Preparation and Self-Healing Properties of Polyurethane with Dual Dynamic Covalent Bonds

Highlights

What are the main findings?

• Optimal R-value elevation enhances SSDA-PU’s mechanical properties.
• The disulfide bonds indirectly contribute to the reinforcement and toughening of the DA-PU system.

What are the implications of the main findings?

• The polyurethane with dual dynamic covalent bonds enables multiple self-healing of the elastomer.
• SSDA-PU’s self-healing stems from synergistic multi-bond effects.

Abstract

Dynamic covalent bonds are commonly used to maintain the self-healing properties of polyurethanes and facilitate resource recycling. However, relying on a single type of dynamic covalent bond often makes it difficult to effectively regulate both mechanical and self-healing properties across a wide temperature range. In this study, a self-synthesized chain extender containing disulfide bonds was introduced into a polyurethane system, leading to the development of a novel dual-dynamic covalent bond self-healing polyurethane (SSDA-PU). Innovatively, this SSDA-PU demonstrates self-healing properties across a wide temperature range. The successful synthesis of the chain extender and the incorporation of both disulfide bonds and Diels–Alder (DA) bonds were confirmed using FTIR and Raman spectroscopy. The physical characteristics and self-healing performance were comprehensively evaluated through multi-scale testing and characterization, including thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), hardness testing, mechanical tensile tests, and self-healing experiments. The underlying synergistic self-healing mechanism was subsequently elucidated. Findings showed that a higher R-value (isocyanate index) in SSDA-PU leads to over-crosslinking, while an R-value of 1.7 achieves the best overall mechanical performance, with tensile strength and elongation at break reaching 21.1 MPa and 755.17%, respectively. Additionally, SSDA-PU demonstrated the capacity for multiple healing cycles, with an initial self-healing efficiency of 90.38%, which remained notably high at 59.21% even after three damage-healing cycles. Importantly, SSDA-PU exhibited healing capabilities even at relatively low temperatures. Cracks in SSDA-PU can be effectively repaired through the synergistic action of disulfide bond exchange, hydrogen bond dissociation, and thermally reversible DA reactions. SSDA-PU also shows excellent recyclability, offering valuable insights for the practical engineering application of functional polyurethanes.

1. Introduction

Polyurethane (PU) are high-performance polymers synthesized via polycondensation reactions involving isocyanates, oligomeric polyols, and chain extenders. They exhibit excellent mechanical properties and tunable structures, along with outstanding characteristics such as abrasion resistance, flexibility, elasticity, and low-temperature tolerance. As a result, PU are widely utilized in sectors including construction engineering, electronic power systems, and advanced manufacturing [1,2,3,4,5]. Nevertheless, during operational use, PU are prone to photo- and thermal aging as well as mechanical damage, which cause irreversible deterioration in performance and substantially reduce their service life. The emergence of self-healing or shape-memory materials addresses these issues of rapid performance deterioration and short service life while reducing economic costs [6,7,8]. Currently, self-healing materials are primarily classified as extrinsic or intrinsic types [9,10,11,12]. Extrinsic self-healing systems face challenges including complex preparation, poor compatibility, limited repair cycles, and susceptibility to external conditions. Conversely, intrinsic self-healing materials facilitate multiple autonomous repairs solely through internal dynamic chemical bonds without requiring external additives, thus emerging as the predominant development direction in this field.

Dynamic chemical bonds are classified into dynamic covalent bonds [13,14,15] and dynamic non-covalent bonds [16,17,18]. Common dynamic covalent bonds include Diels–Alder (DA) bonds, disulfide bonds, diselenide bonds, and imine bonds, whereas dynamic non-covalent bonds comprise hydrogen bonding, host-guest interactions, and ionic interactions. Notably, disulfide bonds possess lower bond dissociation energies relative to other dynamic covalent bonds, enabling activation under milder thermal conditions, making them ideal for low-temperature self-healing materials [19,20,21,22]. Similarly, DA bonds—a thermally reversible dynamic covalent system—are widely adopted in thermally responsive self-healing materials due to their catalyst-free mechanism and minimal byproduct formation [23,24,25,26]. Dong et al. [27] synthesized an aromatic chain extender (ESS) via the reaction of ethylenediamine with salicylaldehyde, incorporating dynamic imine bonds. When used to extend polycaprolactone diol (PCL)/diphenylmethane diisocyanate (MDI) prepolymers, the resulting dynamic covalent polyurethane achieved 498–726% elongation at break and 87.9% self-healing efficiency after 12 h at 40 °C, albeit with compromised mechanical strength. Tian et al. [28] designed polyurethanes with varying -NCO/-OH ratios (R-values) using a diselenide-containing diol (DiSe) as a chain extender. Increasing R-values reduced elongation while achieving a tensile strength of 2.47 MPa; however, healing efficiency was strongly light-dependent. Song et al. [29] introduced disulfide bonds via 2,2′-dithiodibenzoic acid (DTSA) after extending prepolymers with 2,2-bis(hydroxymethyl)propionic acid (DMPA) and isophorone diamine (IPDA). This strategy enhanced hydrogen bonding density and disulfide exchange, yielding elastomers with 5.22 MPa tensile strength, 1820.26% elongation, and 100% single-cycle healing efficiency. Huang et al. [30] constructed triple dynamic networks in polyurethanes using dimethylglyoxime (DMG) and CuCl₂, incorporating oxime-carbamate bonds, metal-ligand coordination, and hydrogen bonds. This system achieved a healing efficiency of 89.49% after 25 h at 90 °C, underscoring the reinforcing effect of Cu²⁺ ions. Zhou et al. [31] incorporated DA bonds via furfurylamine (FM) and bismaleimide (BMI) into polyurethanes with aromatic (TDI) or alicyclic (IPDI) hard segments. The polyurethane containing alicyclic segments demonstrated superior healing performance, attaining 71% healing efficiency after 6 h at 80 °C). From the literature reviewed above, it is evident that the incorporation of a single disulfide bond into polyurethane compromises its mechanical properties, while a single Diels–Alder (DA) bond is constrained by temperature-responsive conditions, indirectly impairing the self-healing performance. Furthermore, introducing metal ionic bonds makes polyurethane susceptible to interference from humidity, pH variations, and chemical agents, causing dissociation in moist or acidic environments and consequent degradation of structural stability. Additionally, synthesizing chain extenders containing such dynamic bonds typically involves complex processes and high costs in current research. Therefore, developing polyurethanes that simultaneously exhibit high mechanical strength and excellent self-healing capability remains a pressing challenge in functional materials research.

