Selective aerobic oxidation of alkyl aromatics on Bi2MoO6 nanoplates decorated with Pt nanoparticles under visible light irradiation

Article abstract of DOI:10.1039/c8cc06909c

Pt/Bi₂MoO₆ nanoplates are efficient photocatalysts for the selective oxidation of saturated C-H bonds in alkyl aromatics under visible light illumination using O₂ as an oxidant. This study opens a new window for direct C-H functionalization through the photocatalytic method based on cheap Bi₂MoO₆ semiconductor materials.

Full text of DOI:10.1039/c8cc06909c

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Materials Chemistry A
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COMMUNICATION
Direct microencapsulation of pure polyamine by
integrating microfluidic emulsion and interfacial
polymerization for practical self-healing materials†
ab
Cite this: J. Mater. Chem. A, 2018, 6,
24092
He Zhang,
Xin Zhang,c Chenlu Bao,d Xin Li,e Dawei Sun,fg Fei Duan,b
Received 27th August 2018
Accepted 12th November 2018
Klaus Friedrichh and Jinglei Yang
f
*
DOI: 10.1039/c8ta08324j
rsc.li/materials-a
Encapsulation of polyamines for the practical application of self-
healing epoxy is promising yet challenging due to their high reac-
tivity and good solubility in water and most organic solvents. Herein,
we developed an innovative method to directly synthesize micro-
capsules containing pure polyamine by integrating microfluidic
emulsion and interfacial polymerization. Using this integration to make
full use of the advantages and avoid the shortcomings of the involved
two techniques, the properties of the fabricated microcapsules could
be delicately tailored according to the practical demands of self-
healing materials. The superiority of the obtained polyamine micro-
1. Introduction
Self-healing materials have the ability to repair damage and
restore the lost or degraded properties or performances using
resources inherently available to a system without external
intervention.1 Since the pioneering work by White et al. in
2001,2 diverse strategies and chemistries have been explored to
realize self-healing function in materials and much progress
has been made in this area.3–10 To date, the development of self-
healing materials is still ongoing.11,12 Although previous works
mainly focused on extending the limits of self-healing area to
verify its feasibility, one trend is to develop fully autonomous
practical self-healing systems with low cost and toxicity, high
healing performance and material compatibility, good thermal
and long-term stability, and fast healing kinetics.13 Intrinsic
self-healing materials by molecular design have great potential
for long-term practical application.14,15 However, to date, this
has only been demonstrated in so and rubbery polymers with
certain human intervention to close cracks due to the local
requirement for molecular interaction, and thus was only
conceived conceptually for primary verication via the bottom-
capsules was demonstrated via
a
dual-microcapsule high-
performance self-healing system with fully autonomous recover-
ability, high thermal and long-term stability, relatively fast healing
kinetics. The highest healing efficiency of 111 Æ 12% in terms of
recovered mode I fracture toughness was achieved at room temper-
ature for 48 h without any external intervention. The high perfor-
mance, environmental stability, and low cost and toxicity introduced
by the robust microcapsules promote the potential practical applica-
tion of this self-healing system.
aThe National Engineering Research Center of Novel Equipment for Polymer up synthesis route. Currently, the faster and more practical way
Processing, The Key Laboratory of Polymer Processing Engineering (SCUT), Ministry
is to integrate extrinsic carriers with healants into commercial
polymeric materials since this strategy can be easily incorpo-
rated without modication of the host matrix, while delivering
a high healing performance for relatively wide cracks.
