Introduction
Polymers that change their absorption and/or fluorescence color in response to mechanical
deformation permit assessing the effects of mechanical forces, without the need for
laborious analyses or expensive equipment.[1 ] Such mechanochromic responses could be useful for the inspection of load-bearing
structures, in which information about the location and extent of damage in the material
is crucial, in particular to avoid catastrophic failure and enable the implementation
of need-based servicing or replacement schemes.[2 ] A popular strategy for fabricating mechanochromic polymer materials is the incorporation
of so-called mechanophores, i.e., molecules featuring a covalent bond that breaks
preferentially when subjected to excessive force. Frequently employed motifs include
spiropyrans, 1,2-dioxetanes, and Diels–Alder adducts, and equipping polymers with
these has allowed for the preparation of a variety of materials that display mechanochromic
responses.[3 ] For example, such mechanophores have been incorporated into polyurethanes that then
display a color change upon tensile or compressive deformation.[4 ] However, mechanophores typically require a high threshold force for activation,
and the reverse reaction generally requires additional stimuli, which impedes the
monitoring of relaxation processes.[5 ]
A particularly straightforward alternative means to create materials that display
a mechanoresponsive luminescence (MRL) behavior involves the blending of excimer-forming
dyes into a polymer matrix, or covalently integrating such dyes into a polymer backbone.[1e ]
[6 ] If the dyes are immiscible with the matrix, phase separation can lead to the formation
of (ideally nanometer-sized) aggregates that fluoresce at a significantly longer wavelength
than the well-solubilized dye molecules. When tensile deformation or compression disrupts
a sufficiently large fraction of such aggregates, monomer emission is restored and
this can lead to a perceptible fluorescence color change.[6a ] Excimer-forming dyes that have been used in such simple blending approaches include
oligo(p -phenylene vinylene) (OPV),[6 ] perylene,[7 ] poly(phenylene ethynylene),[1a ] and bis(benzoxazolyl)stilbene (BBS) derivatives. Polymer matrices that have been
successfully rendered mechanochromic using this approach include poly(ethylene) (PE),[1a ]
[1b ]
[6a ]
[7 ]
[8 ] poly(ethylene terephthalate) (PET),[9 ] poly(1,4-butylene succinate),[10 ] poly(propylene) (PP),[11 ] poly(vinylidene fluoride),[12 ] and polyamide.[13 ] This list reflects that the approach has largely been limited to semicrystalline
polymers, and it appears that this microstructure and the significant local shear
forces that it imparts are necessary to break up microscopic dye aggregates and elicit
a notable fluorescence response.[1e ]
[14 ] Moreover, semicrystalline matrices have been reported to nucleate the formation
of dye aggregates and thereby influence their size in a considerable manner.[8b ] By contrast, matrices with a high fraction of an amorphous phase and a low glass
transition temperature promote the formation of large, hard-to-dissociate dye aggregates,
on account of slow nucleation.[6b ]
[8b ] A notable exception is the use of BBS with a commercial, low-crystallinity polyurethane,
in which the dye aggregates that form upon blending remain small enough to allow for
reversible mechanochromism.[15 ] However, the effectiveness of this strategy for fabricating mechanochromic materials
has thus far been limited to this particular polyurethane/BBS combination, since the
small aggregate size and the force transfer from the polymer host appear to be mediated
by specific hydrogen-bonding interactions between the dye and the polymers' hard segments.[15a ] Alternatively, covalent attachment of dyes to the polymer backbone has been shown
to facilitate force transfer to the aggregates and limit their size, in turn eliciting
a more significant force-dependent fluorescence response.[6b ]
[16 ] For example, a polyurethane containing OPV in the backbone displays a much greater
mechanochromic response than the corresponding melt- or solution-processed blends
with a free OPV dye.[6b ] Despite the enhanced mechanochromic response, covalent dye incorporation significantly
complicates the production of MRL materials and may not be straightforward for a wide
range of polymer types and applications. Moreover, a much higher weight fraction of
the dye is typically required in polymers that covalently integrate the dye, e.g.,
5–20 wt% for OPVs integrated in a polyurethane or a poly(ester),[6b ]
[16 ] whereas blends with semicrystalline PEs featuring only 0.1 wt% of an OPV dye display
a mechanochromic response.[8 ]
To overcome this problem, some of us recently developed a telechelic poly(ethylene-co -butylene) carrying OPV dyes at the termini (tOPV) and demonstrated that blending
