Photoactivated Healable Vitrimeric Copolymers

Article abstract of DOI:10.1021/acs.macromol.8b01898

A composite polymer based on vinylogous urethane vitrimer bonding with both photodimerizable and thermally exchangeable functionalities is described. Polymers containing various ratios of photodimerizable diaminoanthracene monomers and thermally exchangeable diaminoalkyl monomers linked by a common bisacetoacetate group are studied. It is demonstrated that alkyl amines undergo the necessary thermal exchange reactions for vinylogous urethane vitrimers, while aromatic amines do not. UV-induced dimerization of the anthracene units results in changes to the rheological properties and the glass transition temperature due to polymer cross-linking. Rapid and near-complete scratch healing upon heating is demonstrated, with a tunable onset temperature for healing controlled by UV irradiation. The viability of a composite vitrimeric system, wherein vitrimeric monomers are combined with non-vitrimeric, stimuli-responsive monomers to generate random copolymers with new properties, is demonstrated.

Full text of DOI:10.1021/acs.macromol.8b01898

Article
pubs.acs.org/Macromolecules
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Photoactivated Healable Vitrimeric Copolymers
Taylor Wright,† Tanja Tomkovic,‡ Savvas G. Hatzikiriakos,‡ and Michael O. Wolf*,†
†Department of Chemistry, 2036 Main Mall, University of British Columbia, Vancouver, BC V6T 1Z1, Canada
‡Department of Chemical and Biological Engineering, 2360 East Mall, University of British Columbia, Vancouver, BC V6T 1Z3,
Canada
S
* Supporting Information
ABSTRACT: A composite polymer based on vinylogous
urethane vitrimer bonding with both photodimerizable and
thermally exchangeable functionalities is described. Polymers
containing various ratios of photodimerizable diaminoanthracene
monomers and thermally exchangeable diaminoalkyl monomers
linked by a common bisacetoacetate group are studied. It is
demonstrated that alkyl amines undergo the necessary thermal
exchange reactions for vinylogous urethane vitrimers, while
aromatic amines do not. UV-induced dimerization of the
anthracene units results in changes to the rheological properties
and the glass transition temperature due to polymer cross-linking. Rapid and near-complete scratch healing upon heating is
demonstrated, with a tunable onset temperature for healing controlled by UV irradiation. The viability of a composite vitrimeric
system, wherein vitrimeric monomers are combined with non-vitrimeric, stimuli-responsive monomers to generate random
copolymers with new properties, is demonstrated.
matrices.15,16 Here, we integrate photodimerizable anthracene
units into a polymer system based on vinylogous urethane
INTRODUCTION
■
Vitrimers are an emerging class of polymers characterized by
the presence of thermally exchangeable bonds.1−3 This
vitrimers, enabling phototuning of a thermally healable system.
This approach opens alternative routes to control the
network rearrangement imparts high mechanical recyclability,
properties of vitrimeric materials. Irradiation of the polymer
an ability to undergo thermal healing, and rapid stress
relaxation not found in conventional cross-linked systems.2,4,5
permits tuning of the glass transition temperature, rheological
behavior, and scratch-healing dynamics without the need for
These systems can also exhibit shape memory effects allowing
for the autonomous arrangement into complex shapes.6,7
compositional modification.
In our approach, a linear vitrimeric system is cross-linked
post-synthetically through the dimerization of anthracene units
Vitrimeric systems have been modified using additives that
respond to a variety of stimuli including IR light, pH, redox
conditions, electrical current, and solvent.8 The kinetics of the
in the backbone of the polymer chain. The inclusion of
aromatic anthracenes and aliphatic alkyl chains results in a
exchange reaction have also been shown to be controllable
using low amounts of a variety of small molecule additives.9
composite polymer system with anthracenes responding to UV
irradiation and the alkyl monomers undergoing vinylogous
The incorporation of functional molecular groups integrated
urethane exchange.
directly into the polymeric backbone of these systems may
allow for the development of multifunctional and responsive
materials, but this has not been explored extensively to date.
