A New Class of Mechano‐Responsive Polyurethane Via Anthracene ‐TAD Diels‐Alder (DA) Click Chemistry

Abstract

Smart or stimuli‐responsive polymers have garnered significant interest in the scientific community due to their response to different stimuli like pH, temperature, light, mechanical force, etc. Mechanophoric polymer is an intriguing class of smart polymers that respond to external mechanical force by producing fluorescent moieties and can be utilized for damage detection and stress‐sensing assessment. In recent reports on mechanophoric polymers, different mechanophoric motifs such as spiropyran, rhodamine, coumarin, etc. are explored. This investigation reports a new kind of mechanophoric polyurethane (PU) adduct based on Diels‐Alder (DA) click chemistry. Here, an anthracene(An)‐end capped tri‐armed urethane system is synthesized, followed by a DA reaction using bis‐(1,2,4‐triazoline‐3,5‐dione) (bis‐TAD) derivative. The incorporation of bis‐TAD in the urethane system renders the anthracene inactive (“turn‐off”) by dismantling its conjugation as a result of a successful DA reaction. The soft PU translated into a harder material through bis‐TAD linkages between polymer chains as evident from nanoindentation (NINT) analysis. The resulting material reverts back to its fluorescent “turned‐on” mode owing to a force‐accelerated retro‐Diels‐Alder (r‐DA) reaction. Besides the mechanophoric attributes, the material demonstrates self‐healing behavior examined by microscopic investigations. This innovative approach can be a potential route to design responsive polymers with dynamic functionalities for advanced material applications.

Stimuli‐responsive polymers, including mechanophoric variants, are intriguing for their unique properties and found useful for damage assessment. This study introduces a novel mechanophoric Diels‐Alder polyurethane adduct employing Diels‐Alder Click chemistry. Anthracene end‐capped urethane undergoes a D‐A reaction with a bis‐TAD derivative, rendering anthracene inactive. However, mechanical force triggers a r‐Diels‐Alder reaction, restoring fluorescence.

1. Introduction

Mechanophores are interesting functional molecules, which on application of mechanical force, undergo structural alterations, such as conformational shifts or changes in physical properties like absorbance or emission of light.^{[ 1 ]} The inclusion of such kind of material in a polymer system results in a mechanoresponsive polymer. A mechanoresponsive polymer is categorized as a distinct class of polymer that undergoes a physical or chemical change in response to mechanical stimuli, such as stretching, compression, shear, or pressure.^{[ 2 ]} These polymers can exhibit various behaviors, including changes in shape, color, conductivity, or mechanical properties, upon mechanical deformation.^{[ 3 ]} The mechanical impact on the mechanoresponsive polymer can be understood when the species exhibit color and/or fluorescence. Comparing the difference in fluorescence before and after, the deformation or damage can be identified, and thus, it offers a practical method for assessing the potential utility of such material in damage sensing. For example, multiple research groups have demonstrated force‐induced ring opening of spiropyran and naphthopyran to intense colored merocyanines in bulk materials and found their applications in multiple domains, including smart materials, sensors, actuators, and drug delivery systems.^{[ 4} , ^{5 ]}

