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Polymer Degradation and Stability 169 (2019) 108983

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Polymer Degradation and Stability

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

Novel high Tg fully biobased poly(hexamethylene-co-isosorbide-2,5-furan dicarboxylate) copolyesters: Synergistic effect of isosorbideinsertion on thermal performance enhancement

Nejib Kasmi a, Nina Maria Ainali a, Elena Agapiou a, Lazaros Papadopoulos a,George Z. Papageorgiou b, Dimitrios N. Bikiaris a, *

a Laboratory of Polymer Chemistry and Technology, Department of Chemistry, Aristotle University of Thessaloniki, GR-541 24, Greeceb Chemistry Department, University of Ioannina, P.O. Box 1186, 45110, Ioannina, Greece

a r t i c l e i n f o

Article history:Received 4 July 2019Received in revised form14 September 2019Accepted 23 September 2019Available online 25 September 2019

Keywords:2,5-Furandicarboxylic acidIsosorbide1,6-HexanediolCopolyestersMelt polycondensationGlass transition temperatureBiobased polymers

* Corresponding author.E-mail address: [email protected] (D.N. Bikiaris).

https://doi.org/10.1016/j.polymdegradstab.2019.108980141-3910/© 2019 Elsevier Ltd. All rights reserved.

a b s t r a c t

Here an effective solution for overcoming the low glass transition temperature (Tg) of poly(hexa-methylene 2,5-furan dicarboxylate) (PHF), a fully biobased polyester derived from dimethylfuran-2,5-furan-dicarboxylate (DMFD) and 1,6-hexanediol (1,6-HD), is proposed that uses isosorbide (Is) as abicyclic rigid diol comonomer. Incorporating this sugar-derived diol with a broad scope content (3e90mol%) into PHF macromolecular chain, by melt polycondensation and using titanium (IV) isoprop-oxide (TTIP) catalyst, results in the synthesis of highly heat-resistive poly(hexamethylene-co-isosorbide-2,5-furandicarboxylates) copolyesters (PHIsF). The chemical structure and composition of the prepared100% renewable resources-based materials were confirmed in detail by 1H NMR and FTIR spectroscopies.Randommicrostructures were obtained for PHIsF samples, when Is content exceeds 40mol%. As revealedby Wide-Angle X-ray Diffraction (WAXD) patterns, increase of IsF content in the copolymers leads toamorphous materials. The latter exhibited an excellent thermal stability up to 360 �C and very variable Tgvalues oscillating from 10 to 135 �C depending on the comonomer ratio, in which it gradually increaseswith increasing of Is feed content. Results found herewith showed that the high stiff building block Is canbe used as an effective control parameter to spectacularly enhance the thermal properties of polymers,particularly the glass transition temperature. Taking advantage of their features, PHIsF have the potentialto serve as promising fully biobased amorphous materials for practical applications that demand high Tgvalues.

© 2019 Elsevier Ltd. All rights reserved.

1. Introduction

The attention given to sustainable polymers from renewableresources is continuously growing [1e5]. Development of suchenvironmentally friendly polymeric materials to be a promisingalternative for petroleum-derived counterparts is a result of a realwillingness of both the industrial and scientific communities totransition from fossil-based to a Bio-Economy [6e10]. This is due tothe fact that society is becoming more concerned with several cli-matic and environmental global challenges associated with surgingenergy demand, dwindling fossil resources and the difficulty tocontrol the market prices, and global warming, etc. [11]. In recent

3

years, a burgeoning surge in utilization and conversion of biomass-derived feedstock to industrially relevant new building blocks[12e15], in particular bifunctional monomers, in addition to a rapid(bio)technology development in the biorefinery infrastructure[16e18] offer a vast number of opportunities in the synthesis ofnew bioplastics.

2,5-furandicarboxylic acid (FDCA) is such a highly promisingbiobased diacid monomer [19,20]. The latter, recognized as one ofthe twelve most important renewable-based monomers by the USDepartment of Energy [21], has attained a privileged positionowing to its easy accessibility and aromatic character; thereby ahuge research activity has been awakened at industrial and aca-demic circles. In this sense, FDCA called also “The sleeping giant”has drawn a keen interest by many international companies,including Corbion, Avantium, ADM, BASF, and DuPont because of itssimilar chemical and physical properties to those of the petroleum-

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N. Kasmi et al. / Polymer Degradation and Stability 169 (2019) 1089832

based terephthalic acid (TPA) that is extensively exploited inpetrochemical-based polymer industry [22,23]. This renewablearomatic building block, which is deemed as a good substituent toTPA, can be prepared from polysaccharides-derived 5-hydroxymethylfurfural (HMF) intermediate after a selective aero-bic oxidation [24,25].

Thanks to recent successful breakthroughs in the R&D of FDCAthat made it readily available at industrial scale, a paradigm shifthas emerged towards synthesizing of several FDCA-based homo-polyesters [26e36] as well as copolyesters [37e50]. Among them,poly(ethylene furan dicarboxylate) (PEF) is the most promisingfuranoate polyester, which is a 100% renewable-based alternativeto the commercial thermoplastic derived from fossil-based re-sources; poly(ethylene terephthalate) (PET) [51,52]. Another rele-vant example of sustainable poly(alkylene furanoate)s that has notbeen industrially used to date is poly(hexamethylene 2,5-furandicarboxylate) (PHF) [53]. This biobased polyester that possessesreasonably good tensile strength (35.5MPa) and high ductility(elongation at break of 210%) is produced from either FDCA or itsdimethyl ester and a long-chain aliphatic diol; 1,6-hexanediol (1,6-HD) [54]. The latter with a wide range of application in coatings,polyesters, polyurethane, and adhesives can be effectively preparedfrom biomass-derived HMF precursor over double-layered Pd/SiO2 þ IreReOx/SiO2 catalysts [55,56].

