Synthesis and Properties of Polyesteramides Having Short Nylon-610

Article pubs.acs.org/IECR

Synthesis and Properties of Polyesteramides Having Short Nylon-610 Segments in the Main Chains through Polycondensation and Chain Extension Yajiao Hao, Mengyu Chen, Jingbo Zhao,* Zhiyuan Zhang, and Wantai Yang* Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: One kind of nylon-610 oligomer (PrePA) was prepared from the reaction of nylon-610 salt with sebacic acid. Polyesteramide prepolymers (PrePEAs) having amide content from 10 to 60 mol % were synthesized through melt polycondensation from adipic acid, 1,4-butanediol, and the PrePA with the catalysis of stannous chloride. Chain extension of the PrePEAs was carried out at 210 °C using 2,2′-(1,4-phenylene)-bis(2-oxazoline) and adipoyl biscaprolactamate as combined chain extenders. The chain extended polyesteramides (ExtPEAs) with intrinsic viscosity up to 0.70 dL/g were synthesized. The ExtPEAs were characterized by FTIR and 1H NMR spectrum, differential scanning calorimetry, wide-angle X-ray scattering, thermogravimetric analysis, tensile test, and enzymatic degradation. The results showed that the ExtPEAs were biodegradable and had Tm from 95.20 to 155.67 °C, initial decomposition temperature over 325.3 °C, and tensile strength up to 33.1 MPa.

1. INTRODUCTION With the increasing concern for white pollution, more and more researchers have focused their attention on the synthesis, properties, and biodegradation of the biodegradable polymers.1,2 At present, aliphatic polyesters such as poly(butylene succinate) (PBS), polyl(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), and polylactic acid (PLA) are the most important kinds of the synthetic biodegradable polymers and are gradually used as domestic plastics.3 However, low thermal, mechanical, and processing performances greatly restrict the practical use of these materials. By contrast, aliphatic polyamides such as nylon6, nylon-66, and nylon-610 have unique properties such as excellent toughness and strength, high abrasion resistance, and good processing properties, but they cannot be biodegraded in natural environments at an acceptable rate.4,5 As a result, polyesteramides seem to be very promising materials combining the favorable properties of aliphatic polyesters and polyamides.2,6 Polyesteramides are mainly prepared by polycondensation and ring-opening polymerization.7,8 Dijkstra et al.9−11 had synthesized polyesteramides from dimethyl adipate, butanediol, and diamidediols prepared from the reaction of butanediamine with caprolactone. Puiggali et al.12,13 had synthesized a series of polyesteramides derived from diols, dicarboxylic acids, and amino acids. Shalaby et al.14 synthesized the alternating polyesteramides from the reaction of N,N′-bis(ωhydroxyalkylene)oxamide with dicarboxylic acid diester and used them as bioabsorbable materials. Other research has reported the polyesteramides prepared from the copolymerization of ε-caprolactam and ε-caprolactone.15−17 In addition, Bayer Company18,19 had patented polyesteramides synthesized from adipic acid, butanediol, and hexanediamine or caprolactam and commercialized them in BAK series. BAK polyesteramides are competitive at low cost because they can be produced with the industrial equipment used in the production of PET or © 2013 American Chemical Society

Nylon, from cheap commercial starting materials. Most of them have random microstructure. Polyesteramides are usually classified into three kinds: random, alternating, and segmented or block polyesteramides. Compared with the random polyesteramides, alternating and segmented or block polyesteramides have regular structure in the main chains, so they have higher melting point, higher thermal stability, and higher mechanical strength than the random polyesteramides. Chain extension is one of the important methods to enhance the molecular weight in the melt polycondensation. Chain extension is commonly swift and efficient. For some kinds of chain extenders, chain extension can be completed in several minutes.20−22 Chain extenders such as bisoxazolines23−26 and terephthaloyl biscaprolactamate27−29 are often used to enhance the molecular weight of poly(ethylene terephthalate) (PET), aliphatic polyamides, and aliphatic polyesters with the HOOC− or HO− terminal groups, respectively. For the chain extension of the polymers having both the HOOC− and the HO− terminal groups, combined chain extenders were used.30 In this paper, a series of polyesteramides having short nylon610 segments in the main chains were synthesized. Nylon-610 has high Tm; introducing nylon-610 segments in the main chains might increase the intermolecular interaction between the polyamide segments and lead to the increase of the mechanical strength of the polyesteramides obtained. In this work, one kind of nylon-610 oligomer (PrePA) was synthesized through melt polycondensation from nylon-610 salt with sebacic acid in the presence of H3PO3. Several polyesteramide prepolymers (PrePEAs) having short nylon-610 segments in the main chains was synthesized through melt polycondensaReceived: Revised: Accepted: Published: 6410

October 21, 2012 March 16, 2013 April 24, 2013 April 24, 2013 dx.doi.org/10.1021/ie302879t | Ind. Eng. Chem. Res. 2013, 52, 6410−6421

