Synthesis and evaluation of a moisture-promoted healing copolymer

Article abstract of DOI:10.1016/j.polymer.2012.04.048

We report a homogeneous self-repairing copolymer system that exhibits self healing characteristics upon exposure to moisture environment. The copolymer consists of an isocyanate containing methacrylate monomer served as the repairing component and a fluorine-containing monomer as the protection component. Upon a mechanical damage of the coating copolymer, the isocyanate residue groups like the meshes of a zipper will exposure to generate reactive ends with environmental water like a faster of a zipper, thus form new crosslinking and repair the coating film. These materials with merits of easy-preparation and temperate repairing condition, can be used in decorative and anti-corrosion coatings.

Full text of DOI:10.1016/j.polymer.2012.04.048

Polymer 53 (2012) 2979e2990
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and evaluation of a moisture-promoted healing copolymer
*
Zhenhui Zhang, Yuling Hu, Zhengsheng Liu, Tianying Guo
Key Laboratory of Functional Polymer Materials (Nankai University), Ministry of Education, Institute of Polymer Chemistry, College of Chemistry, Nankai University, Weijin Road,
No. 94, Tianjin 300071, China
a r t i c l e i n f o
a b s t r a c t
Article history:
We report a homogeneous self-repairing copolymer system that exhibits self healing characteristics upon
exposure to moisture environment. The copolymer consists of an isocyanate containing methacrylate
monomer served as the repairing component and a fluorine-containing monomer as the protection
component. Upon a mechanical damage of the coating copolymer, the isocyanate residue groups like the
meshes of a zipper will exposure to generate reactive ends with environmental water like a faster of
a zipper, thus form new crosslinking and repair the coating film. These materials with merits of easy-
preparation and temperate repairing condition, can be used in decorative and anti-corrosion coatings.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 4 January 2012
Received in revised form
12 April 2012
Accepted 26 April 2012
Available online 3 May 2012
Keywords:
Self healing
Isocyanate containing polymer
Fluoropolymer
1. Introduction
precursor into the polymeric network, material could be well healed
by the reaction of the active function groups on the both sides of the
As we know, skin plays a role of protection for the body in the
biological systems, and it can also be self healed and recovered its
protection ability again when it suffers damage. Inspired by this
biological repairing system, coating material with self healing ability
may perfectly protect the substrates and itself simultaneously [1e5],
which attracted more and more interests of researchers [6,7].
As the mainstream of self healing systems, the extrinsic healing
system which embedded liquid healing agent into microcapsules
[8e23], hollow fibers [24,25], microvascular [26], core-sheath
structure [27] and so on [28e32] was intensively focused on all
the time. The embedded active healing agent would be released to
the crack once mechanical damage occurred [33]. Afterwards, the
healing agent can polymerize when contacting with the catalyst or
hardener, which either dispersed in the matrix or released from
other ruptured containers, resulting in rebinding of the crack faces.
This approach, despite the sophistication in chemistry and
manufacturing, has been quite successful in achieving high mending
efficiencies and is applicable to various types of polymeric materials.
Aside from the healing agent embedded system, the intrinsic self
healing system can be achieved by physical interaction [34],
reversible chemistry, which can recover itself by the formation of
new covalent bonding [35e38] or weaker interactions, such as
crack under the sun exposure after mechanical damage [49]. Because
there was no heterogeneous “containers” in the polymeric network,
the mechanical property could not be influenced; moreover, the
healing trigger was UV light existed in the open air, which provided
an ambient temperature approach to self healing.
For the metal anti-corrosion self healing coating materials the
material structure designs are mainly focus on the systems with
encapsulated reactive components in polymeric matrices. For
example, Garcia et al. [50] synthesized a water-reactive healing
agent based on a silyl ester and encapsulated it into an organic
coating. The silyl ester could locally released at the damage site,
wets both the metal and polymer surface, reacts with moisture
which protects the metal at the scribe from further corrosion
attack; Samadzadeh et al. [51] encapsulated Tung oil as an oxidative
healing agent the epoxy coating; and Huang et al. [52] reported
a facile microencapsulation preparation approach for liquid isocy-
anates hexamethylene diisocyanate monomer via an interfacial
polymerization method in oil-in-water emulsion, which makes the
instant healing possible.
Herein, we report a novel self healing coating system based on
only one designed copolymer, which can heal itself under moisture
environment. This copolymer consists of two components, isocya-
nate groups containing monomer (ITEGMA) and fluorinated
monomer, 2, 2, 3, 4, 4, 4-hexafluorobutyl methacrylate (HFBMA). In
this system, the fluorine-containing segments in HFBMA compo-
nent, due to their low surface free energy, are prone to gather on
the solideair interface, to form a good water barrier during the
hydrogen bonding [39e44],
pep interactions [45,46] polyelectrolyte
multi layers [47,48] and so on. For example, by incorporated an active
* Corresponding author. Tel./fax: þ86 22 23501597.
E-mail address: tyguo@nankai.edu.cn (T. Guo).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2012.04.048

