Self-Healing Polymers andComposites

Yufei Wang
Sept. 10th. 2012

１．Capsule-based Self-healing Material
Design cycle

Encapsulation-Type Self-Healing Materials

S.R. White et al. Nature, 2001, 409, 794797

Basic Polymerization Reaction:

In this strategy, when catalyst and healing reagent is loaded by 2.5% and 10% (byweight), the average healing efficiency can reach up to 60% (max 75%).

２．Microvascular Self-Healing Material
Design cycle

Microvascular-Type Self-Healing Materials

Image of skin with a cut inthe epidermis layer

Schematic structure of a
microvascular self-healingmaterialS.R. White et al. Nat. Mater., 2007, 6, 581-585

Skeme for self-healing of this type: Crack formation and attracted tomore compliant region created bymicrovascules.
 Healing reagent wicks into crcak(s)through capillary action.
 Polymerization begins once healing
reagent interacts with catalyst coatedon the surface.
Key points to the design: Maximum channel spacing andminimum channel diameter.

 The channel diamete must be large
enough for healing reagent to flow
into the crack plane.

• Healing reagent and catalyst are
DCPD and Grubbs' catalyst, the same as
first generation.
• Multi-time healing can be achieved
due to refillable microvascular structure• Concentration of catalyst does
not significantly influence theaverage healing efficiency.
•The loading amount of catalyst
has great impact on the number ofsuccessful healing cycles achieved.
Healing cycle

3．Intrinsic Self-Healing Material
Design cycle

3-1. Reversible Covalent Bonding Approach
3-1-1 Employing thermally reversible DA cycloaddition

Characteristics of polymer 3:• Easy to synthesize• Having as good mechanicalproperties.
• Fully transparent

(A) A fully polymerizedsample(B) A sample heated to 145C andthen quenched in liquid nitrogen(C) A repolymerized sample(cooled from 120C to 70C in 24h)Pros and cons of this material system• First report for self-healing material via
reversible covalent bonding

Fig. (A) Healing efficiency
obtained by fracture toughness
testing.
Fig. (B)/(C) A broken specimen
before and after thermal
treatment (120C to 150C undernitrogen for 2h then cooled to r.t.).

X. Chen, F. Wudl et al., Science, 2002, 295, 1698-1702

3-1-2. Employing Photochemical [2+2] Cycloaddition
Characteristics of this material system• Both TCE and TCE contained polymer can beeasily prepared.
• Healing can be achieved by UV radiation(>280nm)•Healing time is very short (2~10min) and multitime healing is possible.

Working scheme Just heating or without TCE,
healing did not proceed.
 Combination of irradiationand heating can give higher
healing efficiency.
 Overall healing efficiency
for this system is low.

C-M Chung et al. Chem. Mater. 2004, 16, 3982-3984

3-1-3. Photo-induced Self-healing through Reshuffling ofTrithiocarbonate
Working scheme
First example that achieves both arepeatable self-healing throughphoto-induced covalent crosslink
formation and macroscopic fusionof separate pieces simultaneously.

Model reaction*MeCN, 0.04M, UV iradiation at r.t.

Preparation of TTC polymer

(A) Repetitive self-healing rxns under
UV in MeCN

(B) Bulk state self-healing
experiments for 48h

Characteristics of this system• Healing can be achieved just by UV
irridiation under r.t.
• First report of macroscopic fusion
through photoinduced covalent bondformation.
• Multi-time healing with high healing
efficiency is possible.

A. Takahara, K. Matyjaszewski, ACIE, 2011, 123, 1698-1701

3-1-4. Self-healing through trigger-free radical-derived dynamic
covalent bondCharacteristics of DABBF• Can reach a thermodynamic
equilibium under r.t. without
special stimuli.
• Its radical species is O2
tolerant.

Working skeme

(a)

(b)

(a) Healing exp. at r.t. under air for 24h.
(b) Stress-strain curves of mendedpolymer over time. (3 samples)

A. Takahara, H. Otsuka, ACIE, 2012,
51, 1138-1142

3-1-5. Self healing through anionic siloxane equilibration
Synthesis of this siloxane system

(1)

(2)

(1) BPO derived oxidatative
coupling of Octamethyl
cyclotetrasiloxane(D4)(2)Polymerization from D4and bis-D4 under act ofbis(tetramethylammonium)oli
godimethylsiloxanediolate(BTAODMS).
* BTAODMS acts as anionicpolymerization initiator.
* Bis-D4 plays as source ofcrosslinkings.

Healing experimentCharacteristics of this system Easily accessible heatingconditions (Heating around 90C)
 Multi-time healing is possible
with almost 100% recovery.
 The monomer is very cheap andinnocuous.