Due to differences in chemical structure, polarity, and intermolecular forces between the soft segments (flexible chains) and hard segments (rigid blocks) in polyurethanes, these components exhibit thermodynamic incompatibility, spontaneously separating into discrete microphase-separated domains [32,33,34,35,36]. This inherent characteristic enables the tailoring of polyurethanes’ unique mechanical properties through optimized formulation design that controls the microphase-separated architecture. However, single-dynamic-bond self-healing polyurethanes increasingly fail to meet broader response-range requirements. When disulfide bonds or DA bonds are incorporated into hard segments via chain extenders, both insufficient and excessive bond concentrations reduce healing efficiency. Furthermore, balancing mechanical strength with self-healing capability remains challenging. Therefore, designing and fabricating multi-responsive self-healing materials is essential to accommodate diverse environmental and functional demands.

This study synthesized self-healing polyurethane containing both disulfide and Diels–Alder (DA) bonds by first preparing the disulfide-containing chain extender bis(2-hydroxyethyl) disulfide (B2HD) from low-cost β-mercaptoethanol, then using it to chain-extend the prepolymer of polytetramethylene ether glycol (PTMEG) and 4,4′-diphenylmethane diisocyanate (MDI). Subsequently, furfuryl alcohol (FA) is incorporated to introduce furan groups, followed by reaction with bismaleimide (BMI). Polyurethanes with varying hard-segment contents are designed by controlling different isocyanate indices (R), and their mechanical and self-healing properties are systematically investigated through macro- and micro-scale performance testing. Through the optimized design of multiple dynamic structures in polyurethane, the material achieves excellent mechanical properties while mitigating degradation during service, further enabling recyclability to reduce economic costs and resource waste. This approach offers valuable references for engineering applications of self-healing polyurethanes in diverse fields such as coatings and polymer modifiers.

2. Materials and Methods

2.1. Raw Material

Polytetramethylene ether glycol (PTMEG-2000, Mn = 2000 g/mol) and 4,4′-diphenylmethane diisocyanate (MDI), both of analytical grade, were supplied by Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China), and subjected to dehydration prior to use. Poly(butylene adipate) glycol (PBA-2000, Mn = 2000 g/mol), furfuryl alcohol (FA), and 4,4′-bismaleimidodiphenylmethane (BMI), analytical grade, were obtained from Guangdong Daxiao Chemical Co., Ltd. (Maoming, China) β-mercaptoethanol (ME, analytical grade), 1,4-butanediol (BDO, analytical grade), anhydrous magnesium sulfate (MgSO₄, analytical grade), and ethyl acetate (EA, analytical grade) were provided by Tianjin Damao Chemical Reagent Factory (Tianjin, China), Xuzhou Yihuiyang New Material Co., Ltd. (Xuzhou, China), and Tianjin Zhiyuan Chemical Reagent Co., Ltd. (Tianjin, China), respectively. Dibutyltin dilaurate (DBTDL, 95%) and N, N-dimethylformamide (DMF, analytical grade) were sourced from Shanghai Aladdin Biochemical Technology Co., Ltd. and Chengdu Kelong Chemical Co., Ltd. (Chengdu, China).

2.2. Preparation of Chain Extender Containing Disulfide Bonds (B2HD)

A quantity of 30 g of β-mercaptoethanol (ME) was added to a 250 mL three-necked round-bottom flask immersed in an oil bath maintained at 55 °C with stirring at 200 rpm. To prevent a violent reaction, 23.3 g (21 mL) of 30% hydrogen peroxide solution was added dropwise at a rate of one drop every six seconds. The reaction mixture was stirred under reflux for 4 h. Subsequently, 70 mL of ethyl acetate solvent was added, and stirring continued for an additional hour. The entire process was conducted under a nitrogen atmosphere. The resulting mixture was transferred to a 250 mL separatory funnel and allowed to stand for phase separation. The upper layer was collected in a beaker, mixed with anhydrous magnesium sulfate, and sealed for drying over 24 h. The dried liquid was then vacuum-filtered to remove impurities. The filtered solution was subjected to rotary evaporation to eliminate the ethyl acetate solvent, resulting in the isolation of bis(2-hydroxyethyl) disulfide (B2HD) as a pale yellow, transparent liquid. The synthetic route is depicted in Figure 1, and the reaction afforded a product yield of 91%.

2.3. Preparation of Polyurethane with Dynamic Covalent Bonds

Under a nitrogen atmosphere, 40 g of dehydrated polytetramethylene ether glycol (PTMEG) and 11.01 g of 4,4′-diphenylmethane diisocyanate (MDI) were sequentially added to a 500 mL three-necked flask. The mixture was mechanically stirred at 250 rpm. N,N-dimethylformamide (DMF) was added to adjust the viscosity, followed by the incorporation addition of 0.1 g of dibutyltin dilaurate (DBTDL) as a catalyst. The reaction was maintained at 70 °C with constant stirring for 2 h. Subsequently, the temperature was reduced to 65 °C, and 3.085 g of the disulfide-containing chain extender B2HD was added. Stirring continued for an additional 2 h to yield an NCO-terminated polyurethane prepolymer.

Following the previous reaction, the system temperature was adjusted to 75 °C while maintaining stirring at 250 rpm. Furfuryl alcohol (FA, 0.78 g) was added to the reaction mixture and allowed to react for 2 h. Subsequently, the temperature was lowered to 70 °C, and 1.43 g of bismaleimide (BMI) was introduced for further reaction over 12 h. The stirring speed was then reduced to 100 rpm for 2 h. The resulting polyurethane solution was poured into a polytetrafluoroethylene (PTFE) mold and cured in a forced-air oven at 85 °C for 48 h, yielding SSDA-PU incorporating both disulfide bonds and Diels–Alder (DA) bonds. For the DA bond-only control polyurethane (DA-PU), B2HD was replaced with 1,4-butanediol (BDO) at an R-value of 1.7. All other preparation parameters remained identical to those of SSDA-PU. The synthetic route is depicted in Figure 2, while

Table 1. Formulations of polyurethanes with dynamic covalent bonds at varying R-values.

