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An innovative method combining frontal filtration with ultraviolet (UV) curing has been implemented to design cellulosic nanocomposite films with controlled anisotropic textures from nanometric to micrometric length scales.
Objectives: In this work, an innovative method combining frontal filtration with ultraviolet (UV) curing has been implemented to design nano- to microstructured cellulosic films based on cellulose nanocrystals (CNCs). The objective is to achieve a control over the orientation and organization of anisotropic nanoparticles through frontal filtration, which induces a regular and simultaneous orientation and concentration of the nanoparticles near the membrane surface, and to freeze this structure through a UV crosslinking process.
This project was carried out within the framework of the ANR ANISOFILM project, "Anisotropic and nano- to micro-structured cellulosic thin films produced by cross-flow ultrafiltration and UV crosslinking" (https://anr.fr/Projet-ANR-20-CE43-0015), in collaboration with six partners (LRP, CERMAV, PHENIX, LGP2, LPS, DCM).
To master this innovative processing method, we have characterized the structural organization mechanisms involved in this process. To this end, we have performied in situ characterizations during the fabrication process, at different scales, using small angle X-ray and light scattering (SAXS, SALS) as well as scanning electron microscopy (SEM). The structural properties are then linked to the functional properties measured on the designed nanocomposites.
Results:
A first formulation step aimed at stabilizing cellulose nanocrystals (CNCs) in a UV-curable polymer matrix was successfully completed, yielding stable colloidal suspensions of CNCs in a poly(ethylene glycol) diacrylate (PEGDA) matrix with the addition of a photoinitiator.
A significant achievement is the first time this innovative processing method (frontal filtration and UV curing) has produced a structured cellulosic composite with homogeneously distributed cellulose nanocrystals throughout the composite volume, their directors oriented parallel to the membrane surface on scales ranging from nanometers to micrometers.
Analysis of SAXS (Figures 1 and 2) and SEM (Figure 3) measurements within the nanocomposite revealed the efficiency of the membrane separation process in organizing the densification of cellulose nanoparticles on the membrane surface. Indeed, the advantage of this process compared to other processes such as extrusion or deposition is that it allows the concentration of anisotropic objects from the dilute region, where colloidal interaction forces are weak relative to the external forces of applied transmembrane pressure.
This property of continuous structuring over time, combined with UV curing at the end of filtration step, has made it possible to achieve novel arrangements of cellulose nanocrystals (CNCs) with particle spacings of 12 to 28 nm and thicknesses of 25 to 225 µm from the membrane surface, within crosslinked nanocomposite films, and this for cellulose nanocrystals with lateral dimensions of 11 nm (Figure 4). Furthermore, for the first time, a cholesteric structure of the CNCs has been demonstrated, with their helical axis oriented perpendicular to the membrane surface and possessing a regular pitch gradient established from the membrane surface towards the volume of the nanocomposite (Figures 5 and 6). This cholesteric structure, with a regular pitch gradient over distances on the order of 120 µm, opens the way to applications in the field of photonics.
Figure 1: A) The 70/30 PEGDA/CNC nanocomposite at 2 wt% inital filtered suspension and filtered during tf = 24h and B) corresponding 2D SAXS patterns registered in the nanocomposite as a function of distance z from the membrane surface.
Figure 2: A) Scattered intensity I(q) as a function of the wave vector deduced from the radial integration of the 2D SAXS spectra measured inside the nanocomposite.
B) Kratky plot representation of this scattered intensity, showing the correlation peak corresponding to the average distance between the cellulose nanocrystals, for increasing positions z within the nanocomposite, from the edge of the nanocomposite corresponding to the membrane surface to its bulk. The color scale corresponds to the different distances from the membrane surface. Experimental conditions: PEGDA/CNC 70/30 wt% at CCNC initial = 2 wt% filtered for tf = 24h at ΔP = 1.2 x 10⁵ Pa.
Figure 3: SEM images of 70/30 wt% PEGDA/CNC nanocomposites at 2 wt%: A) unfiltered B) filtered during tf = 24h. Proof of the regular and homogeneous structuring of cellulose nanocrystals, obtained through the innovative processing method by ultrafiltration combined with UV curing.
Figure 4: Variation of the average interparticle distance profile (green) and anisotropy degree (blue) of the CNCs in the concentrated deposit of the 70/30 PEGDA/CNC nanocomposite at CCNC = 2 wt% filtered during tf = 24 h at DP = 1.2 .105 Pa.
Figure5: A) and B) SEM images of a 70/30 wt% PEGDA/CNC nanocomposite at 0.7 wt% obtained by filtration during tf = 24 h at DP = 1.2 .105 Pa. C) Pitch of cholesteric structures as a function of the distance from the membrane measured from image A. The yellow lines highlight the band spacing (or half-pitch). The image in A is a composite of several aligned images that cover a large surface of the sample while preserving the resolution.
Figure 6: Variation of the average interparticle distance and PCA degree (A) and schematic CNC organization in the concentrated deposit (B) scheme of the organization of CNCs in the concentrated deposit comprising 3 distinct structuring regions: (green) cholesteric region with a step gradient, (purple) biphasic region and (pink) isotropic region of the 70/30 PEGDA/CNC nanocomposite at 0.7 wt% filtered during 24 h at DP = 1.2 .105 Pa.
Références :
Mandin S., Metilli L., Karrouch M., Lancelon-Pin C., Putaux J.-L., Chèvremont W., Paineau E., Hengl N., Jean B., Pignon F., “Chiral nematic nanocomposites with pitch gradient elaborated by filtration and ultraviolet curing of cellulose nanocrystal suspensions”, Carbohydrate Polymers, 337, 122162 (2024). https://doi.org/10.1016/j.carbpol.2024.122162
Mandin S., Metilli L., Karrouch M.,Blésès D., Lancelon-Pin C., Sailler P., Chèvremont W., Paineau E., Putaux J.L., Hengl N., Jean B., and Pignon F., ”Multiscale study of the chiral self-assembly of cellulose nanocrystals during the frontal ultrafiltration process”, Nanoscale, 16, 19100, (2024). https://doi.org/10.1039/D4NR02840F
Metilli L., Mandin S., Chazapi I., Paineau E., Chèvremont W., Hengl N., Pignon F., Jean B., “Multi-scale investigation of the effect of photocurable polyethylene glycol diacrylate (PEGDA) on the self-assembly of cellulose nanocrystals (CNCs)”, Journal of Colloid and Interface Science, 685, 476-486 (2025). https://doi.org/10.1016/j.jcis.2025.01.155
Mandin S., Metilli L., Karrouch M., Blésès D., Lancelon-Pin C., Chèvremont W., Putaux J.L., Jean B., Hengl N. and Pignon F. “Hygromorphic nanocomposites elaborated by filtration and ultraviolet curing of cellulose nanocrystal suspensions”, Carbohydrate Polymer Technologies and Applications, 13, 101100, (2026). https://doi.org/10.1016/j.carpta.2026.101100 ⟨hal-05517223⟩
ANR ANISOFILM (https://anr.fr/Projet-ANR-20-CE43-0015)
La cellulose, une ressource à haut potentiel I Focus sciences I CNRS Alpes (https://www.youtube.com/watch?v=EauIZ0wf1EY)
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