Synthesis, structural study and hydrolytic degradation of copolymer based on glycolic acid and bis-2-hydroxyethyl terephthalate

Polymer Degradation and Stability 94 (2009) 221–226

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Synthesis, structural study and hydrolytic degradation of copolymer based on glycolic acid and bis-2-hydroxyethyl terephthalate E. Olewnik*, W. Czerwin´ski Department of Physical Chemistry, Faculty of Chemistry, Nicolaus Copernicus University, Gagarin 7, 87-100 Torun, Poland

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 July 2008 Received in revised form 28 October 2008 Accepted 28 October 2008 Available online 6 November 2008

The current demand for environmentally degradable copolymers has initiated the use of novel degradable copolyesters. One of them is a copolyester based on poly(ethylene terephthalate-co-glycolic acid) (PET–GLA). The copolymer was synthesized by the melt reaction of bis-2-hydroxyethyl terephthalate (BHET) with glycolic acid (GLA) oligomers in the presence of Sb2O3 as a catalyst. Hydrolytic degradation of the copolymer was carried out in two buffered solutions at 45  C: degradation was studied by incubating samples in powder form, in a concentrated solution from 30 to 150 days. The copolymer before and after degradation was characterized by means of different analytical techniques. 1H and 13C NMR spectroscopy was used to confirm the incorporation of glycolide units in PET chains and to observe the structure and decomposition of the novel polyester. The thermal properties and morphology before and during the degradation were studied by scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis for determining melting points as well as melting and decomposition temperatures of investigated copolyester. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: Copolymerization Hydrolysis Glycolic acid Poly(ethylene terephthalate)

1. Introduction Every year, several hundred thousand tons of non-degradable plastic products are introduced into the environment. The continuously increasing extent of pollution of the environment has recently given rise to demands for novel biodegradable polymers mainly for applications related to food packaging [1]. One of the major commercially available polymers with widespread application is poly(ethylene terephthalate) (PET). Post-consumer PET can be recycled into new items or into monomers or oligomers which are generally hydroxy-terminated and can be used for the synthesis of derived polyesters of PET such as PET – poly(lactide), PET – poly(glycolide), or PET – poly(3-caprolactone) [2,3]. In this paper, we describe the synthesis of novel biodegradable copolyester PET– GLA prepared by melt polycondensation from bis-2-hydroxyethyl terephthalate (BHET) and poly(glycolic acid) (PGLA) oligomers. Its structure, microstructure and degradation were investigated. Glycolic acid was chosen because it belongs to a group of popular materials widely used in biomedical applications because of its biocompatibility and bioresorbability [4]. The copolymerization of BHET with GLA is regarded as a good way of obtaining new polymeric materials with original physical, chemical and biological properties adaptable to specific uses. Moreover, because of the formation of ester bonds in the main * Corresponding author. Tel.: þ48 56 611 48 38; fax: þ48 56 654 24 77. E-mail address: [email protected] (E. Olewnik). 0141-3910/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2008.10.026

chain, which are sensitive to hydrolysis, the novel copolymer seems to be biodegradable. It has been reported that during the hydrolysis of the copolymer, water molecules diffuse into the amorphous region and cleave some of the disordered chains [5]. The new copolymer PET–GLA produced by melt polycondensation in our laboratory was studied in two buffered solutions at 45  C. Degradation of copolyester was studied within a period of 30–150 days. The properties of this polymer and its potential for undergoing degradation were investigated by methods of 1H NMR spectroscopy, differential scanning calorimetry (DSC), thermogravimetry (TG) and scanning electron microscopy (SEM). 2. Experimental 2.1. Materials Glycolic acid (70 wt-% aqueous solution) and antimony(III) oxide (Sb2O3) were purchased from Sigma–Aldrich (Germany). Ethane1,2-diol (EG) and cobalt(II) acetate tetrahydrate were obtained from POCH (Poland). Post-consumer soft-drink bottles of PET were obtained from ELANA SA, Torun, Poland. 2.2.

