Abstract
The impact of selected cellulose solvent systems based on the principal constituents tetrabutylammonium fluoride (TBAF), 1-ethyl-3-methyl-1H-imidazolium-acetate, N-methylmorpholine-N-oxide, or calcium thiocyanate octahydrate (CTO) on the properties of cellulose II aerogels prepared from these solvent systems has been investigated as a means towards tailoring cellulose aerogel properties with respect to specific applications. Cotton linters were used as representative plant cellulose. Cellulose was coagulated from solutions with comparable cellulose content, and dried with supercritical carbon dioxide after solvent exchange. The resulting bulk aerogels were comprehensively morphologically and mechanically tested to relate structure and mechanical properties. Different solvent systems caused considerable differences in the properties of the bulk samples, such as internal surface area (nitrogen sorption), morphology, porosity (He pycnometry, thermoporosimetry), and mechanical stability (compression testing). The results of SAXS, WAXS, and solid-state 13C NMR spectroscopy suggest that this is due to different mechanisms of cellulose self-assembling on the supramolecular and nanostructural level, respectively, as reflected by the broad ranges of cellulose crystallinity, fibril diameter, fractal dimension and skeletal density. Both solid state NMR and WAXS experiments confirmed the sole existence of the cellulose II allomorph for all aerogels, with crystallinity reaching a maximum of 46–50 % for CTO-derived aerogels. Generally, higher fibril diameter, degree of crystallinity, hence increased skeletal density were associated with good preservation of shape and dimension throughout conversion of lyogels to aerogels, and enhanced mechanical stability, but somewhat reduced specific surface area. Amorphous, yet highly rigid aerogels derived from TBAF/DMSO mixtures deviated from this trend, most likely due to their particular homogeneous and nanostructured morphology.Electronic supplementary materialThe online version of this article (doi:10.1007/s10570-016-0896-z) contains supplementary material, which is available to authorized users.
Highlights
Aerogels are solids consisting of a coherent openporous network of loosely packed, bonded cellulose particles or nanofibrils whose voids are filled with gases, such as air, and feature very low density and high specific surface area (Liebner et al 2012a)
The dimensional stability of cellulosic lyogels and aerogels along their preparation path and during storage depends on their solid content, the particular cellulose aggregate structure and the presence of hydrophilic cellulose solvent remnants, confined in fibrillary interstices of the lyogel or attached to cellulose
While the rather uniform morphology, fine fibrils and small pore size of CL-tetrabutylammonium fluoride (TBAF) resulted in a brittle structure that proved to be very rigid under compressive stress, the open and more heterogeneous cellulose II network of CL in [EMIm][OAc]/DMSO (CL-EMIm) aerogels was very ductile compared to all other materials and showed a strong density-dependency compared to CL-calcium thiocyanate octahydrate (CTO) aerogels
Summary
Aerogels are solids consisting of a coherent openporous network of loosely packed, bonded cellulose particles or nanofibrils whose voids are filled with gases, such as air, and feature very low density and high specific surface area (Liebner et al 2012a). In case of TBAF/ DMSO, the same mechanism of diffusion-controlled coagulation and gel formation, mediated through gradual replacement of fluoride anions by water (hydrated hydroxide ions) and increasing hydrogen bonding (Ostlund et al 2009), results in dense networks of very small interwoven nanofibers (CLTBAF), and interconnected nanopore systems, which was confirmed by the small fibril and pore radii determined by SAXS and thermoporosimetry, respectively for this sample.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
