Synthesis and characterization of biodegradable aliphatic copolyesters with poly(ethylene oxide) soft segments

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EUROPEAN POLYMER JOURNAL

European Polymer Journal 44 (2008) 904–917

www.elsevier.com/locate/europolj

Synthesis and characterization of biodegradable aliphatic copolyesters with poly(ethylene oxide) soft segments Dragana Pepic a, Ema Zagar b, Majda Zigon b, Andrej Krzan b, Matjaz Kunaver b, Jasna Djonlagic a,* a

Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia b National Institute of Chemistry, Hajdrihova 19, Ljubljana, Slovenia Received 24 April 2007; received in revised form 17 July 2007; accepted 28 November 2007 Available online 14 December 2007

Abstract A series of multiblock poly(ether–ester)s based on poly(butylene succinate) (PBS) as the hard segments and hydrophilic poly(ethylene oxide) (PEO) as the soft segments was synthesized with the aim of developing degradable polymers which could combine the mechanical properties of high performance elastomers with those of flexible plastics. The aliphatic poly(ether–ester)s were synthesized by the catalyzed two-step transesterification reaction of dimethyl succinate, 1,4-butanediol and a,x-hydroxyl terminated poly(ethylene oxide) (PEO, M n = 1000 g/mol) in bulk. The content of soft PEO segments in the polymer chains was varied from about 10 to 50 mass%. The effect of the introduction of the soft PEO segments on the structure, thermal and physical properties, as well as on the biodegradation properties was investigated. The composition and structure of these aliphatic segmented copolyesters were determined by 1H NMR spectroscopy. The molecular weights of the polyesters were verified by gel permeation chromatography (GPC), as well as by viscometry of dilute solutions and polymer melts. The thermal properties were investigated using differential scanning calorimetry (DSC). The degree of crystallinity was determined by means of DSC and wide-angle X-ray scattering. A depression of melting temperature and a reduction of crystallinity of the hard segments with increasing content of PEO segments were observed. Biodegradation of the synthesized copolyesters, estimated in enzymatic degradation tests in phosphate buffer solution with Candida rugosa lipase at 37 °C was compared with hydrolytic degradation in the buffer solution. The weight losses of the samples were in the range from 2 to 10 mass%. GPC analysis confirmed that there were significant changes in molecular weight of copolyesters with higher content of PEO segments, up to 40% of initial values. This leads to conclusion that degradation mechanism of the poly(ether–ester)s based on PEO segments occurs through bulk degradation in addition to surface erosion. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Poly(ether–ester)s; Poly(ethylene oxide); Poly(butylene succinate); Hydrolytic degradation; Enzymatic degradation

1. Introduction * Corresponding author. Tel.: +381 113370 728; fax: +381 113370 387. E-mail address: [email protected] (J. Djonlagic).

The production and consumption of synthetic polymeric materials have grown progressively due to their low cost, as well as their resistance to

0014-3057/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2007.11.035

D. Pepic et al. / European Polymer Journal 44 (2008) 904–917

physical aging and biological attacks. The resistance of synthetic polymers to the degradation action of living systems is becoming problematic especially when they are used for a limited period of time. In several domains, such as surgery, pharmacology and agriculture, as well as for the environment, time-resistant polymeric wastes are less acceptable. From this point of view, biodegradable polymers are good candidates for time-limited applications in agricultural and sanitary fields as well as in packaging [1–5]. Aliphatic polyesters have been recognized as one of the most promising biodegradable materials because they are readily susceptible to biological attack and their degradation products, water-soluble oligomers and the starting diols and acid or hydroxyl acid, are non-toxic and can enter the metabolic cycles of bio-organisms [6,7]. The biodegradability of a polymer is based on the presence of hydrolyzable or oxidizable linkages in the backbone. The rate of biodegradation of polyesters depend on the chemical composition, sequence length, molecular weight, hydrophilic/hydrophobic balance, as well as on the morphology of the sample, e.g. degree of crystallinity, size of spherulites, surface area of the samples, additives, etc. The biodegradability properties of polyesters can be improved by increasing the hydrophilicity, such as by the introduction of hydrophilic segments, such as polyethers, into the backbone of the polymer chains. The introduction of polyether soft segments into copolyesters leads to the formation of segmented polymers, the mechanical properties of which can be easily controlled by the type, the weight percent and the length of the soft segments. For example, poly(ethylene oxide), known as nontoxic, non-antigenic and non-immunogenic, is often used as the component to impart good hydrophilicity and biocompatibility of biomaterials [8]. Poly(ether–ester)s based on poly(butylene terephthalate) PBT as the hard segments and poly(ethylene oxide) as the soft segments, commercially available under the trade-name Polyactive [9], demonstrated longer degradation time and formation of water-insoluble by-products rich in aromatic moieties [10]. The introduction of aliphatic acid residues into poly(ether–ester)s chain allows the attractive combination of the advantages of aromatic polyesters, such as excellent mechanical properties and thermal stabilities and the biodegradability and solubility of aliphatic residues [11–13]. Also, the incorporation of different type of ether soft segments into

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aromatic polyesters could modify or enhance their degradability [14]. There are only a few reports on completely aliphatic poly(ether–ester)s, such as that of Albertsson et al. [15] concerning poly(ethylene succinate) with 8.3 mol% of PEG of molecular weight around 400. Nagata et al. [16] synthesized poly(butylene succinate) copolymers containing poly(ethylene oxide) with molecular weights in the range from 200 to 2000 and reported that with increasing content of PEO, in the range from 10 to 50 mol%, the hydrolysis rate of the copolymers increased but their melting temperatures and degrees of crystallinity decreased. Also, the tensile strength and elongation were significantly reduced in the presence of the hydrophilic PEO segments. Other polyesters based on poly(ethylene succinate) and on L-lactic acid [17] and PEO were also investigated. For example, copolyesters based on e-caprolactone and PEO have found some special biomedical applications [18]. The biodegradability properties were improved, as expected, in all polymers in which PEO was incorporated. However, the oxidative instability of materials containing poly(ethylene oxide) under ambient conditions and exposure of these polymers to daylight are problems which have to be overcome [19]. It is well known that the homogeneous hydrolytic degradation of aliphatic polyesters could proceed by two different mechanisms, i.e., bulk and surface erosion. In the case of bulk erosion, material is lost from the entire polymer volume and the molecular weight changes due to bond cleavage. While, in the case of surface erosion, material is lost but there is no change in the molecular weight of the polymeric material. If diffusion of water into the polymer is faster than the cleavage of polymer bonds, the polymer will undergo bulk erosion. If, however, the cleavage of the polymer bonds is faster than the diffusion of water, the process occurs at the surface of the matrix. Go¨pferich et al. [20] developed a theoretical erosion model which shows that degradable polymers could undergo surface erosion and bulk erosion depending on the erosion conditions and the geometry of the sample. In which manner a polymer matrix will erode depends on the diffusivity of water inside the matrix, the degradation rate of the functional groups and the dimensions of the matrix. In a previous paper [21], the synthesis and characterization of two series of aliphatic poly(ether–ester)s modified with two different types of soft segments, i.e., PEO and poly(tetramethylene oxide) PTMO of