of Education, School of Mechanical and Automotive Engineering, South China
University of Technology, Guangzhou, 510641, China. E-mail: maeyang@ust.hk
bSchool of Mechanical and Aerospace Engineering, Nanyang Technological University,
639798, Singapore
cSchool of Civil and Environmental Engineering, Nanyang Technological University,
639798, Singapore
dSchool of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin,
Aer screening all the self-healing chemistries and the forms
of extrinsic carriers for self-healing purpose,3,16–23 the self-
healing of thermosetting epoxy by encapsulated two-part
amine–epoxy chemistry has the greatest practicability since it
perfectly matches the above-mentioned requirements.24
Previous reports have veried the feasibility of this self-healing
system.25–31 However, a bottleneck still exists for its develop-
ment, that is, albeit epoxy can be encapsulated easily using
various approaches, it is exceedingly challenging to encapsulate
polyamine hardener using the existing techniques due to its
high reactivity and good solubility in water and most organic
solvents. While the former leads to chemical incompatibility of
300387, China
eNational Laboratory of Solid State Microstructures, Department of Materials Science
and Engineering, Nanjing University, Nanjing 210093, China
fDepartment of Mechanical and Aerospace Engineering, Hong Kong University of
Science and Technology, Kowloon, Hong Kong, China
gCollege of Materials Science and Engineering, Beijing University of Technology, Beijing
100124, China
hInstitute for Composite Materials, Technical University of Kaiserslautern,
Kaiserslautern, D-67663, Germany
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ta08324j
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polyamines with many polymer shell-forming chemistries, the microuidic T-junction, (2) formation of preliminary micro-
latter results in difficulty for polyamines to form a stable capsules through instant interfacial polymerization between the
emulsion, which is a prerequisite for traditional encapsulation core polyamine and diisocyanate in reaction solution, and (3)
techniques. Consequently, although there have been several thickening of their shell at elevated temperature. Since the
efforts,25–29,32–34 high-quality polyamine microcapsules still generated non-equilibrium micro-droplets are immediately
cannot be directly synthesized, hindering the further develop- enwrapped via instant interfacial polymerization, this encap-
ment of this system, including its theoretical verication and sulation process does not require a stable emulsion, which is
practical application.
a prerequisite for successful encapsulation via both micro-
Besides, when self-healing materials are targeted for appli- uidic and interfacial polymerization.31,34 Polyamine droplets
cation for encapsulation techniques, it is hard to achieve with a controllable size can be generated by tuning the relative
consistency between application and encapsulation for healants feeding rates of polyamine to co-ow solvent in Teon tubing T1
with different physicochemical properties. Contradictions and T2, respectively, using syringe pumps. As schematically
always exist, where the physicochemical properties preferred for shown in the oval enlargement of the T-junction, the thin
an application are adverse to those required for the encapsu- continuous polyamine stream breaks down into small indi-
lation process, and vice versa. Higher reactivity for the healant is vidual droplets due to the balance between interfacial tension
favorable for autonomous and fast healing without external and shear force.40,41 Once the stream of mixture in T2 ows into
interventions. Nevertheless, it not only challenges the thermal the agitated reaction solution consisting of a non-polar solvent,
and long-term stability of both the healant and the corre- which is miscible with the co-ow solvent, diisocyanate,
sponding carrier, but also leads to difficulty in its encapsulation surfactant, and catalyst, the co-ow solvent mixes with the
via existing techniques. Specically, encapsulability and healing solvent and the polyamine diffuses across interface and reacts
kinetics/speed of a healant fundamentally tend to be mutually instantly with the diisocyanate (reaction scheme in Fig. S1†)
exclusive to each other. Therefore, many explored microcapsule- near the interface to form a polyurea membrane to enwrap the
based self-healing systems either need signicant external polyamine droplet (circle and dashed circle enlargements in
interventions to accelerate the healing reaction or are compro- Fig. 1a). The preliminary shell is then thickened in the reaction
mised by their limited healing performance at room tempera- solution at elevated temperature for a certain duration.
