Study of Diels–Alder reactions between furan and maleimide model compounds and the preparation of a healable thermo-reversible polyurethane

Article abstract of DOI:10.1002/pola.29061

A study of the reactions between various furan and maleimide model compounds and the effects of reaction conditions was performed, allowing for a proper design and preparation of a thermo-reversible polyurethane (PU) material crosslinked via Diels–Alder (DA) bonds. Thus, a linear polyurethane containing furan groups along the main chain was synthesized and crosslinked with a bismaleimide by means of DA reaction. The obtained thermoset exhibited thermo-reversibility as evidenced by DSC and FTIR microscopy, providing the material recyclability and scratch healability. Optical microscopy, SEM and tensile analysis of a scratched PU film revealed that efficient scratch healing was enabled by heating at 110 °C for 30 min and subsequently keeping at room temperature for 24 h, resulting in an approximately 80% recovery of the pristine mechanical strength. This material is a promising candidate for the development of self-healing coatings.

Full text of DOI:10.1002/pola.29061

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Study of Diels–Alder Reactions Between Furan and Maleimide Model
Compounds and the Preparation of a Healable Thermo-Reversible
Polyurethane
1,2
1
Thuy Thu Truong ,1 Ha Tran Nguyen
,
Man Ngoc Phan,1 Le-Thu T. Nguyen
1Faculty of Materials Technology, Ho Chi Minh City University of Technology, Vietnam National University (VNU–HCM), 268 Ly
Thuong Kiet, District 10, Ho Chi Minh City, Vietnam
2Materials Technology Key Laboratory (Mtlab), Ho Chi Minh City University of Technology, Vietnam National University
(VNU–HCM), 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Vietnam
Correspondence to: L.-T. T. Nguyen (E-mail: nguyenthilethu@hcmut.edu.vn)
Received 8 April 2018; accepted 11 May 2018; published online in Wiley Online Library
DOI: 10.1002/pola.29061
ABSTRACT: A study of the reactions between various furan and
maleimide model compounds and the effects of reaction condi-
tions was performed, allowing for a proper design and prepara-
SEM and tensile analysis of a scratched PU film revealed that effi-
cient scratch healing was enabled by heating at 110 8C for 30 min
and subsequently keeping at room temperature for 24 h, resulting
in an approximately 80% recovery of the pristine mechanical
tion of
a
thermo-reversible polyurethane (PU) material
strength. This material is a promising candidate for the develop-
crosslinked via Diels–Alder (DA) bonds. Thus, a linear polyure-
thane containing furan groups along the main chain was synthe-
sized and crosslinked with a bismaleimide by means of DA
reaction. The obtained thermoset exhibited thermo-reversibility
as evidenced by DSC and FTIR microscopy, providing the mate-
rial recyclability and scratch healability. Optical microscopy,
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ment of self-healing coatings.
2018 Wiley Periodicals, Inc. J.
Polym. Sci., Part A: Polym. Chem. 2018, 56, 1806–1814
KEYWORDS: Diels–Alder; healable; polyurethane; thermo-
reversible
desired thermodynamic and kinetic characteristics.27 Thus, the
assessment of thermodynamic and kinetic properties for furan
and maleimide pairings can facilitate the choice of the correct DA
functionalities. In a few previous studies, the reaction kinetics
between furan and maleimide pairs, such as furfuryl acetate and
N-methylmaleimide,28 furfuryl alcohol and bismaleimide,29 and
furoic acid and caproic maleimide,30 were reported. In consider-
ation of the subsequent preparation of thermally reversible
crosslinked polymer materials, in this work, we investigated the
reactions between various furan and maleimide model com-
pounds and the effects of reaction conditions. The model study
has enabled a proper design for the synthesis of a furan-
functional linear polyurethane as well as the selection of malei-
mide crosslinker and appropriate conditions for the preparation
of a thermo-reversible Diels–Alder crosslinked polyurethane,
which was recyclable and capable of scratch healing.
