Rapid, Photomediated Healing of Hexaarylbiimidazole-Based Covalently Cross-Linked Gels

Article abstract of DOI:10.1021/acs.chemmater.7b02640

The intrinsic healing of covalently cross-linked polymer networks is commonly effected via the utilization of backbone-borne functional groups able to reversibly cleave or rearrange, thereby enabling mixing and coreaction of network strands that bridge co

Full text of DOI:10.1021/acs.chemmater.7b02640

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Article
Rapid, photo-mediated healing of hexaarylbiimidazole-
based covalently cross-linked gels
Dowon Ahn, Scott R. Zavada, and Timothy F Scott
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b02640 • Publication Date (Web): 24 Jul 2017
Downloaded from http://pubs.acs.org on July 24, 2017
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Rapid, photo-mediated healing of hexaarylbiimidazole-based cova-
lently cross-linked gels
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Dowon Ahn,† Scott R. Zavada,† and Timothy F. Scott*,†,‡
† Macromolecular Science and Engineering, University of Michigan, Ann Arbor, MI 48109, United States
‡ Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, United States
ABSTRACT: The intrinsic healing of covalently crossꢀlinked polymer networks is commonly effected via the utilization
of backboneꢀborne functional groups able to reversibly cleave or rearrange, thereby enabling mixing and coꢀreaction of
network strands that bridge contacted interfaces; however, such materials often exhibit slow healing rates and are susꢀ
ceptible to creep under load. To address these deficiencies, we incorporated hexaarylbiimidazole (HABI) functionalities,
groups that are homolytically cleavable to yield relatively low reactivity lophyl radicals under UV or visible light irradiaꢀ
tion and which, in the absence of light, spontaneously recombine without significantly participating in deleterious side
reactions, into the backbone of poly(ethylene glycol)ꢀbased polymeric gels. Whereas the network connectivity of these
HABIꢀincorporating gels was stable in the dark, they exhibited significant creep upon irradiation. The influence of
swelling solvent on the reaction kinetics of backboneꢀborne HABI photolysis and lophyl radical recombination were
examined and revealed that gels swollen with 1,1,2ꢀtrichloroethane (TCE) exhibited higher radical concentrations than
those swollen with either acetonitrile or water under equivalent irradiation conditions, attributable to the relative solꢀ
vent affinity for the hydrophobic HABI functionalities affording more rapid HABI cleavage and slower radical recombiꢀ
nation rates in TCE than in water. The fastest healing rates for cleaved samples brought into contact and irradiated with
visible light was observed for TCEꢀswollen gels, although rapid restoration of mechanical integrity was achieved for gels
swollen with any of the solvents examined where tensile strengths approached those of the pristine materials after 1 to 3
minutes of light exposure.
bond rearrangement, but the rearrangement reaction
should ideally occur on a sufficiently long timescale to
allow segmental diffusion and mixing to quickly bridge
the fracture surfaces and effect rapid healing of the damꢀ
aged region.
INTRODUCTION
Significant research attention has been devoted towards
reversible crackꢀhealing and autonomic healing of crossꢀ
linked polymeric matrices in recent years owing to the
potential for successful implementations to yield treꢀ
mendous improvements in reconfigurability, longevity,
and reliability of these materials.1ꢀ3 To date, crackꢀ
healing approaches in covalently crossꢀlinked polymers
typically utilize either systems that incorporate nonꢀload
bearing, liquidꢀfilled inclusions, such as capsules or
channels, whose contents flow upon rupture and polyꢀ
merize in situ to bridge the flaw interfaces,4ꢀ6 or that inꢀ
corporate dynamic covalent functional groups, moieties
able to be rearranged or reverted back to their constituꢀ
ent reactants under specific reaction conditions, in their
backbone to effect transient depolymerization or bond
rearrangement.7 Unfortunately, despite their utility, the
incorporation of continuouslyꢀactive dynamic covalent
chemistries in the network strands of a crossꢀlinked polꢀ
ymer often results in creep under mechanical loads at
ambient temperature,8ꢀ12 a significantly deleterious trait
for stressꢀbearing materials. Additionally, the healing
rates typically exhibited by these materials are relatively
slow, where the healing processes proceed over the
course of hours to days.11ꢀ14 More desirable would be the
incorporation of a dynamic chemistry that only proceeds
upon application of a specific stimulus, mitigating the
potential for creep. Moreover, achieving intrinsic crack
healing in crossꢀlinked polymers necessitates not only
A commonly employed stimulus to afford an intrinsic
healing ability in crossꢀlinked polymers is heat, where
the incorporation of thermallyꢀreversible adducts in
network strands provides a mechanism for bond cleavꢀ
age and rearrangement at raised temperatures which, if
above the material’s glass transition temperature, conꢀ
currently affords the molecular mobility necessary to
bridge defects and effect healing.15 Although the typically
poor spatial and temporal reaction confinement of this
healing stimulus can be addressed in part by phototherꢀ
mal heating,16, 17 enabling heating to be confined to the
vicinity of the defect in the polymer network, intolerance
of the raised temperatures necessary for bond rearꢀ
rangement in these materials precludes their consideraꢀ
tion for many applications.18 In contrast, the incorporaꢀ
tion of functional groups that undergo triggered, reversiꢀ
ble cleavage or bond rearrangement reactions at ambient
temperatures would afford healable materials with
broader utility.
