Article The Thermal Properties and Degradability of Chiral Polyester-Imides Based on Several l/d-Amino Acids

Chen Qi, Wenke Yang, Fuyan He and Jinshui Yao *

Shandong Provincial Key Laboratory of Processing & Testing Technology of Glass and Functional Ceramics, School of Materials Science & Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China; [email protected] (C.Q.); [email protected] (W.Y.); [email protected] (F.H.) * Correspondence: [email protected]; Tel.: +86-531-89631226

 Received: 17 August 2020; Accepted: 4 September 2020; Published: 9 September 2020 

Abstract: Eight kinds of chiral diacid monomers were prepared with amino acids with different side groups or configurations. Polyester-imides (PEIs) were synthesized from these diacid monomers and diphenol monomers through polycondensation reaction, and the performances and properties were compared with the chiral polyamide-imides (PAIs) previously synthesized by our work group. Their thermal properties were analyzed by thermo gravimetric analysis (TGA) and dynamic thermomechanical analysis (DMA), and it was found that the temperature (Tg) of PEI was mainly affected by the volume of side groups. Their degradability was studied through buffer degradation experiments, and the changes in their water contact angle, molecular weight, structure and appearance during the degradation process were characterized by contact angle tester, permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). With degradation, the hydrophilicity of PEI was improved, and when amino acids with larger side groups or D configuration were introduced into the backbone of PEI, the degradability decreased.

Keywords: chiral amino acids; polyester-imides (PEIs); degradation; thermal property

1. Introduction In recent years, materials have been widely used in various fields of our production and life, and have become an indispensable part of our lives. However, the shortcomings of polymer materials, which are difficult to degrade and can easily cause pollution, have caused great harm to the ecological environment [1–8]. Therefore, the development of degradable polymer materials has attracted more and more attention. Polyimide is well known for its excellent heat resistance, chemical resistance, dielectric properties and mechanical properties [9–11], however, its high rigidity, difficulty in processing, and ease with which it can cause pollution, limit its development [12–14]. Polyester is the most concerned material in the field of degradation, such as polylactide (PLA), while its brittleness and poor heat resistance limit its development [15,16]. Polyester-imides (PEIs) obtained by incorporating imide bond and bond into the polymer concurrently can overcome their shortcomings, and the introduction of amino acids improves its biocompatibility [17–19]. The process of experiment is shown in Figure1. The main purpose of this paper is to check the influence of the side group structure and main chain configuration on the properties of PEI. By controlling the types of amino acids incorporated into PEI, eight kinds of PEIs with different side groups or configurations were synthesized, and the influence of amino acids structure and configuration on the thermal performance and degradability of PEI was evaluated through thermal test and degradation experiment [20–22]. Through comparisons with the degradable polyimide (PI)

Polymers 2020, 12, 2053; doi:10.3390/polym12092053 www.mdpi.com/journal/polymers Polymers 2020, 12, 2053 2 of 14 Polymers 2020, 12, x FOR PEER REVIEW 2 of 14

of time, illustrating that the introduction of an ester bond can improve the degradation property of previously studied by our group [20,21], we found that PEI was degraded more in a shorter period of PI. time, illustrating that the introduction of an ester bond can improve the degradation property of PI.

Figure 1. The synthesis and degradation of polyester-imide (PEI). Figure 1. The synthesis and degradation of polyester-imide (PEI). 2. Materials and Methods 2. Materials and Methods 2.1. Materials 2.1. Materialsl-alanine, d-alanine, l-2-aminobutyric acid, d-2-aminobutyric acid, l-Leucine, d-Leucine, l-phenylalanineL-alanine, D and-alanine,d-phenylalanine L-2-aminobutyric were providedacid, D-2-aminobutyric by Aladdin Industrial acid, L-Leucine, Co (Shanghai, D-Leucine, China). L- Dimethylphenylalanine sulfoxide and (DMSO),D-phenylalanineN,N-dimethylformamide were provided by (DMF), Aladdin and Industrial pyridine (Py)Co (Shanghai, were provided China). by TianjinDimethyl Guangfu sulfoxide Fine (DMSO), Chemical N Research,N-dimethylformamide Institute (Tianjin, (DMF), China). and Pyromellitic pyridine (Py) dianhydride were provided (PMDA), by 4,4Tianjin0-dihydroxydiphenyl Guangfu Fine Chemical ether, LiCl Research and diphenyl Institute phosphoryl (Tianjin, chlorideChina). (DPCP)Pyromellitic were provideddianhydride by Macklin(PMDA), Biochemical 4,4′-dihydroxydiphenyl Co (Shanghai, China).ether, LiCl Pyridine and (Py)diph wasenyl further phosphoryl purified chloride by vacuum (DPCP) distillation were beforeprovided using, by otherMacklin chemicals Biochemical were used Co directly(Shanghai, without China). further Pyridine purification. (Py) was further purified by vacuum distillation before using, other chemicals were used directly without further purification. 2.2. General Characterization 2.2. GeneralFourier Characterization transform infrared (FTIR) spectra were generated using a Thermo Fisher Nicolet IS10 1 spectrophotometerFourier transform (New infrared Castle, (FTIR) DE, USA) spectra in KBr.were generated Proton nuclear using magnetica Thermo resonance Fisher NicoletH NMR IS10 spectraspectrophotometer were carried out(New on Castle, a Bruker DE, AVANCE USA) IIin 400KBr. MHz Proton spectrometer nuclear (Swiss)magnetic in deuteratedresonance dimethyl1H NMR sulfoxidespectra were (DMSO-d6) carried asout solvent. on a Bruker The glass AVANCE transition Ⅱ temperatures400 MHz spectrometer and thermomechanical (Swiss) in deuterated properties ofdimethyl polymers sulfoxide were recorded (DMSO-d6) by 850 as dynamicsolvent. The thermomechanical glass transition analysis temperatures (DMA; and TA thermomechanical instruments, New Castle,properties DE, of USA) polymers instrument were at recorded a heating by rate 850 of 5dynamic◦C/min. thermomechanic Thermogravimetrical analysis analysis (DMA; (TGA) TA on polymersinstruments, was New conducted Castle, onDE, a USA) TGA/ SDTA851instrument System at a heating (Setaram, rate Caluire-et-Cuire, of 5 °C/min. Thermogravimetric France) under a analysis (TGA) on polymers was conducted on a TGA/SDTA851 System (Setaram,25 Caluire-et-Cuire, (N2) atmosphere at a flow rate of 10 ◦C/min. Specific rotations ([α] ) were determined D France) under a nitrogen (N2) atmosphere at a flow rate of 10 °C/min. Specific rotations (𝜶 ) were 1 withdetermined concentration with concentration of 0.1010 g dL of− 0.1010in DMF g dL at− 251 in◦C DMF by a at MCP-200 25 °C by polarimeter a MCP-200 (Anton polarimeter Par, Austria). (Anton MeltingPar, Austria). points Melting (mp) were points analyzed (mp) were by Meltinganalyzed Point by Melting Microscope Point SGWMicroscope X-4 (China). SGW X-4 Elemental (China). analysesElemental were analyses recorded were using recorded Elementar using VarioElemen ELtar model Vario element EL model analyses element equipment analyses (Germany).equipment The molecular weights (M ) of polymers were analyzed with gel permeation chromatography (GPC) (Germany). The molecularw weights (Mw) of polymers were analyzed with gel permeation andchromatography multi-angle laser(GPC) and scattering multi-angle (MALLS, laser light Dawn scattering Heleos), (MALLS, using a Dawn linear Heleos), MZGel SDusing Plus a linear GPC column set (two columns, 5-µm particles, 300 8 mm) with DMF as eluent at room temperature with a MZGel SD Plus GPC column set (two columns,× 5-μm particles, 300 × 8 mm) with DMF as eluent at flowroom rate temperature of 1 mL/min with and a flow a concentration rate of 1 mL/min of the polymerand a concentration of ca. 1 mg /ofmL. the The polymer calibration of ca. was 1 mg/mL. based onThe polystyrene calibration standard.was based The on water polystyrene contact standard angles of. polymersThe water were contact characterized angles of inpolymers SL200B were type contactcharacterized angle testerin SL200B (Kenuo, type Wuhan, contact China). angle tester (Kenuo, Wuhan, China).

