TWI852767B - Polyurethane elastomer materials containing polyrotaxane tougheners with mechanochromic fluorescence properties - Google Patents

Abstract

A polyurethane elastomer material containing polyrotaxane tougheners with mechanochromic properties is provided, wherein the reactive monomers include a Rhodamine derivative (Rh-2OH) with a chemical structure represented by Formula (1), a polyol monomer with at least two hydroxyl groups, a polyisocyanate monomer with at least two isocyanate groups, and a polyrotaxane crosslinker. The polyrotaxane crosslinker includes a cyclic molecule with multiple hydroxyl groups for crosslinking with the polyisocyanate monomer, a linear molecule threaded through the cyclic molecule, and two stopper groups bonded to the ends of the linear molecule, wherein the two stopper groups are naphthylimide group with a chemical structure represented by Formula (2).

Description

Polyurethane elastomer material with photochromic properties and polyrotaxane toughening properties

The present disclosure relates to elastomeric materials, and in particular to elastomeric materials having mechanochromic properties.

Elastomers have a wide range of special mechanical applications, such as automobiles, aircraft, buildings, and sports equipment, due to their deformable properties and ability to elastically recover to their original shape after being subjected to external loads. In addition, elastomers have mechanical applications in the development of soft actuated devices (such as grippers, motion and power sensing devices) that can withstand large tensile stress and strain, as well as multi-stimulus responsive behaviors. Polymers with multi-stimulus responsive properties show potential application value in various fields, including soft robots, responsive materials, monitoring and preventing catastrophic damage (or failure) of materials, among which mechanophoric polymers (MPs) show signal changes when mechanical forces are applied. Thus, novel properties of rich mechanoresponsive groups embedded in various polymers have been reported.

One of the purposes of the present disclosure includes providing a polyurethane elastomer material with mechanochromic properties. The toughness of the polyurethane elastomer material is greatly increased by adding a polyrotaxane crosslinker when preparing the polyurethane. Moreover, two different fluorescent chromophores are added to the main chain of the polyurethane and to the end groups of the polyrotaxane crosslinker to allow the polyurethane elastomer material to have multi-stimulus responsive behavior.

The reactive monomers of the polyurethane elastomer material having mechanochromic properties include a rhodamine derivative (Rh-2OH) having a chemical structure as shown in formula (1); a polyol monomer having at least a dihydroxyl group; a polyisocyanate monomer having at least a diisocyanate group; and a polyrotaxane crosslinker.

The polyrotaxane crosslinking agent comprises a cyclic molecule having multiple hydroxyl groups to undergo a crosslinking reaction with the polyisocyanate monomer; a linear molecule passing through the cyclic molecule; and a dicapping group bonded to both ends of the linear molecule, wherein the dicapping group is naphthyl imide and has a chemical structure as shown in formula (2).

According to an embodiment of the present disclosure, the molar ratio of the above-mentioned rhodamine derivative, polyol monomer and polyisocyanate monomer is 1:89:120. The amount of the polyrotaxane crosslinking agent added is 0.1-2.0wt%, for example, 0.5-1.5wt%, 0.6-1.4wt%, 0.7-1.3wt%, 0.8-1.2wt% or 0.9-1.1wt%.

According to another embodiment of the present disclosure, the above-mentioned polyol monomer includes tetraethylene glycol.

According to another embodiment of the present disclosure, the above-mentioned polyisocyanate monomer includes hexamethylene diisocyanate.

According to another embodiment of the present disclosure, the above-mentioned cyclic molecule includes cyclodextrin.

According to another embodiment of the present disclosure, the linear molecule comprises polyethylene glycol.

According to another embodiment of the present disclosure, the polyurethane elastomer material has a chemical structure as shown in formula (3), wherein PR is the polyrotaxane crosslinking agent, and x and y are positive integers. According to some embodiments, the ratio of x to y can be 1:0.9~1:1.1.

According to another embodiment of the present disclosure, the polyrotaxane crosslinker has a chemical structure as shown in formula (4), wherein n is a positive integer. According to some embodiments, n can be a positive integer of 15-30, for example, a positive integer of 16-28, 18-26, 20-25 or 23-24.

As can be seen from the above, since the rhodamine derivative monomer with mechanochromic properties is added to the main chain of polyurethane, when the polyurethane elastomer material is subjected to force, the spiropyran structure in the rhodamine derivative monomer part can be changed from a spiral shape to a planar structure, becoming a red fluorescent chromophore, and the naphthyl imide end group of the polyrotaxane is a fluorescent chromophore that can emit green light. Therefore, when the polyurethane elastomer material with mechanochromic properties is not subjected to force, the green light emitted by the naphthyl imide end group of the polyrotaxane can be seen. However, when the above-mentioned polyurethane elastomer material with mechanochromic properties is subjected to tensile stress, the monomer part of the rhodamine derivative will transform into a red fluorescent chromophore, so it can absorb the green light emitted by the naphthyl imide end group of the polyrotaxane, reduce the intensity of the green light emitted by the naphthyl imide end group, and instead emit red light to increase the intensity of the red light. Therefore, through the clever molecular structure design, the above-mentioned polyurethane elastomer material with mechanochromic properties can emit green and red light of different intensities at different parts of the polyurethane elastomer material according to the different stress conditions at different parts.

