*2.3. Synthesis of TPAEs*

A typical one-pot synthesis procedure is described as follows. To a 250 mL threenecked flask equipped with a mechanical stirrer, an argon inlet, and a condenser, dodecanedioic acid (17.27 g, 0.075 mol), 1,12-dodecanediamine (7.52 g, 0.0375 mol), Jeffamine D2000 (75 g, 0.0375 mol), and sodium hypophosphite monohydrate (1 wt %) were added. The mixture was slowly heated to 220 ◦C and kept for 2.5 h in argon flow at a stirring speed of 200 rpm until no water condensation was observed. Afterwards, the vacuum was gradually applied to 100 Pa within 30 min. After 2 h of reaction, the reaction temperature was raised to 240 ◦C and the vacuum was kept constant at 80–100 Pa for another hour. The resulting product was poured into ice water and dried under reduced pressure.

#### **3. Results and Discussion**

#### *3.1. Synthesis and Structure Characterization of TPAEs*

The one-pot polycondensation for novel TPAEs was performed using monomers of PA1212 and polyetheramine in the presence of sodium hypophosphite monohydrate, as shown in Scheme 1. The molar amount of DA is equal to the sum of DN and PPG diamine to maintain the stoichiometric balance of functional groups. Dodecanedioic acid (DA) can react with both 1,12-dodecanediamine (DN) and PPG diamine, and the reaction of DA and DN produces the PA1212 segment. Practically, the equimolar of DA and DN forms PA1212 and the excess DA limits the molar weight of the PA1212 oligomer on one

hand and links the PPG diamine segment on the other hand. These TPAEs can also be considered as copolyamides due to the amide linkage between polyamide and polyether, thus conferring more resistance to hydrolysis than the most reported TPAEs containing ester linkages. Six TPAEs with varied soft contents were synthesized by adjusting the feed ratio of dodecanedioic acid, 1,12-dodecanediamine, and polyetheramine. The polyetheramine used are long-chain Jeffamine D2000 and short-chain Jeffamine D400 with mole masses of approximately 2000 g·mol−<sup>1</sup> and 400 g·mol−1, respectively, and the polyamide has a regular-length segment. The sample code was expressed as TPAE-x, where x corresponds to the soft content determined according to the 1H NMR spectra.

**Scheme 1.** One-pot synthesis process of TPAEs based on PA1212 and polyetheramine.

The chemical structures of the TPAEs are confirmed by NMR and FT-IR. Figure 1a,b and Figure 2 depict the 1H NMR, 13C NMR, and 1H-1H COSY spectra of TPAE-0.76, respectively. Each peak is assigned to the corresponding hydrogen and carbon in TPAE. The characteristic peaks at δ = 2.75 ppm (H1) and δ = 3.60 ppm (H2) are attributed to CH2 proton next to the carboxyl and nitrogen atom respectively, indicating that the PA1212 segment has formed. The correspondingly CH2 carbons are found at δ = 33 ppm (C1) and δ = 42 ppm in the 13C spectrum, respectively. It is noteworthy that the appearance of resonance at δ = 4.39 ppm (H5) confirms that polyetheramine has been bonded to the PA1212 segment, which agrees with the results of PA6-based copolyetheramide [32]. The corresponding CH carbon appears at δ = 50 ppm. The peaks of CH2 proton (C7, C8, C9) in the aliphatic chain of PA1212 emerge at 1.31–1.81 ppm. The strong signal at 1.31 ppm is attributed to side CH3 (C6) of polyetheramine. The peaks at 3.82 ppm and 3.99 ppm belong to CH2 and CH of the polyetheramine backbone, respectively. The corresponding carbon resonance can be found in a 13C NMR spectrum. Thus, the soft content can be calculated according to peak areas of H5, H2, and H3 in 1H NMR [5,6], and the mass ratios of soft segment are listed in Table 1. The exact compositions of copolymers are close to theoretical calculation based on the feeding ratio, indicating the synthesized copolymers with expected structure. The slight decreases of soft contents calculated by NMR are because the hydrophilic polyetheramine was brought out with water vapor. It is noted that the peak at 180 ppm suggests that there are more residual carboxyl groups in TPAE than amine groups. 1H-1H COSY can demonstrate coupling relations of protons on adjacent carbon atoms, presenting cross dots on the spectrum. The cross-signals found in the COSY spectrum prove the presence of the protons in the copolymer backbone. The polyetheramine segment has been successfully bonded to the PA1212 segment confirmed by the cross-peaks at 5/6 and 5/3.

