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Synthesis and thermal degradation of biodegradable polyesteramide based on ε-caprolactone and 11-aminoundecanoic acid
Polymer Degradation and Stability 81 (2003) 279–286 www.elsevier.com/locate/polydegstab
Synthesis and thermal degradation of biodegradable polyesteramide based on e-caprolactone and 11-aminoundecanoic acid Zhiyong Qian, Sai Li, Yi He, Cao Li, Xiaobo Liu* Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, PR China Received 25 February 2003; accepted 12 March 2003
Abstract A new kind of polyesteramide copolymer based on e-caprolactone and 11-aminoundecanoic acid has been synthesized by melt polycondensation. The copolymers were characterized by FTIR, 1H-NMR, DSC, and WAXD. With the increase in aminoundecanoic acid content, the melting temperature, heat of fusion, and Td,50% increased accordingly. The thermal degradation of the P(CL/AU)x/y copolymers was studied using FTIR, and TG During thermal degradation, the ester bond decomposes at lower temperature, then the amide bond decomposes at higher temperature. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Polyesteramide; Biodegradable polymer; Crystallinity; Thermal degradation; Nylon 11
1. Introduction
2. Experimental
Aliphatic polyesteramide copolymer is a new type of biodegradable material, which was developed recently [1–9]. Due to the polar nature of the amide groups in the polyamide segments and their ability to form hydrogen bonds, these polyamide segments may form intra- as well as inter-molecular hydrogen bonds. Thus, these copolymers may have good thermal and mechanical properties even at relatively low molecular weight. On the other hand, the hydrolytically degradable ester bond in the main chain gives the copolymer good degradability. Several kinds of polyesteramide copolymers have been prepared [7–9] in our laboratory. In this work, a new kind of polyesteramide based on e-caprolactone was synthesized by melt polycondensation and the thermal degradation behaviour was studied.
2.1. Materials
* Corresponding author. Tel.: +86-28-8523-6765; fax: +86-288522-3978. E-mail addresses: [email protected], dragonqzy@hotmail. com (Z.Y. Qian), [email protected] (X.B. Liu).
11-Aminoundecanoic acid was chemically pure grade, and all other materials were analytical reagent grade. The materials were used without further purification. 2.2. Synthesis of P(CL/AU)x/y copolymers P(CL/AU)x/y copolymers were synthesized from e-caprolactone (e-CL) and 11-aminoundecanoic acid (AU) by a melt polycondensation method according to Scheme 1 [9]. The typical P(CL/AU)60/40 copolymer was prepared as follows. A 17.1 g (0.l5 mol) of e-CL, 21 g (0.104 mol) of AU, 0.1 g of titanium dioxide, 0.1 g of Irganox 1010, and 0.05 g of tetrabutyl titanate were added into the reaction vessel under nitrogen atmosphere. The reaction was kept at 110 C for 1 h. It was then gradually raised to 160 C in 30 min, then the mixture was rapidly heated to 240 C under vacuum for another 1 h. At the end, the resultant melt was poured out onto a steel plate, thus P(CL/AU)60/40 copolymer was obtained. Copolymer sheets were prepared by pouring the hot melt
0141-3910/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0141-3910(03)00098-3
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Dh0m ¼ 135:4XPCL þ 196:6XPAU
ð2Þ
where XPCL and XPAU are the mole fraction of e-CL and AU in the copolymer respectively. 2.4. Wide angle X-ray diffraction (WAXD) WAXD was performed at room temperature using a Rigaku DMAX1400 difftactometer (DMAX1400, Rigaku, Japan; l=0.15406 nm) operated at 40 kV and 100 mA with CuKa radiation. The specimens were fixed on the equipment and data were collected with a step interval of 0.02 at a rate of 4 /min.
Scheme 1. Synthesis of P(CL/AU)x/y copolymers.
