Isothermal Crystallization Kinetics of Poly (Ethylene Terephthalate

applied sciences

Article Isothermal Crystallization Kinetics of Poly(ethylene terephthalate) Copolymerized with Various Amounts of Isosorbide

Nicolas Descamps 1, Florian Fernandez 1,2, Pierre Heijboer 3, René Saint-Loup 4 and Nicolas Jacquel 4,*

1 Functional Properties Analysis Department, Roquette Frères, Rue de la Haute Loge, 62080 Lestrem, France 2 École nationale supérieure de chimie de Lille, Avenue Mendeleiev, 59652 Villeneuve-d’Ascq, France 3 Analytical Research Department, Roquette Frères, Rue de la Haute Loge, 62080 Lestrem, France 4 Performance Materials and Cosmetic, Roquette Frères, Rue de la Haute Loge, 62080 Lestrem, France * Correspondence: [email protected]

Received: 19 December 2019; Accepted: 25 January 2020; Published: 5 February 2020

Abstract: Poly(ethylene-co-isosorbide terephthalate) (PEIT) copolyesters could be used in various applications depending on their ability to crystallize. Moreover, the possibility to carry out solid-state post-condensation (SSP) is conditioned by its ability to suﬃciently crystallize. The present study, thus, gives a systematic investigation of isothermal crystallization of these statistical copolyesters with isosorbide contents ranging from 4.8 to 20.8 mol.%. For each copolyester composition, the lowest isothermal half crystallization times and the highest Avrami constant (K) were obtained around 170 ◦C. Over the range of composition that was studied, both melting points and melting enthalpies decreased with increasing amounts of isosorbide (from 250 to 207 ◦C and from 55 to 28 J/g, respectively). On the contrary, half crystallization time displayed an exponential increase when increasing isosorbide contents in the studied range. Finally, structural and thermal analysis of PIT homopolyester are reported for the ﬁrst time, showing that only ET moieties crystallized when PEIT was subjected to isothermal crystallization at 170 ◦C.

Keywords: crystallization; kinetics; poly(ethylene-co-isosorbide terephthalate); isosorbide; copolyester

1. Introduction In recent years, several bio-based monomers were studied for the improvement of polycondensate properties or for the replacement of petrochemical monomers. The 1,4:3,6-dianhydrohexitols (DAHs), namely isomannide, isosorbide, and isoidide are produced in two steps: hydrogenation of glucose and dehydration of the corresponding hexitol. Of the three known isohexide isomers, only dianhydro-1,4:3,6-d-sorbitol (DAS) or isosorbide is currently produced with sufficient purity for polymerization on an industrial scale. Due to their rigidity and their harmlessness, these molecules are gaining more interest for their use as building blocks in various polymers in the field of thermoplastic materials and for curable resin applications [1,2]. As a diol, isosorbide found its place as a monomer suitable for polycondensate synthesis like polyesters, polycarbonates, and thermoplastic polyurethanes [3]. In polycarbonate, isosorbide not only allows the substitution of toxic bisphenol A (BPA), but offers a significant increase in the mechanical strength, heat resistance, UV resistance, and optical properties, with resulting properties between common BPA polycarbonate and PMMA. In polyesters, isosorbide was copolymerized in polymers such as poly(ethylene terephthalate) (PET) [4–6], poly(trimethylene terephthalate) (PTT) [7], poly(butylene terephthalate) (PBT) [8,9], and poly(cyclohexanedimethylene terephthalate) (PCT) [10] for its ability to increase the heat resistance

Appl. Sci. 2020, 10, 1046; doi:10.3390/app10031046 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 14

45 In polyesters, isosorbide was copolymerized in polymers such as poly(ethylene terephthalate) 46 (PET)Appl. Sci. [4–6],2020 ,poly(trimethylene10, 1046 terephthalate) (PTT) [7], poly(butylene terephthalate) (PBT) [8,9],2 and of 14 47 poly(cyclohexanedimethylene terephthalate) (PCT) [10] for its ability to increase the heat resistance 48 of the polymer. As an example, in the case of PET, isosorbide allows for an outstanding increase in 49 Tg.of theIn the polymer. 1990s, AsStorbeck an example, et al. [11] in the demonstrated case of PET, th isosorbideat the Tg allowsfollowed for a an linear outstanding increase increase with a slope in Tg. 50 ofIn 1.2 the °C 1990s, per 1 Storbeck mol.% of et isos al.orbide [11] demonstrated incorporated that in the the subs Tg followedtitution of a ethylene linear increase glycol. with a slope of 51 1.2 ◦CDepending per 1 mol.% on ofthe isosorbide amount of incorporated isosorbide introduced in the substitution in the polymer, of ethylene different glycol. final applications 52 can beDepending targeted. As on thean example amount offor isosorbide the lower introduced amounts of in isosorbide, the polymer, the di ffsemi-crystallineerent final applications properties can 53 ofbe the targeted. polymer As can an examplebe preserved, for the allowing lower amounts its modi offication isosorbide, via solid-state the semi-crystalline post-condensation properties (SSP), of the 54 followedpolymer canby its be use preserved, in injection allowing stretch its blow modification molding via(ISBM) solid-state of a bottle post-condensation or the melt spinning (SSP), of followed fibers. 55 Inby the its final use in shaped injection objects, stretch the blow gain molding in thermal (ISBM) stabil ofity a bottledue to or the the addition melt spinning of isosorbide of fibers. extends In the their final 56 rangeshaped of objects,applicability the gain (i.e., in hot thermal filling stability of bottles due or to making the addition heat-resistant of isosorbide fibers). extends When theirisosorbide range is of 57 usedapplicability in higher (i.e., proportions hot filling (>20 of bottlesmol.%), or it makinginduces heat-resistantlarger variations fibers). in the When crystallization isosorbide behavior is used of in 58 PET,higher thus proportions resulting ( >in20 amorphous mol.%), it induces copolymers larger variationswith a Tg in above the crystallization 100 °C. These behavior fully amorphous of PET, thus 59 polymersresulting incan amorphous then compete copolymers with transparent with a Tg above high 100-Tg◦ C.polymers These fully such amorphous as polycarbonates polymers can(PC) then or 60 poly(methylcompete with methacrylate) transparent high-Tg (PMMA). polymers such as polycarbonates (PC) or poly(methyl methacrylate) 61 (PMMA).The aim of this paper is to provide a deeper study of the crystallization kinetics of various PEITs 62 with isosorbideThe aim of molar this paper contents is to (0% provide to 20%) a deeper in a larg studye temperature of the crystallization range from kinetics 130 °C to of 190 various °C. These PEITs 63 datawith are isosorbide particularly molar of contents interest (0% in order to 20%) to inprec a largeisely temperatureidentify in which range fromrange 130 of ◦isosorbideC to 190 ◦ C.content These 64 coulddata are poly(ethylene- particularlyco of-isosorbide interest in order terephthalate) to precisely be identify used in insemi-crystalline which range of applications. isosorbide content could poly(ethylene-co-isosorbide terephthalate) be used in semi-crystalline applications. 65 2. Experimental Section 2. Experimental Section 66 2.1. Materials 2.1. Materials 67 Terephthalic acid (99%+) was purchased from Acros organics ( Geel, Belgium ), ethylene glycol Terephthalic acid (99%+) was purchased from Acros organics (Geel, Belgium), ethylene glycol 68 (99.8%) was purchased from Aldrich (Saint-Louis, MO, USA), and high-purity Polysorb® isosorbide (99.8%) was purchased from Aldrich (Saint-Louis, MO, USA), and high-purity Polysorb®isosorbide 69 (>99.5%) was obtained from Roquette (, Lestrem, France) was used. Each of these chemicals was used (>99.5%) was obtained from Roquette (Lestrem, France) was used. Each of these chemicals was used 70 as received without further purification. as received without further purification. 71 2.2. General Procedure for PEIT (and PET) Synthesis 2.2. General Procedure for PEIT (and PET) Synthesis 72 Polymer samples were synthesized via a two-step, melt polycondensation reaction Polymer samples were synthesized via a two-step, melt polycondensation reaction (esterification 73 (esterification and transesterification, see Figure 1) in a 7.5-L stainless-steel batch reactor equipped and transesterification, see Figure1) in a 7.5-L stainless-steel batch reactor equipped with a heating 74 with a heating system, a mechanical stirrer with torque measurement, a distillation column, a vacuum system, a mechanical stirrer with torque measurement, a distillation column, a vacuum line, and a 75 line, and a nitrogen-gas inlet. nitrogen-gas inlet.

