Chiang Mai J. Sci. 2014; 41(3) 691

Chiang Mai J. Sci. 2014; 41(3) : 691-703http://epg.science.cmu.ac.th/ejournal/Contributed Paper

Miscibility and Hydrolytic Degradability of Polylacticacid/Poly (ethylene terephthalate-co-lactic acid)BlendsParanee Sriromreun [a], Atitsa Petchsuk [b], Mantana Opaprakasit [c]and Pakorn Opaprakasit*[a][a] School of Bio-Chemical Engineering and Technology, Sirindhorn International Institute of Technology

(SIIT), Thammasat University, Pathum Thani 12121, Thailand.[b] National Metal and Materials Technology Center (MTEC), Pathum Thani 12120, Thailand.[c] Center of Excellence on Petrochemical and Materials Technology, Department of Materials Science,

Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand.*Author for correspondence; e-mail: pakorn@siit.tu.ac.th

Received: 10 June 2012Accepted: 18 February 2013

ABSTRACTPoly(ethylene terephthalate-co-lactic acid) (PET-co-PLA) copolymer with medium

molecular weight was synthesized and blended with commercial polylactic acid (PLLA) bysolution and mechanical blending methods to optimize its mechanical properties.Miscibility and properties of the blends were characterized. Miscible blends are achieved at allcompositions. A decrease in storage modulus and Tg of the blends, compared to neat PLLA,indicates a plasticizing effect of the copolymer in the blends. Hydrolytic degradability of thesamples is examined in phosphate buffer solution (pH 7.2) at 60°C. Appreciable degradabilityof the blends is retained despite the incorporation of aromatic esters. Anomalous degradationbehavior of the samples in buffer medium is examined. Negative weight loss values andflocculation of the samples are clearly apparent at a specific hydrolysis time. This is due tothe re-precipitation of degraded species onto the remaining solid sample. The copolymer andits blends show high potential for use in various applications, especially in packaging andagricultural fields.

Keywords: aliphatic-aromatic copolyester, degradable, hydrolysis, blend, miscibility

1. INTRODUCTIONPolymeric materials are widely used

because of their versatility, low cost, lightweight, and durability, leading to a rapidincrease in the material consumption. This,however, generates large amount of waste,which causes serious environmentalproblems. Petroleum-based polymer wastesrequire long decomposition time, as the

material is highly resistant to environmentinfluences. Various degradable polymers,especially aliphatic polyesters, offer effectivesolutions to this problematic issue. Amongthese, polylactic acid (PLA) has receivedvast attention, because of its renewability,biocompatibility, and ease of processing[1-3]. However, PLA has low impact

692 Chiang Mai J. Sci. 2014; 41(3)

strength and thermal stability, which limitsits use in certain applications. In contrast,aromatic polyesters, especially poly(ethyleneterephthalate) (PET), exhibit excellentmechanical properties and processability,which are suitable for various commercialapplications. PET has been employed in theproperty modification of aliphatic polyestersby copolymerization [4-6]. In addition,this also enhances the degradability of PET,making the material more environmentalfriendly [7-9].

It is challenging to synthesizehigh molecular weight (MW) copolymersof PET and PLA, especially bypolycondensation, which results ininsufficiently-low mechanical propertiesof the products. Several modificationmethods have been practiced to improveproperties of polymers, such as blendingand plasticization. Among these, blending isregarded as a useful and economical way tooptimize the material’s property for specificapplications.This is achieved by formationof specific interactions between the blend’scomponents[10-12]. A compatibilizer can beemployed to enhance compatibility ofpolymer blends. In our previous work,copolymers of lactic acid and terephthalatewere synthesized by employing varioustypes of diols [13-15]. Processability anddegradability of the copolymers and theirblends were assessed [16, 17].

