Photochemical crack healing in cinnamate-based polymers

Article abstract of DOI:10.1166/jnn.2010.2953

Four kinds of cinnamate-type monomers were synthesized as healing agents. Photoirradiation of the monomers gave cyclobutane-containing crosslinked polymers via [2+2] cycloaddition. Cyclobutane cleavage upon cracking of the crosslinked polymers and re-cycl

Full text of DOI:10.1166/jnn.2010.2953

Copyright © 2010 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 10, 6972–6976, 2010
Photochemical Crack Healing in
Cinnamate-Based Polymers
Sung-Youl Cho1, Joong-Gon Kim2, and Chan-Moon Chung1ꢀ∗
1Department of Chemistry, Yonsei University, Wonju, Kangwon-do 220-710, Korea
2Biotechnology Division, Hanwha Chemical R&D Center, Taejon 305-345, Korea
Four kinds of cinnamate-type monomers were synthesized as healing agents. Photoirradiation of the
monomers gave cyclobutane-containing crosslinked polymers via [2+2] cycloaddition. Cyclobutane
cleavage upon cracking of the crosslinked polymers and re-cycloaddition of the cracked polymers
were investigated by FT-IR spectroscopy. Photochemical crack healing was demonstrated by mea-
surement of flexural strength of crosslinked, cracked, and healed polymers. It was observed that
microcracks with width of 200 nm to 2 ꢀm were healed by photoirradiation.
Keywords: Photocycloaddition, Crack Healing, Cinnamate, Cyclobutane.
1. INTRODUCTION
it readily occurs in the solid state. As part of the back-
bone or pendant groups in a polymer, cinnamate structure
has been used to obtain cyclobutane-type crosslinked poly-
mers via its [2 + 2] photocycloaddition. Poly(vinyl cin-
namate) is one of the best known examples that undergo
crosslinking through the photocycloaddition in the film
state.13
One of the major concerns in the development of
our healing system was that the cyclobutane-containing
crosslinks should reverse to the original cinnamate struc-
ture upon crack formation and propagation so that crack
healing can be accomplished by the re-cycloaddition of
cinnamoyl groups as illustrated in Scheme 1. First, it is
necessary that the cleavage of the cyclobutane structure
should be preferred over nonreversible breaking of the
other bonds when cracks form and propagate. The exclu-
sive cleavage of cyclobutane was expected based on the
fact that the bond energy of cyclobutane C–C is known
to be lower than that of the other bonds such as C–O
and the other C–Cs because of the high ring strain of
cyclobutane.14ꢀ15 Second, it is required that mechanical
cleavage of cyclobutane should produce original cinnamoyl
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Polymeric materials are used in a variety of applications,
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12
but they are susceptible to damage induced by various
physical and chemical factors. This results in microc-
rack formation and propagation, ultimately leading to sig-
nificant reduction in mechanical performance and other
desired properties of the materials.1ꢀ2
Copyright: American Scientific Publishers
Recently, much attention has been paid to crack heal-
ing in polymeric matrixes.3ꢀ4 Crack healing in thermoplas-
tics resulted from the interdiffusion of polymer chains and
formation of entanglements by heating a cracked sample
above its glass transition temperature (Tgꢁ.5 Crack healing
induced by small molecules such as methanol and ethanol
has also been reported for thermoplastics.6ꢀ7 A self-repair
system was developed by incorporating hollow fibers into
a polymer composite, so that the fibers can be broken
when cracks propagate, releasing repair chemicals.2 Auto-
nomic healing was accomplished by incorporating micro-
capsules or microvascular networks from which a healing
monomer is released upon crack intrusion and polymer-
izes by contact with an embedded catalyst within a poly-
mer matrix, bonding the crack faces.8ꢀ9 A re-mendable
polymeric material employed a thermally reversible Diels-
Alder (DA) reaction.10 We reported the first example of
photochemical crack healing in a polymeric material.11
Photochemical [2 + 2] cycloaddition of cinnamoyl
groups was chosen as a healing reaction in our work since
groups. The reversion of cyclobutane to original C
C
bonds is known to occur photochemically or thermally.16
The object of this work is to develop a photochemi-
cally mendable polymeric system in which cracks can be
readily healed by photoirradiation. Cinnamate-type heal-
ing monomers were synthesized, and the applicability of
their photocycloaddition to crack healing was investigated.
Preliminary evaluation of crack-healing ability of the cin-
namates was carried out by flexural strength testing.
∗Author to whom correspondence should be addressed.
