Synthesis of tough and fluorescent hydrogels via the synergistic associations of tetraphenylethane fluorogens and polymethylene spacers

Abstract

Incorporating hydrophobic associations into hydrophilic networks as energy dissipation units is an efficient strategy to toughen hydrogels. However, the micro-segregated structures often lead to turbid hydrogels with poor optical properties. Here, we report the synthesis of transparent, tough, and fluorescent hydrogels in which tetraphenylethylene (TPE) fluorogens are linked to the network by a polymethylene spacer. The TPE motif and polymethylene spacer form hydrophobic associations, affording the transparent hydrogels with excellent mechanical properties and strong fluorescence. The mechanical properties of the hydrogels can be tuned by the fraction of hydrophobic units, the length of the polymethylene spacer, and the presence of the TPE motif. A rubbery-to-glassy transition is found in poly(12-(4-(1,2,2-triphenylvinyl)phenoxy)dodecyl acrylate-co-acrylic acid) hydrogels and poly(4-(1,2,2-triphenylvinyl)phenoxy)hexyl acrylate-co-acrylic acid) hydrogels as the fraction of hydrophobic units increases. The increased glass transition temperatures and apparent activation energies of the hydrogels with longer spacers and the TPE motif indicate a synergistic effect between the hydrophobic polymethylene and TPE motifs. Small- and wide-angle X-ray scattering results show that these tough and fluorescent hydrogels have compact hydrophobic domains with a quasi-lamellar structure. The hydrophobic domains are disrupted during stretching to dissipate energy, accounting for the high toughness of the hydrogels. This study presents a novel strategy to construct tough and fluorescent hydrogels by forming synergistic associations, which should be informative for designing other tough materials with specific functions and applications.

---

1. Introduction

Hydrogels are a class of soft materials with characteristics similar to those of biological tissues. They have good biocompatibility and exhibit abundant responsive behaviors, thereby offering extensive applications in diverse fields.^1–4 However, conventional hydrogels usually exhibit poor mechanical properties, which severely limit their applications in load-bearing conditions.^5,6 Over the last two decades, various tough hydrogels have been developed, such as double-network gels,^7–12 slide-ring topological gels,^13–15 nanocomposite gels,^16,17 and tetra-armed polyethylene glycol gels.¹⁸ Among the toughening strategies, introducing physical interactions, such as hydrogen bonding,^19–22 coordination bonds,^23–25 host–guest inclusion,^26,27 π–π stacking,^28–30 and hydrophobic association,^31,32 into polymer networks is an effective way to dissipate energy during deformations of the hydrogels. To further enhance the mechanical performance of hydrogels, different types of associative interactions can be simultaneously incorporated into one network using parallel binding sites on polymer chains. However, the distributed interactions usually have no synergistic effects; thus, the enhancement of the mechanical properties of hydrogels is limited. High-performance hydrogels can be developed by incorporating synergistic associations. For example, hydrophobic interactions are introduced to the network to stabilize neighboring electrostatic interactions or hydrogen bonds, which improve the stability and mechanical properties of hydrogels.^29,30

Hydrophobically modified hydrophilic hydrogels are a typical class of tough hydrogels with hydrophobic associations serving as physical crosslinks and energy-dissipation units.^33,34 Similar to amphiphilic polymer co-network hydrogels, the segregated hydrophobic domains within the network result in a low swelling degree and enhanced mechanical properties.^35,36 Moreover, incorporating hydrophobic moieties into the hydrophilic network to form micro-segregated domains is an effective strategy to toughen hydrogels.^37–39 For example, Shi et al. developed tough hydrogels through the phase separation of polymers with long hydrophilic backbones and short alkyl side chains.⁴⁰ The size of the polymer-rich domains increased with the increasing length of side chains due to enhanced hydrophobic interactions. The hydrogel containing side chains with five carbon atoms exhibited a high tensile strength of 5.3 MPa and a fracture energy of 22 kJ m⁻² when the test temperature approached the glass-transition temperature of the hydrogel. Instead of using hydrocarbons, Weiss et al. prepared tough supramolecular hydrogels using fluorocarbons, which provide stronger hydrophobic interactions. They synthesized tough hydrogels by copolymerization of fluoroacrylates and N-isopropylacrylamide, followed by swelling or solvent-exchange processes; these hydrogels exhibited thermoresponsive behavior and possessed fracture toughness values up to 8 kJ m⁻².⁴¹ Moreover, Okay et al. incorporated surfactants into hydrophobically modified hydrogels to form crystalline domains between the surfactants and long alkyl chains grafted onto the hydrophilic network, endowing the hydrogels with high toughness and shape-memory capacity.^42,43 In the aforementioned studies, single or multiple types of hydrophobic moieties are incorporated into hydrophilic networks to form hydrophobic associations. However, the micro-segregated domains often compromise the transparency of the hydrogels, limiting applications requiring optical functionality. It is a high demand to afford tough hydrogels with other functionalities, yet without altering the microstructures.

Here, we report a novel strategy to synthesize tough and fluorescent hydrogels with synergistic associations by constructing networks with tetraphenylethylene (TPE) fluorogens chemically linked by a polymethylene spacer. The resultant poly(12-(4-(1,2,2-triphenylvinyl)phenoxy) dodecyl acrylate-co-acrylic acid) and poly(4-(1,2,2-triphenylvinyl)phenoxy)hexyl acrylate-co-acrylic acid) hydrogels are transparent and exhibit good mechanical properties as well as strong fluorescence due to a synergistic effect of the hydrophobic spacer and the TPE motif. The hydrophobic spacer enhances the restriction of TPE motion, favoring aggregation-induced emission, while the TPE motifs improve the hydrophobic interactions between polymethylene spacers, increasing the toughness of the hydrogels. Rheological measurements show that hydrogels with relatively large fractions of hydrophobic units are in a glassy state at room temperature because the segmental dynamics are reduced by robust synergistic associations. Small- and wide-angle X-ray scattering measurements disclose the disruption of hydrophobic associations upon stretching, as well as the toughening mechanism of the hydrogels. This work should merit the development of tough hydrogels with additional functional properties by forming synergistic associations between different motifs, which may open opportunities for applications of tough soft materials in flexible electronics, soft robots, etc.

2. Experimental section

2.1. Materials

4-Hydroxybenzophenone, diphenylmethanone, acryloyl chloride, 12-bromo-1-dodecanol, zinc powder, 6-bromo-1-hexanol, titanium(IV) chloride tetrahydrofuran complex, and dodecyl acrylate (C12) were received from Sigma-Aldrich. Acrylic acid (AAc), N,N′-methylenebisacrylamide (MBAA), ammonium persulfate (APS), and N,N,N′,N′-tetramethylethylenediamine (TEMED) were obtained from Aladdin Industrial Corporation. Anhydrous sodium sulfate (Na₂SO₄), potassium carbonate (K₂CO₃), sodium chloride (NaCl), triethylamine (Et₃N), dichloromethane (DCM), tetrahydrofuran (THF), petroleum ether (PE), ethyl acetate (EA), and N,N-dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd. Millipore deionized water (18.2 MΩ cm⁻¹) was used in all experiments.