In this work, a medium-sizedpoly(ethylene terephthalate-co-lactic acid)(PET-co-PLA) copolymer is synthesizedand blended with commercial PLLA(4042D)resin by solution and mechanical blendingmethods. Effect of the blend compositionon miscibility and properties of the blendproducts is investigated by employingNuclear Magnetic Resonance (NMR), FourierTransform Infrared (FTIR) spectroscopy,Scanning Electron microscopy (SEM),

Thermogravimetric Analysis (TGA), X-RayDiffraction (XRD), Differential ScanningCalorimetry (DSC), and Dynamic MechanicalAnalysis (DMA). Hydrolytic degradationbehaviors of the blends are examined byUV-Visible spectroscopy, FTIR, 1H NMRand weight loss measurements.

2. MATERIALS AND METHODS2.1 Materials

PET-co-PLA copolymer (Mn= 3.90 ×103g/mol) was synthesized bypolycondensation of LA, DMT, and EGmonomers, with a molar ratio of 1:1:2, asdescribed earlier[13, 15]. CommercialPLLA resin (4042D, Mn=1.20×105g/mol) was obtained from NatureWork.Dichloromethane (CH2Cl2), trichloroaceticacid (CCl3COOH), potassium dihydrogenphosphate(KH2PO4), and potassium bromide(KBr) were purchased from Carlo Erba.Bis(2-hydroxyethyl terephthalate) (BHET) wasobtained from Sigma-Aldrich. Deuteratedtrichloromethane (CDCl3) and trifluoroaceticacid (CF3COOH)were supplied from Merck.All chemicals were used without furtherpurifications.

2.2 Blending ProcessesThe copolymer was blended with

commercial PLLA by mechanical and solutionblending methods.A Brabender mixer(PLASTI-CORDER® LAB STATION+W50EHT) was employed in a preparationof mechanical blends at 160°C. Solution-blended samples were prepared by usingCH2Cl2 solvent for PLLA and CCl3COOH/CH2Cl2 for the copolymer. Effect of the blendcompositions on propertiesof the materialswas examined by varying the PLLA/PET-co-PLA blend ratio (80/20, 65/35, 50/50, 35/65, 20/80 w/w). The samples are denoted asPL80C20, PL65C35, PL50C50, PL35C65,and PL20C80, respectively. Miscibility,

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physical, mechanical, and thermalproperties of the blends were characterized.

2.3 Hydrolytic Degradation TestsHydrolytic degradation test was

performed in a 0.1M phosphate buffersolution (KH2PO4) pH 7.2, at acceleratedconditions (powder sample, at 60°C)[17].The sample (0.2 g) was placed in vialscontaining the buffer medium (10 ml). Afterinterval times of incubation, the remainingsolid was removed, and dried in a vacuumoven at 60°C for 1 week. The weight ofthe dried sample was recorded and itspercentage weight loss was calculated.The sample chemical structure andcomposition was characterized by 1H NMRand FTIR spectroscopy. The buffer mediumwas not renewed during the degradationtests, where its pH was measured as a functionof time using a pH meter (Ecoscan pH5).The aromatic ester content comprising inthe degraded species, which are soluble inthe buffer medium, was analyzed by UV-Visspectroscopy[17].UV-Vis spectra wererecorded on a Spectronic Genesys 10 UVSpectrophotometer. The buffer mediumobtained at hydrolysis time intervals wastransferred into a quartz cuvette. Absorptionspectra were recorded from 200-400 nm.Solutions of alkali-degraded BHET in thephosphate buffer were used to construct astandard curve in the determination ofsoluble aromatic esters content.

