Convection-assisted assembly of cellulose nanowhiskers embedded in an acrylic copolymer

Quantitative mapping of high modulus materials at the nanoscale: comparative study between atomic force microscopy and nanoindentation

Local mechanical properties of submicron features are of particular interest due to their influence on macroscopic material performance and behaviour. This study is focused on local nanomechanical measurements, based on the latest Atomic Force Microscopy (AFM) mode, where the peak force set point is finely controlled at each pixel. After probe calibration, we evaluate the impact of spring constant of two AFM hand-crafted natural full diamond tips with steel cantilevers, used for mapping. Based on the fast capture of the cantilever deflection at each pixel and real time force curve analysis in the elastic region, AFM local measured contact moduli mappings of the silica beads (>50 GPa) incorporated in an epoxy resin matrix, are compared with those determined using classical instrumented nanoindentation tests. Our analyses show that with the two AFM probes, without local residual deformation, the high moduli of the silica beads measured with this advanced AFM mode are within the standard deviation of the values determined by classical nanoindentation. LAY DESCRIPTION: The knowledge of material properties at the nanometer scale is a key parameter for well understanding and determining the behavior of material at macroscopic scale. In this paper, we compare two methods (an advanced mode and a classical one) based on the analysis of probes in interaction with the surface of studied material. We focus on a latest developed mode for determining local mechanical properties with a very high spatial resolution. For the advanced mode, we also consider two different hand-crafted probes. Our analyses show that with the high spatial resolution advanced mode, local mechanical properties are well determined. We also highlight the impact of the properties of the used probes for this advanced mode. In a final step, the power of the presented investigation lies in the fact that it does not modify the topography of the surface.

Mechanistic Insights on Spontaneous Moisture-Driven Healing of Urea-Based Polyurethanes

Self-healing polymeric materials that can spontaneously repair in a perpetual manner are highly appealing to address safety and restoration issues in different key applications. Usually built from reversible moieties that require to be activated using, for example, temperature, light, or pH changes, most of these self-healing materials rely on energy-demanding processes and/or external interventions to promote self-healing. In this work, we propose to exploit rapid dynamic exchanges between urea-based moieties and moisture as an alternative to promote local and spontaneous healing responses to damage using atmospheric moisture as an external stimulus. Non-hygroscopic urea-based polyurethanes with repetitive moisture-induced healing abilities at different degrees of humidity were thus designed through coupling reactions with non-hygroscopic polypropylene glycol and urea moieties. As supported by density functional theory (DFT) calculations coupled to local FTIR experimental studies, we furthermore established that the healing mechanism is ultimately related to the formation of water-urea clusters. Obviously, this work represents a platform for designing more advanced spontaneous self-healing materials beyond the present study, which hold promise for use in a wide range of technological applications.

The Self-Assembly of Cellulose Nanocrystals: Hierarchical Design of Visual Appearance

By controlling the interaction of biological building blocks at the nanoscale, natural photonic nanostructures have been optimized to produce intense coloration. Inspired by such biological nanostructures, the possibility to design the visual appearance of a material by guiding the hierarchical self-assembly of its constituent components, ideally using natural materials, is an attractive route for rationally designed, sustainable manufacturing. Within the large variety of biological building blocks, cellulose nanocrystals are one of the most promising biosourced materials, primarily for their abundance, biocompatibility, and ability to readily organize into photonic structures. Here, the mechanisms underlying the formation of iridescent, vividly colored materials from colloidal liquid crystal suspensions of cellulose nanocrystals are reviewed and recent advances in structural control over the hierarchical assembly process are reported as a toolbox for the design of sophisticated optical materials.

Increased Surface Roughness in Polydimethylsiloxane Films by Physical and Chemical Methods

Two methods, the first physical and the other chemical, were investigated to modify the surface roughness of polydimethylsiloxane (PDMS) films. The physical method consisted of dispersing multi-walled carbon nanotubes (MWCNTs) and magnetic cobalt ferrites (CoFe₂O₄) prior to thermal cross-linking, and curing the composite system in the presence of a uniform magnetic field H. The chemical method was based on exposing the films to bromine vapours and then UV-irradiating. The characterizing techniques included scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), Fourier transform infrared (FTIR) spectroscopy, optical microscopy, atomic force microscopy (AFM) and magnetic force microscopy (MFM). The surface roughness was quantitatively analyzed by AFM. In the physical method, the random dispersion of MWCNTs (1% w/w) and magnetic nanoparticles (2% w/w) generated a roughness increase of about 200% (with respect to PDMS films without any treatment), but that change was 400% for films cured in the presence of H perpendicular to the surface. SEM, AFM and MFM showed that the magnetic particles always remained attached to the carbon nanotubes, and the effect on the roughness was interpreted as being due to a rupture of dispersion randomness and a possible induction of structuring in the direction of H. In the chemical method, the increase in roughness was even greater (1000%). Wells were generated with surface areas that were close to 100 μm² and depths of up to 500 nm. The observations of AFM images and FTIR spectra were in agreement with the hypothesis of etching by Br radicals generated by UV on the polymer chains. Both methods induced important changes in the surface roughness (the chemical method generated the greatest changes due to the formation of surface wells), which are of great importance in superficial technological processes.

