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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
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].
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
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
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
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.
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.
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
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.
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
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
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).
Figure 2. a) DSC heating scans of the as received samples, b) heating scans of the
melt quenched samples and c) cooling scans.
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
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].
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
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].
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
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].
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.
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)
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.
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)
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
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].
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
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.
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.
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
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.
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).
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
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.
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