Polymers 2020, 12, x FOR PEER REVIEW 3 of 14 Polymers 2020, 12, 2053 3 of 14 2.3. Synthesis of Chiral Diacid Monomers

2.3. SynthesisChiral diacid of Chiral monomers Diacid Monomers (2a–2d′) were synthesized by the condensation of various amino acids and PMDA according to our previous works (see step 1 in Scheme 1) [21–24]. The yields were Chiral diacid monomers (2a–2d ) were synthesized by the condensation of various amino acids obtained by calculating the weight 0ratios of the materials before and after the reaction. Yields and and PMDA according to our previous works (see step 1 in Scheme1)[ 21–24]. The yields were obtained physical properties of the diacid monomers are shown in Table 1. It can be seen that they have high by calculating the weight ratios of the materials before and after the reaction. Yields and physical yields and melting points, and the specific rotation and elemental analysis of them are in line with properties of the diacid monomers are shown in Table1. It can be seen that they have high yields and the expectation. melting points, and the specific rotation and elemental analysis of them are in line with the expectation.

Scheme 1. scheme of PEI3a–PEI3d0.

Table 1. YieldsScheme and physical1. Polymerization properties scheme of the diacid of PEI3a–PEI3d monomers′. (2a–2d0).

Melting 25 Elemental Analysis (%) Diacids (M) YieldTable (%) 1. Yields and physical[α ]properties Formulaof the diacid (Mw) monomers (2a–2d′). Points (◦C) D CHN 25 Elemental Analysis (%) Diacids (M) Yield (%) Melting Points (°C) 𝜶 CFormulaH N (Mw)O Calcd 53.29 3.33 7.78 2a 96 274 60.20 𝐷 16 12 2 8 C H N − (360.28) CalcdFound 53.29 3.3353.22 7.783.29 7.74 2a 96 274 −60.20 C16H12N2O8 (360.28) C16H12N2O8 FoundCalcd 53.22 3.2953.29 7.743.33 7.78 2a0 95 272 68.85 (360.28) CalcdFound 53.29 3.3353.27 7.783.32 7.76 2a′ 95 272 68.85 C16H12N2O8 (360.28) C H N O FoundCalcd 53.27 3.3255.62 7.764.12 7.21 2b 93 231 60.02 18 16 2 8 − (388.33) CalcdFound 55.62 4.1255.66 7.214.20 7.28 2b 93 231 −60.02 C18H16N2O8 (388.33) C H N O FoundCalcd 55.66 4.2055.62 7.284.12 7.21 2b 89 230 61.71 18 16 2 8 0 (388.33) CalcdFound 55.62 4.1255.57 7.214.08 7.20 2b′ 89 230 61.71 C18H16N2O8 (388.33) C H N O FoundCalcd 55.57 4.0859.46 7.205.41 6.31 2c 97 288 52.78 22 24 2 8 (444.48) CalcdFound 59.46 5.4159.50 6.315.42 6.33 2c 97 288 − −52.78 C22H24N2O8 (444.48) C H N O FoundCalcd 59.50 5.4259.46 6.335.41 6.31 2c 94 286 52.70 22 24 2 8 0 (444.48) CalcdFound 59.46 5.4159.44 6.315.43 6.28 2c′ 94 286 52.70 C22H24N2O8 (444.48) Found 59.44 5.43 6.28 C28H20N2O8 Calcd 65.56 3.90 5.46 2d 92 290 176.41 Calcd 65.56 3.90 5.46 2d 92 290 − −176.41 C28H(512.47)20N2O8 (512.47) Found 65.62 3.95 5.50 Found 65.62 3.95 5.50 C28H20N2O8 Calcd 65.56 3.90 5.46 2d0 91 290 169.89 Calcd 65.56 3.90 5.46 2d′ 91 290 169.89 C28H(512.47)20N2O8 (512.47) Found 65.61 3.87 5.42 Found 65.61 3.87 5.42

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2.4. Synthesis of Chiral Polyester-Imides (PEI3a–PEI3d0)

Chiral PEIs (PEI3a–PEI3d0) were synthesized by direct polycondensation of diacid monomers (2a–2d0) with 4,40-dihydroxydiphenyl ether under the reported method (see step 2 in Scheme1)[ 25–27]. Yields and physical properties of PEIs are shown in Table2. It can be seen that they have high yields and molecular weights, and the specific rotation and elemental analysis are in line with the expectation.

Table 2. Yields and physical properties of the PEI3a–PEI3d0.

