A Novel Synthetic Strategy for Preparing Polyamide 6 (PA6)-Based Polymer with Transesterification

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Article A Novel Synthetic Strategy for Preparing Polyamide 6 (PA6)-Based Polymer with Transesteriﬁcation

Shengming Zhang 1, Jingchun Zhang 1, Lian Tang 1, Jiapeng Huang 1, Yunhua Fang 1, Peng Ji 2,*, Chaosheng Wang 1,* and Huaping Wang 1

1 State Key Laboratory for Modiﬁcation of Chemical Fibers and Polymer Materials, Key Laboratory of Textile Science & Technology (Ministry of Education), College of Materials Science and Engineering, Donghua University, Shanghai 201620, China; [email protected] (S.Z.); [email protected] (J.Z.); [email protected] (L.T.); [email protected] (J.H.); [email protected] (Y.F.); [email protected] (H.W.) 2 Co-innovation center for textile industry, Shanghai 201620, China * Correspondence: [email protected] (P.J.); [email protected] (C.W.)

Received: 16 May 2019; Accepted: 30 May 2019; Published: 3 June 2019

Abstract: In the polymerization of caprolactam, the stoichiometry of carboxyl groups and amine groups in the process of melt polycondensation needs to be balanced, which greatly limits the copolymerization modification of polyamide 6. In this paper, by combining the characteristics of the polyester polymerization process, a simple and flexible synthetic route is proposed. A polyamide 6-based polymer can be prepared by combining caprolactam hydrolysis polymerization with transesterification. First, a carboxyl-terminated polyamide 6-based prepolymer is obtained by a caprolactam hydrolysis polymerization process using a dibasic acid as a blocking agent. Subsequently, ethylene glycol is added for esterification to form a glycol-terminated polyamide 6-based prepolymer. Finally, a transesterification reaction is carried out to prepare a polyamide 6-based polymer. In this paper, a series of polyamide 6-based polymers with different molecular weight blocks were prepared by adjusting the amount and type of dibasic acid added, and the effects of different control methods on the structural properties of the final product are analyzed. The results showed that compared with the traditional polymerization method of polyamide 6, the novel synthetic strategy developed in this paper can flexibly design prepolymers with different molecular weights and end groups to meet different application requirements. In addition, the polyamide 6-based polymer maintains excellent mechanical and hygroscopic properties. Furthermore, the molecular weight increase in the polyamide 6 polymer is no longer dependent on the metering balance of the end groups, providing a new synthetic route for the copolymerization of polyamide 6 copolymer.

Keywords: synthetic methodology; polyamide 6; melt polymerization; polyesteramide; transesterification; polyamide 6 (PA6)-based polymer

1. Introduction Due to its high strength, good fatigue resistance, good water absorption, and chemical stability, polyamide 6 is one of the most widely-used engineering thermoplastics. It is also often referred to as nylon 6 or polycaprolactam in industrial production and applications. Polyamide 6 is mainly obtained by hydrolytic polymerization of caprolactam and anionic polymerization. Among these methods, caprolactam hydrolysis polymerization is the main approach for the mass production of polyamide 6 due to its polymerization stability and controllability. During the hydrolysis and polymerization of caprolactam, the molecular chain is grown by dehydration condensation of terminal carboxyl groups and amine groups. Therefore, it is necessary to maintain the stoichiometric balance of carboxyl groups

Polymers 2019, 11, 978; doi:10.3390/polym11060978 www.mdpi.com/journal/polymers Polymers 2019, 11, 978 2 of 17 and amine groups during the polycondensation process. In the copolymerization modification of polyamide 6, the modified polymer component added tends to affect the end group balance in the system, leading to a decrease in the degree of polymerization. Therefore, to achieve copolymerization modification of polyamide 6, the existing research mainly involves copolymerization modification by adding different chain extenders [1–5]. The addition of a chain extender for copolymerization modification mainly relies on an esterification reaction of a terminal carboxyl group on a polyamide 6 molecule with a glycol or a polyether ester or an amidation reaction with a diamine. Camille Bakkali-Hassani et al. [6] prepared a polyamide 6-based copolymer with a benzene structural unit by anionic polymerization using ethyl 4-aminobenzoate as a chain extender. Shumin Peng et al. [7] prepared a polyamide 6-based copolymer by anionic polymerization using ethylenediamine as a chain extender. Thibault Cousin et al. [8] prepared a bio-based polyamide copolymer by adding 2,5-furandicarboxylic acid during the hydrolysis reaction of caprolactam. However, regardless of which polymerization method is used, the addition of a dibasic acid or a diamine as a chain extender changes the reactivity between different substances in the reaction system, thereby destroying the stoichiometric balance between the terminal carboxyl group and the terminal amino group. Finally, the molecular chain is blocked, limiting the growth of molecular weight. Compared with the small-molecule chain extenders modified for polyamide 6, the use of polyether ester as a modifier has a milder polymerization effect and is also more suitable for the caprolactam hydrolysis polymerization system. Li-Kun Xiong et al. [9] prepared cationic dyeable polyamide 6 fibre by using 5-sulfoisophthalic acid monosodium salt as a blocking agent and melt polymerization with polyethylene glycol. Juncheng Huang and Ruchao Yuan et al. [10,11] prepared a polyamide 6 elastomer by using adipic acid and terephthalic acid as blocking agents and esterification of polytetrahydrofuran under melt conditions, respectively. However, in the process of caprolactam hydrolysis polymerization, approximately 8%–10% of oligomers are inevitably formed, also known as hot water extractables [12,13]. Usually, the polyamide 6 slices are extracted with boiling water or superheated water before the product leaves the factory; otherwise, the quality of the material is affected. In the preparation of polyamide 6 matrix materials in the laboratory, we usually use the same treatment methods to ensure the processing performance of the materials [7,14]. After modification copolymerization with a chain extender, an ester group and an ether group are usually introduced in a macromolecular segment, and the two chemical groups are easily broken during high-temperature hydrothermal treatment to cause a decrease in molecular weight [15,16]. This phenomenon has become another problem encountered when using chain extenders for polyamide 6 modification. Compared with the synthetic route for polyamide 6, the synthetic route and modification method for polyethylene terephthalate (PET) are more flexible. At present, terephthalic acid (PTA) and ethylene glycol (EG) are often used in industry to form 2-(hydroxyethyl terephthalate) (BHET) by an esterification reaction, and PET is prepared by transesterification of BHET under high-temperature and vacuum conditions. There is a difference between PET synthesis and polyamide 6 synthesis. The chain growth mode is based on transesterification. The ends of the ester compound are the same and can react with each other. At the same time, transesterification can also occur between the oligomer and the polymer, enhancing the stability of the reaction [17–20]. The modification of PET mainly involves adding a kind of glycol, an oligomer of a glycol, a dibasic acid, an aromatic dibasic acid, or a polyether ester to the system by an esterification reaction or a transesterification reaction. Compared with the copolymerization modification of polyamide 6, the copolymerization modification of PET has the characteristics of high modifier addition content and diversified addition types, and it is easier to design different molecular segments with different complex functions [21–24]. It is worth mentioning that copolymer-modified cationic dyeable PET (CDP/ECDP) has achieved industrial production, indicating that the synthetic route for PET has good stability in copolymerization modification [25]. Inspired by the synthesis route of PET, we propose a simple and flexible melt polymerization route for the preparation of PA6-based polymers. First, with different molar ratios of adipic acid, PTA and other dibasic acid blocking agents are added to the caprolactam hydrolysis polymerization to Polymers 2019, 11, x FOR PEER REVIEW 5 of 17

179 2.2.6. Differential scanning calorimetry (DSC), and thermogravimetric analysis, TGA