INTRODUCTION Since the inception of the “click” chemistry
concept,1 a wide range of synthetic fields of chemical and
polymer science has made use of the versatility of click
chemistry for efficiently constructing linkages for structural
modifications as well as preparation of novel polymeric
materials.2–5 In addition to many irreversible well-known
click reactions such as copper catalyzed azide–alkyne cyclo-
addition and thiol–ene addition,1,6–8 the thermo-reversible
Diels–Alder (DA) reaction has emerged prominently contrib-
uting to the broad click repertoire.9–12
The growing impact of the DA chemistry on the synthesis of re-
mendable, thermally reversible, and recyclable materials has
stimulated studies to gain more insight in the reaction pathway
and kinetics,13–26 which has been the starting point of this
research. The Diels–Alder reaction between maleimide and furan
moieties to form a DA adduct occurs at moderate temperatures,
frequently from room temperature to 100 8C.9 The DA adduct is
cable to revert back to the starting maleimide and furan pair at
elevated temperatures (usually above 1008) via the retro Diels–
Alder (rDA) reaction.9 For a certain application, it is crucial to
properly select the maleimide and furan pairs that exhibit the
EXPERIMENTAL
Materials
All materials were used as received, without further purifica-
tion. Solvents for the synthesis were dried according to
Additional Supporting Information may be found in the online version of this article.
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standard laboratory procedures prior to use. 1,10-(Methyle-
nedi-4,1-phenylene)bismaleimide (bismaleimide, 95%), N-
methylmaleimide (97%), furan (99%1), furfuryl alcohol
(98%), hexyl isocyanate (97%), hexamethylene diisocyanate
(HDI, 99%1), 1,4-butanediol (BDO, 99%1), 2-amino-1,3-
dichloromethane (100 mL), which was washed with H2O (3
3 100 mL). The organic layers were dried over MgSO4 and
concentrated to give an oily intermediate product. This oil
was refluxed in toluene at 110 8C for a couple of hours to
yield bis-(hydroxymethyl)maleimide as a slightly yellow oil.
The product was purified by column chromatography (ethyl
acetate/diethyl ether). Yield: 50%. The 1H NMR spectra of
the intermediate product and bis-(hydroxymethyl)maleimide
are shown in Supporting Information.
propanediol
(97%1),
1,1,2,2-tetrachloroethane
(TCE,
98%1), dimethyl sulfoxide-d6 (99.9%), and dimethyl sulfox-
ide (DMSO, 99%1) were purchased from Sigma-Aldrich. 2,5-
Bis-(hydroxymethyl)furan (99%1) was purchased Poly-
science, Inc. Exo-3,6-epoxy-1,2,3,6-tetrahydrophthalic anhy-
dride (98%1) was purchased from TCI Co. (Japan).
Zirconium(IV) acetylacetonate (98%1) was purchased from
Merck. 2,5-Bis-(hexyl urethane)furan was obtained by react-
ing one equivalent of 2,5-bis-(hydroxymethyl)furan with two
equivalents of hexyl isocyanate in dried chloroform at 60 8C
in the presence of 0.1 mol % of zirconium(IV) acetylaceto-
nate.31 Extra dry dimethyl acetamide (DMA, 99.5%1) was
purchased from Acros Organics.
Synthesis of Furan-Functional Polyurethane (PU)
The synthesis of PU from 2,5-bis-(hydroxymethyl)furan was
performed according to a previously reported procedure.33
In a two-neck round-bottom flask, 2,5-bis-(hydroxymethyl)-
furan (1 equiv.) and BDO (1 equiv.) were added and the flask
was filled with nitrogen gas. Zirconium acetylacetonate
(0.2% mol/mol NCO), dry DMA, and HDI (2.1 equiv.) were
then added. The temperature was raised to 70 8C under stir-
ring and the reaction was allowed to proceed under N2
atmosphere overnight. The clear viscous reaction solution
obtained was dropwise into diethyl ether to precipitate the
PU product. The PU as a powder was filtered and vacuum-
dried to constant weight. Yield: 76%.
