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Polymer 54 (2013) 3774e3783

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Polymer

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Aliphatic/aromatic copolyesters containing biobased u-hydroxyfattyacids: Synthesis and structureeproperty relationships

Annamaria Celli a,*, Paola Marchese a, Simone Sullalti a, Jiali Cai b, Richard A. Gross b,*

aDepartment of Civil, Environmental and Materials Engineering, University of Bologna, Via Terracini 28, 40131 Bologna, ItalybCenter for Biocatalysis & Bioprocessing of Macromolecules, Polytechnic Institute of NYU, Six Metro Tech Center, Brooklyn, NY 11201, USA

a r t i c l e i n f o

Article history:Received 9 March 2013Received in revised form27 April 2013Accepted 1 May 2013Available online 28 May 2013

Keywords:u-Hydroxyfatty acidBiobased copolyestersThermo-mechanical properties

* Corresponding authors.E-mail address: rgross@poly.edu (R.A. Gross).

0032-3861/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymer.2013.05.007

a b s t r a c t

A family of unique aliphaticearomatic copolyesters was prepared by bulk condensation co-polymerizations of bio-derived u-hydroxytetradecanoic acid (H) with 1,4-butanediol (B) and dimethylterephthalate (DMT). 1H NMR analysis showed the copolyesters have random repeat-unit sequencedistributions. Thermal properties strongly depend on molar composition. Melting temperatures arelower than 70 �C for compositions rich in H-units; Tm varies from about 140 to 180 �C for copolymerswith high butylene terephthalate (BT) content. Crystal lattice structures shift from the crystal phase ofpoly(u-hydroxytetradecanoate) (PH) to that of PBT with increasing BT copolymer content, while theminor component is trapped in the crystallizable domains as defects. The amorphous phase is homo-geneous for all compositions and Tg increases from �21 �C (PH homopolymer) to 61 �C (PBT). Also,mechanical properties vary in a continuous way, according to copolymer composition. Therefore, thecombination of aliphatic and aromatic units enables molecular design of partially biobased materialswith adjustable thermal and physicalemechanical properties. Thus, by judicious selection of copolymercomposition, material properties can be fine-tuned to attain the desired balance of material rigidity,ductility, melting point and biobased content.

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1. Introduction

Strong drivers exist for commercial, government and academicscientists and engineers to develop next-generation biobased al-ternatives to petroleum-derived plastics [1e3]. These driversinclude, but are not limited to environmental pollution, climatechange and a finite global supply of fossil fuel that is subject todisruptive price fluctuations. Important fundamental and appliedresearch is focused on developing ‘new to the world’ as well as‘drop in’ replacements for current petroleum based plastics. New tothe world biobased plastics must offer physical properties that areunique, superior or at least on par with existing materials whilebeing cost-competitive [4,5].

Biobased and petroleum-derived aliphatic polyesters can lead touseful materials that also provide the property of biodegradability[6]. Important examples include poly(ε-caprolactone) [7], microbialpolyesters or polyhydroxylalkanoates (PHAs) [8,9], poly(lactic acid)

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(PLA) [10,11], poly(butylene succinate) [12]. These polymers are allcommercially available and offer useful properties. However, forreasons that are well understood, the market penetration of thesealiphatic polyesters has been limited [6]. For example, PLA suffersfrom poor hydrolytic stability that limits its shelf life; it also hasinadequate impact strength and a low heat distortion temperature[13,14]. PCL has excellent ductility, but has lowmelting point (about60 �C) and poor strength (lowmodulus). PHAs vary in structure butgenerally are expensive and require improvement in melt strengthand crystallization rates [15].

A unique family of aliphatic biodegradable polyesters haveemerged that share structural and physical characteristics highlyvalued by polyethylene manufacturers and users. These polyestershave long methylene (e(CH2)xe) sequences separated by estergroups. The PE-like character of such polymers will depend on thelength of e(CH2)xe units [16,17]. Elegant chemical approacheshave been developed to prepare bifunctional building blocks withe(CH2)xe units from readily renewable vegetable oils [1,5,18].Important examples of these methods include the use of metath-esis [19e21], thiol-ene reactions [22,23], methoxycarbonylation[24,25], ozonolysis and reduction [26] chemistries. However, these

ω−Hydroxytetradecanoic acid

Dimethyl terephthalate

1,4-butanediol

(CH2)13C

O

HO OH

C

O

C

O

H3CO OCH3

HO CH2 CH2 CH2CH2 OH

Scheme 1. Structures of the monomers used to synthesize P(H-co-BT) copolyesters.

A. Celli et al. / Polymer 54 (2013) 3774e3783 3775

routes to bifunctional monomers with e(CH2)xe units have one ormore of the following characteristics that limits their commercialutility: i) multistep, ii) harsh reaction conditions, iii) involvingundesirable solvents and reagents.

Our research team has developed a mild and efficient biotech-nological route to convert fatty acids such as tetradecanoic acid(C14) to u-hydroxytetradecanoic acid [27]. For this purpose, anengineered Candida tropicalis strain was developed whose back-ground activity for conversion of u-hydroxyfatty acids to its cor-responding u-carboxydiacids was dramatically reduced. To achievethis goal, 16 genes were identified and removed from theC. tropicalis genome including 6 cytochrome P450s, 4 fatty alcoholoxidases, and 6 alcohol dehydrogenases. After reinsertion of asuitable cytochrome P450 (CYP52A17) enzyme, the new C. tropicalisstrain produced over 160 g/L u-hydroxytetradecanoic acid from theC14-methyl ester with less than 5% formation of the correspondingdiacid [27]. Subsequently, u-hydroxytetradecanoic acid was con-verted by a condensation polymerization catalyzed by titaniumtetraisopropoxide (Ti[OiPr]4) to a polymer (PH) withMw up to 140K[28]. Tensile tests of PH with Mw above 78K showed strain-hard-ening phenomena and tough properties with elongation at breakabout 700% and true tensile strength about 500 MPa. Furthermore,DSC analysis revealed PH has a peak melting temperature at about94 �C and is highly crystalline [28]. Recent work has also shownthat films of PH are readily compostable by ISO14855 (unpublishedwork).

In contrast to most aliphatic polyesters, aromatic polyesterslike poly(ethylene terephthalate) (PET) or poly(butylene tere-phthalate) (PBT) provide excellent material properties and, hence,are widely used commercial products. However, high contents ofaromatic units in these and related copolyesters lead to increasedresistance to microbial degradation in bioactive environments.Furthermore, although work is in progress to develop biobasedterephthalic acid (TPA), commercially viable routes have not yetbeen identified [29e32]. To improve the mechanical propertiesrelative to aliphatic polyesters while retaining material biode-gradability, copolyesters containing aliphatic and aromaticmonomers have been synthesized and studied. For example, atsome compositions, copolymers of TPA with aliphatic co-monomers such as 1,2-ethanediol, 1,3-propanediol, 1,4-butanediol, sebacic acid and adipic acid were shown to bebiodegradable [33,34]. Copolymers of poly[(BT)-co-(1,4-butyleneadipate)], P(BT-co-BA), inoculated with pre-screened mixed mi-crobial cultures from compost at 60 �C [35], are biodegradable atrates that decrease as the fraction of TPA in the copolymer suchthat, at 60 mol% TPA, material compostability would be unlikely.In 1995, BASF AG in Germany began manufacturing a P(BT-co-BA)-based copolyester [36]. Today, BASF markets this copolymerunder the Ecoflex trade name [37]. Ecoflex is completely biode-gradable according to EN 13432 and is used in applications suchas mulch films for agriculture, cling-wrap film or coatings forpackaging and breathable films [38].

