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Polymer Degradation and Stability 98 (2013) 169e176

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate /polydegstab

Standard methods for characterizations of structure and hydrolytic degradation ofaliphatic/aromatic copolyesters

Paranee Sriromreun a, Atitsa Petchsuk b, Mantana Opaprakasit c, Pakorn Opaprakasit a,*a School of Bio-Chemical Engineering and Technology, Sirindhorn International Institute of Technology (SIIT), Thammasat University, Pathum Thani 12121, ThailandbNational Metal and Materials Technology Center (MTEC), Thailand Science Park, Pathum Thani 12120, ThailandcCenter of Excellence on Petrochemical and Materials Technology, Department of Materials Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand

a r t i c l e i n f o

Article history:Received 19 June 2012Received in revised form8 October 2012Accepted 16 October 2012Available online 26 October 2012

Keywords:Biodegradable polymerAliphatic/aromatic copolyestersHydrolysisFTIRUVeVis

* Corresponding author. Tel.: þ66 2 986 9009x1806E-mail address: pakorn@siit.tu.ac.th (P. Opaprakas

0141-3910/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymdegradstab.2012.10.0

a b s t r a c t

Aliphatic/aromatic copolyesters, which possess good mechanical property and degradability, are ofimmense interest. Standard characterization techniques for the copolymer structure and degradationbehaviors have been developed. The techniques are applied to examine hydrolytic degradation ofpoly(ethylene terephthalate-co-lactic acid) in a phosphate buffer solution (pH 7.2) at 60 �C. The weightloss of the copolymer and pH of the medium as a function of time are examined. 1H NMR spectra provideinformation on microstructure and molecular weight of the samples, where deviations of the resultsfrom the actual values are observed, due to low solubility of the copolymer. More accurate results areobtained from TGA and FTIR experiments, as the samples are characterized in bulk. Insight into degra-dation mechanisms of the copolymer is derived from FTIR spectra. The content of aromatic esters in thesoluble degraded species is determined from UVeVis spectroscopy. These standard methods can beapplied to various types of degradable aliphatic/aromatic copolyesters, which are essential in propertyassessment and determination of their potential applications.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The demand for plastic use has rapidly increased, due to thematerial versatility and durability. This, in turn, adversely generateslarge amounts of wastes, which causes serious environmentalproblems. Decomposition of petroleum-based plastic wastesrequires long time periods, because the material is highly resistantto environment influences such as humidity or microbial attack.One of the currentmass-produced synthetic polymers that is highlyresistant to biological and hydrolytic degradation is poly(ethyleneterephthalate) (PET), a thermoplastic aromatic polyester that hasexcellent material properties. PET is commercially used in variousapplications, such as textile fibers, soft-drink bottles, and packagingfilms. Owing to the increased awareness of environmental issues,recycling of PET has been practiced [1,2]. However, this is difficult orinexpedient due to technical and economic considerations.

A promising alternative to replace conventional plastics in massuse is (bio)degradable polymers, in which various types of aliphaticpolyesters play a major role with respect to industrial relevance

; fax: þ66 2 986 9009x1801.it).

All rights reserved.14

[3,4]. Polylactic acid (PLA) offers the advantage of being not onlydegradable, but also renewable since its monomer, lactic acid (LA),is produced from microbial fermentation of agricultural rawmaterials [4e6]. PLA possesses physical and mechanical propertiescomparable to commodity plastics, and can be processed like mostthermoplastics. In addition, its degradability can be incorporatedinto other polymers to produce materials with desired propertiesfor specific applications [7e13]. As a result, PLA and its derivativeshave gained popularity for use in agricultural, biomedical, andpackaging applications.

Given their respective advantages, copolymerization of PET andPLA produces an aliphatic/aromatic copolyester with excellentmechanical properties but still retains its degradability. Manystudies have focused on the synthesis, characterizations, andprocessability of these copolymers [7e9,14e17]. Olewnik et al. re-ported that the synthesis and characterizations of poly(ethyleneterephthalate-co-lactic acid) (PET-co-PLA) by a melt reaction of LAand bis-(2-hydroxyethyl terephthalate) (BHET) in the presence oftin(II)chloride [8]. Acar et al. synthesized a similar copolymer, butfrom a different process, where recycled PET bottles and LAmonomer were employed as starting materials [7].