**Figure 1.** 1H NMR (**a**) and 13C NMR (**b**) spectrum of TPAE-0.76 (\* Solvent: TFA-d).

**Figure 2.** 1H-1H COSY spectrum of TPAE-0.76.



<sup>a</sup> Determined by GPC.

The structure analysis of TPAEs by FT-IR is presented in Figure 3. The characteristic peaks at 3308 cm−<sup>1</sup> (amide A), 1634 cm−<sup>1</sup> (amide I), and 1536 cm−<sup>1</sup> (amide II) are attributed to the PA1212 segment [41]. The incorporation of the polyetheramine segment to the PA1212 backbone is confirmed by the distinct absorption band at 1093 cm−<sup>1</sup> (C-O-C stretching vibration) that became more pronounced with increased soft content, which is ascribed to the polyetheramine segment [41]. The peaks at 2850–2970 cm−<sup>1</sup> are assigned to the stretching vibration of -C-H-.

**Figure 3.** FT-IR spectra of TPAEs.

#### *3.2. Thermal Characterization*

To gain a deep insight into the phase separation behavior and the crystal structure of the copolymers with the different block length of hard and soft segments, the endothermic behavior of different TPAEs was recorded by DSC testing and illustrated in Figure 4. The corresponding characteristic parameters are listed in Table 2.

**Figure 4.** DSC thermograms of different TPAEs.


**Table 2.** DSC and DMA results of TPAEs.

N.D. means not determined.

First, the polyetheramine is completely amorphous, as confirmed by the DSC scan (Figure S1, Supplementary Material). It can be observed that each copolymer sample has only one endothermal peak during the heating scan, which is ascribed to the melting behavior of the semicrystalline polyamide segment. This is much different from the double crystalline TPAEs containing PTMG, PEG, or PPDO soft segments with two distinct endothermal peaks. The amorphous nature of PPG may endow high flexibility and elasticity. Furthermore, the copolymers having long-chain Jeffamine D2000 exhibit similar *Tg* in spite of different compositions (TPAE-0.76, TPAE-0.62, and TPAE-0.43), which suggests that microphase separation occurs in the amorphous region. The *Tg* values of the three samples are close to −69.5 ◦C of the pristine soft block (Figure S1, Supplementary Material). Differently, the copolymers containing short-chain Jeffamine D400 show increasing *Tg* values with increasing polyamide content, which indicates that the flexible polyetheramine segments are compatible with the rigid polyamide segments in the amorphous region [33] (TPAE-0.39 and TPAE-0.25). However, in sample TPAE-0.10, the *Tg* is not detected, suggesting the high miscibility between polyetheramine and polyamide.

For containing different chain lengths and content of polyetheramine, the melting temperature of TPAEs increases obviously as the hard content increases. It can be found that the segmented polyamide has a lower melting temperature than homo PA1212 (184.2 ◦C). The melting temperature of copolymers containing short-chain Jeffamine D400 show a relatively large difference to homo PA1212. It is noted that TPAE-0.43 and TPAE-0.39 have the similar hard content but distinct melting points. This is because short-chain Jeffamine D400 is miscible with polyamide and restricts the growth of polyamide crystals, resulting in a reduction of crystalline degree (from 22.6% to 19.6%). Therefore, it is an effective strategy to decrease the processing temperature of the elastomer by using short chains of polyether. Correspondingly, the crystallization temperature exhibits the same trend as melting temperature (Figure S2, Supplementary Material), which also indicates that soft blocks are compatible to the hard blocks in the amorphous region and that short chain polyetheramine is miscible in this amorphous region. Otherwise, the multiple melting endotherms are caused by (1) the melting of crystallites with different lamellae thickness, (2) the melting of different crystal forms, or (3) the crystallization and remelting of imperfect crystallites [33].

#### *3.3. Morphological Characterizations*

The wide-angle X-ray diffraction of six TPAEs with different compositions are shown in Figure 5. The copolymer films with non-crystalline soft segments show two main diffraction peaks at around *2θ* = 20.1◦ and 24.0◦ that are attributed to the (100) crystal plane and (010)/(110) crystal plane [40,41], suggesting that the films adopt a triclinic α-crystal phase. The corresponding *d* spacing values are 0.44 nm and 0.37 nm, which represent the inter-chain distance within the hydrogen bonded-sheet and the inter-sheet distance, respectively [42]. The microphase separation takes place in the amorphous region of novel TPAEs.