In addition, due to the use of polyrotaxane crosslinking agents, according to experimental results, the length of the polyurethane elastomer after stretching can be up to 80 times the original length, which is amazing.

110: Polyurethane main chain

120: Monomer part of rhodamine derivative (Rh-2OH)

130: Cyclodextrin

140: Straight chain molecule

150: Naphthyl imide end-capping group

FIG1 is a schematic diagram showing a long polymer chain of a polyurethane elastomer material according to an embodiment of the present invention in an amorphous state.

Figure 2A shows the relationship between tensile stress and tensile strain of PU-Rh-PR films with different contents of polyrotaxane PR at a strain rate of 60 mm/min.

Figure 2B shows the effect of different strain rates (60-6000 mm/min) on the mechanical properties of PU-Rh-PR-1.0 film.

Figure 2C shows the relationship curve between tensile stress and tensile strain of PU film after adding various crosslinking agents (PR, PSR, CD, TEA).

Figure 2D shows the toughness of various PU films.

Figure 2E shows the stress-strain curves of the original PU-Rh-PR-1.0 film and the PU-Rh-PR-1.0 film pre-damaged by a 1/4 width notch.

In Figure 3A, the stereochemical structure of the Rh group changes from the distorted spirolactamide form on the left side of the figure to the planar ionic structure on the right side of the figure.

Figure 3B shows a schematic diagram of proportional light emission in a polyurethane film based on polyrotaxane due to force induction, where the left figure is a schematic diagram of the film before force is applied, and the right figure is a schematic diagram of the film after force is applied.

Figure 3C shows the photoluminescence spectrum of PU-Rh-PR-1.0 film under different strains. Figure 3D shows the proportional luminescence intensity of PU-Rh-PR-1.0 film under different strains. Figure 3E shows the CIE graph of photoluminescence from green to red of PU-Rh-PR-1.0 film under different strains.

The present disclosure provides a polyurethane elastomer material with mechanochromic properties. The toughness of the polyurethane elastomer material is greatly increased by adding a polyrotaxane crosslinker when preparing the polyurethane. In addition, two different fluorescent chromophores are added to the main chain of the polyurethane and to the end groups of the polyrotaxane crosslinker, so that the polyurethane elastomer material has multi-stimulus responsive behavior.

Polyurethane elastomer material with mechanochromic properties

The reactive monomer of the polyurethane (PU) elastomer material having mechanochromic properties comprises a rhodamine derivative (Rh-2OH) having a chemical structure as shown in formula (1); a polyol monomer having at least a dihydroxyl group; a polyisocyanate monomer having at least a diisocyanate group; and a polyrotaxane crosslinker. According to an embodiment of the present disclosure, the polyol monomer comprises tetraethylene glycol. According to an embodiment of the present disclosure, the polyisocyanate monomer comprises hexamethylene diisocyanate.

According to another embodiment of the present disclosure, the polyrotaxane crosslinker comprises a cyclic molecule having multiple hydroxyl groups to crosslink with the polyisocyanate monomer; a linear molecule passing through the cyclic molecule; and a dicapping group bonded to both ends of the linear molecule, wherein the dicapping group is a naphthylimide group having a chemical structure as shown in formula (2). According to another embodiment of the present disclosure, the cyclic molecule comprises cyclodextrin. According to another embodiment of the present disclosure, the linear molecule comprises polyethylene glycol.

According to another embodiment of the present disclosure, the molar ratio of the above-mentioned rhodamine derivative, polyol monomer, polyisocyanate monomer and polyrotaxane crosslinker is 1:89:120. According to some embodiments, the amount of polyrotaxane crosslinker added is 0.1-2.0wt%, for example, 0.5-1.5wt%, 0.6-1.4wt%, 0.7-1.3wt%, 0.8-1.2wt% or 0.9-1.1wt%.

According to an embodiment of the present disclosure, the polyurethane elastomer material has a chemical structure as shown in formula (3), wherein PR is the polyrotaxane crosslinker, and x and y are positive integers. According to some embodiments, the ratio of x to y can be 1:0.9~1:1.1.

According to another embodiment of the present disclosure, the polyrotaxane crosslinker has a chemical structure as shown in formula (4), wherein n is a positive integer. According to some embodiments, n can be a positive integer of 15-30, for example, a positive integer of 16-28, 18-26, 20-25 or 23-24.

Polypseudorotaxane and method for preparing polyrotaxane

Please refer to Synthesis Method 1. Synthesis Method 1 shows the synthesis route of polyrotaxane (PR), and the intermediate product of poly pseudo-rotaxane (PSR) is obtained in the middle.

Synthesis method 1

First, polyethylene glycol (PEG-10k; 5.0 g, 5.0 mmol) and triethylamine (4.2 mL, 30.0 mmol) were dissolved in dichloromethane (DCM; 30 mL) and cooled to 0°C. A DCM solution of p-toluenesulfonyl chloride (3.9 g, 20.0 mmol) was slowly added to the above solution. The reaction mixture was stirred at room temperature under _N2 atmosphere for 24 hours. The resulting mixture was concentrated under reduced pressure and dropped into excess cold ether to precipitate the intermediate product. The solid was collected by filtration and dried at 40°C to obtain the intermediate ditoluenesulfonate product (4.8 g) as a white solid.

