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Polymer Testing 137 (2024) 108515
Available online 20 July 2024
0142-9418/© 2024 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-
nc/4.0/).
Thermally stable and self-healable lignin-based polyester
Peter K. Karoki
a
, Shuyang Zhang
a
, Charles M. Cai
b
,
e
, Paul E. Dim
a
,
f
, Arthur J. Ragauskas
a
,
c
,
d
,
*
a
Department of Chemical and Biomolecular Engineering, University of Tennessee Knoxville, Knoxville, TN, 37996, USA
b
Center of Environmental and Research Technology (CE-CERT), University of California, Riverside, CA, 92507, USA
c
Center for Renewable Carbon, The University of Tennessee Knoxville, Institute of Agriculture, Knoxville, TN, 37996, USA
d
Joint Institute for Biological Sciences, Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA
e
Department of Chemical and Environmental Engineering, Bourns College of Engineering, University of California, Riverside, CA, 92521, USA
f
Department of Petroleum and Gas Engineering, Federal University of Technology, Minna, Nigeria
ARTICLE INFO
Keywords:
Bio-based
Lignin
Polyesters
Vitrimers
Thermal stability
Self-healing
ABSTRACT
The increased use of plastics and the associated environmental impact has catalyzed research on the development
of bio-derived polymers. Bio-based polyesters have gained increased attention due to the abundance of their
starting materials and ease of processing. Lignin is naturally occurring in biomass with rich carbon content,
whose functionality and rigidity make it an ideal bio-derived candidate for bio-based polyesters. Herein, a lignin-
based polyester with good thermal stability and self-repairability was synthesized from carboxylated lignin and
epoxidized soybean oil. The synthesized lignin/epoxidized soybean oil (ESO) vitrimer was brittle such that its
mechanical performance could not be recorded. However, when polyethylene glycol (PEG) was incorporated as a
plasticizer, polymer samples exhibited acceptable ductility. From thermomechanical analysis of the synthesized
polyesters, the plasticizer did not impair thermal stability of polymers, but greatly enhanced mechanical prop-
erties. Notably, all samples exhibited stability at high temperatures, and good glass transition temperatures (51.0
±0.9–78.0 ±1.2 ◦C). The highest tensile strength (3.983 ±0.1 MPa) and storage modulus (1463.67 ±12.6
MPa) were recorded for the polyester containing 6 % w/w PEG. Moreover, the polymer samples exhibited self-
healing capability at 180 ◦C. This work expands on valorization of lignin through the synthesis of bio-derived
materials.
1. Introduction
Polymers exhibit varying material properties that are primarily
dependent on the nature of the starting material [1]. After processing
and forming into commercially acceptable form, typically through
curing, polymers can be classied as thermoplastics or thermosets [2].
Thermoplastics are composed of linear chains that associate through
weak intermolecular forces, thereby making them soft and easily
reformed at elevated temperatures. On the contrary, thermosets are
crosslinked by irreversible covalent bonds which do not only impart
thermomechanical strength, but also make them difcult to recycle and
reprocess [3].
Polymers play a critical role in our day-to-day life and are particu-
larly crucial in food, electrical, health, and transportation industries
alongside numerous aspects of life. According to Ritchie et al. [4], the
world annual production of plastics was in excess of 459 million tonnes
in 2019, equivalent to about 230-fold increase compared to the year
1950, and it is expected to increase to over 765 million tonnes by 2040
[4]. Since most of the commercially available plastics are made from
fossil-based feedstock, the growing global concern regarding the envi-
ronmental impact of plastic wastes and the related greenhouse gases
(GHGs) emission has triggered researchers to develop polymeric mate-
rials for a sustainable green economy and minimize the pressure on
depleting fossil fuels.
The preparation of polymers using naturally derived starting mate-
rials is a step towards the development of sustainable materials. Bio-
derived polyesters have gained immense interest partly due to the
abundance of their starting materials, ease of processing, vast range of
application, and tunable properties that allow reprocessability at
elevated temperatures [5,6]. Most reported bio-based polyesters are
primarily derived from plant oil, starch, and cellulose derived feedstock
[7], and thus most do not achieve strong thermomechanical properties
due to lack of aromatic rings [8,9].