2.4 Characterizations1H NMR spectra were recorded at

500 MHz on a Bruker ZHO48201 AV500Dspectrometer.The sample was dissolved in a7% v/v CF3COOD/CDCl3 mixed solvent.The spectra were recorded immediatelyto avoid end-group esterification [4, 18].FTIR spectra were recorded on a ThermoNicolet 6700 spectrometer, with a total of

16 scans at 2 cm-1 resolution. The samplewas cast as a film on a KBr pellet from a2% solution in CCl3COOH/CH2Cl2. Thepeak fitting module integrated in theOMNIC program was employed in aquantitative analysis of the FTIR spectra.DSC measurements were performed in anatmosphere of N2 on a Mettler ToledoDSC 822. Transition temperatures werederived from the second heating curves.Samples were scanned from 0 to 240°Cat a heating rate of 10°C/min. TGAmeasurement was carried out under N2

atmosphere on a Mettler Toledo TGA/SDTA851efrom 20-1000°C at a 20°C/minheating rate. Crystalline characteristics werecharacterized by XRD (JEOL JDX-3530)using Cu Ka1 radiation. The samples werescanned from 2θ of 5-50° at 0.02 step size.Morphology of the blends was analyzedusing SEM microscope (HITASHI S3400N).The samples were prepared by cryogenicallyfracturing in liquid N2. Mechanical propertieswere characterized by DMA (Mettler ToledoDMA/SDTA861e) in a tension mode.The experiment was performed in thetemperature range of 0-100°C, using aheating rate of 3°C/min at 1Hz frequency.Film samples were prepared by acompression molding machine (Lab Tech) at165-170°C, with a pressure of 350 psi.

3. RESULTS AND DISCUSSION3.1 Blends Miscibility

Miscibility of PLLA/PET-co-PLA blendsprepared from mechanical and solutionblending methods were examined. DSCthermograms of the blends as a function ofthe blend compositions are compared withtheir single-component constituents inFigure 1. All spectra (both cooling and 2nd

heating scans) exhibit a single glass transitiontemperature (Tg) characteristic, indicating amiscible blend for all compositions.

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Crystallization (Tc) and melting (Tm)temperature peaks due to the copolymer’saromatic domains are clearly observed ataround 120 and 177°C in those of thesolution-blended samples, which are rarelyfound in the mechanical-blended counterparts.This is due to a higher degree of miscibilityof the mechanical blends, as a result fromthe applied mechanical shear force andtransesterification reaction, which leads toa decrease in the polymer molecularweight [1]. Insufficient dissolution of thecopolymer during the preparation of thesolution blends partly plays a role in theexistent of the aromatic crystallinecharacteristic [17].

Tg values of PLLA/PET-co-PLA blendsas a function of the blend content aresummarized in Figure 2. The observed Tg

of PET-co-PLA (46.3°C) is lower than that

of neat PLLA (59.4°C), due to thecopolymer’s low MW. Tg of mechanical-blended samples varies with the blendcontents, ranging from 40.6-47.1°C, whichis slightly different from those of thesolution blended samples. This is likely dueto the slight differences in degree ofmiscibility and the presence of trace amountof residue solvent in the solution blends.Tg of the blends from both systems is lowerthan those calculated from the Fox theory[19], as shown in the equation below.The theory estimates a Tgvalue of miscibleblends by weighted average, with anassumption that there is no specificintermolecular interaction. The deviation ofthe observed Tg from the theoretical valuesreflects that the blend components bindstrongly to themselves, compared to theinter-association [10].

1 = W1 + W

2 The Fox theory

whereTg = theoretical glass transition

temperature of the blend

Tg1and Tg2 = glass transition temperaturesof the blend components

W1 and W2 = weight fraction of the blendcomponents

Figure 1. DSC thermograms of PLLA/PET-co-PLA blends prepared from (a) solutionblending and (b) mechanical blending methods, as a function of the blend compositions.

Tg

Tg1

Tg2

Temperature (°C) Temperature (°C)

Temperature (°C) Temperature (°C)cooling scan 2nd heating scan

(a)

(b)

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Due to their higher degree of miscibility,detailed investigations are focused on themechanical blended samples. FTIR spectra ofPLLA/PET-co-PLA blends with differentcompositions, as shown in Figure 3, illustratecharacteristic bands of the copolymerat 1720 cm-1 (aromatic C=O stretching),1280 cm-1 (C-O-C aromatic asymmetricstretching), 1207, 1125, 1104 and 1019 cm-1

(ring C-H in-plane bending) and 730 cm-1

(ring C-H out-of-plane bending), andthose of aliphatic PLLA at 1760 cm-1

(C=O stretching), 1455 cm-1 (-CH3

asymmetric bending), 1383 and 1360 cm-1

(C-H bending of lactate group), and 1184,1125 and 1094 cm-1 (C-O stretching)[20].The intensities of the C-O-C stretching bandsat 1280-1100 cm-1 and the aromatic ringmode at 730 cm-1 vary with the copolymercontent. The latter mode is thereforeemployed in a normalization of the spectrafor quantitative analyses.