Elemental Analysis (%) 4 25 PEIs (P) Yield (%) Mw 10 (g/mol) [α] Formula (Mw) × D CHN (C H N O ) Calcd 60.04 3.23 5.03 PEI3a 94 5.423 13.53 28 18 2 9 n − (556.53)n Found 59.77 3.20 4.96 (C28H18N2O9)n Calcd 60.04 3.23 5.03 PEI3a0 91 5.278 14.85 (556.53)n Found 59.73 3.15 4.92 (C H N O ) Calcd 61.64 3.76 4.79 PEI3b 87 4.908 10.02 30 22 2 9 n − (584.58)n Found 60.99 3.73 4.75 (C30H22N2O9)n Calcd 61.64 3.76 4.79 PEI3b0 89 4.762 12.21 (584.58)n Found 69.97 3.70 4.73 (C H N O ) Calcd 66.88 4.91 4.59 PEI3c 93 5.953 15.45 34 30 2 9 n − (610.45)n Found 66.77 4.94 4.53 (C34H30N2O9)n Calcd 66.88 4.91 4.59 PEI3c0 90 5.087 12.45 (610.45)n Found 66.85 4.89 4.55 (C40H26N2O9)n Calcd 67.79 3.67 3.95 PEI3d0 87 6.712 25.12 − (708.72)n Found 67.74 3.65 3.88 (C40H26N2O9)n Calcd 67.79 3.67 3.95 PEI3d0 92 6.458 28.27 (708.72)n Found 67.77 3.64 3.93

2.5. Preparation of PEI Films An amount of 0.25 g of PEI powder (the size of the powder was about 150 µm) was added into 10 mL DMF under stirring until completely dissolved. Then the solution was put into a Teflon mold with a diameter of 5 cm. After that, it was put in an oven with a temperature of 50 ◦C and vacuum drying for 24 h, a piece of clear tawny flexible PEI film was obtained.

2.6. Degradation of PEI Films in Buffer The several kinds of PEI films were cut to quarter sizes, and added into 15 mL Tris-HCl (pH = 7.4) buffer, before being transferred to a constant temperature incubator at 37.5 0.5 C. The films were ± ◦ removed from the buffer and washed with a large quantity of deionized water every week, and then placed into a vacuum drying box at 40 ◦C to dry [21,28].

3. Results and Discussion

3.1. Characterization on Polymers The structures of diacid monomers and PEIs were determined by FTIR and 1H NMR. Figure2 displays FTIR spectra of chiral PEI3a–PEI3d, and FTIR spectra of PEI3a0–PEI3d0 are shown in Figure S3. In the IR spectra, the stretching vibrations of C-H bonds on aromatic and aliphatic groups appear at 1 1 3023 to 2850 cm− . The specific absorption peaks at 1776 and 1724 cm− correspond to the asymmetric stretching vibration and symmetric stretching vibration of carbonyl (C=O) on the imide ring, while the 1 stretching vibration of C=O on the ester bond appears at 1660 cm− . The specific absorption peak of 1 C-C stretching vibration on benzene ring skeleton exists at 1492 cm− . The typical C-N stretching of 1 aromatic-imide is observed at 1384 cm− , and the absorption peak of C-O-C asymmetric vibration on 1 1 1 aromatic nucleus appears at 1180 cm− . In addition, the absorption peaks at 822 cm− and 724 cm− Polymers 2020, 12, x FOR PEER REVIEW 5 of 14 Polymers 2020, 12, 2053 5 of 14 822Polymers cm−1 2020and, 12724, x FORcm− 1PEER can beREVIEW attributed to the out-of-plane bending vibration of the C‒H bond on5 ofthe 14 benzene ring and the C=O bending vibration of the five-membered imide ring, respectively. can822 be cm attributed−1 and 724to cm the−1 can out-of-plane be attributed bending to the vibration out-of-plane of the bending C-H bond vibration on the of benzene the C‒H ring bond and on the the Cbenzene=O bending ring vibrationand the C=O of the bending five-membered vibration imideof the ring,five-membered respectively. imide ring, respectively.

Figure 2. FTIR spectrum of PEI3a–PEI3d.

Figure S4 displays 1H NMRFigureFigure spectra 2. 2.FTIR FTIRof PEI3a-PEI3d spectrum spectrum of of′ PEI3a–PEI3d. .PEI3a–PEI3d. The 1H NMR spectrum of PEI3a is shown in Figure 3. The resonance absorption peaks at 8.28 ppm and 7.05 ppm are provided by the benzene Figure S4 displays 1H NMR spectra of PEI3a-PEI3d . The 1H NMR spectrum of PEI3a is shown in ring protonsFigure S4in displaysPMDA and 1H NMR4,4′-dihydroxydiphenyl spectra of PEI3a-PEI3d ether,0 ′. Therespectively. 1H NMR spectrumIn addition, of PEI3athe hydrogen is shown Figure3. The resonance absorption peaks at 8.28 ppm and 7.05 ppm are provided by the benzene ring protonin Figure on 3.the The chiral resonance carbon absorptionappears at peaks5.42 ppm, at 8.28 and ppm the and methyl 7.05 protonppm are of provided alanine appears by the benzeneat 1.62 protons in PMDA and 4,4 -dihydroxydiphenyl ether, respectively. In addition, the hydrogen proton on ppm.ring protons in PMDA 0and 4,4′-dihydroxydiphenyl ether, respectively. In addition, the hydrogen theproton chiral on carbon the chiral appears carbon at 5.42 appears ppm, at and 5.42 the ppm, methyl and proton the methyl of alanine proton appears of alanine at 1.62 appears ppm. at 1.62 ppm.

Figure 3. Representative 1H NMR spectrum of PEI3a. 1 Figure 3. Representative H NMR spectrum of PEI3a. 3.2. Thermal Properties 3.2. Thermal Properties Figure 3. Representative 1H NMR spectrum of PEI3a. The thermal properties of PEI films were investigated through DMA and TGA. The transition temperatureThe thermal (Tg) properties values were of receivedPEI films from were DMA investigated and the through temperatures DMA correspondingand TGA. The totransition weight 3.2. Thermal Properties losstemperature of 5% (T 5(Tg)) and values 10% (T were10) of received PEIs were from acquired DMA fromand TGA;the temperatures these data are corresponding summarized into Tableweight3. Theloss TGAofThe 5% curves thermal(T5) and of polymersproperties10% (T10) areof of PEIs describedPEI werefilms acquired inwere Figure inve 4from.stigated It can TGA; be through seen these that data DMA the are TGA andsummarized curves TGA. ofThe all in transition PEIsTable are 3. roughlyThetemperature TGA similar. curves (Tg) Asof valuespolymers the volume were are ofreceived described the side from chain in Figure DMA of amino 4.and It acidscanthe betemperatures increases, seen that the the corresponding rigidity TGA curves of the of polymersto all weight PEIs 5 10 increases,areloss roughly of 5% and (T similar.) the and T 510% andAs the(T T10 )volume ofof PEIPEIs become wereof the acquired side larger ch andain from larger,of TGA;amino which th acidsese isdata calledincreases, are thesummarized “rigiditythe rigidity e inffect”. Tableof the 3. polymersTheTake TGA PEI3aincreases, curves as of an andpolymers example, the T5 are theand described T T510of of it PEI is abovein become Figure 410 larger4.◦C, It butcan and afterbe larger,seen heating that which the to 400 TGAis called◦C, curves the the weight of“rigidity all ofPEIs it willeffect”.are droproughly rapidly, similar. after As heating the volume to 800 ◦ofC, the lossside ofch massain of reaches amino moreacids than increases, 80%. the rigidity of the polymers increases, and the T5 and T10 of PEI become larger and larger, which is called the “rigidity effect”.

