56
Poly(ethylene furanoate-co-ethylene terephthalate) biobased copolymers: Synthesis, thermal properties and cocrystallization behavior Maria Konstantopoulou 1 , Zoe Terzopoulou 2 , Maria Nerantzaki 2 , John Tsagkalias 2 , Dimitris S. Achilias 2 , Dimitrios N. Bikiaris 2 *, Stylianos Exarhopoulos 1,3 , Dimitrios G. Papageorgiou 4 , George Z. Papageorgiou 3 * 1 Department of Food Technology, Technological Educational Institute of Thessaloniki, PO Box 141, GR-57400 Thessaloniki, Greece 2 Laboratory of Polymer Chemistry and Technology, Department of Chemistry, Aristotle University of Thessaloniki, GR-541 24, Thessaloniki, Macedonia, Greece 3 Chemistry Department, University of Ioannina, P.O. Box 1186, GR-45110 Ioannina, Greece 4 School of Materials and National Graphene Institute, The University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom Abstract A series of poly(ethylene furanoate-co-terephthalate) (PEFT) copolymers, with compositions ranging from neat poly(ethylene furanoate) (PEF) to poly(ethylene terephthalate) (PET), was synthesized by melt and solid state polycondensation (SSP). 1 HNMR spectra revealed that the copolymers were random, while the WAXD patterns of the copolyesters indicated isodimorphic cocrystallization. A minimum was observed in the plot of

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Poly(ethylene furanoate-co-ethylene terephthalate) biobased copolymers:

Synthesis, thermal properties and cocrystallization behavior

Maria Konstantopoulou1, Zoe Terzopoulou2, Maria Nerantzaki2, John Tsagkalias2,

Dimitris S. Achilias2, Dimitrios N. Bikiaris2*, Stylianos Exarhopoulos1,3, Dimitrios G.

Papageorgiou4, George Z. Papageorgiou3*

1Department of Food Technology, Technological Educational Institute of

Thessaloniki, PO Box 141, GR-57400 Thessaloniki, Greece2Laboratory of Polymer Chemistry and Technology, Department of Chemistry,

Aristotle University of Thessaloniki, GR-541 24, Thessaloniki, Macedonia, Greece3Chemistry Department, University of Ioannina, P.O. Box 1186, GR-45110 Ioannina,

Greece4School of Materials and National Graphene Institute, The University of Manchester,

Oxford Road, Manchester, M13 9PL, United Kingdom

Abstract

A series of poly(ethylene furanoate-co-terephthalate) (PEFT) copolymers, with

compositions ranging from neat poly(ethylene furanoate) (PEF) to poly(ethylene

terephthalate) (PET), was synthesized by melt and solid state polycondensation (SSP). 1HNMR spectra revealed that the copolymers were random, while the WAXD

patterns of the copolyesters indicated isodimorphic cocrystallization. A minimum was

observed in the plot of the melting temperature (Tm) vs composition while the glass

transition temperatures (Tg) varied almost linearly with increasing ET units. The

crystallization rates and degree of crystallinity decreased with comonomer content.

Several thermodynamic models were applied for the analysis of the melting point

depression. A small portion of the comonomer units was found to be introduced into

the homopolymer crystals. It was also realized that it is easier to incorporate the EF

units into the PET crystal than the opposite. PLM was used to observe the spherulitic

morphologies formed during isothermal melt crystallization. Thermogravimetric

analysis (TGA) indicated that the thermal stability of PEFTs decreases slightly with

increasing furanoate content. Finally, the mechanism of decomposition was evaluated

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via Py-GC/MS, which consisted of mostly heterolytic scission and less of homolytic

scission reactions.

Keywords: Poly(ethylene furanoate); poly(ethylene terephthalate); random

copolymers; cocrystallization.

Corresponding author: Dimitrios N. Bikiaris, email: [email protected]; George Z.

Papageorgiou, email: [email protected]

1. Introduction

Poly(ethylene terephthalate) (PET) is one of the most important and highly produced

man-made polymers polymers.[1] It is synthesized from ethylene glycol (EG) and

terephthalic acid (PTA) and the two monomers are both fossil based, currently.

However, a novel process for EG, involving direct conversion of lignocellulose to EG

has been developed recently.[2, 3] Furthermore, p-xylene, the precursor of

terephthalic acid, has been obtained by catalytic conversion of platform chemicals or

raw biomass. So, the total synthesis of green PET from renewable resources seems

feasible.[4-8]

PET has been the most important polymer for beverage packaging for the past

four decades. This was the result of its favorable properties like its optical clarity,

barrier properties, and competitive performance-to-cost ratio. Despite the fact that

PET has met many of the current global packaging needs, there are still some

drawbacks. For example it is characterized by high oxygen transmission rates which

limit its effectiveness for oxygen-sensitive beverages. More importantly, one of its

monomers, terephthalic acid (TA), is fossil based. Coca-Cola Co., in 2009 began to

produce PET bottles based on 30% plant-based renewable material;

monoethylene glycol made from sugarcane ethanol. In fact, the first fully-biobased

PET bottle was shown by the company, at the 2015 World Expo in Milan.[9] Coca-

Cola currently holds collaborations with Virent, Gevo and Avantium for the

production of the bio-based PTA component (or PEF) for PlantBottle®.[10] The new

100% biobased PET bottle is based on technology developed by biofuels and from the

biochemical company Virent, Inc., which enables production of BioFormPX

(paraxylene) from beet sugars instead of fossil fuels [9].

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Copolymerization and reactive blending of polyesters are often used for

adjusting properties through the composition and constitution of the copolyesters.

