Tuning the self-healing behavior of one-component intrinsic polymers

Article abstract of DOI:10.1016/j.polymer.2015.03.027

Novel terpolymers with furan and maleimide units as functional moieties for the reversible crosslinking by the Diels-Alder (DA) reaction with different polar and nonpolar co-monomers and linkers have been prepared for potential applications as self-healing coatings. The synthesized linear one-component systems are able to crosslink via the functional units in the side chain (i.e. furan and maleimide) resulting in a highly crosslinked network. The terpolymers contain different maleimide methacrylates with three different linkers (MIMA 1, MIMA 2, MIMA 3), which vary in the length and the composition of the spacer unit as well as furfuryl methacrylate (FMA) as active units. Moreover, as polar co-monomers hydroxyethyl methacrylate (HEMA), dimethylaminoethyl methacrylate (DMAEMA) as well as 2-(hydroxyethoxy)ethyl methacrylate (DEGMA) and as nonpolar co-monomer butyl methacrylate (BMA) were used. The terpolymers were characterized using ¹H NMR spectroscopy and SEC measurements; the thermal properties were studied by TGA and DSC investigations as well as the self-healing properties by real-time analyses using a microscope equipped with a camera.

Full text of DOI:10.1016/j.polymer.2015.03.027

Accepted Manuscript
Tuning the Self-Healing Behavior of One-Component Intrinsic Polymers
J. Kötteritzsch, Dr. M.D. Hager, Prof. Dr. U.S. Schubert
PII:
S0032-3861(15)00255-4
10.1016/j.polymer.2015.03.027
JPOL 17699
DOI:
Reference:
To appear in: Polymer
Received Date: 9 December 2014
Revised Date: 9 March 2015
Accepted Date: 10 March 2015
Please cite this article as: Kötteritzsch J, Hager MD, Schubert US, Tuning the Self-Healing Behavior of
One-Component Intrinsic Polymers, Polymer (2015), doi: 10.1016/j.polymer.2015.03.027.
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ACCEPTED MANUSCRIPT
Tuning the Self-Healing Behavior of One-Component Intrinsic
Polymers
Julia Kötteritzsch, Martin D. Hager* and Ulrich S. Schubert*
J. Kötteritzsch, Dr. M. D. Hager, Prof. Dr. U. S. Schubert
Laboratory for Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller
University Jena, Humboldtstr. 10, 07743 Jena, Germany
E-mail: ulrich.schubert@uni-jena.de; martin.hager@uni-jena.de
Phone: +49 3641 9-48220
Fax: +49 3641 9-48202
J. Kötteritzsch, Dr. M. D. Hager, Prof. Dr. U. S. Schubert
Jena Center for Soft Matter (JCSM), Friedrich Schiller University Jena,
Philosophenweg 7, 07743 Jena, Germany
1

ACCEPTED MANUSCRIPT
Tuning the Self-Healing Behavior of One-Component Intrinsic
Polymers
Julia Kötteritzsch, Martin D. Hager* and Ulrich S. Schubert*
Novel terpolymers with furan and maleimide units as functional moieties for the
reversible crosslinking by the Diels-Alder (DA) reaction with different polar and
nonpolar co-monomers and linkers have been prepared for potential applications as self-
healing coatings. The synthesized linear one-component systems are able to crosslink
via the functional units in the side chain (i.e. furan and maleimide) resulting in a highly
crosslinked network. The terpolymers contain different maleimide methacrylates with
three different linkers (MIMA 1, MIMA 2, MIMA 3), which vary in the length and the
composition of the spacer unit as well as furfuryl methacrylate (FMA) as active units.
Moreover, as polar co-monomers hydroxyethyl methacrylate (HEMA),
dimethylaminoethyl methacrylate (DMAEMA) as well as 2-(hydroxyethoxy)ethyl
methacrylate (DEGMA) and as nonpolar co-monomer butyl methacrylate (BMA) were
1
used. The terpolymers were characterized using H NMR spectroscopy and SEC
measurements; the thermal properties were studied by TGA and DSC investigations as
well as the self-healing properties by real-time analyses using a microscope equipped
with a camera.
Keywords: copolymerization; crosslinking; Diels-Alder cycloaddition; self-healing
polymers; microscope images; polar co-monomers
2

