Utilizing Furfural-Based Bifuran Diester as Monomer and Comonomer for High-Performance Bioplastics: Properties of Poly(butylene furanoate), Poly(butylene bifuranoate), and Their Copolyesters

Article abstract of DOI:10.1021/acs.biomac.9b01447

Two homopolyesters and a series of novel random copolyesters were synthesized from two bio-based diacid esters, dimethyl 2,5-furandicarboxylate, a well-known renewable monomer, and dimethyl 2,2′-bifuran-5,5′-dicarboxylate, a more uncommon diacid based on biochemical furfural. Compared to homopolyesters poly(butylene furanoate) (PBF) and poly(butylene bifuranoate) (PBBf), their random copolyesters differed dramatically in that their melting temperatures were either lowered significantly or they showed no crystallinity at all. However, the thermal stabilities of the homopolyesters and the copolyesters were comparable. Based on tensile tests from amorphous film specimens, it was concluded that the elastic moduli, tensile strengths, and elongation at break values for all copolyesters were similar as well, irrespective of the furan:bifuran molar ratio. Tensile moduli of approximately 2 GPa and tensile strengths up to 66 MPa were observed for amorphous film specimens prepared from the copolyesters. However, copolymerizing bifuran units into PBF allowed the glass transition temperature to be increased by increasing the amount of bifuran units. Besides enhancing the glass transition temperatures, the bifuran units also conferred the copolyesters with significant UV absorbance. This combined with the highly amorphous nature of the copolyesters allowed them to be melt-pressed into highly transparent films with very low ultraviolet light transmission. It was also found that furan-bifuran copolyesters could be as effective, or better, oxygen barrier materials as neat PBF or PBBf, which themselves were found superior to common barrier polyesters such as PET.

Full text of DOI:10.1021/acs.biomac.9b01447

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Article
Utilizing Furfural-based Bifuran Diester as Monomer and Comonomer
for High-Performance Bioplastics: Properties of Poly(butylene
furanoate), Poly(butylene bifuranoate), and their Copolyesters
Tuomo P. Kainulainen, Terttu I. Hukka, Hüsamettin Deniz Özeren,
Juho Antti Sirviö, Mikael S. Hedenqvist, and Juha Pentti Heiskanen
Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.9b01447 • Publication Date (Web): 02 Dec 2019
Downloaded from pubs.acs.org on December 8, 2019
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Utilizing Furfural-based Bifuran Diester as Monomer and
Comonomer for High-Performance Bioplastics: Properties of
Poly(butylene furanoate), Poly(butylene bifuranoate), and their
Copolyesters
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Tuomo P. Kainulainen,a Terttu I. Hukka,b Hüsamettin D. Özeren,c Juho A. Sirviö,d Mikael S.
Hedenqvist,c Juha P. Heiskanen*,a
aResearch Unit of Sustainable Chemistry, University of Oulu, P.O. Box 4300, FI-90014 Oulu,
Finland
bLaboratory of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541,
FI-33101 Tampere, Finland
cDepartment of Fibre and Polymer Technology, School of Engineering Sciences in Chemistry,
Biotechnology and Health, KTH Royal Institute of Technology, SE-100 44, Stockholm, Sweden
dFibre and Particle Engineering Research Unit, University of Oulu, P.O. Box 4300, FI-90014 Oulu,
Finland
Abstract
Two homopolyesters and a series of novel random copolyesters were synthesized from two bio-based
diacid esters, dimethyl 2,5-furandicarboxylate, a well-known renewable monomer, and dimethyl 2,2’-
bifuran-5,5’-dicarboxylate, a more uncommon diacid based on biochemical furfural. Compared to
homopolyesters poly(butylene furanoate) (PBF) and poly(butylene bifuranoate) (PBBf), their random
copolyesters differed dramatically in that their melting temperatures were either lowered significantly
or they showed no crystallinity at all. However, the thermal stabilities of the homopolyesters and the
copolyesters were comparable. Based on tensile tests from amorphous film specimens, it was
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concluded that the elastic moduli, tensile strengths, and elongation at break values for all copolyesters
were similar as well, irrespective of the furan:bifuran molar ratio. Tensile moduli of approximately 2
GPa and tensile strengths up to 66 MPa were observed for amorphous film specimens prepared from
the copolyesters. However, copolymerizing bifuran units into PBF allowed the glass transition
temperature to be increased by increasing the amount of bifuran units. Besides enhancing the glass
transition temperatures, the bifuran units also conferred the copolyesters with significant UV
absorbance. This combined with the highly amorphous nature of the copolyesters allowed them to be
melt-pressed into highly transparent films with very low ultraviolet light transmission. It was also
found that furan-bifuran copolyesters could be as effective, or better, oxygen barrier materials as neat
PBF or PBBf, which themselves were found superior to common barrier polyesters such as PET.
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Keywords: Barrier, Copolyester, Furan, Renewable
Introduction
Scheme 1. Furan-based polyesters as potential alternatives for current commodity polyesters
Use of biomasses as alternative feedstocks for chemicals is one of the most important aspects of
reducing dependence on fossil-based resources.1 Furans are key bio-based platform-chemicals, as
they are readily prepared from monosaccharides, which plant-based biomasses contain in large
quantities as two important polymerized forms, i.e. cellulose and hemicellulose.2 Moreover, many
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industrial and agricultural processes result in by-products, e.g. sugar-cane bagasse or corncobs that
are rich in cellulose and hemicellulose. After depolymerization, their C5 and C6 sugars can be readily
dehydrated into two important furans, furfural and 5-(hydroxymethyl)furfural (HMF), respectively
(Scheme 1).3,4 Furfural is especially notable, as its commonly derived from the inedible, C5 sugar
containing hemicellulose byproducts of various crops; the industrial production of furfural has been
ongoing since the 1920s.5 The first processes used leftover oat hulls as the feedstock and provided a
way to valorize a “waste” biomass into furfural, which is a highly useful chemical intermediate.