Chemical modification of PET by incorporating various glycol or acid comonomers

has been intensively investigated in the past with the aim of extending the use of PET

in new applications.[11-19]

Interest in polymers from renewable resources has been growing as part of the

general concern for sustainability.[20-22] Biomass is abundant, cheap and one of the

most attractive alternative feedstocks in nature. So, it might be considered as a

suitable replacement of fossil resources, used to produce high value-added chemicals

and fuels. 2,5-Furandicarboxylic acid (FDCA) is one of the most promising

chemicals, readily obtained by oxidation of 5-hydroxymethylfurfural (HMF) which in

its turn can be formed from polysaccharides and sugars.[23-26] In fact, technology

pathways to biobased TPA are still under development, while FDCA is readily

produced from renewable resources. While the structure of FDCA is similar to

terephthalic acid (TA), differences exist in their ring size, polarity, and linearity,

finally resulting in significantly different physicochemical properties. The interatomic

distance between carboxylic acid groups is 5.731 Å in TA, while it is only 4.830 Å in

FDCA. Moreover, the linear p-phenyl connection in TA results in an angle of 180°

between carboxylic acid carbons, while FDCA shows a nonlinear structure which

yields an angle of 129.4°.[27]

FDCA recently gained much interest in polycondensates. It was found to be a

possible substitute of terephthalic acid in aromatic polyesters such as PET, PBT or

PTT. Poly(ethylene 2,5-furandicarboxylate) or poly(ethylene furanoate) (PEF) is

entirely based on renewable resources, as it is produced from FDCA and ethylene

glycol. PEF is new polymer with high performance properties, including barrier,

thermal, and mechanical among others. A surprisingly large 19-fold carbon dioxide

permeability reduction was found for PEF compared to PET.[27] A drastic reduction

in oxygen permeability by a factor of about 11× for PEF compared to PET has also

been stated [28]. PEF and similar furanic polymers have been the subject of recent

research, due to their renewable nature and promising properties.[29-34] PEF is

expected to be a viable candidate for the polyester and food packaging market.[35-38]

Recently, a few studies on PEF-based copolymers PEF have been published

but of course their number is limited compared to those for PET related copolymers.

[22, 38-45] The cocrystallization in random copolyesters has been discussed in the

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past.[46-50] There is some evidence that the copolymer crystal includes different

kinds of comonomer units in a crystalline lattice. This is an isomorphic phenomenon.

[51] In such a case the minor component of crystal should influence the whole

properties of solid copolymers. In fact cocrystallization is easier in aliphatic than in

aromatic copolyesters.[52-57] Cocrystallization in copolymers based on ethylene

terephthalate is of special interest and such copolymers are often used as model

materials.[57-61]

In this work, a full series of eleven PEFT copolymers, with compositions

ranging from neat PEF to neat PET, was synthesized and the thermal and solid state

properties were studied in detail. The cocrystallization behavior of the two

comonomers was investigated, since this can be crucial for the overall performance of

the copolymers and their potential applications as packaging materials.

2. Experimental

2.1. Materials

2,5-furan dicarboxylic acid (2,5-FDCA, purum 97 %), dimethyl terephthalate (purum

99 %), ethylene glycol (EG) and tetrabutyl titanate (TBT) catalyst of analytical grade

were purchased from Aldrich Co. 2,5-dimethylfuran-dicarboxylate (DMFD) was

synthesized from 2,5-FDCA and methanol as described in our previous work.[36] All

other materials and solvents used were of analytical grade.

2.2. Copolymer synthesis

Neat PEF and PET polyesters were prepared by the two-stage melt polycondensation

method (esterification and polycondensation) in a glass batch reactor as described in

previous works [36, 62]. Bis(hydroxyl ethyl-furanoate) (BHEF) was synthesized by

transesterification from DMFD and EG in a molar ratio of diester/diol=1/2.2. Both

reagents were charged into the reaction tube of the polyesterification apparatus with

400 ppm of TBT. The reaction mixture was heated at 150 °C under argon flow for 2h,

at 160 °C for additional 2h and finally at 170oC for 1h. CH3OH byproduct was

removed from the reaction mixture by distillation and at the end of this step

temperature was increased at 200oC and vacuum was applied for 20 min in order to

remove the EG excess, producing BHEF. Bis(hydroxyl ethyl-terephthalate) (BHET)

was synthesized from DMT and EG using a similar procedure as described previously

for BHEF production. The PEFT copolymers were then synthesized by melt

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polycondensation using different BHEF/BHET feeding ratios (Table 1). The mixture

was heated at 220 oC for 2h at stirring speed 720 rpm and vacuum application, at 230 oC for 2h and at 240 oC for additional 1h. Time was remained stable in all copolymers

while used temperatures were gradually increased by 5oC increasing BHET amount

by 15%. After the polycondensation reaction was completed, the polyesters were

easily removed, milled and washed with methanol. Solid state polycondensation was

applied to increase the molecular weight of the samples at temperatures 20oC lower

than melting point of each copolymer for 4h under vacuum application.

2.3. Polyester characterization

2.3.1. Intrinsic viscosity measurement.

Intrinsic viscosity [η] measurements were performed using an Ubbelohde viscometer

at 30 oC in a mixture of phenol/1,1,2,2-tetrachloroethane (60/40, w/w).

2.3.2. Wide angle X-Ray diffraction patterns (WAXD)

X-ray diffraction measurements of the samples were performed using a MiniFlex II

XRD system from Rigaku Co, with CuKα radiation (λ=0.154 nm) in the angle (2θ)

range from 5 to 65 degrees. For the evaluation of the crystalline structure of solvent-

crystallized PEF and PET and copolymers, 10 grams of polymer were milled and

transferred into a glass beaker where 200 mL of trifluoroacetic acid/dichloromethane

(1/4 v/v) were added. The mixtures were stirred mechanically at room temperature

until the polymer was fully dissolved and precipitated in cold methanol. After

filtration, a white material was left in the Gooch filter in both cases. The pure samples

were kept overnight under vacuum in order to remove any final residue of solvents

and the materials were used for the WAXD experiments.

