# Optimizing Epoxy Vitrimer Composites With Sustainable Cellulose Nanofibril Reinforcements: Enhancing Thermal Dimensional Stability While Preserving Dynamic Covalent Behavior

Isis Castro-cabrera, Armand Fahs, Sophie Berlioz, Pascal Carriere

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## To cite this version:

Isis Castro-cabrera, Armand Fahs, Sophie Berlioz, Pascal Carriere. Optimizing Epoxy Vitrimer Composites With Sustainable Cellulose Nanofibril Reinforcements: Enhancing Thermal Dimensional Stability While Preserving Dynamic Covalent Behavior. Polymer Composites, 2025, 10.1002/pc.70135. hal-05389173

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Submitted on 29 Nov 2025

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# Optimizing Epoxy Vitrimer Composites with Sustainable Cellulose Nanofibril Reinforcements: Enhancing Thermal Dimensional Stability while Preserving Dynamic Covalent Behavior

Isis Castro Cabrera<sup>a,1</sup>, Armand Fahs<sup>a</sup>, Sophie Berlioz<sup>a</sup>, Pascal Carriere<sup>a</sup>

<sup>a</sup> Université de Toulon, Laboratoire MAPIEM, Toulon, France.

## Acknowledgements

The authors gratefully thank The Mexican Council of Science and Technology (CONACyT) for their financial support this work.

Corresponding author: Pascal Carriere, [pascal.carriere@univ-tln.fr](mailto:pascal.carriere@univ-tln.fr)

<footnote n="1">¹ Present address : UMR CNRS 5223 Ingénierie des Matériaux Polymères, Université de Lyon, INSA Lyon, 20, Avenue Albert Einstein, 69621, Villeurbanne, France</footnote>
23 Abstract

Vitrimers are networks with dynamic covalent bonds that are promising for recycling composite materials, which often suffer from significant creep. This study explores the integration of cellulose nanofibrils (CNF), without any functionalization, into an epoxy vitrimer matrix to enhance mechanical performance and dimensional thermal stability, while preserving the dynamic covalent network properties. Using an optimized infiltration method of CNF template, CNF was effectively impregnated within the epoxy vitrimer matrix, as demonstrated by SEM and AFM morphological characterizations.

A slight decrease in the composite's glass transition temperature is attributed to a reduced crosslinking density at the CNF-resin interface. A 10 wt% CNF resulted in a 77% increase in the composite's storage modulus compared to the neat resin. This CNF-interpenetrated network within the crosslinked vitrimers leads to a significant reduction in the composite's thermal expansion, which is crucial for applications requiring good dimensional thermal stability. Stress relaxation studies indicate a slight increase in the activation energy of topological rearrangement induced by the CNF, while maintaining complete relaxation, thus enabling the reshaping or recycling of composites.

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38 Keywords

Cellulose nanofibrils, vitrimer, epoxy composite, dimensional thermal stability

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42 Highlights

43 • Optimized CNF template method ensured proper impregnation of the epoxy resin

44 • CNF into the vitrimer slightly modifies the Ea of topological rearrangement

45 • CNF-reinforced vitrimer composites exhibited good thermomechanical stability
46 1. Introduction

47 Epoxy resins are widely used in high-performance coatings, structural adhesives and fiber-reinforced
48 composites due to their excellent mechanical properties, dimensional stability and creep resistance [1].

49 However, the major drawback lies in the difficulty of recycling these networks, which are permanently
50 crosslinked, using low-energy physical processes or environmentally friendly and fast chemical
51 methods [2, 3]. To address this challenge and maintain high-value applications for the recycled
52 polymer network, covalent adaptable networks (CANs) have been largely studied, to enable
53 reprocessing by the introduction of dynamic bonds into the molecular cross-linked structure [4-7].

54 Among these, vitrimers -first introduced by Liebler et al. [8]- feature a constant number of covalent
55 bonds throughout the exchange reactions under external stimulus, such as temperature, allowing
56 reshaping with maintaining crosslink density. Vitrimers are materials exhibiting a truly associative
57 exchange mechanism, while vitrimer-like materials follow a dissociative exchange mechanism, where
58 linkages reform faster than they break. Among them, different chemistries including pioneering work
59 on transesterification [8], imine chemistry [9,10], disulfide exchange [11,12], transamination [13-15],
60 siloxane chemistries [16], dioxaborolane metathesis [17] are classified as vitrimers or vitrimer-like
61 networks. These chemistries have demonstrated promising reprocessability after several recycling
62 cycles while maintaining a constant crosslinking density [18]. Despite these promising properties,
63 vitrimers are susceptible to creep under common service temperatures [19-23]. Strategies to mitigate
64 this include the incorporation of permanent crosslinks into the vitrimer networks [24]. Torkelson et al.
65 [25] demonstrated that, to limit creep while maintaining reprocessability, the fraction of permanent
66 crosslinks must be insufficient to form a percolating permanent network.