R Value	PTMEG (g)	MDI (g)	B2HD/BDO (g)	FA (g)	BMI (g)	Hard Segment Content (%)
1.1	40	11.01	3.085	0.78	1.43	27.3
1.3	40	13.01	3.085	2.35	4.3	34.9
1.5	40	15.02	3.085	3.92	7.17	41.1
1.7	40	17.02	3.085	5.49	10.03	46.2
1.9	40	19.03	3.085	7.06	12.9	50.5
1.7	40	17.02	1.8	5.49	10.03	46.2

details the formulations of SSDA-PU and DA-PU with varying R-values. In the subsequent text, SSDA-PU with different R-value is denoted as R followed by the specific isocyanate index value. For example, R1.1 denotes SSDA-PU with an isocyanate index of 1.1. DA-PU denotes the dynamic covalent polyurethane containing only DA bonds.

2.4. Experimental Methods

2.4.1. Fourier Transform Infrared Spectroscopy Test

The synthesized chain extenders and polyurethanes were characterized using Fourier transform infrared (FTIR) spectroscopy with a Thermo Scientific Nicolet iS5 spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA) equipped with an attenuated total reflection (ATR) accessory. Spectral data were collected over 32 scans at a resolution of 4 cm⁻¹, covering a wavenumber range from 500 to 4000 cm⁻¹.

2.4.2. Raman Spectroscopy Test

To further investigate disulfide bond formation in the polyurethane, the self-healing polyurethane structure was characterized using a Renishaw inVia Raman microscope (Renishaw Inc., Gloucestershire, UK). Spectra were collected employing a 785 nm excitation wavelength, spanning the wavenumber range of 200 to 2000 cm⁻¹.

2.4.3. XRD Crystallinity Characterization

Crystallinity analysis of various dynamic self-healing polyurethanes was performed using a SmartLab X-ray diffractometer (Rigaku Corporation, Tokyo, Japan). Measurements were conducted at 40 kV and 50 mA, with a scanning speed of 5° per minute over an angular range of 10° to 60°.

2.4.4. Thermogravimetric Analysis

The thermal stability of various polyurethanes was evaluated using a Mettler Toledo TGA 2 thermogravimetric analyzer (Mettler Toledo, Zurich, Switzerland). Specimens placed in crucibles were heated from room temperature to 700 °C at a rate of 10 °C per minute under a nitrogen atmosphere, with mass loss continuously recorded.

2.4.5. Dynamic Mechanical Analysis

The thermomechanical properties of dynamic self-healing polyurethanes were evaluated using a Mettler Toledo DMA 1 analyzer (Mettler Toledo, Zurich, Switzerland). Rectangular specimens (30 × 5 × 2 mm³) were prepared from the synthesized polyurethane elastomers and tested in tension mode at a frequency of 1 Hz with an amplitude ranging from 2 to 13 μm. Temperature sweeps were conducted from –80 °C to 160 °C at a heating rate of 5 °C per minute to determine the evolution of storage modulus, loss modulus, and tan δ. For each corresponding R value, three parallel polyurethane samples were tested.

2.4.6. Shore Hardness Test

The hardness of the synthesized self-healing polyurethanes was measured using an LX-A-2 Shore durometer (Shanghai Shuangxu Electronics Co.,Ltd., Shanghai, China) in accordance with ASTM D2240 [37]. Specimens (30 × 30 × 6 mm³) were fabricated to meet standard requirements, with three samples tested per group to analyze the variation trends in hardness.

2.4.7. Tensile Property Test

Tensile properties of polyurethanes were evaluated according to GB/T 528-2009 [38] using a CMT2503 electronic universal testing machine (Jinan Hengruijin Testing Machine Co., Ltd., Jinan, China). Dumbbell-shaped specimens were tested to determine the tensile strength and elongation at break.

2.4.8. Self-Healing Performance Evaluation and Characterization of Polyurethanes

(1)

Scratch recovery performance test

To compare the scratch recovery of SSDA-PU and DA-PU, 0.4 mm-deep scratches were made on the surfaces of polyurethane specimens (with varying isocyanate indices) using a scalpel. The specimens were then sequentially heat-treated in a forced-air oven at 110 °C for 10, 20, and 30 min, followed by 5 h at 70 °C. After turning off the oven and allowing the specimens to cool to ambient temperature, the scratch morphology was observed using an optical microscope.

(2)

Self-healing performance characterization

Dumbbell-shaped polyurethane specimens were incised perpendicular to the tensile direction at the midpoint using a scalpel, with the incision depth of reaching 90% of the specimen thickness. The severed ends were aligned, secured with clamps, and subjected to sequential thermal healing: first at 120 °C for 30 min, followed by a reduction to 70 °C for 24 h. After cooling to ambient temperature, tensile strength was measured. This damage-healing cycle was repeated for three successive iterations, with self-healing efficiency (η) quantified by tensile strength recovery according to Equation (1).

$$\eta ={\displaystyle \frac{{\sigma }_{Healed}}{{\sigma }_{Original}}}\times 100\%$$

(1)

where ${\sigma }_{Healed}$ denotes the tensile strength (MPa) after self-healing, ${\sigma }_{Original}$ represents the tensile strength (MPa) of the undamaged specimen, and $\eta$ indicates the self-healing efficiency (%).

(3)

Thermal reversibility test

Polyurethane specimens cut into strips were immersed in sufficient DMF within test bottles. After vigorous shaking, the bottles were placed in an oven at 110 °C for 20 min. The temperature was then reduced to 70 °C for 3 h, followed by cooling to ambient temperature. The phase evolution of the dynamic covalent bond-containing polyurethane was visually observed throughout the process.