The relatively high temperature (>100 °C) at which these
systems are cured in the presence of the amine, ester, and
EXPERIMENTAL SECTION
Materials. All chemical reagents were purchased from commercial
sources and used without further purification. 1,12-Dodecanediol
(99%), 1,12-dodecanediamine (98+%), 2,2,6-trimethyl-1,3-dioxin-4-
■
ketone functional groups that comprise vinylogous urethane
one (94% containing up to 6% acetone), ethyl acetoacetate (99+%),
vitrimers means careful consideration for functional group
and 1,5-diaminoanthraquinone (90+%) were all purchased from Alfa
tolerance is required to incorporate new responsive molecules
into the polymeric chain.
Aesar. 2-Aminoanthracene (96%) and ammonium cerium nitrate
(≥98.5%) were purchased from Sigma-Aldrich.
Analytical Methods. Spectroscopy. 1H NMR spectroscopic data
were collected on a 400 MHz Bruker Avance 400dir spectrometer at
Polymeric systems containing anthracene units have been
reported to undergo self-healing through a reversible 4 + 4
photocycloaddition reaction, allowing monomers and polymer
chains to be linked and unlinked.10−13 The viscoelastic
properties of hydrogels have also been dynamically altered
using UV-induced dimerization of coumarin groups.14 Tunable
stress relaxation in these hydrogel materials has been
demonstrated to be beneficial for roles as extracellular
1
25 °C. Residual protosolvent peaks were used to reference the H
NMR spectra. Absorption spectra were collected on a Varian Cary
Received: September 3, 2018
Revised: November 10, 2018
© XXXX American Chemical Society
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5000 UV−vis−NIR spectrophotometer. FT-IR measurements were
conducted on a PerkinElmer Frontier FT-IR instrument equipped
with an attenuated total reflectance (ATR) crystal. UV irradiation of
samples was performed using either a Rayonet photochemical reactor
equipped with four 365 nm light sources (35 W each) or an Entela
365 nm hand lamp (6 W). Visible light irradiation of samples was
performed using a commercial CFL bulb (13 W) in tandem with a Xe
arc lamp (150 W).
Table 1. Relative Molar Ratios of Monomers for Polymers
P1a and P2a
Mass Spectrometry and Differential Scanning Calorimetry. Mass
spectrometry measurements were conducted on a Waters ZQ
instrument equipped with an ESCI source. Differential scanning
calorimetry (DSC) experiments were performed using a TA
Instruments DSC Q2000 instrument equipped with a TA Instruments
RCS90 refrigerated cooling system at a ramp rate of 10 °C/min.
Compression Molding and Rheology. Compression molding of
samples was performed using a Carver Laboratory Press, pressing the
samples at 9 psi between two stainless steel plates at 130 °C for 15
min. Parallel plate rheology was performed on an Anton-Parr MCR
501 instrument under open-air conditions. An 8 mm top plate was
used, and the gap was set at ∼1 mm for all measurements.
Temperatures ranged from 70 to 150 °C. Frequency sweep
experiments were conducted at 0.1% strain, measuring from 0.1 to
100 Hz. Strain sweep experiments were conducted at 0.1 Hz,
measuring from 0.001 to 10% strain. Stress relaxation experiments
were conducted at an instantaneous initial strain of 0.5%.
Optical Images. Optical microscope images were collected on
either an Olympus BX41 microscope with a CCD camera using a ×10
objective lens and equipped with an INSTEC heating stage or on a
Senterra Raman microscope and CCD camera capable of stitching
multiple images together using a ×10 objective lens. Samples were
mounted on glass slides and scratched on the top surface using a razor
blade.
and heated to 70 °C in an oil bath until the solvent had evaporated.