Subsequently, the intermediate product was added to 250 mL of ammonia water (28%) and stirred at room temperature for 3 days. The aqueous phase was extracted with DCM (3 x 100 mL) and brine (1 x 100 mL) in sequence. The organic layer was collected, dried over anhydrous Na ₂ SO ₄ , and concentrated under reduced pressure. Then, the concentrated mixture was dropped into an excess of cold ether to precipitate the desired compound. The solid was filtered out, washed with cold ether several times, and vacuum dried to obtain PEG-2NH ₂ as a white powder.

A PEG-2NH ₂ (1.0 g, 0.1 mmol) aqueous solution was added to a saturated aqueous solution of cyclodextrin (α-cyclodextrin, α-CD; 15.0 g, 15.45 mmol). The mixture was stirred at room temperature overnight. The solution was centrifuged, and the precipitate was collected and further freeze-dried to obtain a white powdery poly pseudo-rotaxane (PSR) product.

Naphthylamine-NCS (Nap-NCS; 500 mg) and PSR (5.0 g) were then mechanically mixed. Anhydrous N,N-dimethylformamide (30 mL) was then gradually added to obtain a yellow polymer solution, which was stirred at room temperature for 3 days. The mixture was dissolved in DMSO (50 mL), dialyzed against DMSO, and then added to acetone. After the polyrotaxane precipitated from acetone, it was washed several times with H ₂ O. The solid was filtered and dried under vacuum to produce a polyrotaxane (PR) product as a yellow powder.

Please refer to FIG. 1, which is a schematic diagram showing a long polymer chain of a polyurethane elastomer material according to an embodiment of the present invention in an amorphous state. In FIG. 1, a monomer portion 120 of a rhodamine derivative (Rh-2OH) is incorporated into the polyurethane main chain 110. Cyclodextrin 130 in the polyrotaxane is responsible for cross-linking different polyurethane main chains 110, and the two ends of the linear molecule 140 in the polyrotaxane are each bonded to a naphthyl imide end group 150. Since the -OH group of cyclodextrin 130 undergoes a crosslinking reaction with the main chain of polyurethane, and cyclodextrin 130 can slide back and forth on the linear molecule 140, when the polyurethane elastomer material is stretched, the sliding of cyclodextrin 130 can effectively and significantly increase the stretching properties of the polyurethane elastomer material.

Preparation of polyurethane elastomer film material

Please refer to Synthesis Method 2. Synthesis Method 2 shows the polymerization reaction formula for preparing polyurethane elastomer materials, wherein crosslinkers 1-4 (crosslinker 1-4) are polyrotaxane (PR; crosslinker 1), polyquaternary rotaxane (PSR; crosslinker 2), α-cyclodextrin (α-CD; crosslinker 3) and triethanolamine (TEA; crosslinker 4).

Synthesis method 2

1. Preparation of PU-Rh-PR polyurethane elastomer film material

As shown in Synthesis Method 2, a solution of Rh-2OH (25.0 mg, 0.05 mmol), tetraethylene glycol (TEG) (864.3 mg, 4.45 mmol) and dibutyltin dilaurate (DBTDL; 1 drop) in anhydrous THF (6 mL) was prepared and reacted under nitrogen gas protection for 15 minutes. Hexamethylene diisocyanate (HDI; 1010.2 mg, 6.0 mmol) was then added, and the reaction was continued for 1 hour and refluxed. After one hour of reaction, the solution was added with polyrotaxane (PR; crosslinker 1) in anhydrous DMSO (3 mL) to the above mixed solution and the reaction was continued for 20 minutes. The resulting solution was then poured into a model coated with polytetrafluoroethylene and cured in an oven at 70°C for 1 day.

2. Preparation of PU-Rh-PSR polyurethane elastomer film material

As shown in Synthesis Method 2, a solution of Rh-2OH (25.0 mg, 0.05 mmol), tetraethylene glycol (TEG) (864.3 mg, 4.45 mmol) and dibutyltin dilaurate (DBTDL) (1 drop) in anhydrous THF (6 mL) was prepared and reacted under nitrogen gas protection for 15 minutes. Hexamethylene diisocyanate (HDI) (1010.2 mg, 6.0 mmol) was then added, and the reaction was continued for 1 hour and refluxed. After one hour of reaction, the solution was added with polystyrene (PSR; crosslinker 2) in anhydrous DMSO (3 mL) to the above mixed solution and the reaction was continued for 20 minutes. The resulting solution was then poured into a model coated with polytetrafluoroethylene and cured in an oven at 70°C for 1 day.

3. Preparation of PU-Rh-α-CD polyurethane elastomer film material

As shown in Synthesis Method 2, a solution of Rh-2OH (25.0 mg, 0.05 mmol), tetraethylene glycol (TEG) (864.3 mg, 4.45 mmol) and dibutyltin dilaurate (DBTDL) (1 drop) in anhydrous THF (6 mL) was prepared and reacted under nitrogen gas protection for 15 minutes. Hexamethylene diisocyanate (HDI) (1010.2 mg, 6.0 mmol) was then added, and the reaction was continued for 1 hour and refluxed. After one hour of reaction, α-cyclodextrin (α-CD; crosslinker 3) in anhydrous DMSO (3 mL) was added to the above mixed solution and the reaction was continued for 20 minutes. The resulting solution was then poured into a model coated with polytetrafluoroethylene and cured in an oven at 70°C for 1 day.