Polymers 2020, 12, x FOR PEER REVIEW 6 of 14

Table 3. Thermal properties of PEI3a–PEI3d′. PolymersPolymers2020 2020, ,12 12,, 2053 x FOR PEER REVIEW 6 ofof 1414 Polymer E′ (MPa) a × 103 Tg (°C) T5 (°C) b T10 (°C) c Char Yield (%) d PEI3a 2.389 Table 3. Thermal225 properties413 of PEI3a–PEI3d′424. 17.56 Table 3. Thermal properties of PEI3a–PEI3d . PEI3a′ 1.565a 3 223 384 b 0 407 c 18.66 d Polymer E′ (MPa) × 10 Tg (°C) T5 (°C) T10 (°C) Char Yield (%) PEI3bPEI3a 2.6522.389 a 3 220225 418413 b 431424c 14.8817.56d Polymer E0 (MPa) 10 Tg (◦C) T5 (◦C) T10 (◦C) Char Yield (%) PEI3bPEI3a′ 1.5201.565 × 219223 402384 422407 17.0318.66 PEI3a 2.389 225 413 424 17.56 PEI3bPEI3c 2.1482.652 215220 421418 433431 15.2114.88 PEI3a0 1.565 223 384 407 18.66 PEI3bPEI3c′′ 1.9751.520 216219 417402 430422 15.5317.03 PEI3dPEI3b 2.051 2.652 197 220 418424 431436 14.88 17.55 PEI3cPEI3b 2.1481.520 215 219 402421 422433 17.03 15.21 PEI3d′ 0 2.034 193 423 434 21.97 PEI3cPEI3c′ 1.975 2.148 216 215 421417 433430 15.21 15.53 a Storage modulus measured at 100 °C; b temperature at which 5% weight loss was fully recorded by PEI3dPEI3c 0 2.0511.975 197 216 417424 430436 15.53 17.55 TGAPEI3d inPEI3d a′ N2 atm; c temperature2.034 2.051 at which193 10% 197 weight loss 424423 was fully recorded 436434 by TGA 17.55 in a N21.972 atm; da StorageweightPEI3d percentmodulus0 of measuredthe material2.034 at 100left °Cundecomposed; b temperature 193 after at 423 which TGA at 5% 800 weight 434 °C in lossa N 2was atm. fully 21.97 recorded by a b TGAStorage in a modulus N2 atm; measured c temperature at 100 at◦C; whichtemperature 10% weight at which loss 5% was weight fully lossrecorded was fully by recordedTGA in a by N TGA2 atm; in a c d N2 atm; temperature at which 10% weight loss was fully recorded by TGA in a N2 atm; weight percent of the d weight percent of the material left undecomposed after TGA at 800 °C in a N2 atm. material left undecomposed after TGA at 800 ◦C in a N2 atm.

Figure 4. TGA curves of PEI3a–PEI3d′.

Take PEI3a as an example, the T5 of it is above 410 °C, but after heating to 400 °C, the weight of FigureFigure 4. 4.TGA TGA curvescurves ofof PEI3a–PEI3dPEI3a–PEI3d0.′. it will drop rapidly, after heating to 800 °C, the loss of mass reaches more than 80%.

TheTheTake TGATGA PEI3a curvescurves as an of ofexample, PEIPEI andand the polyamide-imidepolyamide-imide T5 of it is above (PAI) (PAI)410 °C, areare but describeddescribed after heating ininFigure Figure to 4005 .5. Comparing °C,Comparing the weight with with of PEI,PEI,it will thethe drop weightweight rapidly, loss ofafter PAI heating at 800 to°C◦C 800 is is about about°C, the 55%, 55%, loss which whichof mass is is muchreaches much higher highermore thanthan than PEI,80%. PEI, this this is isbecause because at athigh high Thetemperatures, temperatures, TGA curves the theof oxygen PEI oxygen and atoms polyamide-imide atoms on onthe the PEI PEI este (PAI) esterr bond bondare react described react with with carbonin carbonFigure atoms 5. atoms Comparing to generate to generate with CO COandPEI, and COthe CO2weight gas,2 gas, the loss the gas of gas volatilizes PAI volatilizes at 800 through °C through is about the the 55%,exhaust exhaust which port port is inmuch in the the higherTGA TGA furnace, than PEI, while this theisthe because nitrogennitrogen at atomsatomshigh temperatures, onon thethe PAIPAI amideamide the oxygen bondbond reactatomsreact with withon the thethe PEI carboncarbon ester atomsbondatoms react toto generategenerate with carbon solids,solids, atoms theythey to remainremain generate inin theCOthe crucible,crucible,and CO2 so sogas, thethe the weightweight gas volatilizes lossloss ofof PAIPAI through isis lessless than thanthe thatexhaustthat ofof PEI. PEI.port ThisThis in the isis similar similarTGA furnace, toto thethe phenomenon phenomenonwhile the nitrogen whenwhen polyesterpolyesteratoms on fiber fiberthe PAI andand polyamideamidepolyamide bond fiberfiber react burn;burn; with whenwhen the carbon polyesterpolyes teratoms fiberfiber to burns,burns, generate aa lotlot solids,ofof blackblack they smokesmoke remain isis emitted,emitted, in the whilewhilecrucible, whenwhen so polyamide thepolyamide weight burns, lossburns, of it PAIit only only is melts. less melts. than Although Although that of the PEI. the residual Thisresidual is qualities similar qualities to of thesetheof thesephenomenon two two polymers polymers when are quitearepolyester quite different, different, fiber theyand they havepolyamide have already already fiber reacted burn; reacted to when a large to apolyes large extentter extent at fiber high at burns, temperatures.high temperatures.a lot of black The Tsmoke The5 and T 5 Tis and10 emitted,of T PAI10 of arePAIwhile about are when about 310 polyamide◦C 310 and °C 340 and ◦burns,C, 340 respectively, °C, it onlyrespectively, melts. which Although arewhich much are the lower much residual than lower thosequalities than of those PEI, of these so of wePEI, two think so polymerswe that think the thermalthatare quite the stabilitythermal different, ofstability PEIthey is have slightlyof PEI already is better slightly reacted than better thatto a than oflarge PAI. that extent of PAI. at high temperatures. The T5 and T10 of PAI are about 310 °C and 340 °C, respectively, which are much lower than those of PEI, so we think that the thermal stability of PEI is slightly better than that of PAI.

FigureFigure 5.5. TGATGA curvescurves ofof PEIPEI andand polyamide-imidepolyamide-imide (PAI).(PAI).