into a steel mould (1001001 mm). The samples for WAXD, TG and thermal degradation study were cut from the sheets and kept in a desiccator before use. The polyesteramide copolymers prepared in this work are listed in Table 1. 2.3. Differential scanning calorimetry (DSC) Non-isothermal crystallization behaviour of the copolymers was characterized on a DSC TA2910 (TA Instruments, Germany). The specimens were heated from 30 C to 250 C under nitrogen atmosphere at a heating rate of 10 C/min, and cooling rate of 10 C/ min. The crystallinity of copolymers (Xc%) was determined by dividing the observed heat of fusion by the theoretical value for perfectly (100%) crystalline polymer according to Eq. (1). The theoretical hm values for polycaprolactone (PCL) and poly(aminoundecanoic acid) (nylon 11, or PAU) are 135.4J/g [10] and 196.6 J/g [11] respectively. The theoretical AHm values for the copolymers were calculated from heat of fusion of homopolymers taking into account the composition of the copolymer according to Eq. 2 [12]. Xc ð%Þ ¼
Dhm 100 Dh0m
ð1Þ
2.5. Thermogravimetric analysis Thermogravimetric measurements (TGA/DTA) were performed under a steady flow of air or nitrogen atmosphere at a heating rate of 10 C/min in the range of room temperature to 600 C on a thermogravimetric analyzer (Perkin-Elmer, TGA7) coupled with a PerkinElmer data station. 2.6. Thermal degradation study P(CL/AU)60/40 copolymers were put in the hot oven in air atmosphere at different temperature for different periods and then were taken out for FTIR testing. 2.7. Intrinsic viscosity measurement Intrinsic viscosity [] was measured by using an Ubbelohde viscometer at 30 0.1 C. All the copolymers were dissolved in m-cresol to prepare solutions of 0.5 g/dl. [] was calculated using Eq. (3) according to the Solomon–Ciuta method [13]. sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi t t 2
1 ln t0 t0 ð3Þ ½ ¼ C
Table 1 Chemical composition and intrinsic viscosity of the samples used in this paper Polymer code
Molar ratio of e-CL/AU In feed
[] (dl/g)
e-CL (wt.%)
Experimentala Theoretical Experimental valuec valued
P(CLIAU)40/60 1/1.57 –b P(CL/AU)50/50 1/1.04 1/1.06 P(CL/AU)60/40 1/0.69 1/0.70
28.4 38.4 48.3
–b 37.1 47.1
0.61 0.73 0.69
a
Determined by 1H-NMR. Not soluble in CDCl3, so the 1H-NMR was not determined. c Calculated from the molar fraction of e-CL/AU in feed. d Calculated from the molar fraction of e-CL/AU according to the H-NMR asnalysis. b
Fig. 1. FTIR diagram of P(CL/AU)x/y copolymers (the arrows indicate the absorption due to ester moiety).
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where C is the concentration of the solution; t is the flow time of solution, and t0 is the flow time of pure solvent. 2.8. Fourier transform infrared spectroscopy (FTIR)
2.9. 1H-Nuclear magnetic resonance (1H-NMR) 1
H-NMR spectra (in CDCl3) were recorded on a Bruker 300 spectrometer (Bruker, Rheinstetten, Germany) at 300 MHz using trimethylsilane (TMS) as internal reference standard.
FTIR (KBr) spectra of the copolymers were taken with a Nicolet 200SXV spectrophotometer.
Fig. 4. Melting and crystallization temperature against the molar fraction of e-CL in P(CL/AU)x/y copolymers. Fig. 2. 1H–NMR spectrum of P(CL/AU)60/40 copolymer (in CDCI3). Table 2 Thermal properties of P(CL/AU)x/y copolymers Polymer code
Tg ( C)
Tm ( C)
Hm (J/g)
H0m (J/g)
Tc ( C)
Hc (J/g)
Xca (%)
P(CL/AU) 40/60
36.4
37.6
172.1
108.4
17.7
21.8
P(CL/AU) 50/50 P(CL/AU) 60/40
35.5
89.8, 109.4, 126.0 91.7
22.3
166.9
72.9
16.3
13.4
37.5
70.9
22.1
160.6
53.6
14.2
13.8
a
Crystallinity was determined by DSC method. Fig. 5. WAXD patterns of P(CL/AU)x/y copolymers crystallized from the melt. Table 3 Assignment of the PAU (nylon 11) WAXD peaks according to Ref [15,16] Peaks
2( )
dPAU (nm)
hkl
Crystal system
1 2
20.1 23.5
0.44 0.38
100 010, 110
a-Form (triclinic) a-Form (triclinic)
Table 4 Assignment of the PCL WAXD peaks according to Ref. [17,18]
Fig. 3. DSC curves of P(CL/AU)x/y copolymers under nitrogen atmosphere.
Peaks
2y ( )
dPCL (nm)
hkl
1 2
21.5 23.8
0.41 0.37
110 200
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Table 5 Assignment of the PCL and PAU (nylon 11) WAXD peaks in the P(CL/AU)x/y copolymers Peak
1
2
Experimental data
Reference data
P(CL/AU)40/60
P(CL/AU)50/50
P(CL/AU)60/40
P(CL/AU)x/y copolymers
2( )
dexp
2y( )
dexp (nm)
2( )
dexp (nm)
2( )
dref (nm)
Crystal system
20.1
0.44
PAU: a-form, (100)
19.94
0.445
19.88
0.446
19.82
0.448 21.5 23.5
0.41 0.38
PCL: (110) PAU: a-form, (010, 110)
23.8
0.37
PCL: (200)
22.84
0.389
22.94
0.388
22.78
0.390
3. Results and discussion 3.1. Copolymer synthesis
Fig. 6. TG curves of P(CL/AU)x/y copolymers in nitrogen atmosphere.