Lately, the focus has been on HMF, as it is one of the chief precursors to 2,5-furandicarboxylic acid
(FDCA).6 Notably, HMF can be directly oxidized into FDCA, whereas additional steps are necessary
to convert furfural into FDCA.7,8 FDCA continues to attract considerable interest due to its similarity
with terephthalic acid (PTA), which in turn is the precursor to the commodity polyester poly(ethylene
terephthalate) (PET).9,10 Excellent properties of PET have made it especially prevalent in packaging,
and replacing polyesters like PET with furan-based alternatives could help reduce the environmental
impact of non-renewable packaging plastics, which are under high demand.11 Another important
commodity polyester based on PTA is poly(butylene terephthalate) (PBT). It is more commonly used
in manufacture of molded articles where the rapid crystallization of PBT melts is a benefit.12 The
faster crystallization is made possible by the replacement of the stiffer ethylene chain in PET with a
more flexible butylene chain in PBT. However, well-known, widely used technical plastics such as
PET and PBT are not easily displaced by new biomass-based alternatives. For this reason, methods
for producing terephthalic acid from biomass-based routes, including conversion of furfural13, have
been studied and reviewed.14 It has been estimated that some bio-based routes to terephthalic acid
might become feasible soon, and that they could reduce the environmental impact when compared to
the use of non-renewable sources.15
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Still, the straightforward nature of simple furans makes them interesting as bio-based aromatic
polymer precursors. Moreover, FDCA-based polyesters possess value beyond simply their biomass
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origin. In a relatively short time, the potential of furan compounds as precursors for new high-
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performance yet renewable materials has become evident: Poly(ethylene furanoate) (PEF)16,17,18
,
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poly(propylene furanoate) (PPF)19, and poly(butylene furanoate) (PBF)20 have been shown to far
exceed the performance of PET in terms of gas permeability. For example, PEF has been reported to
provide 11x and 19x reductions in oxygen and carbon dioxide permeabilities, respectively, when
compared to PET, potentially enabling furan-based packaging materials with performance
improvements. Reduced permeability can increase the shelf life of foods and beverages, for instance,
and enable the use of thinner and lighter packaging. Thus, the potential of furan compounds as
precursors for new high-performance yet renewable materials has become evident.
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Recently, the use of 2,2’-bifuran-5,5’-dicarboxylic acid (BFDCA) as a precursor for novel bio-based
polyesters has been described.21,22 In addition to this, it has only been demonstrated as a precursor for
novel polyamides.23 BFDCA can be regarded as a furfural-derived C10 monomer with planar
biheteroaryl core, which sets it apart from not only FDCA and PTA but also many other bio-based
bicyclic monomers as well. Polyesters incorporating bio-based bicyclic structures with aliphatic
structures24,25,26,27, especially from isosorbide, are relatively well-known whereas bicyclic aromatic
examples are scarcer and more singular, e.g. indole-based polyesters28. The use of bicyclic monomers
is in large part motivated by their inherent rigidity compared to e.g. linear acyclic monomers, thus
usually leading to polymers with enhanced Tg. Our preliminary investigation showed that BFDCA-
based polyester poly(ethylene bifuranoate) (PEBf) has higher glass-transition temperature than PET
and PEF, meaning BFDCA-based polyesters should maintain higher rigidity at elevated temperatures.
In addition, the highly conjugated molecular structure of BFDCA also inherently provides polyesters
with stronger UV light absorption, again setting it apart from more common bio-based monomers.22
Limited data is available on the barrier properties of bio-derived polyesters containing other rigid
bicyclic structures such as isosorbide, with one such study concerning copolyesters based on
poly(butylene succinate) (PBS) that incorporated isosorbide units.29 The results suggested that the
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effect of the isosorbide units can depend on the unit content: The stiffness of isosorbide moieties at
low content (11 mol%) resulted in reduced oxygen permeability, while higher contents reduced
crystallinity to the point that an increase in oxygen permeability occurred with respect to PBS. In our
investigation, PEBf possessed lower O2 and water vapor permeability than a comparable PET sample,
hinting that bifuran units could be helpful in reducing gas permeability. Due to the initial promise
shown by BFDCA as a new bio-based monomer, it could offer a novel way of utilizing furfural as a
feedstock for high-performance bioplastics.
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In this paper, the synthesis of new random copolyesters of poly(butylene furanoate) (PBF) and
poly(butylene bifuranoate) (PBBf), derived from FDCA and BFDCA, respectively, are presented.
Thermal and mechanical properties of the copolyesters are then compared to the pure homopolyesters
derived from FDCA and BFDCA. Specifically, it is shown that having either FDCA or BFDCA as
the minor comonomer can significantly influence the crystallinity of the resulting copolyester.
However, it is also shown that potentially beneficial UV filtering property can be realized in FDCA-
based polyesters incorporating some BFDCA as the minor component. This allows for
crystallizability from PBF to be retained to some extent. Additionally, the copolyesters are shown to
have promise as excellent barrier against oxygen, much like PBF and PBBf, despite their more
amorphous nature.
