Self-healing polyurethane elastomer with thermally reversible alkoxyamines as crosslinkages

Article abstract of DOI:10.1016/j.polymer.2014.02.033

To provide self-healing ability to polyurethane elastomer, alkoxyamine-based diol was synthesized and reacted with tri-functional homopolymer of hexamethylene diisocyanate (tri-HDI) and polyethylene glycol (PEG). Because alkoxyamines acted as crosslinkers of the resultant polyurethane, the thermally reversible fission/recombination of C-ON bonds in alkoxyamine moieties enabled repeated crosslinking and de-crosslinking of polyurethane chains at certain temperature and hence crack healing. Both qualitative and quantitative characterizations demonstrated that the reversibly crosslinked polyurethane was capable of re-bonding ruptured parts and restoring mechanical strength. The remendability was a function of molecular structure and compositions of the components, which can be purposely tuned according to different application demands.

Full text of DOI:10.1016/j.polymer.2014.02.033

Polymer 55 (2014) 1782e1791
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Self-healing polyurethane elastomer with thermally reversible
alkoxyamines as crosslinkages
,
,
Chan’e Yuan a b, Min Zhi Rong a, Ming Qiu Zhang a
*
a Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, GD HPPC Lab, School of Chemistry and Chemical Engineering,
Sun Yat-Sen University, Guangzhou 510275, PR China
b National Engineering Research Center of Electronic Circuits Base Materials, Shengyi Technology Co., Ltd., Dongguan 523808, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
To provide self-healing ability to polyurethane elastomer, alkoxyamine-based diol was synthesized and
reacted with tri-functional homopolymer of hexamethylene diisocyanate (tri-HDI) and polyethylene
glycol (PEG). Because alkoxyamines acted as crosslinkers of the resultant polyurethane, the thermally
reversible fission/recombination of CeON bonds in alkoxyamine moieties enabled repeated crosslinking
and de-crosslinking of polyurethane chains at certain temperature and hence crack healing. Both qual-
itative and quantitative characterizations demonstrated that the reversibly crosslinked polyurethane was
capable of re-bonding ruptured parts and restoring mechanical strength. The remendability was a
function of molecular structure and compositions of the components, which can be purposely tuned
according to different application demands.
Received 26 October 2013
Received in revised form
25 January 2014
Accepted 8 February 2014
Available online 17 February 2014
Keywords:
Polyurethane
Self-healing
Ó 2014 Elsevier Ltd. All rights reserved.
Thermally reversibility
1. Introduction
When exposed to ultraviolet light, chitosan chain scission occurred,
which formed crosslinks with the reactive oxetane ends, thus of-
Polyurethanes (PU) are known for great property variance,
ranging from rigid and hard to flexible and soft. As a result, they
have been widely used in modern society. Recently, efforts were
made to impart intrinsic self-healing capability to polyurethanes in
hopes of developing next generation versions with smart func-
tionality [1,2].
Automotive industries have introduced self-healing coating
based on PU to attenuate or obscure the scratches [3,4]. By opti-
mizing the crosslink density while keeping the glass transition
temperature low, fine scratch could not result in chain scission as in
the case of conventional paint but plastic deformation. Under the
heat of the sun and outdoor temperature, the deformed parts
would be gradually restored due to the viscoelasticity driven plastic
flow. However, the paint did not self-repair if scratches were deep
enough to sever the bonds. For purposes of creating coating capable
of recovering from fracture, Ghosh and Urban synthesized hetero-
geneous PU networks containing oxetane-substituted derivative of
chitosan [5]. Upon mechanical damage of the network, four-
member oxetane rings opened to generate two reactive ends.
fering one-way repairing capability of crack. Ling et al. further
coupled coumarin with PU as a photoreversible crosslinker [6,7]. By
using its reversible photocyclization habit, cracks on the polymer
films were repeatedly mended by ultraviolet illumination or
sunlight.
Yamaguchi et al. prepared crosslinked PU with different cross-
linking densities by changing the molar ratio of [NCO] to [OH] [8].