Motivated by the useful properties of P(BT-co-BA) as well asthe recent development of a biotechnological route to u-hydroxyfatty acids, this paper explores a unique family of co-polymers consisting of units derived from u-hydroxyte-tradecanoic acid, 1,4-butandiol and dimethyl terephthalate invarious proportions. The copolymers were synthesized by atraditional chemical route (two-step polycondensation), startingfrom monomers described in Scheme 1. Systematic variation incopolymer structure was performed and its effects on physico-mechanical properties of the corresponding materials are dis-cussed. The results of this work support the importance ofexploring integration of chemical and biotechnological feedstocksfor the development of new materials.

2. Experimental

2.1. Materials

u-Hydroxytetradecanoic acid was produced from C14-methylester by a fermentation process in our laboratory by a publishedmethod [27]. 1,4-Butanediol, dimethyl terephthalate (DMT) andtitanium tetrabutoxide (TBT) were all purchased from AldrichChemicals in the highest available purity andwere used as received.PBT (Valox Resin 195, Mw ¼ 47,700, Mw/Mn ¼ 2.4) is a commercialsample furnished by General Electric. Ecoflex F BX 7011(Mw ¼ 121,000, Mw/Mn ¼ 2.3) is a random P(BT-co-BA) copolymer,constituted by 46 mol% BT and 54 mol% BA units (determined by 1HNMR in our laboratories), supplied by BASF.

2.2. Synthesis of the poly(u-hydroxyl tetradecanoic acid) (PH)

PH was prepared by a two stage polycondensation reaction.First, u-hydroxytetradecanoic acid (30 g, 0.12 mol) and TBT(0.045 g, 0.132 mmol) were transferred into a round bottom wide-neck glass reactor (250 mL capacity). The reactor was closed with athree-neck flat flange lid equipped with a mechanical stirrer and atorque meter which gives an indication of the melt reaction vis-cosity. The reactor was immersed into a preheated oil bath at200 �C. A heating band set at 120 �Cwas applied around the lid. Thefirst stage was conducted at atmospheric pressure under nitrogenatmosphere and the mixture was allowed to react for 120 minunder stirring with continuous removal of water. The second stagewas started by gradually reducing the pressure to 0.5 mbar whilethe temperature was raised to the final value of 220 �C. Theseconditions were reached by 60 min using a linear gradient oftemperature and pressure. Thereafter, the temperature was main-tained at 220 �C for 20 min.

2.3. Synthesis of copolyesters

As a representative example, synthesis of the copolyester usinga 60/40 monomer feed ratio of u-hydroxytetradecanoic acid-to-DMT and an excess of 1,4-butanediol was performed as follows:u-hydroxytetradecanoic acid (25.0 g, 0.102 mol), DMT (13.1 g,0.068 mol), 1,4-butanediol (22.0 g, 0.246 mol), and catalyst TBT(0.061 g, 0.179 mmol) were placed into a round bottom wide-neck

Table 1Composition and molecular weight of P(H-co-BT) copolyesters and correspondinghomopolymers.

Samplea H/Tb H/Bb B/Tb Mw$10�3c Mw/Mnc

PH e e e 78.7 2.8P(H-co-BT)-80/20 78/22 77/23 52/48 132.6 2.5P(H-co-BT)-60/40 62/38 61/39 52/48 110.5 2.4P(H-co-BT)-40/60 41/59 40/60 52/48 100.0 2.2P(H-co-BT)-30/70 29/71 28/72 52/48 173.4 2.4P(H-co-BT)-20/80 21/79 21/79 52/48 84.1 2.5PBT e e 50/50 47.7 2.4

a Sample names have the following abbreviations: H is u-hydroxytetradecanoateunits and BT are 1,4-butylene terephthalate units. Values X/Y give the molar feedratio of u-hydroxytetradecanoic acid-to-DMT in copolymerizations.

b Determined by 1H NMR.c Determined by GPC.

A. Celli et al. / Polymer 54 (2013) 3774e37833776

glass reactor (250 mL capacity). The 1,4-butandiol-to-DMT ratiowas 3.6 for this example, but was maintained at 2.4 for the otherpolyester compositions. The reactor was closed with a three-necked flat flange lid equipped with a mechanical stirrer and atorque meter which gives an indication of the melt reaction vis-cosity. The reactor was immersed into a preheated oil bath at200 �C. The first stage was conducted for 120 min at atmosphericpressure under a nitrogen atmospherewith stirring and continuouswater removal. The second stage was begun by gradually reducingthe pressure to 0.2 mbar while the temperature was raised to thefinal value of 230 �C. These conditions were reached by 60 minusing a linear gradient of temperature and pressure. Thereafter, thetemperature was maintained at 220 �C for 20 min. The synthesis ofpolyesters from monomer feed ratios of u-hydroxytetradecanoicacid-to-DMT 20/80, 30/70, 40/60, 60/40 and 80/20 were performedby the above method to obtain copolyesters with varying compo-sitions. Copolyesters prepared herein are abbreviated as P(H-co-BT)-X/Y, where X/Y is the molar feed ratio of u-hydroxyte-tradecanoic acid-to-DMT. Synthesized copolymers from melt-condensation polymerization were analyzed without fraction-ation by precipitation.

2.4. Characterization

NMR spectroscopy: 1H NMR spectra were recorded at roomtemperature on samples dissolved in a CDCl3/CF3COOD (90/10 vol%)mixture using a Varian Mercury 400 spectrometer at the protonfrequency 400 MHz.

Molecular weight analysis: Molecular weights were measured bygel permeation chromatography (GPC) using a Hewlett PackardSeries 1100 liquid chromatography instrument, equipped with a PLgel 5m Mixed-C column, and a refractive index detector. The eluantwas chloroform and a calibration plot was constructed with poly-styrene standards.

Thermal analysis: Measurements by differential scanning calo-rimetry (DSC) were performed using a PerkineElmer DSC6, cali-brated with high purity standards, under nitrogen. In order to erasethe previous thermal history, samples (ca. 10 mg) were initiallyheated at 20 �C min�1 from 40 �C to different temperatures varyingfrom 120 to 250 �C according to the sample characteristics (1stheating scan), kept at high temperature for 1 min and then cooledto �60 �C at 10 �C min�1 (cooling scan). After this thermal treat-ment, samples were heated to 120e250 �C at 10 �C min�1 (2ndheating scan). During the cooling scan, the crystallization temper-ature (Tc) and the enthalpy of crystallization (DHc) were measured.During the 2nd heating scan, the glass transition temperature (Tg),the melting temperature (Tm) and the enthalpy of fusion (DHm)were determined. Tg was not discernible for all samples and, whenpossible, was taken as the midpoint of the heat capacity incrementassociated with the glass-to-rubber transition.

