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Structural polymers are susceptible to damage in the form of cracks, which form deep within the structure where detection is difficult and repair is almost impossible. Cracking leads to mechanical degradation of fibre-reinforced polymer composites; in microelectronic polymeric components it can also lead to electrical failure. Microcracking induced by thermal and mechanical fatigue is also a long-standing problem in polymer adhesives. Regardless of the application, once cracks have formed within polymeric materials, the integrity of the structure is significantly compromised. Experiments exploring the concept of self-repair have been previously reported, but the only successful crack-healing methods that have been reported so far require some form of manual intervention. Here we report a structural polymeric material with the ability to autonomically heal cracks. The material incorporates a microencapsulated healing agent that is released upon crack intrusion. Polymerization of the healing agent is then triggered by contact with an embedded catalyst, bonding the crack faces. Our fracture experiments yield as much as 75% recovery in toughness, and we expect that our approach will be applicable to other brittle materials systems (including ceramics and glasses).
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We thank A. Ben-Kish, J. Bollinger, J. Britton, N. Gisin, P. Knight, P. Kwiat and I. Percival
for useful discussions and comments on the manuscript. This work was supported by the
US National Security Agency (NSA) and the Advanced Research and Development
Activity (ARDA), the US Of®ce of Naval Research, and the US Army Research Of®ce. This
paper is a contribution of the National Institute of Standards and Technology and is not
subject to US copyright.
Correspondence and requests for materials should be addressed to D.J.W.
letters to nature
VOL 409
15 FEBRUARY 2001
Autonomic healing of
polymer composites
S. R. White*, N. R. Sottos
, P. H. Geubelle*, J. S. Moore
, M. R. Kessler
S. R. Sriram
, E. N. Brown
& S. Viswanathan*
* Department of Aeronautical and Astronautical Engineering,
of Theoretical and Applied Mechanics,
Department of Chemistry, University of
Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
Structural polymers are susceptible to damage in the form of
cracks, which form deep within the structure where detection is
dif®cult and repair is almost impossible. Cracking leads to
mechanical degradation
of ®bre-reinforced polymer com-
posites; in microelectronic polymeric components it can also
lead to electrical failure
. Microcracking induced by thermal
and mechanical fatigue is also a long-standing problem in poly-
mer adhesives
. Regardless of the application, once cracks have
formed within polymeric materials, the integrity of the structure
is signi®cantly compromised. Experiments exploring the concept
of self-repair have been previously reported
, but the only
successful crack-healing methods that have been reported so far
require some form of manual intervention
. Here we report a
structural polymeric material with the ability to autonomically
heal cracks. The material incorporates a microencapsulated heal-
ing agent that is released upon crack intrusion. Polymerization of
the healing agent is then triggered by contact with an embedded
catalyst, bonding the crack faces. Our fracture experiments yield
as much as 75% recovery in toughness, and we expect that our
approach will be applicable to other brittle materials systems
(including ceramics and glasses).
Figure 1 illustrates our autonomic healing concept. Healing is
accomplished by incorporating a microencapsulated healing agent
and a catalytic chemical trigger within an epoxy matrix. An
approaching crack ruptures embedded microcapsules, releasing
healing agent into the crack plane through capillary action. Poly-
merization of the healing agent is triggered by contact with the
embedded catalyst, bonding the crack faces. The damage-induced
triggering mechanism provides site-speci®c autonomic control of
repair. An additional unique feature of our healing concept is the use
of living (that is, having unterminated chain-ends) polymerization
catalysts, thus enabling multiple healing events. Engineering this
self-healing composite involves the challenge of combining polymer
science, experimental and analytical mechanics, and composites
processing principles.
We began by analysing the effects of microcapsule geometry
and properties on the mechanical triggering process. For example,
capsule walls that are too thick will not rupture when the crack
approaches, whereas capsules with very thin walls will break during
processing. Other relevant design parameters are the toughness and
the relative stiffness of the microcapsules, and the strength of
the interface between the microcapsule and the matrix. Micro-
mechanical modelling with the aid of the Eshelby±Mura equivalent
inclusion method
has been used to study various aspects of the
complex three-dimensional interaction between a crack and a
microcapsule. An illustrative result from these studies is presented
in Fig. 2a, which shows the effect of the relative stiffness of the
microcapsule on the propagation path of an approaching crack.
The crack, the sphere and the surrounding matrix are subjected to
a far-®eld tensile loading, j
, perpendicular to the crack plane.
healing agent
Healing agent
Figure 1 The autonomic healing concept. A microencapsulated healing agent is
embedded in a structural composite matrix containing a catalyst capable of polymerizing
the healing agent. a, Cracks form in the matrix wherever damage occurs; b, the crack
ruptures the microcapsules, releasing the healing agent into the crack plane through
capillary action; c, the healing agent contacts the catalyst, triggering polymerization that
bonds the crack faces closed.