Industrial & Engineering Chemistry Research

Article

groups was detected by a similar end-group analysis method described in the literature.32 2.6. Intrinsic Viscosity ([η]). As the PrePEAs and the ExtPEAs were insoluble in THF, the intrinsic viscosity of them was determined at 30 °C by Ubbelohde viscometer using mcresol as solvent. 2.7. Infrared (FTIR) and Nuclear Magnetic Resonance (NMR). The chain extended polyesteramides used for the FTIR and NMR detection were purified three times through dissolving-precipitation cycles using 20 mL of DMSO as solvent and 200 mL of methanol as nonsolvent. The FTIR spectrum was recorded on a NICOLET 60SXB FT-IR spectrometer. The 1H NMR spectra were recorded on a Bruker AC-400 spectrometer using dimethylsulfoxide (DMSOd6) as the solvent and tetramethylsilane as the internal standard. 2.8. Differential Scanning Calorimetry (DSC). The glass transition temperature, the melting point, and the crystallization properties of the polymers were performed on a TA DSCQ20 analyzer under a nitrogen atmosphere in three cycles involving heating−cooling−heating. The samples were first heated from room temperature to 250 °C at a heating rate of 60 °C/min and kept for 5 min to eliminate thermal history. Then the samples were cooled to −80 °C at the rate of 40 °C/min. In the second heating scan, the samples were heated from −80 to 250 °C at the rate of 10 °C/min. The cooling DSC scans of the PrePEAs and ExtPEAs were also recorded on a TA DSCQ20 analyzer under a nitrogen atmosphere. The samples were first heated from room temperature to 200 °C at a heating rate of 60 °C/min and kept for 5 min to eliminate thermal history. Then the samples were cooled to −100 °C at the rate of 5 °C/min. 2.9. Thermogravimetric Analysis (TGA). Thermogravimetric analysis was carried out employing TA TGAQ50 analyzer in the temperature range of 25 to 530 °C at a heating rate of 10 °C/min under nitrogen atmosphere. 2.10. Wide Angle X-ray Scattering (WAXS). The wideangle X-ray scattering measurements of the polyesteramides were detected using Rigaku D/Max 2500 VB2+/PC diffractometer with Cu Kα radiation. The samples were continuously scanned over the 2θ range from 5 to 50°. 2.11. Tensile Test. The sample films (50 × 50 × 1 mm) were hot pressed with 70911-24B powder press machine (Tianjin New Technical Instrument Com.). The polyesteramide samples were heated at temperature 20 °C above their Tm for 5 min under 15 MPa, cooled to room temperature under the same pressure, and then cut into dumbbell-shaped bars (50 × 4 × 1 mm). The tensile tests were conducted on Instron 1185 tensile testing machine with crosshead speed at 50 mm/ min. Five measurements were performed for each sample, and the results were averaged to obtain a mean value. 2.12. Enzymatic Degradation. The enzymatic degradation was conducted at 37 °C using protease from Aspergillus as catalyst. Flat-bottomed flasks containing 10 × 10 × 0.1 mm ExtPEA films, 10 mL of sodium phosphate buffer with pH 7.4, and 2 mg of protease were placed on a mechanical shaker with a speed of 100 rpm. After every 12 h, the ExtPEA films were taken out, washed three times with distilled water, and dried overnight at 50 °C. Enzyme and buffer solution were renewed for further degradation. All the dried ExtPEA films were weighed to determine the degradation rate.

tion from adipic acid, 1,4-butanediol, and the PrePA. Chain extension of them was carried out using 2,2′-(1,4-phenylene)bis(2-oxazoline) (PBOX) and adipoyl biscaprolactamate (ABC) as combined chain extenders. The chain extended polyesteramides (ExtPEAs) obtained were characterized by FTIR and 1H NMR spectrum, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), wide-angle X-ray scattering (WAXS), tensile test, and enzymatic degradation.

2. EXPERIMENTAL SECTION 2.1. Materials. Adipic acid (AA) and 1,4-butanediol (BD) were purchased from Tianjin Fuchen Chemical Reagents Factory, China. Sebacic acid (SA) was purchased from Tianjin Guangfu Fine Chemical Research Institute, China. 1,6Hexanediamine (HDA) was purchased from Sinopharm Chemical Reagent Co. Ltd. AA and SA were purified by crystallization in deionized water before used. BD was redistilled under reduced pressure. Stannous chloride (SnCl2·2H2O) was purchased from Beijing Shuanghuan Weiyi Reagent Co. Ltd. Other materials such as phosphorous acid (H3PO3) and dimethyl sulfoxide (DMSO) were all obtained as reagent grade and used directly. Nylon-610 salt was prepared by reacting HAD with SA at a molar ratio of 1:1 in ethanol and crystallized with water−ethanol mixture solvents. The chain extender PBOX was prepared according to the procedure described by Néry et al.24 Its melting point is 249 °C. ABC was prepared according to the procedure by Wilfong.31 Its melting point is 72−73 °C. 2.2. Synthesis of the Nylon-610 Oligomer PrePA. In a 250 mL three-necked flask, 60.00 g (0.19 mol) of nylon-610 salt, 19.06 g (0.094 mol) of sebacic acid, and 0.16 g (0.2 wt %) of H3PO3 were added. The mixture was mechanically stirred under N2 atmosphere, heated from 180 to 210 °C over a period of 2.5 h, and kept for another 1.5 h. Water formed was removed by distillation from the reaction mixture. The concentration of the HOOC− groups ([HOOC−]) of the polyamide obtained, which was determined by titration with 0.05 N NaOH, was 1.98 mol/kg. The concentration of the H2N− groups ([H2N−]) was 0.17 mol/kg. 2.3. Synthesis of the PrePEAs. In a 250 mL three-necked flask, depending on the molar ratio of ester and amide units, different amount of PrePA, adipic acid, 1,4-butanediol, SnCl2 (0.2 wt %), and H3PO3 (0.2 wt %) were added. The mixture was mechanically stirred and heated under N2 atmosphere to 210 °C for 4 h. Then, the pressure in the flask was gradually reduced in stages to 30 mmHg over a period of 2 h and kept 2 h. After that, oil pump was used to reduce the pressure in the flask to 2 mmHg for another 4 h. PrePEAs with both HO− and HOOC− terminal groups were obtained. 2.4. Chain Extension of the PrePEA. In a 100 mL threenecked flask, 6.0 g of PrePEA was stirred and heated under nitrogen to 210 °C. Chain extenders PBOX and ABC were added at molar ratios of (1/2)ABC/−OH = 1.0 and (1/ 2)PBOX/−COOH ≤ 1.0, and the reaction mixture was homogeneously mixed. The chain extension was conducted at normal pressure for 1.5 h and at reduced pressure to 2 mmHg for different times until no further change in the viscosity was observed. The chain extended polyesteramides were poured out and cooled at room temperature. 2.5. Determination of the Concentrations of the HOOC− and HO− or H2N− Terminal Groups. The concentration of the HOOC− and HO− or H2N− terminal