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Z. Zhang et al. / Polymer 53 (2012) 2979e2990
coating preparation and application process [53e55]. And the
isocyanate containing segments in ITEGMA component, due to
their strong polarity, are prone to locate on the solidesubstrate
interface, to form a reparable layer. Once the coating is broken,
isocyanate groups exposed can react with H2O in air and form
a urea crosslink. The reactions are depicted in Equations (1) and (2),
making the moisture self healing process possible.
reactor. The mixture was pre-stirred for 5 min at 450 rpm. When
the temperature was raised to 65 ꢁC, 20 mmol TEG was added drop-
wise to the mixture within 30 min. After that, the reaction was
carried out at 65 ꢁC for 2 h. The final reaction mixture was cooled to
room temperature and poured in hexane-ethyl acetate 2:1 (v:v)
mixture to extract out the unreacted IPDI. The reaction is depicted
in Fig. 1, and the aim product was named as BITEG.
R ꢀ NCO þ H2O/R ꢀ NH ꢀ COOH/R ꢀ NH2 þ CO2
R ꢀ NH2 þ R0 ꢀ NCO/R ꢀ NH ꢀ CO ꢀ NH ꢀ R0
In this work, first, an isocyanate containing metharcylate
monomer ITEGMA was synthesized according to the scheme as
shown in Fig. 1. Then various compositions of poly (HFBMA-co-
ITEGMA) copolymers were synthesized, and then the characteris-
tics and the self healing property of the copolymers as metal
protection coating were investigated. Finally, we speculate as the
possible healing mechanism for this kind of self healing behavior.
(1)
2.3. Preparation of isocyanate containing monomer ITEGMA
(2)
7 g BITEG was dissolved in 10 mL anhydrous THF in a glass
reactor under nitrogen protection and stirred at 40 ꢁC. With a molar
ratio of NCO:OH ¼ 4:1, 0.6 mL HEMA was introduced drop-wise to
the solution of BITEG in THF within 30 min. The reaction was
further carried out for 4 h. The final reaction mixture was cooled to
room temperature and then poured in hexane-ethyl acetate 2:1
(v:v) mixture to extracted out the unreacted BITEG. The reaction is
depicted in Fig. 1. And the aim product was named as ITEGMA.
2.4. Preparation of poly (HFBMA-co-ITEGMA) copolymer
2. Experimental section
HFBMA and ITEGMA with molar ratio of 0:10, 2:8, 3:7, 5:5, 7:3,
10:0, respectively were dissolved in 15 mL dry THF under nitrogen
and then the initiator AIBN was added. The temperature was raised
to 65 ꢁC and the reaction was carried out for 12 h. The final reaction
mixture was cooled down to room temperature and precipitated
twice in hexane-ethyl acetate 1:1 (v:v) mixture. Copolymers with
different copolymerization molar ratio of HFBMA:ITEGMA ¼ 0:10,
2:8, 3:7, 5:5, 7:3 10:0 were named as PITEGMA, poly(HFBMA-co-
ITEGMA)(2-8), poly(HFBMA-co-ITEGMA)(3-7), poly(HFBMA-co-
ITEGMA)(5-5), poly(HFBMA-co-ITEGMA)(7-3), and PHFBMA,
respectively.
2.1. Materials
Isophorone diisocyanate (IPDI, Degussa Co. LTD., German) was
used as received. 2-Hydroxyethyl methacrylate (HEMA, Beijing
Chemical Works, China) and 2, 2, 3, 4, 4, 4-hexafluorobutyl meth-
acrylate (HFBMA, Xeogia Fluorine-Silicon Chemical, Harbin, China)
were purified by vacuum distillation before use. 2, 20-azobis (iso-
butyronitrile) (AIBN, Tianjin Fuchen Chemical, China) was recrys-
tallized from ethanol. Triethylene glycol (TEG, Tianjin No.1
Chemical Reagent Factory, China) was dried under vacuum oven at
80 ꢁC for 4 h. Tetrahydrofuran (THF, Tianjin Chemical Reagent
Wholesaling Company, China) was soaked with CaCl2 overnight,
refluxed with sodium and then distilled. Hexane and acetic ester
(analytical reagent) were dried with 4A molecular sieve overnight
and distilled prior to use. Fluorescein isothiocyanate (FITC,
Shanghai Shize Biotech Company) was used as received.
2.5. Coatings on aluminum sheets
Aluminum sheet treatment: Aluminum sheets were cut into the
same size with 2 cm ꢂ 4 cm, then polished with a 600 mesh
abrasive paper to move off the oxide layer on the surface. After
degreased in a mixture solvent of isopropyl alcohol and acetone
(1:1, v/v), the sheets were flowed and rinsed with deionized water.
Afterwards, the sheets were further etched by ultrasound for
10 min in deionized water [46] and dried in nitrogen atmosphere.
2.2. Preparation of diisocyanate intermediate BITEG
The reaction was carried out in a glass reactor under nitrogen.
80 mmol IPDI and 0.44 mL benzoyl chloride were added to the
NCO
O
O
OH
H
HO
O
+
NCO
N
O
O
C
C
O
O
O
N
H
OCN
NCO
IPDI
TEG
O
BITEG
OH
O
CH3
CH3
C
HEMA
C
C
O
CH2
CH2
O
n
O
C
O
H
O
C
N
O
O
O
C
N
C
O
O
O
N
H
O
O
NCO
H
O
CF2
CF3
NH
O
ITEGMA
C
O
FHC
O
+
O
NH
C
O
F2
C
O
CF3
O
HN
O
C
F
C
O
O
OCN
HFBMA
co
poly(HFMA
ITEGMA)
Fig. 1. Synthetic steps of isocyanate containing monomer and the functional copolymer.

