Force-induced redistribution of a chemical equilibrium

Article abstract of DOI:10.1021/ja106332g

Spiropyran (SP) mechanophores (mechanochemically reactive units) can impart the unique functionality of visual stress detection to polymers and have potential for use in smart materials with self-sensing capabilities. These color-generating mechanophores were incorporated into polyurethane via step growth polymerization. Polyurethane, which is inherently a versatile engineering polymer, possesses an optimized balance of mechanical toughness and elasticity to allow for investigation of the kinetics of the mechanochemical response of the SP mechanophore in the bulk polymer via fluorescence and absorbance measurements. The stress-induced 6-π electrocyclic ring-opening to the colored merocyanine (MC) form of the mechanophore was quantified by measuring the change in absorbance of the polymer, while it was held at constant strain. The closing kinetics of the mechanophore was also studied by fluorescence imaging. Finally, the effects of mechanical strain on the equilibrium between the SP and MC forms are reported and discussed.

Full text of DOI:10.1021/ja106332g

Published on Web 10/26/2010
Force-Induced Redistribution of a Chemical Equilibrium
Corissa K. Lee, Douglas A. Davis, Scott R. White, Jeffrey S. Moore,
Nancy R. Sottos, and Paul V. Braun*
Departments of Materials Science and Engineering, Chemistry, and Aerospace Engineering and
Beckman Institute, UniVersity of Illinois at Urbana-Champaign,
Urbana, Illinois 61801, United States
Received July 16, 2010; Revised Manuscript Received October 4, 2010; E-mail: pbraun@illinois.edu
Abstract: Spiropyran (SP) mechanophores (mechanochemically reactive units) can impart the unique
functionality of visual stress detection to polymers and have potential for use in smart materials with self-
sensing capabilities. These color-generating mechanophores were incorporated into polyurethane via step
growth polymerization. Polyurethane, which is inherently a versatile engineering polymer, possesses an
optimized balance of mechanical toughness and elasticity to allow for investigation of the kinetics of the
mechanochemical response of the SP mechanophore in the bulk polymer via fluorescence and absorbance
measurements. The stress-induced 6-π electrocyclic ring-opening to the colored merocyanine (MC) form
of the mechanophore was quantified by measuring the change in absorbance of the polymer, while it was
held at constant strain. The closing kinetics of the mechanophore was also studied by fluorescence imaging.
Finally, the effects of mechanical strain on the equilibrium between the SP and MC forms are reported and
discussed.
changes in conjugation,14-21 or changes in charge state.22 In
these systems, the mechanically active species has generally
Introduction
been physically dispersed within a bulk polymer matrix.6-13,23
Davis et al.24 and more recently O’Bryan et al.25 have reported
on covalently linked spiropyrans (SP) as highly effective color-
generating mechanophores that can provide visible detection
and mapping of mechanical stresses through their mechanically
induced transformation to the merocyanine (MC) conformation
in glassy and elastomeric chain growth polymers. While the
polymer systems explored by Davis et al.24 were quite successful
in demonstrating a mechanochemically induced visible color
change, the physical properties of these polymers were not ideal
for investigation of the kinetics or thermodynamics of the
mechanically induced transformations of SP mechanophore in
bulk polymers.
Mechanophores, molecules that respond in a productive
fashion to mechanical stimuli, have the potential to dramatically
increase the functionality of polymeric systems.1 A major focus
in this field is the discovery and development of mechanochro-
mic mechanophores, that is, mechanophores that change color
with the application of force. The ability of a material to
autonomically react to changes in its environment lends itself
to many potential applications in damage detection and cata-
strophic failure prevention. A variety of molecules with mecha-
nochromic functionality have been investigated in literature.
Mechanically induced changes in color and/or fluorescence have
been due to the formation2-5 or break up2,6-13 of excimers,
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Lee et al.
Figure 1. (a) Chemical structures of spiropyran (SP) and merocyanine (MC) and the mechanically or optically triggered conversion equilibrium between
the colorless SP and colored MC forms. (b) Schematic of the incorporation of SP mechanophore 1 into PU via step growth polymerization where m . n.