Wide angle X-ray scattering (WAXS): Data from WAXS werecollected in the 2q range 5�e60� using an X’PertPro diffractometer,equipped with a copper anode (Ka radiation, l ¼ 1.5418 �A) and anX’Celerator detector. Measurements were performed on samples atwhich the thermal history had been erased by thermal treatmentin DSC: heating at 20 �C min�1 to different temperatures, varyingfrom 120 to 250 �C, according to the sample characteristics, 1 minof isotherm at this temperature, and then cooling at 10 �C min�1

to 30 �C.Dynamic mechanical thermal analysis (DMTA): Samples for DMTA

were obtained by injection molding in a Mini Max Molder (CustomScientific Instruments) equipped with a rectangular mold(30� 8� 1.6mm3).Molded sampleswere rapidly cooled inwater andthen dried in an oven at 30e50 �C under vacuum overnight. Mea-surements by DMTA were performed on a DMTA IV (Rheometrics

Scientific) operated in the dual cantilever bending mode, at a fre-quency of 3 Hz and a heating rate of 3 �C min�1, over a temperaturerange from�150 �C to a final temperature, varying from40 to 160 �C,according to the sample characteristics.

Tensile testing: For tensile testing, dumbbell shaped sample barswithdimensionsof10.0mm(length)�3.0mm(neckwidth)�1.0mm(thickness) were prepared by press-molding at 20 �C above themelting point. Subsequently, samples were quenched at ambienttemperature. An Instron 5542 tensile testing machine with a 500 Nload cell was used. The crosshead speed was 5 mm/min and the testtemperature was 25 �C. Merlin software was used to collect andanalyze tensile results (stress was calculated according to the initialcross-sectionarea).ValuesofYoung’smodulus, elongationatyieldandbreak and stress at yieldwere obtainedbyaveraging thedata obtainedfrom �5 samples. Young’s modulus was measured as the slope ofstressestrain curves at strain below 1%, using the linear least squaremethod. Elongation and stress at break measurements were deter-mined at the (x, y) coordinates after which the slope of the stressestrain curve became negative.

3. Results and discussion

3.1. Synthesis and molecular characterization

Table 1 summarizes results for bulk condensation polymeriza-tion of copolyesters synthesized herein from monomer feed ratiosof u-hydroxytetradecanoic acid-to-DMT 80/20, 60/40, 40/60, 30/70and 20/80. The novel copolyesters are described by the abbrevia-tion P(H-co-BT)-X/Y, since knowledge of u-hydroxytetradecanoate(H) and 1,4-butylene terephthalate (BT) contents describes thecomposition of copolyesters. The relative amount of these two unitsis given as X/Y, which is the molar feed ratio of u-hydroxyte-tradecanoic acid-to-DMT.

Table 1 describes the experimentally determined molar com-positions of the repeat units derived from u-hydroxytetradecanoic,dimethyl terephthalate and 1,4-butanediol, which are abbreviatedas H, T and B, respectively. Comonomer molar feed ratios (u-hydroxytetradecanoic acid-to-DMT) and corresponding copo-lyester compositions, given by H/T, are all in excellent agreement(Table 1). Feed amounts of 1,4-butanediol were in excess relative toDMT (see Experimental section) since 1,4-butanediol was lost dueto volatilization during polycondensations. Consequently, H/B andB/T correspond to values expected for the chemical structure ofP(H-co-BT) copolyesters. Moreover, the relatively short reactiontimes of about 3.5 h (see Experimental) was used to prepare thecopolyesters in Table 1, in combination with the high Mw valuesattained (generally above 100K), implying that all comonomerswere highly reactive [39]. Furthermore, incorporation of H-units in

A. Celli et al. / Polymer 54 (2013) 3774e3783 3777

copolyesters likely decreases the viscosity of the reaction mediumrelative to PBT synthesis, allowing relatively higher molecularweight P(H-co-BT) copolyesters to be prepared.

Structural analyses of the copolyesters were performed by 1HNMR at 400 MHz in CDCl3/CF3COOD, 90:10 v/v. As an example,Fig. 1a displays the full spectrum of P(H-co-BT)-40/60 andexpanded spectral regions are shown in Fig. 1b and c. Letter as-signments to peaks in Fig. 1aec correspond to letter-designationsfor hydrogens shown in Scheme 2. Assignments were based onliterature reports for PH and PBT homopolymers and PBT-basedcopolymers [28,39e42]. Furthermore, the 1H NMR spectra ofcopolyesters are consistent with that expected according to thecopolymer structures. For terephthalic protons (t), a singlet at8.15 ppm is observed that is independent of whether neighboringgroups are H or B. The other protons (a, b, c, d, e, f, g, h) in Fig. 1a areassigned in expansions of regions 0.7e2.7 ppm and 3.5e5.1 ppmaccording to sequences on the triad level. By considering B-unit

Fig. 1. a) 1H NMR (400 MHz, CDCl3/CF3COOD, 90:10 v/v) spectrum of the P(u-OHC14)-co-PBexpansion of 1H NMR spectrum region from 0.7 to 2.7 ppm. Letters assigned to peaks corr

centered triads, the three possible sequences that may be presentare TBT, TBH, and HBH (Scheme 2). Signals for B-unit CH2eOe(C]O) protons a0, a00, a000 and a0000 are all resolved in expanded region3.5e5.1 ppm (Fig. 1b), according to the assignments reported inRef. [39e42] (a0 centered at 4.50 ppm, a00 centered at 4.45 ppm, a000

centered at 4.24 ppm, and a0000 centered at 4.18 ppm). Resonancescorresponding to B-unit CH2CH2CH2CH2 protons b0, b00, b0 00, b0000 areobserved in expanded region 0.7e2.7 ppm (Fig. 1c). Unlike protonsa, only b0 is well resolved. H-unit protons c0 and c00 (4.14, 4.40 ppm),d0 and d00 (1.65, 1.82 ppm), e0 and e00 (1.28e1.44 ppm), f (about1.62 ppm), g (2.40 ppm) and h (1.24e1.32 ppm) are also observed.H-unit protons a, b and g to the ester oxygen atom (c0 and c00, d0 andd00, e0 and e00, respectively) are all influenced by the neighboring unit(H or T) (see HH and HT diads in Scheme 2). In contrast, neigh-boring group effects for protons at the other end of H-units, that isa, b and g to the ester carbonyl moiety; do not appear to be influ-enced by whether the neighboring group is an H- or T-units.

T-40/60 sample, b) expansion of 1H NMR spectrum region from 3.5 to 5.1 ppm and c)espond to letter-designations for hydrogens shown in Scheme 2.

TBT

TBH

HBH

HH

HT

C

O

C

O

CH2O CH2 CH2 CH2 O C

O

C

Oa' a'b' b'

C

O

C

O

CH2O CH2 CH2 CH2 O C

Oa'' a'''b'' b'''CH2 CH2 (CH2)8 CH2 CH2 CH2 O

C

O

CH2O CH2 CH2 CH2 O C

Oa'''' a''''b '''' b''''CH2 CH2 (CH2)8 CH2 CH2 CH2 OCH2CH2(CH2)8CH2CH2CH2O

C H2 C H2 (C H2)8 C H2 C H2 C H2 Og f h d' c'

C

O e'C H2 C H2 (C H2)8 C H2 C H2 C H2 OC

O

CH2 CH2 (CH2)8 CH2 CH2 CH2 Og f h d'' c''

C

O e''C

O

C

O

Scheme 2. Triads, centered on B units, and selected diads obtainable from the polycondensation reactions.