180 DSC measurements were performed on a TA-Q20 differential scanning calorimeter apparatus 181 calibratedPolymers 2019 with, 11, 978an indium standard. Samples of approximately 5 mg were heated and cooled at a3 rate of 17 182 of 20 °C min−1 under a nitrogen atmosphere. The first cooling scan and second heating scan were 183 selectedprepare for a polyamide studying 6-basedthe thermal prepolymer properties terminated of the samples. by a carboxyl TGA was group performed at both ends. on a German Subsequently, 209- 184 F1ethylene thermogravimetric glycol is added analyzer for esterificationunder a nitrogen to prepare atmosphere a polyamide at a heating 6-based rate of ethylene 20 °C/min glycol from ester. 30 185 toFinally, 600 °C. the transesterification reaction is carried out to obtain a long-chain polymer. In the molecular design process, different functionalization effects can be achieved by changing the blocking agent, 186 2.2.7.which Wide-angle provides a X-ray new synthetic diffraction, route WAXD for the copolymerization modification of the polyamide 6-based polymer. In addition, the polyamide 6-based polymer synthesized by this paper reduces the hot water 187 WAXD data were collected on a D/max-2550 VB X-ray diffractometer (Rigaku, Japan) equipped extractable content by 75%. The excellent mechanical properties and moisture absorption properties of 188 with a filtered Cu Kα radiation source (wavelength λ = 1.542 Å), and the diffraction patterns were the polymer can be maintained by direct processing without hydrothermal extraction, which is also 189 recorded in a range of 2θ = 5° to 60°. difficult to achieve by other synthetic routes for copolymerization modification of polyamide 6.

190 2.2.8.2. Experimental Scanning electron microscope, SEM

191 2.1. MaterialsSEM images were collected on by S-4800 field-emission scanning electron microscope produced 192 by Hitachi G. The samples were treated with gold spray. PA6 chips were purchased from CHTC Sinoﬁber Wuxi Co. Ltd. (Wuxi, China). Caprolactam was 193 2.2.9.purchased Tensile from properties BASF SE (Shanghai, China). Titanium glycolate (Ti(OCH2CH2O)2) was purchased from Xinfengming Group (Zhejiang, China). Ethylene glycol (EG), terephthalic acid (PTA), adipic 194 acid,Tensile deuterated property concentrated data were sulfuric acquired acid at (D room2SO4 temperature), and 98% concentrated using a Changchun sulfuric acidKexin (H WDW30202SO4) were 195 electronicpurchased universal from Sinopharm testing machine. Chemical The Reagent PEA chip Co.s Ltd. were (Shanghai processed branch, into a China).dumbbell-shaped All the materials spline 196 bywere injection analytically molding. pure. The tensile speed was 10 mm min-1, and the spline spacing was 50 ± 0.5 mm. 2.2. Sample Preparation 197 2.2.10. Chips saturated water absorption 2.2.1. Synthesis of Polyamide 6 (PA6)-based Polymer 198 The PA6 and PEA samples (20 ± 0.5 g) were dried in a drying oven for 12 h to a constant weight. 199 The chipsThe syntheticwere removed, route forimmersed the PA6-based in a beaker polymer containing is shown deionized in Scheme water,1. All and reactions soaked werefor 48 carried h. The 200 surfaceout in amoisture 5 L stainless was steeldried, reactor and the with sample a vacuum was weighed. tube, vacuum The weight pump increase and nitrogen is the bottle.chip saturated Speciﬁc 201 watersteps areabsorption as follows. rate.

202 Scheme 1. A simple and flexible synthetic strategy for preparing a polyamide 6 (PA6)-based polymer. 203 Scheme 1. A simple and flexible synthetic strategy for preparing a polyamide 6 (PA6)-based polymer. Step 1: Synthesis of PA6 with binary carboxyl end groups 204 3.• Results and Discussion In a typical polymerization procedure, a mixture of CPL (1695.0 g, 15 mol), adipic acid (146.14 g, 1 mol), and deionized water (44.0 g, 2.44 mol) was transferred into a vessel and stirred at 4 bar gas pressure and 250 ◦C for 3 h. The pressure was then slowly decreased to atmospheric pressure, and then Polymers 2019, 11, 978 4 of 17 nitrogen was used to remove excess water. The reaction was kept at atmospheric pressure for another 1 h, and the products were kept in the vessel for the next experiment. Step 2: Synthesis of PA6 with binary glycol ester end groups (prePA6) • In a typical polymerization procedure, EG (59.53 g, 0.96 mol) and Ti(OCH2CH2O)2 (0.136 g, effective content of 1%) were introduced into the reactor. The reaction was maintained at 2 bar gas pressure and 240 ◦C for 2.5 h. The pressure was then gradually reduced to atmospheric pressure for 1 h. The products were kept in the vessel for the next experiment. The prepolymer molecular weights were controlled at 2000, 3000, 4000, 5000, and 6000, and the samples were coded as prePA6-2K, prePA6-3k, prePA6-4k prePA6-5k, and prePA6-6k, respectively. The prepolymer with its molecular weight controlled at 4000 and terminated by PTA was coded as prePA6-p-4k. Step 3: Synthesis of the polyamide 6 (PA6)-based polymer (polyesteramide, PEA) • In a typical polymerization procedure, the pressure was then gradually reduced to less than 100 Pa, and the temperature was increased to 280 ◦C to remove the generated EG. The reaction was kept at this pressure until the specified power was reached. Then, the resulting products were condensed in cool water. Polymers obtained by polycondensation of prepolymers with different molecular weights were coded as PEA-2k, PEA-3k, PEA-4k, PEA-5k, PEA-6k, and PEA-p-4k.

2.2.2. Preparation of PA6/PEA Plastic Splines For preparation of dumbbell-shaped samples by hot pressing, the pelletized samples were injected using an injection molding machine to obtain the test specimens for each characterization technique. The temperature of the cylinders was 250 ◦C, and the mold temperature was 60 ◦C. The PEA chips are processed without hydrothermal extraction. PA6 chips are processed by hydrothermal extraction before the chips leave the factory.

2.3. Measurements

2.3.1. 1H-NMR and FTIR

1 H-NMR experiments were performed on an AVANCE-600 (Switzerland) spectrometer. D2SO4 was used as the solvent for prePA6 and PEA with a test temperature of 25 ◦C and the frequency of 600 MHz. FTIR spectroscopy is performed with a Nicolet 6700 Fourier transform infrared spectrometer 1 (MA, USA). The scanning range is 4000~600 cm− .

2.3.2. Relative Viscosity The prePA6 samples were tested at (25 0.1) C according to GB/T 12006.1-2006 using a Ubbelohde ± ◦ viscometer with a capillary diameter of 1.03 mm. The solvent was 98% concentrated sulfuric acid. The relative molecular mass (Mη) was calculated according to the following formula (1) and (2).

t ηr = (1) t0

Mη = (ηr 1) 11500 (2) − × In the formula: t0—the outﬂow time of the pure solvent, s; t—the outﬂow time of the concentrated sulfuric acid solution of PA6, s.

2.3.3. Oligomer Content The oligomer content of the prePA6 and PEA samples were tested by hydrothermal extraction. A sample of 20 g 0.5 g was dried in a vacuum oven for 8 h. The dried chips were weighed and ± Polymers 2019, 11, 978 5 of 17 placed in a constant temperature water bath of 97 C for 8 h. Then, the water was removed by a ≥ ◦ centrifuge, the sample was dried in a vacuum oven at 120 ◦C for 8 h, the chips were weighed, and the mass was reduced to the weight of the oligomers. The experiment was repeated 5 times to obtain an average value.

2.3.4. Detection of Amino and Carboxylic End Groups

The concentration of the amino groups [–NH2] and the carboxyl groups [–COOH] of the unesteriﬁed prePA6 was tested by titration. A sample of 0.25 0.01 g was dissolved in 15 mL of a 75:25 volume ± mixture of triﬂuoroethanol and ethanol. The concentration of the amino end group was then tested 1 using a standard hydrochloric acid solution (0.002 mol L− ), and the concentration of the carboxyl end 1 group was tested using a standard sodium hydroxide solution (0.02 mol L− ). The experiment was repeated 5 times to obtain an average value.

