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Multi-scale cellulose based new bio-aerogel composites with thermal super-insulating and tunable mechanical properties
Accepted Manuscript Title: Multi-scale cellulose based new bio-aerogel composites with thermal super-insulating and tunable mechanical properties Author: Bastien Seantier Dounia Bendahou Abdelkader Bendahou Yves Grohens Hamid Kaddami PII: DOI: Reference:
S0144-8617(15)01121-2 http://dx.doi.org/doi:10.1016/j.carbpol.2015.11.032 CARP 10550
To appear in: Received date: Revised date: Accepted date:
3-7-2015 8-11-2015 11-11-2015
Please cite this article as: Seantier, B., Bendahou, D., Bendahou, A., and Kaddami, H.,Multi-scale cellulose based new bio-aerogel composites with thermal superinsulating and tunable mechanical properties, Carbohydrate Polymers (2015), http://dx.doi.org/10.1016/j.carbpol.2015.11.032 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.
Multi-scale cellulose based new bio-aerogel composites with
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thermal super-insulating and tunable mechanical properties
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Bastien Seantiera, Dounia Bendahoua,b, Abdelkader Bendahoua,YvesGrohens*a,
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Hamid Kaddami*b
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Université de Bretagne Sud, Laboratoire Ingénierie des Matériaux de Bretagne, BP 92116, 56321 Lorient Cedex, France. b
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Cadi Ayyad University, Faculty of Sciences and Technologies, Laboratory of Organometallic and Macromolecular Chemistry, Avenue AbdelkrimElkhattabi, B.P. 549, Marrakech, Morocco.
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Corresponding author: [email protected], [email protected].
Abstract
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Bio-composite aerogels based on bleached cellulose fibers (BCF) and cellulose nanoparticles
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having various morphological and physico-chemical characteristics are prepared by a freeze-
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drying technique and characterized. The various composite aerogels obtained were compared
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to a BCF aerogel used as the reference. Severe changes in the material morphology were
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observed by SEM and AFM due to a variation of the cellulose nanoparticle properties such as
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the aspect ratio, the crystalline index and the surface charge density. BCF fibers form a 3D
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network and they are surrounded by the cellulose nanoparticle thin films inducing a
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significant reduction of the size of the pores in comparison with a neat BCF based aerogel.
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BET analyses confirm the appearance of a new organization structure with pores of
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nanometric sizes. As a consequence, a decrease of the thermal conductivities is observed
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from28 mW.m-1.K-1 (BCF aerogel) to 23 mW.m-1.K-1 (bio-composite aerogel), which is below
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the air conductivity (25 mW.m-1.K-1). This improvement of the insulation properties for
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composite materials is more pronounced for aerogels based on cellulose nanoparticles having
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a low crystalline index and high surface charge (NFC-2h). The significant improvement of
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their insulation properties allows the bio-composite aerogels to enter the super-insulating
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materials family. The characteristics of cellulose nanoparticles also influence the mechanical
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properties of the bio-composite aerogels. A significant improvement of the mechanical
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properties under compression is obtained by self-organization, yielding a multi-scale
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architecture of the cellulose nanoparticles in the bio-composite aerogels. In this case, the
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mechanical property is more dependent on the morphology of the composite aerogel rather
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than the intrinsic characteristics of the cellulose nanoparticles.
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Key words: Multi-scale architecture, cellulose, Nanofibrillated cellulose (NFC),
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cellulose nanocrystals (CNC), freeze-drying, porosity, super-insulation, mechanical
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properties.
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1 INTRODUCTION
Recent years have seen an increase in consumer demand for biodegradable products
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designed according to more environmentally friendly methods. Furthermore, the increase in
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oil prices and its scarcity helped to bring forward new products of natural origins. The
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objective of this article is to prepare and characterize an ultra-porous cellulose-based material,
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called a composite aerogel.
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Although Kistler made his discovery in the early 1930s (Kistler, 1931a), aerogels are
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nowadays considered as the most promising materials for various applications. The multitude
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of organic, inorganic or hybrid materials, and the design of new methods for extraction and
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preparation, offer prospects for use in the development of fuel cell separators (R. W. Pekala,
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Mayer, Kaschmitter, & Kong, 1994), in filtering systems and ultra-thin dust capture (Tsou,
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1995; Venkateswara Rao, Hegde, & Hirashima, 2007). Recently, the aerogels have been used
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with success in the field of thermal super-insulation (Baetens, Jelle, & Gustavsen, 2011; Han,
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Zhang, Wu, & Lu, 2015).
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Aerogels can be inorganic (silica-based) (Bonnardel et al., 2006; Fricke, HüMmer,
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Morper, & Scheuerpflug, 1989) or organic (example of resorcinol-formaldehyde) (R. W.
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Pekala, 1989; Richard W. Pekala, 1991). They have a very low thermal conductivity, and are
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often super-insulators. However, they are relatively fragile (silica aerogel) (R. W. Pekala,
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1989) or toxic (organic aerogels) (Lu, Caps, Fricke, Alviso, & Pekala, 1995). A great number
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of researches have been conducted recently to develop innovative aerogel materials, bio-
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sourced for super-insulation. Thus, aerogels made from cellulose or derivatives, or other
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polysaccharides were investigated (Fischer, Rigacci, Pirard, Berthon-Fabry, & Achard, 2006;
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García-González, Alnaief, & Smirnova, 2011; Guilminot et al., 2008; Hall et al., 2012;
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Liebner et al., 2009; Mehling, Smirnova, Guenther, & Neubert, 2009; Rudaz & Budtova,
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2013; Sescousse, Gavillon, & Budtova, 2011).
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Even if Kistler was the first to consider the use of cellulose and nitrocellulose
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(cellulose derivative) to prepare an aerogel in 1930 (Kistler, 1931b), few studies were
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subsequently conducted to develop original nanostructured aerogels made from cellulose
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precursors. Recently, macro-porous materials have been prepared by drying physical gels
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made from cellulose (Hao Jin, Nishiyama, Wada, & Kuga, 2004) or cellulose acetate
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(Pimenov, Drozhzhin, & Sakharov, 2003). Yet, the materials obtained cannot be considered
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as conventional aerogels (i.e. nanostructured and nanoporous) because their pore dimensions
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are too large (>1µm). The first attempt to develop cellulose acetate chemical gels (by
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crosslinking the gel and subsequent supercritical drying)was published in 2001 (Tan, Fung,
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Newman, & Vu, 2001). Mechanically strong nanomaterials were prepared using this method.
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Xiaoyu Gong et al have studied an aerogel based on a preparation by combining the
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NaOH/thiourea/H2O solvent system and the freeze-drying technology with density varying in
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the range of 0.2-0.4 g/cm3 (Gong, Wang, Tian, Zheng, & Chen, 2014). The results showed
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that the main factors affecting thermal conductivity were density and porosity. Thermal
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conductivity decreased with the decrease of density and the increase of porosity, it could be as
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low as 0.029 W/(m.K).
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However, to ensure a sustainable development towards industrialization, research must
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improve the manufacturing processes and promote the use of raw materials that are more
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respectful of the environment. One of the raw materials, which meet these requirements, is
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cellulose in its various forms. It is an ideal material to develop bio-aerogels because of its
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availability, its lightness and its capability of forming a three-dimensional resistant network.
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Additionally, cellulose is easily and cheaply extracted from various sources such as the date
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palm tree in Morocco and may have several aspect ratios (Kaddami et al., 2006). Its low
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intrinsic thermal conductivity, its ability to form a three-dimensional structure involving
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hydrogen bonding and its low molecular weight make cellulose a promising material to
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prepare bio aerogels. Furthermore, the resistance of the produced networks allows the gels to
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support simple manufacturing processes to obtain the final product. Although the organic
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aerogels have good thermal insulation properties, their thermal conductivity remains slightly
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higher than inorganic aerogels like silica. Recently, Kobayashi et al. while studying aerogel
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based liquid-crystalline nanocellulose derivatives reported that these aerogels exhibited very
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low thermal conductivity. The lowest thermal conductivity of 0.018Wm-1K-1 was measured at
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a density of 17 mg.cm-3, and it increased as the density was increased (Kobayashi, Saito, &
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Isogai, 2014).
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In order to obtain various cellulose nanoparticles, the oxidation of cellulose is
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commonly used. This process, used to transform alcohol to aldehyde or a corresponding acid,
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is made according to several methods and reagents (Benhamou, Dufresne, Magnin, Mortha, &
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Kaddami, 2014). Such modifications are usually achieved by converting part of the cellulose
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surface hydroxyl groups into carboxyl groups. At the present time, the catalytic oxidation
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using 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical under mild aqueous conditions
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was found to be the most straightforward and efficient method to selectively oxidize hydroxyl
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functions located at the surface of cellulosic fibers and convert them into sodium carboxylate
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(Okita, Saito, & Isogai, 2010). This method has been widely used on different types of well-
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defined purified cellulose substrates: cotton linters (Saito & Isogai, 2004), cellulose
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microfibrils from sugar beet (Montanari, Roumani, Heux, & Vignon, 2005), tunicin whiskers
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(Habibi, Chanzy, & Vignon, 2006), bleached hardwood kraft pulp (Saito et al., 2009),
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bleached softwood kraft pulp (Dang, Zhang, & Ragauskas, 2007), fibrillated viscose rayon
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fibers (Shibata & Isogai, 2003), lignocellulosic fibers isolated from the leaflets of the date
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palm tree (Sbiai, Kaddami, Sautereau, Maazouz, & Fleury, 2011) and even water soluble
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cellulose acetate (Gomez-Bujedo, Fleury, & Vignon, 2004). It is worth noting that the
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oxidation catalyzed by the TEMPO radical has been developed by several researchers in the
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case of insoluble polysaccharides such as cellulose and chitin (Chang & Robyt, 1996; Isogai
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& Kato, 1998; Tahiri & Vignon, 2000; Zhao et al., 1999), as well as for soluble
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polysaccharides in water (de Nooy, Besemer, & van Bekkum, 1995; Gomez-Bujedo et al.,
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2004). Among various oxidants, sodium hypochlorite/sodium bromide with the TEMPO
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radical is currently the most studied. The hypobromite ion is much more reactive than the
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hypochlorite ion, allowing rapid regeneration of oxoammonium ion, by the introduction of
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sodium bromide.
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Various techniques have been developed to prepare cellulose network based aerogels:
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fiber bonding (Costa-Pinto et al., 2009), freeze-drying (Sudheesh Kumar et al., 2011),
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supercritical fluid technology (Chung & Park, 2007), compression molding and salt leaching
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(Hou, Grijpma, & Feijen, 2003), gas foaming (Chen et al., 2012), rapid prototyping (Yun,
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Kim, Hyun, Heo, & Shin, 2007) and electrospinning (Peng, Shaw, Olson, & Wei, 2011).
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Freeze drying is a dehydration technique in which liquid samples are frozen below their glass
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transition temperature (Tg) or melting point. Then, the frozen solvents are removed by the
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sublimation process, thereby obtaining porous, interconnected structures (Liu, 2006).
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Compared to other techniques, the distinct advantages of freeze-drying are that non-toxic
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organic solvents are being used and the low temperature helps to maintain the activity of bio-
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macromolecules and pharmaceutical products for a long period. Unlike a normal drying
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process, this technique involves low surface tension, which can maintain the pore structure.
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Therefore, the bio-aerogel with a nano-sized pores morphology can be synthesized from
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different cellulose by adjusting various parameters in the freeze-drying process (Qian &
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Zhang, 2011).
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Conscious of the importance of the characteristics of cellulose nanoparticles to the
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performances of their resulting products (Kadimi et al., 2014; Li, Wu, Song, Qing, & Wu,
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2015; Li, Wu, Song, Lee, et al., 2015; Xu et al., 2013), our work aims at designing new
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composite aerogels based on multi-scale cellulose fibers. Combining several aspect ratios and
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varying the formulations, the developed new materials will be optimized to gain the lowest
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thermal conductivity and the best mechanical strength.
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2 MATERIAL AND METHODS
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2-1 Chemical compounds studied in this article:
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TEMPO (Pub Chem CID: 2724126); sodium bromide (Pub Chem CID: 253881);
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sodium hypochlorite solution (15%) (Pub Chem CID: 23665760); HCl (Pub Chem CID: 313);
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NaOH (Pub Chem CID: 14798); methanol (Pub Chem CID: 887); sulfuric acid (Pub Chem
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CID: 1118); chloroform (Pub Chem CID: 6212). More information is available at:
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http://www.elsevier.com/PubChem.
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2.2 Materials
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One aim of this paper was to use local vegetal waste to develop added value materials.
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The date palm tree (Phoenix dactylifera L) is one of the most harvested palm trees in
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Morocco. That is the reason why it was used in this work as an original source of cellulose.
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The procedure to extract cellulose from the rachis of the date palm tree has already been
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described previously (A. Bendahou, Habibi, Kaddami, & Dufresne, 2009; D. Bendahou,
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Bendahou, Seantier, Grohens, & Kaddami, 2015; Hua Jin et al., 2013). In the coming
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sections, the extracted cellulose will be referred to as bleached cellulose fibers (BCF). For the preparation of the cellulosic nanoparticles, the following chemicals were used:
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TEMPO, sodium bromide, sodium hypochlorite solution (15%), HCl, NaOH, methanol,
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sulfuric acid and chloroform. They were all purchased from Sigma-Aldrich and were used
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without further purification.
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2.3 Nanofibrillated cellulose NFCs preparation by TEMPO-mediated oxidation
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The general procedure and reagent ratios used by Sbiai et al. (Sbiai et al., 2011) were
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used for TEMPO-mediated oxidation of cellulose fibers. About 2 g i.e. 2.136 mmol of an
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equivalent anhydroglucose unit (AGU) of cellulose was suspended in water (200 ml) and
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sonicated with a Branson Sonifier for 5min. TEMPO (32 mg, 0.205mmol) and NaBr (0.636 g,
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6.175mmol) were added to the suspension. An additional amount of the NaOCl solution,
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corresponding to 40.5 ml was added in drops to the cellulose suspension. The pH was
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adjusted to10 by the addition of a 0.1 M aqueous solution of HCl. The pH of the mixture was
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maintained at 10 at 4°C by continuously adding 0.1 M NaOH while stirring the suspension.
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When the oxidation had finished, it was terminated by the addition of methanol (5 ml) and the
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pH was adjusted to 7 with 0.1 M HCl. The oxidation times used were 5 min and 2 h.
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After the oxidative treatment, a 1wt% fiber suspension in water was subjected to the
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homogenizing action of a low-pressure slit homogenizer using the Laboratory Homogenizer
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PANDA 2K (GEA Niro Soavi S.P.A), USA. To obtain nanofibrillated cellulose (NFC) the
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oxidized cellulose was disintegrated by pumping the suspension up to 15 times through the
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homogenizer. Efforts were made to keep the pressure constant at 650 MPa for all cycles and
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all samples. During the process, the viscosity and temperature of the suspension increased
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with the increase in the number of cycles. The maximum temperature reached was 70°C.
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Herrik et al. (Herrick, Casebier, Hamilton, & Sandberg, 1983) found that increasing the
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temperature from 20°C to 70–80°C helped the dissociation of physical bonds and facilitates
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homogenization. The prepared NFC-5min and NFC-2h will be denoted as NFCs in the rest of
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this article.
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2.4 Preparation of cellulose nanocrystals
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Cellulose nanocrystals (CNC) were prepared from the rachis of the date palm tree
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(Phoenix dactylifera L) as described in literature (A. Bendahou et al., 2009). Briefly, cellulose
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fibers were treated three times with 3 wt% NaOH solution at 80 °C for 2h under mechanical
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stirring and bleached with a solution made by equal parts of acetate buffer, aqueous chlorite
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(1.7 wt% in water) and distilled water. The bleaching treatment was performed 4 times at
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70°C, under mechanical stirring, at 1h intervals, and bleached cellulose fibers (BCF) were
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obtained. Acid hydrolysis was carried out on the BCF at 45 °C with 65 wt% sulfuric acid
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(pre-heated), for 45 min, using mechanical stirring at 260 tr/min. Successive centrifugations
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were performed at 11000 rpm and 10 °C for 30 min (each step) and the suspensions were
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dialyzed against distillated water. Homogenization was carried out using an Ultra-Turax T25
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homogenizer for 5 min and the suspensions were filtered in a glass filter no. 1 (Size retention
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1.5-1.6 µm). Some drops of chloroform were added to the CNC suspension that was stored at
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4°C to limit bacterial development.