Unlike thermoreversible networks, photoreversible netꢀ
works only undergo bond rearrangement upon irradiaꢀ
tion, otherwise exhibiting little to no creep or adaptation
in the absence of irradiation. Furthermore, photochemiꢀ
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cal approaches enable threeꢀdimensional spatial control
of the healing reaction as well as the ability to remotely
trigger a process on and off. Several approaches to phoꢀ
tochemically triggered rearrangeable polymer networks
have been developed, including photoinduced cyclization
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and
photoꢀmediated
metathesis,
among
other
20
reactions.19, The photocyclization reactions, exhibited
by functional groups such as cinnamate,21, 22 coumarin,23ꢀ
25 and anthracene26, 27 derivatives, exhibit limited relative
response rates as at most a single crossꢀlink is reversibly
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broken for each absorbed photon at relatively low quanꢀ
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tum yields,28,
and require irradiation at ultraviolet
Scheme 1. Mechanism and structures of materials used. (a)
Reversible photocleavage of crossꢀlinked gels incorporating
HABI moieties upon visible light irradiation to afford lophyl
radicals, the recombination of which proceeds over several
minutes, is visualized as a yellow/teal color change. (b) Dialꢀ
kynyl monomers 2,2′ꢀbisꢀ(2ꢀchloroꢀ4ꢀhexyl hexꢀ5ꢀynoateꢀ
phenoxy)ꢀ4,4′,5,5′ꢀtetraphenylꢀ1,2′ꢀbiimidazole (HHyꢀHABI)
and bisphenol A dipropargyl ether (BADPE), and a tetraꢀazide
monomer (PEG tetraꢀazide).
(UV) wavelengths to realize both the forward and reverse
reactions.30 Conversely, photoꢀmediated metathesis reꢀ
actions are effected in covalently crossꢀlinked polymer
networks by incorporating functional groups which unꢀ
dergo a network rearrangement cascade, such as disulꢀ
fides,31 thiuram disulfides,13 allyl sulfides,32, 33 and trithiꢀ
ocarbonates,34 where multiple reversible bond breaking
and reꢀforming reactions occur for each absorbed phoꢀ
ton.33 However, as the chain transfer mechanism does
not afford any significant fraction of network strands
EXPERIMENTAL SECTION
Materials. Fourꢀarm PEG tetraꢀazide (molecular weight
= 10 kg—molꢀ1, Creative PEGWorks), copper(II) sulfate
pentahydrate (SigmaꢀAldrich), (+)ꢀsodium Lꢀascorbate
being cleaved at a given instant,32, 33 this approach again
34
results in sluggish healing rates13,
owing to limited
segmental diffusion. Several other reactions employed
for the photoꢀmediated repair of crossꢀlinked polymers
include the photodissociation of alkoxyamine groups,35
radicalꢀmediated polyureaꢀtoꢀpolyurethane conversion
(SigmaꢀAldrich),
and
2,2ꢀdiphenylꢀ1ꢀpicrylhydrazyl
(DPPH, SigmaꢀAldrich) were used as received. Acetoniꢀ
trile (SigmaꢀAldrich), dimethylformamide (DMF, Fisher
Chemical), and 1,1,2ꢀtrichloroethane (TCE, Acros Organꢀ
ics) were used as solvent for all samples. 2,2′ꢀBis(2ꢀ
chloroꢀ4ꢀhexanꢀ1ꢀolꢀphenoxy)ꢀ4,4′,5,5′ꢀtetraphenylꢀ1,2′ꢀ
biimidazole (1) was synthesized as described
previously,42 whereas the dialkynyl monomers were synꢀ
thesized as described below. Structures of all monomers
used in this study are shown in Scheme 1b.
in
oxetaneꢀ
and oxolaneꢀsubstituted chitosanꢀ
polyurethane networks,36, 37 and Si–O–Si covalent bond
reformation mediated by Cu–O coordination complexꢀ
es,38 each of which proceeds under UV irradiation for
multiple hours.
Here, we describe an approach to address the deficienꢀ
cies of contemporary, intrinsicallyꢀhealable crossꢀlinked
polymers by incorporating hexaarylbiimidazole (HABI)
functionalities, dimers of triphenylꢀsubstituted imidazꢀ
oles,39 in the backbone of polymeric gels. HABIs are phoꢀ
tosensitive groups that undergo reversible, homolytic
cleavage of the carbonꢀnitrogen (C–N) bond between the
imidazole rings to efficiently afford two tealꢀcolored
2,4,5ꢀtriarylimidazoyl (lophyl) radicals which thermally
recombine to reproduce the original imidazole dimer
(see Scheme 1a).40 With some notable exceptions (e.g.,
nitroxides), organic radicals are typically highly reactive
species that recombine at diffusionꢀcontrolled rates;41 in
contrast, the lophyl radicals generated by HABI homolyꢀ
sis are insensitive to atmospheric oxygen and show exꢀ
traordinarily slow recombination rates,42 attributable to
their unique chemical structure affording stabilization by
steric hindrance and electron delocalization.43 Thus, owꢀ
ing to the low reactivity and long lifetime of the lophyl
radicals originating from HABI photolysis, crossꢀlinked
polymers bearing HABIꢀcontaining network strands ofꢀ
fer a unique mechanism for intrinsicallyꢀhealable mateꢀ
rials.
