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

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.

A bibliographic study has been performed to identify several polymers that could fill the project specification, i.e. a water-soluble and photopolymerizable polymer (PHP) and forming a stable suspension in the presence of cellulose nanocrystals (CNC): ACP-SBQ, PVP and PEGDA. This bibliographic study was written by Samuel and provides a useful basis for a journal article on this subject. Two sources of CNC were selected: the first was obtained by Samuel at CERMAV from the sulfuric acid hydrolysis of cotton linters; the second is a commercial source of Celluforce.

STEP 1: Numerous compatibility tests between the polymer and the NCC suspensions were carried out, and made it possible to select the PEGDA at an average molar mass of 700 g/mol, which makes it possible to obtain a homogeneous suspension with a colloidal stability of the CNCs and a maintenance of the nematic chiral organization typical of the liquid crystal behavior of the CNCs in the aqueous phase. This suspension in a mass concentration ratio of 30/70 (aqueous solution of CNC: PEGDA polymer solution) after adding a photo-initiator, is easily photopolymerizable and makes it possible to form a homogeneous cellulosic composite.

This first step is the success of Task 1.1 of the initial program.

Ultrastructural characterization of resting CNCs and PEGDA/NCCs was performed by SAXS and MEB, confirming the preservation of the equivalent interparticle distance of CNC in the presence or absence of PEGDA. Rheo-SAXS measurements on cotton CNCs were made at ESRF in March 2022. The evolution of the anisotropy of CNCs induced by shear flow and its evolution as a function of CNC concentration and under increasing shear gradients were thus characterized. The results are consistent with previous studies of other sources of CNC, including those from the University of Maine.

STEP 2: The PEGDA/CNC/photoinitiator suspensions formulated in step 1 were filtered in front mode and photopolymerized under UV. The nanocomposite obtained was characterized by SAXS and by MEB. These analyzes showed that PEGDA did not put a stop to the structuring of a deposit during filtration and allowed the structure of the deposit to be frozen after polymerization. SAXS measurements on this composite by scanning with a pitch of 50 µm revealed the orientation of the CNCs parallel to the membrane surface with an increasing orientation level following the increase in concentration towards the membrane. The observations by MEB show a lamellar structure in the form of layers oriented parallel to the membrane surface, with a cholesteric organization of the CNCs.

The first major objective of this research project is achieved and constitutes the success of part of Task 2 of the project.

The following objectives are to be achieved:

- Characterize by SAXS in situ the structuring of the composites during the tangential filtration process followed by photopolymerization: a proposal to ESRF has been accepted for this and will be implemented in January 2023.

- Characterize nanocomposite structures at micrometer scales by ex situ MEB.

- Explore different combinations of functional particles (carbon nanotubes, clays) that can bring a synergy of structuring (orientation, organization) or mechanical, optical, electrical or barrier properties.

- Study the rheological behavior of these suspensions, and manufacture films with different proportions of CNCs and of these functional particles, then characterize them from the point of view of their structural properties in relation to their functional properties.

- Develop UV filtration cells to produce composites of suitable shape and size for gas permeation measurements, mechanical tensile tests (a few cm wide and long), and electrical conduction tests.

1. ISPN 2022– International Symposium on Polymer Nanocomposites 28-30 September 2022 – Lorient, France, S. Mandin, N. Hengl, B. Jean, C. Lancelon-Pin, W. Chèvremont, and F. Pignon, Development of cellulosic nanocomposites with controlled structuring by crossflow ultrafiltration and ultraviolet (UV) photopolymerization.

2. XVIII International Small-Angle Scattering Conference 2022- 11-16 Septembre 2022, Campinas, Brésil, F. Pignon, E. Guilbert, S. Mandin, N. Hengl, H. Bodiguel, M. Karrouch, B Jean, J.L. Putaux, T. Gibaud, S. Manneville, T. Narayanan, Typical Three-Layer Orthotropic Organization Of Cartilage Achieved By Frontal Ultrafiltration Under Ultrasound Waves Of Cellulose Nanocrystals Suspensions, Probed By In Situ Time Resolved SAXS.

3. Journées Jeunes Rhéologues 22 au 24 juin 2022 Brest : Mandin. S, Pignon, F., Hengl, N., Jean, B., Lancelon-Pin, C., Chèvremont, W., Narayanan, T., Développement de nanocomposites cellulosiques à structuration contrôlée par ultrafiltration tangentielle et photopolymérisation UV.

4. 2. GDR DUMBIO Grenoble, Mia 2022, F. Pignon, E. Guilbert, S. Mandin, N. Hengl, H. Bodiguel, M. Karrouch, B Jean, J.L. Putaux, T. Gibaud, S. Manneville, T. Narayanan, Organisation orthotrope à trois couches, typique du cartilage, obtenue par ultrafiltration frontale sous ultrasons de suspensions de nanocristaux de cellulose : caractérisation par SAXS in situ.

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.