In this work, lignin-based polyesters were prepared by curing
* Corresponding author. Department of Chemical &Biomolecular Engineering, University of Tennessee Knoxville, Knoxville, TN, 37996, USA.
E-mail address: aragausk@utk.edu (A.J. Ragauskas).
Contents lists available at ScienceDirect
Polymer Testing
journal homepage: www.elsevier.com/locate/polytest
https://doi.org/10.1016/j.polymertesting.2024.108515
Received 6 April 2024; Received in revised form 27 June 2024; Accepted 11 July 2024
Polymer Testing 137 (2024) 108515
2
carboxylated lignin (L-COOH) with epoxidized soybean oil (ESO) in
presence of zinc acetylacetonate catalyst. The lignin used in this study
was extracted from pine using tetrahydrofuan (THF) mixed with 0.5 %
sulfuric acid solution at a 2:1 ratio (THF:acid solution, v/v) at 160 ◦C for
30 min. This type of organosolv technique of lignin extraction is referred
to as cosolvent enhanced lignocellulosic fractionation (CELF), and the
isolated lignin is termed as CELF lignin. The rationale for using CELF
lignin is the fact that, unlike other technical lignins such as kraft lignin,
CELF lignin is more chemically pure and has relatively lower average
molecular weight as well as higher aliphatic OH content [10] which can
easily be modied to carboxylic acid. The rst part of this work focused
on determination of key structural features, purity, and modication of
lignin through carboxylation. The second part was dedicated to syn-
thesis and determination of thermal-mechanical properties of the syn-
thesized polyesters samples.
2. Experimental section
2.1. Materials and chemicals
The lignin used in this study was isolated from pine (pine lignin)
using CELF pretreatment technique. Methylhexahydrophthalic anhy-
dride (MHHPA), tetrahydrofuran (THF), pyridine, polyethylene glycol
(PEG 400), zinc acetylacetate (Zn(acac)
2
), 2-chloro-4,4,5,5-tetramethyl-
1,3,2-dioxaphospholane (TMDP), deuterated chloroform (CDCl
3
), N-
hydroxy-5-norbornene-2,3- dicarboxylic acid imide (NHND) and epox-
idized soybean oil (ESO), were acquired from Sigma Aldrich. All re-
agents were of analytical grade and were used as supplied.
2.2. Percentage purity of lignin
Klason lignin (acid insoluble lignin, AIL) and acid soluble lignin
(ASL) were determined according to the National Renewable Energy
Laboratory (NREL) protocol for the Determination of Structural Carbo-
hydrates and Lignin in Biomass (version 08-13-2012), while the ash
content was determined according to NREL laboratory analytical pro-
cedure for the Determination of Ash in Biomass.
2.3. Molecular weight distribution analysis
Lignin samples (~5 mg) were dissolved under agitation in 5 mL of
THF overnight at room temperature prior to Gel permeation chro-
matographic (GPC) analysis. The average molecular weights were
analyzed using an Agilent GPC SECurity 1200 system equipped with an
UV detector. THF (ow rate of 50
μ
L/min) was used as the mobile phase
and an injection volume set at 30
μ
L. Data processing was done by a
polymer standard service winGPC Unity software.