2.4.9. In Situ Temperature-Dependent FTIR Analysis

In situ heating FTIR spectra of polyurethanes were obtained using a Fourier transform infrared spectrometer equipped with an ATR accessory. Specimens were heated at a rate of 2 °C per minute to target temperatures of 25, 70, 90, 100, 110, 120, 130, 140, 150, and 160 °C, held isothermally for 5 min, and immediately scanned. Spectra data were collected by averaging 32 scans at a spectral resolution of 4 cm⁻¹, covering the wavenumber range from 600 to 4000 cm⁻¹.

3. Results

3.1. FTIR Analysis

The synthesized chain extender and various polyurethanes were characterized using Fourier transform infrared spectroscopy, with the results presented in Figure 3 and Figure 4.

As shown in Figure 3, the absorption bands observed at 3361 cm⁻¹ and 3384 cm⁻¹ are attributed to the O-H stretching vibrations of hydroxyl groups. The peaks detected at 2870 and 2933 cm⁻¹, as well as at 2872 and 2929 cm⁻¹, correspond to C-H stretching vibrations. Additionally, the absorption features at 1054 and 1055 cm⁻¹ are indicative of C-O stretching vibrations. The peak at 2555 cm⁻¹ originates from S-H stretching vibrations in ME [8,20,21]. Crucially, this characteristic S-H peak disappears completely in the FTIR spectrum of B2HD product. Concurrently, a broad new absorption emerges at 641 cm⁻¹, confirming complete oxidation of thiol groups into disulfide bonds and successful synthesis of B2HD [19,29]. Moreover, the successful incorporation of disulfide bonds is further substantiated by the ¹H NMR spectrum of B2HD, as shown in Figure S1 of the Supplementary Materials.

The FTIR spectrum of bismaleimide (BMI) reveals characteristic peaks at 1710 cm⁻¹ (C=O stretching vibration), 1395 cm⁻¹ (imide ring C-N-C stretching vibration), and 688 cm⁻¹ (BMI skeletal vibration absorption). The complete disappearance of the 688 cm⁻¹ peak in both DA-PU and SSDA-PU indicates the conversion of maleimide double bonds into trans-configured adducts via Diels-Alder (DA) reactions. Furfuryl alcohol (FA) shows peaks at 3375 cm⁻¹ (O-H stretching vibration), 2932 cm⁻¹ (symmetric C-H stretching of the furan ring), 1009 cm⁻¹ (symmetric C-O-C stretching vibration), and 745 cm⁻¹ (C-H bending vibration of the furan ring). For 1,4-butanediol (BDO), absorptions appear at 3417 cm⁻¹ (O-H stretching), 2944 cm⁻¹ (C-H stretching), 1445 and 1380 cm⁻¹ (methylene bending vibrations), and 1053 cm⁻¹ (C-O stretching vibration). MDI displays a distinct peak at 2275 cm⁻¹ corresponding to the asymmetric stretching of isocyanate groups (-NCO), which disappears completely in both synthesized polyurethanes, confirming the full reaction between -NCO and -OH groups. Key polyurethane signatures include peaks at 3302 cm⁻¹ and 1537 cm⁻¹ (N-H bending vibration of urethane bonds, -NH-COO-), 2941 and 2853 cm⁻¹ (C-H stretching vibrations), and 1102 cm⁻¹ (C-O-C stretching vibration in polyurethane) [30,39]. Additionally, peaks at 1775 cm⁻¹ (characteristic DA bond) and 641 cm⁻¹ (disulfide bond) are specifically observed in SSDA-PU [23,24], collectively verifying the successful synthesis of both SSDA-PU and DA-PU.

3.2. Raman Spectroscopy Analysis

To further analyze the synthesis status of disulfide bonds in the polyurethane, Raman spectroscopy was employed for characterization, with results presented in Figure 5.

The Raman spectrum of B2HD exhibits prominent characteristic peaks at 511 cm⁻¹ and 647 cm⁻¹, corresponding to disulfide bond (S-S) and C-S stretching vibrations [30], respectively. In contrast, DA-PU shows no detectable peaks in these regions. SSDA-PU displays distinct S-S and C-S peaks at 513 cm⁻¹ and 639 cm⁻¹, respectively, alongside a C=C absorption peak at 1615 cm⁻¹. These spectral features confirm the successful synthesis of both SSDA-PU (containing disulfide and DA bonds) and DA-PU (containing DA bonds only).

3.3. X-Ray Diffraction Characterization and Analysis of Polyurethanes

Polyurethane molecular chains initially adopt random coil conformations; however, under strong hydrogen bonding interactions within the hard segments, the chains reorganize into ordered arrangements that facilitate hard segment crystallization. Due to the inherently short length of hard segments in polyurethane chains, microcrystalline structures typically form and disperse within the soft segment domains, resulting in low overall crystallinity. X-ray diffraction analysis effectively elucidates the microphase-separated structure—comprising both crystalline and amorphous regions—in dynamic covalent bond-containing polyurethanes by detecting distinct diffraction peaks from the crystalline domains.

To characterize the hard segment microcrystalline domains in polyurethanes, Gaussian deconvolution was applied to the XRD curves. Consistent with the literature [40,41,42], amorphous regions manifested near 20° while hard segment crystallites appeared around 27°. Figure 6a,b shows that, with increasing R-values, both SSDA-PU and DA-PU exhibit a prominent amorphous halo at 20° and a crystalline diffuse peak at 28°, forming a continuous broad scattering pattern. This confirms the presence of microcrystalline structures within an amorphous matrix, in agreement with the referenced studies.

Table 2. Peak deconvolution analysis of X-ray diffraction patterns for polyurethanes with varying hard segment content.