© 2001 Macmillan Magazines Ltd
letters to nature
VOL 409
15 FEBRUARY 2001
| 795
As apparent from the j
stress distribution in the equatorial plane
of the sphere in Fig. 2a, the stiffness of the sphere relative to the
matrix strongly affects the stress state in the proximity of the crack
tip and around the sphere itself. In the case of a stiffer inclusion, the
stress ®eld in the immediate vicinity of the crack tip shows an
asymmetry that indicates an undesirable tendency of the crack to be
de¯ected away from the inclusion. The situation is reversed in the
case of a more compliant spherical inclusion, and the crack is
attracted toward the microcapsule, a necessary condition for its
rupture and the triggering of the healing process.
Observations by optical and scanning electron microscopy con-
®rmed the healing concept in Fig. 1 and substantiated the main
®ndings from analytical studies in Fig. 2a. The time sequence of
optical images in Fig. 2b shows the rupture of an embedded
microcapsule ®lled with a red dye and the subsequent release of
the healing agent into the crack plane. The scanning electron
–1 0
–1 0
E*= 3
Figure 2 Rupture and release of the microencapsulated healing agent. a, Stress state in
the vicinity of a planar crack as it approaches a spherical inclusion embedded in a linearly
elastic matrix and subjected to a remote tensile loading perpendicular to the fracture
plane. The left and right ®gures correspond to an inclusion three times stiffer
(E * E
3) and three times more compliant (E * 1=3) than the
surrounding matrix, respectively. The Poisson's ratios of the sphere and matrix are equal
(0.30). b, A time sequence of video images shows the rupture of a microcapsule and the
release of the healing agent. A red dye was added for visualization. The elapsed time from
the left to right image is 1/15 s. Scale bar, 0.25 mm. c, A scanning electron microscope
image shows the fracture plane of a self-healing material with a ruptured urea-
formaldehyde microcapsule in a thermosetting matrix.
Crosslinked polymer
Grubbs' catalyst
1,400 1,200 1,000 800
Wavenumber (cm
100 µm
Figure 3 Chemistry of self-healing. a, Ruthenium-based Grubbs' catalyst initiates
ring-opening metathesis polymerization (ROMP) of dicyclopentadiene (DCPD).
b, Environmental ESEM micrograph and infrared analyses. IR spectra correspond to neat
DCPD (top), an authentic sample of poly(DCPD) prepared with Grubbs' catalyst and DCPD
monomer (middle), and poly(DCPD) ®lm formed at the healed interface (bottom). The
highlighted peak at 965 cm
is characteristic of trans double bonds of ring-opened
© 2001 Macmillan Magazines Ltd
letters to nature
VOL 409
15 FEBRUARY 2001
microscope image of the fracture plane in Fig. 2c further illustrates
the rupture process of an embedded microcapsule.
Completion of the self-healing process requires a suitable chem-
istry to polymerize the healing agent in the fracture plane. We
identi®ed the living ring-opening metathesis polymerization
(ROMP) as meeting the diverse set of requirements of the self-
healing system, which includes long shelf life, low monomer
viscosity and volatility, rapid polymerization at ambient conditions,
and low shrinkage upon polymerization. The ROMP reaction
invokes the use of a transition metal catalyst (Grubbs' catalyst)
that shows high metathesis activity while being tolerant of a wide
range of functional groups as well as oxygen and water
. The
reaction polymerizes dicyclopentadiene (DCPD) at room tem-
perature in several minutes to yield a tough and highly cross-
linked polymer network (Fig. 3a). DCPD-®lled microcapsules (50±
200 mm) with a urea-formaldehyde shell were prepared using
standard microencapsulation techniques. The microcapsule shell
provides a protective barrier between the catalyst and DCPD to
prevent polymerization during the preparation of the composite.
In order to test the stability and activity of the catalyst in an epoxy
matrix, solution-state
H-NMR and solid-state
P-NMR spectro-
scopies of the self-healing polymer composite have been performed.
Solutions of Grubbs' catalyst with the epoxy prepolymer and
diethylenetriamine are chemically compatible as revealed by the
characteristic benzylidene resonance at 23.3 p.p.m. Solid state
NMR measurements of a cured epoxy material with Grubbs' catalyst
shows a characteristic signal corresponding to the tricyclohexyl-
phosphine (PCy
) coordinated to the ruthenium metal present in
the catalyst. Solid state
H-NMR spectroscopy of the self-healing
polymer composite also indicates the presence of a liquid DCPD
monomer phase within the cured epoxy matrix.
