Aromatic thermotropic polyesters based on 2,5-furandicarboxylic acid and vanillic acid

Article abstract of DOI:10.1016/j.polymer.2014.03.033

This paper addresses a route to synthesize bio-based polymers with an aromatic backbone having a liquid crystalline (LC) phase in the molten state. The LC phase is employed to achieve uniaxial orientation during processing required in e.g. fiber spinning. For this purpose 2,5-furandicarboxylic acid (2,5-FDCA) and O-acetylvanillic acid (AVA), obtained from natural resources, are used as monomers. Similar to the 2,6-hydroxynapthoic acid used to perturb the crystalline packing of poly(oxybenzoate) in the Vectran series, these bio-based monomers are used to lower the crystal to liquid crystal transition temperature. Considering that the poly(oxybenzoate) can also be obtained from natural resources, the adopted route provides the unique possibility to synthesize bio-based polymers that can be used for high performance applications. To obtain the desired polymers, a synthetic route is developed to overcome the thermal instability of the 2,5-FDCA monomer. Experimental techniques, such as optical microscopy, FTIR spectroscopy, DSC, and TGA are employed to follow the polymerization, phase transitions and evaluate thermal stability of the synthesized polymers.

Full text of DOI:10.1016/j.polymer.2014.03.033

Polymer 55 (2014) 2432e2439
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Aromatic thermotropic polyesters based on 2,5-furandicarboxylic acid
and vanillic acid
,
,
,c,d,
Carolus H.R.M. Wilsens a d, Bart A.J. Noordover a d, Sanjay Rastogi b
*
a Laboratory of Polymer Materials, Eindhoven University of Technology, 5600MB Eindhoven, The Netherlands
b Department of Biobased Materials, Maastricht University, P.O. Box 616, 6200MD Maastricht, The Netherlands
c Department of Materials, Loughborough University, Loughborough, Leicestershire LE11 3TU, United Kingdom
d Dutch Polymer Institute (DPI), P.O. Box 902, 5600AX Eindhoven, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
This paper addresses a route to synthesize bio-based polymers with an aromatic backbone having a
liquid crystalline (LC) phase in the molten state. The LC phase is employed to achieve uniaxial orientation
during processing required in e.g. fiber spinning. For this purpose 2,5-furandicarboxylic acid (2,5-FDCA)
and O-acetylvanillic acid (AVA), obtained from natural resources, are used as monomers. Similar to the
2,6-hydroxynapthoic acid used to perturb the crystalline packing of poly(oxybenzoate) in the VectranÒ
series, these bio-based monomers are used to lower the crystal to liquid crystal transition temperature.
Considering that the poly(oxybenzoate) can also be obtained from natural resources, the adopted route
provides the unique possibility to synthesize bio-based polymers that can be used for high performance
applications. To obtain the desired polymers, a synthetic route is developed to overcome the thermal
instability of the 2,5-FDCA monomer. Experimental techniques, such as optical microscopy, FTIR spec-
troscopy, DSC, and TGA are employed to follow the polymerization, phase transitions and evaluate
thermal stability of the synthesized polymers.
Received 11 February 2014
Received in revised form
11 March 2014
Accepted 20 March 2014
Available online 27 March 2014
Keywords:
Thermotropic polyester
Renewable resources
2,5-Furandicarboxylic acid
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
from the LC melt challenging and often unattractive from a prac-
tical perspective. [11] Very often, structural modifications are
Over the last decades, extensive research has been performed to
develop fully aromatic liquid crystalline (LC) polymers. The excel-
lent mechanical and thermal properties, chemical resistance, and
ease of processing in the (nematic) LC phase have led to the
development of numerous polymers showing LC behavior. Com-
mercial examples of such high performance LC polymers are the
TwaronÒ or KevlarÒ polyamides and the VectraÒ polyesters. How-
ever, a wide variety of other main-chain LC polymers have been
developed and reviewed extensively by Han and Bhowmik [1],
Ballauff [2], Windle [3], and others [4e8].
Although some examples of lyotropic polyesters are reported by
Lin [9] and Polk [10], most research focuses on the thermotropic
polyesters since these materials can be processed from their melt
and do not require expensive solvents and solvent recovery sys-
tems. However, thermotropic homopolyesters often have a high
crystal to liquid crystal transition temperature, making processing
required to lower the melting temperatures of LC polymers, in or-
der to decrease processing costs and prevent thermal degradation.
