Thermally Reversible and Recyclable Polyethylene Networks via Furan–Maleimide Diels–Alder Dynamic Covalent Chemistry

Abstract

The formation of recyclable polyethylene materials is significantly limited by traditional crosslinking methods, which involve solvent-heavy processes and permanent chemical bonds that cannot be undone. Herein, we report an environmentally friendly and scalable approach to construct a thermo-reversible polyethylene network (PE-g-DA) via solvent-free, one-step melt processing based on furan–maleimide Diels–Alder (D–A) dynamic covalent chemistry. Furan-functionalized polyethylene was dynamically crosslinked with bismaleimide during melt mixing, fully compatible with conventional polyolefin processing techniques. FTIR spectroscopy, temperature-dependent solubility, and differential scanning calorimetry collectively confirm the reversible formation and dissociation of D–A adducts, enabling thermal switching of the network structure. Equilibrium swelling experiments based on the Flory–Rehner model indicate that the crosslink density can be precisely controlled by varying the bismaleimide content. As a result, PE-g-DA exhibits significantly enhanced tensile strength while maintaining high ductility at moderate crosslink densities. Notably, the dynamic network allows efficient thermal reprocessing, with recycled samples retaining approximately 93% and 80% of their original tensile strength after the first and second reprocessing cycles, respectively. Moreover, intrinsic thermal self-healing behavior is directly visualized by scanning electron microscopy at 120 °C. This work demonstrates that combining dynamic Diels–Alder chemistry with solvent-free melt processing offers a practical and sustainable route to recyclable, reprocessable, and self-healable polyethylene materials with clear potential for large-scale industrial production.

1. Introduction

Polyethylene (PE) is one of the most widely produced and consumed polymers due to its low cost, excellent chemical resistance, and outstanding processability. It has been extensively used in packaging, construction, transportation, and daily consumer products. However, to meet the increasing demands for mechanical strength, thermal stability, and dimensional integrity, polyethylene is often chemically crosslinked in practical applications. Conventional crosslinking strategies, such as peroxide or radiation-induced crosslinking, generate permanent covalent networks that severely restrict polymer chain mobility [1,2]. As a result, crosslinked polyethylene materials cannot be remelted or reprocessed, leading to serious challenges in recycling and large accumulations of plastic waste, which raise growing environmental and sustainability concerns [3]. In this context, developing thermo-reversible crosslinked polyethylene that combines the mechanical robustness of covalent networks with the recyclability of thermoplastics is of great significance. Ideally, such materials should be able to form stable covalent crosslinks under service conditions while undergoing reversible de-crosslinking at elevated temperatures, allowing repeated reprocessing and reuse. Achieving this balance would not only extend the lifetime of polyethylene materials but also substantially reduce plastic waste and energy consumption associated with polymer production.

Over the past decade, various self-healable and recyclable polymer networks have been developed based on reversible noncovalent interactions or dynamic covalent bonds. Noncovalent interactions, including hydrogen bonding, ionic interactions, and π–π stacking, have been widely explored to impart self-healing and remolding capabilities [4]. However, these interactions are generally weak and sensitive to temperature, solvents, and external stress, which often limits the mechanical strength and long-term stability of the resulting materials [3].

In contrast, dynamic covalent bonds offer higher bond strength and better environmental stability while maintaining reversible behavior, making them particularly attractive for constructing recyclable and self-healable polymer networks [5]. Among various dynamic covalent chemistries, the Diels–Alder (DA) reaction is one of the most representative thermo-reversible reactions due to its fast kinetics and high efficiency, while its practical reaction conditions strongly depend on the electronic nature of the diene/dienophile pair [6,7]. In general, DA cycloadditions proceed under relatively mild-to-moderate conditions, mainly for reactions between nucleophilic (electron-rich) dienes and electrophilically activated (electron-poor) dienophiles (e.g., maleimide-type alkenes), whereas reactions involving non-activated alkenes typically require substantially harsher conditions, such as higher temperatures and/or elevated pressures. This trend can be rationalized using Conceptual DFT (CDFT) reactivity indices (e.g., electrophilicity/nucleophilicity and related frontier-orbital considerations), which link electronic activation to the activation barrier and thus the reaction severity [8,9,10,11]. Accordingly, for the DA pair employed in this work (electron-rich diene/activated maleimide-type dienophile), the forward DA reaction forms a stable cycloadduct at comparatively moderate temperatures, while the reverse retro-Diels–Alder (rDA) reaction becomes significant at elevated temperatures, enabling bond dissociation and network rearrangement [12].

