Synthesis, characterization, and thermal properties of biodegradable aliphatic copolyester based on ε-caprolactone, adipic acid, and 1,6-hexanediol

Materials Letters 60 (2006) 31 – 38 www.elsevier.com/locate/matlet

Synthesis, characterization, and thermal properties of biodegradable aliphatic copolyester based on q-caprolactone, adipic acid, and 1,6-hexanediol CaiBing Liu a, ZhiYong Qian a,*, YingChun Gu b, LinYu Fan c, Jun Li d, GuoTao Chao a, WenJuan Jia a, MingJing Tu a a

College of Material Science and Engineering, Sichuan University, Chengdu, 610065, China College of Textile and Clothing Engineering, Sichuan University, Chengdu, 610065, China State Key Laborotary of Biotherapy, West China Hospital, Sichuan University. Chengdu, 610041, China d Division of Bioengineering, National University of Singapore, Singapore, 117576, Singapore b

c

Received 2 March 2005; accepted 24 July 2005 Available online 18 August 2005

Abstract In this paper, biodegradable aliphatic copolyesters were synthesized from q-caprolactone, adipic acid, and 1,6-hexanediol by meltpolycondensation method. The chemical structure, and thermal properties of these copolymers were studied in detail. The water absorption behavior and hydrolytic degradation behavior of this copolyester were also studied. This aliphatic copolyester prepolymer would be used to prepare the biodegradable poly(ester urethane), which might have potential application in biomedical field. D 2005 Elsevier B.V. All rights reserved. Keywords: Biodegradable copolymer; Poly(ester urethane); Aliphatic polyester; Thermal property; In vitro degradation

1. Introduction In recent years, biodegradable polymers have attracted considerable attention for the last two decades. Great efforts have also been made to obtain biodegradable polymers with appropriate properties for biomedical and ecological application. Till now many kinds of biodegradable polymers have been synthesized for this purpose, including aliphatic polyester [1,2], poly(a-amino acid)s [3], poly(ortho ester)s [4], polyanhydride [5], polyesteramide copolymer [6– 11], and etc. Among these biodegradable polymers, synthetic aliphatic polyesters have been widely investigated [1,2,12 – 17]; however their limited mechanical property due to its relatively low

* Corresponding author. Tel.: +86 28 85404516; fax: +86 28 85460982. E-mail address: [email protected] (ZY. Qian). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.07.074

molecular weight have restricted their further application. In recent years, the aliphatic polyesters with high molecular weight were produced from ethylene glycol or butanediol and adipic acid or succinic acid, and then coupled with a small amount of chain extenders, by Showa Highpolymer under the trademark of BIONOLLE [14 –16]. In order to obtain biodegradable polymers with enhanced mechanical properties, many polyester based copolymer, including polyesteramide [6– 11], poly(ester urethane) [18 – 23], have been extensively studied. Due to the tunable property of poly(ester urethane), it was widely studied recently. In this article, a new kind of aliphatic copolyester was prepared by melt polycondensation based on caprolactone, adipic acid, and 1,6-hexanediol, using pentaerythritol as the crosslinking agent. And the chemical structure, thermal property, water absorption, and hydrolytic degradation

32

C.B. Liu et al. / Materials Letters 60 (2006) 31 – 38

O

O

CH2OH

O

C O + HO

C

(CH2)4

C

OH + HO

(CH2)6

OH + HOH2C

C

CH2OH

CH2OH

Catalyst Vacuum O

O

C (CH2)4 CH2 O

C (CH2)4 CH2

O O

x

O

C (CH2)4 C O (CH2)6 O

y

CH2O OH2C

C CH2O

CH2O

O

O

C (CH2)4 CH2 O

C (CH2)4 CH2

O O

x

O

C (CH2)4 C O (CH2)6 O

y

Scheme 1. The synthesis scheme of P(CL / AAH) copolyester.

q-Caprolactone (CL, purchased from Alfa Aesar), adipic acid (AA), pentaerythritol (P) and 1,6-hexane diol (HD) are all analytic reagent grade. All these materials were used as received without further purification.

