In this Letter, we study the mechanical and optical response of a thermoplastic polyurethane blended with 0.5 wt. % of bis(benzoxazolyl)stilbene dye. The mechanochromic behavior of the material is characterized in a uniaxial stress-relaxation test by simultaneously acquiring the applied force, mechanical deformation, and fluorescence emission. To offer insight into the stress-strain response of the polymer-dye blend, we adapt a classical nonlinear constitutive behavior for elastomeric materials that accounts for stress-induced softening. We correlate the fluorescent response with the mechanical strain to demonstrate the possibility of accurate strain sensing for a broad range of deformations during both loading and unloading.

Recent advancements in the study of functional materials have promoted an extensive application of soft polymeric materials in the design of active sensors, either by physical dispersion of an active receptor within the polymer matrix or by chemical modification of the polymer backbone.1 Through these structural modifications, the mechanical stimulus is correlated to a predictable, controllable, and measurable change in the properties of the material. For example, mechanical deformations have been associated with variations of electrical capacitance2,3 and resistivity4,5 in conductive polymers and ionic charge redistribution in polymer-metal composites.6 Beyond electromechanical sensing, optomechanical properties of polymers have been leveraged in the study of biological flows through shear sensitive photochromic coatings,7 of biological microswimmers via photoelastic gels,8 and magnetic resonance through the dispersion of polarizable particles in an elastomeric material.9 

Mechanochromic polymers are a novel class of light-sensitive materials, whose properties can be effectively tailored to design highly sensitive non-contact mechanical sensors. Mechanochromic polymers display a detectable change in their optical properties upon reversible or irreversible deformation of their macromolecular structure.10 A stress-dependent optical response is attained either by physical dispersion of a dye in a predetermined polymeric material or by modification of the polymeric backbone through covalent bonding of a chromogenic molecule.11,12 Mechanochromic materials obtained through physical mixing are composed of a polymeric host, usually a thermoplastic polymer, and a fluorescent aggregachromic dye in the form of supramolecular aggregates. Aggregachromic dyes commonly used in these applications10 are characterized by two distinct states with fluorescence emissions at different wavelengths, a monomer state and an aggregate state characterized by excimer emission. Mechanical deformation of the macromolecular backbone results in the progressive reorganization of dye aggregates and creation of a highly dispersed monomeric phase. The progressive disaggregation of the dye is responsible, in turn, for a detectable increase in the monomer to excimer emission ratio. This transition can be effectively utilized to sense mechanical deformation of the polymer.

Mechanochromism of polymer-dye blends has been recently demonstrated for different thermoplastic polymers, such as polyethylene terephthalate,13 polypropylene,14 and thermoplastic polyurethanes.15,16 Thermoplastic polyurethanes (TPU) are particularly interesting for sensing application, since: (i) the mechanochromism of TPU-dye blends shows a certain degree of reversibility, which is desirable in the design of reusable sensors; (ii) the high compliance and low modulus of TPU-dye blends enable their use for sensing small forces at high levels of strain; and (iii) TPU polymers are biocompatible materials.

Here, we demonstrate strain-sensing through a mechanochromic polymer blend composed of TPU doped with a fluorescent aggregachromic dye. The TPU is ESTANE-90AE (purchased from Lubrizol) and the dye is bis(benzoxazolyl)stilbene (BBS) (purchased from Sigma Aldrich); both are used without further purification. The TPU glass transition temperature is −34 °C and the Vicat softening point is 90.6 °C. A polymer-dye film is prepared by solution casting to perform the requisite optomechanical characterization. Briefly, the dye is dispersed by mechanical stirring in a compatible solvent (DMF 99% purity, Sigma Aldrich). The TPU is added to the dye solution and dissolved by vigorous mechanical stirring at 90 °C for 2 h. Upon complete dissolution of the polymer in the solvent, the polymer-dye solution is cast in a steel mold treated with teflon spray as a detaching agent. The solution is left for 24 h at 65 °C to completely evaporate the solvent. Films are fabricated with thickness of 0.2–0.3 mm and dye weight ratio of 0.5%. The material shows an even distribution of dye within the polymer host. Tensile specimens of planar dimension 48 × 5 mm2 are obtained from the solution-cast film using an aluminum template and a cutting tool.