FTIR and 1H-NMR spectra of the copolymers were fully consistent with the anticipated chemical constitution. Fig. 1 shows the FTIR spectra of P(CL/AU)x/y copolymers. The major characteristic absorptions identified in FTIR spectra were: amide A (ca. 3315 cm 1) and amide B (ca. 3078 cm 1), amide I (ca. 1644 cm 1) and amide II (ca. 1540 cm 1), and aliphatic ester groups vC¼O (ca. 1735 cm 1). Their relative intensities varied in the expected way with change in E-CL/AU molar ratio. With the increase in e-CL content, the absorption intensity at 1735 cm 1 increased.
Fig. 7. TGA and DTA thermograms of P(CL/AU)x/y copolymers in nitrogen atmosphere: (a) P(CL/AU)40/60; (b) P(CJ/AU)50/50; (c) P(CL/AU)60/40.
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The typical 1H-NMR spectrum of P(CL/AU)60/40 copolymer is shown in Fig. 2. The characteristic absorption peaks are indicated in this figure. The experimental molar ratio of e-CL/AU was calculated from the integral intensity of peak ‘‘e’’ and peak ‘‘j’’. For these copolymers, the experimental molar ratio of e-CL/AU is very close to the theoretical value, which can be seen from Table 1. 3.2. Thermal properties and crystallization behaviour The results obtained from DSC analysis are summarized in Table 2 and Fig. 3. The melting temperature was determined as the temperature of the main peak in the curve obtained from the first scan. With the increase in e-CL content, the melting temperature, heat of fusion, crystallization temperature, and heat of crystallization decreased accordingly. But glass transition temperature did not show a clear relationship with e-CL content. The insertion of e-CL units into PAU (nylon 11) lattices reduces the density of amide groups, thus diminishing the number of hydrogen bonds and lowering the melting temperature and crystallinity. The melting and crystalTable 6 TGA data of P(CL/AU)x/y copolymers at a heating rate of 10 C/min under nitrogen atmosphere Sample e-CL (Wt.%) (experimental value) Td,5%b ( C) Td,lmaxc ( C)
P(CL/AU) 40/60 28.40a 386.9 416.3
P(CL/AU) 50/50 37.1
47.1
377.8 430.9
364.1 420.3
Thermal degradation behaviour of P(CL/AU)x/y copolymers was studied by TGA and DTA under nitrogen and air atmospheres. 3.3.1. Thermogravimetric analysis in nitrogen TGA/DTA curves of P(CL/AU)x/y copolymers under nitrogen atmosphere are shown in Figs. 6 and 7. P(CL/ Table 7 TGA data of P(CL/AU)x/y copolymers at a heating rate of 10 C/min in air Sample
372.5 374.7
360.3 381.6
Stage 1 Weight loss at stage 1 (%) Td,2maxc ( C)
6.2 414.2
6.3 411.9
12.4 404.3
10.8 473.4
Stage 2 Weight loss at stage 2 (%) Td,3maxc ( C)
13.5 468.1
18.56 471.1
6.3 414.2
9.9 484.0
Stage 3 Weight loss at stage 3 (%) Td,4maxc ( C)
41.1 477.8
71.5 –
13.2 48.0
Stage 4 Weight loss at stage 4 (%) Td,50maxd ( C)
38.1 468.8
– 465.0
66.0 454.4
7.4
6.1
8.5
– –
– –
– –
– –
71.3 –
Stage 3 Weight loss at stage 3 (%) Td,4maxc ( C) Stage 4 Weight loss at stage 4 (%) Td,50maxc ( C)
P(CL/AU) 60/40
380.0 369.6
20.6 457.5
Stage 2 Weight loss at stage 2 (%) Td,3maxc ( C)
28.4a
P(CL/AU) 50/50
47.1
46.4 –
47.3 474.2
P(CL/AU) 40/60
37.1
34.8 428.6
27.1 482.8
Residue at 520 C (%)
3.3. Thermogravimetric analysis
e-CL (Wt.%) (experimental value) Td,5%b ( C) Td,lmaxc ( C)
Stage 1 Weight loss at stage 1 (%) Td,2maxc ( C)
Stage 5 Weight loss at stage 5 (%) Td,50%d ( C)
P(CL/AU) 60/40
lization temperature plotted against the molar fraction of e-CL are shown in Fig. 4. Further information on the crystallization behaviour of these polyesteramide copolymers was obtained by means of WAXD diffractometer. Fig. 5 presents the WAXD patterns of P(CL/AU)x/y copolymers crystallized from the melt. It can be seen that these samples exhibited two strong reflections at the diffraction angle 2 of about 20 and 23 . With the increase in AU content, the diffraction peaks become sharper and higher. This diffraction pattern was very similar to the a-form crystal of PAU (nylon 11) homopolymer [11,14–16]. Tables 3 and 4 list the reference WAXD data of PCL [17,18] and PAU homopolymers respectively. The WAXD data of P(CJJAU)x/y copolymers are presented in Table 5.
– 471.9
– 454.4
15.6 442.3
3.5
3.7
6.2
a Theoretical value calculated from the molar ratio of CL/AU in feed. b Temperature at which 5% of weight was lost. c Temperature of the largest decomposition rate during this stage. d Temperature at which 50% of weight was lost.