In our recent work, copolymers of terephthalate and lactic acidwere synthesized from LA, dimethyl terephthalate (DMT) and

P. Sriromreun et al. / Polymer Degradation and Stability 98 (2013) 169e176170

various diols as monomers. The resulting aliphatic/aromaticcopolymers showed appreciable hydrolytic degradability in phos-phate buffer solutions at room temperature and at 60 �C [16],which is in good agreement with other reports [4,7,9,16,18].Characterization of chemical structures and degradation mecha-nisms of the resulting copolymers are essential for assessment ofthe materials properties. 1H NMR is commonly employed in thestructure characterization of the copolymers. Measurement ofpercentage weight loss of sample as a function of degradation timeis a simple technique that is widely employed [4,7,11,18]. However,the results from this experiment cannot provide insight intomechanisms of degradation and structure of the degraded species.The low solubility of the copolymers also limits the use of NMRmeasurement in some samples, especially for those with higharomatic contents.

In this work, standard characterization techniques utilizing FTIR,TGA, and UVeVis spectroscopy have been developed to assess thechemical structure and degradation behaviors of aliphatic/aromaticcopolyesters. These are applicable to copolymer samples withvarious aromatic contents, as dissolution is not required in thesample preparation. Advantages and disadvantages of individualcharacterization techniques are compared and discussed.

2. Experimental

2.1. Materials

Lactic acid (LA) (88% wt aqueous solution), ethylene glycol (EG),and potassium dihydrogen phosphate (KH2PO4) were purchasedfrom Carlo Erba. DMT and BHET were supplied by Acros. Antimonytrioxide (Sb2O3) was provided by SigmaeAldrich. All chemicalswere used without further purification.

2.2. Synthesis of PET-co-PLA copolymer

PET-co-PLA copolymer was synthesized by polycondensation ofLA, DMT and EG, using a Sb2O3 catalyst, as described earlier[16,19,20]. Properties of the copolymer before and as a function ofdegradation times were characterized by 1H NMR, FTIR, XRD, TGA,and DSC experiments. An overview of properties characterizationsand examinations of degradation behaviors is summarized in Fig. 1.Chemical structure and properties of the original copolymer werecharacterized. The copolymer was then subject to hydrolyticdegradation, where degree of degradation and mechanisms wereexamined by following the weight loss and changes in chemicalstructure of the remaining solid sample. The nature and content ofthe soluble degraded species were also characterized by examiningUVeVis spectra and pH of the medium.

Synthesis of PET-co-PLA

Remaining solid sample

- % weight loss

- NMR

- FTIR

Hydrolytic degradation

Buffer medium

- UV-Vis

- pH

Copolymer characterizations

- NMR

- FTIR

- TGA

- DSC

- XRD

Fig. 1. Overview of experimental procedures.

2.3. Hydrolytic degradation tests

Hydrolytic degradation tests were performed in a 0.1 M phos-phate buffer solution at pH 7.2 by using accelerated conditions, i.e.low-MW copolymer in powder form at high temperature. Thecopolymer powder was placed in vials containing the buffer solu-tion at 60 �C. After certain times of incubation, the remaining solidwas removed from the buffer medium, and dried in a vacuum ovenat 60 �C for 3 days. The dried sample was weighed and its weightloss was calculated. Chemical structure and thermal stability of theremaining sample were characterized by 1H NMR, FTIR, and TGAexperiments. The buffer medium was not renewed during thedegradation period, and its pH was measured as a function ofhydrolysis time using a pHmeter (Ecoscan pH5). The content of thedegraded species dissolved in the buffer medium was analyzed bya UVeVis spectroscopy.

2.4. Characterizations

1H NMR spectra of the original copolymer and its degradedproducts were recorded on a 500 MHz spectrometer (a BrukerZHO48201 AV500D). A 7% v/v trifluoroacetic acid/CDCl3 mixedsolvent was used. The spectrum was recorded immediately afterthe preparation of solutions to avoid end-group esterification ofcopolymer [8,21]. FTIR spectra were recorded on a Thermo Nicolet6700 spectrometer. Solid samples were mixed with KBr powderand pressed into a pellet form. The concentration of sample in KBrpowder was carefully determined to obtain spectra that obey theBeereLambert’s Law, which is essential for quantitative analysis.Each spectrumwas recorded in a transmission mode with a total of16 scans at a 2 cm�1 resolution. Quantitative analysis was per-formed by employing the “Peak resolve” module integrated in theOMNIC program.

