Bioinspired self-healing vitrimer from epoxidised palm oil reinforced with nanofibrillated cellulose and activated carbon

Abstract

A vitrimer composite based on epoxidised palm oil (EPO) was reinforced with nanofibrillated cellulose (NFC) and palm kernel shell (PKS)-derived activated carbon as complementary bio-based fillers. The loadings of NFC and activated carbon were varied to examine their influence on the thermal, mechanical, chemical, and healing-reprocessing behaviour of the EPO vitrimer network. The synergistic interaction between these fillers preserves dynamic bond exchange within the vitrimer network, enabling effective thermal welding in which the welded interface becomes seamless after treatment. This dynamic network behaviour was further reflected in dynamic mechanical and creep-recovery analyses, which revealed the influence of filler content on network mobility. A higher filler content (5 wt% of each filler) enhanced stiffness but restricted network rearrangement, leading to incomplete strain recovery. In contrast, the composite containing 4 wt% of each filler achieved complete strain recovery (100%) while exhibiting strong viscoelastic damping behaviour (tan d ~ 1.36), indicating efficient molecular relaxation during the glass-transition process. FESEM showed improved interfacial continuity within the synergistic system, where NFC extends between activated carbon particles and the vitrimer matrix to maintain local network integrity and facilitate stress transfer. The rigid carbon phase also limits solvent diffusion within the matrix, contributing to improved solvent resistance of the vitrimer composite.

Full Article

Bioinspired Self-Healing Vitrimer from Epoxidised Palm Oil Reinforced with Nanofibrillated Cellulose and Activated Carbon

Chuan Li Lee  ,^a,b Balkis Fatomer A. Bakar,^a,c,* Kit Ling Chin  ,^a,*

Luqman Chuah Abdullah  ,^b,d Xian Foong Lee,^c and Qi Yong Wong ^c

A vitrimer composite based on epoxidised palm oil (EPO) was reinforced with nanofibrillated cellulose (NFC) and palm kernel shell (PKS)-derived activated carbon as complementary bio-based fillers. The loadings of NFC and activated carbon were varied to examine their influence on the thermal, mechanical, chemical, and healing-reprocessing behaviour of the EPO vitrimer network. The synergistic interaction between these fillers preserves dynamic bond exchange within the vitrimer network, enabling effective thermal welding in which the welded interface becomes seamless after treatment. This dynamic network behaviour was further reflected in dynamic mechanical and creep-recovery analyses, which revealed the influence of filler content on network mobility. A higher filler content (5 wt% of each filler) enhanced stiffness but restricted network rearrangement, leading to incomplete strain recovery. In contrast, the composite containing 4 wt% of each filler achieved complete strain recovery (100%) while exhibiting strong viscoelastic damping behaviour (tan δ ~ 1.36), indicating efficient molecular relaxation during the glass-transition process. FESEM showed improved interfacial continuity within the synergistic system, where NFC extends between activated carbon particles and the vitrimer matrix to maintain local network integrity and facilitate stress transfer. The rigid carbon phase also limits solvent diffusion within the matrix, contributing to improved solvent resistance of the vitrimer composite.

DOI: 10.15376/biores.21.2.4457-4489

Keywords: EPO vitrimer; Dynamic covalent network; Activated carbon; NFC; Self-healing behaviour

Contact information: a: Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia; b: Faculty of Biotechnology and Biomolecular Sciences, Universiti Putra Malaysia, Serdang 43400, Selangor, Malaysia; c: Faculty of Forestry and Environment, Universiti Putra Malaysia, Serdang 43400, Selangor, Malaysia; d: Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, Serdang 43400, Selangor, Malaysia; *Corresponding authors: bfatomer@upm.edu.my; c_kitling@upm.edu.my    

Graphical Abstract

INTRODUCTION

Plastics have become globally dominant because they combine high mechanical strength, low density, excellent processability, and low production cost. This combination of performance and affordability has driven global production from only a few million tonnes in the 1950s to more than 400 million tonnes per year, a trend that continues alongside rising carbon emissions and global temperatures (Kelly et al. 2025). However, the inherent properties that make plastics convenient to produce also cause their persistence in the environment. Most plastic waste originates from short-lived consumer products that degrade very slowly, accumulating in landfills, rivers, oceans, and terrestrial ecosystems, with long-term ecological and climatic consequences (Akash et al. 2025). Malaysia faces a comparable challenge, with national assessments indicating that post-consumer plastic waste exceeds 1.07 million tonnes per year (de Jong et al. 2025). Although plastics are technically recyclable, an estimated 81% of their material value is lost due to inefficient recovery and recycling systems, resulting in approximately USD 1.1 billion in lost economic value annually (WWF 2022). If global production continues to grow at around three and a half percent per year, plastics alone could emit nearly three gigatonnes of carbon dioxide annually by 2050, even under scenarios of significant energy sector decarbonisation (de Jong et al. 2025). This projection highlights the urgent need to reduce reliance on fossil-based polymer feedstocks and to develop sustainable polymer composites that combine high structural performance with recyclability, reparability and circular life-cycle management.