The 100% renewable resources-based material PHF, seldom re-ported in the literature, revealed a glass transition temperatureabout 7 �C and a Tm at around 145 �C [57]. This low temperature isan inherent drawback that may limit its use in particular applica-tions requiring high Tg. Although thermal degradation and crys-tallization behavior, and mechanical properties of this polyesterthat have been extensively investigated in few earlier appraisals[57,58], hitherto no data in literature has exclusively dealt with theaforementioned PHF drawback of low Tg. In this context, isosorbide(Is) is one of most promising ecofriendly diol abundantly available[59]. A great deal of attention is given to this bicyclic building blockthat is industrially produced from cornstarch owing to plenty ofbenefits such as nontoxicity, good thermal stability, interestingstiffness, and chirality properties [60]. As a well-proven fact thathas been considerably reported in the relevant literature, Is unitsinsertion in the main chain of polymers confers to them hightoughness [61] and therefore a spectacular increase in their glass-transition temperature, in addition to an obvious thermal stabilityenhancement [62,63]. These outstanding features broadens theapplication window of the resulting polymers [64,65]. In this re-gard, other bicyclic carbohydrate-based difunctional diols such as2,4:3,5-di-O-Methylene-D-glucitol (Glux-OH), 2,4:3,5-di-O-Meth-ylene-D-mannitol (Manx-OH) and 2,3:4,5-di-O-Methylene-galac-titol (Calx-OH) [66e70] as well as octahydro-2,5-pentalenediol(OPD) [71] have been recently used as biobased comonomers incopolyesters synthesis for the same purpose; adding stiffness to theresulting polyesters chains, and leading therefore to a significantincrease in their Tg. To name a few, poly(isosorbide 2,5-furandicarboxylate) (PIsF) is a fully biobased Is-containing polyesterwith a very high Tg value of 157 �C reported byGomes et al. [72]. It isclear that studying the structure/property relationship between1,6-HD and Is diols with FDCA could be a facile and impressive wayto address PHF homopolymer shortcoming via copolymerizationand thereby obtaining fully biobased materials with tunableproperties. Copolymerization is awell-known technique, which hasbeen adopted as simple way to design polymeric materials on de-mand with modified and enhanced properties that indirectly af-fects all other properties of end polymers including themorphology (crystallinity), Tm, Tg, etc. [73,74].

To the best of our knowledge, the incorporation of isosorbidesugar derivative into furanoate polyesters via melt

polycondensation procedure has been assessed for the first time inthe present work. For that, we synthesized in a full range of com-positions a new fully biobased aliphatic-aromatic copolyesters se-ries with high Tg values containing FDCA and a varied flexible andstiff moieties molar ratio like 1,6-HD and Is diols, respectively. Theeffect of the progressive variation of Is content in copolyestercomposition on both the thermal properties and the crystallinityhas been evaluated in detail. The insertion Is diol approach hasappealing features offering a new furanoate polyesters categorywith enhanced thermal properties, thereby developing next-generation sustainable bioplastics from renewable raw materials.This contribution is an advantageous step forward in replacingclassic fossil fuel based polymers that require the concerted effortbetween industrial and academic scientists, civil society, and policymakers.

2. Experimental

2.1. Materials

Dimethyl-2,5-furan dicarboxylate (DMFD) was prepared aspreviously described in our publications [75] using 2,5-furandicarboxylate acid (FDCA) (Aldrich, purum 97%). 1,6-hexanediol(b.p.¼ 250 �C and m.p.¼ 38e42 �C, purum 99%), isosorbide (Is,purum 99%) and titanium (IV) isopropoxide (TTIP, 97%) reagentswere purchased from Aldrich Co. The other solvents used for pu-rification were of analytical grade.

2.2. Copolyester synthesis

As presented in Scheme 1, PHIsF copolyesters of various com-positions were synthesized from DMFD, Is, and 1,6-HD via a three-step melt polycondensation method (transesterification, poly-esterification, and then polycondensation) as follows. In the firststep, transesterification reaction was performed in bulk at160e180 �C under amild nitrogen flow to prepare the bis(hydroxyl-hexamethylene)-2,5-furan dicarboxylate (BHHF) and bis(hydrox-yisosorbide)-2,5-furan dicarboxylate (BHIsF). This reaction lastedfor 4 h until the collection of almost the theoretical amount ofmethanol: DMFD and Is or 1,6-HDwithmolar ratio of diester:diol of1:2 were added in presence of 400 ppm of titanium(IV) isoprop-oxide (TTIP) catalyst into a round-bottomed flask equipped with amechanical stirrer. The mixture was first heated for 2 h at 160 �C.The reaction temperature was gradually increased to 170 �C for 1 hand finally it was left to proceed at 180 �C for 1 h. After coolingdown to room temperature, the reaction mixture was discharged.In the second step of PHIsF synthesis, the polyesterification wascarried out in melt by reacting different molar ratio of BHHF/BHIsFranged from 97/03 to 10/90 and mixed with an equimolar amountof DMFD relative to the total molar ratio of both BHHF and BHIsFmonomers. The mixture was charged after the glass reactor wasvented for 3e5 times with N2 at room temperature to remove air,avoiding thus oxidation during reaction. Polyesterification wasperformed at 160 �C for 2 h and then held for 1 h at 170 �C. Afterthat time, the reaction temperature was raised to 180 �C for 1 h andfinally it was left to proceed for an additional 0.5 h at 190 �C. Finally,the polycondensation step was carried out under a high vacuum of5.0 Pa slowly applied over a time of 0.5 h to minimize the subli-mation of oligomers and to avoid excessive foaming, which con-siders as a potential problem during melt polycondensation. Thetemperature of the melt was raised to 220e250 �C stepwise (1 h at220 �C, 1 h at 230 �C, 1 h at 240 �C, and for an additional 0.5 h at250 �C). At last, after the polymerization of the target copolyesterwas completed, the melt was cooled to ambient temperature. Theobtained crude product was purified by dissolving in a mixture of

Scheme 1. Synthesis route of copolyesters PHIsF from Is, 1,6-HD and DMFD.

N. Kasmi et al. / Polymer Degradation and Stability 169 (2019) 108983 3

TFA/CHCl3 (25/75), and then precipitating in cold methanol.