**Figure 5.** WAXD patterns of the TPAEs at ambient temperature.

Small-angle X-ray scattering was conducted to further explore the microphase separated structure of the TPAEs. Figure 6a presents the Lorentz corrected SAXS profiles (*Iq*<sup>2</sup> vs. *q*) of the TPAEs with different compositions. According to Bragg's law, the corresponding long period (*L*) can be calculated based on the maximum value of the scattering vector (*qmax*), *L* = 2π/*qmax,* and the results are illustrated in Figure 6b. Only one broad scattering peak appears in all the TPAEs samples in Figure 6a. The peak indicates the existence of a periodic structure. The intensity peaks of copolymers with Jeffamine D400 (TPAE-0.39, TPAE-0.25, and TPAE-0.10) are similarly broad, while the ones with Jeffamine D2000 (TPAE-0.76, TPAE-0.62, and TPAE-0.43) are broadened as the soft content increases, suggesting the existence of a long periodic structure in copolymers with Jeffamine D2000, and the structure disappears as soft content increases. Furthermore, due to the noncrystallization of soft segments, the scattering peaks are attributed to the repeat distance between the polyamide crystalline domains. The *L* values decrease from 16.03 nm of TPAE-0.76 to 11.26 nm of TPAE-0.10 with increasing soft content and stay close when copolymers have short-chain Jeffamine D400. The reduction of *L* in value is owing to the crystallization of polyamide [43,44]. It is proof that short-chain polyetheramine is miscible with polyamide amorphous domain.

The surface topographical characteristic of the TPAEs with different compositions was investigated by AFM at ambient temperature, and the height images are shown in Figure 7. The AFM height image of homo PA1212 is included for comparison. The copolymers display well-ordered crystalline structure and the spherulites of the polyamide domain with a clear boundary can be found except for the high soft content of TPAE-0.76 and TPAE-0.62. It should be noteworthy; however, these two copolymers contain a polyamide crystal phase according to DSC and WAXD results. The slender and rigid polyamide domain is observed, which can be explain by the assumption that the polyamide crystalline domain forms randomly ordered lamellar crystals by the dilute solution casting method [5]. For the other samples, the spherulitic morphology consisting of crystalline polyamide and amorphous polyamide segments are well dispersed in the amorphous polyetheramine. It is believed that the amorphous polyetheramine chains are comprised of the spherulites or filled among the lamellar crystals [41].

**Figure 6.** (**a**) Lorentz corrected SAXS profiles of the TPAEs and (**b**) long period vs. soft contents.

**Figure 7.** AFM height images of drop-casting TPAEs: (**a**) TPAE-0.76, (**b**) TPAE-0.62, (**c**) TPAE-0.43, (**d**) TPAE-0.39, (**e**) TPAE-0.25, (**f**) TPAE-0.10, and (**g**) homo PA1212.

In addition, the non-crystalline soft segments have a distinguishing effect on the morphology features. It can be found that when using long-chain Jeffamine D2000, the spherulitic structure forms only in TPAE-0.43 with relative high polyamide content, showing packed down-feather-like lamellar topography; for TPAE-0.39 to TPAE-0.10 with short-chain Jeffamine D400, the bundle-like structure with filamentous polyamide crystals turns into similar spherulite with a feather-like structure. Furthermore, with the increasing of polyamide content, the short-chain polyetheramine has a much greater dilution effect on the crystallization of polyamide than the long-chain polyetheramine because of the difference in the compatibility of distinct polyetheramines with polyamide. As a result, the degree of microphase separation of copolymers with long-chain soft segments is higher than that with short chain. The above deductions are further validated from the polarized optical microscopy (POM) images (Figure S3, Supplementary Material). Only very small crystals can be found in TPAEs with low amount of soft content (containing shortchain polyetheramine), and small size and sparse packed spherulites of polyamidecan be observed in TPAE-0.10.