2.4. NMR characterization
31
P and two-dimensional heteronuclear single quantum correlation
(HSQC) NMR spectra were acquired using Bruker Avance 400 MHz in
accordance with previously published procedures [11,12]. ~ 40 mg of
oven-dry lignin sample was dissolved in deuterated dimethyl sulfoxide
(DMSO‑d
6
), and HSQC spectrum obtained using a pulse sequence of 64
scans, spectral widths of 230 ppm in
13
C (F2) dimension with 2048 data
points, and 12 ppm in 1H (F1) with 256 data points. For quantitative
31
P
NMR, inverse-gated decoupling pulse sequence with 64 scans was used
to acquire the spectra. About 30 mg of lignin samples were phosphity-
lated by dissolving in a mixture of 2-chloro-4,4,5,5-tetramethyl-1,3,
2-dioxaphospholane (TMDP) and deuterated chloroform/pyridine
(1:1.6) solvent system. N-hydroxy-5-norbornene-2,3- dicarboxylic acid
imide (NHND) and chromium acetylacetonate were then added to the
derivatized samples as internal standard and relaxation agent, respec-
tively. All the spectral data obtained was processed using Bruker
TopSpin 4.3.0 software.
2.5. Fourier transform infrared (FTIR)
The FTIR spectra of lignin, L-COOH, and polyester samples were
recorded on PerkinElmer C110864 spectrophotometer equipped with
attenuated total reection (ATR). Each spectrum was obtained from 64
scans ranging from 4000 to 600 cm
−1
at a resolution of 4 cm
−1
.
2.6. Carboxylation of lignin
Carboxylation of CELF lignin was performed following a modied
published method [13]. Briey, 15 g lignin was dissolved in 60 mL of
pyridine to form a homogenous solution with magnetic stirring. 60 mL
of MHHPA was added and the mixture allowed to react for 12 h at 60 ◦C.
After cooling to room temperature, the L-COOH was precipitated in
ice-cold water, ltered under vacuum, and dried overnight under vac-
uum at 50 ◦C.
2.7. Preparation of Lignin/ESO polyesters
The preparation of L-COOH/ESO polyester is schematically shown in
Scheme 1, while the formulations used are as indicated in Table 1. L-
COOH was dissolved in THF (20 mL) by stirring at room temperature. To
the stirred solution, ESO and Zn(acac)
2
were added, and the solution
mixture further stirred for about 1 h, and then transferred onto a Teon
plate of approximately 7 cm by 10 cm. After THF was allowed to
evaporate at room temperature for 1 h, the mold was placed in a vacuum
oven at 65 ◦C for 12 h to further evaporate the THF. The mold was cured
at 120 ◦C for 4 h followed by post-curing at 160 ◦C for 8 h. The same
procedure was used to prepare L-COOH/ESO/PEG polymer samples.
2.8. Characterization of Lignin/ESO polyesters
Thermal properties were determined using a differential scanning
calorimeter (DSC Q2000). The samples were heated from 0 ◦C to 200 ◦C
at 5 ◦C/min. The thermal stability of the synthesized polyesters was
determined by thermal gravimetric analyzer (TGA; TA-Q500) from
30 ◦C to 1000 ◦C at a heating rate of 10
◦
C/min under nitrogen
atmosphere.
The tensile tests of the polymer samples were carried out in line with
the Standard Method for Tensile Properties of Plastics (ASTM D638-22).
The cured samples were cut into dog-bone-shape, and the stress-strain
curves acquired using Tensile Instron 5943 Series with a 5 kN load
cell at a crosshead speed of 1 mm/min. Dynamic mechanical analysis
was performed using ARES-G2 rheometer using samples of approxi-
mately 20 mm length ×5 mm width ×0.3 mm thickness, from −30 ◦C to
200 ◦C at a heating rate of 5 ◦C/min. Frequency and amplitude were set
at 1Hz and 15
μ
m, respectively.
Cross-linking effectiveness was determined through gravimetric gel
content measurements [14,15]. A pre-weighed solid sample (W
o
) was
immersed in THF and left for 24 h at RT. THF was chosen because it is a
good solvent for both lignin and ESO. After 24 h, the remaining sample
in the solvent was ltered, and allowed to dry under vacuum for 24 h
before recording its mass (W
1
). Percent gel content was obtained ac-
cording to equation (1). All measurements were repeated at least three
times, and their average was reported as the experimental value.