this additive with different thermoplastic elastomers renders the latter mechanochromic
([Figure 1a ]).[17 ] The reversible response of such blends to mechanical deformation suggests that the
MRL behavior may be mediated by entanglements between the macromolecular additive
and the polymer matrix, rather than caused by the dissociation of dye aggregates through
shear forces involving crystalline polymer domains. Here, we report that a reliable,
sensitive, and reversible detection of mechanical stresses can also be achieved when
tOPV is used as an additive in thermoplastic polyurethanes (TPUs), a class of materials
that have been difficult to render mechanochromic via simple blending techniques.[6b ] We investigated two TPU matrices that are structurally similar but display very
different mechanical properties. Both polymers feature hard segments that are formed
by the reaction of methylene diphenyl diisocyanate units and butanediol, which form
physical cross-links. One of the polymers (referred to henceforth as PU) features
a poly(tetrahydrofuran) soft segment, while the other (PBA-PU) contains poly(butylene
adipate) (PBA) segments ([Figure S1 ]). The former phase-separates into largely amorphous domains that bestow PU with
a low modulus and high elasticity, whereas the latter crystallizes, so that the deformation
of PBA-PU at ambient temperature is partially plastic. Furthermore, the additional
thermal transition of the PBA domains imparts this material with shape-memory behavior.[18 ] Monitoring the emission during uniaxial tensile deformation or relaxation of the
TPU/tOPV blends allowed us to demonstrate that deformation reliably results in distinct,
visually discernible fluorescence color changes in these different materials. Detailed
characterization by in-situ optomechanical measurements in conjunction with stress-controlled
shape-memory experiments shows that the fluorescence signal can be correlated with
the polymer's response to mechanical deformation, further corroborating that the use
of macromolecular aggregachromic additives enables an efficient force transduction.
We anticipate that the results reported here will serve as a basis for expanding the
use of such additives to a wide range of materials.
Figure 1 a) Chemical structure of the cyano-OPV functionalized telechelic poly(ethylene-co -butylene) (tOPV) that was employed as a strain-sensing additive for thermoplastic
polyurethanes (with M
n = 4500 g mol−1 ; x ≈ 0.36, y ≈ 0.64, z ≈ 44). b, c) Fluorescence spectra of films of (b) a PU/tOPV blend (0.2 wt%) and (c)
a PBA-PU/tOPV blend (0.25 wt%) after being stretched to a strain of up to 400% in
increments of 50%; spectra were recorded after each strain increment and normalized
to the excimer signal at 630 nm. d, e) Images of samples of the (d) PU/tOPV (0.2 wt%)
and (e) PBA-PU/tOPV (0.25 wt%) films taken under UV illumination (λ
ex = 365 nm) after deformation to the indicated strains.
Results and Discussion
Blends of the commercially available TPUs PU and PBA-PU containing small amounts (0.2–1 wt%)
of the cyano-OPV functionalized telechelic poly(ethylene-co -butylene) (tOPV) additive ([Figure 1a ]) were prepared by casting THF solutions of either PU or PBA-PU into poly(tetrafluoroethylene)
(PTFE) molds before a solution of tOPV [c (THF) = 0.33 mmol L−1 ] was added. After evaporation of the solvent, the resulting blends were compression-molded
into films of a uniform thickness of 230–330 µm (see the Supporting Information for
details). Thermogravimetric analyses of the blends show that the onset of mass loss
is at temperatures >300 °C ([Figure S2 ]) and the differential scanning calorimetry (DSC) heating and cooling traces match
those of the neat polymers ([Figure S3 ]). The DSC traces of PU/tOPV feature only a glass transition (T
g ) at ca. –45 °C, confirming the amorphous nature of the phase formed by the poly(tetrahydrofuran)
soft segments. In the case of PBA-PU/tOPV, the DSC traces display reversible transitions
associated with the melting and crystallization of the PBA domains at ca. 50 °C and
–5 °C, respectively.[18h ]
[19 ] The mechanical properties of the blends were probed by uniaxial tensile testing
with dog-bone-shaped samples that were cut from the compression-molded films. The
stress–strain curves recorded for PU/tOPV show the expected elastic behavior of a
soft polyurethane elastomer with a Young's modulus of 19.8 ± 0.3 MPa ([Figure S4 ]). By contrast and on account of the crystalline PBA domains, PBA-PU/tOPV displays
an elastic low-strain regime characterized by a Young's modulus of 118 ± 6 MPa; the
stress–strain curves show a yield point at ca. 20% strain, beyond which plastic deformation
is observed ([Figures S4 ] and [S5 ]). A comparison of the stress–strain curves of the neat polymers and the blends shows
no significant differences, confirming that the addition of small quantities of the
tOPV additive via simple blending does not alter the properties.