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the same molecular weight but different hydrophilicity, were reported. The effect of polymer composition on the physical properties of the segmented poly (ether–ester)s was established. The results obtained in the biodegradation tests with polymers with only 10 mass% (or 2 mol%) of soft segments showed that the introduction of the hydrophilic polyether segments increased the biodegradability and is a good way to tailor the biodegradability of aliphatic polyesters. In a resent paper [22], the synthesis, structure and properties of a series poly(ether–ester)s modified with a relatively high level of PTMO segments, varying from 10 to 50 mass%, was presented. Film samples were subjected to enzymatic attack by lipase from Candida rugosa, whereby it was shown that the enzymatic degradation occurs mainly by the surface erosion mechanism and a dependence on the average length of the hard segments was found. In this article, the synthesis, structure and properties of segmented polyesters modified with a relatively high level of PEO segments, varying between 10 and 50 mass%, (or 2–15 mol%), is presented. High molecular weight poly(ether–ester)s were synthesized which enabled the formation of films with good mechanical properties. Their thermal and physical properties, as well as their biodegradability were investigated in order to gain a better insight into the effect of structure on biodegradability. The film samples were subjected to enzymatic attack by lipase from C. rugosa and the enzymatic and hydrolytic degradation were evaluated from the weight loss and GPC analysis.

component was used in a 15 mol% excess over the dimethyl ester. As an example, the synthesis of a copolyester with 10 mass% of poly(ethylene oxide) is described. A three-necked laboratory reactor equipped with a condenser, nitrogen inlet tube, magnetic stirrer and thermometer was charged with 33.60 g (0.23 mol) of dimethyl succinate, 4.33 g (0.00363 mol) of poly(ethylene oxide) and 23.4 g (0.26 mol) of 1,4-butanediol. The reaction mixture was purged with nitrogen and the reaction was started by the introduction of 0.075 g (0.221 mmol) of Ti(OBu)4, as catalyst. The reaction mixture was heated quickly to 150 °C and gradually (10 °C per 10 min) to the final reaction temperature of 220 °C. The methanol formed during the first stage was distilled off. The second phase of reaction was carried out with a second portion of catalyst (0.221 mmol), under vacuum (p  0.5 mm Hg) and the reaction mixture was maintained under these conditions for 10 h for the polymer with 10 mass% of soft segments up to 34 h for the polymer with 50 mass% of soft segments. After completion of the reaction, the polyester was cooled in the reactor to room temperature under nitrogen without precipitation. All the other polyesters were synthesized in the manner described above. The amount of the poly(ethylene oxide) was varied so as to obtain polyesters with 10, 20, 30, 40 and 50 mass% of soft segments. 2.3. Characterization of the polyesters 1

2. Experimental 2.1. Materials Dimethyl succinate, DMS, (Aldrich) was used as received. a,x-Hydroxyl terminated poly(ethylene oxide), PEO, with a molecular weight M n of 1000 g/mol (from Fluka) was used as obtained. 1,4-Butanediol was purified by vacuum distillation. Titanium-tetrabutoxide, Ti(OBu)4, (Aldrich) was used as a solution in dry n-butanol (1:9 vol.). 2.2. Synthesis of polyesters The aliphatic copolyesters (PBSEO)s were synthesized by a two step transesterification reaction in the bulk, starting from dimethyl succinate, 1,4-butanediol and a,x-hydroxyl terminated poly (ethylene oxide) (PEO, M n = 1000 g/mol). The diol

H NMR spectra were recorded in CDCl3 solution with tetramethylsilane as the reference standard using a Varian Unity Inova 300 MHz instrument. The 1H NMR spectra of these polymers showed characteristic peaks: protons from the succinic acid appear at d = 2.63 ppm, protons from the poly(ethylene oxide) which were attached to the ether group at d = 3.64 – 3.72 ppm, and central and terminal protons from the 1,4-butanediol at d = 1.64–1.77 ppm and d = 4.09–4.12 ppm, respectively. Protons from the poly(ethylene oxide) which were attached to the ester group as well as terminal protons from 1,4butanediol overlap and appear at d = 4.09–4.30. The compositions of the polyesters were calculated from the relative intensities of the peaks characteristics for the diols in the soft and hard segments, i.e., ether protons from poly(ethylene oxide) and butanediol. The viscosities of solutions of the polymers in chloroform were measured at 25 °C using an

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Ubbelohde viscometer. The intrinsic viscosity, [g], was calculated from these measurements. Gel permeation chromatography (GPC) was performed using a Perkin Elmer LC-30 (RID) instrument at 25 °C, with two columns (PLgel Mixed-D with a precolumn) and a refractive index detector. The Plgel Mixed-D columns cover a range of molar masses from 200 g/mol to 4  105 g/mol. Calibration was performed with poly(styrene) standards. Chloroform was used as the eluent at a flow rate of 1 ml/min. The mass of the copolyesters injected onto the column was typically 0.25 mg, as a 10 mg/ml solution. The number-average (M n ) and weight-average molecular weights (M w ) and polydispersity indexes were evaluated from these measurements. Differential scanning calorimetry (DSC) was performed using a Perkin Elmer DSC analyser under a nitrogen atmosphere in the temperature range from 55 to 160 °C at a heating and cooling rate of 10 °C/min. The polyester samples were scanned from 30 to 160 °C, then cooled to 55 °C and heated again to 160 °C. The melting temperatures were determined from the initial scan as the temperatures of the main endothermic peak in the DSC curves. The glass transition temperatures were calculated from the second DSC scans as the middle point of the heat capacity change. A SDT Q600 V7.0 Build 84 (Universal v4.0C TA Instruments) was used for the thermogravimetric measurements. Non-isothermal experiments were performed in the temperature range 0–600 °C at a heating rate of 10 °C/min. The thermal stability of the polyesters was studied under a dynamic nitrogen atmosphere. The average weights of the samples were around 10 mg. Rheological parameters, such as complex dynamic viscosity, g*, were measured on polymer pellets using a Rheometrics RMS-605 instrument, operating in the dynamic mode in the temperature range from 80 to 160 °C. The frequency was varied from 0.1 to 100 rad/s. The radius of the samples was 25 mm and the thickness about 1.5 mm. The polyester pellets were prepared by press molding from the melt at 20 °C above the melting temperature. Wide-angle X-ray scattering (WAXS) measurements of copolyester films were performed using a Philips diffractometer 17 10, in the 2h range from 5 to 50° at a scan speed of 0.04 deg/s. The degree of crystallinity was calculated by peak deconvolution and subsequent determination of the relative areas under the amorphous halo and the crystalline