Table 2Molar fractions of carbonyl functionalities present in H units (FH) and in T units (FT),sequence length (L) and randomness degree (B) for P(H-co-BT)s of varyingcomposition.

Samplea FH FT LTBT LHBH BB LHH

P(H-co-BT)-80/20 0.64 0.36 1.5 2.7 1.02 2.7P(H-co-BT)-60/40 0.45 0.55 2.2 1.8 1.02 1.8

A. Celli et al. / Polymer 54 (2013) 3774e37833778

Measurements of copolymer compositions as given in Table 1were made as follows: i) H/T was determined from the relativeintegration ratios of H-unit g protons (2.4 ppm) and T-unit t-protonsat 8.15 ppm (integrations normalized so intensities correspond toone proton); ii) H/B compared integration intensities for signalscorresponding to H-unit c0 and c00 and B-unit a0, a00, a000 and a0000

protons; and B/T compared integration intensities for signals cor-responding to B-unit a0, a00, a000 and a0000 protons and T-unit t protons.

By applying the method developed by Devaux [43], the proba-bility (f) of finding B-centered triads (TBT, TBH, HBT, and HBH tri-ads) can be determined from integration intensities (I) ofcorresponding NMR signals. For these calculations, the signals dueto a0, a00, a000 and a0000 protons, which depend on neighboring groupsT and H, were used. The equations for these calculations are asfollows:

fTBH ¼ Ia00=�Ia0 þ Ia00 þ Ia000 þ Ia0000

�(1a)

fHBT ¼ Ia000 =ðIa0 þ Ia00 þ Ia000 þ Ia0000 Þ (1b)

fTBT ¼ Ia0=�Ia0 þ Ia00 þ Ia000 þ Ia0000

�(1c)

fHBH ¼ Ia0000=�Ia0 þ Ia00 þ Ia000 þ Ia0000

�(1d)

From these data, the length of TBT and HBH sequences (LTBT andLHBH, respectively) and the parameter BB, which defines therandomness degree around B-units, were determined by thefollowing equations [44]:

LTBT ¼ ðfTBT=fTBHÞ þ 1 (2a)

LHBH ¼ ðfHBH=fHBTÞ þ 1 (2b)

P(H-co-BT)-40/60 0.26 0.74 3.7 1.4 0.99 1.3P(H-co-BT)-30/70 0.17 0.83 4.8 1.3 0.99 1.3P(H-co-BT)-20/80 0.12 0.88 7.5 1.2 0.93 1.1

a Sample names are given as abbreviations, see Table 1 footnote a.

BB ¼ fHBT=FH þ fTBH=FT (2c)

where FT is the molar fraction of carbonyl functionalities of T-unitspresent in the copolymer with respect to the total carbonyl

functionalities. The same concept is used for FH. For this evaluation, itis necessary to consider that a T-unit contains two carbonyl groupswhereas an H-unit only contains one [45]. Therefore, for eachcomposition, FT and FH have been calculated by considering the datainTable1 (H/Tcolumn),where themoles of carbonyl groups inHunitsequal the H value, whereas the moles of carbonyl groups in T unitsequal the 2T-value. For example, for P(H-co-BT)-80/20, from Table 1FH is equal to 78/(78 þ 22 � 2) ¼ 0.64 and FT is equal to (22 � 2)/(78 þ 22 � 2) ¼ 0.36.

Values of FH and FT are listed in Table 2. Moreover, from mea-surements of the relative intensity of c resonances (Ic0 and Ic00), theaverage length of HH sequences (LHH) was calculated. In this case,only diads are considered as they provide sufficient information ofcopolymer microstructure as discussed above (Scheme 2). Tocalculate fHT, fHH and LHH, the following set of equations were used:

fHT ¼ Ic00=ðIc0 þ Ic00 Þ (3a)

fHH ¼ Ic0=ðIc0 þ Ic00 Þ (3b)

LHH ¼ ðfHH=fHTÞ þ 1 (3c)

where fHH and fHT are the molar fractions of HH and HT se-quences, respectively. Table 2 lists the average sequence lengthsand BB values obtained for the copolymers. According to the

0 50 100 150 200

PBT

20/80

30/70

40/60

60/40

80/20

PH

He

at F

lo

w, E

nd

oth

erm

U

P

Temperature

Fig. 2. DSC heating curves (2nd scan) performed at 10 �C min�1.

A. Celli et al. / Polymer 54 (2013) 3774e3783 3779

literature [43], BB is equal to zero in a homopolymer or copolymerthat consists of long homogeneous sequences of B-centered tri-ads, equals 1 when the sequence of B-centered triads approxi-mates a random distribution, and varies between 1 and 2 whenB-centered triads tend to regularly alternate along chains. FromTable 2, for all copolyesters, BB is close to 1 showing that thesequence distribution of copolymers is best approximated asrandom. Furthermore, sequence lengths vary following therandom copolymer model as would be expected with changes incomposition.

3.2. Thermal and structural characterization

Table 3 lists DSC results of PH, P(H-co-BT) and PBT samplessynthesized herein. In addition, as a point of reference, thermalanalysis of a commercial Ecoflex F BX 7011 material was alsoanalyzed. As discussed above, Ecoflex is mainly used for packagingapplications, it is processable on conventional blown film plantsdesigned for polyethylene, complies with food safety requirementsand is compostable.

Fig. 2 displays DSC second heating scans used for determinationof melting temperatures (Tm) and enthalpies of fusion (DHm). Frominspection of DSC thermograms in Fig. 2, all copolymer composi-tions have melting transitions and, therefore, are semicrystalline.As anticipated, Tm and DHm values vary widely with copolyestercomposition. PH exhibits a sharp melting peak at 92 �C with a largeenthalpy (DHm ¼ 131 J g�1), consistent with polymers bearing longmethylene sequences along the main chain. Such polymers, likepolyethylene (PE) and PH, have sufficient flexibility to reach highorder spatial arrangements [16,17,28,46]. According to well-knownbehavior, PE-like polyesters tend to crystallize in the PE crystallinestructure whenever feasible since this arrangement is highly stable.By lengthening the aliphatic chain or the distance between esterfunctionalities that separate methylene sequences, Tm increasesand asymptotically approaches the melting point of linear poly-ethylene (140 �C) [16,47]. This behavior is a consequence of estergroups between methylene sequences that create defects in whatwould otherwise be a polyethylene backbone [47].

In contrast to poly(u-hydroxyfatty acids) such as PH, PBT has ahigher Tm (224 �C) and a relatively low DHm. Indeed, for PBT, and ingeneral for polyesters based on TPA, the coplanarity betweencarbonyl and phenyl groups restricts the rotational angles aboutCphenyl-CO to 0� and 180� [48,49]. The planar conformation favorsthe molecular packing of chains in crystals and enhances theattractive intermolecular interactions between the ester groups ofneighboring chains [50]. As a result, aromatic polyesters exhibit arigid structure and high melting temperatures.

Table 3DSC parameters of melting point (Tm), glass transition temperature (Tg), crystalli-zation temperature (Tc), melting enthalpy (DHm) and crystallization enthalpy (DHc)for P(H-co-BT) copolyesters, their corresponding homopolymers and Ecoflex.

