# Hygromorphic nanocomposites elaborated by filtration and ultraviolet curing of cellulose nanocrystal suspensions

S. Mandin, L. Metilli, M. Karrouch, D. Blésès, C. Lancelon-Pin, W.

Chèvremont, J.-L. Putaux, B. Jean, N. Hengl, Frédéric Pignon

► To cite this version:

S. Mandin, L. Metilli, M. Karrouch, D. Blésès, C. Lancelon-Pin, et al., Hygromorphic nanocomposites elaborated by filtration and ultraviolet curing of cellulose nanocrystal suspensions. Carbohydrate Polymer Technologies and Applications, 2026, 13, pp.101100. 10.1016/j.carpta.2026.101100. hal-05517223

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# Hygromorphic nanocomposites elaborated by filtration and ultraviolet curing of cellulose nanocrystal suspensions

S. Mandin<sup>a</sup>, L. Metilli<sup>b, ID</sup>, M. Karrouch<sup>a</sup>, D. Blésès<sup>a, ID</sup>, C. Lancelon-Pin<sup>b</sup>, W. Chèvremont<sup>c</sup>, J.-L. Putaux<sup>b, ID</sup>, B. Jean<sup>b, ID</sup>, N. Hengl<sup>a</sup>, F. Pignon<sup>a, * ID</sup>

## ARTICLE INFO

**Keywords:**

Cellulose nanocrystals (CNCs)

Nanocomposites

Filtration

UV-curing

Tensile test

Hygromorphic materials

## ABSTRACT

Recent developments in the processing of cellulose nanocrystals (CNCs) and polyethylene glycol diacrylate (PEGDA) suspensions into nanocomposites opened new avenues for producing biosourced materials with attractive functional properties. A recently developed processing method using frontal filtration, followed by UV crosslinking, enabled the fabrication of nanocomposite films with tunable anisotropic textures ranging from nanometric to micrometric scale. This method has been further developed in this work and used to highlight, in manufactured nanocomposite films, an original, highly cohesive bilayer-like structure composed of oriented CNCs with very different concentrations between two nearby regions, inducing original hygromorphic properties. Combined SAXS, WAXD, and SEM characterizations on these nanocomposites revealed a highly organized structure of CNCs oriented parallel to the membrane surface with the presence of an interparticle distance gradient evidenced by SAXS. An improvement of this method consisting in relaxation step (relaxation time of 24 and 66 h) after filtration and before UV crosslinking, has allowed to achieve a more continuous concentration gradient in the prepared nanocomposite films, with cholesteric organizations revealed by WAXD and SEM. This more continuous structuring reduced the hygromorphism but increased the tensile mechanical properties. These results pave the way for developing biobased, humidity-responsive actuators for future emerging technological applications.

## 1. Introduction

Actuator materials are known to respond to external stimuli, such as variations in temperature, pressure, pH, humidity, etc., by movement, deformation, or other specific responses. This type of material is of significant interest both ecologically and in terms of energy savings, as it can move in direct interaction with its environment without the need for an external energy source. This capacity for self-reaction reduces dependence on traditional energy resources, offering promising prospects for developing sustainable technologies. Moreover, the need for actuators in the future is going to increase, but to limit the environmental impact of their production, we should resort to biobased materials instead of synthetic ones containing rare earth metals or circuitry.

Actuator materials can be found in many fields of application, such as robotics (artificial limbs for robots, grippers, propulsion systems, etc.) (Bar-Cohen, 2012; Hines et al., 2017; Kay et al., 2020), medical devices

(insulin pumps, prostheses, controlled drug delivery devices...) (Hoffman, 2012; Lendlein & Langer, 2002), optics (adaptive lenses, deformable mirrors, etc.) (Tyson & Frazier, 2022), transportation (variable-geometry aircraft wings, vibration control devices, etc.) (Boller, 2001; Buehler et al., 1963), electronics (switches, sensors, lens actuators for cameras, etc.) (Judy, 2001; Pelrine et al., 2000) and construction for the development of low-energy, self-regulating buildings using hygromorphic materials (Correa et al., 2015; Poppinga et al., 2018; Reichert et al., 2015). Harnessing water as a stimulus is both economical and eco-friendly, especially compared to using electricity.

Actuators that reversibly change their shape under humidity are called hygromorphic materials. Unlike hygroscopic materials, which only absorb moisture, hygromorphic materials undergo movement with humidity variations. These changes in the shape of the material are generally due to the presence of a bilayer structure at the micrometer scale: a passive, rigid layer, and an active, deformable layer. A well-

<footnote><sup>a</sup> Univ. Grenoble Alpes, CNRS, Grenoble INP (Institute of Engineering Univ. Grenoble Alpes), LRP, F-38000 Grenoble, France</footnote>

<footnote><sup>b</sup> Univ. Grenoble Alpes, CNRS, CERMAV, F-38000 Grenoble, France</footnote>

<footnote><sup>c</sup> ESRF, The European Synchrotron, CS 40220, F-38043 Grenoble, Cedex 9, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: frederic.pignon@univ-grenoble-alpes.fr (F. Pignon).</footnote>
known example is the pine cone, whose scales open and close in response to moisture to disperse seeds (Correa et al., 2020; Le Duigou et al., 2017). Pine cone scales are composed of a bilayer material, with the rigid passive layer based on sclerenchyma fibers, bound to the active layer composed of scleroid fibers that are flexible and with very high hydroexpansion. The difference in mechanical properties and hydroexpansion between these two layers, along with their good adhesion, causes internal stresses and deformations in response to humidity variations, resulting in a spontaneous curvature of the material.

As indicated by Poppinga et al. (Poppinga et al., 2018), the key aspect of these materials is the strong adhesion between the two layers that can be achieved by adding polymers to form covalent bonds between both layers (Mizokuro et al., 2006; Poppinga et al., 2018; Prucker et al., 1999; Schuh et al., 2008; Yanagisawa et al., 2012). However, these techniques are not in line with today's environmental challenges.