Figure 5. TGA curves of PEI and polyamide-imide (PAI).

PolymersPolymers2020 2020, 12, 12, 2053, x FOR PEER REVIEW 77 of of 14 14 Polymers 2020, 12, x FOR PEER REVIEW 7 of 14 The thermomechanical properties and glass transition temperature values of PEIs were characterizedTheThe thermomechanical thermomechanical by DMA. At 100properties properties °C, the storageand and glass glass modulus tr transitionansition of all PEIstemperature temperature are higher values values than 1500of of PEIs PEIsMPa, were werewhich characterizedcharacterizedshows that they by by DMA. have DMA. highAt At 100 mechanical 100 °C,◦ theC, thestorage strength storage modulus [29]. modulus Figure of allof 6PEIs is all the are PEIs DMA higher are curves higher than of1500 thanPEI3a, MPa, 1500 in whichthe MPa, Tan showswhichδ curve, that shows the they transition that have they high between have mechanical high 140 mechanical and strength 170 °C strength[29]. corresponds Figure [29 6]. tois Figure the DMAβ 6transition, is thecurves DMA and of PEI3a, curves the peak in of the at PEI3a, 225 Tan °C δin curve,corresponds the Tan theδ transitioncurve, to the the α between transition, transition 140 that betweenand is, 170 the °C 140 glass corresponds and transition. 170 ◦C to corresponds The the βappearance transition, to the ofandβ thetransition, the β transitionpeak at and 225 is the°C due correspondspeakto a atrelatively 225 ◦toC the correspondsnon-cooperative α transition, to thethat motionα is,transition, the of glass the amtransition. thatorphous is, the The polymer glass appearance transition. segment, of Thethe this β appearance phenomenontransition is of due theonly toβoccurs transitiona relatively in amorphous is non-cooperative due to a relatively polymers. motion non-cooperative As ofthe the degree amorphous motion of crystallinity polymer of the amorphous segment, and orientation this polymer phenomenon increase, segment, onlythe this β occursphenomenontransition in amorphous gradually only occurs polymers.disappears in amorphous As [29–32]. the polymers.degree The ofcr ystallinity Ascrystallinity the degree of andPEI of crystallinity orientationwas investigated increase, and orientation by the XRD β transitionincrease,measurements. thegraduallyβ transition XRD disappears curve gradually of PEI3a [29–32]. disappears is shown The [ 29incr– ystallinity32Figure]. The 7, crystallinity allof ofPEI the wasother of PEIinvestigated PEIs was have investigated similar by XRD XRD by measurements.XRDcurves. measurements. The XRD XRD curve XRDcurve of curve of PEI3a PEI3a of exhibits PEI3a is shown is a shown broad in Figure in band Figure 7,in 7allthe, all of range ofthe the other2 otherθ = 15–30°,PEIs PEIs have have this similar similarindicates XRD that curves.curves.PEI3a Theis The completely XRD XRD curve curve amorphous. of of PEI3a PEI3a exhibits exhibitsThus, there a a broad broad is an band band obvious in in the the β transitionrange range 2 2θθ in= 15–30°, 15–30the Tan◦, this thisδ curve indicates indicates of PEI3a. that that PEI3aPEI3a is is completely completely amorphous. amorphous. Thus, Thus, there there is is an an obvious obvious ββ transitiontransition in in the the Tan Tan δδ curvecurve of of PEI3a. PEI3a.

Figure 6. DMA curves of PEI3a. FigureFigure 6. 6. DMADMA curves curves of of PEI3a. PEI3a.

Figure 7. XRD curve of PEI3a. Figure 7. XRD curve of PEI3a. Figure 7. XRD curve of PEI3a. By comparing different structures of PEIs, it can be seen from the Table3 that PEI3a has the By comparing different structures of PEIs, it can be seen from the Table 3 that PEI3a has the highest Tg value, and the Tg from PEI3a to PEI3c decreases in turn. This is because with the increase in highestBy comparing Tg value, anddifferent the Tg structures from PEI3a of toPEIs, PEI3c it cadencreases be seen in fromturn. theThis Table is because 3 that with PEI3a the has increase the carbon chain in amino acids, the flexibility of polymer side chain will also increase, which leads to the highestin carbon Tg value, chain andin amino the Tg acids, from the PEI3a flexibility to PEI3c of depocreaseslymer side in turn. chain This will is also because increase, with which the increase leads to decrease in Tg, this is called “internal plasticization” [33]. In general, the Tg of polymers increases with inthe carbon decrease chain in in Tg, amino this isacids, called the “internal flexibility plasticization” of polymer side [33]. chain In general, will also the increase, Tg of polymers which leads increases to the increase in rigidity, however, it can be seen that the Tg of PEI3d is the lowest. This is thewith decrease the increase in Tg, this in substituent is called “internal rigidity, plasticization” however, it can [33]. be In seen general, that thethe TgTg of of polymers PEI3d is increasesthe lowest. because the volume of the benzene ring on the side chain of PEI3d is much larger than that of the withThis the is becauseincrease thein substituentvolume of therigidity, benzene however, ring on it the can side be chainseen that of PE theI3d Tg is ofmuch PEI3d larger is the than lowest. that of carbon chain on other polymers. The huge benzene ring leads to a larger distance between the PEIs, Thisthe is carbon because chain the volumeon other of polymers. the benzene The ring huge on benzthe sideene chainring leads of PE toI3d a islarger much distance larger than between that of the resulting in a smaller intermolecular force, which is called “distance effect” [34–36]. Therefore, the Tg thePEIs, carbon resulting chain inon a othersmaller polymers. intermolecular The huge force, benz wheneich ring is called leads “distance to a larger effect” distance [34–36]. between Therefore, the value of PEI3d is the lowest. Furthermore, the reduction from PEI3a to PEI3c is very slow compared PEIs,the resultingTg value inof aPEI3d smaller is intermolecularthe lowest. Furthermore, force, which the is reductioncalled “distance from PEI3aeffect” to [34–36]. PEI3c isTherefore, very slow with the rapid decrease from PEI3c to PEI3d, which also shows that the Tg value of PEI is mainly thecompared Tg value with of PEI3d the rapid is the decrease lowest. from Furthermore, PEI3c to PEI3d, the reduction which also from shows PEI3a that to thePEI3c Tg isvalue very of slow PEI is affected by the distance effect. We can also find that the difference in Tg between PEIs of different comparedmainly affected with the by rapid the decreasedistance fromeffect. PEI3c We can to PEI3d, also find which that also the shows difference that thein TgTg betweenvalue of PEIPEIs is of chirality is not big, which shows that the structure of PEI is the decisive factor of its Tg. mainlydifferent affected chirality by theis not distance big, which effect. shows We canthat alsothe structurefind that ofthe PEI difference is the decisive in Tg factorbetween of its PEIs Tg. of different chirality is not big, which shows that the structure of PEI is the decisive factor of its Tg.

