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Cellulose–silica aerogels
Accepted Manuscript Title: Cellulose-silica aerogels Author: Arnaud Demilecamps Christian Beauger Claudia Hildenbrand Arnaud Rigacci Tatiana Budtova PII: DOI: Reference:
S0144-8617(15)00046-6 http://dx.doi.org/doi:10.1016/j.carbpol.2015.01.022 CARP 9610
To appear in: Received date: Revised date: Accepted date:
26-10-2014 6-1-2015 8-1-2015
Please cite this article as: Demilecamps, A., Beauger, C., Hildenbrand, C., Rigacci, A., and Budtova, T.,Cellulose-silica aerogels, Carbohydrate Polymers (2015), http://dx.doi.org/10.1016/j.carbpol.2015.01.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Submitted to Carbohydrate Polymers 26 October 2014
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Cellulose-silica aerogels
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Revised 6 January 2015
Arnaud DEMILECAMPS1, Christian BEAUGER2, Claudia HILDENBRAND2, Arnaud
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RIGACCI2 and Tatiana BUDTOVA1*
MINES ParisTech, PSL Research University, CEMEF - Centre de Mise en Forme des
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Matériaux, rue Claude Daunesse, CS 10207, 06904 Sophia Antipolis Cedex, France
MINES ParisTech, PSL Research University, PERSEE - Centre procédés, énergies
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renouvelables et systèmes énergétiques, rue Claude Daunesse, CS 10207, 06904 Sophia
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Antipolis Cedex, France
*Corresponding author : [email protected] tel : +33 (0)4 93 95 74 70
fax : +33 (0)4 92 38 97 52
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ABSTRACT
2 Aerogels based on interpenetrated cellulose-silica networks were prepared and
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characterised. Wet coagulated cellulose was impregnated with silica phase,
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polyethoxydisiloxane, using two methods: i) molecular diffusion and ii) forced flow induced
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by pressure difference. The latter allowed an enormous decrease in the impregnation times, by
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almost three orders of magnitude, for a sample with the same geometry. In both cases,
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nanostructured silica gel was in situ formed inside cellulose matrix. Nitrogen adsorption
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analysis revealed an almost threefold increase in pores specific surface area, from cellulose
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aerogel alone to organic-inorganic composite. Morphology, thermal conductivity and
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mechanical properties under uniaxial compression were investigated. Thermal conductivity of
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composite aerogels was lower than that of cellulose aerogel due to the formation of
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superinsulating mesoporous silica inside cellulose pores. Furthermore, composite aerogels
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were stiffer than each of reference aerogels.
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Keywords: aerogels; cellulose; silica; nanostructured composites; specific surface area;
thermal conductivity.
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1. INTRODUCTION
21 Aerogels are light-weight (bulk density 0.003-0.2 g/cm3) and nanostructured materials
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with mean pore size in the mesopores range and very high specific surface area (800-1000
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m2/g) (Brinker & Scherer, 1990; Kocon, Despetis & Phalippou, 1998; Aegerter, Leventis, &
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Koebel, 2011). A new generation of aerogels was developed in the past decade: they are
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biomass-based and can be called bio-aerogels. Inspired by the synthesis of classical aerogels,
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bio-aerogels are prepared via polymer dissolution, gelation (in some cases this step is omitted)
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and coagulation in a non-solvent followed by supercritical (sc) drying with CO2.
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Various polysaccharides can be used for preparing bio-aerogels: for example, cellulose
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(so-called Aerocellulose) (Gavillon & Budtova, 2008; Liebner et al, 2008; Tsioptsias,
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Stefopoulos, Kokkinomalis, Papadopoulou &Panayiotou, 2008; Aaltonen & Jauhiainen, 2009;
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Sescousse, Gavillon & Budtova, 2011a; Sescousse, Gavillon & Budtova, 2011b), cellulose
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esters (Tan, Fung, Newman & Vu, 2011; Fischer, Rigacci, Pirard, Berthon-Fabry, Achard,
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2006), marine polysaccharides (Robitzer, David, Rochas, Di Renzo & Quignard, 2008; Silva,
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Duarte, Carvalho, Mano & Reis, 2011; Ganesan & Ratke, 2014), pectin (White, Budarin. &
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Clark, 2010; García-González, Carenza, Zeng, Smirnova & Roig, 2012; Rudaz et al, 2014)
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and starch (White, Budarin, Luque, Clark & Macquarrie, 2009; García-González, Uy, Alnaief
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& Smirnova, 2012). A special case is bio-aerogels based on cellulose I (nanofibrillated or
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bacterial cellulose): no polymer dissolution or gelation is involved; entangled cellulose
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nanofibrils dispersed in aqueous media are dried with sc CO2 after exchange of water to
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ethanol or acetone (Liebner et al, 2010; Granstroom et al, 2011; Kobayashi, Saito & Isogai,
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2014). In most of the examples cited, bio-aerogels present wide distribution of pore sizes,
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from a few tens of nanometers to a few microns, rather large specific surface area (200-500
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m2/g) and they can be plastically deformed until a strain of 60-80% before complete pore
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collapse (Sescousse et al, 2011a; Kobayashi et al, 2014). Bio-aerogels have a wide range of
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diverse applications, for example, as scaffolds and delivery carriers (Robitzer et al, 2008;
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García-González, Alnaief & Smirnova, 2011), matrices for catalysis (Chtchigrovsky et al,
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2009) and in electro-chemistry when pyrolysed (Budarin, Clark, Luque, Macquarrie & White,
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2008; Rooke et al, 2011).
It is well known that classical silica and some organic synthetic aerogels are thermal
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super-insulating materials, with thermal conductivity λ being lower than that of air in room
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conditions, around 0.015 vs 0.025 W/(m.K) for air (Koebel, Rigacci & Achard, 2012). These
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aerogels are in majority mesoporous which allows decreasing the thermal conductivity of
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gaseous phase below that of air due to Knudsen effect, without needing vacuum technology.
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However, silica aerogels are very brittle which hinders their applications, and synthesis of
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organic aerogels such as resorcinol-formaldehyde based, leads to mechanically strong
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materials but uses toxic components (Pekala, Alviso, & LeMay, 1990). It would be thus
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extremely attractive to develop bio-aerogels that could perform as classical silica aerogels in
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terms of thermal conductivity and as synthetic organic ones regarding mechanical strength.
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Until now, only two examples of thermal super-insulating bio-aerogels has been
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reported in the literature: one is so-called Aeropectin obtained via pectin dissolution-
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coagulation-drying with sc CO2 route and presenting a thermal conductivity between 0.016
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and 0.020 W/(m.K) in room conditions (Rudaz et al, 2014), and the other is surface-
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carboxylated nanofibrillated cellulose with the lowest thermal conductivity of 0.018 W/(m.K)
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(Kobayashi et al, 2014). Because of the wide distribution of pore sizes in most of other bio-
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aerogels and the presence of (very) large macropores, their thermal conductivity is usually
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above that of air. For starch aerogels and foams it stands in the range from 0.025 to 0.050
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W/(m.K) (Glenn & Stern, 1999), and for cellulose aerogels, hydrophobised or not, the best
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values reported up to now are between 0.029 and 0.032 W/(m.K) in room conditions (Shi, Lu,
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Guo, Sun & Cao, 2013; Rudaz, 2013; Nguyen et al, 2014) . 4 Page 4 of 33
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One of the potential ways to decrease the thermal conductivity of bio-aerogels could be “filling” their pores with a super-insulating material which can be silica aerogel.
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Theoretically, it is then expected that the conductivity of the gaseous phase, which represents
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about 60% of the total conductivity (see an example for the Aeropectin in Rudaz et al, 2014),
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would be significantly decreased due to the presence of nanostructured and super-insulating
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silica aerogel in the pores of the bio-aerogel network. This can be performed, for example, by
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impregnating the wet matrix of bio-aerogel (i.e. before drying) with silica sol, followed by in-
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situ silica gelation, formation of an interpenetrated organic-inorganic network and drying with
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sc CO2. It should be noted that the addition of silica phase to cellulose matrix will increase the
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contribution of the skeletal heat conduction of the composite due to an overall density
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increase. There is thus a delicate compromise between the positive and negative effect of
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filling cellulose aerogel pores with silica aerogel.
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The impregnation of wet coagulated cellulose with either tetraethyl orthosilicate
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(TEOS) or with sodium silicate (Na2SiO3), followed by silica cross-linking and drying with sc
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CO2, was recently reported (Cai et al, 2012; Liu, Yu, Hu, Liu & Liu, 2013). When TEOS was
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used, the specific surface area of hybrid aerogel increased significantly, indicating the
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formation of nanostructured silica in the pores of cellulose (Cai et al, 2012). Unfortunately,
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the increase of silica concentration in the pores of cellulose matrix induced the increase of the
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thermal conductivity of hybrids (0.035-0.045 W/(m.K)), and their mechanical properties
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slightly decreased as compared with those of neat Aerocellulose. For the case when sodium
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silicate was used for the impregnation (Liu et al, 2013), the thermal conductivity was not
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reported, but the specific surface area remained the same or even slightly decreased from 320
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m2/g for the neat Aerocellulose to 270 m2/g for the composite aerogels. SEM images suggest
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that distinct silica particles and not an aerogel were formed in the pores of cellulose matrix. A
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similar approach was taken in Sai et al, 2013: freeze-dried bacterial cellulose was
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impregnated by TEOS and then freeze-dried again. The specific surface area significantly
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increased in the presence of silica, from 130 m2/g for the neat freeze-dried bacterial cellulose
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to 800-900 m2/g for composite materials, demonstrating again the formation of nanostructured
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silica aerogel in the porosity of cellulose. The thermal conductivity also increased with the increase of silica content from 0.030 W/(m.K) for the neat bacterial cellulose to 0.033-0.037
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W/(m.K) for the composite. Summarising, a potential decrease of thermal conductivity of
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cellulosic matrix (Aerocellulose) due to the impregnation of silica did not work until now
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probably because of the remaining macropores not filled with silica aerogels and also the
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increase of hybrid material overall density.
