ABSTRACT
We address the limited solubility and early onset of gelation of aqueous sodium hydroxide to position it as a preferred green solvent for cellulose. For this purpose, we expand the concentration window (up to 12 wt%) by using a CO2-depleted air and adjusting the time the dope remains in the given atmosphere, before further processing (holding time) and regeneration conditions. Cellulose solutions are extruded following characteristic (rheology and extrusion) parameters to yield aligned filaments reaching tenacities up to 2.3 cN·dtex-1, similar to that of viscose. Further material demonstrations are achieved by direct ink writing of auxetic biomedical meshes (Poisson's ratio of -0.2, tensile strength of 115 kPa) and transparent films, which achieved a tensile strength and toughness of 47 MPa and 590 kJ·m-3, respectively. The results suggest an excellent outlook for cellulose transformation into bioproducts. Key to this development is the control of the gelation ensuing solution flow and polymer alignment, which depend on CO2 absorption, cellulose concentration, and holding time.
Subject(s)
Carbon Dioxide , Cellulose , Solutions , Sodium Hydroxide , Biocompatible MaterialsABSTRACT
We investigate the interplay between cellulose crystallization and aggregation with interfibrillar interactions, shear forces, and the local changes in the medium's acidity. The latter is affected by the CO2 chemisorbed from the surrounding atmosphere, which, combined with shear forces, explain cellulose gelation. Herein, rheology, nuclear magnetic resonance (NMR), small and wide-angle X-ray scattering (SAXS/WAXS), and focused ion beam scanning electron microscopy (FIB-SEM) are combined to unveil the fundamental factors that limit cellulose gelation and maximize its dissolution in NaOH(aq). The obtained solutions are then proposed for developing green and environmentally friendly cellulose-based materials.
ABSTRACT
One of the main trends in developing bio-based materials is to improve their mechanical and physical properties using MFC derived from sustainable natural sources and compatible low-cost chemicals. The strength of anionic MFC based materials can be increased with addition of multivalent cations. However, direct mixing of solutions of multivalent cations with oxidized MFC may result in immediate, uncontrollable fibril aggregation and flock formation. The aim of this study was to design a method where Ca2+ ions liberate from solid CaCO3 particles on bleached hardwood (birch) kraft pulp, which was mixed with oxidized MFC and crosslink it to tailor the mechanical properties of the dried structure. In few minutes after adding acetic anhydride, pH of the wet film dropped from 7.3-4.8 through liberation of acetic acid and CaCO3 particles solubilized releasing Ca2+. The novel method could be applied on industrial scale for improving the performance of packaging materials.
ABSTRACT
In aqueous solutions the response of polymers and biological matter to external conditions, such as temperature and pH, is typically based on the hydrophobic/hydrophilic balance and its effects on the polymer conformation. In the solid state, related concepts using competing interactions could allow novel functions. In this work we demonstrate that polymeric self-assembly, reversibility of hydrogen bonding, and polymer-additive phase behaviour allow temperature response in the solid state with large and reversible switching of an optical bandgap. A complex of polystyrene-block-poly(4-vinylpyridinium methanesulphonate) and 3-n-pentadecylphenol leads to the supramolecular comb-shaped architecture with a particularly long lamellar period. The sample is green at room temperature, as an incomplete photonic bandgap due to a dielectric reflector is formed. On heating, hydrogen bonds are broken and 3-n-pentadecylphenol additionally becomes soluble in polystyrene, leading to a sharp and reversible transition at approximately 125 degrees C to uncoloured material due to collapse of the long period. This encourages further developments, for example, for functional coatings or sensors in the solid state.