1

H NMR spectroscopy

CDCl3 was used as solvent for BHET and GLA, but copolymer samples PET–GLA were dissolved in CDCl3/trifluoroacetic acid

´ ski / Polymer Degradation and Stability 94 (2009) 221–226 E. Olewnik, W. Czerwin

222

solvent mixture (3/1 vol/vol). The spectrum was recorded immediately in order to avoid end-group esterification by trifluoroacetic acid [6]. 1H NMR spectra were recorded at 300 MHz using a Bruker AC300 spectrometer. The CHCl3 resonance at 7.26 ppm was taken as chemical shift reference. 2.3. Differential scanning calorimetry (DSC) The DSC analyses were carried out under nitrogen with flow rate about 15 ml/min on DSC (Polymer Laboratories, Epson, GB) at heating rates of 10  C/min from room temperature to 200  C, and using samples of 5–9 mg. The pans used during DSC analyses were not hermetically sealed. 2.4. Thermogravimetric analysis (TGA) The thermogravimetric analyses were carried out under nitrogen in the 20–600  C temperature range at heating rates of 10  C/min. TA Instruments, SPT 2960 Simultaneous DSC–TGA was used for studying thermal behaviours of degraded and nondegraded samples which were in powder form. 2.5. Condition of the degradation tests Hydrolytic degradation tests were performed in 100 ml in two buffered solutions (phosphate (pH 7.40) and phosphate-citric (pH 7.35)) at 45  C. The composition of the buffer solutions is as shown in Table 1. The degradation medium was not renewed during the degradation period. After 30th, 60th, 90th, 120th and 150th days of degradation samples were removed from the medium, rinsed thoroughly in distilled water and dried in vacuum for further measurements [7,8]. A 736 GP Titrino pH meter (0.01 resolution) was used to measure the pH of the solutions after degradation time. 2.6. Morphology SEM The morphology of copolyester before and after hydrolysis was observed under a LEO 1430VP scanning electron microscope (SEM). The samples in powder form were covered with gold before the investigation. 2.7. Synthesis of substrates of new copolyester Dehydration of 100 g of glycolic acid aqueous solution (70 wt-%) was performed at 100  C under atmospheric pressure for 2 h, then heated for next 2 h at 170  C under reduced pressure (4.7 kPa). During this time glycolic acid polyesterification took place, resulting in the formation of poly(glycolic acid) (PGLA) oligomers as a solid. The glycolysis of poly(ethylene terephthalate) to obtain bis(2hydroxyethyl terephthalate) (BHET) is presented in earlier our work [9].

Table 1 Composition of the two buffer solutions for degradation. Phosphate buffer solution (pH 7.40)

Phosphate-citric buffer solution (pH 7.35)

Na2HPO4 NaH2PO4 NaCl Sodium azide

0.2 M NaH2PO4 0.1 M citric acid

7.24 g/l 0.75 g/l 5.90 g/l 0.50 g/l

89.9 ml 13.1 ml

According to PN-81/C-06504.

2.8. Synthesis of PET–GLA copolyester Ethylene terephthalate–glycolic acid copolyesters (PET–GLA) were prepared by the melt reaction of bis(2-hydroxyethyl terephthalate) with glycolic acid oligomers (80/20 wt-%; repeating unit mol ratio 0.88:1) in the presence of Sb2O3 (0.1842 wt-%) (Fig. 1). The reaction was maintained for 6 h at 180  C and 0.4 kPa. While the reactants were stirred, glycol and water slowly distilled out. Copolyester was used for further characterization and hydrolytic degradation without purification. 3. Results and discussion 3.1. NMR study of glycolic acid (GLA) oligomers There are only a few references available in the literature presenting the NMR assignments of poly(glycolic acid) (PGLA). The assignments of 1H NMR spectra of GLA oligomers in CDCl3 have been reported by Furch et al. but also in DMSO-d6 [10]. A detailed investigation of the 1H NMR spectra of GLA oligomers in CDCl3 is presented in this study. The 1H NMR spectrum of GLA in CDCl3 is given in Fig. 2. The spectrum allows us to see four characteristic signals. The resonances of chain methylene (H3) are assigned at 4.90 ppm. The –CH2–OH methylene (H2) and –CH2–COOH (H4) give a doublet at 4.30 ppm and at 4.78 ppm, respectively. The very broad signal H1 corresponds to the –CH2–OH end units. The number-average degree of polymerisation of GLA oligomers was determined from the ratio of H2, H3 and H5 integrals and it was 3. The presence of cyclic glycolide which may form during the reaction of polycondensation of glycolic acid (when aiming to obtain oligomers) was not observed. 3.2. NMR study of PET–GLA copolyester The molar masses and compositions of copolymer PET–GLA before and after hydrolysis in the two buffer solutions were determined by 1H NMR spectroscopy using the relative integrated value of the distinctive signals. Because of the problem relating to solubility, NMR spectra of PET–GLA copolyester were recorded in mixture of CDCl3 and trifluoroacetic acid. An 1H NMR spectrum for the PET–GLA copolyester is given in Fig. 3. Atom numbering and assignments are reported in Table 2. The total integrations of aromatic protons (H1 at 8.10 ppm) and methylene groups originating from glycolic acid (H7,14, H17 at 4.90–4.97 ppm and H18 at 4.20 ppm) allowed us to determine the actual terephthalate (T)/ glycolide (G) mol ratio. It can be observed that the T/G mol ratio in the final copolyester is significantly lower than the starting one (T/G)0 (Table 3) which might be caused by partial degradation and elimination of glycolic acid units during synthesis as a result of thermal degradation. Signals derived from methylene groups can be divided in three parts: H14 (4.97 ppm) was assigned to T–G diads, at 4.90 ppm (H7,17) belonging to T–E–G triads and diads G–G, at 4.20 ppm resonances were assigned to –CH2–OH groups according to the 1H NMR spectrum of GLA. The peak at 4.69 ppm is assigned to T–E–T triads (H2, PET blocs). The reaction does not course without the secondary processes like etherification. This conclusion can be based on the resonances at 4.26 ppm (H13,15,19), 3.86 ppm (H9,11,16) and 3.72 ppm (H12). Hydroxyethyl terephthalate and glycolide –CH2–OH give resonances at 3.96 (H4) and 4.47 ppm (H3), respectively. The new signals noted at 4.97 ppm (H14) and at 4.55 ppm (H5,6) correspond to T–G diads and T–E–G triads, respectively. Some peaks are superimposed for example at 4.47 ppm (H3) of methylenes in b-position to OH end groups and to ether oxygens (H8,10). Because of this overlapping it was impossible to calculate of

