Positive effect of functional side groups on the structure and properties of benzoxazine networks and nanocomposites

We have used atomistic simulations to study the network topology and the thermomechanical properties of the methyl-4,4-bis-[6-(3-phenyl-3,4-dihydro-2H-1,3-benzoxazine)]pentanoate based system (MDP-a), in comparison with commercially available bisphenol-A based benzoxazine (Ba-a). In the first part of the work, we focus on the effect of the transesterification reaction of the ester group of the MDP-a molecule on the overall properties of the crosslinked resin. Remarkably, this additional crosslink site shifts the T-g of the MDP-a thermoset by 116 degrees C as compared to the same resin generated with a conventional tetra-functional crosslinking scheme. The rapid increase of the thermomechanical stability is shown to be mostly related to the volume shrinkage of the thermoset upon curing. In the second part of this study, we report on the effect of carbon nanotubes (CNTs) on the properties of both BA-a and MDP-a polybenzoxazines, in particular on the glass transition temperature. We find that the addition of CNTs has a negligible impact on the T-g of the BA-a resin, whereas it induces a surge of 45 degrees C in T-g for the MDP-a nanocomposite. A detailed analysis suggests that the change of the thermomechanical stability of the CNT-nanocomposites is a result of the balance between different structural variations (free volume, crosslink and hydrogen bond densities, chain mobility) and the polymer/CNT interfacial energy.