Cellulose nanocrystals (CNCs) are biosourced nanorods exhibiting a large variety of interesting intrinsic properties such as tunable aspect ratio and surface chemistry, exceptional mechanical properties (axial elastic modulus ranges from 110 to 220 GPa, tensile strength of the order of 7.5 to 7.7 GPa) the ability to self-organize into liquid crystalline phases, non-toxicity, in addition to an abundant and renewable origin (Siqueira et al., 2010; Thomas et al., 2018; Schütz et al., 2020; Trache et al., 2020; Tom et al., 2023). These particles can also be assembled into various types of controlled architectures using different processes (coating, extrusion, deposition, spin coating, electrospinning bio-printing, etc.), yielding nanostructured cellulosic composites exhibiting a wide variety of functional properties (Siqueira et al., 2010; Thomas et al., 2018; Schütz et al., 2020; Lagerwall et al., 2014; Tang et al., 2019; Schwartz et al., 2018; Frost et al., 2019; Kose et al., 2019; De France et al., 2020)

Consequently, the development of processing methods allowing the development of hygromorphic materials with a better continuity or a better adhesion between the two layers and using biobased materials as CNCs for their outstanding properties, could therefore be of high interest to improve the functional properties of biobased, humidity-responsive actuators for future emerging technological applications.

Deposition and evaporation (Chen et al., 2022; Wu et al., 2016), 3D printing (Chabaud et al., 2019; Le Duigou et al., 2016), or frontal filtration are examples of processes that can be used to achieve this goal. Indeed, filtration has the advantage of operating under green conditions, exploiting pressure-driven accumulation and organization of nano-particles across multiple length scales. Previous studies have shown that frontal filtration could structure and orient anisometric particles, such as CNCs in a concentrated deposit or concentration polarization layer (CPL) at the membrane surface (Jin et al., 2015; Mandin et al., 2024a; Metilli et al., 2023; Rey et al., 2019; Semeraro et al., 2020). More recently, Mandin et al. showed that the organized CNC structure obtained during filtration could be fixed by adding the photopolymerizable polymer polyethylene glycol diacrylate (PEGDA) to develop nanocomposites (Mandin et al., 2024b). These nanocomposites have a structure consisting of a CNC concentrated deposit close to the membrane surface, with a concentration gradient over a thickness of *ca.* 500µm. There was a high concentration close to the membrane surface corresponding to an interparticle distance of 12 nm, of the order of the lateral size of the nanoparticles (11 nm), which decreased until reaching a region outside the deposit containing only the initial low concentration filtered suspension (CNC concentration of 2 wt %, corresponding to an interparticle distance d of about 50 nm). These two regions of the deposit, the lower concentrated one (of a thickness of about 500µm) and the upper dilute one of about the same 500µm thickness, could be considered as a bilayer-like material, with a continuous connection between these two regions, allowing the mimicking of a highly cohesive bilayer-like structure potentially having hygromorphic properties.

In the present study, this processing method has been further improved by adding or not a relaxation step after the filtration step and before UV-curing. Furthermore, a new 1-mm high filtration cell, with a

specific quartz glass window along the entire length of the cell, enabling the application of UV radiation along the 10 mm length of the canal, has been developed to prepare 10 mm long nanocomposites with about 1 mm thickness. The aim was to produce in a one-step process, original continuous bilayer-like homogeneous standing films. It was shown that filtration/relaxation/UV-curing enabled the imparting of new hygromorphic properties, thanks to the resulting cohesive bilayer-like structure. To better understand the mechanism that controls these hygromorphic properties, a deep study of the structural organization relating to these properties was performed. The nanocomposites obtained under several experimental conditions (filtration and relaxation time, UV curing step) was characterized over a wide range of length scales, from nano- to micrometers, by wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), and scanning electron microscopy (SEM). Furthermore, the tensile properties of the nanocomposites were measured for the different protocols used with or without relaxation before UV-curing, and for different filtration times.

## 2. Materials and methods

### 2.1. Materials

The CNCs used in this work were purchased from UMaine Development Center (University of Maine, USA), as an aqueous suspension of 11.5 wt %. The CNCs are decorated with sulfate half-ester groups (SO4-) with a sulphur content of 1.1 g per 100 g of CNCs, having $Na^+$ as counterion (Mandin et al., 2024b). This suspension was sonicated using a Branson Digital sonifier (Emerson Electric Company, USA) to break the CNC aggregates ($15.47~\text{kJ.g}^{-1}\cdot\text{L}^{-1}$). The 11.5 wt % sonicated suspension was diluted with ultrapure water. The dimensions of the CNCs were measured from transmission electron microscopy (TEM) images in the previous study (Mandin et al., 2024a). They had a length of $127.4 \pm 34.1$ nm and a width of $10.9 \pm 2.6$ nm, with an aspect ratio of $12.3 \pm 4.3$. The Irgacure 2959 photoinitiator was purchased from Sigma-Aldrich (France), as well as polyethylene glycol diacrylate (PEGDA) with a molar mass $M_w = 700~\text{g.mol}^{-1}$.

### 2.2. Preparation of PEGDA/CNC suspensions

The sonicated CNC suspensions were mixed with PEGDA at a 70/30 wt ratio of (pure PEGDA) / (aqueous CNC suspension) by magnetic stirring at room temperature. This suspension was sonicated using a Branson Digital sonifier (Emerson Electric Company, USA) at $20.5~\text{kJ.g}^{-1}\cdot\text{L}^{-1}$. Then, 0.5 wt % (given as percentages of the mass of PEGDA) of Irgacure 2959 was added under stirring until dissolution. The final CNC concentration in the PEGDA/CNC 70/30 suspension was $C = 2$ wt %.