Experimental
1,4-Butanediol was distilled under reduced pressure onto 3Å molecular sieves prior to use. Otherwise,
1
commercially available solvents were used as received. H NMR chemical shifts are referenced to
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residual solvent peaks in TFA-d and CDCl3 at 11.50 and 7.27 ppm, respectively. C NMR shifts
were referenced to the trifluoromethyl carbon (116.60 ppm, F3CCOOD) in TFA-d. All NMR
measurements were carried out at room temperature.
Dimethyl 2,5-furandicarboxylate (1): 2,5-Furandicarboxylic acid (4.00 g, purity 98%) was refluxed
in large excess of dry methanol (120 mL) overnight with concentrated sulfuric acid (2 equiv) under
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argon. The cooled mixture was then partially evaporated to about 1/3–1/2 volume and diluted with
cold deionized water to precipitate the product. After filtration and drying in air, the raw product was
dissolved in ethyl acetate and passed through a layer of silica gel. After evaporation of the solvent,
dimethyl 2,5-furandicarboxylate (4.25 g, 93%) was collected as a white powder. 1H NMR (400 MHz,
CDCl3, ppm):  7.23 (s, 2H), 3.94 (s, 6H).
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Dimethyl 2,2’-bifuran-5,5’-dicarboxylate (2): The synthesis method we reported previously was
followed to afford dimethyl 2,2’-bifuran-5,5’-dicarboxylate (4.53 g, 91%) as small white needles.22
1H NMR (400 MHz, CDCl3, ppm): δ 7.26 (d, 2H, J = 3.7 Hz), 6.90 (d, 2H, J = 3.7 Hz), 3.93 (s, 6H).
Polyester synthesis: The polyesters were synthesized by weighing the appropriate amount of
diester(s) (8 mmol total) into a 50 mL round-bottom flask equipped with a magnetic stirring bar. 1,4-
Butanediol (3 equiv) was added, together with tetrabutyl titanate (0.1 mol% relative to the total diester
amount) dissolved in small amount of dry toluene (0.03 M). The flask was connected to a short-path
type distillation bridge and the reaction system was evacuated and filled with argon gas. After five
evacuation and filling cycles, the flask was heated to 180 C using an aluminium heating block. After
3 h, the pressure was gradually lowered to 2 mbar over 1 h. Then, the temperature of the system was
increased to 250 C to initiate the polycondensation and distill off 1,4-butanediol from the flask. After
1 h, the polycondensation was stopped, and the cooled polyester was allowed to dissolve in a mixture
of trifluoroacetic acid (TFA) and chloroform (1:5 v/v). The polyester solution (approx. 0.1 g/mL) was
then slowly mixed into 10-fold excess of methanol to yield white-to-off white fibrous solid. After
filtration onto paper, the fibrous solid was rinsed with methanol, and gently pressed dry. Final drying
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was accomplished under vacuum at 60 C to a constant weight. For 1H and C NMR, 5–10 mg of
polyester was dissolved in ca. 0.4 mL of deuterated trifluoroacetic acid or CDCl3-TFA mixture
(approx. 3:1 v/v). For proton decoupled 13C NMR measurements, 1000–5000 scans were collect with
acquisition time of 1.4 s.
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Dilute solution viscometry: Intrinsic viscosities were determined from flow times of pure TFA (t0)
and filtered polyester solutions (0.5 g/dL) in TFA (t) in a micro-Ubbelohde viscometer submerged in
30.0 C water bath. The average of ten flow times were used to calculate t0 and t.
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ATR-FTIR: The vibrational properties of the polymer samples were analyzed using Perkin Elmer
Spectrum One FT-IR Spectrometer with the attenuated total reflection (ATR) technique in the
wavelength region of 650–4000 cm-1, resolution of 2 cm-1, accumulation of eight scans, in
transmission mode.
Differential scanning calorimetry (DSC): Differential scanning calorimeter (Mettler Toledo DSC
821e and DSC1) was used to investigate the phase transition temperatures at the range of 25–300 C
with heating and cooling rates of 10 C/min under nitrogen (N2) gas flow (60 cm3/min). The polymer
samples (ca. 5 mg), were placed into 40 L Al crucibles sealed with pierced lids. Thermal history
was deleted by the first heating.
Thermogravimetric analysis (TGA): Decomposition properties and thermal stability of the polymer
samples (ca. 10 mg) were studied with a thermogravimetric analyzer (Mettler-Toledo TGA851e).
Samples were placed into a 70 L Al2O3 crucible, which was covered with a pierced lid and heated
from 30 to 700 C under nitrogen flow (95 cm3/min) using a heating rate of 10 C/min.
Melt hot pressing: Dried polymer samples were melt-pressed into films suitable for characterization
of mechanical performance and barrier properties. Two aluminium plates coated with thin polyimide
film were used for the pressing, with the appropriate amount of polyester added between the pre-
heated plates. The thickness was controlled using layers of glass fiber mat placed around the sample.