These P(CL/AAH)x / y-Pz copolymers were synthesized from q-caprolactone, adipic acid, and 1,6-hexanediol by melt polycondensation method according to Scheme 1. Pentaerythritol was used as the crosslinking agent. The typical P(CL / AAH)50 / 50-P1 copolymer was prepared as follows: CL(0.15 mol, 17.1 g), AA (0.15 mol, 21.9 g), HD (0.15 mol, 17.7 g), pentaerythritol (0.0045 mol, 0.75 g), and tetrabutyl titanate (0.05 g) were added into the reaction vessel under a nitrogen atmosphere. The mixture was kept at 120– 160 -C for 1.5 h. Then, the mixture was rapidly heated to 240 -C under vacuum for about 1 h. At the end, the resultant hot melt was poured out onto a steel plate, thus P(CL / AAH)50 / 50-P1 copolymer was obtained. All the samples prepared in this work were listed in Table 1.

2.2. Synthesis of P(CL/AAH)x/y-Pz copolymers

2.3. Intrinsic viscosity measurement

In this article, the copolymer was denoted as P(CL/ AAH)x / y-Pz, where x / y represented the molar ration of CL / AA, z represented the molar content of pentaerythritol.

Intrinsic viscosity [g] was measured at 30 T 0.1 -C by using an Ubbelohde viscometer. All the copolymers were dissolved in m-cresol to prepare solutions at a concen-

behavior of this copolyester were studied in detail. This copolyester could be used as the prepolymer for preparing biodegradable poly(ester urethane). And the properties of this copolyester based poly(ester urethane) would be studied in detail later.

2. Experimental 2.1. Materials

Table 1 The samples prepared in this work Sample P(CL / AAH)50 / 50-P1 P(CL / AAH)50 / 50-P2 P(CL / AAH)50 / 50-P3 P(CL / AAH)60 / 40-P3 P(CL / AAH) 40 / 60-P3 a b c

Content of PA (mol%) 0.99 0.50 0.25 0.25 0.25

[g] (dL/g) c

– – c 1.05 0.55 1.02

[COOH] (mgKOH/g)

Mn (a) (g/mol)

Mn (b) (g/mol)

Mw / Mn (b)

2.79 2.72 1.79 4.24 3.52

20,111 20,600 31,300 13,200 15,900

11,500 20,500 18,700 15,100 19,800

5.5 16.5 10.2 3.3 10.5

Determined from COOH titration. Determined from GPC calculation. Due to the lower solubility in m-cresol, the inherent viscosity couldn_t be tested accurately.

C.B. Liu et al. / Materials Letters 60 (2006) 31 – 38

tration of ca.0.5 g/dL. [g] was calculated according to Eq. (1):

½ g ¼

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  ffi t t  1  1n 2 t0 t0 C

33

2.8. Water absorption The copolyester chips were immersed in distilled water at 30 -C for predetermined period, then they were taken out and the surplus surface water were removed by a filter paper. The value was calculated according to Eq. (2):

ð1Þ

:

Water absorption% ¼

W

 Wd  100: Wd

ht

ð2Þ

Where C is concentration of the polymer solution, t is flow time of solution, and t0 is flow time of pure solvent.

Where W d is the initial weight of dry sample, and W ht is the weight of humid sample at time t.

2.4. Differential Scanning Calorimetry (DSC)

2.9. Alkaline hydrolysis behavior

Non-isothermal crystallization behavior of P(CL / AAH)x / y copolymers was characterized on 204 Phoenix differential scanning calorimeter (NETSCZ 204, Germany). The specimens were heated from 20 to 120 -C under nitrogen atmosphere at a heating rate of 10 -C/min, and cooling rate of 10 -C/min.

Alkaline hydrolysis of the chips was conducted in 0.1 mol/L (pH = 13) and 0.01 mol/L (pH = 12) sodium hydroxide (NaOH) aqueous solution at 37 -C. Degradation behavior was studied by weight loss according to Eq. (3). The degree of degradation was calculated by weight loss:

2.5. Thermogravimetric analysis

Weight lossð%Þ ¼

W0  Wt  100: W0

ð3Þ

Thermogravimetric measurements (TGA/DTA) were characterized on thermogravimetric analyzer (TA 2910, DuPont, USA) coupled with Perkin-Elmer computerized data station under nitrogen atmosphere at a heating rate of 10 -C /min in the range of room temperature to 600 -C.

3. Results and discussion

2.6. Fourier transform infrared spectroscopy (FTIR)

3.1. Polymer synthesis and characterization

FTIR (KBr) spectra of copolymers were taken in NICOLET-560 (Nicolet co. USA) Infrared Spectrophotometer.

