Ultrasound-assisted preparation of cellulose nanocrystals-sepiolite bionanocomposites

Oral presentation given at the XVI International Clay Conference, held in Granada (Spain) on July 17-21, 2017.Bionanocomposites are a group of hybrid materials resulting from the combination of naturally occurring polymers and inorganic nanoparticles, which show interesting properties such as non-toxicity, biocompatibility and biodegradability [1]. Cellulose, the most abundant biopolymer on earth, is being widely applied in diverse areas (paper industry, food and pharmaceutical sectors…) and also used in the development of bionanocomposites, in some cases based on its assembly to clay minerals [2-5]. Cellulose is a linear polysaccharide consisting of β(1→4) linked D-glucose units, present as a structural component of the cell wall mainly in higher plants. These individual polysaccharide chains are arranged in crystalline and amorphous regions in elementary fibrils that assemble into cellulose fibers [6]. Several methodologies are applied for defibrillation of wood pulp in order to give the so-called microcrystalline cellulose (50-10 μm length and 10-50 μm diameter) or nanocelluloses, including microfibrillated cellulose (0.5-1 μm length and 10-100 nm diameter) as well as cellulose nanofibers (CNF, 0.5-2 μm length and 4-20 nm diameter) or nanocrystals (CNC, 50-500 nm length and 3-5 nm diameter) [6], which are suitable for preparation of bionanocomposite materials. The current work explores the development of bionanocomposites based on the assembly of two dissimilar fibrous components: sepiolite (Pangel® S9 from Vallecas-Vicálvaro, Tolsa SA) as inorganic component, and CNC obtained from commercial microcrystalline cellulose as the biological counterpart. The methodology used for the preparation of these materials involves the application of shear force and ultrasound energy to aqueous suspensions of microcrystalline cellulose in the presence of sepiolite [5]. CNC are produced during the sonication process [7], as confirmed by FE-SEM and AFM, avoiding the use of concentrated sulfuric acid commonly employed to obtain these nanomaterials. During the sonication process, the interaction of the sepiolite fibers with the produced CNC may help to reduce their re-aggregation, in a similar way than that reported for carbon nanotubes-sepiolite systems prepared under analogous conditions [8]. Thus, the resulting gels are stable and homogeneous, and can be conformed as self-standing films by solvent casting or vacuum filtration. Their characterization by FTIR confirms the good interaction between both types of organic and inorganic fibers, revealing the establishment of hydrogen bonding interactions between the hydroxyl groups in the CNC and the silanol groups in the external surface of sepiolite. The films show improved water resistance as determined from water sorption isotherms. In addition, multifunctional materials can be easily produced by incorporating other components during the preparation step, as for instance carbon nanotubes, or by grafting or adsorbing organic molecules with the desired properties. The CNC-sepiolite films can be promising materials in diverse fields such as environmental remediation, food packaging or biomedicine.MINECO (Spain), projects MAT2012-31759 & MAT2015-71117-R, and PhD fellowship BES-2013-064677 (MMGC)