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peaks of the X-ray diffraction scan. The ratio of the area under the crystalline peaks to the total (amorphous + crystalline) area gives the degree of crystallinity. The water and moisture adsorption of the copolyesters were measured after immersion of a polymer film in distilled water for 1 day and in the chamber above a saturated solution of K2SO4 with relative humidity of 97% during 7 days at room temperature, respectively. 2.4. Hydrolytic and enzymatic degradation tests Enzymatic degradation tests were performed on copolyester films. The polymer films were obtained by hot pressing at 20 °C above the melting temperature. Under these conditions, the compression molding procedure did not affect the polymer properties significantly. In addition, the films were stored at ambient temperature for at least three weeks before characterization in order to attain equilibrium crystallinity. Rectangular pieces for hydrolytic and enzymatic degradation were cut from these films. The polymer films (10  40 mm2 and about 150 lm thick) were incubated in a phosphate buffer solution (pH 7.00 ± 0.01) in the presence of lipase from C. rugosa (SIGMA) in a water bath at 37 °C. The initial weight of the polymer films was in the range from 80 to 100 mg, decreasing with increasing fraction of soft segments. The enzymatic degradation tests of the polyesters films were run in duplicate. Prior to the enzymatic degradation experiments, the activity of the enzyme was determined following the Sigma procedure using an emulsion of olive oil. The activity of the C. rugosa lipase was around 1100 units/mg solids. The enzyme concentration was 2.0 mg/ml, and every seventh day it was replaced with a freshly prepared enzyme solution. Simultaneously, blank experiments without enzyme in phosphate buffer solution were performed. The films were removed either from the enzymatic or buffer solution after selected time intervals, washed with distilled water, and dried under vacuum at room temperature to constant weight. The extent of biodegradation was quantified as the weight loss divided by the initial sample weight. The degraded polymer films were analyzed in terms of change in molecular weight by GPC and change in surface morphology by optical microscopy. The surfaces of the samples were observed using an optical microscope ‘‘Carl Zeiss Jena” with

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reflected light before and after hydrolytic and enzymatic degradation, without any further mechanical treatment.

3. Results and discussion 3.1. Synthesis of the poly(ether–ester)s A series of high-molecular weight aliphatic poly (ether–ester)s based on succinic acid, 1,4-butanediol and a,x-hydroxyl terminated poly(ethylene oxide) was synthesized by transesterification in the bulk without the addition of a heat stabilizer. The level of the soft (PEO) segments in the polyester chains was varied between 10 and 50 mass% or from 2 to 15 mol%. The first step, or transesterification reaction, was carried out in the temperature range 150–220 °C under atmospheric pressure with a 15% stoichiometric excess of 1,4-butanediol. Poly(butylene succinate) PBS, was synthesized by direct polycondenzation in the presence of a highly effective catalyst and without the addition of a heat stabilizer within 6 h, which is in agreement with the results reported by Mochizuchi et al. [23]. In order to obtain high-molecular weight poly(ether–ester)s, long reaction times are required, so the polycondensation was carried out at 220 °C under vacuum for 10–34 h depending on the composition of the reaction mixture, i.e., increasing with increasing content of the introduced soft segments. The chemical structure of synthesized segmented aliphatic poly(ether–ester)s based on succinic acid is O

O

C-CH2-CH2-C-O-(CH2)4-O

O x

to 1,4 butanediol. Therefore, the second phase of the reaction, i.e., polycondensation was prolonged with increasing content of hydroxyl-terminated PEO in order to obtain high molecular weight copolyesters. These results are in agreement with the fact that reaction rate decreases due to the higher transesterification activation energy of dimethyl succinate and PEO compared to dimethyl succinate with 1,4-butanediol [24,25]. Simultaneously, the incorporation of the flexible PEO into the polymer chains decreases the viscosity of the reaction mixture and facilitates the exclusion of by-products, such as methanol, and 1,4-butanediol and favors the formation of products with higher molecular weights, such as in the case of sample PBSEO-50. 3.2. The composition and structure of the poly(ether– ester)s The molecular structure of the poly(ether–ester)s was investigated by 1H NMR spectroscopy. The composition of the poly(ether–ester)s, i.e., the content of hard PBS and soft PEO segments, was calculated as the relative intensities of the proton peaks arising from the methylene groups attached to the ether oxygen from the PEO moiety (d) and the central proton peaks from the butylene moiety (c). Thus, the mole fraction of soft PEO segments was calculated, using the formula: xðflexible segment PEOÞ; mol% ¼

I d =N d I d =N d þ I c =N c

O

C-CH2-CH2-C-O-CH2-CH2-(O-CH2-CH2)22-O

where x is the mole fraction of the hard PBS segments and y the mole fraction of the soft PEO segments with succinate residues. The formation of the polyesters was monitored by measuring the inherent viscosities of the reaction mixture. The molecular weight of the aliphatic homopolyester PBS and the copolyesters, indicated by the change in the inherent viscosity, increased with increasing duration of the polycondensation reaction. The inherent viscosity of poly(butylene succinate), PBS, changed more rapidly than those of the poly(ether–ester)s, indicating a lower reactivity of the a,x-hydroxyl-terminated PEO compared

y

where Ic and Id are the intensities of the corresponding peaks and Nc = 4, and Nd = 88 are the numbers of protons in the corresponding units. The composition of the starting reaction mixture and that of the poly(ether–ester)s, as well as the average length of the hard PBS segments are given in Table 1. The 1H NMR analysis confirmed that succinic moieties are incorporated into polymer chains in the hard as well as in the soft segments. The theoretical and experimental mass% and mol% content of the soft segments of the synthesized copolyesters are presented in Table 1. These results show that the soft segments were incorporated into the copolyesters in

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Table 1 Composition of the reaction mixture, composition of the copolyesters and average length of the hard segments in the synthesized copolyesters Polymer