2.2. Synthesis and characterization of monomers

Synthesis of 12-bromododecyl acrylate. Under a nitrogen atmosphere, a 250 mL three-neck flask was charged with 12-bromo-1-dodecanol (8.0 g, 3.0 mmol), Et₃N (6.2 mL, 4.5 mmol), and dry DCM (150 mL) and stirred in an ice-water bath for 10 min. Subsequently, acryloyl chloride (3.8 mL, 4.5 mmol) was slowly added, and the mixture was stirred at room temperature for 1 day. After the reaction, the mixture was washed with distilled water (3 × 100 mL) and saturated NaCl aqueous solution (2 × 50 mL). The organic layer was dried over Na₂SO₄ and then filtered and concentrated under reduced pressure to obtain 12-bromododecyl acrylate (12.7 g, 89%). 6-Bromohexanyl acrylate was prepared in a similar synthetic route using 6-bromo-1-hexanol in place of 12-bromo-1-dodecanol.

Synthesis of 12-(4-(1,2,2-triphenylvinyl)phenoxy)dodecyl acrylate (TPE_C12). Zinc powder (26.1 g, 400.0 mmol) was first added to THF (400 mL). After lowering the temperature to −10 °C under a dry nitrogen atmosphere, titanium(IV) chloride tetrahydrofuran complex (21.9 mL, 200.0 mmol) was slowly added, and the mixture was refluxed at 65 °C for 2 h. Subsequently, solutions of 4-hydroxybenzophenone (7.9 g, 40.0 mmol) and diphenylmethanone (10.9 g, 60.0 mmol) in THF (30 mL) were added to the stirred solution, and the mixture was further refluxed for 24 h. Upon cooling, the mixture was poured into a large amount of saturated K₂CO₃ aqueous solution (600 mL) and stirred intensely. The solution was then washed with DCM after the solution was layered and clear. The organic layer was dried over Na₂SO₄ and then filtered and concentrated under reduced pressure. The obtained solids were purified by column chromatography on silica gel using PE/EA (4 : 1) as the eluent, yielding white products of 4-(1,2,2-triphenylvinyl)phenol (TPE_OH) (11.8 g, 85%). Under a nitrogen atmosphere, 4-(1,2,2-triphenylvinyl)phenol (5.8 g, 1.66 mmol) and K₂CO₃ powder (6.6 g, 4.8 mmol) were dissolved in dry DMF (200 mL). After 10 min, 12-bromododecyl acrylate (9.7 g, 3.0 mmol) was added to the mixture, and the mixture was stirred at 100 °C for 24 h. Silica gel (20.0 g) was then added to the mixture, and the solvent was removed under reduced pressure. The resulting solids were purified by column chromatography on silica gel using PE/EA = 15 : 1 as the eluent to obtain 12-(4-(1,2,2-triphenylvinyl)phenoxy)dodecyl acrylate (TPE_C12, 8.0 g, 85%). Furthermore, (4-(1,2,2-triphenylvinyl)phenoxy) hexyl acrylate (TPE_C6) was prepared using a similar synthetic route.

2.3. Synthesis of hydrogels

TPE-containing hydrogels with different compositions were synthesized by free-radical copolymerization of hydrophobic monomers (TPE_C12, TPE_C6, or C12) with AAc. The total feeding monomer concentration was fixed at 2.5 M. Prescribed amounts of hydrophobic monomers, AAc, APS (the initiator; 0.5 mol% relative to total monomers), MBAA (the chemical crosslinker; 0.2 mol% relative to total monomers), and TEMED (the reaction accelerator; 0.1 vol% relative to total volume) were dissolved in DMSO to obtain precursor solutions. Subsequently, the precursor solutions were injected into a reaction cell consisting of a pair of polytetrafluoroethylene substrates separated by a 0.5-mm-thick silicone rubber spacer, which was kept in an oven at 55 °C for 8 h to complete polymerization. The resultant organogels were immersed in a large amount of water for one week to eliminate residual chemicals and reach the equilibrium state at room temperature. The hydrogels are denoted as P(TPE_C12-co-AAc)-f_m, P(TPE_C6-co-AAc)-f_m, and P(C12-co-AAc)-f_m, where f_m is the feeding molar fraction of the hydrophobic monomer.

2.4. Characterizations

¹H NMR spectra of the synthesized chemicals were recorded using a Bruker 500 MHz NMR spectrometer in DMSO or CDCl₃. Elemental analysis of the hydrogels was carried out on by a Flash 2000 element analyzer (Thermo Fisher Scientific). The hydrogels were freeze-dried and then ground into powders prior to analysis. Mass fractions of carbon (C), oxygen (O), and hydrogen (H) elements in the hydrogels were measured, and the experimental molar fractions of hydrophobic units in the hydrogels were calculated based on the measured mass ratio of C to O. Gel fractions of the hydrogels were determined by measuring the mass of dried organogels and the mass of dried hydrogels after soaking in DMSO, followed by water to remove residuals. As-prepared organogels were vacuum-dried at 80 °C for 16 h, and the mass was measured and recorded as w_og. The dried organogels were washed thrice with DMSO (4 h for each) and five times with water (6 h each), and then vacuum-dried at 80 °C for 16 h. The mass of the dried hydrogels was measured as w_hg. The gel fraction (F_g) of the hydrogels was calculated as F_g = w_hg/w_og. Mechanical properties of hydrogels were tested using a commercial tensile tester (Instron 3343). The hydrogels were cut into dumbbell-shaped samples with an initial gauge length of 12 mm and a width of 2 mm and were stretched at room temperature at a constant stretch rate of 100 mm min⁻¹. The curves of nominal stress (σ) versus strain (ε) were recorded, while the Young's modulus (E) was extracted from the initial slope of the curves at a strain below 5%. Three separate tests were performed to ensure the accuracy. Tensile tests at different temperatures and stretch rates were performed in a water bath at a certain temperature. The equilibrium time for the hydrogels at different temperatures was 3 min. Tearing experiments were conducted at room temperature to depict the fracture energy of the hydrogels. The hydrogels were sectioned into rectangular samples (35 mm × 12 mm), while a 10 mm initial incision was carefully created at the middle of the short edge. The two arms of the sample were clamped, and the upper arm was gradually pulled upwards at a constant stretch rate of 100 mm min⁻¹. The variation of the tearing force with displacement was recorded. The tearing energy, denoted as G, was calculated using the formula G = 2F/d, where F and d are the steady-state tearing force and the hydrogel thickness, respectively. The water content of the hydrogels, q, was calculated using the formula q = (w_s − w_d)/w_s, where w_s and w_d represent the masses of the hydrogels in the swollen and dried states, respectively. The transmittance of the hydrogels was measured by a UV-Vis spectrophotometer (Shimadzu, UV-2550) in the wavelength range of 200–800 nm. The emission spectra of the hydrogels were recorded with a F-4600 (Hitachi) fluorescence spectrometer.