In polymeric materials, a variety of mechanophoric motifs have been exploited to synthesize mechanoresponsive materials. Spiropyran (SP) is the most explored mechanophoric motif among all the reported ones. Moore's group reported diverse ranges of mechanophoric systems using spiropyran, in which colorless SP was anchored to the backbone of a Polymethylacrylate (PMA) chain. The application of uniaxial extension results in the conversion of spiropyran units into brightly colored merocyanine, leading to a change in the appearance of PMA materials from yellow to purple.^{[ 4} , ⁶ , ^{7 ]} Different pyran analogs, including bis‐naphthopyran (BNP),^{[ 8 ]} spirothiopyran (STP),^{[ 9 ]} and naphthopyran (NP),^{[ 10 ]} have been used to prepare different mechanophoric systems. Many research groups explored rhodamine, a well‐known fluorescent dye, as a mechanophoric motif attached to different functional polymers. Due to mechanically induced conformational changes, the rhodamine showed intense colors during loading and unloading.^{[ 11} , ¹² , ¹³ , ^{14 ]} In our previous work, we used the rhodamine derivative to synthesize an epoxy‐based mechanoresponsive polymer.^{[ 15 ]} Craig and his colleagues included coumarin dimers into various polymer and elastomer backbones and investigated the mechanophoric activation of the dimer upon sonication.^{[ 16} , ^{17 ]} Chen et al., were the first to report bis(adamantyl)−1,2‐dioxetane as a mechanophore, in which the application of force caused the mechanophore to split into two excited adamantanone units. The phenomenon of bright blue fluorescence is observed when the excited adamantanone molecule returns to its ground state.^{[ 18 ]} Anthracene DA adducts are widely used as a mechanofluorescent compound owing to their exceptional fluorescent quantum yield. There are a few anthracene mechanofluorophores that have been successfully included in polymers. These include anthracene dimer^{[ 19 ]} and anthracene‐maleimide (A‐M) DA adduct.^{[ 20} , ²¹ , ²² , ^{23 ]} The fluorescence of the anthracene motif is quenched due to a successful DA reaction. Interestingly, the application of mechanical force triggers ther‐DA reaction of the anthracene DA‐adducts, leading to the release of fluorescent anthracene derivatives.

Bis‐TAD‐based derivatives are recognized for their propensity to undergo a multitude of significant types of reactions, such as click,^{[ 24 ]} Diels‐Alder,^{[ 25} , ²⁶ , ²⁷ , ^{28 ]} Alder‐ene,^{[ 29 ]} electrophilic aromatic substitutions,^{[ 30 ]} and [2+2]‐cycloaddition reactions. In the present scenario, polymer‐TAD mechanochemistry can open up enormous research avenues in the materials science community due to their wide range of reaction possibilities. In 2014, Gossweiler et al., discovered that the release of Phenyl‐TAD originates from the DA adduct of Phenyl‐TAD and anthracene due to the flex‐activated r‐DA reaction.^{[ 31 ]} Recently, Chia‐Chih Chang et al., reported an anthracene‐TAD mechanophore system based on force‐accelerated r‐DA chemistry.^{[ 2 ]}

To date, there is no report based on the combination of anthracene‐TAD mechanochemistry to PU system. Therefore, ample opportunities are accessible to explore a wide range of mechanoresponsive attributes with regard to cycloaddition reactions. The optical features of the r‐DA reaction of PU‐anthracene DA adducts provide a viable foundation for the development of new mechanophores appropriate for stress‐sensing applications together with self‐healing attributes.

In this case, we report a mechanoresponsive polymer capitalizing polyurethane‐based on anthracene‐TAD mechanochemistry. We have chosen PU due to its versatility as a polymer, which allows for a wide range of applications such as paints, varnishes, adhesives, sealants, sports equipment, biomedical devices, and more. By manipulating the functionality and stoichiometry of the reactants during PU synthesis, there exists a wide range of possibilities to suitably design the mechanophoric polymer system. Here, a new class of PU‐ mechanophoric system was prepared based on anthracene‐TAD DA click and r‐DA unclick chemistry, as illustrated in Scheme 1 . A hydroxy‐terminated tri‐arm polycaprolactone (PCL) was synthesized by ring‐opening polymerization (ROP) of ɛ‐Caprolactone (ɛ‐CL) with 1,1,1,‐tris(hydroxymethyl)propane (TMP). This was further reacted with diisocyanate and a chain capping agent viz., 9‐Anthracenemethanol (An‐OH) to obtain a PU having anthracenyl functionality (PCL‐An‐PU) at the end of each macromolecular chain. Subsequently, bis‐TAD was added to the prepolymer to produce the crosslinked DA adduct (PCL‐An‐PU‐TAD). All the synthesized products were comprehensively characterized by ¹H NMR, FT‐IR, and UV–vis spectroscopy. The mechanophoric attributes of the DA adduct were envisioned by using fluorescence spectroscopy. The self‐healing capacities of the PU adducts were examined using optical microscopy (OM) and atomic force microscopy (AFM) analyses. The NINT analysis quantified the increment in surface hardness and modulus of the produced PU DA‐adduct. Thermogravimetric analysis (TGA) was carried out to explain the thermal behavior of the synthesized materials.