The polyesters were first allowed to melt inside the press at temperatures 20–30 C above their
expected melting temperatures (listed in Table 1, e.g. 160 C for PBF50Bf50) for 5 minutes. 20 kN
pressing force was then applied for 1 minute. The samples were cooled inside the press with an
integrated water-cooling circuit (cooling rate approx. 30–40 C/min) to provide low-crystallinity
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films. The plates were then carefully separated, and the films were peeled off to yield flexible,
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transparent films. H NMR samples were also prepared from pressed films to observe possible
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Tensile testing: Tensile test specimens were rectangle-shaped, with 5 mm width and thicknesses
between 100–200 m. The specimens were stored at room temperature for 1–2 weeks prior to the
uniaxial tensile tests and a further 48 h under stable conditions (23 C, 10% RH) before the tensile
tests. The tensile tests were conducted under the same conditions. Tensile modulus, tensile strength
and elongation at break were measured (Instron 5544, USA) using a gage length of 30 mm and
crosshead speed of 5 mm/min.
Dynamic mechanical analysis: The dynamic mechanical properties were obtained in tensile mode
using a DMA Q800 (TA Instruments, USA). Samples with a size of 20×5 mm2 were used. All runs
were performed in a “multi-frequency, strain” mode at 1 Hz, 0.08% strain, from −50 to 120 C at a
heating rate of 3 C/min with a 15 mm gauge length.
Gas permeability analysis: The oxygen transmission rate (OTR) of the melt-pressed films was
measured using a MOCON OxTran 2/20 at 23 C and 50% RH (relative humidity) with a specimen
exposure area of 5 cm2. Oxygen was then introduced to one surface of the sample at atmospheric
pressure. Permeated oxygen was carried away from the downstream surface of the film sample by the
carrier gas (N2). Oxygen permeability (OP) was calculated by multiplying OTR by the thickness of
the film and dividing it by the difference in the partial pressure of the oxygen gas between the two
sides of the film.
XRD: X-ray diffraction was used to probe crystallinity from the melt-pressed films. Measurements
were taken on a PANalytical X’Pert MPD Pro using copper radiation (Kα1 = 1.5405980 Å), equipped
with an X’Celerator-detector.
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UV-vis: Shimadzu UV-1800 spectrophotometer was used to acquire absorption and transmission
spectra from melt-pressed films and solutions of monomers 1 and 2. For both monomers, a 0.01
mg/mL chloroform solution was used to acquire the UV-vis absorption spectra (Figure S15).
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Results and Discussion
To prepare the polyesters, dimethyl esters of FDCA (1) and BFDCA (2) were reacted with 1,4-
butanediol in a two-step procedure (reaction scheme in Table 1). The feed ratio of diesters 1 and 2
was adjusted to provide polyesters with the desired molar ratio of furan and bifuran units.
Titanium(IV)butoxide (TBT) was added as a catalyst for the reaction, as titanium alkoxides are highly
active in both transesterification and polycondensation steps, enabling rapid reactions.30 Due to the
relatively small scale of the reactions (1–2 g) and the low catalyst loading (0.1 mol%), TBT was
added as a solution in dry toluene, enabling more accurate metering of the catalyst.
Table 1. Synthesis results for copolyesters
Ti(O-n-Bu)4 (0.1 mol%)
1,4-BDO (3 equiv)
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
+
x
O
O
y
O
O
O
1. 180 °C, Ar, 4 h
2. 250 °C, 2 mbar, 1 h
O
O
1
2
Polyester
Unit ratio, 1:2 (mol%)
Number-average sequence lengthsc
Yield
IVb
Polyester
Ric
Feed
100:0
90:10
75:25
50:50
25:75
10:90
0:100
Producta
100:0
90:10
76:24
51:49
26:74
10:90
0:100
LFF
-
LBB
0
(%)
93
93
95
98
90
91
97
(dL/g)
PBF
0.77
0.95
0.90
0.87
0.67
0.70
0.72
0
PBF90Bf10
PBF75Bf25
PBF50Bf50
PBF25Bf75
PBF10Bf90
PBBf
8.84
3.65
2.08
1.27
1.12
0
1.16
1.38
1.89
4.13
6.94
-
0.98
1.00
1.01
1.03
1.04
0
aAs determined from 1H NMR integrals (TFA-d). bIntrinsic viscosity evaluated using the Billmeyer relation31, flow
times determined in TFA (30 C) at a concentration of 0.5 g/dL. cDetermined by 13C NMR using Equations 1 and 2,
randomness index Ri calculated from Equation 3.
In all polycondensation reactions with bifuran 2 present, the melts gradually developed light yellow
color during the polycondensation step. After dissolution and precipitation, melt-pressing was used
to obtain films with similar light-yellow appearance (Figure 1). In contrast, the PBF melt had
practically colorless appearance at the end of the polycondensation, and even the melt-pressed films
retained an almost colorless appearance. It seems logical that the increased conjugation and higher
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molar extinction coefficient of 2, when compared to 1, can result in more intensely colored side-
products at elevated temperatures. TBT and similar titanium catalysts are also known to have a
tendency to impart color.30 However, the purity of all synthesized polyesters was comparable based
on 1H NMR analysis (Figure 2). Most importantly, the diester feed ratio was accurately reproduced
in the final polyesters (Table 1). Thus, both diesters must have similar reactivity and stability under
the employed reaction conditions.