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In our work we took the same impregnation-with-TEOS approach, as discussed above.
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First, we significantly improved the impregnation process itself: we developed a so-called
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forced-flow impregnation as opposed to impregnation driven by molecular diffusion reported
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in literature. Second, by a careful selection and combination of cellulose matrix, TEOS
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formulation and gelation conditions, we obtained composite cellulose-silica aerogels with the
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thermal conductivity which was lower than that of the neat Aerocellulose and very close to
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the conductivity of air. We performed a comprehensive characterisation of the morphology
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and properties of composite aerogels by using scanning electron microscopy, nitrogen
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adsorption, thermal conductivity measurement and mechanical uniaxial compression methods.
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2. MATERIALS AND METHODS
2.1. Materials
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Cellulose was kindly provided by Thuringian Institute of Textile and Plastics Research
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(TITK), Rudolstadt, Germany; the degree of polymerization 600, as provided by TITK. Ionic
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liquid 1-ethyl-3-methylimidazolium acetate (EmimAc) was purchased from BASF.
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Dimethylsulfoxyde (DMSO, purity > 99%) and ethanol (purity 98%) were purchased from
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Fischer Scientific. Aqueous solution of ammonium hydroxide, 35wt%, was purchased from 6 Page 6 of 33
Sigma Aldrich. Polyethoxydisiloxane (PEDS) was used as prepolymerized oligomers of
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TEOS in ethanol with SiO2 concentration of 20wt% (diluted with ethanol when needed); it
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was kindly provided by PCAS, Longjumeau, France. All solvents and chemicals were used as
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received.
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2.1.1. Preparation of wet coagulated cellulose for impregnation
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The overall scheme of composite aerogels preparation is shown in Figure 1 and all
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concentrations are in wt% unless otherwise mentioned. First, cellulose powder was dried for 2
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hours in a vacuum oven (50 °C, 50 mbar) before dissolution. Cellulose was dissolved in
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EmimAc:DMSO = 80:20 for 16 h at 70 °C to form a 3wt% cellulose solution. DMSO was
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used to facilitate cellulose dissolution due to solvent viscosity decrease; at this concentration
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of DMSO cellulose is completely dissolved according to the phase diagram obtained (Le,
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Rudaz & Budtova, 2014). Cellulose solution was poured into the moulds (the shape and size
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will be given for each case when a special geometry was requested) and coagulated in
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ethanol. Then several washings with ethanol were performed to fully remove the traces of
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other solvents. Cellulose “alcogels” were ready to be impregnated by silica sol; non-
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impregnated cellulose alcogel was also dried with sc CO2 to obtain reference Aerocellulose
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(for the latter see details below).
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Figure 1.
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A schematic presentation of the preparation route of cellulose-silica composite aerogels and example of 9 cm x 9 cm x 1 cm sample.
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2.1.2. Molecular diffusion impregnation
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Cellulose alcogel was immersed in 16% PEDS solution (malcogel = msol) for 24h at room
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temperature, then extracted and placed in catalyst solution (1.3 wt% of NH4OH in
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ethanol:water = 96:4 w/w) in the proportion mcatalyst solution = mgel for the next 24 h (Figure 1)
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resulting in silica gelation. Most of the samples were disks of 4 cm in diameter and from 0.5
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to 1 cm in thickness, except otherwise mentioned.
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To follow the impregnation kinetics of cellulose alcogels by PEDS via molecular diffusion (before immersion in the catalyst solution), wet disks were extracted from silica
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solution at various impregnation times and dried in a vacuum oven at 80 °C and 150 mbar up
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to constant weight. The evolution of the mass of the disks as a function of impregnation time
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was recorded.
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2.1.3. Forced flow impregnation process In order to accelerate silica impregnation, we developed a home-made set-up to perform
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so-called “forced flow impregnation”. Here we describe the principle of the forced flow
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impregnation; no technical optimisation was performed to further improve the processing.
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Cellulose alcogel disc of the same geometry as for molecular diffusion impregnation was
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placed on a grid fixed in a home-made plastic funnel, the whole setup placed above a Büchner
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flask connected to a primary vacuum pump (Figure 2). 16 wt% PEDS solution was poured
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over the gel (msol = 2×malcogel). A pressure gradient was generated using the vacuum pump to
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force the transport of PEDS through cellulose disk porosity. The impregnated gel was then 8 Page 8 of 33
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immersed in the catalyst solution in the same way as described above for molecular diffusion
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impregnation.
172 Cellulose alcogel
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PEDS solution
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Plastic grid
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To vacuum pump
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Figure 2.
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Scheme of the forced-flow impregnation set-up
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To follow the impregnation kinetics, the filtrate (the liquid phase once passed through
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cellulose alcogel) was collected at various impregnation times, ethanol evaporated and dry
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mass measured. The impregnation was considered completed when the silica concentration in
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the filtrate was equal to two thirds of that of the initial PEDS solution, taking here into
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account the ethanol present in the initial alcogel.
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Reference silica aerogel was prepared by adding catalyst solution to 8wt% PEDS,
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followed by gelation, aging for 24h, washing in ethanol to remove non-reacted products and
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drying with sc CO2 (Bisson, Rigacci, Lecomte, Rodier & Achard, 2003).
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2.1.4. Drying
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Before drying, cellulose-silica impregnated samples were washed in ethanol several
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times to remove unreacted species. Samples were dried with sc CO2 as follows. They were
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placed in a 1 L autoclave containing ethanol to avoid sample drying by evaporation before the 9 Page 9 of 33
system was pressurized at 50 bars and 37 °C with gaseous CO2. The excess of ethanol was
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purged with CO2 maintaining the pressure and temperature constant. Then the system was
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pressurized at 80 bars and 37 °C and dynamic washing step with 5 kg CO2/h was carried out
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for 1 hour. The system is then let in a static mode for 1 to 2 hours at the same pressure and
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temperature to allow supercritical CO2 to diffuse even in the nanometer-size pores. The
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dynamic washing starts with the same sc CO2 output for 2 hours. Afterwards, the system was
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slowly depressurized at 4 bars/h and 37 °C to avoid cracks and cooled down to room
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temperature. The autoclave was then opened and dry samples collected. Composite cellulose-
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silica aerogels were solid, easy to handle and not crumbling materials.
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Specific surface area of pores was determined by N2 adsorption isotherms at 77 K and
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the BET theory using ASAP 2020 apparatus from Micromeritics. Samples were vacuum-
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degassed for 5 hours at 70 °C before analysis. The error was ± 20 m2/g.
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Samples’ bulk densities ρbulk were measured using powder pycnometer Geopyc 1360 from Micromeritics; the error was less than 5%. The porosity ε(%) was calculated as a
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function of bulk and skeletal ρskeletal densities:
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(1)
The morphology of the samples was studied using high resolution scanning electron
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microscope (SEM) SEM-FEG Zeiss Supra 40. Samples were coated with a 7 nm gold-
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palladium layer with a QUORUM Q150T rotating metallizer before observations.
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Philips XL30 environmental SEM coupled with Energy Diffractive Spectroscopy (EDS)
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was used to analyse the distribution of silicon in composite aerogel in order to evaluate the
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distribution of silica.
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Elemental analysis was done in CNRS Service Central d’Analyse laboratory
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(Villeurbanne, France) using atomic absorption spectroscopy. Weight concentration of silicon
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(wt%Si) was measured allowing evaluation of wt% of silica (
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in the dry aerogel (
and silica mass yield
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as follows: (2)
where MSi = 29 g/mol and
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respectively, and
are the molar mass of silicon and silica,
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(3)
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where
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concentration in the wet material in case of total silica conservation in the sample during
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coagulation and washing steps. We thus obtain, for the initial proportion between cellulose
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and silica used,
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being theoretical silica
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72% .
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with
The thermal conductivity was measured with a commercial heat flow meter Laser Comp (Fox 150 from Laser Comp) in room conditions; it is steady state method. The Laser Comp
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Fox 150 was calibrated using a Certified Reference Material IRMM-440 according to
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standard procedures. Before measurements, the samples were polished to obtain flat parallel
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surface squares of 9 cm × 9 cm and 1 cm thickness (see photo in Figure 1). Three samples of
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each formulation were tested and a mean value calculated; the standard deviation was ± 0.002
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W/(m.K).
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Uniaxial compression experiments were carried out on Zwick mechanical testing
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machine. Samples were of cylindrical shape with ratio length/diameter = 3/2, typical length
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was around 3 cm. Because of the restricted samples’ geometry for forced-flow impregnation
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set-up, only diffusion-impregnated samples were studied. Before measurements samples were
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polished to make upper and lower cylinder surfaces planar and parallel which was verified 11 Page 11 of 33
with a micrometric sensor. Two different loads were used, as suggested in Rudaz et al, 2014:
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i) 100 N for precise measurements of Young modulus (E) in the linear visco-elastic regime
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and ii) 2000 N for complete stress-strain curve. The tests were performed at room
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temperature, atmospheric pressure and around 40% relative humidity. The displacement rate
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was 1 mm/min and experiments were performed until sample break. At least three samples
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per formulation were tested and mean values for Young modulus and fracture stress
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calculated. The experimental errors were determined, for each formulation, from standard
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deviation.
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3. RESULTS AND DISCUSSION
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3.1. Impregnation kinetics
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Figure 3 shows the impregnation kinetics for two cases: 1) molecular diffusion and 2)
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forced flow impregnation of PEDS solution into wet cellulose alcogel. In the first case the
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impregnation kinetics is described by the evolution of cellulose alcogel relative weight
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increase wrel, 1(t) in time t as follows:
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(4)
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where w1(t) is the dry weight of the impregnated cellulose at time t and wmax,1 is the
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theoretical maximal dry weight of impregnated cellulose matrix calculated assuming silica
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fully penetrating cellulose and thus the initial silica concentration in silica sol is decreased by
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half (see Experimental section).
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268 269 Wrel,1
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Silica relative weight as a function of time for the case of diffusion impregnation and
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Figure 3.
forced flow impregnation (inset). Sample thickness is 5 mm. Dashed lines are given to guide
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the eye.