Z. Zhang et al. / Polymer 53 (2012) 2979e2990
2981
Coating preparation: 1 g copolymer was dissolved in 10 mL dry
THF under ultrasonic condition. Then polymer solution was
dispensed to the aluminum sheets using a drip pipe under nitrogen
atmosphere in a shielded glove box. After 24 h evaporation, the
samples were transferred to vacuum oven and further dried for 12 h
at room temperature.
Annealing process: The copolymer coated aluminum sheet
samples were transferred to a bake oven and treated at 120 ꢁC for
10 min.
Fluorescence of the cracks was observed using an Olympus BX41
microscope (Japan). The cracked copolymer coatings of PHFBMA,
poly (HFBMA-co-ITEGMA) (7-3), poly (HFBMA-co-ITEGMA) (5-5)
and poly (HFBMA-co-ITEGMA) (3-7) were placed under moisture
for 2 h and 12 h respectively, and then treated with the FITC
solution for 2 h. After washing in deionized water for several times,
and dried, the coatings were observed under both optical and
fluorescent microscopes. Both optical and fluorescent photos were
taken.
Stability experiment: The coating samples were placed in
a sealed beaker with the relative humidity at 95% under 30 ꢁC for
several days to test the stability of the coatings.
3. Results and discussion
3.1. Synthesis of isocyanate containing monomer
2.6. Self healing test
In this work, first, an isocyanate containing metharcylate
monomer ITEGMA was synthesized according to the scheme as
shown in Fig. 1, which is confirmed by infrared (IR) (Fig. 2) and
electrospray ionization spectrometer (ESI-MS). From the ESI-MS
spectrum of BITEG, the molecular ion peak is 617 (M þ Naþ ¼ 617).
The measured molecular mass is in good agreement with calculated
molecular weight of BITEG (M ¼ 594). The molecular ion peak, 742
(M þ NHþ4 ¼ 742), also matches the calculated ITEGMA (M ¼ 724)
molecular mass. From the FT-IR spectrum of BITEG in Fig. 2,
absorption peaks of 1706 cmꢀ1(eC¼O), 2271 cmꢀ1 (eNCO) and
3349 cmꢀ1 (eNHCOOe) can be identified clearly. And peaks at
1703 cmꢀ1 (eC¼O), 2266 cmꢀ1 (eNCO) and 3341 cmꢀ1 (eNHCOOe)
can also be seen in the spectrum of ITEGMA (Fig. 2). Besides, wave
numbers of 3080 cmꢀ1 and 1649 cmꢀ1 incorporated with the ESI-MS
datum can prove that the vinyl group (eC¼CH2) was introduced to
the ITEGMA successfully.
Crack formation: The micro cracks were induced by applying
a flexing load on the coating around a 2 cm radius of round column
[56].
The poly (HFBMA-co-ITEGMA) copolymer coated aluminum
sheets with cracks were maintained in a sealed container and
treated for 12 h with relative humidity RH% ¼ 95% at 30 ꢁC and then
dried for further characterization. After treatment, the situation of
cracks was observed with optical microscope and compared with
that before healing.
2.7. Characterization
The mass spectra of were recorded on electrospray ionization
spectrometer (ESI-MS) Varian QFT-ESI (American). ESI spectra were
obtained by using the (CH3OH þ THF) as a mobile phase. Only the
positive ions were recorded enlarged by the mass of Naþ (23) or
NHþ4 (18) originating essentially from impurities.
3.2. Preparation of poly (HFBMA-co-ITEGMA) copolymers
FT-IR spectra were measured in the range from 4000 to
450 cmꢀ1 with a Bio-Rad FTS-6000 (American) FT-IR spectrometer.
Structures of the copolymers were determined by nuclear
magnetic resonance (1H NMR) spectra on a Varian UNITY-plus 400
spectrometer operated at 400 MHz with CDCl3 as a solvent.
Molecular weights of the copolymers were measured on a gel
permeation chromatograph (GPC) instrument (Equipped with
Waters 2414 refractive index detector and Waters 1525 Binary HPLC
Pump, using Waters styragel HT2, HT3, HT4 THF 7.8 ꢂ 300 mm2
columns). THF was used as an eluent solvent and polystyrene
standard samples were used to calibrate the results.
HFBMA
and
ITEGMA
with
different
molar
ratio
(HFBMA:ITEGMA ¼ 0:10, 2:8, 3:7; 5:5; 7:3 and 10:0) were copoly-
merized using radical polymerization. Their FT-IR and 1H NMR
spectra are shown in Figs. 3 and 4, respectively. From Fig. 3, we can
infer that, the absorption band
n
¼ 1707e1721 cmꢀ1 (eC¼O),
n
¼ 2252e2262 cmꢀ1 (eNCO),
n
¼ 3341e3355 cmꢀ1 (eNHCOOe)
and
n
¼ 2862e2863(eCHeO) are corresponding to the isocyanate
containing component; and the band of
n
¼ 1100e1289 cmꢀ1 is
related to the CeF group. From 1H NMR spectra in Fig. 4, the chemical
shift of 5.1 ppm is related to CF2eCFHeCF3 (PF), which is identified in
all the copolymer spectra but not shown in PITEGMA spectrum; the
Quantera PHI 5300 XPS (American) X-ray photoelectron spec-
troscopy was used to characterize the surface composition of the
copolymer coatings.
Water contact angles (WCA) were measured using the sessile
drop method on Dataphysics OCA ISEC contact angle meter
(German).
BITEG
The surface morphologies of coatings were investigated by
Hirox digital optical microscope KH 7700 (Japan) and scanning
electron microscope (SEM) Hitachi X-650 (Japan).
ITEGMA
2271
3349
1706
3080
The electrochemical testing was carried out at a multichannel
Potentiostat/Galvanostat Wona Tech WMPG1000 (Korea). Linear
sweep voltammetry and chronoamperometry were evaluated in
a three electrode electrochemical cell in 1 M KCl aqueous solution
equipped with Pt as a counter electrode, Calomel electrode as
a reference electrode and coated aluminum substrate as working
electrode, respectively. The test samples were coated with epoxy
resin in order to seal the naked aluminum sheet and just let the
copolymer coating expose to the outside. Linear sweep voltam-
metry was evaluated from ꢀ600 mV to þ1100 mV at a rate of
20 mV sꢀ1 and a chronoamperometric curve was performed at 0.3 V
for 200 s.
3341
2266
1649
1713
4000
3000
2000
1000
0
Wavenumber, cm-1
Fig. 2. FT-IR spectra of ITEGMA and BITEG.

