Synthesis and Characterization of Functional AliphaticCopolyesters

RAMASWAMY MANI,1 MRINAL BHATTACHARYA,1 CHRISTIAN LERICHE,2 LI NIE,3 SUKH BASSI3

1Department of Biosystems and Agricultural Engineering, University of Minnesota, 1390 Eckles Avenue,St. Paul, Minnesota 55110

2Gemplus Corporation, Parc d’Activities de Gemenos, B.P. 100, 13881 Gemenos Cedex, France

3Midwest Grain Products, Incorporated, 1300 Main Street, Atchison, Kansas 66002

Received 20 May 2002; accepted 8 July 2002Published online 00 Month 2002 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pola.10417

ABSTRACT: Functional aliphatic copolyesters of succinic acid (SA) and citric acid (CA)were synthesized via direct copolycondensation in the presence of 1,4-butanediol, withtitanium(IV) butoxide as a catalyst. The effects of the comonomer and comonomer ratioon the polycondensation and the melting and glass-transition temperatures were in-vestigated. The melting temperature was very sensitive to the molar ratio of the SA–CAcomonomer units. The chain extension of this poly(butylene succinate citrate) wascarried out with hexamethylene diisocyanate. The intrinsic viscosity, crystallinitypercentage, and rheological properties of these copolyesters were also studied. © 2002Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3232–3239, 2002Keywords: aliphatic copolyesters; multifunctional monomer; chain extension; co-polymerization; polycondensation; gelation

INTRODUCTION

Biodegradable polymers offer an attractive alter-native to traditional, nonbiodegradable petro-leum-based polymers from an environmental per-spective. A major factor promoting an interest inbiodegradable polymers is the growing concernraised by the recalcitrance and unknown environ-mental fate of many of the currently used syn-thetic polymers. Polyesters, particularly aliphaticpolyesters, are considered to be the most econom-ically competitive of the biodegradable poly-mers.1,2 Witt and coworkers3,4 reported that theproperties of aliphatic and aromatic copolyesterswith a specific monomer distribution in the poly-mer chains resulted in good material propertiesas well as biodegradability. Earlier research from

our laboratories5–11 indicates that blends of anhy-dride functional polymers with natural polymers,such as starch and protein, could lead to productswith useful end properties and biodegradability.The tensile strength was invariant with thestarch content with respect to the original poly-ester for compatibilized blends, whereas it de-creased with an increase in the starch content forthe uncompatibilized blend.12 Compatibilizedblends had finer phase morphology than the un-compatibilized blends. Polyesters blends contain-ing 99% amylopectin starch at the 70% level hadthe lowest crystallinity, which otherwise de-creased with a decreasing amylopectin level inthe starch.

Methods to produce high molecular weightpolymers possessing superior mechanical proper-ties from low molecular weight, unsaturated ali-phatic polyesters produced in a short polymeriza-tion time with a moderate-level vacuum systemare also reported in the literature.13,14 These un-

Correspondence to: M. Bhattacharya (E-mail: bhatt002@umn.edu)Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 40, 3232–3239 (2002)© 2002 Wiley Periodicals, Inc.

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saturated groups offer sites for chain extension orcrosslinking during processing in the presence ofinitiators.13,14 Jin et al.15 introduced unsaturatedgroups into the main chains of aliphatic polyes-ters by condensation polymerization with maleicacid as a comonomer.

Functional aliphatic polyesters may be poten-tially useful as biodegradable compatibilizers andenhancers of processability when blended withnatural polymers such as starch and proteins.There is limited information about the synthesisof polyesters bearing pendant acid and hydroxylgroups in the backbone. Polymers obtained by thetransesterification16 or polymerization of cycliccarbonate with protected pendant hydroxylgroups17 or by the copolymerization of acid anhy-drides with oxiranes carrying substituents con-taining protected hydroxyl groups18 or with allylglycidyl ether19 have been reported in the litera-ture. The reaction of 1,4-dibromobutane with po-tassium salts of tartaric or malic acids also hasbeen reported.20 A series of polyfunctional car-boxyl telechelic microspheres with differentlengths and numbers of oligocaprolactone telech-elic branch chains, which were capped with two(or three) carboxyl groups at one of the telechelicchain extremities, were synthesized by a hydroxyacid [D,L-malic acid or citric acid (CA)] -initiatedpolymerization of caprolactone.21 Shogren22 re-cently studied the effect of coating kraft paperwith vegetable oil-based polyesters on their me-chanical, biodegradation, and weed inhibitionproperties. The polyesters chosen were an oxida-tively polymerized linseed oil and a poly(hydroxyester) formed by the reaction of epoxidized soy-bean oil with CA. Seppala et al.23 studied thepolycondensability of L-lactic acid with D,L-man-delic acid, 4-hydroxybenzoic acid, 4-acetoxyben-zoic acid, D,L-malic acid, and CA and the effect ofthe comonomers on the glass-transition tempera-ture (Tg). All copolymers were completely amor-phous, and only a single glass transition was re-ported.