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t, h
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In the second case the impregnation kinetics is followed by the increase of the relative
silica weight in the filtrate, wrel,2 (t): (5)
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where w2(t) is the silica weight in the filtrate at time t and wmax,2 is the theoretical maximal
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silica weight in the filtrate calculated assuming the initial silica concentration in the sol is
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decreased by 2/3 as far as msol = 2×malcogel, resulting in 10.6%wt of PEDS solution (see
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Experimental section).
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The impregnation by molecular diffusion occurs in two steps (Figure 3): first quick silica penetration into cellulose alcogel and then slow approach to equilibrium. Diffusion is 13 Page 13 of 33
slowed down because of silica “filling” the pores of cellulose matrix close to the sample
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surface thus creating silica concentration gradient and decreasing the pore size. At
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equilibrium, silica weight inside the cellulose matrix is reaching about 90% of the theoretical
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maximal value wmax,1 within 10% experimental error. Overall, the impregnation process by
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molecular diffusion is very slow: at least 7 hours are needed to fully impregnate a 5 mm thick
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cellulose alcogel disk.
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The rate of impregnation via forced flow set-up is much faster as compared to
molecular diffusion process (inset of Figure 3): the time needed to reach 80-95% of the
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theoretical maximal impregnation of cellulose matrix of 5 mm thickness is drastically reduced
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from 7 hours for diffusion to ~ 15-20 minutes for forced flow. As mentioned in Methods
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section, no technical optimisation was performed and pressure difference was not varied to
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study the impregnation kinetics in details and to improve it; our goal was to demonstrate that
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forced flow impregnation allows decreasing the impregnation time by almost three orders of
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magnitude for the same sample thickness. It should also be noted that for diffusion-controlled
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impregnation the effective impregnation distance is equal to the half-thickness of the sample
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while for the forced-flow it is the whole thickness of the disk.
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3.2. Elemental analysis and morphology of composite cellulose-silica aerogels To understand if silica is homogeneously distributed in the cellulose matrix, EDS
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spectroscopy was used. The spectra were taken on a transversal cut of samples prepared via
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molecular diffusion and forced flow impregnation. One spectrum was taken every 100 μm
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over a straight line along the sample cross-section. The weight per cent of Si, wt%Si, was
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determined from the intensity of the silicon Kα peak at 1.71 keV and the concentration of
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SiO2 was calculated according to eq.2. Figure 4 shows a representative example of the
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distribution of SiO2 along the cross-section of composite aerogel,
, as a function of
14 Page 14 of 33
the distance l from the sample surface, for both diffusion and forced flow impregnation. For
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diffusion-controlled impregnation, both sides of the sample are equivalent in terms of silica
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impregnation, while for the forced flow impregnation l = 0 corresponds to the upper sample
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surface in contact with PEDS solution, see Figure 2.
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Figure 4
Example of silica distribution in %wt along the transversal cut of composite aerogels
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impregnated by molecular diffusion for 24h (filled symbols) and forced-flow process (after
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1h) (open symbols) as a function of distance from the upper surface of the sample (with
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thickness equal to 5 mm).
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EDS shows local elemental composition on the surface of the sample cross-section. The
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obtained high values of silica concentration indicate that most of the signals come from Si
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element. This means that cellulose is well coated by silica resulting in the overestimated
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values of silica concentration as compared to the maximal bulk theoretical value of 72wt%, 15 Page 15 of 33
see Experimental section. SEM images (see further) will demonstrate that cellulose network is
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completely covered by silica explaining high local SiO2 concentrations. Despite the fact that
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EDS is not reflecting average bulk silica concentration, it gives valuable information on silica
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penetration and distribution inside the sample. The sample impregnated with forced flow
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process shows slightly higher silica concentration as compared to diffusion process, especially
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near sample upper surface. Diffusion-controlled impregnation is showing a more
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homogeneous silica distribution in the cellulose matrix as compared to forced-flow
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impregnated hybrids: in the latter case, silica concentration gradient appears with the distance
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from the upper sample surface. In overall, forced flow impregnation appears to be a very
345
efficient way to reduce impregnation times while keeping a good filling of the cellulose
346
matrix with silica.
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In order to estimate the average bulk composition of composite aerogels and determine silica yield, classical elemental analysis was performed using atomic absorption spectroscopy:
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wt%Si was determined and recalculated into SiO2 concentration
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of silica wt% in the dry samples obtained with each method is very similar: 51 wt % for
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diffusion and 56 wt % for forced flow impregnation, confirming the observations by EDS
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spectroscopy that cellulose impregnation using both methods results in a similar filling of
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cellulose porosity by silica. Silica yield for both impregnation methods was calculated
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according to eq. 3: it is around 70 and 77% for molecular diffusion and forced flow processes,
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respectively. It is likely that the loss of silica occurs during the catalysis step, when the
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impregnated sample is immersed in the NH4OH solution. The gelation of silica takes about 10
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minutes in these conditions (Achard et al, 2007; Bisson, Rigacci, Lecomte & Achard, 2004)
358
and partial diffusion of silica sol from cellulose matrix into the catalyst solution may occur
359
even in this short duration. A thin layer of silica gel was observed around the impregnated
(eq.2). The amount
Ac ce p
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348
16 Page 16 of 33
360
cellulose after the catalysis step, confirming a partial loss of the silica due to diffusion of
361
silica precursor from the matrix prior to gelation in the porosity.
362
The morphology of composite aerogels obtained with both impregnation methods, as well as the reference samples (Aerocellulose and silica aerogel) was analysed by SEM (Figure
364
5). Aerocellulose (Figure 5a) has a hierarchical structure with “hairy” beads assembled
365
together, as demonstrated previously for Aerocellulose from cellulose-ionic liquid solutions
366
(Sescousse et al, 2011). The beads are supposed to be formed via spinodal decomposition
367
during cellulose coagulation from solution (Sescousse et al, 2011a). The inside morphology of
368
a bead is a “network” of fine cellulose strands. Characteristic pores size varies from several
369
tens of nanometers to several microns, as seen by SEM and also studied by non-intrusive
370
mercury porosimetry (Rudaz, 2013). Silica aerogel morphology (Figure 5b) is typical for
371
base-catalysed silica aerogels (Pierre & Rigacci, 2011). It is a network formed of
372
“nanometric” silica beads with pore diameter being few tens of nanometers, with no
373
macropores observable by SEM.
te Ac ce p
374
1µm
375
d
M
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(b)
(a)
200 nm
17 Page 17 of 33
(d)
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(c)
1µm
1µm
cr
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(e)
200nm
379
Figure 5.
382 383 384
te
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SEM images of Aerocellulose from 3% cellulose-EmimAc-DMSO solution (a); reference silica aerogel (b) and composite aerogels obtained with molecular diffusion (c) and
Ac ce p
380
d
378
M
377
forced flow (d, e) impregnation.
SEM images of composite aerogels are shown in Figure 5c for molecular diffusion-
385
controlled and Figure 5d, e for forced-flow impregnation. In both cases, silica phase appears
386
homogeneously distributed all over cellulose network confirming the formation of organic
387
and mineral interpenetrated networks. As observed by SEM, silica is filling the whole
388
macroporosity of Aerocellulose matrix and is “coating” cellulose backbone itself. The latter
389
explains high values of silica concentration obtained by EDS. Figure 5e shows a zoom on the
390
morphology of composite sample obtained with forced flow impregnation (the same obtained
18 Page 18 of 33
391
with diffusion impregnation): the pore sizes in the silica phase is in the range of few tens of
392
nanometers.
393 3.3. Properties of composite aerogels
395
Bulk density ρbulk of composite aerogels and of cellulose and silica reference samples
ip t
394
was measured as described in Experimental section and porosity ε% was calculated according
397
to eq.1; the values are given in Table 1. Porosity was calculated supposing cellulose and silica
398
skeleton density being roughly 1.5 and 2.0 g/cm3, respectively (Rudaz, 2013; Ayral,
399
Phalippou & Woignier, 1992). The average skeletal density of hybrid aerogels was calculated
400
taking into account the amount of silica for each impregnation case as obtained with
401
elemental analysis, 51 wt% for diffusion and 56 wt% for forced flow. The bulk density of
402
composite materials appears higher, as expected, and porosity slightly lower than that of each
403
of the reference samples. Overall, the porosity of composite aerogels is around 90%.
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404
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396
19 Page 19 of 33
i cr us
Table 1. Bulk density, porosity ε % (eq. 1), specific surface area SBET, Young modulus E, fracture stress σ* and strain S* and thermal conductivity in
406
ambient conditions λ20°C for cellulose and silica reference samples and composite aerogels. ε, %
SBET,
E,
σ*,
S*,
λ20°C,
m²/g
MPa
MPa
%
W/(m.K)
0.92
290
2.8 ± 0.47
9.20 ± 0.12
80 ± 5
0.033
0.130
0.94
975
1.9 ± 0.10
0.07 ± 0.007
4 ± 0.5
0.015
0.225
0.87
810
11.5 ± 1.11
6.30 ± 1.55
60 ± 5
0.026
0.155
0.91
750
-
-
-
0.028
ρbulk,
M
Formulation
an
405
Aerocellulose from 3%
0.125
pt
cellulose solution
ed
g/cm3
ce
SiO2 aerogel
Composite aerogel via diffusion
Ac
impregnation
Composite aerogel via forcedflow impregnation
407
21
Page 20 of 33
Table 1 compares specific surface area of organic and mineral reference samples with
408
that of composites. Both reference samples have SBET similar to what is reported in literature:
409
around 250-300 m2/g for Aerocellulose (Sescousse et al, 2011a; Rudaz, 2013) and around
410
800-1000 m2/g for silica aerogel (Pierre & Rigacci, 2011; Wong, Kaymak, Brunner &
411
Koebel, 2014). As compared to Aerocellulose, the specific surface area of both composites
412
increased tremendously, reaching values that are comparable with the characteristic ones of
413
silica aerogels. This result is a direct confirmation of the presence of nanostructured silica
414
aerogel phase in the pores of cellulose matrix.
us
cr
ip t
407
Silica aerogels are known to be excellent thermal superinsulating materials in room
416
conditions, as also obtained in this work, see Table 1: λ20°C = 0.015 W/(m.K). The main
417
reason for a low-density porous material to fall into superinsulation region is air confinement
418
in the pores of size below the free mean path of air molecule (Knudsen effect); at atmospheric
419
conditions it is around 70 nm. Mesoporous and light-weight silica aerogels satisfy these
420
conditions. Aerocellulose also has low density, but the presence of numerous large
421
macropores (Figure 5a) results in relatively high (as compared to superinsulation) thermal
422
conductivity values, 0.033 W/(m.K) in room conditions. In composite cellulose-silica
423
aerogels, superinsulating silica aerogel is filling Aerocellulose pores in general and
424
macropores in particular: thermal conductivity is thus decreased to 0.027 ± 0.001 W/(m.K).