The escalating accumulation of plastic waste worldwide has intensified the search for sustainable materials capable of repairing themselves, adapting to external stimuli, and extending service lifetimes, thereby reducing disposal rates and minimizing environmental burden. Natural systems offer strong inspiration for such materials, as many biological structures possess intrinsic self-healing and adaptive capabilities that maintain functionality under changing conditions. The thigmonastic movement of Mimosa pudica provides a well-known example of rapid yet reversible deformation: mechanical stimulation redistributes turgor pressure within the pulvinus, driving leaflet closure through the elastic reorientation of cellulose microfibrils in the cell wall. Because the deformation remains within the elastic domain, the leaflet fully recovers its original structure once turgor pressure is restored, without requiring physiological reconstruction (Lou et al. 2025; Nakata and Takahara 2022). Inspired by such mechanisms, biomimetic materials aim to capture functional principles such as self-repair, structural adaptability, and stimulus-responsive behaviour (Wang et al. 2023). In this context, vitrimers provide a suitable polymer platform, as their permanently cross-linked networks can undergo dynamic covalent exchange, allowing reshaping, welding, and recovery under thermal stimulus while maintaining structural integrity (Lee et al. 2025b).

Previous work has demonstrated that vitrimeric behaviour can be successfully achieved using a traditional petrochemical epoxy matrix (Lee et al. 2025a). However, reliance on a petroleum-based epoxy backbone raises significant concerns regarding long-term sustainability, environmental impact, and potential toxicity, reflecting the broader limitations of fossil-derived polymers in achieving circular and environmentally responsible materials (Álvarez et al. 2025). To address these challenges, the present work focuses on epoxidised vegetable oils (EVO) as bio-derived alternatives to fossil-based epoxies. EVO offer several inherent advantages: they are sourced from renewable biomass, exhibit low toxicity, possess biodegradable aliphatic structures, and are highly compatible with dynamic covalent chemistries due to their epoxy bearing triglyceride architecture (Maisonneuve et al. 2013; Ribeiro et al. 2025; Yan et al. 2022). In particular, epoxidised palm oil (EPO) is well suited to Malaysia’s resource landscape. Malaysia’s oil palm sector generates more than 100 million tonnes of dry biomass each year, including trunks, fronds, empty fruit bunches, and palm kernel shells (Anonymous 2025). All of these biomass materials remain largely underutilised despite their potential for value added applications. In parallel, the industry produces substantial quantities of palm oil that can be epoxidised into EPO, a renewable and chemically adaptable precursor. This steady and locally available resource base makes EPO a practical and sustainable alternative to petrochemical epoxy monomers for developing advanced polymer composites.

Unlike conventional vitrimer composites that rely on long-chain polymeric backbones to achieve entanglement-driven stiffness and high glass transition temperatures, EPO consist of short, triglyceride-based molecular architectures (Ogori 2020; Stavila et al. 2023). Prior studies have shown that epoxy networks derived from EPO generally exhibit lower glass transition temperatures and storage moduli than petroleum-based epoxy resins, which can be attributed in part to their relatively low epoxy functionality and the resulting reduced crosslink density (Mauro et al. 2020). Nevertheless, recent reports suggest that vitrimer-like behaviour is not strictly dependent on long polymeric backbones, as heating above T_g produces elastomeric behaviour while exchange reactions remain too slow to rearrange network topology (Fang et al. 2020; Javier and Toro 2021). On this basis, the present work raises the question of whether vitrimer like dynamic network behaviour and self-healing functionality can emerge in a highly crosslinked, short-backbone EPO system.

Bio-based epoxy resins such as EVO typically exhibit lower mechanical properties, including reduced stiffness, strength, and toughness, compared with conventional epoxy composites (Álvarez et al. 2025; Kumar et al. 2020; Zhang et al. 2023). To address these limitations, mechanical reinforcement through the incorporation of suitable fillers is required to improve stiffness, interfacial integrity, and structural recovery in biocomposite networks. In the authors’ previous work, palm kernel shell (PKS)-derived activated carbon was shown to function as an effective thermal-mechanical reinforcement in epoxy-based vitrimer composites, enhancing stiffness, stabilising deformation, and promoting more uniform heat distribution during network rearrangement. However, activated carbon contributes limited catalytic activity and cannot independently regulate nanoscale mobility within the vitrimer structure (Lee et al. 2025a).

To complement this limitation, nanofibrillated cellulose (NFC, which also is called CNF) was incorporated to reinforce the vitrimer composite. NFC has been reported as promising reinforcement agents in polymer nanocomposites owing to their renewable origin and nanoscale fibrillar structure, which contribute to improved structural integrity in polymer matrices (Alwan et al. 2024). Additionally, the synergistic interaction between NFC and activated carbon in shaping the properties of EPO-based vitrimers remains insufficiently explored particularly regarding how NFC-assisted interactions may strengthen the functional role of activated carbon and how this cooperation contributes to the material’s self-healing behaviour. A clearer understanding of this interaction would help guide the future development of fully bio-derived vitrimer materials.

A meaningful research gap remains in understanding the synergistic interactions between NFC, activated carbon, and bio-derived EPO vitrimer networks. Addressing this gap is important for developing renewable vitrimer composites that balance stiffness, molecular mobility, reprocessability, and environmental sustainability. This study develops a bioinspired self-healing vitrimer composite based on EPO, incorporating NFC and PKS-derived activated carbon as reinforcing fillers. By systematically varying the loadings of both fillers, this work provides a foundational and exploratory assessment of their influence on thermal, mechanical, chemical, and reprocessing-healing behaviour. The study therefore provides mechanistic insight into how the integrated effects of these fillers influence the structural and dynamic behaviour of EPO-based vitrimer networks, while highlighting the potential of Malaysia’s oil palm biomass as a feedstock for value-added, sustainable polymer composites within a circular economy framework.