### 2.3. Fabrication of the hygromorphic nanocomposites: filtration and UV-curing

The filtration process resembled the one described in our previous study (Mandin et al., 2024b). However, in the present work, the PEGDA/CNC suspension was filtered in a custom-made Teflon filtration cell ($100 \times 1 \times 4~\text{mm}^3$) with, at the top, a specific quartz glass window along the entire length of the cell, enabling the application of UV radiation. The suspension was filtered in frontal mode at room temperature using a $0.2 \sim \mu m$ PVDF membrane (Orélis Environnement, France) for different times under a transmembrane pressure $\Delta P$ of $1.2 \times 10^5$ Pa. After a fixed filtration time, $t_f$, followed or not by a relaxation time, $t_r$, at $\Delta P = 0$ Pa, the feed channel was UV-cured with an Omnicure S2000 (Polydispensing, France) UV lamp operating at wavelengths between 250 and 450 nm for 50 s. Finally, the UV-cured sample was removed from the filtration cell and air-dried under a weight of 200 g for at least 24 h to prevent deformation.

The influence of different manufacturing process parameters on the final structure of the nanocomposites was studied. Three procedures
have been performed: (i) without any filtration, the suspension being placed in the filtration cell and UV-cured; (ii) filtered during the filtration time, $t_f = 24$ h, without any relaxation step and photopolymerized under pressure; (iii) filtered using a transmembrane pressure DP of $1.2 \times 10^5$ Pa during $t_f = 24$ or 66 h, followed by a relaxation step (DP = 0 Pa) with a fixed duration, $t_r = 24$ h and UV-curing was performed afterwards. Nanocomposite denominations according to nanocomposite filtration conditions are defined in Table 1.

In the following, the different views of the nanocomposites will be presented and discussed as shown in Fig. 1.

## 2.4. Small-angle X-ray scattering (SAXS)

The SAXS measurements were carried out at the TRUSAXS Instrument on the ID02 beamline of the European Synchrotron Radiation Facility (ESRF, Grenoble, France). The measurements were conducted at room temperature, using a sample-detector distance of 10 m and 12.232 keV X-ray photons, corresponding to a q-range of $5.10^{-3}$ to $6.10^{-1}$ nm$^{-1}$, where $q = (4\pi/\lambda) \times \sin(q/2)$, with q being the scattering angle. A beam of approximately 25 µm$^2$ at the sample position was obtained using pinhole collimation (Narayanan et al., 2022). The nanocomposites were placed on a sample holder so that the incident X-ray beam passed through the nanocomposite in the direction parallel to the membrane surface, for observation along the Y-direction in the XZ view plane (Fig. 1). Two-dimensional (2D) scattering intensity patterns were collected using an EIGER2-4 M hybrid pixel detector (Dectris, Switzerland). The recorded data were converted to an absolute intensity scale following the standard protocol outlined by Narayanan and coworkers (Narayanan et al., 2022). Scattering intensity profiles, I(q), were calculated by azimuthally averaging the normalized patterns. Before averaging, the intensity between the detector module gaps was patched using the SAXSUtilities software (Sztucki, 2021) and the central-symmetry property of SAXS patterns.

To measure the average interparticle distance, d, between CNCs as a function of the distance, z, from the membrane surface at position $z_0$, a Kratky plot (corresponding to $q^2$) [I(q)] vs. q) analysis was performed. The relation between the interparticle distance d and characteristic $q_{peak}$ is expressed in the following equation (Schütz et al., 2015; Rey et al., 2019):

$$d = 2\pi/q_{peak} \qquad (1)$$

where $q_{peak}$ is the position of the maximum scattered intensity in nm$^{-1}$.

The average interparticle distance can be related to the CNC concentration. At increasing CNC concentration, one could expect a decrease in interparticle distance, as evidenced by static capillary measurements on CNC suspensions already used in previous works (Mandin, Metilli, Karrouch, Blésés et al., 2024a, 2024b; Metilli et al., 2023; Pignon et al., 2021; Rey et al., 2019; Semeraro et al., 2020).

## 2.5. Wide-angle X-ray diffraction (WAXD)

The nanocomposites were cut into thin strips with a razor blade and fixed to 0.5 mm collimators, with the plane of the strips oriented parallel or perpendicular to the X-ray beam, to observe along the 3 directions shown in Fig. 1. The nanocomposites were analyzed under vacuum using

<table><caption>**Table 1**

Nanocomposite denominations according to the filtration experimental conditions.</caption><thead><tr><td>Sample</td><td>Filtration time t<sub>f</sub> (h)</td><td>Relaxation time t<sub>r</sub> (h)</td></tr></thead><tbody><tr><td>ISO</td><td>0</td><td>0</td></tr><tr><td>FF24</td><td>24</td><td>0</td></tr><tr><td>FF24R24</td><td>24</td><td>24</td></tr><tr><td>FF66R24</td><td>66</td><td>24</td></tr></tbody></table>

a Philips PW3830 generator operating at 30 kV and 20 mA (Ni-filtered CuKα radiation, $\lambda = 0.1542$ nm). Two-dimensional patterns were recorded on Fujifilm imaging plates read with a Fujifilm BAS 1800-II (Fujifilm, Japan) bioimaging analyzer.

## 2.6. Scanning electron microscopy (SEM)

The nanocomposite samples were fractured at ambient temperature, and the resulting fragments were mounted onto carbon-coated sample holders. A thin Au/Pd conductive layer was deposited using a Safematic CCU-010-HV sputter-coater. Fracture surface imaging was carried out in secondary electron mode using a FEI Quanta-FEG 250 (Thermo Fisher Scientific, USA) scanning electron microscope equipped with a field-emission gun, operated at an accelerating voltage of 2.5 kV.

## 2.7. Polarized light optical microscopy (PLOM)

The nanocomposites were observed under polarized light with a Leica DM LM (Leica, Germany) equipped with a cross polarizer and analyzer. Images were acquired with a Basler digital camera, controlled by a pylon Viewer software (Basler, Germany).