The second derivative FTIR spectra(not shown) reveal additional bands inthe C=O stretching region, i.e.,1760 and1740 cm-1, which are characteristic modes of“free” and hydrogen bonded (HB) C=O of

PLLA and the copolymer’s lactates[11, 12].The C=O band of aromatic units in thecopolymer is observed at 1720 cm-1.A vibrational mode located at 1645 cm-1

is due to -COOH end-groups. Curveresolving is applied to quantify these bands,in which that of PL35C65 is shown inFigure 3. Normalized band area of thecorresponding modes as a function of theblend compositions are summarized inTable 1. The intensity ratio of the 1740/1760modes is much higher than that of the neatPLLA and varies with the PLLA content inthe blend, confirming a formation ofhydrogen bonding interaction, probablyfrom PLLA’s carbonyl and carboxylic acid/hydroxyl end groups of the copolymer[11, 12]. It is apparent that a closer match inthe solubility parameters of the blendcomponents is responsible for the miscibilityenhancement [10]. The detected specificinteraction, however, plays some roles inthe blend’s miscibility. As the band area isnormalized based on the characteristic modeof aromatic units, the results confirm thatthese carboxylic acid end-groups are mainlyderived from the medium sized copolymer.

Figure 2. Tg of PLLA/PET-co-PLA blends as a function of the blend compositions.

PLLA weight content (%)

Tg (°

C)

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Phase morphology of PLLA/PET-co-PLA blends is analyzed. Figure 4 shows SEMimages of the blend samples with differentcompositions after cryogenically fracturing.

Homogeneous crack patterns are clearlyobserved in all blends. This indicates miscibleblends, which strongly agrees with thoseobserved from DSC and FTIR experiments.

Figure 3. FTIR spectra of (a) PET-co-PLA, (g) PLLA, and PLLA/PET-co-PLA blendsat various compositions: (b) 20/80, (c) 35/65, (d) 50/50, (e) 65/35, and (f) 80/20. Theinset shows curve resolving of C=O stretching bands of a PL35C65 blend.

Table 1. Normalized band area of C=O modes and normalized crystallinity of PLLA/PET-co-PLA blends as a function of the blend compositions.

Sample

PL20C80PL35C65PL50C50PL65C35PL80C20

PLLA

Aliphaticfree C=O

I1760

0.260.340.450.630.710.94

AliphaticHB C=O

I1740

0.180.150.110.020.010.06

HB/freeC=O

I1740/ I1760

0.680.430.240.030.020.05

AromaticC=OI1720

0.470.460.390.290.17

-

-COOHI1645

0.100.100.110.110.14

-

PET-co-PLA

3745333300

PLLA

0000218

Normalized FTIR band area Normalized crystallinity(%)

wavenumber (cm-1)

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3.2 Thermal DecompositionTGA thermograms and DTGA curves,

as shown in Figure 5, indicate that PET-co-PLA copolymer has 3 main weight losssteps. The decompositions with midpointsat 320 and 390°C are due to aliphatic lactate(L) sequences and the copolymer’s ethyleneglycol (EG) units, respectively. A majorweight loss step at 445°C is associatedwith the copolymer’s aromatic (T) units.The thermogram of PLLA exhibits a singledecomposition step centered at 380°C.For blend samples, L sequences of PLLAstart to decompose at lower temperaturesthan that of the neat PLLA. The degree of

temperature shift increases with the decreaseof PLLA content, as the PLLA domainsare largely disrupted by the copolymercounterparts.This leads to retardation ofPLLA crystallization and its lower thermalstability, i.e., the lower the relative PLLAcontent, the higher the degree of disruption.A slight shift in the decomposition of thearomatic domains to lower temperature isalso observed, i.e., the inflection point shiftsfrom 455 to 445°C with the decrease ofcopolymer content. This clearly indicates amutual effect of crystallization retardationof the 2 components, as a result frommiscible blend formation.