425
This important result demonstrates the feasibility of significantly decreasing thermal
426
conductivity by “incorporating” a superinsulating silica aerogel into cellulose matrix. The
427
thermal superinsulation level is not achieved yet for the present composites most probably
428
because of i) the presence of some remaining macropores not filled with silica aerogel phase
429
and ii) the increase in composite density leading to the increase of phonon conduction through
430
the solid backbone.
Ac ce p
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415
431
Uniaxial compression of reference and composite aerogels is presented in Figure 6 as
432
stress-strain plots; the values of Young modulus and fracture stress and strain are reported in 22 Page 21 of 33
433
Table 1. Each compression experiment was performed to the moment when the sample was
434
starting to break (shoot of stress).
1
2
2
cr
1
ip t
435
M
an
us
3
436
Figure 6
d
437
Example of stress-strain uniaxial compression curves for cellulose-silica composite
439
aerogel obtained via molecular diffusion impregnation (1), reference Aerocellulose (2) and
440
silica (3) aerogels. Inset: zoom at low stresses and strains.
442
Ac ce p
441
te
438
Stress-strain curve for the reference Aerocellulose (Figure 6, curve 1) is typical for bio-
443
aerogels such as Aerocelluloses prepared from cellulose dissolved in aqueous 8%NaOH or in
444
ionic liquid (Sescousse et al, 2011a) or Aeropectins from pectin dissolved in acid medium
445
(Rudaz et al, 2014). Three regions can be distinguished: linear regime at low strains, long
446
plastic deformation region with pore walls bending and finally densification due to pore walls
447
collapse. Aerocellulose shows very high deformability, up to 70-80% strain, but do not
448
recover its shape after being highly compressed.
449
The reference silica aerogel is brittle, breaking at low strains of a few per cent, and with
450
lower (as compared to Aerocellulose) values of fracture stress and Young modulus (Table 1). 23 Page 22 of 33
According to a recent study of the mechanical properties of hydrophobic (silylated) silica
452
aerogels from the same PEDS precursor (Wong et al, 2014), silica aerogels with densities
453
between 0.1 and 0.2 g/cm3 show an elastic-like behavior deforming till 40% strain and
454
partially “springing back” to their original shape when the stress is released. Their Young
455
moduli were from ≈ 1 to 10 MPa and fracture stress from ≈ 0.02 to 2.5 MPa in this density
456
interval. Silica aerogel with densities above 0.2 g/cm3 were shown to exhibit a brittle
457
behavior, while those with the densities lower than 0.1 g/cm3 were highly compressible. In
458
our case, silica aerogels with densities around 0.13 g/cm3 have Young modulus of around 2
459
MPa and fracture stress of 0.07 MPa, but broke at low strains and showed no measurable
460
elastic recovery probably because no hydrophobisation was performed.
an
us
cr
ip t
451
Cellulose-silica composite aerogel shows significantly improved Young modulus as
462
compared to its reference counterparts, both Aerocellulose and silica aerogel (Figure 6, curve
463
1 and Table 1). With a mean value of Young modulus around E = 11.5 MPa the composite is
464
three-four times stiffer than each of reference materials with E = 2.8 and 1.8 MPa for
465
Aerocellulose and silica aerogel, respectively. The composite aerogel can also withstand
466
deformation up to 60% before break, which is similar to Aerocellulose and is ten times higher
467
than what is obtained for the reference silica aerogel. One of the reasons of high Young
468
modulus is hybrid higher density, 0.155 vs 0.125 and 0.130 g/cm3 as compared to each of
469
reference aerogels, Aerocellulose and silica, respectively. However, even if considering that
470
Young modulus is proportional to aerogel density in power 3 (Alaoui, Woignier, Scherer &
471
Phalippou, 2008), the obtained increase in modulus is higher than what could be theoretically
472
expected from density increase. Cellulose is clearly playing an important reinforcing role but
473
more data are needed to quantify this phenomenon (for example, mechanical properties of
474
cellulose strands in Aerocellulose). The synergy of cellulose-silica interpenetrated network
475
provides stiff and ductile aerogel materials despite there is no observable chemical interaction
476
between two components, as shows FTIR spectra (Figure S1 in the Supporting Information).
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24 Page 23 of 33
Cellulose and silica bonds are vibrating in very close energy domains which makes difficult to
478
detect the difference within the bands in the composite. The decrease of the absorption in the
479
[3700-3000] cm-1 region as well as that of the band at 900 cm-1 observed on the Aerocellulose
480
spectrum in composite aerogel is due to the small amount of this component in the composite.
481
We cannot see any disappearance or appearance of any absorption band in the composite
482
spectrum as compared to those of the reference components which could account for a
483
chemical bonding.
cr
ip t
477
4. CONCLUSIONS
an
485
us
484
486
Strong, light (bulk density around 0.2 g/cm3) and monolithic crack-free cellulose-silica
M
487
composite aerogels have been prepared by impregnation of wet coagulated cellulose with
489
polyethoxydisiloxanes solution. The impregnation was performed either by molecular
490
diffusion or by a forced flow process; in the latter PEDS was forced to penetrate inside the
491
cellulose matrix due to a pressure difference. Forced flow impregnation method significantly
492
reduced processing time as compared to impregnation driven by simple molecular diffusion:
493
impregnation times were reduced from ~ 7 hours to less than 30 minutes for a sample with the
494
same geometry. After in situ PEDS gelation and drying with supercritical CO2, aerogels based
495
on interpenetrated cellulose-silica networks were obtained.
te
Ac ce p
496
d
488
BET analysis confirmed the formation of nanostructured silica inside cellulose matrix:
497
specific surface area increased from ≈ 300 m2/g for Aerocellulose to 750-800 m2/g for
498
composites. The thermal conductivity in room conditions was reduced from 0.033 W/(m.K)
499
for Aerocellulose to 0.027 W/(m.K) ± 0.001 for composite aerogels due to the superinsulating
500
properties of silica aerogel itself, demonstrating that the concept of impregnation works for
501
decreasing the total thermal conductivity of a porous matrix with large macropores. Finally,
502
composite aerogels were strongly reinforced as compared with the reference aerogels still 25 Page 24 of 33
keeping high ductility characteristic of Aerocellulose: Young modulus increased in 3-4 times
504
and fracture strain remained very high, about 60%. The results obtained open very promising
505
ways in making high-performance composite organic-inorganic strong and lightweight
506
materials.
ip t
503
507
Acknowledgements
509
The work has received funding from the European Union Seventh Framework
us
cr
508
Programme (FP7/2007-2013) under grant agreement n° 260141, “AEROCOINs” project. We
511
thank P. Ilbizian (PERSÉE, Mines ParisTech, Sophia-Antipolis, France) for supercritical
512
drying and Suzanne Jacomet (CEMEF, Mines ParisTech, Sophia Antipolis, France) for help
513
in SEM and EDS analysis.
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26 Page 25 of 33
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ip t
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625
of monolithic silica aerogels made from polyethoxydisiloxanes. Microporous Mesoporous
629
Mater., 183, 23–29.
630
Ac ce p
628
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Figure captions
631 Figure 1.
633
A schematic presentation of the preparation route of cellulose-silica composite aerogels and
634
example of 9 cm x 9 cm x 1 cm sample.
ip t
632
Figure 2.
637
Scheme of the forced-flow impregnation set-up
us
636
cr
635
an
638 Figure 3.
640
Silica relative weight as a function of time for the case of diffusion impregnation and forced
641
flow impregnation (inset). Sample thickness is 5 mm. Dashed lines are given to guide the eye.
M
639
d
642 Figure 4
644
%wt of silica along the transversal cut of composite aerogels impregnated by molecular
645
diffusion for 24h (filled symbols) and forced-flow process (after 1h) (open symbols) as a
646
function of distance from the upper surface of the sample (with thickness equal to 5 mm).
Ac ce p
647
te
643
648
Figure 5.
649
SEM images of Aerocellulose from 3% cellulose-EmimAc-DMSO solution (a); reference
650
silica aerogel (b) and composite aerogels obtained with molecular diffusion (c) and forced
651
flow (d, e) impregnation.
652 653
Figure 6
32 Page 31 of 33
654
Example of stress-strain uniaxial compression curves for cellulose-silica composite aerogel
655
obtained via molecular diffusion impregnation (1), reference Aerocellulose (2) and silica (3)
656
aerogels. Inset: zoom at low stresses and strains.
