Abstract
Nanocelluloses occur under various crystalline forms that are currently being selectively used for a wide variety of high performance materials. In the present study, two cellulose nanofibers (CF-I) were mercerized by alkaline treatment (CF-II) without degradation, the same molar mass of 560,000 g/mol was measured. Both samples were acid hydrolyzed, leading to cellulose nanocrystals in native (CNC-I) and mercerized (CNC-II) forms. This study focuses on the detailed characterization of these two nanoparticle morphologies (light and neutron scattering, TEM, AFM), surface chemistry (zetametry and surface charge), crystallinity (XRD, 13C NMR), and average molar mass coupled to chromatographic techniques (SEC–MALLS-RI, A4F-MALLS-RI), revealing variations in the packing of the crystalline domains. The crystal size of CNC-II is reduced by half compared to CNC-I, with molar masses of individual chains of 41,000 g/mol and 22,000 g/mol for CNC-I and CNC-II, respectively, whereas the same surface charge density is measured. This study gives an example of complementary characterization techniques as well as results to help decipher the mechanism involved in mercerization.
Highlights
The diffractions patterns of CF-I and cellulose nanocrystals in native (CNC-I) are typical of cellulose I with the presence of diffraction peaks at 15.1°, 16.9°, 20.7° and 22.8° corresponding to (1-10), (110), (012/102) and (200) crystallographic planes respectively
At 12.3°, 20.0° and 21.7° corresponding to the (1-10), (110) and (020) reflections respectively (Nishiyama, Kuga, and Okano 2000; Duchemin 2015; Isogai et al 1989), and traces of cellulose I residuals can be recognized at 15.1° and 16.9° (Figure 1)
Using identical acid hydrolysis on native and mercerized NF, it is shown through a panel of techniques that the mercerization treatment doesn't degrade cellulosic chains (Mw of 560 000 g/mol) but limits the resistance to acid
Summary
Cellulose is a linear homopolysaccharide of D-glucopyranose units connected by β(1-4)glycosidic bonds (Habibi, Lucia, and Rojas 2010; Moon et al 2011; Nishiyama2009)(Nishiyama 2009)(Nishiyama 2009) stabilized by an inter and intramolecular complex network of hydrogen bonds and van der Waals interactions.According to the association type, cellulose exists in six crystalline forms called celluloseI, II, III-I, III-II, IV-I and IV-II (Kroon-Batenburg, Bouma, and Kroon 1996). According to the association type, cellulose exists in six crystalline forms called cellulose. The other forms are obtained by conversion of type I by chemical and/or thermal treatments (Gardner and Blackwell 1974; Atalla and VanderHart 1999; Nishiyama, Langan et al 2002). Cellulose I can undergo an irreversible transition into a more thermodynamically stable crystalline form that is cellulose II by two distinct processes; regeneration or mercerization. Mercerization involves intracrystalline swelling of the cellulose in concentrated aqueous NaOH where the limit concentration depends on the temperature between 8-15%, with lower temperature allowing transformation at lower concentration (Warwicker 1967; Duchemin 2015) where chains change their orientation from original parallel chains of cellulose I to antiparallel chains (opposite polarity)
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