3. RESULTS AND DISCUSSION 3.1. Synthesis of the Nylon-610 Oligomer PrePA. The PrePA with mainly HOOC−terminal groups at the ends was 6411

dx.doi.org/10.1021/ie302879t | Ind. Eng. Chem. Res. 2013, 52, 6410−6421


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Scheme 1. Synthesis of the PrePA via Polycondensation of Nylon-610 Salt with Sebacic Acid in the Presence of Phosphorous Acid

Figure 1. The 1H NMR spectrum of the PrePA.

the reaction temperature was also high. Some 1,6-hexanediamine was lost during the synthesis reaction because its boiling point was just 204 °C, and it had a relative higher vapor pressure in the reaction mixture. Figure 1 shows the 1H NMR spectrum the PrePA obtained. In Figure 1, the peaks at 3.32 and 2.51 ppm corresponded to the H2O and normal DMSO impurity in the DMSO-d6 solvent used in the NMR determination, respectively. The peak at 1.25 ppm corresponded to 1- and 2-CH2− hydrogens in the sebacoyl units. The peak at 1.47 ppm corresponded to the 3CH2− hydrogens in the sebacoyl units and the 6- CH2− hydrogens in the hexylene units. The peak at 2.02 corresponded to the 4-CH2− hydrogens which neighbored the amide groups. The peaks at 1.35 and 3.00 ppm

prepared through melt polycondensation of nylon-610 salt with sebacic acid at a molar ratio of 2:1 in the presence of H3PO3. The synthesis reaction was described in Scheme 1. The PrePA synthesized had [HOOC−] of 1.98 mol/kg and [H2N−] of 0.17 mol/kg. It was mainly terminated with the HOOC− groups. Some H2N− terminal groups still remained. The [HOOC−] and [H2N−] were characterized by end-group analysis. The PrePA had number-average molecular weight (Mn) of 930.81 calculated from the [HOOC−] and [H2N−]. It meant that the number-average number of repeating units per chain (n) of the PrePA was located between 2 and 3, as the HOOC− terminated PrePA having n of 2 and 3 had a theoretical Mn of 766.94 and 1049.41, respectively. The n higher than 2 might be due to that the PrePA had high Tm and 6412



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corresponded to 5- and 7-CH2− hydrogens in the hexylene units. The peak at 7.70 ppm corresponded to the NH hydrogen (8) in amide groups. Peaks at 2.18 and 4.34 ppm corresponded to the 9-CH2− hydrogens neighboring the free −COOH and the 10 hydrogens in the terminal −COOH groups. Peaks at 3.45 and 5.14 ppm corresponded to 11-CH2− hydrogens neighboring the −NH3+ groups and the 12 hydrogens in the terminal −NH3+ groups. The peak at 2.69 ppm corresponded to the 13-CH2− hydrogens neighboring the −COO− groups. Peaks at 2.33 and 11.90 ppm corresponded to the 14-CH2− hydrogens neighboring the −COOH, and the 15 hydrogens in the terminal −COOH groups of unreacted SA remained. From 1 H NMR characterization, it was found that the PrePA was mainly terminated by −COOH groups. A little SA and some −COO−+H3N− salts remained unreacted and left. As SA was still detected, PrePA formed according to Scheme 1 was mainly composed of the HOOC−PA−COOH and H2N−PA− COOH. The H2N−PA−NH2 was in very low level and negligible. The H2N− groups left were in the form of −COO−+H3N− salts. It was assumed that the PrePA detected by 1H NMR spectrum was composed of x mol % of HOOC−PA−COOH, y mol % of +H3N−PA−COO−, and z mol % of SA left. They could be calculated through eqs 1, 2, and 3. The numberaverage number of repeating units per chain of the HOOC− PA−COOH and +H3N−PA−COO− was calculated through eq 4 A9 x = y A11 + A13

(1)

