## 4 Conclusions  
We have prepared a gradient distributing IPN composite using EP and PU simply and inexpensively. We adopted UV-vis, TG, DSC, DMA, AFM and water contact angle to characterize this IPN composite, and found that the absorption in visible light region, glass-transition temperatures (\(T_{\text{g}}\)), thermal decomposition temperatures (\(T_{\text{d}}\)) and DMT modulus were increasing along with the gradient direction from bottom side to top side of the IPN. While the absorption in ultraviolet region and adhesion force were decreasing along with the gradient direction from the bottom side to the top side of the IPN. DMA analysis demonstrates that this continuous gradient IPN has a good balance between the damping temperature range and the loss factor which is suitable for using as a self-supporting damping structure.  
Compared to the traditional homogeneous IPN composites, these gradient IPNs with different shapes and thicknesses can be created simply by adjusting the shape of the mold, the pre-curing temperature and the pre-curing time. Due to the gradually changing modulus, this IPN composite has the potential to replace the constrained damping materials by direct generation on substrates. This gradient composite will minimize the mismatch of the abrupt interfacial tension changes and reduce the adhesion failures of damping structures. The service life of these damping structures will be greatly lengthened.  
The following are available online at www.mdpi.com/2076-3417/7/2/135/s1, Figure S1: Schematic illustrations of the graded/bilayer/gradient materials, (a) the graded materials; (b) the bilayer material; (c) the gradient material, Figure S2: Relationship between the procuring time and the gradient structures.  
The component ratio EP/PU = 60/40. Procuring temperature is 10 \({}^{\circ}\)C. (a) Pre-cured for 0.5 h; (b) pre-cured for 24 h; (c) pre-cured for 48h, Figure S3: SEM picture of the gradient IPN composite (PU/EP = 40/60), Figure S4: Scanning electron microscopy/energy-dispersive X-ray technique (SEM-EDX) analysis of the 70/30 (EP/PU) gradient material. Figure S4a is the fracture surface of the gradient material. Figure S4b is the nitrogen distribution of Figure S4a and the chosen region for the element content analysis. Figure S4c is the whole nitrogen content of the fracture surface. Figure S4d, Figure S4e and Figure S4f are the detailed data of the chosen regions in Figure S4b, Figure S5: ATR-FTIR results of the chosen regions of the gradient sample. Devices applied here have been revised before detection and all curves here have subtracted the baseline, Figure S6: FTIR spectra of the locations chosen from the homogeneous IPN, Table S1: Element distribution of the selected slices from the sample.  
This work supported by the Fundamental Research Funds for the Central Universities (WUT: 2017 IVA093).  
[PERSON] and [PERSON] were responsible for the designing of the experiments; [PERSON] performed the whole experiments; [PERSON] and [PERSON] carried out the AFM and UV-vis test; [PERSON] and [PERSON] analyzed the experimental data; [PERSON], [PERSON] and [PERSON] wrote the whole paper.  
The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.