*3.3. Thermogravimetric Analysis*

The thermal degradation behaviors and DTG curves of Entry 12C are shown in Figure 7, and some crucial data are also displayed. The degradation curve of the crosslinked polyester in the first step was almost the same as the pure polyester, which was reflected by the same 5% weight loss temperature (327 ◦C) and the maximum weight loss rate temperature (403 ◦C). This step degradation was attributed to the ester and aliphatic moieties. The Tmax2 (455 ◦C) was observed in the cross-linked sample, which indicated the cross-linked polyester degraded by a multistep process. The reason might be that the hard segment urethane bonds formed after cross-linking, which reduced the migration of the chain segments. The thermal stability of the material was enhanced after cross-linking [61].

**Figure 7.** TG and DTG of NWCPE and cured polyester film.

#### *3.4. Tensile Properties of the Cross-Linked NWCPE Films*

The tensile properties of cross-linked NWCPE films were summarized in Table 5, and the stress–strain cures were listed in Figure 8. Mechanical properties are crucial for polymer materials which directly affect their industrial applications. The tensile strength increased from 3.1 MPa to 5.9 MPa after CHDM being introduced into the molecule. But when the content of CHDM reached 40%, the strength of the polymer no longer increased with the increase in the CHDM content. In addition, the toughness even decreased slightly due to the excessively high content of this rigid monomer. The result may be related to the following factors. First, the utilization of CHDM enhanced the rigidity of the polymer which could restrict the movement of the molecular chains, and the intermolecular force was increased, resulting in improved mechanical properties. Secondly, the molecular weight of the polymers were slightly different. Polyester with minor *M*<sup>n</sup> contained more terminal hydroxyl groups in the same mass, resulting in a slightly larger cross-linking density during the curing process. The stress–strain curves of all of the films showed typical plastic deformation, and the elongation at the break (ε) of the films were all basically above 70%. The cause might be the following two factors. First, the synthesized polyester has low crystallinity, leading to high mobility between molecular chains. Secondly, the polyetheramine side chains are similar to a spring and increased the ductility of the molecular. The tensile properties of cured NWCPE were better than some cured ionic waterborne polyesters, some of which were about 2–3 MPa [3,62,63].

**Table 5.** Tensile properties of cross-linked films.


**Figure 8.** Stress-strain profiles of cross-linked films.

#### *3.5. The Morphology and Light Transmittance of the Cured Film*

As shown in Figure 9a,c, the neat cross-linked film was transparent, and, at the entire visible wavelength range, the light transmittance of the film was high. In addition, owing to the absence of emulsifier, no surfactant molecules migration during the filmforming process was observed, and the surface of the film was smooth and flat according to Figure 9b [64]. At the same cross-linking density, the CHDM content exerted almost no influence on the transmittance of the film. The SEM and photographic images of cured

Entry 12C (CHDM 40%) were taken as examples. The high transparency was attributed to the following reasons. First, the synthesized NWCPEs have good compatibility with the curing agent WHDIT and there was not significant macrophase separation during the cross-linking procedure. Second, the surface of the film was smooth, which was hard for scattering light [65]. As such, the low crystallinity of the WCPE may also contribute to the high transmittance of the film.

**Figure 9.** The light transmittance (**a**), SEM image (**b**), and photograph (**c**) of cross-linked films.

### **4. Conclusions**

In this paper, MA, HA, CHDM, HG, and NPG were first used as monomers to synthesize linear polyesters by step-growth polycondensation. After this, the hydrophilic segment Jeffamine M-1000 was grafted to the polymer backbone by aza-Michael addition to prepare the NWCPE. The optimal reaction conditions were verified as TsOH as the selected catalyst, a reaction temperature of 180 ◦C, a MA/HA ratio of 5:5, and the content of the rigid monomer CHDM at 40%. In this condition, NWCPE with a molecular weight of about 8000 g mol−<sup>1</sup> was prepared. After dispersing into water, the dispersion with a solid content of 40% and the particle size was appropriate with narrow distribution. The dispersion was good in storage stability and remained stable after 6 months. The good hydrolysis resistance was identified by the slight drop in pH after two months and little change in *M*<sup>n</sup> of the dispersion. After cross-linking with the curing agent, a transparent film with good mechanical properties and water resistance was obtained. The hardness of the film was H, and the adhesion level on the tinplate reaches level 0. The tensile strength of the film was 5.9 MPa, and the *ε* was about 88%. This work provides ideas for the synthesis and functionalization of waterborne polyesters. These synthesized polymers have wide application prospects in environmentally friendly polymer materials.

**Author Contributions:** Conceptualization, methodology, and validation, H.F., L.G. and S.G.; writing—original draft preparation, H.F.; writing—review and editing, H.F., L.G., and S.G.; supervision, and funding acquisition, L.G. and S.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