EXPERIMENTAL

PKS-derived Activated Carbon via Chemical and Physical Activation

Palm kernel shell derived activated carbon was prepared according to the method described in our previous study (Lee et al. 2025a), using biomass milled to an average particle size of approximately 85 µm.

Preparation of EPO-Based Vitrimer Composite

Epoxidised palm oil (EPO), supplied by MPOB (oxirane oxygen 3.2 %; acid value 0.7 mg KOH/g), served as the precursor matrix. The EPO was mixed with hexahydro-4-methylphthalic anhydride (HHMPA) at a stoichiometric ratio of 1:3 and stirred for 15 min to achieve a uniform matrix blend. Nanofibrillated cellulose (NFC) was incorporated at loadings of 3, 4, and 5 wt.%, followed by mechanical stirring at 600 rpm for 15 min to promote adequate dispersion. Citric acid (3 wt.%) was introduced to facilitate the curing process, and the formulation was subsequently mixed for an additional 10 min to enhance uniformity. Pre-dried PKS-derived activated carbon was then added at 3, 4, and 5 wt.% and blended for a further 10 min to ensure homogeneous distribution of the microscale filler. A vitrimer composite containing 4 wt.% activated carbon without NFC was prepared as the control for comparative evaluation. The prepared mixtures were transferred to a humidity-controlled chamber for pre-curing at 90 °C for 24 h. Final curing was then carried out in an oven at 140 °C for an additional 72 h to complete network formation. Following curing, all vitrimer composites were placed in a conditioning room to stabilise temperature and humidity prior to thermal, mechanical, and spectroscopic evaluation.

Characterization and Evaluation

Comprehensive characterization was conducted to examine both the mechanical behaviour and the self-healing capability of the vitrimer composites. The evaluation of self-healing (thermal welding) performance employed a combination of mechanical and microscopic techniques. Following the procedure adapted from Chong et al. (2021), vitrimer composite specimens were cut to a size of 2 mm × 5 mm × 40 mm, as depicted in the dimensional overview in Figure 1(a). Two specimens were then positioned in an overlapping configuration and thermally activated at 140 °C for 3 h while applying a compressive load corresponding to 10% deformation to promote interfacial welding. After heating, the interface formed between the two overlapping samples was examined under an optical microscope to evaluate interfacial rejoining. Qualitative observation of surface recovery was further performed using a Nikon SMZ1270 stereo/photomicroscope to determine the extent of morphological restoration along the welded region. Subsequently, field emission scanning electron microscopy (FESEM) was employed to characterise the microstructural features at the welding line and to confirm interfacial bonding, using a Tescan Clara 2023 instrument.

Fig. 1(a). Dimensions and structural arrangement of the test specimens for the self-healing evaluation

In addition to self-healing studies, a suite of chemical, thermal and mechanical analyses was performed to elucidate structural interactions and property evolution within the composites. Fourier Transform Infrared Spectroscopy (FTIR, PerkinElmer Spectrum 100 IR, Tokyo, Japan) was used to identify functional groups and bond rearrangements, while Thermogravimetric Analysis (TGA, PerkinElmer TGA 8000, USA) provided insight into thermal degradation pathways and residue formation. Creep measurements were performed in tensile film mode using a tension film clamp, following a modified procedure adapted from (Xia et al. 2024). Both pristine and reprocessed vitrimer materials were tested to evaluate the effect of reprocessing on creep-recovery behaviour. The material processing and reprocessing procedure is illustrated in Figure 1(b). The vitrimer composite was fragmented into smaller pieces and subsequently compression moulded at 180 °C for 15 min to produce a homogeneous film. The creep testing setup is presented in Figure 1(c). A constant load of 20 g was applied, and strain was continuously recorded during the creep stage (1-5 min), followed by the recovery stage (5-10 min) after load removal. The strain, ε(t), was calculated using Eq. 1,

ε(t) = ΔL(t)/L₀       (1)

where ΔL(t) is the change in length at time t, and the creep-recovery behaviour was evaluated based on the time-dependent strain response.

Fig. 1. (b) Reprocessing of vitrimer composites; (c) Schematic of creep-recovery test

Dynamic Mechanical Analysis (DMA, TA Instruments Q800 DMA/SDTA e, New Castle, DE, USA) was subsequently employed to determine viscoelastic parameters, including the storage modulus and tan δ (damping factor), which were used to identify the glass-transition region. DMA was also used to examine the viscoelastic response of both pristine and reprocessed vitrimer samples. These analyses enabled a detailed evaluation of the vitrimer network’s thermal stability and temperature-dependent mechanical behaviour. The solvent resistance of the vitrimer composites also was evaluated by immersing the samples in different solvents, including water, toluene, methanol, 1,4-dioxane, ethyl acetate, chloroform, and acetone, at room temperature for 24 h. The swelling ratio and gel content were calculated to assess the solvent stability of the vitrimer network (Lee et al. 2025a).