## 2.8. Tensile tests

The tensile mechanical properties of the nanocomposites were measured using a tensile testing machine developed in the LRP laboratory, equipped with a 100 N load cell (0.1 % accuracy). Tests were carried out on nanocomposites cut to approximately 20 × 3.5 × 0.7 mm$^3$. All compositions were tested in triplicate. The samples were glued to clamps with epoxy adhesive. The stretching speed was set at 100 µms$^{-1}$. For each test, the tensile machine returned the displacement in mm and the force in N. The tensile force F and jaw displacement $\Delta l$ were recorded to estimate the nominal tensile stress $\sigma = F/S_0$ and strain $\epsilon = \Delta l/l_0$ from the knowledge of the initial sample gauge length $l_0$ and cross section $S_0$. This enabled the evaluation of the mechanical properties of the composite, such as the Young's modulus $E$, defined as the slope of the initial linear section of the strain-stress curves corresponding to the elastic deformation part of the material according to Hook's law (eq. (2)),

$$E = \frac{\sigma}{\epsilon} \qquad (2)$$

The maximum stress ($\sigma_{max}$ in Pa) and elongation at break ($\epsilon_{rup}$ in %) were also recovered from the stress-strain curves.

## 2.9. Hygromorphism tests

The nanocomposites were clamped with paper clips to prevent movement and keep them in the air (Figure S1). During the wetting step, the sample was immersed in a tank containing tap water. The observation plane was the XZ view as depicted in Fig. 1. Images were recorded with an acA2000-50gm black and white camera (Basler, Germany) with an AF-S NIKKOR 18-70 mm 1:35-4.5 G lens (Nikon, Japan) combined with a polarizing filter (Circular PL, HOYA) to avoid light reflection on the water, and illuminated by a LED panel (Phlox-LedW-BL-100 × 100-LLUB-Q-1R-24V-V3, PHLOX, France). The setup is shown in Figure S1. Pictures were taken at regular intervals using the pylon Viewer software (Basler, Germany). During the drying step, the water was removed from the tank, and the sample was left in the open air without humidity control. Photographs were also taken at regular intervals. The images were then analyzed with the ImageJ software to determine the X and Y coordinates of the nanocomposite by placing 6 points along the entire length of the material. Then, using the X and Y coordinates, the curvature of the material was determined by fitting a circle (Chemin et al., 2020; Le Duigou et al., 2019) (Figure S2). The radius of the circle gave access to the curvature of the material through the following equation:
<figure><img src="image_3.png" /><figcaption>Fig. 1. Illustration of the observation directions on 1 mm thick nanocomposites. The two letters in front of the eyes refer to the observation planes.</figcaption></figure>

$$ \kappa = 1/R \qquad (3) $$

with $κ$ the curvature in mm⁻¹ and R the radius of the circle in mm.

### 3. Results and discussion

The nanocomposites obtained from a sonicated suspension of 70/30 (wt %) PEGDA/CNC with an initial CNC concentration of 2 wt %, are shown in Fig. 2.

The ISO nanocomposite was highly transparent compared to the others, which tended to whiten and become iridescent due to the concentrated CNC layer formed during filtration or relaxation time. The UV-curing step of the processing induced bending for all samples. Hence, they were compressed during the drying phase to avoid uncontrolled deformation. Nevertheless, the nanocomposites still exhibited some degree of structural deformation. In Fig. 2, all the nanocomposites are shown with the concentrated layer of CNCs (membrane side) at the bottom of the sample. The non-relaxed, 24 h filtered nanocomposite (FF24) remained curved against gravity. In contrast to this sample, the relaxed nanocomposites FF24R24 and FF66R24 maintained a rigid, elongated shape after drying. This difference in curvature was explained by variations in the thickness of the polarization layer (or concentrated

deposit) in the nanocomposites and by a gradient in CNC concentrations across the thickness, which induced differences in local stresses during drying. Indeed, as shown in our previous study (Mandin et al., 2024b), for nanocomposites of larger thickness (8 mm), the thickness of the concentrated deposit increased sharply with relaxation time. This showed that, in the present case of the relaxed nanocomposite obtained using the 1-mm-high filtration cell, the concentrated deposit occupied the entire thickness of the nanocomposite, leading to a more homogeneous distribution of CNCs throughout the sample volume. In contrast, in the case of the FF24 nanocomposite, it occupied a smaller fraction of the total nanocomposite thickness. This implied that the FF24 nanocomposite presents a rigid CNC-rich layer, and a more flexible one, composed of PEGDA and CNCs at an initial CNC concentration of 2 wt %. This difference generated internal stresses during drying, thereby favoring the material's curvature. The ISO nanocomposite, on the other hand, was very flexible due to its low CNC concentration (2 wt %), which caused it to bend downward under gravity, as shown in Fig. 2. Thus, these initial results demonstrated the enhanced structuring of the nanoparticles endowed by the filtration process, and how it significantly affected the macroscopic properties of the nanocomposites.

<figure><img src="image_4.png" /><figcaption>Fig. 2. Photographs of ISO, FF24, FF24R24, and FF66R24 nanocomposites dried under a weight of 200 g at room temperature for 24 h after the filtration process (dry nanocomposites).</figcaption></figure>
### 3.1. Nanocomposite structure

Owing to their relatively small thickness (approx. 1 mm), these nanocomposites were observed by cross-polarized optical microscopy (in the XY view, Fig. 1). Cross-polarizers optical micrographs are shown in Figure S3. These images showed an increase in birefringence with filtration time. Indeed, for the ISO sample, the birefringence was almost absent, whereas for the FF24, FF24R24, and FF66R24 samples, it was significantly higher. This is due to the larger amount of CNCs in the filtered nanocomposites than in the unfiltered ones. The filtered and relaxed samples also appeared to have an oriented structure, particularly visible in the high-magnification images (Figure S3).