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2.5 Porous aerogels preparation from BCF/ NFCs or BCF/CNC mixtures
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In order to obtain aerogels having targeted properties, stable suspensions of each
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constituent have been made to avoid material agglomerates and to obtain the best material
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structure. Ultra-Turrax Ika® was used to homogenize the suspensions by shear forces.
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First of all, date palm tree NFCs or CNC were re-suspended separately in 20 ml of
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distilled water. BCFs were obtained as homogeneous water dispersion. Then, NFCs or CNC
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dispersion, depending on the targeted mixture, was passed through Ultra-Turrax and mixed
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with a specific amount of BCF under magnetic stirring. Solutions are finally freeze-dried
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using liquid nitrogen: after freezing, the dispersions are kept in petri dishes for one night at -
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20°C, the solvent was sublimated (Christ Alpha 1-2 LD Plus) under 80 mBar and at -52°C.
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Freeze-drying was carried out for 24h to remove water traces. The elaborated aerogels were
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stored in an oven under vacuum at 25°C to limit water uptake. The dry mass of the aerogels
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with NFCs or CNC is always equal to 500 mg.
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The concentration of BCF was maintained at a constant level and the amount of NFCs
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or CNC varies in mass as follows: 0%, 1%, 2%, 4%, 5%, 10% and 20% with respect to the
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BCF.
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2.6 Zeta potential measurements
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To obtain information about the surface charge density of the studied particles (BCF,
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NFCs and CNC), the zeta potential was measured. The experiments were carried out using the
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zeta-meter “ZetasizerNano ZS Malvern Instruments” equipped with a 633 nm laser. This
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technique consists of analyzing the phase shifts of the light scattered by the sample while
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submitted to an electric field. It attains information about surface charge density but also
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about the stability of the suspensions. All solutions used to prepare the starting dispersions
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(BCF, NFCs and CNC) were studied. 300 µL of each starting solution, having an ionic
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strength stabilized at 1 mM using NaCl solution, were prepared for all measurements.
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2.7 X-ray diffraction (XRD)
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Nanocellulose films were prepared by a heat-drying method and were examined using
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X-Ray Diffraction (XRD) to evaluate the impact of the treatment on the crystalline index of
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NFCs and CNC. BCF film was prepared and used as the reference.
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An X’PERT (Philips PW3710) wide angle X-ray diffractometer was used to determine
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the crystallinity of the specimens. The source was a Ni-filtered Cu Kα radiation with a
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wavelength of 1.5418 Å. The X-rays were operated at 40 kV and 30 mA.
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X-ray diffractograms were recorded at 0.02°.s-1 over a 2ϴ scan in the range 10-60°.
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Crystalline index (C.I.) values were calculated according to the empirical Segal method (eq.1)
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(Segal, Creely, Martin, & Conrad, 1959): Eq. 1
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Where I0 0 2 is the intensity of the (0, 0, 2) reflection plan (2ϴ between 22° and 23°)
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and IAM is the minimum value at 2ϴ between 18° and 19°, which represents the reflection
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intensity of the amorphous phase. The patterns were recorded, analyzed and phase matched
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by X’PERT software, data collector and graphics.
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2.8 Atomic Force Microscopy (AFM)
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After gluing a cleaved mica surface onto a metallic disk, a droplet of the studied
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dispersion (NFCs and CNC) was deposited and casted. The obtained samples were imaged by
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Atomic Force Microscopy (AFM) with a multimode equipped with the nanoscope III-a
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(Bruckernano, Santa Barbara, USA). The AFM was used in contact mode imaging. The tips
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(DNP–10, Bruckernano) were made of Si3N4 and had a 0.24 N/m nominal spring constant.
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The deflection set point was chosen at the limit of the tip retraction. The imaging speed was
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between 1 and 2 Hz depending on the surface. The pictures obtained were processed with
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Nanoscope 6.13 R1 and Gwyddion v2.29 software.
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2.9 Transmission Electron Microscopy (TEM)
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Transmission electron microscopy (TEM) of the cellulose whiskers was carried out
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with a Philips CM200 transmission electron microscope with an acceleration voltage of 80
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kV. An aqueous dispersion of whiskers was deposited on a microgrid (200 mesh, Electron
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Microscopy Sciences, Hatfield, PA, USA) and covered by a thin carbon film (∼200 nm). The
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deposited whiskers were subsequently stained with a 2% uranyl acetate solution to enhance
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the microscopic contrast.
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2.10 Field Emission Scanning Electron Microscopy (FE-SEM) FE-SEM was used to characterize the BFC and the aerogels structures. The “quanta 200FEI”, with accelerating voltage of 12.5 kV, was used.
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2.11 Fourier Transformed Infrared spectroscopy (FTIR)
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To investigate the effect of chemical pretreatment on NFC, Fourier Transform Infra
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Red spectroscopy was carried out with a Perkin Elmer Spectrum D400 spectrometer (Perkin
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Elmer). For each film prepared by heat-drying NFC suspensions, the spectrometer was used
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in adsorption mode and the wavelength range was from 400 to 4000 cm-1. For each spectrum,
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32 spectra were taken. A separate background spectrum was collected and automatically
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subtracted from the raw spectrum for each specimen.
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2.12 BET analysis
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Nitrogen adsorption/desorption isotherms were obtained using a Micromeritics ASAP
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2420. The samples in the form of aerogel were placed in the measuring cell to degas all water
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molecules adsorbed. The degassing of the material was carried out according to the following
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procedure: by heating at 50°C for 1h and then at 100°C for 15h under 10-2 Torr. We chose
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relatively mild degassing conditions in order not to degrade the sample and to be sure that all
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the water molecules were removed.
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Subsequently, the measuring cell is placed in an insulated tank filled with liquid
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nitrogen. This keeps the sample at -196°C throughout the measurement, the temperature at
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which gaseous nitrogen adsorption is possible on a solid surface. A nitrogen
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adsorption/desorption isotherm represents the evolution of the volume of Nitrogen adsorbed
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per gram of sample extrapolated to standard conditions of temperature and pressure (STP
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cm3/g) depending on the relative pressure of nitrogen (p/p0). Using the BET theory, it is
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possible to evaluate the specific surface and obtain information on the aerogel structure such
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as the pore size.
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2.13 Thermal conductivity measurements The system used to measure the thermal conductivity was the “hot filament” technique
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(Fig.1) (dos Santos, 2005; Scudeller & Bardon, 1991). It is composed of two symmetrical
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cavities having a 1.65 cm3 volume, one with the sample (1) and the other containing a
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reference, the polyurethane foam (2). Both parts surround a thin nickel/chrome heating
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filament (3). The cavities and the filament are maintained in an aluminum box at a
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temperature of 20.7°C controlled by a water circuit (4). The filament (3) is isolated from the
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box (4) with polymer thin films (20 µm thick) (5). The filament (3) is connected to 2 copper
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electrodes (6) for the electric power supply. The temperatures of both sides of the sample (2)
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are measured with 2 thermo-couple K type (7down and 7up) applied in the system.
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The thermal conductivity of the sample (1) is determined in a steady state by the
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measurement of the filament-housing conductance (W.K-1). The ratio of the heating measured
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at the center of the sample and the electric power dissipated by the sample enables the
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calculation of the conductance (eq. 2).
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Caption:
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1: aerogel to be characterized
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2: polyurethane foam
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3: NiCr filament, 2µm x 4mm x 20 mm
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4: isothermal box (aluminum)
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5: thermal insulator (polymer)
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6: metallic electrodes
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7up and 7down: thermocouples K type, diameter 0.1mm
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Figure 1: Scheme of the thermal conductivity measurement system.
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Eq. 2
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In our protocol, the heating, ∆T, is set to 10 °C. The tension, U, and the intensity, I, of the current were measured at the steady state.
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The conductance is related to the sample conductivity and follows a linear law depending on the measurement system as follows (eq. 3):
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Κ = A.λ + B
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Eq. 3
Where Κ (W.K-1) being the conductance, λ (W.m-1.K-1) the thermal conductivity and
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A and B are the experimental constants depending on the sample geometry, the cavities and
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the thermocouples.
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Therefore, the first step of a measurement is the calibration of the system performed
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by measuring the conductance of 5 materials having a known conductivity over the range of
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studied conductivities.
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Then the BCF and composite materials are prepared in the sample holder by freeze-
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drying. Aerogel samples are obtained that exactly fill the sample holder to avoid thermal
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bridge problems while measuring the sample properties.
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2.14 Mechanical characterization
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The mechanical properties of the aerogels were determined with the Dynamic
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Mechanical Analysis (DMA). The experiments were carried out with the DMA 2980 TA
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instrument. The compression modulus was measured on samples having the following
330
dimensions: 25 × 25 × 4 mm3. The tests were carried out at a controlled ambient temperature
331
(23.4°C) and humidity (50%). The mechanical properties were determined for a frequency of
332
1 Hz and displacement of 0.05 mm. This technique has been used to characterize the
333
compression modulus (K) and the Yield stress (σy) of all aerogels (see Figure 7).
334
2.15 The degree of oxidation (DO)
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The degree of oxidation refers to the average normalized number of primary hydroxyl
336
groups oxidized by an anhydroglucose unit of the cellulose sample. As a result, the maximum
337
degree of oxidation is 1, corresponding to an anhydroglucose unit which is completely
338
oxidized. The carboxyl content of the oxidized cellulose samples was determined by
339
conductimetric titration. This titration technique has been used by many researchers (Araki,
340
Wada, Kuga, & Okano, 1998; da Silva Perez, Montanari, & Vignon, 2003) to determine the
341
degree of oxidation. The degree of oxidation is determined from equations 4 and 5.
us
cr
ip t
335
343
an
342
With:
Eq. 5
M
344
Eq. 4
Where [NaOH] is the NaOH concentration (mol.L-1), V0 and V1 are respectively the initial
346
and final volume of NaOH (L), which determine the limits of the plateau observed on the
347
titration curve, and M (g) is the mass of the oven-dried sample. 162 = molar mass (g.mol-1)
348
for a unit of anhydroglucose in a cellulose chain. 36= (198-162) where 198 is the molar mass
349
(g.mol-1) of oxidized an anhydroglucose unit that forms sodium salt in a cellulose chain. In
350
our experiments, the suspensions were titrated without any further purification.
351
3 RESULTS AND DISCUSSION
352
3.1 Physiochemical and structural characterization of raw material suspensions
te
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353
d
345
In this article, we propose the use of cellulosic materials to prepare green insulating
354
aerogels using the freeze-drying method. In order to better understand the obtained structure,
355
the dispersions used to fabricate the aerogel were characterized.
356
3.1.1 Structure and physicochemical properties of pristine bleached cellulose fibers
357
(BCF)
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358
shows a representative BCF micrograph.
360
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359
The cellulose fibers were observed by FE-SEM (see experimental part 2.9). Figure 2a
361 362 363 364
Figure 2: a) Pristine bleached cellulose fibers (BCF) micrograph. b) Contact mode height AFM picture of NFC-5min. c) Contact mode height AFM picture of NFC-2h. Both AFM pictures have the same scale (5 µm x 5µm). d) Transmission electron microscopy (TEM) of a suspension of CNC (whiskers).
365
From 5 micrographs, the fibers dimensions were estimated on 30 fibers. It shows that
366
the fibers have a diameter of 10±5 µm and a length of at least 700±100 µm (Fig. 2). These
367
fibers have a high aspect ratio (over 70, see table 1) and are arranged in a 3D network.
368
In order to better understand this structure, the zeta potential of the BCF solution was
369
measured. The BCF solutions have a negative zeta potential of -28 ± 2 mV, which is expected
370
for cellulose and close to the values obtained in literature (Benhamou et al., 2014).
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371
To complete the characterization of these solutions, the crystalline index has been
372
determined by X-ray diffraction. The pristine BCF crystalline index was 72%. This value is
373
close to the one obtained by K. Benhamou (Benhamou et al., 2014). From this characterization, the structure of the BCF aerogels observed is probably due
375
to the presence of alcoholic functions that may interact together through H bonds allowing the
376
formation of a strong 3D network (see the chemical structure in the supplementary
377
information, Figure S1). The high crystallinity explains the BCF stick like shape. However,
378
their high aspect ratio enables their entanglements that may strengthen the 3D arrangement.
379
The small negative zeta potential probably comes from a negligible native oxidation, which is
380
a result of the bleaching treatment, and explains the suspension stability. It is low enough to
381
allow the formation of the 3D network and high enough to stabilize the BCF dispersion for
382
relatively short period. From the characterization of the pristine BCF dispersions, we
383
demonstrate that these are able to form strong, rigid, 3D networked gels. This arrangement
384
can be used as a strong cellulosic matrix for the formed aerogels.
385
3.1.2 Structure and physicochemical properties of NFCs dispersions
387 388
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The NFC-5min and NFC-2h have different shapes and sizes. Figures 2b and 2c show
Ac ce p
386
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374
AFM pictures of the NFCs.
Compared to the BCF, the dimensions of the cellulosic NFC-5min and NFC-2h are
389
much smaller. On the one hand, the NFC-5min exhibits a mean length of between 1 and 2 µm
390
and a mean width of 40±10 nm (calculation executed on 4 AFM pictures and 50 fibers).
391
These fibrils exhibit a high aspect ratio (estimated at around 50, see Table 1) and are
392
comparable to those obtained by a high mechanical shear using microfluidizer without any
393
oxidative pretreatment (A. Bendahou, Kaddami, & Dufresne, 2010). On the other hand, for
394
the NFC-2h, the mean width is 20±4 nm and the mean length is 300 ± 20 nm (calculation
395
executed under the same conditions as for NFC-5min) (see Fig. 2b and 2c). A reduction of
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their aspect ratio (approximately 15, see Table 1) is also observed. This extreme reduction of
397
the fibril dimensions is the result of the degradation of macromolecular chains during the
398
oxidation reaction and during the homogenization step made by the mechanical treatment
399
(Benhamou et al., 2014).
ip t
396
To confirm the effect of the oxidation time on the NFCs surface charge density, the
401
zeta potential was also measured. After oxidation, the zeta potential decreased from -28 ± 2
402
mV for pristine BCF to-45 ± 3 mV and -64 ± 4 mV for NFC-5min and NFC-2h, respectively.
403
This is in complete agreement with the oxidation process, which produces negative surface
404
charges. These results are confirmed by the FTIR analyses (see supplementary information,
405
Figure S2). The most important change in the spectra concerns the band around 1730 cm-1. As
406
described by Maréchal and Chanzy (Maréchal & Chanzy, 2000), this band corresponds to
407
C=O stretching frequency of carboxyl groups in their acidic form. The intensity of this band
408
increases with the oxidation time indicating an increase of the oxidation degree. Our
409
calculated degree of the average oxidation (DO) for NFCs, extracted from the rachis of the
410
date palm tree, oxidized for 5 min and 2h are 0.036 and 0.129, respectively. These values
411
have the same order of magnitude as that obtained by T. C. F. Silvaet al. (Silva, Habibi,
412
Colodette, Elder, & Lucia, 2012), during the oxidation by TEMPO of nanofibrillated cellulose
413
produced from cellulosic pulp extracted from Eucalyptus urograndis. The DO increasing with
414
the oxidation time confirms what was observed in FTIR.
us
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415
cr
400
The oxidation has another effect on the particle properties: the crystalline index is
416
decreasing from 72% for BCF to 62% for NFC-5min and to 56% for NFC-2h. The oxidation
417
and the mechanical homogenization processes are destroying part of the cellulosic fiber
418
structure through the elimination of the original H bonds. This oxidation process also induces
419
more de-polymerization by β-elimination of the glycoside function (Benhamou et al., 2014;
420
Shinoda, Saito, Okita, & Isogai, 2012).The reduction of H bonds and the de-polymerization
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421
induce a less cohesive structure with more amorphous areas, which explains the decrease of
422
the crystalline index (see the chemical structure in supplementary information Figure S1).