4. Preparation of PU-Rh-TEA polyurethane elastomer film material

As shown in Synthesis Method 2, a solution of Rh-2OH (25.0 mg, 0.05 mmol), tetraethylene glycol (TEG) (864.3 mg, 4.45 mmol) and dibutyltin dilaurate (DBTDL) (1 drop) in anhydrous THF (6 mL) was prepared and reacted under nitrogen gas protection for 15 minutes. Then hexamethylene diisocyanate (HDI) (1010.2 mg, 6.0 mmol) was added, and the reaction was continued for 1 hour and refluxed. After one hour of reaction, triethanolamine (TEA; crosslinker 4) in anhydrous DMSO (3 mL) was added to the above mixed solution and the reaction was continued for 20 minutes. The resulting solution was then poured into a model coated with polytetrafluoroethylene and cured in an oven at 70°C for 1 day.

Mechanical stress test of polyurethane elastomer film

One of the most important parameters of polyurethane elastomer materials is the high tensile strength of its mechanical properties, so the tensile stress test of polyurethane elastomer film (hereinafter referred to as PU film) is carried out here.

When preparing all PU film samples, 864.3 mg of tetraethylene glycol and 1010.2 mg of hexamethylene diisocyanate were added. The addition amounts of Rh-2OH and various crosslinking agents (PR, PSR, α-CD and TEA) are listed in Table 1. For PU films coded as PU-Rh-PR-i, the value of "i" represents the content of PR in weight percentage, and the content is calculated based on the weight of the entire PU film. For example, the film sample PU-Rh-PR-0.1 means that the entire PU film contains 0.1wt% of polyrotaxane (PR).

The estimated molecular weight of polyrotaxane is calculated by the following formula, where Mn, M _{α -CD} , M _Nap-NCS and M _PEG-2NH2 are the molecular weights of polyrotaxane, cyclic molecule α-CD, end-capping group Nap-NCS and linear molecule PEG-2NH ₂ , respectively. When the polyrotaxane has 22 cyclic molecules α-CD, the molecular weight Mn of the polyrotaxane is calculated by the following formula.

Mn=(22×M _{α -CD} )+(2×M _Nap-NCS )+M _PEG-2NH2 =32164.55g/mol

All stress-strain curves were recorded using an MTS Tytron 250 tensile system in all tensile stress experiments. The thickness of the polymer film was measured using a thickness gauge. The polymer film was cut into rectangular sheets with the following dimensions: 15 cm (length) × 2 cm (width) × 0.5-0.65 cm (thickness). Before the tensile test, the sample was placed on the clamp of the tensile tester. The initial length of the tensile machine was 1 mm, and the initial length was the gap between the two clamps. The strain rate of the tensile tester was fixed at 60 mm/min.

The calculation formulas for tensile stress (δ) and tensile strain (ε) are as follows, where N is the applied force, A is the area, △L is the displacement of the length after stretching, and L _o represents the initial length.

δ = N / A

ε (%) = (△ L / L ) × 100

The results of the tensile stress test are listed in Table 1 below and shown in Figures 2A-2E. From the data in Table 1, it can be found that when the amount of polyrotaxane (PR) added is 1wt% (19mg) when synthesizing PU, its tensile strain can be as high as 60 times or more. Among them, the PU-Rh-PR-1.0 film can be stretched from a length of 1mm (transparent color) to a length of 78mm (pink color), indicating that the PU-Rh-PR-1.0 film can withstand a tensile strain of up to 7700%.

Figure 2A shows the relationship between tensile stress and tensile strain of PU-Rh-PR films with different contents of polyrotaxane PR at a strain rate of 60 mm/min. Figure 2A shows the effect of polyrotaxane crosslinker content (0.1-2.0wt%) on the mechanical properties of these elastomers. As the polyrotaxane crosslinker content gradually increases from 0.1wt% to 1.0wt%, it can be observed that the mechanical strength gradually increases and has excellent stretchability.

The content of polyrotaxane crosslinker may affect the modulus and toughness of PU-Rh-PR films for the following reasons. As more polyrotaxane crosslinker is added to PU-Rh-PR film, the crosslinking density increases, which increases the hardness (Young's modulus) of the elastomer. However, the slip effect of more cyclic molecules α-CD in polyrotaxane crosslinker on its linear molecules will reduce the hardness in the opposite way. Therefore, these two balancing effects between the cyclic molecules α-CD and the content of polyrotaxane crosslinker keep the hardness in a similar region. However, the toughness of PU-Rh-PR film is significantly improved due to the slip effect and greater elongation of more polyrotaxane crosslinker. Therefore, in PU elastomers, the hardness does not change significantly regardless of the content of polyrotaxane crosslinker. However, the slip effect of polyrotaxane seems to be more effective, improving the stretchability (strain) and toughness of polyurethane elastomers.