The most common examples of such modifications are the copo-
lymerization of other mesogenic monomers, the usage of bulky
substituents, the incorporation of flexible spacers, or the copoly-
merization of non-linear monomers [12e15]. For example, Cottis
et al. [16] showed in 1972 the perturbing effect of the copolymer-
ization of the linear monomers hydroquinone (HQ) and tereph-
thalic acid (TA) with hydroxybenzoic acid (HBA) and reported
polymers with lower crystal to liquid crystal transition tempera-
tures. The copolymerization of the mesogenic crankshaft monomer
2,6-hydroxynaphthoic acid with HBA was reported in 1978 by
Calundann [17], yielding a random copolymer with a melting
temperature below 330 ꢀC. An example of the usage of the kinked
monomer isophthalic acid was reported by Nagano and Nomura
[18] in 1999 and is commercially available as Sumikasuper LCP.
Examples of the applications of other non-linear monomers such as
m-HBA or different diols are found in the publications of Ballauff
[19], Griffin [20], and Kricheldorf [21]. The last approach to lower
melting temperatures of thermotropic homopolyesters is to include
* Corresponding author.
E-mail addresses: s.rastogi@lboro.ac.uk, sanjay.rastogi@maastrichtuniversity.nl
(S. Rastogi).
http://dx.doi.org/10.1016/j.polymer.2014.03.033
0032-3861/Ó 2014 Elsevier Ltd. All rights reserved.

C.H.R.M. Wilsens et al. / Polymer 55 (2014) 2432e2439
2433
flexible spacers into the polymer backbone. One example of such an
approach was reported by Jackson [22] in 1976, where he showed
the liquid crystalline nature of polymers based on PET and HBA.
Most of the early reports on thermotropic polyesters involve the
usage of substituted benzene, biphenyl or naphthalene derivatives
to perturb the poly(oxybenzoate) (POBA) backbone, leading to the
formation of a non-periodic layer adjacent to the crystalline lattice.
The very nature of the non-periodic layer has been a subject of
independent studies by Blackwell [23] and Windle [24] in the past.
In this publication, inspired by the earlier studies to perturb the
crystal lattice, the possibility to introduce aromatic bio-based
moieties into the POBA backbone is investigated. The perturba-
tion is likely to decrease the melting temperature and promote
processing from the LC melt. The bio-based monomers used in this
study are 2,5-furandicarboxylic acid (2,5-FDCA) [25] and O-ace-
tylvanillic acid (4-acetoxy-3-methoxybenzoic acid, AVA). The
copolymerization of small amounts of AVA in thermotropic poly-
esters has been widely investigated and is known to increase the
uniaxial deformation, mechanical properties, solubility and glass
transition temperature of PET- and HBA-based LC polyesters [26e
28]. The usage of 2,5-FDCA in thermotropic polyesters has been
reported in patent applications by Vriesema [29] and Fujioka [30].
Vriesema showed that the replacement of small amounts of TA by
2,5-FDCA causes a decrease in the melting temperature while
retaining the thermotropic behavior of the polymer. Fujioka copo-
lymerized 2,5-FDCA and a phosphorous containing diol with HBA
to synthesize heat resistant LC polyesters. Both of these inventions
use 2,5-FDCA as a co-monomer in POBA-based LC polyesters.
However, the crystal to LC transition temperature of the designed
thermotropic polyesters exists only above 300 ꢀC.
Fig. 1. Structural formulas of the monomers used in this study.
this study, the thin-film polymerization is used to study different
monomer compositions to find the composition and reaction
parameters suitable for the synthesis of a 2,5-FDCA based ther-
motropic polyester, without degradation of the monomers. To
suppress the melting and polymerization temperatures, the copo-
lymerization of bio-based vanillic acid with 2,5-FDCA is performed.
The composition and the polymerization of the monomers are
monitored as a function of time using high temperature ATReFTIR.
Analysis of the polymers with interesting compositions is per-
formed using TGA and DSC.