2.3. Copolyesters characterization

2.3.1. Intrinsic viscosity measurementIntrinsic viscosity [h] measurements of the prepared PHIsF

copolyesters were carried out using an Ubbelohde viscometer at25 �C in a 1,1,2,2-tetrachloroethane/phenol mixture (40/60, w/w).The polymer sample was kept for 15min in the above mixturesolvents at 70 �C to achieve a complete dissolution. The solutionwas after that cooled to ambient temperature and filtered using adisposable membrane filter (Teflon) to remove any non-solublematerials. The intrinsic viscosity (IV) of each polyester samplewas determined using the SolomoneCiuta equation of a singlepoint measurement [76]:

[h]¼ [2{t/to e ln(t/to) �1}]1/2/c

where c, t, and t0 respectively present the concentration of thesolution; the flow time of solution, and the flow time of pure sol-vent. For each polymer sample, three different measurements wereperformed to determine the average value.

2.3.2. End group analysisCarboxyl end-group content (-COOH) of PHIsF copolyesters was

measured according to Pohl's method by titrating a solution of thecopolyester in chloroform/benzyl alcohol mixture. A standard so-lution of sodium hydroxide (NaOH) in benzyl alcohol and phenolred as indicator were used [77]. For each copolymer sample, threedifferent measurements have been done to determine the averagevalue.

2.3.3. Wide angle X-Ray diffraction patterns (WAXD)Crystalline structure of the PHIsF copolymers was characterized

by wide angle X-Ray diffraction (WAXD), in the powder form, bymeans of a MiniFlex II XRD system from Rigaku Co., operating withCuKa radiation of wavelength (l) 0.154 nm. Samples were scannedin the angle (2q) range of 5e50� at steps of 0.05� and counting timeof 5 s per step.

2.3.4. Nuclear magnetic resonance (NMR)1H NMR spectra of PHIsF were recorded at a frequency of

500MHz on a Bruker spectrometer, and were internally referencedwith TMS. Copolymers samples were prepared by dissolving thepolyester in deuterated trifluoroacetic acid (d-TFA) to get solutionof 5% w/v. The sweep width was equal to 6 KHz and the number ofscans used for 1H NMR was 16.

2.3.5. Thermogravimetric analysis (TGA)Thermal stability of prepared copolymers was determined by

simultaneous thermogravimetry/differential analysis (TG/DTA)with a STA 449C (Netzch-Ger€atebau, GmbH, Germany) thermalanalyser. PHIsF samples were heated in nitrogen atmosphere(50mL/min) from room temperature to 600 �C at a heating rate of10 �C/min.

2.3.6. Differential scanning calorimetry (DSC)DSC measurements of PHIsF copolyesters were conducted on a

Perkin-Elmer, Pyris Diamond DSC differential scanning calorimeter.High purity zinc and indium were used as standards for enthalpyand temperature calibration. In order that the DSC apparatus ach-ieves function at sub-ambient temperatures and high cooling rates,the system is equipped with an Intracooler 2P cooling accessory.The sealed copolyester sample (5± 0.1mg) in aluminium pan was


heated up/cooled down under nitrogen flow with a heating rate of10 �C/min. The sample was held between these two scans at 170 �Cfor 2min to erase any thermal history. The data reported in thiswork was acquired from the second heating and cooling scans.

2.3.7. Fourier transformed-infrared spectroscopy (FTIR)Infrared spectra of PHIsF copolymers were recorded on a Perkin-

Elmer FTIR spectrometer, model SpectrumOne, in the wavenumberranging from 400 to 4000 cm�1 and in absorbance mode. The FTIRspectra were registered at a resolution of 4 cm�1 with a number ofco-added scans of 64.

3. Results and discussion

3.1. Synthesis of PHIsF

In the present work, the synthesis of novel fully biobasedcopolyesters series having a very wide glass transition temperatureTg range, oscillating from 10 �C to 135 �C, is the most appealingaspect of this study. This aliphatic-aromatic copolymers seriesderived fromDMFD,1,6-HD, and Is was successfully prepared via anesterification-polyesterification-polycondensation procedure asshown in Scheme 1. First, bishydroxy products were synthesizedfrom 1,6-HD or Is and DMFD using TTIP as catalyst, and then, thecorresponding bishydroxyalkylene-2,5-furan carboxylate diols(BHIsF and BHHF) were underwent to the polyesterification in thepresence of DMFD, following the molar ratio of 1/1. Due to the highboiling point and bulk structure of 1,6-HD and isosorbide thatprevent their removal from the reactor during the melt poly-condensation step even with high vacuum application [62], DMFDwas added in the polyesterification stage, instead of performing themelt polycondensation step. This innovative trick applied hereinand previously mentioned for the first time by our group [34] en-sures that all added amounts of 1,6-HD and Is are totally reacted. Inthe last step of synthesis, the polycondensation stage was con-ducted at 220e250 �C, and the reaction was proceeded for 3.5 hunder vacuum condition of 5.0 Pa applied slowly over a time of20min to diminish oligomer sublimation and avert excessivefoaming, which is a potential problem during the melt poly-condensation procedure.

Intrinsic viscosity measurements were performed for PHIsFinstead of the determination of their number average molecularweight (Mn) by GPC analysis. This is mainly due to the low solubilityof these resulting polymers in common solvents such as THF, CHCl3,etc. The IV values [h] of purified copolyester samples oscillatedbetween 0.23 and 0.58 dL/g (Table 1). It is very worthy to note thatthe latter are negatively influenced by increasing Is content (high

Table 1Molar composition obtained by 1H NMR, intrinsic viscosity, as well as carboxyl end grou