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doi:10.1166/jnn.2010.2953

Cho et al.
Photochemical Crack Healing in Cinnamate-Based Polymers
speed of 0.75 mm/min according to ISO 4049. A micro-
scope BX-51 (Olympus) was used to take pictures of
cracked surfaces.
O
O
O
O
O
O
2.2. Synthesis of 1,1,1-Tris(cinnamoyloxymethyl)
Ethane (TCE)
O
O
O
1,1,1-Tris(hydroxymethyl)ethane (0.60 g, 5.0 mmol) and
4-(dimethylamino)pyridine (2.20 g, 18 mmol) were dis-
solved in 50 mL of tetrahydrofuran (THF). To the solu-
tion was added a solution of cinnamoyl chloride (3.00 g,
18 mmol) in THF (15 mL). The resultant solution was
refluxed for 5 h. Purification by column chromatography
afforded TCE as a viscous oil in a yield of 70%. 1H
NMR (CDCl3ꢁ: ꢂ 1.17 (s, 3H, CH3ꢁ, 4.29 (s, 6H, OCH2ꢁ,
6.45 (d, 3H, J = 16 Hz, phCH), 7.33–7.51 (m, 15H,
phenyl), 7.70 (d, 3H, J = 16 Hz, phCH CH). 13C NMR
(CDCl3ꢁ: ꢂ 17.57 (CH3ꢁ, 39.05 (CCH3ꢁ, 66.46 (OCH2ꢁ,
117.76, 128.32, 129.04, 134.41 (phenyl), 130.57 (phCH),
145.53 (phCH CH), 166.79 (carbonyl). Elementary anal-
ysis: calculated for C32H30O6, C 75.28, H 5.92, O 18.80;
found C 74.83, H 6.01.
O
O
O
O
O
O
O
Crack
formation and
propagation
hν
Healing
(>300 nm)
O
O
O
O
O
O
O
O
2.3. Synthesis of 1,1,1-Tris(cinnamoyloxymethyl)
Propane (TCP)
O
O
O
O
O
O
To a solution of 1,1,1-tris(hydroxymethyl)propane (0.67 g,
5.0 mmol) in 50 mL of THF was added 4-
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O
(dimethylamino)-pyridine (2.38 g, 20 mmol), and the
Copyright: American Scientific Publishers
O
resultant solution was stirred for 10 min. To the solu-
tion was added a solution of cinnamoyl chloride (3.25 g,
20 mmol) in THF (15 mL). The resultant solution was
refluxed for 24 h. After workup with a saturated aque-
ous NaHCO3 and then a saturated brine, purification
by column chromatography (ethyl acetate: n-hexane =
1:6 in volume) afforded TCP as a pale yellow gum
in a yield of 81% (2.12 g). 1H NMR (CDCl3) ꢂ 1.00
(t, 3H, J = 7ꢃ4 Hz, CH3ꢁ, 1.66 (q, 2H, J = 7ꢃ4 Hz,
CCH2C), 4.32 (s, 6H, OCH2ꢁ, 6.45 (d, 3H, J = 15ꢃ9 Hz,
phCH ), 7.33–7.52 (m, 15H, phenyl), 7.70 (d, 3H, J =
15ꢃ9 Hz, CHC O). 13C NMR (CDCl3) ꢂ 7.61 (CH3ꢁ,
23.56 (CCH2CH3ꢁ, 41.17 (CCH2CH3ꢁ, 64.60 (OCH2ꢁ,
117.69, 128.20, 128.91, 134.27 (phenyl), 130.43 (phCH),
145.35 (phCH CH), 166.65 (carbonyl). Elemental analy-
sis: Calcd. for C33H32O6: C 75.55, H 6.15, O 18.30; Found:
C 75.23, H 6.24.
Scheme 1. Schematic illustration of the healing concept in this study.
2. EXPERIMENTAL DETAILS
2.1. General Methods
Cinnamoyl chloride, 1,1,1-tris(hydroxymethyl)ethane, 1,1,
1-tris(hydroxymethyl)propane, 2,2-bis(hydroxymethyl)-1,
3-propanediol, 2-hydroxyethyl methacrylate, 1,6-bis(2ꢀ-
methacryloyloxyethoxycarbonylamino)-2, 4, 4-trimethyl-
hexane (UDMA), 2-(dimethylamino)ethyl methacrylate
(DMAEMA), and camphorquinone (CQ) were purchased
from Aldrich Chemical Co., and used without purification.