76 FigureFigure 1. 1.General General scheme of of poly(ethylene- poly(ethylene-coco-isosorbide-isosorbide terephthalate terephthalate (PEIT) (PEIT) synthesis synthesis from from 77 terephthalic acid, isosorbide,terephthalic and ethylene acid, glycol.isosorbide, and ethylene glycol.

78 InIn the the ﬁrstfirst step,step, thethe reactor reactor was was charged charged with with 2656 2656 g (16 g moles)(16 moles) of terephthalic of terephthalic acid andacid 19.2 and moles 19.2 79 molesof diol of (comprising diol (comprising isosorbide isosorbide and ethylene and ethylene glycol). glycol). Other Other additives additives such as such anti-diethylene as anti-diethylene glycol, 80 glycol,antioxidants, antioxidants, and catalysts and catalysts were also were fed also into thefed paste.into the To paste. exclude To residual excludeoxygen residual from oxygen the isosorbide from the crystals as much as possible, four vacuum-nitrogen cycles were carried out between 60 and 80 ◦C. The reaction mixture was then heated up to 260 ◦C under a nitrogen pressure of 5.0 bar (270 ◦C and Appl. Sci. 2020, 10, 1046 3 of 14

6.6 bar for PET) and stirred at a constant rate (150 rpm). The esteriﬁcation rate was estimated from the quantity of distillate collected. In the second step, the pressure was reduced in 90 min to 0.7 mbar, and the temperature was gradually increased to 270 ◦C (285 ◦C for PET). Low-pressure conditions were maintained for 90 min, and the viscosity variation of the polymer was measured from the torque applied on the stirrer. A polymer strand was then withdrawn from the bottom drain valve of the reactor and quenched in a water bath. PEIT pellets (15 mg) were obtained after granulation. Using such a procedure avoids contact of the warm polymer with oxygen, thus reducing coloration and thermo-oxidative degradation. For comparison purposes, PIT was synthetized using the same procedure.

3. Polymer Characterization

3.1. Reduced Viscosity

The reduced viscosity (ηred) of the polymers was determined using an automated Ubbelohde capillary at 35 ◦C. The polymer samples were dissolved in ortho-chlorophenol (Aldrich, Saint-Louis, MO, USA) at a concentration of C = 5 g/L at 135 ◦C. Then, the reduced viscosities were calculated using Equation (1), where t0 and ts refer to the neat solvent and the polymer/solvent solution ﬂow times, respectively. (ts t0) η = − (1) red t C 0· 3.2. 1H-NMR Analysis The 1H-NMR analysis of PEIT was carried out using a 400-MHz Bruker liquid-state NMR spectrometer (Billerica, MA, USA) equipped with a QNP probe. The polymer was dissolved in deuterated chloroform and triﬂuoroacetic acid (3:1 v/v) before analysis. All NMR measurements were recorded at 25 ◦C. The calculations of the isosorbide content were done according to method described by Bersot et al. [4].

3.3. 13C-NMR Analysis

13 The distribution of the repetitive units in PEIT was assessed by C-NMR spectrometry at 25 ◦C in a solution of CDCl3 + TFA-d1 (~95 mg of polymer sample in 0.6 mL of CDCl3 + 0.1 mL TFA-d1).