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3.3.3.3. Solubility Solubility

TableTable4 displays 4 displays the the solubility solubility of PEI3a–PEI3dof PEI3a–PEI3d0 in′ in di differentfferent solvents. solvents. The The solubility solubility performances performances 1 ofof PEIs PEIs were were studied studied quantitatively quantitatively at a concentration at a concentration of 0.5 g of dL 0.5− andg dL at− room1 and temperatureat room temperature in different in solvents.different The solvents. results The demonstrate results demonstrate that PEIs are that soluble PEIs inare various soluble solvents in various such solvents as DMSO, such DMF, as DMSO, THF, andDMF, sulfuric THF, acid and atsulfuric room temperature,acid at room whiletemperature, in some while other in solvents, some other such solvents, as toluene, such methylene as toluene, chloride,methylene methanol, chloride, ethanol methanol, and water,ethanol PEIs and are water, insoluble. PEIs are insoluble.

TableTable 4. 4.The The solubility solubility of of PEI3a–PEI3d PEI3a–PEI3d0 and′ and PAI. PAI.

Solvent DMSOSolvent DMSODMF DMF DMAc DMAc NMP NMP THF THF MeOH MeOH EtOH EtOH PEI3a +PEI3a +++ ++ ++ ++ ++ ++ ++ ++ − − − − PEI3a′ PEI3a+ +++ ++ ++ ++ ++ ++ ++ ++ − − 0 − − PEI3b +PEI3b +++ ++ ++ ++ ++ ++ ++ ++ − − − − PEI3b′ PEI3b+ +++ ++ ++ ++ ++ ++ ++ ++ − − 0 − − PEI3c +PEI3c +++ ++ ++ ++ ++ ++ ++ ++ − − − − PEI3c′ PEI3c+ 0 +++ ++ ++ ++ ++ ++ ++ ++ − − − − PEI3d + ++ ++ ++ ++ PEI3d + ++ ++ ++ ++ − −− − PEI3d + ++ ++ ++ ++ PEI3d′ + 0 ++ ++ ++ ++ − −− − PAI + + + + PAI +− + − + + −− − −− − Insoluble; + soluble at heating; + slightly soluble; ++ soluble. − Insoluble; +−− soluble at heating; + slightly soluble; ++ soluble.

InIn general, general, the the solubility solubility of of PI PI in in organic organic solvents solvents is is not not high, high, the the high high solubility solubility of of PEI PEI is is due due to to thethe presence presence of of branches branches on on their their structural structural units, units, which which lead lead to to a lowera lower density density of of polymer polymer molecular molecular arrangements,arrangements, thereby thereby increasing increasing the the solubility solubility of of PEI PEI [37 [37].]. ComparingComparing with with PAI, PAI, PEI PEI has has better better solubility solubility in in di differentfferent organic organic solvents, solvents, especially especially in in THF. THF. ThisThis is becauseis because the the polarity polarity of ester of ester bond bond in PEI in isPEI less is thanless thatthan of that amide of amide bond inbond PAI, in so PAI, PEI hasso PEI better has solubilitybetter solubility in THF within THF smaller with polarity.smaller polarity. In addition, In addition, the molecular the molecular chain flexibility chain flexibility of PEI is of higher, PEI is whichhigher, makes which it havemakes better it have solubility. better solubility.

3.4.3.4. Degradability Degradability Assay Assay FigureFigure8 displays 8 displays the the change change of of the the water water contact contact angles angles before before and and after after the the degradation degradation of of the the PEIPEI films. films. It canIt can be be seen seen that that the the water water contact contact angles angles of the of PEI the films PEI becomefilms become significantly significantly smaller smaller after degradation,after degradation, which iswhich related is related to the increase to the increase in the hydrophilic in the hydrophilic groups exposedgroups exposed on the surface on the ofsurface the PEIof filmsthe PEI after films degradation. after degradation. With the With degradation the degrad processing,ation processing, the carbonyl the carbonyl groups in groups the main in the chains main ofchains the polymers of the werepolymers destroyed were bydestroyed microorganisms by microorganisms and , and which hydrolysis, exposed which more hydroxylexposed andmore carboxylhydroxyl groups and carboxyl on the surface groups of on PEI the films, surface resulting of PE inI films, smaller resulting water contact in smaller angles water and contact significantly angles improvedand significantly hydrophilicity improved [38]. hydrophilicity [38].

Figure 8. Water contact angles of PEI3a–PEI3d . Figure 8. Water contact angles of PEI3a–PEI3d0 ′.

Polymers 2020, 12, x FOR PEER REVIEW 9 of 14

Figure 9 and Table 5 show the weight average molecular changes before and after degradation of PEI films. It can be seen that the weight average molecular remaining rates of PEI3a–PEI3d increase sequentially, which is related to the structure introduced by the polymer. Alkyl groups are hydrophobic groups, solvent molecules and microorganisms that play the role in degradation are

Polymershydrophilic2020, 12 ,substances, 2053 so the hydrophilicity of PEI will increase with the shortening of the carbon9 of 14 chain. The amino acid carbon chain of PEI3a is the shortest, only with one methyl group, so its degradation rate is the highest. The amino acid carbon chains of PEI3a–PEI3c increase sequentially, as aFigure result,9 andtheir Table degradation5 show the performances weight average gradually molecular decrease. changes In before addition, and afterthe degradationbenzene ring of on PEIPEI3d films. is relatively It can be seen stable, that which the weight is not average easily destroyed molecular by remaining hydrolysis rates and of microorganisms, PEI3a–PEI3d increase so its sequentially,degradation whichrate is isthe related lowest. to the amino acid structure introduced by the polymer. Alkyl groups are hydrophobic groups, solvent molecules and microorganisms that play the role in degradation are hydrophilicTable substances, 5. Weight average so the molecular hydrophilicity (Mw × 10 of4) PEI of PEI3a–PEI3d will increase′ at withevery the stage shortening of biodegradation. of the carbon chain. The amino acid carbon chain of PEI3a is the shortest, only with one methyl group, so its Degradation Time PEI3a PEI3a′ PEI3b PEI3b′ PEI3c PEI3c′ PEI3d PEI3d′ degradation rate is the highest. The amino acid carbon chains of PEI3a–PEI3c increase sequentially, 0 week 5.423 5.278 4.908 4.762 5.953 5.087 6.712 6.458 as a result, their degradation4 weeks performances 4.392 gradually4.381 4.221 decrease. 4.142 5.357 In addition,4.680 6.376 the 6.199 benzene ring on PEI3d is relatively stable, which8 isweeks not easily 3.687 destroyed 3.852 by 3.730 hydrolysis 3.762 4.822 and microorganisms,4.273 5.906 5.876 so its degradation rate is the lowest.