423
From this section onwards, we demonstrate that the oxidation of BCF produces small particles that are less crystalline and have a higher surface charge density.
425
3.1.3 Structure and physicochemical properties of Cellulose NanoCrystals
ip t
424
To perform the morphological analysis of CNC, the CNC suspension obtained by
427
hydrolysis with sulfuric acid from the rachis of date palm was visualized by TEM and AFM
428
(data not shown here). One of the micrographs obtained is shown in Figure 2d.
us
cr
426
Using the AFM microscope, we measured the average length of the CNC as being
430
270±30 nm and having an average diameter of 7±2nm. The average aspect ratio is around 38
431
(see Table 1). To carry out these measurements, we measured the length of about 40 sticks.
432
Figure 2d was used to confirm the morphology of the CNC from the date palm tree. The same
433
results were obtained. The aspect ratio calculated for the CNC (about 40, see Table 1) is
434
larger than that extracted from cotton (Dong, Revol, & Gray, 1998) or wood (Beck-
435
Candanedo, Roman, & Gray, 2005) and has a value close to that extracted from sisal (Garcia
436
de Rodriguez, Thielemans, & Dufresne, 2006; Siqueira, Bras, & Dufresne, 2009). The AFM
437
and TEM characterizations show rigid and very small needle-shaped particles. The crystalline
438
index was also determined and, as expected for a CNC, it is very high (around 90%). As
439
shown in the micrographs (Figure 2d) CNCs are usually isolated from each other, although
440
we can note the presence of some aggregates. The latter are probably formed whilst drying on
441
the carbon grid. The presence of negative charges on the surface of crystals (-SO3-) comes
442
from the preparation process (Habibi et al., 2006) and promotes their repulsion and thus their
443
individualization (see the chemical structure in supplementary information, Figure S2).
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444
To verify the expected variation of the surface charge density, the zeta potential of the
445
solutions was measured. The potential is -56 ± 1 mV. The CNC potential is lower than the
446
potential of the initial BCF (-28 ± 2 mV) which confirms the hypothesis. We characterized nanoparticles that have an intermediate aspect ratio, between NFC-
448
5min and NFC-2h. The surface charge density (Zeta potential) is also intermediate; however
449
they have a very high crystalline index. We used these nanoparticles as nanofillers for BCF to
450
prepare BCF/Cellulose nanofillers aerogels. Certainly the differences in the morphological
451
and physicochemical characteristics of these nanofillers will lead to various organizations of
452
the resulting aerogels, inducing several thermal and mechanical properties. Table 1 compiles
453
all the results described previously. This characterization will help to explain the effects
454
observed when mixing the distinct dispersions and some properties of the formed bio-
455
aerogels.
456
Table 1: Geometrical and physical characteristics of the used materials obtained at 25°C.
457
d
Zeta potential (mV) (pH 5-6)
% Crystal
Up to 70 ± 20
- 28 ± 2
72%
40.10-3 ± 10.10-3
Up to 50 ± 15
- 45 ± 3
62%
300.10-3± 20.10-3
20.10-3 ±4.10-3
15 ±5
- 64 ± 4
56%
270.10-3 ± 30.10-3
7.10-3± 2.10-3
38 ± 15
- 56 ± 1
90%
Mean Length (µm)
Mean width (µm)
Cellulose
Up to 700 ±100
10 ±5
Cellulose
Up to 2± 0.5
Cellulose Cellulose
Ac ce p
Pristine Cellulose BCF Nanofibrilated Cellulose oxidized 5 min (NFC-5min) Nanofibrilated Cellulose oxidized 2 h (NFC-2h) Cellulose nanocrystals CNC
Chemical Structure
te
Sample
M
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447
Aspect ratio
458
3.2 Morphological and structural characterization of the bio-aerogels
459
3.2.1 Morphology of the bio-aerogels
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Using the previously described fiber solutions, bio-composite aerogels were prepared
461
by freeze-drying (see experimental part). These aerogels are macroscopic monoliths as shown
462
in Figure 3.
cr
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460
Figure 3: Photograph of a prepared aerogel made from BCF and NFC-5min. The other aerogels look similar and this is why they are not shown.
an
464 465
us
463
A preliminary study made on the bio-aerogels cross section shows that there is likely
467
to be no detectable structural difference between the interior of an aerogel and its surface
468
(data not shown here). Therefore, to have the best overview of an aerogel structure, the SEM
469
observations have been systematically carried out on a cross section of the bulk bio-composite
470
aerogel.
te
d
M
466
In order to determine the effect of NFC-5min on the structure of bio-aerogels, we
472
compared bio-aerogel SEM images made from a BCF (Fig. 4a and 4b) with images of bio-
473
aerogels made from a BCF reinforced by 5% and 10% of NFC-5min (Fig. 4c and 4e).
Ac ce p
471
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474
* The Red arrows show macro porosityand the blue circles highlight peeling from parts of the walls of the fibrils.
NFC-5min
NFC-2h b
ip t
a
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cr
BCF-0%*
c
an
d
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f
Ac ce p
e
d
M
BCF/NFC-5%*
BCF/NFC-10%*
475 476 477
Figure 4: SEM micrographs of cellulose-based materials: (a) and (b) unloaded; (c) loaded by 5wt% of NFC-5min and (e) loaded by 10wt% of NFC-5min; (d) loaded by 5wt% of NFC-2h and (f) loaded by 10wt% of NFC-2h.
478
In Figure 4a, one observes the presence of lamellae around the non-reinforced BCF
479
(blue circles). These sheets appear after the use of the Ultra-Turrax defibrillator. The strong
Page 21
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480
shear force applied by the latter probably causes a detachment of pieces of the walls of fibers,
481
forming these cellulosic films. The SEM micrographs for bio-aerogels based on cellulose and NFC-5min, presented
483
in Figures 4c and 4e, clearly show that the NFC-5min form a network, looking like thin films,
484
in the bio-aerogel. As seen on the micrograph, BCF fibers are forming a 3D network, similar
485
to the one formed without NFC-5min and they are surrounded by the NFC-5min films. The
486
film like structure made from the NFCs comes from strong cellulose nanoparticle interactions
487
that yield particular assemblies in both the liquid and dry states. These liquid crystals like
488
structures are supposed to nucleate on the impurities embedded on the cellulose NFC. The
489
nanofilm formation from CNC solutions has been described by Lagerwall et al. (Lagerwall et
490
al., 2014) in the recent review on Cellulose Liquid Cristal Self Assemblies. CNC has the
491
ability to self-organize in the liquid crystal phase according to the temperature/concentration
492
conditions shown in a phase diagram. This arrangement in solution gives dried films of
493
various thicknesses. As stated in this article, some domains of the phase diagram have not
494
been investigated as yet. It is therefore difficult, with our complex drying and freezing
495
conditions to predict and analyze our CNC nanofilm structure. The formation of NFC films
496
induces the disappearance of part of the macropores and the appearance of organizations
497
having smaller pore sizes is observed. By increasing the amount of NFC-5min from 5wt% to
498
10wt%, the NFC-5min network looked denser, implying that the porosity of the binary system
499
is decreasing (Fig. 4c and 4e).
cr
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M
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Ac ce p
500
ip t
482
Figures 4d and 4f describe the SEM micrographs performed on the cross section of
501
bio-composite aerogels made from BCF loaded with 5wt% and 10wt% of NFC-2h. Similar
502
structures were obtained for BCF/NFC-2h and BCF/NFC-5min composite aerogels. NFC-2h
503
form thin films that are surrounding the BCF. When the concentration of the NFC-2h
504
increases, the film area formed in the network seems denser on the SEM micrographs.
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However, even though they look similar, in fact the area of the thin films is slightly larger for
506
NFC-2h. It might be because of the structural properties of the raw dispersions. With a higher
507
zeta potential difference between BCF and NFC-2h and especially with the difference of
508
surface groups (Hydroxyl groups on the BCF fibers surface and carboxylate groups on the
509
NFC-2h fibers), the interactions might be favored through hydrogen bonding. Furthermore, as
510
the crystalline index is lower for NFC-2h, the particles may change their shape, through
511
modification of the amorphous areas, allowing an easier film formation and optimized
512
interactions with the surface of the BCF fibers through electrostatic forces and entanglement.
us
cr
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505
In order to verify the effect of the zeta potential and the crystalline index on the
514
structure of the aerogels, the morphology of the bio-composite aerogels based on BCF and
515
CNC was characterized by SEM. Figure 5 shows a slice of non-reinforced aerogel (Fig. 5a-c)
516
and bio-aerogels reinforced with 10wt% of CNC (Fig. 5d-f).
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513
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517
* The Red arrows and blue circles show respectively, macro porosity and peeling from parts of the walls of the fibrils. 120µm
b
c
e
f
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a
520 521
M
518 519
Figure 5: SEM micrographs of a cellulose-based composite aerogel: (a), (b) and (c) nonreinforced; (d), (e) and (f) strengthened by 10 wt% CNC with three different scales. Similarly to the addition of NFCs, we observed that when adding CNC to BCF the
523
system is reorganized. The individual BCFs are enveloped by CNCs, forming a structured
524
network looking like films. These results clearly confirm the hypothesis formulated in the
525
previous paragraph. CNC particles are able to form films due to their self-organization, as
526
liquid crystal in water as described by Lagerwall (Lagerwall et al., 2014).
te
Ac ce p
527
d
522
From these SEM images, a decrease of porosity is observed. Arising from the division
528
of macroscopic pores (~120 to 300 µm) due to the presence of CNC films, to pores of smaller
529
sizes, probably meso or micropores (nanometric size pores). The presence of very small size
530
pores has been reported by Fukuzumi et al. (Fukuzumi et al., 2011). Using Positron
531
Annihilation Lifetime Spectroscopy (PALS), they have shown that films based on NFC
532
similar to NFC-2h have pores of nanometric size that can reach diameters of 0.5 nm. Nemoto
533
et al., while studying nanoporous networks prepared by air drying of aqueous NFC
Page 24
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534
dispersions, showed that 90% of formed aerogel pores have sizes between 10 and 100 nm
535
(Nemoto, Soyama, Saito, & Isogai, 2012).
536
However,
bio-composite
aerogels
BCF/CNC
and
BCF/NFCs
cannot
be
characterized in a satisfactory manner by SEM because the expected meso or micropores
538
(nanometric size pores) are too small to be precisely detected. To determine the pore size of
539
such bio-composite aerogels, we used the nitrogen adsorption/desorption technique, which
540
allowed the characterization of meso and nanoporous materials.
541
3.2.2 Structure of aerogels performed by adsorption/desorption of nitrogen (BET)
us
cr
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537
Most of the elaborated bio-composite aerogels were characterized by the technique of
543
nitrogen adsorption. It is typically used to analyze porous materials. The specific surface area
544
has been conventionally estimated using the Brunauer, Emmett and Teller (BET) method
545
(SBET) (Brunauer, Emmett, & Teller, 1938; Haul, 1982). Specific surface areas were measured
546
on bio-aerogels prepared with cellulose fibers (BCF) and reinforced with cellulose
547
nanoparticles (NFCs or CNC) which are between 143 and 162 m2.g-1. These values are higher
548
than that obtained by Silva et al. (Silva et al., 2012) and close to that obtained by Sehaqui et
549
al. (Sehaqui, Zhou, & Berglund, 2011) for the NFC based-aerogels. When compared with
550
most other aerogel type materials (silica and resorcinol-formaldehyde aerogels), these values
551
appear relatively low. However, in the light of specific surface areas and mean diameters of
552
the particles constituting the fibrillary mesoporous network (ranging from 2 to 50 nm), one
553
can qualitatively conclude that the solid skeleton of bio-aerogels is nanostructured.
555
M
d
te
Ac ce p
554
an
542
Table 2 compiles the different textural parameters (SBET, Vméso and DBdB) for bio-
composite aerogels.
Page 25
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Table 2: Textural characteristics of bio-aerogel based cellulose (BCF) reinforced by cellulose nanoparticles (CNC or NFCs) SaBET (m2.g-1)
Vbmeso(cm3.g-1)
DcBdB(nm)
BCF-CNC
142.22
0.0005
5
BCF-NFC5min
161.49
0.0036
13
BCF-NFC2h
116.60
0.0014
ip t
Bio-composite aerogel
cr
556 557
6-7
a: specific surface area determined by the BET method. b: mesoporous volume calculated with the following equation: Vméso=Vp-Vmicro, where Vp is the total porous volume determined for P/P0 ~ 0.85. c: diameter of pores using the BdB method (Broekhoff-de-Boer) (Broekhoff, 1968).
562
Table 2 highlights that bio-composite aerogels have mesopores. The pore size
563
distribution, determined by the Broekhoff-de-Boer method (Broekhoff, 1968), is centered on
564
5 nm in the case of the formulation BCF-CNC10%, and from 6 to 13 nm for formulations
565
BCF-NFCs10%.
d
M
an
us
558 559 560 561
At this stage in the article, we have shown that the mixing of the different cellulosic
567
particles allows specific arrangements to form aerogels. For aerogels formed from BCF, a 3D
568
network with large pores was obtained. When adding cellulosic nanoparticles (NFC-5min,
569
NFC-2h or CNC), films are formed reducing the size of the pores. We also discussed how the
570
properties of each constituent influence the structure of the aerogels. From here on in, the
571
study will focus on the influence of the specific structures obtained on the thermal and
572
mechanical properties.
573
3.3 Thermal characterization of bio-aerogels
574
3.3.1 Influence of density on thermal conductivity
Ac ce p
te
566
575
To investigate the effect of density on the thermal conductivity, the bio-aerogels were
576
compressed in a controlled manner to increase their density and the thermal conductivity was
577
measured immediately afterwards. The results are compiled in Figure 6.
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ip t M
an
us
cr
578
580 581 582 583 584
Ac ce p
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579
Figure 6: Thermal conductivity of bio-aerogels prepared with (a) BCF, (b) NFC-5min, (c) NFC-2h, (d) a mixture of BCF and NFC-2h 10 wt% and (e) a mixture of BCF and CNC 10 wt% depending on the density in kg/m3. The black line shows the thermal air conductivity. The red line shows the optimal thermal conductivity reached depending on the mixture.
585
The results showed that, after compression (inducing an increase in density) the
586
thermal conductivity is changing in a specific way for both composite and single systems and
587
two different zones could be delimited: zone 1 (λ<λminimum) and zone 2 (λ>λminimum).
Page 27
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In zone 1 (λ<λminimum), there is a strong decrease of thermal conductivity. Whilst being
589
compressed, the cellulosic particles are getting closer and the pore size is decreased, reducing
590
the thermal gas conduction via air confinement (Knudsen effect). As a consequence, the
591
effective thermal conductivity is severely reduced. This is true until a threshold density
592
between 30 and 40 kg/m3 is reached. For these densities, a minimum thermal conductivity is
593
observed (λ=λminimum). The pore size induces air confinement and low gas conduction, but the
594
cellulosic network is not yet completely collapsed, which causes a solid conduction that is
595
still low. As a consequence, the overall thermal conductivity is at a minimum. In zone 2
596
(λ>λminimum), the density continues to increase. The cellulosic network starts to collapse
597
severely, allowing the creation of contact nodes between the cellulose nanoparticles. This
598
physical connection between particles will promote solid thermal conduction and induce a
599
significant rise in thermal conductivity known as the thermal percolation phenomenon.
M
an
us
cr
ip t
588
From Figures 6a, 6b and 6c, the minimum value of thermal conductivity λminimum is
601
lower for the two neat NFCs based aerogels, in comparison with the neat BCF based aerogel.