1H and 13C-NMR spectra were taken on a Varian
Gemini spectrometer (300 MHz) in deuteriochloroform
using tetramethylsilane as an internal standard. Infrared
(IR) spectra of the samples were recorded on a Genisis
Fourier transform infrared (FT-IR) spectrometer (Mattson
Instrument Co.). Elemental analysis was done with an
EA 1108 CHNS-O (Fisons Instrument). UV-vis spec-
troscopy was conducted using a Hewlett-Packard 8453-A.
A Sciencetech 500-W high pressure mercury lamp (light
intensity: 72.4 mW/cm2ꢁ was used for photoirradiation in
conjunction with a UV cut off filter (<300 nm, Optosigma
Co). 3-Point flexural test was performed with a universal
testing machine, DTU-900MHA (DT&T), at a cross-head
2.4. Synthesis of Tetra(cinnamoyloxymethyl)
Methane (TECE)
2,2-Bis(hydroxymethyl)-1,3-propanediol
(3.40
g,
25 mmol) reacted with cinnamoyl chloride (21.66 g,
0.13 mol) in THF (370 mL) at 90 ꢁC for 48 h. After
workup with a saturated NaHCO3, a 5% HCl solution, and
a saturated brine, purification by column chromatography
(ethyl acetate: n-hexane = 1:6 in volume) afforded TECE
J. Nanosci. Nanotechnol. 10, 6972–6976, 2010
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Photochemical Crack Healing in Cinnamate-Based Polymers
Cho et al.
as pale yellow powders in a yield of 78% (12.80 g).
1H-NMR (CDCl3ꢁ: ꢂ 4.31 (s, 8H, OCH2ꢁ, 6.47 (d, 4H,
J = 16 Hz, phCH), 7.23–7.52 (m, 20H, phenyl), 7.72 (d,
4H, J = 16 Hz, phCH CH). 13C-NMR (CDCl3ꢁ: ꢂ 40.07
(CCH2ꢁ, 65.21 (OCH2ꢁ, 118.21, 127.29, 129.10, 135.31
(phenyl), 130.59 (phC C), 145.75 (phC C), 166.63
(carbonyl). Elemental analysis: Calcd. For C41H36O8: C
74.98, H 5.53, O 19.49; Found: C 75.23, H 5.41.
O
O
3
3
O
O
TCP
TCE
O
O
O
4
O
O
O
2.5. Synthesis of 2-Methacryloyloxyethyl
Cinnamate (MCE)
TECE
Fig. 1. Structures of the cinnamate monomers.
MCE
2-Hydroxylethyl methacrylate (2.60 g, 20 mmol) reacted
with cinnamoyl chloride (3.33 g, 20 mmol) in the presence
of triethylamine (2.23 g, 22 mmol) in dichloromethane
(10 mL) at room temperature for 3 h. After workup
with water and an aqueous NaOH, MCE was obtained
as a pale yellow oil in a yield of 93% (4.60 g). 1H-
NMR (CDCl3ꢁ: ꢂ 1.95 (s, 3H, CH3ꢁ, 4.44 (s, 4H,
CH2CH2ꢁ, 5.51 (s, 1H, CCH2ꢁ, 6.14 (s, 1H, CCH2ꢁ,
6.45 (d, 1H, J = 16 Hz, phCH CH), 7.30–7.57 (m,
5H, phenyl), 7.71 (d, 1H, J = 16 Hz, phCH CH).
13C-NMR (CDCl3ꢁ: ꢂ 21.8 (CH3ꢁ, 64.6 (CH2CH2ꢁ, 118.96
(phC Cꢁ, 128.92, 129.09 (phenyl), 134.11 (phenyl and
CH3C CH2ꢁ, 132.21 (CH3C CH2ꢁ, 147.21 (phC C),
164.53 (carbonyl in cinnamoyl), 165.12 (carbonyl in
methacryloyl). Elemental analysis: Calcd. For C15H16O4:
chloride with 2,2-bis(hydroxymethyl)-1,3-propanediol.
MCE was prepared from cinnamoyl chloride and 2-
hydroxyethyl methacrylate. The chemical structures of the
1
monomers were confirmed by H NMR, 13C NMR, and
elemental analysis.
The cinnamates showed optical absorption peaks around
280 nm, and photoirradiation was carried out through a
filter that cuts off UV below 300 nm since the UV of
ꢄ < 300 nm would induce cleavage of cyclobutane ring.