2.3.5. Gel Permeation Chromatography, GPC The PEA molecular weight (Mn and Mw) and polymer dispersity index (PDI) were tested by a GPC equipped with a refractive index detector and a PL gel column (5 µm mixed-C). 1 With 1,1,1,3,3,3-hexaﬂuoro-2-propanol as the eluent, the ﬂow rate was 1 mL min− . A calibration curve is generated using polystyrene as a standard.

2.3.6. Differential Scanning Calorimetry (DSC), and Thermogravimetric Analysis, TGA DSC measurements were performed on a TA-Q20 differential scanning calorimeter apparatus calibrated with an indium standard. Samples of approximately 5 mg were heated and cooled at a rate 1 of 20 ◦C min− under a nitrogen atmosphere. The first cooling scan and second heating scan were selected for studying the thermal properties of the samples. TGA was performed on a German 209-F1 thermogravimetric analyzer under a nitrogen atmosphere at a heating rate of 20 ◦C/min from 30 to 600 ◦C.

2.3.7. Wide-angle X-ray Diffraction, WAXD WAXD data were collected on a D/max-2550 VB X-ray diffractometer (Rigaku, Japan) equipped with a filtered Cu Kα radiation source (wavelength λ = 1.542 Å), and the diffraction patterns were recorded in a range of 2θ = 5◦ to 60◦.

2.3.8. Scanning Electron Microscope, SEM SEM images were collected on by S-4800 ﬁeld-emission scanning electron microscope produced by Hitachi G. The samples were treated with gold spray.

2.3.9. Tensile Properties Tensile property data were acquired at room temperature using a Changchun Kexin WDW3020 electronic universal testing machine. The PEA chips were processed into a dumbbell-shaped spline by injection molding. The tensile speed was 10 mm min 1, and the spline spacing was 50 0.5 mm. − ± 2.3.10. Chips Saturated Water Absorption The PA6 and PEA samples (20 0.5 g) were dried in a drying oven for 12 h to a constant weight. ± The chips were removed, immersed in a beaker containing deionized water, and soaked for 48 h. The surface moisture was dried, and the sample was weighed. The weight increase is the chip saturated water absorption rate. Polymers 2019, 11, 978 6 of 17

3. Results and Discussion

3.1. Preparation and Characterization of prePA6 As shown in Scheme1, in the first step of caprolactam hydrolysis polymerization, PA6 segments of different number average molecular weights were prepared by adding a dicarboxylic acid in different molar ratios relative to caprolactam. The dicarboxylic acid can be one or several aliphatic compounds or aromatic compounds. In this paper, adipic acid and PTA were used as representatives of these two kinds of compounds. The molar ratio of caprolactam to dicarboxylic acid [PCL]/[dicarboxylic acid] with different molecular chain lengths of PA6 and the content of terminal amino groups and terminal carboxyl groups are shown in Table1.

Table 1. End groups and [CPL]/[dicarboxylic acid] of the PA6 segment and molecular weight, esteriﬁcation yield and diﬀerential scanning calorimetry (DSC) results of the prePA6 samples.

d,e End Groups MnPrepa6 (mmol/kg) c ( 103 g mol 1) × − [CPL]/ Esteriﬁcation Sample [dicarboxylic T ( C) b T ( C) b Amino Carboxyl M d M e c ◦ m ◦ Yield (%) n η acid] a PrePA6-2k 16 149.8 202.3 93.2 1.76 1192 1.8 2.0 PrePA6-3k 24 159.9 208.9 93.7 2.08 800 2.8 2.9 PrePA6-4k 34 163.9 211.6 92.5 1.92 544 3.7 3.7 PrePA6-5k 43 166.9 214.2 91.6 1.68 448 4.8 4.7 PrePA6-6k 52 172.0 216.5 90.9 1.92 367 5.6 5.5 PrePA6-b-4k 34 158.3 198.1 93.1 2.04 552 3.7 3.7 a The feed ratio of CPL to adipic acid. b Directly measured by DSC. c PrePA6 without esteriﬁcation. d Determined 1 e from H-NMR results. Relative viscosity of prePA6 measured at 25 ◦C in 98% sulfuric acid solution using a Ubbelohde viscometer.

In the second step, ethylene glycol was added as a blocking agent to the prepolymer sample in an amount 2.2 times the carboxyl group content in the PA6 segment. The esterification rate of the polyamide 6 prepolymer is represented by the mass of water produced. The esterification yield, crystallization temperature (Tc), melting temperature (Tm), and relative molecular weight (Mη) of the prepared prePA6 samples are shown in Table1. Di fferential scanning calorimetry (DSC) results of the prePA6 samples are shown in Figure S1. The chemical structure of prePA6 was characterized by 1H-NMR. Figure1a shows the 1H-NMR spectrum of prePA6-2k with D2SO4 as the solvent. The chemical shifts at 3.88 ppm, 3.06 ppm, 2.12 ppm, 2.06 ppm, and 1.80 ppm were from the δ5, δ1, δ4, δ2, and δ3 hydrogen atoms of the polyamide 6 segment, respectively. The chemical shift at 3.12 ppm is the hydrogen on the carbon that is linked to the ester bond from δ1’. The chemical shifts at 2.93 ppm and 2.19 ppm are considered the characteristic hydrogen shifts of adipic acid at δ6, δ7. The chemical shifts at 4.37 ppm, 4.45 ppm, 4.69 ppm, and 4.79 ppm are considered the hydrogen atoms on the ethylene glycol segment. Since the carboxyl groups at both ends of the PA6 segment can be reacted with ethylene glycol, there are three different linkage modes: A-A, A-B, and B-B. Similar to the esterification process between PTA and EG, after the esterification reaction is completed, prePA6 with a polymerization degree of 1 to 4 is formed, and the characteristic peaks of hydrogen atoms correspond to δ8, δ9, δ10, and δ11, respectively. The molecular weight of the PA6 segment can be calculated from the ratio of the integral area of the hydrogen shift characteristic of ε-aminocaproic acid to that of the hydrogen shift of adipic acid [8]. The calculation results are shown in Table1.

M = (I /I 2) 113 + 146 (3) nprePA6 5 6 × × Polymers 2019, 11, x FOR PEER REVIEW 7 of 17

242 In this paper, the transesterification reaction of different molecular weight prePA6 samples is 243 used as the chain growth principle of melt polymerization. It is worth noting that under high vacuum 244 (below 100 Pa), not only the ethylene glycol produced by the transesterification reaction but also the 245 oligomers (mainly cyclic dimers) produced during the caprolactam hydrolysis polymerization are 246 removed. A certain preheating device should be to prevent condensation of the oligomer on the 247 vacuum pipe. Figure 1b shows the 1H-NMR spectrum of PEA-2k prepared by polycondensation with 248 aliphatic adipic acid as a blocking agent. The chemical shifts and the ratio of the characteristic peak 249 integral area are basically the same as those of prePA6, indicating that there is no structural change 250 in the PA6 segment before and after the transesterification, which makes the effect of amide- 251 transesterificationPolymers 2019, 11, 978 negligible. The hydrogen shift ratio of the ethylene glycol segment 7was of 17 252 significantly reduced, which shows that chain growth is achieved by transesterification.

253

Polymers 2019, 11, x FOR PEER REVIEW 8 of 17 254

255 256 Figure 1. 1H-NMR spectra of (a) prePA6-2k, (b) PEA-2k and (c) PEA-p-4k.