Samplea Tc (�C)b DHc

(J g�1)bTm(�C)c

DHm

(J g�1)cTg (�C)d

PH 72 120 92 131 �21P(H-co-BT)-80/20 48 64 66 84 �12P(H-co-BT)-60/40 33 41 61 26 �11P(H-co-BT)-40/60 90 22 140 17 �6P(H-co-BT)-30/70 131 27 174 23 �2P(H-co-BT)-20/80 145 36 188 26 13PBT 189 48 224 43 61Ecoflex 80 21 124 11 �22

a Sample names are given as abbreviations, see Table 1 footnote a. Values X/Yrepresent the weight-% of H-to-BT units.

b Measured by DSC during the cooling scan at 10 �C min�1.c Measured by DSC during the second heating scan at 10 �C min�1.d Measured by DMTA (from E00).

Random P(H-co-BT) shows melting processes whose shape, in-tensity and temperature strongly depend on copolymer composition.P(H-co-BT)-80/20 shows a narrow melting peak reminiscent of PH(Fig. 2). However, introduction of 20 mol% BT units disrupts crystal-lization of H-segments resulting in Tm and DHm values 26 �C and47 J g�1 below that of PH (Table 3). Further increases in the BT unitcontent of copolyesters to 40 and 60 mol% continues the trend ofproducing materials with low crystalline content and poorly orga-nized crystalline phases (see Table 3 and Fig. 2). P(H-co-BT)-60/40 hasa DHm of only 26 J g�1 and Tm at 61 �C whereas P(H-co-BT)-40/60showsabroadmeltingprocesswith aweak transition (DHm¼17 J g�1)at Tm 140 �C. For increased BT content copolyesters P(H-co-BT)-30/70and P(H-co-BT)-20/80,DHm and Tm increase up to 38 J g�1 and 188 �C.Further details of trends inTc, Tm andDHm as a function of copolyestercomposition are given in Fig. 3. All thermal transitions pass throughminimum values at intermediate compositions.

Trends of Tm and Tc suggest the presence of two different crys-talline phases. To confirm this hypothesis, WAXD analysis wasperformed and the diffractograms are shown in Fig. 4. For co-polymers 80/20, the crystalline structure closely resembles that ofPH. The same structure is also present in the 60/40 sample, but thelevel of crystallinity is significantly lower. In contrast, by increasingthemolar content of BT units by synthesizing P(H-co-BT)-40/60 andP(H-co-BT)-20/80, the crystalline structures of copolyesters shift tothat of PBT. This behavior is consistent with shifts in the averagelengths of the HH and TBT triads as a function of copolymercomposition (see Table 2).

This result is in agreement with the correlation found betweensequence lengths and crystalline phases for random copolyesters ofBT and butylene-1,12-dodecanedioate (named 4e12) units [39]. Forthis latter system, triad lengths of about L ¼ 3 are sufficiently longto allow the copolyester to crystallize in the corresponding ho-mopolymeric phase (PBT or 4e12). Moreover, it was found thatcomonomer units 4e12 are not completely rejected from the PBTcrystalline phase, but partially included into crystalline lattice asdefects, resulting in decreases in the Tm relative to homopolymersas intermediate compositions are approached.

To test whether the crystalline phases are formed by purecrystals and the comonomeric unit is completely rejected fromthem, the Baur equation, based on the comonomer exclusionmodel, is used [51]:

mole-% BT Units

0

50

100

150

200

0 20 40 60 80 100

T (°

C)

020406080100120140

ΔHm

(J/g

)

Tc

Tm

Tc

Tm

Fig. 3. Trends of crystallization temperature (Tc), melting enthalpy (DHm) and meltingtemperature (Tm) vs BT mole-% for P(H-co-BT) (full symbols) and P(4e12-co-BT)(empty symbols) [39] copolyesters.

A. Celli et al. / Polymer 54 (2013) 3774e37833780

1=Tm;co ¼ 1=T�m � �

R=DH�m�,½lnxC � 2xC,ð1� xCÞ� (4)

where Tm,co is the melting temperature of a random copolymer ofcrystallizable comonomer C, with mole fraction xC, wherein thecorresponding C-homopolymer has an equilibrium melting tem-perature and heat of fusion with values T

�m and DH

�m, respec-

tively, and R is the gas constant. By plotting 1/Tm,co against �[lnxC � 2xC$(1 � xC)], observation of a linear trend would indicate thatthe co-unit is completely rejected from crystals. In our case, linearextrapolations are characterized by low correlation coefficients,suggesting that for P(H-co-BT), complete exclusion of the como-nomer does not occur. Hence, we conclude that there is partialinclusion of H comonomeric units into the BT crystal phase, and,

10 20 30 40 50

PBT20/8030/7040/6060/4080/20

PH

A.U

.

2θ

Fig. 4. X-ray diffraction patterns of PH, PBT and P(H-co-BT) copolyesters.

correspondingly, BT units within the H crystalline phase. Similarbehavior of partial inclusion of comonomers into the crystal latticeas defects was observed not only for 4e12-co-PBT [39], but also forother copolymers such as 4e12-co-PBCHD [52], poly(butylenesuccinate-co-BT) [40], and poly(butylene isophthalate-co-butyleneadipate) [53] copolyesters.

Further analysis and discussion of results presented in Fig. 3follow. Differences between P(H-co-BT) (filled symbols) and P(4e12-co-BT) [39] (empty symbols) are expected for the left part ofFig. 3, since H- and 4e12-units have significantly different struc-tures. The corresponding PH and 4e12 homopolymers both havelong sequences of methylene (eCH2e) units (13 and 10, respec-tively). However, 4e12 introduces an additional ester unit and ashort e(CH2)4e group due to butanediol repeat units. These char-acteristics result for 4e12 relative to PH in lower Tm and Tc values of(75 vs 92 �C for Tm and 53 vs 72 �C for Tc) as well as lower crys-tallinity content based on DHm values (89 and 131 J g�1, respec-tively). In comparison, high density polyethylene (HDPE),characterized by a very long methylene sequence, has Tm and DHmvalues of 133 �C and 189 J g�1, respectively [46]. For the rightportion of Fig. 3, where data refer to relatively long BT segmentsthat form a PBT crystalline structure, trends for DHm and Tm as afunction of BT content overlap for copolyesters with 4e12 and Hrepeat units. In contrast, trends for Tc differ substantially since P(H-co-BT) crystallizes more slowly than P(4e12-co-BT). Consistentwith this logic, the difference in Tc values for these two copolyestersystems tends to increase with the increment of 4e12- and H-unitscontent, in the right part of Fig. 3. Indeed, the crystallization processof PBT crystals occurs at very high temperatures relative to Tmvalues of 4e12 and PH homopolymers. For example, the crystalli-zation process of P(4e12-co-BT)-30/70 and P(H-co-BT)-30/70 oc-curs at 80 and 40 �C above Tm’s 4e12 and PH, respectively. Thisimplies that, under crystallization conditions, the mobility of 4e12chain segments is higher than that of PH that should better facili-tate the PBT crystallization process. Comparison of crystallizationkinetics at the same undercooling is planned in future work toclarify this behavior.