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Z. Zhang et al. / Polymer 53 (2012) 2979e2990
Table 1
E
D
Data of molecular weight and composition (measured by 1H NMR) of copolymers.
3353
3355
2954
2254
2253
1717
Copolymers
Mn
Mw
PDI
Composition
measured
(1HNMR)
2865
1707
1714
PITEGMA
6660
12,902
1.93
2.23
1.87
2.48
1.80
e
C
Poly (HFBMA-co-ITEGMA)(2-8)
Poly (HFBMA-co-ITEGMA)(3-7)
Poly (HFBMA-co-ITEGMA)(5-5)
Poly (HFBMA-co-ITEGMA)(7-3)
12,143
17,734
43,156
17,817
27,117
33,199
107,176
32,071
22.4/77.6
31.3/68.7
50.1/49.9
69.8/30.2
3343
2862
2862
2252
2254
B
A
1714
3341
3354
2252
2863
1721
4500
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber,cm-1
Fig. 3. FT-IR spectra of poly(HFBMA-co-ITEGMA) copolymers with different copolymer
compositions; A) 7:3; B) 5:5; C) 3:7; D) 2:8 and E) PITEGMA.
chemical shift of 3.25 ppm is attributed to the existence of NCOeCHe
(PI), which is also found in all spectra. All these results have confirmed
the copolymerization of ITEGMA with HFBMA. The real copolymeri-
zation molar ratio of HFBMA and ITEGMA can be calculated from the
1H NMR spectrawith the equation of PF/PI ¼ HFBMA/ITEGMA in molar
ratio in the copolymer. The data of molecular weights and the
copolymer compositions are listed in Table 1.
3.3. Properties of copolymer coating
Coatings were prepared by dispensing the copolymer solution of
THF on aluminum sheets. After placed in an undisturbed circum-
stance of a glove box for 12 h, the coating samples were further
dried in vacuum oven for 24 h and then annealed for 10 min at
120 ꢁC.
The results of water contact angles for the copolymer coatings
were shown in Fig. 5. We can see that the water contact angle of
coatings increases with the increase of HFBMA content. The more is
Fig. 5. Relationship of water contact angles of coatings before and after 120 ꢁC
annealing with HFBMA monomer content of the copolymer.
the HFBMA amount contained in the coatings, the larger is the
water contact angle. But we can also see that when the ratio of
HFBMA component is over 30% in the copolymer, the increase
tendency of the water contact angle slowed down. In other words,
further increase of the HFBMA ratio has only a little help to enlarge
the contact angle. The reason may be attribute to that the content of
fluorine segments have reached a relative high level when the
content of HFBMA is over 30% in the copolymer composition. Thus
further increase of HFBMA content has only a little help to the
decrease of the coating surface energy. As we know, in the coating
formation process, some polymer chains would not be able to
arrange to the most probable conformation with the solvent
evaporation. And the heat annealing treatment will make polymer
side chains move slowly under the enthalpy driven. Results in Fig. 5
show that the contact angles of the copolymer coatings after heat
annealing for poly (HFBMA-co-ITEGMA) (5-5) and poly (HFBMA-
co-ITEGMA) (7-3) can reach to 102.8ꢁ and 106.5ꢁ, respectively,
which indicates that the annealing process could lead the fluorine-
containing segments of the copolymers to gather on the surface
Table 2
Tested data and theoretic data of F% and N% on the surface of copolymer coatings by
XPS.
Poly (HFBMA-co-ITEGMA) F% tested F% theoretic N% tested N% theoretic
data
data
data
data
2：8
3：7
7：3
8.84
10.12
12.51
2.73
4.42
15.85
3.75
2.54
2.07
7.27
6.88
4.53
Fig. 4. 1H NMR spectra of poly(HFBMA-co-ITEGMA) copolymers with different
copolymer compositions; A) 7:3; B) 5:5; C) 3:7; D) 2:8 and E) PITEGMA.