257 Figure 1. 1H-NMR spectra of (a) prePA6-2k, (b) PEA-2k and (c) PEA-p-4k.

258 259 Figure 1c shows the 1H-NMR spectrum of PEA-p-4k prepared by polycondensation of prePA6- 260 p-4k with aromatic terephthalic acid as a blocking agent. The chemical shift at 8.2~8.6 ppm is the 261 characteristic peak of the hydrogen atoms in the benzene ring [11]. Because of the high electron cloud 262 density of benzene, the chemical environment of the hydrogen on the adjacent ε-aminocaproic acid 263 is greatly influenced, leading to a high-field shift in this hydrogen. As the same as Figure 1a, the δ1' is 264 the hydrogen on the carbon that is linked to the ester bond. The integrated areas of the characteristic 265 hydrogen peaks at the same position were combined when calculating the number average molecular 266 weight. In addition, the hydrogen in PTA also exhibits different chemical shifts. This finding indicates 267 that in the first step of the synthesis, a small part of the dibasic acid is connected to the PA6 segment 268 at both ends; however, it can be clearly seen that the proportion of the link mode is much less and 269 that the effect on the overall molecular chain structure can be ignored. The use of two different dibasic 270 acid blocking agents indicates that this synthetic route can be carried out using aliphatic or aromatic 271 dibasic acids and that the molecular chain design is more flexible and controllable. 272 The chemical structure of PEA was further verified in FTIR spectroscopy (as shown in Figure 273 2a). According to the characteristics of the molecular segment design, the characteristic groups (N−H, 274 C=O) are analyzed. The absorption peak at 3304 cm-1 is attributed to the stretching vibration 275 absorption peak of the amide (N−H), the bending vibration peaks of the amide (N–H) appear at 1540 276 cm−1 and 680 cm-1, and the stretching vibration absorption peak of the amide bond (C=O) is at 1638 277 cm-1. These peaks are characteristic peaks of polyamide 6. The stretching vibration absorption peak 278 of the ester bond (C=O) is at 1737 cm-1. As the molecular weight of prePA6 increases, the interval 279 between ester bonds becomes larger, while the characteristic peaks become weaker [26].

Polymers 2019, 11, 978 8 of 17

In the formula (3), I5 and I6 are the characteristic peak integral areas corresponding to δ5 and δ6 in the ﬁgure, respectively. Number 133, 146 represent the relative molecular mass of caprolactam and adipic acid, respectively.

3.2. Preparation and Characterization of PEA In this paper, the transesterification reaction of different molecular weight prePA6 samples is used as the chain growth principle of melt polymerization. It is worth noting that under high vacuum (below 100 Pa), not only the ethylene glycol produced by the transesterification reaction but also the oligomers (mainly cyclic dimers) produced during the caprolactam hydrolysis polymerization are removed. A certain preheating device should be to prevent condensation of the oligomer on the vacuum pipe. Figure1b shows the 1H-NMR spectrum of PEA-2k prepared by polycondensation with aliphatic adipic acid as a blocking agent. The chemical shifts and the ratio of the characteristic peak integral area are basically the same as those of prePA6, indicating that there is no structural change in the PA6 segment before and after the transesterification, which makes the effect of amide-transesterification negligible. The hydrogen shift ratio of the ethylene glycol segment was significantly reduced, which shows that chain growth is achieved by transesterification. Figure1c shows the 1H-NMR spectrum of PEA-p-4k prepared by polycondensation of prePA6-p-4k with aromatic terephthalic acid as a blocking agent. The chemical shift at 8.2~8.6 ppm is the characteristic peak of the hydrogen atoms in the benzene ring [11]. Because of the high electron cloud density of benzene, the chemical environment of the hydrogen on the adjacent ε-aminocaproic acid is greatly influenced, leading to a high-field shift in this hydrogen. As the same as Figure1a, the δ1’ is the hydrogen on the carbon that is linked to the ester bond. The integrated areas of the characteristic hydrogen peaks at the same position were combined when calculating the number average molecular weight. In addition, the hydrogen in PTA also exhibits different chemical shifts. This finding indicates that in the first step of the synthesis, a small part of the dibasic acid is connected to the PA6 segment at both ends; however, it can be clearly seen that the proportion of the link mode is much less and that the effect on the overall molecular chain structure can be ignored. The use of two different dibasic acid blocking agents indicates that this synthetic route can be carried out using aliphatic or aromatic dibasic acids and that the molecular chain design is more flexible and controllable. The chemical structure of PEA was further verified in FTIR spectroscopy (as shown in Figure2a). According to the characteristics of the molecular segment design, the characteristic groups (N H, C=O) 1 − are analyzed. The absorption peak at 3304 cm− is attributed to the stretching vibration absorption 1 peak of the amide (N H), the bending vibration peaks of the amide (N–H) appear at 1540 cm− and 1 − 1 680 cm− , and the stretching vibration absorption peak of the amide bond (C=O) is at 1638 cm− . These peaks are characteristic peaks of polyamide 6. The stretching vibration absorption peak of the 1 ester bond (C=O) is at 1737 cm− . As the molecular weight of prePA6 increases, the interval between ester bonds becomes larger, while the characteristic peaks become weaker [26]. Figure2b shows the spectrum of PEA-p-4k. The C H bending vibration absorption peak of the − 1 1,4-disubstituted benzene structure can be found at 860 cm− , which indicates that the aromatic dibasic acid can be introduced into the macromolecular chain by this synthetic route. In the preparation process, the number average molecular weights are controlled between 18,000 and 21,000 by using constant power discharge, and the PDI is between 2 and 2.3. In addition, the oligomer content of PEA obtained by this synthetic route is approximately 2 wt%, which is far below that of PA6 obtained by the hydrolytic polymerization of caprolactam. The test results are shown in Table2 and Figure S2. Polymers 2019, 11, 978 9 of 17 Polymers 2019, 11, x FOR PEER REVIEW 9 of 17

280

281 FigureFigure 2. FTIR2. FTIR spectra spectra of (aof) polyesteramide(a) polyesteramide (PEA) (PEA) polymerized polymerize byd di byﬀerent different molecular molecular weight weight prePA6 282 samplesprePA6 and samples (b) PEA and polymerized (b) PEA polymerized by prePA6 by containingprePA6 contai diﬀningerent different blocking bl agents.ocking agents.