The cleaved sample sheet made from a proper reaction ratio was
rapidly re-bonded at room temperature within 10 min as a result of
the strong topological interaction (physical entanglement) of
dangling chain ends. Amamoto et al. [9] reported self-healing
crosslinked PU based on radical reshuffling of thiuram disulfide
(TDS) units under the stimulation of visible light at room temper-
ature and without solvent. Since the SeS bonds in the incorporated
TDS units homolytically dissociated in visible light, the radical
reshuffling reactions allowed reorganization of the linking units in
the polymer and self-healing of damage. Recently, Du et al. pro-
duced linear PU through DielseAlder (DA) reaction between a PU
prepolymer end-capped with furan groups and bismaleimide [10].
The filmy specimen was cut and heated at 120 ꢀC for 5 min, then
cooled down to 60 ꢀC and kept for 24 h, recovering 80% tensile
strength taking advantage of reversible DA chemistry [11].
On the whole, not many healing mechanisms have been
involved in PU, as reflected by the above brief summary. Additional
* Corresponding author.
E-mail addresses: ceszmq@mail.sysu.edu.cn, zmq_cst@126.com, ceszmq@126.com
(M.Q. Zhang).
http://dx.doi.org/10.1016/j.polymer.2014.02.033
0032-3861/Ó 2014 Elsevier Ltd. All rights reserved.

C. Yuan et al. / Polymer 55 (2014) 1782e1791
1783
healing chemistries should be explored to meet the application
requirements of PU in different occasions, such as opaque bulk
materials. So far, self-healing approaches can be classified into two
categories: extrinsic and intrinsic self-healing [1e3,11e14]. The
former takes effect by the embedded microcapsules containing
healing agent, while the latter is able to heal cracks by the polymers
themselves without the need of additional healing agent. The
reversible bonds responsible for intrinsic self-healing consist of
covalent and non-covalent ones. Besides DA interactions [11], the
former includes thiuram disulfide units [9], coumarin derivatives
[6,7], alkoxyamine moieties [15], acylhydrazone bonds [16], disul-
fide links [17], and Ru-catalyzed shuffling of CeC bonds [18], while
dilaurate (DBTL) was purchased from Sigma Aldrich Co., whereas
the tri-functional homopolymer of hexamethylene diisocyanate
(tri-HDI) (Desmodur N3300) was supplied by Bayer Materials Sci-
ence AG. All the chemicals were used as received.
2.2. Synthesis of 4-hydroxy-1-(20-hydroxy-10-phenyl-10-methyl)
ethyl-TEMPO (diol)
The alkoxyamine-based diol was synthesized as follows
(Scheme 1). BPO (14.52 g, 0.06 mol) and 4-OH-TEMPO (20.64 g,
0.12 mol) were dissolved with
100 ꢀC under argon for 3 h, and then cooled to room temperature.
The excess -methyl styrene was removed under reduced pressure.
a-methyl styrene (250 mL), heated at
the latter involves hydrogen bonding [19], ionomers [20],
p
ep
a
stacking [21], and van der Waals’ actions [8], etc.
Afterwards, ethanol (300 mL) and KOH aqueous solution (3 N,
120 mL) were added into the system and the mixture was stirred at
room temperature for 12 h, followed by being evaporated to dry-
ness and partitioned between dichloromethane and deionized
water. The aqueous phase was extracted with dichloromethane for
three times and the organic phases were gathered, dried with
anhydrous sodium sulfate for 24 h. After the solvent was removed,
the crude product was purified by column chromatography using
ethyl acetate and petroleum ether as the eluent, to give the diol as a
Among these candidates, thermally reversible CeON bond has
proved to be an effective measure for realizing self-healing of stiff
polymers like alkoxyamine-crosslinked polystyrene [15]. Owing to
synchronous bond fission and radical recombination at certain
hemolytic temperature, cracked parts can be reconnected repeat-
edly, without losing integrity and load bearing ability of the ma-
terial even above Tg. In this context, it is worth verifying whether
CeON bond also enables self-healing of PU elastomer, which has
wider range of application.
