Layered organization of anisometric cellulose nanocrystals and beidellite clay particles accumulated near the membrane surface during cross-flow ultrafiltration: In situ SAXS and ex situ SEM/WAXD characterization

Chiral nematic nanocomposites with pitch gradient elaborated by filtration and ultraviolet curing of cellulose nanocrystal suspensions

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. Namely, an aqueous suspension containing poly (ethylene glycol) diacrylate polymer (PEGDA) as a photocurable polymer and cellulose nanocrystals (CNCs) at a 70/30 mass ratio was processed by frontal filtration, followed by in-situ UV-curing in a dedicated cell. This procedure allowed designing nanocomposite films with highly oriented and densely-packed CNCs, homogeneously distributed in a PEGDA matrix over a thickness of ca. 500 μm. The nanocomposite films were investigated with small-angle X-ray scattering (SAXS), by raster-scanning along their height with a 25 μm vertically-collimated X-ray beam. The CNCs exhibited a high degree of orientation, with their director aligned parallel to the membrane surface, combined with an increase in the degree of alignment as concentration increased towards the membrane surface. Scanning electron microscopy images of fractured films showed the presence of regularly spaced bands lying perpendicular to the applied transmembrane pressure, highlighting the presence of a chiral nematic (cholesteric) organization of the CNCs with a pitch gradient that increased from the membrane surface to the bulk.

Orthotropic organization of a cellulose nanocrystal suspension realized via the combined action of frontal ultrafiltration and ultrasound as revealed by in situ SAXS

HYPOTHESIS

Rodlike cellulose nanocrystals (CNCs) exhibit significant potential as building blocks for creating uniform, sustainable materials. However, a critical hurdle lies in the need to enhance existing or devise novel processing that provides improved control over the alignment and arrangement of CNCs across a wide spatial range. Specifically, the challenge is to achieve orthotropic organization in a single-step processing, which entails creating non-uniform CNC orientations to generate spatial variations in anisotropy.

EXPERIMENTS

A novel processing method combining frontal ultrafiltration (FU) and ultrasound (US) has been developed. A dedicated channel-cell was designed to simultaneously generate (1) a vertical acoustic force thanks to a vibrating blade at the top and (2) a transmembrane pressure force at the bottom. Time-resolved in situ small-angle X-ray scattering permitted to probe the dynamical structural organization/orientation of CNCs during the processing.

FINDINGS

For the first time, a typical three-layer orthotropic structure that resembles the articular cartilage organization was achieved in one step during the FU/US process: a first layer composed of CNCs having their director aligned parallel to the horizontal membrane surface, a second intermediate isotropic layer, and a third layer of CNCs with their director vertically oriented along the direction of US wave propagation direction.

Self-supported MOF/cellulose-nanocrystals materials designed from ultrafiltration

Metal-organic-frameworks (MOFs) are promising materials for addressing critical issues such as petrochemical separation, water purification, energy storage and drug delivery. Their large-scale deployment, however, is hampered by a limited processability due to their powdery nature. Recently, the hybridization of MOFs with biopolymers has emerged as a greener, biocompatible strategy to shape MOFs composites into more processable membranes, films, and porous materials. In this work, cellulose nanocrystals (CNCs) were used in combination with ZIF-8 (a widely used synthetic zeolite) to produce hybrid composites through ultrafiltration. Results showed that small quantities of CNCs (1 to 20 CNC:ZIF-8 volume ratio) were sufficient to form a self-supported, dense deposit with high ZIF-8 loadings. Compared to classical MOF in situ growth strategies, this approach allowed the tuning of the composition of the final nanocomposite by controlling the nature and quantities of particles in the suspension. The fabrication of the deposit was strongly dependent on the physiochemical properties of the suspension, which were fully characterized with a set of complementary techniques, including in situ SAXS. This technique was employed to investigate the filtration process, which exhibited a homogeneous deposition of ZIF-8 particles mediated by CNC self-assembly. Finally, the available pore volume and integrity of the internal porosity of ZIF-8 were characterized by water porosimetry, demonstrating that the presence of CNCs did not alter the properties of the supported ZIF-8.

Small-Angle Neutron Scattering Reveals the Structural Details of Thermosensitive Polymer-Grafted Cellulose Nanocrystal Suspensions

Thanks to the use of small-angle neutron scattering (SANS), a detailed structural description of thermosensitive polymer-grafted cellulose nanocrystals (CNCs) was obtained and the behavior of aqueous suspensions of these derivatized biosourced particles upon temperature increase was revealed. Although literature data show that the surface grafting of thermosensitive polymers drastically enhances the colloidal properties of CNCs, direct space microscopic investigation techniques fail in providing sufficient structural information on these objects. In the case of CNCs decorated with temperature-sensitive polyetheramines following a peptide coupling reaction, a qualitative and quantitative analysis of SANS spectra shows that CNCs are homogeneously covered by a shell comprising polymer chains in a Gaussian conformation with a thickness equal to their radius of gyration in solution, thus revealing a mushroom regime. An increase of the temperature above the lower critical solution temperature (LCST) of the polyetheramine results in the formation of finite size bundles whose aggregation number depends on the particle concentration and suspension temperature deviation from the LCST. SANS analysis further reveals local changes at the CNC surface corresponding to a release of water molecules and a related denser polymer shell conformation. Noticeably, data show a full reversibility at all length scales when a sample was cooled down to below the LCST after being heated above it. Overall, the results obtained by SANS allow an in-depth structural investigation of derivatized CNCs, which is of high interest for the design of functional materials comprising these biosourced colloids.