´ ski / Polymer Degradation and Stability 94 (2009) 221–226 E. Olewnik, W. Czerwin

x OH CH2 CH2 O

C

C O CH2

O

O

CH2 OH + y H

O

CH2 C O

223

OH 3

175°C, 0.4 kPa Sb2O3

O CH2

CH2 O

C

C

O

O m1

O

CH2 C O

O CH2

CH2 O

n

C

C

O

O

m2

Fig. 1. Scheme of synthesis reaction of PET–GLA copolymer.

number fractions FE(TEG) and FE(GEG). Only FE(TET) of TET E-centred triads were determined and it is proportional to I2/4 where In is the integration of Hn resonance. It is interesting that by knowing one of the number fractions it is possible to calculate the other ones. The degree of randomness, B, and the number-average sequence lengths were determined according to Ref. [11] (Table 3). It must also be stressed, that the number-average molar mass ðM n Þ can also be determined from the 1H NMR spectrum. A 3-fold excess of OH to COOH allowed us to assume that the copolyester does not contain –COOH end groups, or contains a negligible amount of it. Therefore, the number-average number of T units per molecule, m, and of G units per molecule, p, are given by Eqs. (1) and (2), and M n by Eq. (3). The results are presented in Table 3.

m ¼

I1 2ðI4 þ I16 þ I18 Þ

(1)

content of easily hydrolysable aliphatic ester bonds decreased significantly and was accompanied by a more notable increase in the length of ET sequences what is surprising but can be explained when we assume that shorter chains are degraded and rinsed by buffer solution. It can also be seen that in the investigated process of hydrolytic degradation the degree of randomness B introduced by Yamadera and Murano [12] increases when glycolide unit content decreases. This proves that the copolymer displays a tendency to form alternating G–E–T sequences which is clearly and in detail explained by Tessie and Fradet [11]. The gradual progress in the reduction of the mass of investigated copolyester in function of time of degradation is presented in Fig. 4. It could be assumed that the hydrolytic degradation of copolymers proceeded faster in phosphate-citric buffer solution in comparison with phosphate buffer solution under the same conditions. 3.3. Thermal properties


I7;14;17 þ I18 p ¼ ðI4 þ I16 þ I18 Þ

(2)

M n ¼ 192m þ 68p þ 62

(3)

1

H NMR spectroscopy was used to determine the value of the molar mass M n , the degree of randomness B and the numberaverage sequence lengths after degradation processes. Chemical structure before and after the hydrolysis process of investigated copolyester is given in Table 3. It can be observed that the overall

Differential scanning calorimetry was used to determine the melting temperature (Tm) of the copolyester before and after different periods of degradation (from 30 to 150 days) in two different buffer solutions (Fig. 5). Incorporation of a short sequence length of aliphatic part in chain still results in the new copolymer PET–GLA being semi-crystalline and it exhibits melting transitions at 166  C before, 162  C and 159  C after degradation in phosphate buffer and in phosphate-citric buffer solutions, respectively. Because of the disruption in the PET chain caused by the

H1

H9,11,16

CDCl3 1 OH

2 CH2 C O

3 CH2 C O

O

4 CH2 C

O

3.9

OH

3.8

3.7

H5,6

O

n

H12

H2

H2

H7,17

H4

H3

H1

7.5

7.0

6.5

6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

ppm Fig. 2. 1H NMR spectrum (300 MHz, CDCl3, ref d (CHCl3) ¼ 7.26 ppm) of GLA.