Scheme 1.

Synthetic route of mechanoresponsive polyurethane based on a) anthracene‐TAD Diels‐Alder click chemistry, b) force‐induced turned‐on r‐DA unclick chemistry of synthesized polymer.

2. Results and Discussion

¹H NMR spectroscopy was utilized to elucidate the structure of tri‐arm hydroxyl‐terminated PCL (Figure 1a). The appearance of peaks at δ (3.66, H9), (2.30, H4), and 0.89 (H1) confirmed the successful ROP and preparation of the hydroxyl‐terminated PCL. The molar mass of PCL was calculated using the characteristic peaks at δ 3.66 (H9), 2.30 (H4), and 0.89 (H1). Figure 1b is the ¹H NMR spectrum of the PCL‐An‐PU polymer, where the emergence of the new peaks at δ 1.08–1.63 ppm corresponding to the methyl protons of Isophorone diisocyanate (IPDI) (H12, H13) and 2.93–2.97 (H11). A tiny resonance at δ 8.35 (H10) and 7.82 (H10’) due to −NH proton in the urethane linkage (─O─CO─NH) indicates the successful reaction of the isocyanate with the hydroxy‐terminated PCL. The resonances at δ 7.25–8.49 (H15–H19) are the characteristic aromatic protons present in anthracene. In this case, IPDI and An‐OH function as hard segments, and PCL is used as a soft segment.

Figure 1.

¹H NMR spectra of a) PCL b) PCL‐An‐PU.

Anthracene moieties were used to functionalize the ‐NCO‐terminated polymers, resulting in anthracene end‐capped (PCL‐PU) polymers, which can be correlated with the M_{n theo} and M_{n NMR} molecular weights illustrated in Table 1 . Subsequently, the PCL‐An‐PU polymers were cross‐linked under ambient conditions using 1.5 mol bis‐TAD. As a result of a very fast DA reaction between the “anthracene” moieties of PCL‐An‐PU and the “azo” groups of bis‐TAD, the color of the solution changed from bright red to pale yellow. The solution was poured into a Teflon Petri dish, and the remaining THF was evaporated at room temperature, followed by drying the system in a vacuum oven for 8 h at 60 °C to obtain the solid film.

Table 1.

Molar ratio and the molecular weight of the prepared polymers.

Sample Name	Reagent used	Molar ratio	Mn theo	Mn NMR
PCL	TMP: ɛ‐CL	1:3	2188	2092
PCL‐An‐PU	PCL: IPDI: An‐OH	1:3:3	3498	3384
PCL‐An‐PU‐TAD	PCL‐An‐PU: bis‐TAD	1: 1.5	7358.3	–