SEM images of the various nanocomposites are shown in Fig. 3. They are observations of the surfaces as shown in the diagram in Fig. 3A and also correspond to the XZ view (Fig. 1). First, the structure of the ISO sample was smooth, unlike the other filtered nanocomposites, which

have a textured surface, due to the presence of CNCs in larger amounts. The structure of the FF24 and FF24R24 nanocomposites was difficult to define. However, white dots pointing towards the observer were visible, which could correspond to CNCs viewed along their long axis. The absence of white dots in the ISO sample highlighted the low CNC concentration, in contrast to the samples obtained under filtration. In both cases, there was no unequivocal evidence of cholesteric structures. However, in the case of the FF66R24 nanocomposite, arcing structures could be seen, strongly resembling the cholesteric structures observed in the article by Majoinen et al. (2012); Mandin et al., (2024a), (2024b). The absence of a well-defined cholesteric structure in the FF24 and FF24R24 nanocomposites may be an artefact induced by sample preparation.

WAXD patterns informed us on the crystallographic structure of the CNCs and the orientation of the CNCs in the volume of the specimen probed by the X-ray beam. The WAXD patterns shown in Fig. 4A were

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** A) Scheme describing the cross-sectional direction of observation (XY view). B) SEM images of the fracture surface of 1-mm-thick nanocomposite films (C: ISO, D-E: FF24, F-G: FF24R24, H-I: FF66R24).</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** A) WAXD patterns of the nanocomposites recorded along different observation directions. The film fragments were oriented horizontally with respect to the 2D patterns. B) 3D illustration of the orientation of a cholesteric structure corresponding to the WAXD patterns in different observation directions.</figcaption></figure>

recorded along the XY, XZ, and YZ directions, as described in Fig. 1. The patterns of the ISO nanocomposite are not shown, as the diffracted intensity was very low due to the very low CNC concentration. In contrast, the WAXD patterns of the other nanocomposites were very intense. In each case, the XY patterns, which represent a top view of the nanocomposite, were circular, indicating an isotropic distribution perpendicular to the direction of observation. Conversely, the XZ and YZ patterns in each case were highly anisotropic, oriented vertically, which is similar to the horizontal orientation of the CNC director axis, i.e., parallel to the membrane. This confirms that the current filtration cell enabled the CNCs to be oriented at the membrane surface under the effect of pressure forces, in agreement with our previous study (Mandin et al., 2024b).

The isotropy of the XY patterns and the anisotropy of the XZ and YZ patterns may support the hypothesis of the presence of cholesteric structures in the nanocomposites, as observed in SEM images for sample FF66R24. Indeed, if cholesteric structures have their helical axis oriented in the direction of applied transmembrane pressure (z-axis), then, when viewed in the XY plane, the CNCs rotate around the helical axis, resulting in an isotropic pattern (Fig. 4B). Conversely, in the XZ and YZ planes, the CNCs were always observed from the side, despite their rotation around the helical axis, resulting in anisotropic patterns (Fig. 4A). These results for FF24 and FF24R24 could support the hypothesis of the presence of cholesteric structures that would not have been visible in SEM due to the fracture of the nanocomposites,

preventing their observation. This remains to be confirmed for these two types of nanocomposites. The small differences between the XZ and YZ patterns for each nanocomposite might have resulted from slight curvature induced by drying. This was probably what also caused the shift in the XY pattern of FF66R24 (Fig. 4B). In summary, combining the SEM and WAXD characterization allowed to conclude that the FF66R24 sample have clearly a cholesteric organization features, while the FF24 and FF224R24 samples needs more investigations to confirm the presence of a cholesteric organization only suggested by WAXD.

SAXS experiments were performed on the various nanocomposites (Fig. 5A). The SAXS images were recorded in XZ view (Fig. 1) at a z position near the bottom of the nanocomposite, i.e., corresponding to a position near the membrane surface during the processing. Fig. 5A shows a major difference in the structural organization between the filtered nanocomposites and the ISO nanocomposite. The SAXS patterns of the filtered nanocomposites were more intense and highly anisotropic as compared to the ISO nanocomposite. This clearly demonstrates the increase in concentration and the structuring and orientation of CNCs parallel to the membrane surface induced by filtration, as shown in our previous studies (Mandin et al., 2024a, 2024b). Furthermore, as the initial filtered suspension, the filtration and UV-curing conditions are the same as in Mandin et al. (2024a), one can estimate the thickness of the deposited concentrated layer. From Figs. 4, and Figure S4 the exit of the CPL is located around z = 500 ± 100 µm (as FF24), where the anisotropy reached a lower value, and the end of the decreasing
<figure><img src="image_7.png" /><figcaption>Fig. 5. A) 2D SAXS patterns; B) *I(q)* curves and C) corresponding Kratky plots for different UV-cured nanocomposites. The beam was focused in the middle of the thickness of the sample, observed along the XZ view.</figcaption></figure>

anisotropy in (Figure S4 A) corresponds to the limit from an anisotropic to an isotropic SAXS pattern (Figure S4B). From these results and the similarity between the two experiments, one can infer that the concentrated layer on the FF24 sample in Fig. 5 was about 500 µm thick. The upper layer out of the CPL with low CNC concentration is thus about 200 µm thick, which define a total thickness layer of this sample of 700 µm

In the 2D scattering patterns, the maximum scattered intensity in the vertical direction (i.e., perpendicular to the membrane surface) extend towards the large wave vectors (away from the beam-stop) as filtration time increased (Fig. 5A). This is explained by the increase in CNC concentration with filtration time. Note that the scattered intensity of the FF66R24 nanocomposite is slightly lower than that of the other nanocomposites, due to the different attenuation filter used in the experiment.

The scattering intensity and corresponding Kratky representation of the 2D spectra are represented in Fig. 5B and C, respectively. The interparticle distance calculated from the intensity peaks of the Kratky plot (Fig. 5C) was *d* = 25 nm for FF24 and *d* = 17 nm for FF24R24. The interparticle distance in the relaxed nanocomposite was smaller than that in the non-relaxed nanocomposite, which was expected. In fact, during relaxation, the most concentrated layers close to the membrane diffused towards the upper layer in the sample (where the SAXS measurement was done), which corresponded to an increase in concentration at the probed *z* position in the nanocomposite and therefore to a decrease in the interparticle distance (Mandin et al., 2024a, 2024b).