When placed under UV illumination, the PU and PBA-PU blends with as little as 0.2–0.25 wt%
of the tOPV additive (equivalent to 0.04–0.05 wt% of the dye) display a homogenous
yellow-orange fluorescence, whereas the neat polymers weakly fluoresce blue ([Figures S6 ] and [S7 ]). As previously reported,[17 ] the neat tOPV additive shows green emission in dilute solution [c (CHCl3 ) = 1.4 µmol L−1 ] with a maximum of λ
max = 505 nm, while orange excimer luminescence characterized by a band centered at 634 nm
is observed after complete evaporation of the solvent. Fluorescence spectra of the
PU/tOPV and PBA-PU/tOPV blends investigated here show the corresponding bands for
the monomer and excimer with maxima at 510 and 630 nm, respectively ([Figures 1b, c ], and [S8 ]). The ratio of monomer-to-excimer emission (I
510 /I
630 ) was used as a metric to monitor the extent of aggregation. In both blend series,
the I
510 /I
630 ratio decreases as the concentration of the tOPV additive is increased, reflecting
a larger fraction of dye aggregates. A comparison of the spectra of solvent-cast and
compression-molded samples indicates that the latter display a slightly higher I
510 /I
630 ratio ([Figure S9 ]), suggesting kinetic trapping of some of the tOPV molecules upon cooling from the
melt. Different cooling rates between 1 and 10°C min−1 were found to furnish samples that exhibit the same I
510 /I
630 ratio. Emission spectra acquired during heating of a PBA-PU/tOPV (0.25 wt%) blend
show that the relative contribution of excimer emission drops upon heating until,
at temperatures above 120 °C, only monomer emission is observed ([Figure S10a ]). DSC measurements show that neat tOPV undergoes a melting transition between 108
and 129 °C ([Figure S11 ]), which is in line with the previously reported values for this additive.[17 ] Monitoring PU/tOPV and PBA-PU/tOPV blend films upon heating to 120 °C (10 °C min−1 ) reveals that the I
510 /I
630 ratio increases above ca. 60 °C, and this increase is expedited above ca. 100 °C
([Figure S10b, c ]), suggesting that tOPV micro-phase-separates from the TPUs. The I
510 /I
630 ratio was only partially recovered upon cooling, presumably due to the photoinduced
isomerizations OPV dyes reportedly undergo when continuously irradiated.[20 ] However, heating a PBA-PU/tOPV (0.25 wt%) sample to 120 °C and acquiring spectra
at temperature increments of 20 °C at different spots of the sample served to demonstrate
that the I
510 /I
630 ratio displays the same trend, while the emission intensity ratio is almost completely
recovered after cooling to 20 °C ([Figure S10d ]). The I
510 /I
630 ratio of samples that were afterwards kept at 20 °C continued to decrease over the
course of 1 h, indicating that a further re-aggregation of a small amount of the tOPV
additive occurs slowly. To ensure a uniform behavior of samples and an equilibrium
aggregation state of the tOPV additive, all compression-molded samples were left at
room temperature for at least 3 h prior to further testing.