The FT‐IR analysis was performed on both the synthesized polymers and the TAD‐derived cross‐linked polymers. The results can be seen in Figures 2 and S1 (Supporting Information). In Figure S1 (Supporting Information), the significant peaks that developed at 3380 and 3540 cm⁻¹ are in line with the stretching vibration of the −NH− group of the urethane linkages in PCL‐An‐PU polymer and the −OH of the PCL, respectively. More significantly, in Figure 2, the drastic decrease in the intensity of 2270 cm⁻¹ suggests the successful reaction between the ─N═C═O group and An‐OH to form the PCL‐An‐PU polymer. New peaks that emerged at 880 cm⁻¹ can be assigned to the >C─H bending vibration of the anthracene rings present in the PCL‐An‐PU polymer.^{[ 32 ]} Similarly, additional peaks appeared at 1638 cm⁻¹, which can be attributed to the stretching of >C═O bond of urethane links as shown in Figure S1 (Supporting Information). It's interesting to point out in Figure 2, that in bis‐TAD, the >C═O stretching vibration band of succinimide appears at 1775 cm⁻¹. But after the addition of the bis‐TAD to the PCL‐An‐PU, the intensity of the >C═O of the succinimide at 1775 cm⁻¹ and the anthracene ring (─C─H) bending at 880 cm⁻¹ considerably decreased in the amplified spectrum of PCL‐An‐PU‐TAD. The drastic decrease in the peak intensity at 1775 and 880 cm⁻¹ indicates the successful reaction of bis‐TAD with PCL‐An‐PU polymers via a DA reaction.

Figure 2.

FT‐IR spectra of a) bis‐TAD, b) PCL‐An‐PU, c) PCL‐An‐PU‐TAD, and amplified region.

We have analyzed the PCL‐An‐PU and the crosslinked PCL‐An‐PU‐TAD via UV–vis spectroscopy. Free anthracene moiety present in PCL‐An‐PU showed an absorption band at 392, 372, 355, and 338 nm. Upon the addition of stoichiometric quantities of bis‐TAD into the prepolymer solution, these characteristic absorption bands of anthracene gradually diminished. This phenomenon can be correlated to the occurrence of a successful DA reaction as depicted in Figure 3 .

Figure 3.

UV–vis spectra of anthracene containing PCL‐An‐PU and PCL‐An‐PU‐TAD with different moles of bis‐TAD.

Here, the mechanoresponsive property of the DA adduct was envisioned using fluorescence spectroscopy. A solution of PCL‐An‐PU‐TAD was subjected to ultrasonic treatment and during this period, measured quantities of aliquots were withdrawn at different time intervals. Fluorescence analysis was performed with all the stock aliquots and the results are presented in Figure 4a and Figure 4b. It is clearly observed that before sonication, the sample (A₀) was fluorescence inactive (turned‐off), however with increasing sonication time, the fluorescence intensity of the samples progressively increased (A₁, A₂, A₃, A₄, A₅, A₆, A₇, A₈) as a consequence of force assisted r‐DA reaction.^{[ 2 ]} Therefore, this result reveals the mechanoresponsive attributes of the newly designed polyurethane DA adduct.

Figure 4.

(a) Fluorescence spectroscopy analysis of DA adduct PCL‐An‐PU‐TAD (A₀), aliquots at different sonication times (A₁ – A₈), and (b) digital photographs of the DA adduct and those aliquots under UV‐light.

Nanoindentation is a highly effective method for evaluating the mechanical properties, such as surface hardness and modulus, of polymer films.^{[ 26 ]} The hardness and modulus were estimated against a fixed nanoindenter penetration depth for PCL‐An‐PU and PCL‐An‐PU‐TAD. Figure S2 (Supporting Information) shows the retained hardness of the PU films before and after the reaction with bis‐TAD (Table  2 ) via NINT analysis. A hardness value of 0.3 MPa resulted in the PCL‐An‐PU, but after modification with the bis‐TAD derivative, the DA adduct film (PCL‐An‐PU‐TAD) led to an increase in its hardness to 0.5 MPa. This suggests that the hardness of the anthracene‐modified polyurethane is increased in comparison to the unmodified material when the material is modified using theDA “click” reaction. Interestingly, when the molar content of bis‐TAD increased, the obtained DA polymer film showed very high hardness, as expected from the DA adduct.

Table 2.

Nanoindentation analysis of the modified and unmodified polyurethane.