Test Specimen	Hard Segment Content (%)	Peak Position (°)		Peak Area Proportion		Crystallinity (%)
		Amorphous Region	Crystalline Region	Amorphous Region	Crystalline Region
R1.1	27.3	20.3	28.4	96.57	3.43	3.4
R1.3	34.9	20.4	28.3	95.66	4.34	4.3
R1.5	41.1	20.3	28.3	94.9	5.1	5.1
R1.7	46.2	20.1	28.2	92.57	7.43	7.4
R1.9	50.5	20.3	28.1	85.55	14.45	14.5
DA-PU	46.2	20.2	28.3	88.82	11.18	11.2

demonstrates the crystallinity increases from 4.3% to 14.5% as the hard segment content rises from 27.3% to 50.5%, with the amorphous halo shifting toward lower angles due to enhanced hydrogen bonding between N-H and C=O groups, expansion of hard segment microdomain sizes, and strengthened intermolecular interactions that promote ordered packing. Notably, at identical hard segment content (R = 1.7), SSDA-PU exhibits 7.4% crystallinity compared to DA-PU’s 11.2%. This difference arises because DA crosslinks in DA-PU restrict chain mobility, facilitating ordered stacking, whereas disulfide bonds in SSDA-PU—with their low bond energy, longer bond length, and conformational isomerism—disrupt hydrogen-bonded alignment and reduce the effective crystalline hard segments, thereby diminishing crystallinity.

3.4. Thermal Stability Analysis of Polyurethanes

Thermogravimetric (TG) and Derivative Thermogravimetric (DTG) curves obtained from thermal analyzer testing objectively reflect mass changes during thermal decomposition of different polyurethanes, thereby revealing the thermal stability characteristics of the polyurethane systems.

As shown in Figure 7a, increasing R-values correspond to a higher content of hard segment content in the polyurethanes. The initial decomposition temperature—defined at 5% mass loss—indicates that R1.9 begins decomposing first at 206 °C, demonstrating the poorest thermal stability among the six polyurethanes. Prior to 5% mass loss, R1.1 demonstrates the optimal thermal stability. At the same Diels-Alder bond content (R = 1.7), SSDA-PU decomposes at lower temperatures than DA-PU, confirming that the incorporation of disulfide bonds reduces thermal stability. This reduction occurs because BDO, a rigid small-molecule chain extender, promotes ordered molecular chain arrangements and stronger intermolecular forces, thereby enhancing overall rigidity. In contrast, disulfide bonds, which have lower bond energy, cleave more readily at lower temperatures; thus, during heating, these bonds act as weak points in SSDA-PU that initiate molecular chain degradation. As the temperature further increases, decomposition rates accelerate significantly between 200 °C and 500 °C, exhibiting three distinct decomposition stages. The highest decomposition rate occurs between 440 °C and 500 °C, with R1.1 decomposing earliest in this range.

From Figure 7b, it can be observed that there is a brief decomposition process of polyurethane near 220 °C, primarily attributed to the breakdown of dynamic covalent bonds within the polyurethane. This minor DTG peak corresponds to the retro Diels-Alder decomposition of DA bonds, during which dissociated furan and bismaleimide components volatilize as small molecules at elevated temperatures, causing slight mass loss (manifested as the minor DTG peak). At R = 1.1, the low DA bond content results in no discernible peak variation near 220 °C in the DTG curve. As DA bond content increases, the intensity of this minor DTG peak gradually rises. At identical R-values, the decomposition temperature of SSDA-PU is lower than that of DA-PU, consistent with the trend observed in Figure 7a. This phenomenon primarily occurs because the introduction of disulfide bonds reduces the ordering and packing density of hard segments, weakens hydrogen bonding within hard segment microdomains, and enhances molecular chain mobility. Consequently, the molecular segments surrounding DA bonds undergo conformational changes more readily during heating, providing more favorable kinetic conditions for DA bond dissociation. This leads to faster decomposition rates in SSDA-PU under identical temperature conditions. As the temperature further increases, the decomposition process of self-healing polyurethanes manifests as three dominant DTG peaks, corresponding to three distinct decomposition stages: Stage 1 primarily involves cleavage of urethane bonds (-NHCOO-), disulfide bonds (S-S), and carbon-sulfur bonds (-C-S-); Stage 2 represents chain scission and degradation of polyether soft segment structures, which exhibit greater thermal stability than hard segments; Stage 3 encompasses further decomposition and carbonization of residues, including fragmented byproducts from prior decomposition, micro-crosslinked structures, and advanced thermal degradation/carbonization of aromatic rings [43], ultimately forming residual char. As shown in

Table 3. Thermal decomposition parameters of different polyurethanes.

Test Specimen	T5% (°C)	TMax1 (°C)	TMax2 (°C)	TMax3 (°C)	Residual Mass Fraction (%)
R1.1	334.8	364.8	401.7	458.0	8.8
R1.3	265.8	368.2	410.3	463.0	10.2
R1.5	218.5	365.7	397.2	464.0	11.5
R1.7	216.0	366.5	402.8	465.5	12.0
R1.9	206.0	365.8	393.3	466.2	12.8
DA-PU	251.6	372.8	404.3	467.8	13.0

, the char residue of dynamic covalent polyurethanes increases with rising hard segment content. Furthermore, DTG peak temperatures progressively shift toward higher values with increasing R-values. In summary, these findings demonstrate that while the introduction of disulfide bonds partially reduces the thermal stability of self-healing polyurethanes, it does not alter the fundamental decomposition pathway. Self-healing polyurethanes initiate decomposition above 206 °C, confirming their overall robust thermal stability.

3.5. Dynamic Mechanical Analysis of Different Polyurethanes

Dynamic Mechanical Analysis (DMA) testing provides the storage modulus (E′), loss modulus (E″), and loss factor (tan δ) of dynamic covalent bond-containing polyurethanes at varying R-values, thereby evaluating extent of microphase separation and the dynamic mechanical properties within the polyurethane system.

As shown in Figure 8a, increasing the R-value from 1.1 to 1.9 leads to a progressive rise in the hard segment content within the polyurethane system. The sample with R = 1.9 exhibits the highest storage modulus at the low temperature of −100 °C, reaching 2453 MPa. However, the storage modulus of all polyurethanes gradually decreases as the temperature elevates to 160 °C. Under identical R-value conditions, SSDA-PU (R = 1.7) demonstrates a slightly higher storage modulus than DA-PU in the low-temperature region. This difference is primarily attributed to the pinning effect of disulfide bonds. Although disulfide bonds have lower bond energy than C-C bonds in the glassy state, they still act as effective crosslinking points, resulting in a higher total crosslink density, denser networks, and enhanced rigidity in the dual dynamic covalent bond system compared to the single dynamic bond system. Additionally, the slightly higher polarity of disulfide bonds relative to the alkyl chains in BDO strengthens hydrogen bonding interactions between hard segments, further forming additional physical crosslinks that synergistically enhance the polyurethane’ s modulus.