FigureFigure 9. 9.Weight Weight average average molecular molecular remaining remaining of of PEI3a–PEI3d PEI3a–PEI3d0 in′ in every every stages stages of of degradation. degradation. 4 Table 5. Weight average molecular (Mw 10 ) of PEI3a–PEI3d0 at every stage of biodegradation. Comparing PEI with different degrees× of chirality, such as PEI3a and PEI3a′, we found that the weightDegradation average Time molecular PEI3a remaining PEI3a 0ratePEI3b of PEI in PEI3b the 0L configurationPEI3c PEI3c is0 lessPEI3d than that PEI3d in the0 D configuration.0 week This enantioselectivity 5.423 5.278 stems 4.908 from the 4.762 spin polarization 5.953 5.087interaction. 6.712 This mechanism 6.458 for the intermolecular4 weeks interaction 4.392 4.381of chiral 4.221molecules 4.142 results in 5.357 a higher 4.680 degradation 6.376 rate of 6.199 the PEI in the L8 configuration weeks [39]. 3.687 Furthermore, 3.852 by 3.730 comparing 3.762 the weight 4.822 average 4.273 molecular 5.906 remaining 5.876 rate of PEI with different chirality and structure, we found that the structure has a greater impact on

degradation.Comparing PEI with different degrees of chirality, such as PEI3a and PEI3a0, we found that the weightFigure average 10 exhibits molecular the IR remainingspectra of ratePEI3a of before PEI in and the Lafter configuration degradation is less(the thanIR spectra that in of the other D configuration.polymers also This exhibit enantioselectivity close changes). stems It is fromnot diffic the spinult to polarization find that after interaction. 8 weeks This of degradation, mechanism forthe theasymmetric intermolecular vibration interaction of C=O of bond chiral at molecules 1776 cm−1 results and the in asymmetric higher degradation vibration rateof C=O of the bond PEI at in 1724 the Lcm configuration−1 on the imide [39]. ring Furthermore, of PEI3a have by comparing almost no thechange, weight while average the stretching molecular vibration remaining of rate C=O of bond PEI withat 1660 diff erentcm−1 almost chirality disappears, and structure, this wedemonstrates found that that the structurein the degradation has a greater process impact of onthe degradation. PEI film, the esterFigure bond 10 is exhibits destroyed the IRfirst, spectra among of PEI3awhich before the carbonyl and after group degradation in the (the ester IR spectrabond is of broken. other polymersAccompanied also exhibitby the breakage close changes). of the carbonyl It is not group, difficult some to find small that molecules after 8 weeksare lost, of which degradation, leads to 1 thethe asymmetric decrease in vibrationthe weight of average C=O bond molecular at 1776 weight cm− andof PEI. the In symmetric addition, vibrationthere are almost of C=O no bond change at 1 1724in other cm− absorptionon the imide peaks ring including of PEI3a the have absorption almost no peak change, of C‒ whileC stretching the stretching vibration vibration on benzene of C =ringO 1 −1 bondskeleton at 1660 at 1492 cm− cmalmost and disappears,the absorption this peak demonstrates of C‒O‒C thatasymmetric in the degradation vibration on process aromatic of thenucleus PEI film,at 1180 the cm ester−1, as bond well is as destroyed the absorption first, among peak of which C‒H outside the carbonyl bending group vibration in the on ester benzene bond isring broken. at 822 Accompaniedcm−1. This is because by the breakage ester bonds of the are carbonyl easily broken group, by some microorganisms small molecules andare hydrolysis, lost, which while leads other to thegroups decrease in PEI, in thesuch weight as C-N average bonds, molecularare still not weight easy to of break PEI. Inunder addition, the action there of are microorganisms almost no change and inhydrolysis, other absorption which leads peaks to including the degradation the absorption of . peak The ofpriority C-C stretching of bond degradation vibration on is benzene much higher ring 1 skeleton at 1492 cm− and the absorption peak of C-O-C asymmetric vibration on aromatic nucleus 1 at 1180 cm− , as well as the absorption peak of C-H outside bending vibration on benzene ring at 1 822 cm− . This is because ester bonds are easily broken by microorganisms and hydrolysis, while other groups in PEI, such as C-N bonds, are still not easy to break under the action of microorganisms and hydrolysis, which leads to the degradation of esters. The priority of bond degradation is much higher Polymers 2020, 12, 2053 10 of 14 Polymers 2020, 12, x FOR PEER REVIEW 10 of 14 Polymers 2020, 12, x FOR PEER REVIEW 10 of 14 thanthan that that of of other other groups, groups, so so after after degradation, degradation, th thethee esterester bondbond changeschanges in inin PEI PEIPEI are areare thethe mostmost obvious,obvious, whilewhile the the changes changes in in other other groups groups areare are minimal.minimal. minimal.

Figure 10. FTIR spectra of the PEI3a in every stages of degradation. Figure 10. FTIRFTIR spectra of the PEI3a in every stages of degradation.

It is well known that polymers based on amide bonds are not favored by microorganisms and ItIt is is well well known known that that polymers polymers based based on on amide amide bonds bonds are are not not favored favored by by microorganisms microorganisms and and hydrolysis [40,41]. Compared with PEI, there is not much difference between the amide bond of hydrolysishydrolysis [40,41]. [40,41]. Compared Compared with with PEI, PEI, there there is is not not much much difference difference between between the the amide amide bond bond of of PAI PAI PAI and other groups in the priority of degradation, which leads to changes in many groups in the andand other other groups groups in in the the priority priority of of degradation, degradation, which which leads leads to to changes changes in in many many groups groups in in the the degradation process of PAI. The magnitude of the change is far less obvious than that of the ester bond degradationdegradation process process of of PAI. PAI. The The magnitude magnitude of of the the change change is is far far less less obvious obvious than than that that of of the the ester ester in PEI. Therefore, we believe that PEI has better degradability than PAI. bondbond in in PEI. PEI. Therefore, Therefore, we we believe believe that that PEI PEI has has better better degradability degradability than than PAI. PAI. Figure 11 displays the SEM images of the degradation process of PEI3a film. Before the degradation FigureFigure 1111 displaysdisplays thethe SEMSEM imagesimages ofof thethe degradationdegradation processprocess ofof PEI3aPEI3a film.film. BeforeBefore thethe started, the surface of the PEI film was smooth, and after 4 weeks and 8 weeks of degradation, degradationdegradation started,started, thethe surfacesurface ofof thethe PEIPEI filmfilm waswas smooth,smooth, andand afterafter 44 weeksweeks andand 88 weeksweeks ofof the surface of the film changed significantly. As the degradation time increased, more and more degradation,degradation, the the surface surface of of the the film film changed changed significantly. significantly. As As the the degradation degradation time time increased, increased, more more particles accumulated on the surface of the film. Moreover, due to the ester bonds on the polymer andand more more particles particles accumulated accumulated on on the the surface surface of of the the film. film. Moreover, Moreover, due due to to the the ester ester bonds bonds on on the the backbone chains were destroyed, the molecular weight of the polymer was decreased, which led the polymerpolymer backbone backbone chains chains were were destroyed, destroyed, the the mole molecularcular weight weight of of the the polymer polymer was was decreased, decreased, which which film became weaker and more easily to be broken. There are more hydrophilic groups on the PEI film, ledled the the film film became became weaker weaker and and more more easily easily to to be be broken. broken. There There are are more more hydrophilic hydrophilic groups groups on on the the such as -COOH and -OH, which made the film more hydrophilic [37]. PEIPEI film, film, such such as as -COOH -COOH and and -OH, -OH, which which made made the the film film more more hydrophilic hydrophilic [37]. [37].