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1
.
Introduction
Self-healing materials are in focus of interest in recent years. In particular, the Diels-
Alder (DA) cycloaddition has been applied in many materials to achieve intrinsic self-
healing abilities without changing significantly the original properties of the materials
[
1–3]. By this manner, it is possible to access self-healing polymers with well-defined
architectures and properties, due to the thermal reversibility (retro-Diels-Alder reaction)
4,5]. The Diels-Alder chemistry was widely utilized in polymer as well as material
[
science [1,6–10]. Later, Wudl and coworkers introduced this concept into the field of
self-healing (remendable) polymers [2,11–13].
The first synthesis of a functional copolymer based on methacrylates was reported by
Wouters and Fischer et al. in 2009 and additionally by Haddleton and coworkers in
2
010 [14,15]. The first investigations on the synthesis of PFMA via ATRP and free
radical polymerization (FRP) and the copolymerization with methyl methacrylate
MMA) were published by Kavitha and Singha as well as the synthesis of a PFMA-b-
(
PEHA-b-PFMA triblock copolymer. The authors also studied the self-healing properties
of the copolymer [16–19].
Our previous work was based on these investigations and we could create a one-
component intrinsic self-healing coating based on the reversible DA reaction. For this
purpose we incorporated both functionalities for the DA reaction, i.e., furan as well as
maleimide units, into linear polymers based on a methacrylate backbone [20]. As a
consequence no additional (low molar mass) crosslinkers are required to crosslink the
polymer and to achieve the desired self-healing properties. The self-healing properties
of the most promising terpolymer P(LMA-co-MIMA-co-FMA) were further
investigated by Bose et al. [21]. Barthel et al. extended the concept to functional block
copolymers [22–24].
3

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One drawback of the Diels-Alder cycloaddition of maleimide and furan is the required
high temperature (120 to 180 °C) of the rDA [20]. In our previous work the DSC
measurements and the self-healing studies of the copolymers P(AMA-co-FMA-co-
MIMA) revealed that the variation of the chain length of the co-monomers influenced
the temperature of the retro-Diels-Alder reaction: More nonpolar environments resulted
in higher temperatures of the retro-Diels-Alder reaction [20]. This effect was also
observed by Kavitha and Singha [16–19]. Additional, other studies revealed that more
polar environments of the maleimide/furan unit resulted in a slightly decreased rDA
temperature [25–28]. Toncelli et al. pointed out that at relatively low maleimide/furan
ratio the RDA transitions become broader and shifts asymmetrically towards lower
temperatures, while the crosslinking density drops significantly. This is due to the
decrease of the furan content and, therefore, an increase of the mobility of the chains
appears [28]. Similar effects of the polarity on the activation energy (E ) of the rDA of
A
small molecules were reported by Wijnen et al. [29], Rispens et al.[30] and Kiseleva et
al. [31]. The authors indicated that polar solvents have a significant influence on the
equilibrium of the Diels-Alder reaction and on the activation energy of the retro-Diels-
Alder reaction. As a consequence we decided to investigate if the incorporation of a
polar co-monomer (HEMA, DMAEMA and DEGMA) into the terpolymer with Diels-
Alder functionalities could also decrease the rDA temperature. Furthermore, this factor
could also be useful to improve the self-healing behavior of the copolymers.
A restriction in this case is the relatively high glass transition temperature of these
monomers compared to lauryl methacrylate (LMA), which revealed up to now the best
self-healing properties [20]. As reported by Mayo and Adronov also the linker unit has a
significant influence on the hardness of the film as well as the film-forming behavior of
the system [32]. An enhanced mobility of the system promotes the self-healing
4

ACCEPTED MANUSCRIPT
properties at lower temperatures due to the required backflow of the material into the
scratch after reaching the rDA temperature.
Therefore, also different linkers of the maleimide methacrylate monomers were used to
improve the flexibility and mobility of the copolymers, because this represents an
important prerequisite for the potential self-healing abilities of our one-component self-
healing system at lower temperatures.
2
.
Experimental Procedures
2
.1
Materials and Instrumentation
All chemicals were purchased from Fluka, Aldrich, Acros Organics as well as Alfa
Aesar and were used without further purification unless otherwise specified. The
solvents were purchased from Biosolve, Aldrich as well as Acros Organics. Dry
dichloromethane and toluene were obtained from a PureSolv-EN Solvent Purification
System (Innovative Technology). Triethylamine was dried over calcium chloride.
1
13
1
D ( H, C) nuclear magnetic resonance spectra were recorded on a Bruker AC 400
(400 MHz), Bruker AC 300 (300 MHz) and Bruker AC 250 (250 MHz) at 298 K.
Chemical shifts are reported in parts per million (ppm, δ scale) relative to the residual
signal of the solvent. Coupling constants are given in Hz.
CHN analysis was carried out on a Vario El III (Elementar) elemental analyzer.
ESI-TOF MS measurements were performed using a micrOTOF (Bruker Daltonics)
mass spectrometer, which was equipped with a syringe pump for sample injection and a
standard electrospray ion source. The mass spectrometer was operating in the positive
ion mode and the data were processed with the micrOTOF control Version 3.0 and Data
5