Here in this work, an alkoxyamine-based diol is synthesized,
whose hydroxyl groups can easily participate in the subsequent
production of PU. As shown in Fig. 1, alkoxyamine moieties are
eventually integrated with PU as intermolecular links attached to
the hard segments of the polymer chains responsible for crack
rehabilitation, while the soft segments are advantage to interaction
of dangling chains across the fracture faces. The design is believed
to benefit strength restoration of the polymer.
pale yellow solid (12.8 g, yield ¼ 34.7%). 1H NMR (CDCl3):
d/ppm
0.89 (3H, CH3), 0.99 (3H, CH3), 1.19 (3H, CH3), 1.20 (3H, CH3), 1.58
(3H, CH3), 1.53e1.84 (4H, 2 CH2), 3.84 (1H, CH), 3.96 (1H, CH), 4.05
(1H, CH), 7.21e7.54 (5H, aromatic); 13C NMR (CDCl3):
d/ppm 21.96,
22.06, 23.93, 34.19, 49.49, 49.75, 61.01, 61.10, 63.04, 70.12, 81.91,
126.88, 127.19, 128.14, 146.20; mass spectrum for C18H29NO3
[M þ 1]þ found 308.2.
2.3. Preparation of polyurethane crosslinked by alkoxyamines
2. Experimental section
PU networks were prepared by reacting tri-HDI with diol and
PEG (Scheme 2). Firstly, the reaction mixtures were completely
dissolved together with the catalyst dibutyltin dilaurate in an
appropriate amount of ethyl acetate, offering homogeneous and
transparent solution. Then, the solution was poured into a silicone
rubber mold at 20 ꢀC under argon atmosphere, allowing reaction
for 12 h. Afterwards, the compounds were put in a vacuum oven at
35 ꢀC for 48 h. The molar ratio of the components in the PU (based
2.1. Materials and reagents
4-OH-TEMPO,
a-methyl styrene, benzoyl peroxide (BPO),
ethanol, polyethylene glycol (PEG) (PEG200, PEG400, PEG600,
Mw ¼ 200, 400, 600, respectively), bisphenol A (BPA) and ethyl
acetate were obtained from Alfa Aesar GmbH, Germany. Dibutyltin
Fig. 1. Healing mechanism of polyurethane crosslinked by alkoxyamines.

1784
C. Yuan et al. / Polymer 55 (2014) 1782e1791
Scheme 1. Synthesis of the alkoxyamine-based diol.
on the stoichiometric ratio of NCO and OH), tri-HDI/PEG/diol, was
1:0.58:0.87. Since PEG200, PEG400 and PEG600 were employed as
chain extenders to tune the networks, respectively, the corre-
sponding polyurethanes were denoted by HDI/PEG200/diol, HDI/
PEG400/diol and HDI/PEG600/diol.
To study the relationship between fission of alkoxyamine moi-
eties and thermal stability of the material when heated, control
polyurethane without PEG was prepared, labeled as HDI/diol. In
addition, control polyurethanes HDI/PEG/BPA (by replacing diol
with BPA) and HDI/PEG were designed to investigate the effect of
entanglement of PEG molecular chains on the self-healing
performance.
of PU, respectively. In this work, E0 values at T ¼ Tg þ 30 ꢀC were
used for the calculation.
Self-healing performance was characterized by tensile tests,
which were performed on rectangle-shaped specimens
(100 ꢂ 10 ꢂ 0.9 mm3) at a crosshead speed of 10 mm/min using a
universal testing machine (SANS CMT 6103). The fractured parts of
the specimens were put together, and treated at 80 ꢀC under argon
atmosphere for 2.5 h. Having been cooled to room temperature, the
fixed specimen was tested to failure again. The ratio of tensile
strength of the healed specimen to that of the virgin one gave
healing efficiency. The tests were repeated for several times to
study the multiple healing ability. Each batch included six speci-
mens to yield average value. It is worth noting that tensile test is
inherently designed to characterize bulk continuum deformation
leading up to failure, but gives little information about failure itself.