67 Another approach to mitigating creep consists in controlling the vitrimer's flow behavior
68 through catalytic control of the exchange reactions. The catalyst must be carefully designed to
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accelerate exchange reactions at processing temperatures while preventing them within the operational
temperature range [21].

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This study explores an alternative approach by reinforcing the vitrimer matrix with an
interpenetrating network of cellulose nanofibrils (CNF) to enhance dimensional stability. This
reinforcement strategy aims to exceed the CNF percolation threshold without compromising vitrimer
properties. CNF possess desirable intrinsic properties like a high Young's modulus (6 GPa in the
amorphous state [26] and 150 GPa in its crystalline form [27]), and high aspect ratio (width of 20-
100 nm and length between 500 and 2000 nm) [28,29], leading to a significant enhancement in the
strength and elastic modulus in composite materials [30,31,32]. From a sustainability perspective, CNF
offer advantages such as lightweight nature, biodegradability, recyclability, renewability, low density,
and cost-effectiveness [33].

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Although numerous polymer matrix composites reinforced with CNF have shown promising
results, including acrylic resins [34], polybutadiene [35], polypropylene [36], polyurethane [37],
polylactic acid [38], melamine-formaldehyde resins [39], and epoxy resins [40], incorporating cellulose
nanofibrils into these polymers poses challenges. Various methods have been developed to address
these challenges in epoxy matrices, including cellulose aerogel impregnation [41], cellulose surface
modification [42], high-speed mechanical stirring [43], and pre-impregnated cellulose suspensions [35,
44]. Among these, the prepreg method adapted by Berglund et al. [35] stands out as particularly promising.
The technique promotes an even distribution of CNF within the matrix, allowing the resin to diffuse
into the cellulose structure. During curing, the fibers are effectively embedded in the epoxy polymer
matrix, resolving common issues such as polarity mismatch between polymer and CNF [45] as well as
the usual need for CNF surface functionalization [46, 47]. Furthermore, this approach helps preserve
the mechanical performance of CNF in both the glassy and rubbery states of the material, while also
enabling the development of cost-effective and eco-friendly composites.

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Research on vitrimer composites reinforced with cellulose has primarily focused on systems where the cellulose undergoes chemical modification, enabling its effective integration into polymer matrices, and participating in the exchange reaction [48-50]. Walther et al. [51] explored vitrimer composites reinforced with TEMPO-oxidated cellulose to facilitate its incorporation in PDMS-diepoxy and transesterification exchange reaction. Jia et al. [52] modified cellulose into dialdehyde cellulose, reducing its polarity and rigidity to enhance chain mobility and enable Schiff-base reactions with diamines. Yet, the effective incorporation of cellulose in a vitrimer system without chemical modification, enhancing mechanical properties without interfering with the dynamic exchange reaction, remains an area of interest. An excellent alternative for producing vitrimer composites is through the disulfide exchange chemistry, using the diglycidyl ether of bisphenol A (DGEBA) and 4-aminophenyl disulfide (4-AFD).

The DGEBA and 4-AFD system has attracted attention due to its high-temperature resistance and the potential applications of epoxy/reinforcement fiber composites documented in packaging, the automotive industry, electronics, and the aerospace industry [53-58]. In the field of high-performance vitrimer composites, Odriozola et al. [59] explored this epoxy-vitrimer system by incorporating carbon reinforcement fibers, demonstrating its damage repair capabilities. The composites dissolved in thiol-containing solutions and were reshaped through thermoforming making them strong candidates for closed-loop recycling. Hu et al. [60] expanded composite research by incorporating silica-based reinforcements into epoxy disulfide networks, improving mechanical properties while restricting chain mobility due to silica functionalization.

This work focuses on developing cellulose-reinforced vitrimer composites by integrating CNFs into DGEBA-4-AFD vitrimer network. The main objectives are i) to adapt and optimize the impregnation method, ensuring strong fiber-matrix interfaces without fiber functionalization, while preserving a CNF network from the CNF template, and ii) to evaluate the mechanical strength, thermomechanical
properties of the resulting composites with a particular focus on enhancing thermal dimensional
stability. Composites were characterized at nano and microscale by AFM and SEM microscopies, and
their thermomechanical behavior were studied through the relaxation spectra of both the vitrimer alone
and the CNF-vitrimer composites. The ultimate goal of this work is to propose a CNF-reinforced
vitrimer composite that combines a high Tg with increasing especially thermal dimensional stability,
while preserving the intrinsic vitrimer characteristics of the composite.