To investigate the MRL response, compression-molded films of the PU blends with 0.2–1.0 wt%
of tOPV were cut into rectangular strips (30 × 3 mm) and subjected to uniaxial tensile
deformation in a mechanical stretching frame that allowed recording fluorescence spectra
at various strains (see the Supporting Information for details). The spectra of PU/tOPV
(0.2 wt%) samples show an increase of the monomer fluorescence upon deformation, indicating
disaggregation of the chromophores ([Figure 1b ]). This effect also manifests itself in a visually discernible change of the emission
color from orange to green when samples are excited with UV light ([Figure 1d ], [Video 1 ]; λ
ex = 365 nm). The I
510 /I
630 ratio of samples with varying tOPV concentrations (0.2–0.5 wt%) increases in all
cases almost linearly with the strain ([Figure S12 ]). The uniaxial deformation of films of the semicrystalline PBA-PU/tOPV (0.25 wt%)
blend beyond the yield point leads to significant necking that starts at the center
of the film and propagates outwards ([Figure 1e ]). An increase in monomer emission can be seen in the parts of the sample that have
undergone plastic deformation and a concomitant, visually discernible irreversible
change of the emission color from orange to green is observed ([Figure 1c, e ]; [Video 2 ]; λ
ex = 365 nm). The monomer emission continues to increase until the material fluoresces
green at a strain of ca. 350% ([Figure 1e ]). PBA-PU/tOPV blends that feature different concentrations of the tOPV additive
(0.25–1.0 wt%) all display similar MRL behavior and plots of the I
510 /I
630 ratio against the applied strain show that the curves are roughly parallel to each
other ([Figure S12 ]). Compositions with a tOPV concentration of between 0.2 wt% (PU/tOPV) and 0.25 wt%
(PBA-PU/tOPV) display the most striking fluorescence color change upon deformation,
since the dominant emission color of the pristine blend films is orange, while green
emission dominates in the deformed samples; therefore, these additive concentrations
were used for most of the subsequent experiments. For compositions with a higher tOPV
content, the changes can be readily monitored spectroscopically, but the change in
the dominant emission is not observed ([Figures S12 ] and [S13 ]). Taken together, these data show that simple blending with low concentrations of
the macromolecular tOPV additive is an effective means to render both amorphous and
semicrystalline PU mechanochromic, and the change in the material's emission during
mechanical deformation can be monitored spectroscopically and is visually detectable
under UV illumination for blends of a suitable composition.
To demonstrate that the MRL response achieved with the telechelic tOPV additive in
the two selected TPUs cannot be achieved with low-molecular-weight aggregachromic
dyes, blends of PU with the OPV derivative 1,4-bis(α-cyano-4-octadecyloxystyryl)-2,5-dimethoxybenzene
(C18-RG) or 4,4′-bis(2-benzoxazolyl)stilbene (BBS) were prepared, since these dyes
were previously used very successfully with a range of different polymer matrices,
including PET,[9 ] polyamide-12,[13 ] PP,[11 ] and some PU polymers.[6b ]
[15 ] Films of PU/BBS blends (0.05–0.5 wt%) only display negligible fluorescence changes
upon uniaxial deformation up to 400% strain ([Figures S14 ] and [S15 ]), while PU/C18-RG blends (0.2–2.0 wt%) showed significant macro- and microscopic
phase separation ([Figures S16 ] and [S17 ]). Blending C18-RG with PBA-PU furnished macroscopically homogeneous films (0.6 wt%
C18-RG; [Figure S17d ]), but no change in fluorescence was observed upon uniaxial tensile deformation ([Figure S17 ]). This comparison unambiguously demonstrates that the limitations of phase separation
and poor responses in elastic matrices that are typically encountered with blends
of aggregachromic dyes can be readily overcome through the use of the tOPV additive.
In order to demonstrate the utility of the tOPV additive as a means to monitor mechanical
deformation, additional in-situ fluorescence measurements were performed during uniaxial
tensile deformation, stress-relaxation, and cyclic loading–unloading experiments.