DSC analysis was performed in N2 atmosphere on a MettlerToledo DSC 822. Transition temperatures were measured on thesecond heating curves. Samples were scanned from 0 to 240 �C ata heating/cooling rate of 10 �C/min. Themogravimetric analysis(TGA) was carried out under N2 atmosphere from 20 to 1000 �Cusing aMettler ToledoTGA/SDTA851e at a heating rate of 20 �C/min.Differential TGA thermogram (DTGA) was generated and used inthe determination of thermal degradation of the samples. Crystal-line characteristics of the copolymer samples were characterized byX-Ray Diffractometer (XRD, JEOL JDX-3530) using Cu Ka1 radiation,scanning from 2q of 5e50� with a 0.02� step size.

UVeVis spectroscopic analysis (Spectronic Genesys 10 UVScanning) was performed to determine hydrolysis mechanisms.The buffer medium before and after specific hydrolysis time wastransferred into a quartz cuvette. Absorption spectra were recordedfrom 200 to 400 nm. UVeVis spectra of low-MW analogues ofaromatic and aliphatic esters and carboxylic acid compounds, i.e.,BHET, LA, and acetic acid were analyzed. Alkali-degraded BHETsolutions were employed in the construction of a standard curve toquantitatively determine the content of degraded aromatic esters(from the copolymer chains) dissolved in the buffer medium.

3. Results and discussion

3.1. 1H NMR and FTIR spectroscopy

The chemical structure of PET-co-PLA copolymer and itssynthesis reaction is shown in Fig. 2. 1H NMR spectrum is shown inFig. 3(a). The band assignments and calculations of MW andmicrostructure were described earlier [16,17,20]. Proton signalsof terephthalate units at 8.1 ppm (aromatic ring), lactate units at5.0e5.5 ppm (CeH) and 1.3e1.8 ppm (eCH3), and those of ethylene

Fig. 2. Synthesis reaction and structure of PET-co-PLA copolymer.

P. Sriromreun et al. / Polymer Degradation and Stability 98 (2013) 169e176 171

glycol units at 4.2e4.8 ppm (eCH2e) indicate an incorporation ofaliphatic and aromatic esters segments in the copolymer chains [8].Molecular weight ðMnÞ and microstructure characteristics of thecopolymer were calculated from 1H NMR spectra [8,16,17]. The

Fig. 3. 1H NMR spectra of PET-co-PLA copolymer (a) befo

average length of ethylene terephthalate and lactate sequences(XET, YL), terephthalate/lactate (T/L) ratio [20], and the degree ofrandomness (B) [22] were analyzed. The calculated values fororiginal copolymer, as summarized in Table 1, indicate a medium

re and (b) after 12 weeks of hydrolytic degradation.

Table 1Molecular weight and microstructure characteristics of PET-co-PLA copolymerbefore and after 12 weeks of hydrolytic degradation at 60 �C, calculated from 1HNMR spectra.

Hydrolytictime (weeks)

Mn Sequential length T/L ratio Degree ofrandomness (B)

XET YL

0 3903 2.72 1.13 1.89 2.8612 1969 3.49 1.00 2.79 3.97

Fig. 5. TGA thermogram and its first derivative (DTGA) of PET-co-PLA copolymer.

P. Sriromreun et al. / Polymer Degradation and Stability 98 (2013) 169e176172

Mn of 3903 g/mol. The values of T/L ratio (1.89) and B (2.86) reflecta blocky nature, which is dominated by longer blocks of aromaticsequences.

FTIR spectrum of PET-co-PLA is compared with those of neat PLAand PET in Fig. 4. Major characteristic bands of PET are observed at1720 (C]O stretching), 1280 (CeOeC asymmetric stretching), 1207,1125, 1104 and 1019 (ring CeH in-plane bending), and 730 cm�1

(ring CeH out-of-plane bending). Characteristic bands of vibra-tional modes associated with lactate units are located at 1760 (C]Ostretching), 1455 (eCH3 asymmetric bending), 1184, 1125 and 1094(CeO stretching), and 1383 and 1362 cm�1 (CeH bending) [23]. Theexistence of these bands in the spectrum of PET-co-PLA confirmsthe presence of PLA and PET segments in the copolymer structure.The copolymer’s aromatic/aliphatic content, i.e., T/L ratio, can bedetermined from FTIR spectra from band ratios of the aromatic andaliphatic C]O stretching modes, employing the peak resolveprogram. Given a similar molar extinction coefficients of the cor-responding bands, an absorption coefficient ratio of 1 is assumed[24]. The T/L ratio of 3.31 is obtained. This is significantly higherthan that calculated from NMR spectra.