PHIsF Samples PHIsF feed molar ratios PHI

PHF 100/0 -PHIsF 97/03 97/03 97.6PHIsF 95/05 95/05 96/4PHIsF 93/07 93/07 94.8PHIsF 90/10 90/10 87.2PHIsF 80/20 80/20 80.8PHIsF 70/30 70/30 72.3PHIsF 60/40 60/40 61.7PHIsF 50/50 50/50 52.1PHIsF 40/60 40/60 41.5PHIsF 30/70 30/70 33/6PHIsF 20/80 20/80 19.1PHIsF 10/90 10/90 11.7PIsF 0/100 -

molar ratio) into the PHIsF macromolecular chains. In other words,an overview inspection of the collected IV data showed that PHIsFsamples with high Is content (50e90%) exhibited quite lower IVvalues (0.23e0.32 dL/g) compared to those of copolymers having Iscontent composition oscillating from 3 to 40% (0.37e0.58 dL/g).Furthermore, despite they have been synthesized following thesame procedure and under the same conditions, the two PHF andPIsF homopolyesters presented a huge difference in their IV values;0.40 vs 0.22 respectively. This finding can be explained on the basisof the significantly lower nucleophilicity of the secondary hydroxylgroups of isosorbide diol with respect to primary eOH moieties ofhexanediol. This outcome could be also boosted by the remarkablesteric hindrance imposed by the endo hydroxyl functionality in theIs, thus resulting in a further reactivity decrease and thereby to alow PHIsF IV values. This is in good agreement with what was re-ported in a previous work of Chen et al. for poly(butyleneterephthalate-co-isosorbide terephthalate) [65]. Characteristiccarboxyl end group values (-COOH) for polyesters synthetized bymelt polycondensation method were obtained for PHIsF co-polymers samples. As can be seen from the data in Table 1, no trendcould be found between eCOOH groups and IV values. The detec-tion of relatively high eCOOH values for the resulting materials canbe associated with the decomposition reactions such as thermaldegradation that are taking place concurrently during the meltpolycondensation stage.

3.2. Structural characterization and composition of PHIsF

1H NMR spectroscopy was used to ascertain the chemicalstructures of PIsF, PHF, and the resulting PHIsF copolyesters asdepicted in Fig. 1. The obtained spectra were perfectly consistentwith the expected structures, where all peaks are correctly attrib-uted to the different protons in the copolymer's chain as elucidatedin Scheme 2. The relative peaks areas of the Is protons e and ftogether to that of peak a of the furan ring bonded to 1,6-HD wasused to calculate the molar composition of PHIsF copolyesters,where the integration of the peaks (Se þ f) was indicated in Fig. 1A.The calculated molar composition of the PHIsF materials compiledin Table 1 were in good consistency with the corresponding feedmolar ratio leading thereby to an easy control of the PHIsFcomposition by varying the feed molar ratio of 1,6-HD to DMFD.This result confirms the successful trick used herein, which statesthe addition of DMFD during the polyesterification step to avoid theremoval of the diols 1,6-HD and Is by distillation from the reactorduring themelt polycondensation process. However, it is noted thatIs molar fraction in the prepared polyesters is generally slightlylower than that in the feed owing to the lower reactivity of Is diol

ps of the purified PHIsF samples.

sF 1H NMR [h] (dL/g) -COOH (eq/106g)

0.40 152.4/2.4 0.41 149.8

0.39 157.2/5.2 0.42 169/12.8 0.58 174.6/19.2 0.53 117.6/27.7 0.37 121/38.3 0.39 113.3/47.9 0.27 103.7/58.5 0.31 110.37 0.32 165.6/80.9 0.25 118.2/88.3 0.23 118.5

0.22 117.6

Fig. 1. (A) The whole 1H NMR spectra of PIsF, PHF and PHIsF. (B) Magnification of protons chemical shifts of PHF, PHIsF, and PIsF.


compared to linear a,u-alkylene diols as frequently reported in theliterature [65]. As clearly depicted in the spectrum of neat PIsF,furan ring protons (a1 and a2) revealed different chemical shifts at7.52 and 7.62 ppm. This is certainly owing to the nonplanar Isstructure and the different special orientations of the two sec-ondary eOH groups: one at C-2 having the exo-orientation withrespect to the V-shaped bicyclic ether ring of Is, whereas the otheris in endo position and is engaged in an intramolecular hydrogenbonding [40]. In other words, due to the endo and exo stereo-chemistry of isosorbide and through the ester linkage formed be-tween this diol and DMFD, two different doublets (7.52e7.62 ppm)corresponding to two non-equivalent furan protons a1 and a2 wereobviously detected. Similar outcome was recently reported forbiobased copolyesters based on isosorbide [78], where for Is-Isdyads, the ester linkage is established between endo- and exo-hy-droxyls of Is and succinic acid-carbonyl groups, hence two different

carbons peaks have been disclosed in the 13C NMR data. Noordoveret al. have also pointed out the same behaviour for isosorbide-derived copolyesters [79], in which broaded chemical shifts cor-responding to the methylene groups signals of succinic acid wereobserved between 2.6 and 2.8 ppm. The Is moiety peaks split intofour multiplets at d 4.48, d 5.12, d 5.52, and d 5.82 ppm wererespectively attributed to the protons gþ h, j, i, and eþ f. In the PHF1H NMR spectrum, the resonance appeared as a singlet signal at7.47 ppm is related to the protons the furanoate ring protons,labelled as (a1). As can be seen, the latter are the most deshieldedprotons due to their highest p-deprotection. The outer CH2 groups(b), middle CH2 (d), and inner methylene groups (d) in 1,6-HDexhibited resonances at 4.65, 2.1, and 1.72 ppm, respectively. Inthe 1H NMR spectra of PHIsF copolyesters, all the proton signalsarose from the homopolymers PIsF and PHF can be readily distin-guished, hence confirming the incorporation of both these units

Scheme 2. Chemical structures of IsFIs, IsFH and HFH units in PHIsF copolyesters.


into the PHIsF macromolecular chains; the resonances at 1.73 and2.12 ppm are assigned to the (d) and (c) protons of the 1,6-HD unit,respectively. The chemical shifts at the 4.44e5.82 ppm range wereattributed to isosorbide protons. The peak corresponding to (g) and(h) protons of Is overlapped those of (b) of the 1,6-HD moiety ataround 4.54e4.61 ppm (4 protons for each fragment, 1,6-HD andIs).

As obviously shown in Fig. 1, the resonances assigned to theidentified proton signals of isosorbide tented to increase withincreasing the Is diol content into the macromolecular chains of theprepared copolyesters. As presented in Fig. 1B, the resonancessignals of the furanic ring protons were split into three differentpeaks, this is due to the three possible environments for furanmoiety: the hetero moiety, Is.H, and the homo moieties, Is.Is andH.H, where Is and H represent the isosorbide and 1,6-HD units,respectively. A new central signal, which is not detected in the 1HNMR data of the corresponding homopolyesters, was divided into amultiplet. This peak, appeared at 7.42e7.49 ppm for all copolyestersamples, was assigned to the CH of furan unit (a4þa5) owned by theIs.H fragment, in which its intensity depicts the amount of theheterolinkage. The detection of the hetero unit confirms thattransesterification reaction between the two bishydroxyalkylene-2,5-furan carboxylate monomers (BHHF and BHIsF) occurred in thesynthesis of the PHIsF copolyesters.