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45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 1. Digital image of a melt-pressed copolyester (PBF90Bf10) film.
a)
b)
d
H
e'
H
H
68.54
O
H
H H H
O
O
O
O
O
O
H H H
O
O
O
O
Fu
O
Fu
O
Bf
O
O
O
O
Fu
Bf
O
Bf
O
H
H H H
O
H
H H
H
O
O
O
H
a
H
H
b
H
c
68.97
e
d'
68.64
69.06
b
c
d'
e'
PBBf
PBBf
e, e'
e, e'
d, d'
d, d'
c
c
PBF10Bf90
PBF10Bf90
b
a
PBF25Bf75
PBF50Bf50
PBF25Bf75
PBF50Bf50
b
a
68.97
68.64
68.54
69.06
e, e'
e, e'
d, d'
d, d'
a
c
b
b
a
a
PBF75Bf25
PBF90Bf10
PBF75Bf25
PBF90Bf10
PBF
c
e, e'
e
d, d'
d
c
b
a
PBF
3
7
6
5
4
2
69.0
68.5
Chemical Shift (ppm)
Chemical Shift (ppm)
Figure 2. a) 1H NMR signal assignments of the polyesters in TFA-d. b) 13C NMR assignment of the sequence sensitive
butylene group carbon in TFA-d (Fu=furan, Bf=bifuran).
A closer look at the 1H NMR spectra (Figures S3–9 in Supporting Info) reveals convolution by the
1
expected random distribution of furan and bifuran units along the polymer chains. The H NMR
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spectrum of PBF50Bf50, for example, clearly shows that the hydrogen atoms on the furan and bifuran
moieties are sensitive to neighboring groups, resulting in overlapping signals and peak broadening
(Figure S6). Utilizing 13C NMR, which allows a more detailed analysis due to the broader chemical
shift range, a reasonable assignment of the chain structure was obtained (Figure 2b, Table 1). Using
areas under the corresponding peaks (AFF, ABB, AFB, and ABF), the number-average sequence lengths
of furan (LFF) and bifuran (LBB) segments were estimated (Equations 1 and 2).32 Previously, such
evaluation of sequence distribution was approximated to result in error of ±10%.32 For the
calculations, the four different butylene carbons indicated in Figure 2b were assumed to always
provide the same peak areas relative to their abundance. This assumption was tested with a 13C NMR
measurement from a 1:1 molar mixture of PBBf and PBF in TFA-d, which yielded peak integrals
with 1:1 ratio for the carbons at 68.97 and 68.64 ppm. Accordingly, calculated randomness indices
(Ri, Equation 3) point to highly random distributions of both furan-based moieties (Ri values near
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unity) in the copolyesters.
1
(
)
퐴
+ ⁄
퐴
+퐴
퐹퐹
퐹퐵 퐵퐹
2
퐿퐹퐹
=
=
(1)
1
(
)
⁄
퐴
+퐴
퐹퐵 퐵퐹
2
1
(
)
퐴
+ ⁄
퐴
+퐴
퐵퐵
퐹퐵 퐵퐹
2
퐿퐵퐵
(2)
(3)
1
(
)
퐵퐹
⁄
퐴
+퐴
퐹퐵
2
1
1
푅푖 =
+
퐿
퐵퐵
퐿
퐹퐹
Due to similar structural features, the FTIR spectra of all prepared polyesters bear notable
resemblance to each other (Figures S3–9). The differences are best illustrated by a direct comparison
of PBF and PBBf (Table 2, Figure S11), where several peaks are seen to appear at different
wavenumbers and intensities. Prominent features of PBBf are the two intense peaks at 798 and 1444
cm-1, which are not observed in PBF. In addition, instead of the two peaks observed at 3152 and 3119
cm-1 in PBF, a single peak is observed in PBBf at 3141 cm-1, with a slight inflection at 3118 cm-1. As
expected, these differences can be linked to the motions of the furan rings.33 Based on simulations,
we can further ascribe the differences to certain specific structural features in monomers 1 and 2
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(Table S1). The FTIR spectra of the copolyesters reflect the structural features present in both
polyesters, only varying in intensity based on the composition, as expected.
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8
9
Table 2. Notable FTIR wavenumbers for PBF and PBBf
Wavenumber (cm-1)
Assignment
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22
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PBF
3153
PBBf
3141
Furans(C–H)
Furanas(C–H)
Methylene C–H
Ester C=O
3119
3118
2965, 2897
1717
2963, 2897
1711
Bifuran s(C=C) +
-
1444
s,ip(C–O–C)
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0.5 W g-1
0.5 W g-1
a)
b)
4
PBF
PBF
5
6
PBF90Bf10
PBF90Bf10
7
8
9
PBF75Bf25
PBF50Bf50
PBF25Bf75
PBF75Bf25
PBF50Bf50
PBF25Bf75
PBF10Bf90
10
11
12
13
14
15
16
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22
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32
33
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59
60
PBF10Bf90
PBBF
PBBF
10 oC/min
-10 oC/min
25
50
75 100 125 150 175 200 225 250
Temperature (oC)
40
60
80 100 120 140 160 180 200
Temperature (oC)
0.5 W g-1
0.5 W g-1
d)
c)
PBF
PBF90Bf10
PBF75Bf25
PBF90Bf10
PBF75Bf25
PBF50Bf50
PBF50Bf50
PBF25Bf75
PBF25Bf75
PBF10Bf90
PBBF
PBF10Bf90
10 oC/min
-5 oC/min
25
50
75 100 125 150 175 200 225 250
Temperature (oC)
40
60
80 100 120 140 160 180 200
Temperature (oC)
0.5 W g-1
e)
PBF90Bf10
PBF75Bf25
PBF50Bf50
PBF25Bf75
PBF10Bf90
5 oC/min
25
50
75 100 125 150 175 200 225 250
Temperature (oC)
Figure 3. a) 1st heating. b) 1st cooling. c) 2nd heating. d) 1st cooling (copolyesters). e) 2nd heating (copolyesters).