The temperature was then raised to 90 °C for 18 h and then to 130
°C for 60 min. The resulting black polymer sheets were removed
intact as one piece from the warm molds.
P1c. 1,5-Diaminoanthracene (0.1 g) was dissolved in 300 mL of
ethanol and ethyl acetate (1:1) and degassed using three freeze−
pump−thaw cycles, followed by irradiation in the same manner as for
1b over 3 days using a Xe arc lamp in conjunction with a CFL bulb.
1,12-Dodecanediamine (0.097 g) and 1,12-dodecanebisacetoacetate
(0.34 g) were then added, and the solution volume was reduced to a
minimum under vacuum. The solution was polymerized in the same
manner as for P1a and P2a.
P1b and P2b. Samples of P1a and P2a were suspended in the
middle of a Rayonet photochemical reactor using clips and irradiated
on all sides for 18 h.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. 1,12-Dodecanebisacetoace-
tate (2). 1,12-Dodecanediol (1.7 g, 8.4 mmol) and m-xylene (4.7 mL)
were combined in an open vial and brought to reflux while stirring.
2,2,6-Trimethyl-1,3-dioxin-4-one (2.6 mL, 18.5 mmol) was added
dropwise, and the solution was stirred at 130 °C for 35 min. The
mixture was cooled to room temperature and then dried under
vacuum to afford a crude oil. The oil was purified using column
chromatography on silica gel with 7:3 ethyl acetate to hexanes as the
mobile phase. Rf = 0.72. Yield: 2.40 g (77%) of a pale yellow solid.
To examine the effects of anthracene photodimerization on the
β-keto enamine systems that comprise vinylogous urethane
vitrimers, a model compound (1a) was synthesized via the
condensation of 2-aminoanthracene with ethyl acetoacetate.
Irradiation of 1a in 1:1 ethanol:ethyl acetate to form 1b
(Scheme 1) was followed using UV−vis spectroscopy (Figure
1
ESCI MS: 371.3 (M−H)+, 393.2 (M−Na)+. H NMR (400 MHz,
Scheme 1. Irradiation of Compound 1a (Enamine) To Give
1b (Imine)
(CD3)2SO, RT): δ 4.03 (t, J = 6.6 Hz, 4H), δ 3.58 (s, 4H), δ 2.17 (s,
6H), δ 1.54 (p, J = 6.7 Hz, 4H), δ 1.27 (m, 16H).
Compound 1a. 2-Aminoanthracene (0.24 g, 1.2 mmol), ethyl
acetoacetate (0.8 g, 6.1 mmol), and ammonium cerium nitrate (0.034
g, 0.06 mmol) were dissolved in CH2Cl2 (2.4 mL) and DMSO (0.8
mL). The mixture was stirred at room temperature for 2.5 h, and the
CH2Cl2 was then removed under vacuum. Deionized water (50 mL)
was added, and the mixture was sonicated for 10 min. The crude solid
was vacuum filtered and rinsed with deionized water. The product was
purified using flash column chromatography on silica gel with hexanes
as the mobile phase. Yield: 0.128 g (34%) of a bright yellow solid.
1
APCI MS: 306.2 (M−H)+, 328.1 (M−Na)+. H NMR (400 MHz,
CD2Cl2, RT): δ 10.66 (s, 1H), δ 8.40 (s, 1H), δ 8.34 (s, 1H), δ 7.99
(m, 3H), δ 7.64 (m, 1H), δ 7.47 (m, 2H), δ 7.25 (dd, J = 9.2, 2.1 Hz,
1H), δ 4.77 (d, J = 0.6, 1H), δ 4.16 (q, J = 7.0 Hz, 2H), δ 2.18 (s,
3H), δ 1.29 (t, J = 7.0 Hz, 3H).
Compound 1b. Compound 1a (0.056 g) was dissolved in 250 mL
of 1:1 ethanol and ethyl acetate. The solution was degassed using
three freeze−pump−thaw cycles and then stirred at room temperature
under nitrogen for 24 h while irradiating with a 365 nm hand lamp.