3.4. Differential Scanning Calorimetry (DSC) The differential scanning calorimetry (DSC, experiments were performed using a TA instruments Q20 calorimeter (New Castle, DE, USA). The instrument was calibrated for temperature and enthalpy using the melting temperature and the heat of fusion of indium (156.6 ◦C and 28.57 J/g). The analyses were carried out on typically 10 mg of material in aluminum pans of 40 µL. Polyesters may take up a little moisture during storage. Therefore, pans were not sealed to ensure that water adsorbed could evaporate to investigate the behavior of dry samples. The DSC oven was flushed with dry nitrogen gas (40 mL/min). In order to analyze and compare the influence of various thermal treatments on the samples, all calorimetric measurements were performed at a standard heating rate of 10 ◦C/min. The reference Tg of each sample was defined as the midpoint of the glass transition heat flow jump. The melting temperature was defined as the peak temperature of the melting endotherm. The crystallization kinetics of PEIT was measured from the melt to erase any thermal history of the sample. For that, a first heating ramp from room temperature to 20 ◦C above the end of the melting endotherm was done. This first scan was followed by an isotherm of five minutes at the temperature reached to ensure that all crystallites were melted and that remaining water was evaporated. The sample was then quickly equilibrated at the crystallization temperature to undergo the crystallization isotherm. According to the isosorbide content, two procedures were followed. Appl. Sci. 2020, 10, 1046 4 of 14

For isosorbide content lower than or equal to 15%, crystallization kinetics was quick enough so that the crystallization exotherm (Hcryst) could be measured directly on the isothermal signal of the DSC. The relative crystallization rate X can be expressed as follows:

R t dH t cryst/dt dt X(t) = 0 , (2) R tf dHcryst/dt dt t0 where t is the time and tf is the time for a complete crystallization. For isosorbide contents higher than 15%, crystallization kinetics was too long to get a precise enough isothermal signal. Therefore, after the crystallization isotherm, samples were cooled down below their Tg and heated up again at 10 ◦C/min up to their melting temperature. During this last step, the melting enthalpy (∆Hmelting) could be recorded. The relative crystallization rate X was calculated as follows: ∆Hmelting(t) X(t) = , (3) ∆Hmelting max where ∆Hmelting max is the maximum melting enthalpy recorded on the considered sample. In Equations (2) and (3), X(t) expresses the relative crystallinity of the sample.

3.5. Powder X-ray diffraction (PXRD) Powder X-ray diffraction experiments were performed on 2-mm-thick polyester sheets with a Brucker D8 Advance diffractometer (Billerica, MA, USA) using a copper anticathode. Prior to measurements, samples were placed at 170 ◦C for the appropriate time to get a complete crystallization.

4. Results Poly(ethylene-co-isosorbide terephthalate) samples with varying isosorbide contents were produced at the pilot scale by Roquette Frères. The isosorbide content is expressed as the molar percentage of ethylene glycol replaced by isosorbide. The characteristics of the studied samples are presented in Table1.

Table 1. Characteristics of studied samples. PEIxT corresponds here to a PEIT containing x% molar of isosorbide.

Average Sequence Isosorbide Reduced Triad Molar Fraction Degree of Coloration Samples Lengths Code in Polymer Viscosity Randomness (dL/g) ITI ITE + ETI ETE n ET n IT(R) L* a* b* PET 0 0.71 ------PEI T 4.9 0.65 0 9.4 90.6 20.3 1.0 1.05 51.8 0.2 1.9 4.9 − PEI6.5T 6.5 0.59 ------55.8 0.1 0.6 PEI T 9.8 0.66 3.2 17.7 79.1 9.9 1.4 0.84 52.1 0.1 1.4 9.8 − PEI T 10.9 0.59 ------57.5 0.2 1.3 10.9 − PEI T 17.1 0.61 4.2 26.7 69.0 6.2 1.3 0.92 52.8 0.2 0.6 17.1 − − PEI T 19.5 0.60 ------56.1 0.6 2.7 19.5 − PEI T 20.8 0.56 6.3 30.2 63.6 5.2 1.4 0.90 50.0 0.6 1.4 20.8 − PIT 100 0.21 ------

The results show that PEIT samples with isosorbide content from 4.9 to 20.8 mol.% were successfully synthetized. Moreover, based on the equation with the polymerization conditions used, the ﬁnal polymer displayed no coloration. Color parameters were evaluated, and L* was higher than 50, a* was between 0.6 and 0.2, and b* was between 1.9 and 2.7. These polymers also displayed reduced − − viscosities ranging from 0.56 to 0.66 dL/g, indicating that high-molar-mass polyesters were achieved. In this small range of reduced viscosity, it was considered that the molar mass of the polymer had a limited eﬀect on the crystallization kinetics of the polymer. For comparison purposes, a polyester Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 14

PIT 100 0.21 ------

149 The results show that PEIT samples with isosorbide content from 4.9 to 20.8 mol.% were 150 successfully synthetized. Moreover, based on the equation with the polymerization conditions used, 151 the final polymer displayed no coloration. Color parameters were evaluated, and L* was higher than 152 50, a* was between −0.6 and 0.2, and b* was between −1.9 and 2.7. These polymers also displayed 153 reduced viscosities ranging from 0.56 to 0.66 dL/g, indicating that high-molar-mass polyesters were 154 achieved. In this small range of reduced viscosity, it was considered that the molar mass of the 155 polymer had a limited effect on the crystallization kinetics of the polymer. For comparison purposes, 156 a polyester made from isosorbide and terephthalic acid only was synthetized. The product displayed 157 a reduced viscosity of 0.21 dL/g and a Tg of 140 °C. 158 In order to see the possible influence of the structure of the polymer on crystallization, 159 copolyester triads were further assessed by 13C-NMR analysis. Identification and quantification of 160 triads were done by analysis of quaternary carbons of terephthalic acid moieties located in the 132– 161 134 ppm region. Depending on the neighboring diols, these carbons presented variable chemical 162 shifts, as shown in Figure 2. 163 Thus, from the integration of these signals, it was possible to evaluate the average sequence 164 length of both ethylene terephthalate (ET) and isosorbide terephthalate (IT) moieties with the 165 following equations [12]: I I + 2 n = , (4) I 2 Appl. Sci. 2020, 10, 1046 5 of 14 I I + 2 n = . (5) made from isosorbide and terephthalic acid onlyI was synthetized. The product displayed a reduced 2 viscosity of 0.21 dL/g and a Tg of 140 ◦C. 166 Thus,In the order degree to see of the randomness possible influence R could of be the evaluated structure as of follows: the polymer on crystallization, copolyester 13 triads were further assessed by C-NMR analysis.1 1 Identification and quantification of triads were done by analysis of quaternary carbons ofR= terephthalic+ acid. moieties located in the 132–134 ppm(6) region. n n Depending on the neighboring diols, these carbons presented variable chemical shifts, as shown in 167 FigureR takes2. 2 as a value for an alternating copolymer, 1 for a random structure, <1 for a sequential 168 block structure, and 0 for a mixture of homopolymers [13].