The thermal properties of the dried, methanol-precipitated polyesters were characterized with DSC
(melting enthalpies corresponding with Fig. 3 are collected in Table S2). Heating and cooling at 10
C/min rate shows that both PBF and PBBf are typical semi-crystalline materials with pronounced
cold-crystallization and melting peaks (Figure 3a–c, Table 3). The thermal properties of PBF34 and
PBBf21 also match the previously reported values, though Tg has not been reported for PBBf
previously. The copolyesters, on the other hand, lack the pronounced crystallization and melting
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peaks. The first heating scans (Figure 3a) do show multiple endothermic peaks (Figure 3a) above Tg,
but these disappear in the subsequent heating scan. As all samples were precipitated from solution
into methanol and dried for several days at 60 C, these endothermic peaks likely result from
annealing effects. Similar annealing at set temperatures is known to cause multiple melting peaks to
appear in similar polyesters such as PBT.35 The endothermic peaks located at the glass transitions
(Figures 3a and 3c) instead result from enthalpic relaxation. Slowing the heating and cooling rates to
allow more crystallization to occur only changes the behavior of PBF90Bf10 and PBF10Bf90 samples
(Figures 3c and 3e). Even at the low scanning rate (5 C/min), neither PBF90Bf10 nor PBF90Bf10, or
the other copolyesters, appear to crystallize from the melt as an exothermic peak is not resolved
(Figure 3d). However, PBF90Bf10 nor PBF90Bf10 samples did cold-crystallize during the subsequent
second heating cycle, as evidenced by the exothermic peak before the corresponding endothermic
melting peak (Figure 3e). From comparison to the neat polyesters PBF and PBBf, it is obvious that
even 10 mol% comonomer incorporation results in hindered crystallization and lowered melting
temperatures. Further comonomer incorporation results in inhibition of crystallization, i.e. random
copolyesters PBF75Bf25, PBF50Bf50, and PBF25Bf75 appear practically amorphous.
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From the data collected into Table 3, it is clear that the bifuran moieties increase the stiffness of the
polyester chains, as Tg increases quite linearly with higher bifuran content. Again, for shorter
methylene chains in poly(ethylene furanoate) (Tg ~79 C34) and poly(ethylene bifuranoate) (Tg ~107
C22), the bifuran units provided similar Tg enhancement over furan units. In summary, furan or
bifuran comonomer incorporation hinders crystallization, but increasing bifuran content results in
monotonously increasing Tg. The impact on crystallinity is lower if the content of defects resulting
from the incorporation of the minor comonomer is kept relatively small, e.g. below 10 mol%.
Compositions approaching 50:50 molar ratio are much more amorphous and are without clear melting
events.
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Table 3. Thermal properties of PBF, PBBf, and their random copolyesters
Tm (C)
Sample
Tg (C)
Tcc (C)
Td5 (C)
Td (C)
1st heating
2nd heating
109
nd (119*)
nd
PBF
39
43
49
53
58
60
62
173
172
156 (158*)
366
365
364
365
367
364
370
391
391
392
393
398
397
402
PBF90Bf10
PBF75Bf25
PBF50Bf50
PBF25Bf75
PBF10Bf90
PBBf
81, 157
82, 130
97, 145
89, 188
202
nd
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11
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20
21
22
23
24
25
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27
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29
30
31
32
33
34
35
36
37
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45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
nd
nd
nd
nd
202 (184*, 202*)
215
144 (134*)
122
217
Tg: Glass transition temperature from 2nd heating at 10 C/min. Tm: Melting temperature from heating at 10 C/min (*:5
C/min) Tcc: Peak of cold-crystallization from 2nd heating at 10 C/min (*:5 C/min). Td5: Temperature at 5% sample mass-
loss. Td: Temperature at peak mass-loss rate. nd: not detected.
Thermogravimetric analysis reveals little variation in the thermal stabilities (Table 3 and Figure 4),
the most significant difference arising from the residual weight observed at 700 C (i.e. char yield).
In general, the bifuran content increased the residual weight, while neat PBF had the lowest residual
mass (10%). Each polyester composition underwent a single-step decomposition in the range of 391–
402 C, which is close to that of related polyesters, e.g. PET, PBT, and PEF.36,37 This result indicates
that the thermal stabilities are high enough to allow the melts to be processed in similar fashion.
100
80
PBF25Bf75
PBF10Bf90
PBF50Bf50
PBBf
PBF75Bf25
PBF90Bf10
PBF
60
40
20
0
Residual mass
at 700 o
C
200
300
400
500
600
700
Temperature (oC)
Figure 4. Thermogravimetric decomposition curves for PBF, PBBf, and their random copolyesters under N2 (heating
rate 10 C/min).
The DMA results (Figure S13) are in a good agreement with the results obtained from DSC. The drop
in storage modulus E’, the peak of loss modulus E’’ and tan  all follow the same pattern as the Tg
values obtained using DSC, though exactly the same values are not obtained with the different
techniques. As can be expected, the polyesters with high Tg retain more of their stiffness at higher
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temperatures, e.g. PBBf retains a storage modulus of more than 1 GPa to up to 62 °C, whereas PBF
drops below 1 GPa after temperature reaches 32 °C. In DMA, clear cold-crystallization is also
observed in PBF, PBF90Bf10, and PBBf samples. The corresponding increases in storage moduli begin
at roughly 70 °C, 90 °C, and 110 °C, respectively. Other polyester samples are evidently not able to
benefit from cold-crystallization, and continue to lose stiffness after 100 °C, having low E’ values at
below 3 MPa.