2.3.3. Differential Scanning Calorimetry (DSC)

A TA Instruments TMDSC (TA Q2000) combined with a cooling accessory was used

for thermal analysis. The instrument was calibrated with indium for the heat flow and

temperature, while the heat capacity was evaluated using a sapphire standard.

Nitrogen gas flow of 50 ml/min was purged into the DSC cell. The sample mass was

kept around 5 mg. The Al sample and reference pans were of identical mass with an

error of ± 0.01 mg. The samples were initially cooled to 0°C and then heated at a rate

of 20°C/min at temperatures 40oC higher than the melting temperature. In order to

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obtain amorphous materials, the samples were held there for 5 min to erase any

thermal history, before cooling in the DSC with the highest achievable rate, which

was 80 oC/min.

Isothermal crystallization experiments of the polymers at various temperatures

below the melting point were performed after self-nucleation of the polyester sample.

Self-nucleation measurements were performed in analogy to the procedure described

by Fillon et al. [63] The protocol used is a modification of that described by Müller et

al. [64, 65] and can be summarized as follows: a) melting of the sample at 40 oC

above the observed melting point for 5 min to erase any previous thermal history; b)

cooling at 20 oC min-1 to room temperature and crystallization; c) cold-crystallization

to create a ‘‘standard’’ thermal history and partial melting by heating at 5oC min-1 up

to a ‘‘self-nucleation temperature’’, Ts which was 224oC for PEF, 262oC for PET and

properly decreased for the copolymers; d) thermal conditioning at Ts for 1 min.

Depending on Ts, the crystalline polyester will be completely molten, only self-

nucleated or self-nucleated and annealed. If Ts is sufficiently high, no self-nuclei or

crystal fragments can remain (Ts Domain I - complete melting domain). At

intermediate Ts values, the sample is almost completely molten, but some small

crystal fragments or crystal memory effects remain, which can act as self-nuclei

during a subsequent cooling from Ts, (Ts Domain II-self - nucleation domain). Finally,

if Ts is too low, the crystals will only be partially molten, and the remaining crystals

will undergo annealing during the 5 min at Ts, while the molten crystals will be self-

nucleated during the later cooling, (Ts Domain III - self-nucleation and annealing

domain); e) cooling scan from Ts at 20 oCmin-1 to the crystallization temperature (Tc),

where the effects of the previous thermal treatment will be reflected on isothermal

crystallization; f) heating scan at 20 oCmin-1 to 40oC above the melting point, where

the effects of the thermal history will be apparent on the melting signals. Experiments

were performed to check that the sample did not crystallize during the cooling to Tc

and that a full crystallization exothermic peak was recorded at Tc. In case that some

other method was applied, this will be discussed in the corresponding part. The self-

nucleation experiments are very useful in this kind of studies since it can provide

information on any interruption of the linear sequence of the crystallizable chains

such as a molecular unit (comonomer) [66]. The melting behavior of all the samples

was recorded on heating at 20oC/min.

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2.3.4. FTIR Spectroscopy

FTIR spectra were obtained using a Perkin-Elmer FTIR spectrometer, model

SPECTRUM 1000, using KBr tablets. The resolution for each spectrum was 2 cm−1,

and the number of co-added scans was 64. The spectra presented were baseline-

corrected and converted to the absorbance mode.

2.3.5. Nuclear magnetic resonance (1H-NMR)

1H-NMR spectra of polyesters were obtained with a Bruker spectrometer operating at

a frequency of 400 MHz for protons at room temperature. A mixture of deuterated

trifluoroacetic acid (DTFA) and chloroform in a ratio of 3:1 (w/w) (DTFA/CDCl3)

was used as a solvent in order to prepare solutions of 5% w/v. The number of scans

was 10 and the sweep width was 6 kHz.

2.3.6. Polarizing Light Microscopy (PLM)

A polarizing optical microscope (Nikon, Optiphot-2) equipped with a Linkam THMS

600 heating stage, a Linkam TP 91 control unit and also a Jenoptic ProgRes C10plus

camera were used for PLM observations.

2.3.7. Thermogravimetric analysis (TGA)

Simultaneous TG/DTA (thermogravimetric/differential thermal analysis)

measurements were carried out by using a STA 449C (Netzch-Gerätebau, GmbH,

Germany) thermal analyzer. The temperature range was from ambient temperature up

to 600 °C, with a heating rate of 20 °C min−1 under a constant flow of N2 (99.9%) at

30 cm3 min−1.

2.3.8. Pyrolysis-Gas Chromatography/Mass spectroscopy

For Py-GC/MS analysis of polyesters a very small amount of each material is

“dropped” initially into the “Double-Shot” EGA/PY‐3030D Pyrolyzer (Frontier

Laboratories Ltd, Fukushima Japan) using a CGS-1050Ex (Japan) carrier gas selector.

For EGA analysis the furnace temperature is programmed from 100 to 600 °C with a

heating rate 20 °C/min using He as purge gas and air as cooling gas. For pyrolysis

analysis (flash pyrolysis) each sample was placed into the sample cup which

afterwards fell free into the Pyrolyzer furnace. The pre-selected pyrolysis temperature

was 400 °C and the GC oven temperature was heated from 70 to 300 °C at 10 °C/min.

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Those two temperatures were selected based on the EGA pyrogram and represent the

sample prior and after thermal decomposition. Sample vapors generated in the furnace

were split (at a ratio of 1/50), a portion moved to the column at a flow rate of 1

mL/min, pressure 53.6 kPa and the remaining portion exited the system via the vent.