A + A13 y = 11 z A14

(2)

x + y + z = 100%

(3)

n=

A8 + A10 +

A12 3

calculated was 0.57, which was lower than that of 0.67 calculated according to the starting materials. So some HAD had lost during the synthesis reaction of the PrePA. The PrePA formed was mainly composed of HOOC−PA−COOH oligomers. It was an easily crystallizable polymer. The Tm detected in the second heating DSC curves was 188.9 °C. 3.2. Synthesis of PrePEA Prepolymers and Their Chain Extension. Through melt polycondensation from AA, BD with PrePA at different molar ratios with SnCl2 as catalyst, and H3PO3 as stabilizing agent, PrePEAs with both HOOC− and HO− terminal groups were synthesized. The major synthesis reaction was described in Scheme 2. As the H2N− terminal Scheme 2. Synthesis of PrePEA via Polycondensation from AA, BD, and PrePA

groups of the H2N−PA−COOH possibly present in the PrePA had higher reactivity than the HO− terminal groups in the BD, after the polycondensation of AA with BD and the PrePA, the H2N− terminal groups were all reacted with AA, and the HOOC− and HO− terminal groups remained. Table 1 shows the properties of the PrePEAs synthesized. The [HOOC−] of PrePEAs was in the range from 0.72 to 1.04 mol/kg, and the concentration of the HO− terminal groups [HO−] was from 0.025 to 0.35 mol/kg. As the PrePEAs were insoluble in THF, their molecular weight was just characterized by the viscometric method using m-cresol as solvent. The [η] value of the PrePEAs was in the range from 0.18 to 0.25 dL/g. The chain extension of PrePEAs was carried out using PBOX and ABC as combined chain extenders with SnCl2 as a catalyst. Table 1 shows the chain extension of the PrePEAs. After the chain extension, the intrinsic viscosity of ExtPEAs was obviously higher than that of the related PrePEAs. ExtPEAs with [η] from 0.33 to 0.70 dL/g were obtained. The chain extension was described in Scheme 3. From Table 1, it was found that as the amide content in the PrePEAs increased from 10 to 30 mol %, the chain extension time under the reduced pressure needed decreased gradually. The reason might be that the increase of the amide content in the PrePEAs led to the increase of the interaction between the amide groups of the nylon-610 segments in the PrePEAs and the chain extenders ABC and PBOX. This interaction through hydrogen bonds had some catalysis effect on the chain extension by ABC or PBOX.23,33 As for the PrePEA-40, PrePEA-50, and PrePEA-60 containing amide units from 40 to 60 mol %, chain extension with a lower amount of SnCl2 catalyst also showed very good chain extending effect. The catalysis effect through the hydrogen bonding interaction between the amide groups of the nylon-610 segments in the PrePEAs and the chain extender ABC or PBOX increased further. Higher molecular weight ExtPEAs with intrinsic viscosity from 0.41 to 0.62 dL/g were obtained, although the chain extension time under the reduced pressure needed was short. Furthermore, in the chain extension of the PrePEA-60, chain extender ABC was added at a molar ratio of (1/2)ABC/− OH = 1.0. When PBOX was added at a molar ratio of (1/

A12 3

+

A13 2

(4)

in which A represented the area of different hydrogens. From eqs 1−4, x, y, and z calculated were 70.81, 21.47, and 7.72%, respectively, and n was 2.73. The Mn of HOOC−PA−COOH (M1) and +H3N−PA−COO− (M2) was 970.73 and 786.73. The [HOOC−] and [H2N−] were calculated according to eqs 5 and 6 [−COOH] =

[−NH3⊕] =

2x + y + 2z xM1 + yM 2 + zM3

y xM1 + yM 2 + zM3

(5)

(6)

in which the M3 represented the molecular weight of SA. From eqs 5 and 6, the [HOOC−] and [H2N−] calculated were 2.05 and 0.25 mol/kg. They were close to those detected by endgroup analysis. Meanwhile, the molar ratio between the hexylene and sebacoyl units could also be calculated through eq 7 A 7 + A11 nH = nS A4 + A 9 + A13 + A14

(7)

in which the nH and nS represented the molecular number of the hexylene and sebacoyl units, respectively. The nH/nS 6413



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Table 1. Synthesis and the Chain Extension of the PrePEA Prepolymers PrePEAa

ExtPEAe

sample

amide content (mol %)

[HOOC−] / mol•kg‑1

[HO−]/ mol•kg−1

Mnc

[η] dL·g−1

SnCl2 /wt%

(1/2)PBOX: COOH (molar ratio)

reaction timef /h

PrePEA-10 PrePEA-20 PrePEA-30 PrePEA-40 rPrePEA-40b PrePEA-50 PrePEA-60

10 20 30 40 40 50 60

0.72 0.73 1.00 0.93 0.95 1.04 1.01

0.025 0.24 0.35 0.26 0.22 0.10 0.31

2685 2062 1481 1681 1709 1754 1515

0.18 0.25 0.20 0.22 0.21 0.24 0.21

0.2 0.2 0.2 -

1.0 1.0 1.0 1.0 1.0 1.0 1.0

4.51 0.75 0.65 0.83 2.79 0.50 0

-

0.83

0.75

d

[η]d / dL·g−1 0.33 0.70 0.56 0.41 0.39 0.62 crosslinked 0.57

The reaction conditions for the PrePEAs: −COOH/(−OH and −NH2) = 1.05:1 (molar ratio); reaction temperature 210 °C; 760 mmHg, 4 h; 30 mmHg (gradually reduced), 4 h; 2 mmHg, 4 h. SnCl2·2H2O: 0.2 wt %; H3PO3: 0.2 wt %. bThe reaction conditions for the rPrePEA-40: First, SA, AA, and BD were reacted at 160−200 °C (gradually increased) 4 h, 760 mmHg; then HDA was added and reacted at 160−200 °C (gradually increased) 4 h; 200 °C, 30 mmHg (gradually reduced), 4 h; 200 °C, 2 mmHg, 4 h; SnCl2·2H2O: 0.2 wt %; H3PO3: 0.2 wt %. −COOH/(−OH and −NH2) = 1.05:1 (molar ratio). cM = (n × 1000)/([−COOH] + [HO−]), n: the functionality of the PrePEAs. dSolvent: m-cresol. e(1/2)ABC/− OH = 1:1 (molar ratio); T = 210 °C; normal pressure (760 mmHg): 1.5 h. fChain extension time under the reduced pressure (2 mmHg). a