The solvent was then removed under vacuum to afford a dark brown
solid.
Polymer Synthesis. P1a and P2a. 1,12-Dodecanediamine, 1,12-
dodecanebisacetoacetate, and 1,5-diaminoanthracene were combined
in the indicated molar ratios (Table 1) and dissolved in a minimum
amount of hot ethyl acetate and ethanol (1:1). The solution was
heated at reflux for 1 h and then reduced to a minimum volume under
vacuum. The solution was transferred to a silicone mold in a beaker
S1). A decrease in the absorbance in the 300−450 nm region
was observed with an isosbestic point at 290 nm when the
sample was irradiated with 365 nm light. Compound 1a in
solution and solid state strongly emitted blue light under 365
nm irradiation while 1b was nonemissive (Figure S2). These
observations are consistent with 4 + 4 photocycloaddition of
the anthracene units resulting in a break in the π-
conjugation.17
FT-IR analysis of solid 1a and 1b showed a shift to higher
wavenumber of the ester CO stretch after irradiation (1649
to 1725 cm−1) (Figure 1). This indicates a decrease in
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further 1a reacting. This result demonstrates that aromatic
amines of this type do not undergo the thermal exchange
reaction necessary for vitrimeric behavior and that inclusion of
anthracenes in vitrimers may result in the introduction of
nonexchangeable cross-links into the system after dimerization.
Composite systems containing vitrimeric and non-vitrimeric
portions have been reported and exhibit similar properties to
systems composed entirely of exchangeable monomers.19
Composite vinylogous urethane systems containing both
exchangeable alkyl diamines and nonexchangeable, photoactive
anthracene diamines linked by a common bis-acetoacetate
monomer were consequently prepared. Exchangeable cross-
linking monomers were omitted to ensure only cross-links
resulting from anthracene dimerization were present.
Polymer Synthesis. The synthesis of solid polymer sheets
was performed via the thermal condensation of acetoacetate
and amine monomers in different ratios, and the effects of UV
irradiation on the polymer samples were studied using FT-IR
spectroscopy as well as thermal and rheological measurements.
1,12-Dodecanebisacetoacetate (2) was synthesized from 1,12-
dodecanediol and 2,2,6-trimethyl-1,3-dioxin-4-one,20 and 1,5-
diaminoanthracene (3) was synthesized by reducing 1,5-
diaminoanthraquinone using Zn/Cu.21 Polymers containing
differing amounts of anthracene were prepared by using a fixed
amount of 2 and altering the ratio of 3 to 4 while maintaining a
5% molar excess of amine functional groups shown to be
beneficial for vitrimeric exchange (Table 1).2
Polymers were synthesized by heating the monomer
mixtures to reflux in 1:1 ethanol:ethyl acetate followed by
transferring the solutions to a silicone mold. After evaporation
of the solvent, the molds were heated at 90 °C overnight and
then cured at 130 °C for 1 h, resulting in nearly black polymer
sheets largely free of surface defects (Figure S5). Thinner
samples from compression molding were seen to be red in
color, consistent with the deep red color of monomer 3 in
solution.
Figure 1. Comparison of the FT-IR spectra of 1a, 1b, and ethyl
acetoacetate.
conjugation with the ester, bringing it closer to the CO
stretching frequency in ethyl acetoacetate (1739 cm−1). These
spectral changes are in good agreement with literature values
for enamine−imine tautomerization observed in quinoxalinone
and pyridopyrazinone systems.18 It is proposed that the
photodimerization of 1a results in a change from the enamine
to imine tautomer.