The ultimate stress and strain values might be misleading as the
material necks prior to failure (which is not applicable for the
polyurethane studied in this work). Crack propagation test focusing
on characterizing the regime of material response is the most
relevant to self-healing functionality. Employment of suitable
specimen geometry is critical for evaluation of self-healing mate-
rials [23].
2.4. Characterization
1H NMR and 13C NMR spectra were collected by VARIAN
Mercury-Plus 300 (300 MHz) with chloroform (CDCl3) as a solvent.
Mass spectrometry (MS) measurement was conducted on Thermo
Finnigan LCQDECA P LC-MS spectrometer. Thermogravimetric
analysis (TGA) was performed on TA Instruments TGA Q50 at a
heating rate of 10 ꢀC/min using a nitrogen purge. Dynamic me-
chanical analysis (DMA) measurements and creep tests were car-
ried out with a 01Db-metravib-DMA25 under nitrogen atmosphere
using tensile mode. Creep stress was set at 0.4 MPa. Electron spin
resonance (ESR) spectra were recorded at different temperatures by
JEOL JES-FA200 to investigate thermal reversibility of the materials.
Molecular weight between crosslinks of the cured PU, Mc, was
calculated from Ref. [22]:
3. Results and discussion
As mentioned in the Introduction (Fig. 1), the planned crack
healing of PU depends on dynamic equilibrium of CeON bond
fission and radical recombination. Therefore, ESR spectra of the
control polymer formed by the reaction of tri-HDI with diol in the
absence of PEG were firstly studied to understand the temperature
dependence of radical concentration.
ꢀ
ꢁ
r
2
4
E0 ¼ 3
RT 1 ꢁ
Mc
Fig. 2(a) shows that radical signal can be observed from 40 ꢀC.
Compared with the crosslinking polystyrene reported in our pre-
vious work [15], the initial fission temperature of the polyurethane
where E0 stands for storage modulus at rubbery plateau zone,
density, R gas constant, T absolute temperature, and functionality
r
f
Scheme 2. Synthesis of polyurethane crosslinked by alkoxyamines.

C. Yuan et al. / Polymer 55 (2014) 1782e1791
1785
increasing with a rise in temperature, meaning crosslinking and de-
crosslinking of the PU via dynamic CeON bonding would be more
effective at elevated temperature. No healing would take place
below 40 ꢀC.
20 C
40 C
60 C
80 C
100 C
(a)
Since the above study reveals that the CeON bond in PU can be
homolyzed as expected, its thermal reversibility should be known
from the ESR spectra collected in the course of repeated heatinge
cooling cycles. Fig. 3 shows the variation of relative ESR signal in-
tensity. Evidently, radical concentration changes up and down over
the thermocycling as a result of reversible CeON bond fission and
radical recombination. A careful survey of the plots indicates that
the ESR signal intensity at 20 ꢀC gradually increases despite the fact
that difference between the data at 80 and 20 ꢀC remains nearly
constant. The trend is more obvious when molecular weight of the
PEG segment decreases. It should result from the permanent
dissociation of a few alkoxyamines as the combination of methy-
lene radicals and nitroxide radicals is somehow obstructed in solid
[25]. The shorter PEG chains lead to higher crosslinking density of
the polyurethane network structure and lower molecular mobility,
which reduces probability of collision of the radicals to re-form Ce
ON bonds. Accordingly, HDI/diol and HDI/PEG200/diol exhibit
relatively poor reversibility.
In fact, all the polyurethanes studied here possess better
reversibility than alkoxyamine-crosslinked polystyrene [15]. This is
because Tg of soft segments in polyurethane is generally lower than
room temperature, which benefits the movement of molecular
chains of the material and hence reconnection of radicals at room
temperature. For alkoxyamine-crosslinked polystyrene, when it is
cooled below its Tg (w120 ꢀC), the molecular chains have difficulty
in moving. Therefore, under the same cooling conditions, higher
reversibility of CeON bond can be achieved in the material with
lower glass transition temperature.