A scan was performed through the thickness of the FF24 nanocomposite with 100 µm intervals. The 2D spectra with *I(q)* curves and associated Kratky plots are shown in Figure S5. These results highlight the presence of an interparticle distance (concentration) gradient (Figure S5) induced by filtration, which corroborates our previous findings (Mandin et al., 2024a). The interparticle distances measured as a function of distance *z* from the membrane surface are consistent with those presented in our previous work (Mandin et al., 2024b).

To summarize the different structural organizations resulting from the various filtration conditions, the following observations can be made: i) ISO exhibited a homogeneous isotropic structure at a low CNC concentration, and the sample tended to collapse under its own weight

(insufficient mechanical strength); ii) FF24 presented two regions of the deposit: the lower concentrated one (about 500 µm-thick) and the upper dilute one of about 200 µm-thick could be considered as a bilayer-like material, with a continuous connection between these two regions. This nanocomposite curled back on itself in the initial dry state, revealing a high internal mechanical stress; iii) FF24R24 and FF66R24 present similar structures, with a more homogeneously oriented distribution of the CNCs and a more concentrated structure than ISO and FF24.

## 3.2. Tensile strength properties

Tensile tests were carried out on the various nanocomposites in order to understand how filtration parameters affect their final mechanical properties. An example of a tensile curve for each nanocomposite is shown in Figure S6. Mechanical properties such as Young's modulus (Fig. 6A), maximum stress (Fig. 6B), and elongation at break (Fig. 6C) were determined, and their values are presented in Table 2.

According to Fig. 6 and Table 2, the filtered nanocomposites displayed enhanced mechanical properties with respect to the ISO nanocomposite, notably with a factor of 16.5 between the Young's modulus of the ISO nanocomposite and that of the FF66R24 nanocomposite. There are several reasons for this major difference in mechanical properties. The first one is CNC concentration. During filtration, the CNC concentration increased close to the membrane surface, and propagated throughout the volume of the composite. As a result, the number of interactions with the PEGDA network increased, thereby enhancing material rigidity (Malo de Molina et al., 2015; Wu et al., 2016; Yang et al., 2013). These interactions likely arise from H-bonds between the polymer and the CNC hydroxyl groups, as in this case, there are no water molecules bound to the surface (Reid et al., 2017). Another reason is the filtration-induced orientation of nanoparticles, which have their director axis parallel to the direction of stretching. This orientation can also enhance mechanical properties in tension. This orientation can also enhance mechanical properties in tension. The effect of the orientation of CNCs on the mechanical properties of thin CNC films has recently been studied in detail by Son et al. (2023). Their observations showed
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** (A) Young's modulus, (B) maximum stress, and (C) elongation at break of the various nanocomposites characterized by tensile tests (at room temperature and 100 µms⁻¹).</figcaption></figure>

<table><caption>**Table 2**

Results of tensile tests on different nanocomposites.</caption><thead><tr><th>Sample</th><th>Young's Modulus (MPa)</th><th>Maximum stress (MPa)</th><th>Elongation at break (%)</th></tr></thead><tbody><tr><td>ISO</td><td>6.76 ± 1.19</td><td>0.70 ± 0.25</td><td>10.35 ± 3.25</td></tr><tr><td>FF24</td><td>58.67 ± 11.81</td><td>5.92 ± 1.07</td><td>18.49 ± 10.45</td></tr><tr><td>FF24R24</td><td>87.67 ± 11.69</td><td>9.39 ± 2.65</td><td>19.13 ± 2.15</td></tr><tr><td>FF66R24</td><td>112.00 ± 4.60</td><td>15.32 ± 1.11</td><td>28.79 ± 5.81</td></tr></tbody></table>

that the modulus of CNC films decreased as the angle between CNC alignment and loading direction increased. Song et al. (2017) have studied the effect of alignment of CNCs in a PVA matrix processed by electrospinning. Their results showed that the uniaxial continuous orientation of CNCs played an important role in the mechanical properties of the nanofibrous composite mats, with higher tensile strength and elongation at break than misaligned, spun nanofibrous mats.

The effect of relaxation also has a major influence on the final mechanical properties of the nanocomposites. When comparing FF24 and FF24R24 nanocomposites, there are marked differences in terms of Young's modulus and maximum stress, with 59 MPa and 6 MPa, respectively, for the FF24 sample versus 88 MPa and 9 MPa, respectively, for the FF24R24 nanocomposites. This difference may be due to the homogeneous distribution of CNCs within the FF24R24 material after relaxation. In fact, as seen previously in the structural analysis, the FF24 sample forms a bilayer-like structure consisting of the concentrated deposit and an upper part of PEGDA with a low CNC content (corresponding to the initial concentration of the suspension, i.e., 2 wt % CNCs). In contrast, the relaxed sample has a more homogeneous structure across the entire thickness of the nanocomposite, giving it greater tensile strength. Thus, a material with a heterogeneous structure may contain defects that weaken it and make it less mechanically resistant.

Filtration time also had an impact on the final mechanical properties of the nanocomposites, particularly in terms of Young's modulus and maximum stress for FF24R24 and FF66R24 samples. These differences can be explained by the increase in CNC concentration with filtration time and their resulting organization in the nanocomposite, inducing improved mechanical properties due to the structural reinforcement role of CNCs.

Finally, regarding the elongation at break, there is little difference between filtered nanocomposites. This may mean that this elongation property mainly derives from PEGDA, and that the structuring provided by filtration did not significantly affect this mechanical property.