Evidence of polymerization of the healing agent induced by
damage is provided by environmental scanning electron microscopy
(ESEM) and infrared spectroscopy (Fig. 3b). ESEM micrographs
reveal the presence of a thin polymer ®lm on the fracture surface.
Infrared spectroscopy of this ®lm indicates an absorption at 965 cm
characteristic of the ring-opened product, poly(DCPD). Control
samples in which the catalyst was excluded showed no ability to
polymerize the DCPD monomer, providing evidence that the
embedded catalyst initiated the polymerization of the healing agent.
To assess the crack-healing ef®ciency of these composite
materials, fracture tests were performed using a tapered double-
cantilever beam (TDCB) specimen (Fig. 4). Self-healing composite
and control samples were fabricated. Control samples consisted of:
(1) neat epoxy containing no Grubbs' catalyst or microspheres; (2)
epoxy with Grubbs' catalyst but no microspheres; and (3) epoxy
with microspheres but no catalyst. A sharp pre-crack was created in
the tapered samples by gently tapping a razor blade into a moulded
starter notch. Load was applied in a direction perpendicular to the
precrack (Mode I) with pin loading grips as shown in Fig. 4a. The
virgin fracture toughness was determined from the critical load to
propagate the crack and fail the specimen. After failure, the load was
removed and the crack allowed to heal at room temperature with no
manual intervention. Fracture tests were repeated after 48 hours to
quantify the amount of healing and in all of the healed samples, the
crack propagated along the original (virgin) crack plane. The
intrinsic ability of the healing agent to rebond epoxy is shown by
the upper horizontal dotted line in Fig. 4a which represents the
average fracture load achieved for control samples (1) manually
healed by injecting a mixture of DCPD and Grubbs' catalyst into the
crack plane.
A representative load±displacement curve for a self-healing
composite sample is plotted in Fig. 4a demonstrating recovery of
about 75% of the virgin fracture load. In great contrast, all three
types of control samples showed no healing and were unable to
carry any load upon reloading. A set of four independently prepared
self-healing composite samples showed an average healing ef®ciency
of 60%. When the healing ef®ciency is calculated relative to the
critical load for the virgin, neat resin control (lower horizontal
dotted line in Fig. 4a), a value slightly greater than 100% is achieved.
The average critical load for virgin self-healing samples containing
microspheres and Grubbs' catalyst was 20% larger than the average
value for the neat epoxy control samples, indicating that the
addition of microspheres and catalyst increases the inherent tough-
ness of the epoxy.
Self-healing composites possess great potential for solving some
of the most limiting problems of polymeric structural materials:
microcracking and hidden damage. Microcracks are the precursors
to structural failure and the ability to heal them will enable
structures with longer lifetimes and less maintenance. Filling
microcracks will also mitigate the deleterious effects of environ-
mentally assisted degradation such as moisture swelling and stress
corrosion cracking. Although the potential bene®ts are quite high,
the speci®c composite described here has some practical limitations
on crack-healing kinetics and the stability of the catalyst to environ-
mental conditions.
We have thus developed a new structural polymeric material that
possesses the ability to heal cracks autonomically and recover
0 200 400 600 800
Load (N)
Displacement (µm)
Neat epoxy
Injected DCPD + catalyst
Figure 4 Self-healing ef®ciency in an epoxy polymer. a, Healing ef®ciency is obtained by
fracture toughness testing of tapered double-cantilever beam (TDCB) specimens. The
virgin fracture toughness is determined by propagating the starter crack along the mid-
plane of the specimen. Subsequently, the load is removed and the crack allowed to heal at
room temperature with no manual intervention. The healed fracture toughness is then
measured by retesting the specimen. b, Post-fracture analysis of the specimens revealed
that the healed crack failed in an interfacial manner from one side of the epoxy/poly(DCPD)
interface to the other. The ESEM image in b shows one area of the fracture plane of a
healed specimen in which the poly(DCPD) ®lm is still attached to the interface on the right
side of the image. The ®lm that originally covered the interface on the left side of the image
is found on the opposite mating surface of the specimen.
© 2001 Macmillan Magazines Ltd
letters to nature
VOL 409
15 FEBRUARY 2001
| 797
structural function. Such materials will increase the reliability and
service life of thermosetting polymers used in a wide variety of
applications ranging from microelectronics to aerospace. These
concepts may also be applicable to a broad class of brittle materials
including ceramics and glasses. We expect that the ®eld of self-
healing, although still in its infancy, will evolve beyond the method
presented here until true biomimetic healing is achieved by incorpor-
ating a circulatory system that continuously transports the necessary
chemicals and building blocks of healing to the site of damage.