Finally, among these PU-Rh-PR films, PU-Rh-PR-1.0 showed the best strain value of more than 6000% (reaching the maximum strain limit of our tensile system), and there was no fracture under a stress of 15.34MPa. However, when the feed weight of polyrotaxane reached 2.0wt%, i.e., PU-Rh-PR-2.0, the tensile strain decreased to 5438% (Figure 2A and Table 1) due to the supersaturated crosslinker density. Therefore, PU-Rh-PR-1.0 showed the best polyrotaxane crosslinker content composition (i.e., PR = 1.0wt%) to give PU-Rh-PR films the highest stretchability and toughness, so further exploration will be carried out using this optimal formulation.

Figure 2B shows the effect of different strain rates (60-6000 mm/min) on the mechanical properties of PU-Rh-PR-1.0 film. It was observed that at slower strain rates, longer strains could be produced, so a strain rate of 60 mm/min was selected throughout the study to obtain the longest strain and reveal the best mechanochromic results.

Figure 2C shows the relationship curve between the tensile stress and tensile strain of the PU film after adding various crosslinking agents (PR, PSR, CD, TEA). In contrast, the PU-Rh-PR-1.0 film in Table 1 showed the most significant strain enhancement. In Figure 2C, in the initial stage, the sliding ring structure of the polyrotaxane reduced the stress, making the modulus lower than that of the PU-Rh-TEA film. However, with further increased strain, the sliding ring structure of the polyrotaxane plays a dominant role in improving the mechanical properties of the polyurethane film. Therefore, the optimal polyrotaxane crosslinker not only releases stress, but also enables the PU-Rh-PR-1.0 polyurethane film to remain unbroken and reach a strain of more than 6000%. This result confirms that the sliding ring structure of polyrotaxane contributes to the mechanical properties of PU-Rh-PR-1.0 film.

18-20MPa)，應變較小(

3800-4200%)， 而不及PU-Rh-PR-1.0薄膜的最佳結果(>6000%的應變，應力>15.65MPa) Due to the lack of crosslinking agent, PU-Rh in Table 1 cannot form a film and appears as powder. On the other hand, PU-Rh-PSR and PU-Rh-CD show almost similar characteristics in mechanical properties, with higher stress values (

18-20MPa), small strain (

3800-4200%), but not as good as the best result of PU-Rh-PR-1.0 film (>6000% strain, stress>15.65MPa)

According to Figure 2C and Table 1, it can be clearly seen that PU-PR shows significant ductility (>6000% strain) and low stress (about 9.45MPa), which is comparable to that of PU-Rh-PR-1.0 film. This further illustrates the importance of the bulky naphthalene diimide (NDI) end-capping agents at both ends of the linear molecules in the polyrotaxane crosslinker added to the PU-Rh-PR-1.0 film. The naphthalene diimide end-capping agent prevents the α-CD ring from slipping off the main chain of the PEG linear polymer, thereby forming a polyrotaxane structure. In PSR, since the PEG polymer has no end-capping agent, the α-CD ring can completely slide out from the main chain of the PEG linear polymer and be released, eliminating the pulley effect at the molecular level in the PU-Rh-PSR film and showing mechanical properties similar to those of PU-Rh-TEA and PU-Rh-CD films with similar non-polyrotaxane crosslinkers.

FIG2D shows the toughness of the various PU films described above. In FIG2D , the PU-Rh-PR-1.0 film No. 4 exhibits an excellent toughness of approximately 550 MJ/m ³ , which is not only higher than any other PU film in the present disclosure (including PU-Rh-TEA No. 1 without polyrotaxane at 350 MJ/m ³ ), but also superior to most of the state-of-the-art technologies reported to date (see Table 2 below). Similarly, the PU-PR film No. 8 exhibits a toughness of 475 MJ/m ³ , confirming the outstanding contribution of polyrotaxane to the toughness of these elastomers. Due to the lack of a polyrotaxane structure, PU-Rh-CD has the lowest toughness of approximately 230 MJ/m ³ .

References:

1. Chen, X.; Zhong, Q.; Cui, C.; Ma, L.; Liu, S.; Zhang, Q.; Wu, Y.; An, L.; Cheng, Y.; Ye, S . ; Chen , , 12 , 30847-30855.

2. Liu, Z.; Guo, W.; Wang, W.; Guo, Z.; Yao, L.; Xue, Y.; Liu, Q.; Zhang, Q. Healable Strain Sensor Based on Tough and Eco- Friendly Biomimetic Supramolecular Waterborne Polyurethane. ACS Appl. Mater. Interfaces 2022, 14 , 6016-6027.

3. Ying, WB; Wang, G.; Kong, Z.; Yao, CK; Wang, Y.; Hu, H.; Li, F.; Chen, C.; Tian, Y.; Zhang, J.; Zhang, R.; Zhu, J. A Biologically Muscle-Inspired Polyurethane with Super-Tough, Thermal Reparable and Self-Healing Capabilities for Stretchable Electronics. Adv. Funct. Mater. 2021, 31 , 2009869.

4. Tan, MWM; Thangavel, G.; Lee, PS Rugged Soft Robots Using Tough, Stretchable, and Self-Healable Adhesive Elastomers. Adv. Funct. Mater. 2021, 31 , 2103097.

5. Wu, J.; Cai, L.-H.; Weitz, DA Tough Self-Healing Elastomers by Molecular Enforced Integration of Covalent and Reversible Networks. Adv. Mater. 2017, 29 , 1702616.