Synthesis of 2,2′ꢀbisꢀ(2ꢀchloroꢀ4ꢀhexyl hexꢀ5ꢀ
ynoateꢀphenoxy)ꢀ4,4′,5,5′ꢀtetraphenylꢀ1,2′ꢀ
biimidazole (HHyꢀHABI). A mixture of 1 (1.00 g, 1.12
mmol), 5ꢀhexynoic acid (0.370 mL, 3.36 mmol), N,N′ꢀ
diisopropylcarbodiimide (0.520 mL, 3.36 mmol), and 4ꢀ
dimethylaminopyridine (74.5 mg, 0.610 mmol) in diꢀ
chloromethane (50 mL) was stirred at room temperature
for 16 hours under nitrogen. The solvent was removed
under reduced pressure, and then the crude product was
collected and purified by silica gel chromatography elutꢀ
ing with hexane/ethyl acetate (2/1, v/v) to afford 0.890 g
1
of the dialkyneꢀsubstituted HHyꢀHABI (73.6% yield). H
NMR (400 MHz, DMSOꢀd6), δ: 1.10ꢀ1.19 (m, 4H), 1.36ꢀ
1.40 (m, 6H), 1.65ꢀ1.70 (m, 10H), 1.99ꢀ2.18 (m, 4H),
2.30ꢀ2.38 (m, 4H), 2.77ꢀ2.79 (m, 2H), 3.85ꢀ3.99 (m,
4H), 4.00ꢀ4.07 (m, 4H), 6.20ꢀ7.54 (m, 26H)
Synthesis of bisphenol
A dipropargyl ether
(BADPE). Bisphenol A (5.00 g, 21.9 mmol) and potasꢀ
sium carbonate (9.08 g, 65.7 mmol) were added to 50
mL of DMF and the mixture stirred. Propargyl bromide
solution (80 wt% in toluene, 7.32 mL, 65.7 mmol) was
then added and the mixture heated at 70°C for 16 hours
under nitrogen. After cooling to room temperature, the
mixture was filtered to remove potassium carbonate,
DMF was evaporated under reduced pressure, and the
residue was purified by silica gel chromatography eluting
with hexane/ethyl acetate (2/1, v/v), yielding 5.45 g of
1
bisphenol A dipropargyl ether (81.8% yield). H NMR
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incorporating gels swollen by water, acetonitrile, and
(400 MHz, DMSOꢀd6), δ: 1.59 (s, 6H), 3.50ꢀ3.51 (t, 2H),
4.74ꢀ4.76 (d, 4H), 6.85ꢀ6.89 (m, 4H), 7.11ꢀ7.15 (m, 4H).
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TCE, respectively, the wavelengths where the visible light
absorbance by the generated lophyl radicals was greatest
(i.e., λmax), while the samples were irradiated with 405
nm for 2.5 min to ensure radical concentration equilibraꢀ
tion and then for a further 7.5 min after the light was
turned off. All kinetics experiments were performed in
triplicate.
Synthesis of crossꢀlinked polymeric gels. Dialꢀ
kyneꢀfunctionalized monomer, either HHyꢀHABI (130
mg, 0.12 mmol) or BADPE (36.5 mg, 0.12 mmol), fourꢀ
arm PEG tetraꢀazide (MW=10 kg—molꢀ1, 0.602 g, 0.060
mmol), copper(II) sulfate pentahydrate (5.94 mg,
0.0238 mmol), and (+)ꢀsodium Lꢀascorbate (23.6 mg,
0.119 mmol) were dissolved in N,Nꢀdimethylformamide
(DMF, 4.32 g). Gel films were fabricated by sandwiching
the formulated monomer solutions between glass microꢀ
scope slides separated by 250 ꢁm shims. After polymeriꢀ
zation overnight, the crossꢀlinked polymeric films were
removed from the glass slides and immersed in acetoniꢀ
trile for 24 hours then in water for a further 24 hours to
ensure thorough extraction of residual solvent, copper
catalyst, and reducing agent. Finally, the sample films
were immersed in the desired solvent for 24 hours to
complete the solvent exchange prior to use. For gels
swollen with 1,1,2ꢀtrichloroethane (TCE), samples were
initially immersed in acetonitrile prior to immersion in
TCE owing to the immiscibility of deionized water and
TCE.
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Electron paramagnetic resonance spectroscopy.