The 1H NMR spectrum of 1a in CD2Cl2 contains peaks due
to the alkene proton (δ 4.77 ppm) and N−H proton (δ 10.66
ppm) that are consistent with only the enamine tautomer
being present at room temperature. The spectrum of 1b under
the same conditions did not show any peaks in these two
regions, and two singlets near δ 2.6 ppm are assigned to the
methylene protons of the imine tautomer (Figure S3). Further
analysis of the spectrum of 1b was not performed due to the
complexity resulting from the high number of possible
stereoisomers.
Liebler et al. have previously examined the kinetics of the
exchange reaction between aliphatic amines and β-keto
enamines.2 The exchange reaction between aromatic amines
and β-keto enamines was examined here using a similar
approach (Scheme 2).
P1a was irradiated and the IR spectrum monitored as a
function of irradiation time (Figure 2). After 5.5 h of
irradiation no further changes were observed, and this
remained the case upon continued UV irradiation up to 18
h. An isosbestic point was observed at 1665 cm−1, and the peak
assigned to the ester CO stretch shifted to higher energy
after irradiation (1648 to 1725 cm−1). The peak at 1602 cm−1
was assigned as the enamine CC stretch and decreased in
Scheme 2. Kinetic Exchange Experiment for Aromatic
Vinylogous Urethanes
Compound 1a was heated at 120 °C in d6-DMSO in the
presence of 5 equiv of p-toluidine and the exchange reaction
followed by 1H NMR spectroscopy (Figure S4). Peaks
matching the expected exchange product were observed, but
no 2-aminoanthracene was present. In addition, the reaction
stopped after only ∼25% of the 1a present had reacted.
Heating the sample at higher temperatures did not result in
Figure 2. IR spectra of P1a after different irradiation times at 365 nm
and sample P1c containing fully dimerized anthracene units.
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intensity with irradiation. A sample with the same monomer
ratios as in P1a was prepared by fully dimerizing 3 prior to
polymerization, and the IR spectrum of the resulting sample
P1c was collected. When compared against P1c in which all
anthracene units are dimerized, the FT-IR spectrum of P1b
shows nondimerized anthracene units that are attributed to
poor spatial overlap of some of the anthracene units preventing
dimerization. The spectral changes between P1a and P1b
closely match those seen in the model compounds 1a and 1b
(Figure S6), and consequently enol to keto tautomerization is
also proposed for the polymeric systems upon irradiation.
Similar changes were observed for irradiation of P2a to P2b,
with no further changes after 12.5 h of UV irradiation. The
higher concentration of anthracene units in P2a requires
longer UV irradiation.
relative molar ratio of anthracene were not tested as the change
in Tg was expected to be negligible in these cases, given there is
only a 5 °C increase in Tg from P1a to P1b.
Rheology. Parallel plate rheological studies were performed
on samples before and after irradiation to probe the effect of
anthracene dimerization on the flow characteristics. Exper-
imental temperatures were adjusted between samples in
accordance with the increase in Tg associated with irradiation.
As discussed above, frequency sweep experiments were
conducted using a constant strain of 0.1% over a frequency
range from 0.01 to 100 rad/s (Figure 4). From the low
frequency region, P2a was observed to behave as a typical
viscoelastic polymer (G″ > G′), consistent with its linear
nature and exchangeable bonds. Although the terminal zone
has not been attained, the tendency of reaching its Newtonian
plateau is evident. After irradiation, the storage modulus, G′,
increased to above the loss modulus, G″ (G″ < G′), indicating
a transition from a viscoelastic fluid to solid-like behavior,
consistent with the formation of anthracene-based cross-links.
Similar results were observed for P1a and P2a (Figure S7).
Previously reported vitrimeric systems exhibited rapid stress
relaxation attributed to the thermal exchange of urethane
bonds.2,9 A sudden deformation of 0.5% was applied to P2a
and P2b, and their relaxation moduli were monitored as a
function of time (Figure 5). Two stages in their stress
Samples of P1a and P2a were compression molded to a
thickness of 200 μm to ensure full UV light penetration and
then irradiated overnight. The resulting samples were
characterized using differential scanning calorimetry (Figure
3).