283 Figure 2b shows theTable spectrum 2. Molecular of PEA-p-4k. weight andThe oligomerC−H bending content vibration of PEA. absorption peak of the 284 1,4-disubstituted benzene structure can be found at 860 cm−1, which indicates that the aromatic a 3 1 a Sample Mn ( 10 g mol ) PDI (Polymer Dispersity Index) Oligomer Content (wt%) 285 dibasic acid can be introduced× −into the macromolecular chain by this synthetic route. 286 PEA-2kIn the preparation 1.96 process, the number average molecular 2.08 weights are controlled 2.21between 18,000 287 andPEA-3k 21,000 by using constant 1.89 power discharge, and 2.12 the PDI is between 2 and 2.3. In 2.15 addition, the 288 oligomerPEA-4k content of PEA 2.04 obtained by this synthetic route 2.18 is approximately 2 wt%, which 2.08 is far below PEA-5k 2.03 2.16 1,94 289 that of PA6 obtained by the hydrolytic polymerization of caprolactam. The test results are shown in PEA-6k 2.06 2.21 1.74 290 TablePEA-p-4k 2 and Figure S2. 1.87 2.08 1.92 PA6 1.84 1.80 8.89 291 Table 2. Molecular weight and oligomer content of PEA. a Determined from Gel Permeation Chromatography (GPC) results. PDI (Polymer Dispersity Index) Sample Mna (×103 g mol−1) Oligomer content (wt%) 3.3. Thermal Properties of PEA a FigurePEA-2k3 shows the DSC1.96 test results of PEA, including 2.08 the first cooling curves (a) 2.21 and the second PEA-3k 1.89 2.12 2.15 heating curves (b). Other thermal performance values are shown in Table3. Figure3a shows that PEA-4k 2.04 2.18 2.08 as the molecular weight of prePA6 increases, both the crystallization peak and the melting peak PEA-5k 2.03 2.16 1,94 move towards higher temperatures. When the molecular weight of prePA6 reaches 6000, both the PEA-6k 2.06 2.21 1.74 melting temperature and the crystallization temperature are close to that of polyamide 6. The thermal PEA-p-4k 1.87 2.08 1.92 propertiesPA6 of polyamide 61.84 are mainly affected by the molar1.80 ratio of the amide group 8.89 to the methylene group in the main chain [27]. The change in the melting point of PEA is due to the presence of an ester 292 a Determined from Gel Permeation Chromatography (GPC) results. bond in the main chain, which destroys the regularity of the local molecular chain and further reduces 293 the3.3. hydrogen Thermal Properties bond density. of PEA This effect leads to a decrease in the force between macromolecules and reduced crystallization. 294 Figure 3 shows the DSC test results of PEA, including the first cooling curves (a) and the second 295 heating curves (b). Other thermal performance values are shown in Table 3. Figure 3a shows that as Table 3. Thermal performance parameters of PEA. 296 the molecular weight of prePA6 increases, both the crystallization peak and the melting peak move a a a b b 297 towards Samplehigher temperatures.Tc (◦C) When theTm molecular(◦C) weXcight(%) of prePA6T5% reaches(◦C) 6000,T bothmax ( ◦theC) melting 298 temperaturePEA-2k and the crystallization 149.7 temperature 199.7 are 15.9close to that 367.4of polyamide 6. 449.2 The thermal 299 propertiesPEA-3k of polyamide 6 155.3are mainly affected 205.3 by the molar 17.4 ratio of the 374.5 amide group to 450.5the methylene 300 group inPEA-4k the main chain [27]. 160.4 The change 210.6 in the melting 18.4 point of PEA 376 is due to the presence 452.0 of an 301 ester bondPEA-5k in the main chain, 161.7 which destroys 212,1 the regularity 19.0 of the local 375.6 molecular chain 454.7 and further 302 reduces PEA-6kthe hydrogen bond 166.8 density. 214.0This effect leads 19.3 to a decrease 380.1 in the force 456.7 between PEA-p-4 151.3 206.1 11.1 376.1 452.1 303 macromolecules and reduced crystallization. PA6 168.2 222.1 23.4 402.5 460.8 a Directly measured by DSC. b Directly measured by TGA.

Polymers 2019, 11, 978 10 of 17 Polymers 2019,, 11,, xx FORFOR PEERPEER REVIEWREVIEW 10 of 17

304 Figure 3. DSC thermographs of cooling (a) PEA polymerized by different molecular weight prePA6 305 Figure 3. DSC thermographs of cooling (a)) PEAPEA polymerizedpolymerized byby differentdifferent molecularmolecular weightweight prePA6prePA6 samples and (c) PEA polymerized by prePA6 containing different blocking agents. DSC thermographs 306 samples and (c)) PEAPEA polymerizedpolymerized byby prePA6prePA6 containicontaining different blocking agents. DSC of heating (b,d). 307 thermographsthermographs ofof heatingheating ((b),), ((d).). There are two kinds of PA6 molecules: cis-aligned and reverse-aligned. The macromolecules only 308 There are two kinds of PA6 molecules: cis-aligned and reverse-aligned. The macromolecules 308 completelyThere combineare two throughkinds of hydrogenPA6 molecules: bonds incis-alig the reversened and alignment. reverse-aligned. The three The diff erentmacromolecules link modes 309 only completely combine through hydrogen bonds in the reverse alignment. The three different link 309 ofonly the completely macromolecular combine chains through are arranged hydrogen in reverse.bonds in The theinfluence reverse alignment. of hydrogen The bonding three different between link the 310 modes of the macromolecular chains are arranged in reverse. The influence of hydrogen bonding 310 molecularmodes of the chains macromolecular is discussed (as chains shown are in arranged Figure4). in The reverse. distribution The influence of C =O of and hydrogen N H in thebonding PEA 311 between the molecular chains is discussed (as shown in Figure 4). The distribution of −C=O and N−H 311 molecularbetween the chain molecular is no longer chains kept is discussed the same (as as thatshow inn the in Figure polyamide 4). The 6 segment distribution after of passing C=O and through N−H 312 in the PEA molecular chain is no longer kept the same as that in the polyamide 6 segment after 312 thein the ester PEA bond molecular position, chain and the is no hydrogen longer bondkept the structure same isas destroyed.that in the A polyamide molecular 6 chain segment that doesafter 313 passing through the ester bond position, and the hydrogen bond structure is destroyed. A molecular 313 notpassing form through a hydrogen the ester bond bond structure position, (as shown and the in hy thedrogen blue frame bond of structure Figure4) is may destroyed. be undergo A molecular folding 314 chain that does not form a hydrogen bond structure (as shown in the blue frame of Figure 4) may be 314 orchain be combinedthat does not with form other a hydrogen molecular bond chains structure to form (as a hydrogen shown in bondthe blue structure. frame of This Figure phenomenon 4) may be 315 undergo folding or be combined with other molecular chains to form a hydrogen bond structure. 315 willundergo increase folding the entanglement or be combined of the with molecular other molecular chain and chains reduce to the form regularity a hydrogen of the molecularbond structure. chain. 316 This phenomenon will increase the entanglement of the molecular chain and reduce the regularity of 316 AsThis the phenomenon molecular weight will increase of prePA6 the graduallyentanglement increases, of the the molecular distribution chain of and ester reduce bonds the in regularity the segment of 317 the molecular chain. As the molecular weight of prePA6 gradually increases, the distribution of ester 317 decreases,the molecular reducing chain. theAs the impact molecular on the weight regularity of prePA6 of the moleculargradually increases, chain and the enhancing distribution the of degree ester 318 bonds in the segment decreases, reducing the impact on the regularity of the molecular chain and 318 ofbonds crystallization. in the segment decreases, reducing the impact on the regularity of the molecular chain and 319 enhancing the degree of crystallization.

320

321 FigureFigure 4.4. AA schematic schematic diagram diagram of of the the hydrogen hydrogen bond bond interaction interactioninteraction model modelmodel between betweenbetween PEA molecular PEAPEA molecularmolecular chains 322 inchains three in di threeﬀerent different linking linking modes: modes: A-A, A-B, A-A, and A-B, B-B and (see B-B the (see text the for text details). for details).

323 TheThe PEAPEA thermalthermal performanceperformance testtest resultsresults followingfollowinging polycondensationpolycondensationpolycondensation withwith didifferentdifferentﬀerent blockingblockingblocking 324 agentsagents andand thethe samesame molecularmolecular weightweight ofof prePA6prePA6 areare shownshown inin FigureFigure3 3c,d.c,d. When When PTA PTA is is used used as as the the 325 blocking agent, the melting temperature, crystallization temperature, and crystallinity of PEA are 326 significantly reduced. Due to the introduction of PTA, the phenyl group from PTA enters the