Z. Zhang et al. / Polymer 53 (2012) 2979e2990
2983
Fig. 6. Stable properties of copolymer coatings with different copolymer compositions under high wet environment. (A, A0): PITEGMA; (B, B0): poly(HFBMA-co-ITEGMA)(2-8);
(C, C0): poly(HFBMA-co-ITEGMA)(3-7); the right two sections b0 and c0 are the SEM photos of the coatings of poly(HFBMA-co-ITEGMA)(2-8) and poly(HFBMA-co-ITEGMA)(3-7),
respectively. (D, D0, D00): coatings of poly (HFBMA-co-ITEGMA) (3-7), poly (HFBMA-co-ITEGMA) (5-5), poly (HFBMA-co-ITEGMA) (7-3) before treatment and after high wet envi-
ronment treatment for 1 and 2 weeks, respectively.

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Z. Zhang et al. / Polymer 53 (2012) 2979e2990
Fig. 7. Optical photos of healing results for three copolymer coating samples before and after healing; (A, A0): poly(HFBMA-co-ITEGMA)(3-7); (B, B0): poly(HFBMA-co-ITEGMA)(5-5);
(C, C0): poly(HFBMA-co-ITEGMA)(7-3). The bar in the Figure is 100
m.
m
areas, so the hydrophobic property of the coating would be
enhanced.
placed in a sealed beaker with the relative humidity at 95% under
30 ꢁC. After 48 h, it can be seen that the color of the coating films
for PITEGMA and poly(HFBMA-co-ITEGMA)(2-8) was turned from
transparent to white, as shown in Fig. 6A0 and B0, which indicates
that the samples were sensitive to the humidity. The samples of the
copolymer coatings would absorb water under high moisture
environment, and the water reacted with the isocyanate groups
inside the coatings to form urea groups and CO2, which then could
make the film become white. The SEM photo of poly (HFBMA-co-
ITEGMA) (2-8) coating film after humidity processing in Fig. 6b0
provides further evidence to the above deduction. There are lots of
micro holes existed on the film which would be generated from the
quickly production of CO2 because of the reaction between isocya-
nate groups and water. But for the samples of HFBMA monomer
content was over 30% (in molar ratio), the coating films were nearly
not influenced and kept transparent, as shown in Fig. 6C and C0. This
phenomenon indicates that the coating films can be protected well
from the high moisture environment by the fluorine-containing
surface barrier. The SEM photo in Fig. 6c0 also shows that the coating
film is still in a good state. In addition, after placed in the sealed
container for 2 weeks under 95% humidity and temperature at 30 ꢁC,
the samples of poly(HFBMA-co-ITEGMA)(3-7), poly(HFBMA-co-
ITEGMA)(5-5) and poly(HFBMA-co-ITEGMA)(7-3) still maintained
in a good state, as shown in Fig. 6D0 and D00, which indicates that the
HFBMA segments have a good protection to the copolymer coatings
from exposing to the high moisture environment when HFBMA
The XPS was used to further characterize the surface composi-
tion of the coatings. The results are shown in Table 2. From the XPS
data, as a whole, the content of N atom is grown larger with the
increase of the ITEGMA content in the copolymer; and the content
of F atom is grown larger with the increase of the HFBMA content.
Furthermore, we can found that the tested F concentration is much
higher than the theoretic data for poly (HFBMA-co-ITEGMA) (2-8)
and poly (HFBMA-co-ITEGMA) (3-7), and the tested N concentra-
tion is much lower than the theoretic data. These data indicate that
there is obvious gathered tendency to the film surface for F
elements, which is in a good agreement with the water contact
angle results. This further indicates that when poly (HFBMA-co-
ITEGMA) copolymer was coated on the aluminum sheet, on one
side, the fluorine-containing segments, because of its low surface
free energy, would be prone to go to the air/solid interface, and
form a moisture barrier layer, which would protect the isocyanate
groups in the bulk matrix away from the outside moisture attack-
ing. On the other side, isocyanate groups, because of its high
polarity and the property to react easily with eOH group on the