Owing to these advantages, DA chemistry has been extensively applied in the preparation of thermo-reversible epoxy resins, polyurethanes, elastomers, and other polymeric systems [13,14]. However, most reported DA-based polymer networks are synthesized through solution-based reactions, which require large amounts of organic solvents, involve long reaction times, and suffer from low production efficiency. For instance, Scholiers et al. reported a two-step upcycling strategy to convert high-molar-mass polybutadiene into reprocessable dynamic covalent networks, yet solvent-assisted modification was required [15]. Thiessen et al. investigated furfuryl/maleimide DA-derived crosslinks and demonstrated structure tunability in dichloromethane [16]. Picchioni et al. employed furfurylamine (FA) as a less odorous diene precursor to construct DA networks [17]. Orozco et al. reported Diels–Alder (furan/maleimide) thermo-reversibly crosslinked polymers with tunable crosslinking densities, prepared by reacting a furan-modified polyketone with a bismaleimide crosslinker in chloroform at a moderate temperature until gelation [18,19]. These representative studies underscore the promise of DA chemistry. They also highlight a persistent bottleneck: industrial translation is hindered when the chemistry relies on solvent-based processing rather than melt processing. From both industrial and sustainability perspectives, an ideal strategy should be solvent-free, melt-processable, rapid, and compatible with existing polyolefin processing equipment [20]. However, to the best of our knowledge, direct grafting of FA on PE in a melt state has been rarely studied comprehensively and reported.

In this study, we report an environmentally friendly and scalable route to prepare a thermo-reversible polyethylene network (PE-g-DA) via one-step melt processing. Polyethylene was first functionalized with furan groups through melt grafting, followed by dynamic crosslinking with bismaleimide (BMI) via the Diels–Alder reaction during melt mixing. The entire preparation process was carried out under solvent-free conditions using conventional melt blending, without complex purification steps, demonstrating excellent potential for large-scale industrial application. The resulting PE-g-DA exhibits a dynamically crosslinked network with tunable crosslink density. The thermo-reversible nature of the DA bonds was systematically confirmed by FTIR spectroscopy, differential scanning calorimetry (DSC), and temperature-dependent solubility behavior. Mechanical testing revealed that PE-g-DA achieves significantly enhanced tensile strength while maintaining high ductility. Importantly, the material can be efficiently recycled through thermal reprocessing, retaining a high percentage of its mechanical performance after multiple cycles. Furthermore, intrinsic thermal self-healing behavior was directly visualized by scanning electron microscopy (SEM). This work demonstrates that dynamic Diels–Alder chemistry combined with solvent-free melt processing provides a practical and sustainable strategy to transform conventional polyethylene into a recyclable, reprocessable, and self-healable material, bridging the gap between advanced dynamic polymer networks and industrially relevant polyolefin manufacturing.

2. Materials and Methods

2.1. Materials

Maleic anhydride-grafted polyethylene (PE-g-MA, maleic anhydride content of 1.2 wt.%) was purchased from China Bluestar Chengrand Co., Ltd. (Chengdu, China). Furfurylamine (FA, >99%), 1,1′-(methylenedi-4,1-phenylene) bismaleimide (BMI, 95%), toluene, absolute ethanol, and o-dichlorobenzene were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). All chemicals were of analytical grade or higher and were used without further purification.

2.2. Preparation of Furan-Functionalized Polyethylene (PE-g-FA)

PE-g-MA (50 g) and furfurylamine (FA) at three equivalents relative to the molar content of maleic anhydride groups in PE-g-MA were introduced into an internal mixer. The mixture was melted and mixed at 160 °C with a rotor speed of 50 rpm for 10 min. During the melt-mixing process, the anhydride groups of PE-g-MA reacted with the amine groups of FA through ring-opening followed by imide formation, resulting in the covalent grafting of furan moieties onto the polyethylene backbone. After completion of the reaction, the product was cooled to room temperature and designated as PE-g-FA.

2.3. Preparation of Diels–Alder Crosslinked Polyethylene (PE-g-DA)

The obtained PE-g-FA (50 g) was subsequently melt-mixed with different molar equivalents of bismaleimide (BMI), calculated based on the theoretical molar content of furan groups in PE-g-FA, using an internal mixer at 170 °C and 50 rpm for 30 min, as displayed in Scheme 1. This process facilitated the Diels–Alder reaction between pendant furan groups and maleimide groups, leading to the formation of a dynamic covalent crosslinked network.