Polymers 2019, 11, 978 11 of 17

blocking agent, the melting temperature, crystallization temperature, and crystallinity of PEA are significantly reduced. Due to the introduction of PTA, the phenyl group from PTA enters the molecular chain.Polymers The 2019 phenyl, 11, x FOR group PEER has REVIEW high steric hindrance, increasing the distance and reducing the hydrogen11 of 17 bonding between the molecular chains [28]. 327 molecularThe TGA chain. test resultsThe phenyl of PEA group are shownhas high in steric Figure hindrance,5. Figure5 aincreasing shows that the the distance higher and the reducing molecular 328 the hydrogen bonding between the molecular chains [28]. weight of prePA6 is, the higher the thermal stability. The initial decomposition temperature (T5%) of 329 The TGA test results of PEA are shown in Figure 5. Figure 5a shows that the higher the molecular PEA-2k is 367.4 ◦C, which is still much higher than the processing temperatures of injection molding 330 weight of prePA6 is, the higher the thermal stability. The initial decomposition temperature (T5%) of and melt-spinning. Therefore, these PEA materials have good thermal stability during processing. 331 PEA-2k is 367.4 °C, which is still much higher than the processing temperatures of injection molding Figure5b shows that the thermal decomposition temperatures of PEA with di fferent dicarboxylic 332 and melt-spinning. Therefore, these PEA materials have good thermal stability during processing. 333 acidsFigure as the5b shows blocking that agent the thermal were almost decomposition identical. temperatures The T5% of of PEA-4k PEA with and different PEA-p-4k dicarboxylic are 376 ◦ C and 376.1 C, respectively. Generally, thermal stability may be improved by the introduction of a 334 acids as ◦the blocking agent were almost identical. The T5% of PEA-4k and PEA-p-4k are 376 °C and 335 phenylated376.1 °C, structurerespectively. into Generally, the polymeric thermal backbone. stability Comparing may be improved the thermal by stabilitythe introduction of PEA-4k of anda 336 PEA-p-4k,phenylated their structureT5% are into not significantlythe polymeric di backbone.fferent. This Comparing phenomenon the thermal is probably stability because of PEA-4k PTA is addedand 337 asPEA-p-4k, a capping agenttheir T with5% are a not low significantly content. In addition,different. comparingThis phenomenon the thermal is probably performances because ofPTA prePA6 is 338 andadded PEA atas thea capping end of theagent synthesis with a low process content. shows In thataddition, they werecomparing basically the similar.thermal Theperformances prePA6-4k of and 339 prePA6-p-4kprePA6 and have PEA almost at the theend same of the molecular synthesis chain proce structure,ss shows that so PEA-4k they were and basically PEA-p-4k similar. have almostThe 340 theprePA6-4k same thermal and prePA6-p-4k stability. We have can speculatealmost the on same the molecular thermal properties chain structure, of PEA so byPEA-4k testing and the PEA-p- thermal 341 properties4k have ofalmost the designed the same prePA6thermal molecules. stability. We can speculate on the thermal properties of PEA by 342 testing the thermal properties of the designed prePA6 molecules.

343 344 FigureFigure 5. 5.TGA TGA thermographs thermographs of of (a(a)) PEAPEA polymerizedpolymerized by by different diﬀerent molecular molecular weight weight prePA6 prePA6 samples samples 345 andand (b )(b PEA) PEA polymerized polymerized by by prePA6 prePA6 containing containing didifferentﬀerent blockingblocking agents.

346 3.4. Crystallization of PEA Table 3. Thermal performance parameters of PEA. α PolyamideSample 6 is a polycrystallineTc(°C) a T polymerm(°C) a with twoXc (%) diff aerent crystalT5%(°C) structures: b T themax(°C)type b and the γ type.PEA-2k Two diffraction149.7 characteristic 199.7 peaks appear15.9 in the α crystal367.4 form, and the (200)449.2 and (002) crystalPEA-3k form di ffraction155.3 peaks of α-PA6205.3 are located at 20.117.4◦ and 23.8◦, respectively.374.5 The characteristic450.5 γ-PA6 diPEA-4kffraction peak is160.4 located at 21.3210.6◦. The single-di18.4ffraction phenomenon376 of PA6 is452.0 attributed to the characteristicPEA-5k diffraction161.7 peaks of (200)212,1 and (001) superimposed19.0 together375.6 [29]. Figure454.76a shows that thePEA-6k characteristic peak166.8 of PEA is exactly214.0 the same as19.3 that of PA6, indicating380.1 that the456.7 crystallinity in PEAPEA-p-4 is completely determined151.3 by the206.1 polyamide 6 segment.11.1 As the376.1 molecular weight452.1 of prePA6 increases,PA6 the scattering168.2 sharpness of the222.1 characteristic peak23.4 of PEA becomes402.5 prominent.460.8 The crystal 347 form gradually changes froma Directly the measuredγ crystal by form DSC. to b theDirectlyα crystal measured form, by TGA. which is a more stable crystal form. Since the crystallinity of PA6 is mainly dependent on the molar ratio of the amino group to the 348 methylene3.4. Crystallization group, the of PEA ester bond in the PEA molecular chain destroys the ordered molar ratio and 349 eventuallyPolyamide leads to 6 ais decrease a polycrystalline in crystallinity. polymer Figure with6 btwo shows different that thecrystal crystal structures: form of the PEA α terminatedtype and 350 withthe PTA γ type. is mainly Two diffraction the γ crystal characteristic form. Compared peaks appear with the in γthecrystal α crystal form, form, the andα crystal the (200) lattice and structure (002) 351 is denser.crystal form After diffraction the introduction peaks of ofα-PA6 the phenyl are located group at 20.1° into theand molecular 23.8°, respectively. chain, the The molecular characteristic chains 352 becomeγ-PA6 separated diffraction from peak eachis located other, at making 21.3°. The it moresingle-diffraction difficult to arrangephenomenon into theof PA6 dense is attributedα-type lattice to 353 the characteristic diffraction peaks of (200) and (001) superimposed together [29]. Figure 6a shows 354 that the characteristic peak of PEA is exactly the same as that of PA6, indicating that the crystallinity 355 in PEA is completely determined by the polyamide 6 segment. As the molecular weight of prePA6

Polymers 2019, 11, x FOR PEER REVIEW 12 of 17

356 increases, the scattering sharpness of the characteristic peak of PEA becomes prominent. The crystal 357 form gradually changes from the γ crystal form to the α crystal form, which is a more stable crystal 358 form. Since the crystallinity of PA6 is mainly dependent on the molar ratio of the amino group to the 359 methylene group, the ester bond in the PEA molecular chain destroys the ordered molar ratio and 360 eventually leads to a decrease in crystallinity. Figure 6b shows that the crystal form of PEA 361Polymers terminated2019, 11, 978 with PTA is mainly the γ crystal form. Compared with the γ crystal form, the α crystal12 of 17 362 lattice structure is denser. After the introduction of the phenyl group into the molecular chain, the 363 molecular chains become separated from each other, making it more difficult to arrange into the 364of thedense polyamide α-type lattice 6 chain. of the The polyamide crystalline 6 chain. form The ofthe crystalline polyamide form 6of segment the polyamide is more 6 segment susceptible is to 365transitioning more susceptible to the metastable to transitioningγ crystalline to the metastable form [30 γ]. crystalline form [30].

366 367 FigureFigure 6. Wide-angle 6. Wide-angle X-ray X-ray diffraction diffraction (WAXD) (WAXD) profiles profiles of (a )of PEA (a) polymerizedPEA polymerized by di ffbyerent different molecular 368 weightmolecular prePA6 weight samples prePA6 and (samplesb) PEA and polymerized (b) PEA polymerized by prePA6 by containing prePA6 containing different different blocking blocking agents. 369 agents. 3.5. Tensile Properties of PEA 370 3.5. Tensile Properties of PEA The mechanical properties of the PEA splines are shown in Figure7. From Figure7a,d, as the 371molecular The weight mechanical of prePA6 properties increases, of the the PEA molecular splines chainsare shown become in Figure ordered, 7. From and Figure more energy7a and isd, neededas 372 to overcomethe molecular intermolecular weight of prePA6 forces increases, to slip the the crystal molecular face chains [31]. become Therefore, ordered, as the and molecular more energy weight is of 373 needed to overcome intermolecular forces to slip the crystal face [31]. Therefore, as the molecular prePA6 increases, the breaking strength and elongation at break gradually improve. The mechanical 374 weight of prePA6 increases, the breaking strength and elongation at break gradually improve. The 375properties mechanical of PEA-6k properties are closeof PEA-6k to those are close of PA6. to those From of PA6. Figure From7b,d, Figure the PEA7b and breaking d, the PEA strength breaking ( σ b) is 376higher strength than the (σb) upper is higher yield than point the upper (σsu). yield The PEApoint molecular(σsu). The PEA chains molecular are oriented chains are in theoriented direction in the of the 377force, direction and the of molecular the force, chainand the arrangement molecular chain is more arrangement regular, is thereby more regular, increasing thereby the increasing crystallinity the and 378improvingPolymers crystallinity 2019 the, 11, mechanicalx andFOR improvingPEER REVIEW strength. the mechanical strength. 13 of 17

379 Figure 7. Mechanical properties of PEA: (a) breaking elongation, (b) upper yield point, (c) lower 380 Figure 7. Mechanical properties of PEA: (a) breaking elongation, (b) upper yield point, (c) lower yield yield point, (d) breaking strength, (e) stress–strain curves of PEA polymerized by diﬀerent molecular 381 point, (d) breaking strength, (e) stress–strain curves of PEA polymerized by different molecular weight prePA6 samples, and (f) stress–strain curves of PEA polymerized by prePA6 containing diﬀerent 382 weight prePA6 samples, and (f) stress–strain curves of PEA polymerized by prePA6 containing blocking agents. 383 different blocking agents.