3.2. TGA and DSC analyses

TGA thermogram and its first derivative (DTGA) of PET-co-PLAare shown in Fig. 5. Three main thermal degradation steps areobserved at 320 �C (L sequences), 400 �C (EG units), and a majordecomposition at 450 �C, due to T sequences. Percentage weightloss of each decomposition step is 9, 22 and 69, which correspondsto molar ratios of 0.13:0.49:0.47. Consequently, the T/L ratio of 3.64is obtained from the TGA experiment, which is comparable to thatfrom FTIR spectra, but significantly higher than that from 1H NMR.

1400 1600 1800

1720

1455

1760

wavenum

Abs

orba

nce

(a.u

.)

Fig. 4. FTIR spectra of (a) PET, (b) PET

Given the sample’s well-separated decomposition characteristics, itis confident that the T/L ratio calculated from this experiment ismore accurate. The lower value obtained from NMR experiment isprobably due to a low solubility of aromatic sequences in thesolvent, which results inweaker aromatic signals and hence a lowercontent of observed aromatic units in the chains. Similar behavior iscommonly found in 1H NMR measurements of copolymers con-sisting of multiple components with different solubility [16]. Incontrast, TGA and FTIR analyze samples in bulk state, without thedissolution process. MALDI-TOF was also used in the determinationof MWof the copolymer (not shown). Various matrix, additive, andsolvent systems were employed. However, it is challenging toobtain reliable results. This agrees with those reported by Olewniket al. [8] that the technique can be applied to only copolymersconsisting of low aromatic contents.

DSC thermogram of the copolymer shows Tg and Tm at 46 and173 �C, which are lower than those of neat PLA [5] and PET [1], dueto its lower MW. The presence of Tm indicates a semi-crystallinematerial. TGA thermogram also indicates high thermal stability,

800 1000 1200

730

(a)

(b)

(c)

1280

ber (cm -1)

-co-PLA copolymer, and (c) PLA.

Fig. 6. XRD spectra of (a) PET, (b) PET-co-PLA, and (c) PLLA.

0

2

4

6

8

0

10

20

30

40

0 4 8 12 16 20 24

pH

wei

ght

loss

(%

)

Time (weeks)

Fig. 7. Percentage weight loss of PET-co-PLA copolymer and pH change of a buffermedium as a function of hydrolytic degradation time.

Fig. 9. FTIR spectra, in the C]O stretching region, of PET-co-PLA solid sample asa function of hydrolysis time and their 2nd derivatives.

P. Sriromreun et al. / Polymer Degradation and Stability 98 (2013) 169e176 173

comparable to that of PET, which is higher than commercial PLA[25]. XRD trace of the synthesized copolymer is dominated bycrystalline characteristic of PET, as shown in Fig. 6. This agrees withthose observed from 1H NMR, FTIR and TGA experiments. Thelonger average sequences of aromatic units, reflected by the high T/L ratio, dominate and form crystal domains, which in turn retardcrystallization of PLA segments.

3.3. Hydrolytic degradation tests

Hydrolytic degradation of PET-co-PLA is examined in a buffersolution pH 7.2 at an accelerated condition (60 �C). Degree ofdegradation is measured in term of percentage weight loss of the

Fig. 8. The proposed hydrolysis mechanism of PET-c

remaining sample as a function of time. Fig. 7 shows a rapidincrease in weight loss, which reaches 15% in the initial 6 weeksperiod. After this, however, the value remains unchanged, indi-cating a steady state or equilibrium degradation reaction. A slightdecrease in pH of the medium is observed during the first 6 weeks,as a result from buffer stabilization. The value rapidly decreasesfrom 6 to 4 during the 6th to 10th week of hydrolysis, and remainssignificantly constant after this period. This indicates that thehydrolytic degradation generates acidic species, which are solublein the buffer solution. Aliphatic ester bonds in the copolymerchains are susceptible to hydrolysis, assisted by the present of eCOOH end groups as catalysts, leading to a formation of degradedspecies consisting of carboxylic acid and hydroxyl end groups[18,26]. These species then dissociate in the aqueous buffermedium and produce acidic protons, resulting in a more acidic pHvalue, as summarized in Fig. 8.

