Preparation of three-dimensional cellulose objects previously swollen in a DMAc/LiCl solvent system

Abstract

International audienceThree-dimensionally shaped cellulosic objects were produced via a two-step procedure: swelling of softwood pulp (93 % cellulose; 4.5 % hemicellulose; 54 % crystallinity) in DMAc/LiCl followed by moulding. Swollen cellulose pulp in the form of gel was solidified with two different anti-solvents: distilled water and a combination of 2-propanol and deionized water. The solid cellulose material was further moulded in a custom-built prototype mould. The role of the anti-solvent was to solidify the swollen cellulose fibres and prepare mouldable solid specimens. The anti-solvent was chosen based on the following criteria, viz., recoverability, stable chemical reactivity, availability, cost and previous research in the anti-solvent area. The choice of solidification solvent had a great influence on the structure and mechanical properties of the final cellulose material. Results of different characterisation techniques showed that when the cellulose gel was washed with distilled water, it had a significantly higher number of lithium cations (ICP-MS and Raman), amorphous structure (X-ray) and lower mechanical properties (nanoindentation) compared to samples washed with a combination of 2-propanol and deionized water. An increase in viscosity as previously reported and changes in the NMR and IR spectra of DMAc upon LiCl suggested the formation of an ion-dipol complex, where lithium cations reside adjacent to the oxygen of the carbonyl group of DMAc. The formed macrocation [DMAcn + Li]+ was preserved between cellulose chains in cellulose specimens washed with distilled water and had an essential role in the disruption of initial bonds, thus enhancing mouldability. Electron microscopy (FE-SEM) studies showed that the surface of cellulose after mechanochemical treatment was rough with no presence of fibre

High-pressure Treatment of DMAc/LiCl Swollen Softwood Pulp

“…As reported in the authors' previous paper (Obradovic et al 2014), adding distilled water to the swollen cellulose mixture induces the formation of a purely amorphous sample, while adding a mixture of 2-propanol and deionized water gave cellulose samples with X-ray patterns typical of cellulose I crystals. Cellulose crystallinity has an impact on the overall transparency of the materials formed either after the addition of water or 2-propanol and deionised water.…”

“…In a present work, cellulose structure was already altered due to breakage of hydrogen bonds by solvent system prior to compression step. As shown by the authors' previous work (Obradovic et al 2014), cellulose samples Cell_DW had amorphous structure, and cellulose samples Cell_2PDW had cellulose I crystallinity with decrease in degree of crystallinity, after swelling treatment. After high compression the crystallinity decreases severely for both set of samples.…”

“…The elastic modulus and hardness, measured by nano-indentation (using Eqs. 1 and 2), are shown in Table 1, as well as the values of the mechanical properties for unpressed samples (Obradovic et al 2014). For both types of regenerating treatments, the elastic modulus and hardness were relatively constant.…”

“…Swelling chopped softwood dissolving pulp pieces in DMAc/LiCl involves a solvent exchange procedure, described elsewhere (Obradovic et al 2014). When the solvent exchange procedure was finished, a transparent gel was formed.…”

“…When fibres are placed in DMAc/LiCl, solvent molecules penetrate the fibre surface, ends of the fibre, and places where the wall structure has been damaged (Pionteck et al 1996). Once regenerated from the DMAc/LiCl solvent system, the X-ray diffraction pattern of cellulose exhibited peaks similar to untreated cellulose (Cell I), or amorphous cellulose depending on the used anti-solvent (Obradovic et al 2014). Cellulose samples were structured with crystallites of cellulose I in a matrix of paracrystalline and amorphous cellulose, indicating that solvent during the swelling process does not penetrate crystalline regions.…”