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Analysis Version 4.0 SP2. The instrument was calibrated by a tunemix solution (m/z 50
to 3,000) from Agilent.
Size exclusion chromatography measurements were performed on an Agilent 1200
system (degasser, isocratic pump, autosampler, RI detector) and two PSS GRAM
(1000/30 Å, 10 µm particle size) columns in series (eluent: DMAc with 2.1 g/L LiCl;
flow rate of 1 mL/min atm 40 °C) using linear poly(methyl methacrylate) standards.
The TGA analysis (TGA) was carried out using a Netzsch TG 209 F1 and the
differential scanning calorimetry (DSC) measurements were performed on a DSC 204
-1
F1 Phoenix by Netzsch under a nitrogen atmosphere with a heating rate of 20 K min .
The simultaneous thermal analysis (STA) was carried out using the Netzsch 449 F3
Jupiter with a Netzsch QMS 403 D Aëolos MS detector and a Bruker Tensor 27 FT-IR
detector.
Optical micrographs of the sample were acquired with the optical detection unit of the
AFM facility. For this purpose, the films were crosslinked for 4 h between 120 to
1
30 °C, scratched with a hollow needle or a knife in a controlled manner and annealed
for individual timescales to obtain the different healing states at different temperatures.
2
.2
Synthesis
The monomers MIMA 1 to MIMA 3 and the corresponding precursors were synthesized
according to Kötteritzsch et al. [20]. The synthesis of the copolymers P1 to P7 was also
performed using similar conditions as described by Kötteritzsch et al. [20]. Only the
reaction conditions for the terpolymers with the polar co-monomers (P1, P2 and P4)
were varied due to the better solubility of the monomers and the resulting polymers in
polar solvents [33,34].
6

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Synthesis
of
,3(2H)-dione (3)
a,4,7,7a-Tetrahydro-4,7-epoxyisobenzofuran-1,3-dione (2.0 g, 12.1 mmol) and
2-(6-hydroxyhexyl)-3a,4,7,7a-tetrahydro-1H-4,7-epoxyisoindole-
1
3
methanol (50 mL) were added into a three-neck round-bottom flask containing a
magnetic stirring bar and a reflux condenser. The solution was purged with nitrogen for
1
0 min in an ice bath. Subsequently, 6-aminohexan-1-ol (1.4 g, 12.1 mmol) and
triethylamine (1.7 mL, 12.1 mmol) were added. The reaction mixture was allowed to
warm to room temperature and the temperature was later increased to 67 °C for 20 h.
Subsequently, 10% of 6-aminohexan-1-ol (0.14 g, 1.2 mmol) was added and stirring
was continued for 2 h at 70 °C. The solution turned dark orange. The flask was cooled
to room temperature and the solution volume was reduced to dryness. Subsequently, the
orange oil was dissolved in dichloromethane, washed with water and dried over MgSO4.
The product was precipitated in ice-cold hexane and stored at –22 °C. The precipitate
was collected by decantation and used without further purification. Yield 1.1 g (35%).
1
H NMR (250 MHz, CDCl ): δ = 6.50 (s, 2H, CH), 5.25 (s, 2H, CH), 3.60 (t, J = 6.3
3
Hz, 2H, CH ), 3.47 (t, J = 7.2 Hz, 2H, CH ), 2.82 (s, 2H, CH), 1.7 – 1.2 (m, 8H, CH ).
2
2
2
1
3
C NMR (62.9 MHz, CDCl ): δ = 176.3 (CO-N), 136.5 (=CH-), 80.9 (>CH-), 62.6 (-
3
CH -), 47.4 (>CH-), 38.8 (-CH -), 32.4 (-CH -), 27.4 (-CH -), 26.1 (-CH -), 25.0 (-
2
2
2
2
2
CH -). (C H NO ) (265.1) : Calcd. C 63.38, H 7.22, N 5.28; Found C 63.62, H 7.01,
2
14 19
4 n
n
N 4.89.
Synthesis of 2-(2-(2-hydroxyethoxy)ethyl)-3a,4,7,7a-tetrahydro-1H-4,7-epoxyiso-
indole-1,3(2H)-dione (4)
3
a,4,7,7a-Tetrahydro-4,7-epoxyisobenzofuran-1,3-dione (1 g, 6 mmol), methanol
(50 mL) and 2-(2-aminoethoxy)ethan-1-ol (0.7 g, 6.7 mmol) were added into a three-
7

ACCEPTED MANUSCRIPT
neck round-bottom flask containing a magnetic stirring bar and a reflux condenser. The
solution was purged with nitrogen for 10 min in an ice bath. Subsequently,
triethylamine (1 mL, 7.2 mmol) was added. The reaction mixture was allowed to warm
to room temperature and the temperature was later increased to 67 °C for 20 h.
Subsequently, 10% of 2-(2-aminoethoxy)ethan-1-ol (0.07 g, 0.7 mmol) was added and
stirring was continued for 2 h at 70 °C. The solution turned dark orange. The flask was
cooled to room temperature and the solution volume was reduced to dryness.
Subsequently, the orange oil was dissolved in dichloromethane, washed with water and
dried over MgSO . The product was precipitated in ice-cold hexane and stored at –22
4
°
C. The precipitate was collected by decantation and used without further purification.
1
Yield 0.59 g (38%). H NMR (300 MHz, CDCl ): δ = 6.50 (s, 2H, CH), 5.28 (s, 2H,
3
1
3
CH), 3.80 – 3.4 (m, 8H, CH ), 2.86 (s, 2H, CH), 2.70 (s, 1H, OH). C NMR (62.9
2
MHz, CDCl ): δ = 176.3 (CO-N), 136.4 (=CH-), 81.0 (>CH-), 72.3 (-CH -), 67.0 (-
3
2
CH -), 61.6 (-CH -), 47.4 (>CH-), 38.5 (-CH -). (C H NO ) (253.3) : Calcd. C
2
2
2
12 15
5 n
n
5
6.91, H 5.97, N 5.53; Found C 56.83, H 6.31, N 5.52.
Synthesis of 6-(1,3-dioxo-1,3,3a,4,7,7a-hexahydro-2H-4,7-epoxyisoindol-2-yl)hexyl
methacrylate (MIMA 2)
2
-(6-Hydroxyhexyl)-3a,4,7,7a-tetrahydro-1H-4,7-epoxyisoindole-1,3(2H)-dione
(0.925 g, 3.6 mmol) was added to a two-neck round bottom flask containing a magnetic
stirring bar. After purging the flask with nitrogen, dry dichloromethane (20 mL) and
triethylamine (0.8 mL, 5.7 mmol) were added and the reaction solution was cooled to
0
°C with an ice bath. To this solution methacryloyl chloride (0.4 mL, 4.1 mmol) was
added dropwise. The reaction was stirred overnight, slowly warming to room
temperature. Subsequently, the reaction mixture was quenched with methanol and
8

