Anisotropic nano-to-micro-structured cellulosic thin films tailored by ultrafiltration and UV curing. – ANISOFILM

The nanocomposite implementation and manufacturing methods followed several steps:

1) Development of colloidal suspension formulations of CNC/functional fillers and a UV-curable polymer.

- Exploration of different combinations of functional particles (clays, carbon nanotubes) that could provide synergy in terms of structure (orientation, organization) or mechanical, optical, electrical, barrier, or hygromorphic properties.

- Study of the rheological behavior of these suspensions and development of films with different proportions of CNCs and these functional particles, then characterization of their structural properties in relation to their functional properties.

2) Fabrication of processing cells with in situ and ex situ characterization:

- UV filtration cells were developed to, first, characterize in situ the induced organization of nanoparticles in the polymer matrix during filtration and crosslinking under UV light using techniques that allow for both nanometric probing using small-angle X-ray scattering (SAXS) and micrometric probing using small-angle light scattering (SALS).

- These UV filtration cells were also designed to fabricate thin films a few millimeters thick and a few centimeters long, suitable for gas permeation measurements, mechanical tensile/compression tests, hygromorphism measurements, and ex situ structural characterization using SEM and WAXS.

- Characterizations of the mechanical behavior in relation to the evolution of the structure were also carried out thanks to the development of an extension-compression cell dedicated to observation by SAXS.

During the first step, which consisted of studying the structuring mechanisms of CNCs in water during ultrafiltration and/or after transmembrane pressure relaxation, the following were demonstrated using SAXS, SALS, and SEM:

1) Controlled orientation of CNCs, parallel to the membrane surface, in concentrated layers reaching 15 to 20% by weight of CNCs over a thickness of about one millimeter.

2) The formation of original liquid crystal structures of the helical cholesteric type, with the axis of the helices oriented perpendicular to the membrane surface, generating an oriented cholesteric structure with a pitch gradient.

In a second step, the study of the interactions between the UV-cured polymer poly(ethylene glycol) diacrylate (PEGDA) and the CNCs revealed the adsorption of PEGDA on the surface of the CNCs. The influence of the polymer addition on the liquid crystal behavior of the CNCs was also observed, and a phase diagram was established.

Several advances were made in the fabrication and characterization of the nanocomposites by filtration and UV photopolymerization of a 70/30 wt% PEGDA/CNC suspension:

1) Controlled and fixed structuring on several scales: Using ex situ SAXS and SEM measurements, we were able to demonstrate that the filtration process enabled the orientation of the CNCs in the composite, resulting in the development of a new cholesteric structure with a pitch gradient, which was preserved during UV crosslinking. The development of these pitch-gradient nanocomposites represents a breakthrough in the fabrication of films with photonic properties that are difficult to reproduce in such a continuous manner using other processes.

2) Improved mechanical properties: With respect to mechanical and functional properties, an increase in Young's modulus and maximum stress with increasing filtration and relaxation times was demonstrated. This improvement in tensile mechanical properties results from an organization of the orientation of the CNCs within the material, but also from an increase in their concentration induced by filtration.

3) A new hygromorphic property of nanocomposites in a continuous bilayer structure was developed through UV filtration/crosslinking. A curvature change of +/- 180° was demonstrated in the resulting nanocomposites, confirming their ability to deform reversibly under humidity variations, opening up applications for actuators such as humidity sensors. This continuous bilayer organization without interface rupture is original compared to previous methods of bonding two independently manufactured layers. The structural continuity between the two layers ensures mechanical reinforcement of these hygromorphic materials.

1) Impact and benefits:

- The development of a new method for processing oriented cellulose nanocomposites using frontal filtration and UV crosslinking has opened the door to applications requiring controlled orientation:
In photonics and optics, the use of the cholesteric structure with a variable pitch obtained can be used as a Bragg-Berry mirror, allowing the manipulation of both the phase and polarization of light, opening the way to applications in advanced optical devices (optical communication systems, polarimeters, or display technologies).

- In food packaging, the anisotropic and homogeneous organization of particles combined with high concentration levels can provide solutions for controlling oxygen and water vapor permeability properties.

- Hygromorphic bi-layer films also find applications in self-regulating buildings or embedded humidity sensors.

2) Scientific and Technological Challenges:

- Use of tangential filtration instead of frontal filtration:

- The methodology implemented using frontal filtration and UV crosslinking has been entirely satisfactory in terms of the organization and orientation of anisotropic NCC particles and/or other particles of interest. However, it is possible to further control the orientation of these nanoparticles by applying shear simultaneously with transmembrane pressure, in a process commonly referred to as tangential filtration, which is widely used in industry.

- Application of other external fields to organize the material under filtration before UV crosslinking:
This project, by extending our previous results on ultrasound-assisted filtration, has shown that it is possible to develop orthotropic structures of CNCs under the simultaneous action of transmembrane pressure and ultrasonic waves.

- A device was developed to generate these two forces simultaneously, and we succeeded, as part of a separately funded thesis, in demonstrating for the first time that it was possible to continuously develop orthotropic cellulose nanocomposites with an organization similar to that of articular cartilage in a single step.