Sample	Hardness [MPa]	Modulus [MPa]
PCL‐An‐PU	0.3 ± 0.0	3.0 ± 1.0
PCL‐An‐PU‐TAD	0.5 ± 0.1	11.8 ± 2.6

Thermogravimetric analysis of the mechanophore was carried out to understand its thermal behavior, especially at higher temperatures. The PCL‐An‐PU‐TAD adduct underwent its initial weight loss at ca. 165 °C, which is due to the cleavage of the bis‐TAD molecule due to the r‐DA reaction as well as its partial decomposition as observed in Figures S3 and S4 (Supporting Information). However, the final decomposition temperature of PCL‐An‐PU‐TAD is at ca. 335 °C, whereas the same of the PCL‐An‐PU occurs at ca. 310 °C. The residue is significantly higher in the case of crosslinked DA‐adduct. The increases in the final decomposition temperature of the crosslinked adduct, as determined by TGA analysis, indicate the successful creation of the anthracene ‐TAD DA‐adduct.

Figure S5 (Supporting Information) presents the differential scanning calorimetry (DSC) traces of both the pristine PCL‐An‐PU and the crosslinked PCL‐An‐PU‐TAD. The DSC traces indicate that the glass transition temperature (T _g ) of the PCL‐An‐PU is −43 °C. On the other hand, a broad endotherm starting from 108 °C and extending up to 145 °C, is associated with the r‐DA reaction temperature in the case of PCL‐An‐PU‐TAD. The stated value for the T _g of the cross‐linked network is −29 °C, which is determined to be greater than the T _g of the PCL‐An‐PU. The existence of an endotherm and the elevation in T _g suggest that the material has been effectively modified and possesses the ability to undergo reversible changes with temperature.

Another important feature of such mechanoresponsive network modified with TAD, namely, the self‐healing ability, was monitored by placing a horizontal cut through the polymeric film using optical microscopy.^{[ 25 ]} The cut was introduced to the polymer film by a razor blade in one stroke to avoid any plastic deformation. Thereafter, the notched sample was kept at its r‐DA temperature of 110 °C for 5 h, followed by cooling down to ambient temperature. Due to the occurrence of r‐DA upon heating to 110 °C, the TAD molecules are released, but once the system was brought back to the ambient temperature, the cut was almost completely healed as a result of a DA reaction, as evident from Figure 5 . The healing effect was also supported by AFM images (Figure S6, Supporting Information). The depth of cut was significantly reduced to ca. 0.014 µm from ca. 0.29 µm over the polymer surface, demonstrating a healing efficiency (E_H) of ca. 95.1%. Thus, the DA reaction and rDA reaction have been proven to be efficient tools for the manifestation of the self‐healing property of such a mechanoresponsive polymeric system. A controlled study was conducted with the PCL‐An‐PU where the dynamic covalent bond (bis‐TAD bridge) is absent. In this case, too, the “cut” and “heal” procedure was conducted following the same protocol maintained for the bridged one. Figure S7 (Supporting Information) clearly shows that the cutting line is still visible even when exposed to the same healing conditions. Thus, the newly designed PU‐based mechanophoric system has proven to be a promising material as damage or crack‐sensing material combined with self‐healing characteristics.

Figure 5.

Optical microscopy images of PCL‐An‐PU‐TAD at different magnifications (a,c) before self‐healing and b,d) after self‐healing.

3. Conclusion

TAD moiety is released from the crosslinked DA‐adduct due to cleavage of the C─N bond in the bridged anthracene and TAD adduct. The mechanoresponsive property was further envisioned by fluorescence spectroscopy and it was found that the synthesized product exhibits mechanosensitivity upon exposure to ultrasonic treatment. Besides the mechanoresponsive attributes, this material also exhibits self‐healing behavior. The interplay between the DA and r‐DA chemistry upon heating and cooling in this functional polyurethane with anthracene‐TAD adduct leads to fascinating properties to heal the cracks and scratches in the polymer. Therefore, such PU‐based mechanoresponsive materials can have potential application prospects in crack sensors or damage‐sensing materials.