Measurements
1H NMR spectra were recorded in deuterated chloroform
(CDCl3) with TMS as an internal reference, on a Bruker
Avance 300 at 300 MHz. Attenuated total reflectance (ATR)
FT-IR spectra were collected as the average of 128 scans
with a resolution of 4 cm21 on a FT-IR Tensor 27 spectrome-
ter equipped with a Pike MIRacle ATR accessory with a dia-
mond/ZnSe element. Size exclusion chromatography (SEC)
measurements were performed on a Polymer PL-GPC 50 gel
permeation chromatograph (GPC) system equipped with an
RI detector, with dimethylformamide as the eluent at a flow
rate of 1.0 mL/min. Molecular weight and molecular weight
distribution were calculated with reference to poly(methyl
methacrylate) (PMMA) standards. Differential scanning calo-
rimetry (DSC) measurements were carried out with a DSC
Q20 V24.4 Build 116 calorimeter under nitrogen flow, at a
heating rate of 10 8C/min. Tensile tests were measured using
a Tensilon RTC-1210A tensile testing machine, making use of
a 1000 N load cell. The rectangular specimens met the
requirements of ASTM D882. The tensile tests were run at a
speed of 0.05 mm/min. At least three specimens were tested
for each material. In the tensile testing of scratched sample,
a scratched was made along the width in the middle of the
sample length. Controlled scratches were performed by slid-
ing a razor or scalpel blade oriented at 458 to the sample
surface, with a motorized constant rate and an applied nor-
mal force of 1 N. Scanning electron microscope (SEM)
images were obtained on a HITACHI S-4800 microscope,
operating at 5 kV. Optical microscopic images were recorded
on an Olympus GX51F microscope.
Crosslinking of Furan-Functional PU
A mixture of furan-functional PU and bismaleimide cross-
linker in a 1:1 furan to maleimide equivalent ratio in DMSO
was cast in a glass Petri disk at 40 8C for 24 h, followed by a
Soxhlet extraction in acetone at 60 8C to eliminate DMSO
and uncrosslinked components.
RESULTS AND DISCUSSION
Study of Diels–Alder Reactions of Model Compounds
Effect of Furan Structure
The equimolar DA reactions between N-methylmaleimide
and four furan model compounds, including furan, furfuryl
alcohol, 2,5-bis-(hydroxymethyl)furan and 2,5-bis-(hexyl ure-
thane)furan were studied by UV–vis spectroscopy in tetra-
chloroethane. Representative UV–vis spectra of a reaction
mixture between the maleimide and furan model compounds
versus time are shown in Figure 1. The reaction of furan
moieties with maleimide derivatives is typically monitored
spectrophotometrically by the decrease in absorbance at
300 nm (conjugated C@CAC@O transitions), attributed to
the disappearance of the maleimide double bond resulting in
breaking of the conjugation between C@C and C@O bonds.
The conversion of the DA reaction at a certain time was esti-
mated by comparing the absorbance intensity at 300 nm
with that at the beginning of the reaction. Figure 2 compares
the kinetics of the reactions of N-methylmaleimide and the 4
furan model compounds (maleimide to furan molar ratio of
1) in tetrachloroethane at 40 8C. Most remarkably, the reac-
tion of N-methylmaleimide and 2,5-bis-(hexyl urethane)furan
achieved a conversion of more than 90% within 5 h,
whereas the conversions of the other reactions were just
above 20% at the same reaction time. The rate of the reac-
tion of 2,5-bis-(hexyl urethane)furan was retarded after 90–
Synthesis of Bis-(Hydroxymethyl)Maleimide
Bis-(hydroxymethyl)maleimide was prepared according
a
previously reported procedure.32 2-Amino-1,3-propanediol
(2.47 g, 27.1 mmol) in methanol (25 mL) was added drop-
wise to a solution of exo-3,6-epoxy-1,2,3,6-tetrahydrophthalic
anhydride (4.5 g, 27.1 mmol) in methanol (200 mL). The
solution was stirred at 56 8C for 3 days, and concentrated to
give a clear oil. This oil was purified by dissolving it in
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FIGURE 1 Progressive decrease in the absorbance of the mal-
eimide absorbance at 300 nm following the progress of the DA
reaction of N-methylmaleimide and 2,5-bis-(hydroxymethyl)-
furan in tetrachloroethane (0.2 M) at 40 8C. [Color figure can be
viewed at wileyonlinelibrary.com]
FIGURE 3 1H NMR spectra of a reaction mixture of 2,5-bis-
(hydroxymethyl)furan and bismaleimide (furan to maleimide
molar ratio of 1) at 2 M concentration in DMSO-d6 after mixing
for 5 min (a) and at 40 8C for 20 h (b). The signals 7–10 and 70–
100 correspond to assigned protons of endo and exo isomer
products, respectively. [Color figure can be viewed at wileyonli-
nelibrary.com]
95% conversion, likely because an equilibrium between the
forward and reverse reactions was established. There were
only slight differences in the reaction rates of the reactions
of furan, furfuryl alcohol and 2,5-bis-(hydroxymethyl)furan.