Although reports on the usage of 2,5-FDCA as a monomer in
thermotropic polyesters are limited, the usage of other five
membered heterocyclic rings such as pyridazine [31], thiadiazole
[32], oxadiazole [33e35], and imidazole [36] are widely applied in
liquid crystals and liquid crystalline polymers. Furthermore, the
usage of 2,5-thiophenedicarboxylic acid (2,5-TDCA), a heterocyclic
monomer similar to 2,5-FDCA, is reported by Cai, Preston and
Samulski [37]. These authors showed that the usage of 2,5-TDCA as
replacement of TA resulted in a decrease in melting temperature
without losing the liquid crystalline nature of the polymer. Inter-
estingly, unlike furan [38], both the thiophene and oxadiazole
moiety are of a mesogenic nature, allowing for the synthesis of non-
linear structures exhibiting thermotropic melt-behavior [26]. Since
the 2,5-furandicarboxylate moiety itself is not a mesogen, we use
4,40-biphenol and hydroxybenzoic acid to ensure the thermotropic
melt-behavior of the polymers synthesized in this study.
It is acknowledged that 2,5-FDCA tends to undergo side re-
actions leading to increasingly discolored products with increasing
reaction temperatures between 200 and 275 ꢀC [39]. As early as
1946, Drewitt and Lincoln [40] noticed that 2,5-FDCA is prone to
decarboxylate and evolve CO2 at the applied reaction temperatures,
which ranged between 220 and 280 ꢀC. Thus the decarboxylation
and degradation of 2,5-FDCA limits the reaction and processing
temperatures. For this reason, we aim to investigate the viability of
the heterocyclic 2,5-FDCA as a monomer in fully aromatic ther-
motropic liquid crystalline polyesters (TLCPs). Considering the
polymerization temperature as the limiting factor, in this publica-
tion, a series of polymers containing 2,5-FDCA exhibiting LC
behavior in the temperature window of 200 ꢀCe300 ꢀC are syn-
thesized and characterized [41].
2. Experimental section
2.1. Materials
4-hydroxybenzoic acid and 4-hydroxy-3-methoxybenzoic acid
were obtained from Sigma. The diol 4,40-biphenol was purchased
from TCI Europe. 2,5-Furandicarboxylic acid (99.5% purity, GCeMS)
was ordered from Atomole (China). All monomers containing hy-
droxyl groups were acetylated and recrystallized at least once
before polymerization. All other chemicals were used as received
unless mentioned otherwise. The monomers used in this study are
shown in Fig. 1.
2.2. General acetylation procedure
Ten grams of a monomer containing one or two hydroxyl groups
were placed in a 100 mL round-bottom flask on a magnetic stirring
plate. Acetic anhydride was added in a slight stoichiometric excess
with respect to the hydroxyl groups together with a catalytic
amount of H2SO4. The mixture was heated to 80 ꢀC and stirred for
4 h. After cooling to 0 ꢀC for 1 h 200 mL water was added to the
mixture and the solution was filtered. The obtained crystals were
washed with water and dried in vacuo at 40 ꢀC overnight. All
acetylated monomers were recrystallized at least once before usage
in polymerization.
2.2.1. Preparation of 4-acetoxybenzoic acid (ABA)
4-acetoxybenzoic acid was prepared from 4-hydroxybenzoic
acid (10 g, 72.4 mmol) and acetic anhydride (10 mL, 106 mmol) as
described in the acetylation procedure. The resulting white powder
was recrystallized twice from methanol to yield 8.44 g of product
(64.7%). 1H NMR (MeOD-d4, d, ppm): 8.06 (d, ArH, 2H), 7.21 (d, ArH,
2H), 4.91 (s, COeOH,1H), 2.30 (s, OeCOeCH3, 3H). 13C NMR (MeOD-
The thin-film polymerization (TFP) method reported by Cheng
et al. [42] is used to screen the incorporation of variable amounts of
2,5-FDCA into the POBA backbone and its influence on the evolu-
tion of the LC phase. A similar approach was performed by Xu and
coworkers [43] to study the incorporation of 2,5-TDCA. However, in
d4,
d, ppm): 170.57 (CH3eCO), 169.00 (COOH), 155.97 (ArCeOAc),
132.27 (ArCeCOOH), 129.49 (ArC), 122.90 (ArC), 20.95 (COeCH3).

2434
C.H.R.M. Wilsens et al. / Polymer 55 (2014) 2432e2439
Fig. 2. Reaction scheme of the acidolysis polycondensation occurring in the 2,5-FDCA/DABP/ABA TFP systems. N.b. The resulting polymer can be of a random or blocky nature.