Preparation of microcapsules by in situ polymerization
In a 600 ml beaker we dissolved urea (0.11 mol, 7.0 g) followed by resorcinol (0.5 g) and
ammonium chloride (0.5 g) in water (150 ml). A 5 wt.% solution of ethylene maleic
anhydride copolymer (100 ml) was added to the reaction mixture and the pH of the
reaction mixture was adjusted to 3.5 using 10% NaOH solution. The reaction mixture was
agitated at 454 r.p.m. and to the stirred solution we added 60 ml of dicyclopentadiene to
achieve an average droplet size of 200 mm. To the agitated emulsion was added 37%
formaldehyde (0.23 ml, 18.91 g) solution and then the temperature of the reaction mixture
was raised to 50 8C and maintained for 2 h. After 2 h, 200 ml of water was added to the
reaction mixture. After 4 h, the reaction mixture was cooled to room temperature and the
microcapsules were separated. The microcapsule slurry was diluted with an additional
200 ml of water and washed with water (3 3 500 ml). The capsules were isolated by
vacuum ®ltration, and air dried. The yield was 80%. Their average size was 220 mm.]
Self-healing epoxy specimen manufacture
The epoxy matrix composite was prepared by mixing 100 parts EPON 828 (Shell
Chemicals Inc.) epoxide with 12 parts DETA (diethylenetriamine) curing agent (Shell
Chemicals Inc.). Self-healing epoxy specimens were prepared by mixing 2.5% (by weight)
Grubbs' catalyst and 10% (by weight) microcapsules with the resin mixture described
above. The resin was then poured into silicone rubber moulds and cured for 24 h at room
temperature, followed by postcuring at 40 8C for 24 h.
TDCB specimens
The TDCB sample was introduced by Mostovoy and co-workers
and is designed so that
the compliance of the specimen changes linearly with crack length during the test. This
tapered geometry enables controlled crack growth across the centre of a brittle sample such
as epoxy. The fracture toughness for TDCB specimens depends only on the applied load
and is independent of the crack length so that K
where a is a function of geometry
and material properties and P
is the critical load at fracture. K
is the experimentally
determined mode-I critical stress intensity factor. A taper angle of 408 was used and a was
measured to be 7,700 m
Quantifying healing ef®ciency
Previous studies of crack healing in thermoplastic polymers quanti®ed healing effects by
comparing the fracture toughness of the virgin material to the fracture toughness
measured after crack closure and healing.
. An ef®ciency of healing was de®ned as the
ratio of the fracture toughness of healed and virgin materials such that h K
where h is the healing ef®ciency.
Received 15 August; accepted 12 December 2000.
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This work has been sponsored by the University of Illinois Critical Research Initiative
Program and the AFOSR Aerospace and Materials Science Directorate. Electron
microscopy was carried out in the Center for Microanalysis of Materials, University of
Illinois, which is supported by the US Department of Energy.
Correspondence and requests for materials should be addressed to S.W.
A chiroselective peptide replicator
Alan Saghatelian, Yohei Yokobayashi, Kathy Soltani & M. Reza Ghadiri
Departments of Chemistry and Molecular Biology and the Skaggs Institute for
Chemical Biology, The Scripps Research Institute, La Jolla, California 92037, USA
The origin of homochirality in living systems is often attributed to
the generation of enantiomeric differences in a pool of chiral
prebiotic molecules
, but none of the possible physiochemical
processes considered
can produce the signi®cant imbalance
required if homochiral biopolymers are to result from simple
coupling of suitable precursor molecules. This implies a central
role either for additional processes that can selectively amplify an
initially minute enantiomeric difference in the starting
, or for a nonenzymatic process by which biopoly-
mers undergo chiroselective molecular replication
. Given that
molecular self-replication and the capacity for selection are
necessary conditions for the emergence of life, chiroselective
replication of biopolymers seems a particularly attractive process
for explaining homochirality in nature
. Here we report that a
32-residue peptide replicator, designed according to our earlier
, is capable of ef®ciently amplifying homochiral
products from a racemic mixture of peptide fragments through
a chiroselective autocatalytic cycle. The chiroselective ampli®ca-
tion process discriminates between structures possessing even
single stereochemical mutations within otherwise homochiral
sequences. Moreover, the system exhibits a dynamic stereochemi-
cal `editing' function; in contrast to the previously observed error
, it makes use of heterochiral sequences that arise
through uncatalysed background reactions to catalyse the pro-
duction of the homochiral product. These results support the idea
that self-replicating polypeptides could have played a key role in
the origin of homochirality on Earth.