Since the repeat units of P(H-co-BT) and P(4e12 co-BT) functionas impurities within the crystal phase formed by each other, itfollows that DHm decreases in Fig. 3 as one starts with the homo-polymers P(H) or P4e12 and then adds increasing contents of BT toabout 50 mol% (left part of plot), or one starts with the homopol-ymer PBT and adds increasing contents of H or 4e12 comonomersto about 50mol% (right side of plot). However, the negative slope ofDHm vs comonomer composition, on the left part of the plot, isgreater than the positive slope on the right side of the plot. Thus,increasing quantities of rigid BT units when the copolymer exists inthe PH crystal phase is more disruptive to crystallization of H-segments than the addition of relatively more flexible H-units tocrystallization of BT-segments.

Attempts to determine Tg values fromDSC scans were successfulonly with low crystallinity copolyesters such as P(H-co-BT)-40/60(Tg ¼ �16 �C), P(H-co-BT)-30/70 (Tg ¼ �10 �C), P(H-co-BT)-20/80(Tg ¼ �2 �C) and Ecoflex (Tg ¼ �31 �C). Also, by using very highcooling rates from themelt, which should inhibit the crystallizationprocess, it was not possible to observe significant shifts of thebaseline in DSC traces for copolymers rich in H-units. Further workby DMTA will be discussed below that provides additional infor-mation on Tg transitions for the copolyesters synthesized. Never-theless, DSC results show that intermediate P(H-co-BT)compositions can be fine-tuned to have thermal transitions similarto Ecoflex. For example, P(H-co-BT)-40/60 and Ecoflex have low Tgvalues (�16 and �31 �C, respectively), Tm values of 140 and 124 �C,respectively, and low crystalline contents based on DHm values (17and 11 J g�1, respectively). Additional comparisons between the

A. Celli et al. / Polymer 54 (2013) 3774e3783 3781

tensile properties of partially biobased P(H-co-BT) and petroleum-derived Ecoflex are provided below.

-100 -50 0 50 100

0.0

0.1

0.2

0.3 PBT 20:80 30:70 40:60 60:40 PH Ecoflex

ta

nδ

Temperature

Fig. 6. Plots for PH, PBT and selected P(H-co-BT) copolyesters of tan d vs temperaturefrom DMTA.

3.3. DMTA analysis

Glass transitions, not discernible for all the samples by DSCanalysis, were determined by dynamicmechanical thermal analysis(DMTA, see Fig. 5), which is a sensitive method to measure storageand loss modulus (E0 and E00) as well as loss factor tan d. Fig. 5ashows that at low temperatures, below �30 �C, E0 is characterizedby different values that depend both on crystallinity and meltingpoint of the samples. The data indicate that the higher the meltingtemperature (i.e. in PBT) and/or the higher the melting enthalpy(i.e. in PH), the higher the sample’s storage modulus. This is aneffect of the degree of crystal perfection and high crystallinity thatcontribute to the rigidity and brittleness of the material. Relativelysmall temperature increases result in steep decreases (one/twoorders of magnitude) in E0. Copolymers with intermediate com-positions and lower degree of crystallinity have the most pro-nounced E0 variation with temperature. The decreases of E0,occurring in the temperature range from�40 to 120 �C, correspondto relaxation peaks of the loss modulus E00 (Fig. 5b) and are due tothe glass-to-rubber transition (Tg). It is notable that each P(H-co-BT)copolyester for all compositions studied herein shows one glasstransition, indicating that copolyester amorphous phases are ho-mogeneous. This is a consequence of the copolymers randomnessdegree discussed above and listed in Table 2. Values of Tg, deter-mined from the maximum of E00 peaks curves and listed in Table 3,range from �21 �C for PH [28] to 61 �C for PBT [39].

The low Tg value for PH is explained by high chain flexibility, dueto long methylene sequences. This explanation is supported by thestill lower Tg of polyethylene (about �36 �C) [54]. In contrast, therigidity of aromatic rings in PBT explains its high Tg (about 60 �C).Change in peak positions of E00 in Fig. 5b from PH towards PBT is notlinearly proportional to the copolymer composition. Indeed, for allcopolyesters, E00 peaks are closer to that of PH than would be ex-pected from theoretical extrapolation (for example, from Fox equa-tion that is applicable to amorphous copolymers [55], see Fig. S2 insupplemental information). This behavior implies that the flexi-bility imparted by methylene sequences of H-units has a greaterinfluence on chain flexibility than rigid T-units. This may simplyreflect that T-units represent a disproportionally small weight frac-tion of the copolyester relative to aliphatic monomers H and B.

Fig. 6 compares the loss factor tan d of Ecoflex with that of PBT,PH and P(H-co-BT)-20/80, -40/60 and -60/40. The tan d or

a)

-100 -50 0 50 100

107

108

109

60:40 40:60

80:20

PH PBT

30:70

20:80

E' (P

a)

Temperature

PBT 20:80 30:70 40:60 60:40 80:20 PH

Fig. 5. Variation of a) E0 and b) E00 with temperat

relaxation (generally called a relaxation) peak intensity is propor-tional to the amorphous content. Thus, since PH is highly crystal-line, it displays a low intensity a relaxation peak. In contrast,intermediate compositions have low and imperfect crystallinephases and, therefore, more intense a relaxation transitions.Moreover, copolymers at intermediate compositions and Ecoflexshow similarities with respect to the intensity of the relaxation andthe temperature at which it occurs.

In the range from �150 to �50 �C, PBT and copolymers rich inBT-units show a secondary relaxation, denoted as a b-relaxation.Generally, relaxations under Tg are associated with restricted chainmotions. For PBT, the relaxation at about �80 �C is generallyattributed to rotations of the methylene units. In other words, tomolecular motions arising from the flexible part of the repeatingunit [56]. Indeed, the literature generally coalesce around thenotion that these motions are responsible for b-relaxations inpoly(methylene terephthalate)s [57] and in polyolefins [58]. How-ever, the nature of secondary relaxation is complex. For example,some believe that motions of carbonyl groups are also involved[59]. The b-relaxation is influenced by a number of factors such asmolecular structure, material crystallinity, the length of thealiphatic sequences and polarity of the chain [59,60]. However, theb-relaxation is primarily a function of properties in noncrystalline

b)

-100 -50 0 50 100

106

107

108

80:20

60:4040:60

PH

PBT20:80

30:70

PBT 20:80 30:70 40:60 60:40 80:20 PH

E''

(P

a)

Temperature

ure for PH, PBT and P(H-co-BT) copolyesters.

Table 4Analysis of strain vs stress curves from tensile tests performed on P(H-co-BT)copolyesters, their corresponding homopolymers and Ecoflex.

Samplea Young’smodulus(MPa)

Elongationat break (%)

Stress atbreak(MPa)

Stress atyield (MPa)

Strain atyield (%)

PH 414 � 51 638 � 128 14 � 2 17 � 2 12 � 1P(H-co-BT)-80/20 61 � 18 613 � 35 5.9 � 0.6 6 � 1 615 � 4P(H-co-BT)-60/40 43 � 10 >1284b 7.9 � 1.2 4 � 1 22 � 2P(H-co-BT)-40/60 66 � 10 757 � 224 12 � 4 8 � 1 30 � 2P(H-co-BT)-30/70 140 � 5 842 � 100 18 � 1 12 � 0 47 � 1P(H-co-BT)-20/80 249 � 36 456 � 31 23 � 1 20 � 0 28 � 4PBT 950 � 104 272 � 71 386 � 3 48 � 2 15 � 1Ecoflex 68 � 8 1204 � 264 18 � 3 8 � 1 36 � 3

a Sample names are given as abbreviations, see Table 1 footnote a. Values X/Yrepresent the weight-% of H-to-BT units.

b P(H-co-BT)-60/40, sample bars were not stretched to break since values werereached that exceeded the instruments limit.