The copolyesters were prepared by melt copolymerization of caprolactone, adipic acid, and 1,6-hexanediol with pentaerythritol as branching agent. Table 1 summarized the parameters for the synthesis of such copolymers, including pentaerythritol content, CL / AA molar ratio, inherent viscosity, and macromolecular weight. According to Table 1, with the increase in pentaerythritol, the macromolecular weight decreased slightly. And the macromolecular weight decreased to some extent with the increase in caprolactone too.

Where W 0 is the dry weight before degradation, W t is the dry weight at time t.

2.7. 1H-Nuclear Magnetic Resonance (1H-NMR) 1

H-NMR spectra (in CDCl3) were recorded using a Varian 400 spectrometer (Varian, USA) at 400 MHz using trimethylsilane as an internal reference standard.

Table 2 The solubility of P(CL / AAH) copolymers Sample

Temperature

P(CL / AAH) 50 / 50-P1 P(CL / AAH) 50 / 50-P2 P(CL / AAH) 50 / 50-P3 P(CL / AAH) 60 / 40-P3 P(CL / AAH) 40 / 60-P3

20 35 20 35 20 35 20 35 20 35

-C -C -C -C -C -C -C -C -C -C

CH3OH

Acetone

CHCl3

THF

DMSO

CHCl2

DMF

Toluene

 +/  +/  +/  +/  +/

++/ ++/ + + + + + + + +

++/ ++/ + + + + + + + +

++/ ++/ + + + + + + + +

 ++/  ++/  ++/  +  +/

++/ ++/ + + + + + + + +

++/ ++/ + + + + + + + +

++/ ++/ + + + + + + + +

+: soluble, : not soluble, +/: part soluble, ++/: a great deal soluble, +/: a few soluble.

34

C.B. Liu et al. / Materials Letters 60 (2006) 31 – 38 100 90

3536.86 3446.98

1394.30

2866.07

80

%Transmittance

%Transmittance

100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25

1465.78 1416.78 1368.91

2948.05

70

3538.79 3447.25

60 50 40

1465.48

30

1416.92

20

2866.22

10 1733.06

0

P(CLO/AAH)-50/50-P1

3000

2000

2948.8

3000

1000

2000

1000

Wavenumbers (cm-1) 100

100 90 70 60 50 2864.29

40

1460.62 1390.88 1357.92

30 20 2941.80

10

90

1421.74

3546.51 3448.29

80

%Transmittance

%Transmittance

1369.19 1738.05

P(CLO/AAH)-50/50-P2

Wavenumbers (cm-1)

80

1395.77

70

3545.82

3448.48 1421.40

60 50 40 30 20 2864.43

10

0

0 1734.13

-10 P(CLO/AAH)-50/50-P3 3000

2000

2942.19

-10 P(CLO/AAH)-60/40-P3 1000

3000

Wavenumbers (cm-1)

1460.76 1390.86 1357.91 1735.21

2000

1000

Wavenumbers (cm-1)

100 90

%Transmittance

80

1421.30

3546.58 3450.08

70 60 2861.74

50

1460.78 1390.93 1358.15

40 30

2935.21

20 10 0 -10

1734.44 P(CLO/AAH)-40/60-P3

3000

2000

1000

Wavenumbers (cm-1) Fig. 1. The FTIR spectra of P(CL / AAH) copolymers.

For these P(CL / AAH) copolymers, the monomers feed ratio was CL : AA : HD = 1 : 1 : 1, and pentaerythritol was added in the range of 0.25 – 0.99% mol. So, the gel point would not be theoretically reached under this given condition. But in our experiment, the polycondensation was conducted at high temperature in high vacuum, loss of monomers was inevitable, thus, the equilibrium was interrupted. According to the solubility experiment, which was shown in Table 2, with the increase in pentaerythritol, the solubility decreased then. This result implied the occurrence of crosslinking between the polymer chains. Because the lower solubility in m-cresol, the inherent viscosity

of P(CL / AAH)50 / 50-P1 and P(CL / AAH)50 / 50-P2 were not tested in this article. According to Table 2, with the increase in molar content of caprolactone, the solubility of the copolymers increased then. The as-polymerized P(CL / AAH)x / y-Pz copolymers were characterized by H1 NMR and FTIR. The results were shown in Figs. 1 and 2. Because the weight fraction of pentaerythritol used in this experiment was lower than 2% wt, the peak due to pentaerythritol couldn’t be observed in FTIR spectra. The major characteristic absorptions identified in FTIR spectra were: aliphatic ester groups c c = o (ca.1735 cm 1). In Fig. 1,