Ac ce p
te
d
M
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657
33 Page 32 of 33
657
Cellulose-silica aerogels
658
Arnaud DEMILECAMPS, Christian BEAUGER, Claudia HILDENBRAND, Arnaud
659
RIGACCI and Tatiana BUDTOVA
661
ip t
660
cr
Highlights
us
662 -
Improved method of making cellulose-silica aerogels is proposed
664
-
Formation of interpenetrated organic-inorganic network of aerogels is confirmed
665
-
Composite aerogel has very high specific surface area due to mesoporous silica
666
-
Composite aerogels are stiffer than each of reference aerogel
667
-
Thermal conductivity of composite aerogel is lower than that of cellulose aerogel
M
d te
669
Ac ce p
668
an
663
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S0144-8617(15)00046-6 http://dx.doi.org/doi:10.1016/j.carbpol.2015.01.022 CARP 9610
To appear in: Received date: Revised date: Accepted date:
26-10-2014 6-1-2015 8-1-2015
Please cite this article as: Demilecamps, A., Beauger, C., Hildenbrand, C., Rigacci, A., and Budtova, T.,Cellulose-silica aerogels, Carbohydrate Polymers (2015), http://dx.doi.org/10.1016/j.carbpol.2015.01.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Submitted to Carbohydrate Polymers 26 October 2014
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Cellulose-silica aerogels
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Revised 6 January 2015
Arnaud DEMILECAMPS1, Christian BEAUGER2, Claudia HILDENBRAND2, Arnaud
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RIGACCI2 and Tatiana BUDTOVA1*
MINES ParisTech, PSL Research University, CEMEF - Centre de Mise en Forme des
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Matériaux, rue Claude Daunesse, CS 10207, 06904 Sophia Antipolis Cedex, France
MINES ParisTech, PSL Research University, PERSEE - Centre procédés, énergies
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renouvelables et systèmes énergétiques, rue Claude Daunesse, CS 10207, 06904 Sophia
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Antipolis Cedex, France
*Corresponding author : [email protected] tel : +33 (0)4 93 95 74 70
fax : +33 (0)4 92 38 97 52
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ABSTRACT
2 Aerogels based on interpenetrated cellulose-silica networks were prepared and
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characterised. Wet coagulated cellulose was impregnated with silica phase,
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polyethoxydisiloxane, using two methods: i) molecular diffusion and ii) forced flow induced
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by pressure difference. The latter allowed an enormous decrease in the impregnation times, by
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almost three orders of magnitude, for a sample with the same geometry. In both cases,
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nanostructured silica gel was in situ formed inside cellulose matrix. Nitrogen adsorption
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analysis revealed an almost threefold increase in pores specific surface area, from cellulose
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aerogel alone to organic-inorganic composite. Morphology, thermal conductivity and
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mechanical properties under uniaxial compression were investigated. Thermal conductivity of
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composite aerogels was lower than that of cellulose aerogel due to the formation of
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superinsulating mesoporous silica inside cellulose pores. Furthermore, composite aerogels
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were stiffer than each of reference aerogels.
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Keywords: aerogels; cellulose; silica; nanostructured composites; specific surface area;
thermal conductivity.
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1. INTRODUCTION
21 Aerogels are light-weight (bulk density 0.003-0.2 g/cm3) and nanostructured materials
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with mean pore size in the mesopores range and very high specific surface area (800-1000
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m2/g) (Brinker & Scherer, 1990; Kocon, Despetis & Phalippou, 1998; Aegerter, Leventis, &
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Koebel, 2011). A new generation of aerogels was developed in the past decade: they are
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biomass-based and can be called bio-aerogels. Inspired by the synthesis of classical aerogels,
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bio-aerogels are prepared via polymer dissolution, gelation (in some cases this step is omitted)
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and coagulation in a non-solvent followed by supercritical (sc) drying with CO2.
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Various polysaccharides can be used for preparing bio-aerogels: for example, cellulose
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(so-called Aerocellulose) (Gavillon & Budtova, 2008; Liebner et al, 2008; Tsioptsias,
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Stefopoulos, Kokkinomalis, Papadopoulou &Panayiotou, 2008; Aaltonen & Jauhiainen, 2009;
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Sescousse, Gavillon & Budtova, 2011a; Sescousse, Gavillon & Budtova, 2011b), cellulose
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esters (Tan, Fung, Newman & Vu, 2011; Fischer, Rigacci, Pirard, Berthon-Fabry, Achard,
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2006), marine polysaccharides (Robitzer, David, Rochas, Di Renzo & Quignard, 2008; Silva,
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Duarte, Carvalho, Mano & Reis, 2011; Ganesan & Ratke, 2014), pectin (White, Budarin. &
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Clark, 2010; García-González, Carenza, Zeng, Smirnova & Roig, 2012; Rudaz et al, 2014)
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and starch (White, Budarin, Luque, Clark & Macquarrie, 2009; García-González, Uy, Alnaief
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& Smirnova, 2012). A special case is bio-aerogels based on cellulose I (nanofibrillated or
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bacterial cellulose): no polymer dissolution or gelation is involved; entangled cellulose
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nanofibrils dispersed in aqueous media are dried with sc CO2 after exchange of water to
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ethanol or acetone (Liebner et al, 2010; Granstroom et al, 2011; Kobayashi, Saito & Isogai,
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2014). In most of the examples cited, bio-aerogels present wide distribution of pore sizes,
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from a few tens of nanometers to a few microns, rather large specific surface area (200-500
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m2/g) and they can be plastically deformed until a strain of 60-80% before complete pore
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collapse (Sescousse et al, 2011a; Kobayashi et al, 2014). Bio-aerogels have a wide range of
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diverse applications, for example, as scaffolds and delivery carriers (Robitzer et al, 2008;
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García-González, Alnaief & Smirnova, 2011), matrices for catalysis (Chtchigrovsky et al,
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2009) and in electro-chemistry when pyrolysed (Budarin, Clark, Luque, Macquarrie & White,
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2008; Rooke et al, 2011).
It is well known that classical silica and some organic synthetic aerogels are thermal
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super-insulating materials, with thermal conductivity λ being lower than that of air in room
52
conditions, around 0.015 vs 0.025 W/(m.K) for air (Koebel, Rigacci & Achard, 2012). These
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aerogels are in majority mesoporous which allows decreasing the thermal conductivity of
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gaseous phase below that of air due to Knudsen effect, without needing vacuum technology.
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However, silica aerogels are very brittle which hinders their applications, and synthesis of
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organic aerogels such as resorcinol-formaldehyde based, leads to mechanically strong
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materials but uses toxic components (Pekala, Alviso, & LeMay, 1990). It would be thus
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extremely attractive to develop bio-aerogels that could perform as classical silica aerogels in
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terms of thermal conductivity and as synthetic organic ones regarding mechanical strength.
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Until now, only two examples of thermal super-insulating bio-aerogels has been
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reported in the literature: one is so-called Aeropectin obtained via pectin dissolution-
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coagulation-drying with sc CO2 route and presenting a thermal conductivity between 0.016
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and 0.020 W/(m.K) in room conditions (Rudaz et al, 2014), and the other is surface-
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carboxylated nanofibrillated cellulose with the lowest thermal conductivity of 0.018 W/(m.K)
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(Kobayashi et al, 2014). Because of the wide distribution of pore sizes in most of other bio-
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aerogels and the presence of (very) large macropores, their thermal conductivity is usually
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above that of air. For starch aerogels and foams it stands in the range from 0.025 to 0.050
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W/(m.K) (Glenn & Stern, 1999), and for cellulose aerogels, hydrophobised or not, the best
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values reported up to now are between 0.029 and 0.032 W/(m.K) in room conditions (Shi, Lu,
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Guo, Sun & Cao, 2013; Rudaz, 2013; Nguyen et al, 2014) . 4 Page 4 of 33
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One of the potential ways to decrease the thermal conductivity of bio-aerogels could be “filling” their pores with a super-insulating material which can be silica aerogel.
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Theoretically, it is then expected that the conductivity of the gaseous phase, which represents
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about 60% of the total conductivity (see an example for the Aeropectin in Rudaz et al, 2014),
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would be significantly decreased due to the presence of nanostructured and super-insulating
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silica aerogel in the pores of the bio-aerogel network. This can be performed, for example, by
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impregnating the wet matrix of bio-aerogel (i.e. before drying) with silica sol, followed by in-
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situ silica gelation, formation of an interpenetrated organic-inorganic network and drying with
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sc CO2. It should be noted that the addition of silica phase to cellulose matrix will increase the
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contribution of the skeletal heat conduction of the composite due to an overall density
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increase. There is thus a delicate compromise between the positive and negative effect of
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filling cellulose aerogel pores with silica aerogel.
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The impregnation of wet coagulated cellulose with either tetraethyl orthosilicate
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(TEOS) or with sodium silicate (Na2SiO3), followed by silica cross-linking and drying with sc
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CO2, was recently reported (Cai et al, 2012; Liu, Yu, Hu, Liu & Liu, 2013). When TEOS was
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used, the specific surface area of hybrid aerogel increased significantly, indicating the
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formation of nanostructured silica in the pores of cellulose (Cai et al, 2012). Unfortunately,
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the increase of silica concentration in the pores of cellulose matrix induced the increase of the
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thermal conductivity of hybrids (0.035-0.045 W/(m.K)), and their mechanical properties
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slightly decreased as compared with those of neat Aerocellulose. For the case when sodium
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silicate was used for the impregnation (Liu et al, 2013), the thermal conductivity was not
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reported, but the specific surface area remained the same or even slightly decreased from 320
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m2/g for the neat Aerocellulose to 270 m2/g for the composite aerogels. SEM images suggest
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that distinct silica particles and not an aerogel were formed in the pores of cellulose matrix. A
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similar approach was taken in Sai et al, 2013: freeze-dried bacterial cellulose was
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impregnated by TEOS and then freeze-dried again. The specific surface area significantly
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increased in the presence of silica, from 130 m2/g for the neat freeze-dried bacterial cellulose
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to 800-900 m2/g for composite materials, demonstrating again the formation of nanostructured
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silica aerogel in the porosity of cellulose. The thermal conductivity also increased with the increase of silica content from 0.030 W/(m.K) for the neat bacterial cellulose to 0.033-0.037
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W/(m.K) for the composite. Summarising, a potential decrease of thermal conductivity of
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cellulosic matrix (Aerocellulose) due to the impregnation of silica did not work until now
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probably because of the remaining macropores not filled with silica aerogels and also the
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increase of hybrid material overall density.
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In our work we took the same impregnation-with-TEOS approach, as discussed above.