Small- and wide-angle X-ray scattering (SAXS/WAXS) measurements were performed on the hydrogels at the BL16B1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF), China. The wavelength of the X-rays was 1.24 Å, and the sample-to-detector distances were 2343 mm and 198 mm for SAXS and WAXS measurements, respectively. Two-dimensional (2D) scattering patterns were acquired by a Pilatus 2M detector with a resolution of 1475 × 1679 pixels and a pixel size of 172 × 172 µm². The data acquisition time was 20 s per frame for the 2D SAXS and WAXS images. All scattering images were analyzed with the Fit2D software, following meticulous background correction. One-dimensional (1D) SAXS and WAXS profiles were extracted by circularly integrating the 2D patterns.

Rheological behavior of the hydrogels was studied using a DHR-2 rheometer (TA instruments) equipped with a parallel plate fixture (diameter = 8 mm) in a small-amplitude oscillatory shear mode. Disk-shaped hydrogel samples with a thickness of 0.5 mm were placed between the parallel plates. The applied axial force was set at 1 N throughout the rheological tests. The edge of the hydrogel sample was surrounded by water to minimize solvent evaporation. Temperature sweeps ranging from 10 to 90 °C were conducted on the samples at a heating rate of 10 °C min⁻¹, a strain amplitude of 0.1% (within the linear range), and an angular frequency of 10 rad s⁻¹. Frequency sweeps were carried out on the hydrogels over an angular frequency range of 100–0.1 rad s⁻¹ at different temperatures. Master curves of the hydrogels were constructed by time-temperature superposition (TTS) with both vertical and horizontal shift factors.

3. Results and discussion

3.1. Mechanical and fluorescent properties of the hydrogels

The synthetic routes of the monomers TPE_C6 and TPE_C12 is depicted in the SI (Fig. S1). The corresponding ¹H NMR spectra are shown in Fig. S2 to S4. A new broad peak at 9.29 ppm representing the hydroxyl protons (H_d) was observed, and the ratio of the integral area of H_d to that of H_a at 6.45–6.43 ppm was calculated to be 1 : 2, indicating TPE_OH was successfully synthesized (Fig. S2). Subsequently, monomers 1 and 2 were characterized by ¹H NMR. Fig. S3a shows the resonance signals of methylene protons (H_c) and double bond groups (H_a+b) at 4.15–4.11 and 6.40–5.78 ppm, respectively, and the integration ratio of H_c to H_a+b was nearly 2 : 3, meaning that the long-chain alkyl groups were connected to acryloyl chloride completely. Lastly, the long alkyl chains containing double bonds were attached to the TPE groups via an etherification reaction. Three sets of peaks corresponding to the double bond appeared in the range of 6.42–5.80 ppm (Fig. S4a). The integral area ratio of the olefin protons (H_a+b) to the aromatic protons (H_g) of the TPE group at 6.93–6.91 ppm and the methylene protons (H_e) of the alkyl chain at 4.17–4.14 ppm was approximately 3 : 2 : 2, indicating the successful preparation of TPE_C12. TPE_C6 was prepared using a similar route (Fig. S4b).

As shown in Fig. 1a, the P(TPE_C12-co-AAc)-f_m, P(TPE_C6-co-AAc)-f_m, and P(C12-co-AAc)-f_m organogels were prepared by polymerizing AAc with the hydrophobic monomers TPE_C12, TPE_C6, or C12 in DMSO (f_m is the feeding molar fraction of the hydrophobic monomer). The organogels are soft and weak (Fig. S5), which are converted into tough and stiff hydrogels by solvent exchange with water. During this process, hydrophobic domains form within the hydrophilic network, where TPE groups are restricted by the polymethylene spacers of the hydrophobic units. A small amount of chemical crosslinkers is added to the precursor solutions to maintain the integrity of the hydrogel networks during solvent exchange. Thus, the obtained samples have a gel fraction of ≥0.94 (Fig. S6), indicating high polymerization efficiency and the successful formation of crosslinked hydrogel networks. Moreover, the real composition of the hydrogels is generally consistent with the feeding molar ratio, with slight differences that should be related to the different reactivities of the monomers (Table S1). The resultant hydrogels exhibit a transparent appearance with different degrees of volume contraction (Fig. 1b). As f_m increases from 4 to 20 mol%, the transmittance of P(TPE_C12-co-AAc)-f_m, P(TPE_C6-co-AAc)-f_m, and P(C12-co-AAc)-f_m hydrogels slightly decreases from 90%, 91%, and 87% to 85%, 85%, and 81%, respectively (Fig. S7). With the presence of TPE groups and the increased length of the spacer, the hydrogels with enhanced hydrophobic interactions become stronger and stiffer, with an increase in breaking stress σ_b and Young's modulus E (Fig. 1c). On the other hand, the hydrogels containing TPE motifs exhibit strong fluorescence. A broad peak at ∼530 nm appears in the fluorescent spectra of P(TPE_C6-co-AAc)-20% and P(TPE_C12-co-AAc)-20% hydrogels (Fig. 1d), corresponding to blue fluorescent emission under UV light (Fig. 1e). The intensity of (TPE_C12-co-AAc)-20% hydrogel is twice that of the P(TPE_C6-co-AAc)-20% hydrogel, demonstrating that a longer spacer can enhance the restriction of TPE motifs within the hydrophobic domains. The polymethylene spacer and the TPE group work cooperatively to provide the hydrogels with good mechanical performance and optical functionality. The hydrophobic association between the spacers should be stabilized by the presence of TPE motifs, while the aggregation and restriction of TPE motifs should also be improved by the hydrophobic spacers.

Fig. 1 (a) Schematic of the preparation of P(TPE_C12-co-AAc)-f_m, P(TPE_C6-co-AAc)-f_m, and P(C12-co-AAc)-f_m hydrogels. (b) Photographs of the P(TPE_C12-co-AAc)-16% hydrogel before and after solvent exchange. (c) Tensile stress–strain curves of P(TPE_C12-co-AAc)-12%, P(TPE_C6-co-AAc)-12%, and P(C12-co-AAc)-12% hydrogels at a stretch rate of 100 mm min⁻¹ at 25 °C. (d) and (e) Emission spectra (d) and photographs under daylight and UV light (e) of P(TPE_C6-co-AAc)-20% and P(TPE_C12-co-AAc)-20% hydrogels. Scale bar: 1 cm.