ITE or ETI

ETE

ITI

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Figure 2. 13 169 Figure 2. 13C-NMRC-NMR chemical chemical shifts shifts of of the the different diﬀerent sequences sequences in in PEIT. PEIT. Thus, from the integration of these signals, it was possible to evaluate the average sequence 170 Results from this sequence analysis show that the ET sequence length decreased from 22.3 to 5.2, length of both ethylene terephthalate (ET) and isosorbide terephthalate (IT) moieties with the following 171 while the isosorbide content increased from 4.2 to 20.8 mol.%. equations [12]: + IITE+ETI 172 4.1. Isothermal crystallization kinetics IETE 2 nET = , (4) IITE+ETI 173 As detailed previously, the isothermal crystallization2 of PEIT with varying isosorbide contents 174 was studied by DSC with two different protocols. IITEFor+ETI low isosorbide contents (≤15% molar of IITI + 2 175 ethylene glycol replaced by isosorbide), thenIT crystal= lization kinetics. could be directly observed on the (5) IITE+ETI 176 isothermal thermogram (see Figure 3). For isosorbide2 content higher than 15%, the crystallization 177 process was too long to be precisely observed in the same way. Therefore, the crystallization kinetics 178 was followed by measuring the melting enthalpy of the crystallized material (see Figure 4). 179 Figure 3 presents DSC thermograms measured on amorphous PEI4.9T during isothermal 180 treatments at different temperatures. These isothermal treatments were obtained after the complete 181 melting of the material. It clearly appears on this figure that all curves presented an exothermic event 182 taking place over a varying period of time depending on the isothermal temperature. This event was 183 attributed to the crystallization of the PEI4.9T. It is to be noted that, for isothermal temperatures from 184 150 °C to 190 °C, the heat flow value recorded at the beginning of the thermogram was lower than 185 that recorded at the end. This particular behavior could be attributed to the beginning of the 186 crystallization process prior to the isothermal start. Actually, for those temperatures, the 187 crystallization started during the cooling stage from the melt. The amount of material that crystallized 188 before reaching the isothermal temperature could not be evaluated, but it was assumed to be small 189 with respect to the whole thermograms. It is, thus, neglected in the rest of this paper. 190 From Figure 3, it was possible to determine the crystallization progress at any time for the 191 studied temperatures. Appl. Sci. 2020, 10, 1046 6 of 14

Thus, the degree of randomness R could be evaluated as follows:

1 1 R = + . (6) nET nIT

R takes 2 as a value for an alternating copolymer, 1 for a random structure, <1 for a sequential block structure, and 0 for a mixture of homopolymers [13]. Results from this sequence analysis show that the ET sequence length decreased from 22.3 to 5.2, while the isosorbide content increased from 4.2 to 20.8 mol.%.

Isothermal crystallization kinetics As detailed previously, the isothermal crystallization of PEIT with varying isosorbide contents was studied by DSC with two diﬀerent protocols. For low isosorbide contents ( 15% molar of ethylene ≤ glycol replaced by isosorbide), the crystallization kinetics could be directly observed on the isothermal thermogram (see Figure3). For isosorbide content higher than 15%, the crystallization process was too long to be precisely observed in the same way. Therefore, the crystallization kinetics was followed by Appl.measuring Sci. 2020, the 10, x melting FOR PEER enthalpy REVIEW of the crystallized material (see Figure4). 7 of 14

192 Figure 3.3.Di Differentialﬀerential scanning scanning calorimetry calorimetry (DSC) signal(DSC) recordedsignal duringrecorded the isothermalduring the crystallization isothermal

193 crystallizationof PEI4.9T at diofﬀ erentPEI4.9 temperaturesT at different (fromtemperatures 130 ◦C to(from 200 ◦130C). The°C to color 200 code°C). The for temperaturescolor code for is 194 temperaturesembedded in is the embedded ﬁgure. in the figure.

195 Figure 4 presents DSC thermograms measured on PEI17.1T after different isothermal treatments 196 at 170 °C. The duration of the isothermal period varies from 60 minutes to 600 minutes. It can be 197 firstly observed on Figure 4 that a heat capacity (Cp) jump attributed to the glass transition of the 198 material appeared at around 95 °C. The amplitude of this Cp jump decreased as isothermal period 199 increased. 200 A second endothermic event appeared between 180 °C and 210 °C, with the size of this event 201 increasing as the isothermal period increased. It is to be noted that this event was itself composed of 202 two sub-events. This endotherm was attributed to the melting of the crystalline part of the material 203 obtained during the isothermal treatment. The opposite and concomitant evolution of both the 204 amplitude of the Cp jump and the enthalpy of the peak confirmed this hypothesis. The presence of 205 several peaks in the melting endotherm revealed that the melting of crystalline PEIT took place in 206 different steps. Such a behavior was already highlighted on many polymers like PET [14], and it was 207 attributed to the presence of several crystal types formed by primary or secondary crystallization. 208 In the present study, the global melting endotherm was considered to evaluate the crystallinity 209 of studied PEIT. Appl. Sci. 2020, 10, 1046 7 of 14 Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 14

210 Figure 4. DSCDSC signal signal recorded recorded upon upon heating heating PEI 17.1TT after after different diﬀerent crystallization crystallization isotherms isotherms at at 170 170 °C.◦C. 211 The duration of the isotherm isothermss is embedded in the ﬁgure.figure.