Chiroselective ampli®cation refers to an autocatalytic process in
which a homochiral template instructs the synthesis of a homo-
chiral polymer of the same handedness
. Past efforts aimed at
establishing the feasibility of nonenzymatic chiroselective ampli®-
cation, using nucleic acids and their analogues, have been hampered
by lack of template turnover
and/or enantiomeric cross-inhibition
processes in template-directed oligomerization of activated mono-
meric building blocks
The peptide used in the present study was identi®ed using
design and mechanistic principles that are similar to previously
© 2001 Macmillan Magazines Ltd
PPARa were determined by chemical-mediated fluorescence energy transfer assays using
the AlphaScreen Technology from Packard BioScience
. The experiments were conducted
with 5 nM PPARa LBD of biotinylated peptide containing individual motifs (Fig. 3a),
following the manufacturer’s instructions for the hexahistidine detection kit in a buffer
containing 50 mM MOPS, pH 7.4, 50 mM NaF, 0.05 mM CHAPS, 0.1 mg ml
serum albumin, and 10 mM dithiothreitol (DTT). The binding signals were detected with
the increasing concentrations of GW6471, and the results from four repeated experiments
were normalized as a percentage of the binding in the absence of GW6471.
The effects of GW6471 on the affinity of the SMRT or N-CoR peptides with purified
PPARa LBD were determined by fluorescence polarization in a buffer containing 10 mM
HEPES, pH 7.4, 0.15 M NaCl, 3 mM EDTA, 0.005% polysorbate-20, 5 mM DTT and 2.5%
DMSO. Varied concentration of PPARa LBD in the presence or absence of 40
M GW6471
were incubated at room temperature with 10 nM of a fluorescein-labelled peptide of N-
CoR2 or SMRT2 (Fig. 3a). The fluorescence polarization values for each concentration of
receptor were determined using a BMG PolarStar Galaxy fluorescence reader with 485 nm
excitation and 520 nm emission filters. The apparent dissociation constant (K
) values
were determined by the binding curves derived from a nonlinear least-squares-fit of the
data for a simple 1:1 interaction.
Mutational analysis of the SMRT co-repressor motif interaction with the PPARa and
LBDs was also performed by fluorescence polarization. To determine the importance
of each amino acid in the SMRT motif for binding to nuclear receptors, SMRT peptides
with alanine substitution at each position were added to inhibit the binding of 1
LBD or 2
MPPARa to the fluorescent N-CoR2 peptide. For the PPARa experiments we
added 10
M GW6471. The inhibition curves were constructed and IC
values were
determined by nonlinear least-squares-fit of the data to a simple 1:1 interaction.
Received 8 November; accepted 12 December 2001.
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We thank B. Wisely and R. Bledsoe for making co-repressor expression constructs in early
crystallization studies; W. Burkart and M. Moyer for protein sequencing; M. Iannone for
compound characterizations; G. Waitt and C. Wagner for mass spectroscopy and amino-
acid content analysis; and J. Chrzas and A. Howard for assistance with data collections at
17-ID. Use of the Advanced Photon Source was supported by the US Department of
Energy, Basic Energy Sciences, and Office of Science. We also thank L. Kuyper and
D. Eggleston for support and critical reading of the manuscript.
Competing interests statement
The authors declare that they have no competing financial interests.
Correspondence and requests for materials should be addressed to H.E.X.
(e-mail: The Protein Data Bank code for the PPARa/GW6471/SMRT
complex and the PPARa/GW409544/SRC-1 complex is 1KQQ and 1K7L, respectively.
Autonomic healing of polymer
S. R. White, N. R. Sottos, P. H. Geubelle, J. S. Moore, M. R. Kessler,
S. R. Sriram, E. N. Brown & S. Viswanathan
Nature 409, 794797 (2001).
In this Letter, the middle infrared spectrum in Fig. 3b, correspond-
ing to an authentic sample of poly(DCPD) prepared with Grubbs’
catalyst and DCPD monomer, was a duplicate of the top spectrum
owing to a formatting error. The corrected spectra are shown
below. A
1,400 1,200 1,000 800
Wavenumber (cm
letters to nature
NATURE | VOL 415 | 14 FEBRUARY 2002 | 817
© 2002 Macmillan Magazines Ltd
... Ideally, the shell needs to be ductile to endure the harsh concrete mixing conditions and brittle to rupture upon concrete cracking. Different shell materials, including glass [5,[21][22][23][24][25][26][27][28][29], ceramics [21,29], and polymers [2][3][4]9,12,[14][15][16]18,19,[30][31][32][33][34][35][36][37][38][39][40][41], have been investigated and tested in the literature. Test results revealed that glass and ceramics have low survivability during mixing [42,43], whereas polymers have "switchable" mechanical properties, with a higher survival ratio [13,[44][45][46][47]. ...