Mechanical properties of the equilibrated P(TPE_C12-co-AAc)-f_m, P(TPE_C6-co-AAc)-f_m, and P(C12-co-AAc)-f_m hydrogels are examined by tensile tests at room temperature (Fig. 2). For P(TPE_C12-co-AAc)-f_m hydrogels, as f_m increases from 4 mol% to 20 mol%, E and σ_b increase from 2.75 MPa and 4.04 MPa to 101.8 MPa and 9.16 MPa, respectively, whereas the breaking strain ε_b decreases from 752% to 97% (Fig. 2a). When f_m reaches 20 mol%, a prominent yielding behavior appears in the stress–strain curve of the hydrogel. The water content q of the hydrogels decreases from 71 wt% to 36 wt% with the increase in f_m (Fig. 2b). The increased mechanical performance of the hydrogels with increasing f_m is associated with enhanced hydrophobic associations, which result in a denser hydrogel matrix.^28,44,45 Moreover, the length of the spacer and the presence of the TPE motif also influence the mechanical properties of the hydrogels. For P(TPE_C6-co-AAc)-f_m hydrogels with a shorter spacer, the variations of σ_b, E, ε_b, and q with the increase in f_m exhibit similar trends as those observed for P(TPE_C12-co-AAc)-f_m hydrogels (Fig. 2c and d). Meanwhile, at a certain f_m, the values of σ_b and E of P(TPE_C6-co-AAc) hydrogels are smaller than those for P(TPE_C12-co-AAc) hydrogels; this result indicates that stronger associations are formed in hydrogels containing hydrophobic units with longer spacers. As shown in Fig. 2e and f, in the absence of TPE, P(C12-co-AAc)-f_m hydrogels exhibit higher water content (50 wt% to 95 wt%) and drastically reduced mechanical properties (E = 0.05–0.72 MPa; σ_b = 0.048–1.98 MPa; ε_b = 152–1990%) due to the formation of weaker hydrophobic associations. The tearing fracture energy (G) of the hydrogels also varies with the components (Fig. S8). As f_m increases from 4 mol% to 20 mol%, the G values of P(TPE_C12-co-AAc)-f_m and P(TPE_C6-co-AAc)-f_m hydrogels increases from 1.6 kJ m⁻² to 5.3 kJ m⁻² and from 0.8 kJ m⁻² to 3.5 kJ m⁻², respectively, comparable to the value of native cartilages (∼1.0 kJ m⁻²).^46,47 This increase in G of the hydrogels with the increasing f_m is mainly attributed to the formation of denser hydrophobic domains that dissipate more energy during rupture. As expected, P(TPE_C12-co-AAc)-f_m hydrogels exhibit larger G than hydrogels with shorter spacers (P(TPE_C6-co-AAc)-f_m gels) or without the TPE motif (P(C12-co-AAc)-f_m gels). By contrast, the poly(acrylic acid) hydrogels with similar water content are mechanically soft and weak (Fig. S9). These results suggest that the TPE motif and the long polymethylene spacer are both indispensable for the formation of robust hydrophobic interactions and thus for the good mechanical performance of the hydrogels.

Fig. 2 Tensile stress–strain curves (a), (c) and (e) and the corresponding mechanical parameters (b), (d) and (f) of P(TPE_C12-co-AAc)-f_m (a) and (b), P(TPE_C6-co-AAc)-f_m (c) and (d), and P(C12-co-AAc)-f_m (e) and (f) hydrogels with different f_m at 25 °C (stretch rate: 100 mm min⁻¹). The inset in (e) is the tensile stress–strain curve of P(C12-co-AAc)-4% hydrogel. Error bars represent the standard deviation of the mean (n = 3).

The dependence of the mechanical properties of TPE-containing hydrogels on the stretch rate and test temperature was examined. Since water content has a strong influence on the mechanical properties of the hydrogel, P(TPE_C12-co-AAc)-8%, P(TPE_C6-co-AAc)-12%, and P(C12-co-AAc)-16% hydrogels with a similar water content of ∼60 wt% were chosen to investigate the effects of spacer length and the presence of TPE on the properties of hydrogels. As the stretch rate increased from 20 mm min⁻¹ to 500 mm min⁻¹, the E and σ_b of the P(TPE_C12-co-AAc)-8% hydrogel increased from 2.4 MPa and 2.07 MPa to 10.3 MPa and 9.83 MPa, respectively (Fig. 3a and b). The value of ε_b first increased from 349% to 445% with the rise in stretch rate from 20 to 100 mm min⁻¹, and then becomes insensitive to further increases in stretch rate. The rate-dependent mechanical properties should be related to the dynamic nature of the hydrophobic interactions and variations in segmental mobility. At a higher stretch rate, the hydrophobic interactions become stronger, leading to improved σ_b, E, and ε_b. The values of σ_b and E of the P(TPE_C6-co-AAc)-12% hydrogel with a shorter spacer and the P(C12-co-AAc)-16% hydrogel without TPE exhibit weaker dependence on stretch rate due to relatively weak hydrophobic interactions (Fig. S10). Test temperature also influences the mechanical behaviors of the hydrogels. The rise in temperature leads to increased segmental mobility.^48,49 As the temperature increases from 10 °C to 70 °C, σ_b and E of the P(TPE_C6-co-AAc)-12% hydrogel decrease from 5.9 MPa and 8.0 MPa to 1.5 MPa and 0.4 MPa, respectively, while ε_b first increases from 326% to 1152% and then decreases to 690% (Fig. 3c and d). With the increase in temperature from 10 °C to 50 °C, segmental mobility is improved, leading to increased ε_b. Further increase in temperature results in decreased ε_b due to the dissociation of hydrophobic interactions. We should note that the temperature at which the decrease in E for P(TPE_C6-co-AAc)-12% and P(C12-co-AAc)-16% hydrogels becomes more gradual is lower than that of the P(TPE_C12-co-AAc)-8% hydrogel (Fig. S11). This is because the P(TPE_C6-co-AAc)-12% hydrogel has a relatively high glass transition temperature, which will be discussed in Section 3.2.

Fig. 3 Tensile stress–strain curves (a) and (c) and the corresponding mechanical properties (b) and (d) of P(TPE_C12-co-AAc)-8% hydrogels at different stretch rates (a) and (b) and temperatures (c) and (d). Error bars represent the standard deviation of the mean (n = 3).