212 FromFigure Figures3 presents 3 and DSC 4, thermograms the evolution measured of the weight on amorphous fraction crystallinity PEI 4.9T during (Xt) isothermalcould be determined treatments 213 accordingat different to temperatures. Equations (2) These and isothermal(3), respectively. treatments The wereXt evolution obtained as after a function the complete of the melting isothermal of the 214 crystallizationmaterial. It clearly time appearsand temperature on this figure for PEI that4.9 allT and curves PEI17.1 presentedT is presented an exothermic in Figures event 5a,b. taking In Figures place 215 5c,d,over the a varying plot of periodLog(−ln(1 of time − Xt)) depending as a function on of the Log(t) isothermal is alsotemperature. given for the Thissame event experimental was attributed data. to the crystallization of the PEI4.9T. It is to be noted that, for isothermal temperatures from 150 ◦C to 190 ◦C, the heat flow value recorded at the beginning of the thermogram was lower than that recorded at the end. This particular behavior could be attributed to the beginning of the crystallization process prior to the isothermal start. Actually, for those temperatures, the crystallization started during the cooling stage from the melt. The amount of material that crystallized before reaching the isothermal temperature could not be evaluated, but it was assumed to be small with respect to the whole thermograms. It is, thus, neglected in the rest of this paper. From Figure3, it was possible to determine the crystallization progress at any time for the studied temperatures.

Figure4 presents DSC thermograms measured on PEI 17.1T after different isothermal treatments at 170 ◦C. The duration of( thea) isothermal period varies from 60 minutes to 600(b)minutes. It can be firstly observed on Figure4 that a heat capacity (Cp) jump attributed to the glass transition of the material appeared at around 95 ◦C. The amplitude of this Cp jump decreased as isothermal period increased. A second endothermic event appeared between 180 ◦C and 210 ◦C, with the size of this event increasing as the isothermal period increased. It is to be noted that this event was itself composed of two sub-events. This endotherm was attributed to the melting of the crystalline part of the material obtained during the isothermal treatment. The opposite and concomitant evolution of both the amplitude of the Cp jump and the enthalpy of the peak confirmed this hypothesis. The presence of several peaks in the melting endotherm revealed that the melting of crystalline PEIT took place in different steps. Such a behavior was already highlighted on many polymers like PET [14], and it was attributed to the presence of several crystal types formed by primary or secondary crystallization. In the present study,(c) the global melting endotherm was considered to( evaluated) the crystallinity of studied PEIT. 216 Figure 5. Weight fraction crystallinity Xt versus isothermal crystallization time t (a,b) and Log(−ln(1 − From Figures3 and4, the evolution of the weight fraction crystallinity (X t) could be determined 217 Xt)) versus Log(t) (c,d) for several crystallization temperatures. The two PEIT samples presented here according to Equations (2) and (3), respectively. The X evolution as a function of the isothermal 218 are PEI4.9T (a,c) and PEI17.1T (b,d). t crystallization time and temperature for PEI4.9T and PEI17.1T is presented in Figure5a,b. In Figure5c,d,

219 the plotFrom of Log(the plotsln(1 inX t))Figures as a function 5c,d, the of Log(t) crystallization is also given half for time the same t1/2, experimentalAvrami index data. n, and − − 220 crystallization rate constant K were obtained using the following Avrami equation: X(t) =1−e() , (7) Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 14

210 Figure 4. DSC signal recorded upon heating PEI17.1T after different crystallization isotherms at 170 °C. 211 The duration of the isotherms is embedded in the figure.

212 From Figures 3 and 4, the evolution of the weight fraction crystallinity (Xt) could be determined 213 according to Equations (2) and (3), respectively. The Xt evolution as a function of the isothermal

214 Appl.crystallization Sci. 2020, 10, time 1046 and temperature for PEI4.9T and PEI17.1T is presented in Figures 5a,b. In Figures8 of 14 215 5c,d, the plot of Log(−ln(1 − Xt)) as a function of Log(t) is also given for the same experimental data.

(a) (b)

216 Figure 5. Weight fraction crystallinity Xt versus isothermal crystallization time t (a,b) and Log(−ln(1 − Figure 5. Weight fraction crystallinity Xt versus isothermal crystallization time t (a,b) and Log( ln(1 217 Xt)) versus Log(t) (c,d) for several crystallization temperatures. The two PEIT samples presented here− − Xt)) versus Log(t) (c,d) for several crystallization temperatures. The two PEIT samples presented here 218 are PEI4.9T (a,c) and PEI17.1T (b,d). are PEI4.9T(a,c) and PEI17.1T(b,d).

219 From the plots in Figures 5c,d, the crystallization half time t1/2, Avrami index n, and From the plots in Figure5c,d, the crystallization half time t 1/2, Avrami index n, and crystallization 220 ratecrystallization constant K rate were constant obtained K were using obtained the following using the Avrami following equation: Avrami equation: X(t) =1−e() , (7) (K t)n X(t) = 1 e− × , (7) − 1 where X(t) is the relative crystallinity at time t, K is the crystallization rate constant (min− ), and n is the Avrami exponent linked to the geometry of crystals. This equation may also be expressed in the following form: log [ ln(1 X)]= nlog(t) + nlog(K). (8) − − This second form of the Avrami equation gives easy access to parameters n and K, as well as t1/2, corresponding to the half crystallization time. Those three essential parameters to describe the crystallization kinetics of polymers are gathered in Table2. Table2 gathers the Avrami parameters measured on PET and PEIT with varying isosorbide contents. Those parameters were obtained by fitting experimental crystallization kinetics with the Avrami equation (Equation (7)). Examples of such a fit are presented in Figure5c,d. It firstly appears from Table1 that the isosorbide content slowed down the crystallization kinetics of PEIT. The t1/2 was actually multiplied by 100 between PEI4.9T and PEI20.8T (t1/2 = 8 min and t1/2 = 813 min, respectively at 170 ◦C). In parallel, it is to be noted that the isosorbide content did not clearly have an impact on the value of parameter n. Actually, n always remained in the range 2–3. Higher values of parameter n were obtained for PEI20.8T at 150 ◦C and 180 ◦C (3.73 and 3.41, respectively). However, for those particular cases, crystallization kinetics were so long that a limited number of points were taken into account for Avrami equation fitting. This limited number of points was most probably at the origin of a less accurate fitting, leading to higher values of parameter n. More generally, n values shown in Table2 tended to show that the geometry of PET crystals was not modified by the introduction of isosorbide molecules in the structure of the polymer (for the isosorbide content range between 0 and 20 mol.%). Appl. Sci. 2020, 10, 1046 9 of 14

Table 2. Avrami parameters obtained on PET and PEIT with diﬀerent isosorbide contents for varying isothermal crystallization temperatures. Crystallization was performed on previously melted materials.