Composition of reaction mixture (BS/PEOS)a

Composition of copolyester (PBS/PEOS)b

Mol% of soft segments Theoretical

Experimental

PBS PBSEO-10 PBSEO-20 PBSEO-30 PBSEO-40 PBSEO-50

100/0 89/11 79/21 67/33 58/42 47/53

100/0 90.2/9.8 79.8/20.2 69.5/30.5 58.8/41.2 48.0/52.0

0 1.9 4.1 7.2 10.3 14.8

0 1.7 3.8 6.4 9.9 14.5

a b

Ln – 58 25 15 9 6

BS – weight fraction of butylene succinate units and PEOS – weight fraction of poly(ethylene oxide) succinate units. Determined by 1H NMR spectroscopy.

an amount which were from 2 to 11 mol%, in deficit to that which was to be expected from the compositions of the starting reaction mixtures. For all the samples, except for PBSTMO 30, the deficit was in the range of experimental error. It could be concluded that the composition of the copolyesters were in good agreement with those expected from the composition of the feed, except for PBSEO-30. Another important parameter of the polymer structure having an influence on the chain flexibility, which also determines the biodegradability potential of poly(ether–ester)s, is the average sequence length of the hard segments. The values of the average sequence length of the hard PBS segments, Ln, are also shown in Table 1. The average sequence length was calculated from the mole fraction of PBS in the copolymer assuming random copolymerization of the comonomers, using the following formula: Ln ¼

1 1 1  xPBS

The average degree of polymerization of the hard PBS segments in the polymer chains were between 58 and 6, indicating a decrease in the length of the hard segments with increasing incorporation of soft PEO segments.

The intrinsic viscosities of the synthesized polyesters were between 56.0 and 92.6 cm3/g for the copolyesters and 180.3 cm3/g for the homopolyester, PBS. The values of the intrinsic viscosities of synthesized copolyesters and PBS are considerably higher than those reported in a previous paper. The higher vacuum and lower molar ratio of diol to diester compared to those in the previous work resulted in these higher viscosity values. The values of the Newtonian complex dynamic viscosity, g*, at 120 °C, which could be used as an indicator of the molecular weight of the poly (ether–ester)s, were in the range from 8 to 179 Pa s (Table 2.) and were significantly lower compared to homopolyester, PBS (754 Pa s). The complex dynamic viscosity and the results obtained from viscosity measurements of dilute polymers solutions followed the trend in molecular weight change in the series. The number average molecular weights, M n , of PBS and the copolyesters in these series ranged from 41,000 to 22,000 g/mol, while the polydispersity index, M w =M n , was in the range 1.9–3.4 (Table 2). The molecular weights of the synthesized copolyester were lower compared to PBS and the polydispersity of the copolyesters increased with increasing molecular weight.

Table 2 The intrinsic viscosities, complex dynamic viscosities and average molecular weights determined by GPC of the poly(ether–ester)s Polymer

[g] (cm3/g)

g* (Pa s) 120 °C, 1 Hz

M n  104 g/mol

M w  104 g/mol

M w =M n

PBS PBSEO-10 PBSEO-20 PBSEO-30 PBSEO-40 PBSEO-50

180.3 69.4 67.0 56.0 64.3 92.6

754 17 45 36 8 179

4.1 3.1 2.8 2.6 2.2 2.6

14.1 5.8 5.5 5.8 4.9 8.5

3.5 1.9 2.0 2.6 2.2 3.3

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In this study, it was shown that relatively highmolecular weight poly(ether–ester)s can be synthesized at 220 °C using a highly effective catalyst, such as tetra-n-butyl-titanate, without a heat stabilizer. The molecular weight M n of the polyesters except that of PBSEO-40 exceed 25,000 and are considerably higher than those reported in a previous work [21]. The higher vacuum and lower molar ratio of diol to diester as well as the prolongation of the duration of the polycondensation with increasing content of PEO glycol promote chain extension. Therefore, the copolymers exhibited macromolecular behavior and were suitable for the preparation of flexible and tough films by the melt-pressed method. 3.3. Thermal properties of poly(ether–ester)s The synthesized poly(ether–ester)s were partly crystalline polymers, for which melting and glass transition temperatures were observed by DSC analysis. The DSC thermograms of the synthesized poly(ether–ester)s recorded during heating and cooling are presented in Figs. 1a and b, respectively. The melting of the crystallites of the poly(ether–ester)s occurs in the temperature region 80–114 °C, while crystallization occurs in the range 19–69 °C. The melting temperature of PBS homopolymer was 116 °C and it crystallized from 78 °C. The melting temperature was determined as the temperature of the main peak in the DSC thermograms from the first run. The DSC thermograms illustrate a significant shift of the melting temperatures to lower temperatures with increasing fraction of soft segments, from 116 °C (PBS) to 80 °C (PBSEO-50). However, the melting temperatures of the synthesized poly

a

(ether–ester)s copolymerized with 10–30 mass% of the PEO segments are above 100 °C, which candidates them for potential application as biodegradable copolymers. In the thermograms of the second heating run, (not presented), multiple endothermic peaks were observed. This behavior can be explained by the melt-crystallization model, which was earlier reported for PBS and its copolyesters [26,27], and by the presence of crystallites of different size and perfection, due to the irregularity of the length of the PBS segments in the poly(ether–ester)s. The enthalpies of melting, DHm, were calculated from the melting thermograms. The values of DHm increased with increasing length of the PBS segments and size of crystallites from 37.1 to 78.5 J/g. In the second run, the enthalpies of melting were lower than in the first scan, which indicated a decrease in the degree of crystallinity of the poly(ether–ester)s. The results obtained from DSC analysis, the melting temperature (Tm), the enthalpy of melting (DHm), the crystallization temperature (Tc), the glass transition temperature (Tg) and the degree of crystallinity of the poly(ether–ester)s and, for sake of comparison, of PBS homopolymer are given in Table 3. The degree of crystallinity of PBS hard segments of the poly(ether–ester)s were calculated by means of the following equation: XcPBS ¼

DH m DH m  wPBS

where DH m is the theoretical value of the enthalpy of melting of perfectly crystalline PBS homopolymer, calculated on the basis of the group

b

PBS

PBS PBSEO 10

PBSEO 20

EGZO

ENDO

PBSEO 10 PBSEO 20 PBSEO 30

PBSEO 30

PBSEO 40 PBSEO 40 PBSEO 50 PBSEO 50

50

100

Temperature, 0C

150

0

50

100

150

Temperature, 0C

Fig. 1. DSC thermograms of the aliphatic copolyesters with different contents of PEO soft segments in the polymer chain: (a) heating at 10 °C/min and (b) cooling at 10 °C/min.