We now demonstrate the incorporation of SP mechanophore
into step-growth polyurethane (PU). The inherent mechanical
toughness, elasticity, and low glass transition temperature (Tg
≈ -60 °C) of PU enable the effects of mechanical force on the
SP-MC equilibrium (Figure 1a) to be studied. The initial
polymer systems studied were not amenable to kinetic analysis
because they were either too soft [elastomeric poly(methyl
acrylate)], resulting in mechanical activation only at high strains
close to the strain of failure, or too glassy [poly(methyl
methacrylate) (PMMA)], Tg ) 105 °C. In PMMA, because of
the high Tg, the rate of conversion from MC to SP form could
not be investigated at room temperature.24 We show that the
SP mechanophore is mechanochromic in PU and that the
equilibrium between the colored MC and colorless SP form can
be directly controlled by mechanical strain. Because of the low
Tg, equilibrium is reached in experimentally accessible times
at room temperature. Here we report for the first time on the
kinetics of the mechanically activated SP to MC conversion as
well as the thermally activated conversion of MC to SP in a
bulk polymer. PU is of specific interest not only because it is a
ubiquitous engineering polymer but also because it is synthe-
sized via step growth polymerization, so the mechanophore
concentration can be modulated independently of molecular
weight or cross-linking density. Finally, from a practical
standpoint, the synthesis of the mechanochromic PU can be
scaled to large volumes, simplifying mechanical testing studies.
Experimental and Methods
See also Supporting Information for details.
Polyurethane Synthesis. Dihydroxyspiropyran 1 (R ) H) was
first reacted with an excess of methylene diphenyldiisocyanate
(MDI) in anhydrous tetrahydrofuran (THF), resulting in a diiso-
cyanate-functionalized mechanophore. Still in solution, the func-
tionalized spiropyran was then reacted with a hydroxyl-terminated
poly(tetramethylene glycol) (PTMG, Mn ) 650). The polymer was
then put under vacuum at 70 °C to remove the solvent. Finally,
hexamethylene diisocyanate (HDI) was added as a chain extender.
Tensile specimens were prepared by pouring the polymer into Delrin
“dog bone”-shaped molds and cured under a N2 atmosphere at 60
°C for 2 days to yield the mechanophore-containing polymer (PU-
1) with the general structure shown in Figure 1b. Different
concentrations of 1 were incorporated (0.015-0.06 wt %). The
resulting mechanochromic PUs all had similar molecular masses
of 50-70 kDa and polydispersity indices of 3-6. For the following
discussions, PU-1 with an optimized concentration of 0.03 wt % 1
was used for fluorescence and absorbance experiments.
Figure 2. (a) Optical images of prestretched PU-1 “dog bone” containing
0.03 wt % 1 before (left) and after (right) being stretched to a stretch ratio
(final length/initial length) of 2.0 and released. (b) Optical images of PU-2
before and after stretching compared to UV irradiation-induced conversion
of the control SP mechanophore. (c) Difunctional control SP mechanophore.
Polymer connectivity occurs at the hydroxyl functional groups and therefore
does not bridge the spiro junction. (d) Absorbance spectra of PU-2 showing
no mechanical activation but a clear MC peak at about 580 nm after UV
irradiation.
capacity Futek LSB300 load cell via a panel meter (Omega
Engineering Inc., DP25B-S-A), DAQ card (National Instruments),
and Labview software from National Instruments. Before absor-
bance and fluorescence testing, the tensile specimens were pre-
strained to a length 5 times the initial gauge section, which caused
the samples to neck and cold draw (Figure 2a, left). The prestrained
specimens were then reloaded to increasing levels of stretch, where
stretch is defined as the final length over the initial length (after
prestraining). Typical load-stretch plots (see Supporting Informa-
tion) of PU-1 and plain PU were very similar, suggesting that the
Mechanical Testing. Tensile testing was done at room temper-
ature on a rail frame with loads data determined by use of a 50 lb
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Figure 3. (a) Absorbance spectra of PU-1 as a function of increasing stretch ratio (1.0, 1.2, 1.4, 1.6, 1.8, and 2.0). (b) Peak absorbance (average of
absorbance between 570 and 580 nm) of PU-1 and difunctional control mechanophore (PU-2) as a function of stretch ratio. (Inset) Peak absorbance as a
function of true stress. Sufficient time was allowed between stretch steps for the mechanically induced SP to MC conversion to reach equilibrium. (c)
Absorbance spectra of a mechanically activated PU-1 stretched and held at a constant stretch of 2.0 compared to the same PU-1 sample irradiated with UV
(365 nm) for 5-10 min. The peak absorbance (570-580 nm) of the mechanically activated sample is, on average, ≈20-30% of the UV-activated peak
absorbance.