The pyrolyzates were separated using temperature programmed capillary column of a

Shimadzu QP-2010 Ultra Plus (Japan) gas chromatograph and analysed by the mass

spectrometer MS‐QP2010SE of Shimadzu (Japan) use 70 eV. Ultra ALLOY® metal

capillary column from Frontier Laboratories LTD (Fukushima Japan) was used

containing 5% diphenyl and 95% dimethylpolysiloxane stationary phase, column

length 30 m and column ID 0.25 mm. For the mass spectrometer the following

conditions were used: Ion source heater 200 °C, interface temperature 320 °C, vacuum

10-4-100 Pa, m/z range 45-500 amu and scan speed 10.000. The chromatograph and

spectra retrieved by each experiment are subject to further interpretation through

Shimadzu and Frontier post-run software.

3. Results and discussion

3.1. Synthesis and molecular characterization.

Poly(ethylene furanoate-co-ethylene terephthalate) copolymers were synthesized from

dimethyl furanoate, dimethyl terephthalate and ethylene glycol by applying the two

step polycondensation method as was described in the experimental part. Solid state

polycondensation was also applied in the case of the samples with low molecular

weights (SSP) prior to increase them. The intrinsic viscosity values after the SSP are

shown in Table 1 for the copolymers and neat polyesters. The FTIR spectra of the

copolymers are shown in Figure S1-Supplementary Information. It can be seen that as

the copolymer composition is varied, the corresponding spectra reflect the expected

decrease of the aromatic content in favor of the furan counterpart. More specifically, a

peak at 765 cm-1 rises with the introduction of EF units, indicative of the deformation

vibration of the methylene group. Also, the broad peak at 1115 cm-1, which

corresponds to the ester C-O-C stretching bonds of PET splits at two components

starting from the PEFT 10-90 sample, namely 1100 cm-1 and 1120 cm-1, from the

vibrations of the ester C-O bond and the aliphatic ether C-O bond of the copolymers.

Furthermore, the shoulder at 1384 cm-1, also starting from the PEFT 10-90 sample

originates from the weak methyl band and it is indicative of a long chain, linear

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aliphatic structure. The peak at 2910 cm-1 is another indication of the stretching of C-

H groups, while the appearance of the characteristic peak of the furan ring can be seen

at 3125 cm-1, which increases with increasing the EF content. Regarding the samples

with high EF content (higher than 50 mol%), the peaks at 735 and 876, 1450 and 1530

cm-1 vanish on the spectrum of neat PEF, as a result of the disappearance of the

aromatic compounds that are found on the ET units of the copolymers.

The 1HNMR spectra of the copolymers were also recorded. The chemical structures

and 1HNMR spectra of PEF, PET and the PEFT 50/50 copolymer are shown in

Scheme 1 and Figure 1 respectively. The protons of the furanoate ring are the most

deprotected in the macromolecules due to the carbonyl groups and the π electron

system of the ring and they appear at about 7.52 ppm (a protons). At lower values

protons of ethylene diol part are recorded at 4.8 ppm (b protons), while these protons

in PET due to their similarity are recorded at 4.90 ppm (d protons). Finally, the

aromatic protons of terephthalic acid are recorded at 8.32 ppm (c protons). All these

are also recorded in prepared copolymers with their intensity, mainly these of furanic

and terephthalic groups, to be dependent on the used molar ratio between

furanic/terephthalic acids.

Scheme 1. Chemical structures of PEF, PEFT 50-50 copolymer and PET.

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Figure 1. 1H NMR spectra of PEF, PEFT 50-50 copolymer and PET.

The composition and degree of randomness (R) in the PEFT copolyesters was

calculated using the resonance peaks of the ethylene units’ aliphatic protons. The

degree of randomness is defined as [56]:

R = PFT + PTF (1)

PFT =

( f FT +f TF)2

( f FT+ f TF )2

+ f FF =

1LnF (2)

PSF =

( f FT+ f TF )2

( f FT+ f TF )2

+ f TT =

1LnT (3)

where PFT and PTF are the probability of finding a Furanoate (F) unit next to a

Terephthalate (T) unit and the probability of finding a Terephthalate unit next to a

Furanoate unit, respectively. Also fFF, fFT, fTF, fTT represent the dyads fraction,

calculated from the integral intensities of the resonance signals FF, FT, TF and TT,

correspondingly [67]. LnT and LnF stand for the number average sequence length, the

so-called block length, of the T and F units, respectively. For random copolymers the

degree of randomness R should be equal to 1, for alternate copolymers equal to 2 and

for block copolymers close to zero. Table 1 shows the calculated values for the degree

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of randomness. Practically these were equal to 1, indicating that the prepared PEFS

copolymers were essentially random.

The number average sequence length or block length for Furanoate (LnF) and

Terephthalate (LnT) units was calculated using equations 2 and 3 respectively,

according to the work of Yamadera and Murano [67]. Table 1 summarizes the

corresponding values and it can be seen that by increasing the EF ratio in copolymers,

the corresponding block length also increased while the same picture was formed for

ET blocks, when the ET ratio was higher.

Table 1. Intrinsic viscosity [η], calculated compositions, degree of randomness (R)

and average block length for terephthalate (LnT) and furanoate (LnF) blocks of the

prepared copolymers.