RESULTS AND DISCUSSION

Effect of Activated Carbon and NFC Loading on Thermal Welding of the Vitrimer Composites

A class of thermoplastic like thermosets termed vitrimers has been developed with extensive applications, in which welding plays a central and fundamental role, effectively functioning as a form of self-healing through dynamic covalent bond exchange (Shi et al. 2023). Figure 2 presents the influence of activated carbon and NFC loading on the thermal welding behaviour of EPO-based vitrimer composites, which proceeds through interfacial welding, revealing how variations in filler loading modulate network reconfiguration during thermal activation. A vitrimer with 4 wt.% activated carbon without NFC was used as the control. It exhibited a visible interface after thermal treatment, indicating limited interfacial reconnection. Rigid particulate fillers such as carbon particles tend to restrict polymer chain mobility at the filler-matrix interface, leading to hindered chain interdiffusion and reduced interfacial entanglements, which in turn limit interface healing and weld strength in polymer nanocomposites (Bonardd et al. 2023).

Fig. 2. Surface morphology of welded vitrimer composites for the 4wt.% activated carbon control and NFC containing composites (3-5 wt.%) under 10× optical magnification (scale bar: 100 µm)

A systematic comparison of Figure 2 elucidates the manner in which activated carbon loading governed the interfacial morphology and welding behaviour of the vitrimer when the catalytic environment was held constant. At an activated carbon loading of 3 wt.%, as depicted in Figure 2(a), the welded interface retained a sharply defined fracture boundary, and the welding line remained distinctly visible. This persistence of the crack trace indicates that only limited interfacial wetting and structural reconstruction occurred during the thermal activation. Low loading of carbon-based fillers does not allow the formation of an effective mechanical support network and therefore contributes minimally to the overall structural rigidity of the matrix, as reported by (Syduzzaman et al. 2024). Thus, at low activated carbon loading (3 wt.%), the vitrimer network received only limited microstructural reinforcement, as the sparse distribution of rigid carbon particles was insufficient to constrain segmental mobility or provide meaningful dimensional stabilization.

When the activated carbon loading was increased to 4 wt.%, the morphology in Figure 2(d) shows a notable improvement in the coherence of the welded interface, with the welding line becoming less pronounced and the surface appearing smoother and more continuous. Although the interface remained visible, its reduced sharpness and the enhanced surface homogeneity suggest that the matrix was able to undergo partial but more orderly topological rearrangement under these conditions. This indicates that the intermediate filler loading provided a balanced combination of confinement and mobility, thereby allowing bond exchange reactions and segmental motion to proceed in a more coordinated manner along the welded interface. At this loading, the activated carbon particles began to form a more effective stress transfer microstructure, enhancing local stiffness and reducing the amplitude of strain gradients along the fracture plane. Activated carbon, which contains nanostructured and partially graphitic domains, operates as a low dimensional carbon filler; phonons propagate efficiently along the localised graphitic planes but are hindered at pore boundaries, resulting in anisotropic thermal conduction (Aigaje et al. 2023; Dai et al. 2019; Zhang et al. 2022). This directional heat transport behaviour further improves local thermal activation near the crack interface, promoting a more uniform temperature distribution and contributing to the coordinated realignment of the opposing crack faces during welding.

At the highest filler loading in this series, 5 wt.%, the morphology in Figure 2(g) became markedly irregular, with a discontinuous and roughened welded region indicative of pronounced deformation heterogeneity. The suppression of chain mobility at this elevated activated carbon loading aligns with molecular dynamics findings showing that rigid nanoscopic fillers substantially reduce polymer free volume and impose strong interfacial confinement, thereby slowing segmental diffusion and relaxation (Starr et al. 2000). The confined polymer matrix exhibited localised roughening, shear deformation traces, and embedded particulate residues. Such features may arise from non-uniform surface flow during thermal activation rather than from void formation or filler debonding. These morphological characteristics collectively reflect the diminished molecular mobility and heightened structural heterogeneity induced by the high filler loading, conditions that restrict dynamic bond interchange and impede the formation of a coherent and continuous healed interface.

Across the NFC loading series, welding behaviour of the vitrimer reflected the synergistic interplay between activated carbon and NFC. This combined mechanism becomes evident when comparing the 3, 4, and 5 wt.% NFC formulations, each demonstrating a different balance between catalytic activity, mobility restriction, and nanoscale reinforcement within the vitrimer network. The cellulose backbone is stabilised by intrachain hydrogen bonding between hydroxy groups and the ring oxygen atoms of neighbouring glucose units, a structural feature that gives the nanofibers a stiff and linear configuration capable of promoting effective stress transfer in polymer matrices (Collard and Blin 2014; Shaghaleh et al. 2018). A distinct improvement in welding performance arose at 4 wt.% NFC, where the catalytic and structural functions of NFC began to work in concert with the thermal-modulating role of activated carbon. As NFC loading increased, a more effective fibrillar network develops within the vitrimer matrix, improving crack-face compatibility. Such intermolecular interactions associated with nanocellulose can generate reversible hydrogen-bonded crosslinking in polymer composites, a mechanism that has been widely exploited in self-healing materials (Lamm et al. 2022). Activated carbon, present at moderate levels, improves heat uptake and contributes to a more uniform temperature distribution along the fracture interface, thereby supporting NFC mediated bond exchange without imposing excessive confinement. This balanced interaction is evident in Figure 2(e), where the crack line becomes faint or nearly indistinguishable from the surrounding matrix. The resulting morphology reflects a highly coordinated topological rearrangement in which NFC govern chemical and structural reconfiguration, while activated carbon facilitates thermal activation in a supportive manner.

