**1. Introduction**

Thermoplastic elastomers (TPEs) are usually multiblock segmented copolymers consisting of thermodynamic incompatible hard blocks and soft blocks, normally leading to a microphase separated structure. The soft blocks are often polyether or aliphatic polyester with low glass transition temperature (*T*g), enabling elastomeric properties to the copolymers. Moreover, the hard blocks containing crystalline units form a physical cross-linked site. Chemical covalent links between the two blocks can avoid macro-phase separation. These materials bridge the gap between thermoplastics and elastomers. Thermoplastic polyurethanes (TPUs), thermoplastic polyether ester elastomers (TPEEs), and thermoplastic polyamide elastomers (TPAEs) are the three main types of segmented TPEs. TPEs have received much industrial and commercial interest duo to their good thermomechanical properties, excellent chemical resistance, wide service temperature range, and ease of processing.

**Citation:** Jiang, J.; Tang, Q.; Pan, X.; Li, J.; Zhao, L.; Xi, Z.; Yuan, W. Facile Synthesis of Thermoplastic Polyamide Elastomers Based on Amorphous Polyetheramine with Damping Performance. *Polymers* **2021**, *13*, 2645. https://doi.org/10.3390 /polym13162645

Academic Editor: Angels Serra

Received: 7 July 2021 Accepted: 4 August 2021 Published: 9 August 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

TPAEs have been recently developed as a relatively new family of engineering plastics because of their highly efficient energy return, good performance at low temperature, and selective gas permeability. They are widely used in sports equipment, such as ski boots, cleated shoes, and running shoes. Generally, TPAEs are segmented poly(ether-*block*-amide) copolymers composed of polyether as soft segments and polyamide as hard segments. The performance of the material depends not only on the ratio of soft and hard segments but also the nature of a soft block when using a certain polyamide. Currently, the main soft blocks for TPAEs are PEG [1] and PTMG, enabling antistatic property and good flexibility, respectively. Commercially available TPAEs have been mainly developed by Arkema and Evonik under the trade name of PEBAX and VESTAMID, respectively. The copolymers are synthesized by the polycondensation of carboxyl terminated polyamide (PA6, PA11, or PA12 oligomers) and hydroxyl terminated polyether (PTMG or PEG), resulting in ester linkage [2,3], which is the so-called "two-step method". The structure–property relationships of TPAEs have been extensively studied [4–10]. For example, TPAEs consisting of PTMG has been mostly used for footwear [11–14] and gas separation [15,16], while those containing PEG as a flexible phase are employed to selectively separate CO2 [17–20] and permanent antistatic packaging [21]. It should be noted that PTMG and PEG are a semicrystalline C4 building block and C2 building block, respectively. The C3 building block has been hardly used in the preparation of TPAEs. In our previous work, a linear semicrystalline poly(trimethylene glycol) (PPDO) has been utilized to prepare TPAEs with high elasticity [22]. Nevertheless, for these semicrystalline blocks, the melting of the corresponding segments will cause the variation of storage modulus, which is not conducive to the use of materials in this temperature range [2]. To enrich the TPAE family, poly(propylene glycol) (PPG, C3) is an alternative amorphous polyether with a methyl side group on each repeat unit, which has weak dispersion and a low dipole moment, resulting in the superior elasticity to polyurethane elastomers [23,24]. However, the reactivity of secondary hydroxyl groups in PPG is relatively low, and its poor compatibility with polyamide oligomers prevents the formation of high molecular weight copolymers. Considering the condensation of hydroxyl or amino groups with carboxyl groups, PPG diamine (trade name Jeffamine D) is an alternative C3 building block. It is a non-crystalline aliphatic polyetheramine with low viscosity and low tendency to form a hydrogen bond, which is generally used as a gasoline additive, curing agents of epoxy resins [25,26], unit of epoxy-imine vitrimer [27], precursor of main-chain polybenzoxazines resin [28], or soft segments of Pus [29], providing high toughness, thermal stability, or flexibility. For PPG-based TPAEs, monomer casting PA6 modified with maximum 6 wt % of this polyetheramine shows an obvious decrease in crystallization ability and an elastic deformation behavior [30]. Grymans et al. selected PA46 salt and PPG diamine to prepare PA46-PPG segmented copolymers via condensation followed by solid-phase polycondensation [31]. However, severe macro-phase separation occurs despite adding *m*-cresol as solvent. The solid products are inhomogeneous after postpolymerization when using mole mass 2000 of Jeffamine D. Jo et al. synthesized PA6-PPG copolymers with soft content from 10 wt % to 56 wt % by melt copolymerization [32–34], whose microphase separation structure was systematically investigated. Shibasaki et al. reported a monodisperse poly(N-methyl benzamide) copolymerized with PPG diamine in solution to afford non-hydrogen-bondable block copolymers, which are non-crystalline materials but showing weak tensile modulus [35]. Luo et al. prepared fluorene-based PA-PPG membranes via solution polycondensation with high carbon dioxide permeability [36,37].