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Another notable feature in the DMA curves is the small size of the tan  peak for PBF compared to
the other polyesters investigated. This suggests a higher crystallinity in PBF initially; despite the
broad amorphous halo observed in XRD (Figure S16) consistent with the other samples. Polarized
microscopy, coupled with Bertrand lens diffraction, revealed a birefringent pattern due to the presence
of spherulites in the melt pressed PBF sample (Figure S14). The melt pressed PBBf sample showed
a less clear birefringence due to a very low degree of crystallinity, and the Bertrand lens pattern
revealed therefore less developed spherulites. The birefringent pattern in the PBBf sample became
significantly clearer after cold crystallization (after the DMA run) and the Bertrand lens pattern
indicated more well developed spherulites after the cold crystallization. The birefringent pattern
became also clearer after cold crystallization of the PBF sample, due to, as in the case of PBBf, a
higher crystallinity.
According to the tensile test data (Table 4), all polyesters were characterized by high rigidity at room
temperature. While the Et was almost constant at around 2 GPa for the range of polyesters, the bifuran
units appeared to endow PBBf and the copolyesters with higher tensile strength (≥65 MPa) compared
to PBF (59 MPa). In the case of PBF, the mechanical properties can also be compared with previous
values found in the literature, though a very wide range of results can be found, e.g. b = 3–
200%38,39,40, most likely due to variations in samples and testing methods (e.g. pressed film instead
of an injection molded piece). Our results roughly fall in line with previous characterizations, though
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the measured elongation at break is lower than we expected. However, within this test series, PBBf
and the copolyesters offered similar or superior mechanical properties compared to PBF.
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Table 4. Measured tensile properties of copolyesters
Samplea
Et (GPa)
2.0±0.1
2.1±0.02
2.0±0.1
2.2±0.1
2.0±0.1
2.1±0.04
2.0±0.1
m (MPa)
58.9±2.2
65.1±2.5
66.6±3.1
66.0±1.3
65.8±4.2
65.5±2.7
66.0±3.0
b (%)
4.0±0.3
4.2±0.2
5.0±0.2
4.3±0.4
5.0±0.2
5.0±0.4
5.4±0.2
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33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
PBF
PBF90Bf10
PBF75Bf25
PBF50Bf50
PBF25Bf75
PBF10Bf90
PBBf
aAt least five amorphous specimens were evaluated for each polyester composition. Et = Tensile modulus. m =
maximum tensile stress. b = elongation at break.
As discussed in the introduction, polyesters derived from FDCA generally have low gas permeability
and are considered particularly promising as barrier materials that could replace fossil-based plastics,
e.g. PET in some applications. On this basis, O2 transmission rates of PBF, PBBf, PBF90Bf10, and
PBF75Bf25 were evaluated from melt-pressed, low-crystallinity film specimens (Table 5, entries 1–4).
The permeability coefficient for PBF has been reported previously (entry 7), and our sample yielded
a somewhat higher value, though they are still in relatively good agreement. Despite the apparent
incompatibility of furan and bifuran units in regards to crystallization, it is interesting that the
amorphous copolyester films yielded the lowest gas permeability coefficients. It is also important to
note that the measured coefficients compare very favorable even against previous data from PET
(entries 5 and 6) and PBT (entries 8–10), including data from biaxially oriented, crystalline PBT
(entry 10). Comparison to high-performance barrier polyesters poly(ethylene naphthalate) (PEN) and
poly(butylene naphthalate) (PBN) shows that further work is still required to challenge these
materials: (PEN) is reported to have O2 permeability as low as 0.008 barrer41, while bottles made
from poly(butylene naphthalate) (PBN) have shown 3x and 6x lower O2 permeability than PEN and
PET bottles, respectively42. However, the barrier performance of PEN and PBN comes at a price,
since the principal monomer, 2,6-naphthalanedicarboxylic acid, lacks close link to biomass.43,44
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Based on our results, both FDCA and BFDCA can be almost equally effective for reducing the
transmission of oxygen, which is promising for the further development of new bio-based oxygen
and gas barrier polymers.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 5. O2 barrier properties of low crystallinity/amorphous PBF, PBBf, PBF90Bf10, and PBF25Bf75 films.
Permeability coefficient
Barrier improvement
Entry
Sample
Conditions
(barrer)
factor vs. PET
1
2
PBF
PBF90Bf10
PBF75Bf25
PBBf
0.0246±0.0002
0.0225±0.0004
0.0217±0.0015
0.0285±0.0021
0.099
2.4–4
25 C at 50% relative
humidity (RH)
2.7–4.4
3
2.8–4.6
4
5
2.1–3.5
PETa
1
1b
3.3b
23 C, 0% RH
30 C, 50% RH
30 C, 50% RH
23 C, 50% RH
25 C, 0% RH
25 C, 0% RH
23 C, 50% RH
6
PETb
PBFb
PBTc
PBTc,d
PBTc,e
PENf
0.060
7
0.018
8
0.231
-
-
-
-
9
0.0456
10
0.0258
11
0.008
aref 22. bref 34. cref 45. dUnoriented film. eBiaxially oriented film. fref 41.