1 Samples Code T (◦C) K (min− ) n t1/2 (min) PET 110 0.03 2.08 27.7 115 0.07 2.07 10.8 120 0.15 2.12 5.7 130 0.47 2.01 1.8 140 1.11 1.93 0.7 217 0.24 2.43 3.6 220 0.19 2.74 4.7 224 0.12 2.62 7.5

PEI4.9T 150 0.071 2.65 12.2 160 0.09 2.46 9.2 170 0.11 2.43 8.1 180 0.11 2.66 8.3 190 0.076 2.51 11.3 200 0.043 2.5 20.2

PEI6.5T 150 0.041 2.76 21.4 160 0.062 2.58 13.9 170 0.07 2.52 12.3 180 0.063 2.62 13.9 190 0.039 2.61 22.1

PEI9.8T 140 0.0103 2.86 85.1 150 0.019 2.51 44.3 160 0.029 2.63 31.1 170 0.025 2.44 34.4 180 0.018 2.87 49.1

PEI10.9T 150 0.0081 2.72 107.8 160 0.012 2.26 69.6 170 0.013 2.66 67.3 180 0.01 2.26 82.1

PEI17.1T 140 0.00091 3.11 976.5 160 0.0025 2.76 348.7 170 0.0028 2.91 318.9 180 0.0019 2.31 455.5 190 0.00067 2.03 1247.5

PEI20.8T 150 0.00081 3.73 1122 160 0.00116 3.03 765 170 0.00109 2.99 813 180 0.00082 3.41 1094

Figure6 presents the evolution of t 1/2 values given in Table2 as a function of temperature for isothermal crystallization of PEIT with varying isosorbide contents. Results presented in Figure6 give a good overview of the impact of the isosorbide content on the crystallization kinetics of PEIT. It also clearly appears from Figure6 that the crystallization kinetics of PET and PEIT presented a maximum speed at around 170 ◦C. This temperature remained the same over the isosorbide range studied. This variation of the half crystallization time is well known for polymers. It is actually the result of the competition between the thermodynamic driving force and the mobility of the molecules. When decreasing the temperature from the melt, the first increases while the second decreases. On the one hand, between the melting temperature and the maximum crystallization speed temperature, the thermodynamic effect was predominant which was caused by an increase in crystallization speed. On the other hand, below 170 ◦C, the molecular mobility became so low that the overall crystallization speed was affected and decreased. Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 14

170 0.0028 2.91 318.9 180 0.0019 2.31 455.5 190 0.00067 2.03 1247.5

PEI20.8T 150 0.00081 3.73 1122 160 0.00116 3.03 765 170 0.00109 2.99 813 180 0.00082 3.41 1094

230 Table 2 gathers the Avrami parameters measured on PET and PEIT with varying isosorbide 231 contents. Those parameters were obtained by fitting experimental crystallization kinetics with the 232 Avrami equation (Equation (7)). Examples of such a fit are presented in Figures 5c,d. 233 It firstly appears from Table 1 that the isosorbide content slowed down the crystallization 234 kinetics of PEIT. The t1/2 was actually multiplied by 100 between PEI4.9T and PEI20.8T (t1/2 = 8 min and 235 t1/2 = 813 min, respectively at 170 °C). 236 In parallel, it is to be noted that the isosorbide content did not clearly have an impact on the 237 value of parameter n. Actually, n always remained in the range 2–3. Higher values of parameter n 238 were obtained for PEI20.8T at 150 °C and 180 °C (3.73 and 3.41, respectively). However, for those 239 particular cases, crystallization kinetics were so long that a limited number of points were taken into 240 account for Avrami equation fitting. This limited number of points was most probably at the origin 241 of a less accurate fitting, leading to higher values of parameter n. More generally, n values shown in 242 Table 2 tended to show that the geometry of PET crystals was not modified by the introduction of 243 isosorbide molecules in the structure of the polymer (for the isosorbide content range between 0 and 244 20 mol.%).

245 Appl. Sci.Figure2020, 106 ,presents 1046 the evolution of t1/2 values given in Table 2 as a function of temperature10 of for 14 246 isothermal crystallization of PEIT with varying isosorbide contents.

Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 14

249 Results presented in Figure 6 give a good overview of the impact of the isosorbide content on 250 the crystallization kinetics of PEIT. It also clearly appears from Figure 6 that the crystallization 251 kinetics of PET and PEIT presented a maximum speed at around 170 °C. This temperature remained 252 the same over the isosorbide range studied. This variation of the half crystallization time is well 253 known for polymers. It is actually the result of the competition between the thermodynamic driving 254 force and the mobility of the molecules. When decreasing the temperature from the melt, the first 255 increases while the second decreases. On the one hand, between the melting temperature and the 256 maximum crystallization speed temperature, the thermodynamic effect was predominant which was

257 causedFigure by 6.an Evolutionincrease ofin tcrystallization1/2 with temperature speed. for On isothermal the other crystallization hand, below of PEIT170 °C, with the varying molecular 258247 mobilityisosorbideFigure became 6. contents.Evolution so low ofthat t1/2 the with overall temperature crystallization for isothermal speed wascrystallization affected andof PEIT decreased. with varying 259248 Figureisosorbide 7 presents contents. the impact of the isosorbide molar content on the crystallization half time at Figure7 presents the impact of the isosorbide molar content on the crystallization half time at the 260 the maximum crystallization speed temperature (170 °C). This graph highlights that the increase in maximum crystallization speed temperature (170 ◦C). This graph highlights that the increase in the 261 the molar content of isosorbide induced an exponential increase of t1/2. The embedded equation with molar content of isosorbide induced an exponential increase of t1/2. The embedded equation with a 262 a regression coefficient R² almost equal to 1 confirmed this effect of isosorbide. regression coefficient R2 almost equal to 1 confirmed this effect of isosorbide.

263 FigureFigure 7.7.Evolution Evolution of of the the half half crystallization crystallization time time as aas function a function of the of isosorbidethe isosorbide molar molar content content of PEIT of 264 forPEIT isothermal for isothermal temperature temperature of 170 of◦C. 170 °C.