310
315
320
325
330
335
Magnetic field [mT]
20 oC
40 oC
60 oC
80 oC
100 oC
(b)
To further understand the fission/recombination behavior of
alkoxyamine groups in the polyurethanes, isothermal ESR spectra
of each sample at 80 ꢀC were scanned for every 5 min. As shown in
Fig. 4, the relative radical signal intensities nearly do not change
with time. It demonstrates that dynamic equilibrium between
homolysis and rebonding of CeON bonds can be quickly achieved.
When the polymer is healed at 80 ꢀC, concentration of the radicals
would stay almost the same, which ensures sufficient amount of
radicals are involved in the healing action.
310
315
320
325
330
335
Magnetic Field [mT]
(c)
Fig. 2 shows that radical concentration of the material increases
with increasing temperature. It implies more and more CeON
bonds are involved in the dynamic reversible reaction when heated.
Therefore, thermal stability of the PU should be understood to
reveal the effect of CeON bonds.
It is found in Fig. 5(a) that all the polyurethanes possess
acceptable thermal stability, as their temperatures of 1% weight loss
are higher than 200 ꢀC. Relatively, HDI/PEG400/diol and HDI/
PEG600/diol are more stable, showing 1% weight loss at about
260 ꢀC. This is because the concentration of CeON bond in the two
polyurethanes is lower due to the appearance of PEG with higher
molecular weight, and hence the reversible crosslinkages are less.
The negative influence of homolysis of CeON bonds on their py-
rolytic resistance becomes less significant. Nevertheless, when
comparing HDI/PEG400 and HDI/PEG400/diol, we can see that the
onset thermal degradation temperature of the latter is about 30 ꢀC
higher. The result suggests that the soft PEG segments, rather than
thermo-sensitive alkoxyamine moieties, are responsible for the
initial decomposition of the polyurethane at lower temperature. In
fact, heat resistance of polyurethane itself is not high. After incor-
poration of alkoxyamines into the molecular chains, the radicals
generated by the reversible reaction of CeON bonds might prob-
ably combine with the radicals of pyrolysis products of other bonds,
so that thermal stability of the polyurethane is somewhat
20
40
60
80
100
Temperature [oC]
Fig. 2. (a) Typical ESR spectra of HDI/diol measured at various temperatures. (b)
Normalized absorption lines converted from the data of (a). (c) Temperature depen-
dence of relative ESR signal intensities calculated from integral areas of the normalized
absorption lines in (b).
is much lower because of the methyl group of a-methyl styrene
attached to the C atom in CeON bond. It increases steric hindrance
and lowers bonding energy of CeON bond [24]. By using the data in
Fig. 2(a), normalized ESR curves can be calculated (Fig. 2(b)), from
which the relative ESR signal intensities are plotted as a function of
temperature in Fig. 2(c). It can be seen that the signal keeps on

1786
C. Yuan et al. / Polymer 55 (2014) 1782e1791
80 C
(a)
80 oC
80 C
80 C
(d)
20 oC
80 oC
20 C
20 C
20 C
(c)
(b)
20 oC
20 C
Cycle 1
Cycle 2
Cycle 3
80 oC
80 oC
80 oC
(b)
80 oC
20 oC
80 oC
(a)
20 oC
20 oC
20 oC
20 oC
0
10
20
30
40
50
Time [min]
20 oC
Fig. 4. Relative ESR signal intensities of (a) HDI/diol, (b) HDI/PEG200/diol, (c) HDI/
Cycle 1
Cycle 2
Cycle 3
PEG400/diol and (d) HDI/PEG600/diol measured at 80 ꢀC as a function of time.
improved. On the basis of the above discussion, it is known that the
multi-peaks on the derivative thermogravimetric curves (Fig. 5(b))
can be respectively attributed to the weight loss dominated by PEG,
alkoxyamines and HDI segments with a rise in temperature.
Since the dynamic reversibility of CeON bonds is temperature
dependent, not only thermal stability but also dynamic mechanical
property of the polyurethanes should be affected by the alkoxy-
amine moieties. Fig. 6 plots the results of DMA tests, indicating that
longer PEG segments leads to lower Tg of the polymer, narrower
80 oC
(c)
80 oC
80 oC
20 oC
20 oC
width and higher intensity of tan
d peak. It is understandable
20 oC
because in the case of identical molar ratio of hard and soft seg-
ments in the polyurethane networks, the higher molecular weight
of the chain extender, the lower crosslinking density. Accordingly,
20 oC
Tg of the polyurethane is reduced, and tan
d peak becomes higher.