The TPE-containing hydrogels also exhibit different self-recovery abilities due to different segmental mobilities at room temperature. As shown in Fig. S12, the P(TPE_C12-co-AAc)-8% hydrogel shows significant hysteresis in the loading–unloading curve, indicating large energy dissipation during the loading process.⁵⁰ However, a large residual strain of 50.4% and a hysteresis ratio (calculated as the ratio of the second loop area to the first one) of 11.7% are observed. After the waiting time (t_w) reaches 24 h, there is still a residual strain of 12% and a hysteresis ratio of 58%, reminiscent of the poor self-recovery property of glassy gels.⁴⁸ The P(TPE_C6-co-AAc)-12% hydrogel shows a similar self-recovery process, with a residual strain of 10% and a hysteresis ratio of 62% after waiting for 24 h. However, the residual strain of the P(C12-co-AAc)-16% hydrogel completely disappears, and the hysteresis ratio reaches 98% after waiting for 30 min. This fast self-recovery behavior of the P(C12-co-AAc)-16% hydrogel is attributed to the high segmental mobility and the fast reformation of weak associations.

In addition to the synergistic effect of the hydrophobic spacer and TPE motifs on the mechanical performance of the hydrogels, the spacer also enhances the aggregation and restriction of TPE groups for efficient aggregation-induced emission.^51–53 The fluorescence spectra of P(TPE_C12-co-AAc)-f_m and P(TPE_C6-co-AAc)-f_m hydrogels are shown in Fig. 4a. Upon excitation at 395 nm, the hydrogels exhibit strong fluorescence emission. As f_m increases from 4 mol% to 20 mol%, the emission wavelength of P(TPE_C12-co-AAc)-f_m hydrogels slightly shifts from 527 nm to 538 nm (Fig. 4a). This red shift is associated with the increased density of fluorogens within the hydrogel. The fluorescent behavior is also influenced by the spacer length of the hydrophobic unit. As shown in Fig. 4b, the fluorescence intensity of P(TPE_C6-co-AAc)-f_m gels is lower than that of P(TPE_C12-co-AAc)-f_m gels due to weaker hydrophobic associations, which reduce the restriction of TPE groups. Specifically, the P(TPE_C12-co-AAc)-8% and P(TPE_C6-co-AAc)-12% hydrogels with similar water content were prepared for the comparison of their fluorescent behaviors. The P(TPE_C12-co-AAc)-8% hydrogel exhibits higher fluorescence intensity than the P(TPE_C6-co-AAc)-12% hydrogel (Fig. 4a and b), manifesting the positive influence of the polymethylene spacer on the restriction and emission of TPE motifs.

Fig. 4 Emission spectra of P(TPE_C12-co-AAc)-f_m (a) and P(TPE_C6-co-AAc)-f_m (b) hydrogels. The excitation wavelength is 395 nm.

3.2. Rheological properties of the hydrogels

To investigate the influence of hydrophobic associations on the dynamics of the network, rheological measurements were performed on the hydrogels with different compositions. The temperature sweep spectra of the hydrogels are shown in Fig. 5. The storage modulus G′ and loss modulus G″ of the hydrogels dramatically decrease with increasing temperature, accompanied by a broad peak in the spectrum of loss factor (tan δ) that corresponds to the glassy-to-rubbery transition process. For the hydrogels with the same spacer length, the glass transition temperature (T_g) increases from 41 °C to 55 °C as f_m rises from 8 mol% to 20 mol%. This is because more hydrophobic associations are formed and act as physical crosslinks, which reduce segmental dynamics and increase T_g of the hydrogel. The P(TPE_C12-co-AAc)-20% hydrogel is in a glassy state at room temperature, therefore showing pronounced yielding during the tensile test. Furthermore, it is found that the T_g of the P(TPE_C12-co-AAc)-8% hydrogel is higher than that of the P(TPE_C6-co-AAc)-12% hydrogel (33 °C) and the P(C12-co-AAc)-16% hydrogel (15 °C), even with similar water content (60 wt%). The different viscoelastic behaviors further highlight the synergistic effect of the polymethylene spacer and hydrophobic TPE motifs on the formation of robust physical associations.

Fig. 5 Temperature sweep spectra (a)–(d) and glass transition temperature (e) of P(TPE_C12-co-AAc)-8%, P(TPE_C12-co-AAc)-20%, P(TPE_C6-co-AAc)-12%, and P(C12-co-AAc)-16% hydrogels measured at a heating rate of 10 °C min⁻¹, an angular frequency of 10 rad s⁻¹, and a strain amplitude of 0.1%.

To extend the observation time scales, dynamic moduli spectra of the hydrogels across wide frequency ranges were acquired through time-temperature superposition (TTS). The horizontal shift factors (a_T) and vertical shift factors (b_T) were obtained via

b_TG′(ω, T) = G′(ωa_T, T₀)

and

b_TG″(ω, T) = G″(ωa_T, T₀),

where T₀ is the reference temperature, and G′(ωa_T, T₀) and G″(ωa_T, T₀) correspond to the storage and loss modulus at T₀ and the shifted frequency ωa_T, respectively.⁵⁴ As shown in Fig. 6, the master curves of the hydrogels can be divided into three regions with increasing frequency: (i) the rubbery plateau region, where G′ slightly changes while G″ first decreases and then increases; (ii) the transition region, where both G′ and G″ drastically increase; (iii) the glassy plateau region, where G′ becomes stable and G″ decreases.^54,55 Additionally, the loss factor (tan δ) displays a valley–peak–valley trend with increasing frequency, suggesting that the hydrogels undergo a solid-like-viscoelastic-solid-like process.⁵⁴ As f_m increases from 8 to 20 mol%, the master curve of the P(TPE_C12-co-AAc)-f_m hydrogel shifts to lower frequencies, indicating a longer relaxation time of polymer segments arising from the increased density of hydrophobic associations (Fig. 6a and b). Moreover, the master curve of the P(TPE_C6-co-AAc)-12% hydrogel with a shorter spacer and the P(C12-co-AAc)-16% hydrogel without the TPE motif both shift to higher frequencies compared to the spectra of the P(TPE_C12-co-AAc)-8% hydrogel (Fig. 6c and d), indicating a faster relaxation of polymer segments. Moreover, the plateau modulus, determined by the value of G′ at the frequency when G″ reaches the valley value, is smaller for P(TPE_C6-co-AAc)-12% and P(C12-co-AAc)-16% hydrogels than for P(TPE_C12-co-AAc)-f_m hydrogels. The above observations demonstrate that the synergistic effect of the long polymethylene spacer and the hydrophobic TPE motif increases the robustness of physical associations, reducing the dynamics of polymer segments.

Fig. 6 (a)–(d) Spectra of storage modulus G′, loss modulus G″, and loss tangent (tan δ) of P(TPE_C12-co-AAc)-8% (q = 60 wt%) (a), P(TPE_C12-co-AAc)-20% (q = 36 wt%) (b), P(TPE_C6-co-AAc)-12% (q = 61 wt%) (c), and P(C12-co-AAc)-16% hydrogels (q = 51 wt%) (d) obtained via TTS with a reference temperatures of 40 °C and a strain amplitude of 0.1%. (e) and (f) Arrhenius fitting of ln a_Tversus 1/T (e) and the ascertained apparent activation energy (E_a) (f) of the TPE-containing hydrogels.

