Epoxy resins exhibit excellent mechanical and heat resistance properties making them viable for numerous industrial applications. However, the brittle fracture caused by restriction of the molecular motions as a result of the crosslinks between the molecular chains and constraining of the energy dissipation in the cured products is a big threat to their development. Thus, there is need to increase the toughness of the epoxy thermosets. Presently, many epoxy / block copolymer blend compositions have been researched to obtain different micelle structures including curved lamella, spheres, and worm-like micelles. Despite the formation of several types of nanostructures of epoxy / block copolymer blends using different chemical compositions, little have been reported about the effects of epoxy curing catalysts on the formation of the nanostructures in the epoxy / block copolymer blends.
To this note, Professor Hajime Kishi, Kazuyoshi Yamada and Jin Kimura from the University of Hyogo investigated the effects of curing catalysts on the nanostructures and mechanical properties of epoxy/acrylic block copolymer blends. In particular, they purposed to determine the role of curing catalyst less than 1wt% in manipulating the reaction type in the epoxy matrix as well as controlling the nanostructures and mechanical properties of the resulting blends. The work is published in the journal, Polymer.
Phenol novolac with curing catalysts less than 1wt% was used in curing the blends. Eventually, a comparison was made between three different catalysts: triphenylphosphine (TPP), 2,4,6-tris (dimethyl amino methyl) phenol (DMP), and 1,2-dimethylimidazole (DMIz). The authors observed that even at 1wt% of curing catalyst, significant effects on the reaction types and linkage structures of the phenol novolac-cured epoxy matrices as well as the nanostructures of the epoxy/acrylic block copolymer blends was noted. When TPP was used, a polyol-type linkage structure was formed at the cured epoxy matrix. On the other hand, both polyol-type linkage and polyether-type linkage structures were formed when DMP and DMIz were used. Therefore, different nanostructures were formed in response to different linkage structures in the epoxy matrix obtained from the same blend composition.
It was worth noting that the compatibility of the cured epoxy matrix with polymethyl methacrylate-blockchains of the acrylic block copolymer was influenced by the reaction types of the epoxy matrices. A decrease in the compatibility of the polymethyl methacrylate chains led to a corresponding decrease in the interfacial curvature between the epoxy matrices. This factor provided an efficient tool for controlling mechanical properties and nanostructures.
Specifically, the fracture toughness and modulus of elasticity of the epoxy/acrylic block copolymer blends could be altered by selecting the preferable catalyst type without changing the chemical structures and composition of the block copolymers. For instance, the TPP catalyst produced spherical nanostructures while DMP and DMIz produced branched cylindrical nanostructures. Based on the obtained results, Professor Kishi the lead author in a statement to Advances in Engineering highlighted that manipulation of epoxy matrix reaction type is a promising approach for the control of nanostructure and toughness of epoxy/acrylic block copolymer blends. This will thus advance their use in various applications.
Kishi, H., Yamada, K., & Kimura, J. (2019). Control of nanostructures and fracture toughness of epoxy/acrylic block copolymer blends using in situ manipulation of the epoxy matrix reaction type. Polymer, 176, 89-100.