602
Additionally, the NFC-2h based aerogel exhibits the lowest thermal conductivity (below 24
603
mW.m-1.K-1). It is worth noting that we could not perform similar experiments on a CNC
604
based aerogel because it is too brittle to be handled.
te
Ac ce p
605
d
600
Figures 6d and 6e show thermal conductivity depending on the density of bio-
606
composite aerogels prepared from BCF filled with cellulose nanoparticles (NFCs, CNC). The
607
same behavior as for the previous materials is observed. However, the thermal conductivity of
608
these bio-composite aerogels can reach lower values (~23 mW.m-1.K-1 under ambient
609
conditions of pressure and temperature). Thus the minimum value of thermal conductivity
610
decreases from 28 ± 1 mW.m-1.K-1 for a BCF based aerogel to 23 ± 1 mW.m-1.K-1 for the
611
composite aerogel prepared from BCF and 10wt% NFC-2h. The thermal conductivity values
612
(~23 mW.m-1.K-1) obtained for these bio-composite aerogels are lower than those obtained by
Page 28
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S. T. Nguyen et al., (32 mW.m-1.K-1) (Nguyen et al., 2014) for cellulose aerogels prepared
614
from paper waste or wool (30-40 mW.m-1.K-1). However, they are comparable to those of
615
aerogels produced by the Aspen group (21 mW.m-1.K-1) (“Pyrogel XT-E,” 2015; Sequeira,
616
Evtuguin, & Portugal, 2009). This low value of thermal conductivity leads to a promising
617
material for thermal insulation. In comparison with air thermal conductivity (~ 25 mW.m-1.K-
618
1
cr
), the studied aerogels are classified as thermal super-insulators.
ip t
613
The obtained thermal insulation properties of the binary aerogels result from the
620
structure of the bio-aerogel. The morphology illustrated in Figures 4 and 5 highlight the fact
621
that the cellulose nanoparticles form films that reduce the pore size. The air, which is
622
abundant in these highly porous aerogels, is confined in pores smaller than the mean free path
623
(~70 nm at atmospheric pressure and ambient temperature), resulting in a decrease of the gas
624
molecule mobility. The thermal conductivity is thus reduced. We refer to the Knudsen effect
625
(Notario et al., 2015). The decrease of the pore size is confirmed by the BET analysis.
d
M
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us
619
To conclude, we demonstrated that the density of the aerogel has a significant impact
627
on their thermal properties. By controlling the bio-aerogel density, it is possible to tune the
628
pore size and thus the thermal insulation properties to lower thermal conductivities.
629
3.3.2 Influence of cellulose particle properties on thermal conductivity
Ac ce p
630
te
626
To assess the impact of the cellulosic particle properties on the thermal conductivity,
631
six formulations of aerogels were prepared. Diverse weight fractions of the nanofillers were
632
selected for each type of cellulose nanoparticle. Table 3 summarizes the results for thermal
633
conductivities of the prepared aerogels.
634
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634 635
638
BCF/CNC λ aver (mW.m-1.K-1)
0%
28 ± 1
1%
27 ± 2
2%
27 ± 2
5%
25 ± 0.5
10%
25 ± 0.5
20%
25 ± 1
BCF/NFC-5min λ aver (mW.m-1.K-1) 28 ± 1
28 ± 1
27 ± 1
an
us
27.5 ± 0.05 27 ± 1
BCF/NFC-2h λ aver (mW.m-1.K-1)
cr
Aerogel (%)
ip t
Table 3: Evolution of thermal conductivity (λ) depending on the proportion of cellulose nanoparticles. The density of the aerogels is the same and is approximately ~40 kg.m-3.
26 ± 1
25 ± 1
23 ± 1
23 ± 1
M
25 ± 0.5
23 ± 0.5
25 ± 0.5
d
636 637
From Table 3 we can clearly see that the conductivity decreases when the weight
640
fraction of nanofiller increases. However, using the same fraction of nanofiller the
641
conductivity reached for the aerogels prepared with NFCs is slightly, but not significantly,
642
lower than those achieved on CNC. The minimum thermal conductivity is reached when the
643
nanofillers fraction is between 10 and 20 wt%. The minimal value measured for these
644
concentrations are 25 mW.m-1.K-1 for BCF/CNC composite aerogel and 23 mW.m-1.K-1 for
645
the BCF/NFCs ones. These results clearly show that adding cellulose nanoparticles to BCF
646
decreases the apparent thermal conductivity because of the decrease of the pore sizes in the
647
resulting aerogels. This induces the formation of small cavities as confirmed by BET
648
analyses. In addition to the reduction of the space between the BCF due to the formation of a
649
tight 3D network, the films of nanoparticles contain pores of nanometric sizes (Fukuzumi et
650
al., 2011). All these changes in the aerogel structure are responsible for air confinement and
Ac ce p
te
639
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651
the decrease of gas conduction in the aerogels. The presence of pores smaller than the mean
652
free path of the air molecules significantly reduces the contribution of the gas phase by the
653
Knudsen effect, and reaches super-insulating properties. Upon introduction of the NFC-2h in the BCF, we find that conductivity decreases up
655
to the fraction of 10wt%. Beyond this fraction the conductivity of the binary aerogel
656
increases. The behavior is different in the case of BCF/CNC aerogels. The conductivity
657
decreases when the weight fraction CNC increases up to 5wt% and then remains constant for
658
the higher fractions of CNC at 25 mW.m-1.K-1. While comparing the structure and physical
659
characteristics of CNC and NFC-2h, one can clearly see that the two nanoparticles have
660
similar sizes and aspect ratios (see Table 1). Their zeta potentials are slightly different (-64
661
mV for NFC-2h vs. -56 mV for CNC). However the main difference is their crystallinity
662
(56% for NFC-2h vs. 90% for CNC). This makes NFC-2h stiffness very low compared to
663
CNC. It results in differences, not only in terms of interaction towards a BCF but also in
664
terms of increasing entanglement and more importantly in terms of the morphology and
665
porosity of the walls between the BCF formed by the nanofillers. The flexibility and the
666
amorphousness of NFC-2h are very likely to have nanometric size pores as reported by
667
Fukuzumi et al. (Fukuzumi et al., 2011).
cr
us
an
M
d
te
Ac ce p
668
ip t
654
In conclusion, the properties of the used particles are largely influencing the aerogel
669
thermal behavior through morphological changes. However, from an application point of
670
view, it is necessary to verify the mechanical performances of the aerogels formed.
671
3.4 Mechanical characterization of bio-aerogels
672
The mechanical properties of aerogels for thermal insulation applications are very
673
relevant and make them attractive. Due to their large pore volume, many aerogels, which have
674
good insulation properties, have low mechanical properties, which make them difficult to use
675
in industry. An agreeable compromise between the stiffness and mechanical strength is
Page 31
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676
required. Therefore, it is necessary to optimize the mechanical properties and understand how
677
to improve them. In order to characterize the mechanical properties of the aerogels, compression
679
experiments were carried out. Using these curves, one can distinguish three different domains:
680
1)
681
modulus (K). The compression modulus can be calculated from the compression curve (Eq.
682
6). It is the slope of the linear elastic portion of the stress-strain curve:
ip t
678
us
cr
A domain that corresponds with the elastic domain characterized by the compression
Eq. 6
683
2)
The second domain is a long, almost horizontal plastic plateau corresponding to the
685
progressive irreversible collapse of the pores.
686
3)
687
densification of the material due to the contact between adjacent cavity walls (Gibson &
688
Ashby, 1997).
an
684
te
d
M
Finally, the third domain relates to a steep rise of the constraint. This step matches the
From the compression stress-strain curves, we determined the main mechanical
690
characteristics, namely the compression modulus (K), the stress yield and deformation at the
691
end of the domain 1(Gibson & Ashby, 1997). All mechanical characteristics have been
692
plotted in Figure 7.
693
Ac ce p
689
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us
cr
ip t
693 694
695
Figure 7: Variation of the mechanical properties with the weight fraction of nanofillers contained in the bio-composite aerogel. a) Compression modulus and b) Yield stress is varying with the amount of CNC, NFC-2h and NFC-5min in the BCF composite aerogel.
an
696 697 698
From Figure 7a, we can observe that the compression stiffness (or compression
700
modulus) of the composite aerogels is higher than the neat BCF based aerogel. The
701
compression modulus increases from 13 kPa for the neat BCF based aerogel to 176 kPa and
702
62 kPa for the BCF/NFC-2h and BCF/CNC with 10wt% of load, respectively. Furthermore,
703
the stiffness increases with the fraction of cellulose nanoparticles in the aerogel. Whilst
704
comparing the three families of aerogels, one can observe that NFC-2h causes a greater
705
improvement in the stiffness. No significant difference could be observed while comparing
706
the stiffness with the same nanofillers fraction of BCF/NFC-5min and BCF/CNC composite
707
aerogels. In addition, from Figure 7b, the yield stress increases significantly with the fraction
708
of the cellulose nanoparticles in the BCF/NFC-2h composite aerogels. However an opposite
709
trend is observed in the case of BCF/NFC-5min and BCF/CNC aerogels. In these later cases,
710
the aerogels become brittle after a cellulose nanoparticle addition. Indeed, the yield stress
711
increases from 860 Pa for the neat BCF based aerogel to 2.9 kPa for BCF/NFC-2h loaded
712
with 5 wt% of nanofiller and decreases to about 300 Pa for BCF/NFC-5min and BCF/CNC
713
loaded with 10 wt% of nanofiller. The values of the compression modulus for the bio-aerogels
Ac ce p
te
d
M
699
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714
developed here (62-176 kPa) are higher than that obtained by Nguyen et al. (Nguyen et al.,
715
2014), with values around 11 kPa. From these results, the type of nanocellulose particles significantly influences the bio-
717
composite aerogels from a mechanical point of view. We can see that the NFC-2h has a much
718
more efficient strengthening effect for the BCF bio-aerogel compared to the CNC and NFC-
719
5min charges: the higher compression modulus and higher compression yield stress in the
720
range of studied fractions of the nanofillers. b)
us
a)
cr
ip t
716
Long fibers (BCF)
an
Long fibers (BCF)
Mesoporos ity
Macroporo sity
d
M
Short fibers (NFC or CNC)
725
Figure8: Conceptual scheme of the arrangement of the fibers in the aerogel. (a) Pristine long fibers BCF aerogels with macro porosity. (b) Binary mixture of BCF (black) and Cellulose nanoparticles (red) with mesoporosity.
Ac ce p
722 723 724
te
721
BCFs have a surface containing hemicelluloses and amorphous cellulose chains. The
726
high surface charge density of NFC-2h and its softness due to its less crystalline character
727
confer a more important interaction with the BCF through physical interactions and
728
entanglement, compared to the other two nanofibers (CNC and NFC-5min) (D. Bendahou et
729
al., 2015).
730
Furthermore, the higher NFC-2h load induces a greater number of physical
731
interactions with BCF, compared to the other two nanofibers (CNC and NFC-5min). As
732
described in Figure 8b, the formation of the nanofiber films surrounding the cellulose fibers
733
increased the number of entanglements in the BCF network. Thus the interaction number
Page 34
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increases and the arrangement becomes stronger. This reinforcement was predictable. Lavoine
735
et al. showed in their work that incorporated cellulose nanofillers improve the mechanical
736
properties of paper (Lavoine, Desloges, Dufresne, & Bras, 2012). Yet, if the load is higher
737
than a threshold concentration, in our case, around 10%, the BCF surface is fully covered by
738
the nanofillers. It generates disorders when adding more nanoparticles yielding a decrease in
739
mechanical properties.
cr
ip t
734
In this section, we demonstrated that the type of nanocellulose particles influences the
741
mechanical properties of bio-aerogels independently of their intrinsic properties. For example
742
the NFC-2h gives the lowest crystalline index and the lowest aspect ratio but the best
743
mechanical properties. The architecture of the bio-composite aerogels due to chemical and
744
physical interactions is the significant factor influencing their mechanics. By controlling the
745
cellulosic nanoparticle properties, it is possible to adjust the interaction forces and to obtain
746
aerogel materials having mechanical properties higher than those obtained so far.
747
4 CONCLUSION
te
d
M
an
us
740
Bio-aerogel materials were prepared using various combinations of bleached cellulose
749
fibers (BCF) and cellulose nanofibers (NFC or CNC). It was demonstrated that these bio-
750
sourced materials have very useful thermal conductivity and mechanical properties. A thermal
751
conductivity value as low as 23 mW.m-1.K-1 was obtained for BCF/NFCs systems. The
752
structure observed could be described as stacked nanofiber films around BCF. It has been
753
described how the combination of BCF and nanofillers promotes the creation of meso and
754
nanopores resulting from the interactions between the two types of fibers. The nanofiller films
755
have been proven to be efficient to confine air in the bio-aerogel through the Knudsen effect
756
and significantly reduce the thermal conductivity of the binary bio-aerogels. The mechanical
757
properties of the BCF aerogel are also affected by the introduction of cellulose nanoparticles.
758
In comparison to the neat BCF based aerogel, the compression modulus and the yield stress
Ac ce p
748
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are significantly increased in the case of BCF/NFC-2h. However the compression modulus is
760
slightly increased and the yield stress is decreased in the case of BCF/NFC-5min and
761
BCF/CNC aerogels. The physical properties, especially the crystalline index and the surface
762
charge density seem to be a determinant in the obtained mechanical and insulating properties
763
of these binary aerogels. The benefit of combining cellulose fibers of different sizes
764
(micrometric and nanometric sizes) is that multi-scale aerogels can offer improved thermal
765
insulation and mechanical properties. This study will certainly open a new field of
766
investigation to understand and optimize the thermal conductivity and mechanical properties
767
of nanocellulose based aerogels. Furthermore, this study opens up new opportunities of
768
developing relatively low cost and biodegradables thermal insulators practical for commercial
769
purposes.
770
ACKNOWLEDGEMENTS
M
an
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cr
ip t
759
The authors are grateful for Anthony Magueresse’s assistance with the SEM and FE-
772
SEM experiments. The authors appreciatively acknowledge to Center of Analyses and
773
Characterizations (CAC) of Cadi Ayyad University - Morocco, Bretagne region, the European
774
Union (FEDER) and the French Ministry for Research for rendering possible the financial
775
support for conducting the studies.
te
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cr
783
Baetens, R., Jelle, B. P., & Gustavsen, A. (2011). Aerogel insulation for building applications: A state-of-the-art review. Energy and Buildings, 43(4), 761–769. http://doi.org/10.1016/j.enbuild.2010.12.012
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Bendahou, A., Habibi, Y., Kaddami, H., & Dufresne, A. (2009). PhysicoChemical Characterization of Palm from Phoenix Dactylifera–L, Preparation of Cellulose Whiskers and Natural Rubber–Based Nanocomposites. Journal of Biobased Materials and Bioenergy, 3(1), 81–90. http://doi.org/10.1166/jbmb.2009.1011
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HIGHLIGHTS:
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1. Aerogels preparation by freezing drying
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2. Multi-scale cellulose aerogels for super thermal–insulation properties
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3. Reinforcing effect of CNF and CNC as ultra-thin films in aerogels
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4. Effect of TEMPO oxidation time of CNF on the structure and properties of aerogels
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S0144-8617(15)01121-2 http://dx.doi.org/doi:10.1016/j.carbpol.2015.11.032 CARP 10550
To appear in: Received date: Revised date: Accepted date:
3-7-2015 8-11-2015 11-11-2015
Please cite this article as: Seantier, B., Bendahou, D., Bendahou, A., and Kaddami, H.,Multi-scale cellulose based new bio-aerogel composites with thermal superinsulating and tunable mechanical properties, Carbohydrate Polymers (2015), http://dx.doi.org/10.1016/j.carbpol.2015.11.032 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.