After melt-mixing, the samples were transferred to a convection oven and annealed at 120 °C for 24 h to promote the completion of the Diels–Alder crosslinking reaction and to stabilize the network structure. The resulting thermally reversible Diels–Alder crosslinked polyethylene was denoted as PE-g-DA. Figure 1 illustrates the reaction scheme and functionalization of PE-g-MA with furfurylamine and the subsequent construction of a thermally reversible dynamic covalent network via the Diels–Alder reaction between furan groups and bismaleimide.

2.4. Characterization and Measurements

2.4.1. Fourier Transform Infrared (FTIR) Spectroscopy

Fourier transform infrared (FTIR) spectra were recorded using a Nicolet 6700 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) equipped with an attenuated total reflectance (ATR) accessory. All spectra were collected in the wavenumber range of 400–4000 cm⁻¹ with 32 scans at a resolution of 4 cm⁻¹. FTIR analysis was employed to identify the characteristic functional groups and monitor the chemical structure evolution of PE-g-MA, PE-g-FA, and PE-g-DA.

2.4.2. Solubility and Thermal Reversibility Tests

Solubility tests were conducted to evaluate the degree of crosslinking and decrosslinking of PE-g-MA, PE-g-FA, and PE-g-DA, thereby assessing the thermal reversibility of the Diels–Alder (D-A) network. In a typical experiment, 0.5 g of sample and 15 mL of o-dichlorobenzene (DCB) were placed in a 25 mL glass vial, which was then heated in an oven at 125 °C for 24 h. Digital photographs were taken before heating (t = 0 h) and after heating (t = 24 h) to visually record the dissolution behavior of each sample. To further investigate the reversibility of the D-A crosslinked network, PE-g-DA samples (0.5 g) were immersed in DCB (15 mL) at 160 °C for different time intervals to induce the retro-Diels–Alder reaction. The solutions were subsequently cooled to temperatures below 60 °C, and the reformation of the crosslinked network was visually examined to confirm network reconstruction.

2.4.3. Crosslink Density Determination

The crosslink density S (mol·cm⁻³) of PE-g-DA was determined using equilibrium swelling experiments in o-dichlorobenzene, followed by calculation based on the Flory–Rehner equation. Approximately 0.5 g of dried PE-g-DA sample was weighed and placed in a 25 mL glass vial containing 15 mL of DCB. The sample was allowed to swell at 125 °C for 72 h until swelling equilibrium was reached, after which the swollen mass (W₁) was recorded. The sample was then dried in an oven at 80 °C to constant weight, and the dried mass (W₂) was measured. The crosslink density was calculated according to the Flory–Rehner Equation (1):

$$S={\displaystyle \frac{\ln \left(1-V_R\right)+V_R+\chi {V_R}^2}{2V_S(0.5V_R-{V_R}^{\frac{1}{3}})}},V_R={\displaystyle \frac{W_2}{W_2+\left(W_1-W_2\right)\frac{{\rho }_{\mathrm{PE}\text{-}\mathrm{g}\text{-}\mathrm{DA}}}{{\rho }_{DCB}}}}$$

(1)

where V_R is the volume fraction of PE-g-DA in the swollen sample, V_S is the molar volume of the solvent (o-dichlorobenzene, 113.3 mL·mol⁻¹ at room temperature), χ is the polymer–solvent interaction parameter (taken as 0.40 for the PE-g-MA/DCB system), and the densities of PE-g-DA and DCB were assumed to be 0.95 g·cm⁻³ and 1.30 g·cm⁻³, respectively.

2.4.4. Differential Scanning Calorimetry (DSC)

Differential scanning calorimetry (DSC) measurements were performed using a TA Instruments DSC Q2000 (TA Instruments, New Castle, DE, USA) to investigate the thermal reversibility of the D-A crosslinked network. Samples of PE-g-MA and PE-g-DA were sealed in aluminum pans and heated from 70 to 200 °C at a heating rate of 1 °C·min⁻¹ under a nitrogen atmosphere. High-purity nitrogen (≥99.999%) was used as the purge gas at a flow rate of 50 mL·min⁻¹.