ture. A similar paradox occurs for the molecular structure of
Thus ingenious integration tackles the previously mentioned
healants when traditional encapsulation techniques are adop- paradoxes between the encapsulability during encapsulation
ted. This is especially true for polyamines. Polyamines (such as and delivered performance for the application of polyamines
polyetheramines) that can deliver high healing performances when traditional encapsulation techniques are applied. Due to
are difficult or even unable to be encapsulated,25,26,29 while those the complete separation of generating micro-droplets and
that can be encapsulated with moderate success (such as forming the shell into two independent steps, the encapsula-
ethylenediamine-based condensates or derivatives) are not tion approach is not selective to the targeting cores and a wide
good candidates for self-healing applications.28,31–35 The high variety of liquid polyamines can be encapsulated regardless of
selectivity of traditional encapsulation techniques for the their molecular structures, solubility, reactivity, and composi-
physicochemical properties of healants greatly restricts the easy tions. In this study, tetraethylenepentamine (TEPA) and its
tailoring of the core composition of microcapsules according to mixtures with a commercial trifunctional primary amine with
the practical demands of self-healing materials.
so oxypropylene units in its backbone, i.e. JEFFAMINE T403,
Considering the intrinsic limits of existing encapsulation were adopted for encapsulation. TEPA and JEFFAMINE T403
techniques and the wide applications of encapsulated poly- represent two different types of polyamines and their molecular
amines, including healants for self-healing, pH triggers for self- structures are shown in Fig. S2.† The reason why the solubility
reporting,36–38 and latent curing agents,33,34 it is necessary to of polyamine in the solvents is not crucial is that the diffused
explore new approaches for the synthesis of high-quality poly- polyamine can instantly react with diisocyanate at the interface
amine microcapsules. Herein, by integrating microuidic to thicken the shell rather than dissolve in the solvents, which is
emulsion and interfacial polymerization,39 we develop a smart attributed to the extremely high reactivity of amine with isocy-
strategy to directly fabricate robust microcapsules containing anate and the adopted high concentration of diisocyanate in the
pure polyamine with great potential for practical self-healing reaction solution. Based on this strategy, polyamines with
applications.
a higher reactivity are preferred for the encapsulation process,
which evidently differs from the prior methods and can solve
the paradox about the reactivity of healants for encapsulation
and practical application.
The fabricated device functioned as designed and expected
(Videos S1–S4†). The typical freeze frames (Fig. 1b) from Videos
2. Results and discussion
2.1. Mechanism for encapsulation by integrating
microuidic emulsion and interfacial polymerization
The encapsulation system was elaborately designed to make full S1–S3† display the three different zones of the T-junction,
use of the advantages but avoid the shortcomings of the showing a thin continuous stream in T2 right aer the T-
involved two traditional encapsulation processes. It consists of junction and breaking down of the stream into individual
three successive steps in the device (Fig. 1): (1) generation of droplets in T2 before owing into the reaction solution. Fig. 1c
individual polyamine micro-droplets in non-equilibrium by shows the freeze frames with a time interval of 40 ms from
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Fig. 1 (a) Mechanism for the encapsulation of pure polyamine by integrating microfluidic T-junction (T1/T2) and interfacial polymerization. (b)
Evolution of liquid polyamine from thin continuous stream to individual droplets. The two arrows indicate the thin continuous stream and
droplet, respectively. (c) Sequential shell formation around the amine droplet in the flowing reaction solution. Time interval between two
neighboring images is 40 ms. The arrows in the figures indicate the evolution of the same TEPA droplet, and the area in dashed curve indicates
flowing and mixing of the co-flow solvent with the solvent in the reaction solution. Feeding rates for TEPA and co-flow solvent were 0.02
mL minÀ1 and 0.5 mL minÀ1, respectively, which were the same for the figures below.
Video S4† for the evolution of a droplet into a microcapsule with feeding rates of 0.02 mL minÀ1 and 0.5 mL minÀ1 for the
with the preliminary polyurea membrane upon owing of the polyamine and co-ow solvent were imaged via eld emission
amine droplet into the reaction solution. The area in the scanning electronic microscopy (FESEM, Fig. 2c). The
dashed curve indicates owing and mixing of the co-ow microcapsules were able to survive through the high vacuum
solvent with the solvent in the reaction solution. The arrows during FESEM imaging and keep their shapes; whereas, the
in the images illustrate the change of a certain droplet from fractured ones collapsed immediately once cut by a razor
the end of T2 to the reaction solution. In addition, other (inset in Fig. 2c). These phenomena demonstrate that the
individual microcapsules can be seen, which were generated preliminary microcapsules have walls with excellent
early. The preliminary microcapsules generated at this step tightness.