384 An interesting phenomenon was found in the text. Figure 6c shows the lower yield point (σsl) of 385 the polymer, which is the stress during the yielding process. The lower yield point of PEA-4k is 386 basically the same as that of PA6. As the molecular weight of prePA6 increases, the resistance of the 387 polymer in the yield process becomes higher than that of traditional PA6. The main reason for this is 388 that 8~10 wt % of oligomers are formed during the hydrolysis of caprolactam. For better molding, the 389 PA6 slices need hydrothermal extraction to remove the oligomers, while the PEA samples were 390 processed without hydrothermal extraction. Approximately 2 wt% of the oligomers contained in PEA 391 are mainly composed of cyclic dimers [12]. In PEA, the cyclic dimer is present in the matrix in the 392 form of a rod [32]. In the uniaxial stretching process, the rod crystal is parallel to the stretching 393 direction. It is easy to generate hydrogen bonds with the macromolecular chain, which compensates 394 for a portion of the strength lost due to the decrease in molecular chain regularity (the schematic 395 model is shown in Figure 8). The interaction between the oligomer and the macromolecular chain 396 increases the entanglement between the molecular chains, making mutual movement more difficult. 397 It is manifested in PEA showing an increase in resistance during plastic yielding [33,34]. At the same 398 time, the presence of oligomers also reduces the viscosity during melting and improves the fluidity 399 [35].

400 401 Figure 8. Schematic diagram of the hydrogen bond interaction model between the oligomer and the 402 PEA molecular chain after orientation-induced crystallization (see the text for details).

Polymers 2019, 11, x FOR PEER REVIEW 13 of 17

379

380 Figure 7. Mechanical properties of PEA: (a) breaking elongation, (b) upper yield point, (c) lower yield 381 point, (d) breaking strength, (e) stress–strain curves of PEA polymerized by different molecular 382 weight prePA6 samples, and (f) stress–strain curves of PEA polymerized by prePA6 containing 383 Polymersdifferent2019, 11 ,blocking 978 agents. 13 of 17

384 An interesting phenomenon was found in the text. Figure 6c shows the lower yield point (σsl) of 385 the polymer,An interesting which phenomenon is the stress wasduring found the inyieldi theng text. process. Figure 6Thec shows lower the yield lower point yield of pointPEA-4k ( σ slis) 386 ofbasically the polymer, the same which as that is the of stressPA6. As during the molecular the yielding weight process. of prePA6 The lower increases, yield the point resistance of PEA-4k of the is 387 basicallypolymer thein the same yield as process that of PA6. becomes As the higher molecular than that weight of traditional of prePA6 PA6. increases, The main the resistancereason for ofthis the is 388 polymerthat 8~10 in wt the % yieldof oligomers process are becomes formed higher during than the that hydrolysis of traditional of caprolactam. PA6. The For main better reason molding, for this the is 389 thatPA6 8~10slices wt need % of hydrothermal oligomers are extraction formed during to remo theve hydrolysis the oligomers, of caprolactam. while the ForPEA better samples molding, were 390 theprocessed PA6 slices without need hydrothermal hydrothermal extraction. extraction Approx to removeimately the oligomers,2 wt% of the while oligomers the PEA contained samples in were PEA 391 processedare mainly without composed hydrothermal of cyclic dimers extraction. [12]. In Approximately PEA, the cyclic 2 dimer wt% ofis thepresent oligomers in the containedmatrix in the in 392 PEAform are of mainlya rod [32]. composed In the ofuniaxial cyclic dimersstretching [12 ].proc In PEA,ess, the the rod cyclic crystal dimer is isparallel present to in the the stretching matrix in 393 thedirection. form of It ais rod easy [32 to]. generate In the uniaxial hydrogen stretching bonds with process, the macromolecular the rod crystal is chain, parallel which to the compensates stretching 394 direction.for a portion It is easyof the to strength generate hydrogenlost due to bonds the decre withase the macromolecularin molecular chain chain, regularity which compensates (the schematic for 395 amodel portion is ofshown the strength in Figure lost 8). due The to interaction the decrease be intween molecular the oligomer chain regularity and the (the macromolecular schematic model chain is 396 shownincreases in Figurethe entanglement8). The interaction between between the molecular the oligomer chains, and making the macromolecular mutual movement chain more increases difficult. the 397 entanglementIt is manifested between in PEA the showing molecular an increase chains, making in resistance mutual during movement plastic more yielding diﬃcult. [33,34]. It is At manifested the same 398 intime, PEA the showing presence an increaseof oligomers in resistance also reduces during the plastic viscosity yielding during [33, 34melting]. At the and same improves time, the the presence fluidity 399 of[35]. oligomers also reduces the viscosity during melting and improves the ﬂuidity [35].

400

401 FigureFigure 8.8. SchematicSchematic diagramdiagram ofof thethe hydrogenhydrogen bondbond interactioninteraction modelmodel betweenbetween thethe oligomeroligomer andand thethe 402 PEAPEA molecularmolecular chainchain afterafter orientation-inducedorientation-induced crystallizationcrystallization (see(see thethe text text for for details). details).

3.6. Surface Topography of PEA We unexpectedly discovered the presence of oligomers when SEM was used to observe the cross-section of the material. The PA6 slices before and after hydrothermal extraction and PEA-4k slices without hydrothermal extraction are melted and extruded into round filaments. After liquid nitrogen extraction, the cross-sectional morphology of the samples is observed by SEM, as shown in Figure9a–c. Some oligomers can be observed in Figure9a, which are similar in morphology to the rod-shaped cyclic dimers previously reported in the literature [32]. Most of their directions are the same as the forming direction, and a large amount of oligomers are observed in the cross-section, indicating that too many oligomers will be concentrated in a part of the matrix [32]. This occurrence can lead to stress concentrations that cause the material to appear brittle. There are obvious voids in the cross-section, indicating that the caprolactam will overflow from the matrix during processing, leaving voids to cause defects in the material. This result also indicates the necessity of hydrothermal extraction of the PA6-based polymer before processing. In Figure9b, it can be seen that PA6 after hydrothermal extraction has a flat profile and no obvious holes or other oligomers. In the PEA-4k sample shown in Figure9c, only one or several white substances are distributed in the cross-section, similar to the rod-shaped oligomers in Figure9a. This result proves our speculation on the enhancement in material resistance. Polymers 2019, 11, x FOR PEER REVIEW 14 of 17

403 3.6. Surface Topography of PEA 404 We unexpectedly discovered the presence of oligomers when SEM was used to observe the 405 cross-section of the material. The PA6 slices before and after hydrothermal extraction and PEA-4k 406 slices without hydrothermal extraction are melted and extruded into round filaments. After liquid

407 Polymersnitrogen2019 extraction,, 11, 978 the cross-sectional morphology of the samples is observed by SEM, as shown14 of 17in 408 Figure 9a–c.