Scheme 3. Chain Extension of PrePEA through the −COOH and the −OH Terminal Groups with PBOX and ABC

catalyze chain extension of ABC or PBOX through the hydrogen bonding interaction, so chain extension of rPrePEA-40 was much slower. 3.3. FT-IR and 1H NMR Characterization of the ChainExtended PEA. Figure 2 shows the FTIR spectrum of chain

2)PBOX/−COOH = 1.0, cross-linking occurred immediately. The reason might be that the PrePEA-60 had so many amide groups that the interaction between the amide groups and the chain extender ABC or PBOX was strong enough and made the chain extension very fast. Before PBOX was completely dissolved in the reaction mixture, some oxazoline terminal groups of the PBOX and the ExtPEA chains had polymerized through cationic polymerization under the catalysis of the HOOC−terminal groups of the PrePEAs, because monomers having oxazoline groups can be initiated by cationic initiators and polymerize through a cationic mechanism.34 As chain extension along the main chains and polymerization of the oxazoline terminal groups between the PBOX and the ExtPEA chains took place simultaneously, fast chain extension and polymerization of the oxazoline groups made the ExtPEA-60 cross-linked. When PBOX was added at a molar ratio of (1/ 2)PBOX/−COOH = 0.83, chain extension took place smoothly, and ExtPEA with [η] of 0.57 dL/g was obtained. In Table 1, a random polyesteramide prepolymer (rPrePEA40) was also synthesized. SA, AA, and BD were first polymerized at 160−200 °C at normal pressure for 4 h, and then HDA was added. The mixture was further polymerized at normal pressure for 4 h and at reduced pressure for 8 h to get the rPrePEA-40. Its chain extension was also carried out. At the same conditions, chain extension of rPrePEA-40 needed a longer reaction time to get [η] of 0.39 dL/g similar to that of the segmented PrePEA-40. Maybe the rPrePEA-40 lacked the nylon-610 segments, which were the effective structural units to

Figure 2. FT-IR spectrum of the ExtPEA-40.

extended polyesteramide with 40 mol % amide (ExtPEA-40). In Figure 1, the wide absorption peaks at 3424.07 and 3319.42 cm−1 were assigned to the N−H stretching vibration of the amide groups in the main chains of the PrePEA, and the amide groups in the terephthalamide diesters units (I) formed from the chain extension between the HOOC− terminal groups of the PrePEA and the chain extender PBOX. The peak at 3080.02 cm−1 corresponded to the H−C stretching vibration of the aromatic −C6H4− units in the terephthalamide diesters units (I). Two peaks at 2932.46 and 2852.66 cm −1 corresponded to the stretching vibration of C−H bonds in the −CH2− groups. The characteristic peak at 1733.92 cm−1 was assigned to the CO stretching vibration of the ester linkages in the butylene adipate (BA) units, and those in the terephthalamide diesters units (I) formed during the chain 6414



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Figure 3. 1H NMR spectrum of the PrePEA-40.

In Figure 4, the peaks at 3.32 and 2.50 ppm corresponded to the H2O and normal DMSO impurity in the DMSO-d6 solvent used in the NMR determination, respectively. The peaks at 2.29 and 1.52 ppm belonged to 1- and 2-CH2− hydrogens in the adipoyl structure; the peaks at 4.03 and 1.62 ppm corresponded to the 3- and 4-CH2− hydrogens in the butylene units. The weak peaks at 4.16 and 3.50 ppm corresponded to the 5- and 6CH2− hydrogens in the terephthalamide diesters units (I) formed from the chain extension between the HOOC− terminal groups of the PrePEA and PBOX, and their area ratio was almost 1:1. The peak at 7.70 ppm corresponded to the NH hydrogen (7) in amide groups of the nylon-610 segments and that in the terephthalamide diesters units (I) formed in the chain extension from PBOX. The peak at 7.90 ppm corresponded to the 8-C6H4− hydrogens in the terephthalamide diesters units (I). The peaks at 3.00, 1.48, and 1.35 ppm corresponded to 9-, 10-, and 11-CH2− hydrogens in the hexylene units of the nylon-610 segments. The peaks at 2.01, 1.48, and 1.22 ppm corresponded to 12-, 13-, 14-, and 15-CH2− hydrogens in the sebacoyl units of the nylon610 segments. As the chain extension between the HO− terminal groups of the PrePEA-40 and ABC resulted in the adipate linkages which were the same as those in the butylene adipate units, no other structural units were detected in the 1H NMR and FTIR spectra. Peaks observed in both 1H NMR and FTIR spectra were fully consistent with the ExtPEA-40 and verified the chemical structure of it.