Electron paramagnetic resonance (EPR) spectroscopy
was performed with a Bruker EMX spectrometer. The
spectrometer was equipped with a TE102 cavity (Bruker
model ER 4102ST), and a frequency of 9.712 GHz, 2.05
mW microwave power, 5.02 × 104 receiver gain, 100 kHz
modulation frequency, and 1 G modulation amplitude
were used for all experiments. Optical access to the caviꢀ
ty was afforded by a 10 mm × 23 mm grid providing 50%
light transmittance to the sample. For the sample prepaꢀ
ration, a 1.1 mm inner diameter glass capillary tube was
filled with 20 ꢁL of sample formulation and allowed to
react for 24 hours. After polymerization, each sample
was rigorously extracted with acetonitrile and water to
remove residual solvent, copper catalyst, and reducing
agent, then immersed in the desired solvent to complete
the solvent exchange prior to use. Each sample capillary
tube was then inserted into a 3.2 mm inner diameter
quartz sample tube and inserted again into the specꢀ
trometer cavity for analysis. The samples were irradiated
in situ with 405 nm light and spectra were collected after
90 s when the radical concentration had reached steady
state. Kinetics of photoꢀdissociation and dark recombiꢀ
nation was performed by monitoring at a 3453 G static
field, the first derivative signal intensity maximum for
the HABIꢀincorporating gel under irradiation. All experꢀ
iments were performed at room temperature. Radical
concentrations were quantified by calibrating the EPR
spectrum integral against known concentration solutions
of DPPH in three solvents (water, acetonitrile, and TCE)
using the same sample geometry, volume, and acquisiꢀ
tion conditions employed for the HABIꢀincorporating
polymeric gels (see Figure S1).
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Cylindrical gels were fabricated by injecting the formuꢀ
lated monomer solutions described above into 6 mm
inner diameter poly(tetrafluoroethylene) tubing. After
polymerization overnight, approximately 1 cm long secꢀ
tions of tubing were carefully removed from around the
enclosed gels and the cylindrical samples were subjected
to the extraction and solvent exchange steps described
above. The residual copper content in HABIꢀ
incorporating gels digested by nitric acid prior to and
after extraction, determined by inductively coupled
plasma optical emission spectroscopy (ICPꢀOES, Perkinꢀ
Elmer Optima 2000 DV), revealed that the extraction
procedure yielded a fiveꢀfold copper concentration deꢀ
crease.
Light sources and intensity measurement. For the
spectroscopy experiments, violet light was provided by a
collimated, LEDꢀbased illumination source (Thorlabs
M405L2ꢀC) with an emittance centered at 405 nm
(FWHM 13 nm), used in combination with a currentꢀ
adjustable LED driver (Thorlabs LEDD1B) for intensity
control. For the photoꢀmediated healing experiments,
violet light was provided by 405 nm LEDꢀbased illumiꢀ
nation source (Arroyo Instruments, 226 TEC LED Laꢀ
Rheometry. Rheological measurements to monitor the
polymerization of HABIꢀ and bisphenol Aꢀincorporating
crossꢀlinked gels were performed using a TA Instruꢀ
ments ARꢀG2 rheometer equipped with a 60 mm diameꢀ
ter parallel plate geometry. Immediately after preparaꢀ
tion as described in the Experimental Section, 500 ꢁL of
a monomer formulation incorporating either HHyꢀHABI
or BADPE was pipetted onto the lower plate of the rheꢀ
ometer fixture and the upper plate was lowered to
achieve a gap of 300 ꢁm. Time sweep measurements
were recorded at a rate of approximately one point every
9 seconds while applying a strain of 1% at a frequency of
1 Hz.
sermount), used in combination with
a currentꢀ
adjustable LED driver (Arroyo Instruments, 6340 Comꢀ
boSource Laser Diode Controller) and delivered via a
bifurcated light guide. Irradiation intensities were measꢀ
ured with an International Light IL1400A radiometer
equipped with a broadband silicon detector (model
SEL033), a 10× attenuation neutral density filter (model
QNDS1), and a quartz diffuser (model W).
Photoꢀmediated creep tests were performed using the TA
Instruments ARꢀG2 rheometer equipped with a parallel
plate, UVꢀLED curing accessory consisting of a 20 mm
diameter upper plate and a transparent, a quartz bottom
plate, below which an LED array provides irradiation to
the sample at a wavelength of 365 nm. HABIꢀcontaining
crossꢀlinked polymeric gel films were prepared as deꢀ
scribed in the Experimental Section, although 500 ꢁm
Ultravioletꢀvisible spectroscopy. UVꢀvis spectrosꢀ
copy was performed on 250 ꢂm thick film samples using
an Agilent Technologies Cary 60 UVꢀvis spectrophotomꢀ
eter. Spectra were collected from 200 to 800 nm both in
the dark and under irradiation once the radical concenꢀ
tration reached equilibrium. HABI photodissociation
and subsequent recombination were examined by moniꢀ
toring 609 nm, 597 nm, and 604 nm for the HABIꢀ
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thick shims were used to separate the microscope slides, thereby ensuring incorporation of either the HABI or
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immersed in acetonitrile for solvent exchange, and
mounted on the UVꢀLED test fixture. During the measꢀ
urement, no shear stress was applied to the sample from
t = 0 to 1 minute; however, from 1 minute through to the
end of the experiment, a constant shear stress of 100 Pa
was applied. From 3 minutes through 6 minutes, the gel
films gels were subjected to alternating irradiation and
dark periods of 15 and 45 seconds, respectively.
bisphenol A functional groups in the backbone of the
resultant crossꢀlinked, monolithic gels. The progress of
these CuAAC polymerizations were monitored by parallel
plate rheometry (see Figure 1a and 1b), revealing that
gelation, determined here by the crossover in the storage
and loss moduli,48 occurred after approximately 10
minutes and 75 minutes for the HABIꢀ and bisphenol Aꢀ
based formulations, respectively. Although unexpected,
the relatively rapid reaction rate observed for the HABIꢀ
based system is perhaps a result of the imidazole moiety
9
Swelling measurements.
