Cellulose nanofibers-sepiolite biohybrid materials

Abstract

Bionanocomposites are a type of nanostructured biohybrid materials resulting from the assembly of naturally occurring polymers and inorganic solids with particle size in the nanometer scale [1], as for instance diverse clay minerals with layered or fibrous morphology [2,3]. Amongst biopolymers, there is a special interest in the use of polysaccharides for the preparation of bionanocomposites due to their abundancy in nature and their renewable and biodegradable character. In the recent years, many works are focusing on the use of cellulose, the most abundant polysaccharide on Earth, to develop new hybrid materials involving clay minerals [4-7]. Cellulose chains consisting of β(1→4) linked D-glucose units are arranged in crystalline and amorphous regions forming elementary fibrils, which are in turn assembled forming the cellulose fibers present in the cell wall of plants [8]. In this context, the current work explores the development of biohybrids based on the assembly of cellulose nanofibers (CNF) to the fibrous clay sepiolite. For this purpose, Pangel® S9, sepiolite from Vallecas-Vicálvaro of rheological grade commercialized by Tolsa SA, was used together with CNF (dimensions around 0.5-2 μm in length and 4-20 nm in diameter), which was prepared following a reported procedure [9] and gently provided by Prof. P. Mutjé (LEPAMAP group, Univ. of Girona, Spain). Aqueous suspensions of CNF and sepiolite were combined by means of shear force and ultrasounds energy in order to produce homogenous and stable gels, which can be dried by solvent casting or vacuum filtration leading to self-standing films [7]. The sonication treatment seems to be compulsory in order to obtain uniform films with a considerable degree of transparency. The characterization of these materials reveals the good interaction between both types of fibers of biological and inorganic nature. The decrease in intensity of the IR vibration band at 3720 cm-1, related to the O-H stretching vibration of silanol groups in sepiolite, suggests a perturbation due to their hydrogen bonding interaction with hydroxyl groups in CNF. Accordingly, XPS results show a large perturbation in the C1s signal from CNF as a consequence of its interaction with sepiolite. Tensile modulus values of hybrid films show slightly higher values than those of the individual components, reaching a maximum value of 3.4 GPa for the material containing 20% sepiolite. The thermal stability was also improved as the sepiolite content increased, showing a shift in the thermal decomposition temperature towards higher values. The degree of hydrophobicity can be also controlled with the sepiolite content, and thus, water adsorption was reduced in hybrid films containing a small percentage of inorganic fibers in comparison to pristine CNF. The possibility of incorporating other nanoparticles allows the preparation of multifunctional materials [10]. For instance, multi-walled carbon nanotubes (MWCNTs) can be included in the CNF-sepiolite hybrids in order to provide them with electrical conductivity, allowing their application as active phase in sensor devices.