As shown in Figure 8b, the loss modulus of polyurethanes decreases with increasing R-values, primarily due to the rise in crosslink density, which further restricts molecular chain movement and makes relative slippage between chain segments more difficult. This reduction in chain mobility lowers frictional energy dissipation. Additionally, the denser packing structure formed at higher R-values reduces the molecular free volume, further limiting chain mobility and consequently decreasing viscous energy loss.

As shown in Figure 8c, as the temperature increases from −100 °C to 160 °C, the loss factor curves of all polyurethanes exhibit two distinct peaks in the low- and high-temperature regions, indicating significant microphase separation within the polyurethane systems. These peak positions correspond to the glass transition temperatures associated with molecular chain motions. When the R-value increases from 1.1 to 1.9, the low-temperature peak shifts from −54.4 °C to −31.9 °C, accompanied by a gradual decrease in peak height. This low-temperature peak represents the glass transition temperature (Tg) of the soft segments. An increase in hard segment content leads to partial dissolution or interpenetration of the hard segments into soft phase domains, restricting soft segment mobility and requiring higher temperatures to initiate motion, thereby raising Tg. The reduction in peak height indicates that fewer soft segments participate in the glass transition due to the increased crosslink density. Meanwhile, the high-temperature peak height progressively intensifies, and its temperature rises from 78.6 °C to 101.9 °C. This behavior is attributed to the retro-DA reaction of DA bonds occurring around 80–120 °C, which consumes substantial energy; this peak corresponds to the dissociation of these dynamic crosslinks and the Tg of hard segment microdomains. At the same R-value, DA-PU exhibits a higher peak temperature (108.2 °C) than R1.7 SSDA-PU because the absence of dynamic disulfide bonds in DA-PU the stability of the crosslinking network and increases hard segment rigidity, requiring higher temperatures for large-scale segmental motion and DA bond cleavage. At elevated temperatures, dynamic disulfide exchange in R1.7 SSDA-PU facilitates chain mobility, enabling DA bond dissociation at lower temperatures.

3.6. Shore Hardness Analysis

Shore hardness testing was conducted on polyurethanes under varying R-values, with results presented in Figure 9.

As shown in Figure 9, increasing the R-values leads to a rise in the hard segment content of various self-healing polyurethanes from 27.3% to 50.3%. Concurrently, Shore hardness progressively increases from 52 to 84, representing a 61.5% improvement. This enhancement primarily results from the elevated hard segment content, which intensifies hydrogen bonding interactions within the inherently rigid hard segments, thereby increasing physical crosslinking points that aggregate to form larger rigid microdomains, and simultaneously. Simultaneously, a higher DA bonds establishes a dual network combining physical and dynamic covalent crosslinks, further increasing the crosslink density, which macroscopically manifests as improved hardness. Additionally, at identical hard segment content, SSDA-PU exhibits lower hardness than DA-PU. This difference is mainly because BDO—a short-chain extender with a symmetric, ordered structure—forms well-aligned hydrogen-bonded arrays with MDI, creating stronger rigid microdomains. In contrast, B2HD’s longer molecular chains, combined with low rotational hindrance and the reduced bond energy of -S-S- bonds [39], decrease hard segment ordering, making the hard domains more susceptible to perturbation.

3.7. Tensile Performance Evaluation

The tensile fracture strength and elongation at break of the synthesized self-healing polyurethanes were measured using an electronic universal testing machine. The results are presented in Figure 10.

As shown in Figure 10, increasing the R-value from 1.1 to 1.9 results in the tensile strength of polyurethanes rising from 7.82 MPa to 18.91 MPa. However, the maximum tensile strength of 21.1 MPa is observed at R = 1.7. This is because high-R polyurethane systems contain more polar groups, such as benzene rings and urethane bonds. These groups not only form hard segment microdomains that act as physical crosslinks but also create additional DA chemical crosslinks through FA and BMI. The increased crosslink density raises the number of effective chain segments bearing external forces per unit area during stretching, thereby requiring greater tensile stress to cause fracture. At R = 1.9, the tensile strength decreases compared to R = 1.7, indicating over-crosslinking that leads to network inhomogeneity and localized stress concentrations, which cause premature failure before reaching the theoretical maximum strength. The elongation at break decreases from 1085.4% to 540.65% because rising R-values increase physical and chemical crosslinks, restricting chain slippage and extension and limiting segmental deformation. Additionally, at identical R-values, SSDA-PU exhibits a tensile strength 0.51 MPa higher than DA-PU with an elongation at break of 755.17% (65.84% higher). This improvement is attributed to the polar disulfide bonds introduced via B2HD, which enhance dipole interactions and hydrogen-bonding capability within the hard segments, thereby strengthening physical crosslinking in hard domains compared to nonpolar BDO-chain-extended systems. Compared to studies featuring only one type of dynamic covalent bond, the tensile strength of SSDA-PU is significantly higher than that of polyurethanes containing only disulfide bonds, as reported in references [19,30], which exhibit strengths below 6 MPa. However, it is lower than that of the carbon fiber-reinforced polyurethane with DA bonds described in reference [23], which reaches 30 MPa. Notably, the elongation at break for the polyurethane in reference [23] is only 76.7%, considerably less than that of SSDA-PU. This suggests that relying on a single dynamic covalent bond limits the ability to dynamically regulate the mechanical properties of polyurethane.

3.8. Self-Healing Property Testing and Evaluation of Dynamic Covalent Polyurethanes

3.8.1. Characterization of Scratch Self-Healing in Polyurethanes

SSDA-PU and DA-PU samples with varying R-values were subjected to heat treatment at 110 °C for different durations, followed by an additional heat treatment at 70 °C for 5 h. Scratch recovery was then observed.