To this end, an optical fiber connected to a spectrometer was aligned with the center
of samples that were mounted in a tensile tester and emission spectra were acquired
at a rate of 0.33 Hz during and, optionally, after deformation. After screening strain
rates from 5% min−1 to 300% min−1 with samples of PBA-PU ([Figure S18 ]), an intermediate strain rate of 50% min−1 was chosen for all subsequent experiments. At lower strain rates, the deformation
was less homogeneous, whereas only a limited number of fluorescence measurements can
be acquired at higher strain rates. A comparison of the stress–strain curves of PU/tOPV
(0.2–0.5 wt%) and PBA-PU/tOPV (0.25 wt%) blend films and the in-situ measured I
510 /I
630 values shows that the mechanochromic response of the two systems varies considerably,
indicative of different deformation processes in the two TPUs. In the case of the
elastic PU/tOPV blends, the changes of the I
510 /I
630 ratio correlate well with the applied strain ([Figures 2a ] and [S19a, b ]). Interestingly, two distinct linear regimes (R
2 ≥ 0.92) are discernible in the I
510 /I
630 vs. strain plot. The transition between these regimes at a strain of ca. 130% coincides
with an inflection point in the stress–strain curve that marks the onset of the strain-hardening
region, suggesting that the I
510 /I
630 ratio reflects the changing viscoelastic response of the PU matrix. In the case of
the PBA-PU/tOPV blends, a pronounced change in the I
510 /I
630 ratio only occurs after deformation beyond the yield point ([Figures 2b ] and [S19c, d ]), i.e., in the plastic deformation regime. Thus, a steep increase in I
510 /I
630 is observed at a strain of ca. 15–50%; due to the experimental setup, the value varies
from sample to sample and depends on where the neck forms and when it propagates through
the center of the sample where fluorescence is monitored. Further deformation leads
to a moderate increase of the I
510 /I
630 ratio that linearly correlates (R
2 = 0.97) with the applied strain and follows a slope that is comparable to the one
observed for PU/tOPV in the strain-hardening region.
Figure 2 Fluorescence color change of PU/tOPV (0.2 wt%) and PBA-PU/tOPV (0.25 wt%) blend films
during (a, b) uniaxial tensile deformation and (c, d) equilibration under continuously
applied strain. a, b) The comparison of the stress–strain curves (blue lines) and
the monomer-to-excimer intensity ratio (I
510 /I
630 , red dots) as a function of strain shows a correlation between the strain and the
emission characteristics. Linear fits of the I
510 /I
630 data appear to indicate different extension regimes (1, 2; represented by black dashed
lines). Blends containing tOPV at other concentrations show a similar response ([Figure S19 ]). c, d) Plots of the I
510 /I
630 ratio (red dots; dark red lines represent a smoothened 5-point average) and stress
(blue lines) recorded during extension of films to a strain of ca. 250%, subsequent
equilibration under a fixed strain over the course of 2.5 h, and finally release of
the stress and relaxation of the samples. The evolution of I
510 /I
630 in the holding segments is similar for blend films containing different concentrations
of tOPV ([Figure S20 ]). In all experiments a strain rate of 50% min−1 was applied.
The different responses of the two matrix polymers upon mechanical deformation are
also apparent in stress-relaxation experiments. When films of the elastic PU/tOPV
blend (0.2 wt%) were deformed to and maintained at 250% strain, stress relaxation
occurs over the course of ca. 60 min ([Figures 2c ] and [S20a ]). The I
510 /I
630 ratio mirrors this behavior, showing a steady decrease of the monomer emission over
time that follows the reduction in stress and approaches a plateau ([Figure 2c ]), likely as a consequence of chain relaxations that facilitate re-aggregation of
a fraction of the tOPV additives. The same experiment was also carried out with PBA-PU/tOPV
(0.25 wt%) blends. In this case a plot of the I
510 /I
630 ratio shows that the tOPV monomer signal increases during the stress-relaxation step in a logarithmic manner ([Figures 2d ] and [S20 ]
[S21 ]