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Table 3 Thermal properties and degree of crystallinity of the synthesized polymers Polymer

Tm (°C) I run (II run)

DHm (J/g) I run (II run)

Tc (°C)

DHc (J/g)

Tg (°C)

Xcb, (%) DSC

XcPBS (%) DSC

Xca, (%) WAXS

PBS PBSEO-10 PBSEO-20 PBSEO-30 PBSEO-40 PBSEO-50

116.0 (114.9) 114.0 (113.2) 110.2 (109.3) 102.8 (102.3) 97.4 (95.0) 79.7 (78.4)

87.3 78.5 57.2 46.2 37.1 40.2

78.3 68.7 61.1 66.0 27.1 18.9

73.8 67.1 58.0 47.9 34.0 13.9

26.6 33.2 38.0 41.6 42.6 42.2

79.4 71.0 51.8 41.8 33.6 36.4

79.4 78.7 64.9 60.1 57.1 75.8

55.9 36.6 43.6 36.1 31.7 33.6

The degree of crystallinity was determined by the X-ray diffraction method. Calculated by dividing the observed heat of fusion by the theoretical value calculated on the basis of the group contribution.

contribution method [28] (110.5 J/g), and wPBS is the weight fraction of hard segments in the sample, determined by 1H NMR spectroscopy. The total degree of crystallinity of the poly (ether–ester)s, (Xc), was in the range 33.6–71.0% (in the first run), i.e., lower than the degree of crystallinity of the homopolyester (PBS) (79.4%), and decreased with increasing content of soft PEO segments. The degree of crystallinity relative to the weight fraction of PBS segments in the poly(ether– ester)s (XcPBS), were in the range 57–79%, which means that only 57–79 wt% of the PBS segments in the poly(ether–ester)s samples crystallized. The value of XcPBS tended to decrease with increasing content of soft PEO segments, from which it can be concluded that the presence of the soft segments disturbs the crystal growth of the PBS hard segments. The difference in the melting and crystallization temperature (supercooling) (DTh = Tm – Tc) is an indication of the rate of crystallization of different block copolymers. PBS, which is considered to be a fast crystallizing polymer, has a DTh value of 38 °C. The DTh values for the hard segments of the synthesized aliphatic poly(ether–ester)s lie in the range from 45 and 70 °C, indicating that the rate of crystallization decreases with increasing content of soft segments. The results show that the crystallization rate is dependent on both the block length and composition in the examined range. The glass transition temperature of the amorphous phase in poly(ether–ester)s lie between 27 to 42 °C. The glass transition temperature varies slightly with the composition of the copolyesters decreasing with increasing content of soft PEO segments and approaches that of PEO homopolymers of about 50 °C [29]. PBS homopolyester has a glass transition temperature of 27 °C, which is higher than the Tg values of the poly(ether–ester)s.

This result indicates the existence of a mixed amorphous phase containing both PEO soft segments and uncrystallized PBS segments.

3.4. X-ray analysis of poly(ether–ester)s Further information on the crystalline structure and degrees of crystallinity of the synthesized poly (ether–ester)s was obtained from X-ray analysis of polymer films. The diffraction diagrams for the PBS homopolymer and the copolyesters with different contents of PEO soft segments are shown in Fig. 2. Characteristic peaks for PBS appear at 2h = 19.68°, 22.15°, 22.72°, 26.15° and 29.24°. An electron diffraction study revealed that PBS crystallites have a monoclinic crystal lattice, as was reported previously [30]: a = 0.523 nm, b = 0.908 nm, c = 1.079 nm and b = 123.87°. The characteristic peaks for the copolyester with the highest content of soft segments are at 2h = 19.72°, 21.86°, 22.44°, 26.02° and 28.81°. With increasing content of soft segments, the peak at 2h = 21.83° is more pronounced, while the peak

Arbitrary Intensity

a b

(73.3) (73.5) (62.0) (43.4) (41.5) (33.9)

PBS PBSEO 10 PBSEO 20 PBSEO 30 PBSEO 40 PBSEO 50

10

15

20

25

30

2θ Fig. 2. X-ray diffractograms of PBS and the copolyesters with different content of PEO soft segments.

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3.5. Thermogravimetry of the poly(ether–ester)s The thermal stability of the synthesized poly (ether–ester)s was studied by non-isothermal thermogravimetry in a nitrogen atmosphere. The TG curves of the synthesized polyesters with different content of flexible segments in the integral and differential form are shown in Fig. 3. The thermal behaviors of PBS and the copolyester modified with 10 and 20 mass% of flexible polyether segments are very similar and less stable than the other three copolyesters containing more polyether segments. The thermal stability of aliphatic polyesters is an important parameter which could limit their practical application. The characteristic temperatures for weight losses of 5%, 50% and 90%, T5%, T50% T90%, respectively, as well as the residual weight at 500 °C in nitrogen are considered. The T5% value is considered to represent the beginning of degradation. The degradation of the PBS and PBSEO-10 and PBSEO-20 polymers commences between 327 and 338 °C, while for the samples with 30–50 mass% of soft PEO segments, the degradation starts at 347–364 °C. Also, the temperatures of 50 and 90 mass% loss in the series increased with increasing content of soft PEO segments. The maximum thermal decomposition rate

100 0

2

Derivative residual, % / C

at 2h = 22.44° decreases. The position of the peaks in the diffractograms of the copolyesters shows that there is no change in the type of the crystal lattice and that aliphatic poly(ether–ester)s crystallites have a monoclinic crystal lattice similar to that of PBS. With increasing content of soft segment in the polymers, the peaks of these copolymers are shifted towards lower values of 2h, indicating a small increase in the unit cell dimensions. The degrees of crystallinity of the synthesized poly(ether–ester)s were in the range from 32% to 43%, as determined from the WAXS measurements (Table 3), which are lower than the values obtained from DSC measurements (33–71%). The degree of crystallinity of PBS determined by WAXS (55.9%) was also lower than the value obtained from DSC measurements (79.4%). This can be explained as a consequence of possible distortions in the crystal lattice [31] as well as due to the methods of calculation and detection [32]. The degree of crystallinity for PBS was compared to the copolyesters containing soft polyether segments and again there was a trend to lower degrees of crystallinity with increasing content of soft segment.