6. Kim, S.-M.; Jeon, H.; Shin, S.-H.; Park, S.-A.; Jegal, J.; Hwang, SY; Oh, DX; Park, J. Superior Toughness and Fast Self-Healing at Room Temperature Engineered by Transparent Elastomers. Adv. Mater. 2018, 30 , 1705145.

7. Wang, X.; Zhan, S.; Lu, Z.; Li, J.; Yang, X.; Qiao, Y.; Men, Y.; Sun, J. Healable, Recyclable, and Mechanically Tough Polyurethane Elastomers with Exceptional Damage Tolerance. Adv. Mater. 2020, 32 , 2005759.

8. Li, Z.; Zhu, Y.-L.; Niu, W.; Yang, X.; Jiang, Z.; Lu, Z.-Y.; Liu, X.; Sun, J. Healable and Recyclable Elastomers with Record-High Mechanical Robustness, Unprecedented Crack Tolerance, and Superhigh Elastic Restorability. Adv. Mater. 2021, 33 , 2101498.

9. Zhou, W.; Chen, X.; Yang, K.; Fang, H.; Xu, Z.; -complexation. Applied Surface Science 2022, 572 , 151393.

10.Liu, X.; Liu, X.; Li, W.; Ru, Y.; Li, Y.; Sun, A.; Wei, L. Engineered Self-Healable Elastomer with Giant Strength and Toughness Via Phase Regulation and Mechano-Responsive Self-Reinforcing. Chem. Eng. J. 2021, 410 , 128300.

11.Li, Y.; Li, W.; Sun, A.; Jing, M.; Liu, X.; Wei, L.; Wu, K.; Fu, Q. A Self-Reinforcing and Self-Healing Elastomer with High Strength, Unprecedented Toughness and Room-Temperature Reparability. Mater. Horiz. 2021, 8 , 267-275.

12.Eom, Y.; Kim, S.-M.; Lee, M.; Jeon, H.; Park, J.; Lee, ES; Hwang, SY; Park, J.; Oh, DX Mechano-Responsive Hydrogen -Bonding Array of Thermoplastic Polyurethane Elastomer Captures Both Strength and Self-Healing. Nat. Commun. 2021, 12 , 621.

13.Chen, S.; Sun, L.; Zhou, X.; Guo, Y.; Song, J.; Qian, S.; Liu, Z.; Guan, Q.; Meade Jeffries, E.; Liu, W.; Wang, Y.; He, C.; You, Z. Mechanically and Biologically Skin-Like Elastomers for Bio-Integrated Electronics. Nat. Commun. 2020, 11 , 1107.

Mechanochromic mechanism of PU-Rh-PR film

To the best of our knowledge, there are very limited reports on polyrotaxane-based mechanophoric polymer films. There are some reports involving the generation of multicolor emission through stretch-induced photoinduced electron transfer processes [Sagara, Y.; Karman, M.; Verde-Sesto, E.; Matsuo, K.; Kim, Y.; Tamaoki, N.; Weder, C. Rotaxanes as Mechanochromic Fluorescent Force Transducers in Polymers. J. Am. Chem. Soc. 2018 , 140 , 1584-1587] or monochromatic emission [Shen, Z.; Zhu, FX; Yong, K.-T.; Gu, G. Stimuli-responsive Functional Materials for Soft Robotics. J. Mater. Chem. B 2020 , 8 , 8972-8991; Wang, T.; Zhang, N.; Dai, J.; Li, Z.; Bai, W.; Bai, R. Novel Reversible Mechanochromic Elastomer with High Sensitivity: Bond Scission and Bending-Induced Multicolor Switching. ACS Appl.Mater.Interfaces 2017 , 9 , 11874-11881].

However, in this disclosure, for the first time, a polyrotaxane-based mechanofluorescent PU elastomer was demonstrated, which caused dual-color ratio emission through a stress-induced molecular ring-opening reaction. This force-induced emission color change is caused by the stereochemical structure transformation of the Rh group. In Figure 3A, the stereochemical structure of the Rh group changes from the distorted spirolactamide form on the left side of the figure to the planar ionic structure on the right side of the figure. Here, we constructed a polyrotaxane-based polyurethane film using a naphthyl imide (Nap) end group that emits green light and a mechanofluorescent Rh derivative (Rh-2OH) that emits red light to observe the dual fluorescence emission change during its stretching process.

Figure 3B shows a schematic diagram of proportional luminescence induced by force in a polyrotaxane-based polyurethane film, where the left figure is a schematic diagram before the film is subjected to force, and the right figure is a schematic diagram after the film is subjected to force. In Figure 3B, the polyrotaxane-based polyurethane film contains a sliding effect between the naphthyl imide end group as an energy supplier and the mechanical fluorescence Rh group as an energy acceptor, realizing the force-induced proportional luminescence change and energy transfer (ET) process.

Figure 3C shows the photoluminescence spectrum of PU-Rh-PR-1.0 film under different strains. Figure 3D shows the proportional luminescence intensity of PU-Rh-PR-1.0 film under different strains. Figure 3E shows the CIE graph of photoluminescence from green to red of PU-Rh-PR-1.0 film under different strains. In Figures 3C and 3D, the decrease in green light intensity of the naphthamide end group at 503nm is inversely proportional to the increase in red mechanical fluorescence intensity of the Rh group at 597nm. Under 8000% strain, the red light emitted by Rh is the strongest, while the green light emitted by the naphthamide end group is the weakest, and the green light is reduced by 75% compared to when no force is applied. This result explains the proportional luminescence behavior caused by the transfer of green light energy emitted by the naphthamide end group to the Rh acceptor under 8000% strain. Subsequent time-resolved photoluminescence experiments also confirmed that the energy transfer efficiency between the naphthamide end group and the mechanofluorescent Rh group can reach 25%.