[S22 ]), even though the stress relaxation is similar to that of the PU/tOPV blends. The
increase of the monomer emission when holding samples of the PBA-PU blend under tension
is possibly caused by local strains that are related to strain-induced crystallization
of the PBA, a phenomenon that is known to occur when polyurethane elastomers containing
polyester or polyether soft segments are placed under tensile stress.[18e ]
[21 ] Indeed, samples of PBA-PU/tOPV (0.25 wt%) that were deformed to a strain of 300%
display a pronounced melting transition in the first DSC heating trace ([Figure S23 ]), while the DSC trace of a PU/tOPV (0.25 wt%) film that was repeatedly deformed
to 300% strain mirrors the one of a pristine sample. Sample fluorescence was also
monitored while extending films to 250% strain and subsequently releasing them from
the clamps of the tensile tester. To directly compare the fluorescence changes, PU/tOPV
and PBA-PU/tOPV blends with 0.25 wt% of the additive were used. In the case of PU/tOPV
(0.25 wt%), only a slight initial increase of the monomer emission was observed that
can be ascribed to photoinduced reactions of the OPV dyes ([Figure S21 ]),[20 ] particularly since this increase is also observed for pristine samples ([Figure S22 ]). By contrast, the same experiment with a PBA-PU/tOPV (0.25 wt%) blend film shows
that sample relaxation is followed by a logarithmic increase in the I
510 /I
630 ratio over the course of 1.5 h ([Figures S20 ] and [S21 ]). The fluorescence response reflects that relaxation processes continue to occur
in plastically deformed samples of the semicrystalline PBA-PU. Indeed, extension and
stress-relaxation experiments with PBA-PU/tOPV (0.25 wt%) samples carried out at 55
°C, i.e., above the melting transition of the PBA domains, show that the mechanical
properties and fluorescence response closely resemble the behavior of PU/tOPV blends
([Figure S24 ]).
To further demonstrate the utility of the tOPV additive in enabling a reversible mechanochromic
response in these different polyurethanes, cyclic loading–unloading experiments with
both blends were carried out. The results show that (dis)aggregation of the additive
is fully (PU/tOPV) or partially (PBA-PU/tOPV) reversible over at least five deformation
and relaxation cycles ([Figures 3 ] and [S25 ]
[S26 ]
[S27 ]), mirroring the behavior of the respective polymer matrices in their corresponding
stress–strain curves ([Figure S28 ]). To facilitate the detection of minor changes in the I
510 /I
630 ratio during cyclic deformation, PU/tOPV blends with 0.5 wt% were used. Following
the changes in the I
510 /I
630 ratio of these samples upon extension to a strain of 250%, the original fluorescence
color and I
510 /I
630 ratio are almost completely recovered after relaxation ([Figure 3a ]). Interestingly, in subsequent extension and contraction cycles, the I
510 /I
630 ratio follows the strain even more closely than in the first deformation cycle ([Figures 3a ] and [S25 ]
[S26 ]). A detailed analysis of in-situ fluorescence spectroscopy data recorded during
deformation of pristine PU/tOPV (0.5 wt%) to a maximum strain of only 30% shows that
a reversible mechanochromic response is observed before significant plastic deformation
occurs ([Figures 3c ] and [S29 ]). The increase of the I
510 /I
630 ratio can be detected at strains at least as low as 5% ([Figures 3d ] and [S29 ]), suggesting an efficient transfer of the small mechanical stresses of the surrounding
matrix to the tOPV aggregates.
Figure 3 Plots showing the results of cyclic deformation experiments of (a, c, and d) PU/tOPV
(0.5 wt%) and (b) PBA-PU/tOPV (0.25 wt%) blends. Shown are the time–stress (blue)
and time–strain (green) curves and the corresponding fluorescence response (I
510 /I
630 ratio, red points). Experiments were conducted by controlling the strain (a: 250%,
b: 300%, c, d: 30%). d) The comparison of the stress–strain curves (blue lines) and
the I
510 /I
630 ratio (each red dot represents the average of data points from all six cycles; error
bars correspond to one standard deviation) as a function of strain recorded during
each extension cycle of (c) shows a very good correlation.