As observed from Figure 11, polyurethanes exhibit initial self-healing after 10 min of heat treatment, further repair at 20 min, and nearly complete scratch recovery after 30 min. Scratch recovery initially increases with rising R-values, but then decreases; notably, SSDA-PU at R = 1.7 demonstrates near-total restoration. This decline at higher R-values is attributed to excessive R = 1.1 to 50.5% at R = 1.9 in SSDA-PU. The elevated hard segment content leads to greater molecular chain entanglement density and the formation of dense physical crosslinks. Although disulfide and DA bonds cleave at elevated temperatures, the molecular chains remain confined within the rigid network, reducing the mobility of small segments and hindering the flow necessary to repair scratched regions. Furthermore, at identical R-values, SSDA-PU exhibits superior scratch recovery compared to DA-PU. This is because disulfide bonds have lower bond energy, enabling earlier cleavage at 110 °C, which significantly reduces the initial resistance to chain movement. Prolonged heating further dissociates hydrogen and DA bonds, and the incorporation of disulfide bonds facilitates smoother chain mobility and accelerates the dynamic recombination of DA bonds.

3.8.2. Self-Healing Properties of Different Polyurethanes

To analyze the self-healing properties of different polyurethanes, specimens were 90% severed and placed in a 120 °C constant-temperature oven for 30 min, followed by maintenance at 70 °C for 24 h; this three-step cycle was repeated three times to observe the self-healing efficiency of SSDA-PU and DA-PU, with results presented in Figure 12 and

Table 4. Self-healing efficiency of polyurethanes with different dynamic covalent bonds.

Polyurethane	Original (MPa)	${\mathsf{\eta }}_{{\mathbf{1}}^{\mathbf{s}\mathbf{t}}}$ (%)	${\mathsf{\eta }}_{{\mathbf{2}}^{\mathbf{n}\mathbf{d}}}$ (%)	${\mathsf{\eta }}_{3^{\mathbf{r}\mathbf{d}}}$
R1.7	21.11	90.38	78.21	59.21
DA-PU	20.6	83.93	68.78	48.10

.

As observed from Figure 12a,b, both the tensile strength and elongation at break of R1.7 SSDA-PU and DA-PU decrease with increasing stretching cycles. According to the specific parameters in Table 4, R1.7 achieves a healing efficiency of 90.38% after the first repair and retains 59.21% after the third repair, whereas DA-PU exhibits 83.93% after the first repair, declining to 48.1% after the third repair. These results clearly demonstrate the superior healing capability of SSDA-PU compared to DA-PU. This enhancement arises because at 120 °C, not only do DA bonds dissociate, but disulfide bonds also undergo dynamic exchange reactions. The disulfide exchange accelerates the movement of broken short molecular chains, facilitating their diffusion and migration toward fracture surfaces, where molecular chains on both sides rearrange and interpenetrate. Additionally, disulfide exchange dissipates local stress at interfaces, promoting intimate molecular chain contact and resulting in stronger interfacial bonding after healing. However, with increasing repair cycles, repeated high-temperature exposure causes partial oxidation or irreversible side of furan or maleimide groups, reducing the number of effective functional groups available for rebonding and progressively diminishing healing efficiency. A comparative analysis of the literature [16,23,24] indicates that, under equivalent thermal response conditions, the polyurethane reported in reference [16] demonstrates a self-healing rate of 87%, which is lower than that observed for SSDA-PU following a 24 h treatment at 95 °C. Furthermore, the self-healing temperatures of the polyurethanes described in references [23,24] reach up to 160 °C, substantially exceeding that of SSDA-PU. The self-healing efficiency of 89.49% reported for the polyurethane containing three dynamic bonds in reference [30] is comparable to that of SSDA-PU. These findings suggest that modulating both the content and types of multiple dynamic covalent bonds can enhance the self-healing performance of polyurethanes.

To investigate the self-healing capability of SSDA-PU at lower temperatures, the R1.7 specimen after the third tensile test, was clamped at the incision and held statically at 60 °C for 24 h. Its self-healing properties were then observed.

As shown in Figure 13, the polyurethanes can lift a 500 g weight after 24 h of healing, demonstrating that SSDA-PU retains its self-healing capability at lower temperatures even after three repair cycles. This confirms the persistence of disulfide bonds within the material. This behavior occurs primarily because, when fracture surfaces maintain intimate contact, disulfide bonds at the interface undergo chain exchange reactions—a process that requires no high temperatures and proceeds slowly under mild conditions. Additionally, the PTMEG soft segments in polyurethanes have a Tg well below 60 °C, enabling chain diffusion and mutual entanglement across the incision interface. Although physical entanglements are weaker than covalent bonds, they facilitate interfacial fusion synergistically with disulfide bond reorganization.

3.8.3. Thermal Reversibility Analysis of Different Polyurethanes

To analyze the recyclability of polyurethanes, the sol–gel method was employed to characterize phase transition processes across different polyurethanes under varying temperatures and durations.

As shown in Figure 14, both SSDA-PU and DA-PU dissolve and exhibit favorable fluidity after heating at 110 °C for 20 min following solvent addition. This behavior occurs primarily because most DA bonds undergo retro-DA reactions at this elevated temperature, rapidly decomposing into furan and maleimide components. These linear molecular chains are then solubilized and dispersed by solvent molecules. When subsequently maintained at 60 °C for 3 h, the polyurethanes exhibit increased viscosity and loss of fluidity. This change is attributable to the re-initiation of DA addition reactions between furan and maleimide groups as the temperature decreases. The originally dispersed linear chains progressively reconstruct a three-dimensional network via DA bonds, causing a sharp rise in solution viscosity. Simultaneously, hydrogen bonding interactions between hard segments gradually recover upon cooling, forming a physical crosslinking network that ultimately results in gelation. These observations demonstrate that both dynamic covalent polyurethanes possess excellent thermal reversibility.