Sample[η]

(dL/g)

Ethylene

Furanoate in

Feed (mol%)

Ethylene

Furanoate1HNMR (mol%)

R LnT LnF

PEF 0.464 100 100

PEFT 95-05 0.446 95 96 1.05 1.0511.1

2

PEFT 90-10 0.436 90 92 1.03 1.09 9.09

PEFT 85-15 0.623 85 82 1.04 1.21 4.00

PEFT 70-30 0.682 70 67 1.04 1.51 2.63

PEFT 60-40 0.449 60 63 1.02 1.74 2.28

PEFT 50-50 0.773 50 45 0.98 2.23 1.87

PEFT 40-60 0.465 40 42 0.99 2.37 1.75

PEFT 30-70 0.478 30 35 0.97 2.87 1.62

PEFT 15-85 0.587 15 19 1.02 5.17 1.21

PEFT 10-90 0.522 10 14 1.03 7.06 1.12

PEFT 05-95 0.502 5 8 0.98 12.11 1.11

PET 0.596 0 0

3.2. DSC study

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The DSC thermograms recorded at 20oC/min for the as received PEFT copolymers

and the neat PEF and PET are shown in Figure 2a. The copolymers with intermediate

compositions showed broad or multiple melting peaks on heating scans, which is an

indication of isodimorphism, as some comonomer inclusion can be observed in both

phases. The downshift of the PEF melting peak is due to the fact that the PEF linear

sequences are interrupted by the ET repeating units. Moreover, the respective

enthalpies and degrees of crystallinity were calculated, given the fact that the enthalpy

of PET is 140 J/g [68] and that of PEF is 137 J/g [36]. The values can be seen in

Table 2. Obviously the homopolymers exhibited higher degrees of crystallinity than

the copolymers, while at intermediate compositions the degree of crystallinity was

very low, as expected, indicating the amorphous nature of the copolymers. The DSC

traces of the melt quenched samples can be observed in Figure 2b. As it can be seen

neat PET and the PEFT 5-95, 10-90 and 15-85 samples show the characteristic cold-

crystallization, indicative of the high ET content. For neat PEF only a slight

exothermic peak and a subsequent small melting peak were observed, proving the

slower crystallization kinetics compared to PET. This was also proved by the cooling

scans of the samples at 10oC/min (Figure 2c).

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Figure 2. a) DSC heating scans of the as received samples, b) heating scans of the

melt quenched samples and c) cooling scans.

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Figure 3. Melting temperature and glass transition temperatures as a function of the

copolymer composition.

Table 2. Heat of fusion and degree of crystallinity for all samples under study.

Sample ΔΗm (J/g) XC (%)PEF 38.6 28.2

PEFT 95/5 34.3 25.0PEFT 90/10 29.7 21.7PEFT 85/15 23.4 17.1PEFT 70/30 8.7 6.4PEFT 60/40 3.1 2.3PEFT 50/50 3.9 2.8PEFT 40/60 3.7 2.6PEFT 30/70 9.2 6.7PEFT 15/85 24.3 17.4PEFT 10/90 44.1 31.5PEFT 5/95 46.7 33.4

PET 52.8 37.7

The plots of the melting (Tm) and glass transition (Tg) temperatures as a function

of the ethylene terephthalate content are exhibited in Figure 3. It is important to note

that the melting temperature of the copolymers exhibits a very broad window, from

100 to 240oC, indicating that the materials can be used in a wide spectrum of

applications. A minimum in the Tm vs composition plot can be seen, consistent with a

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pseudoeutectic behavior, which is indicative of the random character of the

copolymers. On the other hand, the Tg values varied almost linearly, exhibiting a

decrease with increasing ET content. This is an indication of the increase of the steric

hindrance of the macromolecular chains, therefore the copolymers with increasing ET

content should possess higher activation energy and thermal stability.

The isothermal crystallization from the melt after self-nucleation was studied

for the PEFT copolymers in comparison with neat PET and PEF. As it can be seen in

Figures 4a and b the half-times of crystallization after self-nucleation increase with

increasing temperature. Furthermore, the crystallization of the copolymers with low

terephthalate content even after self-nucleation was quite slow as can be concluded

from the plot in Figure 4a for the PEFT 95/5 sample, for which the crystallization half

times are quite longer compared to the also slowly crystallizing PEF. In contrast, the

respective values for PET and the copolymers with high terephthalate content were

shorter (Figure 4b). In both graphs it is obvious that the values of t1/2 increase

exponentially with increasing temperature. This is an indication that crystallization

takes place by a nucleation-controlled mechanism [69].

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Figure 4. Half-times of isothermal crystallization after self-nucleation for a) PEF and

PEFT 95/05 and b) PET and copolymers with high ethylene terephthalate content.

3.3. WAXD study

WAXD patterns of the samples were recorded after solvent treatment (Figure 5a). The

patterns of PEF and the copolymers with high ethylene furanoate content were

consistent with that of β-type crystals of PEF reported in a previous work [70]. It can

also be observed that the initial introduction of ET units (up to the 85-15 sample) in

the EF structure, shifts the characteristic peaks of PEF towards slightly lower angles.

This is an indication that the spacing between the layers in the atomic lattice of PEF

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increases with the introduction of the ET units, while further increase of ET leads to

the appearance of wide peaks and large amorphous background. Therefore, the

introduction of ET units loosens the crystal packing and decreases the crystallite size

[71]. The solvent-treated ET-rich samples are characterized by diffraction peaks of

low intensity, signifying low crystallinity, as it can be seen in Figure 8a. Figure 5b

shows the WAXD patterns of PEF after different treatments, i.e. after solvent

treatment, melt quenching and melt quenching and annealing. It can be seen that

different crystal phases are observed after solvent and thermal treatment, since β-

crystals were obtained by solvent treatment and α-crystals were formed after

annealing of the quenched sample, as elaborated in a previous work from our group

[70].

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Figure 5. WAXD patterns a) for the copolymers after solvent treatment and b) for

PEF after solvent treatment, after quenching and after cold crystallization.