Full-text available
Autonomous healing is a very promising technique in self-healing concrete systems. For capsules to achieve their anticipated performance, they should be able to survive the harsh mixing conditions of concrete, yet rupture upon concrete cracking. At present, there are no standard test methods, either experimental or analytical, for determining the capsule survival rate during concrete mixing. This study investigates the correlation between the capsules’ shell properties, concrete rheological properties, the capsules’ external forces, and capsule survival rate during concrete mixing. Finite element and statistical modeling techniques were employed to evaluate the capsule performance and predict the survival rate of capsules during concrete mixing, with 68% confidence. The results revealed that the capsules’ survivability during concrete mixing is highly influenced by the capsule’s radius-to-thickness ratio, the rheological properties of the fresh concrete, the average-paste-thickness (APT) of the concrete mix, the aggregate content and angularity, and the speed of the mixer. In brief, capsules with a radius-to-thickness ratio between 30 and 45 are likely to survive concrete mixing and yet still rupture upon concrete cracking.
... Concerning epoxy resins employed for structural composites, the main approach, proposed to overcome the non-trivial issue of poor impact damage resistance, has been the integration into the polymeric matrix of auto-repair functionality through the employment of different strategies, such as those based on the storage of a healing agent within microcapsules [4][5][6][7][8][9][10][11][12] and vascular networks [13,14] or those inspired by supramolecular chemistry [15][16][17][18][19]. ...
Full-text available
This paper undertakes the thermal and electrical characterization of three commercial unsaturated polyester imide resins (UPIR) to identify which among them could better perform the insulation function of electric motors (high-power induction motors fed by pulse-wide modulation (PWM) inverters). The process foreseen for the motor insulation using these resins is Vacuum Pressure Impregnation (VPI). The resin formulations were specially selected because they are one-component systems; hence, before the VPI process, they do not require mixing steps with external hardeners to activate the curing process. Furthermore, they are characterized by low viscosity and a thermal class higher than 180 °C and are Volatile Organic Compound (VOC)-free. Thermal investigations using Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) techniques prove their excellent thermal resistance up to 320 °C. Moreover, impedance spectroscopy in the frequency range of 100 Hz–1 MHz was analyzed to compare the electromagnetic performance of the considered formulations. They manifest an electrical conductivity starting from 10−10 S/m, a relative permittivity around 3, and a loss tangent value lower than 0.02, which appears almost stable in the analyzed frequency range. These values confirm their usefulness as impregnating resins in secondary insulation material applications.
Van der Waals-driven self-healing in copolymers with "lock-and-key" architecture has emerged as a concept to endow engineering-type polymers with the capacity to recover from structural damage. Complicating the realization of "lock-and-key"-enabled self-healing is the tendency of copolymers to form nonuniform sequence distributions during polymerization reactions. This limits favorable site interactions and renders the evaluation of van der Waals-driven healing difficult. Here, methods for the synthesis of lock-and-key copolymers with prescribed sequence were used to overcome this limitation and enable the deliberate synthesis of "lock-and-key" architectures most conducive to self-healing. The effect of molecular sequence on the material's recovery behavior was evaluated for the particular case of three poly(n-butyl acrylate/methyl methacrylate) [P(BA/MMA)] copolymers with similar molecular weights, dispersity, and overall composition but with different sequences: alternating (alt), statistical (stat), and gradient (grad). They were synthesized using atom transfer radical polymerization (ATRP). Copolymers with alt and stat sequence displayed a 10-fold increase of recovery rate compared to the grad copolymer variant despite a similar overall glass transition temperature. Investigation with small-angle neutron scattering (SANS) revealed that rapid property recovery is contingent on a uniform microstructure of copolymers in the solid state, thus avoiding the pinning of chains in glassy MMA-rich cluster regions. The results delineate strategies for the deliberate design and synthesis of engineering polymers that combine structural and thermal stability with the ability to recover from structural damage.
Although self-healing elastomers have been developed in a great breakthrough, it is still a challenge to develop one kind of material that can respond to the fracture instantly even though this characteristic plays an essential role in emergency circumstances. Herein, we adopt free radical polymerization to construct one polymer network equipped with two weak interactions (dipole-dipole interaction and hydrogen bonding). The elastomer we synthesized has a high self-healing efficiency (100%) and a very short healing time (3 min) in an air atmosphere, and it can also self-heal in seawater, showing an ideal healing efficiency of >80%. Additionally, on account of its high elongation (>1000%) and antifatigue capacity (no rupture after loading-unloading 2000 times), the elastomer can be utilized in a wide range of applications, including e-skin and soft robot fields.