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## 2. Materials and methods

### 2.1 Materials

The vitrimer matrix was synthesized using diglycidyl ether of bisphenol A (DGEBA) Epikote 828, acquired from Delta Resins Ltd, with a molecular weight of 365.33 g.mol⁻¹ and 4-aminophenyl disulfide (4-AFD) supplied by TCI, in a stoichiometric amine to epoxide ratio of 1:1. 4,4'-diaminodiphenylmethane (DDM) was obtained from Sigma-Aldrich. Cellulose Nanofibrils (CNF) were sourced as a 1% (w/w) aqueous suspension from INOFIB (Grenoble-France) with a fiber diameter estimated by SEM microscopy to range from 20 to 70 nm (see Fig. S1 in supplementary information).

### 2.2 Characterization Techniques

#### 2.2.1 Thermal analysis

Thermogravimetric analysis (TGA) was performed using a Q600 TA Instruments with a sample mass of approximately 10 mg in alumina pans. The analysis was conducted under an N₂ flow of 100 mL.min⁻¹, with a temperature ramp from ambient to 800 °C. Differential Scanning Calorimetry (DSC) was performed using a Q100 TA Instruments to determine the glass transition temperature (Tg) of CNF-
139 epoxy vitrimer composite and neat epoxy resin samples. Samples weighing 7-10 mg were placed in an
140 aluminum pan and subjected to a two-heating cycle from 30 °C to 220 °C at a rate of 10 °C.min⁻¹ under
141 an N₂ flow of 50 mL.min⁻¹. The Tg was defined as the inflection point observed on the second heating
142 trace. This procedure was repeated twice for each sample to ensure repeatability.

143 2.2.2 Mechanical analysis

144 Mechanical testing of epoxy resins and CNF composites were performed using a INSTRON 5982 with
145 Extensometer controlled by Bluehill 3 software. Tensile strength measurements were carried out using
146 rectangular samples 150 mm x 10 mm x 0,8 mm at an elongation rate of 2 mm.min⁻¹.

147 2.2.3 Thermomechanical analysis

148 Dynamic Mechanical Analysis (DMA) was performed using a Q800 TA Instruments in single
149 cantilever mode. The experiments were conducted at a heating rate of 3 °C.min⁻¹ at a frequency of 1
150 Hz and a displacement amplitude of 20 µm. The dimensions of the composite and neat epoxy samples
151 were 20 × 12 × 1 mm³, while the thickness of the pristine CNF template was 0.2 mm.

152 Stress relaxation tests and dilatometry experiments were performed with the same instrument in tensile
153 mode with sample dimensions of 9 x 7 x 0.7 mm³. During the stress relaxation test, the sample was
154 first equilibrated at the target temperature for 30 min, followed by an instantaneous 0.1% strain applied
155 within 2 s. Dilatometry experiments were carried out at 3 °C/min from 50 °C to 220 °C at a controlled
156 force of 0.010 N.

157 2.2.4 Morphology and nanomechanical characterizations

158 The morphology of CNF samples and CNF-epoxy vitrimer composite was examined using a Field
159 Emission Scanning Electron Microscope FE-SEM (Zeiss Supra 40 VP) equipped with a Gemini
160 Column. To expose the transversal morphology of the epoxy composite, the samples were prepared via
161 cryofracture after immersion in liquid nitrogen. The fractured surfaces were then coated with a
162 conductive carbon layer for SEM imaging. Imaging was performed in secondary electron detection
163 mode, using a field emission gun operated at 15 kV.

164 Nanoscale characterization of epoxy and composite samples was conducted using an Atomic Force
165 Microscope (AFM, Bruker Multimode 8) in air at ambient conditions using PeakForce Quantitative
166 Nano-Mechanics (PF-QNM) mode. This mode was used to acquire topography, mechanical, and
167 adhesion maps. Cross-sections of the neat epoxy and composites were prepared by successively
168 polishing with diamond and alumina oxide suspensions down to a final grain size of 0.04 µm. The
169 surfaces were then cleaned with water and dried before measurement.