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Fig. 2 (a) Appearance of fabricated TEPA microcapsules and the microcapsules after evacuation at 80 ꢁC for 24 h. (b) Trend and theoretical fitting
of microcapsule size with respect to feeding rate of TEPA when the feeding rate of co-flow solvent was fixed at 0.5 min minÀ1. (c) Preliminary
microcapsules generated right after contact of mixture from the end outlet of T2 with reaction solution. Inset shows a collapsed microcapsule
after fracture. Morphology and structure of the TEPA microcapsules including (d) Overall appearance, (e) core–shell structure, (f) double-walled
structure of shell, and (g) thin dense inner wall.
Notably, the location of T-junction on T2 highly inuences the rate of 0.005 mL minÀ1 (Fig. 2b and ESI†). The morphology and
morphology and quality of the products. In this study, a distance structure of the pure TEPA microcapsules were characterized via
of ꢀ30 mm from the end outlet of T2 was adopted for the FESEM (Fig. 2d–g). The imaged microcapsules were obtained
generation of spherical microcapsules. When it is too close, the with feeding rates of 0.02 mL minÀ1 and 0.5 mL minÀ1 for TEPA
thin continuous polyamine stream does not have enough time to and the co-ow solvent, respectively. Due to the gradually
break down into individual droplets before getting into the diffused polyamine during shell formation process, shrinkage
reaction solution, resulting in the formation of tube-like struc- occurred for almost all the microcapsules (Fig. 2d), which is
tures containing a liquid polyamine core rather than spherical a typical feature of microcapsules synthesized via interfacial
microcapsules (Fig. S3a†). This observation offered the opportu- polymerization.42 The cross-section of a typical microcapsule
nity to in situ synthesize brous carriers containing polyamine for shows it has a clear core–shell structure with a big chamber
self-healing applications. However, when the T-junction is too far inside to reserve polyamine (Fig. 2e). The shell, with a uniform
away, the interactions between the polyamine and co-ow solvent thickness of ꢀ10 mm (Fig. 2f), has a double-walled structure
signicantly inuence the encapsulation process, especially consisting of a thick porous outer wall and a thin dense inner
when the polyamine has a relatively high solubility in the co-ow wall of thickness ꢀ100 nm (Fig. 2g). This type of structure is
solvent. Under this circumstance, a large amount of polyurea reasonable since the shell grows in two steps by rstly forming
spongy debris was generated due to the reaction between diiso- the preliminary polyurea membrane upon contact of the droplet
cyanate and the diffused/dissolved polyamine (Fig. S3b†). This with the reaction solution (Fig. 2c) and then thickening at
debris not only caused the easy agglomeration of the microcap- elevated temperature. The robustness of this microcapsule
sules in the subsequent encapsulation process, but also originates from both the support from its thicker outer wall and
decreased the quality of nal microcapsules dramatically.
the tightness of its thin but dense inner wall.
The presence of polyamine in the microcapsules was iden-
tied via Fourier-transform infrared (FTIR, Fig. 3a) and proton
nuclear magnetic resonance (1H NMR, Fig. 3b) spectroscopy.