Multi-scale cellulose based new bio-aerogel composites with
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thermal super-insulating and tunable mechanical properties
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Bastien Seantiera, Dounia Bendahoua,b, Abdelkader Bendahoua,YvesGrohens*a,
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Hamid Kaddami*b
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Université de Bretagne Sud, Laboratoire Ingénierie des Matériaux de Bretagne, BP 92116, 56321 Lorient Cedex, France. b
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Cadi Ayyad University, Faculty of Sciences and Technologies, Laboratory of Organometallic and Macromolecular Chemistry, Avenue AbdelkrimElkhattabi, B.P. 549, Marrakech, Morocco.
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Corresponding author: [email protected], [email protected].
Abstract
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Bio-composite aerogels based on bleached cellulose fibers (BCF) and cellulose nanoparticles
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having various morphological and physico-chemical characteristics are prepared by a freeze-
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drying technique and characterized. The various composite aerogels obtained were compared
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to a BCF aerogel used as the reference. Severe changes in the material morphology were
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observed by SEM and AFM due to a variation of the cellulose nanoparticle properties such as
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the aspect ratio, the crystalline index and the surface charge density. BCF fibers form a 3D
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network and they are surrounded by the cellulose nanoparticle thin films inducing a
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significant reduction of the size of the pores in comparison with a neat BCF based aerogel.
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BET analyses confirm the appearance of a new organization structure with pores of
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nanometric sizes. As a consequence, a decrease of the thermal conductivities is observed
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from28 mW.m-1.K-1 (BCF aerogel) to 23 mW.m-1.K-1 (bio-composite aerogel), which is below
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the air conductivity (25 mW.m-1.K-1). This improvement of the insulation properties for
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composite materials is more pronounced for aerogels based on cellulose nanoparticles having
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a low crystalline index and high surface charge (NFC-2h). The significant improvement of
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their insulation properties allows the bio-composite aerogels to enter the super-insulating
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materials family. The characteristics of cellulose nanoparticles also influence the mechanical
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properties of the bio-composite aerogels. A significant improvement of the mechanical
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properties under compression is obtained by self-organization, yielding a multi-scale
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architecture of the cellulose nanoparticles in the bio-composite aerogels. In this case, the
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mechanical property is more dependent on the morphology of the composite aerogel rather
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than the intrinsic characteristics of the cellulose nanoparticles.
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Key words: Multi-scale architecture, cellulose, Nanofibrillated cellulose (NFC),
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cellulose nanocrystals (CNC), freeze-drying, porosity, super-insulation, mechanical
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properties.
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1 INTRODUCTION
Recent years have seen an increase in consumer demand for biodegradable products
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designed according to more environmentally friendly methods. Furthermore, the increase in
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oil prices and its scarcity helped to bring forward new products of natural origins. The
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objective of this article is to prepare and characterize an ultra-porous cellulose-based material,
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called a composite aerogel.
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Although Kistler made his discovery in the early 1930s (Kistler, 1931a), aerogels are
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nowadays considered as the most promising materials for various applications. The multitude
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of organic, inorganic or hybrid materials, and the design of new methods for extraction and
45
preparation, offer prospects for use in the development of fuel cell separators (R. W. Pekala,
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Mayer, Kaschmitter, & Kong, 1994), in filtering systems and ultra-thin dust capture (Tsou,
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1995; Venkateswara Rao, Hegde, & Hirashima, 2007). Recently, the aerogels have been used
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with success in the field of thermal super-insulation (Baetens, Jelle, & Gustavsen, 2011; Han,
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Zhang, Wu, & Lu, 2015).
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Aerogels can be inorganic (silica-based) (Bonnardel et al., 2006; Fricke, HüMmer,
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Morper, & Scheuerpflug, 1989) or organic (example of resorcinol-formaldehyde) (R. W.
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Pekala, 1989; Richard W. Pekala, 1991). They have a very low thermal conductivity, and are
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often super-insulators. However, they are relatively fragile (silica aerogel) (R. W. Pekala,
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1989) or toxic (organic aerogels) (Lu, Caps, Fricke, Alviso, & Pekala, 1995). A great number
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of researches have been conducted recently to develop innovative aerogel materials, bio-
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sourced for super-insulation. Thus, aerogels made from cellulose or derivatives, or other
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polysaccharides were investigated (Fischer, Rigacci, Pirard, Berthon-Fabry, & Achard, 2006;
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García-González, Alnaief, & Smirnova, 2011; Guilminot et al., 2008; Hall et al., 2012;
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Liebner et al., 2009; Mehling, Smirnova, Guenther, & Neubert, 2009; Rudaz & Budtova,
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2013; Sescousse, Gavillon, & Budtova, 2011).
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Even if Kistler was the first to consider the use of cellulose and nitrocellulose
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(cellulose derivative) to prepare an aerogel in 1930 (Kistler, 1931b), few studies were
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subsequently conducted to develop original nanostructured aerogels made from cellulose
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precursors. Recently, macro-porous materials have been prepared by drying physical gels
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made from cellulose (Hao Jin, Nishiyama, Wada, & Kuga, 2004) or cellulose acetate
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(Pimenov, Drozhzhin, & Sakharov, 2003). Yet, the materials obtained cannot be considered
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as conventional aerogels (i.e. nanostructured and nanoporous) because their pore dimensions
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are too large (>1µm). The first attempt to develop cellulose acetate chemical gels (by
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crosslinking the gel and subsequent supercritical drying)was published in 2001 (Tan, Fung,
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Newman, & Vu, 2001). Mechanically strong nanomaterials were prepared using this method.
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Xiaoyu Gong et al have studied an aerogel based on a preparation by combining the
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NaOH/thiourea/H2O solvent system and the freeze-drying technology with density varying in
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the range of 0.2-0.4 g/cm3 (Gong, Wang, Tian, Zheng, & Chen, 2014). The results showed
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that the main factors affecting thermal conductivity were density and porosity. Thermal
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conductivity decreased with the decrease of density and the increase of porosity, it could be as
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low as 0.029 W/(m.K).
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However, to ensure a sustainable development towards industrialization, research must
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improve the manufacturing processes and promote the use of raw materials that are more
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respectful of the environment. One of the raw materials, which meet these requirements, is
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cellulose in its various forms. It is an ideal material to develop bio-aerogels because of its
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availability, its lightness and its capability of forming a three-dimensional resistant network.
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Additionally, cellulose is easily and cheaply extracted from various sources such as the date
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palm tree in Morocco and may have several aspect ratios (Kaddami et al., 2006). Its low
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intrinsic thermal conductivity, its ability to form a three-dimensional structure involving
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hydrogen bonding and its low molecular weight make cellulose a promising material to
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prepare bio aerogels. Furthermore, the resistance of the produced networks allows the gels to
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support simple manufacturing processes to obtain the final product. Although the organic
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aerogels have good thermal insulation properties, their thermal conductivity remains slightly
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higher than inorganic aerogels like silica. Recently, Kobayashi et al. while studying aerogel
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based liquid-crystalline nanocellulose derivatives reported that these aerogels exhibited very
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low thermal conductivity. The lowest thermal conductivity of 0.018Wm-1K-1 was measured at
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a density of 17 mg.cm-3, and it increased as the density was increased (Kobayashi, Saito, &
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Isogai, 2014).
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In order to obtain various cellulose nanoparticles, the oxidation of cellulose is
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commonly used. This process, used to transform alcohol to aldehyde or a corresponding acid,
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is made according to several methods and reagents (Benhamou, Dufresne, Magnin, Mortha, &
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Kaddami, 2014). Such modifications are usually achieved by converting part of the cellulose
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surface hydroxyl groups into carboxyl groups. At the present time, the catalytic oxidation
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using 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical under mild aqueous conditions
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was found to be the most straightforward and efficient method to selectively oxidize hydroxyl
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functions located at the surface of cellulosic fibers and convert them into sodium carboxylate
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(Okita, Saito, & Isogai, 2010). This method has been widely used on different types of well-
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defined purified cellulose substrates: cotton linters (Saito & Isogai, 2004), cellulose
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microfibrils from sugar beet (Montanari, Roumani, Heux, & Vignon, 2005), tunicin whiskers
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(Habibi, Chanzy, & Vignon, 2006), bleached hardwood kraft pulp (Saito et al., 2009),
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bleached softwood kraft pulp (Dang, Zhang, & Ragauskas, 2007), fibrillated viscose rayon
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fibers (Shibata & Isogai, 2003), lignocellulosic fibers isolated from the leaflets of the date
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palm tree (Sbiai, Kaddami, Sautereau, Maazouz, & Fleury, 2011) and even water soluble
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cellulose acetate (Gomez-Bujedo, Fleury, & Vignon, 2004). It is worth noting that the
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oxidation catalyzed by the TEMPO radical has been developed by several researchers in the
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case of insoluble polysaccharides such as cellulose and chitin (Chang & Robyt, 1996; Isogai
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& Kato, 1998; Tahiri & Vignon, 2000; Zhao et al., 1999), as well as for soluble
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polysaccharides in water (de Nooy, Besemer, & van Bekkum, 1995; Gomez-Bujedo et al.,
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2004). Among various oxidants, sodium hypochlorite/sodium bromide with the TEMPO
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radical is currently the most studied. The hypobromite ion is much more reactive than the
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hypochlorite ion, allowing rapid regeneration of oxoammonium ion, by the introduction of
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sodium bromide.
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Various techniques have been developed to prepare cellulose network based aerogels:
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fiber bonding (Costa-Pinto et al., 2009), freeze-drying (Sudheesh Kumar et al., 2011),
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supercritical fluid technology (Chung & Park, 2007), compression molding and salt leaching
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(Hou, Grijpma, & Feijen, 2003), gas foaming (Chen et al., 2012), rapid prototyping (Yun,
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Kim, Hyun, Heo, & Shin, 2007) and electrospinning (Peng, Shaw, Olson, & Wei, 2011).
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Freeze drying is a dehydration technique in which liquid samples are frozen below their glass
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transition temperature (Tg) or melting point. Then, the frozen solvents are removed by the
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sublimation process, thereby obtaining porous, interconnected structures (Liu, 2006).
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Compared to other techniques, the distinct advantages of freeze-drying are that non-toxic
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organic solvents are being used and the low temperature helps to maintain the activity of bio-
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macromolecules and pharmaceutical products for a long period. Unlike a normal drying
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process, this technique involves low surface tension, which can maintain the pore structure.
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Therefore, the bio-aerogel with a nano-sized pores morphology can be synthesized from
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different cellulose by adjusting various parameters in the freeze-drying process (Qian &
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Zhang, 2011).
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Conscious of the importance of the characteristics of cellulose nanoparticles to the
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performances of their resulting products (Kadimi et al., 2014; Li, Wu, Song, Qing, & Wu,
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2015; Li, Wu, Song, Lee, et al., 2015; Xu et al., 2013), our work aims at designing new
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composite aerogels based on multi-scale cellulose fibers. Combining several aspect ratios and
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varying the formulations, the developed new materials will be optimized to gain the lowest
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thermal conductivity and the best mechanical strength.
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2 MATERIAL AND METHODS
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2-1 Chemical compounds studied in this article:
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TEMPO (Pub Chem CID: 2724126); sodium bromide (Pub Chem CID: 253881);
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sodium hypochlorite solution (15%) (Pub Chem CID: 23665760); HCl (Pub Chem CID: 313);
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NaOH (Pub Chem CID: 14798); methanol (Pub Chem CID: 887); sulfuric acid (Pub Chem
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CID: 1118); chloroform (Pub Chem CID: 6212). More information is available at:
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http://www.elsevier.com/PubChem.
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2.2 Materials
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One aim of this paper was to use local vegetal waste to develop added value materials.
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The date palm tree (Phoenix dactylifera L) is one of the most harvested palm trees in
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Morocco. That is the reason why it was used in this work as an original source of cellulose.
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The procedure to extract cellulose from the rachis of the date palm tree has already been
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described previously (A. Bendahou, Habibi, Kaddami, & Dufresne, 2009; D. Bendahou,
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Bendahou, Seantier, Grohens, & Kaddami, 2015; Hua Jin et al., 2013). In the coming
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sections, the extracted cellulose will be referred to as bleached cellulose fibers (BCF). For the preparation of the cellulosic nanoparticles, the following chemicals were used:
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TEMPO, sodium bromide, sodium hypochlorite solution (15%), HCl, NaOH, methanol,
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sulfuric acid and chloroform. They were all purchased from Sigma-Aldrich and were used
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without further purification.
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2.3 Nanofibrillated cellulose NFCs preparation by TEMPO-mediated oxidation
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The general procedure and reagent ratios used by Sbiai et al. (Sbiai et al., 2011) were
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used for TEMPO-mediated oxidation of cellulose fibers. About 2 g i.e. 2.136 mmol of an
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equivalent anhydroglucose unit (AGU) of cellulose was suspended in water (200 ml) and
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sonicated with a Branson Sonifier for 5min. TEMPO (32 mg, 0.205mmol) and NaBr (0.636 g,
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6.175mmol) were added to the suspension. An additional amount of the NaOCl solution,
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corresponding to 40.5 ml was added in drops to the cellulose suspension. The pH was
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adjusted to10 by the addition of a 0.1 M aqueous solution of HCl. The pH of the mixture was
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maintained at 10 at 4°C by continuously adding 0.1 M NaOH while stirring the suspension.
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When the oxidation had finished, it was terminated by the addition of methanol (5 ml) and the
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pH was adjusted to 7 with 0.1 M HCl. The oxidation times used were 5 min and 2 h.
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After the oxidative treatment, a 1wt% fiber suspension in water was subjected to the
171
homogenizing action of a low-pressure slit homogenizer using the Laboratory Homogenizer
172
PANDA 2K (GEA Niro Soavi S.P.A), USA. To obtain nanofibrillated cellulose (NFC) the
173
oxidized cellulose was disintegrated by pumping the suspension up to 15 times through the
174
homogenizer. Efforts were made to keep the pressure constant at 650 MPa for all cycles and
175
all samples. During the process, the viscosity and temperature of the suspension increased
176
with the increase in the number of cycles. The maximum temperature reached was 70°C.
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Herrik et al. (Herrick, Casebier, Hamilton, & Sandberg, 1983) found that increasing the
178
temperature from 20°C to 70–80°C helped the dissociation of physical bonds and facilitates
179
homogenization. The prepared NFC-5min and NFC-2h will be denoted as NFCs in the rest of
180
this article.
181
2.4 Preparation of cellulose nanocrystals
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177
Cellulose nanocrystals (CNC) were prepared from the rachis of the date palm tree
183
(Phoenix dactylifera L) as described in literature (A. Bendahou et al., 2009). Briefly, cellulose
184
fibers were treated three times with 3 wt% NaOH solution at 80 °C for 2h under mechanical
185
stirring and bleached with a solution made by equal parts of acetate buffer, aqueous chlorite
186
(1.7 wt% in water) and distilled water. The bleaching treatment was performed 4 times at
187
70°C, under mechanical stirring, at 1h intervals, and bleached cellulose fibers (BCF) were
188
obtained. Acid hydrolysis was carried out on the BCF at 45 °C with 65 wt% sulfuric acid
189
(pre-heated), for 45 min, using mechanical stirring at 260 tr/min. Successive centrifugations
190
were performed at 11000 rpm and 10 °C for 30 min (each step) and the suspensions were
191
dialyzed against distillated water. Homogenization was carried out using an Ultra-Turax T25
192
homogenizer for 5 min and the suspensions were filtered in a glass filter no. 1 (Size retention
193
1.5-1.6 µm). Some drops of chloroform were added to the CNC suspension that was stored at
194
4°C to limit bacterial development.
195
2.5 Porous aerogels preparation from BCF/ NFCs or BCF/CNC mixtures
us
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196
cr
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In order to obtain aerogels having targeted properties, stable suspensions of each
197
constituent have been made to avoid material agglomerates and to obtain the best material
198
structure. Ultra-Turrax Ika® was used to homogenize the suspensions by shear forces.