In terms of DA reactions with N-methylmaleimide in tetra-
chloroethane, the overall comparison of the reactivity of the
furan derivatives is as follows: 2,5-bis-(hexyl urethane)furan
ꢀ
2,5-bis-(hydroxymethyl)furan ꢁ furfuryl alcohol > furan.
bond vicinal to the furan group, which limits the dissociation
of DA linkages favorable for the forward reaction. Indeed,
the role of hydrogen-bonding interaction in enhancement of
the DA reaction rate has also been observed for other
systems.34,35
The dominantly higher reaction rate of 2,5-bis-(hexyl uretha-
ne)furan could be explained by the hydrogen-bonding
between the C@O bond of maleimide and the urethane NAH
Effect of Maleimide Structure
The DA reactions of 2,5-bis-(hydroxymethyl)furan and vari-
ous maleimide model compounds, including 1,10-(methyle-
nedi-4,1-phenylene)bismaleimide
(bismaleimide),
bis-
(hydroxymethyl)maleimide, and N-methylmaleimide were
compared using the 1H NMR method. Representative 1H
NMR spectra of these reaction mixtures in DMSO-d6 with
assigned characteristic peaks are presented in Figures 3–5.
The occurrence of the DA reaction was evidenced by the
decrease in intensity of the peaks corresponding to the furan
and maleimide groups at around 6.2 (peak 3) and 6.95–7.1
ppm (peak 4) ppm, respectively. Accompanying this is the
simultaneous evolution of peaks attributed to the DA bonds in
the regions of 6.25–6.5 ppm (peak 9/90) and 2.95–3.55 ppm
(peak 10/100). The relative reaction conversion was determined
by a comparison of the ratio between the integral of peak 3
(furan group) and the total integral of peaks 9 and 90 (DA
bond). The comparison result is shown in Figure 6. The overall
comparison of the reactivity of the maleimide derivatives is as
follows: bismaleimide ꢀ bis-(hydroxymethyl)maleimide > N-
methylmaleimide. Bismaleimide exhibits superior reactivity
compared to the other studied maleimide model compounds,
attributed to the high electron deficiency of the double bond as
FIGURE 2 Conversion versus reaction time from UV spectros-
copy data for the stoichiometric DA reaction between N-meth-
ylmaleimide (0.2 M) and various furan model compounds in
tetrachloroethane at 40 8C. [Color figure can be viewed at
wileyonlinelibrary.com]
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FIGURE 4 1H NMR spectra of a stoichiometric reaction mixture
of 2,5-bis-(hydroxymethyl)furan and bis-(hydroxymethyl)malei-
mide at 1.1 M concentration in DMSO-d6 at 40 8C for 0.5 h (a)
and 20
h
(b). The signals 8–11 and 80–110 correspond to
assigned protons of endo and exo isomer products, respec-
tively. [Color figure can be viewed at wileyonlinelibrary.com]
a result of the aromatic N-substituted unit in the bismaleimide
structure.
Effect of Concentration and Temperature
A comparison of Figure 6(a,b) showing the DA reaction
kinetics at concentrations of 1.1 and 0.2 M indicates that the
DA reaction rate was considerably accelerated with increased
concentration. Moreover, the DA reaction was also affected
strongly by the temperature. As shown in Figure 7, the DA
reaction rate was enhanced with increasing the temperature
from 24 to 40 8C, and was fastest at 40 8C. The DA reaction
FIGURE 6 Conversion versus reaction time from 1H NMR spec-
troscopy data for the DA reactions (maleimide to furan molar
ratio of 1) of 2,5-bis-(hydroxymethyl)furan with bismaleimide,
bis-(hydroxymethyl)maleimide and N-methylmaleimide in
DMSO-d6 at 40 8C at concentrations of 1.1 M (a) and 0.2 M (b).