Experiments are conducted on an in-house developed miniature tensile test machine assembled on an optical table. The setup consists of two continuous rotation HS-805BB servo-motors connected to an Arduino Uno board (www.arduino.cc) to control the distance between two clamping grips (SC-8 Imada Force Gauge Attachments). One clamp is actuated by the motors, while the other clamp is connected to a FUTEK LSB303 25 lb S-beam type load cell. During testing, the distance between the clamps holding the sample is progressively increased, and the tensile force is measured through the load cell. The initial distance between the clamps is 36 mm, and the clamping length is set to 6 mm. A Point-Grey FLEA FL3-U3-13E4C-C high speed camera is used to acquire sample deformations during the test. To reduce the effect of the clamping conditions, we focus on a portion of the sample that is sufficiently far from the edges of the clamps. For this purpose, we place two fiduciary marks on the polymer surface 6 mm from the edge of each clamp, with a relative distance of l0 = 24 mm from each other. The marks are gently traced with a marker on the surface of the samples to assure the absence of scratches that may lead to stress intensification during loading. In the post-processing phase, we use a built-in feature tracking technique in Proanalyst software (www.xcitex.com) to determine the positions of the marks across consecutive frames and compute the actual sample length. As the film deforms, its fluorescent emission is acquired in real time by an Ocean Optics USB2000+ portable spectrometer. The spectrometer is connected to a compatible fiber optic probe (600 μm diameter silica core with numerical aperture of 0.22) positioned 20 mm from the sample, facing the sample surface at an angle of 90°. Four 385 nm light emitting diodes are positioned on the opposite side of the sample with respect to the probe and utilized as monochromatic excitation sources. The fluorescence signal is filtered before acquisition of the spectrum by a dichroic filter with cut-on wavelength at 409 nm. To improve the quality of the fluorescence signal, the acquisition is performed in a dark room.

Experiments are conducted at room temperature (approximately 23 °C) and the strain rate is set to 0.025 s−1. The output of the load cell is acquired using a USB-6221 National Instruments data acquisition board controlled through Labview (www.ni.com). During the test, the camera acquisition frequency is 7.5 frames per second, and video is acquired for the entire duration of the test. The acquisition of the fluorescence spectra is performed with an integration time of 3 s for the photon count process. A complete loading-unloading cycle of the sample is performed to assess reversible mechanochromism of the material. Following Ref. 17, the load is applied in a sequence of steps, allowing the stress field to relax toward equilibrium after each step. During the loading phase, the distance between the clamps is increased in 8 steps of 5 s for a total of 40 s, which corresponds to a final strain of approximately 100%. After each step, the stress is allowed to relax for 60 s. An equivalent number of steps is used during the unloading phase with 60 s relaxation time. Results reported in this Letter pertain to nine virgin samples, which have no prior strain history. First, we focus on a single specimen to illustrate the physics of mechanochromism of TPU-BBS blends, and then we present statistical evidence for the whole batch of samples.

The stretch and nominal axial stress measured during the stress relaxation experiment for a sample are reported in Figure 1. The stretch λ is computed by dividing the gauge length l by the initial gauge length l0. The nominal stress FA0 is the ratio between the force reading of the load cell F and the original cross-sectional area of the unstressed sample A0 = 1.5 mm2. Figure 1(a) displays the stretch of the sample as a function of time during the cyclic test. A residual stretch λr = 1.18 is measured when the tension is completely released, which is related to plastic deformations. As shown in Figure 1(b), during each step, we observe an initial sharp increase of the stress in the material, which is followed by a progressive decrease during stress relaxation. In the unloading phase, the process is reversed, and the stress increases during relaxation after an initial sharp decrease. This transient behavior has been previously found in compression tests of TPU17 and should be related to the non-equilibrium rate dependent response of the polymer. Here, we focus on the equilibrium constitutive behavior of the polymer.

FIG. 1.

(a) Stretch as a function of time during the loading-unloading cycle. (b) Nominal stress in the material as a function of time during the loading-unloading cycle.

FIG. 1.

(a) Stretch as a function of time during the loading-unloading cycle. (b) Nominal stress in the material as a function of time during the loading-unloading cycle.