Cycle 1
Cycle 2
Cycle 3
Additionally, with increasing molecular weight of PEG, the fraction
of hard segments in the polyurethane decreases and their contri-
bution to the chains motion is weakened. The spectra of internal
friction are gradually governed by the characteristics of PEG, so that
80 oC
80 oC
(d)
80oC
the tan
and diol resemble each other in HDI/PEG200/diol, which results in
two maxima on its tan curve representing motions of both soft
d peak narrows down. In contrast, chain lengths of PEG200
d
and hard segments, respectively. With respect to HDI/diol that
excludes the chain extender PEG, it has the highest crosslinking
density. Consequently, its tan
highest.
d peak is the lowest, while Tg the
In accordance with the thermal reversibility study by means of
ESR spectroscopy, repeated DMA scans of typical polyurethanes
20 oC
20 oC
20 oC
20 oC
Cycle 1
Fig. 3. Relative ESR signal intensities of (a) HDI/diol, (b) HDI/PEG200/diol, (c) HDI/
PEG400/diol and (d) HDI/PEG600/diol measured during heatingecooling cycles be-
tween 20 and 80 ꢀC.
Cycle 2
Cycle 3

C. Yuan et al. / Polymer 55 (2014) 1782e1791
1787
By using the data of storage moduli of Figs. 6e8, molecular
weights between crosslinks, Mc, of the polyurethanes were esti-
mated (refer to Experimental for more details). It can be seen from
Table 1 that crosslinking densities marginally decrease with the
scan numbers due to appearance of a few permanently cleft radi-
cals, which coincides with the results of Fig. 3.
Fig. 9 shows tensile stressestrain curves of the polyurethanes.
Owing to different concentrations of the components, the poly-
urethanes exhibit different mechanical responses. For the polymers
containing PEG with proper molecular weight, balanced perfor-
mance is acquired offering higher strength and toughness (see the
curves of HDI/PEG200/diol and HDI/PEG400/diol in Fig. 9). When
PEG600 is used, however, crosslinking density of the PU decreases
and the networks have to be more inhomogeneous. As a result, HDI/
PEG600/diol possesses the lowest tensile properties. On the other
hand, HDI/diol is completely made of hard segments, which pro-
vide the material with higher strength and stiffness but lower
elongation to break. On the basis of these results and application
prospects as well, HDI/PEG400/diol is selected for the healing
performance evaluation hereinafter.
100
80
60
40
20
0
(a)
HDI/diol
HDI/PEG200/diol
HDI/PEG400/diol
HDI/PEG600/diol
HDI/PEG400
0
100
200
300
400
500
600
Temperature [oC]
(b)
Fig. 10 plots creep curves of the polyurethanes, which exhibits
their mechanical performance from another angle. In general, the
time dependence of creep strain of plastics consists of four stages
[26]: (i) initial rapid elongation, which is independent of time and is
attributed to the elastic and plastic deformation of the polymer; (ii)
primary creep, the creep rate starts at a relatively high value, and
then decreases with time; (iii) secondary creep and (iv) tertiary
creep, in which the creep rate increases rapidly and final creep
fracture occurs. For the polyurethanes shown in Fig. 10, no sec-
ondary and tertiary creeps are perceived within the time of interest.
HDI/diol has the lowest creep strain because of lack of soft PEG. In
contrast, creep strain of HDI/PEG600/diol is the highest owing to its
longest soft segment PEG600, which results in low crosslinking
density (Table 1) and high deformability. Compared to other ma-
terials, the appropriate combination of soft and hard segments of
HDI/PEG400/diol provides the best recovery ability of deformation.
Its permanent deformation is quickly reduced to zero after removal
of creep stress, indicating its application potentiality as elastomeric
material.