aluminum surface would be prone to locate on the substrate/solid
interface, to form a healing function layer.
The environment stability of the copolymer coatings was tested,
in order to validate if the fluorinated component in copolymers can
support the practical protection for the coatings. Samples were

Z. Zhang et al. / Polymer 53 (2012) 2979e2990
2985
Fig. 8. SEM photos of copolymer coating of poly (HFBMA-co-ITEGMA) (5-5) after healing.
segments is over 30% in the copolymer composition. So the samples
of poly (HFBMA-co-ITEGMA) (3-7), poly (HFBMA-co-ITEGMA) (5-5)
and poly (HFBMA-co-ITEGMA) (7-3) were chosen for the following
self healing test experiments.
only a little content of fluorine (30% in molar ratio) containing
monomer employed, the fluorine segments of the surface layer
can reach a relative high value. So the self healing ability of the
coatings is mainly decided by the isocyanate group content of the
coatings once the fluorine segment reaches an appropriated
amount.
3.4. Self healing performances
Fig.
7
shows the optical microscope photographs of the
3.5. Electrochemical testing
morphology of cracks for the copolymer coatings before and after
healing. We can see that all the cracks for poly (HFBMA-co-
ITEGMA) (3-7) were repaired completely with almost no scar left
(Fig. 7A0), indicating the good self healing ability of the coating.
Fig. 7B and B0 refer to the coating of poly (HFBMA-co-ITEGMA)
(5-5). The crack was also healed, but a slight healing scar can be
found. SEM photos in Fig. 8 for poly (HFBMA-co-ITEGMA) (5-5)
coating after healing process also confirmed the self healing result.
From Fig. 8A, the crack was almost healed, only a little light scar left.
And with further amplification, Fig. 8B shows that a small amount
of white protuberance existed. With a further amplification of the
protuberance, Fig. 8C and D show that some place of the crack is not
Electrochemical testing can provide further evidence of passiv-
ation property for the self healed coatings [27]. The test was con-
ducted in a three electrode system by using electrochemical
workstation. In the three electrode system, Pt equipped as
a counter electrode, Calomel electrode equipped as a reference
electrode and coated aluminum substrate as working electrode,
respectively. When the voltage swept from ꢀ0.6 V to 1.1 V, the
coated aluminum sheet was under an oxidation process which is
charging to the coated aluminum. Results of linear sweep voltam-
metry and chronoamperometry are shown in Fig. 9. Three samples
including self healing processed coating samples of poly(HFBMA-
co-ITEGMA)(7-3), poly(HFBMA-co-ITEGMA)(3-7), and coating
sample of poly(HFBMA-co-ITEGMA)(3-7) just after crack formation
as a control were tested. From Fig. 9a, we can infer that the sample
of poly (HFBMA-co-ITEGMA) (3-7) after self healing exhibited
a passivation property with current of 1.35 ꢂ 10ꢀ9 A. Meanwhile,
the current of poly (HFBMA-co-ITEGMA) (7-3) counterpart is
0.05 A, and the current of control sample new cracked poly
(HFBMA-co-ITEGMA) (3-7) is linear with voltage.
healed completely, and the crack is approximate 1 mm in width. For
samples poly (HFBMA-co-ITEGMA) (7-3), as shown in Fig. 7c and c0,
there is no healing phenomena occurred.
From the experimental data shown above, we can conclude
that among the three different ratios of HFBMA and ITEGMA for
the copolymer coatings, poly (HFBMA-co-ITEGMA) (3-7) shows
the best self healing behavior. The self healing properties of the
copolymer systems are mainly related to two factors. On one hand,
the more isocyanate groups the coating contains, the better the
self healing effect is. On the other hand, the more fluorine groups
the coatings contains, the better the protection effect is. But with
The chronoamperometrycurves are shownin Fig. 9b, The currents
of healed samples poly(HFBMA-co-ITEGMA)(3-7), sample
poly(HFBMA-co-ITEGMA)(7-3), and thecontrol samplepoly(HFBMA-