Close modal

For the sample considered in Figure 1, we report in Figure 2 the stress-strain response of the material by plotting the true stress σ=λFA0 as a function of the strain (λ − 1). Therein, we isolate the equilibrium path as the locus of the equilibrium states following the procedure in Ref. 17 and neglecting the presence of the plastic deformation. In the stress-strain response of the TPU/BBS blend, we identify an initial stiff response for deformations below λ = 1.2 and a more compliant response at higher stretches, followed by an increase in the tensile modulus for λ close to 2. The initial modulus of the material E=FA0(λ1) is computed on the first loading step of the stress relaxation experiment by fitting the stress-strain curve with an affine function. In Figure 1, we identify E = 16.30 MPa for the TPU-BBS blend sample, which is in line with other studies on TPU blends.18–20 

FIG. 2.

(a) Experimental stress-strain curve for a TPU-BBS blend sample as determined from the stress-relaxation test (crosses) and equilibrium path identified from the value of the stress after 60 s relaxation time (red dashed line). For ease of visualization, the equilibrium points are connected by straight segments. (b) Equilibrium path and material behavior as predicted from the model in Eq. (1). In the inset, the variation of the soft domains volume fraction with the strain λ – 1.

FIG. 2.

(a) Experimental stress-strain curve for a TPU-BBS blend sample as determined from the stress-relaxation test (crosses) and equilibrium path identified from the value of the stress after 60 s relaxation time (red dashed line). For ease of visualization, the equilibrium points are connected by straight segments. (b) Equilibrium path and material behavior as predicted from the model in Eq. (1). In the inset, the variation of the soft domains volume fraction with the strain λ – 1.

Close modal

As suggested in Refs. 17 and 21, the mechanical properties of TPU blends are controlled by the interaction between soft and hard domains that characterizes the microstructure of TPU polymers. Experimental findings in Figure 2 demonstrate that the TPU/BBS blend presents a significant hysteretic behavior, with consistent softening during the unloading phase. In particular, softening in the unloading phase can be related to an increase of the volume fraction of the soft domains during the deformation due to a structural reorganization of soft and hard domains.17 The extent of the the hysteresis loop can be measured through the hysteresis ratio HR, defined as the ratio between the difference of the strain energies during loading and unloading scaled by the strain energy during loading (both these quantities are calculated based upon a piecewise constant equilibrium path, as illustrated in Figure 2(a)). For the sample in Figure 1, we compute HR = 0.42.

Following Ref. 17, we assume that the constitutive response of the TPU blend is well approximated by a modified version of the Arruda-Boyce model22 of the form

σ=vsXμ3NΛchainL1(ΛchainN)(λ21λ),
(1)

where μ and N are parameters of the polymeric network,17,23 vs is the volume fraction of soft domains, X=1+2.5(1vs)+18(1vs)2 is the stretch amplification factor, and Λchain=X(13(λ2+2λ1)1)+1 is the amplified chain stretch. The function L−1(β) is the inverse of the Langevin function, that is, L(β)=coth(β)1β. Different from a classical hyperelastic model, the stress may vary between the loading and the unloading cycles. Specifically, during tensile loading of the virgin material, the volume fraction of soft domains evolves as follows:

vs=vss(vssvs0)exp(AΛchain1NΛchain),
(2)

where vs0 and vss are the initial and maximum volume fraction of soft domains in the polymer, respectively, and A is a parameter that regulates the evolution of vs. Note that vs cannot be computed directly from (2) since the amplified chain stretch is a function of vs itself through X. Thus, the calculation of vs requires the solution of a nonlinear algebraic equation. During the material unloading, the volume fraction of the soft domains does not change and equals the value assumed for the maximum chain stretch Λchainmax attained during the loading phase.17,23 We note that if a second loading cycle were considered, the evolution of vs would be re-activated upon reaching an amplified stretch greater than Λchainmax.

To illustrate the possibility of explaining the equilibrium constitutive response through the adapted Arruda-Boyce model in Eqs. (1) and (2), we refer to the parameters reported in Ref. 17 for TPU. Specifically, we set N = 6, vs0 = 0.4, vss = 0.8, and A = 1.4, as in Ref. 17, and for replicating the value of the peak stress after relaxation σmax = 6.84 MPa in Figure 2(a), we modify the constant μ to 0.431 MPa. The theoretical stress-strain behavior is reported in Figure 2(b), along with the evolution of the volume fraction of soft domains as predicted from Eq. (2). Interestingly, the model is successful in anticipating a robust softening effect, which, in turn, causes a significant hysteresis loop in the loading-unloading cycle of the virgin material. More specifically, the model underestimates the initial tensile modulus during the loading cycle, consistently with observations in Ref. 17, while it accurately predicts the modulus during the unloading phase, which is expected to be associated with the response of the film in further cyclic loading.