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afterwards extracted with aqueous NH Cl, H O and brine. The organic layer was dried
4
2
over MgSO , filtered and reduced in vacuum. The product was further purified via
4
1
column chromatography (silica, ethyl acetate as eluent). Yield 0.667 g (55%). H NMR
(
300 MHz, CDCl ): δ = 6.51 (s, 2H, CH), 6.09 (s, 1H, CH), 5.55 (s, 1H, CH), 5.26 (s,
3
2
1
H, CH), 4.11 (t, J = 6.6 Hz, 2H, CH ), 3.46 (t, J = 7.3 Hz, 2H, CH ), 2.84 (s, 2H, CH),
2 2
13
.93 (s, 3H, CH ), 1.8 – 1.5 (m, 4H, CH ), 1.5 – 1.2 (m, 4H, CH ). C NMR (62.9
3
2
2
MHz, CDCl ): δ = 176.3 (-CO-N), 167.5 (>CO), 136.5 (=CH-), 136.5 (>C=), 125.2
3
(
>C=), 80.9 (>CH-), 64.5 (-CH -), 47.4 (>CH-), 38.8 (-CH -), 28.4 (-CH -), 27.4 (-
2 2 2
CH -), 26.2 (-CH -), 25.5 (-CH -), 18.3 (-CH ). (C H NO ) (333.2) : Calcd. C 64.85,
2
2
2
3
18 23
5 n
n
H 6.95, N 4.20; Found C 64.77, H 7.08, N 3.97.
Synthesis
yl)ethoxy)ethyl methacrylate (MIMA 3)
-(2-(2-Hydroxyethoxy)ethyl)-3a,4,7,7a-tetrahydro-1H-4,7-epoxyisoindole-1,3(2H)-
of
2-(2-(1,3-dioxo-1,3,3a,4,7,7a-hexahydro-2H-4,7-epoxyisoindol-2-
2
dione (0.5 g, 2 mmol) was added into a two-neck round bottom flask containing a
magnetic stirring bar. After purging the flask with nitrogen, dry dichloromethane
(10 mL) and triethylamine (0.4 mL, 2.9 mmol) were added and the reaction solution
was cooled to 0 °C with an ice bath. To this solution methacryloyl chloride (0.3 mL, 3.1
mmol) was added dropwise. The reaction was stirred overnight, slowly warming to
room temperature. Subsequently, the reaction mixture was quenched with methanol and
afterwards extracted with aqueous NH Cl, H O and brine. The organic layer was dried
4
2
over MgSO , filtered and reduced in vacuum. The product was further purified via
4
1
column chromatography (silica, ethyl acetate as eluent). Yield 0.5 g (79%). H NMR
(
300 MHz, CDCl ): δ = 6.52 (s, 2H, CH), 6.13 (s, 1H, CH), 5.58 (s, 1H, CH), 5.27 (s,
3
2
H, CH), 4.25 (t, J = 4.8 Hz, 2H, CH ), 3.8 -3.6 (m, 6H, CH ), 2.87 (s, 2H, CH), 1.95
2 2
9