The FTIR and NMR spectra of the extracted core liquid from the
TEPA microcapsules were identical to those of pure TEPA,
which demonstrates the encapsulated substance is TEPA. As
mentioned previously, this proposed method is not selective to
the input polyamine for encapsulation. Besides pure TEPA,
amine mixtures of 50TEPA50T403 (50 wt% TEPA and 50 wt%
JEFFAMINE T403, similarly referred to hereinaer) and
25TEPA75T403 were also encapsulated (FESEM images in
2.2. Properties of achieved microcapsules containing pure
polyamine
Microcapsules containing pure polyamine were successfully
synthesized using the proposed device and process. Aer
collection, the microcapsules could ow free like sand without
any leakage (Fig. 2a). The size of the microcapsule was tunable
by varying the feeding rate of polyamine, and predictable
according to the theoretical tting based on volume trans-
formation between the feeding rate and the reference feeding
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Fig. 3 (a) FTIR spectra of pure TEPA and squeezed liquid from synthesized TEPA microcapsules. (b) NMR spectra of pure TEPA, pure T403,
squeezed TEPA, squeezed 50TEPA50T403, and squeezed 25TEPA75T403 from corresponding microcapsules. (c) TGA curves of pure TEPA,
TEPA microcapsules, TEPA microcapsules treated isothermally at 170 ꢁC for 60 min, and pure polyurea shell. (d) Isothermal TGA curves at 170 ꢁC
for 60 min of the original and thermally-treated TEPA microcapsules. (e) Absolute and relative weight percentage of TEPA in the original and
thermally-treated microcapsules.
Fig. S4†) and characterized via NMR (Fig. 3b). Aer encapsula- mbar). Furthermore, they could still ow freely without evident
tion, both TEPA and JEFFAMINE T403 were retained in the difference aer thermal treatment at 80 ꢁC for 24 h (Fig. 2a) or
microcapsules. While TEPA with higher reactivity can promote 100 ꢁC for up to 16 h. The isothermal TGA curves of the
healing speed, JEFFAMINE T403 with lower functionality and thermally-treated TEPA microcapsules at 80 ꢁC for 24 h and
soer oxypropylene units can increase the fracture toughness of 100 ꢁC for different durations are shown in Fig. 3d, and the
the healed epoxy wedge in cracks. The ease of tailoring the core absolute and relative remaining fraction of TEPA in the micro-
composition of the microcapsules enables the convenient capsules aer these thermal treatments were plotted in Fig. 3e.
ꢁ
control of healing behavior for self-healing materials according It shows that TEPA evaporated slowly by only ꢀ4 wt% at 80 C
to practical demands.
for 24 h and ꢀ10 wt% at 100 ꢁC for 16 h under vacuum. The nal
ꢁ
The composition and thermal stability of the microcapsules absolute TEPA in the microcapsules treated at 100 C for 16 h
were characterized via thermogravimetric analysis (TGA, was maintained above 75 wt%, which is still comparable to that
Fig. 3c–e). It showed that the polyurea shell is quite stable of other freshly prepared microcapsules used for self-healing
before 200 ꢁC and its fraction in the microcapsules is only applications. When JEFFAMINE T403 was adopted, its thermal
ꢁ
ꢀ7 wt%, which is very low compared to microcapsules obtained stability was even higher at 100 C in contrast to that of TEPA
via other processes.28,43–46 The isothermal curve at 170 ꢁC for 1 h (Fig. S5†). Polyamine microcapsules with high thermal stability
for the microcapsule indicates the effective core fraction of are benecial to self-healing epoxy based on it, since epoxy-
TEPA is ꢀ85 wt%. Due to the high thermal stability of both based composites are commonly thermally treated at elevated
TEPA and the polyurea shell, these microcapsules are quite temperatures during manufacturing and occasionally suffer
stable at temperatures up to 100 ꢁC under high vacuum (<1 from thermal shocks in their life cycle.
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Thus, through this innovative approach, robust pure poly-
This system exhibited a high healing performance due to the
amine microcapsules were successfully achieved with a perfect adopted dual microcapsules. Firstly, the inuence of the ratio of
core–shell structure, tunable and predictable size, uniform shell amine microcapsules to epoxy microcapsules was investigated
thickness with tight inner wall, high core content with tailorable when the total concentration of the microcapsules was xed at
composition, and high thermal stability. The superiority of 10 wt% (Fig. 4b) since the stoichiometry of the released healants
these microcapsules for the practical application of self-healing is crucial for any two-part self-healing system. The highest
materials was demonstrated by a self-healing epoxy with dual healing efficiency (h by healed peak load over original peak load,
microcapsules containing polyamine hardener and epoxy 100% Â Phealed/Poriginal) of 111 Æ 12% was achieved at ratio of
monomer.