extension of PBOX. There were two characteristic peaks that corresponded to the amide groups: one at 1642.07 cm−1 was assigned to the CO stretching vibration of the amide groups, and the other at 1537.43 cm−1 was assigned to the N−H bending vibration in the secondary amide groups (amide II band). Figure 3 shows the 1H NMR spectrum of the PrePEA-40, in which the assignments of the peaks were illustrated. In Figure 3, the peaks at 3.33 and 2.51 ppm corresponded to the H2O and normal DMSO impurity in the DMSO-d6 solvent. The peaks at 2.28 and 1.51 ppm belonged to the 1- and 2-CH2− hydrogens in the adipoyl structure; the peaks at 4.02 and 1.61 ppm corresponded to the 3- and 4-CH2− hydrogens in the butylene units. The peak at 7.70 ppm corresponded to the NH hydrogen (5) in amide groups of the nylon-610 segments. The peaks at 3.00, 1.48, and 1.35 ppm corresponded to 6-, 7- and 8-CH2− hydrogens in the hexylene units of the nylon-610 segments. The peaks at 2.08, 1.48, and 1.22 ppm corresponded to 9-, 10-, 11-, and 12-CH2− hydrogens in the sebacoyl units of the nylon610 segments. The structure of the PrePEA-40 was confirmed, and no H2N− as well as −COO−+H3N− salts were detected in the 1H NMR spectrum. The HOOC− and HO− terminal groups of the PrePEA-40 were not detected because their resonance was weak and sensitive to the solvents and environments. The 1H NMR spectrum of the ExtPEA-40 was shown in Figure 4, in which the assignments of the peaks were illustrated. 6415



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Figure 4. 1H NMR spectrum of the ExtPEA-40.

Figure 5. The second heating DSC curves of the PrePEAs (a) and the ExtPEAs (b) (heating rate = 10 °C/min).

3.4. DSC Characterization of PrePEAs and ExtPEAs. The PrePEAs and the ExtPEAs containing amide from 10 to 60 mol % were also characterized by DSC spectra. Figure 5(a),(b) shows the second heating DSC curves of the PrePEAs and the ExtPEAs, respectively. The glass transition temperature (Tg), cold crystallization temperature (Tcc), enthalpy of cold crystallization (ΔHcc), melting point (Tm), and enthalpy of

melting (ΔHm) of them were compiled in Table S1. From Figure 5(a),(b) and Table S1, it was found that the Tg of the PrePEAs was in the range of −48.95 to −27.56 °C, and that of the ExtPEAs was from −48.83 to −11.38 °C. The Tg increased with the increase of the amide content in the polyesteramides. As amide groups showed higher rigidity and were easier to form hydrogen bonds, increasing the amide content brought about 6416



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Figure 6. The cooling DSC scans of the PrePEAs (a) and the ExtPEAs (b) (cooling rate = 5 °C/min).

the increase of the Tg. Meanwhile, after chain extension, the Tg of the ExtPEAs increased. The higher the acid value of the PrePEAs was, the bigger the Tg increased. Chain extension between the HOOC− terminal groups of PrePEAs and PBOX brought about the rigid terephthalamide diesters units (I), which led to the increase of the Tg. Meanwhile, chain extension between the HO− terminal groups and ABC brought about the butylene adipate linkages which were the same as those in the PrePEAs and had little effect on the flexibility of the polymers. So after the chain extension, the Tg of the polymers increased. From Figure 5 and Table S1, it was found that the PrePEAs and the ExtPEAs were all crystallizable polyesteramides. The PrePEAs had a typically microphase-separated structure,35 including polybutylene adipate (PBA)-rich phase, a middle phase composed of BA units and short polyamide 610 (PA 610) segments, and a PA 610-rich phase. PrePEA-10 had a strong peak at 45.64 °C and a very weak peak at 109.65 °C. After chain extension, the ExtPEA-10 showed a strong peak at 44.20 °C and a very weak peak at 95.20 °C. The Tms decreased after the chain extension. Meanwhile, ExtPEA-10 showed a Tcc at −13.88 °C. PrePEA-10 and ExtPEA-10 mainly crystallized in the similar crystallites to the PBA, whose Tm was about 58 °C.36 The amide components just formed a very little portion of PA 610 crystallites, so the PrePEA-10 and ExtPEA-10 just showed a very weak peak at 109.65 and 95.20 °C, respectively. PrePEAs from PrePEA-20 to PrePEA-60 had three T m s. They corresponded to Tms of the PBA-rich phase, the middle phase, and the PA 610-rich phase, respectively. After chain extension, the ExtPEAs containing amide from 20 to 60 mol % just showed one Tm from 117.31 to 155.67 °C. These ExtPEAs mainly crystallized in the crystallites of nylon-610, whose Tm was about 205 °C.37 After chain extension, the Tm decreased compared with that of the related PrePEAs. Chain extension between the HOOC− terminal groups of PrePEAs and PBOX brought about the terephthalamide diesters units (I), which lowered the regularity of the ExtPEAs. So the Tm decreased. Meanwhile, the ExtPEAs from ExtPEA-20 to ExtPEA-60 all showed an obvious Tcc peak. It meant that after chain extension, the crystallization of the ExtPEAs became difficult to some extent compared with the PrePEAs. The PrePEAs crystallized faster than the ExtPEAs. In the heating−cooling− heating DSC detection, the crystallization of the PrePEAs had