In this study, functional aliphatic polyesterswere synthesized from 1,4-butanediol (BD), suc-cinic acid (SA), and multifunctional monomerssuch as CA. These hydroxyl and acid functionalgroups in the polyester backbone could providebetter compatibility and processability, particu-larly when blending with natural polymers suchas starch and protein. Poly(butylene succinatecitrate)s (PBSCs) containing different contents ofCA units were synthesized. Chain extension withhexamethylene diisocyanate and the properties of

these copolyesters were investigated with respectto the CA content.

EXPERIMENTAL

Materials

SA, BD, CA, titanium(IV) butoxide, and 1,6-hexa-methylene diisocyanate were reagent-grade (Al-drich) and were used as received. Organic sol-vents were analytical-grade and were used with-out further purification.

Polymerization Procedures

The polymerization reactions of BD, SA, and CAwere carried out in bulk under a nitrogen atmo-sphere with 0.15 mol % titanium(IV) butoxide asa catalyst in a two-step process. In the first step,esterification was carried out at 170 °C for 3 hunder atmospheric pressure, and then the reac-tion temperature was gradually increased to 200°C with the simultaneous reduction of pressure to5 mmHg with continuous stirring throughout thereaction. In the second step, polycondensationwas followed for an additional 5 h at 200 °C under5 mmHg with continuous stirring. Polymeriza-tions were performed in a three-necked reactionflask equipped with a stirrer, a nitrogen gas inlet,and a condensate collector with an air-locked vac-uum system. Different stoichiometric composi-tions of CA, SA, and BD were charged into thereaction flask to obtain various copolymer compo-sitions (Table 1), and the polymerization reactionwas carried out at 200 °C for different times invacuo. The copolyesters were purified by dissolu-tion in chloroform and precipitated in a fivefoldamount of ice-cold methanol. They were filtered,washed with methanol, and dried at 25 °C in avacuum oven so that a constant weight was ob-tained. The yields after purification were appro-priate: 95–98%.

A typical experimental procedure was as fol-lows. BD (44.31 mL, 0.5 mol), 45.03 g of SA (0.45mol), 9.61 g of CA (0.05 mol), and 0.3 mL ofTi(OBu)4 (0.15 mol %) as a catalyst were chargedinto the reaction flask. The system was purgedwith nitrogen, and the bath temperature wasgradually raised to 170 °C, with continuous stir-ring throughout the reaction. The reaction wascarried out for 3 h under atmospheric pressurewith a nitrogen blanket, and then the reactiontemperature was gradually increased to 200 °C

FUNCTIONAL ALIPHATIC COPOLYESTERS 3233

with the simultaneous reduction of pressure to 5mmHg with continuous stirring throughout thereaction. The polycondensation was followed foran additional 5 h at 200 °C under 5 mmHg withstirring. The polyester was purified by dissolutionin chloroform and precipitated in ice-cold metha-nol. A similar procedure was used to synthesizethe copolymers with higher CA molar ratios (0.2,0.3, 0.5, and 1) in the polymerization reactions.

Chain Extension of PBSC

For the chain extension of the copolymers (PBSC),40 g of the copolyester was charged into the reac-tion flask. The flask was heated to 200 °C. Afterthe polyester had melted, the system was purgedwith nitrogen, and 0.4 mL of hexamethylene di-isocyanate (1 wt%) was added; the reaction wascontinued for an hour with continuous stirring.

Characterization

A PerkinElmer differential scanning calorimeter(DSC 7) was used to determine the melting tem-

perature (Tm) and Tg of the copolyesters contain-ing CA contents of 0, 10, 20, and 30 mol %. Thesample size was 10–15 mg, with a heating rate of10 °C/min.

Intrinsic viscosity measurements were carriedout in a constant-temperature bath with an Ub-belohde viscometer at 30 °C. The purified copoly-ester samples were dissolved in chloroform andthen diluted to the required concentrations.

The crystallinity percentage of the copolyesterwas determined from the wide-angle X-ray scat-tering pattern with a Siemens D5005 X-ray pow-der diffractometer with nickel-filtered Cu K� ra-diation (� � 0.154 nm). The rotating anode gen-erator was operated at 45 kV and 40 mA. Thescanning regions of the diffraction angle 2� were5–35°, with a step size of 0.04 and dwell of 1.0,which covers all the significant diffraction peaksof polyester crystallites. Samples of the copolyes-ter were compression-molded into thin sheets at125 °C.