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First, we significantly improved the impregnation process itself: we developed a so-called
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forced-flow impregnation as opposed to impregnation driven by molecular diffusion reported
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in literature. Second, by a careful selection and combination of cellulose matrix, TEOS
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formulation and gelation conditions, we obtained composite cellulose-silica aerogels with the
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thermal conductivity which was lower than that of the neat Aerocellulose and very close to
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the conductivity of air. We performed a comprehensive characterisation of the morphology
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and properties of composite aerogels by using scanning electron microscopy, nitrogen
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adsorption, thermal conductivity measurement and mechanical uniaxial compression methods.
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2. MATERIALS AND METHODS
2.1. Materials
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Cellulose was kindly provided by Thuringian Institute of Textile and Plastics Research
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(TITK), Rudolstadt, Germany; the degree of polymerization 600, as provided by TITK. Ionic
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liquid 1-ethyl-3-methylimidazolium acetate (EmimAc) was purchased from BASF.
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Dimethylsulfoxyde (DMSO, purity > 99%) and ethanol (purity 98%) were purchased from
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Fischer Scientific. Aqueous solution of ammonium hydroxide, 35wt%, was purchased from 6 Page 6 of 33
Sigma Aldrich. Polyethoxydisiloxane (PEDS) was used as prepolymerized oligomers of
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TEOS in ethanol with SiO2 concentration of 20wt% (diluted with ethanol when needed); it
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was kindly provided by PCAS, Longjumeau, France. All solvents and chemicals were used as
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received.
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2.1.1. Preparation of wet coagulated cellulose for impregnation
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The overall scheme of composite aerogels preparation is shown in Figure 1 and all
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concentrations are in wt% unless otherwise mentioned. First, cellulose powder was dried for 2
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hours in a vacuum oven (50 °C, 50 mbar) before dissolution. Cellulose was dissolved in
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EmimAc:DMSO = 80:20 for 16 h at 70 °C to form a 3wt% cellulose solution. DMSO was
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used to facilitate cellulose dissolution due to solvent viscosity decrease; at this concentration
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of DMSO cellulose is completely dissolved according to the phase diagram obtained (Le,
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Rudaz & Budtova, 2014). Cellulose solution was poured into the moulds (the shape and size
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will be given for each case when a special geometry was requested) and coagulated in
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ethanol. Then several washings with ethanol were performed to fully remove the traces of
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other solvents. Cellulose “alcogels” were ready to be impregnated by silica sol; non-
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impregnated cellulose alcogel was also dried with sc CO2 to obtain reference Aerocellulose
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(for the latter see details below).
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Figure 1.
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A schematic presentation of the preparation route of cellulose-silica composite aerogels and example of 9 cm x 9 cm x 1 cm sample.
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2.1.2. Molecular diffusion impregnation
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Cellulose alcogel was immersed in 16% PEDS solution (malcogel = msol) for 24h at room
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temperature, then extracted and placed in catalyst solution (1.3 wt% of NH4OH in
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ethanol:water = 96:4 w/w) in the proportion mcatalyst solution = mgel for the next 24 h (Figure 1)
153
resulting in silica gelation. Most of the samples were disks of 4 cm in diameter and from 0.5
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to 1 cm in thickness, except otherwise mentioned.
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To follow the impregnation kinetics of cellulose alcogels by PEDS via molecular diffusion (before immersion in the catalyst solution), wet disks were extracted from silica
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solution at various impregnation times and dried in a vacuum oven at 80 °C and 150 mbar up
158
to constant weight. The evolution of the mass of the disks as a function of impregnation time
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was recorded.
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2.1.3. Forced flow impregnation process In order to accelerate silica impregnation, we developed a home-made set-up to perform
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so-called “forced flow impregnation”. Here we describe the principle of the forced flow
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impregnation; no technical optimisation was performed to further improve the processing.
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Cellulose alcogel disc of the same geometry as for molecular diffusion impregnation was
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placed on a grid fixed in a home-made plastic funnel, the whole setup placed above a Büchner
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flask connected to a primary vacuum pump (Figure 2). 16 wt% PEDS solution was poured
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over the gel (msol = 2×malcogel). A pressure gradient was generated using the vacuum pump to
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force the transport of PEDS through cellulose disk porosity. The impregnated gel was then 8 Page 8 of 33
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immersed in the catalyst solution in the same way as described above for molecular diffusion
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impregnation.
172 Cellulose alcogel
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PEDS solution
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Plastic grid
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To vacuum pump
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Figure 2.
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Scheme of the forced-flow impregnation set-up
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To follow the impregnation kinetics, the filtrate (the liquid phase once passed through
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cellulose alcogel) was collected at various impregnation times, ethanol evaporated and dry
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mass measured. The impregnation was considered completed when the silica concentration in
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the filtrate was equal to two thirds of that of the initial PEDS solution, taking here into
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account the ethanol present in the initial alcogel.
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Reference silica aerogel was prepared by adding catalyst solution to 8wt% PEDS,
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followed by gelation, aging for 24h, washing in ethanol to remove non-reacted products and
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drying with sc CO2 (Bisson, Rigacci, Lecomte, Rodier & Achard, 2003).
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2.1.4. Drying
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Before drying, cellulose-silica impregnated samples were washed in ethanol several
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times to remove unreacted species. Samples were dried with sc CO2 as follows. They were
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placed in a 1 L autoclave containing ethanol to avoid sample drying by evaporation before the 9 Page 9 of 33
system was pressurized at 50 bars and 37 °C with gaseous CO2. The excess of ethanol was
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purged with CO2 maintaining the pressure and temperature constant. Then the system was
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pressurized at 80 bars and 37 °C and dynamic washing step with 5 kg CO2/h was carried out
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for 1 hour. The system is then let in a static mode for 1 to 2 hours at the same pressure and
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temperature to allow supercritical CO2 to diffuse even in the nanometer-size pores. The
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dynamic washing starts with the same sc CO2 output for 2 hours. Afterwards, the system was
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slowly depressurized at 4 bars/h and 37 °C to avoid cracks and cooled down to room
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temperature. The autoclave was then opened and dry samples collected. Composite cellulose-
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silica aerogels were solid, easy to handle and not crumbling materials.
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205 2.2. Methods
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Specific surface area of pores was determined by N2 adsorption isotherms at 77 K and
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the BET theory using ASAP 2020 apparatus from Micromeritics. Samples were vacuum-
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degassed for 5 hours at 70 °C before analysis. The error was ± 20 m2/g.
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Samples’ bulk densities ρbulk were measured using powder pycnometer Geopyc 1360 from Micromeritics; the error was less than 5%. The porosity ε(%) was calculated as a
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function of bulk and skeletal ρskeletal densities:
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(1)
The morphology of the samples was studied using high resolution scanning electron
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microscope (SEM) SEM-FEG Zeiss Supra 40. Samples were coated with a 7 nm gold-
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palladium layer with a QUORUM Q150T rotating metallizer before observations.
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Philips XL30 environmental SEM coupled with Energy Diffractive Spectroscopy (EDS)
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was used to analyse the distribution of silicon in composite aerogel in order to evaluate the
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distribution of silica.
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Elemental analysis was done in CNRS Service Central d’Analyse laboratory
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(Villeurbanne, France) using atomic absorption spectroscopy. Weight concentration of silicon
221
(wt%Si) was measured allowing evaluation of wt% of silica (
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in the dry aerogel (
and silica mass yield
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as follows: (2)
where MSi = 29 g/mol and
225
respectively, and
are the molar mass of silicon and silica,
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(3)
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where
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concentration in the wet material in case of total silica conservation in the sample during
229
coagulation and washing steps. We thus obtain, for the initial proportion between cellulose
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and silica used,
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being theoretical silica
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= 8% and
72% .
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with
The thermal conductivity was measured with a commercial heat flow meter Laser Comp (Fox 150 from Laser Comp) in room conditions; it is steady state method. The Laser Comp
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Fox 150 was calibrated using a Certified Reference Material IRMM-440 according to
234
standard procedures. Before measurements, the samples were polished to obtain flat parallel
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surface squares of 9 cm × 9 cm and 1 cm thickness (see photo in Figure 1). Three samples of
236
each formulation were tested and a mean value calculated; the standard deviation was ± 0.002
237
W/(m.K).
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Uniaxial compression experiments were carried out on Zwick mechanical testing
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machine. Samples were of cylindrical shape with ratio length/diameter = 3/2, typical length
240
was around 3 cm. Because of the restricted samples’ geometry for forced-flow impregnation
241
set-up, only diffusion-impregnated samples were studied. Before measurements samples were
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polished to make upper and lower cylinder surfaces planar and parallel which was verified 11 Page 11 of 33
with a micrometric sensor. Two different loads were used, as suggested in Rudaz et al, 2014:
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i) 100 N for precise measurements of Young modulus (E) in the linear visco-elastic regime
245
and ii) 2000 N for complete stress-strain curve. The tests were performed at room
246
temperature, atmospheric pressure and around 40% relative humidity. The displacement rate
247
was 1 mm/min and experiments were performed until sample break. At least three samples
248
per formulation were tested and mean values for Young modulus and fracture stress
249
calculated. The experimental errors were determined, for each formulation, from standard
250
deviation.
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3. RESULTS AND DISCUSSION
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3.1. Impregnation kinetics
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Figure 3 shows the impregnation kinetics for two cases: 1) molecular diffusion and 2)
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forced flow impregnation of PEDS solution into wet cellulose alcogel. In the first case the
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impregnation kinetics is described by the evolution of cellulose alcogel relative weight
258
increase wrel, 1(t) in time t as follows:
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te
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(4)
260
where w1(t) is the dry weight of the impregnated cellulose at time t and wmax,1 is the
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theoretical maximal dry weight of impregnated cellulose matrix calculated assuming silica
262
fully penetrating cellulose and thus the initial silica concentration in silica sol is decreased by
263
half (see Experimental section).
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268 269 Wrel,1
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Silica relative weight as a function of time for the case of diffusion impregnation and
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Figure 3.
forced flow impregnation (inset). Sample thickness is 5 mm. Dashed lines are given to guide
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the eye.