A
15 wt% HABIꢀ
incorporating gel was prepared and its swelling behavior
in water, acetonitrile, and TCE was calculated from the
volume of the polymeric gels in their dry and swollen
states at room temperature. The swelling degree (Q),
defined as the ratio of the volume of absorbed solvent to
that of the dry polymer, was determined using the equaꢀ
tion (eq. 1):
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acting as a ligand for the generated Cu(I) species,49,
increasing its stability and improving its catalytic activiꢀ
ty. Nevertheless, a storage modulus plateau of ~10 kPa
was achieved within 4 hours for both systems, indicating
reaction completion after that time and assuring that the
overnight reaction used to synthesize crossꢀlinked samꢀ
ples was sufficient for full polymerization.
(1)
where Vwet and Vdry are the volumes of the swollen and
dry polymeric gels, respectively. The volume of absorbed
solvent was determined from both the mass change of an
initially dry sample after immersion in solvent for 24
hours in the dark and the density of the solvent.
Photoꢀmediated healing and mechanical testing.
6 mm diameter cylindrical gel samples were sectioned
perpendicular to their axes using a razor blade and the
exposed surfaces were reꢀcontacted immediately. The
samples were then irradiated at room temperature with
405 nm light at 10 mW—cmꢀ2, delivered to the vicinity of
the reꢀcontacted gel surfaces via a bifurcated light guide
to ensure sufficient light penetration through the sample
thickness, for different periods of time.
Pristine or healed polymeric gel samples were deformed
in tension under air at room temperature using an elecꢀ
tromechanical test machine (TestResources 100Q1000)
equipped with a 25 lb load cell (TestResources SMꢀ25ꢀ
294) at a crosshead speed of 0.1 mm—secꢀ1. All experiꢀ
ments were performed in triplicate.
Figure 1. Rheological characterization of covalently crossꢀ
linked gels. The CuAACꢀmediated polymerization between a 4ꢀ
arm PEG tetraꢀazide and either (a) HHyꢀHABI or (b) BADPE in
DMF solution, monitored by oscillatory parallel plate rheomeꢀ
try (storage modulus, solid black line; loss modulus, dashed red
line). The photoꢀinduced creep of gel films, swollen with aceꢀ
tonitrile and incorporating (c) HABI or (d) bisphenol A funcꢀ
tional groups in their backbone, under 100 Pa shear stress (iniꢀ
tially applied at 1 min) and intermittently irradiated with 365
nm light at 0.5 (black), 1 (red), 3 (blue), and 5 (green) mW—cmꢀ2.
The periods of irradiation are indicated by the shaded regions,
and the experiments were performed in triplicate.
RESULTS AND DISCUSSION
The utilization of reactive functional groups such as HAꢀ
BIs to afford intrinsicallyꢀhealable, covalently crossꢀ
linked polymers necessitates the incorporation of these
functionalities in the network backbone. Crossꢀlinked
polymers incorporating HABI functional groups have
been synthesized previously,44ꢀ47 one approach being to
treat polymers bearing triarylꢀsubstituted imidazole
pendant groups, synthesized via a radicalꢀmediated,
chain growth polymerization mechanism, with potassiꢀ
um ferricyanide to oxidize the imidazole groups and
yield HABI crossꢀlinks.44, 46, 47 Notably, this synthetic apꢀ
proach affords crossꢀlinked particles which would reꢀ
quire further treatment to yield monolithic materials. In
contrast, the crossꢀlinked gels examined here were synꢀ
thesized by treating solutions of PEG tetraꢀazide and a
dialkynyl monomer, where the polymerizable moieties
flank either side of the monomer core, in DMF with
CuSO4 and sodium ascorbate to effect polymerization by
Cu(I)ꢀcatalyzed azideꢀalkyne cycloaddition (CuAAC),
As noted above, covalently crossꢀlinked, photoꢀreversible
networks undergo network connectivity rearrangement
exclusively under irradiation and consequently should
undergo plastic deformation upon irradiation while not
exhibiting significant creep in the absence of exposure to
light. To confirm whether the HABIꢀincorporating gels
exhibited this property and thus held promise as photoꢀ
healable materials, gel films swollen with acetonitrile
were mounted in a parallel plate rheometer, equipped
with a UV curing accessory to enable irradiation of the
sample, and subjected to a constant shear stress with
alternating periods of irradiation and darkness (see Figꢀ
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ure 1c and 1d). The initial application of shear stress in absence of light, significant radical formation proceeded
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6
7
8
the dark to the HABIꢀincorporating films resulted in
consistent elastic deformation of the samples with no
discernable creep for the duration of the experiment;
however, significant plastic deformation proceeded imꢀ
mediately upon exposure of the samples to UV irradiaꢀ
tion for all intensities examined (Figure 1c), indicating
that the network rearrangement necessary for the photoꢀ
mediated healing of this material does proceed under
light exposure. Increasing the irradiation intensity afꢀ
forded raised rates and extents of plastic deformation,
attributable to the photolysis reaction equilibrium proꢀ
gressively tending towards the cleaved state (see Scheme
1a), increasing the network rearrangement rate. Notably,
the observed creep under irradiation did not discontinue
immediately upon cessation of light exposure; rather,
although the creep rate did rapidly decrease, plastic deꢀ
formation continued for several seconds after terminatꢀ
ing the irradiation, an effect most apparent at the highest
light intensity examined. This delayed cessation of plasꢀ
tic deformation is indicative of the stability and slow reꢀ
combination of lophyl radicals, resulting in a continued,
though progressively diminished, capacity for network
rearrangement immediately after exposure of the samꢀ
ples to light is ceased. The bisphenol Aꢀbased gel films,
employed here as a lightꢀinsensitive, negative control
material, similarly exhibited constant elastic deforꢀ
mation upon application of shear stress; however, in
contrast to the HABIꢀincorporating films, they did not
exhibit any discernable creep under irradiation (Figure
1d), indicating an absence of photoꢀinduced network
rearrangement in this material.