Residue at 520 C (%)
a Theoretical value calculated from the molar ratio of CL/AU in feed. b Temperature at which 5% of weight was lost. c Temperature of the largest decomposition rate during this stage. d Temperature at which 50% of weight was lost.
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AU)40/60 and P(CL/AU)50/50 copolymers show a twostage degradation pattern in nitrogen, but P(CL/AU)60/ 40 copolymer shows a complicated degradation pattern. The two-stage degradation pattern suggests that the structure of P(CL/AU)x/y copolymers could be partially blocked. Indeed, the degradation of the PCL block proceeds at a temperature lower than that of PAU block; the peak at ca. 420 C could be mainly due to the degradation of PCL blocks while the last degradation peak at ca. 480 C should be essentially due to
Fig. 8. TG curves of P(CL/AU)x/y copolymers in air.
that of PAU blocks. With the increase in e-CL content, Td,5% decreased and Td,1max increased accordingly. The P(CL/AU)40/60 copolymer, has a typical microphase-separated structure, including PCL-rich phase, PAU-rich phase, and a middle phase composed of e-CL and AU segments. According to the DSC curve in Fig. 3, there are three melting points at 89.8 C, 109.4 C, and 126 C respectively. The lowest melting temperature might be due to PCL-rich phase, the highest one to a PAU-rich phase, and the middle one to the middle phase. Fig. 7a and Table 6 present the TGA and DTA results of this P(CL/AU)40/60 copolymer. The weight loss in stages 1 and 2 might be due to the degradation of PCL-rich phase and PAU-rich phase respectively, because the weight loss at both stages is very close to the theoretical weight fraction of PCL or PAU respectively. This suggests that P(CL/AU)40/60 copolymer is a kind of block copolymer. But for P(CL/AU)50/50 and P(CL/ AU)60/40 copolymers, there is a complicated thermal degradation pattern. Because of the good miscibility between PCL and PAU segments, they just have one melting transition temperature. So, the weight loss at every stage is not equivalent to the corresponding weight content of PCL or PAU segments. This phenomenon is similar to the thermal degradation behaviour of poly[(e-caprolactam)-(e-caprolactone)] copolymers [19].
Fig. 9. TGA and DTA thermograms of P(CL/AU)x/y copolymers in nitrogen atmosphere: (a) P(CL/AU)40/60; (b) P(CL/AU)50/50; (c) P(CJJAU)60/40.
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285
Fig. 10. FTIR spectra of P(CL/AU)60/40 copolymers degraded in air at 390 C for different period (the arrows indicate the absorption due to ester moiety).
3.3.2. Thermogravimetric analysis in air Thermal degradation in air is more complicated than that in nitrogen atmosphere. Table 7 and Figs. 8 and 9 present the TGA/DTA results of P(CL/AU)x/y copolymers in air. When the copolymers were exposed to air, the decomposition temperature decreased because of the thermal oxidation. The peaks at 367, 376, and 382 C might be due to decomposition of PCL segment, and the peaks at 471, 478, and 480 C might be due to PAU segments. This proposition is supported by the FTIR analysis of the degradation products after degradation in air. Fig. 10 presents FTIR spectra of the degraded products at 390 C. It shows that the ester moiety decreased to some extent, but the amide moiety remained unchanged in 8 min. Fig. 11 presents FTIR spectra of the degraded products at 469 C. At this temperature both the ester and amide moieties decreased quickly. Table 7 suggests that with the increase of e-CL content, the decomposition temperature of PCL segment (Td,1max) increased, but Td,5% decreased. This phenomenon is similar to the thermal degradation behaviour in nitrogen. In Tables 6 and 7, the involatile residue yielded at 520 C in air increased compared with that under nitrogen atmosphere, which might be due to the thermal oxidation process in air.
Fig. 11. FTIR spectra of P(CL/AU)60/40 copolymers degraded in air at 469 C for different time (the arrows indicate the absorption due to ester moiety).
4. Conclusion Biodegradable polyesteramide P(CL/AU)x/y copolymers were synthesized from e-CL and AU by melt polycondensation. FTIR and 1H–NMR results suggest that the experimental value of e-CL/AU molar ratio is very close to the theoretical value. DSC and WAXD results suggest that the crystallinity increases with AU content. The WAXD patterns are very similar to the a-form crystal of PAU homopolymer. Thermal degradation behaviour was studied in nitrogen and air atmosphere respectively. The two-stage degradation pattern suggests that the copolymers are partially blocked. These results are in good agreement with DSC results of these copolymers.
Acknowledgements This work was sponsored by ‘‘Outstanding young scientist fund of Sichuan Province’’ and the ‘‘Hundreds of Talents Program’’ of the Chinese Academy of Sciences.
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[14] Kim KG, Newman BA, Scheinbeim Ji. J Polym Sci: Polym Phys 1985;23:2477. [15] Gogolewski S, Pennings AJ. Polymer 1977;18:660. [16] Zhang Qingxin, Mo Zhishen, Liu Siyang, Cheng SZD. Polymer 2001;42:5543.