To further investigate the morphological evolution with different compositions of the TPAEs, TEM was performed, and the representative images are shown in Figure 8. The TPAE-0.76 and TPAE-0.62 with high soft content grow long rigid-rod polyamide crystals distributed in a disordered form in amorphous soft domain. The rigid-rod crystals are assembled into a bundle-like structure in TPAE-0.39 as the increasing of polyamide content. The densely packed spherulites with feather-like structure can be seen in TPAE-0.10 with high polyamide content. These results are agreeable with the above deductions by AFM measurements.

**Figure 8.** TEM images of (**a**) TPAE-0.76, (**b**) TPAE-0.62, (**c**) TPAE-0.43, (**d**) TPAE-0.39, (**e**) TPAE-0.25, and (**f**) TPAE-0.10.

#### *3.4. Dynamic Thermomechanical Analysis*

To analyze the relationships between the microstructure and thermal mechanical properties of elastomers, DMA measurements were carried out on all the synthesized novel TPAEs. Figure 9 illustrates the temperature dependence of the storage modulus (*E* ) and loss factor (*tan δ*) for the TPAEs with variable compositions. The glass transition temperature *Tg* (peak value of *tan δ*) and *E'* at −25 ◦C and 25 ◦C are listed in Table 2, correspondingly. It can be found that the copolymers have decreased *E'* with decreasing soft content at low temperature ca. −70 ◦C and exhibit a plateau of *E'* in the glassy state. Then, *E'* drops with the glass transition behavior. The *E'* values of TPAE-0.76 and TPAE-0.62 at −25 ◦C are far less than those of the other three samples, which are lower than commercial PEBAX at the similar compositions [5]. That means that these two copolymers with high soft content are well flexible at low temperature. Following the transition, the copolymers exhibit a rubbery state and a low dependence on temperature, providing elastic performance. The *E'* value at room temperature increases with the decreasing of soft content, which indicates the corresponding stiffness increases. It is worth noting that the *E'* value of TPAE-0.43 is smaller than that of TPAE-0.39, while the soft contents of two TPAE are very close. The *E'* of TPAE-0.43 has little change from −25 to 25 ◦C. To obtain TPAE with high elastic property, in addition to considering increasing the soft content, it is more significant to apply long-chain soft segments.

**Figure 9.** Thermomechanical analysis of TPAEs: (**a**) storage modulus (*E*') vs. temperature and (**b**) loss factor (*tan δ*) vs. temperature.

Figure 9b shows the thermal transitions of the TPAEs. The peak values of *tan δ* decrease with the increasing of polyamide content, indicating that polyamide crystals restrain the chain mobility of amorphous PPG. This is because the polyamide domain can act as a physical crosslinker in TPAEs [45]. The copolymers containing long-chain soft segments exhibit similar *Tg* values that are independent of the soft content, whereas those having short-chain polyetheramine show increasing *Tg* values as the soft content increases. The DMA results are in good agreement with the DSC results. The values of *Tg* measured by DMA are higher than those by the DSC method, which is because the copolymer chain begins to relax at the temperature before the *tan δ* peak, which is measured by mechanical response [46].

*Tan δ* is defined using the ratio of loss modulus (*E"*) and storage modulus (*E'*) and is used as an assessment of the ability of energy dissipation by elastomers. The strain hysteresis caused by the viscoelasticity of copolymers can result in energy absorption. Therefore, TPAEs are widely used as damping materials for attenuating vibration and reducing noise due to their significantly viscoelastic properties. Damping materials should meet the requirement of *tan δ* > 0.3, and the corresponding temperature region is defined as

the damping temperature range [47,48]. The *tan δ* peak values of PEBAX are reported to be lower than 0.16, which is indicative of good energy return when using as an elastomer [5]. However, the *tan δ* peak values of the synthesized TPAEs are over 0.14 except for TPAE-0.10. Especially, the *tan δ* peak values of TPAE-0.76 and TPAE-0.62 with high PPG content are distinctly high (>0.5), which indicates that these two TPAEs have excellent shock absorbing capacity. It is because the energy dissipation by internal friction can be promoted since this C3 building block bears a methyl side group in the backbone [48]. As *tan δ* > 0.3, the damping temperature range for TPAE-0.76 and TPAE-0.62 are from −61 to −47 ◦C (ΔT = 14 ◦C) and from −58 to −45 ◦C (ΔT = 13 ◦C), respectively. It can be found that the synthesized TPAEs have excellent viscoelastic properties and damping performance by introducing a PPG block, which is important for toughness applications such as ski boots and rail gasket in an extremely cold situation. Thus, the novel TPAEs with long-chain semicrystalline polyamide and a high content of amorphous polyetheramine enrich the family of TPAEs for damping materials.