2.2.2. Preparation of 4-acetoxy-3-methoxybenzoic acid (AVA)
4-acetoxy-3-methoxybenzoic acid was prepared from 4-
hydroxy-3-methoxybenzoic acid (10 g, 60.9 mmol) and 10 mL of
acetic anhydride (106 mmol) as described in the acetylation pro-
cedure. The resulting slightly yellow powder was recrystallized
twice from toluene to yield 9.34 g of product (72.9%). 1H NMR
(MeOD-d4, d, ppm): 7.69 (d, ArH,1H), 7.64 (dd, ArH,1H), 7.12 (d, ArH,
1H), 3.86 (s, OeCH3, 3H), 2.28 (s, COeCH3, 3H). 13C NMR (MeOD-d4,
without a
l
wave plate and CD achorplan objectives (20ꢁ or 32ꢁ
Zoom). A THMS 600 heating stage connected to a Linkam TMS 94
control unit was mounted on the optical microscope. The mono-
mers were dried overnight in vacuo at 40 ꢀC, mixed and grinded
twice before usage. Samples were prepared by placing 2e5 mg of a
monomer mixture in between two glass slides with a ring spacer,
where the sample was placed on the top glass slide. Samples were
heated from room temperature with 100 ꢀC/min to the desired
reaction temperature and all reactions were conducted under a N2
flow. The reaction time (tr) is set to 0 when the desired reaction
temperature was reached. Calibration of the temperature was
performed using the melting temperatures of well-known com-
pounds. Temperature calibration showed a 20 ꢀC ꢂ 3 ꢀC difference
from the input temperature and all temperatures mentioned in this
publication are corrected values. The reactions were followed on-
line and images were acquired every second.
d, ppm): 170.20 (CH3eC]O), 169.02 (COOH), 152.62 (ArCeOCH3),
145.11 (ArCeOAc), 130.75 (ArCeCOOH), 123.88 (ArC), 123.74 (ArC),
114.51 (ArC), 56.51 (OeCH3), 20.46 (COCH3).
2.2.3. Preparation of 4,40-diacetoxybiphenyl (DABP)
4,40-dihydroxydiphenyl (10 g, 54 mmol) was acetylated as
described in the general procedure to yield 11,2 g (77%) 4,40-diac-
etoxybiphenyl after recrystallization from toluene. 1H NMR (CDCl3,
ATReFTIR measurements were performed on a Bio-rad FTS6000
FTIR Spectrometer with an ATR-plate connected to a high temper-
ature golden gate with a maximum temperature of 300 ꢀC. Samples
were measured by the placement of 2e5 mg sample onto the hot
plate at a fixed reaction temperature and spectra were collected on-
line with intervals of 2 s using a liquid nitrogen-cooled WB MCT
detector with a resolution of 4 cmꢃ1. A cover was placed on the top
of the ATR-plate and was connected to N2 flow to prevent evapo-
ration and degradation of the monomers.
d
, ppm): 7.55 (d, ArH, 4H), 7.16 (d, ArH, 4H), 2.33 (s, C]OeCH3, 6H).
13C NMR (CDCl3,
d, ppm): 169.4 (CH3eC]O),150.1 (ArCeOAc),138.1
(ArCeArC), 128.1 (ArC), 121.9 (ArC), 21.1 (COeCH3).
2.3. Methods
Thin-film polymerizations (TFP)s monitored by polarization
optical microscopy (POM) were conducted on a Zeiss Axioplan 2
Imaging optical microscope under crossed polarizers with or
Fig. 3. Optical micrographs (in between cross-polars and red wave plate) showing the transitions from isotropic to the liquid crystalline and to the crystalline phase during the TFP
of the 2,5-FDCA/DABP/ABA 15/15/70 reaction system at 300 ꢀC. The mentioned time on the micrographs refers to the reaction time (tr).