The IsFH, IsFIs and HFH sequence distribution in PHIsF could bedefined from the 1H NMR spectra of the aromatic protons in furanmoiety (Fig. 1B). The sensitivity of the latter to the different con-nected ester bond obviously effects on their chemical shifts. Thearomatic protons of the furan ring of PHIsF were split into foursignals in the 7.42e7.49 ppm chemical shift, which are assigned tofive different types of aromatic protons (a1, a2þa3 and a4þa5). Theintensity of their proton peaks strongly depends on the 1,6-HD andIs molar content in PHIsF. The detailed attribution of the differentrepeating units into copolyester's backbone is depicted in Scheme2.

The degree of randomness (R) of the PHIsF copolyesters wascalculated using the following Equations (1)e(6):

R¼ PH.Isþ PIs.H (1)

PHIs¼ [(!H.Is þ !Is.H)/2] / [(!H.Is þ !Is.H)/2 þ !H$H]¼1/LnH (2)

PIsH¼ [(!Is.H þ !H.Is)/2] / [(!Is.Hþ!H.Is)/2 þ !Is.Is]¼1/LnIs (3)

!Is.Is¼ (Ia2 þ Ia3) / (Ia1 þ Ia2 þ Ia3 þ Ia4 þ Ia5) (4)

!H.Is¼ (Ia4 þ Ia5) / (Ia1 þ Ia2 þ Ia3 þ Ia4 þ Ia5) (5)

!H$H¼ Ia1 / (Ia1 þ Ia2 þ Ia3 þ Ia4 þ Ia5) (6)

Where PIs.H and PH.Is refer to the probability of finding hetero-linkage unit: an isosorbide (Is) unit next to a hexanediol (H) unitand the probability of finding an H unit next to a Is unit respectively,while LnH and LnIs are the block length, the so-called numberaverage sequence of the repeating units, determined according toYamadera and Muand by Equations (2) and (3). Furthermore, !H.Is,!Is.Is and !H$H stand for the triads fractions of IsFH, IsFIs and HFHand determined from the integral intensities of the resonancesignals using Equations (4)e(6), wherein the integration of shift Iwas labelled as Ii. The corresponding results were gathered inTable 2. According to the Bernoullian statistics, the degree ofrandomness (R) values were close to 1 for the prepared PHIsFsamples having Is feed content ranging from 50% to 90%, suggestingthat random copolymers were the preferred structures.

In contrast, the prepared copolymers with a Is molar ratiooscillating between 3% and 40%, respectively, revealed R values of0.70 and 0.82, suggesting that a complete randomization has notbeen reached. Bernoullian statistics [80] declare that for randomcopolymers, R is⩽ 1, while for block copolyesters, R is close to 0;and for alternating copolyester, R is equal to 2. As can be obviouslyseen, R values of the PHIsF copolyesters constantly and graduallyincreased from 0.70 to 1.08 with increasing of isosorbide feedcontent from 3% to 90%. This result exhibited that neat polyester(PIsF) during polyesterification step is much easier to undergotransesterification than the neat PHF.

The chemical structure of PHIsF copolyesters was furtherassessed by FT-IR spectroscopy. The obtained data, used as sup-portive and complementary evidence for the 1H NMR results, isdepicted in Fig. 2. Two new characteristic absorption bandsassigned to the C]O stretching modes (v C]O) and (v CeOeC) ofthe ester moiety are observed at 1720-1730 cm�1 and 1285 cm�1,respectively. Two absorption bands arising from the CeH sym-metrical and asymmetrical stretching modes of both (CH2) groupsof isosorbide and 1,6-HD fragments are detected at 2840 and2925 cm�1. A further characteristic band related to these moietieshas been revealed at 1455 cm�1 in the fingerprint region. In addi-tion, near 3125 and 3175 cm�1, there are twoweak bands attributedto the¼ CeH stretching vibration of furan moieties (v¼ CeH) andtypical bands at 1580 and 1540 cm�1 related to the double bond ofthe furan fragment were observed for all PHIsF samples. Thepresence of a broad detectable band in the 3300-3550 cm�1 range,

Table 2Experimentala Sequence Distribution and Randomness in PHIsF copolyesters.

Copolyesters samples Triads (mol %)b Probability Block length Degree of Randomness (R)

!H$H !H.Is !Is.Is PH.Is PIs.H LnH LnIs

PHF - - - - - - - -PHIsF 97/3 95.3 10.2 6.7 0.096 0.603 10.41 1.65 0.700PHIsF 95/5 91.7 12.3 7.8 0.118 0.611 8.47 1.63 0.729PHIsF 93/7 87.9 13.8 9.1 0.135 0.602 7.40 1.66 0.737PHIsF 90/10 84.4 15.9 11.4 0.158 0.582 6.32 1.71 0.740PHIsF 80/20 72.1 17.8 13.9 0.198 0.562 5.05 1.77 0.760PHIsF 70/30 61.2 19.5 16.4 0.241 0.543 4.14 1.84 0.784PHIsF 60/40 55.1 23.4 21.4 0.298 0.522 3.35 1.91 0.820PHIsF 50/50 47 27 26 0.364 0.509 2.74 1.96 0.873PHIsF 40/60 34 31 35 0.476 0.469 2.10 2.13 0.945PHIsF 30/70 25 32 43 0.561 0.426 1.78 2.34 0.987PDIsF 20/80 18 33 49 0.647 0.402 1.54 2.48 1.049PCIsF 10/90 11 31 58 0.738 0.348 1.35 2.87 1.086PIsF - - - - - - - -

a Experimental values were calculated from integrations of 1H NMR peaks.b Triad contents relative to the total of sequences centered at the Isosorbide and 1,6-HD units.