low concentration of SP had negligible effects on the tensile
properties of the polymer.
about 580 nm. PU-2 was also tested with varying strain levels
and resulted as expected with no change in absorbance (Figure
3b).
Absorbance and Fluorescence Measurements. After prestrain-
ing of the tensile specimen, absorbance data were collected by use
of an Ocean Optics HL-2000 tungsten halogen light source and
Ocean Optics HR2000+ spectrometer. For collection of absorbance
data, the samples were mechanically tested on a custom-built rail
frame. Displacement control was established through a bidirectional
screw-driven Lintech rail table. A fluorescence light microscope
(Zeiss Axiovert 200M) was used to capture fluorescence images,
and ImageJ was used for image analysis.
The increase of absorbance with strain as well as with stress
for PU-1 appears linear, at least up to a stretch of 2.0 (Figure
3b). Higher stretch values of about 2.3 were tested and resulted
in continued increases in absorbance (see Supporting Informa-
tion); however, higher stretch ratios led to slippage of the
polymer samples in the testing grips. The linear increase of
absorbance with strain leads us to believe we are far from
activating all the mechanophore incorporated into the PU, since
the absorbance must asymptotically reach some value. When
the already mechanically activated PU-1 was held at a stretch
of 2.0 and then irradiated with UV light for 5-10 min, the
polymer became noticeably darker purple and the MC absor-
bance peak increased (Figure 3c). Analysis of these results
indicates that the mechanical activation of PU-1 activates ca.
20-30% of the SP relative to the amount activated by UV
irradiation.
Nearly complete mechanophore activation must simply
require levels of mechanical force on the mechanophore not
attainable with the current experimental system. In part, this is
an experimental problem in that the sample slips out of the grips
at high strain, but also, on the molecular level, this partial
activation could reflect on the force distribution on individual
mechanophores due to both the finite chain lengths in the sample
and the distribution in mechanophore alignment in the sample.
Significant color change was generally observed only after the
necking event (Video S1 in Supporting Information) during the
prestrain step. Even though substantial plastic deformation and
thus chain alignment takes place upon prestraining, some
fraction of mechanophores are likely to be oriented in such a
direction that they do not activate under uniaxial tension. Overall
this lends evidence to the hypothesis that chain alignment may
play a key role in activation of the SP mechanophore as
postulated by O’Bryan et al.25 It is also quite likely that the
stress required for nearly complete activation would result in
fracture of the polymer dog bone.
Results and Discussion
Mechanical Activation and Controls. When the polymer was
uniaxially stretched, a deep purple coloration appeared along
the entire gauge section of the PU-1 sample (Figure 2a),
demonstrating the stress-induced formation of the open, MC
form of the mechanophore (Figure 1a). To confirm that
activation of the SP mechanophore was indeed mechanical and
not due to thermal or photo means of activation, a difunctional
control SP molecule 2 (Figure 2c) was synthesized and
incorporated into PU (PU-2) by the same synthetic procedure
as PU-1. The control SP was a hydroxyl-functionalized version
of the difunctional control used by Davis et al.,24 where the SP
is coupled into the polymer backbone in such a fashion such
that force is not transferred to the spiro C-O bond, and thus
conversion from the SP to MC form is not mechanically
triggered. Upon uniaxial stretching, no color change was
observed for plain PU (containing no mechanophore) or PU
containing the difunctional control SP (PU-2) (Figure 2b).
Absorbance measurements confirmed the lack of mechanical
activation displaying no MC peak for both unstretched and
stretched spectra (Figure 2d). In the control system, the
difunctional control samples could be activated with ultraviolet
(UV) light, thus confirming the presence and photochemical
activity of the spiropyran (Figure 2b,d).