C.H.R.M. Wilsens et al. / Polymer 55 (2014) 2432e2439
2435
220 ꢀC and 380 ꢀC, and the morphological changes were followed
via polarization optical microscopy. Fig. 2 shows the reaction
scheme of the polycondensation reaction occurring in the thin-film
polymerization of 2,5-FDCA/DABP/ABA systems. During this reac-
tion, polymerization proceeds through an acidolysis reaction,
forming acetic acid as the condensate. Fig. 3 shows an example of
the development of the morphology during the ongoing polymer-
ization of the 2,5-FDCA/DABP/ABA 15/15/70 system at a reaction
temperature of 300 ꢀC. All micrographs were taken from the same
area of the sample over time, under isothermal condition. The
micrographs show the typical morphological changes occurring
during the synthesis of high melting, thermotropic polyesters.
During this process, transformation of the isotropic phase to the LC
phase (tr w 20 s) is observed, followed by the coalescence and
homogenization of the LC phase (tr w 20e60 s) and crystallization
(tr > 4 min). Melting of the crystals thus obtained in the 2,5-FDCA/
DABP/ABA 15/15/70 system is observed at 380 ꢀC upon heating at
10 ꢀC/min. Unfortunately, because of the rapid degradation of the
monomers, TFP to follow the development and morphology of the
LC melt of the 2,5-FDCA/DABP/ABA 15/15/70 at 380 ꢀC could not be
performed.
Fig. 4. Overview of the ATReFTIR spectra collected with the increasing polymerization
time between 2000 and 650 cmꢃ1 obtained during a TFP of the 2,5-FDCA/DABP/ABA
15/15/70 system at 300 ꢀC.
Fig. 3 shows that the formation of the LC droplets occurs
(tr ¼ 19e23 s) prior to the full dissolution of the 2,5-FDCA crystals,
indicating that the 2,5-FDCA is poorly soluble in the monomer/
oligomers mixture. It is expected that the poor solubility of the 2,5-
FDCA allows the reaction to progress simultaneously with the
dissolution of the 2,5-FDCA crystals, i.e. polymerization initiates on
the surface of the 2,5-FDCA crystals. Hence, during polymerization,
the crystals react and dissolve from the outside in. The rate of the
reaction will thus be limited by the reaction temperature, by the
availability of reactive groups, and by the surface to volume ratio of
the 2,5-FDCA crystals.
Similar behavior was observed by Economy during co-
polymerizations of ABA with terephthalic acid (TA), and 4,40-
biphenol [11]. This author showed that the poor solubility of TA
resulted in the formation of oxybenzoate/4,40-biphenol-rich do-
mains. Rapid crystallization of the system prohibited the trans-
esterification in the LC melt and led to a high degree of phase
segregation of the monomers in the form of blocks in the final
polymer. This slow dissolution behavior of TA was confirmed by
Cheng et al. [44] during the TFP of ABA/BP/TA systems.
Thermal decomposition studies were performed on a TA in-
struments TGA Q500 machine under N2 rich atmosphere. Samples
were heated at 20 ꢀC/min from 20 ꢀC to 800 ꢀC. Temperature cali-
bration was performed using the Curie points of high-purity
aluminum, nickel and perkalloy standards.
The thermal behavior of the samples was studied using a TA
instruments Q1000 DSC. The apparatus was calibrated using in-
dium and aluminum oxide. The normal heating and cooling-rates of
the samples were 10 ꢀC/min and measurements were performed
under a N2 rich atmosphere.
3. Results and discussion
3.1. Phase behavior of 2,5-FDCA/DABP/ABA system
The thin-film polymerization (TFP) method, described by Cheng
[42], was used to follow the influence of the copolymerization of
2,5-furandicarboxylic acid (2,5-FDCA) and 4,4-diacetoxybiphenyl
(DABP) on the LC behavior during a polymerization of p-acetox-
ybenzoic acid (ABA). Different compositions of 2,5-FDCA, DAPB and
ABA were screened along a broad temperature range between
It is expected that the polymers obtained during the TFP of 2,5-
FDCA/DABP/ABA systems exhibit a high degree of block formation,
similar to the systems studied by Economy. The LC droplets formed
Fig. 5. Optical micrographs showing the morphological changes of the LC phase formation during the TFP of the 2,5-FDCA/DABP/ABA 35/35/30 reaction system under isothermal
condition at a) 280 ꢀC and b) 300 ꢀC.