Figure 3. DSC traces of polymer samples before and after irradiation,
measured on the second heating cycle at 10 °C/min. Solid circles
indicate Tg.
The glass transition temperature (Tg) increases from 30 °C
for P1a to 35 °C for P1b, consistent with the UV irradiation
resulting in cross-linking of the polymer chains. A 25 °C
increase in Tg was observed between P2a and P2b, consistent
with a higher concentration of anthracene units in P2b and
therefore a greater concentration of cross-links after irradiation
than in P1a. No decrease in Tg was observed for P1b and P2b
when heated to higher temperatures, suggesting that the
photodimerization is irreversible. Samples containing <0.5
Figure 5. Stress relaxation curves for P2a and P2b under a 0.5% initial
strain.
relaxation were observed for both P1 (Figure S9) and P2.
Rapid relaxation occurred for the initial 300 s, during which
approximately 50% (P2a) and 60% (P2b) of the applied stress
Figure 4. G′, G″, and η for P2a (left, 60 °C) and P2b (right, 85 °C) measured at 0.1% strain.
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were relaxed nearly reaching a plateau that represents an initial
elastomeric plateau modulus of the anthracene portion that
initially showed solid-like behavior. After this, a slower
relaxation period was observed; however, the remaining stress
did not fully relax during the duration of the experimental
period. The two-stage relaxation is attributed to mixed
polymerization of the flexible, thermally exchangeable
diaminoalkyl along with the rigid, nonexchangeable diami-
noanthracene monomer. The alkyl portion of the polymer
initially relaxed, followed by the slow stress relaxation of the
anthracene component. This two-stage relaxation process is
also evident from Figure 4 (right) where G′ tends to attain a
plateau before the second mode of relaxation begins. Stress
relaxation at higher temperatures (>150 °C) was not pursued
as the polymers became too fluid to load properly into the
parallel-plate rheometer.
Samples were observed using an optical microscope and heated
in 5 °C increments using a heating stage until the scratch
(≈100 μm wide initially) was observed to begin healing (Table
2). By use of optical microscopy, scratches were initially
Table 2. Composition, Glass Transition Temperature, and
Scratch Healing Onset Temperature of Polymers with and
without Anthracene Incorporated
sample
mol % anthracene
Tg (°C)
healing onset (°C)
P1a
P1b
P2a
P2b
P3
25
25
50
50
0
30
34
23
49
−15
45
50
60
80
35
To test the proposal that the two-stage relaxation arises from
the mixed polymerization of two different amine monomers, a
polymer (P3) composed only of monomers 4 and 2 was
prepared (Tg = −15 °C). This sample exhibited rapid stress
relaxation between 120 and 150 °C, relaxing fully after 36.5
min at 120 °C and 10 min at 150 °C (Figure 6). The 120 °C
observed as dark lines crossing the sample, which were seen to
decrease in width during healing through a series of expansions
and contractions of the material (Video S1). This behavior is
consistent with previously reported examples of visually
observed scratch healing.3,22,23
The onset of scratch healing was observed to be a slow
expansion and contraction of the polymer sample, gradually
resulting in closing of the scratch (Video S2). Below the
healing onset temperature (Table 2), no significant changes in
scratch width or sample volume were observed. At higher
temperatures (∼70 °C above Tg) scratches in all tested
polymers healed to a width of <5 μm in under 5 min with no
changes in width observed on cooling. An increase in healing
onset temperature is consistent with the increase in Tg caused
by UV irradiation. Despite the low Tg for P3, the onset of
healing did not begin until ∼35 °C, close to the healing onset
of polymers with a much higher Tg. The topological freezing
point transition (Tv) for vitrimers is the temperature at which
network rearrangement begins to occur and is commonly in
the range 30−70 °C for rigid vitrimers.2,24,25 Scratch healing
necessitates heating to both above the Tg, to enable mobility of
the polymer chains and above the Tv, to allow for network
rearrangement to heal the scratch. In the case of P3, Tg is
below room temperature and the healing onset above room
temperature; thus, Tv can be estimated at between 25 and 35
°C.