C.B. Liu et al. / Materials Letters 60 (2006) 31 – 38

35

Fig. 2. The H-NMR spectra of P(CL / AAH) copolymer.

there was no evident difference between the P(CL / AAH) copolymers. A typical H1 NMR spectrum of the P(CL / AAH)x / y-P3 copolymer was shown in Fig. 2, and the characteristic absorption peaks are also indicated in this figure. 3.2. Non-isothermal crystallization behavior of copolyesters For these P(CL / AAH)x / y block copolymers, there were no evident crystallization and melt peaks found during the heating and cooling run, and the DSC spectra were not shown in this article. That’s to say, this P(CL / AAH) copolymers were non-crystalline. 3.3. Thermal degradation behavior of P(CL/AAH)x/y copolyesters The thermal degradation behavior of P(CL / AAH) copolymers were studied by TGA/DTA method. For these P(CL / AAH) copolymers, the thermal degradation behavior was mainly determined by pentaerythritol content and the caprolactone

Table 3 TGA/DTA data of P(CL / AAH)50 / 50-Pz copolymers at a heating rate of 10 -C /min in nitrogen Sample T d, T d, T d, T d, T d, a b c d e

a

(-C) (-C) c 95% (-C) d (-C) onset e max (-C) 5%

b

50%

Temperature Temperature Temperature Temperature Temperature

P(CL / AAH)50 / 50-P1

P(CL / AAH)50 / 50-P2

P(CL / AAH)50 / 50-P3

304.79 400.19 437.50 371.98 415.72

319.25 398.77 420.19 390.30 416.96

307.48 403.63 424.19 387.58 417.97

at which weight loss of 5%. at which weight loss of 50%. at which weight loss of 95%. at which the decomposition started. of the largest decomposition rate during the stage.

content. For these P(CL / AAH)x / y-Pz copolymers, there was just one thermal degradation peak in the DTA curves, which suggested that these copolymers might be random copolymers. 3.3.1. Effect of pentaerythritol content As was shown in Table 3 and Fig. 3, with the decrease in pentaerythritol, the decomposition temperature T d, onset, T d, 5%, and T d, max increased then, but T d, 95% decreased to some extent. 3.3.2. Effect of CL / AAH molar ratio According to Table 4 and Fig. 4, with the increase in qcaprolactone, the decomposition temperature T d, onset, T d, 50%, T d, 95% and T d, max increased then, but T d, 5% decreased to some extent. 3.4. Water absorption of P(CL/AAH)x/y copolymers The water absorption of P(CL / AAH) copolymers were mainly determined by the pentaerythritol content and the molar ratio of CL / AAH. According to Fig. 5, with the increase in pentaerythritol, the water absorption increased accordingly, which might be due to the increase in crosslinking between the polymer chains due to the increase in pentaerythritol. As was shown in Fig. 6, the water absorption of the copolymers didn_t change distinctly with the change in caprolactone content, which might be due to the similarity of the chemical structure between q-caprolactone, adipic acid, and 1,6-hexane diol. Thus there is no distinct difference between the hydrophilicity of these P(CL / AAH)x / y-P3 copolymers with different molar ratio of CL / AAH. 3.5. Alkaline degradation behavior of P(CL / AAH)x / y copolymers In this article, the alkaline degradation behavior of P(CL / AAH) copolymers were studied. And the degradation behavior was

36

C.B. Liu et al. / Materials Letters 60 (2006) 31 – 38 2.5

1.0

60 40

0.5

20 0.0

0 100

200

300

400

500

1.5

60

1.0

40 20

0.5

0

0.0

600

0

100

Temperature (oC)

200

300

400

500

600

Temperature (oC) 3.0

100 P(CLO/AAH)50/50-P3

Weight Loss (%)

2.0

P(CLO/AAH)50/50-P2

2.5

80

2.0 60 1.5 40

1.0

20

0.5

0

Deriv. Weight (%/oC)

0

80

Deriv. Weight (%/oC)

P(CLO/AAH)50/50-P1

Deriv. Weight (%/oC)

Weight Loss (%)

80

100

1.5

Weight Loss (%)

100

0.0 0

100

200

300

400

50 0

600

Temperature (oC) Fig. 3. Effect of pentaerythritol on TGA/DTA traces of P(CL / AAH) 50 / 50 copolymers in nitrogen.

mainly determined by the pentaerythritol content and the molar ratio of CL / AAH. According to Fig. 7, with the increase in pentaerythritol, the degradation rate increase then, which might be due to the decrease of macromolecular weight. According to Fig. 8, the degradation rate increased accordingly with the increase in caprolactone content, which might be due to the relatively higher degradation rate of polycaprolactone homopolymer compared with the aliphatic polyester based on adipic acid, 1,6-hexane diol.