4. Experimental Section

Materials

1,1,1‐tris(hydroxymethyl) propane (TMP, 98%), tin (II) 2‐ethyl hexanoate (Sn (Oct)₂, 95%), ɛ‐Caprolactone (ɛ‐CL, 97%), isophorone diisocyanate (IPDI), 9‐Anthracenemethanol (An‐OH, 98%), and dibutyltin dilaurate (DBTDL, 98%) were purchased from Sigma–Aldrich, USA, and used without any further purification. The deuterated solvents namely, CDCl₃ and DMSO‐d₆ were also procured from Sigma–Aldrich (USA). The synthesis procedure of bis‐TAD is described in the Supporting Information. The solvents, for instance, tetrahydrofuran (THF), toluene, chloroform (CHCl₃), dichloromethane (DCM), and acetone, were obtained from Merck India. Prior to use, THF and toluene were purified by distillation using sodium metal and benzophenone.

Characterization Techniques and Measurements

The ¹H nuclear magnetic resonance (NMR) spectra were collected in CDCl₃ solvent using a Bruker 500 MHz spectrometer with tetramethyl silane (TMS) serving as an internal standard. The acquired spectra were analyzed by using MestReNova software. The synthesized samples were then subjected to molecular weight (M_n) and dispersity (D̵) determination employing an Agilent Gel Permeation Chromatography (GPC) apparatus equipped with a refractive index (RI) detector. In each case, THF was used as the eluent at a constant flow rate of 1 mL·min⁻¹. Agilent GPC software was utilized to analyze the data, with PS calibration standards (molar mass range: 2000–600000 g·mol⁻¹) of narrow dispersity (D̵). The produced samples were also characterized using the PerkinElmer (Model spectrum‐2) Fourier transform infrared spectroscopy (FT‐IR) in ATR mode. In each case, 16 scans were taken covering a scanning range of 400–4000 cm⁻¹. The TA Discovery DSC instrument was used for differential scanning calorimetry (DSC) analysis. Three temperature scans covering a temperature range of −50–+180 °C were recorded at a heating/cooling ramp of 10 °C·min⁻¹. A PerkinElmer Lambda 35 UV–vis spectrometer was used to record the UV–vis absorption spectra of the synthesized polymers. A Shimadzu RF‐6000 Spectro fluorophotometer was employed to acquire the fluorescence spectra. The ultrasound experiment was conducted in dry THF using a Labman PRO650 probe sonicator having a frequency of 20 kHz equipped with an 8 mm titanium probe. The hardness of the polymeric materials was examined via quasistatic nanoindentation modes by depth sensing indentation (DSI) with a TribroIndenter TI 950 (Hysitron Inc., Minneapolis, MN, USA) that had a diamond indenter tip with a radius of 150 nm. A Cilika BT‐E Benchtop Biological Digital optical microscope was used to assess the self‐healing characteristics of the synthesized polyurethane system. The thermal stability of the synthesized materials was analyzed by the TGA method employing TA instruments (model: Q5000) operated under a nitrogen atmosphere at a heating ramp of 10 °C per min. Atomic force microscopy was performed in tapping mode by using the Agilent 5500 SPM instrument, Agilent Technologies, Inc. The polymeric film was cast on a silicon wafer, and a horizontal cut was made using a razor blade. The step heights were estimated before and after a self‐healing operation.

Synthesis of Hydroxy‐Terminated Caprolactone (PCL)

In a 50 mL dried one‐necked reaction vessel, 6 g (52.5 mmol) of ɛ‐caprolactone, 0.39 g (2.91 mmol) of 1,1,1‐tris(hydroxymethyl) propane (TMP), and 0.064 g (1 wt.% of the total weight of ɛ‐CL and TMP) of catalyst [Sn (Oct)₂] were added and purged with N₂. Then, the reaction vessel was stirred at 110 °C for 24 h under a nitrogen environment. Then, the reaction vessel was cooled down to room temperature and the polymer was precipitated in n‐hexane. The formed polymer was purified by repeated dissolution and precipitation in THF and n‐Hexane, respectively. Finally, the resulting polymer was collected and dried at 45 °C under vacuum and was used for further analyses.