As exposure time was increased, absorbance of the cin-
namates decreased at 280 nm (Fig. 2). This implied that
cinnamoyl C C bonds underwent [2+2] cycloaddition to
form cyclobutane crosslinks.18 It was confirmed by FT-IR
spectroscopy that radical polymerization of cinnamates did
not occur under those conditions.19 The photoreaction was
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C 69.22, H 6.19, O 24.59; Found: C 68.86, H 6.35.
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Copyright: American Scientific Publishers
almost completed within 30 min and gave hard, transpar-
ent films. The crosslinked films were insoluble in common
organic solvents such as chloroform and acetone while the
original monomers were quite soluble in those solvents.
The photocrosslinking of the cinnamates was also veri-
fied by FT-IR spectroscopy. As shown in Figure 3(a), TCP
(coated on a KBr disk) showed absorption bands of cin-
namoyl C O and C C at 1713 and 1637 cm−1, respec-
tively. Upon irradiation with UV (>300 nm), the carbonyl
absorption shifted to 1734 cm−1 and C C absorption
almost disappeared (Fig. 3(b)), indicating the photocy-
cloaddition of cinnamoyl groups. The photocycloaddition
conversion of the cinnamates was measured based on the
ratios of the areas of the two absorption bands (C C and
2.6. Crack-Healing Experiment
Photocurable pastes were formulated using a mass ratio
of a cinnamate:UDMA:DMAEMA:CQ of 40:57:2:1. The
mixtures were stirred for 1 h, and placed under vacuum
to remove interior bubbles. Each solution was inserted
into a mold made of stainless steel and Teflon. The open
end of the mold was covered with a polyethylene film
and the solution was photoirradiated for 20 h. The pho-
tocured samples had an average dimension of 25 × 2 × 2
mm. About twenty specimens were prepared for each for-
mulation. Cracks were generated with a universal testing
machine by pressing the center part of a specimen to a
given extent. A part of the cracked samples were pho-
toirradiated to induce healing reaction. Flexural strength
was measured for crosslinked, cracked, and healed samples
using the universal testing machine. About five specimens
were used to obtain an average strength value.
3. RESULTS AND DISCUSSION
Chemical structures of the cinnamate monomers
used in this work are shown in Figure 1. TCE and
TCP were prepared by reacting cinnamoyl chloride
with 1,1,1- tris(hydroxymethyl)-ethane and 1,1,1-
tris(hydroxymethyl)propane, respectively.11ꢀ17 Tetrafunc-
tional TECE was synthesized by reaction of cinnamoyl
Fig. 2. UV spectral change of a TCE film upon photoirradiation for
0, 20, 40, 60, 80, 100, and 120 s.
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Cho et al.
Photochemical Crack Healing in Cinnamate-Based Polymers
O
H
C
H
C
(a)
O
O
1713
O
H
C
H
C
(b)
(c)
1637
1734
hν
(d)
O
O
O
O
O
O
c
a
d
b
Fig. 3. IR spectra of (a) TCP, (b) a TCP polymer obtained by irradi-
ation, (c) a ground TCP polymer, and (d) a ground TCP polymer after
re-irradiation.
O
O
(head-to-tail)
a, b
(head-to-head)
c
C
O as an internal standard) before and after exposure.11
d
Cleavage
The conversion of TCE and TCP reached almost 100%,
but TECE and MCE showed much lower conversion values
(62 and 52%, respectively). Because TECE is somewhat
powdery in room temperature, TECE molecules inside a
particle or at bottom of a film might have not been exposed
enough to light because of light scattering. In the case of
O
O
H
C
H
C
O
O
O
CH
CH
CH
O
MCE, both the methacryloyl C C and cinnamoyl C
C
CH
H
C
H
C
bonds showed absorption at 1637 cm−1. The low con-
O
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version (52%) of MCE is due to the methacryloyl group
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that is unreactive under the experimental conditions.
O
Copyright: American Scientific Publishers
Structural change of the cinnamate polymers upon
Stilbene
cracking was studied by FT-IR spectroscopy. A TCP
polymer was ground into fine particles to create ‘a
large amount of cracks’ for easy detection of structural
change. A KBr pellet containing the powders showed re-
appearance of the cinnamoyl C C band at 1637 cm−1
(Fig. 3(c)). This indicated that the cyclobutane crosslinks
were cleaved to reverse to the original cinnamate moieties.