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Z. Zhang et al. / Polymer 53 (2012) 2979e2990
Fig. 9. (a) Linear sweep voltammetry curves. A): poly (HFBMA-co-ITEGMA) (3-7) after crack formation as control; B): poly (HFBMA-co-ITEGMA) (7-3) and C): poly (HFBMA-co-
ITEGMA) (3-7) after moisture healing process. The in section is the magnification of healed poly (HFBMA-co-ITEGMA) (3-7) with voltage from from ꢀ600 mV to þ1100 mV at a rate
of 20 mV sꢀ1; (b) Chronoamperometry curves. A): poly (HFBMA-co-ITEGMA) (3-7) after crack formation as control; B): poly (HFBMA-co-ITEGMA) (7-3) and C): poly (HFBMA-co-
ITEGMA) (3-7) after moisture healing process. The in section is the magnification of healed poly (HFBMA-co-ITEGMA) (3-7) with evaluation time for 200 s.
co-ITEGMA)(3-7) just after crack formation are nearly 1.45 ꢂ 10ꢀ9 A,
0.04 A and 0.19 A, respectively, which also reveals the same tendency
consistent with the voltammetry test results. The obtained similar
results by the above two types of different electrical testing models
suggest that the current dataunder the same testingconditionsas the
quantization index cannot only illustrate the healing effects of the
coating materials before and after repairing process, but also be acted
as the quantization index of repairing degree of the materials. For the
well repaired sample of poly(HFBMA-co-ITEGMA)(3-7), the current is
very small (10ꢀ9 A), and for the partly repaired sample of
poly(HFBMA-co-ITEGMA)(7-3), the current has an outbreak
increases (to 0.04 A). And for the un-repaired sample, there is
a complete conduction path in the sample, and the current value is
the biggest one correspondingly.
3.6. Analysis of self healing mechanism
In this homogenous copolymer system, the preconditions for
self healing process are that the isocyanate groups inside the
coating matrix are exposed to the outside when the coatings suffer
crack damage and the surface fluorine-containing barrier layer is
opened up. And then isocyanate groups reacted with moisture
water in air and generated amine groups, finally the amine groups
reacted with another isocyanate group and form the urea crosslink
to heal the crack. In order to characterize this process more clearly,
FITC which is an amine-reactive fluorophore was used to func-
tionalize the post-moisture cracks on the copolymer coatings. From
the photos in Fig. 10, coating film of PHFBMA was observed no
fluorescence phenomena; however, obvious fluorescence
phenomena was observed for all the other three copolymer coat-
ings, which proves the generation of amine groups after the
isocyanate groups exposed to the outside moisture. From Fig. 11B
and B0, the cracks on poly (HFBMA-co-ITEGMA) (7-3) coating were
almost not healed, and the fluorescence signal was observed all
The electrochemical testing results indicate that the coating film
of poly (HFBMA-co-ITEGMA) (3-7) is healed well after healing
process under moisture environment; moreover, the healed coating
film still has good electrochemical protection ability to the metal
substrate.

Z. Zhang et al. / Polymer 53 (2012) 2979e2990
2987
Fig. 10. Optical and fluorescent photos on cracks of the different coating after treated under moisture for 2 h; then treated with FITC: (A, A0): PHFBMA; (B, B0): poly(HFBMA-co-
ITEGMA)(7-3); (C, C0): poly(HFBMA-co-ITEGMA)(5-5) and (D, D0):poly(HFBMA-co-ITEGMA)(3-7). The multiple of zoom in is 100.
over the cracks. And we can also infer from the Fig. 11C and C0, the
cracks on poly (HFBMA-co-ITEGMA) (5-5) coating were partly
healed in some place where a little amount of fluorescence signal
was observed. However, from Fig. 11D and D0, we can see that after
healing process under moisture for 12 h, the cracks on
poly(HFBMA-co-ITEGMA)(3-7) coating were well healed, and
almost no fluorescence phenomena was observed, which indicates
that the amine groups were consumed by isocyanate groups to
form urea groups.
a whole crack, the possible contact areas between the two sides of
the crack may be the ends, the bottom and some possible areas
nearby between the both sides of the crack. These effective contact
areas are called part A, and the other areas of the crack are called
part B. Area A and area B can be summarized in Fig. 12a. Part A
includes the bottom and the end of the crack, as shown in Fig. 12a
(1) and (2), which are the main reactive areas where the self healing
reactions are carried out. There are also some other possible part A
areas existed as shown in Fig. 12a (3) and (4). These possible parts A
areas will contribute greatly to the self healing process because
they could increase the amount of reactive spots. In part B areas, the
isocyanate groups between two sides of the crack cannot contact
Another important factor to heal the crack is that the isocyanate
groups on the two sides of the crack can contact with each other
effectively, and then react with H2O to form urea crosslink. In