It is noteworthy that for the sporting applications of TPAEs, the attenuation of vibration is an important property. The methyl side group in the PPG backbone allows strong energy dissipation by internal friction, enabling the shock-absorbing capacity of the TPAEs when using them as potential damping materials, which has not been extensively investigated. PA1212 is an important semicrystalline engineering plastic, which owns high toughness and strength, low temperature resistance, and low shrinkage [38]. It has close properties with PA12, and the polymerization of PA1212 does not require high temperature and high pressure of PA12, which is mostly used to produce commercial PEBAX. Further-

more, it is not easy to prepare the high molecular weight of TPAEs using the "two-step method" due to the incompatibility between polyamide and polyether oligomer and the difficulties in adjusting the stoichiometric balance of reactive groups [39]. In this work, one-pot melt polycondensation was proposed to firstly synthesize a series of segmented TPAEs with variable compositions of PA1212 block (C12) and PPG block (C3). The TPAEs were prepared directly from monomers of PA1212 and PPG diamine. This procedure is feasible with the advantages of low starting viscosity, ease of stoichiometric balance of acid and amine groups, and high tolerance of compatibility between polyamide and polyether compared with the two-step method. This one-pot strategy is its simplicity, high efficiency, and low cost, especially using commercial PPG diamine (C3) to manufacture advanced materials. The chemical structures of novel TPAEs were studied using nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FT-IR). The microphase-separated structures were characterized by differential scanning calorimetry (DSC), dynamic thermomechanical analysis (DMA), and X-ray diffraction and confirmed by microscopy technics. Additionally, tensile tests and cyclic tensile tests were performed to evaluate the mechanical properties of the series of new copolymers, indicating that the synthesized TPAEs have high elasticity. The novel TPAEs have the damping properties at low temperature, especially those with high flexible content due to the incorporation of the amorphous PPG segment. The effects of content and length of the PPG segment on microphase separated morphology were systematically investigated. Thermal gravimetric analysis (TGA) was applied to find that the TPAEs have good thermal stability. These novel family of TPAEs are believed to have great potential for low-temperature applications.

#### **2. Experiment Section**

#### *2.1. Materials*

Dodecanedioic acid (≥99%), deuterated trifluoroacetic acid (TFA-d, 99.5 atom %D), *m*-cresol (≥99%), and 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP, 99%) were purchased from Shanghai Titan Science and Technical Company (Shanghai, China). Polyetheramine JeffamineD400 (Mn ≈ 400 g·mol<sup>−</sup>1) and JeffamineD2000 (Mn ≈ 2000 g·mol<sup>−</sup>1) (PPG diamine) were purchased from Shanghai Aladding Biochemical Technology Co., Ltd. (Shanghai, China), and 1,12-dodecanediamine (≥99%) was supplied by Zibo Guangtong Chemical Company (Shandong, China). Sodium hypophosphite monohydrate (NaH2PO2·H2O, ≥98%) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All the reagents were used as received without further purification.

#### *2.2. Characterization*