170 The silicon probe RTESPA-300 was selected based on the AFM manufacturer's recommendation. The
171 spring constant of the cantilever was measured using the Sader method [61]. The AFM probe was
172 calibrated on a polystyrene reference sample (2.8 GPa), and the surface elastic modulus was calculated
173 using the Derjaguin–Muller–Toporov (DMT) contact mechanic's model [62]. AFM images were
174 captured from three different regions on the surface with a resolution of 256 pixels and a scan rate of
175 0.8 Hz
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# 3. Results and discussion

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## 3.1 Composite impregnation method and curing step

178 The dispersion of CNF in acetone, followed by direct mixing with epoxy resin and curing
179 process, often results in heterogeneous samples with significant voids [63]. To mitigate these issues, a
180 two-step process, inspired by the work of Berglund et al., was implemented.
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182 The first step consists of preparing a CNF template from an aqueous suspension. The CNF was
183 filtered through a polyamide membrane with a pore diameter of 1 µm, resulting in a CNF template
184 comprising 10 wt% of CNF and 90 wt% of water. In our case, a higher concentration of CNF than that
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reported by Berglund et al. led to poor resin impregnation. To replace the water with acetone, the template underwent sequential solvent exchange steps, each lasting 4 h. A total of four to five acetone exchanges were performed to ensure the complete removal of water.

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The second step involves the diffusion of epoxy resin into the CNF template. For the final composite, a composition of a 10 wt% CNF and 90 wt% epoxy-vitrimer was established. The impregnation process was carried out in an acetone solution by mixing the epoxy prepolymer and the curing agent. The optimal solvent-to-resin ratio of acetone was set at 70 wt% and 30% of epoxy resin. Indeed, different CNF concentrations and solvent-to-resin ratios were explored, but none were successful as reported in the work of Berglund et al. with DGEBA and Jeffamine D-400 polyetheramine. The templates immersed in the solution were cast in a silicone mold and placed in a desiccator. This step promotes a saturated acetone atmosphere, allowing a complete impregnation of the template while acetone slowly evaporates overnight until the mass loss is complete. Fig. 1 illustrates the procedure to produce CNF templates, and their immersion in the solvent-to-resin mixture to elaborate the CNF-epoxy vitrimer composites. Fig. S2 shows an example of the resin impregnation process and the visual changes during the drying phase.

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<caption>Fig. 1. Schema of the elaboration of the CNF-epoxy vitrimer composite.</caption>

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The impregnated CNF templates in epoxy were subjected to a multi-stage curing process: 60 °C for 1h, 80 °C for 1 h, 130 °C for 2 h, 150 °C for 2 h, and post-cured under vacuum at 180 °C for 1 h. The neat epoxy resin system was also subjected to the same curing protocol. This curing process is
optimized (Fig. S3) to achieve full conversion of epoxy and amine whilst avoiding thermal degradation to reach the highest Tg observed in the literature [60, 64].

## 3.2 Morphological and nanomechanical characterization

The interaction between the CNF and the epoxy resin is critical to the composite's performance. Imaging techniques were applied to the cross-sectional microstructure, enabling the assessment of the microstructural features and structural integrity of the filler within the matrix.

Fig. 2 displays the SEM cross-section images of the CNF-reinforced epoxy composite at different magnifications. It also includes an SEM micrograph of the CNF template before impregnation with the epoxy resin. At low magnification (Fig. 2a and b), the morphology of the CNF-epoxy composite appears predominantly homogeneous, suggesting an overall uniform distribution of CNF within the epoxy matrix. However, at higher magnification (Fig. 2c), fibril aggregates with entangled structures become evident. The branch-like fibril aggregates are likely formed by flocculation during the filtration step in the preparation of the CNF template. This observation is consistent with the micrograph of the CNF template shown in fig. 2d.

The branch-like morphology of aggregates suggests that the resin impregnation may have been incomplete during the process. As a result, these areas might exhibit weak interactions with the resin, leading to poor interfacial adhesion between the resin and the CNF fillers. This could potentially reduce the efficiency of stress transfer at the interface. Nonetheless, the presence of these aggregates is limited, resulting in a moderate impact on the composite overall mechanical properties and performance. This underlines the importance of the sample preparation in achieving both effective filler dispersion and maintaining the structural integrity of the CNF template.
<figure><img src="image_4.png" /><figcaption>Fig. 2. SEM cross-section images of the CNF-reinforced epoxy composite (a, b, c) at different scales.
SEM image of the CNF template (d) before impregnation into the epoxy resin.</figcaption></figure>

While the SEM analysis provided valuable insights regarding the distribution and morphology of CNF fillers at the microscale, it is insufficient for fully characterizing the composite material at the nanoscale. To address this limitation, AFM analysis was performed. Fig. 3 presents AFM images of both the epoxy resin and the CNF-reinforced epoxy composite, captured at a 5 µm scale using PeakForce QNM mode. This mode enables the simultaneous acquisition of high-resolution topographical, mechanical, and adhesion maps.