By plotting ln a_Tversus 1/T, it is revealed that the TPE-containing hydrogels display Arrhenius behavior over the probed temperature range (Fig. 6e). The apparent activation energy (E_a) can be extracted from the slope of the fitted lines based on the following equation:⁵⁴

ln a_T = E_a/RT,

where R is the gas constant. As shown in Fig. 6f, with the increase in f_m from 8 to 20 mol%, the E_a values of the hydrogels increase from 195.3 kJ mol⁻¹ to 285.1 kJ mol⁻¹. Additionally, the E_a values of P(TPE_C6-co-AAc)-12% and P(C12-co-AAc)-16% hydrogels are 143.7 kJ mol⁻¹ and 92.7 kJ mol⁻¹, respectively, which are both smaller than that of the P(TPE_C12-co-AAc)-8% hydrogel. This result demonstrates that hydrogels with longer polymethylene spacers and TPE motifs have a larger energy barrier for segmental motion, which contributes to higher T_g as well as enhanced mechanical performances.

3.3. Microstructures and toughening mechanism of the hydrogels

To elucidate the synergistic effect of polymethylene spacers and TPE motifs on the microstructures of hydrophobic domains in the hydrogels, SAXS measurements were performed on the gels with different compositions.^56–59 For the organogels before solvent exchange, no scattering peaks were found in the 1D SAXS profiles (Fig. S13). When f_m ≥ 8 mol%, scattering peaks appear at a q of ∼0.88 nm⁻¹ in the 1D SAXS profiles of P(TPE_C12-co-AAc)-f_m hydrogels, indicating the formation of compact hydrophobic domains with a characteristic spacing (d) of ∼7.13 nm (Fig. 7a). This spacing corresponds to the sum of the thickness of the quasi-lamellar structure formed by the polymethylene spacer as well as TPE motifs, and the thickness of the amorphous domain composed of backbones.⁴² When f_m < 8 mol%, a scattering peak appears at a q of 1.21 nm⁻¹, corresponding to a d of 5.19 nm. The smaller value of d indicates partial penetration of the polymethylene spacers into neighboring layers within the hydrophobic domains.^45,60 The scattering peaks of P(TPE_C6-co-AAc)-f_m and P(C12-co-AAc)-f_m hydrogels become broader and shift to the high-q region compared to P(TPE_C12-co-AAc)-f_m hydrogels (Fig. 7b and c). The variation of d with f_m for P(TPE_C6-co-AAc)-f_m hydrogels shows a trend similar to that of P(TPE_C12-co-AAc)-f_m hydrogels (Fig. 7d). The equilibrium value of d is smaller than that of P(TPE_C12-co-AAc)-f_m hydrogels due to the shorter polymethylene spacer. However, as f_m increases, the d of P(C12-co-AAc)-f_m hydrogels continuously increases from 3.85 nm to 5.29 nm, indicating a less compact structure because of weaker hydrophobic associations and the lack of steric hindrance in the absence of TPE. Note that the f_m at which d starts reaching equilibrium for P(TPE_C12-co-AAc)-f_m hydrogels is smaller than that for P(TPE_C6-co-AAc)-f_m hydrogels because of stronger hydrophobic associations formed within gels with longer polymethylene spacers and in the presence of TPE motifs. The different trends in d with the increased f_m of hydrogels highlight the synergistic effect of the polymethylene spacer and the hydrophobic TPE motif on the formation of quasi-lamellar structures.

Fig. 7 (a)–(c) 1D SAXS profiles of P(TPE_C12-co-AAc)-f_m (a), P(TPE_C6-co-AAc)-f_m (b), and P(C12-co-AAc)-f_m (c) hydrogels. (d) Characteristic spacing of hydrophobic domains in the hydrogels. Insets in (a) and (c) are schematic of the structures of hydrophobic domains in P(TPE_C12-co-AAc)-f_m and P(C12-co-AAc)-f_m hydrogels.

To further reveal the toughening mechanism, SAXS and WAXS measurements were performed on the P(TPE_C12-co-AAc)-8% hydrogel to reveal the microstructure evolution during stretching. The tensile stress–strain curve shows a strain-softening behavior beyond the linear region, followed by strain-hardening at an elongation ratio of λ ≥ 3 (λ = 1 + ε) (Fig. 8a). The 2D SAXS scattering pattern of the hydrogel changes from isotropic to ellipse with increased λ, indicating the orientation of the backbones (Fig. 8b). The structural evolution of hydrophobic domains during stretching is shown in Fig. 8c. 1D scattering profiles of the gel at different λ values were obtained by integrating the 2D patterns perpendicular (⊥) and parallel (‖) to the stretching direction (Fig. 8d and e). In the perpendicular direction, the scattering peak shifts to the high-q region as λ increases from 1 to 3.5, and then stabilizes at a q of ∼1.14 nm⁻¹ with a further increase in λ. In the parallel direction, the scattering peak also shifts to the high-q region as λ increases from 1 to 2, and then disappears. The values of d are summarized in Fig. 8f. As λ increases from 1 to 3.5, d_⊥ decreases from 7.45 nm to 5.63 nm due to the orientation and alignment of polymer backbones. When λ > 3.5, d_⊥ reaches equilibrium, indicating that a part of the hydrophobic domains maintains a compact structure during the strain-hardening stage due to the robust synergistic associations between TPE motifs and the polymethylene spacer. On the other hand, the value of d_‖ decreases from 7.65 to 6.54 nm as λ increases from 1 to 2. The absence of the scattering peak at higher λ indicates the disruption of hydrophobic domains during stretching, dissipating energy and toughening the hydrogels.⁶⁰ 1D profiles of the P(TPE_C12-co-AAc)-8% hydrogel, obtained by integrating the 2D WAXS patterns perpendicular and parallel to the stretching direction at various λ values, are shown in Fig. 8g–i. In the parallel direction, the scattering peaks appear at a q of ∼15.1 nm⁻¹ with increasing λ, corresponding to a constant distance between two polymethylene spacers (d_3‖) of 0.41 nm.^45,60,61 In the perpendicular direction, the peak corresponding to d_⊥ disappears when λ is beyond 2, confirming the disruption of the hydrophobic domains during stretching.

Fig. 8 (a) Tensile stress–strain curve of the P(TPE_C12-co-AAc)-8% hydrogel during SAXS measurements at room temperature (stretch rate = 9 mm min⁻¹). (b) 2D SAXS patterns of the hydrogel at various elongation ratios (λ). (c) Schematic of the structural evolution of hydrophobic domains during stretching. (d) and (e) 1D SAXS profiles perpendicular (d) and parallel (e) to the stretching direction at different λ values. (f) Variations in d in perpendicular (⊥) and parallel (//) directions with increasing λ. (g) and (h) 1D WAXS profiles perpendicular (g) and parallel (h) to the stretching direction at different λ values. (i) Variation in d₃ in the perpendicular and parallel directions with increasing λ.

