Impact of selected solvent systems on the pore and solid structure of cellulose aerogels
Cellulose
Impact of selected solvent systems on the pore and solid structure of cellulose aerogels
0 L. Carbajal J.-M. Nedelec UMR 6296, Institute of Chemistry of Clermont-Ferrand, Centre National de la Recherche Scientifique , 24 Avenue des Landais, 63171 Aubie`re , France
1 L. Carbajal J.-M. Nedelec Institute of Chemistry of Clermont-Ferrand, Clermont Universite ́, Ecole Nationale Supe ́rieure de Chimie de Clermont-Ferrand , BP 10448, 63000 Clermont-Ferrand , France
2 N. Pircher C. Schimper M. Bacher T. Rosenau F. Liebner (&) Division of Chemistry of Renewable Resources, University of Natural Resources and Life Sciences Vienna , Konrad-Lorenz-Straße 24, 3430 Tulln , Austria
3 H. Rennhofer H. C. Lichtenegger Institute of Physics and Material Sciences, University of Natural Resources and Life Sciences Vienna , Peter Jordan Straße 82, 1190 Vienna , Austria
The impact of selected cellulose solvent systems based on the principal constituents tetrabutylammonium fluoride (TBAF), 1-ethyl-3-methyl-1Himidazolium-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.
Introduction
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)
.
Depending on the source material and preparation
method, their solid structure can consist of the
cellulose polymorphs I (Ia: dominating in bacterial
cellulose; Ib: principal crystal phase of native plant
cellulose) or II, the sole allomorph of cellulose
coagulated from solution state. In any case, intrinsic
characteristics of the cellulosic source material, such
as molecular weight average and distribution or
purity—for example in terms of residual lignin content
in wood pulp or content of carboxyl- and carbonyl
groups—substantially govern the material properties
of the derived aerogels. For cellulose II aerogels—the
target material of this study—morphology and
mechanical properties can be furthermore controlled
by the type of solvent used to shape aerogels from
solution state.
Cellulose solvents share the capability to disrupt
and rearrange the complex intra- and inter-molecular
hydrogen bond networks in the different cellulose
allomorphs, thereby achieving cellulose solubility.
Several solvents have previously been used for the
preparation of cellulosic aerogels, including molten
Ca(SCN)2 nH2O, an inorganic salt hydrate
(Hoepfner
et al. 2008; Jin et al. 2004)
, aqueous alkali hydroxides,
such as aq. NaOH
(Cai et al. 2008; Gavillon and
Budtova 2008)
or aq. LiOH (Cai et al. 2008) [often
under addition of solubilizing additives such as urea
(Cai et al. 2008)
or thiourea
(Zhang et al. 2012)
], a
range of low-melting organic salts (ionic liquids)
(Aaltonen and Jauhiainen 2009; Sescousse et al.
2011)
, and the tertiary amine oxide
N-methylmorpholine-N-oxide monohydrate (NMMO H2O), which is
used in the large-scale production of Lyocell fibers
(Innerlohinger et al. 2006; Liebner et al. 2009)
. A
comprehensive summary of cellulose solvents with
focus on manufacture of cellulosic aerogels, including
particularities regardin (...truncated)