3.4. Cocrystallization behavior

In copolymers, if the two crystallizable components A and B, are compatible in each

crystal lattice, cocrystallization can be observed. The first case of cocrystallization is

isomorphism, where only one crystalline phase which contains both comonomer units

is observed at all compositions, alongside a distinct melting temperature and

appearance of crystallinity over the entire copolymer composition [51]. In the second

case of cocrystallization named isodimorphism, two crystalline phases are observed

and this behavior is accompanied by a minimum in the plot of melting temperature

versus copolymer composition (pseudo-eutectic behavior) and also with lowering in

the degree of crystallinity [51].

For PEF, the crystal structure was estimated in an early study by Kazaryan and

Medvedeva [72]. According to the authors, the α crystal modification of PEF exhibits

a triclinic unit cell, with dimensions a=0.575 nm, b=0.535 nm, c=2.010 nm, α=133.3ο,

β=90ο and γ=112ο, comprising of two repeating units and a crystal density of 1.565

g /cm3. The density of the amorphous phase is 1.4299 g /cm3 [72]. However, a very

recent work from Mao et al. [73] gave slightly different values regarding the unit cell

dimensions and the density of PEF. In detail, the authors suggested that the space

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group was P21, with a monoclinic unit cell where a=0.578 nm, b=0.678 nm, c = 2.029

nm and γ=103.3°, giving a unit cell density of the crystal phase of 1.562 g/cm3. In a

previous work it was shown that when PEF crystallizes from solution or after solvent

treatment it shows a different crystal modification, called β modification [70]. For

PET, the most commonly cited crystal structure was determined by Daubeny et al.

[74] using X-ray diffraction measurements on drawn PET fibers. The crystal structure

of PET is triclinic with dimensions a=0.456 nm, b=0.594 nm, c=1.075 nm, α=98.5ο,

β=118ο and γ=112ο, which comprises one repeating unit and yields a crystal density of

1.455 g/cm [75].

As PEFT copolymers are of practical interest and given that processing involves

melt crystallization, the WAXD patterns of the copolymers were also recorded after

melt-quenching and annealing at temperatures 30oC below the corresponding melting

temperatures. These patterns are shown in Figure 6a. Similarly to the PEFT

copolymers prepared previously from Sousa et al. [19], the materials exhibited the

characteristic semi-crystalline diffractograms, with the PEFTs ranging from 70-30 to

40-60 being essentially amorphous. It is important to note here that a continuous shift

in the peak positions was observed, especially in case of ethylene terephthalate-rich

copolymers. The variation of the interplanar spacings can be seen in Figure 6b. This

variation is consistent with the cocrystallization of the two comonomers, meaning

ethylene terephthalate and ethylene furanoate units.

Assuming comonomer exclusion alone, the length of crystallizing sequences is

supposed to decrease steadily as the comonomer content increases in copolymers.

This causes a decrease of the lamellar thickness, and thus a depression of the melting

point. However, for PEFT copolymers the linear variation in the interplanar spacings

of PET crystal with comonomer content indicates that comonomer units are not

completely excluded from the crystallizing chain segments. Especially for those

random copolymers with intermediate compositions, the probability for long

homopolymer sequences to occur along the macromolecular chains is limited, as was

proved by 1HNMR spectra. It seems that sequences random but similar yet can

crystallize. Such assumptions are common for most theories for copolymer

crystallization [16, 17]. It has also been reported for copolymers containing aromatic

moieties that chain segments with random but similar sequences can crystallize by

parallel alignment [17].

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It should also be reported that for cases of strict isomorphism, the

crystallinities of the copolymers are not affected by the composition of the material

and the crystallinity is rather constant at all composition range. However, in our case

the crystallinities of PEFT copolymers are rather low when the compositions of EF

and ET units are comparable. This is similar to the general non-isomorphic

copolymers and means that the degree of isomorphism in PEFT is rather low. In fact,

for PEFT copolymers the system is isodimorphic.

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Figure 6. WAXD patterns a) for the copolymers after melting and annealing and b)

variation of the interplanar spacing with composition.

3.5. Thermodynamics of melting point depression

Several theories have been introduced for the thermodynamics analysis of

cocrystallization in random copolymers, such as those of Flory [75, 76] or Baur [77]

which assume comonomer exclusion and those of Inoue[78], Helfand-Lauritzen [79]

and Sanchez-Eby [80] which assume comonomer inclusion in the crystal.

In the equation of Flory [75]:

1Tm

o− 1

T m( XB)= R

ΔH mo

ln(1−XB )(1)

ΧΒ is the concentration of the minor comonomer B units in the polymer and

ln (1−X B)equals the collective activities of Α sequences in the limit of the upper

bound of the melting temperature. T mo

and ΔH mo

are the homopolymer equilibrium

melting temperature and heat of fusion and R is the gas constant.

The Sanchez-Eby model assumes that B comonomer units are included into

the crystals of A forming defects. The corresponding equation is [80]:

1Tm

o− 1

T m( XB)= R

ΔH mo

ln(1−XB+X B e−∈ /RT )(8)

where, X B e−∈ /RTis the equilibrium fraction of repeat units B that are able to

crystallize, and is the excess free energy of a defect created by the incorporation of

one B unit into the crystal.

The basic concept in the theory of Baur, is that homopolymer sequences of

length ξ may be included into crystals of lamellar thickness corresponding to that

length [77]:

1Tm

o− 1

T m( XB)= R

ΔH mo[ ln (1−XB )−⟨ξ ⟩−1 ]

(9)

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where ⟨ξ ⟩=[ 2 XB(1−X B) ]−1 (10)

is the average length of homopolymer sequences in the melt.