2.4.5. Mechanical Properties and Reprocessing Tests

Tensile tests were conducted at room temperature using a universal tensile testing machine to evaluate the mechanical properties of PE-g-MA and PE-g-DA. Dog-bone-shaped specimens were prepared according to standard testing protocols and tested at a crosshead speed of 50 mm·min⁻¹. For each sample, at least 10 specimens were measured, and the stress–strain curve corresponding to the median value was selected as representative. The tensile strength at break and elongation at break were obtained from the stress–strain curves to quantitatively assess the mechanical enhancement induced by D-A crosslinking. Reprocessing experiments were performed to evaluate the recyclability of PE-g-DA. After tensile testing, the fractured specimens were cut into small pieces and reprocessed in a hot press at 200 °C under 10 MPa for 30 min. After cooling to room temperature, the reprocessed samples were subjected to tensile testing under identical conditions.

2.4.6. Scanning Electron Microscopy (SEM)

The self-healing behavior of PE-g-DA was examined using field-emission scanning electron microscopy (FE-SEM, JSM-7610F; JEOL Ltd., Tokyo, Japan) at an accelerating voltage of 5 kV. Specimens of PE-g-MA and PE-g-DA films were first cut to create well-defined fracture interfaces. The separated film surfaces were then brought into contact and subjected to thermal treatment at 120 °C for different healing times (0, 2, and 4 h). The healed interfaces were subsequently observed by SEM to evaluate the morphological evolution and healing efficiency.

3. Results and Discussion

3.1. Preparation of PE-g-DA

The successful grafting of furfurylamine (FA) onto maleic anhydride grafted polyethylene was confirmed by Fourier transform infrared (FTIR) spectroscopy, as shown in Figure 2. For pristine PE-g-MA, a distinct absorption band at 1718 cm⁻¹ is observed, which is characteristic of the carbonyl stretching vibration of the anhydride groups [2,21]. This peak serves as a clear spectroscopic signature of the maleic anhydride functionality grafted onto the polyethylene backbone. After reaction with FA, the FTIR spectrum of PE-g-FA exhibits several notable changes. A new absorption band appears at 737 cm⁻¹, which can be assigned to the C–H out-of-plane bending vibration of the furan ring, indicating the successful introduction of furan moieties [22]. In addition, the emergence of a band at approximately 1145 cm⁻¹ is attributed to C–O stretching vibrations associated with the furan structure and imide formation. Meanwhile, the carbonyl absorption shifts slightly from 1718 to 1714 cm⁻¹, suggesting the conversion of anhydride groups into imide linkages upon reaction with FA. These spectral features collectively confirm that furfurylamine was successfully grafted onto PE-g-MA, yielding furan-functionalized polyethylene (PE-g-FA), which provides reactive diene sites for subsequent Diels–Alder crosslinking.

Following furan functionalization, the PE-g-FA was further reacted with bismaleimide (BMI) to construct a dynamic covalent network via the Diels–Alder (D-A) reaction. As shown in Figure 2, the FTIR spectrum of the resulting PE-g-DA displays additional characteristic absorption bands that are absent in PE-g-FA. A new absorption band at 1513 cm⁻¹ is observed, which is attributed to the stretching vibration of the aromatic C=C bonds in the BMI moiety, confirming the incorporation of BMI into the polymer system [22]. More importantly, a distinct band appears at 1187 cm⁻¹, which can be assigned to the coupled C–O and C–N stretching vibrations associated with the D-A adduct structure formed between the furan ring and the maleimide group [20]. The presence of this characteristic D-A absorption provides direct evidence for the occurrence of the Diels–Alder reaction. The simultaneous appearance of BMI-related aromatic vibrations and D-A adduct signals demonstrates that a dynamic covalent crosslinked network was successfully formed through the reversible Diels–Alder reaction between furan side groups and bismaleimide crosslinkers. This spectroscopic evidence lays the foundation for the thermally reversible behavior, enhanced mechanical performance, and self-healing capability of the PE-g-DA system discussed in the subsequent sections.

3.2. Temperature-Dependent Solubility and Gelation Behavior

The temperature-dependent solubility behaviors of PE-g-MA, PE-g-FA, and PE-g-DA in o-dichlorobenzene (DCB) provide direct macroscopic evidence for the formation and reversibility of the Diels–Alder (D-A) crosslinked network. Representative photographs taken before and after heating at 125 °C for 24 h are shown in Figure 3.