1 : 1 for the dual microcapsules when the specimens were
healed at room temperature (RT ꢀ 22–25 C) for 48 h without
ꢁ
any external interventions. Efficiency over 100% means full
recovery, which indicates the formed epoxy wedge in cracks
aer healing has excellent mechanical property and good
compatibility with the host epoxy matrix. Notably, a high effi-
ciency of over 90% was obtained within a wide ratio range (2 : 3
to 2 : 1), although the released healants do not stoichiometri-
cally match each other. This suggests the recovery is not very
sensitive to the ratio of the dual microcapsules. This is impor-
tant for practical application, considering the local non-
uniform distribution of microcapsules and mixing issue of
the released two-part healants in cracks. In contrast to the self-
healing system with full recoverability using amine microcap-
sules containing 25TEPA75T403, the systems using microcap-
sules containing pure TEPA or 50TEPA50T403 showed much
lower healing performances (Fig. S10†). With an increase in the
concentration of JEFFAMINE T403 in the amine microcapsules,
the healing efficiency increased dramatically due to the better
fracture toughness of the epoxy cured by JEFFAMINE T403
compared to that by TEPA. This observation demonstrates the
importance of tailoring the core composition of the adopted
2.3. Quantication of performance for practical self-healing
epoxy
The self-healing epoxy adopted amine microcapsules with
25TEPA75T403 and epoxy microcapsules with F10B (bisphenol
F diglycidyl ether (BFDGE) diluted with 10 wt% butyl glycidyl
ether (BGE)). The epoxy microcapsules were fabricated
following the process established by Jin et al.25 and strength-
ened using the coating process developed by Sun et al.47 Fig. S6
and S7† show the molecular structures of BFDGE and BGE,
respectively, and the synthesized epoxy microcapsules, which
have good thermal stability and tightness, as veried by TGA
(Fig. S8†). The healing behavior was characterized via the
recovered mode I fracture toughness using a tapered double
cantilever beam (TDCB, Fig. 4a) specimen.48,49 This test was
carried out because the quasi-static test for self-healing using
TDCB is simple but well-established and comparable to other
self-healing systems and fracture toughness is the most essen-
tial property for the brittle thermosetting epoxy. Both the amine
microcapsules and epoxy microcapsules were dispersed
uniformly in the host epoxy matrix (Fig. S9†).
Fig. 4 Healing performance at RT in terms of recovered mode I fracture toughness. (a) Schematic TDCB specimen of self-healing epoxy by dual
microcapsules using amine–epoxy chemistry. Healing efficiency regarding: (b) Ratio variation of amine microcapsules to epoxy microcapsules
when total concentration is fixed at 10 wt%, (c) Total concentration of the dual microcapsules at the optimized ratio (1 : 1), (d) post-curing
conditions at optimized conditions (10 wt%, 1 : 1), (e) aging duration at RT and (f) healing duration at RT at optimized conditions.
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microcapsules for self-healing, which is a signicant advantage latter, the fracture surface was only covered by a layer of epoxy
of the invented encapsulation technique.
liquid with the absence of polyamine hardener.
The healing performance of this self-healing epoxy regarding
total concentration of microcapsules at the optimized ratio
(1 : 1) is shown in Fig. 4c, with the highest efficiency at 10 wt%.