- A thesis was conducted in collaboration with the RMeS Laboratory at INSERM in Nantes to develop these nanocomposites and study their mechanical behavior in relation to their structural changes under compression, in order to improve our knowledge of the properties of these orthotropic materials that mimic articular cartilage.

- These results have been the subject of publications (Pignon et al. J. Colloid and Interface Sci., 659, 914-925 (2024); Bosson et al. Nanoscale, 17, 14381-14393 (2025).

- 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). 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). doi.org/10.1039/D4NR02840F

- Pignon F., Guilbert E., Mandin S., Hengl N., Karrouch M., Jean B., Putaux J.L., Gibaud T., Manneville S., Narayanan T., “Orthotropic organization of a cellulose nanocrystal suspension realized via the combined action of frontal ultrafiltration and ultrasound as revealed by in situ SAXS”, Journal of Colloid and Interface Science, 659, 914-925 (2024). doi.org/10.1016/j.jcis.2023.12.164

- 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). doi.org/10.1016/j.jcis.2025.01.155

- Bosson F., Chèvremont W., Karrouch M., Blésès D., Delplace V., Hengl N. and Pignon F., “In situ multiscale characterization of cellulose nanocrystals orthotropic organization achieved by combining ultrasound and frontal ultrafiltration”, Carbohydrate Polymers, 362, 123680 (2025). doi.org/10.1016/j.carbpol.2025.123680

- Bosson F., Challamel M., Karrouch M., Hengl N., Djeridi, H. and Pignon F., “Rayleigh streaming phenomena at the physical origin of cellulose nanocrystals orientations during combined ultrasound and ultrafiltration processes”, Nanoscale, 17, 14381-14393 (2025). doi-org.insis.bib.cnrs.fr/10.1039/D5NR00521C

- Bosson F., Karrouch M., Blésès D., Chèvremont W., Gibaud T., Michot L., Jean B., Delplace V., Hengl N. and Pignon F.,” Structural Mechanisms of Cellulose-Based Nanocomposites Mimicking the Structure of Articular Cartilage under Uniaxial Compression probed by in situ SAXS, Nanoscale Communication, (2025). doi.org/10.1039/D5NR01942G

The ANISOFILM project aims at developing a new and scalable method of processing by combining crossflow ultrafiltration with frontal photopolymerization to produce innovative cellulosic composite films with controlled anisotropic textures from nanometric to micrometric lengthscale.

Although nanocelluloses have a large potential as building blocks in biosourced composites, it remains challenging to achieve optimal performance of the materials due to the necessity of a near-perfect organization, ideally from nanometer to macroscopic scales. When applying current processing methods (extrusion, injection molding, compression molding, film casting, electrospinning and spin coating), one of the main difficulties is to achieve a control of the orientation of cellulosic nanoparticles and to maintain their homogeneous organization over a wide range of lengthscales.

Currently, the homogeneity of the spatial distribution and the degree of orientation imparted by these processes are limited because they are implemented at the final nanoparticle content of the designed nanocomposites. This concentration is too high for colloidal interactions to predominate over pressure, shear flow, or capillary forces applied during these processes, preventing an effective control of the orientation.

This limitation can be overcome using cross-flow ultrafiltration. Indeed, membrane separation processes have the advantage of starting the process from a low initial volume fraction at which the shear flow and pressure forces are sufficiently high to overcome the internal colloidal interactions between nanoparticles. This allows achieving a regular deposition and alignment of the colloids near the membrane surface. It has been recently demonstrated that this process followed by air drying of the deposit allows generating well-defined layered structures with a uniform orientation over distances from nanometers to several tens of micrometers. (Semeraro et al. Colloids Surf. A 584, 124030 (2020) - doi.org/10.1016/j.colsurfa.2019.124030).

Nevertheless, our previous studies have also evidenced that partial relaxation phenomena could occur upon switching off cross-flow and transmembrane pressure. (Rey C. et al. J. Membrane Sci. 578, 69-84 (2019) - doi: 10.1016/j.memsci.2019.02.019). Consequently, we propose a challenging strategy that involves a cross-flow ultrafiltration to reach a well-oriented state followed by the impregnation under frontal filtration of the deposit by a UV-curable polymer and finally in situ photopolymerization to freeze the resulting texture.

The nanocomposite thin films will be designed from nanocelluloses (nanofibrils or nanocrystals) combined with conductive nanoparticles like multiwalled carbon nanotubes or nanofillers like natural clays. UV-curable polymers will consist of an aqueous polyester acrylate oligomer and a photo-initiator.

Aiming at understanding the organization mechanisms involved during this processing method over a wide spatial range, both during ultrafiltration and photopolymerization, the materials will be characterized by in situ small-angle X-ray and light scattering (SAXS-USAXS and SALS) coupled with ex situ direct observation by electron microscopy (SEM and TEM) and X-ray diffraction (WAXS). Their functional properties (conductive, barrier, mechanical or optical properties) will be evaluated as well in relation with the texture of the nanocomposites.

This new processing method will allow industrial activities to emerge for designing new nano-to-micro nanocellulose-biobased composite materials with superior functional properties such as: i) improved dielectric and conductivity properties, ii) enhanced oxygen or water vapor barrier properties, iii) reinforced mechanical properties, or iv) broad-band reflecting, scattering and transparent optical properties.