In Figure 7a, b the experimental excess crystallization Gibbs energy obtained

as [ ΔHmo /( RT m) ]⋅(1−Tm /T m

o ) is plotted together with the theoretical values

calculated as a function of copolymer composition for the copolyesters of this work.

The temperature corresponding to the end of the melting peak was used, as an

approach of the equilibrium melting temperature. Also, in calculations, the

equilibrium melting enthalpy was taken to be 25 kJ/mol [36] for PEF and 26.9 kJ/mol

for PET [81]. The excess free energy was calculated as ln (1−X B)−⟨ξ ⟩−1 in the case

of the Baur model and ln (1−X B)in the case of the Flory model. In the case of

copolymers with very low ET content, Baur model rather fits the experiment, showing

that the comonomer exclusion model may hold in case of PEF crystal. The deviation

is more obvious as the comonomer content increases. In case of PET crystal, the

deviation between the experimental and the values predicted by the model of Baur is

larger. Consequently, comonomer exclusion alone cannot account for the observed

melting point depression in PEFTs, although there is an ambiguity for PEFTs with the

very low ethylene terephthalate content.

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Figure 7. Excess Gibbs energy/RT as obtained from experiment and from Flory and

Baur models a) in case of copolymer crystallization in PEF crystal and b) in case of

copolymer crystallization in PET crystal.

Wendling and Suter combined both inclusion and exclusion models to arrive

to the following equation [58-60]:

1Tm

o − 1T m( XB)

= RΔH m

o [ ∈XCB

RT+(1−XCB) ln

1−XCB

1−X B+ XCB ln

X CB

XB+⟨ξ

~⟩−1 ]

(11)

where XCB is the concentration of the B units in the crystal. In the equilibrium

comonomer inclusion, the concentration of B units in the crystal is given by:

XCBeq =

XB e−∈/ RT

1−XB+ XB e−∈/ RT(9)

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Substitution ofXCB in equation 8 by equation 9, gives a simplified equation following

equilibrium inclusion model:

1Tm

o − 1T m( XB)

= RΔH m

o {ln(1−X B+X Be−∈ /RT )−⟨ξ~⟩−1}

(12)

where

⟨ξ~⟩−1=2( X B−X B e−∈ /RT )⋅(1−X B+X Be−∈ /RT ) (13)

Uniform inclusion is reached if XCB=X B while for XCB=0 equation 8 is reduced to

the exclusion model.

Various thermodynamic models were tested for the melting point depression

of PEFT (Figure 8a). As can be seen the Flory and Sanchez-Eby models cannot

predict realistic values. The Baur model seemed to be rather reliable. However, the

Wendling-Suter model showed the best fit to the experimental data in the range of low

comonomer content. The value of the function /RT is determined as an adjustable

parameter. A constant /RT value is given by the model regardless of the comonomer

composition. For PEF best fit was found for /RT=2.5 which results in a value =

10.5 kJ/mol of defects, for the average defect free energy in case of incorporation of

ET unit into the PEF crystal, in the limiting case of X comonomer=0 . In the opposite

case of incorporation of EF units in the PET crystal a value /RT=2.2 was needed for

best fit, giving = 9.8 kJ/mol for the average defect free energy. The in this case is

slightly lower than in case of incorporation of ET units in the PEF crystal, indicating

that the bulkier ET unit is more difficult to be included in the PEF crystal, while from

the eutectic composition is estimated to be ca. 36 mol% ET from the intersection of

the two melting temperature curves .

In an attempt to estimate the percentage of the minor comonomer units which

were practically incorporated in the crystals, the results from the Wendling-Suter

model were further analyzed. Using equation 9 the minor comonomer equilibrium

concentrations XCEF and XCET of ethylene furanoate and ethylene terephthalate units

in the PET and PEF crystals, respectively, were calculated. The plot of the

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equilibrium concentration comonomer units in the crystal (XC) versus the

concentration of ET/EF units in the copolymer (X) is shown in Figure 8b. The

comonomer concentration in crystals increases with increasing the comonomer

composition in bulk. The comonomer concentration in crystal is small and much

lower than the copolymer concentration corresponding to uniform inclusion, i.e.

XC=X . The different defect free energies of the different units in the crystals result

in different equilibrium concentrations at a given comonomer concentration in bulk.

Curvature with an increasing slope is observed in the plots of XC versus X and the

physical meaning of this observation is that it is easier to create the excess volume

necessary for a comonomer unit in an already imperfect crystal lattice [59].

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Figure 8. a) comparison of the theoretical melting temperatures and experimental

values and b) equilibrium concentrations of the minor comonomer units in the crystal

of the homopolymer corresponding to the major commonomer, as a function of

copolymer composition.

3.6. Spherulitic morphologies

The crystallization of the two homopolymers and the copolymers with high content in

EF or ET units was studied by means of Polarized Light Microscopy. The

photographs of Figure 9 show the morphologies obtained after isothermal

crystallization. In specific, Figure 9a shows the spherulites of PEF generated at 200oC.

In this case the crystallization of the solvent treated polyester was slow, but resulted

in rather moderate spherulite sizes, larger than those observed in our previous studies

for untreated samples [32, 33, 36, 82, 83]. Furthermore, the morphologies were rather

diffuse. Figure 9b shows the same sample after heating to 218oC; that is up to the

melting temperature region. The diffuse nature of the spherulites is confirmed in this

case. For the copolymers PEFT 95/05 and PEFT 90/10 reduced spherulite sizes and

slower growth rates were observed. For PET larger spherulites were formed at about

equivalent temperatures, regarding the supercooling, with those for PEF. For the

copolymers with high ET content both reduced sizes growth rates were recorded. As

can also be seen for the PEFT 05/95, 10/90 and 15/85 at 180oC. The case of 15/85 is

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different compared to the others as spherulites are not dense and clearly observed.