In our previous study, the low UV light transmittance of poly(ethylene bifuranoate) films was
discussed as a part of the barrier properties. That in mind, the UV-vis transmittance curves of the
various films here were also measured (Figure 5, Table 6). As expected, the bifuran moieties provided
the copolyesters intrinsic UV light filtering ability up to about 400 nm wavelengths. The PBF film
clearly did not provide significant absorption of UV light in the range of 300–400 nm. PET, often
used for its excellent clarity, lacks significant UV-light absorption above 300 nm as well.22 In the
very recent literature, varying strategies for realizing UV-protective green materials have been
reported; lignin being a green and bio-based choice.46,47 However, low transmission at UV
wavelengths has been accompanied by low transmission in the visible wavelengths as well due to
crystallinity in the aliphatic polyester matrices and most notably the broad absorbance of the lignin
used. Here, the strong UV absorption of copolyester films was combined with excellent transmittance
values in the visible range (e.g. 82% at 450 nm for PBF90Bf10). As discussed previously21,22, the
bifuran monomer 2 has its absorption maximum at longer wavelengths (325 nm) than the furan
monomer 1 (265 nm) in chloroform solution (Figure S15). However, in actual melt-pressed polyester
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films, the cutoff-wavelength was shifted by almost a 100 nm when enough bifuran moieties were
introduced. Most likely, the more conjugated bifuran structures results in more effective
chromophore-chromophore interactions with both furan and bifuran units of the polymer chains in
solid state that causes the red-shift of absorption. This effect is commonly observed with highly
conjugated organic materials.48 The prepared films with thicknesses of ca. 0.15 mm were remarkably
UV-opaque at 10 mol% or higher bifuran content. At a low concentration of 1 mol% (PBF99Bf1 in
Figure 5; 1H NMR spectrum in Figure S10), the transmittance at 400 nm is similar to PBF. However,
UV opacity was retained in the region of 300–350 nm, which is near the peak absorbance of the
bifuran monomer in solution. As a conclusion, the presence of bifuran units in the polymer structure
can effectively provide UV filtering ability, reaching the maximum filtration with 1–10 mol% bifuran
content for these particular films.
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60
100
80
PBF
60
PBF99Bf1
PBF90Bf10
40
20
0
PBF75Bf25
PBBf
300
400
500
600
700
800
Wavelength (nm)
Figure 5. UV-vis transmittance curves of melt-pressed polyester and copolyester films (ca. 0.15 mm thick).
Table 6. UV-vis transmittance values at given wavelengths from Fig. 5.
Samplea
PBF
T350 (%)
T380 (%)
64.8
44.7
2
T400 (%)
T450 (%)
47.7
0.1
0
69.7
76.8
PBF99Bf1
PBF90Bf10
PBF75Bf25
PBBf
64.2
73.2
47.7
82.3
0
1
35.2
80.3
0
0.1
13.9
74.5
T350 = Transmittance at 350 nm. T380 = Transmittance at 380 nm.
T400 = Transmittance at 400 nm. T450 = Transmittance at 450 nm.
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FDCA and BFDCA in combination with 1,4-butanediol were used for the first time to prepare
copolyesters, and the properties were compared to the corresponding homopolyesters, PBF and PBBf.
In addition, the properties of PBBf were for the first time characterized from melt pressed films,
confirming its excellent O2 barrier properties. The stiff bifuran units in PBBf and the copolyesters
endowed them with improved mechanical properties and elevated glass transition temperatures over
PBF. The presence of bifuran units also provided intrinsic UV light barrier due to the highly
conjugated biheteroaryl moiety, a feature that is not found in PBF. Most notably however, it was
shown that the mixtures of furan and bifuran moieties present in the tested copolyesters did not
adversely affect the O2 barrier properties. Instead, the barrier properties were slightly improved
compared to either PBF or PBBf. Thus, FDCA-co-BFDCA copolyesters were shown function as
superior O2 barriers compared to fossil-based polyesters like PET and PBT. Based on results obtained
here, FDCA-co-BFDCA polyesters appear to be an interesting class of mechanically strong, semi-
crystalline to amorphous materials with combination of excellent oxygen and UV light barrier
properties. In addition, the strategy of upgrading C5 sugar-derived furfural into BFDCA, a C10
monomer, appears attractive due to the emerging observations of good properties in bifuran
containing polyesters. However, further studies are needed to fully uncover and understand the
behavior of BFDCA as a component of different polymers, as this strategy could allow the use of
furfural as the feedstock for next generation bio-based polymeric materials.
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Associated Content
Supporting Information Available: 1H NMR spectra of dimethyl 2,5-furandicarboxylate and dimethyl
1
2,2’-bifuran-5,5’-dicarboxylate (Figures S1 and S2), H NMR and FTIR spectra for synthesized
polyesters (Figures S3–11, Table S1), Melting enthalpies obtained via DSC (Table S2), DMA curves
for synthesized polyesters (Figure S13), POM images of PBF and PBBf films (Figure S14), UV-vis
absorption spectra for dimethyl 2,5-furandicarboxylate and dimethyl 2,2’-bifuran-5,5’-dicarboxylate
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in CHCl3 (Figure S15), XRD-diffractograms of melt-pressed films (Figure S16). This material is
available free of charge via the Internet at http://pubs.acs.org.
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Author Information
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*Corresponding Author: Juha P. Heiskanen. E-mail: juha.heiskanen@oulu.fi
Conflicts of interest
There are no conflicts of interest to declare
Acknowledgements
The Alfred Kordelin foundation and Magnus Ehrnrooth foundation are acknowledged for providing
funding. University of Oulu is thanked for providing financial support (Proof of Concept grant). Dr.
Tao Hu is thanked for providing guidance in obtaining the XRD data.
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