Cellulose nanofibrils prepared by gentle drying methods reveal the limits of helium ion microscopy imaging.

TEMPO-oxidized cellulose nanofibrils (TCNFs) have unique properties, which can be utilised in many application fields from printed electronics to packaging. Visual characterisation of TCNFs has been commonly performed using Scanning Electron Microscopy (SEM). However, a novel imaging technique, Helium Ion Microscopy (HIM), offers benefits over SEM, including higher resolution and the possibility of imaging non-conductive samples uncoated. HIM has not been widely utilized so far, and in this study the capability of HIM for imaging of TCNFs was evaluated. Freeze drying and critical point drying (CPD) techniques were applied to preserve the open fibril structure of the gel-like TCNFs. Both drying methods worked well, but CPD performed better resulting in the specific surface area of 386 m2 g-1 when compared to 172 m2 g-1 and 42 m2 g-1 of freeze dried samples frozen in propane and nitrogen, respectively. HIM imaging of TCNFs was successful but high magnification imaging was challenging because the ion beam tended to degrade the TCNFs. The effect of the imaging parameters on the degradation was studied and an ion dose as low as 0.9 ion per nm2 was required to prevent the damage. This study points out the differences between the gentle drying methods of TCNFs and demonstrates beam damage during imaging like none previously reported with HIM. The results can be utilized in future studies of cellulose or other biological materials as there is a growing interest for both the HIM technique and bio-based materials.

The sol-gel transition of ultra-low solid content TEMPO-cellulose nanofibril/mixed-linkage ß-glucan bionanocomposite gels.

We present the preparation, morphological analysis, and rheological characterization of ultra-low solid content gels prepared by physically cross-linking TEMPO-oxidized cellulose nanofibrils (TEMPO-CNF) with the soluble plant-cell-wall polysaccharide, mixed-linkage ß-glucan (MLG). Of particular note, gel formation was rapidly induced by very small amounts of MLG (e.g. 0.125% w/v) at extremely low TEMPO-CNF concentration (0.05% w/v), which independently were otherwise fluid and thus easily handled. Rheology of these bionanocomposite gel systems as a function of MLG and TEMPO-CNF concentrations revealed that the critical gel concentration of MLG and TEMPO-CNF followed a power-law relation of the concentration of the other component. Surprisingly, these systems also exhibited an additional transition to thick gels at high TEMPO-CNF and MLG concentrations that was visible only at low frequencies. Cryogenic scanning electron microscopy (cryo-SEM) imaging of admixture solutions and gels revealed increased network crowding with increasing MLG amounts. The data are consistent with the hypothesis that non-covalent cellulose-MLG interactions, analogous to those occurring within plant cell walls, drive gel formation. The ability to tune gel physical properties simply by controlling CNF (a promising forest bioproduct) and MLG (a readily available agricultural polysaccharide) fractions at very low solid and polymer content opens new possibilities for material applications in diverse industries.

Dual Light- and pH-Responsive Composite of Polyazo-Derivative Grafted Cellulose Nanocrystals.

As a type of functional group, azo-derivatives are commonly used to synthesize responsive materials. Cellulose nanocrystals (CNCs), prepared by acid hydrolysis of cotton, were dewatered and reacted with 2-bromoisobuturyl bromide to form a macro-initiator, which grafted 6-[4-(4-methoxyphenyl-azo) phenoxy] hexyl methacrylate (MMAZO) via atom transfer radical polymerization. The successful grafting was supported by Fourier transform infrared spectroscopy (FT-IR) and Solid magnetic resonance carbon spectrum (MAS 13C-NMR). The morphology and surface composition of the poly{6-[4-(4-methoxyphenylazo) phenoxy] hexyl methacrylate} (PMMAZO)-grafted CNCs were confirmed with Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The grafting rate on the macro-initiator of CNCs was over 870%, and the polydispersities of branched polymers were narrow. The crystal structure of CNCs did not change after grafting, as determined by X-ray diffraction (XRD). The polymer PMMAZO improved the thermal stability of cellulose nanocrystals, as shown by thermogravimetry analysis (TGA). Then the PMMAZO-grafted CNCs were mixed with polyurethane and casted to form a composite film. The film showed a significant light and pH response, which may be suitable for visual acid-alkali measurement and reversible optical storage.

The effect of oxyalkylation and application of polymer dispersions on the thermoformability and extensibility of paper.

Wood fiber-based packaging materials, as renewable materials, have growing market potential due to their sustainability. A new breakthrough in cellulose-based packaging requires some improvement in the mechanical properties of paper. Bleached softwood kraft pulp was mechanically treated, in two stages, using high- and low-consistency refining, sequentially. Chemical treatment of pulp using the oxyalkylation method was applied to modify a portion of fiber material, especially the fiber surface, and its compatibility with polymer dispersions including one carbohydrate polymer. The results showed that the compatibility of the cellulosic fibers with some polymers could be improved with oxyalkylation. By adjusting mechanical and chemical treatments, and the thermoforming conditions, the formability of paper was improved, but simultaneously the strength and stiffness decreased. The results suggest that the formability of the paper is not a direct function of the extensibility of the applied polymer, but also depends on the fiber network structure and surface energy.

Chemical modification of cellulosic fibers for better convertibility in packaging applications.

Cellulose fiber has been modified by mechanical and chemical means in order to improve paper properties, which respond to moisture and temperature. When the cellulose is first refined and then etherified using hydroxypropylation under dry conditions, the paper sheets prepared from the hydroxypropylated cellulose show improved elongation. When the level of hydroxypropylation is high enough, the paper sheets also become transparent. Additionally, the effect of cellulose activation using different mechanical methods has been compared by esterification reactions. It is shown that removal of water is the most crucial step for the esterification reactions while other methods have a lesser impact. The paper sheets prepared from the esterified cellulose fibers show an increase in contact angles and high hydrophobicity.