As shown in Figure 3a, PE-g-MA rapidly dissolved in DCB and formed a homogeneous solution after 24 h, indicating that no covalently crosslinked network was present in the system. Similarly, PE-g-FA (Figure 3b) exhibited complete dissolution after 24 h, although the dissolution rate was slightly slower than that of PE-g-MA. This behavior can be attributed to the introduction of furan moieties, which increases intermolecular interactions but does not result in permanent or dynamic covalent crosslinking. In both cases, polymer chains are held together only by physical entanglements, enabling full dissolution upon heating. In sharp contrast, PE-g-DA showed markedly different behavior (Figure 3c). After heating at 125 °C for 24 h, PE-g-DA did not dissolve completely but instead formed a swollen, partially soluble gel. This observation indicates the presence of a covalently crosslinked network that restricts polymer chain diffusion into the solvent. The inability of PE-g-DA to fully dissolve at this temperature suggests that the D-A adducts remain largely intact, maintaining the integrity of the crosslinked structure [20,23]. To further elucidate the thermal reversibility of the D-A crosslinks, the sol–gel transition behavior of PE-g-DA in DCB was examined upon temperature cycling, as illustrated in Figure 4. When the PE-g-DA/DCB system was heated to ≥160 °C, the initially swollen gel gradually transformed into a clear and homogeneous solution. This transition is attributed to the retro-Diels–Alder (rDA) reaction, which is activated at elevated temperatures and leads to the cleavage of D-A adducts, thereby releasing polymer chains from the crosslinked network and restoring solubility [24]. Upon subsequent cooling to ≤60 °C, the homogeneous solution underwent gelation again, accompanied by the reappearance of insoluble fractions and a macroscopic gel. This reversible sol–gel transition demonstrates the reformation of D-A bonds between furan and maleimide groups and the reconstruction of the dynamic covalent network. The repeatable dissolution–gelation process observed upon heating and cooling provides compelling macroscopic evidence for the dynamic and thermally switchable nature of the D-A crosslinks in PE-g-DA [23,25]. These results are fully consistent with the FTIR analysis, which confirmed the formation of furan–maleimide D-A adducts and their reversible dissociation upon thermal stimulation. Together, the solubility and temperature-dependent gelation behaviors confirm that PE-g-DA possesses a thermally reversible covalent network, in which rDA-induced decrosslinking at high temperatures enables dissolution and processing, while D-A recombination upon cooling restores the crosslinked structure. Such a reversible network architecture is highly advantageous for polyethylene-based materials, as it combines the mechanical robustness associated with covalent crosslinking with the recyclability and reprocessability characteristics of thermoplastics. This dynamic sol–gel behavior underpins the potential of PE-g-DA for sustainable applications requiring both enhanced performance and thermal recyclability.

3.3. Crosslink Density and Swelling Behavior

The crosslink density of the Diels–Alder crosslinked polyethylene (PE-g-DA) was quantitatively evaluated by equilibrium swelling experiments in o-dichlorobenzene (DCB), followed by calculation using the Flory–Rehner equation. Samples with different crosslink densities were prepared by varying the amount of bismaleimide (BMI) while keeping the PE-g-FA content constant. As shown in Figure 5, the calculated crosslink density increases monotonically with increasing BMI content. When a low amount of BMI (0.0005 mol) was introduced, PE-g-DA exhibited a relatively low crosslink density of approximately 6.5 × 10⁻⁴ mol cm⁻³. Increasing the BMI content to 0.001 mol led to a noticeable rise in crosslink density, indicating more effective formation of Diels–Alder junctions. A further increase in BMI content to 0.003 and 0.006 mol resulted in a pronounced enhancement of crosslink density, reaching values on the order of 2.0 × 10⁻³ and 2.8 × 10⁻³ mol cm⁻³, respectively. This trend can be directly attributed to the increased availability of maleimide functional groups, which promotes the formation of a higher number of furan–maleimide Diels–Alder adducts. As a result, the network connectivity and effective crosslinking points within the polymer matrix are significantly enhanced, leading to a more densely crosslinked structure. Correspondingly, higher crosslink densities restrict solvent penetration and polymer chain mobility, which is consistent with the reduced solubility and pronounced gel-like swelling behavior observed for PE-g-DA in the solubility experiments. Importantly, the crosslink density results provide quantitative support for the conclusions drawn from FTIR analysis, where characteristic absorption bands associated with Diels–Alder adduct formation intensified with increasing BMI content, and from the temperature-dependent solubility tests, which demonstrated stronger resistance to dissolution at lower temperatures for more highly crosslinked samples. Together, these findings confirm that the Diels–Alder reaction efficiency and network architecture of PE-g-DA can be effectively tuned by adjusting the BMI content. Overall, the swelling and crosslink density analysis demonstrates that PE-g-DA possesses a controllable dynamic covalent network, in which the density of reversible crosslinks can be systematically regulated. This tunability is critical for balancing mechanical robustness, thermal reversibility, and processability, thereby laying the structural foundation for the enhanced mechanical performance and recyclability discussed in the subsequent sections.