[Color figure can be viewed at wileyonlinelibrary.com]
FIGURE 5 1H NMR spectra of a stoichiometric reaction mixture
of 2,5-bis-(hydroxymethyl)furan and N-methylmaleimide at
1.1 M concentration in DMSO-d6 after mixing (a) and at 20 8C
for 24 h (b). [Color figure can be viewed at wileyonlinelibrary.
com]
FIGURE 7 Conversion versus reaction time from 1H NMR spec-
troscopy data for the DA reactions of 2,5-bis-(hydroxymethyl)-
furan with bismaleimide (furan to maleimide molar ratio of 1)
at various temperatures in DMSO-d6 at a concentration of 2 M.
[Color figure can be viewed at wileyonlinelibrary.com]
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SCHEME 1 Chemical structures of model compounds investigated in this work and the formation of thermo-reversible PU.
still occurred at 24 8C, although with a significantly lower
rate. Compared to the reactions at 20 and 30 8C, the reaction
rate at 60 8C was slightly enhanced at the beginning but was
slightly retarded after around 80% conversion. This could be
because that at a relatively high DA conversion, the rDA
reaction was more favored at 60 8C.
recyclability of thermoset materials incorporating these moi-
eties as crosslinks.
Preparation of Thermoreversible Polyurethane (PU)
Thermoset
Based on the above kinetic study, 2,5-bis-(hexyl urethane)-
furan (which arises from 2,5-bis-(hydroxymethyl)furan) and
bismaleimide appear to be the most reactive furan and mal-
eimide model compounds, respectively. Moreover, the pres-
ence of urethane groups with hydrogen-bonding ability had
an effect on favoring the forward DA reaction. Therefore, a
crosslinked polyurethane (PU) with thermal reversibility via
the reversible DA crosslinking was designed and prepared. A
linear polyurethane bearing furan groups along the backbone
was synthesized from 2,5-bis-(hydroxymethyl)furan, 1,4-
butanediol (BDO), and hexamethylene diisocyanate (HDI),
followed by a curing process using bismaleimide as the
crosslinker (Scheme 1).
Feasibility of rDA Reactions
In addition, the feasibility of the reversibility of the DA reac-
tions was briefly assessed. The stoichiometric reaction solu-
tion of 2,5-bis-(hydroxymethyl)furan and bismaleimide in
DMSO-d6 (2 M) after being conducted at 40 8C for 24 h
(attaining a 95.2% conversion) was heated at either 90 8C or
110 8C for 2 h to initiate the rDA reaction, followed by
immediate 1H NMR analysis. The result showed that about
80% of the DA bonds were dissociated generating the furan
and maleimide groups upon heating the solution at 110 8C
for 2 h, while a lower temperature of 90 8C resulted in a
lower rDA reaction yield (about 60%). The feasibility of the
rDA reaction of 2,5-bis-(hydroxymethyl)furan and bismalei-
mide suggests a good possibility for the reversibility and
Synthesis of Linear Furan-Functional PU
1
Figure 8 shows the H NMR spectrum of the synthesized PU.
All characteristic peaks were well assigned to the structure
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FIGURE 8 1H NMR spectrum of the furan-functional PU in DMSO-d6. [Color figure can be viewed at wileyonlinelibrary.com]
of the PU. A peak at 6.43 ppm (peak g) corresponding to the
furan double bond protons appears. In addition to the peaks
in the range of 7–7.3 ppm attributed to the urethane OCONH
proton, there are peaks at 5.7 and 4.4 ppm which are
assigned to the urea NHCONH protons.36,37 In the polycon-
densation process of polyurethanes, it is commonly known
that the highly reactive isocyanate groups of the diisocyanate
can undergo side reactions, for example, a trace of water can
react with isocyanate groups leading to the formation of
urea bonds with the liberation of carbon dioxide.