PeakForce QNM AFM study of chitin-silica hybrid films

Chitin-silica hybrid thin films, prepared through the colloidal self-assembly of chitin nanorods and siloxane oligomers, have been studied for the first time by PeakForce QNM AFM mode to explore their structure and mechanical behaviour. The change in structure and mechanical properties of chitin-silica hybrids is mainly driven by the relative quantities in chitin nanorods and silica, expressed as the chitin volume fraction ϕ_chi. The coating of the chitin polysaccharide by silica leads to an increase of the nanorods diameter and films surface roughness at small ϕ_chi values. The DMT (Derjaguin-Muller-Toporov) modulus increased both at small ϕ_chi due to a large amount of silica and at very high ϕ_chi→1 due to an incomplete tip penetration between nanorods. The local parallel orientation of nanorods observed at different ϕ_chi values resulted in a modulus increase due to an enhancement of the cohesion between nanorods.

Epoxy Monomers Cured by High Cellulosic Nanocrystal Loading

The present study focuses on the use of cellulose nanocrystals (CNC) as the main constituent of a nanocomposite material and takes advantage of hydroxyl groups, characteristic of the CNC chemical structure, to thermally cross-link an epoxy resin. An original and simple approach is proposed, based on the collective sticking of CNC building blocks with the help of a DGEBA/TGPAP-based epoxy resin. Scientific findings suggest that hydroxyl groups act as a toxic-free cross-linking agent of the resin. The enhanced protection against water degradation as compared to neat CNC film and the improvement of mechanical properties of the synthesized films are attributed to a good compatibility between the CNC and the resin. Moreover, the preservation of CNC optical properties at high concentrations opens the way to applying these materials in photonic devices.

Probing nanomechanical properties with AFM to understand the structure and behavior of polymer blends compatibilized with ionic liquids

The PeakForce QNM AFM mode was used to investigate the nanoscale mechanical properties of poly(butylene-adipate-co-terephthalate)/poly(lactic acid) (PBAT/PLA) blends successfully compatibilized with phosphonium-based ionic liquids (ILs).

Electric field alignment of nanofibrillated cellulose (NFC) in silicone oil: impact on electrical properties

This work aims to study how the magnitude, frequency, and duration of an AC electric field affect the orientation of two kinds of nanofibrillated cellulose (NFC) dispersed in silicone oil that differ by their surface charge density and aspect ratio. In both cases, the electric field alignment occurs in two steps: first, the NFC makes a gyratory motion oriented by the electric field; second, NFC interacts with itself to form chains parallel to the electric field lines. It was also observed that NFC chains become thicker and longer when the duration of application of the electric field is increased. In-situ dielectric properties have shown that the dielectric constant of the medium increases in comparison to the randomly dispersed NFC (when no electric field is applied). The optimal parameters of alignment were found to be 5000 Vpp/mm and 10 kHz for a duration of 20 min for both kinds of NFC. The highest increase in dielectric constant was achieved with NFC oxidized for 5 min (NFC-O-5 min) at the optimum conditions mentioned above.

Simultaneous reinforcing and toughening of polyurethane via grafting on the surface of microfibrillated cellulose

In the present work, a series of thermoplastic polyurethane (TPU)/microfibrillated cellulose (MFC) nanocomposites were successfully synthesized via in situ polymerization. TPU was covalently grafted onto the MFC by particular association with the hard segments, as evidenced by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis. The adequate dispersion and network structure of MFC in the TPU matrix and the strong interfacial interaction through covalent grafting and hydrogen bonding between MFC and TPU resulted in significantly improved mechanical properties and thermostability of the prepared nanocomposites. The tensile strength and elongation-at-break of the nanocomposite containing only 1 wt % MFC were increased by 4.5-fold and 1.8-fold compared with that of neat TPU, respectively. It was also very interesting to find that the glass transition temperature (Tg) of TPU was decreased significantly with the introduction of MFC, indicating potential for low-temperature resistance applications. Most importantly, compared with TPU nanocomposites reinforced with other nanofillers, the TPU/MFC nanocomposites prepared in this work exhibited excellent transparency and higher reinforcing efficiency.