409

410 FigureFigure 9.9. SEMSEM of of cross-sectional cross-sectional appearance: appearance: (a ()a PA6) PA6 without without hydrothermal hydrothermal extraction, extraction, (b) (bPA6,) PA6, (c) 411 (PEA-4kc) PEA-4k without without hydrot hydrothermalhermal extraction. extraction. 3.7. Water Absorption Performance of PEA 412 Some oligomers can be observed in Figure 9a, which are similar in morphology to the rod- 413 shapedThe cyclic saturated dimers water previously absorption reported of PEA in slices the liter is shownature [32]. in Figure Most of10 .their The directions ﬁgure shows are thatthe same PEA 414 isas more the forming hydrophilic direction, than and PA6. a large The amount reason isof thatoligomers water are in observed the free state in the is cross-section, mainly present indicating in the 415 amorphousthat too many region oligomers of the material. will be concentrated The sample within a pa strongrt of crystallizationthe matrix [32]. ability This occurrence does not have can enough lead to 416 amorphousstress concentrations areas to retain that cause free water. the material When phenyl to appear groups brittle. are There introduced are obvious into the voids molecular in the chain,cross- 417 Polymersthesection, crystallinity 2019 indicating, 11, x FOR of PEA PEERthat is theREVIEW drastically caprolactam lowered, will causingoverflow a largefrom increasethe matrix in waterduring absorption. processing, 15leaving of 17 418 voids to cause defects in the material. This result also indicates the necessity of hydrothermal 419 extraction of the PA6-based polymer before processing. In Figure 9b, it can be seen that PA6 after 420 hydrothermal extraction has a flat profile and no obvious holes or other oligomers. In the PEA-4k 421 sample shown in Figure 9c, only one or several white substances are distributed in the cross-section, 422 similar to the rod-shaped oligomers in Figure 9a. This result proves our speculation on the 423 enhancement in material resistance.

424 3.7. Water Absorption Performance of PEA 425 The saturated water absorption of PEA slices is shown in Figure 10. The figure shows that PEA 426 is more hydrophilic than PA6. The reason is that water in the free state is mainly present in the 427 amorphous region of the material. The sample with strong crystallization ability does not have 428 enough amorphous areas to retain free water. When phenyl groups are introduced into the molecular 429 chain, the crystallinity of PEA is drastically lowered, causing a large increase in water absorption. 430 431 Figure 10. Slice saturated water absorption absorption. 4. Conclusions 432 4. Conclusions In this paper, a simple and flexible polymerization route was proposed. The combination of 433 In this paper, a simple and flexible polymerization route was proposed. The combination of caprolactam hydrolysis polymerization, an esterification reaction, and a transesterification reaction was 434 caprolactam hydrolysis polymerization, an esterification reaction, and a transesterification reaction carried out to prepare a polyamide 6-based copolymer with a molecular structure similar to polyamide 435 was carried out to prepare a polyamide 6-based copolymer with a molecular structure similar to 6. PA6 segments of different molecular weights were prepared by changing the type and content of 436 polyamide 6. PA6 segments of different molecular weights were prepared by changing the type and dicarboxylic acid used as a blocking agent. Furthermore, PEA was prepared by esterification and 437 content of dicarboxylic acid used as a blocking agent. Furthermore, PEA was prepared by 438 esterification and transesterification. PEA prepared by this synthetic route not only retains the 439 excellent properties of PA6 but also increases the possibilities of molecular design during the 440 polymerization process. At the same time, the limitation of the measurement balance of the chemical 441 groups at both ends in the copolymerization modification of polyamide 6 is broken. By 1H-NMR and 442 FTIR spectroscopy, it can be observed that adjusting the molecular weight of PA6 is effective in 443 controlling the molecular structure of PEA. In addition, by using PTA as a blocking agent, it was 444 verified that both aliphatic and aromatic dicarboxylic acid can be applied to this polymerization 445 method, which broadens the selectivity of the synthetic raw materials. The effects of prePA6 with 446 different molecular weights and different blocking agents on the molecular structure and material 447 properties of PEA were studied. 448 The thermodynamic properties, crystal structure, mechanical properties, and water absorption 449 performance of PEA obtained by polycondensation of different prePA6 samples were compared with 450 those of polyamide 6. The results show that as the molecular weight of the PA6 segment continues to 451 increase, the molecular chain regularity increases. When the molecular weight of the PA6 segment 452 reaches 6000, its thermodynamic properties, crystal structure and crystallinity are basically similar to 453 those of PA6. PEA maintains good mechanical properties and water absorption after processing. It is 454 a thermoplastic semi-crystalline polymer with good performance. Moreover, PEA has a low content 455 of oligomers, so it can be processed directly without hydrothermal extraction, which reduces the 456 energy consumption during processing of the PA6-based polymers. It was unexpectedly discovered 457 in this study that a small amount of oligomers also played a certain role in plasticizing, which 458 enhanced the mechanical properties of PEA. This synthetic route can be performed with conventional 459 polyester equipment and has certain potential for industrialization.

460 Supplementary Materials: The following are available online at www.mdpi.com/link.

461 Author Contributions: Conceptualization, L.T. and H.W.; methodology, C.W. and S.Z.; validation, J.Z., Y.F., and 462 J.H.; Formal analysis, S.Z.; Investigation, S.Z.; Writing—Original Draft preparation, S.Z.; Writing—Review and 463 Editing, S.Z. and P.J.; Supervision, C.W., P.J.; Project administration, H.W.; Funding acquisition, C.W.

464 Acknowledgments: This work was financially supported by the National Key Research and Development 465 Program of China (2016YFB0302700), and National Natural Science Foundation of China (51703023).

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

Polymers 2019, 11, 978 15 of 17 transesterification. PEA prepared by this synthetic route not only retains the excellent properties of PA6 but also increases the possibilities of molecular design during the polymerization process. At the same time, the limitation of the measurement balance of the chemical groups at both ends in the copolymerization modification of polyamide 6 is broken. By 1H-NMR and FTIR spectroscopy, it can be observed that adjusting the molecular weight of PA6 is effective in controlling the molecular structure of PEA. In addition, by using PTA as a blocking agent, it was verified that both aliphatic and aromatic dicarboxylic acid can be applied to this polymerization method, which broadens the selectivity of the synthetic raw materials. The effects of prePA6 with different molecular weights and different blocking agents on the molecular structure and material properties of PEA were studied. The thermodynamic properties, crystal structure, mechanical properties, and water absorption performance of PEA obtained by polycondensation of different prePA6 samples were compared with those of polyamide 6. The results show that as the molecular weight of the PA6 segment continues to increase, the molecular chain regularity increases. When the molecular weight of the PA6 segment reaches 6000, its thermodynamic properties, crystal structure and crystallinity are basically similar to those of PA6. PEA maintains good mechanical properties and water absorption after processing. It is a thermoplastic semi-crystalline polymer with good performance. Moreover, PEA has a low content of oligomers, so it can be processed directly without hydrothermal extraction, which reduces the energy consumption during processing of the PA6-based polymers. It was unexpectedly discovered in this study that a small amount of oligomers also played a certain role in plasticizing, which enhanced the mechanical properties of PEA. This synthetic route can be performed with conventional polyester equipment and has certain potential for industrialization.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/11/6/978/s1. Author Contributions: Conceptualization, L.T. and H.W.; methodology, C.W. and S.Z.; validation, J.Z., Y.F., and J.H.; Formal analysis, S.Z.; Investigation, S.Z.; Writing—Original Draft preparation, S.Z.; Writing—Review and Editing, S.Z. and P.J.; Supervision, C.W., P.J.; Project administration, H.W.; Funding acquisition, C.W. Acknowledgments: This work was financially supported by the National Key Research and Development Program of China (2016YFB0302700), and National Natural Science Foundation of China (51703023). Conflicts of Interest: The authors declare no conflict of interest.

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