Fig. 2. FTIR spectra of prepared PHIsF copolyesters.


for copolymers samples having high Is molar content compositions,originating from the stretching vibration of a hydroxyl functionality(-OH) corroborates the reasonable amount of hydroxyl terminalgroups. This thus results in obtaining low molecular weight co-polymers that was proved by the obtained IV values listed inTable 1. To sum up, 1H NMR and FT-IR data confirmed that the PHIsFcopolyesters were successfully prepared.

Fig. 3. DSC thermograms of PHIsF copolyesters with different compositions: a) Secondheating scans of the purified samples, and b) second cooling scans (by 10 �C.min�1).

3.3. Thermal properties of PHIsF copolyesters

Differential scanning calorimetry (DSC) was used to thoroughlyinvestigate the thermal behaviour of the prepared PHIsF co-polymers series. The DSC thermograms of the purified copolyesterssamples, obtained from the second heating and cooling scans, aredepicted in Fig. 3. The related thermal transition parameters,including the glass transition temperature (Tg), melting and crys-tallization temperatures (Tm and Tc) with their correspondingnormalized enthalpies values (DHm and DHc), as well as the coldcrystallization temperature (Tcc), are all compiled in Table 3. Ac-cording to previous studies [53,54,57,58], Tg of the neat PHF

Table 3Thermal properties of the purified copolyesters PHIsF with different compositions.

PHIsF samples Td,5% Td,10% Td, max R 500�C (%) Tg (�C) Tcc (�C) Tca (�C) DHca (J/g) DHma (J/g) Tma (�C)

PHF 339.7 377.3 398.5 3 7 - 112.8 69.9 88.3 146.1PHIsF 97/03 365.5 384.1 406.4 3.7 10 - 111.2 73.8 70.2 145.1PHIsF 95/05 359.2 384.3 408.8 4 13 - 110.4 71.2 65.8 144.9PHIsF 93/07 314.4 375.9 408.1 3 17 - 108.5 69.1 61.4 141PHIsF 90/10 368.5 375.5 396.5 6.8 20 92.4 - - 36.8 132.3PHIsF 80/20 370.1 378.8 404.2 8.5 27 - - - - -PHIsF 70/30 370.3 379.9 406.1 8.9 34 - - - - -PHIsF 60/40 370.5 380.1 403.5 9 49 - - - - -PHIsF 50/50 369.8 380.9 404.5 12.4 58 - - - - -PHIsF 40/60 379.3 386.2 408.4 13 85 - - - - -PHIsF 30/70 354.4 380.3 411.1 8.1 101 - - - - -PHIsF 20/80 349.3 379.5 416.3 11.2 114 - - - - -PHIsF 10/90 373.5 390.2 421.6 14.8 135 - - - - -PIsF 354.3 382 421 6.7 157 - - - -

a These values refer to second calorimetric scan. Controlled cooling rate scan is 10 �C.min�1.

Fig. 4. Theoretical predictions and experimental glass transition temperatures (Tg) ofthe PHIsF copolyesters versus IS content.


determined to be in the range of 7e28.1 �C and it crystallizes fromits melt at Tc value oscillating from 81 to 95.2 �C. These differencesmainly arise from the different molecular weights of the resultingmaterial. Because of its rapid melt crystallization, it is obvious thatTg value of PHF, reported in the literature, is overestimated to someextent. The semicrystalline furanoate PHF polyester preparedherein exhibits a melting point (Tm) appearing at around 146.1 �Cand a Tg of 7 �C. In contrast, the DSC traces of PIsF homopolyesterconfirms its wholly amorphous nature by the presence of a veryhigh glass transition close to 157 �C. It was found that with theincreasing molar content of Is fraction from 3% to 10%, the crys-tallizability of these copolyesters decreased continuously andgradually, exhibiting thereby a drop in both melting point and thecorresponding enthalpy of fusion, respectively, from 145.1 to132.3 �C and from 70.2 to 36.8 J/g. This is understandable owing tothe disturbing effect of the bulky and asymmetric isosorbidestructure on crystal packing of PHF homopolyester, which henceleads to chain regularity destruction of the prepared PHIsF samplesand thus to a poor crystallizability. Due to its fast crystallizationrate, only the PHIsF 90/10 sample could crystallize upon heatingexhibiting a noticeable cold crystallization peak (Tcc) at 92.4 �C.Results (Fig. 3b) showed that PHIsF containing very low Is fractionpossess remarkable exothermic crystallization values (Tc) withenthalpies (DHc) following both the same trend of Tm and DHmdiscussed above, that is, respectively decreasing from 111.2 to108.5 �C and from 73.8 to 69.1 J/g, with isosorbide molar contentincrease from 3 to 7%. This finding indicates the fairly good crys-tallization ability of these copolymers from the melt. For copo-lyesters containing isosorbide molar amount ranging from 20 to90%, neither melting/crystallization phenomena nor cold crystalli-zation can be respectively detected during heating/cooling scans,hence revealing a completely amorphous nature of all PHIsF sam-ples. This is a consequence of the replacement of the highly flexiblehexamethylene moiety by the much more rigid isosorbide frag-ment, resulting in an obvious increase in Tg. All the PHIsF copo-lyesters showed a single Tg, indicating a good compatibilitybetween HF and IsF segments in addition to a random distributionof the Is and 1,6-HD comonomers. As clearly shown in Fig. 3a, theobtained Tg values of the PHIsF samples steadily and significantlyincrease from 10 to 135 �C with Is molar content increase from 3 to90%. This firmly confirms the very high rigidity of Is moiety thatdramatically boosts the glass transition temperature of the pre-pared copolymers, suggesting hence a significant improvement inhigh temperature resistance.

To sum up, it is worth mentioning that the aforementioned DSCresults did not show only the profound influence of isosorbideinsertion on the glass-transition temperature increase of resulting

materials by reducing chain mobility of the end products, but itseffective and facile tuning has been also proved. In other words,results proved that isosorbidemoieties could be positively used as acontrol parameter to enhance the thermal properties of resultingpolymers. This unique feature of Is opens the doors towards thesynthesis of high-performance polymers from fully renewable re-sources, offering thus a wide range of potential practicalapplications.