Activation versus Stretch. The mechanical activation of PU-1
as a function of stretch was studied. It was observed that the
MC absorbance peak centered on 580 nm grew as a function
of strain (Figure 3a,b), supporting the hypothesis that mechanical
forces are directly responsible for converting the mechanophore
from SP to MC form. Increasing levels of strain correlate to
larger amounts of mechanical force on the mechanophore, which
in turn opens an increasing percentage of the mechanophores
from SP to MC, resulting in the growing MC peak centered at
Kinetics Studies. In PU-1, which has a Tg ≈ -60 °C (see
Supporting Information), the mechanically induced 6-π elec-
trocyclic ring-opening conversion from SP to MC fully reverts
to the SP form after about 1 h of exposure to fluorescent room
light; this behavior can be repeated multiple times with the same
sample. We previously reported that the MC to SP reversion in
elastomeric poly(methyl acrylate) (Tg ≈ 10 °C) occurred after
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Lee et al.
just over an hour, kT at room temperature is sufficient to allow
the MC form to revert back to the SP form favored at
equilibrium.
Fluorescence-based characterization was also performed on
a mechanically activated PU-1 “dog bone”-shaped sample. The
sample was stretched to a stretch ratio of 2.0 and then unloaded,
placed in a dark container, and taken to the fluorescence
microscope. The time at which the sample was unloaded and
removed from the rail frame was defined as time zero. Results
for the mechanically activated PU-1 sample were similar except
for a slightly longer time constant, τ ≈ 38 min (Figure 4b),
when fitted to a single-exponential decay function and slightly
noisier due to thickness and imperfect surface of the “dog bone”
shape. We suspect that the slight increase in time constant τ
could be a result of slowly relaxing residual strain in the polymer
preventing the mechanophore from closing as quickly.
Mechanically Biased Equilibrium. The relatively rapid in-
terconversion between SP and MC forms, coupled with the
robust mechanical properties of PU-1, enable a quantitative
analysis of the effects of strain on the equilibrium between the
SP and MC forms. Visible color change of PU-1 occurs
immediately after mechanical activation, but the full color
change is not instantaneous. When the sample is held at constant
strain, over time it becomes increasingly purple and the MC
peak (570-580 nm) increases (Figure 5a). The peak absorbance
versus time of the PU-1 sample held in the dark at a stretch of
2.0 shows the absorbance approaches a steady-state value after
1-2 h. In contrast to the mechanically activated PU-1 sample
used for the fluorescence decay measurements (Figure 4b),
which demonstrates the reversion of stress-induced MC form
back to the more thermally stable SP form; the mechanically
activated MC in the PU-1 sample held under constant load does
not revert. The plateau in absorbance seen in Figure 5a and the
lack of MC to SP reversion suggest that the mechanophore has
reached a new mechanically biased equilibrium in the strained
state.
Figure 4. (a) Mean fluorescence intensity decay of UV-activated PU-1
plotted against time and fitted to a single-exponential decay function with
τ ≈ 30 min. (b) Mean fluorescence intensity decay of mechanically activated
PU-1 fitted to a single exponential with τ ≈ 38 min.
approximately 6 h of exposure to fluorescent room light, whereas
reversion in glassy poly(methyl methacrylate) (Tg ≈ 105 °C)
required several weeks.24 The MC to SP ring closing kinetics
appears to vary greatly with the local environment, with Tg being
a major determinant.
Fluorescence imaging was used to quantify the time required
for MC to revert to SP under no load within PU. We took
advantage of the photochromic properties of SP.26 UV light was
used to activate a solution-cast film of PU-1, converting the SP
mechanophore to the colored/fluorescent MC conformation. A
thick film was used to eliminate any signal variation from the
slightly different thicknesses of the “dog bone” sample as well
as scratches and defects on the surface, which could lead to
scattering. Over several hours, fluorescence images of the
activated region were captured by brief exposures in the dark.