3.4. Differential Scanning Calorimetry (DSC) Analysis

Differential scanning calorimetry (DSC) was employed to further elucidate the thermal reversibility and characteristic temperature window of the Diels–Alder (D-A) crosslinked network in PE-g-DA. The DSC heating and cooling curves of PE-g-MA and PE-g-DA samples with different BMI contents are presented in Figure 6, while the temperature-cycling DSC behavior of PE-g-DA with the highest BMI content is shown in Figure 7. As shown in Figure 6, pristine PE-g-MA exhibits only a sharp endothermic peak around 110–115 °C, which can be attributed to the melting of the polyethylene crystalline domains [3]. No additional thermal events are observed over the investigated temperature range, confirming the absence of thermally reversible covalent interactions in PE-g-MA. For PE-g-DA samples with relatively low BMI contents (≤0.001 mol), the DSC traces remain similar to those of PE-g-MA, and no distinct additional thermal transitions are detected. This suggests that at low BMI concentrations, the density of D-A junctions is insufficient to generate a detectable thermal signature by DSC, consistent with the lower crosslink densities obtained from swelling experiments. In contrast, when the BMI content is increased to 0.003 mol and 0.006 mol, a new and well-defined endothermic peak appears at approximately 150 °C during the heating process. This endothermic event is attributed to the retro-Diels–Alder (rDA) reaction, corresponding to the thermally induced cleavage of furan–maleimide D-A adducts. Upon subsequent cooling, a distinct exothermic peak emerges at around 130 °C, which can be assigned to the reformation of D-A bonds between furan and maleimide groups [25]. The appearance of these reversible endothermic and exothermic transitions provides direct calorimetric evidence for the dynamic nature of the D-A crosslinked network [24]. To unambiguously confirm that these thermal events originate from the reversible D-A chemistry rather than irreversible thermal degradation or crystallization effects, a heating–cooling–reheating DSC experiment was performed on the PE-g-DA sample with the highest BMI content (0.006 mol), as shown in Figure 7. Notably, the characteristic endothermic peak associated with the rDA reaction appears reproducibly during both heating cycles, while the exothermic peak corresponding to the D-A recombination is consistently observed during the cooling step. The reproducibility of these thermal transitions over multiple cycles confirms the fully reversible nature of the D-A and rDA reactions within the investigated temperature range. Importantly, the temperature window identified by DSC correlates well with the macroscopic solubility and gelation behavior observed in the temperature-dependent dissolution experiments. The rDA-induced endothermic transition near 150–160 °C coincides with the temperature at which PE-g-DA undergoes dissolution in o-dichlorobenzene, while the exothermic D-A recombination around 130 °C is consistent with the reappearance of the crosslinked gel upon cooling [23]. Furthermore, the presence and intensity of these DSC transitions are in good agreement with the FTIR results, which revealed the formation and reversible dissociation of furan–maleimide D-A adducts. Overall, the DSC analysis provides compelling thermal evidence that PE-g-DA possesses a thermally reversible covalent network, with a well-defined switching temperature range governed by the D-A/rDA equilibrium. The dependence of the DSC response on BMI content further demonstrates that the dynamic behavior of the network can be effectively tuned by controlling the crosslink density. This thermally switchable crosslinking behavior underpins the material’s reprocessability, recyclability, and potential for repeated thermal healing, which are explored in the subsequent mechanical and recycling studies.