The results of cyclic loading–unloading experiments with films of the PBA-PU/tOPV
(0.25 wt%) blend show that the first deformation of a pristine sample to a strain
of 300% causes significant plastic deformation ([Figure 3b ]), with a recoverable strain of ca. 100% ([Figures 3b ], [S28c ], and [Video 2 ]). The limited recovery is clearly reflected by the I
510 /I
630 ratio, which increases from 0.8 (as-prepared film) to 1.1 after the first loading
and unloading cycle. In subsequent deformation cycles, the sample fully recovers,
and the I
510 /I
630 ratio traces the applied stress and strain reasonably well, although a “drift” towards
monomer emission is evident. Indeed, the increase in monomer emission observed for
the PBA-PU/tOPV films in stress-relaxation experiments ([Figure 2d ]) is reflected in the increasing baseline of the I
510 /I
630 ratio after unloading to 0.1 MPa ([Figures 3b ] and [S27 ]). The in-situ optomechanical characterization of the PU/tOPV and PBA-PU/tOPV blends
unambiguously demonstrates that the use of tOPV as an additive imparts MRL behavior
to very different types of polymer matrices. Moreover, the observed response at relatively
low strains in an elastomer strongly supports that chain entanglement between the
matrix and the additive's poly(ethylene-co -butylene) backbone is what enables the efficient transfer of mechanical stresses,[17 ] and the precise mechanism of force transfer is currently under investigation.
To further demonstrate the usefulness of the tOPV additive to monitor complex thermomechanical
processes, shape-memory programming and release experiments were carried out with
films of the semicrystalline PBA-PU/tOPV (0.25 wt%) blend while simultaneously monitoring
the fluorescence response. Thus, films were heated to 55 °C (i.e. , above the T
m of the PBA domains) and uniaxially deformed at this temperature (using stress control;
see the Supporting Information for details) to a strain of 45% ([Figures 4 ], [S30 ], and [Video 3 ]). The temporary shape was then fixed by keeping the stress constant (0.9 MPa), while
the sample was cooled to 0 °C and maintained at this temperature for 5 min. Due to
the crystallization of the PBA segments in the fixing step, the sample further elongated
to a strain of ca. 70%. The sample showed little contraction (1.5% strain) when the
stress was reduced to the lowest possible level (a preload of 0.15 MPa), indicating
good fixity of the temporary shape (98%). Finally, the recovery of the permanent shape
was triggered by heating the sample again to 55 °C while keeping the stress at a minimum
level (0.15 MPa). Gratifyingly, the original shape was practically fully recovered,
with a recovery ratio of 90% for the first cycle and 100% for all subsequent cycles.
This cycle was repeated three times ([Figure S30 ]) and the fluorescence response was spectroscopically monitored during the last cycle.
Thus, when PBA-PU/tOPV (0.25 wt%) was heated to 55 °C and deformed to 45% strain,
the I
510 /I
630 ratio was found to correlate with the applied strain ([Figure 4 ]). During shape fixing, the PBA-PU/tOPV sample is held under tension while being
cooled to 0 °C, which induces PBA crystallization and leads to a concomitant elongation
and increase in the I
510 /I
630 ratio. One may speculate that the increased monomer emission is caused by internal
strains that arise when the crystallization of PBA domains leads to a local contraction,
in line with the above-discussed observations during stress relaxation at room temperature
([Figures 2d ] and [S20 ]
[S21 ]
[S22 ]
[S23 ]). While no significant changes to the I
510 /I
630 ratio were observed when samples were unloaded in the programmed shape, melting the
PBA domains by heating to 55 °C and subsequent release of the strain for shape recovery
led to the reformation of dye aggregates and a recovery of the I
510 /I
630 ratio. Thus, the fluorescence response shows that the tOPV additive provides a reliable
way to directly monitor the complex, internal stresses occurring in PBA-PU during
a shape-memory cycle.
Figure 4 Shape-memory cycle carried out on a PBA-PU/tOPV (0.25 wt%) blend film while monitoring
fluorescence in situ. a) The fluorescence response was followed via the I
510 /I
630 ratio, which is represented using the displayed color code, during the last of three
consecutive shape-memory cycles. The minor signal fluctuations that occur at the end
of the recovery and deformation phases are due to instrument artifacts, stemming from
the fact that the tensile equipment used is not optimized for stress-controlled shape-memory
cycling. b) Two-dimensional representation of the data in part (a). The lower panel
plots the strain (blue line) and temperature (green line) profiles applied during
the shape-memory cycle. The upper panel plots the corresponding fluorescence response
(red dots; dark red line represents a smoothened 20-point average).