The topographical structure of both the resin and the composite (Fig. 3a and d) reveals granular features with nanometric roughness of approximately 10 nm, resulting from the super-polishing protocol applied during AFM sample preparation. The mechanical mapping of the resin (Fig. 3b) shows overall homogeneity, with a mean DMT modulus of 2.4 ± 0.1 GPa. Similarly, the adhesion mapping (Fig. 3c) indicates uniform adhesion forces of 14 ± 1 nN between the AFM tip and the epoxy surface.
<figure><img src="image_5.png" /><figcaption>Fig. 3. AFM topographic images (a, d) and corresponding mechanical (b, e) and adhesion maps (c, f) of the epoxy resin (DGEBA-4-AFD) and the CNF-epoxy composite (DGEBA-4-AFD-CNF).</figcaption></figure>

In contrast, the composite sample exhibits an heterogeneous mechanical mapping (Fig. 3e), with regions of high DMT modulus (circled in red) corresponding to the CNF fillers observed in SEM images (Fig. 2). The CNF-rich areas display modulus values around 8 GPa, while the surrounding matrix-dominated regions align with the neat resin, showing a DMT modulus of 2.5 GPa. Concerning the composite adhesion mapping (Fig. 3f), significant differences were observed. Notably, the adhesion force increased from 14 nN in the resin regions to approximately 50 nN in the CNF-rich areas. This increase in adhesion at the locale scale underlines the highly hydrophilic character of the CNF, which
influences the composite's hygroscopic behavior. These findings suggest that further optimization such as, surface treatment may be required to balance adhesion and performance depending on the target application. Nevertheless, the incorporation of CNF into epoxy resin without any chemical functionalization yields promising results. By incorporating structural insights from the micro to nanoscale, it was demonstrated that CNFs provide localized reinforcement and play a crucial role in modulating the performance of the composites.

## 3.3 Mechanical properties

The effect of CNF fibrils was tested on the epoxy vitrimer resin, and the resulting mechanical properties from tensile test were summarized in Table 1.

<table><caption>Table 1. Mechanical properties of the neat epoxy resin and the CNF-epoxy composite</caption><thead><tr><th>System</th><th>Elastic modulus (GPa)</th><th>Tensile strength (MPa)</th><th>Strain at break (%)</th></tr></thead><tbody><tr><td>DGEBA-4-AFD</td><td>2.2 ± 0.2</td><td>56 ± 3</td><td>3.3 ± 0.3</td></tr><tr><td>DGEBA-4-AFD-CNF</td><td>3.3 ± 0.6</td><td>67 ± 13</td><td>2.4 ± 0.2</td></tr></tbody></table>

The addition of 10 % wt. CNF into the DGEBA-4-AFD epoxy vitrimer significantly increases the Young modulus (E), from 2.2 ± 0.2 GPa to 3.2 ± 0.5 GPa. This enhancement confirms the reinforcing role of CNFs for the epoxy resin, which can be attributed to their high intrinsic stiffness and good interaction with the epoxy matrix.

An increase of the tensile strength (σ) of the vitrimer also shows a considerable increase, from 56 ± 3 MPa for the neat resin to 67 ± 13 MPa for the CNF-reinforced composite. This result can be explained by the efficient stress transfer from the epoxy matrix to the rigid CNF fibrils, despite the absence of functionalization of the CNF fibres. This good interfacial adhesion with the epoxy matrix can be explained by interactions of OH functional groups of CNF and the matrix during the post-reticulation step. As expected for filler reinforced polymer composites, a reduction of strain at break (ε) was
observed for the CNF-reinforced composite, from 3.3 ± 0.3 for the epoxy resin to 2.4 ± 0.2 for the composite. This result indicates that the CNF-epoxy composite becomes more brittle, and presents a limitation of plastic deformation induced by the presence of the filler. Higher E and σ but lower ε compared to neat epoxy vitrimer matrix are in agreement with the principle of reinforced polymer composites by fillers.

## 3.4 Thermal and thermomechanical properties

The thermal and thermomechanical behaviors of the CNF-epoxy vitrimer composite were thoroughly studied and compared with those of the epoxy vitrimer network (DGEBA-4-AFD) at the same stoichiometric ratio between the DGEBA and the 4-AFD hardener. The thermal stability, assessed via TGA (Fig. 5), revealed no significant change in degradation temperature after incorporating CNF into the polymer matrix. The addition of CNF does not adversely affect the thermal stability of the composite.