Although a large increase in weight loss is observed in an earlystage of degradation, the slight change in pH is because theresulting Hþ are taken up by the buffer. An excess amount of Hþ isaccumulated, in the later stage of degradation (after 6 weeks).When the content of the newly generated species is higher than thebuffer capacity, a rapid decrease in pH of the buffer medium is

o-PLA copolymer in phosphate buffer solution.

0.00

0.05

0.10

0.15

0.20

0.25

0 4 8 12 16 20 24

band

fra

ctio

n

Time (weeks)

1600 cm-11685 cm-1

0

5

10

15

20

0 4 8 12 16 20 24

T/L

rat

io

Time (weeks)

a b

Fig. 10. T/L ratio calculated from FTIR band intensity ratio of the 1720/1760 cm�1 modes (a), and contents of carboxylate (1610 cm�1) and carboxylic acids (1685 cm�1) of PET-co-PLAcopolymer (b) as a function of hydrolytic time.

P. Sriromreun et al. / Polymer Degradation and Stability 98 (2013) 169e176174

observed. It is noted that the equilibrium pH ofw4 (after 10 weeks)is comparable to a pKa value of carboxylic acids. This likely indicatesan equilibrium state of the acid dissociation, as themediumwas notrenewed during the experiment, which also agrees with the satu-ration of weight loss value.

The 1H NMR spectrum of the degradation product after 12weeks of hydrolysis, as shown in Fig. 3(b), exhibits similar peakpattern to the original sample. A new signal appears at 4.2 ppm,reflecting new species generated from the scission of ester bonds,i.e., eCH(CH3)OH from lactate sequence [8]. Results on micro-structure of the remaining solid after 12 weeks (Table 1), indicatethat Mn decreases from 3903 to 1969 g/mol, where YL alsodecreases. In contrast, the T/L ratio and XET increase, reflecting thatlactate units or aliphatic (L) sequences are more susceptible todegradation than ET, leading to higher aromatic content in theremaining chain. An increase in the B value from 2.86 to 3.97indicates a higher blocky characteristic. It is noted that the valuesobtained from this experiment are lower than the actual values,due to the incomplete dissolution of the aromatic sequence. As thedegradation leads to a higher aromatic content in the copolymerchain, a higher degree of deviation of the results is expected, due tolower dissolubility.

Normalized FTIR spectra, in the C]O stretching region, of theremaining solid sample at different hydrolysis time and their 2ndderivatives are compared in Fig. 9. A decrease in intensity of bothC]O stretching modes is observed, where that of the aliphaticmode (1760 cm�1) decreases at a higher rate than the aromaticcounterpart (1720 cm�1). The T/L ratio of the copolymer, calculatedfrom the intensity ratio of the 1720/1760 cm�1 modes, as a function

Fig. 11. UV absorption spectra of analogue model compounds (a) degraded PET film, (b) alkaland PET-co-PLA copolymer at (f) 2, (g) 4, (h) 6, (i) 10, (j) 16, (k) 24 weeks of hydrolysis.

of time is summarized in Fig. 10(a). The value increases from 3.31 to18.64 at 24 weeks of hydrolysis, indicating a higher degree of bondscission of the aliphatic sequences. An increase in intensity asa function of time is also observed in the 1685 cm�1 band, which isassigned to C]O mode of carboxylic acids [23], confirming that eCOOH end groups are generated as a result from hydrolysis of thecopolymers. A new band, associated with carboxylate groups,appears at 1610 cm�1 [23], whose intensity varies with the degra-dation time.