This is obviously the result of the higher EF comonomer content in the bulk and the

incorporation of a higher portion of EF units in the crystals.

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Figure 9. PLM photographs showing the spherulitic morphologies after isothermal

crystallization of a) PEF at 200oC, b) PEF at 200oC and heated to 218oC, c) PEFT

95/05 at 200oC, d) PEFT 90/10 at 200oC, e) PET at 225oC, f) PEFT 05/95 at 180oC, g)

PEFT 10/90 at 180oC and h) PEFT 15/85 at 180oC.

3.7. Thermal stability

The potential uses of polymers in applications is dependent on their thermal stability.

So, the thermal stability of the PEFT copolymers was studied by thermogravimetric

analysis (TGA). In general, the decomposition occurred at higher temperatures with

increasing the ET content. Indicative plots are shown in Figure 10. As was found in

previous studies, such as the similar one from Sousa et al. [19] thermal decomposition

of PEF begins at lower temperatures compared to PET. Moreover, the decomposition

of EF-rich copolymers proceeds in three steps, since in the early two steps the

degradation of the furanic and benzene units takes place [19, 84]. As a matter of fact

some copolymers showed a further decrease in the temperature of decomposition

initiation, but this can also be associated with loss of remaining solvent.

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Figure 10. TGA curves for PEF, PET and the PEFT 70-30, 30-70 and 15-85

copolymers a) in the whole mass scale range and b) in the range corresponding to up

5% mass loss.

3.8 Decomposition mechanism study by Py-GC/MS

In order to fully understand the effect of comonomer composition on thermal stability,

the mechanism of degradation under pyrolytic conditions was studied with

Py-GC/MS. First, the polyesters were subjected to EGA analysis, and the resulting

profiles are presented in Figure 11. The curves arise from the evolution of pyrolysis

products in a certain temperature range. While neat PEF starts producing gaseous

compounds at a temperature range between 300-400 °C, increasing the ET

comonomer content shifts the peak to higher temperatures, up to 400-480 °C for neat

PET. This increase in thermal stability is associated with the higher viscosity and

therefore the molecular weight of the copolyesters that are rich in ET units. These

results are in agreement with TGA measurements, and bibliography [38]. The

temperature chosen for single-shot pyrolysis experiments was 400 °C, in order to

compare the pyrolysis products under same conditions.

Figure 11. EGA thermograms of PEF, PET, PEFT 85-15 and PEFT 15-85

Decomposition profiles of terephthalate polyesters have been widely studied

in the literature [85], and it is generally agreed that the main decomposition

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mechanism consists of hererolytic scission via a six-membered ring intermediate,

where the hydrogen from a β-carbon to the ester group is transferred to the ester

carbonyl, followed by scission at the ester links, producing compounds with

carboxylic and vinyl end groups. In higher pyrolysis temperatures, radical (homolytic)

degradation pathways may also occur [86]. Regarding the decomposition of furanic

polyesters, our research group was the first to study its degradation mechanism

through extensive Py-GC/MS studies, and the findings suggested that the reaction

pathway followed is similar to that of PET [87-89] . Heterolytic scission products are

mainly detected at lower temperatures, and homolytic scission is found to occur in

higher temperatures for furanic polyesters as well.

Figure 12 presents the recorded chromatographs after pyrolysis of the

polyesters at 400 °C. As the ET content increases, the chromatographs exhibit less

peaks and simpler patterns, because the more thermally stable the polyester, fewer

degradation products are released. The pyrolysis products that correspond to the main

peaks of the chromatographs were identified through their MS spectra and are

presented in Table 1 – Supplementary Information.

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Figure 12. Chromatographs of the polyesters after pyrolysis at 400 °C.

For all polyesters, pyrolysis products derived from both heterolytic and

homolytic processes were detected. Most of the compounds have carboxylic or vinylic

end groups, suggesting β-scission is the main degradation mechanism (Scheme 2).

The carboxylic end group can undergo decarboxylation, thus the detection of

compounds with end benzene or furan rings. Similar volatile compounds were

reported by Buxbaum [90] concerning the degradation of PET, such as vinyl, carboxyl

and aromatic-ring terminated derived from heterolytic scission, and aldehyde, methyl,

or hydroxyl ended compounds that result from homolytic scission mechanisms

(Scheme 3).

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Scheme 2: Heterolytic scission mechanism of PEFT polyesters

Scheme 3: Homolytic scission mechanism of PEFT polyesters

Conclusions

Poly(ethylene furanoate-co-terephthalate) random copolymers were successfully

synthesized by melt and solid state polycondensation (SSP). WAXD patterns of the

copolyesters as well as the pseudoeutectic behavior evidenced isodimorphic

cocrystallization. The crystallization rates and final degree of crystallinity were found

to decrease with increasing comonomer content. In general copolymers with high

terephthalate content crystallized faster. The thermodynamic analysis of the melting

point depression proved that only a small portion of the comonomer units can be

introduced into the homopolymer crystals. The spherulitic morphology of the

copolymers generated upon isothermal crystallization was investigated using PLM.

Thermal stability of PEFTs was better for the copolymers with higher terephthalate

content. The main pyrolysis products for PEF, PET and their copolymers resulted

mainly from heterolytic scission and less from homolytic scission processes.

The copolymers prepared in this work exhibited characteristics which can be

ultimately comparable with petroleum-based polymers, such as their adequate thermal

stability, their high glass transition temperature, their average crystallization times and

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their melting tempertures. These facts indicate their applicability in several

applications and their potential to be used as eco-friendly alternatives to the

environmentally harmful terephthalate-based polyesters.

References

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