Residual mass, %

912

80 60 40

PBS PBSEO 10 PBSEO 20 PBSEO 30 PBSEO 40 PBSEO 50

1

20 0 0 200

300

400

500

0

Temperature, C Fig. 3. TG and DTG curves for PBS and the aliphatic poly(ether–ester)s with different content of soft PEO segment in nitrogen.

was in the range from 1.81 to 2.03%/°C, varying slightly with composition of the copolyesters, while the corresponding temperature of the maximum decomposition rate of the copolyesters increased with increasing content of PEO segments, from 403 to 427 °C. The residual mass of the poly (ether–ester)s at 500 °C increased from 0.2% to 4.1% (5.6% for PBS homopolymer) with increasing weight fraction of PBS segments. From these results it could be concluded that the residual mass in nitrogen originated mainly from the PBS fraction. Thus it could be concluded that thermal stability of the synthesized poly(ether–ester)s compared to homopolyester PBS is improved by modification with PEO segments and that it increased with their content.

3.6. Hydrolytic and enzymatic degradation of the poly(ether–ester)s Hydrolytic degradation of the poly(ether–ester)s in phosphate buffer solution and in the presence of lipase C. rugosa, on melt-pressed polymer films was followed by mass loss during degradation and changes in molecular weight by GPC analysis, as well as by optical microscopy of the surface of the degraded samples. The absorption of water by the PBS and poly (ether–ester)s films after 24 h and 7 days was measured. The PBS sample absorbed about 0.7 mass% of water, confirming the low hydrophilicity of the homopolyester film, while the poly(ether–ester)s modified with PEO soft segments absorbed in the range from 1.6 to 44 mass%, increasing with

D. Pepic et al. / European Polymer Journal 44 (2008) 904–917

increasing content of hydrophilic soft segments. Also, the moisture uptake at a relative humidity of 97% after 7 days increased from 1.4 to 17 mass% with increasing content of PEO soft segments due to the increased hydrophilicity of the copolyesters (Fig. 4). The weight losses of the homopolyester PBS films in the buffer solution and in the presence of the enzyme were similar and around 0.18 and 0.29 mass% after 7 days, respectively and increased slightly with time. The weight losses of the poly (ether–ester)s based on PEO in the hydrolytic tests increased from 1 to 4 mass% with increasing content of PEO after 1 day to 2 to 10 mass% after 28 days (Fig. 5). The increase of the content of the incorporated PEO soft segments provides more amorphous domains and imparts good chain hydrophilicity, which would facilitate the diffusion of water molecules and attack at the ester bond and hence accelerates the hydrolytic degradation. Furthermore, a high degree of water absorption generally results in a swollen polymer matrix with a greater free volume for mass transfer, leading to an increase in the transport of soluble degradation products from material bulk. These results of hydrolytic degradation tests also confirm, as has been reported, that the hydrolytic degradation of segmented copolyester films is influenced by the concentration of degradable ester linkages between the hard and soft polyether segments [13]. It is known that hydrolytic degradation of polyesters results in morphology changes, topological changes, the formation of soluble degradation products and changes in mechanical properties. In most

moisture uptake water uptake

Weight increase, %

40

30

20

10

0 10

20

30

40

50

Mass % of soft segment Fig. 4. Change in the water and moisture uptake after 7 days with changing content of soft PEO segments in the aliphatic poly(ether–ester)s.

913

Fig. 5. Weight losses of the aliphatic poly(ether–ester)s in buffer solution.

cases, the primary enzymatically catalyzed hydrolysis of ester bonds occurs in a surface erosion process and leads at least to water-soluble intermediates. Enzymes can not penetrate into the polymer bulk, but the significant decrease of the molecular weight indicates that a significant hydrolysis of the copolyesters chains occurred in the bulk [33]. Incorporation of PEO soft segments leads to an increase in the hydrolytic degradability of the copolymers, which increased steadily with increasing content of soft segments. On the other hand, incorporation of just 10 mass% of soft segments results in a modest increase in the degree of enzymatic degradation. The biodegradability then remains almost constant in the investigated range of the content of soft segments incorporated into the polymer chains (Fig. 6). The enzymatic degradation was, within experimental error at same level as the hydrolytic degradation or even lower. The hindered biodegradation of two copolymers, PBSEO-30 and PBSEO40, could be the consequence of the hard PBS segments containing an insufficient number of degradable ester bonds for the catalytic enzyme action. According to the results of Hercog et al. [34], the rate of enzymatic degradation of aliphatic copolyesters in the presence of lipase depends mainly on the availability of the active center of the lipase, which is located inside the protein and, therefore, the copolyester chains must be able to reach the catalytic active centers. It has been reported that each molecule of lipase cover approximately 70–90 ester

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D. Pepic et al. / European Polymer Journal 44 (2008) 904–917 1.4

12 28 days blank 28 days with enzyme

1.2

8

Hydrolitic degradation

1.0

6

Mn/(Mn)o

Weight loss,%

10

4 2

0.8 0.6 Enzymatic degradation

0.4 0 PBSEO 10 PBSEO 20 PBSEO 30 PBSEO 40 PBSEO 50

Fig. 6. Comparison of the weight losses of the aliphatic poly(ether–ester)s in the presence of enzyme and due to hydrolytic degradation in buffer solution.

bonds. The capability of the bonds to reach the active center is assumed to depend mainly on the flexibility of the polymer chain. In the case of poly (ether–ester)s, this idea could be extended to include not only dependence on chain mobility but also on the dimensions of the hard PBS segments. The results of enzymatic degradation confirmed that the poly(ether–ester)s with a degree of polymerization of the hard segments in the range of 59–27 (PBSEO-10 and 20), each containing 2 ester bonds in the PBS repeating units, could be more available to catalytic action than the poly(ether–ester)s with the higher contents of PEO segments and shorter PBS segments (PBSEO-30 and 40). The copolyester PBSEO-50 with the highest content of PEO segments showed the highest degree of both hydrolytic and enzymatic degradation of the same magnitude. This could be explained by the results of optical microscopy which confirmed the presence of very small size spherulites of 3 lm diameter (compared to other polymer films), which then decreases after hydrolytic and enzymatic degradation, implying that degradation occurs preferentially in the amorphous domains or at the circumference of the spherulites. The GPC measurements performed on polymer films before and after exposure to hydrolytic and enzymatic degradation showed a change in the mechanism of degradation in the series with increasing content of PEO segments (Fig. 7). The poly(ether–ester) with 10 mass% of PEO segments exhibits an increase in molecular weight after both hydrolytic and enzymatic degradation. The reason for this phenomenon could be preferential degradation at the surface of segregated amorphous phase consisting of the low molecular weight polymer fraction. While, in the samples with a content of PEO soft segments larger than 30 mass%, there were

0.2 0.0 10

20

30

40

50

Weight fraction of soft PEOs segments, mass% Fig. 7. Relative number-molecular weight, M n =ðM n Þo, as a function of the content of soft PEO segments after 28 days of hydrolytic and enzymatic degradation in buffer solution.