As clearly shown in Fig. 4, the benefit of incorporating rigidisosorbide moieties into PHF homopolyester can be reflected byalmost linear dramatic increase of the glass transition temperatures(Tg). Several equations have been proposed to describe the Tg-composition dependence of miscible polymer blends. In theequation of Fox [81] (7):

1Tg

¼w1Tg1

þ w2Tg2

(7)

w1 and w2 are the weight fractions of the comonomers and Tg1 andTg2 the glass transition temperatures of the respective homopoly-mers. As can be seen in Fig. 4, the Fox equation results in somehowhigher values Tg values compared to the experimental ones.


3.4. Isothermal crystallization and subsequent melting

Isothermal crystallizations for the neat PHF and the PHIsF 97/3,95/5 and 93/7 were performed at temperatures in the range110e135 �C. The relative degree of crystallinity can be obtained if itis assumed that the evolution of crystallinity is linearly propor-tional to the heat released during the isothermal crystallization:

XðtÞ ¼

ðt0ðdHc=dtÞdtð∞

0ðdHc=dtÞdt

(8)

where dHc denotes the measured enthalpy of crystallization duringan infinitesimal time interval dt. The limits t and∞ on the integralsdenote the elapsed time during the crystallization and at the end ofthe crystallization process, respectively. Fig. 5a shows the crystal-lization exothermic curves for the PHIsF 95/5 and Fig. 5b shows theevolution of the relative degree of crystallinity for the samecopolymer.

The most common macrokinetic theory which is applied onisothermal crystallization is that of Avrami [82]. The relative degreeof crystallinity X(t) in relation to the crystallization time can begiven by:

XðtÞ¼1� expðktnÞ

where n is the Avrami exponent which is related to the nucleationprocess and k is the growth function which is dependent to thenucleation and crystal growth [83]. The values of n, k can becalculated from the fitting of the experimental data using thedouble logarithmic form of the above equation:

logf� ln½1�XðtÞ�g¼ log kþ n log t (9)Plots of log[-ln(1-X(t))] versus logt were constructed for the

isothermal crystallization of PHIsF copolymers and neat PHF atdifferent temperatures. Fig. 6a exemplifies the Avrami treatmentfor the PHIsF 95/5 in the interval 5e90% relative degree of crysta-linity. It can be seen that the Avrami plots can be fitted by straightlines, indicating primary crystallization for all degrees of conver-sion and from the slope and the intersect of the plots, the values ofn and k can be calculated. The results for all the studied samples arepresented in Table 4. The values of the Avrami exponent n are found

Fig. 5. Isothermal crystallization peaks for PHIsF 95/05, and evolution of the rela

between 1.88 and 2.6, while the growth parameters k decreasewithincreasing temperature, since they represent the crystallizationrates which are significantly slower at elevated temperatures. Thedependence of the crystallization half-time, on the crystallizationtemperature for the studied samples is shown in Fig. 6b. The in-verse of crystallization half times can be used as a relative measureof the overall crystallization rate. As was expected, the crystalliza-tion half times increase with temperature that is the crystallizationrates decrease with increasing crystallization temperatures.

3.4.1. Multiple melting behaviorThe copolymers, like PHF, showed multiple melting behavior

(Fig. 7). Multiple melting behaviors have been reported for manypolymers and especially for thermoplastic polyesters [84,85]. Forthe interpretation of multiple melting different theories have beenproposed. The most acceptedmodels are that involving the meltingof crystals of different stability (dual morphology mechanism) andthat referring to the melting, re-crystallization, re-melting process(reorganization mechanism) [86].

For the copolymers of this work triple melting behaviors wereobserved during heating after isothermal crystallization at lowtemperatures, while the number of the melting peaks was reducedto two and finally to one with increasing the crystallization tem-perature (Fig. 7). This behavior was attributed to the partial meltingrecrystallization and final melting of the recrystallized material.

3.5. Wide angle X-ray diffraction patterns (WAXD) of PHIsFcopolymers

In order to probe and to make clear how the rigid bicyclic Ismoiety affect the crystal structure of the PHIsF copolymers, wide-angle X-ray diffraction analysis (WAXD) was used. The detailedWAXD patterns of the samples under investigation are presented inFig. 8. As previously evidenced in different work of Juang et al. [53],the neat semicrystalline PHF polyester showed two main sharpsignals at 24.31� and 16.74�, and a less intense diffraction peak at13.00�, suggesting its high degree of macromolecular order. Thisresult is in full accordance with what was found in the presentwork. PIsF homopolymer didn't display any diffraction peaks,confirming its amorphous nature by the presence of a halo at2q¼ 17.57� in its diffractogram. This is mainly associated with theasymmetric exo and endo configuration of the two-hydroxyl func-tions of Is unit that readily hinder the special molecule

tive degree of crystallinity with time at various crystallization temperatures.

Fig. 6. Avrami plots for PHIsF 95/5 and crystallization half times vs temperature for PHIsF copolymers compared to neat PHF.

Table 4Results of the Avrami analysis for the isothermal crystallization of PHF.

Sample Tc (oC) logk k n t1/2

PHF 115 0.63731 4.33820 2.273 0.45120 �0.12056 0.75760 2.231 0.96125 �0.83186 0.14728 2.049 2.13130 �1.84638 0.01424 2.063 6.57135 �2.70026 0.00199 2.082 16.62

PHIsF 97/3 115 0.33424 2.15894 2.482 0.63120 �0.68375 0.20713 2.479 1.63125 �1.82328 0.01502 2.571 4.44130 �3.05288 0.00088 2.522 14.04135 �4.14637 0.00007 2.557 36.25

PHIsF 95/5 115 0.06169 1.15263 2.187 0.79120 �0.86856 0.13534 2.418 1.96125 �1.72206 0.01896 2.173 5.23130 �3.0834 0.00082 2.542 14.13135 �4.86658 0.00001 2.807 47.53

PHIsF 93/7 110 0.81818 6.5793 2.531 0.41115 �0.43211 0.36973 2.044 1.36120 �1.34252 0.04544 2.004 3.00125 �2.12372 0.00752 1.882 11.06130 �4.27050 0.00005 2.601 38.08

Fig. 7. DSC heating scans after isothermal crystallization at the


arrangement. Diffractograms of the copolyesters containing low Iscontent (�10mol %) highly resemble that of PHF homopolymer byexhibiting very similar diffraction peak positions and shapes,denoting an adoption of similar crystal unit cells. These samplesshowed progressively decreased diffraction peaks intensity withincreasing of Is molar content from 3 to 10%, hence indicating acontinuous reduction of crystallinity. This outcome could bestrengthened by the reported decrease of melting and crystalliza-tion enthalpies presented in DSC section. It can be concluded fromthe latter that the insertion of small amounts of Is (3e10mol%) intoPHF homopolyester backbone apparently does not alter thediffraction patterns of the resulting copolymers but solely results ina reduction in their crystal size.