3.5. Tensile Properties

The tensile stress–strain behaviors of PE-g-MA and PE-g-DA samples with different bismaleimide (BMI) contents are shown in Figure 8, providing insight into the effect of dynamic Diels–Alder (D-A) crosslink density on the mechanical performance of the materials. Pristine PE-g-MA exhibits relatively low tensile strength and moderate elongation at break, which is typical of polyethylene systems modified only by grafted functional groups without covalent crosslinking. The mechanical response is dominated by chain entanglements and crystalline domain deformation, resulting in limited load-bearing capability. Upon incorporation of BMI, the mechanical properties of PE-g-DA are significantly enhanced. As the BMI content increases, the tensile strength shows a pronounced increase, while the elongation at break exhibits a non-monotonic dependence on crosslink density. Specifically, compared with PE-g-MA, the PE-g-DA sample containing 0.006 mol BMI shows a 94% increase in tensile strength, accompanied by a 12% reduction in elongation at break. This behavior is indicative of a densely crosslinked network, in which the high density of D-A junctions effectively restricts chain mobility, leading to improved stiffness and load transfer but reduced deformability [24]. For intermediate BMI contents, a more balanced mechanical performance is observed. The PE-g-DA sample with 0.003 mol BMI exhibits a 72% increase in tensile strength along with a 43% increase in elongation at break, while the sample containing 0.001 mol BMI shows a 62% enhancement in tensile strength and a remarkable 80% increase in elongation at break. This simultaneous improvement in strength and ductility suggests that an optimal density of dynamic D-A crosslinks can reinforce the polymer network while still allowing sufficient chain rearrangement under tensile deformation [19,26]. The observed mechanical trends can be directly correlated with the crosslink density results obtained from swelling experiments. Higher BMI contents lead to higher crosslink densities, which increase the number of effective load-bearing junctions and enhance tensile strength. However, excessive crosslinking suppresses chain extensibility, resulting in reduced elongation at break. In contrast, at lower BMI contents, the dynamic nature of the D-A bonds allows partial bond dissociation and reformation under stress, enabling energy dissipation and large deformation without catastrophic failure [27,28]. Importantly, the dynamic covalent nature of the D-A crosslinks distinguishes PE-g-DA from permanently crosslinked polyethylene. The reversible bond exchange, confirmed by DSC analysis and temperature-dependent solubility tests, provides additional molecular mobility during deformation, which contributes to the improved toughness observed at moderate crosslink densities. Overall, the tensile test results demonstrate that the mechanical properties of PE-g-DA can be effectively tailored by adjusting the BMI content. By balancing crosslink density and dynamic bond reversibility, PE-g-DA achieves a desirable combination of enhanced strength and ductility, outperforming PE-g-MA while retaining the advantages of thermal reversibility and recyclability. These findings further highlight the effectiveness of Diels–Alder dynamic crosslinking as a versatile strategy for tuning the structure–property relationships of polyethylene-based materials.

The reprocessability and mechanical stability of the Diels–Alder crosslinked polyethylene were evaluated through repeated tensile tests on recycled PE-g-DA specimens. Owing to the reversible nature of the D-A bonds, PE-g-DA is expected to undergo network dissociation and reconstruction during thermal reprocessing. As a representative example, the PE-g-DA sample containing 0.003 mol BMI was subjected to hot-press recycling, and the corresponding stress–strain curves of the original, first-recycled, and second-recycled specimens are shown in Figure 9. As illustrated in Figure 8, the recycled samples retain mechanical behaviors that are highly comparable to those of the original material. After the first recycling cycle, the tensile strength of the reprocessed specimen remains at approximately 93% of the original value, indicating that the majority of effective load-bearing crosslinks are preserved during thermal reprocessing. Even after a second recycling cycle, the tensile strength retention remains around 80%, demonstrating the robustness and reversibility of the dynamic covalent network. Although a gradual decrease in elongation at break is observed with increasing recycling cycles, the recycled PE-g-DA samples still exhibit elongation values that are higher than those of the unmodified PE-g-MA. This behavior suggests that partial network rearrangement and possible microstructural heterogeneities introduced during repeated processing may slightly limit chain extensibility. Nevertheless, the dynamic exchange of D-A bonds enables effective stress redistribution and prevents catastrophic degradation of mechanical performance [20]. The observed mechanical retention can be rationalized by the reversible D-A/retro-D-A equilibrium, as evidenced by the DSC analysis. During hot-press reprocessing at elevated temperatures, the rDA reaction is activated, allowing temporary decrosslinking and polymer flow. Upon cooling, D-A recombination occurs, reconstructing the crosslinked network and restoring mechanical integrity. This mechanism is further supported by the temperature-dependent solubility and gelation behavior, which demonstrated repeatable sol–gel transitions upon heating and cooling. Overall, the recycling experiments confirm that PE-g-DA combines the mechanical advantages of covalent crosslinking with the reprocessability typically associated with thermoplastic materials. The ability to retain a high level of tensile strength over multiple recycling cycles highlights the effectiveness of Diels–Alder dynamic covalent chemistry in enabling sustainable, recyclable polyethylene-based materials with durable mechanical performance.