At 5 wt.% NFC, the welding performance began to deteriorate as excessive nanofiber loading disrupted the stability of the vitrimer network. While the initial addition of filler can enhance thermal stability, further increases in NFC loading may lower the degradation temperature, suggesting that excessive filler disturbs the matrix structure. In addition, hydroxy groups on the NFC surface can form strong hydrogen bonds through hornification, which promotes irreversible agglomeration and reduces the ability of the nanofibers to disperse effectively within the polymer matrix (Beaumont et al. 2017). Achieving homogeneous NFC dispersion is widely recognised as essential for optimizing the mechanical and interfacial properties of nanocellulosic composites; however, NFC inherently tend to aggregate through hydrogen bonding and van der Waals interactions, making uniform distribution difficult to maintain (Chu et al. 2020). As a result, these agglomerates disrupt the structural continuity of the vitrimer network and produce heterogeneous surface flow, as observed in Figure 2(c), (f), and (i). The formation of such localised clusters creates spatially uneven regions within the vitrimer, where some domains experience excessive confinement while others lack sufficient catalytic activity, ultimately disturbing the coordinated bond exchange required for effective interfacial welding. Moreover, NFC clustering might reduce stress transfer efficiency and limits interfacial cohesion, which further undermines the ability of the vitrimer network to undergo smooth topological rearrangement (Jose et al. 2025). As a result, the morphology at 5 wt.% NFC displays incomplete crack closure and irregular surface reconstruction, reflecting a composition where excessive catalytic activation and nanoscale aggregation collectively impair the welding performance.

Effect of Activated Carbon and NFC Loading on the Thermal Stability of EPO-based Vitrimer Composites

Figure 3 shows the thermogravimetric behaviour of the EPO-based vitrimer composites containing different loadings of NFC and activated carbon. All formulations exhibited a two-step thermal degradation pattern, beginning with an initial mass-loss region around 300 to 350 °C, followed by a dominant degradation stage between 350 and 470 °C associated with the rupture of ester linkages and the aliphatic epoxy-derived backbone. This relatively high temperature for the first degradation event reflects the molecular architecture of EVO networks: their long aliphatic chains and ester functionalities require greater activation energy for scission, which delays the onset of decomposition. Malburet et al. (2020) reported that EVO monomers only begin to degrade above 250 °C, supporting the result that EVO-based resin possess intrinsically higher initial thermal stability than conventional petrochemical epoxy monomers. This interpretation aligns with a previous study on a petrochemical epoxy vitrimer, which exhibited three distinct degradation stages, including an early event between 250 to 350 °C attributed to pendant-group scission, followed by aromatic-backbone decomposition (Lee et al. 2025a). The absence of this early degradation stage in the EPO vitrimer and the upward shift in its primary decomposition region provide clear evidence that EPO composites can exhibit superior initial thermal stability. This advantage is limited to the onset region, as EPO-based networks continue to generate lower char residues. This behaviour arises from their flexible aliphatic chains and relatively low crosslink density, which limit the formation of thermally stable carbonaceous structures during pyrolysis (Jin and Park 2008).

Fig. 3. Effect of NFC and activated carbon loading on the thermal properties of the vitrimer composites

The highest char yields were obtained in the vitrimer composites containing elevated activated carbon loading combined with lower NFC loading, particularly the formulations with 5 wt.% activated carbon and 3 wt.% NFC and with 4 wt.% activated carbon and 3 wt.% NFC, which retained 6.30% and 6.21% residue respectively. Activated carbon is a carbon rich material, partially aromatised material that undergoes minimal mass loss during high temperature pyrolysis and therefore increases the residual mass of the composite (Chin et al. 2020). This trend was further reinforced by the behaviour of the PKS-derived activated carbon produced in this study, which exhibited exceptionally high thermal stability, retaining approximately 69% residue even at 1000 °C. This remarkable resistance to thermal degradation can be attributed to the presence of thermally robust oxygen-containing functional groups introduced during the activation process, including anhydrides, lactones, and epoxy-derived C–O–C linkages, which are known to withstand decomposition until considerably elevated temperatures and thereby enhance the char forming capability of bio-derived activated carbons (Lee et al. 2018). Collectively, these observations demonstrate that the high temperature stabilisation of the vitrimer composites is strongly governed by the amount of activated carbon in the formulation and by the intrinsic thermal robustness of the PKS-based activated carbon itself.