Prior to quantitative characterization, self-healing performance
of the materials was qualitatively characterized (Fig. 11). Firstly, the
strip-like specimen of HDI/PEG400/diol was cut apart in the mid-
dle. Then, the broken parts were brought together in alignment and
treated at 80 ꢀC for 2.5 h in argon atmosphere. It could be found
that the crack line disappeared, and the healed specimen can bear a
weight of 200 g. In order to verify the repeated healing ability of the
material, the repaired specimen was cut again from the upper left
to the lower right across the trace of the first cut, followed by the
HDI/diol
HDI/PEG200/diol
HDI/PEG400/diol
HDI/PEG600/diol
HDI/PEG400
0
100
200
300
400
500
600
Temperature [oC]
Fig. 5. Pyrolytic behaviors of polyurethanes.
were carried out (Figs. 7 and 8). With increasing the scan numbers,
the temperature dependences of storage modulus and tan are
almost the same, except that the tan peaks slightly shift to higher
d
d
temperature. Moreover, storage moduli of the polyurethanes at
elastomeric state do not decay during the multiple scans, demon-
strating that the CeON bonds are not completely debonded even at
elevated temperature and the polymers still possess certain load-
bearing ability in the course of dynamic reversible reaction of Ce
ON bonds. It can be regarded as a technical merit of the material,
which avoids possible deformation of the end-products even when
crack healing is being conducted above Tg.
1.2
HDI/PEG200/diol
HDI/PEG400/diol
HDI/PEG600/diol
(b)
(a)
1000
0.9
HDI/diol
0.6
0.3
0.0
100
HDI/PEG200/diol
HDI/PEG400/diol
HDI/PEG600/diol
HDI/diol
10
-80
-40
0
40
80
120
-80
-40
0
40
80
120
Temperature [oC]
Temperature [oC]
Fig. 6. Temperature dependences of (a) storage modulus and (b) tan
d
of polyurethanes.

1788
C. Yuan et al. / Polymer 55 (2014) 1782e1791
0.6
(b)
(a)
1000
100
10
0.4
0.2
0.0
Round 1
Round 2
Round 3
Round 4
Round 1
Round 2
Round 3
Round 4
-80
-40
0
40
80
120
160
200
-80
-40
0
40
80
120
160
200
Temperature [oC]
Temperature [oC]
Fig. 7. Temperature dependences of (a) storage modulus and (b) tan
d of HDI/diol measured by repeated scans.
1.0
(a)
(b)
0.8
1000
0.6
0.4
0.2
0.0
Round 1
Round 2
Round 3
Round 1
Round 2
Round 3
100
10
-80
-40
0
40
80
120
-80
-40
0
40
80
120
Temperature[oC]
Temperature [oC]
Fig. 8. Temperature dependences of (a) storage modulus and (b) tan
d of HDI/PEG400/diol measured by repeated scans.
second healing as mentioned above. The healed version is also
recovered as evidenced by the regained load bearing ability. Finally,
the third cut was made on the specimen from the upper right to the
lower left in hopes of intersecting the traces of the former two cuts.
Having been healed under the same circumstances, the specimen
can also be stretched and folded.
Fig. 12 gives the tensile stressestrain curves of HDI/PEG400/diol
specimen, which experienced three successive rupture-healing
tests. Like the result of above qualitative assessment, the self-
healing capability of the polymer is evidenced once more. Tensile
Table 1
Molecular weight between crosslinks, Mc, of the polyurethanes estimated from
repeated DMA scans.
PU
Mc (g/mol)
Cycle 1
Cycle 2
Cycle 3
Cycle 4
HDI/diol
568
524
605
696
605
e
605
e
635
e
HDI/PEG200/diol
HDI/PEG400/diol
HDI/PEG600/diol
643
e
660
e
e
e
15
12
9
0.12
HDI/PEG600/diol
HDI/PEG400/diol
HDI/PEG600/diol
HDI/diol
HDI/PEG200/diol
HDI/PEG400/diol
HDI/PEG600/diol
HDI/diol
0.10
0.08
0.06
0.04
0.02
0.00
6
3
0
0
0
100
200
300
400
500
600
50
100
150
200
250
300
Strain [%]
Time [s]
Fig. 9. Tensile stressestrain curves of polyurethanes.