upon irradiation, the concentration of which increased
with raised irradiation intensity (Figure 2b and S2), esꢀ
tablishing the visible absorbance peaks as attributable to
HABIꢀderived lophyl radical formation.42, 53 In contrast,
no radicals were observed in the bisphenol Aꢀ
incorporating gels under any of the irradiation intensiꢀ
ties at 405 nm examined, irrespective of the solvent emꢀ
ployed to swell the gels (see Figure S3), confirming the
stability of this polymer network under visible light irraꢀ
diation.
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60
To further characterize the influence of solvent and irraꢀ
diation intensity on network strand cleavage of the
HABIꢀincorporating gels, reaction kinetics experiments
to examine photodissociation and lophyl radical recomꢀ
bination rates were performed. Here, HABIꢀ
incorporating gels were subjected to cycles of alternating
light and dark periods at progressively raised irradiation
intensities while monitoring either λmax by UVꢀvis specꢀ
trophotometry (Figure 2c) or the radical concentration
by EPR spectroscopy (Figure 2d). Whereas UVꢀvis specꢀ
trophotometry data were used to examine reaction rate
orders owing to their low noise, data obtained by EPR
spectroscopy were used to determine kinetic rate conꢀ
stants given the direct measurement of radicals afforded
by that technique. As is apparent from this spectroscopic
characterization, the solvent used to swell the gels greatꢀ
ly affected the concentration of lophyl radicals generatꢀ
ed. The equilibrium functional group dissociation in TCE
was approximately double that in acetonitrile and an
order of magnitude greater than that in water (Figure
2d), attributable to a higher affinity of the hydrophobic
HABI groups and, upon photolysis, resultant lophyl radiꢀ
cals for TCE than for acetonitrile or water. This is supꢀ
ported by swelling measurements performed in the dark
where, in contrast to previouslyꢀreported swelling behavꢀ
ior of crossꢀlinked, PEG diacrylate gels which were most
swollen by water and less so by acetonitrile and
toluene,54 the HABIꢀincorporating gels were most swolꢀ
len by, and hence had the highest affinity for, TCE and
least by water (see Figure S4). Notably, since two lophyl
radicals result from every HABI bond broken, the maxiꢀ
mum percentage of cleaved network strands was 0.07,
0.35, and 0.64% for HABIꢀincorporating gels swollen
with water, acetonitrile, and TCE, respectively, under
405 nm light irradiation at 50 mW—cmꢀ2, one to two orꢀ
ders of magnitude greater than contemporary polymer
networks incorporating radicalꢀmediated, dynamic covaꢀ
lent bonds such as those incorporating diarylbibenzoꢀ
furanone (DABBF) groups in their backbone where less
than 0.01% of the DABBF network strands are dissociatꢀ
ed even at 50°C.55 A small decrease in the absorbance at
λmax for the HABIꢀincorporating gels swollen with water
and TCE while under the highest irradiation intensity
investigated indicated that, instead of exclusively particꢀ
ipating in selfꢀrecombination, some of the generated
lophyl radicals were being consumed in an irreversible
side reaction, potentially owing to the susceptibility of
the residual copper azideꢀalkyne cycloaddition catalyst to
radicalꢀmediated reduction from Cu(II) to Cu(I).56
The photoꢀmediated cleavage of HABI moieties within
polymeric gels in response to visible light irradiation at
405 nm was further examined using UVꢀvis spectrophoꢀ
tometry by in situ sample irradiation to determine the
influence of varying incident irradiation intensity and
swelling solvent (see Figure 2). Prior to irradiation, the
HABIꢀincorporating gels exhibited weak absorption tails
extending into the visible spectral region; however, irraꢀ
diation at 405 nm irradiation resulted in a dramatic colꢀ
or change from yellow to teal (see Scheme 1a) and the
emergence of an absorbance peak in the visible region
(λmax = 609, 597, and 604 nm in water, acetonitrile, and
TCE, respectively), which became progressively stronger
with raised irradiation intensity (Figure 2a and S2), atꢀ
tributable to generation of the visible lightꢀabsorbing
lophyl radical.42 Subsequently ceasing irradiation of the
gels resulted in complete reversion of their color from
teal back to yellow and corresponding disappearance of
the visible region absorption peak over the course of sevꢀ
eral minutes. The capacity of the gels in the three solꢀ