The effect of NFC addition on the thermal behaviour of the vitrimer composites is clearly evident from the TGA results. At a fixed activated carbon loading of 4 wt.%, the incorporation of NFC led to an approximately 1.9-fold increase in char residue compared with the composite without NFC, indicating a pronounced enhancement in condensed-phase stability. Moreover, NFC loading introduced a distinct influence to the vitrimer network, revealing a clear structure property relationship in the thermal behaviour of the composites. Lower NFC incorporation within the EPO-based vitrimer produced higher char residue, indicating that limited nanofiber content enhanced network cohesion and delayed thermal degradation. Increasing NFC loading introduced a significantly larger surface area and smaller particle dimensions, which generated a higher number of thermally labile end-chains and promoted early-stage decomposition. This trend corresponds to the behaviour described by Yildirim and Shaler (2017), where increased chain end availability in small cellulose particles enhances early thermal breakdown and contributes to elevated char at moderate loadings. However, at highest NFC loading (5 wt.%), the char residue decreased to approximately 3.4 to 4.8%, suggesting a shift in the dominant degradation mechanism. At this point, the intrinsic devolatilisation behaviour of NFC became more influential, where NFC released combustible fragments instead of contributing to char formation. Under dry, hot, and energetic conditions, cellulose combustion generates levoglucosan that rapidly decomposes into CH₄, H₂O, CH₂O, CH₃OH, CO₂, and HCOOH, with major gas evolution occurring between 250 and 400°C which is the pyrolysis region of cellulose, hemicellulose, and lignin (Turku et al. 2024). The release of these volatiles accelerates matrix degradation and reduces solid residue, leading to lower char values despite higher solid biomass content (Xu et al. 2022). Collectively, these observations reveal a new thermal interplay in NFC reinforced vitrimers, where moderate NFC loadings enhance char yield through end-chain decomposition, while excessive NFC promote volatile driven mass loss and suppresses final residue formation. This behaviour mirrors bioinspired thermal responses commonly seen in natural cellulose structures, where protective charring transitions to volatile release as temperature increases.

Effect of NFC and Activated Carbon Loadings on the Functional Group of Vitrimer Composites

Figure 4 presents the FTIR spectra of the EPO-based vitrimer composites. The strong absorbance band at 1736 cm⁻¹ appears consistently across all samples, confirming the formation of an ester-rich network through epoxy-anhydride curing. This ester carbonyl band is characteristic of fatty acid-derived oil structures (Silva et al. 2025) and is associated with the generation of new ester linkages during network development (Aliyeva et al. 2023). These structures constitute the labile dynamic bonds required for associative transesterification-based vitrimer behavior. This assignment is further supported by the pronounced C–O stretching band at 1179 cm⁻¹ (Jawad et al. 2016), together with additional C–O absorbances in the 1220 to 1280 cm⁻¹ region, which are characteristic of the C–O bonds in ester linkages within the cured network (Smith and Northrop 2014; Vardamides et al. 2006). Aside from that, the bands at 2916 cm⁻¹ and 2850 cm⁻¹ correspond to the asymmetric and symmetric –CH₂– stretching vibrations, respectively (Salih et al. 2015), while the absorption near 1470 cm⁻¹ is attributed to the bending (deformation) vibration of methyl groups (–CH₃–) in the aliphatic backbone (Gheje et al. 2025). Together, these bands reflect the aliphatic hydrocarbon backbone of the epoxidised triglyceride structure. Although EPO contains long fatty-acid segments, its triglyceride-based architecture remains relatively short and less entangled than conventional polymeric vitrimer backbones, highlighting the important role of fillers in enhancing network stiffness and structural integrity.

Across the NFC-containing vitrimer composites, a broad O–H stretching band is observed in the region of 3200 to 3600 cm⁻¹, arising from hydroxy groups in the cured epoxy-anhydride network together with contributions from cellulose surface –OH functionalities. A feature near 3836 cm⁻¹ becomes apparent after NFC incorporation and may be attributed to additional hydroxy environments associated with the cellulose filler (Wang et al. 2018). These hydroxy groups enhance filler-matrix cohesion through interfacial hydrogen bonding, contributing to mechanical reinforcement (Karoki et al. 2025). Nevertheless, overly high NFC loading can be detrimental. The formulation containing 4 wt.% NFC exhibited several distinguishable O–H stretching components in the higher wavenumber region, with features around 3525, 3564, and 3801 cm⁻¹, suggesting a balanced hydroxy interaction environment and effective interfacial organisation. In contrast, when the NFC loading was increased to 5 wt.%, these sharper hydroxy bands became less resolved or disappeared and the overall O–H envelope appeared more broadened, implying increased hydroxy clustering and partial filler aggregation at excessive loading. Such aggregation can locally stiffen the network and reduce chain mobility, consistent with the rougher healed interface observed for the highest NFC formulation (Figure 2(f)), thereby limiting efficient network reorganisation during self-healing.

These hydroxy functionalities not only participate in interfacial hydrogen bonding with the surrounding polymer matrix but also act as reactive sites within the dynamic covalent network. In epoxy-anhydride composites, curing proceeds through the ring opening reaction between epoxide groups and cyclic anhydrides, generating repeating units containing both ester linkages and alcohol groups through reaction pathway (a). The coexistence of these functionalities is essential for vitrimer behaviour. At elevated temperatures, the alcohol groups can react with neighbouring ester linkages through catalytic ester-alcohol transesterification, as represented by reaction pathway (b) (Denissen et al. 2016). In the present composite, citric acid acts as an acid catalyst that promotes this exchange process. Under catalytic conditions, the ester carbonyl becomes activated, enabling nucleophilic attack by a neighbouring hydroxy group through an alcoholysis pathway, which results in the formation of a new ester linkage and the release of another alcohol molecule (Su et al. 2024). Because bond formation precedes bond cleavage, the overall crosslink density of the network is maintained during the exchange. Consequently, this dynamic covalent mechanism allows continuous network rearrangement under thermal activation, providing the molecular basis for the welding behaviour observed in the EPO-based vitrimer composites.

Meanwhile, Figure 4 also compares vitrimer composites with varying activated carbon loadings, both in the absence and presence of NFC.