Conclusions
In summary, we have shown that blending TPU elastomers with a small weight fraction
of a telechelic additive carrying two excimer-forming OPV dyes at the termini provides
straightforward access to MRL elastomers. The fluorescence response due to dissociation
of dye aggregates is highly sensitive to the deformation and relaxation processes
that occur in the herein investigated different types of polymer matrices. A detailed
analysis of the MRL behavior shows that the application of low strains already furnishes
an easily detectable and reliable mechanochromic response, regardless of the crystallinity
of the host matrix. The fluorescence response was found to scale with the applied
strain and its release during multiple cycles of deformation or relaxation, respectively.
Moreover, complex processes upon stress relaxation under tension or during a shape-memory
cycle were readily followed by monitoring the fluorescence changes. To achieve a defined
MRL response with low-molecular-weight aggregachromic dyes, the latter generally need
to be tailored to a given host polymer or covalently integrated to avoid excessive
phase segregation and the formation of bulky, persistent aggregates. Given that tOPV
fluorescence behavior appears to be intricately linked to the mechanically induced
rearrangements of the surrounding matrix, such additives should be able to provide
information about the nature and extent of (very small) deformations experienced by
a priori any elastomeric material. We anticipate that similar mechanochromic additives
bearing different excimer-forming dyes at the termini or comprising different cores
may further expand the tunability of such deformation sensors.
Experimental Section
The telechelic tOPV additive and C18-RG dye used in this study were prepared as previously
reported.[9 ]
[17 ] BBS (97%) was obtained from Sigma Aldrich and used without further purification.
The TPU elastomers PBA-PU (Desmopan 2795A; number-average molecular weight M
n = 93 kDa, Ð = 2.1) and PU (Texin 985; M
n = 111 kDa, Ð = 2.0) were received in the pellet form from Covestro AG and were used as received.
The polyurethane/tOPV blends were prepared by solvent casting the polymer and tOPV
from THF solutions into a PTFE mold with a diameter of 6 cm, drying overnight, and
subsequently compression-molding the materials at 170–180 °C to obtain 230–330 µm
thick films. Blend films containing C18-RG and BBS dyes were fabricated following
the same process. Films were deformed in a stepwise manner using a custom-made stretching
frame, stress–strain curves were measured on a Zwick/Roell Z010 tensile testing machine
with a 200 N load cell, and mechanical tests with in-situ fluorescence measurements
were carried out using a Linkam TST350 microtensile stage equipped with a 20 N load
cell. Unless otherwise noted, a strain rate of 50% min−1 was used. Films for mechanical testing were prepared either as rectangular films
(with dimensions of 3.0 × 30 mm for manual stretcher and microtensile experiments)
or as ASTM standard dog-bone-shaped samples (with dimensions of 5.0 × 40 mm for experiments
conducted on the Zwick/Roell Z010 tensile tester). In-situ fluorescence measurements
were performed using an Ocean Optics USB 4000 spectrometer connected to an Ocean Optics
LS-450 LED light source with an excitation wavelength of λ
ex = 380 nm and an Ocean Optics QR230-7-XSR SMA 905 optical fiber positioned perpendicular
to the film at a distance of approximately 2 mm. Fluorescence microscopy images were
acquired in reflectance mode using an Olympus BX51 microscope with an integrated Olympus
DP72 camera. Photographs of as-prepared and stretched films were taken with a Nikon
D7100 digital camera equipped with an AF-S DX Zoom-NIKKOR 18-135mm lens (f/3.5-5.6G
IF-ED).
Video 1 Reversible deformation of a PU/tOPV (0.2 wt%) sample. (Repeated extension and contraction
cycles recorded under UV light illumination (365 nm).)
Video 2 Deformation of a PBA-PU/tOPV (0.25 wt%) sample. (Manual sample deformation recorded
under UV light illumination (365 nm).)
Video 3 Shape memory cycle with a sample of PBA-PU/tOPV (0.25 wt%). (Movie recorded under
UV light illumination (365 nm) during the shape memory cycle shown in [Fig. 4 ].)