% Gel =(W
1
/W
0
)×100 % (1)
The self-healing ability experiment was conducted by observing the
recovery of scratches on LEP-6 and LEP-8 sample using an optical mi-
croscope (NIKON SMZ 800 N). Knife-scratched samples were then
placed in a pre-heated Thermo Precision Compact oven at 180 ◦C for 60
min, and the scratch width versus time for each sample was observed on
a microscope. The self-healing efciencies were determined according
to equation (2).
% healing efciency after 30 min =(D
0
–D
t
/D
0
]×100% (2)
P.K. Karoki et al.
Polymer Testing 137 (2024) 108515
3
Scheme 1. Synthetic route for L-COOH/ESO polyester.
Table 1
Formulation and thermomechanical properties of LCOOH/ESO polyesters.
Polymernetwork L-COOH/ESO/
PEG
% w/w
PEG
Storage modulus
(MPa)
Young modulus
(MPa)
Tensile strength
(MPa)
T
g
(DMA)
(◦C)
Tg (DSC)
(◦C)
%Gel
content
LEP-0 500:479:0 0 ….…… …… …. 69.7 ±0.2 79.68 ±0.6
LEP-5 500:479:51.5 5 525.22 ±12.2 178.92 ±3.8 1.323 ±0.1 78.0 ±1.2 57.8 ±0.6 90.04 ±0.9
LEP-6 500:479:62.5 6 1463.67 ±12.5 178.69 ±1.2 3.983 ±0.1 67.8 ±1.5 56.4 ±0.4 88.52 ±0.4
LEP-8 500:479:85.1 8 1298.51 ±9.8 67.00 ±1.3 2.268 ±0.2 53.0 ±1.9 54.6 ±0.3 82.12 ±0.8
LEP-10 500:479:108.8 10 1052.36 ±27.2 37.98 ±1.0 2.1220.1 51.0 ±0.9 54.3 ±0.4 81.48 ±0.6
Zn(acac)
2
was loaded at 5 mol% based on the total phenolic and carboxylic hydroxy groups.
Fig. 1. The most dominant ether linkages and sub-unit in pine lignin [18,19].
P.K. Karoki et al.
Polymer Testing 137 (2024) 108515
4
Where D
0
and D
t
represent the width of the scratch after 0 min and at
any given time, respectively.
3. Results and discusion
3.1. Purity and structural composition of lignin
From the quantitative composition analysis of lignin, the purity of
the lignin used was 91.11 % with the acid-insoluble lignin (AIL) and
acid-soluble lignin (ASL) accounted for 86.92 wt% and 4.19 wt%,
respectively (Table S1 in the supporting information), while the mois-
ture and ash content in the sample were 3.07 wt% and 0.20 wt%,
respectively. To gain insight on the structure of lignin used, HSQC
analysis was done to determine its linkages and sub-unit composition.
The most important HSQC regions in the study of lignin structures are
the unsaturated (aromatic) and the oxygenated-aliphatic regions. Lignin
isolated from softwood (such as pine lignin) consists almost entirely of
guaiacyl (G) unit (>95 %), with β-0-4 linkages dominating (i.e., about
45–50 %) followed by phenylcoumaran (β-5) linkages (9–12 %). Resinol
(β-β
′
), biphenyl (5-5’), and diaryl ether (4-O-5) are also present in very
small amounts [16,17]. The key sub-units and linkages in the pine lignin
are shown in Fig. 1.
Fig. 2 shows the oxygenated and aromatic regions of the HSQC NMR
for the pine lignin sample. The three unsubstituted positions of G unit,
alongside some linkages were assigned. Several peaks were not assigned
as they did not match any of the key reported structures of lignin. They
may be due to new structures that may have formed during the CELF
extraction process, or signals of degradation products.
3.2. Modication of CELF lignin
The carboxylation of CELF lignin was accomplished by an esteri-
cation reaction between methylhexahydrophthalic anhydride (MHHPA)
and hydroxyl groups of lignin. Excess amount of MHHPA was used to
avoid the formation of diesters that would arise from the reaction of OH
from lignin and newly formed carboxylic groups in L-COOH. Since
carboxylated lignin (L-COOH) is insoluble in water while the MHHPA
and pyridine are water soluble, L-COOH was precipitated from the re-
action mixture in large amount of ice-cold water. In this reaction, pyr-
idine acted both as a solvent and as a catalyst [20] as shown in the
reaction mechanism in Scheme S1 (in the supporting information).