ACCEPTED MANUSCRIPT
1
3
(
s, 3H, CH ). C NMR (62.9 MHz, CDCl ): δ = 176.1 (-CO-N), 167.3 (>CO), 136.5
3 3
(
=CH-), 136.1 (>C=), 125.7 (>C=), 80.9 (>CH-), 68.6 (-CH -), 67.1 (-CH -), 63.8 (-
2
2
Na+
CH -), 47.4 (>CH-), 38.1 (-CH -), 18.3 (-CH ). HR-ESI-TOF MS: [C H NO ]
2
2
3
16 19
6
calcd.: m/z = 344.1105; found: m/z = 344.1102; error: 0.8 ppm.
General procedure for the synthesis of copolymers P(RMA-co-FMA-co-MIMA)
P1 to P7)
(
CuBr (2 eq.), HMTETA (2 eq.), RMA, FMA, the maleimide monomer (MIMA 1,
MIMA 2 or MIMA 3) and toluene or DMF (for P1, P2 and P4) (2 mL) were added to a
vial containing a stirring bar. The polymerization was initiated by adding EBiB (1 eq. or
2
eq.) and was carried out at 70 °C or room temperature (for P1, P2 and P4) under a
nitrogen atmosphere. The reaction was stopped after 2 to 4 h. The polymer was purified
by passing the solution through an alumina column to remove the catalyst, precipitation
in ice-cold diethylether or n-hexane and drying in an IR Dancer (Hettlab) at room
temperature. Also later purification by preparative size exclusion chromatography using
®
a Biobeads S-X1 column was performed for P4 to P7. Parts of the polymer samples
were analyzed by size exclusion chromatography (SEC) to determine the molar mass
and molar mass distribution.
3
.
Results and Discussion
3
.1
Synthesis of the Maleimide Methacrylates (MIMA 1 to MIMA 3)
The synthesis of the new maleimide methacrylate monomers (MIMA 1, MIMA 2 and
MIMA 3) is depicted in Scheme 1. The first step is a Diels-Alder reaction between
furan and maleimide as described by Kötteritzsch et al. [20]. Subsequently, compound 1
was reacted via an amine insertion with different amino alcohols. The last reaction step
1
0

ACCEPTED MANUSCRIPT
was the esterification of the compounds 2 to 4 with methacryloyl chloride to yield
MIMA 1, MIMA 2 and MIMA 3. The characterization of the monomers was performed
1
13
via H NMR and C NMR spectroscopy as well as by elemental analysis.
3
.2
Synthesis and Characterization of the Copolymers
The reversible crosslinkable terpolymers, bearing both furfuryl and maleimide
functionalities with different linkers, were prepared by a copolymerization of different
polar and nonpolar methacrylates (RMA), respectively, with furfuryl methacrylate
(FMA) and MIMA 1, MIMA 2 or MIMA 3 (Scheme 2). The atom transfer radical
polymerization (ATRP) procedure was applied using ethyl 2-bromoisobutyrate (EBiB)
as initiator, CuBr as catalyst and 1,1,4,7,10,10-hexamethyltriethylene-tetramine
(HMTETA) as ligand at a temperature of 70 °C in toluene as solvent or at room
temperature in DMF, respectively. The reaction conditions for the terpolymers with the
polar co-monomers (P1, P2 and P4) were varied due to the better solubility in polar
solvents [33,34]. But it was also possible to synthesize a terpolymer with DMEAMA
with toluene as solvent at a temperature of 70 °C (P6). A co-monomer to functional
monomer ratio of 8:1:1 was used, following our previous work as the best ratio to
obtain optimal self-healing properties [20]. Selected properties of the polymers with
different maleimide methacrylate monomers, polar and nonpolar co-monomers, the
reaction conditions and the calculations of the molar masses and molar mass
distributions, obtained from SEC measurements, are listed in Table 1.
1
H NMR spectroscopy was utilized for the structural characterization of the RMA-co-
1
FMA-co-MIMA 1, MIMA 2 or MIMA 3 copolymers. Figure 1 shows the H NMR
spectra of P6 and P7, respectively. The signals from the saturated protons (-CH3
and -CH ) of the methacrylate backbone are correlated to the signals 0.4 to 2.1 ppm.
2
1
1

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Additionally, the signals of the aromatic protons of the furfuryl groups are assignable to
the signals 7.4 and 6.4 ppm. The signals corresponding to the protecting furan groups of
the maleimide functionalities are visible at 6.6 and 5.3 ppm, respectively. Additionally,
further signals at 3.0 ppm could also be assigned to the maleimide group. Moreover, the
signal at 3.9 ppm belongs to the methylene groups of the co-monomers. The signals “4”,
“6” and “9” have been utilized to determine the composition of the polymers. The
different monomer ratios of the copolymers and the theoretical crosslinking densities
are summarized in Table 2.
3
.3
Thermal Properties of the Copolymers
The potential of the self-healing abilities of the copolymers P1 to P7 was investigated
by DSC measurements. The presence of specific transitions during the heating and
cooling cycles represents an important precondition for the suitability of the polymers
of self-healing applications. In the first heating curve a large endothermic peak should
be visible, assignable to the energy which is required for the retro-Diels-Alder reaction
and the subsequent evaporation of the maleimide protecting group (i.e. furan). This
behavior could be observed by using a simultaneous thermal analysis (STA) of the
copolymer P7* of the previous work [20], were the mass loss of the furan is visible
between 120 to 160 °C and the corresponding MS spectra shows two additional peaks at
3
9 and 68 m/z at a temperature of 151 °C corresponding to the evaporating furan
molecules (Figure 2) [35]. The lower mass signals belong to the background
measurement.
Moreover, the glass transition temperatures (T ) of the protected and, therefore, non-
g
crosslinked copolymers should be observed. The T of the polymer will also have an
g
influence on the mechanical film properties of the resulting polymer film. In the non-
1
2