Based on the results of the studied model reactions, a mini-
mum amount of solvent required to completely dissolve the
components and a curing temperature of 40 8C were
employed. Figure 9 shows the gelation behavior of a mixture
of PU and bismaleimide in DMSO. The initial solution gelled
after 24 h at 40 8C as a result of crosslinking via DA reac-
tion. The gel sample went back to solution at 110 8C for 2 h
and then again gelled at 40 8C for 24 h, indicating the occur-
rence of reversible rDA and DA reactions. Soxhlet extraction
of the obtained gel in warm acetone was performed to
remove DMSO, resulting a crosslinked content of 92%. The
differential scanning calorimetry data of the crosslinked PU
after removal of solvent is presented in Figure 10. The sam-
ple displayed a broad endothermic transition in the range of
110–160 8C corresponding to the rDA reaction indicating the
cleavage of crosslinked bonds.38 The appearance of this ther-
mal transition indicates that the gelation process occurred as
a result of the formation of DA crosslinks.
By comparing the ratio between the integral of the ethylene pro-
tons arising from HDI (total integrals of peaks d and e) and the
total integrals of methylene protons arising from BDO and 2,5-
bis-(hydroxymethyl)furan moieties (peaks a and f, respectively),
the molar ratio between urethane and urea groups could be
estimated. The molar contents of urethane and urea bonds were
determined to be 69% and 31%, respectively. Moreover, the
furan content was estimated from the integral ratio between the
2,5-bis-(hydroxymethyl)furan methylene protons (peak f) and
the BDO methylene protons (peak a). Accordingly, a furan equiv-
alent weight of 1089.8 g mol21 was determined. GPC analysis
showed a number average molecular weight (Mn) of 13,000 g
Thermo-Reversibility Behavior of the Cured PU
The maintenance of the DA thermo-reversibility on material
level was studied by ATR-FTIR tests evidencing the occur-
rence of the rDA and DA reactions in the cured PU. As
shown in Figure 11, the IR spectrum of the material after
heating at 110 8C for 2 h, without subsequent quenching,
reveals the evolution of maleimide bands at 691 and
828 cm21 and simultaneous decrease in intensity of the
band at 861 cm21 ascribed to the furan–maleimide
mol21 and a polydispersity index (D) of 2.7.
-
Curing of the Furan-Functional PU via DA Reaction
The synthesized furan-functional PU was then crosslinked by
bismaleimide, using a furan to maleimide molar ratio of 1.
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FIGURE 9 Gelation behavior of a mixture of furan-functional
PU and bismaleimide (furan/maleimide molar ratio 51) in
DMSO. [Color figure can be viewed at wileyonlinelibrary.com]
FIGURE 11 ATR-FTIR spectra evolution of the crosslinked PU
during the heating cycles. The material was heated at to 110 8C
(rDA) and at 40 8C (DA) in a cyclic way. [Color figure can be
viewed at wileyonlinelibrary.com]
cycloadduct,14,39 suggesting the presence of disconnected DA
bonds. Although the ATR FTIR spectrum of the pre-heated
sample is not representative of what actually occurred at the
temperature of cleavage because some re-crosslinking could
occur during the measurement at ambient temperature, such
an observation still clearly suggests the partial cleavage of
the DA linkages. Then, keeping the sample at room tempera-
ture (30 8C) for 24 h reformed the DA bonds, as evidenced
by the decrease in the absorption band at 691 and
828 cm21 and the recovery of the band at 861 cm21. The
repeated occurrence of the rDA and DA reactions was proven
for a couple of cycles (Fig. 11), suggesting the recyclability of
the material. A test showing that the PU thermoset was recy-
clable is demonstrated in Figure 12.