As illustrated in Scheme 2, the [2 + 2] cycloaddition
of cinnamoyl groups might give two types of cyclobu-
tane product, head-to-tail and head-to-head adducts.20ꢀ21
Cleavage of the head-to-tail adduct would generate two
cinnamate groups (routes a and b). The head-to-head type,
on the other hand, would be cleaved to form either two
cinnamate groups (route d) or unsaturated ester and stil-
bene (route c).21 The IR spectrum of Figure 3(c) showed
no band for unsaturated ester C C (1644 cm−1ꢁ or stil-
bene (1600 cm−1ꢁ, implying that route c does not occur
Scheme 2. Formation and cleavage of cyclobutane.
mostly occurred between adjacent cinnamoyl groups on
the surface of each particle. This result, however, indicated
that re-cycloaddition can occur between cinnamoyl groups
formed by cyclobutane cleavage.
Preliminary evaluation of the crack-healing ability of the
cinnamates was conducted. Test specimens were prepared
by adding a dimethacrylate monomer, UDMA. In the pres-
ence of a radical photoinitiator system (CQ+DMAEMA),
UDMA undergoes crosslinking; thus, it has been used as
a photocurable monomer, especially in restorative den-
tal composites.22 A photocurable paste was prepared by
mixing a cinnamate, UDMA, DMAEMA and CQ. Upon
photoirradiation (ꢄ > 300 nm), visible light-induced rad-
ical polymerization of the methacrylate and UV-induced
cycloaddition of a cinnamate occurred simultaneously, and
very hard, transparent specimens were obtained. Microc-
racks were generated in each specimen using a universal
testing machine. Cracked specimens were photoirradiated
to induce re-cycloaddition of cinnamoyl groups. It was
observed that microcracks with width of 200 nm to 2 ꢅm
(Fig. 4, left) were healed by photoirradiation for several
minutes (Fig. 4, right). Flexural strength was measured for
original, cracked, and healed specimens and the results are
1
upon crack propagation. This was also supported by H
NMR spectroscopy and thin-layer chromatography (TLC):
the ground polymer particles were extracted with CDCl3,
and stilbene was not detected from the extract.
When the ground sample was re-irradiated, the C
C
absorption completely disappeared (Fig. 3(d)), implying
that [2+2] cycloaddition between cinnamoyl C C bonds
occurred again. Since the polymer particles were dispersed
inside a KBr pellet, the cycloaddition reaction might have
J. Nanosci. Nanotechnol. 10, 6972–6976, 2010
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Photochemical Crack Healing in Cinnamate-Based Polymers
Cho et al.
re-photocycloaddition of resulting cinnamoyl groups were
confirmed by FT-IR spectroscopy. The crack healing using
the cinnamates was successfully demonstrated by flexural
strength testing and optical microscopy. The trifunctional
TCE and TCP showed greater crack-healing ability than
TECE and MCE. The photochemical healing proceeded
very fast, and does not require any catalyst, additive,
or severe heat treatment. Our results could give material
chemists a valuable insight into the development of effi-
cient polymeric healing systems.
Fig. 4. Microscopic images of a TCP-based sample after crack forma-
tion (left) and after healing by re-irradiation (right).
Table I. Flexural strength and healing efficiency of cinnamate-based
polymer specimens.
References and Notes
Flexural strength (MPa)
Healing
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Monomer
Original
Cracked
Healed
Efficiency (%)a
TCE
TCP
TECE
MCE
42.1
46.1
26.0
54.1
3.1
1.5
1.2
1.7
5.8
8.2
2.0
3.5
13.8
17.8
7.7
6.5
aHealing efficiency = healed/original.
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summarized in Table I. The flexural strength of the pho-
tocrosslinked specimens drastically decreased when cracks
were created. The cracked samples showed recovery of
flexural strength upon re-irradiation, while a control sam-
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based samples showed the lowest strength and a very low
healing efficiency. This might be due to a higher ten-
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because of larger number of cinnamoyl groups in TECE
molecule compared to the other cinnamates.20 The highest
flexural strength of MCE-based samples can be ascribed
to methacryloyl group in MCE capable of binding to the
polymer network through radical polymerization. But the
MCE-containing samples showed the lowest healing effi-
ciency probably due to more nonreversible breaking of
bonds upon cracking associated with the lowest concentra-
tion of cinnamoyl groups in the matrix, compared to the
other samples.
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4. CONCLUSION
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Novel cinnamate-type monomers, TCE, TCP, TECE, and
MCE, were synthesized and their photochemical crack-
healing ability was demonstrated. The crosslinking of the
monomers via [2 + 2] photocycloaddition, reversion of
cyclobutane crosslinks to original cinnamoyl groups, and
Received: 26 September 2008. Accepted: 11 June 2009.
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J. Nanosci. Nanotechnol. 10, 6972–6976, 2010