P. Zheng, T.J. McCarthy, JACS, 2012, 134, 2024-2027

3-2. Chain Reentanglement Approach
Basic polymerization reaction(1) Introduction of oxetane(OXE)
group to chitin(CHI)(2) Incorporation of OXE-CHI intotrifunctional hexamethylene
diisocyanate(HDI) in presence ofpolyethyleneglycol(PEG).

Working mechanism(1) Upon mechanicaldamage of the network,
oxetane rings open tocreate two reactive ends.
(2) Chitin chain scission
occurs when exposed to UV.
(3) Above two kinds ofreactive species form newcrosslinks.

Healing experiment (I)IR(upper) and optical(lower) images of OXE-CHI-PUR networks recorded as a
UV exposure time. A1, 0 min; A2, 15 min; A3, 30 min by 120W fluororescent
UV lamp at 302nm wavelength.

Healing experiment (II)(A) HDI:PEG:CHI =1: 1.4: 0.57*10-4

A1, 0 min; A2, 15min; A3, 30 min to
UV irradiation(B) HDI:PEG:OXECHI =1: 1.4: 0.57*10-4
B1, 0 min; B2, 15min; B3, 30 min
to UV irradiation(C) HDI:PEG:OXECHI =1: 1.33: 1.17*10-4
C1, 0 min; C2, 15min; C3, 30 min toUV irradiation

B. Ghosh, M.W. Urban, Science, 2009, 323, 1458-1460

3-３. Noncovalent Bond Approach
3-3-1. Self-healing via metal-ligand interactions
Working mechanism

UV-induced temporary
disengagement inmetal-ligand motifs

Surface rearrangement
and reentanglement ofpolymer chains

Synthesis of this metal-ligands polymer system* La(NTf2)3 is also applicable to this system.

(a) Stress-strain curves ofpolymers of 3 and various
amount of Zn(NTf2)2 orLa(NTf2)3(b) Toughness of polymers of
3 and various amount ofZn(NTf2)2 or La(NTf2)3(c) Stress-strain curves offilms of 3-[Zn(NTf2)2]0.7 of
the original, damaged
and healed examples.
(d) Healing efficiency of filmsof 3 and various amount ofZn(NTf2)2 or La(NTf2)3

S.J. Rowan, C. Weder, Nature, 2011, 472, 334-337

3-3-2. Self-healing through crown ether based host-guest
interaction

MixingGel 4 can beobtained just bymixing 1 and 2
under r.t. in
CHCl3/MeCN (1:1)

Gel 5 is formed
from 1 and 3 byrefluxing in
CHCl3/MeCN(1:1)
for 30d thenstirring at r.t. for45d.

Partial 1H-NMR(c) Polymer 1(b) Mixture of cross-linker 2(3.6 mM) and polymer 1(1.0 mM)(a) Cross-linker 2* "c" and "u" denotecomplexed and uncomplexedmoieties respectively.

Healing experiment for Gel 4

Healing experiment for Gel 5(a/e) Original Gel 4/5(b/f) Right after damage(c/g) After free standing for 2 min(d/h) After free standing for 4 minGel 4 can proceed reversible
sol-gel transition by pH control

Gel 5 can be irreversibly
degraded by TEA.

*Gel 4's healing property is attributed to reversible interlocked
crosslinking between crown ether and bisammonium salt.
*Gel 5's healing property is attributed to electrostatic and hydrogen-bonding
interactions between 1 and 3 but not crosslinking between crown ether andbisammonium salt.

M. Zhang, F. Huang, ACIE, 2012, 51, 7011-7015

3-3-3. Self-healing through electrostatic interaction

Mechanism for hydrogel formation(a) Clay nanosheets(CNSs)
entangled with each other atfirst(b) CNSs are dispersed
homogeneously by interactionof their positived charged edge
parts with sodium
polyacrylate(ASAP).
(c) Exfoliated CNSs are crosslinked to each other by adendritic macromolecule(G3binder) via electrostaticinteraction.

Structure of G3-binder and its "half" analogue

Gn-binder compounds are reported to play as an efficient "molecular glue" via
te interaction of its guanidinium groups with oxyanions on target compounds.

Healing experimentHydrogels with or without
0.01% methylene blue werecut into 7 cuts. Then, placethem one to another.
*(Hydrogel prepared by CNS
3.0%, G3-binder 0.21%,
ASAP 0.09%, water 96.7%)(c) Original heart-shaped
sample of hydrogel(d) Sample after being
immersed for 6h threetimes in fresh THF at r.t.

Stability test for pH, buffer and NaCl aq.