2436
C.H.R.M. Wilsens et al. / Polymer 55 (2014) 2432e2439
The presence of the insoluble 2,5-FDCA limits the polymeriza-
360
340
320
300
280
260
240
220
tion rate at low reaction temperatures or in reactions with a very
high 2,5-FDCA content. The time required for the 2,5-FDCA to fully
dissolve and participate in the reaction increases with the
increasing 2,5-FDCA concentrations and thus extends the reaction
time. However, with the extended reaction times, the oxybenzoate/
4,40-biphenol rich blocks will have more time to build-up molecular
weight and will be able crystallize during polymerization. Fig. 5a
shows an example of the polymerization of the 2,5-FDCA/DABP/
ABA 35/35/10 system at 280 ꢀC where, due to the presence of un-
dissolved 2,5-FDCA crystals, polymerization cannot be completed.
Fig. 5a shows that at 280 ꢀC, the formation of an LC phase is visible
after 120 s, but does not develop over the whole view area. Instead,
the LC domains crystallize and the undissolved 2,5-FDCA crystals
remain present (tr ¼ 480 s). When the same reaction is conducted
at 300 ꢀC, a full dissolution and simultaneous reaction of the 2,5-
FDCA monomers is achieved over time and the development of a
homogeneous LC phase is observed (Fig. 5b). On further polymer-
ization (beyond tr ¼ 180 s) the LC phase transforms into the crys-
talline phase.
The effect of reaction temperature and composition on the LC
layer formation and early precipitation in the 2,5-FDCA/DABP/ABA
systems was determined, and is shown in a phase diagram as a
function of ABA content (Fig. 6). Region I in Fig. 6 indicates the
regime where a full dissolution and reaction of the 2,5-FDCA
monomers occurs, while region II indicates the regime where
crystallization occurs prior to the full reaction of the 2,5-FDCA
monomers. Generally, the reaction temperature required to reach
full conversion increases with the increasing 2,5-FDCA concentra-
tion to a maximum of 330 ꢀC for systems having 41 mol% or higher
amounts of 2,5-FDCA. Apparently, the reaction temperature of
330 ꢀC lies close enough to the melting temperature of 2,5-FDCA of
342 ꢀC to allow rapid dissolution and reaction [46].
*
T
m
II
I
200
0
Critical dissolution temperature
40 30 20 10
2,5-FDCA content (mol %)
50
0
Fig. 6. The decrease of the critical dissolution/reaction temperatures as a function of
the 2,5-FDCA content in the 2,5-FDCA/DABP/ABA systems. The ratio of 2,5-FDCA/DABP
is kept equimolar. The Tm* denotes the temperature where all visible 2,5-FDCA crystals
are molten/dissolved at the start of the reaction, independent of the amount of 2,5-
FDCA present in the system (>330 ꢀC).
in the initial phase of polymerization have an oxybenzoate/4,40-
biphenol rich composition, due to the unavailability of 2,5-FDCA
in the liquid phase. The phase transformation from LC to crystal
phase occurs once the 2,5-FDCA monomers have fully reacted. The
blocky nature of the formed polymer is maintained since trans-
esterification from the melt is prevented by the rapid crystallization
(>4 m, Fig. 3).
To follow the incorporation of the 2,5-FDCA monomer into the
polymer backbone, a TFP was followed with time using ATReFTIR
at 300 ꢀC (Fig. 4). The FTIR results show a rapid decrease of the
signal for the CH3 vibration band of the acetoxy end-group at
1376 cmꢃ1 (C]OeCH3), the carbonyl stretch band of the acetoxy
end-group at 1770 cmꢃ1, and carbonyl stretch band of the free acid
group at 1703 cm-1. This indicates that the concentration of the
reactive end-groups decreases with polymerization. Simulta-
neously, the increase of the signal of the aromatic ester carbonyl
stretch band at 1735 cmꢃ1 indicates that the aromatic ester groups
are formed, thus polymerization occurs. The incorporation of 2,5-
FDCA in the final polymer can be detected by the presence of
bands at 943 and 1567 cmꢃ1 corresponding to the vibrations of the
furan ring [45].
Interestingly, the 2,5-FDCA/DABP/ABA system shows LC
behavior along the whole range of compositions. However, the
temperature required to observe the LC phase increases steadily
from 200 to 210 ꢀC for pure POBA to 340 ꢀC for 2,5-FDCA/DABP/ABA
40/40/20 systems. Above this temperature, the LC phase is
observed, but if the reaction temperature is too low, crystallization
from the isotropic melt occurs. For example, for the 2,5-FDCA/DABP
50/50 system, the LC phase is only observed above 360 ꢀC.