A. Celli et al. / Polymer 54 (2013) 3774e37833782

regions. Therefore, the absence of a b peak for PH is likely due to itslow content of an amorphous phase. Secondary relaxations, due tothe main chain, are beneficial for impact resistance and are believeto facilitate macroscopic shear yielding [61].

3.4. Tensile testing

Fig. 7 shows typical stressestrain curves for P(H), PBT and theircopolymers. The stressestrain curve of Ecoflex was added to Fig. 7for comparison purpose. Stressestrain curves for all investigatedpolymers exhibit four regions: I e linear and nonlinear viscoelas-ticity, II e neck region/strain softening, III e plastic flow, and IV e

strain hardening [62]. For P(H-co-BT)-60/40, sample bars were notstretched to break since values were reached that exceeded theinstruments limit. With increasing weight ratio of H-units from 0 to60 wt.%, stress values during plastic flow of the copolymer de-creases. Stress values during plastic flow for both PH and Ecoflexare close to those of P(H-co-BT)-40/60 and P(H-co-BT)-30/70.

Parameters of Young’s modulus, elongation at break, stress atbreak, stress at yield and strain at yield determined from stressestrain curves for P(H-co-BT) are listed in Table 4 as the mean plus orminus one standard deviation (see also Fig. S4 for bar plot of data).For PH, values of Young’s modulus, elongation at break, and stressat break are consistent with those reported previously by us for thehomopolymer with Mw ¼ 78K [28]. With respect to PBT, PH has alower Young’s modulus (414 vs 950MPa) and a higher elongation atbreak (638 vs 272%).

We attribute this difference to the relatively higher flexibility of(CH2)13-units separated by ester moieties despite PHs' high crys-tallinity (w50%) [63].

For PBT and P(H-co-BT)-20/80, 20% H-units cause a largedecrease in Young’s modulus (950e249 MPa). Further increase incopolyester H-unit content results in decreases in Young’s modulusuntil a minimumvalue 43MPa at 60% H-units is reached. Moreover,P(H-co-BT)-80/20 is also a relatively low modulus material(61 MPa) due to the large disruption in the materials crystallineorder relative to PH (Tm decreased by 26 �C, DHm decreased by47 J g�1). The Young’s moduli of P(H-co-BT)-40/60 and 80/20 areclose to that of Ecoflex (68 MPa).

Stress at break decreases from 38 to 6 MPa as the content of H-units in copolyesters increases from 0 to 80%. This is a logicalconsequence of decreasing the content of rigid BT-units in

0 300 600 900 1200 1500

0

10

20

30

40

50

100:0

80:20

60:40

40:60

30:70

20:80

0:100

Ecoflex

Stress

Strain, %

0:100

20:80

30:70

40:60

60:40

80:20

100:0

Ecoflex

PH:PBT

Fig. 7. Stressestrain curves from tensile tests of PH, PBT, Ecoflex and P(H-co-BT)copolyesters.

copolyesters. Relative to Ecoflex, the stress at break of P(H-co-BT)-30/70 is closest in value (both 18 MPa).

Both Young’s modulus and stress at break exhibited minimumvalues at about 60% H-units. Conversely, the %-elongation at breakincreases to its maximum value of >1284% at this copolymercomposition (Table 4). The minimum %-elongation at break valuewas 456 MPa for P(H-co-BT)-20/80. Hence, all P(H-co-BT) compo-sitions studied herein showed good ductility. Ecoflex has excellentductility with %-elongation at break of 1204% similar to P(H-co-BT)-60/40.

Variation in the stress at yield with copolymer composition issimilar to that of Young’s modulus, discussed above. Strain at yieldvaries irregularly with copolymer composition. Nevertheless, strainat yield for all the copolyester compositions is higher than those ofthe corresponding homopolymers.

The above benchmarking of P(H-co-BT) tensile properties to thatof Ecoflex was to provide insights into the potential utility of thesenew copolyesters. However, much remains to be learned about thephysico-mechanical properties of P(H-co-BT) materials that willprovide greater insights into their values as stand-alone materials oras components in blend systems. What is evident from this work isthat u-hydroxyfatty acids such as u-hydroxytetradecanoic acid usedherein are interesting comonomers with aromatic building blocksthat can provide a wide range of interesting properties. This workrepresents a first study of others thatwill follow that explores the useof biobasedu-hydroxyfatty acids asmonomers for random, block andgraft copolymer systems. The extent that P(H-co-BT) and othercopolyesters containingu-hydroxyfattyacidsmonomers retains theirbiodegradabilitywill also be a subject of great interest in futurework.

4. Conclusions

This work reports the preparation and characterization of aunique family of aliphaticearomatic copolyesters from 1,4-butane-diol, dimethyl terephthalate and u-hydroxytetradecanoic acid. Thelatter is a new biobased monomer made available by the develop-ment of an engineered C. tropicalis strain that efficiently convertsmethyl myristate to u-hydroxytetradecanoic acid. 1H NMR analysisshows that polycondensation reactions result in random co-polymers. Thus, new macromolecular structures were synthesizedby combining polymer building blocks frombiotechnological routeswith those from petrochemical-derived feedstocks.

Thermal and mechanical properties of the copolyesters areinterpreted based on the combination of flexible long aliphaticsegments with more rigid aromatic moieties. The novel copo-lyesters are semicrystalline materials, characterized by the pres-ence of a single crystalline phase (PH or PBT crystals), according to

A. Celli et al. / Polymer 54 (2013) 3774e3783 3783

the composition. The minor component is included in crystals asdefects. Consequently, melting temperatures and crystallinitydecreasewith respect to those of the corresponding homopolymersas the percentage of the second component increases, reachingminimum values at intermediated compositions. The amorphousphase is fully homogeneous, as confirmed by DMTA spectrawhere asingle glass transition process is evident for all compositions. Themechanical properties are varied to a large extent by the selectedmonomer feed composition based on the fact that PBT has a higherYoung’s modulus and a lower elongation at break whereas PH has amoderate modulus and high ductility. The copolymer with 60 mol%H units has similar tensile properties as the commercial productEcoflex. Thus P(H-co-BT)-60/40, containing 60 mol% biobased u-hydroxytetradecanoic acid repeat units, is an interesting alternativestructure to that of Ecoflex which to the best of our knowledge hasno renewable carbon content.

Acknowledgment

We are grateful for funding received from the National ScienceFoundation Office of International Science and Engineering (NSF-OISE) under award #1214469 for their financial support of thisresearch.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.polymer.2013.05.007.