To ascertain the effectiveness of carboxylation, FTIR,
31
P NMR and
GPC analyses of both the modied and unmodied lignin were accom-
plished. From the FTIR spectra, Fig. 3a, a broad O–H peak at 3100 -
3600 cm
−1
is apparent, and increased intensity of C
–
–
O peak at 1735-
1750 cm
−1
in L-COOH spectrum show evidence of successful grafting of
–COOH onto lignin. This was supported by
31
P NMR and GPC. From the
NMR data shown in Fig. 4a and Fig. S1 (in the supporting information),
aliphatic and phenolic hydroxyl groups decreased signicantly, from
2.48 to 0.11 mmol/g and 2.56 to 1.28 mmol/g, respectively, while
carboxylic OH groups increased from 0.43 to 3.03 mmol/g (Table S2 in
the supporting information). Moreover, the molecular weight of LCOOH
was higher than that of the initial lignin (Fig. 4b). This increase was due
to the added anhydride moiety onto the lignin.
3.3. Synthesis of LCOOH/ESO polyesters
Epoxidized soybean oil with an oxirane content of 4.5 mmol/g was
cured with L-COOH using Zn(acac)
2
as a catalyst. Since phenolic and
carboxylic acid hydroxyl groups in L-COOH can react with epoxy groups
to form ester linkages [21], the stoichiometric molar amount of epoxy
groups was based on the sum of carboxylic and phenolic hydroxyl
groups. L-COOH:ESO ratio of 1:1 was used in all formulations as shown
in Table 1. The epoxy groups in ESO react with hydroxyl groups to form
ester bonds and new hydroxyl groups. These newly formed hydroxyl
groups can undergo further reaction with ester groups and unreacted
carboxylic groups to form a cross-linked network. In this work, the
synthesized neat polyester was so brittle that it was easily breaking
(Video S1 and Fig. S3 in the supporting information), and thus difcult
to determine its mechanical properties. This phenomenon was related to
the observed low cross-linking density (Table 1) that was attributed to
high steric hindrance of internal epoxy groups in ESO [22], and lower
functionality of softwood derived lignin [23]. To enhance the mechan-
ical performance, PEG was incorporated as a plasticizer, and the
resulting samples exhibited improved ductility, thus enabling mechan-
ical testing.
3.4. FTIR characterization of lignin-based polyesters
A series of FTIR spectra (Fig. 3b) for non-plasticized and plasticized
LCOOH/ESO samples were obtained to reveal the occurrence of esteri-
cation reaction and interaction between PEG and polymer network.
The absence of a characteristic transmission peak at 826 cm
−1
corre-
sponding to epoxy C–O–C stretching vibration in polymer samples is
evidence that most epoxy groups were successfully reacted with OH
groups from modied lignin. The notable increase in the intensity of OH
transmission band between 3100 and 3600 cm
−1
in plasticized samples
was related to OH groups contribution from PEG. The peaks at
approximately 1750 cm
−1
are due to C
–
–
O of the ester groups, while
those between 1050 cm
−1
and 1200 cm
−1
are due stretching vibration of
C–O bond of ester. These C–O stretching vibrations peaks are broader in
samples with PEG than in non-plasticized samples. According to litera-
ture, electron density in C–O increases as a result of dipole interaction
Fig. 2. The aromatic region (top) and the side chain region (bottom) of HSQC
NMR spectrum of lignin obtained from pine. Peaks were assigned by compar-
ison with literature [12,18,19]. Blue contours could not be assigned. (For
interpretation of the references to colour in this gure legend, the reader is
referred to the Web version of this article.)
P.K. Karoki et al.