[17] Lefevre C, Villers D, Koch MHJ, David C. Polymer 2001; 42:8769. [18] Rhee SH, Choi JY, Kim HM. Biomaterials 2002;23:4915. [19] Draye AC, Persenaire O, Brozek J, Roda J, Kosek T, Dubois Ph. Polymer 2001;42:8325.
Synthesis and thermal degradation of biodegradable polyesteramide based on e-caprolactone and 11-aminoundecanoic acid Zhiyong Qian, Sai Li, Yi He, Cao Li, Xiaobo Liu* Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, PR China Received 25 February 2003; accepted 12 March 2003
Abstract A new kind of polyesteramide copolymer based on e-caprolactone and 11-aminoundecanoic acid has been synthesized by melt polycondensation. The copolymers were characterized by FTIR, 1H-NMR, DSC, and WAXD. With the increase in aminoundecanoic acid content, the melting temperature, heat of fusion, and Td,50% increased accordingly. The thermal degradation of the P(CL/AU)x/y copolymers was studied using FTIR, and TG During thermal degradation, the ester bond decomposes at lower temperature, then the amide bond decomposes at higher temperature. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Polyesteramide; Biodegradable polymer; Crystallinity; Thermal degradation; Nylon 11
1. Introduction
2. Experimental
Aliphatic polyesteramide copolymer is a new type of biodegradable material, which was developed recently [1–9]. Due to the polar nature of the amide groups in the polyamide segments and their ability to form hydrogen bonds, these polyamide segments may form intra- as well as inter-molecular hydrogen bonds. Thus, these copolymers may have good thermal and mechanical properties even at relatively low molecular weight. On the other hand, the hydrolytically degradable ester bond in the main chain gives the copolymer good degradability. Several kinds of polyesteramide copolymers have been prepared [7–9] in our laboratory. In this work, a new kind of polyesteramide based on e-caprolactone was synthesized by melt polycondensation and the thermal degradation behaviour was studied.
2.1. Materials
* Corresponding author. Tel.: +86-28-8523-6765; fax: +86-288522-3978. E-mail addresses: [email protected], dragonqzy@hotmail. com (Z.Y. Qian), [email protected] (X.B. Liu).
11-Aminoundecanoic acid was chemically pure grade, and all other materials were analytical reagent grade. The materials were used without further purification. 2.2. Synthesis of P(CL/AU)x/y copolymers P(CL/AU)x/y copolymers were synthesized from e-caprolactone (e-CL) and 11-aminoundecanoic acid (AU) by a melt polycondensation method according to Scheme 1 [9]. The typical P(CL/AU)60/40 copolymer was prepared as follows. A 17.1 g (0.l5 mol) of e-CL, 21 g (0.104 mol) of AU, 0.1 g of titanium dioxide, 0.1 g of Irganox 1010, and 0.05 g of tetrabutyl titanate were added into the reaction vessel under nitrogen atmosphere. The reaction was kept at 110 C for 1 h. It was then gradually raised to 160 C in 30 min, then the mixture was rapidly heated to 240 C under vacuum for another 1 h. At the end, the resultant melt was poured out onto a steel plate, thus P(CL/AU)60/40 copolymer was obtained. Copolymer sheets were prepared by pouring the hot melt
0141-3910/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0141-3910(03)00098-3
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Dh0m ¼ 135:4XPCL þ 196:6XPAU
ð2Þ
where XPCL and XPAU are the mole fraction of e-CL and AU in the copolymer respectively. 2.4. Wide angle X-ray diffraction (WAXD) WAXD was performed at room temperature using a Rigaku DMAX1400 difftactometer (DMAX1400, Rigaku, Japan; l=0.15406 nm) operated at 40 kV and 100 mA with CuKa radiation. The specimens were fixed on the equipment and data were collected with a step interval of 0.02 at a rate of 4 /min.
Scheme 1. Synthesis of P(CL/AU)x/y copolymers.