Figure 11. SEM images of PEI3a degradation in buffer. ((a): 0 week; (b): 4 weeks; (c): 8 weeks). Figure 11. SEM images of PEI3a degradation in bubuffer.ffer. ((((aa):): 0 week; (b): 4 4 weeks; weeks; ( c): 8 8 weeks). weeks). Figure 12 shows the SEM images of PEIs after degradation for 8 weeks. It can be seen that the Figure 12 shows the SEM images of PEIs after degradation for 8 weeks. It can be seen that the surface of all PEI films has undergone significant changes, of which PEI3a has the most obvious surface ofof allall PEI PEI films films has has undergone undergone significant significan changes,t changes, of which of which PEI3a PEI3a has the has most the obvious most obvious change, change, which indicates that PEI3a has the best degradation performance. From PEI3a–PEI3d′, the whichchange, indicates which indicates that PEI3a that has PEI3a the best has degradation the best degradation performance. performance. From PEI3a–PEI3d From PEI3a–PEI3d0, the number′, the of number of particles on the surface gradually decreased, which indicated that their degradation particlesnumber onof particles the surface on gradually the surface decreased, gradually which decreased, indicated which that their indicated degradation that their performance degradation also performance also decreased sequentially. performancedecreased sequentially. also decreased sequentially.

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Figure 12.12. SEMSEM images images of of PEI3a–PEI3d PEI3a–PEI3d0 after′ after 8 weeks 8 weeks of degradation of degradation in buff iner (buffer(a) = PEI3a ((a) =; (PEI3a;a0) = PEI3a (a′) =0; (PEI3ab) = PEI3b;′; (b) = (PEI3b;b0) = PEI3b (b′) =0 PEI3b;(c) =′;PEI3c; (c) = PEI3c; (c0) = PEI3c(c′) = PEI3c0;(d) =′; (PEI3d;d) = PEI3d; (d0) =(dPEI3d′) = PEI3d0). ′).

4. Conclusions Eight series of chiral PEIsPEIs (PEI3a–PEI3d(PEI3a–PEI3d0′) based on eighteight diaciddiacid monomersmonomers (2a–2d(2a–2d0′) have been successfully synthesized synthesized through through dire directct polycondensation. polycondensation. By Byobserving observing the thestructure structure of PEI of and PEI PAI, and itPAI, is found it is found that thattheir their difference difference is only is only in inthe the ester ester bond bond and and amide amide bond, bond, while while there there are are many similarities andand didifferencesfferences in in their their properties. properties. The The degradation degradation of PEI of andPEI PAIand begins PAI begins with the with broken the brokencarbonyl carbonyl group at group the end at the of aminoend of acid.amino The acid. diff Therencee difference is that PEI is that has PEI a higher has a degradationhigher degradation degree, whiledegree, the while degradation the degradation of PEI only of PE occursI only on occurs the carbonyl on the groupcarbonyl of the group ester of bond. the ester This bond. makes This it possible makes itto possible develop materialsto develop with materials specific with degradation specific groups.degradation By conducting groups. By thermal conducting tests, wethermal found tests, that thewe T and T of PEI are higher than those of PAI, while PAI has a higher weight remaining rate than PEI found5 that10 the T5 and T10 of PEI are higher than those of PAI, while PAI has a higher weight remaining rateat 800 than◦C. PEI In terms at 800 of °C. solubility, In terms PEI of hassolubility, higher solubilityPEI has higher in organic solubility solvents. in organic These phenomenasolvents. These can phenomenabe attributed can to the be diattributedfferent properties to the different of ester bondsproperties and amideof ester bonds. bonds This and provides amide ideasbonds. for This the providesdevelopment ideas of for degradable the development materials of with degrad diffableerent materials functions. with different functions.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/12/9/2053/s1. TheSupplementary following is theMaterials: supplementary The following data related are available to this article. online Figures at www.mdpi.com/xxx/s1. S1 and S2: Additional The component following analysis is the supplementaryinclude FTIR and data1H related NMR to spectra this ar ofticle. diacid Figures monomers S1 and S2: 2a–2d, Additional Figure component S3: FTIR spectra analysis of include polyester-imides FTIR and 1H NMR spectra of diacid 1monomers 2a–2d, Figure S3: FTIR spectra of polyester-imides PEI3a′–PEI3d′, Figure PEI3a0–PEI3d0, Figure S4: H NMR spectra of polyester-imides PEI3a–PEI3d0. S4: 1H NMR spectra of polyester-imides PEI3a–PEI3d′. Author Contributions: Conceptualization, C.Q.; Data curation, C.Q., W.Y., F.H.; Formal analysis, C.Q.; Funding Authoracquisition, Contributions: J.Y.; Methodology, Conceptualization, C.Q.; Writing—original C.Q.; Data curation, draft, C.Q.; C.Q. Writing—review, W.Y., F.H.; Formal and analysis, editing, J.Y. C.Q.; All Funding authors acquisition,have read and J.Y.; agreed Methodology, to the published C.Q.; Writing—original version of the manuscript. draft, C.Q.; Writing—review and editing, J.Y. All authors haveFunding: read andThis agreed research to the was published funded version by the of Shandongthe manuscript. Provincial Natural Science Foundation, China, grant number ZR2012EMZ003. Funding: This research was funded by the Shandong Provincial Natural Science Foundation, China, grant Acknowledgments:number ZR2012EMZ003.This project was supported by the Shandong Provincial Natural Science Foundation, China, grant number ZR2012EMZ003. We are particularly grateful to the funding supporting from Qilu University of TechnologyAcknowledgments: for talents. This project was supported by the Shandong Provincial Natural Science Foundation, China, grantConflicts number of Interest: ZR2012EMZ003.The authors We declare are particularly no conflict grateful of interest. to the funding supporting from Qilu University of Technology for talents.

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

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