1. Introduction Over the past decades, polymer blends also known as multiphase polymeric systems have been employed in a broad number of design applications that require high resistance to fracture, a highly desired charac- teristic for virtually all engineering materials [1]. These polymeric materials became appealing due to their low density when compared with other engi- neering materials, due to the ability to synergistically incorporate properties of their individual compo- nents and achieve materials with better mechanical properties than the original constituents, as well as for their transparency [2]. The challenges with these multiphase systems are to improve their compatibil- ity and interfacial adhesion between phases, so as to guarantee the desired performance of the final mate- rial. It is known that only a relatively small number of polymer pairs form miscible blends, mostly because of these blends have low entropy of mixing. Although the entropy of mixing favors the miscibil- ity of a given system, it also depends on the number of molecules per unit volume. Thus, the higher the molecular weight of the polymers involved, the fewer molecules per unit volume and the lower the entropy of mixing. And since the heat of mixing of poly- mers pairs is generally unfavorable, polymer blends tend to macroscopically phase separate, leading to systems with poor mechanical properties [3–5]. In order to overcome this problem, many efforts have been made to find different ways to improve the mis- cibility of multiphase systems, one of this approaches involves the use of interfacial agents. These agents decrease the average domain size of the disperse phase by acting as steric barriers at the interphase 204 Sequential graft-interpenetrating polymer networks based on polyurethane and acrylic/ester copolymers R. Ballestero 1 , B. M. Sundaram 2 , H. V. Tippur 2 , M. L. Auad 1* 1 Deparment of Chemical Engineering, Auburn University, 36849 Auburn, United States 2 Deparment of Mechanical Engineering, Auburn University, 36849 Auburn, United States Received 22 June 2015; accepted in revised form 4 October 2015 Abstract. Highly transparent and tough graft-interpenetrating polymer networks (graft-IPNs) were synthesized using an elastomeric polyurethane phase (PU) and a highly stiff acrylate-base copolymer phase. The grafting points between the two networks were generated with the purpose of minimizing the phase separation process of the polymeric systems. In order to generate the grafting between the networks, an acrylic resin capable of undergoing both free radical and poly-addition poly- merization was employed. The thermo-mechanical properties, fracture toughness properties as well as network and surface phase morphology of the graft-IPNs synthesized were evaluated in this work. Data obtained suggested that the minimiza- tion of the phase separation was achieved by the generation of crosslinking points between both networks. High trans- parency was obtained in all samples as an indication of the high level of interpenetration achieved. The relative high values obtained for the fracture toughness tests suggest that generating chemical crosslinks between networks is a good approach for increasing the fracture toughness of polymeric materials. Keywords: thermal properties, graft-IPNs, fracture toughness, BisGMA, sequential polymerization eXPRESS Polymer Letters Vol.10, No.3 (2016) 204–215 Available online at www.expresspolymlett.com DOI: 10.3144/expresspolymlett.2016.19 * Corresponding author, e-mail: auad@auburn.edu © BME-PT