Fig. 4. FTIR spectra of vitrimer composites with varying activated carbon and NFC loadings: (a) AC4%/N0%, (b) AC3%/N5%, (c) AC3%/N4%, (d) AC3%/N3%, (e) AC4%/N5%, (f) AC4%/N4%, (g) AC4%/N3%, (h) AC5%/N5%, (i) AC5%/N4%, and (j) AC5%/N3%.

In the mid-IR region of the vitrimer formulation containing activated carbon only, the band at 1843 cm⁻¹ is assigned to the asymmetric C=O stretching vibration of the five member cyclic anhydride ring (Zhang et al. 2025). Additionally, bands at 2085 cm⁻¹, 2317 cm⁻¹, and 1569 cm⁻¹ are associated with activated carbon surface related species and graphitic aromatic (C=C) structures (Aribam et al. 2025; Cherik and Louhab 2017; Rajasekaran and Raghavan 2022). These activated carbon related features become less distinguishable upon the incorporation of NFC into the vitrimer composites, which may be attributed to masking by the stronger polymer and cellulose absorbance bands.

Effect of Formulation Variables on the Solvent Resistance of EPO-based Vitrimer Composites

In practical applications, vitrimer materials must exhibit strong solvent resistance during service while retaining the potential for controlled solvolysis under specific end-of-life conditions (Xia et al. 2024). Therefore, the solvent resistance and degradation behaviour of the palm oil derived epoxy vitrimer composites were evaluated based on swelling ratio, gel content, and visual observations after 24 h of solvent immersion, as presented in Table 2 and Figure 5, to assess network stability and recyclability. The swelling ratio reflects the extent of solvent uptake and network expansion, indicating network density and solvent resistance, while gel content represents the insoluble fraction, reflecting the degree of crosslinking and structural integrity of the network. As shown in Table 2, all formulations exhibited very low swelling values (<6%), confirming the formation of a chemically stable vitrimer network. In contrast, many synthetic epoxy composites, particularly DGEBA-based vitrimers with lower effective crosslink density or network defects, can exhibit swelling ratios exceeding 100% in aggressive organic solvents due to extensive chain plasticization and solvent penetration (Lee et al. 2025a). In the present composite, epoxy anhydride curing between EPO and HHMPA forms an ester-rich crosslinked network through ring opening reactions that generate ester linkages and hydroxy groups, contributing to the observed solvent resistance (Khairkkar et al. 2025; Paramarta et al. 2016). This behaviour is further supported by the molecular structure of vegetable oil-based epoxies, where monounsaturated fatty acid segments and long aliphatic chains promote flexible yet stable network architectures (Li et al. 2026) while reducing solvent affinity, resulting in improved chemical stability compared with conventional petroleum-based epoxy composites (Chong et al. 2021).

Furthermore, the presence of NFC and activated carbon introduces additional physical constraints that reduce free volume and create tortuous diffusion pathways, effectively restricting solvent penetration. As a result, solvent uptake is confined to minor network expansion rather than large scale swelling, explaining why the swelling ratios remain well below 6% for the EPO-based vitrimer, in contrast to the excessive swelling commonly observed in conventional synthetic epoxy materials. In particular, water (H₂O) induces negligible swelling across all samples, which is further confirmed by the absence of dimensional or colour changes in the immersed specimens (refer to Figure 5). This behaviour indicates that water interaction is largely confined to surface absorption and is insensitive to variations in activated carbon and NFC loadings, reflecting the limited affinity of the vitrimer network toward water. Slightly higher swelling is observed in methanol and acetone, consistent with their small molecular size and moderate polarity, which allow partial diffusion into the polymer matrix without causing significant network disruption (Momin et al. 2025).

Table 2. Swelling Ratio and Gel Content of Vitrimer Composites at Different Activated Carbon and NFC Loadings in Various Solvents

Fig. 5. Digital photograph of EPO-based vitrimer soaked in different solvents

In contrast, chloroform and 1,4-dioxane induce noticeably higher swelling, clearly highlighting the influence of formulation on solvent resistance. This trend is evident both quantitatively in the swelling rate data and visually through increased sample softening and solvent coloration, as shown in Table 2. The stronger swelling response in these solvents arises from their higher affinity toward epoxy-based networks, making them effective probes for assessing network integrity. Importantly, increasing activated carbon loading leads to a consistent reduction in swelling in chloroform and 1,4-dioxane, demonstrating that activated carbon enhances solvent resistance by physically restricting polymer chain mobility and increasing the tortuosity of solvent diffusion pathways. Similarly, formulations containing higher NFC loading (5 wt.%) generally exhibit lower swelling than those with 3 to 4 wt.% NFC, reflecting improved network compactness and more effective chain immobilization. The slightly higher swelling observed at intermediate NFC loading can be attributed to localised heterogeneity or dispersion effects, as also suggested by subtle visual differences among the samples.