3.6. Self-Healing Behavior of PE-g-DA

The self-healing capability of PE-g-DA was investigated by scanning electron microscopy (SEM) to directly visualize the morphological evolution of surface cracks under thermal stimulation. Owing to the presence of reversible Diels–Alder (D-A) dynamic covalent bonds, PE-g-DA is expected to exhibit intrinsic self-healing behavior. For comparison, PE-g-MA, which lacks dynamic crosslinks, was examined under identical conditions. As illustrated in Figure 10a, a distinct and sharp surface crack remains clearly visible on the PE-g-MA film even after heating at 120 °C for 2 h and 4 h. The crack width and morphology show negligible changes over time, indicating that thermal softening alone is insufficient to induce effective crack closure or healing in the absence of dynamic covalent interactions. In contrast, the PE-g-DA film exhibits pronounced self-healing behavior, as shown in Figure 10b. Immediately after scratching, a well-defined crack is observed on the surface. After heating at 120 °C for 2 h, the crack width is significantly reduced, accompanied by partial material flow and interfacial smoothing. Upon extending the healing time to 4 h, the crack becomes barely distinguishable, and the surface appears nearly continuous, demonstrating effective crack healing at the microscopic level. The observed self-healing behavior of PE-g-DA can be attributed to the thermally activated D-A/retro-D-A equilibrium [29]. At 120 °C, partial dissociation of D-A adducts occurs, as confirmed by DSC analysis, leading to a temporary reduction in crosslink density and enhanced chain mobility near the damaged region [30]. This increased molecular mobility allows polymer chains to diffuse across the crack interface. Upon cooling, the reformation of D-A bonds reconstructs the crosslinked network, effectively sealing the crack and restoring structural integrity. This self-healing mechanism is consistent with the temperature-dependent solubility and gelation behavior, where PE-g-DA exhibits reversible sol–gel transitions, and with the mechanical recycling results, which demonstrated substantial retention of tensile strength after repeated thermal processing. Together, these results confirm that the dynamic covalent network in PE-g-DA enables not only reprocessability and recyclability but also intrinsic self-healing functionality.

Overall, the SEM observations provide direct microscopic evidence that PE-g-DA possesses an efficient thermal self-healing capability around 120 °C, arising from the reversible Diels–Alder crosslinked network. This combination of mechanical robustness, recyclability, and self-healing behavior highlights the potential of PE-g-DA for advanced, sustainable polymer applications.

3.7. Conclusions

In this work, a thermally reversible and recyclable polyethylene network was successfully constructed via furan–maleimide Diels–Alder (D–A) dynamic covalent chemistry. By grafting furan functionality onto polyethylene and subsequently introducing bismaleimide crosslinkers, a dynamic crosslinked PE-g-DA system with tunable network architecture was obtained. The following key conclusions can be drawn: A thermally switchable covalent network was unambiguously demonstrated across multiple length scales. FTIR spectroscopy confirmed the formation and reversible dissociation of furan–maleimide D–A adducts, while temperature-dependent solubility and sol–gel transition experiments provided direct macroscopic evidence of network decrosslinking at elevated temperatures and reconstruction upon cooling. DSC analysis further established a well-defined D–A/rDA temperature window, enabling precise thermal control of network dynamics. The crosslink density of PE-g-DA can be quantitatively tuned by BMI content, enabling structure–property regulation. Equilibrium swelling experiments combined with the Flory–Rehner model revealed a monotonic increase in crosslink density with increasing BMI content. This quantitative network characterization bridges molecular design and macroscopic behavior, providing a solid structural basis for interpreting the observed thermal, solubility, and mechanical responses. Dynamic D–A crosslinking enables a rare combination of enhanced strength, high ductility, and mechanical recyclability. Tensile tests demonstrated that PE-g-DA exhibits significantly improved tensile strength compared to PE-g-MA, while maintaining or even enhancing elongation at break at moderate crosslink densities. Importantly, recycled samples retained up to ~93% of their original tensile strength after the first reprocessing cycle and ~80% after the second cycle, confirming the robustness and reversibility of the dynamic covalent network. Intrinsic thermal self-healing behavior was directly visualized by SEM. Surface cracks in PE-g-DA films gradually disappeared upon heating at 120 °C, whereas no healing was observed in PE-g-MA under identical conditions. This self-healing behavior arises from thermally activated D–A bond exchange and chain diffusion, further highlighting the multifunctionality imparted by dynamic covalent crosslinks.

Overall, this study demonstrates that Diels–Alder dynamic covalent chemistry provides an effective and versatile strategy to transform conventional polyethylene into a recyclable, reprocessable, and self-healable material without sacrificing mechanical performance. The insights gained from the systematic correlation between network structure, thermal reversibility, and mechanical behavior offer valuable guidance for the design of next-generation sustainable polyolefin materials.