Figure 5. TGA and DTGA thermograms of (a) PET-co-PLA, (g) PLLA, and PLLA/PET-co-PLA blends: (b) 20/80, (c) 35/65, (d) 50/50, (e) 65/35, and (f) 80/20.

Figure 4. SEM images of (a) PET-co-PLA, (e) PLLA, and PLLA/PET-co-PLA blends:(b) 20/80, (c) 50/50, and (d) 80/20.

Temperature (°C)

% w

eigh

t

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3.4 Crystalline CharacteristicsFigure 7 shows XRD traces of mechanical

blended samples, where a variation incrystalline characteristics as a function of theblend contents is observed. The crystallinepatterns are dominated by those of thecopolymer, indicating that the crystallinedomains of PLLA are largely interrupted bythose of the copolymer. Normalizedcrystallinity of each blend components is

calculated based on their correspondingweight fractions, as summarized in Table 1.Crystallinity of the blends consisting ofcopolymer-rich compositions is similar, whichis approximately 25% lower than that of theneat copolymer, whereas those of PLLA arenot detectable. The PL80C20 blend showsalmost no crystalline patterns of both blendconstituents, reflecting a complete mutualdisruption of their crystalline formation.

Figure 6. (a) Storage modulus and (b) tan δ values, measured at 1 Hz, of PLLA/PET-co-PLA blends, as a function of the blend compositions and temperature.

3.3 Dynamic Mechanical PropertiesDynamic mechanical properties of

PLLA and PLLA/PET-co-PLA blends areexamined. Figure 6(a) shows that commercialPLLA exhibits the highest storage modulusvalue. Upon blending with the copolymer,the modulus of the blends (in glassy state)decreases with the copolymer content.This is likely because the medium-sizedcopolymer acts as a plasticizer in PLLAmatrix, leading to a softening of the material.This phenomenon is observed even thoughthe copolymer contains stiff aromaticsequences, indicating that an effect of MWis more significant. A large decrease in thestorage modulus is observed at 30-80°C,reflecting the glass transition process.

In a rubbery state, however, the blendswith higher PET-co-PLA contents showhigher storage modulus, reflecting that role ofstiff aromatic sequences. Loss tangent (tan δ)value, a ratio of loss and storage moduli, asa function of temperature, is shown inFigure 6(b). The placement of its maximumvalue indicates a temperature whererelaxation process takes place, especiallythe glass transition process. All samplesexhibit a single Tg, where commercialPLLA has the highest value at around65°C. Tg of the blends decreases as afunction of the copolymer compositiondue to the plasticizer effect. This is ingood agreement with those observed fromDSC experiments.

Temperature (°C)

log

(Tanδ)

Stor

age m

odul

us (l

og M

Pa)

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3.5 Hydrolytic Degradation BehaviorsDegradation behaviors of the blends and

their single component counterparts areexamined by monitoring their percentageweight loss as a function of hydrolysis time.Figure 8(a) indicates that commercial PLLApossesses higher weight loss value than thatof PET-co-PLA, as the copolymer contains

hydrophobic aromatic units in its structure.All blends show an increase in weight loss withan increase in the PLLA composition, wherethe highest weight loss of 30% is observedin PL80C20 at 24 weeks of hydrolysis.This is because aliphatic lactate sequencesare major components that undergohydrolytic degradation[7].

Figure7. XRD traces of (a) PET-co-PLA, (g) PLLA, and PLLA/PET-co-PLA blends: (b)20/80, (c) 35/65, (d) 50/50, (e) 65/35, and (f) 80/20.

