Article pubs.acs.org/cm
Strain Hardening and Pore Size Harmonization by Uniaxial Densification: A Facile Approach toward Superinsulating Aerogels from Nematic Nanofibrillated 2,3-Dicarboxyl Cellulose Sven F. Plappert,† Jean-Marie Nedelec,‡ Harald Rennhofer,§ Helga C. Lichtenegger,§ and Falk W. Liebner*,† †
Division of Chemistry of Renewable Resources, University of Natural Resources and Life Sciences Vienna, Konrad-Lorenz-Straße 24, 3430 Tulln, Austria ‡ Université Clermont Auvergne, CNRS, SIGMA Clermont, ICCF, F-63000 Clermont-Ferrand, France § Institute of Physics and Material Sciences, University of Natural Resources and Life Sciences Vienna, Peter-Jordan-Straße 82, 1190 Vienna, Austria S Supporting Information *
ABSTRACT: Dissolving pulp has been subjected to consecutive periodate/chlorite treatments to afford 2,3-dicarboxyl cellulose (DCC, 1.02 mmol g−1 COOH). Subsequent nanofibrillation afforded stable nematic nf-DCC dispersions (average particle size 2.1 nm × 525 nm) at significantly lower energy input compared to TEMPO-oxidation. Acid-induced gelation triggered by extensive hydrogen bonding sets the ordered state and affords free-standing hydrogels that can be converted to highly transparent birefringent aerogels by scCO2 drying. Uniaxial compression of the obtained ultra-lightweight ductile nf-DCC aerogels down to 5% of their original volume intriguingly preserves nematic orientation and transparence. Simultaneously, strain hardening translates into exceptionally good mechanical properties, such as toughness at nearly zero Poisson’s ratio. Uniaxial compression has been furthermore demonstrated to be a facile and efficient means for converting nf-DCC aerogels of broad, multiscale pore size distribution into entirely micro/mesoporous scaffolds of narrow size distribution at far-reaching preservation of porosity. Following this approach, thermally superinsulating nf-DCC aerogels (λ = 0.018 W m−1 K−1) have been prepared, whose intriguing mechanical properties, transparence, and nematic ordering bear great potential for other applications as well.
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INTRODUCTION Self-assembly and liquid-crystalline ordering of nanoscale cellulose fibers promoted by charged surfaces moieties is an intriguing approach toward nanostructured bio-based materials. Surface charges are capable of increasing the effective volume of such anisometric nanoparticles, promoting their self-assembly into liquid crystal phases as first described by Onsager.1 This phenomenon has so far been used in the preparation of liquid crystal templates,2,3 photonic materials,4,5 nanocomposites6 and transparent aerogels.7 Cellulose aerogels of narrow pore size distribution in the mesopores range (