Few CNC-based nanocomposites have been produced under similar conditions. The closest example was the study of Hou et al., with the manufacture of fibers with an anisotropic structure obtained by extrusion combined with UV photopolymerization (Hou et al., 2017). These authors used a 70/30 (wt %) PEGDA/CNC ratio, varying the CNC concentration from 0 to 1.27 wt %. They observed an increase in Young's modulus with increasing CNC concentration up to 0.96 wt % (approx. 8 MPa without CNC to 11.5 MPa at 0.96 wt %), followed by a decrease for the sample at 1.27 wt % (10.5 MPa). According to these authors, this was due to the stress-defect point formed by an excessive amount of CNC in the fiber matrix, which is normally observed in nanocomposite fibers. The samples illustrated in this work were comparable to the ISO nanocomposites presented in our study. Young's moduli are therefore quite comparable and of the same order of magnitude (10 MPa vs. 6.75 MPa for the ISO nanocomposite). The slight difference may be due to the orientation of the CNCs imposed by the extrusion, which was not present for the ISO nanocomposite. However, what is remarkable are the higher Young's modulus values for the nanocomposites structured by ultrafiltration, and UV-curing developed in this work, even though other authors have succeeded in developing CNC-based nanocomposites with higher Young's moduli (Rescignano et al., 2014; Germiniani et al., 2019; Missale et al., 2024). One interesting point in this work is the enhancement of mechanical properties induced by the continuous structuring and specifically the orientation of CNCs and their concentration gradient distribution induced by the transmembrane pressure forces applied during ultrafiltration, fixed by the UV-curing, and evidenced by local SAXS measurements with an accuracy of 25 µm in the thickness of the nanocomposite film.

### 3.3. Hygromorphic properties

As previously illustrated, the FF24 nanocomposite, consisting of two layers-like regions with one being more humidity-sensitive (rich in PEGDA), exhibited a structure similar to that of a hygromorphic material (Fig. 7). A concentration gradient of CNCs close to the membrane formed the first region of nanoparticles oriented parallel to the membrane surface. Moving away from the surface of the membrane, at a distance z of 500 µm and above, the sample contains the initial isotropic PEGDA/CNC suspension with a 70/30 (wt %) concentration ($C_{CNC} = 2$ wt %) (Fig. 5). As a result, the highly concentrated CNC region near the membrane surface was much stiffer than the upper region composed of PEGDA and CNC above the deposit. Moreover, the upper PEGDA-rich region was more sensitive to moisture due to a hydrogel behavior (Malo de Molina et al., 2015) than the CNC-concentrated part built in the vicinity of the membrane, notably due to its more open polymer network. The fact that the material was manufactured in a single step (an original and novel feature of the filtration manufacturing process) means that there is no interface discontinuity between these two regions. As indicated by Poppinga et al. (2018), the crucial requirement for these materials is to control the strong adhesion between the two layers, which was perfectly reached in the nanocomposites developed in the present work.

The hygromorphism property of sample FF24 was investigated by recording the deformation of the nanocomposite during immersion (Fig. 8A) and drying (Fig. 8B) with a digital camera. It is worth mentioning that first, the sample was fully immersed in water, and
<figure><img src="image_9.png" /><figcaption>Fig. 7. Scheme of the FF24 nanocomposite bilayer-like structure.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 8. Photographs of the FF24 nanocomposite over time during immersion in water (A) and air drying at room temperature (B).</figcaption></figure>

second, that it was air-dried without humidity control. Nevertheless, this first attempt allowed us to clearly demonstrate that the nanocomposite responded as a hygromorphic material. Starting from the nanocomposite in the dry state (upward curvature), upon immersion in water, the sample started deforming in the opposite direction, becoming linear after 8 min. Afterwards, the deformation progressed, reaching a downward curvature (Fig. 8A). This motion resulted from the hygroexpansion of the upper region, containing mostly the polymer, in which PEGDA swelled with water and generated an internal stress blocked by the lower CNC-rich region, thus triggering the bending of the composite in the other direction (up to down in Fig. 8A). Then, as the material was air dried, the top PEGDA-rich layer shrank under the effect of water loss,

inducing curvature in the opposite direction (down to up in Fig. 8B) and returning to the initial curvature (Fig. 8B). Note that the curvature of the nanocomposite after drying appears to be more pronounced than the initial curvature. This may be because after its formation, the nanocomposite was dried between two plates under a weight for 24 h to hold it in place, thereby limiting its initial curvature.

The curvature of this nanocomposite was evaluated by measuring the X and Y coordinates of the material in each image. Fig. 9 shows the evolution of the nanocomposite coordinates during immersion and drying. The curvature variation over time was assessed using image analysis. Since curvature is an arc of a circle, it was measured by fitting a circle (Figure S2), with determination of the radius of curvature

<figure><img src="image_11.png" /><figcaption>Fig. 9. X-Y coordinate plot of the time course of the bending of the FF24 nanocomposite upon immersion in water and air drying at room temperature. Y and X correspond to the coordinates of the nanocomposite in the space.</figcaption></figure>
(Figure S2). This method has been used in several publications on hygromorphic materials. Le Duigou et al. have worked on composite materials based on flax fiber and polypropylene molded by hot pressing (Le Duigou et al., 2019). Closer to our system, the work of Chemin et al. featured chemically modified cellulose nanofiber films with a curvature response following pH variations (Chemin et al., 2020).

Fig. 10 shows the curvature of the FF24 nanocomposite as a function of time during immersion and drying. When immersed in water, curvature decreased until reaching a plateau in the negative values ($κ = -0.5 mm^{-1}$), corresponding to a curvature in the opposite direction to the initial curvature. However, during drying, curvature values increased continuously until they reached a plateau. In both cases, the curves passed through the equilibrium position of the two regions corresponding to a curvature equal to 0. This point where the nanocomposite is no longer curved corresponds to the release of residual stresses and thermal and hygroscopic stresses (Le Duigou et al., 2019).

The deformation kinetics seemed to follow the same shape as the water sorption curve (Figure S7), with an initial rapid curvature followed by a plateau during which the curvature was maximal (Fig. 10). This phenomenon was described in the literature and corresponds to the typical behavior of an actuator in a humid environment or after drying (Le Duigou et al., 2019).