The gel content results presented in Table 2 further highlight the dominant role of NFC loading in governing the solvent resistance and network integrity of the palm oil derived epoxy vitrimer composites. While all samples remain insoluble in the tested solvents, clear NFC dependent variations in gel content were observed. Formulations containing higher NFC loading generally exhibited higher gel content across most solvents, indicating a greater fraction of polymer chains were effectively incorporated into the EPO-based vitrimer network. This behaviour reflects the reinforcing role of NFC, which promotes physical entanglement and interfacial interactions with the epoxy matrix, thereby restricting chain mobility and reducing the extractable sol fraction. In aggressive solvents such as chloroform and 1,4-dioxane, samples with lower NFC loading showed a more pronounced reduction in gel content, suggesting that a less reinforced network allows partial extraction of loosely bound chains. In contrast, increasing NFC loading mitigates this effect, maintaining higher gel content even under strong solvent exposure. This NFC-dependent trend correlates well with the swelling results in Table 2, where higher NFC loading corresponded to reduced swelling. The photographic evidence in Figure 5 further supports this interpretation: samples with higher NFC loading retained their shape and colour after solvent immersion, whereas those with lower NFC loading exhibited slight softening or edge deformation, particularly in chloroform. Overall, these results demonstrate that NFC played a critical role in enhancing network compactness, stabilizing the vitrimer structure, and improving solvent resistance in palm oil derived epoxy vitrimer composites.

Morphological Analysis of Vitrimer Composites

Based on the chemical resistance results discussed in the previous section, vitrimer composites reinforced with NFC and activated carbon exhibited notable stability under chemical exposure. To further elucidate the structural features responsible for this behaviour, morphological analysis was performed using FESEM to examine the surface and interfacial morphologies of the vitrimer composites, with particular attention to filler distribution and matrix continuity. The hybrid composite containing 4 wt.% NFC and 4 wt.% activated carbon was analysed (Figure 7), while a vitrimer containing only 4 wt.% activated carbon was examined as a reference sample (Figure 6). A comparison of these samples revealed clear differences in interfacial architecture between the activated carbon only composite and the hybrid NFC and activated carbon composites, highlighting the role of NFC in improving interfacial connectivity within the vitrimer matrix. These structural differences correlated with the welding-assisted self-healing behaviour observed in vitrimer composites. Dynamic covalent chemistries enable vitrimer segments to be reconnected through welding after curing via thermally activated bond exchange (Shi et al. 2023; Wu et al. 2023). In vitrimer composites, topological rearrangement at elevated temperatures allows previously cured materials to be rejoined under heat and pressure, effectively restoring interfacial continuity despite limited matrix flow (Masten-davies et al. 2025; Vashchuk and Kobzar 2022). This welding capability therefore contributes to the durability of vitrimer composites by enabling repeated interfacial repair.

The interface in the activated carbon only vitrimer (Figure 6(b)) was characterised by a distinct linear feature accompanied by interstitial regions, indicating limited continuity between adjoining vitrimer domains. This morphology reflects the influence of rigid particulate fillers on polymer chain interdiffusion and interfacial reconnection, processes that are required for effective welding or self-healing in polymer composites. The restricted interfacial continuity is further evident in Figure 6(c), where localised gaps appear between activated carbon particles and the surrounding vitrimer matrix, indicating incomplete polymer and filler contact and constrained local chain mobility during thermal activation (Dubey et al. 2020). The microstructural origin of this behavior is revealed in Fig. 6(d), which shows exposed activated carbon platelets with a lamellar, flake like morphology arising from weak interactions between overlapping carbon sheets. When incorporated into a rigid amorphous vitrimer network, these loosely stacked carbon structures disrupt uniform matrix penetration and dense polymer packing, generating regions of reduced network continuity (Kotan and Bayrakçeken 2022).

In contrast, the hybrid vitrimer composite exhibited a fundamentally different interfacial morphology. At the same magnification, Figure 7(b) shows no discernible interface line, indicating effective structural continuity between adjoining vitrimer regions at the microscopic scale. This absence of a visible boundary reflects enhanced interfacial connectivity associated with the presence of NFC, which promotes polymer chain mobility and network interpenetration during thermal activation. As polymer chains become mobile and interdiffuse across the interface, intimate contact between the vitrimer matrix and NFC are established, enabling the development of interfacial interactions (Ge et al. 2012). This process is evident in Figure 7(c), where NFC particles were distributed within the interfacial region and extended between activated carbon particles and the vitrimer matrix, occupying regions that appear as gaps in the activated carbon only vitrimer and promoting local network continuity. The resulting interfacial reorganization is further manifested in Figure 7(d), which reveals a compact, crumpled lamellar morphology dominated by activated carbon platelets that are folded and constrained within the vitrimer matrix. This morphology arises from thermally activated rearrangement of lamellar carbon sheets under matrix confinement, where the presence of NFC and the surrounding vitrimer network suppress platelet separation and pull-out, favoring folding and compaction instead. Such confinement maintains intimate matrix filler contact and prevents the formation of interfacial voids, allowing the interfacial region to behave as a consolidated, load-bearing zone. Consequently, polymer chain interdiffusion and dynamic covalent bond exchange can proceed across the interface without disruption, leading to the formation of an interpenetrating polymer network-like interfacial structure in which the vitrimer network and NFC associated polymer regions are topologically intertwined (Feng et al. 2025). This stabilised interfacial architecture provides a microstructural basis for the disappearance of a distinct interface line and supports effective welding assisted self-healing behavior in the hybrid vitrimer composite.

Fig. 6. Surface morphology of the welded vitrimer composite containing 4 wt% activated carbon.(a) Optical microscopy image of the welding line region formed at the interface between two vitrimer samples after thermal welding. (b) Scanning electron microscopy image of the welding surface at higher magnification. (c) Higher magnification view of the interfacial region between the vitrimer matrix and activated carbon particles along the welding line. (d) Microstructural features of the vitrimer matrix and activated carbon in the welded region.