In the discussion of the data obtained thus far via the TFP, we
have neglected the effect of the thermal instability of the 2,5-FDCA
monomer on the polymerization. According to the data obtained by
Gruter [39] and Drewitt [40], it should be realized that the
Fig. 7. Optical micrographs showing formation of a stable LC melt in a TFP of the 2,5-FDCA/DABP/ABA/AVA 15/15/60/10 reaction system at 300 ꢀC.

C.H.R.M. Wilsens et al. / Polymer 55 (2014) 2432e2439
2437
conclusively demonstrated that on fixing the 2,5-FDCA/DABP
content to 30 mol% and by varying the amount of AVA between
0 and 20 mol% a series of polymers having thermotropic LC
behavior can be obtained. Fig. 7 shows an example of the
morphological changes observed in the 2,5-FDCA/DABP/ABA/AVA
15/15/60/10 copolymer at 300 ꢀC. The formation of the LC phase
occurs prior to the complete dissolution of the 2,5-FDCA crystals.
These observations are similar to the 2,5-FDCA/DABP/ABA 15/15/
70 system, where in the initial stages of polymerization 2,5-FDCA
monomers are not distributed randomly along the polymer
chains. To recall, in the 2,5-FDCA/DABP/ABA 15/15/70 system, the
distribution of the monomers along the main-chain is thought to
be of a blocky nature due to rapid crystallization of the polymer
chains during polymerization, preventing transesterification.
However, in the 2,5-FDCA/DABP/ABA/AVA 15/15/60/10 copolymer
the absence of crystallization during polymerization allows for
the transesterification in the melt, promoting a random distri-
bution of the monomers along the chain and inhibiting the block
formation.
100
80
60
FDCA/DABP/ABA/AVA
15/15/70/0
40
15/15/60/10
15/15/50/20
20
0
200
400
Temperature (°C)
600
800
Fig. 8. TGA thermograms showing a decrease of the weight loss temperature with the
incorporation of AVA in 2,5-FDCA/DABP/ABA/AVA 15/15/70/0, 15/15/60/10 and 15/15/
50/20 polymers synthesized via the TFP route at 300 ꢀC.
In Fig. 7, with the increasing polymerization time, the LC drops
coalesce rapidly and a typical Schlieren texture is formed while
the 2,5-FDCA monomer dissolves and reacts. The LC phase is
observed after 2 min of reaction time in full view of the optical
micrograph. The LC melt does not show any phase transition
within 60 min of reaction time and remains in the mobile melt
phase. The absence of crystallization allows for build-up of mo-
lecular weight by transesterification in the LC melt. Upon cooling
at 10 ꢀC/min, solidification of the sample is observed at 230e
240 ꢀC (Tc), while upon heating at 10 ꢀC/min again, the crystal to LC
transition is observed around 270e280 ꢀC (Tm). Similar experi-
ments are performed with the 2,5-FDCA/DABP/ABA/AVA 15/15/50/
20 system, at different reaction temperatures, showing the for-
mation of a homogeneous LC phase and no crystallization until
120 min of reaction time. The system shows crystallization at
200 ꢀC upon cooling at 10 ꢀC/min and on heating shows the
transition from the crystalline to the LC phase at 240 ꢀC. This data
clearly indicates that AVA decreases the melting temperature of
the 2,5-FDCA/DABP/ABA systems and provides opportunities for
increasing the molecular weight in the LC phase at temperatures
below 300 ꢀC.
maximum polymerization temperature of 2,5-FDCA is 280 ꢀC or
lower, since severe discoloration occurs at 280 ꢀC or higher. Pref-
erably, reactions containing 2,5-FDCA should be conducted below
220 ꢀC to fully prevent any side reaction. According to Fig. 6, only
the 2,5-FDCA/DABP/ABA systems with w30 mol% 2,5-FDCA or less
can be successfully synthesized at 280 ꢀC or below, without having
the reaction mixture crystallizing prior to the full dissolution of the
2,5-FDCA monomers.