265 FigureFigure8 8shows shows the the e effectﬀect ofof thethe isosorbideisosorbide contentcontent onon thethe meltingmelting behaviorbehavior ofof PEITPEIT (melting(melting 266 temperaturetemperature andand meltingmelting enthalpy).enthalpy). ItIt actuallyactually appearsappears fromfrom FigureFigure8 8 that that the the increase increase in in isosorbide isosorbide 267 contentcontent ledled toto aa decreasedecrease inin bothboth meltingmelting temperaturetemperature andand meltingmelting enthalpy.enthalpy. InIn thethe isosorbideisosorbide contentcontent 268 rangerange studiedstudied (0%(0% toto 20%),20%), thethe meltingmelting temperaturetemperature decreaseddecreased byby moremore thanthan 4040 ◦°CC (from(from 249249 ◦°CC toto 269 207207◦ °C).C). InIn thethe meantime,meantime, thethe maximummaximum meltingmelting enthalpyenthalpy waswas reducedreduced byby aroundaround 50%50% (from(from 54.754.7 toto 270 28.428.4 JJ/g)./g). 271 The decrease in maximum melting enthalpy described the reducing capability of the material to 272 crystallize when adding isosorbide. As seen in Table 1, replacing ethylene glycol by isosorbide 273 induced a decrease in both the ET sequence length in the polymer and the percentage of those 274 sequences. At the same time, the molar fraction of ITI sequences increased but with an almost 275 constant average sequence length (between 1 and 1.4). As mentioned by Aoki et al. [15], the minimum 276 sequence length for possible crystallization is about five. This means that ITI sequences are too small 277 to crystallize. Therefore, the crystallinity of PEIT should only come from the ETE sequences. To 278 validate this hypothesis, X-ray diffraction analyses were carried out on all crystallized samples 279 prepared in this study. Pure PET and pure PIT were used as reference samples. Appl. Sci. 2020, 10, 1046 11 of 14 Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 14

Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 14

280 FigureFigure 8.8.Evolution Evolution of of the the melting melting temperature temperature and and melting melt enthalpying enthalpy as a function as a function of the molar of the content molar

281 ofcontent isosorbide. of isosorbide. Melting enthalpyMelting enthalpy was measured was meas afterured complete after complete crystallization crystallization at 170 ◦C. at 170 °C.

282 TheFirstly, decrease contrary in maximum to what is melting commonly enthalpy admitted described in the the literature reducing [1,16], capability PIT ofwas the found material to tobe 283 crystallizesubjected whento crystallization adding isosorbide. when As precipitated seen in Table in1, replacingcold methanol ethylene from glycol a bypolymer isosorbide solution induced in 284 achloroform. decrease in bothAs shown the ET sequencein Figure length9, this in polymer the polymer displayed and the a percentage multiple melting of those sequences.peak with Ata first the 285 sameendotherm time, the around molar 194 fraction °C (10.4 of ITIJ/g) sequences and a second increased at 241 but °C with(5.5 J/g). an almost The annealing constant of average that polymer sequence at 286 length200 °C (between for 7 h induced 1 and 1.4). changes As mentioned in the melting by Aoki profil et al.e which [15], the led minimum to modificati sequenceons of length the crystals. for possible Two 287 crystallizationmelting peaks isat about206 °C five. (9.4 ThisJ/g) and means 234 that°C (4.2 ITI J/ sequencesg) were recorded. are too small As anticipated, to crystallize. these Therefore, endotherms the 288280 crystallinitydisappearedFigure of 8.on PEITEvolution the shouldsecond of the onlyheating melting come scan temperature from due the to ETE andthe sequences. meltlowing ability enthalpy To of validate the as apolymer function this hypothesis, toof theorganize molar X-ray for 289281 dicrystallization.ffractioncontent analyses of Thisisosorbide. were sample carried Melting was, out thus,enthalpy on not all was crystallizedconsidered measured in samples after this complete thermal prepared crystallization crystallization in this study. at study.170 Pure °C. PET and pure PIT were used as reference samples. 282 Firstly,Firstly, contrary contrary to to what what is is commonly commonly admitted admitted in in the the literature literature [ 1[1,16],,16], PIT PIT was was found found to to be be 283 subjectedsubjected to crystallizationto crystallization when when precipitated precipitated in cold in methanol cold methanol from a polymer from a solution polymer in chloroform.solution in 284 Aschloroform. shown in Figure As shown9, this in polymer Figure displayed 9, this polymer a multiple displayed melting a peak multiple with amelting first endotherm peak with around a first 285 endotherm around 194 °C (10.4 J/g) and a second at 241 °C (5.5 J/g). The annealing of that polymer at 194 ◦C (10.4 J/g) and a second at 241 ◦C (5.5 J/g). The annealing of that polymer at 200 ◦C for 7 h induced 286 200 °C for 7 h induced changes in the melting profile which led to modifications of the crystals. Two changes in the melting profile which led to modifications of the crystals. Two melting peaks at 206 ◦C 287 melting peaks at 206 °C (9.4 J/g) and 234 °C (4.2 J/g) were recorded. As anticipated, these endotherms (9.4 J/g) and 234 ◦C (4.2 J/g) were recorded. As anticipated, these endotherms disappeared on the 288 seconddisappeared heating on scan the due second to the heating low ability scan of thedue polymer to the low to organize ability forof the crystallization. polymer to Thisorganize sample for 289 was,crystallization. thus, not considered This sample in this was, thermal thus, not crystallization considered study.in this thermal crystallization study.

290 Figure 9. DSC thermograms of PIT crystallized from solution and after annealing treatment of 7 h at 291 200 °C.

292 Figure 10 presents X-ray diffractograms obtained on semi-crystalline PET, PEI5T, PEI9.5T, and 293 PEI19.5T crystallized from melt, as well as that of PIT measured on a polymer crystallized from 294 solution.

Figure 9. DSC thermograms of PIT crystallized from solution and after annealing treatment of 7 h at

290 200Figure◦C. 9. DSC thermograms of PIT crystallized from solution and after annealing treatment of 7 h at 291 200 °C.