Le but de ce travail de thèse a été d’explorer de nouveaux concepts et des molécules innovantes pour développer différentes techniques de polymérisation ou différents matériaux visant l’impression 3D/4D comme application finale. Pour cela, trois principaux axes de recherche ont été définis. Tout d’abord, des recherches ont été menées pour le décalage des longueurs d’onde d’irradiation vers le proche-infrarouge (NIR), justifié principalement par une plus grande pénétration de la lumière dans les substrats. Pour surmonter la faible énergie apportée au système, le principe d’Upconversion par Annihilation Triplet-Triplet a notamment été étudié, avant de tirer parti des propriétés photothermiques (conversion de la lumière incidente en énergie thermique) de différentes molécules (e.g. cyanines ou complexes photo-sensibilisateurs) sous ce type d’irradiation. Dans un deuxième temps, des stratégies complémentaires ont été étudiées pour accéder à une polymérisation en profondeur (ou pour des échantillons présentant des zones d’ombres) : la polymérisation frontale photo-amorcée et la chimiluminescence. Enfin, les réseaux dynamiques basés sur des liaisons covalentes et supramoléculaires ont été étudiés au travers de deux molécules synthétisées par des laboratoires partenaires, l’une présentant des liaisons de type imine et hydrogène (avec l’eau et la température étudiés comme stimuli) et l’autre des liaisons de type azo photoisomérisables.
The investigation of the crack propagation behavior and fatigue-life extension was conducted by using a tapered double cantilever beam (TDCB) specimen consisting of neat epoxy with diethylene triamine (DETA) as a curing agent, epoxy-DETA with 15% dicyclopentadiene (DCPD) and Grubb’s catalyst, and epoxy-DETA with 15% ethyl phenyl acetate (EPA) microcapsules. Both the epoxy-DETA with EPA microcapsules and epoxy-DETA with DCPD and Grubb’s catalyst demonstrated a significant fatigue life extension. The healing efficiency was found to be about 3500% for TDCB specimen with epoxy-EPA microcapsules and about 410% for a TDCB specimen with DCPD/catalyst microcapsule system, which is about 9 times for epoxy-EPA relative to DCPD/catalyst system. Two healing mechanisms contributed to the fatigue life extension: the reaction between the matrix material and the healing agent and the capillary effect of the liquid healing agent.
For several years, I have been responsible for organizing and teaching in the fall a short course on "Fundamentals of Adhesion: Theory, Practice, and Applications" at the State University of New York at New Paltz. Every spring I would try to assemble the most pertinent subjects and line up several capable lecturers for the course. However, there has always been one thing missing-an authoritative book that covers most aspects of adhesion and adhesive bonding. Such a book would be used by the participants as a main reference throughout the course and kept as a sourcebook after the course had been completed. On the other hand, this book could not be one of those "All you want to know about" volumes, simply because adhesion is an interdisciplinary and ever-growing field. For the same reason, it would be very difficult for a single individual, especially me, to undertake the task of writing such a book. Thus, I relied on the principle that one leaves the truly monumental jobs to experts, and I finally succeeded in asking several leading scientists in the field of adhesion to write separate chapters for this collection. Some chapters emphasize theoretical concepts and others experimental techniques. In the humble beginning, we planned to include only twelve chapters. However, we soon realized that such a plan would leave too much ground uncovered, and we resolved to increase the coverage. After the book had evolved into thirty chapters, we started to feel that perhaps our mission had been accomplished.
This book stems from a course on Micromechanics that I started about fifteen years ago at Northwestern University. At that time, micromechanics was a rather unfamiliar subject. Although I repeated the course every year, I was never convinced that my notes have quite developed into a final manuscript because new topics emerged constantly requiring revisions, and additions. I finally came to realize that if this is continued, then I will never complete the book to my total satisfaction. Meanwhile, T. Mori and I had coauthored a book in Japanese, entitled Micromechanics, published by Baifu-kan, Tokyo, in 1975. It received an extremely favorable response from students and re­ searchers in Japan. This encouraged me to go ahead and publish my course notes in their latest version, as this book, which contains further development of the subject and is more comprehensive than the one published in Japanese. Micromechanics encompasses mechanics related to microstructures of materials. The method employed is a continuum theory of elasticity yet its applications cover a broad area relating to the mechanical behavior of materi­ als: plasticity, fracture and fatigue, constitutive equations, composite materi­ als, polycrystals, etc. These subjects are treated in this book by means of a powerful and unified method which is called the 'eigenstrain method. ' In particular, problems relating to inclusions and dislocations are most effectively analyzed by this method, and therefore, special emphasis is placed on these topics.