into a steel mould (1001001 mm). The samples for WAXD, TG and thermal degradation study were cut from the sheets and kept in a desiccator before use. The polyesteramide copolymers prepared in this work are listed in Table 1. 2.3. Differential scanning calorimetry (DSC) Non-isothermal crystallization behaviour of the copolymers was characterized on a DSC TA2910 (TA Instruments, Germany). The specimens were heated from 30 C to 250 C under nitrogen atmosphere at a heating rate of 10 C/min, and cooling rate of 10 C/ min. The crystallinity of copolymers (Xc%) was determined by dividing the observed heat of fusion by the theoretical value for perfectly (100%) crystalline polymer according to Eq. (1). The theoretical hm values for polycaprolactone (PCL) and poly(aminoundecanoic acid) (nylon 11, or PAU) are 135.4J/g [10] and 196.6 J/g [11] respectively. The theoretical AHm values for the copolymers were calculated from heat of fusion of homopolymers taking into account the composition of the copolymer according to Eq. 2 [12]. Xc ð%Þ ¼
Dhm 100 Dh0m
ð1Þ
2.5. Thermogravimetric analysis Thermogravimetric measurements (TGA/DTA) were performed under a steady flow of air or nitrogen atmosphere at a heating rate of 10 C/min in the range of room temperature to 600 C on a thermogravimetric analyzer (Perkin-Elmer, TGA7) coupled with a PerkinElmer data station. 2.6. Thermal degradation study P(CL/AU)60/40 copolymers were put in the hot oven in air atmosphere at different temperature for different periods and then were taken out for FTIR testing. 2.7. Intrinsic viscosity measurement Intrinsic viscosity [] was measured by using an Ubbelohde viscometer at 30 0.1 C. All the copolymers were dissolved in m-cresol to prepare solutions of 0.5 g/dl. [] was calculated using Eq. (3) according to the Solomon–Ciuta method [13]. sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi t t 2
1 ln t0 t0 ð3Þ ½ ¼ C
Table 1 Chemical composition and intrinsic viscosity of the samples used in this paper Polymer code
Molar ratio of e-CL/AU In feed
[] (dl/g)
e-CL (wt.%)
Experimentala Theoretical Experimental valuec valued
P(CLIAU)40/60 1/1.57 –b P(CL/AU)50/50 1/1.04 1/1.06 P(CL/AU)60/40 1/0.69 1/0.70
28.4 38.4 48.3
–b 37.1 47.1
0.61 0.73 0.69
a
Determined by 1H-NMR. Not soluble in CDCl3, so the 1H-NMR was not determined. c Calculated from the molar fraction of e-CL/AU in feed. d Calculated from the molar fraction of e-CL/AU according to the H-NMR asnalysis. b
Fig. 1. FTIR diagram of P(CL/AU)x/y copolymers (the arrows indicate the absorption due to ester moiety).
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where C is the concentration of the solution; t is the flow time of solution, and t0 is the flow time of pure solvent. 2.8. Fourier transform infrared spectroscopy (FTIR)
2.9. 1H-Nuclear magnetic resonance (1H-NMR) 1
H-NMR spectra (in CDCl3) were recorded on a Bruker 300 spectrometer (Bruker, Rheinstetten, Germany) at 300 MHz using trimethylsilane (TMS) as internal reference standard.
FTIR (KBr) spectra of the copolymers were taken with a Nicolet 200SXV spectrophotometer.
Fig. 4. Melting and crystallization temperature against the molar fraction of e-CL in P(CL/AU)x/y copolymers. Fig. 2. 1H–NMR spectrum of P(CL/AU)60/40 copolymer (in CDCI3). Table 2 Thermal properties of P(CL/AU)x/y copolymers Polymer code
Tg ( C)
Tm ( C)
Hm (J/g)
H0m (J/g)
Tc ( C)
Hc (J/g)
Xca (%)
P(CL/AU) 40/60
36.4
37.6
172.1
108.4
17.7
21.8
P(CL/AU) 50/50 P(CL/AU) 60/40
35.5
89.8, 109.4, 126.0 91.7
22.3
166.9
72.9
16.3
13.4
37.5
70.9
22.1
160.6
53.6
14.2
13.8
a
Crystallinity was determined by DSC method. Fig. 5. WAXD patterns of P(CL/AU)x/y copolymers crystallized from the melt. Table 3 Assignment of the PAU (nylon 11) WAXD peaks according to Ref [15,16] Peaks
2( )
dPAU (nm)
hkl
Crystal system
1 2
20.1 23.5
0.44 0.38
100 010, 110
a-Form (triclinic) a-Form (triclinic)
Table 4 Assignment of the PCL WAXD peaks according to Ref. [17,18]
Fig. 3. DSC curves of P(CL/AU)x/y copolymers under nitrogen atmosphere.
Peaks
2y ( )
dPCL (nm)
hkl
1 2
21.5 23.8
0.41 0.37
110 200
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Table 5 Assignment of the PCL and PAU (nylon 11) WAXD peaks in the P(CL/AU)x/y copolymers Peak
1
2
Experimental data
Reference data
P(CL/AU)40/60
P(CL/AU)50/50
P(CL/AU)60/40
P(CL/AU)x/y copolymers
2( )
dexp
2y( )
dexp (nm)
2( )
dexp (nm)
2( )
dref (nm)
Crystal system
20.1
0.44
PAU: a-form, (100)
19.94
0.445
19.88
0.446
19.82
0.448 21.5 23.5
0.41 0.38
PCL: (110) PAU: a-form, (010, 110)
23.8
0.37
PCL: (200)
22.84
0.389
22.94
0.388
22.78
0.390
3. Results and discussion 3.1. Copolymer synthesis
Fig. 6. TG curves of P(CL/AU)x/y copolymers in nitrogen atmosphere.