already completed in the cooling run before the second heating run started, but that of the ExtPEAs in the cooling did not complete and still progressed in the second heating run. So a Tcc peak emerged in the DSC curves of the ExtPEAs. The reason was also that the chain extension between the HOOC− terminal groups of PrePEAs and PBOX lowered the regularity of the ExtPEAs. So the crystallization of the ExtPEAs became difficult. From Table S1, it was found that the Tm of PrePEAs and ExtPEAs all increased with the amide content increased. As the amide content increased, more intra- as well as intermolecular hydrogen bonds formed between the short nylon-610 segments, so the Tm increased. Chain extension also led to the decrease of the crystallinity or ΔHm for the PrePEA20, PrePEA-30, and PrePEA-40 but did not influence that of PrePEA-50 and PrePEA-60. The reason might be that the PrePEAs from PrePEA-20 to PrePEA-40 had low portion of the PA 610 segments. They crystallized slowly and were easily influenced by introducing the terephthalamide diesters units (I) through chain extension, but PrePEA-50 and PrePEA-60 had a higher portion of the middle and PA 610-rich phase; they had higher Tm and crystallized fast. So the crystallinity or ΔHm of ExtPEA-50 and ExtPEA-60 did not decrease after chain extension. From Figure 5 and Table S1, it was also found that the random rPrePEA-40 just showed two Tms at 67.46 and 138.49 °C, which were obviously lower than those of the segmented PrePEA-40. After chain extension, the rExtPEA-40 showed obviously lower Tm than that of the ExtPEA-40. This might be an evidence to verify that the PrePEAs had segmented structure composed of PA 610 segments. The Tms of the rPrePEA-40 corresponded to the ester-rich phase and the middle phase. They were more randomly composed of BA or butylene sebacate (BS) units and hexylene adipamide (HA) or hexylene sebacamide (HS) units at low or high ester/amide ratio. The PrePEA-40 was composed of BA units and short nylon-610 segments. They formed the PBA-rich phase, the middle phase, and the PA 610-rich phase at different ester/amide ratios. So PrePEA-40 showed three Tms and ExtPEA-40 had a higher Tm than rExtPEA-40. Although the reaction temperature was high, most of the nylon-610 segments remained unchanged, and no serious transesterification happened during the synthesis and chain extension of the PrePEAs. 6417



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Figure 6(a),(b) shows the cooling DSC scans of the PrePEAs and ExtPEAs. The crystallization temperature (Tc) was compiled in Table S2. From Figure 6(a),(b) and Table S2, it was found that the Tc of the PrePEA-10 was 21.20 °C. It mainly crystallized in the PBA crystallites. After chain extension, the Tc of the ExtPEA-10 crystallized in the PBA crystallites was still detected. Its Tc decreased because the chain extension lowered the regularity of the PBA segments. PrePEA-20, PrePEA-30, and PrePEA-40 showed three Tcs. The lowest Tc corresponded to the crystallization of the PBA-rich phase, the second one corresponded to the middle phase composed of BA units and short PA 610 segments, and the third or highest one corresponded to the PA 610-rich phase. Increasing the amide content resulted in the decrease of the PBA-rich phase and the increase of the middle and the PA 610-rich phase. So the lowest Tc decreased, but the second and the highest Tc increased. The PrePEAs turned to crystallize in the PA 610 crystallites as the amide content increased. The PrePEA-50 and PrePEA-60 just showed the highest Tc which corresponded to the PA 610-rich phase. They mainly crystallized in the PA 610 crystallites. After the chain extension, the Tcs of the ExtPEAs from ExtPEA-20 to ExtPEA-60 decreased obviously. Chain extension between the HOOC− terminal groups of PrePEAs and PBOX brought about the terephthalamide diesters units (I), which lowered the regularity of the ExtPEAs. So the Tcs decreased. 3.5. TGA Characterization of ExtPEA. The thermal stability of the ExtPEAs containing amide units from 10 to 60 mol % was studied with the thermogravimetric analyses and was shown in Figure 7. The initial decomposition temperature

Figure 8. X-ray diffraction diagrams of PrePEAs.

Figure 9. X-ray diffraction diagrams of ExtPEAs.

Figure 8 and Figure 9, it was shown that the peaks corresponding to PBA at 2θ of Miller indices 21.3° (110), 29.9° (120) and the peaks corresponding to the nylon-610 at 20.3 (100), 24.3 (010, 110)36,38 were present in the X-ray diffraction diagrams of the PrePEA-10 and ExtPEA-10. The PrePEA-10 mainly crystallized in the crystallites of the PBA, with some PA 610-rich phase crystallized in the crystallites of the PA 610. After chain extension, the crystallization of ExtPEA-10 in the crystallites of the PBA decreased and that in the crystallites of the PA 610 increased. As the chain extension lowered the regularity of the ExtPEA-10, the crystallization in the crystallites of the PBA became difficult; but the PA 610 segments had strong intermolecular interaction, so the ExtPEA10 crystallized in the crystallites of the PA 610 became a little obvious. As for the PrePEAs with amide content from 20 to 60 mol %, with the amide content increased, the intensity of the peaks corresponding to PA 610 became stronger, while that corresponding to PBA became weaker, because the amount and the interaction between the short nylon-610 segments increased. These PrePEAs mainly crystallized in the nylon-610 crystallites. From Figure 9, it was found that the ExtPEAs with amide content from 20 to 60 mol % also mainly crystallized in the crystallites of nylon-610. The peaks corresponding to PA 610 at 2θ of 20.0 (100), 23.9 (010, 110) were present in the Xray diffraction diagrams. The reason was that the introduced terephthalamide diesters units (I) into the PEA main chains by chain extension with PBOX just formed the amorphous phase and had little effect on the interaction of the short nylon-610

Figure 7. TGA curves of the ExtPEAs containing amide units from 10 to 60 mol % (heating rate: 10 °C/min; atmosphere: N2).