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t, h
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In the second case the impregnation kinetics is followed by the increase of the relative
silica weight in the filtrate, wrel,2 (t): (5)
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where w2(t) is the silica weight in the filtrate at time t and wmax,2 is the theoretical maximal
288
silica weight in the filtrate calculated assuming the initial silica concentration in the sol is
289
decreased by 2/3 as far as msol = 2×malcogel, resulting in 10.6%wt of PEDS solution (see
290
Experimental section).
291 292
The impregnation by molecular diffusion occurs in two steps (Figure 3): first quick silica penetration into cellulose alcogel and then slow approach to equilibrium. Diffusion is 13 Page 13 of 33
slowed down because of silica “filling” the pores of cellulose matrix close to the sample
294
surface thus creating silica concentration gradient and decreasing the pore size. At
295
equilibrium, silica weight inside the cellulose matrix is reaching about 90% of the theoretical
296
maximal value wmax,1 within 10% experimental error. Overall, the impregnation process by
297
molecular diffusion is very slow: at least 7 hours are needed to fully impregnate a 5 mm thick
298
cellulose alcogel disk.
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The rate of impregnation via forced flow set-up is much faster as compared to
molecular diffusion process (inset of Figure 3): the time needed to reach 80-95% of the
301
theoretical maximal impregnation of cellulose matrix of 5 mm thickness is drastically reduced
302
from 7 hours for diffusion to ~ 15-20 minutes for forced flow. As mentioned in Methods
303
section, no technical optimisation was performed and pressure difference was not varied to
304
study the impregnation kinetics in details and to improve it; our goal was to demonstrate that
305
forced flow impregnation allows decreasing the impregnation time by almost three orders of
306
magnitude for the same sample thickness. It should also be noted that for diffusion-controlled
307
impregnation the effective impregnation distance is equal to the half-thickness of the sample
308
while for the forced-flow it is the whole thickness of the disk.
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3.2. Elemental analysis and morphology of composite cellulose-silica aerogels To understand if silica is homogeneously distributed in the cellulose matrix, EDS
312
spectroscopy was used. The spectra were taken on a transversal cut of samples prepared via
313
molecular diffusion and forced flow impregnation. One spectrum was taken every 100 μm
314
over a straight line along the sample cross-section. The weight per cent of Si, wt%Si, was
315
determined from the intensity of the silicon Kα peak at 1.71 keV and the concentration of
316
SiO2 was calculated according to eq.2. Figure 4 shows a representative example of the
317
distribution of SiO2 along the cross-section of composite aerogel,
, as a function of
14 Page 14 of 33
the distance l from the sample surface, for both diffusion and forced flow impregnation. For
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diffusion-controlled impregnation, both sides of the sample are equivalent in terms of silica
320
impregnation, while for the forced flow impregnation l = 0 corresponds to the upper sample
321
surface in contact with PEDS solution, see Figure 2.
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Figure 4
Example of silica distribution in %wt along the transversal cut of composite aerogels
328
impregnated by molecular diffusion for 24h (filled symbols) and forced-flow process (after
329
1h) (open symbols) as a function of distance from the upper surface of the sample (with
330
thickness equal to 5 mm).
331 332
EDS shows local elemental composition on the surface of the sample cross-section. The
333
obtained high values of silica concentration indicate that most of the signals come from Si
334
element. This means that cellulose is well coated by silica resulting in the overestimated
335
values of silica concentration as compared to the maximal bulk theoretical value of 72wt%, 15 Page 15 of 33
see Experimental section. SEM images (see further) will demonstrate that cellulose network is
337
completely covered by silica explaining high local SiO2 concentrations. Despite the fact that
338
EDS is not reflecting average bulk silica concentration, it gives valuable information on silica
339
penetration and distribution inside the sample. The sample impregnated with forced flow
340
process shows slightly higher silica concentration as compared to diffusion process, especially
341
near sample upper surface. Diffusion-controlled impregnation is showing a more
342
homogeneous silica distribution in the cellulose matrix as compared to forced-flow
343
impregnated hybrids: in the latter case, silica concentration gradient appears with the distance
344
from the upper sample surface. In overall, forced flow impregnation appears to be a very
345
efficient way to reduce impregnation times while keeping a good filling of the cellulose
346
matrix with silica.
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In order to estimate the average bulk composition of composite aerogels and determine silica yield, classical elemental analysis was performed using atomic absorption spectroscopy:
349
wt%Si was determined and recalculated into SiO2 concentration
350
of silica wt% in the dry samples obtained with each method is very similar: 51 wt % for
351
diffusion and 56 wt % for forced flow impregnation, confirming the observations by EDS
352
spectroscopy that cellulose impregnation using both methods results in a similar filling of
353
cellulose porosity by silica. Silica yield for both impregnation methods was calculated
354
according to eq. 3: it is around 70 and 77% for molecular diffusion and forced flow processes,
355
respectively. It is likely that the loss of silica occurs during the catalysis step, when the
356
impregnated sample is immersed in the NH4OH solution. The gelation of silica takes about 10
357
minutes in these conditions (Achard et al, 2007; Bisson, Rigacci, Lecomte & Achard, 2004)
358
and partial diffusion of silica sol from cellulose matrix into the catalyst solution may occur
359
even in this short duration. A thin layer of silica gel was observed around the impregnated
(eq.2). The amount
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16 Page 16 of 33
360
cellulose after the catalysis step, confirming a partial loss of the silica due to diffusion of
361
silica precursor from the matrix prior to gelation in the porosity.
362
The morphology of composite aerogels obtained with both impregnation methods, as well as the reference samples (Aerocellulose and silica aerogel) was analysed by SEM (Figure
364
5). Aerocellulose (Figure 5a) has a hierarchical structure with “hairy” beads assembled
365
together, as demonstrated previously for Aerocellulose from cellulose-ionic liquid solutions
366
(Sescousse et al, 2011). The beads are supposed to be formed via spinodal decomposition
367
during cellulose coagulation from solution (Sescousse et al, 2011a). The inside morphology of
368
a bead is a “network” of fine cellulose strands. Characteristic pores size varies from several
369
tens of nanometers to several microns, as seen by SEM and also studied by non-intrusive
370
mercury porosimetry (Rudaz, 2013). Silica aerogel morphology (Figure 5b) is typical for
371
base-catalysed silica aerogels (Pierre & Rigacci, 2011). It is a network formed of
372
“nanometric” silica beads with pore diameter being few tens of nanometers, with no
373
macropores observable by SEM.
te Ac ce p
374
1µm
375
d
M
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(b)
(a)
200 nm
17 Page 17 of 33
(d)
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(c)
1µm
1µm
cr
376
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(e)
200nm
379
Figure 5.
382 383 384
te
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SEM images of Aerocellulose from 3% cellulose-EmimAc-DMSO solution (a); reference silica aerogel (b) and composite aerogels obtained with molecular diffusion (c) and
Ac ce p
380
d
378
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377
forced flow (d, e) impregnation.
SEM images of composite aerogels are shown in Figure 5c for molecular diffusion-
385
controlled and Figure 5d, e for forced-flow impregnation. In both cases, silica phase appears
386
homogeneously distributed all over cellulose network confirming the formation of organic
387
and mineral interpenetrated networks. As observed by SEM, silica is filling the whole
388
macroporosity of Aerocellulose matrix and is “coating” cellulose backbone itself. The latter
389
explains high values of silica concentration obtained by EDS. Figure 5e shows a zoom on the
390
morphology of composite sample obtained with forced flow impregnation (the same obtained
18 Page 18 of 33
391
with diffusion impregnation): the pore sizes in the silica phase is in the range of few tens of
392
nanometers.
393 3.3. Properties of composite aerogels
395
Bulk density ρbulk of composite aerogels and of cellulose and silica reference samples
ip t
394
was measured as described in Experimental section and porosity ε% was calculated according
397
to eq.1; the values are given in Table 1. Porosity was calculated supposing cellulose and silica
398
skeleton density being roughly 1.5 and 2.0 g/cm3, respectively (Rudaz, 2013; Ayral,
399
Phalippou & Woignier, 1992). The average skeletal density of hybrid aerogels was calculated
400
taking into account the amount of silica for each impregnation case as obtained with
401
elemental analysis, 51 wt% for diffusion and 56 wt% for forced flow. The bulk density of
402
composite materials appears higher, as expected, and porosity slightly lower than that of each
403
of the reference samples. Overall, the porosity of composite aerogels is around 90%.
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404
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396
19 Page 19 of 33
i cr us
Table 1. Bulk density, porosity ε % (eq. 1), specific surface area SBET, Young modulus E, fracture stress σ* and strain S* and thermal conductivity in
406
ambient conditions λ20°C for cellulose and silica reference samples and composite aerogels. ε, %
SBET,
E,
σ*,
S*,
λ20°C,
m²/g
MPa
MPa
%
W/(m.K)
0.92
290
2.8 ± 0.47
9.20 ± 0.12
80 ± 5
0.033
0.130
0.94
975
1.9 ± 0.10
0.07 ± 0.007
4 ± 0.5
0.015
0.225
0.87
810
11.5 ± 1.11
6.30 ± 1.55
60 ± 5
0.026
0.155
0.91
750
-
-
-
0.028
ρbulk,
M
Formulation
an
405
Aerocellulose from 3%
0.125
pt
cellulose solution
ed
g/cm3
ce
SiO2 aerogel
Composite aerogel via diffusion
Ac
impregnation
Composite aerogel via forcedflow impregnation
407
21
Page 20 of 33
Table 1 compares specific surface area of organic and mineral reference samples with
408
that of composites. Both reference samples have SBET similar to what is reported in literature:
409
around 250-300 m2/g for Aerocellulose (Sescousse et al, 2011a; Rudaz, 2013) and around
410
800-1000 m2/g for silica aerogel (Pierre & Rigacci, 2011; Wong, Kaymak, Brunner &
411
Koebel, 2014). As compared to Aerocellulose, the specific surface area of both composites
412
increased tremendously, reaching values that are comparable with the characteristic ones of
413
silica aerogels. This result is a direct confirmation of the presence of nanostructured silica
414
aerogel phase in the pores of cellulose matrix.
us
cr
ip t
407
Silica aerogels are known to be excellent thermal superinsulating materials in room
416
conditions, as also obtained in this work, see Table 1: λ20°C = 0.015 W/(m.K). The main
417
reason for a low-density porous material to fall into superinsulation region is air confinement
418
in the pores of size below the free mean path of air molecule (Knudsen effect); at atmospheric
419
conditions it is around 70 nm. Mesoporous and light-weight silica aerogels satisfy these
420
conditions. Aerocellulose also has low density, but the presence of numerous large
421
macropores (Figure 5a) results in relatively high (as compared to superinsulation) thermal
422
conductivity values, 0.033 W/(m.K) in room conditions. In composite cellulose-silica
423
aerogels, superinsulating silica aerogel is filling Aerocellulose pores in general and
424
macropores in particular: thermal conductivity is thus decreased to 0.027 ± 0.001 W/(m.K).