4. Conclusions

In summary, we developed tough and fluorescent hydrogels in which TPE fluorogens are linked to the network via polymethylene spacers. By tailoring the feeding ratio of hydrophobic monomers, the length of spacer, and the presence of TPE fluorogens, the transparent hydrogels exhibit strong fluorescence and tunable mechanical properties, with σ_b of 4.0–9.2 MPa, ε_b of 97–752%, and E of 2.8–101.8 MPa. The TPE-containing hydrogels undergo a glassy-to-rubbery transition as the temperature increases. The glass transition temperature and the apparent activation energy of the hydrogels increase with the spacer length and the fraction of hydrophobic units. We found that the polymethylene spacer and TPE motif have a synergistic effect on the formation of robust hydrophobic associations, favoring the toughening and luminescence of the hydrogels. The hydrogels have compact hydrophobic domains with quasi-lamellar structure, which are disrupted during stretching to dissipate energy and thus toughen the gels. This study provides a novel strategy for synthesizing tough and fluorescent hydrogels by synergistic associations, which should merit designing other tough soft materials with applications in load-bearing conditions.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: additional experimental results include the 1H NMR spectra, UV-Vis spectra, tearing force–displacement curves, and cyclic tensile stress–strain curves of the hydrogels. See DOI: https://doi.org/10.1039/d5sm00950b.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (52325302, 52173012), Shanxi-Zheda Institute of Advanced Materials and Chemical Engineering (2022SZ-FR004), and Fundamental Research Funds for the Central Universities (226-2025-00031). We thank the staff at BL 16B1 beamline of Shanghai Synchrotron Radiation Facility (SSRF) for the SAXS measurements and also Xin Ning Zhang from the Testing and Analysis Center of Department of Polymer Science and Engineering, Zhejiang University, for the help with tensile measurements.

