*3.2. Structure Analysis*

Figure 3 are FTIR spectra and XPS survey spectra of the linear WPU-SS samples. In Figure 3a, all curves have no absorption peak at 2270 cm<sup>−</sup>1, which indicates that −NCO groups in the system are all involved in the reaction [16,17]. Compared with the three curves, the distribution of all characteristic peaks was essentially the same, indicating that −OH and −NCO on HEDS reacted to form carbamate, and no new chemical bond was formed. The three curves are in line with the characteristic peaks of waterborne polyurethane. The band located at 3321 cm−<sup>1</sup> corresponds to the N−H stretching vibration and the band at 1531 cm−<sup>1</sup> is assigned to the N–H in-plane bending vibration [18,19]. The absorption bands at 1460 and 1360 cm−<sup>1</sup> are assigned to the –CH2– bending vibrations. The bands at 2940 and 2856 cm−<sup>1</sup> are associated with the C–H asymmetry and symmetric stretching vibration of methylene in polyurethane molecular chain [20,21]. The band at 1110 cm−<sup>1</sup> corresponds to bending vibration of C−O (aliphatic ether) in polyether polyol [22]. The characteristic band about 1707 and 1631 cm−<sup>1</sup> is related with nonhydrogen bond C=O hydrogen bond C=O in urea, respectively [23–25]. HEDS in polymer chain has a characteristic absorption peak of SS bond at 637 [26]. With the increase of disulfide bond content, the absorption peak of the corresponding SS bond at 637 cm−<sup>1</sup> became more obvious, which proved that SS bond was successfully introduced into waterborne polyurethane to form WPU-SS.

**Figure 3.** (**a**) FTIR spectra of WPU-SS samples, (**b**) XPS survey spectra of WPU2, (**c**) C 1s of WPU2, and (**d**) O 1s of WPU2.

The XPS measurement was used to further confirm the construction by analyzing the chemical compositions in the WPU-SS system. The peaks in XPS survey spectrum indicated the present of S element (163.5 eV) in the samples due to disulfide bond should be successfully chemically linked to the matrix of waterborne polyurethane. In addition, the C 1s spectrum was further measured to explore the types of carbon bonds to analyze the existence of interfacial interactions, as shown in Figure 3c. The C 1s peak was curvedfitted into four main components including C-C/C-H, C-N, C-O, C-S and C=O bonds corresponding to the peaks at 284.8, 285.7, 286.5, 287.7 and 288.8 eV, respectively. The O 1s peak was curved-fitted into four main components including C-O-C and C=O bonds. The peak of C-S bond appeared implying that disulfide bond was successfully grafted onto polyurethane chains, as supported by FTIR results.

#### *3.3. Dynamic Thermodynamic Analysis*

The DMA results are shown in Figure 4. During the heating process of WPU-SS at −100~120 ◦C, the polymer with microphase separation structure showed two glass transition temperatures. The storage modulus and loss modulus of WPU-SS films are shown in Figure 4a,b, respectively. It could be found that the storage modulus (E ) decreased significantly at about −70 to −60 ◦C, while the loss modulus (E") increased significantly at the same temperature range, which corresponding to the glass transition of the hard segments, moreover, with the increase of SS bonds, the E' did not change much while the peak value of E" changed obviously, which meant the move of the polymer chains took more energy. The decrease of disulfide bond content leads to the increase of soft segment content, which reduced the E and E" of the films at room temperature. This is mainly due to the less restriction of the slip of the soft segment molecular chain, which leads to the easier entanglement of the molecular chain and the easier interaction with the main chain [27]. At the same time, the degree of microphase separation is reduced but the damping performance is improved.

**Figure 4.** (**a**) Storage modulus, (**b**) loss modulus and (**c**) tan delta of WPU-SS samples with different contents of SS as a function of temperature.

The link between tan delta and temperature is determined in Figure 4c. With the loss of SS bond loading, the soft segment rises, and the microphase separation diminishes gradually. When the microphase separation is cut down to a certain extent, the glass transition temperature of the hard segment cannot be shown in the experiment. Since the glass transition temperature of the hard segment is much lower than room temperature, the film is in a highly elastic state and exhibits the properties of an elastomer at room temperature.