significant decreases in the molecular weight, up to 40% of the initial values. This leads to conclusion that besides surface erosion, bulk degradation occurred. The significant decrease of the number average molecular weights of the residual polymers in comparison with the initial molar masses indicates that in addition to enzymatic degradation at the surface, a significant chemical hydrolysis of the copolyester chains in the bulk also occurs. The surface of the copolyesters films after hydrolytic and enzymatic degradation was observed using an optical microscope in reflected light. The optical microscopy photographs of the surface of the specimens before and after hydrolytic and enzymatic degradation after 28 days for PBSEO-10, PBSEO40 and PBSEO-50 are shown in Fig. 8. On the all micrographs a spherulitic superstructure is clearly seen. The diameter of the spherulites was about 3–50 lm, depending on the copolymer composition. In the series of poly(ether–ester)s, the diameter of the spherulites decreases with increasing the content of the soft PEO segments in the polymer chain. It has been reported that the degradation of PBS depends not only on the degree of crystallinity but also on the internal structure of the spherulites [35]. Since the diameter of the spherulites decreased because of both hydrolytic and enzymatic degradation, this result implies that both hydrolytic and enzymatic degradation occurs preferentially in the amorphous part of the surface or at the circumference of the spherulites. The spherulitic texture of the samples with the lowest content of the soft segments is clearly observed as a consequence of hydrolytic and enzymatic degradation. The spherulites of

D. Pepic et al. / European Polymer Journal 44 (2008) 904–917

915

Fig. 8. Optical micrographs of the surface of the poly(ether–ester)s before degradation (control sample), after incubation in buffer (hydrolytic degradation) and enzyme solution (enzyme degradation) for 28 days.

these samples have a larger diameter than the others. On the surface of polymer films with the lowest content of PEO segments, only holes are observed, while on the surface of PBSEO-50 with the highest degradation, scratches and fracture were present. These results suggest that besides the flexibility and hydrophilicity of the polymer chains and the degree of crystallinity, the size and internal structure of the spherulites are also very important parameters which could control the enzymatic and hydrolytic degradation. The results of the hydrolytic and enzymatic degradation of the samples showed that the introduction of soft PEO segments up to 50 mass% into the polymer chains increased the degradability compared to

PBS in the examined period of four weeks. The difference in the hydrophilicity in the series of synthesized poly(ether–ester)s results in their different biodegradation properties. It was shown that the crystallinity and degradation kinetics could be tailored by the content of soft PEO segments. The results also indicate that the biodegradability depends, in addition to hydrophilicity and flexibility of the chain backbone strongly on the average size of the hard segments as well as on the surface microstructure. 4. Conclusions High molecular weight poly(ether–ester)s based on poly(butylene succinate) as the hard segments

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and poly(ethylene oxide) as the soft segments were successfully synthesized by a catalyzed transesterification reaction in the melt. The number average molecular weights, M n , of the PBS and copolyesters in this series ranged from 22,000 to 41,000 g/mol, while the polydispersity index, M w =M n , was in the range 1.9 to 3.4. The molecular weights of synthesized copolyester were lower compared to PBS and the polydispersity of the copolyesters increased with increasing molecular weight. The average sequence length of the PBS hard segments was between 6 and 58, depending on the molar fraction of the soft PEO segments, which was varied from 2 to 15 mol%. The melting temperatures of the poly (ether–ester)s were lower than that of PBS and decreased with increasing content of soft segments. However, except for the copolymer with 40 and 50 mass% of PEO segments, all the other poly (ether–ester)s had melting temperatures above 100 °C, which is important for their possible application. The copolymers exhibited macromolecular behavior and were suitable for the preparation of flexible and tough films by the melt-pressed method. The total degree of crystallinity of the poly(ether– ester)s was in the range of 34–71%, i.e., lower than the degree of crystallinity of the homopolyester (PBS) (79.4%), and decreased with increasing content of PEO segments. The degree of crystallinity calculated with respect to the weight fraction of PBS segments (XcPBS) in the poly(ether–ester)s indicated a decreasing tendency of crystallization of the hard segments with increasing content of soft PEO segments. The X-ray diffraction analysis performed on the copolymer films prepared for the biodegradability tests confirmed the trend decreasing crystallinity with increasing content of soft segments in the series as observed by DSC measurements. The X-ray diffraction patterns indicated that in the hard phase PBS in the poly(ether–ester)s crystallized in a monoclinic crystal lattice, i.e., similar to PBS homopolymer, but with a small increase in the unit cell dimensions. Incubation in buffer solution for 4 weeks resulted in a mass loss from 2% to 10%, depending on the content of soft segments. Enzymatic degradation in the presence of C. rugosa showed that the introduction of the soft PEO segments into the polymer chains increased the degradability compared to PBS in the examined period of four weeks, showing significant dependence on the chain structure, hydrophilicity and surface morphology concerning size of the spherulites. The biodegradation of the poly

(ether–ester)s was similar or even hindered (PBSEO-30 and PBSEO-40) in the presence of C. rugosa lipase compared to hydrolytic degradation, although the degree of crystallinity decreased and the hydrophilicity increased, suggesting that it depends, in addition to hydrophilicity and flexibility of the chain backbone strongly on the average size of the hard segments as well as on the surface microstructure. High molecular weight poly(ether–ester)s based on PBS and hydrophilic poly(ethylene oxide) show promise as biodegradable elastomers, having satisfactory thermal and mechanical properties and simultaneously good biodegradability. Acknowledgements This work was financially supported by the Ministry of Science and Environmental Protection of the Republic of Serbia (Project no. 142023) and by the Ministry of Science, Higher Education and Technology of Slovenia as part of a bilateral project. References [1] Albertsson A-C, Varma IK. Aliphatic polyesters: synthesis, properties and applications. Adv Polym Sci 2002;157:1–40. [2] Huang JS. Encyc Polym Sci Eng, Biodegradable polymers, vol. 2. New York: Wiley-Interscience; 1985. p. 220–43. [3] Mohanty AK, Misra M, Hinrichsen G. Biofibres, biodegradable polymers and biocomposites: an overview. Macromol Matter Eng 2000;276/277:1–24. [4] Vert M. Aliphatic polyesters (Great degradable polymers that cannot do everything. Biomacromolecules 2005;6: 538–46. [5] Okada M. Chemical synthesis of biodegradable polymers. Prog Polym Sci 2002;27:87–133. [6] Rizzarelli P, Impallomeni G, Montaudo G. Evidence for selective hydrolysis of aliphatic copolyesters induced by lipase catalysis. Biomacromolecules 2004;5:433–44. [7] Lindstro¨m A, Albertsson A-C, Hakkarainen M. Quantitative determination of degradation products, an effective means to study early stages of degradation in linear and branched poly(butylene adipate) and poly(butylene succinate). Polym Degrad Stab 2004;83:487–93. [8] Tirelli N, Lutolf MP, Napoli A, Hubbell JA. Poly(ethylene glycol) block copolymers. Rev Mol Biotechnol 2002;90:3–15. [9] Bezemer JM, Grijpma DW, Dijkstra PJ, van Blitterswijk CA, Feijen J. A control release system for proteins based on poly(ester ether) block copolymers (polymer network characterisation). J Control Rel 1999;62:393–405. [10] Deschamps AA, van Apeldoorn AA, Hayen H, de Bruijn JD, Karst U, Grijpma DW, et al. In vivo and in vitro degradation of poly(ester-ether) block copolymers based on poly(ethylene glycol) and poly(butylene terephthalate). Biomaterials 2004;25:247–58.