Normalized emission spectra collected during the stress relaxation tests in Figures 1 and 2 are displayed in Figure 3. Figure 3(a) reports the normalized emission during the material loading as a function of the progressive stretch of the TPU/BBS sample. The spectrum of the TPU/BBS blend shows two distinct peaks in the vicinity of 436 and 493 nm. The first peak (436 nm) is associated with fluorescence of a monomeric state of the dye, and the peak at higher wavelength (493 nm) corresponds to the excimer fluorescence of dye aggregates.14,15 The emission spectra are normalized with respect to the value at 436 nm to highlight the relative intensity of the excimer emission with respect to the monomer emission in the polymer-dye blend. By increasing the stretch of the specimen, we observe a progressive reduction of the peak at 493 nm with respect to the peak at 436 nm. This phenomenon can be related to a progressive dissolution of the dye aggregates in the TPU matrix, with a relative increase of the fluorescence emission of the monomeric dye phase. The process is inverted during the unloading phase, as displayed in Figure 3(b). The excimer emission progressively increases as the polymer stretch is reduced. Notably, the process is not completely reversible as the initial ratio between excimer to monomer emissions is not recovered after the load is completely released. This is likely associated with the residual deformation after stretching observed in Figure 1.

FIG. 3.

(a) Normalized emission spectra at 436 nm of a TPU-BBS blend as a function of the progressive stretch of the sample during the loading phase. Blue (top) to purple (bottom) are spectra obtained for increasing stretch levels (blue, λ = 1.00; green, λ = 1.21; red, λ = 1.39; cyan, λ = 1.89; and purple, λ = 2.04). (b) Normalized emission spectra at 436 nm at decreasing stretch levels during the unloading phase. Blue (bottom) to purple (top) are spectra obtained for decreasing stretch levels (blue, λ = 2.01; green, λ = 1.84; red, λ = 1.58; cyan, λ = 1.27; and purple, λ = 1.18). (The emission at wavelengths below 409 nm is blocked by the filter.)

FIG. 3.

(a) Normalized emission spectra at 436 nm of a TPU-BBS blend as a function of the progressive stretch of the sample during the loading phase. Blue (top) to purple (bottom) are spectra obtained for increasing stretch levels (blue, λ = 1.00; green, λ = 1.21; red, λ = 1.39; cyan, λ = 1.89; and purple, λ = 2.04). (b) Normalized emission spectra at 436 nm at decreasing stretch levels during the unloading phase. Blue (bottom) to purple (top) are spectra obtained for decreasing stretch levels (blue, λ = 2.01; green, λ = 1.84; red, λ = 1.58; cyan, λ = 1.27; and purple, λ = 1.18). (The emission at wavelengths below 409 nm is blocked by the filter.)

Close modal

In Figure 4(a), we report the monomer to excimer emission ratio I436/I493 measured during the tensile test as a function of time. The emission ratio shows a progressive increase as a function of the polymer stretch in the loading phase, with a relative increase of approximately 60% as the stretch increases from λ = 1 to λ = 2. As the process is inverted, I436/I493 decreases with a reduction of comparable intensity as the stretch of the sample is varied from λ = 2 to λ = 1.18. The initial value of the emission ratio is approximately (I436/I493)0 = 0.52, and the final value is (I436/I493)1 = 0.56. These values are characteristic of the specific polymer-dye blend, the weight ratio, and the fabrication process. We expect that different emission ratios would be obtained by varying these parameters.

FIG. 4.

(a) Monomer to excimer emission ratio I436/I493 as a function of time during the loading and unloading phases. (b) Emission ratio as a function of the increasing stretch during the loading (black crosses) and unloading (red dots) phases. Lines are fitting of experimental data for loading (solid black) and unloading phases (dashed red). The coefficients of determination are R2 = 0.71 and R2 = 0.95 for loading and unloading, respectively.

FIG. 4.