For the samples PHIsF 80/20, PHIsF 70/30 and PHIsF 60/40,detectable reflective peaks similar to those of neat PHF could bespotted, which highlighted the crystallization of these samples.Nevertheless, no melting or crystallization peak was observed intheir DSC thermograms. This result pointed out the extremely lowcrystallinity degree of these materials that can't be detected in DSCscans using the given heating and cooling rate. Similar finding wasrecently reported for an isosorbide-derived aliphatic copolyesters

indicated temperatures for: PHIsF 97/03, and PHIsF 93/07.

Fig. 8. WAXD patterns of the PHIsF copolyesters.


series [78]. For PHIsF with ⩾ 50mol% Is content, the samples turn tofully amorphous materials, confirming that the chain regularitywas strongly undermined and thus the total absence of crystallinitycapacity of the prepared copolymers. This outcome is mainlyascribed to the predominantly incorporating of the sugar-derived Iscomonomer into the amorphous domains. The WAXD results are inline with the above-mentioned DSC findings.

Fig. 9. TGA thermograms of the pure copolyesters (PHIsF), a) remaining mass and b)first derivative of mass loss.

3.6. Thermal degradation of copolymers

It is obviously crucial to evaluate the thermal degradation be-haviors and stability of polymers for determining their potentialapplication. For that, thermogravimetric analysis was performed ina nitrogen atmosphere to assess the thermal stability of PHIsFsamples. Their TGA curves as well as the derivative traces (DTG)measured from 30 to 600 �C are depicted in Fig. 9. Decompositiontemperatures at 5% and 10% weight loss (Td,5% and Td,10%),maximum degradation temperature (Td,max), and the residueweight percentage at 500 �C are listed in Table 3. On the basis ofthese data, it is found that most of PHIsF copolyesters samples,except PHIsF 93/07, revealed high thermal stability up to 360 �Cwith Td,5% appearing in the 349e379 �C range. The same samplesexhibited much higher Td,5% and Tmax than those of PHF homo-polyester, suggesting that the thermal stability of the latter hasbeen enhanced by incorporation of Is into its backbone. This isreasonable because the cyclic aliphatic chain is less vulnerable tothermal degradation than the straight aliphatic chain. Thedecomposition profiles of PHIsF copolymers essentially undergoone major single stage with Tmax values oscillating from 396.5 to421.6 �C, leaving approximately 3e14.8% of degradation residuesupon heating to 500 �C. It is worthy to note that PHIsF containinghigh Is contents showed high residual weight percentages. This ismainly associated with the fact that the bicyclic Is fragment pos-sesses much better carbon-forming properties compared withlinear aliphatic chain. To summarize, the above results highlightedthe benefit of isosorbide insertion into the macromolecular chainof polymers in improving their thermal properties and delayingthe onset of degradation (Td,5%). This is in full accordance withwhat was reported in the literature.

4. Conclusions

In this work, aromatic-aliphatic poly(hexamethylene-co-iso-sorbide-2,5-furan dicarboxylate) (PHIsF) copolyesters with a broadrange of glass transition temperatures were successfully synthe-sized for the first time by melt polycondensation of DMFD, Is, and1,6-HD. 1H NMR and FT-IR spectroscopy were useful to confirmtheir molecular structures and to determine their compositionsthat could be readily controlled by the feeding stoichiometry. Theprepared copolymers in the entire range of compositions exhibiteda great resistance to heat up to 360 �C and they are wholly amor-phous materials except for extremely low contents in Is. The mostoutstanding feature of prepared isosorbide-derived copolyesters istheir high Tg, which continuously increases with the Is molar ratioincrease to reach a value of 135 �C for a 90mol % composition.


Results attained here corroborated the determining role of the stiffIs segments insertion in overcoming the drawback of PHF homo-polyester (low Tg¼ 7 �C), which may restrict it for wide applica-tions. The overall conclusion is that such kind of copolyestersdeveloped herein, which combines both the sustainability andimproved thermal performance, is highly desired. This is under-standable owing to the current global challenge facing traditionalfossil fuel-based polymers. Hence, this study is a concrete contri-bution towards the development of fully biobased materials withhigh glass transition temperatures that have crucial importance formyriad practical applications, therefore building a greener mate-rials world in the future.

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Novel high Tg fully biobased poly(hexamethylene-co-isosorbide-2,5-furan dicarboxylate) copolyesters: Synergistic effect of ...1. Introduction2. Experimental2.1. Materials2.2. Copolyester synthesis2.3. Copolyesters characterization2.3.1. Intrinsic viscosity measurement2.3.2. End group analysis2.3.3. Wide angle X-Ray diffraction patterns (WAXD)2.3.4. Nuclear magnetic resonance (NMR)2.3.5. Thermogravimetric analysis (TGA)2.3.6. Differential scanning calorimetry (DSC)2.3.7. Fourier transformed-infrared spectroscopy (FTIR)

3. Results and discussion3.1. Synthesis of PHIsF3.2. Structural characterization and composition of PHIsF3.3. Thermal properties of PHIsF copolyesters3.4. Isothermal crystallization and subsequent melting3.4.1. Multiple melting behavior

3.5. Wide angle X-ray diffraction patterns (WAXD) of PHIsF copolymers3.6. Thermal degradation of copolymers

4. ConclusionsReferences

polymer degradation and stability...copolyester synthesis as presented in scheme 1, phisf...

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