<figure><img src="image_6.png" /><figcaption>Fig. 5. TGA curves of epoxy vitrimer (DGEBA-4-AFD in black), CNF-epoxy vitrimer composite (blue), and pristine CNF (green).</figcaption></figure>

However, the glass transition temperature Tg of the composite (162 °C), determined by the DSC (Fig. 6), displays a decrease of 5 °C compared to the neat polymer matrix (167 °C). This reduction in Tg may be attributed to a secondary esterification reaction between the hydroxyl groups of cellulose and the epoxide ring of the DGEBA, which likely reduces the crosslink density at the interface with
cellulose. Similar behavior has been observed by Berglund *et al.* [35] and Lin *et al.* [50] in their epoxy-
cellulose resin composites prepared at a stoichiometric ratio. Berglund's work [35] evidenced that the
Tg of epoxy-cellulose resin composites is significantly influenced by the cellulose content in the
system. In the present work, the decrease in Tg is relatively modest compared to systems with higher
cellulose content.

<figure><img src="image_7.png" /><figcaption>Fig. 6. DSC curves showing the Tg of epoxy vitrimer (black) and the CNF-epoxy vitrimer composite (blue).</figcaption></figure>

Following the thermal analysis, which provided information on the thermal stability and the main
transition temperature, the thermomechanical analysis allows us to evaluate the mechanical
performance across temperature ranges. By examining the storage modulus and damping factor, this
test highlights the effect of CNF on the stiffness, viscoelastic properties (Fig. S4), and crosslinking
homogeneity of the vitrimer matrix. Table 2 summarizes the thermal and thermomechanical properties
of both the vitrimer system and its composite.

**Fig. 7a** shows the storage modulus (G') of both the epoxy vitrimer and the CNF-reinforced epoxy
composite. Incorporating CNF into the resin significantly enhances the storage modulus in the glassy
state. At 30 °C, the composite displays a G' increase of approximately 77% with only 10% (w/w) CNF
addition, demonstrating the reinforcing effectiveness of cellulose nanofibrils.

These results are in line with the reinforcing effects of cellulose nanofibrils measured by tensile tests.
309

This result is consistent with other studies that report similar improvements in composites using natural fibers
in epoxy resins [65-67].

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<figure><img src="image_8.png" /></figure>

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<caption>Fig. 7. Storage Modulus (G') and damping factor (Tan δ) as a function of the temperature of the epoxy vitrimer and CNF-epoxy vitrimer composite systems.</caption>

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However, the storage modulus is lower than expected from a mixing law calculation between the CNF and the epoxy vitrimer. Indeed, the storage modulus of CNF fibrils reported in the literature ranges between 30 and 150 GPa [68,69], depending on the source and processing method. Applying the mixing law, the theoretical storage modulus of the epoxy vitrimer composite with 10 wt% of fibrils should range between 5 and 17 GPa. More precisely, using Lavengood and Gottler's [70] averaging technique, which suggest multiplying by 3/8, the estimated range is 2 to 6 GPa, not to mention the chemical interactions between the resin and the available functional groups of the nanofibrils [71]. In particular, etherification between the epoxy and alcohol groups is limited, resulting in a less dense crosslinked network near the CNFs. This hypothesis aligns with the slight reduction in Tg observed for the epoxy system.

Despite the limited covalent bonding, CNF effectively reinforces the matrix by restricting the mobility of the polymer chains. These segmental restrictions induce a significant rise in storage modulus at the glassy state [72,73]. Furthermore, the composite displayed an increase in modulus at the rubbery plateau, suggesting the presence of an additional stable network generated by the nanofibrils. The CNF
328 interacts favorably with the matrix even without chemical functionalization.

329 Concerning the damping factor, Tan δ, Fig. 7b illustrates the behavior of the epoxy vitrimer system
330 both with and without CNF. The broader Tan δ peak in the composite indicates a more heterogeneous
crosslinking network, characterized by regions of varying mobility, particularly near the CNF interface.

331 The slight reduction in the temperature of Tan δ further suggests the presence of less dense crosslinking
332 around the nanofibrils. Remarkably, the composite with impregnated CNF showed enhanced
333 mechanical properties, while maintaining an elevated Tg. These results indicate the effectiveness of
334 CNF reinforcement even without chemical functionalization, and present a promising approach for the
335 development of robust and efficient composites.
336

<table><caption>Table 2. Thermal and thermomechanical properties of epoxy vitrimer, pristine CNF, and CNF-epoxy vitrimer composite.</caption><thead><tr><th>System</th><th>Tg(°C)</th><th>Degradation temperature (°C)</th><th>G' @ 30°C (GPa)</th><th>G' @ Tg + 30°C (MPa)</th><th>Tα (°C)</th></tr></thead><tbody><tr><td>DGEBA-4-AFD</td><td>167 ± 1</td><td>359 ± 1</td><td>2.7 ± 0.1</td><td>268 ± 98</td><td>171 ± 3</td></tr><tr><td>Pristine CNF</td><td>-------</td><td>351 ± 3</td><td>8.1 ± 0.1</td><td>735 ± 40</td><td>------</td></tr><tr><td>DGEBA-4-AFD-CNF</td><td>162 ± 2</td><td>354 ± 1</td><td>4.6 ± 0.1</td><td>313 ± 52</td><td>169 ± 1</td></tr></tbody></table>