vents examined (i.e., water, acetonitrile, and TCE) to
generate radicals in response to 405 nm irradiation was
confirmed by EPR spectroscopy. Whereas studies examꢀ
ining photoinitiator photolysis commonly employ spin
traps51 or the severely restricted molecular mobility at
low temperatures52 to attain detectable radical concenꢀ
trations, the low recombination rates of lophyl radicals
generated by HABI photolysis affords sufficient radical
concentrations
for
facile
detection
at
room
The reversible HABI photocleavage mechanism deꢀ
scribed in Scheme 1a suggests that the lophyl radical reꢀ
combination should proceed as a second order reaction;
temperature.40, 42, 53 Although no radicals were observed
in the EPR spectra of the HABIꢀincorporating gels in the
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Chemistry of Materials
Page 6 of 10
darkness and irradiation with 405 nm light at 1 (2.5 – 5
minutes), 5 (12.5 – 15 minutes), 10 (22.5 – 25 minutes), and 50
(32.5 – 35 minutes) mW—cmꢀ2.
however, we previously established that this reaction can
be described by either 1.5 or second order reaction kinetꢀ
ics, depending on the structure of the parent HABI comꢀ
pound.42 Although fitting of the λmax absorbance curves
(presented in Figure 2c) during lophyl radical recombiꢀ
nation in the dark revealed that lophyl recombination
was well described as a nearꢀsecond order reaction for
HABIꢀincorporating gels swollen with TCE (see Figure
S5), the recombination reaction for acetonitrileꢀ and waꢀ
terꢀswollen gels was better fit by 1.6 and 1.2 order kinetꢀ
ics, respectively (Figure S5). Notably, radical diffusionꢀ
1
2
3
4
5
6
7
8
Table 1. Subꢀsecondꢀorder lophyl radical recombination (krec
)
and first order HABI photodissociation (kdis) rate constants for
photolysis under varying irradiation intensities (I0) with 405
nm light and in the dark of HABIꢀincorporating gels swollen
with varying solvents.
krec × 101
I0 (mW—cmꢀ
Water
Acetonitrile
TCE
9
2
(mMꢀ0.2—secꢀ
(mMꢀ0.6—secꢀ
(mMꢀ0.9—secꢀ
)
10
11
12
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58
59
60
1
1
1
)
)
)
inhibited HABI derivatives have exhibited rapid, first
58
order lophyl radical recombination kinetics.57,
Thus,
5
10
50
1.13 ± 0.03
1.06 ± 0.03
0.94 ± 0.02
1.96 ± 0.04
1.54 ± 0.02
1.36 ± 0.03
kdis (secꢀ1) × 104
Acetonitrile
1.55 ± 0.03
2.19 ± 0.03
6.75 ± 0.13
1.86 ± 0.09
1.45 ± 0.07
1.46 ± 0.05
the respective reaction orders for the recombination of
network backboneꢀborne lophyl radicals suggest that,
whereas the HABI groups for gels swollen with TCE are
not subject to significant diffusion inhibition, HABIs in
gels swollen with acetonitrile or water may be subject to
partial diffusion inhibition, yielding subꢀsecond order
recombination behavior. Thus, despite the structural
similarity between the respective chromophores of HHyꢀ
HABI and 1, a HABIꢀderived compound which affords
radicals that recombine according to second order kinetꢀ
ics,42 we analyzed the HHyꢀHABI photolysis and lophyl
radical recombination results shown in Figure 2d by asꢀ
suming first and subꢀsecond order reactions, respectively
(see Table 1). As expected, the dissociation rate constant
I0 (mW—cmꢀ
2
)
Water
TCE
5
10
50
0.58 ± 0.02
0.81 ± 0.02
1.32 ± 0.03
3.88 ± 0.20
4.96 ± 0.23
14.06 ± 0.53
Having established the solventꢀdependent backbone
cleavage behavior in the presence of light and recombiꢀ
nation in its absence, the photoꢀmediated healing capaꢀ
bility of the HABIꢀincorporating gels was investigated at
room temperature and under aerobic conditions. To
demonstrate this, the exposed surfaces of sectioned gel
cylinders, swollen in one the three solvents examined
(i.e., water, acetonitrile, or TCE) were brought into conꢀ
tact and irradiated with visible light to effect cleavage
and rearrangement of network strands at the interface of
contacted surfaces; these healed materials, as well as the
pristine gels, were subsequently subjected to tensile testꢀ
ing. The exposed region of each HABIꢀincorporating gel
again turned from yellow to teal upon irradiation and
back to yellow upon irradiation cessation (Supplemenꢀ
tary Video S1), confirming the backbone cleavage and
subsequent recombination necessary for healing, whereꢀ
as no lightꢀinduced color change was observed for the
bisphenol Aꢀincorporating gels. Although many backꢀ
boneꢀborne, dynamic functional groups have been emꢀ
ployed to achieve intrinsicallyꢀhealable polymeric gels