8.0

7.5

7.0

H13,15,19 H18 4 H

H14

CDCl3 8.5

H3,8,10

6.5

6.0

5.5

5.0

4.5

4.0

3.5

3.0

ppm Fig. 3. 1H NMR spectrum (300 MHz, CDCl3, ref d (CHCl3) ¼ 7.26 ppm) of PET–GLA copolyester.

´ ski / Polymer Degradation and Stability 94 (2009) 221–226 E. Olewnik, W. Czerwin

224 Table 2 Atom numbering in PET-GLA copolyesters.

1 C

2 C O CH2 CH2 O C

C

O

O

O

O

C

3 4 C O CH2 CH2 OH

O

O

C

14 C O CH2 C O

O

O

O CH2 C O

O

15 16 CH2 CH2 OH

O

C

7 5 6 C O CH2 CH2 O C CH2 O

O

O

17 O CH2 C O CH2 C O CH2 C O

O

O

O

O

C

8 9 C O CH2 CH2 O CH2 CH O C 2

C

18 C CH2 OH

O

O

O

O

O

C

12 13 10 11 C O CH2 CH2 O CH2CH2 O C CH2 O

O

O

19 O CH2 C O CH2 CH2 O

O

O

C CH2 O O

Table 3 1 H NMR characterization of PET–GLA copolyester before and after degradation: mole fraction FT/FG, number-average block length of T and G units in ethylene terephthalate and glycolide blocks Ln;EþT and Ln;G , degree of randomness, B and molar mass M n . Time of degradation (days)

Phosphate buffer

0 30 60 90 120 150

Phosphate-citric buffer

FT/FG

Ln;EþT

Ln;G

B

Mn

FT/FG

Ln;EþT

Ln;G

B

Mn

64.3/35.7 67.8/32.2 69.0/31.0 70.1/29.9 70.4/29.6 71.7/28.3

4.40 5.08 4.96 4.96 5.00 4.92

1.20 1.14 1.12 1.08 1.05 0.97

1.06 1.07 1.10 1.13 1.15 1.23

2120 2110 1950 1870 1820 1770

64.3/35.7 68.6/31.4 71.7/28.3 72.0/28.0 72.7/27.3 77.2/22.8

4.40 4.82 4.85 4.96 5.04 4.50

1.20 1.03 0.96 0.96 0.94 0.66

1.06 1.17 1.25 1.25 1.26 1.73

2120 2070 1810 1790 1640 1610

incorporation of aliphatic units the melting temperature of PET– GLA copolymer is much lower than that of the original PET (265  C) and PGA (220  C). Characteristic differences were observed in the thermal behaviour of the copolyester, where the melting temperature decreases with time of degradation as is clearly shown in Fig. 5. This

2400

Temperature (oC)

Mn

2000

1600

1200

0

40

80

120

160

Hydrolysis time (days) Fig. 4. The mass losses of copolymer PET–GLA during hydrolysis in phosphate buffer (:) and in phosphate-citric buffer (,) solutions.

167 166 165 164 163 162 161 160 159 158

0

20

40

60

80

100

120

140

160

Hydrolysis time (days) Fig. 5. Changes in melting temperature of copolymer PET–GLA during hydrolytic degradation in phosphate buffer (:) and in phosphate-citric buffer (,) solutions.

´ ski / Polymer Degradation and Stability 94 (2009) 221–226 E. Olewnik, W. Czerwin

copolymers increases from 238  C to 298  C after 150 days of hydrolysis in phosphate-citric buffer solution.