1,6-hexanediol were prepared by melt polycondensation method. With the increase in pentaerythritol, the water absorption behavior of the copolymers increased then, but the water absorption didn_t change distinctly with the content of caprolactone. The alkaline degradation behavior of these copolyesters were also studied, the degradation rate increased with the increase in pentaerythritol and the caprolactone content. References

4. Conclusion In this article, a series of biodegradable aliphatic copolyesters based on e-caprolactone, adipic acid, and Table 4 TGA/DTA data of P(CL / AAH)x / y-P3 copolymers at a heating rate of 10 -C /min in nitrogen Sample T d, T d, T d, T d, T d, a b c d e

a

(-C) (-C) c 95% (-C) d (-C) onset e max (-C) 5%

b

50%

Temperature Temperature Temperature Temperature Temperature

P(CL / AAH)40 / 60-P3

P(CL / AAH)50 / 50-P3

P(CL / AAH)60 / 40-P3

314.46 383.23 397.42 370.59 393.27

307.48 403.63 424.19 387.58 417.97

277.85 387.62 402.42 374.77 395.74

at which weight loss of 5%. at which weight loss of 50%. at which weight loss of 95%. at which the decomposition started. of the largest decomposition rate during the stage.

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C.B. Liu et al. / Materials Letters 60 (2006) 31 – 38

40

1

20

80

0

100

200

300

400

500

2.5 2.0

60 1.5 40 1.0 20

0

0

P(CLO/AAH)50/50-P3

0.5

0

Deriv. Weight (%/oC)

2

60

Deriv. Weight (%/oC)

Weight Loss (%)

80

3.0

100

3

P(CLO/AAH)40/60-P3

Weight Loss (%)

100

37

0.0

600

0

100

Temperature (oC)

200

300

400

500

600

Temperature (oC) 3.0

Weight Loss (%)

2.5 80

P(CLO/AAH)60/40-P3

2.0

60

1.5

40

1.0

20

0.5

0

0.0 0

100

200

300

400

500

Deriv. Weight (%/oC)

100

600

Temperature (oC) Fig. 4. Effect of CL / AAH on TGA/DTA traces of P(CL / AAH)-P3 copolymers in nitrogen.

PCL/AAH-50/50-P1 PCL/AAH-50/50-P2 PCL/AAH-50/50-P3

5

P(CLO/AAH)-50/50-P1 P(CLO/AAH)-50/50-P2

4

6

Weight loss (%)

Water Absorption (%)

8

4

2

0

P(CLO/AAH)-50/50-P3

3 2 1 0

0

20

40

60

0

80

1

2

3

Fig. 5. Effect of pentaerythritol on water absorption behavior of P(CLO / AAH) copolymers in distilled water at 37 -C.

3.0

Weight loss (%)

Water Absorption (%)

1.0 0.8 0.6 0.4 P(CL/AAH)40/60-P3 P(CL/AAH)50/50-P3 P(CL/AAH)60/40-P3

0

20

40

6

7

8

P(CLO/AAH)-40/60-P3 P(CLO/AAH)-50/50-P3 P(CLO/AAH)-60/40-P3

2.5

0.0

5

Fig. 7. Effect of pentaerythritol on alkaline degradation behavior of P(CLO/ AAH) copolymers in 0.1M NaOH solution at 37 -C.

1.2

0.2

4

Time (day)

Time (Hour)

60

80

Time (Hour) Fig. 6. Effect of CL / AAH on water absorption behavior of P(CLO-AAH) copolymers in distilled water at 37 -C.

2.0 1.5 1.0 0.5 0.0 0

1

2

3

4

5

6

7

8

Time (day) Fig. 8. Effect of CL / AAH on alkaline degradation behavior of P(CLOAAH) copolymers in 0.1 M NaOH solution at 37 -C.

38

C.B. Liu et al. / Materials Letters 60 (2006) 31 – 38

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