3. Conclusions
Notably, a healing efficiency of ꢀ90% (88 Æ 11%) could be ob- In conclusion, robust microcapsules containing pure poly-
tained at a concentration as low as 5 wt% and almost full amine were directly synthesized through the integration of
recovery (97 Æ 10%) could be achieved at 7.5 wt%. Since microuidic emulsion and interfacial polymerization, and were
extrinsic inclusions such as microcapsules compromise some of successfully applied in a high-performance self-healing epoxy
the intrinsic mechanical properties of the host matrix,30,50,51 with great potential for practical application. This integration
high healing efficiency with less inclusions is vital for practical makes full use of the advantages but avoids the shortcomings of
self-healing systems. The excellent performance of this self- the involved two traditional encapsulation techniques, and thus
healing epoxy with microcapsules at a low concentration requires no stable emulsion and is not selective to the physi-
proves the feasibility and potential of this system for practical cochemical properties of polyamines. It reconciles the aim
applications.
between encapsulation and application of polyamines, and
The environmental robustness of this two-part self-healing delivers microcapsules with tailorable properties according to
system regarding thermal stability and long-term stability also the practical demands of self-healing materials. The fabricated
benets from the high quality of the microcapsules. The healing dual-microcapsule self-healing epoxy shows fully autonomous
performance of the system under optimized conditions recoverability with low inclusion, high thermal and long-term
(10 wt%, 1 : 1) aer thermal treatment (60 ꢁC or 80 ꢁC for 24 h) stability, and relatively fast healing kinetics at RT without any
or RT aging for different durations was compared to that of the external interventions, which demonstrate the superiority of the
freshly prepared system (Fig. 4d and e, respectively). Although achieved microcapsules and practicability of the self-healing
the healing efficiency decreased slightly, full recovery was still system. Besides this practical self-healing system, the ach-
observed under both post-curing conditions and long-term ieved polyamine microcapsules from this compounding
aging up to 12 months. While this high thermal stability is method also enable an instant self-healing material via two-part
attributed to the low weight loss of the encapsulated healants amine-isocyanate chemistry and a multi-functional smart
upon thermal treatment and insensitivity of healing to a varia- material that can fully autonomously visualize and repair
tion in the ratio of the two healants, the outstanding long-term damages simultaneously using the encapsulated polyamine as
stability originates from the high chemical stability of the two a multi-role trigger for both color indication and self-healing.
healants isolated separately in carriers and the high tightness of
the two carriers, especially the polyamine microcapsules.
Conflicts of interest
The healing kinetics of this system via the encapsulated
amine–epoxy chemistry was characterized in term of the healing There are no conicts to declare.
efficiency with respect to healing duration at RT (Fig. 4f). Due to
the high reactivity of TEPA and relatively high reactivity of
Acknowledgements
JEFFAMINE T403 towards epoxy monomer, ꢀ85% (84.9 Æ 5.0%)
of fracture toughness was reestablished within 24 h and full We thank the SUG Grant from HKUST (R9365), the Key Program
recovery (105 Æ 13%) was obtained within 36 h at RT.
of National Natural Science Foundation of China (Grant No.
Finally, a fractograph of a crack at optimized condition was 51435005), the National Instrumentation Program (Grant No.
observed to visually examine the healing behavior (Fig. S11†). It 2012YQ230043), the Science and Technology Program of
shows the facture surfaces were perfectly wet and covered by the Guangzhou (Grant No. 201607010240), and the Science and
released healants due to the good compatibility between the Technology Planning Project of Guangdong Province (Grant No.
cured healants and host epoxy matrix. The absence of a newly 2015B090904004, 2016A030313486, 2018A030313264), for
formed epoxy wedge in Fig. S11a† can be found in Fig. S11b,† nancial support.
which means adhesive failure, i.e. failure along the interface
between the newly formed epoxy wedge and the fracture
surface, rather than cohesive failure, i.e. failure in the newly
References
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test. This failure mode suggests that the fracture toughness of
the cured healants is high, attributed to the use of JEFFAMINE
T403 as the encapsulated polyamine hardener. For comparison,
the unhealed fracture surface with the healants rinsed away by
acetone before healing and fracture surface of the specimen
with only 10 wt% epoxy microcapsules were also imaged in
Fig. S12.† For the former, only fractured empty microcapsules
and a smooth fracture surface were observed, while for the
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