In this paper the possible role of chain entanglements in stress transfer is discussed. By the new method of crack healing, chain diffusion coefficients down to 1 multiplied by 10** minus **2**2 m**2/s have been determined. On the basis of these results it can be estimated that the full short-time stresses are transferred across an interface in PMMA and SAN (of M//w equals 130 000) once the molecular coils have penetrated by about 10 nm into the opposite matrix. A description of the healing process is also given in terms of the reptation model, leading to a tube diffusion coefficient of D//t(390 K) equals 32. 5 multiplied by 10** minus **2 **0 m**2 s and to curvilinear diffusion distances of 200 nm at the moment of full apparent crack healing.
The reactions of RuCl2(PPh3)3 with a number of diazoalkanes were surveyed, and alkylidene transfer to give RuCl2(=CHR)(PPh3)2 (R = Me (1), Et (2)) and RuCl2(=CH-p-C6H4X)(PPh3)2 (X = H (3), NMe2 (4), OMe (5), Me (6), F (7), Cl (8), NO2 (9)) was observed for alkyl diazoalkanes RCHN2 and various para-substituted aryl diazoalkanes P-C6H4XCHN2. Kinetic studies on the living ring-opening metathesis polymerization (ROMP) of norbornene using complexes 3-9 as catalysts have shown that initiation is in all cases faster than propagation (ki/kp = 9 for 3) and that the electronic effect of X on the metathesis activity of 3-9 is relatively small. Phosphine exchange in 3-9 with tricyclohexylphosphine leads to RuCl2=CH-P-C6H4X)(PCy3)2 10-16, which are efficient catalysts for ROMP of cyclooctene (PDI = 1.51-1.63) and 1,5-cyclooctadiene (PDI = 1.56-1.67). The crystal structure of RuCl2(=CH-p-C6H4Cl)(PCy3) 2 (15) indicated a distorted square-pyramidal geometry, in which the two phosphines are trans to each other, and the alkylidene unit lies in the Cl-Ru-Cl plane. The benzylidenes RuCl2-(=CHPh)(PR3)2 (R = Cy (cyclohexyl) (10), Cp (cyclopentyl) (17), i-Pr (18)) are quantitatively available via one-pot synthesis with RuCl2(PPh3)2, PhCHN2, and PR3 as reaction components. 10 is an efficient catalyst for metathesis of acyclic olefins: On reaction with excess ethylene, the methylidene complex RuCl2(=CH2)(PCy3)2 (19) is formed quantitatively, and various alkylidene compounds RuCl2(=CHR)(PCy3)3 (R = Me (20), Et (21), n-Bu (22)) are isolated as the kinetic products from the reaction of 10 with an excess of the corresponding terminal or disubstituted olefins. Metathesis of conjugated and cumulated olefins with 10 results in the formation of vinylalkylidene and vinylidene complexes, as shown by the synthesis of RuCl2=CHCH=CH2)(PCy3)2 (23) and RuCl2(=C=CH2)(PCy3)2 (24) from 1,3-butadiene or 1,2-propadiene, respectively. Also, functional groups such as -OAc, -Cl, and -OH can be introduced into the alkylidene moiety via cross metathesis with the appropriate alkene.
Using a double torsion loading configuration, crack closure and repropagation was studied in soda-lime-silica, borosilicate, and silica glasses and in an epoxy resin. Closure and repropagation characteristics in a given material were reproducible but depended on the environment, and in some cases the strain energy release rate required to repropagate a closed crack increased with time and heat treatment. Water from the environment appears to play a central role in this process.
The nuclear receptor PPARγ/RXRα heterodimer regulates glucose and lipid homeostasis and is the target for the antidiabetic drugs GI262570 and the thiazolidinediones (TZDs). We report the crystal structures of the PPARγ and RXRα LBDs complexed to the RXR ligand 9-cis-retinoic acid (9cRA), the PPARγ agonist rosiglitazone or GI262570, and coactivator peptides. The PPARγ/RXRα heterodimer is asymmetric, with each LBD deviated ∼10° from the C2 symmetry, allowing the PPARγ AF-2 helix to interact with helices 7 and 10 of RXRα. The heterodimer interface is composed of conserved motifs in PPARγ and RXRα that form a coiled coil along helix 10 with additional charge interactions from helices 7 and 9. The structures provide a molecular understanding of the ability of RXR to heterodimerize with many nuclear receptors and of the permissive activation of the PPARγ/RXRα heterodimer by 9cRA.