The deformation kinetics clearly shows that the curvature was driven by the phenomenon of water transfer within the layers, and in particular by the PEGDA-rich region (Fig. 10). During drying, the curvature kinetics slowed down, which can be explained by the fact that drying was not under controlled humidity conditions but also by the phenomenon of internal stress induced by CNCs (Fig. 10). Indeed, a recent study investigated the internal drying stress of a CNC suspension (Klockars et al., 2023). By drying a CNC suspension on the surface of a cantilever, the increase in residual stresses was shown to result from the increase in capillary forces. The cantilever thus underwent compressive and tensile stresses in the upper and lower parts, respectively. A bending moment was thus generated and, due to the high length/width ratio and small substrate thickness, the cantilever underwent upward bending. In our case, the nanocomposite was oriented in the opposite direction: the CNC-concentrated region was at the bottom, so drying this region should produce downward bending, the opposite of what we observe. This could be one explanation for the slower curvature of the material during the drying stage. Another explanation for the slower curvature could be the transfer of water inside the material. Water diffused very quickly

during immersion, whereas upon drying, the transfer took place via water evaporation. Moreover, as it is a two-layer-like material, the diffusivity of water inside could be different, affecting the curvature kinetics.

Thus, during the immersion stage, the CNC-poor part rapidly expanded as it contains an open network, which generates the curvature. Upon drying, on the other hand, the slower water transfer and the opposing curvature forces from the shrinkage of the PEGDA-rich part and the CNCs could explain the slower curvature kinetics. It should be noted that the internal stresses generated by drying CNCs are insufficiently high to prevent curvature.

Measurements were carried out with the other filtration conditions, i.e., the FF24R24 and FF66R24 nanocomposites (Figure S8). The curvature of the relaxed samples was much less pronounced than that of the non-relaxed nanocomposite (FF24), due in particular to the relaxed nanocomposites having a more continuous concentration gradient with no marked separation between a concentrated region and a much less concentrated region, which moved them away from a bilayer-type material. It should also be noted that the higher the filtration time-to-relaxation ratio, the greater the curvature, due to a structure tending towards a bilayer-like structure. This clearly shows that the structural organization of nanocomposites controlled by the filtration conditions can govern the transfer phenomena, hence the curvature features.

## 4. Conclusion

Thanks to a recently developed process of frontal filtration/relaxation/UV-curing, hygromorphic properties of CNC/PEGDA nanocomposites films have been highlighted. This one-step manufacturing process allowed producing bilayer-like homogeneous standing films, which is crucial for the development of high-performance hygromorphic materials. By changing the experimental conditions of filtration, several structures have been shown to coexist in 1-mm-thick nanocomposites. They show a highly organized structure of CNCs oriented parallel to the membrane surface, as characterized by SAXS and WAXD. In addition, the presence of an interparticle distance gradient was observed by SAXS on sample FF24, which can be assimilated to a cohesive bilayer-like structure. Finally, the presence of cholesteric organizations was evidenced in the FF66R24 nanocomposite by the WAXD patterns and SEM images.

The mechanical and hygromorphism properties of these

<figure><img src="image_12.png" /><figcaption>Fig. 10. Evolution of the FF24 nanocomposite curvature over time during immersion and drying.</figcaption></figure>
nanocomposites were tested. For tensile mechanical properties, a considerable increase in stiffness was shown owing to the structuring induced by filtration, from a Young's modulus of 7 to 106 MPa for the ISO nanocomposite and the FF66R24 nanocomposite, respectively. It was also shown that increasing filtration time and using relaxation improved the mechanical properties. Finally, it was shown that the FF24 nanocomposite has hygromorphic properties due to its shape change under humidity variation. The curvature of the nanocomposite during immersion and drying was characterized by image analysis. The demonstration of the ability of the frontal filtration/relaxation/UV-curing process to obtain hygromorphic thin films paves the way for developing biobased, humidity-responsive nanocomposites for future emerging technological applications.

### CRediT authorship contribution statement

S. Mandin: Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis. L. Metilli: Writing – review & editing, Investigation, Formal analysis. M. Karrouch: Investigation. D. Bléses: Investigation. C. Lancelon-Pin: Investigation, Formal analysis. W. Chèvremont: Investigation, Formal analysis. J.-L. Putaux: Writing – review & editing, Investigation, Formal analysis. B. Jean: Writing – review & editing, Investigation, Formal analysis. N. Hengl: Writing – review & editing, Investigation, Formal analysis. F. Pignon: Writing – review & editing, Writing – original draft, Validation, Supervision, Project administration, Methodology, Investigation, Formal analysis.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

ESRF is acknowledged for the provision of synchrotron beamtime (proposal IHSC-1832). We thank Theyencheri Narayanan (ESRF, Grenoble), Fanny Bosson, Francois Truong, Frédéric Hugenell and Eric Faivre (Laboratoire Rhéologie et Procédés) for technical assistance. The authors acknowledge the support of the French Agence Nationale de la Recherche (ANR), under grant agreement #ANR-20-CE43-0015 (ANISOFILM) for financial support of S. Mandin's PhD thesis and L. Metilli's postdoctoral position. The authors would like to thank the NanoBio-ICMG Platform (UAR 2607, Grenoble) for granting access to the electron microscopy facility. LRP is part of LabEx Tec21 (Investissements d'Avenir - grant agreement #ANR-11-LABX-0030). LRP and CERMAV are part of Institut Carnot PolyNat (Investissements d'Avenir - grant agreement #ANR-16-CARN-0025-01), and the Glyco@Alps programme (Investissements d'Avenir - grant agreement #ANR-15-IDEX-02).

### Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.carpta.2026.101100.

### Data availability

Data from ESRF can be found online at https://doi.esrf.fr/10.15151/ESRF-ES-1030392887.

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