339 3.5 Vitrimer composite properties

340 Thermal and thermomechanical analyses highlight the reinforcing effect of CNF on the glass transition
341 temperature and viscoelastic properties. However, these characterizations do not demonstrate the
342 vitrimer behavior. Unlike classical crosslinked materials, vitrimers are characterized by the full
343 relaxation of their network, which is evidenced through stress relaxation. This analysis allows an
344 understanding of the material's response to deformation over time, a critical factor for evaluating the
345 potential recyclability of vitrimers. The following discussion aims to illuminate the interplay between
346 the dynamic disulfide network and the physical constraints imposed by the CNF on the vitrimer
347 behavior.
Fig. 8 shows the normalized stress relaxation curves of the epoxy resin and the CNF-epoxy vitrimer composite at temperatures above Tg. Both systems exhibit complete stress relaxation and flow, demonstrating the dynamic vitrimer behavior enabled by the reversible disulfide bonds within the network. Notably, the incorporation of CNFs introduces valuable features such as enhanced composite reprocessing and recycling capabilities.

<figure><img src="image_9.png" /><figcaption>Fig. 8. Normalized stress relaxation behavior of a) DGEBA-4-AFD and b) DGEBA-4-AFD-CNF</figcaption></figure>

$$ \frac{G(t)}{G_o} = e^{-\frac{t}{\lambda}} \quad (1) $$

The relaxation times are determined from Maxwell's model, as given in equation (1), at the maximum of the relaxation, i.e., when $\lambda = t$, corresponding to a value of $e^{-1}$ (approximately 0.37). The presence of cellulose nanofibrils notably impacts the relaxation behavior of the system. For instance, at 180 °C the relaxation time exceeds 145 min, compared to 30 min for the neat matrix. Similar behavior was reported by Wang et al. [60] when adding nano-silica to their vitrimer system. The extended relaxation times suggest that CNF restricts the matrix diffusion and topological rearrangement induced by dynamic covalent bonds. This delays the relaxation behavior without blocking the complete flow of the composite. This phenomenon is likely due to the hindered interaction between disulfide bonds in the network restricted by the CNF, which reduces their contact probability.
Fig. 9 shows the relaxation time as a function of temperature following Arrhenius' law, as expressed in equation (2). For the neat dynamic disulfide network, the activation energy (Ea) is calculated to be 96 kJ/mol, while for the CNF-epoxy vitrimer composite the Ea slightly increased to 112 kJ/mol. These results suggest that the presence of CNF does not disrupt the chemical exchange in the composite. Instead, the CNF fillers limit the interactions between the dynamic bonds, allowing the composite to reorganize and flow. Therefore, the relaxation time increases due to the steric and topological constraints induced by the cellulose nanofibrils.

$$ \tau(T) = \tau_0 e^{\left(\frac{E_a}{RT}\right)} \qquad (2) $$

<figure><img src="image_10.png" /><figcaption>Fig. 9. Arrhenian fitting of the Angell plot of the relaxation time as function of the inverse temperature of DGEBA-4-AFD system (black) and DGEBA-4-AFD-CNF system (blue).</figcaption></figure>

### 3.6 Increasing thermal dimensional stability

Integrating cellulose nanofibrils into dynamic networks offers eco-friendly, re-processable materials that require superior thermal and mechanical stability. Nonetheless, the temperature-dimensional stability is also a critical property to ensure the material's ability to maintain its shape under load and at elevated temperatures. To confirm the benefits of CNFs in controlling temperature dimensional stability, dilatometry experiments were performed (Fig. 10). The permanently crosslinked DGEBA-DDM reference network shows a slight and constant strain increase with temperature, displaying a
383 non-linear region that corresponds to the glass transition temperature.

<figure><img src="image_11.png" /></figure>

384

<caption>Fig. 10. Dilatometry experiments of epoxy vitrimer (black), epoxy non-vitrimer (orange) and CNF-epoxy vitrimer composite (blue).</caption>

386

387 In contrast, the neat vitrimer network displays a significant increase in strain at temperatures above Tg.
388 This last slope change in this curve is associated with topological rearrangement of the network induced
389 by the acceleration of the dynamic bonds with temperature. Dilatometry experiments on vitrimers not
390 only measure the coefficient of thermal expansion but also capture the creep or flow behavior induced
391 by thermal stress [74].