199
First of all, date palm tree NFCs or CNC were re-suspended separately in 20 ml of
200
distilled water. BCFs were obtained as homogeneous water dispersion. Then, NFCs or CNC
201
dispersion, depending on the targeted mixture, was passed through Ultra-Turrax and mixed
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with a specific amount of BCF under magnetic stirring. Solutions are finally freeze-dried
203
using liquid nitrogen: after freezing, the dispersions are kept in petri dishes for one night at -
204
20°C, the solvent was sublimated (Christ Alpha 1-2 LD Plus) under 80 mBar and at -52°C.
205
Freeze-drying was carried out for 24h to remove water traces. The elaborated aerogels were
206
stored in an oven under vacuum at 25°C to limit water uptake. The dry mass of the aerogels
207
with NFCs or CNC is always equal to 500 mg.
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The concentration of BCF was maintained at a constant level and the amount of NFCs
209
or CNC varies in mass as follows: 0%, 1%, 2%, 4%, 5%, 10% and 20% with respect to the
210
BCF.
211
2.6 Zeta potential measurements
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To obtain information about the surface charge density of the studied particles (BCF,
213
NFCs and CNC), the zeta potential was measured. The experiments were carried out using the
214
zeta-meter “ZetasizerNano ZS Malvern Instruments” equipped with a 633 nm laser. This
215
technique consists of analyzing the phase shifts of the light scattered by the sample while
216
submitted to an electric field. It attains information about surface charge density but also
217
about the stability of the suspensions. All solutions used to prepare the starting dispersions
218
(BCF, NFCs and CNC) were studied. 300 µL of each starting solution, having an ionic
219
strength stabilized at 1 mM using NaCl solution, were prepared for all measurements.
220
2.7 X-ray diffraction (XRD)
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M
212
Nanocellulose films were prepared by a heat-drying method and were examined using
222
X-Ray Diffraction (XRD) to evaluate the impact of the treatment on the crystalline index of
223
NFCs and CNC. BCF film was prepared and used as the reference.
224
An X’PERT (Philips PW3710) wide angle X-ray diffractometer was used to determine
225
the crystallinity of the specimens. The source was a Ni-filtered Cu Kα radiation with a
226
wavelength of 1.5418 Å. The X-rays were operated at 40 kV and 30 mA.
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227
X-ray diffractograms were recorded at 0.02°.s-1 over a 2ϴ scan in the range 10-60°.
228
Crystalline index (C.I.) values were calculated according to the empirical Segal method (eq.1)
229
(Segal, Creely, Martin, & Conrad, 1959): Eq. 1
ip t
230
Where I0 0 2 is the intensity of the (0, 0, 2) reflection plan (2ϴ between 22° and 23°)
232
and IAM is the minimum value at 2ϴ between 18° and 19°, which represents the reflection
233
intensity of the amorphous phase. The patterns were recorded, analyzed and phase matched
234
by X’PERT software, data collector and graphics.
235
2.8 Atomic Force Microscopy (AFM)
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After gluing a cleaved mica surface onto a metallic disk, a droplet of the studied
237
dispersion (NFCs and CNC) was deposited and casted. The obtained samples were imaged by
238
Atomic Force Microscopy (AFM) with a multimode equipped with the nanoscope III-a
239
(Bruckernano, Santa Barbara, USA). The AFM was used in contact mode imaging. The tips
240
(DNP–10, Bruckernano) were made of Si3N4 and had a 0.24 N/m nominal spring constant.
241
The deflection set point was chosen at the limit of the tip retraction. The imaging speed was
242
between 1 and 2 Hz depending on the surface. The pictures obtained were processed with
243
Nanoscope 6.13 R1 and Gwyddion v2.29 software.
244
2.9 Transmission Electron Microscopy (TEM)
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245
M
236
Transmission electron microscopy (TEM) of the cellulose whiskers was carried out
246
with a Philips CM200 transmission electron microscope with an acceleration voltage of 80
247
kV. An aqueous dispersion of whiskers was deposited on a microgrid (200 mesh, Electron
248
Microscopy Sciences, Hatfield, PA, USA) and covered by a thin carbon film (∼200 nm). The
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249
deposited whiskers were subsequently stained with a 2% uranyl acetate solution to enhance
250
the microscopic contrast.
251
2.10 Field Emission Scanning Electron Microscopy (FE-SEM) FE-SEM was used to characterize the BFC and the aerogels structures. The “quanta 200FEI”, with accelerating voltage of 12.5 kV, was used.
254
2.11 Fourier Transformed Infrared spectroscopy (FTIR)
cr
253
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To investigate the effect of chemical pretreatment on NFC, Fourier Transform Infra
256
Red spectroscopy was carried out with a Perkin Elmer Spectrum D400 spectrometer (Perkin
257
Elmer). For each film prepared by heat-drying NFC suspensions, the spectrometer was used
258
in adsorption mode and the wavelength range was from 400 to 4000 cm-1. For each spectrum,
259
32 spectra were taken. A separate background spectrum was collected and automatically
260
subtracted from the raw spectrum for each specimen.
261
2.12 BET analysis
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Nitrogen adsorption/desorption isotherms were obtained using a Micromeritics ASAP
263
2420. The samples in the form of aerogel were placed in the measuring cell to degas all water
264
molecules adsorbed. The degassing of the material was carried out according to the following
265
procedure: by heating at 50°C for 1h and then at 100°C for 15h under 10-2 Torr. We chose
266
relatively mild degassing conditions in order not to degrade the sample and to be sure that all
267
the water molecules were removed.
Ac ce p
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te
262
Subsequently, the measuring cell is placed in an insulated tank filled with liquid
269
nitrogen. This keeps the sample at -196°C throughout the measurement, the temperature at
270
which gaseous nitrogen adsorption is possible on a solid surface. A nitrogen
271
adsorption/desorption isotherm represents the evolution of the volume of Nitrogen adsorbed
272
per gram of sample extrapolated to standard conditions of temperature and pressure (STP
273
cm3/g) depending on the relative pressure of nitrogen (p/p0). Using the BET theory, it is
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274
possible to evaluate the specific surface and obtain information on the aerogel structure such
275
as the pore size.
276
2.13 Thermal conductivity measurements The system used to measure the thermal conductivity was the “hot filament” technique
278
(Fig.1) (dos Santos, 2005; Scudeller & Bardon, 1991). It is composed of two symmetrical
279
cavities having a 1.65 cm3 volume, one with the sample (1) and the other containing a
280
reference, the polyurethane foam (2). Both parts surround a thin nickel/chrome heating
281
filament (3). The cavities and the filament are maintained in an aluminum box at a
282
temperature of 20.7°C controlled by a water circuit (4). The filament (3) is isolated from the
283
box (4) with polymer thin films (20 µm thick) (5). The filament (3) is connected to 2 copper
284
electrodes (6) for the electric power supply. The temperatures of both sides of the sample (2)
285
are measured with 2 thermo-couple K type (7down and 7up) applied in the system.
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The thermal conductivity of the sample (1) is determined in a steady state by the
287
measurement of the filament-housing conductance (W.K-1). The ratio of the heating measured
288
at the center of the sample and the electric power dissipated by the sample enables the
289
calculation of the conductance (eq. 2).
310
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286
291
Caption:
293
1: aerogel to be characterized
7up
295
2: polyurethane foam
297 299
3: NiCr filament, 2µm x 4mm x 20 mm
301
4: isothermal box (aluminum)
303
5: thermal insulator (polymer)
305
6: metallic electrodes
307 309
7up and 7down: thermocouples K type, diameter 0.1mm
7down
Figure 1: Scheme of the thermal conductivity measurement system.
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Eq. 2
311
313 314
In our protocol, the heating, ∆T, is set to 10 °C. The tension, U, and the intensity, I, of the current were measured at the steady state.
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312
The conductance is related to the sample conductivity and follows a linear law depending on the measurement system as follows (eq. 3):
316
Κ = A.λ + B
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315
Eq. 3
Where Κ (W.K-1) being the conductance, λ (W.m-1.K-1) the thermal conductivity and
318
A and B are the experimental constants depending on the sample geometry, the cavities and
319
the thermocouples.
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Therefore, the first step of a measurement is the calibration of the system performed
321
by measuring the conductance of 5 materials having a known conductivity over the range of
322
studied conductivities.
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320
Then the BCF and composite materials are prepared in the sample holder by freeze-
324
drying. Aerogel samples are obtained that exactly fill the sample holder to avoid thermal
325
bridge problems while measuring the sample properties.
326
2.14 Mechanical characterization
Ac ce p
327
te
323
The mechanical properties of the aerogels were determined with the Dynamic
328
Mechanical Analysis (DMA). The experiments were carried out with the DMA 2980 TA
329
instrument. The compression modulus was measured on samples having the following
330
dimensions: 25 × 25 × 4 mm3. The tests were carried out at a controlled ambient temperature
331
(23.4°C) and humidity (50%). The mechanical properties were determined for a frequency of
332
1 Hz and displacement of 0.05 mm. This technique has been used to characterize the
333
compression modulus (K) and the Yield stress (σy) of all aerogels (see Figure 7).
334
2.15 The degree of oxidation (DO)
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The degree of oxidation refers to the average normalized number of primary hydroxyl
336
groups oxidized by an anhydroglucose unit of the cellulose sample. As a result, the maximum
337
degree of oxidation is 1, corresponding to an anhydroglucose unit which is completely
338
oxidized. The carboxyl content of the oxidized cellulose samples was determined by
339
conductimetric titration. This titration technique has been used by many researchers (Araki,
340
Wada, Kuga, & Okano, 1998; da Silva Perez, Montanari, & Vignon, 2003) to determine the
341
degree of oxidation. The degree of oxidation is determined from equations 4 and 5.
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335
343
an
342
With:
Eq. 5
M
344
Eq. 4
Where [NaOH] is the NaOH concentration (mol.L-1), V0 and V1 are respectively the initial
346
and final volume of NaOH (L), which determine the limits of the plateau observed on the
347
titration curve, and M (g) is the mass of the oven-dried sample. 162 = molar mass (g.mol-1)
348
for a unit of anhydroglucose in a cellulose chain. 36= (198-162) where 198 is the molar mass
349
(g.mol-1) of oxidized an anhydroglucose unit that forms sodium salt in a cellulose chain. In
350
our experiments, the suspensions were titrated without any further purification.
351
3 RESULTS AND DISCUSSION
352
3.1 Physiochemical and structural characterization of raw material suspensions
te
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353
d
345
In this article, we propose the use of cellulosic materials to prepare green insulating
354
aerogels using the freeze-drying method. In order to better understand the obtained structure,
355
the dispersions used to fabricate the aerogel were characterized.
356
3.1.1 Structure and physicochemical properties of pristine bleached cellulose fibers
357
(BCF)
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358
shows a representative BCF micrograph.
360
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359
The cellulose fibers were observed by FE-SEM (see experimental part 2.9). Figure 2a
361 362 363 364
Figure 2: a) Pristine bleached cellulose fibers (BCF) micrograph. b) Contact mode height AFM picture of NFC-5min. c) Contact mode height AFM picture of NFC-2h. Both AFM pictures have the same scale (5 µm x 5µm). d) Transmission electron microscopy (TEM) of a suspension of CNC (whiskers).
365
From 5 micrographs, the fibers dimensions were estimated on 30 fibers. It shows that
366
the fibers have a diameter of 10±5 µm and a length of at least 700±100 µm (Fig. 2). These
367
fibers have a high aspect ratio (over 70, see table 1) and are arranged in a 3D network.
368
In order to better understand this structure, the zeta potential of the BCF solution was
369
measured. The BCF solutions have a negative zeta potential of -28 ± 2 mV, which is expected
370
for cellulose and close to the values obtained in literature (Benhamou et al., 2014).
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371
To complete the characterization of these solutions, the crystalline index has been
372
determined by X-ray diffraction. The pristine BCF crystalline index was 72%. This value is
373
close to the one obtained by K. Benhamou (Benhamou et al., 2014). From this characterization, the structure of the BCF aerogels observed is probably due
375
to the presence of alcoholic functions that may interact together through H bonds allowing the
376
formation of a strong 3D network (see the chemical structure in the supplementary
377
information, Figure S1). The high crystallinity explains the BCF stick like shape. However,
378
their high aspect ratio enables their entanglements that may strengthen the 3D arrangement.
379
The small negative zeta potential probably comes from a negligible native oxidation, which is
380
a result of the bleaching treatment, and explains the suspension stability. It is low enough to
381
allow the formation of the 3D network and high enough to stabilize the BCF dispersion for
382
relatively short period. From the characterization of the pristine BCF dispersions, we
383
demonstrate that these are able to form strong, rigid, 3D networked gels. This arrangement
384
can be used as a strong cellulosic matrix for the formed aerogels.
385
3.1.2 Structure and physicochemical properties of NFCs dispersions
387 388
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The NFC-5min and NFC-2h have different shapes and sizes. Figures 2b and 2c show
Ac ce p
386
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374
AFM pictures of the NFCs.
Compared to the BCF, the dimensions of the cellulosic NFC-5min and NFC-2h are
389
much smaller. On the one hand, the NFC-5min exhibits a mean length of between 1 and 2 µm
390
and a mean width of 40±10 nm (calculation executed on 4 AFM pictures and 50 fibers).
391
These fibrils exhibit a high aspect ratio (estimated at around 50, see Table 1) and are
392
comparable to those obtained by a high mechanical shear using microfluidizer without any
393
oxidative pretreatment (A. Bendahou, Kaddami, & Dufresne, 2010). On the other hand, for
394
the NFC-2h, the mean width is 20±4 nm and the mean length is 300 ± 20 nm (calculation
395
executed under the same conditions as for NFC-5min) (see Fig. 2b and 2c). A reduction of
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their aspect ratio (approximately 15, see Table 1) is also observed. This extreme reduction of
397
the fibril dimensions is the result of the degradation of macromolecular chains during the
398
oxidation reaction and during the homogenization step made by the mechanical treatment
399
(Benhamou et al., 2014).
ip t
396
To confirm the effect of the oxidation time on the NFCs surface charge density, the
401
zeta potential was also measured. After oxidation, the zeta potential decreased from -28 ± 2
402
mV for pristine BCF to-45 ± 3 mV and -64 ± 4 mV for NFC-5min and NFC-2h, respectively.
403
This is in complete agreement with the oxidation process, which produces negative surface
404
charges. These results are confirmed by the FTIR analyses (see supplementary information,
405
Figure S2). The most important change in the spectra concerns the band around 1730 cm-1. As
406
described by Maréchal and Chanzy (Maréchal & Chanzy, 2000), this band corresponds to
407
C=O stretching frequency of carboxyl groups in their acidic form. The intensity of this band
408
increases with the oxidation time indicating an increase of the oxidation degree. Our
409
calculated degree of the average oxidation (DO) for NFCs, extracted from the rachis of the
410
date palm tree, oxidized for 5 min and 2h are 0.036 and 0.129, respectively. These values
411
have the same order of magnitude as that obtained by T. C. F. Silvaet al. (Silva, Habibi,
412
Colodette, Elder, & Lucia, 2012), during the oxidation by TEMPO of nanofibrillated cellulose
413
produced from cellulosic pulp extracted from Eucalyptus urograndis. The DO increasing with
414
the oxidation time confirms what was observed in FTIR.
us
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415
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400
The oxidation has another effect on the particle properties: the crystalline index is
416
decreasing from 72% for BCF to 62% for NFC-5min and to 56% for NFC-2h. The oxidation
417
and the mechanical homogenization processes are destroying part of the cellulosic fiber
418
structure through the elimination of the original H bonds. This oxidation process also induces
419
more de-polymerization by β-elimination of the glycoside function (Benhamou et al., 2014;
420
Shinoda, Saito, Okita, & Isogai, 2012).The reduction of H bonds and the de-polymerization
Page 17
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421
induce a less cohesive structure with more amorphous areas, which explains the decrease of
422
the crystalline index (see the chemical structure in supplementary information Figure S1).
423
From this section onwards, we demonstrate that the oxidation of BCF produces small particles that are less crystalline and have a higher surface charge density.