Table 4 The temperature ranges of decomposition of copolyester PET–GLA. Time of degradation (days)

0 30 60 90 120 150

Phosphate buffer

Phosphate-citric buffer

T5%

T10%

Tmax

T5%

T10%

Tmax

238 276 278 280 286 287

309 328 330 331 331 330

437 433 433 433 435 435

238 272 276 278 282 298

309 321 323 328 325 330

437 433 433 433 433 434

7,6 7,4 7,2 7

pH

225

6,8 6,6

3.4. Changes in pH of buffer solutions The degradation studies of PET–GLA copolyester were focused on hydrolysis in two kinds of buffer till 150 days. pH of used solutions was measured before and after degradation time. It is well known that the ester bonds originate from aliphatic units broken randomly to generate carboxyl groups. This carboxyl groups accelerate the degradation rate by the autocatalytic action of the carboxylic acid end groups of glycolic acid. It is easy to observe that the changes of pH are not only a function of hydrolysis time but also depend on the kind of buffer solution and its power (capacity) (Fig. 6). Comparison value of pH buffers showed that pH of the phosphate-citric buffer has been changing faster than the one of phosphate buffer. For this reason we can assume that phosphate-citric solution possesses lower capacity and in this way it can influence the kinetics of hydrolysis. 3.5. Morphology

6,4 6,2 6

0

20

40

60

80

100

120

140

160

Hydrolysis time (days) Fig. 6. Changes in pH during hydrolysis in phosphate buffer (:) and in phosphatecitric buffer (,) solutions.

phenomenon can be explained by the scission of the ester bonds in the amorphous regions of the copolymer where the segments can be more easily attacked by buffer solutions. The largest drop of Tm was about 7  C after incubating in phosphate-citric buffer solutions. The other technique which was used to measure the thermal stability of polymers, the influence and efficiency of additives and the behaviour of copolymers is thermogravimetry. In the present work, the thermal properties of copolyester PET–GLA before and after degradation were investigated. The main criteria to indicate the thermal stability of the copolymers are the percentage weight loss as a function of temperature. T5, T10 (temperature for 5% and 10% weight loss, respectively) and the temperature at maximum of weight loss (Tmax) (Table 4). Based on the obtained results it can be observed that the thermal stability of investigated copolyester improves as the degradation progresses for all of hydrolysis reactions, in both buffer solutions. This phenomenon can be explained in view of the fact that during hydrolysis the content of GLA decreases and in this case the temperature for 5% weight loss

Scanning electron microscopy is a useful technique for analysing the morphology of the copolymer before and after degradation. The photographs of one of the samples before and after hydrolysis in phosphate-buffered solution are presented in Fig. 7. Before hydrolysis the sample displays an initial smooth surface. After 150 days of degradation some spot defects are observed as well as microcracks covering the whole surface. This may well be the reason why the copolyester looked different before and after degradation. On the basis of the above-mentioned results it can be supposed that the hydrolysis of PET–GLA can be accelerated by the presence of hydrophilic water-insoluble aliphatic part (GLA) [13]. Probably, the water concentration around PET–GLA molecules was raised by the presence of hydrophilic phases, which promoted the autocatalytic hydrolysis of GLA [14,15]. 4. Conclusions – PET–GLA copolyester was prepared by the melt/solid polycondensation of glycolic acid oligomers and BHET obtained by glycolysis of post-consumer PET. – The degree of randomness of copolyester is slightly higher than 1 which indicates that the copolymer presents alternating characteristics both before and after degradation. – The hydrolytic degradation was studied by DSC, TG, SEM and 1 H NMR. The thermal behaviour as well as microcracks appearing on the surfaces suggested the degradation of the copolyesters.

Fig. 7. Surface of copolyester PET–GLA (a) before and (b) after degradation.

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´ ski / Polymer Degradation and Stability 94 (2009) 221–226 E. Olewnik, W. Czerwin

– 1H NMR analysis allowed establishing that the mass of copolymer and length of aliphatic units decrease significantly during degradation processes. – The hydrolytic decomposition showed a time dependence, as expected, and the parameter, which had major effect on the rate of decomposition was the kind of buffer solution. – According to earlier studies, the hydrolysis in citric-phosphate buffer was faster than in phosphate buffer solution. This might relate to bigger capacity of phosphate buffer. – The degradation of PET–GLA copolymer could be significantly accelerated by the autocatalytic action of the carboxylic acid end groups of glycolic acid.

Acknowledgement This research was supported by the Grant of JM Rector of Nicolaus Copernicus University. References [1] Olewnik E, Czerwin´ski W, Nowaczyk J. Hydrolytic degradation of copolymers based on L-lactic acid and bis-2-hydroxyethyl terephthalate. Polym Degrad Stab 2007;92:24–31. [2] Kint D, Munoz-Guerra S. A review on the potential biodegradability of poly(ethylene terephthalate). Polym Int 1999;48:346–52. [3] Grzebieniak K, Slusarczyk C, Wlochowicz A, Janicki J. Microstructure transformation due to hydrolytic degradation of ethylene terephthalate/lactic acid copolyesters. Polimery 2002;47:528–33.

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