**5. Polyurethane/epoxy interpenetrating polymer network**

Due to high modulus, strength, and mechanical properties, epoxy resins have been used in high‐performance structural composites. However, engineering applications of epoxy resins have been limited in several cases such as damping materials [35]. In contrast, polyurethane is a flexible and elastic polymer with low mechanical strength [36]. PU prepolymers have been prepared and incorporated into epoxy resin to form IPNs. The mechanical properties of PU/epoxy IPNs largely depend on the amount of polyurethane in the blend network. Jin et al. [6] investigated tensile properties of soybean oil‐based PU/epoxy IPNs. The tensile strength and tensile modulus of PU/EP IPNs with 5–20 mass% PU were lower than pure epoxy. This means that the addition of PU turned epoxy to the rubbery elastomer increased the elon‐ gation at break by 13‐fold as compared to pure epoxy. However, increasing the amount of epoxy increased tensile strength and tensile modulus drastically. IPNs basically integrate the structure and properties of these versatile polymers (epoxy and polyurethane). Integrated per‐ formance of epoxy/polyurethane networks have been improved by the structural modification. The glass transition temperature of PU/epoxy IPNs provides important information about the miscibility of the blend components and blend compatibility. Moreover, the glass transition temperature of PU/epoxy IPNs also provides information about the cure rate of the reaction. At the beginning of cure reaction, *T*<sup>g</sup> of PU/epoxy IPNs is usually lower than that of neat epoxy. The cure rate of epoxy is usually slower than that of PU. Some of the epoxide groups remain unreacted and act as plasticizer leading to lower *T*<sup>g</sup> values of IPNs at the beginning of the cure reaction. When reaction proceeds and an IPN is formed, this may result in an increase in *T*<sup>g</sup> of IPNs. The increase in *T*<sup>g</sup> can be attributed to the miscibility/formation of graft struc‐ ture through the reaction of hydroxyl groups of epoxy with isocyanate [37].