ACCEPTED MANUSCRIPT
crosslinked state and, therefore, also during the self-healing process, a lower T will
g
result in softer films compared to a high T monomer, which is beneficial for the self-
g
healing. However, the crosslinking will lead to harder polymer films. A co-monomer
with a low T still results in crosslinked films which feature mechanical properties
g.
comparable to non-crosslinked PMMA [20,21]. Most significant in the second heating
curve is the presence of the endothermic peak of the retro-Diels-Alder reaction, even
though the area below the curve will be smaller, because the energy of the evaporation
of the furan is missing and only the energy of the retro-Diels-Alder reaction is
observable. It was anticipated that the polar surrounding of the functional groups could
influence the TRDA. Nevertheless, the appearance of this peak in the second heating
curve represents also an important precondition for the self-healing properties of the
copolymers. These properties depend on the structural composition of the copolymer,
like the co-monomer and the linker of the maleimide. They influence the
flexibility/mobility of the chain and the glass transition temperature. The summary of
the DSC results of the copolymers is listed in Table 3.
The DSC measurements of copolymer P1 reveal in the first heating curve a usual TRDA
between 120 to 180 °C, only in the second heating curve the TRDA already started at
1
00 °C. The T of copolymer P1 is slightly lower than in HEMA homopolymers due to
g
the flexible furan and the maleimide monomers [36]. For P2 no change of the TRDA
could be observed. Also in case of P3 and P4 with MIMA 2 no significant decrease of
the TRDA could be detected. Only the T of the DMAEMA copolymer is low comparable
g
to the BMA copolymer. This effect should be beneficial for the self-healing abilities of
the copolymer. The T of DMAEMA homopolymers is around 4 °C and for the BMA
g
homopolymers around 17 °C [37,38]. The same trend is visible for the materials P5 and
P6 with BMA and DMAEMA and the MIMA 3 with the oxygen-containing linker. Also
1
3

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in this case the TRDA is not significantly decreased (only 10 °C for the BMA copolymer
P5). The glass transition temperature is also lower for copolymer P5 compared to P3.
This behavior is caused by the oxygen-containing linker of the maleimide. Copolymer
P3 revealed the same glass transition temperature as copolymer P5*, in spite of the
longer linker; only copolymer P5 with the oxygen-containing linker showed a lower Tg.
However, the temperature of the retro-Diels-Alder reaction for copolymers P3 to P6 is
slightly decreased compared to copolymer P5*. Copolymer P7 with the DEGMA as co-
monomer has the lowest glass transition temperature of –10 °C, but also in that case the
TRDA could not be decreased significantly (only 5 °C lower than the usually observed
temperature between 120 to 180 °C). An interesting finding is the presence of a glass
transition temperature in the second heating curve for copolymers P5 to P7. In the case
of copolymer P5 and P6 the T in the second heating curve is higher due to the
g
crosslinking; but this is not the case for copolymer P5. Nevertheless, the glass transition
temperatures of the copolymers P1 to P6 are not lower as the T of P7*, which revealed
g
the best self-healing properties in the earlier published work [20]; only copolymer P7
has a comparable glass transition temperature. Also the TRDA of the new copolymers are
comparable to the TRDA of copolymer P7*, only P3, P5 and P7 showed a slightly
decreased TRDA in the second heating curve. The DSC traces for copolymer P5 are also
depicted in Figure 3. In the second heating curve there are also two peaks visible for the
retro-Diels-Alder reaction, because the retro-Diels-Alder reaction for the endo-product
occurs at lower temperatures than for the exo-product, due to the fact that the endo-
product is the kinetically and the exo-product the thermodynamically preferred state
[
39].
In the TGA measurements of the copolymers the mass loss of the maleimide protecting
1
group (furan) is observable. By using H NMR spectroscopy the theoretical mass loss
1
4

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was estimated from the calculated monomer ratios. In Table 3 the comparison of the
temperature of the evaporation with the theoretical and the measured mass loss are
listed. The temperatures of the mass loss are in the expected range between 120 to
1
80 °C. Only for copolymer P1 the deprotection occured at lower temperatures. The
theoretical and the calculated mass losses correlate very well, only copolymer P1
revealed larger differences, which are presumably due to degradation processes, which
take place at higher temperatures.
3
.4
Self-healing studies of the Copolymers
Due to the used co-monomers resulting in very high T and T
values no self-healing
RDA
g
studies were performed with P1 and P2. The study of the self-healing properties of
copolymers P3 to P7 was performed by using a microscope with a camera for
visualization, because with this setup also real-time self-healing measurements of large
scratches could be visualized. Copolymers P3 and P5 with the BMA co-monomer and
the MIMA 2 and 3 with the longer linker showed good self-healing properties. The
surface is not as smooth as obtained in the previously published work with the LMA co-
monomer [20], but an acceptable film quality could be obtained. The self-healing
studies of copolymer P3 is depicted in Figure 4. The scratch in a millimeter range could
be healed completely at 140 °C after 3 min and also at 110 °C after 3 h the scratch was
nearly healed. Copolymer P3 shows improved self-healing properties compared to
copolymer P5*, because in the previous case only scratches in the nanometer range
could be healed using higher temperatures of 170 °C after 2 h [20]. The materials have
also slightly better self-healing properties compared to copolymer P7* (20 °C lower
healing temperature). Copolymer P4 showed no self-healing behavior in the
investigated temperature range because the copolymer was too stiff with many defects
1
5