#### *3.5. Mechanical Properties*

Figure 10 presents the uniaxial tensile stress–strain curves of the TPAEs with different compositions at room temperature. The typical elastic behavior of the specimen is determined by the lack of a yield point during the deformation except for TPAE-0.10, which has the highest hard content and shows thermoplastic character. The polyetheramine-based TPAEs have good elastic properties according to the manner in which TPAE-0.25 reveals an intermediate stress–strain behavior. The Young' moduli (the slope of initial linear elastic domain) increase with increasing polyamide content due to the increase in interconnectivity within the rigid polyamide domain [49]. Moreover, the tensile strength increases as the rigid content increases, which is independent of the polyetheramine length. The specimens exhibit good elongation at break that increases with the increase of soft content. Based on the testing results, the tensile property of TPAEs highly depends on the soft content instead of the segment length.

**Figure 10.** Tensile stress–strain curves of TPAE films.

To further study the elastic properties of the TPAEs, mechanical hysteresis was measured by a cyclic tensile test and the representative stress–strain curves are presented in Figure 11. The residual strain was calculated after 10 cycles of tensile testing. As we expected, the residual strain increases with the decreasing of soft content and increasing of testing deformation (Table S1, Supplementary Material). The novel TPAEs using diamine-terminated PPG present good elastic recovery and flexibility. In fact, the specimens after testing just have a small extent of residual strain after storing at room temperature overnight (Figure S4, Supplementary Material).

**Figure 11.** Selected cyclic tensile stress–strain curves of TPAE films.

#### **4. Conclusions**

Novel TPAEs consisting of long-chain semicrystalline PA1212 (C12) as the hard segment and amorphous diamine terminated PPG (C3) as the soft segment were successfully synthesized via one-pot melt polycondensation. The chemical structures of the copolymers were characterized by NMR and FT-IR, and the results prove that the diamine terminated PPG segments have been incorporated to PA1212 blocks with expected composition. The molecular weights of the TPAEs were approaching the commercial grade. The microphaseseparated morphologies and tensile properties of novel TPAEs have been systematically investigated by DSC, X-ray diffraction, AFM, SEM, and DMA. It has been found that microphase separation occurs in the amorphous region, and short-chain PPG diamine is miscible with the amorphous phase of PA1212. The copolymers adopt the same α-crystal phase as homo PA1212, and the crystallinity of the copolymers (PA1212 segments) varies with the content and segment length of PPG diamine. The microphase separation structure, associated with the macro properties, is tunable according to the contents and length of the flexible PPG diamine. Due to the amorphous PPG segment and long-chain semicrystalline PA1212, the new family of TPAEs shows high elasticity and damping performance, especially at low temperature for TPAE-0.76 and TPAE-0.62. The novel TPAEs show balanced properties of tremendous lightness, elastic return, and attenuation of vibration, suggesting their potential applications such as running shoes, ski boots, and tennis rackets even products used in extreme cold condition such as rail gaskets.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/polym13162645/s1, Figure S1: DSC thermogram of Jeffamine D2000, Figure S2: DSC thermogram of TPAEs, Figure S3: POM images of (a) (b) TPAE-0.39, (c) TPAE-0.25 (d) TPAE-0.10, (e) homo PA1212, Figure S4: Photos of specimens before and after cyclic tensile testing for (a) TPAE-0.76 and (b) TPAE-0.43 (stored at room temperature overnight), Figure S5: Thermal gravimetric curves of TPAEs, Table S1: The residual strain of the specimens after cyclic tensile testing, Table S2 Characteristic thermal degradation parameters of TPAEs.

**Author Contributions:** Conceptualization, J.J. and Z.X.; Formal analysis, J.J., Q.T. and X.P.; Funding acquisition, L.Z. and Z.X.; Investigation, J.J.; Supervision, W.Y.; Writing—original draft, J.J.; Writing review and editing, X.P., J.L. and Z.X. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was financially supported by the National Natural Science Foundation of China (Grant No. 21978089 and No. 21878256), the Fundamental Research Funds for the Central Universities (Grant No. 22221818010), and the 111 Project (Grant No. B20031).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** There are no conflict of interest to declare.

#### **References**

