Layer structure formation of mesogenic liquid crystalline epoxy resin during curing reactions: A reactive coarse-grained molecular dynamics study

Liquid crystalline epoxy resins have been reported to show superior mechanical and thermal properties. This high functionality is attributed to their meso- or macroscopic layered domain structure and their molecular scale ordering; hence, elucidation of the mechanism of layer formation during curing reactions is necessary for material development. In this study, the layer formation process has been investigated by the reactive coarse-grained molecular dynamics method. A mesogenic liquid crystalline epoxy and a typical non-mesogenic epoxy (diglycidyl ether of bisphenol A), mixed with a curing agent 4,4'-DDS (diaminodiphenyl sulfone), were considered. The results clearly showed that the mesogenic epoxy molecules formed a layer structure, whereas the non-mesogenic epoxy molecules remained as the amorphous structure even after the curing reactions. This difference was assumed to be caused by the straight structure of the mesogenic epoxy molecules and the strong attractive interactions between mesogenic parts. The interlayer spacing calculated by the simulation was in close agreement with the X-ray measurement. The details of the curing reaction process, e.g. the conversion and the molecular size in the mesogenic epoxy system, were also investigated. A large layer structure covered the simulation system after the stage in which the first reaction of the amine group was dominant, but the molecules were not large: the molecules were mainly attracted by the non-bond interactions. With the progress of the second reaction of the amine group, by which the tertiary amine was formed from the secondary amine, the molecular size became large and a rigid layer structure was formed over the whole simulation system. These results clearly indicated the role of the curing reactions: the first reaction produced small molecules from monomers and made them align in a layer form, and the second reaction connected them by cross-linking bonds and that produced rigid and large domains.