Figure 6. Stress relaxation curves and Arrhenius plot for P3
containing no anthracene units.
stress relaxation for P3 at 100 s is comparable to the amount
P2b relaxes over the same time interval. The stress relaxation
of vitrimers can be described by the Maxwell law (eq 1).
To demonstrate the overall effect of UV irradiation on
scratch healing, samples of P2a and P2b were scratched and
heated on a hot plate at 60 °C (the healing onset for P2a but
below that for P2b) for 10 min followed by heating at 80 °C
(the healing onset for P2b) for 10 min (Figure 7).
G(t)/G0 = exp(−t/τ)
(1)
Relaxation times at 37% (1/e) relaxation were fit to an
Arrhenius plot for P3, and the activation energy was found to
be 47 kJ/mol. The reported activation energy for cross-linked
vitrimers is typically ∼60 kJ/mol.2 The lower value for P3 is
attributed to the linear nature of the polymer in conjunction
with thermal bond exchange. The rapid relaxation of P3 in
comparison with P1 and P2 demonstrates that the anthracene
units are responsible for deviations from expected vitrimeric
stress relaxation profiles.
Polymer Healing. UV penetration depth is a limiting
factor for potential applications of the polymers studied here.
Modification of surface properties was examined wherein the
degree of anthracene dimerization can be considered
consistent as a function of depth. Scratch healing experiments
were conducted to demonstrate the photocontrol of vitrimeric
properties. Polymer samples before and after irradiation were
mounted on glass slides and scratched with a razor blade.
The scratch in polymer P2a had an initial width of 135 μm
and healed 93% to a width of 9 μm after treatment at 60 °C.
Polymer P2b had an initial scratch width of 118 μm and healed
10% to a width of 106 μm after treatment at 60 °C and healed
98% (2 μm) after treatment at 80 °C. A decrease in the
darkened area of the scratch for P2b after treatment at 60 °C
was observed (Figure 7d), but the width of the scratch
measured from the onset of deformation does not decrease
significantly. This suggests that closing of the scratch is likely
occurring slowly at this temperature for the irradiated sample
and from the bottom of the scratch. Depending on the depth
of the scratch, this may be consistent with a reduced amount of
anthracene photodimer present at the bottom of the scratch,
resulting in a lower scratch healing temperature compared to
the photodimerized surface. Near complete rapid healing of
the scratch in P2b at elevated temperatures (Figure 7e) is
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Figure 7. Optical microscope image of (a) sample P2a initial scratch, (b) P2a after 60 °C 10 min, (c) sample P2b initial scratch, (d) P2b after 60
°C 10 min, and (e) P2b after 80 °C 10 min. Black scale bar is 300 μm. Value in parentheses indicates approximate scratch width. Images were
assembled from individual photos focused at each position.
consistent with the thermal healing of the nonirradiated sample
P2a and demonstrated that the introduction of nonexchange-
able cross-links does not disable the thermal healing of these
vitrimer copolymers.
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ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.8b01898.
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Spectral details and rheological data (PDF)
Video S1 (AVI)
Video S2 (AVI)
AUTHOR INFORMATION
Corresponding Author
*E-mail: mwolf@chem.ubc.ca.
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ORCID
Taylor Wright: 0000-0002-7224-6574
Tanja Tomkovic: 0000-0002-1144-2723
Savvas G. Hatzikiriakos: 0000-0002-1456-7927
Michael O. Wolf: 0000-0003-3076-790X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The Natural Sciences and Engineering Research Council of
Canada is acknowledged for financial and scholarship support.
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DOI: 10.1021/acs.macromol.8b01898
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