Polymer Testing 137 (2024) 108515
5
Fig. 3. a) FTIR of CELF lignin and modied lignin (L-COOH) and methylhydrohexaphthalic anhydride (MHHPA), b) FTIR of epoxidized soybean oil (ESO), poly-
ethylene glycol (PEG), and polymer networks.
Fig. 4. a) Hydroxyl content and (b) molecular weight of Lignin and L-COOH as determined by
31
P NMR and GPC, respectively.
P.K. Karoki et al.
Polymer Testing 137 (2024) 108515
6
between PEG and ester groups [24]. This increase in electron density can
result in peak shift or peak broadening [25].
3.5. Mechanical properties of polyesters
DMA coupled ARES-G2 Rheometer and tensile Instron were used to
characterize the mechanical properties of polymer samples. Non-
plasticized samples were too brittle for mechanical characterization.
Upon modication with PEG, mechanically useful lms were obtained.
Among all the plasticized networks, LEP-6 exhibited the best perfor-
mance in terms of storage modulus (1463.67 ±12.56 MPa) and tensile
strength (3.983 ±0.091 MPa). The young modulus for LEP-5, LEP-6,
LEP-8 and LEP-10 were 178.92 ±3.80, 178.69 ±1.24, 67.00 ±1.37
and 37.98 ±1.04 MPa, respectively (Table 1). This trend of decrease in
modulus with increase in plasticizer content has also been observed in
other studies [26,27]. As shown in Fig. 5 and Fig. S2 (in the supporting
information), above 6 % w/w plasticizer, both the tensile strength and
storage modulus decreased with an increase in PEG content. Since PEG is
a soft component, increase in its content decreases the stiffness of the
polymer backbone, thus decreasing the storage modulus and strength.
However, at 5 % w/w PEG content, the polymer sample exhibited poor
ductility, thus low tensile strength, and storage modulus.
3.6. Thermal properties of lignin polyester
Glass transition temperature (Tg) is a key property that shows the
maximum temperature under which thermoset plastic can be utilized
effectively. The Tg values of plasticized samples prepared in this study
were determined using DMA and DSC. From the DMA analysis of LEP-5,
a Tg value of 78.0 ±1.2 ◦C was exhibited, while that of LEP-6, LEP-8 and
LEP-10 were 67.8 ±1.5 ◦C, 53.0 ±1.9 ◦C and 51.0 ±0.9 ◦C respec-
tively. The observed difference was attributed to modications in
crosslinking density as shown by their percentage gel content. A similar
trend was observed in Tg values obtained from DSC analysis, where LEP-
5 recorded the highest among the plasticized networks (Table 1 and
Fig. S2 in the supporting information). Notably, the Tg of non-
plasticized sample was higher than those of other polymer samples
prepared in this study. The plasticizer effect of lowering the Tg has been
demonstrated elsewhere in the literature [28,29].
From the TGA thermograms, summarized in Fig. 6, all samples
exhibited a single-step decomposition, implying that there was no
unreacted, or chemically unbound component in the polymer networks
[30,31]. Results further show that the lignin-based polyesters possess
higher degradation temperatures and are thermally stable up to 291 ◦C.
This makes lignin-based polyester suitable for higher temperature ap-
plications compared to bio-based polyesters derived from plant oil and
cellulose. For instance, Fonseca and coworkers reported a set of low
glass transition temperature (−37.4 ◦C to −13.0 ◦C) polyesters whose
onset degradation temperatures ranged between 152.1 and 164.6 ◦C
from a mixture of diacids (fumaric acid and sebacic acid) and diols
(diethylene glycol and propylene glycol). This implies that the poly-
esters were more prone to thermal degradation at elevated tempera-
tures, and thus not suitable for high-temperature applications [9].
In this study, most of the polymer networks decomposed between
300 and 400 ◦C, which is typical in most of the reported vitrimeric
polyesters [32,33]. The onset degradation temperature, taken as the
temperature for 5 % weight loss (T
5%
), for LEP-0, LEP-5, LEP-6, LEP-8
and LEP-10 were 309 ◦C, 293 ◦C, 291 ◦C, 288 ◦C and 290 ◦C, respec-
tively. The temperature for the maximum rate of degradation was in the
range of 352–362 ◦C for all samples. This data shows that integration of
PEG does not impair the thermal stability of LCOOH/ESO polymers.
