*2.6. Characterization*

Chemical structure: the chemical structure of PET-DA-PU was examined by ATR-FTIR and NMR. ATR-FTIR was performed on a Bruker Tensor 27 instrument (KBr pellet) in the range 4000–400 cm−1. NMR spectra were performed on a Bruker 500 MHz instrument with CDCl3 as the solvent at 25 ◦C and tetramethylsilane as the internal standard. Thermal property: the thermal property of PET-DA-PU was investigated by differential scanning calorimetry (DSC). DSC was performed in a TA Instruments Q1000 equipment in the range of −100~0 ◦C at a heating rate of 10 ◦C min−<sup>1</sup> under a nitrogen flow. POM (Zeiss Axioskop 2 plus) with a heating stage was employed to observe the evolution of cracks on PET-DA-PU film. Mechanical property: AG-X Plus universal testing machine (Shimadzu, Kyoto, Japan) was used to measure the tensile property of PET-DA-PU films in accordance with GB/T528- 1998 with a loading rate of 500 mm min−1. The specimens were cut into dumbbell-like with the sizes of 20 mm (neck length) × 4 mm (width) × 2 mm (thickness) and kept at 0% humidity for 7 days before measurement. Samples were conducted independently in quintuplicate, and the results were presented with the average values. Fracture morphology of the films which were prepared by freeze-fractured (films were frozen in the liquid nitrogen and ruptured) and tensile-fractured were observed on a Tescan Vega 3 LMU scanning electron microscope (Tescan, Brno, Czech Republic).

#### **3. Result and Discussion**

#### *3.1. Preparation and Characterization of PET-DA-PU*

PET-DA-PU was synthesized via a prepolymer process of PET, using DA diol which prepared though cycloaddition reaction as chain extender, and TDI as a curing agent, as demonstrated in Schemes 1 and 2. The chemical structure of as-synthesized PET-DA-PU was first investigated by ATR-FTIR (as shown in Figure 1). In the ATR-FTIR of PET-DA-PU, the shoulder peaks at 1775 and 1513 cm−<sup>1</sup> specific were due to the characteristic absorption of DA ring on PET-DA-PU [27]. The peak at 1107 cm−<sup>1</sup> is ascribed to the characteristic absorption of C–O from PET. As compared to PET, the intensity decreases in –OH peak (3490 cm<sup>−</sup>1) of PET and the absence of adsorption peak at around 2270 cm−<sup>1</sup> indicated that there were no residual hydroxyl and isocyanate groups in PET-DA-PU. It is worth noting the additional peaks at 3295, 1714 and 1540 cm−<sup>1</sup> which were due to the –NH stretching vibration, –C=O band and –N–H stretching bands of the urethane group on PET-DA-PU, respectively [28]. Therefore, it could be confirmed that PET based polyurethane containing DA bonds had been synthesized successfully.

The chemical structure of as-synthesized PET-DA-PU was also characterized by 1H NMR and 13C NMR (as shown in Figure 2). The peaks at 1.61, 3.46 and 3.62 ppm were assigned to the methylene protons of PET as shown in Figure 2a, and the corresponding carbon atom signals appearing at 26.6 and 70.3 ppm in Figure 2b. Meanwhile, the peaks at 3.04, 3.15, 5.15 and 6.54 ppm due to DA ring on the PET-DA-PU. The signals at 1.25 and 7–8 ppm were attributed to the methyl protons and hydrogen protons on the benzene rings of TDI, respectively, and the corresponding carbon signals appeared at 16.9 ppm and 127–135 ppm [29]. All of the results above confirmed that PET-DA-PU has been synthesized successfully via the prepolymerization of PET.

**Figure 1.** ATR-FTIR spectra of PET-DA-PU and PET.

**Figure 2.** 1H NMR (**a**) and 13C NMR (**b**) spectra of PET-DA-PU. ('\*' for CDCl3).

#### *3.2. Glass Transition Temperature of PET-DA-PU*

Glass transition temperature (*T*g) is an important property of polymeric binders as it determines the application temperature range. In this study, DSC was used to investigate the *T*<sup>g</sup> of PET-DA-PU, and the *T*<sup>g</sup> was measured by the cooling DSC curves of the PET-DA-PU. As shown in Figure 3, in comparison of PET prepolymer (*T*<sup>g</sup> = −84 ◦C), the PET-DA-PU showed a slightly higher *T*<sup>g</sup> of −59 ◦C, which allows it to work in a lowtemperature environment. It is due to the decrease in chain flexibility of PET as to the introduction of DA diol and TDI, and the formation of urethane group [30]. Moreover, PET-DA-PU exhibits only one *T*g, which may be attribute to the fact that PET and TDI-extended DA diol segments are well-dispersed and blended within the films, thus the polyurethane consists of one phase. The same phenomenon was also observed in other literature [31].

**Figure 3.** DSC curves of PET-DA-PU and PET prepolymer.