Figure 10 presents X-ray diﬀractograms obtained on semi-crystalline PET, PEI5T, PEI9.5T, and PEI19.5T crystallized from melt, as well as that of PIT measured on a polymer crystallized Appl.from Sci. solution. 2020, 10, x FOR PEER REVIEW 13 of 14

295 FigureFigure 10. 10. X-rayX-ray diffractograms diﬀractograms of of semi-crystalline semi-crystalline PET, PET, PI PIT,T, and and PEIT. PEIT. The The isosorbide isosorbide content content of of PEIT PEIT 296 samplessamples is is embedded embedded in in the the figure. ﬁgure.

297 ForFor PEIT, the peak intensity decreased with increasingincreasing isosorbideisosorbide levels. This This confirmed confirmed the 298 decreasedecrease inin thethe degree degree of of crystallinity crystallinity measured measured by theby meltingthe melting enthalpy enthalpy in DSC. in TheDSC. di ffTheraction diffraction spectra 299 spectraobserved observed for PEITs for werePEITs close were to close those to those of PET. of AsPET. suggested As suggested by Zhang by Zhang [17], [17], this this indicates indicates that that the 300 thecrystalline crystalline parts parts of the of the PEITs PEITs are relatedare related to PET to PE patterns.T patterns. However, However, we cannotwe cannot rule rule out theout factthe thatfact 301 thatthe presencethe presence of isosorbide of isosorbide units can units disrupt can disrupt the crystallization the crystallization of the PET of units, the PET with units, this disturbance with this 302 disturbancereflected by reflected the observed by the peak observed offsets peak as a offsets function as ofa function the isosorbide of the isosorbide level. Finally, level. the Finally, spectrum the 303 spectrumrecorded onrecorded pure crystalline on pure crystalline PIT had di PITffraction had diffraction peaks very peaks different very from different those from of PET those and of PEIT. PET Thisand 304 PEIT.led to This confirming led to confirming the previous the hypothesis previous accordinghypothesis to according which, for to the which, isosorbide for the contents isosorbide studied contents here 305 studied(up to 20%), here the(up PITto 20%), patterns the presentPIT patterns in the presen PEIT samplest in the PEIT were samples not crystallized. were not crystallized.

306 5.5. Conclusion Conclusions 307 InIn this this study, study, the crystallization of poly(ethylene-co-isosorbide terephthalat terephthalate)e) copolyesters was was 308 assessed.assessed. For that, PEITPEIT samplessamples with with various various amounts amounts of isosorbideof isosorbide going going from from 4.8 to4.8 20.8 to mol.%20.8 mol.% were 309 weresuccessfully successfully synthetized synthetized with a randomwith a random distribution dist ofribution isosorbide of isosorbide moieties in moieties the polymer in the backbone polymer as 13 310 backbonerevealed byas revealedC-NMR. by 13C-NMR. 311 FirstFirst results results showed showed clearly clearly that that the the presence presence of of isosorbide isosorbide modified modified the the crystallization crystallization behavior behavior 312 ofof PET. PET. Actually, Actually, increasing the isosorbide content led to an an exponential exponential increase increase in in the the crystallization crystallization half time (t = 1.946 exp(0.297.x ) at 170 C). This increase in crystallization time went together 313 half time (t1/21/ 2= 1.946·exp(0.297.x· ISO)ISO at 170 °C).◦ This increase in crystallization time went together with 314 thewith decrease the decrease in melting in melting temperature temperature (from (from 250 250°C for◦C PET for PET to 207 to 207°C for◦C forPEI PEI20T)20 andT) and the thedecrease decrease in 315 overallin overall crystallinity crystallinity (from (from 55 55J/g J/ gto to28 28 J/g J/ gmelting melting enthalpy enthalpy for for PET PET and and for for PEI PEI2020TT crystallized crystallized at at the the 316 maximum,maximum, respectively). respectively). 317 InIn addition, addition, comparative comparative X-Ray X-Ray diffraction diffraction analysis analysis of of crystallized crystallized PET, PET, PEIT, PEIT, and PIT samples 318 clearlyclearly indicated indicated that that ethylene ethylene terephthalate mo moietiesieties were the only ones to crystallize. 319 Finally,Finally, mathematical fit fit of the Avrami model wa wass done on experimental data of crystallization 320 kineticskinetics forfor PETPET and and PEIT. PEIT. In allIn cases,all cases, the valuethe value of the of Avrami’s the Avrami’s exponent exponent (n) was close(n) was to 3,close indicating to 3, 321 indicatinga three-dimensional a three-dimens (3D) spheruliticional (3D) spherulitic growth of thegrowth crystals. of the crystals. 322 AllAll parameters parameters studied studied here are here essential are essentialknowledge knowledge for further processing for further of poly(ethylene- processingco of- 323 isosorbidepoly(ethylene- terephthalate)co-isosorbide copolyesters. terephthalate) They copolyesters. show that, Theywith showisosorbide that, withcontents isosorbide higher contentsthan 15 324 mol.%,higher thanthese 15 copolyesters mol.%, these copolyestersare difficult areand di ffilongcult to and crystallize. long to crystallize. Potential Potential applications applications are, thus, are, 325 different depending on the isosorbide content. Polymers that are able to “quickly” crystallize could 326 be used for injection stretch blow molding of hot filling bottle or for making fibers, whereas 327 amorphous ones would be more perfectly suitable for making high-transparency polymers for optical 328 application or as a substitute of polycarbonate in food contact applications. In addition these 329 isothermal kinetics give some important parameters for the solid-state post-condensation of these Appl. Sci. 2020, 10, 1046 13 of 14 thus, different depending on the isosorbide content. Polymers that are able to “quickly” crystallize could be used for injection stretch blow molding of hot filling bottle or for making fibers, whereas amorphous ones would be more perfectly suitable for making high-transparency polymers for optical application or as a substitute of polycarbonate in food contact applications. In addition these isothermal kinetics give some important parameters for the solid-state post-condensation of these copolyesters, i.e., optimized crystallization temperature and time, and maximal temperature to be used for the solid state.

Author Contributions: Investigation, F.F.; writing—original draft, N.D. and N.J.; writing—review and editing, P.H. and R.S.-L. All authors have read and agreed to the published version of the manuscript Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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