During the stretching process, the red luminescence of PU-Rh-TEA was activated at a strain of about 3000%, and the maximum red luminescence was reached at a strain of about 4000%. In contrast, PU-Rh-PR-1.0 still exhibited the initial green luminescence at a strain of 3000%, and was not activated to an open Rh group until a strain of 5000%, producing yellow luminescence (mixed luminescence of green Nap and open red Rh groups). In addition, the maximum red luminescence of PU-Rh-PR-1.0 was obtained at a strain of more than 7000%, and it broke at a strain of about 8000%, which further indicated the dissipation of mechanical force due to the pulley effect of the sliding polyrotaxane crosslinker (i.e., PR).

It is noteworthy that the observed energy transfer in PU-Rh-PR-1.0 films upon stretching can be further verified by the spectral overlap between the emission spectrum of the naphthyl imide end-group donor and the absorption spectrum of the Rh acceptor and by time-resolved photo-luminescence (TRPL) measurements. Therefore, UV-Vis spectroscopy measurements were performed on PU-Rh-PR-1.0, PU- Rh-TEA, and PU-PR films to show the absorption changes of the naphthyl imide end-group and the Rh open form in the unstretched and stretched states.

In all the films tested, the absorption band at 365-400nm was attributed to the absorption of the naphthyl imide end groups, while the new absorption band at 480-580nm (maximum wavelength of 547nm) was attributed to the open-ring Rh groups in the PU-Rh-TEA film, which changed from slightly yellow to pink when stretched, which was due to the photochromic behavior of the Rh open form. In addition, the PU-Rh-PR-1.0 film contained naphthyl imide end groups and Rh groups, and the absorption band of the naphthyl imide end group unit was between 365-400nm before stretching, and after stretching, it showed a clear absorption peak at 480-580nm (maximum wavelength of 547nm) to indicate the Rh open form. In contrast, the PU-PR film shows an absorption spectrum similar to that of PU-Rh-PR-1.0 during the stretching process due to the lack of Rh groups. Therefore, after stretching, the absorption spectrum of the Rh open form in the stretched PU-Rh-PR-1.0 film (λabs=547nm) partially overlaps with the green emission spectrum of the unstretched PU-Rh-PR-1.0 film (λem=503nm), thereby transferring energy from the emission energy of the naphthyl imide end group donor to the Rh open form acceptor, realizing the energy transfer process.

In addition, the fluorescence lifetime and energy transfer efficiency of the PU-Rh-PR-1.0 film before and after stretching were obtained by TRPL measurement to confirm the energy transfer from the emission of the naphthalene imide-terminated group donor to the Rh-open acceptor. As a comparison, the TRPL results of the PU-PR film without the mechanoluminescent Rh group before and after stretching showed similar fluorescence lifetime values (7.34 and 7.11ns, respectively); compared with the fluorescence intensity during stretching, this shows that the film thickness has little effect on the TRPL fluorescence lifetime value. Obviously, the fluorescence lifetime of the naphthyl imide-terminated group donor in the stretched PU-Rh-PR-1.0 film (τ _DA = 6.16ns, after energy transfer) is shorter than that in the initial PU-Rh-PR-1.0 film (τ _D = 8.11ns, without energy transfer); this indicates that energy transfer occurs between the naphthyl imide-terminated group donor and the ring-opened Rh acceptor in the stretched PU-Rh-PR-1.0 film. Therefore, the energy transfer efficiency (E,%) of the stretched PU-Rh-PR-1.0 film can be estimated by the following formula.

E = 1-( τ _DA / τ _D ) where τ _DA and τ _D represent the fluorescence lifetime of the corresponding stretched and pristine PU-Rh-PR-1.0 films, respectively, with an estimated value of about 24%. This suggests that the novel material embeds two fluorescent chromophores into the stimulus-responsive polymer, exhibiting an interesting proportional mechanochromic fluorescence switching behavior. In the stretched PU film, the decay of the red Rh mechanofluorescence should be stable and long enough (only about 5% decay in 15 minutes and about 20% decay in 1 hour) to demonstrate its novel mechanical property behavior in the environment for future applications.

Shape memory effect of PU-Rh-PR materials

It is now found that the stretched state of PU film of PR can still recover well to its original state after exceeding the yield point (about 40%). Recently, polymers with shape memory effect (such as PU) have attracted great attention due to their wide application in the field of soft robotics. Generally, polymers can easily produce strains in their elastic region, and when these materials are quenched to a temperature below their glass transition temperature (Tg), the mechanical strain can be stored. Then, as the temperature increases, the polymer can release the stored energy and recover its original shape. In addition, the shape of polyurethane can be quickly recovered at elevated temperatures (e.g., 60°C relative to room temperature), where the hydrogen bonds strengthened by oriented stretching caused by stress can be eliminated by heating.