References

[1] Gandini A. Green Chem 2011;13:1061e83.[2] Gallezot P. Chem Soc Rev 2012;41:1538e58.[3] Tschan MJ-L, Brulé E, Haquette P, Thomas CM. Polym Chem 2011;3:836e51.[4] Mathers RT, Meier MAR, editors. Green polymerization methods. Wiley; 2011.[5] Belgacem MN, Gandini A, editors. Monomers, polymers and composites from

renewable resources. Elsevier; 2008.[6] Gross RA, Kalra B. Science 2002;297(5582):803e7.[7] Dash TK, Konkimalla VB. J Control Release 2012;158(1):15e33.[8] Braunegg G, Bona R, Koller M. Polym Plast Technol Eng 2004;43(6):1779e93.[9] Lee Sang Yup, Choi Jong-Il. Biopolyesters 2001;71:183e207.

[10] Domenek S, Courgneau C, Ducruet V. Kalia S, Averous L, editors. Biopolymers2011:183e223.

[11] Vert M. Biomacromolecules 2005;6(2):538e46.[12] WuD,YuanL, LaredoE,ZhangM,ZhouW. IndEngChemRes2012;51(5):2290e8.[13] Bhardwaj R, Mohanty AK. J Biobased Mater Bioenergy 2007;1:191e209.[14] Garlotta DJ. J Polym Environ 2002;9(2):63e84.[15] Sudesh K, Abe H, Doi Y. Prog Polym Sci 2000;25:1503e55.[16] Focarete ML, Scandola M, Kumar A, Gross RA. J Polym Sci Part B Polym Phys

2001;39:1721e9.[17] Berti C, Celli A, Marchese P, Marianucci E, Barbiroli G, Di Credico F. e-Polymers

2007. n. 057.[18] Biermann U, Bornscheuer U, Meier MAR, Metzger JO, Schäfer HJ. Angew Chem

Int Ed 2011;50:3854e71.[19] Meier MAR. Macromol Chem Phys 2009;210:1073e9.[20] Rybak A, Fokou PA, Meier MAR. Eur J Lipid Sci Technol 2008;110:797e804.[21] Trzaskowtski J, Quinzler D, Bahrle C, Mecking S. Macromol Rapid Commun

2011:1352e6.

[22] Turunc O, Meier MAR. Green Chem 2011;13:314e20.[23] Desroches M, Caillol S, Lapinte V, Auvergne R, Boutevin B. Macromolecules

2011;44:2489e500.[24] Quinzler D, Mecking S. Angew Chem Int Ed 2010;49:4306e8.[25] Furst MRL, Le Goff R, Quinzler D, Mecking S, Botting CH, Cole-Hamilton DJ.

Green Chem 2012;14(2):472e7.[26] Petrovi�c ZS, Mili�c J, Xu Y, Cvetkovi�c I. Macromolecules 2010;43:4120e5.[27] Lu W, Ness JE, Xie W, Zhang X, Minshull J, Gross RA. J Am Chem Soc 2010;132:

15451e5.[28] Liu C, Liu F, Cai J, Xie W, Long TE, Turner SR, et al. Biomacromolecules 2011;12:

3291e8.[29] Deligio T. Plastics today. Published, http://www.plasticstoday.com/articles/

coke-and-pepsi-foresee-biobased-pet-future-bottles; October 3, 2011.[30] Berti C, Binassi E, Colonna M, Fiorini M, Kannan G, Karanam S, et al. PCT Int

Appl 2010. WO 2010078328 A2 20100708.[31] Colonna M, Berti C, Fiorini M, Binassi E, Mazzacurati M, Vannini M, et al. Green

Chem 2011;13:2543e8.[32] Burk MJ, Osterhout RE, Sun J. WO2011017560. PCT Int Appl 2011.[33] Witt U, Muller RJ, Deckwer W-D. J Macromol Sci A Pure Appl Chem 1995;32:

851.[34] Witt U, Muller R-J, Augusta J, Widdecke H, Deckwer W-D. Macromol Chem

Phys 1994;195:739e45.[35] Muller R-J, Kleeberg I, Deckwer W-D. J Biotech 2001;86:87e95.[36] Witt U, Yamamoto M, Seeliger U, Muller R-J, Warzelhan V. Angew Chem Int Ed

1999;38:1438.[37] http://www.bioplastics.basf.com/pdf/Ecoflex-Ecovio_Brochure_2008.pdf.[38] Audic J-L, Legentil L, Nugier-Chauvin C, Ferrieres V, Potel M, Roisnel T. Mater

Sci 2012;47:5806e14.[39] Berti C, Celli A, Marchese P, Barbiroli G, Di Credico F, Verney V, et al. Eur Polym

J 2008;44:3650e61.[40] Li F, Xu X, Hao Q, Li Q, Yu J, Cao A. J Polym Sci Part B Polym Phys 2006;44:

1635e44.[41] Shi XQ, Aimi K, Ito H, Ando S, Kikutani T. Polymer 2005;46:751e60.[42] Park SS, Chae SH, Im SS. J Polym Sci Part A Polym Chem 1998;36:147e56.[43] Devaux J, Godard P, Mercier JP. J Polym Sci Polym Phys 1982;20:1875e80;

Devaux J, Godard P, Mercier JP. J Polym Sci Polym Phys 1982;20:1881e94.[44] Opaprakasit M, Petchsuk A, Opaprakasit P, Chongprakobkit S. eXPRESS Polym

Lett 2009;3:458e68.[45] Tessier M, Fradet A. e-Polymers 2003. no. 030.[46] Celli A, Barbiroli G, Berti C, Di Credico F, Lorenzetti C, Marchese P, et al. J Polym

Sci Part B Polym Phys 2007;45:1053e67.[47] Wunderlich B. Macromolecular physics: crystal melting, vol. 3. New York:

Academic; 1980.[48] Tonelli AE. Polym Lett Ed 1973;11:441e7.[49] Tonelli AE. J Polym Sci Part B Polym Phys 2002;40:1254e60.[50] Gonzales CC, Riande E, Bello A, Pere�na JM. Macromolecules 1988;21:3230e4.[51] Baur H, Baltorowicz M. Makromol Chem 1966;98:297e301.[52] Berti C, Celli A, Marchese P, Barbiroli G, Di Credico F, Verney V, et al. Eur Polym

J 2009;45:2402e12.[53] Montané J, Armelin E, Asín L, Rodríguez-Galán A, Puiggalí J. J Appl Polym Sci

2002;85:1815e24.[54] Gaur U, Wunderlich B. Macromolecules 1980;13:445e6.[55] Fox TG. Bull Am Phys Soc 1956;1:123e5.[56] Radusch H-J. Poly(butylene terephthalate). In: Fakirov S, editor. Hand-

book of thermoplastic polyesters, vol. 1. Weinheim: Wiley-VCH; 2002.p. 389e418.

[57] Farrow G, McIntosh J, Ward IM. Makromol Chem 1960;38:147e58.[58] Ilers KH, Breuer H. J Colloid Sci 1963;18:1e31.[59] Soccio M, Nogales A, Lotti N, Munari A, Ezquerra TA. Phys Rev Lett 2007;98:

037801/1e037801/4.[60] McCrum MG, Read BE, Williams G. Anelastic and dielectric effects in poly-

meric solid. London: Wiley; 1967.[61] Chen LP, Yee AF, Goetz JM, Schaefer J. Macromolecules 1998;31:5371e82.[62] Ward IM. Mechanical properties of solid polymers, vol. 399. JohnWiley & Sons

Pub; 1983 [chapter 12].[63] Cai JL, Liu C, Cai MM, Zhu J, Zuo F, Hsiao BS, et al. Polymer 2010;51:1088e99.