Fourier transform infrared (FTIR) spectrawere recorded with a Nicolet 7000 series instru-ment. Copolyester samples were dissolved in chlo-

Scheme 1. Synthesis of the PBSC copolyester.

Table 1. Compositions and Thermal Properties of the PBSC Copolyesters

Sample Molar Ratio (BD/SA/CA) COOH/OH Tm (°C)a Tg (°C)a

PBS 1.0/1.0/0.0 1 115 �32PBSC10 1.0/0.9/0.1 1 96 �25PBSC20 1.0/0.8/0.2 1 76 �25PBSC30 1.0/0.7/0.3 1 65 �22PBSC50 1.0/0.5/0.5 1 ND NDPBC [poly(butylene citrate)] 1.0/0.0/1.0 1 ND ND

a Measured by DSC. ND � not determined.

3234 MANI ET AL.

roform and cast into thin films onto a KBr disc.They were then dried in a nitrogen chamber be-fore the spectra were taken.

The NMR spectra of the copolyesters in deuter-ated chloroform were recorded with a VarianVXR300 instrument with a 12.2-�s (90°) pulseand an acquisition time of 2.0 s with tetrameth-ylsilane as a reference.

The rheological properties, that is, the lossmodulus (G�) and storage modulus (G�), of thesecopolyesters were measured with a RheometricScientific ARES at a temperature of 120 °C, at astrain of 1%, and at a frequency of 1 rad/s.

RESULTS AND DISCUSSION

CA was copolymerized with BD and SA to producePBSC copolyesters with hydroxyl and acid func-tional side groups (Scheme 1). PBSCs containingdifferent stoichiometric contents of SA and CAare shown in the Table 1. As mentioned in the

Experimental section, the polymerization wascarried out in a two-step process. In the first step,esterification was carried out at 170 °C for 3 hunder atmospheric pressure, and then the reac-tion temperature was gradually increased to 200°C with the simultaneous reduction of pressure to5 mmHg with continuous stirring throughout thereaction. In the second step, polycondensationwas followed for an additional 5 h at 200 °C under5 mmHg with stirring. Copolymers with CA mo-lar ratios of 0.2 and greater became gels an hourafter the polymerization reaction started. As themolar ratio of CA in the reaction increased, thegelling time decreased. This was probably due tothe presence of free hydroxyl/carbonyl groups inCA. Although the reactivity of the hydroxyl andcarboxylic acid groups at the tertiary carbonshould be lower than the other reactive groups inthe system, the higher concentrations of CA in thesystem probably led to the formation of low mo-lecular weight cyclic structures and self-conden-sation reactions at longer reaction times.

Figure 1. FTIR spectra of the copolyesters: (a) PBSC10, (b) PBSC20, and (c) PBSC30.

FUNCTIONAL ALIPHATIC COPOLYESTERS 3235

Characterization of the Copolyesters

Table 1 also demonstrates the thermal propertiesof PBSC copolyesters with CA molar ratios of 0.1,0.2, and 0.3, which were samples taken before gelformation. Copolyesters with higher CA molarratios (0.5 and 1) were not measured, as thesesamples were very sticky viscous liquids. The de-pendence of Tm on the amount of CA in the co-polyesters is shown in Table 1. Tm is very sensi-tive to the molar ratio of the SA–CA comonomerunits, as expected in random copolymers. Therelation between the copolymer composition andthe melting behavior is not yet adequately re-solved.24 One question concerns the distributionof the comonomer units in the crystalline andamorphous regions of a semicrystalline copolymerconsisting of A and B units. There are two basicmodels by Flory25 and Eby26,27 describing thissituation. The model proposed by Flory assumesthat one of the comonomeric units, B, is not in-cluded in the crystallites and remains in an amor-

phous phase. In Eby’s model, there is completecompatibility between the monomers of bothtypes A and B to build up the crystalline lattice.Also, it is pointed out that morphological effectsshould be responsible for the melting behavior ofthe copolyesters rather than thermodynamic ef-fects.25,28 A high density of branching, or long-chain branching, with higher CA concentrationswill have the same effect as side groups in re-stricting chain mobility, and so Tm decreases.

FTIR showed the changes in the followingbands according to the concentration of each con-stituent (Fig. 1). An IR vibrational band charac-teristic of ester bond formation is indicated by thepresence of carbonyl peaks at 1715 and 1733cm�1. Overlapped broad bands corresponding tothe free and hydrogen-bonded OH and COOHgroups as well as CH2 groups (symmetric andasymmetric) were observed at 2800–3600 cm�1.An increase in the intensity of the bands in thisregion was observed as the CA concentration in-

Figure 2. 1H NMR spectra of the copolyesters: (a) PBSC10, (b) PBSC20, and (c)PBSC30.