Figure 8. Percentage weight loss values (a) obtained from weight loss experiments and(b) equivalent weight loss calculated from content of degraded aromatic species of PLLA/PET-co-PLA blends at various blend compositions, as a function of hydrolyticdegradation time. Appearance of a PL35C65 blend during hydrolytic degradation at(c) 4, (d) 16, and (e) 24 weeks.

2θ (deg)

Inte

nsity

Time (week)

Wei

ght l

oss (

%)

Equi

vale

nt w

eigh

t los

s (%

)

700 Chiang Mai J. Sci. 2014; 41(3)

The cumulative weight loss increaseswith the degradation time, where a sharpincrease is observed during the first 4 weeksand levels off after this period. This indicatesa saturation of degraded species soluble inthe buffer solution, as the medium is notrenewed during the hydrolysis. An anomalousdecrease in the cumulative weight loss ofthe copolymer and all blended samples isobserved at 16 weeks of hydrolysis, wheresome blends even exhibit negative weightloss values. This is in concert with anobservation of flocculants formation, whichis not observed during the first 16 weeks ofmeasurements, as shown in Figure 8(c-e).The origin of this phenomenon is likelybecause the degraded species from the blendscan partially dissolve in the buffer mediumdue to the interplay between ionic charges atthe scission points and the hydrophobic natureof the copolymer fragments. The content ofthese species increase with hydrolysis timeand reaches a saturation concentration.At this point, the degraded species flocculateby binding with or absorbing water moleculesinto their structures and precipitate onto thesolid samples. The strong interaction in thebound structure prevents the evaporation ofwater molecules during the drying process(60°C for 1 week), resulting in a large increasein the weight of the remaining “dried”solid samples, which is reflected in thenegative weight loss values. After 20 weeksof hydrolysis, however, further degradationof the soluble degraded species generatessmaller-sized species, which exhibit a highercharge/hydrophobic ratio and hence ahigher solubility in the buffer medium.This is evident from the disappearance ofthe flocculation and a reappearance of theincreasing trend of the cumulative weightloss values, as shown in Figure 8(e).

To gain further insight into changes inchemical structures of the blends as a function

of hydrolysis time, UV-Vis spectroscopy isemployed to examine the content of solublespecies in the buffer solution. BHET isemployed as a standard model for quantitativeanalysis of aromatic esters, as the compoundshows similar absorption patterns to thoseof the soluble degraded products of theblends[17]. Figure 8(b) illustrates the contentof degraded aromatic species in the buffermedium, calculated from UV-Vis spectra.The concentration is dependent on thecopolymer composition, as this is a directmeasurement of the aromatic esters degradedfrom the copolymer in the blends. All samplesshow an increase in the concentration ofsoluble aromatic ester fragments during thefirst 10 weeks, which reaches a saturationstage after this period. This agrees with thoseobserved from the weight loss experiments,implying the maximum degradability ofthe solid sample. Subsequently, the solubledegraded fragments undergo furtherdegradation by chain scission of theirremaining aliphatic ester bonds. However,this does not alter the concentration of thearomatic units soluble in the buffer medium.

FTIR spectra of the remaining solidsample of PL35C65 blend obtained atdifferent hydrolysis time are shown inFigure 9(a). The spectra are normalized basedon the aromatic ring mode at 730 cm-1.Band area of the normalized C=O bands ismeasured and compared across differentsamples, as illustrated in Figure 10(a).The relative intensity (band area) of the C=Oband of PLLA (1760 cm-1) decreaseswith time, reflecting an increase in the degreeof hydrolytic degradation. An unusualincrease in the band intensity, however,is observed at 16 weeks of measurement,which subsequently decreases to be aweak band after 24 weeks.The origin of thisanomalous behavior is investigated bywashing the degraded solid samples with a