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of the film due to the furan evaporation after crosslinking and, therefore, no reflow of
the damaged parts was possible, despite of the low glass transition temperature.
The self-healing studies of copolymer P5 is depicted in Figure 5. The scratch could be
healed completely at 140 °C after 5 min and also at 80 °C after 48 h the scratch was
nearly gone. Copolymer P6 showed also self-healing behavior at temperatures above
1
20 °C in a timescale of 5 min, but the surface of the coating was not as smooth as the
surface of P5. Copolymers P5 and P6 revealed also better self-healing properties
compared to copolymer P5* with regard to the size of the healed scratch and the lower
healing temperatures, also in comparison to copolymer P7* in respect to the lower self-
healing temperature.
The self-healing studies of copolymer P7 is depicted in Figure 5. Compared to the
previous copolymer P7 was annealed after deprotection of the maleimde group
overnight at 40 °C to improve the crosslinking density and for hardening of the film.
The longer hardening times for oxygen-containing system were also observed by Mayo
et al. [32]. They pointed out that their oxygen-containing linker requires more time for
the subsequent crosslinking compared to the alkyl, cyclic as well as aryl linker and also
reveals excellent film-forming characteristics. The surface of the copolymer P7 is also
as smooth as obtained by copolymer P7*. As showed in Figure 6 the smaller scratch
could be healed completely at 100 °C after 2 h and the larger ones after 4 h. Also at
lower temperatures of 80 °C the scratch could be healed after 24 h partially. The
copolymer P7 has the best film-forming and self-healing properties of all synthesized
polymers. Additionally, P7 features also a lower healing temperature than copolymer
P7* due to the higher flexibility of the linker unit in combination with the DEGMA.
1
6

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4
.
Conclusion
The synthesis of new terpolymers with MIMA 1, MIMA 2 and MIMA 3 with three
different linkers and FMA as functional units for Diels-Alder reactions and different
polar and nonpolar co-monomers could be accomplished. As polar co-monomers
hydroxyethyl methacrylate (HEMA), dimethylaminoethyl methacrylate (DMAEMA)
and 2-(hydroxyethoxy)ethyl methacrylate (DEGMA) and as nonpolar co-monomers
butyl methacrylate (BMA) were utilized. The polymers were prepared via ATRP with
CuBr as catalyst, HMTETA as ligand and EBiB as initiator at 70 °C in toluene or at
1
room temperature in DMF. Furthermore, the characterization was carried out using H
NMR spectroscopy, SEC, TGA and DSC measurements. The self-healing properties of
the copolymers were studied using a microscope with a camera for real-time analyses.
Copolymers P3 and P5 with BMA and the MIMA 2 and 3 with the longer alkyl chain
and the oxygen-containing linker revealed superior self-healing behavior compared to
the previously described “all-in-one” system P7*. These materials were able to
completely heal scratches on the millimeter range at 140 °C. Copolymer P7 with
DEGMA and the oxygen-containing linker reveals the best self-healing performance of
all synthesized polymers. A low healing temperature of 100 °C could be achieved by
the introduction of the linker unit featuring a higher flexibility in combination with
DEGMA. In contrast to other examples described, polar co-monomers did not
significantly influence the temperature of the retro-Diels-Alder reaction in these
polymeric systems. Nevertheless, the required healing temperature could be decreased
significantly compared to copolymer P7*. In summary, an alternative for the one-
component self-healing system of the previously published work could be developed
with shorter side chains (BMA) and a polar side chain (DEGMA) combined with a
longer as well as an oxygen-containing linker for the MIMA. In contrast to the rather
1
7

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small influence of the nature of the co-monomer (as long as it features a low T ) the
g
nature of the linker unit revealed a significant influence on the resulting properties
enabling an enhancement of the self-healing properties.
Acknowledgements:
This
study
was
supported
by
the
Deutsche
Forschungsgemeinschaft (SPP 1568).
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Scheme and Figure Captions:
Scheme 1: Schematic representation of the synthesis of the maleimide methacrylate
monomers (MIMA 1, MIMA 2 and MIMA 3).
Scheme 2: Schematic representation of the copolymerization of different polar and
nonpolar methacrylates (RMA), FMA and MIMA 1, MIMA 2 and MIMA 3 by ATRP.
1
Figure 1: H NMR spectra of P6 (left) and P7 (right) (in CDCl ).
3
Figure 2: STA measurement of copolymer P7*: TGA (left), MS spectra at 29 and
1
51 °C (right).
Figure 3: DSC analysis of copolymer P5.
Figure 4: Self-healing experiment of copolymer P3 using a microscope for
visualization. a) to c) Healing experiments at 140 °C, a) scratch before annealing, b)
scratch after annealing for 1 min and c) scratch after annealing for 3 min and d) to f)
healing experiments at 110 °C, d) scratch before annealing, e) scratch after annealing
for 30 min and f) scratch after annealing for 3 h.
Figure 5: Self-healing experiment of copolymer P5 using a microscope for
visualization. a) to c) Healing experiments at 140 °C, a) scratch before annealing, b)
scratch after annealing for 2 min and c) scratch after annealing for 5 min and d) to f)
healing experiments at 80 °C, d) scratch before annealing, e) scratch after annealing for
2
h and f) scratch after annealing for 48 h.
Figure 6: Self-healing experiment of copolymer P7 using a microscope for
visualization. a) to d) Healing experiments at 100 °C, a) scratch before annealing, b)
scratch after annealing for 1 h, c) scratch after annealing for 2 h, d) scratch after
annealing for 4 h and e) to f) healing experiments at 80 °C, e) scratch before annealing,
2
1