Fig. 10(b) illustrates fractions of normalized band area of thecarboxylate and carboxylic acid modes, which reflects the contentsof the two species, as a function of time. The value for carboxylicacids increases with degradation time, indicating a generation of eCOOH end groups by hydrolysis. The corresponding values forcarboxylate also increase in an early stage and reaches the highestvalue at 10 weeks. An adverse decreasing trend of the content isobserved after this period, however. An explanation on mecha-nisms of this degradation behavior is as follows. In the earlydegradation stage, hydrolysis reaction generates degraded specieswith longer chain length, whose acidic chain-ends dissociate andbecome dissoluble in the buffer medium. This rapidly reachesa saturation concentration, due to the interplay between main-chain hydrophobicity and the carboxylate hydrophilicity. Asa result, the degraded fragments re-precipitate onto the remainingsolid sample, reflected by the early-increasing trend of thecarboxylate content. This agrees with the weight loss results andthe pH change of the buffer medium. In the later stage, further bondscissions of the precipitated species produces smaller-sizedcarboxylate-capped fragments, which possess higher solubility.

i-degraded BHET, (c) degraded PLA, (d) alkali-neutralized LA, (e) acetic acid sodium salt;

0

2

4

6

8

10

12

0

6

12

18

24

0 4 8 12 16 20 24

perc

enta

ge w

eigh

t lo

ss

Con

tent

of

arom

atic

est

ers

(mol

/l *

10

-3)

Time (weeks)

Fig. 12. Concentration of aromatic ester comprising in soluble degraded species fromhydrolysis of PET-co-PLA at 60 �C as a function of time, and the correspondingpercentage weight loss values.

P. Sriromreun et al. / Polymer Degradation and Stability 98 (2013) 169e176 175

This leads to a decrease in the carboxylate content of the remainingsolid samples.

The nature and content of degraded species soluble in the buffermedium are examined. UVeVis spectra of aqueous solutions ofanalogue models are compared in Fig. 11(a), where those consistingof aromatic esters, i.e., alkali-degraded products of BHET and PET,show an absorption band with lmax at 243 nm. This is corre-sponding to an absorption mode of conjugated aromatic carboxylgroups. In contrast, no absorption peak is found in this region formodel compounds containing aliphatic esters, such as lactic acid,acetic acid, and alkali-degraded products of PLA. The buffermedium obtained from hydrolysis of PET-co-PLA copolymer showa similar absorption pattern to that of the aromatic ester models.The intensity of the absorption peak is dependent on the hydrolysistime, as shown in Fig. 11(b).

Given that chain scission of the copolymer mainly takes place atthe aliphatic ester bonds, it is likely that the soluble speciesconsists of aliphatic ester terminals with enveloped aromaticesters. The absorption band at 243 nm is, therefore, employed inthe determination of aromatic ester content of degraded speciessoluble in the buffer medium by using a standard curve con-structed from BHET solutions. The results, as shown in Fig. 12,indicate a rapid increase in the content of the soluble aromaticesters during the first 10 weeks, which is in good agreement withother experiments. It is noted that the calculated weight content ofthe aromatic esters is lower than those obtained from the weightloss measurement, due to the mass of aliphatic ester fragments inthe solutions, which cannot be detected by this technique.Nevertheless, this clearly supports our discussion that the solublespecies consist of aromatic units capped by aliphatic units at pointof degradation.

4. Conclusions

Standard characterization techniques for chemical structure anddegradation behaviors of aliphatic/aromatic copolyesters have beendeveloped. The techniques are applied to examine hydrolyticdegradation of PET-co-PLA copolyester. The percentage weight lossof the sample and pH of the buffer medium as a function of time isexamined. Conventional 1H NMR experiments provide informationon microstructure and MW of the samples. There are, however,some deviations of the results from the actual values, due to lowsolubility of the copolymer. More accurate results can be obtainedfrom TGA and FTIR experiments, as the samples are characterized inbulk, where the aromatic and aliphatic features are clearly

separated. Insight into degradation mechanisms of the copolymeris also derived from FTIR spectra of the remaining solid sample bymeasuring the compositions of newly generated carboxylic acidsand carboxylates. The content of aromatic esters containing in thesoluble degraded species is determined from UVeVis spectroscopy.These standard methods can be applied to various types ofdegradable aliphatic/aromatic copolyesters, which are essential inproperty assessment and determination of their potentialapplications.

Acknowledgements

Financial support of this work is provided by The ThailandResearch Fund (TRF) and The Commission on Higher Education(CHE), under grant number RSA5280029. P. Sriromreun is sup-ported by The Commission on Higher Education under the CHE-PhD-THA scholarship program and a Thammasat Universityresearch grant for graduate students.

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