Fig. 10. Typical tensile creep strainetime curves of polyurethanes at 20 ꢀC.

C. Yuan et al. / Polymer 55 (2014) 1782e1791
1789
Fig. 11. Repeated macro-repairing of crossed ruptures of HDI/PEG400/diol.
strength of the specimen can be repeatedly restored, and the effi-
ciency of the first healing event is about 70%. It is worth noting that
the healing efficiency decreases after each test. The main reason
should lie in the softness of the polyurethane. It was hard to force
the fractured surfaces of the broken specimen to have intimate
contact during rehabilitation [27]. Therefore, insufficient amount of
CeON bonds were involved in the radical recombination across the
interface. In addition, the appearance of permanently disconnected
radicals discussed above would also reduce the reaction efficiency.
When PEG400 in HDI/PEG400/diol is replaced by PEG600, the
material is also self-healable, but the healing efficiencies deter-
mined by successive rupture-healing tests are 51.8 ꢃ 7.3, 40.5 ꢃ 6.4

1790
C. Yuan et al. / Polymer 55 (2014) 1782e1791
down to room temperature, they were recombined and treated at
80 ꢀC for 2.5 h in air again. No healing effect was detected due to
deactivation of the cleft CeON bonds.
4. Conclusions
Polyurethane elastomer was successfully coupled with self-
healing ability by crosslinking the backbone made of tri-HDI and
PEG with alkoxyamine-based diol. The thermally dynamic revers-
ible equilibrium between fission/recombination of CeON bonds in
alkoxyamine moieties enables repeated crosslinking and de-
crosslinking of polyurethane chains and hence multiple crack
healing. Structure of alkoxyamine-based diol is closely related to
homolysis temperature of CeON bonds, while molecular weight of
the chain extender PEG exerts important influence on thermal
reversibility, thermal stability and remendability. According to the
information revealed in this study, not only mechanical properties
but also healing performance can be optimized by careful design of
the polyurethane molecules.
Fig. 12. Typical tensile stressestrain curves of virgin and repaired HDI/PEG400/diol.
Compared to self-healing based on reversible DA bonds, which
has to be heated up to a certain temperature for disconnecting the
intermonomer linkages and then cooled down for reconnection,
the present strategy completes the crack healing only by one step
dynamic equilibrium of CeON bonding. As a result, the possible
dimensional change of materials due to creep occurring after retro-
DA reaction is avoided. However, the reversible reaction of
alkoxyamines is somewhat sensitive to oxygen, so that the healing
of our polyurethane elastomer has to be conducted in argon. The
shortcomings of air sensitivity and manual intervention can be
overcome by tuning the structure of alkoxyamines as revealed in a
recent work of the authors [28], which helps to further expand the
area of application of the healing chemistry.
Acknowledgment
The authors thank the support of the Natural Science Founda-
tion of China (Grants: 51273214, 20874117, 51073176 and
51333008), Doctoral Fund of Ministry of Education of China (Grant:
20090171110026), the Science and Technology Program of Guang-
dong Province (Grants: 2010B010800021, 2010A011300004,
2011A091102001, 2011A091102003 and S2013020013029), and the
Basic Scientific Research Foundation in Colleges and Universities of
Ministry of Education of China (Grant: 12lgjc08).
Fig. 13. Effect of healing time on healing efficiency of HDI/PEG400/diol.
and 33.3 ꢃ 5.1%, respectively. It demonstrates that the longer
molecular chain of PEG600 leads to lower content of alkoxyamines
in the polyurethane. Naturally, lower healing efficiency than the
case of HDI/PEG400/diol is observed.
Fig. 13 examines the effect of healing time on healing efficiency
of HDI/PEG400/diol. The healing efficiency increases with healing
time, and then levels off, which follows the general law of other
self-healing materials. The maximum is achieved at 2 h, and further
increase of time makes no evident change. It means that the dy-
namic equilibrium is established after 2 h of healing.
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