425
3.1.3 Structure and physicochemical properties of Cellulose NanoCrystals
ip t
424
To perform the morphological analysis of CNC, the CNC suspension obtained by
427
hydrolysis with sulfuric acid from the rachis of date palm was visualized by TEM and AFM
428
(data not shown here). One of the micrographs obtained is shown in Figure 2d.
us
cr
426
Using the AFM microscope, we measured the average length of the CNC as being
430
270±30 nm and having an average diameter of 7±2nm. The average aspect ratio is around 38
431
(see Table 1). To carry out these measurements, we measured the length of about 40 sticks.
432
Figure 2d was used to confirm the morphology of the CNC from the date palm tree. The same
433
results were obtained. The aspect ratio calculated for the CNC (about 40, see Table 1) is
434
larger than that extracted from cotton (Dong, Revol, & Gray, 1998) or wood (Beck-
435
Candanedo, Roman, & Gray, 2005) and has a value close to that extracted from sisal (Garcia
436
de Rodriguez, Thielemans, & Dufresne, 2006; Siqueira, Bras, & Dufresne, 2009). The AFM
437
and TEM characterizations show rigid and very small needle-shaped particles. The crystalline
438
index was also determined and, as expected for a CNC, it is very high (around 90%). As
439
shown in the micrographs (Figure 2d) CNCs are usually isolated from each other, although
440
we can note the presence of some aggregates. The latter are probably formed whilst drying on
441
the carbon grid. The presence of negative charges on the surface of crystals (-SO3-) comes
442
from the preparation process (Habibi et al., 2006) and promotes their repulsion and thus their
443
individualization (see the chemical structure in supplementary information, Figure S2).
Ac ce p
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444
To verify the expected variation of the surface charge density, the zeta potential of the
445
solutions was measured. The potential is -56 ± 1 mV. The CNC potential is lower than the
446
potential of the initial BCF (-28 ± 2 mV) which confirms the hypothesis. We characterized nanoparticles that have an intermediate aspect ratio, between NFC-
448
5min and NFC-2h. The surface charge density (Zeta potential) is also intermediate; however
449
they have a very high crystalline index. We used these nanoparticles as nanofillers for BCF to
450
prepare BCF/Cellulose nanofillers aerogels. Certainly the differences in the morphological
451
and physicochemical characteristics of these nanofillers will lead to various organizations of
452
the resulting aerogels, inducing several thermal and mechanical properties. Table 1 compiles
453
all the results described previously. This characterization will help to explain the effects
454
observed when mixing the distinct dispersions and some properties of the formed bio-
455
aerogels.
456
Table 1: Geometrical and physical characteristics of the used materials obtained at 25°C.
457
d
Zeta potential (mV) (pH 5-6)
% Crystal
Up to 70 ± 20
- 28 ± 2
72%
40.10-3 ± 10.10-3
Up to 50 ± 15
- 45 ± 3
62%
300.10-3± 20.10-3
20.10-3 ±4.10-3
15 ±5
- 64 ± 4
56%
270.10-3 ± 30.10-3
7.10-3± 2.10-3
38 ± 15
- 56 ± 1
90%
Mean Length (µm)
Mean width (µm)
Cellulose
Up to 700 ±100
10 ±5
Cellulose
Up to 2± 0.5
Cellulose Cellulose
Ac ce p
Pristine Cellulose BCF Nanofibrilated Cellulose oxidized 5 min (NFC-5min) Nanofibrilated Cellulose oxidized 2 h (NFC-2h) Cellulose nanocrystals CNC
Chemical Structure
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Sample
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Aspect ratio
458
3.2 Morphological and structural characterization of the bio-aerogels
459
3.2.1 Morphology of the bio-aerogels
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Using the previously described fiber solutions, bio-composite aerogels were prepared
461
by freeze-drying (see experimental part). These aerogels are macroscopic monoliths as shown
462
in Figure 3.
cr
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460
Figure 3: Photograph of a prepared aerogel made from BCF and NFC-5min. The other aerogels look similar and this is why they are not shown.
an
464 465
us
463
A preliminary study made on the bio-aerogels cross section shows that there is likely
467
to be no detectable structural difference between the interior of an aerogel and its surface
468
(data not shown here). Therefore, to have the best overview of an aerogel structure, the SEM
469
observations have been systematically carried out on a cross section of the bulk bio-composite
470
aerogel.
te
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466
In order to determine the effect of NFC-5min on the structure of bio-aerogels, we
472
compared bio-aerogel SEM images made from a BCF (Fig. 4a and 4b) with images of bio-
473
aerogels made from a BCF reinforced by 5% and 10% of NFC-5min (Fig. 4c and 4e).
Ac ce p
471
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474
* The Red arrows show macro porosityand the blue circles highlight peeling from parts of the walls of the fibrils.
NFC-5min
NFC-2h b
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a
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BCF-0%*
c
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f
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e
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M
BCF/NFC-5%*
BCF/NFC-10%*
475 476 477
Figure 4: SEM micrographs of cellulose-based materials: (a) and (b) unloaded; (c) loaded by 5wt% of NFC-5min and (e) loaded by 10wt% of NFC-5min; (d) loaded by 5wt% of NFC-2h and (f) loaded by 10wt% of NFC-2h.
478
In Figure 4a, one observes the presence of lamellae around the non-reinforced BCF
479
(blue circles). These sheets appear after the use of the Ultra-Turrax defibrillator. The strong
Page 21
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480
shear force applied by the latter probably causes a detachment of pieces of the walls of fibers,
481
forming these cellulosic films. The SEM micrographs for bio-aerogels based on cellulose and NFC-5min, presented
483
in Figures 4c and 4e, clearly show that the NFC-5min form a network, looking like thin films,
484
in the bio-aerogel. As seen on the micrograph, BCF fibers are forming a 3D network, similar
485
to the one formed without NFC-5min and they are surrounded by the NFC-5min films. The
486
film like structure made from the NFCs comes from strong cellulose nanoparticle interactions
487
that yield particular assemblies in both the liquid and dry states. These liquid crystals like
488
structures are supposed to nucleate on the impurities embedded on the cellulose NFC. The
489
nanofilm formation from CNC solutions has been described by Lagerwall et al. (Lagerwall et
490
al., 2014) in the recent review on Cellulose Liquid Cristal Self Assemblies. CNC has the
491
ability to self-organize in the liquid crystal phase according to the temperature/concentration
492
conditions shown in a phase diagram. This arrangement in solution gives dried films of
493
various thicknesses. As stated in this article, some domains of the phase diagram have not
494
been investigated as yet. It is therefore difficult, with our complex drying and freezing
495
conditions to predict and analyze our CNC nanofilm structure. The formation of NFC films
496
induces the disappearance of part of the macropores and the appearance of organizations
497
having smaller pore sizes is observed. By increasing the amount of NFC-5min from 5wt% to
498
10wt%, the NFC-5min network looked denser, implying that the porosity of the binary system
499
is decreasing (Fig. 4c and 4e).
cr
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Ac ce p
500
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482
Figures 4d and 4f describe the SEM micrographs performed on the cross section of
501
bio-composite aerogels made from BCF loaded with 5wt% and 10wt% of NFC-2h. Similar
502
structures were obtained for BCF/NFC-2h and BCF/NFC-5min composite aerogels. NFC-2h
503
form thin films that are surrounding the BCF. When the concentration of the NFC-2h
504
increases, the film area formed in the network seems denser on the SEM micrographs.
Page 22
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However, even though they look similar, in fact the area of the thin films is slightly larger for
506
NFC-2h. It might be because of the structural properties of the raw dispersions. With a higher
507
zeta potential difference between BCF and NFC-2h and especially with the difference of
508
surface groups (Hydroxyl groups on the BCF fibers surface and carboxylate groups on the
509
NFC-2h fibers), the interactions might be favored through hydrogen bonding. Furthermore, as
510
the crystalline index is lower for NFC-2h, the particles may change their shape, through
511
modification of the amorphous areas, allowing an easier film formation and optimized
512
interactions with the surface of the BCF fibers through electrostatic forces and entanglement.
us
cr
ip t
505
In order to verify the effect of the zeta potential and the crystalline index on the
514
structure of the aerogels, the morphology of the bio-composite aerogels based on BCF and
515
CNC was characterized by SEM. Figure 5 shows a slice of non-reinforced aerogel (Fig. 5a-c)
516
and bio-aerogels reinforced with 10wt% of CNC (Fig. 5d-f).
Ac ce p
te
d
M
an
513
Page 23
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517
* The Red arrows and blue circles show respectively, macro porosity and peeling from parts of the walls of the fibrils. 120µm
b
c
e
f
an
us
d
cr
ip t
a
520 521
M
518 519
Figure 5: SEM micrographs of a cellulose-based composite aerogel: (a), (b) and (c) nonreinforced; (d), (e) and (f) strengthened by 10 wt% CNC with three different scales. Similarly to the addition of NFCs, we observed that when adding CNC to BCF the
523
system is reorganized. The individual BCFs are enveloped by CNCs, forming a structured
524
network looking like films. These results clearly confirm the hypothesis formulated in the
525
previous paragraph. CNC particles are able to form films due to their self-organization, as
526
liquid crystal in water as described by Lagerwall (Lagerwall et al., 2014).
te
Ac ce p
527
d
522
From these SEM images, a decrease of porosity is observed. Arising from the division
528
of macroscopic pores (~120 to 300 µm) due to the presence of CNC films, to pores of smaller
529
sizes, probably meso or micropores (nanometric size pores). The presence of very small size
530
pores has been reported by Fukuzumi et al. (Fukuzumi et al., 2011). Using Positron
531
Annihilation Lifetime Spectroscopy (PALS), they have shown that films based on NFC
532
similar to NFC-2h have pores of nanometric size that can reach diameters of 0.5 nm. Nemoto
533
et al., while studying nanoporous networks prepared by air drying of aqueous NFC
Page 24
Page 24 of 46
534
dispersions, showed that 90% of formed aerogel pores have sizes between 10 and 100 nm
535
(Nemoto, Soyama, Saito, & Isogai, 2012).
536
However,
bio-composite
aerogels
BCF/CNC
and
BCF/NFCs
cannot
be
characterized in a satisfactory manner by SEM because the expected meso or micropores
538
(nanometric size pores) are too small to be precisely detected. To determine the pore size of
539
such bio-composite aerogels, we used the nitrogen adsorption/desorption technique, which
540
allowed the characterization of meso and nanoporous materials.
541
3.2.2 Structure of aerogels performed by adsorption/desorption of nitrogen (BET)
us
cr
ip t
537
Most of the elaborated bio-composite aerogels were characterized by the technique of
543
nitrogen adsorption. It is typically used to analyze porous materials. The specific surface area
544
has been conventionally estimated using the Brunauer, Emmett and Teller (BET) method
545
(SBET) (Brunauer, Emmett, & Teller, 1938; Haul, 1982). Specific surface areas were measured
546
on bio-aerogels prepared with cellulose fibers (BCF) and reinforced with cellulose
547
nanoparticles (NFCs or CNC) which are between 143 and 162 m2.g-1. These values are higher
548
than that obtained by Silva et al. (Silva et al., 2012) and close to that obtained by Sehaqui et
549
al. (Sehaqui, Zhou, & Berglund, 2011) for the NFC based-aerogels. When compared with
550
most other aerogel type materials (silica and resorcinol-formaldehyde aerogels), these values
551
appear relatively low. However, in the light of specific surface areas and mean diameters of
552
the particles constituting the fibrillary mesoporous network (ranging from 2 to 50 nm), one
553
can qualitatively conclude that the solid skeleton of bio-aerogels is nanostructured.
555
M
d
te
Ac ce p
554
an
542
Table 2 compiles the different textural parameters (SBET, Vméso and DBdB) for bio-
composite aerogels.
Page 25
Page 25 of 46
Table 2: Textural characteristics of bio-aerogel based cellulose (BCF) reinforced by cellulose nanoparticles (CNC or NFCs) SaBET (m2.g-1)
Vbmeso(cm3.g-1)
DcBdB(nm)
BCF-CNC
142.22
0.0005
5
BCF-NFC5min
161.49
0.0036
13
BCF-NFC2h
116.60
0.0014
ip t
Bio-composite aerogel
cr
556 557
6-7
a: specific surface area determined by the BET method. b: mesoporous volume calculated with the following equation: Vméso=Vp-Vmicro, where Vp is the total porous volume determined for P/P0 ~ 0.85. c: diameter of pores using the BdB method (Broekhoff-de-Boer) (Broekhoff, 1968).
562
Table 2 highlights that bio-composite aerogels have mesopores. The pore size
563
distribution, determined by the Broekhoff-de-Boer method (Broekhoff, 1968), is centered on
564
5 nm in the case of the formulation BCF-CNC10%, and from 6 to 13 nm for formulations
565
BCF-NFCs10%.
d
M
an
us
558 559 560 561
At this stage in the article, we have shown that the mixing of the different cellulosic
567
particles allows specific arrangements to form aerogels. For aerogels formed from BCF, a 3D
568
network with large pores was obtained. When adding cellulosic nanoparticles (NFC-5min,
569
NFC-2h or CNC), films are formed reducing the size of the pores. We also discussed how the
570
properties of each constituent influence the structure of the aerogels. From here on in, the
571
study will focus on the influence of the specific structures obtained on the thermal and
572
mechanical properties.
573
3.3 Thermal characterization of bio-aerogels
574
3.3.1 Influence of density on thermal conductivity
Ac ce p
te
566
575
To investigate the effect of density on the thermal conductivity, the bio-aerogels were
576
compressed in a controlled manner to increase their density and the thermal conductivity was
577
measured immediately afterwards. The results are compiled in Figure 6.
Page 26
Page 26 of 46
ip t M
an
us
cr
578
580 581 582 583 584
Ac ce p
te
d
579
Figure 6: Thermal conductivity of bio-aerogels prepared with (a) BCF, (b) NFC-5min, (c) NFC-2h, (d) a mixture of BCF and NFC-2h 10 wt% and (e) a mixture of BCF and CNC 10 wt% depending on the density in kg/m3. The black line shows the thermal air conductivity. The red line shows the optimal thermal conductivity reached depending on the mixture.
585
The results showed that, after compression (inducing an increase in density) the
586
thermal conductivity is changing in a specific way for both composite and single systems and
587
two different zones could be delimited: zone 1 (λ<λminimum) and zone 2 (λ>λminimum).
Page 27
Page 27 of 46
In zone 1 (λ<λminimum), there is a strong decrease of thermal conductivity. Whilst being
589
compressed, the cellulosic particles are getting closer and the pore size is decreased, reducing
590
the thermal gas conduction via air confinement (Knudsen effect). As a consequence, the
591
effective thermal conductivity is severely reduced. This is true until a threshold density
592
between 30 and 40 kg/m3 is reached. For these densities, a minimum thermal conductivity is
593
observed (λ=λminimum). The pore size induces air confinement and low gas conduction, but the
594
cellulosic network is not yet completely collapsed, which causes a solid conduction that is
595
still low. As a consequence, the overall thermal conductivity is at a minimum. In zone 2
596
(λ>λminimum), the density continues to increase. The cellulosic network starts to collapse
597
severely, allowing the creation of contact nodes between the cellulose nanoparticles. This
598
physical connection between particles will promote solid thermal conduction and induce a
599
significant rise in thermal conductivity known as the thermal percolation phenomenon.
M
an
us
cr
ip t
588
From Figures 6a, 6b and 6c, the minimum value of thermal conductivity λminimum is
601
lower for the two neat NFCs based aerogels, in comparison with the neat BCF based aerogel.