3.7. Self-healing of L-COOH/ESO polyesters
The self-healing ability was investigated by monitoring the size of the
scratch on the surface of the sample with time at 180 ◦C. Knife-scratched
samples were placed in a pre-heated oven at 180 ◦C for 1 h. Because of
dynamic transesterication between hydroxyl groups and ester linkages
that occur in presence of Zn(ac)
2
catalyst at high temperatures [34],
thermal healing was observed in the polymer sample, a characteristic
property of vitrimers [6,13,32,35]. Fig. 7 and Table S3 (in the sup-
porting information) show that the width of LEP-6 and LEP-8 sample
reduced by over 42 % and 66 %, respectively in 60 min. The higher
efciency recorded in LEP-8 was attributed to higher PEG content that
resulted to more OH groups which accelerated the transesterication
reaction.
4. Conclusion
Due to its high heat resistance and natural abundance, lignin is an
ideal material for synthesis of mechanically strong and heat resistant
bio-derived materials. In this study, lignin derived polycarboxylic acid
(L-COOH) was prepared from a reaction between CELF lignin and
methyl hexahydrophthalic anhydride. The carboxylated lignin was then
reacted with ESO to prepare bio-based polyesters with good thermal
stability and self-repairability. Unlike some reported bio-based poly-
esters from plant oil and cellulose-derived materials, the synthesized
lignin-based polyesters were stable at elevated temperatures (>290 ◦C),
with most degradation happening between 300 ◦C and 400 ◦C. Presence
of residual hydroxyls groups and zinc acetylacetonate catalyst enabled
self-healing at elevated temperature (180 ◦C).
During this study, it was found that neat L-COOH/ESO polymer
Fig. 5. a) Storage modulus (E
′
) and (b) Tan δof L-COOH/ESO polyesters with different %w/w PEG content.
P.K. Karoki et al.
Polymer Testing 137 (2024) 108515
7
sample was very brittle and fractured easily so that its mechanical per-
formance could not be studied. This brittleness was attributed to the
rigidity of the lignin [36] and low crosslinking density. However, upon
incorporation of PEG as a plasticizer, the mechanical performance of the
samples was enhanced with the highest tensile strength and storage
modulus achieved at 6 % w/w PEG. Furthermore, the plasticizer was
found to lower the glass transition temperature of the polymers but did
not compromise on thermal stability. The ndings from this study
regarding neat and plasticized L-COOH/ESO polymer samples will open
new fronts for further investigation on the effects of structural properties
of lignin on the mechanical performance of lignin-based polymers.
Funding sources
The Southeastern Sun Grant Center.
CRediT authorship contribution statement
Peter K. Karoki: Writing –review &editing, Writing –original draft,
Investigation, Data curation. Shuyang Zhang: Writing –original draft,
Formal analysis, Data curation. Charles M. Cai: Writing –original draft,
Investigation. Paul E. Dim: Investigation. Arthur J. Ragauskas:
Writing –review &editing, Funding acquisition, Conceptualization.
Declaration of competing interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Data availability
The authors declare that the data supporting the ndings of this
Fig. 6. TGA thermograms of polymer networks: a) weight loss versus temperature, (b) rate of weight loss (dTG) versus temperature.
Fig. 7. Thermal repair of L-COOH/ESO polymer for LEP-6(a,b,c) and LEP-8 (d,e,f). The mple, the width of the scratch at different heating times were recorded at the
same position.
P.K. Karoki et al.
Polymer Testing 137 (2024) 108515
8
study are available within the paper,
Acknowledgement
The authors wish to thank the Southeastern Sun Grant Center for
funding “Lignin Based Polyester Vitrimers”which supported this
project.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.polymertesting.2024.108515.
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