To observe the shape memory effect of the PU-Rh-PR 1.0 film, a thermomechanical cycle experiment was conducted. Initially, the pale pink and green luminescence of the PU-Rh-PR-1.0 film in its original state was observed. After stretching, the film exhibited a distinct pink color and a strong yellow-orange luminescence. Then, the specimen was twisted into a spiral shape and frozen in liquid nitrogen to temporarily maintain the shape and retain the stretched yellow-orange luminescence. Afterwards, once the film was immersed in a hot water bath at 60°C, the spiral shape and yellow-orange luminescence that had just temporarily maintained the specimen recovered to its original shape (about >90%) and green luminescence. Similarly, the PU-Rh-PR-1.0 film (about 20 mg) was stretched to 12 cm and loaded with a 10-gram 50-NTD coin, which is almost 500 times heavier than the elastic PU film. Once a few drops of 60°C hot water were added, the PU film was able to lift the weight by about 5.1 cm, and the stretched yellow-orange luminescence also turned back to the unstretched green luminescence. The ultra-fast shape recovery properties of this polyrotaxane-based polyurethane film with shape memory effect and luminescence color change make it attractive for potential soft artificial muscle actuation applications.

In summary, a polyrotaxane (PR)-based polyurethane (PU) film has been successfully designed and synthesized. Notably, a very small amount (1wt%) of PR can greatly enhance the mechanical strength, with a tensile capacity of more than 6000% and a tensile strength of about 16MPa. This enhancement is attributed to the pulley structure of the PR, which can release the stress applied to the PU film. In addition, compared with similar PU films without PR, PU-Rh-PR-1.0 has the largest tensile capacity (8000%) and more than 60% toughness (550MJ/m ³ ) when the PR content increases from 0.1wt% to 1wt%. In addition, two luminescent behaviors of the emission spectra of green-luminescent naphthylimide and red-mechanical luminescence Rh (at 503 and 597 nm, respectively) were successfully introduced into the elastic PU film. Therefore, a remarkable and reversible dual luminescent switching behavior can be achieved during the mechanical stretching and relaxation of PU-Rh-PR-1.0. In addition, the energy transfer efficiency of the stretched PU-Rh-PR-1.0 film was studied, and it was about 24% from the green-luminescent Nap donor to the red-mechanical luminescent Rh acceptor, which indicates an interesting proportional mechanoluminescent switching behavior of the PR-based PU film during the stretching process.

Finally, a simple soft robot with ultrafast shape recovery behavior and reversible colorimetric mechanoluminescence switching in PR-based PU films heated at 60 °C accompanied by color changes in the emission spectrum is also proposed. These results reveal the potential of integrating polyrotaxane crosslinkers and mechanoluminescent materials with shape memory effects, which will open a new path for future soft, bio-integrated, and mechanoluminescent electronic sensor devices based on functionalized elastomers.

110: Polyurethane main chain

120: Monomer part of rhodamine derivative (Rh-2OH)

130: Cyclodextrin

140: Straight chain molecule

150: Naphthyl imide end-capping group

Claims (12)

A polyurethane elastomer material with mechanochromic properties, wherein the reactive monomers include: a rhodamine derivative (Rh-2OH) having a chemical structure as shown in formula (1);

A polyol monomer having at least a dihydroxyl group; a polyisocyanate monomer having at least a diisocyanate group; and a polyrotaxane crosslinking agent, comprising: a cyclic molecule having a plurality of hydroxyl groups for crosslinking with the polyisocyanate monomer; a linear molecule passing through the cyclic molecule; and a dicapping group bonded to both ends of the linear molecule, wherein the dicapping group is a naphthyl imide group having a chemical structure as shown in formula (2);

The molar ratio of the rhodamine derivative, the polyol monomer, and the polyisocyanate monomer is 1:89:120. The polyurethane elastomer material as described in claim 1, wherein the amount of the polyrotaxane crosslinking agent added is 0.1-2.0wt%. The polyurethane elastomer material as described in claim 1, wherein the amount of the polyrotaxane crosslinking agent added is 0.5-1.5wt%. The polyurethane elastomer material as described in claim 1, wherein the polyol monomer comprises tetraethylene glycol. The polyurethane elastomer material as described in claim 1, wherein the polyisocyanate monomer comprises hexamethylene diisocyanate. The polyurethane elastomer material as described in claim 1, wherein the cyclic molecule comprises cyclodextrin. The polyurethane elastomer material as described in claim 1, wherein the linear molecule comprises polyethylene glycol. The polyurethane elastomer material as described in claim 1, wherein the polyol monomer comprises tetraethylene glycol, the polyisocyanate monomer comprises hexamethylene diisocyanate, the cyclic molecule comprises cyclodextrin, and the linear molecule comprises polyethylene glycol. The polyurethane elastomer material as described in claim 1 has a chemical structure as shown in formula (3), wherein PR is the polyrotaxane crosslinking agent, and x and y are positive integers.

Formula (3). The polyurethane elastomer material as described in claim 9, wherein the ratio of x to y is 1:0.9~1:1.1. The polyurethane elastomer material as claimed in claim 1, wherein the polyrotaxane crosslinking agent has a chemical structure as shown in formula (4), wherein n is a positive integer

The polyurethane elastomer material as described in claim 11, wherein n is a positive integer between 15 and 30.