## 3 Results and Discussion
### DMA Analysis  
Damping curves of EP, PU and the three kinds of IPN materials are shown in Figure 8.  
The best damping performance of EP and PU appears at \(-\)19.5 and 97.6 \({}^{\circ}\)C, respectively, thus they are not suitable for using alone. The IPN modification technology is one of the most widely used methods for polymer modification. As is shown in Figure 8, the damping temperature \(\Delta\)T (in which loss factor value is larger than 0.3) of the homogeneous IPN locates in the range of 23.2-116.7 \({}^{\circ}\)C with a distinct peak (value: 1.21) at 65.8 \({}^{\circ}\)C, which can be used as traditional polymer damping material. However, though the damping temperature \(\Delta\)T of this homogeneous IPN has been shifted into the general service temperature regions, their storage modules are not high enough to provide the self-supporting strength within the \(\Delta\)T. The common improvement is pasting several IPN layers with different \(T_{\text{g}}\) together, which is named the graded IPN. DMA curves show that the damping temperature range of this graded IPN is from \(-\)25.8 to 120.7 \({}^{\circ}\)C, which is wider than the other IPNs. However, the tan\(\delta\) of this graded IPN is less than 0.65, which is lower than others. What is more interesting, the gradient IPN has a good balance between the damping temperature range and the loss factor. It is suitable for using as a self-supporting damping structure which can provide self-supporting strength and keep its shape in working temperatures. When the soft side is pasted to substrates and the hard surface is exposed outside, this kind of structure can be used as a replacement of constrained damping structure, reducing stress concentration and risk of joint failure [48]. Besides, comparing to the graded IPN composite, the fabricating method of the gradient IPN is simplified. There are three reasons that are responsible for the good damping performance of this continuous gradient IPN composite. Firstly, interface friction is the main factor to influence the damping property [49]. Numbers of the interfaces are directly affected by the solubility of the components. Because of the relatively poor  
Figure 8: Damping curves of the EP, PU and the three kinds of IPN materials. Specimens were measured under the shear mode from \(-\)40 to 130 \({}^{\circ}\)C with a heating rate of 1.5 \({}^{\circ}\)C/min. Specim dimension were \(\Phi\) 8.79 mm \(\times\) 2.17 mm. compatibility of the resins, phase separation (Figure 7) of the IPN would occur unavoidably and result in a multiphase structure  
Secondly, curing reactions of the components happen immediately the time when resins are mixed. The mixed resins will gel and cure before the complete phase separation, creating the gradient structure. The gradient IPN can be regarded as a kind of composite stacked up together by \"infinite layers of IPN materials\" [50; 51] (as is shown in Figure 7, especially in Figure 7c). Each \"IPN layer\" has different glass transition temperatures (\(T_{\text{g}}\)), and those \(T_{\text{g}}\) points transit from the glass transition temperature of PU to that of EP. Due to the \"transition regions\" in the gradient composite always exist and transfer along with the temperature change, this gradient composite show better damping performances comparing to homogeneous polymers. Besides, as is shown in Figure 7, there are innumerable phase running through the adjacent layers. Under a certain temperature, one \"IPN layer\" may be under its \(T_{\text{g}}\) point while the other nearby is not; hence even the adjacent layers also have different responses during vibration. Phase that going across the adjacent layers will increase the vibration friction greatly [52]. Friction exists not only in each layer, but also in the adjacent layers. Thirdly, the good damping properties of the gradient IPN composite can also be explained by the molecular structure.