Chiang Mai J. Sci. 2014; 41(3) 701

0.1 M NaOH solution and excess amountof water. The corresponding results afterwashing are shown in Figures 9(b) and10(b).The aliphatic C=O band at 1760 cm-1

almost disappears after 10 weeks ofhydrolysis, which confirms that the maximumdegree of hydrolysis has been reached.The resulting large content of degradedproducts in the buffer medium subsequently

leads to a precipitation onto the remainingsolid sample, as discussed earlier. Theprecipitate can be eliminated by washingthe solid sample with an alkali solution.In addition, carboxylate groups are largelygenerated as a result from the hydrolysis,reflected by an increase in intensity of abroad band centered at 1590 cm-1, as afunction of time.

Figure 9. FTIR spectra of the remaining solid samples of PL35C65 blend, as a functionof hydrolysis time: (a) original degraded sample and (b) the degraded sample after washingwith NaOH solution.

Figure 10. Normalized band area of ester’s C=O and carboxylate modes of PL35C65blend, as a function of hydrolysis time, (a) original degraded sample and (b) the degradedsample after cleaning with a NaOH solution.

Time (week)Time (week) (a) Time (week) (b)

Band

are

a

Band

are

a

wavenumber (cm-1) wavenumber (cm-1)

702 Chiang Mai J. Sci. 2014; 41(3)

Table 2. Aromatic/aliphatic ratio of PL35C65 blend, determined from 1H NMRspectra, as a function of hydrolytic time.

Hydrolytic time(week)

04101624

aromatic/aliphaticratio0.420.530.640.420.71

1H NMR spectra of solid sample ofPL35C65 blend before and after 10 weeksof hydrolysis(not shown) exhibit signalsdue to aromatic at 8.1 ppm (H1), lactate at5.4 ppm (H2), and -CH2CH2 at 4.5 and4.7 ppm (H5,6). The resonances of lactatein PLLA homopolymer are observed at5.3 ppm (H8) and 1.6-1.7 ppm (H4,9).The signal of the copolymer chain end-CH2(H

3) islocated at 4.2 ppm, and that ofthe PLLA chain end-CH3(H

7) is at 1.3 ppm[13, 15].The intensities of H3 and H7 signalsincrease after hydrolysis, as these signals areassociated with species generated by scissionof the ester bonds.

Chemical compositions of PLLA/PET-co-PLA blends were also analyzed from 1HNMR spectra, as summarized in Table 2.The aromatic/aliphatic molar ratios of allsamples are lower than those obtained fromFTIR and TGA, due to the low solubility ofthe samples which contain aromatic estersequences[17].On hydrolysis of the PL35C65blend, the values increase with time from0.42 to 0.71 at 0 and 24 weeks, indicatingthat lactate sequences are degraded at a fasterrate than the aromatic counterparts. Theresults also show an unusual decrease in theratio at 16 weeks, which agrees with thosediscussed earlier. After 20 weeks, the normalincreasing trend of the ratio is observed.

4. CONCLUSIONSPET-co-PLA copolymer was synthesized

and blended with commercial PLLA bymechanical and solution blending methodsto optimize its mechanical properties.Miscible blends are obtained for all blendcompositions. A decrease in storage modulusand Tg of the blends, despite the inclusionof the stiff aromatic copolymer, indicatesa plasticizer effect of the copolymer in theblends.The blend’s hydrolytic degradability isretained, as the copolymer has appreciabledegradability. Anomalous degradationbehavior of the samples in phosphatebuffer solution was observed, due to there-precipitation of degraded species onto theremaining solid sample. Given the improvedphysical properties and degradability of thecopolymer and its blends, the materials showhigh potential for use in various applications,especially in packaging and agricultural fields,which require good mechanical properties andhigh degradation rates.

ACKNOWLEDGEMENTSFinancial support of this work is

provided by the Thailand Research Fund (TRF)and the Office of Higher EducationCommission (OHEC), under grant numberRSA5280029. P. S. thanks the Office ofHigher Education Commission under theCHE-PhD-THA scholarship program and aThammasat University research grant forgraduate students.

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