## 3 Results and Discussion
### Characterization of Epoxy Compositions with Polyurethanes  
In addition, the properties of the prepared polyurethanes as a toughener were studied by adding them to epoxy compositions composed of an epoxy resin and a curing agent: dicy. The epoxy compositions include various amounts of PU, except for EPPU-2 because it has a high viscosity; it was cured as described previously to prepare the test specimens for the measurements of the mechanical and thermal properties. Figure 8a shows that epoxy compositions with synthetic EPPU had higher tensile strengths than those of the epoxy compositions with Ref PU. Furthermore, epoxy compositions containing EPPU have higher flexural strength than epoxy compositions containing Ref PU (Figure 8b). This indicates that the rigid EPPU in the epoxy compositions compensated for the flexibility of the overall PU and enhanced the mechanical strength. In addition, the impact resistance of the epoxy binder with 10 phr of EPPU increased by \(\sim\)17% compared to that of neat epoxy (Figure 8c). However, when more than 10 phr of EPPU was included in the epoxy compositions, the impact resistance became lower than that of the epoxy with Ref PU due to the increased rigidity from aromatic groups in EPPU [21]. Figure 8: _Cont._  
Figure 7: TGA data for Ref PU and EPPU in air. The viscoelastic properties of each epoxy composition were measured using DMA. The storage modulus and tan \(\delta\) curves are displayed in Figure 9a,b, and the calculated crosslink density values are shown in Table 4. The storage modulus (\(E^{\prime}\)) of neat epoxy exhibited the highest value, while the \(E^{\prime}\) of the epoxy compositions decreased as the amount of PU in the epoxy matrix increased. Meanwhile, the storage moduli of the EPPU epoxy compositions were higher than those of the Ref PU epoxy compositions. The high crosslink density of the EPPU epoxy matrix can cause this, because the diglycidylI ethers of EPPU can form a polymer network structure with an amine hardener [22]. To calculate the crosslink density of each epoxy composition in the rubbery plateau region in the range of 150-250 \({}^{\circ}\)C, the following equation was used:  
\[\mathrm{V}_{c}=\frac{E^{\prime}}{3 RT} \tag{1}\]  
where \(V_{c}\) is the crosslink density, \(E^{\prime}\) is the tensile storage modulus in the rubbery plateau, \(T\) is the temperature in K corresponding to the storage modulus value, and \(R\) is the gas constant. The calculated values for the EPPU epoxy compositions were higher than those of the Ref PU epoxy binder with the same amount of polyurethane. These results correspond to the storage modulus values listed in Table 4. However, the \(V_{c}\) values of the epoxy compositions with EPPU also decreased as the content of polyurethane increased. This was caused by the decreased amount of epoxy resin forming network  
Figure 8: Mechanical strength data for epoxy composites including various amounts of Ref PU or EPPU: (**a**) tensile strength–(**b**) flexural strength–and (**c**) impact resistance. phr: parts per hundred resin. polymers [23; 24]. In Figure 9b, one can see that the curves in the \(\tan\delta\) plot of the epoxy compositions tend to decrease owing to the increase of the flexible segment of the polyurethane, giving rise to chain motion. Specifically, it is obvious that epoxy binders containing EPPU have higher glass transition temperatures than those of the epoxy compositions with Ref PUs because both phenanthrene and the benzene group of EPPU function as hard segments that restrain the chain motion of PU in the polymer matrix. In addition, parameters such as the aromatic density can change the thermal properties of the polymer, increasing \(T_{g}\) or improving the degradation characteristics [25]. The thermal stabilities of the epoxy compositions containing PUs were measured using TGA and observing the weight loss as a function of temperature (Figure 10).