scratched sample was heated at 110 8C for 0.5 h, followed
by keeping at room temperature (30 8C) for different time
periods. The first heating step at 110 8C was to partially
break the crosslinks, resulting in enhanced network mobility
without losing the structure integrity. As a result, the poly-
mer chains could locally flow giving rise to a reduction of
the scratch gap. This consequently enabled the DA reaction
between disconnected furan and maleimide moieties in the
next healing step at room temperature (30 8C). The optical
microscopic and SEM images in Figure 13 show that follow-
ing the first heating step at 110 8C for 0.5 h, the width of
the scratch was reduced significantly after 10 min at room
temperature, and was healed almost completely after 24 h
As the thermo-reversibility of the DA bonds enables the
breaking and reformation of the crosslinks, the PU
thermoset also exhibited scratch healability. To assess the
mendability of the obtained material, a scratch was made in
a thin film of the PU using a razor blade. Afterward, the
FIGURE 12 Demonstration of the recycling of the PU cross-
linked via DA bonding: (a) the original sample was cut into
small pieces; (b) the pieces of original sample was immersed
in a small amount of DMSO and heated at 110 8C until a clear
solution was obtained; (c) the solution in image b was kept at
40 8C for 24 h to give a re-crosslinked sample; (d) the recycled
thermoset sample after Soxhlet extraction to remove the sol-
vent. [Color figure can be viewed at wileyonlinelibrary.com]
FIGURE 10 DSC scans of the furan-functional PU (a) and the
crosslinked furan-functional PU/bismaleimide system (b).
[Color figure can be viewed at wileyonlinelibrary.com]
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FIGURE 13 Optical microscopic (a–c) and SEM (d, e) images of scratches before (a, d) and after healing via a process including
firstly heating at 110 8C for 0.5 h and secondly at room temperature (30 8C) for 0.5 h (b) or 24 h (c, e). [Color figure can be viewed
at wileyonlinelibrary.com]
CONCLUSIONS
resulting in a slight scar. Correspondingly, the tensile perfor-
mance of the scratched film after healing at 100 8C for 0.5 h
and subsequently at room temperature for 0.5 h showed a
In this study, the reactions between various furan and malei-
mide model compounds were studied for elucidating the
maximum stress recovery of 60% (Fig. 14). Longer second-
effects of reaction conditions as well as maleimide and furan
step healing periods of 12 and 24 h led to higher tensile
structures on the formation of DA adducts. This allowed for
strength recovery efficiencies of around 75% and 79%,
respectively.
a
suitable design and successful preparation of a PU
FIGURE 14 (a) Stress–strain curves of the initial film and the scratched film after heating at 100 8C for 0.5 h followed by keeping at
room temperature (30 8C) for 0.5, 12, and 24 h; (b) tensile strength recoveries of the scratched film after corresponding healing
processes. [Color figure can be viewed at wileyonlinelibrary.com]
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16 D. H. Turkenburg, Y. Durant, H. R. Fischer, Prog. Org. Coat.
2017, 111, 38.
thermoset with DA bonds as thermo-reversible crosslinks.
The thermo-reversibility and recyclability of the cured PU
were visualized by FTIR and DSC analysis and qualitatively
demonstrated. Scratch healing was possible via a two step
heating process at 110 8C for 0.5 h to increase network
mobility as a result of partial de-crosslinking via rDA reac-
tion and subsequently at room temperature to reform the
DA crosslink bonds. Increasing the time of the second step
enhanced the healing efficiency, reaching a tensile strength
recovery of approximately 80% after 24 h.
17 K. Yang, J. C. Grant, P. Lamey, A. Joshi-Imre, B. R. Lund, R.
A. Smaldone, W. Voit, Adv. Funct. Mater. 2017, 27, 1700318.
18 P. Berto, A. Pointet, C. Le Coz, S. Grelier, F. Peruch, Macro-
molecules 2018, 51, 651.
ꢀ
19 O. Guaresti, C. Garcıa-Astrain, R. H. Aguirresarobe, A.
Eceiza, N. Gabilondo, Carbohydr. Polym. 2018, 183, 278.
20 J.-W. Kim, D. H. Lee, H.-J. Jeon, S. I. Jang, H. M. Cho, Y.
Kim, Appl. Surf. Sci. 2018, 429, 128.
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21 I. C.-Y. Hou, Y. Hu, A. Narita, K. Mullen, Polym. J. 2018, 50,
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22 S. Li, L. Wang, X. Yu, C. Wang, Z. Wang, Mater. Sci. Eng. C.
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ACKNOWLEDGMENTS
23 S. Jung, T. Patel, J. K. Oh, Macromol. Rapid Commun. 2018,
39, 1700575.
This research was fully supported by Vietnam National Founda-
tion for Science and Technology Development – NAFOSTED
(104.02–2015.95).
24 K. Luo, J. Li, G. Duan, Y. Wang, J. Yu, J. Zhu, Z. Hu, Poly-
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