The mean fluorescence intensity of the activated region was
plotted versus time and fitted to a single-exponential decay with
a time constant of τ ≈ 30 min (Figure 4a). This shows that in
After PU-1 is allowed to come to equilibrium, no change is
expected to occur without changes to the ambient conditions,
such as light exposure and temperature. For instance, irradiation
with visible light should drive the equilibrium toward the closed
SP form, even if the strain in the sample is held constant. If
indeed the equilibrium ratio of MC to SP in the absence of light
is determined by the force on the sample, upon removal of the
visible light source, the amount of MC present should return to
the no-light levels. A PU-1 sample was mechanically activated
in the dark at a stretch of 2.0 and allowed to equilibrate in the
Figure 5. (a) Peak normalized absorbance at 580 nm of mechanically activated PU-1 held at a constant stretch ratio of 2.0 vs time. (b) Typical stress
relaxation of PU-1 held under constant strain. Plot represents “dog bone”-shaped PU-1 with initial gauge length of 10.2 cm stretched at 1 mm/s to a
displacement of 6 cm, and then held for over 90 min. (c) Peak absorbance (570-580 nm) plotted versus time. The PU-1 sample had been held at constant
stretch of 2.0 for 2 h, and at time ) 0 min a bright halogen light source is continuously irradiated onto the PU-1 sample for 1 h and then removed for 2 h;
this is repeated, demonstrating the altering of the equilibrium between SP and MC by strain.
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A R T I C L E S
strained state (approximately 1-2 h). Then, while still strained,
the equilibrated sample was exposed to a bright visible light
driving the sample toward the SP form. This could be observed
as the color rapidly faded (≈30 min). Upon removal of the light,
the color returned in 1-2 h (the strain on the sample was
constant over these experiments). This could be repeated
multiple times, for as long as the sample stays in the strained
state (Figure 5c). This not only provides further indication that
the SP to MC conversion is induced by force and not some
other effect (such as some effect during the initial mechanical
deformation) but also shows that mechanical force is altering
the potential energy surface such that the MC form becomes
more favored under strain compared to the no-strain situation.
Stress relaxation of the PU-1 tensile specimen was also
monitored at different levels of strain (Figure 5b). PU-1 was
stretched at a strain rate of 1 mm/s to different levels of
displacement. Between each increasing step of displacement,
sufficient time was allowed to ensure that the sample had relaxed
and that the absorbance measurements were indeed taken at the
new equilibrium. After 90 min in constant strain, the stress
values indeed were close to a steady-state value of stress
relaxation, establishing that the dynamical values of stress in
the tensile specimen had also equilibrated.
constant with only a slight increase in volume with stretch ratio
(see Supporting Information). The minimal volume change
indicates that although the sample is held under strain for long
periods of time, there is minimal grip slippage and also further
supports that the measured equilibrium values are valid.
In conclusion, we have developed a new method of incor-
porating mechanophore into step-growth polymers, specifically
PU, and have demonstrated bulk mechanical activation of the
SP mechanophore in this system. By use of visible spectroscopy,
the absorbance was shown to increase linearly with strain, and
when held at constant strain, the MC form did not revert back
to the thermodynamically preferred (under no load) SP form,
indicating a strain-induced change in the energy landscape of
the SP mechanophore system. We envision that the transfer of
this mechanophore incorporation method to other step growth
polymers is possible, leading to the possibility of developing a
variety of different engineering polymers with local stress-
sensing capabilities.
Acknowledgment. This work was supported by the Army
Research Office MURI (Grant W011NF-07-1-0409). We also
acknowledge Dr. Yuxiang Liu and Dr. Jinglei Yang for carrying
out preliminary experiments on this material system.
The thickness, cross-sectional area, and overall volume were
monitored throughout the mechanical testing to ensure that
measured physical dimensions of the sample over time remained
constant. Thickness and cross-sectional area decreases correlated
consistently with increases in gauge length of the tensile
specimen, and the sample volume change remained fairly
Supporting Information Available: Text, two schemes, six
figures, and two videos describing experimental details, synthetic
procedures, mechanical testing procedures, and absorbance and
fluorescence procedures and spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
(26) Minkin, V. I. Chem. ReV. 2004, 104, 2751–2776.
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