(a) Monomer to excimer emission ratio I436/I493 as a function of time during the loading and unloading phases. (b) Emission ratio as a function of the increasing stretch during the loading (black crosses) and unloading (red dots) phases. Lines are fitting of experimental data for loading (solid black) and unloading phases (dashed red). The coefficients of determination are R2 = 0.71 and R2 = 0.95 for loading and unloading, respectively.

Close modal

In Figure 4(b), we display I436/I493 as a function of the sample strain (λ − 1) during the loading and unloading phases. We notice that the emission ratio does not directly correlate with the stress in the material, and no transient in the fluorescence emission is observed. This suggests that structural modification related to viscoelastic phenomena are secondary with respect to time-independent network deformations in controlling the aggregation state of dye molecules dispersed within the polymer. We observe that a linear relation holds between the stretch of the polymer network and the emission ratio. This behavior shares similarities with previous observations for polyurethane-based mechanochromic materials,16 where a comparable linear relation can be observed for values of λ between 1 and 2. On the other hand, a higher reversibility of the mechanochromic response is observed in this study with respect to previous investigations.15,16 Figure 4(b) reports the fitting with an affine function I436/I493 = α0 + α1(λ − 1) of the data recorded during the loading and unloading phases. The values of the coefficients identified are α0 = 0.528 and α1 = 0.351 for the loading phase, and α0 = 0.538 and α1 = 0.264 for the unloading phase. Notably, a reduction in the slope of the curve is found during the unloading phase, which can be attributed to the permanent modification of the polymeric network generated by the large deformation. However, the large extent of the reversible mechanochromic response of TPU/BBS blends open the door to the design of reusable, non-contact, corrosion- and chemical-resistant, bio-compatible, large strain optical sensors. To offer an estimate of the efficiency of the optical sensor, we compute the equivalent optical gauge factor Kopt=1λ1α1(I436/I493)0, which yields Kopt = 0.67 from Figure 4(b), a highly comparable value to resistive gauge factors of traditional metal strain sensors.4,5

Table I reports aggregated data for the entire set of nine specimens considered in this work. Importantly, we find that mechanical properties of the TPU-BBS blend, such as the peak stress after relaxation, residual strain, elastic modulus, and hysteresis ratio display limited variations of the order of 20% of the nominal values across the samples. Similar variations are found on salient optical properties, such as the initial emission ratio and the coefficient α1, which quantifies strain sensing during loading and unloading.

TABLE I.

Mean and standard deviation for salient mechanical and optical properties measured during stress-relaxation experiments for the nine virgin TPU-BBS samples.

σmaxλr – 1E (MPa)HR(I436/I493)0α1, loadingα1, unloading
Mean 8.12 0.18 18.87 0.42 0.457 0.355 0.197 
Standard deviation 1.13 0.04 2.61 0.03 0.070 0.057 0.061 
σmaxλr – 1E (MPa)HR(I436/I493)0α1, loadingα1, unloading
Mean 8.12 0.18 18.87 0.42 0.457 0.355 0.197 
Standard deviation 1.13 0.04 2.61 0.03 0.070 0.057 0.061 

In this Letter, we studied a mechanochromic polymer-dye blend composed of a thermoplastic polyurethane and bis(benzoxazolyl)stilbene aggregachromic dye. The material was tested on a dedicated setup to characterize optical and mechanical response simultaneously during a uniaxial stress-relaxation test. To provide further understanding of the material behavior, we have adopted a classical constitutive model for the stress-strain behavior of elastomers. We have shown that a linear relation holds between the stretch of the polymeric network and the variation of the monomer to excimer emission ratio during uniaxial tension. We expect that this new class of soft optical sensors will find application in fluid mechanics and environmental sensing, where the use of electromechanical sensors is often limited by the presence of wet environments or corrosive agents.

This research was supported by the National Science Foundation under Grant No. CBET-1332204. The authors also acknowledge the support of the Office of Naval Research through Grant No. N00014-10-1-0988 that has allowed the acquisition of equipment used in this study. The authors would like to thank Dr. Avi Ulman, Dr. Stephen Arnold, and Dr. Charles Martucci for inspiring talks and advice, the Institute of Engineering Interfaces at the New York University Polytechnic School of Engineering for granting access to the experimental facilities, and Steven Osma for help on assembling the experimental setup.

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