• Hydrogel samples
are prepared by 2.0%
CNS, 0.06% ASAP,
0.15% G3-binder and
0.01% methylene
blue in water.
• Then 3 ml water with
pH= 2.0[a], 4.0[b],6.0[c], 8.0[f], 10.0[g],12.0[h] or phosphatebuffer(pH=7.4, 10mM)[d], or 1M NaClaq.[e]Catalytic activities of myoglobin in hydrogels
Hydrogels were prepared by mixing 2.0%
CNS with 0.04% G3-binder in a 5.0mM
aqueous solution of myoglobin with (green)or without (blue) 0.06% ASAP, and suspened
in phosphate buffer containing ophenylenediamine(10 mM) followed byH2O2 addition.

Characteristics of this system Easy to prepare Stable and very environmentallyfriendly Excellent self-healing and shaperetaining ability Mouldable into various shapeswith satisfying mechanical strength.

T. Aida et al., Nature, 2010, 463, 339-343

3-3-4. Self-healing through hydrogen bond formation (1)Key components & network forming scheme* H-bond acceptors are shown in red, donors in green.

Healing experiment• Cut parts can heal by themselves by just be brought into contact at r.t.

• The higher the healing temperature is, the lower healing efficient is obtained.
• The longer time passes before putting the cutting ends togather, the lower
healing efficient is obtained.

(a) Stress-strain curves of thissupramolecular rubber (Data of 3 samples)(b) Stress-strain curves of sample after
different healing times at 20oC

Characteristics of this system• Very simple healing conditions and free of external energy input• Multi-time healing is possible with high healing efficiency• Low cost of raw ingredients• Easily being synthesized, re-used and recycled

L. Leibler et al. Nature, 2008, 451, 977-980

3-3-4. Self-healing through hydrogen bond formation (2)Working scheme• Unique hard-soft
multiphase systemmerging propertiesof stiff and toughpolymers withdynamic assemblies.

• Dynamic healing
motifs are designed
as the soft phase ina hydrogenbonding brushpolymer (HBP).

Synthesis of HBP material system(1) Styrene is first copolymerizedwith an ATRP co-monomer 4 via
free-radical polymerization.
(2) Following ATRP
polymerization with monomer 1
is carried out to form brushes.
* ATRP stands for "atom transfer
radical polymerization".
Ref. : K. Matyjaszewski et al.
Science, 1996, 272, 866

Healing experiment*Sample cut into completely separate pieces
and then brought togather to heal at room
temperature.

Characteristics of this system
 Very simple healing conditions and freeof external energy input
 Multi-time healing is possible with highhealing efficiency.
 Excellent mechanical properties
 Easily being synthesized, re-used andrecycled

Z. Guan et al. Nat. Chem., 2012, 4, 467

3-3-5. Self-healing through pi-pi stacking and hydrogen-bonding
interactions
Key components in this systemcompound 1

polyimde with a p-electron-deficient bis(diimde) motif

compound 2

polyurethane with p-electron-rich pyrene moiety

compound 3

polyurethane with terminal benzyl group instead of pyrenyl moiety incompound 2, used in control experiment

Working schemep-p stacking and Hbonds get
disengaged atheating, after random
diffusion, newnetwork reforms
when cooling.

Minimized computational model of theinteraction between diimde moieties andpyrenyl group in compound 1 and 2.
• Electronically complementary triple p-stack
was formed.
• A pair of strong, convergent hydrogenbonds from the urea unit to a diimide groupwere formed.

Healing experiment (1)(a) Healing efficiency of network [1+2] asfunction of time(Healing temp is 100C)(b) Recovery of the tensile modulus
for network [1+2] sample overbreak-heal cycles.

Healing experiment (2)* False-color ESEM images at different healing temperature.

S.J. Rowan et al. JACS, 2010, 132, 12051-58

Summary and perspective

 New materials that employ cross category design are expected.

Materials towards more biomimetic manner may come into trend.

3-1-2. Single-Component Thermally Remendable Polymer Network
Working scheme• Retro-DA reactions first take placewhen heating to 120C to generate
di-cyclopentadiene monomer.
• Then heating up to 150C for 10hfollowed by slow cooling to r.t. to
get polymer network.
• Not only dimer but also trimer
formed which enabled cross-linking
formation.

Characteristics of this system: Healing could be achieved byjust heating at 120C.
 Multi-time healing is possible
but only moderate healing efficiency.
 Especially efficient in shaperecovery Single component but with highcross-links.

(A) Polymer specimen rightafter compression testing(B) After healing, identicalshape to the pretest state

E.B. Murphy, F. Wudl et al., Macromolecules, 2008, 41, 5203-5209