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f) scratch after annealing for 24 h. Pretreatment: Annealing at 130 °C for 3 h and at
4
0 °C overnight.
2
2

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Tables:
Table 1: Comparison of the different maleimide methacrylates, the co-monomers and
the reaction conditions of the copolymers, the theoretical molar mass and the molar
mass and molar mass distributions of the copolymers measured by SEC.
a)
Polymer MIMA
Co-monomer Reaction
conditions
Mn
M (SEC)
PDI
n
(theo)
[g/mol]
12,090
9,860
4,910
7,490
4,330
4,440
3,100
(SEC)
[
g/mol]
P1
P2
P3
P4
P5
P6
P7
MIMA 1
MIMA 1
MIMA 2
MIMA 2
MIMA 3
MIMA 3
MIMA 3
HEMA
DMAEMA
BMA
DMF,
r. t.
3,710
1.48
1.69
1.33
1.69
1.36
1.33
1.41
DMF,
r. t.
8,510
8,190
8,790
4,060
4,360
4,980
toluene,
7
0 °C
DMAEMA
BMA
DMF,
r. t.
toluene,
7
0 °C
toluene,
0 °C
toluene,
0 °C
SEC conditions: DMAc with 2.1 g/L LiCl as solvent, PMMA as standard for the
calibration)
DMAEMA
DEGMA
7
7
a)
(
2
3

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Table 2: Comparison of the theoretical and calculated ratios of the co-monomers and
the theoretical crosslinking density.
a)
a)
Polymer
Ratio of the co-
monomers
Theoretical
Calculated
Crosslinking
b)
densitytheo
[%]
P1
P2
P3
P4
P5
P6
P7
HEMA:FMA
HEMA:MIMA 1
DMAEMA:FMA
DMAEMA:MIMA 1
BMA:FMA
8:1
8:1
8:1
8:1
8:1
8:1
8:1
8:1
8:1
8:1
8:1
8:1
8:1
8:1
7:1
5:1
6:1
7:1
6:1
7:1
5:1
11:1
8:1
9:1
7:1
9:1
6:1
14:1
23
26
30
15
25
22
11
BMA:MIMA 2
DMAEMA:FMA
DMAEMA:MIMA 2
BMA:FMA
BMA:MIMA 3
DMAEMA:FMA
DMAEMA:MIMA 3
DEGMA:FMA
DEGMA:MIMA 3
a)
b)
The theoretical and calculated molar ratios.
The crosslinking density is calculated by weight.
2
4

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Table 3: Comparison of the glass transition temperatures (T ) and the temperature of
g
the retro-Diels-Alder reaction (TRDA) of the copolymers measured by DSC as well as
the temperature of evaporation of the protecting group (Tevap) and the theoretical as well
as the calculated ratios of the co-monomers obtained by TGA.
Polymer
DSC measurements
TGA measurements
Heating
curve
Tg [°C]
TRDA [°C] Tevap [°C]
mass
mass
losstheo lossexp
[%]
[%]
st
P1
P2
P3
P4
P5
P6
P7
1
80
-
120 to 180 100 to 180
100 to 180
5
19
nd
2
st
1
50
-
125 to 170 130 to 160
120 to 170
7
5
5
4
4
4
5
4
6
6
4
4
4
3
6
4
nd
2
st
1
50
-
125 to 180 120 to 180
115 to 180
nd
2
st
1
25
-
125 to 180 120 to 160
120 to 180
nd
2
st
1
40
60
35
40
-10
-10
50
-
120 to 180 135 to 170
110 to 180
nd
2
st
1
120 to 180 130 to 180
120 to 180
nd
2
st
1
120 to 180 125 to 175
115 to 160
nd
2
a)
st
P5*
1
140 to 180 140 to 180
140 to 180
nd
2
a)
st
P7*
1
-16
-
120 to 180 132 to 163
nd
2
120 to 190
a)
*
Polymers of the previous publication [20]; P5 represents P(BMA-co-FMA-co-MIMA
) and P7* represents P(LMA-co-FMA-co-MIMA 1) with 20 mol-% functionality.
1
2
5

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