Toughening of Epoxy Resin by Methyl Methacrylate/2-Ethylhexyl Acrylate Copolymers: The Effect of Copolymer Composition Soraia Zaioncz, Adriana A. Silva, Alex S. Sirqueira, Bluma G. Soares * Introduction Cured epoxy resins constitute important thermosetting polymers, which exhibit good engineering properties such as high stiffness, good adhesion strength, low cure shrinkage, low creep, and excellent corrosion resistance. However, they are highly brittle and notch sensitive because of the high network density. It is well known that a small amount of low molar mass liquid reactive rubbers can greatly improve the fracture toughness of epoxy networks. [1] The use of low molar mass polymers is very important to avoid excessive increase in viscosity, so that the processability of the system is not impaired. [2] Initially the rubber must be able to dissolve in the epoxy/hardener system. As the curing reaction proceeds, the molar mass increases and phase separation occurs at some stage, leading to the formation of a two-phase morphology. [3] The balance between phase separation and polymeriza- tion (crosslinking) depends upon the solubility of the rubber in the epoxy resin, the amount of rubber, and the cure conditions. In addition, the chemical bonding between the rubber particles and the epoxy matrix is necessary to improve the interfacial adhesion and to achieve efficient stress transfer. All these parameters influence the particle size, particle size distribution, and the degree of interactions between the phases. As a consequence, an improvement in the impact resistance is usually achieved. Liquid rubbers containing reactive end groups such as carboxyl- (CTBN) [4–6] or amine-terminated butadiene acrylonitrile copolymers (ATBN) [7] are frequently used to improve the fracture properties of epoxy networks. These systems undergo phase separation on curing, leading to outstanding impact strength as the rubber particles, dispersed and bonded to the epoxy matrix, act as center for dissipation of mechanical energy by cavitation and shear Full Paper MMA-EHA copolymers with different compositions and with a low amount of AA were synthesized and used as impact modifier for epoxy networks. The effect of the copolymers on the tensile and dynamic mechanical properties as well as impact resistance of the epoxy network was evaluated. The addition of 10 phr of low-molar-mass MMA-EHA copolymer with defined composition resulted in a significant increase in impact resistance without any signifi- cant changes in the tensile strength, modulus, and glass transition temperature. The morphology of the modified epoxy network depends upon the copolymer composition. S. Zaioncz, A. A. Silva, A. S. Sirqueira, B. G. Soares Instituto de Macromole ´culas, Universidade Federal do Rio de Janeiro, Centro de Tecnologia, Bl. J, Ilha do Funda ˜o, 21945-970 Rio de Janeiro, RJ, Brasil Fax: þ55 21 2562 7207; E-mail: bluma@ima.ufrj.br Macromol. Mater. Eng. 2007, 292, 1263–1270 ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/mame.200700227 1263