FTIR and 1H-NMR spectra of the copolymers were fully consistent with the anticipated chemical constitution. Fig. 1 shows the FTIR spectra of P(CL/AU)x/y copolymers. The major characteristic absorptions identified in FTIR spectra were: amide A (ca. 3315 cm 1) and amide B (ca. 3078 cm 1), amide I (ca. 1644 cm 1) and amide II (ca. 1540 cm 1), and aliphatic ester groups vC¼O (ca. 1735 cm 1). Their relative intensities varied in the expected way with change in E-CL/AU molar ratio. With the increase in e-CL content, the absorption intensity at 1735 cm 1 increased.
Fig. 7. TGA and DTA thermograms of P(CL/AU)x/y copolymers in nitrogen atmosphere: (a) P(CL/AU)40/60; (b) P(CJ/AU)50/50; (c) P(CL/AU)60/40.
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The typical 1H-NMR spectrum of P(CL/AU)60/40 copolymer is shown in Fig. 2. The characteristic absorption peaks are indicated in this figure. The experimental molar ratio of e-CL/AU was calculated from the integral intensity of peak ‘‘e’’ and peak ‘‘j’’. For these copolymers, the experimental molar ratio of e-CL/AU is very close to the theoretical value, which can be seen from Table 1. 3.2. Thermal properties and crystallization behaviour The results obtained from DSC analysis are summarized in Table 2 and Fig. 3. The melting temperature was determined as the temperature of the main peak in the curve obtained from the first scan. With the increase in e-CL content, the melting temperature, heat of fusion, crystallization temperature, and heat of crystallization decreased accordingly. But glass transition temperature did not show a clear relationship with e-CL content. The insertion of e-CL units into PAU (nylon 11) lattices reduces the density of amide groups, thus diminishing the number of hydrogen bonds and lowering the melting temperature and crystallinity. The melting and crystalTable 6 TGA data of P(CL/AU)x/y copolymers at a heating rate of 10 C/min under nitrogen atmosphere Sample e-CL (Wt.%) (experimental value) Td,5%b ( C) Td,lmaxc ( C)
P(CL/AU) 40/60 28.40a 386.9 416.3
P(CL/AU) 50/50 37.1
47.1
377.8 430.9
364.1 420.3
Thermal degradation behaviour of P(CL/AU)x/y copolymers was studied by TGA and DTA under nitrogen and air atmospheres. 3.3.1. Thermogravimetric analysis in nitrogen TGA/DTA curves of P(CL/AU)x/y copolymers under nitrogen atmosphere are shown in Figs. 6 and 7. P(CL/ Table 7 TGA data of P(CL/AU)x/y copolymers at a heating rate of 10 C/min in air Sample
372.5 374.7
360.3 381.6
Stage 1 Weight loss at stage 1 (%) Td,2maxc ( C)
6.2 414.2
6.3 411.9
12.4 404.3
10.8 473.4
Stage 2 Weight loss at stage 2 (%) Td,3maxc ( C)
13.5 468.1
18.56 471.1
6.3 414.2
9.9 484.0
Stage 3 Weight loss at stage 3 (%) Td,4maxc ( C)
41.1 477.8
71.5 –
13.2 48.0
Stage 4 Weight loss at stage 4 (%) Td,50maxd ( C)
38.1 468.8
– 465.0
66.0 454.4
7.4
6.1
8.5
– –
– –
– –
– –
71.3 –
Stage 3 Weight loss at stage 3 (%) Td,4maxc ( C) Stage 4 Weight loss at stage 4 (%) Td,50maxc ( C)
P(CL/AU) 60/40
380.0 369.6
20.6 457.5
Stage 2 Weight loss at stage 2 (%) Td,3maxc ( C)
28.4a
P(CL/AU) 50/50
47.1
46.4 –
47.3 474.2
P(CL/AU) 40/60
37.1
34.8 428.6
27.1 482.8
Residue at 520 C (%)
3.3. Thermogravimetric analysis
e-CL (Wt.%) (experimental value) Td,5%b ( C) Td,lmaxc ( C)
Stage 1 Weight loss at stage 1 (%) Td,2maxc ( C)
Stage 5 Weight loss at stage 5 (%) Td,50%d ( C)
P(CL/AU) 60/40
lization temperature plotted against the molar fraction of e-CL are shown in Fig. 4. Further information on the crystallization behaviour of these polyesteramide copolymers was obtained by means of WAXD diffractometer. Fig. 5 presents the WAXD patterns of P(CL/AU)x/y copolymers crystallized from the melt. It can be seen that these samples exhibited two strong reflections at the diffraction angle 2 of about 20 and 23 . With the increase in AU content, the diffraction peaks become sharper and higher. This diffraction pattern was very similar to the a-form crystal of PAU (nylon 11) homopolymer [11,14–16]. Tables 3 and 4 list the reference WAXD data of PCL [17,18] and PAU homopolymers respectively. The WAXD data of P(CJJAU)x/y copolymers are presented in Table 5.