602
Additionally, the NFC-2h based aerogel exhibits the lowest thermal conductivity (below 24
603
mW.m-1.K-1). It is worth noting that we could not perform similar experiments on a CNC
604
based aerogel because it is too brittle to be handled.
te
Ac ce p
605
d
600
Figures 6d and 6e show thermal conductivity depending on the density of bio-
606
composite aerogels prepared from BCF filled with cellulose nanoparticles (NFCs, CNC). The
607
same behavior as for the previous materials is observed. However, the thermal conductivity of
608
these bio-composite aerogels can reach lower values (~23 mW.m-1.K-1 under ambient
609
conditions of pressure and temperature). Thus the minimum value of thermal conductivity
610
decreases from 28 ± 1 mW.m-1.K-1 for a BCF based aerogel to 23 ± 1 mW.m-1.K-1 for the
611
composite aerogel prepared from BCF and 10wt% NFC-2h. The thermal conductivity values
612
(~23 mW.m-1.K-1) obtained for these bio-composite aerogels are lower than those obtained by
Page 28
Page 28 of 46
S. T. Nguyen et al., (32 mW.m-1.K-1) (Nguyen et al., 2014) for cellulose aerogels prepared
614
from paper waste or wool (30-40 mW.m-1.K-1). However, they are comparable to those of
615
aerogels produced by the Aspen group (21 mW.m-1.K-1) (“Pyrogel XT-E,” 2015; Sequeira,
616
Evtuguin, & Portugal, 2009). This low value of thermal conductivity leads to a promising
617
material for thermal insulation. In comparison with air thermal conductivity (~ 25 mW.m-1.K-
618
1
cr
), the studied aerogels are classified as thermal super-insulators.
ip t
613
The obtained thermal insulation properties of the binary aerogels result from the
620
structure of the bio-aerogel. The morphology illustrated in Figures 4 and 5 highlight the fact
621
that the cellulose nanoparticles form films that reduce the pore size. The air, which is
622
abundant in these highly porous aerogels, is confined in pores smaller than the mean free path
623
(~70 nm at atmospheric pressure and ambient temperature), resulting in a decrease of the gas
624
molecule mobility. The thermal conductivity is thus reduced. We refer to the Knudsen effect
625
(Notario et al., 2015). The decrease of the pore size is confirmed by the BET analysis.
d
M
an
us
619
To conclude, we demonstrated that the density of the aerogel has a significant impact
627
on their thermal properties. By controlling the bio-aerogel density, it is possible to tune the
628
pore size and thus the thermal insulation properties to lower thermal conductivities.
629
3.3.2 Influence of cellulose particle properties on thermal conductivity
Ac ce p
630
te
626
To assess the impact of the cellulosic particle properties on the thermal conductivity,
631
six formulations of aerogels were prepared. Diverse weight fractions of the nanofillers were
632
selected for each type of cellulose nanoparticle. Table 3 summarizes the results for thermal
633
conductivities of the prepared aerogels.
634
Page 29
Page 29 of 46
634 635
638
BCF/CNC λ aver (mW.m-1.K-1)
0%
28 ± 1
1%
27 ± 2
2%
27 ± 2
5%
25 ± 0.5
10%
25 ± 0.5
20%
25 ± 1
BCF/NFC-5min λ aver (mW.m-1.K-1) 28 ± 1
28 ± 1
27 ± 1
an
us
27.5 ± 0.05 27 ± 1
BCF/NFC-2h λ aver (mW.m-1.K-1)
cr
Aerogel (%)
ip t
Table 3: Evolution of thermal conductivity (λ) depending on the proportion of cellulose nanoparticles. The density of the aerogels is the same and is approximately ~40 kg.m-3.
26 ± 1
25 ± 1
23 ± 1
23 ± 1
M
25 ± 0.5
23 ± 0.5
25 ± 0.5
d
636 637
From Table 3 we can clearly see that the conductivity decreases when the weight
640
fraction of nanofiller increases. However, using the same fraction of nanofiller the
641
conductivity reached for the aerogels prepared with NFCs is slightly, but not significantly,
642
lower than those achieved on CNC. The minimum thermal conductivity is reached when the
643
nanofillers fraction is between 10 and 20 wt%. The minimal value measured for these
644
concentrations are 25 mW.m-1.K-1 for BCF/CNC composite aerogel and 23 mW.m-1.K-1 for
645
the BCF/NFCs ones. These results clearly show that adding cellulose nanoparticles to BCF
646
decreases the apparent thermal conductivity because of the decrease of the pore sizes in the
647
resulting aerogels. This induces the formation of small cavities as confirmed by BET
648
analyses. In addition to the reduction of the space between the BCF due to the formation of a
649
tight 3D network, the films of nanoparticles contain pores of nanometric sizes (Fukuzumi et
650
al., 2011). All these changes in the aerogel structure are responsible for air confinement and
Ac ce p
te
639
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Page 30 of 46
651
the decrease of gas conduction in the aerogels. The presence of pores smaller than the mean
652
free path of the air molecules significantly reduces the contribution of the gas phase by the
653
Knudsen effect, and reaches super-insulating properties. Upon introduction of the NFC-2h in the BCF, we find that conductivity decreases up
655
to the fraction of 10wt%. Beyond this fraction the conductivity of the binary aerogel
656
increases. The behavior is different in the case of BCF/CNC aerogels. The conductivity
657
decreases when the weight fraction CNC increases up to 5wt% and then remains constant for
658
the higher fractions of CNC at 25 mW.m-1.K-1. While comparing the structure and physical
659
characteristics of CNC and NFC-2h, one can clearly see that the two nanoparticles have
660
similar sizes and aspect ratios (see Table 1). Their zeta potentials are slightly different (-64
661
mV for NFC-2h vs. -56 mV for CNC). However the main difference is their crystallinity
662
(56% for NFC-2h vs. 90% for CNC). This makes NFC-2h stiffness very low compared to
663
CNC. It results in differences, not only in terms of interaction towards a BCF but also in
664
terms of increasing entanglement and more importantly in terms of the morphology and
665
porosity of the walls between the BCF formed by the nanofillers. The flexibility and the
666
amorphousness of NFC-2h are very likely to have nanometric size pores as reported by
667
Fukuzumi et al. (Fukuzumi et al., 2011).
cr
us
an
M
d
te
Ac ce p
668
ip t
654
In conclusion, the properties of the used particles are largely influencing the aerogel
669
thermal behavior through morphological changes. However, from an application point of
670
view, it is necessary to verify the mechanical performances of the aerogels formed.
671
3.4 Mechanical characterization of bio-aerogels
672
The mechanical properties of aerogels for thermal insulation applications are very
673
relevant and make them attractive. Due to their large pore volume, many aerogels, which have
674
good insulation properties, have low mechanical properties, which make them difficult to use
675
in industry. An agreeable compromise between the stiffness and mechanical strength is
Page 31
Page 31 of 46
676
required. Therefore, it is necessary to optimize the mechanical properties and understand how
677
to improve them. In order to characterize the mechanical properties of the aerogels, compression
679
experiments were carried out. Using these curves, one can distinguish three different domains:
680
1)
681
modulus (K). The compression modulus can be calculated from the compression curve (Eq.
682
6). It is the slope of the linear elastic portion of the stress-strain curve:
ip t
678
us
cr
A domain that corresponds with the elastic domain characterized by the compression
Eq. 6
683
2)
The second domain is a long, almost horizontal plastic plateau corresponding to the
685
progressive irreversible collapse of the pores.
686
3)
687
densification of the material due to the contact between adjacent cavity walls (Gibson &
688
Ashby, 1997).
an
684
te
d
M
Finally, the third domain relates to a steep rise of the constraint. This step matches the
From the compression stress-strain curves, we determined the main mechanical
690
characteristics, namely the compression modulus (K), the stress yield and deformation at the
691
end of the domain 1(Gibson & Ashby, 1997). All mechanical characteristics have been
692
plotted in Figure 7.
693
Ac ce p
689
Page 32
Page 32 of 46
us
cr
ip t
693 694
695
Figure 7: Variation of the mechanical properties with the weight fraction of nanofillers contained in the bio-composite aerogel. a) Compression modulus and b) Yield stress is varying with the amount of CNC, NFC-2h and NFC-5min in the BCF composite aerogel.
an
696 697 698
From Figure 7a, we can observe that the compression stiffness (or compression
700
modulus) of the composite aerogels is higher than the neat BCF based aerogel. The
701
compression modulus increases from 13 kPa for the neat BCF based aerogel to 176 kPa and
702
62 kPa for the BCF/NFC-2h and BCF/CNC with 10wt% of load, respectively. Furthermore,
703
the stiffness increases with the fraction of cellulose nanoparticles in the aerogel. Whilst
704
comparing the three families of aerogels, one can observe that NFC-2h causes a greater
705
improvement in the stiffness. No significant difference could be observed while comparing
706
the stiffness with the same nanofillers fraction of BCF/NFC-5min and BCF/CNC composite
707
aerogels. In addition, from Figure 7b, the yield stress increases significantly with the fraction
708
of the cellulose nanoparticles in the BCF/NFC-2h composite aerogels. However an opposite
709
trend is observed in the case of BCF/NFC-5min and BCF/CNC aerogels. In these later cases,
710
the aerogels become brittle after a cellulose nanoparticle addition. Indeed, the yield stress
711
increases from 860 Pa for the neat BCF based aerogel to 2.9 kPa for BCF/NFC-2h loaded
712
with 5 wt% of nanofiller and decreases to about 300 Pa for BCF/NFC-5min and BCF/CNC
713
loaded with 10 wt% of nanofiller. The values of the compression modulus for the bio-aerogels
Ac ce p
te
d
M
699
Page 33
Page 33 of 46
714
developed here (62-176 kPa) are higher than that obtained by Nguyen et al. (Nguyen et al.,
715
2014), with values around 11 kPa. From these results, the type of nanocellulose particles significantly influences the bio-
717
composite aerogels from a mechanical point of view. We can see that the NFC-2h has a much
718
more efficient strengthening effect for the BCF bio-aerogel compared to the CNC and NFC-
719
5min charges: the higher compression modulus and higher compression yield stress in the
720
range of studied fractions of the nanofillers. b)
us
a)
cr
ip t
716
Long fibers (BCF)
an
Long fibers (BCF)
Mesoporos ity
Macroporo sity
d
M
Short fibers (NFC or CNC)
725
Figure8: Conceptual scheme of the arrangement of the fibers in the aerogel. (a) Pristine long fibers BCF aerogels with macro porosity. (b) Binary mixture of BCF (black) and Cellulose nanoparticles (red) with mesoporosity.
Ac ce p
722 723 724
te
721
BCFs have a surface containing hemicelluloses and amorphous cellulose chains. The
726
high surface charge density of NFC-2h and its softness due to its less crystalline character
727
confer a more important interaction with the BCF through physical interactions and
728
entanglement, compared to the other two nanofibers (CNC and NFC-5min) (D. Bendahou et
729
al., 2015).
730
Furthermore, the higher NFC-2h load induces a greater number of physical
731
interactions with BCF, compared to the other two nanofibers (CNC and NFC-5min). As
732
described in Figure 8b, the formation of the nanofiber films surrounding the cellulose fibers
733
increased the number of entanglements in the BCF network. Thus the interaction number
Page 34
Page 34 of 46
increases and the arrangement becomes stronger. This reinforcement was predictable. Lavoine
735
et al. showed in their work that incorporated cellulose nanofillers improve the mechanical
736
properties of paper (Lavoine, Desloges, Dufresne, & Bras, 2012). Yet, if the load is higher
737
than a threshold concentration, in our case, around 10%, the BCF surface is fully covered by
738
the nanofillers. It generates disorders when adding more nanoparticles yielding a decrease in
739
mechanical properties.
cr
ip t
734
In this section, we demonstrated that the type of nanocellulose particles influences the
741
mechanical properties of bio-aerogels independently of their intrinsic properties. For example
742
the NFC-2h gives the lowest crystalline index and the lowest aspect ratio but the best
743
mechanical properties. The architecture of the bio-composite aerogels due to chemical and
744
physical interactions is the significant factor influencing their mechanics. By controlling the
745
cellulosic nanoparticle properties, it is possible to adjust the interaction forces and to obtain
746
aerogel materials having mechanical properties higher than those obtained so far.
747
4 CONCLUSION
te
d
M
an
us
740
Bio-aerogel materials were prepared using various combinations of bleached cellulose
749
fibers (BCF) and cellulose nanofibers (NFC or CNC). It was demonstrated that these bio-
750
sourced materials have very useful thermal conductivity and mechanical properties. A thermal
751
conductivity value as low as 23 mW.m-1.K-1 was obtained for BCF/NFCs systems. The
752
structure observed could be described as stacked nanofiber films around BCF. It has been
753
described how the combination of BCF and nanofillers promotes the creation of meso and
754
nanopores resulting from the interactions between the two types of fibers. The nanofiller films
755
have been proven to be efficient to confine air in the bio-aerogel through the Knudsen effect
756
and significantly reduce the thermal conductivity of the binary bio-aerogels. The mechanical
757
properties of the BCF aerogel are also affected by the introduction of cellulose nanoparticles.
758
In comparison to the neat BCF based aerogel, the compression modulus and the yield stress
Ac ce p
748
Page 35
Page 35 of 46
are significantly increased in the case of BCF/NFC-2h. However the compression modulus is
760
slightly increased and the yield stress is decreased in the case of BCF/NFC-5min and
761
BCF/CNC aerogels. The physical properties, especially the crystalline index and the surface
762
charge density seem to be a determinant in the obtained mechanical and insulating properties
763
of these binary aerogels. The benefit of combining cellulose fibers of different sizes
764
(micrometric and nanometric sizes) is that multi-scale aerogels can offer improved thermal
765
insulation and mechanical properties. This study will certainly open a new field of
766
investigation to understand and optimize the thermal conductivity and mechanical properties
767
of nanocellulose based aerogels. Furthermore, this study opens up new opportunities of
768
developing relatively low cost and biodegradables thermal insulators practical for commercial
769
purposes.
770
ACKNOWLEDGEMENTS
M
an
us
cr
ip t
759
The authors are grateful for Anthony Magueresse’s assistance with the SEM and FE-
772
SEM experiments. The authors appreciatively acknowledge to Center of Analyses and
773
Characterizations (CAC) of Cadi Ayyad University - Morocco, Bretagne region, the European
774
Union (FEDER) and the French Ministry for Research for rendering possible the financial
775
support for conducting the studies.
te
Ac ce p
776
d
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784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808
ip t
cr
783
Baetens, R., Jelle, B. P., & Gustavsen, A. (2011). Aerogel insulation for building applications: A state-of-the-art review. Energy and Buildings, 43(4), 761–769. http://doi.org/10.1016/j.enbuild.2010.12.012
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Beck-Candanedo, S., Roman, M., & Gray, D. G. (2005). Effect of Reaction Conditions on the Properties and Behavior of Wood Cellulose Nanocrystal Suspensions. Biomacromolecules, 6(2), 1048–1054. http://doi.org/10.1021/bm049300p
an
781
Bendahou, A., Habibi, Y., Kaddami, H., & Dufresne, A. (2009). PhysicoChemical Characterization of Palm from Phoenix Dactylifera–L, Preparation of Cellulose Whiskers and Natural Rubber–Based Nanocomposites. Journal of Biobased Materials and Bioenergy, 3(1), 81–90. http://doi.org/10.1166/jbmb.2009.1011
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d
779
Araki, J., Wada, M., Kuga, S., & Okano, T. (1998). Flow properties of microcrystalline cellulose suspension prepared by acid treatment of native cellulose. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 142(1), 75–82. http://doi.org/10.1016/S0927-7757(98)00404-X
te
778
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817
Chang, P. S., & Robyt, J. F. (1996). Oxidation of Primary Alcohol Groups of Naturally Occurring Polysaccharides with 2,2,6,6-Tetramethyl-1-Piperidine Oxoammonium Ion. Journal of Carbohydrate Chemistry, 15(7), 819–830. http://doi.org/10.1080/07328309608005694
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HIGHLIGHTS:
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1. Aerogels preparation by freezing drying
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2. Multi-scale cellulose aerogels for super thermal–insulation properties
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3. Reinforcing effect of CNF and CNC as ultra-thin films in aerogels
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4. Effect of TEMPO oxidation time of CNF on the structure and properties of aerogels
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