(Ti), 50% mass decomposition temperature (T50), end decomposition temperature (Tend), and the end mass loss were summarized in Table S3. From Figure 7 and Table S3, it was found that the Ti of the ExtPEAs was all above 325.3 °C. With the increase in amide content of the ExtPEAs, the Ti increased. As the amide content of the ExtPEAs increased, more intra- as well as intermolecular hydrogen bonds formed, thus the thermal stability increased. The ExtPEAs had thermal stability high enough to be used as thermoplastics and were suitable to be processed with normal thermally processing machines. 3.6. WAXS Characterization of PrePEAs and ExtPEAs. The crystalline structure of the PrePEAs and the ExtPEAs were also determined by WAXS measurements. Figure 8 and Figure 9 show the diffraction curves of the PrePEAs and the ExtPEAs containing amide from 10 to 60 mol % respectively. From 6418



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segments, so the ExtPEAs mainly crystallized in the crystallites of nylon-610. 3.7. Tensile Testing. The tensile testing of the ExtPEAs was conducted, and the stress−strain curves were shown in Figure 10. The results of the tensile testing were shown in

3.8. Enzymatic Degradation. The enzymatic degradation of the ExtPEAs from ExtPEA-10 to ExtPEA-60 was conducted at 37 °C using protease from Aspergillus as an enzyme. Figure 11 shows their degradation diagrams. It was found that as the

Figure 10. Stress−strain curves of ExtPEAs containing amide from 10 to 60 mol %.

Figure 11. Enzymatic degradation of the ExtPEAs with protease from Aspergillus (10 mL phosphate buffer, pH 7.4, 37 °C).

degradation time increased, the weight loss increased. With the increase of the amide amount in the ExtPEAs, the degradation rate decreased. They all showed biodegradable characteristics. Protease catalyzed the hydrolytic breakage of the ester and amide linkages and led to the enzymatic degradation of the ExtPEAs.

Table S3. From Table S3, it was found that the tensile strength was in the range of 5.1 to 33.1 MPa, and the strain at break was in the range of 1274.6 to 41.1%. The tensile strength of ExtPEA-10 was just 5.1 MPa and was lower than PBA.32 As the ExtPEA-10 mainly crystallized in the PBA crystallites, introduction of a little amount of the nylon-610 segments lowered the regularity of the PBA segments and made the crystallization difficult. So the ExtPEA-10 just had low tensile strength. As for the ExtPEAs with amide content from 20 to 50 mol %, as the amide content increased from 20 to 50 mol %, the tensile strength increased from 11.2 to 33.1 MPa, and the strain at break decreased from 1274.6 to 472.1%. From Figure 5, Figure 6, and Table S1, it was found that although ExtPEA10 crystallized in the PBA crystallites and ExtPEA-20 in the nylon-610 crystallites, ExtPEA-10 crystallized much easier with higher ΔHm than the ExtPEA-20. As ExtPEA-20 had higher nylon-610 segments, it showed higher tensile strength than the ExtPEA-10; but ExtPEA-20 had lower crystallinity, so it had much higher flexibility and showed much higher strain at break than the ExtPEA-10. As the amide content increased, the intermolecular interaction between the short nylon-610 segments increased. So the tensile strength of ExtPEAs increased. As the amide content increased from 20 to 50 mol %, the crystallinity of the ExtPEAs increased as showed from the ΔHm of them. Higher crystallinity lowered the flexibility of the ExtPEAs, so the strain at break decreased. As for the ExtPEA-60 with an amide content of 60 mol %, maybe too small of an amount of the soft BA segments was introduced in the main chains. It made the ExtPEA-60 lack enough flexibility and hampered it crystallizing well. So the tensile strength and the strain at break decreased. Even so, characterization of the mechanical properties showed that ExtPEAs with amide content from 30 to 60 mol % were strong thermoplastic materials. From Table S3, it was also found that the ExtPEA-10 had higher Young modulus than the ExtPEA-20, maybe because ExtPEA-10 had higher crystallinity and lower flexibility than the ExtPEA-20. From ExtPEA-20 to ExtPEA-60, as the nylon-610 segments increased, the Young modulus increased, also because of the increase of the stiffness and crystallinity.

4. CONCLUSION A PrePA oligomer was prepared through polycondensation from nylon-610 salt with sebacic acid in the presence of H3PO3. PrePEAs with amide content from 10 to 60 mol % and intrinsic viscosity from 0.18 to 0.25 dL/g were prepared through polycondensation from AA, BD, and the PrePA. ExtPEAs with intrinsic viscosity up to 0.70 dL/g were synthesized through chain extension of the PrePEAs at 210 °C with PBOX and ABC as combined chain extenders. FT-IR and 1H NMR characterization verified the polyesteramide structure of the ExtPEA-40. DSC and WAXS study showed that PrePEA-10 and ExtPEA-10 mainly crystallized in the PBA crystallites. ExtPEAs with amide content from 20 to 60 mol % all mainly crystallized in the nylon-610 crystallites. ExtPEAs with amide content from 20 to 60 mol % had Tm from 117.31 to 155.67 °C. They had tensile strength up to 33.1 MPa, strain at break from 1274.6 to 41.1%, and thermal stability with initial decomposition temperature over 325.3 °C. They were biodegradable. ExtPEAs with amide content from 30 to 60 mol % were strong, thermally stable, and tough thermoplastic polymers.

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ASSOCIATED CONTENT

S Supporting Information *

Table S1, DSC data of PrePEAs and ExtPEAs in the second heating scans; Table S2, DSC data of the PrePEAs and the ExtPEAs in the cooling scans, Table S3, TGA data and the stress−strain data of the ExtPEAs. This material is available free of charge via the Internet at http://pubs.acs.org. 6419



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AUTHOR INFORMATION

Corresponding Author

*Phone: +8610-6443-4864. Fax: +8610-6441-6338. E-mail: [email protected] (J.Z.), [email protected] (W.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 21244006 and 50873013).

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Synthesis and Properties of Polyesteramides Having Short Nylon-610

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