3.2. Tuning the melting temperature of 2,5-FDCA based polymers
Since the high melting temperatures of the 2,5-FDCA/DABP/
ABA polymers do not allow molecular weight build-up, mainly
due to rapid crystallization from the melt, O-acetylvanillic acid
(AVA) has been used as a co-monomer during polymerization. It
was anticipated that the presence of the methoxy group of AVA
perturbs the crystal lattice of the 2,5-FDCA/DABP/ABA systems
and lowers the melting temperature. A small fraction of ABA was
replaced by AVA to determine the influence of AVA on the melting
temperature. These systems were investigated using the TFP. It is
The thermal stability of the 2,5-FDCA15/DABP15/ABA/AVA
polymers with 0% AVA, 10% AVA and 20% AVA, synthesized through
Fig. 9. Second heating and cooling DSC traces showing the decrease of the melting temperature with the increasing amount of AVA in the 2,5-FDCA/DABP/ABA/AVA a) 15/15/70/0,
b) 15/15/60/10 and c) 15/15/50/20 TFP systems synthesized at 300 ꢀC.

2438
C.H.R.M. Wilsens et al. / Polymer 55 (2014) 2432e2439
Table 1
Acknowledgment
Thermal data showing the influence of AVA on the Tg, Tm
obtained from 2,5-FDCA/DABP/ABA/AVA systems.
,
D
Hmelt and Tons of polymers
This work is part of the research program of the Dutch Polymer
Institute (DPI), #739 BioLCP. The authors acknowledge financial
support by Teijin Aramid, The Netherlands at the initial stage of this
project.
a
a
a
b
c
AVA content
Tg (ꢀC)
Tm (ꢀC)
D
Hmelt (J/g)
Tm (ꢀC)
Tons (ꢀC)
0 mol%
10 mol%
20 mol%
96.7
103.5
109.2
336.6
275.3
230.3
6.80
1.77
0.66
360e380
270e280
240e250
460
411
402
a
Obtained from the second heating runs in DSC experiments during heating and
cooling at a rate of 10 ꢀC/min.
Abbreviations
b
Obtained from optical microscopy experiments during heating and cooling at a
ABA
AVA
BP
p-acetoxybenzoic acid
rate of 10 ꢀC/min.
c
Obtained from TGA experiments with a heating rate of 20 ꢀC/min.
4-acetoxy-3-methoxybenzoic acid or O-acetylvanillic acid
4,40-biphenol
DABP
4,40-diacetoxybiphenyl
TFP at 300 ꢀC, is evaluated by TGA and the thermograms are shown
in Fig. 8. The 2,5-FDCA/DABP/ABA 15/15/70 system shows an onset
of weight loss (Tons) at 475 ꢀC, but partial weight loss of 5 wt% oc-
curs around 350e370 ꢀC. The partial weight loss of the polymer is
likely to be a result of the limited thermal stability of the low molar
mass component of the polymer that crystallizes during polymer-
ization. The introduction of AVA into the 2,5-FDCA/DABP/ABA
copolymer decreases the onset temperature for thermal degrada-
tion from 475 ꢀC to 411 ꢀC. Furthermore, the polymers showed no
weight loss for at least 1 h at the isothermal temperature of 300 ꢀC
as observed by TGA.
DSC analysis of the different 2,5-FDCA/DABP/ABA/AVA systems
synthesized using the TFP at 300 ꢀC is performed and the infor-
mation obtained from the second heating runs is depicted in Fig. 9.
It should be noted that the peak melting temperature (Tm) of the
first heating run of the 2,5-FDCA/DABP/ABA/AVA 15/15/70/0 system
is observed at 369 ꢀC in the first heating run, but shifts to 330 ꢀC
during the second heating run. This shift is expected to be a result
of the partial degradation, as observed in the TGA analysis. How-
ever, also the transesterification occurring upon melting of the
sample in the DSC can lower the melting temperature observed in
the second heating run. From the data shown in Fig. 9, it is evident
that the addition of AVA lowers the peak melting temperature of
the polymers, causes a broadening of the melting endotherm, and
2,5-FDCA2,5-furandicarboxylic acid
HBA p-hydroxybenzoic acid or 4-hydroxybenzoic acid
Hmelt melt-enthalpy
D
POBA
LC
poly(oxybenzoic acid)
liquid crystalline
TA
terephthalic acid
TFP
Tg
TLCP
Tr
thin-film polymerization
glass transition temperature
Thermotropic Liquid Crystalline Polyesters
reaction temperature
Tm
peak melting temperature (DSC) or crystal to liquid
crystal transition (POM)
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