D. Pepic et al. / European Polymer Journal 44 (2008) 904–917 [11] Park YH, Cho CG. Synthesis and characterization of poly(butylenes succinate)-co-(butylenes terephthalate)-bpoly(tetramethylene glycol). J Appl Polym Sci 2001;79:2067–75. [12] Zhang Y, Feng Z, Zhang A. Synthesis and characterisation of polu(butylene terephthalate)-co-poly(butylene succinate)b-poly(ethylene glycol) segmented block copolymers. Polym Int 2003;52:1351–8. [13] Nagata M, Kiyotsukuri T, Minami S, Tsutsumi N, Sakai W. Enzymatic degradation of poly(ethylene terephthalate) copolymers with aliphatic dicarboxylic acids and/or poly(ethylene glycol). Eur Polym J 1997;42:1701–5. [14] Nagata M, Kiyotsukuri T, Minami S, Tsutsumi N, Sakai W. Biodegradability of poly(ethylene terephthalate) copolymers with poly(ethylene glycol)s and poly(tetramethylene glycol). Polym Int 1996;39:83–9. [15] Albertsson A-C, Ljungquist O. Degradable polymers. II. Synthesis, chracterization and degradation of an aliphatic thermoplastic block copolyester. J Macromol Sci Chem A 1986;23(3):411–22. [16] Nagata M, Kiyotsukuri T, Takeuchi S, Tsutsumi N, Sakai W. Hydrolytic degradation of aliphatic polyesters copolymerized with poly(ethylene glycol)s. Polym Int 1997;42:33–8. [17] Li SM, Rashkov I, Espartero JL, Manolova N, Vert M. Synthesis, chracterisation and hydrolytic degradation of PLA/PEO/PLA triblock copolymers with long poly(L-lactic acid). Macromolecules 1996;27:57–62. [18] Chen D, Chen H, Bei J, Wang S. Morphology and biodegradation of microstructures of polyester–polyether block copolymer based on polycaprolactone/polylactide/ poly(ethylene oxide). Polym Int 2000;49:269–76. [19] Deschamps AA, Grijpma DW, Feijen J. Poly(ethylene oxide)/poly(butylene terephthalate) segmented block copolymers (the effect of copolymer composition on physical properties and degradation behavior). Polymer 2001;42:9335–45. [20] Von Burkersroda F, Schedl L, Go¨pferich A. Why degradable polymers undergo surface erosion or bulk erosion. Biomaterials 2002;23:4221–31. [21] Jovanovic D, Nikolic MS, Djonlagic J. Synthesis and characterisation of biodegradable aliphatic copolyesters with hydrophilic soft segments. J Serb Chem Soc 2004;69: 1013–28. [22] Pepic D, Nikolic SM, Djonlagic J. Synthesis and characterization of biodegradable aliphatic copolyesters with poly(tet-

[23]

[24]

[25]

[26]

[27]

[28] [29]

[30]

[31]

[32]

[33]

[34]

[35]

917

ramethylene oxide) soft segments. J Appl Polym Sci 2007;106:1777–86. Mochizuki M, Mukai K, Yamada K, Ichise N, Murase S, Iwaya Y. Stuctural effects upon enzymatic hydrolysis of poly(butylenes succinate-co-ethylene succinate)s. Macromolecules 1997;30:7403–4. Zhang Y, Feng Z, Feng Q, Cui F. The influence of soft segment length on the properties of poly(butylene terephthalate-co-succinate)-b-poly(ethylene glycol) segmented random copolymers. Eur Polym J 2004;40:1297–308. Hsu J, Chio KY. Transesterification of dimethyl terephthalate with poly(tetramethylene ether) glycol and 1,4-butanediol catalyzed by tetra butyl titanate. J Appl Polym Sci 1987;33:329–51. Sweet GE, Bell JP. Multiple endotherm melting behavior in relation to polymer morphology. J Polym Sci Part A-2 1972;10:1273–83. Yasuniwa M, Satou T. Multiple melting behavior of poly(butylenes succinate). I. Thermal analysis of meltcrystallized samples. J Polym Sci Part B 2002;40:2411–20. Van Krevelen DW. Properties of Polymers. Amsterdam: Elsevier; 1990. Wang M, Zhang L, Ma D. Degree of microphase separation in segmented copolymers based on poly(ethylene oxide) and poly(ethylene terephthalate). Eur Polym J 1999;35:1335–43. Ihn KJ, Yoo ES, Im SS. Structure and morphology of poly(tetramethylene succinate) crystals. Macromolecules 1995;28:2460–4. Zong X-H, Wang Z-G, Hsiao BS, Chu B, Zhou JJ, Jamiolkowski DD, et al. Structure and morphology changes in absorbable poly(glycolide) and poly(glycolide-co-lactide) during in vitro degradation. Macromolecules 1999;32:8107–14. Ruth JP. In: Kroschwitz JI, editor. Encyclopedia of polymer science and engineering. Crystallinity determination, vol. 4. New York: John Wiley & Sons; 1986. p. 482–519. Mu¨ller R-J, Kleeberg I, Decker W-D. Biodegradation of polyesters containing aromatic constituents. J Biotechnol 2001;86:87–95. Hercog K, Mu¨ller R-J, Deckwer W-D. Mechanism and kinetics of the enzymatic hydrolysis of polyester nanoparticles by lipases. Polym Degrad Stab 2006;91:2486–98. Cho K, Lee J, Kwon KJ. Hydrolytic degradation behavior poly(butylenes succinate)s with different crystalline morphologies. Appl Polym Sci 2001;79:1025–33.