3.9. In Situ FTIR Spectroscopy Analysis of Dynamic Covalent Polyurethanes

In situ FTIR spectroscopy enables the characterization of structural changes in polyurethane materials at varying. It analyzes the thermal response of dynamic covalent bonds over a temperature range from 25 °C to 160 °C with specific intervals at 70 °C, 80 °C, 90 °C, 100 °C, 110 °C, 120 °C, 130 °C, 140 °C, and 150 °C.

As observed from Figure 15a, as the temperature increases from 25 °C to 160 °C, the stretching vibration peak of N-H in NHCOO bonds at 3390 cm⁻¹ gradually broadens and shifts to lower wavenumbers, with a reduction in peak intensity due to the disruption of N-H hydrogen bonding within the polyurethane system. Concurrently, the absorption peak at 3438 cm⁻¹ intensifies, indicating an increase in free N-H groups. The C-H stretching vibration peaks of methyl and methylene groups at 2935 cm⁻¹ and 2853 cm⁻¹ persist without disappearance upon heating. The absence of characteristic peaks at 2243 cm⁻¹ confirms that no isocyanate group (-NCO) are formed, demonstrating the material’s integrity without degradation. The peak at 1775 cm⁻¹ corresponds to C=C bonds from DA adducts in the synthesized polyurethane. Figure 15b shows that at approximately 80 °C, the 1775 cm⁻¹ peak begins to weaken while the maleimide characteristic peak emerges at 690 cm⁻¹. Upon further heating to 90 °C, the 1775 cm⁻¹ peak initially shifts to lower wavenumbers, then shifts upward, gradually attenuating with increasing temperature. This behavior occurs because elevated temperature weakens intermolecular hydrogen bonding, causing carbonyl stretching vibrations to shift toward lower frequencies. Additionally, intensified molecular thermal motion increases average bond lengths and reduces force constants, collectively driving the characteristic peak toward lower wavenumbers. At 110 °C, the retro-DA reaction accelerates significantly, reaching a maximum at 150 °C where maximal DA bond cleavage minimizes the 1775 cm⁻¹ peak intensity. When the temperature reaches 160 °C, although retro-DA continues, side reactions (e.g., maleimide self-polymerization or thermal degradation) partially consume the cleaved components, shifting the reversible equilibrium toward DA bond reformation and consequently restoring characteristic peak intensity. The peak at 1706 cm⁻¹ represents hydrogen-bonded carbonyl groups in polyurethane, whose intensity moderately decreases with rising temperature.

Due to the characteristic peak of disulfide bonds residing in the fingerprint region of infrared spectra, detecting their temperature-dependent changes can be challenging; however, comparative analysis can be performed using DA-PU as a reference. As observed from Figure 15a, most characteristic peaks in DA-PU exhibit trends similar to those in SSDA-PU—the intensity of the carbonyl peak at 1710 cm⁻¹ gradually decreases with increasing temperature. This is primarily because the introduction of disulfide bonds accelerates hydrogen bond dissociation and lowers its disassociation temperature in polyurethanes. Disulfide bonds alter the dynamic environment of molecular chains and the packing state of hard segments, indirectly promoting hydrogen bond cleavage. In contrast, Figure 15b reveals that the peak intensity at 1775 cm⁻¹ in DA-PU is significantly weaker than in SSDA-PU. This difference arises mainly because B2HD, used as a chain extender, has a more distorted and flexible structure compared to linear BDO, which modifies the microphase separation architecture and segmental mobility of the polyurethane. This structural variation brings furan and maleimide groups into closer spatial proximity, providing more thermodynamically favorable conditions for DA bond formation.

3.10. Analysis of Self-Healing Mechanism

The self-healing process of SSDA-PU is not driven solely by a single dynamic covalent bond; rather, dynamic disulfide bonds, DA bonds, and hydrogen bonds each respond differently to external stimuli, contributing through distinct healing mechanisms.

As observed from Figure 16, when microcracks or damage occur in polyurethane under load, molecular chains fracture, and the molecular structures and hydrogen bonds are disrupted. Thermal treatment of the damaged specimen induces the dissociation of hydrogen bonds and exchange of disulfide bonds within the polyurethane. Further heating to 110 °C activates retro-DA reactions in the DA bonds, cleaving long chains into shorter segments or small molecules. Driven by thermal motion, these fragments diffuse toward crack interfaces, initiating preliminary healing at the fracture surfaces. Subsequent treatment at 60 °C facilitates the diffusion of DA bonds, disulfide bonds, and hydrogen bonds along the fracture interface through increased chain mobility. When the diffusing species reach a critical proximity, DA bonds, disulfide linkages, and hydrogen bonds successfully reconfigure, forming new molecular architectures. The reconstituted polyurethane chains develop elongated, denser, and more stable configurations, progressively repairing microcracks and ultimately restoring mechanical properties. Consequently, polyurethane self-healing results from the synergistic interaction of DA bonds, disulfide bonds, and hydrogen bonds.

4. Conclusions

(1)

XRD analysis reveals microcrystalline structures in dynamic covalent polyurethanes; however, disulfide bonds reduce the quantity of effective hard segments available for crystallization, thereby diminishing crystallinity.

(2)

With increasing R-values, the mechanical properties of polyurethanes progressively enhance, but excessive crosslinking occurs when R exceeds 1.7. At identical R-values, SSDA-PU exhibits superior mechanical performance compared to DA-PU, indicating that disulfide bonds indirectly provide auxiliary strengthening and toughening effects.

(3)

Polyurethanes with dual dynamic covalent bonds achieve multiple self-healing cycles across a broad temperature range while retaining self-repair capability at lower temperatures.

(4)

In situ variable-temperature FTIR spectroscopy demonstrates that the incorporation of disulfide bonds alters the chain conformation and flexibility of polyurethane molecules, where B2HD provides more flexible segmental structures, spatially facilitating proximity between furan and maleimide groups.

(5)

The damage self-healing in SSDA-PU results from the synergistic interaction of disulfide bond exchange, hydrogen bond dissociation, and thermally reversible DA reactions.

This study designed and synthesized a polyurethane incorporating multiple dynamic covalent bonds, providing valuable references for the engineering application of functional polyurethanes; future research could further investigate aspects such as its durability.