392 Remarkably, the incorporation of CNF strongly reduces the thermal expansion and/or creep behavior
393 of the vitrimer composite under the same thermal stress conditions. This reduction is particularly
394 notable above Tg (Table 3), indicating a stable CNF network within the polymer. Controlling
395 dimensional thermal stability by introducing CNFs into vitrimers is a promising approach for
396 applications requiring good performance materials with robust and reliable thermal and mechanical
397 properties.

<table><thead><tr><th>System</th><th>Thermal strain at glassy state (µm.K<sup>-1</sup>)</th><th>Thermal strain at rubbery state (µm.K<sup>-1</sup>)</th><th>Strain at Tg (µm)</th></tr></thead><tbody><tr><td>DGEBA-DDM</td><td>0.7</td><td>2.3</td><td>74</td></tr><tr><td>DGEBA-4-AFD</td><td>0.6</td><td>4.0</td><td>90</td></tr><tr><td>DGEBA-4-AFD-CNF</td><td>0.1</td><td>0.9</td><td>22</td></tr></tbody></table>

398

<caption>Table 3. Thermal expansion in the glassy and rubbery states of epoxy networks and CNF-epoxy</caption>
## 3.7 Chemical recycling

Although the main objective of this article is not the composite recycling, a classical proof of concept of disulfide matrix degradation [75] has been implemented on the vitrimer composite [76] in presence of thiols. Due to the thiol-disulfide exchange reaction, the matrix network is disrupted. A small piece of epoxy vitrimer composite 1 cm³ was immersed in a solution of 2-mercaptoethanol at 1% v/v in DMF. After 48H of magnetic stirring at 35°C, the matrix was completely dissolved (Fig. 11.c). This first result underlines the potential of vitrimer nanocomposites recyclability expected.

<figure><img src="image_12.png" /><figcaption>Fig. 10. a) Initial immersion of CNF-epoxy vitrimer sheet in a thiol-containing solution of 2-mercaptoethanol at 1% v/v in DMF b) Partially dissolve resin of CNF-epoxy vitrimer sheet after 24 h immersion c) Completely dissolve resin of CNF-epoxy vitrimer sheet after 48 h immersion</figcaption></figure>

## 4. Conclusion

We have developed a new strategy for improving dimensional thermal stability of epoxy vitrimers through a controlled two-stage composite processing method. In the first stage, a wet NFC template is obtained via controlled filtration. The composite is then prepared by dissolving the epoxy resin and
416 hardener in acetone, which effectively impregnates the wet NFC template. Controlled solvent 417 evaporation yields a high-quality composite. This synthesis method eliminates the need for 418 functionalizing cellulose nanofibrils to achieve good mechanical properties, thus preserving the 419 composites's potential recyclability.

420 After optimizing the composite processing, this study examined the influence of cellulose nanofibrils 421 on the thermal and thermomechanical properties of an epoxy vitrimer material containing disulfide- 422 based dynamic bonds. The objective was to maintain the vitrimer characteristics while benefiting from 423 the reinforcing effects of the CNFs. The results demonstrated that CNFs significantly reinforced the 424 mechanical properties of the vitrimer, with Young's modulus improving by 77% with the addition of 425 10 wt% of cellulose nanofibrils. This work highlights that substantial reinforcement by cellulose 426 nanofibrils can be achieved without the need for interfacial functionalization. The limited decrease in 427 the glass transition temperature reaffirms thermal stability for further applications. Additionally, 428 incorporating CNF into the vitrimer resin markedly improved dimensional stability, effectively 429 mitigating issues related to creep and thermal expansion. This enhanced stability was balanced by a 430 pronounced increase in relaxation time without compromising complete flow behavior.

431 Moreover, since the CNF were not functionalized, the fiber-matrix interaction had minimal impact on 432 the activation energy of bond exchange, thereby preserving the intrinsic dynamic adaptability of the 433 vitrimer. As a result, this bio-based reinforced composite remains fully recyclable.

434

435

436 Funding Sources

437 This work was supported by The Mexican Council of Science and Technology (CONACyT), 438 [scholarship No. 471702].

439 CRediT authorship contribution statement
440 **Isis Castro Cabrera**: Writing – original draft, Investigation, Formal analysis, Methodology,

441 Visualization.

442 **Armand Fahs**: Writing - review & editing, Investigation, Formal analysis, Visualization.

443 **Sophie Berlioz**: Writing - review & editing, Formal analysis.

444 **Pascal Carriere**: Writing - review& editing, Data curation, Formal analysis, Conceptualization,

445 Supervision.

446 **Data availability**

447 The data that supports the findings of this study are mostly available in the manuscript, and other data

448 will be available from the corresponding author upon reasonable request.

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