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Z. Zhang et al. / Polymer 53 (2012) 2979e2990
Fig. 11. Optical and fluorescent photos on cracks of the different coating after treated under moisture for 12 h; then treated with FITC: (A, A0): PHFBMA; (B, B0): poly(HFBMA-co-
ITEGMA)(7-3); (C, C0): poly(HFBMA-co-ITEGMA)(5-5) and (D, D0):poly(HFBMA-co-ITEGMA)(3-7). The multiple of zoom in is 100.
easily and the reactions have no contribution to the self healing
behavior.
(HFBMA-co-ITEGMA) (7-3). For the system with low content of
isocyanate groups, the consuming of isocyanate groups in part B
will be a fatal side reaction, which leads to no healing phenomena
(Fig. 7c and c0). And for poly (HFBMA-co-ITEGMA) (5-5) copolymer
coating film, the side reaction influences to a certain degree, which
results in that some places in the crack are also unhealed. However,
the side reaction to the coating of poly (HFBMA-co-ITEGMA) (3-7)
nearly does not affect to the healing process due to the existence of
the enough amount of isocyanate groups.
Along with the repairing process going, part A areas are grad-
ually extending to part B areas. The part A areas are enlarged and
part B areas are reduced gradually, which makes the crack grow
narrower and shorter, thus the healing process are carrying on.
However, when the parts A areas are conducting the healing
process, part B areas are also exposed to the moisture. Then, the
isocyanate groups on the same side will react with H2O. Under this
situation, isocyanate groups are consumed but no healing effect
shows. And this will bring down the healing effect especially to the
coating systems with low content of isocyanate groups such as poly
The above self healing process can be described as a slider
meshes together the opposing rows of teeth in a zipper as shown in
Fig. 12b. The isocyanate groups on the both sides of the crack act as

Z. Zhang et al. / Polymer 53 (2012) 2979e2990
2989
Fig. 12. a) Illustration of the crack self healing process. A: 1) the bottom of the crack; 2) the ends of the crack; 3, 4) possible contact areas between the two sides of the crack; B: no
contact areas. b) Illustration of the zipper model mechanism.
the teeth whereas the H2O acts as the slider in the zipper. Along
with the self healing process conducting, the slider H2O meshes the
isocyanate group teeth together automatically to form a stable urea
crosslink. Comparing with the zipper in real world, this “zipper”
may consist of several sliders, which are in the different part A areas
in the crack.
This zipper-like model can also be used to explain other self
healing systems, such as the active precursor containing system of
Ghosh et al. [49] In their work, the crack was self healed under UV
exposure by couple reaction between two crack faces. The width of
the crack grew narrower, and the length of the crack became
shorter along with the self healing process, until the crack is healed
completely finally. Thus the “zipper model” should be a universal
mechanism to explain the self healing systems, which can heal
themselves in virtue of contained active functional groups in the
homomatrix.
components has been synthesized. The self healing property was
demonstrated when using the copolymer as a protect coatings on
aluminum sheet. When the fluorine segment containing monomer
is over 30%, the coating film of the copolymer can be protected well
against the environment moisture. The optimized ratio of the self
healing coating film was found out to be HFBMA:ITEGMA ¼ 3:7. A
zipper-like self healing mechanism has been proposed to explain
the healing behavior for this copolymer system. The isocyanate
groups on the two sides of the crack within the effective contact
areas are coupled under the moisture mediated condition and this
process can be extended to the areas which are ineffective contact,
and finally the entire crack is healed. This work provides a simple
and effective method with a fine prospect to prepare a kind of anti-
corrosion coatings with high self healing performances.
Acknowledgments
4. Conclusion
This work was supported by National Natural Science Founda-
tion of China (Proj. No. 50978138) and State Key Lab for Modifica-
tion of Chemical Fibers and Polymer Materials, Donghua University,
China (Proj. No. LK 0808) for financial support.
In this work, a novel poly (HFBMA-co-ITEGMA) copolymer
composed of fluorine segments and isocyanate groups containing

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