigations have been undertaken for two different interpenetrating polymer networks (IPNs) based on a model vinyl ester (VER, composed of bisphenol A diglycidyl ether dimethacrylate, bisGMA, in 30 wt % styrene) cured with azobis(isobutyronitrile) (AIBN) and an epoxy monomer (diglycidyl ether of bisphenol A, DGEBA) cured either with 1-methylimidazole (1-MeI) or with a stoichiometric quantity of 4,4'-diaminodiphenylmethane (DDM). NIR studies showed that the vinyl ester resin component cured more quickly than the epoxy component. The rate of vinyl conversion within the 50:50 wt % VER/AIBN:DGEBA/DDM IPN system was slower than in the 50:50 wt % VER/AIBN:DGEBA/1-MeI IPN and was much slower than in the neat VER/AIBN system due to the dilution of the VER components by the epoxy system and retardation of the radical polymerization by the amines, as previously observed in DSC and mid-FTIR studies of similar systems. The rate of epoxy conversion in the 50:50 wt % VER/AIBN:DGEBA/1-MeI IPN was slower than in the neat 1-MeI cured epoxy, indicating that the presence of the vinyl ester component also had a dilutional effect within the IPN reducing the concentration of epoxy and imidazole species and hence slowing the reaction. In contrast, the rate of epoxy conversion in the 50:50 IPN of VER/AIBN:DGEBA/DDM was faster than in the neat DDM-cured epoxy, possibly due to the catalysis of the epoxy-amine reaction by the hydroxy groups in the bisGMA. The similarity between the rheology and gel times for the VER/AIBN system and the IPNs indicated that the gelation of the VER component determined the overall gel behavior of the IPN. The gel times correlated well with the vinyl NIR conversion data in that the gelation occurred first for the neat VER/AIBN resin and last for the VER/AIBN:DGEBA/DDM IPN. These observations are also consistent with the effects of dilution of the VER reactants by the epoxy component in the IPN and were confirmed by studies of VER/AIBN diluted with xylene. All systems appeared to vitrify after a period of time as the cure temperature was well below the maximally attainable glass transition temperature of all resins systems. The vinyl group conversion at the vitrification point was marginally higher in the IPNs than in the neat resin systems, but the epoxy conversions in the IPNs were significantly lower at vitrification. The neat resins approached vitrification faster than the IPNs which showed a much slower rise in modulus to the glassy plateau.