References

1. D. Zhou, W. T. Wang, W. Z. Ma, Y. W. Xian, Z. J. Zhang, Z. Pan, Y. X. Li, L. Huang, L. Liu, Z. M. Zheng, H. M. Liu and D. Wu, Adv. Mater., 2025, 37, 2414081.
2. J. Zhu, Y. T. Wei, G. M. Yang, J. N. Zhang, J. Y. Liu, X. Zhang, Z. Zhu, J. Y. Chen, X. B. Pei, D. D. Wu and J. Wang, Nat. Commun., 2025, 16, 7090.
3. B. Wu, R. W. Lewis, G. Li, Y. Gao, B. Fan, B. Klemm, J. Huang, J. Wang, M. A. C. Stuart and R. Eelkema, Chem. Sci., 2023, 14, 1512–1523.
4. D. Jiao, Q. L. Zhu, C. Y. Li, Q. Zheng and Z. L. Wu, Acc. Chem. Res., 2022, 55, 1533–1545.
5. Y. S. Zhang and A. Khademhosseini, Science, 2017, 356, eaaf3627.
6. X. Kuang, M. O. Arıcan, T. Zhou, X. Zhao and Y. S. Zhang, Acc. Mater. Res., 2023, 4, 101–114.
7. X. Li and J. P. Gong, Nat. Rev. Mater., 2024, 9, 380–398.
8. Z. J. Wang, W. Li, X. Y. Li, T. Nakajima, M. Rubinstein and J. P. Gong, Nat. Mater., 2025, 24, 607–614.
9. J. Y. Sun, X. Zhao, W. R. K. Illeperuma, O. Chaudhuri, K. H. Oh, D. J. Mooney, J. J. Vlassak and Z. Suo, Nature, 2012, 489, 133–136.
10. H. C. Yu, C. Y. Li, M. Du, Y. Song, Z. L. Wu and Q. Zheng, Macromolecules, 2019, 52, 629–638.
11. M. Hua, S. Wu, Y. Ma, Y. Zhao, Z. Chen, I. Frenkel, J. Strzalka, H. Zhou, X. Zhu and X. He, Nature, 2021, 590, 594–599.
12. J. Kim, G. Zhang, M. Shi and Z. Suo, Science, 2021, 374, 212–216.
13. X. Q. Wang, C. J. Yu, M. L. Wang, S. Y. Zhao, Q. M. Mao and Y. P. Wang, Angew. Chem., Int. Ed., 2025, e202511493.
14. C. Liu, N. Morimoto, L. Jian, S. Kawahara, T. Noritomi, H. Yokoyama, K. Mayumi and K. Lto, Science, 2021, 372, 1078–1081.
15. M. Tang, D. Zheng, J. Samanta, E. H. R. Tsai, H. Qiu, J. A. Read and C. F. Ke, Chem, 2023, 9, 3515–3531.
16. S. Tamesue, M. Ohtani, K. Yamada, Y. Ishida, J. M. Spruell, N. A. Lynd, C. J. Hawker and T. Aida, J. Am. Chem. Soc., 2013, 135, 15650–15655.
17. P. Das, S. Ganguly, P. K. Marvi, M. Sherazee, S. R. Ahmed, X. Tang, S. Srinivasan and A. R. Rajabzadeh, Adv. Funct. Mater., 2024, 34, 2314520.
18. H. Kamata, Y. Akagi, K. Y. Kayasuga, U. I. Chung and T. Sakai, Science, 2014, 343, 873–875.
19. X. Hu, M. Vatankhah-Varnoosfaderani, J. Zhou, Q. Li and S. S. Sheiko, Adv. Mater., 2015, 27, 6899–6905.
20. Z. L. Han, P. Wang, Y. C. Lu, Z. Jia, S. X. Qu and W. Yang, Sci. Adv., 2022, 8, eabl5066.
21. I. Jeon, J. Cui, W. R. K. Illeperuma, J. Aizenberg and J. J. Vlassak, Adv. Mater., 2016, 28, 4678–4683.
22. Y. Q. Wu, Y. Q. Shi and H. L. Wang, Macromolecules, 2023, 56, 4491–4502.
23. J. H. Liu, C. M. Xie, A. Kretzschmann, K. Koynov, H. Butt and S. Wu, Adv. Mater., 2020, 32, 1908324.
24. Y. W. Huang, S. Y. Qian, J. Zhou, W. J. Chen, T. Liu, S. Yang, S. J. Long and X. F. Li, Adv. Funct. Mater., 2023, 33, 2213549.
25. S. C. Grindy, R. Learsch, D. Mozhdehi, J. Cheng, D. G. Barrett, Z. Guan, P. B. Messersmith and N. Holten-Andersen, Nat. Mater., 2015, 14, 1210–1216.
26. C. J. Lu, M. M. Zhang, D. T. Tang, X. Z. Yan, Z. Y. Zhang, Z. X. Zhou, B. Song, H. Wang, X. P. Li, S. C. Yin, H. Sepehrpour and P. J. Stang, J. Am. Chem. Soc., 2018, 140, 7674–7680.
27. D. J. Zhang, M. A. Soto, L. Lewis, W. Y. Hamad and M. J. MacLachlan, Angew. Chem., Int. Ed., 2020, 59, 4705–4710.
28. C. Jiao, Y. Y. Chen, T. Q. Liu, X. Peng, Y. X. Zhao, J. A. Zhang, Y. Q. Wu and H. L. Wang, ACS Appl. Mater. Interfaces, 2018, 10, 32707–32716.
29. M. T. I. Mredha, S. K. Pathak, V. T. Tran, J. X. Cui and I. Jeon, Chem. Eng. J., 2019, 362, 325–338.
30. Z. Jiang, M. L. Tan, M. Taheri, Q. Yan, T. Tsuzuki, M. G. Gardiner, B. Diggle and L. A. Connal, Angew. Chem., Int. Ed., 2020, 59, 7049–7056.
31. X. Liu, H. J. Zhang, S. S. Xi, Y. F. Zhang, P. Rao, X. Y. You and S. X. Qu, Adv. Funct. Mater., 2025, 35, 2413464.
32. H. Chen, F. Yang, Q. Chen and J. Zheng, Adv. Mater., 2017, 29, 1606900.
33. O. E. Philippova, D. Hourdet, R. Audebert and A. R. Khokhlov, Macromolecules, 1997, 30, 8278–8285.
34. J. K. Hao and R. A. Weiss, Macromolecules, 2011, 44, 93909398.
35. C. S. Patrickios and K. Matyjaszewski, Polym. Int., 2021, 70, 10–13.
36. D. E. Apostolides, G. Michael, K. Andronikou, C. S. Patrickios, S. Pásztor, G. Szarka, A. Petróczy, B. Iván, T. Sakai, S. Prévost, D. G. Tsalikis and M. Gradzielski, Chem. Mater., 2025, 37, 5515–5528.
37. D. E. Apostolides, G. Michael, C. S. Patrickios, B. Notredame, Y. Zhang, J. Gohy, S. Prévost, M. Gradzielski, F. A. Jung and C. M. Papadakis, ACS Appl. Mater. Interfaces, 2024, 16, 23813–23825.
38. Y. Sun, S. Liu, G. Du, G. Gao and J. Fu, Chem. Commun., 2015, 51, 8512–8515.
39. J. Y. Hu, L. X. Hou, A. Zhu, H. N. Qiu, Z. R. Zhang, C. Du, K. Cui, Q. Zheng and Z. L. Wu, Macromolecules, 2024, 57, 11007–11019.
40. J. G. Pan, W. Zhang, J. Zhu, J. Y. Tan, Y. Huang, K. L. Mo, Y. Tong, Z. H. Xie, Y. B. Ke, H. D. Zheng, H. Ouyang, X. T. Shi and L. Gao, Adv. Mater., 2023, 35, 2207750.
41. F. Wang and R. A. Weiss, Macromolecules, 2018, 51, 7386–7395.
42. C. Bilici, S. Ide and O. Okay, Macromolecules, 2017, 50, 3647–3654.
43. C. Bilici, V. Can, U. Nöchel, M. Behl, A. Lendlein and O. Okay, Macromolecules, 2016, 49, 7442–7449.
44. A. Argun, U. Gulyuz and O. Okay, Macromolecules, 2018, 51, 2437–2446.
45. Y. Geng, X. Y. Lin, P. Pan, G. Shan, Y. Bao, Y. Song, Z. L. Wu and Q. Zheng, Polymer, 2016, 100, 60–68.
46. J. Liu, W. Zhao, Z. C. Ma, H. W. Zhao and L. Q. Ren, Nat. Commun., 2025, 16, 2309.
47. C. Fan, B. Liu, Z. Xu, C. Cui, T. Wu, Y. Yang, D. Zhang, M. Xiao, Z. Zhang and W. Liu, Mater. Horiz., 2020, 7, 1160.
48. X. N. Zhang, Y. J. Wang, S. Sun, L. Hou, P. Wu, Z. L. Wu and Q. Zheng, Macromolecules, 2018, 51, 8136–8146.
49. X. X. Lin, X. L. Wang, L. P. Zeng, Z. L. Wu, H. Guo and D. Hourdet, Chem. Mater., 2021, 33, 7633–7656.
50. L. Jiang, Z. Sha, Y. Zheng, R. Zhu, C. Yu, Q. Chen, R. Ran and W. Cui, Prog. Mater. Sci., 2025, 152, 101459.
51. Y. Hong, J. W. Lam and B. Z. Tang, Chem. Soc. Rev., 2011, 40, 5361–5388.
52. W. Zhao, Z. He and B. Z. Tang, Nat. Rev. Mater., 2020, 5, 869–885.
53. G. M. Y. Su, N. Wang, Y. K. Liu, R. Y. Zhang, Z. Li, Y. L. Deng and B. Z. Tang, Adv. Mater., 2024, 36, 2400085.
54. X. N. Zhang, C. Du, Y. J. Wang, L. X. Hou, M. Du, Q. Zheng and Z. L. Wu, Macromolecules, 2022, 55, 7512–7525.
55. C. Du, X. N. Zhang, T. L. Sun, M. Du, Q. Zheng and Z. L. Wu, Macromolecules, 2021, 54, 4313–4325.
56. K. Cui, T. L. Sun, X. Liang, K. Nakajima, Y. N. Ye, L. Chen, T. Kurokawa and J. P. Gong, Phy. Rev. Lett., 2018, 121, 185501.
57. H. N. Qiu, X. P. Hao, L. X. Hou, K. Cui, M. Du, Q. Zheng and Z. L. Wu, Macromolecules, 2023, 56, 8887–8898.
58. J. Y. Hu, D. Jiao, X. P. Hao, X. Kong, X. N. Zhang, M. Du, Q. Zheng and Z. L. Wu, Adv. Funct. Mater., 2023, 33, 2307402.
59. C. N. Zhu, S. Y. Zheng, H. N. Qiu, C. Du, M. Du, Z. L. Wu and Q. Zheng, Macromolecules, 2021, 54, 8052–8066.
60. C. Bilici, D. Karaarslan, S. Ide and O. Okaya, Polymer, 2018, 151, 208–217.
61. X. Chang, Y. Geng, H. Cao, J. Zhou, Y. Tian, G. Shan, Y. Bao, Z. L. Wu and P. J. Pan, Macromol. Rapid Commun., 2018, 39, 1700806.

---

Footnote

† They contributed equally to this work.

---