425
This important result demonstrates the feasibility of significantly decreasing thermal
426
conductivity by “incorporating” a superinsulating silica aerogel into cellulose matrix. The
427
thermal superinsulation level is not achieved yet for the present composites most probably
428
because of i) the presence of some remaining macropores not filled with silica aerogel phase
429
and ii) the increase in composite density leading to the increase of phonon conduction through
430
the solid backbone.
Ac ce p
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d
M
an
415
431
Uniaxial compression of reference and composite aerogels is presented in Figure 6 as
432
stress-strain plots; the values of Young modulus and fracture stress and strain are reported in 22 Page 21 of 33
433
Table 1. Each compression experiment was performed to the moment when the sample was
434
starting to break (shoot of stress).
1
2
2
cr
1
ip t
435
M
an
us
3
436
Figure 6
d
437
Example of stress-strain uniaxial compression curves for cellulose-silica composite
439
aerogel obtained via molecular diffusion impregnation (1), reference Aerocellulose (2) and
440
silica (3) aerogels. Inset: zoom at low stresses and strains.
442
Ac ce p
441
te
438
Stress-strain curve for the reference Aerocellulose (Figure 6, curve 1) is typical for bio-
443
aerogels such as Aerocelluloses prepared from cellulose dissolved in aqueous 8%NaOH or in
444
ionic liquid (Sescousse et al, 2011a) or Aeropectins from pectin dissolved in acid medium
445
(Rudaz et al, 2014). Three regions can be distinguished: linear regime at low strains, long
446
plastic deformation region with pore walls bending and finally densification due to pore walls
447
collapse. Aerocellulose shows very high deformability, up to 70-80% strain, but do not
448
recover its shape after being highly compressed.
449
The reference silica aerogel is brittle, breaking at low strains of a few per cent, and with
450
lower (as compared to Aerocellulose) values of fracture stress and Young modulus (Table 1). 23 Page 22 of 33
According to a recent study of the mechanical properties of hydrophobic (silylated) silica
452
aerogels from the same PEDS precursor (Wong et al, 2014), silica aerogels with densities
453
between 0.1 and 0.2 g/cm3 show an elastic-like behavior deforming till 40% strain and
454
partially “springing back” to their original shape when the stress is released. Their Young
455
moduli were from ≈ 1 to 10 MPa and fracture stress from ≈ 0.02 to 2.5 MPa in this density
456
interval. Silica aerogel with densities above 0.2 g/cm3 were shown to exhibit a brittle
457
behavior, while those with the densities lower than 0.1 g/cm3 were highly compressible. In
458
our case, silica aerogels with densities around 0.13 g/cm3 have Young modulus of around 2
459
MPa and fracture stress of 0.07 MPa, but broke at low strains and showed no measurable
460
elastic recovery probably because no hydrophobisation was performed.
an
us
cr
ip t
451
Cellulose-silica composite aerogel shows significantly improved Young modulus as
462
compared to its reference counterparts, both Aerocellulose and silica aerogel (Figure 6, curve
463
1 and Table 1). With a mean value of Young modulus around E = 11.5 MPa the composite is
464
three-four times stiffer than each of reference materials with E = 2.8 and 1.8 MPa for
465
Aerocellulose and silica aerogel, respectively. The composite aerogel can also withstand
466
deformation up to 60% before break, which is similar to Aerocellulose and is ten times higher
467
than what is obtained for the reference silica aerogel. One of the reasons of high Young
468
modulus is hybrid higher density, 0.155 vs 0.125 and 0.130 g/cm3 as compared to each of
469
reference aerogels, Aerocellulose and silica, respectively. However, even if considering that
470
Young modulus is proportional to aerogel density in power 3 (Alaoui, Woignier, Scherer &
471
Phalippou, 2008), the obtained increase in modulus is higher than what could be theoretically
472
expected from density increase. Cellulose is clearly playing an important reinforcing role but
473
more data are needed to quantify this phenomenon (for example, mechanical properties of
474
cellulose strands in Aerocellulose). The synergy of cellulose-silica interpenetrated network
475
provides stiff and ductile aerogel materials despite there is no observable chemical interaction
476
between two components, as shows FTIR spectra (Figure S1 in the Supporting Information).
Ac ce p
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461
24 Page 23 of 33
Cellulose and silica bonds are vibrating in very close energy domains which makes difficult to
478
detect the difference within the bands in the composite. The decrease of the absorption in the
479
[3700-3000] cm-1 region as well as that of the band at 900 cm-1 observed on the Aerocellulose
480
spectrum in composite aerogel is due to the small amount of this component in the composite.
481
We cannot see any disappearance or appearance of any absorption band in the composite
482
spectrum as compared to those of the reference components which could account for a
483
chemical bonding.
cr
ip t
477
4. CONCLUSIONS
an
485
us
484
486
Strong, light (bulk density around 0.2 g/cm3) and monolithic crack-free cellulose-silica
M
487
composite aerogels have been prepared by impregnation of wet coagulated cellulose with
489
polyethoxydisiloxanes solution. The impregnation was performed either by molecular
490
diffusion or by a forced flow process; in the latter PEDS was forced to penetrate inside the
491
cellulose matrix due to a pressure difference. Forced flow impregnation method significantly
492
reduced processing time as compared to impregnation driven by simple molecular diffusion:
493
impregnation times were reduced from ~ 7 hours to less than 30 minutes for a sample with the
494
same geometry. After in situ PEDS gelation and drying with supercritical CO2, aerogels based
495
on interpenetrated cellulose-silica networks were obtained.
te
Ac ce p
496
d
488
BET analysis confirmed the formation of nanostructured silica inside cellulose matrix:
497
specific surface area increased from ≈ 300 m2/g for Aerocellulose to 750-800 m2/g for
498
composites. The thermal conductivity in room conditions was reduced from 0.033 W/(m.K)
499
for Aerocellulose to 0.027 W/(m.K) ± 0.001 for composite aerogels due to the superinsulating
500
properties of silica aerogel itself, demonstrating that the concept of impregnation works for
501
decreasing the total thermal conductivity of a porous matrix with large macropores. Finally,
502
composite aerogels were strongly reinforced as compared with the reference aerogels still 25 Page 24 of 33
keeping high ductility characteristic of Aerocellulose: Young modulus increased in 3-4 times
504
and fracture strain remained very high, about 60%. The results obtained open very promising
505
ways in making high-performance composite organic-inorganic strong and lightweight
506
materials.
ip t
503
507
Acknowledgements
509
The work has received funding from the European Union Seventh Framework
us
cr
508
Programme (FP7/2007-2013) under grant agreement n° 260141, “AEROCOINs” project. We
511
thank P. Ilbizian (PERSÉE, Mines ParisTech, Sophia-Antipolis, France) for supercritical
512
drying and Suzanne Jacomet (CEMEF, Mines ParisTech, Sophia Antipolis, France) for help
513
in SEM and EDS analysis.
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26 Page 25 of 33
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629
Mater., 183, 23–29.
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628
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Figure captions
631 Figure 1.
633
A schematic presentation of the preparation route of cellulose-silica composite aerogels and
634
example of 9 cm x 9 cm x 1 cm sample.
ip t
632
Figure 2.
637
Scheme of the forced-flow impregnation set-up
us
636
cr
635
an
638 Figure 3.
640
Silica relative weight as a function of time for the case of diffusion impregnation and forced
641
flow impregnation (inset). Sample thickness is 5 mm. Dashed lines are given to guide the eye.
M
639
d
642 Figure 4
644
%wt of silica along the transversal cut of composite aerogels impregnated by molecular
645
diffusion for 24h (filled symbols) and forced-flow process (after 1h) (open symbols) as a
646
function of distance from the upper surface of the sample (with thickness equal to 5 mm).
Ac ce p
647
te
643
648
Figure 5.
649
SEM images of Aerocellulose from 3% cellulose-EmimAc-DMSO solution (a); reference
650
silica aerogel (b) and composite aerogels obtained with molecular diffusion (c) and forced
651
flow (d, e) impregnation.
652 653
Figure 6
32 Page 31 of 33
654
Example of stress-strain uniaxial compression curves for cellulose-silica composite aerogel
655
obtained via molecular diffusion impregnation (1), reference Aerocellulose (2) and silica (3)
656
aerogels. Inset: zoom at low stresses and strains.
Ac ce p
te
d
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33 Page 32 of 33
657
Cellulose-silica aerogels
658
Arnaud DEMILECAMPS, Christian BEAUGER, Claudia HILDENBRAND, Arnaud
659
RIGACCI and Tatiana BUDTOVA
661
ip t
660
cr
Highlights
us
662 -
Improved method of making cellulose-silica aerogels is proposed
664
-
Formation of interpenetrated organic-inorganic network of aerogels is confirmed
665
-
Composite aerogel has very high specific surface area due to mesoporous silica
666
-
Composite aerogels are stiffer than each of reference aerogel
667
-
Thermal conductivity of composite aerogel is lower than that of cellulose aerogel
M
d te
669
Ac ce p
668
an
663
34 Page 33 of 33