3236 MANI ET AL.

creased from 10 to 30% in the reaction, indicatingthe presence of hydroxyl and acid functional sidegroups.

1H NMR spectra for PBSC10, PBSC20, andPBSC30 are shown in Figure 2. In the NMR spec-tra of these copolyesters with CA molar ratios of0.1, 0.2, and 0.3, three signals are observed at �� 1.7, 2.6, and 4.1 ppm. Methylene protons of BDunits show their peaks at � � 1.7 and 4.1 ppm(OCH2OCH2OCH2OCH2O and OCH2OCH2OCH2OCH2O). The peaks at � � 2.6 ppm are dueto the methylene protons of succinyl and CAunits. An increased intensity of the peak at �� 2.6 ppm with an increasing CA feed ratio in thereaction indicates that CA is incorporated to agreater extent into the polymer. Also, the meth-ylene protons of BD units next to citrate unitsshift downfield to � � 4.25 ppm (small peak) asthe CA molar ratio increases in the polymer [Fig.2(b,c)].

Figure 3 shows a comparison of the wide-angleX-ray diffraction (WAXD) patterns of the com-pression-molded copolyester samples [poly(buty-

lene succinate) (PBS) and PBSC10]. The X-raydiffraction patterns are indicative of the presenceof crystallites in the copolyester (PBSC10). How-ever, there is no essential difference in the X-raydiffraction angles of both samples, and bothshowed peaks at 2� values of 19.6 and 22.6° witha shoulder peak at 22° and with a small peak at29°. The crystallinity percentage was measuredfrom the relative areas of crystalline and amor-phous regions, which were computed by the draw-ing of a smooth curve.12 The crystallinity percent-ages of PBS and PBSC10 are shown in Table 2.There is no essential difference in the crystallin-ity percentage at the CA molar ratio of 0.1, indi-cating that the aggregation of crystallizable seg-ments and the formation of crystallites are notsignificantly affected by noncrystallizable units.

As mentioned earlier, copolyesters with highermolar ratios of CA (�0.2) became gels after anhour; we tried to get a higher molecular weightcopolyester with a 0.1 molar ratio of CA. Also, at0.1 CA molar ratios in the reaction, these copoly-esters became a gel after 3 h during the second

Figure 3. WAXD patterns of compression-molded copolyester samples.

FUNCTIONAL ALIPHATIC COPOLYESTERS 3237

step of the polycondensation reaction. To increasethe molecular weight of these copolyesters with-out forming a gel hexamethylene, we used diiso-cyanate (1 wt %) as a chain extender (Scheme 2).Table 2 lists the intrinsic viscosities for PBSC10before and after chain extension. The intrinsicviscosity of PBSC10 was increased to 0.363 dLg�1 from 0.225 dL g�1 as a result of the chainextension.

Table 2 also demonstrates the rheological prop-erties of these copolyesters. The rheological prop-erties (i.e., G� and G�) of these copolyesters weremeasured as an indication of the molecularweight. As a result of chain extension, G� re-mained practically unchanged, but G� increasedto 3100 Pa from 1300 Pa. G� is a measure ofmaterial elasticity and is due to the intermolecu-lar and intramolecular hydrogen bonding be-tween the hydroxyl and acid functional sidegroups. This would indicate that during the chainextension reaction, there is significant bond for-mation rather than any significant increase inmolecular weight, which would result in an in-crease in G� as well. This study also indicates thatthe incorporation of a small amount of CA (0.1molar ratio) into the polymer will increase thematerial elasticity to a greater extent.

CONCLUSIONS

In this study, functional aliphatic copolyesterscontaining different stoichiometric contents of SAand CA were investigated. This study revealedthe dependence of the material characteristics onthe chemical composition of the aliphatic copoly-esters with multifunctional monomers such asCA. It appears that there is an optimum concen-tration of CA (0.1 molar ratio) to be used to pro-duce functional copolyesters. Also, these copoly-esters with increased molecular weight could beused as impact modifiers and compatibilizers inpolyester/starch/protein blends.

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Table 2. Results of the Characterization of the Synthesized PBSC Copolyesters

Sample Intrinsic Viscosity (dL g�1) Crystallinity (%)b G� (Pa) G� (Pa)

PBS 0.852 41.5 175 1550PBSC10 0.225 40.3 1300 304PBSC10-NCOa 0.363 ND 3100 300

a Hexamethylene diisocyanate (1 wt %) was used as a chain extender.b Measured by WAXD. ND not determined.

Scheme 2. Synthesis of the high molecular weightPBSC copolyester with a chain extender.

3238 MANI ET AL.

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FUNCTIONAL ALIPHATIC COPOLYESTERS 3239