– 471.9
– 454.4
15.6 442.3
3.5
3.7
6.2
a Theoretical value calculated from the molar ratio of CL/AU in feed. b Temperature at which 5% of weight was lost. c Temperature of the largest decomposition rate during this stage. d Temperature at which 50% of weight was lost.
Residue at 520 C (%)
a Theoretical value calculated from the molar ratio of CL/AU in feed. b Temperature at which 5% of weight was lost. c Temperature of the largest decomposition rate during this stage. d Temperature at which 50% of weight was lost.
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AU)40/60 and P(CL/AU)50/50 copolymers show a twostage degradation pattern in nitrogen, but P(CL/AU)60/ 40 copolymer shows a complicated degradation pattern. The two-stage degradation pattern suggests that the structure of P(CL/AU)x/y copolymers could be partially blocked. Indeed, the degradation of the PCL block proceeds at a temperature lower than that of PAU block; the peak at ca. 420 C could be mainly due to the degradation of PCL blocks while the last degradation peak at ca. 480 C should be essentially due to
Fig. 8. TG curves of P(CL/AU)x/y copolymers in air.
that of PAU blocks. With the increase in e-CL content, Td,5% decreased and Td,1max increased accordingly. The P(CL/AU)40/60 copolymer, has a typical microphase-separated structure, including PCL-rich phase, PAU-rich phase, and a middle phase composed of e-CL and AU segments. According to the DSC curve in Fig. 3, there are three melting points at 89.8 C, 109.4 C, and 126 C respectively. The lowest melting temperature might be due to PCL-rich phase, the highest one to a PAU-rich phase, and the middle one to the middle phase. Fig. 7a and Table 6 present the TGA and DTA results of this P(CL/AU)40/60 copolymer. The weight loss in stages 1 and 2 might be due to the degradation of PCL-rich phase and PAU-rich phase respectively, because the weight loss at both stages is very close to the theoretical weight fraction of PCL or PAU respectively. This suggests that P(CL/AU)40/60 copolymer is a kind of block copolymer. But for P(CL/AU)50/50 and P(CL/ AU)60/40 copolymers, there is a complicated thermal degradation pattern. Because of the good miscibility between PCL and PAU segments, they just have one melting transition temperature. So, the weight loss at every stage is not equivalent to the corresponding weight content of PCL or PAU segments. This phenomenon is similar to the thermal degradation behaviour of poly[(e-caprolactam)-(e-caprolactone)] copolymers [19].
Fig. 9. TGA and DTA thermograms of P(CL/AU)x/y copolymers in nitrogen atmosphere: (a) P(CL/AU)40/60; (b) P(CL/AU)50/50; (c) P(CJJAU)60/40.
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Fig. 10. FTIR spectra of P(CL/AU)60/40 copolymers degraded in air at 390 C for different period (the arrows indicate the absorption due to ester moiety).
3.3.2. Thermogravimetric analysis in air Thermal degradation in air is more complicated than that in nitrogen atmosphere. Table 7 and Figs. 8 and 9 present the TGA/DTA results of P(CL/AU)x/y copolymers in air. When the copolymers were exposed to air, the decomposition temperature decreased because of the thermal oxidation. The peaks at 367, 376, and 382 C might be due to decomposition of PCL segment, and the peaks at 471, 478, and 480 C might be due to PAU segments. This proposition is supported by the FTIR analysis of the degradation products after degradation in air. Fig. 10 presents FTIR spectra of the degraded products at 390 C. It shows that the ester moiety decreased to some extent, but the amide moiety remained unchanged in 8 min. Fig. 11 presents FTIR spectra of the degraded products at 469 C. At this temperature both the ester and amide moieties decreased quickly. Table 7 suggests that with the increase of e-CL content, the decomposition temperature of PCL segment (Td,1max) increased, but Td,5% decreased. This phenomenon is similar to the thermal degradation behaviour in nitrogen. In Tables 6 and 7, the involatile residue yielded at 520 C in air increased compared with that under nitrogen atmosphere, which might be due to the thermal oxidation process in air.
Fig. 11. FTIR spectra of P(CL/AU)60/40 copolymers degraded in air at 469 C for different time (the arrows indicate the absorption due to ester moiety).
4. Conclusion Biodegradable polyesteramide P(CL/AU)x/y copolymers were synthesized from e-CL and AU by melt polycondensation. FTIR and 1H–NMR results suggest that the experimental value of e-CL/AU molar ratio is very close to the theoretical value. DSC and WAXD results suggest that the crystallinity increases with AU content. The WAXD patterns are very similar to the a-form crystal of PAU homopolymer. Thermal degradation behaviour was studied in nitrogen and air atmosphere respectively. The two-stage degradation pattern suggests that the copolymers are partially blocked. These results are in good agreement with DSC results of these copolymers.
Acknowledgements This work was sponsored by ‘‘Outstanding young scientist fund of Sichuan Province’’ and the ‘‘Hundreds of Talents Program’’ of the Chinese Academy of Sciences.
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