Synthesis of Anthracene End‐Capped Polyurethane (PCL‐An‐PU)

The anthracene end‐capped trifunctional polyurethane was synthesized by reacting PCL, IPDI, and 9‐Anthracenemethanol serving as an end‐capper. Prior to commencing the reaction, the complete reaction set‐up was dried at 110 °C followed by purging with N₂ to get rid of any moisture and air. Next, 2.0 g (1.1 mmol) of dry PCL 0.742 g (3.3 mmol) of IPDI, and 0.1% (2.74 mg) of DBTDL catalyst were added into the reaction vessel and dissolved in 25 mL of dry THF. Then, the mixture was stirred at 65 °C for 6 h while maintaining the N₂ environment. The anthracene end‐capped polyurethane (PCL‐An‐PU) was then produced by dropwise addition of the chain capper 9‐Anthracenemethanol [An‐OH, 0.677 g (3.3 mmol)] to the polymer solution using a syringe under N₂ atmosphere. The reaction continued to run for an additional 18 h. Once the reaction was completed, the mixture was cast on a Teflon Petri dish and allowed to cool down to room temperature. A pale‐yellow film was obtained upon drying the resulting polymer at room temperature, followed by drying in a vacuum oven at 60 °C for 48 h.

Preparation of 1,2,4‐triazoline‐3,5‐dione (bis‐TAD)

The crosslinker of 1,2,4‐triazoline‐3,5‐dione (bis‐TAD) was synthesized as per the previous report.^{[ 33} , ^{34 ]} In brief, DABCO‐Br was synthesized by bromination of 1,4‐Diazabicyclo 2.2.2 octane (DABCO), followed by a reaction of DABCO‐Br with urazole to obtain the pink‐colored bis‐TAD crystals. After the synthesis of bis‐TAD, it was stored under inert conditions inside the refrigerator.

Preparation of Anthracene‐Based Crosslinked PU by Anthracene‐TAD DA Click Chemistry

Prior to the reaction, 0.06 g of the PCL‐An‐PU film was dissolved in dry THF under N₂ conditions. Similarly, 0.0146 g bis‐TAD (1.5 mole) solution was also prepared in the same solvent and under the same conditions. Next, the bis‐TAD solution was added to the polymer solution dropwise under N₂ conditions and stirred for a few minutes at room temperature. The addition of bis‐TAD caused a noticeable change in the color of the solution within 2 min. The red color transformed into a pale yellow, suggesting a complete reaction between the azo group and the anthracene moiety of the bis‐TAD and polyurethane, respectively. This reaction resulted in the formation of crosslinked networks designated as PCL‐An‐PU‐TAD (DA adduct). The solution was cast in a Teflon petri dish and kept at room temperature for 24 h, followed by a vacuum oven at 60 °C for another 48 h to get a film.

Ultrasonic Examination of the Crosslinked Polyurethane (Turned‐Off to Turned‐On Mode)

A Labman PRO650 ultrasonicator was used for the ultrasonication studies. The PCL‐An‐PU‐TAD adduct polymer solutions were prepared in a specially designed two‐neck glass cell, and before sonication, they were purged with N₂ for 15 min to remove any air present in the system. The glass cell was maintained in an ice‐water bath throughout the sonication process. The ultrasonic pulse was configured with a 1 s ON and 2 s OFF cycle, operating at an amplitude of 20%. Samples were withdrawn from the glass cell at different time intervals during sonication for analysis through fluorescence spectroscopy.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Chakraborty S., Choudhury S., Singha N. K., A New Class of Mechano‐Responsive Polyurethane Via Anthracene ‐TAD Diels‐Alder (DA) Click Chemistry. Small 2024, 20, 2406866. 10.1002/smll.202406866

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request