and elastomers,59 effective restoration of mechanical
properties upon contacting fracture surfaces can take
several hours or days,13, 14 while more rapid healing rates
afforded by materials with a more dynamic network conꢀ
nectivity are generally accompanied by correspondingly
faster stress relaxation or creep.10 In contrast, by inducꢀ
ing reversible, photoꢀmediated cleavage of network
strands, far more rapid healing rates can be attained in
polymer networks that, in the absence of light, are not
susceptible to creep under load. As shown in Figure 3
and tabulated in Table 2, the healing rate order correꢀ
sponds to the radical under equivalent irradiation condiꢀ
tions, where the HABIꢀincorporating gel with the highest
lightꢀinduced radical concentration (i.e., the TCEꢀ
swollen gel) afforded the most rapid tensile strength reꢀ
covery upon healing while the waterꢀswollen gel recovꢀ
ered its tensile strength slowest. Nevertheless, all of the
HABIꢀincorporating gels had recovered over half of their
tensile strength after only 30 seconds of visible light irꢀ
radiation and were approaching the tensile strength of
k
dis increased with raised irradiation intensity. Moreover,
under equivalent irradiation conditions, kdis was lowest
for water and highest for TCE, attributable to the relative
affinities the HABI functional group had for the examꢀ
ined solvents as described above. Although their dissimiꢀ
lar reaction orders impedes direct comparison of the
recombination rate constants (krec) for each respective
gel, only minor krec variation was observed for each gel as
the irradiation intensity was varied.
0.8
0.6
0.4
0.2
0.0
6000
4000
2000
0
(a)
(b)
-2000
-4000
-6000
350 400 450 500 550 600 650
3440
3460
3480
3500
Wavelength (nm)
Field (Gauss)
(c)
(d)
Figure 2. UVꢀVis and EPR with kinetics. (a) UVꢀvis and (b)
EPR spectra of HABIꢀincorporating gels swollen with acetoniꢀ
trile at equilibrium under irradiation with 405 nm light at 0
(black), 1 (red), 5 (blue), 10 (green), and 50 (magenta) mW—cmꢀ
2. (c) Absorbance at λmax as determined by UVꢀvis spectrophoꢀ
tometry, and (d) lophyl radical concentration and network
strands dissociated as determined by EPR spectroscopy, of
HABIꢀincorporating gels swollen with water (black), acetoniꢀ
trile (red), and TCE (blue) under alternating conditions of
the pristine materials after
3 minutes irradiation,
demonstrating the rapid healing attainable in these maꢀ
terials even under a moderate irradiation intensity. No
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Page 7 of 10
Chemistry of Materials
photoꢀmediated healing was observed for the bisphenol provide a route to achieve subꢀglass transition temperaꢀ
1
2
3
4
5
6
7
8
Aꢀincorporating material upon irradiation at any of the
conditions examined (Figure 3d), establishing that
cleavage of the backboneꢀborne HABI functional group
was responsible for the observed photoꢀmediated healing
of these covalently crossꢀlinked networks.
ture healing of vitrified thermosets.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on
the ACS Publications website.
EPR radical concentration calibration curves, UVꢀvis and
EPR spectra, swelling characterization, and reaction order
plots. (PDF) Video of photoꢀmediated healing of acetoniꢀ
trileꢀswollen HABIꢀincorporating gel. (AVI)
Table 2. Tensile strength (TS) of 15 wt% HABIꢀincorporating
gels prior to and after photoꢀmediated healing.
% recovery of TS after irradiaꢀ
TS of prisꢀ
tion
9
Solvent
Water
tine gel
(kPa)
10
11
12
13
14
15
16
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19
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60
1 miꢀ
nute
3
30 s
minutes
56.2
±1.8
AUTHOR INFORMATION
71.6±1.3
83.9±1.4 94.7±1.0
Corresponding Author
Acetonitrile
TCE
(a)
58.2±1.8
58.6±1.8
65.9±1.7 88.4±1.1
97.6±1.1
>99
67.1±2.0 97.1±1.3
* Eꢀmail: tfscott@umich.edu.
(b)
ORCID
Dowon Ahn: 0000ꢀ0003ꢀ4837ꢀ0038
Timothy F. Scott: 0000ꢀ0002ꢀ5893ꢀ3140
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We gratefully acknowledge funding from the 3M Nontenꢀ
ured Faculty Award, the National Institutes of Health (NIH
U01, award number U01DE023771), the National Aeroꢀ
nautics and Space Administration (NASA Space Technology
Research Fellowship, award number NNX12AM31H), and
the University of Michigan Chapter of Phi Kappa Phi. We
also thank the Putnam group and Dr. Yen Kong for their
assistance with rheometry instrumentation.
(c)
(d)
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